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.