# Selective recognition of fluoride anion in water by a copper(ii) center embedded in a hydrophobic cavity

Andrea Brugnara, Filip Topić, Kari Rissanen, Aurélien de La Lande, Benoit Colasson, Olivia Reinaud

► To cite this version:

Andrea Brugnara, Filip Topić, Kari Rissanen, Aurélien de La Lande, Benoit Colasson, et al.. Selective recognition of fluoride anion in water by a copper(ii) center embedded in a hydrophobic cavity. Chemical Science, 2014, 5 (10), pp.3897-3904. 10.1039/c4sc01457j. hal-05423099

HAL Id: hal-05423099

https://hal.science/hal-05423099v1

Submitted on 18 Dec 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

CrossMark

← click for updates

Cite this: *Chem. Sci.*, 2014, 5, 3897

# Selective recognition of fluoride anion in water by a copper(II) center embedded in a hydrophobic cavity†

Andrea Brugnara,ª Filip Topić,ª Kari Rissanen,ª Aurélien de la Lande,ᵇ Benoit Colassonª and Olivia Reinaudª

The ability of a water-soluble pentacationic calix[6]arene-based CuII complex to bind anions in water has been explored. Quite remarkably, the complex exhibits strong and selective fluoride binding in the pH range of 6–7. The binding constant at pH 5.9 was evaluated to be 85 000 M⁻¹, which is one of the highest values ever reported for a fluoride probe in water and at this pH. The complex also binds chloride ions, but 1000 times less efficiently. The combination of the calix[6]arene hydrophobic cavity with the CuII complex, presenting its labile site in the *endo* position, is the reason for the selective recognition process. The single crystal X-ray structure of the organo-soluble parent complex revealed a strong interaction between the coordinated fluoride anion and a hosted CHCl₃ solvent molecule. Molecular modeling applying an aqueous environment suggests that a water cluster, [F·H₂O·H₂O]⁻, is the species recognized by the host, which provides an appropriate environment for the stabilization of such a hydrated fluoride guest/species.

Received 18th May 2014

Accepted 18th June 2014

DOI: 10.1039/c4sc01457j

www.rsc.org/chemicalscience

## Introduction

Anion recognition and complexation by hosting molecules is an active field of research in supramolecular chemistry, as witnessed by the intense activity in reviewing the subject in the last few years.¹ Indeed, the design of anion receptors is particularly challenging: anions are larger than isoelectronic cations,² which leads to less effective electrostatic binding interactions; they may be sensitive to pH, becoming protonated at low pH; and hydroxylic solvents can strongly solvate anions through multiple hydrogen bonds. The major tools for building an anion receptor are charge-charge interactions, H-bonding, coordination bonds to metal ions and the hydrophobic effect. Whereas anion recognition has been relatively well mastered in organic solvents, major difficulties arise in water, due to strong competition with the solvent. In particular, binding highly solvated anions such as fluoride and sulfate (the more hydrophilic anions of the Hofmeister series)³ in water is a challenge *per se*, as reflected by the small number of systems that have been reported in the literature. Fluoride recognition by

synthetic hosts has been receiving particular interest, not only because of the scientific challenge, but also for societal reasons.⁴ While a low concentration of fluoride in the human diet has beneficial health effects, particularly in dental care, it seems to be toxic at high doses and has been related to several human pathologies (osteosarcoma, neurological dysfunctions etc.). Hence, despite the interest in fluoride recognition in water, only a few cases in which the binding can be achieved in this medium have been reported in the literature. This recognition process was actually achieved either with polyprotonated azacryptands in acidic media⁵ᵃ⁻⁶⁾ or by coordinatively unsaturated rare earth⁵⁸⁻⁷⁾ or uranyl⁵⁸ᵏ metal complexes, with one example employing a foldameric structure⁵⁹⁾ and only two examples employing copper center(s).⁵ᵃ The existence of only a few examples in the literature is probably due to the peculiar properties of this anionic species. Indeed, its high hydration energy (ΔrG°hyd = −429 kJ mol⁻¹)⁶⁾ makes its recognition process energetically highly demanding for a host, and as a hard base, it is difficult to distinguish from water.

Inspired by nature, we have been developing a set of biomimetic systems associating a binding site for a metal ion with a hydrophobic cavity. Our recent efforts have aimed at synthesizing water-soluble “cavity-complexes”. Our first example was obtained with calix[6]arenes functionalized at the small rim by three imidazole groups providing a tridentate coordination core, and at the large rim by hydrophilic substituents to ensure water-solubility. Initial studies showed that the calix[6]arene-based tris(imidazole) core was able to stabilize and protect a potentially reactive metal center (CuI), while

<footnote>ªLaboratoire de Chimie et de Biochimie Pharmacologiques et Toxicologiques, CNRS UMR 8601, Université Paris Descartes, 45 rue des Saints Pères, 75006 Paris, France</footnote>

