Hydrogen elimination from a hydroxycyclopentadienyl ruthenium(II) hydride: Study of hydrogen activation in a ligand-metal bifuncfional hydrogenation catalyst

Article abstract of DOI:10.1021/ja043460r

At high temperatures in toluene, [2,5-Ph₂-3,4-Tol ₂(η⁵-C₄COH)]Ru(CO)₂H (3) undergoes hydrogen elimination in the presence of PPh₃ to produce the ruthenium phosphine complex [2,5-Ph₂-3,4-Tol₂- (η⁴-C₄CO)]Ru(PPh₃)(CO)₂ (6). In the absence of alcohols, the lack of RuH/OD exchange, a rate law first order in Ru and zero order in phosphine, and kinetic deuterium isotope effects all point to a mechanism involving irreversible formation of a transient dihydrogen ruthenium complex B, loss of H₂ to give unsaturated ruthenium complex A, and trapping by PPh₃ to give 6. DFT calculations showed that a mechanism involving direct transfer of a hydrogen from the CpOH group to form B had too high a barrier to be considered. DFT calculations also indicated that an alcohol or the CpOH group of 3 could provide a low energy pathway for formation of B. PGSE NMR measurements established that 3 is a hydrogen-bonded dimer in toluene, and the first-order kinetics indicate that two molecules of 3 are also involved in the transition state for hydrogen transfer to form B, which is the rate-limiting step. In the presence of ethanol, hydrogen loss from 3 is accelerated and RuD/OH exchange occurs 250 times faster than in its absence. Calculations indicate that the transition state for dihydrogen complex formation involves an ethanol bridge between the acidic CpOH and hydridic RuH of 3; the alcohol facilitates proton transfer and accelerates the reversible formation of dihydrogen complex B. In the presence of EtOH, the rate-limiting step shifts to the loss of hydrogen from B.

Full text of DOI:10.1021/ja043460r

Published on Web 02/12/2005
Hydrogen Elimination from a Hydroxycyclopentadienyl
Ruthenium(II) Hydride: Study of Hydrogen Activation in a
Ligand-Metal Bifunctional Hydrogenation Catalyst
Charles P. Casey,* Jeffrey B. Johnson, Steven W. Singer, and Qiang Cui*
Contribution from the Department of Chemistry, UniVersity of WisconsinsMadison,
Madison, Wisconsin 53706
Received October 28, 2004; E-mail: casey@chem.wisc.edu
Abstract: At high temperatures in toluene, [2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(CO)2H (3) undergoes hydro-
gen elimination in the presence of PPh3 to produce the ruthenium phosphine complex [2,5-Ph2-3,4-Tol2-
(η4-C4CO)]Ru(PPh3)(CO)2 (6). In the absence of alcohols, the lack of RuH/OD exchange, a rate law first
order in Ru and zero order in phosphine, and kinetic deuterium isotope effects all point to a mechanism
involving irreversible formation of a transient dihydrogen ruthenium complex B, loss of H2 to give unsaturated
ruthenium complex A, and trapping by PPh3 to give 6. DFT calculations showed that a mechanism in-
volving direct transfer of a hydrogen from the CpOH group to form B had too high a barrier to be considered.
DFT calculations also indicated that an alcohol or the CpOH group of 3 could provide a low energy pathway
for formation of B. PGSE NMR measurements established that 3 is a hydrogen-bonded dimer in toluene,
and the first-order kinetics indicate that two molecules of 3 are also involved in the transition state for
hydrogen transfer to form B, which is the rate-limiting step. In the presence of ethanol, hydrogen loss from
3 is accelerated and RuD/OH exchange occurs 250 times faster than in its absence. Calculations indicate
that the transition state for dihydrogen complex formation involves an ethanol bridge between the acidic
CpOH and hydridic RuH of 3; the alcohol facilitates proton transfer and accelerates the reversible formation
of dihydrogen complex B. In the presence of EtOH, the rate-limiting step shifts to the loss of hydrogen
from B.
Introduction
The development of new ligand-metal bifunctional hydro-
genation catalysts is dramatically changing the face of reduction
chemistry.1 These catalysts provide attractive “green” alterna-
tives to stoichiometric LiAlH4 and NaBH4 reductions and utilize
environmentally friendly terminal reductants, such as hydrogen
and 2-propanol.2 The active reducing agent in each of these
catalytic systems contains electronically coupled acidic and
hydridic hydrogens, which work in concert to efficiently hy-
drogenate polar unsaturated substrates. The first ligand-metal
bifunctional catalyst, the hydroxycyclopentadienyl diruthenium
bridging hydride (1), was reported by Shvo in the mid 1980s.3
Since then, Noyori has led the development of these catalysts,
including the ruthenium(diamine)(BINAP) catalyst (2), which
has displayed extraordinary activity in the asymmetric reduction
of ketones (Figure 1).1,4
Figure 1. Examples of ligand-metal bifunctional catalysts.
of primary deuterium isotope effects on both hydridic and acidic
hydrogens, our group has provided experimental evidence sup-
porting a concerted hydrogen transfer mechanism from the active
reducing species 4 to carbonyls and of 3 to carbonyls and imines
(Figure 2).6,7 Noyori has performed theoretical calculations that
support this concerted hydrogen transfer from species 4.8
Although the mechanism for reduction in these systems is
well understood, evidence for the mechanism of heterolytic
Our group has reported studies on both the tolyl-substituted
analogue of Shvo’s hydroxycyclopentadienyl ruthenium hy-
dride (3) and Noyori’s arene-Ru(II)-TsDPEN [(N-p-toluene-
sulfonyl)-1,2-diphenylethylenediamine] hydride (4).5 On the
basis of detailed mechanistic studies, including the observation
(4) The active species has been identified as the trans-dihydride structure: (a)
Abdur-Rashid, K.; Faatz, M.; Lough, A. J.; Morris, R. H. J. Am. Chem.
Soc. 2001, 123, 7473. (b) Abdur-Rashid, K.; Clapham, S. E.; Hadzovic,
A.; Harvey, J. N.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2002,
124, 15104.
(5) (a) Haack, K.-J.; Hashiguchi, S.; Fujii, A.; Ikariya, T.; Noyori, R. Angew.
Chem., Int. Ed. Engl. 1997, 36, 285. (b) Noyori, R.; Hashiguchi, S. Acc.
Chem. Res. 1997, 30, 97. (c) Matsumura, K.; Hashiguchi, S.; Ikariya, T.;
Noyori, R. J. Am. Chem. Soc. 1997, 119, 8738.
