Chlorine Anion Encapsulation by Molecular Capsules Based on Cucurbit[5]uril and Decamethylcucurbit[5]uril

Three barrel-shaped artificial molecular capsules 1-3, based on normal cucurbit[5]uril (Q[5]) and decamethylcucurbit[5]uril (Me 10 Q[5]), were synthesized and structurally characterized by single-crystal X-ray diffraction. Encapsulation of a chlorine anion in the cavity of a Q[5] or Me 10 Q[5] to form closed a molecular capsule with the coordinated metal ions or coordinated metal ions and water molecules in the crystal structures of these compounds is common. The three complexes


Introduction
In past several decades, the coordination chemistry of cations has become well developed, while the chemistry of anion encapsulation is still in its infancy [1].Recently, a new host-guest chemistry of anions is becoming a field of great interest [2][3].Because anions are ubiquitous throughout biological systems [4], and they also play important roles in the areas of templation (or directed self-assembly), catalysis, environmental pollution and so on [5][6].
Many host molecules for including halide anions have been designed, synthesized and studied.The earliest macrocyclic receptor for anions, which coordinates to halide anions by a combination of electrostatic interactions and hydrogen bonds, was synthesized by Park and Simmons in 1968 [7].More recently, calix[n]pyrroles and their derivatives, some imidazole derivatives and cryptands were used as receptors for halide anions through hydrogen bonds [8][9][10][11][12], and mercury was used as a Lewis-acidic center in a number of halide anion receptors [13].
To investigate the capability of Q[n] inclusion anions and their potential applications, we have designed and synthesized three complexes of Q [5] and Me 10 Q [5] in which chlorine anions are encapsulated, and herein we described this synthesis and their crystal structures.

Crystal structure of the complex
Crystals of the fully methyl substituted cucurbit [5]uril Me 10 Q [5] with KCl and ZnCl 2 solution were prepared in 2 M HCl.In the unit cell of the X-ray crystals structure of 3, three metal ions -K1, Zn1 and Zn2 -are coordinated.The K1 cation is hepta-coordinated by five carbonyl oxygens (O1, O2, O3, O4 and O5), with bond lengths of 2.437(7), 2.449(7), 2.587(8), 2.539(8) and 2.481(7) Å. respectively, by one water molecule (O17), with a bond length of 2.273 (10), and one chlorine anion (Cl1), with a bond length of 2.677(4) Å.Like the complex 1, the K1 cation plays a role of a lid on a Me 10 Q [5] portal, while the coordinated chlorine Cl1 is encapsulated in the cavity of the Me 10 Q [5].However, the Zn1 and Zn2 atoms are tetra-coordinated by four chlorine anions (Cl2, Cl3, Cl5 and Cl6) and two chlorine anions (Cl4 and Cl7) and two water molecules (O11 and O12), respectively.The experiments revealed that the Zn 2+ transition metal cations were not involved in direct coordination to Me 10 Q [5].A possible explanation for this result is that the Zn 2+ ion diameter is not effectively coordinated to the carbonyl oxygens of the portals of the Me 10 Q [5].It is common that cucurbituril (Q [6]) displays rather weak ligation towards transition metals (as opposed to s and f-elements), and does not replace water, as in Cr(H  4).As has been mentioned, all three coordinated compounds encapsulate anions to form molecular capsules.The major reason for formation of these molecular capsules is electrostatic interactions.The carbonyl groups at the portal of Q [5] or Me 10 Q [5] can provide metal ion binding sites, and when a metal ion is chelated to the carbonyl oxygen, the molecular capsule shell offers a high affinity environment for a anion, such as Cl -.The strong electrostatic interactions attract anions entering the cavity of the Q [5]s and lead to the formation of open or closed molecular capsules that encapsulate an anion.In the present cases of the simultaneous encapsulation of chlorine anions, the driving force appears to be electrostatic interactions.In addition, hydrophobic effect of the cavity of the Q [5]s must be taken into account, and complementarily between the receptor and the anion is clearly crucial in determining selectivity.

General
Chemicals, such as zinc chloride, potassium chloride, strontium chloride, hydrochloric acid and praseodymium oxide were of commercial quality and used without further purification.Q [5] and Me 10 Q [5] were prepared and purified according to the published procedures or procedures developed in our laboratory [16,33].Elemental analysis was carried out on a EURO EA-3000 element analyzer.

Synthesis
To a solution of Q [5] (0.42 g, 0.51 mmol) in HCl (3 M), Pr 2 O 3 (0.83 g, 2.5 mmol) was added.The solution was heated in boiling water for 30 min and cooled to room temperature.Then the solution was filtered and the filtrate was left standing for about three months to give green crystals.Anal.Calcd.for C 30

Crystal Structure Determination
The data were collected on a Bruker Apex-II CCD diffractometer using graphite monochromated Mo Kα radiation (λ= 0.71073 Å) with ω and φ scan mode.Lorentz polarization and absorption corrections were applied.Structural solution and full matrix least-squares refinement based on F2 were performed with the SHELXS-97 and SHELXL-97 program packages [34][35], respectively.All the non-hydrogen atoms were refined anisotropically.Analytical expressions of neutral-atom scattering factors were employed, and anomalous dispersion corrections were incorporated.Crystallographic data, data collection and refinement parameters for compounds 1-3 are given in Table 1.Crystallographic data (excluding structure factors) for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as deposition Nos.CCDC 647406, 648721 and 648722.Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: + 44-1223/336-033; E-mail: deposit@ccdc.cam.ac.uk).

Figure 2 .
Figure 2. a) X-ray crystal structure of 1 (Symmetry code: (A) x, −y+1/2, z).Dashed lines indicate hydrogen bonding interactions.b) The water molecule O21 as a lid on the other portal of the Q[5].Pr cations, Cl anions and water molecules are all omitted for clarity, except for O21.

Figure 3 .
Figure 3. X-ray crystal structure of 2. Solvated water molecules and anions are omitted for clarity.

Figure 4 .
Figure 4. X-ray crystal structure of 3, anions and solvated water molecules and anions are omitted for clarity, except for O13.