N-Site Regulation of Pyridyltriazole in Cp*Ir(N̂N)(H₂O) Complexes Achieving Catalytic FA Dehydrogenation

A series of novel Cp*Ir complexes with nitrogen-rich N̂N bidentate ligands were developed for the catalytic dehydrogenation of formic acid in water under base-free conditions. These complexes were synthesized by using pyridyl 1,2,4-triazole, methylated species, or pyridyl 1,2,3-triazole as a N-site regulation ligand and were fully characterized. Complex 1-HO bearing 1,2,4-triazole achieved a high turnover frequency of 14192 h^-1 at 90 °C in 4 M FA aqueous solution. The terminal and bridged Ir-H intermediates of 1-HO were successfully detected by ¹H NMR and mass spectrometry measurements. Kinetic isotope effect experiments and density functional theory (DFT) calculations were performed; then a plausible mechanism was proposed involving the β-hydride elimination and formation of H₂. Water-assisted H₂ release was proven to be the rate-determining step of the reaction. The distribution of Mulliken charges on N atoms of triazole ligand internally revealed that the ortho site N2 of 1-HO with a higher electron density was conducive to efficient proton transfer. Additionally, the advantage of water-assisted short-range bridge of 1,2,4-triazole moieties led to a higher catalytic activity of 1-HO. This study demonstrated the effectiveness of nitrogen-rich ligands on FA dehydrogenation and revealed a good strategy for N site regulation in the development of new homogeneous catalysts.