<footnote>ᵇLaboratoire de Chimie Physique, CNRS UMR 8000, Université Paris-Sud 11, Campus Orsay, 15 Rue Georges Clemenceau, 91400 Orsay, France</footnote>

<footnote>Department of Chemistry, Nanoscience Center, University of Jyväskylä, P.O. Box 35, 40014 Jyväskylä, Finland</footnote>

<footnote>† Electronic supplementary information (ESI) available: Materials and methods, supporting data, XRD details. CCDC 997677. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4sc01457j</footnote>
maintaining a coordination site open to exogenous ligand binding.⁷ We also described a ZnII complex that can selectively host hydrophobic primary amines in water thanks to the embedment of the amino-guest inside the cone of the calixarene.⁸ More recently, we succeeded in obtaining an open-shell copper complex that is stable in water in both of its oxidation states, CuI and CuII. The latter complexes are based on a calix[6]-arene core covalently capped at its small rim by a TMPA unit that allows strong binding of one isolated copper ion at the bottom of the conical hydrophobic cavity [see Fig. 1, calix-ligands 1 and 2(NO₃)₃; TMPA: tris(2-pyridylmethyl)amine].⁹ The water solubility of the system is ensured by the presence of three quaternary ammonium substituents implemented at the large rim of the calix.

These mononuclear copper complexes $[Cu^{II}/2]^{5/4+}$ have an unsaturated coordination site available for exogenous ligand binding, and directed toward the cavity: the binding site is located in a protected microenvironment, shielded from the aqueous solvent by the calix[6]arene moiety. In this paper, we report the capacity of the CuII complex of 2(NO₃)₃ to bind the highly hydrophilic fluoride anion in water, highlighting the importance of surrounding the binding site by a hydrophobic cavity in the recognition process.

## Complexation of fluoride anion by complex $[Cu^{II}/2]^{5+}$ in water

Compound [H2](NO₃)₄ and complex $[Cu^{II}/2](NO₃)₅$ (ref. 10) were prepared according to the procedure reported in the literature.⁹ The interaction of $[Cu^{II}/2]^{5+}$ with anions was investigated in buffered water by UV-Vis spectroscopy by measuring absorption bands due to d-d transitions. Previous studies on $[Cu^{II}/2]^{5+}$ showed that the molecule of water coordinated to the copper center has a pKₐ value of 6.7. Therefore, we conducted the studies in a slightly acidic medium (pH = 5.9), using a MES ([2-(*N*-morpholino)ethanesulfonic acid]/KOH) buffer, in order to ensure the aqua species was predominant in solution.¹¹ In a

preliminary study, we noticed that the affinity of the aqua complex for the fluoride anion was very high. In order to measure the binding constant accurately, we evaluated this affinity through competition experiments with the chloro complex. Hence, we looked first at the coordination of chloride in water. To an aqueous buffered solution (pH = 5.9, [MES] = 30 mM) of $[Cu^{II}/2]^{5+}$, the progressive addition of NaCl up to 70 equivalents resulted in spectral changes of the d-d absorptions, displayed in Fig. 2A. The resulting new species shows two absorption bands with maxima at 742 and 974 nm ($ε$ = 107 and 120 M⁻¹ cm⁻¹, respectively). These values are in agreement with the formation of complex $[Cu^{II}(Cl)2]^{4+}$, in which the chloride anion is coordinated to the metal center, since comparable values were obtained for $[Cu^{II}(Cl)1]⁺$ in an organic solvent.¹²

The presence of isosbestic points clearly shows that the two cupric species involved in the equilibrium are linearly related by stoichiometry. Nonlinear least-squares treatment of the titration data, using dedicated software,¹³ indicated the formation of a complex $[Cu^{II}(Cl)2]^{4+}$ with an association constant of $76 \pm 4 \text{ M}^{-1}$.

In an analogous way, to an aqueous buffered solution (pH = 5.9, [MES] = 30 mM) of $[Cu^{II}/2]^{5+}$ containing an excess of NaCl (70 eq.),¹⁴ the progressive addition of NaF resulted in spectral changes of the d-d bands, displayed in Fig. 2B. After the addition of 1.3 eq. of NaF (vs. $[Cu^{II}(Cl)2]^{4+}$), the two absorption bands characteristic of the $[Cu^{II}(Cl)2]^{4+}$ species completely vanished, being replaced by two new absorption bands with maxima at 696 and 872 nm ($ε$ = 96 and 153 M⁻¹ cm⁻¹, respectively). The presence of isosbestic points again shows a simple ligand exchange at the metal center.

