Acid-promoted homogeneous hydrogenation of alkenes catalyzed by the ruthenium-hydride complex (PCy3)2(CO)(Cl)RuH: Evidence for the formation of 14-electron species from the selective entrapment of the phosphine ligand

Article abstract of DOI:10.1021/om000315n

The addition of 1.0 equiv of HBF₄·OEt₂ led to a ca. 2-3-fold increase in the catalyst activity of (PCy₃)₂(CO)(Cl)RuH (1a) toward the hydrogenation of alkenes. The stoichiometric reaction of 1a with HBF₄·OEt₂ produced a 1:3.5 mixture of the new ruthenium-hydride species 2 and Cy₃PH⁺BF₄^-. The catalyst activity of the isolated 2/Cy₃PH⁺ mixture was found to be similar to 1a/HBF₄·OEt₂. The complex 2 slowly decomposed in C₆H₆ solution to give a novel tetrameric complex 3. The treatment of 1a with HBF₄·OEt₂ in CH₃CN led to the selective formation of the monophosphine species 4, which was converted to the stable complex 5 upon treatment with excess PPh₃. The reaction of 1a with a weak acid, HSiCl₃, led to the formation of the anionic silyl complex 7. The structures of 3, 5, and 7 were determined by X-ray crystallography. The formation of η²-H₂ complex 8 was observed by NMR at low temperature. These results suggested that the 14-electron species [(PCy₃)(CO)RuHCl], generated from the selective entrapment of the phosphine ligand, is the key species involved in the catalytic hydrogenation reaction.

Full text of DOI:10.1021/om000315n

Organometallics 2000, 19, 2909-2915
2909
Acid -P r om oted Hom ogen eou s Hyd r ogen a tion of Alk en es
Ca ta lyzed by th e Ru th en iu m -Hyd r id e Com p lex
(P Cy3)2(CO)(Cl)Ru H: Evid en ce for th e F or m a tion of
14-Electr on Sp ecies fr om th e Selective En tr a p m en t of
th e P h osp h in e Liga n d
Chae S. Yi,* Do W. Lee, and Zhengjie He
Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201-1881
Arnold L. Rheingold, Kin-Chung Lam, and Thomas E. Concolino
Department of Chemistry, University of Delaware, Newark, Delaware 19716
Received April 13, 2000
The addition of 1.0 equiv of HBF4‚OEt2 led to a ca. 2-3-fold increase in the catalyst activity
of (PCy3)2(CO)(Cl)RuH (1a ) toward the hydrogenation of alkenes. The stoichiometric reaction
of 1a with HBF4‚OEt2 produced a 1:3.5 mixture of the new ruthenium-hydride species 2
and Cy3PH+BF4-. The catalyst activity of the isolated 2/Cy3PH+ mixture was found to be
similar to 1a /HBF4‚OEt2. The complex 2 slowly decomposed in C6H6 solution to give a novel
tetrameric complex 3. The treatment of 1a with HBF4‚OEt2 in CH3CN led to the selective
formation of the monophosphine species 4, which was converted to the stable complex 5
upon treatment with excess PPh3. The reaction of 1a with a weak acid, HSiCl3, led to the
formation of the anionic silyl complex 7. The structures of 3, 5, and 7 were determined by
X-ray crystallography. The formation of η2-H2 complex 8 was observed by NMR at low
temperature. These results suggested that the 14-electron species [(PCy3)(CO)RuHCl],
generated from the selective entrapment of the phosphine ligand, is the key species involved
in the catalytic hydrogenation reaction.
In tr od u ction
addition of CuCl to the reaction mixture.4b The forma-
tion of an adduct PCy3‚CuCl has been suggested for
generating a highly reactive 14-electron monophosphine
ruthenium-alkylidene complex. Also reported was up
to a 10-fold rate increase in the catalyst activity when
HCl was added to a water-soluble analogue of the
ruthenium-alkylidene complex.5 Using BPh3 as a Lewis
acid, Berry and co-workers recently demonstrated an
effective entrapment of PMe3 ligand in forming a novel
ruthenium-silene complex (PMe3)3Ru(η2-CH2dSiMe3)-
H2.6 Lewis acids have also been well-known to promote
the hydrogenation of unsaturated compounds.7-9 Some
of the representative examples include the transfer
hydrogenation of aryl-substituted olefins by Pd/C and
The dissociation of a phosphine ligand is one of the
most common ways to activate metal-phosphine com-
plexes.1 For example, metal-phosphine complexes such
as (PPh3)3RhCl (hydrogenation),2 (PPh3)3(CO)RhH (hy-
droformylation),3 and (PCy3)2(Cl)2RudCHPh (metath-
esis)4 have been well-known to generate catalytically
active species via an initial dissociation of the phosphine
ligand. As a way to increase the catalyst activity,
considerable efforts have been directed to develop
methods for selectively promoting the dissociation and
trapping of phosphine ligands. Grubbs and co-workers
observed a substantial rate enhancement of the me-
tathesis catalytic activity of (PCy3)2(Cl)2RudCHPh upon
8
AlCl3 and the hydrogenation of aromatic compounds
(1) (a) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987. (b) Homogeneous
Catalysis with Metal Phosphine Complexes; Pignolet, L. H., Ed.;
Plenum Press: New York, 1983.
(2) (a) Chaloner, P. A.; Esteruelas, M. A.; J oo´, F.; Oro, L. A.
Homogeneous Hydrogenation; Kluwer: Boston, 1994. (b) Brunner, H.
In Applied Homogeneous Catalysis with Organometallic Compounds;
Cornils, B., Herrmann, W. A., Eds.; VCH: New York, 1996; Vol. 1.
