Transition metal complexes of reactive alkynes, arynes and cumulenes


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Transition metal complexes of reactive alkynes, arynes and cumulenes
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x, 217 leaves : ill. ; 29 cm.
Klosin, Jerzy, 1967-
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Thesis (Ph. D.)--University of Florida, 1995.
Includes bibliographical references (leaves 209-216).
Statement of Responsibility:
by Jerzy Klosin.
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To my wife Magdalena and daughter Sylvia


First of all I would like to express my gratitude to Professor William M. Jones, my

supervisor, for his wise guidance and support throughout graduate school. I am grateful to

Dr. Khalil A. Abboud for his help in obtaining the X-ray crystal structures in my work and

for his patience in teaching me crystallography. I also thank Dr. Jian-guo Yin for

introducing me to new experimental techniques and for his help and contribution to the

work described in chapter 6. I am indebted to the University of Florida and the Graduate

School at the University of Florida for accepting me as a graduate student and enabling me

to be part of this excellent university. The University of Florida is by far the best school I

have ever attended. I would also like to express many thanks to all of the people who

created such a wonderful learning and research environment in the Department of

Chemistry of the University of Florida. I must also acknowledge all of the professors in the

Department of Chemistry from whom I took classes, for their enthusiasm and excellent



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

LIST OF TABLES................................................................................ vi

LIST OF FIGURES .............................................................................. vii

ABSTRACT....................................................................................... ix



Reactive Alkynes, Arynes and Cumulenes............................................. 3
Transition Metal Complexes of Cyclic Alkynes, Arynes and Cumulenes.......... 7
The Nature of Bonding between Transition Metal and Carbon-Carbon 7t
Bond................................................................................. 7
Transition Metal Complexes of Cyclic Alkynes................................... 8
Transition Metal Complexes of Arynes ............................................10
Transition Metal Complexes of Cyclic Cumulenes ..............................12

TROPYNE................................................................................ 15

Attempted Synthesis of Palladium Complex of 1,2,4,6-cycloheptatetraene
Synthesis of Palladium Complexes of Cyclohepta-3,5-dien- 1-yne (2-12)
and Cyclohepta-3,6-dien-1-yne (2-12)...........................................19
X-Ray Crystal Structure Analysis of 2-12 ........................................... 21
Synthesis of Palladium Complex of Tropyne (2-16)..................... 24

AND TROPYNES .......................................................................29

Synthesis of Platinum Complex of 2,3,4,6-Cycloheptatetraeneone (3-8).........30
Synthesis of Platinum Complex of Dibenzannelated Didehydrotropone (3-
X-Ray Crystal Structure Analysis of 3-10 ............................................35
Reactions of Complex 3-10............................................................38
Synthesis of trimer 3-14.................................................................42
X-Ray Crystal Structure Analysis of 3-14 ............................................44
X-Ray Crystal Structure Analysis of 3-23 ............................................53

Synthesis of Platinum Tropyne Complex 3-24.......................................56
Reactions of Platinum Tropyne Complex 3-24.......................................60

CUMULENES AND TROPYNES .................................................... 62

Synthesis and Spectroscopy of Bimetallic Complexes of Cyclohepta-3,5
dien-1-yne (102) and Cyclohepta-3,6-dien-l-yne (103)......................63
X-Ray Crystal Structure Analysis of (PPh3)2Pt [12(j16-C7H6)Mo(CO)3]
(4-5) ...................................................................................70
Preparation of a Bimetallic Complex of Tropyne (4-9) .............................72
Hydride reduction of (PPh3)2Pt[r12(l 7-C7H5)Mo(CO)3] (4-9)...................77
X-Ray Crystal Structure Analysis of (PPh3)2Pt [q2(1l6-C7H6)Mo(CO)3]
Some Reactions of 4-5, 4-6 ...........................................................85

1,2,4,6-CYCLOHEPTATETRAENE ..................................................87

Synthesis, Spectroscopy and Fluxional Behavior of 5-1............................91

6 RUTHENIUM COMPLEXES OF 1-SILAALLENE ............................... 101

Introduction............................................................................. 101
Preparation of the Precursor to a 1-Silaallene Ligand.............................. 104
Synthesis and Spectroscopy of 6-24................................................ 108
X-Ray Crystal Structure Analysis of 6-24 ......................................... 111

7 EXPERIMENTAL ..................................................................... 123

Materials and Methods................................................................. 123
Synthesis................................................................................ 124
Crystallographic Studies............................................................... 146


A SINGLE CRYSTAL X-RAY STUDIES............................................ 154

LIST OF REFERENCES .......................................................................209

BIOGRAPHICAL SKETCH...................................................................217


2-1. Selected Bond Lengths (A) and Angles (deg) for Complex 2-12................23
3-1. Selected Bond Lengths (A) and Angles (deg) for Complex 3-10................38
3-2. Selected Bond Lengths (A) and Angles (deg) for Compound 3-14................46
3-3. Selected Bond Lengths (A) and Angles (deg) for Complex 3-23................55
4-1. Selected Bond Lengths (A) and Angles (deg) for Complex 4-5 ...................72
4-2. Selected Bond Lengths (A) and Angles (deg) for Complex 4-10................84
6-1. Selected Bond Lengths (A) and Angles (deg) for Complex 6-24.............. 113


2-1. 1H NMR spectrum of complexes 2-12 and 2-13............................... 20

2-2. Thermal ellipsoid drawing of complex 2-12.....................................22

2-3. 1H NMR spectrum of complex 2-16.............................................. 26

3-1. 1H NMR spectrum of complex 3-8 ............................................... 32

3-2. 31P (1H) NMR spectrum of complex 3-8 ....................................... 33

3-3. 1H NMR spectrum of complex 3-10................................................ 36

3-4. Thermal ellipsoid drawing of complex 3-10 ..................................... 37

3-5. 1H NMR spectrum of complex 3-11.............................................. 40

3-6. 1H NMR spectrum of compound 3-14 ........................................... 43

3-7. Thermal ellipsoid drawing of complex 3-14......................................45

3-8. Distortion of the central benzene ring in 3-14................................... 46

3-9. 1H NMR spectrum of complexes 3-18 and 3-20............................... 49

3-10. 1H NMR spectrum of complex 3-23............................................. 51

3-11. 195Ppt(H) and 31P{ H)NMR spectra of complex 3-23....................... 52

3-12. Thermal ellipsoid drawing of complex 3-23.....................................54

3-13. 1H NMR spectrum of complex 3-24............................................. 58

3-14. 195ptp 1H) and 31p( 1H)NMR spectra of complex 3-24....................... 59

4-1. 1H NMR spectrum of complexes 4-5 and 4-6 .................................. 66

4-2. 2D COSY NMR spectrum of complexes 4-5 and 4-6.......................... 67

4-3. 195Pt 1H}) and 31P('H)NMR spectra of complexes 4-5 and 4-6............ 68

4-4. Thermal ellipsoid drawing of complex 4-5........................................71

4-5. 1H NMR spectrum of complex 4-9 ............................................... 75

4-6. 195Pt(H}) and 31p( H)NMR spectra of complex 4-9......................... 76

4-7. 1H NMR spectrum of complex 4-10...............................................79

4-8. 195Pt{ 1H) and 31P{ 1H}NMR spectra of complex 4-10....................... 80

4-9. 13C{ 1H) NMR spectrum of complex 4-10 ...................................... 81

4-10. Thermal ellipsoid drawing of complex 4-10 ..................................... 83

5-1. 1H NMR spectrum of complex 5-1............................................... 92

5-2. 195Pt(pH) and 31p(1H}NMR spectra of complex 5-1......................... 93

5-3. 2D COSY NMR spectrum of complex 5-1....................................... 94

5-4. NOE difference spectrum and part of 1H NMR spectrum of

complex 5-1.......................................................................... 96

6-1. 1H NMR spectrum of complex 6-24................................................ 109

6-2. Thermal ellipsoid drawing of complex 6-24 ..................................... 112

6-3. Comparison of bond lengths and angles between complexes 6-24 and

6-19................................................................................... 113

6-4. 1H NMR spectrum of complex 6-27................................................ 118

6-5. 13C({H) NMR spectrum of complex 6-27...................................... 119

6-6. 29Si and 29Si { H) NMR spectra of complex 6-27 ............................. 120

B-1. Stereoview drawings of 2-12, 3-10, 3-14 ......................................206

B-2. Stereoview drawings of 3-23, 4-5, 4-10....................................... 207

B-3. Stereoview drawing of 6-24 ....................................................... 208

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



Jerzy Klosin

December, 1995

Chairman: William M. Jones
Major Department: Chemistry

Research presented in this dissertation concerns the synthesis and study of

transition metal complexes which contain reactive organic molecules such as strained

alkynes, tropynes and cumulenes as one of their ligands.

Palladium complexes of cyclohepta-3,5-dien-1-yne (2-12) and cyclohepta-3,6-

dien-1-yne (2-13) have been prepared and converted to a palladium complex of tropyne

(2-16) by treatment with triphenylcarbenium tetrafluoroborate. Complex 2-12 was

characterized by X-ray crystal structure analysis. The physical and chemical properties of

these new complexes have been investigated.

Platinum complexes of dibenzannelated seven membered ring alkynes (3-10, 3-
18, 3-19) have been synthesized in good yields by base induced dehydrobromination
from the corresponding bromoalkenes in the presence of Pt(PPh3)3. Complex 3-10 was

characterized by X-ray diffraction analysis. Complex 3-10 was shown to react with
compounds such as t-butylisocyanide, HBF4, HBr and bis(dicyclohexylphosphino)ethane.

Complex 3-10 reacts with tetracyanoethylene to give the highly distorted substituted

benzene 3-14 which was characterized by X-ray diffraction. Complex 3-18 reacts with

TCNE to produce the platinacyclopent-2-ene 3-23. Complex 3-23 was characterized by

X-ray diffraction analysis. Reaction of 3-19 with triphenylcarbenium tetrafluoroborate

gives dibenzannelated tropyne complex 3-24 which can be converted back to 3-19 with
KBEt3H. Reaction of 3-24 with bis(dicyclohexylphosphino)ethane gives a dimer resulting

from nucleophilic attack of the phosphine on the tropyne ring whereas reaction with HBr

gives the oxidative addition product 3-26 which slowly isomerizes to 3-27.
The platinum complex of tropyne 2-5 reacts rapidly with (116-p-xylene)Mo(CO)3 in

THF-d8 solution to give the bimetallic tropyne complex of tropyne 4-9. Reduction of 4-9
with LiAI(Ot-Bu)3H or KBEt3H gives a mixture of three isomers 4-5, 4-6 and 4-10. The

major isomer (4-10) contains a 1,2,3,5-cycloheptatetraene ring, a ring system that has

previously been inaccessible either free or completed to a transition metal by

dehydrobromination of bromocycloheptatrienes. The structures of the two minor isomers

(4-5 and 4-6) were confirmed by independent synthesis from 2-2 and 2-3 by reaction
with (rl6-p-xylene)Mo(CO)3. Solid X-ray crystal structures of 4-5 and 4-10 have been

determined. The physical and chemical properties of all new complexes have been studied.

The bis(triphenylphosphine)platinum complex of 1,2,4,6-cycloheptatetraene 1-51
was found to react with (116-p-xylene)Mo(CO)3 to produce a bimetallic, thermally unstable

allene complex 5-1. Complex 5-1 was shown by an NOE experiment to be fluxional at

room temperature and possible mechanisms of this fluxional process are discussed.
The first transition metal complex of 1-silaallene has been prepared by P-hydrogen
elimination from Cp*(PCy3)Ru(qil-Ph2C=C-SiMe2H) (6-23). In contrast to an analogous

silene complex 6-19 where hydrogen transfer to the metal is complete, the X-ray crystal
structure analysis and the 29Si-1H coupling constant of 6-24 reveals that an 1r2-Si-H

bonding mode is present in the complex.


Highly reactive and usually non-isolable organic molecules have, over the years,
held a special attraction for organic chemists. Some of the more popular include carbenes,
nitrenes, carbynes, radicals, antiaromatics and strained ring compounds.1 Representatives
of each of these classes of compounds are shown on Scheme 1-1. Because of their very
high instability (reasons for instability vary for each class of compounds), their transient
existence has been mainly deduced from trapping and kinetic experiments. However, some

Scheme 1-1




methylene (1-1)

t-butylnitrene (1-2)

methylmethyne (1-3)




trimethylenemethane (1-4) cyclobutadiene (1-5) A''4-bicyclo[2,2,0]-hexene (1-6'
radical antiaromatic strained ring


of these species have been successfully generated in matrices at very low temperature or in

the gas phase at low pressure which has permitted them to be studied by appropriate

spectroscopic techniques.1'7

Early in the history of modem organometallic chemistry scientists discovered that

transition metals are capable of stabilizing reactive, short-lived organic intermediates by

coordinating them to the metal center. A number of complexes that contain organic

intermediates such as carbenes,2-3 nitrenes,4 carbynes,5 radicals,3 antiaromatics6 and

strained ring molecules7-10 were synthesized and studied. When the organic intermediate is

coordinated to the metal center its reactivity is greatly altered as compared to that of the

parent organic fragment and as a result many complexes with organic species in the

coordination sphere have been isolated at room temperature and their solid crystal structures

Scheme 1-2


ON ,,.eI CH2



Rhenium carbene
complex (1-7)

H2C C-,CH2
H2C ,'


Iron trimethylene-
methane complex (1-10)

,.- -Ir N-tBu

Iridium nitrene
complex (1-8)

OCe'" A CO

Ruthenium cyclobutadiene
complex (1-11)

Chromium carbyne
complex (1-9)


Platinum bicyclohexene
complex (1-12)

have been determined. The chemistry of such complexes has been studied intensively
because of the novelty of the bonding modes, their structural diversity and their

involvement in catalytic processes. Examples of transition metal complexes of some

organic intermediates are shown in Scheme 1-2.

