Hydrogen bonding-driven elastic bis(zinc)porphyrin receptors for neutral and cationic electron-deficient guests with a sandwich-styled complexing pattern

This Letter reports the design and synthesis of a new type of hydrogen bonding-mediated foldamer-derived tweezer receptors that are incorporated with two peripheral (zinc)porphyrin units. Due to the existence of four intramolecular hydrogen bonds, the (zinc)porphyrin units are forced to approach and stack with each other. ¹H NMR and fluorescent studies revealed that the new receptors could form 1:1 complexes with planar electron-deficient molecules such as naphthalene and benzene diimides and paraquat through a unique sandwich-styled binding pattern. The association constants of the new complexes have been evaluated by the ¹H NMR or fluorescent titration methods.     

Strapped porphyrin rosettes based on the melamine-cyanuric acid motif. Self-assembly and supramolecular recognition

This paper describes studies on the synthesis, self-assembly behavior, and complexing properties of several strapped porphyrin-incorporated melamine-cyanuric or melamine-barbiturate-based rosette supramolecules in chloroform-d. Strapped porpyrin cyanuric acid H₂1 and its Zn (II) complex Zn1 were designed and synthesized. Both H₂1 and Zn1 could combine melamine derivatives 11 or 12 to afford porphyrin rosettes, which are more stable than the model rosette initially reported by Whitesides due to the larger size of the porphyrin unit. The new porphyrin rosettes could efficiently complex tripyridyl derivative 13 through intermolecular, cooperative coordination between Zn (II) and pyridine. Two new pyridine-bearing barbiturates 18a and 18b were also synthesized. Mixing the identical amount of 18a or 18b with 11 or 12 in chloroform-d led to the formation of new isomeric rosettes as a result of different orientation of the pyridine unit of 18a or 18b in the rosettes. ¹H NMR study also revealed that porpyrin-bearing rosette Zn1₃·11₃ could complex pyridine-bearing rosette 11₃·18a₃, leading to the formation of new two-layer-typed supramolecular architectures.     

Selective recognition of sodium cyanide and potassium cyanide by diaza-crown ether-capped Zn-porphyrin receptors in polar solvents

Two new ditopic porphyrin receptors Zn1, incorporating a diaza-15-crown-5 unit, and Zn2, incorporating a diaza-18-crown-6 unit, have been prepared and characterized. UV-vis study in polar methanol has revealed that Zn1 is able to selectively recognize sodium cyanide over potassium cyanide (the ratio of their binding constant is ca. 56), whereas Zn2 exhibits a higher binding affinity for potassium cyanide over sodium cyanide (the ratio of their binding constant is ca. 12). In contrast, both receptors display substantially weaker binding affinity for sodium thiocyanate and potassium thiocyanate presumably due to a monotopic binding fashion.     

‘Two-point’-bound supramolecular complexes from semi-rigidified dipyridine receptors and zinc porphyrins

Two linear compounds 1 and 2 have been designed and synthesized as new receptors for zinc porphyrins. Both compounds consist of two folded aromatic amide moieties, which are connected with an acetylene linker in 1 or directly in 2. The rigid conformations of their folded moieties are stabilized by intramolecular tri-centered hydrogen bonding, while the whole molecule adopts a ‘S’- or ‘C’-styled conformation depending on the relative orientation of the two rigid moieties. Two pyridine units are introduced at the ends of 1 and 2 for the complexation of zinc porphyrin guests. Although the ¹H NMR investigation indicated that both compounds can bind two zinc porphyrin guests at high concentrations (≥5 mM) in chloroform, the UV-vis studies revealed that, at low concentration of 1 and 2 (4 μM), both compounds complex one zinc porphyrin guest to form structurally unique ‘two-point’-bound 1:1 complexes. The association constants of the 1:1 complexes have been determined with the UV-vis titration experiments.     

Pyridine-based receptors with high affinity for carbohydrates. Influence of the degree of steric hindrance at pyridine nitrogen on the binding mode

Remarkable changes in the binding affinity and selectivity of pyridine-based receptors toward monosaccharides have been observed when the degree of steric hindrance at pyridine nitrogen atom decreases.     

Hydrogen-bonding-induced oligoanthranilamide foldamers. Synthesis, characterization, and complexation for aliphatic ammonium ions

The self-assembly of a novel series of intramolecular hydrogen bonding-driven foldamers have been described. Five linear aromatic amide oligomers 1-5, which bear two to six repeating benzoyl amide subunits, respectively, have been prepared by continuous amide-coupling reactions. The existence of three-centered hydrogen bonds in the oligomers and consequently, the folding conformation of the oligomers in the solid state and solution have been proved by the X-ray analysis (for 2) and the ¹H NMR and IR experiments. Molecular modeling reveals a planar and rigid conformation for the oligomers and a cavity of 0.86 nm in diameter for 6-mer 5. Fluorescent and ¹H NMR experiments have demonstrated that the new aromatic oligo-amide foldamers can bind primary and secondary alkyl ammonium ions in chloroform and the associated binding constants have been determined. It is revealed that 5-mer 4 exhibits the largest binding ability. A face-to-face binding mode has been proposed for the complexes.     

