Hydrosilylation with bis(alkynyl)(1,5-cyclooctadiene)platinum catlysts: A density functional study of the initial activation

Article abstract of DOI:10.1021/om0200196

At elevated temperatures bis(alkynyl)(1,5-cyclooctadiene)platinum complexes catalyze the cross-linking of polyorganosiloxanes containing Si - H and vinyl groups. Density functional calculations with medium-size basis sets and effective core potentials are reported for reactions that may activate these precatalysts for hydrosilylation. For a model system consisting of the bis(ethynyl) complex, trimethylsilane, and ethylene, the computations provide two plausible pathways for gaining access to the Chalk - Harrod cycle. The first one involves a sequence of four oxidative additions and reductive eliminations, while the second one requires a reductive coupling that is induced by olefin coordination. In both cases, the initial step is rate-determining, with a computed barrier of 27 kcal/mol. Experiments for polysiloxane systems of industrial interest favor the first pathway and yield barriers of 25-30 kcal/mol. Substituents in the alkynyl groups affect the measured barriers and the barriers computed for the rate-determining initial step of the first pathway in a qualitatively similar manner. We propose that the activation of the precatalysts is initiated by oxidative addition of Si - H.

Full text of DOI:10.1021/om0200196

2076
Organometallics 2002, 21, 2076-2087
Hyd r osilyla tion w ith
Bis(a lk yn yl)(1,5-cycloocta d ien e)p la tin u m Ca ta lysts: A
Den sity F u n ction a l Stu d y of th e In itia l Activa tion
Mavinahalli N. J agadeesh and Walter Thiel*
Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1,
D-45470 Mu¨lheim an der Ruhr, Germany
J utta Ko¨hler
Consortium fu¨r elektrochemische Industrie GmbH, Zielstattstrasse 20,
D-81379 Mu¨nchen, Germany
Armin Fehn
Wacker-Chemie GmbH, Werk Burghausen, J ohannes-Hess-Strasse 24,
D-84480 Burghausen, Germany
Received J anuary 9, 2002
At elevated temperatures bis(alkynyl)(1,5-cyclooctadiene)platinum complexes catalyze the
cross-linking of polyorganosiloxanes containing Si-H and vinyl groups. Density functional
calculations with medium-size basis sets and effective core potentials are reported for
reactions that may activate these precatalysts for hydrosilylation. For a model system
consisting of the bis(ethynyl) complex, trimethylsilane, and ethylene, the computations
provide two plausible pathways for gaining access to the Chalk-Harrod cycle. The first one
involves a sequence of four oxidative additions and reductive eliminations, while the second
one requires a reductive coupling that is induced by olefin coordination. In both cases, the
initial step is rate-determining, with a computed barrier of 27 kcal/mol. Experiments for
polysiloxane systems of industrial interest favor the first pathway and yield barriers of
25-30 kcal/mol. Substituents in the alkynyl groups affect the measured barriers and the
barriers computed for the rate-determining initial step of the first pathway in a qualitatively
similar manner. We propose that the activation of the precatalysts is initiated by oxidative
addition of Si-H.
In tr od u ction
A large variety of effective catalysts are available, most
notably transition metal complexes of group VIII ele-
ments (Co, Rh, Ni, Pd, and Pt). The most commonly used
catalysts are platinum compounds such as chloroplatinic
acid in alcohol,8 platinum(II) complexes with olefins, and
platinum(0) complexes with phosphines.1-4
We are interested in a typical industrial application
of hydrosilylation, namely, the construction of siloxane
networks from organopolysiloxanes containing aliphatic
CdC double bonds and Si-H bonds.1,5-7 Addition cur-
able silicone rubber compositions cross-link by hydrosi-
lylation, which is normally catalyzed by small amounts
of highly active platinum compounds (10-100 ppm). To
avoid premature cross-linking, addition curing silicone
compositions are generally handled as two-component
systems consisting of the unsaturated polyorganosilox-
ane plus catalyst and the cross-linker with Si-H bonds.
Hydrosilylation reactions involve the addition of hy-
drosilanes to unsaturated bonds.1-4 Hydrosilylation of
alkenes is one of the most important methods of forming
silicon-carbon bonds and is therefore widely used for
the synthesis of organosilicon compounds. In the silicone
industry, this reaction is applied for making organo-
functional silicon monomers and for cross-linking silicon
polymers. Industrial products include silicone rubber,
liquid injection molding compounds, paper release coat-
ings, pressure sensitive adhesives, binders, and coupling
agents.1,5-7
Hydrosilylation reactions are normally carried out
under mild conditions in the presence of a catalyst.1-4
(1) Marciniec, B.; Gulinski, J .; Urbaniak, W.; Kornetka, Z. W.
Comprehensive Handbook on Hydrosilylation; Pergamon Press: Ox-
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Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, W.
E., Eds.; Pergamon Press: Oxford, 1982; Vol. 2, pp 306-362.
(7) Noll, W. Chemistry and Technology of Silicones; Academic
Press: New York, 1968.
(8) (a) Speier, J . L.; Webster, J . A.; Barnes, G. H. J . Am. Chem. Soc.
1957, 79, 974. (b) Saam, J . C.; Speier, J . L. J . Am. Chem. Soc. 1958,
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(5) Lewis, L. N.; Stein, J .; Gao, Y.; Colborn, R. E.; Hutchins, G.
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10.1021/om0200196 CCC: $22.00 © 2002 American Chemical Society
Publication on Web 04/18/2002

