Energy and Geometry of Cooperative Hydrogen Bonds in p-Substituted Calix[n]- and Thiacalix[n]arenes: A Quantum-Chemical Approach

Abstract

(Thia)calix[n]arenes have been widely applied as molecular platforms and host molecules in supramolecular chemistry due to their high level of preorganization and well-detectable conformational preferences. Here we report on quantum-chemical calculations allowing the conformational analysis of p-substituted calix[4]-, calix[6]-, thiacalix[4]-, and thiacalix[6]arenes. To this effect, ab initio and density functional theory (DFT) calculations with the aid of RHF/3-21G, B3LYP/6-31G, B3LYP/6-31G(d,p), and B3LYP/6-311G(d,p) have been applied. The obtained structural data and the estimated energies of the intramolecular hydrogen bonds give clear evidence of the presence of cooperative effects of the hydrogen bonding. Multiple correlations between the pairs of Hammett constants of substituents and the calculated values of hydrogen bond energies in the corresponding p-substituted (thia)calix[n]arenes have been found. These energies can be considered as descriptors of a chemical reactivity of the p-substituted derivatives of (thia)calix[n]arenes. For example, the reaction of nucleophilic substitution, involving p-substituted calix[6]arenes in the presence of weak bases and in aprotic solvents or in the gas phase, under orbital control conditions should proceed through the diastereomeric transition states. Here, the achiral p-substituted calix[6]arene derivative mainly forms as an intermediate product of the reaction with a substrate without asymmetric centers.

Hydrogen Bond Energies and Cooperativity in Substituted Calix[n]arenes (n= 4, 5)

“…As shown, the HBEs in substituted CX[4] hosts are predicted to be nearly 30–33 kJ mol –1 . It should be remarked here that HBE values in S-CX[4] and S-CX[4]- t- Bu due to Novikov and Shapiro are ∼4 times larger owing to bifurcated hydrogen-bonded interactions of phenolate oxygen and charge delocalization subsequent to deprotonation. The CX[4]- t -Bu and CX[4]-NH 2 hosts reveal strong hydrogen-bonding interactions compared to those in CX[4]-CH 2 Cl, CX[4]-NO 2 , and CX[4]-SO 3 H or in S-CX[4].…”

“…45 Moreover, Fourier transform IR spectroscopic measurements on CX[n] (n = 4, 6, 8) have shown that cyclic cooperative intramolecular hydrogen bonding is responsible for a stable conformation which is not affected by the presence of bulky substituents. 47 In a very recent work, Novikov and Shapiro 18 have studied the energetics and cooperative hydrogen bonding in para-substituted calix[n]arenes and thiacalix[n]arenes employing quantum chemical and density functional calculations. Intramolecular HBEs in calixarene hosts were calculated in terms of the energy difference between the monoanion and the neutral system and by further computing the energy of phenol relative to its anion, which enables one to elucidate the mechanism of nucleophilic substitution reactions.…”

“…13−16 Hydrogen bonding and the underlying cooperativity therein govern the energetics of CD or CX conformers. 17,18 With this view intra-and intermolecular hydrogen bond energies (HBEs) are analyzed by 1 H NMR, vibrational spectroscopy experiments and molecular modeling. 19−22 The varying definitions of an intermolecular hydrogen bond pose certain difficulties for estimation of the corresponding bond energies.…”

“…The family of cyclodextrins (CDs), and higher generation cucurbituril and calixarene (CX) − hosts, has attracted significant attention in molecular recognition and supramolecular chemistry owing to their selective and efficient binding toward neutral or cationic guests and macromolecules as well. − The electronic structure of such novel macrocyclic receptors should prove useful in understanding host-guest interactions at the molecular level. − Hydrogen bonding and the underlying cooperativity therein govern the energetics of CD or CX conformers. , With this view intra- and intermolecular hydrogen bond energies (HBEs) are analyzed by 1 H NMR, vibrational spectroscopy experiments and molecular modeling. − The varying definitions of an intermolecular hydrogen bond pose certain difficulties for estimation of the corresponding bond energies . Recently, Gadre and co-workers − proposed a direct method to derive HBEs based on the molecular tailoring approach (MTA) wherein a molecule is scissored into a set of fragments and HBEs are subsequently estimated by appropriate addition or subtraction of the overlapping part from each fragment.…”

Recent developments of thiacalixarene based molecular motifs

“…On the other hand, in the case of partially substituted thiacalix [4]arenes, the activating ability of the -OH groups of unsubstituted phenol rings is stronger than that of -OR groups and sulfide bonds, resulting in the substitution at the para positions of the -OH groups. Novikov et al 75 evaluated the conformational analysis of p-substituted (thia)calix [4]arenes and (thia)calix [6]arenes by using ab initio density functional theory (DFT) calculations. The calculated hydrogen bond energies indicated strong cooperative effects of intramolecular hydrogen bonds on the conformational distribution and chemical reactivity pattern of p-substituted derivatives of (thia)calix [4]arenes and (thia)calix [6]arenes.…”