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^IIIO-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^IIIO-H BDFEs.