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

The degradation pathways of highly active [Cp*Ir(κ 2 - 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 eq. of HNO 3 ), 2 undergoes partial protonation of the amide moiety, inducing rapid κ 2 - N,N to κ 2 - N,O ligand isomerization. Consistently, DFT modelling on the simpler complex 1 showed that the κ 2 - N,N key intermediate of FA dehydrogenation ( I NH ), bearing a N-protonated pica, can easily transform into the κ 2 - N,O analogue ( I NH2 ; Δ G # ≈ 11 kcal/mol, Δ G ≈ -5 kcal/mol). Intramolecular hydrogen liberation from I NH2 is predicted to be rather prohibitive (Δ G # ≈ 26 kcal/mol, Δ G ≈ 23 kcal/mol), indicating that FA dehydrogenation should involve mostly κ 2 - 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%) was 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 be also 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 1c . DFT calculations confirmed that this side reaction is kinetically and thermodynamically accessible.