Studies on cobalt-tris(bipyridyl) complexes in acetonitrile–water mixtures

The Gibbs energies of transfer, ΔtG°, for [Co(bipy)3]3+ and [Co(bipy)3]2+ in the full range of acetonitrile–water mixtures were derived from solubility measurements of the picrates and partitioned into single-ion contributions based on the tetraphenylarsonium tetraphenylborate assumption. The enthalpies of solution were obtained from calorimetry of the perchlorates of [Co(bipy)3]3+ and [Co(bipy)3]2+. The single-ion transfer entropies were calculated ia the tetraphenylarsonium tetraphenylborate assumption. In addition polarography, cyclic voltammetry and UV–visible spectroscopy were also performed on these complexes in water–acetonitrile mixtures. Gibbs energies of transfer of [Co(bipy)3]+ were derived from the ΔtG° values for [Co(bipy)3]2+ and from electrochemical measurements. The TΔtS° values for [Co(bipy)3]3+ and[Co(bipy)3]2+ were calculated from the enthalpies and the Gibbs energies of transfer. The TΔtS° values of [Co(bipy)3]+ were obtained from the respective data for [Co(bipy)3]2+ and from the assumption of a negligible thermal diffusion potential. The Gibbs energies of transfer are accounted for in terms of hydrogen–hydrogen interactions between the methyl group of acetonitrile and the hydrogen atoms of the ligands of the studied complexes. The interpretations of TΔtS° values and the enthalpies of transfer were based on different regions for the structure of acetonitrile–water mixtures and on the possibility of the Co–bipy cations entering cavities in the water structure of water-rich mixtures.