Coordination frameworks constructed from bipyridyl piperazine and MCl ₂ (M = Co, Ni, Zn): Structural characterization and optical properties

Three metal-organic polymers, CoCl₂(bpfp)]_n 1, (NiCl₂(bpfp)₂(H₂O)₃)_n 2 and ZnCl₂(bpfp)_n 3 (bpfp = N,N′-bis(3-pyridylformyl) piperazine), are formed by the self-assembly of the flexible bpfp with MCl ₂ (M = Co, Ni, Zn), respectively. X-Ray single-crystal structural analysis reveals that polymer 1 exhibits a novel grid network, in which the grid is composed of segments of bpfp and cobalt ions. Polymer 2 consists of 2D rhombohedral grids, the dimensions of the grid are 15.782 × 12.434 ² and the diagonal-to-diagonal distances are 13.186 × 25.169 ². In polymer 3, infinite wavelike chains are extended to 2D supramolecular arrays via C-H⋯Cl hydrogen bonds. The third-order nonlinear optical (NLO) behaviors of 1-3 and bpfp were investigated in dilute DMF solution by Z-scan measurement. The results show that 1, 2 and 3 exhibit good third-order NLO properties, which are quite different from bpfp that shows weak NLO behavior. This paper demonstrates that metal ions can strongly influence the crystal structures and third-order NLO properties of polymers.