Elsevier

Tetrahedron

Volume 71, Issue 47, 25 November 2015, Pages 8965-8974
Tetrahedron

Hexapodal pyrazole-based receptors: complexes with ammonium ions and solvent molecules in the solid state

Dedicated to Professor Gerhard Roewer on the occasion of his 75th birthday
https://doi.org/10.1016/j.tet.2015.09.058Get rights and content

Abstract

New crystalline complexes of hexapodal pyrazole-based hosts with NH4+PF6and/or solvent molecules were obtained and detailed analysed. The binding motifs observed between the binding partners give valuable insights into the phenomena of molecular recognition processes. Analyses of the conformations of the hexasubstituted benzene derivatives have shown that both ababab as well as abbabb and aaabbb conformations of the haxapodal compounds 13 are observed in the crystal structures 1ad, 2, 2a, 3a and 3b.

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Key topics: Molecular recognition (Complexes of ammonium ions with artificial receptors).

Introduction

Pyrazole-based compounds that enforce tripodal or hexapodal topologies were reported to be effective in the molecular recognition of ions1, 2 and neutral molecules, such as carbohydrates.3 Tripodal compounds based on a triethylbenzene scaffold were described to be interesting receptors for cations,2(a), 2(b) whereas the hexapodal compound 1 (Fig. 1) was shown to be an effective receptor in the recognition of ion-pairs.4 The potential of pyrazole-based compounds in the complexation of ionic and neutral substrates was evaluated both in the solution and in the solid state.

In this paper we describe new crystalline complexes of hexapodal pyrazole-based compounds with NH4+ and/or solvent molecules, showing interesting binding motifs between the binding partners. The analyses of the binding motifs give valuable insights into the phenomena of molecular recognition processes. The interactions of compound 1 with NH4+PF6 and solvent molecules provided four new multi-component crystals 1ad (see Fig. 1). In addition to compound 1, the new bromo-substituted analogues 2 and 3 were prepared and used for the crystallographic studies. On the base of these bromo derivatives three multi-component crystals with NH4+PF6 and/or solvent molecules could be obtained and analysed in detail. Moreover, the solvent-free crystal structure of 2 is described in this work.

Multi-component crystals including solvent molecules are described as solvates, however, this definition is controversial discussed in the literature (for a discussion on the definitions ‘solvate’ and ‘co-crystal’, see Ref. 5). In the case of the complexes of 1 and 3 with NH4+PF6 (crystals 1a, 1b, 1c and 3a) such solvents as ethanol (1a), methanol/water (1b), chloroform/acetonitrile (1c) and methanol (3a) are involved in the crystal structures. Crystals 1d, 2a and 3b represent solvates of 1, 2 and 3 with acetonitrile, dimethylformamide and toluene, respectively.

It should be noted that solvate formation is of high interest to different research areas. As mentioned by Griesser,6 ‘solvate formation opens perspectives for supramolecular chemistry and crystal engineering. Crystal structures of different solvates enable the study of molecular conformations, recurring patterns in hydrophobic aggregation and inclusion as well as molecular recognition phenomena in general’. Furthermore, the formation of solvates is of particular interest to the pharmaceutical industry.7

Analyses of the conformations of different hexasubstituted benzene derivatives,8 including analyses in solvate inclusion compounds, have been reported in the literature.9 Although the ababab conformation (a=above, b=below) of the hexapodal molecules is most commonly observed in the crystal structures, unusual conformations such as aaabbb and aaabab are also described in the literature. Such conformations are, for example, assumed by hexakis(4-cyanophenyloxy)benzene in its different solvates.9(b), 9(c) Substrate-dependent conformational changes (including also the formation of the aaaaaa conformation) were observed for hexasubstituted benzenes with amide functionalities, designed to recognize anionic substrates.9(e), 9(f), 9(g), 9(h) In the case of the present studies, both the ababab as well as the abbabb and aaabbb conformations of the haxapodal hosts 13 are observed in the crystal structures 1ad, 2, 2a, 3a and 3b.

Section snippets

Results and discussion

The crystal structures of four complexes of the hexapodal hosts (receptors) 1 and 3 with NH4+PF6 and solvent molecules [1·NH4+PF6·EtOH (1:1:1) (1a), 1·NH4+PF6·MeOH·H2O (1:1:1:1) (1b), 1·NH4+PF6·CHCl3·CH3CN (1:2:2:2) (1c), 3·NH4+PF6·MeOH (1:2:2) (3a)] as well as of the solvate inclusion compounds 1d, 2a and 3b [1·CH3CN (1:2) (1d), 2·DMF (1:2) (2a), 3·toluene (1:2) (3b)] and the crystal structure of the solvent-free host 2 have been elucidated by X-ray diffraction experiments (basic

Conclusion

New crystalline complexes of pyrazole-bearing hexapodal hosts with NH4+/PF6 and solvent molecules (complexes 1a, 1b, 1c and 3a) were obtained and used for detailed analysis of the noncovalent interactions between the binding partners. In addition, the analysis of the binding motifs in the solvates of 1, 2 and 3 with acetonitrile, dimethylformamide and toluene, respectively (crystals 1d, 2a and 3b), was carried out and is described in this work. The selected host molecules involve either

General

Compounds 13 were prepared by the reaction of hexabromomethylbenzene with 1H-pyrazole, 4-bromo-1H-pyrazole and 4-bromo-3,5-dimethyl-1H-pyrazole.4(a), 10, 11 Crystals suitable for X-ray diffraction were grown by slow evaporation of solutions of 13 from acetonitrile, methanol, ethanol, chloroform, toluene or dimethylformamide. The intensity data were collected at 100 K on a Kappa APEX II diffractometer (Bruker AXS) with MoKα radiation (λ=0.71073 Å) using ω- and ϕ-scans. Data integration and

Acknowledgements

Financial support of the Dr.-Erich-Krüger-Stiftung is gratefully acknowledged.

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