(1) Noyori, R.; Ohkuma, T. Angew. Chem., Int. Ed. 2001, 40, 40.
(2) Fehring, V.; Selke, R. Angew. Chem., Int. Ed. 1998, 37, 1827.
(3) (a) Shvo, Y.; Czarkie, D.; Rahamim, Y.; Chodash, D. F. J. Am. Chem.
Soc. 1986, 108, 7400. (b) Menashe, N.; Shvo, Y. Organometallics 1991,
10, 3885. (c) Menashe, N.; Salant, E.; Shvo, Y. J. Organomet. Chem. 1996,
514, 97.
(6) Casey, C. P.; Johnson, J. B. J. Org. Chem. 2003, 68, 1998.
(7) Casey, C. P.; Singer, S. W.; Powell, D. R.; Hayashi, R. K.; Kavana, M. J.
Am. Chem. Soc. 2001, 123, 1090.
(8) Yamakawa, M.; Ito, H.; Noyori, R. J. Am. Chem. Soc. 2000, 122, 1466.
9
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J. AM. CHEM. SOC. 2005, 127, 3100-3109
10.1021/ja043460r CCC: $30.25 © 2005 American Chemical Society

Hydrogen Activation in a Ligand−Metal Bifunctional Catalyst
A R T I C L E S
Figure 3. Proposed mechanism of hydrogen formation in iron-only
Figure 2. Concerted hydrogen transfer from ligand-metal bifunctional
catalysts.
hydrogenases.
Scheme 2
Scheme 1
Scheme 3
Results
cleavage of hydrogen has remained elusive. In catalytic reduc-
tion by the Shvo catalyst, the rate of substrate reduction depends
on the rate of hydrogen cleavage. A detailed understanding of
this process may assist in the development of more active
catalysts.
Loss of H2 from Ruthenium Hydride 3. During catalytic
reduction of an unsaturated polar species, a proton and hydride
are transferred from ruthenium hydride 3 through a concerted
mechanism. In addition to the alcohol product, unsaturated
ruthenium species A is proposed to result from transfer of
hydrogen (Scheme 1). Due to the reactive nature of this proposed
intermediate, it has been neither isolated nor spectroscopically
observed. As a result, hydrogen cleavage by this species cannot
be studied directly. During the course of our previous studies
on the Shvo catalytic system, we have observed the slow loss
of hydrogen from ruthenium hydride at high temperatures in
toluene. We initiated a detailed study on the loss of hydrogen
from ruthenium hydride 3 in order to gain greater mechanistic
understanding of its microscopic reverse, the hydrogen cleavage
process.
Upon loss of hydrogen from ruthenium hydride 3, we
observed the simultaneous formation of diruthenium bridging
hydride 5, which is proposed to occur via the trapping of
unsaturated intermediate A by a second equivalent of 3 (Scheme
2). Use of 1H NMR spectroscopy in toluene-d8 allows observa-
tion of the disappearance of the hydride resonance of 3 [δ -9.55
(RuH)] and the concurrent appearance of the hydride resonance
from dimer 5 [δ -17.82 (RuHRu)]. The resulting hydride
disappearance follows first-order kinetics, resulting in a rate of
kobs ) 7.13 × 10-4 s-1 at 95 °C. This rate corresponds to twice
the rate of hydrogen loss from ruthenium hydride 3, as one
equivalent loses hydrogen while a second equivalent functions
to trap A.
Noyori’s arene-Ru(II)-TsDPEN catalyst 4 is a transfer
hydrogenation catalyst which utilizes 2-propanol or a formic
acid/triethylamine system as the terminal reductant. The catalytic
cycle is completed by regenerating the active reducing species
4 by hydrogen transfer from 2-propanol to the stable 16 electron
species via the microscopic reverse of ketone reduction.
Shvo’s hydroxycyclopentadienyl ruthenium system is a direct
hydrogenation system, using hydrogen to regenerate active
species 3. This regeneration presumably occurs through hydro-
gen cleavage by intermediate A, which is formed upon dis-
sociation of the tolyl-substituted diruthenium bridging hydride
5 or by transfer of hydrogen from 3 to an unsaturated substrate
(Scheme 1).
The microscopic reverse of molecular hydrogen cleavage,
hydrogen formation, has been proposed to occur at the crys-
tallographically characterized active sites of iron-only hydro-
genases isolated from DesulfoVibrio desulfuricans and Clostrid-
ium pasteurianum.9 The active site of Fe-only hydrogenases is
a six Fe cluster containing an Fe4S4 subsite and an organome-
tallic 2Fe subsite where reversible hydrogen formation may
occur by ammonium cation protonation of an intermediate Fe-H
(Figure 3).
Although there are several proposed mechanisms for hydro-
gen cleavage and hydrogen evolution, experimental informa-
tion is limited and will be addressed in the discussion. To gain
further understanding of hydrogen cleavage, as well as additional
insight into its overall catalytic cycle, we initiated study of the
mechanism of heterolytic hydrogen cleavage by the Shvo
catalyst.
Addition of an excess of PPh3 to the reaction mixture resulted
in the formation of triphenylphosphine complex 6, rather than
diruthenium complex 5, upon loss of hydrogen from 3. This
observation is consistent with the formation of intermediate A
and subsequent trapping by triphenylphosphine (Scheme 3).
These reactions were monitored at 95 °C using 1H NMR
spectroscopy to observe the disappearance of ruthenium hydride
resonances and the appearance of those consistent with ruthe-
(9) Evans, D. J.; Pickett, C. J. Chem. Soc. ReV. 2003, 32, 268 and references
therein.
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A R T I C L E S
Casey et al.
Table 1. Rate of Hydrogen Loss from 3 in the Presence of
Different Phosphines in Toluene-d8
1
phosphine
T (°
C)
k (104 s-
)
P(m-Tol)3
PPh3
PMe3
99
99
95
95
5.95
5.57
14.23
3.52
PPh3
nium phosphine complex 6 [δ 7.75 (aryl) and 1.90 (tolyl)].
Disappearance of the ruthenium hydride follows first-order
kinetics for over three half-lives.
The rate of hydrogen loss was independent of PPh3 concen-
tration over the range of 0.01-0.06 M, remaining constant at
half that of the reaction without phosphine (kobs ) 3.52 × 10-4
s-1). This is expected as intermediate A, formed upon hydrogen
loss from 3, is trapped by PPh3 rather than a second equivalent
of 3. These results indicate a zero-order dependence on PPh3
concentration when present in superstoichiometric quantities.
These observations established a first-order rate law for loss of
hydrogen from 3 (eq 1).
