Cobalt complexes, electron-transfer reactions hydrides

Cobalt(II) complexes of three water-soluble porphyrins are catalysts for the controlled potential electrolytic reduction of H O to Hi in aqueous acid solution. The porphyrin complexes were either directly adsorbed on glassy carbon, or were deposited as films using a variety of methods. Reduction to [Co(Por) was followed by a nucleophilic reaction with water to give the hydride intermediate. Hydrogen production then occurs either by attack of H on Co(Por)H, or by a disproportionation reaction requiring two Co(Por)H units. Although the overall I easibility of this process was demonstrated, practical problems including the rate of electron transfer still need to be overcome. " " ... 

On the basis of these observations the draft mechanism shown in Scheme 3.160 has been proposed for the catalytic reaction, by analogy with the previous reaction. The reaction of CoCl2(dpph) with M( jSiCH2MgOl gives complex 168, which is electron-rich, because of coordination of the Grignard reagent. Complex 168 effects single-electron transfer to an alkyl halide to yield an anion radical of the halide and cobalt complex 169. Immediate loss of halide from the anion radical affords an alkyl radical intermediate, which adds to styrene to yield a benzyl radical. Cobalt species 169 would then recombine with the carbon-centered radical to form cobalt species 170. Finally, /i-hydride elimination provides... 

The oxidation of coordinated formate in the complex cis-[Co(en)2(NH2R)(02CH)] (R = H, Me, Et) by peroxydisulfate is catalyzed by silver(I). The reaction is first order in the complex, Ag(I) and the oxidant. The major product of the reaction is cw-[Co(en)2(NH2R)OH2] cobalt(II) was produced in less than 5%. The oxidation mainly involves two-electron transfer from the formate ligand, which is consistent with a hydride transfer mechanism. 

A direct hydrogen atom transfer has been shown to occur, on the other hand, from neutral 19-electron transients obtained by one-electron reduction of saturated cationic hydrides. These derivatives apparently have a reduced acidity relative to cationic 17-electron complexes, while the presence of an unpaired electron in an orbital which may have an M-H a component may further contribute to weaken the M-H bond homolytically. An example is provided by complex CpCoH(dppe), which forms as a transient by one-electron reduction of [CpCoH(dppe)]". Cobaltocene is a strong radical scavenger, and has sufficient reducing power to carry our the reduction of the cationic cobalt hydride. Thus, reaction of the Co(III) hydride complex with two equivalents of cobaltocene affords CpCoCCjHg) in 80 % isolated yields, according to the proposed mechanism of Scheme 19 [25]. 

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