Catalytic Transfer Hydrogenation with Terdentate CNN Ruthenium Complexes: The Influence of the Base



The catalytic activity of the terdentate complex [RuCl(CNN)(dppb)] (A) [dppb=Ph2P(CH2)4PPh2; HCNN=6-(4′-methylphenyl)-2-pyridylmethylamine] in the transfer hydrogenation of acetophenone (S) with 2-propanol has been found to be dependent on the base concentration. The limit rate has been observed when NaOiPr is used in high excess (A/base molar ratio > 10). The amino-isopropoxide species [Ru(OiPr)(CNN)(dppb)] (B), which forms by reaction of A with sodium isopropoxide via displacement of the chloride, is catalytically active. The rate of conversion of acetophenone obeys second-order kinetics v=k[S][B] with the rate constants in the range 218±8 (40 °C) to 3000±70 M−1 s−1 (80 °C). The activation parameters, evaluated from the Eyring equation are ΔH=14.0±0.2 kcal mol−1 and ΔS=−3.2 ±0.5 eu. In a pre-equilibrium reaction with 2-propanol complex B gives the cationic species [Ru(CNN)(dppb)(HOiPr)]+[OiPr] (C) with K ≈2×10−5M. The hydride species [RuH(CNN)(dppb)] (H), which forms from B via β-hydrogen elimination process, catalyzes the reduction of S and, importantly, its activity increases by addition of base. The catalytic behavior of the hydride H has been compared to that of the system A/NaOiPr (1:1 molar ratio) and indicates that the two systems are equivalent.