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

This study deals with the unprecedented reactivity of a [(cyclam)Mn^II(OTf)₂] (3-cis; OTf = CF₃SO_3^-) 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 Mn^IV₂ (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 Mn^IVMn^III 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.