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J. Phys. Chem. B 2002, 106,
Heterogeneous Inhibition of Homogeneous Reactions: Karstedt Catalyzed Hydrosilylation
Francesco Faglioni,† Mario Blanco, William A. Goddard, III,* and Dennis Saunders
Materials and Process Simulation Center, Beckman Institute (139-74), Caltech, Pasadena, California 91125,
Very Research Center, Pasadena, California 91107
Ved: July 26, 2001; In Final Form: No
Vember 26, 2001
The Karstedt-catalyzed hydrosilylation reaction used in curing of silicone release coatings was investigatedusing first-principle quantum mechanical techniques (density functional theory) as well as semiempiricalmethods to estimate solubility parameters. The results we obtain for the catalytic cycle indicate, in agreementwith experimental results, that hydrosilylation occurs easily at room temperature. The detailed mechanismwe suggest contains the key features of the models previously proposed in the literature by Lewis and Chalk-Harrod and adds quantitative estimates of reaction energies and barriers. On the basis of the energy profilefor the catalytic cycle in the presence of inhibitor molecules and on solubility parameters for the speciesinvolved in the reaction, we conclude that the role of the inhibitors we considered is to phase-separate thecatalyst from the substrate. The reaction is thus quenched by introducing a microscopic second phase thatinterferes with the homogeneous reaction.
elementary steps commonly observed in organometallic chem-istry, such as oxidative addition and reductive elimination. A
Hydrosilylation reactions (eq 1) are among the most important
variation of the Chalk-Harrod mechanism was also proposed
reactions in silicone and siloxane chemistry,1 and they are
to explain the formation of vinylsilanes.2,7 This version assumes
extensively used as a means to form Si-C bonds. One importantapplication of these reactions is to cross link hydrosiloxanes
an olefin attack on the Pt-Si bond that has never been observed
with vinyl terminated dimethylsiloxanes1 for the production of
experimentally. Although both the original and the modified
protective backing surfaces in pressure sensitive adhesive labels.
Chalk-Harrod mechanisms correctly describe the hydrosilyla-
This often involves catalysis by late transition metals, most
tion reaction, they fail to account for a number of phenomena
observed mainly at the beginning (induction period) and theend (change in color) of the reaction.
To explain the induction period, color change, and several
other effects, Lewis proposed in 19868,9 a radically different
Typical catalysts used for the cross linking are low valent Pt
mechanism. He suggested that the catalytic species are colloidal
compounds. Of these, one of the best characterized is Karstedt’s
platinum particles rather than organometallic complexes. He
regarded the reaction as a surface, or edge, catalyzed hetero-geneous process. His original mechanism became known as theLewis mechanism. Later, he realized that although mostexperiments result in the formation of colloidal particles,hydrosilylation does not depend on their existence and mustthus be regarded as a homogeneous process.10 When the originalLewis mechanism is translated from the platinum surface to anorganometallic compound, it becomes similar to the older
Chalk-Harrod mechanism, the main difference being the order
in which different ligands are coordinated and activated.
dCH2)]3. Upon solvation, the bridging ligand is released,
yielding the more active form Pt[(H2CdCH)(CH3)2SiOSi(CH3)2-
The structure of several compounds resulting from the
reaction of inhibitor molecules with catalyst 2 are known from
The catalyzed hydrosilylation reaction is typically fast at
EXAFS or NMR analysis.11 To date, however, no clear model
ambient conditions and, in the case of cross linking polymer-
has been formulated to rationalize the role of the inhibitors. In
ization, results in solidification of the substrate. It is hence
particular, the problem of why certain additives are better
common practice to quench the reaction by adding inhibitors
inhibitors than others has not been addressed in the literature.
to allow easy handling and storage of the bath as a liquid. Once
In the next section, we report our computational results for
the bath is applied to the backing surface support, it is cured
each of the elementary steps involved in the Chalk-Harrod,
by removing the inhibitor, which activates the cross linking
Lewis, and a third, similar, mechanism. On the basis of these
reaction. Typical curing conditions are 5-20 s at 100-150 °C.
results, we conclude that all three mechanisms represent minor
In 1964, Chalk and Harrod5,6,7 proposed a mechanism for
variations of the same catalytic cycle and that the activation
platinum catalyzed hydrosilylation reactions based on simple
energy is small. In the following section, we investigate possibleroles of the inhibitors and come to the conclusion that inhibitors
* To whom correspondence should be addressed.
cannot block the active site by binding to it. In the fourth section,
Current address: Dip. di Chimica, Universita` di Modena, Via Campi
we report the results from solubility simulations of the various
Heterogeneous Inhibition of Homegeneous Reactions
J. Phys. Chem. B, Vol. 106, No. 7, 2002 1715
Chalk-Harrod mechanism as originally proposed (A) and our implementation to compute the reaction profile (B).
species involved in the reaction and conclude that inhibitors
basis set on Pt and Si.16,17 Hay and Wadt’s
effective core potentials (ECP) were used to replace the innerelectrons of Pt and Si.16,17 In the case of Pt, we used the small
2. Computational Model
core (core-valence) ECP, whereas for Si, we employed the largecore (valence-only) ECP.
