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 31 P NMR spectroscopy covering a wide range of pH (from 0 to 8) and varying the nature of the CD (a-, b-, and g-form).A shift of ca. two pH units of the stability domains of these POMs were found in the presence of two eq. g-CD compared to pure water, leading to keep intact the PW12O40 3anion without any decomposition up to pH 3.5 (versus 1.5 in pure water) and pH 2.5 for PMo12O40 3which begins to decompose even at pH 0 in pure water. The effect of the smaller CDs (a-and b-form) is much less pronounced (only 0.5 pH units shift of 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 183 W and 1 H NMR spectroscopy. Finally, using quantitative 31 P NMR analyzes, the new speciation and formation constants of phosphomolybdates and phosphotungstates in the presence of cyclodextrins are determined and compared to those previously reported in pure water or in 50:50 water/1,4-dioxane mixture.

NMR spectroscopy to study cyclodextrin-based host–guest assemblies with polynuclear clusters

“…Fig. 6 51 The evolution of the spectra of γ-CD is very similar to that observed with [PW 12 O 40 ] 3− (see Fig. 5), where H2 and H4 show only a negligible effect by comparison to H3, and to a lesser extent to H5 and H6, experiencing all strong deshielding effects.…”

“…11 shows the effect of presence of α-, β-, and γ-CD on the 183 W NMR of [PW 12 O 40 ] 3− . 51 The Keggin POM [PW 12 O 40 ] 3− is characterized by a single sharp 183 W signal at −95.4 ppm corresponding to the 12 W equivalent nuclei of the POM structure (Fig. 11).…”

{2-Phases 2-reactions 1-catalyst} concept for the sustainable performance of coupled reactions

“…The addition of a base, in contrast, is known to lead to the PW's decomposition as PW is only stable at high acidities ( pH < 2.5). 46,85 The decomposition products of PW (e.g. lacunary POMs, PO 4 3− and WO 4 2− ) exhibit a higher charge density with a higher hydrophilicity and consequently a lower distribution coefficient to hydrophobic alcohols.…”

Stabilization of Octahedral Metal Halide Clusters by Host–Guest Complexation with γ-Cyclodextrin: Toward Nontoxic Luminescent Compounds

“…Native cyclodextrins (CDs) are cheap drug carriers widely used in pharmacology . Recent reports demonstrated that γ-CD, an macrocyclic oligosaccharide built on eight glucopyranose units defining a cavity of 10 Å (Figure ), can host various types of inorganic polynuclear species − and even stabilize hydrolytically fragile polyoxometalates and Fe–S clusters in water. − The chaotropic effect has been identified as the main contributor of such supramolecular associations, where water solvation plays prominent role in stabilizing these types of hybrid complexation. , [{M 6 X 8 }­L 6 ] n +/– octahedral-cluster-based units represent multifunctional nanosized building blocks composed of a robust {M 6 X 8 } 4+ cluster core (M = Mo or W and X = halides) decorated by six interchangeable apical ligands (L) such as halides, H 2 O, or organic ligands (Figure ). These clusters exhibit both bright phosphorescence in the therapeutic window upon photoirradiation or X-ray irradiation and the ability to photoinduce singlet oxygen production. − The photophysical features of [{M 6 Br 8 }­Cl 6 ] 2– and [{M 6 I 8 }­Cl 6 ] 2– cluster-based units make them particularly appealing for biological applications. − Nevertheless, the low stability of the octahedral halide molybdenum or tungsten cluster complexes in aqueous media is a serious obstacle to their use in biomedicine.…”