Metal-mediated self-assembly of pyridylcalixarenes: prevention of intramolecular metal chelation is essential in constructing molecular capsules

Facile synthetic routes to bridge-functionalised calix[4]arenes

Ring-opening of furans at the equatorial methylene bridge positions of a calix[4]arene gives access to a range of new molecules (in good yield) that have widespread potential impact in supramolecular chemistry amongst other areas.

Metal-dependent selective formation of calix[4]arene assemblies based on dynamic covalent chemistry

The reaction of calix[4]arene derivatives 1a and 1b bearing four salicylaldehyde moieties with 1,3-propanediamine gave macrocyclic trimers 5a and 5b, respectively, which have intramolecular bridges formed via the flattened cone conformation. In contrast, a capsular-shaped dimeric cage [7a·2Na]²⁺ was selectively formed when the conformation of the calix[4]arene moiety of 1a was fixed in the spread cone conformation by complexation with Na⁺ at the lower-rim amide groups.

Modular synthesis of linear bis- and tris-monodentate fused [6]polynorbornane-based ligands and their assembly into coordination cages

A modular approach has been developed for the synthesis of rigid linear di- and tritopic ligands based on a fused [6]polynorbornane scaffold. The design provides up to three sites for installing functionality, including both "ends" and a "central" position with the advantage that each region can be independently addressed during synthesis. To illustrate the utility of the approach, both pyridyl and picolyl units were incorporated to provide six new ligands, with centers and ends either matched or mismatched. Indeed, both [M2L4] cages with endohedral functionality and [M3L4] complexes were cleanly produced from these ligands with assembled structures confirmed by using (1)H NMR spectroscopy, HRMS, and molecular modelling.

Supramolecular fluorescence enhancement via coordination-driven self-assembly in bis-picolylcalixarene blue-emitting M2L2Xn macrocycles

Photoactive macrocycles are obtained through coordination-driven self-assembly between calixarene L and Zn²⁺, Pd²⁺, Ag⁺, and Cd²⁺. Blue emission enhancement of L is observed upon assembly. M₂L₂X_n act as turn-off sensors for picric acid.

Diphosphine capsules for transition-metal encapsulation

Self-assembly and characterization of novel heterodimeric diphosphine capsules formed by multiple ionic interactions and composed of one tetracationic diphosphine ligand and one complementary tetraanionic calix[4]arene are described. Encapsulation of a palladium atom within a diphosphine capsule is achieved successfully by using the metal complex of the tetracationic diphosphine ligand for the assembly process. In this templated approach to metal encapsulation, the transition-metal complex is an integrated part of the capsule with the transition metal located inside the capsule and is not involved in the assembly process. We present two approaches for capsule assembly by mixing solutions of the precharged building blocks in methanol and mixing solutions of the neutral building blocks in methanol. The scope of the diphosphine capsules and the metallodiphosphine capsules is easily extended by applying tetracationic diphosphine ligands with different backbones (ethylene, diphenyl ether, and xanthene) and cationic binding motifs (p-C(6)H(4)-CH(2)-ammonium, m-C(6)H(4)-ammonium, and m-C(6)H(4)-guanidinium). These tetracationic building blocks with different flexibilities and shapes readily associate into capsules with the proper capsular structure, as is indicated by (1)H NMR spectroscopy, 1D NOESY, ESI-MS, and modeling studies.

Different metal-ion-induced dimeric self-assembling cavities based on thiacalix[4]benzocrown-4 isomers

To construct the self-assembly of metal-ion-induced well-ordered architectures based on calixcrowns, isomeric thiacalix[4]benzocrowns-4 1 and 2 with rigid and small crown units were employed as the new scaffolds. They all show remarkable selectivity for Ag(+) and their complexation ability towards Ag(+) results in two novel dimeric aggregates of calixcrowns, which were first evidenced by ESI-MS, (1)H and DOSY-NMR spectra. Ultimately, X-ray diffraction experiments confirmed unambiguously the existence of the two metal-ion-induced dimers in lower rim/lower rim mode, and showed that dimerization of calixcrown 1 or 2 in the presence of Ag(+) could form dimeric supramolecular cavity with a small inner room. Moreover, the positional isomerism of their crown units (o-benzocrown unit for ligand 1 and m-benzocrown unit for 2) led to a dramatic change in the configuration of the two dimeric cavities 1·Ag(+) and 2·Ag(+). For dimeric cavity 1·Ag(+), two silver centers seamed two thiacalix[4]crown molecules together and resided at the edge of the dimeric self-assembling cavity; for dimeric cavity 2·Ag(+), one Ag(+) stitched two thiacalixcrowns together and was encapsulated in the center of the self-assembling cavity, while the other Ag(+) is tied down to one end of the dimer. Consequently, as a result of the different construction of dimeric cavities 1·Ag(+) and 2·Ag(+) the extended structures of the complexes are also different. The neighbour self-assembling cavities 1·Ag(+) are mutually oriented side-by-side and form a 1-D rectilineal polymeric chain. While, the neighbour self-assembling cavities 2·Ag(+) arrange themselves in a typical head-to-tail fashion to form a zig-zag polymeric chain.

