<i>N,N</i>-Coordinated π Radical Anions of <i>S</i>-Methyl-1-phenyl-isothiosemicarbazide in Two Five-Coordinate Ferric Complexes [Fe^III(L^Me•)₂X] (X = CH₃S^-, Cl^-)

The reaction between [FeIII(dmf)6](ClO4)3 and the ligand S-methyl-1-phenyl-isothiosemicarbazide, H2[LMe], and triethylamine (1:3:6) in methanol under an argon blanketing atmosphere at elevated temperatures (reflux) yields a purple solution from which upon cooling to 20 °C dark green crystals of [FeIII(LMe•)2(SCH3)] (1) were obtained in 15% yield. From a similar reaction mixture using FeCl3 as starting material in the solvent acetone under anaerobic conditions at −80 °C, dark green crystals of [FeIII(LMe•)2Cl] (2) were obtained in 21% yield. The structures of complexes 1 and 2 have been determined by single-crystal X-ray crystallography at 100 K. Both complexes are five-coordinate square base pyramidal ferric species containing two N,N-coordinated, monoanionic π radicals, (LMe•)1-, of the parent S-methyl-1-phenyl-isothiosemicarbazide(2−) dianion in the basal positions whereas the axial position is occupied by methylthiolate in 1 and chloride in 2, respectively. The electronic structure of both species has been elucidated by their electronic spectra, magnetic properties, and X-band EPR and Mössbauer spectra. Both possess an St = 1/2 ground state which is attained via an antiferromagnetic coupling between the spins of an intermediate spin ferric ion (SFe = 3/2) and two ligand π radical anions (Srad = 1/2).