Reduction of imines by hydroxycyclopentadienyl ruthenium hydride: Intramolecular trapping evidence for hydride and proton transfer outside the coordination sphere of the metal

Article abstract of DOI:10.1021/ja053956o

Reduction of imines by [2,5-Ph₂-3,4-Tol₂(η ⁵-C₄COH)]Ru(CO)₂H (2) produces kinetically stable ruthenium amine complexes. Reduction of an imine by 2 in the presence of an external amine trap gives only the complex of the newly generated amine. Reaction of 2 with H₂N-p-C₆H₄N=CHPh (11), which contains an intramolecular amine trap, gave a 1:1 mixture of [2,5-Ph ₂-3,4-Tol₂(η⁴-C₄CO)](CO) ₂RuNH(CH₂Ph)(C₆H₄-p-NH₂) (8), formed by coordination of the newly generated amine to the ruthenium center, and [2,5-Ph₂-3,4-Tol₂(η⁴-C ₄CO)](CO)₂RuNH₂C₆H ₄-p-NHCH₂Ph (9), formed by coordination of the amine already present in the substrate. These results require transfer of hydrogen to the imine outside the coordination sphere of the metal to give a coordinatively unsaturated intermediate that can be trapped inside the initial solvent cage. Amine diffusion from the solvent cage must be much slower than coordination to the metal center. Mechanisms requiring prior coordination of the substrate to ruthenium would have led only to 8 and can be eliminated.

Full text of DOI:10.1021/ja053956o

Published on Web 09/21/2005
Reduction of Imines by Hydroxycyclopentadienyl Ruthenium
Hydride: Intramolecular Trapping Evidence for Hydride and
Proton Transfer Outside the Coordination Sphere of the Metal
Charles P. Casey,* Galina A. Bikzhanova, Qiang Cui, and Ilia A. Guzei
Contribution from the Department of Chemistry, UniVersity of Wisconsin,
Madison, Wisconsin 53706
Received June 15, 2005; E-mail: casey@chem.wisc.edu
Abstract: Reduction of imines by [2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(CO)2H (2) produces kinetically stable
ruthenium amine complexes. Reduction of an imine by 2 in the presence of an external amine trap gives
only the complex of the newly generated amine. Reaction of 2 with H2N-p-C6H4NdCHPh (11), which contains
an intramolecular amine trap, gave a 1:1 mixture of [2,5-Ph2-3,4-Tol2(η4-C4CO)](CO)2RuNH(CH2Ph)(C6H4-
p-NH2) (8), formed by coordination of the newly generated amine to the ruthenium center, and [2,5-Ph2-
3,4-Tol2(η4-C4CO)](CO)2RuNH2C6H4-p-NHCH2Ph (9), formed by coordination of the amine already present
in the substrate. These results require transfer of hydrogen to the imine outside the coordination sphere of
the metal to give a coordinatively unsaturated intermediate that can be trapped inside the initial solvent
cage. Amine diffusion from the solvent cage must be much slower than coordination to the metal center.
Mechanisms requiring prior coordination of the substrate to ruthenium would have led only to 8 and can be
eliminated.
Introduction
Density functional theory (DFT) calculations at the B3LYP/
LANL2DZ level of theory on the simplified model reaction of
(C5H4OH)Ru(CO)2H (2-H) with formaldehyde support a con-
certed mechanism (Figure 1).6 We have found a concerted
pathway with an early transition state in which the Ru-H bond
has lengthened by 0.11 Å and the C-H distance is 1.42 Å; the
CpO-H bond has lengthened by 0.11 Å, and the forming
H-OMe distance is 1.35 Å. The calculated activation barrier
of 13.8 kcal mol-1 is similar to the experimental ∆Hq of 12
kcal mol-1 found for the reaction of 2 with benzaldehyde.
Details of the calculations of both H2CdO and H2CdNCH3
reduction are presented in Supporting Information. Noyori has
provided theoretical support for concerted hydrogen transfer
from his ruthenium(diamine)(arene) catalysts to carbonyl com-
pounds.7
Several years ago, Shvo discovered that diruthenium hydride
complex {[2,3,4,5-Ph4(η5-C4CO)]2H}Ru2(CO)4(µ-H) (1-S) is an
efficient catalyst for the hydrogenation of aldehydes and
ketones1 and is also very useful in the transfer hydrogenation
of ketones using alcohols as the reducing agent.2,3 In the
reduction of ketones to alcohols, the mononuclear [2,3,4,5-
Ph4(η5-C4COH)Ru](CO)2H (2-S) was proposed to be the active
reducing agent and has been shown to reduce aldehydes and
ketones (Scheme 1).1b,c
On the basis of detailed mechanistic studies on the related
tolyl analogue [2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(CO)2H (2),
including observation of primary deuterium isotope effects for
transfer of both OH and RuH, we proposed a mechanism
involving concerted transfer of proton and hydride to aldehyde
outside the coordination sphere of the metal (Scheme 2).4
Related mechanisms for aldehyde reduction have been proposed
by Noyori for his very active ruthenium-diamine-diphosphine
hydride catalysts.5
Ba¨ckvall has proposed an alternative mechanism involving
Cp ring slippage and aldehyde coordination prior to concerted
transfer of the hydrogens (Scheme 3).8 In this mechanism, an
alcohol complex is the initial product of reduction. In contrast,
in our proposed mechanism, an alcohol is formed outside the
(1) (a) Shvo, Y.; Czarkie, D.; Rahamim, Y. 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.
(2) (a) Almeida, M. L. S.; Beller, M.; Wang, G.-Z.; Ba¨ckvall, J. E. Chem.s
Eur. J. 1996, 2, 1533. (b) Larsson, A. L. E.; Persson, B. A.; Ba¨ckvall, J.
E. Angew. Chem., Int. Ed. Engl. 1997, 36, 1211. (c) Persson, B. A.; Larsson,
A. L. E.; Le Ray, M.; Ba¨ckvall, J. E. J. Am. Chem. Soc. 1999, 121, 1645.
(d) Persson, B. A.; Huerta, F. F.; Ba¨ckvall, J. E. J. Org. Chem. 1999, 64,
5237. (e) Huerta, F. F.; Laxmi, Y. R. S.; Ba¨ckvall, J. E. Org. Lett. 2000,
2, 1037. (f) Laxmi, Y. R. S.; Ba¨ckvall, J. E. Chem. Commun. 2000, 611.
