2,5-Dichloro-1-(ROSO2)benzene [R = C6H5, C6F5, and CH2(CF2)4H]: Synthesis, molecular structure, and solubility in supercritical CO2

Highly crystalline phenyl 2,5-dichlorobenzenesulfonate (PDBS, Tmelt = 86–87 °C) and pentafluorophenyl 2,5-dichlorobenzenesulfonate (FPDBS, Tmelt = 120–122 °C) were synthesized. Single-crystal X-ray molecular structure determinations show that both compounds have similar three-dimensional molecular structures; however, PDBS crystals are thin platelets and FPDBS crystals form hexagonal tube-like structures that are predominately hollow at one end. PDBS crystals exhibit offset π-stacking of the phenoxy-rings that form complete two-dimensional layers each two molecules thick. Hydrogen-bonding interactions are calculated at ∼3.2 Å between the C6-hydrogen and the sulfonyl-oxygen of a neighboring molecule. On the other hand, for FPDBS, π-stacking of the dichloro-substituted ring as well as dipole–dipole interactions of the fluorinated-phenoxy rings appears to be the predominate intermolecular interactions. Neither structure exhibits any kind of side-on interaction of the phenyl rings. PDBS and FPDBS exhibit melting point depressions of 26 and 40 °C, respectively, in the presence of supercritical CO2. Although both sulfonates exhibit high solubility in CO2, much lower pressures are needed to dissolve FPDBS compared to PDBS. For example, at 100 °C FPDBS dissolves at 4750 psia and PDBS dissolves at 11,000 psia. The solubility data reinforce the observation that fluorinating a compound can significantly lower the conditions needed to dissolve that compound in CO2.

Substitution of fluorine in the phenoxy ring of 2,5-dichlorobenzenesulfonates affords crystals of very different morphologies and behavior in supercritical CO2. With R = phenyl, mica-like crystals are produced with complete two-dimensional planes in the crystal defined by π-stacking of the phenoxy-rings. Fluorination induces a more helical-like packing and hollowed out crystals as shown to the right. Solubility in supercritical CO2 for the latter compound is significantly increased. Figure optionsDownload as PowerPoint slide