(3) (a) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis, 2nd ed.;
Wiley & Sons: New York, 1992. (b) Frohning, C. D.; Kohlpaintner, C.
W. In Applied Homogeneous Catalysis with Organometallic Com-
pounds; Cornils, B., Herrmann, W. A., Eds.; VCH: New York, 1996;
Vol. 1.
by Ni(acac)2/AlEt3.9
While studying the ruthenium-catalyzed hydrovinyl-
ation reactions of alkynes, we recently observed the
formation of the phosphonium salt Cy3PH+BF4- in the
reactions catalyzed by the cationic ruthenium complex
[(PCy3)2(CO)(Cl)RudCHCHdC(CH3)2]+BF4- 10 The for-
.
(5) Lynn, D. M.; Mohr, B.; Grubbs, R. H. J . Am. Chem. Soc. 1998,
120, 1627-1628.
(6) Dioumaev, V. K.; Plo¨ssl, K.; Caroll, P. J .; Berry, D. H. J . Am.
Chem. Soc. 1999, 121, 8391-8392.
(7) Olah, G. A.; Molna´r, A. Hydrocarbon Chemistry; Wiley: New
York, 1995.
(4) (a) Nguyen, S. T.; Grubbs, R. H.; Ziller, J . W. J . Am. Chem. Soc.
1993, 115, 9858-9859. (b) Dias, E. L.; Nguyen, S. T.; Grubbs, R. H. J .
Am. Chem. Soc. 1997, 119, 3887-3897. (c) Dias, E. L.; Grubbs, R. H.
Organometallics 1998, 17, 2758-2767. (d) Ulman, M.; Grubbs, R. H.
Organometallics 1998, 17, 2484-2489.
(8) Olah, G. A.; Prakash, G. K. S. Synthesis 1978, 397-398.
(9) (a) Lapporte, S. J .; Schuett, W. R. J . Org. Chem. 1963, 28, 1947-
1948. (b) Sloan, M. F.; Matlack, A. S.; Breslow, D. S. J . Am. Chem.
Soc. 1963, 85, 4014-4019.
10.1021/om000315n CCC: $19.00 © 2000 American Chemical Society
Publication on Web 06/21/2000

2910 Organometallics, Vol. 19, No. 15, 2000
Yi et al.
Ta ble 1. Effect of Ad d ition of Acid s on th e
Hyd r ogen a tion of Cycloocten e Ca ta lyzed by 1a
coordinating anions such as CF3CO2H and HCl‚OEt2
(entries 6, 7).
turnover rateb
We next surveyed the catalyst activity of 1a /HBF4‚
OEt2 toward the hydrogenation of a number different
alkenes (Table 2). In general, the rate of hydrogenation
catalyzed by 1a /HBF4‚OEt2 was found to be ca. 2-3
times higher than the reactions catalyzed by 1a alone.
Furthermore, the increased activity was observed with-
out significantly increasing the isomerization rate for
terminal alkenes with â-hydrogens. More than a 4-fold
rate increase has been observed for 5-hexen-2-one
(entries 7, 8). The terminal alkene is preferentially
hydrogenated over the internal one for the 4-vinyl-1-
cyclohexene case (entries 9, 10). Since the major portion
of isomerization products was found to be formed during
the sample preparation period,11 the formation of the
isomerization products can be effectively suppressed by
applying H2 pressure to the solution containing the
catalyst 1a before adding alkene substrates.
entry
catalyst
acid
1
2
3
4
5
6
7
8
9
1a
1b
1a
1b
1a
1a
1a
1a
1a
1a
none
none
940
760
2200
600
HBF4‚OEt2
HBF4‚OEt2
HOTf
CF3CO2H
HCl‚OEt2
CuCl2
2200
310
∼0c
700
BBh3
PdCl2
1080
150
10
a
Reaction conditions: 5.7 mmol of alkene (1.0 M); 0.69 µmol of
the catalyst (0.12 mM; alkene:1 ) 8300:1); 0.7-1.4 µmol of acid;
b
1.0 atm H2; 5 mL of C6H6; 22 ( 1 °C. Turnover rate ) (mol of
product)(mol of catalyst)-1 h-1
the addition of acid.
.
c The catalyst decomposed upon
-
mation of Cy3PH+BF4 appeared to promote the cata-
lytic activity of the ruthenium complex. These results
suggested that the phosphonium salt formation may be
a viable way for sequestering dissociated phosphine
ligands. Herein we wish to report an acid-induced
selective entrapment of the phosphine ligand on the
ruthenium-hydride complex (PCy3)2(CO)(Cl)RuH (1a ).
We also present evidence of the formation of a mono-
phosphine intermediate species and its role in the
catalytic hydrogenation of alkenes.
