Synthesis and study of platinum silylene complexes of the type (R 3P)2Pt=SiMes2 (Mes = 2,4,6-trimethylphenyl)

Article abstract of DOI:10.1139/v03-018

The synthesis and characterization of neutral platinum silylene complexes (R₃P)₂Pt=SiMeS₂ (R = i-Pr (1) or cyclohexyl (2), Mes = 2,4,6-trimethylphenyl) is reported. The dimesitylsilylene ligand in 2 is displaced by a number of ligands including phosphines, alkenes, alkynes, and O₂. Complex 2 reacts with ROH substrates (R = H, Me, Et) to give (Cy₃P)₂Pt and Mes₂Si(OR)(H) and with H ₂ to give trans-(Cy₃P)₂Pt(H)SiHMeS₂ (3). Reaction of H₂SiMeS₂ with (Cy₃P)2Pt gave cis-(Cy₃P)₂Pt(H)SiHMeS₂ (4). EXSY NMR experiments of 4 reveal that exchange of silicon and platinum hydrides occurs via reductive elimination - oxidative addition and not via a silylene intermediate.

Full text of DOI:10.1139/v03-018

1127
Synthesis and study of platinum silylene
complexes of the type (R3P)2Pt=SiMes2
(Mes = 2,4,6-trimethylphenyl)1
Jay D. Feldman, Gregory P. Mitchell, Jörn-Oliver Nolte, and T. Don Tilley
Abstract: The synthesis and characterization of neutral platinum silylene complexes (R3P)2Pt=SiMes2 (R = i-Pr (1) or
cyclohexyl (2), Mes = 2,4,6-trimethylphenyl) is reported. The dimesitylsilylene ligand in 2 is displaced by a number of
ligands including phosphines, alkenes, alkynes, and O2. Complex 2 reacts with ROH substrates (R = H, Me, Et) to give
(Cy3P)2Pt and Mes2Si(OR)(H) and with H2 to give trans-(Cy3P)2Pt(H)SiHMes2 (3). Reaction of H2SiMes2 with (Cy3P)2Pt
gave cis-(Cy3P)2Pt(H)SiHMes2 (4). EXSY NMR experiments of 4 reveal that exchange of silicon and platinum hydrides
occurs via reductive elimination – oxidative addition and not via a silylene intermediate.
Key words: silylene, EXSY, platinum, hydride.
Résumé : On a effectué la synthèse et la caractérisation de complexes neutres de silylène de platine, (R3P)2Pt=SiMes2
(R = isopropyle (1) ou cyclohexyle (2); Mes = 2,4,6-triméthylphényle). Le ligand diméthylsilylène du composé 2 est dé-
placé par un grand nombre de ligands, dont les phosphines, les alcènes, les alcynes et le dioxygène. Le complexe 2 réagit
avec des substrats de formule ROH (R = H, Me, Et) pour conduire à la formation de (Cy3P)2Pt et de Mes2Si(OR)(H); il
réagit aussi avec H2 pour donner le trans-(Cy3P)2Pt(H)SiHMes2 (3). La réaction du H3SiMes2 avec le (Cy3P)2Pt conduit à
la formation du cis-(Cy3P)2Pt(H)SiHMes3 (4). Des expériences de RMN de type « EXSY » effectuées sur le composé 4
révèlent qu’il se produit un échange des hydrures de silicium et de platine par le biais d’une réaction d’élimination ré-
ductrice – addition oxydante et non pas par le biais d’un intermédiaire silylène.
Mots clés : silylène, « EXSY », platine, hydrure.
[Traduit par la Rédaction] Feldman et al. 1136
Introduction
the latter mechanism have precedent (e.g., Si–H oxidative
addition and reductive elimination), there is little evidence
to support the direct addition of an Si—C bond to platinum.
In 1971 Kumada and co-workers discovered the first
hydrosilane oligomerization and redistribution reactions cat-
alyzed by a platinum complex (3b, 5). In the presence of
trans-(Et3P)2PtCl2 as a precatalyst, Me3SiSiMe2H was trans-
formed into Me3SiH (redistribution product) and Me-
(SiMe2)nH (n = 3–6, oligomerization products). The investi-
gators subsequently found that introducing diphenyl-
acetylene into the reaction resulted in formation of a
disilacyclohexadiene (eq. [1]), which was proposed to result
from the trapping of dimethylsilylene (5).
Platinum complexes play an important role as catalysts in
transformations of organosilanes. These metal species are
used to effect olefin hydrosilylation (1), dehydrogenative
coupling of silanes to produce oligomers (2), and redistribu-
tion of substituents on silicon (3, 4). The latter two transfor-
mations occur extensively with low-valent platinum catalysts
and are usually observed concurrently. Because these pro-
cesses occur together it is likely that they involve a common
intermediate. One possible mechanism for this redistribu-
tion–oligomerization reaction involves an intermediate
silylene complex (LnPt=SiR2) and migration chemistry
(Scheme 1), although a mechanism involving reductive elim-
ination – oxidative addition (Scheme 2) is preferred by some
(4). While many of the proposed steps for redistribution via
[1]
Received 6 May 2002. Published on the NRC Research Press
Web site at http://canjchem.nrc.ca on 23 May 2003.
