Calix[n]arenes - Powerful Building-Blocks of Supramolecular Chemistry

Pavel Lhotak and Seiji Shinkai*

Department of Chemical Science & Technology, Faculty of Engineering, Kyushu University,

Abstract: Current methodology provides ready access to the bowl-shaped calix [n]arenes. Further chemical manipulation of these unique molecular structures provided a means to modifying their basic skeleton. These molecules were found to be extremely useful in host-guest chem istly due to their propensity to complex the ions and/or neutral molecules inside their cavities. They are thus ideal building blocks for the construction of various types of more sophisticated host molecules, sensors, and larger molecular systems with well-predefined structures. Some recent advances made in this field concerning the usage of calix[nlarenes in supramolecular chemistry are summarized in this review.

1. Introduction

Calix[n]arenes 1 are cyclic oligophenols from the class of Prdmetacyclophanes readily

accessible by simple condensation of para-substituted phenols with formaldehyde under basic

catalysis-see Figure 1 (ref. 1, 2). By careful choice of reaction conditions (phenol/base ratio, solvent,

temperature., and reaction time) we can selectively prepare calix[n]arenes with an even number of

phenolic units in surprisingly high yields-60% (la) to 85% (le) (ref. 3-5). On the other hand,

preparation of calix[n]arenes containing an odd number of phenolic units (lb, 1d) often poses a

synthetic obstacle requiring laboriuos chromatographic isolation of products. Until recently these

calix[n]arenes were usually obtained in very poor yields (•ƒ 10% ). However, an improved procedure

has appeared, allowing calix[5]arene lb to be prepared in an enhanced yield (18%, ref. 6,7).

By virtue of the fact that such an unique molecular structure can be constructed in a single-step preparation, the chemistry of calix[n]arenes has been arousing the chemists' interest in last two decades and especially throughout the 1990's an explosion of the calix[n]arene literature has taken effect. The most important properties of

calix[n]arenes are their easy derivatization in a well-defined manner and their unique molecular architecture suitable for complexation of ions and/or neutral molecules (ref. 8,9). In this context calix[4]arene la is surely the most significant member hitherto of the calix[n]arene family and its chemistry is by far the most advanced (ref. 10). This molecule represents a well-preorganized cavity,

Figure 1. Preparation of calix[n]arenes

Vol.53, No.11 (November 1995) ( 41 ) 963

the shape of which is tunable by suitable substitution of hydroxyl groups (substitution on the "lower rim"). Figure 2 shows four basic conformers (isomers) which we can prepare from la, and any of

them has its own specific

properties and characteristic utilization in host-guest chemistry. This makes

calix[4]arene a very attractive compound that we can use as a starting platform for designing more sophisticated structures which in turn can be used for the binding of

ions and neutral molecules. In this review we shall present some examples of such "higher level" derivatives together with the usage of calix[n]arenes in supramolecular chemistry.

2. Chemistry of multiple calix[n]arenes.

By the connection of two or more than two calix[n]arene units together (via the reactionwith bi- or polyfunctional reagents) we can prepare more elaborated structures belonging to the family of multiple calix[n]arenes (ref. 11). The assembly of two calixarene moieties into one bis-calix[n]arene can lead to a receptor having new

properties unknown in simple calix[n]arene monomers. For instance, compound 2 represents bis-calix[n]arene

(carceplex) exhibiting a novel type of stereoisomerism, which is schematically depicted in Figure 3 (ref. 12). Carceplex 2 was found to hold a guest

molecule inside the cavity and because of its unsymmetrical

structure there are two basic mutual orientations of host and guest molecules. The presence of two isomers, named by the authors,

carceroisom ers, was unambiguously proven by temperature-

2

dependent 11-I NMR spectra and thus the coalescence temperature for the N-methyl-2-pyrrolidinone

3

Figure 2. Possible conformers of calix[4]arenes

Figure 3. Carceroisomerism

Figure 4. Intra- and intermolecular hopping of cations

964 ( 42 ) J. Synth. Org. Chem., Jpn.

guest was aproximately 50 •Ž (activation enthalpy of rotation is 16±1 kcal moll). Another interesting

function -metal hopping between two calix[n]arene subunits - was discovered (ref. 13,14,) in bis-

calix[4]arene 3, where both calix[4]arene moieties are bound together via the lower rims. When a

sodium cation was used for the complexation study, two different coalescence temperatures were

found in 1H NMR spectra. While the former seems to be spacer independent, the later depends on

the number of methylene units. They are assignable to the inter- and intramolecular hopping of

cations, respectively (Figure 4). Similar metal oscillation was observed also in doubly-bridged bis-

calix[4]arenes (ref. 15).

