In situ characterization of cofacial Co(IV) centers in Co4O4 cubane: Modeling the high-valent active site in oxygen-evolving catalysts
In situ characterization of cofacial Co(IV) centers in Co4O4 cubane: Modeling the high-valent active site in oxygen-evolving catalysts
复制标题
DOI：
10.1073/pnas.1701816114
复制
发表时间：
2017-04-11
影响因子：
11.1
通讯作者：
Nocera, Daniel G.
中科院分区：
文献类型：
--
作者：
Brodsky, Casey N.;Hadt, Ryan G.;Nocera, Daniel G.
立方烷Co_4O_4是氧化型钴析氧催化剂（Co-OECs）的典型结构模型。Co-OEC在高于全Co（III）静止状态的两个氧化水平时具有活性。这种双重氧化的Co（IV）（2）态可以在Co（III）（2）（IV）（2）立方烷中捕获。我们证明，钴（III）（2）（IV）（2）立方烷可以电化学生成和这种独特的高价态的电子性质可以探测原位光谱。在近红外光谱中观察到Co（III）（2）（IV）（2）立方烷的价间电荷转移（IVCT）带，光谱分析和电化学动力学测量表明，与单氧化立方烷相比，双氧化立方烷具有更大的重组能和更小的电子转移速率常数.光谱电化学X-射线吸收数据进一步揭示了系统的光谱变化，从立方烷静止状态的连续氧化。与实验数据相关的电子结构计算表明，这种状态最好表示为局域化的，反铁磁耦合的Co（IV）（2）二聚体。共面Co（IV）（2）位的交换耦合使得Co_4O_4立方烷模型体系的电子结构与Co-OEC的高价活性位之间存在平行关系，特别强调了双氧化Co（IV）（2）中心在O-O键形成上的表现。
The Co4O4 cubane is a representative structural model of oxidic cobalt oxygen-evolving catalysts (Co-OECs). The Co-OECs are active when residing at two oxidation levels above an all-Co(III) resting state. This doubly oxidized Co(IV)(2) state may be captured in a Co(III)(2)(IV)(2) cubane. We demonstrate that the Co(III)(2)(IV)(2) cubane may be electrochemically generated and the electronic properties of this unique high-valent state may be probed by in situ spectroscopy. Intervalence charge-transfer (IVCT) bands in the near-IR are observed for the Co(III)(2)(IV)(2) cubane, and spectroscopic analysis together with electrochemical kinetics measurements reveal a larger reorganization energy and a smaller electron transfer rate constant for the doubly versus singly oxidized cubane. Spectroelectrochemical X-ray absorption data further reveal systematic spectral changes with successive oxidations from the cubane resting state. Electronic structure calculations correlated to experimental data suggest that this state is best represented as a localized, antiferromagnetically coupled Co(IV)(2) dimer. The exchange coupling in the cofacial Co(IV)(2) site allows for parallels to be drawn between the electronic structure of the Co4O4 cubane model system and the high-valent active site of the Co-OEC, with specific emphasis on the manifestation of a doubly oxidized Co(IV)(2) center on O-O bond formation.