A novel
triruthenium hydrido cluster, (Cp*Ru)3(μ3-H)(μ-H)2(μ-CO) (4; Cp*
= η5-C5Me5), having a bridging
CO ligand was synthesized by the reaction of {Cp*Ru(μ-H)}3(μ3-O) (3) with methanol. Upon
introduction of a CO ligand, the cyclic voltammogram of 4 demonstrated a significant shift of the redox potential in the positive
direction in comparison to a pentahydrido complex, {Cp*Ru(μ-H)}3(μ3-H)2 (1), which
adopts the same 44-electron configuration. Owing to its coordinatively
unsaturated nature, 4 readily reacted with butadiene,
terminal alkynes, and alkenes similarly to the parent pentahydrido
complex 1. However, the reactivity was slightly different
from that of 1, and the influence of the CO ligand at
the triruthenium site was evaluated by the reaction of various unsaturated
hydrocarbons. The reaction of 4 with 1,3-dienes yielded
a diene adduct, {Cp*Ru(μ-H)}3(μ-η2:η2-s-cis-H2CCRCHCH2)(CO) (5a, R = H; 5b, R = Me) without elimination of dihydrogen.
In the same manner as for 1, 4 reacted with
phenylacetylene to yield a μ3-η2:η2(⊥)-alkyne complex, (Cp*Ru)3(μ-H){μ3-η2:η2(⊥)-PhCCH}(μ3-CO) (7a); in contrast,
an isomeric μ3-pentenylidene complex, (Cp*Ru)3(μ-H){μ3-η2-CC(nPr)H}(μ-CO) (9b), was obtained by reaction
with 1-pentyne. A series of products was obtained by the reaction
of 4 with ethylene molecules. A μ3-ethylidyne
complex, (Cp*Ru)3(μ-H)2(μ3-CMe)(μ-CO) (11), was initially formed, accompanied
by the formation of ethane at ambient temperature. The treatment of 11 with ethylene resulted in the removal of hydrido ligands,
affording a μ3-vinylidene complex, (Cp*Ru)3(μ-H)(μ3-η2-CCH2)(μ-CO) (9a). At higher temperatures, a
second ethylene molecule was incorporated in the triruthenium plane
and an equilibrated mixture of a μ3-ethylidyne−μ-ethylidyne
complex, (Cp*Ru)3(μ-H)(μ-CMe)(μ3-CMe)(μ-CO) (13), and μ3-ethylidyne−μ-vinyl
complex, (Cp*Ru)3(μ-H)(μ3-CMe)(μ-η2-CHCH2)(μ-CO) (12),
was obtained. The formation of a μ3-η3-C3 ring on the Ru3 plane was observed upon
the thermolysis of the equilibrated mixture at 180 °C, which
clearly demonstrates the coupling of the two C2 moieties
placed on each face of the triruthenium plane.