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Article

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

1
Key Laboratory of Macrocyclic and Supramolecular Chemistry of Guizhou Province, Guizhou University, Guiyang 550025, People’s Republic of China
2
Institute of Applied Chemistry, Guizhou University, Guiyang, 550025, People’s Republic of China
*
Author to whom correspondence should be addressed.
Molecules 2007, 12(7), 1325-1333; https://doi.org/10.3390/12071325
Submission received: 2 June 2007 / Revised: 3 July 2007 / Accepted: 3 July 2007 / Published: 6 June 2007

Abstract

:
Three barrel-shaped artificial molecular capsules 1-3, based on normal cucurbit[5]uril (Q[5]) and decamethylcucurbit[5]uril (Me10Q[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 Me10Q[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 [Pr2(C30H30N20O10)Cl3(H2O)13]3+3Cl-·5H2O (1), [Sr2(C40H50N20O10)(H2O)4Cl]3+3Cl-·2(HCl) 19H2O (2) and [K(C40H50N20O10)(H2O)Cl]·[Zn(H2O)2Cl2]·[ZnCl4]2-·2(H3O)+·8H2O (3) all crystallize as isolated molecular capsules.

1. 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].
Cucurbit[n=5-8]urils (Q[n=5-8]) are new members within the macrocyclic compound family; they are made up of glycoluril units interconnected by methylene bridges and have a macrocyclic cage rimmed by a number of carbonyl oxygens (cf. Figure 1) [14,15,16,17]. Recently some efforts have been made to introduce substitution in order to achieve ready solubility in both aqueous systems and common organic solvents, and a number of fully and partially substituted Q[n]s have been reported [18,19,20,21,22,23]. Decamethylcucurbit[5]uril (Me10Q[5]) was the first fully substituted Q[n] discovered in 1992 by Stoddart and co-workers [19]. The novel structure and the capability of forming complexes with molecules and ions make the cucurbit[n]uril family attractive, not only as synthetic receptors but also as building blocks for supramolecular assemblies such as rotaxanes, catenanes, and molecular machines[24,25,26,27,28,29,30]. Nevertheless research on Q[n]s as receptors for anions is scarce [17, 31].
To investigate the capability of Q[n] inclusion anions and their potential applications, we have designed and synthesized three complexes of Q[5] and Me10Q[5] in which chlorine anions are encapsulated, and herein we described this synthesis and their crystal structures.
Figure 1. Structures of the cucurbit[n=5-8]urils.
Figure 1. Structures of the cucurbit[n=5-8]urils.
Molecules 12 01325 g001

2. Results and Discussion

2.1 Crystal structure of [Pr2(C30H30N20O10)Cl3(H2O)13]3+·3Cl -·5H2O (1)

In the X-ray crystals structure of Q[5] complexes with praseodymium, two praseodymium ions (Pr1 and Pr2) are coordinated to the two portals of a Q[5] molecule because the carbonyl oxygens at the Q[5] portal can provide metal ion binding sites. The Pr1 cation is nona-coordinated by five carbonyl oxygens (O1, O1A, O2, O2A and O3) with bond lengths of 2.529(4), 2.529(4), 2.577(4), 2.577(4), and 2.509(5) Å, respectively, one water molecule (O23) with a bond length of 2.530(7), and three chlorine anions (Cl1, Cl4 and Cl4A), with bond lengths of 2.766(2), 2.546(4) and 2.546(4) Å, respectively. Thus, the Pr1 cation plays a role of a lid on a Q[5] portal, while the coordinated chlorine Cl1 is encapsulated in the cavity of the Q[5]. The Pr2 atom is disordered over two positions (represented by Pr2 and Pr2A in Figure 2a). The Pr2 or Pr2A cation is octa-coordinated by two carbonyl oxygens (O5 and O6) with bond lengths of 2.358(5) and 2.774(5) Å, respectively, and six water molecules (O21, O24, O25, O26, O28 and O31), with bond lengths of 2.509(4), 2.608(8), 2.164(6) 2.768(7), 2.612(11), and 2.33(3) Å (Figure 2a). Among these coordinated atoms or water molecules, the O21 water molecule plays an important role as a lid on the other portal of the Q[5] molecule, and the distances of this water molecule to the five carbonyl oxygens (O4, O4A, O5, O5A and O6) are 2.870, 2.870, 2.830, 2.830 and 2.881 Å (Figure 2b). Thus, the Cl1 chlorine anion coordinated to the Pr1 cation is encapsulated in the Q[5] cavity and forms a molecular capsule (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 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.
Molecules 12 01325 g002

