Enhanced Understanding of Structure-Function Relationships for Oxomanganese(IV) Complexes.

A series of manganese(II) and oxomanganese(IV) complexes supported by neutral, pentadentate ligands with varied equatorial ligand-field strength (N3pyQ, N2py2I, and N4py Me2 ) were synthesized and then characterized using structural and spectroscopic methods. On the basis of electronic absorption spectroscopy, the [Mn IV (O)(N4py Me2 )] 2+ complex has the weakest equatorial ligand field among a set of similar Mn IV -oxo species. In contrast, [Mn IV (O)(N2py2I)] 2+ shows the strongest equatorial ligand-field strength for this same series. We examined the influence of these changes in electronic structure on the reactivity of the oxomanganese(IV) complexes using hydrocarbons and thioanisole as substrates. The [Mn IV (O)(N3pyQ)] 2+ complex, which contains one quinoline and three pyridine donors in the equatorial plane, ranks among the fastest Mn IV -oxo complexes in C-H bond and thioanisole oxidation. While a weak equatorial ligand field has been associated with high reactivity, the [Mn IV (O)(N4py Me2 )] 2+ complex is only a modest oxidant. Buried volume plots suggest that steric factors dampen the reactivity of this complex. Trends in reactivity were examined using density functional theory (DFT)-computed bond dissociation free energies (BDFEs) of the Mn III O-H and Mn IV ═ O bonds. We observe an excellent correlation between Mn IV ═O BDFEs and rates of thioanisole oxidation, but more scatter is observed between hydrocarbon oxidation rates and the Mn III O-H BDFEs.