Solubility of two metal-organic ruthenium precursors in supercritical CO2 and their application in supercritical fluid technology

The solubility of ruthenium(III) acetylacetonate [Ru(acac)3] and bis(2,2,6,6-tetramethyl-3,5-heptanedionato)(1,5-cyclooctadiene) ruthenium(II) [Ru(tmhd)2(COD)] in supercritical CO2 (scCO2) was measured using a high-pressure variable volume view cell between the temperatures 313 K and 353 K and pressures up to 20.0 MPa. Both compounds have been proposed as precursors in the synthesis of ruthenium containing materials using scCO2, therefore the need of accurate solubility data. The use of scCO2 as the solvent in metallization processes yields materials of exceptional properties and, at the same time, represents a sustainable alternative to conventional techniques thus bringing environmental and economic benefits. These metal-organic compounds exhibit different solubility behaviour in scCO2, being the solubility of Ru(tmhd)2(COD) at the same pressure and temperature almost five times larger than that of Ru(acac)3. The dissolution rate is also much faster for Ru(tmhd)2(COD). The shielding of the metal centre by the bulkier ligands in Ru(tmhd)2(COD) favours dissolution of this compound in scCO2. At the conditions of this study, Ru(tmhd)2(COD) mole fraction solubility values vary from 2 × 10−4 to 9 × 10−4 whilst those for Ru(acac)3 vary from 2 × 10−5 to 13 × 10−5. For a given temperature, the solubility increases with pressure due to the higher density of the solvent. At constant pressure, the solubility decreases with temperature due to the decrease in density of the fluid. The crossover phenomenon was not observed in the pressure range studied in both ruthenium precursors. Solubility data were satisfactorily correlated to semi-empirical equations which allow the interpolation of solubility data at different conditions.

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► Solubilities of Ru(acac)3 and Ru(tmhd)2(COD) in supercritical CO2 are reported between T = 313 K and T = 353 K up to P = 20.0 MPa.
► Their application in clean metallization processes using supercritical CO2 is reviewed.
► Ru(tmhd)2(COD) solubility is large due to the good shielding of the metal centre by the bulky ligands.
► Ru(acac)3 is five times less soluble than Ru(tmhd)2(COD) and its dissolution rate is very slow.
► Data are satisfactorily correlated using the Chrastil and Mendez-Santiago–Teja models.