Synthesis and Characterization of Positively Charged tris‐Imidazolium Calix[6]arene Hosts for Anion Recognition

Abstract

The syntheses of new tripodal ligands with effective C 3 symmetry and anion recognition properties, are reported herein. The ligands have a 1,3,5-tris-methoxy-2,4,6-tris-(α,ωbromoalkoxy)calix- [6]arene as a backbone, and were prepared from 1,3,5-tris-methoxy-p-tert-butyl-calix [6]arene by alkylation with a series of α,ω-dibromo-alkanes [Br(CH 2 ) n Br] of different chain lengths (n = 2, 4 or 6). Further substitution of the bromo groups, via the use of 1-methyl-, 1-mesityl-or 2,6-diisopropylphenyl-1H-imidazole, led to alkylation by imidazole units and afforded tripodal and tricationic 1,3,5-tris-methoxy-2,4,6-trisimidazolium-p-tert-butyl-calix[6]arenes. These scaffolds provide a framework for H-bonded and electrostatic interactions with various anions and their anion binding properties were investigated by 1 H NMR and UV-visible spectroscopy. A sizeable selectivity for HSO 4 -(present as the n-Bu 4 N + salt) was demonstrated for most of the tripodal ligands considered in this work. Other anions such BF 4 -, ClO 4 -, PF 6 -, NO 3 -, Clexhibited weak to medium associations with the investigated ligands. A single crystal X-ray study, unfortunately of poor quality, corroborated the geometry of the calix [6]arenes that was previously established by 1 H NMR spectroscopy. The bindingsite geometry was more definitively probed by DFT calculations. IntroductionOver the last decades, the design of molecular receptors with high selectivity has been one of the goals and challenges in the field of supramolecular chemistry: [1] different functionalities and shapes [2][3][4] have been investigated to induce selective and efficient [5][6][7][8] recognition of charged and/or neutral species. [9] Due to their potential applications in the biological, environmental and industrial domains, [10][11][12][13][14] interest has also been focused on pre-organized hosts for anion coordination. [15,16] Nevertheless the selective coordination of anions remains a research area which has been less explored compared to that of cations. [17][18][19][20] The designed receptors can be neutral or cationic organic scaffolds or even metal-ligand complexes. [21][22][23][24][25] Calix[4]arenes have attracted interest due to their concave macrocyclic structure and hydrophobic cavity, despite their small cavity size, which can hamper the design of systems for anion recognition. Following subtle substitution and preorganization on the upper and lower rims, [26][27][28][29][30] charged systems have been prepared and investigated. Different type of positively charged sites can be grafted onto the calixarene. [31] Imidazolium groups, bound to different positions have been recently used. [32][33][34] The imidazolium proton's high acidity can promote efficient and strong C-H .… anion interactions and, in addition, the cationic imidazolium groups, may also induce strong electrostatic interactions with negatively charged anions. Secondary effects, such as anion-π interactions [35,36] could also take place. Overall, this series of complementary interactions makes c...

Synthesis of Imidazolium Cations Linked to Para-t-Butylcalix[4]arene Frameworks and Their Use as Synthons for Nickel-NHC Complexes Tethered to Calix[4]arenes

“…on Suzuki-Miyaura catalysis. We have already reported similar studies with related calix [6]arenes linked to CpNi(NHC) groups and have investigated their catalytic activity, as mentioned earlier [29,30]. Other groups have published examples of catalysis by nickel complexes attached to a supramolecular framework [45][46][47].…”

“…It was of interest to us to graft nickel NHC complexes onto a calixarene backbone to see whether the catalytically active organometallic fragment close to the calixarene cavity would lead to any enhanced catalytic activity. Indeed, we recently described the synthesis of nickel-NHC complexes grafted onto a calix [6]arene framework [29,30]. Thus, we decided to prepare similar complexes in which the metal carbene moiety was linked to a calix [4]arene backbone.…”

“…These species in turn could be reacted with [Co 2 (CO) 8 ] to yield a series of cobalt carbonyl species linked to calixarene units via cobalt π-alkyne-bond interactions [28]. We also recently reported on the syntheses and anion-complexing ability of a series of calix [6]arenes that were functionalized with imidazolium ligands [29]. We are still interested in combining the relatively undeveloped organometallic chemistry of calixarenes [30] with our current research on the chemistry of N-heterocyclic carbene complexes of nickel [31][32][33][34][35][36][37][38][39].…”

In silico exploration of O-H…X2+ (X = Cu, Ag, Hg) interaction, targeted adsorption zone, charge density iso-surface, O-H proton analysis and topographic parameters theory for calix[6]arene and calix[8]arene as model

“…The last thirty years have shown an important interest for the calix [n]arene chemistry due to their excellent interaction behavior of various chemical functions that permits to address very diverse fields of sensor application [1][2][3][4]. Even if they remain somewhat confidential, the opto-electronic applications of the calix [6,8]arenes molecules are constantly increasing in very different fields such as cation or anion recognition [5][6][7][8], cell transfection [9], biopharmaceutical field [10], but also recent amphiphilic species cumulating both transfective 3 and antibacterial properties [11,12].…”