Clathration of Two-Dimensional Coordination Polymers: Synthesis and Structures of [M(4,4‘-bpy)2(H2O)2](ClO4)2·(2,4‘-bpy)2·H2O and [Cu(4,4‘-bpy)2(H2O)2](ClO4)4·(4,4‘-H2Bpy) (M = CdII, ZnII and bpy = Bipyridine)

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Abstract
In the presence of guest 2,4‘-bpy molecules or under acidic conditions, three compounds, [Cd(4,4‘-bpy)2(H2O)2](ClO4)2·(2,4‘-bpy)2·H2O (1), [Zn(4,4‘-bpy)2(H2O)2](ClO4)2·(2,4‘-bpy)2·H2O (2), and [Cu(4,4‘-bpy)2(H2O)2](ClO4)4·(4,4‘-H2bpy) (3), were obtained from the reactions of the metal salts and 4,4‘-bpy in an EtOH−H2O mixture. 1 has a 2-D square-grid network structure, crystallizing in the monoclinic space group P2/n, with a = 13.231(3) Å, b = 11.669(2) Å, c = 15.019(3) Å, β = 112.82(3)°, Z = 2; 2 is isomorphous with 1, crystallizing in the monoclinic space group P2/n, with a = 13.150(3) Å, b = 11.368(2) Å, c = 14.745(3) Å, β = 110.60(3)°, Z = 2. The square grids superpose on each other into a channel structure, in which each layer consists of two pairs of shared edges, perfectly square-planar with an MII ion and a 4,4‘-bpy at each corner and side, respectively. The square cavity has dimensions of 11.669(2) × 11.788(2) and 11.368(2) × 11.488(2) Å for 1 and 2, respectively. Every two guest 2,4‘-bpy molecules are clathrated in each hydrophobic host cavity and are further stabilized by π−π stacking and hydrogen bonding interactions. The NMR spectra clearly confirm that both 1 and 2 contain 4,4‘-bpy and 2,4‘-bpy molecules in a 1:1 ratio, which have stacking interaction with each other in the solution. 3 crystallizes in the orthorhombic space group Ibam, with a = 11.1283(5) Å, b = 15.5927(8) Å, c = 22.3178(11) Å, Z = 4. 3 is made up of two-dimensional square [Cu4(4,4‘-bpy)4] grids, where the square cavity has dimensions of 11.13 × 11.16 Å. Each [4,4‘-H2bpy]2+ cation is clathrated in a square cavity and stacks with one pair of opposite edges of the host square cavity in an offset fashion with the face-to-face distance of ca. 3.95 Å. Within each cavity, the [4,4‘-H2bpy]2+ cation forms twin three-center hydrogen bonds with two pairs of ClO4- anions. The results suggest that the guest 2,4‘-bpy molecules and protonated [4,4‘-H2bpy]2+ cations present in the reaction systems serve as structure-directing templates in the formation of the crystal structures and exclude self-inclusion of the networks having larger square cavities.