Beschreibung:
<jats:title>Abstract</jats:title><jats:p>The degradation pathways of highly active [Cp*Ir(κ<jats:sup>2</jats:sup>‐<jats:italic>N</jats:italic>,<jats:italic>N</jats:italic>‐R‐pica)Cl] catalysts (pica=picolinamidate; <jats:bold>1</jats:bold> R=H, <jats:bold>2</jats:bold> R=Me) for formic acid (FA) dehydrogenation were investigated by NMR spectroscopy and DFT calculations. Under acidic conditions (1 equiv. of HNO<jats:sub>3</jats:sub>), <jats:bold>2</jats:bold> undergoes partial protonation of the amide moiety, inducing rapid κ<jats:sup>2</jats:sup>‐<jats:italic>N</jats:italic>,<jats:italic>N</jats:italic> to κ<jats:sup>2</jats:sup>‐<jats:italic>N</jats:italic>,<jats:italic>O</jats:italic> ligand isomerization. Consistently, DFT modeling on the simpler complex <jats:bold>1</jats:bold> showed that the κ<jats:sup>2</jats:sup>‐<jats:italic>N</jats:italic>,<jats:italic>N</jats:italic> key intermediate of FA dehydrogenation (<jats:bold>I<jats:sub>NH</jats:sub></jats:bold>), bearing a <jats:italic>N</jats:italic>‐protonated pica, can easily transform into the κ<jats:sup>2</jats:sup>‐<jats:italic>N</jats:italic>,<jats:italic>O</jats:italic> analogue (<jats:bold>I<jats:sub>NH2</jats:sub></jats:bold>; Δ<jats:italic>G</jats:italic><jats:sup>≠</jats:sup>≈11 kcal mol<jats:sup>−1</jats:sup>, Δ<jats:italic>G</jats:italic> ≈−5 kcal mol<jats:sup>−1</jats:sup>). Intramolecular hydrogen liberation from <jats:bold>I<jats:sub>NH2</jats:sub></jats:bold> is predicted to be rather prohibitive (Δ<jats:italic>G</jats:italic><jats:sup>≠</jats:sup>≈26 kcal mol<jats:sup>−1</jats:sup>, Δ<jats:italic>G</jats:italic>≈23 kcal mol<jats:sup>−1</jats:sup>), indicating that FA dehydrogenation should involve mostly κ<jats:sup>2</jats:sup>‐<jats:italic>N</jats:italic>,<jats:italic>N</jats:italic> intermediates, at least at relatively high pH. Under FA dehydrogenation conditions, <jats:bold>2</jats:bold> 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 <jats:bold>1</jats:bold> was obtained, as further confirmed by spectroscopic and catalytic tests on the independently synthesized degradation product <jats:bold>1 c</jats:bold>. DFT calculations confirmed that this side reaction is kinetically and thermodynamically accessible.</jats:p>