Triplet 4-Nitrenemethyl-Pyridine-N-Oxide: A Model Alkyl Nitrene Isolated in Cryogenic Matrices

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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 > 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 > 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.