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Hofmeister effect in the Keggin-type polyoxotungstate series

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Abstract

The chaotropic character of Keggin-type polyoxotungstate anions was evaluated with respect to their ability to bind to γ-cyclodextrin (γ-CD) by varying the global charge density of the nanometer-sized polyanion. The strengths of the host–guest association were analyzed within the series of isostructural [XW12O40]n anions where the ionic charge varies from 6- to 3- depending on the heteroatom, respectively, X = H22+, B3+, Si4+ or P5+. Titration experiments using complementary techniques (ITC, DOSY NMR, and electrochemistry) revealed that the affinity between γ-CD and polyoxometalates (POMs) is directly correlated to the charge density of the Keggin anion as reflected in the values of the binding constants K1:1. These constants increase dramatically following the order: [H2W12O40]6− < [BW12O40]5− < [SiW12O40]4− < [PW12O40]3−. Additionally, cloud point experiments on a non-ionic surfactant resulted in the same series of POMs, emphasizing the general affinity of these inorganic Keggin-ions to non-ionic organic soft matter due to a general solvent effect arising from the weakening of the hydration sphere with decreasing ionic charge. Furthermore, single crystal X-ray diffraction analysis showed distinct organizations of the POMs with γ-CD in the solid-state, where the moderate chaotrope [BW12O40]5− interacts with the external wall of the γ-CD, while on the other side of the series, [PW12O40]3− penetrates deeply into the cavity of the γ-CD through its secondary rim offering optimal contact area. Finally, our investigations revealed the unique behavior of [PW12O40]3−, which displayed not only the highest affinity of the POMs to γ-CD (K1:1 > 105 M−1), but also the ability to interact with both CD faces resulting in a wide variety of supramolecular aggregates.

Graphical abstract: Hofmeister effect in the Keggin-type polyoxotungstate series

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Supplementary files

Article information


Submitted
27 Jul 2020
Accepted
12 Sep 2020
First published
18 Sep 2020

Inorg. Chem. Front., 2020, Advance Article
Article type
Research Article

Hofmeister effect in the Keggin-type polyoxotungstate series

S. Yao, C. Falaise, A. A. Ivanov, N. Leclerc, M. Hohenschutz, M. Haouas, D. Landy, M. A. Shestopalov, P. Bauduin and E. Cadot, Inorg. Chem. Front., 2020, Advance Article , DOI: 10.1039/D0QI00902D

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