Mechanistic and site isolation studies of transition metal oxidation catalysts

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Mechanistic and site isolation studies of transition metal oxidation catalysts
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vii, 145 leaves : ill. ; 28 cm.
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Pribich, David Chappel, 1957-
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Transition metal catalysts   ( lcsh )
Chemistry thesis Ph. D
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Thesis (Ph. D.)--University of Florida, 1985.
Bibliography:
Bibliography: leaves 139-144.
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by David Chappel Pribich.
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Typescript.
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Vita.

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Full Text













MECHANISTIC AND SITE ISOLATION STUDIES
OF TRANSITION METAL OXIDATION CATALYSTS





By

DAVID CHAPPEL PRIBICH


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


1985
























To Susan, Jean, Mitch and Pat Pribich
















TABLE OF CONTENTS

PAGE
ABSTRACT . . .

CHAPTER

I GENERAL INTRODUCTION . ... 1

II THE SPECIFIC OXIDATION OF [Rh(CO) C1] BY
O VIA THE COORDINATION OF IN SITO GENERATED
HiDROGEN PEROXIDE . 3

Introduction . . 3
Background . . 5
Results and Discussion . .. 8
Characterization of the Catalyst
as a Rhodium(III) Chloride Complex 8
Mechanism of the O Oxidation of
[Rh(CO) C12 to Rh~dium(III)
Chlorid . . 19
Conclusion . . .. 43
Experimental Section . ... 44
Catalytic Oxidation of 1-Hexene .... 45
Preparation of RhCl3(H20)2CH3-
CH OH (II) . . 46
Determination of Acetone Production. 46
Titration of [Rh(CO)2C112 with
HOOH . .. .. . 47

III ENHANCED SITE ISOLATION ON SILICA GEL AND
IMPROVED LIFETIMES OF SITE ISOLATED CATALYSTS 49

Introduction . ......... 49
Results and Discussion ... 55
Polymer Support of the Catalyst 55
Catalytic Oxidations of 1-Hexene by
Supported Complexes . .. 59
Alkyl Covering of the Silica
Surface to Improve Site Isolation
and Catalysis . 67
Catalytic Oxidations Using Alkylated
Silica Gels as Solid Supports .. .. .70
Possible Effects of a Different
Rhodium Catalyst Characterization .. 84
Conclusions . 84


iii












Experimental . .
General Procedures . .
Preparation of Silica Gel Supports .
Determination of -SH on Silica Gel .
Catalytic Oxidations of 1-Hexene .

IV THE SYNTHESIS AND CATALYTIC APPLICATIONS OF A
MULTIDENTATE LIGAND AND CORRESPONDING METAL
COMPLEXES BOUND TO SILICA GEL .


Introduction ..
Results and Discussion ..
Synthesis of a Salen Ligand on
Silica Gel . .
Oxygen Transfer Using [SGl-Fe(III)-
SalDPT and [SG]-Mn(II)SalDPT ..
Synthesis of [SG]-Fe(II)SalDPT .
Incorporation of an Active Metal
into a Functionalized Support .
Synthesis of a Silica Gel Anion


Exchange Resin .
Titanium Carbide as a Solid
Support . .
Fenton Chemistry .


Experimental . .
Silica Gel . .
Functionalization of Silica Gel
with l-trimethoxysilyl-2-(p,m,-
chloromethylphenylethane) .
Preparation of Silica Gel Bound
3,3'-Iminodipropionitrile .
Preparation of Silica Gel Bound
bis-(3-aminopropyl)amine [SG -DPT
Condensation of Salicylaldehyde with
[SG -DPT . .
Incorporation of Metal Ions into
iSGl-SalDPT .
Incorporation of Fe(II) into [SG]-
SalDPT . .
Incorporation of Cu(II) into the
Silica Gel Matrix .
Preparation of Silica Gel Anion
Exchange Resins . .
Titanium Carbide as a Solid
Support . .
Fenton Chemistry . .


101
107

110

117

118
127
128
128


129


. 129

. 130

. 131

. 132

. 132

S 133

134


135
137


S. 138


139

145


V GENERAL CONCLUSION . .


REFERENCES . . .


BIOGRAPHICAL SKETCH


iv


* *


. .
. .








* .
. .


.*
.*


* .


* .
. *














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


MECHANISTIC AND SITE ISOLATION STUDIES
OF TRANSITION METAL OXIDATION CATALYSTS





By

David Chappel Pribich

May, 1985


Chairman: Russell S. Drago
Major Department: Chemistry


Mechanistic work reported here involves the investi-

gation of the oxidation of 1-hexene to 2-hexanone catalyzed

by Rh(III)/Cu(II) mixtures. A number of results are obtained

that indicate that [Rh(CO)2C1]2 as a catalyst precursor must

be converted to a rhodium(III) complex before catalysis

occurs. The oxidation of the rhodium(I) precursor to the

active rhodium(III) catalyst in the absence of copper is

reported in detail. An unusual mechanism results which

involves the in situ production of hydrogen peroxide from the

alcohol solvent reduction of 02. The hydrogen peroxide then

oxidizes [Rh(CO)2C1]2 to an unstable [Rh(CO)(OOH)(?)]










intermediate. This oxidation occurs only in solvents

capable of reducing 02.

Cofunctionalization of silica gel with trialkylchloro-

silanes and (CH30)3SiCH2CH2CH2SRh(CO)2 produces a catalyst

for the oxidation of 1-hexene that can be compared with a

catalyst that does not have an R3Si- covering. Physical

methods are used to determine the concentration of groups on

the silica surface and also to determine the loading at

which site separation occurs. The alkylated supports can be

loaded with a greater concentration of the site-isolated

rhodium complex producing catalysts which have greater

activity per gram of catalyst and longer lifetimes than the

corresponding catalysts produced with non-alkylated silica

gels. Increasing the length of the alkyl group used to cover

the surface decreases the effectiveness of the silica as a

catalytic support and eventually leads to a catalyst surface

that is not wetted by ethanol solvent.

A three nitrogen, two oxygen salen-type ligand was

successfully synthesized on silica gel. Cobalt, copper,

manganese and iron metal ions were incorporated into the

supported ligand system. The iron(III) and manganese(II)

systems achieved oxygen atom transfer from iodosylbenzene to

a cyclohexene substrate producing different product ratios

than homogeneous tetraphenylporphyrinato metal systems have

yielded and are also the first supported systems to

achieve this process.










Several other results are reported, including the

preparation of an air sensitive iron(II) silica gel bound

complex, incorporation of copper into the silica gel matrix,

preparation of a silica gel based anion exchange resin, and

examination of titanium carbide as a catalytic support.


vii
















CHAPTER I
GENERAL INTRODUCTION




The efficient conversion of ubiquitous substances on

this planet into other useful species is a desirable and

perhaps even honorable goal which correspondingly attracts

the efforts of large segments of the scientific community.

Specifically, a subset of this broad area is the efficient

catalytic use of naturally occurring dioxygen via interaction

with transition metal centers. The following chapters will

describe investigations which in some way involve this area

of research. A rather detailed study on the formation and

nature of a specific homogeneous rhodium oxidation catalyst

is presented in the second chapter.

The general interest in the Chapter II catalyst led to

work which supported the catalytic system on an inorganic

polymeric solid. Subsequent efforts to maximize the

efficiency of this supported system while gaining detailed

information on the support medium itself are represented in

the third chapter.









2

A general broadening of study to include different

metals, ligands, substrates and even solid supports led to

further research; the various results of which are detailed

in the fourth chapter.

Specific background information and introduction is

included with each topic as it is discussed. The general

introduction provided by this brief overview should then

allow comprehension of the pattern of research on

interrelated topics presented in the following.














CHAPTER II
THE SPECIFIC OXIDATION OF [Rh(CO) Cl]
BY 0 VIA THE COORDINATION OF IN %ITL
GENERATED HYDROGEN PEROXIDE




Introduction



Homogeneous catalysis has developed rapidly during the

last twenty years as organometallic chemistry has been firmly

established as a major discipline within inorganic chemistry.

The search for new and more efficient catalysts was

intensified as the finite nature of the crude oil supplies

which provide the feedstocks of many important chemical

processes was more closely realized. Oxidations have

specifically received much attention; there have been

estimates that an oxidation step is involved in the

production of over 50% of the chemicals manufactured

industrially.1 Water, alkylperoxides and dioxygen are

frequently used as oxidation reagents. Dioxygen is the most

desirable one, not only due to its obvious availability, but

also because of its favorable energetic.








4

Homolytic and heterolytic are the two major categories

into which metal catalyzed homogeneous oxidations have most

often been divided.2'3 Homolytic oxidations are those in

which radical intermediates are produced during the oxidation

process. However, the drawback of most radical processes is

a lack of product selectivity, and that is also a major

problem in these oxidations. One reason for this lack of

specificity is that in many homogeneous metal catalyzed

oxidations of hydrocarbon by 02 the metals are involved in
4-9
just the first step of reaction.4-9 These metals decompose

peroxides and thus initiate free-radical autoxidations.

These free radicals then react with substrates such as

olefins leading to the formation of several products.

Heterolytic oxidations, also known as nonfree-radical

oxidations, involve binding of a substrate to a metal center,

rearrangement, and later release of products. The Pd/Cu

catalyzed oxidation of ethylene to acetaldehyde in the

presence of water0 (the Wacker process) is one well-studied

example. The systems which undergo this type of oxidation

have been the focus of much research aimed at adjusting the

metal centers in order to improve catalytic activity. This

has been especially true since the discovery of Vaska's

complex, Ir(P(C6H5)3)2(CO)Cl, and the realization that it is

able to reversibly bind dioxygen.11 This "oxygen-atom

transfer process" does not involve free radicals and results

in product specificity. Since Vaska's complex was discovered

several 02 oxidations of non-organic substrates have been










12-19
judged to occur by the O-atom transfer process.12-19 General

knowledge of this type of reaction is largely dependent upon

the ability to procure detailed mechanistic information. A

mechanistic investigation of one reaction will be presented

in the following sections.



Background



Considering specifically rhodium catalyzed oxidations

of hydrocarbons by dioxygen, several examples have been

reported which seemingly do not proceed by either free

radical or simple Wacker processes.20-24 One of these is

the rhodium/copper co-catalyzed oxidation of terminal olefins

to 2-ketones with >98% specificity20 (reaction 1).



2CH2=CHR + 02 2CH3C(O)R (1)



The Wacker-Smidt process refers to the palladium(II)/-

copper(II) co-catalyzed oxidation of olefins to ketones and

aldehydes using water as the direct oxidant.25

Palladium(0) is produced as an intermediate in this process,

and the copper(II) is necessary to oxidize it back to Pd(II),

the active catalyst. Only a stoichiometric reaction occurs

and Pd(0) metal is produced if no Cu(II) is used. The

general steps of the mechanism and the net reaction are shown











below (reactions 2-5) for the catalytic oxidation of ethylene

to acetaldehyde.