(3) For excellent reviews of ligand-metal bifunctional catalysis, see: (a) Noyori,
R.; Kitamura, M.; Ohkuma, T. Proc. Natl. Acad. Sci. U.S.A. 2004, 101,
5356. (b) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. ReV.
2004, 248, 2201.
(5) (a) Noyori, R.; Ohkuma, T. Angew. Chem., Int. Ed. 2001, 40, 40. (b)
Sandoval, C. A.; Ohkuma, T.; Mun˜iz, K.; Noyori, R. J. Am. Chem. Soc.
2003, 125, 13490.
(6) The values were obtained with the B3LYP method associated with a
double-ú quality basis set (LANL2DZ) with an f polarization function added
to Ru and 6-31++G(d,p) for the main group elements; Ru was treated
with the effective core potential developed by Hay and Wadt. Single point
energies were also calculated with Moller-Plesset (MP2) method at the
B3LYP optimized geometries. To account for solvation effects, single point
solvation free energies were estimated with a continuum model at the IEF-
PCM level. For further details, see the Supporting Information.
(7) Yamakawa, M.; Ito, H.; Noyori, R. J. Am. Chem. Soc. 2000, 122, 1466.
(8) Csjernyik, G.; EÄ ll, A. H.; Fadini, L.; Pugin, B.; Ba¨ckvall, J. E. J. Org.
Chem. 2002, 67, 1657.
(4) Casey, C. P.; Singer, S. W.; Powell, D. R.; Hayashi, R. K.; Kavana, M. J.
Am. Chem. Soc. 2001, 123, 1090.
9
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J. AM. CHEM. SOC. 2005, 127, 14062-14071
10.1021/ja053956o CCC: $30.25 © 2005 American Chemical Society

Reduction of Imines by Hydroxycyclopentadienyl Ru Hydride
A R T I C L E S
Scheme 1
Scheme 2
Results
For a trapping experiment to be mechanistically significant,
it is necessary that the trap form a kinetically stable product
and that the trap be able to effectively compete for a reactive
intermediate. Therefore, control experiments were performed
to determine the exchange rates of the amine complexes and to
determine the relative ability of the two amines to compete for
an independently generated unsaturated intermediate A.
Amine complexes were synthesized by reaction of the amines
with cyclopentadienone dimer 3, which serves as a synthetic
source of unsaturated intermediate A (Scheme 5).
Kinetics of Isopropylamine Dissociation from 5. At the
start, we knew that aliphatic amines dissociate slowly from Ru
since we had found that PPh3 displacement of isopropylamine
from ruthenium complex 5 required heating at 90 °C (Scheme
6).9 The kinetics of this ligand exchange reaction were measured
by adding a >10-fold excess of PPh3 to a toluene-d8 solution
of 5 and monitoring the disappearance of tolyl methyl reso-
coordination sphere of the metal, and an alcohol complex might
or might not be formed following hydrogen transfer to a
noncoordinated aldehyde (Scheme 2).
1
nances (δ 1.87) of 5 by H NMR spectroscopy in a probe
A clear distinction between our proposed concerted mecha-
nism and Ba¨ckvall’s ring slip mechanism centers on whether
the reduced species is initially coordinated to the metal. The
question cannot be addressed for aldehyde reduction since Ru-
alcohol complexes have never been observed and are expected
to be extremely kinetically labile.9 In contrast, imine reduction
produces kinetically stable ruthenium amine complexes and
offers the possibility of determining whether the reduced amine
is initially “born coordinated to the metal”.
To distinguish between these two mechanisms, we set out to
perform what we thought was a simple trapping experiment.
Would reduction of an imine by 2 in the presence of an added
amine trap give only the amine complex derived from the
reduced imine, as suggested by Ba¨ckvall, or would the added
amine give rise to a second amine complex, as suggested by
our proposed mechanism (Scheme 4)?
Here we describe the results of our trapping experiments using
either an intermolecular amine trapping agent or an intramo-
lecular amine trapping agent. These trapping experiments
provide persuasive evidence that reductions proceed outside the
coordination sphere of the metal to give an amine and a
coordinatively unsaturated intermediate inside a solvent cage
and that amine coordination occurs more rapidly than diffusion
from the solvent cage.
preheated to 90 °C. The disappearance of 5 followed first-order
kinetics to over 80% conversion, indicating a first-order
dependence on 5. No dependence of kobs on PPh3 concentration
was observed: kobs ) 1.1 × 10-3 s-1 at 0.20 M PPh3; 0.97 ×
10-3 s-1 at 0.26 M; 1.3 × 10-3 s-1 at 0.39 M; 0.91 × 10-3 s-1
at 0.68 M. This establishes a dissociative mechanism for
substitution reactions of ruthenium amine complexes. It also
demonstrates that alkylamine complexes are very kinetically
stable at room temperature and only undergo substitution upon
heating.
Kinetics of N-Phenylbenzylamine Dissociation from 6.
Aniline displacement of N-phenylbenzylamine from ruthenium
complex 6 was more rapid and occurred at room temperature
(Scheme 6). The kinetics of this ligand exchange reaction were
measured by adding a >10-fold excess of aniline to a CD2Cl2
solution of 6 at low temperature and monitoring the disappear-
1
ance of tolyl methyl resonances (δ 1.78 and 1.82) of 6 by H
NMR spectroscopy at 25 °C. The disappearance of 6 followed
first-order kinetics to over 80% conversion, indicating a first-
order dependence on 6. No dependence of kobs on aniline
concentration was observed: kobs ) 1.5 × 10-3 s-1 at 0.20 M
aniline; 1.8 × 10-3 s-1 at 0.26 M; 1.4 × 10-3 s-1 at 0.32 M;
1.4 × 10-3 s-1 at 0.40 M. Thus, while arylamine complexes
are less kinetically stable than alkylamine complexes, they are
kinetically stable below room temperature and undergo substitu-
tion at moderate rates at room temperature.
(9) Casey, C. P.; Bikzhanova, G. A.; Ba¨ckvall, J.-E.; Johansson, L.; Park, J.;
Kim, Y. H. Organometallics 2002, 21, 1955-1959.
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A R T I C L E S
Casey et al.
Figure 1. Calculated transition state for reaction of (C5H4OH)Ru(CO)2H (2-H) with formaldehyde supports a concerted mechanism with an early transition
state.
Scheme 3
Scheme 5
Scheme 6
Scheme 4
stable and would give reliable information in trapping experi-
ments.
Competition of Amines for Unsaturated Intermediate A.
