Bridged calixarenes; X-ray crystal and molecular structures and spectroscopic studies
Single crystals of two macrocyclic compounds of the calix arene type, in which two opposite para-positions are connected by an additional aliphatic chain where n= 5 (1a) or n= 7 (1c) respectively, were obtained from acetone without incorporation of solvent. Crystals of compound (1a) are monoclinic, space group P21/n, a= 8.879(1), b= 17.216(4), c= 18.167(3)Å, β= 98.16(1)°, V= 2 748.2 Å3, Z= 4, Dx= 1.258 g cm–3, final R-value 0.046 (2 476 unique reflections); crystals of compound (1c) are monoclinic, space group P21/c, a= 18.105(3), b= 9.117(2), c= 19.123(2)Å, β= 97.84(1)°, V= 3127.0 Å–3, Z= 4, Dx= 1.166 g cm–13, final R-value 0.043 (4 470 unique reflections). In both cases the arrangement of the four phenolic residues corresponds to the cone conformation. This cone is strongly distorted in the case of compound(1a), where two aromatic rings are nearly parallel. For the whole series of bridged calixarenes (n= 5–14) FTIR spectra were measured in CCl4. A decreasing OH-stretching frequency with increasing chain length n shows an increasing strength of the intramolecular hydrogen bonds. These results, which are also supported by the 1H NMR data, are discussed in connection with the molecular conformations found in the crystalline state.