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, [Fe^II₂(LS)₂] (LS^2- = 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^II₂(LS)(LSH)]⁺. When reaction with O₂ occurs in the absence of protons or in the presence of 1 equiv of proton (i.e., from [Fe^II₂(LS)(LSH)]⁺), unsupported μ-oxo or μ-hydroxo Fe^III dinuclear complexes ([Fe^III₂(LS)₂O] and [Fe^III₂(LS)₂(OH)]⁺, respectively) are generated. [Fe^III₂(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. Density functional theory calculations provide insight into the formation of the μ-oxo or μ-hydroxo Fe^III dimers, suggesting that a dinuclear μ-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.