A fascinating discovery in the chemistry of ribonucleotide reductases (RNRs) has been the identification of a dimanganese (Mn ) active site in class I b RNRs that requires superoxide anion (O ), rather than dioxygen (O ), to access a high-valent Mn oxidant. Complex 1 ([Mn (O CCH )(N-Et-HPTB)](ClO ) , 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 O at -40 °C resulting in the formation of a metastable species (2). 2 displayed electronic absorption features (λ =460, 610 nm) typical of a Mn-peroxide species and a 29-line EPR signal typical of a Mn Mn entity. Mn K-edge X-ray absorption near-edge spectroscopy (XANES) suggested a formal oxidation state change of Mn in 1 to Mn Mn for 2. Electrospray ionisation mass spectrometry (ESI-MS) suggested 2 to be a Mn Mn -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 O activation at class I b Mn RNRs.
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