Analysis of the association of cobalt(III) chelates with fluorine-containing phenols in supercritical carbon dioxide.

The solubilities of several cobalt(III) chelates (CoL3) with in supercritical carbon dioxide (SC-CO2) were investigated in the presence of fluorine- and trifluoromethyl-substituted phenols (PhOH) using UV-vis spectrophotometry. Solubility enhancement of CoL3 complexes in SC-CO2 was accomplished by adding PhOH. The association constants for tris(pentane-2,4-dionato)cobalt(III) (Co(acac)3) with various PhOH in SC-CO2 were determined from the relationship between the solubility enhancement and the PhOH concentration for each corresponding phenol. For compounds with no fluorine or trifluoromethyl groups in the ortho position, the association constants decreased as the pKa values of the PhOH compounds increased. In addition, the association constants for the PhOH compounds bearing a fluorine or trifluoromethyl group in the ortho position were less than the values expected based on their pKa values. These results are ascribed to the occurrence of the steric hindrance of the ortho-fluorine or trifluoromethyl group. Furthermore, the association constants for various CoL3 complexes and 3,5-bis(trifluoromethyl)phenol (BTMP) increased as the basicity of the chelating ligand increased. Finally, the thermodynamic parameters and the numbers of CO2 molecules released upon association of Co(acac)3 and BTMP were calculated using data for the dependence of the association constant on the pressure and temperature.