Low-temperature homogeneous oxidation of alkanes using hydrogen peroxide

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Low-temperature homogeneous oxidation of alkanes using hydrogen peroxide
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xv, 157 leaves : ill. ; 29 cm.
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Gonzalez, Michael A., 1969-
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Alkanes   ( lcsh )
Cryochemistry   ( lcsh )
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Chemistry thesis, Ph. D   ( lcsh )
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Thesis:
Thesis (Ph. D.)--University of Florida, 1998.
Bibliography:
Includes bibliographical references (leaves 151-156).
Statement of Responsibility:
by Michael A. Gonzalez.
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Typescript.
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Vita.

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University of Florida
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LOW-TEMPERATURE HOMOGENEOUS OXIDATION
OF ALKANES USING HYDROGEN PEROXIDE












By

MICHAEL A. GONZALEZ


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA































To my dear wife, Stacy Michelle, for all your support and belief in me.
You complete me.



















ACKNOWLEDGMENTS


No one has ever said life way easy, but it has been said that it can be extremely

rewarding. My decision to attend college and graduate school has made my life very

fulfilling, and I have only lived less than one-third of it. Although college and graduate

school has been filled with late night study sessions, overnighters at the lab and numerous

classes (as well as migraines), I would do it all again if given the opportunity.

My undergraduate freshman year at the University of Texas El Paso (UTEP) was

a time of indecisiveness and uncertainty about my future career aspirations. In my

sophomore year all these worries had changed for the better, this was the result of

receiving a scholarship and being given the opportunity to conduct undergraduate

research in the area of inorganic structural and polymeric chemistry. For this I would like

to thank Dr. Phillip Goodell, Dr. L.W. ter Haar and the Research Careers for Minority

Students (RCMS) committee for the opportunity to allow me to demonstrate my

academic and research wares. My adventures at UTEP would not have been possible

without the aid of Tony "Hubcap" Sanchez, Manuel "Manny" Garbalena, Anthony

"Skate" Guarero, Joseph "Little Joe" Janisheck and Gerardo "Ardo" Fuentes. To my

friends I say thanks for the memories and the friendship; you will never be forgotten.








From Texas my quest for higher learning landed me in sunny Florida, where

graduate life began at the University of Florida. I chose to work for Dr. Russell "Doc"

Drago, the second best decision I have made in my life. Doc is the reason I remain in

chemistry; he is the most intelligent person I know, and his fervor for chemistry is

undaunting. I can only hope to achieve this level in my chemistry career. "Doc, thank

you for all your patience and instilling a portion of your scientific knowledge in me." I

would also like to offer thanks to Ruth Drago, she has made my four-year stay

memorable. I would like to thank her and Doc for taking Stacy, Mikey and me into their

"scientific" as well as personal family and making us feel like family. This, in turn, made

our transition to Florida effortless.

My graduate school life was also made memorable by the friendships I formed

with members of the Drago group, both past and present. To the past members, Mike

"Bevis" Robbins, Todd "I can fix it" Lafrenz, Phil Kaufmnan, Garth "Ogre" Dahlen, Chris

Chronister, Don Ferris "Bueller" and Mike Naughton, I offer my thanks for showing me

how to survive in graduate school on both an academic and personal level. The current

Drago group members, especially the oxidation sub-group, are the greatest. Ken "Kato"

Lo, has been a great friend and always listened to me, plus I will never forget our Friday

lunches. Alfredo "Alf" Mateus was a great help with understanding heterogeneous

oxidation chemistry. Without Ben "Hubcap" Gordon I would not have completed the last

three months of research. Cheng "Tiger" Xu helped me understand the iron-oxo system.

I also want to thank Steve "Jorg" Joerg for bailing me out of trouble, answering my

philosophical and ethical questions, and running my NMR's. I would also like to thank

Krystoff Jurzyk and Nick "Corn on the" Kob for insight on high temperature








heterogeneous chemistry. I would also like to say thanks to Maribel and Diane for their

help in office matters.

The biggest thanks go to Stacy, my best decision, for standing by me, believing in

me and giving me the best things in life a person could ask for: love and a child. I thank

her for putting up with me on late nights of writing research summaries, being at the lab

and tutoring. This dissertation is as much hers as it is mine.

I thank my parents for their love and support. Dad always told me to set high

goals and never settle for second best. And mother was always there when I needed an

ear, and for this I say thanks.

No one knows what the future holds, but with my wife, family and friends, I know

I can approach it and control my own destiny.




























v














TABLE OF CONTENTS


a.gg


ACKNOWLEDGMENTS ............................................................ ......................... iii

LIST OF TABLES........................................ ...................................................... ix

LIST OF FIGURES ................................. ..............xii

A B ST R A C T .............................................................................. ....................... .......... xiv

CHAPTERS

1 GENERAL INTRODUCTION TO ACTIVATION OF ALKANES ..........................
Importance of Homogeneous Catalysis .............................................. ........................1
General Classification of Homogeneous Oxidation Reactions...................................3
General Classification of Alkane Activation Reactions ..............................................8
Examples of Alkane Activation by Homogeneous Catalysts ......................................9
Designing a Homogeneous Catalyst......................................................................... 12
Five Classes of Metal-Oxo Reactions..................................................................13
Homolytic or Heterolytic Pathway?.......................... ...........................19
Effect of Ligand on Metal Center .......................................................................21
H202 as an Oxidant ....................................................... ............................23

2 HOMOGENEOUS CATALYZED PARTIAL OXIDATION OF METHANE
WITH HYDROGEN PEROXIDE AND OXYGEN ..................................................27
Introduction.......................................................................... ..................................27
Experim ental................................................................................. ...................................30
Materials and Methods............................ ...... ..........................30
Physical Measurements...................... ...............................................33
Synthesis of Com pounds .......................................................................................33
Oxidation Procedure .................................. ... ..........................34
Results and D discussion ....................................................... .. .........................37
A union M odification.............................................................. ...............................37
Ruthenium Catalyzed Oxidation of Methane with Hydrogen Peroxide ................40
Peracid Formation and Reactivity....................................................................44
Oxidation of Methane with Molecular Oxygen.....................................................48
C onclusions............................. ...... ................................................................ 51









3 OXIDATION OF ALKANES WITH HYDROGEN PEROXIDE USING A
RUTHENIUM METAL-OXO CATALYST............................................................53
Introduction......................................................................... ...................................53
Experim ental............................................................................... .................................55
Materials and Methods............................ ...................... ..........................55
Physical Measurements....................... .............................................55
Synthesis of Com pounds ..................................................... ............................... 55
Oxidation Procedure ................... .. ..... ............................56
Results and Discussion .......................... ............................ .............................58
Alkane Oxidation.......................... ................................... ..........................58
Mechanism of Oxidation.......................... ....... ..........................62
A addition of CuC ........................................... ............... ................... ...............64
Effect of Temperature ....................... .....................................72
C onclusions.......................................................................... ..................................77

4 SYNTHESIS AND CHARACTERIZATION OF IRON DIMETHYL
PHENANTHROLINE COMPLEXES.......................... ...........................78
Introduction.......................................................................... ..................................78
Experim ental........................................................................... ...............................81
Materials and Methods............................ ....... ...........................81
Physical Measurements....................................................... ...........................81
Synthesis of Compounds ............................................................82
Results and Discussion ......................................................... ...........................83
Characterization .............................................................. ...........................83
Single Crystal X-ray Diffraction............................ .................................. 83
FAB Analysis.................... ........................................................94
IR Analysis..................................... ...................................................... 94
N M R Analysis ........................................ ............................................97
High Valent Iron-Oxo Formation Studies..............................................103
C onclusions........................................................................ ..................................106

5 OXIDATION OF ALKANES WITH HYDROGEN PEROXIDE USING AN
IRON METAL-OXO CATALYST.......................................... ...........................109
Introduction ......................................................................... ................................. 109
Experim ental............................................... ...................................................110
M materials and M ethods..................................... ......... ............. 10
Physical Measurements.................................................... 11
Synthesis of Com pounds ............................................... .......................... 111
Oxidation Procedure ........................................................... ..........................1. 12
Oxidation of Methane ........................ .....................................................112
Oxidation of Higher Alkanes..................................................................... 113
Results and Discussion ........................................................................................ 114
Oxidation of Methane with H202 ............................... ............................... 114
Mechanism for Oxidation of Methane............................................................ 118
Oxidation of Methane with 02 ........................................................................ .... 118


vii









Alkane Oxidations with cis-[Fe(dmp)2(H20),](CF3SO3), ....................................122
Alkane Oxidation with [Fe(dmp)C12]............................. ................................126
Mechanism for Higher Alkane Oxidation...................................................... 130
Addition of CuC ........................................................... ...............................131
Effect of Temperature ................................................................................... 141
Conclusions......................................................................... .................................146

6 CONCLUSIONS...........................................................................................148

GLOSSARY .......................................................................... ....................................150

REFERENCES ......................................................................... .................................151

BIOGRAPHICAL SKETCH ........................ ............................157





































viii













LIST OF TABLES


Table pag

1-1: Advantages of a Homogeneous Catalysts .......................... .......................... 14

1-2: Disadvantages of a Homogeneous Catalyst........................ .......................... 15

2-1: Oxidation Results for Methane @ 750C with cis-[Ru(dmp)2(H20)2](CF3S03)2 Using
H 20 2................................... ..................................................................................43

2-2: Oxidation Results for Methane @ 750C with cis-[Ru(dmp),(H20)23(CF3SO,)2 Using
H 20 and 0 ............................................ ......................................................49

3-1: Oxidation Results for Ethane, Propane and Butane @ 750C with cis-
[Ru(dmp),(H20)](CF3S03)2 using H202........................... .......................... 59

3-2: Oxidation Results for Iso-Butane and Pentane @ 750C with cis-
[Ru(dmp)2(HO0)](CF3S03)2 using H20....................................................... 60

3-3: Oxidation Results for Ethane, Propane and Butane @ 75C with cis-
[Ru(dmp)2(H20)](CF3SO3)2 and CuCI2 using H202 ...................................... ...67

3-4: Oxidation Results for Iso-Butane and Pentane @ 75C with
[Ru(dmp),(H20)](CFSO03)2 and CuCI2 using H202 ...................................... ...68

3-5: Oxidation Results for Propane @ 75C with cis-[Ru(dmp),(H20)](CF3S03)2 and
Varying Mole Equivalents of CuC12 using H202 .....................................................70

3-6: Oxidation Results for Propane @ Varying Temperatures with cis-
[Ru(dmp)2(H20)](CF3SO3)2 using H202.......................... ............................73

3-7: Oxidation Results for Propane @ Varying Temperatures with cis-
[Ru(dmp)2(H20)](CF3S03)2 and CuC12 using H202 ............................... .............75

4-1: Crystal Data and Structure Refinement for Fe(dmp)Cl2...................................85

4-1: (C ont'd). ................................................................................. .............................86

4-2: Atomic Coordinates (x 104) and Equivalent Isotropic Displacement Parameters (A2 x
10 ) for Fe(dm p)C 2........................................ ........................... ................... 87








4-3: Bond Lengths [A] for Fe(dmp)C12.................................................... 89

4-4: Bond Angles [o] for Fe(dmp)C2. .................................. ........................90

4-4: (Cont'd) ........................................... .................... 91

4-5: Anisotropic Displacement Parameters (A2 x 103) for Fe(dmp)Cl. ...........................92

4-6: Hydrogen Coordinates (x 104) and Isotropic Displacement Parameters
(A2 x 10 ) for Fe(dmp)C .................................................. ......................... 93

5-1: Oxidation Results for Methane @ 750C with cis-[Fe(dmp)2(H20)2](CF3SO,), Using
H 20 2 ................ ........................... .. .............................115

5-2: Oxidation Results for Methane @ 750C with cis-[Fe(dmp)2(H20)2](CF3SO3)2 Using
H 20 2 and 0 ............................................ ............................ ......... ...............119

5-3: Oxidation Results for Ethane, Propane and Butane @ 750C with cis-
[Fe(dmp),(H20)](CF3SO3), using H2................. ............................ 123

5-4: Oxidation Results for Iso-Butane and Pentane @ 750C with cis-
[Fe(dmp)(H2O)](CF3SO3)2 using H202.................................... ..................124

5-5: Oxidation Results for Ethane, Propane and Butane @ 750C with Fe(dmp)Cl2 using
H 20 2 ....... ........................................ .................................127

5-6: Oxidation Results for Iso-Butane and Pentane @ 750C with Fe(dmp)C12 and H202128

5-7: Oxidation Results for Ethane, Propane and Butane @ 750C with cis-
[Fe(dmp)2(H20)](CF3SO3)2 and CuCI2 using H202..................................................132

5-8: Oxidation Results for Iso-Butane and Pentane @ 750C with cis-
[Fe(dmp)2(H20)](CF3SO3)2 and CuCI2 using H ,0...............................................133

5-9: Oxidation Results for Ethane, Propane and Butane @ 75C with Fe(dmp)C1, and
CuCl2 using H 20 2 .......................................... .............................................135

5-10: Oxidation Results for Iso-Butane and Pentane @ 750C with Fe(dmp)C1, and CuC12
using H 20 ....... ........ ...... .... ....................................... 136

5-11: Oxidation Results for Propane @ 750C with cis-[Fe(dmp),(H20)](CF3SO3), and
Varying Mole Equivalents of CuCI2 using H202 ............. ..................... 137

5-12: Oxidation Results for Propane @ 750C with Fe(dmp)Cl2 and Varying Mole
Equivalents of CuCl2 using H202............................................................................139








5-13: Oxidation Results for Propane @ Varying Temperatures with cis-
[Fe(dmp),(H20)](CF3SO3)2 using H ........................... ........................142

5-14: Oxidation Results for Propane @ Varying Temperatures with cis-
[Fe(dmp)2(H20)](CF3SO3)2 and CuCI2 using H202 ...............................................143

5-15: Oxidation Results for Propane @ Varying Temperatures with Fe(dmp)CI2 using
H202........................... .. .... ..... ................................. 144

5-16: Oxidation Results for Propane @ Varying Temperatures with Fe(dmp)C1, and
CuC12 using H 20 2 .................................................... ............................ 145













LIST OF FIGURES


Fgigue page

1-1: Catalytic Cycle for the Hydroformylation Reaction....................................................11

1-2: Desired Reaction Pathway for Substrate Oxidation .................................................. 20

1-3: Detailed Reaction Pathways of Metal-Oxygen Species...........................................22

1-4: Methods for Activating Hydrogen Peroxide ........................ .......................26

2-1: Structure for 2,9-dimethyl-l,10-phenanthroline........................................................27

2-2: Proposed Catalytic Cycle for Alkane Oxidation via cis-[Ru(dmp)2(O)2]2 ...............29

2-3: Detailed Catalytic Cycle for Ruthenium Analogue....................................................31

2-4: Distribution of Worldwide Natural Gas Resources....................................................32

2-5: Diagram of Batch Type Hydrogenation Reactor................................................35

2-6: 'H NMR Spectrum for cis-[Ru(dmp)2(H20)2](PF6)2.............. ...........................38

