Optimised synthesis and further structural diversity of ytterbium benzene-1,4-dicarboxylate MOFs

Abstract

The optimisation of the crystallisation of the hydrothermally-stable metal–organic framework Yb₆-MOF (Yb₆(BDC)₇(OH)₄(H₂O)₄) to provide a reproducible one-step synthesis is achieved by use of the sodium salt of benzene-1,4-dicarboxylate (Na₂BDC) as ligand precursor and control of pH with aqueous NaOH at 190 °C over 3 days. Phase purity is confirmed using powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA). During exploration of synthesis conditions from the same set of chemical reagents, three further ytterbium benzene-1,4-dicarboxylates have been isolated and structurally characterised using single-crystal X-ray diffraction, with phase purity assessed by PXRD and TGA. UOW-3 (Yb₂(H₂O)₆(BDC)₃) crystallises by lowering pH, and has a relatively dense three-dimensionally connected structure with no Yb–O–Yb linkages but dimers of Yb bridged by BDC linkers lying in the ab plane with a pseudo, pillared-layered structure, where BDC connects along c. UOW-4 (Yb₄(BDC)₆(H₂O)₆) forms under the same chemical conditions but upon lowering the temperature to 100 °C, and this material again contains no Yb–O–Yb linkages, but chains of BDC-bridged Yb centres cross-linked to give a dense three-dimensional structure. Upon increasing pH of the synthesis mixture, the material UOW-5 forms, Yb₅O(OH)₈(BDC)₂(HBDC), consisting of dense inorganic layers of ytterbium oxyhydroxide, cross linked by BDC and HBDC pillars. The formulation is supported by infrared spectroscopy, which provides evidence for the HBDC monoanion, and also the presence of a short O–O distance indicative of hydrogen bonding between a carboxylate OH and an oxide anion of the inorganic layer. UOW-3 and UOW-4 both convert to Yb₆-MOF upon heating in water above their synthesis temperature, whereas UOW-5 is hydrothermally stable at 240 °C. The structures of the new materials are discussed in terms of ligand binding modes, and connectivity of metal centres, with comparison to other reported Yb-BDC phases in order to relate structural chemistry to their synthesis conditions and the hydrothermal stability of the materials.