The PPh3-substituted hydroxycyclopentadienyl ruthenium hydride [2,5-Ph2-3,4-Tol2(η5-C4COH)] Ru(CO)(PPh3)H is a more efficient catalyst for hydrogenation of aldehydes

Article abstract of DOI:10.1021/om0509387

The phosphine-substituted hydroxycyclopentadienyl ruthenium hydride [2,5-Ph₂-3,4-Tol₂(η⁵-C₄COH)]- Ru(CO)(PPh₃)H (8) displayed behavior significantly different from that of the dicarbonyl analogue, including the failure to form an unreactive diruthenium complex analogous to the Shvo catalyst 5. Complex 8 shows no apparent reduction of aldehydes in the absence of a trap but exchanges deuterium between the hydride of 8-RuDOD and the aldehydic hydrogen of p-tolualdehyde. This provides evidence that aldehyde reduction by 8 occurs but is reversible. 8 catalyzes the hydrogenation of benzaldehyde under mild temperature and pressure conditions. A rate law of -d[RCH=O]/dt = κ [RCH=O][8][H₂] ⁰ was obtained.

Full text of DOI:10.1021/om0509387

1230
Organometallics 2006, 25, 1230-1235
The PPh3-Substituted Hydroxycyclopentadienyl Ruthenium Hydride
[2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(CO)(PPh3)H Is a More Efficient
Catalyst for Hydrogenation of Aldehydes
Charles P. Casey,* Neil A. Strotman, Sharon E. Beetner, Jeffrey B. Johnson,
David C. Priebe, Thomas E. Vos, Babak Khodavandi, and Ilia A. Guzei
Department of Chemistry, UniVersity of WisconsinsMadison, Madison, Wisconsin 53706
ReceiVed NoVember 1, 2005
The phosphine-substituted hydroxycyclopentadienyl ruthenium hydride [2,5-Ph2-3,4-Tol2(η5-C4COH)]-
Ru(CO)(PPh3)H (8) displayed behavior significantly different from that of the dicarbonyl analogue, in-
cluding the failure to form an unreactive diruthenium complex analogous to the Shvo catalyst 5. Complex
8 shows no apparent reduction of aldehydes in the absence of a trap but exchanges deuterium between
the hydride of 8-RuDOD and the aldehydic hydrogen of p-tolualdehyde. This provides evidence that alde-
hyde reduction by 8 occurs but is reversible. 8 catalyzes the hydrogenation of benzaldehyde under mild
temperature and pressure conditions. A rate law of -d[RCHdO]/dt ) k[RCHdO][8][H2]0 was obtained.
Introduction
Recent advances in hydrogenation catalysts are revolution-
izing carbonyl reduction chemistry. This wave of new research
was initiated by Shvo’s development of his hydroxycyclopen-
tadienyl ruthenium hydride catalyst (1),1 a species which utilizes
electronically coupled acidic and hydridic hydrogens to ef-
ficiently catalyze the reduction of polar unsaturated species using
hydrogen as the terminal reductant (Figure 1). Noyori has
spearheaded the development of this new series of catalysts,
terming the hydrogen cooperativity “ligand-metal bifunctional
catalysis.” Ru(arene)(diamine) (2) and Ru(BINAP)(diamine) (3)
catalysts have become the most prominent members of this class,
each capable of the reduction of ketones in high yields and
enantioselectivities with over a million turnovers.2-5 These
catalysts utilize 2-propanol or hydrogen as the terminal reduc-
tant, providing “green” alternatives to stoichiometric reducing
agents such as LiAlH4 and NaBH4.6
Figure 1. Ligand-metal bifunctional catalysts.
4 and on Noyori’s catalyst 2 support the outer-sphere concerted
transfer of hydrogen to carbonyl compounds and to imines
(Figure 2).8,9
Computational studies on Noyori’s Ru(p-cymene)(TsDPEN)
catalyst (2) have indicated that the concerted transfer of the
hydride and proton to carbonyls without prior coordination to
the metal center is the lowest energy pathway for hydrogen
transfer.7 Our kinetic isotope effect observations on the tolyl
analogue of Shvo’s hydroxycyclopentadienyl ruthenium hydride
Figure 2. Concerted hydrogen transfer mechanism for ligand-
metal bifunctional catalysts 2 and 4.
* To whom correspondence should be addressed. E-mail: casey@
chem.wisc.edu.
(1) (a) Shvo, Y.; Czarkie, D.; Rahamim, Y.; Chodosh, 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.
(2) (a) Haack, K.-J.; Hashiguchi, S.; Fujii, A.; Ikariya, T.; Noyori, R.
Angew. Chem., Int. Ed. 1997, 36, 285. (b) Noyori, R.; Hashiguchi, S. Acc.
Chem. Res. 1997, 30, 97. (c) Marsumura, K.; Hashiguchi, S.; Ikariya, T.;
Noyori, R. J. Am. Chem. Soc. 1997, 119, 8738.
(3) Noyori, R.; Ohkuma, T. Angew. Chem., Int. Ed. 2001, 40, 40 and
references therein.
