Complexation of carboxylic acids and anions by alpha and beta cyclodextrins

A pH potentiometric method is used to measure complex formation constants of aqueous alpha- and/or beta-cyclodextrin with several carboxylic acids and carboxylate anions: butyric acid/butyrate; valeric acid/valerate; hexanoic acid/hexanoate; octanoic acid/octanoate; decanoic acid/decanoate; cyclohexanecarboxylic acid/cyclohexanecarboxylate and benzoic acid/benzoate. Standard enthalpies and entropies of complex formation are calculated from the temperature dependencies of the equilibrium constants. These thermodynamic parameters of the alpha-cyclodextrin complexes largely conform to a correlation based on complexes with other substrate species previously reported. Both standard enthalpies and entropies of formation of beta-cyclodextrin complexes are found to be more positive than the corresponding complexes of alpha-cyclodextrin with the same substrates. These observations lead to insights into the bonding mechanism of cyclodextrin complexation.     

A Room‐Temperature Luminescent AgCF3COO Complex Consisting of Cationic Complex Chains and Anionic Guests

Reaction of p‐phenylenediacetonitrile (p‐phda) with AgCF₃COO afforded the coordination polymer, {[Ag₂(p‐phda)₂] [Ag₄(CF₃COO)₆]}_n ( 1 ), where the 1D cationic [Ag₂(p‐phda)₂]²⁺ chain acts as host and the anionic [Ag₄(CF₃COO)₆]^2– as guest molecules occupy the channel between neighboring host chains. This is a rare crystal example of AgCF₃COO complex consisting of cationic complex chains and anionic guests. In addition, complex 1 exhibits luminescence at room temperature in solid state.     

Crystalline complexes of N,N'-ditritylurea (DTU) and N-tritylurea (NTU) hosts with molecular guests

This paper reports crystalline complexes of the new hosts N,N'-ditritylurea (DTU) and N-tritylurea (NTU) with various uncharged molecular guests. The crystal structures of the following complexes were elucidated by single crystal X-ray diffraction analysis at 115^oK: (I) 1:1 DTU-propanamide — space group C2/c, a=15.839Å, b=9.088Å, c=24.584Å, =111.05^o, Z=4; (II) 1:1 DTU-ethyl N-acetylglycinate — space group P1, a=9.010Å, b=10.800Å, c=19.810 Å, =105.29^o =94.33^o, =93.03^o, Z=2; (III) 2:1 NTU-N, N-dimethylformamide — space group Cc, a=29.614Å, b=8.906Å, c=16.127Å, =121.04^o, Z=4. The three crystal structures are stabilized mainly by a cooperative effect of hydrogen bonding between amide fragments displaced along the shortest axis of each crystal. This interaction occurs between host and guest in complexes I and II, and between host and host in complex III. The latter also represents a cage-type clathrate in which the guest molecules are accommodated in voids between the hydrophobic fragments of four neighboring NTU hosts. On the other hand, complexes of DTU are characterized by a more specific interaction between the two components, each guest molecule being inserted between two adjacent hosts (related by translation) and strongly bound to them via hydrogen bridges. These results illustrate a useful concept in the design of molecular species which can be potential hosts upon crystallization with neutral molecular guests. Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82022 (7 pages).     

Complexation of adamantane-ammonium substrates by beta-cyclodextrin and itsO-methylated derivatives

A spectrophotometric method is used to determine formation constants of complexes of-cyclodextrin, the 2,6-di-O-methylated and 2,3,6-tri-O-methylated derivatives of-cyclodextrin as hosts with adamantan-1-ylammonium, adamantan-2-ylammonium and adamantan-1-ylmethylammonium as substrate species. The spectrophotometric method uses methyl orange anion and acid forms as indicator species. Complexes of the cyclodextrins with these species are determined as well as with the adamantane derivatives. Standard enthalpies and entropies of formation of all complexes are calculated from the temperature variation of the equilibrium constants.-Cyclodextrin and its 2,6-O-methyl derivative have comparable complex strengths with adamantaneammonium substrates and these strengths are about two orders of magnitude stronger than the corresponding complexes of the permethylated derivative. Thermodynamic parameters are interpreted in terms of differing intramolecular properties of the cyclodextrin complexes.     

