A mixed metallic and mixed valence MoV4VIV2VIII2 complex as an electron transfer photo-catalyst for the hydrogen evolution reaction

Ligand-directed structure modulation is observed during the synthesis of mixed-metallic vanado-molybdate complexes functionalized with pyridinyl diphosphonate ligand(s). The structural assembly is determined by the location of the nitrogen-atom at the aromatic ring of the pyridinyl-ligands, forming either a tetra-nuclear mixed-valence V(III/IV)-containing complex (1) having a butterfly-like structure, or a monocentric S-shaped complex (2) containing an octahedral V(III). Photocatalytic hydrogen evolution from water (HER) studies show higher catalytic activity by complex 1, as compared with complex 2, attributed to the delocalization of spin densities between V(III) and V(IV) in 1. The transfer of electrons from vanadium(III and IV) mixed-valence complex 1 to H⁺ is more facile than from the vanadium(III) homo-valent complex 2. A dominant reductive quenching mechanism has been further established by detailed photophysical studies of 1.