Relations between Structural and Luminescence Properties of Novel Lanthanide Nitrate Complexes with Bis-phosphoramidate Ligands.

Five new bisphosphoramide-based LnIII nitrate complexes [La2(NO3)6L3I] n (1), [Ce2(NO3)6L3I] n (2), [Sm2(NO3)6L3II] n (3), Sm2(NO3)6L3III (4), and Er(NO3)3L2III (5) [LI = piperazine-1,4-diylbis(diphenyl phosphine oxide), LII = N, N'-(ethane-1,2-diyl)bis( N-methyl- P, P-diphenylphosphinic amide, and LIII = N, N'-(ethane-1,2-diyl)bis( P, P-diphenylphosphinic amide)] have been synthesized and characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis (TGA), and single crystal X-ray and powder diffractions. The results of the X-ray diffraction analysis revealed the new polymorph of LIII, and the structural diversity of the synthesized complexes in the solid state. Complexes 1-3 display two-dimensional coordination polymers (2D-CP), containing layers with honeycomb (6, 3) topology. In these 2D-CPs, each Ln center (La, Ce, and Sm in 1, 2, and 3, respectively) could be considered as a triconnected node, linked by three bridging bisphosphoramide ligands as two-connecting linkers. In contrast, 4 is a discrete binuclear complex, in which bidentate LIII ligand has two entirely different conformations: the syn chelating and the anti bridging. Cationic complex 5 shows the monomeric structure, where bidentate LIII adopts the syn-chelating conformation. A comprehensive luminescence investigation has been performed on free ligands and their corresponding complexes as well. The synthesized compounds display a variety of luminescence behavior, including the ligand-centered fluorescence in 1, 2, and 5, two distinct emission peaks in 1 and 2, characteristic Sm-centered f-f emission in 3 and 4, and excitation-dependent emission in LIII, 1, and 2. Furthermore, the time-dependent density functional theory (TD-DFT) study was carried out on the reported compounds to understand the nature of the emission peaks and the observed luminescence properties. The solid-state emission quantum yields of lanthanide complexes were also determined at different excitation wavelengths.