Hetero-Coencapsulation within a Supramolecular Cage: Moving away from the Statistical Distribution of Different Guests

Beside sensing and delivery, another peculiar property arising from confinement in discrete molecular hosts comes from the possibility to have in close proximity, and in defined position, two different molecules (hetero-coencapsulation). This phenomenon can be tuned considering steric and electronic properties of the guests. In this work, a study on the parameters affecting homo- and hetero-coencapsulation processes within a supramolecular cage is reported. In particular, different benzoate guests were bound within a supramolecular cage containing two metal-binding sites and the experimental binding thermodynamics measured. Unexpectedly, from competition experiments it was observed that the maximum concentration of hetero-coencapsulation is achieved if a weakly binding guest is used to partially displace a strongly binding guest.     

The role of charge transfer interactions in the inclusion complexation of anionic guests with α-cyclodextrin

Two-parameter correlation models for the inclusion complexation of 30 carboxylic acids and their conjugated basis with α-cyclodextrin were obtained in terms of guest desolvation free energy (G_desolv), guest global softness (S) and a host-guest charge transfer parameter (ΔN) defined in the frame of Density Functional Theory (DFT). From the obtained models, it was concluded that charge transfer interactions are relevant in the stabilization of anionic inclusion complexes with α-cyclodextrin. Finally, an orbital picture is proposed in order to account for the charge transfer process between α-cyclodextrin and anionic species.     

Inclusion of neutral guests by water-soluble macrocyclic hosts – a comparative thermodynamic investigation with cyclodextrins,calixarenes and cucurbiturils

Abstract

The driving forces of association between three different families of macrocycles as hosts, namely cyclodextrins (α-, β-, and γ-), p-sulfonatocalix[n]arenes (n = 4–6) as well as cucurbit[n]urils (n = 6–8), and three different bicyclic azoalkane homologues as guests, namely 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH), 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) as well as 2,3-diazabicyclo[2.2.3]non-2-ene (DBN), were examined by means of calorimetric titrations, NMR spectroscopy and molecular dynamics simulation, all in aqueous solution. The small, spherical and uncharged guests preferably bind inside the cavities of the medium sized hosts. The inclusion complexation by β-cyclodextrin and p-sulfonatocalix[4]arene shows medium binding affinities (millimolar), while cucurbit[7]uril macrocycle shows very strong binding (micromolar). For all types of macrocycles, the complex formation is enthalpically driven (ΔH° < 0), accompanied by slightly unfavourable entropy changes (ΔS° < 0). The results are discussed in terms of the flexibility of the hosts, the hydrophobic character of their cavities and the release of high-energy water upon binding, and generalised by including two additional guests, the ketones cyclopentanone and (+)-camphor.     

Simultaneously bound guests and chiral recognition: a chiral self-assembled supramolecular host encapsulates hydrophobic guests

Driven by the hydrophobic effect, a water-soluble, chiral, self-assembled supramolecular host is able to encapsulate hydrophobic organic guests in aqueous solution. Small aromatics can be encapsulated in the supramolecular assembly, and the simultaneous encapsulation of multiple species is observed in many cases. The molecular host assembly is able to recognize different substitutional isomers of disubstituted benzenes with ortho substitution leading to the encapsulation of two guests, but meta or para substitution leading to the encapsulation of only one guest. The scope of hydrophobic guest encapsulation is further explored with chiral natural products. Upon encapsulation of chiral molecules into the racemic host, diastereomeric host-guest complexes are formed with observed diastereoselectivities of up to 78:22 in the case of fenchone.     