This information, together with that from EPR (Fig. 3) and ESI-MS (Fig. S1†) spectra, unambiguously attest to the

<figure><img src="image_3.png" /><figcaption>Air stable open-shell complexes</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Fig. 1** Open-shell CuI/CuII funnel complexes of ligand [H2](NO₃)₄, a water-soluble version of 1.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 2** UV-Vis spectra (298 K) in water (pH 5.9 buffered solution, [MES] = 30 mM) of: (A) $[Cu^{II}/2](NO₃)₅$ (2.4 mM) with stepwise addition of NaCl (from 0 to 70 eq.) and (B) $[Cu^{II}/2](NO₃)₅$ (1.7 mM) in the presence of NaCl (120 mM) with stepwise addition of NaF (from 0 to 1.3 eq.).</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 3 EPR spectra (100 K, H₂O) of: (A) [CuII2](NO₃)₅ (15 mM); (B) [CuII2](NO₃)₅ (11 mM) in the presence of NaF (24 mM), (g⊥ = 2.21, g‖ = 1.96, A‖ = 146 G).</figcaption></figure>

substitution of the chloride anion by fluoride at the copper center. The affinity of complex [CuII2]⁵⁺ for the fluoride anion was evaluated to be 85 000 ± 4500 M⁻¹,¹³ which is more than three orders of magnitude higher than for chloride.

As expected, [CuII(F)2]⁴⁺ was revealed to be pH-sensitive: increasing the pH of the solution induced spectral changes in the pH window from 6.9–11. At pH = 11, the spectroscopic features observed by UV-Vis absorption (maxima at 672, 830 and 1015 nm with ε = 156, 126 and 86 M⁻¹ cm⁻¹, respectively) and EPR (A‖ = 190 G) spectroscopies indicate the exclusive formation of [CuII(OH)2]⁴⁺ (Fig. S2 and S3†). The thermodynamic constant for the substitution of fluoride by hydroxide is 250 (Scheme 1).¹⁵

In order to evaluate the various contributions to fluoride binding by the metal center and the calix[6]arene cavity in [CuII(F)2]⁴⁺, titration experiments with NaF in water were conducted with [H2]⁴⁺ and [CuII3]²⁺ as possible hosts, under similar experimental conditions (Fig. 4). The addition of [H2](NO₃)₄ to a D₂O solution of NaF did not affect the ¹⁹F NMR spectrum, indicating that the two partners do not interact. Hence, the

<figure><img src="image_7.png" /><figcaption>Fig. 4 Evidence for the synergistic contributions of the CuII complex and the calixarene cavity for fluoride binding in water by [CuII2]⁵⁺. ¹⁹F NMR spectra (300 MHz, D₂O, 293 K) of: NaF 4.2 mM (A); NaF 4.0 mM and [H2](NO₃)₄ 4.0 mM (B). UV-Visible spectra (298 K) in water (pH 5.9 buffered solution, [MES] = 30 mM) of [CuII3](OTf)₂ (2.31 mM) in the presence of 0, 1 and 10 eq. of NaF (C).</figcaption></figure>

metal ion plays a crucial role in fluoride binding. The addition of up to 10 eq. of NaF to an aqueous buffered solution (pH = 5.9, [MES] = 30 mM) of [CuII3]²⁺ did not affect the UV-Vis absorption spectrum, suggesting that these two partners do not interact either. This demonstrates that the cavity also plays a crucial role in binding. Hence, as illustrated in Fig. 4, only the complexation of the cupric center within the calix[6]arene framework provides a receptor that efficiently recognizes fluoride in aqueous solution.

## Crystal structure of the organo-soluble version of the fluoride complex, [CuII(F)1]⁺

In order to rationalize the exceptional affinity of the calix-complex for fluoride, we first tried to obtain insights into the molecular structure of the complex. All our efforts to obtain single crystals of the water-soluble [CuII(F)2]⁴⁺ were unsuccessful, however, we succeeded in crystallizing the corresponding organo-soluble complex, [CuII(F)1](ClO₄)·† This complex was obtained by reacting one equivalent of KF dissolved in water with the dicationic aqua complex [CuII(OH₂)1](ClO₄)₂ dissolved in dichloromethane. Its spectroscopic data (EPR and UV-Vis) have been previously reported¹² and are very similar to those reported for the water-soluble complex herein. Single crystals of

<figure><img src="image_8.png" /><figcaption>Scheme 1 Anion binding by complex [CuII2]⁵⁺ in aqueous solution.</figcaption></figure>
[CuII(F)1](ClO₄) were grown by slow diffusion of hexane into a chloroform solution of the complex at 4 °C. Four different conformers of the host were observed within the crystal structure, out of which three are displayed in Fig. 5. The asymmetric unit contains three [CuII(F)1]⁺ moieties, each in a markedly different conformation; A, B or C. While conformers A and B were modeled with full occupancy, C was modeled as disordered with an occupancy of 56.3(7)%, with the minor

component B' (occupancy 43.7(7)%) exhibiting a conformation analogous to that of B (see ESI†).

