Ionic liquid Keggin polyoxometallates with the tris(dihexylamino)cyclopropenium cation

Abstract

Five new Keggin polyoxometalate (POM) salts were prepared with the tris(dihexylamino)cyclopropenium cation [C3(NHex2)3]+ in an effort to investigate and understand the impact of combining triaminocyclopropenium (TAC) cations with POMs on their thermochemical and thermophysical properties. The tri-anions [PM12O40]3– (M = Mo, W) have high melting salts (Tm = 140 and 129 °C, respectively), whereas the tetra-anions [PVMo11O40]4– and [HPV2M10O40]4– (M = Mo, W), despite the increased electrostatic charge, have lower and very similar melting points (Tm ∼ 82 °C). The solid-state structures are all very similar, however, the tetra-anionic salts have a double layer of cations (compared to a single layer for the tri-anionic salts) and the electrostatic repulsion between these layers is likely responsible for the lower and similar melting points, as well as their increased solubility in non-polar solvents. The salts were also characterised by TGA, DSC, solubility studies, X-ray crystallography, NMR and infrared spectroscopy.