Photoinduced Metallonitrene Formation by N2 Elimination from Azide Diradical Ligands

Transition metal nitrides/nitrenes are highly promising reagents for catalytic nitrogen atom transfer reactivity. They are typically prepared in situ upon optically induced N2-elimination from azido precursors. A full exploitation of their catalytic potential, however, requires in-depth knowledge of the primary photo-induced processes and the structural/electronic factors mediating the N2-loss with birth of the terminal metal-nitrogen core. Using femtosecond infrared-spectroscopy, we elucidate here the primary molecular-level mechanisms responsible for the formation of a unique platinum(II) nitrene with a triplet ground state from a closed-shell platinum(II) azide precursor. The spectroscopic data in combination with quantum-chemical calculations provide compelling evidence that product formation requires the initial occupation of a singlet excited state with an anionic azide diradical ligand that is bound to a low-spin d8-configured PtII ion. Subsequent intersystem-crossing generates the Pt-bound triplet azide diradical, which smoothly evolves into the triplet nitrene via N2-loss in a near barrierless adiabatic dissociation. Our data highlight the importance of the productive, N2-releasing state possessing azide ππ* character as a design principle for accessing efficient N-atom-transfer catalysts.