Template synthesis, crystal structures, and optical properties of Er3+, Y3+, Dy3+ and Gd3+ nitrate complexes based on a quinoline–pyridine Schiff base

Five rare-earth nitrate coordination complexes were synthesized through a template condensation reaction between 2-quinolinecarbohydrazide, 2,6-diacetylpyridine, and nitrate salts of Y 3+ , Gd 3+ , Er 3+ and Dy 3+ : [Er(Hpbqh)(NO 3 ) 2 ]·MeOH ( 1 ) [Y(Hpbqh)(NO 3 ) 2 ]·2MeOH ( 2 ), [Y(H 2 pbqh)(H 2 O)(NO 3 ) 2 ]NO 3 ( 3 ), [Dy(H 2 pbqh)(H 2 O)(NO 3 ) 2 ]NO 3 ( 4 ) and [Gd(H 2 pbqh)(NO 3 ) 3 ] ( 5 ). Single-crystal X-ray diffraction revealed that all complexes exhibit mononuclear structures, in which the Schiff base ligand coordinates in a pentadentate ONNNO fashion. In complexes 1 and 2 , the ligand coordinates in the mono-deprotonated Hpbqh − form, whereas in complexes 3 – 5 it remains in the protonated H₂pbqh form. The nitrate anions coordinate to the metal centers in a bidentate chelating mode. Complexes 1 and 2 are isostructural and feature channel-like voids generated upon molecular packing through π–π stacking interactions, which accommodate methanol solvent molecules. Ionic complexes 3 and 4 are also isostructural, they pack together in the crystal through a network of hydrogen bonds. Thermal desolvation of complexes 1 and 2 afforded the corresponding desolvated phases 1d and 2d , while preserving crystallinity, as confirmed by single-crystal X-ray diffraction. Solid-state photoluminescence investigations revealed that complex 3 exhibits the most intense emission, characterized by two blue-shifted bands at 410 and 432 nm compared to the H₂pbqh ligand. Heat treatment at 130 °C causes luminescence quenching for this complex. Complex 5 also displays blue-region emission centered at 418 nm, while complexes 1 , 2 and 4 show weaker emissions in the green spectral region, with maxima at 544 and 565 nm, respectively. Nitrogen adsorption measurements performed on the activated complexes 1d and 2d indicate low porosity, consistent with weak adsorbate–adsorbent interactions. The complexes were further characterized by PXRD, FTIR spectroscopy, UV–Vis absorption, and thermal analysis (TGA/DTA/DSC). • Synthesis of Y(III), Gd(III), Dy(III) and Er(III) nitrate complexes. • Mononuclear complexes with pentadentate Schiff base ligand. • Compounds reveal blue emission. • Ligand-based emission. • Disturbed planarity of organic ligand.