FUNDAMENTAL ASPECTS OF THE COORDINATION CHEMISTRY OF TRANSITION METALS WITH FUNCTIONALLY SUBSTITUTED THIOAMIDES (PART 1)
Authors: Orysyk Svitlana , Pekhnyo Vasyl , Orysyk Viktor , Zborovskii Yuri , Borovyk Polina , Mykhailo Vovk 
Published: April 2022

The influence of competitive coordination, a tautomeric form of functionally substituted thioamides, conditions of synthesis and nature of the metal on the course of the reaction and structure of mono-, bi, and polynuclear complexes of 3d, 4d-metals is considered based on results obtained in the Department of "Chemistry of Complex Compounds" of the V.I. Vernadsky Institute of General and Inorganic Chemistry NAS of Ukraine, together with the staff of the Department of “Chemistry of Heterocyclic Compounds” of the Institute of Organic Chemistry NAS of Ukraine.

The influence of ligand denticity, as well as conditions of complex formation on the structure of obtained complexes and their polymorphic modifications, was studied based on the reaction of d-metals with functionally substituted N, S- and O, N, S-containing thioamides. In addition, it is proved the influence of tautomeric forms of thioamides on the stereochemistry of coordination polyhedra: it is found that the thionic tautomeric form promotes the transposition of thioureas, while the thiol form promotes its cis-position in the square-planar of a polyhedron of 3d, 4d-metals in the structure of complexes. However, it was found that the thion tautomeric form leads to the formation of octahedral, while the thiol form to the square-planar of coordination nodes in complexes of Cu(II) and Ni(II), which are characterized by a change in coordination polyhedra (from square-planar and tetrahedron to octahedron) that depending on the strength of the ligand field. It is obvious that this effect of tautomeric forms of thioamides is associated with the formation of a conjugate system of double bonds in their molecules. In this case, the transition of thioamide to thiol form depends on the pH and the nature of the organic solvent: in a weakly alkaline medium or polar organic solvent (pyridine, chloroform) there is a shift of equilibrium towards to the dominance of thiol tautomeric form.

It was found that the thionic tautomeric form of thioamides (depending on pH and substituent composition) reacts with metal salts mainly in neutral form or in the monoanionic form, forming complexes of molecular or ionic nature, while thiol form reacts in the form of dianion, forming complexes preferably anionic type. Ionic compounds are usually soluble or sparingly soluble in water in low concentrations (10-3–10-5 mol/l), while compounds of the molecular type are soluble only in DMSO and DMF.

It is shown that the stereoselective synthesis of various ligand complexes is carried out mainly in three ways: 1) by the interaction of the initial components in the corresponding stoichiometry. In this case, the vacancy in the metal environment is occupied by either the anions of the starting metal salt (Hal-, SO42-, NO3-, CH3COO-, etc.) or other organic molecules (triphenylphosphine, pyridine, etc.); 2) carrying out parallel reactions (hydrolysis and oxidation of thioureas), which lead to participation in the coordination of by-products of the reaction; 3) carrying out reactions with intraligand rearrangements, which leads to the cyclization of organic ligands and coordination of the products of their transformation to the central metal ion. However, it was found that hydrolysis / oxidation or intraligand cyclization of substituted polydentate thioamides can occur both under the action of synthesis conditions and under the action of complexing metals as promoters of organic reactions.

It was found that depending on the temperature and time of interaction of the starting reagents, different polymorphic modifications of complexes (triclinic or monoclinic) are formed, which differ in packing density and the nature of intermolecular interactions. As a result, such polymorphic modifications have different solubilities in water, which is important for the controlled synthesis of appropriate structures and their practical application.