Influence of H-bond competitors on the solvent-dependent structures of an octaurea-calix[4]tube

A novel octaurea-calix[4]tube (UC4T) has been synthesized in three steps from the original Beer's p-tert-butylcalix[4]tube ionophore. In a polar solvent (DMSO-d6), UC4T rapidly interconverts between two identical conformations with C2v symmetry for the two calix[4]arene subunits. However, in a less polar solvent mixture (CDCl3/DMSO-d6, 98 : 2), UC4T adopts a highly distorted asymmetric structure, which hinders the formation of typical tetraurea calix[4]arene capsular assemblies. The complexation of potassium (or barium) cations inside the dioxyethylene ionophoric binding site of UC4T triggers a C2v to C4v symmetry rearrangement of the two calix[4]arene subunits. This rearrangement leads to the formation of a transient capsular dimeric species observed in solution upon the addition of KI or BaCl2·2H2O to a solution (CDCl3/DMSO-d6, 98 : 2) of the macrocycle. X-ray studies confirm UC4T's ability to adopt different asymmetric conformations, depending on its interactions with solvent molecules. Two distinct crystal forms (α and β) of UC4T have been obtained, each displaying divergent calix[4]arene subunits with pinched-cone conformations. These conformations exhibit distinctive head-to-tail (α) or head-to-head/tail-to-tail (β) orientations of the ureido groups, which are involved in hydrogen bonding with solvent molecules. Notably, the pseudo-capsular 1D supramolecular polymeric arrays observed in the β form of UC4T assemble to create large parallel solvent channels.

Aryl-Extended and Super Aryl-Extended Calix[4]pyrroles: Design, Synthesis, and Applications

ConspectusProteins exhibit high-binding affinity and selectivity, as well as remarkable catalytic performance. Their binding pockets are hydrophobic but also contain polar and charged groups to contribute to the binding of polar organic molecules in aqueous solution. In the past decades, the synthesis of biomimetic receptors featuring sizable aromatic cavities equipped with converging polar groups has received considerable attention. "Temple" cages, naphthotubes, and aryl-extended calix[4]pyrroles are privileged examples of synthetic scaffolds displaying functionalized hydrophobic cavities capable of binding polar substrates. In particular, calix[4]pyrroles are macrocycles containing four pyrrole rings connected through their pyrrolic 2- and 5-positions by tetra-substituted sp³ carbon atoms (meso-substituents). In 1996, Sessler introduced the meso-octamethyl calix[4]pyrrole as an outstanding receptor for anion binding. Independently, Sessler and Floriani also showed that the introduction of aryl substituents in the meso-positions produced aryl-extended calix[4]pyrroles as a mixture of configurational isomers. In addition, aryl-extended calix[4]pyrroles bearing two and four meso-aryl substituents (walls) were reported. The cone conformation of "two-wall" αα-aryl-extended calix[4]pyrroles features an aromatic cleft with a polar binding site defined by four converging pyrrole NHs. On the other hand, "four-wall" αααα-calix[4]pyrrole isomers possess a deep polar aromatic cavity closed at one end by the converging pyrrole NHs. Because of their functionalized interior, aryl-extended calix[4]pyrroles are capable of binding anions, ion-pairs, and electron-rich neutral molecules in organic solvents. However, in water, they are restricted to the inclusion of neutral polar guests.Since the early 2000s, our research group has been involved in the design and synthesis of "two-wall" and "four-wall" aryl-extended calix[4]pyrroles and their derivatives, such as aryl-extended calix[4]pyrrole cavitands and super aryl-extended calix[4]pyrroles. In this Account, we mainly summarize our own results on the binding of charged and neutral polar guests with these macrocyclic receptors in organic solvents and in water. We also describe the applications of calix[4]pyrrole derivatives in the sensing of creatinine, the facilitated transmembrane transport of anions and amino acids, and the monofunctionalization of bis-isonitriles. Moreover, we explain the use of calix[4]pyrrole receptors as model systems for the quantification of anion-π interactions and the hydrophobic effect. Finally, we discuss the self-assembly of dimeric capsules and unimolecular metallo-cages based on calix[4]pyrrole scaffolds. We comment on their binding properties, as well as on those of bis-calix[4]pyrroles having a fully covalent structure.In molecular recognition, aryl-extended calix[4]pyrroles and their derivatives are considered valuable receptors owing to their ability to interact with a wide variety of electron-rich, neutral, and charged guests. Calix[4]pyrrole scaffolds have also been applied in the development of molecular sensors, ionophores, transmembrane carriers, supramolecular protecting groups and molecular containers modulating chemical reactivity, among others. We believe that the design of new calix[4]pyrrole receptors and the investigation of their binding properties may lead to promising applications in many research areas, such as supramolecular catalysis, chemical biology and materials science. We hope that this Account will serve to spread the knowledge of the supramolecular chemistry of calix[4]pyrroles among supramolecular and nonsupramolecular chemists alike.

