Molybdenum carbonyl complexes in the solvent-free catalytic hydrogenation of ketones

Article abstract of DOI:10.1021/om050564h

The heterodifunctional ligand Li[η⁵-C₅H ₄(CH₂)₂PR₂] (R = Ph, Cy, and Bu) reacts with Mo(CO)₃(diglyme) to give the molybdenum anion complex Li{Mo(CO)₃[η⁵-C₅H₄(CH ₂)₂PR₂]}. Protonation with HOAc gives the metal hydride complexes HMo(CO)₂[η⁵:η¹- C₅H₄(CH₂)₂PR₂], in which the phosphine and cyclopentadienyl ligands are linked by a two-carbon bridge. Crystal structures of HMo(CO)₂[η⁵:n¹-C ₅H₄CH₂)₂PR₂] with all three R groups (R = Ph, Cy, and ^tBu) are reported. Syntheses of the C₃-bridged complex, HMo(CO)₂[η⁵: η¹-C₄H₅(CH₂)₃- PPh₂], and a W analogue, HW(CO)₃[η⁵-C ₅H₄(CH₂)₂P^tBu ₂], were carried out by analogous routes. Hydride transfer to Ph ₃C⁺BAr′₄; [Ar′ = 3,5-bis(trifluoromethyl)phenyl] from the catalyst precursors HMo(CO) ₂[η⁵:η¹-C₅H ₄(CH₂)₂PR₂] leads to homogeneous catalysts for hydrogenation of ketones, with the best performance being found for R = Cy. Protonation of HMo(CO)₂[η⁵: η¹-C₅H₄(CH₂)₂PR ₂] by HOTf leads to metal triflate complexes (TfO)Mo(CO) ₂[η⁵:η¹-C₅H ₄(CH₂)₂PR₂], which are used in ketone hydrogenation. Compared to the previously prepared complexes that did not have the phosphine and Cp linked together, these new complexes provide catalysts that have much longer lifetimes (up to about 500 turnovers) and higher thermal stability. Solvent-free ketone hydrogenation can be carried out with these complexes at catalyst loadings as low as 0.1 mol%.

Full text of DOI:10.1021/om050564h

6220
Organometallics 2005, 24, 6220-6229
Molybdenum Carbonyl Complexes in the Solvent-Free
Catalytic Hydrogenation of Ketones
Barbara F. M. Kimmich,‡ Paul J. Fagan,§ Elisabeth Hauptman,§
William J. Marshall,§ and R. Morris Bullock*,‡
Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973,
and Central Research and Development Department, The DuPont Company,
Experimental Station, P.O. Box 80328, Wilmington, Delaware 19880
Received July 6, 2005
t
The heterodifunctional ligand Li[η5-C5H4(CH2)2PR2] (R ) Ph, Cy, and Bu) reacts with
Mo(CO)3(diglyme) to give the molybdenum anion complex Li{Mo(CO)3[η5-C5H4(CH2)2PR2]}.
Protonation with HOAc gives the metal hydride complexes HMo(CO)2[η5:η1-C5H4(CH2)2PR2],
in which the phosphine and cyclopentadienyl ligands are linked by a two-carbon bridge.
Crystal structures of HMo(CO)2[η5:η1-C5H4(CH2)2PR2] with all three R groups (R ) Ph, Cy,
t
and Bu) are reported. Syntheses of the C3-bridged complex, HMo(CO)2[η5:η1-C4H5(CH2)3-
PPh2], and a W analogue, HW(CO)3[η5-C5H4(CH2)2PtBu2], were carried out by analogous
-
routes. Hydride transfer to Ph3C+BAr′4 [Ar′ ) 3,5-bis(trifluoromethyl)phenyl] from the
catalyst precursors HMo(CO)2[η5:η1-C5H4(CH2)2PR2] leads to homogeneous catalysts for
hydrogenation of ketones, with the best performance being found for R ) Cy. Protonation of
HMo(CO)2[η5:η1-C5H4(CH2)2PR2] by HOTf leads to metal triflate complexes (TfO)Mo(CO)2-
[η5:η1-C5H4(CH2)2PR2], which are used in ketone hydrogenation. Compared to the previously
prepared complexes that did not have the phosphine and Cp linked together, these new
complexes provide catalysts that have much longer lifetimes (up to about 500 turnovers)
and higher thermal stability. Solvent-free ketone hydrogenation can be carried out with
these complexes at catalyst loadings as low as 0.1 mol %.
