First Synthesis and Structure of β-Ketoimine Calix[4]arenes: Complexation and Extraction Studies

Supramolecular Handshakes: Characterization of Urea-Carboxylate Interactions Within Calixarene Frameworks

The research of molecular capsules offers high application potential and numerous benefits in various fields. With the aim of forming supramolecular capsules which can be reversibly assembled and dissociated by simple external stimuli, we studied interactions between calixarenes containing urea and carboxylate moieties. To this end two ureido-derivatives of p-tert-butylcalix[4]arene comprising phenylureido-moieties and diacetate-calix[4]arenes were prepared. The binding of acetate by ureido-derivatives of calixarene in acetonitrile was characterized, revealing high affinity of ureido-calixarenes for carboxylates. This suggested high potential for uniting the complementary calix[4]arenes via H-bonds between carboxylic groups and urea moieties. The assembly of calixarenes was examined in detail by means of UV, 1H NMR, ITC, DOSY, MS, and conductometry providing insight in the structure-stability relationship. The tetraureido-calixarene derivative formed the most stable heterodimers with diacetate-calix[4]arenes featuring practically quantitative association upon mixing the two calixarene counterparts. The possibility of controlling the formation of the heterodimer by protonating the carboxylates, thereby hindering the interactions critical for capsule assembly, was investigated. Indeed, the reversibility of breaking and re-forming the heterodimer by addition of an acid and base to the solution containing urea- and carboxylate-derivative calix[4]arene was demonstrated using NMR spectroscopy.

Selective Chromogenic Recognition of Copper(II) Ion by Thiacalix[4]arene Tetrasulfonate and Mechanism

Detection of biologically important transition metal ions such as copper by using a simple method is desirable and of great importance. In this work, we firstly reported that water-soluble thiacalix[4]arene tetrasulfonate (TCAS) exhibited selective chromogenic recognition towards copper(II) ion over other transition metal ions. Color change from colorless to salmon pink was observed in TCAS solution, weak bathochromic shift was induced in UV absorption spectrum of TCAS upon addition of copper(II) ion, and the absorbance of characteristic absorption band at 312 nm increased linearly with copper(II) ion concentration. The recognition mechanism of TCAS to copper(II) ion was investigated by a comparative study with calix[4]arene tetrasulfonate (CAS) and time-dependent density functional theory(TD-DFT) study, and the absorption bands were assigned based on transition orbital analysis.

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.

Photoluminescence properties of tetrahedral zinc(ii) complexes supported by calix[4]arene-based salicylaldiminato ligands

The synthesis and photophysical properties of four new hybrid salicylaldiminato-calix[4]arene ligands and their corresponding zinc(ii) complexes are described. The Schiff bases were obtained from condensation reactions between cone-25,27-di(aminoethoxy)-26,28-dihydroxy-calix[4]arene and salicylaldehyde (H2L1) or o-vanillin (H2L2) and 1,3-alt-25,27-di(aminoethoxy)-26,28-di(n-propyloxy)-calix[4]arene and 3,5-di-tert-butyl-salicylaldehyde (H2L3) or o-vanillin (H2L4). Complexation reactions were investigated by ESI-MS, IR, NMR, UV-vis absorption and steady-state and time resolved fluorescence spectroscopy, and X-ray crystallography. All ligands support 1 : 1 complexes (ZnL1-ZnL4), with equilibrium constants derived from absorption spectrophotometry in the range log K11 = 5.5-8.2 (MeCN or MeOH/CH2Cl2, I = 0.01 M). The zinc complexes show blue fluorescence, both in solution as well as in the solid state, with λem, Φf, and τ ranging from 472-504 nm, 0.11-0.60, and 2-9 ns, respectively. The nature of the substituents on the salicylaldiminato fragments was found to be the main parameter that influences the photophysical properties of the zinc complexes. Insights into the electronic nature of the UV-vis transitions were obtained with time dependent density functional theory (TD-DFT) calculations.

Synthesis, structure and conformational mobility of tetra-substituted cyanomethoxy p-tert-butylcalix[4]arenes

A range of p-tert-butylcalix[4]arenes are functionalized at the lower rim with nitrile and amide functional groups, and the conformational mobility of macrocycles is investigated.

Easy and selective method for the synthesis of various mono-O-functionalized calix[4]arenes: de-O-functionalization using TiCl4

An efficient and selective method for the monofunctionalization of p-tert-butylcalix[4]arene is described. A mono-de-O-functionalization of disubstituted p-tert-butylcalix[4]arenes using titanium tetrachloride was developed to synthesize a series of monosubstituted p-tert-butylcalix[4]arenes with the pendant functions being ethoxycarbonylmethyloxy, 3-ethoxycarbonylpropyloxy, cyanomethyloxy, 3-cyanopropyloxy, 4-bromobutyloxy, 3-hydroxypropyloxy, propyloxy, 2-methylpropyloxy, 3-butynyloxy, and 3-cyanopropyloxy groups. The reaction mechanism of the formation of 5,11,17,23-tetra-tert-butyl-26,27,28-trihydroxy-25-(3-ethoxycarbonylpropyloxy) calix[4]arene was studied by (1)H NMR and GC/mass spectroscopy monitoring. Reaction of TiCl4 with the disubstituted p-tert-butylcalix[4]arene produced the corresponding dioxocalix[4]arene titanium dichloride complex, which undergoes elimination of ethyl 4-chlorobutyrate, leading to a trioxocalix[4]arene titanium dichloride complex and to monosubstituted calix[4]arene after hydrolysis. These two complexes were also synthesized, isolated, and fully characterized.

Calix[4]azacrown and 4-aminophthalimide-appended calix[4]azacrown: synthesis, structure, complexation and fluorescence signaling behaviour

The synthesis and single crystal structure of 25,27-(diamidomonoazacrown-5)-calix[4]arene (L), calix[4]arene capped by a diamide bridge in the 1,3-position on the lower rim, which forms a supramolecular dimer in the solid state via intermolecular hydrogen bonding, are described. A 4-aminophthalimide (AP) fluorophore has been regioselectively linked to the secondary amino function of the azacrown unit with a dimethylene spacer to construct N-(4-aminophthalimidoethyl)calix[4]azacrown (APL), a fluorescent sensor, via a fluorophore-spacer-receptor architecture. Fluorescence quantum yield and lifetime of APL have been measured to be lower than those of the bare fluorophore (AP) due to the photoinduced intramolecular electron transfer (PIET) between the fluorophore and the receptor moieties of the molecule. In the presence of transition metal ions, fluorescence enhancement of is observed suggesting the binding of the metal ion to the sensor. Complexation properties of APL with transition metal ions are investigated using UV-vis spectroscopy. A 1 : 1 stoichiometry of the complex is determined from a Job plot and the corresponding association constants for the various metal ions are evaluated. Fe(3+) and Cu(2+) ions have the largest association constants (K= 2.3 x 10(5) M(-1) and 1.6 x 10(5) M(-1) respectively) compared to other metal ions indicating that they form complexes selectively with APL.