Rea ction of 1a w ith HBF 4‚OEt2. We thought that
the increased catalyst activity of 1a /HBF4‚OEt2 might
be due to the selective entrapment of the phosphine
ligand and the formation of the 14-electron ruthenium-
monophosphine species. The stoichiometric reactions of
1a with different acids were examined to detect/isolate
reactive intermediate species. Thus, the treatment of
1a with 1.2 equiv of HBF4‚OEt2 in C6H6 at room
temperature led to the formation of the new complex 2
and Cy3PH+BF4 in 1:3.5 ratio, as determined by 31P
-
NMR (eq 2). The product mixture of 2/Cy3PH+BF4- was
Resu lts a n d Discu ssion
Effect of Acid s on th e Ra te of Hyd r ogen a tion of
Alk en es. We recently reported that the ruthenium-
hydride complex 1a is an effective catalyst for the
hydrogenation of alkenes and proposed the mechanism
of reaction via an initial dissociation of the phosphine
ligand.11 In an effort to increase the catalyst activity,
we explored the effect of adding acids on the catalyst
activity toward the hydrogenation reaction of cyclo-
octene (eq 1). Among the selected group of acids, the
most significant rate enhancement was observed with
protic acids with weakly coordinating anions, HOTf and
HBF4‚OEt2 (Table 1). For example, the treatment of
cyclooctene with 1.0 atm of H2 in the presence of 1a and
1.0 equiv of HBF4‚OEt2 at room temperature resulted
in a turnover rate of 2200 h-1, which was more than 2
times higher than the reaction catalyzed by 1a under
similar reaction conditions (entry 3). In contrast, the
addition of HBF4‚OEt2 to the PPri analogue (PPri ) -
isolated after simple precipitation/filtration procedures
from the solution. Unfortunately, numerous attempts
to separate 2 from the phosphonium salt thus far have
not been successful because of the similar solubility
property for both 2 and Cy3PH+BF4 and due to the
-
instability of 2 in solutions. The NMR spectroscopic data
of 2, the metal-hydride peak at δ -10.52 (d, J HP ) 25.2
Hz), and the carbonyl carbon peak at δ 196.9 (d, J CP
)
17.6 Hz) clearly indicated that the complex contains only
one phosphine ligand per each ruthenium center. Fur-
thermore, a relatively high CO stretching band (νCO
)
1969 cm-1) suggested a cationic nature of the complex.
The greater amount of the phosphonium salt compared
to 2 indicated that most of the Cy3PH+BF4 resulted
-
from the protonation of the second phosphine ligand.13
3
3 2
-
The isolated 2/Cy3PH+BF4 mixture was found to
(CO)(Cl)RuH (1b) did not lead to a rate increase
compared to 1b alone (entry 4).12 Only a marginal
increase in catalyst activity was observed with Lewis
acids, CuCl2 and BPh3 (entries 8, 9), while a decrease
in catalyst activity was observed for protic acids with
exhibit activity similar to the in-situ generated 1a /
HBF4‚OEt2 under similar reaction conditions (TON )
2750 h-1 for cyclooctene at 1.0 atm of H2).
-
As mentioned, the C6H6 solution of the 2/Cy3PH+BF4
mixture was not stable at room temperature and slowly
decomposed into a novel tetrameric ruthenium complex
3 along with other unidentified ruthenium products.14
(10) Yi, C. S.; Lee, D. W.; Chen, Y. Organometallics 1999, 18, 2043-
2045.
(11) Yi, C. S.; Lee, D. W. Organometallics 1999, 18, 5152-5156.
(12) For the synthesis and reactions of 1b, see: (a) Buil, M. L.; Elipe,
S.; Esteruelas, M. A.; On˜ate, E.; Peinado, E.; Ruiz, N. Organometallics
1997, 16, 5748-5755, and references therein. (b) Esteruelas, M. A.;
Werner, H. J . Organomet. Chem. 1986, 303, 221-231. (c) Esteruelas,
M. A.; Valero, C.; Oro, L. A.; Meyer, U.; Werner, H. Inorg. Chem. 1991,
30, 1159-1160. (d) Esteruelas, M. A.; Oro, L. A.; Valero, C. Organo-
metallics 1992, 11, 3362-3369. (e) Esteruelas, M. A.; Herrero, J .; Oro,
L. A. Organometallics 1993, 12, 2377-2379.
(13) The IR spectra of isolated 2/Cy3PH+ showed little of other
carbonyl-containing species, but we cannot rigorously rule out the
presence of byproducts having no phosphine ligands. The elemental
analyses on several samples of 2/Cy3PH+ gave disparate results.
(14) Complex 3 was isolated in 15-20% yields based on 1a . Selected
spectroscopic data of 3: 31P{1H} NMR (CD2Cl2, 121.6 MHz) δ 76.1 (s,
PCy3); IR (CH2Cl2) νCO ) 1997 cm-1
.

Acid-Promoted Homogeneous Hydrogenation of Alkenes
Organometallics, Vol. 19, No. 15, 2000 2911
F igu r e 2. Molecular structure of the cation part of 5
drawn with 30% thermal ellipsoids. Hydrogen atoms except
the metal-hydride, the solvent molecule, and the BF4- ion
are omitted for clarity.
F igu r e 1. Molecular structure of 3 drawn with 30%
thermal ellipsoids. Hydrogen atoms, BF4 ion, solvent
-
molecules, and the carbon atoms of cyclohexyl group except
ipso carbons are omitted for clarity.
coordinating solvent. For example, when the reaction
of 1a (100 mg, 0.14 mmol) with HBF4‚OEt2 (25 µL, 1.2
equiv) was conducted in CH3CN, the formation of a new
monophosphine adduct 4 was initially observed by NMR
(Scheme 1). The ratio between 4 and Cy3PH+BF4- was
found to be ∼1:1, indicating that the selective entrap-
ment of PCy3 has been effected by the acid in this case.