Pd(II) + H20 + CH2=CH2 Pd(0) + H3CCHO + 2H+ (2)

Pd(0) + 2Cu(II) Pd(II) + 2Cu(I) (3)

2Cu(I) + 1/2 02 + 2H 2 Cu(II) + H20 (4)



Net: CH2=CH2 + 1/2 02 + CH3CHO (5)



Water is the direct oxidant here, even though 02 is consumed.

Dioxygen is reduced to H20 by Cu(I), which reforms the Cu(II)

necessary for oxidizing Pd(0) to Pd(II). Other processes

which are thought to proceed in a similar manner are referred

to as having a "Wacker" mechanism.

Mimoun and co-workers put forth great effort to rule

out a Wacker-type mechanism for reaction 1. Several

experimental observations could not be rationalized by a

Wacker mechanism.20 Thus, despite apparent similarities

between Mimoun's system and the earlier rhodium/copper

catalyzed system which is reported to have a Wacker-type
27
mechanism,27 the differences are great enough to rule out the

hydrolysis process found in Wacker chemistry for Mimoun's

system.

Mimoun proposed that the catalyst in the Rh(III)/Cu(II)

oxidation system is a rhodium(I) complex produced from the

ethanol reduction of RhC13 and Cu(II) in the initiation step

shown in reaction 6.












RhCl3 + 2CH2=CHR + Cu(Cl04)2 + 1.5CH3CH OH -

[Rh(CH2=CHR)2]C104 + CuCl() + HCIO4 + 2HC1 +

1.5 CH3CHO (6)



The observations which lead to this proposal by Mimoun

are the following: (1) active catalysts are formed by using

[Rh(CsH14)2C1]2 in the presence of 2 equivalents of HC1

instead of RhC13.3H20; (2) an amount of acetaldehyde

approximately equal to the moles of catalyst used is produced

from ethanol oxidation at the beginning of the catalytic

reaction; and (3) 85% of the Cu(II) precipitates as CuCl at

the beginning of the catalytic 1-hexene oxidation.

Initiation of the catalysts would then be possible by the

binding of dioxygen to the reduced rhodium species. From

reaction 6 it can be seen that there are at least two

possible needs for an equivalent of Cu(II) to produce the

best catalytic conditions. The copper(II) is either

necessary for the reduction of RhCl3 3H20 to Rh(I) with the

stoichiometric production of CuC1, or for the removal of

chloride ion from solution to yield a more unsaturated

rhodium center. A combination of both roles for copper is

possible, and Mimoun did not eliminate some other possible

need for copper.

The mechanism Mimoun proposed involves a metal centered

oxygen atom transfer involving coordination of dioxygen and










olefin to the rhodium(I) cation from reaction 6, a

rearrangement to a peroxometallocycle, and then a decom-

position to the rhodium(I) complex and the oxidation

products. Figure 1 shows this mechanism. It was based upon

Mimoun's suggestion that the active catalyst was a rhodium(I)

complex that led to the preparation of a silica gel organo-

sulfide supported rhodium(I) complex. This heterogenized

complex produced an active and stable catalyst for the

oxidation of 1-hexene to 2-hexanone28 videe infra, Chapter

III).



Results and Discussion



Characterization of the Catalyst as a Rhodium(III) Chloride
Complex

The rhodium carbonyl dimer (A), [Rh(CO)2C11]2 was

selected as the catalyst precursor due to the facile loss of

the CO ligands in the presence of 02, the presumed need for a

rhodium(I) catalyst to coordinate 02 and olefin, and the

usefulness of the CO ligands for infrared analyses.

Employing A as a homogeneous catalyst for reaction 1 at

various chloride concentrations and in the presence of Cu(II)

at 70 C, the data in Figure 2 were obtained. Note that the

maximum initial 1-hexene oxidation rate (indicated by the

amount of 2-hexanone produced) is achieved using a

chloride/rhodium mole ration of 3:1, as was originally

































Figure 1.


The mechanism proposed by Mimoun to account
for the Rh(III)/Cu(II) catalyzed 0
oxidation of 1-hexene to 2-hexanona.












RhCIt


-CH3CHO
-2HCI
-CuCt
R-CH=CH


+CH3CH2OH
*RCHzCHa
+CuX


R-C-CHM t
S Rh X(R-CH=CHI'
0

H C-R
C-H
x1 'H
c



XRh
y H %H


R-CHsCH IOH
XRh OH
Y Y


+HY


X.ClO;or No;
Y.Ct or CtO;

r +02

0
IO
0
X~h R
H Xih R
-c'c
H 'H



0-0 R
XRh C
C H
H' H


/
R-C-CH3
0I
0





























(C
C0
Q)
) 00
* .C,
O :: -
.Crl 0C






)41 -40 c
U O -4L
. O




OCU











0.,'4 U)
41 0- *

X, 0





-4 4 0 ->
w -4 0 CM4


OC) 0
0 r-O 4 (O
0 -4 U r-4
4-) r-4 03
U 0-*o >
a) -4 ja r- -4
44 41 (0
(d 4- j






--4
tnl~w












































uE
LL
W


uouex&U-Z opujuO








13

reported for this system at 400C. However, increasing the

chloride/rhodium ratio to 5:1 at 700C causes a considerable

increase in catalyst stability with little change in the

initial rates, in contrast to the result obtained at 400C,

where a large drop in activity was found for the 5:1 ratio.

(For example, after 20 h at 70C with a 3:1 Cl/Rh mole ratio,

the catalytic activity is only 7.5% of the initial value,

while after the same time period with a 5:1 Cl/Rh ratio the

activity is 34% the initial rate.) Increasing the

chloride/rhodium ratio to 10 causes an even further increase

in catalyst stability at 70C (the activity after 20 h is 51%

of the initial rate). A similar beneficial effect of

increasing the Cl:Rh ratio to 5:1 was observed in analogous

experiments with Cu(II) absent. This suggests that a

specific interaction of chloride with rhodium and not copper

leads to catalyst improvement. Furthermore, the marked

dependence of both initial rate and catalyst stability on

such large chloride concentrations suggest a rhodium(III)-

chloride interaction. These results, along with others to be

presented, indicate that a rhodium(III) chloride complex is

the catalyst or the immediate precursor.

Experiments were conducted investigating the effect of

dioxygen pressure on 1-hexene oxidation at 70C. The use of

[Rh(CO)2C112, H2SO4 and NaC1 to catalyze the production of

2-hexanone from 1-hexene can be accomplished in the absence

of Cu(II). These results, along with the existence of an

induction period, are shown in Figure 3. Also demonstrated



























Figure 3.


Demonstration of the dependence of the length
of the induction period on the pressure of
O in the absence of Cu(II) at 70 C,
using [Rh(CO)2C1]2 as the precursor. The 02
pressures are for A, 3.7 atm; B, 1.9 atm; C,
1.4 atm; and D, 1.0 atm.












6.0




4.0


2-0


12.0


4.0 8.0
hours










is the first-order dependence of the induction period on

dioxygen pressure. During the induction period the solution

color changes from light yellow (the color of [Rh(CO)2C1]2)

to bright orange (normal for rhodium(III) chloride

complexes).29 No induction period is seen when RhCl3 3H20

is used rather than A, nor when 0.1 equivalent or more of

Cu(II) is added to the reaction mixture. A proposal

suggested by these experiments is that of the dioxygen

oxidation of [Rh(CO)2Cl]2 to a rhodium(III) chloride complex

(which is either the active catalyst or an immediate

precursor) during the induction period.

Further investigation of this system involved the

reaction of A, dioxygen, and HC1 in ethanol, at 400C in the

absence of 1-hexene substrate. A bright orange complex (C)

was isolated as RhC13(H20)2-CH3CH2OH after 12 hours of

reaction. This process is described by reaction 7.


[Rh(CO)2Cl]2 + 4HC1 + 202 + 4CH3CH20H -

2RhC13(H20)2'CH3CH2OH + 2CH3CHO + 4CO (7)



The rhodium product (C) was characterized by elemental

analysis, molecular weight determination, and its infrared

and visible spectra.30 The stoichiometry for reaction 7 was

verified in both [Rh(CO)2C1]2 and HC1, but this was not

possible for 02, CH3CH2OH, and CH3CHO due to the catalysis of

reaction 8 by C videe infra). The addition of an equivalent










2CH3CHROH + 02 2CH3CRO + 2H20 (8)

R CH3 or H



of Cu(II) or the use of [Rh(C8H14)2C1]2 as the rhodium(I)

precursor considerably speeds up reaction 7 videe infra) but

does not affect its outcome. Compound C catalyzes the

oxidation of 1-hexene to 2-hexanone as efficiently as

RhC13.3H20 and without the occurrence of the induction period

observed when using [Rh(CO)2C1]2 as the catalyst precursor

under identical conditions (see Figure 3). The observed

solution color changes and other results described above

indicate that under reaction conditions for the catalytic

oxidation of 1-hexene (reaction 1), the rhodium(III) complex

(C) is produced from the oxidation of [Rh(CO)2C112. An

explanation for the role of copper(II) and a more detailed

study of reaction 7 will be discussed in the second part of

this chapter.

To rule out the possibility that RhC13 3H20 (or C) may

be subsequently reduced to a rhodium(I) complex by alcohol

solvent to initiate catalysis (as proposed by Mimoun in

reaction 6), we investigated the initiation step using

RhCl 33H20 and Cu(NO3)2.2.5H20 as precursors. For our study

we chose isopropyl alcohol as solvent since upon oxidation

this alcohol forms acetone which is much easier to

quantitatively measure than the more volatile acetaldehyde

(produced from ethanol). Using GLC, we found that in the










presence of 1 equivalent of Cu(II) only 0.5 equivalents of

acetone is produced immediately on mixing all reagents

necessary for the catalytic oxidation of 1-hexene and that

its concentration remains constant for at least 0.5 h. From

this result we can rule out the simultaneous reduction of

Rh(III) and Cu(II) proposed in reaction 6 because this would

require the oxidation of 1.5 equivalents of isopropyl

alcohol. We propose that reaction 9 initiates the catalytic

cycle for this system




2+ 1 atm 0d at 40C
Cl + Cu + 0.5(CH3)2CHOH isopropyl alcohol
3 2 isopropyl alcohol
1-hexene

CuCl(s) + 0.5(CH3)2CO + H+ (9)



when RhC13.3H20 is used as the catalyst precursor. Our

proposal requires that only enough reducing equivalents are

provided by isopropyl alcohol for the reduction of Cu2+ to

form CuCl. Copper(I) chloride may be isolated from the

reaction mixture in 85% yield without affecting the catalytic

oxidation.