Homogeneous catalysts for hydrogenation of ketones
mensely useful for many reductions, their use produces
stoichiometric amounts of waste. One of the principles
of green chemistry7,8 is that catalysts are preferred over
stoichiometric reagents.
traditionally use ruthenium or rhodium complexes that
operate by a mechanism where binding of the ketone
to the metal and subsequent insertion of the ketone into
a M-H bond are key steps.1 An alternative mechanism
for ketone hydrogenation that does not require an
insertion into an M-H bond is ionic hydrogenation, in
which H2 is delivered to the ketone in the form of a
proton (H+) and a hydride (H-).2 Ionic hydrogenations
offer the possibility of removing the need for precious
metals, since inexpensive metals such as Mo and W
hydride complexes are well-established to function as
proton donors3 and as hydride donors.4-6
Several studies have reported the use of Cr, Mo, or
W complexes as homogeneous catalysts for hydrogena-
tion of CdO bonds. Darensbourg and co-workers re-
ported extensive studies on the reactivity of anionic
metal carbonyl hydrides9 and found that anionic cata-
lysts such as [(CO)5M(OAc)]- (M ) Cr, Mo, W) or the
bimetallic complexes [(µ-H)M2(CO)10]- served as catalyst
precursors for ketone hydrogenations.10 Typical condi-
tions were 5 mol % catalyst at 125 °C for 24 h, leading
to a maximum of about 18 turnovers. Related anionic
catalysts were reported by Marko´11 and by Fuchikami.12
Transfer hydrogenation of ketones was reported by
Brunetandco-workers,whoused20mol%K+[(CO)5CrH]-
as the catalyst precursor.13 Tyler and co-workers have
One of our goals is to develop catalysts based on
inexpensive metals that could eventually become com-
petitive with stoichiometric reagents such as LiAlH4 or
NaBH4. While these main group reagents are im-
* To whom correspondence should be addressed. E-mail: bullock@
bnl.gov.
‡ Brookhaven National Laboratory.
(7) Anastas, P. T.; Kirchhoff, M. M. Acc. Chem. Res. 2002, 35, 686-
694.
§ The DuPont Company.
(1) Chaloner, P. A.; Esteruelas, M. A.; Joo´, F.; Oro, L. A. Homoge-
neous Hydrogenation; Kluwer Academic Publishers: Boston, 1994.
(2) Bullock, R. M. Chem. Eur. J. 2004, 10, 2366-2374.
(3) For a review of proton transfer reactions of metal hydrides, see:
Kristja´nsdo´ttir, S. S.; Norton, J. R. In Transition Metal Hydrides;
Dedieu, A., Ed.; VCH: New York, 1991; Chapter 9, pp 309-359.
(4) For a review of nucleophilic reactions of metal hydrides, see:
Labinger, J. A. In Transition Metal Hydrides; Dedieu, A., Ed.; VCH:
New York, 1991; Chapter 10, pp 361-379.
(8) Anastas, P. T.; Kirchhoff, M. M.; Williamson, T. C. Appl. Catal.
A 2001, 221, 3-13.
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27, 1-50.
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J. Am. Chem. Soc. 1985, 107, 2428-2434 (b) Tooley, P. A.; Ovalles,
C.; Kao, S. C.; Darensbourg, D. J.; Darensbourg, M. Y. J. Am. Chem.
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193-203.
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Soc. 1998, 120, 13121-13137.
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10.1021/om050564h CCC: $30.25 © 2005 American Chemical Society
Publication on Web 11/02/2005

Molybdenum Carbonyl Complexes
Organometallics, Vol. 24, No. 25, 2005 6221
Scheme 1
developed the aqueous chemistry of molybdenocene
complexes and have reported transfer hydrogenation of
ketones using isopropyl alcohol as the source of hydro-
gen.14 Kuo and co-workers carried out studies on ketone
hydrogenations using Cp2MoH(OTf) (OTf ) OSO2CF3)
in water.15 These examples show that Mo and other
inexpensive metals can function as homogeneous cata-
lysts for hydrogenation of aldehydes or ketones, but this
area is still not well-developed compared to catalysis by
precious metals, so further mechanistic information and
new classes of catalysts are being sought.