Figure 4. Plot of the rate of hydrogen loss from 3-RuHOH versus
concentration of PMe3 in dry toluene-d8 at 90 °C, while maintaining constant
total phosphine concentration ([PPh3] + [PMe3]).
Scheme 4
d[3]
dt
-
) k[3][PPh3]0
(1)
The temperature dependence of the rate of hydrogen loss in
the presence of PPh3 was monitored between 83 and 110 °C (k
) 1.03 × 10-4 s-1 at 83 °C; k ) 1.87 × 10-4 s-1 at 90 °C; k
) 3.54 × 10-4 s-1 at 95 °C; k ) 6.90 × 10-4 s-1 at 100 °C;
k ) 15.5 × 10-4 s-1 at 110 °C). An Eyring plot gave the
activation parameters: ∆Hq ) 26.1 ( 1.4 kcal mol-1 and ∆Sq
) -3.7 ( 3.6 eu.
for exchange between the ruthenium hydride and the hydroxyl
hydrogen.
Initial preparation of the isotopologs of 3 utilized water to
exchange the acidic proton from 3 and 3-RuDOD, which were
prepared by literature procedure.7 The use of water in the
exchange led to complications in rate measurements. Despite
attempts to remove all water following proton exchange, the
presence of residual water led to inconsistent rates of hydrogen
loss.
Due to inconsistencies resulting from the presence of residual
water, an alternative method was utilized to prepare the
isotopologs. Under ∼4 atm of D2 in THF, the ruthenium hydride
selectively exchanges with D2 without exchange of the acidic
proton.10 Using 3-RuHOH and 3-RuDOD as our starting
compounds, the final two isotopologs, 3-RuDOH and 3-Ru-
HOD, were prepared by exchange with D2 and H2, respectively
(Scheme 4).
Hydrogen Loss from 3 in the Presence of Different
Phosphines. Other phosphines were also employed to trap the
unsaturated intermediate (A) formed by hydrogen loss from 3.
For each phosphine, the rate of hydrogen loss was independent
of phosphine concentration when present in stoichiometric
concentrations. Loss of hydrogen in the presence of P(m-Tol)3
resulted in the formation of tri(m-Tol)phosphine ruthenium
complex 7. The rate constant at 99 °C (k ) 5.95 × 10-4 s-1
)
was similar to that in the presence of PPh3.
Surprisingly, however, the presence of PMe3 accelerated
hydrogen loss and formation of PMe3 complex 8. This rate was
about four times faster (k ) 14.23 × 10-4 s-1 at 95 °C) than
that in the presence of PPh3 (Table 1). Loss of hydrogen from
3 in the presence of a mixture of PMe3 and PPh3 gave a
statistical mixture of PPh3 complex 6 and PMe3 complex 8. The
rate of hydrogen loss from 3 (0.011 M) was measured in the
presence of variable PMe3 concentration and approximately
constant total phosphine concentration ([PPh3] + [PMe3] ≈ 0.06
M) (Figure 4). The rate increased nearly linearly with PMe3
concentration until reaching approximately stoichiometric quan-
tities relative to 3, at which point the rate plateaued.
At this point, we did not understand how PMe3 could affect
the rate and yet not appear in the rate law. If both PPh3 and
PMe3 are simply trapping agents, how could different rates be
observed? We will return to this point later.
Minimal Interchange of Ruthenium Hydride and Hy-
droxyl Hydrogen Prior to Loss of Hydrogen. The possible
reversible formation of ruthenium dihydride or ruthenium
dihydrogen intermediates prior to formation of the unsaturated
intermediate A was investigated using deuterium labels to probe
Hydrogen loss from 3-RuDOH and 3-RuHOD was fol-
1
lowed by H NMR spectroscopy. Less than 10% of hydrogen
exchange between the RuH and CpOH hydrogens was observed.
HD (δ 4.47) was the major hydrogen species seen in solution.
Deuterium Isotope Effects On Hydrogen Elimination from
Isotopologs of 3. As little or no exchange of deuterium was
observed, the rate constant for hydrogen loss from each
isotopolog in the presence of PPh3 was readily determined in
toluene-d8 at 95 °C. These rate constants [3-RuHOH, k ) (3.52
( 0.35) × 10-4 s-1; 3-RuDOH, k ) (2.74 ( 0.14) × 10-4
s-1; 3-RuHOD, k ) (2.13 ( 0.22) × 10-4 s-1; and 3-Ru-
DOD, k ) (1.69 ( 0.12) × 10-4 s-1] were compared to
determine the kinetic isotope effects (Table 2).
(10) The mechanism of this exchange reaction is not well understood and is
currently under investigation.
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Hydrogen Activation in a Ligand−Metal Bifunctional Catalyst
A R T I C L E S
Scheme 5
Table 2. Kinetic Isotope Effects for Loss of Hydrogen from
Isotopologs of 3 at 95 °C under Alcohol-Free Conditions in
Toluene
ruthenium-hydrogen bonds, consistent with observed isotope
effects on the labeling of each (Scheme 5).
Calculations of Hydrogen Loss from 3. To provide further
information about the loss of hydrogen from 3, a computational
study was performed on a simplified model of 3, in which
hydrogens were substituted for the aryl groups on the cyclo-
pentadienyl ring. Computational investigation of the activation
energy of uncatalyzed proton transfer from 3 to form B predicted
a barrier of 43.2 kcal mol-1 (including zero-point corrections).
Although the imaginary frequency at the transition state (Figure
5) is rather high, 1972i cm-1, indicative of a narrow barrier
and potentially significant tunneling contribution that are
characteristic of hydrogen/hydride transfer reactions, the dif-
ference between the computed barrier (43.2 kcal mol-1) and
the experimentally determined activation enthalpy (26.1 kcal
mol-1) strongly suggests that alternative mechanistic pathways
are operative.
kRuHOH/kRuHOD
kRuDOH/kRuDOD
kRuHOH/kRuDOH
kRuHOD/kRuDOD
kRuHOH/kRuDOD
1.65 ( 0.18
1.63 ( 0.14
1.28 ( 0.14
1.26 ( 0.16
2.08 ( 0.21
Initial Proposed Mechanism of Hydrogen Elimination
from Ruthenium Hydride 3 under Dry Conditions. On the
basis of the first-order rate law, we presumed that the loss of
hydrogen from ruthenium hydride 3 is a unimolecular process
(see below for revised mechanism). The activation entropy for
this process, ∆Sq ) -3.7 ( 3.6 eu, indicates that the transition
state is of similar entropy to the starting materials, consistent
with a unimolecular process with neither an associative nor a
dissociative process prior to the transition state.
