Complexation Between (O‐Methyl)6‐2,6‐Helic[6]arene and Tertiary Ammonium Salts: Acid/Base‐ or Chloride‐Ion‐Responsive Host–Guest Systems and Synthesis of [2]Rotaxane

Complexation between (O ‐methyl)₆‐2,6‐helic[6]arene and a series of tertiary ammonium salts was described. It was found that the macrocycle could form stable complexes with the tested aromatic and aliphatic tertiary ammonium salts, which were evidenced by ¹H NMR spectra, ESI mass spectra, and DFT calculations. In particular, the binding and release process of the guests in the complexes could be efficiently controlled by acid/base or chloride ions, which represents the first acid/base‐ and chloride‐ion‐responsive host–guest systems based on macrocyclic arenes and protonated tertiary ammonium salts. Moreover, the first 2,6‐helic[6]arene‐based [2]rotaxane was also synthesized from the condensation between the host–guest complex and isocyanate.     

Nano‐Saturn: Experimental Evidence of Complex Formation of an Anthracene Cyclic Ring with C60

An anthracene cyclic hexamer was synthesized by the coupling reaction as a macrocyclic hydrocarbon host. This disk‐shaped host included a C₆₀ guest in 1:1 ratio to form a Saturn‐type supramolecular complex in solution and in crystals. X‐ray analysis unambiguously revealed that the guest molecule was accommodated in the middle of the host cavity with several CH???π contacts. The association constant K_a determined by NMR titration measurements was 2.3×10³ L mol^?1 at 298 K in toluene. The structural features and the role of CH???π interactions are discussed with the aid of DFT calculations.     

Bis-thiourea macrocycles incorporating fused [n]polynorbornanes: binding of flexible versus rigid dicarboxylates

Two macrocyclic bis-thiourea hosts 5 and 6 were constructed and their interactions with two dicarboxylates of similar size (pimelate = flexible and terephthalate = rigid) were evaluated using ¹H NMR titration techniques. In contrast to previous work with thiourea functionalised [n]polynorbornanes (where a notable increase in H:G affinity was noted for the rigid guest), the new macrocyclic hosts, in particular host 6, bind pimelate more strongly than terephthalate (for 6 binding pimelate log K_a = 4.7, terephthalate log K_a = 3.7). A binding arrangement in which the flexible dicarboxylate is ‘perched’ above the macrocycle is proposed to justify these results.     

New polydentate macrocyclic ligands of hybrid amine-imine and amide-imine types as artificial anion receptors. Synthesis and study of anion binding

Three new macrocyclic Schiff bases containing an amine or amide structural fragment along with imine groups were synthesized by condensation of 2,6-bis(2-aminophenyliminomethyl)pyridine (1) and N, N’-bis(2-aminophenyl)pyridine-2,6-dicarboxamide (2) with 2,5-diformylpyrrole (₃) and 2,2-bis(5-formylpyrrol-2-yl)propane (4). The reaction of compound 1 with 3 proceeds abnormally and is accompanied by redox disproportionation of compound 1 in the first step. The structure of the macrocyclic product of this reaction was established by X-ray diffraction analysis. Spectrophotometric titration showed that hybrid macrocycle 10, which was prepared by condensation of compound 2 with 4, possesses the properties of an anion receptor and selectively binds hydrosulfate and dihydrophosphate anions in the presence of bromide and nitrate anions. The structures of 10 and its adduct with the hydrosulfate anion were calculated by density functional theory.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 161–168, January, 2005.     

Host–guest complexation of antioxidative caffeic and ferulic acid amides with a functionalized cyclophane

Host?guest complexation has been studied by ¹H NMR on the benzyl and phenethyl amides of ferulic and caffeic acids as the guests in chloroform and acetonitrile; the counter host is a cyclophane which integrates four phenylene rings, amino and amide groups in the macrocyclic framework and bears four pendant methyl acetate ester arms. CAPE, one of the best known natural antioxidants, also has been studied for comparison. Among the guests studied, ferulic acid benzyl amide shows NMR shifts due to the formation of a host?guest complex in chloroform. The complexation occurs in two steps with the formation constants K ₁?=?[HG]/[H][G]?=?6?M^?1 and β ₂?=?[HG₂]/[H][G]²?=?87?M^?2. Two guest molecules are bound on the surface of the macrocyclic framework of a host molecule by two hydrogen bonds, NH(host amide)···O=C(guest amide) and C=O(host ester)···HO(guest phenol). The latter hydrogen bond may protect the bioactive site, i.e., phenol OH, of guest molecules captured in the complex against undesirable oxidation. This feature is observed only for ferulic acid benzyl amide in chloroform; the cyclophane ester interacts with this amide, distinctively from the other hydroxycinnamic acid derivatives.     

