Shaping the cavity of calixarene architecture for molecular recognition: synthesis and conformational properties of new azocalix[4]arenes

Synthesis of Upper-Rim Sulfanylpropyl- and p-Methoxyphenylazo-Substituted Calix[4]arenes as Chromogenic Sensors for Hg2+ and Ag+ Ions

A series of calix[4]arenes with upper-rim sulfanylpropyl and p-methoxyphenylazo groups (compounds 8-10) were synthesized and found to be effective chromogenic sensors for selectively detecting Hg2+, Hg+, and Ag+ ions among 18 screened metal perchlorates. In comparison to previously reported diallyl- and dithioacetoxypropyl-substituted calix[4]arenes (5, 6, 14, 15, and 16) and the newly synthesized compound 7, the distal (5,17)-disulfanylpropyl-substituted di-p-methoxyphenylazocalix[4]arene 9 demonstrated superior performance with a limit of detection of 0.028 μM for Hg2+ ions in a chloroform/methanol (v/v = 399/1) cosolvent. Job's plot revealed 1:1 binding stoichiometry for all these upper-rim sulfanylpropyl- and p-methoxyphenylazo-substituted calix[4]arenes 8-10 with Hg2+ ions, and Benesi-Hildebrand plots from ultraviolet/visible (UV-vis) titration spectra were used for the determination of their association constants. Our findings indicated that the distal orientation of two p-methoxyphenylazo and two sulfanylpropyl groups in calix[4]arenes 8-10 is more favorable for binding Hg2+ ions than the proximal (5,11-) orientation; moreover, the adjacent sulfanylpropyl groups exhibited superior coordination as ligands compared to the allyl and thioacetoxypropyl groups. Notably, compounds 8-10 displayed a comparable trend in their association with Ag+ ions, albeit with 1 order of magnitude lower binding constants and a distinct binding mode compared to Hg2+ ions. UV-vis spectroscopy, Job's plots, high-resolution mass spectrometry, and 1H nuclear magnetic resonance titration studies are presented and discussed.

A self-assembled nanotube supported by halogen bonding interactions

Upper-rim halogenation of a calix[4]arene modulates the packing of self-assembled nanotubes through the formation of complementary halogen bonding interactions.

Azo group(s) in selected macrocyclic compounds

Azobenzene derivatives due to their photo- and electroactive properties are an important group of compounds finding applications in diverse fields. Due to the possibility of controlling the trans-cis isomerization, azo-bearing structures are ideal building blocks for development of e.g. nanomaterials, smart polymers, molecular containers, photoswitches, and sensors. Important role play also macrocyclic compounds well known for their interesting binding properties. In this article selected macrocyclic compounds bearing azo group(s) are comprehensively described. Here, the relationship between compounds' structure and their properties (as e.g. ability to guest complexation, supramolecular structure formation, switching and motion) is reviewed.

Synthesis and Characterisation of Asymmetrically Bridged Calix[4]Arene and Tetrathiacalix[4]Arene Mono Amido Crown Derivatives

The synthesis of asymmetrically bridged calix[4]arene and tetrathiacalix[4]arene amido crown derivatives has been achieved by the aminolysis of distal diester derivatives of calix[4]arene and tetrathiacalix[4]arene with 1,2-diamino-propane or 1,2-diaminocyclohexane (stereoisomeric mixture, mainly trans). The title compounds have been characterised by detailed analysis of their NMR spectra. The assignment of NMR signals and stereo differentiation in the amido crown ring formation has been studied using a chiral shift reagent.

Synthesis and structure elucidation of 25,26,27,28-tetramethylcalix[4]arene tetraketone using 1D and 2D NMR spectroscopies

The (1)H and (13)C NMR spectra of the new calix[4]arene-based tetraketone were completely assigned by one- and two-dimensional homo- and heteronuclear experiments ((1)H-(1)H COSY, (1)H-(13)C HMQC, and HMBC) at 400 and 100MHz, respectively, at 25 degrees C standard pulse sequence. (1)H and (13)C spectra were measured at room temperature for 25,26,27,28-tetramethylcalix[4]arene tetraketone (1). (13)C[(1)H], DEPT and NMR techniques were used to distinguish the methyl, methylene and methine carbon resonance signals of calix[4]arene 1. Correlation of 1D ((1)H, (13)C[(1)H], DEPT) and 2D (HMQC and HMBC) NMR data was used to completely assign various overlapping and broad signals of calix[4]arene 1. Heteronuclear multibond correlation (HMBC) studies were used to completely assign various carbon resonances. Our results show that the conformation of calix[4]arene 1 in the solution is very similar to the cone conformation reported in the literature.

Synthesis and binding characteristics of novel calix[4]arene(amidocrown) diquinones

A series of new calix[4]arene(amidocrown) diquinones (3a–3d) have been synthesized, characterized, and evaluated for cation recognition. It has been observed that 3b interacts with alkali metal ions (Li+, Na+, and K+) and ammonium ions to induce an unprecedented downfield shift in the NH proton resonance, which can be attributed to polarization of the amidocrown ring of the calix[4]arene diquinone receptor. The observation has been confirmed by a significant anodic shift (Li+ > NH4+ > Na+> K+) of the corresponding amidocrown-diquinone redox couple in cyclic and square wave voltammetric experiments. Both NMR and electrochemical studies of the binding characteristics of 3b with alkali metal cations and ammonium ions revealed a 1:1 binding stoichiometry for all and exhibited the highest association constant for lithium ions. This indicated that the receptor 3b selectively binds the lithium ion in preference to other alkali metal cations and ammonium ions.