Mechanism of [Rh(PCH2XRCH2P)2]+ (XR = CH2, N-CH3, CF2) catalyzed CO2 hydrogenation was studied to obtain the insight into the effect of outer coordinate sphere of diphosphine ligand on the catalytic efficiency. The most favorable reaction mechanism was found to be composed by three steps: 1) Oxidative addition of dihydrogen onto the Rh of the catalyst species; 2) the first hydride abstraction by base from the Rh dihydride complexes; 3) the second hydride transfer from the Rh hydride complexes to the CO2. It was found that the first hydride abstraction from Rh dihydride complex was the rate-determining step in this favorable mechanism. The activation energy of this rate-determining step was found related with the thermodynamic hydricity of the Rh dihydride complex. The calculated results of model catalyst with electron withdrawing group on the diphosphine ligand showed higher hydricity of the Rh dihydride complex and lower activation energy when compared with the other two catalysts. Our study found that it is the electron withdrawing group rather than the electron donating group that enhanced the catalytic efficiency of CO2 hydrogenation. This finding will benefit the study of ligand design in transition metal catalyst and lead to more efficient CO2 transformation.

Outer coordination sphere effect on Rh(diphosphine)+ complexes catalyzed CO2 hydrogenation

251st National Meeting (2016)

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