Chemical Water Oxidation by Nonheme Iron Catalysts Supported by Di-and Trinucleating N4 Donor Ligands: Unveiling the Role of Coordinating Buffer in Directing Cooperative Catalysis

Abstract

Three iron(II)-triflate complexes supported by trinucleating (L1), dinucleating (L2) and mononucleating tris(2-pyridylmethyl)amine (TPA) nitrogen donor ligand framework, [(L1)Fe3(OTf)6] (1), [(L2)Fe2(OTf)4] (2), and [(TPA)Fe(OTf)2] (3) were synthesized and evaluated as catalysts for water oxidation. All complexes efficiently catalyse the water-oxidation reaction at near-neutral pH in borate buffer, using Oxone (KHSO₅) as the sacrificial oxidant. Among them, complex 1 exhibits the highest catalytic activity, with a turnover frequency (TOF) of 2.1 × 10-4 h-1, underscoring the role of nuclearity in dictating the kinetics of the water-oxidation reaction. Complex 1 represents one of the active homogeneous iron-based catalysts for chemical water oxidation reported to date. An iron(V)-oxo oxidant is implicated in the O-O bond formation step, with borate buffer facilitating cooperative interactions between the iron centres. Oxone mediates the iron-oxo generation, and the subsequent nucleophilic attack of water at the iron-oxo unit leads to dioxygen formation. The isolation of a borate-bridged diiron(III) complex from 2 offers insights into the nature of the active species responsible for initiating the catalytic cycle. It also highlights how multiple metal centres, engaged in cooperative interactions, enhance catalytic efficiency and improve the structural robustness of flexible molecular systems in iron-based water-oxidation catalysis.