19

Mechanism of the 02 Oxidation of [Rh(CO)2C1]2 to Rhodium(III)
Chloride


The oxidation of [Rh(CO)2C1]2 (A) to rhodium(III)

trichloride (C) by 02 was originally studied because of our

interest in characterizing the active catalyst for the

Rh/Cu-co-catalyzed 1-hexene oxidation (reaction 1).

Copper(II) was excluded in these initial investigations to

facilitate the interpretation of the electronic absorption

spectra, elemental analysis and molecular weight data. The

substrate 1-hexene was excluded from these solutions to avoid

its catalytic oxidation to 2-hexanone after the formation of

the rhodium(III) chloride product. As described in the

previous section, the exclusion of Cu(II) and 1-hexene during

these studies proved to be quite useful for characterizing

the rhodium catalyst. The oxidation of [Rh(CO)2C1]2 to

rhodium(III) chloride was generally monitored by electronic

absorption spectroscopy in order to determine the reaction's

end point. In the course of these studies it was found that

at elevated 02 pressures (3-5 atm) an unusual intermediate

exhibiting a visible absorption band at 385 nm could be

detected. This observation led to a detailed investigation

of the mechanism of the 02 oxidation of [Rh(CO)2C112 to

rhodium(III) chloride in reaction 7, and the results are

presented below.

The electronic spectral changes accompanying the

oxidation of the [Rh(CO)2C1]2 (A) to rhodium(III) chloride

(C) in the absence of both Cu(II) and 1-hexene is shown in










Figure 4 as a series of spectra recorded over the course of

the reaction. The growth and decay of an intermediate (B)

with an absorbance at 385 nm is noted. The charge-transfer

band beginning at the shortest wavelength is a result of A,

while the band at 480 nm is produced by C. As shown in

Figure 5, intermediate B is more stable in the presence of

excess Bronsted acid (8HC104/A). The presence of two

isosbestic points (at 377 and 435 nm) in both Figures 4 and 5

suggests the formation and subsequent reactivity of only one

intermediate. The isosbestic point at 377 nm is a result of

the reaction of A to form B at the beginning of reaction 7

(before much final product C has been formed). Early in the

reaction the decomposition of B to C becomes quite

pronounced, and this point disappears. The second isosbestic

point emerges at 435 nm when all A has been consumed and

results from the exclusive reaction of B to form C.

The initial rate of formation of B was found by visible

spectroscopy to follow the rate expression shown in eq. 10,


-d[A] or -d[B] = k[H][2] (10)
dt dt




which is independent of the concentration of A. The

zero-order dependence on [A] rules out a mechanism involving

initially the formation of a rhodium hydride followed by the

insertion of dioxygen to form a rhodium hydroperoxide
intermediate, which occurs with K[RhH(CN) 0)],31e and is
intermediate, which occurs with K2tRhH(CN)4(H20)], and is






























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25
32
proposed in two other studies. The rate law in eq. 10 is

surprising because it indicates that the initial step or

steps in reaction 7 (those including and preceding the

rate-determining step) involve a reaction between H+, 02, and

possibly CH3CH2OH, forming an intermediate oxidant which

subsequently reacts with [Rh(CO)2C1]2.

We investigated the possibility that peroxide could

play the role of direct oxidant in reaction 7. This was

confirmed by experiments involving the titration of A with

HOOH to form C in the absence of dioxygen, monitored by

visible spectroscopy (reaction 11, in which S' is solvent).



[Rh(CO)2C1]2 + 4HC1 + 2HOOH + S' -

2RhCl3(H20)2"S' + 4CO (11)

Because HOOH slowly disproportionate or is reduced in

ethanol, as well as being consumed in a competing side

reaction videe infra), the oxidation of A to C in this

solvent goes to only 70% completion. Very significantly the

oxidation of A by HOOH proceeds with the formation of the

same intermediate [B] as produced with 02 as oxidant,

indicating that reaction 11 and reaction 7 occur through

similar mechanisms. The kinetic rate law for reaction 11 was

determined by using Fourier transform infrared spectroscopy

(FT-IR) to monitor the consumption of A, and is shown in

equation 12. Several reports describe the reaction between a








26

-d[A]/dt = k( [Rh(CO) C112)[HOOH] (12)



metal complex and peroxide to form a coordination

compound.3ad The reaction in eq. 11 proceeding as

described above would exhibit the rate law shown in eq. 12.

On the basis of these arguments and those that follow, we

propose that intermediate B results from the coordination of

HOOH to [Rh(CO)2C]22. A proton NMR analysis at -70C showed

no evidence for B possessing a hydride ligand. Any hydride

species generated from the oxidative addition of HOOH to A

apparently are acidic, and fast exchange of all protons with

the alcohol solvent is very likely occurring.

The characterization of B was assisted by an FT-IR

spectroscopic study of reaction 11. Immediately on adding

HOOH to a solution of A and HC1 in ethanol, the formation of
-l
a complex exhibiting a CO stretching band at 2102 cm- is

observed, and this band increases in intensity at the expense
-i
of the bands due to A (at 1995 and 2069 cm ; Figure 6). The

growth and subsequent decay of the CO band at 2102 cm- was

found to correlate directly with the band due to B at 385.nm

observed in the electronic absorption spectra. Therefore,

intermediate B retains one CO ligand. The assignment of the
-i
band at 2102 cm to coordinated CO was confirmed by use of
1CO in the experiment.

That B is a hydroperoxo rather than a u-peroxo complex

is evidenced by our ability to substitute tert-butyl

hydroperoxide (t-BuOOH) for HOOH in reaction 11 to obtain a



























Figure 6.


Series of infrared spectra obtained from the
HOOH oxidation of [Rh(CO) C] 2 in ethanol.
(A) Spectrum of [Rh(SO)2Cf] Reaction was
run using 0.84 x 10- M [RhCO) C] .
Spectra were recorded at (B) 0.35 hgurs; (C)
0.55 hours; and (D) 1.83 hours.






28






A





8



C





D
nocar

*M^r~a








29

much slower reaction and the formation of an intermediate

analogous to B with an electronic absorption band at about

385 nm33 and an infrared CO stretch at 2099 cm-1, 3 cm-1

lower than that found for B with HOOH. Since t-BuOOH is not

known to bridge two rhodium species in the y-peroxo

configuration, we suggest that B is not a 1-peroxo complex.

Only a limited number of stable hydroperoxide and alkyl

peroxide complexes of the platinum metals have been reported5

(not including those with Schiff base or bio-type ligands),

and some of these are capable of oxidizing terminal olefins

to 2-ketones.31a,b Intermediate B is only formed in the

presence of HC1 and is not formed on substituting either

HC104 or N(C2H5) C1lH20 for HC1. Thus, both a proton and an

additional chloride are required in the formation of the

intermediate. We propose that intermediate B is

H2 [Rh(CO)Cl2- (OOH)], produced as shown in eq. 13. Use of

the oxidation



[Rh(CO)2C1]2 + 2HOOH + 2HCI -

2H2[Rh(CO)C12(OOH) ] + 2CO (13)



state formalism to describe these species is potentially

misleading. An oxidative addition of HOOH to a Rh(I) complex

produces HRh OOH. Deprotonation generates Rh OOH After

a number of attempts, we have not been able to observe the

0-0 stretch between 800 and 900 cm-1 expected for a

coordinated peroxo group in the infrared spectrum. This is










due to the low concentrations necessary to stabilize this

intermediate, the poor window in this region of the infrared

spectrum for ethanol solvent, and the hydrogen bonding in

this system which would broaden this band. When the

intermediate is generated with 02 and excess 02 is removed, it

spontaneously decomposes to rhodium(III) chloride over

several hours. This is consistent with the formulation of

this intermediate as H2[Rh(CO)Cl2(OOH)], for this species

possesses two oxidizing equivalents in the peroxo (or

hydroperoxo) ligand.

Intermediate B is not formed in the absence of HC1 in

reaction 11, but the reaction in eq. 14 occurs under argon in

ethanol solvent,


Ar
[Rh(CO)2C1]2 + HOOH 2CO2 + D (14)



producing free CO2 from the oxidation of a CO ligand and a

very deep brown rhodium(I) complex (D) exhibiting a broad CO

stretching band at 2048 cm-1. Thus, [Rh(CO)2Cl]2 could be

reformed from D by exposure to a CO atmosphere, and the

production of CO2 from the CO oxidation by HOOH is catalytic

in A in a CO atmosphere. This reaction has been previously
34
reported in benzene,34 where D precipitates. It is

reported34 that redissolving this solid in ethanol and

exposing it to CO leads to the formation of a rhodium(I)

carbonyl complex with a spectrum similar to A. Reforming

[Rh(CO)2C1]2 from D under CO is significant because it










indicates that D is a rhodium(I) complex and that the

irreversible oxidation of rhodium(I) to rhodium(III) by HOOH

in ethanol is slow relative to the oxidation of the CO

ligands to CO2 (which takes several hours). These results

further suggest that in the presence of HC1 the reaction

between A and HOOH produces the relatively stable rhodium(I)

hydroperoxo coordination complex B as an intermediate. In

the presence of HC1 reaction 3 is faster than reaction 14,

and the majority of HOOH is consumed to form B rather than

CO2 and D. That reaction 14 does occur to a small extent in

the presence of HC1 can be seen in Figure 6, in which the
-i
band at 2334 cm due to CO2 is evident. This competing

reaction accounts, at least in part, for the incompleteness

noted for the titration of A with HOOH in reaction 11.

The stabilization of B by excess Bronsted acid,

evidenced by a comparison of the electronic absorption

spectra in Figures 3 and 4, can be explained by considering

the deprotonation of the RhOOH group as the initiation step

for the reaction of B to form C. A further investigation of

the decomposition characteristics of B also proved to be

useful. Upon isolation of a mixture of B and C by quick

evaporation of a dilute solution to dryness under vacuum, B

decomposes within a minute to a wet solid. An infrared

spectrum of this product revealed a large concentration of

water in addition to a high-energy CO stretching band at

2132 cm- (shifted from 2102 cm- for B). These are the

results expected from the fast, autocatalytic decomposition








32

of a hydroperoxo complex. The reduction of the hydroperoxo

ligand by Rh(I) will produce H20 and a rhodium(III) carbonyl.

