Integrating Homogeneous and Heterogeneous Catalytic Systems for Synergistic Water Oxidation: Role of Geometrically Distinct Bispidine 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, Ni L1 Flex, and a rigid backbone, Ni L2 Rigid, act as homogeneous catalysts, whereas β-Ni(OH) 2 @NF nanoflowers serve as a heterogeneous catalyst counterpart. Ni L2 Rigid, featuring an octahedral geometry, exhibits superior electrocatalytic performance with an overpotential of 330 mV at 10 mA/cm 2 and a Tafel slope of 97 mV/dec in 0.1 M phosphate buffer solution (pH 7), outperforming Ni L1 Flex with a distorted tetrahedral geometry. Single-crystal XRD reveals that the octahedral geometry with cis-positioned labile water sites of Ni L2 Rigid promotes O–O coupling for O 2 generation, explaining its lower overpotential and enhanced charge transfer kinetics compared to the distorted tetrahedral Ni L1 Flex complex, which is devoid of such attached cis-labile water molecules. The enhanced activity of Ni L2 Rigid is attributed to its minimized potential difference across the redox couple (Ni 2+ /Ni 3+ ), aiding favorable synchronization among homogeneous and heterogeneous catalytic systems. Mullikan spin density calculations reveal that the rigid complex Ni L2 Rigid 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 Ni L1 Flex 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.