Electronic Structure Description of a [Co(III)₃Co(IV)O₄] Cluster: A Model for the Paramagnetic Intermediate in Cobalt-Catalyzed Water Oxidation

Abstract

Multifrequency electron paramagnetic resonace (EPR) spectroscopy and electronic structure calculations were performed on [Co(4)O(4)(C(5)H(5)N)(4)(CH(3)CO(2))(4)](+) (1(+)), a cobalt tetramer with total electron spin S = 1/2 and formal cobalt oxidation states III, III, III, and IV. The cuboidal arrangement of its cobalt and oxygen atoms is similar to that of proposed structures for the molecular cobaltate clusters of the cobalt-phosphate (Co-Pi) water-oxidizing catalyst. The Davies electron-nuclear double resonance (ENDOR) spectrum is well-modeled using a single class of hyperfine-coupled (59)Co nuclei with a modestly strong interaction (principal elements of the hyperfine tensor are equal to [-20(±2), 77(±1), -5(±15)] MHz). Mims (1)H ENDOR spectra of 1(+) with selectively deuterated pyridine ligands confirm that the amount of unpaired spin on the cobalt-bonding partner is significantly reduced from unity. Multifrequency (14)N ESEEM spectra (acquired at 9.5 and 34.0 GHz) indicate that four nearly equivalent nitrogen nuclei are coupled to the electron spin. Cumulatively, our EPR spectroscopic findings indicate that the unpaired spin is delocalized almost equally across the eight core atoms, a finding corroborated by results from DFT calculations. Each octahedrally coordinated cobalt ion is forced into a low-spin electron configuration by the anionic oxo and carboxylato ligands, and a fractional electron hole is localized on each metal center in a Co 3d(xz,yz)-based molecular orbital for this essentially [Co(+3.125)(4)O(4)] system. Comparing the EPR spectrum of 1(+) with that of the catalyst film allows us to draw conclusions about the electronic structure of this water-oxidation catalyst.

Catalytic Oxygen Evolution by Cobalt Oxido Thin Films

“…2) and at active sites during turnover (Sect. 4) are coupled to proton loss from bound OH x ligands, the increase in ligand-field strength accompanying this proton loss could result in substantial localization of unpaired spin density [58]. This localization of the oxidized equivalent is consistent with subsequent reactivity of the Co IV moieties in CoP i compared to the molecular model.…”

“…Additional multifrequency EPR studies and electronic structure calculations were performed on the Co IV -containing cubane compound to investigate further the electronic and geometric structure relationships of MCCs such as those found in CoP i [58]. The magnetic parameters determined by electron nuclear double resonance (ENDOR) and spectroscopic results from electron spin echo envelope modulation (ESEEM) combined with density functional theory (DFT) calculations revealed that the unpaired spin of the cubane is highly delocalized.…”

Towards a Molecular Level Understanding of the Multi-Electron Catalysis of Water Oxidation on Metal Oxide Surfaces

“…Future time resolved (pulsed) EPR and transient X-ray spectroscopy studies on such systems will likely provide new mechanistic insights about the precise electronic structure of the metal centers at each step of the catalytic cycle. This is particularly important in light of possible extensive charge delocalization across multiple Co centers as suggested by recent multi-frequency EPR studies of a Co 4 (III 3 ,IV)O 4 oxobridged Co cubane model compound [41]. For understanding the dynamics of the wider domain structure of Co oxide electrodeposits under catalytic conditions which may influence reaction pathways, pair distribution function analysis of X-ray scattering probes longer range distances not accessible by EXAFS spectroscopy.…”

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

“…[21][22][23][24][25] Starting with Nocera's oxygen evolving catalyst, 26 cobalt oxide has become a subject of interest to many research groups focusing their attention on Co(III)-based cubane type WOCs. [27][28][29][30][31] Wang and Van Voorhis explored a Co(III)-based cubane model complex using a quantum mechanics/molecular mechanics approach investigating an oxo-oxo coupling mechanism. 32 Calculations have also been performed by Li and Siegbahn for a Co(III) 4 O 4 complex.…”