Electrocatalytic water oxidation by nickel complexes: the effect of coordination number and ligand backbone structure on their catalytic properties

Abstract

Two novel Ni complexes, [Ni(MePy₂tacn)(H₂O)](ClO₄)₂ (1, MePy₂tacn = 1-methyl-4,7-bis(2-picolyl)-1,4,7-triazacyclononane) and [Ni(Py₃tacn)](ClO₄)₂ (2, Py₃tacn = 1,4,7-tris(2-picolyl)-1,4,7-triazacyclononane), are developed as electrocatalysts for water oxidation under near neutral conditions. It is found that the number of picolyl functional groups on the ligand backbone significantly influences their redox behavior and catalytic performance. The pentadentate MePy₂tacn ligand in 1 provides an exchangeable coordination site occupied by a water molecule, which is crucial for facilitating proton-coupled electron transfer (PCET) processes during water oxidation, resulting in a low catalytic onset overpotential of 510 mV and considerable catalytic current, while the hexadentate Py₃tacn ligand in 2 results in a saturated coordination sphere, necessitating structural reorganization that leads to a higher overpotential of 610 mV and diminished catalytic activity. Furthermore, the catalytic performance of 1 was compared with that of the analogous complex [Ni(tmbptn)(H₂O)](ClO₄)₂ (3, tmbptn = 1,9-bis(2-pyridyl)-2,5,8-triazanonane), which contains an acyclic triazanonane backbone with two picolyl groups. Electrochemical studies reveal that the macrocyclic structure of MePy₂tacn in 1 confers superior catalytic activity compared to the acyclic analogue 3, which exhibits a high onset overpotential of 710 mV and lower catalytic current. This work highlights the importance of the backbone structure of the ligand, demonstrating that the labile coordination site and the suitable spatial orientation of alkyl on the ligand backbone are critical factors governing the water oxidation activity of Ni complexes. These findings provide important insights for the rational design of efficient molecular catalysts for artificial photosynthesis.