Formation of tetrameric water clusters driven by a cavitand template

Abstract

We describe an innovative approach to the generation of tetrameric water clusters in the solid state. The specific H-bond pattern induced by the tetraphosphonate cavitand template via its rigidly preorganized P=O acceptor groups leads to the exclusive formation of the unique cyclic homodromic water tetramer of C₄ symmetry.

---

The molecular origins of the physical properties of water continue to intrigue the scientific community. Despite many theoretical and experimental efforts, understanding the behaviour of solid and liquid water at the molecular level remains a challenge.¹ The investigation of small water clusters is crucial to mimic water’s properties in the bulk phase.² The possibility of controlling the formation of new water cluster motifs allows investigations to be extended to the condensed phase in a stepwise manner. Various patterns of water arrangements (chains, rings, layers, etc.) in organic molecules are possible,³ and an extensive classification of the existing structures has been previously reported.⁴ Among these arrangements, the cyclic water tetramer, as a simple two-structure model for liquid water,⁵ is of particular significance. In the past decades, theoretical studies have predicted several configurations of cyclic water tetramers,⁶ some of which have been verified experimentally in the solid state.⁷ Nevertheless, the predictability and rational design of discrete water structures remain elusive.

Here we report an innovative approach for generating tetrameric water clusters in the solid state by using a tetraphosphonate cavitand template. The specific H-bond interaction pattern induced by the host via its rigidly preorganized platform of four P=O acceptor groups leads to the exclusive formation of a cyclic water tetramer with C₄ symmetry, which has never been characterized previously.

Phosphonate cavitands are extremely versatile molecular receptors, capable of binding both positively charged species such as inorganic cations,⁸ ammonium and pyridinium salts⁹ and neutral molecules such as alcohols.¹⁰ H-bonding, cation–π and CH–π interactions are the dominant binding modes of phosphonate cavitands. Each of them can be activated either individually or in combination, according to the guest’s requirements.^9a,b

The template molecule chosen was the tetraphosphonate cavitand Tiiii[H,CH₃,CH₃]¹¹ (Scheme 1), featuring four converging P=O bridges, located at the upper rim of a compact resorcinarene skeleton.¹² In this specific case, only the multiple H-bonding ability of the receptor was exploited to induce water clustering in the solid state.

Scheme 1 Top and side views of the Tiiii[H,CH₃,CH₃] template.

In the first experiment, crystals of Tiiii[H,CH₃,CH₃]·4H₂O§ were obtained by adding water to a suspension of the tetraphosphonate cavitand in acetonitrile, followed by slow evaporation. The complex crystallizes in the P4/n space group; it lies on a molecular fourfold axis, which coincides with a crystallographic fourfold axis. This leads to a highly symmetric structure in which every water molecule acts as: (i) a hydrogen bond donor to a P=O group (blue dotted lines in Fig. 1, O⋯O=P 2.801(5) Å); (ii) a hydrogen bond donor and acceptor towards two adjacent water molecules, respectively (green dotted lines in Fig. 1, O⋯O 2.858(5) Å). In this way, the four water molecules, linked to each other through a homodromic arrangement of H-bonds,¹³ form a discrete tetrameric cluster having full C₄ symmetry (see Table S2 for geometrical parameters of the hydrogen bonds‡). It must be emphasized that the cluster formation is fully reproducible under different crystallization conditions, indicating the strong templating effect exerted by the cavitand.

Fig. 1 Top and side view of Tiiii[H,CH₃,CH₃]·4H₂O. (Colour code: P, orange; O, red; C, grey; H, white; weak intermolecular interactions, blue and green). Only the hydrogen atoms involved in the interactions are shown.

Theoretical calculations^5,6c and experimental data^7d reported so far, show that in the minimum-energy geometries of tetrameric clusters, the four oxygen and the four hydrogen atoms involved in the cyclic array of hydrogen bonds are almost coplanar. Conversely, the other four “free” water hydrogens can adopt different configurations above or below this plane. Most of the time, the occurrence of a specific pattern is unpredictable and it depends both on energetic and kinetic parameters difficult to control. In our case, the orientation of the “free” hydrogens on one side of the plane of the water cluster is induced by the formation of four O–H⋯O=P hydrogen bonds.