Due to the thermal instability of 4 in solution, its
structure was tentatively established by spectroscopic
methods at low temperature.17 The NMR data of 4,
which exhibited the ruthenium-hydride peak at δ
-13.73 (d, J PH ) 22.2 Hz) and the carbonyl peak at δ
202.3 (d, J PC ) 17.4 Hz), clearly indicated that the
complex contained only one phosphine ligand. Also, a
Sch em e 1
The single crystals of 3 grown from C6H6 solution were
found to be suitable for X-ray crystallography (Figure
1). The molecular structure of 3 showed a tetrameric
ruthenium center, in which two symmetric [(Cy3P)(CO)-
RuCl]2 units are joined by three bridging chlorides and
where all four CO ligands are in syn configuration with
a overall pseudo C2v symmetry. One of the most unusual
structural features of 3 is the tetrabridging chloride
ligand on the complex. While a few examples of tetra-
bridging chloride on late metal complexes of Cu, Cd, Ag,
and Hg have been reported,15 to the best of our knowl-
edge, this is a unique example of having a µ4-Cl ligand
for group VIII metal species. Hawthorne and Pud-
dephatt independently observed the preferential µ4-
coordination of the chloride ion by tetrameric Ag and
Hg complexes.15a,b One possible route to the formation
of 3 is from the addition of Cl- to an initially formed
tetrameric unit of Ru42+, in which the trapping of a Cl-
might have been driven by a highly electrophilic tet-
rameric Ru42+ metal center. It is also worthwhile to note
that a tetrazirconium complex with a nonclassical,
planar µ4-phosphonium ion has been recently reported.16
In an attempt to further establish the structure of 2,
we next explored the reaction of 1a with acids in a
relatively high CO stretching band (νCO ) 1951 cm-1
)
is consistent with a cationic metal complex. Interest-
ingly, dissolution of the isolated 2/Cy3PH+BF4- mixture
in CH3CN formed the same complex 4, suggesting that
a common 14-electron fragment is involved in both
cases.
Further treatment of 4 with excess PPh3 at room
temperature produced the stable adduct 5 in 75% yield.
One of the most diagnostic spectroscopic features of 5
was the AB pattern of two phosphorous peaks (δ 47.5,
J AB ) 238 Hz) resulting from their inequivalent envi-
ronments. The metal-hydride peak at δ -13.57 with a
relatively small coupling constant (t, J PH ) 18.0 Hz) was
consistent with the hydride ligand cis to both phosphine
ligands. The structure of 5 was further established by
X-ray crystallography (Figure 2). The molecular struc-
ture of 5 clearly showed an octahedral arrangement
around the metal center with two trans phosphine
ligands and two cis CH3CN ligands.
Rea ction of 1a w ith a Wea k Acid , HSiCl3. Since
a relatively strong acidity of HBF4‚OEt2 might have
caused the protonation of the second PCy3 ligand, we
explored the reaction of 1a with weak acids in the hope
of effecting a more selective entrapment of the single
phosphine ligand. When the reaction of 1a with the
(15) (a) Xu, W.; Vittal, J . J .; Puddephatt, R. J . J . Am. Chem. Soc.
1993, 115, 6456-6457. (b) Yang, X.; Knobler, C. B.; Zheng, Z.;
Hawthorne, M. F. J . Am. Chem. Soc. 1994, 116, 7142-7159. (c) Oliver,
K. J .; Waters, T. N.; Cook, D. F.; Rickard, C. E. F. Inorg. Chim. Acta
1977, 24, 85-89. (d) Wuest, J . D.; Zacharie, B. Organometallics 1985,
4, 410-411. (e) Beauchamp, A. L.; Oliver, M. J .; Wuest, J . D.; Zacharie,
B. J . Am. Chem. Soc. 1986, 108, 73-77. (f) Yang, X.; Knobler, C. B.;
Hawthorne, M. F. Angew. Chem., Int. Ed. Engl. 1991, 30, 1507-1508.
(g) Gonza`lez-Durarte, P.; Clegg, W.; Casals, I.; Sola, J .; Rius, J . J . Am.
Chem. Soc. 1998, 120, 1260-1266.
(17) Selected spectroscopic data of 4: 1H NMR (CD2Cl2, 300 MHz,
-45 °C) δ -13.73 (d, J PH ) 22.2 Hz, Ru-H), 2.05 (s, CH3CN); 13C{1H}
NMR (CD2Cl2, 75.6 MHz, -30 °C) δ 202.3 (d, J PC ) 17.4 Hz, CO);
31P{1H} NMR (CD2Cl2, 121.6 MHz, -30 °C) δ 64.6 (s, PCy3); IR (CH2Cl2)
(16) (a) Driess, M.; Aust, J .; Mertz, K.; van Wu¨llen, C. Angew. Chem.,
Int. Ed. Engl. 1999, 38, 3677-3680. (b) Stephan, D. W. Angew. Chem.,
Int. Ed. Engl. 2000, 39, 501-502.
νCO ) 1951 cm-1
.

2912 Organometallics, Vol. 19, No. 15, 2000
Yi et al.
Sch em e 2
weak acid HSiCl3 (2.2 equiv) in toluene-d8 was moni-
tored by NMR at room temperature, the formation of
the new complex 6 was initially observed along with
extensive H2 gas evolution (Scheme 2). In the 31P NMR
spectra, the phosphorus signal of 6 (δ 36.9) initially
appeared at the expense of 1a , which gradually con-
verted to an insoluble yellow-orange solid, 7, within 30
min at room temperature. Due to the instability of the
complex, we tentatively assigned the structure of 6
based on the available spectroscopic data. The 31P NMR
of the isolated 7 in CD2Cl2 exhibited two different
phosphorus peaks, one assignable to the phosphonium
ion (δ 28.4) and the other bonded to the ruthenium
center, as indicated by the presence of 29Si satellites (δ
30.1, J PSi ) 51.3 Hz).
F igu r e 3. Molecular structure of the anion part of 7 drawn
with 30% thermal ellipsoids. Hydrogen atoms and the
phosphonium ion are omitted for clarity.
