Transition metal quinone–thiosemicarbazone complexes 3: Spectroscopic characterizations of spin-mixed iron (III) of naphthoquinone–thiosemicarbazones

An interesting series of iron (III) complexes with naphthoquinone–thiosemicarbazones are synthesized and physico-chemically characterized by elemental analysis, UV–vis, IR, EPR and magnetic susceptibility measurements. They possess a cationic octahedral [FeL2]+ species and a tetrahedral [FeCl4]− anion and exhibit unusual spin-mixed states involving high-spin and low-spin ferric centers as revealed from magnetic behavior with significant amount of exchange interactions mediated by intermolecular associations. The magnetic susceptibility data is fitted with S1=52 and S2=12 Heisengberg's exchange coupled model; Hˆ=−2JS1S2 and the magnetic exchange interactions are found to be of the order of −13.6 cm−1 indicating the moderate coupling between two paramagnetic centers present in different chemical and structural environment. The presence of spin-paired iron (III) cation having dxz2dxz2dxz1 ground state is revealed from the EPR spectra with three prominent peaks while the high-spin tetrahedral iron (III) anion exhibits characteristics g = 4 signal whose intensity increases with lowering the temperature suggesting its influence on the magnetic properties of the complex molecule. FTIR measurements indicate tridentate ONS donor systems involving quinone/hydroxyl oxygen, imine/hydrazinic nitrogen and thione/thiol sulfur atoms as binding sites for naphthoquinone–thiosemicarbazones.