A High Spin Mn(IV)-Oxo Complex Generated via Stepwise Proton and Electron Transfer from Mn(III)-Hydroxo Precursor: Characterization and C-H Bond Cleavage Reactivity.

The oxomanganese(IV) complex [(dpaq)Mn(O)]-M (, M = redox-inactive metal ion, H-dpaq = 2-[bis(pyridin-2-ylmethyl)]amino--quinolin-8-ylacetamide), generated in the reaction of the precursor hydroxomanganese(III) complex 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 using various combinations of protic acids and oxidants at 293 K is reported. The reaction of with triflic acid and the one-electron-oxidizing agent [Ru(bpy)] leads to the formation of the oxomanganese(IV) complex. The putative species has been identified as a mononuclear high-spin ( = 3/2) nonheme oxomanganese(IV) complex () 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 = 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-OH precursor and not from water. A mechanistic investigation reveals an initial protonation step forming the Mn-OH 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-oxo complex cleaves the C-H bonds of xanthene ( = 5.5 M s), 9,10-DHA ( = 3.9 M s), 1,4-CHD ( = 0.25 M s), and fluorene ( = 0.11 M s) at 293 K. The electrophilic character of the nonheme Mn-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-oxo complexes bearing anionic ligands.