[RuCl₂(PPh₃)(PNN‘)] Complexes as Efficient Catalysts in Transfer Hydrogenation of Ketones

Ruthenium complexes of the general formula [RuCl 2 (PPh 3 )(PNN‘)] have been obtained from tridentate PNN‘ ligands containing phosphine (P), amine or imine (N), and pyridyl donor groups (N‘). The imino ligand Ph 2 P( o, o ‘-C 6 H 4 CH NCH 2 C 5 H 4 N) ( a ) has been synthesized from Ph 2 P(2-C 6 H 4 CHO) and 2-(aminomethyl)pyridine, whereas amino Ph 2 P( o, o ‘-C 6 H 4 CH 2 NHCH 2 C 5 H 4 N) ( b ) is prepared by the reduction of a with NaBH 4 . The complexes trans -[RuCl 2 (PPh 3 )(PNN‘)] [PNN‘ = b, ( 1 ); a, ( 2 )] containing a five-membered NN‘ cycle have been isolated in high yield by the reaction of RuCl 2 (PPh 3 ) 3 with b and a, respectively. By the same route and using the ligand Ph 2 P( o, o ‘-C 6 H 4 CH NCH 2 CH 2 C 5 H 4 N)] ( c ), the complex cis -[RuCl 2 (PPh 3 )( c )] ( 3 ) was isolated, and it displays a different stereochemistry as a result of the different size of the tridentate ligand. For the amino derivative 1, an X-ray diffraction experiment was carried out. Treatment of [RuHCl(PPh 3 ) 3 ] with the ligands a or b leads to the monohydride complexes trans -[RuHCl(PPh 3 )(PNN‘)] [PNN‘ = b, ( 4 ); a, ( 5 )]. Complexes 1 − 5 have been proven to catalyze the transfer hydrogenation of linear, cyclic, and aromatic ketones to secondary alcohols in 2-propanol at reflux and in the presence of (CH 3 ) 2 CHONa with a very high rate (TOF values up to 250 000 h -1 ). The trans derivatives 1 and 2 containing the amino and imino functions catalyze the reduction of acetophenone with the same activity (TOF = 190 000 and 185 000 h -1, respectively), suggesting that the C N group is reduced during catalysis. A lower activity has been observed for complexes 3 − 5 .