The significant role of the solvent in high selectivity of symmetrical calix[4]tubes for potassium ion in solution: A DFT study Sadegh Salehzadeh ⇑ , Yasin Gholiee, Mehdi Bayat Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran article info Article history: Received 15 July 2014 Received in revised form 4 September 2014 Accepted 5 September 2014 Available online 16 September 2014 Keywords: Density functional theory Host–guest chemistry Selectivity Calix[4]tubes Alkali metal ions abstract In this work the formation of p-tert-butylcalix[4]tube (1) and p-H-calix[4]tube (3) complexes with Na + , K + , Rb + and Cs + cations are studied theoretically. Two different modes of complexation, poly-ether tube (p.e.t.) and benzene-rings pocket (b.r.p.), were investigated and the results showed that the complexes of p.e.t. form are significantly more stable. Whereas K + selectivity is observed for calix[4]tubes 1 and 3 in aqueous environments, calculations show that both calix[4]tubes have larger bonding interaction with Na + . Thus, the K + selectivity of calix[4]tubes in solution is not an intrinsic property of the calix[4]tube itself. Indeed, our calculations show that the selectivity is reversed for Na + and K + because the K + leaves much easier the solvent molecules and has a greater chance to go into the calix[4]tubes. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction In traditional supramolecular chemistry the selectivity is mainly characterized by the hole-size fitting concept [1–4]. How- ever, nowadays, the high selectivity in host–guest complexes has been redefined and it not only depends on ‘‘hole-size fitting’’ but also on ‘‘affinity’’, ‘‘strain energy’’ and ‘‘solvent effects’’ [5–10]. Interestingly, due to solvation and likely entropic factors a higher binding constant is observed if the guest molecule enjoys more freedom, and occupies only part of the space inside a host cavity [11]. In recent years, the theoretical approaches have been used to justification of systems including host–guest complexes [12], selective recognition of alkali metals [13] and ionophores with cat- ion-p interactions [14]. Furthermore, it has been shown that the hydrogen-bonding interactions formed by the well-designed moi- eties, can be used to improve the binding affinity of the receptor for the anion [15,16]. More recently, we theoretically showed that the change in solvation energies for the reaction between a recep- tor and an anion in solution, producing an anion-receptor complex, is very important in designing a suitable receptor for a specific anion [17]. Our data clearly showed that a better receptor is not always the one whose cavity size has better compatibility with the anion size or the one that has larger interaction energy with the anion. In this work we like to extend our DFT studies on host–guest complexation but where the guest is a metal cation. Thus for this study the symmetric calix[4]tubes 1 and 3 (see Fig. 1) and alkali metal ions were selected. Calixarenes have been widely used as backbones in the design of supramolecular receptors [18–20]. These are well organized vases like molecular entities recognized for their utility in building units for molecular recognition [19–23]. Although several calixa- rene based receptors have been reported and studied for their applicability in supramolecular chemistry, the more popularly studied one is the calix[4]arene [24,25]. Indeed, calix[4]arenes are the simplest and most common members of the calixarene family, with four phenolic residues being present in the macrocy- clic ring [23]. In recent years, several reviews have been published on biological applications, (structural aspects) and ion and molec- ular recognition of the calix[4]arene derivatives [26–38] and a few reviews deal with computational studies of calixarene host–guest complexes [39–41]. On the other hand, the selective recognition and extraction of alkali metals is an area of great interest [42,43]. Several crypt- and-like ionophores based on a bis calix[4]arene containing either alkyl or phenyl substituents at the upper rim (calix[4]tubes) have been developed for selective complexation of K + over the alkali ions and Ba 2+ by Beer and co-workers [44,45] (see Fig. 1, com- pounds 1–9). The single crystal structures of 1, 2 and [1K] + have been established. The structure of 1 contains a crystallographic center of symmetry with two calix[4]arene units exhibiting flat- tened cone conformation. The four ethylene linkages present between the calix[4]arenes exists in two different conformations (tgtg, t = trans, g = gauche). In such a case, the cage composed of http://dx.doi.org/10.1016/j.comptc.2014.09.013 2210-271X/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +98 811 828 2807; fax: +98 811 838 0709. E-mail address: saleh@basu.ac.ir (S. Salehzadeh). Computational and Theoretical Chemistry 1048 (2014) 62–68 Contents lists available at ScienceDirect Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc