(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.