2.2. Crystal structure of the complex [Sr2(C40H50N20O10)·(H2O)4·Cl]3+·3Cl -·2(HCl)·19H2O (2)

Crystals of the fully methyl substituted cucurbit[5]uril Me10Q[5] with SrCl2 were prepared and the crystal structure showed that two Sr2+ cations are located at the center of both portals of a Me10Q[5] molecule. Each strontium ion is octa-coordinated by five carbonyl oxygens. The bond lengths of the Sr1 cation with O1, O2, O3, O4 and O5 are 2.522(3), 2.574(3), 2.574(3), 2.510(4), and 2.592(3) Å, respectively; the coordination unit also has two water molecules (O21 and O24) with bond lengths of 2.547(4) and 2.663(4) Å, respectively, and one chlorine anion (Cl1) with a bond length of 3.270 (2) Å. The two lids on each portal of the Me10Q[5] are coordinated to the chlorine anion, which is located in the center of the Me10Q[5] cavity. Thus, a typical molecular capsule is formed with a Me10Q[5] shell capping two coordinated Sr2+ cations (Figure 3).

2.3. Crystal structure of the complex [K(C40H50N20O10)(H2O)Cl]·[Zn(H2O)2·Cl2]·[ZnCl4]2-·2(H3O)+·8H2O (3)

Crystals of the fully methyl substituted cucurbit[5]uril Me10Q[5] with KCl and ZnCl2 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 Me10Q[5] portal, while the coordinated chlorine Cl1 is encapsulated in the cavity of the Me10Q[5].
Figure 3. X-ray crystal structure of 2. Solvated water molecules and anions are omitted for clarity.
Figure 3. X-ray crystal structure of 2. Solvated water molecules and anions are omitted for clarity.
Molecules 12 01325 g003
Figure 4. X-ray crystal structure of 3, anions and solvated water molecules and anions are omitted for clarity, except for O13.
Figure 4. X-ray crystal structure of 3, anions and solvated water molecules and anions are omitted for clarity, except for O13.
Molecules 12 01325 g004
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 Zn2+ transition metal cations were not involved in direct coordination to Me10Q[5]. A possible explanation for this result is that the Zn2+ ion diameter is not effectively coordinated to the carbonyl oxygens of the portals of the Me10Q[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(H2O)63+, Ni(H2O)63+, Al(H2O)63+, In(H2O)63+, which remain uncoordinated [32]. At other portal of the Me10Q[5], a water molecule (O13) is coordinated to the five carbonyl oxygens (O6, O7, O8, O9 and O10) of the remaining portal of the Me10Q[5] and play a role of another Me10Q[5] lid. The distances of O13 to O6-O10 are 2.722, 2.646, 2.592, 2.645 and 2.627 Å respectively. Thus, a typical molecular capsule is formed with a Me10Q[5] shell capping a coordinated K+ cation and a coordinated water molecule (Figure 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 Me10Q[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.

3. Experimental

3.1 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 Me10Q[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.

3.2. Synthesis

3.2.1. Preparation of [Pr2(C30H30N20O10)Cl3·(H2O)13]3+·3Cl -·5H2O (1)

To a solution of Q[5] (0.42 g, 0.51 mmol) in HCl (3 M), Pr2O3 (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 C30H67Cl6N20O28.50Pr2: C, 21.71; H, 4.04; N, 16.89. Found: C, 21.74; H, 4.01; N, 16.79.