The water cluster also becomes the structure-directing factor for the self-assembly of the complexes in the crystal lattice. The complexes form infinite columns parallel to the crystallographic c axis as shown in Fig. 2. The four oxygens of the water cluster act as acceptors of weak hydrogen bonds towards the methylene hydrogen atoms of the bridging carbons at the lower rim of the cavitand (O⋯C 3.598(5), O⋯H 2.669(4) Å; see grey dotted lines in Fig. 2). Considering these contacts, each oxygen atom shows a slightly distorted tetrahedral geometry similar to that observed in the structure of ice.¹⁴

Fig. 2 Self-assembly of the Tiiii[H,CH₃,CH₃]·4H₂O complexes in the infinite up–up columnar arrangement running parallel to the crystallographic c axis. Each up–up column is surrounded by four nearest neighbouring down–down parallel columns of complexes. Only relevant hydrogen atoms have been reported. Water molecules are represented as red balls.

To test the resilience of the water cluster in the presence of competing H-bonding guests, acetonitrile was replaced by methanol as crystallization solvent. The affinity of this cavitand towards alcohols has already been shown in previous work,¹¹ where the crystal structure of the anhydrous Tiiii[H,CH₃,CH₃]·CH₃OH complex proves that methanol is located inside the cavitand. Addition of water to the methanol solution of Tiiii[H,CH₃,CH₃] led to the formation of the new Tiiii[H,CH₃,CH₃]·CH₃OH·4H₂O§ complex where, in addition to the water tetramer, a methanol molecule fills the intramolecular cavity of the host (see Fig. 3). Both complexes are isostructural and crystallize in the P4/n space group, showing that general packing is not disturbed when the cavity is occupied. In the anhydrous compound methanol lies on the fourfold axis and it is stabilized inside the host through four alternative equivalent weak interactions between the alcoholic OH group and the P=O moieties at the upper rim. On the other hand, in the hydrate complex the C–O_guest bond does not lie on the fourfold axis. However, the OH group is disordered over four alternative equivalent orientations, experiencing weak hydrogen bond contacts with the P=O groups (O⋯O, 2.958(8) Å) and strong hydrogen bonds with the oxygen atoms of the water molecules (O⋯O, 2.796(9) Å) (Table S3‡). The self-assembly of this complex in the crystal lattice (Fig. S4‡) is of columnar type, as the one shown in Fig. 2.

Fig. 3 Top and side view of Tiiii[H,CH₃,CH₃]·CH₃OH·4H₂O. (Colour code: P, orange; O, red; C, grey; H, white; weak intermolecular interactions, blue and green). Hydrogen atoms are omitted for clarity. All the four equivalent orientations of the methanol molecule inside the cavity are shown.

It is noteworthy that the water tetramer still survives even when a bulkier ditopic guest, like ethylene glycol, fills the intramolecular cavity. By dissolving the cavitand in a mixture of dichloromethane, trifluoroethanol, ethylene glycol and water (the first two solvents used only for solubility purposes), the Tiiii[H,CH₃,CH₃]·HOCH₂CH₂OH·4H₂O§ complex was isolated.¶ The host cavity becomes elliptical to accommodate the ethylene glycol (the P=O⋯O=P separations between opposite P=O groups are 6.954(2) and 7.896(3) Å, respectively). The glycol is stabilized within the cavity by two hydrogen bonds with two adjacent P=O groups (Table S4‡) and by two CH–π interactions between two methylene hydrogens of the guest and two aromatic rings of the host (C–H⋯centroid 2.798(4) and 2.977(2) Å; C–H⋯centroid 155.19(2) and 157.03(8)°).

