Reversible-irreversible approach to Schiff base macrocycles: access to isomeric macrocycles with multiple salphen pockets

We have developed methodology for the formation of a new family of metal-free Schiff base macrocycles utilizing the differential exchange rates of aldimines and ketimines. The more robust ketimine bond is kinetically inert under the milder conditions used for aldimine bond formation. In particular, this route enables access to the first conjugated macrocycles with four unsymmetrical N2O2 salphen-like pockets.     

Schiff base macrocycles containing pyrroles and pyrazoles

A double nucleophilic substitution reaction of 3,5-bis(chloromethyl)pyrazole with pyrroles generates a novel pyrrole-pyrazole hybrid building block, the pyrazole analogue to tripyrrane. Vilsmeier-Haack formylation produces the corresponding dialdehyde, which was used in the formation of a series of nonaromatic Schiff base macrocycles. NMR and UV/Vis spectroscopy and single-crystal diffractometry were used to characterize the novel macrocycles. The solid-state structures of select free bases and protonated members of this class of macrocycles display a range of intra- and intermolecular hydrogen-bonding patterns that suggest their use in molecular-recognition systems. They also contain an acid-sensitive chromophore. Their acid-base and anion-recognition properties were ascertained; alas, only modest anion-selective spectroscopic signatures could be detected by using UV/Vis and (1)H NMR spectroscopy. The macrocycles proved resistant toward oxidation to their aromatic congeners. The pyrrole-pyrazole building blocks presented are potentially useful for the synthesis of a range of pyrazole analogues of all-pyrrole macrocycles.     

Synthesis, crystal structures and photochromic properties of novel chiral Schiff base macrocycles

Enantiomerically pure and racemic forms of calixsalen-type macrocycles 1 and 2 were synthesized and their crystal structures were determined. The enantiomerically pure crystals of (S,S,S,S,S,S)-1 exhibited thermally reversible photochromism from yellow to orange-red upon photoirradiation in the solid state, while rac-crystals of 2 with the guest CH₃CN did not show any photocolouration.     

Tunable organic particles: An efficient approach from solvent-dependent Schiff base macrocycles

The synthesis of cyclopolymers upon controlling the degree of macrocyclic polymerization, followed by the discovery of new properties has attracted increasing attention in supramolecular chemistry. Herein, a Schiff-base condensation method performed at room temperature was used to control the formation of [1 + 1] and [2 + 2] macrocycles. In pure MeOH, the isomer [1 + 1] macrocycles were synthesized and organic particles such as dendritic, rods, and solid microspheres were directly precipitated from the reaction solution. The [1 + 1] macrocycles can be efficiently converted into their corresponding [2 + 2] macrocycles accompanied by the tunable morphology of the organic particles when n-hexane was added to the MeOH solution. Further studies showed that these organic particles have potential application toward the selective removal of Cd²⁺ ions with different adsorption ability in MeOH solution.     

Gels of shape-persistent macrocycles: the role of the interior

The aggregation of shape-persistent macrocycles with an empty cavity, an undecyldiether strand and a tetraethylene glycol strand leads in all cases to a macroscopic gelation of the solvent. However, the gelation temperatures are fine-tuned by the intraannular substituents.     

Novel chiral Schiff base macrocycles containing azobenzene chromophore: gelation and guest inclusion

A novel chiral Schiff base macrocycle 1 was synthesized by [3+3] condensation of enantiomerically pure trans-1,2-diaminocyclohexane with azobenzene-4,4′-dicarbaldehyde. Subsequent reduction of 1 afforded macrocyclic hexamine 2 having three azobenzene units. The former could be converted into a benzene gel, while the latter could include several aromatic guest molecules.     

Social and antisocial [3 + 3] Schiff base macrocycles with isomeric backbones

Two new conjugated macrocycles have been prepared in high yield using Schiff base condensation. These are the first Schiff base macrocycles to incorporate phenanthrene, and they contain 66 and 78 atoms, respectively, in their smallest closed ring. Although the backbones of the two macrocycles have nearly the same constitution, one aggregates in chloroform while the other does not. This is rationalized based on the differential overlap of aromatic components in the dimers.     