Recent progress has been made in the use of inex-
pensive, nontoxic metals as homogeneous catalysts for
hydrogenation of CdC bonds. Chirik and co-workers
recently discovered a series of Fe complexes that serve
as very efficient catalyst precursors for the hydrogena-
tion of alkenes under 1 atm H2 at room temperature.16
Daida and Peters reported a series of Fe precatalysts
with tris(phosphino)borate ligands that hydrogenate
alkenes under mild conditions.17
nium cations (HPR3+) in the catalytic reactions sug-
gested that a phosphine ligand was dissociating and was
being protonated under the reaction conditions. Recog-
nition of this decomposition pathway suggested that
catalysts designed to suppress phosphine dissociation
might provide longer lifetimes. Here we report the
synthesis, structures, and catalytic reactivity of com-
plexes in which the Cp and phosphine ligands are linked
by a hydrocarbon bridge.25 These complexes have sev-
eral advantages over the first-generation nonbridged
complexes reported previously. These improved catalyst
precursors exhibit enhanced thermal stability and can
be used at low catalyst loading (0.1-0.4 mol %).
Furthermore, these catalysts can be used under solvent-
free26 conditions, with a ketone as both substrate and
solvent. Recent progress in solvent-free ketone hydro-
genation was reported by O¨ zkar and Finke, who found
that Ir(0) nanocluster catalysts hydrogenate acetone.27
We have shown that metal hydrides are capable of
functioning as hydride donors in the presence of acids
and have reported the use of HW(CO)3Cp and other
metal hydrides in the stoichiometric ionic hydrogenation
of alkenes,18 alkynes,19 ketones,20 and acyl chlorides.21
Related reactions involving hydride transfer showed
that ether complexes resulted from reactions of acetals
with metal hydrides and acids.22 These stoichiometric
reactivity studies led to the development of a series of
Mo and W catalysts for homogeneous hydrogenation of
ketones.23,24 Hydride abstraction from HM(CO)2Cp(PR3)
(M ) Mo, W; R ) Me, Ph, Cy; Cy ) cyclohexyl) in the
presence of Et2CdO gave ketone complexes [M(CO)2-
-
Cp(PR3)(η1-Et2CdO)]+BAr′4 [Ar′ ) 3,5-bis(trifluoro-
methyl)phenyl], which are catalyst precursors for the
homogeneous hydrogenation of ketones.23,24 The Mo
catalysts were faster than the analogous W complexes,
and the dependence of turnover rate as a function of
phosphine was found to be PCy3 > PPh3 > PMe3.
Mechanistic experiments supported the proposed ionic
mechanism shown in Scheme 1, involving proton trans-
fer from a cationic dihydride and hydride transfer from
a neutral metal hydride. While these catalysts func-
tioned under mild conditions (23 °C, 4 atm H2) the
turnover rates were slow. The formation of phospho-
Results and Discussion
Synthesis and Characterization of HMo(CO)2[η5-
C5H4(CH2)2PR2]. Heterodifunctional ligands that have
a hydrocarbon chain linking phosphine and cyclopen-
tadienide ligands have been used in many organo-
metallic complexes and catalysts.28 The reaction of
Mo(CO)3(diglyme)29 with Li[C5H4(CH2)2PR2]30,31 (R )
Ph, Cy, and tBu) at room temperature formed the
anionic intermediates Li{Mo(CO)3[η5-C5H4(CH2)2PR2]}
(1, Scheme 2) in high yield. The five metal carbonyl
bands observed in the IR spectrum between 1902 and
1715 cm-1 are very similar to those previously observed
in the spectrum of the unsubstituted Cp complex Li-
(14) (a) Balzarek, C.; Tyler, D. R. Angew. Chem., Int. Ed. 1999, 38,
2406-2408. (b) Balzarek, C.; Weakley, T. J. R.; Tyler, D. R. J. Am.
Chem. Soc. 2000, 122, 9427-9434.
(15) (a) Kuo, L. Y.; Weakley, T. J. R.; Awana, K.; Hsia, C. Organo-
metallics 2001, 20, 4969-4972 (b) Kuo, L. Y.; Finigan, D. M.; Tadros,
N. N. Organometallics 2003, 22, 2422-2425.
(16) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004,
126, 13794-13807.
(17) Daida, E. J.; Peters, J. C. Inorg. Chem. 2004, 43, 7474-7485.
(18) (a) Bullock, R. M.; Rappoli, B. J. J. Chem. Soc., Chem. Commun.
1989, 1447-1448 (b) Bullock, R. M.; Song, J.-S. J. Am. Chem. Soc.