(4) (a) Abdur-Rashid, K.; Faatz, M.; Lough, A. J.; Morris, R. H. J. Am.
Chem. Soc. 2001, 123, 7473. (b) Abdur-Rashid, K.; Lough, A. J.; Morris,
R. H. Organometallics 2001, 20, 1047. (c) Abdur-Rashid, K.; Clapham, S.
E.; Hadzovic, A.; Harvey, J. N.; Lough, A. J.; Morris, R. H. J. Am. Chem.
Soc. 2002, 124, 15104.
Our detailed kinetic studies, which provided accurate activa-
tion parameters for each step of the Shvo catalytic system, have
enabled simulation of the overall rate of the Shvo catalytic cycle
and of the relative concentrations of the ruthenium species
present during catalysis (Scheme 1).10 Due to the reactive nature
of the two species directly involved in the catalytic cycle
(ruthenium hydride 4 and the oxidized, coordinatively unsatur-
ated intermediate A), the most stable species in the Shvo
catalytic system is the diruthenium bridging hydride 5, which
(7) Yamakawa, M.; Ito, H.; Noyori, R. J. Am. Chem. Soc. 2000, 122,
1466.
(5) For a recent review of hydrogenation of polar bonds catalyzed by
ruthenium hydride complexes, see: Clapham, S. E.; Hadzovic, A.; Morris,
R. H. Coord. Chem. ReV. 2004, 248, 2201.
(8) Casey, C. P.; Singer, S. W.; Powell, D. R.; Hayashi, R. K.; Kavana,
M. J. Am. Chem. Soc. 2001, 123, 1090.
(9) Casey, C. P.; Johnson, J. B. J. Org. Chem. 2003, 68, 1998.
(10) Casey, C. P.; Johnson, J. B. Unpublished results.
(6) Fehring, V.; Selke, R. Angew. Chem., Int. Ed. 1998, 37, 1827.
10.1021/om0509387 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 01/25/2006

Efficient Ru Catalyst for Aldehyde Hydrogenation
Organometallics, Vol. 25, No. 5, 2006 1231
Scheme 2
is in equilibrium with the active reducing agent 4 and coordi-
natively unsaturated A. At low temperatures, the rate of
dissociation of 5 severely limits the overall rate of reduction.
At high temperatures (>80 °C), the reversible dissociation to 5
becomes rapid enough to support efficient catalysis, but the
unreactive diruthenium compound 5 still constitutes a large
fraction of the ruthenium species.
[2,5-Ph2-3,4-Tol2(η5-C4COH)Ru(CO)(PPh3)H] (8). Reac-
tion of the triphenylphosphine chloride complex 7 with sodium
methoxide in refluxing methanol for 4 h led to the isolation of
the phosphine-substituted ruthenium hydride complex 8 in 83%
yield as an air-sensitive orange powder. In the 1H NMR
spectrum of 8, a doublet (JHP ) 34.2 Hz) at δ -10.37 established
the presence of a ruthenium hydride coupled to phosphorus. In
the X-ray crystal structure of 8 (Figure 4), the bulky PPh3 ligand
lies directly below the hydroxyl group on the Cp ligand. The
phosphine hydride complex 8 is significantly more air sensitive
than the corresponding chloride complex 7. Upon exposure to
air, a solution of 8 decomposes within seconds to unidentified
dark material.
Scheme 1
To develop more active ruthenium catalysts, we sought to
minimize the amount of ruthenium present as an unreactive diru-
thenium hydride. Previously, we reported use of CpNHPh analo-
gues that prevented dimerization but required acid catalysis for
fast rates.11 Here we report a promising alternative strategy in-
volving replacement of a carbonyl with a phosphine ligand. Two
benefits were anticipated from phosphine substitution: (1) the
steric bulk of the phosphine was expected to destabilize the diru-
thenium bridging hydride, and (2) its electron-donating proper-
ties were expected to increase the hydride donor ability of the
hydridic active species. In addition, the availability of large num-
bers of phosphines with varying steric and electronic properties
provides many possibilities for future catalyst development.
Here we describe the preparation of a triphenylphosphine-
substituted hydroxycyclopentadienyl ruthenium hydride catalyst,
which does not form diruthenium bridging hydride species. We
also report its activity as an aldehyde hydrogenation catalyst
under mild conditions and discuss the catalytic cycle involved.
Figure 4. X-ray crystal structure of 8. Selected bond lengths (Å)
and angles (deg): Ru(1)-P(1), 2.2792(10); P(1)-Ru(1)-H(1),
81.6(13); C(32)-Ru(1)-H(1), 78.6(13); C(32)-Ru(1)-P(1),
88.18(13); P(1)-Ru(1)-C(1)-O(1), 8.9(3); C(32)-Ru(1)-C(1)-
O(1), 90.0(4).
