Mutual transformation between crystalline phases and dielectric properties of coordination polymers with the formula [Cd(N-methylimidazole)2(H2O)x(glutarate)]·nH2O (x = 0 or 1; n = 0 or 4)

Abstract

We have successfully achieved crystals of two pseudo-polymorphs, Cd(NMIZ)₂(glutarate) (1) and [Cd(NMIZ)₂(H₂O)(glutarate)]·4H₂O (2) where NMIZ represents N-methylimidazole, through controlling the crystallization temperature. The Cd²⁺ ions form a distorted trigonal prism coordination geometry with two nitrogen atoms from two NMIZ ligands and four oxygen atoms from two carboxylates in 1, while in 2 they form a pentagonal bipyramid coordination geometry with two nitrogen atoms from two NMIZ ligands, four oxygen atoms from two carboxylates and one oxygen atom from coordinated water. The glutarates acted as μ₂-bridging ligands, connecting the coordination polyhedra to form coordination polymeric chains in both 1 and 2. The coordination polymeric chains are arranged in parallel arrays and held together via intermolecular van der Waals force in a three-dimensional structure in 1, whereas two-dimensional bi-layers are observed in 2, where the lattice water molecules locate between bi-layers and are archored to the coordination polymeric chain via intermolecular H-bonds. The crystalline phase transformation between 1 and 2 is reversible, as well as the transformation between 2 and 3. The three different crystalline phases show distinct dielectric features. The dielectric permittivity is related to the amount of water in the lattice with the order 2 > 3 > 1. This is because the orientation of polar water molecules in the crystal is able to adjust to the change in the external electric field and transfer the polarization.