Hydrophilic Microporous Lanthanide-Organic Frameworks Based on 4,4'-Biphenyldiacetate: Synthesis, Crystal Structures and Sorption Properties

Abstract

Fifteen microporous coordination polymers of symmetrical and flexible 4,40-biphenyldiacetate (bpda2–) with lanthanide(III) cations (Ln3+) of formula {[Ln2(bpda)3(H2O)] 2H2O}n (Ln = La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8), Dy (9), Ho (10), Er (11), Tm (12), Yb (13), Lu (14), Y (15)), have been synthesized and characterized. Single crystal X-ray diffraction (for 1, 3 and 4) in combination with powder X-ray diffraction showed isostructural compounds with an infinite secondary building unit (SBU). In this SBU, two crystallographically independent lanthanide ions are connected by three carboxylate groups in chelating-bridging mode to zig-zag Ln-carboxylate chains. These chains are assembled by the biphenyl part of the ligand to a 3D network with water molecules in channels with rectangular cross-sections of 1.6 3 Å2. Thermogravimetric analysis indicates complete removal of the water molecules at 150 C and thermal stability up to 380 C. Stability studies on compound 1 show no degeneration in crystallinity after refluxing in water or different organic solvents. The dehydrated compound 1 exhibited no N2 or H2 uptake at 77 K and no CH4 or CO2 uptake at 273 K up to 1 bar. High-pressure (17 bar, 273 K) CO2 uptake of dehydrated [Ln2(bpda)3]n corresponded to the formation of {[Ln2(bpda)3] CO2}n with about 40% of the adsorbed CO2 retained upon desorption. Water vapor sorption of activated and hygroscopic [Ln2(bpda)3]n reformed {[Ln2(bpda)3(H2O)] 2H2O}n. 4,40-Biphenyldiacetic acid (H2bpda) shows blue emission, its europium 6 and terbium 8 compounds show red and weak green luminescence under UV light irradiation, with the bpda2 ligand acting as an antenna ligand for the europium(III) ions.