Bond metal hydride cation radical

Scheme 5. Thermochemical cycle for the determination of metal-hydride cation radical bond-dissociation energies.

Metal-Hydride Bond-dissociation Energies in Cation Radicals... 

Subsequent oxidation and C-H activation steps, however, tend to control the selectivity and the slate of final products that form. C-H activation can proceed through either ho-molytic or heterolytic processes depending upon the catalyst that is used and the nature of the active site. Homolytic C-H activation processes usually lead to the production of free radical intermediates whereas heterolytic activation leads to the formation of charged complexes. The heterolytic activation of a C-H bond can occur through the formation of a carbanion intermediate which is bound to a surface metal cation and a proton which binds to a nearby surface oxygen atom, thus forming a surface hydroxyl intermediate. Alternatively, the heterolytic activation of the C-H bond can lead to the formation of a metal hydride (M-H) and a surface alkoxide intermediate. 

As demonstrated in this chapter, there have always been the fundamental mechanistic questions in oxidation of C-H bonds whether the rate-determining step is ET, PCET, one-step HAT, or one-step hydride transfer. When the ET step is thermodynamically feasible, ET occurs first, followed by proton transfer for the overall HAT reactions, and the HAT step is followed by subsequent rapid ET for the overall hydride transfer reactions. In such a case, ET products, that is, radical cations of electron donors and radical anions of electron acceptors, can be detected as the intermediates in the overall HAT and hydride transfer reactions. The ET process can be coupled by proton transfer and also by hydrogen bonding or by binding of metal ions to the radical anions produced by ET to control the ET process. The borderline between a sequential PCET pathway and a one-step HAT pathway has been related to the borderline between the outer-sphere and inner-sphere ET pathways. In HAT reactions, the proton is provided by radical cations of electron donors because the acidity is significantly enhanced by the one-electron oxidation of electron donors. An electron and a proton are transferred by a one-step pathway or a sequential pathway depending on the types of electron donors and acceptors. When proton is provided externally, ET from an electron donor that has no proton to be transferred to an electron acceptor (A) is coupled with protonation of A -, when the one-electron reduction and protonation of A occur simultaneously. The mechanistic discussion described in this chapter will provide useful guide to control oxidation of C-H bonds. 

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