Mimicking Class Ib 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 (Mn 2 ) active site in class Ib RNRs that requires superoxide anion (O 2 • − ), rather than dioxygen (O 2 ), to access a high-valent Mn 2 oxidant. Complex 1 ([Mn 2 (O 2 CCH 3 )(N-Et-HPTB)](ClO 4 ) 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 O 2 • − 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 II Mn III entity. Mn K-edge X-ray absorption near-edge spectroscopy (XANES) suggested a formal oxidation state change of Mn II 2 in 1 to Mn II Mn III for 2 . Electrospray ionisation mass spectrometry (ESI-MS) suggested 2 to be a Mn II Mn 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 O 2 • − activation at class Ib Mn 2 RNRs.