Hofmeister effect in the Keggin-type polyoxotungstate series

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 K_1:1. 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 (K_1:1 > 10⁵ M⁻¹), but also the ability to interact with both CD faces resulting in a wide variety of supramolecular aggregates.

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Introduction

Polyoxometalates (POMs) are a remarkable class of anionic polynuclear metal oxide clusters of transition metal ions like W^VI or Mo^VI.¹ POMs are able to reversibly and massively exchange electrons without structural change, making them promising electro-active subcomponents of hybrid systems relevant for applications in the energy field.² Recent studies highlighted that POMs are an appealing molecular ‘electron-reservoir’ to construct lithium-, sodium-ion and redox flow batteries,^3–7 supercapacitors,⁸ fuel cells^9,10 or catalytic systems^11–14 relevant for sustainable processes. The development of straightforward preparation routes to hybrid materials including POM units through supramolecular contacts has led to unique molecular assemblies with novel properties and reactivity.¹⁵ In context, the exploration of ionic and other non-covalent interactions (hydrogen bonding, van der Waals forces, etc.) between POMs and other types of molecular systems has attracted considerable interest, revealing the remarkable supramolecular behavior of POMs to self-assemble across multiple length scales ranging from discrete systems to gigantic spherical single-layered nanostructures known as “blackberries”.^12,16–20

One of the most striking supramolecular properties of POMs arises from their ability to form in solution resilient non-bonding contacts with neutral surfaces such as organic macrocycles, micelles, proteins or polymers.^21–30 Actually, their solution behavior appears “counter-intuitive” because POMs are anionic inorganic discrete species considered generally as hydrophilic. Others and the authors established that association phenomena between POMs and organic moieties are driven by a solvent mediated effect arising from a water structure recovery upon release of the POM's and the moiety's hydration water.^22,26,31 POMs as well as other polynuclear species (dodecaborates or electron-rich octahedral clusters) have been identified as water structure breakers, also named chaotropic entities after the Hofmeister classification.^22,32–34 Based on the current understanding of the Hofmeister series, the chaotropicity of an ion should increase when its charge density decreases, but regarding POMs more insight is needed into the influence of the ionic charge, especially the effect of redox change on their chaotropic character.^22,31 From a thermodynamic point of view, processes involving chaotropes are usually considered to be enthalpically driven accompanied by an entropic penalty, i.e. enthalpy–entropy compensation.^26,35

Cyclodextrins (CDs) are natural cyclic oligosaccharides with a hydrophobic internal cavity and a hydrophilic external surface, and thus exhibit interesting properties as host molecules for a wide variety of inorganic species including polyoxometalates.³⁶ CDs are readily available as α-, β- and γ-forms comprising six, seven, or eight glucose units, respectively. The host–guest assembly of the toroidal CDs and spherical POM clusters has attracted much attention and is a fast-growing topic. In 2015, Stoddart and coworkers published the first examples of host–guest complexes between γ- and β-CDs and phosphomolybdate [PMo₁₂O₄₀]³⁻.³⁰ Since then, several hybrid (organic/inorganic) adducts or materials have also been reported with various archetypal polyoxotungstates including the Lindqvist-type [W₆O₁₉]²⁻,^37,38 the Keggin-type ions [PW₁₂O₄₀]³⁻ and [SiW₁₂O₄₀]⁴⁻,^38–43 and the Dawson-type [P₂W₁₈O₆₂]⁶⁻.^25,26 Some of these hybrid CD/POM materials are promising for application in catalysis^39,41 and energy storage,⁴⁴ where surface effects and hybrid structures would play a crucial role. Therefore, detailed characterization of the interactions between CDs and POMs and a better understanding of the phenomena occurring at their interfaces are essential for further innovative development of redox-responsive systems and related applications.

Herein, we studied the formation of host–guest inclusion complexes of γ-CD and Keggin-type POM anions along the [XW₁₂O₄₀]ⁿ⁻ series with anionic charges from 3- to 6- (Fig. 1) while keeping constant the shape and size of the host and of the guest. Using a set of complementary techniques (NMR, ITC and electrochemistry), we show in this article that affinity between γ-CD and Keggin-type anions increases dramatically with decreasing ionic charge. Simultaneously, we evaluate the chaotropic behavior using a simple experimental procedure based on the propensity of the POM species to adsorb on the non-ionic ethylene glycol-based surfactant C₈E₄. Thus, we establish unambiguously in this report the direct correlation between the chaotropic nature and the formation of highly stable supramolecular complexes. The present study might open new opportunities for the development of smart supramolecular devices with responsive behavior based on redox changing of the POM unit.

Fig. 1 Structural representations of the molecular entities used as building blocks. (a) γ-cyclodextrin C₄₈H₈₀O₄₀ (γ-CD) resulting from the condensation of eight glycopyranose units. The toroidal macrocycle delimits a hydrophobic cavity of 9 Å in diameter. (b) The Keggin-type POM, [XW₁₂O₄₀]ⁿ⁻, is composed of twelve octahedral WO₆ polyhedra that form an anionic tetrahedral cage {W₁₂O₄₀}⁸⁻ with a central cavity occupied by the heteroatom X (P⁵⁺, Si⁴⁺, B³⁺ or H₂²⁺). The choice of X allows the control of the charge density of the nano-sized inorganic POM.

Results and discussion

Synthesis

Attempts for the isolation of the supramolecular adducts have been carried out systematically using different ratios of sodium salt of POMs Na_m[XW₁₂O₄₀]·nH₂O with X = P⁵⁺, Si⁴⁺, B³⁺, H₂²⁺ and varying procedures of crystallization such as slow evaporation of aqueous solution or ethanol vapor diffusion into aqueous solution containing cesium chloride. In all cases, crystalline products were obtained but only [BW₁₂O₄₀]⁵⁻ and [PW₁₂O₄₀]³⁻ led to single crystals suitable for XRD analysis. Their composition was established combining different complementary techniques (elemental analysis, EDS, XRD, and TGA). The compounds CsK₂H₂{[BW₁₂O₄₀]·2(γ-CD)}·29H₂O (BW₁₂·2CD) and Na₃{[PW₁₂O₄₀]@(γ-CD)}·10H₂O (PW₁₂@CD) present different molecular organization as further discussed in the next section.

