Electrochemical Water Oxidation and CO₂ Reduction with a Nickel Molecular Catalyst

Mimicking the photosynthesis of green plants to combine water oxidation with CO₂ reduction is of great significance for solving energy and environmental crises. In this context, a trinuclear nickel complex, [Ni^II₃(paoH)₆(PhPO₃)₂]·2ClO₄ (1), with a novel structure has been constructed with PhPO₃^2- (phenylphosphonate) and paoH (2-pyridine formaldehyde oxime) ligands and possesses a reflection symmetry with a mirror plane revealed by single-crystal X-ray diffraction. Bulk electrocatalysis demonstrates that complex 1 can homogeneously catalyze water oxidation and CO₂ reduction simultaneously. It can catalyze water oxidation at a near-neutral condition of pH = 7.45 with a high TOF of 12.2 s^-1, and the Faraday efficiency is as high as 95%. Meanwhile, it also exhibits high electrocatalytic activity for CO₂ reduction towards CO with a TOF of 7.84 s^-1 in DMF solution. The excellent electrocatalytic performance of the water oxidation and CO₂ reduction of complex 1 could be attributed to the two unique µ₃-PhPO₃^2- bridges as the crucial factor for stabilizing the trinuclear molecule as well as the proton transformation during the catalytic process, while the oxime groups modulate the electronic structure of the metal centers via π back-bonding. Therefore, apart from the cooperation effect of the three Ni centers for catalysis, simultaneously, the two kinds of ligands in complex 1 can also synergistically coordinate the central metal, thereby significantly promoting its catalytic performance. Complex 1 represents the first nickel molecular electrocatalyst for both water oxidation and CO₂ reduction. The findings in this work open an avenue for designing efficient molecular electrocatalysts with peculiar ligands.