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Determining the Fate of a Non-Heme Iron Oxidation Catalyst Under Illumination, Oxygen, and Acid

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journal contribution
posted on 25.10.2016, 19:24 by Samuel L. Esarey, Joel C. Holland, Bart M. Bartlett
We analyze the stability of the non-heme water oxidation catalyst (WOC), Fe­(bpmcn)­Cl2 toward oxygen and illumination under nonaqueous and acidic conditions. Fe­(bpmcn)­Cl2 has been previously used as a C–H activation catalyst, a homogeneous WOC, and as a cocatalyst anchored to WO3 for photoelectrochemical water oxidation. This paper reports that the ligand dissociates at pH 1 with a rate constant k = 19.8(2) × 10–3 min–1, resulting in loss of catalytic activity. The combination of UV–vis experiments, 1H NMR spectroscopy, and cyclic voltammetry confirm free bpmcn and Fe2+ present in solution under acidic conditions. Even under nonaqueous conditions, both oxygen and illumination together show slow oxidation of iron over the course of a few hours, consistent with forming an Fe3+–O2 intermediate as corroborated by resonance-enhanced Raman spectroscopy, with a rate constant of k = 3.03(8) × 10–3 min–1. This finding has implications in both the merits of non-heme iron complexes as WOCs as well as cocatalysts in photoelectrochemical schemes: the decomposition mechanisms may include both anchoring group hydrolysis and instability under illumination.