Review of Keggin/γ-CD interactions in the solid-state

The crystal structure of PW₁₂@CD was found to be isostructural to the recently published compounds NaAH₆[(PW₁₂O₄₀)₃(γ-CD)₃]·nH₂O (A = Co^II or Cu^II).⁴⁵ Zhan and coworkers introduced Co(NO₃)₂ or CuSO₄ to aqueous solutions containing γ-CD and Na₃[PW₁₂O₄₀] to obtain crystalline products.⁴⁵ In our case, only Na⁺ and H⁺ were present evidencing that the POM-CD packing in PW₁₂@CD is highly flexible to accommodate various types of counter-cations without structural change. PW₁₂@CD's structure consists of a supramolecular 1 : 1 adduct involving a [PW₁₂O₄₀]³⁻ unit partially incorporated into the cavity of γ-CD through its secondary rim (Fig. 2a). This configuration is quite unique among the few known polyoxometalate/γ-CD composites that mostly feature interactions with the primary face.^26,30 Of particular interest, comparison with the isostructural Keggin phosphomolybdate [PMo₁₂O₄₀]³⁻ reveals a different complexation mode with γ-CD involving the opposite face, namely, the primary rim (Fig. 2b).³⁰ Furthermore, the stoichiometry of the [PMo₁₂O₄₀]³⁻ : CD adduct is 1 : 2, while a 1 : 1 stoichiometry was found with [PW₁₂O₄₀]³⁻ although higher stoichiometry numbers should be expected as observed with Dawson-type POMs with a stoichiometry of 1 : 1 up to 1 : 3.²⁶ Indeed, while PW₁₂@CD crystallized in a 1 : 1 solution mixture, other crystalline products were obtained with higher γ-CD contents (2 γ-CDs per POM unit), but we were unable, despite numerous repeated attempts, to solve the solid-state structures of these crystals featuring a tetragonal unit cell (a = b = 23.94 and c = 18.54 Å). The 1 : 1 adduct observed in PW₁₂@CD exhibits several non-bonding contacts involving the terminal O^t and bridging O^b atoms of the POM and H3 and H5 protons (see Fig. 1a for H labeling) of the inner cavity of the γ-CD (see Fig. S1, in the ESI†). The short interatomic distances H3⋯O^t=W (from 2.69 to 3.31 Å), H3⋯O^b–W (from 2.41 to 2.88 Å), H5⋯O^t=W (from 2.29 to 3.38 Å), and H5⋯O^b–W (from 3.13 to 3.88 Å) reflect a strong and deep host–guest inclusion. As PW₁₂@CD crystallizes in a cubic crystallographic system, the 1 : 1 host–guest adduct is surrounded by four host–guest units giving additional non-bonding contacts between the exposed face of the Keggin ion and the external surface of each of the neighboring γ-CDs (see Fig. S2, in the ESI†).

Fig. 2 Illustration of the different supramolecular assemblies observed in the solid-state involving γ-CD and the Keggin anions (a) [PW₁₂O₄₀]³⁻ in PW₁₂@CD, (b) [PMo₁₂O₄₀]³⁻ from reference,³⁰ and (c) [BW₁₂O₄₀]⁵⁻ in BW₁₂·2CD.

In the solid-state, the organization of the [BW₁₂O₄₀]⁵⁻ and γ-CD is significantly different since the crystalline structure of BW₁₂·2CD reveals that the two components co-crystallize without forming an inclusion complex. Indeed, the [BW₁₂O₄₀]⁵⁻ is positioned near the γ-CD in close contact with its external wall (Fig. 2c). One [BW₁₂O₄₀]⁵⁻ unit is surrounded by eight γ-CDs and each γ-CD is in contact with four [BW₁₂O₄₀]⁵⁻ through the three outward protons H1, H2, and H4 as well as the H6 of the methoxy arm (see Fig. S3, ESI†). This leads to a global stoichiometry of γ-CD/POM of 2:1. Two neighboring γ-CDs are face-to-face connected through a hydrogen bonding network from their secondary rims. The [BW₁₂O₄₀]⁵⁻ is therefore positioned near the primary face with H6⋯O^t=W and H6⋯O^b–W distances in the 2.66–2.91 Å and 2.73–3.91 Å range, respectively. This arrangement with empty γ-CDs is unique in POM/γ-CD composites although it could occur with the smaller α-CD.⁴⁴ Indeed, the [PW₁₂O₄₀]³⁻ Keggin anion led to an extended framework with α-CD forming double layers of interconnected [PW₁₂O₄₀]³⁻ and CD motifs through bridging alkali cations.⁴⁴ This highlights the importance of the size-matching effect upon the molecular recognition process,⁴⁶ however this could not explain the difference of binding interactions observed in the Keggin/γ-CD arrangement.

Finally, it is surprising to see how γ-CD interacts so differently with isostructural nano-sized objects, namely [PW₁₂O₄₀]³⁻, [PMo₁₂O₄₀]³⁻, and [BW₁₂O₄₀]⁵⁻. By keeping the charge constant, the basicity of Keggin molybdates is known to be stronger compared to their tungstate analogues as the charge density is higher on the oxygen atoms at the surface of the molybdates.^47,48 Furthermore, the ionic charge must also play a crucial role in the supramolecular association.²¹ Both these aspects are expected to affect significantly solvent structuration around the POM, and should have important consequences in the formation and stability of the resulting POM-based supramolecular arrangement. Systematic studies in solution are thus needed to shed light on such effects.

Stability in solution

To investigate the complexation of the Keggin-type anions by γ-CD, titration experiments were conducted in solution by means of a set of complementary techniques including NMR spectroscopy (¹H 1D and DOSY), electrochemistry, and isothermal titration calorimetry (ITC).

Isothermal titration calorimetry

ITC experiments were carried out to determine the thermodynamic fingerprint of the interaction of γ-CD respectively with the four Keggin-type heteropolytungstates. ITC thermograms and isotherms are shown in the ESI (see Fig. S4–6, ESI†), and a summary of binding constants, enthalpy Δ_rH* and entropy Δ_rS* changes at 298 K are given in Table 1. Besides the 1 : 1 complexation, a second 1 : 2 step process is systematically considered, according to eqn (1) and (2) shown below.

POMⁿ⁻ + CD → [POM@CD]ⁿ⁻ (1)

[POM@CD]ⁿ⁻ + CD → [POM@2CD]ⁿ⁻ (2)

Table 1 Binding constants (M⁻¹) of γ-CD with the Keggin-type POMs and associated thermodynamic parameters (kJ mol⁻¹) at T = 298 K measured by ITC. Calculated uncertainties are inferior to 10%

POM	POM : CD	K	ΔrH*	TΔrS*	ΔrG*
[H2W12O40]^6−	—	—	—	—	—
[BW12O40]^5−	1 : 1	1032	−53.5	−36.3	−17.2
[SiW12O40]^4−	1 : 1	17 209	−56.4	−32.3	−24.2
	1 : 2	459	−55.0	−39.8	−15.2
[PW12O40]^3−	1 : 1	2919	−35.1	−15.3	−19.8

