Supramolecular chemistry of substituted cucurbit[n]urils

Abstract

Since the first example of substituted cucurbit[n]urils (Q[n]), decamethylcucurbit[5]uril (Me₁₀Q[5]), was reported in 1992, numerous substituted Q[n]s have been synthesized and studied. In this review article, we focus on the recognition properties of the substituted Q[n]s toward a variety of organic species as well as the coordination chemistry of substituted Q[n]s with different metal ions, including alkali/alkaline-earth metals, transition metals, and lanthanides. Applications of substituted Q[n]s, and the self-assembly processes affording mechanically interlocked molecules (MIMs) are also described. The main purpose of this review is to highlight important advances in the blossoming field of substituted Q[n]s, which will likely be of interest to researchers in supramolecular chemistry.

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Rui-Lian Lin

Rui-Lian Lin graduated from Gannan Normal University in 1995, and received her MS degree from Anhui University of Technology in 2012. She became an associate professor at Anhui University of Technology in 2016. Her research focuses on supramolecular chemistry and coordination chemistry based on cucurbit[n]urils.

Jing-Xin Liu

Jing-Xin Liu graduated from Gannan Normal University in 1995, received his MS degree from Guizhou University in 2004, and received his PhD degrees from Xiamen University in 2007. He became a professor of chemistry at Anhui University of Technology in 2011. His current research focuses on properties and applications of cucurbit[n]urils.

Kai Chen

Kai Chen studied Chemistry at the Guizhou University, where he completed his Master degree with honors in 2011 under the supervision of Prof. Zhu Tao. And then he received his Ph.D. from Nanjing University in 2015 under the supervision of Prof. Wei-Yin Sun. In May 2015 he joined Nanjing University of Information Science & Technology as a lecture, where he is working on the development of new chemical environmental materials based on cucurbit[n]urils.

Carl Redshaw

Carl Redshaw studied chemistry (BSc, PhD) at Newcastle University, was a Welch Fellow at the University of Texas (Austin), a PDRA with Sir G. Wilkinson at Imperial College, a PDRA at Durham and later a Levehulme Fellow at Imperial College. He was Lecturer, Senior lecturer and Reader at UEA before becoming Chair of Inorganic Materials at the University of Hull. He holds a Visiting Professorship at Northwest University, Xi'an (100 Talents Plan), and has >350 research publications, mostly in coordination chemistry and catalysis. His group also has interests in calixarenes, cucurbiturils and anti-cancer agents.

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1. Introduction

Since the first macrocyclic polyether, dibenzo[18]crown-6, was reported by Pedersen back in 1967,^1,2 host–guest/supramolecular chemistry has experienced rapid development. Host–guest/supramolecular chemistry usually involves the molecular recognition and inclusion of substrates by macrocyclic compounds, such as cyclodextrins (α, β and γ), calix[n]arenes, pillar[n]arenes, and various cyclophanes.^3–9 Cucurbit[n]uril (n = 5–8, 10, 13–15, commonly abbreviated as Q[n]s or CB[n]s, Fig. 1),^5,10–20 a family of artificial macrocyclic hosts, have in recent years played a central role in supramolecular chemistry, which reflects their excellent properties in molecular recognition and molecular assembly. The journey for Q[n]s started in 1905, when a white solid (Q[6]), was prepared by Behrend and co-workers, although its composition and structure was not known at that time.²¹ Seventy-five years later, Freeman et al. restudied the synthesis of the white solid and ascertained the structure of Q[6] via the use of X-ray diffraction.²² Since then, Q[6] has been used for both metal ion coordination and small molecule inclusion. After the discovery of a series of Q[n] homologues in 2000 and 2001 by Kim and Day,^23,24 research activity in Q[n] chemistry progressed greatly, and has impacted on a wide range of fields, including sensing, catalysis, drug delivery, nano-materials and stimuli-responsive systems.

Fig. 1 X-ray crystal structures of cucurbit[n = 5–8, 10, 13–15]urils.

The common Q[n] homologues comprise n glycoluril units linked by 2n methylene (Fig. 1). They feature a hydrophobic cavity with different sizes, two polar carbonyl portals with negative electrostatic potential, and a circular outer surface with positive electrostatic potential. Based on these structural features, Q[n]s can accommodate hydrophobic molecules or groups of suitable size in their cavities, coordinate to various metal ions at their portals, and connect with electronegative species through their outer surfaces. As a result, many research groups have reported a large number of Q[n] complexes with different metal ions, anions and charged organic molecules.^5,10–20 Although Q[n] homologues are outstanding macrocyclic hosts for fundamental and applied molecular recognition, three main shortcomings still hinder their research and applications. Firstly, Q[n] homologues have poor solubility in pure water and in most common organic solvents, especially the Q[6], Q[8], and Q[10] systems. Most host–guest chemistry research on Q[n] homologues has to be performed in acid solution or in the presence of alkali metal ion, which strongly affects the binding affinity between the host and the guest, as illustrated by Buschmann,²⁵ Kaifer,²⁶ Nau,²⁷ and Kim²⁸et al. Secondly, although the sizes and shapes of the guests are diverse, Q[n] homologues (except for Q[13–15]) possess high symmetrical cavities. Consequently, Q[n] homologues fail to selectively recognize some guest molecules with special shape.^{75–77,142,152} Thirdly, in comparison with other macrocyclic hosts, for example the calix[n]arenes, Q[n] homologues are difficult to functionalize. To overcome these shortcomings, significant efforts have been invested on the design and synthesis of substituted Q[n]s.

Over the past two decades or so, various substituted Q[n]s have been prepared.^29–60 As will be discussed later, the host–guest/supramolecular properties exhibited by substituted Q[n]s reveal advantages versus Q[n]s. (1) Substitutions greatly increase solubility of substituted Q[n]s by decreasing solid-state packing. (2) Unsymmetrical substitutions deform the cavity symmetry and open up better selectivity to guest sequestration.^39,40,50,56 It should be noted that Q[n]s are amongst some of the most selective hosts because of their different cavity sizes. (3) Substitutions greatly increase the functionalizability of Q[n] homologues. (4) By creating substituted Q[n]s and increasing the diversification of the structures, more complex chemical systems such as complicated molecular machines and smart sensors can be built.

Although great progress has been achieved in Q[n]s chemistry and numerous reviews have appeared,^10–20 few focus on the field of substituted Q[n]s. This review article will highlight advances in field of substituted Q[n]s. We will begin by discussing the synthesis of classical substituted Q[n]s, such as fully and partially methyl-substituted Q[n]s, cyclohexano-substituted Q[n]s, cyclopentano-substituted Q[n]s, hydroxy-substituted Q[n]s, and substituted Q[n]s with mixed substitutions. We will then focus on the host–guest complexation properties of the substituted Q[n]s, their inclusion and coordination compounds, plus mechanically interlocked molecules (MIMs) and molecular devices, including many examples from our own group's efforts.

2. Synthesis of substituted Q[n]s

Back in 1992, Stoddart and co-workers reported the first example of a substituted Q[n], namely decamethylcucurbit[5]uril (Me₁₀Q[5]).²⁹ The reaction of 2,3-butanedione with urea under acidic conditions leads to dimethylglycoluril. Condensation of dimethylglycoluril with formaldehyde in acidic medium generates a white precipitate of Me₁₀Q[5] (Table 1). Since then, a variety of Q[n] derivatives, including fully and partially alkyl-substituted Q[n], hemicucurbit[n]uril,^30,31 biotin[6]uril,³² bambus[6]uril,³³ and acyclic Q[n] congeners,^34,35 have been synthesized by Day, Isaacs, Kim, Sindelar, Tao and other groups. Note that the present review focuses on substituted Q[n]s, those Q[n] derivatives where the basic skeleton is preserved, as shown in Table 1.

