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

Two novel P-N ligands, 1-[2-(diphenylphosphanyl)ethyl]-1<i>H</i>-benzo-1,2,3-triazole (<b>1</b>) and its N2-isomer (<b>2</b>), were synthesized. Reaction of <b>1</b> and <b>2</b> 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-<i>P</i>,<i>N</i> complexes, <b>Ir-1</b>, <b>Ir-2</b>, <b>Rh-1</b> and <b>Rh-2</b>. Density functional theory studies provided insights into the electronic structure and bonding of the complexes. Complex <b>Ir-3</b> was also prepared for comparison, employing ligand <b>3</b>, 1-[2-(diphenylphosphanyl)methyl]-1<i>H</i>-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 (Δ<i>G</i>^‡) for the isomerization equilibrium of [IrCl(cod)(PN)] complexes featuring ligands <b>1</b>, <b>2</b> and <b>3</b> 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 <b>3</b> > <b>1</b> > <b>2</b>. 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 <b>Ir-1</b>, <b>Ir-2</b>, and <b>Ir-3</b>, respectively. This indicates that <b>Ir-3</b>, 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.