Constructing Penrose-like tilings with 7-fold symmetry

Penrose-like tilings with higher-order rotational symmetry are discussed. Two new examples of recursive rhombic tilings with 7-fold symmetry are derived. The entangled inflation/deflation rules cyclically permute the characteristic 7-fold patches and provide a possibility of expanding the tilings up to infinity without additional ad hoc rearrangements. The derived tilings are self-similar and fully coincide with themselves after inflations and deflations. The main structural motif is presented by repeatedly appearing seven-pointed stars surrounded by similar seven-pointed stars. Characteristic patches demonstrate mutual metamorphoses. The patches cyclically morph each into another while the tilings as a whole remain self-consistent. The results of the study can be used for the development of new artificial materials with heptagonal symmetry.

     

Cationic shape-persistent macrocycles: the unexpected formation of a nano-size supramolecular dimer

Shape-persistent macrocycles with a rigid pyridyl core and a flexible oligo-alkyl corona aggregate to some extent in nonpolar solvents to form large (tubular) aggregates. To increase the assembling tendency, the intraannular pyridyl groups of the macrocycles were alkylated. Unexpectedly, the quarternized macrocycles show no tendency at all to form tubular micellar-like structures but form well-defined dimers, as determined by X-ray and dynamic light scattering.     

Chelating behaviour of new 12-membered Schiff base macrocycles containing pendant groups

Summary The free Schiff base macrocycles 1,4,7,10-tetraazacyclo-dodeca-5,6,11,12-tetraaminoacetic acid-4,6,10,12-tetraene (L¹), 1,4,7,10-tetraazacyclododeca-5,6,11,12-tetraamino-phenyl-4,6,10,12-tetraene (L²) and 1,4,7,10-tetraazacyclo-dodeca-5,6,11,12-tetraaminopyridyl-4,6,10,12-tetraene (L³) have been prepared by the reaction of oxamidediacetic acid, oxamidediphenyl and oxamidedipyridyl, respectively, with ethylenediamine. Their complexes with first row transition metal ions, of types [MLCl₂] and [MLCl₂]Cl, have also been prepared and characterized by physico-chemical and spectroscopic methods. The complexes of divalent metal ions are non-ionic while those of trivalent metal ions appear to be 11 electrolytes. An octahedral geometry is proposed for all the complexes.     

Synthesis and anion binding properties of 2,5-diamidothiophene polypyrrole Schiff base macrocycles

[reaction: see text] Two easy-to-synthesize polypyrrolic 2,5-diamidothiophene Schiff base macrocycles are reported, along with their anion binding properties as determined via UV-vis spectroscopic titrations carried out in dichloroethane. There is a striking difference between the interactions with anions of the two macrocycles, a finding ascribed to differences in their rigidity. For example, the more flexible dipyrromethane-derived macrocycle displays a 1.2:1 hydrogen sulfate versus nitrate selectivity, while its more rigid bipyrrole-derived congener shows a 7.4:1 selectivity in favor to hydrogen sulfate.     

Calix[4]pyrrole Schiff base macrocycles: novel binucleating ligands for Cu(I) and Cu(II)

New bimetallic copper(I) and copper(II) complexes of dipyrromethane-derived Schiff base macrocycles are reported. Two different structural motifs were identified, providing support for the notion that ligands of this type can support a variety of coordination modes. In the case of the Cu(I) complexes, the metal centers were found to have a distorted tetrahedral geometry and be coordinated to two imine nitrogens on each side of the ligand, with the exact structure depending on the choice of Schiff base macrocycle. In contrast to what is seen for Cu(I), with Cu(II) as the coordinated cation the Cu(II) metal centers assumed distorted square planar geometries, and both pyrrole N-Cu and imine N-Cu interactions were confirmed by single-crystal X-ray diffraction analysis. This structural analysis revealed a copper-copper distance of 3.47 A, while SQUID magnetic susceptibility data provided evidence for antiferromagnetic coupling between the two metal centers.     

Synthesis, aggregation, and adsorption phenomena of shape-persistent macrocycles with extraannular polyalkyl substituents

The synthesis of shape-persistent macrocycles based on the phenyl-ethynyl backbone containing various extraannular alkyl side chains is described. Although compound solubility increases with increasing size of the side groups, decreasing the solvent polarity induces aggregation of the rings by nonspecific interactions. This was investigated by proton NMR spectroscopy. The magnitude of aggregation can be varied by using solvent mixtures of different hexane content, supporting the model of a solvophobic effect. From 1,2,4-trichlorobenzene solution the macrocycle 1c adsorbs at the surface of highly oriented pyrolitic graphite (HOPG). The two-dimensional order of the structure was investigated by scanning tunneling microscopy (STM) revealing the formation of a two-dimensional lattice of p1(2)mm symmetry with lattice parameters A = 3.6 nm, B = 5.7 nm, and Gamma = 74 degrees.     

