IntegratingHomogeneous and Heterogeneous CatalyticSystems for Synergistic Water Oxidation: Role of Geometrically DistinctBispidine Metal Complexes

This study investigates the integration of two geometrically distinct nickel complexes with a heterogeneous catalytic counterpart to enhance the water oxidation performance. Bispidine-based Ni(II) complexes with a flexible backbone, <b>Ni</b>_<b>L1</b>^<b>Flex</b>, and a rigid backbone, <b>Ni</b>_<b>L2</b>^<b>Rigid</b>, act as homogeneous catalysts, whereas β-Ni(OH)₂@NF nanoflowers serve as a heterogeneous catalyst counterpart. <b>Ni</b>_<b>L2</b>^<b>Rigid</b>, featuring an octahedral geometry, exhibits superior electrocatalytic performance with an overpotential of 330 mV at 10 mA/cm² and a Tafel slope of 97 mV/dec in 0.1 M phosphate buffer solution (pH 7), outperforming <b>Ni</b>_<b>L1</b>^<b>Flex</b> with a distorted tetrahedral geometry. Single-crystal XRD reveals that the octahedral geometry with cis-positioned labile water sites of <b>Ni</b>_<b>L2</b>^<b>Rigid</b> promotes O–O coupling for O₂ generation, explaining its lower overpotential and enhanced charge transfer kinetics compared to the distorted tetrahedral <b>Ni</b>_<b>L1</b>^<b>Flex</b> complex, which is devoid of such attached cis-labile water molecules. The enhanced activity of <b>Ni</b>_<b>L2</b>^<b>Rigid</b> is attributed to its minimized potential difference across the redox couple (Ni²⁺/Ni³⁺), aiding favorable synchronization among homogeneous and heterogeneous catalytic systems. Mullikan spin density calculations reveal that the rigid complex <b>Ni</b>_<b>L2</b>^<b>Rigid</b> has a highly localized spin density (∼80%) on the nickel center, whereas the spin density population mainly lies between the metal and oxygen bonds in <b>Ni</b>_<b>L1</b>^<b>Flex</b> reducing the redox activity of nickel in a flexible backbone. This research work establishes a model system that integrates homogeneous and heterogeneous catalysis for efficient water oxidation, leveraging the advantages of both systems while addressing their limitations through their integration.