A novel pentadentate redox-active ligand and its iron(III) complexes: electronic structures and O₂ reactivity

A novel redox-active ligand, H4(Ph2S)L(AP) (1) which was designed to be potentially pentadentate with an O,N,S,N,O donor set is described. Treatment of 1 with two equivalents of potassium hydride gave access to octametallic precursor complex [H2(Ph2S)L(AP)K2(thf)]4 (2), which reacted with FeCl3 to yield iron(III) complex [H2(Ph2S)L(AP)FeCl] (3). Employing Fe[N(SiMe3)2]3 for a direct reaction with 1 led to ligand rearrangement through C-S bond cleavage and thiolate formation, finally yielding [HL(AP)Fe] (5). Upon exposure to O2, 3 and 5 are oxidized through formal hydrogen-atom abstraction from the ligand NH units to form [(Ph2S)L(SQ)FeCl] (4) and [L(SQ)Fe] (6) featuring two or one coordinated iminosemiquinone moieties, respectively. Mössbauer measurements demonstrated that the iron centers remain in their +III oxidation states. Compounds 3 and 5 were tested with respect to their potential as models for the catechol dioxygenase. Thus, they were treated with 3,5-di-tert-butyl-catechol, triethylamine and O2. It turned out that the iron-catecholate complexes react with O2 in dichloromethane at ambient conditions through C-C bond cleavage mainly forming extradiol cleavage products. Intradiol products are only side products and quinone formation becomes negligible. This observation has been rationalized by a dissociation of two donor functions upon coordination of the catecholate.