Results
[2,5-Ph2-3,4-Tol2(η5-C4COH)Ru(CO)(PPh3)Cl] (7). Reac-
tion of ruthenium chloride 6 with excess PPh3 in refluxing THF
for 4 days produced the corresponding triphenylphosphine-
substituted ruthenium chloride complex 7 in 91% yield (Scheme
2). 7 was characterized by spectroscopy and by X-ray crystal-
lography (Figure 3).
Acidity of PPh3-Substituted Ruthenium Complexes. The
acidity of the hydroxyl proton is vital to comparing the reactivity
of phosphine-substituted ruthenium hydrides to that of the parent
dicarbonyl complex 4. The pKa of the unsubstituted ruthenium
hydride 4 was previously determined to be 17.5 in CH3CN using
Norton’s IR method.12 The same procedure was used to deter-
mine the pKa values of the hydroxyl proton in the PPh3-substi-
tuted chloride 7 (pKa ) 18.1) and hydride 8 (pKa ) 20.7). These
phosphine-substituted complexes are significantly less acidic
than the related dicarbonyl compounds (pKa ) 12.0 and 17.5,
respectively). The diminished acidity influences the reactivity
of 8 and may alter the mechanism of hydrogen transfer.
Loss of H2 from 8 occurs more slowly than from dicarbonyl
ruthenium hydride 4, which loses H2 upon heating at 80 °C for
several hours. When the triphenylphosphine-substituted ruthe-
nium hydride 8 (15 mM) was heated at 95 °C in toluene-d8
under 200 Torr of CO for 72 h, 1H NMR spectroscopy showed
about 30% loss of H2 from 8 and appearance of a resonance (δ
(11) Casey, C. P.; Vos, T. E.; Singer, S. W.; Guzei, I. A. Organometallics
2002, 21, 5038.
(12) (a) Jordan, R. F.; Norton, J. R. J. Am. Chem. Soc. 1982, 104, 1255.
(b) Moore, E. J.; Sullican, J. M.; Norton, J. R. J. Am. Chem. Soc. 1986,
108, 2257.
Figure 3. X-ray crystal structure of 7. Selected bond lengths (Å)
and angles (deg): Ru-P, 2.3423(13); P-Ru-Cl, 86.73(5); C(32)-
Ru-Cl, 93.09(16); C(32)-Ru-P, 89.79(14); P-Ru-C(1)-O(1),
1.2(4); Cl-Ru-C(1)-O(1), 89.0(4).

1232 Organometallics, Vol. 25, No. 5, 2006
Casey et al.
1.90) consistent with the formation of the ruthenium dicarbonyl
phosphine [2,5-Ph2-3,4-Tol2(η4-C4CO)]Ru(CO)2(PPh3).
Exchange of Ruthenium Hydride with D2. In contrast to
the dicarbonyl ruthenium hydride 4, which undergoes complete
exchange of D2 with RuH (but not with CpOH) within 1 h at
room temperature,13 the triphenylphosphine-substituted ruthe-
nium hydride 8 showed no exchange of RuH with D2 (4 atm)
at room temperature over 48 h. When it was heated at 95 °C
for 96 h, approximately 50% exchange of ruthenium hydride
with D2 was observed, which is similar to the rate of hydrogen
loss (30% over 72 h).
Stoichiometric Aldehyde Reduction by 8. When the reaction
of 8 with excess benzaldehyde in toluene-d8 at room temperature
was monitored by 1H NMR spectroscopy, partial disappearance
of the ruthenium hydride resonance (δ -10.37) and concurrent
appearance of resonances due to benzyl alcohol (δ 4.30) were
seen within 30 min, indicating that some reduction had occurred.
However, little additional reduction was seen over the following
2 h, indicating that equilibrium was being approached. No
bridging hydride resonances were observed, but new tolyl
methyl resonances were seen at δ 1.61 and 1.81 for the
ruthenium product 9.
Figure 6. X-ray crystal structure of 9. Selected bond lengths (Å)
and angles (deg): Ru-P, 2.3362(14); C(32)-Ru-O(3), 97.49(18);
O(3)-Ru-P, 84.52(11); C(32)-Ru-P, 87.73(16); P-Ru-C(1)-
O(1), 3.1(4).
Scheme 3
To isolate this new ruthenium complex (9), a large excess of
benzaldehyde (>100 equiv) was added to a toluene solution of
8. After several hours, solvent was evaporated and the crude
product was washed with pentane to remove excess aldehyde
1
and alcohol. The resulting yellow solid displayed two tolyl H
NMR resonances at δ 1.61 and 1.81.
Spectroscopy was little help in determining the structure of
9. The absence of a hydride resonance eliminated a diruthenium
bridging hydride as a possible structure. No resonances for
coordinated benzaldehyde or coordinated benzyl alcohol were
present. The largest fragment seen in the electrospray mass
spectrum corresponded to (8 - H)+. IR spectroscopy showed
the presence of only a single metal carbonyl stretching frequency
at 1960 cm-1, and 31P NMR spectroscopy indicated only a single
phosphine resonance at δ 52.4. 13C NMR spectroscopy showed
a resonance at δ 173.7, which is consistent with the carbonyl
carbon of an η4-dienone moiety.