An inclusion complex of β-cyclodextrin with mnt anion (mnt = maleonitriledithiolate) studied by induced circular dichroism

A new inclusion complex of β-cyclodextrin with sodium maleonitriledithiolate (Na₂mnt) was investigated by electronic spectra, induced circular dichroism (ICD), and quantum mechanics (QM) methods. The orientation of the guest anion inside the host cavity was studied by ICD spectra and analyzed by structural optimization using PM3 quantum chemical method. Finally, the inclusion constant was determined by both a linear and a non-linear fitting methods, which were based on the variation of ICD signals of the guest upon inclusion complexation with the host. The inclusion constant of Na₂mnt/β-cyclodextrin was estimated to be (2.45 ± 0.15) × 10³ or (3.10 ± 0.11) × 10³ M^?1 in solution by these two fitting methods.     

Syntheses,crystal structures,and magnetic properties of 1-D coordination polymers

Three coordination polymers of Robson-type macrocycles, {[Cu₄L¹(4,4′-bipy)₂]·4ClO₄·H₂O}_∞ (1), {[Cu₄L²(4,4′-bipy)₄]·2CH₃CN·4ClO₄·2H₂O}_∞ (2), and {[Zn₂L²(4,4′-bipy)₂]·(ClO₄)₂}_∞ (3) (where H₂L¹ and H₂L² are the [2?+?2] condensation products of 1,3-diaminopropane with 2,6-diformyl-4-methylphenol and 2,6-diformyl-4-fluorophenol, respectively), have been synthesized and characterized. Magnetic susceptibility was measured for 1 and 2 from 2 to 300?K. The optimized magnetic data were J?=?–368.5?cm^?1, J′?=?40.5?cm^?1 with R?=?1.69?×?10^?6 for 1 and J?=?–291.22?cm^?1, J′?=?83.74?cm^?1, ρ = 0.00168 with R?=?1.8?×?10^?11 for 2, respectively. The data reveal strong antiferromagnetic interactions between two Cu(II) ions in the macrocyclic unit and ferromagnetic interaction between the Cu(II) ions in two adjacent macrocyclic units for 1 and 2.     

Voltammetric Characterization of Redox‐Inactive Guest Binding to LnIII[15‐Metallacrown‐5] Hosts Based on Competition with a Redox Probe

A novel competitive binding assay was implemented to monitor the binding of a redox inactive substrate to a redox inactive metallacrown host based on its competition with ferrocene carboxylate (FcC^?) using cyclic voltammetry (CV). First, the binding of FcC^? to Ln^III[15‐MC‐5] (LnMC) hosts was characterized by cyclic voltammetry. It was shown that the voltammetric half wave potentials, E_1/2, shifted to more positive potentials upon the addition of LnMC. The explicit dependence of E_1/2 with the concentration of LnMC was used to determine the association constants for the complex. The FcC^? binding strength decreased with larger central lanthanide metals in the LnMC hosts, and substantially weaker binding was observed with La^III. X‐ray crystallography revealed that the hydrophobic host cavity incompletely encapsulated FcC^? when the guest was bound to the nine‐coordinate La^III, suggesting the LnMC’s ligand side chains play a substantial role in guest recognition. With knowledge of the MC‐FcC^? solution thermodynamics, the binding affinity of a redox inactive guest was then assessed. Addition of sodium benzoate to a LnMC and FcC^? mixture resulted in E_1/2 shifting back to the value observed for FcC^? in the absence of LnMC. The association constants between benzoate and LnMC’s were calculated via the competitive binding approach. Comparison with literature values suggests this novel assay is a viable method for determining association constants for host–guest systems that exhibit the proper electrochemical behavior. Notably, this CV competitive binding approach does not require the preparation of a modified electrode or a tethered guest, and thus can be generalized to a number of host–guest systems.     

