Imidazole versus pyridine as ligands for metalloporphine immobilization in ligninolytic peroxidases-like biomimetic catalysts

This paper describes the immobilization of 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphine-manganese(III) through coordinative bond onto imidazolyl- and pyridyl-grafted silica, emulating the active site of ligninolytic peroxidases. To this purpose, plain silica gel was functionalized with two organosilanes: the first bearing an imidazolyl functionality, the other one bearing a pyridyl functionality. Manganese-porphine was then immobilized onto the two modified silicas. The two catalytic adducts have been fully characterized to point out the effect of the ligands on their catalytic features. Pyridine-immobilized metalloporphine was found to be a much more performant catalyst, giving significantly higher conversion rates with all the tested substrates: a lignin-model compound such as veratryl alcohol, the lignin peroxidase well-known substrate azure B, and recalcitrant textile dyes such as alizarin red S, phenosafranine, methylene blue, methyl green, xylenol orange, and methyl orange. Imidazole-immobilized metalloporphine on the contrary has proved to be more stable than its pyridine-based counterpart. Besides, the ligands seem to promote different catalytic pathways in the two adducts. These results allow giving rational and rigorous insights about the effect of the ligands during the immobilization of metalloporphines, thus helping in the design of catalysts with specific feature (i.e. stability, reactivity).