This oxidized metal complex will be much poorer at

back-bonding into the CO than was the rhodium(I) in B, and

therefore its stretching frequency would be shifted nearer to

that for free CO (at 2143 cm-). Finally, in Figure 7 are

presented data which illustrate the effect of increasing the

rhodium concentration on the rate of decomposition of the

intermediate. This rate was measured by monitoring the

electronic absorbance for C at 480 nm. It is evident that at

a threshold concentration in A between 0.98 x 101 M and 1.47
-3
x 10 M, the reaction of B to form C becomes autocatalytic

in character. At 0.98 x 10- and 0.68 x 10 M this reaction

follows a much more regular course. These results indicate

that a free radical decomposition of intermediate B is a

sustained process only above the threshold rhodium

concentration. Indeed, the electronic absorption spectra

indicate that at much high concentrations, reaction 7 is less

specific to the exclusive formation of C. In this case the

band at 480 nm for C is present only as a plateau due to the

absorbance of a secondary rhodium(III) product which absorbs

at lower wavelengths.

The characterization of intermediate B and the rate law

in eq. 10 have an importance not yet discussed. They

indicate that in reaction 7, hydroperoxide is initially

formed from the Bronsted acid catalyzed reduction by dioxygen

by ethanol or isopropyl alcohol solvents (eq. 15). The


























C
0
,-4 3M f

(4 0 .





*0 0 1




tOO *4



O00 O-
4 ) C
1 0u r- -

4-I .104 0
O4) 01
-44O .0

U 0 OC


0 0 0 0
0-444 C







-4 c*4 C4
0000U

0i m( Uo




W 44 0 C
0 0 U- -













































0 6


aouoqJosqV


N




- O












CH3CHROH + 02 ) CH3CRO + HOOH (15)

R E CH3 or H




oxidation of primary and secondary alcohols to aldehydes and

ketones using dioxygen has been known for some time.35'36

These oxidations may be divided into two categories: (1)

free radical initiated reactions, using asoisobutyronitrile

(AIBN) for example, and (2) metal-catalyzed oxidations. The

principal difference is that oxidations of the first type may

produce HOOH stoichiometrically (eq. 15) as long as

precautions are taken to stabilize hydrogen peroxide while

oxidations of the second type produce waterl0 (eq. 8),

because the metals which catalyze HOOH formation also

catalyze decomposition of hydroperoxides to H20 and 02 very

efficiently. Thus, there is literature precedence supporting

our proposal that HOOH is produced (albeit inefficiently)

with a Bronsted acid as a catalyst.37

Under our reaction conditions the HOOH concentration

reaches a very low steady-state value, accounting for the

slow initial rate of oxidation of A (eq. 7) and our inability

to detect HOOH by iodometric techniques in acidic ethanol

solutions under 02. However, we have obtained indirect

evidence of HOOH production in two series of experiments in

which H Cu(II) and Rh(III) were checked for their ability

to (1) speed up the oxidation of A (eq. 7) by producing HOOH








36

catalytically from the alcohol reduction of 02 and (2)

catalyze the oxidation of isopropyl alcohol (eq. 8) as

measured by acetone production. For the investigation of the

effect of these three reagents on the rate of oxidation of A

(eq. 7), we worked at 1 atm rather than at 80 psi of 02

(5.4 atm) because the reaction is much slower at this reduced

pressure. As shown in experiment D in Table I, no final

product is observed at 480 nm in the electronic absorption

spectra in the absence of HC104, Cu(II) or Rh(III) after 1

hour, and the reaction take 36 hours to come to completion.

In contrast, the addition of 0.1 equiv of Cu(II) or Rh(III)

at the beginning of the oxidation of A (eq. 7) results in a

much faster reaction, with Cu(II) being most efficient

(experiments A and B). Doubling the acid concentration also

speeds up the reaction but to a much lesser extent

(experiment C). From these data it appears Cu(II) and

Rh(III), and to a much lesser extent H catalyze the

production of HOOH from ethanol reduction of 02, and this

causes the increased rates observed for the oxidation of A

(eq. 7) in their presence.

Substantiation of this was found from our investigation

of the effectiveness of Cu(II), Ru(III) and H+ in catalyzing

the oxidation of isopropyl alcohol (eq. 8). Their

effectiveness at the beginning of this reaction follows the

order Cu(II) ^ Rh(III) >>H+, with rhodium resulting in 19

turnovers (acetone/rhodium) in 25 h and H producing only a

trace amount of acetone.38 The consumption of dioxygen when










Rh(III) was used for reaction 8 was also followed on a gas

burette for several hours and is linear over that time,

indicating this catalyst is not slowly.rendered inactive. It

is reasonable to suggest that HOOH, or peroxo metal

complexes, are formed as intermediates in reaction 8 and that

the efficient catalysis of this oxidation is evidence of

the ability of a reagent to catalyze the production of

peroxide from the alcohol reduction of 02. Because both

Cu(II) and Rh(III) are efficient hydroperoxide decomposition

catalysts, the HOOH is produced at a very low steady-state

concentration and may in fact never leave the coordination

sphere of the catalysts.

Experiment E in Table I demonstrates unequivocably that

an intermediate oxidant (HOOH) is formed in the presence of

only HC1, HC104, ethanol, and 02. This has been labled an

"incubation" experiment because it involved stirring 3.2 x
-3
10 M each of HC1 and HC104 in ethanol under 1 atm of 02 at
4 2
40C for 48 hours, followed by the addition of the [Rh(CO)2

C1]2 to initiate reaction 7. In this case upon adding A no

induction period was observed and the reaction was complete

in only 7 hours compared to the 36 hours necessary when

mixing all reagents from the start. In agreement with the

rate law in eq. 10, this further indicates that the first

step in the oxidation of A involves the production of HOOH

from H 02 and CH3CH2OH and that hydroperoxide is the

reagent directly responsible for the oxidation of [Rh(CO)2











Table I. Oxidation of [Rh(CO)2Cl]2.


Relative
Absorbance Time for
S for Complex C Completion of
Experiment Catalyst at 480 nm at 1.0 h Reaction 7


A 0.15 mM Cu(II) 14 4


B 0.17 mM Rh(III) 3 10


C 4.2 mM HC104 0b 14


D None 0 36


E "Incubation"c 1 7



aThese reactions were run at 40 C and 1 atm of 02, using
-3 -3
0.80 x 10 M [Rh(CO)2C1]2, 3.2 x 103 M HC1, and 22.5 mL of
ethanol as solvent, in addition to the catalyst listed.
bNo final product II could be detected after 1.0 h.

cThis run involved stirring 3.2 x 10-3 M HCl and
3.2 x 10-3 M HC104 in ethanol at 40 C and 1 atm of 0 for
48 h, followed by the addition of [Rh(CO)2Cl]2 to a
-3
0.90 x 10 M concentration.










C1]2 to rhodium(III) chloride in ethanol via a hydroperoxo-
rhodium complex intermediate. Our proposed mechanism for

this oxidation is shown in Scheme I and is substantiated by

all of the preceding evidence and arguments.




Scheme I


CH CHROH + O2 CH CRO + HOOH (a)



[Rh(CO)2Cl]2 + 2HC1 + 2HOOH
2- +
2[Rh(CO)Cl2(OOH)]2- + 4H + 2CO (b)


[Rh(CO)C1(OOH)]2- + 2H+ [Rh(CO)C(OO)]3- + 3H+ (c)



[Rh(CO)C12(00)]3- + 4H + Cl- + CH3CHROH -

RhCl3(H20)2 CH3CHROH (d)


An intermediate analogous to B is not produced when

[Rh(C8H14)2C1]2 is used instead of [Rh(CO)2C1]2 as the

rhodium(I) starting material in a reaction similar to that in

eq. 7. In this case the reaction to form rhodium(III)

chloride is complete in only 40 min (compared to 12 hours

under identical conditions with A), and no evidence for

intermediates is found in the electronic absorption spectra.

In contrast, [Rh(P(tolyl)3)(CO)C1]2 is not oxidized to

rhodium(III) chloride even after 48 hours.












Since Scheme I requires solvent reducing equivalents to

produce HOOH and subsequently [Rh(CO)C12(OOH)] from 02, any

solvent capable of this 02 reaction could lead to the

oxidation of A by this mechanism. Indeed, we have found that

methanol, ethanol, isopropyl alcohol, and to a lesser extent

tetrahydrofuran (THF), all produce B as an intermediate in

the oxidation of A by 02. In THF this could appear through

the intermediacy of THF hydroperoxide, produced from the

abstraction of an a-hydrogen atom by 02 and a subsequent

radical coupling reaction. The THF-hydroperoxide may react

with [Rh(CO)2Cl]2 as do both HOOH and t-BuOOH. In contrast,

use of the typically nonreducing solvents tert-butyl alcohol,
39
acetone,39 and N,N-dimethylformamide (DMF) does result in

oxidation of [Rh(CO)2C1]2 without forming B as a stable

intermediate. In tert-butyl alcohol and acetone the reaction

is very fast, finishing in 40 min under conditions for which

it takes 10 hours for completion in ethanol. In DMF the

reaction is much slower (also taking 10 hours) and was

monitored by electronic absorption spectroscopy. In this

solvent only one isosbestic point is observed (at 390 nm) due

to the production of rhodium(III) trichloride from the

oxidation of [Rh(CO)2C1]2 without the formation of any stable

intermediates (Figure 8).

It is interesting that the oxidation of A (eq. 7)

proceeds through the coordination of HOOH to [Rh(CO)2C1]2 in

primary and secondary alcohol solvents rather than proceeding

































1-1



1 -i
*1




00
,-4
ono





ur
LU) *
a) -1M












4- 4
>O>





44C 4
S-4 0
41 r.(l-4

0U


















*


S-
0 0 H
*^i 0
(fcl 41 e
(U (0 J









cMc




aD

*^c
tx












CM

I
U
t-
nr


C.

0
CID


cO

00
I


eoueqJosqe


I



U

8
LJ


0



3t








43

by the alternative mechanism that seems to occur in tert-

butyl alcohol, acetone and DMF. When THF is used as solvent

the reaction is much faster and only poor resolution of the

electronic absorption band due to B is observed. This is due

to a lower concentration of B relative to that found in the

alcohols, and possibly a low energy absorption band.40 In

THF it may be that both mechanisms are functioning, which

would explain the low concentration of B observed. It is

important to note that we cannot rule out the alternate

mechanism occurring in methanol, ethanol, and isopropyl

alcohol solvents to a small degree. However, it is evident

from the magnitude of the bands due to B in the electronic

absorption and infrared spectra, as well as data from the

kinetic rate law in eq. 10, that Scheme I involving the

coordination of HOOH to [Rh(CO)2Cl]2 is dominant in primary

and secondary alcohols.