Preparation and structural,spectroscopic, thermal,and electrochemical characterizations of iron(III) compounds containing dipicolinate and 2-aminopyrimidine or acridine

Two new iron(III) compounds, (Hamp)[Fe(pydc)₂]?·?2H₂O (1) and (Hacr)[Fe(pydc)₂]?·?2H₂O (2) (pydc^2??=?pyridine-2,6-dicarboxylic acid, amp?=?2-aminopyrimidine, acr?=?acridine), have been hydrothermally synthesized. Both compounds were characterized by spectroscopic methods (IR, UV/Vis), and their molecular and crystal structures were determined by X-ray crystal structure analysis and their thermal stability by thermogravimetric analysis/differential thermal analysis (TGA/DTA) methods. Compound 1 consists of Hamp⁺ cation and [Fe(pydc)₂]^? anion and 2 consists of Hacr⁺ cation and [Fe(pydc)₂]^? anion. Crystallographic characterization revealed an octahedron as a coordination polyhedron for the complex anion in 1 and 2 and the same O,N,O′-chelated coordination mode of pyridine-2,6-dicarboxylate. The crystal structures of 1 and 2 are stabilized by a complicated network of hydrogen bonds between the crystallization water molecules, counter ion, and carboxylates of pydc^2?. Thermogravimetric (TG) analyses of the two compounds were carried out to examine their thermal stabilities. Cyclic voltammetric response of bare glassy carbon electrode surface in 0.10?mol?L^?1 phosphate buffer containing 1 and 2 at different pH values indicated that they have the same voltammograms at all pH values and the electrochemical behavior of 1 and 2 has not been affected by different ion pairs. The formal potential of the solutions of 1 and 2 at the glassy carbon electrode surface was also pH-dependent with a slope of ?57.0?mV/pH unit at 25°C. This shows that the number of electrons and protons involved in the electrode process is equal.     

Helicenoid-based bis-1,2,3-triazole tweezer: Synthesis and selective iodide sensing

Abstract

Novel helicenoid based 1,2,3-triazole tweezer 4 was synthesized using a multistep synthetic protocol with high yields from 2,7-Dihydroxynaphtlene 3 as a precursor. This helicenoid-based bis-1,2,3-triazole tweezer 4 selectively causes non-covalent interaction with iodide anion. The UV-vis absorption exhibited enhancement while fluorescence spectra exhibited significant quenching. ¹H NMR titration showed shift of 1,2,3-triazole C-H signal with an increase in iodide concentration. Association constant of 3.818?×?10⁴ M^?1 was recorded for the host interaction with iodide ions. This value of association constant for iodide sensing using 1,2,3-triazole is the best reported so far for hosts with 1,2,3-triazole moiety and suggests that the helicenoid geometry is responsible for this remarkable behavior.     

Synthesis and Molecular Recognition of Water‐Soluble S6‐Corona[3]arene[3]pyridazines

We report the efficient and scalable synthesis and molecular‐recognition properties of novel and water‐soluble S₆‐corona[3]arene[3]pyridazines. The synthesis comprises a one‐pot nucleophilic aromatic substitution reaction between diesters of 2,5‐dimercaptoterephthalate and 3,6‐dichlorotetrazine followed by the inverse electron‐demand Diels–Alder reaction of the tetrazine moieties with an enamine and exhaustive saponification of esters. The resulting S₆‐corona[3]arene[3]pyridazines, which adopt a 1,3,5‐alternate conformation in the crystalline state, are able to selectively form stable 1:1 complexes with dicationic guest species in water with association constants ranging from (1.10±0.06)×10³ M ^?1 to (1.18±0.06)×10⁵ M ^?1. The easy availability, large cavity size, strong and selective binding power render the water‐soluble S₆‐corona[3]arene[3]pyridazines useful macrocyclic hosts in various disciplines of supramolecular chemistry.     