Great attention has been devoted to the synthesis of cage molecules based on bis-

calix[4]arenes during the last several years (ref. 11, 16-21). Until now, three basic approaches have been reported in the literature; a) connection through the upper rims, b) cyclization via the lower rims, and c) upper rim-lower rim connection of calix[4]arene subunits (ref. 11). Since a review surveying the literature concerning bis-calix[4]arenes up to 1993 has appeared (ref. 11), we shall only concern ourselves with the most recent examples.

Reaction of calix[4]arene with bifunctionalalkylation (acylation) agents seems to be the simplest way to access derivatives connected by the lower rims, but only limited success has been achieved by this method. In this system, the drawback of the reaction is that several side reactions (polymerization, intramolecular bridging, etc.) are

competing and thus purification of the resultant mixture requires time-consuming chromatographic isolation. Therefore, the choice of appropriate spacer molecule, solvent, base, stoichiometry of the reactants, etc., is of high importance

(ref. 11). While the prepa-

45

ration of double-bridged (schematically shown in Figure 5) cage derivatives was achieved by the reaction of p-tert-butylcalix[4]arene with di-, tri-, and tetraethylene glycol ditosylate (ref. 11), tothe best of our knowledge, the similar tetrabridged derivative has not been prepared by direct alkylation. In the contrast to the carcerands reported by Cram et al.

(ref. 22), the intramolecular cross-linking dominates over the inter-molecular dimerization in calix[4]- arenes. For instance, cali^[4]- barreland 5, being designed for Eu3+ complexation, was prepared by additional alkylation of half-ban-eland 4 by 5,5'-bis(bromo-methyl)-2,2'-bipyridine in the pre-sence of NaH in 4% yield. Direct

Figure 5. Calik[4]arene cage molecules,X=spacer.

Figure 6. Preparation of cage molecule 8

Vol.53, No.11 (November 1995) ( 43 ) 965

alkylation of la did not lead to the proposed product at all (ref. 17) .A different approach was used for the preparation of an upper rim-upper rim linked cage

molecule (Figure 6). The authors (ref. 20) have increased the yield of 8, when conformationally mobile (R= Me) starting tetrachloromethyl compound was used instead of an immobilized (R=Pr) one. Since in this case the "miss-stitched" derivative 7 can transform itself into 6 by simple ring inversion of the mobile portion of the molecule, the desired cage 8 was obtained in areasonable 12% yield. A similar reaction with immobilized starting compound (R=Pr) afforded 8 in very low yield (less than 1%).

The lower rim-upper rim connected cage derivative 10 was prepared (ref 19) by a fragment condensation method from la, as illustrated in Figure 7. One calix[4]arene moiety was used as a

"holder" where -

upon a second calix[4]arene unit was built up by alkylation with a suitable tosylate, benzyl group deprotection and reaction with 2,6-bis(bromo-

methyl)-p-cresol. However, the low yield (4%) of the macrocyclization step is a significantdrawback of this strategy.

Not only cone conformers have been utilized for the construction of multiple calix[4]arenes, but there is also one example (ref 23) of bis-and Iris-derivatives based on 1,3-alternate confor-mations. Starting from a readily available tetrachlo-romethyl calix[4]arene (1,3 -alternate) derivative, the target compound 11 was

prepared by sequential reaction with catechol and 4,4'-isopropylidenediphenol. A similar bis-calix[4]arene

12

13

14

15

Figure 7. Lower rim-upper rim connected cage derivative

Figure 8. Synthesis of oligo-calix[4]arenes, R = H or But

966 ( 44 ) J. Synth. Org. Chem., Jpn.

has been prepared in an analogous manner. Both compounds are believed to exhibit Agthopping from one calix[4]arene unit to another (analogy to derivative 3 ).

