Effect of N,N′-dimethylformamide solvent on structure and thermal properties of lanthanide(III) complexes with flexible biphenyl-4,4′-dioxydiacetic acid

Abstract

The N,N′-dimethylformamide (DMF) was used in the microwave-assisted (MW) solvothermal method for the preparation of lanthanide(III) complexes with biphenyl-4,4′-dioxydiacetic acid (H2L) of the formula Ln2L3·xDMF⋅yH2O where Ln = lanthanide(III) ions from La(III) to Lu(III) with the Pm(III) exception x = 1.5–3; y = 2 for Yb and Lu or 0 for the remaining complexes and L = C14H12O2(COO) 2 2− . The obtained complexes were characterized by the elemental analysis, infrared spectroscopy (ATR-FTIR), X-ray diffraction patterns and different methods (TG-DSC and TG-FTIR) of thermal analysis. Crystalline complexes contain N,N′-dimethylformamide molecules in their structures. Additionally, water molecules are present in the structures of Yb(III) and Lu(III) complexes. Coordination of lanthanide ions occurs through carboxylate oxygen atoms from the biphenyl-4,4′-dioxydiacetate ligand and most probably carbonyl oxygen atom from N,N′-dimethylformamide. The investigated compounds exhibit different thermal stability, and removal of solvent molecules takes place in a more complex way in the group of heavy lanthanide complexes. Their desolvated forms are an unstable [except Yb(Lu)2L3 compounds], and their further heating leads to the formation of suitable lanthanide oxides (air atmosphere). Thermal stability of the investigated compounds is higher in nitrogen atmosphere. The TG-FTIR methods allowed to propose way of thermal decomposition of investigated compounds through identification of volatile products such as: DMF, H2O, CO2, CO and CH4 evolved during heating of complexes. The luminescent properties of terbium(III) and europium(III) complexes were also investigated.