Diffusion NMR for the characterization, in solution, of supramolecular systems based on calixarenes, resorcinarenes, and other macrocyclic arenes

The use of diffusion NMR in studying calixarenes and other arene-based supramolecular systems is described, emphasizing the pivotal role played by the calixarene community in transforming the methods into a routine tool used in supramolecular chemistry.

Molecular Tectonics with Di- and Trinuclear Organotin Compounds

Di- and trinuclear organotin(IV) complexes, in which the metal atoms are separated by large aromatic connectors, are useful building blocks for self-assembly. This is demonstrated by the preparation of [1+1], [2+2], and [2+3] macrocyclic and cage-type structures in combination with organic aromatic dicarboxylates. The linkage of the metal atoms by organic binders and the option of varying the number of reactive M-X sites generate versatile building blocks enabling molecular tectonics instead of the node-based strategy generally employed in metallo-supramolecular self-assembly.

Diffusion NMR of molecular cages and capsules

In the last decade diffusion NMR and diffusion ordered spectroscopy (DOSY) have become important analytical tools for the characterization of supramolecular systems in solution. Diffusion NMR can be used to glean information on the (effective) size and shape of molecular species, as well as to probe inter-molecular interactions and can be used to estimate the association constant (Ka) of a complex. In addition, the diffusion coefficient, as obtained from diffusion NMR, is a much more intuitive parameter than the chemical shift for probing self-association, aggregation and inter-molecular interactions. The diffusion coefficient may be an even more important analytical parameter in systems in which the formed supramolecular entity has the same symmetry as its building units, when there is a large change in the molecular weight, where many molecular species are involved in the formation of the supramolecular systems, and when proton transfer may occur which, in turn, may affect the chemical shift. Some of the self-assembled molecular capsules and cages prepared in the last decade represent such supramolecular systems and in the present review, following a short introduction on diffusion NMR, we survey the contribution of diffusion NMR and DOSY in the field of molecular containers and capsules. We will first focus on the role played by diffusion NMR in the field of hydrogen bond driven self-assembled capsules. We then survey the contributions of diffusion NMR and DOSY to the study and characterization of metal-ligand cages and capsules. Finally, we describe a few recent applications of diffusion NMR in the field of hydrophobic, electrostatic and covalent containers.

Probing the inner space of salt-bridged calix[5]arene capsules

A combined DOSY and XRD study indicates that a carboxylcalix[5]arene receptor is able to encapsulate α,ω-diamines of appropriate length by means of a proton-transfer-mediated recognition process followed by salt-bridge-assisted bis-endo-complexation.

Through-the-annulus threading of the larger calix[8]arene macrocycle

A complete study of the through-the-annulus threading of the larger calix[8]arene macrocycle with di-n-alkylammonium cations has been performed in the presence of the "superweak" TFPB counterion. Thus, it was found that such threading occurs only upon partial preorganization of the calix[8]arene macroring by intramolecular bridging. In particular, 1,5-bridged calix[8]arenes with a meta- or para-xylylene bridge (2 and 3) gave pseudo[2]rotaxanes in which one dialkylammonium axle (4a-4e(+)) was threaded into one of the two subcavities of the calix[8]-wheel. Conformational studies by using chemical shift surface maps and DFT calculations evidenced a 3/4-cone geometry for these subcavities. Higher pseudorotaxane K(ass) values were obtained for calix[8]-wheels 2 and 3, with respect to calix[6]-host 1a, due to the cooperative effect of their two subcavities. Dynamic NMR studies on calix[8]-pseudorotaxanes evidenced a direct correlation between K(ass) (and ΔG(ass)) values and energy barriers for calix inversion due to the effectiveness of thread templation. In accordance with DFT calculations, an endo-alkyl preference, over the endo-benzyl one, was observed by threading calix[8]-wheel 3 with the directional n-butylbenzylammonium axle 4d(+).

