Structure–activity relationship study of half-sandwich metal complexes in aqueous transfer hydrogenation catalysis†
A systematic structure–activity relationship study was performed to identify the factors that are important to enhancing the transfer hydrogenation efficiency of half-sandwich metal complexes, which is an important class of compounds recently shown to be useful as artificial enzyme cofactors, catalytic drugs, and intracellular catalysts. In general, electron-rich ligands provided catalysts with greater activity and chemical stability than electron-poor ligands under physiologically relevant aqueous conditions. Complexes containing Ir were better catalysts than those containing Ru, Rh, or Os. Kinetic studies by UV-vis absorption spectroscopy revealed that ligand substituent effects could dramatically accelerate the rates of both hydride formation and hydride transfer processes, the latter by up to a remarkable 28-fold. The thermodynamic hydricities of several iridium–hydride species were determined using a hydride exchange method, which showed a strong correlation between the hydride donor strengths of the iridium–hydride species and the transfer hydrogenation yields of their parent complexes. Specifically, this work provides a practical guide for further elaboration of the pentamethylcyclopentadienyl iridium pyridinecarboxamidate catalyst platform. We found that electron-donating groups on the pyridyl ring are conducive to achieving high catalytic rates whereas a wide variety of alkyl and aryl groups on the N-amide moiety are well-tolerated, suggesting that this position might be the best site for conjugation to other functional entities for biological applications.