1
in 1:1 toluene-d8/CD3CN was monitored by H NMR at
The structure of 7 was further established by X-ray
crystallography (Figure 3). The molecular structure of
7 showed that the complex still retained a square
pyramidal geometry on the ruthenium center, with two
silyl groups occupying each basal and apical positions.
A considerably longer basal Ru-Si bond distance
(2.316(2) Å) compared to the apical one (2.249(2) Å)
indicated a strong trans influence from the phosphine
ligand to the basal silyl group. In this case, a selective
transformation of the phosphine ligand into the phos-
phonium ion has been achieved with 2 equiv of HSiCl3.
The initial H2 gas formation suggested that the proton-
ation at the metal was preferred over the direct proton-
ation at the phosphine ligand.
low temperature. At -45 °C, the new broad metal-
hydride peak appeared at δ -9.80, which was consistent
with the formation of the η2-H2 complex 8. The hydride
signal of 8 was gradually disappeared within 30 min at
-45 °C as the peaks due to 4 began to appear. The
formation of the η2-H2 complex was further established
from the reaction of deuterated 1a -d1 with HBF4‚OEt2.
In this case, the hydride peak turned into a character-
istic 1:1:1 triplet (J HD ) 29.3 Hz), which is consistent
with the formation of the η2-HD complex 8-d1 (Figure
4). Though not completely resolved, each 1:1:1 triplet
peak appeared to be further split into small triplets (J HP
< 7 Hz) due the coupling with two phosphorus atoms.20
Previously, numerous stable η2-H2 complexes of ruthe-
nium and osmium complexes have been reported.21
The spectroscopic observation of the η2-H2 complex 8
in the reaction with 1a -d1 is consistent with the rapid
protonation at the metal center prior to the elimination
of Cy3PH+. The apparent statistical distribution of the
deuterium atom (approximately 20% deuterium incor-
poration into the phosphonium salt with 4 equiv of the
acid) also suggests that the initial H/D exchange via the
formation of the η2-H2 complex 8 is much faster than
the subsequent elimination of Cy3PH+ salt. The addition
of excess PPh3 to the solution containing 4 led to 5 with
the same amount of deuterium (20%), indicating that
no further H/D exchange had occurred during the
formation of 5. As evidenced by the formation of the
cationic complex 5, the Cl- ligand dissociation became
a competitive process especially in the coordinating
solvent CH3CN.
Deu ter iu m -La belin g Stu d ies a n d th e F or m a tion
of th e η2-H2 Com p lex. In an attempt to elucidate the
source of the metal-hydride of 2, the reaction of the
deuterated 1a -d1 (>95% D) with 4 equiv of HBF4‚OEt2
in CD2Cl2 was monitored by NMR at room temperature
(Scheme 3).18 Initially formed 2 contained ∼10% deu-
terium on the metal-hydride, while the phosphonium
salt had a ∼4:1 ratio of Cy3PH+BF4 to Cy3PD+BF4
.
-
-
When the reaction of 1a -d1 with HBF4‚OEt2 was con-
ducted in CD3CN, reappearance of the ruthenium-
hydride peak of 1a was initially observed by H NMR
1
at room temperature. Again, a 4:1 ratio of Cy3PH+ to
Cy3PD+ was observed for the phosphonium salt.19 The
H/D ratio of the phosphonium salt was found to depend
on the amount of acid; for example, addition of 10 equiv
of HBF4‚OEt2 led to a ∼10:1 ratio of Cy3PH+ to Cy3PD+.
The deuterium content of 4 could not be accurately
estimated in this case, since the complex 4 rapidly
decomposed in solution, which prevented obtaining a
2
meaningful H NMR.
(20) Several attempts for T1 measurement of 8 were not successful
because of a relatively short lifetime of the complex at low temperature.
(21) (a) Chinn, M. S.; Heinekey, D. M. J . Am. Chem. Soc. 1990, 112,
5166-5175. (b) Gusev, D. G.; Vymenits, A. B.; Bakhmutov, V. I. Inorg.
Chem. 1992, 31, 2-4. (c) Esteruelas, M. A.; Lahoz, F. J .; On˜ate, E.;
Oro, L. A.; Valero, C.; Zeier, B. J . Am. Chem. Soc. 1995, 117, 7935-
7942. (d) Bakhmutov, V. I.; Bertra´n, J .; Esteruelas, M. A.; Lledo´s, A.;
Maseras, F.; Modrego, J .; Oro, L. A.; Sola, E. Chem. Eur. J . 1996, 2,
815-825. (e) Gusev, D. G.; Kuhlman, R. L.; Renkema, K. B.; Eisenstein,
O.; Caulton, K. G. Inorg. Chem. 1996, 35, 6775-6783. (f) Luther, T.
A.; Heinekey, D. M. Inorg. Chem. 1998, 37, 127-132. (g) Gru¨ndemann,
S.; Ulrich, S.; Limbach, H.-H.; Golubev, N. S.; Denisov, G. S.; Epstein,
L. M.; Sabo-Etienne, S.; Chaudret, B. Inorg. Chem. 1999, 38, 2550-
2551.
The evolution of H2 gas during the reaction of 1a with
HSiCl3 and the rapid H/D exchange implied the forma-
tion of a η2-H2 intermediate species. In an effort to detect
the η2-H2 complex, the reaction of 1a with HBF4‚OEt2
(18) Approximately 4 equiv of the acid was required to complete the
reaction.
(19) The H/D ratio of the phosphonium salt was determined by 31P
NMR. The phosphorous signal of Cy3PD+ at δ 29.3 (1:1:1 triplet, J PD
) 71 Hz) was approximately 1 ppm upfield-shifted compared to the
signal of Cy3PH+ (δ 30.2).

