Understanding the Deactivation Pathways of Iridium(III) Pyridine-Carboxiamide Catalysts for Formic Acid Dehydrogenation

The degradation pathways of highly active [Cp*Ir(κ2-N,N-R-pica)Cl] catalysts (pica=picolinamidate; 1 R=H, 2 R=Me) for formic acid (FA) dehydrogenation were investigated by NMR spectroscopy and DFT calculations. Under acidic conditions (1 equiv. of HNO3), 2 undergoes partial protonation of the amide moiety, inducing rapid κ2-N,N to κ2-N,O ligand isomerization. Consistently, DFT modeling on the simpler complex 1 showed that the κ2-N,N key intermediate of FA dehydrogenation (INH), bearing a N-protonated pica, can easily transform into the κ2-N,O analogue (INH2; ΔG≠≈11 kcal mol−1, ΔG ≈−5 kcal mol−1). Intramolecular hydrogen liberation from INH2 is predicted to be rather prohibitive (ΔG≠≈26 kcal mol−1, ΔG≈23 kcal mol−1), indicating that FA dehydrogenation should involve mostly κ2-N,N intermediates, at least at relatively high pH. Under FA dehydrogenation conditions, 2 was progressively consumed, and the vast majority of the Ir centers (58 %) were eventually found in the form of Cp*-complexes with a pyridine-amine ligand. This likely derived from hydrogenation of the pyridine-carboxiamide via a hemiaminal intermediate, which could also be detected. Clear evidence for ligand hydrogenation being the main degradation pathway also for 1 was obtained, as further confirmed by spectroscopic and catalytic tests on the independently synthesized degradation product 1 c. DFT calculations confirmed that this side reaction is kinetically and thermodynamically accessible.