Catalytic Four-Electron Reduction of Oxygen to Water
by a Molecular Cobalt Complex Consisting of a Proton Exchanging Site
at the Secondary Coordination Sphere
Controlling
the selectivity of 4e–/4H+ reduction
of oxygen over 2e–/2H+ reduction
is a key challenge in making efficient catalysts for fuel cell cathodes.
A tyrosine residue poised over the active site of cytochrome c oxidase
(CcO) has been demonstrated to control the hydrogen
atom transfer reactions and cleavage of the O–O bond of a Fe–O–O–Cu
moiety to yield water. In a couple of small-molecule iron complexes
supported by porphyrin derivatives, it was shown that the presence
of protonation sites at the secondary coordination sphere plays an
important role in directing the selectivity and rate of the oxygen
reduction reaction (ORR). In this study, we designed and synthesized
a mononuclear CoIII complex (1) of a bis-pyridine-bis-oxime
ligand where the oxime site can participate in reversible proton exchange
reactions. Electrocatalytic ORR of 1 was investigated
in aqueous buffer solutions and acetonitrile containing trifluoroacetic
acid as the proton source. We observed that in a 0.1 M phosphate buffer
solution (PBS), 1 is selective for 4e–/4H+ reduction of O2 at pH 4, and the selectivity
decreases with increasing the buffer medium’s pH, producing
ca. 75% H2O at pH 7. However, in a 0.1 M acetate buffer
solution (ABS), 1 remained highly selective for the cleavage
of the O–O bond to produce H2O at pH 4 and pH 7.
The overpotential (η) of H2O formation (ca. 0.8–0.65
V) decreased proportionally with increasing pH in PBS and ABS. In
acetonitrile, 1 remained highly selective for 4e–/4H+ reduction for electrocatalytic and
chemical ORR. An overpotential of 760 mV was estimated for H2O production in acetonitrile. Kinetic analysis suggests the first-order
dependence of catalyst concentration on the reaction rate at 25 °C.
However, the formation of a peroxo-bridged dinuclear cobalt(III) complex
was noted as a reaction intermediate in the ORR pathway in acetonitrile
at −40 °C. We conjecture that the oxime scaffold of the
ligand works as a proton exchanging site and assists in the proton-coupled
electron transfer (PCET) reactivity to cleave the O–O bond
in the acidic buffer solutions and acetonitrile, further corroborated
by theoretical studies. Density functional theory (DFT) calculation
suggests that the acetate ion works as a mediator at pH 7.0 for transferring
a proton from the oxime scaffold to the distal oxygen of the CoIII(OOH) intermediate, responsible for high selectivity toward
4e–/4H+ reduction of O2.