Reinvestigation of the mechanism of dioxygen activation at a MnII (cyclam) center

This study deals with the unprecedented reactivity of a [(cyclam)MnII(OTf)2] (3-cis; OTf = CF3SO3−) with O2, which, depending on the presence or absence of a hydrogen atom donor like 1-hydroxy-2,2,6,6-tetramethyl-piperidine (TEMPO-H), selectively generates di-μ-oxo Mn(III)Mn(IV) (1) or MnIV2 (2) complexes, respectively. Both dimers have been characterized by different techniques including single-crystal X-ray diffraction, X-ray absorption spectroscopy, and electron paramagnetic resonance. Oxygenation reactions carried out with labeled 18O2 and Resonance Raman spectroscopy unambiguously show that the oxygen atoms present in the MnIVMnIII dimer originate from O2. Experimental evidences are provided for a novel method of dioxygen activation involving three Mn ions or two Mn ions and TEMPO-H to generate the bis(μ-oxo)dimanganese(IV) or bis(μ-oxo) dimanganese(III, IV) cores, respectively.