Kinetic isotope effects were observed upon deuteration of
either the acidic or the hydridic hydrogen. These isotope effects,
k
RuHOH/kRuHOD ) 1.65 and kRuHOH/kRuDOH ) 1.28, indicate that
both the O-H and the Ru-H bonds are broken during either a
single step or two steps with similar activation barriers. The
product of the individual isotope effects (1.65 × 1.28 ) 2.11)
lies within error of the observed isotope effect of D2 loss from
the doubly labeled material (kRuHOH/kRuDOD ) 2.08); this
relationship is consistent with a process in which both bonds
are broken in a single transition state.
Hydrogen loss from ruthenium hydride 3 is proposed to occur
through rate-limiting transfer of the acidic hydrogen. Since no
hydrogen exchange between the acidic and hydridic hydrogens
was seen, this proton transfer must be irreversible. There are
two distinct and distinguishable ways hydrogen transfer might
occur: (1) transfer to the ruthenium hydrogen bond could
generate ruthenium dihydrogen complex B, or (2) transfer to
ruthenium could generate ruthenium dihydride C.
Kinetic isotope effects allow distinction between these
processes. If the rate-determining step had involved transfer of
the hydroxyl proton to ruthenium to generate ruthenium
dihydride complex C, then a significant isotope effect should
have been observed only for the hydroxyl group since the
ruthenium hydride bond would be largely unchanged. Since we
observed a significant isotope effect for the ruthenium hydride
(kRuHOH/kRuDOH ) 1.28), this mechanism can be excluded. In
contrast, proton transfer to form ruthenium dihydrogen complex
B involves a change in both the oxygen-hydrogen and
Figure 5. Calculated transition state of (a) uncatalyzed proton transfer and
(b) water-facilitated transfer of the acidic proton to the ruthenium hydride
of 3.
Motivated by the Grotthus mechanism proposed for proton
translocation in water and biological systems,11 we explored
the possibility of water acting as the proton relay group.
Calculations indeed confirmed such a catalytic role of water;
the corresponding transition state (Figure 5) was found to be
only ∼20 kcal mol-1 above the hydrogen-bonding complex
formed by 3 and a water molecule. Further calculations indi-
cated that a ruthenium dihydrogen complex is a likely inter-
mediate in this process (see Supporting Information for further
details).
These results suggest that an agent that can simultaneously
donate and accept a proton can effectively catalyze the transfer
of the acidic proton to the hydride in order to form a dihydrogen
complex, which loses hydrogen to ultimately result in the
(11) von Grotthus, C. J. D. Ann. Chim. 1806, 58, 54.
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A R T I C L E S
Scheme 6
Casey et al.
ruthenium phosphine complex. No intermediate dihydride
complex was found in the calculations.
At 95 °C in the presence of ethanol and PPh3, complete
exchange between the hydroxyl proton and ruthenium deuteride
of 3-RuDOH was observed prior to significant hydrogen loss.
The rate of phosphine complex formation from 3-RuDOH was
identical to that from 3-RuHOH in the presence of similar
concentrations of alcohol, providing further indication that fast
exchange between ruthenium deuteride and the acidic proton
had occurred.
These insights prompted us to extend our study to include
the observation of hydrogen loss from ruthenium hydride 3 in
the presence of water and to re-examine our initial proposal of
the hydrogen elimination mechanism under completely dry
conditions.
H2 Loss from 3 is Accelerated by H2O or Ethanol. Since
calculations suggested an intimate role of water in the transfer
of hydrogen to form a dihydrogen complex, we systematically
studied the effect of water and ethanol on the rate of hydrogen
loss from 3. In the presence of approximately 5 mM water, the
rate increased by a factor of 2. Since the dissolution of water
in toluene is troublesome, ethanol was used to obtain quantitative
kinetic data.
The rate of loss of hydrogen from ruthenium hydride 3
(0.0145-0.0175 M) in the presence of PPh3 was determined
for ethanol concentrations between 0.005 and 0.05 M. The
rate increased as ethanol concentration was increased up to
approximately 0.03 M, at which point the rate plateaued
(Figure 6).
Determination of Isotope Effects on Hydrogen Loss in the
Presence of Ethanol. Although the fast exchange between
ruthenium hydride and the hydroxyl proton in the presence of
alcohol prevents the determination of deuterium kinetic isotope
effects with mixed isotopologs 3-RuHOD and 3-RuDOH,
the isotope effect for the loss of deuterium from 3-RuDOD
can be determined in the presence of an excess of deuterated
EtOD, again using PPh3 as the trapping agent. The rate of
deuterium loss was measured at two different concentrations
of EtOD, and in each case, these rates (kobs ) 4.00 ( 0.38 s-1
at 0.051 M EtOD and kobs ) 3.92 ( 0.12 s-1 at 0.032 M EtOD)
were significantly slower than those of the corresponding
reactions of 3-RuHOH with ethanol. Isotope effects were
determined by direct comparison of the rates (kRuHOH/kRuDOD
)
3.3) and were approximately the same at both 0.032 and 0.051
M EtOD.
Mechanism of Hydrogen Elimination in the Presence of
Ethanol. The mechanism of hydrogen elimination from ruthe-
nium hydride 3 in the presence of EtOH must account for the
exchange of ruthenium deuteride, faster rate of hydrogen loss,
a plateau in rate at high concentrations of EtOH, and larger
isotope effects (kRuHOH/kRuDOD ) 3.3) in the presence of EtOH
than in its absence (kRuHOH/kRuDOD ) 2.08). A mechanism
involving fast and reversible formation of ruthenium dihydrogen
complex B catalyzed by a bridging alcohol molecule, followed
by rate-limiting hydrogen loss, is consistent with each of the
three phenomena (Scheme 6).13
Figure 6. Rate of H2 loss from 3 versus ethanol concentration in toluene-
d8 at 95 °C.
Fast Ruthenium Deuteride Exchange in the Presence of
Ethanol. To determine whether ruthenium deuteride exchanges
in the presence of alcohol, the reaction of 3-RuDOH (∼0.01
M) in the presence of excess ethanol (>0.030 M) was monitored
The fast exchange of hydrogen into the ruthenium deuteride
occurs via rapid and reversible alcohol-assisted formation of
ruthenium dihydride complex B. The two hydrogens of the
dihydrogen ligand are equivalent; thus either can be transferred
back to the oxygen. The result is that the acidic and hydridic
hydrogens exchange 250 times more rapidly than the rate in
which hydrogen is lost.
