Di- and Tetrairon(III) mu-Oxido Complexes of an N3S-Donor Ligand : Catalyst Precursors for Alkene Oxidations

Abstract

The new di- and tetranuclear Fe(III) mu-oxido complexes [Fe-4(mu-O)(4) (PTEBIA)(4)]CF3SO3)(4)(CH3CN)(2)] (1a) , [Fe-2(mu-O)Cl-2(PTEBIA)(2)(CF3SO3)(2) (1b), and [Fe-2(mu-O)(HCOO)(2)(PTEBIA)(2)](ClO4)(2) (MeOH) (2) were prepared from the sulfur-containing ligand (2-((2,4-dimethylphenyl)thiO)-N,N-bis ((1-methyl-benzimidazol-2-yl)methyl)ethanamine (PTEBIA). The tetrairon complex 1a features four mu-oxido bridges, while in dinuclear 1b, the sulfur moiety of the ligand occupies one of the six coordination sites of each Fe(III) ion with a long Fe-S distance of 2.814(6) angstrom . In 2, two Fe(III) centers are bridged by one oxido and two formate units, the latter likely formed by methanol oxidation. Complexes 1a and 1 b show broad sulfur-toiron charge transfer bands around 400-430 nm at room temperature, consistent with mononuclear structures featuring Fe-S interactions. In contrast, acetonitrile solutions of 2 display a sulfur-to-iron charge transfer band only at low temperature (228 K) upon addition of H2O2/CH3COOH, with an absorption maximum at 410 nm. Homogeneous oxidative catalytic activity was observed for 1a and 1b using H2O2 as oxidant, but with low product selectivity. High valent iron-oxo intermediates could not be detected by UV-vis spectroscopy or ESI mass spectrometry. Rather, evidence suggest preferential ligand oxidation, in line with the relatively low selectivity and catalytic activity observed in the reactions.