A study on the coordination of cyclohexanocucurbit[6]uril with copper, zinc, and magnesium ions


 Herein, we report the supramolecular complexes of cyclohexanocucurbit[6]uril (CyH6Q[6]) with Cu(ClO4)2, Zn(ClO4)2, and Mg(ClO4)2 in formic acid solution. The crystal structure was determined using single crystal X-ray diffraction. The analysis results showed that CyH6Q [6] formed a one-dimensional supramolecular chain with Cu(ClO4)2 and formed a supramolecular assembly with a mixture ratio of 2:3 with Zn(ClO4)2 and Mg(ClO4)2. In this system, 
 
 
 
 
 
 ClO
 
 
 4
 
 
 −
 
 
 
 {\text{ClO}}_{4}^{-}
 
 induced CyH6Q[6] and metal ions to form supramolecular complexes through ion–dipole interactions.


Introduction
Cucurbit[n]uril [1][2][3][4][5][6][7][8][9][10][11][12][13] is the fourth generation of macrocyclic compounds after cyclodextrin, crown ether, and calixarene. In 1905, German chemists Behrend et al. obtained a white solid compound through the reaction of glycoluril and paraformaldehyde under acidic conditions [14]. However, because of the problem of solubility, it had not been further studied. It was not until 1981, when the Mock team determined its structure using single crystal X-ray diffraction, that the structure of substance was known [11]. It is a ring compound with a hydrophobic cavity having a neutral potential, two carbonyl ports with a negative potential, and an outer surface with a positive potential. Since then, various cucurbit[n]uril have been discovered one after another and are still being explored [15][16][17][18].
However, because the cucurbit[n]uril itself can dissolve only in solutions of formic acid, concentrated acid, and concentrated alkali, the development of cucurbit [n] uril is greatly limited. Through the efforts of some researchers, some modified cucurbit[n]urils such as methyl-, hydroxyl-, cyclopentyl-, and cyclohexyl-substituted cucurbit[n]urils have been reported [19][20][21][22][23][24][25]. In the presence of inducers, it is easy to form complexes with various metal ions due to the effect of the outer surface of cucurbit[n]uril [26]. Moreover, cyclohexyl alkyl groups have a stronger ability to bind metal ions due to their electron-pushing effects. Therefore, it leads to an increase in electron cloud density in the ports and an increase in electrostatic repulsion. This increases the cavity of the cucurbit[n]uril and has better performance than ordinary cucurbit[n]uril and the solubility is greatly improved. Therefore, cyclohexyl-substituted cucurbit[n]uril is a kind of cucurbit[n]uril with great generalizing significance. Kim's research group, which was the first to introduce cyclohexyl into cucurbit[n]uril, reported a series of fully substituted cyclohexyl cucurbits. The experimental results show that fully substituted cyclohexyl cucurbit [5]uril and fully substituted cyclohexyl cucurbit [6]uril have good water solubility [27]. In the Key Laboratory of Macrocyclic and Supramolecular Chemistry of Guizhou Province, a series of Ln-CyH 5 Q [5] complexes formed by the interaction of fully substituted cyclohexyl cucurbit [5]uril with rare earth metal ions has been reported [28]. A series of Ln-CyH 6 Q[6] complexes formed by the interaction of fully substituted cyclohexyl cucurbit [6]uril with rare earth metal ions has been reported [29]. There are few studies on the supramolecular complexes constructed by CyH 6 Q[6] and metal perchlorates.
In this study, CyH 6 Q[6] (Figure 1) was used as the ligand with Cu(ClO 4 ) 2 , Zn(ClO 4 ) 2 , and Mg(ClO 4 ) 2 to construct three kinds of supramolecular complexes in formic acid solution. Finally, their structures were determined using single crystal X-ray diffraction.

