Results and Discussion - New Synthetic Routes to Polyoxometalate Containing Ionic Liquids - page 24

Scheme 2 Reaction equation for the formation of the precursor [ D -SiW 11 O 39 ] 8- from an acidified solution
containing silicate and tungstate in a stoichiometric molar ratio of 1:11.
Isolation of the cluster in form of its potassium salt is done by salting out the reaction
mixture with KCl, prior to which the solution should be filtered or centrifuged once in
order to remove solid traces of insoluble paratungstate. Washing the product with cool
water removes remaining KCl. Nevertheless the product should be recrystallized once or
twice from warm water to ensure that the precursor is as pure as possible, because
impurities at this stage lead to wrong molar ratios when the cluster is converted into a
ionic liquid in following synthetic steps. Work-up of the final POM-IL is practically not
possible as it can´t be distilled or recrystallized.
An important factor for stability plays the pH that should be controlled with a precision
of about 0.3 units on a calibrated pH meter. The potassium salt of the
undecatungstosilicate is stable in aqueous solution between pH 4.5 to 7.0. When the pH
increases, hydrolytic cleavage of W-O bonds occurs. [56] It may well be reasoned that
mechanically first the [SiW 12 O 40 ] 4- is formed which upon the influence of the acidic pH is
converted into its monolacunary polyanion [SiW 11 O 39 ] 8- . Other well defined lacunary
polyanions like the decatungstosilicate or nonatungstosilicate can be obtained in a
similar fashion by choosing a pH in the right range. [56]
Interconversion of the dodecatungstosilicate and its various lacunary polyanions is
possible and occurs readily. As these ions differ only slightly in their composition
elemental analysis is not a reliable criterion for purity and it is more convenient to
characterize the product by its physiochemical properties like its vibrational spectrum
(see chapter 3.1.3.1).
Incorporation of a d-element into the void of the flexible framework of the {W 11 } cluster
is relatively straight forward. The lacuna may accommodate cations of widely varying
size ranging from 53 pm (AI 3+ ) to at least 88 pm (Tl 3+ ) ionic radius [59] which is an
important prerequisite for the aim of this work to vary the lacunary heteroatom. The