1994, 116, 8602-8612.
(25) For a preliminary report of this work, see: Kimmich, B. F. M.;
Fagan, P. J.; Hauptman, E.; Bullock, R. M. Chem. Commun. 2004,
1014-1015.
(26) (a) DeSimone, J. M. Science 2002, 297, 799-803. (b) Tanaka,
K.; Toda, F. Chem. Rev. 2000, 100, 1025-1074. (c) Tanaka, K. Solvent-
free Organic Synthesis; Wiley-VCH: Weinheim, 2003. (d) Cave, G. W.
V.; Raston, C. L.; Scott, J. L. J. Chem. Soc., Chem. Commun. 2001,
2159-2169. (e) Thomas, J. M.; Raja, R.; Sankar, G.; Johnson, B. F.
G.; Lewis, D. W. Chem. Eur. J. 2001, 7, 2973-2978.
(27) O¨ zkar, S.; Finke, R. G. J. Am. Chem. Soc. 2005, 127, 4800-
4808.
(19) Luan, L.; Song, J.-S.; Bullock, R. M. J. Org. Chem. 1995, 60,
7170-7176.
(20) (a) Song, J.-S.; Szalda, D. J.; Bullock, R. M.; Lawrie, C. J. C.;
Rodkin, M. A.; Norton, J. R. Angew. Chem., Int. Ed. Engl. 1992, 31,
1233-1235 (b) Song, J.-S.; Szalda, D. J.; Bullock, R. M. Organometallics
2001, 20, 3337-3346.
(21) Song, J.-S.; Szalda, D. J.; Bullock, R. M. Inorg. Chim. Acta 1997,
259, 161-172.
(28) Butenscho¨n, H. Chem. Rev. 2000, 100, 1527-1564.
(29) Kralik, M. S.; Rheingold, A. L.; Hutchinson, J. P.; Freeman, J.
W.; Ernst, R. D. Organometallics 1996, 15, 551-561.
(30) Kauffmann, T.; Ennen, J.; Lhotak, H.; Rensing, A.; Steinseifer,
F.; Woltermann, A. Angew. Chem., Int. Ed. Engl. 1980, 19, 328-329.
(31) Kettenbach, R. T.; Bonrath, W.; Butenscho¨n, H. Chem. Ber.
1993, 126, 1657-1669.
(22) Song, J.-S.; Szalda, D. J.; Bullock, R. M. J. Am. Chem. Soc. 1996,
118, 11134-11141.
(23) Bullock, R. M.; Voges, M. H. J. Am. Chem. Soc. 2000, 122,
12594-12595.
(24) Voges, M. H.; Bullock, R. M. J. Chem. Soc., Dalton Trans. 2002,
759-770.

6222 Organometallics, Vol. 24, No. 25, 2005
Kimmich et al.
Scheme 2
Hz, 98%) for the trans isomer, while the resonance for
2
the cis-hydride is observed at -6.17 (d, JHP ≈ 53 Hz,
2%; partially overlapped with trans isomer). In contrast,
the unbridged complex HMo(CO)2Cp(PCy3) favors the
cis isomer by a 9:1 ratio.5 All resonances in the 13C NMR
spectrum of the C2-PCy2 hydride, 2Cy, were assigned
using a combination of 1H NMR, 13C NMR, and 2D
HMQC experiments (see Figure S1 in the Supporting
Information).
The W analogues of these Mo complexes were inves-
tigated. The reaction of W(CO)3(NCEt)336 with Li[C5H4-
(CH2)2PtBu2] at room temperature formed the anionic
intermediate Li{W(CO)3[η5:η1-C5H4(CH2)2PtBu2]}. Pro-
tonation by acetic acid led to the formation and isolation
of the tungsten hydride complex trans-HW(CO)2[η5:η1-
C5H4(CH2)2PtBu2], which was fully characterized by
spectroscopy and elemental analysis. Synthetic details
and spectroscopic characterization are provided in the
Supporting Information.
Structural Characterization of HMo(CO)2[η5:η1-
C4H5(CH2)2PR2] Complexes (2Ph, 2Cy, and 2tBu).
The X-ray crystal structures of 2Ph, 2Cy, and 2tBu
were determined, and ORTEP diagrams are shown in
Figures 1-3. Table 1 provides information on the data
collection and structure refinement; additional crystal-
lographic information is provided in the Supporting
Information. Selected bond lengths and bond angles are
given in Table 2.