2-7: 'H NMR Spectrum for cis-[Ru(dmp)2(H20)2](CF3SO3)2 ..........................................39

3-1: Proposed Hydrogen Atom Abstraction Mechanism for cis-[Ru(dmp)2(H20)2]2
C om plex ................................................................ ..............................................63

3-2: Proposed Oxygen Atom Insertion Mechanism for cis-[Ru(dmp)2(H20)212+
C om plex ......................................................... .....................................................65

3-3: Oxidation of Propane with cis-[Ru(dmp)2(H20)](CF3S03)2 and Varying Mole
Equivalents of CuC1, using H202........................................ .............................. 71

3-4: Oxidation of Propane @ Varying Temperatures with cis-[Ru(dmp)2(H20)](CF3SO3)2
using H 20 .......................................................... ..........................................74

3-5: Oxidation of Propane @ Varying Temperatures with cis-[Ru(dmp)2(H20)](CF3SO3)2
and CuC using H202 ........................................ ..........................................76

4-1: Crystal Structure for Fe(dmp)C2.......................... ..... ......................84








4-2: FAB' Spectral Results for cis-[Fe(dmp)2(H20)2](CFSO3)2.......................................95

4-3: Infrared Spectrum for cis-[Fe(dmp)2(H20),](CF3SO3). ............................................96

4-4: 'H NMR Spectrum for Fe(dmp)C .................................... ......................98

4-5: 'H NMR Spectrum for cis-[Fe(dmp)2(H20)2](CF3S3)2...........................................100

4-6: Crystal Structure for cis-Fe(dmp)2(NCS)...........................................................101

4-7: UV-VIS Spectra for the Addition of H202 to cis-[Fe(dmp)2(H20)2](CF3SO3). ........105

4-8: UV-VIS Spectra for the Addition of H202 to Fe(dmp)Cl2......................................107

5-1: Oxidation of Propane with cis-[Fe(dmp)2(H20)](CF3SO3)2 and Varying Mole
Equivalents of CuC12 using H202 ................................ .............138

5-2: Oxidation of Propane with Fe(dmp)C12 and Varying Mole Equivalents of CuCl2
using H 20 2......................... ............... ............. ........................140














Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy


LOW-TEMPERATURE HOMOGENEOUS OXIDATION
OF ALKANES USING HYDROGEN PEROXIDE

By

Michael A. Gonzalez

May 1998


Chairman: Dr. Russell S. Drago
Major Department: Chemistry

The activation of saturated hydrocarbons by metal complexes in the liquid phase

became an active research area towards the end of the 1960. Stringent conditions such as

the use of powerful oxidants, corrosive superacids, elevated pressures and high

temperatures are used for the selective oxidation of low molecular weight paraffins. As

products from these oxidations gain increased industrial importance, environmentally

friendly and cost-efficient catalysts and processes are sought.

Natural gas reserves, which are approximately 90% methane, are abundant and a

source of a valuable feedstock. The direct oxidation of methane to methanol provides the

first step towards a route to an alternative large-scale transportation fuel replacing

dwindling petroleum reserves. For this reason, the development of a direct one-step

process for the oxidation of methane to methanol becomes of interest. Also of








importance is the ability to synthesize valuable organic compounds selectively (alcohols,

aldehydes, ketones and carboxylic acids) from higher alkane feed stocks.

Previous reports from this laboratory have demonstrated the use of the cis-

[Ru(dmp)2(H20),](PF6), precursor, where dmp is 2,9-dimethyl-l,10-phenanthroline, for

the activation of methane in acetonitrile. The nitrogen-based dmp ligand not only

increases the electrophillicity of the metal center, but also imparts a steric hindrance

about the metal, forcing a cis geometry, allowing the catalyst to be a more potent oxidant.

Using the properties from this complex, a number of derivatives have been successfully

synthesized and characterized.

The synthesis and characterization of each newly synthesized catalyst: cis-

[Ru(dmp)2(H20)2](CF3SO3)2, cis-[Fe(dmp)2(H20)2](CF3SO3)2 and [Fe(dmp)C12], as well as

the ability of each to hydroxylate C2-C5 alkanes in acetonitrile and methane in a glacial

acetic acid/acetic anhydride solvent mixture at 750C and 40 psi are presented. Also

investigated is the ability to modify and increase the selectivity to the alcohol product

upon addition of a metal chloride, and the effect of temperature on each catalyst

precursor. Preliminary mechanistic data suggesting a pathway for methane and higher

alkane oxidation are also presented.


XV













CHAPTER 1
GENERAL INTRODUCTION TO ACTIVATION OF ALKANES

Importance of Homogeneous Catalysis


The end of the 1960s confronted the field of homogeneous catalysis with a

potential problem. This obstacle was the inability for a soluble transition-metal complex

to effectively activate a saturated hydrocarbon in the liquid phase.' As large resources of

natural gas were discovered, the need for improvements to the current homogeneous

catalytic systems became apparent, as well as the demand for alternative catalysts.

One solution to this issue was the development of new classes of metal

complexes, which were capable of undergoing an oxidative addition across the C-H bond

of a paraffin (Equation 1-1). The application of this catalyst would allow for alkanes to

be utilized as feedstocks in a number of industrial chemical reactions. This oxidative

addition process produces the desired oxidized products in selectivity ratios, which are

more favorable than those achieved using typical free radical reagents.2,3,4



(Equation 1-1) M + R-H R-M-H



As advances in coordination chemistry and catalysis were developed, reports of

activating hydrogen, olefins, aromatics, carbon monoxide, and molecular nitrogen with

soluble transition-metal complexes became commonplace.' The term activation is used








to imply "the molecule or its part becoming a ligand in the coordination sphere of the

complex and, as a result, undergoing a subsequent chemical transformation"(p. 2).1

The primary deterrent one is confronted with in the activation of alkanes is the

chemical inertness these molecules possess. Alkanes are benign, to the point they can be

safely used as a non-interacting solvent. Therefore, any such activation of an alkane

would require notably stringent conditions. These conditions typically require active

particles, which include strong oxidants, superacids, free atoms, radicals, carbenes, high

temperatures (500-1000C) and other sources of energy (i.e. radiation chemistry). As

environmental and monetary constraints become more significant, a more desirable

oxidation technology needed to be sought.2-3,4

Interest in the activation of methane, a primary component in natural gas, also

heightened prompting the development of a new catalyst that was able to impart

selectivity in its oxidized product under relatively mild conditions. The typically inert

methane molecule, along with its alkane counterparts, required designing a catalyst that

was able to effectively activate these substrates which did not contain double or triple

bonds or lone electron pairs and possessing relatively strong covalent C-H and C-C

a-bonds.

Shilov' had presented evidence for the activation of alkanes; he expressed that if

other more selective reactions of alkanes under "comparatively mild conditions" exist,

there is an inherent ability to oxidize the alkanes in question. The statements that follow

provide a basis for his proclamation:








Numerous examples of homogeneous H2 activation are reported.2 One example

is the H-D exchange for hydrogenation. Yet, the cr bond in molecular

hydrogen is not weaker than the a C-H bond in alkanes.

Many metal complexes (ML, or M) are capable of reacting with substrates

containing "activated" C-H bonds.2 One example is the C-H bond in aromatics

or in the a-position to double bonds. As the C-H bond is broken, formation of

a M-C bond results. However, the C-H bond energy which is greater in

aromatics than alkanes does not hinder the reaction.

Also demonstrated is the involvement of a "nonactivated" aliphatic C-H bond

within the coordination sphere. This occurs when a suitable position on the

ligand of the complex becomes available.2 This results in a process defined as

"cyclometallation" and provides evidence for the possibility of a reaction

occurring with alkanes, possibly at elevated temperatures.

Hydroxylation of C-H bonds in saturated hydrocarbons catalyzed by metal

enzymes is known.2 One published example is the oxidation of methane to

methanol (primarily) by methane monooxygenase.

General Classification of Homogeneous Oxidation Reactions


This research is directed towards developing a series of homogeneous catalysts

able to activate saturated hydrocarbons, olefins and aromatics with mild oxidants. In

order to do so, an understanding of the fundamental chemical reactions must be

understood. These liquid phase (homogeneous) transition metal catalyzed oxidations can

be categorized into three areas.5 These categories include the following:








1. Free Radical Autoxidation

The Mid-Century/Amoco process for the conversion ofp-xylene to terepthalic

acid is one example of this category. This reaction is exhibited in Equation 1-2.

CH3 CO2H

Co(OAc)2
(Equation 1-2) + 02 -a 0
NaBr
200 OC
H3 02H

In this process air (20psi) is utilized as the oxidant, with a bromide-promoted

cobalt salt as the catalyst in an acetic acid solvent. The primary oxidation step, i.e.

p-xylene to p-toluic acid, occurs quite readily in the presence of a small amount of the

cobalt salt. The secondary oxidation step, formation of the di-acid, necessitates the

presence of higher concentrations of the catalyst and/or addition of the bromide

promoter.2

Oxidations of alkyl aromatics involving a cobalt (III) catalyst appear to proceed

almost exclusively via an electron transfer mechanism.' The reaction of the alkyl

aromatic with Co(III) results in formation of [ArCH3]*. and a Co(III) radical. Loss of a

proton results in producing ArCH2, and HW. In the presence of air or oxygen, a peroxy

species is formed producing ArCH2O2', which then reacts to produce the normal

oxidation products. The Co(II)* catalyst can be regenerated either by reacting with 0,,

combined with ArCH2O,*, or by reacting with the hydroperoxy species, ArCH202H, to

produce ArCHO, Co(III) and OH-.2








The corresponding aromatic acids are produced by subsequent aldehyde oxidation

via a peroxy acid intermediate. The addition of the bromide allows for formation of

bromine atoms (Br-) via electron transfer oxidation of bromide by Co(III). The bromine

atom has been demonstrated to be extremely efficient at hydrogen atom abstraction,

which can allow for rapid formation of the benzyl radical, thus initiating the autoxidation

sequence.'

2. Nucleophillic Attack on Coordinated Substrates

An example of this classification is the Wacker Process for oxidizing ethylene to

vinyl acetate. This reaction is provided in Equation 1-3.



(Equation 1-3) H,C=CH2 + 2 02 + HOAc H2C=CHOAc + H,O



PdCI2 and CuCI2 are two catalysts necessary for the above reaction to proceed in

an acetic acid (CH3COOH) solvent. The initial stage of the oxidation involves the t-

coordination of the ethene substrate with concurrent or subsequent loss of chloride to

form a sixteen-valence electron palladium (II) anion, which in turn undergoes a ligand

replacement reaction to yield the neutral palladium (II) aquo species. Coordination of

ethene to the palladium (II) metal center results in a decrease in the double bond electron

density, allowing it to become more susceptible to nucleophillic attack by either OH or

H2O,.2

The palladium metal is re-oxidized to palladium chloride by the co-catalyst, either

CuCl2 or FeCl3, as described above. In this case, the co-catalyst, CuCI2 is the reduction








product of CuCl that is readily oxidized with either air or oxygen. Therefore, this catalyst

combination gives rise to a catalytic system which allows for air or oxygen oxidation of

ethene to ethanal.2

3. Metal-Catalyzed Oxygen Atom Transfer Reactions from Coordinated
Hydroperoxides or Metal-Oxo Species to Organic Substrates

The reaction ofpropylene with alkylhydroperoxides to yield propylene oxide falls

within this category. This reaction is illustrated in Equation 1-4.



(Equation 1-4) Mo Cat O
H3CHC=CH2 + ROOH H3CHC H2 + ROH



This process involves the reaction of a homogeneous metal catalyst, molybdenum,

with an organic hydroperoxide and an alkene. As a result this reaction produces an

epoxide product in a relatively high yield. This oxidation process proceeds with the

hydroperoxide becoming "activated by coordination" to the metal center. As a result, the

peroxide's oxygen experiences a decrease in electron density, rendering it susceptible to

nucleophillic attack by the alkene. Additional metal catalysts for this reaction are those

belonging to the second row, positioned early in the series and possessing high attainable

oxidation states. Metals with these characteristics include Mo(VI), W(VI) and Ti(VI).2

Transfer of the oxygen from the metal hydroperoxide complex to the alkene is

suggested to occur via a cyclic transition state, as detailed in Equation 1-5.









(Equation 1-5) ---- + MRO,H

R



The success of the Mo(VI), W(VI) and Ti(VI) catalysts for this type of oxidation

reaction is attributed to their being weak oxidants with poor one electron redox potentials:

-0.21, -0.03 and -0.37eV respectively. Therefore, they are poor catalysts for homolytic

hydroperoxide decomposition, as exhibited in Equation 1-6, which is detrimental to the

desired epoxidation reaction.'



(Equation 1-6) [M"ROH] M'"" + RO,. + H'.



It is important to emphasize that only the autoxidation, category number I,

utilizes the direct reaction of molecular oxygen with the organic substrate.6

Homogeneous catalysis is now a relatively mature field with a number of diverse

reactions being investigated for informative studies as well as mechanism and theoretical

analysis. Along with the growth in this field, a relationship to other important areas has

been firmly established. These areas include heterogeneous catalysis, organometallic

chemistry and bio-catalysis.

Homogeneous catalysis is a resource, which is virtually untapped, and its

importance cannot be overestimated. In the age of industry becoming oriented towards

specialty chemicals, methods to catalyze functional group transformation, hydrocarbon








activation, polymerization and asymmetrical catalysis, the need and importance for

effective homogeneous catalytic systems will continue to grow.

The activation of C-H bonds in alkanes by transition metal complexes, not long

ago thought to be the most difficult challenges facing chemists, is now almost

commonplace. Many, examples, often under remarkably mild conditions, have

appeared.',3,7 Problems arise in the transformation of these fundamentals into the

practical alkane conversion process. These reasons include thermodynamics, a reaction

being uphill at ambient conditions, catalyst stability, and catalyst incompatibility with 02

to activate alkanes. Other problems include the issue of selectivity. Regioselectivity is

an obvious problem, one such case is if terminal alcohols are preferred over their isomeric

products. More important, the products (alcohols and aldehydes) tend to be considerably

more reactive than the starting alkane. This then places a limit on the achievable yield of

the desired hydroxylated product.

General Classification of Alkane Activation Reactions


Three categories of remarkably facile alkane activation can be described:

oxidative addition (Equation 1-7), c-bond metathesis (Equation 1-8) and electrophillic

substitution (Equation 1-9).1.3.7



(Equation 1-7) L,M + R-H s L,M(H)(R)

(Equation 1-8) LM-X + R-H LnM-R + H-X

(Equation 1-9) LM"~ + R-H LM-R"(-' + H'








The first in this classification of reactions, oxidative addition reactions, has been

found to occur in low valent electron rich, coordinatively unsaturated metal centers

towards the right end of the transition metal series (Groups VI-X).3

Sigma-bond metathesis reactions, the second classification, are observed for

complexes containing Group III transition metals (including lanthanides and actinides), as

well as metals belonging to Group IV. Also exhibited in this category are examples of

intramolecular reactions with metals belonging to Group V. These reactions lead to

stable organometallic products; however, very limited examples of alkane

functionalization have been achieved. Each of the active species involved is highly

sensitive to 02 and the oxygenated products that are produced.3

Finally, electrophillic substitution reactions have been observed with the

"traditional" or "classic" coordination complexes. An example of a such complex is

PtClx(H20)4,,1-'2.89 In the duration of the mechanistic process, a stable organometallic

species is not obtained, but net functionalization of the alkane is achieved. This category

of catalysis appears attractive in that the species involved will utilize dioxygen. This

property allows in principle for the closing of the catalytic cycle.

