A Novel Pentadentate Redox-Active Ligand and Its Iron(III) Complexes: Electronic Structures and O-2 Reactivity

A novel redox-active ligand, (H4LAP)-L-Ph2S (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 [(H2LK2)-L-Ph2S-K-AP(thf)](4) (2), which reacted with FeCl3 to yield iron(III) complex [(H2LFeCl)-L-Ph2S-Fe-AP] (3). Employing Fe[N(SiMe3)(2)](3) for a direct reaction with 1 led to ligand rearrangement through CS bond cleavage and thiolate formation, finally yielding [(HLFe)-Fe-AP] (5). Upon exposure to O-2, 3 and 5 are oxidized through formal hydrogen-atom abstraction from the ligand NH units to form [(LFeCl)-L-Ph2S-Fe-SQ] (4) and [(LFe)-Fe-SQ] (6) featuring two or one coordinated iminosemiquinone moieties, respectively. Mossbauer 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 O-2. It turned out that the iron-catecholate complexes react with O-2 in dichloromethane at ambient conditions through CC 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.