Mechanistic Insights into Selective Hydrogenation of C=C Bonds Catalyzed by CCC Cobalt Pincer Complexes: A DFT Study

The mechanistic insights into hydrogenations of hex-5-en-2-one, isoprene, and 4-vinylcyclohex-1-ene catalyzed by pincer ((CCC)-C-Mes)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 ((CCC)-C-Mes)Co-H-2 as the catalyst, we found that its less stable dihydride isomer, ((CCC)-C-Mes)Co(H)(2), is the real catalyst in those catalytic cycles. The generations of final products with H-2 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 PPh3 could be explained by a 15.8 kcal/mol free energy barrier for the dissociation of PPh3 from the precatalyst.

Automated summary

Computational investigation using density functional theory revealed that the less stable dihydride isomer ((CCC)-C-Mes)Co(H)(2) is the real catalyst in hydrogenations of hex-5-en-2-one, isoprene, and 4-vinylcyclohex-1-ene. The turnover-limiting steps in all three reactions involve the formations of C-H bonds through H-2 cleavages. The selectivity of hydrogenation in different C=C bonds primarily depends on steric effects, while the selectivity between C=C and C=O bonds may be influenced by electronic effects.