Chemical Characterization of Ruthenium Complex 9. As
expected for a highly reactive four-coordinate Ru(0) complex
such as B, the new complex 9 rapidly reacted with H2, rapidly
dehydrogenated alcohols, and rapidly reacted with benzylamine
to produce the amine complex 11 (Scheme 3). When excess
benzyl alcohol (0.311 M) was added to a solution of 9 (0.0105
M) in toluene-d8, nearly complete conversion of 9 to ruthenium
hydride 8 was observed within 10 min at room temperature.
These products result from the microscopic reverse of reduction,
the transfer of hydrogen from benzyl alcohol to the unsaturated
species.
X-ray Crystal Structure of Reactive Complex 9. Eventu-
ally, a single crystal of the new complex 9 was obtained and,
much to our surprise, X-ray crystallography showed that it was
the benzoic acid adduct [2,5-Ph2-3,4-Tol2(η5-C4COH)]-
Ru(O2CPh)(PPh3)(CO) (9) (Figure 6).
At this point, we considered two possible structures for 9,
the unsaturated 16e dienone carbonyl triphenylphosphine com-
plex [2,5-Ph2-3,4-Tol2(η4-C4CO)]Ru(PPh3)(CO) (B) and its
dimer C, analogous to the well-known dienone dicarbonyl dimer
10 (Figure 5). Since cis and trans isomers of dimer C are
1
possible, four H NMR tolyl resonances and two 31P NMR
resonances would have been expected. However, the possibility
that only a single isomer formed could not be excluded. The
electrospray mass spectrum would be compatible with dimer
C if dissociation occurred during ionization. The monoruthenium
nature of complex 9 was later established by PGSE NMR
experiments (see the Supporting Information). At this point, we
favored structure B, even though four-coordinate Ru(0) com-
plexes are rare and are expected to be highly reactive.
Since most of the yellow powder did not form crystals, we
could not be sure that the bulk material was the same as that of
the single crystals of 9. In principle, 1H NMR spectroscopy could
distinguish between 9 and dienone B or dienone dimer C on
the basis of the ratio of integrals for the aryl hydrogen and
methyl hydrogen resonances (33:6 vs 38:6). However, due to
the low solubility of the material, this was not a reliable
technique. Unfortunately, neither 13C NMR spectroscopy nor
HRMS could distinguish between 9 and B either. The frequency
of the carbon resonance at δ 173.7 is consistent with both a
dienone carbonyl and a carboxylate carbonyl, while the exact
mass obtained could be formulated as either [B + H]+ or as
[9 - O2CPh]+.
Instead, we turned to making a similar derivative starting from
p-tolualdehyde and 8. If reduction of p-tolualdehyde by 8 gave
a product with exactly the same spectral properties as from
benzaldehyde, then a carboxylic acid complex could be ruled
out as the bulk material. However, if a p-toluic acid adduct were
formed, it would contain an additional methyl group which could
Figure 5.
(13) The mechanism of this exchange reaction is not well understood
and is currently under investigation: Casey, C. P.; Johnson, J. B.; Singer,
S. W.; Cui, Q. J. Am. Chem. Soc. 2005, 127, 3100.

Efficient Ru Catalyst for Aldehyde Hydrogenation
Organometallics, Vol. 25, No. 5, 2006 1233
be easily observed by NMR spectroscopy. 8 was combined with
a large excess (80 equiv) of p-tolualdehyde in toluene; after 24
h, the toluene was evaporated and the residue was washed with
pentane to remove p-tolualdehyde and 4-methylbenzyl alcohol.
or that reduction was reversible and thermodynamically unfa-
vorable in the absence of a trap for unsaturated species B.
To test the hypothesis of reversible reduction, we monitored
the reaction of p-tolualdehyde (0.215 M) with the dideuterated
analogue [2,5-Ph2-3,4-Tol2(η5-C4COD)]Ru(CO)2D (8-RuDOD;
1
The resulting yellow powder (9-Me) showed H NMR reso-
1
nances in CD2Cl2 for tolyl methyl groups at δ 2.06, 2.24, and
2.30 and for a hydroxyl proton at δ 3.68. 1H NMR spectroscopy
in CD2Cl2 showed that treatment of 9-Me with H2 produced 8
and p-toluic acid (δ 2.44).
This spectroscopic data established that 9-Me is the p-toluic
acid adduct [2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(O2CTol)(PPh3)-
(CO) (9-Me) and that the bulk material from the benzaldehyde
reduction was the benzoic acid adduct 9. We suggest that
benzoic acid adduct 9 is formed during reduction of benzalde-
hyde by 8, due to the presence of small amounts of benzoic
acid in the excess benzaldehyde employed. The reduction of
benzaldehyde by 8 produces the unsaturated reactive intermedi-
ate B, which is then trapped by benzoic acid to give 9 (Scheme
4).
