Activation Si-H bonds

The active catalytic species appears to involve a metal hydride complex, as added H2 enhances the reaction rate. Additionally, H4Ru4(CO)12 is observed during the course of the reaction and may also be used as an effective entry into the catalytic system. The oligomerization mechanism may proceed via a nucleophilic attack of the amine on a metal-activated Si-H bond, resulting in Si-N bond formation. 

Calculations on an unchelated model without the phenylene bridge give shorter dSiH of about 1.75 A hence the chelation gives more activated Si - H bonds in accordance with the decrease in JSiH as the Si-Ru-Si bite angle shrinks smaller X in RuH2[(SiHMe2)2X](P)2. ... 

However, recently, a theoretical paper has been published, which provides interesting arguments for a conventional silylmetal hydride rather than for a 3c2e M(H)Si bond [132,133]. For the great implications of these compounds for Si —H bond activation reactions consult, e.g., on the work of Crabtree [129]. 

This synthetic approach is known from the synthesis of L M(alkene)H compounds from LnM(CO)alkane precursors and can easily be applied to the analogous silyl complexes. The Si—H bond even shows an increased activity for oxidative addition reactions [42, 43]. 

Many catalytic reactions involve the activation of Si—H bonds and consequently the reaction of silanes with platinum(0) has been studied extensively. The reactions of hydrosilanes with transition-metal complexes, including platinum, has been extensively reviewed by Corey and Braddock-Wilking.58... 

An Arrhenius plot of the hydrogen penetration depth is shown in Fig. 7. The activation energy derived from the Arrhenius plot is 0.39 eV. This activation energy is believed to be the energy for breaking an Si—H bond near a B atom. Heating above 40°C appears to be necessary to cause the... 

It is well known that the comparatively inert Si-H bonds of triorganosilane R3SiH (R = alkyl, aryl) can be activated by transition metal complexes. Chatgililaloglu et al. have used Et3SiH in palladium-catalyzed dehalogenation reactions, which occur with the involvement of free radicals.245... 

For this reaction, the reaction coordinate is simply the Si-H bond distance. Since the reverse of this reaction is radical-radical recombination and should have no activation energy barrier, the E for forward reaction would be expected to be AH,. 

The reaction coordinate again is simple it is the H-Si-H bond angle. In this case, however, the reverse reaction is a radical-molecule reaction, and we cannot make the a priori assumption that its activation energy would be zero. In fact, the literature is full of examples of radical-molecule reactions with large activation energies (Benson, 1976). As a result, we cannot also make the assumption that for the forward reaction E = AH as we did in the case of the Si-H bond fission reaction. At this point, we must resort to either quantum chemical calculations or experiments to resolve this issue. 

As seen from Fig. 12, quantum chemical calculations indeed support the fact that the activation energy for the SiHj-H bond scission reaction corresponds to the bond dissociation energy or the heat of reaction, i.e., AH, % s 90 kcal/mol. In addition, at the transition state, the Si-H bond distance is calculated to be about 2.5 A, a clear indication of the loosness and the increase in entropy associated with the formation of the transition state. 

The formation of a silane a-complex 7 can be viewed as the result of incomplete Si-H bond activation ( arrested oxidative addition ) by an electrophilic metal... 

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