Table 1 Synthesis of substituted Q[n]s

No.	Synthetic strategy	Yields	Ref.
1		Me10Q[5]: 16%	29
2		CyH5Q[5]: 16%	36
		CyH6Q[6]: 2%
3		CyP5Q[5]: 3.2%	37
		CyP6Q[6]: 5.2%
		CyP7Q[7]: 1.4%
4		CyB5Q[5]: 10%	38
		CyB6Q[6]: 13%
		CyB7Q[7]: 4%
		CyB8Q[8]: 1%
5		TMeQ[6]: 30%	39
6		OMeQ[6]: 3.4%	40
7		o-TMeQ[6]: 3.4%	41
8		m-Me4Q[8]: 4%	42
9		HMe5Q[5]: 6.4%	43
		HMe6Q[6]: 5.8%
10		CyH2Q[6]: 30%	46
11		CyH3Q[6]: 39.6%	47
12		o-Me4Q[8]: 11%	50
		o-CyH2Q[8]: 4%
13		(HO)10Q[5]: 42%	51
		(HO)12Q[6]: 45%
		(HO)14Q[7]: 5%
		(HO)16Q[8]: 4%
14		Q[7]-NHCO-CH2CH3: 96%	52
		Q[7]-NHCO-CHCH2: 87%
		Q[7]-NHCO-CH2CN: 94%
		Q[7]-NHCO-CH2CO2H: 84%
		Q[7]-NHCO-C6H5: 63%
		Q[7]-NHCO-(CH2)2Br: 73%
		Q[7]-O-(CH2)3OH: 76%
		Q[7]-O-(CH2)3Br: 63%
15		(HO)1Q[6]: 12%	54
16		Me2Q[7]: 31%	55
		CyHQ[7]: 18%
		MePhQ[7]: 3%
17		(HO)1Q[5]: 95%	57
		(HO)1Q[6,7]: 95–100%
		(HO)1Q[8]: 90%
18		PheQ[6]: 0.2%	60

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In 2001, Kim et al. synthesized fully substituted cyclohexanoQ[n]s (abbreviated as CyH_nQ[n]s, n = 5, 6), which are soluble not only in water but also in some organic solvents.³⁶ Day and coworkers synthesized two other kinds of fully substituted Q[n], the cyclopentanoQ[n]³⁷ (abbreviated as CyP_nQ[n], n = 5, 6, 7) in 2012 and the cyclobutanoQ[n]³⁸ (abbreviated as CyB_nQ[n], n = 5–8) in 2017. The syntheses of these fully substituted Q[n]s have similar procedures: the synthesis of the substituted glycoluril (precursor), and condensation of substituted glycoluril with formaldehyde in acidic medium.

The Tao group prepared the first example of a partially methyl-substituted Q[n], the symmetrical tetramethylcucurbit[6]uril (TMeQ[6]) in 2004.³⁹ The synthetic procedure involved the diether of dimethylglycoluril and the glycoluril dimer. Using similar strategies and appropriate building blocks, the Tao group later prepared a series of partially methyl-substituted Q[n]s, such as the symmetrical octamethyl-substituted cucurbit[6]uril⁴⁰ (OMeQ[6]), ortho-tetramethyl-substituted cucurbit[6]uril⁴¹ (o-TMeQ[6]), meta-tetramethyl-substituted cucurbit[8]uril⁴² (m-Me₄Q[8]). Moreover, the same group has recently synthesized a series of hemimethyl-substituted cucurbit[n]urils (HMe_nQ[n]s, n = 5, 6, 7) using the precursor 3α-methylglycoluril.^43–45 A series of partially cyclohexano-substituted cucurbit[6]uril such as the para-dicyclohexanocucurbit[6]uril⁴⁶ (CyH₂Q[6]) and meta-tricyclohexanocucurbit[6]uril⁴⁷ (CyH₃Q[6]) and some substituted Q[n]s with mixed substituents⁴⁸ were isolated. Concomitantly, the toxicity of HMe₇Q[7] was studied by Wang and co-workers, revealing that HMe₇Q[7] exhibited a good biocompatibility profile.⁴⁹ Other groups also reported the synthesis of methyl- and cyclohexano-substituted Q[n]. The Isaacs group in 2015 presented a building block strategy for the synthesis of ortho-tetramethyl-substituted cucurbit[8]uril (o-Me₄Q[8]) and ortho-dicyclohexano-substituted cucurbit[8]uril (o-CyH₂Q[8]) by condensation of glycoluril hexamer with bis(cyclic ethers) under well-defined conditions.⁵⁰

One of the shortcomings of the Q[n] homologues is that they are difficult to functionalize. To overcome this long-standing problem, Kim et al. created an effective procedure in 2003 via the oxidation the Q[n] homologues with K₂S₂O₈ in water to produce their corresponding hydroxy-substituted (HO)_mQ[n]s (n = 5–8, m = 10, 12, 14 and 16), which can then be further modified to afford tailored Q[n] derivatives.⁵¹ The Kim group recently demonstrated a superacid-mediated conversion of the hydroxyl group on Q[n]s (n = 6 and 7) to yield other important functional groups in high yields.⁵² Additionally, they found that the resulting substituted Q[n] can be readily conjugated to an enzyme to afford a Q[n]-conjugated enzyme which is useful in protein blotting assays.⁵³

Other groups also have made significant progress towards functionalizing Q[n] homologues. In 2012, Scherman and co-workers prepared monohydroxylated Q[6] through the controlled oxidation of Q[6] in the presence of a tailor-made bisimidazolium guest.⁵⁴ In the same year, the Isaacs group presented a building-block approach for mono-functionalization of some substituted Q[7]s.⁵⁵ Interestingly, mono-functionalized Q[7] exhibits self-association into tetrameric aggregates in aqueous solution, as shown in Fig. 2. In 2015, the Isaacs group synthesized hydrophobic monofunctionalized Q[7], which can undergo self-inclusion complexation and form vesicle-type assemblies.⁵⁶ Ouari and Bardelang et al. reported a photochemical method for producing hydroxy-substituted (HO)_mQ[n]s using hydrogen peroxide and UV light.^57,58 Very recently, Kaifer, Dong and coworkers obtained monohydroxylated cucurbit[7]uril ((HO)₁Q[7]) through the direct oxidation of Q[7].⁵⁹

Fig. 2 Mono-functionalized Q[7] self-associated into tetrameric aggregates.

By simply mixing glycoluril, paraformaldehyde, and another aldehyde, Sindelar and coworkers prepared a novel type of monosubstituted Q[6], mono(2-phenylethyl)cucurbit[6]uril (PheQ[6]).⁶⁰ Different from the above mentioned substituted Q[n]s, the substituent is attached to one methylene bridge of the PheQ[6] (Fig. 3a and b). Impressively, the monosubstituted Q[6] macrocycles self-assembled into tetrameric aggregates (Fig. 3c) in the solid state. Compared to the PheQ[6], it is impossible for unsubstituted Q[n]s to self-associate into polymers without the assistance of other compounds.

Fig. 3 The crystal structures of mono(2-phenylethyl)cucurbit[6]uril (PheQ[6], top (a) and side (b) views) and the tetramer (c) based on PheQ[6].

3. Molecular recognition of substituted Q[n]s

3.1. Binding behaviour of alkyldiammonium ions toward substituted Q[n]s

In their pioneering work, Collet, Cram, Rebek, et al. observed that molecular behaviour including conformations and reactivity in small spaces can be quite different from that in dilute solution.^61–64 With this in mind, the molecular behaviour of different alkyl chains in the hydrophobic cavities of the Q[n] derivatives can also be regarded as another study of a kind of small space. Therefore, the binding interactions between a series of alkyldiammonium ions and a variety of substituted Q[6]s, including TMeQ[6],^65,66 CyH₂Q[6],⁶⁷ CyH₆Q[6],⁶⁷ and CyP₆Q[6]s⁶⁸ has been investigated both in aqueous solution and in the solid state. NMR spectra and single-crystal X-ray diffraction analyses revealed that the binding behaviour varies depending upon the alkyl chain length. Here, the TMeQ[6] is taken as a representative example for discussion (Fig. 4). 1,2-Ethanediammonium resides outside the portals of the TMeQ[6] and binds exo to the host portals forming an exclusion complex. Other longer alkyldiammonium guests can be accommodated in the cavity of the TMeQ[6] forming 1 : 1 inclusion complexes. Interestingly, if the alkyl chain length is longer than the height of the cavity of the TMeQ[6], then the alkyl chains usually take a contorted conformation. Isothermal titration calorimetry (ITC) experiments indicated that the binding of the alkyldiammonium guests with the substituted Q[6] is mainly enthalpy driven, which benefits from hydrophobic effects and host–guest interactions. It is accepted that the main driving force for the assembly of the inclusion complex based on Q[n] is the hydrophobic effect.⁶⁹ However, the binding of the charged alkyldiammonium guests with the substituted Q[6] should also take into account the host–guest interactions, including ion–dipole interactions and van der Waals interactions.

Fig. 4 Extended and contorted conformations of alkanediammonium ions in the cavity of TMeQ[6]: (a) 1,2-ethanediammonium∩TMeQ[6]; (b) 1,4-butanediammonium@TMeQ[6]; (c) 1,6-hexanediammonium@TMeQ[6]; (d) 1,8-octanediammonium@TMeQ[6]; (e) 1,10-decanediammonium@TMeQ[6].

HMe₆Q[6] is soluble in water as well as in DMSO. The binding behaviour of HMe₆Q[6] towards alkyldiammonium ions in aqueous solution is completely different from that observed in the organic solvent DMSO.⁷⁰ In aqueous solution, the alkyl chains of the alkyldiammonium ions were encapsulated into the HMe₆Q[6] cavity, forming an inclusion complex. In contrast, in DMSO, the HMe₆Q[6] engulfs only the NH₃⁺ groups of the alkyldiammonium ions, forming a head-inclusion complex, which can be switched to supramolecular polymers upon heating (Fig. 5). Recently, we also compared the binding behaviour of HMe₇Q[7] with alkyldiammonium ions (protonated guests) and their corresponding uncharged alkyldiamines in aqueous solution.⁷¹ The results suggested that the driving force for the host–guest binding is related to the features of the guests.

