Competition between Hydrogen Bonding and Proton Transfer during Specific Anion Recognition by Dihomooxacalix[4]arene Bidentate Ureas

Competition between hydrogen bonding and proton transfer reactions was studied for systems composed of electrogenerated dianionic species from dinitrobenzene isomers and substituted dihomooxacalix[4]­arene bidentate urea derivatives. To analyze this competition, a second-order E_rC_rC_i mechanism was considered where the binding process is succeeded by proton transfer and the voltammetric responses depend on two dimensionless parameters: the first related to hydrogen bonding reactions, and the second one to proton transfer processes. Experimental results indicated that, upon an increase in the concentration of phenyl-substituted dihomooxacalix[4]­arene bidentate urea, voltammetric responses evolve from diffusion-controlled waves (where the binding process is at chemical equilibrium) into irreversible kinetic responses associated with proton transfer. In particular, the 1,3-dinitrobenzene isomer showed a higher proton transfer rate constant (∼25 M^–1 s^–1) compared to that of the 1,2-dinitrobenzene (∼5 M^–1 s^–1), whereas the 1,4-dinitrobenzene did not show any proton transfer effect in the experimental conditions employed

Site-Specific Description of the Enhanced Recognition Between Electrogenerated Nitrobenzene Anions and Dihomooxacalix[4]arene Bidentate Ureas

Electron transfer controlled hydrogen bonding was studied for a series of nitrobenzene derivative radical anions, working as large guest anions, and substituted ureas, including dihomooxacalix[4]­arene bidentate urea derivatives, in order to estimate binding constants (<i>K</i>_b) for the hydrogen-bonding process. Results showed enhanced <i>K</i>_<i>b</i> values for the interaction with phenyl-substituted bidentate urea, which is significantly larger than for the remaining compounds, e.g., in the case of 4-methoxynitrobenzene a 28-fold larger <i>K</i>_b value was obtained for the urea bearing a phenyl (<i>K</i>_b ∼ 6888) vs <i>tert</i>-butyl (<i>K</i>_b ∼ 247) moieties. The respective nucleophilic and electrophilic characters of the participant anion radical and urea hosts were parametrized with global and local electrodonating (ω^–) and electroaccepting (ω⁺) powers, derived from DFT calculations. ω^– data were useful for describing trends in structure–activity relationships when comparing nitrobenzene radical anions. However, ω⁺ for the host urea structures lead to unreliable explanations of the experimental data. For the latter case, local descriptors ω_<i>k</i>⁺(<i><b>r</b></i>) were estimated for the atoms within the urea region in the hosts [∑_<i>k</i>ω_<i>k</i>⁺(<i><b>r</b></i>)]. By compiling all the theoretical and experimental data, a <i>K</i>_b-predictive contour plot was built considering ω^– for the studied anion radicals and ∑_<i>k</i>ω_<i>k</i>⁺(<i><b>r</b></i>) which affords good estimations

Selective Binding of Spherical and Linear Anions by Tetraphenyl(thio)urea-Based Dihomooxacalix[4]arene Receptors

Three novel tetra­(thio)­ureido dihomo­oxa­calix­[4]­arene anion receptors (phenylurea <b>4a</b>, phenylthiourea <b>4b</b>, and <i>tert</i>-butylurea <b>4c</b>) were synthesized and obtained in the cone conformation in solution, as shown by NMR studies. The X-ray crystal structure of <b>4c</b> is reported. The host–guest properties of these receptors toward several anions were investigated by ¹H NMR titrations. Phenylurea <b>4a</b> displayed a very efficient binding toward the spherical F^– and Cl^– anions, and the linear CN^– (log <i>K</i>_ass = 3.46, 3.50, and 4.02, respectively). In comparison to related bidentate phenylurea dihomooxacalix[4]­arenes, tetraphenylurea <b>4a</b> is more preorganized and the higher number of hydrogen bond donor sites provides a remarkable enhancement of its binding efficiency

Alkylammonium Cation Complexation into the Narrow Cavity of Dihomooxacalix[4]arene Macrocycle

