Synthesis and reactivity of a ruthenium(III) bis(anilide) dimer by oxidative addition of an N,N′-disubstituted hydrazine

The Ru ^III bis(anilide) dimer [Cp*RuCl(μ-NHPh)] ₂ (1a) is formed upon reaction of (Cp*RuCl) ₄ with 1,2-diphenylhydrazine in benzene. An X-ray crystal structure shows a centrosymmetric (C _i) dimer with a planar Ru ₂N ₂ core. Reaction of (Cp*RuCl) ₄ with a mixture of PhNHNHPh and TolNHNHTol gives only the homodimers la and [Cp*RuCl(μ-NHTol)] ₂ (Toi =p-tolyl). The lack of crossover indicates that cleavage of the hydrazine N - N bond is an intramolecular process. In polar solvents such as acetonitrile, the d dimer la isomerizes to the C _2i, isomer 1b, in which the anilido substituents are cis across the Ru ₂N ₂ core. Both isomers appear to be kinetic products of the synthesis, but 1b isomerizes to la in the benzene solvent. In the presence of la, 1,2-diphenylhydrazine disproportionates into azobenzene and aniline, and labeling studies show that this occurs without participation of the anilido ligands of la. Complex la shows slow exchange of the anilide ligands with toluidine (H ₂NTol) by NMR spectroscopy and is unreactive toward methanol, triethylphosphine, styrene, and phenylacetylene. The addition of silver triflate and phenylacetylene to la leads to a new species, assigned as the acetylene complex [{Cp*Ru(μ-NHPh)} ₂(μ-n ²:n ²-PhC≢CH)](OTf) ₂ (6a).