Electrocatalytic Water Oxidation by a Water-Soluble Copper(II) Complex with a Copper-Bound Carbonate Group Acting as a Potential Proton Shuttle

Abstract

Water-soluble copper(II) complexes of the dianionic tridentate pincer ligand N,N'-2,6-dimethylphenyl-2,6-pyridinedicarboxamidate (L) are catalysts for water oxidation. In [L-Cu^II-DMF] (1, DMF = dimethylformamide) and [L-Cu^II-OAc]^- (2, OAc = acetate), ligand L binds Cu^II through three N atoms, which define an equatorial plane. The fourth coordination site of the equatorial plane is occupied by DMF in 1 and by OAc^- in 2. These two complexes can electrocatalyze water oxidation to evolve O₂ in 0.1 M pH 10 carbonate buffer. Spectroscopic, titration, and crystallographic studies show that both 1 and 2 undergo ligand exchange when they are dissolved in carbonate buffer to give [L-Cu^II-CO₃H]^- (3). Complex 3 has a similar structure as those of 1 and 2 except for having a carbonate group at the fourth equatorial position. A catalytic cycle for water oxidation by 3 is proposed based on experimental and theoretical results. The two-electron oxidized form of 3 is the catalytically active species for water oxidation. Importantly, for these two oxidation events, the calculated potential values of E_p,a = 1.01 and 1.59 V vs normal hydrogen electrode (NHE) agree well with the experimental values of E_p,a = 0.93 and 1.51 V vs NHE in pH 10 carbonate buffer. The potential difference between the two oxidation events is 0.58 V for both experimental and calculated results. With computational evidence, this Cu-bound carbonate group may act as a proton shuttle to remove protons for water activation, a key role resembling intramolecular bases as reported previously.