Preparation and X-ray structures of complexes of 18-membered crown ethers with polyfunctional guests: Urea and (O-alkyliso)uronium salts

The preparation and X-ray structure determinations of six complexes of urea and (O-n-butyliso)uronium salts with crown ethers are presented. Urea forms isostructural 5:1 adducts with 18-crown-6 (1) and aza-18-crown-6 (2), in which two urea molecules are each hydrogen bonded to two neighbouring hetero atoms of the macroring. The remaining urea molecules form two-dimensional layers alternating with crown ether layers. In both complexes the macroring has theg ⁺ g ⁺ a ag ^– a ag ^– a g ^– g ^– a ag ⁺ a ag ⁺ a conformation withC _i symmetry. In the solid 1:1 complex of O-n-butylisouronium picrate with 18-crown-6 (3) two types of conformations of the macroring were observed: theg ⁺ g ⁺ a ag ^– a ag ⁺ a ag ^– g ^– ag ^– a ag ⁺ a conformation with approximateC _m symmetry and to a lesser extent theg ⁺ g ⁺ a ag ^– a ag ⁺ a g ⁺ g ⁺ a ag ^– a ag ⁺ a conformation with approximateC ₂ symmetry. Both conformations allow the guest to form three hydrogen bonds to the macrocyclic host. Three complexes of 18-crown-6 and uronium salts have been prepared and characterized by X-ray crystallography. The 1:1 complexes with uronium nitrate (4) and uronium picrate (5) both exhibit the sameC ₂ conformation and the same hydrogen bonding scheme as in the least occupied form of the previous complex. A 1:2 complex with uroniump-toluenesulphonate (6) has a different hydrogen bonding scheme (two hydrogen bonds per cation to neighbouring oxygen atoms of the macroring) and a different conformation of the host molecule (theag ⁺ a ag ^– a ag ⁺ a ag ^– a ag ⁺ a ag ^– a conformation with almostD _3d symmetry). An attempt to prepare a solid uronium nitrate complex with diaza-18-crown-6 in the same way as the 18-crown-6·uronium nitrate (1:1) complex did not yield the expected result. Instead X-ray analysis revealed that the uronium ion is dissociated, resulting in the nitrate salt of the diprotonated diaza crown ether (7). Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82058 (26 pages).     

A Neutral Supramolecular Hyperbranched Polymer Fabricated from an AB2‐Type Copillar[5]arene

A heterotritopic copillar[5]arene monomer by introducing effective neutral guest moieties (methylene chains end‐capped with cyano and triazole groups) to a pillar[5]arene macrocycle is prepared. This well‐designed AB₂‐type copillar[5]arene contains strong host–guest recognition motifs that are connected with relatively flexible and long linkers, thus efficiently assembles to form supramole­cular hyperbranched polymer (SHP) in chloroform solution, which is characterized by various techniques including ¹H NMR, DOSY, viscosity, DLS, and TEM. Particularly, this supramolecular polymer can be effectively depolymerized by adding a competitive butanedinitrile guest.

     

The complexation of ferrocene derivatives by a water-soluble calix[6]arene

The complexation of several ferrocene derivatives by the water-soluble hostp-sulfonato-calix[6]arene was investigated using electrochemical and¹H-NMR spectroscopic techniques. The electrochemical results indicate that both oxidation states of the guests are bound to the calixarene host, although the oxidized (ferrocenium) forms are complexed more strongly than the reduced (ferrocene) species.¹H-NMR spectroscopic data indicate that the complexation phenomena involves the inclusion of the guest's ferrocene moiety into the flexible calixarene cavity.This paper is dedicated to the commemorative issue on the 50th anniversary of calixarenes.     

