The kinetics of the reaction
(μ-H)2Ru3(CO)8(μ-P(t-Bu)2)2
+ H2 ⇄
(μ-H)2Ru3(CO)8(H)2(μ-P(t-Bu)2)2
have
been studied. The reaction of
(μ-H)2Ru3(CO)8(μ-P(t-Bu)2)2
with H2 has a rate law which is first-order in
cluster
concentration and in hydrogen pressure and inverse order in CO
pressure; on the basis of the rate law, activation
parameters, and deuterium kinetic isotope effect, hydrogen addition is
proposed to involve rapid, reversible dissociation
of a carbonyl ligand, followed by rate-determining oxidative addition
of hydrogen through a three-center transition
state at a single metal atom. Loss of hydrogen from
(μ-H)2Ru3(H)2(CO)8(μ-P(t-Bu)2)2
also involves reversible loss
of a carbonyl, followed by rate-determining reductive elimination of
molecular hydrogen. The reaction is highly
sensitive to the steric bulk of the phosphido substituents, as
(μ-H)2Ru3(CO)8(μ-PR2)2,
R = cyclohexyl and phenyl,
do not react with hydrogen. In addition, the rate of exchange with
13CO is much faster for R = t-Bu than for
R =
cyclohexyl. Based upon the temperature dependence of the
equilibrium constant for hydrogenation, the energy for
the unbridged Ru−Ru bond of
(μ-H)2Ru3(CO)8(μ-P(t-Bu)2)2
is estimated to be 47−59 kJ/mol, the low value being
attributed to steric strain.
The kinetics of the isomerizations of
(μ-H)3Ru3(μ3-CCO2Me)(CO)9
to
(μ-H)2Ru3(μ3-η2-CHCO2Me)(CO)9 and of
(μ-H)3Ru3(μ3-CSEt)(CO)9
to
(μ-H)Ru3(μ3-η2-CH2SEt)(CO)9,
including the
measurements of activation volumes, are reported. An earlier study
of reductive elimination
of CH3X from
H3Ru3(μ3-CX)(CO)9
found that the reaction was promoted by CO for X = Ph,
Cl, and Et and was unaffected by CO pressure for X =
CO2Me. The activation volume
ΔV
⧧
for the intramolecular isomerization of
(μ-H)3Ru3(μ3-CCO2Me)(CO)9
to
(μ-H)2Ru3(μ3-η2-CHCO2Me)(CO)9 was determined to be −0.3(0.7)
cm3/mol at 57.0 °C. For the isomerization of
(μ-H)3Ru3(μ3-CSEt)(CO)9
to
(μ-H)Ru3(μ3-η2-CH2SEt)(CO)9,
the rate is inhibited by CO; activation
parameters (ΔH
⧧=121(3) kJ/mol,
ΔS
⧧= +36(10) J/(mol K),
ΔV
⧧= +22.0(1.4)
cm3/mol) are
consistent with a mechanism involving reversible CO dissociation prior
to the rate-determining step but following an intramolecular rearrangement. The
change in mechanism
of reductive elimination of a C−H bond from CO associative to CO
independent to CO
dissociative is due to anchimeric assistance by the methylidyne
substituent. These results
may have relevance to C−H formation occurring on metal surfaces.
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