Control experiments were carried out prior to trapping experi-
ments to determine whether the two amines can effectively
compete for reactive coordinatively unsaturated intermediate A
when it is generated independently. Cyclopentadienone dimer
3 was selected as a synthetic source of unsaturated intermediate
A for these competition studies. In the first competition
experiment, the reaction of dimer 3 with a mixture of N-
methylbenzylamine and isopropylamine at room temperature in
CD2Cl2 produced a 1:1 ratio of amine complexes 4 and 5
(Scheme 7). These two amine complexes are stable indefinitely
in the presence of the other amine at room temperature. For
example, isolated 4 showed no reaction with isopropylamine at
These kinetic studies establish that both aryl- and alkylamine
ruthenium complexes are kinetically stable at low temperature.
Arylamine complexes are less kinetically stable than alkylamine
complexes probably because of their lower basicity. If these
amine complexes were generated during the reduction of imines
well below room temperature, then they would be kinetically
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14064 J. AM. CHEM. SOC. VOL. 127, NO. 40, 2005

Reduction of Imines by Hydroxycyclopentadienyl Ru Hydride
A R T I C L E S
Scheme 7
Scheme 8
Scheme 9
room temperature. At 90 °C, both 4 and 5 exchange amine
ligands in the presence of the other amine. These competition
experiments were carried out in CH2Cl2 because of the low
solubility of 3 in toluene, the solvent employed for imine
reduction.
In a second competition experiment, the reaction of ruthenium
cyclopentadienone dimer 3 with a mixture of N-phenylbenzy-
lamine and aniline at 0 °C in CD2Cl2 produced a 1:3 ratio of
amine complexes 6:7 (Scheme 7). The amine ratio remained
the same upon warming up to room temperature. Arylamine
complexes 6 and 7 are stable indefinitely below 0 °C and
interconvert only slowly at room temperature.
corresponding to the inequivalent tolyl methyl groups of 4. Other
peaks for the newly generated amine complex were too broad
to assign. However, upon warming up to -20 °C, resonances
due to the diastereotopic benzyl hydrogens of 4 at δ 3.58 (ABX,
3
2
3JAB ) 13.1 Hz, JAX ) 11.3 Hz, NHCHH), 3.77 (ABX, JAB
) 13.1 Hz, 3JBX ) 2.4 Hz, NHCHH), and to the N-methyl group
3
of 4 at δ 2.38 (d, J ) 5.8 Hz, NHCH3) were observed. None
of the isopropylamine complex 5 was seen, and 5% would have
been easily detected. No change in the 1H NMR spectrum was
seen upon warming up to room temperature. However, upon
heating at 90 °C for 1 h, resonances due to isopropylamine
3
complex 5 were observed at δ 0.96 (d, J ) 6.5 Hz) and 2.85
These control experiments establish that the selected amine
pairs could fairly compete for the reactive coordinatively
unsaturated intermediate A, if it were generated in the proposed
trapping experiments carried out well below room temperature.
MeNdCHPh Reduction in the Presence of Isopropylamine
as a Possible Trapping Reagent. When a 1:1 mixture of MeNd
CHPh and H2NCHMe2 was added to a toluene-d8 solution of
3
(nonet, J ) 6.5 Hz). The final ratio of 4:5 was 1:1.
The initial exclusive formation of 4, the complex of the newly
generated amine, is in agreement with Ba¨ckvall’s ring slip
mechanism involving prior coordination of the imine. This result
is inconsistent with our mechanism involving hydrogen transfer
to an imine outside the metal coordination sphere, unless
coordination of the newly generated amine is faster than its
diffusion from the solvent cage.
1
ruthenium hydride 2 at -60 °C, low temperature H NMR
spectroscopy showed that the only product was 4, the complex
of the newly generated amine (Scheme 8). The 1H NMR
spectrum showed complete disappearance of the hydride (δ
-9.76) and the hydroxyl (δ 8.60) resonances of 2 within 2 min
at -60 °C.10 New resonances were observed at δ 1.80 and 1.82
PhNdCHPh Reduction in the Presence of Aniline as a
Possible Trapping Reagent. When a mixture of PhNdCHPh
and H2NPh (1:1) was added to a toluene-d8 solution of 2 at
-60 ° C, the only product was 6, the complex of the newly
1
generated amine (Scheme 9). The H NMR spectrum showed
(10) In contrast, the reduction of the less basic N-aryl imine 11 by 2 is greatly
retarded by isopropylamine (see below).
complete disappearance of 2 within 2 min at -60 °C and
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A R T I C L E S
Scheme 10
Casey et al.
Scheme 11
formation of 6.11 None of the aniline complex 7 was seen, and
5% would have been easily detected. Upon warming to room
temperature, a single tolyl methyl resonance at δ 1.80 and a
single NH2 resonance at 3.87 due to aniline complex 7 slowly
appeared. The final ratio of 6:7 was 1:5.
Again, the initial exclusive formation of 6, the complex of
the newly generated amine, is in agreement with Ba¨ckvall’s ring
slip mechanism involving prior coordination of the imine, and
again, this result is consistent with our mechanism involving
hydrogen transfer to an imine outside the metal coordination
sphere only if coordination of the newly generated amine is
faster than its diffusion from the solVent cage.
Intramolecular Amine Trapping Experiments. If coordina-
tion of the newly generated amine is faster than its diffusion
from the solvent cage, then placing the imine and the amine
trap within the same molecule should be able to trap the
coordinatively unsaturated ruthenium intermediate A if it indeed
was generated. We chose H2N-p-C6H4NdCHPh (11) for in-
tramolecular trapping experiments.
in a 25:75 ratio at 0 °C (Scheme 10). Upon warming to room
temperature, the ratio decreased to 20:80. This indicates that
both amine centers can trap A, and that the product amine
complexes 8 and 9 are stable below 0 °C.
