Synthesis, structure, and properties of oligo-tridentate ligands; Covalently assembled precursors of coordination arrays

Oligo-tridentate ligands based on alternating pyridines and pyrimidines were synthesised by Stille-type carbon-carbon bond-forming reactions. The terpyridine-like sites are designed to coalign upon metal complexation, giving rise to organized and rigidly spaced metal ions. Peripheral functionalization of the basic bis-tridentate framework was explored. The heterocycles in the ligands are in an all-trans conformation about the interannular bonds as indicated by comparison of their H-1 NMR spectra. An X-ray crystal structure analysis of the nonchiral tris-tridentate ligand 2a reveals a helical structure in the solid state. The seven heterocycles form a helical structure with resulting overlap of the terminal pyridines. Their centroid-to-centroid distance is 4.523 Angstrom with 38.8 degrees between the planes. NMR investigations support a helical conformation in solution as well. Electrochemical and UV absorption measurements indicate that the LUMO resides on the pyrimidine moiety of the ligands.