Heterobimetallic Trihydrido Complex of Ruthenium and Rhodium Supported by Cyclopentadienyl Groups with Different Steric Demands

The reactivity of heterobimetallic trihydrido complexes
of Ru and
Rh supported by two kinds of cyclopentadienyl groups that provide
steric differentiation in the dinuclear reaction field, [CpsRu(μ-H)3RhCps] (3b: CpsRuCp‡Ru, CpsRhCp*Rh, 3c: CpsRuCp*Ru, CpsRhCp‡Rh; Cp‡1,2,4-tBu3C5H2, Cp*C5Me5), was examined to elucidate the role of each
metal center. Since the bulky Cp‡ group on the Ru
center in 3b retarded the uptake of diphenylacetylene,
complex 3b reacted with diphenylacetylene at 60 °C
to yield benzorhodacyclopentadiene complex 7b via the
cyclometalation at the Rh center. In contrast, complex 3c, which contains a less congested Ru center, reacted with diphenylacetylene
smoothly at 25 °C and afforded μ-alkenyl and μ-alkyne
complexes, 5c and 6c. The rhodacyclopentadiene
complex 7c was also obtained upon thermolysis of 6c but required heating at 180 °C. The reactivity toward
acetylene did not differ from each other due to its high reactivity
and μ-s-cis-η2:η2-butadiene-μ-vinyl complexes, 10b and 10c, were obtained. Although no intermediate was
observed in the reaction, DFT calculations for a model compound supported
by unsubstituted cyclopentadienyls suggested the exclusive uptake
of acetylene at the Ru center. Complex 3b reacted with
benzene at 25 °C under photo-irradiating conditions to yield
μ-phenyl complex 11b in 52% yield, although 3c decomposed under the same conditions. The reason why a
μ-phenyl complex was not formed in the photoreaction of 3c is that the Cp‡ group on the Rh center
suppressed the intramolecular migration of the phenyl group from the
Ru to the Rh center. Instead, the decomposition induced by the coordination
of additional benzene molecules to the Ru center was not suppressed.
These results demonstrated that the Ru center acts as an initial binding
site, although it is difficult to determine directly from the structure
of the product.