文章編號10005269（2024）06002608

DOI：10.15958/j.cnki.gdxbzrb.2024.06.05

Abstract：

Thispaper has investigated the coordination and supramolecular assemblies of alkali metal ions， cucurbit［5］uril （Q［5］）， and ［CdCl4］2- to confirm whether ［CdCl4］2- can produce the “honeycomb effect”， induce coordination of alkali metal ions to Q［5］， and form linear coordination polymers. In this work， the effect of alkali metal ions on the construction of Q［5］Cd2+ ion system under acidic conditions was investigated. Five complexes were successfully obtained by solvent evaporation method. Among the five crystal structures obtained， it can be observed that the presence of ［CdCl4］2- did not result in the complexation of alkali metal ions by the Q［5］ molecule. Instead， a bowllike Cd2+@Q［5］ complex was formed. Indeed， ［CdCl4］2- did not produce the honeycomb effect but led to the formation of Q［5］based honeycomb frameworks with hexagonal cellsoccupied by ［CdCl4］2-. The experimental results show that cadmium ion showed stronger ability to coordinate to Q［5］ in HCl solution.

Key words：

alkali metal ion; cucurbit［5］uril; tetrachloridecadmium anion; supramolecularassemblies

CLC number：O614.1;O641.3

Document code：A

In recent ten years， Tao’s group has systematically studied the supramolecular assemblies of cucurbit［n］urils （Q［n］s）［15］ with metal ions in the environment of structuredirecting agents such as small aromatic molecules and polychloridometallate complexes. Metals in these complexes include Cd， Zn， Cu， Co， Ni， and Pt［6］. Such supramolecular assemblies are characterized by honeycombpatterns in which ［MdblockClx］nanions or ［MdblockCly］m+cations of the polychlorideMdblock complexform the honeycomb， and Mn+/Q［n］based linear coordination polymers occupy the hexagonal cells［717］. Mn+ ions include alkali and alkalineearth metal ions， lanthanides， andeven transitionmetal ions， while the Q［n］s include Q［5］， Q［6］， Q［7］， and Q［8］. The driving forces for these assemblies are interactions of electrondeficient carbon sites of the carbonyl dipole of Q［n］ molecules and the chloride of MdblockCl bonds in ［MdblockClx］n， as well as the unusual HBond between methine and methylene groups on the outer surface of adjacent Q［n］ molecules and chlorides of ［MdblockClx］n. This has been named as “outersurface interactions” of Q［n］s by TAO et al［2， 56］.

Among the polychloridometallate complexes， ［CdCl4］2- was first found to present a structure direction function that results in the formation of Ln3+/Q［7］based linear coordination polymers［11，13］. We found that ［CdCl4］2- also induces the formation of Q［6］ and Q［8］based onedimensional coordination polymers which insert into the cells of the honeycomb of ［CdCl4］2- units［1819］. We described such phenomenon as the “honeycomb effect” of ［CdCl4］2- anions. To understand the coordination of Q［5］ with metal ions and the corresponding supramolecular assemblies in the presence of ［CdCl4］2-， we investigated coordination and supramolecular assemblies of alkali metal ions， Q［5］， and ［CdCl4］2- in aqueous HCl solution. The structure direction function of ［CdCl4］2- as well as the competitive coordination of alkali metal and cadmium ions to Q［5］ were considered in our investigation. The experimental results reveal that the alkali metal ions exhibited a similar ability to cadmium cation for coordination to the Q［5］ molecule in neutral medium. In HCl solution， the cadmium ion showed stronger ability to coordinate to Q［5］， leading to only the Cd2+Q［5］ complex in the solid state. We did not observe the formation of the ［CdCl4］2- honeycomb， but molecular bowllike Cd2+@Q［5］ complexes formed a honeycomblike framework， and ［CdCl4］2- anions occupied the cells of the ［CdCl4］2- honeycomb.

1Experimental

1.1Materials and physical measurements

All metal salts used in this paper were of reagent grade and were used directly without any purification. Q［5］ was synthesized in the laboratory as reported in the previous literature procedure［2］. Elemental analyses were tested on a EURO EA3000 elemental analyzer.