Hydrosilylation with Platinum Catalysts
Organometallics, Vol. 21, No. 10, 2002 2077
Prior to use, the two components must be mixed
together in a well-defined ratio to initiate the cross-
linking reaction which yields the desired silicone rubber
material. A major disadvantage of such addition curing
systems is the necessity to handle two separate com-
ponents during the whole process from manufacturing
and packaging to mixing.
rod mechanism.15 Recent ab initio studies16-19 of the bis-
(phosphine)platinum(0)-catalyzed hydrosilylation of eth-
ylene by silane have covered the full Chalk-Harrod
cycle (oxidative addition of SiH4 to Pt(PH3)2, coordina-
tion of C2H4, ethylene insertion into Pt-H, and reduc-
tive elimination of H3Si-C2H5). They have provided
much detailed insight into these reactions and have
shown in particular that the Chalk-Harrod cycle is
energetically feasible (with a maximum barrier of about
22 kcal/mol) and preferred over a modified Chalk-
Harrod cycle with ethylene insertion into the Pt-Si
bond even if cis-trans isomerization of the intermedi-
ates is taken into account.16-18
The present paper investigates possible initial trans-
formations of the (pre)catalyst (COD)Pt(CCR)2 that
allow access to the Chalk-Harrod cycle. The next
section describes the computational methods used. In
the following sections, we report the theoretical results
and discuss them in the context of the available experi-
mental evidence.
Consequently there have been many attempts to
provide an addition-cross-linking silicone material as a
one-component formulation with an extended work life
at ambient temperature and a rapid cure at elevated
temperature. The central problem in the development
of one-component systems is to suppress any premature
cross-linking, which usually occurs already at room
temperature in the presence of the platinum catalyst.
One possible solution is the use of inhibitors1,9,10 which
can form complexes with the catalyst; this reduces its
activity at room temperature considerably, but may also
impair the activity at higher temperatures due to
incomplete decomplexation, which may cause low cross-
linking rates or even incomplete cure. A second ap-
proach is the encapsulation of the platinum catalyst in
a finely divided material such as a thermoplastic silicone
resin, an organic thermoplast, or a cyclodextrine,1,11,12
which does not release the catalyst until a certain
temperature is reached; problems with this approach
include the large size of the capsules (preventing the
formation of stable suspensions) and the sudden release
of locally high catalyst doses (resulting in inhomoge-
neous cross-linking). A third method is the design of new
classes of platinum catalysts that show essentially no
hydrosilylation activity at ambient temperatures and
high curing rates above a tunable kick-off temperature,
thus combining pot lives of several months with high
curing rates.13 One of the best examples for such
catalysts are bis(alkynyl)(1,5-cyclooctadiene)platinum
complexes (COD)Pt(CCR)2.14
Com p u ta tion a l Meth od s
The quantum-chemical calculations were carried out using
density functional theory (DFT).20 They employed the gradient-
corrected BP86 functional which combines the Becke ex-
change21 and Perdew correlation22 functionals. For platinum
we used a small-core quasirelativistic effective core potential
with the associate (8s7p6d)/[6s5p3d] valence basis set con-
tracted according to a (311111/22111/411) scheme.23 The other
elements were represented by the 6-31G(d) basis24 with one
set of d polarization functions at all non-hydrogen atoms.
Spherical d functions were used throughout. Unless noted
otherwise, geometries were optimized without symmetry
constraints. Transition states were normally located using the
synchronous transit-guided quasi-Newton (STQN) method.25
Force constants were computed for all stationary points to
establish their character. All calculations were performed
using the Gaussian98 program.26
For a rational optimization of these new platinum
(pre)catalysts, it is essential to understand how they
work and, in particular, how they are activated at
higher temperatures. Given the complexity of the real
systems that are used industrially, an experimental
study of the underlying reaction mechanisms is difficult.
We have therefore decided to investigate suitable model
systems computationally using density functional theory
in order to identify possible modes of reaction.
Resu lts
In our computational studies, the precatalyst is
represented by the parent (1,5-cyclooctadiene)bis(ethy-
nyl)platinum complex (COD)Pt(CCH)2, and the reac-
tants are modeled by trimethylsilane, HSiMe3, and
ethylene, C2H4. This is clearly a severe simplification
of the real system, which contains a substituted pre-
catalyst (COD)Pt(CCR)2 (where R is normally aryl) and
It is commonly accepted that the transition metal
catalyzed hydrosilylation proceeds by the Chalk-Har-
(15) Chalk, A. J .; Harrod, J . F. J . Am. Chem. Soc. 1965, 87, 16-21.
(16) Sakaki, S.; Mizoe, N.; Sugimoto, M. Organometallics 1998, 17,
2510-2523.
(9) Examples of inhibitors from the patent literature: (a) Hatanaka,
M.; Kurita, A. U.S. Pat. 4,329,275, 1982. (b) Sasaki, S.; Hamada, Y.
U.S. Pat. 4,603,168, 1986. (c) Michel, U.; Delker, J . R. U.S. Pat.
4,530,989, 1985. (d) Shirahata, A.; Shosaku, S. U.S. Pat. 4,465,818,
1984. (e) Cavezzan, J . U.S. Pat. 4,595,739 (1986). (f) Kniege, W.; Michel,
W.; Ackerman, J .; Randolph, K. H. U.S. Pat. 4,487,906, 1984. (g) J anik,
G.; Buentello, M. U.S. Pat. 4,487,906, 1984. (h) Lo, P. Y. K. U.S. Pat.
4,774,111, 1988.
(17) Sakaki, S.; Mizoe, N.; Musashi, Y.; Sugimoto, M. J . Mol. Struct.
(THEOCHEM) 1999, 462, 533-546.
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(19) Sugimoto, M.; Yamasaki, I.; Mizoe, N.; Antai, M.; Sakaki, S.
Theor. Chem. Acc. 1999, 102, 377-384.
(10) Examples of recent work on inhibitors: (a) Kishi, K.; Ishimaru,
T.; Ozono, M.; Tomita, I.; Endo, T. J . Polym. Sci. A: Polym. Chem.
2000, 38, 35-42. (b) Kishi, K.; Ishimaru, T.; Ozono, M.; Tomita, I.;
Endo, T. J . Polym. Sci. A: Polym. Chem. 2000, 38, 804-809.
(11) Examples of encapsulation from the patent literature: (a)
Togashi, A.; Kasuya, A. Eur. Pat. Appl. 449,181, 1991. (b) Fujioka, K.;
Yashida, Y. U.S. Pat. 5, 525,425, 1996.
(12) (a) Lewis, L. N.; Sumpter, C. A.; Davis, M. J . Inorg. Organomet.
Polym. 1995, 5, 377-390. (b) Lewis, L. N.; Sumpter, C. A.; Stein, J . J .
Inorg. Organomet. Polym. 1996, 6, 123-144.
(13) (a) Sumpter, C. A.; Lewis, L. N.; Lawrence, N. B. U.S. Pat.
5,331,075, 1994. (b) Fehn, A.; Achenbach, F.; Dietl, S. U.S. Pat. 6,-
187,890, 2001.
(20) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and
Molecules; Oxford University Press: Oxford, 1989.
(21) Becke, A. D. Phys. Rev. A 1988, 38, 3098-3100.
(22) Perdew, J . P. Phys. Rev. B 1986, 33, 8822-8824.
(23) Andrae, D.; Ha¨ussermann, U.; Dolg, M.; Stoll, H. Theor. Chim.
Acta 1990, 77, 123-141.
(24) (a) Hehre, W. J .; Ditchfield, R.; Pople, J . A. J . Chem. Phys. 1972,
56, 2257-2261. (b) Hariharan, P. C.; Pople, J . A. Theor. Chim. Acta
1973, 28, 213-222. (c) Francl, M. M.; Pietro, W. J .; Hehre, W. J .;
Binkley, J . S.; Gordon, M. S.; DeFrees, D. J .; Pople, J . A. J . Chem.
Phys. 1982, 77, 3654-3665.
(25) (a) Peng, C. Y.; Schlegel, H. B. Isr. J . Chem. 1994, 33, 449-
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(14) Fehn, A.; Achenbach, F. Eur. Pat. Appl. 994,159, 2000.

