Synthesis, crystal structure and third-order nonlinear optical properties of a novel coordination polymer [Cd(en)(NO₃)₂(4,4′-bpy)]_n containing three kinds of ligands (en=ethylenediamine)

In this paper a new Cd-4,4′-bpy co-ordination framework formulated as [Cd(en)(NO₃)₂(4,4′-bpy)]_n has been prepared and characterized by single crystal X-ray diffraction. The polymer exhibits a zigzag chain structure bridged by 4,4′-bpy. Cd atoms are in a distorted octahedronal coordination environment, binding to two NO₃^-, one μ₂-en and two 4,4′-bpy. In the solid state structure, all the zigzag chains are connected by two kinds of N-H⋯O hydrogen bonds to lead to a three-dimensional network. The polymer also exhibits strong NLO refraction behavior, its n₂ value is 6×10^-18 m² W^-1 in a 2.2×10^-4 mol dm^-3 DMF solution, and gives self-focusing effect. The susceptibilities χ⁽³⁾ and the hyperpolarizability |γ| value were calculated to be 1.5×10^-11 esu and 1.7×10^-29 esu, respectively. The |γ| is comparable to those of the known cluster compounds, and better than those observed in other compounds, such as ferrocene derivatives, Group 10 metal alkynyl polymers and some octopolar alkynylruthenium complexes.