[Mo(CO)3Cp].32 The appearance of five bands for three
CO ligands is due to the presence in THF solution of
ion pairs, such as a Mo-CO‚‚‚Li+ interaction.33 The 31
P
NMR resonances of Li{Mo(CO)3[η5-C5H4(CH2)2PR2]} at
δ -13.2 (1Ph, R ) Ph), -1.8 (1Cy, R ) Cy) and 31.8
(1tBu, R ) tBu) are shifted only slightly downfield (<3
ppm) from the starting ligands, supporting the assign-
ment of “dangling phosphine” complexes.
Addition of acetic acid to complexes 1 led to the
formation of the hydride complexes trans-HMo(CO)2-
[η5:η1-C5H4(CH2)2PR2] (2Ph, R ) Ph; 2Cy, R ) Cy;
The Mo-P bond lengths of 2 range from 2.4343(5) to
t
2tBu, R ) Bu). Use of Mo(CO)3(diglyme) was signifi-
t
2.5256(5) Å going from R ) Ph to Cy to Bu. This
cantly cleaner and gave higher yields than reactions
using Mo(CO)3(NCMe)3 as the starting Mo complex. The
trans-hydrides are pale yellow air-sensitive crystalline
solids. The hydrides appear to be stable in solution,
showing only minor signs (<5%) of decomposition after
many days in CD2Cl2 at room temperature.
variation and changes in the P(1)-C(3)-C(4) bond
angles reflect an increase in the steric bulk of the
phosphine ligands. The Mo-H distances are listed here
but are subject to the normal uncertainties of metal
hydride positions determined by X-ray crystallography,
which often provides M-H distances that are 0.1-0.2
Å shorter than their true distances.37
Coordination of the phosphine to the metal is ac-
companied by a substantial (72-87 ppm) downfield shift
from the resonance of the “dangling phosphine” inter-
mediates 1. The 31P NMR spectra of hydrides had
resonances at δ 74.4 (2Ph) 80.6 (2Cy), and 103.8 (2tBu).
At room temperature, the 31P NMR resonances are
broad, with ω1/2 ≈ 30 Hz for complexes 2Ph and 2tBu,
and ω1/2 ≈ 200 Hz for 2Cy. At -89 °C the 31P NMR
resonance of 2Cy has decoalesced to give two sharper
peaks (ω1/2 ≈ 7 Hz) at δ 78.7 (98%) and 89.3 (2%). This
provides evidence that the hydride complex 2Cy under-
goes cis-trans isomerization.
Synthesis and Characterization of HMo(CO)2-
[η5:η1-C5H4(CH2)3PPh2]. The related ligand with a C3-
Isomerization between trans and cis isomers was
studied previously for closely related complexes with
unsubstituted Cp ligands such as HMo(CO)2Cp(PPh3),34
HMo(CO)2Cp(PMe3),35 and HMo(CO)2Cp(PCy3).5 For
5 2
HMo(CO)2Cp(PCy3),
J
) 20 Hz for the trans isomer
HP
2
and JHP ) 60 Hz for the cis isomer. The magnitude of
the JHP (2JHP ) 26.8 Hz) observed at δ -5.81 at 25 °C
for 2Cy suggests a predominance of the trans isomer,
and low-temperature spectra provide further support for
1
this assignment. The H NMR of 2Cy recorded at -89
°C exhibits a hydride resonance at -6.13 (d, 2JHP ) 21.9
Figure 1. Molecular structure of HMo(CO)2[η5:η1-C5H4-
(CH2)2PPh2]‚toluene, 2Ph. Thermal ellipsoids were drawn
at the 50% probability level. The toluene molecule and all
hydrogens except H(1) were removed for clarity.
(32) Darensbourg, M. Y.; Jimenez, P.; Sackett, J. R.; Hanckel, J.
M.; Kump, R. L. J. Am. Chem. Soc. 1982, 104, 1521-1530.
(33) Darensbourg, M. Y. Prog. Inorg. Chem. 1985, 33, 221-274.
(34) Faller, J. W.; Anderson, A. S. J. Am. Chem. Soc. 1970, 92, 5852-
5860.
(35) Kalck, P.; Pince, R.; Poilblanc, R.; Roussel, J. J. Organomet.
Chem. 1970, 24, 445-452.
(36) Kubas, G. J. Inorg. Chem. 1983, 22, 692-694.

