Cadmium(II) coordination polymers based on substituted malonic acid: Synthesis, characterization and photoluminescence properties

Four novel complexes of cadmium(ii) with alkyl/aryl-substituted malonate ligands of formulae {[Cd(Memal)(H₂O)]₆·0.5H₂O}_n (1), [Cd(Etmal)(H₂O)]_n (2), [Cd(Butmal)(H₂O)]_n (3) and [Cd(Bzmal)(H₂O)]_n (4), (Memal = methylmalonate, Etmal = ethylmalonate, Butmal = butylmalonate and Bzmal = benzylmalonate) have been prepared and characterized by single crystal X-ray analysis. Their luminescence, UV-Vis absorption properties and thermal behaviour were also investigated. Complex 1 is a three-dimensional compound where each metal centre is connected to four other ones leading to a sodalite network with the point symbol {4²·6⁴}. Each cadmium(ii) ion in 1 is seven-coordinate with a water molecule and six oxygen atoms from three Memal ligands building a somewhat distorted pentagonal bipyramidal surrounding. The Memal ligand in 1 adopts a μ₃-κO:κ²O′:κ²O′′:κO′′′ coordination mode. 2-4 are isomorphous compounds with a two-dimensional structure where centrosymmetric double oxo(carboxylate-substituted malonate)-bridged dicadmium(ii) units are linked to four other dinuclear entities through anti-anti and anti-syn carboxylate bridges to form a Shubnikov tetragonal layered [4⁴6²]-sql net. Each cadmium(ii) ion in 2-4 is six-coordinate in a distorted octahedral environment with four carboxylate-oxygen atoms building the basal plane and a water molecule plus another carboxylate-oxygen atom in the axial positions. The substituted malonate ligands in 2-4 exhibit the μ₄-κO:κ²O′:κ²O′′:κO′′′ coordination mode. The tests of the emission spectra of 1-4 show that the length of the side chain influences the photoluminescence properties of the complexes. Finally, the decays of the luminescence of 1-4 and the respective free substituted malonic acids reveal that the same value of the average life time is observed for 1 (H₂Memal), whereas in the case of 2-4, they are greater than that of 1 and those of their corresponding acids. The longer decay rates in 2-4 are consistent with the lack of water molecules in the environment of the cadmium(ii) ion in these complexes, indicating that the sheet-like structure of 2-4 provides a chemical surrounding with less non-radiative relaxation processes.