Examples of Alkane Activation by Homogeneous Catalysts


Many examples of homogeneous catalytic reactions are published in literature,

although this number is too large to provide one example for each area of activation.

Two important examples are exhibited to provide a general idea of the catalytic chemistry

being performed on the industrial and laboratory scale with homogeneous catalysts.








The hydroformylation or Oxo reaction discovered in 1938 by Roelen is utilized on

a large industrial scale.10.1,12 This process employs a homogeneous catalyst based on

cobaltl0'1 ,12 or rhodium.13 Most commonly the HCo(CO)4 pre-catalyst is employed, the

product generated by the in-situ hydrogenation of CO2(CO),.

In 1961, Heck and Breslow14 proposed the mechanism for the cobalt-based

Hydroformylation process, as illustrated in Figure 1-1. Five elementary processes

comprise the complete mechanism.

The first step involves the dissociation of the carbonyl ligand from HCo(CO)4 to

produce the catalytically active species tricarbonylhydridocobalt, [HCo(CO)3]. This is

followed by the combination of the active catalyst with the olefin to generate a n-olefin

complex. The migratory insertion of the olefin into the Co-H bond is the third step,

proceed by the alkyl undergoing a migratory insertion into the Co-CO bond. In the final

step an H2 induced aldehyde elimination occurs, producing the desired oxidized product

and regeneration of the active catalyst.

Another cobalt based catalytic system is the reductive carbonylation of methanol

(Equation 1-10). This example illustrates how cobalt-based catalysts have dominated this

area of industrial chemistry for the past fifty years.15-22 Alternative metals are capable of

catalyzing the identical reaction; however, these are generally inferior to those of the

cobalt-based systems.


(Equation 1-10) CH3OH + CO + H2 CH3CHO + H20














HCo(CO)4

1 CO
RCOH H ,CCH2




5 2
HCo(CO)3 -- --)



H-H--




R-CO-Co(CO)3 OCH,





4
3


RCo(CO)4


Figure 1-1: Catalytic Cycle for the Hydroformylation Reaction








Improvements to the cobalt-based catalyst have been achieved through the use of

co-catalysts and promoters. Such promoters employed include iodide, phosphines and

transition metals.18 Iodide, by far the most important of this group, is almost always

utilized as a promoter. Despite advances in catalyst performance, high pressure and

temperatures are required. Conditions for these catalysts are typically operated at

pressures of 4000-8000 psi and temperatures of 175-200*C. The increased pressures

required can pose obvious difficulties for reactor design and operation, as well as for

safety considerations. The elevated temperatures required can lead to the formation of

heavy by-products via the Aldol condensation reaction of acetaldehyde.19,20

It is shown both rhodium and cobalt are capable of catalyzing a variety of

carbonylation reactions.23,24 Rhodium catalysts have been demonstrated to be

significantly more active than their cobalt counterparts and allow reactions to proceed at

much lower extremes of temperature and pressure. However, iodide is necessary as a

promoter. In the presence of this promoter and CO, rhodium is an extremely proficient

catalyst for the carbonylation of methanol to acetic acid. This process is known as the

Monsanto Acetic Acid Process.25-28

Designing a Homogeneous Catalyst


The focal point of our research is directed towards designing and developing a

catalyst able to undergo chemistry involving a transition-metal catalyzed oxygen atom

transfer from a metal-oxo complexes or species. The activation of alkanes, primarily

methane, utilizing this newly developed metal-oxo complex is our primary goal. The

development of this catalyst can be hindered due to the following conditions:








1. The role of the transition metal complex for activation of molecular oxygen

can be complicated by numerous competing reaction pathways which are

present.

2. In order to selectively oxidize a variety of substrates, a number of different

conditions may be required.

3. The lack of any obvious pattern or role portrayed by the metal, as well as lack

of a detailed reaction mechanism.

A catalyst can either be homogeneous or heterogeneous in nature. A homo-

geneous catalyst was selected on the basis of its advantages in a reaction mixture and

underlying chemistry. The advantages and disadvantages of utilizing a homogeneous

catalyst are listed in Table 1-1 and Table 1-2 respectively.

Five Classes of Metal-Oxo Reactions

Oxidation pathways have been exhibited to occur in a number of general reaction

pathways. Drago29 has formulated five classes of metal-oxo reactions, which are based

on the role of the transition metal complex. In order to effectively categorize these

reactions, the mechanism for substrate oxidation was omitted. The five classes are listed

below and each reaction mechanism summarized. An example of each reaction class is

also provided.



Class I: Metal Bound 0,

(Equation 1-11) M + 02 M-O-O

(Equation 1-12) M-O-O + RH M-O-O-H + R-

























Table 1-1: Advantages of Homogeneous Catalysts
Advantage Description
Homogeneity Uniform active structure
Efficiency Theoretically all the atoms or molecules of the catalyst are
available for reactants
Reproducibility A result of well-controlled active sites
High Selectivity Often more specific than a heterogeneous catalyst even for
asymmetric induction
Mild Reaction High activity often achieved under mild conditions
Conditions
Controllability If chemical information is readily available, modification
allowing the control of electronic and steric properties of the
metal center can be attained.
Reaction Mechanism Investigation of the molecular reaction can be performed.

























Table 1-2: Disadvantages of a Homogeneous Catalyst
Advantage Description
Separation The separating of an expensive catalyst from the reaction
mixture poses a problem and can be costly, and may also require
special treatment, which usually destroys the catalyst.
Solubility The range of solvents suitable for a homogeneous catalyst is
often limited by the solubility of the catalyst; a compatible
solvent is not necessarily the most suitable for a high reaction
rate.








(Equation 1-13) M-O-O-H +R- M + ROOH

(Equation 1-14) ROOH ROH or R=O



Molecular oxygen is coordinated to the metal (Equation 1-11) to increase its

basicity and radical reactivity. The substrate then undergoes hydrogen atom abstraction

by the metal-peroxo species to generate a metal hydroperoxo species and an alkyl radical

(Equation 1-12). The metal hydroperoxo species and the alkyl radical then undergo

recombination (Equation 1-13) to generate an alkylhydroperoxide species and the reduced

metal species. The final step (Equation 1-14) involves decomposition of the

alkylhydroperoxide to yield the corresponding alcohol and ketone. Cobalt (II)

coordination compounds that reversibly bind dioxygen are examples of this group.



Class II: Metal-Oxo via 0,

(Equation 1-15) M + 02 M-O-O

(Equation 1-16) M-O-O + M M-O-O-M

(Equation 1-17) M"N-O-O-M"* 2 M""(O)

(Equation 1-18) M=O + S SO + M



Once again the metal coordinates dioxygen (Equation 1-15); however, in this

series another metal enters the coordination sphere and forms a L-peroxo dimer (Equation

1-16). This p-peroxo dimer then undergoes cleavage to form two high-valent metal-oxo

complexes (Equation 1-17). The metal-oxo complex is then able to undergo oxygen atom








transfer or another similar type reaction (Equation 1-18), thus regenerating the lower

oxidation-state of the metal and producing the oxidized species of the substrate. Metal

oxidation states normally involved in this cycle are two to four, three to five or four to

six. One system to be discussed, cis-[Ru(dmp)2(H2O)2]2', is an example of this category.



Class III: Metal-Oxo via Peroxides

(Equation 1-19) M"' + H20, M7+2(0) + H,O

(Equation 1-20) M"+(O) + S SO + M"'



This class of reactions takes advantage of the more powerful oxidizing ability of

hydrogen peroxide to oxidize lower-valent metal complexes to metal-oxo complexes.

The low-valent metal complex is oxidized with hydrogen peroxide (Equation 1-19) to a

higher valency producing a metal-oxo complex and a benign by-product, water. The

metal-oxo species is then able to undergo oxygen atom transfer or a similar reaction

(Equation 1-20), thus regenerating the lower oxidation-state of the metal and producing

the oxidized species of the substrate. A candidate for this class is a monooxygenase

enzyme, which involve metal-oxo oxidants.



Class IV: Metal-Peroxo Systems

IVa- Metal Catalyzed Peroxide Decomposition

(Equation 1-21) M"' + H,02 M" + OH + OH-

(Equation 1-22) M" + H202 M" + I+ + HO2-








IVb: Metal-Peroxo Formation

(Equation 1-23) M=O + H202 MO2 + H20

(Equation 1-24) MO, + S I SO + M=O



Metal peroxo complexes are the reactive intermediates for this group. Several

reactions with substrates yield further divisions of this class. In class IVa, the metal

complex reacts with hydrogen peroxide to generate radical species, which are the result of

peroxide decomposition (Equations 1-23 and 1-24). An example of this class is Haber-

Weiss and Fenton Chemistry. The radial species, the active oxidant, are then able to

oxidize the substrate. Class IVb is the result of the bound metal peroxo or alkylperoxo

complex attacking the substrate. The metal peroxo is generated by the reaction of a

metal-oxo species with hydrogen peroxide. The alkylhydroperoxo species is generated in

the same fashion as illustrated in Equation 1-22. Asymmetric epoxidation of alkenes

with alkylperoxides are examples of this class of chemistry.



Class V: Metal Centered Oxidizing Agents

Metal center is active electron acceptor in the oxidation.

Often involves coordination of substrate to metal center, "activation".

The Wacker process, Equation 1-3, is an example of this class.

The reduced complex is re-oxidized by 02 or peroxide.








The approach taken in this research is to develop a catalyst that possesses

properties pertaining to either Class II or III. The developed catalyst could also possess a

mechanism of regeneration with dioxygen to generate a high-valent metal-oxo complex,

as in Class V, necessary for alkane oxidation. This catalyst must be able to perform the

key step, that being the generation of a strongly oxidizing high-valent metal center.30

The desired reaction pathway, illustrated in Figure 1-2, exhibits the need for a

metal center, which can be activated by hydrogen peroxide or molecular oxygen to form

the mono-oxo species. This activated complex may be a powerful enough oxidant to

activate the substrate or be further oxidized by hydrogen peroxide or dioxygen to generate

a di-oxo species, a complex with greater oxidizing strength than the mono-oxo species.

Upon delivery of its one or two oxygen atoms, the reduced metal center could be

re-oxidized by hydrogen peroxide or oxygen to regenerate the mono or di-oxo species,

thereby closing the catalytic cycle

Homolvtic or Heterolvtic Pathway?

There are two pathways, which exist for a metal center to be activated by

dioxygen or hydrogen peroxide. These pathways are known as homolytic and heterolytic

oxidation. The former31 normally involves the following transition metal couples:

VV/V", Crv/Crv, Mn'n/Mn", Fe"/Fe1, Co '/Con and Cu1/Cu'. Characteristics of this

reaction include the production of a free radical intermediate, oxidation occurring in the

outer-sphere via bimolecular steps, non-coordination of the substrate. Oxidation products

are generally not very selective or stereospecific, and the metal center undergoes a one-

electron oxidation and reduction step. Examples of relevant oxidations employing this















Substrate Oxidized Substrate



[M(lig)2(H20)2]x+
S H202 or
( 02
OH2

[M(lig)2(0)2]x+ [M(lig)2(H20)(O)]x+


OH2 H202
or
02


Figure 1-2: Desired Reaction Pathway for Substrate Oxidation








oxidation pathway include the non-stereoselective epoxidation of olefins occurring with

transition metal complexes of vanadium, manganese and iron and the hydroxylation of

alkanes and arenes occurring with the transition metal complexes of vanadium,

chromium, manganese, iron, cobalt and copper.

The latter, heterolytic oxidations,31 involves the following transition metals: Ti",

V", Cr", Mo"1, Mo", Mnv". Ru", Os"v'", Rhm/Rh, Irm/Ir', Pd"/Pdo, pt/Pt.

Characteristics of this reaction pathway include the inability to produce a free radical

intermediate, the reaction occurring in the coordination sphere of the metal, substrate

coordination resulting in activation, high selectivity and stereospecificity of the oxidized

products and no net change in the oxidation state of the metal. However if a change in

the oxidation state occurs, it is via a two electron step, i.e. M" to M4. Examples of

heterolytic oxidations include the stereoselective epoxidation of olefins with transition

metal complexes of titanium, vanadium and tungsten and the ketonization of olefins with

transition metal complexes of rhenium, iridium, palladium and platinum. Mimoun30 has

also categorized a reaction coordinate pathway involving metal-oxygen species, which is

illustrated in Figure 1-3.

Effect of Ligand on Metal Center

Numerous and thorough studies for polypyridyl ligands with ruthenium metal

centers by Drago31, Meyer32, Takeuchi33 and Che34 have been performed. These studies

have revealed certain features that make these complexes capable of catalyzing oxidations

of hydrocarbons with molecular oxygen. These complexes display a series of reversible

















hydroxo

Mn+2-OH


+ 02
0.
Q + Mn

0M^


superoxc









peroxo

peroxo


O Mn+1
M \ / 2 M+2=O
Mn~l 0


'N


o 1I-peroxo oxo

+ HO H20



S+ R M+2-0OR +0ROOH
alkylperoxo -HX Mn2
+ H++ H 0
Mn2-00H -
hydroperoxo


+ M"



M nloM n+

Pl-OXo


Figure 1-3: Detailed Reaction Pathways of Metal-Oxygen Species








and accessible oxidation states (i.e. from 2' to 6') to reduce dioxygen completely.

The poylpyridyl ligand provides the stability for the metal center to achieve

higher oxidation states and impart stability to the complex. It also forms stable

ruthenium oxo-species, {Ru(O)}2 and {Ru(0),}2', which are known to be excellent metal

oxygen atom-transfer reagents towards a variety of substrates. The activation of methane

and other saturated alkanes is the field where the majority of our interest lies.

HO, as an Oxidant

The need for an inexpensive, environmentally friendly and benign by-product oxidant is

of top concern. Many oxidants are available which include dioxygen, hydrogen peroxide,

alkyl hydroperoxides, peracids, sodium sulfate, oxone, nitric acid, dichromate, bromine,

manganate and Caro's acid (H2SO,). However, the last five are costly, and involve

expensive disposal problems. Nitric acid also produces noxious NO, compounds as by-

products.

Peroxygen reagents have been utilized in chemical synthesis for a number of

years.35 As the public, chemical producers, and governmental agencies direct their efforts

towards "greener" pathways, the interest in the use ofperoxygen oxidants has increased.

A reagent, hydrogen peroxide (H202), can offer environmental and economic benefits.