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Nonetheless, only ITC data for silicotungstate were found to be consistent with the 2 : 1 binding model involving a sequential process. No significant heat exchange was observed in the case of [H₂W₁₂O₄₀]⁶⁻ indicating negligible interactions with γ-CD. For all other complexes, supramolecular binding revealed to be enthalpically driven. The enthalpic gain is accompanied by an entropic penalty according to the usual enthalpy–entropy compensation observed within CD-based host–guest series.⁴⁹ These thermochemical fingerprints together with low values of both enthalpic and entropic changes suggest that Keggin anions behave as chaotropic species (water structure breakers).³¹ In such a case, the magnitude of the chaotropic character and the related binding constants are expected to increase with decreasing global charge. However, the highest enthalpic and entropic changes (and binding constants) were observed for [SiW₁₂O₄₀]⁴⁻ and the lowest values were found for [PW₁₂O₄₀]³⁻. This result represents therefore an anomaly within the expected series, as confirmed by the electrochemical and NMR studies discussed below. The discrepancy here may have appeared for two reasons: (i) the partial hydrolysis of the [PW₁₂O₄₀]³⁻ anion under the ITC conditions (non-buffered and diluted solution) or (ii) the simultaneous formation of numerous complexes with various stoichiometries, e.g. 1 : 1, 1 : 2, 2 : 1, etc., (overparametrization).

Electrochemical studies

Interactions of the Keggin-type ions with γ-CD have been studied further using cyclic voltammetry. As shown below, the presence of γ-CD alters significantly the redox properties of the POMs. Besides, the POM's redox state is expected to have a high impact on the binding constants. As shown in Fig. 3, CVs with γ-CD appear to be strongly dependent on the nature of the Keggin anion.

Fig. 3 Cyclic voltammetry of the [XW₁₂O₄₀]ⁿ⁻ anions with (a) X = P⁵⁺, (b) Si⁴⁺, (c) B³⁺ or (d) H₂²⁺ (0.5 mmol L⁻¹, glassy carbon working electrode, scan rate 50 mV s⁻¹) in the presence of increasing amounts of γ-CD (from 0 to 8 equivalents). The experiments involving [PW₁₂O₄₀]³⁻, [SiW₁₂O₄₀]⁴⁻ and [BW₁₂O₄₀]⁵⁻ have been performed in 25 mmol L⁻¹ HClO₄ aqueous solution, while acetate buffer solution (0.5 M CH₃COONa; 0.5 M CH₃COOH) has been used for the electrochemical measurements of [H₂W₁₂O₄₀]⁶⁻.

For instance, as the amount of γ-CD increases up to 8 equivalents, the half-wave potential related to the two first monoelectronic transfers of the [PW₁₂O₄₀]³⁻ ion decreases continuously (see Fig. 3). Actually, the electrochemical behavior of the [PW₁₂O₄₀]³⁻ ion gives the most representative effect reflected by a rough dependency of about −60 mV/pCD where pCD = −Log[CD] (see Fig. S9, ESI†). This variation indicates that γ-CD is involved in the predominant redox process written in eqn (3), with two consecutive monoelectronic transfers, n = 3 or 4 for the first and the second redox processes, respectively. Furthermore, the complexation behavior of γ-CD should be rather consistent with x = 2 or 1 corresponding to a 2 : 1 and 1 : 1 supramolecular adduct, respectively.

[POM(CD)_x]ⁿ⁻ + 1e⁻ → [POM(CD)_x−1]⁽ⁿ⁺¹⁾⁻ + CD (3)

Along the Keggin series, the variation of the redox properties becomes less pronounced as the ionic charge of the Keggin ion increases. Thus, in the case of [SiW₁₂O₄₀]⁴⁻, the first mono-electronic redox wave appears strongly affected, whereas the second one varies weakly. For the boron derivative, only the first redox wave is moderately shifted and finally, the redox features of [H₂W₁₂O₄₀]⁶⁻ remain nearly unchanged in the presence of γ-CD (see Fig. 3). With the exception of [PW₁₂O₄₀]³⁻, the redox behavior reflects the trend highlighted by the ITC analysis.

Quantitative analysis of these electrochemical data can yield the equilibrium constants (K_1:1 and K_1:2) regarding the binding of the Keggin anions at various redox states by the γ-CD. Considering the three available oxidation states of POMⁿ⁻ (ox), POM⁽ⁿ⁺¹⁾⁻ (red₁), POM⁽ⁿ⁺²⁾⁻ (red₂), associated with the three related complexed forms POM@xCD with x = 0, 1 or 2, the 3X3 matrix diagram can be drawn (see Scheme 1).

Scheme 1 Equilibria involved in redox-dependent supramolecular association of γ-cyclodextrin (CD) and the Keggin type anions (POM).

Including the six redox components involved in each redox process, the Nernst equations are written as followed:

where E⁰₁ and E⁰₂ are the standard potentials of each redox process and [CD]_eq is the concentration in free γ-CD at the equilibrium. The values K_1:1 and K_1:2 correspond to the binding constants associated with eqn (1) and (2), respectively, wherein the POM can be in its three redox states, oxidized (ox), one-electron reduced (red1) or two-electron reduced (red2) (see Scheme 1). Depending on the Keggin-type POM, eqn (4) or (5) were adapted and then used to extract the binding constants through the fitting of experimental shift of the apparent potential upon free γ-CD concentration (see the ESI, Fig. S7–9†). The case of the metatungstate ion [H₂W₁₂O₄₀]⁶⁻ is trivial as neither ITC nor electrochemistry evidences any interaction with γ-CD. For the [BW₁₂O₄₀]⁵⁻ ion, ITC experiments showed the formation of a 1 : 1 adduct. Furthermore, the second wave is nearly independent of the γ-CD concentration meaning that the one-electron and the two-electron reduced forms do not interact with γ-CD. Under such conditions, the expression of the apparent potential E_peak,1 is adapted from eqn (4) by using only the binding constant K^ox_1:1. Decreasing further the ionic charge produces a richer and more complex behavior. The first wave of the [SiW₁₂O₄₀]⁴⁻ anion undergoes a shift of about −60 mV per pCD unit (see the ESI, Fig. S8†) meaning the Nernst Equation can be derived from eqn (3). The second wave exhibits only moderate variation, consistent with a weak complexation of the one-electron reduced form (red₁) and no complexation for the two-electron reduced form (red₂). Then eqn (5) is derived by using only the binding constant and matches the experimental data for . Considering the first monoelectronic exchange, both the 1 : 1 and the 1 : 2 adduct must be considered for the oxidized form as shown by ITC. Then eqn (4) should include K^ox_1:1, K^ox_1:2 and = 130 M⁻¹. Suitable fitting with experimental data was obtained with numerical values 9500 and 150 M⁻¹ for K^ox_1:1 and K^ox_1:2, respectively. Finally, the electrochemical behavior of the [PW₁₂O₄₀]³⁻ ion consists of two CD-dependent monoelectronic waves which move both by about −60 mV per pCD unit in accordance with eqn (3). The second wave is satisfactory modeled by using = 6000 M⁻¹ and = 30 M⁻¹ while calculation of the first redox potentials requires the inclusion of K^ox_1:1 and K^ox_1:2 in addition to the binding constants related to the reduced form [PW₁₂O₄₀]⁴⁻, namely red₁. From eqn (4), good agreement was found between calculated and experimental data for K^ox_1:1 = 90 000 M⁻¹ and K^ox_1:2 = 1500 M⁻¹. The values of the overall binding constants are reported in Table 2 and the comparison between experimental data and the simulated evolution of the observed apparent potential is reported in the ESI (see Fig. S7–9†). The binding constants determined by electrochemistry for [BW₁₂O₄₀]⁵⁻ and [SiW₁₂O₄₀]⁴⁻ ions are significantly lower compared to those found by ITC (see Table 1). This difference is likely due to the different medium used in the ITC experiments (pure water) and in the electrochemical measurements (aqueous solution of 0.025 mol L⁻¹ HClO₄) and may arise from the presence of hydronium ions H₃O⁺ or perchlorate ion ClO₄⁻ involved in competition processes with the POM-CD aggregation. Importantly, analysis of the electrochemical data allows us to determine the K_1:1 binding constants of the [PW₁₂O₄₀]³⁻ ion. With a value of 9 × 10⁴ M⁻¹, the affinity of [PW₁₂O₄₀]³⁻ for γ-CD is about one order of magnitude higher than the affinity of [SiW₁₂O₄₀]⁴⁻. Such a high value corresponds to one of the largest affinity constants reported so far with γ-CD.^32,50,51 Moreover, the second binding constant K_1:2 is also quite large, around one order of magnitude higher than the K_1:2 value observed for [SiW₁₂O₄₀]⁴⁻.