This paper is dedicated to John Harrod, in recognition of his
many contributions to homogeneous catalysis and metal–
silicon chemistry.
More recently, Tanaka suggested that the formation of a
platina-sila-cyclohexadiene, by the addition of phenyl-
acetylene to a similar oligomerization–redistribution system,
resulted from the trapping of a Pt0 silylene complex (6)
(eq. [2]). Although these results suggested a mechanism in-
volving intermediate platinum silylene species, the goal of
isolating and probing the reactivity of such a complex still
remained. Additionally, given the utility of carbene com-
J.D. Feldman, G.P. Mitchell, J.-O. Nolte, and T.D. Tilley.2
Department of Chemistry, University of California at
Berkeley, Berkeley, CA 94720–1460, U.S.A.
1This article is part of a Special Issue dedicated to Professor
John Harrod.
2Corresponding author (email: tdtilley@socrates.berkeley.edu).
Can. J. Chem. 81: 1127–1136 (2003)
doi: 10.1139/V03-018
© 2003 NRC Canada

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Can. J. Chem. Vol. 81, 2003
Scheme 1.
plexes as catalysts and synthons in organic and polymer
chemistry (7), it is likely that development of silylene chem-
istry will lead to similar advancements in organosilicon
chemistry.
Many synthetic efforts have targeted the isolation of com-
plexes featuring silylene ligands. In recent years, these ef-
forts have culminated in the syntheses of a number of
transition metal compounds containing sp2-hybridized sili-
con (8). For the most part, these compounds are cationic,
having been obtained via an “anion-abstraction” method
(8a–e). However, some neutral silylene complexes have been
prepared via coordination of a stable free diamidosilylene
(of which there are very few) to a transition metal (8k–o).
The free silylenes are conjugated, five-membered rings in
which the Si atoms are stabilized via heteroatom coordina-
tion to nitrogen.
[2]
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Feldman et al.
1129
Scheme 2.
Only two platinum silylene complexes have been previ-
ously reported, [trans-(Cy3P)2(H)Pt=Si(SEt)2][BPh4] (8c)
and [(i-Pr2PCH2CH2P-i-Pr2)(H)Pt=SiMes2][MeB(C6F5)3] (8f).
Neither of these cationic compounds is a good model for
silylenes of the type (R3P)2Pt=SiR2, which have been the pri-
mary focus of speculation regarding catalytic silylene inter-
mediates (9). In an effort to prepare netural platinum(0)
silylene complexes, we have explored reactions of meta-
stable silylenes generated in situ with zero-valent platinum
compounds. Some of this work has been communicated
(10).
small amounts (<10%) of new products were observed (by
31P NMR spectroscopy) with Pt(P-i-Pr3)3 and Pt(PCy3)2. It is
notable that in these two cases the reacting platinum species
is 2-coordinate, since Pt(P-i-Pr3)3, unlike Pt(PEt3)3, readily
dissociates one of its phosphines in solution to give Pt(P-i-
Pr3)2 and an equivalent of free phosphine. Thus, further ex-
periments concentrated on the use of these 2-coordinate spe-
cies. The generation of Me2Si: or Ph2Si: in the presence of
Pt(P-i-Pr3)2 (+ free phosphine) or Pt(PCy3)2 gave no new
complexes (by 31P NMR spectroscopy). However, Mes2Si:
generated from Mes2Si(SiMe3)2 gave >85% of a new, blood-
red complex (1) as observed by 31P NMR spectroscopy
(eq. [3]). Furthermore, use of
Mes2Si(SiMe3)2 (1.2 equiv) gave 1 as the only platinum
a slight excess of
Results and discussion
phosphine species observed.
Survey of reactions and observation of (i-Pr3P)2-
Pt=SiMes2
[3]
Silylenes were generated photolytically from linear or cy-
clic trisilanes, including Mes2Si: (Mes = mesityl) from
Mes2Si(SiMe3)2 (11) or (Mes2Si)3 (12), Ph2Si: (13) from
Ph2Si(SiMe3)2, and Me2Si: from (Me2Si)6 (14). The
photolyses were carried out using a quartz tube and in hex-
anes with a 1:1 stoichiometry of silylene precursor to plati-
num substrate. The first reactions surveyed involved the
generation of Mes2Si: from (Mes2Si)3 in the presence of
Pt(PEt3)3 (15), (C2H4)Pt(PPh3)2 (15), Pt(P-i-Pr3)3 (15),
Pt(PCy3)2 (15), or Pd(PCy3)2 (16). While no reaction was
observed with Pt(PEt3)3, (C2H4)Pt(PPh3)2, or Pd(PCy3)2,
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Can. J. Chem. Vol. 81, 2003
Fig. 1. 29Si spectrum of 1.
