Computational study of calix[4]arene derivatives and complexation with Zn2+
Introduction
Molecular recognition plays an extremely important role in chemistry and biochemistry, especially in the fast growing field of supramolecular host–guest chemistry [1]. One interesting class of supramolecular systems is the macrocyclic calixarenes that are widely used as effective host compounds for various guest species such as cations, anions, and neutral molecules [2], [3], [4]. The increasing interest on calixarenes is certainly due to their versatility in terms of complexation capability, conformational flexibility and reactivity, and they have been applied in a wide range of areas as catalysis, host–guest chemistry, molecular recognition, separation chemistry, selective ion transport and sensors [2], [3], [5]. The most well known member of this class is calix[4]arene which comprises cyclic tetramers of phenol units linked by ortho-methylene bridges and reveals a basket-shaped molecular structure [6]. In addition, calix[4]arenes have been widely used as a molecular platform for the design and construction of different kinds of new excellent receptors in molecular recognition by easy chemical modifications [3], [7]. By complete or partial functionalization of the lower (phenolic OH groups) and/or upper rims (the p-position), a wide variety of calix[4]arenes has been obtained [2], [3], [6], [8]. The replacement of methylene junctions between the phenolic moieties by hetero atoms has been recently reported [9], [10], in particular the remarkable thiacalix[4]arenes where sulfur atoms are used in place of the four methylene bridges [11], [12], [13], [14], [15]. NMR study has shown that thiacalix[4]arene is much more flexible in solution than parent calix[4]arene [11].The flexibility increases with the number of additional binding sites at sulfur bridges. The complexation ability of thiacalix[4]arenes towards neutral molecules [12], [13], or metal ions, especially transition metal ions [14], [15], [16], has also been investigated. A study of the extraction with transition metal ions shows that thiacalix[4]arene formed 1:1 complexes with divalent metal ions M2+ [Co2+, Cu2+, Zn2+] by ligating sulfurs in addition to the adjacent two phenoxide oxygens to form two sets of five-membered chelated rings as evidenced by the NMR study of the diamagnetic Zn2+ complex [14c].
Theoretical calculations based on full geometry optimizations by density functional theory (DFT) methodology are now feasible and are being applied successfully to investigate the properties of calix[4]arene derivatives and, in particular, the complexation with charged species [17], [18], [19], [20], [21], [22]. Structure and conformational equilibrium of thiacalix[4]arene derivatives namely tetraaminothiacalix[4]arene (1) and tetramercaptothiacalix[4]arene (2) have been the subject of a previous theoretical study in gas-phase [22]. In this work, apart from the application of new DFT functionals in gas phase, we have studied the influence of two different solvents (chloroform and dichloromethane) on the conformational equilibrium of these thiacalix[4]arenes by means of a polarizable continuum model. On the other hand, structures of the corresponding methylene bridged compounds, namely tetraaminocalix[4]arene (3) and tetramercaptocalix[4]arene (4) were also investigated to analyze the effect of bridges counterparts. The structures of the four main conformers of each compound are presented in Fig. 1. The main goal of this study is to understand how the presence of different substituents at the lower rim and methylene junctions modifies their conformational flexibilities and binding abilities and selectivities, which is a fundamental aspect in complexation of supramolecular compounds. The interest on the transition metal zinc in the chemical and biochemical systems grew due to its ability to coordinate with a variety of ligands in different structural patterns, and it has been used in a wide diversity of chemistry fields as catalysis, metalloenzymes, complexation of macrocyclic ligands, organometallic and bioinorganic systems [23], [24]. In this study, structural and energetic data for the complexes formed between the zinc cation and each conformer of compound 1 are also reported, which may open new perspectives in host–guest supramolecular chemistry based on aniline rather than phenol. High level DFT calculations are applied for the first time to study the complexation of this thiacalix[4]arene derivative with a transition metal ion. In addition, we hope that the understanding of the complexation/recognition process from a microscopic point of view will provide a new progress for the design and development of sophisticated sensors at molecular level in future.
Section snippets
Computational methodology
The structures of the four main conformers of 1, 2, 3 and 4 were initially obtained by complete optimization without any constraints at the restricted Hartree–Fock (RHF) level with the 6-31G(d) basis set. These geometries were then used as input for final optimizations with the 6-31G(d,p) basis set at DFT level, using the Becke’s three parameter hybrid exchange functional [25] combined with the Lee, Yang and Parr correlation functional [26] (B3LYP). The electronic energies of these optimized
Structure and hydrogen bonding
The DFT optimized structures of the four conformers cone, partial cone (paco), 1,2-alternate (1,2-alt) and 1,3-alternate (1,3-alt) of all the compounds were obtained at B3LYP/ 6-31G(d,p) level. Unfortunately, none of them have been experimentally achieved yet, though very similar compounds with tert-butyl groups at para position were synthesized, namely the tetraamino-p-tert-butylthiacalix[4]arene [34], tetramercapto-p-tert-butylthiacalix[4]arene [35], and tetramercapto-p-tert
Conclusion
The four main conformers of the sulfur bridged compounds, namely tetraaminothiacalix[4]arene (1) and tetramercaptothiacalix[4]arene (2), and the corresponding methylene bridged compounds, namely tetraaminocalix[4]arene (3) and tetramercaptocalix[4]arene (4) have been studied by a DFT methodology. Our results predict the cone to be the most stable conformer for both sulfur bridged derivatives, which is in a good agreement with a previous theoretical work on parent thiacalix[4]arene. On the other
Acknowledgement
Financial Support from the Fundação para a Ciência e a Tecnologia (Lisbon) through PhD scholarship SFRH/BD/7077/2001 is gratefully acknowledged.
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