10.8 µM) in toluene-d8 at 26 °C by H NMR spectroscopy
(Scheme 6). The hydride resonance of 8 grew in with a half-
life of ∼25 min. The growth of the hydride resonance followed
pseudo first-order kinetics; assuming second-order kinetics (vide
infra) gave k ) 2.24 × 10-3 M-1 s-1. When the reaction of
p-tolualdehyde (0.215 M) with the singly labeled isotopologue
[2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(CO)2D (8-RuDOH) (10.8 µM),
obtained by shaking 8-RuDOD with H2O, was monitored by
1H NMR spectroscopy at 26 °C in toluene-d8, the hydride
resonance of 8 appeared with a half-life of ∼14 min (k ) 4.07
× 10-3 M-1 s-1).
Scheme 6
Scheme 4
These isotope exchange experiments clearly demonstrate that
8 reduces aldehydes reversibly and that the equilibrium lies far
on the side of 8 and the aldehyde, rather than B and alcohol,
due to the inherent instability of the unsaturated 16e intermediate
B. These isotope exchange experiments also provided an initial
measure of the deuterium kinetic isotope effect for aldehyde
reduction: kOH/kOD ) 1.8.
Hydrogenation of Benzaldehyde Catalyzed by 8. Having
observed reversible aldehyde reduction by 8, we decided to
investigate the catalytic hydrogenation of benzaldehyde. If H2
trapped the unsaturated intermediate B and converted it back
to hydride 8, then aldehyde reduction would be possible. The
hydrogenation of benzaldehyde (0.530 M) under 2.5 atm of H2
in toluene-d8 was catalyzed by 8 (18 mM, 3 mol %). 1H NMR
spectroscopy showed that disappearance of benzaldehyde fol-
lowed a first-order decay and was completely converted to
benzyl alcohol (29 turnovers) over 10 h at room temperature.
The ruthenium hydride 8 was the only ruthenium compound
observed during the course of the hydrogenation.
Previously, we reported the ruthenium dicarbonyl formate
complex [2,5-Ph-3,4-Tol(η5-C4COH)]Ru(CO)2(O2CH) (12),14
which underwent rapid exchange with DCO2H at -20 °C. This
exchange was proposed to involve reversible formation of
unsaturated intermediate A (Scheme 5). It seems likely that the
reactions of 9 and 9-Me with H2 or alcohols involve fast,
reversible dissociation of the carboxylic acid to give the
unsaturated intermediate B, followed by reaction of H2 or
alcohol with B to form 8.
Scheme 5
The hydrogenation of benzaldehyde (0.97 M) catalyzed by 8
(5.2-8.0 mM, 0.5-0.8 mol %) in toluene was also monitored
in a high-pressure vessel (11-55 atm H2) at 22 and 45 °C using
in situ IR spectroscopy with a REACT-IR probe. The hydro-
genations were followed by monitoring the disappearance of
the benzaldehyde CO band at 1709 cm-1. These reactions
followed first-order decay of benzaldehyde and were complete
in 5-20 h. Plots of ln [PhCHO] vs time were linear to over
90% reaction, indicating a first-order rate dependence on
aldehyde and no inhibition by the product benzyl alcohol (see
Supporting Information).
Stoichiometric Aldehyde Reduction by 8 in the Absence
of a Carboxylic Acid. When an excess of freshly distilled
p-tolualdehyde (<0.01% p-toluic acid) was added to a solution
of 8 in CD2Cl2 or C6D6, no formation of 4-methylbenzyl alcohol
was observed, even after 20 h. Addition of p-toluic acid led to
rapid formation of 4-methylbenzyl alcohol and 9-Me.
The failure to observe 4-methylbenzyl alcohol suggested
either that an acid catalyst was necessary for aldehyde reduction
Table 1 shows the dependence of the rates of these catalytic
hydrogenations on the concentration of 8 and on H2 pressure.
This process shows a first-order dependence on the catalyst
concentration (entries 4-6) and essentially no dependence on
(14) Casey, C. P.; Singer, S. W.; Powell, D. R. Can. J. Chem. 2001, 79,
1002.

1234 Organometallics, Vol. 25, No. 5, 2006
Casey et al.
Table 1. Catalytic Hydrogenation of Benzaldehyde (0.97 M)
in Toluene
Discussion
In an effort to develop a more active ruthenium hydrogenation
catalyst related to the Shvo system, we sought to minimize the
fraction of ruthenium present as an unreactive diruthenium
hydride by adding a bulky triphenylphosphine ligand. This
strategy produced a significantly different catalyst system in
which ruthenium shuttles between hydride 8 and an unseen
unsaturated intermediate B without the involvement of any
unreactive diruthenium species.
