Anion Recognition by Heterocalixarenes: Understanding the Physical Nature of the Interaction

Non-covalent interactions are the main forces involved in the recognition, transport and regulation of virtually all events in biological systems [1]. Although the chemical relevance of anionic molecules is very defined, the chemistry of anion receptors is still a growing area [2]. Host molecules which can interact with anionic species through non-covalent interactions have some advantages due to intrinsic characteristics of the anions, such as size, polarizability and coordination number [3]. Recently, Wang and co-workers [4] prepared a family of heterocalixarenes containing rings of 1,3,5-triazine, bridged by either oxygen or nitrogen atoms. Using the Wang’s compound bis-cloro-tetraoxacalix[2]arene[2]triazine as a reference model compound, we carefully addressed the nature and the magnitude of the anion-π interactions with the anionic guests Cl, Br, I, BF4, CH3CO2, H2PO4, HSO4, NCS, NO3, PF6, and SO4 [5]. The role of the bridging heteroatoms (N(H)-, O-, and S-) were also studied. The magnitude and the physical nature of the interaction in these hostguest complexes were analyzed through the energy decomposition analysis (EDA) [6], natural bond orbitals (NBO) [7] and the non-covalent index (NCI) [8] as fundamental tools. The most stable situation in the host-guest systems was observed when the anions are disposed between the triazine rings, leading to different sorts of non-covalent interactions, as well as hydrogen bond formation (Fig. 1a). Anions containing hydrogen atoms also acts as weak hydrogen bond donors to the nitrogen atoms of the triazine units. Non-covalent regions due dispersion interactions are observed between the anions and the triazine unities, specially for the sulfur-bridged heterocalixarene, Fig 1c. The 12 a 17/Nov, 2017, Águas de Lindóia/SP, Brasil EDA results reveal stabilizing interactions in all host-guest complexes. The electrostatic interaction plays a fundamental role in oxygen-bridged complexes, whereas the orbital term is the most significant contribution in sulfurand nitrogen-bridged heterocalixarene complexes. Since in all complexes the Pauli repulsion and the electrostatic term are very similar in magnitude, they cancel each other to a large extent, which suggests that is the orbital interactions the driven forces to the host-guest complex formation. Two different orbital contributions to the host-guest interaction was found in the NBO analysis: (a) the donation from lone pairs of the anion to the σ* antibonding orbital of C-H bonds, which is related to the hydrogen bonds and (b) the donation from the anion lone pair to the triazine C-N π* antibonding orbital, which is the orbital part of the anion-π interaction, Fig. 1b. Figure 1: (a) Minima structure with selected geometrical parameters; (b) NBO donor-acceptor interactions with their respective energy; and the (c) Perspective and superior view of NCI analysis for the host-guest complex of oxygen-bridged heterocalixarene with chloride. Key-words: Anion Recognition. Heterocalixarenes. DFT. EDA. NBO. NCI. Support: This work has been supported by CNPq, CAPES, NWO and FAPESP. References: [1] A. Frontera, P. Gamez, M. Mascal, T. J. Mooibroek, and J. Reedijk, Angew. Chemie Int. Ed., 50, 9564 (2011). [2] J-M. Lehn, Supramolecular Chemistry. (1995) Wiley-VCH, Weinheim, Germany. [3] J. W. Steed and J. L. Atwood, Supramolecular Chemistry. (2009) Wiley-VCH, Weinheim, Germany [4] D. X. Wang and M. X. Wang, J. Am. Chem. Soc., 135, 892 (2013). [5] Full article submitted to Physical Chemistry Chemical Physics (PCCP) journal. [6] F. M. Bickelhaupt and E. J. Baerends, Rev. Comput. Chem.; 15, 1 (2000). [7] E. D. Glendening, C. R. Landis, and F. Weinhold, Wiley Interdiscip. Rev. Comput. Mol. Sci., 2, 1 (2012). [8] E. R. Johnson, S. Keinan, P. Mori-Sánchez, J. Contreras-García, A. J. Cohen, and W. Yang, J. Am. Chem. Soc., 132, 6498 (2010).