Table 2 Binding constants K_1:1 and K_1:2 involving γ-CD with the Keggin-type anions at different oxidation states as determined from electrochemistry. The accuracy is estimated to be within 10% of the corresponding value

X	POM : CD	[XW12O40]^3−	[XW12O40]^4−	[XW12O40]^5−
P	1 : 1	90 000	6 000	0
	1 : 2	1 500	30	0
Si	1 : 1	—	9 500	130
	1 : 2	—	150	0
B	1 : 1	—	—	600
	1 : 2	—	—	0

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This electrochemical study enabled to restore a logical sequence within the Keggin-type series, corroborating the fact that the POM/γ-CD association is mainly related to the global charge of POMs. For instance, plotting LogK_1:1versus the ionic charge of the Keggin-type anion results in a quite perfect linear relationship (see the ESI, Fig. S10†). Furthermore, analysis of the electrochemical data provides access to the binding constants of the lower oxidation states of the Keggin anions (see Table 2). The binding constants of the reduced POM⁽ⁿ⁺¹⁾⁻ exhibit lower values compared to those determined for the oxidized parent (POMⁿ⁻), evidencing again the preponderant effect of the charge on the association process with the macrocyclic host. For instance, the binding constants of the one-electron reduced [PW₁₂O₄₀]⁴⁻ anion (K_1:1 = 6000 M⁻¹) fall in a similar range order to that found for the oxidized silicotungstate [SiW₁₂O₄₀]⁴⁻ (K_1:1 = 9500 M⁻¹) (see Table 2). A similar observation is made for the one electron-reduced silicotungstate [SiW₁₂O₄₀]⁵⁻ and the oxidized borotungstate [BW₁₂O₄₀]⁵⁻, which both exhibit comparable affinity toward γ-cyclodextrin. Besides, such a trend can be compared to results reported in a previous study which demonstrated that decreasing the ionic charge of a rhenium-selenide cluster [Re₆Se₈(CN)₆]⁴⁻ through a one-electron oxidative process provokes a dramatic increase of the K_1:1 binding constant from 1.5 × 10³ to 2.2 × 10⁵ M⁻¹.³⁴ Nevertheless, alteration of the redox properties through supramolecular interactions remains quite intriguing in the case of the Keggin-type polyoxometalates. γ-CD being a non-ionic component, its interaction with POM does not change the ionic charge as observed generally with protons or lithium ions. In context, proton-coupled electron transfer (PCET) involving POM species is a well-known pH dependent process featured by an increase of the standard potential as pH decreases. Such an effect can even be extended to the POM-lithium aggregation process under reducing conditions in non-aqueous solvents.⁵² In the presence of γ-CD, the origin of the redox potential variation upon γ-CD interaction should arise mostly from a change in the solvation shell of the POM corresponding to a shed of surrounding water molecules upon binding to γ-CD resulting in a more hydrophobic environment around the POMs. The influence of the medium, i.e. solvent and electrolyte support, on the redox properties of the POM species is well documented experimentally as well as through theoretical calculations and related studies reinforce our interpretation of the γ-CD effect on the redox properties of Keggin-type POMs.^53,54 For instance, the first one-electron wave of the [SiW₁₂O₄₀]⁴⁻ ion undergoes a 600–700 mV potential shift toward the negative potential when the medium is changed from aqueous to organic such as acetonitrile or dimethyl sulfoxide. This effect has been attributed to the capacity of the solvent to stabilize the reduced derivatives and has been quantified by the acceptor number (AN) of the solvent showing that the reduction process becomes difficult as solvent AN decreases. Water exhibits a high AN value (58.4) while less hydrophilic or less polar media feature lower AN parameters.⁵⁵ Furthermore, the intensity of the peak current decreases significantly in the presence of γ-CD. This behavior accounts for the complexation process which lowers the diffusion coefficient of the parent oxidized POM and then consecutively affects the peak current intensity, according to the Randles–Sevcik equation.⁵⁶ This effect on the current peak intensity appears quite pronounced in the CVs of the [PW₁₂O₄₀]³⁻ ion, which exhibits the strongest affinity with the γ-CD and diminishes as the ionic charge of the POM increases. The diffusion coefficient change upon γ-CD complexation is also nicely supported by the ¹H DOSY NMR study presented below.

To sum up, electrochemical investigations demonstrate that the binding constants K_1:1 and K_1:2 increase by at least one order of magnitude as the global ionic charge decreases along the Keggin-type series [XW₁₂O₄₀]ⁿ⁻. From this observation, the Keggin/γ-CD association appears as an appealing component for redox switches and could be integrated into the design of stimuli-responsive supramolecular systems controlled by the redox state of the POMs.

