Synthesis, Structure, Electrochemical Properties, and Antioxidant Activity of Organogermanium(IV) Catecholate Complexes.

A series of novel organogermanium(IV) catecholates - of the general formula R'Ge(Cat), where R' = Ph, Et, have been synthesized. Compounds were characterized by H, C NMR, IR spectroscopy, and elemental analysis. The molecular structures of -, , and in crystal state were established using single-crystal X-ray analysis. The complexes are tetracoordinate germanium(IV) compounds containing a dioxolene ligand in a dianion (catecholato) form. Electrochemical transformations of target germanium(IV) complexes have been studied by cyclic voltammetry. The electro-oxidation mechanism of complexes -, , and (the related complex PhGe(3,5-Cat) where 3,5-Cat is 3,5-di--butylcatecholate) involves the consecutive formation of mono- and dicationic derivatives containing the oxidized forms of redox-active ligands. The stability of the generated monocations depends both on the hydrocarbon groups at the germanium atom and on the substituents in the catecholate ring. Compounds , , and are oxidized irreversibly under the electrochemical conditions with the formation of unstable complexes. The radical scavenging activity and antioxidant properties of new complexes were estimated in the reaction with DPPH radical, ABTS radical cation, and CUPRAC assay. It has been found that compounds and with benzothiazole or phenol fragments are more active in DPPH test. The presence of electron-rich moieties in the catecholate ligand makes complexes and - more reactive to ABTS radical cation. The value of CUPRAC for organogermanium(IV) catecholates varies from 0.23 to 1.45. The effect of compounds - in the process of lipid peroxidation of rat liver (Wistar) homogenate was determined in vitro. It was found that most compounds are characterized by pronounced antioxidant activity. A feature of complexes , , and - is the intensification of the antioxidant action with the incubation time. In the presence of additives of complexes , , , and , an induction period was observed during the process of lipid peroxidation.