Water Oxidation Catalysis by Co(II) Impurities in Co(III)₄O₄ Cubanes

Abstract

The observed water oxidation activity of the compound class Co4O4(OAc)4(Py–X)4 emanates from a Co(II) impurity. This impurity is oxidized to produce the well-known Co-OEC heterogeneous cobaltate catalyst, which is an active water oxidation catalyst. We present results from electron paramagnetic resonance spectroscopy, nuclear magnetic resonance line broadening analysis, and electrochemical titrations to establish the existence of the Co(II) impurity as the major source of water oxidation activity that has been reported for Co4O4 molecular cubanes. Differential electrochemical mass spectrometry is used to characterize the fate of glassy carbon at water oxidizing potentials and demonstrate that such electrode materials should be used with caution for the study of water oxidation catalysis.

Rate and Stability of Photocatalytic Water Oxidation using [Ru(bpy)3]2+ as Photosensitizer

“…In all cases, the photosensitizer was [Ru(bpy)3]Cl2 (PS), the sacrificial electron acceptor was Na2S2O8, and the solvent was a phosphate buffer at pH=7.0. Under such conditions, [1] is reported to be a homogeneous catalyst, 22 [2] is reported to form nanoparticles, 3,4,71 and the nature of catalyst [3] seems to depend strongly on the conditions. 35,39,72,73 A lag phase in the photocatalytic water-oxidation reaction might be present as the active catalyst must first be formed by oxidation of the precatalyst (see the grey arrow in Scheme 1).…”

Mechanistic Investigations of Water Oxidation by a Molecular Cobalt Oxide Analogue: Evidence for a Highly Oxidized Intermediate and Exclusive Terminal Oxo Participation

“…In agreement with the results of Nocera and co-workers, purified 1 does not exhibit noticeable catalysis over this pH range, indicating that these samples do not contain Co(II) impurities. 29 This reversible couple shifts linearly with pH to more oxidizing potentials below pH 4, at a rate (63 mV/pH unit) that indicates a 1H + , 1e − redox process ( Figure S2). The peak potential remains constant below pH 0.7 (to 0), and this behavior is consistent with protonation of cubane 1 to 1H + , with pK a = 3.5 for 1.…”

“…Indeed, the oxidized cubane [1]PF 6 has previously been isolated from the oxidation of 1 in acetonitrile. 29 In water, the reaction of 1 with 1 equiv of ceric ammonium nitrate, followed by addition of NH 4 PF 6 , resulted in precipitation of [1]PF 6 as a black solid in 75% yield (Scheme 1, reaction A). Recrystallization from CH 2 Cl 2 /hexane afforded analytically pure single crystals of [1]PF 6 ·CH 2 Cl 2 for X-ray diffraction (XRD) analysis ( Figure S8b).…”

A Bioinspired Molecular Polyoxometalate Catalyst with Two Cobalt(II) Oxide Cores for Photocatalytic Water Oxidation

“…[9] The result of their research is that the water oxidation activityo ft he compound class [Co 4 ]c ontains no terminal aquo ligands.I nspired by the proposed PS II-oxygen evolution complex (OEC) mechanisms, the implementation of transition metals coordinated to terminala quo, hydroxo, or oxo ligands is am ajor step toward OÀOf ormation pathways via water attack and exchange processes. [10] Recently,m any robust,m olecular polyoxometalates (POMs) as efficient WOCs have been developed, because this kind of catalyste liminates the problem of ligand oxidative instability, while being thermally stable and allowing for structuralfine-tuning.…”