Reduction of H2N-p-C6H4NdCHPh (11). When a toluene-
d8 solution of H2N-p-C6H4NdCHPh (11) was added to a
1
toluene-d8 solution of 2 at -60 °C, H NMR spectroscopy
showed resonances for a 1:1 mixture of secondary amine
complex 8 and primary amine complex 9 (Scheme 11). The 1H
NMR spectrum showed complete disappearance of the hydride
(δ -9.76) and the hydroxyl (δ 8.60) resonances of 2 within 2
min at -60 °C. New peaks corresponding to the inequivalent
tolyl methyl groups of secondary amine complex 8 at δ 1.52
and 1.74 and to the single tolyl methyl group of primary amine
complex 9 at δ 1.60 appeared. Other peaks for both 8 and 9
were too broad at -60 °C to assign. Nonetheless, upon warming
to -20 °C, new resonances were observed at δ 3.95 (ABX,
2JAB ) 12.0 Hz, 3JBX ) 2.0 Hz, NHCHH) and 4.19 (ABX, 2JAB
3
) 12.0 Hz, JAX ) 10.0 Hz, NHCHH), corresponding to the
diastereotopic benzyl hydrogens of 8. In addition, a new benzyl
resonance at δ 3.68 and a new NH2 resonance at δ 3.75 for 9
were also observed. The 1:1 ratio of 8:9 remained unchanged
between -60 and -20 °C. Upon warming to 0 °C, secondary
amine complex 8 slowly isomerized to primary amine complex
9 (kobs ) 5.2 × 10-4 s-1, t1/2 ) 22 min at 0 °C). The slow
isomerization of 8 at 0 °C establishes its kinetic stability at lower
temperature. The final ratio of 8:9 was 6:94 in toluene-d8 at
room temperature. This is somewhat different from the 20:80
ratio seen in CD2Cl2.
The formation of both amine complexes 8 and 9 in this
intramolecular trapping experiment requires the intervention of
an intermediate, such as the coordinatively unsaturated species
A. Either the newly formed secondary amine or the primary
amine of the reduction product 12 can coordinate to the
ruthenium of A. This is consistent with the mechanism we
proposed earlier (Scheme 4). The fact that intermolecular amine
traps are incapable of trapping coordinatively unsaturated
intermediate A, therefore, suggests that the newly reduced amine
Control experiments were performed to determine whether
the two different amino groups of the trapping agent could fairly
compete for the reactive coordinatively unsaturated intermediate
A if it were generated and to determine whether the amine
complexes were kinetically stable under their conditions of
formation. Reaction of ruthenium cyclopentadienone dimer 3
with the diamine 4-(N-benzylamino)aniline 12 was performed
at -60 °C in CD2Cl2 to determine the ratio of amine complexes
formed when both amine groups compete for unsaturated
intermediate A (Scheme 10). The reaction proceeded only after
the temperature was increased to 0 °C. Secondary amine
complex [2,5-Ph2-3,4-Tol2(η4-C4CO)](CO)2RuNH(CH2Ph)-
(C6H4-p-NH2) (8) and primary amine complex [2,5-Ph2-3,4-
Tol2(η4-C4CO)](CO)2RuNH2C6H4-p-NHCH2Ph (9) were formed
(11) New resonances were observed at -60 °C at δ 1.78 and 1.82 for the
inequivalent tolyl methyl groups of 6. Other peaks for 6 were too broad at
-60 °C to assign. After warming up to -20 °C, new peaks corresponding
to the diastereotopic benzyl hydrogens of 6 appeared at δ 4.02 (ABX, 2JAB
) 13.5 Hz, 3JAX ) 2.5 Hz, NHCHH), 4.24 (ABX, 2JAB ) 13.5 Hz, 3JBX
11.5 Hz, NHCHH).
)
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Reduction of Imines by Hydroxycyclopentadienyl Ru Hydride
A R T I C L E S
Scheme 12
and A are generated in a solvent cage, and that complexation
is faster than diffusion from the cage. It should be noticed that
more of the newly generated secondary amine complex 8 (50%)
is formed in this imine reduction in toluene (Scheme 11) than
from the trapping of intermediate A in the reaction of dienone
dimer 3 in CH2Cl2 (25%) (Scheme 10). This may not be simply
a solvent effect (see Discussion).
Mechanisms, including Ba¨ckvall’s ring slip mechanism
(Scheme 3), which involve prior coordination of the substrate
to ruthenium would have led only to 8, in which the newly
formed secondary amine is coordinated to ruthenium. Such
mechanisms are inconsistent with the intramolecular trapping
experiments leading to 1:1 mixtures of 8:9 and can be
eliminated.
mixture of amine complexes 8 and 9; no isopropylamine
complex 5 was seen.
Upon warming to room temperature, isomerization of 8 to 9
occurred in less than 15 min without the appearance of
isopropylamine complex 5. After 24 h at room temperature, 1H
NMR spectroscopy showed the formation of 5. The 60:40 ratio
of 8:5 seen after 24 h shifted slowly to 20:80 after 7 days at
room temperature.
Again, isomerization of 8 to 9 occurred without trapping by
an external amine. This supports the view that amine dissociation
and recoordination from 8 and 9 is faster than diffusion from
the solvent cage. At longer times, trapping by the external amine
occurs.
Rate Retardation by Isopropylamine. Unusually slow imine
reduction by 2 was seen only with the combination of strongly
basic isopropylamine with the weakly basic N-aryl imine 11.
Both the combination of weakly basic aniline with the weakly
basic N-aryl imine 11, and the combination of strongly basic
isopropylamine with the strongly basic N-alkyl imine, MeNd
CHPh, led to rapid reactions with 2 at -60 °C. We hypothesized
that the less basic N-aryl imine 11 cannot effectively compete
with isopropylamine for hydrogen bonding to 2; this drastically
decreases the amount of free 2 available for reaction with 11
and greatly decreases the rate of reaction of 2 with 11. In
contrast, the more basic N-alkyl imine, MeNdCHPh, is better
able to compete with isopropylamine for hydrogen bonding to
2 and reacts rapidly. Similarly, the less basic N-aryl imine 11
effectively competes with the less basic aniline for hydrogen
bonding to 2; aniline does not hydrogen bond as strongly to 2
and is less effective at slowing the reaction with 11. Since the
reaction of 2 with 11 is too fast to measure at -60 °C, we cannot
determine whether the reaction is somewhat slower in the
presence of aniline.
Interaction of Isopropylamine with Hydroxycyclopenta-
dienyl Ruthenium Hydride 2. To explain why isopropylamine
dramatically slows the reduction of H2N-p-C6H4NdCHPh (11)
by 2, we hypothesized that hydrogen bonding between isopro-
pylamine and the hydroxyl group of the hydroxycyclopentadi-
enyl ligand of 2 would reduce the concentration of 2 available
for reaction with 11. To test this hypothesis, we investigated
the interaction of 2 with isopropylamine by NMR and IR
spectroscopy.
