Synthesis, structures, thermal decomposition mechanism, and luminescence properties of Sm(iii), Pr(iii), Nd(iii), and La(iii) complexes with diverse carboxylate coordination modes

Abstract

A series of four novel lanthanide complexes was obtained: [Ln(3-MBA)₃(5,5′-DM-2,2′-bipy)]₂·(H₂O) (Ln = Sm (1), Pr (2), Nd (3)) and [La(3-MBA)₄(5,5′-DM-2,2′-bipy)]₂ (4) (3-MBA = 3-methylbenzoate; 5,5′-DM-2,2′-bipy = 5,5′-dimethyl-2,2′-bipyridine). Single-crystal X-ray diffraction structural studies established that complexes 1–3 crystallize in the triclinic system with the space group P and the metal center adopts a nine coordinate contiquration. In contrast, complex 4 crystallizes in the monoclinic with space group P2₁/n. Detailed crystallographic data indicate that in complexes 1–3, the 3-MBA ligands bind to the Ln³⁺ through diverse coordination modes, specifically exhibiting chelating bidentate, bridging bidentate, and bridging tridentate modes. Surprisingly, there are eight 3-MBA ligands in the structure of complex 4, which adopt monodentate, bridging bidentate, and bridging tridentate coordination modes, which is very rare in the study of similar complex structures. A suite of complementary techniques, including elemental analysis, IR and Raman spectroscopy, and PXRD, was employed to characterize the complexes. The thermal decomposition behavior of all four complexes was studied using a coupled simultaneous TG-DSC/FTIR/MS system. The results indicate that the complexes initially lose crystal water or neutral ligand molecules at relatively low temperatures, followed by the stepwise elimination of organic carboxylate ligands at higher temperatures. The final thermal decomposition products were identified as the corresponding lanthanide metal oxides. Additionally, the fluorescence spectrum of complex 1 was measured. DFT calculations reveal an energy transfer mechanism between the triplet states of both ligands and the Sm³⁺.