EXAFS Study into the Speciation of Metal Salts Dissolved in Ionic Liquids and Deep Eutectic Solvents

The
speciation of metals in solution controls their reactivity,
and this is extremely pertinent in the area of metal salts dissolved
in ionic liquids. In the current study, the speciation of 25 metal
salts is investigated in four deep eutectic solvents (DESs) and five imidazolium-based
ionic liquids using extended X-ray absorption fine structure. It is
shown that in diol-based DESs MI ions form [MCl2]− and [MCl3]2– complexes,
while all MII ions form [MCl4]2– complexes, with the exception of NiII, which exhibits
a very unusual coordination by glycol molecules. This was also found
in the X-ray crystal structure of the compound [Ni­(phen)2(eg)]­Cl2·2eg (eg = ethylene glycol). In a urea-based
DES, either pure chloro or chloro–oxo coordination is observed.
In [C6mim]­[Cl] pure chloro complexation is also observed,
but coordination numbers are smaller (typically 3), which can be explained
by the long alkyl chain of the cation. In [C2mim]­[SCN]
metal ions are entirely coordinated by thiocyanate, either through
the N or the S atom, depending on the hardness of the metal ion according
to the hard-soft acid-base principle. With weaker coordinating anions,
mixed coordination between solvent and solute anions is observed.
The effect of hydrate or added water on speciation is insignificant
for the diol-based DESs and small in other liquids with intermediate
or strong ligands. One of the main findings of this study is that,
with respect to metal speciation, there is no fundamental difference
between deep eutectic solvents and classic ionic liquids.