1
by H NMR spectroscopy between 56 and 63 °C. The rate of
ruthenium deuteride exchange was monitored by the appearance
of the ruthenium hydride resonance (δ -9.55). Although the
reaction rate was somewhat difficult to measure consistently,
the rate was independent of ethanol concentration and displayed
first-order dependence upon ruthenium. Approximate rate
constants were determined at 56 (k ) 2.03 × 10-3 s-1) and 63
°C (k ) 3.66 × 10-3 s-1). The activation energy of the exchange
In the absence of alcohol, the rate is limited by the formation
of dihydrogen complex B. The rate of hydrogen loss from 3 is
faster in the presence of alcohol because this rate limitation is
removed by rapid alcohol-mediated formation of B. At ethanol
concentrations above 0.03 M, the formation of B is rapid and
was determined from these two values (∆Gq ) 23.5 kcal mol-1
)
(12) Assumption of ∆Sq ) 5.0 eu extrapolates to a rate constant of 107 × 10-3
s-1 at 95 °C, while assumption of ∆Sq ) -5.0 eu extrapolates to a rate
constant of 69 × 10-3 s-1 at 95 °C.
and was used to estimate the rate constant for this reaction at
95 °C. When an activation entropy of ∆Sq ) 0 eu is assumed,
the rate constant of this process is 86 × 10-3 s-1, approximately
250 times faster than hydrogen loss from 3-RuHOH in the
absence of ethanol.12
(13) A mechanism involving reversible formation of dihydride intermediate C
prior to rate-limiting formation of B cannot be definitively excluded,
although kinetic isotope effect evidence, calculations, and better understand-
ing of the mechanism under dry conditions render this mechanism unlikely.
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Hydrogen Activation in a Ligand−Metal Bifunctional Catalyst
A R T I C L E S
Figure 8. Possible dimerized structure of ruthenium hydride in toluene
and transition state for hydrogen transfer to form dihydrogen intermediate
B.
Figure 7. Calculated transition state for formation of dihydrogen intermedi-
ate B from ruthenium hydride 3 catalyzed by a second equivalent of 3.
reversible and the rate is determined by hydrogen loss from a
very low equilibrium concentration of B. The equilibrium
between 3 and B is responsible for the saturation phenomenon
observed at high concentrations of ethanol.
Figure 9. Ruthenium compounds used for PGSE models.
The observed deuterium isotope effect of 3.3 for D2 loss from
3-RuDOD with EtOD results from a combination of an equi-
librium isotope effect on the formation of dihydrogen complex
B and a kinetic isotope effect on loss of D2 from B. This is
different from the 2.08 isotope effect on hydrogen loss from
3-RuDOD in the absence of EtOH, which is a kinetic isotope
effect on the formation of dihydrogen complex B from 3.
Problems with Proposed Hydrogen Loss Mechanism in
the Absence of Ethanol. Our initial mechanism for hydrogen
loss from 3 in the absence of alcohols proposed rate-limiting
transfer of the hydroxyl proton to the hydride, forming the H-H
bond of dihydrogen complex B, followed by rapid loss of
dihydrogen (Scheme 5). This proposal was called into question
by a calculated barrier for the hydrogen transfer (43.2 kcal
mol-1) that was much greater than the experimental value, ∆Hq
) 26.1 kcal mol-1. Calculations also suggested that formation
of B would be greatly facilitated by water or alcohol mediation
of transfer of hydrogen from the CpOH group. As outlined
above, we have obtained experimental evidence to confirm the
predicted role of alcohols. This suggested that in the absence
of alcohol, the CpOH unit of a second molecule of 3 might
serve as the mediator for the hydrogen transfer leading to
dihydrogen complex B; calculations on this process predicted
an activation barrier of 25.5 kcal mol-1 (Figure 7).
While calculations favored the presence of 2 equiv of 3 in
the transition state, the kinetic rate law for loss of hydrogen
displayed first-order dependence on 3. The apparent contradic-
tion between two molecules of 3 being involved in the rate-
limiting hydrogen transfer and a first-order dependence on 3
could be reconciled if 3 were associated as a hydrogen-bonded
dimer in toluene. If the starting material and the transition state
both have two ruthenium species, then a kinetic order of one
would be observed.
Aggregation State of 3 in Toluene. Pulsed gradient spin-
echo (PGSE) NMR measurements were used to determine the
state of aggregation of 3 in toluene. The PGSE method provides
a means of monitoring the translational motion of a molecule
through solution, thereby allowing determination of the diffusion
coefficient, D, which is inversely related to its hydrodynamic
volume.14 This method has been successfully utilized to assess
monomer-dimer equilibria in several organometallic systems.15
Figure 10. Plot of results of PGSE experiments on 3 (O), 5 (0), and 9
(4). Note that the points and lines for 3 and 5 overlap.
PGSE experiments were performed on toluene-d8 solutions
of 3-RuHOH and on methoxy-protected ruthenium hydride 9
as a model for a monomeric ruthenium compound and on diru-
thenium bridging hydride complex 5 as a model for a dimer
(Figure 9).
The intensity of the Cp tolyl methyl resonances (δ 1.87) of
3-RuHOH in toluene-d8 (6.6 mg mL-1, 0.0115 M)16 was
measured as a function of diffusion time, which was varied
between 25 and 400 ms. Similar measurements were performed
on solutions of 5 (δ 1.79) and 9 (δ 1.85). Figure 10 shows plots
of ln(I/Io) (where I is the intensity in the presence of the gradient,
and Io is the intensity in the absence of a gradient) versus a
function of diffusion time with a constant gradient strength. Each
point on the plot represents the resonance intensity utilizing a
different diffusion time. The slopes of the resulting plots provide
the diffusion coefficient, D, for each compound, as shown in
eq 2. The absolute value of the slope decreases with increasing
hydrodynamic volume, thus providing a means of determining
the relative volume of each species. The slope for 3-RuHOH
(D ) 2.01 × 10-11 m2 s-1) is nearly equal to that of ruthenium
dimer 5 (D ) 1.98 × 10-11 m2 s-1) and substantially different
from that of the methoxy-protected monomer 9 (D ) 3.65 ×
10-11 m2 s-1). These results indicate that species 3 and 5 have
(15) (a) Williams, C. K.; Breyfogle, L. E.; Choi, S. K.; Nam, W.; Young, V.
G., Jr.; Hillmyer, M. A.; Tolman, W. B. J. Am. Chem. Soc. 2003, 125,
11350. (b) Geldbach, T. J.; Pregosin, P. S.; Albinati, A.; Rominger, F.
