New chiral receptors based on dibenzotetraaza[14]annulenes

Reactions of dibenzotetraaza[14]annulene Ni(II) complexes 1 and 2 with oxalyl chloride and chiral terpene alcohols ((−)menthol, (−)borneol), and the Cinchona alkaloid (quinine) afforded new mono and disubstituted derivatives bearing corresponding ester groups at the meso positions. The demetallation of di(−)menthyloxycarbonyl and di(−) bornyloxycarbonyl derivatives has been accomplished by means of gaseous HCl, leading to corresponding free bases. Single-crystal X-ray diffraction of the free ligand equipped with two (−)menthyloxycarbonyl substituents revealed a saddle-like shape of the molecule resulting in the non-equivalence of two axial coordination sites of the macrocycle. The (−) menthyloxycarbonyl substituents were found to define the ‘walls’ of a cavity on one side of the macrocyclic platform. The two menthyl rings belonging to the meso substituents appeared to be non-equivalently arranged on both propanediiminate parts of the macrocycle, relative to their phenyl and methyl substituents. The molecules of the ligand are arranged in stacking columns and form cavities in the crystal lattice. The molecules of solvent (benzene) were found to reside in these cavities. The amine protons of the central tetraaza fragment of the macrocycle are involved in two asymmetric intramolecular N−H⋯N hydrogen bonds. The 1H and 13C NMR spectra measured at room temperature, in CDCl3 solution, provided evidence of conformational non-equivalence within both meso-disubstituted propanediiminate fragments of the macrocycle. Additionally, two non-equivalent NH protons have been detected in the 1H NMR spectra of both free ligands. The new products have been characterized by elemental analyses, ESI MS, IR, 1H and 13C NMR data.