Hydrogen peroxide is available for chemical synthesis as an aqueous solution in

concentrations ranging from 35% to 90% by weight. Stabilizers are normally present in

the part per million levels to prevent decomposition. With no additional additives being

present, a minimization in unwanted side reactions is an additional advantage.








Hydrogen peroxide can also be stabilized for extended periods of times under a

variety of conditions. As an aqueous solution, hydrogen peroxide will lose less than one

percent of its active oxygen content per year, when stored in compatible containers at

ambient temperatures. This stability allows hydrogen peroxide to be a more efficient

oxidant than dichromate or permanganate on a weight for weight basis.

Reactions involving hydrogen peroxide are traditionally performed under mild

conditions of temperature and pressure, preventing an increase in peroxide

decomposition. By performing under these conditions the need for a large capital

investment is eliminated as well as a decrease in safety concerns. Reactions, which use

hydrogen peroxide as an oxidant, can also proceed in either an aqueous, organic or neat

solvent. This allows for reaction and reactor design to be engineered allowing one to

avoid the use of solvents, thus eliminating costly start-up expenses and production of

waste streams.36

The major benefit of using hydrogen peroxide is its environmental acceptability.

When the oxidizing power of the peroxide is spent, only water remains as the by-product,

eliminating the need for expensive effluent disposal and treatment. Hydrogen peroxide is

also relatively inexpensive as a reagent, when compared on the basis of oxidizing power

and by-product production. Although chlorine and oxygen oxidants are less expensive,

since these are gases, phase separation concerns are raised as well as the inability to

achieve selectivity towards oxidation products.

Hydrogen peroxide can be utilized in a variety of methods: direct activation,

catalytic activation and activation via peroxides. All require the use of hydrogen

peroxide to be activated to an even more potent oxidant. Figure 1-4 summarizes methods








for activating hydrogen peroxide. Our use of hydrogen peroxide falls under the area of

catalytic activation. Hydrogen peroxide is activated with a metal complex by forming a

metal-oxo or metal-peroxo species.36

The Einchem Corporation has demonstrated the feasibility of using hydrogen

peroxide as an oxidant on a commercial scale. Einchem has developed a heterogeneous

titanium silicate zeolite catalyst, commercial tradename of TS-1.37 This catalyst, totally

heterogeneous in nature, has a wide synthetic use for hydrogen peroxide oxidations.

Einchem has commercialized this catalyst for the hydroxylation of phenol to

hydroquinone and catechol.

The catalysts used in this research will employ hydrogen peroxide and dioxygen.

Hydrogen peroxide was chosen as the primary oxidant for hydrocarbons because of its

ease of handling and oxidizing strength. Dioxygen is the desired oxidant but hydrogen

peroxide can be used safely with hydrocarbons, while dioxygen requires one to remain

within the explosion limits.
















































Figure 1-4: Methods for Activating Hydrogen Peroxide













CHAPTER 2
HOMOGENEOUS CATALYZED PARTIAL OXIDATION OF METHANE WITH
HYDROGEN PEROXIDE AND OXYGEN

Introduction


Cis-ruthenium oxo complexes were previously reported to be effective catalysts to

activate hydrogen peroxide and molecular oxygen for the selective oxidation ofalkenes38

and alkanes.39,40 The sterically hindered complex cis-[Ru(dmp)2(H0)J](PF6,) (II), where

dmp is 2,9-dimethyl-l,10-phenanthroline (Figure 2-1), is a precursor that can be oxidized

with hydrogen peroxide to form cis-[Ru(dmp)2(O)(HO2)](PF,), or cis-

[Ru(dmp)(O)j2(PF6)2 as shown in Equations 2-1 and 2-2.






N N
H3C CH3


Figure 2-1: Structure for 2,9-dimethyl-l,10-phenanthroline


Unsuccessful attempts to isolate these oxo complexes led to their characterization based



(Equation 2-1) cis-[Ru"(dmp),(S)2](PF6)2 2

cis-[Ru'(dmp)2(S)(O)](PF6)2








(Equation 2-2) cis-[RuV(dmp)2(S)(O)](PF,),

cis-[Ru"'(dmp)2(O)2(PF )2



on isobestic points in the electronic spectra and changes in solution NMR as hydrogen

peroxide is added to II. The electronic spectra of the oxidized complex are similar to

those of analogous ruthenium-oxo complexes.41 Cis-{Ru(O),}2 complexes isomerize to

the trans-complexes which are weaker oxidants than the cis analogues.41,42,4344.45 The

novel aspect of the dmp ligand is its steric requirement which prevents isomerization of

the cis-[Ru(O),]2* complex and inhibits formation of both tris complexes and stable t-oxo

dinuclear species. The intra-ligand repulsions and neutral ligand charge also make the

high oxidation state ruthenium center electropositive.4'

Preliminary results describe the use of this complex to catalyze the oxidation of

alkanes using H202 as the oxidant.40 Figure 2-2 summarizes the proposed46 catalytic

cycle for alkane oxidation via cis-[Ru(dmp),(0)2 2. The cis-[Ru(II)(dmp)2(H20)2]2+

precursor is oxidized rapidly by H202 to form the (III),(IV) and (VI) oxidation state

complexes. In alkane oxidations, cis-[Ru(III)(dmp)2(H20)(OH)]2 reacts with dioxygen to

produce the cis-[Ru(IV)(dmp),(HO0)(O)]2' but cis-[Ru(II)(dmp)2(H20)2]2 is reported to

be unreactive with molecular oxygen.46 When appreciable concentrations of Ru(IV)

exist, the Ru(II) complex reacts with it to form cis-[Ru(III)(dmp)2(H20)(OH)]2*. For

kinetic reasons, alkane oxidations require the electrophillic cis-[Ru(VI)(dmp),(O)2]2+

















Oxidized Substrate
Substrate [Ru(dmp)2(H20)]2
H202

1< H20
[Ru(dmp)20,J]2 [Ru(dmp)2(H20)0]2*




H2O H202


Figure 2-2: Proposed Catalytic Cycle for Alkane Oxidation via cis-[Ru(dmp),(O)2]2-







complex. The cis-[Ru(IV)(dmp)2(H20)(Oh)2+ formed after oxygen atom transfer must be

oxidized back to the Ru(VI) complex with hydrogen peroxide40 to make the system

catalytic in CH3CN solvent. Figure 2-3 illustrates the catalytic cycle of the ruthenium

analogue.

Methane was selected as the substrate for study because the oxidation of natural

gas into a liquid product is of great industrial and economic importance.47 Figure 2-4

depicts the worldwide distribution of natural gas resources. Currently, methanol is

produced from methane by the steam reforming of methane into synthesis gas, followed

by the catalytic conversion of the syn gas into methanol. In order to increase efficiency,

the direct partial oxidation of methane to methanol is the desired pathway.48 However,

hydrogen atom abstraction, commonly involved in methane oxidation, occurs more

rapidly with methanol than methane resulting in the over-oxidation of methanol to CO,.

This research extends earlier reports of the use ofcis-[Ru(VI)(dmp)2(0)2]2' as a

catalyst for methane oxidation. Greatly improved conversions to oxygenates result by

trapping methanol and slowing its over-oxidation. Also demonstrated is an improved

catalyst precursor, evidence for oxidation of methane by 02 with this catalyst, and

demonstrate oxidation of methane by peracids.

Experimental


Materials and Methods

RuC13 x HO0, 2,9-dimethyl-l,10-phenanthroline, LiCI, NaPF6, 60% HPF6,

NH4PF6, NaCF3SO3, were all used as received from Aldrich. Ethylene glycol,

acetonitrile, glacial acetic acid, acetic anhydride, 4A molecular sieves and hydrogen



















H202 (Fast) H202 (Fast)
02 (V. Slow) or 02 H202 (Fast)


Ru" Ru" + RuV Ru + Ru Rv() Ru(0)
Ru Rui "' RuV(O) RuV(O),






Alkane
Epoxide Alkene Oxidized
Alkane


Figure 2-3: Detailed Catalytic Cycle for Ruthenium Analogue






















Africa
Middle East 10%
25%




Russia
30%

Asia-Oceania
13%

N. America Am W Europe
12% 5% 5%


Figure 2-4: Distribution of Worldwide Natural Gas Resources








peroxide (30%) were all used as received from Fisher Scientific (ACS grade).

Acetonitrile was distilled over P205 under dinitrogen and was stored over 4A activated

molecular sieves. Methane (99.99%) was purchased from Matheson and used as

received.

Physical Measurements

UV-Vis measurements employed a Perkin Elmer lambda-6 spectrophotometer.

The pH measurements were made with a Fisher Accumet model 630 pH meter. NMR

spectra were recorded on a Varian VXR300 spectrometer. FAB mass spectral data were

obtained by Dr. David Powell (U.F.) in a m-nitrobenzyl alcohol matrix. U.F. Analytical

Services performed elemental analyses.

Synthesis of Compounds

Cis-Ruthenium(II) Bis(chloride)bis(2,9-dimethyl-l,10-phenanthroline)

Monohydrate, [Ru(dmp),C1,] H2O (I). Cis-[Ru(dmp)2Cl2] H,O was synthesized as

reported.40 Analysis: Calculated for C2,H26N4OC12Ru; C, 55.45; H, 4.29; N, 9.24. Found:

C, 55.65; H, 4.35; N, 9.29.

Cis-Ruthenium(II) Bis(aquo)bis(2,9-dimethyl-l,10-phenanthroline)

bis(hexafluorophosphate), Cis-[Ru(dmp),(HO0)2](PF6) (II). A 1.0 g (1.7 mmol)

portion of I was dissolved in 150 ml of deionized H20, under N2 by heating 500C for 30

minutes. After adding 50 ml of a saturated NaPF, (aq), the solution is cooled, placed in

an ice bath for two hours, and filtered. The precipitate is redissolved in H20 by heating to

500C; a saturated NaPF6 (aq) solution is added and reprecipitated as above. Re-

crystallization and exchange of Cl for PF6 is repeated until the filtrate affords a negative







chloride test with AgNO3. The product is dried in vacuum at 600C overnight. Analysis:

Calculated for C2,H2,N402P2F12Ru: C, 39.85; H, 3.32; N, 6.64. Found: C, 39.46; H 3.29;

N, 6.72.

For comparison purposes cis-[Ru(dmp)2(H20)2](PF6)2 was also synthesized using

the previously reported procedure.40 Analysis: Calculated for C28H2,N4O2P2F,2Ru: C,

39.85; H, 3.32; N, 6.64. Found: C, 39.57; H 3.27; N, 6.63.

Cis-Ruthenium(II) Bis(aquo)bis(2,9-dimethyl-l,10-phenanthroline)

bis(trifluoromethanesulfonate), Cis-[Ru(dmp)2(HI0),](CF3SO3)2 (III). A 1.0g (1.7

mmol) portion of I is slowly dissolved in H20 as above. Following the addition of 50 ml

of a saturated aqueous solution of NaCF3SO,, the resulting solution is cooled, and placed

in an ice bath to complete precipitation. The product is collected, the filtrate tested for

chloride, and recrystallized with CF3SO3" until the filtrate gives a negative chloride test.

The resulting solid is dried under vacuum at 600C overnight. Analysis: Calculated for

C30H34N4OSSFRu, C, 42.25; H, 3.31; N, 6.57. Found: C, 41.40, H, 3.26; N 6.39.

Oxidation Procedure

The pressurized oxidations were carried out as previously described49, in glass,

batch hydrogenation reactors. Figure 2-5 provides an illustration of such batch reactor.

The reaction mixtures were varied as described in the table footnotes. Blank runs

omitting certain reactants and solvent components are also described in the tables.

Oxidations with oxygen use 30 psig of a 5% 02 in helium mixture, to remain outside the

explosion limits, and 20 psig methane. Reaction temperatures normally were maintained

















Pressure
Gauge


Gas Inlet/Purge


Gas Centrifuge
Bottle


Figure 2-5: Diagram of Batch Type Hydrogenation Reactor







between 75 to 770C. To remove air initially present, nitrogen gas is purged through the

reactor, the apparatus is pressurized with the substrate, which is then released, and re-

pressurized to the desired pressure.

The oxygenated products of the reaction were analyzed and quantified with a

Hewlett-Packard 5890 Gas Chromatograph equipped with a FID detector and outfitted

with a 30m Alltech RSL 160 column (5im thickness). Helium was employed as the

carrier gas. Carbon dioxide and carbon monoxide analyses were performed with a Varian

3700 Gas Chromatograph equipped with a TCD detector outfitted with a 15' Carboxen

Column. Methane was quantified by gas chromatography (TCD) using N2 as an internal

standard. Three chromatograms were measured for each sample using injection volumes

of 0.lml for gas and 0.1Al for liquid samples.

The following definitions describe terms used in the presentation of the results.

Selectivity to any product is the moles of a given product divided by the total moles of all

products formed expressed in percent. The percent CO2 produced is moles of CO2

divided by the total moles of all products. The selectivity to oxygenates is the moles of

HCO, CH3OH and CH3C(O)OCH3 divided by the total moles of products in percent.

Traces of CH,(OCH3),, HCOOCH3 are formed, but not quantified. Percent peroxide

efficiency is the moles of H202 needed to account for all oxidized products including CO2

divided by the moles ofH202 consumed. The percent conversion ofCH4 is the moles

carbon in the oxidized products divided by the moles of CH4 added to the reactor.

Safety Precautions, the combination of molecular oxygen with organic

compounds and solvents at elevated temperatures and pressures are potentially explosive.







Extreme caution should be taken during the charging and disassembly of the

experimental apparatus. Equipment which can generates sparks must be avoided, a safety

shield and cooling of the batch reactor in an ice bath for 30 minutes prior to disassembly

is recommended.

Results and Discussions


Anion Modification

Reproducibility for the oxidation of CH4 cis-[Ru(dmp),(H20)2](PF6)2 catalyst

precursor40 depends on the purity of the complex. Chloro complexes are inactive41 and

excess AgPF, used to remove the chloride from the dichloro precursor inhibits3839,40 the

reaction. Using NaPF, instead of AgPF, requires a large amount of NaPF6 and repeated

recrystallizations. The hexafluorophosphate anion of the final product is hydrolytically

unstable and extensive etching of the glass vial occurs during storage. With more than

20% fluorine, verification of the complex purity by elemental analysis is difficult.

Complex degradation is evident as well as the inability to obtain an appropriate NMR

spectrum for complex II as illustrated in Figure 2-6.

In an attempt to overcome these problems, the anion was changed to

trifluoromethanesulfonate, a poorly coordinating anion. A shortened synthesis time, need

for less anion in the synthesis, more reliable elemental analyses, and stability towards

hydrolysis are immediate advantages of this new complex. The direct effects of altering

the anion are exhibited in the lack of complex degradation and the ability to obtain an

identifiable NMR spectrum. A NMR for this newly synthesis complex III is provided in

Figure 2-7.
