The main interest in our laboratory in the area of stabilization of reactive

intermediates has been devoted to the synthesis and study of new complexes with reactive

alkynes, arynes and cumulenes in their coordination sphere.

Reactive Alkvnes. Arynes and Cumulenes

High reactivity of strained alkynes,7,11,12 arynes7,12,13 and carbocyclic
cumulenes7'14 arises from distortion of the multiple bond(s) from its "normal" bond

lengths) and angles. Such distortion, which induces severe strain energy is achieved by

incorporation of multiple bond(s) into small carbocyclic rings. The smallest cyclic alkyne
that has been isolated in its free state is cyclooctyne (1-13). Cycloheptyne (1-14),

cyclohexyne (1-15) and cyclopentyne (1-16) (Scheme 2-3) can only exist as transients

and their lifetime, as expected, diminishes with decreasing ring size.11 Structural and

spectroscopic studies show that severe cis-bending in cyclic alkynes causes the partial
breakage of the in-plane x-bond as is evident by increase in an alkyne bond lengths with

the decrease in ring size. There are two important consequences of in-plane bending in

Scheme 1-3

1-13,n6 nC 6
1-14, n= 5 (CHA
1-15, n = 4
1-16, n = 3
A 1-17 B

alkynes. First, the two alkyne 7x-bonds become non-degenerate as observed by

photoelectron spectroscopy and second, noticeable lowering of the alkyne LUMO occurs
due to efficient mixing of the x* and o*-orbitals as shown by ab-initio calculations.15 The

latter property accounts for the high electrophilicity of small ring alkynes and benzynes.

Benzyne (1-17) is the best known representative of a broad class of reactive intermediates

known as arynes.1Z13 Arynes (didehydroaromatics) are somewhat similar to cyclic alkynes

because one of their resonance forms (A) has a cyclic alkyne-like structure (Scheme 1-3).

It is still not entirely clear which resonance form, alkyne A or cumulene B, is the major

contributor in benzyne. Theoretical calculations indicate that a biradical is not the ground

state of benzyne. This suggestion is supported by the experimental observation that [2+4]

cycloaddition reactions of benzyne are stereospecific, whereas [2+2] additions are not. The

strain energy incorporated into ortho-benzyne (1-17) has been estimated to be ca. 63

kcal/mol explaining the extremely high reactivity of 1-17. ortho-Benzyne reacts with furan

in a Diels-Alder fashion at temperatures even as low as 50 OKI

Cumulenes are hydrocarbons which contain consecutive double bonds sharing
common atoms.16 The first three members of this series are 1,2-propadiene (1-18) (more

frequently called allene), 1,2,3-butatriene (1-19) and 1,2,3,4-pentatetraene (1-20).

Scheme 1-4

H, C C= H HN C C C= CH C C C C C
H O" < H H, \ H H \ L
1-18 1-19 1-20

The reactivity of unsubstitiuted linear, higher cumulenes is quite high, for example

molecules such as 1-19 and 1-20 readily polymerize if not stored in dilute solutions. The

smallest cyclic allene that has been prepared in its free state is 1,2-cyclononadiene (1-21).

Smaller cyclic allenes, eight (1-22), seven (1-23) and six (1-24)-membered rings exist

only as intermediates (Scheme 1-5).14 To date there is no hard evidence for the existence of

Scheme 1-5






a five-membered ring allene (1-25). An interesting result which was obtained by molecular

orbital calculations predicts that the allene C=C=C bond can be bent for up to 20 with only

a slight increase in its strain energy (ca. 4 kcal/mol). However, beyond 200 the bending

potential rises steeply.17 Much less is known about cyclic 1,2,3-butatrienes than cyclic

allenes. The nine membered-ring cumulene (1-26) is a relatively stable molecule at room

temperature. The seven (1-27)18 and six (1-28)19 membered-ring 1,2,3-butatrienes are

Scheme 1-6




also known (Scheme 1-6) but only as transient intermediates. The strain energy in cyclic
cumulenes arises from the weakened 7t-bonds resulting from in plane bending.

The instability of the molecules described above derives from the ring induced
strain energy. In certain cases, however, high reactivity of a multiple bond is caused by

factors other than strain energy. For example, silaethene (1-29)20 and 1,1-dimethyl-3-

phenyl-3-trimethylsilyl-l-silaallene (1-30)21 (Scheme 1-7) are highly reactive and unstable

organosilicon compounds although these molecules are not strained. In this case instability

Scheme 1-7

Me. Me / SiMe3 Me3Si\
Si==CH2 Si=C=C Si-'
Me Me Ph Me3Si \OSiMe3

1-29 1-30 1-31

is caused by a weak 7t-bond between silicon and carbon.22 This generality has been

frequently challenged in recent years with the preparation of several stable compounds

containing carbon-silicon double bonds.23 However, isolation of compounds with Si=C

bonds at room temperature is only possible when very bulky substituents are present on

both the silicon and the carbon atoms (e.g. 1-3023c).

Transition Metal Complexes of Cyclic Alkynes. Arynes and Cumulenes

Transition metal complexes with reactive alkyne, aryne and cumulene ligands are
the focus of the research presented in this dissertation. In the following section the
bonding, synthesis and reactivity of such complexes will be briefly discussed.

The Nature of Bonding between Transition Metals and Carbon-Carbon c Bonds

The bonding interaction of alkynes, arynes and cumulenes with transition metal
fragments is often described by the Dewar-Chatt-Duncanson model24 (Scheme 1-8). There
are two components in this model. In the first one (a) the 7t bonding orbital of the ligand
donates election density to the vacant orbital(s) (e.g. dx2y2, dz2, Py, s) of the transition
metal whereas in the second component (b) the filled orbital(s) (e.g. dxy, Px) of the
transition metal donates election density to an empty x* anti-bonding orbital of the ligand.
Both bonding components strengthen the metal-carbon bond but simultaneously weaken
the carbon-carbon multiple bond as is evident by the elongation of the carbon-carbon

Scheme 1-8

n donor

dx2y2 acceptor

/tm, NOl

x* acceptor

dxy donor


--- y--- x


bond upon coordination to the metal center. Acceptor/donor properties of the transition
metals toward multiple C-C bonds described by this model correlate quite well with
electron affinities and promotion energies of the transition metals. In contrast to cumulenes
and alkenes, alkynes have two orthogonal double bonds which can be utilized in bonding
with transition metals. The alkyne ligand is regarded as either a 2 electron or a 4 electron
donor depending on whether one or two of the 7t-bonds interact with the metal center. The
chemical shift of an alkyne carbon in 13C NMR is sensitive to the number of it-electrons

donated by the alkyne to the metal. For 2e donors it typically appears in the range of 100-
140 ppm whereas the chemical shift for 4e donors falls in the range of 200-240 ppm.25

Transition Metal Complexes of Cyclic Alkynes

One of the most thoroughly studied transition metal cycloalkyne systems is that of
bis(triphenylphosphine)platinum(0). Platinum complexes of cycloheptyne (1-35),
cyclohexyne (1-36)26 and even cyclopentyne (1-37)8 were successfully made by alkali
metal reduction of the corresponding 1,2-dibromocycloalkenes in the presence of
Pt(PPh3)3 as shown on scheme 1-9. It is believed that the first step of this reaction involves
coordination of the dibromoalkene to the Pt(PPh3)2 fragment followed by sodium amalgam

Scheme 1-9

S0NH Br g [ C PPh3
(CH2n II + Pt(PPh3)3 Na/Hg a (CH2 -Pt


1-32, n = 5 1-35, n = 5
1-33, n = 4 1-36, n = 4
1-34, n = 3 1-37, n = 3

reduction to generate the cycloalkyne at the platinum center. Complexes 1-35 and 1-36
were also obtained by LDA induced dehydrobromination from the corresponding 1-
bromoalkenes in the presence of Pt(PPh3)3.27 The reactivity of platinum complexes
increases dramatically with decrease in the size of the coordinated cyclic alkyne. Complex
1-35 is quite unreactive whereas 1-36 reacts with a variety of reagents such as carbon
monoxide, tert-butyl isocyanide, methyl iodide and weak acids (H20, CH3OH, CH3CN)
all of which result in breaking the metal-alkyne bond.26 Complex 1-37 is the most
reactive, decomposing in THF or in CH30H to give a biplatinum complex with loss of a
C5H6. This high reactivity of complexes of the smaller cycloalkynes (1-36, 1-37)
indicates that the coordinated alkynes are still substantially strained. The palladium
equivalent of 1-36 has been synthesized and was found to be much more reactive than 1-
36.26 Another transition metal system that has been successfully used for coordination of a
variety of organic intermediates is dicyclopentadienyl zirconium(II).9 Preparation of the
cyclohexyne zirconium complex28 is displayed in Scheme 1-10. Reaction of 1-38 with

Scheme 1-10

Cp. .. }Me 'i 1
cp.Zz o + cp

1-38 1-39 CH4

S CP -, PMe3 Cp" \.




1-lithiocyclohexene yields a-complex 1-39 which decomposes via O3-H elimination to

form methane and the 16 electron complex 1-40 which, in turn, can be trapped with PMe3
to form the stable cyclohexyne complex 1-41. The mechanism of P3-H elimination in

zirconocene a-complexes such as 1-39 is thought to proceed through a four-membered

ring transition state. Complex 1-41 reacts with a wide variety of unsaturated organic

compounds, including nitriles, ketones, aldehydes, ethylene, acyclic alkynes and dienes to

form oxidative coupling products.28 A synthetic approach similar to that used for 1-41

was also used for the preparation of seven and eight membered-ring zirconocene alkyne

complexes. Five membered-ring alkyne complexes of zirconocene were also prepared29 but

only after installing two methyl groups on the carbon adjacent to the triple bond in order to
facilitate 3-H elimination.

The shift in the stretching frequency in the IR of completed alkynes relative to free
alkynes gives an approximate estimate of the extent to which the triple bond is modified by
coordination. The stretching frequency of the C=C in free cyclooctyne (CsH12) is

2260/220630 cm-1 whereas in two cyclooctyne complexes CuBr(C8gH2)30 and
Pt(PPh3)2(C8Hi2)26,31 the i (C=C) is 2060 cm-1 and 1810 cm-1 respectively. The larger

shift in the platinum complex can be explained by the lower value of the promotion energy
in Pt(0) (3.28 eV)32 as compared to that of Cu(I) (8.25 eV). The C=-C distance in

cycloalkyne complexes falls in the range of 1.28-130 A which is significantly longer than

that of free alkynes (1.20-1.22 A) and implies significant reduction of triple bond character.

Transition Metal Complexes of Arynes

The first aryne complexes that were synthesized were those of tantalum (1-42) and
niobium (1-43) (Scheme 1-11).33 The former complex was characterized by X-ray crystal

structure analysis34 which revealed noticeable bond alternation in the benzyne ligand.

