Electronic Structure Description of a [Co(III)3Co(IV)O4] Cluster: A Model for the Paramagnetic Intermediate in Cobalt-Catalyzed Water Oxidation
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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DOI：
10.1021/ja202320q
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发表时间：
2011-10-05
影响因子：
15
通讯作者：
Britt, R. David
中科院分区：
文献类型：
--
作者：
McAlpin, J. Gregory;Stich, Troy A.;Britt, R. David
用多频电子顺磁共振（EPR）方法研究了[Co_4O_4（C_5H_5N）（4）（CH_3CO_2）（4）]（+）（1（+））的电子结构。其钴和氧原子的立方排列类似于钴-磷酸盐（Co-Pi）水氧化催化剂的分子钴酸盐簇的拟议结构。Davies电子-核双共振（ENDOR）谱使用具有适度强相互作用的单类超精细耦合Co-59核进行了良好建模（超精细张量的主元素等于[-20（+/-2），77（+/-1），-5（+/-15）] MHz）。1（+）与选择性氘代吡啶配体的Mims H-1 ENDOR光谱证实，钴键合配偶体上的未配对自旋量从1显著减少。多频N-14 ESEEM光谱（在9.5和34.0 GHz获得）表明，四个几乎相等的氮原子核耦合到电子自旋。累积起来，我们的EPR光谱研究结果表明，未成对的自旋是离域的八个核心原子几乎相等，从DFT计算结果证实了这一发现。每个八面体配位的钴离子被迫进入一个低自旋电子配置的阴离子氧代和羧基配体，和分数电子空穴是本地化的Co 3d（xz，yz）为基础的分子轨道，这个基本上[Co 4 +3.125O4]系统中的每个金属中心。比较1（+）的EPR谱和催化剂膜的EPR谱，我们可以得出关于这种水氧化催化剂的电子结构的结论。
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-phosphate (Co-Pi) water-oxidizing catalyst. The Davies electron-nuclear double resonance (ENDOR) spectrum is well-modeled using a single class of hyperfine-coupled Co-59 nuclei with a modestly strong interaction (principal elements of the hyperfine tensor are equal to [-20(+/- 2), 77(+/- 1), -5(+/- 15)] MHz). Mims H-1 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 N-14 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 [Co4+3.125O4] 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.