Yang Xiaofan, Baik Mu-Hyun
Department of Chemistry, Indiana University, Bloomington, Indiana 47405, USA.
J Am Chem Soc. 2008 Dec 3;130(48):16231-40. doi: 10.1021/ja8034043.
The resting state of the recently reported water oxidation catalyst tpyRu(II)-OH(2)L(3+) (tpy = terpyridine; L = bipyridylpyrazolylic anion) (2,2) must be activated by a series of proton-coupled oxidations in which four protons and four electrons are removed overall to afford the catalytically competent species tpyRu(IV)OL(3+) (4,4). We have examined all of the plausible redox intermediates utilizing density functional theory coupled to a continuum solvation model. Our calculations reproduce well the first three redox potentials under pH = 1 conditions, and a reasonable correlation between theory and experiment is found for the fourth irreversible redox process that accompanies O(2) generation. The computed oxidation potentials to access 5,4 and 5,5, 1.875 and 2.032 V vs NHE, respectively, exclude the otherwise plausible possibilities of the catalytically active species having a higher oxidation state. 4,4 has an antiferromagnetically coupled ground state in which one ruthenium has two unpaired electrons antiparallel to those of the other ruthenium. As we found in our previous work, two radicaloid terminal oxygen moieties with different spin orientations that are induced by spin polarization from the electron-deficient Ru(IV) centers are found. Two mechanistic scenarios are relevant and interesting for the key O-O bond formation event: intramolecular oxo-oxo coupling and coupling between one terminal oxo and the oxygen atom of the incoming water substrate. The intramolecular oxo-oxo coupling is facile, with a low barrier of 13.9 kcal mol(-1), yielding a peroxo intermediate. The necessary subsequent addition of water in an associative substitution mechanism to cleave one of the Ru-peroxo bonds, however, is found to be impractical at room temperature, with a barrier of DeltaG(double dagger) = 30.9 kcal mol(-1). Thus, while plausible, the intramolecular oxo-oxo coupling is unproductive for generating molecular dioxygen. The intermolecular O-O coupling is associated with a high barrier (DeltaG(double dagger) = 40.2 kcal mol(-1)) and requires the assistance of an additional proton, which lowers the barrier dramatically to 24.5 kcal mol(-1).
最近报道的水氧化催化剂tpyRu(II)-OH(2)L(3+)（tpy = 三联吡啶；L = 联吡啶吡唑阴离子）(2,2)的基态必须通过一系列质子耦合氧化来激活，在此过程中总共去除四个质子和四个电子，以生成具有催化活性的物种tpyRu(IV)OL(3+) (4,4)。我们利用密度泛函理论结合连续溶剂化模型研究了所有可能的氧化还原中间体。我们的计算很好地再现了pH = 1条件下的前三个氧化还原电位，并且对于伴随O(2)生成的第四个不可逆氧化还原过程，发现理论与实验之间存在合理的相关性。计算得出的生成5,4和5,5的氧化电位分别为相对于标准氢电极1.875和2.032 V，排除了催化活性物种具有更高氧化态的其他可能情况。4,4具有反铁磁耦合基态，其中一个钌有两个未成对电子，与另一个钌的未成对电子反平行。正如我们在之前的工作中发现的，由缺电子的Ru(IV)中心的自旋极化诱导出两个具有不同自旋取向的类自由基末端氧部分。对于关键的O - O键形成事件有两种相关且有趣的机制：分子内氧 - 氧偶联以及一个末端氧与进入的水底物的氧原子之间的偶联。分子内氧 - 氧偶联很容易发生，势垒低至13.9 kcal mol(-1)，生成过氧中间体。然而，发现在室温下通过缔合取代机制随后添加水以断裂其中一个Ru - 过氧键是不切实际的，势垒为ΔG‡ = 30.9 kcal mol(-1)。因此，虽然看似合理，但分子内氧 - 氧偶联对于生成分子态二氧是无成效的。分子间O - O偶联具有高势垒（ΔG‡ = 40.2 kcal mol(-1)），并且需要额外质子的协助，这将势垒大幅降低至24.5 kcal mol(-1)。