[8]
(mM)
temp
(°C)
pressure
(atm)
kobs
k
entry
(10-5 s-1
)
(10-3 M-1 s-1
)
1
2
3
4
5
6
7
18.1
8.0
5.5
5.2
7.2
8.0
5.9
22
22
45
45
45
45
45
2.5a
35
11
35
35
35
55
12
5.6
16
16
21
23
18
6.7
7.0
29
31
29
29
31
No evidence for a phosphine-substituted diruthenium bridging
hydride analogous to 5 was obtained in studies of the phosphine-
substituted hydride 8. In the all-carbonyl system, the unseen
coordinatively unsaturated intermediate A either reacts with
ruthenium hydride 4 to form the unreactive bridging hydride 5
or reacts with H2 to re-form the monoruthenium hydride 4
(Scheme 1). In contrast, the phosphine-substituted coordinatively
unsaturated intermediate B reacts only with H2 to re-form
ruthenium hydride 8 (Scheme 7). The steric bulk of the PPh3
ligand is probably the crucial factor in destabilizing diruthenium
species. As a consequence, the phosphine-substituted catalyst
system does not have an unreactive sink for ruthenium and all
the ruthenium is present as the active reducing agent 8.
While no reduction of p-tolualdehyde by 8 was seen in the
absence of a trap, isotopic exchange between ruthenium
deuteride (8-RuDOD) and p-tolualdehyde provided evidence
for reversible reduction. In the absence of a trap, the equilibrium
lies far in the direction of aldehyde and 8. Hydrogen gas proved
to be an efficient trap for reactive intermediate B, and
hydrogenation of benzaldehyde catalyzed by 8 occurred under
very mild temperature and pressure conditions (22 °C, 2.5 atm
of H2) (Scheme 7).
a This hydrogenation of benzaldehyde (0.530 M) was carried out in a
resealable NMR tube and monitored by H NMR spectroscopy.
1
Scheme 7
H2 pressure over a wide range (entries 1 and 2; entries 3, 4,
and 7). There is a moderate temperature dependence, and an
increase of 23 °C produced a rate acceleration of ∼4. Addition
of benzoic acid gave rates of catalytic hydrogenation which were
within experimental error of those obtained in the absence of
benzoic acid.
These experiments established the rate law -d[PhCHO]/dt
) kobs[PhCHO][H2]0 ) k[8][PhCHO][H2]0. This rate law is
consistent with the catalytic cycle shown in Scheme 7. Rate-
limiting hydrogen transfer from 8 to the aldehyde produces the
corresponding alcohol and the unsaturated intermediate B. Under
the catalytic conditions, B reacts much more rapidly with H2
than with alcohols to complete the catalytic cycle by regenerat-
ing 8.
Experimental Section
[2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(CO)(Cl)(PPh3) (7). A solu-
tion of PPh3 (458 mg, 1.75 mmol) and ruthenium chloride 6 (330
mg, 0.54 mmol) in THF (35 mL) was refluxed under N2 for 4 days
with a periodic nitrogen purge. Solvent was evaporated under
reduced pressure, and the residual solid was recrystallized from
THF/pentane (4:1) at -30 °C to afford orange crystals of 7 (413
mg, 91% yield). Slow evaporation of a THF/hexane solution yielded
X-ray-quality crystals. While crystalline 7 is stable for extended
Hydrogenation of Propanal Catalyzed by 8. To evaluate
the utility of catalyst 8 for hydrogenation of aliphatic aldehydes,
the hydrogenation of propanal (1.358 M) with 8 (5.2 mM) in
toluene at 45 °C under 35 atm of H2 was followed by monitoring
the aldehyde CO band at 1733 cm-1. The disappearance of
propanal followed first-order decay. Using the rate law estab-
lished for hydrogenation of benzaldehyde with 8, a rate constant
of k ) 4.6 × 10-2 M-1 s-1 was determined. Catalyst 8
hydrogenated propanal ∼50% faster than it did benzaldehyde.
1
periods in air, its solutions are air sensitive. H NMR (360 MHz,
CD2Cl2): δ 2.21 (s, CpTolCH3), 3.48 (s, OH), 6.90-7.50 (m,
aromatic). 13C{1H} NMR (90 MHz, CD2Cl2): δ 21.3 (CpTolCH3);
21.5 (CpTolCH3); 76.1, 86.0 (C 3,4 of Cp); 102.1, 105.1 (C 2,5 of
Cp); 128-138 (31 resonances, aromatic); 137.6 (C1 of Cp); 206.8
(d, JCP ) 18.1 Hz, CO). 31P{1H} NMR (146 MHz, CD2Cl2) δ 46.5.
IR (CH2Cl2): 1950 cm-1. HRMS (ESI): calcd (found) for
[C50H39ClO2PRu]- 839.1420 (839.1416).