Although there are many examples of silylene-bridged,
dimeric bis(phosphine) platinum complexes (dimeric analogs
of 1) (17), the observation of only a single resonance with
1
195Pt satellites in the 31P NMR spectrum (δ: 82.2 JPPt
=
3119 Hz) is consistent with a monomeric structure for 1.
Apparently, the steric protection provided by the Mes and i-
Pr substituents is sufficient to prevent the formation of a
more thermodynamically stable 4-coordinate silicon species
via dimerization. The 29Si NMR data for 1 are also consis-
tent with the characterization of this complex as monomeric,
with an sp2-hybridized silicon atom. Silylene complexes ex-
hibit characteristically large downfield chemical shifts, typi-
cally in the range of 250 to 340 ppm (8). Significantly, the
resonance for 1 (Fig. 1) was observed at δ: 367 (2JSiP
=
1
107 Hz; JSiPt = 2973 Hz). For comparison, the only other
platinum silylene complexes to be reported have 29Si reso-
nances at δ: 309 for [trans-(Cy3P)2(H)Pt=Si(SEt)2][BPh4]
(8c) and δ: 338 for [(i-Pr2PCH2CH2P-i-Pr2)(H)Pt=SiMes2]
[MeB(C6F5)3] (8f). Complexes related to 1 include several
dimeric analogs of the type [(R3P)2Pt(µ-SiR′2)]2, which were
prepared from reactions of secondary silanes with Pt(PR3)n
complexes (17), as well as the trimer [Pt(µ-SiPh2)(PMe3)]3,
which was derived from loss of Ph2SiH2 and PMe3 from cis-
(Me3P)2Pt(SiHPh2) (18). While the [(R3P)2Pt(µ-SiR′2)]2 com-
plexes exhibit 29Si NMR shifts that are over 400 ppm upfield
of that for 1 (in the range of δ: –65 to –95), [Pt(µ-
SiPh2)(PMe3)]3 has a 29Si chemical shift of 279 ppm, which
is uncharacteristically downfield for a 4-coordinate silicon
species.
Fig. 2. ORTEP diagram of 2.
Repeated attempts to isolate 1 were unsuccessful. Re-
moval of the solvent afforded Pt(P-i-Pr3)3, along with a vari-
ety of unidentified silicon-containing products. When Pt(P-i-
Pr3)2 (19) (without a third equivalent of P-i-Pr3 present) was
employed as a trapping reagent, the only platinum product
observed upon photolysis was trans-H2Pt(P-i-Pr3)2 (20) (by
1H and 31P NMR spectroscopy) presumably resulting from
C–H activation of the hexanes solvent. When the photolysis
reaction was performed in benzene-d6, neither solvent acti-
vation nor efficient formation of 1 were observed.
downfield chemical shift is evidence for the presence of an
sp2-hybridized silicon atom, confirming the assignment of 2
as a silylene complex.
Synthesis and structure of (Cy3P)2Pt=SiMes2 (2)
X-ray quality crystals of 2 were grown from dilute
(<0.04 M) reaction solutions over a 3–4 h period.3 An
ORTEP diagram of 2 is shown in Fig. 2. The Pt—Si bond
distance of 2.210(2) Å is the shortest yet reported and is
about 6% shorter than typical Pt—Si single bonds (2.30–
2.40 Å) (1, 21). For comparison, the Pt—Si distance in
[trans-(Cy3P)2(H)PtSi(SEt)2][BPh4] is 2.270(2) Å, but note
that the latter complex was characterized as being somewhat
Fischer-like, with both Si—S and Pt—Si π-bonding (8c, 22).
The summation of angles about Si, 359.8(6)°, confirms the
presence of a planar, sp2-hybridized silicon atom, and the
angles about platinum also sum within experimental error to
360°. The least-squares plane of the silylene ligand (including
Pt, Si, and the two ipso carbons) and the coordination plane of
platinum (Si, Pt, P, P) intersect at a dihedral angle of 68.6° (see
Fig. 3), which is not optimal for Pt-to-Si π-donation. The ideal
dihedral angle for overlap between the HOMO of the L2Pt
fragment and the empty p-orbital on silicon is 90°, which is the
Photolysis of a mixture of Mes2Si(SiMe3)2 and Pt(PCy3)2
in hexanes gave
a green, supersaturated solution of
Me3SiSiMe3 (by H and 29Si NMR spectroscopy) and the
silylene complex (Cy3P)2Pt=SiMes2 (2). Cooling the solu-
tion to 0°C, concentrating the solution in vacuo, or allowing
the solution to stand for a few hours facilitated precipitation
of microcrystalline 2 (54% isolated yield). Complex 2 is
completely insoluble in alkane solvents, dichloromethane,
fluorobenzene, and tetrahydrofuran (THF) and is only
1
1
slightly soluble in toluene or benzene. The H NMR spec-
trum of 2 in benzene-d6 consists of resonances for each of
the three types of Mes protons (o-Me, p-Me, ArH) as well as
a group of multiplets for the cyclohexyl protons. Likewise,
the 31P NMR spectrum contains a single peak with 195Pt sat-
1
ellites (δ: 71.7, JPPt = 3068). Because of its low solubility, a
long accumulation time was required to observe the 29Si
NMR resonance for 2 at δ: 358 (2JSiP = 112 Hz). This large
3 Supplementary data may be purchased from the Depository of Unpublished Data, Document Delivery, CISTI, National Research Council
Canada, Ottawa, ON K1A 0S2, Canada (http://www.nrc.ca/cisti/irm/unpub_e.shtml for information on ordering electronically).
