The role of a hydroxycyclopentadienyl ruthenium dicarbonyl formate in formic acid reductions of carbonyl compounds catalyzed by Shvo's diruthenium catalyst

Article abstract of DOI:10.1139/v00-201

Addition of excess HCO₂H to {2,5-Ph₂-3,4-Tol₂(η⁵-C₄CO)] Ru(CO)₂}₂ (6) at -20°C led to the formation of [2,5-Ph₂-3,4-Tol₂{η⁵-C₄COH)] Ru(CO)₂(η¹-OCHO) (5), a proposed intermediate in catalytic transfer hydrogenations developed by Shvo. Hydroxycyclopentadienyl formate 5 undergoes rapid reversible dissociation of HCO₂H at -20°C, and undergoes decarboxylation at 1°C to form a 1:10 mixture of {[2,5-Ph₂-3,4-Tol₂(η⁵-C₄CO)] ₂H}Ru₂(CO)₄(μ-H) (3):[2,5-Ph₂-3,4-Tol₂(η⁵-C₄ COH)Ru(CO)₂H] (4). 5 does not reduce PhCHO below the temperature at which 5 is converted to hydride 4. The catalytic production of benzyl alcohol from 5 and PhCHO in the presence of excess HCO₂H is not accelerated by higher concentrations of PhCHO, indicating that 5 does not directly reduce PhCHO. Formate complex 5 is the precursor of hydride 4 which transfers hydrogen to PhCHO. A crucial role for the CpOH proton in the decarboxylation of 5 was indicated by the much slower decarboxylation of the methoxycyclopentadienyl analog [2,5-Ph₂-3,4-Tol₂(η⁵-C₄COCH ₃)]Ru(CO)₂(η¹-OCHO) (7). A mechanism for decarboxylation of 5 is proposed which involves reversible dissociation of formic acid to form the unsaturated dienone dicarbonyl ruthenium intermediate C, followed by simultaneous transfer of hydride to ruthenium from the formic acid carbon and of proton to the carbonyl of C from the formic acid OH group.

Full text of DOI:10.1139/v00-201

1002
The role of a hydroxycyclopentadienyl ruthenium
dicarbonyl formate in formic acid reductions of
carbonyl compounds catalyzed by Shvo’s
diruthenium catalyst
Charles P. Casey, Steven W. Singer, and Douglas R. Powell
Abstract: Addition of excess HCO2H to {2,5-Ph2-3,4-Tol2(η5-C4CO)]Ru(CO)2}2 (6) at –20°C led to the formation of
[2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(CO)2(η1-OCHO) (5), a proposed intermediate in catalytic transfer hydrogenations de-
veloped by Shvo. Hydroxycyclopentadienyl formate 5 undergoes rapid reversible dissociation of HCO2H at –20°C, and
undergoes decarboxylation at 1°C to form a 1:10 mixture of {[2,5-Ph2-3,4-Tol2(η5-C4CO)]2H}Ru2(CO)4(µ-H) (3):[2,5-
Ph2-3,4-Tol2(η5-C4COH)Ru(CO)2H] (4). 5 does not reduce PhCHO below the temperature at which 5 is converted to
hydride 4. The catalytic production of benzyl alcohol from 5 and PhCHO in the presence of excess HCO2H is not ac-
celerated by higher concentrations of PhCHO, indicating that 5 does not directly reduce PhCHO. Formate complex 5 is
the precursor of hydride 4 which transfers hydrogen to PhCHO. A crucial role for the CpOH proton in the
decarboxylation of 5 was indicated by the much slower decarboxylation of the methoxycyclopentadienyl analog [2,5-
Ph2-3,4-Tol2(η5-C4COCH3)]Ru(CO)2(η1-OCHO) (7). A mechanism for decarboxylation of 5 is proposed which involves
reversible dissociation of formic acid to form the unsaturated dienone dicarbonyl ruthenium intermediate C, followed
by simultaneous transfer of hydride to ruthenium from the formic acid carbon and of proton to the carbonyl of C from
the formic acid OH group.
Key words: Shvo catalyst, ruthenium formate, decarboxylation.
Résumé : L’addition d’un excès de HCO2H au complexe {2,5-Ph2-3,4-Tol2(η5-C4CO)][Ru(CO2}2 (6), à –20°C, conduit
à la formation de [2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(CO)2(η1-OCHO) (5), un intermédiaire présumé dans les hydrogéna-
tions par transfert catalytique développées par Shvo. Le formate d’hydrocyclopentadiényle, 5, subit une dissociation ré-
versible rapide en HCO2H à –20°C et il subit une décarboxylation à 1°C conduisant à la formation d’un mélange 1:10
de {[2,5-Ph2-3,4-Tol2(η5-C4CO)]2H}Ru2(CO)4(µ-H) (3) : [2,5-Ph2-3,4-Tol2(η5-C4COH)Ru(CO)2H] (4). Le composé 5 ne
réduit pas le PhCHO en dessous de la température à laquelle ce composé est transformé en hydrure 4. La vitesse de la
production catalytique d’alcool benzylique à partir de 5 et de PhCHO, en présence d’un excès de HCO2H n’est pas ac-
célérée par des concentrations plus élevées de PhCHO; ce résultat indique que 5 ne réduit pas le PhCHO directement.
Le complexe 5 à base de formate est le précurseur de l’hydrure 4 qui transfère de l’hydrogène au PhCHO. Un rôle
crucial pour le proton du CpOH dans la décarboxylation du composé 5 est mis en évidence par la décarboxylation
beaucoup plus lente de l’analogue méthyoxycyclopentadiényle, [2,5-Ph2-3,4-Tol2(η5-C4COCH3)]Ru(CO)2(η1-OCHO) (7).
On propose un mécanisme de décarboxylation du composé 5; il implique une dissociation réversible de l’acide for-
mique conduisant à la formation de l’intermédiaire insaturé C, ruthénium dicarbonyle diénone, suivi par un transfert si-
multané d’hydrure du carbone de l’acide formique vers le ruthénium et du proton du groupe OH de l’acide formique
vers le carbonyle de l’intermédiaire C.
Mots clés : catalyseur de Shvo, formate de ruthénium, décarboxylation.
[Traduit par la Rédaction] Casey et al. 1011
Introduction
(1), which catalyzes the reduction of a wide variety of unsat-
urated organic substrates (1). Shvo has proposed that
[2,3,4,5-Ph4(η5-C4COH)Ru(CO)2H] (2), a unique organo-
metallic complex containing both acidic OH and hydridic
Recently, we began investigating Shvo’s fascinating hydro-
genation catalyst, {[2,3,4,5-Ph4(η5-C4CO)]2H}Ru2(CO)4(µ-H)
Received September 26, 2000. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on July 14, 2001.
Dedicated to Brian James on the occasion of his 65th birthday in recognition of his insightful research on homogeneous catalysis
of hydrogenation.