Supramolecular Assemblies of Sulfonatocalixarenes with Phenanthroline: Factors Governing Capsule Formation versus Bilayer Arrangements

Four crystalline complexes were prepared by the inclusion complexation of the 1,10-phenanthrolinium ion (Phen) with p-sulfonatothiacalix[4]arene (TCAS) (2 from a solution at pH 1-2 and 4 from 1 M HCl) and with p-sulfonatocalix[5]arene (C5AS) (3 from a solution at pH 1-2 and 5 from 1 M HCl) upon varying the acidity of the solution. By combining the results obtained for complexes 2-5 with those for our previously reported complex (1), p-sulfonatocalix[4]arene (C4AS) complexed to Phen, it was revealed that p-sulfonatocalixarenes (CASs) form "bis-molecular" capsules (1, 2, and 3) around Phen at pH 1-2, whereas complexes 4 and 5 display distinct host-guest inclusion behavior at higher acid concentrations. The degree of compactness of the capsules increases with the enlargement of the calixarene cavity, which is affected significantly by both the penetration depth of Phen and the structure of the Phen dimer. Furthermore, the complexation behavior of TCAS/C5AS with Phen in 1 M DCl was investigated by using NMR spectroscopy, and was discussed in comparison with the previously reported results obtained from solutions at pH 2.0.

Calix[6]arene Derivatives Selectively Functionalized at Alternate Sites on the Smaller Rim with 2-Phenylpyridine and 2-Fluorenylpyridine Substituents to Provide Deep Cavities

The synthesis is described of calix[6]arene derivatives 4, 9, and 14 functionalized at alternate sites on the smaller rim with 4'-(pyrid-2' '-yl)phenylmethoxy, (6'-phenylpyrid-3'-ylmethoxy), and {6'-[2-(9,9-di-n-hexylfluorenyl)]pyrid-3'-ylmethoxy} substituents, respectively. They were obtained by 3-fold reactions of 2-[4-(bromomethyl)phenyl]pyridine (3), 5-(bromomethyl)-2-phenylpyridine (8), and 5-(bromomethyl)-2-(9,9-di-n-hexylfluorenyl)pyridine (13) with the 1,3,5-trimethylether of the t-Bu-calix[6]arene in the presence of sodium hydride in THF in 56-75% yields. Detailed analysis of the 1H NMR spectra (including variable-temperature data for 4) has established that 4, 9, and 14 exist predominantly in the C3v cone conformation with minor Cs isomers also observed. The X-ray crystal structure of 4 reveals two molecules of similar cone conformation, with all three 4'-(pyrid-2' '-yl)phenylmethoxy substituents stretched in the axial direction. Molecule I has a dimeric capsule structure with (pyrid-2' '-yl)phenylmethoxy substituents of one molecule interpenetrating those of its inversion equivalent to form a deep enclosed intermolecular cavity, which contains a CH2Cl2 guest molecule. Molecule II forms no such pair: the intramolecular cavity is filled with solvent molecules.

Supramolecular blueprint approach to metal-coordinated capsules

An important problem in designing any large network is the assembly of systems that are resilient to change. From a chemical point of view, an analogy can be used where one requires supramolecular assemblies to maintain their dimensionality combined with limited structural perturbation in response to variation in its intermolecular framework. The identification of hydrogen-bonded framework patterns within experimentally known supramolecular assemblies that are structurally robust to disruption and selective hydrogen substitution are envisioned to act as a supramolecular blueprint or template for metal-ion retroinsertion. Here, we report the formation of a large neutral discrete pseudo-spherical coordination capsule assembled from 6 pyrogallol[4]arene ligands and 24 Cu(II) metal ions. Amazingly, this coordination capsule is structurally analogous to its hydrogen-bonded counterpart. This result shows a robust ability of pyrogallol[4]arene molecules to self-assemble into large hexameric cage structures from either the hydrogen-bonding or metal-ligand coordination process. The identification of robust supramolecular assemblies that conserve their structure in response to interchangeability between hydrogen-bonded networks for metal coordination, or inversely, represents an important advancement in supramolecular design.