Acid-Promoted Homogeneous Hydrogenation of Alkenes
Organometallics, Vol. 19, No. 15, 2000 2913
Sch em e 3
is most effective for complexes with a sterically demand-
ing PCy3 ligand. It is interesting to note that analogous
reaction of 1b with HBF4‚OEt2 did not give clean
products; several unidentified phosphorus peaks ap-
peared along with a significant amount of decomposi-
tion, as judged by the 31P NMR. It was previously
reported that the analogous reaction of (PPh3)3(CO)(Cl)-
RuH with aqueous HBF4 formed the cationic complex
{[(PPh3)2(CO)Ru(µ-Cl)]2(µ-H)}+ (9) with a bridging
metal-hydride,24 but the complex 9 was found to be
completely inactive toward the hydrogenation reaction.
Several similar dimeric structures with terminal metal-
hydrides, which are consistent with the spectroscopic
data of 2, are currently being considered as a possible
structure.
F igu r e 4. 1H NMR of the metal-hydride region of 8-d1
in toluene-d8/CD3CN (1:1) at -45 °C.
Con clu sion s
These results implicate the involvement of a 14-
electron species as the key intermediate in the catalytic
hydrogenation reaction. The substantial rate increase
in the reactions catalyzed by 1a /HBF4‚OEt2 can be
rationalized via the formation of a transient species,
[(Cy3P)(CO)RuHCl], generated from the selective en-
trapment of the phosphine ligand. The selective forma-
tion of 4 and 7 from the reactions of 1a with HBF4‚OEt2
in CH3CN and with the weak acid HSiCl3 also supports
the formation of a monophosphine species. Caulton and
co-workers recently reported the synthesis of stable
In summary, an acid-induced selective entrapment of
the phosphine ligand from 1a was found to give a
substantial rate increase toward the hydrogenation of
alkenes. The stoichometric reaction of 1a with HBF4‚
OEt2 led to the isolation of the mixture of 2 and
Cy3PH+BF4-. The selective entrapment of the phos-
phine ligand has been demonstrated in the stoichiomet-
ric reactions of 1a with HBF4‚OEt2 in CH3CN and with
the weak acid HSiCl3. These results together with the
observation of the η2-H2 complex 8 suggested the
involvement of the monomeric 14-electron species in the
catalytic reaction. Further structure elucidation and the
exploration of the catalyst activity of 2 toward other
reactions are currently being pursued.
cationic 14-electron ruthenium complexes, [(PBut Me)2-
2
-
(CO)RuX]+BAr′4 (X ) H, CH3, Ph, BC6H4O2).22 These
ruthenium complexes were shown to have a nonplanar
“sawhorse” geometry, with M-H-C agostic interactions
from the t-Bu groups of the two phosphine ligands.
Baratta and co-workers reported a neutral 14-electron
ruthenium complex, [PPh2(2,6-Me2C6H5)]2RuCl2, which
was also shown to be stabilized by having agostic
interactions with the phosphine ligand.23
Exp er im en ta l Section
Gen er a l In for m a tion . All reactions were carried out in
an inert-atmosphere glovebox or by using standard high-
vacuum and Schlenk line techniques unless otherwise noted.
Benzene, hexanes, and Et2O were distilled from purple solu-
tions of sodium and benzophenone immediately prior to use.
CH2Cl2 was distilled from CaH2. The NMR solvents were dried
from activated molecular sieves (4 Å). All organic alkene
substrates were purchased from commercial sources and
vacuum-distilled from either molecular sieves or sodium prior
to use. The acids, HBF4‚OEt2 and HSiCl3, were purchased from
Aldrich Chemical Co. and used without further purification.
The complexes 1a and 1b were prepared according to the
Preliminary investigations indicated that the acid-
induced selective entrapment of the phosphine ligand
previously reported procedure.10,12 The H, 13C, and 31P NMR
1
(22) (a) Huang, D.; Huffman, J . C.; Bollinger, J . C.; Eisenstein, O.;
Caulton, K. G. J . Am. Chem. Soc. 1997, 119, 7398-7399. (b) Huang,
D.; Oliva´n, M.; Huffman, J . C.; Eisenstein, O.; Caulton, K. G.
Organometallics 1998, 17, 4700-4706. (c) Huang, D.; Streib, W. E.;
Bollinger, J . C.; Caulton, K. G. Winter, R. F.; Scheiring, T. J . Am.
Chem. Soc. 1999, 121, 8087-8097.
spectra were recorded on a GE GN-Omega 300 MHz FT-NMR
spectrometer. Infrared and mass spectra were recorded from
Nicolet Magna 560 and Hewlett-Packard HP 5890 GC/MS
(24) Sa´nchez-Delgado, R. A.; Thewalt, U.; Valencia, N.; Andriollo,
A.; Ma´rquez-Silva, R.-L.; Puga, J .; Scho¨llhorn, H.; Klein, H.-P.; Fontal,
B. Inorg. Chem. 1986, 25, 1097-1106.
(23) Baratta, W.; Herdtweck, E.; Rigo, P. Angew. Chem., Int. Ed.
Engl. 1999, 38, 1629-1631.

