Synthesis, Structures, and Luminescent and Magnetic Properties of Ln−Ag Heterometal−Organic Frameworks

Abstract

A series of Ln−Ag heterometal−organic frameworks based on 4-hydroxylpyridine-2,6-dicarboxylic acid (H3CAM) with formulas {LaAg2(CAM)(HCAM)(H2O)2}n (1), {LnAg(HCAM)2(H2O)3}n (Ln = Pr, 2; Nd, 3; Sm, 4; Eu, 5), and {LnAg3(CAM)2(H2O)}n (Ln = Gd, 6; Tb, 7; Dy, 8; Tm, 9; Yb, 10), have been synthesized with the hydrothermal reaction of Ln(OH)3, Ag2O, and H3CAM at 160 °C. The single-crystal X-ray diffraction analyses reveal that three kinds of structures are exclusively governed by the size of lanthanide ions and the progression of structures is mainly ascribed to the lanthanide contraction effect. Compound 1 consists of a 3D network with an α-polonium-like Ag+-homometallic net and helical La3+ chain. Compounds 2−5 display a 2D honeycomb-like structure with 18-membered Ln3Ag3O12 motifs, and compounds 6−10 can be described as a sandwich-like 3D framework built of a 3D Ag+-homometallic net and 2D Ln3+−44 layer. In 4 (Sm), 5 (Eu), 7 (Tb), and 8 (Dy) samples, the efficient energy transfer from CAM to Ln(III) ions was observed, which results in the typical intense emissions of corresponding Ln(III) ions in the visible region, and the strongest emissions are 4G5/2 → 6H7/2 (602 nm), 5D0 → 7F2 (614 nm), 5D4 → 7F5 (548 nm), and 4F9/2 → 6H13/2 (576 nm) transitions. Variable-temperature magnetic susceptibility measurements of 6−10 show that the ferromagnetic interaction between gadolinium(III) ions appears in 6, whereas the μeff values of 7−10 smoothly decrease on cooling. For the orbital contribution of Ln(III) ions, it is very difficult to determine the intrinsic magnetic interactions between Ln(III) ions