2078 Organometallics, Vol. 21, No. 10, 2002
Sch em e 1. Ch a lk -Ha r r od Mech a n ism
J agadeesh et al.
1). The hydrogen migration from Pt to COD has been
considered as a side reaction of intermediate 2 which
leads to 5.
Scheme 3 outlines an alternative conversion of the
precatalyst 1 that starts with the decoordination of one
COD double bond to yield the tricoordinated species 9.
This species can formally undergo a large number of
reactions. It turns out, however, that 9 is relatively high
in energy, and we have thus only considered the
oxidative addition of trimethylsilane leading to 10 and
the loss of COD from 9 and 10.
Scheme 4 shows another conceivable pathway that
involves the initial addition of ethylene to the precata-
lyst 1. The resulting π complex 11 may then undergo
reductive elimination of diacetylene to yield compound
12, followed by an oxidative addition of trimethylsilane.
The complex 13 thus formed is an entry point to the
Chalk-Harrod cycle (see Scheme 1) and can rearrange
to 14 by ethylene insertion.
We have carried out DFT calculations on most of the
reactions in Schemes 2-4. The platinum complexes
studied are collected in Scheme 5 for easy reference.
Total energies and zero-point vibrational energies of all
species considered are given as Supporting Information.
Table 1 lists the relevant reaction and activation ener-
gies. Figures 1-3 present reaction profiles for the
pathways investigated, while Figures 4-11 show the
most important transition structures (others being
available as Supporting Information, Figures S1-S4).
Table 2 contains selected geometrical parameters of the
optimized minimum structures. In the following we
shall discuss the theoretical results in the order sug-
gested by the reaction sequences in Schemes 2-4. Since
the relative energies without and with zero-point vi-
brational corrections do not differ much (see Table 1),
we shall only quote the former in the text.
The precatalyst 1 (C2 symmetry) contains two σ-bond-
ed ethynyl ligands (Pt-C bond length of 1.97 Å) and
an η4-COD ligand with two π bonds (Pt-C 2.25-2.28
Å, CdC 1.40 Å). Oxidative addition of HSiMe3 (see
Scheme 2) leads to a hexacoordinated species 2 with the
silyl and one ethynyl group trans to the COD double
bonds, while the hydride and the other ethynyl group
occupy the remaining two coordination sites; hence the
two ethynyl groups remain cis to each other in 2, and
the bulky SiMe3 ligand points away from COD. The
alternative addition product 2a , with reversed positions
of the H and SiMe3 ligands, is less stable than 2 by 12.0
kcal/mol due to unfavorable steric interactions between
SiMe3 and COD. There are two further isomers 2b and
2c with a trans-arrangement of the ethynyl groups
(energies of -4.5 and 15.3 kcal/mol relative to 2 depend-
ing on the position of SiMe3) which are irrelevant for
our purposes since they cannot be reached easily from
the cis-bis(ethynyl) precatalyst 1. The structures of 2
and its isomers 2a -c (see Table 2) provide clear
evidence of the trans-influence (structural trans-effect)
which increases in the order ethynyl < hydride < SiMe3:
27,28 for example, the Pt-C distances to the COD double
bonds increase from 2.27-2.32 Å (trans to CCH) via
polyorganosiloxane reactants with Si-H bonds and
vinyl groups. We expect, however, that the DFT calcula-
tions on the model system will still be helpful to assess
the intrinsic reactivities in the real system.
Scheme 1 depicts the Chalk-Harrod mechanism15 for
the platinum-catalyzed hydrosilylation of alkenes. After
oxidative addition of the silane and coordination of the
alkene, three coordination sites at platinum are oc-
cupied by the reactants. Assuming that platinum can
provide at most six coordination sites, the active cata-
lytic species can accommodate other ligands at no more
than three sites. Since the bis(alkynyl)(1,5-cyclooctadi-
ene)platinum complexes (COD)Pt(CCR)2 are tetracoor-
dinated, it is obvious that some decoordination is
required for the activation of the precatalyst.
The most direct path from the precatalyst to the
Chalk-Harrod cycle would involve decoordination of
1,5-cyclooctadiene (COD) or reductive elimination of
diacetylene to yield dicoordinated platinum complexes.
These reactions are computed to be endothermic by 61
and 45 kcal/mol, respectively, and can therefore be ruled
out. Hence we need to consider more complicated
pathways involving interactions between the precatalyst
and the reactants.
Scheme 2 shows a possible conversion of the precata-
lyst 1 by oxidative addition of trimethylsilane to yield
the hexacoordinated adduct 2, which may then be
transformed to the tetracoordinated species 8 either
directly by reductive coupling (elimination of diacety-
lene) or by a three-step sequence of reductive elimina-
tion (HCCSiMe3 or HCCH), oxidative addition (HSiMe3),
and reductive elimination (HCCH or HCCSiMe3). Spe-
cies 8 can enter the Chalk-Harrod cycle (see Scheme
(26) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J . A.,
J r.; Stratmann, R. E.; Burant, J . C.; Dapprich, S.; Millam, J . M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J .;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J . B.; Cioslowski, J .; Ortiz, J . V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J .; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; J ohnson, B. G.; Chen,
W.; Wong, M. W.; Andres, J . L.; Head-Gordon, M.; Replogle, E. S.;
Pople, J . A. Gaussian 98, revision A.7 and A.9; Gaussian, Inc.:
Pittsburgh, PA, 1998.
(27) Coe, B. J .; Glenwright, S. J . Coord. Chem. Rev. 2000, 203,
5-80.
(28) Belluco, U. Organometallic and Coordination Chemistry of
Platinum; Academic Press: New York, 1974.

Hydrosilylation with Platinum Catalysts
Organometallics, Vol. 21, No. 10, 2002 2079
Sch em e 2. Con ver sion of P r eca ta lyst by Oxid a tive Ad d ition of HSiMe3 a n d Su bsequ en t Rea ction s
Sch em e 3. Con ver sion of P r eca ta lyst by Dien e Decoor d in a tion a n d Su bsequ en t Rea ction s
Sch em e 4. Con ver sion of P r eca ta lyst by Olefin
Coor d in a tion a n d Su bsequ en t Rea ction s
transition structure (maximum Pt-C distance of 2.35
Å to the COD double bonds).
The initial adduct 2 can undergo at least four exo-
thermic reactions (see Scheme 2). Reductive elimina-
tion of trimethylsilylacetylene HCCSiMe3 leads to
the most stable product 3 (20 kcal/mol below 2), while
the elimination of acetylene HCCH to yield 4 is some-
what less exothermic (by 14 kcal/mol). The barriers
for these two eliminations are quite low (13-14 kcal/
mol), and the corresponding transition states are al-
most isoenergetic with the transition state for the
initial oxidative addition (slightly lower, see Figure 1).
Both transition structures resemble 2. This holds true
particularly for the elimination of HCCH (see Figure
5), where the breaking Pt-H and Pt-C bonds show
only minor elongations (by 0.10 and 0.07 Å, respec-
tively), while the other Pt-ligand distances remain
almost unchanged (forming C-H bond: 1.41 Å). The
transition structure for elimination of HCCSiMe3 (see
Figure 6) deviates somewhat more from 2, especially
with regard to the weaker interactions with one of
the COD double bonds, but the breaking Pt-Si and
Pt-C bonds are again not much longer than in 2
(by 0.24 and 0.03 Å, respectively), and the SiCC angle
of 127° is still far from linearity (forming Si-C bond:
2.10 Å).
2.36-2.49 Å (trans to H) to 2.59-2.79 Å (trans to
SiMe3), with similar trends in the other Pt-ligand bond
lengths.
The oxidative addition of HSiMe3 to 1 is endothermic
by 12 kcal/mol and has to overcome a barrier of 27 kcal/
mol. The corresponding transition structure is already
rather product-like (see Figure 4). The Pt-Si and Pt-H
distances are only slightly larger than in 2 (2.67 vs 2.43
Å and 1.71 vs 1.62 Å, respectively), whereas the Si-H
distance is elongated more strongly compared with
HSiMe3 (1.81 vs 1.51 Å). On the other hand, the trans-
influence of SiMe3 is not yet very pronounced in the

