Formation and conversion of six temperature-dependent fluorescent ZnII-complexes containing two in situ formed N-rich heterocyclic ligands

Abstract

Six temperature-dependent Zn(II) complexes 1–6 based on the simple Schiff base L¹ from condensation of equivalent 2-pyridine formaldehyde (2-Pfd) and 2-pyridylethylamine (2-Pea) were systematically studied for the first time. These six complexes are: two complexes of ZnL²X₂ (1 and 2) involving in situ formed N-rich heterocyclic ligand L² at 80 °C; two complexes of [Zn₂(cis-L³)X₄·S] (S = H₂O for 3, S = 0 for 5) involving in situ formed azaheterocyclic ligand cis-L³ at 100 °C; and the rest two ones of [[Zn₂(trans-L³)X₄] (4 and 6) involving in situ formed azaheterocyclic ligand trans-L³ at 120 °C (where X = Cl (1, 3, 4) and N₃ (2, 5, 6), L¹ = N-(2-pyridylmethyl)-pyridine-2-carbaldimine, L² = 1-pyridineimidazo-[1,5-a]pyridine, L³ = 1-(1,2-di(pyridin-2-yl)-2-(3-(pyridin-2-yl)H-imidazo-[1,5-a]-pyridin-1-yl)-ethyl)-3-(pyridin-2-yl)H-imidazo-[1,5-a]pyridine). Interestingly, three Cl-based complexes 1, 3, 4 under appropriate conditions can be irreversibly translated into the corresponding N₃-based 2, 5, 6, respectively. The possible formation/conversion mechanism shows that the α-H activation in –CHN–CH₂– moiety of L¹ coordinated to Zn²⁺ ion should be the original driving force for the intermolecular C–C/C–N coupling and ring formation reactions, meanwhile reaction temperature also plays a very important role during the formation/conversion of 1–6. Moreover, the results indicate that complexes 1–6 have good fluorescence properties as potential fluorescent materials.