NMR spectroscopy

The formation and stability of Keggin POM/γ-CD complexes in aqueous solution were further probed by ¹H NMR spectroscopy in D₂O. In previous separate studies, the interaction of γ-CD with both [PW₁₂O₄₀]³⁻ and [SiW₁₂O₄₀]⁴⁻ has been characterized by ¹H NMR.^39,45 Here, we conducted a comparative titration study of 2 mM aqueous γ-CD solution with the four Keggin-type anions. Fig. 4 shows the resulting ¹H NMR spectra in the range 3.7–4.5 ppm corresponding to the resonance domain of the most exposed protons to guest species, namely H3 and H5 are located inside the cavity and H6 on the top of the secondary rim (see Fig. 1 for the proton numbering scheme and ESI Fig. S11–14† for the complete ¹H NMR spectra). These protons are very sensitive to host–guest contact and are usually used as indicators to probe the inclusion phenomenon.

Fig. 4 ¹H NMR spectra in the chemical shift range for H3, H5, and H6 resulting from the titration of 2 mmol L⁻¹ aqueous γ-CD solution with [XW₁₂O₄₀]ⁿ⁻ POMs. (a) [PW₁₂O₄₀]³⁻; (b) [SiW₁₂O₄₀]⁴⁻; (c) [BW₁₂O₄₀]⁵⁻ and (d) [H₂W₁₂O₄₀]⁶⁻.

Depending on the Keggin anion, the spectra underwent more or less strong alteration evidencing POM-specific recognition. Table 3 summarizes the extent of shift variations observed for H3, H5, and H6 after adding eight equivalents of each Keggin-type POM. As expected, the ¹H NMR spectrum of the macrocyclic host remains nearly unchanged when [H₂W₁₂O₄₀]⁶⁻ was added to the γ-CD solution, and the maximum shifts observed were ca. 0.07 ppm for both H5 and H6 indicating very weak interaction. Notably, much larger effects were observed in the cases of [BW₁₂O₄₀]⁵⁻ and [SiW₁₂O₄₀]⁴⁻, where shifts up to 0.33 and 0.18 ppm were observed for H6 and H5, respectively. These changes in the spectra are similar suggesting comparable behavior in solution for [BW₁₂O₄₀]⁵⁻ and [SiW₁₂O₄₀]⁴⁻. Nevertheless, close inspection revealed some differences. First, the H6 (and also H5) signal evolves much more significantly with increasing [SiW₁₂O₄₀]⁴⁻ amount compared to [BW₁₂O₄₀]⁵⁻ although the final chemical shift at the highest ratio is the same. As the observed chemical shift corresponds to a weighted average of free and complexed γ-CD species, this means the binding constant of POM-CD complexation should be higher with [SiW₁₂O₄₀]⁴⁻ than with [BW₁₂O₄₀]⁵⁻. The second observation is the splitting of the initial singlet of the H6 protons into two unresolved doublets in [BW₁₂O₄₀]⁵⁻ containing solution. This may be due to constrained dynamic rotation of the methoxy arms which narrows the signal of the two methylenic diastereotopic H6 protons. Such steric hindrance would result from the interaction of the POM specifically with the primary rim of the γ-CD since the H3 protons present in the secondary rim did not show any significant perturbation. The situation is totally different with the [PW₁₂O₄₀]³⁻ anion where, besides the shift of H6, we can also appreciate strong effects on both H5 and H3 resonances with up to 0.53 and 0.36 ppm downfield shifts, respectively, indicating involvement of the secondary face of the γ-CD in the strong interaction with [PW₁₂O₄₀]³⁻. Furthermore, we note that the line broadening of the H3 signal for POM:γ-CD ratio below two is the signature of strong POM/γ-CD interactions involved in a 1 : 2 stoichiometry supramolecular adduct.⁵⁷

Table 3 Differences in chemical shifts (ppm) of protons H3, H5, and H6 of γ-CD before and after the addition of eight equivalents of POM

POM	ΔδH3	ΔδH5	ΔδH6
[H2W12O40]^6−	0.02	0.06	0.07
[BW12O40]^5−	0.03	0.18	0.33
[SiW12O40]^4−	0.03	0.16	0.30
[PW12O40]^3−	0.53	0.36	0.28

---

By analyzing the chemical shift variation arising from the fast chemical exchange regime, quantitative treatment allows for a determination of the binding constants for 1 : 1 POM:γ-CD complexes (for further details, see Fig. S15 and 16, ESI†). The values obtained for K_1:1 are 23 and 1100 M⁻¹ for [H₂W₁₂O₄₀]⁶⁻ : CD and [BW₁₂O₄₀]⁵⁻ : CD, respectively, that compare well with the ITC and electrochemistry results (see Tables 1 and 2). In summary, NMR results are fully consistent with electrochemistry showing that the interaction strength with γ-CD increases in the following order: [H₂W₁₂O₄₀]⁶⁻ < [BW₁₂O₄₀]⁵⁻ < [SiW₁₂O₄₀]⁴⁻ < [PW₁₂O₄₀]³⁻.

Further quantitative investigations were also carried out using DOSY NMR. This technique is a valuable tool to study dynamic equilibria, and similarly to the chemical shift, the observed diffusion coefficient D in the fast exchange limit can be described as a weighted average of a given species evolving between different states.⁵⁸ Thus, the observed NMR of γ-CD in the presence of POM corresponds to an average situation weighted on residency times between free (solvated) and bound states. The observed diffusion coefficients D as a function of the POM : γ-CD molar ratio produce different profiles depending on the nature of the Keggin-type anions (see Fig. 5). From the initial value for solvated γ-CD measured at 254 μm² s⁻¹, the self-diffusion D drops more or less abruptly upon addition of POM and reaches a plateau at a large excess of POM at 226 μm² s⁻¹ in the case of [SiW₁₂O₄₀]⁴⁻ and [BW₁₂O₄₀]⁵⁻, while for the [H₂W₁₂O₄₀]⁶⁻ ion, the decrease of the self-diffusion coefficient draws a smooth slope featured by larger self-diffusion values as an indication of very weak POM-γ-CD interactions. For the [PW₁₂O₄₀]³⁻ ions, the D profile is characteristic of strong interactions. The decrease of D is sharper, reaching even the lower limit value measured at 219 μm² s⁻¹. We note also a minimum value for the curves of both [SiW₁₂O₄₀]⁴⁻ and [PW₁₂O₄₀]³⁻ at ca. POM : γ-CD = 0.5 suggesting the presence of bulkier intermediate species at these compositions. Both curve profiles have been modeled satisfactorily using either a one-step complexation model for [H₂W₁₂O₄₀]⁶⁻ and [BW₁₂O₄₀]⁵⁻ or a two-step complexation model for [SiW₁₂O₄₀]⁴⁻ and [PW₁₂O₄₀]³⁻. Simulations of these curves considering binding constants consistent with those determined previously from ITC, electrochemistry and also 1D NMR studies allowed to estimate diffusion coefficients of γ-CD within 1 : 1 and 1 : 2 adducts. Fair agreements between calculated and experimental data were found using D values of 226 and 163 μm² s⁻¹ for POM@CD and POM@2CD adducts, respectively. Noteworthy, the same D values were used for the 1 : 1 or 1 : 2 adduct independently of the nature of the Keggin-type anion (see the ESI for details of modeling, Fig. S17–20†), considering similar volume/size for the POM-CD adduct along the Keggin-type series. The observed lower limit value for D with [PW₁₂O₄₀]³⁻ (219 vs. 226 μm² s⁻¹, see the ESI for modeling, Fig. S20†) highlights the remarkable solution behavior of the phosphotungstate ion as reported by Antonio et al.⁵⁹ In this composition domain, it may result from some further aggregation which could be consistent with a γ-CD unit sandwiched by two [PW₁₂O₄₀]³⁻ ions interacting with both primary and secondary faces of the γ-CD. This hypothesis is also quite consistent with the ¹H NMR titration which reveals interactions involving simultaneously both rims of the γ-CD.