© 2003 NRC Canada

Feldman et al.
1131
Fig. 3. ORTEP view of 2 in which most of the cyclohexyl car-
bon atoms have been removed for clarity. The dihedral angle be-
tween the silicon coordination plane and platinum coordination
plane is 68.6°.
bonds. Given the high steric congestion surrounding the
Pt=Si bond, small substrates, including 2-butyne, propyne,
and ethylene, were employed.
Addition of 2-butyne to a benzene-d6 solution of 2 initially
resulted in the formation of an unsymmetrical complex with
inequivalent resonances in the 31P NMR spectrum. This is
consistent with a complex derived from either alkyne addition
across the Pt=Si double bond (eq. [4], structure B) or coordi-
nation of the alkyne to the silylene complex (structure C).
[4]
This species appears to be an intermediate and is only ob-
served in very small concentrations (<5% by 31P NMR spec-
troscopy), as the silylene complex is converted into the
alkyne complex (Cy3P)2Pt(η2-MeCCMe).4 The 31P NMR
spectrum of the intermediate consists of two doublets with
2
1
195Pt satellites (δ: 41.6, d JPP = 34 Hz, JPPt = 3483 Hz; δ:
1
33.3, d JPPt = 2899 Hz). An analogous process is observed
observed angle of the related germylene and stannylene com-
plexes (Et3P)2PtGe[N(SiMe3)2]2 (23) and (i-Pr2PCH2CH2P-i-
Pr2)PdSn[CH(SiMe3)2]2 (24), respectively. The deviation
from 90° for 2 appears to result from steric interactions be-
tween the Mes and Cy substituents.
It is interesting that although complexes of the type
(R3P)2PtER′2 are known for tin, germanium, and now silicon,
no such analogs are known with carbon. The complexes with
germanium (23) and tin (24) were prepared by reacting a
stable, free germylene or stannylene with a platinum com-
plex of the type (R3P)2PtL, where L is a leaving group such
as an alkene, alkyne, or oxalate (which forms 2 equiv of
CO2). It is noteworthy that while (Et3P)2PtGe[N(SiMe3)2]2
decomposes in ambient laboratory light (25), silylene com-
plexes 1 and 2 are generated under intense UV–vis irradia-
tion. Also, no mention has been made of the sensitivity of
stannylene complexes to light.
when propyne or ethylene (1 atm (1 atm = 101.325 kPa)) is
used as the reactant in place of 2-butyne. In this ligand-
displacement reaction the dimesitylsilylene is lost as multi-
ple uncharacterized silicon-containing products. The appar-
ent low stability of the intermediate species is likely due to
the steric congestion associated with 2 and its derivatives.
Hydrosilanes do not react cleanly with 2; however, NMR
tube reactions indicate that redistribution and, to a small ex-
tent, oligomerization processes take place. For example, ad-
dition of PhSiH3 (4 equiv) to a green benzene-d6 solution of
2 resulted in bleaching of the color and formation of
Mes2SiH2 as the only mesityl-containing product. Redistri-
bution products Ph2SiH2, SiH4, and other Si-H containing
products, as well as trace amounts of H2 (presumably from
1
dehydrocoupling), were also observed (by H NMR spec-
troscopy). The platinum-containing product from this reac-
tion has equivalent phosphines, as well as a hydride ligand
(by NMR spectroscopy). These data are consistent with its
formulation as trans-(Cy3P)2Pt(H)(SiHPh2), although this
could not be confirmed as it was not isolated. Furthermore,
the symmetrical complex trans-(Cy3P)2PtH2 (20) (also con-
sistent with the NMR data) could be ruled out based on
comparison of NMR data to an authentic sample.
Reactivity of 2
Given that 2 is a member of the (R′3P)2Pt=SiR2 class of
compounds proposed as intermediates in hydrosilane oligo-
merization and redistribution, we have searched for similari-
ties between the reactivity of 2 and that of the proposed
intermediate species. Alkyne coupling with extruded
silylenes is the primary evidence supporting such an inter-
mediate species (vide supra); thus we have examined reac-
tions of 2 with substrates containing unsaturated C—C
Complex 2 quantitatively transfers its silylene group to
water and alcohols, giving O—H bond insertion products
(eq. [5]).