## Description of the host structures

The three conformers are all very different, each of them providing insights into the specific interactions of the fluoride anion and the surrounding host macrocycle. Conformer A presents a 1,3-alternate conformation where two anisole moieties are completely upside-down relative to the neighboring aromatic rings, while that in position 5 is nearly perpendicular to the neighboring rings. The conformers B and C are more symmetrical, with the calixarene core in a cone conformation. Structures B and B' (see ESI†) have an encapsulated CHCl₃ guest molecule in the calixarene cavity.

- In structure A, despite the differing orientations of the respective anisole moieties, all three methoxy groups are endo-oriented. They close the cavity opening, forming a closed hydrophobic pocket around the fluoride anion. One of the methoxy groups is hydrogen bonded to the fluoride anion with moderate strength [$d(C···F) = 3.09$ Å, with a C-H···F angle of 143.5°, Fig. 5 (A)]. No other H-bonds occur in this structure, the other H₃C···F distances being 4.05 and 4.47 Å.

- In structure B, a chloroform molecule occupies the center of the calix cavity completely. The CHCl₃ fits very snugly into the cavity, the Cl···C contact distances to the methoxy-substituted benzene ring carbon atoms being only 3.22, 3.28 and 3.38 Å, somewhat shorter than the sum (3.45 Å) of the van der Waals radii of the chlorine (1.75 Å) and aromatic carbon (1.7 Å). The CHCl₃ is strongly hydrogen bonded to the fluoride anion ($d(C···F) = 2.84$ Å, with a C-H···F angle of 162.2°). The calixarene structure displays a flattened cone conformation with the aromatic units alternately in the endo and exo positions relative to the center of the macrocycle. All methoxy groups point towards the outside, away from the CuF center.

- In structure C, and in contrast to structures A and B, one of the methoxy groups is oriented in the endo position. This allows the methyl group to interact with the fluoride anion. The corresponding $d(C···F) = 3.16$ Å and C-H···F angle of 137.4° indicate hydrogen bonding, albeit weaker than in the case of the structure A.

These molecular structures are very informative regarding the possible stabilizing interactions between F⁻ and the calixarene host: structures A and C for the self-inclusion of the methoxy groups through weak H-bond interactions without guest entrapment, and structure B for the solvent entrapment via strong H-bonding.

## Molecular modeling

In order to obtain some insight into the interaction energies, the relative stability of the structures and solvent interaction, molecular modeling studies were performed. With the focus of our efforts on understanding the strong affinity of the complex for fluoride in water, all calculations reported below were performed with the fluoride complex based on 2. Three different structures for the [CuII2(F)]⁴⁺ complex were geometrically

<figure><img src="image_9.png" /><figcaption>Fig. 5 Conformers of [CuII(F)1]⁺ as observed in the crystal structure. The groups inside the calixarene cavity are depicted in the space filling model. The calixarene hydrogen atoms are excluded for clarity. Color code for the atoms: hydrogen in white, carbon in grey, oxygen in red, nitrogen in blue, copper in orange, fluorine in green and chlorine in light green.</figcaption></figure>
optimized at the Density Functional Theory (DFT) level of theory, modeling the solvent with a polarizable continuum: a C₃ cone conformation with an empty cavity and all methoxy groups in the *exo* position; a cone conformation with one methoxy group in the *endo* position interacting with F⁻ (analogous to structure C); and a cone conformation with a chloroform molecule sitting in the cavity (analogous to structure B). As expected, in each case, the cupric center in the complex adopts a trigonal bipyramidal geometry (Fig. S6†). The Cu–F bond length is short for all structures (1.85 Å), suggesting a strong Cu–F bond in all three cases. The minimized structure obtained with a methoxy group hydrogen bonded to F⁻ is very similar to structure C obtained by XRD. However, this structure lies 7.5 kJ mol⁻¹ above the one with all methoxy groups in the *exo* position. This is indicative of a cost associated with the *exo* → *endo* re-orientation of the methoxy group, probably through structural tension of the macrocycle. The structure minimized with a chloroform guest resembles structure B. All these results validate the computational methodology. In light of these results, we further explored the putative role of solvent inclusion on the complex stabilization with water, in water.

## Binding energies

We estimated the Gibbs free energy of binding of different ligands (X⁻) to the [CuII3]²⁺ complexes (ΔGbind, Table 1). For this purpose, a key point is to define a theoretical method enabling a quantitative reproduction of the free energies of hydration of the halides. Our methodology (DFT CAM-B3LYP-D with implicit solvation model, see ESI†) leads to values of −424 and −308 kJ mol⁻¹ for X⁻ = F⁻ or Cl⁻ ions, respectively, in excellent agreement with the experimental values reported by Tissandier *et al.* (−429 and −304 kJ mol⁻¹).⁶ Turning to the Gibbs free energy of binding ΔGbind, this amounts to −109 kJ mol⁻¹ and −84 kJ mol⁻¹ for X⁻ = F⁻ or Cl⁻ ions, respectively, which indicates a stronger intrinsic coordination of fluoride ions to the cupric center within the first coordination sphere provided by the TMPA core. With the [CuII2(X)]⁴⁺ complex, the values are −110 kJ mol⁻¹ and −52 kJ mol⁻¹, for X = F and Cl, respectively. This indicates that the second coordination sphere provided by the calixarene core slightly strengthens the Cu–F bond and

