Rim, Side Arms, and Cavity: Three Sites for the Recognition of Anions by Tetraazolium Resorcinarene Cavitands

Abstract

Two tetrabenzoimidazolium-resorcinarene cavitands were prepared and used for the recognition of chloride, bromide, iodide, cyanide, nitrate, perchlorate, hexanoate, benzenesulfonate, and p-toluenesulfonate. Binding affinities of the two cavitands were determined by H NMR titration and computational analysis. The observed spectral changes were related to specific interaction sites, which were supported by the computational studies. In the case of the C2-H tetrabenzoimidazolium-resorcinarene, the recognition region of the inorganic anions and hexanoate was located at the rim of the cavitand, although chloride and bromide also interacted with the aromatic C-H bonds located between adjacent arms of the cavitand. By contrast, the recognition of the two anions with an aromatic ring (benzenesulfonate and p-toluenesulfonate) results from encapsulation of the aromatic part of the anions inside the hydrophobic cavity of the host. In the case of the C2-Me tetrabenzoimizazolium-resorcinarene receptor, the ability of the molecule to bind all inorganic anions and hexanoate was suppressed, but the receptor maintained its ability to strongly bind benzenesulfonate and p-toluenesulfonate. This is interpreted in terms of suppression of the ability of the cavitand to form hydrogen bonds at the rim of the molecule due to replacement of the C2-H proton by a methyl group, while the hydrophobic pocket of the molecule maintains its binding abilities.

‘Pincer-tweezer’ tetraimidazolium salts as hosts for halides

“…In addition, the use of the two hosts with the two different relatively rigid spacers should provide interesting information about the influence of the separation of the two pincers on the binding properties of the receptors. The design of these pincer-tweezer anion receptors connects well with our previous studies on the synthesis of molecular tweezers [52][53][54][55][56] and azolium-based anion receptors [57][58][59][60]. The present study describes the use of these two tetraimidazolium-based molecular tweezers as receptors for a selection of three anion guests (Cl À , Br À and I À ).…”

Microsolvation and Encapsulation Effects on Supramolecular Catalysis: C–C Reductive Elimination inside [Ga4L6]12– Metallocage

“…Theoretical analysis is nowadays indispensable to acquire a deeper understanding at a molecular level of catalytic processes. , However, few theoretical studies of the rate acceleration by supramolecular hosts have been reported. Some examples are cycloaddition inside cucurbit[6]­uril , and inside Rebek’s capsule and Diels–Alder reactions inside β-cyclodextrin (β-CD) and in Rebek’s “softball”, among others. − For metallocages, and more particular for the case of the Raymond [Ga 4 L 6 ] 12– metallocage, the orthoformate hydrolysis and aza-Cope rearrangement have been theoretically investigated; in a recent work by Vaissier-Welborn and Head-Gordon the effect of the overall charge of the [M 4 L 6 ] metallocage on the C–C reductive elimination process (by comparing [Ga 4 L 6 ] 12– with [Si 4 L 6 ] 8– , respectively) was also computationally investigated. Most of these works concluded that the electrostatic environment is essential for increasing the reaction rate.…”