9 8 7


3 2 1 PPM

Figure 2-6: 'H NMR Spectrum for cis-[Ru(dmp)2(H20)(PF6)2





39







,,
m 7






0 7


Figure 2-7: 'H NMR Spectrum for cis-[Ru(dmp)2(H0)2](CF3SO,)2


1 3 2 PPH


a


-








ivUII"IWlUII 4Yi IvLVU JAIULkVUII Ui IVIcUIOAUi WIUI IIYUIUFII FUULVAIU


The trifluoromethanesulfonate derivative, complex III, is a potent catalyst for the

oxidation of CH4 by hydrogen peroxide in an acetonitrile solvent. Within 48 hours, at

75C, CO,, CO and traces of both methanol and formaldehyde appear which correspond

to 60% of the initial concentration of methane. A more active catalyst, less methanol and

more CO2 result with III than with the PF." derivative (II).

Previous reports suggest the di-oxo complex is necessary for the activation of the

C-H bond in methane40. In water, the hydrogen peroxide potential46 gives a negative free

energy for the formation of the di-oxo species only when the solution pH is less than 5.

However, at a pH of 2 in water, the oxidation of CH4 by H202 with this catalyst is not

observed at 750C.46 The relevant potentials are not known in acetonitrile but spectral

studies indicate a high oxidation state complex is formed upon addition of H202.

The best condition reported for methane oxidation40 with complex II in

acetonitrile yield only 5 turnovers for methanol accompanied by extensive over-oxidation

of methane to CO2 occurs. This prompted an experiment to determine the activity of this

catalyst for methanol oxidation to CO2. Methanol, catalyst (6.6x10" moles) and

hydrogen peroxide (5.0x10"2 moles) reacted in 4 hours at 750C to convert 97% of the

methanol to carbon dioxide and carbon monoxide. Thus, the main challenge to the use of

these ruthenium catalysts for the selective oxidation of methane to methanol is to inhibit

the over-oxidation. The successful oxidation of methane in HS0450,51 and

CF3C(O)OHS2,53,54 to large quantities of available methanol can be attributed to trapping

methanol as CH3OSO3H and CF3C(O)OCH3 respectively, and stabilizing it from over-








oxidation. This led to an attempt to trap methanol in our system as an ester by reaction

with acetic acid and acetic anhydride. Acetic acid was used since it is an acidic solvent,

and relatively difficult to oxidize. With aqueous H202 as the oxidant and water formed in

the reaction, the anhydride can function to keep the water concentration low enhancing

ester formation, as demonstrated in Equations 2-3, 2-4 and 2-5.



(Equation 2-3) CH3OH + CH3C(O)OH CH3C(O)OCH3 + H20

(Equation 2-4) H20 + CH3C(O)OC(O)CH3 2 CH3C(O)OH

(Equation 2-5) CH3C(O)OC(O)CH3 + CH3OH CH3C(O)OCH3 +

CHC(O)OH



The anticipated product, methyl acetate, could be subsequently hydrolyzed to produce

methanol and regenerate acetic acid.

The oxidative stability of methyl acetate to over-oxidation by our catalyst was

shown with an experiment in which methyl acetate, catalyst (6.6x10-5 moles) and

hydrogen peroxide (5.0x10-2 moles) dissolved in acetonitrile were reacted for 4 hours at

750C. An 8% decrease in methyl acetate and formation of small amounts of carbon

monoxide and carbon dioxide result, indicating that methyl acetate has the necessary

stability to be an effective trap for methanol. The 8% oxidation observed could proceed

through methanol formed from the equilibrium shown in Equation 2-3.

Using catalyst III and a solvent mixture composed of equal volumes of

CH3CN/CH3COOH/(CHCO)20, 7.4 millimoles of oxygenates formed in 24 hours







representing a significant improvement over the previously reported 0.5 millimoles in

acetonitrile.3839,40 Methyl acetate, methanol, formaldehyde, formic acid, methyl format,

carbon monoxide and carbon dioxide are all detected as products. As mentioned above,

in the absence of acetic acid and acetic anhydride, only CO and CO, and trace amounts of

formaldehyde are obtained with III as the catalyst.

Increasing the reaction time and hydrogen peroxide concentration in a series of

experiments, gave increased amounts of CO2, but led to an upper limit of 1.5x102 M

methyl acetate in this solvent mixture. Apparently when methyl acetate approaches this

concentration, the equilibrium concentration of CH3OH reaches a level at which its rate of

oxidation becomes equal to its rate of formation from methane. Decreasing the water in

solution should produce a lower steady state concentrations of methanol (i.e. the position

of the equilibrium in Equations 2-3 and 2-5 is shifted towards ester formation) and

increase the trapping efficiency leading to decreased CO, and increased methyl acetate.

To remove water, 4A molecular sieves were added to the reaction mixture and shown to

have a significant effect on the conversion of methane to methyl acetate. In comparable

24 hour runs, the conversion to oxygenates increased from 3 millimoles to 7 millimoles

(Table 2-1, Experiment 1) with the addition of sieves even though some peroxide

decomposition by the sieves occurs, vide infra. This result reinforces the proposal that

efficient trapping is the main challenge for effective methanol synthesis.

Variation of the reaction time in Experiments 2 and 3 show that in four hours the

maximum amount of methyl acetate and minimum amount of CO2 form. The shorter

time also led to an increase in the amount of formaldehyde. The methyl acetate formed

corresponds to 61 turnover numbers (5 mmoles, 6.6x102 M) with a selectivity of 55%















Table 2-1: Oxidation Results for Methane @ 750C with cis-[Ru(dmp)2(H20)2(CF3SO,3)
Using H202
Experiment MeCOOMe CO, Total CH,
Number mmol,(%) mmol,(%) Oxygenates Consumed
mmol,(%) (%)
1" 5,(44) 5,(44) 7,(62) 30

2 4,(40) 3,(30) 7,(69) 26

3V 5,(57) 1,(11) 7,(80) 19

4Af 10,(49) 9,(44) 10,(49) 62

5bs 4,(70) 1,(17) 5,(87) 25

6b- 9,(80) 3,(27) 8,(71) 49

7b-1 2,(100) 0 2,(100) N/A

8i 3,(47) 0 4,(62) 28

a. Reactions 1-4 used 6.6 x 10' moles cis-[Ru(dmp),(H20)2](CF3SO,)2 catalyst, 5 ml 30% H20,
(5.0xl0-2 moles). The solvent mixture is 20 ml acetonitrile, 20 ml glacial acetic acid, 20 ml
Acetic Anhydride, Initial methane pressure was 40 psig corresponding to 23 millimoles. 3.5g
4A molecular sieves (MS) were used. The percent products are based on total products seen
from all sources.
b. Reactions 5-8 used 2.4 x 10'5 moles cis-[Ru(dmp)2(H20)](CF3SO,), catalyst, 1 ml 35% H20,
(l.lxl0"' moles). The solvent mixture is 3 ml a-dichlorobenzene, 5 ml glacial acetic acid, 10
ml Acetic Anhydride, Initial methane pressure was 40 psig corresponding to 23 millimoles.
1.0g 4A molecular sieves (MS) were used.
c. Reaction Time 24 hours. The percent products are based on total products from all sources.
d. Reaction Time 12 hours. The percent products are based on total products from all sources.
e. Reaction Time 4 hours. The percent products are based on total products from all sources.
f. Blank experiment, performed in the absence of catalyst, Reaction Time 1 hour.
g. Blank experiment, performed in the absence of catalyst and acetonitrile, Reaction Time 4 hours.
h. Reaction Time 4 hours.
i. Blank experiment, performed in the absence of catalyst and methane, Reaction Time 4 hours.
j. Blank experiment, performed in the absence of catalyst, Reaction time 4. Hours.








while formaldehyde corresponds to 24 turnover numbers (2 mmoles) with a selectivity of

28%. The CO and CO2 produced correspond to 9 and 67 turnover numbers (0.2 mmoles

and 5 mmoles) respectively. The total turnover numbers for all the products is 198,

which if they all arose from methane would correspond to a methane conversion of 38%.

The total moles of oxidant used are 1.3x10"2, so 21% of the peroxide has decomposed.

Peracid Formation and Reactivity

In view of the difficulty of oxidizing methane and its low concentration in

solution, the importance of running control experiments to determine the source of

products cannot be overemphasized. When the catalyst was eliminated from the reaction

mixture, methyl acetate formed in significant amounts, Experiment 4 of Table 2-1. In

earlier research from this laboratory it was shown that hydrogen peroxide is catalytically

activated by forming peracids with organic acids55. To determine if peracetic acid forms,

acetic acid, acetic anhydride, and 35% aqueous hydrogen peroxide were stirred at 250C

for thirty minutes in the absence of sieves. The difference in an iodometric titration of the

products for total oxidant, and a cerium titration, for hydrogen peroxide, indicates by

difference that 20% of the HO,0 was converted to peracid (1.0xl0O2 moles). At this point

methane (40 psig) was introduced and the reaction vessel placed in an oil bath at 750C.

Experiment 4 (Table 2-1) shows that, in one hour of reaction time 10 mmoles of methyl

acetate (49% selectivity), traces of formaldehyde, 0.4 mmoles of carbon monoxide and a

large amount of carbon dioxide (9 mmoles or 44% of the products) were formed. This

reaction led to about 19 mmoles of total products which, if they all arose from methane

would correspond to 89% of the methane. The decrease observed in the amount of








methane by TCD is approximately 60% (Table 2-1) implying some of the oxygenates

arise from peracid decomposition.

To limit over-oxidation by peracid, the reaction time was decreased to twenty

minutes. The reaction produced 6.4x10" moles of methyl acetate (69% selectivity)

(1.1x10-' M), 1.6x0l moles of formaldehyde (0.04% selectivity), 3.9x10" moles of

carbon monoxide (0.06% selectivity) and 2.9x103 moles of carbon dioxide (31.1%). The

peroxide utilization was 97% and 1.0x10.2 moles of products were observed. The

decrease in reaction time leads to less products, but increased selectivity to methyl

acetate.

Blank runs were performed with and without sieves to determine if hydrogen

peroxide and acetonitrile form a peracid, CH3C(OOH)NH, which oxidizes methane to

produce methyl acetate. In these experiments, acetonitrile, hydrogen peroxide and

methane were added together and allowed to react for 48 hours at 750C. Iodometric

titrations also confirmed the absence of peracid.

The next concern is the source of the products in the blank, as shown in

Experiment 5. Oxidations of paraffins by 3,5-dinitroperbenzoic acid, perbenzoic acid and

perfluoroperacetic acid result56 in more than 50% conversion of the paraffin. A radical

mechanism is suggested57.58 for the formation of the hydroxylation products that is

initiated by the decomposition of peracids as shown in Equations 2-6, 2-7, and 2-8.



(Equation 2-6) R-CO3H RCO2 + 'OH

R'+ CO2 +'OH








(Equation 2-7) R-H + R" 'R" + R-H

(Equation 2-8) 'R + R-CO3H 'R-OH + R-CO,'



The R-CO," radical decomposes to form CO, and the alkyl radical (R') which

abstracts a hydrogen atom from the paraffin R'H to form the 'R' alkyl radical which

reacts with the peracid to form the alcohol generating R'OH and another RCO2". In our

system, with CH4 as the alkane and peracetic acid as the oxidant, decomposition of

CH3CO3H would generate methyl radicals which may exchange with methane in

Equation 2-7 and would have to react as shown in Equation 2-8 to produce CHOH.

Methanol formation would in effect arise from peracid decomposition.

Alternatively, RCO2*can react with R'H to form R" which then reacts as shown

in Equation 2-8. In this case CH4 would be converted to methanol by CH3CO2, formed

in Equation 2-5, reacting with methane, as shown in Equation 2-6, to form methanol via

Equation 2-8. Methane oxidation by peracid results, in contrast to the literature

mechanism where the products would derive from the solvent. In view of the possible

exchange of methyl radical with labeled methane, Equation 2-7, labeling experiments will

not be definitive. One also notes that less methyl acetate is produced with the ruthenium

complex present (Experiment 3, Table 2-1), than with it absent in Experiment 4,

suggesting metal complex decomposition of H202. Crude estimates of the methane that

disappeared in the experiments described above, indicate the metal complex is involved

in Experiments 1-3 but more compelling evidence is required.








The question concerning the reactivity of methane with peracid raised above

impacts not only on the metal catalyzed oxidations in acetic acid-acetic anhydride solvent

but also on peracid oxidation ofalkenes.56 Blank runs at 75*C, with catalyst, hydrogen

peroxide, acetic acid/acetic anhydride, acetonitrile and no methane, produced one

millimole of methane and traces of methanol after four hours of reaction. A similar

experiment in a-dichlorobenzene, showed only traces of methane and methanol. The

source of methane is thought to arise from solvent facilitated peracid decomposition in

acetonitrile. This proposal was not investigated further, but the solvent was switched

from acetonitrile to a-dicholorbenzene and Experiment 5-8 were carried out. Since

labeling experiments are uninformative, efforts were expended to accurately determine

the methane disappearance to provide a material balance. To facilitate the metal

catalyzed path, the catalyst and hydrogen peroxide concentrations were increased by

20%. Experiments 5 and 7 indicate that the beneficial effect of the sieves removing water

is accompanied by the deleterious decomposition of hydrogen peroxide. In Experiment 6,

methane oxidation is catalyzed by III, giving an amount of products equivalent to 49%

conversion to oxygenates. Nine millimoles of methyl acetate, two millimoles of other

oxygenates and 3 millimoles of CO, were formed. In Experiments 5 and 6 a good

material balance for methane disappearance and product formation resulted.

When an identical blank experiment with no catalyst or methane was carried out,

two millimoles of methyl acetate and no other products were formed from peracid

decomposition, Experiment 7. When no catalyst was used and methane added,

Experiment 8, three millimoles of methyl acetate and one millimole of other oxygenates





48

were formed. Experiment 7 indicates that methyl acetate can arise from the solvent via

peracid decomposition. Since the amounts of oxygenates produced are doubled when

methane is added, Experiment 8, methyl acetate is produced by the reaction of the peracid

with methane. However, in Experiment 6 when the ruthenium catalyst is present, the

almost three-fold increase in oxygenates compared to Experiment 7 establishes the metal

catalyzed path.

All methane conversions given in Table 2-1 is based on the amount of methane

consumed in the reaction. In the absence of a material balance for runs 1-4, an upper

estimate results for conversions. Since peracid formation is not expected for oxidations

with 02, trapping with the acetic acid-acetic anhydride mixture was investigated next with

this oxidant.

