Copper-2,2′-bipyridines: Catalytic performance and structures in aqueous alkaline solutions

Abstract

2,2′-Bipyridine and ten modified 2,2′-bipyridines were complexed in situ with copper(II) sulphate in aqueous alkaline solution. In oxidation studies with these complexes, the substituents of the bipyridine ring were found to have a significant effect on the catalytic activity. Parallel screening revealed that Cu-complexes with the bipyridine ligands substituted in 4,4′- and 5,5′-positions were highly active in the oxidation of veratryl alcohol with molecular oxygen. More detailed studies with 4,4′-disubstituted bipyridines showed that the electron donating methyl and methoxy substituents enhance the oxidation activity of Cu–bipyridine. The most effective Cu-complexes in the oxidation of veratryl alcohol also catalysed the oxidation of 2,2′-biphenol and the oxidative depolymerisation of the carbohydrate model compound dextran. The performance of Cu-2,2′-bipyridines in oxygen delignification of softwood pulp were in accordance with the model compound studies; the catalysts lowered the lignin content of the pulp, but they also tended to damage the cellulose fibres. According to the oxidation results, the complex distribution calculations and the UV–vis studies the structures of the active species were [Cu(2,2′-bipyridine) 2OH] + and [Cu(4,4′-dimethoxy-2,2′-bipyridine) 2OH] +.