SYNTHESIS, STRUCTURE, AND PROPERTIES OF OLIGO-TRIDENTATE LIGANDS - COVALENTLY ASSEMBLED PRECURSORS OF COORDINATION ARRAYS

Oligo-tridentate ligands based on alternating pyridines and pyrimidine s were synthesised by Stille-type carbon-carbon bond-forming reactions . The terpyridine-like sites are designed to coalign upon metal comple xation, giving rise to organized and rigidly spaced metal ions. Periph eral functionalization of the basic bis-tridentate framework was explo red. The heterocycles in the ligands are in an all-trans conformation about the interannular bonds as indicated by comparison of their H-1 N MR spectra. An X-ray crystal structure analysis of the nonchiral tris- tridentate ligand 2a reveals a helical structure in the solid state. T he seven heterocycles form a helical structure with resulting overlap of the terminal pyridines. Their centroid-to-centroid distance is 4.52 3 Angstrom with 38.8 degrees between the planes. NMR investigations su pport a helical conformation in solution as well. Electrochemical and UV absorption measurements indicate that the LUMO resides on the pyrim idine moiety of the ligands.