Copper/Zinc-Directed Heterometallic Uranyl-Organic Polycatenating Frameworks: Synthesis, Characterization, and Anion-Dependent Structural Regulation

By employing a multidentate ligand, 2,2′-bipyridine-5,5′-dicarboxylic acid (H₂bpdc), with both O-donors and N-donors, five uranyl-Cu­(II)/Zn­(II) heterometallic coordination polymers, (UO₂)­Cu­(μ₄-bpdc)­(μ₃-bpdc) (1-Cu), (UO₂)­Zn­(μ₄-bpdc)­(μ₃-bpdc) (1′-Zn), (UO₂)­CuCl­(μ₃-bpdc)­(μ₂-Hbpdc)­(H₂O) (2-Cu), (UO₂)₂Cu₂Cl₂­(μ₃-bpdc)₂­(μ₂-Hbpdc)₂­(H₂O)₃·2H₂O (2-Cu′), and (UO₂)₂Zn­(μ₃-SO₄)­(μ₄-bpdc)­(μ₃-bpdc)­(H₂O)₃ (3-Zn), were prepared under hydrothermal conditions. Thermal stability and luminescent properties of 1-Cu, 1′-Zn, 2-Cu, and 3-Zn were also investigated. Isostructural compounds 1-Cu and 1′-Zn both have a three-dimensional (3D) framework built by polycatenating of two sets of paralleling two-dimensional (2D) grids with octahedral transition metal cations (Cu or Zn) as the cross-linking nodes. As far as we know, compounds 1-Cu and 1′-Zn are the first two cases that possess polycatenated networks in heterometallic uranyl-organic coordination polymers. Compound 2-Cu contains 3-fold interpenetrated 2D networks which are built by the connection of [(UO₂)₂(bpdc)₂­(Hbpdc)₂]^2– secondary building units and Cu­(II). A one-dimensional tilted ladder-like structure in 2-Cu′ is constructed by uranyl-bpdc chains connected by Cu­(II) and Hbpdc^–. Compound 3-Zn displays a layered-like 2D network contain an unusual [(UO₂)₂Zn­(μ₃-SO₄)] unit. Interestingly, different anions could lead to the change of coordination sites of transition metal cations, resulting in structural diversity of heterometallic uranyl-organic frameworks.