Triplet 4‐Nitrenemethyl‐Pyridine‐ <i>N</i> ‐Oxide: A Model Alkyl Nitrene Isolated in Cryogenic Matrices

ABSTRACT Alkyl nitrenes are highly reactive intermediates that are particularly challenging to study experimentally. Here, we report the first investigation of the simplest triplet alkyl nitrene bearing a pyridine‐ N ‐oxide moiety. Direct irradiation (λ &gt; 350 nm) of 4‐azidomethyl‐pyridine‐ N ‐oxide in an Ar matrix (15 K) generated the target triplet 4‐nitrenemethyl‐pyridine‐ N ‐oxide 3 2 , along with E and Z isomers of 4‐iminomethyl‐pyridine‐ N ‐oxide 3 . Results from subsequent irradiations (λ &gt; 325 nm) enabled the discrimination of the IR spectroscopic signatures of these species, allowing their unequivocal identification with support from DFT‐B3LYP computations. Triplet nitrene 3 2 was found to be stable in dark Ar matrices, demonstrating that quantum tunneling 1,2‐H shift to imine 3 is not operative on the experimental timescale. Computations show that this reaction is thermodynamically favorable on the triplet surface but involves a high‐energy barrier (~36 kcal mol −1 ), explaining the absence of H‐tunneling. As the pyridine‐ N ‐oxide moiety can act as an anchoring site for complexation with calix[4]pyrrole derivatives, our findings open new perspectives for investigating alkyl nitrene reactivity and tunneling phenomena under supramolecular confinement conditions.