Subtle Modification of Imine-linked Helical Receptors to Significantly Alter their Binding Affinities and Selectivities for Chiral Guests

Aromatic helical receptors P- 1 and P- 2 were slightly modified by aerobic oxidation to afford new receptors P- 7 and P- 8 with right-handed helical cavities. This subtle modification induced significant changes in the binding properties for chiral guests. Specifically, P- 1 was reported to bind d -tartaric acid (K_a=35500 M⁻¹), used as a template, much strongly than l -tartaric acid (326 M⁻¹). In contrast, its modified receptor P- 7 exhibited significantly reduced affinities for d -tartaric acid (3600 M⁻¹) and l -tartaric acid (125 M⁻¹). More dramatic changes in the affinities and selectivities were observed for P- 2 and P- 8 upon binding of polyol guests. P- 2 was determined to selectively bind d -sorbitol (52000 M⁻¹) over analogous guests, but P- 8 showed no binding selectivity: d -sorbitol (1890 M⁻¹), l -sorbitol (3330 M⁻¹), d -arabitol (959 M⁻¹), l -arabitol (4970 M⁻¹) and xylitol (4960 M⁻¹) in 5% (v/v) DMSO/CH₂Cl₂ at 25±1 °C. These results clearly demonstrate that even subtle post-modifications of synthetic receptors may significantly alter their binding affinities and selectivities, in particular for guests of long and flexible chains.     

Two 3-D Neutral Zn(Ⅱ)-MOFs Generated from the Asymmetric Schiff-base Ligand

Two new Zn(Ⅱ)-MOFs were synthesized based on an asymmetric Schiff-base ligand LH, which is obtained by the condensation of 5-formyl-8-hydroxyquinoline and 3-pyridinecarboxylic acid hydrazide. In different mixed solvents, we get two new isomorphism compounds, [Zn(L)2]·C4H8O2·2MeOH(1) and [Zn(L)2]·CH2Cl2·2MeOH(2). Compounds 1 and 2 have been characterized by single-crystal X-ray diffraction, elemental analysis and IR. Compound 1 crystallizes in monoclinic, space group P21/c, with a = 13.856(4), b = 14.156(4), c = 9.620(3) , β = 110.167(5)°, V = 1771.3(9) 3, Dc = 1.500 g/cm3, C38H38N8O8 Zn, Mr = 800.13, F(000) = 832, μ(MoKα) = 0.761 mm-1, Z = 2, the final R = 0.0596 and wR = 0.1221 for 2112 observed reflections(I 2σ(I)). Compound 2 belongs to monoclinic, space group P21/c, with a = 13.575(3), b = 14.130(3), c = 9.485(2) , β = 107.908(3)°, V = 1731.2(6) 3, Dc = 1.529 g/cm3, C35H32Cl2N8O6 Zn, Mr = 796.96, F(000) = 820, μ(MoKα) = 0.926 mm-1, Z = 2, the final R = 0.0527 and wR = 0.1395 for 2501 observed reflections(I 2σ(I)). The Zn(Ⅱ) centers in both 1 and 2 display a distorted octahedral coordination geometry. The octahedral Zn(Ⅱ) nodes and L linkages build up 2-D nets consisting of parallelogram-like grids in the ab plane. The neighboring nets stack through hydrogen bonds to generate 3-D rhombic channels. In 1, the 1,4-dioxane and MeOH guests alternatively arrange in the channels and effectively take up the free space; while in 2, the CH2Cl2 guests are arranged in the channels. In addition, the encapsulated MeOH molecules are bonded to the framework by hydrogen bonds.     

Template-Directed Quantitative One-Pot Synthesis of Homochiral Helical Receptors Enabling Enantioselective Binding

Template-directed synthesis and dynamic covalent chemistry were implemented to achieve quantitative one-pot syntheses of homochiral helical cavities inside aromatic foldamers. One-handed helical receptors P- 1 , M- 1 , P- 2 and M- 2 were assembled from their precursors in the presence of appropriate templates (d - and l -tartaric acid, and d - and l -sorbitol, respectively) via three sequential steps in one pot: imine-linked chain elongation, template-induced folding and [4+2] cycloaddition between helical turns. These helical receptors were proven to enantioselectively bind chiral guests used as the templates, and the differences between the association constants of enantiomeric guests were up to more than two orders of magnitude. The structures and binding modes of the receptors were fully characterized by single-crystal X-ray crystallography and ¹H NMR spectroscopy.     

