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 κ³-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 Cu^II/Cu^I reduction at -0.930 V (vs Fc⁺/Fc⁰, MeCN), and an irreversible Cu^II/Cu^I 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 Cu^I 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 Cu^I complex is feasible with either release of H₂O₂ or O-O cleavage resulting in the liberation of H₂O. The Cu^I complex is regenerated by subsequent reduction and protonation to close the cycle.