Computational Monitoring of Oxidation States in Olefin Metathesis

Ruthenium-based catalysts play a pivotal role in the formation of carbon–carbon double bonds in olefin metathesis. The reaction mechanism always involves the formation of a four-membered metallacycle where the Ru centers should exhibit a formal +4 oxidation state. The steric and electronic properties of the so-called spectator N-heterocyclic carbene (NHC) ligand of Ru-benzylidene complexes are, presumably, responsible for the ability/inability of the metal center to achieve the necessary formal oxidation state for the catalytic process run on, and hence should be an essential ingredient in the design of efficient catalysts. In the present study we make use of density functional theory (DFT) calculations combined with the so-called effective oxidation state (EOS) analysis to examine the evolution of the metal oxidation state along the reaction mechanism. In particular, we tackle the Ru2+ versus Ru4+ debate in Grubbs catalysts, deepening into details about the different nature of the intermediates of the reaction pathway. We also analyze the picture of (cheaper and cleaner) iron- and osmium-based catalysts. Moreover, the nature and occupation number of the frontier effective fragment orbitals of the ligands sheds light into the subtle electronic differences between first- and second-generation Grubbs catalysts that nevertheless do not affect the assignment of oxidation states.

钌基催化剂在烯烃复分解反应的碳-碳双键构建过程中发挥关键作用。该反应的机理始终涉及四元金属环的生成，此过程中钌中心需呈现形式+4氧化态。钌亚苄基配合物中所谓的旁观型氮杂环卡宾（N-heterocyclic carbene, NHC）配体的空间与电子性质，推测决定了金属中心能否达到催化过程所需的形式氧化态，因此是设计高效催化剂的核心要素。本研究采用密度泛函理论（DFT）计算结合有效氧化态（effective oxidation state, EOS）分析，探究了反应全程中金属氧化态的演化规律。具体而言，本研究针对格拉布斯催化剂中Ru²+与Ru⁴+的争议展开探讨，深入剖析反应路径中间体的不同本质。此外，本研究还分析了（更廉价且更环保的）铁基与锇基催化剂的相关情况。进一步地，配体的前线有效片段轨道的性质与占据数，可为第一代与第二代格拉布斯催化剂之间的细微电子差异提供阐释——尽管这类差异并不影响氧化态的归属。