Heterometallic Effects in Nitrogen–Hydrogen Bond Cleavage by Trinuclear Mixed-Metal Polyhydrido Clusters Containing Ruthenium and Osmium

Trinuclear heterometallic mono­(μ3-oxo)-
and bis­(μ3-oxo)­hydrido clusters, (Cp*Ru)n(Cp*Os)3–n(μ3-O)­(μ-H)3 (8: n =
2; 10: n = 1; 12: n = 0) and (Cp*Ru)n(Cp*Os)3–n(μ3-O)2(μ-H) (Cp* = η5-C5Me5; n = 2, 1, and 0), were synthesized by the reaction
of the corresponding trinuclear metal pentahydrido complexes, (Cp*Ru)n(Cp*Os)3–n(μ-H)5, with the appropriate oxidizing agents. The
mono­(μ3-oxo) complex is proven to be highly active
toward nitrogen–hydrogen bond cleavage of ammonia. While the
reaction of (Cp*Ru)3(μ-H)3(μ3-H)2 with ammonia proceeded slowly and reached
a plateau in its conversion, the mono­(μ3-oxo) complex
reacted smoothly with ammonia to generate a μ3-imido
complex and an equivalent of water. A notable heterometallic effect,
namely, the acceleration of the formation rate of the μ3-imido complex, was observed in the reactions of 8 and 10 with ammonia. The relative ratios of the pseudo-first-order
rate constants kobs for (Cp*Ru)3(μ3-O)­(μ-H)3 (3), 8, and 10 are 1:8:11, and polarization of the
metal–metal bond induced in the metal core of the cluster is
responsible for the acceleration of the reaction rate.