Benzyne in 1-42 is oriented perpendicular to the Cp* ring in contrast to analogous ethylene
complex. These observations suggest that both orthogonal 7t-orbitals in benzyne are

involved in bonding with tantalum. Thus benzyne behaves as 4 election donor. Both 1-42

and 1-43 react with ethylene to form benzometallacyclopentene complexes. The

zirconocene complex of benzyne (1-44) was prepared in a similar way to the cyclohexyne

complex 1-41 described above.35 The chemistry of 1-44 is dominated by coupling

reactions with unsaturated reagents which include alkenes, alkynes, ketones, nitriles and

tungsten hexacarbonyl. The crystal structure of 1-44 showed that the bonds in the

Scheme 1-11



M=Ta, Nb

1-42, 1-43

Cp h... "'C
Cp' \




Cy Cy Cy Cy

Ni- Ni-

Cy Cy Cy Cy

1-46 1-47

benzyne ring are the same length, within experimental error. The ruthenium-benzyne

complex 1-45 was prepared by thermolysis of (PMe3)4RuPh2 in ether solution.36 The

mechanism of this reaction is different from that of early transition metals and involves

initial reversible dissociation of phosphine followed by oxidative addition of an ortho-C-H

and finally elimination of benzene. The chemistry of 1-45, however, resembles that of
early transition metals. Complex 1-45 undergoes a-bond metathesis with benzene and

toluene solvents, reacts with weak acids such as water, aniline, 2-propanol and undergoes
coupling reactions with unsaturated organic compounds. The nickel benzyne complex37 1-
46 and very recently the naphthalyne38 1-47 complex have been prepared. Complex 1-46
reacts readily with electrophiles such as iodide, methyl iodide and undergoes insertion
reactions with CO2, ethylene and dimethyl acetylenedicarboxylate. Several other
mononuclear benzyne complexes have been synthesised.8 Interesting extensions of
benzyne complexes are bimolecular nickle39 (1-48) and zirconium40 (1-49) complexes of
tetradehydrobenzene (Scheme 1-12). There are many metal cluster complexes of arynes and
cyclic alkynes known.8

Scheme 1-12

C y Cy Cy Cy

Ni-p 1: 71 Cp
/ / \ Me3P PMe3
Cy Cy Cy Cy
1-48 1-49

Transition Metal Complexes of Cyclic Cumulenes

In contrast to transition complexes of acyclic cumulenes41 there are only very few
known complexes of cyclic cumulenes. Platinum complexes of 1,2-cycloheptadiene (1-
51)42 and 1,2,4,6-cycloheptatetraene (1-52)43 have been prepared by trapping reactive

allenes which were generated in situ with Pt(PPh3)3. One interesting feature of 1-51 is that

it reacts with another allene to form a platinacyclopentane. Another example of a seven-

membered ring allene complex is 1-52 which was prepared by methoxy abstraction from
dicarbonyl(rs5-cyclopentadienyl)(Xll-7-methoxycycloheptenyl)iron.44,45 The carbonyl

ligand in 1-52 can be photolytically substituted by triphenylphosphine to form 1-53.46

Scheme 1-13

Ph3P, /


Ph3Pk /






n=l, 2
1-54, 1-55

Both complexes show dynamic behavior46 which will be discussed in more detail in
chapter 5. Recently seven- (1-55) and six- (1-54) membered ring allene complexes have

been prepared and structurally characterized.47 Complex 1-54 is the first and the only

example of a six-membered ring allene stabilized by a metal center. There are only three

7 +


known complexes of cyclic 1,2,3-butatrienes. Complex 1-56 was prepared by reacting

isolated 1,2,3-cyclononatriene with Wilkinson's catalyst (PPh3)3RhCl in benzene

solution.48 Complex 1-57 is unique in that the seven-membered ring cumulene contains

zirconium as a part of the ring.49 Recently a six-membered ring butatriene coordinated to

zirconium (1-58) was prepared by a methodology similar to that used to prepare

zirconocene benzyne and cycloalkyne complexes.50

Scheme 1-14

PPh3 C-C Cp,

0/p" Me3P

1-56 1-57 1-58

This dissertation presents research on transition metal complexes of reactive

alkynes, arynes and cumulenes is presented. It includes 1) the preparation and a study of

the reactivity of a palladium tropyne complex, a series of platinum benzannelated tropyne

complexes and a platinum-molybdenum tropyne complex; 2) the preparation and a study of

platinum complexes of benzannulated seven-membered ring alkynes; 3) the synthesis of a

platinum-molybdenum complex of a seven-membered ring 1,2,3-butatriene; 4) the

preparation of the first transition metal complex of a 1-silaallene.



It was found in our laboratory that the product of dehydrobromination of

bromocycloheptatrienes in the presence of Pt(PPh3)3 is strongly dependent on the base

used.27,43 If K-OtBu is used as a base proton loss from the sp3 carbon takes place leading

to the formation of the platinum complex of 1,2,4,6-cycloheptatetraene 1-51.43 However,

if the base employed is LDA (lithium diisopropylamide) the regiochemistry of elimination is
reversed (the P-hydrogen is abstracted from vinylic carbons only) leading to platinum

complexes of cyclohepta-3,5-dien-1-yne (2-2) and cyclohepta-3,6-dien-1-yne (2-3).27 It

is important to note that neither reaction produced a platinum complex of 1,2,3,5-

cycloheptatetraene 2-4 (Scheme 2-1). It is not clear why there is such a substantial

difference in the regiochemistry of dehydrohalogenation reactions by K-OtBu and LDA.

Complex 1-51 was of interest for two reasons; first, it is the only complex in which the

seven-membered monocyclic C7H6 exists in an allene form (other known complexes of

C7H6 favor the carbene form)51.52 and, second, it exhibits a dynamic behavior: the

platinum undergoes a 1,2-shift between allene bonds, a process that was shown to be

intermolecular.51 Alkyne complexes 2-2 and 2-3 afforded a unique opportunity to

synthesize a new type of aryne complex. Treatment of a mixture of 2-2 and 2-3 with

triphenylcarbenium tetrafluoroborate generated a complex of tropyne 2-5, a tropylium

equivalent of ortho-benzyne (Scheme 2-2).53 Complex 2-5 was of special interest because

it was the first known example of a tropyne molecule (which is unknown in the free state).

It was found that the chemical shift of protons that were not bonded to the platinum atom in

Scheme 2-1

LDA or
Pt3 K-OtBu

L2Pt- LDA H 5H
PtL3 Br
.- Br

L = PPh3


K-OtBu PtL3


Scheme 2-2

Ph3C+ L2Pt

L2Pt- O + L2Pt-I0

2-3 L = PPh3


2-5 are shifted to a higher field than those of the tropylium ion. This upfield shift was
attributed to possible electron donation from the platinum atom to a symmetry allowed
LUMO of the tropyne 7r-system. Complex 2-5 is air-stable at room temperature and its

reactivity is rather moderate. It does not react with acetone, methanol or acetonitrile;
however, it reacts with strong acids such as HBr and HCI to form platinum(II)
cycloheptatrienylidene complexes. Complex 2-5 can be reduced with KBEt3H to produce
a mixture of 2-2 and 2-3 although this reaction is not clean. A zirconium complex of
tropyne (2-9) was also prepared by the same method as that of the platinum analog

Scheme 2-3

Cp CZr- +


-1 +

(Scheme 2-3).54 Complex 2-9 is very thermally unstable and could only be observed at
-78 oC by NMR spectroscopy. The high instability of 2-9 hindered further reactivity
study. To further explore this kind of chemistry, we undertook the preparation of the

palladium equivalent of platinum allene complex 1-51 primarily because it was interesting

to us to compare how a change in the metal from platinum to palladium would alter the

fluxional behavior in 1-51 (the rate and mechanism of fluxionality) and its reactivity. We

also planned to prepare a palladium tropyne complex because we reasoned that such a

complex should be more reactive than its platinum analog 2-5 but less reactive than

zirconium tropyne complex 2-9, therefore providing an opportunity to explore possible

new and interesting reactions of the tropyne ligand.

Attempted Synthesis of a Palladium Complex of (2-11)

The approach attempted for the synthesis of a palladium complex of 1,2,4,6-

cycloheptatetraene was the same as that for its platinum counterpart 1-51. To a mixture of

tris(triphenylphosphine)palladium(O) and K-OtBu in THF was added a mixture of three

isomers of bromocycloheptatrienes (2-1) at room temperature. After 1 h, addition of

hexane caused precipitation of a yellow compound which turned out to be the palladium

starting material. No evidence for the formation of the desired allene complex 2-11 was

obtained; the only organic compound that was identified in the filtrate was heptafulvalene

(2-10) (Scheme 2-4). Heptafulvalene is a known product of K-OtBu induced

dehydrobromination of 2-1.55 These observations indicate that either the reactive allene

Scheme 2-4


Br :KCOtBu
S Pd(PPh3)3

not found
2-1 2-10

formed during the reaction of 2-1 with K-OtBu does not react with Pd(PPh3)3, but instead

dimerizes to give 2-10 or it reacts, presumably to give the palladium allene complex 2-11

which rapidly dissociates at room temperature releasing allene from the palladium

coordination sphere to form 2-11. This failure to form 2-11 is somewhat surprising,

especially in the view of fact that the same reaction with IDA instead of K-OtBu generates

the two expected palladium cycloheptadienyne complexes (2-12 and 2-13).

Synthesis of Palladium Complexes of Cyclohepta-3.5-dien-1-yne (2-12) and Cyclohepta-
3.6-dien-1-yne (2-13).

Complexes 2-12 and 2-13 were prepared in an analogous way to the platinum

counterparts with a modification in the workup procedure. Addition of a mixture of bromo-

cycloheptatrienes 2-1 to a mixture of Pd(PPh3)3 and LDA in THF at room temperature

followed by workup and crystallization afforded a mixture of palladium cycloheptadienyne

complexes 2-12 and 2-13 in a ratio of ca. 7:1 in 52% yield. New complexes were

Scheme 2-5

EBr LDA Ph3R" + Ph3P" i
LDA I P-\ 3 + Pd- 3'
Pd(PPh3)3 Ph3PP 2 Ph3P'
1 1'
2-1 2-12 2-13

characterized by multinuclear spectroscopy, HRMS, IR, elemental analysis and in the case

of 2-12 by single crystal X-ray analysis. The 1H NMR of the mixture of 2-12 and 2-13

displays clearly the protons attached to the sp3 carbon of the seven-membered ring as a
doublet at 8 3.67 ppm (H5) for 2-12 and a triplet at 85 2.6 ppm (H3') for 2-13 (Figure 2-

1). Full proton assignment of each isomer was accomplished based on a 2D cosy




I nf

2 28
+ -

2 a 3 V
/ \ (S


experiment. Even more informative is the 31P{ 1H) NMR which exhibits two doublets at 8

31.20 and 30.77 ppm (2Jp.p = 4.6 Hz) and a singlet at 8 31.52 ppm as expected for

complexes with Ci (2-12) and Cs (2-13) symmetry, respectively. The IR spectrum of the

mixture shows a medium size peak at 1,769 cm-1 which is assigned to the coordinated triple
bond of 2-12. The corresponding peak in the platinum analog 2-2 appears at 1712 cm-1.

The significantly lower value for the stretching frequency of the coordinated triple bond in

the platinum complex might reflect a stronger bond between the platinum and the alkyne

ligand. A similar trend in stretching frequencies is seen in palladium and platinum

complexes of cyclohexyne (Pd vs. Pt : 1780 vs. 1721 cm-1) and cycloheptyne (Pd vs. Pt :

1848 vs. 1770 cm-1).26 It has also been found that the stretching frequency in platinum and
palladium cycloalkyne complexes decreases with an increase in ring strain.8 It is therefore
interesting to note that the V (C=C) in 2-12 (1769 cm-1) is slightly lower than that of the

palladium cyclohexyne complex (1780 cm-1) although the cylcoheptadienyne complex is

certainly less strained than its cyclohexyne counterpart (the strain energies calculated by
MMX for cycloheptadienyne and cyclohexyne are 22.52 and 28.66 kcal/mol respectively).
Probably, the low value of V (C-C) in 2-12 is due to conjugation of the two double bonds

in the ring with the triple bond. The stretching frequency V (C=C) in 2-12 is considerably

lower than that of the corresponding palladium cycloheptyne complex (1848 cm-1) which,

presumably, reflects both greater ring strain in cycloheptadienyne relative to cycloheptyne

(MMX calculations indicate that cycloheptadienyne is ca. 5 kcal/mol more strained than

cycloheptyne) and the conjugation with the triple bond. Since no palladium complex

containing a strained organic carbocyclic ligand had previously been structurally

characterized, an X-ray single crystal analysis was carried out on 2-12.

X-Ray Crystal Structure Analysis of 2-12

Complex 2-12 was crystallized from a toluene/hexane mixture at -16 oC. It
crystallizes in a monoclinic P 21/c space group. The structure of 2-12 is isomorphic to that

Ph3P / PPh3

Figure 2-2. Thermal ellipsoid drawing of complex 2-12.

Table 2-1. Selected Bond



Lengths (A) and Angles (deg)

Bond Lengths (A)

2.305(1) Pd-P2
2.031(4) Pd--C2
1.834(4) P2-C51
1.838(4) P2--C61
1.826(4) P2-C71
1.258(6) C2-C3
1.364(8) C4-C5
1.360(11) C6---C7

Bond Angles (deg)

109.53(4) P1-Pd--C1
104.0(1) P2-Pd--C2
36.0(2) C2-Pd-P1
112.5(5) C1--C2--C3
134.7(5) C4-C3-C2
128.1(6) C6-C5-C4

for Complex 2-12.



of platinum complex 2-2.27 The thermal ellipsoid drawing of 2-12 is displayed in Figure

2-1 while a stereoview of the structure is shown in Figure B-1 (Appendix B). Selected

bond lengths and angles are listed in Table 2-1. Crystal structure data and final fractional

atomic coordinates are provided in Appendix A in Tables A-1 and A-2, respectively. The

coordination geometry around the palladium atom in 2-12 is square planar. The alkyne

bond distance (C1---C2) in 2-12 is 1.258(6) A and is equal, within experimental error, to

alkyne distances in platinum cycloheptyne (1-35)56 (1.283(5) A), cycloheptadienyne (2-

2)27 (1.31(2) A) and cyclohexyne (1-36)56 (1.297(8) A) complexes. The bond lengths of

C3--C4 and C5-C6 are 1.364(8) A and 1.360(11) A, respectively, which indicate as

expected the presence of the double bonds. The bond angle C1---C7--C6 of 112.5(5)0 is

consistent with C7 being sp3 hybridized. The seven-membered ring has a shape of a

shallow boat as shown by a small dihedral angle (35.0(6)0) between planes Cl---C7-C6

and C2-C3--C4--C5. Corresponding dihedral angles in cycloheptatriene derivatives are

much larger [for example for 2,5-dimethyl-3,4-diphenyl-l,3,5-cycloheptatriene57 (2-14)

and 3-anilino-2-(2,4,6-cycloheptatrien-1-yl)-2-propenal58 (2-15) (Scheme 2-6) these

values are 93.10 and 98.30 respectively]. This substantial difference between the dihedral

angle in 2-12 and, for example, in 2-14 and 2-15 is presumably caused by the

hybridization difference of carbon atoms in these compounds. A boat conformation in

cycloheptatriene derivatives (e.g. 2-14, 2-15) is a result of maintaining approximate 1200

bond angles of sp2 hybridized carbons. In complex 2-12, however, the carbon atoms

(Cl and C2) that are bonded to the palladium atom have substantial sp character,

Scheme 2-6

7 NHPh

Me Me

Ph Ph

2-14 2-15

therefore,they tend to have a larger bond angle than 1200. The bond angles around Cl1 and

C2 are 134.7(5) and 134.4(4) respectively (Table 2-1). The consequence of increased bond

angles of C1 and C2 is then considerable flattening of the seven-membered ring in 2-12.