2.1. Computational Model.
All quantum computations were
obtained using the JAGUAR program suite.12 We performed
To reduce the cost of the computations, especially of
density functional theory computations using Becke’s hybrid
geometry optimizations, we studied the effect of the catalyst
three parameter functional for the exchange13 and Lee, Yang,
only on monomers of silicon hydrides and siloxo vinyls. These
and Parr functional14 for the correlation. We used a 6-31G**
were further simplified by replacing all methyl groups with
double- plus polarization basis set15 on all H, C, and O atoms
hydrogens and all inactive O-SiR3 groups with hydroxides. The
1716 J. Phys. Chem. B, Vol. 106, No. 7, 2002
Energy profile for the Chalk-Harrod mechanism.
molecules used are H2Si(OH)2 and CH2CHSiH2OH. Being
The energetics for the modified Chalk-Harrod mechanism
aware of the approximations made, we consistently verified that
are found to be less favorable than those for the original. We
the artificial hydrogens and hydroxides so introduced would not
could not find a stable structure for the intermediate which
move to sterically hindered regions or participate in hydrogen
follows insertion in the Pt-Si bond. All calculational attempts
at such an intermediate resulted in compounds where one olefin
The original Chalk-
inserts in the Pt-Si bond and a second olefin inserts in the Pt-H
Harrod mechanism5-7 is reported schematically in Figure 1A.
bond, i.e., olefin insertion in the Pt-Si bond activates the Pt-H
Starting from a Pt(0) species, it involves oxidative addition of
bond. The barrier for this process was found to be 31.4 kcal/
Si-H, olefin insertion in the Pt-H bond, reductive elimination
mol, which is higher than the 24.0 kcal/mol obtained for
of the product, and regeneration of the catalyst. Each one of
these steps has been observed in similar compounds and is
According to our profile, the oxidative addition and the
generally regarded as plausible in the organometallic com-
catalyst regeneration (olefin coordination) steps are either
munity. A modified version of the Chalk-Harrod mechanism
completely or practically barrierless. Possible rate-determining
has been proposed to account for the formation of vinyl-silane,
steps are the olefin insertion and reductive elimination. On the
which may be found in the products depending on catalyst and
basis of this model, we thus expect most of the catalyst in
reactive conditions.7 The modified version is similar to the
solution to be found in one of the intermediates preceding these
original, except that the olefin inserts into the Pt-Si bond and
steps, i.e., with oxidation state II.
the C-H bond is formed during the reductive elimination step.
2.3. Lewis Mechanism.
The original Lewis mechanism is
To estimate the energy profile associated with the catalytic
reported in Figure 3A. Our implementation is reported in Figure
cycle, it is necessary to add explicit ligands. We thus performed
3B. It is apparent that in our implementation the Lewis
computations on cycle 1B. The corresponding energy profile is
mechanism differs from the Chalk-Harrod mechanism only in
reported in Figure 2. It must be noted that the pentacoordinated
the order of addition of the olefin and the hydride. Since these
Pt moiety is unstable and the olefin in the axial position
two steps are not rate-determining, the two mechanisms are
effectively comes off the complex. This is due to the fact that
essentially the same. The energy profile, reported in Figure 4,
we are performing computations for gas-phase molecules
differs from the one for Chalk-Harrod (Figure 2) only for the
whereas the real system is solvated by olefins or siloxanes. To
circumvent this problem, one should modify the catalytic cycle
2.4. “Practical” Mechanism.
To avoid pentacoordinated
to maintain fewer ligands around the central platinum atom.
species, we modified the Chalk-Harrod and Lewis mechanisms
We take this approach later in this document. The energy barrier
by performing the insertion step before adding the fifth (olefin)
reported in Figure 2 refers to the insertion of the equatorial
ligand. This yields the catalytic cycle reported in Figure 5. We
olefin, the insertion of the loosely bound axial olefin being
emphasize the fact that, although it is more practical to perform
gas-phase computations on this cycle, once the catalyst is
Heterogeneous Inhibition of Homegeneous Reactions
J. Phys. Chem. B, Vol. 106, No. 7, 2002 1717
Lewis mechanism as originally p
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