EaStCHEM School of Chemistry, University of Edinburgh, Edinburgh, EH9 3FJ, UK.
Dalton Trans. 2018 Oct 7;47(37):13106-13112. doi: 10.1039/c8dt02919a. Epub 2018 Aug 31.
Three phenoxyimine Fe(iii)Cl complexes bearing electronically diverse -Cl, -H or -Bu substituents in the ortho position were synthesised. X-ray crystallographic analysis of the complexes reveals mononuclear structures with pentacoordinate iron centres and trigonal bipyramidal geometries. All three complexes demonstrated excellent catalytic activities towards CO/epoxide coupling to selectively form cyclic carbonates, with catalyst activity correlating with the electron withdrawing nature of the ortho-substituent (Cl > H > Bu) and thus the Lewis acidity of the metal centre. The chloro-substituted complex displayed remarkable activity in the synthesis of propylene carbonate from propylene oxide and CO, reaching turnover frequencies (TOF) up to 760 h in the presence of TBABr co-catalyst at 120 °C and 20 bars of CO pressure. Importantly, the catalyst is also very robust, functioning with high substrate loading, under air or in the presence of water. The substrate scope was successfully extended to other terminal epoxides including epichlorohydrin (TOF = 900 h) and to the more challenging internal epoxide, cyclohexene oxide (TOF = 80 h). These are amongst the highest TOF values reported for an iron CO/epoxide coupling catalyst to date.
三种带有邻位不同取代基的邻位取代基为-Cl、-H 或 -Bu 的苯氧基亚胺 Fe(iii)Cl 配合物被合成。配合物的 X 射线晶体学分析揭示了具有五配位铁中心和三角双锥几何形状的单核结构。所有三种配合物都对 CO/环氧化物偶联具有优异的催化活性，可选择性地形成环状碳酸酯，催化剂活性与邻位取代基的吸电子性质（Cl>H>Bu）以及金属中心的路易斯酸度相关。氯取代的配合物在由环氧丙烷和 CO 合成碳酸丙烯酯方面表现出显著的活性，在 TBABr 共催化剂存在下，在 120°C 和 20 巴 CO 压力下，达到高达 760 h 的转化频率（TOF）。重要的是，该催化剂也非常稳健，在高底物负载、空气或水存在下都能很好地发挥作用。该底物范围成功扩展到其他末端环氧化物，包括环氧氯丙烷（TOF = 900 h）和更具挑战性的内环氧环己烯氧化物（TOF = 80 h）。这些是迄今为止报道的用于铁 CO/环氧化物偶联催化剂的最高 TOF 值之一。