2080 Organometallics, Vol. 21, No. 10, 2002
Sch em e 5. P la tin u m Com p lexes Stu d ied
J agadeesh et al.
The elimination products 3 and 4 can be converted
to the tetracoordinated species 8 by oxidative addition
of HSiMe3 and subsequent elimination of HCCH or
HCCSiMe3, respectively (see Scheme 2). The pathway
3 f 8 should be favored since it starts from the more
stable compound and avoids the destabilization from
steric hindrance that is expected to occur between the
two bulky SiMe3 ligands on the alternative pathway 4
f 8. On the first route, the intermediate cis-dihydride
6 is more stable than the corresponding trans-dihydride
6a , while the complex 7 with two SiMe3 groups trans
to COD is the lowest intermediate on the second route
(see Schemes 2 and 5). Relative to a common reference
system (2 + HSiMe3), the energies of the intermediates
increase in the order 6 < 6a < 7 (i.e., -16 < -11 < -6
kcal/mol). Hence, as expected on steric grounds, the
pathway 3 f 8 is strongly favored already at the level
of the intermediates (by 10 kcal/mol), and we have
therefore located transition states only for this trans-
formation. The oxidative addition of HSiMe3 to 3 may
proceed in two orientations, with barriers of 17 and 22
kcal/mol, and the resulting adducts 6 and 6a can
eliminate HCCH easily, with barriers of 18 and 16 kcal/
mol, respectively. The alternative reductive eliminations
of HCCSiMe3 from 6 and 6a should have similar
barriers (see the results for 2), but they do not provide
direct access to the Chalk-Harrod cycle and were
therefore not studied. The overall conversion 3 f 8 is
endothermic by 3 kcal/mol and requires an activation
of 22 kcal/mol via 6 (or 24 kcal/mol via 6a ). It is facile
in the sense that each transition state is energetically
far below the initial transition state for 1 f 2 (see
Figure 2).
This is not true for the other two transformations of
the initial adduct 2 (see Scheme 2), which have to
overcome larger barriers, i.e., 21 kcal/mol for the rear-
rangement 2 f 5 by hydrogen migration from Pt to COD
and 25 kcal/mol for the direct one-step conversion 2 f
8 by reductive elimination of diacetylene. The transition
structure for the latter reaction (see Figure 7) is
reactant-like since the coordination geometry of the
remaining ligands (COD, H, and SiMe3) is similar to

Hydrosilylation with Platinum Catalysts
Organometallics, Vol. 21, No. 10, 2002 2081
Ta ble 1. Ca lcu la ted Rea ction En er gies a n d
Activa tion En er gies (k ca l/m ol)a
and 2 (by 28 and 16 kcal/mol, respectively), and it must
be separated from 2 at least by a small barrier (given
the fact that it could be located by an unconstrained
energy minimization). Likewise, loss of COD from 10
must require some activation. This latter reaction is
exothermic by 6 kcal/mol and yields Pt(CCH)2(H)SiMe3
(a tetracoordinated complex 20 which could be a suitable
entry into a Chalk-Harrod cycle). Concerning the
overall conversion 1 f 2 f 10 f 20 the second or third
step must be rate-determining since 10 is energetically
already slightly above the transition state for the first
step. We have not located the transition states for the
second and third step, but we would assume on the basis
of the data for related reactions that the overall barrier
for the three-step conversion will be well over 30 kcal/
mol.
q
reactionb
∆E
∆E0
∆Eq
∆E0
1 + HSiMe3 f 2
2 f 3 + HCCSiMe3
2 f 4 + HCCH
2 f 5
2 f 8 + HCCCCH
3 + HSiMe3 f 6
3 + HSiMe3 f 6a
4 + HSiMe3 f 7
6 f 8 + HCCH
6a f 8 + HCCH
7 f 8 + HCCSiMe3
1 f 9
11.8
-19.9
-14.1
-10.1
-16.8
3.9
8.5
7.8
-0.8
-5.4
-10.6
37.7
13.1
-21.1
-15.5
-8.7
-18.4
5.1
9.3
9.3
-2.4
-6.6
-12.2
36.2
26.8
13.4
14.0
21.4
24.6
17.0
22.3
26.7
12.1
11.5
20.3
22.7
17.5
22.1
17.8
15.4
15.1
13.1
40.0
38.2
9 f 10
-9.5
-7.7
9 f 16 + COD
1 + C2H4 f 11
11 f 12 + HCCCCH
12 + HSiMe3 f 13
13 f 14
23.5
22.4
12.0
14.3
13.1
14.4
12.1
14.8
13.1
13.1
-21.9
-23.2
Hence, the reactions in Scheme 3 should not be rele-
vant mechanistically. Compared with the rate-limiting
barrier of 27 kcal/mol for Scheme 2, the intermediate
species 9 is significantly higher in energy (at 38 kcal/
mol), and alternative pathways via 10 are also expected
to be less favorable.
5.6
7.0
-19.9
-17.8
a
Computed from the energies given in Table S1 of Supporting
Information; reaction energies ∆E and activation energies ∆Eq
refer to total energies without zero-point vibrational corrections,
b
while ∆E0 and ∆E0q include these corrections. See Schemes 2-4.
We now turn to reactions that might be initiated by
interactions between the precatalyst and ethylene (see
Scheme 4). It is experimentally known in nickel chem-
istry that olefins with electron-withdrawing substitu-
ents activate dialkyl(dipyridyl)nickel complexes to un-
dergo reductive coupling, which yields the corresponding
olefin complexes and alkanes.29 A number of such
reactions have been reported,30 and there is both
experimental and theoretical evidence that they proceed
by an associative mechanism via pentacoordinated
olefin complexes.29,31 We have therefore studied the
analogous reaction between our platinum-based pre-
catalyst and ethylene.
Initially a weak van der Waals complex is formed
between 1 and ethylene, with Pt-C distances to ethyl-
ene of more than 4 Å and a binding energy of less than
1 kcal/mol (see Figure 3). Closer approach of the
ethylene leads to the formation of the π complex 11,
which lies in a very shallow minimum (12 kcal/mol
above the reactants) and can dissociate again easily
(barrier of 1 kcal/mol). The corresponding transition
structure (see Figure 9) looks like the π complex, except
that the Pt-C distances to the incoming ethylene are
elongated by about 0.2 Å. Reductive elimination of
diacetylene from the π complex requires an additional
activation of 14 kcal/mol (i.e., 26 kcal/mol relative to the
separated reactants). The transition structure (see
Figure 10) differs from the π complex 11 mainly with
regard to the ethynyl moieties: the breaking Pt-C
bonds are still quite short (2.02 and 2.09 Å), but the
two ethynyl ligands have approached each other to allow
for the formation of the new central bond in diacetylene
(C-C 1.83 Å).
that in 2. The breaking Pt-C bonds are only slightly
longer than in 2 (by 0.02-0.05 Å), but the leaving
ethynyl groups have moved much closer together to
initiate the formation of the new central bond in
diacetylene (C-C 1.75 Å).
Summarizing the results for Scheme 2, we have found
a feasible four-step path 1 f 2 f 3 f 6 f 8 that
converts the precatalyst (COD)Pt(CCH)2 to the tetra-
coordinated species (COD)Pt(H)SiMe3, which can enter
the Chalk-Harrod cycle. This path involves an alter-
nating sequence of oxidative additions (HSiMe3) and
reductive eliminations (HCCSiMe3, HCCH). The rate-
determining step is the initial oxidative addition 1 f 2
with a barrier of 27 kcal/mol.
We have also considered transformations of the pre-
catalyst 1 that start with the decoordination of one COD
double bond (see Scheme 3). The resulting tricoordi-
nated species 9 contains a twisted COD ligand and is
38 kcal/mol less stable than 1. It is separated from 1 by
a small barrier of 2 kcal/mol and can therefore revert
to 1 easily. The barrier for decoordination 1 f 9 thus
amounts to 40 kcal/mol. As expected from the Hammond
postulate, the transition structure (see Figure 8) re-
sembles the product 9, with two short Pt-C distances
to the COD double bonds (2.26 Å, 2.35 Å) and two long
ones (3.32 Å, 3.57 Å).
Full decoordination of COD from 9 is endothermic by
another 23 kcal/mol and therefore energetically prohibi-
tive (products 61 kcal/mol above 1). We have checked
oxidative addition of HSiMe3 to 9 as an alternative
pathway and found a corresponding adduct 10, which
is analogous to 2 in the orientation of the ligands (except
that one of the COD double bonds is not coordinated).
We could not locate the transition state for the reaction
9 f 10, but exploratory calculations indicate that the
barrier must be sizable (see also reaction 1 f 2). Hence,
we expect that the overall conversion 1 f 9 f 10 should
require a larger activation than the value of 40 kcal/
mol obtained for the first step. This is a moot issue,
however, since the adduct 10 can also be reached via
the path 1 f 2 f 10, with a barrier of 27 kcal/mol for
the first step (see above). Energetically, 10 lies above 1
The reductive coupling yields the complex (COD)Pt-
(C2H4), 12, which is best characterized as a metalla-
cycle. In the optimized structure (C2 symmetry) the CC
(29) Yamamoto, T.; Yamamoto, A.; Ikeda, S. J . Am. Chem. Soc. 1971,
93, 3350-3359, 3360-3363.
(30) Examples: (a) Kohara, T.; Yamamoto, T.; Yamamoto, A. J .
Organomet. Chem. 1980, 192, 265-274. (b) Semmelhack, M. F.; Ryono,
L. S. J . Am. Chem. Soc. 1975, 97, 3873-3875. (c) Yamamoto, T.; Abla,
M. J . Organomet. Chem. 1997, 535, 209-211.
(31) Tatsumi, K.; Nakamura, A.; Komiya, S.; Yamamoto, A.; Yama-
moto, T. J . Am. Chem. Soc. 1984, 106, 8181-8188.