Fig. 5 Diffusion coefficients of γ-CD as a function of the POM:γ-CD molar ratio, measured from ¹H DOSY NMR of a solution of 2 mM of γ-CD with various POM contents. Lines correspond to calculated curves with one-step 1 : 1 POM : CD complexation for [H₂W₁₂O₄₀]⁶⁻ and [BW₁₂O₄₀]⁵⁻, and with two-step 1 : 2 POM : CD complexation for [SiW₁₂O₄₀]⁴⁻ and [PW₁₂O₄₀]³⁻.

Furthermore, ¹H NMR can also characterize the proton-containing [H₂W₁₂O₄₀]⁶⁻ anion exhibiting a resonance at ca. 6.0 ppm. Very small changes in the diffusion coefficient D (245–260 μm² s⁻¹) were observed over the titration experiment, reflecting very weak interaction with γ-CD as previously identified.

Driving force of the aggregation processes

This comparative study evidences clearly that Keggin-type anions bind to γ-CD to different extents despite their iso-structure. Although γ-CD is non-ionic, the charge of the POM plays a dramatic role in the molecular recognition process, both in its thermodynamic stability and in its supramolecular structure. All experimental measurements (ITC, electrochemistry and NMR) have shown that the interaction with [H₂W₁₂O₄₀]⁶⁻ is very weak (K_1:1 ≈ 2.3 × 10¹ M⁻¹), while it is very strong with [PW₁₂O₄₀]³⁻ (K_1:1 ≈ 1.6 × 10⁵ M⁻¹). It was proposed that the binding affinity is related to the charge density of the polyanion, which has an effect on the hydration strength, i.e. hydration free energy of the ions. The monotonous trend observed along the oxidized derivatives of the Keggin-type series could be understood by thermodynamic features of the hydration shell around the POM, which is directly related to its chaotropic nature. The weak charge density of the Keggin-type POM generates a loosely bound solvation shell of disordered water molecules with high structural entropy. The energy gain associated with the release of this hydration water into the bulk upon binding to non-ionic moieties (at micellar surfaces, water surfactant monolayer or macrocycles) was proposed as the main driving force of the superchaotropic effect of POMs and more generally of low charge density nano-ions.²² The evolution of the cloud point of a non-ionic surfactant, tetra-ethylene glycol mono-octyl ether (C₈E₄), upon addition of POMs was proposed as a tool to evaluate their chaotropic behavior.²² The higher the observed cloud point increase, the stronger the interaction with the non-ionic surfactant and the stronger the chaotropic nature of the tested ion. This procedure was carried out for the Keggin-type anions investigated here. Corresponding to the respective increase in the cloud point by each POM, Fig. 6 illustrates an increase in the chaotropic nature of the polyanions from [H₂W₁₂O₄₀]⁶⁻ to [PW₁₂O₄₀]³⁻, i.e. from high to low charge density. The superchaotropic P-, Si- and B-Keggin POMs produce much stronger increases in the cloud point compared to the classical chaotrope SCN⁻. The resulting ordering of POMs according to their chaotropic behavior is in full agreement with the series obtained by electrochemistry and NMR measurements.

Fig. 6 Cloud point evolution of 60 mM C₈E₄ as a function of salt concentration for the four isostructural Keggin-ions [PW₁₂O₄₀]³⁻, [SiW₁₂O₄₀]⁴⁻, [BW₁₂O₄₀]⁵⁻ and [H₂W₁₂O₄₀]⁶⁻. The classical salts NaCl (neutral salt) and NaSCN (classical chaotropic salt) are included as references.

SAXS measurements provided further insight into the superchaotropic character of the Keggin POMs (see Fig. S21 and S22, ESI†). [H₂W₁₂O₄₀]⁶⁻in the presence of C₈E₄ shows identical scattering as in bare water, while with [BW₁₂O₄₀]⁵⁻ a strong oscillation appears, which is even more pronounced with [SiW₁₂O₄₀]⁴⁻. This oscillation is characteristic of core–shell structuration and typical of POM-decorated micelles. The increase of the oscillation with the superchaotropic character of the Keggin-ion is a direct indicator of the extent of adsorption onto the C₈E₄-micelles resulting in the same ordering as in other experiments: [SiW₁₂O₄₀]⁴⁻ > [BW₁₂O₄₀]⁵⁻ > [H₂W₁₂O₄₀]⁶. It is worth noting that in the case of [PW₁₂O₄₀]³⁻ precipitation occurs in the presence of C₈E₄ micelles, preventing SAXS measurement. Nevertheless, precipitate formation hints at a particularly strong interaction of Na₃[PW₁₂O₄₀] with the surfactant.

Overall, we showed herein that the Keggin-type POMs interact with the γ-CD through various recognition modes depending on the POM's chaotropic power. The different interaction types previously identified can be classified along the Keggin ion series related to the magnitude of the binding constants. As tentatively depicted in Fig. 7, lowering of the ionic charge gives rise to multimodal supramolecular processes consistent with our experimental observations. On the left side of the scale, the [H₂W₁₂O₄₀]⁶⁻ ion shows nearly no ability to interact with γ-CD and the [BW₁₂O₄₀]⁵⁻ species reveals only weak interactions that involve preferably the outer surface of the CD as observed in the crystal structure of BW₁₂·2CD (see Fig. 2c). Lowering further the ionic charge down to [SiW₁₂O₄₀]⁴⁻ ion leads to the formation of 1 : 1 and 1 : 2 inclusion complexes involving mostly the primary face featuring higher binding constants. Finally, on the right side, the lowest ionic charge anion [PW₁₂O₄₀]³⁻ gives rise to the highest binding constants reflected in the large diversity of supramolecular binding modes. As shown previously by ¹H NMR, both primary and secondary faces contribute to the POM's complexation and the complementary DOSY NMR study suggested even the presence of an unusual supramolecular assembly corresponding to a γ-CD sandwiched by two [PW₁₂O₄₀]³⁻ anions. Despite the lack of any structural model for such an arrangement, NMR showed clearly that in excess of POM, the primary face represented by H5 and H6 resonances and the secondary face detected by H3 resonances are both simultaneously involved in low self-diffusion coefficient assembly compatible with a {POM⋯CD⋯POM} arrangement. The binding on the primary and secondary faces of the CD, which results in a large contact surface with the POM, is favored over the binding of POMs on the exterior side of the CD

Fig. 7 Illustration of the most representative binding modes involved in the supramolecular assemblies of γ-CD (green torus) and the Keggin anions (spheres).