Very recently, we have developed a simple but attractive strategy enabling preparation of oligo-calix[4]arenes fixed in the cone conformation. The starting tripropoxy derivative was alkylated

with an excess amount of ƒ¿,ƒÖ-dibromoalkanes to yield "higher order"

bromoalkyl derivatives of calix[4]arene 12, which were used for

construction of appropriate bis-(13), tris-(14), and pentakis-calix[4]arenes

(15), as schematically depicted in Figure 7. These derivatives represent one

of the first series of intermediates for the synthesis of calix[4]arene-based

dendrimers. It is noteworthy that this method is suitable for the synthesis of unsymmetrically substi-tuted bis-calix[4]arenes

(ref. 24) as exemplified in the novel syntheses of derivatives 13 (R=H, n=3,6), whereby tert-

16

17

butyl and de-tert-butylated calix[4]arene moieties are linked together within one molecule.

It has been shown, that la (R=But) can react with some metal halides (ref. 25-27) to form dimeric structures of type 16, where two calix[4]arene moieties are connected (fused) by two metal atoms. In particular, tetracoordinate silicon (ref. 26, 27) or titanium (ref. 25) were able to fulfil the ideal coordination geometry required for such type of dimers.

The analogous derivative 17 is based on the self-association of calix[4]arenes bearing two acetylacetone groups and a Cu' ceion. The resultant structure is held together through Cu' chelation, while the cavity shows an interesting recognition ability for diamines (ref. 28, 29).

3. Calixporphyrins and the others

Starting from a dialdehyde derivative of la, the double cofacial porphyrin 18 linked by two calix[4]arene units was prepared ina, one-pot synthesis (ref. 30). However, the yield of this reaction is very low

(0.4%), and moreover, other authors were unable (ref. 31) to reproduce the described procedure. Interesting models for the multifunctional recogni-

18

19

20

tion of anions or neutral molecules have been designed. The central metalloporphyrin (ref. 31) unit and the two calix[4]arenes, appended either via their lower (19) or upper (20) rims, create together

Vol.53, No.11 (November 1995) ( 45 ) 967

well a preorganised cavity. Also in this case, the low yield of cyclization steps (3-5%) seems to be a drawback of this approach.

Compound 21 is another example (ref. 32) of calix[4]arene "capped" porphyrin. This derivative possesses several interesting features: 1) it has the same C4 symmetry as porphyrin itself, 2) the linkage between calix[4]arene and porphyrin is provided by four chiral pillars , and 3) there are both

alkali-metal and transition-metal binding sites within the molecule-hard and soft binding sites. It was found that the shift of UV absorption maxima in the presence of I- can be used for its recognition.

Recently, a similar type of calix-capped derivative

(without chirality) has been prepared (ref. 33) and complexation ability along with the oxygen carrierfunction of the Fe- salt has been investigated.

There still remains numerous other examples of calix[n]arene-based compounds which we can refer to. However the sheer volume of this work appearing in the literature entails that we have to be

21

22

selective in our discussion here. Restricting ourselves to certain examples we have: calix[4]arene

connected (ref. 34) with ƒÀ-cyclodextrin (22), could be a very useful receptor for the inclusion of

guest molecules due to the presence of two cavities with different size and hydrophobicity. Another attractive class of molecule were

prepared by O-glyco-sylation of la (R=H) at the phenolic groups (ref. 35) or la (R=CH2OH) at the upper rim. Such compounds are of interest because they are

23

24

the first calix[4]arene derivatives with polyhydroxylated chiral substituents. Moreover, compound 23 represents the water-solubility with potential use as enzyme mimics and molecular receptors in aqueous solution. Last but not least example of calix[n]arene derivatives (ref. 36) with interesting functionalization could be molecule 24, where calix[6]arene is triply bridged by molecule of cyclotriveratrylene. The resultant derivative, due to its increased rigidity, has well-defined conformational behaviour, as proven by 11-1 NMR spectroscopy.