Fig. 5 Supramolecular polymer aggregated from head-inclusion complex.

It is also noteworthy that ammonium ions can act as templates in Q[n] formation, influencing both the kinetics and the thermodynamics of the process. In particular, Anzenbacher Jr. and Isaacs and coworkers showed that the p-xylylenediammonium ion was capable of acting as a template in the reaction between glycoluril and formaldehyde (<2 equiv.) affording a methylene bridged glycoluril hexamer and bis-nor-seco-Q[10].⁷² Importantly, this methodology could be readily scaled up to deliver, without the need for chromatography, multigram quantities of the hexamer. Further reaction (macrocyclization) with phthalaldehydes allowed access to monofunctionalized Q[6] derivatives, for example derivatives containing NO₂ or CO₂H motifs, thereby opening up the possibility for further functionalization. The size of the template proved crucial in determining the product formed during the macrocyclization. For example, both the p-xylylenediammonium and hexanediammonium ions were found to slow down the formation of Q[6], instead preferring to favour formation of an intermediate containing an NCOCN-bridge which contains two stereogenic centers. Conducting the reaction between the hexamer and o-phthaldehyde in CF₃CO₂H allowed the intermediate to be characterized by mass spectrometry and ¹H NMR spectroscopy.

Cao and Isaacs designed a monofunctionalized Q[6] derivative (symbol 1 in Fig. 6) that possessed a covalently bonded (added by Click chemistry via a propargyloxy substituent) isobutylammonium group.⁷³ The structure comprised a mixture of cyclic dimers as evidenced by DOSY and ESI-MS experiments, and in water, they self-assemble into a cyclic daisy chain. This structure proved to be responsive to specific competing hosts, for example whilst Q[6] and Q[7] had little effect, the addition of Q[8] (1.5 equiv.) resulted in the formation of a new complex involving the monomeric isobutylammonium and Q[8]. ¹H NMR studies (upfield shifts for the triazole and isobutylammonium protons) indicated that a back-folded conformation was present in the Q[8] cavity. Further addition of Q[6] reverses the process and gives back the daisy chain. Obviously, substituents in 1 also act as competitive guests.

Fig. 6 Depiction of the three isomers of 1₂ and their response to addition of spermine (symbol 8), Q[8], and Q[6].

3.2 Binding behaviour of N-alkylated viologens with substituted Q[n]s

The series of N-alkylated viologens (methyl viologen MV²⁺, ethyl viologen EV²⁺, propyl viologen PV²⁺, butyl viologen BV²⁺, pentyl viologen FV²⁺ and heptyl viologen HV²⁺) are special redox-active organic guests with an alkyl chain and an aromatic group. A host–guest study involving N-alkylated viologens can help to understand the interdependence of redox processes and molecular selectivity. We and others have performed a series of experiments to study the binding interactions between a series of N-alkylated viologen guests and substituted Q[6]s, including (HO)₁Q[7],⁵⁵ CyH₆Q[6],⁷⁴ TMeQ[6],⁷⁵ CyH₂Q[6]⁷⁶ and CyH₃Q[6].⁷⁶ As shown in Fig. 7, MV²⁺ and EV²⁺ form 1 : 1 complexes in which the bipyridinium aromatic nucleus is partially included inside the CyH₆Q[6] cavity. PV²⁺, BV²⁺, FV²⁺, and HV²⁺ form 2 : 1 complexes with CyH₆Q[6], in which each of the viologen aliphatic chains is included by a host molecule. The results suggested that the inclusion of the alkyl chains is favored compared to inclusion of the aromatic nucleus.⁷⁴

Fig. 7 Binding behaviour of N-alkylated viologens with CyH₆Q[6].

3.3. Aniline-containing guests recognized by substituted Q[n]

In these Q[n] systems, it is usual to encounter shielding and deshielding effects of the host. For example, the hydrophobic cavity of a Q[n] is a proton-shielding region. When the guest is encapsulated into the Q[n] cavity, the proton signals of the guest undergo an upfield shift. In contrast, the outside of the portals of a Q[n] is a proton-deshielding region which would induce a downfield shift of the proton signals of guests located outside of the Q[n] portal. However, we recently encountered shielding and deshielding effects induced by a guest.^75–77 When the aromatic motif of a guest, such as aniline-containing guests, is located inside of the TMeQ[6] cavity, the ellipsoidal cavity of the TMeQ[6] is complementary in both size and shape to the aromatic group of the guest. As a result, remarkable chemical shifts for the TMeQ[6] host protons were observed (Fig. 8). Similar situations were observed for other substituted Q[n]s with ellipsoidal cavity.⁷⁶

Fig. 8 Aromatic ring-induced shielding and deshielding zones.

3.4. Recognition of enantiomeric amino acids by substituted Q[n]s

Amino acid recognition and determination is of great significance in fields as diverse as food testing, nutritional analysis, and medical diagnostics.^78–80 In 2017, we reported a family of supramolecular complexes of TMeQ[6] which interacted with enantiomeric amino acids (D,L-Gln; D,L-Glu; D,L-Met; D,L-Ser; D-Val).⁸¹ The seven supramolecular complexes are classified into two structural types, inclusion and exclusion structures (Fig. 9), the adoption of which mainly depends on the length of the alkyl chain of the enantiomeric amino acid. Interestingly, the reaction of TMeQ[6] with L-Val doesn't produces a crystalline material, suggesting that TMeQ[6] can be used to separate D-Val from its enantiomer.

Fig. 9 X-ray crystal structure of TMeQ[6] complexes with different amino acids showing inclusion and exclusion structural types: (a) D-Gln@TMeQ[6]; (b) D-Met@TMeQ[6]; (c) D-Ser∩TMeQ[6].

3.5. Recognition of other organic guests by substituted Q[n]s

Haloalkanes are an important class of organic compound which possess various applications. In 2015, we reported the first example of haloalkane encapsulation inside the cavities of Q[n]s.⁸² X-ray crystallography and NMR spectroscopy were used to establish that the haloalkane 1-(3-chlorophenyl)-4-(3-chloropropyl)-piperazinium (PZ⁺) dihydrochloride forms a highly stable inclusion complex PZ⁺@TMeQ[6], with half of the chloropropyl group of the PZ⁺ residing within the TMeQ[6] cavity (Fig. 10).

Fig. 10 ORTEP diagram of the inclusion complex PZ⁺@TMeQ[6].

The binding behaviour of HMe₇Q[7] with 1,10-phenanthroline and its derivative 4,7-dimethyl-1,10-phenanthroline was investigated by NMR spectroscopy and ITC techniques, which suggested the formation of a half-inclusion complex and a double binding complex (Fig. 11).⁸³ The latter complex exhibits two binding modes simultaneously: endo binding and exo binding, which is not observed in conventional host–guest complexes.

Fig. 11 Schematic representation of two binding modes.

As more substituted Q[n]s are synthesized, more organic guests are reported to be recognized and accommodated by these new substituted Q[n]s. For example, hymexazol,⁸⁴ thiabendazole,⁸⁵ 6-benzyladenine⁸⁶ were reported to form inclusion complexes with HMe₆Q[6], and TMeQ[6]. Kaifer, Dong and coworkers recently studied the binding properties of (HO)₁Q[7] with a series of selected guests.⁵⁹ The results indicated that the binding affinities of the guests with the (HO)₁Q[7] were slightly smaller than those of the parent Q[7].

4. Coordination chemistry of substituted Q[n]s with metal ions

The two identical carbonyl-laced portals that Q[n]s possess makes them fascinating coordination ligands. The coordination affinities of substituted Q[n]s are different from those of their parent Q[n]s because of the effects of the substituents. This is in part due to the new affinities displayed by the introduced substituents, which will become evident from the examples described below, but also because these substituents impart new properties such as increased solubility which can greatly enhance chemical reactivity. For fully and partially alkyl-substituted Q[n]s, the alkyl groups on the waist belong to electron-donating groups, which can enhance the polar properties of the carbonyl oxygen atoms at the portal, and enhance the electrostatic interactions between substituted Q[n]s and metal ions. In this section, we describe the outstanding coordination properties of the substituted Q[n]s, and their coordination compounds with a wide range of metal ions, including alkali metals, alkaline-earth metals, transition metals and lanthanides. These coordination compounds (Table 2) have led to many interesting structural features and applications, some of which are discussed herein.