Conclusions

A comparative study allowed for an establishment of the relationship between the ionic charge of Keggin-type polyoxotungstate anions and their affinity towards a non-ionic substrate, γ-cyclodextrin. As the charge is lowered systematically keeping the molecular size and shape, the ability to form supramolecular complexes increases continuously as was also observed for the interaction of the POMs with non-ionic surfactant micelles. The effects governing such supramolecular assemblies are suggested to be directly related to the solvation properties of the Keggin ions and to their propensity to shed hydration water upon the formation of POM-substrate contact. While the most charged POMs, [H₂W₁₂O₄₀]⁶⁻ and [BW₁₂O₄₀]⁵⁻ showed weak to moderate affinity towards γ-CD, interacting mainly with its exterior wall, the lowest charge density species formed sandwich-type host–guest complexes involving either the primary face or the secondary face, featuring larger shared interfaces. Thus, in the Keggin-type series, the [PW₁₂O₄₀]³⁻ ion previously identified as a superchaotrope⁶⁰ exhibits a remarkable capacity to bind γ-CD through its two faces, thus presenting unique aggregation behavior. The host–guest adaptability allows maximizing contact interfaces through weak dispersion forces, highly amplified through a solvent effect arising from the pronounced superchaotropic nature of the [PW₁₂O₄₀]³⁻ ion. This remarkable solution behavior of the [PW₁₂O₄₀]³⁻ ion is not restricted to the systems involving cyclodextrins and the present study gives a better understanding of other self-assembly processes involving non-ionic substrates in water.^21,59,61,62 Furthermore, POM/CD association has dramatic consequences on the POM's redox properties, i.e. a decrease in the standard redox potential. This effect can be understood as a partial removal of the hydration shell of the redox active ions arising from the presence of bulky γ-CD in the close vicinity of the POM. It is worth noting that opposite effects corresponding to a redox potential increase were commonly observed for interaction with cations such as protonation or ion-pairing with lithium cation.

Finally, the propensity of the Keggin-type anions to bind to γ-CD can be dramatically tuned by reversible electron transfer as demonstrated herein. This opens opportunities for the design of smart molecular materials like sensors, artificial muscles, and actuators, responsive to redox or possibly light stimuli.

Experimental section

Chemicals

All reagents were purchased from commercial sources and used without further purification. K_4.3Na_0.7[BW₁₂O₄₀]·17H₂O,⁶³ H₅[BW₁₂O₄₀]·29H₂O,⁶⁴ K₄[SiW₁₂O₄₀]·6H₂O,⁶⁵ Na_0.6H_3.4[SiW₁₂O₄₀]·16H₂O,⁶⁶ K₃[PW₁₂O₄₀]·5H₂O, Na_1.4H_1.6[PW₁₂O₄₀]·12H₂O, and H₃[PW₁₂O₄₀]·14H₂O^65,67 were prepared according to literature procedures and checked by routine analyses prior use. The synthesis of Rb₆[H₂W₁₂O₄₀]·11H₂O is described below. Solutions were prepared in Milli-Q water.

Materials characterization

Fourier transform infrared (FT-IR). Spectra were recorded on a 6700 FT-IR Nicolet spectrophotometer, using the diamond ATR technique. The spectra were recorded on non-diluted compounds and ATR correction was applied.

Energy-dispersive X-ray spectroscopy (EDS). EDS measurements were performed using a SEM-FEG (scanning electron microscope enhanced by a field emission gun) equipment (JSM 7001-F, Jeol). The measures were acquired with a SDD XMax 50 mm² detector and the Aztec (Oxford) system working at 15 kV and 10 mm working distance. The quantification is realized with the standard library provided by the constructor using Lα lines.

Thermal gravimetric analysis (TGA). To determine water and organic contents, a Mettler Toledo TGA/DSC 1, STAR^e System apparatus was used under oxygen or nitrogen flow (50 mL min⁻¹) at a heating rate of 5 °C min⁻¹ up to 700 °C.

Single-crystal X-ray diffraction (XRD). Intensity data collections were carried out at T = 200(2) K with a Bruker D8 VENTURE diffractometer equipped with a PHOTON 100 CMOS bidimensional detector using high brilliance IμS microfocus X-ray Mo Kα monochromatized radiation (λ = 0.71073 Å). Crystals were glued in paratone oil to prevent any loss of crystallization water. Data reduction was accomplished using SAINT V7.53a. The substantial redundancy in data allowed a semi-empirical absorption correction (SADABS V2.10) to be applied, on the basis of multiple measurements of equivalent reflections. Using Olex2,⁶⁸ the structure was solved with the ShelXT⁶⁹ structure solution program using Intrinsic Phasing and refined with the ShelXL⁷⁰ refinement package using Least Squares minimization. Heavier atoms (W) for each structure were initially located by direct methods. The remaining nonhydrogen atoms were located from Fourier differences and were refined with anisotropic thermal parameters. Positions of the hydrogen atoms belonging to the γ-CDs were calculated and refined isotropically using the gliding mode. In the compound PW₁₂@CD, numerous free water molecules (H₂O) located inside the voids are disordered. Thereby the contribution of solvent-electron density was removed using the SQUEEZE routine in PLATON, producing a set of solvent-free diffraction intensities. Crystallographic data for single-crystal X-ray diffraction studies are summarized in Table S1.†

Isothermal titration calorimetry (ITC). Formation constants and inclusion enthalpies were simultaneously determined for each γ-CD/POM system by the use of an isothermal calorimeter (ITC200, MicroCal Inc., USA). Degassed deionized water solutions were used in both cell (V₀ = 202.8 μL) and syringe (40 μL). After the addition of an initial aliquot of 2 μL, 10 aliquots of 3.5 μL of the syringe solution were delivered over 7 s for each injection. The time interval between two consecutive injections was 70 s, which proved to be sufficient for a systematic and complete return to the baseline. The agitation speed was set to 1000 rpm. The resulting heat flow was recorded as a function of time. ITC titrations were realized at three temperatures (288, 298 and 308 K), with 5 mM γ-CD solution in the syringe and 0.25 mM POM solution in the cell. Values and uncertainties of formation constants and inclusion enthalpies were determined by global analysis of the binding isotherms, by means of a dedicated homemade program,⁷¹ implementing 1 : 1 and 1 : 1 + 1 : 2 binding polynomials.

