Experimental and Computational Determination of a M-Cl Homolytic Bond Dissociation Free Energy: Mn(III)Cl-Mediated C-H Cleavage and Chlorination

This study confirms the hypothesis that [MnCl₃(OPPh₃)₂] (1) and acetonitrile-solvated MnCl₃ (i.e., [MnCl₃(MeCN)_x]) can be used as synthons to prepare Mn(III) chloride complexes with facially coordinating ligands. This was achieved through the preparation and characterization of six new {Mn^IIICl} complexes using anionic ligands Tp^H (tris(pyrazolyl)borate) and Tp^Me (tris(3,5-dimethylpyrazolyl)borate). The Mn^III-chloride dissociation and association equilibria (K_eq) and Mn^III/II reduction potentials were quantified in DCM. These two thermochemical parameters (K_eq and E_1/2), in addition to the known Cl-atom reduction potential in DCM, enabled the quantification of the Mn-Cl bond dissociation (homolysis) free energy of 21 and 23 ± 7 kcal/mol at room temperature for R = H and Me, respectively. These are in reasonable agreement with the bond dissociation free energy (BDFE_M-Cl) of 34 ± 6 kcal/mol calculated using density functional theory. The BDFE_M-Cl of 1 was also calculated (25 ± 6 kcal/mol). These energies were used in predictive C-H bond reactivity.