Hemilability Modulation via Phosphane-Triazole Ligand Design: Impact on Catalytic Formic Acid Dehydrogenation

Two novel P-N ligands, 1-[2-(diphenylphosphanyl)ethyl]-1H-benzo-1,2,3-triazole (1) and its N2-isomer (2), were synthesized via alkylation of benzo-1,2,3-triazole followed by reaction with KPPh2. Reaction of 1 and 2 with [Ir(-Cl)(cod)]2 and [Rh(-Cl)(cod)]2 in a 2:1 molar ratio followed by chloride abstraction with AgBF4 led to the formation of square-planar κ2-P,N complexes, Ir-1, Ir-2, Rh-1 and Rh-2, as confirmed by NMR spectroscopy and by single-crystal X-ray diffraction. DFT studies provided insights into the electronic structure and bonding of the complexes. Calculated geometries match well with experimental results, supporting the proposed bonding model. Complex Ir-3 was also prepared for comparison, employing ligand 3, 1-[2-(diphenylphosphanyl)methyl]-1H-benzo-1,2,3-triazole, previously prepared by us. Variable-temperature NMR studies on [IrCl(cod)(P-N)] complexes revealed fluxional behavior attributed to ligand hemilability. Activation Gibbs free energies (ΔG‡) for the isomerization equilibrium of [IrCl(cod)(PN)] complexes, determined from the variable temperature 1H NMR spectra of ligands 1, 2 and 3 are 10.24, 10.60, and 8.87 kcal·mol−1, respectively. This enabled us to propose a coordination-ability scale that follows the trend 3 > 1 > 2. The relative activities of the iridium complexes were evaluated in the dehydrogenation of formic acid. Under optimized conditions, in an HCOOH/Et3N mixture, the initial TOFs are 186, 828, and 948 h−1 for Ir-1, Ir-2, and Ir-3, respectively. These results indicate that complex Ir-3, bearing the most strongly coordinating ligand, exhibits the highest catalytic activity, reaching a TON value of 444 after 7h. This study demonstrates the tunability of the hemilability of benzo-1,2,3-triazole-based P-N ligands and their potential in modulating catalytic activity.