Synthesis and self-assembling behavior of a porphyrin bearing multiple meso-conjugated barbiturates

An efficient procedure through deprotection of 2,4,6-trimethoxypyrimidine derivative afforded porphyrinato zinc bearing multi-barbiturates acting as multiple hydrogen bonding sites at the meso positions. Addition of excess amphiphilic triaminopyrimidine derivative, as a complementary motif to the barbiturate, in an aqueous solution of porphyrin conjugated with multiple barbiturates at the meso positions resulted in precipitation. The cast film from the chloroform solution of the precipitate indicated the formation of a well-defined porphyrin assembly.     

Template-assisted control of porphyrin aggregation by ladder-type supramolecular assemblies

A new strategy to control the orientation and aggregation numbers of porphyrins by alignment along well-defined templates has been demonstrated. Porphyrin-bridged bisPYBOX ligands were arranged along the oligomeric secondary dialkylammonium cations as templates to form well-defined supramolecular complexes. The templates controlled the aggregations of the porphyrins.     

Hydrogen bonding-mediated foldamer-bridged zinc porphyrin-C60 dyads: ideal face-to-face orientation and tunable donor-acceptor interacion

Four porphyrin-bridge-C₆₀ dyads have been synthesized by covalently linking the chromophores at the opposite ends of a hydrogen bonded arylamide-derived foldamer bridge. For comparison, four C₆₀-free porphyrin derivatives of the same frameworks have also been prepared. The fully hydrogen bonded bridges enable the appended porphyrin and C₆₀ moieties to contact in a face-to-face manner. ¹H NMR, UV-vis and fluorescent investigations in chloroform indicate that such a structural matching remarkably facilitates the intramolecular energy and electron transfer and charge separation between the two chromophores and also retards the recombination of the charge-separated state. Removing one hydrogen bond considerably reduces the energy and electron transfer. Further removing another one leads to no important interaction between the chromophores to occur.     

Recognition of a dicarboxylic acid with dipicolyl urea in solution and in solid phases: intramolecular hydrogen bond inhibiting both pyridine nitrogens from binding carboxyl groups

Recognition of a dicarboxylic acid in solution as well as in the solid phase by the pyridyl urea based pseudoditopic receptor 1 has been studied. The X-ray structures of both the receptor and its complex with 1,4-phenylenediacetic acid are also presented. Intramolecular hydrogen bonding inhibits both the pyridine ring nitrogens from forming hydrogen bonds with the carboxyl group and force the receptor to behave in a monotopic manner, using the syn urea amide moiety to bind carboxyl group of a dicarboxylic acid to form a 2:1 complex. Binding of receptor 1 with a monocarboxylic acid is also compared.     

Complexes between hydrogen bonded bisporphyrin tweezers and cholesterol-appended fullerenes as organogelators and liquid crystals

This paper reports the gelation and liquid crystal properties of a class of new complexes between zinc and copper bisporphyrin and C₆₀ derivatives. The bisporphyrins are induced by intramolecular hydrogen bonding to adopt a preorganized ‘U’-shaped conformation and therefore efficiently complex the C₆₀ derivatives. As a result, the capacity of their mixtures to gelate alkanes is increased notably. The bisporphyrins themselves and their complexes with the C₆₀ derivatives form the smectic liquid crystal phase. However, the glassy transition temperature of the complexes decreases considerably.     

Anion binding and transport by steroid-based receptors

The steroid nucleus is well-established as a scaffold for anion receptors. The bile acids are especially useful, providing inexpensive starting points with helpful substitution patterns. This article describes developments since an earlier review in 2003. Included are podand and cyclic structures, uncharged and positive receptors, and various arrays of H-bond donor and other binding functionality. Applications have been found in anion sensing, selective extractions, transport across bilayer membranes, and the discovery of antibiotics.     

2,6-Bis(porphyrin)-substituted pyrazine: a new class of supramolecular synthon binding to a transition-metal ion and fullerene (C60)

2,6-Disubstituted-3,5-dimethylpyrazines have been synthesized via biased acetal synthesis from symmetric 2,3,5,6-tetrakis(chloromethyl)pyrazine. The pyrazine ligands coordinated to trans-dichloropalladium(II) at the nitrogen whose neighboring carbons were connected to less hindered methyl groups. 2,6-Bis(porphyrin)-substituted pyrazine bound C₆₀ to yield 1:1 inclusion complex. The binding of C₆₀ with the pyrazine ligand and its zinc complex was determined by ESI-MS, NMR, and fluorescence spectroscopic analyses.     

