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

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• 作者：J. Gregory McAlpin ; Troy A. Stich ; C. Andr茅 Ohlin ; Yogesh Surendranath ; Daniel G. Nocera ; William H. Casey ; R. David Britt
• 刊名：Journal of the American Chemical Society
• 出版年：2011
• 出版时间：October 5, 2011
• 年：2011
• 卷：133
• 期：39
• 页码：15444-15452
• 全文大小：943K
• 年卷期：v.133,no.39(October 5, 2011)
• ISSN：1520-5126

文摘

Multifrequency electron paramagnetic resonace (EPR) spectroscopy and electronic structure calculations were performed on [Co₄O₄(C₅H₅N)₄(CH₃CO₂)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鈥損hosphate (Co鈥揚i) water-oxidizing catalyst. The Davies electron鈥搉uclear double resonance (ENDOR) spectrum is well-modeled using a single class of hyperfine-coupled ⁵⁹Co nuclei with a modestly strong interaction (principal elements of the hyperfine tensor are equal to [鈭?0(卤2), 77(卤1), 鈭?(卤15)] MHz). Mims ¹H 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 ¹⁴N 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^+3.125₄O₄] 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.