Characterization and chemical reactivity of room-temperature-stable Mn^III-alkylperoxo complexes

While alkylperoxomanganese(iii) (Mn^III-OOR) intermediates are proposed in the catalytic cycles of several manganese-dependent enzymes, their characterization has proven to be a challenge due to their inherent thermal instability. Fundamental understanding of the structural and electronic properties of these important intermediates is limited to a series of complexes with thiolate-containing N₄S^- ligands. These well-characterized complexes are metastable yet unreactive in the direct oxidation of organic substrates. Because the stability and reactivity of Mn^III-OOR complexes are likely to be highly dependent on their local coordination environment, we have generated two new Mn^III-OOR complexes using a new amide-containing N₅ ^- ligand. Using the 2-(bis((6-methylpyridin-2-yl)methyl)amino)-N-(quinolin-8-yl)acetamide (H^6Medpaq) ligand, we generated the [Mn^III(OO Bu)(^6Medpaq)]OTf and [Mn^III(OOCm)(^6Medpaq)]OTf complexes through reaction of their Mn^II or Mn^III precursors with BuOOH and CmOOH, respectively. Both of the new Mn^III-OOR complexes are stable at room-temperature (t _1/2 = 5 and 8 days, respectively, at 298 K in CH₃CN) and capable of reacting directly with phosphine substrates. The stability of these Mn^III-OOR adducts render them amenable for detailed characterization, including by X-ray crystallography for [Mn^III(OOCm)(^6Medpaq)]OTf. Thermal decomposition studies support a decay pathway of the Mn^III-OOR complexes by O-O bond homolysis. In contrast, direct reaction of [Mn^III(OOCm)(^6Medpaq)]⁺ with PPh₃ provided evidence of heterolytic cleavage of the O-O bond. These studies reveal that both the stability and chemical reactivity of Mn^III-OOR complexes can be tuned by the local coordination sphere.