Conclusion



It is usually necessary to include bio-type or

Schiff-base ligands in order to prepare a hydroperoxide

complex, which makes the characterization of B as H2[Rh-

(CO)C12(OOH)] noteworthy. The CO ligand seems to be

responsible for at least some amount of the stability of this

complex. It is this stability which does not lead to the

characterization of the complex as an intermediate in the

catalytic oxidation of 1-hexene to 2-hexanone. Complex B








44

appears to have only slight (if any) reactivity as a catalyst

for 1-hexene oxidation. This result is not unexpected when a

comparison is made between the rhodium(III) active catalyst

and the rhodium(I) hydroperoxo complex, B.

The fact that primary and secondary alcohols can form

low concentrations of HOOH from the reduction of 02 may also

have several implications. Other metal complexes may have to

be included with H Cu(II) and Rh(III) as catalysts for the

alcohol reduction of 02 to HOOH. Systems which require long

term exposure to alcohol and 02 may find the low

concentrations of HOOH produced to be quite significant.

Substrate oxidations of 02 via HOOH formation may also be

more prevalent than now realized.

Further results have shown HOOH and t-BuOOH to both be

effective reagents in the RhC13'3H20 catalyzed oxidation of

1-hexene to exclusively 2-hexanone in the absence of 02. The

results are the same whether the solvent is ethanol or tert-

butyl alcohol (which has no reducing equivalents). These

facts provide still further justification for the character-

ization of the active catalyst for reaction 1 as a

rhodium(III) chloride complex.



Experimental Section



All solvents and reagents were of reagent grade and

used without further purification. Literature methods were

used to prepare [Rh(CO) C1]2.41 Hydrogen peroxide and tert-
use t pepae RhCO 2CI2










butyl peroxide were used as 30% and 70% aqueous solutions,

respectively, and were standardized iodometrically. The

acids HC1, H2SO4, and HC104 were used as their concentrated

aqueous solutions.

Infrared spectra were recorded on a Nicolet 7000 Series

Fourier transform infrared spectrometer. GLC spectra were

obtained with a Varian Model 940 FID instrument using a

3-m,1/16 in. i.d. copper column packed with Chromasorb P

supported diethylene glycol adipate. For the detection of

acetone a column temperature of 600C was employed, and the

measurement of 2-hexanone was quantitated by using

2-heptanone as an internal standard. The electronic

absorption spectra were recorded on a Cary 14, and all

samples were run in air at ambient temperature and pressures.

Care was taken to verify that the intermediates monitored by

this technique were stable over the course of the measure-

ments. The molecular weight of C was determined in methanol

by vapor pressure osmometry.

Catalytic Oxidations of 1-Hexene

All catalytic 1-hexene oxidations were run in 250-mL

Parr pressure bottles equipped with bras Swagelok pressure

heads. These were constructed to allow purging with 02, as

well as sampling of the solution under reaction conditions

during the course of the reactions. 2-Hexanone production

was measured by GLC.

A typical catalytic reaction was run as follows: to a

250-mL Parr bottle were added 0.074 mmol of [Rh(CO)2S'n]BF4










(prepared as earlier reported2 and used immediately),

0.0171.g of Cu(NO3)2'2.5H2 (0.074 mmol), 0.0219 g of NaCI

(0.375 mmol for the case in which 5:1 mole ratio chloride/

rhodium was desired), 0.41 mL of 0.36 M H2SO4 (0.148 mmol as

an ethanol solution prepared from aqueous concentrated

H2SO4), 0.568 mmol of 2-heptanone, 45 mL of absolute ethanol,

and 15 mL of 1-hexene (purged through alumina to remove

peroxides). This mixture was purged 5 times with 60 psi of

02, set to 40 psi of 02, and the reaction initiated by
placing in a 70 C oil bath.



Preparation of RhCl (H2O)2CH CH OH (II)

Compound C was prepared for characterization studies

most easily under 40 psi of 02 at 70 C by mixing 0.0514 g of

[Rh(CO)2C112 (0.132 mmol), 1.10 mL of 0.48 M HC1 (in ethanol,

0.528 mmol), and 15 mL of ethanol. This produced a bright

orange solution after a reaction overnight, from which C was

isolated by rotovapping to dryness and drying in vacuo. The

Mr for H402Cl3Rh calcd. 245; found, 226. Anal. Calcd for

C2H1003C13Rh: C, 8.24; H, 3.46; Cl, 36.50. Found: C, 8.41;

H, 2.69; Cl, 37.17.



Determination of Acetone Production

In Presence of 1-Hexene. The measurement of acetone

produced in the initial stage of the Rh/Cu-catalyzed

oxidation of 1-hexene with isopropyl alcohol as solvent was

made by GLC as follows: To a 50-mL round-bottom flask were









added to 0.159 g of RhCl3 3H20 (0.605 mmol), 0.137 g of

Cu(NO3)2'2.5H20 (0.589 mmol), and stir bar. This was purged

20 min with 02 at 400C and 30 mL of an 02-purged, 9/1 (v/v)

solution of isopropyl alcohol/l-hexene added to initiate the

reaction. GLC's were recorded after 4, 8, 17, 25 and 40 min.

The amount of acetone produced was determined by comparison

of peak heights with standards at the same time under

identical conditions. When this 1:1 Cu/Rh ratio was used,

0.5 mol of acetone was formed per mole or rhodium in the

first 4-8 min. No further production was observed. When a

2:1 Cu/Rh mole ratio was used, continuous, catalytic

production of acetone was observed.

In Absence of 1-Hexene. The catalytic production of

acetone from the 02 oxidation of isopropyl alcohol was

observed when 1-hexene was excluded from the solutions. Both

RhC13*3H20 (0.140 mmol) and a stir bar were placed in a 15-mL

round-bottom flask and purged 20 min with 02 at 400C. Into this

was syringed 7 mL of isopropyl alcohol, purged itself with

02 at 40 C, to initiate the reaction. Acetone production was

measured as described above.



Titration of [Rh(CO)2Cl]2 with HOOH

Because the reaction of HOOH with Rh(CO)2C12 (A) is

quite slow in ethanol, it was run at 40C. The visible

spectrum of the intermediate, H2[Rh(CO)C12(OOH)] (B), is not

clearly observed by first adding 1.0 equiv of HOOH, followed

an hour later by 0.5 equiv. The first addition causes the










reaction of much of the starting material A (which overlaps

the band at 385 nm), so that after the second addition the

band due to C at 385 nm is easily observable in the

electronic absorption spectrum. Intermediate B is easily

detected in the oxidation of A by HOOH by using FT-IR. Since

there is no overlap of the carbonyl bands of A and B, the

intermediate is detected in the first addition of HOOH.

Immediately after use HOOH was diluted in ethanol. Aqueous

dilution causes the addition of too much H20, which retards

the reaction considerably.
















CHAPTER III
ENHANCED SITE ISOLATION ON SILICA GEL
AND IMPROVED LIFETIMES OF SITE ISOLATED CATALYSTS




Introduction



Functionalized polymers have been increasingly employed

to support transition metal complexes as catalysts in recent

years. There have been several reviews in the literature

involving both organic supports4247 and inorganic

solids. 50 The system of interest here involves

functionalized supports which usually contain molecularly

definable species. Such systems are largely in the minority,

because they do not include either (1) the deposition of

metals or metal oxides by the oxidation or reduction of metal

complexes, or (2) the intercalation of metals in

unfunctionalized inorganic oxides.

The lack of physical methods which can routinely

provide identification of surface supported metal complexes

hinders the development of new catalytic systems. As a

result, attempts to prepare supported catalysts are often

based upon known homogeneous systems. The approaches for








50

attaching metal complexes is often guided by the ligand

system of the homogeneous catalyst. The methods which have

been utilized include ligand exchange or substitution of the
51,52
metal complex to the support,5152 addition to unsaturated
53,54
metal complexes, 4 and ionic attachment.55'56

Immobilization of metal complexes on solid supports can

provide advantages over both homogeneous and heterogeneous

catalysts. Included in these are (1) the ease of separating

solid catalyst from the reaction mixture, which encourages

the use of flow reactors, (2) the ability to greatly increase

dispersion of the metal on the surface, allowing the use of

less metal (and finances), and (3) reduced contact with the

reaction vessel leading to reduced corrosion. This potential

for gaining the advantages of both homogeneous and

heterogeneous catalysts has many industrial applications

which are being investigated.

Following below is a brief discussion of the general

properties of the inorganic solid, silica gel, which is used

as a catalytic support in the research presented in this

chapter.

Silica gel has a surface which is very irregular

consisting of hydroxyl and silicon bridging oxide groups. It

is quite rigid and contains many channels throughout its

structure and only the silica gel surface may be functional-

ized. Silica gel samples which vary widely in surface area

are available. Several characteristics of silica gel make it

potentially preferable to organic polymers as a support for








51

metal complexes. The rigidity of its silicon oxide tetra-

hedra is often an advantage for immobilizing and site

isolating catalytic centers. The stability of silica gel at

higher temperatures than organic polymers may also be

important. Finally, the large number of organosilanes which

are becoming commercially available and the mild conditions

under which functionalization is achieved makes the number

of possible silica supported moieties quite large.

As mentioned above, organosilanes are employed to

functionalize the surface of silica gel. It has been

reported that the reaction proceeds by hydrolysis, hydrogen

bonding and final bond formation as shown in Figure 9 as

reactions a-c. The symbol [SG]-OH is used to represent

unfunctionalied silica gel.

An investigation of the nature of the binding of

organosilanes to the silica surface was recently reported.58

Before this study was reported, it was generally believed

that three bonds from the silane to the silica surface were

formed. However, in this report Waddell et al. state that most

commonly only one bond is formed from silane to silica, while

the formation of two bonds is possible, and tridentate

bonding seems unlikely due to steric limitations.

Another important characteristic of polymer bound

metals is the ability to achieve site isolation of the metal

complex on the polymer surface. Solid supports can function

to avoid molecular aggregation and prevent the formation of

multinuclear complexes or clusters. This usually requires




























Figure 9.


General reaction scheme for the functional-
ization of the surface of silica gel,
[SG ]-OH.




















R3SiCI + H20


R Si(OH) + [SG]-OH



[SG]-O--H---OH(Si)R3


R3Si(OH) + HC1



- [SG ]----H---OH(Si)R3



[SG -O(Si)R3 + H20


(a)



(b)


(c)








54

dilution of active sites on the surface in order to minimize

contact of supported metal complexes, although another method

has been successful at achieving site isolation videe infra).

Most of the work reported in supporting metal catalysts has

employed very large loadings of the polymeric surface with

the desired functional group and metal complex. In these

cases elemental analyses can often be utilized, which is

advantageous since there are so few physical methods avail-

able. However, this has led to a lack of research on systems

with definite site isolation of active species, or possible

improvement of existing catalysts by the establishment of

site isolation. Rigid, non-flexible polymeric supports such

as silica gel are usually required when attempts to obtain

site isolation of a supported species are undertaken.

