Toward Rational Design of 3d Transition Metal Catalysts for CO2 Hydrogenation Based on Insights into Hydricity-Controlled Rate-Determining Steps

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• 作者：Bhaskar Mondal ; Frank Neese ; Shengfa Ye
• 刊名：Inorganic Chemistry
• 出版年：2016
• 出版时间：June 6, 2016
• 年：2016
• 卷：55
• 期：11
• 页码：5438-5444
• 全文大小：479K
• 年卷期：0
• ISSN：1520-510X

文摘

Carbon dioxide functionalization attracts much interest due to the current environmental and energy challenges. Our earlier work (Mondal, B.; Neese, F.; Ye, S. Inorg. Chem. 2015, 54, 7192–7198) demonstrated that CO₂ hydrogenation mediated by base metal catalysts [M(H)(η²-H₂)(PP₃^Ph)]ⁿ⁺ (M = Co(III) and Fe(II), n = 1, 2; PP₃^Ph = tris(2-(diphenylphosphino)phenyl)phosphine) features discrete rate-determining steps (RDSs). Specifically, the reaction with [Co^III(H)(η²-H₂)(PP₃^Ph)]²⁺ passes through a hydride-transfer RDS, whereas the conversion with [Fe^II(H)(η²-H₂)(PP₃^Ph)]⁺ traverses a H₂-splitting RDS. More importantly, we found that the nature and barrier of the RDS likely correlate with the hydride affinity or hydricity of the dihydride intermediate [M(H)₂(PP₃^Ph)]⁽ⁿ⁻¹⁾⁺ generated by H₂-splitting. In the present contribution, following this notion we design a series of potential Fe(II) and Co(III) catalysts, for which the respective dihydride species possess differential hydricities, and computationally investigated their reactivity toward CO₂ hydrogenation. Our results reveal that lowering the hydrictiy of [Co^III(H)₂(PP₃^Ph)]⁺ by introducing anionic anchors in PP₃^Ph dramatically decreases the hydride-transfer RDS barrier, as shown for the enhanced reactivity of [Co(H)(η²-H₂)(CP₃^Ph)]⁺ and [Co(H)(η²-H₂)(SiP₃^Ph)]⁺ (CP₃^Ph = tris(2-(diphenylphosphino)phenyl)methyl, SiP₃^Ph = tris(2-(diphenylphosphino)phenyl)silyl), while the same ligand modification increases the H₂-splitting RDS barriers for [Fe(H)(η²-H₂)(CP₃^Ph)] and [Fe(H)(η²-H₂)(SiP₃^Ph)] relative to that for [Fe(H)(η²-H₂)(PP₃^Ph)]⁺. Conversely, upon increasing the hydricity of [Fe^II(H)₂(PP₃^Ph)] by adding an electron-withdrawing group to PP₃^Ph, the transformation with [Fe(H)(η²-H₂)(PP₃^PhNO2)]⁺ (PP₃^PhNO2 = tris(2-(diphenylphosphino)-4-nitrophenyl)phosphine) is predicted to encounter a lower barrier for H₂-splitting and a higher barrier for hydride transfer than those for [Fe(H)(η²-H₂)(PP₃^Ph)]⁺. Thus, we have shown that hydricity can be used as a guide to direct the rational design and development of more efficient catalysts.