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

Abstract

The oxomanganese(IV) complex [(dpaq)Mn^IV(O)]⁺-M^<i>n</i>+ (<b>1-M</b>^{<b><i>n</i>+</b>}, M^<i>n</i>+ = redox-inactive metal ion, H-dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-<i>N</i>-quinolin-8-ylacetamide), generated in the reaction of the precursor hydroxomanganese(III) complex <b>1</b> 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 <b>1</b> using various combinations of protic acids and oxidants at 293 K is reported. The reaction of <b>1</b> 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 (<i>S</i> = 3/2) nonheme oxomanganese(IV) complex (<b>1-O</b>) 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 <i>S</i> = 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 (<i>k</i>_<i>2</i> = 5.5 M^-1 s^-1), 9,10-DHA (<i>k</i>_<i>2</i> = 3.9 M^-1 s^-1), 1,4-CHD (<i>k</i>_<i>2</i> = 0.25 M^-1 s^-1), and fluorene (<i>k</i>_<i>2</i> = 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.