Ligand-Field Directed Electronic Effects in Heterogenized Bifunctional Co(II) Molecular Clusters Accomplish Efficient Overall Water Splitting
Aditya Borah, Jayeeta Saha, Sunita Sharma, Savi Chaudhary, Sandeep K. Gupta, Gopalan Rajaraman, Chandramouli Subramaniam, Ramaswamy Murugavel
Abstract
We demonstrate a hitherto unknown approach of employing cobalt organophosphate D4R cage-clusters, [(RO) PO 3 Co(L)] 4 ( 1 – 4 ) and [nBu 4 N][F@{(RO)PO 3 Co(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.