Imine Hydrogenation Catalyzed by 8. The hydrogenation
of N-benzylideneaniline (PhNdCHPh) (0.252 M) under ∼4 atm
of H2 in toluene-d8 at 63 °C catalyzed by 8 (9.5 mM, 4 mol %)
[2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(CO)(H)(PPh3) (8). A de-
gassed solution of 7 (250 mg, 0.297 mmol) and NaOMe (9 mL,
0.5 M) in MeOH was refluxed under N2 for 4 h. After the mixture
was cooled, a 5% HCl solution (7.5 mL) was added under a stream
of N2, leading to formation of an off-white precipitate. This
suspension was extracted with toluene (3 × 10 mL), and the toluene
extract was evaporated under vacuum to afford 8 (199 mg, 83%
yield) as a pale yellow powder. Slow diffusion of pentane into a
toluene solution of 8 at -30 °C gave bright yellow crystals suitable
for X-ray crystallographic analysis. 1H NMR (toluene-d8, 300
MHz): δ -10.37 (d, JHP ) 34.2 Hz, RuH), 1.83 (s, CpTolCH3),
1.89 (s, CpTolCH3), 4.25 (s, OH), 6.63-7.54 (aromatic). 1H NMR
(300 MHz, C6D6): δ -10.07 (d, JPH ) 34.5 Hz, RuH), 1.80 (s,
CpTolCH3), 1.84 (s, CpTolCH3), 3.28 (s, OH), 6.67-6.91 (m, 15H),
1
was monitored by H NMR spectroscopy. The disappearance
of imine (δ 8.09) followed pseudo first-order kinetics; a
nonlinear least-squares fit of the plot of [imine] versus time
gave kobs ) 1.10 ( 0.03 × 10-5 s-1; k ) (1.17 ( 0.03) × 10-3
M-1 s-1. After 86 h, the reaction was complete (27 turnovers).
Throughout the reaction, the only appreciable ruthenium species
in solution was 8 (δ 1.87, 1.82) and there was no evidence for
ruthenium amine complexes. However, ∼29% catalyst decom-
position to several unidentified products was seen. For com-
parison, the hydrogenation of benzaldehyde catalyzed by 8 at
45 °C (k ) 3.0 × 10-2 M-1 s-1) occurs ∼26 times faster than
imine hydrogenation at a temperature 18 °C higher.
1
6.97-7.10 (m, 4H), 7.45-7.56 (m, 10H), 7.71 (m, 4H). H NMR
(300 MHz, CD2Cl2): -10.71 (d, JPH ) 33.6 Hz, RuH), 2.19 (s,

Efficient Ru Catalyst for Aldehyde Hydrogenation
Organometallics, Vol. 25, No. 5, 2006 1235
CpTolCH3), 2.22 (s, CpTolCH3), 3.20 (s, OH), 6.85 (d, J ) 8.0
Hz, 2H), 6.90 (d, J ) 7.7 Hz, 2H), 6.98 (m, 6H), 7.08 (m, 4H),
7.14-7.37 (m, 19 H). 13C{1H} NMR (90 MHz, C6D6): δ 21.3
(CpTolCH3); 21.4 (CpTolCH3); 88.8, 89.0 (C 3,4 of Cp); 103.4,
103.5 (C2,5 of Cp); 125-139 (31 resonances, aromatic); 137.1 (C1
of Cp); 208.4 (d, JCP ) 19 Hz, CO). 31P{1H} NMR (145 MHz,
C6D6) δ 63.4. IR (CH2Cl2): 1918 cm-1. HRMS (ESI): calcd
(found) for [C50H40O2PRu]- 805.1809 (805.1808).
the residue was washed with pentane to give 9-Me (10.5 mg, 90%)
as a pale yellow powder.
Exchange of Deuterium from 8-RuDOD into p-Tolualdehyde.
Excess distilled p-tolualdehyde (15.2 µL, 129 µmol) was added to
a solution of 8-RuDOD (150 µL (0.0430 M solution in toluene-
d8), 6.45 µmol) in toluene-d8 (0.45 mL), and the appearance of a
1
hydride resonance of 8 (δ -10.37) was monitored by H NMR
spectroscopy at 26 °C over 140 min. Pseudo first-order appearance
of the hydride resonance was observed.
[2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(O2CPh)(PPh3)(CO) (9). In
an inert-atmosphere glovebox, a toluene solution of 8 (40.0 mg,
0.050 mmol) and benzaldehyde (0.40 mL, 3.94 mmol) was stirred
at room temperature for 24 h. Solvent was evaporated under
vacuum, and the residual solid was redissolved in toluene. Ben-
zaldehyde (0.40 mL, 3.94 mmol) was added, and the solution was
stirred for 24 h. Solvent was evaporated under vacuum, and the
resulting solid was washed twice with pentane to afford 9 (29.9
mg, 65% yield) as a pale yellow solid. X-ray-quality crystals of 9
were obtained by recrystallization from toluene. 1H NMR (toluene-
d8, 360 MHz): δ 1.61 (s, CpTolCH3), 1.81 (s, CpTolCH3), 3.88
(s, OH), 6.6-8.1 (38 H, aromatics). 13C{1H} NMR (THF-d8, 125
MHz): δ 20.9, 22.0 (CpTolCH3); 73.3, 85.5 (C3, 4 of Cp); 102.2,
105.5 (C2, 5 of Cp); 126-138 (24 resonances, Ar); 173.7 (PhCO2);
209.0 (d, JPC ) 20.4 Hz, CO). 31P{1H} NMR (toluene-d8, 145
Catalytic Hydrogenation of Benzaldehyde by 8 Monitored
by NMR Spectroscopy. A solution of benzaldehyde (20 µL, 0.1
mmol, 0.526 M) and ruthenium hydride 8 (7.3 mg, 9.1 µmol, 18
mM) in 0.5 mL of toluene-d8 in a resealable NMR tube was
degassed by three freeze-pump-thaw cycles, cooled to -196 °C,
and placed under 0.66 atm of H2. Benzaldehyde disappearance (δ
9.59, aldehyde CH) and benzyl alcohol appearance (δ 4.30, CH2OH)
1
were followed by H NMR spectroscopy at room temperature.