2914 Organometallics, Vol. 19, No. 15, 2000
Ta ble 2. Hyd r ogen a tion of Alk en es Ca ta lyzed by 1/HBF 4‚OEt2
Yi et al.
a
entry
alkene
1-hexene
catalyst
additive
H2 (atm)
turnover rateb
1
2
3
4
5
6
7
8
9
1a
1a
1b
1b
1a
1a
1a
1a
1a
1a
none
HBF4‚OEt2
none
HBF4‚OEt2
none
HBF4‚OEt2
none
HBF4‚OEt2
none
1.0
1.0
1.0
1.0
2.0
2.0
2.0
2.0
2.0
2.0
1200 (180)
2250 (500)
700 (200)
800 (200)
2000 (390)
3000 (240)
240 (24)
1100 (60)
1300
1-hexene
allylbenzene
5-hexen-2-one
4-vinyl-1-cyclohexene
10
HBF4‚OEt2
3100
a
Reaction conditions: 5.7 mmol of alkene (1.0 M); 0.69 µmol of the catalyst (0.12 mM; alkene:1a ) 8300:1); 1-2 equiv of the acid; 5
mL of C6H6; 22 ( 1 °C. b Turnover rate ) (mol of product)(mol of catalyst)-1 h-1; the numbers in parentheses correspond to the isomerization
rate.
-
-
Ta ble 3. Cr ysta llogr a p h ic Da ta for [Ru 4(Cl)7(P Cy3)4(CO)4]+BF 4 (3), [(P Cy3)(P P h 3)(CH3CN)2(CO)Ru H]+BF 4
(5), a n d Cy3P H+[(P Cy3)(CO)(SiCl3)2Ru Cl]- (7)
C88H144BCl7F4O4P4Ru4 (3)
C43.5H58.5BCl2F4N2OP2Ru (5)
C37H67Cl7OP2RuSi2 (7)
formula wt
space group
a, Å
b, Å
c, Å
2129.15
P1h
946.14
P1h
995.25
P1h
14.9933(2)
17.6503(2)
21.6833(2)
84.3204(5)
84.7235(5)
72.5387(2)
5418.13(9)
2, 1
10.288(2)
17.352(4)
26.729(5)
76.517(5)
82.826(4)
79.354(4)
4543(3)
11.7530(2)
20.0661(2)
21.2648(3)
110.1073(9)
92.4954(8)
91.5251(5)
4700.30(13)
4, 2
R, deg
â, deg
γ, deg
V, Å3
Z, Z′
4
cryst color, habit
D(calc), g cm-3
µ(Mo KR), cm-1
temp, K
red brick
1.301
8.24
colorless block
1.383
5.84
orange block
1.406
8.79
220(2)
173(2)
173(2)
diffractometer
radiation
R(F), %a
R(wF2), %a
Siemens P4/CCD
Mo KR(λ ) 0.71073 Å)
8.94
9.90
20.19
4.70
16.07
19.01
a
2
2
Quantity minimized ) R(wF2) ) ∑[w(Fo - Fc2)2]/∑[(wFo2)2]1/2; R ) ∑∆/ ∑(Fo), ∆ ) |(Fo - Fc)|, w ) 1/[σ2(Fo2) + (aP)2 + bP], P ) [2Fc
+ max(Fo, 0)]/3.
spectrometers, respectively. Gas chromatographs were re-
corded from a Hewlett-Packard HP 6890 GC spectrometer.
Gen er a l P r oced u r e of t h e Ca t a lyt ic H yd r ogen a t ion
Rea ction . In a glovebox, 1.0 mL of a predissolved 0.69 mM
C6H6 solution of complex 1a (0.5 mg, 0.69 µmol) and HBF4‚
OEt2 (0.1 µL, 1.0 equiv) was placed in a 100 mL Schlenk flask
equipped with a stirring bar. The solution was diluted with
4.0 mL of C6H6. Excess alkene (5.7 mmol) was added, and the
bottle was attached to a vacuum line. The reaction bottle was
evacuated while it was cooled in a liquid N2 bath. The 1.0 atm
of H2 gas was applied to the reaction bottle, and the reaction
mixture was vigorously stirred at room temperature (22-23
°C) for 0.5-2 h. For reactions at higher pressure of H2, a
Fisher-Porter pressure bottle was used. The product yield was
determined by GC.
yellow homogeneous solution within 1 min. The acid HBF4‚
OEt2 (54 wt %, 25 µL, 1.2 equiv) was added via a syringe into
the solution, and the solution was stirred for 5 min. The 31P
NMR of the solution showed the clean formation of 4 at this
time. Excess PPh3 (45 mg, 0.17 mmol) was added to this
solution at room temperature, and the reaction mixture was
stirred for 10 min. Solvent was removed under vacuum, and
the residue was filtered through a short silica column using
CH3CN as an eluent. The filtrate was concentrated to ∼1 mL
and was precipitated by adding ∼10 mL of hexanes. The
resulting solid was filtered and washed with hexanes (3 × 5
mL). Drying under vacuum gave the product 5 as an off-white
solid (75%).
For 5: 1H NMR (CD2Cl2, 300 MHz) δ -13.57 (pseudo t, J PH
) 18.0 Hz, Ru-H), 7.9-7.4 (m, PPh3), 2.2-1.2 (m, PCy3), 1.95
(s, CH3CN); 13C{1H} NMR (CD2Cl2, 75.6 MHz) δ 203.4 (pseudo
t, J CP ) 13.9 Hz, CO), 125.1 (s, CH3CN), 3.2 (pseudo q, J )
4.0 Hz, CH3CN); 31P{1H} NMR (CD2Cl2, 121.6 MHz) δ 47.5
(AB quartet, J AB ) 236 Hz, PPh3 and PCy3); IR (CH2Cl2) νCO
) 1952 cm-1; FAB-MS 669.2 (M+ - (H + 2 CH3CN)).
P r ep a r a tion of Cy3P H+[(P Cy3)(CO)(SiCl3)2Ru Cl]- (7).
