Novel Ba²⁺ and Pb²⁺ metal–organic frameworks based on a semi-rigid tetracarboxylic acid: syntheses, structures, topologies and luminescence properties

Multidentate carboxylate ligands have been widely used in the construction of metal–organic frameworks (MOFs) owing to the rich variety of their coordination modes, which can lead to crystalline products with interesting structures and properties. Two new main-group MOFs, namely, poly[[di-μ-aqua-diaqua(dimethylformamide)[μ₇-5,5′-methylenebis(2,4,6-trimethylbenzene-1,3-dicarboxylato)]dibarium(II)] trihydrate], {[Ba₂(C₂₃H₂₀O₈)(C₃H₇NO)(H₂O)₄]·3H₂O}_n or {[Ba₂(BTMIPA)(DMF)(H₂O)₄]·3H₂O}_n (1), and poly[[diaqua[μ₆-5,5′-methylenebis(2,4,6-trimethylbenzene-1,3-dicarboxylato)]dilead(II)] 2.5-hydrate], {[Pb₂(C₂₃H₂₀O₈)(H₂O)₂]·2.5H₂O}_n or {[Pb₂(BTMIPA)(H₂O)₂]·2.5H₂O}_n (2), were prepared by the self-assembly of metal salts with the semi-rigid tetracarboxylic acid ligand 5,5′-methylenebis(2,4,6-trimethylisophthalic acid) (H₄BTMIPA). Both structures were characterized by elemental analysis (EA), single-crystal X-ray diffraction, powder X-ray diffraction (PXRD), IR spectroscopy and thermogravimetric analysis (TGA). Complex 1 reveals a three-dimensional (3D) flu network formed via bridging tetranuclear secondary building units (SBUs) and complex 2 displays a 3D framework with an sqp topology based on one-dimensional metal chains. The BTMIPA⁴⁻ ligands adopt a rare coordination mode in 2, although the ligands in both 1 and 2 are X-shaped. The luminescence properties of both complexes were investigated in the solid state.