Redox-active metal(II) complexes of sterically hindered phenolic ligands: Antibacterial activity and reduction of cytochrome c. Part II. Metal(II) complexes of o-diphenol derivatives of thioglycolic acid

The synthesis and physico-chemical characterization of Fe(II) and Mn(II) complexes of 2-[4,6-di(tert-butyl)-2,3-dihydroxyphenylsulfanyl]acetic acid (HLI) and 2-[4,6-di(tert-butyl)-2,3-dihydroxyphenylsulfinyl]acetic acid (HLII) were carried out. The investigation of the molecular and electronic structure of Cu(II), Ni(II), Zn(II), Fe(II) and Mn(II) complexes has been performed within the density functional theory (DFT) framework. The computed properties were compared to the experimental ones, and molecular structures of the compounds were proposed based on the array of spectral data and quantum chemical calculations. Antibacterial activity of the Fe(II) and Mn(II) complexes was evaluated in comparison with Cu(II), Co(II), Ni(II) and Zn(II) complexes and three standard antibiotics; it was found to follow the order: (1) Сu(LI)2 > Mn(LI)2 > HLI > Ni(LI)2 > Zn(LI)2 > Fe(LI)2 > Co(H2O)2LI; (2) Cu(LII)2 > Сo(LII)2 > Ni(LII)2 > Mn(H2O)2(LII)2 > Fe(LII)2 > HLII > Zn(LII)2; their reducing ability (determined electrochemically) followed the same order. Spectrophotometric investigation was carried out in order to estimate the rate of the reduction of bovine heart сytochrome c with the ligands and their metal(II) complexes. The complexes Сu(LI)2, Mn(LI)2 and Co(LII)2 with the high reducing ability were found to be characterized by the highest rates of Cyt с reduction. NADPH:cytochrome P450-reductase had no substantial effect on the rate of сytochrome c reduction with HLI and HLII ligands.

Graphical abstractThe synthesis, physico-chemical characterization and biological evaluation of metal(II) complexes of 2-[4,6-di(tert-butyl)-2,3-dihydroxyphenylsulfanyl]acetic acid (HLI) and 2-[4,6-di(tert-butyl)-2,3-dihydroxyphenylsulfinyl]acetic acid (HLII) were carried out. The investigation of the molecular and electronic structure of metal(II) complexes has been performed within the density functional theory (DFT) framework. The complexes Сu(LI)2, Mn(LI)2 and Co(LII)2 with the high reducing ability were found to be characterized by the highest rates of Cyt с reduction.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Synthesis and characterization of Fe(II) and Mn(II) complexes of thioglycolic acid derivatives. ► The phenolic ligands coordinate in their singly deprotonated forms. ► Coordination cores of the metal(II) complexes are octahedral or square planar. ► The antibacterial activity of the compounds correlates with their reducing ability.