Solubility of β-diketonate complexes for cobalt(III) and chromium(III) in supercritical carbon dioxide

The solubilities of tris(2,2,6,6-tetramethyl-3,5-heptanedionate) cobalt(III) (Co(thd)3) and chromium(III) (Cr(thd)3) in supercritical carbon dioxide (scCO2) were measured at temperatures ranging from 313 to 343 K. The measurements were carried out using a circulation-type apparatus with a UV–vis spectrometer. The solubilities of both Co(thd)3 and Cr(thd)3 increased as both the density of scCO2 and the temperature increased, which has the same tendency as cobalt(III) acetylacetonate (Co(acac)3) and chromium(III) acetylacetonate (Cr(acac)3) had in our previous work. The solubilities of Cr(thd)3 were higher than that of Co(thd)3, and the solubilities of Co(thd)3 and Cr(thd)3 were about 50- and 70-fold higher than those of Co(acac)3 and Cr(acac)3, respectively. The measured solubilities of the metal complexes were correlated with the equation based on Chrastil's equation. The parameters were determined by correlating the experimental data for each metal complex, and the correlated results well reproduced the experimental data, especially Co(thd)3. Moreover, the charge density distributions on the molecular surface of CO2 and the metal complexes used in the measurement were estimated by the quantum chemical calculation and the COSMO-RS to clear the effect of the molecular structure of the metal complexes on the affinity for CO2.