Poly[diaqua(mu-4,4'-bipyridine-kappa2N:N')bis(mu-cyanido-kappa2C:N)bis(cyanido-kappaC)nickel(II)copper(II)]: a metal-organic cyanide-bridged framework.

The structure of the title compound, [NiCu(CN)4(C10H8N2)(H2O)2]n or [{Cu(H2O)2}(mu-C10H8N2)(mu-CN)2{Ni(CN)2}]n, was shown to be a metal-organic cyanide-bridged framework, composed essentially of -Cu-4,4'-bpy-Cu-4,4'-bpy-Cu- chains (4,4'-bpy is 4,4'-bipyridine) linked by Ni(CN)4 anions. Both metal atoms sit on special positions; the Cu(II) atom occupies an inversion center, while the Ni(II) atom of the cyanometallate sits on a twofold axis. The 4,4'-bpy ligand is also situated about a center of symmetry, located at the center of the bridging C-C bond. The scientific impact of this structure lies in the unique manner in which the framework is built up. The arrangement of the -Cu-4,4'-bpy-Cu-4,4'-bpy-Cu- chains, which are mutually perpendicular and non-intersecting, creates large channels running parallel to the c axis. Within these channels, the Ni(CN)4 anions coordinate to successive Cu(II) atoms, forming zigzag -Cu-N[triple-bond]C-Ni-C[triple-bond]N-Cu- chains. In this manner, a three-dimensional framework structure is constructed. To the authors' knowledge, this arrangement has not been observed in any of the many copper(II)-4,4'-bipyridine framework complexes synthesized to date. The coordination environment of the Cu(II) atom is completed by two water molecules. The framework is further strengthened by O-H...N hydrogen bonds involving the water molecules and the symmetry-equivalent nonbridging cyanide N atoms.