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 MnIV-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 (MnO)-O-III-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 (MnO)-O-III-H BDFEs.

Automated summary

A series of manganese(II) and oxomanganese(IV) complexes with different ligand-field strengths were synthesized and characterized. The [Mn-IV(O)(N4py(Me2))]2+ complex showed the weakest equatorial ligand field, while the [Mn-IV(O)(N2py2I)]2+ complex exhibited the strongest equatorial ligand-field strength. The [Mn-IV(O)(N3pyQ)]2+ complex with quinoline and pyridine donors showed high reactivity in C-H bond and thioanisole oxidation reactions.