X-ray Diffraction, FT-IR, and 13 C CP/MAS NMR Structural Studies of Solvated and Desolvated C-Methylcalix[4]resorcinarene Rafal Kuzmicz, † Violetta Kowalska, † Slawomir Domagala, ‡ Marcin Stachowicz, ‡ Krzysztof Woz ´niak, ‡ and Waclaw Kolodziejski* ,† Department of Inorganic and Analytical Chemistry, Faculty of Pharmacy, Medical UniVersity of Warsaw, Banacha 1, Warszawa 02-097, Poland, and Department of Chemistry, UniVersity of Warsaw, Pasteura 1, Warszawa 02-093, Poland ReceiVed: February 21, 2010; ReVised Manuscript ReceiVed: June 16, 2010 Solid C-methylcalix[4]resorcinarene solvated by acetonitrile and water (CAL-Me) and then modified by slow solvent evaporation (CAL-Me*) was studied using single-crystal and powder X-ray diffraction, FT-IR, and 13 C CP/MAS NMR. The CAL-Me solvate crystallizes in the monoclinic P2 1 /n space group with three CH 3 CN and two H 2 O molecules in the asymmetric part of the unit cell. The CAL-Me molecules adopt a typical crown conformation with all of the hydroxyl groups of the aryl rings oriented up and all of the methyl groups disposed down (the rccc isomeric form). The crystalline network is formed by resorcinarene, CH 3 CN, and H 2 O molecules and assembled by intermolecular hydrogen bonds and weak C-H ··· A or C-H ··· π interactions. The desolvated CAL-Me* loses its crystalline character and becomes partly amorphous. It is devoid of CH 3 CN and deficient in water. However, the resorcinarene molecules still remain in the crown conformation supported by intramolecular hydrogen bonds, while intermolecular hydrogen bonds are considerably disintegrated. The work directs general attention to the problem of stability and polymorphism of resorcinarene solvates. It shows that the joint use of diffractometric and spectroscopic methods is advantageous in the structural studies of complex crystalline macromolecular systems. On the other hand, the solid-state IR and NMR spectroscopic analyses applied in tandem have been found highly beneficial to elucidate the disordered structure of poorly crystalline, desolvated resorcinarene. Introduction Calix[4]resorcinarenes are macrocyclic compounds (Figure 1) formed by acid-catalyzed condensation of benzene-1,3-diol (resorcinol) with acetaldehyde or higher aliphatic aldehydes. 1-4 They are of great interest in the host-guest chemistry as molecular receptors for many useful molecules. 5-8 Unsubstituted calix[4]resorcinarenes may exist in one of the five conformations: crown (C 4V ), boat (C 2V ), chair (C 2h ), saddle (D 2d ), and the least discussed diamond (C s ). 1,4 Four intramo- lecular hydrogen bonds between the OH groups on neighboring aromatic rings support the rigid structure of the crown conformer (Figure 1). 1,9-13 In the solid state, the rccc isomer (epimer all- cis) of the crown conformation 1 is the prevalent form (Figure 1). 2,10-22 Chemical properties of unsubstituted calix[4]resorcinarene in the crown conformation are generally determined by the presence of eight hydroxyl groups in the so-called “upper rim” and by its internal hydrophobic cavity. 1 Calix[4]resorcinarenes have unusual complexing properties toward organic molecules such as solvents, 10-13 aliphatic and aromatic amines, 16 neu- rotransmitter acetylcholine, 7,17 and sugars. 18-21 The complexes were comprehensively studied using single-crystal X-ray dif- fraction (XRD) and NMR in solutions. So far, little is known about the structure of desolvated resorcinarenes in the solid state. 23 The reason is that they do not form single crystals suitable for the XRD studies. Hence, in order to solve this problem, one has to apply methods, which do not rely on the material crystallinity. In this study, we use IR and NMR spectroscopic techniques in the solid state. The solid-state high-resolution 13 C NMR spectroscopy comprises three major techniques: magic-angle spinning (MAS) of a sample, high-power decoupling of protons, and signal enhance- ment by cross-polarization (CP) from protons. However, the kinetics of 1 H f 13 C CP has not yet been used to investigate the structure and phase homogeneity of calixarene samples. C-Methylcalix[4]resorcinarene (Figure 1), hereafter abbrevi- ated CAL-Me, is the simplest member of the resorcinarene compounds. A solid-state structure of CAL-Me cocrystallized with various small molecules, e.g., solvents, has been exclusively studied using X-ray crystallography. 12-14,23-55 Recent X-ray diffraction studies 56 showed a dramatic effect of water and CH 3 CN exerted on C-alkylpyrogallol[4]arene assemblies by means of intermolecular hydrogen bonding. * To whom correspondence should be addressed. E-mail: waclaw.kolodziejski@wum.edu.pl. † Medical University of Warsaw. ‡ University of Warsaw. Figure 1. Hydrogen bonds stabilizing the crown conformer of C-methylcalix[4]resorcinarene and the systematic enumeration of its carbon atoms. For other resorcinarenes, the methyl groups are replaced by various substituents. J. Phys. Chem. B 2010, 114, 10311–10320 10311 10.1021/jp1015565  2010 American Chemical Society Published on Web 07/26/2010