Organometallics 2001, 20, 1932. (c) Pichota, A.; Pregosin, P. S.; Valentini,
M; Wo¨rle, M.; Seebach, D. Angew. Chem., Int. Ed. 2000, 39, 153.
(16) Concentration values provided assuming a monomeric state for 3.
(14) Price, W. S. Concepts Magn. Reson. 1997, 9, 299.
9
J. AM. CHEM. SOC. VOL. 127, NO. 9, 2005 3105

A R T I C L E S
Scheme 7
Casey et al.
similar hydrodynamic volumes, which is consistent with species
3 existing as a dimer in toluene.17
individual labeling (1.65 × 1.28 ) 2.11), indicating that all
hydrogen transfers occur in a single step.
Nature of Ruthenium Hydride 3 in Solution with Phos-
phines. Earlier, we noted that we were puzzled by the faster
rates seen in the presence of PMe3 than in PPh3 since the
phosphines did not appear in the rate law. To further investigate
the mechanism of hydrogen loss from 3 in the presence of
phosphines, the state of aggregation of 3 in toluene in the
presence of added PMe3 and PPh3 was investigated by PGSE.
In the presence of excess PPh3 (0.187 M, with 0.0103 M 3),
the intensity of the tolyl methyl group (δ 1.87) was measured
as a function of diffusion time, and a plot of ln(I/Io) yielded a
diffusion coefficient of 1.88 × 10-11 m2 s-1. This diffusion
coefficient is very similar in value to that for ruthenium dimer
5, consistent with the proposal that complex 3 exists as a dimer
in solution in the presence of PPh3. With an excess of PMe3
(0.215 M, with 0.0103 M 3), PGSE gave a diffusion constant
of 3.32 × 10-11 m2 s-1. This diffusion constant is similar in
value to that observed for monomeric complex 9, establishing
that in the presence of trimethylphosphine, ruthenium hydride
3 exists as a monomer, presumably hydrogen bonded to PMe3.
I
Io
δ
3
ln ) γ2δ2G2 ∆ -
D
(2)
[
( )
]
where D is the diffusion coefficient, I the intensity of resonance,
Io the intensity of resonance in the absence of gradient, G the
gradient strength (4.45 G cm-1), γ the gyromagnetic ratio (4.258
× 103 s-1 G-1), δ the length of the gradient pulse (10 ms), and
∆ the delay between gradient pulses ) diffusion time ) varied
from 25 to 400 ms.
To determine whether 3 maintains a dimeric structure in the
presence of EtOH in toluene-d8 or whether the dimer is broken
up by ethanol solvation, PGSE measurements of 3 (6.3 mg
mL-1, 0.0110 M) were carried out in a 0.04 M solution of
ethanol in toluene-d8. The diffusion coefficient (D ) 3.40 ×
10-11 m2 s-1) was similar to that obtained for the monomeric
model compound 9 (D ) 3.65 × 10-11 m2 s-1). Therefore,
ruthenium hydride 3 exists as a monomer in the presence of
excess ethanol in toluene-d8.
A Revised Mechanism of Hydrogen Loss from 3 in the
Absence of Ethanol. The PGSE experiments establishing that
3 is a dimer in toluene require a revision of our initial
mechanism. The first-order kinetic rate law requires that, as the
starting material is dimeric, the transition state must also have
two ruthenium species involved. Calculations support the role
of the CpOH group of a second molecule of 3 in mediation of
proton transfer to form B and predict an activation barrier of
∆Hq ) 25.5 kcal mol-1 for this bimolecular process, very close
to the experimental value of ∆Hq ) 26.1 kcal mol-1 (Figure
7). Since very little exchange between RuH and OD was seen
in the absence of ethanol, formation of intermediate B must be
irreversible and is the rate-limiting step of the overall reaction.
Loss of hydrogen from B and trapping of the resulting
unsaturated intermediate A by phosphine occur after the rate-
limiting step (Scheme 7).
Hydrogen Scrambling in 3-RuDOH in the Presence of
PMe3. When the loss of hydrogen from isotopically labeled
3-RuDOH in the presence of PMe3 was monitored by 1H NMR
spectroscopy at 95 °C, the observation of a resonance at δ -9.55
indicated that RuD/OH scrambling had occurred. After 10 min,
the ratio of RuH:RuD was approximately 1:1, based upon the
intensity of the ruthenium hydride resonance relative to that of
the tolyl groups of 3 and 6. This exchange occurred quickly
relative to HD loss, and the ratio of RuH:RuD remained near
1:1 throughout the reaction.
Mechanism of Hydrogen Elimination from 3 in the
Presence of PMe3. Although there is no discernible differ-
ence in the rate of hydrogen loss from 3 in the presence of
PPh3 and in the complete absence of phosphines, a rate
acceleration is observed in the presence of PMe3. The observa-
tion of rate acceleration and saturation in the presence of
increasing concentrations of PMe3, as well as fast exchange
between ruthenium hydride and hydroxyl positions, indicates
the presence of a mechanism similar to that observed in the
presence of alcohol.
The RuH kinetic isotope effect (kRuHOH/kRuDOH ) 1.28) results
from the conversion of the ruthenium hydride bond to a
dihydrogen complex. The OH kinetic isotope effect (kRuHOH
/
kRuHOD ) 1.65) is now seen as involving the two OH bonds
that mediate the proton transfer in the formation of dihydrogen
complex B. All three of these bonds are altered during rate-
limiting proton transfer, thus resulting in a doubly labeled
isotope effect (kRuHOH/kRuDOD ) 2.08). This is consistent with
the product of the kinetic isotope effects observed upon
PMe3 is significantly more basic than PPh3 or P(m-Tol)3 and
may transfer the acidic hydrogen to the ruthenium hydride via
an acid-base reaction to form dihydrogen intermediate B.
Calculations suggest that PMe3 can assist in proton transfer and
predict a rate-limiting barrier of 25.8 kcal mol-1. Observation
of deuterium scrambling and saturation behavior is consistent
with the reversible formation of dihydrogen complex B,
followed by the rate-limiting loss of hydrogen (Scheme 8).
Although the means of equilibration between 3 and B differ,
this mechanism mirrors that of alcohol-catalyzed hydrogen loss.
(17) In principle, ruthenium hydride 3 may also dimerize through metal hydride
to acidic proton hydrogen bonding. PGSE studies have indicated that the
ruthenium methyl analogue, [2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(CO)2CH3,
also exists as a dimer in toluene, suggesting that dimerization does not
involve the metal hydride unit. The structure involving dimerization through
hydrogen bonding of OH groups to one another, shown in Figure 8, is also
supported by calculations.
