Assemblies of Two New Metal−Organic Frameworks Constructed from Cd(II) with 2,2‘-Bipyrimidine and Cyclic Oxocarbon Dianions C<i>_n</i>O<i>_n</i>^2- (<i>n</i> = 4, 5)

Two extended networks of Cd(II) with 2,2‘-bipyrimidine (bpym) and cyclic oxocarbon dianions CnOn2- (n = 4, 5) with formulas [Cd(C4O4)(bpym)0.5(H2O)] (1) and [Cd(C5O5)(bpym)0.5(H2O)] (2) have been synthesized and characterized by single-crystal X-ray diffraction studies. Structural determination reveals that, in compounds 1, each Cd center lies in a seven-coordinate environment bonded to two bpym nitrogen atoms and five oxygen donors from four squarate and one water molecules. The squarate acts as a bridging ligand with two different binding modes, a tetramonodentate (μ4) and a bis-bridging (μ4) coordination mode, linking the Cd(II) ions to form a two-dimensional (2D) metal−squarate layer. Adjacent layers are then mutually linked via bridges of bis-bidentate bpym ligands constructing a three-dimensional (3D) triangular metal−organic framework (MOF). In compound 2, each Cd center lies in an eight-coordinate environment bonded to two bpym nitrogen atoms and six oxygen donors from three croconate ligands and one water molecule. Each croconate (C5O52-) adopts a new bis-bidentate/monodentate (μ5) bridging mode and links crystallographically identical Cd ions, forming a one-dimensional (1D) bichain. Adjacent bichains are then mutually linked via the bridges of bis-bidentate bpym ligands constructing a 2D layered MOF, which is then extended to a 3D supramolecular architecture by two intermolecular π−π interactions between the pyrimidyl rings of bpyms and between cyclic five-membered rings of croconates. Both MOFs are thermally stable, as evidenced by thermogravimetric analysis and in-situ powder X-ray diffraction measurements.