2082 Organometallics, Vol. 21, No. 10, 2002
J agadeesh et al.
F igu r e 1. Calculated energy profile for conversion of precatalyst by oxidative addition of trimethylsilane: initial steps
(see Scheme 2).
F igu r e 2. Calculated energy profile for conversion of precatalyst by oxidative addition of trimethylsilane: overview (see
Scheme 2).
bond of the C2H4 moiety lies in the coordination plane
of platinum, the Pt-C bonds in the three-membered
ring are quite short (2.11 Å), and the C-C bond in the
ring is rather long (1.45 Å). By contrast, in the π
complexes studied (11, 13, 19, 19a ; see Scheme 5), the
CC bond of ethylene tends to be perpendicular to the
coordination plane of platinum, the Pt-C distances to
the C2H4 ligand are longer (typically 2.25 Å), and the
CC bond in C2H4 is shorter (around 1.40 Å). These
differences are consistent with the commonly accepted
distinction between the σ- and π-bonding mode of
olefins.32

Hydrosilylation with Platinum Catalysts
Organometallics, Vol. 21, No. 10, 2002 2083
F igu r e 3. Calculated energy profile for conversion of precatalyst by ethylene coordination and subsequent reactions (see
Scheme 4).
F igu r e 4. Transition structure for oxidative addition: 1
F igu r e 5. Transition structure for reductive elimination:
+ HSiMe3 f 2 (bond lengths in Å, bond angles in deg).
2 f 4 + HCCH (bond lengths in Å).
The formation of the metallacycle 12 is exothermic
by insertion of C2H4 into the Pt-H bond to form (COD)-
overall (by 10 kcal/mol relative to the separated reac-
Pt(SiMe3)(C2H5), 14. Both 13 and 14 are more stable
tants). The subsequent oxidative addition of HSiMe3 to
than the separated reactants (by 4 and 24 kcal/mol,
yield the pentacoordinated complex 13 is facile and
respectively).
requires an activation of only 12 kcal/mol. The corre-
Looking at the overall energy profile for the reactions
sponding transition structure (see Figure 11) has rather
in Scheme 4 (see Figure 3), it is clear that the π complex
normal Pt-COD distances and clearly moves toward the
11 will not be relevant mechanistically because it
product geometry in the three-membered ring (Pt-C
resides in a very shallow minimum. The rate-limiting
2.17 Å, 2.21 Å) and in the reactive region (Pt-Si 2.85
barrier for entering the Chalk-Harrod cycle via Scheme
Å, Pt-H 1.78 Å, and Si-H 1.63 Å). The adduct (COD)-
4 amounts to 26 kcal/mol and refers to the conversion
Pt(H)SiMe3(C2H4), 13, is an entry point to the Chalk-
1 f 12 by reductive coupling.
Harrod cycle (see Scheme 1), which will then continue
Our calculations thus suggest two different pathways
for the activation of the precatalyst 1 that have almost
the same effective barriers. In both cases, the initial
(32) Hartley, F. R. In Comprehensive Organometallic Chemistry;
Wilkinson, G., Stone, F. G. A., Abel, W. E., Eds.; Pergamon Press:
Oxford, 1982; Vol. 6, pp 614-680.
reaction of the precatalyst corresponds to the rate-

