Effect of substituent, metal and pH on the peroxidase activity of metal complexes with Schiff-base ligands

FERREYRA, JOAQUÍN; PALOPOLI, CLAUDIA M.; SIGNORELLA, SANDRA R.; COLAGIOIA, REGINA

Fortaleza

Congreso; XIX Brazilian Meeting on Inorganic Chemistry - VI Latin American Meeting on Biological Inorganic Chemistry - VIII Brazilian Meeting on Rare Earths; 2018

Lignin is the largest renewable raw material containing aromatic fragments. However, no processes have been developed to obtain aromatic products from this natural source. Metal complexes of Schiff-base ligands show a rich redox chemistry modulated by ligand substituents and reaction conditions and can afford high-valent metal-oxo complexes able to act as O-atom transfer agents with a variety of substrates. This kind of complexes can mimic ligninolytic enzymes that employ hydrogen peroxide for lignin degradation.In this work, complexes of Mn(III), Co(II) and Cu(II) with Schiff-base ligands derived from 1,3-bis(salicylidenamino)propane (X-salpn, X = aromatic substituent) were used as peroxidase mimics for the oxidation of lignin models using hydrogen peroxide as terminal oxidant, at pH 7 and 9 and room temperature. The H2O2-based oxidation of phenol catalyzed by [M(X-salpn)]0/+ in the presenceof 4-aminoantipyrine, was monitored following the absorbance increase at 500nm due to the quinoneimide adduct, while the oxidation of 2,6-dimethoxyphenol was monitored at 470 nm, corresponding to 3,3',5,5?-tetramethoxydiphenoquinone. While Mn(III) catalysts are more active than Co(II) and Cu(II) ones at pH 7, Co(II) complexes were the best at pH 9. The behaviour of Mn(III) complexes at pH 9 was interpreted in terms of the formation of oxo-bridged diMn(IV) dimers. This dimers catalyze H2O2 dismutation, which in turns leads to a loss of activity.