Mechanistic Insights into Selective Hydrogenation of C=C Bonds Catalyzed by CCC Cobalt Pincer Complexes: A DFT Study
oleh: Zheng Zuo, Xinzheng Yang
| Format: | Article |
|---|---|
| Diterbitkan: | MDPI AG 2021-01-01 |
Deskripsi
The mechanistic insights into hydrogenations of hex-5-en-2-one, isoprene, and 4-vinylcyclohex-1-ene catalyzed by pincer (<sup>Mes</sup>CCC)Co (Mes = bis(mesityl-benzimidazol-2-ylidene)phenyl) complexes are computationally investigated by using the density functional theory. Different from a previously proposed mechanism with a cobalt dihydrogen complex (<sup>Mes</sup>CCC)Co-H<sub>2</sub> as the catalyst, we found that its less stable dihydride isomer, (<sup>Mes</sup>CCC)Co(H)<sub>2</sub>, is the real catalyst in those catalytic cycles. The generations of final products with H<sub>2</sub> cleavages for the formations of C−H bonds are the turnover-limiting steps in all three hydrogenation reactions. We found that the hydrogenation selectivity of different C=C bonds in the same compound is dominated by the steric effects, while the hydrogenation selectivity of C=C and C=O bonds in the same compound could be primarily influenced by the electronic effects. In addition, the observed inhabition of the hydrogenation reactions by excessive addition of PPh<sub>3</sub> could be explained by a 15.8 kcal/mol free energy barrier for the dissociation of PPh<sub>3</sub> from the precatalyst.