Structural and theoretical study of π-stacking interactions in new complexes based on CuCl2 and 3-sulfonamide-substituted imidazo[2,1-b]thiazoles

CONTEXT

At present, sulfonamides and their metal complexes have received a new impetus for development. Of particular interest is the study of molecular and crystal structures, which takes into account weak non-valent interactions. Despite the low energy of such interactions, in many cases, they act collectively, and the sum of their actions can play a significant role. As a result, the spectrum of medical and biological activity of new metal complexes is expanded. In this regard, the synthesis and study of the molecular and crystal structure of sulfonamides and their metal complexes is of undoubted relevance. In this work, we studied non-valent intra- and intermolecular interactions in ligands of sulfonamide-substituted imidazo[2,1-b]thiazoles and their previously unknown complexes with CuCl₂. The performed analysis of the data obtained by X-ray diffraction analysis made it possible to establish the intramolecular π-stacking interaction in imidazothiazole ligands, which is retained in their complexes with CuCl₂. Within the framework of QTAIM topological analysis of electron density and DORI analysis, stereoelectronic and topological structures were studied. In the complexes, tetral, chalcogen, and pnycogen new interligand non-valent interactions were established. The energies of all established types of non-valent interactions have been calculated, and their comparative evaluation has been made.

METHODS

X-ray data of new arylsulfonylamino-substituted derivatives of imidazo[2,1-b]thiazoles and their metal complexes with CuCl₂ have been studied. To determine the theoretical prerequisites for the occurrence of π-stacking in the molecules under study, the QTAIM method was used in the framework of the DFT/B3LYP/6-311 + G(d) calculation using the GAUSSIAN 09 program. In addition, the DORI electron density region overlap indicator and the Multiwfn program were used to analyze non-valent interactions.

Characteristics of 2-chloropyridine and thiourea condensation. Structure of the as-formed products and their effect on coating properties during electrochemical nickel plating

The synthesis and spectral characterisation of 2-pyridylisothiuronium chloride were performed by regulating the rate of feeding 2-chloropyridine into a thiourea solution in ethyl alcohol for ensuring its low concentration in the reaction zone. According to the data of NMR spectroscopy (1H,13С,15N), the obtained compound represents an approximately equimolar mixture of two tautomers: the expected isothiuronium salt and pyridinium chloride with an isothiocarbamide substituting group in the 2nd position. The ability of isothiuronium salt to transit tautomerically to pyridinium salt is determined by the presence of two main centres, including nitrogen atoms of the isothiourea unit and a nitrogen atom of the pyridine ring. A quantum chemical analysis performed using the DFT method showed that the free energy values of the tautomers were similar, with the tautomer protonated on the nitrogen imido-atom being 2.9 (in the gas phase) and 4.7 kcal/mole (taking into account the dimethyl sulphoxide solvent DMSO at the PCM level) more advantageous as compared to the pyridinium salt. A small difference in the tautomer energies determines their formation in an approximately equimolar quantity. A rapid addition (5–10 mL/min) of 2-chloropyridine to the thiourea solution in the reaction zone creates the surplus of the reagent, acting as a base and causing splitting of the isotiuronium salt. This leads to an additional formation of bis(2-pyridyl)sulphide in the reaction medium, representing a valuable ligand for obtaining coordination compounds. The synthesised mixture of tautomers was examined as an additive to the standard nickel-plating electrolyte. In the concentration of 0.3–0.5 g/L, this additive ensured the production of bright low-porous nickel coatings at a sufficiently high current density of 5–10 A/dm2 and a current yield of 98–99 %.