H2N-p-C6H4NdCHPh Reduction in the Presence of p-
Isopropylaniline. We next investigated trapping experiments
in which both an intramolecular and an intermolecular trap were
available. When a solution of H2N-p-C6H4NdCHPh (11) and
p-isopropylaniline (1:1) in toluene-d8 was added to a toluene-
d8 solution of 2 at -60 °C, a 1:1 mixture of two amine
1
complexes 8 and 9 was observed by H NMR spectroscopy.
No resonances for p-isopropylaniline complex 10 were seen.
Reduction was complete within 2 min at -60 °C. Upon warming
to -20 °C, resonances for the 1:1 mixture of the two amine
complexes 8 and 9 became sharper, but their ratio did not change
and no 10 was seen.
Upon warming to room temperature, isomerization of 8 to 9
occurred in less than 15 min to give a 6:94 equilibrium ratio of
8:9. No p-isopropylaniline complex 10 was seen at this point.
After 24 h at room temperature, 1H NMR spectroscopy provided
evidence for the formation of 10: δ 1.12 (d, 3J ) 7.0 Hz, CH-
(CH3)2), 2.75 (septet, 3J ) 7.0 Hz, CH(CH3)2), 4.39 (NH2). The
final ratio of 8:9:10 was 6:75:19.12
The reduction of H2N-p-C6H4NdCHPh (11) in either the
presence or absence of p-isopropylaniline was too fast to
measure and was complete within 2 min at -60 °C.10
It is interesting that the external trapping agent p-isopropy-
laniline does not compete with intramolecular trapping. It is
also intriguing that isomerization of 8 to 9 occurs without
trapping of coordinatively unsaturated intermediate A by
p-isopropylaniline. Amine dissociation and recoordination from
8 and 9 is apparently faster than diffusion from the solvent cage.
Reduction of H2N-p-C6H4NdCHPh (11) in the Presence
of Isopropylamine was dramatically slower than in its absence.
When a solution of H2N-p-C6H4NdCHPh (11) and isopropy-
lamine (1:1) in toluene-d8 was added to a toluene-d8 solution
of 2 at -60 °C, no reduction occurred. Reduction slowly
proceeded only after increasing the temperature to -20 °C. 1H
NMR spectroscopy showed that the only products were a 1:1
An inversion-saturation T1 NMR experiment was performed
in order to determine whether isopropylamine is hydrogen
bonded to 2 (Scheme 12). T1 is a measure of the spin-lattice
relaxation rate and is dependent on the tumbling rate, which
should be faster for free rather than hydrogen bonded isopro-
pylamine.13 T1 times are correlated between the isopropylamine
and organometallic fragments and different from the T1 of the
free isopropylamine in solution.14 The T1 for the isopropyl
methyl resonance of 2-H2NCHMe2 was 0.4 s, while the T1 time
(12) The observed 6:75 ()7:93) ratio of 8:9 is within experimental error of the
6:94 equilibrium ratio reported above.
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A R T I C L E S
Casey et al.
tadienone oxygen atom and the H atom on the coordinated
primary amine. The five-membered ring of the dienone ligand
is not planar; the C(O) carbon atom is displaced from the plane
defined by atoms C(2), C(3), C(4), and C(5) away from the Ru
center by 0.139(10) Å. Thus, the Ru-C(1) distance of 2.407-
(8) Å is longer than the Ru-C distances to C(2) and C(5) (av.
2.240(10) Å), which are adjacent to C(1), and then the Ru-C
bond lengths to the other two cyclopentadienone carbon atoms
C(3) and C(4) (av. 2.179(17) Å). The envelope flap angle in
the cyclopentadienone ligand between the planes defined by
atoms C(2)-C(3)-C(4)-C(5) and C(2)-C(1)-C(5) measured
9.1(9)°. O(1) is not coplanar with atoms C(2)-C(1)-C(5) and
is displaced toward the amine ligand as a consequence of the
hydrogen bonding interaction, with the angle between the Cd
O vector and the C(2)-C(1)-C(5) plane being 4.2(9)°.
Figure 2. A molecular drawing of 9 shown with 30% probability ellipsoids.
Discussion
for the free isopropylamine was 2.8 s at -60 °C. This provides
compelling evidence for association between 2 and isopropy-
lamine.
Two mechanisms have been proposed for the reduction of
aldehydes by hydroxycyclopentadienyl ruthenium hydride 2. We
proposed a mechanism involving concerted transfer of proton
and hydride to aldehyde outside the coordination sphere of the
metal that generates the coordinatively unsaturated intermediate
A (Scheme 2).4 Ba¨ckvall proposed an alternative mechanism
involving Cp ring slippage and aldehyde coordination prior to
concerted transfer of the hydrogens (Scheme 3). These mech-
anisms are not readily distinguished in the case of aldehyde
reduction because the alcohol complexes are too labile and
undetected. Imine reduction offered the possibility of determin-
ing whether the reduced amine is initially “born coordinated to
the metal” since Ru-amine complexes are kinetically stable.
Intermolecular Trapping Experiments were carried out that
involved reaction of 2 with an imine in the presence of an added
amine trap (Scheme 4). In both cases studied, only the amine
complexes from the reduced imine were observed at low
temperature (Schemes 8 and 9); amine complexes expected from
trapping of an unsaturated intermediate, such as A, by the added
amine were not seen. While these results are consistent with
prior imine coordination proposed in Ba¨ckvall’s ring slip
mechanism, they can also be explained by imine reduction
outside the coordination sphere to give a new amine and
coordinatively unsaturated species A inside a solvent cage
followed by more rapid coordination of the amine to ruthenium
than diffusion apart.
An Intramolecular Trapping Experiment was devised to
distinguish these possibilities that involved reduction of H2N-
p-C6H4NdCHPh (11) by 2 at -60 °C (Scheme 11). In this case,
a 1:1 mixture of two amine complexes 8 and 9 was formed. 8
is the complex of the newly generated amine, while 9 is the
complex of the intramolecular amine trap, and both are
kinetically stable at low temperature. The formation of 9 requires
the intervention of a coordinatively unsaturated intermediate A
that can be trapped. Since Ba¨ckvall’s mechanism would have
led to 8 as the exclusive kinetic product, this ring slippage
mechanism can be ruled out.
