Terdentate RuX(CNN)(PP) (X = Cl, H, OR) complexes: Synthesis, properties, and catalytic activity in fast transfer hydrogenation

Terdentate ruthenium(II) complexes of general formula RuX(CNN)(dppb) (X = chloride, hydride, alkoxide; dppb = Ph2P(CH2)(4)PPh2), where CNN is a deprotonated 2-aminomethyl-6-arylpyridine ligand, have been prepared. The orthometalated derivative RuCl(b)(dppb) (1) has been obtained by reaction of RuCl2(PPh3)(dppb) with N,N-dimethyl-2-aminomethyl-6-(4-methylphenyl)pyridine (Hb) in 2-propanol and in the presence of triethylamine by elimination of PPh3 and HCl. Similarly, RuCl(a)(dppb) (2) and the chiral analogue RuCl(c)(dppb) (3), containing primary amine ligands, have been isolated starting from 2-aminomethyl-6-(4-methylphenyl)pyridine (Ha) and (R)-2,2-dimethyl-1-(6-phenylpyridin-2-yl)propylamine (Hc), respectively. The synthesis of the functionalized pyridines Ha-Hc is here described, whereas the crystal structure of 3 has been determined through an X-ray diffraction experiment. Treatment of 1-3 with sodium or potassium isopropoxide gives the corresponding hydrides RuH(b)(dppb) (4), RuH(a)(dppb) (5), and RuH(c)(dppb) (6) from the ruthenium isopropoxide complexes, via a beta-H elimination process. Studies in solution show that the isopropoxides bearing a NH donor group are in equilibrium with the corresponding hydrides (5 and 6). Reaction of 5 with benzophenone leads to the alkoxide Ru(OCHPh2)(a)(dppb) (7), which has been proven to interact with benzhydrol in C6D6, leading to the adduct 7 center dot(HOCHPh2), the alkoxide ligand, and the alcohol being in rapid exchange. Complexes 2 and 3 display a remarkable high catalytic activity for the transfer hydrogenation of ketones to alcohol in 2-propanol using a very small amount of catalyst. With the chiral complex 3 (0.005 mol %) methyl-aryl ketones can be quickly reduced (TOF ranging from 5.4 x 10(5) to 1.4 x 10(6) h(-1)) with an enatiomeric excess up to 88%.