Synthesis of a Palladium Complex of Tropyne (2-16)

With the palladium cycloheptadienyne complexes in hand, the preparation of a

corresponding tropyne complex was attempted. The mixture of 2-12 and 2-13 was

dissolved in methylene chloride and the solution was cooled to -78 OC. To this solution

triphenylcarbenium tetrafluoroborate dissolved in a minimum amount of methylene chloride

was added very slowly via a syringe. Reaction occurred rapidly at -78 oC as was evident

from the color change of the reaction mixture from yellow to dark red. A similar color
change was observed during the preparation of the platinum tropyne complex.59 After the
reaction mixture was stirred for a few h it was slowly warmed to room temperature.
Addition of ether caused precipitation of a dark solid which was shown by 1H and
31P 1H} NMR to be a mixture of unidentified products, none of which even slightly

resembled the spectroscopic characteristics of the platinum tropyne complex. The filtrate
was evaporated and the residue was dissolved in CDCl3. This 1H NMR also showed a
mixture of products among which the resonances corresponding to triphenylmethane were
clearly identified (this was confirmed by obtaining a 1H NMR spectrum of commercial
triphenylmethane under the same conditions) indicating that hydride abstraction had taken
place at some point in the reaction. These observations seemed to suggest that the tropyne
complex might have been formed but upon warming to room temperature it simply
decomposed. To check this possibility a low temperature NMR experiment was attempted.
For this purpose a mini Schlenk tube consisting of an NMR tube with an attached stopcock
was constructed. The reaction was carried out in CD2Cl2 at -78 oC. The NMR tube was
sealed and maintained at -78 oC for 1 h. After that time multinuclear NMR spectra
revealed unequivocally the formation of the palladium tropyne complex (2-16) in a high

Scheme 2-7

Ph3P\ Ph3l\ Ph3C+ Ph3\ -
Pd- )+ Pd- Pd- 0 3
Ph3P Ph3P/ -78 C Ph3P/ 2












yield (ca. 90% by 1H and 31P {1H) NMR) (Scheme 2-6). The 1H NMR of 2-16 (Figure
2-3) is almost identical to that of the platinum tropyne complex 2-5. All of the proton

resonances of the tropyne ligand are shifted downfield as compared to those of 2-12 and
2-13. The most downfield resonance in 2-16 at 8 8.52 ppm is displayed as a triplet

(3JH1-H2 = 9.6 Hz) and this resonance is assigned to proton H3 based on: (a) the area is
one half the area of the remaining vinyl peaks and (b) comparison with the platinum analog.
Decoupling of this peak caused a change in the peak at 8 8.32 ppm which therefore, was

assigned to proton H2. The highest field resonance at 8 7.83 ppm is displayed as a doublet

(3JH3-H2 = 6.3 Hz) belonging to proton Hl. Triphenylmethane, a second product of the
reaction, is clearly seen in the IHNMR at 8 5.58 ppm. Resonances in the 13C (1H) NMR

are also very similar to those of the platinum analog. Probably the most characteristic

resonance in the 13C 1H} NMR of complex 2-16 is that of the carbons attached to the

palladium atom which are displayed as a doublets of doublets due to coupling to two non-

equivalent phosphorous nuclei (2JC4-Ptrans = 85.3 Hz, 2JC4-Pcis = 4.2 Hz). The 31P
{1H)NMR and the 19F NMR exhibit, as expected, singlets at 8 24.32 and -151.84 ppm,


When the NMR tube was warmed to -50 oC in the NMR probe some

decomposition of the tropyne complex was already visible after 15 min. The half life of 2-

16 at -35 oC was determined to be ca. 3 h. We attempted to carry out some reactions with

2-16 at low temperature such as reduction with KBEt3H and LiAl(C(CH3)3)3 and reaction

with HBr. All of these reactions were performed at -78 OC followed by warming to room

temperature. In all three reactions only complex mixtures of products (probably

decomposition products) were obtained.

NMR measurements indicate that the tropyne ligand in 2-16 has very similar

electronic properties to those of its platinum analog. It is very surprising then that platinum

and palladium tropyne complexes are so different in terms of their relative thermal


stabilites. It seems that the reason for palladium trypyne complex thermal instability might

lie in a weaker bond to the tropyne ligand compared to that of platinum.



The last step in the preparation of tropyne complexes is hydride abstraction from the
corresponding cycloheptadienyne complexes and is analogous to one way tropylium ions
have been synthesized.60 Derivatives of tropylium ions (3-2, 3-4) can also be prepared
by addition of electrophiles to the carbonyl oxygen of tropone, free60 (3-1) or completed
to a transition metal (3-3)61 (Scheme 3-1). It occurred to us that by analogy to the known
tropone chemistry it should be possible, in principle, to prepare alkoxy- and hydroxy-

Scheme 3-1.

EI f


-- 1+
>- OE



substituted tropyne complexes (3-6) by alkylation or protonation of didehydrotropone (3-
5) (Scheme 3-2). We decided, therefore, to investigate the possibility of using metal

complexes of didehydrotropone to prepare new tropyne complexes. The metal system that

was chosen for exploration of this idea was bis(triphenylphosphine)platinum(0) because it

was known that the platinum complex of tropyne, which had already been prepared by
hydride abstraction, is a stable molecule.53

Scheme 3-2




OE +



Synthesis of a Platinum Complex of (3-8)

The plan was to generate didehydrotropone by dehydrobromination of 4-bromo-

tropone and traping the resulting alkyne with Pt(PPh3)3. Since dehydrobromination with

LDA has been successfully used for the preparation of platinum complexes of other
cycloalkynes27 it was initially chosen for the preparation of 3-5. Slow addition of 4-

bromotropone62 in THF to a mixture of LDA and Pt(PPh3)3 in THF however led only to
recovered starting platinum complex. When potassium t-butoxide was used in place of
LDA, an interesting platinum cumulene complex (3-8) was isolated, however in only 5%


yield (Scheme 3-3). The identity of 3-8 is based on IR, HRMS, 1H and 31P{ 1H} NMR.

The IR spectrum reveals a very strong peak at 1587.8 cm-1 which is assigned to the

carbonyl group of the ligand. The strongest signal in the HRMS (FAB) which appeared at

824.173 (m/e), is that of the molecular ion of 3-8 (calcd. for (M+1)+= 824.181). Apart

from PPh3 peaks the 1H NMR displays four different resonances which is consistent with

the absence of symmetry in the ligand (Figure 3-1) (if dehydrobromination of 3-7 resulted

Scheme 3-3


Pt (PPh3 )3 3 pt---PPh3
3-7 3-8 PPh3

in formation of the alkyne complex only two resonances would be observed due to the C2
symmetry of the ligand). The resonance at 8 6.31 ppm clearly shows platinum satellites

(3JH3-Pt = 58.5 Hz) and could be assigned to either proton H3 or H4 because both are

separated from the platinum by three bonds. However, the chemical shift of H4 should

appear at lower field then H3 due to the electron-withdrawing ability of the carbonyl group.
The resonance at 8 6.31 ppm is therefore assigned to proton H3. This was confirmed by

selective decoupling experiments and the magnitude of coupling constants between

protons. Assignment of the remaining peaks in the spectrum is based on selective

decoupling experiments. The 31P{ 1H) NMR of 3-8 (Figure 3-2) shows, as expected from
the 1H NMR, two doublets at 8 27.22 and 26.97 ppm (2Jp..p,- = 14.7 Hz) flanked


~1~~ I
*1 F

.--.j K ; I

~7:7I7 ...- ...-j-.I F
j I
F- ---I--i
___ ~ ;*j~ij*; I: *~

__ 9L -I


- - -


- ... .

-. m

9 Pea z

by 195Pt satellites ( = 3043.1 Hz, 1Jp, = 3085.5 Hz). Another peak in the
31P 1H} spectrum (Figure 3-2) with 195Pt satellites appears at 8 25.87. This peak might be

due to the symmetrical alkyne complex which may have been formed in even lower yield
than 3-8 although there is no other experimental support for this idea.

Although it is not clear why the reaction in Scheme 3-3 did not work with LDA or
is so inefficient when K-OtBu is used, it is possible that the base, rather than inducing
elimination of HBr, may react with 4-bromotropone in a different way, leading to

decomposition. The second possible explanation is that the produced reactive intermediate
(cumulene and/or alkyne) is so reactive that its side reactions are faster than reaction with
the platinum starting material. The very small amounts of 3-8 that could be isolated did not
allow us to investigate its reactivity towards electrophiles; therefore; we decided to focus on
the synthesis of a dibenzannelated analog of 3-7 with the hope that its dehydrobromination
reaction in the presence of Pt(PPh3)3 would allow the synthesis of benzannelated

didehydrotropone platinum complex in good yields.63

Synthesis of a Platinum Complex of Dibenzannelated Didehydrotropone (3-10)

It has been shown in the past that 3-9 reacts with K-OtBu generating the
corresponding alkyne which, although it could not be isolated, was successfully trapped
with both nucleophiles and dienes.64 The question was whether Pt(PPh3)3 would be an

Scheme 3-4

0 4

/ \ K-OtBu

Pt(PPh3)3 Pt
r Ph3P PPh3


efficient trapping reagent of the generated cyclic alkyne. Slow addition of 3-9 in THF to a

mixture of K-OtBu and Pt(PPh3)3 in THF resulted in formation of a dark yellow solution

which, after workup, gave the desired complex 3-10 as yellow crystals in 65% yield.

Complex 84 was characterized by multinuclear NMR spectroscopy, IR, HRMS and X-ray
single crystal analysis. In addition to PPh3 resonances the 1H NMR of 84 (Figure 3-1)

shows three different resonances of protons on the alkyne ligand (the resonance

corresponding to proton H3 is hidden under the PPh3 signals) and the methyl group of

toluene (3-10 crystallizes with toluene). The chemical shift of H1 of 3-10 appears at an

uncommonly high field for aromatic compounds. This upfield shift of HI, as will be

discussed in more detail later in this chapter, is caused by the diamagnetic shielding by

triphenylphosphine coordinated to the platinum atom. The 195Ptp H) and 31P({ H) NMR
exhibit a triplet at 8 -4745.2 ppm (JPt-p = 3391.4 Hz) and a singlet at 5 23.73 ppm,

respectively, which is consistent with structure proposed for 3-10. The most characteristic

resonances in the 13C({ H) NMR are those of the carbonyl group (195.66 ppm) and the
coordinated alkyne carbons which are displayed as doublets of doublets at 8 132.43 ppm

(dd, 2Jc-pcis = 8.23 Hz, 2Jc-ptrans = 84.56 Hz). The IR spectrum shows two strong

absorptions, one at 1622 cm-1 which is assigned to the C=O group and the other at 1708

cm-1 which is due to a coordinated triple bond. As might be expected from conjugation
with the aryl rings the C=C v in 3-10 is lower than those of platinum complexes of

cycloheptyne (1771 cm-1) and cyclohexyne (1721 cm-1).26 Finally complex 3-10 was also

investigated by X-ray diffraction analysis.

X-Ray Crystal Structure Analysis of 3-10

Crystals of 3-10 were obtained from a mixture of toluene and hexane as yellow
plates. Complex 3-10 crystallizes in Pi space group together with one molecule of toluene
which is disordered in two different positions. A thermal ellipsoid drawing of the structure

is given in Figure 3-4 while a stereoview of the structure is shown in Figure B-1

~: A



Figure 3-4. Thermal ellipsoid drawing of complex 3-10.

Ph3P /PPh3

Table 3-1. Selected Bond Lengths (A) and Angles (deg) for Complex 3-18.