Highly substituted Schiff base macrocycles via hexasubstituted benzene: a convenient double Duff formylation of catechol derivatives

The synthesis of soluble, shape-persistent macrocycles is important for developing new materials. Double Duff formylation of 4,5-dialkylcatechol derivatives yields 3,6-diformyl-4,5-dialkylcatechols in moderate yields. These hexasubstituted aromatics are useful precursors to highly substituted Schiff base macrocycles. We illustrate convenient routes to these compounds as well as structural studies of two new macrocycles with alkyl substituents.     

SAMs of shape-persistent macrocycles: structure and binding on HOPG and Au(111)

Self-assembled monolayers of shape-persistent macrocycles have been adsorbed on highly oriented pyrolytic graphite and Au(111) substrates. Their structure and binding characteristics have been investigated by scanning tunneling microscopy, X-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy experiments. Special functionalization of the macrocycles for the first time has enabled their self-assembling and binding on a metal substrate and opened a new route for new functional nanostructures.     

Calix[4]pyrrole Schiff base macrocycles. Novel binucleating ligands for mu-oxo iron complexes

New bimetallic mu-oxo diferric complexes of several previously reported calix[4]pyrrole Schiff base macrocycles are described. The synthesis of a new member of this class of macrocycles is also reported; it was prepared via an acid-catalyzed condensation between 1,9-bisformyl-5,5-dipropyldipyrromethane and o-phenylenediamine. Reactions of the free base macrocycles or their bis-HCl salts with Fe(II) mesitylene, followed by air oxidation, gave the binuclear mu-oxo bis-Fe(III) compounds 6-10 in moderate yield. X-ray crystallography data reveal two different coordination environments for the Fe-O-Fe subunit in 6-10 that it is suggested can be controlled by altering the reaction conditions. Structural properties of these metalated pyrrolic macrocycles are also compared to those of mu-oxo diferric porphyrins and mu-oxo diferric texaphyrin. Complexes 6-10 exhibit two distinct types of M-N bonds that are similar in length to the bonds observed in metallotexaphyrin complexes. However, the electronics of the present systems are very different from those of texaphyrins and porphyrins in that no delocalized bonding patterns are observed within the ligands as a whole.     

A novel 5-mercapto triazole Schiff base as a selective chromogenic chemosensor for Cu2+

A novel colorimetric cation sensor bearing phenol, thiol and HCN groups was designed and synthesized. In a DMSO/H2O (9:1, v/v) solution, the sensor exhibited highly selective recognition of Cu2+ among a range of metal ions tested. In the presence of Cu2+, solutions of the sensor underwent a dramatic color change from colorless to yellow, while the presence of other metal cations such as Zn2+, Pb2+, Cd2+, Ni2+, Co2+, Fe3+, Hg2+, Ag+ and Ca2+ had no effect on the color. The detection limit of the sensor toward Cu2+ is 8.0×10(-7) M and an association constant Ka of 4.3×10(5) M(-1) was measured. The sensing of Cu2+ by this sensor was found to be reversible, with the Cu2+-induced color being lost upon addition of EDTA.     

Spectrophotometric and spectrofluorimetric studies of novel heterocyclic Schiff base dyes

Novel heterocyclic Schiff base dyes were prepared by the reaction of salicylaldehyde/2-Hydroxy-1-naphthaldehyde with different heterocyclic amines. Results of the newly synthesized compounds established by FT-IR, ¹H-NMR, ¹³C-NMR and GC–MS spectroscopic experiments were consistent with their chemical structures. Resulted heterocyclic Schiff base dyes were found to be pure from data obtained by the elemental analysis. In addition, spectrophotometric and spectrofluorimetric studies showed that these dyes were good absorbent and fluorescent. Fluorescence polarity study data revealed that some of these compounds were sensitive to the polarity of the microenvironment provided by different solvents. Time-based fluorescence steady-state measurements also showed that these heterocyclic Schiff base dyes have high photostability against photobleaching.