Spectroscopic Studies on Inclusion Properties of Fullerenocalix[4]arene Conjugates with Metal Ions

Metal-chelating properties-in the ground and excited states-of fullerenocalix[4]arenes containing two malonamide substituents at the upper rim and four alkyl ester chains at the lower rim have been studied by means of steady-state absorption, fluorescence spectroscopy, and time-resolved transient absorption spectra. In particular, the influence that Ag+ enforces on the fullerene electronic spectra is due to direct interactions between Ag+ and the surface of C60. The effects stemming from Na+, Mg2+, and Ba2+, on the other hand, are indirect and are introduced through chelating the metal ions to the calix[4]arene moiety. They strongly depend on the molecular structure of the fullerenocalix[4]arenes. No spectroscopic evidence was obtained for any influence caused by Mn2+, although the malonamide groups provide good chelating ability even for this transition metal ion.

Deliberate synthesis of the preselected enantiomer of an enantiorigid molecule with pure rotational symmetry T

A carceplex with T symmetry of the type A(4)B(6) has been synthesized enantiospecifically by employing the trianion of benzene-1,3,5-tricarboxylic (trimesic) acid as A and the R-cis-Rh(2)(C(6)H(4)PPh(2))(2)(2+) cation as B. The chiral (C(2)) B units abolish the symmetry planes required for T(d) symmetry, thus leaving only the eight C(3) and three C(2) rotations of the group T. Racemization is impossible under any chemically interesting conditions. This would appear to be only the third case where both preselection of the desired enantiomer and enantiostability based on covalent bonding are achieved.

A Calix[4]arene Carceplex with Four Rh24+ Fasteners

It is shown that a spheroidal carceplex can be assembled by linking two bowl-shaped calix[4]arenes via four dimetal units, (DAniF)(2)Rh(2) (DAniF = N,N'-di-p-anisylformamidinate), with a molecule (diethyl ether) or a cation (tetraethylammonium ion) trapped inside. The tetraethylammonium carceplex, 1b, has been characterized by X-ray crystallography, (1)H NMR, IR, and mass spectrometry. The tetraethylammonium ion fits snugly in the interior of the spheroidal carceplex. A two-fold axis of the tetrahedral cation coincides with the idealized four-fold axis of the cage, and the cation is disordered over two rotational orientations. Only one axial position on each dirhodium unit is occupied by a ligand, CH(3)CN or H(2)O. The carceplex is very stable, and the axial ligands can be exchanged in single crystals without disrupting the crystallinity of the samples. In this way, a red crystal of the complex with all axial positions occupied by acetonitrile can be transformed to a green crystal of the complex with two axial positions having acetonitrile and the other two having water by simply putting the crystal in contact with distilled water. The calix[4]arene used to make the carciplex structure is 25,26,27,28-tetra-n-propoxycalix[4]arene-5,11,17,23-tetracarboxylic acid. By employing 25,26,27,28-tetrapropoxy-5,17-dibromo-calix[4]arene-11,23-dicarboxylic acid, two 1:1 dimetal:calixarene compounds have also been made and characterized: 2, cis-Rh(2)(DAniF)(2)(calix)(CH(3)OH), and 3, cis-Mo(2)(DAniF)(2)(calix). The Rh-Rh distances in 1b are in the range of 2.410(2)-2.428(2) A, that in 2 is 2.4383(4) A, and the Mo-Mo distance in 3 is 2.0931(4) A.

Capsule-like assemblies in polar solvents

Calix[4]arene derivatives with four anionic groups at their upper rim form discrete 1:1 complexes with complementary calix[4]arene derivatives bearing four cationic groups at their upper rim. Each cation is bound by two anions, and vice versa, in a mutual chelate arrangement, reinforced by a network of ionic hydrogen bonds. These multiple electrostatic interactions lead to the formation of highly stable capsule-like assemblies even in polar protic solvents such as methanol and water. In the capsule interior a cavity is formed that is in principle large enough for the encapsulation of small aliphatic and aromatic guests (170-230 A(3)). Monte Carlo simulations in water reproducibly lead to the same regular opimized structures. These differ mainly by their inner volume and flexibility, as demonstrated by molecular dynamics calculations. Most half-spheres can be synthesized by way of the tetrakis(chloromethyl) or the tetrabromocalix[4]arene intermediate. Oppositely charged calix[6]arenes also form strong complexes, but no indication was found for a lock in the cone conformation. The formation of the ball-shaped complexes from calix[4]arene building blocks was studied with Job plots, NMR titrations, NOESY, and variable-temperature experiments, as well as ESI-MS measurements. Investigations aimed at the inclusion of various guest molecules were carried out with alcohols, sulfoxides, benzene derivatives, and ammonium, as well as pyrazinium guests. Although binding isotherms were generated with cationic guests, these must be considered to be loosely associated around the seam rather than included inside the capsule.

