Solubilities of Cerium(IV), Terbium(III), and Iron(III) β-Diketonates in Supercritical Carbon Dioxide

Abstract

The solubilities of the cerium(IV), terbium(III), and iron(III) chelates of the anions of 2,2,7-trimethyl-3,5-octanedionate, H(tod), 2,2,6,6-tetramethyl-3,5-octanedionate, H(thd), and 2,4-pentanedionate, H(acac), were measured in supercritical carbon dioxide by near-infrared spectroscopy from 313 to 333 K and 10 to 35 MPa with a high-pressure optical cell. Solubilities increased in the order acac < thd < tod for all metal β-diketonates, with Ce(tod)4, Tb(tod)3, and Fe(tod)3 an order of magnitude more soluble than Ce(thd)4, Tb(thd)3, and Fe(acac)3, respectively. For the larger metal ions, Ce4+ and Tb3+, the flexibility of the isobutyl group in the tod ligand creates a more lipophilic shell around the central metal ion, increasing the solubility of the chelate in CO2. These observations indicate that the isobutyl group is at least as CO2-philic as the tert-butyl moiety. The experimental solubilities were correlated using the model proposed by Chrastil.