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

The [(p-cymene)RuCl(κ²C,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 κ2-(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)₃RuCl(κ²C,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 ¹H and ¹³C{¹H} 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 (TOF_50%(h^-1) = 1286).