(PNCP)Ir vs (PNNP)Ir: Neutral Iridium Complex for Direct Hydrogenation of Carboxylic Acids

Herein, we demonstrate the significant impact of tetradentate-ligand-coordinated metal complexes, which have not yet been exploited for the direct catalytic hydrogenation of carboxylic acids (CAs). Our previously developed cationic iridium complex coordinated with a PNNP-ligand [(PNNP)Ir] is effective for hydrogenating esters and carboxylic anhydrides generated in situ from CAs but unsuitable for the direct hydrogenation of CAs. In sharp contrast, the corresponding neutral iridium complex with a PNCP-ligand [(PNCP)Ir] developed in this study facilitates the direct hydrogenation of CAs, including biorelevant and pharmaceutical compounds, under not more than 1 MPa of H₂. Quantum-chemical calculations indicated that (PNCP)Ir is kinetically a far more competent catalyst than (PNNP)Ir, particularly for the C–H bond formation via hydride transfer from Ir–H to the carbonyl carbon of CA, which was identified as the rate-determining step. While Ir-carboxylates are in resting states throughout the catalytic cycle, CA itself barely interacts with the Ir center during the hydride transfer process.