Highly Stable Mn(V)-Nitrido and Nitrogen-Atom Transfer Reactivity within a De Novo Protein.

High-valent metal-nitrido species are powerful nitrogen-atom transfer intermediates but remain difficult to access and control due to intrinsic instability and bimolecular N-N coupling pathways. Herein, we report the first formation of a high-valent Mn(V)-nitrido complex within a de novo designed protein scaffold and demonstrate that a reactive precursor to this species can be catalytically intercepted for enantioselective aziridination. A Mn(V)≡N unit derived from an abiological diphenyl porphyrin is confined within a designed helical bundle protein, where the protein environment suppresses bimolecular decay and enables detailed spectroscopic characterization. Electron paramagnetic resonance, resonance Raman, and circular dichroism spectroscopies confirm formation of a low-spin Mn(V)-nitrido species that is stable for weeks at room temperature and exhibits minimal perturbation of the Mn≡N unit upon modulation of the axial histidine ligand, while catalytic activity and stereochemical outcome are sensitive to its presence. Mechanistic studies identify monochloramine (NH 2 Cl) as the operative nitrogen-atom donor and support the involvement of a transient Mn-bound N-transfer intermediate en route to nitrido formation. Under catalytic conditions, this intermediate is inter-cepted to perform aziridination with TON ≈ 180 and an enantiomeric ratio of 65:35. Together, these results establish de novo protein design as a platform for stabilizing high-valent metal-nitrido species and harnessing their reactivity for nitrogen-atom transfer chemistry beyond the limits of natural metalloenzymes and small-molecule catalysts.