Recognition of guests bearing donor and acceptor hydrogen bonding groups by heteroditopic calix[4]arene receptors

The synthesis of new hosts specifically designed for the recognition of neutral guests bearing donor-acceptor hydrogen bonding groups is described. These hosts are characterized by the presence of two distinct binding region in close proximity: the rigid π-donor cavity and the H-bond donor N-methylene-N′-phenylureido group inserted onto the upper rim of the calix[4]arene skeleton. The binding abilities of these receptors were investigated toward a series of neutral ditopic organic molecules in CDCl₃ solution by ¹H NMR spectroscopy. The results obtained show that rigidity of the calix[4]arene apolar cavity is the control element in determining efficiency. In fact, compared with the more rigid 2, host 10, where the rigidity of the cone structure is maintained by hydrogen bonding of the OH of the lower rim, a decrease of efficiency of almost one order of magnitude was observed. The cooperative effect of the two binding region of host 2 was verified with different classes of ditopic guests. Good efficiency in the recognition of urea derivatives and dimethylsulfoxide was achieved.     

Halide‐Anion Water Clusters in Cucurbit[6]uril Supramolecular Systems

Three types of dihalide water clusters [X₂(H₂O)₈]^2? (X=Cl, Br) and [I₂(H₂O)₁₀]^2? have been observed in cucurbit[6]uril supramolecular systems with same guest. According to the single crystal data and the quantitative theory computation, with the increase of electronegativity of halide ions, dihalide water clusters become more stable. A further comparison of three kinds of guests with different molecule length but the same anions, [p‐phenylenediamine salt, hexamethylenediamine salt and N,N′‐hexamethylenebis(pyrazinylbromide)], gives the result that the dihalide water clusters collapse from 2D to 1D with the increase of the lengths.     

Inclusion Compounds of Bulky Binaphthyl-type Bis-fluorenol Hosts

Inclusion properties of a new family of clathrate hosts (1–4) containing two 9-hydroxy-9-fluorenyl units or chloro-, bromo- and t-butyl-substituted derivatives of this group attached in the 3,3′-position to a basic 2,2′-binaphthyl construction element are reported (115 examples of clathrates). The crystal structures of six selected clathrates, involving dimethylsulfoxide, cyclopentanol, diethylether, benzylamine, chloroform and acetone as the guest, have been determined by X-ray diffraction, showing varied modes of supramolecular interaction dependent on the host and guest constitutions, while the formation of an intramolecular hydrogen bond between the hydroxy groups of the fluorenol units is a common structural feature (except in 3a) controlling twisted conformation of the host molecules.     

New Exo-Functional Cyclophane Hosts. Synthesis and Crystal Structures of Inclusion Compounds

The new cyclophane type host compounds 1–3, containing rigid aromatic units and two exo-topic carboxylic acid functions, have been synthesized. Crystalline solvent inclusions, involving the dicarboxylic acid hosts and their corresponding ester intermediates 6 (a–c), namely 1·DMSO (1:4), 2·pyridine (1:3), 3·pyridine (1:4), 6a·pyridine (1:2) and 6a·benzene (1:2), have been prepared and studied using X-ray diffraction on single-crystals. Moreover, X-ray structure analyses of the solvent-free crystals of the 6 (a, b) intermediates were also carried out for comparison. Co-crystals of the carboxylic hosts 1–3 contained H-bonded 1:2 host-guest associates as building blocks, together with additional space-filling guests, whereas only loosely bounded space-filling solvent molecules were found in the two solid inclusion compounds of the 6a cyclophane ester host. In addition to the mentioned conventional H-bond interactions between carboxylic hosts and their guests, the crystal structures proved to be held together by relatively weak C–H…O bonds besides the ordinary van der Waals' interactions. Packing relations, and the effects of structural variations, guest molecules and anisotropic packing forces on the conformation of the semi-rigid cylcophane ring have been discussed and compared in seven crystal structures.