1.2Synthesis of compounds

Generally， a similar procedure was used to synthesize compounds of the reaction of alkali metal salts（0.12 mmol）with Q［5］·10H2O（20 mg， 002 mmol）in the presence of cadmium salts（29.3 mg， 016 mmol） in aqueous HCl（6.0 mol/L， 4 mL）. For example， NaCl（7.1 mg， 0.12 mmol） and CdCl2（29.3 mg， 0.16 mmol） were dissolved in 2.0 mL 60 mol/L HCl（solution A）.Q［5］ was dissolved in 2.0 mL 6.0 mol/L HCl（solution B）， and was then added in the solution A with stirring.Xray quality crystals were obtained from the solution over a period of 3-7 days. Summarizing the preparations： {CdCl2·Q［5］}·CdCl4·2H3O·7H2O（1） was obtained from NaCl （7.1 mg）; {CdCl2·Q［5］}·CdCl4·2H3O·7H2O （2） was obtained from KCl （8.9 mg）; {CdCl2·Q［5］}·CdCl4·2H3O·6H2O （3） was obtained from RbCl （14.5 mg）; {CdCl2·Q［5］}·CdCl4·2H3O·6H2O （4） was obtained from CsCl （20.0 mg）; the compound 2 with chemical formula of {（CdCl）［K（H2O）］（Cl@Q［5］）}·［CdCl3（H2O）2］·7H2O （5） was obtained from neutral water but 6.0 mol/L HCl. Elemental analysis results for the five compounds are given in Tab.1.

1.3Crystal structure determination

Suitable single crystals were mounted on a Bruker SMART Apex II CCD di ractometer equipped with a graphite monochromator and MoKα （λ=0710 73 ， 1 =1×10-10 m，293 K） radiation. Data collection was performed using φ and ω scan. The structure was solved using ShelXT followed by full matrix leastsquares renements against F2 with the ShelXS97 and ShelXL97 program packages， respectively［2021］. Subsequent difference Fourier synthesis and leastsquares renement revealed the positions of the remaining nonhydrogen atoms. Determinations of the crystal system， orientation matrix， and cell dimensions were performed according to the established procedures. Lorentz polarization and multiscan absorption correction were applied. Nonhydrogen atoms were rened with hydrogen atoms placed geometrically and rened using the riding model. Most of the water molecules in the compounds were omitted using the SQUEEZE option of the PLATON program. Details of the crystal parameters， data collection conditions， and refinement parameters for the five compounds were collected in Tab.2. In addition， the crystallographic data for the five structures have been deposited at the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC980415（1， 2， 3， 4） and 980416 （5）. These data could be obtained free of charge via http：//www.ccdc.cam.ac.uk/data_request/cif， or by emailing data_request@ccdc.cam.ac.uk.

2Results and discussion

It was unexpected that the four compounds obtained from aqueous solutions of M+（M = Na， K， Rb， Cs）， Q［5］， and CdCl2 in 6 mol/L HCl are isomorphous which is similar to that obtained from Q［5］， and CdCl2 in 3 mol/L HCl as reported by Liu and coworkers in 2006［22］. However， in the Liu et al. work， they only focused on the coordination of Q［5］ with Cd2+ cation， and did not discuss the role of the outersurface interaction of Q［n］s（Fig.1）［2， 6， 18］. Singlecrystal Xray diffraction analysis revealed the coordination feature of compound 1 obtained from the Na+Q［5］CdCl2 system（a representative example is shown in Fig.2（a））.The Q［5］ molecule coordinates with one cadmium ion at one portal and forms a molecular bowl conformation. The Cd2+ can fully cover five portal carbonyl oxygens of Q［5］ and coordinate with two chloride anions， one of which occupies the cavity of the Q［5］ molecule， and one is outside the cavity. The bond distances between Cd ion and carbonyl oxygen atoms are 0.246 1-0.260 6 nm，and the distances between the Cd cation and coordinated chloride anions are 0.241 4-0.247 5 nm. A water molecule covers the other portal of the Q［5］ molecule attached through HBond. The bond distances between the water molecule and the carbonyl oxygen atoms are 0.254 3-0.289 4 nm. ［CdCl4］2-， which formed in the CdCl2/HCl solution， is another important component in these isomorphous compounds. Interactions between Cd2+@Q［5］ complexes and ［CdCl4］2- drew our attention to the driving forces that result in formation of novel supramolecular assemblies based on these two species. The crystal structure analysis further shows the detailed noncovalent interactions between the Cd2+@Q［5］ complexes and ［CdCl4］2-. Each Q［5］ molecule is surrounded by four ［CdCl4］2- anions. Two Cd2+@Q［5］ complexes interact through “outersurface interaction” of the Q［5］ molecules（Fig.2（b））， namely， （1）Hbond of =CH2 or ≡CH groups on the outersurface of Q［5］ molecules with Cl of ［CdCl4］2-; （2）Hbond of =CH2 or ≡CH groups on the outersurface of Q［5］ molecules with Cl coordinated to adjacent Q［5］ molecules; and （3）Hbond of =CH2 or ≡CH groups on the outersurface of Q［5］ molecules with portal carbonyl oxygens of adjacent Q［5］ molecules.In turn， each ［CdCl4］2- anioninteracts with four Q［5］ molecules through HBond of =CH2 or ≡CH groups on the outersurface of Q［5］ molecules with Cl of ［CdCl4］2-（Fig.2（c））. Every two of the four Q［5］ molecules interact through the aforementioned interactions（（2） and （3））. In addition to the above forces， it should be pointed out that dipoledipole interactions originated from portal carbonyl oxygens and portal carbonyl carbons of the adjacent Q［5］ molecule are also present， as shown in Fig.2（c）. Thus， these outersurface interactions of Q［n］ molecules lead to the formation of novel supramolecular assemblies of Cd2+@Q［5］ complexes. These complexes comprise honeycomblike frameworks with large channels （Fig.3） filled with numerous ［CdCl4］2- anions linked by water molecules through HBond（Fig.2（d）–（f））. Although Liu and coworkers reported Cd2+@Q［5］ complex.

