Direct Observation of Contact Ion-Pair Formation in Aqueous Solution

摘要

The formation of ion pairs in aqueous solution orchestrates vital processes in nature and engineered systems, including electron transfer and other chemical reactions and crystallization/precipitation of synthetic and natural matter. Polyoxometalates (POMs) have received special attention with regard to ion pairing. POMs are highly charged anions with ideal topologies for binding and associating metal cations, as their assembly, redox, and catalytic properties in aqueous solution are influenced by ion pairing. Methods for determining ion pairing are largely considered to be indirect in that they observe physical changes in a solution that results from ion association. Spectroscopic methods are the exception, but these are prone to insufficient resolution, and are unable to detect solvent-separated ion pairing. Herein we show that small-angle X-ray scattering (SAXS) of the Lindqvist Nb6O_(19)~(8-) POM provides a model example of direct measurement of the formation of ion pairs in aqueous solution, and reveals remarkable similarity between solution and solid-state ion contact. This study provides evidence that the structures of Lindqvist hexaniobate salts described with extreme accuracy in the solid state can be used to model the ion-pairing behavior in solution.