4 The alkyne complex, (Cy3P)2Pt(η2-MeCCMe), was generated via the addition of 2-butyne to (Cy3P)2Pt in order to compare its 31P spectrum
1
with the product mentioned here. (Cy3P)2Pt(2-butyne) 31P NMR (C6D6) δ: 36.8, JPPt = 3302 Hz.
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1132
Can. J. Chem. Vol. 81, 2003
trans-(Cy3P)2PtH2 (20) (ca. 50%; without formation of
Mes2SiH2). At higher temperatures (refluxing benzene-d6), 3
was hydrogenated to give trans-(Cy3P)2PtH2 and Mes2SiH2.
In contrast to the germylene complex (Et3P)2PtGe[N-
(SiMe3)2]2 (33), the addition of hydrogen to 2 is irreversible.
Complex 3 is fluxional in solution, as observed by vari-
able-temperature 31P NMR spectroscopy. At –29°C, two
[5]
1
2
doublets (δ: 26.9, d JPtP = 2632 Hz, JPP = 346 Hz; δ: 33.6,
d
1JPtP = 2715 Hz) are observed, which, upon heating,
1
broaden and coalesce to a singlet (δ: 28.9, JPtP = 2707 Hz)
at 91°C. This behavior is consistent with restricted rotation
about the sterically congested Pt—Si bond; the rotational
barrier (∆Gr‡ot) was calculated to be 15.8 0.4 kcal mol–1.
The mechanism of hydrogen addition to 2 could involve oxi-
dative addition to the platinum center followed by migration
of hydride to the silylene ligand, or direct addition of hydro-
gen to the Pt=Si double bond. At this point the mechanism is
unknown, but it is interesting to note that whereas the trans
silyl hydride is formed by this reaction, the oxidative addi-
tion of Mes2SiH2 to Pt(PCy3)2 exclusively gives cis-(Cy3P)2-
Pt(H)(SiHMes2) (vide infra).
Thus, addition of ROH (R = H, Me, Et) to 2 resulted in
the clean formation of Pt(PCy3)2 and the corresponding
siloxane Mes2Si(OR)H. No intermediates were observed in
the reaction of 1 with ethanol at –60°C (by 1H and 31P NMR
spectroscopy in toluene-d8). Related reactivity was previ-
ously reported for the base-stabilized silylene complex
[Cp*(Me3P)2RuSiPh2(NCMe)]+, which was presumed to dis-
sociate acetonitrile prior to reaction with an alcohol (26).
This reactivity contrasts with what has been observed for (i-
Pr2PCH2CH2P-i-Pr2)PdSn[CH(SiMe3)2]2, which reversibly
adds H2O across the Pd—Sn bond to form a palladium hy-
dride species (27).
Many reactions of 2 result in loss of the silylene ligand.
For example, O2 reacts with 2 to give Pt(PCy3)2(η2-O2) (28),
and the displaced silylene ligand is observed to decompose
to a number of species (but not to significant amounts of the
silylene dimerization product, Mes2Si=SiMes2). This reactiv-
ity is unlike that of the related germylene complex, (Et3P)2-
PtGe[N(SiMe3)2]2, which cleanly adds O2 across its double
bond (29). Reactions of 2 in benzene-d6 with an excess
amount of i-Pr2PCH2CH2P-i-Pr2, Ph2PCH2CH2PPh2, and
PMe3 led to PCy3 and Mes2Si: displacement, with quantita-
tive formation of the corresponding PtL4 (30) complex (by
31P NMR spectroscopy). Addition of 1 equiv of phosphine
resulted in partial conversion to PtL4, and no other products
were observed. These results suggest that the dimesityl-
silylene ligand in 2 is rather labile. However, attempts to
trap the silylene by reacting 2 with 2 equiv of 2,3-dimethyl-
butadiene (which is known to react with free dimesityl-
silylene) (31) in benzene-d6 over 3 days at room tempera-
ture failed to give the expected cycloaddition product,
Synthesis and EXSY NMR Studies of cis-(Cy3P)2Pt-
(H)SiHMes2 (4)
The cis isomer of 3 is obtained via the addition of
Mes2SiH2 to Pt(PCy3)2 in 48% isolated yield. Competing
cis–trans isomerization and decomposition took place upon
thermolysis (80°C, 6 h) or photolysis (2.5 h) of a benzene
solution of 4.
The Pt–H and Si–H resonances of 4 are broad at room
temperature, suggesting that exchange of these protons takes
place. Cooling the sample to –70°C (toluene-d8) only par-
tially resolved the broad peaks. The cyclohexyl resonances
prevent observation of the Pt-H–Si-H proton coalescence at
high temperatures, and thus exchange NMR spectroscopy
1
(EXSY) was used to study this process. The H EXSY ex-
periment confirms the exchange of the platinum and silicon
hydrides. The rate constant for exchange was determined to
be 23 4 Hz (27°C).5
Given the stability of silylene ligands in this system, it
seemed possible that this exchange could occur via an α-H
migration process to give a 5-coordinate intermediate
(eq. [6]).
.
