The azomethine group is the common structural feature of SBs, where the substituents can be alkyl, cycloalkyl, aryl, or heterocyclic groups. The carbon atom of the C=Nimine bond is prone to nucleophilic addition, while the nitrogen atom possesses a highly reactive free electron pair that can form stable complexes with metal ions. SBs are among the most widely used organic compounds, showing a wide range of applications as intermediates in organic synthesis, chemosensors, and polymeric stabilizers, in the food, dye, and pigment industry, as well as as catalysts and, in recent years, for their recognized biological properties. The use of tridentate SBs ligands in different organometallic and coordination complexes containing main-group metals and transition metals has been an option to study the biological activity of new possible metallopharmaceuticals that contribute to increase activity and to counteract the effect of microbial resistance.
To prepare the metal complexes derived from SBs, triethylamine is generally used to guarantee a basic medium and thus promote the deprotonation of ligands, while the metal is protected by adding weak acids that prevent the precipitation.
Medicinal inorganic chemistry is a growing field that has proven to be very effective in the treatment and diagnostic of many diseases . Nowadays, transition metal complexes are the most known and most used compounds in the design of metallodrugs. However, main group elements have received extensive attention since the discovery of salvarsan, also known as arsphenamine or Ehrlich 606, a mixture of 3-amino-4-hydroxyphenyl-As(I) and As(V) compounds synthetized by Paul Ehrlich as an effective cure for syphilis. Salvarsan is acknowledged as the first pharmaceutic cure for a disease and opened the door to the research of new molecules that target specific cells to treat many infections . Arsenic was widely used to treat parasite infections, skin diseases, and anaemia in the early 20th century. Livingstone reported the effective action of arsenic in trypanosomiasis, and subsequent reports showed the beneficial action of arsenic against the parasites in the blood stream, but also acknowledged that since the known forms of arsenic were toxic, its use could be lethal to the patient. Organic arsenicals were recognized as less toxic that their inorganic counterpart and were used to treat syphilis and sleeping sickness . Atoxyl, melarsoprol, and melarsonyl are examples of chemotherapeutic drugs for the treatment of infectious diseases . Furthermore, antimony-based chemotherapy drugs are the main treatment for Leishmaniosis . The pentavalent antimonial compounds that had traditionally been used as treatment for Leishmaniosis (meglumine antimoniate and sodium stibogluconate) had important side effects, and over time the parasite became resistant, so the available drug was not safe enough nor effective .
Research about the synergic effect of SBs with metals has been addressed mainly for transition metals, and the main group has received less attention, mostly due to the toxicological effects that these kinds of compounds have historically shown. However, in recent decades, the number of reports of medicinal research of SBs with the main group have been increasing. Tin is a main group metal that shares some characteristics with the transition metals. It can form complexes with several organic molecules bound through donor heteroatoms as oxygen, sulphur, or nitrogen, in a wide range of geometries.
SBs with Sn(IV) in vitro antimicrobial activities have been studied against Gram-positive bacteria, Gram-negative bacteria, and fungal strains. In many cases, these complexes showed better activity against Gram-positive strains, such as S. aureus and B. subtilis, than Gram-negative strains, such as E. coli and P. aeruginosa . This result can be explained by the chelation theory that sustains that chelation increases the lipophilic character of the complexes, allowing the permeation of the compound through the cell wall of the bacteria. Through chelation, the metal polarity is reduced by sharing its positive charge with the heteroatom in the ligand, originating an electron delocalization over the metallo-ring. As such, the biological activity of organotin(IV) complexes depend on the donor ligand, the geometry, and the coordination number over the tin atom .
Along with the chemistry of organotin(IV) complexes, the biological activities of hypercoordinated silicon compounds with SB ligands have also been studied . Silicon compounds with tridentate SBs are scarce, with some early reports by Puri et al. looking to examine the physical and chemical properties of these compounds. In this report, a series of complexes, which exhibit a hexacoordinate Si(IV) atom and the ligand N,N′-diethylenetriamine-bis(salicylideneimine), are reported .