Vitamin B1-based thiazol-2-ylidene-Ru(ii) complexes: recyclable transfer hydrogenation catalysts in water

Abstract

The [(p-cymene)RuCl(kappa C-2,N-{C-NHC-NH})]X-+(-) (C-NHC = thiazol-2-ylidene) complexes with a bidentate ligand (2: X = Cl and 3: X = PF6) were prepared by a one-pot reaction of vitamin B1 (VB1, 1), Ag2O and [(p-cymene)RuCl2](2). In the complexes, VB1 coordinates through C2 and the exocyclic N in the imino form with the kappa 2-(C,N) coordination mode. The complexes 2 and 3 are stable in the solid state, but slowly release p-cymene in solution. Furthermore, upon heating in polar solvents, 2 or 3 can be converted by ligand exchange reactions to produce [(L)(3)RuCl(kappa C-2,N-{C-NHC-NH})]+PF6- (4: L = py).Robustness was improved remarkably for 4. The complex 4 is stable in the solid state and in solution. The complexes 2-4 have been identified by H-1 and C-13{H-1} 2D NMR spectroscopy and 2 and 4 were studied by X-ray crystallography. In an effort to develop a recyclable catalyst in water, 2-4 were evaluated for TH of ketones and aldehydes with an azeotropic mixture of HCOOH/Et3N in water. The complexes 3 and 4 exhibited very good catalytic activity and 4 could be reused nine times without significant loss of activity, giving a high turnover frequency (TOF50%(h(-1)) = 1286).