There are several reports of functionalized silica gel

being used to achieve site isolation, and perhaps other

examples from industrial laboratories which have not reached

any available literature. Among the reported examples are a

bound imidazole iron tetraphenylporphyrin which is used to

reversibly bind dioxygen,59 and cyclopentadiene groups

supported on silica gel which form stable mononuclear iron

and cobalt carbonyl complexes.60












Results and Discussion



Polymer Support of the Catalyst

The oxidation of terminal olefins to 2-ketones

catalyzed by rhodium and copper was discussed in some detail

in the previous chapter in relation to its homogeneous

mechanisms. Here will be described an investigation to

increase that catalyst's relatively short lifetime by

attempting to provide an ideal supported environment for the

catalytic species. Prevention of the suspected deactivating

oxidative aggregation of the rhodium catalyst was the

rationale for this approach.

Required of a solid support is a solvent independent

rigidity which could improve prospects for obtaining site

isolation. Silica gel, unlike organic polymers such as poly-

styrene, meets these requirements. Initial attempts at using

an organosulfide ligand on silica gel to bind the

rhodium/copper cocatalysts were successful.61 Goals of

improving the activity and lifetimes of supported catalysts

while gaining a detailed knowledge of the silica surface

precipitated the research described in this chapter.

Silica gel supported organosulfide samples (abbreviated

[SG]-SH) were synthesized using (3-mercaptopropyl)trimethoxy-

silane [(CH30)3Si(CH2CH2CH2SH)] reacted with plain silica

gel. A simple model was used to help gain insight into the

surface coverage on the silica gel. Approximating the










silicon-oxygen bond distance as two Angstroms and assuming

tetrahedral coordination around silicon, then the distance
0
between oxygen atoms of the surface silanol groups is 6.5 A.

In order to obtain an upper limit it is assumed that a flat

surface exists and that all of the surface groups are

silanols (-SiOH). Since the Davison Grade 62 silica gel

employed has a surface area of 340 m2/g [SG], then there is
-9 2
approximately one surface hydroxyl group per 4.3 x 10 m of

surface. Thus, it can be calculated that as an upper limit

there are about one mmol of surface silanol groups per gram

[SG]. With this value it can be calculated that the [SG]-SH

samples prepared contain approximately 1/40, 1/20, 1/10 and

1/5 mmol sulfide/mmol surface Si (S/Si) which correspond to

0.025, 0.5, 0.10 and 0.20 mmol S/g[SG]. The number of mmol

S/gLSG] was determined by electronic absorption for all

samples as described in the experimental section and is

independent of the model and calculations discussed above.

The values for the number of mmol S/g[SG] varied only a small

amount between preparations of functionalized silica gels.

It was these experimental values which were employed to

determine the amount of silica gel necessary for each

catalytic oxidation. The general labels used to represent

[SG]-SH samples of different sulfide loadings are shown in

Table II.

Also worthy of consideration is the relatively large

potential surface area over which the supported rhodium

complex may cover due to its attachment to a chain of six
















Table II. Representations used to designate [SG]-SH samples
of varying sulfide concentrations.



SH Loadings on [SG] Surface


0.025 mmole S/g [SG]


0.05 mmole S/g [SG]

0.10 mmole S/g [SG]

0.20 mmole S/g [SG]


-0 1 surface -SH/ 40 surface


-SiOH


+ 1lS/20Si

- IS/10Si

IS/ 5Si










atoms. Despite not being linear, this chain still allows a

380 square Angstrom area over which each rhodium may migrate.

This number, along with the surface Si concentration, allows

the calculation that the silica surface must have a

concentration less than 1 S/g surface Si in order for site

isolation of the rhodium to exist. This is a convenient

number to which properties of actual [SG]-SH samples may be

compared.

Preparation of supported rhodium complexes on silica

gel was achieved by utilizing freshly generated [Rh(CO)2S'n]-

BF4 (S' = solvent) in ethanol62 or tetrahydrofuran (THF) as

shown in reaction 16.



[Rh(CO)2C112 + 2AgBF4 2[Rh(CO)2S' ]BF4 + 2AgCl (16)



This rhodium cation readily binds to [SG]-SH samples at room

temperature producing silica gels varying from bright yellow

to red-orange as the concentration of surface sulfide is

increased.

The carbonyl ligands in the rhodium complex bound to

the silica gel allows the use of infrared spectroscopy to

investigate the functionalized surface. In these studies

freshly prepared [Rh(CO)2S'n]BF4 (S' = solvent) was reacted

with [SG]-SH sample under nitrogen and an infrared spectrum

quickly taken. The supported rhodium complex produced on

many dilute functionalized gels (e.g., 1S/40Si) yields a

two-band infrared spectrum (2055 and 2005 cm- ) characteristic










of [SG]SRh(CO) S' (S' = solvent). More concentrated
2 n
[SG]-SH samples (e.g., lS/5Si) treated identically produce

three-band spectra (2075(m), 2055(s), and 2005(s) cm-1)

indicative52 of a supported dimer, ([SG]-S)2Rh2(CO)4. These

different surface species are represented in Figure 10. It

seems very evident that concentration of surface sulfide

groups past a certain point removes site isolation and

permits supported rhodium dimerization. These trends and

their implications will be discussed further in later

sections.



Catalytic Oxidations of 1-Hexene by Supported Complexes

Silica gel organosulfide supports ([SG]-SH) as

described previously were used to immobilize rhodium and

copper in order to investigate in detail the heterogeneous

catalytic oxidation of 1-hexene to 2-hexanone (reaction 17).



0

2CH2=CH(CH2)3CH3 + 02 2CH3C(CH2)3CH3 (17)



Electronic absorption (as described in the Experimental

Section) was used to quantify the amount of sulfide in each

silica gel sample. The amount of sulfide present determined

the quantity of [SG]-SH used as a support in each catalytic

oxidation. Each oxidation to be discussed employs 0.074 mmol

of both [Rh(CO)2S' ]BF4 (S' = solvent) and Cu(NO3)2"3H20.

The quantity of [SG]-SH which provides 0.16 mmol of sulfide



























I



41

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a U



- CO
04J

41
41 4'




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414 I
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sites (a 10% excess) is used in each oxidation. Also added

to each reaction were 2 equivalents of HC104 (based on moles

of rhodium), 15 equivalents of 2-haptanone internal standard,

45 mL absolute ethanol solvent and 15 mL 1-hexene substrate.

An apparatus of 250 mL Parr pressure bottles was used to

conduct the experiments along with a pressure head which

allowed the removing of aliquots of the reaction mixture

under reaction conditions. Reactions were initiated by

purging the pressure bottle containing all components of the

reaction five times with 60 psi 02, setting the pressure to

40 psi 02 and placing the apparatus in a 70C oil bath. The

HC104 was used because Mimoun found a Bronsted acid to be

necessary when using rhodium(I) in his homogeneous

investigation, and it is given a role in his proposed

mechanism. Gas-liquid chromatography (GLC) was employed to

follow the catalytic oxidations. A calibration curve of

2-hexanone product versus 2-heptanone standard was used to

calculate 2-hexanone production in millimoles.

In preparing [SG]-SH supports, if it is assumed that

all of the added (CH3)3SiCH2CH2CH2SH reacts completely, one

obtained values for [SG]-SH of 0.025, 0.050, 0.100 mmol

S/g[SG] corresponding to 1S/40Si, 1S/ 20Si, and 1S/lOSi,

respectively. The analysis for sulfide (using a rhodium

complex and electronic absorption as in the Experimental

Section) for a series of [SG]-SH samples produces values of

0.020, 0.041, and 0.076 mmol S/g[SG]. These values are

reasonably close to those expected for complete reaction and










also have relative ratios to one another very similar to

those which assume complete reaction. Again, it is the

experimental values for sulfide content which are used in

determining the quantity of [SG]-SH necessary for each

catalytic oxidation.

Results of the heterogeneous catalytic oxidation of

1-hexene to 2-hexanone are most clearly displayed as graphs

of mmol of product (2-hexanone) versus time (in hours).

Figure 11 shows the oxidation results characteristic of three

[SG]-SH samples which vary in sulfide loading. The samples

are referred to by a ratio of sulfide to surface silicon

atoms in the silica gel. These values are obtained by

assuming complete reaction when functionalizing the gels.

However, the experimental sulfide concentration values were

always used when determining the mass of silica gel necessary

for reaction. Each curve in the oxidation graphs represents

many experiments repeated with the same supply of

functionalized silica gel. Also, other samples were prepared

and compared to previous gels. The experimental sulfide

concentrations were very similar between silica gel batches.

The idealized S/Si ratios are then used for convenience in

referring to several different supplies of similar silica

gels.

In Figure 11, if all of the rhodium and copper is

effectively site isolated on these samples, then very similar

activity should result from each of the catalysts. The

lS/40Si (0.025 mmol S/g[SG]) sample, curve A, is less active





















.0
4U 4J
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0 4-4


= > a) *W


44- -4
C 0-0


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-1 .4 *-4

0 0 144

mJ
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X 0 4- (

C 0, r-
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66

for 2-hexanone production than the 1S/20Si and IS/lOSi

samples (0.05 and 0.10 mmol S/g[SG]). This results from

diffusion problems which will be discussed further later in

this chapter. Curves B and C do show similar activity for 24

hours as shown. Catalytic activity resulting from immobili-

zation of the rhodium and copper complexes on silica gel

certainly has been demonstrated, and several comparisons to

these results will be made in following sections.

After establishing supported activity, it is important

to check for leaching of the metal complexes from the solid

into solution. Catalytic oxidations were run for three hours

and the solutions removed by syringe under reaction

conditions. The filtrate, after measuring the volume, was

then placed in a new Parr bottle with another equivalent of

Cu(II) and the oxidation re-started in the original manner.

The filtrate had activity ranging from 5 to 15 percent of the

total normal supported activity depending on the silica gel

sample being studied. Since the homogeneous rhodium/copper

oxidation catalyst is much more active than the corresponding

supported system, then it is apparent that the vast majority

of the heterogeneous catalytic activity is due to [SG]-SH

bound catalyst.

It is relevant to stress that experiments to check for

leaching must be done under reaction conditions. The

chemical equilibria involved in leaching may be greatly

affected by conditions milder than those during reaction and











thus leaching should be investigated only under reaction

conditions.


Alkyl Covering of the Silica Surface to Improve Site
Isolation and Catalysis


The silica gel supported catalytic system as described

until this point offers several characteristics which make it

ideally suited for further investigation of site separation

in a quantitative manner. Already demonstrated has been a

method for determining the surface loading of organosulfide

functional groups, a spectroscopic technique for establishing

the identity of silica gel immobilized rhodium species, and

an oxidation reaction which can accurately measure catalytic

activities and catalyst lifetimes. Thus, any further

attempts to alter the silica surface may be monitored rather

precisely.

