Trapping of Octameric Water Cluster by the Neutral Unclosed

Communication pubs.acs.org/crystal

Trapping of Octameric Water Cluster by the Neutral Unclosed Cryptand Environment Kajetan Dąbrowa,† Magdalena Ceborska,‡ and Janusz Jurczak*,† †

Institute of Organic Chemistry and ‡Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, Warsaw 01-224, Poland S Supporting Information *

ABSTRACT: Unclosed cryptands (UCs) are neutral and easily accessible compounds with precisely defined spatial arrangement of various functional groups. They readily form crystals with a unusually high packing efficiency (≥0.8), thus providing a well-suited 3Dframework for study of supramolecular assemblies. Herein, one example of the discrete eight-membered water cluster trapped in the matrix of UC 1 is presented. X-ray, ATR FT-IR, and DFT studies indicate extraordinary stability of this assembly, which originated from the cumulative interplay of many noncovalent interactionshydrogen bonds and London dispersion forces.

W

ater is crucial for most of the chemical and biological processes occurring in Nature. For instance, nucleic acids show no biological activity in the absence of water1 and protein folding is induced by water.2 Nevertheless, the structure of bulk water is still not fully understood and remains one of the greatest mysteries for Science.3 Moreover, no currently known theoretical model is able to precisely explain all of water’s atypical properties.4 According to the most recent research, bulk water can be approximated by the interactions of clusters of different size, predominantly (H2O)n, where n ≤ 10.5−7 Data obtained from structural studies for such clusters may provide better insight into the nature and behavior of bulk water,7−9 which may broaden our knowledge of certain interface interactions, e.g., the hydration shell of biomolecules.10−13 There are a number of reported structures of small clusters in both the solid and gas phases, whereas large ones, which are mainly aggregates, have been reported only sparsely.14 It is noteworthy that water clusters are predominantly studied as guests in metal−organic frameworks (MOF), stabilized by strong and nondirectional electrostatic interactions, which cannot be compared with those in a biological environment. Although stabilization of clusters, both discrete and polymeric, by systems not containing metals or salts are extremely rare,15 there are evidence that structured water is crucial part of many important biomolecules.11,16−18 In the course of our recent studies in the field of anion recognition,19−21 we reported a novel class of neutral anion receptors of type 1, called unclosed cryptands (UCs).21 They contain a minimum of three amide groups connected by alkyl linkers which enable high structural flexibility, resulting in better shape adjustment toward anionic and some neutral guests as compared to cryptands. In solution as well as in the solid state, unclosed cryptands exhibit high water affinity owing to the dual role of UC amide groups and water, which are both H-bond donors and acceptors. Furthermore, conformation of © 2014 American Chemical Society

that macrocyclic systems, containing amide groups, is very sensitive even to traces of water as has recently been reported by us22 and others.23 For the above reasons, UCs seem to be a well-suited platform for studying water interactions in biological-like environment. However, during our structural studies with UCs having ring size of less than 24-member we noticed that they preferentially crystallize as monohydrates without any clustered water.21,22 We assumed that increasing the size of the UC’s macrocyclic cavity would expand the H-bond network with neighboring water molecules; thus, the formation of more complex hydrates should be facilitated both in solution and in the solid state. In solution, however, the exchange of water molecules between bulk and shell is extremely fast.13 Therefore, to trap this transient assembly we turned to experiments in the solid state. From a number of trials, crystals suitable for the Xray measurements were obtained only when the dihydrogenphospate or chloride anions were present in solution.24 However, a structure containing water clusters was obtained only in the former case, while the remaining ones gave simple monohydrate.25 Interestingly, a similar observation was reported by Bowman-James in the studies of structurally related cryptands.14d,26 This indicates that highly solvated anionic species may participate in the formation of water Received: September 2, 2014 Published: September 17, 2014 4906

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Figure 1. (a) Structure and H-bond parameters of water octamer (in gray symmetry equivalent part of cluster) and (b) packing of clusters along a axis.

structure of methane clatrate built from cyclic water tetramers.35 To gain deeper insight into the nature of water− receptor interactions we decided to carry out attenuated total reflectance (ATR) FT-IR studies at RT (Figure 2). The IR spectrum of a dried sample of 1·(H2O)4 is identical to that recorded for an amorphous sample of 1·(H2O), which

clusters, which is currently underway in our laboratory. Nevertheless, in Figure 1a we present the discrete water octamer trapped by tailored H-bonds which are supported by dispersion interactions originating from a matrix formed by eight molecules of UC 1, as shown in Figure 1b. X-ray single crystal diffraction analysis of the assembly at 100 K reveals that receptor 1 crystallizes in the triclinic P1̅ space group with an asymmetric unit containing 1 in the form of a pair of enantiomers and eight water molecules.27 The cluster consists of cyclic water tetramer of D2h symmetry and four dangling water molecules which are placed in alternate up-anddown positions (udud), different from the global minimum28 characterized by uudd conformation with S4 symmetry. Half of the water molecules in the cluster are three-coordinated while the other half are tetra-coordinated. The average distance between the oxygen atoms of water molecules is 2.872 Å for core cyclic tetramer, which is substantially larger than the values obtained from both ab inito calculations (2.74 Å)29,30 and gasphase measurements (2.78 Å).31 On the other hand, the average value of H-bonds for all water molecules composing a cluster (including contacts with 1) is 2.856 Å, which is similar to the value reported for bulk water (2.854 Å).32,33 The cluster is tightly bound with receptor by 12 strong H-bonds and the resulting assembly is additionally stabilized by dispersion forces originating from receptor n-butyl chains. The distance between interacting water and alkyl groups (dmin = 3.62 Å) is similar to that found experimentally for propane clatrates34 (davg = 3.69 Å). Comparable distances were found for the calculated

