Chiral Silver–Lanthanide Metal–Organic Frameworks Comprised of One-Dimensional Triple Right-Handed Helical Chains Based on [Ln₇(μ₃-OH)₈]¹³⁺ Clusters

Abstract

Three new isostructural chiral silver-lanthanide heterometal-organic frameworks [Ag₃Ln₇(μ₃-OH)₈(bpdc)₆(NO₃)₃(H₂O)₆](NO₃)·2H₂O [Ln = Eu (1), Tb (2, Sm (3); H₂bpdc = 2,2'-bipyridine-3,3'-dicarboxylic acid] based on heptanuclear lanthanide clusters [Ln₇(μ₃-OH)₈]¹³⁺ comprised of one-dimensional triple right-handed helical chains were hydrothermally synthesized. Various means such as UV-vis spectroscopy, IR spectroscopy, elemental analysis, powder X-ray diffraction, and thermogravimetric/differential thermal analysis were used to characterize the compounds, wherein compound 3 was crystallographically characterized. In the structure of compound 3, eight μ₃-OH^- groups link seven Sm³⁺ ions, forming a heptanuclear cluster, [Sm₇(μ₃-OH)₈]¹³⁺, and the adjacent [Sm₇(μ₃-OH)₈]¹³⁺ clusters are linked by the carboxylic groups of bpdc^2- ligands, leading to the formation of a one-dimensional triple right-handed helical chain. The adjacent triple right-handed helical chains are further joined together by coordinating the pyridyl N atoms of the bpdc^2- ligands with Ag⁺, resulting in a chiral three-dimensional silver(I)-lanthanide(III) heterometal-organic framework with one-dimensional channels wherein NO₃^- anions and crystal lattice H₂O molecules are trapped. The compounds were studied systematically with respect to their photoluminescence properties and energy-transfer mechanism, and it was found that H₂bpdc (the energy level for the triplet states of the ligand H₂bpdc is 21505 cm^-1) can sensitize Eu³⁺ luminescence more effectively than Tb³⁺ and Sm³⁺ luminescence because of effective energy transfer from bpdc^2- to Eu³⁺ under excitation in compound 1.