Instead, several unidentified
1
mesityl-containing species were observed by H NMR spec-
troscopy. This suggests that complex decomposition path-
ways are operative in silylene displacement reactions. Unlike
cationic silylene complexes, 2 is stable in the presence of 1
equiv of the nitrogen bases para-dimethylaminopyridine and
acetonitrile and does not form a base-stabilized silylene
complex (32). Formation of the four-coordinate adduct may
be disfavored due to the significant steric protection of the
electrophilic, sp2-hybridized silylene.
[6]
Formation of trans-(Cy3P)2Pt(H)(SiHMes2) (3) from 2
and H2
Hydrogen (1 atm) reacted with a benzene solution of 2
over 2 days to form trans-(Cy3P)2Pt(H)(SiHMes2) (3,
Scheme 3). With this slow conversion there was competing
decomposition of 2 via loss of the silylene ligand to produce
However, a reductive elimination – oxidative addition
couple could also account for this observation. An inter-
molecular mechanism was suggested upon close examina-
5 EXSY NMR experiments (phase sensitive NOESY) were performed at two different mixing times of 12 and 25 ms, and exchange rate con-
stants were determined using the program D2DNMR.
© 2003 NRC Canada

Feldman et al.
1133
Scheme 3.
1
Fig. 5. 31P EXSY NMR spectrum of 4.
Fig. 4. Pt-H region of H EXSY NMR spectrum of 4 showing
exchange cross peaks between different Pt isotopes.
tion of the Pt–H region of the EXSY spectrum, Fig. 4, which
contains cross-peaks corresponding to the exchange of
195Pt–H hydrides with molecules of complex 4 containing
other Pt isotopes. More significantly, EXSY experiments re-
vealed the exchange of Pt–H hydrides with the Si–H hydrides
of free H2SiMes2 (25 equiv) with a rate constant of 23 ± 4 Hz
(27°C). Also, addition of D2SiMes2 to 4 resulted in concur-
rent incorporation of deuterium into the Pt–H and Si–H posi-
tions of 4. In addition, 31P EXSY NMR experiments provided
a similar rate constant (27 ± 4 Hz) for phosphine exchange.
This process was shown to be intramolecular, based on the
absence of cross peaks between 195Pt–P phosphines with
those bound to other isotopes of platinum (Fig. 5), as well as
the lack of exchange with free phosphine (5 equiv). Coales-
cence of the phosphorus resonances of 4 into a broad singlet
occurred at 135°C (toluene-d8), yielding an energy of activa-
tion of 16.1 ± 0.5 kcal mol–1. Thus, hydrogen exchange in
cis-(Cy3P)2Pt(H)(SiHMes2) occurs via the rate-determining
reductive elimination of dimesitylsilane, which does not
require prior phosphine dissociation. These results are consis-
tent with the recent observation that hydrogen appears not to
migrate to platinum in 4-coordinate (i-Pr2PCH2CH2P-i-Pr2)Pt-
(Me)SiHMes2, whereas the 3-coordinate cation [(i-Pr2PCH2-
CH2P-i-Pr2)PtSiHMes2]+ rapidly converts to [(i-Pr2PCH2CH2-
P-i-Pr2)(H)Pt=SiMes2]+(8f).
NMR specroscopy (1 and 2) as well as by X-ray crystallog-
raphy and UV–vis, IR, and Raman spectroscopies (2). Their
preparation was achieved via a novel route: trapping of a
metastable silylene with electron-rich bis(phosphine)plati-
num compounds. These are the first silylene complexes to
closely resemble proposed catalytic intermediates, in that the
L2Pt=SiR2 compounds (1 and 2) resemble species proposed
in the redistribution and oligomerization of hydrosilanes by
platinum catalysts. Steric protection of the reactive Pt=Si
double bond not only stabilizes 1 and 2, thus allowing for
their isolation, but apparently also limits their reactivity.
Dihydrogen is the only reactant to yield a product with an
intact Si—Pt bond (3). Other substrates either displace the
silylene ligand or fail to react with 2. Although no direct
analogies can be drawn between the reactivity of the isolated
and proposed intermediate silylenes, the very existence of 1
and 2 lends some credence to mechanisms which invoke this
class of compounds. Studies of the closely related and dy-
namic molecule cis-(Cy3P)2Pt(H)SiMes2H (4) point to a Pt-
H–Si-H exchange mechanism that does not involve a
silylene intermediate, but rather a reductive elimination –
oxidative addition couple. This observation is consistent
with the notion that a metal center must be unsaturated for
Conclusion
The first examples of neutral, platinum(0) silylene com-
plexes have been synthesized and characterized by 29Si
© 2003 NRC Canada

1134
Can. J. Chem. Vol. 81, 2003
silylene complex formation by α-migration to be favorable
(see ref. 8f).
