ORIGINAL RESEARCH Fine-tuning of packing architecture: symmetrically bridge-disubstituted tetramethoxycalix[4]arenes Conrad Fischer • Petra Bombicz • Wilhelm Seichter • Edwin Weber Received: 12 June 2012 / Accepted: 18 July 2012 / Published online: 4 August 2012 Ó Springer Science+Business Media, LLC 2012 Abstract Three acetonitrile solvates of tetramethoxyca- lix[4]arenes equally substituted on opposite methylene bridges are described with respect to their conformation and packing behaviour. All of the host molecules adopt a 1,2- alternate conformation, their packing architecture seems to be affected by the spatial demand of the bridge substituents only. This results in the synthetically implemented fine- tuning of the molecular arrangement. The engineering of the relevant packing motif, the ‘‘synthon’’ may be discussed most appropriate by the term ‘‘synthon engineering’’ fol- lowing the expression of crystal engineering. Keywords Calix[4]arene Á Inclusion compounds Á Single crystal X-ray analysis Á Isostructural calculations Á Crystal Engineering Á Bridge substitution Introduction Since decades, calixarenes act as prominent building blocks for the design of supramolecular inclusion com- pounds [1, 2]. Whereas it is a well-known fact that the chemical modification of the upper and lower rim of the chalice clearly influences the inclusion potential, until recently less was known about the effect of a bridge modification on the supramolecular properties. In this latter respect, we showed that either one [3–8] or two [9, 10] bridge substituents on opposite methylene units of the parent tetramethoxycalix[4]arene 1 (Fig. 1) exercise a distinct influence on the molecular conformation of the host as well as on the supramolecular architecture of the crystal packing. In a more detailed examination, a single small lateral substituent like ethyl or COOH seems to reduce the close packing of the crystal due to the higher dissymmetry of the chalice leading to straightened guest channels capable of solvent inclusion [8]. A conforma- tional change upon attachment of a second lateral substituent transform symmetry from partial cone to 1,2-alternate, resulting in dense packing with highly reduced solvent accessible voids [10]. In this study involving subsequent substitution of the bridge monosub- stituted calixarenes 2–5, we compare the recently described bridge dimethylated derivative 6 [10] with three new ace- tonitrile solvates of equal bridge-disubstituted calix[4]are- nes (7–9). Thereby, the focus is on comparison of their crystal packing depending on growing spatial demand of the substituent, i.e. ethyl (7), allyl (8) and benzyl (9) including isostructurality and packing density calculations. It can be applicable in a special case of crystal engineering being adequately described as ‘‘synthon engineering’’. Experimental Melting points were measured on a microscope heating stage Nagema (Wa ¨getechnik Rapido Radebeul) and are uncor- rected. IR spectra were recorded on a Nicolet Avatar 370 DTGS (Thermo) using ATR technique (wave numbers given in cm -1 ). 1 H- and 13 C-NMR spectra were obtained from a Bruker Avance 500 at 500.1 MHz ( 1 H) and 125.8 MHz C. Fischer (&) Á W. Seichter Á E. Weber Institut fu ¨r Organische Chemie, TU Bergakademie Freiberg, Leipziger Str. 29, 09596 Freiberg, Saxony, Germany e-mail: conrad.fischer@chemie.tu-freiberg.de P. Bombicz (&) Institute of Organic Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, P.O. Box 17, Budapest 1525, Hungary e-mail: bombicz.petra@ttk.mta.hu 123 Struct Chem (2013) 24:535–541 DOI 10.1007/s11224-012-0104-1