Molybdenum Carbonyl Complexes
Organometallics, Vol. 24, No. 25, 2005 6223
Scheme 3
M-FBF3 complexes, but in CH3CN, the acetonitrile
coordinates to the metal, giving M(NCCH3)+BF4- com-
plexes. The C2-bridged molybdenum hydrides 2 cleanly
Figure 2. Molecular structure of HMo(CO)2[η5:η1-C5H4-
(CH2)2PCy2], 2Cy. Thermal ellipsoids were drawn at the
50% probability level. All hydrogens except H(1) were
removed for clarity.
-
transfer hydride upon reaction with Ph3C+BF4 in
CD3CN to give complexes with acetonitrile ligands,
trans- and cis-[Mo(CO)2[η5:η1-C5H4(CH2)2PR2](NCCD3)]+
-
BF4 (eq 1). Further details and characterization of
these complexes are in the Supporting Information.
Figure 3. Molecular structure of HMo(CO)2[η5:η1-C5H4-
(CH2)2PtBu2], 2tBu. Thermal ellipsoids were drawn at the
50% probability level. All hydrogens except H(1) were
removed for clarity.
Formation of Isolable Mo-OTf Complexes. The
C2-hydrides, 2, react cleanly with HOTf (OTf )
OSO2CF3) at room temperature to form cis- and trans-
(TfO)Mo(CO)2[η5:η1-C5H4(CH2)2PR2] (4, eq 2) presum-
ably through the unobserved dihydride or dihydrogen
complexes that eliminate H2. Tungsten phosphine com-
bridge linking the phosphine and cyclopentadienyl, Li-
[C5H4(CH2)3PPh2], was prepared from Cl(CH2)3Br, by
sequential replacement of the two halogens through
reaction with LiPPh2 and NaCp, followed by deproto-
nation with n-BuLi to give the Li+ salt of the substi-
tuted cyclopentadienide.31 The heterodifunctional ligand,
Li[C5H4(CH2)3PPh2], reacts with Mo(CO)3(diglyme) to
give the molybdenum anion complex Li{Mo(CO)3[η5-
C5H4(CH2)3PPh2]}, which subsequently reacts with
HOAc to produce the hydride trans-HMo(CO)2[η5:η1-
C4H5(CH2)3PPh2] (3; Scheme 3). Additional details of the
NMR data, including complete assignments of 13C NMR
resonances based on HMQC data, are found in the
Supporting Information (Figure S2).
Formation of Cationic Acetonitrile Complexes.
Hydride transfer from metal hydrides to Ph3C+ is well-
plexes such as HW(CO)2Cp(PMe3) are protonated by
HOTf to give dihydrides; the structure of [(H)2W-
(CO)2Cp(PMe3)]+OTf- was determined by X-ray crystal-
known.38 We previously reported the kinetics of hydride
- 5,6
transfer of a series of metal hydrides to Ph3C+BF4
.
When carried out in CH2Cl2, these reactions produce
(38) Bau, R.; Teller, R. G.; Kirtley, S. W.; Koetzle, T. F. Acc. Chem.
Res. 1979, 12, 176-183.
(37) Beck, W.; Su¨nkel, K. Chem. Rev. 1988, 88, 1405-1421.

6224 Organometallics, Vol. 24, No. 25, 2005
Kimmich et al.