(i-Pr3P)2Pt=SiMes2 (1)
Complex 1 was prepared by the same method employed
for 2 (vide infra), using Pt(P-i-Pr3)3. This red compound was
observed in solution but was not isolated. 31P NMR δ: 82.2
1
(s with 195Pt satellites, JPPt = 3119 Hz). 29Si NMR δ: 367 (t
Experimental
2
1
with 195Pt satellites, JPSi = 107 Hz, JSiPt = 2973 Hz).
General information
Unless otherwise noted, all manipulations were performed
under an atmosphere of nitrogen using standard Schlenk
techniques and (or) in a glovebox. Dry, oxygen-free solvents
were employed throughout. Diethyl ether (Et2O), pentane,
hexane, and toluene were distilled from sodium benzophe-
none ketyl and stored under N2 prior to use. Benzene-d6 and
toluene-d8 were purified by vacuum distillation from Na–K
alloy. Unless otherwise specified, all reagents were pur-
chased from commercial suppliers and used without further
purification. Pyridine was distilled from sodium and stored
under N2. Pt(P-i-Pr3)3 (15), Pt(PCy3)2 (15), Mes2Si(SiMe3)2
(11), and Mes2SiH2 (34) were prepared by literature meth-
ods. Lithium aluminum hydride was recrystallized from
Et2O and dried in vacuo prior to use. Ethylene (99.9%) was
obtained from Airgas, and 2-butyne (Aldrich) was vac-
transferred and stored over molecular sieves prior to use.
Photolysis reactions were carried out in a Rayonet RPR-
100 Hg lamp reactor.
NMR spectra were recorded with a Bruker AMX-400 at
400.1 MHz (1H) and 162.0 MHz (31P) or a DRX-500 at
125.8 MHz (13C) instrument and at room temperature and in
benzene-d6 unless otherwise noted. 29Si NMR spectra were
obtained with the Bruker DRX-500 instrument at 99.4 MHz
using 10 mg of Cr(acac)3 as a relaxation agent. EXSY NMR
experiments (phase sensitive NOESY) were performed at
two different mixing times of 12 and 25 ms, and exchange
rate constants were determined using the program D2DNMR
(35). Melting points were taken under N2 in sealed capillar-
ies and are uncorrected. Elemental analyses were performed
by the UCB Microanalytical Laboratory. Infrared (IR) spec-
tra were obtained on a Mattson Galaxy 3000 FT-IR spec-
trometer, and all absorptions are reported in cm–1. The UV–
vis spectrum of 2 in toluene were recorded on a Hewlett
Packard 8452A Diode Array spectrophotometer, respec-
tively.
(Cy3P)2Pt=SiMes2 (2)
A fused silica Schlenk tube containing a magnetic stir bar
was charged with Pt(PCy3)2 (314 mg, 0.415 mmol),
Mes2Si(SiMe3)2 (212 mg, 0.499 mmol), and 12 mL of hex-
ane. The resulting mixture was stirred rapidly until all solids
dissolved (10 min). Within minutes of beginning photolysis,
the color of the solution changed from colorless to green.
This color persisted and darkened over the photolysis period
of 120 min at which time a green precipitate was observed
on the sides of the tube. The hexane and hexamethyldisilane
byproduct were removed under vacuum. The resulting mix-
ture was washed with 2 × 5 mL of hexane to remove tetra-
mesityldisilene (the silylene dimerization byproduct) as well
as unreacted Pt(PCy3)2. The remaining green solid was dried
in vacuo to give analytically pure 2 (mp 104°C decomposi-
tion (dec)). Yield: 231 mg (54%). X-ray quality crystals of 2
were grown from dilute (<0.04 M) reaction solutions over a
3–4 h period during photolysis. UV–vis (ε (M–1 cm–1)): 282
(14 000), 316 (14 000), 392 (2900). IR: 3033 (w), 1601 (w),
1448 (s), 1265 (w), 1173 (w), 1046 (w), 1002 (w), 850 (m),
1
738 (w), 723 (w), 613 (w), 512 (w), 464 (m). H NMR δ:
6.83 (s, 4H, ArH), 2.72 (s, 12H, ortho-Me), 2.19 (s, 6H,
para-Me), 0.8–2.1 (br m, Cy). 31P NMR δ: 71.7 (s with 195Pt
1
2
satellites, JPPt = 3068 Hz). 29Si NMR δ: 358 (t, JPSi
=
112 Hz). Anal. calcd for C54H88P2PtSi: C 63.44, H 8.68;
found: C 63.35, H 8.87.
Reaction of 2 with 2-butyne
In an NMR tube, 2 (11 mg, 0.01 mmol), 2-butyne (1 µL,
0.01 mmol), and benzene-d6 were combined, and the tube
was inverted several times to mix the reactants. The reaction
was monitored by 31P NMR spectroscopy and produced
(Cy3P)2Pt(η2-MeCCMe) via an unsymmetrical intermediate
over several hours.4 The 31P NMR spectrum of the interme-
diate consists of two doublets with 195Pt satellites (δ: 41.6, d
1
1
2JPP = 34 Hz, JPPt = 3483 Hz; δ: 33.3, d JPPt = 2899 Hz).