significantly weakens the Cu–Cl bond, with, as a net result, an enhanced discrimination in the binding of these two halides. We attribute the origin of this increased affinity and selectivity in favor of fluoride to a delicate balance of electrostatic interactions at the small rim: attractive with the CH₂ arms linking the TMPA cap to the calixarene [d(X···C) = 3.00/3.17 Å for X = (F/Cl)] and repulsive with the oxygen atoms of the anisole units [d(X···O) = 3.73/3.66 for X = (F/Cl)]. However, we found that the water molecule binds to the [CuII3]²⁺ and [CuII2]⁵⁺ complexes with ΔGbind values of −135 and −140 kJ mol⁻¹, respectively. As a consequence, the standard free energies associated with the displacement of a water molecule by either F⁻ or Cl⁻ (ΔGdisp) are thermodynamically unfavorable. The computational results are thus in line with the experiment performed on the [CuII3]²⁺ complex (Fig. 4). Knowing that the calix[6]arene cavity defines a space that can be occupied by two water molecules,¹⁶ we explored the structures of hydrated complexes. We performed optimizations of [CuII(F·H₂O·H₂O)₂]⁴⁺ starting from different initial geometries, in which between one and three water molecules interact directly with the halide anion. It turned out that all structures converged towards the same type of geometry, displayed in Fig. 6 for [CuII(F·H₂O·H₂O)₂]⁴⁺. A single water molecule is hydrogen bonded to the F⁻ anion. This interaction is very strong considering the length of the hydrogen bond [d(F···O) = 2.60 Å]. The water molecule also interacts with the aromatic unit through an OH–π interaction [the shortest C(Ar)···O(OH) distance equals 3.6 Å], and with a second water molecule through a hydrogen bond (d(O···O) = 2.73 Å). The second water molecule is located lower within the cavity and interacts through OH–π interactions [the shortest C(Ar)···O(OH) distance equals 3.4 Å]. A favorable value of −32 kJ mol⁻¹ for ΔGdisp is found upon replacing a water ligand by the hydrated (F(H₂O)₂)⁻ fluoride ions within the [CuII2]⁵⁺ complex. Importantly, ΔGdisp remains positive with the [CuII3]²⁺ complex (5 kJ mol⁻¹), highlighting the capability of the calixarene core to stabilize the water cluster within the cavity.

<table><caption>**Table 1** Standard Gibbs free energies of binding (ΔGbind) of various ligands to the [CuII2]⁵⁺ and [CuII3]²⁺ complexes and standard Gibbs free energies (ΔGdisp) of displacement of a water molecule by these ligands. ΔGdisp values are corrected to take into account the fact that water is the solvent (see ESI†). Values are given in kJ mol⁻¹. All the calculations were performed at the CAM-B3LYP-D/6-311(+)G** level of theory in implicit aqueous solvent¹⁷</caption><thead><tr><th rowspan="2"></th><th colspan="2">[CuII2]⁵⁺</th><th colspan="2">[CuII3]²⁺</th></tr><tr><th>ΔG<sub>bind</sub></th><th>ΔG<sub>disp</sub></th><th>ΔG<sub>bind</sub></th><th>ΔG<sub>disp</sub></th></tr></thead><tbody><tr><td>F<sup>-</sup></td><td>-110</td><td>39</td><td>-109</td><td>35</td></tr><tr><td>(F(H<sub>2</sub>O)<sub>2</sub>)<sup>-</sup></td><td>-182</td><td>-32</td><td>-139</td><td>5</td></tr><tr><td>Cl<sup>-</sup></td><td>-52</td><td>98</td><td>-84</td><td>60</td></tr><tr><td>H<sub>2</sub>O</td><td>-140</td><td>—</td><td>-135</td><td>—</td></tr></tbody></table>

<figure><img src="image_10.png" /><figcaption>**Fig. 6** DFT optimized structure of the [CuII(F·H₂O·H₂O)₂]⁴⁺ complex. The calixarene hydrogen atoms are not shown for clarity. Color code: hydrogen in white, carbon in grey, oxygen in red, nitrogen in blue, copper in orange and fluorine in green.</figcaption></figure>
A bis-aqua fluoride complex