BROMOHYDROXYLATION OF GLYCALS—AN INVESTIGATION INTO THE REACTION OF SOME 4-N-ACYLATED DERIVATIVES OF METHYL 5-ACETAMIDO-7,8,9-TRI-O-ACETYL-2,6-ANHYDRO-3,4,5-TRIDEOXY-D-GLYCERO-D-GALACTO-NON-2-ENONATE AND ITS 4-EPIMER

Bromohydroxylation of some 4-N-acylated derivatives of the glycals of N-acetylneuraminic acid, methyl 5-acetamido-7,8,9-tri-O-acetyl-2,6-anhydro-3,4,5-trideoxy-D-glycero-D-galacto-non-2-enonate (4) and methyl 5-aceta-mido-7,8,9-tri-O-acetyl-2,6-anhydro-3,4,5-trideoxy-D-glycero-D-talo-non-2-enonate (the 4-epimer of 4), with N-bromosuccinimide (NBS) and water in the presence of a co-solvent has provided a range of new glycosyl donors. The stereoselectivity of the halohydroxylation reaction was found to be governed by solvent composition, reaction temperature and the stereoelectronic nature of the substituent at C-4.     

Chromogenic amides of pyridine-2,6-dicarboxylic acid as anion receptors

The synthesis of simple, chromogenic pyridine-2,6-dicarboxylic acid amides, derivatives of isomeric nitroanilines and aminonitrophenols, and their ion binding properties are described. The ligands' response to ionic species was examined by naked eye and was studied with the use of UV–vis spectroscopy in DMSO and its mixture with water. The effect of the localisation and the type of the substituents in aromatic rings were discussed. ¹H NMR experiments were carried out to probe the mechanism of anion recognition, i.e. complexation via hydrogen bond formation versus ligand deprotonation. A selective response of N,N′-bis(2-hydroxy-4-nitrophenyl)pyridine-2,6-dicarboxamide (L5) towards dihydrogen phosphate was found in both DMSO and DMSO–water (95:5) solvent mixture. The structure of N,N′-bis(2-hydroxy-5-nitrophenyl)pyridine-2,6-dicarboxamide (L4) was confirmed by X-ray crystallography.     

Guest Binding via N−H⋅⋅⋅π Bonding and Kinetic Entrapment by an Inorganic Macrocycle

Modern supramolecular chemistry is overwhelmingly based on non‐covalent interactions involving organic architectures. However, the question of what happens when you depart from this area to the supramolecular chemistry of structures based on non‐carbon frameworks remains largely unanswered, and is an area that potentially provides new directions in molecular activation, host–guest chemistry, and biomimetic chemistry. In this work, we explore the unusual host–guest chemistry of the pentameric macrocycle [{P(μ‐N^tBu}₂NH]₅ with a range of anionic and neutral guests. The polar coordination site of this host promotes new modes of guest encapsulation via hydrogen bonding with the π systems of the unsaturated C≡C and C≡N bonds of acetylenes and nitriles as well as with the PCO^? anion. Halide guests can be kinetically locked within the structure by oxidation of the phosphorus periphery by oxidation to P^V. Our study underscores the future promise of p‐block macrocyclic chemistry.     

Importance of aliphatic C–H hydrogen bonding on anion recognition

We have designed and synthesised new anion receptors 1 and 2, both of their C–H groups were at the α positions to carbonyl groups and further polarised by the attached polarising substituents. This enabled us to study hydrogen bonding donor ability of C–H bonds. The polarising substituents are electron withdrawing cyano group for host 1, while charged pyridinium group for host 2. As expected from charge effects, host 2 shows roughly an order of magnitude higher binding constants against various anion guests than those of receptor 1. Since the magnitude of polarisation change should be greatest for C–H group among various hydrogen bonding groups, this indicates the importance of C–H hydrogen bonding. In contrast, the relative order of binding constants was the same for both host 1 and 2. The order of association constants was found to be (CH₃)₂POO^? > CH₃COO^? > C₆H₅COO^? > Cl^? > Br^?. DFT calculation results were in good agreement with experimental binding constants and confirmed the importance of charged group substitution. In addition, receptor 1 showed the highest association constant for dimethyl phosphinate, which is implicated in many metabolic diseases.     

