The Metal-Hydride Protonation Mechanism of a Platinum(II) Pyridylidene Amide (PYA) Complex with Brookhart's Acid

Protonation of transition-metal hydrides is commonly considered as an instantaneous acid-base process, often omitting mechanistic details due to the fleeting nature of any potential intermediates. Herein, we report the unusual resistance of a platinum(II) hydride complex ([Pt]-H) supported by a pyridylidene amide (PYA) ligand toward protonation, even with Brookhart's acid [(Et₂O)₂H][BAr^F₄]. No reaction was observed over 7 days in Et₂O solvent, underscoring the remarkable kinetic and thermodynamic stability of [Pt]-H. Reactivity is unlocked with the addition of 50 equiv of MeCN, with protonation completed in ∼60 min at 25 °C. Accompanying ¹H NMR chemical shift changes (Δδ from 0.79 to 0.50 ppm) indicate a transition from L- to X-type bonding of the PYA donor sites within [Pt]-H that is induced by MeCN addition, which in turn enables hydride protonation reactivity. Kinetic modeling revealed a pre-equilibrium step consistent with [Pt]-H···acid adduct formation. H/D exchange and kinetic isotope effect studies with [(Et₂O)₂D][BAr^F₄] show complete deuteration in only 30 min at 25 °C (k_H/k_D = 0.40). This data supports a pre-equilibrium associated with adduct formation and suggests an inverse equilibrium isotope effect as the rate-enhancing factor. These results establish the pivotal role of ligand electronics in governing hydride protonation and provide rare kinetic and mechanistic insight into metal-hydride activation by strong acids.