Ring Nitrogenation of Aromatic Amines by the Excited State of an Osmium(VI) Nitrido Complex

The reactivity of electrophilic Os^VI and Ru^VI nitrides toward various aliphatic and aromatic amines have been previously reported; these reactions all go through an initial nucleophilic addition of the amine nitrogen to M^VI≡N (M = Os, Ru) to generate a M^IV hydrazido species. Herein, we report that the excited state of a luminescent osmium(VI) nitrido complex, [Os^VI(N)(L)(CN)₃]⁻ (OsN, HL = 2-(2-hydroxy-5-nitrophenyl)benzoxazole), undergoes unprecedented ring nitrogenation of aromatic amines. Visible-light irradiation of OsN generates OsN*, which predominantly attacks the aromatic ring of 2,6-dimethylaniline (Me₂PhNH₂) to give an Os(II) benzoquinone diimine compound (PPh₄)[Os^II(L)(CN)₃(p-NH═Me₂Ph═NH₂)] [(PPh₄)2] in 60% yield, while nucleophilic addition of the amine N to OsN* also occurs to give the osmium(II) diazonium compound (PPh₄)[Os^II(L)(CN)₃(N = N-Me₂Ph)] [(PPh₄)1] as a minor product (10% yield). On the other hand, OsN* undergoes exclusive ring nitrogenation of diphenylamine, indole, and carbazole to give the corresponding osmium(II) benzoquinone diimines. All products have been characterized by various spectroscopic techniques and by X-ray crystallography. The reaction between OsN* and Ar₂N is proposed to proceed via an initial 1e^- transfer (ET) followed by proton transfer (PT) to generate Os^VNH and Ar₂N^• intermediates, which then further combine to give the product. The benzoquinone diimine ligands are susceptible to oxidation. Oxidation of 2 with H₂O₂ at ambient conditions affords [Os^IV(L)(CN)₃(N═PhMe₂(O)═O)]⁻, which bears the previously unknown (epoxy)benzoquinone iminato ligand.