Synthesis and calcium channel antagonist activities of new derivatives of dialkyl 1,4-dihydro-2,6-dimethyl-4-(5-phenylisoxazol-3-yl)pyridine-3,5-dicarboxylates

The new analogues of nifedipine, in which 2-nitrophenyl group at position 4 is replaced by phenylisoxazolyl substituent, were synthesized. The symmetrical dialkyl 1,4-dihydro-2,6-dimethyl-4-(5-phenylisoxazol-3-yl)pyridine-3,5-dicarboxylates were prepared by classical Hantzsch condensation, and the asymmetrical analogues were synthesized using a procedure reported by Dagnino that involved the condensation of alkyl acetoacetate with alkyl 3-aminocrotonate and 5-phenylisoxazole-3-carboxaldehyde. The structure of all compounds was confirmed by IR, ¹H NMR and Mass spectra. In vitro calcium channel antagonist activities were evaluated as calcium channel antagonists using the high K⁺ concentration of guinea-pig ileum longitudinal smooth muscle (GPILSM) assay. These compounds exhibited moderate calcium antagonist activity (IC₅₀ = 10^?7 to 10^{? 5} M range) relative to the reference drug nifedipine (IC₅₀ = 1.10 ± 0.40 × 10^?8 M).     

Complexation of thiacalix[4]arene methylphosphonic and sulphonic acids with amino acids

In this paper, host–guest complexation process of thiacalix[4]arene tetrakis–methylphosphonic and tetrakis–sulphonic acids with amino acids by HPLC and molecular modelling methods has been studied. It was shown that thiacalix[4]arene tetrakis–methylphosphonic acid due to transformability of macrocyclic skeleton and flexibility of methylphosphonic substituents can adopt its conformation for strong multicentre binding of the amino acids with association constant values 530–10,140 M^? 1 in water.     

Clip[5]arenes: A new family of molecular clips

Here we report the design and syntheses of two new triptycene-based rigid acyclic C-shaped hosts, clip[5]arenes C[5]OH and C[5]ME, and the strong host–guest complexation between C[5]OH and an electron-poor bipyridinium salt, paraquat G. The K_a value for the host–guest complex C[5]OH???G was calculated to be (1.09?±?0.36)??×??10⁵?M^?1 in acetone by using a non-linear curve-fitting method based on the UV–vis absorption titration experiments. Furthermore, based on this new host–guest recognition motif, a novel pseudopolyrotaxane-like supramolecular structure was constructed with C[5]OH threaded on polyviologen polymer VP-10.     

Acyclic cucurbit[n]uril type receptors: secondary versus tertiary amide arms

ABSTRACT

Two acyclic CB[n]-type hosts (1 and 2) which possess four 2° or 3° amide arms are reported. Host 2 has four 3° amide arms that exist as a mixture of E- and Z-isomers. ¹H NMR was used to qualitatively investigate the binding properties of 1 and 2 which indicates they retain the essential binding features of macrocyclic CB[n] hosts. We measured the K_a values of 1 and 2 toward guests 6–14 by ITC. Neutral hosts 1 and 2 bind less tightly than tetraanionic hosts M1, ACB1, and ACB2. We attribute the lower K_a values to the absence of secondary ion-ion electrostatic interactions for host?guest complexes of 1 and 2. The secondary amide functionality on 1 decreases affinity by the formation of intramolecular NH???O=C H-bonds. Tertiary amide host 2 binds even more weakly than 1 due to backfolding of the amide N-CH₃-groups of 2 into its own cavity.     

NMR Studies of Host–guest Complexes Between Monocarboxylic Acids and Amide-based Cyclophanes in Chloroform

Formation of host–guest complexes with acetic acid and benzoic acid was studied by NMR for amide-based octaazacyclophanes having pendant methyl ester arms; the cyclophanes were tetramethyl 2,9,18,25-tetraoxo-1,4,7,10,17,20,23,26-octaaza[10.10]paracyclophane-4,7,20,23-tetraacetate, its meta-isomer and analogues. Amide NH proton and CH₂ proton adjacent to amide C = O in every cyclophane host showed down-field NMR shifts in the presence of the guest acids in CHCl₃-d, suggesting the formation of 1:1 complexes in which the carboxyl group of an acid molecule formed two hydrogen bonds with the amide NH and C = O moieties of a host molecule. Since the complex formation competed with the dimerization of the guest acids, the monomer–dimer equilibrium was restudied by NMR and the equilibrium constant was determined to be 330 M^? 1 for acetic acid and 518 M^? 1 for benzoic acid. By using these values, the formation constants of the host–guest complexes were determined to be 8–51 M^? 1. The close contact between the host and guest molecules via hydrogen bonding was consistently confirmed by NMR shifts due to the ring current of aromatic group.