Hydride complexes cobalt porphyrins

In contrast to the rhodium porphyrin hydride complexes, Rh(Por)H, which play a central role in many of the important developments in rhodium porphyrin chemistry, the corresponding cobalt porphyrin hydride complexes have been implicated as reaction intermediates in a variety of processes, but a stable, i.solable example has yet to be achieved. 

The corresponding reactions of transient Co(OEP)H with alkyl halides and epoxides in DMF has been proposed to proceed by an ionic rather than a radical mechanism, with loss of from Co(OEP)H to give [Co(TAP), and products arising from nucleophilic attack on the substrates. " " Overall, a general kinetic model for the reaction of cobalt porphyrins with alkenes under free radical conditions has been developed." Cobalt porphyrin hydride complexes are also important as intermediates in the cobalt porphyrin-catalyzed chain transfer polymerization of alkenes (see below). 

Gridnev, A.A. Ittel, S.D. Fryd, M. Wayland. B. B. Formation of Organocobalt Porphyrin Complexes from Reactions of Cobalt(II) Porphyrins and Dialkylcyanomethyl Radicals with Organic Substrates Chemical Trapping of a Transient Cobalt Porphyrin Hydride, Organometallics 1993,12 (12), 4871-80. 

In some cases where a reaction involving a radical species occurred within cobalt porphyrin complexes, it has been possible to trap transient cobalt porphyrin hydride species. This was indeed observed during the synthesis of organocobalt porphyrin that resulted from the reaction of cobalt(n) porphyrin and dialkylcyanomethylradicals with alkenes, alkynes, alkyl halides, and epoxide. A transient hydride porphyrin complex was also involved in the cobalt porphyrin-catalyzed chain transfer in the free-radical polymerization of methacrylate. The catalytic chain transfer in free-radical polymerizations using cobalt porphyrin systems has been extensively investigated and will not be treated in this section. Gridnev and Ittel have published a comprehensive overview of the catalytic chain transfer in free-radical polymerizations. ... 

The chemistry of organorhodium and -iridium porphyrin derivatives will be addressed in a separate section. Much of the exciting chemistry of rhodium (and iridium) porphyrins centers around the reactivity of the M(ll) dimers. M(Por) 2-and the M(III) hydrides, M(Por)H. Neither of these species has a counterpart in cobalt porphyrin chemistry, where the Co(ll) porphyrin complex Co(Por) exists as a monomer, and the hydride Co(Por)H has been implicated but never directly observed. This is still the case, although recent developments are providing firmer evidence for the existence of Co(Por)H as a likely intermediate in a variety of reactions. 

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. " " ... 

Photocatalytic reductions may concern organic unsaturations (C=0, C=C, etc.), or inorganic CO2, or bicarbonate. Even carbon tetrachloride is very efficiently photo-reduced to chloroform by alcohols with mc.9o-tetra(2,6-dichlorophenyl)porphyrin [108]. Intermolecular hydrogen transfer is catalyzed by cobalt-phosphine complexes [109]. In this reaction photoirradiation generates the active hydride species CoH[PPh(OEt)2]3 for the reduction of ketones with secondary alcohols. The... 

Square planar cobalt complexes including porphyrin and cobaloxime derivatives rapidly and reversibly add carbon-centred radicals. An alternative pathway to the reverse reaction is an elimination to form the olefin and a cobalt hydride species, which can initiate polymerization by donating hydrogen to monomer.24... 

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