Dimeric capsules formed by tetra-CMPO derivatives of (thia)calix[4]arenes

Thiacalix[4]arene 2, calix[4]arene 3a and its tetraether fixed in the cone conformation 3b form homo- and heterodimeric capsules in apolar solvents, which are held together by a seam of NH...O=P hydrogen bonds between carbamoylmethyl phosphine oxide functions attached to their wide rim. Their internal volume of approximately 370 A(3) requires the inclusion of a suitable guest. Although neutral molecules such as adamantane (derivatives) or tetraethylammonium cations form kinetically stable complexes ((1)H- and (31)P-time scale), the included solvent is rapidly exchanged. The internal mobility of the included tetraethylammonium cation is distinctly higher (DeltaG=42.5 and 49.7 kJ mol(-1) for 3a and 3b) than that for similar capsules of tetraurea calix[4]arenes 1. Mixtures of 1 with 2, 3a, or 3b contain only the two homodimers but the heterodimerization occurs with the tetraloop tetraurea 6, which cannot form homodimers. Two dimers with cationic guests (2.(C(5)H(5))(2)Co(+).2 and 3a.Et(3)NH(+).H(2)O.3a) were confirmed by single-crystal X-ray analysis.

Molecular recognition and self-assembly special feature: Self-assembly of dimeric tetraurea calix[4]pyrrole capsules

Calix[4]pyrroles having extended aromatic cavities have been functionalized with 4 ureas in the para position of their meso phenyl substituents. This elaboration of the upper rim was completed in 2 synthetic steps starting from the alpha,alpha,alpha,alpha-tetranitro isomer of the calix[4]pyrrole obtained in the acid catalyzed condensation of p-nitrophenyl methyl ketone and pyrrole. In dichloromethane solution and in the presence of 4,4'-bipyridine N-N'-dioxide the tetraurea calix[4]pyrrole dimerizes reversibly forming a cyclic array of 16 hydrogen bonds and encapsulating 1 molecule of bis-N-oxide. The encapsulated guest is bound in the cavity by hydrogen bonding to the 2 endohedral calix[4]pyrrole centers. Further evidence for dimerization of the tetraurea calix[4]pyrroles is provided by (1)H-NMR experiments and by the formation of mixed capsules.

Octahydroxypyridine[4]arene Self-Assembles Spontaneously To Form Hexameric Capsules and Dimeric Aggregates

In recent years it has been observed that resorcin[4]arenes and pyrogallol[4]arenes form hydrogen-bonded hexameric capsules in nonpolar solvents. In the present study we have used NMR spectroscopy, with an emphasis on diffusion NMR, to investigate the self-assembly and the aggregation mode of solutions of octahydroxypyridine[4]arene (1 b) in chloroform. In spectroscopic studies, the hexameric capsule of C-undecylresorcin[4]arene (2 b) was used as a reference compound and in some cases also as an internal reference. The current diffusion NMR spectroscopy study shows, in contrast to a previous report, that compound 1 b self-assembles spontaneously into hexameric and dimeric aggregates in solutions in chloroform. The (1)H NMR and diffusion NMR spectroscopic studies on a solution of 1 b in CHCl(3) show the presence of new upfield-shifted peaks, which diffuse with the same diffusion coefficient as the hexameric peaks in the spectrum. Therefore, these new upfield-shifted peaks were attributed to encapsulated CHCl(3) molecules. Interestingly, diffusion NMR measurements showed that the addition of trifluoroacetic acid (6.7 equiv), which had no effect on the hexameric capsules of 2 b, led to the disassembly of the hexamer and the dimer of 1 b into its monomers. Therefore, we conclude that compound 1 b self-assembles spontaneously into hexameric capsules in nonpolar organic solvents, as do resorcin[4]arenes 2 b and 2 c and pyrogallol[4]arenes 3 a and 3 b.

Calix[4]arene-Based Bis[2]catenanes: Synthesis and Chiral Resolution

The exclusive formation of hydrogen-bonded dimers between tetraaryl and tetratosylurea calix[4]arenes has been used to prepare a series of ten "bisloop" tetraurea calix[4]arenes 3, in which adjacent phenylurea groups are covalently linked through alpha,omega-dioxyalkane chains. This dimerization with tetratosylurea 2 as template preorganizes the alkenyl residues of tetra(m-alkenyloxyphenyl) ureas 1 and enables their selective connection in high yield (up to 95 %) by olefin metathesis followed by hydrogenation. The "bisloop" calixarenes 3 also exclusively form heterodimers with 1. Thus, in a separated metathesis/hydrogenation sequence, a series of 14 cyclic bis[2]catenanes 4, in which two calix[4]arenes are connected through their wide rims by two pairs of interlocked rings (total size 29 to 41 atoms), were prepared in yields of up to 97 %. Optical resolution of these chiral bis[2]catenanes was studied by HPLC on chiral stationary phases. The single-crystal X-ray structure of one example (4(P,10)) confirmed the interlocking rings and revealed that the hydrogen-bonded dimeric capsule of the calix[4]arene can be "completely" opened.

