Transition metal complexes bearing flexible N3 or N3O donor ligands: Reactivity toward superoxide radical anion and hydrogen peroxide

Mononuclear complexes of N-methylpropanoate-N,N-bis-(2-pyridylmethyl)amine (MPBMPA) and N-propanoate-N,N-bis-(2-pyridylmethyl)amine (HPBMPA) with first row transition metals from Mn to Cu were synthesized and characterized by spectroscopy (infrared, UV–visible), electrochemistry (cyclic voltammetry), microanalysis and in four cases X-ray crystallography. Structure of the complexes revealed high flexibility of these ligands that can adopt facial (Fe) and meridional (Cu) geometry. Activity in the degradation of reactive oxygen species (superoxide radical anion: superoxide dismutase (SOD)-like activity and hydrogen peroxide: catalase-like activity) was tested throughout the complex series in aqueous solutions. In connection with the catalytic dismutation of H2O2, bleaching tests with morin were also conducted in water. Comparison of the two ligands helped in elucidating the possible role of the carboxylate moiety in the different catalytic reactions. Although no general trends could be revealed between reactivity and constitution of the first coordination sphere, plausible explanations for differences are discussed individually for SOD like, catalase-like and bleaching activity.

Graphical abstractFacial (Fe) and meridional (Cu) geometry extremes are either possible with flexible N3 and N3O ligands in complexes that show various activity in SOD-like, catalase-like reactions and bleaching reaction of morin. Considerable catalase-like activity is paired with efficient oxidation of morin. Common intermediates may be responsible for each reactivity.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Flexible ligand framworks that can adopt facial (Fe) and meridional (Cu) geometry. ► Mn, Fe and Cu complexes are efficient against reactive oxygen species (O2·- and H2O2). ► Complexes with low catalase activity are poor catalysts of the oxidation of morin. ► Considerable catalase activity is paired with efficient oxidation of morin.