M-H bond activation

Of special interest is the behavior of 1-aza-zirconacyclopentene complexes which allows electrophilic sp, sp2, and sp3 C-H bond activations as well as M-H bond activations (M=N, O, P, S) and the preparation of the first C-metallated diazoalkanes LnMC(N2)R with an early transition metal. 

Scheme 9. Examples for Si chemical shifts (c5 Si) of silane metal complexes, in which metal-H and Si-H bonding is present (agostic Si-M-H bonding activation of Si-H bonds).

Shubina, E. Belkova, N. Filippov, O. Epstein, L. Weak interactions and M-H bond activation. In Advances in Organometallic Chemistry and Catalysis, John Wiley Sons, Hoboken, NJ, 2013, pp. 97-109. 

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]. 

Fuchita, Y., Utsunomiya, Y. and Yasutake, M. (2001) Synthesis and reactivity of arylgold(III) complexes from aromatic hydrocarbons via C—H bond activation. Journal of the Chemical Society, Dalton Transactions, (16), 2330. 

Holthausen, M. C., Koch, W., 1996b, Mechanistic Details of the Fe+-Mediated C-C and C-H Bond Activations in Propane A Theoretical Investigation , Helv. Chim. Act., 19, 1939. 

In Scheme 1 is represented an idealized picture of the two possibilities for the hydrogenation of alkenes by metal complexes not containing an M—11 bond. One possibility involves initial coordination of the alkene followed by activation of H2 (alkene route). The other (more general) possibility is the hydride route, which involves initial reaction with H2 followed by coordination of the alkene. The second general mechanism, usually adopted by catalysts containing an M—H bond, is shown in Scheme 2. 

The bonding of H2 in metal complexes was described in Chapter 16. In connection with the oxad reaction in which the bonding is not static, it can be presumed that the o orbital on the hydrogen molecule functions as an electron pair donor to an orbital on the metal atom. Simultaneously, the o orbital on the H2 molecule receives electron density from the populated d orbitals on the metal atom as a result of back donation. The result is that two M-H bonds form as the H-H bond is broken in a process that is accompanied by a very low activation energy. 

Synthetic Reactions via C-H Bond Activation C-C and C-E Bond Formation, Pages 101-166, M. Pfefferand J. Spencer... 

Synthetic Reactions via C-H Bond Activation Carbene and Nitrene C-H Insertion, Pages 167-212, Huw M L Davies and Xing Dai... 

A large number of catalysts or catalyst precursors for the reaction in Eq. (1) have been identified (Table 17.1). Almost all of these are complexes of Rh and Ru, although there are a few, less active, examples of fr, Pd, Ni, Fe, Ti, and Mo. The most active catalysts are RhCl(TPPTS)3, Rh(hfacac)(dcpb) and RuCl(OAc) (PMe3)4, with little difference between them after rough correction of the turnover frequencies (TOF) for pressure and temperature differences [38]. Musashi and Sakaki [39] argue that the barrier to C02 insertion into M-H bonds increases in the order Rh(I) < Ru(II) < Rh(I II) because Rh(lll)-formate bonds are too weak and Ru(II)-H bonds are too strong compared to the case of Rh(I). 

Insertion of the C=C bond into an M-H bond precedes frequently the C-X bond activation in halogenated alkenes. Such a pathway has been suggested for the cobalamin-mediated dechlorinations of cis- and trans-DCE, as well as VC with Ti(III) citrate as a reducing agent [141]. 

The silanol complex 57 exhibits a Si H M agostic interaction characterized by a /(Si-H) of 41 Hz and a Si-H distance of 1.70(7) It would be incautious to interpret such a low value of the Si-H coupling in terms of a significant Si-H bond activation, because the Si-H bond forms rather acute angles with the Si-C and Si-Si bonds (about 82 and 101°, respectively) and thus must have a considerable p character on silicon, which should contribute to the decrease of /(Si-H). The silanol ligand is -coordinate to ruthenium and the Ru-Si bond of 2.441(3) A is not exceptional, but the Si(SiMe3)3 deviates from the silanol plane by 19.0°, probably as a result of the Si-H interaction. Deprotonation of 57 by strong bases affords a neutral ruthenocene-like product. 

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