C.P. Casey,1 S.W. Singer, and D.R. Powell. Department of Chemistry, University of Wisconsin-Madison, Madison, WI 53706,
U.S.A.
1Corresponding author (telephone: (608) 262-0584; fax: (608) 265-4534; e-mail: casey@chem.wisc.edu).
Can. J. Chem. 79: 1002–1011 (2001)
© 2001 NRC Canada
DOI: 10.1139/cjc-79-5/6-1002

Casey et al.
1003
Scheme 1.
served at δ 2.16 and 2.18 in 1:2 ratio in addition to the tolyl
resonance of 6 at δ 2.11. The resonance at δ 2.16 and a hy-
Ph
O
H
O
H2
Ph
Ph
dride resonance at
δ
–9.76 were assigned to
Tol
Tol
OH
Ph
Ph
hydroxycyclopentadienyl hydride 4. The resonance at δ 2.18
and a characteristic (7) formate resonance at δ 7.76 were as-
signed to the hydroxycyclopentadienyl formate complex
[2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(CO)2(η1-OCHO) (5). Af-
ter 10 min, a 1:1 ratio of 4:5 was observed and a small hy-
dride resonance at δ –18.25 indicated the formation of some
diruthenium hydride 3. After 30 min, the only species seen
were a 1:1 ratio of 3:4; all of diruthenium complex 6 was
consumed and there was no evidence for remaining interme-
diate formate complex 5. The ratio of 3:4 remained constant
for 10 h at room temperature. The mixture was converted to
a solution of 4 by heating for 10 min at 80°C.
Ph
H
Ru
H
3
HCO2H
Tol
Ru
Ru
Tol
Tol
OC
Tol
OC
OC
CO
CO
CO
4
RuH functions, is the active species in the reductions. Our
long term interest in complexes containing acidic and
hydridic hydrogens and their growing importance in homo-
geneous catalysis prompted us to initiate a detailed mecha-
nistic study of Shvo’s catalytic system (2, 3). Our studies of
the
tolyl-substituted
analogs
{[2,5-Ph2-3,4-Tol2(η5-
C4CO)]2H}Ru2(CO)4(µ-H) (3) and [2,5-Ph2-3,4-Tol2(η5-
C4COH)Ru(CO)2H] (4) (Scheme 1) provided persuasive evi-
dence that 4 reduced carbonyls and imines by a concerted
hydrogen transfer mechanism (4). Alkenes appear to be re-
duced by a very different mechanism, probably involving
CO dissociation from 4 to form a reactive 16e species.
One puzzling aspect of the chemoselectivity of catalysis
by 1 was that carbonyls were efficiently reduced by either
H2 or HCO2H, but alkenes were reduced only by H2 (5).
Shvo’s proposal that hydroxycyclopentadienyl hydride 2 was
the common active species in all the catalytic reactions ap-
peared to be at odds with the observation that
chemoselectivity could be affected by the choice of the re-
ducing agent used to generate 2. We considered the possibil-
ity that the hydroxycyclopentadienyl ruthenium formate B
might play a direct role in reductions catalyzed by 1 that use
HCO2H as the reducing agent. Shvo had proposed the
intermediacy of formate B but had never observed it
(Scheme 2). Formate complexes have also been proposed as
important intermediates in transfer hydrogenation reactions
where HCO2H is the hydrogen source and in the transition-
metal-catalyzed reductions of CO2 (6).
The reaction was complicated by the low solubility of
dimer 6 in THF-d8; all solid material was not dissolved until
after 30 min of reaction. To circumvent these solubility
problems, the reaction was also studied in CD2Cl2, in which
6 is more soluble. HCO2H (0.4 M) was injected into a
CD2Cl2 solution of 6 (0.02 M) at room temperature and the
reaction was immediately monitored by H NMR spectros-
copy. After 2 min, >95% of 6 was consumed, and the major
product was formate 5, identified by a formate resonance at
1
δ
7.87 and
a
tolyl resonance at
δ
2.23.
Hydroxycyclopentadienyl hydride 4 and diruthenium hy-
dride 3 were present in a 4:5 ratio. The tolyl methyl reso-
nance of 3 overlaps the resonance of 5, so the relative
amounts of 3, 4, and 5 were estimated by comparison of the
intensity of the formate resonance to that of the hydride res-
onances of 3 (δ –18.63) and 4 (δ –9.81). After 5 min, dimer
6 was completely consumed, there was no evidence for re-
maining formate intermediate 5, and the major products
were a 2.5:1 mixture of 3:4. After 20 min, the ratio of prod-
ucts was steady at 2.5:1. The ratio of 3:4 slowly changed
over time; after 10 h, the ratio of 3:4 was 4:1.
We initiated studies of the interaction of HCO2H with the
Shvo catalytic system to answer three questions: (i) Are
formate complexes formed? (ii) Do they reduce carbonyls
and (or) alkenes? (iii) Are they converted to hydrides, and if
so, how? Here, we report the low-temperature observation of
Clean conversion of ruthenium dimer 6 to
hydroxycyclopentadienyl ruthenium formate 5 at –20°C
Since evidence for the transient formation of
hydroxycyclopentadienyl ruthenium formate 5 from 6 and
excess HCO2H was obtained at room temperature, low-
temperature studies were initiated in an effort to observe
high concentrations of 5. HCO2H (0.2 M) was injected into
an NMR tube containing a CD2Cl2 solution of 6 (0.03 M) at
–78°C. The tube was inserted into an NMR probe precooled
a
tolyl-substituted analog of hydroxycyclopentadienyl
formate B, its failure to reduce benzaldehyde, and mechanis-
tic studies of its decarboxylation. These studies afford in-
sight into the decarboxylation of formate complexes and
clarify the role of HCO2H in Shvo’s catalytic system.
1
to –20°C and the reaction was followed by H NMR spec-
troscopy. After 30 min, 5% 6 (tolyl methyl δ 2.06) remained
and 95% of the material had been converted to 5 (tolyl
methyl δ 2.20, formate hydrogen δ 7.47). No further change
in the equilibrium ratio (Keq = [5]2/[6][HCO2H]2 ≈ 55) was
seen over the next 30 min. In the 13C NMR spectrum, the
formate resonance of 5 was observed at δ 172. At concentra-
tions ≥0.4 M HCO2H, no resonances for dimer 6 were ob-
served, consistent with the estimated equilibrium constant.
Results
Observation of a hydroxycyclopentadienyl ruthenium
formate
We previously reported that heating diruthenium hydride
3 with excess HCO2H at 80°C in THF-d8 for 10 min led to
the formation of a solution of hydroxycyclopentadienyl ru-
thenium hydride 4 (4). A possible intermediate in this reac-
tion is the hydroxycyclopentadienyl formate 5. In an effort
to observe this species directly, we monitored the reaction of
excess HCO2H with a suspension of the more reactive
diruthenium complex {2,5-Ph2-3,4-Tol2(η5-C4CO)Ru(CO)2}2
(6) in THF-d8 by 1H NMR spectroscopy at room temperature
(Scheme 3). After 5 min, two new tolyl resonances were ob-
Isotope labeling evidence for reversible dissociation of
HCO2H from 5
The observation of a steady ratio of 5:6 in the presence of
excess HCO2H requires an equilibrium process. Equilibra-
tion is readily explained by invoking dienone carbonyl inter-
© 2001 NRC Canada

1004
Can. J. Chem. Vol. 79, 2001
Scheme 2.
