Piano-Stool Benzothiazol-2 Ru(II) Complexes for Effective Transfer Hydrogenation of Carbonyls

Abstract

Benzothiazolium bromides (NSHC.HBr) (1a-j) with a variety of alkyl chain or benzyl substituents on N atom were prepared. The synthesis of ruthenium(II) (NSHC) complexes, (2a–j) could be achieved by in situ deprotonation of benzothiazolium salts with Ag<inf>2</inf>O and [RuCl<inf>2</inf>(p-cymene)]<inf>2</inf>. They were characterized by 1H, 13C, 19F NMR, IR spectroscopy and elemental analysis. The molecular structures of 2d, 2e and 2g were ascertained by single-crystal X-ray diffraction studies. The catalytic properties of complexes, (2a–j), with electron-donating or -withdrawing groups were investigated in transfer hydrogenation (TH) of ketones, and aldehydes with good to excellent yields. The presence of the CH<inf>2</inf>(CH<inf>2</inf>)<inf>16</inf>CH<inf>3</inf> or CH<inf>2</inf>C<inf>6</inf>F<inf>5</inf> on the N atom of the benzothiazol-2-ylidene complexes (2g, 2j) showed the highest activity for TH reaction. The catalytic properties of the N-alkyl substituted ruthenium(II) (NSHC) complexes(2h–j) may be interpreted by micelle effects. © 2016 Elsevier B.V.