Department of Biochemistry and Biophysics, Arrhenius Laboratory, Stockholm University , SE-106 91 Stockholm, Sweden.
J Am Chem Soc. 2013 Sep 18;135(37):13804-13. doi: 10.1021/ja4053448. Epub 2013 Sep 5.
Hybrid DFT model calculations have been performed for some cobalt complexes capable of oxidizing water. Since a very plausible mechanism for the oxygen-evolving complex involving the cuboidal Mn4Ca structure in photosystem II (PSII) has recently been established, the most important part of the present study concerns a detailed comparison between cobalt and manganese as water oxidation catalysts. One similarity found is that a M(IV)-O(•) state is the key precursor for O-O bond formation in both cases. This means that simply getting a M(IV) state is not enough; a formal M(V)═O state is required, with two oxidations on one center from M(III). For cobalt, not even that is enough. A singlet coupled state is required at this oxidation level, which is not the ground state. It is shown that there are also more fundamental differences between catalysts based on these metals. The favorable low-barrier direct coupling mechanism found for PSII is not possible for the corresponding cobalt complexes. The origin of this difference is explained. For the only oxygen-evolving cubic Co4O4 complex with a defined structure, described by Dismukes et al., the calculated results are in good agreement with experiments. For the Co4 models of the amorphous cobalt-oxo catalyst found by Nocera et al., higher barriers are found than the one obtained experimentally. The reasons for this are discussed.
已针对一些能够氧化水的钴配合物进行了杂化 DFT 模型计算。由于最近已经建立了涉及光合作用系统 II（PSII）中立方 Mn4Ca 结构的放氧复合物的非常合理的机制，因此目前研究的最重要部分涉及钴和锰作为水氧化催化剂之间的详细比较。发现的一个相似之处是，M(IV)-O(•) 态是两种情况下 O-O 键形成的关键前体。这意味着仅仅获得 M(IV) 态是不够的；需要一个形式上的 M(V)═O 态，其中一个中心有两个来自 M(III)的氧化。对于钴来说，甚至这还不够。在这个氧化水平上需要一个单重态耦合态，而不是基态。事实表明，基于这些金属的催化剂之间也存在更根本的差异。对于 PSII 发现的有利的低势垒直接偶联机制，对于相应的钴配合物是不可能的。解释了这种差异的起源。对于 Dismukes 等人所描述的具有明确结构的唯一放氧立方 Co4O4 配合物，计算结果与实验结果吻合良好。对于 Nocera 等人发现的无定形钴-氧催化剂的 Co4 模型，发现的势垒高于实验获得的势垒。讨论了出现这种情况的原因。