O
Ph
O
Ph
Ph
Ph
Ru
H
H
OH
OC
OC
A
OH
O
O
R
R
Ph
Ph
Ph
Ph
OH
Ph
Ph
Ph
Ph
O
Ru
OC
H
1
Ru
Ru
Ph
Ph
Ph
OC
Ph
CO
O
OC
H
CO
CO
B
O
R
R
Ph
Ru
Ph
Ph
O
H
Ph
H
CO2
OC
OC
2
spectroscopy at 1°C. The major product of the
decarboxylation was diruthenium hydride and
Scheme 3.
3
Ph
Ph
CO
hydroxycyclopentadienyl hydride 4 was the minor product
(10:1 ratio). Duplicate kinetic runs on 0.06 M solutions of 5,
formed from 6 and 0.2 M HCO2H, gave kobs = 5.9 and 5.8 ×
10–4 s–1. Rates were slower in the presence of higher con-
centrations of HCO2H. Duplicate runs on solutions of 5 pre-
pared from 6 and 0.4 M HCO2H gave kobs = 3.2 and 4.2 ×
10–4 s–1. Duplicate runs on solutions of 5 prepared from 6
and 1.0 M HCO2H gave kobs = 2.0 and 1.7 × 10–4 s–1.
Isotope effects on decarboxylation were investigated using
0.06 M solutions of 5-d prepared by adding 0.2 M DCO2H
to CD2Cl2 solutions of 6 at –20°C. Duplicate kinetic runs on
solutions of 5-d at 1°C gave kobs = 5.6 and 5.2 × 10–4 s–1.
The isotope effect on decarboxylation, k5/k5-d = 1.1 ± 0.1,
was negligible.
Tol
Tol
O
Tol
Tol
OH
CO
HCO2H
Ph
O
Ph
O
Ru
O
Ru
OC
Ru
OC
-CO2
Ph
Tol
Tol
OC
OC
H
Ph
5
6
O
H
O
Ph
Ph
Ph
Ph
Ph
Tol
Tol
O
H
Ph
H
H
3
+
Ru
Ru
Ru
Tol
Tol
Tol
OC
Tol
CO
OC
CO
CO
OC
4
mediate C which can either dimerize to form 6 or react with
HCO2H to give 5, possibly via an intermediate formic acid
complex D (Scheme 4).
Previous studies on decarboxylation of CpFe(CO)2(η1-
OCHO) had shown that reversible CO dissociation was the
first step in the decarboxylation (8). To test whether CO dis-
sociation was involved in the decarboxylation of 5, the rate
of decarboxylation of 5 under 1.4 atm (1 atm = 101.325 kPa)
of 13CO was measured in the presence of 0.2 M HCO2H at
1°C. No inhibition of decarboxylation (kobs = 5.7 × 10–4 s–1)
was observed and there was no incorporation of 13CO into
diruthenium hydride 3 or hydroxycylopentadienyl hydride 4
(10:1 ratio of products). The formation of [2,5-Ph2-3,4-
Tol2(η4-C4CO)]Ru(CO)2(13CO) as a byproduct (<5%) of the
decarboxylation was indicated by a characteristic tolyl
Exchange of DCO2H with 5 occurred rapidly at –20°C and
is consistent with the intervention of intermediate C. A solution
of deuterated formate complex was prepared by addition of
DCO2H (0.3 M) to a CD2Cl2 solution of 6 (0.03 M) at –20°C.
After 30 min, the 1H NMR spectrum showed >98% conversion
to [2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(CO)2(η1-OCDO) (5-d); no
formate 1H NMR resonance was observed. The tube was
cooled to –78°C and an equal amount of HCO2H (0.3 M) was
added. When the tube was replaced in the NMR spectrometer
at –20°C, a formate resonance was seen at δ 7.75. The inte-
grated intensity of this resonance relative to the tolyl methyl
resonance of 5 at δ 2.20 was 0.5:6, indicating a 1:1 equilibrium
mixture of 5:5-d. Equilibration is rapid at –20°C.
1
methyl resonance at δ 2.25 in H NMR.
Hydroxycyclopentadienyl ruthenium formate 5 does not
reduce benzaldehyde
Shvo proposed that hydroxycyclopentadienyl hydride 4 is
the active hydrogen transfer agent in all the reductions cata-
lyzed by diruthenium hydride 3. For reductions with
HCO2H, we wished to explore the possibility that the
Kinetics of decarboxylation of hydroxycyclopentadienyl
ruthenium formate 5
The rate of decarboxylation of 5 generated from 6 and ex-
1
cess HCO2H in CD2Cl2 at –20°C was measured by H NMR
© 2001 NRC Canada

Casey et al.
1005
Scheme 4.
O
O
Ph
O
Ph
Ph
Tol
Tol
Tol
Tol
Tol
Tol
O
H
O
H
Ph
Ph
Ph
Ru
Ru
D
Ru
C
O
OC
OC
OC
OC
OC
O
OC
H
5
H
Ph
CO
Tol
O
CO
Tol
Ph
O
Ru
+ 2HCO2H
Ru
OC
Ph
Tol
OC
Ph
Tol
6
Scheme 5.
O
Ph
Ph
H
???
H
H
Tol
Tol
O
Tol
Tol
O
C
O
C
Ph
O
Ph
+
Ru
OC
Ru
C
OC
OC
H
Ph
-CO2
H
OC
H
Ph
O
5
hydroxycyclopentadienyl formate 5 might be the active re-
ducing agent (Scheme 5) (Formic acid and formate salts are
well-known reducing agents, see ref. 9).
that of decarboxylation. To test for this possibility,
decarboxylation of 5 was carried out at 10°C in the presence
of varying amounts of PhCHO. If the rate of hydrogen trans-
fer from 5 to PhCHO is competitive with decarboxylation,
the rate of PhCH2OH formation should increase at higher
PhCHO concentration. The rate of formation of PhCH2OH
We devised a method for producing 5 in the presence of
benzaldehyde at low temperature patterned after our earlier
observation that reaction of 4 with PhCHO in the presence
of CF3CO2H leads to the formation of benzyl alcohol and
1
(δ 4.68 in H NMR) was monitored in solutions containing
[2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(CO)2(η1-O2CCF3).
