Crystal forms of highly “dynamic” 18-crown[6] complexes with M[HSO4] and M[H2PO4] (M+ = NH4+, Rb+, Cs+): thermal behaviour and solid-state preparation

Abstract

The hydrated complexes 18-crown[6]·[NH₄][H₂PO₄]_0.5[HSO₄]_0.5·H₂O (1), 18-crown[6]·Rb[H₂PO₄]_0.5[HSO₄]_0.5·3H₂O (2), 18-crown[6]·Cs[H₂PO₄]_0.5[HSO₄]_0.5·3H₂O (3) and 18-crown[6]·Cs[HSO₄]·2H₂O (4) have been quantitatively prepared by the mechanical mixing of crystalline 18-crown[6], and M[H₂PO₄] and/or M[HSO₄] inorganic salts. The structures of the four complexes have been determined from single crystals, obtained by recrystallization of the products from water. On heating the complexes to ca. 70 °C, water loss takes place. The dehydration processes have been investigated by DSC, TGA and variable temperature X-ray powder diffraction; in all of the compounds, water loss leads to the formation of their corresponding anhydrous phases. On further heating of complexes 1 and 2 to 120 °C, decomplexation is observed, with formation of liquid crown ether and H₃PO₄, as well as crystalline [NH₄]₂[SO₄] and Rb₂[SO₄], while on heating compound 3 to 140 °C, decomplexation is observed, with formation of liquid crown ether and crystalline Cs[H₂PO₄]_0.5[HSO₄]_0.5. On heating complex 4, the loss of water is followed by a reversible phase transition, that is very likely to be associated with the onset of a solid-state dynamic process, followed at 145 °C by the extrusion of the crown ether from the complex, with formation of crystalline Cs[HSO₄].