Reaction landscape of a pentadentate N5-ligated MnII complex with O2˙− and H2O2 includes conversion of a peroxomanganese(iii) adduct to a bis(μ-oxo)dimanganese(iii,iv) species

Abstract

Herein we describe the chemical reactivity of the mononuclear [Mn^II(N4py)(OTf)](OTf) (1) complex with hydrogen peroxide and superoxide. Treatment of 1 with one equivalent superoxide at −40 °C in MeCN formed the peroxomanganese(III) adduct, [Mn^III(O₂)(N4py)]⁺ (2) in ∼30% yield. Complex 2 decayed over time and the formation of the bis(μ-oxo)dimanganese(III,IV) complex, [Mn^IIIMn^IV(μ-O)₂(N4py)₂]³⁺ (3) was observed. When 2 was formed in higher yields (∼60%) using excess superoxide, the [Mn^III(O₂)(N4py)]⁺ species thermally decayed to Mn^II species and 3 was formed in no greater than 10% yield. Treatment of [Mn^III(O₂)(N4py)]⁺ with 1 resulted in the formation of 3 in ∼90% yield, relative to the concentration of [Mn^III(O₂)(N4py)]⁺. This reaction mimics the observed chemistry of Mn-ribonucleotide reductase, as it features the conversion of two Mn^II species to an oxo-bridged Mn^IIIMn^IV compound using O₂⁻ as oxidant. Complex 3 was independently prepared through treatment of 1 with H₂O₂ and base at −40 °C. The geometric and electronic structures of 3 were probed using electronic absorption, electron paramagnetic resonance (EPR), magnetic circular dichroism (MCD), variable-temperature, variable-field MCD (VTVH-MCD), and X-ray absorption (XAS) spectroscopies. Complex 3 was structurally characterized by X-ray diffraction (XRD), which revealed the N4py ligand bound in an unusual tetradentate fashion.