Sulfonamido-Pincer Complexes of Cu(II) and the Electrocatalysis of O2 Reduction

Heteroleptic copper complexes of an asymmetrical pincer ligand containing a central anionic sulfonamide donor (pyridine-2-yl-sulfonyl)­(quinolin-8-yl)-amide (psq), which contains a central anionic sulfonamido donor have been prepared. Meridional κ 3-N,N″,N‴ binding with the co-ligands acetate, chloride, or acetonitrile (MeCN), trans to the central sulfonamido N-donor, is revealed by the X-ray crystal structures of [Cu­(OAc)­(psq)­(H2O)], [CuCl­(psq)]2, and [Cu­(psq)­(MeCN)]­(PF6). Either overall distorted square pyramidal or octahedral geometries of the copper atom are satisfied by coordinated water in the case of the acetate complex or interactions with periphery sulfonamido oxygen atoms on adjacent molecules in the dimeric chloride and 1D polymeric acetonitrile complexes. The cyclic voltammogram (CV) of [Cu­(OAc)­(psq)­(H2O)] shows a quasi-reversible CuII/CuI reduction at −0.930 V (vs Fc+/Fc0, MeCN), and an irreversible CuII/CuI reduction for [Cu­(psq)­(MeCN)]­(PF6) is seen at −0.838 V. This signal is split into two quasi-reversible redox processes on the addition of 2,2,2-trifluoroethanol (TFE). This suggests that TFE pushes a solution equilibrium toward a dimeric acetate complex analogous to [CuCl­(psq)]2, which shows two quasi-reversible waves at −0.666 V and −0.904 V vs Fc+/Fc0 consistent with its dimeric solid-state structure. A comparison of the CVs of [Cu­(OAc)­(psq)­(H2O)] under either a N2 or an O2 atmosphere revealed that this complex catalyzes turnover electro-reduction of O2 to H2O2 and H2O. The rate of reaction increases on addition of a weak organic acid, and a coulombic efficiency of 48% for H2O2 was determined by iodometric titration. We propose that a CuI complex formed on electroreduction binds O2 to yield an intermediate superoxide complex. On electron and proton transfer to this species, a bifurcated route back to the O2-activating CuI complex is feasible with either release of H2O2 or O–O cleavage resulting in the liberation of H2O. The CuI complex is regenerated by subsequent reduction and protonation to close the cycle.