It is well known that the calixarene macrocycle is flexible and
can adopt many more conformations with fluctuating water
content. In order to further investigate the dynamics of the
water molecules within the flexible cavity, we have performed
120 ns of classical molecular dynamics simulations of the
[CuII(F·H₂O·H₂O)₂]⁴⁺ complex solvated in a box of water
molecules. We observed two types of structure in the course of
the MD simulations (Fig. 7). In the first type, the cavity is filled
by two water molecules. The structure is very similar to that
described above for the DFT optimized [CuII(F·H₂O·H₂O)₂]⁴⁺
complex. In the second type of structure, the cavity of the
fluoride complex is empty. The exchanges between the two types
of structures, one filled with two water guests, the other empty,
are easily seen by following the distances between the tBu
groups (Fig. 7).¹⁸ When the cavity is empty, it is shrunk as seen
by inter-tBu distances fluctuating around 6.6 Å. When the cavity
is filled, its volume is inflated and the distances increase by
almost 1 Å. Note that the situations where three or four water
molecules are present are rare. The cavity may eventually be
filled by only one water molecule. In fact, we found that the
content of the cavity fluctuates, with frequent exchanges of
water molecules between the solvent and the cavity. These
exchanges occur on the nanosecond time scale. The mechanism
of exchange is dissociative, with the departure of one water
molecule from the cavity before another molecule enters. The
situation where only one water molecule is present corresponds
to these intermediate situations.

When the cavity is filled, the average interaction energy (AIE<sup>MM</sup>) between the water cluster and the calixarene complex calculated at the MM level along the MD simulation amounts to -50 kJ mol<sup>-1</sup>. This AIE<sup>MM</sup> can be decomposed into contributions arising from the different molecular fragments of the complex. The main sources of stabilization of the water cluster are the fluoride ion and the cavity with only little contribution from the NMe<sub>3</sub><sup>+</sup> groups (Table S2†). Hence, the MD simulations

<figure><img src="image_11.png" /><figcaption>Fig. 7 Fluctuations along the MD simulations of the three distances between the tBu central carbon atoms of the fluoride CuII complex based on the tri-cationic ligand 2. For clarity, we have plotted the running averages over time windows of 1.25 ns. Corresponding modeled structures (from top to bottom): two water molecules in; empty cavity. To count the water molecules inside the cavity, the edge of the cavity is identified as the plane defined by the three tBu central carbon atoms.</figcaption></figure>

indicate that the complex can adopt either a packed confor-
mation, in which the access of water molecules to the cavity is
hindered by the tBu groups, or an open conformation, which
allows a water molecule to interact with the bound fluoride
anion and the aromatic rings. When the cavity is open, the
optimal number of water molecules inside the cavity is two.
Quite remarkably, this shows that the macrocycle offers a well-
defined cavity space suitable for stabilizing fluoride as a
[F·H₂O·H₂O]⁻ cluster. The presence of a couple of water
molecules embedded in the calix[6]arene core was previously
observed with a ZnII funnel complex, in the solid state as well as
in solution.¹⁶

Discussion

XRD analysis of the fluoride complex crystallized from chloro-
form led to the characterization of three different conforma-
tions for the calix-complex, which highlights the flexibility of
the calix[6]arene macrocycle, in spite of the constraints imposed
by the TMPA cap. In these structures, interaction of F⁻ with
either a solvent molecule or a methoxy group belonging to the
anisole units of the ligand is observed. Most interestingly, a very
strong H-bond was evidenced between the embedded solvent
molecule CHCl₃ and F⁻, highlighting the well-known propen-
sity of F⁻ for H-bonding. This suggests that in water too, the F⁻
anion can be stabilized through H-bonding to the hosted
solvent. Molecular modeling further supports this possibility,
with the synergistic binding of a cluster of two water solvents in
the heart of the cone.

The possibility of binding partially hydrated F⁻ species has been highlighted by several solid-state investigations. Single crystal X-ray structures of fluoride–protein complexes (e.g.: pyruvate decarboxylase,¹⁹ butyrylcholinesterase²⁰) often show the anion interacting *via* hydrogen bonding with NH backbones and side chain residues (typically Ser, Thr, Asp...) and having a water molecule which completes its coordination sphere. In the case of synthetic receptors defining a protected and appropriate environment, hydrated fluoride species have been similarly observed for host–guest complexes in the solid state.²¹ For these receptors, the number of water molecules directly coordinated to F⁻ depends on the host design (nature of coordinating groups, geometry, size *etc*. ) and may reach quite high values in some cases. For example, a tetrahydrated cluster, [F(H₂O)₄]⁻, encapsulated in a tricyclic organic molecular capsule has been reported.²¹c All these “biotic and abiotic” examples of hosts binding a cluster of fluoride and water as a guest illustrate the high hydration energy of this anion. We calculated that the free energies of hydration of a “naked” fluoride and of a [F(H₂O)₄]⁻ anion amount to -424 and -371 kJ mol⁻¹, respectively. Hence, it is reasonable to think that receptors able to recognize a partially hydrated [F(H₂O)₄]⁻ species should be more efficient.

