Beckman Institute, California Institute of Technology, Pasadena, California 91125, USA.
Acc Chem Res. 2009 Dec 21;42(12):1995-2004. doi: 10.1021/ar900253e.
Natural photosynthesis uses sunlight to drive the conversion of energy-poor molecules (H(2)O, CO(2)) to energy-rich ones (O(2), (CH(2)O)(n)). Scientists are working hard to develop efficient artificial photosynthetic systems toward the "Holy Grail" of solar-driven water splitting. High on the list of challenges is the discovery of molecules that efficiently catalyze the reduction of protons to H(2). In this Account, we report on one promising class of molecules: cobalt complexes with diglyoxime ligands (cobaloximes). Chemical, electrochemical, and photochemical methods all have been utilized to explore proton reduction catalysis by cobaloxime complexes. Reduction of a Co(II)-diglyoxime generates a Co(I) species that reacts with a proton source to produce a Co(III)-hydride. Then, in a homolytic pathway, two Co(III)-hydrides react in a bimolecular step to eliminate H(2). Alternatively, in a heterolytic pathway, protonation of the Co(III)-hydride produces H(2) and Co(III). A thermodynamic analysis of H(2) evolution pathways sheds new light on the barriers and driving forces of the elementary reaction steps involved in proton reduction by Co(I)-diglyoximes. In combination with experimental results, this analysis shows that the barriers to H(2) evolution along the heterolytic pathway are, in most cases, substantially greater than those of the homolytic route. In particular, a formidable barrier is associated with Co(III)-diglyoxime formation along the heterolytic pathway. Our investigations of cobaloxime-catalyzed H(2) evolution, coupled with the thermodynamic preference for a homolytic route, suggest that the rate-limiting step is associated with formation of the hydride. An efficient water splitting device may require the tethering of catalysts to an electrode surface in a fashion that does not inhibit association of Co(III)-hydrides.
自然光合作用利用阳光将低能分子（H(2)O、CO(2)）转化为高能分子（O(2)、(CH(2)O)(n)）。科学家们正在努力开发高效的人工光合作用系统，以实现太阳能驱动水分解的“圣杯”。挑战之一是发现能有效催化质子还原为 H(2)的分子。在本报告中，我们介绍了一类有前途的分子：带有二肟配体的钴配合物（钴卟啉）。化学、电化学和光化学方法都被用于探索钴卟啉配合物的质子还原催化作用。Co(II)-二肟的还原生成 Co(I)物种，该物种与质子源反应生成 Co(III)-氢化物。然后，在均裂途径中，两个 Co(III)-氢化物在双分子步骤中反应以消除 H(2)。或者，在异裂途径中，Co(III)-氢化物的质子化产生 H(2)和 Co(III)。对 H(2)演化途径的热力学分析为 Co(I)-二肟还原质子涉及的基本反应步骤的障碍和驱动力提供了新的见解。结合实验结果，该分析表明，沿着异裂途径的 H(2)演化的势垒在大多数情况下远大于均裂途径的势垒。特别是，沿着异裂途径形成 Co(III)-二肟会产生一个巨大的障碍。我们对钴卟啉催化 H(2)演化的研究，以及对异裂途径热力学偏好的研究表明，限速步骤与氢化物的形成有关。一个高效的水分解装置可能需要将催化剂以一种不会抑制 Co(III)-氢化物结合的方式键合到电极表面上。
