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

Abstract

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.

Dehydrogenation of formic acid using iridium-NSi species as catalyst precursors

“…These data evidenced that the C–H bond activation of FA is the rate-determining step of the process. 25…”

Substituent Effects on the Activity of Cp*Ir(pyridine-carboxylate) Water Oxidation Catalysts: Which Ligand Fragments Remain Coordinated to the Active Ir Centers?

“…H 2 O was used as an internal standard to account for temperature and viscosity fluctuations; its D t was calibrated using an external sample of H 2 O . The hydrodynamic dimensions were derived from the D t data according to previously reported protocols, , as applied to aqueous systems. ,, …”

“…82 The hydrodynamic dimensions were derived from the D t data according to previously reported protocols, 80,81 as applied to aqueous systems. 54,63,83 Catalytic Tests. Manometric measurements were performed using two homemade jacketed glass reactors coupled to a Testo 521-1 manometer, following previously benchmarked procedures.…”

An ETS‐NOCV‐based computational strategies for the characterization of concerted transition states involving CO2

“…We encountered a concerted TS in the decomposition of formic acid to CO 2 catalyzed by a 16e iridium bifunctional complex 14 . In fact, formic acid interacts at the same time with the metal vacancy and the amido moiety, donating one hydride and one proton in a concerted way and liberating CO 2 .…”

“…We encountered a concerted TS in the decomposition of formic acid to CO 2 catalyzed by a 16e iridium bifunctional complex. 14 In fact, formic acid interacts at the same time with the metal vacancy and the amido moiety, donating one hydride and one proton in a concerted way and liberating CO 2 . In principle, the same mechanism could be active for the reverse reaction, that is the synthesis of formic acid from CO 2 and the 18e iridium hydride complex (Scheme 1 ).…”