Mimicking Class I b Mn₂-Ribonucleotide Reductase: A Mn^II₂ Complex and Its Reaction with Superoxide

A fascinating discovery in the chemistry of ribonucleotide reductases (RNRs) has been the identification of a dimanganese (Mn₂) active site in class I b RNRs that requires superoxide anion (O₂^.−), rather than dioxygen (O₂), to access a high-valent Mn₂ oxidant. Complex 1 ([Mn₂(O₂CCH₃)(N-Et-HPTB)](ClO₄)₂, N-Et-HPTB=N,N,N′,N′-tetrakis(2-(1-ethylbenzimidazolyl))-2-hydroxy-1,3-diaminopropane) was synthesised in high yield (90 %). 1 was reacted with O₂^.− at −40 °C resulting in the formation of a metastable species (2). 2 displayed electronic absorption features (λ_max=460, 610 nm) typical of a Mn-peroxide species and a 29-line EPR signal typical of a Mn^IIMn^III entity. Mn K-edge X-ray absorption near-edge spectroscopy (XANES) suggested a formal oxidation state change of Mn^II₂ in 1 to Mn^IIMn^III for 2. Electrospray ionisation mass spectrometry (ESI-MS) suggested 2 to be a Mn^IIMn^III-peroxide complex. 2 was capable of oxidizing ferrocene and weak O−H bonds upon activation with proton donors. Our findings provide support for the postulated mechanism of O₂^.− activation at class I b Mn₂ RNRs.