Validation of Speciation Techniques: A Study of Chlorozincate(II) Ionic Liquids

The speciation of chlorozincate(II) ionic liquids, prepared by mixing 1-octyl-3-methylimidazolium chloride, [C8mim]Cl, and zinc(II) chloride in various molar ratios, χZnCl2, was investigated using Raman spectroscopy and differential scanning calorimetry; the Gutmann acceptor number, which is a quantitative measure of Lewis acidity, was also determined as a function of the composition. These results were combined with literature data to define the anionic speciation; in the neat liquid phase, the existence of Cl–, [ZnCl4]2–, [Zn2Cl6]2–, [Zn3Cl8]2–, and [Zn4Cl10]2– anions was confirmed. From two chlorozincate(II) ionic liquids with [C2mim]+ cations (χZnCl2 = 0.33 and χZnCl2 = 0.50), crystals have been obtained, revealing the structures of [C2mim]2[ZnCl4] and [C2mim]2[Zn2Cl6] forming three-dimensional hydrogen-bond networks. The compound [C2mim]2{Zn4Cl10} was crystallized from the χZnCl2 = 0.75 composition, showing an open-framework structure, with the first example of zinc in a trigonal-bipyramidal chloride coordination. Reinvestigation of the electrospray ionization mass spectrometry of these systems demonstrated that it is an unreliable technique to study liquid-phase speciation.