How big should a calixarene macrocycle be for <i>endo</i>-cavity complexation to occur or to allow <i>through-the-annulus</i> threading? To answer these questions, a complete study on the complexation of primary and secondary (di)­alkylammonium cations by 18-membered <i>p</i>-<i>tert</i>-butyldihomooxacalix­[4]­arene macroring has been performed in the presence of the “superweak” TFPB counterion. Thus, it was found that this macrocycle is currently the smallest calixarene able to host linear and branched alkylammonium guests inside its aromatic cavity. Molecular mechanics calculations revealed that this recognition event is mainly driven by a H-bonding interaction between the guest ammonium group and the host CH₂OCH₂ bridge. The <i>endo</i>-cavity complexation of chiral <i>s</i>-BuNH₃⁺ guest results in an asymmetric complex in the NMR time scale. The chirality transfer from guest to host is likely due to a restricted guest motion inside the tight cavity. The complexation study with secondary di-<i>n</i>-alkylammonium·TFPB salts revealed that the 18-membered dihomooxacalix[4]­arene macroring cannot give the <i>through-the-annulus</i> threading with them because of its small dimension. However, the macrocycle is able to complex such ions, which can only be accommodated in an hook-like conformation characterized by two unfavorable gauche interactions around the CH₂–NH₂⁺ bonds. The strain generated by this unfavorable folding is very likely compensated by stronger H-bonds and more favorable CH/π interactions between guest and host

Experimental and computational studies of the binding properties of lower rim tetra- and di-substituted calix[4]arene amide derivatives with metal ions

<p>Experimental and theoretical binding studies of representative alkali, alkaline earth, transition, heavy metal and lanthanide cations by tetra- and di-substituted calix[4]arene amide derivatives (diethyl amide <b>1a</b>–<b>c</b> and morpholide amide <b>2a</b>–<b>c</b>) in the cone conformation were carried out. Binding was assessed by extraction experiments of the metal picrates from water to dichloromethane and proton NMR titrations. Density functional theory calculations were also performed to determine the binding energy of the complexes formed and to analyse the host–guest interaction modes. In the cases of ligands <b>1b</b> and <b>2c</b> with Na⁺ and Ag⁺ picrates, the extraction energy was also determined using the polarisable continuum model. The results are discussed in terms of the nature of the amide residue and the substitution pattern (1,3 vs. 1,2). Both tetra-amide derivatives are good extractants, showing preference for Na⁺, Ca²⁺, Ag⁺ and Pb²⁺ cations, mainly di-ethylamide <b>1a</b>. Concerning di-amide derivatives, the relative position of the substituents seems to be more important than the nature of the amide group in the extraction process. Proton NMR studies indicate the formation of 1:1 complexes between the amides and the cations studied, and DFT data show that all ligands form the most stable complexes with La³⁺.</p> <p>The binding properties of <b>1a-c</b> and <b>2a-c</b> towards several metal cations were determined by UV spectrophotometry, proton NMR spectrometry and DFT methods.</p

Bidentate Urea Derivatives of <i>p‑tert</i>-Butyldihomooxacalix[4]arene: Neutral Receptors for Anion Complexation

Three new bidentate ureidodihomooxacalix[4]­arene derivatives (phenyl <b>5a</b>, <i>n</i>-propyl <b>5b</b>, and <i>tert</i>-butyl <b>5c</b>) were synthesized in four steps from the parent compound <i>p-tert</i>-butyldihomooxacalix­[4]­arene and obtained in the cone conformation, as shown by NMR studies. The binding ability of these neutral receptors toward spherical, linear, trigonal planar, and tetrahedrical anions was assessed by ¹H NMR and UV–vis titrations. The structures and complexation energies of some complexes were also studied by DFT methods. The data showed that the association constants are strongly dependent on the nature of the substituent (aryl/alkyl) at the urea moiety. In general, for all the receptors, the association constants decrease with decrease of anion basicity. Ph-urea <b>5a</b> is the best anion receptor, showing the strongest complexation for F^– (log <i>K</i>_assoc = 3.10 in CDCl₃) and also high binding affinity for the carboxylates AcO^– and BzO^–. Similar results were obtained by UV–vis studies and were also corroborated by DFT calculations