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

The Gibbs energies of transfer, DtGo, 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 via 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 DtGo values for [Co(bipy)3]2+ and from electrochemical measurements. The TDtSo-values for [Co(bipy)3]3+ and[Co(bipy)3]2+ were calculated from the enthalpies and the Gibbs energies of transfer. The TDtSo-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 TDtSo-values and the enthalpies of transfer was 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.