Under these conditions, the tetrameric water cluster (almost square) is still formed, but, due to the presence of the bulky guest and to the significant deformation of the cavitand, it is now located asymmetrically with respect to the pseudo-twofold axis of the cavitand (Fig. 4). The water cluster is simultaneously linked to the cavitand via two hydrogen bonds involving two adjacent P=O groups (P=O⋯O_water 2.762(3), 2.817(3) Å, see blue dotted lines in Fig. 4) and to the ethylene glycol with two other hydrogen bonds (O_water⋯O_glycol 2.692(3), 2.660(3) Å, see pink dotted lines in Fig. 4 and Table S4 for details‡). The array of hydrogen bonds in the water cluster is still homodromic and the “free” water hydrogens are again located on the same side of the plane of the water cluster.

Fig. 4 Top and side view of Tiiii[H,CH₃,CH₃]·HOCH₂CH₂OH·4H₂O. (Colour code: P, orange; O, red; C, grey; H, white; weak intermolecular interactions, blue, green and pink). Only relevant hydrogen atoms are shown.

In summary we have shown that we can induce the formation of tetrameric water clusters in the solid state. The structural key is the use, as templating agent, of a cavitand bearing four square-planar arranged P=O groups. These groups act as a template of suitable shape and size for the organization of hydrogen bonded water tetramers. The crystallization processes are reproducible and lead univocally to the formation of water clusters, with or without a guest inside the cavity. Moreover, the role of the tetrameric water cluster, when formed, is also pivotal as the structure-directing factor for the columnar ordering of the complexes in the crystal lattice.

This work was supported by the EU through projects BION (ICT 213219) and FINELUMEN (ITN 215399).

Notes and references

1. M. H. Mir and J. J. Vittal, Angew. Chem., Int. Ed., 2007, 46, 5925.
2. R. Ludwig, Angew. Chem., Int. Ed., 2001, 40, 1808.
3. (a) J. L. Atwood, L. J. Barbour, T. J. Ness, C. L. Raston and P. L. Raston, J. Am. Chem. Soc., 2001, 123, 7192; (b) M. Yoshizawa, T. Kusukawa, M. Kawano, T. Ohhara, I. Tanaka, K. Kurihara, N. Niimura and M. Fujita, J. Am. Chem. Soc., 2005, 127, 2798.
4. L. Infantes and S. Motherwel, CrystEngComm, 2002, 4, 454.
5. S. W. Benson and E. D. Siebert, J. Am. Chem. Soc., 1992, 114, 4269.
6. (a) B. R. Lentz and H. A. Scheraga, J. Chem. Phys., 1973, 58, 5296; (b) T. P. Radhakrishnan and W. C. Herndon, J. Phys. Chem., 1991, 95, 10609; (c) K. Liu, J. D. Cruzan and R. J. Saykally, Science, 1996, 271, 929; (d) J. K. Gregory and D. C. Clary, J. Phys. Chem., 1996, 100, 18014 and references therein; (e) J. M. Ugalde, I. Alkorta and J. Elguero, Angew. Chem., Int. Ed., 2000, 39, 717; (f) A. Sediki, F. Lebsir, L. Martiny, M. Dauchez and A. Krallafa, Food Chem., 2008, 106, 1476.
7. (a) S. Pal, N. B. Sankaran and A. Samanta, Angew. Chem., Int. Ed., 2003, 42, 1741; (b) S. Supriya and S. K. Das, New J. Chem., 2003, 27, 1568; (c) L.-S. Long, Y.-R. Wu, R.-B. Huang and L.-S. Zheng, Inorg. Chem., 2004, 43, 3798; (d) M. Zuhayra, W. U. Kampen, E. Henze, Z. Soti, L. Zsolnai, G. Huttner and F. Oberdorfer, J. Am. Chem. Soc., 2006, 128, 424; (e) J. Martínez-Lillo, D. Armentano, G. De Munno, N. Marino, F. Lloret, M. Julve and J. Faus, CrystEngComm, 2008, 10, 1284 and references therein.
8. P. Delangle, J.-C. Mulatier, B. Tinant, J.-P. Declercq and J.-P. Dutasta, Eur. J. Org. Chem., 2001, 3695.
9. (a) E. Biavardi, G. Battistini, M. Montalti, R. M. Yebeutchou, L. Prodi and E. Dalcanale, Chem. Commun., 2008, 1638; (b) E. Biavardi, M. Favazza, A. Motta, I. L. Fragalà, C. Massera, L. Prodi, M. Montalti, M. Melegari, G. G. Condorelli and E. Dalcanale, J. Am. Chem. Soc., 2009, 131, 7447; (c) R. M. Yebeutchou and E. Dalcanale, J. Am. Chem. Soc., 2009, 131, 2452; (d) R. M. Yebeutchou, F. Tancini, N. Demitri, S. Geremia, R. Mendichi and E. Dalcanale, Angew. Chem., Int. Ed., 2008, 47, 4504.
10. L. Pirondini and E. Dalcanale, Chem. Soc. Rev., 2007, 36, 695.
11. M. Melegari, M. Suman, L. Pirondini, D. Moiani, C. Massera, F. Ugozzoli, E. Kalenius, P. Vainiotalo, J.-C. Mulatier, J.-P. Dutasta and E. Dalcanale, Chem.–Eur. J., 2008, 14, 5772.
12. For the stereochemical notation and nomenclature adopted for phosphonate cavitands see: R. Pinalli, M. Suman and E. Dalcanale, Eur. J. Org. Chem., 2004, 451.
13. G. A. Jeffrey and W. Saenger, Hydrogen Bonding in Biological Structures, Springer-Verlag, Berlin, Heidelberg, 1991, p. 38.
14. N. H. Fletscher, The Chemical Physics of Ice, Cambridge University Press, Cambridge, 1970.

