Hydrogen-transfer catalyzed by half-sandwich Ru(ii) aminophosphine complexes

Abstract

The syntheses of and catalytic studies on some Ru(II) complexes bearing the aminophosphine ligands N,N-dimethyl-2-diphenylphosphinoethylamine (PN), optically pure (R_C,S_pl)-2- {1-(N,N-dimethylamino)ethyl}-1-diphenylphosphinoferrocene (PPFA), and N,N-dimethyl-2-diphenylphosphinoaniline (DBD) are described, [RuCp(CH₃CN)₃]⁺ reacts with these ligands to give the cationic complexes [RuCp(PN-κN,κP)(CH₃CN)]⁺ (1a), [(S_Ru,R_C,S_pl)-RuCp(PPFA-κN,κP)(CH₃CN)]⁺ (1b), and [RuCp(DBD-κN,κP )(CH₃CN)]⁺ (1c), respectively, in high yields. From these, in turn, the residual CH₃CN ligand can be replaced by Br⁻ upon addition of NEt₄Br in CH₂Cl₂, resulting in the formation of the neutral complexes RuCp(PN)Br (2a), (S_Ru,R_C,S_pl)-RuCp(PPFA)Br (2b), and RuCp(DBD)Br (2c), again in good yields. Similarly, [Ru(η⁶-p-cymene)Cl₂]₂ reacts with 1 equiv. of PN or PPFA to give Ru(η⁶-p-cymene)(PN-κP)Cl₂ (3a) and Ru(η⁶-p-cymene)(PPFA-κP)Cl₂ (3b). Furthermore, treatment of 3 with TlCF₃SO₃ in THF at room temperature affords the cationic complexes [Ru(η⁶-p-cymene)(PN-κN,κP)Cl]CF₃SO₃ (4a) and [(R_Ru,R_C,S_pl)-Ru(η⁶-p-cymene)(PPFA-κN,κP)Cl]CF₃SO₃ (4b). The absolute configuration at the metal center of 2b and 4b′ (BPh₄⁻ salt of 4b) was determined by X-ray crystallography. Catalytic studies were performed with the racemic complexes 1a, 2a, 2c, and 4a and the diastereopure complexes 1b, 2b, and 4b. All of these proved to be excellent precatalysts for the transfer hydrogenation of acetophenone and derivatives thereof, and cyclohexanone. With the enantiomerically pure systems 2b and 4b, only racemic products were obtained. This testifies to the hemilabile nature of the aforementioned aminophosphine ligands giving transient κ-P-bonding coordination. Since diastereoface selection of incoming substrates is based on the planar chirality of the ferrocene moiety, rather than the metal centered chirality, no enantioselective reaction takes place.