General materials
All materials and reagents are analytically pure and purchased on the Aladdin platform, used without any further purification. The cyclohexanocucurbit [6]uril (CyH 6 Q[6]) was prepared and purified in accordance with a literature method [24]. The synthesis process of CyH 6 Q[6] is shown in Scheme 1. were obtained following the method described above for complex 1. The yield of complex 2 was about 50% and of complex 3 was about 46% based on CyH 6 Q[6].

Instrument characterization methods and test conditions
We selected crystals of an appropriate size and fixed them to a glass filament with Vaseline. Crystal data were collected using a Bruker D8 Venture X-ray single-crystal diffraction machine in scan mode using a graphite monochromatic Mo-K ray (λ = 0.71073 Å, μ = 0.828 mm −1 ) in ω-scan mode. Lorentz polarization and absorption corrections were applied. Structural solutions and full-matrix least-squares refinements based on F 2 were performed using the SHELXT-14 and SHELXL-14 program packages, respectively. All non-hydrogen atoms were refined anisotropically. Analytical expressions for the neutral-atom scattering factors were used and anomalous dispersion corrections were incorporated. Most of the water molecules in the compounds were omitted using the SQUEEZE option in the PLATON program. The main crystal structure parameters are shown in Table 1.   3 Results and discussion 3.1 Description of the crystal structure of complexes 1-3 Complex 1 exhibited the triclinic P1̄space group. The asymmetric unit structure contained one CyH 6 Q[6] molecule, two counter ClO 4 − ions, two central Cu 2+ ions, and four coordinating water molecules (Figure 2a). One central Cu 2+ ion of the complex was coordinated by one carbonyl oxygen atom (O1) and two water molecules (O1W and O2W) from the ligand CyH 6 Q[6] molecule. The coordination bond length of Cu1-O1 was 2.356 Å. In addition, the coordination bond lengths of Cu1-O1W and Cu1-O2W were 1.975 and 1.978 Å, respectively. Another central Cu 2+ ion of the complex was coordinated by one carbonyl oxygen atom (O7) and two water molecules (O3W and O4W) from the ligand CyH 6 Q[6] molecule. The coordination bond length of Cu2-O7 is 2.288 Å. In addition, the coordination bond lengths of Cu2-O3W and Cu2-O4W were 1.980 and 1.971 Å, respectively. The bond angle around Cu 2+ was between 86.74°and 91.97° (Figure 2b).
The port of CyH 6 Q [6] and Cu 2+ was bridged by coordination bonds to form a one-dimensional supramolecular chain (Figure 2b). The ion-dipole interaction between counter ClO 4 − ions and the positive potential outer surface of the CyH 6 Q[6] leads to distance of 2.589-2.700 Å (Figure 2c). Complex 1 formed a two-dimensional stacking model in the presence of coordination and ion dipoles as shown in Figure 2d. Complex 2 exhibited the triclinic P1̄space group. The asymmetric unit structure contained one CyH 6 Q[6] molecule, three counter ClO 4 − ions, two central Zn 2+ ions, five coordinating water molecules, and one coordinating chloride ion (Figure 3a). One central Zn 2+ ion of the complex was coordinated by one carbonyl oxygen atom (O1) and two water molecules (O4W and O5W) from the ligand CyH 6 Q[6] molecule. The coordination bond length of Zn1-O1 was 2.146 Å. In addition, the coordination bond lengths of Zn1-O4W and Zn1-O5W were 2.027 and 2.074 Å, respectively. Another central Zn 2+ ion of the complex was coordinated by two carbonyl oxygen atoms (O7 and O8), one chloride ion (Cl14), and three water molecules (O1W, O2W, and O3W) from the ligand CyH 6 Q[6] molecule. The coordination bond lengths of Zn2-O7 and Zn2-O8 were 2.077 and 2.153 Å, respectively. The coordination bond length of Zn2-Cl14 was 2.187 Å. In addition, the coordination bond lengths of Zn2-O1W, Zn2-O2W, and Zn2-O3W were 2.108, 2.033, and 2.043 Å, respectively. The bond angle around Zn 2+ was between 84.20°and 98.66° (Figure 3b). In fact, because of this outer surface interaction of the cucurbit[n] uril, the ion-dipole interaction between counter ClO 4 − ions and the positive potential outer surface of the CyH 6 Q [6] produces distance of 2.388-3.179 Å (Figure 3c). Complex 2 formed a two-dimensional stacking model in the presence of coordination and ion dipoles as shown in Figure 3d. Complex 2 was different from complexes 1 and 3 in that it not only coordinates with carbonyl oxygen and water molecules but also with chloride ions.
Complex 3 exhibited the triclinic P1̄space group. The asymmetric unit structure contained one CyH 6 Q[6] molecule, three counter ClO 4 − ions, two central Mg 2+ ions, and six coordinating water molecules (Figure 4a). One central Mg 2+ ion of the complex was coordinated by one carbonyl oxygen atom (O1) and two water molecules (O1W and O2W) from the ligand CyH 6 Q[6] molecule. The coordination bond length of Mg1-O1 was 2.092 Å. In addition, the coordination bond lengths of Mg1-O1W and Mg1-O2W were 2.033 and 2.068 Å, respectively. Another central Mg 2+ ion of the complex was coordinated by two carbonyl oxygen atoms (O7 and O12) and four water molecules (O3W, O4W, O5W, and O6W) from the ligand CyH 6 Q [6] molecule. The coordination bond lengths of Mg2-O7 and Mg2-O12 were 2.062 and 2.113 Å, respectively. In addition, the coordination bond lengths of Mg2-O3W, Mg2-O4W, Mg2-O5W, and Mg2-O6W were 2.080, 2.070, 2.045, and 2.033 Å, respectively. The bond angle around Mg 2+ was between 85.59°and 92.47° (Figure 4b). In fact, because of this outer surface interaction of CyH 6 Q [6], the ion-dipole interaction between counter ClO 4 − ions and the positive potential outer surface of CyH 6 Q[6] results in distance of 2.419-3.215 Å (Figure 4c). Complex 3 formed a two-dimensional stacking model in the presence of coordination and ion dipoles (Figure 4d). The coordination configurations of complexes 3 and 2 were similar. The difference was that the central Mg 2+ of complex 3 did not form coordination bonds with chloride ions, whereas the central Mg 2+ of complex 2 formed coordination bonds with chloride ions.