---

Footnotes

† This article is dedicated to Professor Javier de Mendoza on the occasion of his 65th birthday.

‡ Electronic supplementary information (ESI) available: ORTEP views, details of crystallographic data collection and crystallographic data. CCDC 737537, 737538 and 737539 for Tiiii[H,CH₃,CH₃]·4H₂O, Tiiii[H,CH₃,CH₃]·CH₃OH·4H₂O and Tiiii[H,CH₃,CH₃]·HOCH₂CH₂OH·4H₂O respectively. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b917931c/

§ Crystallographic data: compound Tiiii[H,CH₃,CH₃]·4H₂O: C₃₆H₄₄O₁₆P₄, M = 856.59, tetragonal, P4/n (Z = 2), a = b = 14.396(8), c = 9.799(5) Å, α = β = γ = 90°, V = 2031(2) ų, T = 293(2) K, 11 151 total reflections, 2354 unique reflections [R(int) = 0.0994]. Final R indices [I > 2σ(I)]: R₁ = 0.0481, wR₂ = 0.0765. Compound Tiiii[H,CH₃,CH₃]·CH₃OH·4H₂O: C₃₇H₄₈O₁₇P₄, M = 888.63, tetragonal, P4/n (Z = 2), a = b = 14.352(2), c = 9.822(2) Å, α = β = γ = 90°, V = 2023.2(6) ų, T = 293(2) K, 12 598 total reflections, 2626 unique reflections [R(int) = 0.0260]. Final R indices [I > 2σ(I)]: R₁ = 0.0467, wR₂ = 0.1417. Compound Tiiii[H,CH₃,CH₃]·HOCH₂CH₂OH·4H₂O: C₄₀H₅₄O₁₈P₄Cl₄, M = 1088.51, triclinic, P1̄ (Z = 2), a = 10.3676(4), b = 13.1307(5), c = 19.1132(8) Å, α = 69.980(1), β = 88.713(1), γ = 82.935(1)°, V = 2425.6(2) ų, T = 173(2) K, 25 175 total reflections, 11 553 unique reflections [R(int) = 0.0385]. Final R indices [I > 2σ(I)]: R₁ = 0.0515, wR₂ = 0.1288.

¶ The presence of some disordered dichloromethane molecules in the lattice is not relevant to the discussion, and has been omitted from the formula.

---