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 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-1s-1 (80 °C). The activation parameters, evaluated from the Eyring equation are ΔH‡ = 14.0 ± 0.2 kcal/mol 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 × 10-5 M. 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.