Metal-ion mediated regioselective complexation for bis(pyridine-2,6-dione) dyes with different deprotonated states

Bis(pyridine-2,6-dione) dyes with different pyridine N-tails are first used as the multidentate ligands to prepare a series of Ca-II, Na-I, Cu-II and Ni-n complexes. H-1 NMR and UV-Vis spectral as well as X-ray single-crystal analyses reveal that they have mononuclear, dinuclear and one-dimensional structures bearing bis(pyridine-2,6-dione) based ligands with more coordination sites, distinguishable coordination modes and deprotonated states. In addition, the transformations from the hydrazone form to deprotonated hydrazone form and deprotonated azo form have been observed before and after metal-ion complexation. pH-controlled syntheses and pH-induced conversion have been achieved for a pair of alkali-earth metal Ca-II complexes, which can be verified by (1) their different colors and crystalline forms in syntheses; (2) different chemical shifts of pyridine methyl groups in the H-1 NMR spectra; (3) the presence of an isosbestic point during the pH-titration experiments; (4) comparisons on the coordination bond lengths and dihedral angles between the two pyridine rings for related complexes. It is very interesting to mention that regioselective complexation has been found for bis(pyridine-2,6-dione) based ligands, which could be explained by the competition between coordination abilities of metal ions and strength of intramolecular hydrogen-bonding ring involving the central hydrazone proton.