Pt(P-i-Pr3)2
A previous report described the low-yield conversion of
Pt(P-i-Pr3)3 to Pt(P-i-Pr3)2 in vacuo (19). An improved syn-
thesis is reported here. In a PTFE-sealed vessel, (i-
Pr3P)2PtCl2 (1.94 g, 3.30 mmol) was dissolved in 25 mL of
THF, and 1% sodium amalgam (55 g, 30 equiv Na) was
added. The pale yellow solution turned red over the reaction
time of 3 days. The solution was transferred away from the
remaining amalgam–NaCl mixture via cannula. The amal-
gam was washed with 2 × 5 mL pentane, and the washings
were combined with the original supernatant. All solvent
was removed in vacuo to give a viscous red oil. To this,
30 mL of pentane were added, giving a red solution and pur-
ple precipitate. The solution was filtered and concentrated to
Reaction of 2 with ethylene
A sealable PTFE-capped NMR tube containing 2 (10 mg,
0.01 mmol) and benzene-d6 was degassed via three freeze-
pump-thaw cycles, and 1 atm of ethylene was introduced.
The reaction was monitored by 31P NMR spectroscopy as it
was converted to (Cy3P)2Pt(η2-C2H4) via an unsymmetrical
intermediate over several hours.
Reaction of 2 with PhSiH3
Compound 2 (11 mg, 0.01 mmol), PhSiH3 (5 µL,
0.04 mmol), and benzene-d6 were combined in an NMR
tube, which was then inverted several times to mix the reac-
tants. The green color of 2 was rapidly bleached, and H
NMR spectroscopy revealed the presence of Mes2SiH2,
Ph2SiH2, SiH4, and trace H2.
1
dryness to give red crystals of Pt(P-i-Pr3)3 (1.62 g, 96%). H
2
1
1
NMR δ: 1.99 (m, 1H, CH), 1.31 (virtual q JHP = JHH
=
7 Hz, 3H, CH3). 31P NMR δ: 72.2 (s with 195Pt satellites,
1JPPt = 4179 Hz).
© 2003 NRC Canada

Feldman et al.
1135
Formation of trans-(Cy3P)2Pt(H)SiMes2H (3) from 2
and H2
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A sealable PTFE-capped NMR tube containing a solution
of 2 (7 mg, 6 µmol) and benzene-d6 was degassed via two
freeze-pump-thaw cycles, and 1 atm of H2 was introduced.
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cis-(Cy3P)2Pt(H)SiMes2H (4)
A
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0.302 mmol), a magnetic stir bar, and 25 mL of hexane was
stirred and cooled to 0°C. Simultaneously, another tube con-
taining Mes2SiH2 (81 mg, 0.302 mmol) and 10 mL of hex-
ane was likewise cooled and stirred. The dimesitylsilane
solution was added to the first vessel dropwise via cannula
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Recrystallization from diethyl ether afforded 147 mg (48%)
of 4 (mp 159°C dec). IR (KBr): 3014 (m), 2932 (m), 2660
(w), 2150 (w), 2064 (m), 1596 (m), 1546 (w), 1446 (s), 1410
(w), 1328 (w), 1260 (m), 1173 (m), 1105 (m), 1011 (m), 928
(w), 887 (m), 846 (s), 737 (m), 710 (w), 599 (w), 519 (m),
1
496 (m), 432 (w). H NMR δ: 6.89 (s, 4H, ArH), 5.86 (br,
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2
1–2.2 (br m, Cy), –3.98 (br dd with 195Pt satellites, JHPcis
=
37 Hz, JHPtrans = 183 Hz, JHP1t = 1050 Hz, PtH). 31P NMR
2
2
δ: 45.1 (s with 195Pt satellites, JPPt = 1580 Hz), 31.6 (s with
195Pt satellites, JPPt = 2740 Hz). 13C NMR δ: 144.2 (br,
1
ortho-C), 143.1 (br, ipso-C), 136.0 (br, para-C), 129.3 (s,
meta-C), 39.6, 37.1, 31.1, 28.6, 28.2, 27.4, 27.2 (br, Cy),
25.4 (br, ortho-Me), 21.6 (s, para-Me). Anal. calcd for
C54H90P2PtSi: C 63.31, H 8.85; found: C 63.84, H 9.28.
Reactions of 1 with ROH (R = H, Me, Et)
In three separate reactions, water (ca. 0.2 µL, 0.01 mmol),
methanol (0.3 µL, 0.01 mmol), or ethanol (0.6 µL,
0.01 mmol) was injected into a septum-capped NMR tube
containing 1 (10 mg, 0.01 mmol) and benzene-d6. The tube
was then inverted several times to assure mixing. The result-
1
ing colorless solutions were then analyzed by H and 31P
NMR spectroscopy, which revealed quantitative formation of
Pt(PCy3)2 and Mes2Si(OR)H, which were identified by com-
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Acknowledgment
Acknowledgment is made to the National Science Foun-
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