Copper(II) bis-oxamidate(L1) complex undergoes selective water oxidation while the analogous nickel(II) complex demonstrate both homogeneous water oxidation and catalyst deactivation due to the 2e⁻ oxidized anionic intermediate [(L1⋅)Ni^III(OH)]¹⁻ which promote both nucleophilic (OH⁻) and electrophilic (H⁺) attack while the corresponding copper intermediate, [(L1⋅)Cu^II(OH⋅)]¹⁻, display radical character on the hydroxyl ligand with greater electrophilicity and avoids catalyst deactivation.

Abstract

Water splitting is a potential pathway for hydrogen gas evolution and thereby realization of a carbon-neutral sustainable energy scheme. However, oxidation of water to dioxygen is the major impediment in conversion of solar energy to fuel. Herein, density functional studies are conducted to explore the reactivity conduits of two molecular electro-catalysts consisting of nickel and copper tetra-anionic tetradentate amide ligand complexes of the type [(L1)M^II]²⁻, where L1=o-phenylenebis(oxamidate), and their substitutionally modified analogues. While nickel complexes demonstrate complex borderline chemistry between homogeneous and heterogeneous pathways, showing competition between water oxidation and molecular species degradation, copper complexes display robust and efficient molecular water oxidation behavior. Our analysis predict that this disparity is primarily due to the reversible O−O bond formation in nickel complexes, which provide the platform necessary for a direct attack of OH⁻/H⁺ on the metal and terminally accessible amidate groups of the 2e⁻ oxidized anionic intermediate, [(L1⋅)Ni^III(OH)]¹⁻, respectively. This intermediate streamline ligand deactivation with a comparatively higher driving force for nickel complexes in acidic medium. Contrarily, the copper complexes display radical character on the hydroxyl ligand in the corresponding intermediate, [(L1⋅)Cu^II(OH⋅)]¹⁻, that expedite O−O interaction, leading to predominant homogeneous water oxidation under all conditions.