3.2.2. Preparation of [Sr2(C40H50N20O10)·(H2O)4·Cl]3+·3Cl -·2(HCl)·19H2O (2)

SrCl2 (0.53 g, 2.5 mmol) was dissolved in a solution of Me10Q[5] (0.77 g, 0.50 mmol) in HCl (3 M), after stirring for 10 min and heating for 30 min. Colorless crystals of 2 separated after two weeks. Anal. Calcd. for C40H96N20O33Sr2Cl2: C, 27.07; H, 5.42; N, 15.79. Found: C, 27.12; H, 5.38; N, 15.72.

3.2.3. Preparation of [K(C40H50 N20O10)(H2O)·Cl]·(ZnCl4)[Zn(H2O)2·Cl2]2-·2(H3O)+·8(H2O) (3)

KCl (0.29 g, 3.9 mmol) and ZnCl2 (0.51 g, 3.9 mmol) was added to a solution of Me10Q[5] (0.77g, 0.5 mmol) in HCl (3 M) and kept at room temperature in an open beaker. X-ray quality colorless crystals of 3 separated after two weeks. Anal. Calcd. for C40H78N20O23KZn2Cl7: C, 29.54; H, 4.80; N, 17.23. Found: C, 29.59; H, 4.78; N, 17.16.

3.3. 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.
Table 1. Crystallographic data for complexes.
Table 1. Crystallographic data for complexes.
Complex123
Empirical formulaC30H67N20O28.5Pr2 Cl6C40H96N20O33Sr2Cl2C40H78N20O23KZn2Cl7
Formula weight1658.261772.941624.99
Crystal systemOrthorhombicTriclinicMonoclinic
Space groupP nmaP -1P 21/n
a, Å15.6055(13)16.100(4) 15.4926(11)
b, Å17.8941(13)16.105(4) 25.7015(18)
c, Å20.3494(16)16.465(4) 17.5866(12)
α,deg9065.370(3) 90.00
β, deg9080.916(3) 114.2380(10)
γ, deg9065.687(3) 90.00
V, Å 35682.5(8)3536.0(16)6385.4(8)
Z424
Dcalcd, g cm-31.8951.6221.661
T, K22223223
μ, mm-12.0801.8341.199
unique reflns52461213911198
obsd reflns448478366264
params421910834
Rint0.05080.0560.0866
R[I> 2σ(I)]a0.05220.06160.1135
wR[I> 2σ(I)]b0.14500.16990.3021
R(all data)0.06190.10990.1753
WR(all data)0.15310.19270.3312
GOF on F21.0801.0311.039
a Conventional R on Fhkl: Σ||Fo| - |Fc||/Σ|Fo|. b Weighted R on |Fhkl|2: Σ[w(Fo2-Fc2)2]/Σ[w(Fo2)2]1/2
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: [email protected]).

Acknowledgments

This work was supported by the National Natural Science Foundation of China (NSFC; No. 20662003), the International Collaborative Project of the Ministry of Science and Technology (ICPMST; No. 2003DF000030), and the Foundations of the Governor of Guizhou Province.

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MDPI and ACS Style

Zhang, Y.-Q.; Zhu, Q.-J.; Xue, S.-F.; Tao, Z. Chlorine Anion Encapsulation by Molecular Capsules Based on Cucurbit[5]uril and Decamethylcucurbit[5]uril. Molecules 2007, 12, 1325-1333. https://doi.org/10.3390/12071325

AMA Style

Zhang Y-Q, Zhu Q-J, Xue S-F, Tao Z. Chlorine Anion Encapsulation by Molecular Capsules Based on Cucurbit[5]uril and Decamethylcucurbit[5]uril. Molecules. 2007; 12(7):1325-1333. https://doi.org/10.3390/12071325

Chicago/Turabian Style

Zhang, Yun-Qian, Qian-Jiang Zhu, Sai-Feng Xue, and Zhu Tao. 2007. "Chlorine Anion Encapsulation by Molecular Capsules Based on Cucurbit[5]uril and Decamethylcucurbit[5]uril" Molecules 12, no. 7: 1325-1333. https://doi.org/10.3390/12071325

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