Chlorostannate(II) Ionic Liquids: Speciation, Lewis Acidity, and Oxidative Stability

The anionic speciation of chlorostannate­(II) ionic liquids,
prepared
by mixing 1-alkyl-3-methylimidazolium chloride and tin­(II) chloride
in various molar ratios, χSnCl2, was investigated
in both solid and liquid states. The room temperature ionic liquids
were investigated by 119Sn NMR spectroscopy, X-ray photoelectron
spectroscopy, and viscometry. Crystalline samples were studied using
Raman spectroscopy, single-crystal X-ray crystallography, and differential
scanning calorimetry. Both liquid and solid systems (crystallized
from the melt) contained [SnCl3]− in
equilibrium with Cl– when χSnCl2 < 0.50, [SnCl3]− in equilibrium
with [Sn2Cl5]− when χSnCl2 > 0.50, and only [SnCl3]− when χSnCl2 = 0.50. Tin­(II)
chloride
was found to precipitate when χSnCl2 >
0.63. No evidence was detected for the existence of [SnCl4]2– across the entire range of χSnCl2, although such anions have been reported in the literature
for chlorostannate­(II) organic salts crystallized from organic solvents.
Furthermore, the Lewis acidity of the chlorostannate­(II)-based systems,
expressed by their Gutmann acceptor number, has been determined as
a function of the composition, χSnCl2,
to reveal Lewis acidity for χSnCl2 >
0.50
samples comparable to the analogous systems based on zinc­(II). A change
of the Lewis basicity of the anion was estimated using 1H NMR spectroscopy, by comparison of the measured chemical shifts
of the C-2 hydrogen in the imidazolium ring. Finally, compositions
containing free chloride anions (χSnCl2 < 0.50) were found to oxidize slowly in air to form a chlorostannate­(IV)
ionic liquid containing the [SnCl6]2– anion.