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

Abstract

Artificial photosynthesis (AP) promises to replace society's dependence on fossil energy resources via conversion of sunlight into sustainable, carbon-neutral fuels. However, large-scale AP implementation remains impeded by a dearth of cheap, efficient catalysts for the oxygen evolution reaction (OER). Cobalt oxide materials can catalyze the OER and are potentially scalable due to the abundance of cobalt in the Earth's crust; unfortunately, the activity of these materials is insufficient for practical AP implementation. Attempts to improve cobalt oxide's activity have been stymied by limited mechanistic understanding that stems from the inherent difficulty of characterizing structure and reactivity at surfaces of heterogeneous materials. While previous studies on cobalt oxide revealed the intermediacy of the unusual Co(IV) oxidation state, much remains unknown, including whether bridging or terminal oxo ligands form O2 and what the relevant oxidation states are. We have addressed these issues by employing a homogeneous model for cobalt oxide, the [Co(III)4] cubane (Co4O4(OAc)4py4, py = pyridine, OAc = acetate), that can be oxidized to the [Co(IV)Co(III)3] state. Upon addition of 1 equiv of sodium hydroxide, the [Co(III)4] cubane is regenerated with stoichiometric formation of O2. Oxygen isotopic labeling experiments demonstrate that the cubane core remains intact during this stoichiometric OER, implying that terminal oxo ligands are responsible for forming O2. The OER is also examined with stopped-flow UV-visible spectroscopy, and its kinetic behavior is modeled, to surprisingly reveal that O2 formation requires disproportionation of the [Co(IV)Co(III)3] state to generate an even higher oxidation state, formally [Co(V)Co(III)3] or [Co(IV)2Co(III)2]. The mechanistic understanding provided by these results should accelerate the development of OER catalysts leading to increasingly efficient AP systems.

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

“…In general, as was exhibited by our catalyst through that distribution of electron and spin density across the cubane cage, stabilization of high oxidation states is very important. 93 Furthermore, for the Co(IV)-O and Co(IV)-OO species, we have found large spin densities and hence strong radical character on the oxygen ligands. Although systems with implicit and explicit solvation showed similar solute configurations and (for the single-site pathway) similar lowest energy multiplicities and spin populations, the difference in relative free energies of the catalytic states between the two solvation methods was significant.…”

In situ characterization of cofacial Co(IV) centers in Co 4 O 4 cubane: Modeling the high-valent active site in oxygen-evolving catalysts

“…Although the ligand field of Co 4 O 4 cubanes differs slightly from that of Co-OEC arising from the presence of pyridine N and acetate O donors, cobalt-oxo molecular species that stabilize one or more Co(IV) centers are rare. The Co(III) 3 Co(IV) state of the cubane has been shown to be a Robin-Day class II mixed-valence (MV) complex (30), and an increase in formal oxidation state to the doubly oxidized cubane has been shown to be active for OER (31,32). In a Co 4 O 4 (py) 4 (OAc) 4 cubane (1) (33), the doubly oxidized Co(III) 2 Co(IV) 2 core may be electrochemically accessed (30).…”

Electrocatalytic Water Oxidation by Single Site and Small Nuclearity Clusters of Cobalt

“…In general, mechanistic studies suggest that water oxidation on cobalt oxide can proceed through both mononuclear and dinuclear pathways. [29][30][31][32][33] At neutral pH, a dinuclear (dicobalt) pathway for oxygen formation appears to be preferred, which in principle could involve intermolecular coupling of adjacent oxyl radicals, or the coupling of hydroxyl and oxo ligands on neighboring cobalt centers to form a hydroperoxo intermediate. 29,30 By comparison, a mononuclear pathway, involving nucleophilic attack of water or hydroxide onto a cobalt oxo species, is reported to be slow at neutral pH because of the difficulty in deprotonating the water molecule.…”

“…31 It is important to note; however, that even though the latter mechanism proceeds through a single terminal oxo species, all four cobalt centers in the cubane appear to play a role in supporting the formal, high oxidation state of Co(V), so in this case multiple cobalt atoms are required for efficient catalysis.…”