Synthesis, X-ray structure, spectroscopic (IR, NMR) analysis and DFT modeling of a new polymeric Zinc(II) complex of cystamine, [Zn(Cym-Cym)Cl₂]_n

A novel polymeric Zn–cystamine complex, [Zn(Cym-Cym)Cl2]_n(1) was obtained by the reaction of ZnCl₂and the disulfide bridged cystamine dihydrochloride ligand (Cym-Cym·2HCl) in the presence of 1,2-diaminoethane (en). A similar reaction of the corresponding monomeric thiol, cysteamine hydrochloride (CymH·HCl) yielded a tetrameric cyclic complex, [Zn₄Cym₄Cl₄] (2). The X-ray diffraction analysis revealed that each zinc atom in 1 is coordinated by two nitrogen atoms of the bridging Cym-Cym ligands and two chloride ions, which complete the tetrahedral environment of Zn(II). During the reaction the cystamine ligand retained the disulfide linkage and the sulfur atoms remained uncoordinated. Combined experimental and theoretical IR,¹H, and¹³C NMR spectroscopic studies of the isolated ligands and their complexes, 1 and 2 were applied to derive structure-spectra correlation for the complexes. The assignment of the solid state IR spectra of 1 and 2 was performed by means of periodic DFT modeling and frequency calculations using DFT/PAW-PBE methodology. The vibrational modes sensitive to the N- and S ligand coordination to Zn(II) and to the type of the coordination polyhedron were determined. The C–S stretching vibrations were assumed to be the characteristic modes for prediction of different binding behavior of the CS donor group in complexes 1 and 2. The¹³C NMR data are able to distinguish the ligand N-binding to Zn(II) in 1 and S-binding in 2.