We
hydroxycyclopentadienyl
0.4 M and 0.8 M PhCHO. Plots of the concentration of
PhCH2OH vs. time were linear. (The constant rate of alcohol
formation is due to the nearly constant concentrations of 5,
which is continually regenerated, and of benzaldehyde,
which is present in excess.) The rate of formation of benzyl
alcohol did not depend on the concentration of benzaldehyde
(k = 1.20 × 10–1 s–1 for 0.4 M PhCHO, and k = 1.16 × 10–1 s–1
for 0.8 M PhCHO). Therefore, the hydroxycyclopentadienyl
ruthenium formate 5 does not directly reduce benzaldehyde.
have
proposed
that
the
trifluoroacetate complex is formed by addition of CF3CO2H
to the coordinatively unsaturated intermediate C generated
by hydrogen transfer from 4 to PhCHO.
PhCHO (0.8 M) was added to a CD2Cl2 solution of 4
(0.03 M) containing HCO2H (0.5 M) at –78°C and the solu-
tion was monitored by 1H NMR spectroscopy at –29°C.
Rapid disappearance (t0.5 ≈ 1 min) of the δ –9.74 hydride res-
onance of 4 and rapid appearance of resonances for
PhCH2OH and 5 were observed. No further reduction of
PhCHO by 5 was observed even after the temperature was
raised to –15°C. These observations indicate that 4 is a
much more reactive reducing agent than 5; indeed, no obser-
vation requires that 5 act as a hydride donor.
When the above solution was warmed to 0°C, the onset
temperature for decarboxylation of 5, the resonance for
PhCH2OH began growing slowly. Upon further warming to
20°C, rapid catalytic reduction of PhCHO was observed,
along with the appearance of characteristic NMR resonances
for diruthenium hydride 3. After 20 min, the ratio of 5 to 3
was 1:6, unreacted HCO2H (0.06 mmol) and PhCHO
(0.10 mmol) remained, and 10 equiv of benzyl alcohol had
been produced.
Synthesis of a methoxycyclopentadienyl ruthenium
formate
Our studies of the reduction of carbonyls and imines by
hydroxycyclopentadienyl ruthenium hydride 4 highlighted
the crucial importance of the CpOH proton in promoting hy-
drogen transfer. The triethylsiloxy derivative of 4, [2,5-Ph2-
3,4-Tol2(η5-C4COSiEt3)]Ru(CO)2H, was unreactive towards
carbonyls and imines under the same conditions that 4 was
highly reactive (4). The possible role of the CpOH proton of
5 in decarboxylation and hydrogen transfer reactions
prompted us to investigate the reactivity of its methoxy-
substituted analog.
[2,5-Ph -3,4-Tol ( 5-C COCH )]Ru(CO) ( 1-OCHO) (7)
η
η
2
2
3
was synthesized in a t4wo-step procedur2e from dimer 6
(Scheme 6). Reaction of excess CH3I with 6 in CH2Cl2 led
to the isolation of the methyoxycyclopentadienyl ruthenium
The above experiment does not exclude the possibility
that 5 may transfer hydrogen to substrates at a rate similar to
© 2001 NRC Canada

1006
Can. J. Chem. Vol. 79, 2001
Scheme 6.
Ph
Ru
Ph
CO
Tol
Tol
OCH3
Tol
Tol
O
CO
Ph
I
CH3I
Ph
O
Ru
Ru
Ph
OC
Tol
Tol
OC
OC
8
OC
Ph
6
O
H
Ag
O
Ph
Ru
Ph
Ph
O
Tol
Tol
OCH3
Tol
Tol
Tol
Tol
OCH3
CH3OTf
Ph
Ph
H
Ph
H
-CO2
80°C
O
Ru
OC
Ru
OC
O
OC
OC
OC
H
OC
7
10
9
Fig. 1. X-Ray crystal structure of [2,5-Ph2-3,4-Tol2(η5-
position of the formate and the carbon monoxide ligands. In
the ordered molecule, the O(2A)—C(33A) distance (1.27 Å)
and C(33A)—O(4A) distance (1.23 Å) are similar to struc-
turally characterized η1-formate complexes (12). The
Ru(1A)—O(1A) distance (2.30 Å) and C(1A)—O(1A) dis-
tance (1.35 Å) are characteristic of η5 coordination of the
CpOMe ligand.2
C4COCH3)]Ru(CO)2(η1-OCHO) (7). Thermal ellipsoids are drawn
at 50% probability.
Very slow decarboxylation of methoxycyclopentadienyl
ruthenium formate 7
The high thermal stability of 7 is remarkable in compari-
son to the rapid decarboxylation of 5 at room temperature.
Decarboxylation of 7 required heating at 80°C in THF-d8
(kobs = 1.7 × 10–4 s–1; t0.5 = 67 min) (Scheme 6). The major
product
formed
in
75%
yield
was
the
methoxycyclopentadienyl ruthenium hydride [2,5-Ph2-3,4-
Tol2(η5-C4COCH3)]Ru(CO)2H (9), which was characterized
1
by spectroscopy (RuH at δ –9.78 in H NMR) and by inde-
pendent
synthesis
from
NEt4[2,5-Ph2-3,4-Tol2(η4-
C4CO)Ru(CO)2H] (10) and excess CH3OTf. The
decarboxylation of 7 was not clean and produced small
amounts of four unidentified side products. In an independ-
ent control experiment, 9 was stable for 1 h at 80°C in THF-
d8. When the decarboxylation was attempted under 1.7 atm
iodide [2,5-Ph2-3,4-Tol2(η5-C4COCH3)]Ru(CO)2I (8) as a
red powder in 67% yield. Following a procedure similar to
that used for the synthesis of Pd(II) formates (10), reaction
of 8 with a suspension of AgOCHO in CH2Cl2 gave the de-
sired methoxycyclopentadienyl ruthenium formate 7 as an
off-white powder in 56% yield. 7 was thermally stable in the
solid state and solution at room temperature. Addition of ex-
cess DCO2H to a CD2Cl2 solution of 7 resulted in no dimi-
nution of its formate 1H NMR resonance (δ 7.81) after 24 h.
Complex 7 has spectroscopic features characteristic of a
metal formate complex: an IR band at 1618 cm–1 assigned to
1
(1 atm = 101.325 kPa) of 13CO at 80°C and the formate H
NMR resonance of 7 was monitored, <5% decarboxylation
was observed. Over the same time, the 13C NMR resonance
at δ 201 for the CO ligands of 7 grew relative to the aromatic
resonances, indicating 13CO incorporation into 7.
Discussion
1
the νasym (CO2) absorption (11), a formate H NMR reso-
nance at δ 7.81, and a formate 13C NMR resonance at
δ 168.3. The X-ray crystal structure of 7 confirmed its struc-
tural assignment as an O-bound η1-formate complex (Fig. 1).
The unit cell contained three molecules per asymmetric unit
and two of the molecules were disordered with respect to the
Direct observation of hydroxycyclopentadienyl
ruthenium formate 5
Shvo
had
proposed
the
intermediacy
of
hydroxycyclopentadienyl ruthenium formate B in reductions
catalyzed by 1 that use HCO2H as the reducing agent (5b),
2 Copies of material on deposit may be purchased from the Depository of Unpublished Data, Document Delivery, CISTI, National Research
Council Canada, Ottawa, ON K1A 0S2, Canada (http:/www.nrc.ca/cisti/irm/unpubl_e.shtml for information on ordering electronically).
