Phenyl Radical Activates Molecular Hydrogen Through Protium and Deuterium Tunneling

Activating dihydrogen, H₂, is a challenging endeavor typically achieved using transition metal centers. Pure main-group compounds capable of this are rare and have emerged in recent decades. These systems rely on synergistic donor-acceptor interactions with H₂'s antibonding σ* and bonding σ orbital. An alternative (hydrocarbon) radical-mediated activation is problematic because the H-H bond is stronger (104.2 kcal mol^-1) than most C-H bonds. Here, we explore using the phenyl radical to tackle this, forming benzene with a C-H bond energy (112.9 kcal mol^-1) that provides a thermodynamic driving force. We mainly observe a benzene-HI complex upon photolysis of iodobenzene in an H₂-doped neon matrix at 4.4 K despite a barrier of 7.6 kcal mol^-1, while phenyl radical forms in case of the heavier D₂ isotopologue. When D₂ molecules are allowed to diffuse, mono-deuterated benzene accumulates within hours. Computations using path integral-based instanton theory highlight that primarily the transferred hydrogen atom is moving during the reaction which greatly increases the tunneling probability. In excellent agreement with the experimental results, we predict significant tunneling rate constants for both isotopologues, H₂ and D₂, featuring a strong kinetic isotope effect of up to four orders of magnitude at the lowest temperatures.