Conclusion
Complexes 1-3 were constructed using CyH 6 Q [6] and Cu(ClO 4 ) 2 , Zn(ClO 4 ) 2 , and Mg(ClO 4 ) 2 , respectively, in formic acid aqueous solution. The experimental results showed that complex 1 formed a two-dimensional stacking model in the presence of coordination and ion-dipole interaction. The coordination configurations of complexes 3 and 2 were similar. The difference was that the central Mg 2+ of complex 3 did not form coordination bonds with chloride ions, whereas the central Zn 2+ of complex 2 formed coordination bonds with chloride ions. Complex 2 was different from complexes 1 and 3 in that it not only coordinated with carbonyl oxygen and water molecules but also with chloride ions. This study further helped to fill the research gap of CyH 6 Q [6]. At the same time, it provided a theoretical basis for cucurbit[n]urils in the fields of carrier catalyst, collection, precipitation, adsorption, enrichment, and recovery. Author contributions: Jun Zheng: writingoriginal draft, writingreview and editing, methodology, formal analysis; Lin Zhang: writingoriginal draft, formal analysis; Xinan Yang: visualization, project administration; Yanmei Jin: resources, writingreview and editing, supervision, data curation; Jie Gao: resources, writingreview and editing, supervision, data curation; Peihua Ma: writingreview and editing.

Conflict of interest: Authors state no conflict of interest.
Data availability statement: The X-ray crystallographic data for structures reported in this study have been deposited in the Cambridge Crystallographic Data Center under accession numbers CCDC: 2055928 (1), 2055951 (2), and 2055947 (3). These data can be obtained free of charge via https://www.ccdc.cam.ac.uk//data_request/cif.