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

A novel redox‐active ligand, H 4 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 [H 2 Ph2S L AP K 2 (thf)] 4 ( 2 ), which reacted with FeCl 3 to yield iron(III) complex [H 2 Ph2S L AP FeCl] ( 3 ). Employing Fe[N(SiMe 3 ) 2 ] 3 for a direct reaction with 1 led to ligand rearrangement through CS bond cleavage and thiolate formation, finally yielding [HL AP Fe] ( 5 ). Upon exposure to O 2 , 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 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.