Table 2 A list of the substituted Q[n]-based coordination compounds. For the sake of clarity, they are named according to the type of the metal ions contained in the compounds: A for alkali metal, AE for alkali-earth metal, TM for transition metal, and Ln for lanthanide metals

Compounds	Formula	Structural type	Ref.
A-1	{[K(H2O)](NO3@(HO)10Q[5])[K(μ-NO3)]}Cl(H3O)2^2+·7H2O	2-D	88
A-2	{[Rb3(μ2-H2O)(μ3-H2O)(H2O)5SO4]^+(H2O@(HO)10Q[5])}Cl(H3O)^+·2H2O	1-D	88
A-3	[Cs2(H2O@(HO)10Q[5])]SO4^2−(H3O)4^4+	2-D	88
A-4	{Sr2(Cl@Me2Q[5]}3Cl·19H2O	2-D	89
A-5	{K2(H2O@1,2,4-CyH3Q[5])}2Cl·15.5H2O.	3-D	89
A-6	{[K2(1,2,4-Me6Q[5])]3(H2O)4}^6+·6Cl^−·47(H2O)	3-D	90
A-7	{[K2(CyH5Q[5])]34(H2O)}^6+·6I^−·23(H2O)	3-D	91
A-8	{K6(H2O)4[H2O@Me2Q[5]]3}·6I·39H2O	2-D	91
A-9	{K6(H2O)4[H2O@1,3-CyH2Q[5]]2[H2O@1,2,4-CyH3Q[5]]}·6I·46H2O	2-D	91
A-10	{K6(H2O)4[H2O@1,2,4-CyH3Q[5]]3}·6Cl·55H2O	2-D	91
A-11	{K6(H2O)4[H2O@1,2,3-CyH3Q[5]]3}·6I·26H2O	2-D	91
A-12	{K6(H2O)4[H2O@CyH5Q[5]]3}·6Cl·53H2O	2-D	91
A-13	{[Na2(H2O)6(H2O@TMeQ[6])]·2(C6H5NO3)·Cl2(H2O)10}	1-D	92
A-14	{[Na2(H2O)6(H2O@TMeQ[6])]·2(C6H5NO3)·Br2(H2O)10}	1-D	92
A-15	{[Na2(H2O)6(C6H5NO3)2(2H2O@TMeQ[6])]·6(C6H5NO3)}F2(H2O)4}	1-D	92
A-16	[K2(H2O)3((H2O)0.5@Me10Q[5])]·Cl2	1-D	93
A-17	[K2(H2O)2(H2O@Me10Q[5])]·(ClO4)2·3H2O	0-D	93
A-18	[K2(H2O)2(SCN)(Me10Q[5])]·(SCN)·H2O	0-D	93
A-19	[Sr2Me10Q[5](H2O)4Cl]^3+·3Cl^−·2(HCl)·19H2O	0-D	94
A-20	[K(H2O)Me10Q[5]Cl]·[Zn(H2O)2Cl2]·[ZnCl4]^2−·2(H3O)^+·8H2O	0-D	94
A-21	[Na(H2O)(H2O)2@Me10Q[5]](C6H6O2)2Cl·8H2O	0-D	95
A-22	[K2(H2O)2(H2O)@Me10Q[5]](C6H6O2)2Cl2·7H2O	1-D	95
A-23	[Rb2(H2O)2(H2O)@Me10Q[5]](C6H6O2)2Cl2·7H2O	1-D	95
A-24	[Cs(H2O)2(H2O@Me10Q[5]](C6H6O2)2Cl·6H2O	0-D	95
A-25	{Na2(H2O)2Cl@CyP5Q[5]}·[ZnCl4]·H3O·9H2O	0-D	96
A-26	{K2(H2O)2Cl@CyP5Q[5]}·[ZnCl4]·H3O·10H2O	0-D	96
A-27	{Rb2CyP5Q[5]}·[ZnCl4]·3H2O	0-D	96
A-28	{Cs2CyP5Q[5]}·[ZnCl4]·3H2O	1-D	96
A-29	OMeQ[6]·[Cd2Cl7]·3H3O·10H2O	0-D	97
A-30	{K4(H2O)10OMeQ[6]}·[Cd2Cl8]·2H2O	1-D	97
A-31	OMeQ[6]·[CdCl4]·2H3O·11H2O	0-D	97
A-32	{CsOMeQ[6]}·[Cd2Cl7]·2H3O·5H2O	0-D	97
A-33	{Na2(H2O)6(p-Hyb)2OMeQ[6]}·2(p-Hyb)·2Cl·H2O	0-D	97
A-34	{Cs(H2O)OMeQ[6]}·4(p-Hyb)·3H3O·7H2O	0-D	97
A-35	{Na2(H2O)10(HO)1OMeQ[6]}·2[CdCl4]·2H3O·3H2O	0-D	98
A-36	{K2(H2O)10(HO)1OMeQ[6]}·2[CdCl4]·2H3O·2H2O	1-D	98
A-37	{Rb(H2O)8(HO)1OMeQ[6]}·2[CdCl4]·3H3O·H2O	1-D	98
A-38	{K2(H2O)6(1,3,5-Me6Q[6])}^2+·Cr2O7^2−·15H2O	0-D	99
A-39	{Rb2(H2O)6(1,3,5-Me6Q[6])}^2+·2Cl^−·14H2O	0-D	99
A-40	{Cs2(H2O)6(1,3,5-Me6Q[6])}^2+·2Cl^−·11H2O	1-D	99
A-41	[Li2(H2O)4{H2O@Me10Q[5]}·PdCl4·4H2O	0-D	124
A-42	[Na2(H2O)5{H2O@Me10Q[5]}·PdCl4·5H2O	0-D	124
A-43	[K2(H2O)3{(H2O)0.5@Me10Q[5]}]·PdCl4	2-D	124
A-44	[Rb2(H2O)3{(H2O)0.5@Me10Q[5]}]·PdCl4	2-D	124
A-45	[Cs2(H2O)2{H2O@Me10Q[5]}]·PdCl4	0-D	124
A-46	{[Ag(H2O)]2(H2O@Me10Q[5])}·2NO3·2H2O	0-D	125
AE-1	2{Sr2(H2O)8(HO)1OMeQ[6]}·3[CdCl4]·2Cl·31H2O	1-D	97
AE-2	{Ba2(H2O)8Cl[CdCl4](HO)1OMeQ[6]}·2Cl·H3O·22H2O	1-D	97
AE-3	2[Ca2(H2O)8(NO3@o-TMeQ[6])]·2(CdCl4)·4Cl·2H3O·21H2O	1-D	41
AE-4	2[Sr2(H2O)9(NO3@o-TMeQ[6])]·2(CdCl4)·2Cl·22H2O	1-D	41
AE-5	{(CaCl)(TMeQ[6])}·Cl·17.5H2O	0-D	111
AE-6	{(Sr2Cl2)[TMeQ[6]@H2O]}·Cl2·10H2O	1-D	111
AE-7	{Ca2(H2O)4[(NO3)@CyH5Q[5]]}2(NO3)6·26H2O	0-D	102
AE-8	{Sr2(H2O)3(NO3)[(NO3)@CyH5Q[5]]}(NO3)2·7H2O	1-D	102
AE-9	{Ba(H2O)[(H2O)2@CyH5Q[5]]}(NO3)2·11H2O	1-D	102
AE-10	{Mg(H2O)(Cl@CyP5Q[5])}·2[ZnCl4]·3H3O·15H2O	0-D	103
AE-11	{Ca2(Cl@CyP5Q[5])}·2[ZnCl4]·H3O·16H2O	0-D	103
AE-12	2{Sr2(H2O)5(Cl@CyP5Q[5])}·2[ZnCl4]·[Zn(H2O)Cl3]·Cl·H3O·8H2O	0-D	103
AE-13	2{Ba2(H2O)3(Cl@CyP5Q[5])}·3[ZnCl4]·23H2O	1-D	103
AE-14	{Mg(H2O)6@CyP6Q[6])}·2[Zn(H2O)Cl3]·9H2O	0-D	104
AE-15	{Ca2(H2O)8CyP6Q[6]}·2[ZnCl4]·13(H2O)	1-D	104
AE-16	{Sr2(H2O)10CyP6Q[6]}·2[ZnCl4]·6(H2O)	1-D	104
AE-17	{Ba2(H2O)10CyP6Q[6]}·2[ZnCl4]·6(H2O)	1-D	104
AE-18	{Ca(H2O)4@HMe6Q[6]}[CdCl4]·7H2O	1-D	105
AE-19	{Ba2(H2O)8@HMe6Q[6]}[CdCl4]·2Cl·12H2O	1-D	105
AE-20	{Ca1.5(H2O)5(HO)1Q[7]}·2[CdCl4]·H3O·35H2O	1-D	106
AE-21	{Sr1.5(H2O)5(HO)1Q[7]}·2[CdCl4]·H3O·35H2O	1-D	106
TM-1	{Cd6Cl14CyH6Q[6]}·(H3O)2·(H2O)2	0-D	108
TM-2	(TMeQ[6])·(H3O)4^4+·(CuCl4)2^4−·8H2O	0-D	109
TM-3	[(TMeQ[6])(CuCl2)2(H2O)4]·10H2O	1-D	109
TM-4	TMeQ[6]·14H2O	0-D	109
TM-5	[Cu(H2O)4(C4H8O2)@(CyH6Q[6])]·2(NO3)·4(H2O)	1-D	110
TM-6	[Cu2(H2O)6Cl2(TMeQ[6])]·2Cl·15(H2O)	1-D	110
TM-7	[Cu2(H2O)9(TMeQ[6])]·2(SO4)·16(H2O)	0-D	110
TM-8	[(C6H10N2)@(TMeQ[6])]·CuCl4·7(H2O)	0-D	110
TM-9	[Cu(H2O)3·CyP5Q[5]]·(ClO4)2	1-D	111
TM-10	[Zn2(H2O)4·(CyP5Q[5])2]·(ClO4)4	1-D	111
TM-11	[Cu(H2O)2·CyP6Q[6]]·(ClO4)2	1-D	111
TM-12	[Zn(H2O)2·CyP6Q[6]]·(ClO4)2·H2O	0-D	111
Ln-1–3	{LnCl2(H2O)3CyH5Q[5]}·NO3·18.5H2O Ln = La, Ce, Pr	1-D	115
Ln-4,5	{LnCl2(H2O)2CyH5Q[5]}·NO3·14H2O Ln = Nd, Sm	1-D	115
Ln-6	{Dy(H2O)2[Cl@CyH5Q[5]]Dy(H2O)6}·5Cl·20H2O	0-D	115
Ln-7,8	{LnCl(H2O)[Cl@CyH5Q[5]]Ln(H2O)6}·4Cl·13H2O Ln = Ho, Er	0-D	115
Ln-9–17	[Ln(H2O)4](NO3)3·CyH6Q[6]·nH2O, Ln = Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb	1-D	116
Ln-18	{Lu2(H2O)12CyH6Q[6]}·4NO3·2Cl·20H2O	1-D	116
Ln-19–21	{Ln4(H2O)16[NO3@CyH6Q[6]]2}·10NO3·nH2O, Ln = La, Ce, Nd	0-D	116
Ln-22	{Pr(H2O)3Cl2[NO3@CyH6Q[6]]}·25H2O	0-D	116
Ln-23	{Pr(H2O)3Cl2[NO3@CyH6Q[6]]}·23H2O	0-D	116
Ln-24–27	{Ln2(H2O)6(Cl@HMe5Q[5])}·2[CdCl4]·2H2O Ln = La, Ce, Pr, Nd	0-D	117
Ln-28–32	{HMe6Q[6]·[Ln(H2O)8]}·3Cl·nH2O n = 11–14, Ln = Tb, Dy, Ho, Er, Tm	0-D	117
Ln-33	{HMe6Q[6]·[Sm(H2O)8]}·3Cl·10H2O	0-D	117
Ln-34–40	{o-TMeQ[6]·[Ln(H2O)8]}·2(NO3)·Cl·9H2O Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu	0-D	118
Ln-41	La(H2O)4OMeQ[6]}2·2(CdCl4)·[CdCl2(H2O)4]·16H2O	1-D	119
Ln-42	{(NO3)2@OMeQ[6]·Ce2(H2O)10}·2(CdCl4)·10H2O	0-D	119
Ln-43,44	{Cl@OMeQ[6]·Ln2(H2O)9}·2(CdCl4)·3Cl^−·nH2O Ln = Pr, Eu	0-D	119
Ln-45	{OMeQ[6]·Nd2(H2O)12}·2(CdCl4)·2Cl^−·4H2O	0-D	119
Ln-46	OMeQ[6]·Sm(H2O)8·(CdCl4)·NO3^−·3H2O	0-D	119
Ln-47–49	OMeQ[6]·Ln(H2O)8·(CdCl4)·Cl^−·nH2O Ln = Gd, Dy, Ho	0-D	119
Ln-50	{(C6H6O2)@OMeQ[6]·Ho2(H2O)10}·2(CdCl4)·(C6H6O2)·4Cl^−·4(H3O)^+·8H2O	0-D	119
Ln-51–53	{Ln(H2O)6·CyP6Q[6]}·2[ZnCl4]·Cl·2(H3O)·n(H2O) Ln = La, Ce, Sm	0-D	120
Ln-54–59	{Ln(H2O)5·CyP6Q[6]}·2[ZnCl4]·(H3O)·n(H2O) Ln = Pr, Nd, Eu, Gd, Tb, Dy	0-D	120
Ln-60	CyP6Q[6]·[Ho(H2O)8]·[ZnCl4]·[Zn(H2O)Cl3]·15(H2O)	0-D	120
Ln-61–69	{t(HO)2OMeQ[6]·[Ln(H2O)8]}·x(NO3)·yCl·zH2O Ln = Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu	0-D	121
Ln-70–72	{Ln(H2O)5(NO3)(o-TMeQ[6])}·(NO3)2·6H2O Ln = Nd, Sm, Eu	0-D	122
Ln-73–80	{Ln(H2O)6(o-TMeQ[6])}·2(NO3)·Cl·nH2O Ln = Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu	0-D	122
Ln-81–84	{Ln(H2O)6CyH6Q[6]}·2(CdCl4)·H3O·nH2O Ln = La, Ce, Pr, Nd	1-D	123
Ln-85	{Sm(H2O)5CyH6Q[6]}·2(CdCl4)·H3O·10H2O	1-D	123
Ln-86–88	{Ln(H2O)5(NO3)@CyH6Q[6]}·2(CdCl4)·2H3O·nH2O Ln = Gd, Tb, Dy	1-D	123

