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 N(4)S(−) 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 N(5)(−) ligand. Using the 2-(bis((6-methylpyridin-2-yl)methyl)amino)-N-(quinolin-8-yl)acetamide (H(6Me)dpaq) ligand, we generated the [Mn(III)(OO(t)Bu)((6Me)dpaq)]OTf and [Mn(III)(OOCm)((6Me)dpaq)]OTf complexes through reaction of their Mn(II) or Mn(III) precursors with (t)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 CH(3)CN) 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)((6Me)dpaq)]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)((6Me)dpaq)](+) with PPh(3) 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.