Bond Lengths (A)
Pt-P1 2.290(3) Pt-P2 2.277(3)
Pt-C10 2.056(13) Pt--C 11 2.031(13)
P1-C51 1.839(11) P2--C21 1.840(11)
P1-C61 1.826(10) P2-C31 1.836(12)
P1-C71 1.844(13) P2-C41 1.826(11)
C5-O 1.27(2) C11-C10 1.283(15)
C14-C10 1.42(2) C12-C11 1.43(2)

Bond Angles (deg)
P1-Pt-P2 102.04(12) P1-Pt -C10 144.4(3)
P2-Pt ---C10 113.0(3) P2-Pt-C11 148.9(3)
C10---Pt-C1 1 36.6(4) Cll-Pt-P1 109.0(3)
C13--C5-O 115.7(13) C15-C5--O 116.3(13)
C11-C10---C14 135.8(14) C14-C10---Pt 152.3(10)
C12---C11-Pt 154.1(8) Pt-C11-C10 72.8(8)

(Appendix B). Selected bond lengths and angles are listed in Table 3-1. Crystal structure
data and final fractional atomic coordinates are provided in Appendix A in Tables A-6 and
A-7, respectively. The alkyne bond length in 3-10 is 1.283(15) A and is equal within
experimental error, to those in platinum complexes of cyclohexyne and cycloheptyne.56
The alkyne ligand is bent, with a dihedral angle between the phenyls of 146.8(5)0. This
value is almost the same as in dibenzotropone65 [142.8(6)o] which indicates that
coordination of platinum has virtually no effect on the geometry of the ligand. Complexes
of this type have an essentially square planar geometry with the donor phosphine groups
occupying cis coordination sites. The coordinated alkyne is slightly rotated from the P1-
Pt-P2 plane. In complex 3-10 the dihedral angle between planes defined by Pt, PI, P2
and Pt, C10, C11 is 12.3(7)0 and is one of the largest among all known platinum alkyne

Reactions of Complex 3-10

Reactions of 3-10 with electrophiles are summarized in Scheme 3-5. Reaction of
3-10 with HBF4 at -78 oC gave only one isolable product which was identified as the

aqua complex 3-11. The 'H NMR (Figure 3-5) shows a few peaks that correspond to
protons on the alkyne ligand and a broad peak at 8 3.8 ppm which is assigned to a

coordinated water molecule. Other spectroscopic measurements (195Pt { H), 31(1H)

NMR, IR, HRMS) and elemental analysis are also consistent with the proposed structure

of 3-11. An analogous product was obtained from reaction of the platinum cyclohexyne

complex (1-36) with HBF4.26 Reaction of 3-10 with HBr also does not lead to an

oxygen-protonated product but instead yields 3-12 which is consistent with oxidative

addition of HBr to the platinum center. The 31p( 1H) NMR of 3-12 exhibits two doublets
at 8 15.2 and 16.77 ppm (2Jpl-p2 = 15.4) for P1 and P2, respectively, which is consistent

Scheme 3-5

IC= N-1-

Ph3P PPh3

HBF4, H20





Ph3P- Pt PPh3



Ph3P2- Pt Br










, j?

* V

- p.o

with a cis-configuration for 3-12. 195Ptp 1H} NMR shows a doublet of doublets centered

at 8 -4554.3 ppm with two different coupling constants to Pl(1Jpt-P1 = 1686.2 Hz) and

P2 (1JPt-P2 = 4432.2 Hz). This difference in coupling constants is due to a stronger trans

influence of the alkenyl group than that of the chlorine.66 Complex 3-12 does not undergo

conversion to thetrans isomer even after two months in methylene chloride. Complex 3-

13 was also obtained by oxidative addition of 3-9 to Pt(PPh3)3 in THF. These products

are presumed to result from attack of the proton on the platinum center. Attempts to alkylate

3-10 with (Me3O)+BF4 were also unsuccessful; no reaction was observed. However,

when water was added to a reaction mixture, complex 3-11 was formed presumably as a

result of electrophilic attack of H30+ formed by hydrolysis of the oxonium salt.

Although our initial goal was not realized we decided to investigate the reactivity of

3-10 in more detail. Complex 3-10 is relatively inert; it does not react with weak acids

such as ethanol or acetonitrile even when warmed to 80 oC for 2 days. It also does not react

with methyl iodide, dimethyl acetylenedicarboxylate or phenylacetylene. This lack of

reactivity is very similar to that of the platinum complex of cycloheptyne (1-37).26

Complex 3-10 reacts rapidly, however, with tert-butyl isocyanide to form a single

phosphine displacement product 3-13 (Scheme 3-5). Further ligand exchange or other

reactions with tert-butyl isocyanide could not be induced, even at elevated temperature.

This result is somewhat different from platinum cyclohexyne (1-36) and cycloheptyne (1-

37) complexes. Complex 1-37 was reported to be entirely inert towards tert-butyl

isocyanide whereas 1-36 underwent both phosphine displacement and isocyanide insertion

between platinum and the alkyne carbon.26 The IR spectrum of 3-13 shows bands at

1710.9 and 2155.5 cm-1 due to the coordinated triple bond and the isocyanide ligand,

respectively. The 195Pt( 1H) NMR spectrum shows a resonance centered at -4694 ppm as

a broad doublet which is a result of coupling to one phosphine and the 14N nucleus.

Synthesis of Trimer 3-14

Bennett and coworkers have found that tetracyanoethylene (TCNE) reacts with
bis(triphenylphosphine)platinum complexes of cyclohexyne (1-36) and cycloheptyne (1-
37) to give the bis(triphenylphosphine)platinum complex of TCNE quantitatively and
presumably, although not identified, free cycloheptyne and cyclohexyne.26 It occurred to
us that it might be possible to release dibenzotropynone from 3-10 in the same way and
observe its reactivity. Treatment of a C6D6 solution of 3-10 with one equivalent of TCNE

led to a rapid change in color from yellow to dark red-brown. The 1H NMR spectrum of
the crude reaction mixture showed complete loss of 3-10. Addition of hexane precipitated
a complex which after analysis was identified as the TCNE adduct of
bis(triphenylphosphine)platinum. The filtrate was passed through a short silica-gel column
using methylene chloride/hexane mixture and the resulting solution was evaporated leaving
a yellow compound. The 1H NMR spectrum of this material (Figure 3-6) shows four
resonances: one doublet, one doublet of doublets and two triplets of doublets. The
13C(1H) NMR spectrum displayed 7 peaks in the aromatic region and one at 197.42 ppm

indicating the presence of a carbonyl group. Based on these NMR data the structure of the

Scheme 3-6

T o + Pt
Pt /
S/Ph3P PPh3
Ph3P PPh3




new compound can be formulated as that of dibenzannelated dibenzotropynone, its dimer,
trimer or tetramer etc.(because of symmetry all of these would have exactly the same

number of resonances in both the 1H and the 13C{ 1H) NMR). The problem of which of

these is actually formed in the reaction of 3-10 with TCNE was solved by HRMS (FAB)

which showed an intensive peak at 613.1835 (m/e). This peak corresponds to the mass of

trimer 3-14 (calcd. for (M+1)+ = 613.1804 ) (Scheme 3-6). The structure of 3-14 was

confirmed by an X-ray crystal structure analysis.

X-Ray Crystal Structure Analysis of 3-14

Crystals of 3-14 were grown by slow evaporation of a methylene chloride/hexane

solution at room temperature. The structure of 3-14 was solved in a triclinic space group

Pi using Direct Methods. Thermal ellipsoid and stereographic drawings of 3-14 are

presented in Figures 3-7 and B-i (Appendix B), respectively, while selected bond lengths

and angles are listed in Table 3-2. Crystal data and final fractional atomic coordinates are

provided in Tables A- 11 and A-12, respectively. The structure shows three dibenzotropone

fragments fused to the benzene ring located in the center of the molecule. To minimize non-

bonding interactions between the large dibenzotropone fragments, two of them bend in

opposite directions forming dihedral angles between phenyl rings in the two fragments of

112.30 and 115.90, respectively. The third fragment with a dihedral angle between phenyl

rings of 158.20 is positioned in a unique way with one phenyl ring above and the other

below the middle benzene ring. The consequence of such a spatial arrangement of these

three fragments is a substantial twisted boat deformation (Figure 3-8) of the benzene ring in

the center of the molecule. This distortion is quite severe and is comparable to other

previously reported distorted benzenes. For example, the dihedral angle between planes

C10--C11---C40 and C26-C25--C41 is 38.4(4)0 is equal, within experimental error, to

the corresponding dihedral angle in 8,9-dicarbomethoxy-[6]-para-cyclophane67 (38.90) but

is less than the most highly distorted ring reported to date, perchlorotriphenylene68

Figure 3-7. Thermal ellipsoid drawing of compound 3-14.

Table 3-2. Selected Bond



Lengths (A) and Angles (deg) for Complex 3-14.

Bond Lengths (A)
1.404(5) C26-C10 1.433(4)
1.398(4) C41-C25 1.424(5)
1.394(4) C40-C1 1 1.430(5)
1.490(5) C12-C10 1.488(4)
1.224(5) C20-02 1.210(5)
1.214(4) C13-C5 1.482(6)
1.405(4) C14-C15 1.418(5)

Bond Angles (deg)
119.0(3) C26-C10--C11 117.6(3)
119.5(3) C41--C25--C26 118.4(2)
119.1(3) C40-C11--C10 116.3(3)
124.5(3) C28-C20--C30 114.3(3)

which has a dihedral angle of 54.10. Other more severely distorted benzenes include
tetramethyl [6](9,10)anthracenophane69 (49.30 dihedral) and [6](1,4)anthracenophane7o
(420 dihedral). Theoretical work on deformed benzenes has also recently appeared.71 There
is a slight bond alternation (0.03 A) in the middle benzene ring of 3-14, presumably as a
result of the distortion. In perchlorotriphenylene this value is larger (0.06 A) which is
consistent with more severe distortion in this molecule.72

C11 C25

C10 C41

Figure 3-8. Distortion of the central benzene ring in 3-14.

The trimer 3-14 exhibits approximately C2 geometry in the solid state. If this

structure is maintained in solution, however, it must undergo rapid conformational
equilibriation because the 1H NMR clearly shows only four different kinds of aromatic

hydrogens (if 3-14 were not equilibrating it should show twelve) and the 13C NMR

spectrum shows only eight kinds of carbon while non-equilibrating 3-14 should show
twenty three. Furthermore, if it has C2 symmetry in solution, the inversion barrier must be

quite low since the 1H NMR spectrum at -80 OC (C6D5CD3) showed no significant


In order to find out how general this trimerization reaction is synthesis of two other

platinum alkyne complexes were considered, one (3-18) in which the carbonyl is replaced

by oxygen and a second (3-19) in which CH2 replaces C=O. Initial attempts to prepare 3-

18 and 3-19 were exactly analogous to those used for 3-10. In an attempt to synthesize

the starting material for preparation of 3-19, Clemmensen reduction of 3-9 was attempted.

Unfortunately, although this reduction produced the desired product, it was highly impure

and formed in very low yield. As an alternate approach to the preparation of a suitable

precursor to 3-19, dibenzocycloheptatriene (which we found could be easily prepared in

good yield from reduction of dibenzosuberenol with NaI/Me2SiCl2)73 was treated with

bromine to give 3-17. Initial attempts to mono dehydrobrominate 3-17 to give an

analogue to 3-9 were not very successful. However, double dihydrobromination with

Scheme 3-7


/ K-OtBu

Pt(PPh3 )3 Pt
Br Br Ph3P PPh3

X= 0, CH2 X= 0 CH2
3-16, 3-17 3-18, 3-19

potassium t-butoxide in the presence of Pt(PPh3)3 cleanly gave the disired complex 3-19.
The same method was also successfully used to prepare 3-18 (Scheme 3-7).
Spectroscopic features of 3-18 and 3-19 are similar to those of 3-10. The infrared
spectra of 3-18 and 3-19 showed alkyne absorptions at 1691 and 1689 cm-1,
respectively. The 1H NMR spectra of both complexes show an uncharacteristic upfield
shift of the aromatic protons nearest the triple bond (HI). This upfield shift was shown in
two ways to be due to diamagnetic shielding by triphenylphosphine coordinated to the
platinum.74 First an NOE study of 3-18 was carried out. Irradiation of the ortho protons
of the triphenylphosphine led to an 11.9% enhancement of H1 indicating close proximity of
HI and the phenyl groups of PPh3. We reasoned that if triphenylphosphine is really the
cause of this upfield shift then replacing PPh3 with a ligand which is incapable of
significant shielding should have a substantial effect on the chemical shift of the high field
protons. New alkyne complexes (3-20, 3-21) were therefore prepared by PPh3 exchange
with 1,2-bis(dicyclohexylphosphino)ethane75 (Scheme 3-8) and their 1H NMR measured.
Indeed, the chemical shifts of all ligand protons appeared below 7 ppm (Figure 3-9). The V

Scheme 3-8

Pt Pt

Ph3/ PPh3 CY2 .PCy2

X= C=0, 0 X= C=0, 0
3-10, 3-18 3-20, 3-21






















of the C=C in 3-21 shows a red shift of 18 cm-1 as compared with 3-18 which could be

expected from the increased basicity of the phosphine ligand. However, the stretching
frequency of the same bond in 3-20 is almost identical to that of 3-10. The 195Pt( IH}
NMR of 3-20 and 3-21 display triplets centered approximately 300 ppm higher field than
those of 3-10 and 3-18.
With the new platinum alkyne complexes in hand their reactions with TCNE were
studied. First, complex 3-13 was treated with TCNE in C6D6, giving trimer 3-14 in

yields comparable to those of complex 3-10 together with two unidentified platinum
products which were detected in the 31P([H) NMR. Treatment of complex 3-20 under
the same condition gave a mixture of organic products containing only about 5-10% (by 1H
NMR) of 3-14. [(Cy2PCH2)2Pt(TCNE)] (3-22) was formed quantitatively in this

reaction. The reason for the difference in behavior is not clear. Reaction of 3-19 with
TCNE generated 3-15 quantitativaly, however, no organic product could be isolated.
Treatment of 3-18 with TCNE (Scheme 3-9) led to an interesting product. In
addition to 10% of the TCNE complex of bis(triphenylphosphine)platinum(0) (3-15) (31p
NMR), 3-18 gave the coupling product 3-23 in 55% isolated yield. Such oxidative

Scheme 3-9

Pt\ p CN
Ph3P PPh3 Ph3Pl P2Ph3





ma B


ma 0

ma 0..
ma U)

* z









- m







coupling reactions are very common for early transition metals9 but quite unusual for

platinum alkyne complexes.76 The initial structural assignment to 3-23 was based on

elemental analysis, IR and multi-nuclear NMR. The IR spectrum showed a weak

absorption at 2216 cm-1 indicating the presence of cyano groups in the complex. The 1H

NMR spectrum of this material showed eight different protons belonging to the ligand.