Norton’s IR method for determination of the pKa of metal
carbonyl hydrides was adapted to study hydrogen bonding
between 2 and H2N(CHMe2).15 When isopropylamine (1.3
equiv) was added to a toluene solution of 2 (0.022 M), IR bands
due to 2 at 2015 and 1956 cm-1 disappeared and new bands at
1999 and 1946 cm-1 appeared and were assigned to
2-H2NCHMe2. These shifts provide evidence for strong hy-
drogen bonding in toluene. Previously, in determining the pKa
of hydroxyl proton of 2 in acetonitrile, we observed IR bands
at 1985 and 1920 cm-1 for NEt3H+[2,5-Ph2-3,4-Tol2(η5-C4CO)]-
Ru(CO)2H]-.4
The inversion-saturation T1 NMR and IR experiments
provide strong evidence for hydrogen bonding between isopro-
pylamine and the hydroxyl group of the hydroxycyclopentadi-
enyl ligand of 2. This supports the hypothesis that this interaction
reduces the concentration of 2 available for reaction with 11
and is responsible for the much slower rates seen in the presence
of isopropylamine.
The X-ray Structure of 916,17 is similar to that of the
isopropylamine complex (η4-C4Ph4CO)(CO)2RuNH2CHMe2.9
The Ru metal center in 9 is formally five-coordinate with a
bidentate (η4-Ph2Tol2C4CdO) ligand, two carbonyls, and an
amine (Figure 2). The short C(1)-O(1) distance of 1.264(8) Å
is best described as a CdO unit. The Ru-C(1) bond in 9 is
significantly shorter than the average corresponding distance
of 2.58(7) Å in related complexes18-20 due to a moderately
strong O(1)‚‚‚H-N(1) hydrogen bond between the cyclopen-
(13) (a) Sanders, J. K. M.; Hunter, B. K. Modern NMR Spectroscopy, A Guide
for Chemists; Oxford University Press: London, 1993. (b) Abragam, A.
The Principles of Nuclear Magnetism; Oxford University Press: London,
1971.
(14) Bakhmutov, V. I.; Vorontsov, E. V.; Antonov, D. Yu. Inorg. Chim. Acta
1998, 278, 122.
(15) (a) Jordan, R. F.; Norton, J. R. J. Am. Chem. Soc. 1982, 104, 1255. (b)
Moore, E. J.; Sullivan, J. M.; Norton, J. R. J. Am. Chem. Soc. 1986, 108,
2257.
(16) X-ray crystal data for C46H38N2O3Ru (9): monoclinic, P21, a ) 11.719(3)
Å, b ) 9.906(2) Å, c ) 17.403(5) Å, â ) 97.938(10)°, V ) 2000.9(10)
Å3, Z ) 2, T ) 100(2) K, Dcalcd ) 1.274 Mg/m3, R(F) ) 0.0466 for 5737
independent reflections. All non-hydrogen atoms were refined with
anisotropic displacement coefficients. All hydrogen atoms were included
in the structure factor calculation at idealized positions and were allowed
to ride on the neighboring atoms with relative isotropic displacement
coefficients.
The combination of the failure of intermolecular trapping with
successful intramolecular trapping provides evidence for hydride
(19) Bruno, I. J., Cole, J. C., Edgington, P. R., Kessler, M., Macrae, C. F.,
McCabe, P., Pearson, J., Taylor, R. Acta Crystallogr. 2002, B58, 389-
397.
(20) The distances from ruthenium to the cyclopentadienone carbon atoms were
compared with the corresponding values in 29 related complexes reported
to the Cambridge Structural Database (CSD).
(17) Bruker-AXS: SADABS V.2.05, SAINT V.6.22, SHELXTL V.6.10 and
SMART 5.622 Software Reference Manuals, Madison, Wisconsin, USA,
2000-2003.
(18) Allen, F. H. Acta Crystallogr. 2002, B58, 380-388.
9
14068 J. AM. CHEM. SOC. VOL. 127, NO. 40, 2005

Reduction of Imines by Hydroxycyclopentadienyl Ru Hydride
A R T I C L E S
Scheme 13
Scheme 14
and proton transfer to the imine outside the metal coordination
sphere to produce the new amine and intermediate A inside a
solvent cage; coordination of the new amine to ruthenium occurs
before diffusion apart.
the dienone oxygen in B to give diamine and A within the
solvent cage and re-formation of a different hydrogen bond and
complexation before diffusion out of the solvent cage can occur.
As pointed out earlier, more of the newly generated secondary
amine complex 8 (50%) was formed in reduction of imine 11
in toluene than in the reaction of dienone dimer 3 with diamine
12 in CH2Cl2 (25%). This might be due to collapse of the initial
hydrogen bonded intermediate B to 8 before breaking the
hydrogen bond to give A and diamine 12 inside the solvent
cage.
When the reduction of H2N-p-C6H4NdCHPh (11) by 2 was
performed in the presence of an external amine, such as
p-isopropylaniline or isopropylamine, only intramolecular trap-
ping products 8 and 9 were formed, confirming that only amines
inside the initial solvent cage can compete for coordination to
ruthenium. Interestingly, the isomerization of 8 to 9 in the
presence of an external amine occurred intramolecularly without
formation of complexes of the added amines. This is consistent
with amine dissociation from 8 to give A and the diamine within
a solvent cage. Again, coordination of the amine to A is more
rapid than diffusion apart. At longer times and higher temper-
ature, 8 and 9 do react with external amines to give new
products. Escape from the solvent cage can occur, but it is
slower than coordination.
Is it really believable that diffusion from the solvent cage
can be slower than coordination of the newly produce imine to
intermediate A? What might slow escape from the cage? We
suggest that the newly reduced amine is initially hydrogen
bonded to the dienone oxygen as B (Scheme 13). This is
expected to be a weak hydrogen bond between a weakly acidic
amine hydrogen and a weakly basic dienone oxygen. (The
dienone oxygen acts as a base in hydrogen bonding to a proton
on a complexed amine in all of the amine complexes reported
here.) In a nonpolar solvent, such as toluene, hydrogen bonding
between these two units has few competitors. The barrier for
escape from the cage is the sum of the hydrogen bond energy
and activation energy for diffusion. We suggest that the barrier
for coordination of an amine to B is lower than this combined
barrier. The intramolecular isomerization of 8 to 9 occurred
without trapping by an external amine. This isomerization
requires breaking the hydrogen bond between the diamine and
In other work on the mechanism of imine reduction by 2 in
THF, a shift in the rate-determining step was seen as a function
of imine basicity.21 For imines with electron-withdrawing
substituents on nitrogen, significant kinetic isotope effects were
observed for concerted transfer of hydride and proton from 2
to the imine. For imines with electron-donating alkyl substituents
on nitrogen, we observed imine isomerization, deuterium
exchange, and inverse equilibrium isotope effects that estab-
lished a mechanism involving reversible hydrogen transfer to
the imine followed by rate-determining coordination of the
amine to ruthenium (Scheme 14).
