Oxygen Reduction Catalysis by a Binuclear Cu(II) Complex: Impact of Carboxamido Coordination on Mechanism and Rate

The oxygen reduction efficacy of a binuclear Cu(II) complex [Cu₂(PaPy₃H)₂](ClO₄)₄ (1) (H-PaPy₃ = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide) is reported. In this complex, the ligand featuring a carboxamido moiety coordinates two Cu(II) centers in a [3 + 2] ligand-sharing motif. Notably, the carboxamido moiety binds to Cu(II) via the oxygen donor atom, in contrast to the more typical nitrogen coordination observed with a structurally related N5-carboxamido ligand, H-dpaq (H-dpaq = 2-[bis(pyridine-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate). Chronoamperometry (CA) and computational analyses clearly indicated two consecutive one-electron-reduction events, Cu(II)/Cu(II) to Cu(II)/Cu(I) or Cu(I)/Cu(II), followed by the appearance of the Cu(I)/Cu(I) state. Despite a large Cu(II)···Cu(II) separation, a degenerate set of d-orbitals enables efficient electron sharing between the two Cu centers, facilitating reduction while preserving structural integrity. Complex 1 efficiently catalyzes 4e^-/4H⁺ reduction of O₂ to H₂O using decamethylferrocene (Fc*) as the electron donor and perchloric acid (HClO₄) as the proton source in dimethylformamide (DMF) at 298 K. Compared to the mononuclear Cu(II) complex, [Cu^II(dpaq)](ClO₄) (2), complex 1 exhibits significantly faster ORR kinetics. Mechanistic studies, supported by spectrokinetic analysis, reveal a key difference in the rate-determining step (RDS) of the dioxygen reduction by these complexes. For complex 2, the RDS involves a proton-coupled electron transfer (PCET) to the Cu(II)-superoxo intermediate, while for complex 1, the formation of the putative Cu(II)-superoxo species itself constitutes the RDS. This study highlights the crucial role of binuclearity, wherein one Cu binds O₂ while the second Cu supplies the electron required for Cu(II)-hydroperoxo formation, unveiling how the ligand design and nuclearity govern the ORR mechanism and efficiency.