Design, synthesis and spectroscopic characterization of mixed N- and O-donor Schiff base ligands (N2O3): Crystal structures of free ligands

Abstract

Five Schiff base ligands containing the same bridging spacer group but differing in the substituents on the phenyl ring were synthesised and fully characterized by various spectroscopic and analytical techniques. Three of these ligands are new and all have been synthesised by an improved, optimised method. The 1H NMR spectra of these ligands were shown to differ from that of their N-donor counterparts (ON3O-type ligands) as confirmed by the systematic use of two-dimensional and variable temperature NMR measurements. The effect of the added substituents were monitored via IR analysis of the ligands and showed that the methoxy substituted ligand, L2 resulted in the highest wavenumber shift of the imine signal whilst the naphthylaldehyde ligand, L3 was observed to cause the lowest wavenumber shift. The crystal structures of four of these ligands showed differing low energy conformations and in addition to all other analytical data confirms the structural analysis of these ligands. All ligands show intramolecular hydrogen bonding while two of the ligands show intermolecular hydrogen bonding as seen in their packing diagrams. Interestingly, the ligand with the naphthylaldehyde group, L3, underwent tautomerism and the quinoid form of the ligand is reported here.