The assignment (Figure 3-10) of the protons in the dibenzannelated oxepin fragment relies

on two assumptions. First, that the highest field doublet at 8 6.37 ppm corresponds to

proton H8. This assumption is based on the close proximity of H8 to PPh3 which causes

its upfield shift. The X-ray crystal structure of 3-23 videe infra) clearly shows that one of

the PPh3 phenyl rings is positioned parallel to a phenyl ring of dibenzannelated oxepin

fragment bearing H5-H8 (Figure 3-12). All these protons are therefore shifted upfield

relative to protons (H1-H4) on the other phenyl ring. The assignment of protons H5 to H7

was made by selective decoupling experiments. The second assumption is that the most
downfield doublet at 8 7.88 ppm corresponds to proton H4. The downfield shift of H4 is

believed to be caused by the electron withdrawing oxygen in the ligand. The assignment of

the remaining protons was again made by selective decoupling experiments. In the
31P({H) NMR, two phosphorus nuclei (PI and P2) were observed as nonequivalent

doublets at 8 13.7 and 14.6 ppm, respectively (2Jpi.P2 = 20.5 Hz) (Figure 3-11). The
195pt{ 1H) NMR exhibited a doublet of doublets centered at 8 -4347.3 ppm with two

different coupling constants to P1 (1Jpt.p1 = 3259.4 Hz) and P2 (1Jptp2 = 1918.2 Hz).

This difference in coupling constants is presumably due to a stronger trans influence on P2

than on Pl. The structure of complex 3-23 was confirmed by X-ray crystal structure


X-Ray Crystal Structure Analysis of 3-23

Crystals of complex 3-23 were obtained by slow evaporation of a methylene

chloride/hexane solution. Complex 3-23 crystallizes in a monoclinic space group la

Figure 3-12. Thermal ellipsoid drawing of complex 3-23.

Ph3P / PPh3
Pt N


Table 3-3. Selected Bond


P2-Pt -C9

Lengths (A) and Angles (deg) for Complex 3-23.
Bond Lengths (A)
2.305(3) Pt-P2 2.370(3)
2.090(9) Pt-C16 2.161(9)
1.375(14) C14-O 1.42(2)
1.10(2) C17-N2 1.11(2)
1.14(2) C20-N4 1.14(2)
1.327(14) C13-C9 1.474(15)
1.475(14) C16-C15 1.609(14)
1.51(2) C18-C15 1.51(2)
1.465(14) C20-C16 1.46(2)
Bond Angles (deg)
97.50(11) P2-Pt -C16 93.2(3)
168.2(2) P1-Pt-C16 165.0(3)
77.0(3) Pt--C16--C15 98.7(6)
117.0(7) C16-C15--C10 104.7(8)
108.5(9) C17-C15-C18 106.6(9)
108.2(9) N2--C17-C15 177.4(12)
174.4(13) N3-C19-C16 175.3(13)

together with one molecule of methylene chloride solvent. Thermal ellipsoid and
stereographic drawings of 3-23 are presented in Figures 3-12 and B-2 (Appendix B),
respectively, while selected bond lengths and angles are listed in Table 3-3. Crystal data
and final fractional atomic coordinates are provided in Tables A-16 and A-17, respectively.
Complex 3-23 can be viewed as a platinacyclopent-2-ene with the platinum bonded to two
phosphorous and two carbon atoms. The Pt--P1 bond [2.305(3) A] is shorter than Pt-P2
[2.370(3) A] as expected from two different 195Pt--31P coupling constants. Correlation
between bond distances of Pt-P and 195Pt-31P coupling constants is well established.66
The Pt-C9 bond [2.090(9) A] is shorter than Pt-C16 [2.161(9) A], a difference which is
attributed to the different hybridization of the two carbons bonded to the platinum atom.
From these data it is clear that C9 (sp2) exerts a stronger trans influence than does C16
(sp3). The Pt-C9 and Pt--C16 bond lengths are equal, within experimental error, to the
corresponding bond distances observed in Pt(cyclohexenyl)(CH2COC6H5) (diphos) [Pt-

C(sp2) = 2.068(10) A, Pt-C(sp3) = 2.175(10) A].77 Complex 3-23 has a distorted

square planar geometry around the Pt atom with the dihedral angle between the planes

defined by Pt, Pl, P2 and Pt, C9, C16 equal to 12.5(3)0. The dihedral angle between the

phenyl ring planes of the dibenzoxepin ligand [109.6(4)0] is much smaller than that of free

dibenzoxepin78 [134(2)]. The most pronounced feature of the structure of 3-23 is the

presence of a platinacyclopent-2-ene ring which has a geometry of a half-chair. The atoms

Pt, C9, C10 and C15 are coplanar (max. deviation from the least square plane is 0.016 A

for C10) while C16 lies 0.97 A below the plane. The dihedral angle between the Pt--C9--

C10--C15 and the Pt-C16-C15 plane is 54.7(6)0. Such distortion from planarity is a

common feature of metallacyclopent-2-enes.79 This distortion, however, is significantly

larger in 3-23 than in any other known metallacyclopent-2-enes known for which crystal

structure data is available.80 It is not clear why 3-10 and 3-18 react differently with

Synthesis of Platinum Tropyne Complex 3-24

As was discussed earlier, attempts to prepare platinum tropyne complexes by
alkylation of a carbonyl group were not successful. However, the platinum complex of

dibenzannelated didehydrocycloheptatriene (3-19) provided the opportunity to synthesize a

benzannelated tropyne complex by hydride abstraction, a method which was successfully

employed for the preparation of platinum53 (2-5) zirconium54 (2-9) and palladium (2-16)

complexes of tropyne. Treatment of 3-19 with triphenylcarbenium tetrafluoroborate in

methylene chloride at -78 oC, followed by warming to room temperature gave a deep blue

solution. Addition of diethyl ether gave 3-24 as deep blue-black needles (Scheme 3-10).

Complex 3-24 was characterized by 1H, 13C{1H), 19F, 31P[1H), 195Ptp1H) NMR,

HRMS and elemental analysis. In addition to PPh3 signals, the 1H NMR (Figure 3-13)
shows five different resonances which are substantially deshielded relative to 3-19. The
31P({H) and 195Pt(pH) NMR display a singlet at 5 19.96 ppm and a triplet centered at 8

-4060.2 ppm, respectively (Figure 3-14). The electronic structure of 3-24 is somewhat

Scheme 3-10

Ph3P PPh3

CPh3 BF4



7 +

Ph3P PPh3





Ph3P- Pt Br




_- +

L = PPh3

Ph3P- Pt -PPh3








-2 .


different from that of the parent tropyne complex 2-5 in that the positive charge in 3-24

resides to a greater extent on the ligand. This is best shown by the 1H and 195Pt{ 1H}

NMR. The chemical shift of H5 in 3-24 is the same (10.45 ppm) as that of the

corresponding proton in the dibenzotropylium ion.81 However, the same proton in

complex 2-5 (8.64 ppm) shows a significant upfield shift when compared to the tropylium

ion (9.55 ppm).82 Similarly, the chemical shift in the 195Ptp lH) NMR of 3-24 is 280 ppm

upfield relative to 2-5. Both of these differences are expected if the platinum atom more

effectively delocalizes the positive charge in 2-5 than in 3-24.

Reactions of Platinum Tropyne Complex 3-24

Tropyne complexes of platinum have two reaction sites, one on the ring which is
susceptible to nucleophilic attack and the other on the metal center where electrophilic attack

would be expected. Examples of these two reaction types for 3-24 are shown in Scheme
3-10. Reaction with KBEt3H is rapid and gives 3-19 in 70% yield (1H and 31P NMR).

Three minor phosphorus containing platinum products were formed in this reaction as

shown by 31P 1H) NMR, but they were not characterized. The bidentate phosphine,

bis(dicyclohexylphosphino)ethane also reacts rapidly (within seconds) with 3-24 to form

the bis alkyne complex 3-25. No triphenylphosphine displacement was observed in this
reaction. The 31P( 1H) NMR of this material exhibits two peaks, one centered at 8 21.95

ppm and the other at 5 34.93 ppm. The former resonance belongs to a PPh3 as is evident

by the presence of its 195pt satellites whereas the latter belongs to PCy2. The ratio of

cyclohexyl to phenyl protons in the 1H NMR is consistent with the stoichiometry of
complex 3-25. This reaction parallels the reaction of metal Tr7-tropylium complexes with

diphosphines.83 Electrophilic addition to the metal center was demonstrated by addition of

HBr in acetic acid to a THF solution of 3-24 which led to a rapid color change from deep

blue to purple. Addition of ether gave 3-26 as a purple precipitate. The 31P{ 1H) NMR of
3-26 showed two doublets at 5 13.96 and 15.03 ppm with 195pt satellites indicating


formation of the cis-isomer. Upon standing, this complex slowly isomerized to the trans-
isomer (3-27) (ca. 70% conversion after 4 weeks at rt in CD2Cl2). The same types of

products were obtained from reaction of 2-5 with HBr and HCI.53



The chemistry of the tropyne complexes that have been prepared has not been

explored in detail, mainly because two of them, complexes of zirconium54 (2-9) and

palladium (2-16) are very thermally unstable. Since the platinum analog (2-5) is the only

known tropyne complex that is stable at room temperature, this molecule was the best

candidate for further reactivity study.

In the tropyne and cycloheptadienyne complexes described thus far, the ligands are
bonded to a transition metal in an r12 fashion. Each of these ligands, however, possesses a

set of x-orbitals perpendicular to the plane of the ligand. The tropylium ion and

cycloheptatriene, which are structurally similar to the completed tropyne and
cycloheptadienyne, are known in organometallic chemistry as good X-ligands and many

complexes of both ligands have been prepared and characterized.3,22 By analogy to the

tropylium ion and cycloheptatriene complexes we decided to investigate the possibility of
coordinating a second transition metal to a 7i-system in platinum complexes of tropyne and


The transition metals that have been used most often in the preparation of tropylium

ion and cycloheptatriene complexes are those of group VI, chromium, molybdenum and

tungsten. Cycloheptatriene complexes are typically prepared by substitution reactions

whereas tropylium ion complexes are formed by hydride abstraction from the

corresponding cycloheptatriene complexes. Of the group VI metals, molybdenum has been

found to undergo substitution reactions faster than tungsten or chromium.84 We chose it as

a potential candidate for coordination to complexes of cycloheptadienyne and tropyne

Synthesis and Spectroscopy of Bimetallic Complexes of Cyclohepta-3.5-dien-1-yne (4-5)
and Cyclohepta-3.6-dien-1-yne (4-6)

It has been reported that treatment of (T16-arene)Mo(CO)3 with a more basic

aromatic ring in THF or acetone leads to rapid arene exchange at room temperature.85
Cycloheptadienyne complexes 2-2 and 2-3 can be viewed as substituted cycloheptatrienes
and since cycloheptatriene is known to bind to the Mo(CO)3 fragment more strongly than
arenes (by ca. 7 kcal/mol)86 it occurred to us that it might be possible to use such an arene
exchange reaction to attach the Mo(CO)3 fragment to the seven membered ring in 2-2 and
2-3 (Scheme 4-2). A model reaction was carried out first in order to determine if the
Mo(CO)3 fragment could be transferred from p-xylene to cycloheptatriene at room
temperature. When to a THF solution containing one equiv. of (qT6-p-xylene)Mo(CO)3 (4-

2) was added to one equiv. of cycloheptatriene (4-1) a sudden color change from yellow
to red was observed. After 15 min. of stirring the THF was removed in vacuum and the
residue was dissolved in C6D6. The 1H and 13C{ 1H) NMR showed, within limits of
detection, quantitative formation of complex 4-3 (Scheme 4-1) (the spectral data of the