Oxidation of Methane with Molecular Oxygen

The direct conversion of methane to methanol using molecular oxygen is even a

greater challenge than using hydrogen peroxide as the oxidant. Table 2-2 provides results

for experiments using 30 psig of 5% 0O in helium as the oxidant. The moles of oxidized

products formed clearly exceed the moles hydrogen peroxide (2.5x105) added to initiate

the reaction by oxidizing III. After 48 hours (Experiment 9), methyl acetate (5 turnover

numbers, 0.32 mmoles, 19% selectivity), and relatively large amounts of formaldehyde

(20 turnover numbers, 0.65 mmoles, 38% selectivity) methanol and methyl format were

obtained. GC analysis (TCD) shows carbon monoxide (9 turnover numbers, 0.19

mmoles, 11% selectivity) and carbon dioxide (32 turnover numbers, 0.52 mmoles, 30%

selectivity). At most a total of 0.4 turnovers (2.5x10" moles) for methyl


























Table 2-2: Oxidation Results for Methane @ 750C with cis-[Ru(dmp)2(H20)](CF3SO3)2
Using H202 and 02


a. 6.6 x 10-5 moles cis-[Ru(dmp)2(HO0)](CF,SO,)2, 20 ml acetonitrile, 20 ml glacial acetic
acid, 20 ml Acetic Anhydride, 20 psig CH, (1.0x102 moles), 3.5g Molecular Sieves (MS).,
750C,
b. Reaction time 48 hours 2.5 MI 30% H,20 (2.5x105 moles).
c. Reaction time 24 hours 2.0 ui 30% H20, (2.0x 10' moles).
d. Blank Experiment, performed in the absence of H20, 48 hours.
e. Blank Experiment, performed in the absence of catalyst 48 hours.







acetate could have resulted from the 2.5x105 moles ofH202 assuming 100% peroxide

efficiency. Clearly this catalytic system utilizes 02 for the oxidation of methane at 75C.

The total moles of products correspond to 1.7 mmoles, corresponding to a 17% percent

conversion of methane. The formation of methyl acetate indicates the trapping of

methanol is occurring.

The reaction of methane with 0O was carried out for 24 hours (Experiment 10).

Less CO2 and less methane conversion results. The products are methyl acetate (5

turnover numbers, 0.30 mmoles, 50% selectivity), carbon monoxide (3 turnover numbers,

0.06 mmoles, 10% selectivity) and carbon dioxide (16 turnover numbers, 0.26 mmoles,

40% selectivity). A total of 0.63 mmoles are formed and the percent methane conversion

was 6%. Since the same amount of methyl acetate forms in 24 or 48 hours, the extended

time results in the oxidation of methanol that is in equilibrium with methyl acetate

(Equation 2-2).

Experiment 11 was performed to determine if di-oxygen could catalyze the

reaction without hydrogen peroxide to initiate formation of the ruthenium-oxo species.

After 48 hours no oxidation products were observed. The [Ru(dmp)2(H20),2+ catalyst

precursor utilizes molecular oxygen in epoxidations38.39 only after an initiation period in

which an alkyl hydroperoxide is generate to oxidize the complex to the (IV) oxidation

state. Upon oxygen atom transfer to the substrate from the (IV) oxidation state the (II)

oxidation state forms, which reacts with the Ru (IV) to generate Ru (III). The (III)

oxidation state reacts with molecular oxygen to regenerate the higher oxidation state

metal-oxo species.46 Since 02 will not form methylhydroperoxide from methane, the

inactivity of the ruthenium (II) complex with 02 (Experiment 11), and the required







initiation by H202 suggests that a metal-oxo species generated from ruthenium (III) by 02

is involved by either reacting with ruthenium (III) to form a metal-oxo species, catalyzing

the reduction of O to H202 by methane or inhibiting radical chain termination steps.

Conclusions


The use of the sterically hindered cis-[Ru(dmp)2(H20)2,] complex to catalyze the

oxidation of methane has been investigated. Using the poorly coordinating

trifluoromethanesulfonate anion in place ofhexafluorophosphate an improvement in

reproducibility and catalytic activity of the complex has been demonstrated. This

complex is a precursor to the proposed di-oxo catalyst that oxidizes methane to CO2 with

hydrogen peroxide at 750C in acetonitrile. In order to stop the reaction at the methanol

intermediate, an acetic acid/acetic anhydride mixture was chosen to trap methanol as

methyl acetate. This ester, which is not as readily oxidized as the alcohol, increases the

selectivity and inhibits the over-oxidation of methanol.

In oxidations with hydrogen peroxide, control experiments indicate the formation

of peracetic acid from the solvent mixture of acetic acid/acetic anhydride. In the absence

of methane, the peracid decomposes to produce methyl acetate. When the reaction is

carried out in identical conditions under methane, a doubling of the methyl acetate,

produced shows that methane is oxidized by peracid to form methyl acetate. A

substantial increase in methyl acetate, for identical reaction conditions with metal

complex added, clearly demonstrates that metal catalyzed oxidation occurs. The material

balance shows that the predominant source of the methyl acetate is methane with at most

a small contribution from peracid decomposition.





52

The cis-[Ru(dmp)2(H20)2]2* complex is a catalytic precursor for the oxidation of

methane with dioxygen in acetonitrile at 750C. It is necessary to initiate the reaction by

oxidation of the complex with trace amounts of hydrogen peroxide. With molecular

oxygen as the oxidant, a 6% conversion of methane and 50% selectivity to methyl acetate

can be obtained after 24 hours of reaction.














CHAPTER 3
OXIDATION OF ALKANES WITH HYDROGEN PEROXIDE USING A
RUTHENIUM METAL-OXO CATALYST

Introduction


The transformation of saturated hydrocarbons into their oxygentated derivatives

(i.e. alcohols, aldehydes, ketones and carboxylic acids) has been the subject of intense

investigation over the past two decades. Alkanes given their great abundance offer a cost

effective and ideal feedstock for an industrial process.59 However, the chemical inertness

of saturated hydrocarbons make activation extremely difficult at mild conditions. The

oxygenation products, vital intermediates in many industrial processes, are then converted

into commercial products. In order for a process to be feasible on the industrial scale the

catalyst must demonstrate two properties: selectivity towards the partial oxygenates and

exhibit a specific regioselectivity. To achieve these requirements a commercial industrial

application must maintain high temperatures and pressures.60 Currently, research efforts

are being directed towards the development of new efficient catalytic systems, which are

able to oxygenate, saturated hydrocarbons under mild conditions using hydrogen

peroxide and/or molecular oxygen.

Reagents with the capacity to oxidize paraffins and arylalkanes have been known

for well over a century.61,62 Two such compounds are chromyl chloride (CrO2-Cl) and

permanganate (MnO").61.62 However, these oxidants are stoichiometric and a catalytic







one is desired. Numerous researchers63,64.65 have reported organic radical mechanisms

via homogeneous or heterogeneous oxidation. The essential properties that allow these

oxidants to oxidize a relatively inert C-H bond are not fully understood.

A number ofmetal-oxo and metal-oxide surfaces perform as reagents or catalysts

for the oxidation of hydrocarbons, on industrial and laboratory scales.1,66,67,68 Metallo-

enzyme sites also activate hydrocarbons, two well-documented examples being

Cytochrome P-45069 and methane monooxygenase47 (MMO).

Current research in the area of hydrocarbon activation has utilized a variety of

oxidants, catalysts and reaction conditions. Barton et al.70,71 has developed the Gif

system, as well as a number of variations (i.e. Barton-Gif). Gif uses a pyridine/acetic

acid solution, substrate, Fe catalyst and an oxidant, usually t-BuOOH. The mechanism of

oxidation has been reported as proceeding through the involvement of free radicals.

Que72 has been investigating the use of di-iron complexes, in this case a high valent

Fe2(9-O), moiety. This complex has been proposed as the key oxidizing species for

methane monooxygenase, as well as for other non-heme di-iron enzymes.

Ribonucleotide reductase and fatty acid desaturase are mentioned.

This research demonstrates the cis-[Ru(dmp)2(H20)2(CF3SO,)2 as a precursor for

the catalytic oxidation of higher linear and branched alkanes with hydrogen peroxide in

acetonitrile at 750C. The complex generates a large fraction of products oxygenated at

the primary carbon position. An increase in selectivity towards the alcohol product is

observed upon addition of CuCI2. The effect of temperature on reactions with this catalyst







is also investigated to determine the effect on overall hydrocarbon conversion, selectivity

distribution of oxygenates.

Experimental


Materials and Methods

RuCI x H20, 2,9-dimethyl-l,10-phenanthroline, LiCI, NaCF3SO3, hydrogen

peroxide (35%) and pentane (99.9%) were all used as received from Aldrich. Ethylene

glycol and acetonitrile were all used as received from Fisher Scientific (ACS grade).

Acetonitrile was distilled over P205 under dinitrogen and was stored over 4A activated

molecular sieves. Ethane (99.9%), propane (99%), butane (99%) and iso-butane (99.5%)

was purchased from Matheson and used as received.

Physical Measurements

UV-Vis measurements employed a Perkin Elmer lambda-6 spectrophotometer.

The pH measurements were made with a Fisher Accumet model 630 pH meter. NMR

spectra were recorded on a Varian VXR300 spectrometer. FAB mass spectral data were

obtained by Dr. David Powell (U.F.) in a m-nitrobenzyl alcohol matrix. U.F. Analytical

Services performed elemental analyses.

Synthesis of Compounds

cis-Ruthenium(I) Bis(chloride)bis(2,9-dimethyl-l,10-phenanthroline)

Monohydrate, [Ru(dmp)2C1, H,0 (I). Cis-[Ru(dmp),C1l] H20 was synthesized as

reported. Analysis: Calculated for C2,H,6N4OC12Ru; C, 55.45; H, 4.29; N, 9.24. Found:

C, 55.65; H, 4.35; N, 9.29.







cis-Ruthenium(II) Bis(aquo)bis(2,9-dimethyl-l,10-phenanthroline)

bis(trifluoromethanesulfonate), Cis-[Ru(dmp),(HzO)2](CF3SO3), (II). A 1.0g (1.7

mmol) portion of I is slowly dissolved in 150 ml of deionized H20 under N2 by heating at

500C for 30 minutes. After adding 50 ml of a saturated NaCF3SO3 (aq), the solution

cooled, placed in an ice bath for two hours and filtered. The precipitate is redissolved in

H20 by heating at 500C, a saturated NaCF3SO3 (aq) (9g NaCF3SO3/ 25ml deionized H20)

solution is added and reprecipitated as above. Recrystallization and exchange of Cl for

CF3SO3 is repeated until the filtrate affords a negative chloride test with AgNO3. The

product is dried in vacuum at 60C overnight. Analysis: Calculated for

C3oH34N40OS2,FRu, C, 42.25; H, 3.31; N, 6.57. Found: C, 41.40, H, 3.26; N 6.39.

Oxidation Procedure

The pressurized oxidations were carried out as previously described49, in batch

type hydrogenation reactors, Figure 2-5. The reaction mixtures were varied as described

in table footnotes. Blank runs were performed omitting certain reactants and are also

described in the text. Reaction temperatures were normally maintained at 750C unless

stated otherwise. To remove air initially present, nitrogen gas is purged through the

reactor, the apparatus is pressurized with the substrate, which is then released, and re-

pressurized to the desired pressure.

The oxygenated products of the reaction were analyzed and quantified with a

Hewlett-Packard 5890 Series II gas chromatograph equipped with an FID detector and

outfitted with a 30m HP 50+ (50% Ph Me Silicone Gum; 1 im thickness). Helium was

employed as the carrier gas. Carbon dioxide and carbon monoxide analysis were







performed with a Varian 3700 gas chromatograph equipped with a TCD detector outfitted

with a 15' Carboxen Column, I prm thickness. Helium was utilized as the carrier gas.

Concentrations of the substrate and oxidized products were quantified using acetonitrile

as an internal standard. Three chromatograms were measured for each sample using

injection volumes of 0.1 ml for gas and 0.1 gl for liquid samples.

A typical reaction mixture consisted of 60 ml of acetonitrile, 40 psig total pressure

for gaseous reactants, 1.6x10' moles of catalyst and 5ml of 35% hydrogen peroxide

(5.0x10'" moles). Experiments in the absence of oxidants were performed as blanks.

The following definitions describe terms used in the presentation of the results.

Selectivity to any product is the moles of a given product divided by the total moles of all

products formed expressed as a percent. The percent peroxide efficiency is the moles of

H20, needed to account for all oxidized products divided by the moles of H20,

consumed. The percent conversion of alkane is the moles carbon in the oxidized products

divided by the moles ofalkane added to the reactor.

Safety Precautions, the combination of molecular oxygen with organic

compounds and solvents at elevated temperatures and pressures are potentially explosive.

Extreme caution should be taken during the charging and disassembly of the

experimental apparatus. Equipment which can generates sparks must be avoided, a safety

shield and cooling of the batch reactor in an ice bath for 30 minutes prior to disassembly

is recommended.








Results and Discussion


Alkane Oxidation

Attempts to oxidize alkanes with hydrogen peroxide in H20 as the solvent at 750C

were unsuccessful after 48 hours with or without catalyst at solution pH values of 1 to 7.

As suggested previously46, the Ru(VI) oxidation state is necessary for the activation of

the C-H bond of alkanes. The [Ru(dmp)2(H20)2(CF3SO3)2 complex II has been shown to

be a potent catalyst in an acetonitrile solvent.73 Consequently alkane oxidations were

performed in acetonitrile.

Each alkane (ethane, propane, butane, iso-butane and pentane) was reacted in

acetonitrile (60ml) with hydrogen peroxide (5.0x 10mole) and catalyst (1.6x10" moles)

for 15 hours at 750C. The results are provided in Table 3-1 and Table 3-2. Propane, iso-

butane and pentane allowed us to determine catalyst activity in terms of selectivity and

regioselectivity.

After 15 hours, a 20.8% conversion of propane was observed corresponding to 3.4

mmoles of oxidized products. The selectivities of the oxidized products obtained are of

interest. Oxidation at the primary carbon position accounts for 65.1% or 2.2 mmoles of

the total oxidized products (1-propanol: 0.1 mmoles, 1.6% selectivity, and

propionaldehyde: 2.1 mmoles, 63.6% selectivity). Trace amounts ofpropionoic acid

were detected by gas chromatography analysis, however peak broadening and tailing

make quantification difficult. Oxidation at the secondary carbon position was also

observed, accounting for 34.9% or 1.2 mmoles of the total oxidized products (2-propanol:











Table 3-1: Oxidation Results for Ethane, Propane and Butane @ 750C with cis-
[Ru(dmp)2(H20)](CF3SO3)2 using H202
Oxidized Total Percent
Substrate" Products mmoles Peroxide Percent
mmoles Oxidant Efficiency Conversionb
(Selectivity)
Ethanol:
0.08 (2.3)
Ethane Acetaldehyde: 8.10 16.2 20.1
2.50 (69.3)
Acetic Acid:
1.00 (28.4)
1-Propanol:
0.05 (1.6)
2-Propanol:
0.07(1.9)
Propane Propanal: 6.80 13.5 20.8
2.10 (63.6)
Acetone:
1.20 (32.9)
Propionoic Acid:
Trace
1-Butanol:
0.08 (4.8)
2-Butanol:
0.94 (5.3)
Butane Butanal: 5.40 10.7 26.2
0.10 (5.5)
2-Butanone:
0.31 (18.1)
Butanoic Acid:
1.20 (66.3)
a: 1.6x10" mole cis-[Ru(dmp)2(H20)](CFSOz), 60ml acetonitrile, 5.0xl02 moles 35% HO2,
40psi substrate (1.9x10'2 moles), 15 hours at 750C.
b: Percent conversion is based on the total of oxidized products divided by the initial amount
of substrate.
















