Université Paris Diderot, Sorbonne Paris Cité, Laboratoire d'Electrochimie Moléculaire, Unité Mixte de Recherche Université, CNRS 7591, 75205 Paris Cedex 13, France;
LCM, CNRS, Ecole Polytechnique, Université Paris-Saclay, 91128 Palaiseau Cedex, France
Proc Natl Acad Sci U S A. 2018 Sep 11;115(37):9104-9109. doi: 10.1073/pnas.1810255115. Epub 2018 Aug 24.
The world of coordination complexes is currently stimulated by the quest for efficient catalysts for the electrochemical reactions underlying modern energy and environmental challenges. Even in the case of a multielectron-multistep process, catalysis starts with uptake or removal of one electron from the resting state of the catalyst. If this first step is an outer-sphere electron transfer (triggering a "redox catalysis" process), the electron distribution over the metal and the ligand is of minor importance. This is no longer the case with "chemical catalysis," in which the active catalyst reacts with the substrate in an inner-sphere manner, often involving the transient formation of a catalyst-substrate adduct. The fact that, in most cases, the ligand is "noninnocent," in the sense that the electron density and charge gained (or removed) from the resting state of the catalyst are shared between the metal and the ligand, has become common-place knowledge over the last half-century. Insistent focus on a large degree of noninnocence of the ligand in the resting state of the catalyst, even robustly validated by spectroscopic techniques, may lead to undermining the essential role of the metal when such essential issues as kinetics, mechanisms, and product selectivity are dealt with. These points are general in scope, but their discussion is eased by adequately documented examples. This is the case for reactions involving metalloporphyrins as well as vitamin B12 derivatives and similar cobalt complexes for which a wealth of experimental data is available.
目前，配位化合物领域的研究受到了电化学反应高效催化剂的刺激，这些催化剂是现代能源和环境挑战的基础。即使是在多电子多步骤过程的情况下，催化作用也是从催化剂的静止状态中吸收或去除一个电子开始的。如果这第一步是一个外部电子转移（触发“氧化还原催化”过程），那么金属和配体上的电子分布就不重要了。对于“化学催化”来说，情况就不再如此了，在这种催化中，活性催化剂以内球方式与底物反应，通常涉及催化剂-底物加合物的瞬时形成。在过去的半个世纪里，大多数情况下配体“非中性”，即催化剂静止状态下获得的电子密度和电荷（或去除）在金属和配体之间共享，这已成为常识。即使光谱技术强有力地验证了配体在催化剂静止状态下具有很大程度的非中性，但在处理动力学、机制和产物选择性等基本问题时，可能会削弱金属的重要作用。这些观点具有广泛的范围，但通过充分记录的示例可以更容易地讨论这些观点。这适用于涉及金属卟啉以及维生素 B12 衍生物和类似的钴配合物的反应，这些反应都有大量的实验数据。