9
3106 J. AM. CHEM. SOC. VOL. 127, NO. 9, 2005

Hydrogen Activation in a Ligand−Metal Bifunctional Catalyst
A R T I C L E S
Scheme 8
Scheme 9
Discussion
Hydrogen Loss from 3 Catalyzed by Ethanol. Determina-
tion of the mechanism of hydrogen loss from ruthenium hydride
3 in the presence of ethanol is key to understanding the
hydrogenation of carbonyl compounds catalyzed by 3 since
alcohols are necessarily present as hydrogenation products. As
predicted by our calculations, the presence of alcohol facilitates
proton transfer to form dihydrogen complex B. This faster rate
of proton transfer results in fast RuH/OD exchange, an overall
rate increase, and a rate of hydrogen loss independent of ethanol
concentration above 0.03 M. PGSE studies have established that
in the presence of ethanol in toluene, the hydrogen-bonded dimer
of 3 is broken and 3 exists as a monomer, presumably hydrogen
bonded to ethanol.
Hydrogen loss from 3 in toluene-d8 at 95 °C occurs at a
similar rate in the presence of high concentrations of ethanol (k
) 13.9 × 10-4 s-1) or PMe3 (k ) 14.2 × 10-4 s-1). The
similarity of these rate constants is consistent with their related
mechanisms. Since both EtOH and PMe3 catalyze the fast and
reversible formation of dihydrogen complex B, both rates are
expected to be limited in rate by the loss of hydrogen from B.
At low concentrations of EtOH or PMe3, we observed an
increase in rate as the concentration was increased (Figures 4
and 6). At these concentrations, ruthenium hydride 3 exists as
a mixture of monomer and dimer, there are multiple hydrogen
transfer mechanisms, and the kinetics are undoubtedly complex.
The rate increase reflects the shift from the slower reaction of
dimeric 3 to the faster reaction of monomeric 3 with increasing
concentrations of EtOH or PMe3.
Benefits of Calculations in this Study. Calculations have
often provided important support for and insight into organo-
metallic reaction mechanisms. In this case, calculations played
a more crucial role in crushing our preconceived mechanism in
which the CpOH directly protonates the hydride of 3 to produce
dihydrogen complex B. Calculations were also important in
suggesting the importance of an alternative mechanism involving
proton transfer mediated by alcohols in the conversion of 3 to
B. Computations are particularly valuable when they dispel
incorrect mechanistic preconceptions.
Tautomeric Equilibria Involving Metal Hydrides. The
equilibrium between ruthenium hydride 3 and dihydrogen
intermediate B is an example of a tautomeric equilibrium in-
volving a metal hydride. Other examples include (1) metal
dihydrogen complexes and their tautomeric metal dihydrides,
and (2) complexes in which a metal hydride is intramolecularly
hydrogen bonded to an acidic hydrogen and their tautomeric
dihydrogen complexes.18,19
with a growing body of data that implicate alcohols or alkoxides
as promoters of heterolytic H2 cleavage by homogeneous
reduction catalysts operating through a metal-ligand bifunc-
tional mechanism. Ikariya proposed an alcohol-assisted H2
cleavage for the generation the chiral Cp*Ru diamine hydride
complex 10, which serves as the active species in catalytic
asymmetric hydrogenation of ketones (Scheme 9).20
Morris and Rautenstrauch have also reported faster acetophe-
none hydrogenation by ruthenium dihydride 11 in 2-propanol
compared to benzene (Figure 11). Heterolytic hydrogen cleavage
Figure 11. Ruthenium hydrogenation catalyst 11.
is the rate-limiting step of catalysis, and acceleration is proposed
to result from the stabilization of the polar transition state of
dihydrogen cleavage by the hydrogen bonding ability and the
higher dielectric constant of 2-propanol.21 The possible role of
alcohol as a transfer agent to catalyze the cleavage of hydrogen
was not mentioned.
On the basis of theoretical studies of Ru(II)BINAP(diamine)
(1), Noyori also proposed that the active reducing species is
re-formed by cleaving hydrogen either by direct addition across
the Ru-N bond or by alcohol-mediated hydrogen transfer via
a six-membered transition state (Figure 12).22 Morris has also
contributed to this issue, reporting DFT calculations of proposed
intermediates and transition states for the direct cleavage of
hydrogen by a model of catalyst 1.4b
(18) Lee, J. C., Jr.; Peris, E.; Rheingold, A. L.; Crabtree, R. H. J. Am. Chem.
Soc. 1994, 116, 11014.
(19) Lough, A. J.; Park, S.; Ramachandran, R.; Morris, R. H. J. Am. Chem.
Soc. 1994, 116, 8356.
(20) Ito, M.; Hirakawa, M.; Murata, K.; Ikariya, T. Organometallics 2001, 20,
379.
Alcohol/Alkoxide Promotion of Heterolytic Hydrogen
Cleavage during Metal-Ligand Bifunctional Catalysis. The
observation of alcohol-promoted elimination from 3 is consistent
(21) Rautenstrauch, V.; Hoang-Cong, X.; Churlaud, R.; Abdur-Rashid, K.;
Morris, R. H. Chem.sEur. J. 2003, 9, 4954.
(22) Sandoval, C. A.; Ohkuma, T.; Mun˜iz, K.; Noyori, R. J. Am. Chem. Soc.
2003, 125, 13490.
9
J. AM. CHEM. SOC. VOL. 127, NO. 9, 2005 3107

A R T I C L E S
Casey et al.
3-RuDOH. Ruthenium species 3 was dissolved in toluene in a
resealable NMR tube and degassed with three freeze-pump-thaw
cycles. The NMR tube was cooled to -196 °C in liquid nitrogen, and
1 atm of D2 was added. This mixture was shaken over a period of 2 h
at ambient temperature for complete conversion to 3-RuDOH.10
3-RuHOD. Ruthenium species 3-RuDOD was dissolved in
toluene in a resealable NMR tube and degassed with three freeze-
pump-thaw cycles. The NMR tube was cooled to -196 °C in liquid
nitrogen, and 1 atm of H2 was added. This mixture was shaken over a
period of 2 h at ambient temperature for complete conversion to
3-RuHOD.
Figure 12. Possible hydrogen cleavage mechanisms for catalyst 1.