For compound [CuII2]5+, we noticed a good selectivity (ca. 10³) for fluoride over chloride complexation, which is in agreement with a stronger CuII–F bond compared to a CuII–Cl bond. DFT calculations point to the enhancement of the binding selectivity by the calixarene cavity. The selectivity may also be related to the encapsulation of water molecules, since
the free energy of hydration for fluoride is higher than for chloride [$\Delta_r G^{\circ}_{hyd}(F^-) = -429$ kJ mol$^{-1}$, $\Delta_r G^{\circ}_{hyd}(Cl^-) = -304$ kJ mol$^{-1}$].⁶ As suggested by molecular modeling studies of compound [CuII(F)2]⁴⁺, this result can be rationalized by taking into account [F·H₂O·H₂O]⁻ as the guest species for the host [CuII2]⁵⁺, in which water molecules facilitate the fluoride extraction from water in two ways (Fig. 8): (i) the dehydration cost is considerably lowered (by ca. 80 kJ mol⁻¹ for the coordination of one remaining water; possibly more, due to the participation of water in the second sphere), and (ii) water molecules present in the cavity interact with the aromatic walls of the macrocycle *via* OH-π interactions, further stabilizing the host-guest complex.

Conclusion

In summary, complex [CuII2]⁵⁺ exhibits an exceptionally high affinity for the fluoride anion in a medium of 100% water (log K = 4.93 ± 0.02). It is a rare example of a transition metal-based receptor,⁵ª and as far as we know, it displays the highest association constant value among the monometallic systems described in the literature.⁵ª⁻⁵e In contrast to most azacryptand-based systems,⁵ª⁻e in which the recognition process is typically optimal at low pH values in order to obtain a polyprotonated host, complex [CuII2]⁵⁺ strongly binds F⁻ in the pH window of 6–7.²²

Binding studies monitored with the CuII complex deprived of a cavity and the calixarene ligand deprived of CuII have revealed that no complexation of fluoride takes place under the same experimental conditions. These observations bring to light the synergistic roles of CuII and the calixarene core in the binding process. XRD analysis of the organo-soluble fluoride complex based on ligand **1** highlights the possible embedment of one molecule of solvent (CHCl₃) inside the calix cavity with an associated strong hydrogen bond with F⁻. Molecular modeling calculations have shown that the anionic guest F⁻ can strongly interact with two water molecules within the cavity. Hence, a possible explanation to rationalize the observed synergy between CuII coordination and cavity embedment for fluoride extraction from water may stem from the complexation of a partially hydrated species, likely [F·H₂O·H₂O]⁻, which, compared to the complexation of a “naked” F⁻ ion, is energetically less demanding for the host.

<figure><img src="image_12.png" /><figcaption>**Fig. 8** Proposed mechanism for F⁻ binding in water by a calixarene CuII complex, according to molecular modeling.</figcaption></figure>

Acknowledgements

The authors thank Dr Elise Prost for ¹⁹F NMR spectra (Paris Descartes Univ.). This work was supported by the CNRS (Institut de Chimie), the Ministère de l'Enseignement Supérieur et de la Recherche, the Agence Nationale pour la Recherche [Cavityzyme(Cu) Project ANR-2010-BLAN-7141], the Academy of Finland (KR, grant no. 265328 and 263256), and the National Doctoral Program in Nanoscience, Finland (F. T., PhD fellowship) and the COST Action “Supramolecular Chemistry in Water” (CM 1005). This work was performed using HPC resources from GENCI [CCRT/CINES/IDRIS] (Grant 2013-2014, project number c2014076913).

Notes and references

1. For a selection of examples see: (a) A. Frontera, *Coord. Chem. Rev.*, 2013, **257**(11-12), 1716-1727; (b) M. Wenzel, J. R. Hiscock and P. A. Gale, *Chem. Soc. Rev.*, 2012, **41**, 480-520; (c) P. Ballester, *Chem. Soc. Rev.*, 2010, **39**, 3810-3830; (d) S. Kubik, *Chem. Soc. Rev.*, 2010, **39**, 3648-3663; (e) J. W. Steed, *Chem. Soc. Rev.*, 2009, **38**, 506-519.

2. R. D. Shannon, *Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr.*, 1976, **32**, 751.

3. F. Hofmeister, *Arch. Exp. Pathol. Pharmakol.*, 1888, **24**, 247.

4. (a) M. Cametti and K. Rissanen, *Chem. Commun.*, 2009, 2809-2829; (b) M. Cametti and K. Rissanen, *Chem. Soc. Rev.*, 2013, **42**, 2016-2038.

