Hydrogen Activation by a σσ*-Carbene Through Quantum Tunneling

The electronic structure of carbenes arises from the occupation of a σ and a π frontier orbital. While parent methylene possesses a triplet ground state (σ¹π¹), substituents are capable of stabilizing the singlet as the ground state (σ²π⁰ or σ⁰π²) by altering the frontier orbital energies. Here, we reveal that the 1,2[I]-shift isomer of 2-iodopyridine, the N-iodo Hammick intermediate, features a resonance between its carbene σ and N–I bond σ* orbitals, rendering them frontier orbitals. This singlet carbene is efficiently generated via UV photolysis of 2-iodopyridine in solid neon at 4.4 K and reacts with molecular hydrogen – but not deuterium – via N–I bond cleavage enabled by quantum tunneling. Instanton theory computations demonstrate the preference for a concerted hydrogen addition mechanism at elevated temperatures, while hydrogen atom abstraction dominates below 100 K despite a higher kinetic barrier for this process. Our findings introduce an unprecedented carbene class, unlocking new opportunities for reactivity and electronic structure explorations.