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

The speciation of chlorozincate(II) ionic liquids, prepared by mixing 1-octyl-3-methylirnidazolium chloride, [C(8)mim]Cl, and zinc(II) chloride in various molar ratios, chi zncl(2), 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(H) ionic liquids with [C(2)mim](+) cations (chi zncl(2) = 0.33 and chi zncl(2) = 0.50), crystals have been obtained, revealing the structures of [C(2)mim)(2)[ZnCl4] and [C(2)mim](2)[Zn2Cl6] forming three-dimensional hydrogen-bond networks. The compound [C(2)mim](2){Zn4Cl10} was crystallized from the chi zncl(1) = 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.