2084 Organometallics, Vol. 21, No. 10, 2002
J agadeesh et al.
F igu r e 8. Transition structure for first step of diene
decoordination: 1 f 9 (bond lengths in Å, bond angles in
deg).
F igu r e 6. Transition structure for reductive elimination:
2 f 3 + HCCSiMe3 (bond lengths in Å, bond angles in deg).
F igu r e 9. Transition structure for ethylene coordination:
1 + C2H4 f 11 (bond lengths in Å).
F igu r e 7. Transition structure for reductive elimination:
2 f 8 + HCCCCH (bond lengths in Å).
determining step: either oxidative addition of HSiMe3
(Scheme 2 and Figure 1, barrier of 26.8 kcal/mol) or
coordination of ethylene inducing reductive elimination
of diacetylene (Scheme 4 and Figure 3, barrier of 26.4
kcal/mol).
Having completed the discussion of the reactions in
Schemes 2-4, we briefly address some thermochemical
aspects. Table 3 collects computed dissociation energies
for the platinum complexes studied. These energies refer
to the following reactions: decoordination of COD
(breaking of two π bonds), loss of ethylene from a
metallacycle (breaking of two σ bonds in 12 and 17) or
from a π complex (breaking of one π bond in 11, 13, and
19), and reductive elimination of HCCCCH or HSiMe3
(breaking of two σ bonds). It is obvious that the
dicoordinated platinum complexes show the highest
dissociation energies (55-83 kcal/mol), indicating that
the electronic structure of the metal can adapt to the
ligands in an optimum manner in this case. By contrast,
in the pentacoordinated and hexacoordinated species,
F igu r e 10. Transition structure for reductive coupling:
11 f 12 + HCCCCH (bond lengths in Å).
only the COD ligand is still weakly bound (by 7-16 kcal/
mol), while the loss of the other ligands is exothermic
(by 5-22 kcal/mol in the case of the reductive elimina-
tions); this is probably due to the relief of steric

Hydrosilylation with Platinum Catalysts
Organometallics, Vol. 21, No. 10, 2002 2085
articles on reactions of transition metal complexes34 and
on palladium and platinum molecular chemistry.35
These reviews33-35 complement previous surveys and
validation studies for the BP86 functional.36-38 In the
current context, specific comparisons between the re-
sults at different theoretical (ab initio and DFT) levels
are available for the bis(phosphine)platinum(0)-cata-
lyzed hydrosilylation of ethylene16-18 and for the eth-
ylene binding energy in (PH3)2Pt(C2H4).39,40 On the basis
of this material16-18,33-40 we expect that the chosen
theoretical approach will provide realistic results, with-
out claiming quantitative accuracy.
We refrain from making more precise error estimates
because the use of model systems introduces an ad-
ditional uncertainty that is hard to quantify. The real
systems contain organopolysiloxanes with Si-H bonds
and CdC double bonds which are sterically much more
demanding than the model reactants (HSiMe3 and
H2CdCH2). In addition, the neighboring substituents
in the real systems also exert electronic effects: elec-
tronegative oxygen atoms next to the Si-H bond should
be electron-withdrawing, while silicon atoms next to the
CdC bond should be electron-donating (even though the
next nearest atoms in the substituents will counteract
these trends to some extent). The electronic effects may
be expected to lower the rate-determining barriers for
the initial oxidative addition 1 f 2 (see Scheme 2) and
the initial reductive elimination 1 f 12 (see Scheme
4), respectively. On the other hand, the increased steric
demand would tend to increase these barriers since the
corresponding transition states are more highly coor-
dinated than the reactants. Hence it is difficult to
predict how much the barriers will change when going
from the model to the real system. Under these circum-
stances, we have decided to use the computational
results for the model system at face value and to make
an attempt to relate them to experimental evidence
available for the real systems.
To distinguish between the two modes of activation
suggested by the DFT calculations, two substituted
siloxanes were heated in the presence of the precatalyst
(COD)Pt(CCR)2 with R ) p-trimethylsilylphenyl (1b )
(see Experimental Section). Pentamethyldisiloxane
Me3Si-O-SiHMe2 was found to react already at 60 °C,
and tempering at 120 °C for 1 h led to a complex mixture
of products including COD, cyclooctene, R-C2H5, and
a resin of unidentified composition. By contrast, the
divinyl-substituted polysiloxane (CH2dCH)SiMe2-O-
[SiMe2-O]20-SiMe2(CHdCH2) could be heated to 120
°C for 1 h without any signs of a reaction; in particular,
no diacetylene product could be detected. These experi-
ments indicate that the vinyl group in a polysiloxane is
less reactive toward the precatalyst than the Si-H
group, implying that the initial activation of the pre-
catalyst should involve the oxidative addition of an
Si-H bond. It should also be noted in this context that
the reported reductive eliminations in dialkyl(dipyridyl)-
F igu r e 11. Transition structure for oxidative addition: 12
+ HSiMe3 f 13 (bond lengths in Å).
congestion upon dissociation, and partly also to elec-
tronic effects.
When comparing different dissociation channels for
a given complex, the reductive elimination of diacetylene
is normally favored thermodynamically (least endo-
thermic for 1, 19, and 20; most exothermic for 11 and
nearly so for 2). This does not mean, of course, that this
reaction is most facile: both for 2 and 11, there are
thermodynamically less favorable channels with smaller
barriers (see Figures 2 and 3).
Finally, it is evident that the dissociation energies for
ethylene are significantly higher for the metallacycles
(17 and 12) than for the π complexes (11, 13, and 19).
The difference is seen most clearly for (COD)Pt(C2H4),
12, and Pt(CCH)2(C2H4), 19, since two more coordina-
tion sites are occupied in each case: the corresponding
ethylene binding energies are 55 and 33 kcal/mol,
respectively. The π complex Pt(CCH)2(C2H4) is related
to the precatalyst (COD)Pt(CCH)2 in that the ethylene
ligand of 19 replaces the COD ligand of 1 (occupying
one of the coordination sites of the COD double bonds).
The ethylene binding energy of 33 kcal/mol in 19 is
consistent with the energy of 38 kcal/mol needed to
decoordinate one COD double bond of 1 (reaction 1 f
9, see Table 1); the latter value is somewhat higher
probably because of the internal strain of COD in 9. In
the tricoordinated complex 19, the ethylene ligand can
easily migrate to the vacant site in the coordination
plane of platinum (via a C2v transition state 19a , barrier
of 7 kcal/mol).
Discu ssion
The relevance of the theoretical results reported in
the preceding section is limited by the use of model
systems and by the error margins of the DFT calcula-
tions. Concerning the latter, the application of gradient-
corrected density functionals (such as BP86) in combi-
nation with medium-size basis sets and effective core
potentials has become an established tool in computa-
tional transition metal chemistry over the past years.
A comprehensive assessment of this field can be found
in a recent issue of Chemical Reviews,33 which includes
(34) Niu, S.; Hall, M. B. Chem. Rev. 2000, 100, 353-406.
(35) Dedieu, A. Chem. Rev. 2000, 100, 543-600.
(36) Ziegler, T. Chem. Rev. 1991, 91, 651-667.
(37) Ziegler, T. Can. J . Chem. 1995, 73, 743-761.
(38) J onas, V.; Thiel, W. J . Chem. Phys. 1995, 102, 8474-8484.
(39) Uddin, J .; Dapprich, S.; Frenking, G.; Yates, B. F. Organome-
tallics 1999, 18, 457-465.
(40) Yates, B. F. J . Mol. Struct. (THEOCHEM) 2000, 506, 223-
232.
(33) Davidson, E. R., Ed. Chem. Rev. 2000, 100, 351-818.

