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

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 P-31 NMR spectroscopy covering a wide range of pH (from 0 to 8) and varying the nature of the CD (alpha-, beta-, and gamma-form). A shift of ca. two pH units of the stability domains of these POMs was found in the presence of two equivalents of gamma-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 (alpha- and beta-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 W-183 and H-1 NMR spectroscopy. Finally, using quantitative P-31 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.

Automated summary

Keggin-type molybdo- and tungsto-phosphate polyoxoanions suffer from limited stability at low pH, but their stability can be improved by forming supramolecular complexes with cyclodextrins (CDs). Gamma-CD shows the strongest stabilization effect on these polyoxoanions, allowing them to remain intact at higher pH values. In addition, the stability enhancement of the complexes formed with smaller alpha- and beta-CDs is less pronounced.