Table 3-2: Oxidation Results for Iso-Butane and Pentane @ 750C with cis-
[Ru(dmp)2(H20)](CF3SO3)2 using H202

Oxidized Total Percent
Substrate' Products mmoles Peroxide Percent
mmoles Oxidant Efficiency Conversion'
(Selectivity)
Iso-Butanol:
Iso-Butane 3.90 (99.0) 3.90 7.8 20.5
Iso-Butyl-ol:
0.04 (0.9)
1-Pentanol:
0.58 (5.7)
2-Pentanol:
1.20 (11.5)
3-Pentanol:
Pentaneb 1.20 (17.1) 16.10 32.4 23.4
Pentanal:
2.20(21.7)
2-Pentanone:
1.90 (18.4)
3-Pentanone:
2.60 (25.7)
Pentanoic Acid:
Trace
a: 1.6xl0" mole cis-[Ru(dmp),(H20)](CFSO3), 60ml acetonitrile, 5.0x 10 moles 35% H202,
40psi substrate (1.9x 10" moles), 15 hours at 750C.
b: 2ml pentane (2.0x10"2 moles) used.
c: Percent conversion is based on the total of oxidized products divided by the initial amount
of substrate.







0.1 mmoles, 1.9% selectivity and acetone: 1.1 mmoles, 32.9% selectivity). Activation of

propane by this catalyst produces a selectivity ratio of primary:secondary carbon

oxidation of 2:1. A peroxide efficiency of 13.5% and 43 turnover numbers (TON's) was

obtained for this reaction.

Ethane and butane were oxidized under identical conditions giving a 20.1% and

26.2% conversion respectively. The results are shown in Tables 3-1 and 3-2. Over-

oxidation of the hydroxylated products to the corresponding aldehyde, ketone and

carboxylic acid were also exhibited. As a result selectivity, overall conversion of the

alkane, peroxide efficiency and catalyst lifetime are decreased by over-oxidation.

Iso-butane was investigated to demonstrate the ability of this catalyst to produce

oxidation products at the tertiary and primary carbon position. After 15 hours of reaction,

3.9 mmoles of oxidized products were obtained. Oxidation at the tertiary carbon

accounted for 99.0% (3.9 mmoles) of the oxidized products. Also detected were trace

amounts (0.04 mmoles, 0.9% selectivity) ofisobutyl alcohol (2-methyl-l-propanol),

demonstrating some oxidation at the primary position.

Pentane can lead to oxidation at the CI, C2 and C3 positions. This is also the first

liquid alkane investigated. After 15 hours, a 23.4% conversion ofpentane was observed

accounting for 10 mmoles of oxidized products. Oxidation at the primary carbon position

(Ci) afforded: 1-pentanol (0.6 mmoles, 5.7% selectivity), valeraldehyde (pentanal) (2.2

mmoles, 21.7% selectivity) and a trace amount of valeric acid (pentanoic acid), once

again quantification was difficult due to peak broadening and tailing. Oxidation at the

secondary carbon (C,) position produced: 2-pentanol (1.2 mmoles, 11.5% sel.) and 2-

pentanone (1.9 mmoles, 18.4% sel.). Activation at the C3 position produces: 3-pentanol








(1.2 mmol, 17.1% sel.) and 3-pentanone (2.6 mmoles, 25.7% sel.). A total of 16 mmoles

of oxidant was consumed in this reaction yielding a peroxide efficiency of 32.4% and 50

TON.

Mechanism of Oxidation

Two mechanisms, hydrogen abstraction or "rebound" and oxygen atom insertion,

have been proposed for the oxidation of hydrocarbons with cis-[Ru(dmp),(H2O)j]2.40o74

These mechanisms are illustrated in Figures 3-1 and 3-2 respectively.

The hydrogen abstraction or "rebound" mechanism produces an alkyl free radical.

The free radical produced in the first step of the mechanism is the result of the cis-

{Ru(0)2)2 moiety abstracting a hydrogen atom from the alkane to produce a reduced

ruthenium-hydroxide species. The active metal center, Rum)=O functions as a free

radical4o which is capable of abstracting a hydrogen atom from the hydrocarbon. The

second step proceeds with the transfer of the hydroxo ligand to the formed radical to yield

the hydroxylated product. The catalytic cycle involves regeneration of the high valent

metal center by oxidation with 02 or H202. Other ruthenium complexes have been

proposed to proceed through this or a similar mechanism.75,76,77

The oxygen atom insertion mechanism has been proposed for the epoxidation of

olefins.40'78,79 It has also been reported that ruthenium-oxo complexes are able to

epoxidize alkenes via a non-radical mechanism.40 The first step in this mechanism for

alkane oxidation is the insertion of the oxygen bound to ruthenium into the C-H bond of

the substrate. This adduct forms a three coordinate oxygen and a five coordinate carbon





63





















0 0 0
Ru:O + H-CH RuoH + C + HO-CH3
3ORu:OH HCH3 Ruu


O
II
- Ru:O


+ H202


+ H20


Figure 3-1: Proposed Hydrogen Atom Abstraction Mechanism for cis-
[Ru(dmp)2(H20)]2 Complex








similar to CH,5 obtained in an S,2 mechanism. The next step is scission of the C-H bond

forming a coordinated alcohol followed by displacement of the alcohol from the

ruthenium complex. The final step is the regeneration of the high valent metal-oxo as

previously described in the hydrogen abstraction mechanism.

Results from this laboratory4o,73 indicate that the activation ofalkanes with cis-

[Ru(dmp)Oj2+ is proceeding via the hydrogen abstraction mechanism.

Addition of CuCl,

The majority of the oxidized products obtained are aldehydes, ketones and

carboxylic acids. In order to obtain increased amounts of the partially oxidized products

its direct over-oxidation must be prevented. Addition of a metal chloride could offer a

solution. Addition of NiCl2 to a methanol solution produces complexes with methanol

bound to the nickel.80 In this investigation formation of this metal-alcohol adduct was

determined by IR analysis. Coordination of the alcohol produced in our oxidations to a

metal could possibly retard over-oxidation and improve selectivity for the hydroxylated

product. To determine if an increase in selectivity towards the hydroxylated products can

be obtained one mole equivalent of CuCl2 (1.6xl04) was added to the previously

described reaction with all other conditions held constant. The results are shown in Table

3-3 and Table 3-4.

After 15 hours at 750C, an 18.2% conversion of propane was observed in the

presence of CuCI. The addition of CuCI2 led to a 2.4% decreased in conversion.

However, an increase in alcohol selectivity to 6.1% for 1-propanol and a 18.5% for























H-CH3


H TCH3
0+
Ru:O


0
____ I
Ru


O
II
------ Ru:O


+ HO-CH3


+ H,O


Figure 3-2: Proposed Oxygen Atom Insertion Mechanism for cis-
[Ru(dmp)2(H2O0)2] Complex


0
II
Ru:O +



H CH,


Ru:O


+ H202







2-propanol is observed, compared to selectivities of 1.6% and 1.9% when CuCI2 is

omitted. With the increase in alcohol selectivity a decrease in selectivity to the aldehyde

and ketone results. A 61.0% selectivity to propionaldehyde and 14.4% selectivity for

acetone is exhibited with the addition of CuCl,. CuC12 addition leads to a decrease in the

peroxide efficiency attributed to known peroxide decomposition by first row transition

metal salts.29 Increases in selectivity to the alcohol are also found for each of the alkanes

studied. These results are presented in Table 3-3 and 3-4.

To determine the role of CuCI2 a series of blank experiments were performed.

When 60 ml of acetonitrile, 1.6x104 moles CuCI2, 5.0x10" mole H20, and 1.9x10'2 moles

of propane were reacted for 15 hours at 750C no oxygenates were observed suggesting

CuCI2 is not participating in the oxidation of propane.

Experiments were performed to determine the stability of alcohol under reaction

conditions. Ethanol (Iml, 1.9x102 mol), acetonitrile (60ml), 1.6x10' moles catalyst and

5.0x102 moles H202 were reacted for 15 hours at 750C. A 77% decrease in ethanol and

formation of acetaldehyde and acetic acid was observed. TCD gas chromatography

analysis exhibited the trace amounts of carbon monoxide and carbon dioxide. The

quantity of oxidized products corresponds to the decrease in moles of ethanol. Analysis

of the resulting solution for peroxides with sodium metavanadate and iodometric titration

showed none was present. An identical experiment was performed with the addition

CuC12 (1.6x10" mol) to the reaction solution. After 15 hours the amount of alcohol

present in the final reaction solution corresponded to only a 43% decrease. Aldehyde and

carboxylic acid also formed in decreased amounts according to mass balance calculations.











Table 3-3: Oxidation Results for Ethane, Propane and Butane @ 75C with cis-
[Ru(dmp)2(H20)](CF3SO3)2 and CuCl2 using H20,

Oxidized Total Percent
Sub- Products mmoles Peroxide Percent
state' mmoles Oxidant Efficiency Conversionb
(Selectivity)
Ethanol:
0.18 (4.31)
Ethane Acetaldehyde: 9.10 18.1 23.2
3.10(74.7)
Acetic Acid:
0.88 (21.0)
1-Propanol:
0.21 (6.1)
2-Propanol:
0.64 (18.5)
Propane Propanal: 6.10 12.1 18.2
2.10 (61.0)
Acetone:
0.49 (14.4)
Propionoic Acid:
Trace
1-Butanol:
0.26 (12.5)
2-Butanol
0.15(7.1)
Butane Butanal: 4.90 9.7 30.9
0.11 (5.3)
2-Butanone:
0.46 (21.8)
Butanoic Acid:
1.10(53.2)
a: 1.6x10' mole cis-[Ru(dmp)2(H20)](CF3S03),, 1.6x10" mole CuCl, 2 H20,
60ml acetonitrile, 5.0xl0-2 moles 35% H202, 40psi substrate (1.9xl02 moles),
15 hours at 750C.
b: Percent conversion is based on the total of oxidized products divided by the initial amount
of substrate.
















Table 3-4: Oxidation Results for Iso-Butane and Pentane @ 750C with
[Ru(dmp)2(HO0)](CF3SO3)2 and CuC12 using H202
Oxidized Total Percent Percent
Substrate' Products mmoles Peroxide Conversion'
mmoles Oxidant Efficiency
(Selectivity)
Iso-Butanol:
Iso-Butane 3.50(99.2) 3.50 7.1 18.8
Iso-Butyl ol:
0.01 (0.8)
1-Pentanol:
0.94(10.4)
2-Pentanol:
1.60 (17.5)
3-Pentanol:
Pentaneb 2.70 (30.1) 13.10 25.5 20.9
Pentanal:
1.30 (13.8)
2-Pentanone:
1.30(13.7)
3-Pentanone:
1.30 (14.5)
Pentanoic Acid:
Trace
a: 1.6x 10 mole ci-[Ru(dmp),(H20)](CF3SO3)2, 1.6x 10 mole CuC 2 H,0,
60ml acetonitrile, 5.0x10"2 moles 35% H202, 40psi substrate (1.9xl0" moles),
15 hours at 75C.
b: 2ml pentane (2.0xl10" moles) used.
c: Percent conversion is based on the total of oxidized products divided by the initial amount
of substrate.







calculations. No peroxide was found in the resulting solution. These experiments

confirm the role of CuC12 in producing more alcohol in the oxidation of alkanes.

In the absence of the catalyst and CuCIl, a mixture of H202 (5.0x02 moles) and

ethanol (1.9x102 moles) in acetonitrile (60 ml) leads to a 21% decrease of ethanol after

15 hours. Hydrogen peroxide an alcohol under these type of reactive conditions. When

the reaction is carried out with CuCI (1.6x10' moles) added, the hydrogen peroxide

(5.0xl0-2 moles) in acetonitrile (60 ml) oxidizes only 9% of the ethanol to acetaldehyde

and acetic acid after 15 hours. This experiment demonstrates CuCI2 either complexes the

alcohol preventing oxidation or increases the rate of decomposition of the peroxide

preventing oxidation of the products.

The effect of increased amounts of CuC12 on selectivity to the alcohol was

investigated using the oxidation of propane at 750C. The results of these experiments are

given in Table 3-5 and illustrated in Figure 3-3.

Successive additions of CuC12 increased selectivity towards the alcohol with the

accompanying decrease in selectivity to the aldehyde and ketone. With increased

selectivity, a decrease in overall conversion is observed. This is most likely attributed to

peroxide decomposition by the increased copper concentration present. Without CuCI2 a

20.80% conversion is achieved. Addition of 1 mole equivalent decreases the conversion

decreases to 18.12%. Additional CuCI2 continues the trend reaching a 6.52% conversion

with 5 mole equivalents added. This decrease in conversion is the result of increased

peroxide decomposition, leading to a lower concentration of peroxide present to perform

catalysts and over-oxidize the products.


















Table 3-5: Oxidation Results for Propane @ 750C with cis-[Ru(dmp)2(H20)](CF3SO3)2
and Varying Mole Equivalents of CuCI2 using H202

Mole mmoles mmoles mmoles Total Percent
Equivalents Alcohol Aldehyde Ketone mmoles Conversionb
CuCl,"' ___Products

0 0.12 2.14 1.18 3.44 20.80


1 0.59 1.45 0.34 2.38 18.24


2 0.84 0.14 0.33 1.30 7.19


3 0.84 0.13 0.32 1.29 7.14


4 0.90 0.11 0.22 1.22 6.73


5 0.91 0.10 0.18 1.18 6.52

a: 1.6x0L mole cis-[Ru(dmp),(H0O)](CF3SO3)2, 1.6x10" x mole eq. moles CuCl, 2 H20, 60ml
acetonitrile, 5.0x102 moles 35% H202, 40psi substrate (1.9x l02 moles), 15 hours at 750C.
b: Percent conversion is based on the total of oxidized products divided by the initial amount
of substrate.




















Effect of CuCl2 on Products with the
cis-[Ru(dmp)2(H20)J2] Precursor


-
14.00

0
11.00
g.


0 1 2 3 4 5
Mole Equivalents CuCIl
---Alcohol ---Aldehyde --Ketone -A-Total Moles -- Percent Conversion (2nd)


Figure 3-3: Oxidation of Propane with cis-[Ru(dmp),(H20)](CF3SO3)2 and Varying Mole
Equivalents of CuClz using H202







Effect of Temperature

Increased temperature is expected to increase the rate on an oxidation reaction.

Increase in substrate reactivity and solubility of the catalyst is also expected with an

increase in reaction temperature. Disadvantages include an increased rate of peroxide

decomposition. A series of experiments was performed at 25, 50, 75 and 1000C with and

without CuCI2 were performed to determine the effect of temperature on this oxidation

reaction. The results provided in Tables 3-6 and 3-7, are illustrated in Figures 3-4 and

3-5.