Generally， Q［5］ molecules have no special coordination selectivity for metal ions because of the concentrated distribution of their portal carbonyl oxygen. They can coordinate to alkali metal ions， alkaline earth metal ions， lanthanides， and even transitionmetal ions［6］. In particular， Q［5］ strongly coordinates to potassium cation. The coordinated K+ could not even be isolated by using common column chromatography.

（a） a complex of Q［5］ with cadmium cation; （b） details of the interactions of ［CdCl4］2– anions and Cd2+@Q［5］ complexes with the central Q［5］ molecule; （c） details of the interactions of Cd2+@Q［5］ complexes with the central ［CdCl4］2–anion; （d） overall view of the supramolecular assembly based on the ［CdCl4］2–/Cd2+@Q［5］ complex; （e） an isolated “cell” based on the Cd2+@Q［5］ complex， viewed along the crystallographic a axis; （f） ［CdCl4］2- anions linked by water molecules（O4W） through HBond.

Although Cd2+（in CdCl2） does not form ［CdCl4］2- anions in neutral aqueous solution， the spare Cl- anion from KCl leads to the formation of a trigonal bipyramidal complex anion ［Cd（H2O）2Cl3］- which has a function similar to that of ［CdCl4］2-（Fig.4（a））. Bond distances between Cd ion and coordinated chloride anions are 0.243 3-0.248 2 nm，and the distances between Cd ion and coordinated water oxygens are 0.237 6-0.253 4 nm. Similar to the structural features of compounds 1-4， the interactions between ［Cd（H2O）2Cl3］- and Q［5］ molecules in 5 also belongs to the outersurface interactions of Q［5］ molecules. Fig.4（b） and Fig.4（c）show details of the noncovalent interaction of the central Cd2+/K+@Q［5］ complex or the central ［Cd（H2O）2Cl3］- anion with the adjacent ［Cd（H2O）2Cl3］- or Cd2+/K+@Q［5］ complex， respectively. The ≡CH or =CH2 groups on the outersurface of the Q［5］ molecules form hydrogen bonds with Cl of ［Cd（H2O）2Cl3］- or coordinated chloride anions and with portal carbonyl oxygens of adjacent Q［5］ molecules.There is also a dipoledipole interaction between the portal carbonyl oxygens and portal carbonyl carbons of the adjacent Q［5］ molecule， as shown in Fig.4（c）. A similar honeycomb framework is constructed from the Cd2+/K+@Q［5］ complexes. Large channels of hexagonal cells are filled with ［CdCl4］2- anions that form hydrogen bonds between the coordinated water molecules（Fig.4（d）-（f））.

（a）one ［Cd（H2O）2Cl3］-complex; （b）details of the interactions of ［Cd（H2O）2Cl3］- anions and adjacent Cd2+/K+@Q［5］ complexes with the central Cd2+/K+@Q［5］ complex; （c）details of the interactions of Cd2+/K+@Q［5］ complexes with the central ［Cd（H2O）2Cl3］- anion; （d）overall view of the supramolecular assembly based on the ［Cd（H2O）2Cl3］-/Cd2+/K+@Q［5］ complex; （e）one 1D channel in the Cd2+/K+@Q［5］ complex， viewed along the crystallographic a axis;（f）［Cd（H2O）2Cl3］- anions linked by HBond.

