Cyclopentadienone Iron Complex-Catalyzed Hydrogenation of Ketones: The Influence of the Charge-Tag on Catalytic Performance

The reason for the discrepancy in reaction rate in hydrogenation reactions using cyclopentadienone iron complexes as catalysts, depending on the absence or presence of a negative charge-tag (sulfonate or phosphonate), was investigated experimentally. Based on NMR and kinetic experiments, the direct binding of the charge-tag to the active site of the catalyst and electric field effects influencing transition state energies could be excluded. Preactivation of the catalysts as the monoacetonitrile dicarbonyl complexes was found to be superior to the in situ activation of the tricarbonyl complex with trimethylamine oxide with an up to 32-fold rate enhancement. CO ligand removal from the tricarbonyl iron complexes with Me₃NO was found to be disfavored in protic solvents compared to polar, aprotic solvents. Micelle formation was observed for the negatively charge-tagged complexes, with critical micelle concentrations in the range of 1.75–17 mM depending on the alkali metal counterion. The presence of the tertiary amine moiety in the charge-tagged catalyst was found to be responsible for the decrease in reaction rate. The presence of micelles was found to increase the reaction rate compared to noncharged complexes bearing a tertiary amine group.