Zhang Pan, Ni Shao-Fei, Dang Li
Department of Chemistry, South University of Science and Technology of China, ShenZhen, 518055, P.R. China.
Chem Asian J. 2016 Sep 20;11(18):2528-36. doi: 10.1002/asia.201600611. Epub 2016 Aug 31.
The reactivity difference between the hydrogenation of CO2 catalyzed by various ruthenium bidentate phosphine complexes was explored by DFT. In addition to the ligand dmpe (Me2 PCH2 CH2 PMe2 ), which was studied experimentally previously, a more bulky diphosphine ligand, dmpp (Me2 PCH2 CH2 CH2 PMe2 ), together with a more electron-withdrawing diphosphine ligand, PN(Me) P (Me2 PCH2 N(Me) CH2 PMe2 ), have been studied theoretically to analyze the steric and electronic effects on these catalyzed reactions. Results show that all of the most favorable pathways for the hydrogenation of CO2 catalyzed by bidentate phosphine ruthenium dihydride complexes undergo three major steps: cis-trans isomerization of ruthenium dihydride complex, CO2 insertion into the Ru-H bond, and H2 insertion into the ruthenium formate ion. Of these steps, CO2 insertion into the Ru-H bond has the lowest barrier compared with the other two steps in each preferred pathway. For the hydrogenation of CO2 catalyzed by ruthenium complexes of dmpe and dmpp, cis-trans isomerization of ruthenium dihydride complex has a similar barrier to that of H2 insertion into the ruthenium formate ion. However, in the reaction catalyzed by the PN(Me) PRu complex, cis-trans isomerization of the ruthenium dihydride complex has a lower barrier than H2 insertion into the ruthenium formate ion. These results suggest that the steric effect caused by the change of the outer sphere of the diphosphine ligand on the reaction is not clear, although the electronic effect is significant to cis-trans isomerization and H2 insertion. This finding refreshes understanding of the mechanism and provides necessary insights for ligand design in transition-metal-catalyzed CO2 transformation.
通过密度泛函理论（DFT）研究了各种钌双齿膦配合物催化二氧化碳加氢反应的活性差异。除了之前已进行实验研究的配体dmpe（Me2PCH2CH2PMe2）外，还从理论上研究了一个空间位阻更大的双膦配体dmpp（Me2PCH2CH2CH2PMe2）以及一个吸电子能力更强的双膦配体PN(Me)P（Me2PCH2N(Me)CH2PMe2），以分析空间位阻和电子效应对这些催化反应的影响。结果表明，双齿膦钌二氢配合物催化二氧化碳加氢的所有最有利途径都经历三个主要步骤：钌二氢配合物的顺反异构化、二氧化碳插入Ru-H键以及氢气插入甲酸根钌离子。在这些步骤中，与每个优选途径中的其他两个步骤相比，二氧化碳插入Ru-H键的势垒最低。对于由dmpe和dmpp的钌配合物催化的二氧化碳加氢反应，钌二氢配合物的顺反异构化势垒与氢气插入甲酸根钌离子的势垒相似。然而，在由PN(Me)PRu配合物催化的反应中，钌二氢配合物的顺反异构化势垒低于氢气插入甲酸根钌离子的势垒。这些结果表明，尽管电子效应对于顺反异构化和氢气插入具有重要意义，但双膦配体外层变化引起的空间位阻效应在反应中的作用并不明确。这一发现刷新了对该反应机理的认识，并为过渡金属催化二氧化碳转化中的配体设计提供了必要的见解。
