Catalytic Activity of Aliphatic PNP Ligated CoIII/I Amine and Amido Complexes in Hydrogenation Reaction—Structure, Stability, and Substrate Dependence

Abstract

Structures, energies, and stability of aliphatic PNP ligated CoIII/CoI amine and amido complexes in different spin (singlet, triplet, and open-shell singlet) states and coordination spheres have been computed. Spin exchange is found for the interconversion between the singlet CoIII amine and the triplet CoI amine complexes as well as the singlet CoIII amido and the triplet CoI amido complexes. For the hydrogenation of CH2O, PhCHO, and PhCOCH3 having lower Gibbs free energy barriers than that of the interconversion from the singlet CoIII amine and the triplet CoI amine complexes, both singlet CoIII amine and triplet CoI amine complexes are active catalysts and both catalytic cycles are independent, and the triplet CoI amine complex is more active than the singlet CoIII amine complex. For the hydrogenation of PhCOOCH3 having higher barrier than that of catalyst interconversion, the singlet CoIII amine complex is the sole active catalyst and more active than the triplet CoI amine complex. For the hydrogenation of CH3COCH3, both catalytic cycles can be competitive depending on reaction temperature. These reveal the substrate dependent mechanisms. The correlation between the hydride affinity of the substrate and the H– transfer barrier for aldehydes and ketones as well as between the deprotonation energy of alcohol and the reverse barrier of H+ transfer indicates that the energy of the transition state of the H–/H+ transfer via the corresponding most favorable mechanism can be estimated by the hydride affinity of substrate and the deprotonation energy of the product.