Synthesis of a nitro complex of RuIII(salen): Unexpected aromatic ring nitration by a nitrite salt

Reaction of silver nitrite with Ru(salen)(PPh3)(Cl) (salen2− = N,N′-ethylenebis(salicylideneiminato dianion) using [NEt4]OH as a phase transfer agent led to the formation of a stable, N-coordinated nitrite ion complex (RuIII–NO2) that was characterized by single crystal X-ray diffraction. This reaction also resulted in nitration of the two phenolic rings of the salen ligand, which was unexpected given the basic conditions. With sodium nitrite, the Ru(III)-nitro complex was formed, but no ring nitration was observed. Thus, in a non-acidic medium, the combination of nitrite with redox active metal centers may provide a route to the nitration of phenolic derivatives. Possible mechanisms of this unusual reaction having potential relevance to aromatic nitrations in biological systems are discussed. This is also the first report of the crystal structure of a Ru(III)-nitro complex, a species that has been suggested to be unstable and prone to rapid disproportionation.