The general supposition that increased site isolation

of the silica gel supported rhodium species is directly

related to increased catalytic activity is supported by the

results obtained using [SG]-SH as the support. Very highly

loaded [SG]-SH samples produce catalytic systems of very low

activity. Further manipulation of the silica gel supports in

order to produce better and/or longer lived site isolation

was greatly desired. It was assumed that the general

deactivation of the supported catalysts was dependent to some

degree upon the aggregation, over time, of rhodium species on

the silica surface. A mechanism could be logically

postulated whereby sulfide bound rhodium species exchange








68

with surface silanol protons and thus migrate along the

surface until aggregation occurs. As a result, a

modification of the silica surface was sought which could

remove this possible path of aggregation. One other goal was

to obtain a method for increasing the loadings of site

isolated species per gram of silica support.

The functionalization of unused surface silanol (-SiOH)

groups with alkylsilane moieties results in an effective

method for achieving the previously stated goals. Chloro-

alkylsilanes are utilized to produce alkyl groups bound to

the silica surface. The reaction of the alkylchlorosilanes

with the silica surface is identical to the surface reaction

with (3-mercaptopropyl)trimethoxysilane as described in

reactions 1-3. The chloro group here reacts as does the

methoxy group in those reactions, forming HC1 instead of

CH3OH. Chlorotrimethylsilane, chlorotriethylsilane and

chlorotripropylsilane were each used to prepare silica gel

samples, along with the same (3-mercaptopropyl)trimethoxy-

silane in each case. Samples prepared with alkyl groups and

the sulfide functionality on the silica surface will be

generally referred to as alkylated [SG]-SH, while those with

only sulfide will be called either non-alkylated [SG]-SH or

merely [SG]-SH. The functionalized silica gels will also be

identified by a ratio of surface sulfur to surface silicon

(S/Si) as described previously (e.g., 1S/10Si, 1S/20Si). The

alkylated gels will also be referred to by a percent

alkylation which is the percent of all unused surface silicon










atoms reacted with chloroalkylsilane assuming complete

reactions. The assumption of complete reaction is generally

supported by elemental analyses for carbon which became

possible at levels of substantial alkylation.

Quantitative analyses for sulfide were also done for

alkylated [SGI-SH samples. The values of mmol S/g [SG] for

these were generally slightly lower than for their

non-alkylated analogs. Again, the values obtained for

sulfide content were used to determine the necessary quantity

of silica gel for catalytic oxidations.

Infrared spectroscopy provided a good deal of

information concerning the properties of the alkylated [SG]

-SH gels. As described earlier, reaction of Rh(CO)2S' BF4

(S' = solvent) with [SG]-SH samples provides species whose

infrared spectra are quite useful. Rhodium species bound to

lS/40Si and lS/20Si [SG]-SH gels produce the two band

infrared spectrum indicative of the monomeric supported

complex using both non-alkylated and alkylated [SG]-SH

samples. The IR spectrum which results using IS/lOSi

non-alkylated [SG]-SH indicates the presence of primarily

dimeric rhodium complex on the silica surface. This

generally supports the estimation discussed earlier in this

chapter that the silica surface must be on the average about

lS/9Si or more dilute in order to produce a system in which

most of the rhodium is site isolated. The alkylated [SG]-SH

gels (e.g., 80% methylated) produce the monomeric rhodium

complex on the surface as indicated by their infrared










spectra. The alkyl surface covering allows the formation of

a different surface species than its non-alkylated analog at

identical loadings of metal per gram of silica gel. A

mixture of monomeric and dimeric rhodium complexes is present

on the surface of 80% methylated 1S/5Si [SG]-SH, while

primarily the dimer is produced on 80% methylated lS/2.5Si

[SG]-SH. These infrared spectra are shown in Figure 12. It

may then be concluded that the alkyl covering cannot prevent

rhodium dimerization on the silica surface past a certain

level of metal loading on the silica gel.



Catalytic Oxidations Using Alkylated Silica Gels as Solid
Supports


The experimental procedures for conducting catalytic

oxidations using alkylated [SG]-SH supports are identical to

those for the non-alkylated systems described in Section B.2.

Again, the most effective way to present data for these

catalytic oxidations is in the form of graphs of millimoles

of 2-hexanone product versus time as measured in hours.

The oxidations obtained using [SG]-SH with 50% of its

available surface covered with trimethylsilyl group (50%

methylated) as supports for the rhodium and copper

cocatalysts are shown in Figure 13. Not only do these

catalysts demonstrate much greater initial activity in some

cases than their unmodified (non-alkylated) counterparts in

Figure 11, but they are still more active after 24 hours.

The 1S/20Si (0.05 mmole S/G[SG]) 50% methylated gel in curve


























Figure 12.


Infrared spectra for immobilized [Rh(CO) -
(C H OH) ]BF using [SG]-SH supports witA
(AT IS/lUSi 50% Me; (B) 1S/5Si 80% Me; and
(C) lS/2.5Si 80% Me.












A 'si 50% Me









B S/5si 80% Me






C S/2,5s 80% Me









1 I
1800


I I
2300


I 2
2000


(cm'')

























0




40)

.l > E




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0 03dP
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m > cnfi







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B produces virtually linear 2-hexanone production over 24

hours. Compared to the non-alkylated catalyst curve B of

Figure 11 on almost two-fold increase in product has been

obtained after 24 hours and a very active catalyst remains.

At the end of four hours essentially the same amount of

2-hexanone had been produced by curve B in Figures 11 and 13.

Infrared spectra indicated that in both catalysts the

essential species was monomeric rhodium complex before the

start of catalysis. Thus, alkylation of silica gel in this

instance has greatly enhanced catalyst lifetime.

Comparison of IS/lOSi catalyst systems from curve C of

Figures 11 and 13 reveals that the methylated catalyst is

superior almost from the very start of the oxidation. This

result correlates well with the fact that the non-alkylated

catalyst is composed of rhodium dimer, while the alkylated

one contains monomeric rhodium before reaction. This

demonstrates that higher loadings of catalytically active

complex can be obtained under site isolated conditions by

diluting the functionalized reagent in the hydrocarbon matrix

formed by -Si(CH3)3. A very active catalyst is still present

after 24 hours.

A surprising result is represented by curve A in

Figures 11 and 13. According to infrared spectroscopy site

isolation (rhodium monomer) exists in both systems before

reaction. The diffusion problem described in relation to

Figure 12 has apparently been decreased by the greater

affinity of alkene for the alkylated surface than for








76

solvent. Such a rationalization explains the greater initial

activity of curve A in Figure 13 versus Figure 11 as well as

the increased activity after 24 hours.

Figure 14 shows the results of 1-hexene using catalysts

supported on 50% and 80% ethylated silica gels. They are

generally more active than the non-alkylated gel catalysts

and are still very active after 24 hours. The IS/lOSi 80%

ethylated catalyst (curve E) is less active and the 1S/20Si

80% ethylated gel (curve D) more active than might have been

predicted. Samples which were 80% methylated yielded results

similar to those of the 50% methylated gels. It is also seen

in Figure 14 that increasing the percent alkylation does not

have a drastic effect on catalytic activity.

The 80% propylated [SG]-SH samples produce catalytic

results very much like those of the non-alkylated silica gels

of Figure 11. Figure 15 displays oxidations run for longer

time periods, each done with lS/20Si [SGI-SH gels which have

different surface modifications. The 80% methylated gel

(curve B) is still active after 140 hours. It can be seen

that the 80% ethylated gel is less active than the methylated

one, while the propylated catalyst is nearly identical to the

non-alkylated sample. The propylated silica gels, and to a

lesser extent the ethylated ones, are markedly slow in

"wetting" by ethanol. Perhaps it is this increased

hydrocarbon-like nature of the surface which accounts for

their decreased efficiency as catalytic supports relative to

the methylated gels. Thus, even though enhanced solubility


























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of the alkene in the surface layer may occur, exclusion of

the solvent would inhibit the reaction.

An investigation was undertaken to determine whether

under conditions in which the large amounts of silica gel

used in 1-hexene oxidations when the 1S/40Si (0.025 mmol S/g

[SG]) gels are employed as catalyst supports diffusion might

become rate-controlling. A typical 1-hexene oxidation

employing 1S/40Si non-alkylated [SGI-SH was run. A second

oxidation using one-half the amount of rhodium and copper,

and thus one-half of the silica gel support, was also run.

All other initial conditions and amounts of reactants were

identical. Assuming that diffusion is not rate-controlling

the rate of this oxidation should have been one-half of the

original rate since the oxidation is first order in rhodium

concentration.63 Figure 16 clearly shows the reaction with

one-half of the silica gel supported catalyst to have a rate

greater than one-half of the normal oxidation. The dashed

line C is one-half of the curve A. Thus, there seems to be

some reaction inhibition due to the large amounts of silica

gel support used in the 1S/40Si oxidations (9-12 g), which is

near the limit of the mass of gel which may be stirred in

this experimental apparatus. This result may be ued to help

explain why in general several 1S/40Si (0.025 mmol S/g[SG])

supported catalysts are not as active as their lS/20Si (0.05

mmol S/g[SG]) counterparts (e.g., Figure 11).


























4) 0-
m4



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(D CO 0 (D 4
Q-- O

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Possible Effects of a Different Rhodium Catalyst
Characterization

The design and implementation of the supported

catalytic systems have been based on Mimoun's

characterization of the active catalyst as a rhodium(I)

complex. The possible characterization of the catalyst as a

rhodium(III) species as discussed in Chapter II may have

several implications. Chloride ion may also be quite

important since 3-5 equivalents of chloride ion promote

better homogeneous oxidations using [Rh(CO)2S' BF4 (S' =

solvent) and given that the most stable homogeneous catalyst

species may be rhodium chloride complexes of same type.64

Chloride ion cannot be added to the [SG]-SH systems without

extensive leaching occurring. The possible use of a

supported multi- dentate ligand for the immobilization of a

rhodium(III) catalyst should be considered for future use.

It would also be of interest to compare the effects of

alkylation of a multidentate ligand silica gel support to

those presented here for a monodentate ligand system.




Conclusions

The spectral properties and catalytic behavior of the

described systems demonstrate that isolating functionalized

[SG]-SH groups with [SG]-SiR3 (where R is alkyl) leads to

some very pronounced changes. The stability of the

methylated catalysts is greatly enhanced over that of the












non-alkylated systems. On non-alkylated [SG]-SH it is

possible that the derivatized silanol groups may exchange

with protons on nearby silanol groups and thus migrate along

the surface, aggregate and become inactive. The alkylated

gels remove -OH groups and would be able to effectively

suppress such a process leading to a longer lifetime for

site-isolated conditions. The mixing of reagents prior to

functionalization also ensures a better distribution of

functional groups over the surface under site isolated

conditions and enables one to attain a more concentrated site-

isolated catalyst.