Synthesis of covalently bonded nanostructure from two porphyrin molecular wires leading to a molecular tube

Molecular wire, diacetylene-linked porphyrin dimer 6 having terminal alkenes, was synthesized. Porphyrin dimer 6 formed the 1:1 double-stranded ladder complex with 1,4-diazabicyclo[2.2.2]octane (DABCO). The co-planar stacked two porphyrin molecular wires in the ladder complex were connected by olefin metathesis in the presence of the Grubbs catalyst in order to make a covalently bonded tubular nanostructure. The obtained molecular tube 7 was characterized by ¹H, ¹³C NMR spectroscopy, and MALDI-TOF MS spectrometry.     

Hydrogen bonded aromatic hydrazide foldamers for the self-assembly of vesicles and gels

This paper describes an investigation of the structural and side-chain factors for the formation of vesicles and gels by hydrogen bonding-mediated aromatic hydrazide foldamers. Six foldamers and one straight analog that bear discrete side chains have been synthesized. SEM and AFM studies reveal that the molecules with the appended 2-(2-(dialkyl-amino)-2-oxoethylamino)-2-oxoethoxyl chains form vesicles, hydrogels or organogels, depending on the solvents. Both the inner amide units and the terminal N,N-dialkylamide units in the chains are revealed to play essential roles in controlling the self-assembly. The former facilitates it by forming the intermolecular hydrogen bonding, while the latter modulates it by providing solubility and balancing the hydrophobicity of the whole molecules in solvents of varying polarity.     

Highly selective imine-linked fluorescent chemosensor for adenine employing multiple hydrogen bonding

We investigated an imine-linked fluorescent receptor bearing both the hydrogen bond donor and the hydrogen bond acceptor motifs as recognition sites in the design of the receptor. The recognition behavior of the receptor toward various nucleobases was evaluated in CH₃CN/H₂O (95:5, v/v) solution. The receptor showed significant changes in fluorescent intensity with adenine, but it showed no such changes on the addition of other nucleobases.     

Complexation of two macrocycles for amide,saccharide, and halide derivatives: the capacity of 1,2,3-triazole as hydrogen and halogen bonding acceptors

Two 1,2,3-triazole- and amide-incorporated macrocycles have been prepared by 1,3-dipolar cycloaddition of the corresponding dialkyne and diazide precursors. Intramolecular C–H?O hydrogen bonding is introduced to lock the C⁵–H atoms of the 1,2,3-triazole rings. The binding of the two macrocycles to amide, monosaccharide, and halide derivatives in chloroform or dichloromethane has been investigated. It is revealed that the amide units dominate their binding toward the amide and monosaccharide guests through forming intermolecular hydrogen bonding and 1,2,3-triazole is as weak as an intermolecular hydrogen bonding acceptor, but it forms intermolecular halogen bonding when cooperative effect exists.     

Hydrogen bonding-mediated self-assembly of rigid and planar metallocyclophanes and their recognition for mono- and disaccharides

A hydrogen bonding approach has been developed to facilitate the self-assembly of a new series of rigid and planar metallocyclophanes. Two new anthranilamide derivatives 1 and 2, which are incorporated with two acetylene units, respectively, have been synthesized and characterized. X-ray analysis (for 1), 1D and 2D ¹H NMR and IR experiments reveal that, due to the formation of intramolecular three-centered hydrogen bonding, both compounds adopt rigid and planar conformations with the two acetylene units located at the same side of the anthranilamide skeleton. Two new metallocyclophanes 17 and 18 have been constructed in moderate yields from the reaction of 1 and 2 with trans-Pt(PEt₃)₂Cl₂, respectively, in dichloromethane in the presence of diethylamine and cupric chloride. Fluorescent and ¹H NMR investigations reveal that both 17 and 18 can efficiently complex mono- and disaccharide derivatives in chloroform, with a binding selectivity for disaccharides, which is driven by intermolecular hydrogen bonding.     

Hydrogen bonding in molecular recognition by HIV-1 protease

Molecular recognition of the cleavage sites of the substrates by HIV-1 protease is analyzed in terms of hydrogen bonding. Crystal structures of an inactive enzyme complexed with six different substrates were used as reference structures. Applying molecular mechanics calculations it can be shown that the interaction energies between the real substrate and the enzyme are larger than with other peptides. From the analysis, it can be concluded that water molecules are essential in the recognition process. Moreover, the hydrogen bonds between the protease and various substrates are characterized in detail.     

4-Amino-1,8-naphthalimide-based anion receptors: employing the naphthalimide N-H moiety in the cooperative binding of dihydrogenphosphate

The 4-amino-1,8-naphthalimide-based anion receptor 3 binds dihydrogenphosphate with 1:1 stoichiometry through cooperative hydrogen bonding to a naphthalimide N-H and thiourea N-H groups. This was clearly established from ¹H NMR titration experiments in DMSO-d₆ where a substantial shift in the resonance for the naphthalimide N-H was observed concomitant with the expected thiourea N-H chemical shift migration upon successive additions of H₂PO₄⁻. However, whilst ¹H NMR titration experiments indicate that 3 was capable of binding other anions such as acetate, the naphthalimide N-H does not participate and the N-H resonance was essentially invariant during the titration. The lack of cooperative binding in this instance was justifiable on steric grounds.