Multi-functional metal–organic frameworks assembled from a tripodal organic linker

Abstract

The reaction between (benzene-1,3,5-triyltris(methylene))triphosphonic acid (H₆bmt) and lanthanide chlorides, under typical hydrothermal conditions (180 °C for 3 days) or using microwave heating (5 minutes above 150 °C), led to the isolation of an isotypical series of compounds formulated as [Ln₂(H₃bmt)₂(H₂O)₂]·H₂O [where Ln³⁺ = La³⁺ (1), Ce³⁺ (2), Pr³⁺ (3), Nd³⁺ (4), (La_0.95Eu_0.05)³⁺ (5) and (La_0.95Tb_0.05)³⁺ (6)]. Compounds 1 to 4 have been readily isolated as large single-crystals and their structures determined in the monoclinic C2/c space group using single-crystal X-ray diffraction. All compounds were thoroughly characterized in the solid-state using powder X-ray diffraction, FT-IR spectroscopy, thermogravimetry, scanning electron microscopy (SEM and EDS) and elemental analysis. Solid-state NMR (³¹P MAS and ¹³C{¹H} CP MAS) and thermodiffractometry studies have been performed on the La³⁺-based material. [Ln₂(H₃bmt)₂(H₂O)₂]·H₂O were found to be three-dimensional frameworks with water molecules (both of crystallization and coordinated to the lanthanide center), which could be reversibly removed by either heating the materials or by applying high vacuum. This typical zeolitic behaviour was confirmed experimentally by determining the crystal structure of the evacuated La³⁺-based material (1-dehyd) using single-crystal X-ray diffraction. This series of materials was found to exhibit dual functionality: photoluminescence and catalytic activity. Small amounts (5%) of Eu³⁺ and Tb³⁺ cations were engineered into the La³⁺-based matrices, promoting the isolation of optically active materials. The H_6−xbmt^x− residues were found to be good sensitizers of Tb³⁺, with 6 having the remarkable absolute emission quantum yield of ca. 46% (at 280 nm excitation). The zeolitic properties of the Eu³⁺-based material allowed an increase of the quantum efficiency from ca. 15% to 54% by removing under vacuum all water molecules in the material. Based on studies of the La³⁺-based material, these compounds can be employed as effective heterogeneous catalysts in the ring-opening reaction of styrene oxide with methanol, showing excellent regioselectivity, recyclability and structural stability in consecutive catalytic runs.