2086 Organometallics, Vol. 21, No. 10, 2002
J agadeesh et al.
Ta ble 2. Op tim ized Bon d Len gth s (Å) in Selected P la tin u m Com p lexes (see Sch em e 5)
bonda,b
(ligand) (COD)
Pt-C1 Pt-C2 Pt-C3 Pt-C4 C1dC2 C3dC4 Pt-C5 Pt-C6 Pt-H
Pt-Si Pt-C7 Pt-C8 C7dC8
(COD)
(COD)
(COD)
(COD)
(COD)
(CCH)
(CCH)
(C2H4)
(C2H4)
(C2H4)
1
2
2a
2b
2c
3
4
5
6
6a
7
8
9
11
12
13
14
2.25
2.32
2.29
2.48d
2.28
2.22
2.21
2.99
2.34d
2.26
2.56c
2.26d
3.65
2.23
2.30
2.28
2.47c
2.28
2.30
2.32
2.49d
2.28
2.24
2.24
2.13f
2.38d
2.27
2.60c
2.29d
3.97
2.21
2.27
2.33
2.38d
2.28
2.25
1.40
1.40
1.41
1.38
1.41
1.41
1.41
1.55
1.40
1.41
1.38
1.40
1.35
1.43
1.40
1.41
1.38
1.40
1.37
1.40
1.37
1.41
1.38
1.38
1.40
1.38
1.38
1.38
1.39
1.40
1.40
1.40
1.43
1.40
1.97
1.99e
2.00e
2.03
2.00
1.97
1.98
1.96
2.07
1.99
2.06
1.97
2.08d
2.10c
2.03
2.00
2.59c
2.37d
2.79c
2.30
2.65c
2.36d
2.70c
2.27
1.62
1.58
1.57
1.66c
1.59
2.43
2.54
2.42
2.59d
-
2.35d
2.50c
2.38
2.31d
2.43c
2.30
2.39
2.44
2.40
2.42
2.41i
2.38
2.07
2.57c
2.53c
2.64c
2.44c
2.24
2.53c
2.47c
2.55c
2.37c
2.38
1.58g
1.65h
1.63
1.61
1.93e
1.98e
1.91
2.08
2.33
2.27
2.28
2.26
2.33
2.30
2.22
2.29
2.29
2.11
2.28
2.10j
2.24
2.11
2.23
1.41
1.45
1.42
1.60
2.47
2.39
a
C1, C2, C3, C4: unsaturated carbon atoms of COD (see Scheme 5: C1 and C2 upper double bond, C1 and C3 frontside atoms); C5,
b
C6: carbon atoms of ethynyl group; C7, C8: carbon atoms of ethylene moiety. The CC triple bond lengths in the ethynyl group are not
given since they are almost constant (1.23 Å). c Trans to SiMe3. Trans to H. e Trans to one of the COD double bonds. f σ-bonded to Pt.
d
g
h
i
Second Pt-H bond trans to CCH: 1.63 Å. Same value for both Pt-H bonds (difference of 0.002 Å). Same value for both Pt-Si bonds
j
(difference of 0.003 Å). Pt-C(ethyl).
Ta ble 3. Ca lcu la ted Dissocia tion En er gies
(k ca l/m ol)a ,b
) H (1). For further comparison, additional calculations
were done on this reaction using the precatalysts 1a ,
1b, and 1c. In the case of R ) phenyl (1a ), full geometry
optimizations were performed as well as single-point
calculations which employed the optimized structures
of the parent system, except for replacement of the
substituent R ) H in the ethynyl groups by R ) Ph
(standard bond lengths and angles): both approaches
led to the same41 activation energy of 28.2 kcal/mol for
1a . In the case of 1b and 1c, only the less expensive
single-point calculations were carried out, yielding
barriers of 28.0 and 26.5 kcal/mol, respectively. It is
evident that the computed DFT barriers for 1a -c are
consistent with the experimental data with regard to
both the barrier heights and the substituent effects
encountered. This supports the notion that the precata-
lysts under study are indeed activated by the initial
oxidative addition of a silane.
complex
label coordc oxd
loss of
∆Ee
Pt(COD)
Pt(CCH)2
Pt(C2H4)
Pt(H)SiMe3
Pt(CCH)2(C2H4)
15
16
17
18
19
2
2
2
2
3
0
2
2
2
2
COD
HCCCCH
C2H4
HSiMe3
C2H4
HCCCCH
HSiMe3
HCCCCH
COD
HCCCCH
COD
C2H4
COD
HSiMe3
COD
71.2
54.7
75.8
82.5
33.0
11.9
38.9
11.1
61.2
44.7
50.0
54.6
38.0
49.3
10.5
-11.8
-14.1
Pt(CCH)2(H)SiMe3
(COD)Pt(CCH)2
20
1
4
4
4
4
6
4
2
2
2
4
(COD)Pt(C2H4)
12
8
(COD)Pt(H)SiMe3
(COD)Pt(CCH)2(H)SiMe3
2
HSiMe3
HCCH
HCCCCH -16.5
HCCSiMe3 -19.9
(COD)Pt(C2H4)(CCH)2
(COD)Pt(C2H4)(H)SiMe3
11
13
5
5
2
2
COD
C2H4
16.2
Con clu sion s
-12.0
The DFT calculations on a model system provide two
plausible pathways for the activation of precatalyst 1
(see Schemes 2 and 4, Figures 1-3). The first one
involves a sequence of four oxidative additions and
reductive eliminations to gain access to the Chalk-
Harrod cycle, while the second one accomplishes this
goal more directly by a reductive coupling that is
induced by olefin coordination. In both cases, the initial
step is rate-determining, with a computed barrier of 27
kcal/mol. The available experimental evidence for the
real polysiloxane systems favors the first pathway.
Therefore we propose that the activation of the precata-
lysts (COD)Pt(CCR)2 is initiated by oxidative addition
of Si-H. The corresponding barrier, and hence the kick-
off temperature of the catalyst, can be tuned by proper
choice of the substituent R in the alkynyl groups.
HCCCCH -22.0
COD
7.3
-0.3
-5.6
C2H4
HSiMe3
a
Computed from the total energies given in Table S1 of
Supporting Information; zero-point vibrational corrections are not
b
included. J udging from the calculated geometries and energies,
the ethylene ligand forms a metallacycle in two cases (12, 17) and
a π complex in the other cases (11, 13, 19). The coordination and
oxidation numbers are assigned accordingly. c Coordination num-
d
ber of Pt. Formal oxidation number of Pt. e Negative values
indicate exothermic dissociation.
nickel complexes29,30 are activated by olefins with
electron-withdrawing substituents.
Kinetic measurements were carried out for an unsat-
urated polysiloxane using three different precatalysts
(COD)Pt(CCR)2 with R ) phenyl (1a ), R ) p-trimeth-
ylsilylphenyl (1b), and R ) pentafluorophenyl (1c) (see
Experimental Section). The resulting Arrhenius activa-
tion energies are 29.5, 26.6, and 25.4 kcal/mol, respec-
tively. These values are close to the computed barrier
of 26.8 kcal/mol (see Table 1) for the initial oxidative
addition 1 + HSiMe3 f 2 in the model system with R
Exp er im en ta l Section
All experiments were conducted in a dry nitrogen atmo-
sphere using standard Schlenk techniques. Solvents and
(41) Test calculations with other small substituents R show that
the barriers from single-point calculations and from full optimizations
normally differ somewhat (by about 0.5 kcal/mol).

