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(IIIIV) Species

摘要

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.