Some of this supplementary material has also been deposited with the Cambridge Crystallographic Data Centre. Copies of the data can be
obtained, free of charge, on application to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (Fax: 44-1223-336033 or
e-mail: deposit@ccdc.cam.ac.uk).
© 2001 NRC Canada

Casey et al.
1007
Scheme 8.
Scheme 7.
Ph
Tol
Tol
OH
-CO2
+CO
-CO
Fe
Fe
Fe
Ph
O
OC
OC
OC
O
H
OC
O
Ru
Ph
OC
HCO2H
6
HCO2H
OC
H
Tol
Tol
H
11
O
5
OC
O
Ph
Ru
O
H
CO
O
E
CO
-CO2
O
Re
Re
Re
ON
O
ON
ON
H
H
Ph3P
Ph3P
Ph3P
O
but had never observed it. We have found that the
hydroxycyclopentadienyl ruthenium formate 5 is readily
formed by addition of excess HCO2H to Ru dimer 6 at low
temperature. The hydroxycyclopentadienyl formate is unsta-
ble and loses CO2 at 0°C to form hydroxycyclopentadienyl
ruthenium hydride 4. In addition, 5 undergoes rapid ex-
change with DCO2H at –20°C, presumably by reversible dis-
sociation of formic acid and formation of the unsaturated
H
12
O
of a vacant coordination site. Darensbourg et al. (8) showed
that CpFe(CO)2(η1-OCHO) (11) exchanges 13CO faster than
it undergoes decarboxylation, and proposed a mechanism in-
volving CO dissociation to give an unsaturated species, fol-
lowed by β-hydride elimination concurrent with CO2
formation (Scheme 8). Based upon kinetic studies and deute-
rium isotope effects, Merrifield and Gladysz (15) proposed
that decarboxylation of CpRe(NO)(PPh3)(η1-OCHO) (12)
proceeds by dissociation of formate ion to form a tight ion
pair, followed by hydride transfer from the formate ion to
the unsaturated rhenium cation. Our evidence suggests that
the decarboxylation of 5 does not proceed by either type of
mechanism.
The hydroxy group on the cyclopentadienyl ring of 5
plays a crucial role in the decarboxylation since 5 decom-
poses at temperatures 79° lower (t0.5 = 19 min at 1°C) than
the corresponding methoxycyclopentadienyl formate com-
plex 7 (t0.5 = 67 min at 80°C). CO inhibition of the
decarboxylation of 7 and 13CO exchange into 7 at 80°C pro-
vides persuasive evidence that CO dissociation is the initial
step in the decarboxylation of 7, just as it is in the case of
iron formate 11. However, since CO does not inhibit
decarboxylation of 5 and since 13CO does not exchange into
5 and is not incorporated into its decarboxylation products, it
is clear that decarboxylation of 5 does not involve CO disso-
ciation.
The huge difference in the rates of decarboxylation be-
tween 5 and 7 is also inconsistent with a simple ion pair
mechanism as proposed by Merrifield and Gladysz (15) for
rhenium formate 12. If 5 decomposed by an ion-pair mecha-
nism, then the same pathway should have been available to 7
and the compounds should have undergone decarboxylation
at similar rates.
Any viable mechanism for the decarboxylation of 5 must
account for: (i) the much faster decarboxylation of 5 com-
pared to its CpOMe derivative 7; (ii) the lack of CO inhibi-
tion of the rate of decarboxylation of 5; (iii) the lack of 13CO
incorporation into 5 or its decarboxylation products; (iv) the
rapid exchange of DCO2H with 5; (v) a negligible deuterium
isotope effect on decarboxylation of k5/k5-d = 1.1 ± 0.1;
(vi) and slower decarboxylation at higher HCO2H concentra-
tion.
dienone
dicarbonyl
intermediate
C.
The
rapid
decarboxylation of 5 is consistent with the failure to observe
it under catalytic conditions.
Mays et al. (13) reported that scrambling of two symmet-
ric Ru dimers analogous to 6 required heating at 52°C for
55 min. The rate of dissociation of dimer 6 at –20°C should
be very slow while the reaction between 6 and HCO2H has a
half-life of about 15 min at this temperature. Since the rate
of addition of HCO2H to dimer 6 is much faster than disso-
ciation of the dimer, the mechanism of formation of 5 must
involve direct addition of HCO2H to dimer 6, instead of dis-
sociation of 6 to dienone intermediate C followed by oxida-
tive addition of HCO2H. A possible mechanism proceeding
through the diruthenium HCO2H adduct E is outlined in
Scheme 7.
Hydroxycyclopentadienyl ruthenium formate 5 does not
reduce benzaldehyde
At the beginning of this work, we hypothesized that the
hydroxycyclopentadienyl formate 5 might be the active re-
ducing agent in reductions of carbonyls by formic acid using
the Shvo catalyst. This hypothesis seemed plausible since
the formate anion has been demonstrated to act as a hydride
donor in a number of organic reactions and has been shown
to donate hydride directly to NAD+ in reactions catalyzed by
formate dehydrogenase (9, 14). However, we have obtained
no evidence that 5 directly transfers hydrogen to carbonyls.
The hydroxycyclopentadienyl formate 5 does not reduce
benzaldehyde below the temperature at which it
decarboxylates to give the hydroxycyclopentadienyl hydride
4, which we have previously shown to reduce benzaldehyde
very rapidly. Moreover, higher concentrations of PhCHO do
not cause more rapid formation of benzyl alcohol from 5.
Therefore, the role of hydroxycyclopentadienyl formate 5 is
to decarboxylate to 4 which is the active hydrogen transfer
agent.
The mechanism shown in Scheme 9 provides an attractive
explanation for these observations. The CpOH proton plays
a special role in the formation of unsaturated dienone inter-
mediate C and formic acid via a formic acid complex D. The
Mechanism of decarboxylation of
hydroxycyclopentadienyl ruthenium formate 5
Two types of mechanism have been proposed for metal
formate decarboxylations and both involve initial generation
© 2001 NRC Canada

1008
Can. J. Chem. Vol. 79, 2001
Scheme 9.
Scheme 10.
