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 (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 tunnelling. Our findings introduce an unprecedented carbene class, unlocking new opportunities for reactivity and electronic structure explorations.