Electrocatalytic Oxidation of Alcohol with Cobalt Triphosphine Complexes

Coordination of the tridentate ligand bis(2-diphenylphosphinoethyl)phenylphosphine (P-3) to cobalt forms [(CH3CN)(2)(CoP3)-P-II](BF4)(2) ((CoP3)-P-II). In the presence of the Bronsted base (Pr2EtN)-Pr-i, (CoP3)-P-II electrocatalytically oxidizes benzyl alcohol (BnOH) to benzaldehyde at an applied potential of -630 mV vs Fc(+/0) with a TON of 19.9. In a noncatalytic reaction with excess BnOH and (Pr2EtN)-Pr-i, (CoP3)-P-II is reduced by one electron to [(CH3CN)(2)(CoP3)-P-I]BF4 ((CoP3)-P-I) with concomitant formation of half an equivalent of benzaldehyde. This stoichiometric oxidation of BnOH suggests electron transfer occurs between intermediate cobalt species and starting (CoP3)-P-II. Kinetics and computational studies support an unfavorable alcohol binding preequilibrium step followed by favorable deprotonation of the bound alcohol.