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

• Published in: Journal of the American Chemical Society 2023-06, Vol.145 (24), p.13384-13391
• Main Authors: Saju, Ananya, Gunasekera, Parami S., Morgante, Pierpaolo, MacMillan, Samantha N., Autschbach, Jochen, Lacy, David C.
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.3c03651

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 {MnIIICl} complexes using anionic ligands TpH (tris­(pyrazolyl)­borate) and TpMe (tris­(3,5-dimethylpyrazolyl)­borate). The MnIII–chloride dissociation and association equilibria (K eq) and MnIII/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 (BDFEM–Cl) of 34 ± 6 kcal/mol calculated using density functional theory. The BDFEM–Cl of 1 was also calculated (25 ± 6 kcal/mol). These energies were used in predictive C–H bond reactivity.