Functionalized pyridyl diazole iridium complex catalyzed FA dehydrogenation: synergistic effect of adjacent versus long-range interaction†
Abstract
Catalytic dehydrogenation of formic acid (FA) in a green and efficient manner remains interesting and challenging. A series of novel Cp*Ir complexes with nitrogen-rich ligands were developed for catalytic FA dehydrogenation in water under base-free conditions. These complexes were synthesized using pyridyl pyrazole or imidazole as an N atom regulation ligand and fully characterized. Complex 5-H2O bearing a trifluoromethyl substituted pyridyl pyrazole achieved the highest turnover frequency of 33 084 h−1 at 90 °C in 2 M FA aqueous solution among the azole-based catalyst systems reported thus far. The terminal Ir–H intermediates were successfully detected by 1H NMR and mass spectrometry. Kinetic isotope effect (KIE) experiments and density functional theory (DFT) calculations demonstrated a plausible mechanism involving the decarboxylation and formation of H2, and the latter was proved to be the rate-determining step of the reaction for these two complexes. In contrast to the imidazole 2-H2O, the ortho NH action in the pyrazole led to the higher catalytic activity of 5-H2O, which unveiled the triumph of the synergistic effect of an adjacent proton over long-range interaction. This study demonstrated the effectiveness of introducing diazole ligands in FA dehydrogenation and revealed new strategies for developing new homogeneous catalysts.