STUDIES ON SELECTED MACROCYCLES

Abstract

The macrocycle 2, 17-dioxa-5,14-dithia[6,6](1.2)cyclophane was successfully synthesized. It acts as a bridging ligand in a mercury(II) complex confirmed by X-ray crystallographic studies. 1H NMR studies of the mercury(II) complex indicated that the Hg2 + readily dissociates from the ligand in solution. 1H NMR studies of the silver complex however suggested that the macrocycle behaves as a tetradentate ligand with the Ag+ coordinating to all four heteroatoms. The same macrocycle in a palladium(II) complex was shown, by X-ray crystallography and 1H NMR spectroscopy in the solid state and in solution respectively, to exhibit a symmetrically bidentate configuration in which the Pd2+ coordinates to only the two sulphur and two chlorine atoms. 12, 13,25,26-Tetraaza-2,15-dithia[3.3]phenanthrolinophane was obtained as a mixture of the anti and syn conformers. Irreversible thermal conversion of the syn to the anti conformer was observed. Results from semiempirical molecular orbital PM3 calculations also indicate that the anti confonmer is thermodynamically more stable. The zinc(II) and cadmium(II) complexes of anti-12, 13,25,26-tetraaza-2, 15-dithia[3.3]phenanthrolinocyclophane could be prepared but failed to yield single crystals suitable for structural determinations. The 1H NMR spectrum of the zinc(II) complex is consistent with a highly symmetrical structure with the Zn2+ expected to coordinate to all the four nitrogen atoms in the two phenanthroline units. Although it is uncertain whether the two sulphur atoms coordinate to Zn2+, the bridges are believed to be held rigid in the complex. 9,21,22-Triaza-2, 11-dithia[3.3]pyridinophenanthrolinophane was synthesized and found to be conformationally mobile at room temperature. Only the anti conformer was observed at the low temperature limit in the dynamic 1H NMR studies. At room temperature, a fast interconversion between the anti conformers is expected with the syn conformer present as a high-energy intermediate. The conformational energy barrier for the interconversion process is estimated at 38.1 kJ moI-1. Complexation reactions of 9,21,22- traaza-2, 11-dithia[3.3]pyridinophenanthrolinophane with Mn2+ and Zn2+ afforded the respective complexes. Only the zinc(II) complex gave a satisfactory 1H NMR spectrum. The spectral data suggest that the ligand coordinates to Zn2+ only via the two nitrogen atomes in the phenanthroline unit and the two sulphur atoms in the bridges. The pyridine moiety and its adjacent methylene units could still enjoy a certain degree of conformational mobility. 2,9-Bis(2-amino-ethyl)-1, 10-phenanthroline was found to form the respective complexes with Cr3+ and Ni2+ in a 1: 1 metal to ligand ratio. A reaction of 2,9-bis(2-amino - ethyl)-1, 10-phenanthroline with 2,6-diacetylpyridine in the presence of the template metal ion Zn2+ afforded the zinc(II) complex of 10,23,24-triaza-4, 11-diimino-4, 11-dimethyl[4.4]pyridinophenanthrolinophane. The elemental analysis supports a 1 :1 metal to ligand stoichiometry with two molecules of water of crystallization.