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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry