Improvement of the Hydrolytic Stability of the Keggin Molybdo- and Tungsto-Phosphate Anions by Cyclodextrins.

Abstract

Keggin-type molybdo- and tungsto-phosphate polyoxoanions are among the most popular polyoxometalates (POMs) but suffer from their limited stability at low pH in aqueous solution. Their superchaotropic properties generate strong supramolecular complexes with cyclodextrins (CDs), which significantly affect the hydrolytic stability of POM. This chaotropically driven stabilization effect was systematically monitored by 31P NMR spectroscopy covering a wide range of pH (from 0 to 8) and varying the nature of the CD (α-, β-, and γ-form). A shift of ca. two pH units of the stability domains of these POMs was found in the presence of two equivalents of γ-CD compared to pure water, leading to keep intact the PW12O403- anion without any decomposition up to pH 3.5 (versus 1.5 in pure water) and pH 2.5 for PMo12O403-, which begins to decompose even at pH 0 in pure water. The effect of the smaller CDs (α- and β-form) is much less pronounced (only 0.5 pH units shift of the stability domain) confirming the importance of host-guest size matching to form a sandwich-type inclusion complex and thus protect the POM structure against basic hydrolysis. Such complexation was further supported by 183W and 1H NMR spectroscopy. Finally, using quantitative 31P NMR analyses, the new speciation and formation constants of phospho-molybdates and phospho-tungstates in the presence of cyclodextrins are determined and compared to those previously reported in pure water or in the 50:50 water/1,4-dioxane mixture.