In Situ IR Spectroscopy Monitoring of Hydrogenation of
Benzaldehyde Catalyzed by 8. This process will be illustrated with
a specific example. A toluene solution (5.1 mL) containing
benzaldehyde (0.5 mL, 4.9 mmol, 0.97 M) and 8 (25.0 mg, 0.031
mmol, 6.1 mM, 0.6 mol %) was prepared in a high-pressure vessel
equipped with an attenuated total reflection element (ReactIR). The
reaction vessel was heated to 45 °C under a nitrogen atmosphere.
The vessel was flushed with H2 three times, pressurized to 35 atm
of H2, and maintained at that pressure during the course of the
hydrogenation. The hydrogenation of benzaldehyde was followed
by measuring the intensity of the CO stretching frequency of
benzaldehyde (1709 cm-1) every 4 min.
MHz): δ 52.4. IR (CH2Cl2): 1958 cm-1. IR (toluene): 1960 cm-1
.
HRMS (ESI): calcd (found) for [C50H40O2PRu]+ (9-O2CPh)+:
805.1809 (805.1819).
[2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(O2CTol)(PPh3)(CO) (9-Me).
A solution of p-tolualdehyde (0.50 mL, 4.2 mmol) and 8 (50 mg,
0.062 mmol) in toluene (5 mL) was stirred for 24 h. The toluene
was evaporated under vacuum, the residue was washed with pentane
and redissolved in toluene, and p-tolualdehyde (0.50 mL, 4.2 mmol)
was added. After an additional 24 h, toluene was evaporated and
the residue was washed with pentane to give 9-Me as a pale yellow
powder. 1H NMR (300 MHz, C6D6): δ 1.57 (s, 3H), 1.77 (s, 3H),
2.08 (s, 3H), 3.87 (s, 1H), 6.61-7.75 (m, 33H), 7.93 (d, J ) 8.4
Hz, 2H), 8.09 (d, J ) 8.4 Hz, 2H). 1H NMR (500 MHz, CD2Cl2):
δ 2.06 (s, 3H), 2.24 (s, 3H), 2.30 (s, 3H), 3.68 (s, 1H), 6.71 (d, J
) 8.1 Hz, 2H), 6.89 (td, J ) 7.7, 1.6 Hz, 6H), 6.97 (d, J ) 8.1 Hz,
4H), 7.02-7.08 (m, 7H), 7.15-7.21 (m, 8H), 7.27-7.32 (m, 6H),
7.43-7.47 (m, 4H). 13C{1H} NMR (126 MHz, CD2Cl2): δ 21.3,
21.5, 21.6, 73.17, 85.3 (d, JPC ) 2 Hz), 102.1 (d, JPC ) 5 Hz),
104.9 (d, JPC ) 8 Hz), 127.4-134.4 (48C, 21 resonances), 137.4,
138.0, 140.1, 173.9, 208.6 (d, JPC ) 22 Hz). 31P{1H} NMR (203
Acknowledgment. Financial support from the Department
of Energy, Office of Basic Energy Sciences, is gratefully
acknowledged. J.B.J. thanks the ACS Organic Division Em-
manuil Troyansky Graduate Fellowship for support. Grants from
the NSF (No. CHE-962-9688) and NIH (No. I S10 RR04981-
01) for the purchase of NMR spectrometers are acknowledged.
Supporting Information Available: Text, tables, and figures
detailing measures of acidity of 7 and 8, the disproportionation of
benzaldehyde catalyzed by 8 and benzoic acid, the loss of H2 from
8, the PGSE NMR of 9, the oxidation of benzyl alcohol by 9,
reactions of 9 and 9-Me with H2, the preparation of 11, the
preparation of 8-RuDOD, catalytic hydrogenation of an imine by
8, plots of rate vs pH2, of rate vs [8], and of ln [ald] vs time for
benzaldehyde hydrogenation by 8, and X-ray crystal structure data
(the corresponding CIF files are also given) for 7-9. This material
is available free of charge via the Internet at http://pubs.acs.org.
MHz, CD2Cl2): δ 50.8. IR (CH2Cl2): 1953 cm-1
.
Alternatively, 9-Me was prepared by combining 8 (10 mg, 12
µmol), p-tolualdehyde (13 µL, 11 µmol), and p-toluic acid (3.3
mg, 24 µmol) in benzene (0.9 mL). After 24 h, most of the 9-Me
had precipitated. After half of the benzene was evaporated under
reduced pressure, the remaining liquid was decanted by pipet and
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