Propane was again selected as the substrate for these experiments. A mixture of

catalyst (1.6x 10 moles), acetonitrile (60 ml), H02, (5.0xl0" moles) and propane

(1.9xl0"2) were allowed to react for 15 hours at each temperature. Even at 250C the

ruthenium produces oxygenates from propane with a 3.71% conversion. The moles of

aldehyde and ketone produced are greater than the moles of alcohol generated. As the

temperature increases, overall conversion increases along with an increase in the relative

amounts of the aldehyde and ketone. The over-oxidation of alcohol present is also more

rapid with the catalyst as the temperature is raised.

An identical series of experiments were performed with the addition of 1 mole

equivalent of CuC12 (1.6x10" mole). The result are given in Table 3-7 and graphically

represented in Figure 3-5. The conversions obtained at high temperatures with CuCI,

present are decreased when compared to those in the absence of CuC12. Again, a higher

selectivity to the alcohol is obtained.























Table 3-6: Oxidation Results for Propane @ Varying Temperatures with cis-
[Ru(dmp),(H20)](CF3SO3)2 using HAO2

Total
Temp. Mmoles mmoles mmoles mmoles Percent
(C)' Alcohol Aldehyde Ketone Products Conversionb


25 0.14 0.28 0.26 0.67 3.71


50 0.13 0.55 0.40 1.08 5.52


75 0.12 1.58 0.49 2.19 12.09


100 0.10 2.04 0.89 3.03 16.72

a: 1.6xl0' mole cis-[Ru(dmp),(H1O)](CFSO3),, 60ml acetonitrile, 5.0xl0" moles 35% HO22,
40psi substrate (1.9x10"2 moles), 15 hours.
b: Percent conversion is based on the total of oxidized products divided by the initial amount
of substrate.






74


















Temperature Dependence of cis-[Ru(dmp)2(H20)2]2+


18.00


15.00


12.00 .


9.00 0


6.00


3.00


U.UU II 0.00
0 25 50 75 100
Temperature (Degrees Celcius)

---Alcohol ----Aldehyde -Ar-Ketone --Total Moles --- Percent Conversion (2nd)






Figure 3-4: Oxidation of Propane @ Varying Temperatures with cis-
[Ru(dmp),(HzO)](CF3SO3)2 using H202


















Table 3-7: Oxidation Results for Propane @ Varying Temperatures with cis-
[Ru(dmp),(H20)](CFSO), and CuC1, using H202

Total
Temp. Mmoles mmoles mmoles mmoles Percent
(C) Alcohol Aldehyde Ketone Products Conversionb


25 0.23 0.28 0.40 0.91 5.03


50 0.50 0.99 0.43 1.93 9.89


75 0.58 1.45 0.54 2.58 14.23


100 0.66 1.70 0.67 3.03 16.72

a: 1.6x104 mole cis-[Ru(dmp)2(H,0)](CFSO3)2, 1.6x10' mole CuCl 2 HO, 60ml acetonitrile,
5.0x10' moles 35% HzO2, 40psi substrate (1.9x10-2 moles), 15 hours.
b: Percent conversion is based on the total of oxidized products divided by the initial amount
of substrate.























Temperature Dependence of
cis-[Ru(dmp)2(H20)]2* with CuCl2 (1 mol eq)


0 25 50 75 100
Temperature (Degrees Celcius)
---Alcohol ---Aldehyde ---Ketone ---Total Moles -- Percent Conversion (2nd)


Figure 3-5: Oxidation of Propane @ Varying Temperatures with cis-
[Ru(dmp)2(H20)](CF3SO3), and CuC12 using H202








Conclusions


The oxidizing ability of the cis-[Ru(dmp)2(H20)21(CF3SO3)2 complex to activate

linear and branched saturated hydrocarbons has been investigated. Using hydrogen

peroxide as the oxidant, production of the respective alcohols and their over-oxidized

products are seen after 15 hours at 750C in an acetonitrile solution. An example of the

oxidizing power of this system is shown by the 20.1% conversion of ethane to ethanol

(2.3% selectivity, 0.08 mmoles), acetaldehyde (69.3% selectivity, 2.5 mmoles) and acetic

acid (28.4% selectivity, 1.0 mmoles).

To deter production of the over-oxidized products and increase the selectivity of

the hydroxylated products addition of one mole equivalent of CuC12 was performed. The

addition of CuCIl, illustrates a pronounced effect towards increasing the selectivity to the

alcohols. Successive additions of CuCl2 mole equivalents also exhibit a further increase

in alcohol selectivity. Blank experiments performed also illustrate the ability of CuC1 to

retard further oxidation of the alcohol. Additional blank experiments performed in the

absence of catalyst, produce small quantities of oxidized products demonstrating the

catalyst is responsible for the majority of the oxidation.

Temperature dependence studies were also performed with and without the

addition of CuCl. The effect observed is the ability of this catalyst to produce

oxygenates of propane at 250C (3.2% conversion) to 1000C (16.7% conversion). The

addition of CuCI2 to the oxidations performed at variable temperatures also allowed for

increased selectivity of the alcohol.













CHAPTER 4
SYNTHESIS AND CHARACTERIZATION OF IRON DIMETHYL
PHENANTHROLINE COMPLEXES

Introduction


As previously discussed, a high valent metal-oxo complex is required for the

activation of alkanes. The cis-[Ru(dmp)2(H20)2]2+ precursor is converted to cis-

[Ru(0)2)2+to attain this high oxidation state moiety.40,74 This oxidant effectively oxidizes

alkanes with hydrogen peroxide, and methane with hydrogen peroxide and dioxygen.73

The success with this complex prompted research directed towards the synthesis of

analogous complexes in an attempt to prepare a more robust catalyst. This complex

should be able to utilize dioxygen without the need of a co-oxidant and be less expensive.

First row transition metals might produce a complex with these advantages, therefore the

synthesis of an iron based catalyst was investigated.

Iron containing complexes including heme and non-heme enzymes are reported to

effectively oxidize alkenes and alkanes.81,82,83 The development of catalytic systems to

mimic a variety of biological systems has been the subject of intense investigation.

Cytochrome P-450, peroxidases, catalysases and high valent iron-oxo porphyrin

complexes, all involve a 2-electron oxidation producing the reactive intermediate in

hydrocarbon oxidation reactions.69,84








The use of iron porphyrins as model catalysts has allowed for further

understanding of significant steps, which are involved in many enzymatic oxidation

reaction mechanisms.85 More recently, Watanabe and Morishima82 have demonstrated

the use of a (TMP)Fem(RCO2), where TMP is 5,10,15,20-tetramesityl porphyrin, complex

for the epoxidation of norborylene and a-methylstyrene at -780C with a variety of

peracids. This research has suggested the O=Fe"(TMP) 7-cation radical as being the

active oxidant for this oxygenation reaction.

Catalysts based on iron complexes in the absence of porphyrins have also been

investigated. Que83 has demonstrated the reactivity of (j-oxo) di-ferric complexes with

t-BuOOH for the activation of cyclic hydrocarbons in acetonitrile. The p-oxo di-ferric

complex was synthesized as an effort to model the dinuclear sites found in non-heme

iron-enzymes. This [Fe(TPA)20(OAc)](C103)3, where TPA is tris-(2-pyridyl)-methyl

amine and OAc is acetate, complex is demonstrated to be a robust catalyst for the

activation of cyclohexane at ambient temperature and pressure. The oxidation products

produced from this reaction are cyclohexanol, cyclohexanone and (t-butylperoxy)-

cyclohexane. Que has suggested the formation of the alcohol and ketone occur by

catalyst initiated decomposition of t-BuOOH to afford a high valent metal-oxo complex

via a heterolytic pathway. Additional research presented by Que72 has been directed

towards the further investigation of these non-heme iron centers. Furthermore, Que has

proposed a high valent Fe2(P-O)2 diamond core structure, which is believed to be the key

oxidizing species of methane monooxygenase (MMO).








Further research using iron coordinated ligand type complexes have also been

reported. Sawyers' has reported a number of iron (II) complexes [FenL.); Fe(DPAH)2,

where DPAH2 is 2,6-dicarboxylpyridine, Fe"(PA),, where PA is picolinic acid, and

Fe"(bpy)2', where bpy is 2,2'-bipyridine. Each of these complexes with the addition of a

reductant, [DH2:PhNHNHPh] for example, are able to catalytically activate 02 (1 atm) for

the hydroxylation of phenol and substituted phenol. Results provided indicate the

mechanism of oxidation is proceeding via a Fenton like intermediate.

The synthesis of an iron "analogue" based on the cis-[Ru(dmp)2(H20)2](CF3SO3)2

complex is the focus of our research. By altering the metal center it is anticipated a more

robust catalyst will result. The ligand was not altered due to its ability to impart a cis

geometry40, with respect to the two remaining coordination sites, when the metal is

coordinated by two dmp ligands. This geometry, the result of steric hindrance created by

the methyl groups a to the nitrogen atoms results in the cis isomer being a more powerful

oxidant when compared to the trans counterpart.74 The lesser oxidizing power of the

trans isomer, measured to be on the order of 35 kcal mol', 86 is attributed to the lower

energy of the HOMO d,.40 Additional benefits of this ligand include its ability to

prevent the condensation of hydroxo or oxo bridged dinuclear species when two ligands

are present on the metal center.

Based on accounts reported in literature and the success of the cis-

[Ru(dmp)2(H2O),]2 complex, the synthesis of the iron analogues was attempted.

Characterization of these newly formed complexes, as well as investigation for the







production of a high valent iron-oxo species, for the eventual use as an oxidation catalyst,

was also performed.

Experimental


Materials and Methods

FeCI2 4 HO0, 2-9-dimethyl-l,10-phenanthroline, NaCF3SO3 and LiCI were all

used as received from Aldrich. Acetonitrile, acetone and 30% hydrogen peroxide (aq)

were all used as received from Fisher Scientific (ACS Grade). Acetonitrile was distilled

over P20, and stored over 4A molecular sieves.

Physical Measurements

UV-Vis measurements employed a Perkin Elmer lambda-6 spectrophotometer, all

spectra were background corrected. Infrared Spectroscopy (IR) analysis were all

collected either as Nujol mulls or KBr pellets on a Nicolet 5DXB spectrometer and were

background corrected. Nuclear Magnetic Resonance (NMR) measurements were

recorder on a multinuclear Varian VXR 300 MHz spectrometer or a multinuclear Gemini

300 MHz spectrometer. All samples analyzed by NMR were performed in deutrated

solvents with 1% w/w TMS. Electrochemical studies were performed with a PAR model

175 Universal Programmer connected to a model 173 potentiostat/galvanostat. The

electrodes used were a platinum working and auxiliary electrode, with an Ag/AgCI

reference electrode. The pH measurements were made with a Fisher Accumet model 630

pH meter. FAB (+ and -) mass spectral data were obtained by Dr. David Powell (U.F.) in

a m-nitrobenzyl alcohol matrix. Single crystal X-ray diffraction analysis were performed

by Dr. Kalih Aboud using a Nicolet diffractometer equipped with a graphite-








monochromated Mo-Ka radiation source. The Nicolet Structure Determination Package

was used for data collection, data recovery and structure elucidation. U.F. Analytical

Services performed elemental analysis.

Synthesis of Compounds

Iron(II)bis(chloride)mono(2,9-dimethyl-1,10-phenanthroline), [Fe(dmp)Cl2]

(I). A 2.25 g (12 mmol) of 2,9-dimethyl-l,10-phenanthroline was dissolved in 100 ml of

acetonitrile under nitrogen at 700C. Next, 1.25 g (6 mmol) FeClI 4H20 was added to

the solution. Upon addition of the metal a precipitate is formed immediately. The

resulting mixture is allowed to react for 30 minutes, the solution cooled and filtered. The

product is then dried under vacuum at 600C overnight. Analysis: Calculated for

C,4H,2N2C12Fe: C, 50.29; H, 3.59; N, 8.38. Found: C, 50.12; H, 3.46; N, 8.23.

Cis-Iron(II)bis(aquo)bis(2,9-dimethyl-1,10-phenanthroline)bis(trifluoro-

methanesulfonate), cis-[Fe(dmp),(HIO):(CF3SO3)2 (II). This complex was

synthesized using modification of a prior method.86 A 4.5g (24 mmol) of 2,9-dimethyl-

1,10-phenanthroline was added to 150 ml of deionized H20 at 900C and allowed to stir

vigorously for 20 minutes under nitrogen. Upon complete dissolution of the ligand, 2.5g

(12 mmol) of FeC12 4 H20 was added, the resulting solution was then stirred for 2 hours.

The solution is then filtered (hot) and the filtrate immediately added dropwise to a chilled

saturated NaCF3SO3 (aq) solution (25ml H20 / 9g NaCF3SO3). The resulting precipitate

is allowed to stand in ice for 2 hours and filtered. The product is dried under vacuum at

600C overnight. Analysis: Calculated for C301HN40,F6S,Fe: C, 44.66; H, 3.47; N, 6.94.

Found: C, 44.87; H, 3.33; N, 7.01.








Results and Discussion


Characterization

A number of analytical techniques were utilized to characterize the complexes (I

and II) synthesized in this chapter. The conditions employed for each spectroscopic

technique are described within the experimental section of this chapter. Any variation in

analysis will be described in the appropriate section. Elemental values obtained are

provided at the end of each synthesis procedure.

Single Crystal X-ray Diffraction

Single crystal X-ray diffraction analysis was collected for the mono-2,9-dimethyl-

1,10-phenanthroline (dmp) complex, Fe(dmp)Cl2 (I) to provide structural information and

confirm results obtained from elemental analysis. A brown-orange needle of I (0.26 x

0.19 x 0.16mm) was mounted on the end of a glass capillary tube and analysis performed.

The crystal structure and crystal data obtained are provided in Figure 4-1 and Table 4-1.

Additional information for this structure is given in Tables 4-2, 4-5 and 4-6.

In this structure iron has a four coordinate tetrahedral geometry with two

coordination sites occupied by the nitrogen atoms of the dmp ligand and the remaining

bound to the chlorine atoms leading to a 2* oxidation state. The coordination of only one

dmp ligand leads to the possibility of u-oxo dimer formation when this complex is

oxidized in the presence of others.
















































Figure 4-1: Crystal Structure for Fe(dmp)C1,.








Table 4-1: Crystal Data and Structure Refinement for Fe(dmp)Cl2.


Empirical formula

Formula weight

Temperature

Wavelength

Crystal system

Space group

Unit cell dimensions



Volume, Z

Density (calculated)

Absorption coefficient

F(000)

Crystal size

Theta range for data collection

Limiting indices

Reflections collected

Independent reflections

Absorption correction

Max. and min. transmission


C14H12CI2FeN2

335.01

173(2) K

0.71073 A

Orthorhombic

Pnma

a =11.2265(7) A a =90*
B = 7.4630(5) A p = 90"
C= 17.788(1)A y =90"

1490.3(2) A3, 4

1.493 Mg/m3

1.356 mm-1

680

0.26 x 0.19 x 0.16 mm

2.15 to 27.50 deg.

-5<=h<=15, -4<=k<=10, -24<=1<=18

5761

1841 [R(int) = 0.0451]

Semi-empirical from psi-scans

0.9660 and 0.7631




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