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

Ruthenium complexes of the general formula [RuCl2(PPh3)(PNN')] have been obtained from tridentate PNN' ligands containing phosphine (P), amine or imine (N), and pyridyl donor groups (N'). The imino ligand Ph2P(o,o'-C6H4CH=NCH2C5H4N) (a) has been synthesized from Ph2P(2-C6H4CHO) and 2-(aminomethyl)pyridine, whereas amino Ph2P(o,o'-C6H4CH2NHCH2C5H4N) (b) is prepared by the reduction of a with NaBH4. The complexes trans-[RuCl2(PPh3)(PNN')] [PNN' = b, (1); a, (2)] containing a five-membered NN' cycle have been isolated in high yield by the reaction of RuCl2(PPh3)(3) with b and a, respectively. By the same route and using the ligand Ph2P(o,o'-C6H4CH=NCH2CH2C5H4N)] (c), the complex cis-[RuCl2(PPh3)(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(PPh3)(3)] with the ligands a or b leads to the monohydride complexes trans-[RuHCl(PPh3)(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 (CH3)(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.