4. Interactions with fulleitne

It is without a doubt that the most spectacular and probably most important progress in host-

guest chemistry of calix[n]arenes has been achieved in the study of interaction with

968 ( 46 ) J. Synth. Org. Chem., Jpn.

buckminsterfullerenes (C60). The first paper concerning this topic has appeared in 1992 wherein itdescribed (ref. 37) an interesting phenomenon - formation of a water-soluble complex of C60 with the octakis-sulphonate derivative of calix[8]arene 25. Here the fullerene molecule acts as a "core" which is "packed" into the cavity of calix[8]arene and the authors have

proposed extensive electronic charge-transfer type interactions between n-systems of the constituent fullerene and calix[8]arene units. Interestingly, C70 (slightly greater relative to C60) was not included into the similar complex at all.

25

This discovery led directly to the ultimate goal-simple isolation of C60. At the same time two scientific groups (ref. 38, 39) described a very simple and efficient method for large-scale

purification of C60 from carbon arc soot. The process is based on the selective formation of a 1:1

complex of le (R = But) with C60. Due to its low solubility in organic solvents, the complex can be obtained as precipitate from toluene solution of carbon arc soot and le, and pure fullerene can

be again released by decomposition of the complex with CHC13 (C60 remains as

precipitate while le is dissolved in chloroform). Repetition of the simple precipitation-decomposition operations can yield C60 fullerene in multigram amounts, high purity, and moreover, the minimum effort

26

need only be expended. The whole procedure is schematically depicted in Figure 9. While to date the X-ray of the solid complex is yet to be measured, the solid state 13C NMR measurements (ref. 40) also indicate a 1:1 stoichiometry. C70 does not form the similar complex with le at all. In order to deeper our understanding of the complex structure, compound 26 was synthesized (ref. 41) and the self-inclusion complex (intramolecular) has been studied by 1H NMR spectroscopy. Very recently, the influence of para-substituents to the precipitation process was studied (ref. 42). Surprisingly, it was found that le (R =But) can create a 1:2 complex with C70 in benzene, the

Figure 9. Purification of Cho-buckminsterfullerene

Vol.53, No.11 (November 1995) ( 47 ) 969

structure of which presently remains unclear.

5. Self-assembly of calix[n]arenes

Calix[4]arene in the cone conformation, due to the relatively rigid bowl-shaped structure , wasused as a starting material for an interesting self-assembled molecular capsule 27 (ref. 43). Both hemispheres are complementarily bonded through suitably designed substituents at the upper rim with the aid of a hydrogen-bonding interaction . Probably the rigidity of such a system was insufficient to allowing the detection of interactions by a NMR. technique. Fluorescence spectra along with vapor pressure osmometry revealed the creation of a 1:1 complex in THE solution.

Recently, we have described another self-assembled system based on the interaction between calix[4]arene derivatives and 5,5-dialkyl barbituric acids (Figure 10). The 2,6-Diaminopyridine

portion of the molecule is "locked" by the intramolecular hydrogen bonds and cannot interact with complementary barbituric acid (ref. 44). After addition of 1 equivalent of NaC104 this bond is disrupted because of the change in the conformation caused by metal complexation. This results in the exposure of

27

the hydrogen-bonding sites to the medium and the creation of intermolecular hydrogen bonds . The resultant clusters were detected with the help of light scattering.

Calix[n]arenes having long acyl groups at the upper rim were found to behave as an excellent and unique gelators for various organic solvents (ref. 45-47). For instance, recrystallization of compound le (R=COC,H,5) from benzene, n-hexane, or cyclohexane results in gels. On the contrary, similar structures (ref. 46) [le (R=CH,C1,1-15) or le (R=COC,H25) methylated at OH

groups] without either carbonyl or free hydroxyl groups failed to gelatinize any of the tested organic solvents. These findings established that the network of gels is formed by intermolecular C=0<=>H-0 interactions. In some cases, resulting aggregates (network formed from fibrillar substructures with 11_,Em diameter) could be observed directly by optical microscopy. Interestingly, this system (ref. 33)

Figure 10. Metal-controlled self-assembly of calix[4]arenes

970 ( 48 ) J. Synth. Org. Chem., Jpn.

is capable of operating even in protic solvents such as isopropanol, n-butanol, or n-hexanol. Calix[n]arenes can form also liquid crystalline phases as described in case of compound 28