Table 1. Crystallographic Data and Details of Refinement HMo(CO)2[η5:η1-C5H4(CH2)2PR2], where R ) Ph,
t
Cy, Bu (2Ph, 2Cy, and 2tBu)
HMo(CO)2[C5H4(CH2)2PPh2],
HMo(CO)2[C5H4(CH2)2PCy2],
HMo(CO)2[C5H4(CH2)2PtBu2],
2Ph
2Cy
2tBu
formula
fw
temp (°C)
cryst syst
space group
a (Å)
C
21H19MoPO2‚C7H8
C
21H31MoPO2
C17H27MoPO2
522.44
442.39
390.32
-100
-100
-100
triclinic
P1h (No. 2)
9.076(1)
9.946(1)
14.473(1)
97.07(1)
94.46(1)
111.60(1)
1194.8
monoclinic
P21/c (No. 14)
8.182(1)
monoclinic
P21/c (No. 14)
9.185(1)
b (Å)
c (Å)
15.618(1)
15.861(1)
14.562(1)
13.358(1)
R (deg)
â (deg)
γ (deg)
V, Å3
100.90(1)
98.94(1)
1990.2
1764.9
Z
2
4
4
µ(Mo), cm-1
6.22
7.33
8.16
F calc (g cm-1
)
1.452
1.476
1.469
cryst size (mm)
cryst character
2θ range (deg)
total no. of reflns
no. of unique reflns,
Ig3.0σ(I)
0.23 × 0.25 × 0.41
yellow prism
2.9-56.5
7505
0.17 × 0.37 × 0.42
light gold wedge
2.6-56.6
12880
0.27 × 0.35 × 0.42
irregular gold block
4.2-56.6
10796
4727
3015
3322
no. of params
R1
293
350
194
0.024
0.030
1.43
0.026
0.025
0.76
0.022
0.026
1.04
Rw
goodness of fit
Table 2. Selected Bond Lengths (Å) and Angles
tungsten complexes with monodentate phosphines hy-
drogenate ketones such as Et2CdO at room temperature
under 4 atm H2.23,24 The ketone complexes, [M(CO)2Cp-
(PR3)(Et2CdO)]+BAr′4- (M ) Mo, W; R ) Ph, Me), were
isolated and used as catalyst precursors. Catalysis with
the PCy3 complexes was carried out by hydride transfer
to Ph3C+BAr′4- from HM(CO)2Cp(PCy3) in the presence
of Et2CdO. Ketone complexes were not isolated for these
PCy3 complexes, and NMR data suggested exchange of
free and bound ketone. Catalytic reactions (eq 3) were
initiated by reacting the hydride complexes with Ph3C+
in the presence of Et2CdO, followed by addition of
hydrogen. Our initial studies using the bridged C2-PR2
(deg) HMo(CO)2[η5:η1-C5H4(CH2)2PR2] (R ) Ph
t
(2Ph) Cy (2Cy), and Bu (2tBu)
R
Ph (2Ph)
Cy (2Cy)
tBu (2tBu)
Mo(1)-H(1)
1.680(23)
2.4343(5)
1.959(2)
1.953(2)
103.14(5)
108.2(1)
112.3(1)
1.595(31)
2.4664(7)
1.960(3)
1.955(3)
105.0(1)
111.7(2)
111.3(2)
1.660(26)
2.5256(5)
1.959(2)
1.959(2)
103.31(7)
113.1(1)
112.1(2)
Mo(1)-P(1)
Mo(1)-C(1)
Mo(1)-C(2)
Mo(1)-P(1)-C(3)
P(1)-C(3)-C(4)
C(3)-C(4)-C(5)
lography.39 In contrast, Poli and co-workers found that
H2 was produced rapidly when the Mo hydride complex
HMo(CO)2Cp(PMe3) was protonated, and they suggested
that the initial product of protonation was an unob-
served dihydrogen complex rather than a dihydride.40
The Mo dihydrides or dihydrogen complexes are not
directly observed, so it is possible that the Mo example
forms a dihydrogen complex even though the W ana-
logue forms an isolable dihydride.
These triflate complexes with C2-PR2 bridges were
isolated as intensely colored red or purple solids. In con-
trast to the C2-hydrides 2 and the cationic C2-acetoni-
trile complexes, the triflates 4Cy and 4tBu preferen-
tially form the cis isomer. The bright red solid (TfO)-
Mo(CO)2[η5:η1-C5H4(CH2)2PCy2] (4Cy) was found to
have a cis:trans product ratio of 6:1 in CD2Cl2. The ratio
does not significantly change after 2 days in solution.
The triflate complex 4Cy was also formed in an NMR
tube reaction by hydride abstraction from 2Cy using
Ph3COTf.
complexes were designed to directly compare their
catalytic activity to those of the previously reported
unbridged PR3 catalyst systems under the same reaction
conditions. The reactions were conducted in CD2Cl2 with
initial concentrations of 30 mM catalyst precursor and
300 mM Et2CdO, under 4 atm H2 at room temperature
(23 °C). 1H NMR indicated the appearance of the
product alcohol, Et2CHOH, as well as smaller amounts
of its ether, (Et2CH)2O, arising from condensation of two
alcohols (eq 4). Figure S3 in the Supporting Information
shows the time profile of product formation for a typical
catalytic reaction.
Hydrogenations of Et2CdO in CD2Cl2 under 4
atm H2. We previously showed that molybdenum and
(39) Bullock, R. M.; Song, J.-S.; Szalda, D. J. Organometallics 1996,
15, 2504-2516.
