Solubilities of cerium(IV), terbium(III), and iron(III) ss-diketonates in supercritical carbon dioxide

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-tetrameth yl-3,5-octanedionate, H(thd), and 2,4-pentanedionate, H(acac), were measure d 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 inc reased in the order acac < thd < tod for all metal beta -diketonates, with Ce(tod)(4), Tb(tod)(3), and Fe(tod)(3) an order of magnitude more soluble t han Ce(thd)4, Tb(thd)(3), and Fe(acac)(3), respectively. For the larger met al ions, Ce4+ and Tb3+, the flexibility of the isobutyl group in the tod li gand creates a more lipophilic shell around the central metal ion, increasi ng the solubility of the chelate in CO2. These observations indicate that t he isobutyl group is at least as CO2-philic as the tert-butyl moiety. The e xperimental solubilities were correlated using the model proposed by Chrast il.