Electrochemical carboxylation of arylmethyl chlorides catalysed by [Co(salen)] [H2salen = N,N'-bis(salicylidene)ethane-1,2-diamine]

The electrochemical carboxylation of some arylmethyl chlorides RCl [R = benzyl, 4-methoxybenzyl, 4-(trifluoromethyl)benzyl or diphenylmethyl] catalysed by [Co(salen)] [H(2)salen = N,N'-bis(salicylidene)-ethane-1,2-diamine] was studied in acetonitrile. Comparable amounts of carboxylic acids and saturated hydrocarbons were obtained when R = PhCH2 or 4-MeOC6H4CH2 whereas carboxylic acids were the main products with R = 4-F3CC6H4CH2 or Ph2CH. Electrogenerated [Co(I)(salen)]- reacts with RCl to give an organometallic complex, [Co(III)(salen)R], with a rate constant which, for all chlorides investigated, is of the order of 10(5) dm(3) mol(-1) s(-1). The one-electron-reduced complex [Co(II)(salen)R]- is unstable and its decomposition in the presence of CO2 is the key step of the electrocatalytic process. Different decomposition pathways are considered and their mechanistic implications discussed. In the presence of proton donors [Co(II)(salen)R]- undergoes rapid hydrolysis to RH and [Co(salen)].