Another very rapid process was uncovered in our unpublished
studies of the stereochemistry of imine reduction by 2.22 When
hydrogen is transferred to an imine, two stereocenters are
formed, one at carbon and a second at nitrogen. We established
that hydrogen is transferred via a stereospecific addition of
deuterium from 2-RuDOD to PhNdCHPh and related imines,
resulting in a trans addition of deuterium (Scheme 15). The
observed stereospecificity requires that complexation of the
stereospecifically formed amine to ruthenium be much faster
than nitrogen inversion, which has a very low barrier (∼7.5
kcal mol-1).23
(21) Casey, C. P.; Johnson, J. B. J. Am. Chem. Soc. 2005, 127, 1883.
(22) Casey, C. P.; Bikzhanova, G. A. Unpublished results.
9
J. AM. CHEM. SOC. VOL. 127, NO. 40, 2005 14069

A R T I C L E S
Scheme 15
Casey et al.
(CD2Cl2, 500 MHz): δ 2.18 (s, tolyl CH3), 2.24 (s, tolyl CH3), 3.69
(br d, 3J ) 11.0 Hz, NH), 3.80 (ABX, 2JAB ) 13.5 Hz, 3JAX ) 2.5 Hz,
In summary, the combination of failed intermolecular and
successful intramolecular trapping of a coordinatively unsatur-
ated intermediate in the reduction of imines by hydroxycyclo-
pentadienyl ruthenium hydride 2 provides convincing evidence
for hydrogen transfer outside of the metal coordination sphere.
In addition, we have uncovered a remarkable array of extremely
fast reactions of the hydrogen bonded unsaturated species B.
“Slow” reactions of B include breaking of the hydrogen bond
and diffusion from the solvent cage and inversion of the nitrogen
lone pair. These reactions are only slow in comparison with
the even faster coordination of nitrogen to ruthenium and
reversible dehydrogenation of the amine.
2
3
NHCHH), 4.39 (ABX, JAB ) 13.5 Hz, JBX ) 11.5 Hz, NHCHH),
6.8-7.4 (m, 26 H, aromatic), 7.79 (dd, 3J ) 8.0, 1.0 Hz, 2 H, aromatic).
13C{1H} NMR (CD2Cl2, 125 MHz): δ 21.1, 21.2 (tolyl CH3); 63.3
(NCH2Ph); 83.5, 85.1 (C 3, 4 of Cp); 103.8, 103.9 (C 2, 5 of Cp);
119.9, 125.1, 126.6, 126.8, 127.9, 128.2, 128.6, 128.8, 129.0, 129.1,
129.5, 130.5, 131.0, 132.2, 132.3, 132.8, 133.1, 137.2, 138.0, 138.3,
150.7 (aromatic);24 163.8 (C 1 of Cp); 199.1, 201.2 (CO). HRMS (ESI)
(M + H)+ calcd for C46H37NO3102Ru, 754.1895; found, 754.1885.
[2,5-Ph2-3,4-Tol2(η4-C4CO)](CO)2RuNH(CH2Ph)(C6H4-p-NH2) (8)
and [2,5-Ph2-3,4-Tol2(η4-C4CO)](CO)2RuNH2C6H4-p-NHCH2Ph (9).
A solution of 4-amino-N-(benzylidene)aniline (4.0 mg, 0.02 mmol) was
added to a solution of 2 (11.4 mg, 0.02 mmol) in toluene-d8 (0.5 mL)
at -78 °C. The solution was slowly warmed to -30 °C. Both 8 and 9
were formed in a 1:1 ratio at -30 °C. The amine complex 8 was not
isolated since it isomerized to 9 upon warming to room temperature.
1H and 13C{1H} NMR spectra for 8 were obtained by subtracting
Experimental Section
[2,5-Ph2-3,4-Tol2(η4-C4CO)](CO)2RuNH(CH3)(CH2Ph) (4). Pro-
cedure A. A solution of N-benzylidenemethylamine (3.4 µL, 0.02
mmol) was added to a solution of 2 (11.4 mg, 0.02 mmol) in toluene-
d8 (0.5 mL) at -78 °C. The solution was slowly warmed to room
temperature, and solvent was evaporated under vacuum to give a green
solid which was recrystallized from hexane at -10 °C to give pure 4
as a pale green precipitate (8 mg, 62% yield).
1
resonances belonging to 9 from the H and 13C{1H} NMR spectra of
the mixture of 8 and 9. For 8: 1H NMR (CD2Cl2, 500 MHz, -30 °C)
δ 2.16 (s, tolyl CH3), 2.22 (s, tolyl CH3), 3.47 (ABX, 3JAX ) 10.0 Hz,
2
3JBX ) 2.0 Hz NHCHH), 3.52 (br s, NH2), 3.63 (ABX, JAB ) 12.0
3
2
3
Hz, JBX ) 2.0 Hz, NHCHH), 4.27 (ABX, JAB ) 12.0 Hz, JAX
)
Procedure B. N-Methylbenzylamine (26 µL, 0.2 mmol) was added
via syringe to a CD2Cl2 suspension of ruthenium cyclopentadienone
dimer (3) (114 mg, 0.1 mmol), and the mixture was stirred for 30 min,
until all the material dissolved. Solvent was evaporated under vacuum
to give a green solid which was recrystallized from hexane at -10 °C
to give pure 4 as a pale green precipitate (109 mg, 79%), mp 174-
10.0 Hz, NHCHH), 6.80-7.48 (m, 25 H, aromatic), 7.72 (d, 3J ) 7.5
Hz, 2 H, aromatic). 13C{1H} NMR (CD2Cl2, 125 MHz, -30 °C): δ
20.88, 20.94 (tolyl CH3); 63.26 (NCH2Ph); 82.63, 84.25 (C 3, 4 of
Cp); 102.86, 102.91 (C 2, 5 of Cp); 116.09, 119.39, 126.18, 126.24,
127.56, 127.68, 127.78, 128.25, 128.34, 128.38, 129.17, 129.90, 130.38,
131.68, 132.26, 132.50, 136.79, 137.56, 137.66, 141.45, 143.35, 144.76
(aromatic);25 162.74 (C1 of Cp); 198.53, 200.92 (CO).
1
177 °C (dec). IR (CH2Cl2): 2011 (s), 1952 (s) cm-1. H NMR (CD2-
Cl2, 250 MHz): δ 1.66 (br m, NH), 2.22 (s, tolyl CH3), 2.24 (s, tolyl
[2,5-Ph2-3,4-Tol2(η4-C4CO)](CO)2RuNH2C6H4-p-NHCH2Ph (9).