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4.1. With alkali metal ions

Generally, the coordination of Q[n] homologues with alkali metal ions leads to discrete (closed or opened) molecular capsules or 1D chains because the Q[n] homologues contain only two opposite coordination orientations. For instance, we previously reported a series of discrete molecular capsules of unsubstituted Q[5] with various metal ions, such as alkali (K⁺), alkaline earth (Ba²⁺), transition (Cd²⁺), and lanthanide (La³⁺) metal ions.⁸⁷ However, (HO)_mQ[n] are a series of multi-dentate ligands with different coordination orientations. In 2011, we prepared and characterized three coordination polymers of alkali metal ions (K⁺, Rb⁺ and Cs⁺) with the (HO)₁₀Q[5] ligand.⁸⁸ Their crystal structures revealed that not only the carbonyl groups of the portals but also the hydroxy groups at the waist position of the (HO)₁₀Q[5] ligand participated in the coordination, which led to 1D to 2D coordination polymers A-1–3 (Fig. 12).

Fig. 12 2D coordination polymers of (HO)₁₀Q[5] with alkali metal ions K⁺ (a), Rb⁺ (b) and Cs⁺ (c).

In 2008 and 2010, Day, Lindoy, Tao, and Wei et al. reported three attractive catenane structures, which resulted from the coordination of three substituted Q[5]s (1,2,4-Me₆Q[5], 1,2,4-CyH₃Q[5] and CyH₅Q[5]) with potassium ions.^89,90 These catenated structures A-5–7 featured trigonal-planar units, each one involving three substituted Q[5] ligands and six bound K⁺ ions. Adjacent trigonal-planar basic units connected with each other through their bound K⁺ ions, generating a 10-ligand “bracelet” framework. Interpenetration of the “bracelet” framework led to a complicated 3D catenane structure (Fig. 13). Using a similar strategy, the Tao group also prepared a series of 2D networks A-8–12 by using the coordination of K⁺ ion with other substituted Q[5]s, including Me₂Q[5], 1,3-CyH₂Q[5], 1,2,3-CyH₃Q[5] and 1,2,4-CyH₃Q[5].⁹¹ Apparently, the electron donating effect of the substituents increased the electron density at the portals of these substituted Q[5] ligands, and hence enhanced their binding affinities.

Fig. 13 (a) Schematic illustrating that the dihedrals between any two K1 junction planes in the trigonal planar branch are identical at 70.53. (b) A single 10-membered 1,2,4-HMeQ[5] “bracelet”. (c) Catenation of two 10-membered 1,2,4-HMeQ[5] bracelets.

We and others have also reported coordination compounds of alkali metal ions (Na⁺, K⁺, Rb⁺ and Cs⁺) with both fully and partially alkyl-substituted Q[n]s, including TMeQ[6],⁹² Me₁₀Q[5],^93–95 CyP₅Q[5],⁹⁶ OMeQ[6],⁹⁷ (HO)₁OMeQ[6],⁹⁸ 1,3,5-Me6Q[6].⁹⁹ For example, Chen, Tao and coworkers investigated the coordination of OMeQ[6] with alkali metal ions in the presence of polychloride cadmium anions, such as [Cd₂Cl₈]⁴⁻, [CdCl₄]²⁻, and [Cd₂Cl₇]³⁻ anions, or in the presence of p-hydroxybenzoic acid.⁹⁷ The resulting structures A-29–34 indicated the presence of “outer-surface interactions”, which are a unique type of noncovalent interaction first proposed by Tao.¹⁰⁰ For the OMeQ[6] molecules and the selected inorganic anions or aromatic molecules (Fig. 14), such interactions play an important role in the formation of different OMeQ[6]/A⁺-based supramolecular assemblies.

