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. Reaction of 1 and 2 with [Ir(μ-Cl)(cod)]₂ and [Rh(μ-Cl)(cod)]₂ in a 2:1 molar ratio followed by AgBF₄ led to the formation of square-planar κ²-P,N complexes, Ir-1, Ir-2, Rh-1 and Rh-2. Density functional theory studies provided insights into the electronic structure and bonding of the complexes. Complex Ir-3 was also prepared for comparison, employing ligand 3, 1-[2-(diphenylphosphanyl)methyl]-1H-benzo-1,2,3-triazole. 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 featuring 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. This indicates that Ir-3, bearing the most strongly coordinating ligand, exhibits the highest catalytic activity, reaching a TON value of 444 after 7 h. This study demonstrates the tunability of the hemilability of benzo-1,2,3-triazole-based P-N ligands and their potential for modulating catalytic activity.