Scheme 4-1

O/ \THF + O

Mo Mo
OC' k CO OC' 0 CO

4-1 4-2

newly formed complex are identical to those obtained from an independently synthesized

sample of 4-2).87 This result encouraged us to perform an analogous reaction with 2-2
and 2-3. Addition of THF-d8 to a mixture of 2-2, 2-3 (6:1; total one equiv.) and (T16-p-

xylene)Mo(CO)3 (4-2) in an NMR tube led to a rapid color change from yellow to deep

red. After 15 min., analysis (1H and 31P NMR) showed total disappearance of starting

materials and formation of 4-5 and 4-6 in about 95% yield in a 6: ratio. This was the

same as the ratio of 2-2 to 2-3. The mixture of 4-5 and 4-6 was crystallized from

benzene/hexane in ca. 65 % yield. In addition to the PPh3 signals, the 1H NMR of 4-5
(the major isomer) exhibited six different proton resonances, two of which (at 8 5.55 and

3.51 ppm) showed coupling to the 195Pt nucleus (Scheme 4-1). The peak at 8 5.55 ppm is

Scheme 4-1

Ph3P"131. / H4

H1 H2

Ph3P or

Ph3Pi. pt- H4-
Ph3PO H. 2
H1 H 3

4-5 Co co

Ph3P'i... _H
+ Ph3P. Pt/ / H
Hl' Hi

(TI6-p-xylene)Mo(CO)3. (4-2)


Ph3Ps,. "; H Hu'
PhP Pt-// H
HPh3P* < Hdi

4-6' VC co
4-6 co co

assigned to the vinyl proton H1 while the peak at 8 3.51 ppm is assigned to the methylene

proton Hd. The second methylene proton Hu (8 3.21 ppm) is displayed as a doublet due to

geminal coupling to Hd (2JHu-Hd = 15.7). The X-ray crystal structure videe infra) shows

that bond vector C-Hu is almost orthogonal to the C-H4 and Pt-C vectors explaining the

absence of coupling to either H4 or the 195Pt nucleus. The minor isomer 4-6 shows four
peaks with only one (at 8 4.99 ppm) coupled to the 195Pt nucleus. The peak at 8 4.5 is

therefore assigned to HI'. The remaining signals in the spectrum of 4-5 and 4-6 are

assigned based on a 2D cosy experiment (Figure 4-2). The proton resonances of both 4-5

and 4-6 are shifted upfield relative to 2-2 and 2-3. The 195Pt 1H) NMR of the mixture
of 4-5 and 4-6 shows a doublet of doublets centered at 8 -4480.4 ('Jpt-p' = 3326.4 Hz,

1Jpt-p" = 3418 Hz) and a triplet at 5 -4449 (1Jpt-p = 3323 Hz) as expected for complexes

with Ci (4-5) and Cs (4-6) symmetry, respectively (Figure 4-3). The 31p( 1H} NMR

displays two doublets and a singlet for 4-5 and 4-6, respectively (Figure 4-3). The room

temp. 13C{ 1H) NMR spectrum of 4-5 and 4-6 does not exhibit any signals in the

carbonyl region. The spectrum obtained at -20 OC however, reveals three carbonyl signals

of equal intensity that are assigned to the major isomer 4-5. This observation is a result of

rapid rotation of the molybdenum tricarbonyl moiety around the axis perpendicular to the

seven membered ring. The coalescence temperature for this fluxional process must lie in the

range of 10-45 oC although we were unable to determine precisely its value due to limited

solubility of 4-5 and 4-6 in organic solvents. The IR spectrum of the mixture shows a

medium size absorption at 1610 cm-1 which is assigned to the triple bond coordinated to the

metal center in 4-5. The same absorption in 2-227 is at 1710 cm-1 which indicates that

coordination of molybdenum to 2-2 causes a decrease in the bond order of the triple bond.
The IR spectrum also shows three very strong bands (1956.7, 1886.8, 1845.7 cm-1) in the

metal carbonyl region. Interestingly these carbonyl stretches have a lower frequency (ca. 15
cm-1) than those of tricarbonyl molybdenum cycloheptatriene (4-3) (1970, 1908, 1856 cm-
1),88 which suggests that the seven membered ring in 4-5 is more electron rich than

-.4 U
~YXJ~ /0

\'~4 "8



* *0

* 0
* 0

* 0



'-"8 8

0 0





B 0 < *

*o $ CON








* bO


cycloheptatriene (4-1) itself.89 To test this suggestion, a mixture of 2-2 and 2-3 ( 6:1;
total one equiv.) was added to two equiv. of 4-3 in CD2C12 in an NMR tube at room
temperature and formation of 4-5 and 4-6 was monitored by 1H and 31p{ 1H) NMR
(Scheme 4-3). Within experimental detection, both platinum complexes showed complete
conversion to the corresponding Mo(CO)3 complexes, although at different rates. Reaction
of 4-5 was complete within 12 h whereas the minor isomer required four days. The
reason for this reactivity difference is not clear. In a control experiment one equivalent of
4-5, 4-6 was mixed with three equiv. of cycloheptatriene and monitored as above. As

Scheme 4-3


+ LPt--/



+ 2Z
i' V -co



L = PPh3


expected, no reaction was observed even after one week at room temperature. The fact that

the Mo(CO)3 moiety transfers from 4-3 to 2-2, 2-3 and that the equilibrium lies, within

detection, exclusively toward the more sterically hindered reagents is clear evidence of the
greater basicity of the cycloheptadienyne ring in the bis(triphenylphosphine)platinum

complexes when compared with cycloheptatriene. The structure of complex 4-5 was

determined by X-ray single crystal analysis.

X-Ray Crystal Structure Analysis of (PPh 2Pt L __iC2H )Mo(CO)- (4-5)

A deep red crystal suitable for X-ray diffraction study was obtained from a mixture
of hexane and benzene at 4 oC. Thermal ellipsoid and stereographic drawings of 4-5 are

depicted in Figures 4-4 and B-2 (Appendix B) respectively while selected bond lengths and

angles are listed in Table 4-1. Crystal data and final fractional atomic coordinates are

provided in Tables A-21 and A-22, respectively. Complex 4-5 crystallized in monoclinic,

centrosymmetric space group P 21/n. The geometry around the Pt atom is square planar

with Pt atom coordinated to two P atoms and two carbon atoms belonging to the seven-

membered ring. The C1-C2 distance is 1.292(14) A and is comparable to other transition

metal cycloalkyne complexes.26,60 Six carbons (C1-C6) of the seven membered ring are

almost coplanar (max. deviation from the least square plane is 0.014 A for C4) whereas the

C7 is 0.44 A above this plane. The bond angle C1---C7--C6 of 107.8(10)0 is consistent

with C7 being sp3 hybridized. The dihedral angle between planes defined by C1-C2-C3-

C4-C5-C6 and C1-C7-C6 is 150(1)o. This angle is significantly larger than the

corresponding angle (from 1280 to 1380)90 found in any other reported molybdenum

cycloheptatriene complexes. It is thought that the reason for this difference lies in the

hybridization at Cl and C2, which is intermediate between sp and sp2. Because of the sp

character of C1 and C2 the bond angles about these atoms tend to be larger than 1200

which results in flattening of the seven-membered ring.









Figure 4-4. Thermal ellipsoid drawing of complex 4-5. Phosphine phenyl rings are
omitted for clarity.

Table 4-1. Selected Bond Lengths (A) and Angles (deg)

Bond Lengths (A)

Pt-P1 2.275(2) Pt-P2
Pt-C1 2.033(10) Pt-C2
Mo--C1l 2.453(10) Mo-C2
Mo-C3 2.379(10) Mo-C4
Mo--C5 2.35(2) Mo-C6
Mo-C8 1.950(14) Mo--C9
Mo--Cl 2.008(11) C1-C2
C1--C7 1.524(15) C2--C3
C3-C4 1.39(2) C4-C5
C5--C6 1.36(3) C6-C7

Bond Angles (deg)
Pl-Pt-P2 102.42(8) Pl-Pt --Cl
P2-Pt -C1 114.4(3) P2-Pt-C2
Cl -Pt-C2 37.1(4) C2-Pt-Pl1
C2-C1-C7 133.9(11) C2-Cl-Pt
C3-C2-Pt 152.0(8) C3-C2-C1
C4-C3--C2 121.6(11) C5--C4--C3
C6-C5-C4 134.(2) C7-C6--C5
C1-C7-C6 107.8(10) Mo-CS-01
Mo-C9--C2 175.7(11) Mo-ClO--C3

for Complex 4-5.



Preparation of a Bimetallic Complex of Tropyne (4-9)

Successful preparation of bimetallic complexes of cycloheptadienynes (4-5, 4-6)

raised the question of whether the Mo(CO)3 fragment can also be transferred in the same

way to the tropyne complex 2-5. The properties of the tropylium ion videe infra) suggest

that such a Mo(CO)3 transfer should be feasible from a thermodynamic standpoint. It has
been found91 that complex [Mo(T7-C7H7)(T16-C6H6)]+ reacts readily with nucleophiles

causing smooth displacement of the benzene and formation of new tropylium complexes
[Mo(Tl7-C7H7)L3]+, (L = MeCN, PMe2Ph) in high yields. These experiments suggest that

the tropylium ion may be bonded more strongly than arenes to molybdenum. A crystal
structure92 of Mo(T17-C7H7)(CO)3 shows that the bond length between molybdenum and

the ring carbon is one of the shortest (2.314 A) found in arene complexes also indicating a

strong bond between molybdenum and the tropylium ion. A model reaction was carried out
in order to: a) determine if the tropylium ion (4-7) can efficiently displace p-xylene from
complex 4-2, and b) to confirm the greater bond strength between Mo-C7H7 relative to
Mo-C6H6. When white tropylium ion 4-7 was added to a yellow solution of 4-2 an
immediate color change was observed. White 4-7 disappeared in a few minutes and a dark
orange crystalline precipitate appeared. After 1 h of stirring the solution was decanted and
the precipitate was washed several times with hexane. The properties (IR, mp.) of the
orange complex are identical to authentic Mo(T17-C7H7)(CO3)]+ (4-8) which was

Scheme 4-4

+ + 0
Mo Mo

4-7 4-2 4-8 4-4

synthesized by a literature method87 (Scheme 4-4). With this result in hand the Mo(CO)3
transfer to tropyne complex 2-5 was attempted (Scheme 4-5). Addition of a mixture of
CD2Cl2 and THF-d8 to one equiv. of 2-5 and one equiv. of (Ti6-p-xylene)Mo(CO)3 (4-2)

at room temperature led to an essentially instantaneous color change from red to brown-
red. Comparison of the NMR spectra of this solution with the tropyne complex
synthesized more conveniently by hydride abstraction from a mixture of 4-5 and 4-6 videe
infra) confirmed the essentially quantitative yield of 4-9 (Scheme 4-5). Unfortunately, we
were unable to grow crystals of this red-brown solid that were suitable for X-ray.

However, the bimetallic complex was completely characterized by multinuclear NMR
spectroscopy, IR and HRMS. In the 1H NMR, in addition to PPh3 signals, three different
resonances are displayed in the range of 8 5.55-6.1 ppm (Figure 4-5) with the one at 8

Scheme 4-5

PPh3W Pt-^
H, H2

(p-xylene)Mo(CO)3 (4-2)

PPh3,. Pt -... H3

4-9 co

I + -
Ph3C BF4

PPh3t1...Pt// PPh31,,,,.. H'
PPh3'" 'P3 PPh3, Pt --/ Hd.
H | H; H,' H,'
Mo Mo
4-5 / co 4-6 "/ co
co co co co



fu u


ui w



J *
a <_ L. a

I n "(8- u





5.55 ppm showing coupling to the 195Pt nucleus (3JH1-Pt = 36.7 Hz). The multiplicity of

these three signals is the same as that of 2-5 but each is shifted ca. 3 ppm upfield from 2-
5. Both the 19F('H) and the 31p 1H) NMR show singlets whereas the 195Pt (1H) NMR
exhibits a triplet centered at 8 -4087.6 ppm (Figure 4-6). The chemical shift of the 195Pt

nucleus in 4-9 is 308 ppm upfield from the corresponding resonance for complex 2-5.
The IR spectrum displays two very strong bands (2036, 1976 cm-1) in the metal carbonyl
region. These stretching frequencies are ca. 40 cm'1 lower than the Mo(CO)3 complex of

the tropylium cation (4-8) which is consistent with a somewhat more electron rich tropyne
ring (from electron donation from bistriphenylphosphineplatinum)53 relative to a tropylium
ion. The carbonyl region in the 13C({H) NMR shows only one peak, even at -100 C
indicating a very low barrier for rotation of the molybdenum tricarbonyl rotation around the
molybdenum-seven-membered ring axis. Complex 4-9 was also prepared in a more
traditional way by hydride abstraction from a mixture of 4-5, 4-6. Complexes 4-5, 4-6
reacted rapidly with triphenylcarbenium tetrafluoroborate in CD2C2 at room temperature to

give 4-9 in high yield (90%, confirmed by 31p(1H) and 'H NMR). From a synthetic
point of view hydride abstraction from 4-5 and 4-6 was more convenient then
displacement and is therefore the preferred method to prepare 4-9.

