Ruthenium complexes with naphthyridine ligands. Synthesis, characterization and catalytic activity in oxidation reactions†

Abstract

New ruthenium complexes with 1,8-naphthyridine (napy) or derivatives thereof as ligands have been prepared and characterized. Three groups of complexes were obtained. The first consists of three dinuclear ruthenium complexes with two ligands (1,8-naphthyridine and pyridopyrazine) co-ordinated to two ruthenium ions in a bridging fashion. The second consists of two ruthenium dinuclear complexes having one ligand (2,7-dimethoxy- or 2,7-dichloro-1,8-naphthyridine, abbreviated to dmnapy and dcnapy respectively) co-ordinated to two ruthenium atoms. Proton NMR spectra for both complexes in aqueous solution and in acetonitrile revealed the conversion of a symmetrical form, suggesting dinucleating behaviour of the ligand, into an asymmetrical form, suggesting mononucleating behaviour of the ligand. The third group consists of a mono- and a di-nuclear complex with the ligand 2,7-di(phenylazo)-1,8-naphthyridine. The catalytic activity of the novel naphthyridine complexes in oxidation reactions has been studied. The catalytic oxidation of alcohols and the epoxidation of trans-stilbene were examined and the different reaction rates and selectivities are discussed in a comparative way. The active high-valent species resulting from the [Ru₂(napy)₂(H₂O)₄Cl(OH)]⁴⁺ complex is discussed in more detail.