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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-H 2 O 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-H 2 O were successfully detected by 1 H 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 H 2 . Water-assisted H 2 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-H 2 O 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-H 2 O . 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.

Interconversion between CO 2 + H 2 and FA/formate is the most promising strategy for the fixation of carbon dioxide and reversible hydrogen storage; however, FA dehydrogenation and CO 2 hydrogenation are usually studied separately using different catalysts for each reaction. This report describes of the catalysis of [Cp*Ir(N∧N)(X)] n + (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; X = Cl, n = 0; X = H 2 O, n = 1) bearing a proton-responsive N∧N pyridylpyrrole ligand for both reactions. Complex 2-H 2 O catalyzes FA dehydrogenation at 90 °C with a TOF max of 45 900 h -1 . Its catalysis is more active in aqueous solution than in neat solution under base-free conditions. These complexes also catalyze CO 2 hydrogenation in the presence of base to formate under atmospheric pressure (CO 2 /H 2 = 0.05 MPa/0.05 MPa) at 25 °C with a TOF value of 4.5 h -1 in aqueous solution and with a TOF value of 29 h -1 in a methanol/H 2 O mixture solvent. The possible mechanism is proposed by intermediate characterization and KIE experiments. The extraordinary activity of these complexes are mainly attributed to the metal-ligand cooperative effect of the the pyrrole group to accept a proton in the dehydrogenation of formic acid and assist cooperative heterolytic H-H bond cleavage in CO 2 hydrogenation.