DFT Study on Mn-H Bond Dissociation Enthalpies of Manganese Hydrides in Manganese-Catalyzed Hydrogen Atom Transfer Reactions

In recent years, metal-catalyzed hydrogen atom transfer (MHAT) reactions have become integral to many catalytic reactions due to their stereoselectivity, chemical fidelity and selectivity. As the first transition metal element in abundant global reserves, manganese has the advantages of lower price and less toxicity, and a series of manganese complexes are used in MHAT reactions. In the reaction, the prominent catalytic intermediate manganese hydride formation is involved in the whole catalytic cycle, therefore the rupture of manganese-hydrogen bonds denoted by one of the thermodynamic parameter of the Mn-H bond dissociation enthalpy (BDE) is essential. In the present research, the Mn-H BDEs of some manganese hydrides which have experimental values were computed by using several DFT functionals, and the data showed that the B3P86 functional provided the highest precision, which has the least root mean square error (RMSE) of 3.3 kcal/mol. In the following, the BDEs and substituent effects of ten kinds of manganese hydrides involved in manganese-catalyzed hydrogen atom transfer reactions were further explored, and the calculations showed that the ligand has a significant effect on Mn-H BDEs, and the effect of substituents on Mn-H BDEs varies in different types of manganese hydrides. Moreover, the natural bond orbital (NBO) as well as the energies of frontier orbitals analyses aimed at revealing the intrinsic factors of different patterns of change of the Mn-H BDE were accomplished.