O2 activation by non-heme thiolate-based dinuclear Fe complexes

Iron centers featuring thiolates in their metal coordination sphere (as ligands or substrates) are well known to activate dioxygen. Both heme and non-heme centers that contain iron-thiolate bonds are found in nature. Investigating the ability of iron-thiolate model complexes to activate O2 is expected to improve the understanding of the key factors that direct reactivity to either iron or sulfur. We report here the structural and redox properties of a thiolate-based dinuclear Fe complex, [FeII2(LS)2], (LS2- = 2,2’-(2,2’-bipyridine-6,6’-iyl)bis(1,1diphenylethanethiolate) and its reactivity with dioxygen, in comparison with its previously reported protonated counterpart, [FeII2(LS)(LSH)]+. When reaction with O2 occurs in the absence of protons or in the presence of one equivalent of proton (i.e. from [FeII2(LS)(LSH)]+), unsupported μ-oxo or μ-hydroxo FeIII dinuclear complexes ([FeIII2(LS)2O] and [FeIII2(LS)2(OH)]+, respectively) are generated. [FeIII2(LS)2O], reported previously but isolated here for the first time from O2 activation, is characterized by single crystal X-ray diffraction and Mössbauer, Resonance Raman and NMR spectroscopies. The addition of protons leads to the release of water and the generation of a mixture of two Fe-based "oxygen-free" species. DFT-calculations provide insight into the formation of the μ-oxo or μ-hydroxo FeIII dimers, predicting that a dinuclear μ-peroxo FeIII intermediate is key to reactivity, and the structure of which changes as a function of protonation state. Compared to previously reported Mn-thiolate analogues, the evolution of the peroxo intermediates to the final products is different and involves a comproportionation vs a dismutation process for the Mn and Fe derivate, respectively.