Ph
O
O
Ph
Ru
Ph
Ph
Tol
Tol
OH
Tol
Tol
OH
Tol
Tol
Tol
H
O
H
H
Ph
H
Ph
H
Ph
Tol
Ph
-CO
O
5
Ru
Ru
C
Ru
D
OC
OC
OC
OC
OC
OC
O
O
O
OC
4
H
-CO2
R
O
H
Ph
Ph
Ph
Ru
Ph
Ru
Ph
Ph
Ph
Tol
OH
Tol
C
Tol
Tol
OH
Tol
Tol
OH
R
Tol
Ph
H
Tol
Tol
Ru
Ru
Tol
OC
HCO2H
Ph
Ph
CO
Ru
F
H
OC
CO
3
CO
OC
H
OC
4
OC
R
very rapid and reversible dissociation of formic acid from D
explains DCO2H exchange with 5. The key step in the
decarboxylation is a slower concerted transfer of hydride
from the carbon of formic acid to ruthenium of C and of
proton from the oxygen of formic acid to the dienone car-
bonyl of C that produces hydroxycyclopentadienyl hydride 4
and CO2. The slower decarboxylation at higher formic acid
concentration can be explained by stabilization of 5 by hy-
drogen bonding between formic acid and the formate ligand
of 5 that must be broken up in the formation of C (16).3
A primary deuterium isotope effect would be expected for
this mechanism. In other formate decarboxylations, kH/kD
was 1.55 for the decarboxylation of rhenium formate 12
(15), 3.1 for the Rh-catalyzed transfer hydrogenation of
itaconic acid with HCO2H (17), and 2.2 for hydride transfer
from HCO–2 to NAD+ catalyzed by formate dehydrogenase
(13b). While the small to negligible isotope effect observed
in the decarboxylation of 5 is troublesome, we have not been
able to devise a satisfactory alternative mechanism.
diate F. H2 addition to F leads to alkene reduction but
HCO2H interaction with intermediate F does not lead to
alkane formation (Scheme 10). However, further experi-
ments are needed to more fully understand the catalytic re-
actions of 4 with alkenes.
The much more rapid decarboxylation of hydroxycyclo-
pentadienyl formate
5
than methoxycyclopentadienyl
formate 7 highlights the crucial importance of an acidic pro-
ton in promoting decarboxylation. In related work on the
hydrogenation of CO2 to formic acid catalyzed by
[Rh(norbornadiene)(PMe2Ph)3]BF4 and by Ru(PMe3)4H2,
Tsai and Nicholas (18a) and Noyori and co-workers (18b)
have independently proposed that the acidic proton of a co-
ordinated water interacts with CO2 to promote hydride addi-
tion to generate formate intermediates. Earlier we reported
that the acidic CpOH proton of 4 is required for concerted
hydride addition to carbonyl compounds (4). A deeper un-
derstanding of the role of acidic protons in mediating the
formation and consumption of CO2 by transition metal com-
plexes should lead to more efficient catalysts for both trans-
fer hydrogenation and CO2 reduction.
Relevance to Shvo catalytic system
Our studies show how HCO2H reduces carbonyl com-
pounds in the Shvo catalytic system. Formic acid first reacts
with ruthenium species to generate the transient
hydroxycyclopentadienyl formate complex 5. This formate
complex is not itself a hydride transfer agent but it under-
Experimental
goes
rapid
decarboxylation
to
generate
the
General methods
hydroxycyclopentadienyl hydride 4. This hydride then rap-
idly reduces carbonyl compounds. The unsaturated dienone
dicarbonyl intermediate C generated in this process rapidly
reacts with formic acid to regenerate 5 and continue the cat-
alytic cycle or reacts with 4 to generate the diruthenium hy-
dride complex 3, which can slowly reenter the catalytic
cycle by reaction with HCO2H.
Our results also help explain why the Shvo system does
not catalyze the reduction of alkenes by HCO2H. We have
shown that while hydroxycyclopentadienyl hydride 4 rapidly
reduces aldehydes, it reduces alkenes only under a hydrogen
atmosphere. We suggest that alkene reduction proceeds via
CO dissociation from 4, coordination of an alkene, and re-
versible hydride migration to give unsaturated alkyl interme-
Manipulations of air and moisture sensitive compounds
were performed in a nitrogen atmosphere glovebox or using
standard high-vacuum line techniques. THF-d8, toluene, and
hexane were distilled from sodium–benzophenone. CH2Cl2
and CD2Cl2 were distilled from CaH2. PhCHO (Amend) was
distilled immediately before use. 96% HCO2H (Aldrich) and
95% DCO2H (Aldrich) were stored under nitrogen. Synthe-
ses of {[2,5-Ph2-3,4-Tol2(η5-C4CO)]2H}Ru2(CO)4(µ-H) (3),
[2,5-Ph2-3,4-Tol2(η5-C4COH)Ru(CO)2H] (4), {2,5-Ph2-3,4-
Tol2(η5-C4CO)]Ru(CO)2}2 (6), and NEt4[2,5-Ph2-3,4-
Tol2(η4-C4CO)Ru(CO)2H] (10) were previously described
(4). AgOCHO was synthesized by the method of Yamamoto
and co-workers (19). All other reagents were used as re-
ceived.
3 Puddephatt and co-workers observed slower [Ru2(µ-CO)(CO)4(µ-dppm)2] catalyzed decarboxylation of formic acid at higher HCO2H con-
centrations. His proposed mechanism involved HCO–2 in the rate determining step; higher concentration of formic acid was suggested to
shift the equilibrium to a less reactive hydrogen bonded adduct [(HCO2)2H]–.
© 2001 NRC Canada

Casey et al.
1009
1H NMR were obtained on a Bruker AC 250, Bruker AC
300, or Varian Unity 500 spectrometer. 13C NMR spectra
were obtained on a Varian Unity 500 spectrometer operating
at 125 MHz. IR spectra were measured on a Mattson Polaris
FT-IR spectrometer. Mass spectra were determined on a VG
AutoSpec M mass spectrometer. Elemental analysis was per-
formed by Desert Analytics of Tucson, Arizona.
Kinetics of decarboxylation of 5
HCO
2H (3 µL, 0.1 mmol, 0.2 M) was added to a CD2Cl2
solution of 6 (15 mg, 0.01 mmol, 0.03 M) at –78°C. The
tube was placed in an NMR probe precooled to –20°C and
1
the reaction monitored by H NMR as described above. The
NMR tube was removed from the probe and cooled to –78°C.
The NMR probe was warmed to 1 ± 0.5°C and the tube rein-
serted. Decarboxylation was monitored by the disappearance
of the resonance for phenyl protons at δ 7.45 in the H NMR
spectrum. The reaction followed first order kinetics to >80%
1
Addition of HCO2H to 6 in THF-d8
HCO2H (6.8 µL, 0.18 mmol, 0.4 M) was added to a THF-
d8 solution of 6 (11 mg, 0.01 mmol, 0.02 M) at –78°C. The
conversion. See Results.
1
reaction was monitored by H NMR spectroscopy at room
Decarboxylation of 5 under 13CO
temperature. Spectra were obtained at 5 min intervals for
35 min, until the solution was homogeneous and 6 had been
completely consumed. After 10 h, the relative amounts of 3
and 4 were measured. During the reaction, the tube was peri-
odically shaken to dissolve the remaining solid material.
HCO2H (2.9 L, 0.08 mmol, 0.2 M) was added to a
µ
CD2Cl2 solution of 6 (14 mg, 0.01 mmol, 0.03 M) at –78°C.
