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 O-2 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, [Fe-2(II)(LS)(2)] (LS2- = 2,2'-(2,2'-bipyridine-6,6'-iyl)bis(1,1-diphenylethanethiolate)), and its reactivity with dioxygen, in comparison with its previously reported protonated counterpart, [Fe-2(II)(LS)(LSH)](+). When reaction with O-2 occurs in the absence of protons or in the presence of 1 equiv of proton (i.e., from [Fe-2(II)(LS)(LSH)](+)), unsupported mu-oxo or mu-hydroxo Fe-III dinuclear complexes ([Fe-2(III)(LS)(2)O] and [Fe-2(III)(LS)(2)(OH)](+), respectively) are generated. [Fe-2(III)(LS)(2)O], reported previously but isolated here for the first time from O-2 activation, is characterized by single crystal X-ray diffraction and Mossbauer, 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. Density functional theory calculations provide insight into the formation of the mu-oxo or mu-hydroxo Fe-III dimers, suggesting that a dinuclear mu-peroxo Fe-III 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.