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

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.