Structure, reactivity, luminescence and magnetism of dinuclear Ln3+ complexes produced by the Ln3+-assisted hydrolysis of 3,6-bis(2-pyridyl)tetrazine

Twelve Pr3+, Sm3+, Eu3+, Gd3+, Tb3+ and Dy3+ complexes of N-(pyridin-2-ylmethylene)picolinohydrazonate (phzp−) were synthesized by the Ln3+ assisted hydrolysis of 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine. Crystallographic characterization of Pr3+, Sm3+ and Eu3+ hydrazonate complexes with nitrate or trifluoroacetyleacetonate co-ligands shows them to have binuclear structures with a planar Ln2O2 core and the ligand to be in its mono-anionic form. Exception is the Pr3+ complex with nitrate co-ligand, which is a mononuclear species containing the neutral hydrazone. Paramagnetic 1D 1H and 2D gCOSY and gNOESY–gEXSY 1H NMR spectroscopies revealed the presence of different isomers exhibiting a sterically controlled fluxional behavior. In addition, phzp− significantly enhances the luminescence of Eu3+ and Tb3+. Magnetic measurements show antiferromagnetic coupling between the Ln3+ ions in the dinuclear complexes. The structural characterization of the complexes in both solid state and solution show the versatility of phzp− ligand to adopt various ligating motifs.

Graphical abstractPr3+, Sm3+, Eu3+, Gd3+, Tb3+ and Dy3+ complexes of N-(pyridin-2-ylmethylene)picolinohydrazonate were synthesized by the metal ion assisted hydrolysis of 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine. Crystallographic characterization of Pr3+, Sm3+ and Eu3+ hydrazonate complexes with nitrate or trifluoroacetyleacetonate co-ligands shows them to have binuclear structures with a planar Ln2O2 core and the ligand present in its mono-anionic form. Exception is the Pr3+ complex with nitrate co-ligand, which is a mononuclear species containing the neutral hydrazone. The size of the Ln3+ and the co-ligands steric hindrance define the structure of the complexes and the geometric characteristics of the Ln2O2 core. Paramagnetic NMR spectroscopy in solution revealed the presence of isomers exhibiting a sterically controlled fluxional behavior. Preliminary results show hydrazone to enhance the luminescence of Eu3+ and Tb3+, and the two nuclei inside the Ln2O2 core to be antiferromagnetically coupled in the Gd3+ and the Dy3+ complexes.Figure optionsDownload full-size imageDownload as PowerPoint slide