A 3D coordination polymer of Cd(II) with conformationally flexible mixed ligands as an active filler for silicone elastomers

The electronic configuration of d(10) metals enables for a wide range of coordination numbers and geometries which, together with the use of mixtures of ligands for their coordination, allows fine-tuning of the properties of the complexes formed. By treating Cd(ClO4)(2) with 1,3-bis(carboxypropyl)tetramethyldisiloxane (H(2)Cx) and 4,4 '-azopyridyne (AzPy) in solution, a 3D coordination polymer, [Cd(Cx)(AzPy)<middle dot>1.85H(2)O](n) (1), was mostly formed. The compound was investigated by elemental and spectral analysis, as well as by powder and single crystal X-ray diffraction. Thermal behavior of compound 1 was evaluated by TGA and DSC, while the moisture behavior was studied by dynamic vapor sorption (DVS) analysis. Compound 1 contains a five-dentate Cx(2-) ligand with Cd atoms in a pentagonal-bipyramidal geometry, as determined by X-ray analysis. The conformational flexibility of the ligands allows the establishment of all possible intermolecular interactions in a very dense packing, the resulting structure having an extremely low free volume (14.8%). The compound is hydrophobic, adsorbing up to 2.4 wt% water vapor, thermally stable up to 160 degrees C, and has a phase transition around 6 degrees C, which, based on analysis by polarized optical microscopy (POM), is considered as a second-order crystal-to-crystal transition, facilitated by the mobility of the siloxane fragment. Due to the constraints imposed by the covalent 3D architecture, the conformational photoisomerization of 1 is extremely limited, as indicated by UV-Vis spectroscopy studies on solutions with solvents of different polarities as well as dielectric spectroscopy studies on silicone composites in which it is in solid state with 5 and 20 wt% loading. Instead, it was found that compound 1 significantly increases the dielectric strength of the resulting materials (up to 46 V/mu m compared to 18 V/mu m in the empty matrix) by incorporating it as a filler, making them suitable for dielectric elastomer generators, as the results of theoretical calculations indicate. In the purification process, a 2D coordination polymer, [Cd(Cx)(AzPy)<middle dot>H(2)Cx](n) (2), was also identified in traces.