Fig. 14 Outer-surface interactions of OMeQ[6] with inorganic anions (left) and aromatic moieties (right).

4.2. Interaction with alkaline earth metal ions

As early as 2008, the Tao group reported the coordination compounds AE-5 and AE-6 of TMeQ[6] which incorporated two alkaline earth metal ions, Ca²⁺ and Sr²⁺, which coordinated directly to the TMeQ[6] ligand.¹⁰¹ In 2012, we prepared and characterized three coordination compounds AE-7–9 of the CyH₅Q[5] ligand with alkaline-earth metal ions (Ca²⁺, Sr²⁺, Ba²⁺).¹⁰² Analysis of the crystallographic data indicated that the radius (size) of the coordinated metal ions determines whether the molecular capsule is closed or opened (Fig. 15).

Fig. 15 Crystal structures of three coordination compounds of CyH₅Q[5] with alkaline-earth metal ions (Ca²⁺ (a), Sr²⁺ (b) and Ba²⁺ (c)) showing three kinds of molecular capsules.

Tao and Ma et al. investigated the coordination compounds AE-10–17 of CyP₅Q[5] and CyP₆Q[6] with a series alkaline-earth metal ions (AE²⁺ = Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) in the presence of [ZnCl₄]²⁻ anions.^103,104 X-ray diffraction analysis revealed that the [ZnCl₄]²⁻ anion acts as an inducer in the self-assembly of the CyP₅Q[5]/CyP₆Q[6]-based complexes. As shown in Fig. 16, for example, due to the ion–dipole interaction between the [ZnCl₄]²⁻ anions and the electropositive outer surface of the CyP₅Q[5], the [ZnCl₄]²⁻ anions form a honeycomb-like framework, and the CyP₅Q[5]-Mg²⁺ coordination complex occupy the cells of this framework. For coordination compounds of alkaline earth metal ions with other substituted Q[n]s, such as (HO)₁OMeQ[6],⁹⁹o-TMeQ[6],⁴¹ CyP6Q[6],¹⁰⁴ HMe6Q[6]¹⁰⁵ and (HO)₁Q[7],¹⁰⁶ the honeycomb effect is also observed, which originates from the inorganic anions [CdCl₄]²⁻ or [ZnCl₄]²⁻.

Fig. 16 Binding interaction of Mg²⁺ with CyP₅Q[5]. (a) Supramolecular assembly of Mg²⁺ cations, CyP₅Q[5] molecules and [ZnCl₄]²⁻ anions. (b) Occupying the cells of the framework. (c) Honeycomb-like framework constructed of [ZnCl₄]²⁻ anions, with CyP₅Q[5]-based chains.

Coordination compounds of substituted Q[n]s with alkaline-earth metal ions constructed in the presence of inorganic anions usually possess different porous structures, and display differing absorption of volatile compounds.^103,105,106 For example, the HMe₆Q[6]-Ca²⁺/Ba²⁺-[CdCl₄]²⁻ based coordination compounds AE-18 and AE-19 exhibit excellent thermal stability as well as permanent porosity. They also show high adsorption capacity and selectivity to Et₂O and CH₃OH.¹⁰⁵ The porous materials constructed by the coordination compounds of (HO)₁Q[7] with Ca²⁺ and Sr²⁺, AE-20 and AE-21, revealed a large sorption capacity for CH₃OH.

4.3. Interaction with transition metal ions

Transition metal coordination complexes usually display special optical, electrical and magnetic properties. However, only a few examples of Q[n]-transition metal coordination complex have been reported.^107–110 More often than not, transition metal ions prefer to form aqua complexes rather than to coordinate with Q[n] ligands in aqueous solution. This section presents some examples of substituted Q[n]s complexes with transition metal ions.

In 2010, the complexation behaviour of CyH₆Q[6] with Cd²⁺ ions was investigated by electrochemical methods and X-ray crystallography.¹⁰⁸ The host CyH₆Q[6] displayed extraordinary binding affinity towards Cd²⁺ ions. It can form a 1 : 6 CyH₆Q[6]/Cd²⁺ complex TM-1 in both aqueous solution and the solid state (Fig. 17). The strong binding affinity of CyH₆Q[6] toward Cd²⁺ ions may be derived from to the electron donating property of its six substituted cyclohexano groups. The study suggests the potential utility of CyH₆Q[6] as an effective chelator and extractant for toxic heavy metals.

Fig. 17 X-ray structure of CyH₆Q[6]/Cd²⁺ complex: (a) top view; (b) side view.

Three years later, we studied the effect of chloride anion concentration on the self-assembly of supramolecular architectures based on TMeQ[6] and copper chloride.¹⁰⁹ Under different chloride anion concentrations, three supramolecular architectures TM-2–4 were obtained through the reaction of TMeQ[6] and copper chloride. X-ray diffraction analysis revealed three tubular structures (Fig. 18), one of which is attributed to coordination bonds and the other two resulted from weak noncovalent interactions. In 2013, Zhang et al. described coordination compounds of CyP₆Q[6] (TM-5) and TMeQ[6] (TM-6–8) with Cu²⁺ in the absence or presence of a third species.¹¹⁰ The resulting coordination compounds showed how the corresponding supramolecular assemblies can depend on the addition of the third species. Very recently, Ma and co-workers studied coordination compounds of CyP₅Q[5] (TM-9 and TM-10) and CyP₆Q[6] (TM-11 and TM-12) with two transition metal ions, namely Cu²⁺ and Zn²⁺.¹¹¹

Fig. 18 Three tubular structures self-assembled from TMeQ[6] and copper chloride under different chloride anion concentrations: (a) 1.0 M HCl; (b) 3.0 M HCl; (c) 6.0 M HCl.

4.4. Interaction with lanthanide metal ions

Due to their unique physicochemical properties, lanthanide ions are widely used in numerous technological devices.^112–114 However, lanthanide ions are difficult to separate because of the poor shielding of the nuclear charge by filling of the 4f electron shell. The study of lanthanide coordination complexes here may lead to fascinating topological structures and promising applications. On comparison with other metal ions, lanthanide ions are the most studied in the coordination chemistry of substituted Q[n]s.^115–123

Because of the effect of lanthanide contraction, the lanthanide complexes with substituted Q[n]s usually display interesting structural progressions. As early as 2012, we prepared eight coordination compounds Ln-1–8 of CyH₅Q[5] with a number of lanthanide ions (La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Dy³⁺, Ho³⁺, and Er³⁺) by reactions of the corresponding lanthanide species with the CyH₅Q[5] in aqueous solution.¹¹⁵ Crystal structure analysis revealed two coordination modes (Fig. 19). Close inspection of these crystal structures revealed some interesting structural variations, which were ascribed to the effect of lanthanide contraction.

Fig. 19 Two kinds of coordination mode observed for CyH₅Q[5] with lanthanide ions.

In the following year, the effect of lanthanide contraction induced coordination architectures of CyH₆Q[6] was also studied by Ni and Tao et al.¹¹⁶ The slightly different ionic radii of the lanthanide metal ions plays a key role in the formation of different coordination modes for CyH₆Q[6]. As a result, fifteen coordination architectures Ln-9–23 of CyH₆Q[6]-Ln were structurally classified into four groups, as shown in Fig. 20.

Fig. 20 Coordination complexes of Cy6Q[6] with lanthanide ions, which were structurally classified into four groups.

On the other hand, each substituted Q[n] displays a different binding affinity to the same lanthanide ions. Some substituted Q[n]s only coordinate with light lanthanide ions, while others only coordinate with heavy lanthanide ions, and this provided a new method for separating lanthanide ions. For example, Zhu et al. investigated the interactions between a series of lanthanide ions and HMe₅Q[5] and HMe₆Q[6] in 2015.¹¹⁷ X-ray diffraction analysis revealed that HMe₅Q[5] and HMe₆Q[6] selectively interacted with certain lanthanide ions. In the presence of [CdCl₄]²⁻, HMe₅Q[5] crystallized with four light lanthanides, La³⁺, Ce³⁺, Pr³⁺, and Nd³⁺ and formed the coordination capsules Ln-24–27, whereas the other lanthanide ions remained in solution. Under the same conditions, HMe₆Q[6] crystallized with aqua complexes of lanthanide cations ([Ln(H₂O)₈]³⁺, Ln = Gd–Lu) and formed the adducts Ln-28–32, whilst La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, and Eu³⁺ remained in solution. It should be noted that no satisfactory data were collected for the ions Gd³⁺, Yb³⁺, and Lu³⁺. In neutral solution and in the absence of CdCl₂, HMe₆Q[6] crystallized with aqua complexes of lanthanide cations (Ln = Sm–Lu) to form adducts. Here, Ln-33 is a representative adduct. No solid crystals of HMe₆Q[6] with La³⁺, Ce³⁺, Pr³⁺, and Nd³⁺ could be obtained. Energy-dispersive spectrometry (EDS) studies indicated that the lighter or heavier lanthanide ions could be isolated from their counterparts through the interaction with HMe₅Q[5] and HMe₆Q[6].