Electrochemistry. Purified water was used throughout. It was obtained by passing water through a RiOs 8 unit followed by a Millipore-Q Academic purification set. All reagents were of high-purity grade and were used as purchased without further purification. Cyclic voltammetry (CV) experiments were carried out with an Methrohm Autolab PGSTAT12 potentiostat/galvanostat associated with a GPES electrochemical analysis system (EcoChemie). Measurements were performed at room temperature in a conventional single compartment cell. A glassy carbon (GC) electrode with a diameter of 3 mm was used as the working electrode. The auxiliary electrode was a Pt plate placed within a fritted-glass isolation chamber and potentials are quoted against a saturated calomel electrode (SCE). The solutions were deaerated thoroughly for at least 30 minutes with pure argon and kept under a positive pressure of this gas during the experiments.

Nuclear magnetic resonance (NMR). All solution NMR spectra were measured in D₂O at 26 °C. ¹H NMR spectra were recorded on a Bruker Avance 400 spectrometer at a Larmor frequency of 400.1 MHz, using 5 mm standard NMR tubes. The 1D ¹H spectra were recorded with one pulse sequence at 30° flip angle (pulse duration 2.7 μs), using 0.1 s recycle delay, 3 s acquisition time, and 8 number of scans. Translational diffusion measurements were performed using Bruker's “ledbpgs2s” stimulated echo DOSY pulse sequence including bipolar and spoil gradients. Apparent diffusion coefficients were obtained using an adapted algorithm based on the inverse Laplace transform stabilized by maximum entropy.⁷² Chemical shifts are reported relative to tetramethylsilane (TMS).

Cloud point measurements (CP). Aqueous solutions containing 60 mM C₈E₄ and a given amount of POM were prepared in 1 mL glass vials. The vials were placed in a temperature controlled water bath. The heating rate was constant at 1 K min⁻¹ and the CP was detected by visual inspection after the occurrence of turbidity. The choice of the counter-ion has negligible effects on the cloud point evolution.²²

Syntheses

Rb_4.5Na_1.5[H₂W₁₂O₄₀]·11H₂O. The synthetic procedure is inspired from the syntheses of tetraalkylammonium (TAA) salts, TAA₆[H₂W₁₂O₄₀]·9H₂O.⁷³ Na₂WO₄·2H₂O (10 g, 30 mmol) is dissolved in 30 mL of deionized water, and the resulting solution is acidified with HCl (6 mol L⁻¹) until pH ∼ 3. The solution is kept under boiling conditions for 24 h and the pH is maintained at 3. After cooling to room temperature, the solution is then filtered, and rubidium chloride (5.5 g, 45 mmol) is added. The white solid product is collected, and finally dried in air. Yield: ∼3.8 g (33%). IR (cm⁻¹): 956 (sh), 935 (s), 896 (s), 878 (s), 766 (vs), ¹⁸³W NMR: −117.6 ppm; ¹H NMR: 5.99 ppm. ICP-OES Anal. Calcd for H₂₄Na_1.5O₅₁Rb_4.5W₁₂: Na, 1.00; Rb, 11.10; W, 63.66. Found: Na, 0.9; Rb, 10.5; W, 58.0. TGA showed a weight loss of 5.5% in the 20–220 °C temperature range corresponding to the eleven hydration water molecules (calculated 5.7%).

CsK₂H₂{[BW₁₂O₄₀]·2(C₄₈H₈₀O₄₀)}·29H₂O (BW₁₂·2CD). A mixture of K_4.3Na_0.7[BW₁₂O₄₀]·17H₂O (68 mg; 0.02 mmol) and γ-CD (85 mg; 0.06 mmol) in 3.5 mL of aqueous solution of CsCl (0.05 mol L⁻¹) and KCl (0.1 mol L⁻¹) was stirred until a clear solution was obtained. The solution was then allowed to stand for crystallization in air. Colorless crystals of BW₁₂·2CD were obtained by slow ethanol vapor diffusion into water solution (10 mL vessel containing 2 mL of water solution was placed into a 250 mL vessel with a tight-fitting lid containing 50 mL of ethanol) after 3 days, and were collected, washed with a water/ethanol mixture and dried in air. Yield 37 mg, 30%. The FT-IR spectrum is given in Fig. S23.† ICP-OES Anal. Calcd for BC_1.5CsH₂₂₀K₂O₁₄₉W₁₂: B, 0.17; C, 18.64; K, 1.26; W, 35.66. Found: B, 0.2; C, 18.5; K, 1.2; W, 32.5. EDS showed Cs : K : W ratio = 1.7 : 2.1 : 12. TGA showed a weight loss of 8.4% in the 20–220 °C temperature range corresponding to the twenty-nine hydration water molecules (calculated 8.4%) and a weight loss of 42% in the 220–700 °C range assigned to the loss of 2 CD (calculated 41.1%).

Na₃{[PW₁₂O₄₀]·(C₄₈H₈₀O₄₀)}·10H₂O (PW₁₂@CD). Na_1.4H_1.6[PW₁₂O₄₀]·12H₂O (0.3 g; 0.096 mmol) was dissolved in 10 mL aqueous solution of NaCl (0.2 mol L⁻¹) and γ-CD (0.138 g; 0.096 mmol) was added. The clear solution was stirred for 15 min. The solution was then allowed to stand for crystallization in air. Needle-like colorless crystals of PW₁₂@CD suitable for single crystal X-ray diffraction appeared within 5 days. They were isolated by filtration and washed with cold water. Yield 0.35 g, 76%. The FT-IR spectrum is given in Fig. S24.† ICP-OES Anal. Calcd for C₄₈H₁₀₀Na₃O₉₀PW₁₂: C, 13.03; Na, 1.56; P, 0.70; W, 49.87. Found: C, 12.8; Na, 1.6; P, 0.7; W, 48.9. EDS showed Na : P : W ratio = 2.7 : 1.1 : 12. TGA showed a weight loss of 3.9% in the 20–220 °C temperature range corresponding to the ten hydration water molecules (calculated 4.1%) and a weight loss of 26.8% in the 220–700 °C range assigned to the loss of 1.3 CD (calculated 27%).

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work is supported by a public grant overseen by the French National Research Agency as part of the “Investissements d'Avenir” program (Labex Charm3at, ANR-11-LABX-0039-grant) as well as the CNRS-MOMENTUM. This research is also supported by the China Scholarship Council (CSC) to S. Y. (201904910419). The authors are grateful to Flavien Bourdreux and Cyrielle Rey for their help with elemental analyses.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Additional experimental data (single crystal XRD, ITC, electrochemistry, NMR, SAXS and FT-IR spectroscopy). CCDC 2015681 and 2015682. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0qi00902d

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