Electrocatalytic Oxidation of Alcohol with Cobalt Triphosphine Complexes

Coordination
of the tridentate ligand bis­(2-diphenylphosphinoethyl)­phenylphosphine
(P3) to cobalt forms [(CH3CN)2CoIIP3]­(BF4)2 (CoIIP3). In the presence of the Brönsted base iPr2EtN, CoIIP3 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 iPr2EtN, CoIIP3 is reduced
by one electron to [(CH3CN)2CoIP3]­BF4 (CoIP3) with concomitant formation of
half an equivalent of benzaldehyde. This stoichiometric oxidation
of BnOH suggests electron transfer occurs between intermediate cobalt
species and starting CoIIP3. Kinetics and computational studies
support an unfavorable alcohol binding preequilibrium step followed
by favorable deprotonation of the bound alcohol.