Reasons for the exclusive formation of heterodimeric capsules between tetra-tolyl and tetra-tosylurea calix[4]arenes

The selective heterodimerization of tetra-tolyl () and tetra-tosylurea () calixarenes, serendipitously found by Rebek et al. (R. K. Castellano, B. H. Kim and J. Rebek, Jr., J. Am. Chem. Soc., 1997, 119, 12671-12672), has been used for the construction of highly sophisticated macrocycles and well-defined supramolecular assemblies. Regrettably, hitherto, neither the exact structure of these heterodimers nor the reason for their exclusive formation is known. We present molecular dynamics simulations using the AMBER force field in explicit chloroform solvent for the two homodimers, the heterodimer and the two uncomplexed tetra-urea calixarenes. The rigid rotation about the C-S-N-C bond of the tosylurea group has been calculated for a model compound (N-mesylformamide) at the RHF/6-31G* level of theory. The calculations suggest that the heterodimer . is energetically favored over the homodimers by a sterically relaxed conformation of the tosylurea hemisphere in ., by a moderate degree of reorganization of the hemispheres from the uncomplexed to the complexed state and by favorable interactions between the hemispheres. The tosylurea S=O groups are involved in the hydrogen bonding system which results in different sizes of the three capsules in increasing order . < . < .. To prove the computational predictions, 1H NMR experiments have been carried out with solvents/guests differing in shape and size. The largest capsule . prefers the larger guests toluene and p-xylene while the latter is not encapsulated in the smallest capsule ..

Tetraurea calix[4]arenes with sulfur functions: synthesis, dimerization to capsules, and self-assembly on gold

Various calix[4]arene derivatives, fixed in the cone conformation by decylether groups and functionalized at their wide rim by urea residues, were synthesized. In two compounds (,) sulfur functions were attached to the urea groups via different spacers in order to allow binding to metal surfaces. While they exist as single molecules in polar solvents, tetraurea calix[4]arenes of this type () combine to form dimeric capsules in aprotic, apolar solvents. A solvent molecule is usually included in such a capsule, if no guest with a higher affinity is present. In the presence of an equimolar amount of the tetratosylurea, the exclusive formation of heterodimers, consisting of one molecule of and, is observed. The homo- and heterodimerization of the newly prepared derivatives, were studied by 1H NMR to establish the conditions under which they exhibit the desired dimerization behaviour. Self-assembled monolayers (SAMs) were formed using the single calix[4]arenes, and the heterodimeric capsules. Chloroform, dichloromethane and ferrocenium cations were used as guests in these immobilized heterodimeric capsules. The particular supramolecular architecture of the heterodimers should ensure that, after the immobilization on the metal surface, decomposition of the capsules and release or exchange of the guest is impossible or at least hindered. The self-assembly process and the stability of SAMs formed by capsules filled with ferrocenium cations in electrolyte solutions were tested with surface plasmon spectroscopy. The inclusion of guests, such as dichloromethane or ferrocenium, in the immobilized capsules were confirmed by classical surface plasmon spectroscopy, by gold nanoparticle absorption spectroscopy and by time-of flight secondary ion mass spectrometry (ToF-SIMS). The film stability and quality was tested by cyclic voltammetry.

Dimolybdenum-Containing Molecular Triangles and Squares with Diamidate Linkers: Structural Diversity and Complexity

By employing cis-Mo2(DAniF)2(2+) (DAniF = N,N'-di(p-anisyl)formamidinate) as the vertex building block and terephthaloyldiamidate as the linker, four dimolybdenum-containing cyclic oligomers have been synthesized and structurally characterized. In these compounds, described by the general formula [cis-Mo2(DAniF)2((ArNOC)2C6H4)2]n, n = 3 and 4, the geometry and composition of the products are affected by the identity of the aromatic groups of the linker. When Ar = phenyl, n = 3 (1a and 1b); however, n = 4 for Ar = p-trifluoromethylphenyl (2) and when Ar = m-trifluoromethylphenyl (3). All these compounds have a central cavity, shaped by the diamidate linker, that is capable of serving as host to guest molecules in a selective manner. For compounds 2 and 3, self-assembly that takes place in the crystalline state entails intermolecular C-H...F-C interactions. Such interactions generate a one-dimensional network with a tunnel cross section of 10 x 10 A(2) in 2, whereas in 3, they result in a cage in which two THF molecules are encapsulated. The F...H distances vary in a broad range from 2.38 to 2.70 A.