Catalyst Design for the Ionic Hydrogenation of C=N Bonds

New chiral half-sandwich Ru hydride enantiomers with asymmetric disubstitution on the Cp ligand have been successfully synthesized and resolved. An enantiopure thiolate ligand was installed on the Ru center to form a pair of diastereomers, which were separated by crystallization via vapor diffusion of pentane into their saturated Et2O solution. Racemization occurred at elevated temperatures, but a room temperature conversion pathway was developed to remove the chiral thiolate ligand and generate the enantiopure hydride complex. Two new Rh(III) hydride complexes and their Ir analogues have been synthesized and characterized. The hydride complexes readily transfer H– to the N-carbophenoxypyridinium cation at room temperature, giving mixtures of 1,2- and 1,4-dihydropyridine products. In CD3CN, all four hydrides give nearly the same product ratio, demonstrating that the hydride transfer mechanism is outer sphere. In weak or non-coordinating solvents, the resulting 16-electron cations catalyze the isomerization of 1,2- to 1,4-dihydropyridine at rates that depend upon the cation and the solvent. The fastest isomerization was observed with the Rh(III) cation [Cp*Rh(2-(2-pyridyl)phenyl)]+, Acetonitrile can trap the 16-electron cations resulting from hydride transfer, dramatically slowing the isomerization process. The thermodynamics and kinetics of hydride, hydrogen atom and proton transfer reactions of the Rh(III) hydride, Cp*Rh(2-(2-pyridyl)phenyl)H, were studied both thermodynamically and kinetically. This hydride is both a good hydride and hydrogen atom donor, but a poor proton donor. This previously unobserved combination of properties is due to the high energy of the hydride’s conjugate base, [Cp*Rh(2-(2-pyridyl)phenyl)]−. Its exceptional hydride donor ability makes Cp*Rh(2-(2-pyridyl)phenyl)H a very efficient catalyst for the ionic hydrogenation of iminium cations.