Polydentate analogues of 8-hydroxyquinoline and their complexes with ruthenium

Selective reduction of 2-nitro-3-methoxybenzaldehyde provides 2-amino-3-methoxybenzaldehyde that undergoes the Friedländer condensation with a variety of acetyl-substituted derivatives of pyridine and 1,10-phenanthroline. After cleavage of the methyl ether, the resulting polydentate analogues of 8-hydroxyquinoline are excellent ligands for ruthenium. The resulting oxidation state of the metal center depends on the anionic character of the ligands. The presence of two electron donating anionic ligands results in a Ru(III) complex as evidenced by paramagnetic NMR behavior. The electronic absorption and redox properties of the complexes were measured and found to be consistent with the anionic character of the 8-HQ moieties. A planar pentadentate ligand provides two Ru-O and two Ru-N bonds in the equatorial plane. An X-ray structure shows that the central pyridine of the ligand is oriented toward the metal but held at a distance of 2.44 Å.