Piano-stool benzothiazol-2-ylidene Ru(II) complexes for effective transfer hydrogenation of carbonyls

• A series benzothiazolium bromides with a variety of alkyl chain or benzyl substituents on N atom were synthesized and metalated with ruthenium precursors.
• The catalytic properties of the complexes in transfer hydrogenation reaction of ketones and aldehydes were investigated.
• Their catalytic properties may be interpreted that electronic influences and micele effects of the ligand.

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 Ag2O and [RuCl2(p-cymene)]2. 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 CH2(CH2)16CH3 or CH2C6F5 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.

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