Hydride Reduction of (PPh12Pt IL7)C Mo(COL (L4-9

One of the few reactions studied with platinum (2-5)53 and zirconium (2-9)54
tropyne complexes was hydride reduction with KBEt3H and LiAl(Ot-Bu)3H. These

reactions were shown to produce the corresponding cycloheptadienyne complexes,
however, in poor yields. We decided to investigate hydride reduction reactions with 4-9 in
order to determine the effect of a coordinated molybdenum on both the yields and
regiochemistry of reduction. All reduction reactions were carried out by adding the
reducing agent to a THF solution of 4-9. Reduction of 4-9 with either KBEt3H or
LiAl(Ot-Bu)3H was very clean (ca. 95% yield as shown by the 31P NMR) in contrast to

that of 2-5 which gave a complex mixture of products. The reductions gave a mixture of
three products (4-10: 4-6: 4-5 = 85:10:5 for LiAl(Ot-Bu)3H and 71:21:8 for KBEt3H)

(Scheme 4-6). The two minor products (4-5, 4-6) were identified by comparison with

authentic samples synthesized as described above. Their formation is a result of hydride
addition to the tropyne carbons indicated by arrows a (4-5) and c (4-6) (Scheme 4-6).

Selective formation of major isomer (4-10) resulting from hydride addition to a tropyne

carbon indicated by arrowb was unexpected because this kind of cumulene product had not

been observed in the reductions of 2-553 and 2-954. The cumulene complex 4-10 was

Scheme 4-6

aPb --] +

PPh3P ^S-/

4-9 C& CO

Ph B LiAl[OC(CH3)3]3H, or
L =PPh3

L2Pt-/ / LPt-//

Mo Mo MO



~ '8

z -


- I
-y 0











? U

isolated by crystallization from the reaction mixture to give red crystals in 42% yield. Its

1H NMR showed six different resonances with two showing coupling to the 195Pt nucleus
(Figure 4-7). One of these signals (at 8 3.85) showed coupling to one of the methylene

protons and was therefore assigned to H4. Consequently, the other signal at 5 5.84 ppm

was assigned to HI. The remaining signals in the spectrum were assigned based on a 2D

cosy experiment. The 195Pt(pH) and 31P{1H} NMR (Figure 4-8) appeared as doublets of
doublets centered at 8 -4420.2 (dd, 1Jpt-p = 3124 Hz, 1Jpt,," = 3282.5 Hz) and a pair of

doublets at 8 23.81, 23.55 (d, 2Jp,.p. = 9.2 Hz) respectively which is consistent with the

C1 symmetry for 4-10. The 13C( 1H) NMR (Figure 4-9) exhibited a broad singlet in the

carbonyl region which decoalesced at lower temperature (-35 oC) to three resonances of

equal intensity. This observation indicates that the molecule is fluxional and that the origin

of this process is a rapid spinning of the Mo(CO)3 fragment around the metal-seven-

membered ring axis. The IR spectrum showed three very strong peaks in the carbonyl
region with stretching frequencies (1947, 1870.8, 1847 cm-1) which are lower than 4-3

(1970, 1908, 1856 cm-1)88 and interestingly even lower (ca. 10 cm-1) than 4-5 videe

supra). As expected 4-9 readily undergoes hydride abstraction reaction to re-generate 4-9

in high yield. The structure of 4-10 was further confirmed by an X-ray diffraction study.

X-Ray Crystal Structure Analysis of (PPh3)2Pt (PC2H)MofCO) (4-10)

Suitable crystals for X-ray diffraction analysis were obtained from a mixture of
benzene and hexane at -16 oC. Thermal ellipsoid and stereographic drawings of 4-10 are

depicted in Figures 4-10 and B-2 (Appendix B) respectively while selected bond lengths

and angles are listed in Table 4-2. Crystal data and final fractional atomic coordinates are

provided in Tables A-26 and A-27, respectively. Complex 4-10 crystallized in monoclinic,

non-centrosymmetric space group P21 with one molecule of benzene located in the general

position. Complex 4-10 is chiral and since it crystallized in a non-centrosymmetric space

Ph5l Ph41


Ph6lP2 C1 C7 C6

Ph7l C4

01 03



Figure 4-10. Thermal ellipsoid drawing of complex 4-10. Phosphine phenyl rings are
omitted for clarity.


co Co

Table 4-2. Selected Bond


P2-Pt -Cl
C1 -Pt-C2
C2--C 1-C7

Lengths (A) and Angles (deg)

Bond Lengths (A)

2.287(5) Pt-P2
2.084(13) Pt-C2
2.36(2) Mo--C2
2.33(2) Mo-C4
2.45(2) Mo--C7
1.98(2) Mo--C9
2.05(3) C1-C2
1.33(2) C2-C3
1.42(3) C4-C5
1.58(4) C6--C7

Bond Angles (deg)

106.7(2) P1-Pt --Cl
111.0(7) P2-Pt---C2
37.0(9) C2-Pt-P1
139.(2) C2-C1-Pt
152.(2) C3-C2-C1
125.(2) C5-C4-C3
128.(2) C7-C6--C5
110.(2) Mo-C8---01
178.(2) Mo-C10-03

for Complex 4-10.



group only one enantiomer can be present in the crystal. By refining both enantiomers with

the same data set and comparing the refinement results, we were able to determine the

absolute configuration of the complex in the crystal. The geometry around the Pt atom is

square planar. The platinum atom is bonded to two phosphorous and two carbon atoms.

Six carbon atoms belonging to the seven-membered ring are coplanar (max. deviation from

the least square plane is 0.053 A for C3) whereas C6 is 0.66 A above this plane. The

dihedral angle between the planes defined by C7-C6-C5 and C7-C1-C2-C3-C4-C5 is equal

to 127(2) oC. This value is in the normal range for molybdenum cycloheptatriene

complexes.90 A bond length analysis could not be made due to low data quality.

Unfortunately all attempts to get better data sets failed.

Complex 4-10 is of particular interest because the 1,2,3,5-cycloheptatetraene ring

has not been previously reported. To date the only recorded butatriene confined to a seven-

membered ring is 4-13 which was prepared by desilabromination of 4-11 with KF

followed by isomerization.18 As described in chapter 2, attempts to prepare a platinum

Scheme 4-7



4-11 4-12 4-13

complex of 1,2,3,5-cycloheptatetraene (2-4) by base induced P-elimination from

bromcyloheptatrienes in the presence of Pt(PPh3)3 were not successful (Scheme 2-1).27

Hydride reduction of the tropyne ligand seems to be the best way to generate this new ring


Some Reactions of 4-5.4-6

As mentioned above, methods that were successful for the preparation of 2-2 and
2-3 failed to yield 2-4. As a possible source of 2-4, Mo(CO)3 removal from 4-10 was

therefore explored. From the outset it was recognized that this goal provided a special
challenge since any reagent used to effect Mo(CO)3 displacement must either be inert to

platinum or give a non-productive reaction. 1,10-phenanthroline appeared to qualify in the
first category because it does not displace PPh3 from platinum but is known to react with

4-3 displacing cycloheptatriene.84 Indeed, upon addition of 1,10-phenanthroline to a

mixture of 4-5 and 4-6, displacement occurred to give a good recovery of 2-2 and 2-3

(Scheme 4-1). This reaction is much slower (tn/2 for 4-5 at room temp. = ca. 2 days, t/2

for 4-6 = ca. 6 days) than the corresponding reaction with 4-3 which, under the same

conditions, is complete within minutes. Unfortunately, 4-10 showed no detectable

reaction with phenanthroline, even after several days at room temperature. Similarly,

treatment of a mixture of 4-5 and 4-6 with 3.5 equiv. of PPh3 led to displacement of

Mo(CO)3 (presumably concomitant with non-productive PPh3 exchange) but, again, much

slower than displacement from 4-3; tl/2 = ca. 3 days at rt. for 4-5and ti/2 = ca. 2 weeks

at rt. for 4-6 while reaction with 4-3 was complete within minutes. However, as with the

phenanthroline, 4-10 showed no detectable reaction with PPh3, even after a week at room

temperature. These reactivity differences qualitatively correlate with the CO absorptions in

the infrared which, in turn, presumably reflect differences in triene basicities. In addition,

steric resistance from the (Ph3P)2Pt moiety might also retard Mo(CO)3 displacement from

4-5, 4-6 and 4-10 relative to 4-3.



Allenes are linear organic compounds with two consecutive and orthogonal 1c

molecular orbitals (double bonds). There is a large number of transition metal complexes in

which the metal is coordinated to one of the double bonds of the allene.41 The number of

transition metal complexes containing reactive, small cyclic allenes is however very

small.27,4247,93 One of the most important physical properties of some allene complexes is

the ability of the transition metal to migrate between the double bonds of the allene (1,2-

metal shift). The fluxional behavior of allene complexes was first observed by Pettit and

Ben-Shoshan in 1967 in the tetracarbonyliron complex of tetramethylallene and was shown

to be an intramolecular process.94 Later, other allene complexes such as those of cationic

Fe(II)95 and neutral Pt(II)96 were found to undergo intramolecular fluxionality. The rate of

migration in allene complexes is dependent on the steric congestion around the allene and

the metal center as well as the electron density on the metal center. Thus, in a series of

PtCl2(tetramethylallene)(p-XC5H4N) complexes where X = NH2, Me, Et, H, Br, CN, the

rate of migration was found to increase with more electron withdrawing groups on the

pyridine ligand indicating that reduction of electron density at the platinum center decreases

the activation energy of the fluxional process.97 There are essentially two possible

mechanisms for intramolecular fluxionality in allene complexes. The most significant

difference between these two mechanisms is that chirality of the allene is retained in the

transition state in one (Vrieze-Rosenblum mechanism) whereas in the other one, known as

the allyl cation mechanism, the allene loses chirality due to formation of a planar allyl cation

in the transition state. The former mechanism was demonstrated unequivocally by
Rosenblum by studying the dynamic behavior of cationic dicarbonyl(r5S-cyclopentadienyl)

iron complexes of allenes.95 The allyl cation mechanism was observed by Jones in the case
of the cationic carbonyl(r15-cyclopentadienyl) (triphenylphosphine) iron complex of 1,2-

cycloheptadiene (1-53).46 and was shown to be of a higher energy (5-6 kcal/mol) than

that of Vrieze-Rosenblum mechanism. Successful detection of the allyl cation mechanism

was possible due to raising of the ground state energy (a result of a considerable ring strain

energy incorporated in 1,2-cycloheptadiene) of the allene complex. An intermolecular

mechanism involving dissociation-recombination of the metal from the allene ligand has

been observed in the neutral palladium complex (PPh3)2Pd(PhCH=C=CHPh).98

One allene complex that has been extensively studied in our laboratory in recent
years is bis(triphenylphosphine)platinum(1,2,4,6-cycloheptatetraene)43,51,99 (1-51) which

was first prepared by Jones and coworkers43 as described on scheme 5-1. Treatment of

three isomers of bromocycloheptatriene with K-OtBu in the presence of Pt(PPh3)3

generates complex 1-51. The 1H NMR spectrum of 1-51 exhibits six different protons

indicating that the molecule is not symmetrical with respect to the seven-membered ring.

Scheme 5-1

Ph3PP h / PPh3
Br K-O'Bu H,.O H6

Pt(PPh3)3 H,



The 31p(1H} NMR and 195Pt{1H) NMR spectra show two doublets and doublet of

doublets, respectively, which are also consistent with the lack of symmetry in the

complex. The absence of symmetry in the 1-51 indicates that if the molecule is fluxional

(1,2-shift) the fluxional process is slow on the NMR time scale because otherwise C2

symmetry would be observed in all NMR spectra. The fluxionality of complex 1-51 was

studied thoroughly and it was demonstrated that even at temperatures as high as 105 oC no

coalescence of proton resonances was seen in the 1H NMR spectrum.51'59 This indicated

that the possible fluxional process was slow even at high temperatures. A one dimensional

NMR technique that allows detection of slower exchange processes is spin saturation

transfer (SST). The main idea of SST is to saturate specifically one of the exchanging spin

protons and observe its effect on the remaining resonances in the spectrum. The SST

technique was employed to study the fluxionality of complex 1-51. Irradiation of proton

HI (Scheme 5-2) at room temperature did not affect any of the other resonances. However

at 80 OC the transfer of saturated spin to proton H6 was clearly observed. Kinetic studies

revealed the following activation parameters for the fluxional process: AHI* = 26.8 1.3

Scheme 5-2

Ph3P PPh3
PPPh3 / Ph3P t PPh
PhP / \ PPh3

fH, H, H, A4

kcal/mol and ASt = 13.1 1.3 eu.51 Taking advantage of the high natural abundance

(33.8%) of magnetically active 195Pt and the spin saturation technique the researchers were

also able to determine that this fluxional process is operating by a dissociation-

recombination mechanism (Scheme 5-2). One important aspect of this mechanism is that

due to chirality of the seven-membered ring allene the 1,2-platinum shift must be

accompanied by a change in the face of the seven-membered ring to which the platinum is

coordinated. When the bis(triphenylphosphine)platinum(0) moiety dissociates from the

allene ligand it may return either to its original face of coordination (A) without

equilibrating of H1 and H6 or to the other allene double bond which changes the face of

coordination (C) equilibrating HI and H6 protons. This phenomenon is represented in

Scheme 5-3

Ph3P /PPh3
PhsP Pt / PPh3
Pt Pt
Ph3P \ / t PPh3

A \ B / C

1,2 shift