To a C6H6 (20 mL) suspension of 1a (300 mg, 0.41 mmol) was
added HSiCl3 (100 µL, 1.0 mmol, 2.4 equiv) dropwise via a
syringe at room temperature. The reaction mixture was stirred
for 2 h at room temperature. The resulting pale yellow solution
was concentrated to ∼5 mL and was precipitated by adding
hexanes (10 mL). The resulting solid was filtered through a
frit and washed with hexanes (3 × 10 mL). After drying under
vacuum, the product 7 was isolated in 85% yield as a pale
yellow solid.
P r ep a r a tion of 2. To a C6H6 (5 mL) suspension of complex
1a (100 mg, 0.14 mmol) was added HBF4‚OEt2 (54 wt %, 25
µL, 1.2 equiv) via syringe in three portions at 1 h intervals at
room temperature. After the solution was concentrated to 1
mL, it was triturated with 10 mL of hexanes. The resulting
brick-colored precipitate was washed three times with hexanes.
The product mixture of 2 and Cy3PH+BF4- (1:3.5) was isolated
after the removal of solvents under vacuum (yield of 2 )
∼15%).
Spectroscopic data of 2: 1H NMR (CD2Cl2, 300 MHz) δ
-10.52 (d, J PH ) 25.2 Hz, Ru-H); 13C{1H} NMR (CD2Cl2, 75.6
MHz) δ 196.9 (d, J PC ) 17.6 Hz, CO); 31P{1H} NMR (CD2Cl2,
121.6 MHz) δ 73.8 (s, PCy3); IR (CH2Cl2) νCO ) 1969 cm-1
.
P r ep a r a tion of [(P Cy3)(P P h 3)(CH3CN)2(CO)Ru H]+BF 4
-
(5). To a suspension of 1a (100 mg, 0.14 mmol) in CH2Cl2
solution was added 0.1 g of CH3CN (2.3 mmol) via a syringe
at room temperature. The reaction mixture turned into a pale
For 7: 1H NMR (CD2Cl2, 300 MHz) δ 6.03 (dq, J PH ) 462
Hz, J HH ) 0.9 Hz, HPCy3); 13C{1H} NMR (CD2Cl2, 75.6 MHz)

Acid-Promoted Homogeneous Hydrogenation of Alkenes
Organometallics, Vol. 19, No. 15, 2000 2915
δ 198.9 (d, J PC ) 8.4 Hz, CO); 31P{1H} NMR (CD2Cl2, 121.6
MHz) δ 30.1 (s, J PSi ) 51.3 Hz, PCy3), 28.4 (s, HPCy3+); IR
the SADABS were applied to both 5 and 7. One full molecule
of benzene and two-half molecules of benzene were present in
a unit cell of 3. The BF4- anion was positionally disordered in
a 61:39 distribution about F(1). The asymmetric unit of 5
(CH2Cl2) νCO ) 1946 cm-1
.
NMR Rea ction of 1a w ith HBF 4‚OEt2. In a NMR tube
capped with rubber septum, complex 1a (6 mg, 8.3 µmol) was
dissolved in a 1:1 mixture of toluene-d8/CD3CN at room
temperature, and the tube was cooled in a dry ice/acetone bath.
The acid HBF4‚OEt2 (54 wt %, 4.8 µL, 4 equiv) was carefully
added to the top of the solution via a syringe. The tube was
shaken several times while it was immersed in a dry ice/
acetone bath. The tube was inserted into a precooled NMR
probe at -50 °C.
-
contained two cationic ruthenium complexes, two BF4 ions,
-
and a half benzene molecule. The BF4 anion and one of the
Cy groups were positionally disordered. The asymmetric unit
of 7 contained two ruthenium anion and two Cy3PH+ cation
parts and was disordered over two positions with a 80:20
distribution. All non-hydrogen atoms were refined with aniso-
tropic displacement coefficients except B(1′) and C(58) of 3.
All hydrogen atoms were treated as idealized contributions,
except for the hydrogen atoms of the disordered Cy group of 5
and the Cy3PH+ cation of 7, which were not located. All
software and sources of the scattering factors are contained
in the SHELXTL (version 5.10) program library (G. Sheldrick,
Siemens XRD, Madison, WI).
Selected spectroscopic data of 8: 1H NMR (1:1 toluene-d8/
CD3CN, 300 MHz, -45 °C) δ -9.80 (br s); 31P{1H} NMR (1:1
toluene-d8/CD3CN, 121.6 MHz, -45 °C) δ 35.3 (s, PCy3).
Cr ysta llogr a p h ic Str u ctu r a l Deter m in a tion . Single
crystals of 3, 5, and 7 suitable for X-ray crystallographic
analysis were grown from benzene/hexanes and CH2Cl2/
hexanes solutions, respectively. The data collections were
performed on a Siemens P4/CCD diffractometer. The crystal-
lographic data for complexes 3, 5, and 7 are summarized in
Table 3. Systematic absences and diffraction symmetry were
uniquely consistent for the reported space groups, whose
correctness was subsequently confirmed by chemically reason-
able and computationally stable results of refinement. The
structures were solved by using direct methods, completed by
subsequent difference Fourier synthesis and refined with full-
matrix, least-squares procedures. The DIFABS empirical
absorption corrections were applied to the data sets of 3, while
Ack n ow led gm en t. Financial support from the Na-
tional Institutes of Health (R15 GM55987) and the
donors of the Petroleum Research Fund, administered
by the American Chemical Society, is gratefully ac-
knowledged.
Su p p or tin g In for m a tion Ava ila ble: X-ray crystallo-
graphic data of 3, 5, and 7 (PDF). This material is available
free of charge via the Internet at http//:pubs.acs.org.
OM000315N