Scheme 10
General Kinetic Procedure. The general kinetic procedure will be
illustrated with a specific example. A standard THF solution of
ruthenium species 3 containing ferrocene as an internal standard (0.25
mL, 0.0121 M, prepared in a nitrogen atmosphere glovebox from 8.3
mg of 5, ∼2 mg of ferrocene, and 0.9 mL of THF) in a resealable
NMR tube was degassed with three freeze-pump-thaw cycles. This
sample was cooled to -196 °C in liquid nitrogen, and 1 atm H2 was
added. The sample was heated at 85 °C for 8 h to ensure complete
formation of ruthenium hydride 3. The solvent was removed under
low pressure. The NMR tube was refilled with N2, and the solids
were dissolved in a standard toluene-d8 solution of triphenylphos-
phine (0.35 mL, 0.0543 M, prepared in a nitrogen atmosphere glovebox
from 17.1 mg of triphenylphosphine and 1.2 mL of toluene-d8) added
from a 500 µL gastight syringe. The NMR tube was resealed and
inserted into an NMR spectrometer preheated to 95 °C. After locking
and shimming (∼2 min), we began the data acquisition. The disap-
pearance of ruthenium hydride complex 3 and the appearance of the
ruthenium phosphine complex 6 were both followed by 1H NMR
spectroscopy for over 2.5 half-lives (∼90 min). The concentration of
3 was followed by measuring the integrations from δ 1.79 to 1.84 and
from δ -9.5 to -9.6 (tolyl methyl and hydride resonances, re-
spectively) compared to the integration of the ferrocene internal
standard. The concentration of product 6 was followed by measuring
the integrations between δ 1.87-1.93 and δ 7.70-7.80 (tolyl methyl
and aryl resonances, respectively) compared to the integration of the
ferrocene internal standard.
Chen demonstrated that an alkali metal cation is required to
achieve high activity in catalytic asymmetric ketone reductions
using Noyori’s RuH2(diphosphine)(diamine) system. Chen sug-
gested two possible mechanisms for H2 cleavage by the
unsaturated precursor 12 to form the active reducing species 1
(Scheme 10),23 one involving direct amido deprotonation of a
dihydrogen ligand and the other involving deprotonation of a
dihydrogen complex by an alkoxide bound through an alkali
metal cation to the amide group on ruthenium. Recently, Noyori
has proposed alcohol-assisted H2 cleavage for catalytic asym-
metric ketone reductions under neutral conditions with RuH-
(η1-BH4)[(S)-tolbinap][(S,S)-dpen] as the precatalyst.22
Conclusion
Computational Methods. The geometries for all critical species
(reactants, intermediates, transition states, and products) were optimized
in the gas phase using the hybrid density functional theory, B3LYP.25
To simplify calculations, the tolyl and phenyl substituents on the
cyclopentadiene of 3 were replaced by hydrogens. The effective core
potential (ECP) of Hay and Wadt26 and the corresponding basis set
(augmented by an f function) were used for Ru; the 6-31++G(d,p)
basis27 was used for all other main group elements. Vibrational
frequency calculations were subsequently carried out to verify the
character of the optimized structures and to obtain the zero-point
vibrational energy corrections to barrier heights. Solvation effect was
estimated to be small (so results not included), using the polarizable
continuum model (IEF-PCM28) on gas-phase-optimized structures; a
dielectric constant of 7.58 was used for tetrahydrofuran (THF) as
the solvent. All calculations were performed with the Gaussian98
program.29
Our studies of hydrogen loss from ruthenium hydride 3 have
provided significant insight into its mechanistic reverse process,
the heterolytic cleavage of hydrogen in the presence of ethanol.
Reaction begins with addition of hydrogen to ruthenium
unsaturated intermediate A, forming dihydrogen complex B.
Alcohol mediates proton transfer to the carbonyl from B to form
the active reducing species 3. This work contributes to a growing
body of data underscoring the importance of alcohols in
promoting heterolytic hydrogen cleavage in metal bifunctional
catalysis.
Experimental Section
General. All syntheses and sample preparations were performed
following Schlenk techniques or in a nitrogen atmosphere glovebox.
Toluene-d8 and THF were dried over sodium and benzophenone and
distilled prior to use. Solvents were dried with activated alumina
purification columns.24 NMR spectra were recorded on a 360 MHz
Acknowledgment. Financial support from the Department
of Energy, Office of Basic Sciences, is gratefully acknowledged.
1
spectrometer. H NMR spectra are referenced to residual protons in
deuterated solvent; 13C NMR spectra are referenced to carbon-13 in
the deuterated solvent, and 31P NMR are referenced to an external
standard of H3PO4.
(25) (a) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (b) Lee, C.; Yang, W.; Parr,
R. G. Phys. ReV. B 1988, 37, 785. (c) Becke, A. D. J. Chem. Phys. 1993,
98, 5648.
(26) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
(27) (a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54,
724. (b) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972,
56, 2257. (c) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28,
213.
Ruthenium complexes 5, 3, and 3-RuDOD were prepared according
to literature procedure.7 Independent synthesis and characterization of
6, 7, 8, and 9 are provided in the Supporting Information.
(23) (a) Hartmann, R.; Chen, P. Angew. Chem., Int. Ed. 2001, 40, 3581. (b)
Hartmann, R.; Chen, P. AdV. Synth. Catal. 2003, 345, 1353.
(24) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers,
F. J. Organometallics 1996, 15, 1518.
(28) (a) Miertus, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55, 117. (b)
Cammi, R.; Tomasi, J. J. Comput. Chem. 1995, 16, 1449. (c) Mennucci,
B.; Cance`s, E.; Tomasi, J. J. Phys. Chem. B 1997, 101, 10506. (d) Cance`s,
E.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 107, 3032.
9
3108 J. AM. CHEM. SOC. VOL. 127, NO. 9, 2005

Hydrogen Activation in a Ligand−Metal Bifunctional Catalyst
A R T I C L E S
J.J. thanks the ACS Organic Division Emmanuil Troyansky
Graduate Fellowship for support. S.S. thanks the NIH (5T32
GM 08505) for support under a Chemistry Biology Interface
Training Grant. We also thank Dr. Charles Fry for his valuable
assistance with PGSE experiments. Grants from NSF (che-962-
9688) and NIH (I S1- RR04981-01) for the purchase of NMR
spectrometers are acknowledged.
kinetic runs. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA043460R
(29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-
Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.6;
Gaussian, Inc.: Pittsburgh, PA, 1998.
Supporting Information Available: General experimental
methods, experimental details, characterization of complexes
7, 8, and 9, further computational details, and summary of
9
J. AM. CHEM. SOC. VOL. 127, NO. 9, 2005 3109