(ref. 48, 49). Such azomethine-type derivatives, immobilized in a cone conformation, were found to create a columnar mesophase with calix[4]arene units aligned in a columnar arrangement - Figure11. The same arrangement was ob-served for 29, where rigidification of ca-lix[4]arene derivatives has been achieved by binding of a tungsten atom (ref. 50, 51). The behaviour of meso-

phase in 29 can be dramatically influenced by host-guest inte-ractions: for instance, inclusion of DMF

28

29

results in suppression of the mesomorphism and lowering of the isotropic point by 85 °C (compared with uncomplexed 29). This type of columnar mesogens could be interesting for preparation of new materials with ferroelectric properties (ref. 52). Liquid crystals constructed from the intermolecular hydrogen bonds between a calix[4]arene derivative having four pyridyl groups and four p-alkoxybenzoic acid units (similar to 27) was described recently (ref. 53).

6. Langmuir-Blodgett films, monolayers and micelles

Calix[n]arenes bearing sulphonate groups at the upper or lower rim were proved to be water soluble and critical micelle concentrations have been evaluated with the help of surface tension, conductance, and spectroscopic measurements (ref. 54,55). The aggregation properties are closely related to the length of introduced alkyl groups and can be classified into three categories: a) non-micellar (water soluble) calix[n]arenes,

b) micelle-forming calix[n]arenes, and

c) unimolecular micellar calix[n]arenes

(ref. 54).

Very recently, self-assembled

monolayers of calix[4]resorcinarenes

on gold surface have been

characterized (ref. 56, 57). While the

monolayer prepared by the adsorption

of thioether derivative 30 at room

temperature is kinetically disordered

(Figure 12 a), after heating at 60 •Ž,

the highly ordered structure (Figure 12

b) can be obtained. A rationale for this

change is that there is monolayer

reorganization to an energetically more

favorable structure with all alkyl

30

a)

b)

groups oriented parallel to each other (thermodynamically controlled process). The use of such

Figure 11. Liquid crystals based on calix[4]arenes

Figure 12. Gold surface assembly of calix[4]arenes

Vol.53, No.11 (November 1995) ( 49 ) 971

assemblies as receptors and sensors is now in progress. There are several examples of stable monolayers at the air-water interface based on

calix[n]arenes alkylated (acylated) at lower (ref. 58, 59) or upper rims (ref. 60, 61), and the similar behaviour has been described also in case of some n-alkyl calix[4]resorcinarenes (ref. 62, 63). Since the monolayer behaviour differs as a function of the specific metal-ion or neutral molecule binding, one of the possible applications of calix[n]arenes mentioned above is molecular recognition at theair-water interface. The selective recognition of metal ions (ref. 59), ammonium cations (ref. 58, 61), or even sugars (ref. 63) has indeed been demonstrated.

Calix[n]arene monolayers have been also used for construction of fundamentally new classes of membranes suitable for molecular separation (ref. 64-70). Monolayers of

31

calix[n]arene derivatives can be cross-linked to form highly ordered polymeric membranes with an

accurate and defined internal pore size by virtue of the calix[n]arene incorporated into the polymer

(Figure 13). It was shown by using a Langmuir-Blodgett technique that we can create the carrier-

supported multilayers, representing a new type of porous composite material suitable for molecular "filtration

." For instance, selectivity towards He and SF, using this type of composite membranes

(12-layers of polymerized 31) is very high (He/SF6•„440) and even in case of He and N, the

selectivity factor (He/N,) was found to be 28 (ref. 65).

7. Conclusions

During the past decade, calix[n]arenes have been becoming an increasingly important and "indispensable" tool in host-guest chemistry. The unique molecular architecture has predetermined them for "catching" ions and/or neutral molecules in a well-defined manner. Furthermore, calix[n]arenes can be useful not only in pure "scientific" chemistry, but also in various industrial applications (for example, isolation of C60). Their advancement as building-blocks for the construction of new types of highly sophisticated host molecules, sensors, and larger molecular assemblies with well-predefined structures and functionalities seems to be almost unrelenting. In the above review we tried to demonstrate several examples of calix[n]arenes' important role in supramolecular chemistry.

8. References and Notes

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Figure 13. "Molecular sieves" based on calix[n]arenes

972 ( 50 ) J. Synth. Org. Chem., Jpn.

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(Received June 14, 1995)

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