Inclusion complexes of triphenylphosphine derivatives and peracetylated-β-cyclodextrin in supercritical carbon dioxide

Authors

Edward C. Navarre

Alessandro Galia

Onofrio Scialdone

Giuseppe Filardo

Eric Monflier

Published

January 1, 2007

Abstract
Viewed as an alternative solvent system for industrial chemical syntheses, supercritical carbon dioxide (scCO2) presents many opportunities and challenges in its application. The insufficient solubility of common organometallic catalysts impedes the use of scCO2 for many reactions employing homogeneous catalysis. In practice, limited solubility is a common problem for a wide range of solutes in scCO2 and is often resolved by modifying the compound to make it more CO2-philic. We have demonstrated an enhanced solubility of triarylphosphines in scCO2 using a host-guest interaction between the phosphine and peracetylated-β-cyclodextrin (perAc-β-CD) [G. Filardo, M. Di Blasi, A. Galia, A. Ponchel; H. Bricout, A. D. Sayede, E. Monflier, J. Supercrit. Fluids, 173 (2006)]. Considering the high solubility of perAc-β-CD in scCO2, it presents a means to increase the solubility of organometallic catalysts without changing the structure of the catalyst and potentially altering its activity. For this approach to be practical, the inclusion constant (K) should be large, for example in similar aqueous systems K is often 103 to 105. In this work, the supramolecular chemistry of perAc-β-CD as a host for triphenyphosphine derivatives has been studied in scCO2 using UV absorption spectroscopy. It was found that the inclusion constant in scCO2 at 40 °C and 300 bar is 10 to 1000 times smaller compared to analogous systems in aqueous solvent. This striking difference is attributed to the absence of the hydrophobic effect in scCO2 due to the much smaller polarity of scCO2 versus water. To further explore the effect of the solvent on the inclusion constant, values of solvent polarity intermediate between the limits of scCO2 and water are needed. To this end, methanol is added to the scCO2 as a cosolvent to modify the overall polarity. By taking advantage of the tunability of the supercritical phase, a wide range of solvent polarity can be accessed by varying the temperature (35 to 50 °C), pressure (100 to 400 bar), and mole fraction of methanol. This study is pursued with the goal of enhancing the phosphine–perAc-β-CD inclusion constant to make it comparable to those observed in aqueous systems.