Anion Recognition by Partial Cone Dihomooxacalix[4]arene‐Based Receptors Bearing Urea Groups: Remarkable Affinity for Benzoate Ion

Abstract

Tetraureido‐dihomooxacalix[4]arenes (tert‐butyl 4a and phenyl 4b) were synthesised and obtained in a partial cone conformation in solution and in the solid state, as confirmed by NMR and X‐ray diffraction studies. Their complexation ability towards halides, pseudo‐halides and oxoanions was assessed by 1H NMR and UV/Vis titrations. Structural and energetic insights of phenylurea 4b complexes were also obtained using molecular dynamics (MD) simulations. The binding data showed that the association constants are strongly dependent on the nature of the substituent (alkyl/aryl) at the urea unit. tert‐Butyl urea 4a is a much weaker receptor than phenylurea 4b, and showed association constants that decrease with decreasing of anion basicity. Phenylurea 4b is a highly efficient anion receptor, exhibiting a remarkable binding ability towards BzO– ion (log Kass = 4.81). In comparison to the phenylurea analogue bearing a butyl spacer and a cone conformation, receptor 4b containing a shorter spacer (three carbon atoms) and a partial cone conformation is more pre‐organized, displaying a strong enhancement of its binding efficiency. MD simulations have shown that the anions are preferentially bound to the urea moieties of the macrocycle lower rim, in agreement with the ROESY spectrum carried out with phenylurea 4b and BzO– anion.

Ditopic Receptors Based on Dihomooxacalix[4]arenes Bearing Phenylurea Moieties With Electron-Withdrawing Groups for Anions and Organic Ion Pairs

“…The facing butoxy substituted phenyl ring (D) is inclined slightly outwards, with a dihedral angle of 101°. Consistently with previous reports, the ureido groups form an intramolecular N–H···O bifurcated hydrogen bond (Marcos et al, 2014b; Augusto et al, 2018). In this case the H-bond is rather asymmetric, with the N···O distance of the NH directly bonded to the phenyl ring, para to the electron-withdrawing CF 3 group, shorter with respect to the other N···O distance (2.89 vs. 3.06 Å).…”

Recognition of Anions, Monoamine Neurotransmitter and Trace Amine Hydrochlorides by Ureido‐Hexahomotrioxacalix[3]arene Ditopic Receptors

“…The association constants, as log K ass and reported in Table , were determined following the urea NH chemical shifts through the WinEQNMR2 program . In the case of the partial cone receptors, the data were calculated using the NH chemical shifts of the two (thio)ureido groups oriented on the same side of the macrocyclic ring …”

“…As part of our interest in the study of homooxacalixarenes (calixarene analogues in which the CH 2 bridges are partly or completely replaced by CH 2 OCH 2 groups) we have reported the anion binding affinity of several di‐, and tetra‐substituted (thio)ureido‐dihomooxacalix[4]arenes, in the cone conformation incorporating a four carbon atom spacer. Tetra‐substituted dihomooxa receptors in a partial cone conformation and bearing a propyl spacer were also studied . In general, we observed that phenylurea receptors are more efficient than phenylthioureas, the substituents (alkyl/aryl) on the urea moiety deeply affect the complexation ability, the phenylurea derivatives are the strongest receptors, and a higher number of potential hydrogen‐bond donors enhances the binding affinity.…”

Dihomooxacalix[4]arene-Based Fluorescent Receptors for Anion and Organic Ion Pair Recognition

“…As part of our on-going interest on the host-guest properties of substituted dihomooxacalix[4]arenes (calix[4]arene analogues in which one CH 2 bridge is replaced by one CH 2 OCH 2 group) with (thio)urea units [ 19 , 20 , 21 , 22 , 23 ], we have extended our research into the study of fluorescent receptors for anion [ 24 ] and ion-pair recognition. Thus, dihomooxacalix[4]arene-based fluorescent sensors bearing (thio)urea groups as a binding site and naphthalene moieties as a fluorophore unit were obtained for the first time.…”