Structural investigation and application of ruthenium(II)-benzimidazole complexes for N-alkylation

Abstract

The N-substituted ligands (1a-d) were synthesized through the reaction of benzimidazole and a base with alkyl halides. After that, a new series of N-coordinated benzimidazole ruthenium(II) complexes (2a-d) was synthesized in this study. ¹H and ¹³C NMR spectroscopy, elemental analysis and FT-IR spectroscopy methods were applied to examine the structures of the complexes. The structures of complexes 2a-c were additionally elucidated by X-ray diffraction spectroscopy. Arylmethyl alcohols have been alkylated with these complexes in the presence of KO^tBu under solvent-free mild conditions at 120 °C. These complexes catalyzed the N-alkylation process, yielding secondary amines from primary amines with great selectivity. Without using solvents, these complexes are found to be effective catalysts for the alkylation of aromatic amines with various alcohols. Given the structural similarity of the ruthenium complexes, it is clear that there is no substantial difference in their catalytic activity; however, the selectivity of the reaction is dependent upon the characteristics of the amines and alcohols involved.