In the past five years, the coordination chemistry of lanthanide metal ions with other substituted Q[n]s, such as o-TMeQ[6],^118,122 OMeQ[6],¹¹⁹ CyP6Q[6],¹²⁰t(HO)₂OMeQ[6],¹²¹ and CyH6Q[6],¹²³ under different conditions were also studied. Table 3 lists the coordination conditions and results of all substituted Q[n]s on interaction with lanthanide metal ions.

Table 3 The coordination conditions and results of substituted Q[n]s with lanthanide ions^a

Ligand	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	Ref.
a Promethium is not discussed because it is a radioactive element. × No crystals were obtained, √ Obtained crystals contain lanthanide ions, — no information, ○ The obtained crystals without lanthanide ions.
CyH5Q[5] in water	√	√	√	√	√	—	—	—	√	√	√	—	—	—	115
CyH6Q[6] in water	√	√	√	√	√	√	√	√	√	√	√	—	√	√	116
HMe5Q[5] in [CdCl4]^2−	√	√	√	√	×	×	×	×	×	×	×	×	×	×	117
HMe6Q[6] in [CdCl4]^2−	×	×	×	×	×	×	√	√	√	√	√	√	√	√	117
HMe6Q[6] in water	×	×	×	×	√	√	√	√	√	√	√	√	√	√	117
o-TMeQ[6] in neutral sol.	×	×	×	×	×	×	×	√	√	√	√	√	√	√	118
OMeQ[6] in [CdCl4]^2−	√	√	√	√	√	√	√	—	√	√	×	×	×	×	119
CyP6Q[6] in [ZnCl4]^2−	√	√	√	√	√	√	√	√	√	√	√	○	○	○	120
(HO)2OMeQ[6] in water	×	×	×	×	×	√	√	√	√	√	√	√	√	√	121
o-TMeQ[6] in Cd(NO3)2	×	×	×	√	√	√	√	√	√	√	√	√	√	√	122
CyH6Q[6] in [CdCl4]^2−	√	√	√	√	√	√	—	√	√	○	×	○	○	×	123

---

5. Applications of substituted Q[n]s

It must be noted that applications of hydroxylated Q[n]s and their derivatives have been reviewed in our previous work,¹⁹ and will not be discussed in this review.

5.1. Catalysis

In 2013, the Cao group prepared a series of M-Pd-Me₁₀Q[5] (M = Li, Na, K, Rb, and Cs) hybrid solid materials (Fig. 21), which displayed excellent catalytic performance and good recyclability as phosphine-free pre-catalysts for Heck cross-coupling reactions.¹²⁴ Their studies revealed that the activated Pd(II) species were released from the crystalline hybrid pre-catalysts and were transformed into catalytically active Pd nanoparticles during the catalytic reactions. In the following year, the Cao group prepared a composite material Ag@Me₁₀Q[5], in which spherical silver nanoparticles (Ag⁰ NPs) with an average size of ca. 4.4 nm were observed.¹²⁵ As a heterogeneous catalyst for the reduction of various nitrophenols, the composite material Ag@Me₁₀Q[5] exhibited excellent catalytic performance and remained active after several consecutive cycles.

Fig. 21 Crystal structures of a series of M-Pd-Me₁₀Q[5] (M = Li (a), Na (b), K (c), Rb (d), and Cs (e)) hybrid solid materials.

5.2. Inorganic–organic hybrid materials

The redox catalysis and magnetism exhibited by inorganic polyoxometalates (POMs) has made them attractive for research.¹²⁶ In 2009, Kögerler and coworkers reported an example of a hybrid supramolecular architecture based on POMs and Q[n].¹²⁷ Since then, Cao, Tao, Zhang, Zhu and co-workers have demonstrated numerous hybrid compounds of POMs with substituted Q[n] derivatives.^128–137 Very recently, Zhang and co-workers found that the self-assembly of (HO)₁₂Q[6] and Keggin-type POMs (such as H₃[PMo₁₂O₄₀] and H₃[PW₁₂O₄₀]) under different concentrations of hydrochloric acid yields two types of supramolecular assemblies: one being hybrid compounds (HO)₁₂Q[6]-POMs-HCl and the other (HO)₁₂Q[6]-HCl (Fig. 22).¹³⁷

Fig. 22 Two types of supramolecular assemblies in different directions: hybrid compounds (HO)₁₂Q[6]-POMs-HCl ((a) and (b)) and accumulation body of (HO)₁₂Q[6]-HCl ((c) and (d)).

They exhibited various interesting chemical and physical properties. It is worth mentioning that the complexation of POMs with Q[n]s or substituted Q[n]s is driven by “outer-surface interactions”, including ion–dipole interactions, hydrogen bonding, C–H⋯π interactions, as well as π⋯π stacking. Compared to Q[n]s, HO groups of the (HO)₁₂Q[6] facilitate the formation of the “outer-surface interactions”, and enhance crystal stability of the supramolecular assemblies.

5.3. Fluorescent chemosensors

Using the host–guest interactions of TMeQ[6] and HMe₆Q[6] with a fluorophore guest (Fig. 23) as a fluorescence indicator displacement (FID) system, Ni and co-workers constructed a new type of ratiometric fluorescent chemosensor for the sensing, detection, and recognition of two α-amino acids, namely lysine and methionine.¹³⁸ As indicated in Fig. 23, the differing binding interactions of the two substituted Q[6]s towards the target analytes led to a useful ratiometric detection signal output for the discrimination of lysine and methionine versus the other tested α-amino acids.

Fig. 23 Illustration of the plausible fluorescence indicator displacement process based on host–guest interactions.

As part of their template studies (see section 3.1), Anzenbacher Jr. and Isaacs and coworkers isolated a Q[6] derivative with a covalently attached 2,3-dialkylnaphthalene fluorophore (symbol 19 in Fig. 24), which can emit fluorescence in response to UV irradiation.⁷² When combined with a number of different metals, only in the case of Eu³⁺ or Dy³⁺ was significant fluorescence quenching observed. The Eu³⁺ system was found to act as a sensor for the histamine shown below (Fig. 24, bottom, left). It was postulated that the histamine occupies the cavity and one ureidyl C=O portal and adopts a 1 : 1 binding model; the Eu³⁺ was thought to occupy the other ureidyl C=O portal. The addition of the histamine leads to displacement of the quenching Eu³⁺ or Dy³⁺, and recovers fluorescence.

Fig. 24 Structure and Schematic representation of fluorescence assay based on 19.⁷²

Urbach and coworkers reported an optical sensor based on the combination of Q[7] and tetramethylrhodamine.¹³⁹ Equilibrium dissociation constants for this system matched those of the parent Q[7] despite the presence of the fluorophore. Cellular uptake was demonstrated using HT22 neurons, with localization at the cytoplasm and no disruption of cell growth over a 4 day period (concentration ≤2.2 μM).

5.4. Controlled “smart” SERS hot spot

One of the challenges that has faced single-molecule surface-enhanced Raman spectroscopy (SERS) is the ability to place the single molecule of interest reliably within a hot spot. In their pioneering work, Kim and co-workers demonstrated a novel strategy for locating and securing a single target analyte in a SERS hot spot at a plasmonic nanojunction.¹⁴⁰ As shown in Fig. 25, a thiol-functionalized cucurbit[6]uril, mercaptopropyloxy-Q[6] (thiol-Q[6]), acts as dual-function building block: a molecular spacer to generate a nanogap between a Ag nanoparticle and a Ag substrate, and a binding pocket to accommodate the target molecule through host–guest interactions. It is noteworthy that the substituent of the thiol-Q[6] play an important role in linking a single silver nanoparticle (AgNP) to the Ag substrate. The position of the hot spot is controllable by adjusting the length of the polymethylene linker separating the target from the binding moiety, spermine.

Fig. 25 (a) Schematic representation of a SERS hot spot. (b) Fabrication process of a SERS hot spot.

5.5. Gas inclusion and adsorption

Huber et al. recently investigated the binding of three substituted Q[5]s with dioxygen O₂.¹⁴¹ The study revealed that the (HO)₁₀Q[5] is able to significantly bind dioxygen gas at physiological temperature, even in the presence of sodium chloride at the concentration of injectable solution in blood. The study suggests the potential utility of (HO)₁₀Q[5] as a precursor host to transport O₂ in a hemoglobin substitute solution.