Construction of monomeric and polymeric porphyrin compartments by a Pd(II)-pyridine interaction and their chiral twisting by a BINAP ligand

The construction of chirally twisted porphyrin-based molecular capsule 6 and polymeric capsule 8 was investigated by means of scanning electron microscopy (SEM) and (1)H NMR, UV-visible, and CD spectroscopic observations. Molecular capsule 6 and polymeric capsule 8 were constructed by the reaction of chiral cis-Pd(II) complex 4 bearing a (R)-(+)-2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (BINAP) ligand with porphyrin 1 bearing four pyridyl groups and porphyrin 2 bearing eight pyridyl groups, respectively. The peak-splitting pattern of the beta-pyrrole protons in the (1)H NMR spectrum and the specific CD spectral pattern bearing an exciton coupling band indicate that both molecular capsule 6 and polymeric capsule 8 are chirally twisted. Moreover, it was found that the CD intensity of the polymeric capsule plotted against [4]/([4] + [3]) shows a sigmoidal curvature, reflecting a unique cooperativity among the ligand groups; that is, the ligand existing in excess over the other dominates the twisting direction. These results consistently demonstrate that "chirality" in these molecular assembly systems is conveniently controlled by the use of chiral ligands.

Molecular discrimination of N-protected amino acid esters by a self-assembled cylindrical capsule: spectroscopic and computational studies

A self-assembled, cylindrical capsule was used to bind N-alpha-protected amino acid esters. The reversible encapsulation was studied using NMR spectroscopy in deuterated mesitylene solution and by computer-aided molecular modeling. BOC-L-alanine alkyl esters and BOC-beta-alanine alkyl esters were tested as guests, and the relative binding affinities were established by direct competition experiments. A good correlation was found between the experimental and calculated relative binding affinities in these two series. Guests that were slightly longer than the internal dimensions of the cavity were accommodated by adopting compacted conformations.

Guest encapsulation and self-assembly of molecular capsules in polar solvents via multiple ionic interactions

Herein we report the formation and characterization of a novel type of capsules resulting from the self-association between oppositely charged complementary building blocks in MeOH/H2O. The assembly is based on the interaction between tetraamidinium calix[4]arenes 1a-d and tetrasulfonato calix[4]arene 2. Evidence for the formation of the expected 1:1 assemblies is provided by proton NMR, ESI-MS, and ITC. The association process is fast on the NMR time scale and strongly entropy driven, with association constants in the range of 10(6) M-1. The system 1a.2 shows binding affinity toward acetylcholine, tetramethylammonium, and N-methylquinuclidinium cations.

Design and self-assembly of wide and robust coordination cages

The self-assembly of a new class of coordination cages formed from two tetrapyridyl-substituted cavitands connected through four square-planar palladium or platinum complexes is reported. The shape of the internal cavity resembles an ellipsoid with a calculated volume of 840 A(3). The four lateral portals have a diameter of about 6 A, large enough to allow the fast entrance/egress of counterions in solution. The platinum cages 3a,e cannot be disassembled using triethylamine as competitive ligand and they are kinetically stable at room temperature, whereas the palladium cages 3b-d, 3f-h are disassembled and kinetically labile under the same conditions. The different solubility properties of the cage components have allowed the extension of this self-assembly protocol to the liquid-liquid interface.

Dynamic covalent chemistry

Dynamic covalent chemistry relates to chemical reactions carried out reversibly under conditions of equilibrium control. The reversible nature of the reactions introduces the prospects of "error checking" and "proof-reading" into synthetic processes where dynamic covalent chemistry operates. Since the formation of products occurs under thermodynamic control, product distributions depend only on the relative stabilities of the final products. In kinetically controlled reactions, however, it is the free energy differences between the transition states leading to the products that determines their relative proportions. Supramolecular chemistry has had a huge impact on synthesis at two levels: one is noncovalent synthesis, or strict self-assembly, and the other is supramolecular assistance to molecular synthesis, also referred to as self-assembly followed by covalent modification. Noncovalent synthesis has given us access to finite supermolecules and infinite supramolecular arrays. Supramolecular assistance to covalent synthesis has been exploited in the construction of more-complex systems, such as interlocked molecular compounds (for example, catenanes and rotaxanes) as well as container molecules (molecular capsules). The appealing prospect of also synthesizing these types of compounds with complex molecular architectures using reversible covalent bond forming chemistry has led to the development of dynamic covalent chemistry. Historically, dynamic covalent chemistry has played a central role in the development of conformational analysis by opening up the possibility to be able to equilibrate configurational isomers, sometimes with base (for example, esters) and sometimes with acid (for example, acetals). These stereochemical "balancing acts" revealed another major advantage that dynamic covalent chemistry offers the chemist, which is not so easily accessible in the kinetically controlled regime: the ability to re-adjust the product distribution of a reaction, even once the initial products have been formed, by changing the reaction's environment (for example, concentration, temperature, presence or absence of a template). This highly transparent, yet tremendously subtle, characteristic of dynamic covalent chemistry has led to key discoveries in polymer chemistry. In this review, some recent examples where dynamic covalent chemistry has been demonstrated are shown to emphasise the basic concepts of this area of science.