Metal-Hydride Cation Radical Acidities

Scheme 4. Thermochemical cycle for the determination of metal-hydride cation radical acidities.

The thermochemical cycle in Scheme 4 can be used to estimate the effect of one-electron oxidation on metal-hydride acidities. The method is analogous to one that has been extensively used to investigate organic cation radicals [10c]. Eq. 29 shows that measurements of °ox(MH) and °ox(M ) provide relative p a data for metal hydrides and their cation radicals. Absolute values for p a(MH +) are obtained if the acidities of the neutral hydrides are known. The oxidation potentials of 18-electron hydrides can be readily obtained by cyclic voltammetry. In our experience, the waves that are obtained are frequently chemically irreversible, even at rather high scan rates. Consequently, the oxidation peak potentials will be kinetically shifted and represent minimum values for the true °ox(MH) data, the estimates for p a(MH +) represent maximum values, and calculated Ap a are minimum values. 

Another route to productive CT photochemistry can involve proton transfer reactions between A and D +, which are favored in the redox pair arising from the enhanced acidity of D + (relative to D) and basicity of A (relative to A). Numerous examples of these reactions exist in the organic literature [240-241], and such a pathway should be particularly important for transition-metal hydrides with significantly enhanced acidities of their (metastable) cation radicals [242]. Thus irradiation of the EDA complex of fumaronitrile (as acceptor) with the hydridic donor (CP2M0H2) [35] leads to CT hydro-metallation. 

It is as difficult to distinguish between direct H transfer and elctron transfer followed by H atom transfer (H = e -i- H ) as it is to distinguish between direct H atom transfer and electron transfer followed by proton transfer (H = e" -t H+). It is possible to favor the electron-transfer reaction by using an anode or a powerful monoelectronic oxidant that is not an H atom acceptor. We have noted above that the acidity of a metal hydride is enormously increased by monoelectronic oxidation. Indeed, the monoelectronic oxidation of a neutral 18-electron metal-hydride complex to a 17-electron radical cation is systematically followed by the... 

The same mechanistic dichotomy for HAT reactions, one-step (concerted) HAT versus sequential (stepwise) electron and proton transfer (Scheme 2.1), is applied to hydride transfer reactions, one-step (concerted) hydride transfer versus sequential (stepwise) ET followed by proton-electron (or hydrogen) transfer.13,40 64 68 Such one-step versus multistep pathways have been discussed extensively in hydride transfer reactions of dihydronicotinamide coenzyme (NADH) and analogues, particularly including the effect of metal cations and acids, 69-79 because of the essential role of acid catalysis in the enzymatic reduction of carbonyl compounds by NADH.80 In contrast to the one-step hydride transfer pathway that proceeds without an intermediate, the ET pathway would produce radical cation hydride donors as the reaction intermediates, which have rarely been observed. The ET pathway may become possible if the ET process is thermodynamically feasible. 

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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