In the specific catalytic system studied, increasing

the length of the alkyl chain covering the silica surface to

ethyl and propyl decreases catalytic activity over the methyl

covering. This may be due to solubility properties as

discussed earlier, or perhaps due to increased efficiency at

keeping the two cocatalysts apart. This approach should

still find general application and utility in the area of

hybrid catalysts where site isolation is a desired feature.










Experimental



General Procedures

All solvents and reagents were of reagent grade and

used without further purification unless otherwise specified.
The [Rh(CO) Cl 241 and [Rh(CO)2S' ]BF 62 (S' = solvent) were
S2I 2 n 4
prepared as reported in the literature or purchased from

Aldrich and recrystallized from n-hexane. The (CH30)3Si(CH2CH2-

CH2SH) and ClSi(CH3)3 were obtained from Aldrich and

ClSi(C2H )3 and ClSi(C3H7)3 from Petrarch Systems. Silica

gel was grade 62 from Davison Chemical.

Infrared spectra were recorded on a Perkin-Elmer model

283B infrared spectrometer. The GLC were obtained with a Varian

model 940 FID instrument using a 3m, 1/16 in. i.d. copper

column packed with Chromasorb P supported diethylene

glycoladipate or with a Varian model 3700 FID chromatograph

using an 8 ft, 1/8 in. column of the same material. The

2-hexanone production was quantified using 2-heptanone as an

internal standard. Electronic absorption spectra were taken

on a Cary 14, and all samples were run in air at ambient

temperatures and pressures.



Preparation of Silica Gel Supports

All reactions of functionalizing silanes with silica

gel were done under argon with xylenes as solvent. Silica

gel was stirred in xylenes under argon, followed by heating,

and then a solution of one or more silanes in xylenes was








87

added dropwise to the hot silica gel slurry. The non-

alkylated [SG]-SH gel was made by adding (CH3O)3Si(CH2CH2-

CH2SH) to the gel. The [SG]-SH alkylated gels were made by

adding a solution of the mercaptosilane mixed with R3SiCl (R=

CH3, C2H5 or C3H7). This technique is preferable to adding

each silane in separate steps. After refluxing 24 hours, the

silica gels were thoroughly washed with xylenes, ethanol and

then dried at A90oC. The silica gels remain white in

appearance.

Determination of -SH on Silica Gel

Silica gel and its surface silanol groups were reacted

with (CH30)3Si(CH2CH2CH2SH) to produce the [SG]-OSi(OCH3)2-

(CH2CH2CH2-SH)(abbreviated as [SG]-SH). R3SiC1 (R=methyl,

ethyl or propyl) was used to complete reaction of the silanol

groups and cover the silica gel surface with alkyl groups.

Evidence that trialkylsilyl groups are bound to the surface

is obtained by a substantial increase in the percent carbon

found in the elemental analyses and by an increase in the

lifetime of the functionalized catalyst.

Electronic absorption was used to determine the amount

of sulfide on each silica gel sample ([SG]-SH). The [SG]-SH

samples were stirred with freshly prepared [Rh(CO)2S' ]BF4

(S' = solvent), the solid filtered off and the electronic

absorption spectrum of the filtrate taken. From a

calibration curve of the rhodium complex concentration versus

its absorbance at 390 nm the amount of rhodium in the

filtrate is measured. The amount of rhodium on the gel is










determined by the difference of the known initial amount of

rhodium and the amount of rhodium in the filtrate. Assuming

complete reaction of one rhodium per sulfide, the amount of

rhodium is equal to the amount of sulfide on the silica

surface. An analysis of this type was done for all samples.

Samples were prepared which varied in surface sulfide

concentration and in percent of available surface silanol

groups which were "alkylated" (reacted with trialkylchloro-

silane). Assuming tetrahedral coordination around silicon

and oxygen atoms with an approximate silicon-oxygen bond

distance of two Angstroms, and assuming that as an upper

limit a flat surface exists with all surface groups being
0
silanols, a separation of 6.5 A exists between the oxygens of

surface hydroxyl groups. This corresponds to one surface
-19 2
hydroxyl per 4.3 x 1019 m of surface so it can be

calculated that as an upper limit there are about one mmol

surface silanol group atoms per gram [SG]. With this value

it can be determined that the [SG]-SH samples prepared

contain approximately 1/40, 1/20, 1/10 and 1/5 millimoles

sulfide/millimole surface Si (S/Si) which corresponds to

0.025, 0.05, 0.10, 0.20 and 0.40 mmol S/g[SG]. Silica gel

samples used as catalytic supports will be referred to by a

(S/Si) ratio and by a number of mmole S/g[SG]. In some

instances the percent alkylation (methylation, ethylation or

propylation) of the surface silicon sites will also be used.










Catalytic Oxidations of 1-Hexene

All catalytic 1-hexene oxidations were run in 250 mL

Parr pressure bottles equipped with brass Swagelok pressure

heads. These were constructed to allow purging with 02, as

well as sampling of the solution under reaction conditions

during the course of the reactions. The 2-hexanone

production was measured by GLC.

All catalytic reactions were run using 0.074 mmol

[Rh(CO)2S'n]BF4 (prepared as reported earlier and used

immediately) and 0.0179 g Cu(NO3)2*3H20 (0.074 mmol). The

amount of silica gel employed was varied to provide enough

-SH sites to bind ail of the rhodium and copper with a 10%

excess (e.g., 4.9 g of 1S/20Si 80% Et [SG]-SH). Both metal

ions are attached to the [SG]-SH samples through the -SH

moiety producing a lemon-yellow to yellow-green supported

catalyst. The catalyst was added to a 250 mL Parr bottle

along with 0.32 mL 0.47 M HC104 (0.150 mmol, as an ethanol

solution prepared from aqueous cone. HC1O4), 1.136 mmol

2-heptanone, 45 mL absolute ethanol, and 15 mL 1-hexene

(purged through alumina to remove peroxides). This mixture

was purged 5 times with 60 psi 02' set to 40 psi 02, and the

reaction initiated by placing the system in a 70C oil bath.



















CHAPTER IV
THE SYNTHESIS AND CATALYTIC APPLICATIONS
OF A MULTIDENTATE LIGAND AND CORRESPONDING METAL COMPLEXES
BOUND TO SILICA GEL




Introduction



The use of a polymer as a support for catalysts was

discussed in Chapter III for a specific system which employed

a monodentate chelate on silica gel. The general use of

polymers as supports for catalysts and chelates, and also as

synthetic reagents, has developed enormously since their use

in peptide synthesis was shown by Merrifield approximately
65
twenty years ago. It was generally assumed for several

years that there was little, if any, interaction between

functionalized sites on resins such as polystyrene. This is

now known not to be the case, and Chapter III demonstrated

some of the rather stringent conditions which must be met in

order to achieve site isolation on a support such as silica

gel.










Another general approach to preparing stable polymer

bound metal complexes is that of first attaching a

multidentate ligand system to a polymer backbone and then

adding the metal ion to the previously formed ligand. There

have been many reports discussing polymers which contain

chelating groups of various sizes and donor atoms.66-69 An

excellent review of this area is cited here as reference 70.

Most of the polymer bound chelates studied are mono- or

bidentate, with only a few tri- and tetradentate systems

having been investigated.69b71 Resins with bidentate

chelate groups often must provide a metal ion with two or

three chelating groups since many metal ions prefer forming

four or six coordinate complexes. This led to the common use

of uncrosslinked or lightly crosslinked polymers. The

ambiguity involved in trying to characterize these polymer

bound metal complexes makes it very difficult to interpret

data from catalytic reactions or physical methods.

In virtually all of the literature reports cited in

this area, there is no attempt made to isolate functionalized

sites from one another and in many cases two or more sites

must interact in order to provide successful chelation.

Chapter III discussed in detail the conditions required

to achieve site isolation on silica gel and surface

modifications which may enhance that isolation. The initial

sections of this chapter will discuss the preparation of a

stable multidentate ligand site isolated on silica gel which

can complex a variety of metal ions into polymer bound










complexes of a known geometry. Investigations of several

applications of these polymer bound complexes will then be

presented, followed by investigations into the use of

different solid matrices as chelate metal complex supports.

The ligand prepared on silica gel is a pentadentate

ligand which complexes several different metal ions. The

silica gel supported complex of iron(III) was used to

investigate some known homogeneous reactions.

Biological oxidation processes are generally quite

selective and sensitive and as a result attract the attention

of many chemists.72 Cytochrome P-450 is one enzymatic system

which can catalyze several types of oxidative

transformations, including selective alkane hydroxylations

and has therefore been an object of much research effort.

Several studies have proposed intermediate high-valent

oxometalloporphyrin species in the catalytic cycles of
73,74
several heme-containing oxygenase enzymes.7374 Simple

chemical models for cytochrome P-450 were sought in order to

provide insight into some of the basic processes involved in

its chemical reactivity. A significant finding was the

realization that iron,75 chromium,76 and manganese77

porphyrins could catalyze oxygen transfer from iodosylbenzene
78
to simple hydrocarbons.7 In all of these cases, a metal-oxo

species is proposed as an intermediate, although complete

mechanistic details have not been determined. However, it

was the possibility of generating silica gel supported iron

or manganese oxo species to achieve some type of oxidative











transformation which was the impetus behind the work reported

here. The ability to form supported cobalt-dioxygen

complexes (vida infra) helped to encourage the idea that it

would be sterically possible to generate the supported iron

or manganese oxo intermediates. Another goal was to

determine the effect of limiting or eliminating interaction

between metal centers via their support and site isolation on

silica gel.

Another topic briefly investigated is the long known

"Fenton chemistry" of iron salts and hydrogen peroxide

affecting the hydroxylation of organic substrates.79 This

area has recently been reinvestigated by Cheves Walling and

ne gives an excellent overview of this area in reference 80.

The original goal of achieving this type of peroxide induced

oxidation with a supported iron complex was not realized due

to the leaching of metal complex into solution in the

presence of acid and peroxide. Thus, no mechanistic or

catalytic information is provided, although some further

knowledge of the silica surface is obtained.

Catalytic processes which require two different metals

have not achieved much success in being supported on

polymers. A method which may successfully attain support of

both metals is that of incorporating one active metal into

the structure of the support while supporting the other metal

on the surface of the solid. Incorporation of copper(II)

ions into silica gel is described along with attempts at

catalysis by resulting bimetallic systems.