The tube was placed in an NMR probe precooled to –20°C
1
and the reaction monitored by H NMR spectroscopy until 5
and 6 had reached equilibrium. The tube was removed from
the probe, cooled to –78°C, degassed, and 13CO (1 atm)
(1 atm = 101.325 kPa) was added. The tube was placed in an
NMR probe precooled to 1 ± 0.5°C and the rate of de-
Addition of HCO2H to 6 in CD2Cl2
HCO2H (6.3 µL, 0.17 mmol, 0.4 M) was added to a
CD2Cl2 solution of 6 (12 mg, 0.01 mmol, 0.02 M) at –78°C.
The homogeneous solution was monitored by 1H NMR spec-
troscopy at room temperature. Spectra were obtained at
5 min intervals for 30 min, until 6 had been completely con-
sumed. After 10 h, the relative amounts of 3 and 4 were
measured.
carboxylation of 5 was measured as described above (kobs
=
5.7 × 10–4 s–1, t0.5 = 20 min).
Competition between 4 and 5 in PhCHO reduction
Diruthenium hydride 3 (12 mg, 0.01 mmol) in THF was
degassed and placed under 1 atm (1 atm = 101.325 kPa) of
H2 at –78°C. The tube was sealed at –78°C and heated to
80°C for 8 h. THF was evaporated and CD2Cl2 was con-
densed in to dissolve the yellow, oily solid. The solution was
cooled to –78°C and HCO2H (10 µL, 0.26 mmol, 0.5 M) and
PhCHO (30 µL, 0.29 mmol, 0.8 M) were added successively
via syringe. The tube was placed in an NMR probe
precooled to –29°C. The reaction was monitored at –29°C
for 25 min, at –15°C for 10 min, at 0°C for 10 min, and at
21°C for 20 min. See Results.
[2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(CO)2(η1-OCHO) (5)
HCO2H (3.2 µL, 0.08 mmol, 0.2 M) was added to a
CD2Cl2 solution of 6 (13 mg, 0.01 mmol, 0.03 M) and the
tube inserted into an NMR probe precooled to –20°C. After
30 min, the area ratio of the tolyl methyl resonances at
1
δ 2.20 (5):δ 2.06 (6) was 20:1. For 5, H NMR (CD2Cl2,
500 MHz) δ: 2.20 (s, 6H, tolyl CH3), 6.84 (AA′BB′, 8H,
tolyl), 7.32 (m, 6H, phenyl), 7.42 (m, 4H, phenyl), 7.47 (s,
OCHO). 13C NMR (CD2Cl2,125 MHz) δ: 21.8 (tolyl CH3),
87.9 (C 3,4 of Cp), 99.1 (C 2,5 of Cp), 126.7–138.7 (8 res-
onances aromatic), 147.0 (C1 of Cp), 172.0 (OCHO),
199.2 (CO). Four aliquots of 3.2 µL of HCO2H were added
at –78°C to the solution of 5 in CD2Cl2 containing 0.2 M
HCO2H. After the addition of each aliquot, the NMR tube
was reinserted into the probe cooled to –20°C and a 1H
Effect of PhCHO concentration on rate of benzyl
alcohol formation
Two identical solutions of 5 were prepared by addition of
HCO2H (7.8 µL, 0.2 mmol, 0.5 M) to CD2Cl2 solutions of
Ru dimer 6 (15 mg, 0.01 mmol, 0.025 M) at –78°C. Tubes
containing the solutions were placed into an NMR probe
1
NMR spectrum was obtained. The H NMR chemical shifts
1
of the formate hydrogen of 5 changed by 0.36 ppm with
concentration: [HCO2H], δ 5; 0.2 M, 7.47; 0.4 M, 7.67; 0.6
M, 7.76; 0.8 M, 7.80; 1.0 M, 7.83. Over the same concentra-
tion range, the formate hydrogen of free formic acid
changed by 0.18 ppm from δ 7.92 to 8.10.
precooled to –20°C and the reaction monitored by H NMR
1
until no 6 was observable in the H NMR spectrum. The
tubes were cooled to –78°C and 17 µL PhCHO (0.16 mmol,
0.4 M) was syringed into one tube and 34 µL PhCHO
(0.32 mmol, 0.8 M) was syringed into the second tube. The
tubes were placed in an NMR probe precooled to 10 ± 0.5°C
1
Exchange of [2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(CO)2(η1-
OCDO) (5-d) with HCO2H
and the rate of appearance of the H NMR resonance for
PhCH2OH (δ 4.68) was monitored for 40 min. Plots of the
concentration of PhCH2OH were linear. For 0.4 M PhCHO,
k = 1.20 × 10–1 s–1; for 0.8 M PhCHO, k = 1.16 × 10–1 s–1.
DCO2H (5.2 µL, 0.13 mmol, 0.3 M) was added to a
CD2Cl2 solution of 6 (15 mg, 0.01 mmol, 0.03 M) at –78°C.
The tube was placed in an NMR probe precooled to –20°C
and the reaction monitored by 1H NMR. After 30 min,
>98% conversion to 5-d was observed. No formate hydrogen
resonance in the region between δ 7.50 and δ 8.00 was visi-
ble. The tube was removed and cooled to –78°C. HCO2H (5µL,
0.12 mmol, 0.3 M) was added via syringe and the tube re-
placed in the spectrometer at –20°C. A formate resonance at
[2,5-Ph2-3,4-Tol2(η5-C4COCH3)]Ru(CO)2I (8)
Reaction of 6 (200 mg, 0.17 mmol) with CH3I (400 µL,
6.40 mmol) in CH2Cl2 (10 mL) for 40 h at room temperature
followed by evaporation of solvent under vacuum gave a red,
oily solid. Chromatography on silica gel using 1:1
CH2Cl2:hexane afforded 8 as a red powder (167 mg, 67%),
mp 195–202°C (dec). IR (CH2Cl2) (cm–1): 2033 (s), 1988
1
δ 7.75 was observed in the H NMR spectrum.
© 2001 NRC Canada

1010
Can. J. Chem. Vol. 79, 2001
1
(s). H NMR (CD2Cl2, 300 MHz) δ: 2.21 (s, 6H, tolyl CH3),
202.1 (CO). HR-MS (EI) calcd. for C34H28O3Ru: 586.1075;
found: 586.1029.
3.45 (s, OCH3), 6.87 (AA′BB′, 8H, tolyl), 7.32 (m, 6H,
phenyl), 7.53 (m, 4H, phenyl). 13C NMR (CD2Cl2,125 MHz)
δ: 21.2 (tolyl CH3), 62.9 (OCH3), 88.0 (C3,4 of Cp), 102.4
(C2,5 of Cp), 126.8–138.8 (8 resonances, aromatic), 147.0
(C1 of Cp), 198.2 (CO). Anal. calcd. for C34H27O3RuI: C
57.39, H 3.82; found: C 57.56, H 3.77.
