Spectrophotometric and thermodynamic studies of the charge transfer complexation of nitroimidazoles with chloranilic acid following metal hydride reduction

The spectrophotometric and thermodynamic properties of the charge-transfer (CT) complexation formed between reduced nitroimidazoles and chloranilic acid has been carried out. The possibility of room temperature reduction of metronidazole and tinidazole via hydride transfer using LiBH4 and NaBH4 was examined and the physicochemical properties of the resulting complex of the reduced nitroimidazoles with chloranilic acid were studied. The energy changes accompanying the formation of the CT band were studied over four temperature ranges of 30, 50, 60 and 80°C. The formation constants and molar absorptivities over these temperature ranges were obtained using the Benesi-Hildebrand plot. Other physicochemical parameters such as energy of transition, transition dipole, oscillator frequency and ionization energies and thermodynamic properties were estimated and related to the stability of the formed charge-transfer band. Reduction of nitroimidazoles with metal hydrides was completed at room temperature within 10 min with the formation of purple-coloured solution with chloranilic acid (CAA). The transition energies were of the order of 2.303 eV with ionization energy of 6.054 eV for the complexes. Variation existed in the oscillator frequency and transition dipole of the formed complexes with higher values obtained for MZ-CAA (metronidazole- chloranilic acid) complex. The Gibbs energy and entropy varied with the temperature and room temperature values favoured formation of stable complexes. The superiority of the metal hydride reduction was clearly evident from well defined physicochemical properties of the CT band and this is the first assessment of the utilization of the room temperature metal hydride reduction of nitroimidazoles.   Key words: Nitroimidazoles, metal-hydride reduction, physico-chemical studies, thermodynamic studies.