Bullock R Morris, Appel Aaron M, Helm Monte L
Center for Molecular Electrocatalysis (efrc.pnnl.gov), Physical Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, K2-57, USA.
Chem Commun (Camb). 2014 Mar 25;50(24):3125-43. doi: 10.1039/c3cc46135a.
Generation of hydrogen by reduction of two protons by two electrons can be catalysed by molecular electrocatalysts. Determination of the thermodynamic driving force for elimination of H2 from molecular complexes is important for the rational design of molecular electrocatalysts, and allows the design of metal complexes of abundant, inexpensive metals rather than precious metals ("Cheap Metals for Noble Tasks"). The rate of H2 evolution can be dramatically accelerated by incorporating pendant amines into diphosphine ligands. These pendant amines in the second coordination sphere function as protons relays, accelerating intramolecular and intermolecular proton transfer reactions. The thermodynamics of hydride transfer from metal hydrides and the acidity of protonated pendant amines (pK(a) of N-H) contribute to the thermodynamics of elimination of H2; both of the hydricity and acidity can be systematically varied by changing the substituents on the ligands. A series of Ni(II) electrocatalysts with pendant amines have been developed. In addition to the thermochemical considerations, the catalytic rate is strongly influenced by the ability to deliver protons to the correct location of the pendant amine. Protonation of the amine endo to the metal leads to the N-H being positioned appropriately to favor rapid heterocoupling with the M-H. Designing ligands that include proton relays that are properly positioned and thermodynamically tuned is a key principle for molecular electrocatalysts for H2 production as well as for other multi-proton, multi-electron reactions important for energy conversions.
通过两个电子还原两个质子来生成氢气可由分子电催化剂催化。确定从分子配合物中消除H₂的热力学驱动力对于分子电催化剂的合理设计很重要，并且允许设计使用丰富、廉价金属而非贵金属的金属配合物（“用廉价金属完成重要任务”）。通过将侧链胺引入二膦配体中，可显著加速H₂的析出速率。这些处于第二配位层的侧链胺起到质子中继的作用，加速分子内和分子间的质子转移反应。金属氢化物中氢化物转移的热力学以及质子化侧链胺的酸度（N-H的pK(a)）有助于H₂消除的热力学；通过改变配体上的取代基，水合性和酸度都可以系统地变化。已经开发出了一系列带有侧链胺的Ni(II)电催化剂。除了热化学方面的考虑外，催化速率还受到将质子传递到侧链胺正确位置的能力的强烈影响。胺在金属内部的质子化导致N-H的位置适当，有利于与M-H快速异质偶联。设计包含位置适当且经过热力学调节的质子中继的配体，是用于H₂生产的分子电催化剂以及对能量转换很重要的其他多质子、多电子反应的关键原则。