We recently compared the binding abilities of the hosts CyH₂Q[6] and CyH₆Q[6] with the guests aniline, p-methylaniline, and p-phenyldiamine.¹⁴² The results suggested that both hosts can accommodate all guests to form stable inclusion complexes. Entrapment tests and thermogravimetric analyses showed that the CyH₂Q[6] possessed higher removal efficiency for aniline than did the CyH₆Q[6]. Presumably the size and shape of the CyH₂Q[6] is more complementary to the aromatic rings of the guests than those of CyH₆Q[6] (Fig. 26).

Fig. 26 X-ray crystal structures of CyH₂Q[6] with guests aniline (a), p-methylaniline (b), and p-phenyldiamine (c).

In 2015, the Tao group synthesized HMe₅Q[5]-Hydroquinone-based¹⁴³ and HMe₆Q[6]-based¹⁴⁴ supramolecular assemblies which had large porous structures. These porous materials demonstrated selective sorption for methanol and ethanol. Zhang and Chen et al. recently prepared two supramolecular assemblies of (HO)₁₂Q[6] under different concentrations of hydrochloric acid, which were found to possess different kinds of channels.¹⁴⁵ These activated desolvated (HO)₁₂Q[6]-based solid supramolecular assemblies demonstrated high absorption selectivities and capacities for polychloromethanes, including tetrachloromethane, trichloromethane, and dichloromethane.

6. Self-assembly processes using substituted Q[n]s

6.1. Self-assembly of (pseudo)rotaxane and poly(pseudo)rotaxane

(Pseudo)rotaxanes, poly(pseudo)rotaxanes, catenanes, dentrimers, and other MIMs have attracted much attention because they are precursors for molecular machines. From a structural point of view, Q[n]s and substituted Q[n]s are perfect candidates for MIMs because they possess different hydrophobic cavities and different binding affinities. About twenty years ago, Kim and co-workers demonstrated a useful approach for constructing polyrotaxanes.¹⁴⁶ By using the same approach with minor modifications, some examples of MIMs based on substituted Q[n]s have been reported.

In 2011, we presented a novel strategy for synthesizing chiral helical polyrotaxanes. The achiral N,N′-bis(2-pyridylmethyl)-1,6-hexanediamine “string” contains two typical functional pyridyl groups and one 1,6-hexanediamine chain.¹⁴⁷ In aqueous solution, the TMeQ[6] bead was held in the middle position of the “string”. The addition of AgNO₃ resulted in a helical polyrotaxane, which was characterized by X-ray crystallography (Fig. 27).¹⁴⁸ The chirality of the polyrotaxane originated from the twist of the long alkyl chain when bound within the TMeQ[6] cavity. Two opposite chiral helical polyrotaxanes were observed in the crystal structure, indicating that they crystallized as a racemic compound. When KI was added to the solution of helical polyrotaxane, a mass of white precipitate was formed immediately and all of the ¹H NMR signals of the helical polyrotaxane disappeared, indicating that the polyrotaxane structure was thoroughly destroyed.

Fig. 27 X-ray crystal structure of helical polyrotaxanes of TMeQ[6] with N,N′-bis(2-pyridylmethyl)-1,6-hexanediamine and Ag⁺.

Zhang et al. reported the synthesis of two polyrotaxanes by using TMeQ[6] (Fig. 28).¹⁴⁹ The TMeQ[6] “bead” first reacted with the “string's” of N,N′-bis(3-pyridylmethyl)-1,4-butanediamine dichloride and N,N′-bis(3-pyridylmethyl)-1,6-hexane-diamine dichloride to form stable pseudorotaxanes. The reaction of the pseudorotaxanes with various transition metal ion Cd²⁺ then produced 1D polyrotaxanes. Another kind of polypseudorotaxanes, which involve the combination of multiple noncovalent interactions, including host–guest interactions, π–π stacking interactions, C–H⋯π interactions and metal–host coordination, were also reported.¹⁵⁰

Fig. 28 X-ray crystal structure of two polyrotaxanes of TMeQ[6] with N,N′-bis(3-pyridylmethyl)-1,4-butanediamine (a) and N,N′-bis(3-pyridylmethyl)-1,6-hexane-diamine (b).

6.2. Self-assembly of heterowheel pseudorotaxanes

The Day group designed and synthesized an intriguing tetraammonium chain, which performs the molecular machine function of contraction and expansion when bound with CyP₆Q[6] and Q[7], respectively.¹⁵¹ They also demonstrated the construction of a hetero-[4]-pseudorotaxane (Fig. 29), which combined the binding function of the “wheels” CyP₆Q[6] and Q[7] with the designed molecular “axle”.

Fig. 29 Schematic representation of the construction of a hetero[4]pseudorotaxane based on CyP₆Q[6] and Q[7].

Very recently, we constructed two novel heterowheel [4]pseudorotaxanes consisting of TMeQ[6] and Q[7] in which the Q[7] can rotate freely around the horizontal axis, while the TMeQ[6] cannot. In the construction process of the [4]pseudorotaxanes, the dethreading and movement of the wheels along the axle was observed (Fig. 30).¹⁵² Under alkaline conditions, the [4]pseudorotaxanes is destroyed. Thus, the construction and dissociation of the [4]pseudorotaxanes can be controlled through acid/base regulation.

Fig. 30 Schematic representation of the construction of a hetero[4]pseudorotaxane based on TMeQ[6] and Q[7].

6.3. Molecular encapsulation and release by substituted Q[n]

Artificial macrocyclic hosts have potential applications in various fields such as fragrance sustained-release, nutrient preservation, and drug delivery, molecular encapsulation and controlled release. Very recently, we have synthesized 1-(4-carboxybenzyl)-4-[2-(4-pyridyl)-vinyl]-pyridinium chloride with trans- and cis-isomers.¹⁵³ The trans- and cis-form of the guest can be controlled by encapsulation and release by the molecular container TMeQ[6] under light irradiation and heating (Fig. 31). The encapsulation of the trans-form into TMeQ[6] is attributed to strong host–guest interactions and hydrophobic effects while cis-form release from TMeQ[6] is attributed to a size effect.

Fig. 31 Guest encapsulation and release by TMeQ[6] under light irradiation and heating.

Very recently, we found another molecular encapsulation and release system based on CyH₆Q[6] and 1,ω-bisbenzimidazolyl derivatives.¹⁵⁴ Experimental results revealed that all guests can form 1 : 1 or 1 : 2 inclusion complexes with CyH₆Q[6] residing over benzoimidazole groups of the guests. Interestingly, the guest was released from the CyH₆Q[6] cavity at high pH value and encapsulated back into the CyH₆Q[6] cavity at low pH value.

6.4. Molecular switches

In 2017, we designed and successfully constructed an electrochemically-driven molecular switch involving a special axle guest hexyldimethyl(ferrocenylmethyl)ammonium bromide.¹⁵⁵ As shown in Fig. 32, both hosts CyH₆Q[6] and Q[7] can form different stable [2]pseudorotaxanes with the axle guest in its different redox states. Most importantly, the combination and dissociation of the hosts with the guest as well as the binding location can be controlled by electrochemical means.

Fig. 32 Electrochemically-driven molecular switch constructed by axle guest hexyldimethyl(ferrocenylmethyl)ammonium bromide with hosts CyH₆Q[6] and Q[7].

7. Summary and outlook

Almost two decades have passed since the first example of substituted Q[n] was reported by the Stoddart group. Numerous substituted Q[n]s have been designed and successfully synthesized, which addressed many long-standing concerns in cucurbituril chemistry. These substituted Q[n]s possess enhanced solubility in aqueous and organic media, good selectivity in molecular recognition, and high binding affinities to different metal ions. In particular, the synthesis of monohydroxylated Q[n]s provides an opportunity to further functionalize the substituted Q[n]s. In this review article, the recognition properties of the substituted Q[n]s toward many organic species have been demonstrated. The coordination structures and characteristics of substituted Q[n] with different metal ions, including alkali/alkaline-earth metals, transition metals, and lanthanides have also been identified. The supramolecular properties of the substituted Q[n]s have led to their applications in areas as diverse as catalysis, sensors, hybrid materials, and gas inclusion and adsorption. The substituted Q[n]s also have been used in the construction of MIMs and molecular devices.

With the synthesis of more substituted Q[n]s, cucurbituril chemistry has ushered in a new era of development. The intrinsic properties of the substituted Q[n]s-high binding affinities, high selectivity, fine solubility in aqueous and organic media, diversity of cavity size and shape-suggest that substituted Q[n]s will become important components in MIMs. New molecular machines involving substituted Q[n]s with unprecedented characteristics are expected. The potential applications of substituted Q[n]s can be envisioned in pharmaceutical sciences, functional material science and beyond.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

We thank the National Natural Science Foundation of China (Grant No.: 21371004 and 21601090), Natural Science Foundation of Anhui Province of China (1808085MB43, 2008085MB36), the Natural Science Foundation of Jiangsu Province (BK20160943) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) for financial support. CR thanks the EPSRC for financial support (Grant no. EP/L012804/1 and EP/S025537/1).

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