Studies of metal complexes with organic ligands at Kazan Federal University were initiated back in the 1970s by Professor Andrey Popel. Current research, headed by Associate Professor Tamara Bychkova and Associate Professor Valery Shtyrlin, concentrates on complex compounds of transition metals in aqueous and aqueous-organic solutions. So far, data has been obtained about the composition and stability of complexes of cobalt (II), nickel (II), copper (II) and vanadium oxide (IV) with mono-, di- and tetrahydrazides of various nature (aliphatic, aromatic, and macrocyclic) in aqueous, aqueous-dimethylsulfide, aqueos-dimethylformamide, aqueous-ethanol, and other media. Special software tools created at the Department of Inorganic Chemistry - CPESSP and STALABS - are used to calculate equilibrium parameters in solutions. In some cases, based on the analysis of stability constants and kinetic characteristics of complex formation, conjectures have been put forth about the structure of complexes; lately, quantum chemistry apparatus is widely used for this purpose.
The article presents the results of a study of complex formation processes in aqueous solutions containing copper (II) as a complexing agent and two ligands - dihydrazide (malonic or adipic) acid and amino acid - L-histidine. Heteroligand complexes of 1: 1: 1 composition with the zwitterionic form of L-histidine and adipic or malonic acid dihydrazide in neutral form were found. Despite the pH restrictions created by the solubility of the ligands (hydrazides are salted out from solution above pH 5), a heteroligand complex with adipic acid and the anionic form of L-histidine was also detected. The stability of the first two heteroligand complexes is consistent with statistics (we always carry out a statistical analysis of equilibrium constants, guided by the generally accepted approach), but for the latter complex there is some extrastabilization. In the general case, this means that the presence of one ligand in the internal sphere of the complexing ion favors the entry of another, that is, one ligand "positively" affects the other. The next question is what is this influence? Unable to isolate the complexes unchanged from the solution, we performed quantum-chemical calculations of the structures. From these calculations, several interesting conclusions follow: 1) copper (II) ions in heteroligand complexes with the indicated ligands are pentacoordinated (a discussion over the coordination number of copper (II) ions in water has been going on for many years), 2) a chain is realized in some isomeric forms of complexes hydrogen bonds between the protonated imidazole group of histidine and the oxygen atom of the hydrazide fragment through an axially coordinated water molecule; 3) the cis-structure of heteroligand complexes with malonic or adipic dihydrazide acid and negatively charged L-histidine are energetically more beneficial than the corresponding trans-structure. This fact is explained by the manifestation of trans-influence in copper (II) complexes, in which the strongest trans-agents (NH2 groups in this case) avoid being located on the same coordinate. Such an effect was previously established for homoligand complexes of copper (II) with L / DL-histidine. The manifestation of transfusion in heteroligand complexes of copper (II) with histidine and dicarboxylic acid hydrazides was established for the first time in this work.
Information on the presence of trans-influence as a type of stereoselectivity, which manifests itself in the formation of copper (II) complex compounds with malonic and adipic acid dihydrazides and L-histidine, contributes to a deeper understanding of the properties of coordination bonds, structural features of complex compounds in general and their existence in solutions in particular.
"The material accumulated to date in coordination chemistry allows us to speak about many aspects of the complex formation of transition metals with confidence, but there are still many unsolved problems. We plan to continue research on heteroligand complex compounds of essential 3d metals with biologically significant organic ligands," explains co-author Nikita Troshanin. Such complexes are interesting both from the point of view of fundamental chemistry (they can serve as models of biological systems, for example, compounds of proteins with metals), and for solving a number of practical problems (primarily in pharmacology). The literature describes many examples of how the biologically active properties of organic substances, mainly bacteriostatic, are enhanced by complex formation. However, not all complexes formed in solution can be isolated in solid form; therefore, biological tests are either impossible or impractical. "We hope that someday we will be able to get at least a few crystals of our heteroligand complexes, and then they will be studied comprehensively," concludes Mr. Troshanin.