X-ray crystal structure determination of [2,5-Ph2-3,4-
Tol2(η5-C4COCH3)]Ru(CO)2(η1-OCHO) (7)
X-ray quality crystals were grown by slow evaporation of
a CH2Cl2–hexane solution of 7. A yellow plate-shaped crys-
tal of dimensions 0.38 × 0.25 × 0.21 mm was selected for
structural analysis. Intensity data for this compound were
collected using a Bruker SMART ccd area detector mounted
on a Bruker P4 goniometer using graphite-monochromated
Mo Kα radiation (λ = 0.71073 Å). The sample was cooled to
133(2) K. The intensity data, which nominally covered 1.5
hemispheres of reciprocal space, were measured as a series
of f oscillation frames each of 0.4° for 60 s per frame. The
detector was operated in 512 × 512 mode and was posi-
tioned 5.00 cm from the sample. Coverage of unique data
was 98.8% complete to 25.00 deg in θ. Cell parameters were
determined from a nonlinear least-squares fit of 5158 peaks
in the range 3.30 < θ < 25.32°. The first 50 frames were re-
peated at the end of data collection and yielded 459 peaks
showing a variation of –0.15% during the data collection. A
total of 30 480 data were measured in the range 3.28 < θ <
25.00°. The data were corrected for absorption by the empir-
ical method (20) giving min and max transmission factors of
0.8084 and 0.8871. The data were merged to form a set of
15 000 independent data with R(int) = 0.0586.
[2,5-Ph2-3,4-Tol2(η5-C4COCH3)]Ru(CO)2(η1-OCHO) (7)
A suspension of AgOCHO (150 mg, 0.87 mmol) in a so-
lution of 8 (130 mg, 0.18 mmol) in CH2Cl2 (10 mL) was
stirred for 18 h at room temperature until IR monitoring in-
dicated complete consumption of 8. The mixture was filtered
to give a light green solution. Evaporation of solvent and
chromatography of the resulting green solid on silica gel us-
ing 3:1 CH2Cl2:acetone gave 7, as an off-white powder
(75 mg, 56%), mp 130–135°C (dec). IR (CH2Cl2) (cm–1):
1
2043 (s), 1993 (s), 1618 (m). H NMR (CD2Cl2, 300 MHz)
δ: 2.20 (s, 6H, tolyl CH3), 3.46 (s, OCH3), 6.84 (AA′BB′,
8H, tolyl), 7.32 (m, 6H, phenyl), 7.48 (m, 4H, phenyl), 7.81
(s, OCHO). 13C NMR (CD2Cl2,125 MHz) δ 21.2 (tolyl CH3),
62.0 (OCH3), 88.4 (C 3,4 of Cp), 102.5 (C 2,5 of Cp),
126.7–138.7 (8 resonances, aromatic), 143.7 (C1 of Cp),
168.3 (OCHO), 198.8 (CO). Anal. calcd. for C35H28O5Ru: C
66.76, H 4.48; found: C 66.56, H 4.38.
Decarboxylation of 7
The orthorhombic space group P2(1)2(1)2(1) was deter-
mined by systematic absences and statistical tests and veri-
fied by subsequent refinement. The structure was solved by
direct methods and refined by full-matrix least-squares
methods on F2 (21). Hydrogen atom positions were initially
determined by geometry and were refined using a riding
model. Non-hydrogen atoms were refined with anisotropic
displacement parameters. A total of 1210 parameters were
refined against 216 restraints and 15 000 data to give
wR(F2) = 0.1067 and S = 1.010 for weights of w = 1/[σ2 (F2) +
A THF-d8 solution of 7 (0.03 M) containing ferrocene as
an internal standard was heated at 80 ± 0.2°C. The disap-
1
pearance of the H NMR resonance for -OCHO (δ 7.68) was
monitored at 20, 66, 119, and 194 min; kobs = 1.7 × 10–4 s–1,
t0.5 = 67 min. The major product was [2,5-Ph-3,4-Tol(η5-
C4COCH3)]Ru(CO)2H (75%); four unidentified minor prod-
ucts were also observed.
Decarboxylation of 7 in the presence of 13CO
A THF-d8 solution of 7 (0.03 M) containing ferrocene as
an internal standard was heated to 80 ± 0.2°C under 13CO
(1 atm added at –78°C (1 atm = 101.325 kPa)). The disap-
2
2
(0.0300P)2 + 2.2000P], where P = [Fo + 2Fc ]/3. The final
R(F) was 0.0483 for the 11 005 observed, [F > 4σ(F)], data.
The largest shift/s.u. was 0.003 in the final refinement cycle.
The final difference map had maxima and minima of 0.581
and –0.505 e Å–3, respectively. The absolute structure was
determined by refinement of the Flack parameter (22).
1
pearance of the H NMR resonance for -OCHO (δ 7.68) was
1
monitored at 30, 60, and 120 min. H NMR showed that
<5% of 7 had reacted after 2 h. 13C NMR showed dramatic
growth of the CO resonance of 7 (δ 201) relative to aromatic
resonances of 7.
Acknowledgments
[2,5-Ph2-3,4-Tol2(η5-C4COCH3)]Ru(CO)2H (9)
Addition of CH3OTf (250 µL, 2.2 mmol) to a suspension
of NEt4[2,5-Ph2-3,4-Tol2(η4-C4CO)Ru(CO)2H] (10) (225 mg,
0.31 mmol) in toluene (10 mL) gave a green solution with a
white precipitate. The solution was filtered and solvent evap-
orated under vacuum. The residue was dissolved in hexane
(5 mL), filtered through a glass wool, and cooled to –30°C
for 18 h to give 9 (40 mg, 22%) as a yellow precipitate. IR
(CH2Cl2) (cm–1): 2018 (s), 1958 (s). 1H NMR (CD2Cl2,
300 MHz) δ: –9.80 (s, RuH), 2.21 (s, 6H, tolyl CH3), 3.42 (s,
3H, OCH3), 6.86 (d, J = 8.4 Hz, 4H, tolyl), 6.96 (d, J =
8.4 Hz, 4H, tolyl), 7.25 (m, 6H, phenyl), 7.37 (m, 4H,
phenyl). 13C NMR (CD2Cl2,125 MHz) δ: 21.2 (tolyl CH3),
66.9 (OCH3), 94.9 (C 3,4 of Cp), 104.6 (C 2,5 of Cp),
127.9–137.5 (8 resonances, aromatic), 138.9 (C1 of Cp),
Financial support from the Department of Energy, Office
of Basic Energy Sciences, is gratefully acknowledged. Ste-
ven Singer thanks the NIH (5T32 GM 08505) for support
under a Chemistry Biology Interface Training Grant. Grants
from NSF (CHE-9629688) and NIH (I S10 RR04981–01)
for the purchase of NMR spectrometers, and from NSF
(CHE-9105497) for the purchase of X-ray instruments are
acknowledged.
3
3
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