For coordination and supramolecular assemblies in the metalQ［n］［MdblockClx］n-system with Q［n］ larger than Q［5］， ［MdblockClx］n- anions induce honeycomb frameworks. Q［n］s（ngt;5） coordinate to metal ions and form linear coordination polymers， filling the ［MdblockClx］n-based honeycomb cells［617，19］. Such supramolecular assemblies are characterized by Q［n］ molecules being surrounded with ［MdblockClx］n- anions through outersurface interactions of Q［n］ molecules. However， we did not observe the honeycomb effect of the ［MdblockClx］n- anions［714］. The Q［5］based complexes comprise honeycomb frameworks and ［CdCl4］2- or ［Cd（H2O）2Cl3］- anions occupy the hexanocells. Close inspection revealed that the driving forces for the formation of such supramolecular assemblies are also the outersurface interactions of Q［n］s. Both ［CdCl4］2- or ［Cd（H2O）2Cl3］- seem to attract Q［5］ complexes around them， causing Q［5］ complexes to adopt honeycomb conformations. The reason for this could be the smaller size of the Q［5］ molecule and the repulsion between the ［CdCl4］2- or ［Cd（H2O）2Cl3］- anions， which generally frustrate the formation of such anionbased honeycomb frameworks.

Coordination of Q［5］ to alkali metal ions or cadmium ion in various media such as neutral aqueous solutions or HCl aqueous solutions presents different coordination abilities. In the neutral aqueous solutions， Q［5］ shows no special selectivity for alkali metal ions or cadmium ion， whereas in concentrated solutions of HCl， Q［5］ shows preference to cadmium ion and forms a bowllike complex. The essence of why these isomerization phenomena are caused has not yet been explored， but it is most likely speculated that they are related to the size and charge of these metal ions.

3Conclusion

To understand better the influence of ［CdCl4］2- on the coordination of Q［5］ to metal ions and to the corresponding supramolecular assemblies， we introduced a series of common alkali metal ions into the Q［5］/［CdCl4］2- system. Unexpectedly， a Cd2+@Q［5］ bowllike complex instead of a M+alkali@Q［5］ complex was obtained in HCl aqueous solution. A heterometallic complex based on K+ and Cd2+@Q［5］ was obtained as a competitive product in neutral aqueous solution. The results of isothermal titration calorimetry confirmed that there is no difference in the coordination constants of Q［5］ with cadmium cations and alkali metal ions. Importantly， although ［CdCl4］2- could not produce the honeycomb effect to induce formation of linear， Q［n］based coordination polymers［34］， it could lead to the formation of Q［5］based honeycomb frameworks with ［CdCl4］2- anions occupying the hexagonal cells. The outersurface interactions of the Q［5］ molecules are the main driving force for the construction of Q［5］based supramolecular selfassemblies in this work. And these interactions have been proven to be the most important and common force in Q［n］based supramolecular assemblies.

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（責任編輯：周曉南）

收稿日期：20230320

基金項目：江蘇省高等學?；A科學（自然科學）研究重大資助項目（22KJA150002）

作者簡介：鄭園園（1998—），女，在讀碩士，研究方向：瓜環(huán)基超分子框架材料制備及其功能，Email：azhengyy@163.com.

*通訊作者：陳凱，Email：kaichen85@nuist.edu.cn.

［CdCl4］2-導向構筑堿金屬基五元瓜環(huán)超分子自組裝體

鄭園園" 李婕" 張文宇" 吳陶然" 黃琳" 陳凱*

摘要：

為了考察［CdCl4］2-無機陰離子在堿金屬離子與五元瓜環(huán)（Q［5］）體系中能否通過“瓜環(huán)外壁”作用產生“蜂巢效應”，且促進五元瓜環(huán)端口碳基氧與堿金屬離子直接配位，形成Q［5］M+線性配位聚合物填充于“蜂巢”中，因此，在五元瓜環(huán)Cd2+離子體系中，考察在酸性條件下堿金屬離子對Q［5］Cd2+的構筑影響，通過溶劑揮發(fā)法成功獲得5種配合物。X射線衍射分析表征結果顯示：在該體系中，［CdCl4］2-未形成“蜂窩”框架，而是形成了“碗狀”復合物Cd2+Q［5］，在外壁作用下構筑形成了具有近似“六邊形”通道的超分子自組裝體，且其被［CdCl4］2-離子所占據(jù)。實驗結果表明，鎘離子在鹽酸溶液中與Q［5］配位的能力較強。

關鍵詞：堿金屬離子；五元瓜環(huán)；四氯鎘酸根離子；超分子自組裝體