A hemiaminal–ether structure stabilized by lanthanide complexes with an imidazole-based Schiff base ligand

Abstract

Imidazole-based lanthanide complexes bearing a hemiaminal-ether pendant, [Ln(L′-OR)(NO₃)₂](NO₃)·solvent (Ln = Tb and Gd; L′-OR = hemiaminal-ether ligand, 1,9-bis(1H-imidazole-4-yl)-5-[(R-1H-imidazole-4-yl)alkyl]-2,5,8-triazanon-1,8-dien; R = Me and Et; solvent = MeOH, EtOH, and H₂O; abbreviated as LnL′-OR), were unexpectedly obtained from the reaction of diethylenetriamine with 4-imidazolecarbaldehyde in the presence of lanthanide nitrate in alcohol. These LnL′-OR structures were also obtained by treating dipodal imidazole-based lanthanide complexes [Ln(HL)(NO₃)₂(MeOH)](NO₃)·MeOH (HL = bis{[2-{(imidazole-4-yl)methylidene}amino]ethyl}amine; abbreviated as LnHL) independently with (i) 4-imidazolecarbaldehyde, (ii) HNO₃, or (iii) NO in alcohol. The results indicate that the hemiaminal-ether structure is stabilized by the lanthanide complex. The LnL′-OR structures were confirmed by X-ray analysis and cold electrospray ionization mass spectrometry in solution. The photophysical properties indicate that the L′-OR ligands can sensitize the Tb³⁺ luminescence. A significant spectral change was observed by the ligand component exchange. The exchange of the ligand component of the non-luminescent imidazole-based Tb complex, [Tb(MeL)(NO₃)₂](NO₃) (MeL = bis{[2-{(1-methylimidazole-2-yl)methylidene}amino]ethyl}amine; abbreviated as TbMeL), results in a significant spectral change.