Organogermanium(IV) complexes with O,N,O’-tridentate Schiff bases: synthesis, electrochemical transformations, photophysical properties, and anti/prooxidant activity

Abstract

A series of novel diorganogermanium(IV) complexes of the type [R2Ge(Ln)] (where R is Et, Ph) with O,N,O’-tridentate redox-active Schiff bases has been synthesized. Compounds 1–7 were characterized by 1H, 13C{1H} NMR, IR spectroscopy, and elemental analysis. The molecular structures of complexes 1, 3, and 7 in crystal state have been established using single-crystal X-ray analysis. The complexes are pentacoordinate mononuclear compounds of germanium(IV). The bond lengths in tridentate ligands correspond to their diamagnetic dianionic forms. The electrochemical properties of complexes 1–7 were studied by cyclic voltammetry. A feature of the electrochemical behavior of target complexes is the ability to form relatively stable reduced/oxidized species as a result of the one-electron transfer. The stability of electrogenerated forms is influenced by both the organic groups at the germanium(IV) atom and the substituents in the Schiff bases. The energy gap (ΔEel) between the boundary redox orbitals obtained from electrochemical data for the complexes varies from 2.43 to 2.60 eV, while the optical ΔE was observed in the narrow range (2.46 – 2.50 eV). Luminescent activity was detected for some complexes in the solution. The maximal values ​​of the relative quantum yield were determined for complexes containing an electron-withdrawing nitro group in the redox-active ligand. In the reaction with ABTS radical cation, compound 1 only possessed the radical scavenging activity while the other compounds are characterized by weak neutralizing properties. The dual anti/prooxidant action was observed for compounds 1–7 in the process of rat liver (Wistar) homogenate lipid peroxidation.