Water Oxidation by the [Co₄O₄(OAc)₄(py)₄]⁺ Cubium is Initiated by OH^– Addition

Abstract

The cobalt cubium Co4O4(OAc)4(py)4(ClO4) (1A(+)) containing the mixed valence [Co4O4](5+) core is shown by multiple spectroscopic methods to react with hydroxide (OH(-)) but not with water molecules to produce O2. The yield of reaction products is stoichiometric (>99.5%): 41A(+) + 4OH(-) → O2 + 2H2O + 41A. By contrast, the structurally homologous cubium Co4O4(trans-OAc)2(bpy)4(ClO4)3, 1B(ClO4)3, produces no O2. EPR/NMR spectroscopies show clean conversion to cubane 1A during O2 evolution with no Co(2+) or Co3O4 side products. Mass spectrometry of the reaction between isotopically labeled μ-(16)O(bridging-oxo) 1A(+) and (18)O-bicarbonate/water shows (1) no exchange of (18)O into the bridging oxos of 1A(+), and (2) (36)O2 is the major product, thus requiring two OH(-) in the reactive intermediate. DFT calculations of solvated intermediates suggest that addition of two OH(-) to 1A(+) via OH(-) insertion into Co-OAc bonds is energetically favored, followed by outer-sphere oxidation to intermediate [1A(OH)2](0). The absence of O2 production by cubium 1B(3+) indicates the reactive intermediate derived from 1A(+) requires gem-1,1-dihydoxo stereochemistry to perform O-O bond formation. Outer-sphere oxidation of this intermediate by 2 equiv of 1A(+) accounts for the final stoichiometry. Collectively, these results and recent literature (Faraday Discuss., doi:10.1039/C5FD00076A and J. Am. Chem. Soc. 2015, 137, 12865-12872) validate the [Co4O4](4+/5+) cubane core as an intrinsic catalyst for oxidation of hydroxide by an inner-sphere mechanism.

Synthesis and reactivity of a mononuclear non-haem cobalt(IV)-oxo complex

“…Previous computational and in situ or ex situ spectroscopic studies during catalytic alkane hydroxylation and water oxidation reactions mediated by non-haem cobalt catalysts, including cobalt–oxygen clusters (for example, Co x O y ), have frequently proposed Co IV –O species as the active oxidants that effect the stereospecific C–H activation and O–O bond formation reactions2021222324252627282930. However, despite the ubiquity of putative Co IV –O intermediates in cobalt-catalysed redox reactions, direct spectroscopic characterization of the Co IV –O species under catalytic conditions has been achieved in only extremely rare cases.…”

“…Nevertheless, owing to their importance in a plethora of oxygen-dependent processes, inorganic compounds containing terminal metal–oxo moieties have been discussed or sought for this curious class of LTMOs19, which are thought to be reactive intermediates in a number of oxygen-dependent processes. Of particular relevance to this study are the cobalt(IV)–oxo species, which have also been proposed as reactive intermediates in a number of cobalt-mediated oxidation reactions2021222324252627. In particular, for catalytic oxidation of water to give molecular oxygen, a topic of intensive research aimed at developing artificial photosynthesis and efficient water splitting catalysis, ex situ electron paramagnetic resonance (EPR)28, X-ray absorption29 and time-resolved Fourier-transform infrared30 spectroscopic methods have provided direct evidence for the involvement of terminal and bridging cobalt(IV)–oxo species as key intermediates in the water oxidation reactions.…”

What Influences the Water Oxidation Activity of a Bioinspired Molecular CoII4O4 Cubane? An In-Depth Exploration of Catalytic Pathways

“…In a very recent study, Dismukes and coworkers suggested that O-O bond formation involving a [Co 4 O 4 ] 5+ -based "cubium" happens via geminal coupling of two terminal oxo ligands on the same Co center. 75 This event is preceded by partial removal of a bridging bidentate acetate ligand and attack of hydroxide. Their inspiring study certainly motivates future research into the catalytic mechanism of 1, for which we have already computationally investigated ligand mobility of bridging, as well as monodentate acetate.…”

Manganese–Cobalt Oxido Cubanes Relevant to Manganese-Doped Water Oxidation Catalysts

“…8−14 Functional models that are structurally similar to the OEC, or to an active site of an oxide catalyst, are far more rare. Only recently was the catalytic edge site of the layered oxide–hydroxide CoOOH successfully modeled by the molecular tetracobalt oxido cubane Co 4 ( μ 3 -O) 4 (OAc) 4 py 4 , to reveal a surprising mechanism for OER that implicates an unusually high formal oxidation state for cobalt.…”