Figure 2. ATR IR spectra on the crystal of ZnSe, a = 3460 cm−1, b = 3415 cm−1, c = 3332 cm−1, and d = 3297 cm−1; for detailed analysis of the peaks see Supporting Information. 4907

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Figure 3. Hirshfeld fingerprint plot for 1·(H2O)4 with resolved H···H contacts (left, blue dots) and Hirshfeld surface mapped with dnorm (right).

Figure 4. Electrostatic potential surfaces for 1·(H2O) side-view (left) and front-view (right) calculated at DFT ωb97x-d41 6-31+G* level of theory.

Figure 5. X-ray structure of 1·(H2O)4 showing intermolecular H-bonds, π···π and, C−H···π interactions, and the shortest (red dashed line) and average (green) recognized interactions.

indicates loss of water accompanied by destruction of the crystal lattice of 1·(H2O)4. The spectra of 1 are characterized by a lack of broad signals typical for free amidic (∼3600 cm−1) and hydroxy groups (3720 cm−1). The spectrum obtained for 1·(H2O)4 shows a sharp peak at 3415 cm−1 which is attributed to clustered water with a pattern identical to that of cyclic water

tetramer obtained from infrared molecular beam depletion and fragment spectroscopy at RT (3416 cm−1).36 Moreover, peaks at 3460 and 3332 cm−1 could be attributed to three and four coordinated water molecules, respectively. This led us to the assumption that the clustered water in the crystal lattice is characterized by high thermal stability. This observation turned 4908

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analysis, details of DFT calculations, structures, and Cartesian coordinates of optimized structures. This material is available free of charge via the Internet at http://pubs.acs.org.

our attention again to X-ray studies. The structure measured at 293 K gave results close to those obtained at 100 K.37 The parameters of elemental cell and crystal packing coefficients CPk38 are virtually identical, but the distances between H-donor and H-acceptor were slightly longer, with average difference value of 0.013 Å. This means that the crystal lattice is robust; to ascertain the origin of this stability, Hirshfeld analysis (Figure 3)39 and DFT calculations (Figure 4) were therefore performed. Analysis of the Hirshfeld fingerprint unequivocally identifies the dominant interactions as nondirectional H···H (45%) and highly directional O−H···O and N−H···O (36%) interactions. Additionally, less directional C−H···π interactions, considered to be weak H-bonds, account for an 11% share. The contribution of other types of interactions is much smaller, e.g., 0.9% for Cπ···Cπ interaction, describing the parallel arrangement of aromatic rings. The distance between the di and de parameters (the left-hand part of Figure 3) in the Hirshfeld fingerprint does not exceed 4.8 Å, and the surface is compact, which means that packing of the crystalline form is very efficient. This can be explained by the local nature of stacking interactions40 which interact by complementary polarized surfaces, i.e., positive surfaces of water protons interacting with negative surfaces of carbonyl oxygens and/or aromatic rings (Figure 4). Moreover, there are no H-bonds between molecules of 1 which interact only by means of strong π···π interactions, as shown in Figure 5. All of this allows for very dense packing with a near maximum number of close contacts, corresponding with the exceptionally high crystal packing coefficient CPk for 1·(H2O)4 measured at both 100 and 293 K (0.792). Furthermore, CPk for 1·(H2O) is even higher (0.808) which indicates very efficient packing in this case, too. Nevertheless, values for both hydrates are much higher than the average reported for crystal organic molecules (0.68−0.75).42,43 DFT calculations show that the structure and energy of the host 1 are similar in both the solid and gas phases. On the other hand, energy calculated for water clusters is higher by 35.3 kcal·mol−1 than the global minimum with cage symmetry which indicates large energetic compensation of this transient assembly by the crystal lattice (see Supporting Information). In conclusion, we have reported the first example of a discrete water octamer stabilized by a neutral supramolecular environment containing amide functionalities, typical for efficient anion receptors.44,45 The octamer remains locked in a semi-liquid-ice state in the crystal lattice over a wide range of temperatures. Such extraordinary durability of the octamer is provided by the high structural stability of the receptor 1 while maintaining strong intermolecular noncovalent interactions, which as a consequence allow efficient crystal packing and prevent decomposition of this transient assembly. This means that under appropriate conditions the water clusters may be very stable and resistant to freezing in a hybrid solid−liquid state. Additionally, our work shows that water octamer inside unclosed cryptand 1 may mimic the behavior of water molecules in the cleft of biological macromolecules.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

We would like to acknowledge the National Center of Science (project 2011/02/A/ST5/00439) for financial support. Notes

The authors declare no competing financial interest.

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ABBREVIATIONS UC, unclosed cryptand; ATR, Attenuated Total Reflectance REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data (CIF), additional structures of the host, monohydrate, and water cluster, Cambridge Structural Database (CSD) survey for similar water clusters; FTIR spectra and 4909

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