Mimicking ClassIb Mn2-Ribonucleotide Reductase: A MnII2 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 classIb 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 (λmax=460, 610nm) typical of a Mn-peroxide species and a 29-line EPR signal typical of a MnIIMnIII entity. Mn K-edge X-ray absorption near-edge spectroscopy (XANES) suggested a formal oxidation state change of MnII2 in 1 to MnIIMnIII for 2. Electrospray ionisation mass spectrometry (ESI-MS) suggested 2 to be a MnIIMnIII-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 classIb Mn2 RNRs.