5. Fluoride receptors in 100% aqueous solution: (a) R. J. Motekaitis, A. E. Martell, I. Murase, J.-M. Lehn and M.-W. Hosseini, *Inorg. Chem.*, 1988, **27**, 3630-3636; (b) B. Dietrich, B. Dilworth, J.-M. Lehn, J.-P. Souchez, M. Cesario, J. Guilhem and C. Pascard, *Helv. Chim. Acta*, 1996, **79**, 569-587; (c) M. A. Hossain, J. M. Llinares, S. Mason, P. Morehouse, D. Powell and K. Bowman-James, *Angew. Chem., Int. Ed.*, 2002, **41**, 2335-2338; (d) C. A. Ilioudis, D. A. Tocher and J. W. Steed, *J. Am. Chem. Soc.*, 2004, **126**(39), 12395-12402; (e) B.-G. Zhang, P. Cai, C.-Y. Duan, R. Miao, L.-G. Zhu, T. Niitsu and H. Inoue, *Chem. Commun.*, 2004, 2206-2207; (f) J. Suk and K. Jeong, *J. Am. Chem. Soc.*, 2008, **130**(36), 11868-11869; (g) E. L. Yee, O. A. Gansow and M.-J. Weaver, *J. Am. Chem. Soc.*, 1980, **102**, 2278-2285; (h) R. Tripier, C. Platas-Iglesias, A. Boos, J.-F. Morfin and L. Charbonnière, *Eur. J. Inorg. Chem.*, 2010, 2735-2745; (i) L. M. P. Lima, A. Lecointre, J.-F. Morfin, A. de Blas, D. Visvikis, L. Charbonnière, C. Platas-Iglesias and R. Tripier, *Inorg. Chem.*, 2011, **50**, 12508-12521; (j) A. Dalla Cort, G. Forte and L. Schiaffino, *J. Org. Chem.*, 2011, **76**, 7569-7572; (k) M. Cametti, A. Dalla Cort and K. Bartik, *ChemPhysChem*, 2008, **9**, 2168-2171.

6. M. D. Tissandier, K. A. Cowen, W. Y. Feng, E. Gundlack, M. H. Cohen, A. D. Earhart and J. V. Coe, *J. Phys. Chem. A*, 1998, **102**, 7787-7794.

7. Y. Rondelez, G. Bertho and O. Reinaud, *Angew. Chem., Int. Ed.*, 2002, **41**, 1044-1046.

8. O. Bistri, B. Colasson and O. Reinaud, *Chem. Sci.*, 2012, **3**, 811-818.
9 G. Thiabaud, A. Brugnara, M. Carboni, N. Le Poul, B. Colasson, Y. Le Mest and O. Reinaud, *Org. Lett.*, **2012**, *14*, 2500–2503.

10 Unless otherwise mentioned, the fifth binding site in the cavity is occupied by a water molecule.

11 G. Thiabaud, PhD thesis, Université Paris Descartes, 2010.

12 G. Izzet, X. Zeng, H. Akdas, J. Marrot and O. Reinaud, *Chem. Commun.*, **2007**, 810–812.

13 Affinity constants were evaluated by processing titration experiments with the HyperQuad package: P. Gans, A. Sabatini and A. Vacca, *Talanta*, **1996**, *43*, 1739–1753. http://www.hyperquad.co.uk/index.html.

14 Under these conditions, detailed in Fig. 2, more than 90% of the complex is a chloro species.

15 Evaluated with the $pK_a$ for the water molecule coordinated to the cupric centre in $[Cu^{II}2]^{5+}$ (6.7) and the log K for water substitution by fluoride (4.9) on the cupric centre.

16 (a) O. Sénèque, M.-N. Rager, M. Giorgi and O. Reinaud, *J. Am. Chem. Soc.*, **2001**, *123*, 8442–8443; (b) D. Coquière, J. Marrot and O. Reinaud, *Chem. Commun.*, **2006**, 3924–3926.

17 See the ESI† for the detailed computational protocol.

18 Even though we observe some exchange between the free cavity and the water-filled cavity conformations, the

number of exchanges during these MD simulations is too small to allow a calculation of the Gibbs free energy of inclusion of a water cluster from the bulk. Here, the MD simulations must be considered as the means of identification of mainly two types of cavities: either empty or filled by 2 water molecules.

19 X. Y. Pei, K. Erixon, B. F. Luisi and F. J. Leeper, *Biochemistry*, **2010**, *49*, 1727–1736.

20 F. Nachon, E. Carletti, M. Wandhammer, Y. Nicolet, L. M. Schopfer, P. Masson and O. Lockridge, *Biochem. J.*, **2011**, *434*, 73–82.

21 For a selection of examples see: (a) I. Ravikumar, S. Saha and P. Gosh, *Chem. Commun.*, **2011**, *47*, 4721–4723; (b) S. K. Kim, J. L. Sessler, D. E. Gross, C.-H. Lee, J. S. Kim, V. M. Lynch, L. H. Delmau and B. P. Hay, *J. Am. Chem. Soc.*, **2010**, *132*(16), 5827–5836; (c) Q.-Q. Wang, V. W. Day and K. Bowman-James, *Angew. Chem., Int. Ed.*, **2012**, *51*, 2119–2123.

22 At these pH values, only one example, a poly-protonated hepta-aza cryptand, displaying slightly higher affinity constants ($\log K \approx 5.5$, estimated at pH 7), has been previously reported. See ref. 5d.