Ruthenium-cymene containing pyridine-derived aldiimine ligands: synthesis, characterization and application in the transfer hydrogenation of aryl ketones and kinetics studies

Abstract

Five electron-rich Nsingle bondN′−pyridylimine ligands, which form five-membered chelate ring when coordinate to a metal center, have been synthesized by the easily accessible condensation between 2-pyridinecarboxyaldhehyde and the corresponding substituted aniline in diethyl ether solution, in the presence of catalytic amount of p-toluenesulfonic acid. The reaction of Nsingle bondN′–pyridylimine ligands with [RuCl2(p-cym)]2 (1) in the presence of excess of KPF6 led to the formation of mononuclear adducts with general formula [RuCl(p-cym)(Nsingle bondN′)]PF6, where Nsingle bondN′ = N-phenyl-1-(pyridin-2-yl)methanimine (Amp) (2), N-(4-chlorophenyl)-1-(pyridin-2-yl)methanimine (Clmp) (3), N-(4-methylphenyl)-1-(pyridin-2-yl)methanimine (Memp) (4), N-(4-tert-butylphenyl)-1-(pyridin-2-yl)methanimine (Tbump) (5), N-(2,6-diethylphenyl)-1-(pyridin-2-yl)methanimine (Diemp) and N-[2,6-bis(propan-2-yl)phenyl]-1-(pyridin-2-yl)methanimine (Diipmp) (7). Additionally, the reaction of 2-acetylbenzenolate (Osingle bondO′) with (1) produces a less bulky neutral congener [RuCl(p-cym)(Osingle bondO′)] (8). All complexes were isolated and fully characterized by molar conductivity, elemental analysis, cyclic voltammetry and spectroscopic methods, including single crystal X-ray diffraction of (4), (7) and (8). The structures (2)–(7) were also optimized with the Density Functional Theory (DFT) and theoretical absorption spectra were obtained with TD-DFT formalism. The complexes (2)–(8) were applied as pre-catalyst in the transfer hydrogenation of the acetophenone and 4-methylacetophenone, showing good catalytic activity, with productivity up to 90%, and TOF up to 302 h−1 within 3 h of reaction; except for (8), which has no activity for the catalytic reduction performed in this work. A kinetic investigation using (3) and (7) as pre-catalysts showed that the time dependence for the production of 1-phenylethanol has a sigmoidal shape, with an induction time of 20 min. The catalytic activity of (3) is more sensitive to the temperature variation and its rate constant (k) increases faster than for (7). An Arrhenius plot reveals a smaller activation barrier (Ea) for the rate-determining step for (7) than for (3), Ea = 14.75 and 49.38 kJ mol−1, respectively. The ΔS≠ values obtained from an Eyring plot suggest an associative mechanism in the transition state of the catalytic reactions.