Selectivity and Cooperativity in the Binding of Multiple Guests to a Pillar[5]arene–Crown Ether Fused Tricyclic Host

Crystal structure and supra-molecular features of a host-guest inclusion complex based on A1/A2-hetero-difunctionalized pillar[5]arene

A host-guest supra-molecular inclusion complex was obtained from the co-crystallization of A1/A2-bromo-but-oxy-hy-droxy difunctionalized pillar[5]arene (PilButBrOH) with adipo-nitrile (ADN), C47H53.18Br0.82O10·C6H8N2. The adipo-nitrile guest is stabilized within the electron-rich cavity of the pillar[5]arene host via multiple C-H⋯O and C-H⋯π inter-actions. Both functional groups on the macrocyclic rim are engaged in supra-molecular inter-actions with an adjacent inclusion complex via hydrogen-bonding (O-H⋯N or C-H⋯Br) inter-actions, resulting in the formation of a supra-molecular dimer in the crystal structure.

Noncovalently bound and mechanically interlocked systems using pillar[n]arenes

Pillar[n]arenes are pillar-shaped macrocyclic compounds owing to the methylene bridges linking the para-positions of the units. Owing to their unique pillar-shaped structures, these compounds exhibit various excellent properties compared with other cyclic host molecules, such as versatile functionality using various organic synthesis techniques, substituent-dependent solubility, cavity-size-dependent host-guest properties in organic media, and unit rotation along with planar chiral inversion. These advantages have enabled the high-yield synthesis and rational design of pillar[n]arene-based mechanically interlocked molecules (MIMs). In particular, new types of pillar[n]arene-based MIMs that can dynamically convert between interlocked and unlocked states through unit rotation have been produced. The highly symmetrical pillar-shaped structures of pillar[n]arenes result in simple NMR spectra, which are useful for studying the motion of pillar[n]arene wheels in MIMs and creating sophisticated MIMs with higher-order structures. The creation and application of polymeric MIMs based on pillar[n]arenes is also discussed.

Covalently bridged pillararene-based oligomers: from construction to applications

Covalently bridged pillararene-based oligomers (CBPOs) are formed by covalent bonding of pillararene monomers, and they play a critical role in expanding the multi-disciplinary application of pillararenes due to their excellent molecular complexing ability, specially designed geometry and multifunctional linking groups. This article provides a comprehensive review of the synthesis and applications of CBPOs. The design and synthetic strategies of a series of CBPOs (dimers, trimers, tetramers and others) are first introduced. Many CBPOs with multi-cavities and unique geometry are very attractive and efficient building blocks for constructing novel smart supramolecular polymers (SPs) with different topological structures through host-guest interactions. We describe the methods of constructing various SPs based on CBPOs in detail. Furthermore, the extensive applications of CBPOs and CBPO-based SPs in recognition and detection of ions and organic small molecules, selective adsorption and separation, artificial light-harvesting systems, catalysis, drug delivery systems, and others are systematically introduced. Finally, the future challenges and perspectives for CBPOs are also highlighted.

Crown ether-pillararene hybrid macrocyclic systems

A combination of Nobel macrocycle-crown ether and star macrocycle-pillararenes together in organic synthesis and material science is significant in obtaining hybrid systems, with rigid/flexible structural architecture, induced planar chirality, a negative cooperative effect and multiple fused cyclic hosts. In this review, we will discuss the synthesis/preparation of crown ether-pillararene hybrid macrocyclic systems by covalent bonds, supramolecular interactions and mechanical bonds, leading to hybrid compounds, supramolecular assemblies and mechanically interlocked molecules. The practical applications of crown ether-containing pillararenes will also be discussed in diverse areas, such as molecular recognition via fused multiple macrocycles and ion channels as well as external stimuli-responsive smart materials. We also call the attention of related researchers towards academic and technical issues about topological structures and applied functions in this fresh new fused macrocyclic field.

Oxacalix[4]arene-bridged pillar[5]arene dimers: syntheses, planar chirality and construction of chiral rotaxanes

Conformationally fixed oxacalix[4]arene-bridged pillar[5]arene dimers comprising a pair of enantiomers and a meso isomer were designed and synthesized. Furthermore, chiral [2]rotaxanes and [3]rotaxanes were constructed. Two pairs of enantiomers for [2]rotaxanes, one pair of enantiomers and a meso isomer for [3]rotaxanes were found, respectively.

Systematic rim cyano-functionalization of pillar[5]arene and corresponding host–guest property varieties

Synthetic investigation of the systematic replacement of alkoxy groups in pillar[5]arene by cyano ones was carried out. Host–guest property studies of nCN-pillar[5]arenes and 1,4-dicyanobutane showed the functionization number related host–guest interaction varieties.

Strong positive allosteric cooperativity in ternary complexes based on hydrogen-bonded aromatic amide macrocycles

Three new hydrogen-bonded aromatic amide macrocycles with eight residues were synthesized. The first single crystal structure of this class of larger macrocycles was obtained, which reveals a saddle-like conformation. Interestingly, in sharp contrast to previous negative cooperativity in binding paraquat with cyclo[6]aramide, strong positive allosteric cooperativity in ternary complexes was observed. This may open an avenue for the construction of mechanically interlocked molecules with these larger H-bonded macrocycles.

Guest-regulated chirality switching of planar chiral pseudo[1]catenanes

Pillar[5]arene-derived pseudo[1]catenanes in dichloromethane exist in an equilibrium between a self-included conformational state and a de-threading one.

Bis- and mono(m-benzoic acid)-functionalized pillar[5]arenes

Installation of m-benzoic acid functionalities on pillar[5]arene rims resulted in bis- and mono(m-benzoic acid)-functionalized pillar[5]arenes 1 and 2. Bis(m-benzoic acid)-functionalized pillar[5]arene 1 was able to self-assemble to form one-dimensional channels with DMF molecules residing in pillar[5]arene cavities. Esterification of two carboxylic acids in 1 with decane-1,10-diol did not afford a [1]catenane, but a bicyclic compound. Although 1-decanol esterification of mono(m-benzoic acid)-functionalized pillar[5]arene 2 did not form a self-included [1]pseudorotaxane-like structure, a mono(decyl m-benzoate)-functionalized pillar[5]arene bearing an ethyl acetate chain was found to form a self-included complex with the ethyl acetate moiety residing inside the pillar[5]arene cavity.

Nature of intramolecular interactions of vitamin C in view of interacting quantum atoms: the role of hydrogen bond cooperativity on geometry

The conformational analysis of six dihedral angles was calculated by second-order Moller-Plesset perturbation theory (MP2) with the correlation-consistent aug-cc-pVDZ basis set. The quantum theory of atoms in molecules (QTAIM) was applied to gain a description of the atoms and chemical bonds. A high content of hydroxyl groups in vitamin C's (VC) structure leads to a wide range of intramolecular interactions. The nature of these interactions within the selected VC conformers was studied in view of the interacting quantum atom (IQA) approach. Complete IQA analysis of the atomic and interatomic interaction energies indicated hydrogen bond formation was responsible for the stability of most of the local minima in the potential energy surface. In these conformers, the tandem participation of interactions was operating by way of two- or three-centered (bifurcated) cooperative networks. For the intramolecular hydrogen bond interplay in cooperativity, changes of the IQA atomic and interatomic interaction energies of the participant interactions were monitored during the formation of cooperative networks. The results of the intramolecular cooperativity were evaluated with changes of the delocalization index and bond distances.

Multicavity macrocyclic hosts

Multicavity macrocyclic hosts are host molecules comprising more than one macrocyclic guest binding components connected through multipoint linkages.