A Ru(I) Metalloradical That Catalyzes Nitrene Coupling to Azoarenes from Arylazides

Unusual N-N coupling of aryl azides to yield azoarenes is demonstrated by the Ru(I) metalloradical, [SiP3iPr]Ru(N-2) (4) ([SiP3iPr] = (2-iPr(2)PC(6)H(4))(3)Si-). The yield of the azoarene is dependent on the substituent on the aryl azide, and the reaction is catalytic for p-methoxy and p-ethoxy phenyl azides, while no azoarene is observed for p-trifluoromethylphenyl azide. Studies aimed at probing the viability of a bimolecular couling mechanism of metal imide species, as shown in the related [SiP3iPr]Fe system, have led to the isolation of several structurally unusual complexes including the ruthenium(IV) imide, 7-OMe, as well as the Ru(II) azide adduct 8-OMe. One electron reduction of 7-OMe complex led to the isolation of the formally Ru(III) imide complex, [SiP3iPr]Ru(NAr) (Ar = p-MeOC6H4, 5-OMe). EPR spectroscopy on 5-OMe suggests that the complex is electronically similar to the previously reported imide complex, [SiP3iPr]Ru(NAr) (Ar = p-CF3C6H4, 5-CF3), and features radical character on the NAr moiety, but to a greater degree. The stability of 5-OMe establishes that bimolecular coupling of 5-OMe is kinetically inconsistent with the reaction. Further studies rule out mechanisms in which 5-OMe reacts directly with free aryl azide or a transient Ru(I) azide adduct. Together, these studies show that 5-OMe is likely uninvolved in the catalytic cycle and demonstrates the influence of the metal center on the mechanism of reaction. Instead, we favor a mechanism in which free aryl nitrene is released during the catalytic cycle and combines with itself or with free aryl azide to yield the azoarene.