Hydrosilylation with Platinum Catalysts
Organometallics, Vol. 21, No. 10, 2002 2087
phenylacetylene were obtained from Aldrich and used without
further purification. The compounds were characterized by a
combination of elemental analysis and spectroscopic methods.
C and H analyses were determined with a Leco-CHN 800
microanalyzer. Infrared spectra (range 400-4000 cm-1) were
recorded as KBr disks on a Nicolet Magna System 750. 1H and
13C NMR spectra were recorded in CDCl3 solution on Bruker
Advance DPX 360 (1H, 360.13 MHz) and DPX 400 (13C, 100.62
MHz) machines. The starting material (COD)PtCl2 was pre-
pared as described in the literature.42
Anal. Calcd for C24H12F10Pt: C, 42.1; H, 1.8. Found: C, 41.6;
H, 1.7.
Siloxa n e Com p ou n d s for Kin etic Stu d ies. The SiH
cross-linking agent was a copolymer containing Me3SiO,
Me2SiO, and MeHSiO units, with a viscosity of 310 mPa‚s and
a weight content of 0.46% of Si-bonded hydrogen; 12.0 g of this
cross-linker was mixed with 200.0 g of a poly(dimethylsiloxane)
terminated at both ends with a vinyl group (viscosity 1000
mPa‚s) and 4.24 mg of 1-ethynyl-1-cyclohexanol. Thereafter,
a solution of the platin compound in 3 mL of CH2Cl2 was added
and mixed homogeneously. The platinum content in the
mixture was chosen as 10 ppm, corresponding to 5.5 mg of
1a , 7.1 mg of 1b, and 7.5 mg of 1c.
(COD)P t(CtCP h )2 (1a ). The synthesis followed published
1
procedures.43 IR: ν(CtC) 2124 (s). H NMR: δ 7.38-7.10 (m,
2
5 H, ArH), 5.68 (t, 4 H, J HPt ) 45 Hz, CHdCH), 2.55 (br s, 8
H, CH2). 13C NMR: phenyl, 121.1 (C-CtC), 132.3 ((CH)2C-
), 128.2 ((CH-CH)2C-), 126.8 ((CH)2CH), COD, 30.8 (CH2),
104.6 (CH); others, 94.9 (Pt-Ct), 108.8 (Pt-CtC-). Anal.
Calcd for C24H22Pt: C, 57.0; H, 4.4. Found: C, 56.7; H, 4.4.
(COD)P t(CtC-C6H4SiMe3-p )2 (1b).44 A suspension of 0.50
g of (COD)PtCl2 in 20 mL of methanol was cooled to -20 °C.
A freshly prepared solution of 0.77 g of (p-trimethylsilylphen-
ylethynyl)trimethylsilane45 and sodium methanolate (61.5
mg of sodium, 15 mL of methanol) was then slowly added
dropwise. After 20 min, the mixture was heated to room
temperature, and the precipitate was filtered off and washed
five times with acetone. Yield: 0.78 g (90%). IR: ν(CtC) 2124
(s). H NMR: δ 7.36 (s, 8 H, ArH), 5.68 (br t, 4 H, J HPt ) 44
Hz, CHdCH), 2.55 (br s, 8 H, CH2), 0.20 (s, 18 H, Si(CH3)3).
13C NMR: aryl group, 126.5 (C-CtC), 130.5 ((CH)2C-), 132.5
((CH-CH)2C-), 138.0 ((CH)2C-Si(CH3)3); COD, 30.0 (CH2),
103.8 (CH); others, 94.8 (Pt-Ct), 108.2 (Pt-CtC-), -1.5
(Si(CH3)3). Anal. Calcd for C30H38Si2Pt: C, 55.5; H, 5.9.
Found: C, 54.8; H, 5.9.
Kin etic Stu d ies. Measurements were done at seven tem-
peratures between 75 and 90 °C. The curing times were deter-
mined at each temperature using a gel timer (following DIN
1694). After completing a series of consecutive measurements
at different temperatures, the curing time was determined
again at the initial temperature which reproduced the initially
measured value to within 5% and thereby confirmed the
stability of the silicone mixture during these experiments.
Rea ctivity Stu d ies for 1b. A mixture of 4.0 g of a
vinyldimethylsilyloxy-terminated polysiloxane (CH2dCH)-
SiMe2-O-[SiMe2-O]20-SiMe2(CHdCH2) and 2.0 g of 1b was
stored at 120 °C for 1 h, but no reaction occurred (neither in
air nor in a vacuum). Likewise, a mixture of 4.0 g of pentam-
ethyldisiloxane Me3Si-O-SiHMe2 and 2.0 g of 1b was stored
at 120 °C for 1 h; in this case, a reaction took place. The
reaction mixture was analyzed by NMR, IR, and GC: cyclooc-
tadiene, cyclooctene, and (p-ethylphenyl)trimethylsilane could
be identified as reaction products, along with a dark brown
resin of unknown composition (no ethynyl groups left according
to IR).
1
2
(COD)P t(CtC-C6F 5)2 (1c).46 A suspension of 2.0 g (5.35
mmol) of (COD)PtCl2 in 30 mL of methanol was cooled to -30
°C. A freshly prepared solution of 2.21 g (11.5 mmol) of
(pentafluorophenyl)acetylene47 was added to this suspension
followed by 12.75 mL (10.0 mmol) of sodium methanolate
(0.784 M in methanol). After 60 min at -30 °C, the mixture
was allowed to warm to room temperature, and the precipitate
was filtered off and washed five times with acetone. Yield: 3.20
Ack n ow led gm en t. We thank Prof. P. W. J olly for
pointing out previous experimental work on reductive
coupling in nickel complexes and for suggesting analo-
gous calculations in our system. We are also grateful to
Dr. F. Achenbach, Dr. V. Frey, Dr. O. Scha¨fer, and Dr.
W. Schmitt-Sody for many helpful discussions, and to
Debasis Koley for his help in the final stage of the
computational work.
1
2
g (87%). IR: ν(CtC) 2143 (m). H NMR: δ 5.66 (t, 4 H, J HPt
) 40.5 Hz, CHdCH), 2.55 (s, 8 H, CH2), 2.50 (s, 6 H, CH3).
(42) McDermott, J . X.; White, J . W.; Whitesides, G. M. J . Am. Chem.
Soc. 1976, 98, 6521-6528.
Su p p or tin g In for m a tion Ava ila ble: Total energies with-
out and with zero-point vibrational corrections of all relevant
species (Table S1), further optimized bond lengths and angles
(Table S2), and additional transition structures (Figures
S1-S4). This material is available free of charge via the
Internet at http://pubs.acs.org.
(43) Cross, R. J .; Davidson, M. F. J . Chem. Soc., Dalton Trans. 1986,
1987-1992.
(44) Achenbach, F.; Fehn, A. DE 19,938,338, 1999.
(45) Eaborn, C.; Thompson, A. R.; Walton, D. R. M. J . Chem. Soc.
(C) 1967, 1364-1366.
(46) (a) Achenbach, F.; Fehn, A. DE 19,847,097, 1998. (b) Achenbach,
F.; Fehn, A. Eur. Pat. Appl. 994,159, 1998.
(47) Waugh, F.; Walton, D. R. M. J . Organomet. Chem. 1972, 39,
275-278.
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