Structural And Electrochemical Properties Of Two Novel Cdx2 (X = Br, I) Picolinamide Complexes

Two novel discrete cadmium(II) complexes, namely [CdBr2(pia)(2)] (1) and [CdI2(pia)(2)] (2) were prepared by reactions of aqueous solutions of CdX2 (X = Br, I) salts with picolinamide (pia) in the 2:1 ligand to metal stoichiometric ratio. Both compounds were characterized by elemental analysis, IR-spectroscopy, TG/DSC analyses and electrochemical methods. The electrochemical characteristics of both ligand (pia) and prepared complexes were studied by cyclic and (cyclic) square-wave voltammetry, on a static mercury drop electrode (SMDE), in aqueous media over a wide pH range. The molecular and crystal structure of the compounds was determined by the single crystal X-ray diffraction method. X-ray structure analysis of 1 and 2 have shown that the compounds are isostructural with minor differences in the bond angles of the coordination sphere. In both compounds the Cd(II) ion is coordinated by two halide atoms and two mutually orthogonal picolinamide ligands that act as N,O-chelators in a distorted octahedral arrangement. In the crystal structure, the molecules of 1 and 2 are primarily linked via strong head-to-head amide hydrogen bond interactions forming dimers. In 1 the adjacent dimers are connected via N-H center dot center dot center dot Br hydrogen bonds and offset face to face pi center dot center dot center dot pi interactions that involve pyridine rings, while in the structure of 2, the dimers are connected via C-H center dot center dot center dot O, C-H center dot center dot center dot N and N-H center dot center dot center dot I hydrogen bonds into the final 3D structure. The intermolecular interactions in both crystal structures were further studied by Hirshfeld surface analysis. Electrochemical analysis of 2-picolinamide indicates the irreversible nature of its electro-reduction reaction on SMDE at pH 2. To provide better insight into the redox mechanism and electrokinetic properties of 2-picolinamide, the study of the effect of signal frequency on CSWV response was carried out, too. The electrochemical reduction of complex 2 involves two electron transfer reactions at -0.55 V and -0.83 V, indicating two redox active centers in the molecule, while complex 1 appears to be apparently electro-inactive in the studied potential range. (C) 2020 Elsevier Ltd. All rights reserved.