A High Spin Mn(IV)-Oxo Complex Generated via Stepwise Proton and Electron Transfer from Mn(III)-Hydroxo Precursor:...

The oxomanganese(IV) complex [(dpaq)Mn^IV(O)]⁺-M (1-M, M = redox-inactive metal ion, H-dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-ylacetamide), generated in the reaction of the precursor hydroxomanganese(III) complex 1 with iodosylbenzene (PhIO) in the presence of redox-inactive metal triflates, has recently been reported. Herein the generation of the same oxomanganese(IV) species from 1 using various combinations of protic acids and oxidants at 293 K is reported. The reaction of 1 with triflic acid and the one-electron-oxidizing agent [Ru^III(bpy)₃]³⁺ leads to the formation of the oxomanganese(IV) complex. The putative species has been identified as a mononuclear high-spin (S = 3/2) nonheme oxomanganese(IV) complex (1-O) on the basis of mass spectrometry, Raman spectroscopy, EPR spectroscopy, and DFT studies. The optical absorption spectrum is well reproduced by theoretical calculations on an S = 3/2 ground spin state of the complex. Isotope labeling studies confirm that the oxygen atom in the oxomanganese(IV) complex originates from the Mn^III-OH precursor and not from water. A mechanistic investigation reveals an initial protonation step forming the Mn^III-OH₂ complex, which then undergoes one-electron oxidation and subsequent deprotonations to form the oxomanganese(IV) transient, avoiding the requirements of either oxo-transfer agents or redox-inactive metal ions. The Mn^IV-oxo complex cleaves the C-H bonds of xanthene (k = 5.5 M^-1 s^-1), 9,10-DHA (k = 3.9 M^-1 s^-1), 1,4-CHD (k = 0.25 M^-1 s^-1), and fluorene (k = 0.11 M^-1 s^-1) at 293 K. The electrophilic character of the nonheme Mn^IV-oxo complex is demonstrated by a large negative ρ value of 2.5 in the oxidation of para-substituted thioanisoles. The complex emerges as the "most reactive" among the existing Mn^IV/V-oxo complexes bearing anionic ligands.