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Abstract

The development of efficient catalysts with base metals for CO₂ hydrogenation has always been a major thrust of interest. A series of experimental and theoretical work has revealed that the catalytic cycle typically involves two key steps, namely, base-promoted heterolytic H₂ splitting and hydride transfer to CO₂, either of which can be the rate-determining step (RDS) of the entire reaction. To explore the determining factor for the nature of RDS, we present herein a comparative mechanistic investigation on CO₂ hydrogenation mediated by [M(H)(η²-H₂)(PP₃^Ph)]ⁿ⁺ (M = Fe(II), Ru(II), and Co(III); PP₃^Ph = tris(2-(diphenylphosphino)phenyl)phosphine) type complexes. In order to construct reliable free energy profiles, we used highly correlated wave function based ab initio methods of the coupled cluster type alongside the standard density functional theory. Our calculations demonstrate that the hydricity of the metal–hydride intermediate generated by H₂ splitting dictates the nature of the RDS for the Fe(II) and Co(III) systems, while the RDS for the Ru(II) catalyst appears to be ambiguous. CO₂ hydrogenation catalyzed by the Fe(II) complex that possesses moderate hydricity traverses an H₂-splitting RDS, whereas the RDS for the high-hydricity Co(III) species is found to be the hydride transfer. Thus, our findings suggest that hydricity can be used as a practical guide in future catalyst design. Enhancing the electron-accepting ability of low-hydricity catalysts is likely to improve their catalytic performance, while increasing the electron-donating ability of high-hydricity complexes may speed up CO₂ conversion. Moreover, we also established the active roles of base NEt₃ in directing the heterolytic H₂ splitting and assisting product release through the formation of an acid–base complex.