Synthesis and N–H Reductive Elimination Study of Dinuclear Ruthenium Imido Dihydride Complexes

Diruthenium imido dihydride complexes [(Cp*Ru)2(μ-NAr)­(μ-H)2] (Ar = Ph (2a), p-MeOC6H4 (2b), p-ClC6H4 (2c), 2,6-Me2C6H3 (2d); Cp* = η5-C5Me5) have been synthesized by hydrogenation of the corresponding bis­(amido) complexes [Cp*Ru­(μ-NHAr)]2 (1a–d). Reductive elimination of the N–H bond from 2a–c in the presence of arene yields the amido hydride complexes [(Cp*Ru)2(μ-NHAr)­(μ-H)­(μ-η2:η2-arene)] containing a π-bound arene. The rate and kinetic isotope effect for this reaction are consistent with a mechanism involving initial rate-determining reductive elimination of an N–H bond to produce the coordinatively unsaturated amido hydride species {(Cp*Ru)2(μ-NHAr)­(μ-H)} (A) followed by rapid trapping of this species by an arene. The existence of A is also supported by the reversible interconversion of [(Cp*Ru)2(μ-NHPh)­(μ-H)­(μ-η2:η2-C7H8)] with the tetranuclear complex [(Cp*Ru)4(μ4-NHPh)­(μ-NHPh)­(μ-H)2] (4), a dimerization product of A through a μ4-NHPh bridge. DFT calculations provide structures of A and transition states for the N–H reductive elimination. Two distinct reaction pathways are found for the N–H reductive elimination, one of which involves direct migration of a μ-hydride to the μ-NAr ligand, and the other involves formation of a transient terminal hydride species.