Procedure A. A solution of 4-amino-N-(benzylidene)aniline (4.0 mg,
0.02 mmol) was added to a solution of 2 (11.4 mg, 0.02 mmol) in
toluene-d8 (0.5 mL) at -78 °C. The solution was slowly warmed to
-30 °C to give a 1:1 mixture of 8:9. Upon warming to room
temperature, 8 isomerized to 9 within 20 min. The final ratio of 8:9
was 4:96. Solvent was evaporated under vacuum to give a green solid
which was recrystallized from hexane at -10 °C to give pure 9 as a
yellowish green precipitate (10 mg, 67% yield).
CH3), 2.40 (d, 3J ) 5.8 Hz, NCH3), 3.58 (ABX, 2JAB ) 13.1 Hz, 3JAX
2
3
) 11.3 Hz, NHCHH), 3.77 (ABX, JAB ) 13.1 Hz, JBX ) 2.4 Hz,
NHCHH), 6.95 (t, 3J ) 8.0, 4 H, aromatic), 7.06-7.31 (m, 15 H,
aromatic), 7.56 (m, 2 H, aromatic), 7.72 (m, 2 H, aromatic). 13C{1H}
NMR (CD2Cl2, 75 MHz): δ 21.1, 21.2 (tolyl CH3); 45.1 (NCH3); 66.2
(NCH2Ph); 83.5, 84.2 (C 3, 4 of Cp); 103.7, 104.0 (C 2, 5 of Cp);
126.6-138.0 (20 resonances, aromatic); 163.8 (C1 of Cp); 200.9, 201.0
(CO). HRMS (EI) calcd for C41H35NO3102Ru, 691.1654; found,
691.1785.
Procedure B. 4-(N-Benzylamino)aniline (20.0 mg, 0.1 mmol) was
added to a CD2Cl2 suspension of ruthenium cyclopentadienone dimer
(3) (57 mg, 0.05 mmol), and the mixture was stirred for 30 min, until
all the material dissolved. Solvent was evaporated under vacuum to
give a green solid which was recrystallized from hexane at -10 °C to
give pure 9 as a yellowish green precipitate (6.5 mg, 84%), mp 176-
178 °C (dec). IR (CH2Cl2): 2015 (s), 1956 (s) cm-1. 1H NMR (C6D6,
500 MHz): δ 1.78 (s, 6 H, tolyl CH3), 3.06 (t, 3J ) 5.8 Hz, NH), 3.69
[2,5-Ph2-3,4-Tol2(η4-C4CO)](CO)2RuNH(Ph)(CH2Ph) (6). Proce-
dure A. A solution of N-benzylideneaniline (3.6 mg, 0.02 mmol) was
added to a solution of 2 (11.4 mg, 0.02 mmol) in toluene-d8 (0.5 mL)
at -78 °C. The solution was slowly warmed to room temperature, and
solvent was evaporated under vacuum to give a green solid which was
recrystallized from hexane at -10 °C to give pure 6 as a pale green
precipitate (9 mg, 60% yield).
Procedure B. N-phenylbenzylamine (35 µL, 0.20 mmol) was added
via syringe to a CD2Cl2 suspension of ruthenium cyclopentadienone
dimer (3) (114 mg, 0.10 mmol), and the mixture was stirred for 30
min, until all the material dissolved. Solvent was evaporated under
vacuum to give a green solid which was recrystallized from hexane at
-10 °C to give pure 6 as a dark green precipitate (112 mg, 75%), mp
3
3
(d, J ) 5.5 Hz, CH2), 3.75 (br s, NH2), 5.91 (d, J ) 8.5 Hz, 2 H,
3
3
aromatic), 6.18 (d, J ) 8.5 Hz, 2 H, aromatic), 6.65 (d, J ) 8.0 Hz,
3
4 H, aromatic), 6.97-7.12 (m, 11 H, aromatic), 7.21 (d, J ) 8.5 Hz,
4 H, aromatic), 8.14 (d, J ) 8.5 Hz, 4 H, aromatic). 13C{1H} NMR
3
(CD2Cl2, 125 MHz): δ 21.17 (tolyl CH3); 48.69 (NCH2Ph), 84.39 (C
1
191-193 °C (dec). IR (CH2Cl2): 2016 (s), 1957 (s) cm-1. H NMR
(24) If none of the aryl resonances were accidentally equivalent, then 24 peaks
would be expected; 21 were seen.
(25) If none of the aryl resonances were accidentally equivalent, then 24 peaks
would be expected; 22 were seen.
(23) Bushweller, C. H.; O’Neil, J. W.; Bilofsky, H. S. Tetrahedron 1972, 28,
2697.
9
14070 J. AM. CHEM. SOC. VOL. 127, NO. 40, 2005

Reduction of Imines by Hydroxycyclopentadienyl Ru Hydride
A R T I C L E S
3, 4 of Cp); 103.46 (C 2, 5 of Cp); 113.19, 120.03, 126.70, 127.50,
127.79, 128.02, 128.60, 128.88, 129.03, 131.03, 132.25, 133.25, 137.70,
137.90, 139.84, 145.61 (aromatic); 163.55 (C1 of Cp); 200.25 (CO).
HRMS (ESI) (M + H)+ calcd for C46H38N2O3102Ru, 769.2005; found,
769.2015.
Acknowledgment. Financial support from the Department
of Energy, Office of Basic Energy Sciences, and from NSF
(CHE-9629688) and NIH (I S10 RR04981-01) for the purchase
of NMR spectrometers is gratefully acknowledged.
Procedure for Trapping Experiments will be illustrated with a
specific example. A 50 µL aliquot of a standard toluene-d8 solution of
N-benzylidenemethylamine (0.40 M, 20 µmol) and isopropylamine (0.40
M, 20 mmol) was added via a gastight syringe to a solution of 2 (11.4
mg, 20 mmol, 0.044 M) in toluene-d8 (0.45 mL) and cooled to -78
°C. The sample was inserted into an NMR spectrometer precooled to
-60 °C, and spectra were acquired.
Supporting Information Available: General experimental
information, preparation of solutions of 2, preparation of 7 and
10, kinetics of amine exchange reactions, T1 measurement of
2, DFT calculations, and X-ray structure of 9. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA053956O
9
J. AM. CHEM. SOC. VOL. 127, NO. 40, 2005 14071