Thermodynamics of the ion association of potassium cobalt(III) bis (1,2-dicarbollide) in acetonitrile, methanol and dimethyl sulfoxide

The molar conductivities, Lambda, of dilute solutions of potassium cobalt(III) bis(1,2-dicarbollide) (K[COSAN]) in methanol (MeOH), acetonitrile (MeCN) and dimethyl sulfoxide (DMSO) were determined at different temperature ranges. The data were analysed in the frame of Barthel's low-concentration chemical model (lcCM) to obtain the limiting molar conductivities,Lambda infinity, and standard association constants, KoA, of K[COSAN]. To confirm the ion association of oppositely charged ions K+ and [COSAN]-, molecular dynamics (MD) simulations were performed, in which the values of KoA from the potential of the mean force between the pair of particles (ions) were determined. From temperature dependence of KoA, the thermodynamic parameters for ion association, namely standard free enthalpy Delta GoA, enthalpy Delta HoA, and entropy Delta SoA, were estimated. The dilution enthalpies of concentrated solutions of K[COSAN] in organic solvents were also determined by using isothermal titration calorimetry (ITC). Our experimental results show that K+ and [COSAN]- ions form ion pairs in all the investigated solvents. Ion association is most pronounced in MeOH and MeCN and the least expressed in DMSO, the process is endothermic and is accompanied by an increase in entropy. The agreement between the calculated values of KoA and the thermodynamic parameters obtained by conductivity and MD simulations follow the same trend.