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

Structures, energies, and stability of aliphatic PNP ligated Co-III/Co-I 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 Co-III amine and the triplet Co-I amine complexes as well as the singlet Co-III amido and the triplet Co-I amido complexes. For the hydrogenation of CH2O, PhCHO, and PhCOCH3 having lower Gibbs free energy barriers than that of the interconversion from the singlet Co-III amine and the triplet Co-I amine complexes, both singlet Co-III amine and triplet Co-I amine complexes are active catalysts and both catalytic cycles are independent, and the triplet Co-I amine complex is more active than the singlet Co-III amine complex. For the hydrogenation of PhCOOCH3 having higher barrier than that of catalyst interconversion, the singlet Co-III amine complex is the sole active catalyst and more active than the triplet Co-I 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.

Automated summary

The structures, energies, and stability of aliphatic PNP ligated Co-III/Co-I amine and amido complexes with different spin states and coordination spheres have been calculated, showing spin exchange between singlet Co-III and triplet Co-I complexes. The catalytic activity of these complexes in hydrogenation reactions depends on the substrate, with singlet Co-III amine being more active than triplet Co-I amine for certain substrates. The study also highlights the correlation between substrate properties and reaction mechanisms.