CO₂ Hydrogenation and Formic Acid Dehydrogenation Using Ir Catalysts with Amide-Based Ligands

A series of Ir catalysts bearing amide-based ligands generated by a deprotonated amide moiety was prepared with the hypotheses that the strong electron-donating ability of the coordinated anionic nitrogen atom and the proton-responsive OH group near the metal center will improve the catalytic activity for CO₂ hydrogenation and formic acid (FA) dehydrogenation. The effects of the modifications of the ligand architecture on the catalytic activity were investigated for CO₂ hydrogenation at ambient conditions (25 °C with 0.1 MPa H₂/CO₂ (v/v = 1/1)) and under slightly harsher conditions (50 °C with 1.0 MPa H₂/CO₂) in basic aqueous solutions together with deuterium kinetic isotope effects (KIEs) with selected catalysts. Cp*Ir(L12)(H₂O)HSO₄ (L12 = 6-hydroxy-N-phenylpicolinamidate) that has an anionic coordinating N atom and an OH group in the second coordination sphere, exhibits a turnover frequency (TOF) of 198 h^-1 based on the initial 1 h of reaction. This TOF which, to the best of our knowledge, is the highest value ever reported under ambient conditions in basic aqueous solutions. However, Cp*Ir(L10)(H₂O)HSO₄ (L10 = (4-hydroxy-N-methylpicolinamidate) performs better in long-term CO₂ hydrogenation (up to a TON of 14 700 with [Ir] = 10 μM after 348 h and the final formate concentration of 0.643 M with [Ir] = 250 μM) at ambient conditions. Further, the catalytic activity for FA dehydrogenation was examined under three different conditions (pH 1.6, 2.3, and 3.5). The Cp*Ir(L12)(H₂O)HSO₄ complex in any of these conditions is less active compared to the picolinamidate catalysts without ortho-OH, owing to its instability. The complex without OH group, Cp*Ir(L8)(H₂O)HSO₄ (L8 = N-phenyl-picolinamidate), exhibits a high TOF (up to 118 000 h^-1) at 60 °C. Theoretical calculations were performed to examine the catalytic mechanism, and a step-by-step mechanism has been proposed for both CO₂ hydrogenation and FA dehydrogenation reactions. Density functional theory calculations of [Cp*Ir(L3)(H₂O)]HSO₄ (L3 = picolinamidate) and the X-ray structure of the [Cp*Ir(L7)(H)]·H₂O (L7 = N-methylpicolinamidate) complex imply a pH-dependent conformational change from N,N coordination to N,O coordination upon lowering the pH of the aqueous solution.