Ligand-Field Directed Electronic Effects in Heterogenized Bifunctional Co(II) Molecular Clusters Accomplish Efficient Overall Water Splitting

Abstract

We demonstrate a hitherto unknown approach of employing cobalt organophosphate D4R cage-clusters, [(RO) PO3Co(L)]4 (1–4) and [nBu4N][F@{(RO)PO3Co(L)}4], (1F–4F), where L = DMSO (1 or 1F), pyridine (2 or 2F), 4-formylpyridine (3 or 3F), and 4-cyanopyridine (4 or 4F), as bifunctional heterogeneous catalysts for electrochemical water splitting in near-neutral conditions. Electronic and steric modifications on the clusters are achieved through a synergistic combination of endohedral trapping of fluoride ion and exohedral functionalization of pyridines. This two-pronged approach significantly reduces the charge density at the metal leading to energetically favorable water coordination with a perceptible enhancement in the kinetics of overall water splitting, with 4F exhibiting the lowest cell overpotential of 0.949 V (overall cell potential 2.179 V), observed among molecular catalysts. Density functional calculations highlight the importance of cooperativity between the cobalt centers. The unique D4R geometry stabilizes the high-valent metal-oxo/hydroperoxo species, emphasizing the need for polynuclear clusters to catalyze such challenging reactions, as has been well-established in the evolutionary PS-II.