(40) (a) Quadrelli, E. A.; Kraatz, H.-B.; Poli, R. Inorg. Chem. 1996,
35, 5154-5162 (b) Galassi, R.; Poli, R.; Quadrelli, E. A.; Fettinger, J.
C. Inorg. Chem. 1997, 36, 3001-3007.
We thought that avoiding an alkyl chloride such as
CD2Cl2 might produce a more stable catalyst, because

Molybdenum Carbonyl Complexes
Organometallics, Vol. 24, No. 25, 2005 6225
cies observed during catalysis. In addition, the alcohol
complexes were observed during the catalytic reaction
and were isolated in some cases.24 The mechanism of
catalysis with the bridged complexes is presumed to be
the same as that established for the nonbridged sys-
tems, but the evidence is less definitive for the C2-
bridged complexes studied here.
Ionic hydrogenations were also studied in a solution
of 90% Et2CdO/10% benzene-d6 (as a lock solvent for
the NMR); i.e., 8.5 M Et2CdO. This avoided the use of
chlorinated solvents but still enabled the reactions to
be monitored by NMR. These hydrogenations gave
catalytic activity in the order C2-PCy2 > C3-PPh2
>
C2-PPh2; see Figure S4 in the Supporting Information.
Solvent-Free Hydrogenations. The ideal solvent
for hydrogenation of ketones is the neat ketone.7 Much
lower catalyst loadings were used in these solvent-free
reactions, with some experiments employing about 0.1
mol % catalyst. As in the NMR tube experiments, the
catalysts using BAr′4-, BF4-, or PF6- counterions were
generated in situ by reaction of HMo(CO)2[η5:η1-C5H4-
(CH2)nPR2] (2 or 3) or HMo(CO)2Cp(PR3) with Ph3C+A-
(A- ) BAr′4-, BF4-, or PF6-). The corresponding triflate
complexes, (TfO)Mo(CO)2[η5:η1-C5H4(CH2)2PR2] (4Cy
and 4tBu) or (TfO)Mo(CO)2Cp(PR3) (R ) Ph, Cy), were
added as isolated solids to the ketone.
Results for the hydrogenations of neat Et2CdO are
summarized in Table 3. An arbitrary sampling time of
t ) 10 days was chosen, and most reactions were
sampled again at t ) 30 days. Separate variation of
ligands, temperature, pressure, counterion, and catalyst
loading allowed a comparison of the results of indepen-
dently varying each of these parameters.
For the bridged complexes, 2Cy is a better catalyst
precursor than the related 2Ph at all temperatures
studied. The C2-PtBu2 complex 2tBu gives an improved
performance compared to 2Cy at 23 °C, but the trend
is reversed at 50 and 75 °C, suggesting thermal insta-
bility of 2tBu at elevated temperatures. Comparison of
C2 vs C3 bridge lengths for 2Ph vs 3 shows that their
performance is similar.
Increasing the reaction temperature from 23 °C to 50
°C results in about 6 times as many turnovers for 2Cy
at t ) 10 days and an even larger increase for 2Ph.
While an increase from 23 °C to 50 °C improved the
turnover frequency for all catalysts, a further increase
to 75 °C was not always beneficial. The increase in TON
on going from 50 °C to 75 °C was modest in some cases.
Furthermore, in some of the examples conducted at 75
°C, little or no additional turnovers were detected at t
) 30 days compared to t ) 10 days, suggesting that the
catalyst was deactivated after 10 days.
Figure 4. Hydrogenation of Et2CdO by a series of C2- and
C3-bridged catalysts, generated in situ by reaction of 2 or
-
3 with Ph3C+BAr′4 in the presence of Et2CdO. Initial
reaction conditions: 30 mM MoH, 300 mM Et2CdO, 4 atm
H2 in CD2Cl2 at 23 °C. Additional Et2CdO was added in
10 equiv increments. Hydrogen was replenished as needed.
of the possibility of decomposition of the catalyst through
reactivity with CD2Cl2. But a reaction conducted in
C6D5Cl instead of CD2Cl2 gave a significant decrease
in the catalysis rate, compared to reactions in CD2Cl2.
When the hydrogenation was carried out at 70 °C
instead of 23 °C in CD2Cl2, the reaction was worse,
apparently due to limited thermal stability of the
catalyst at elevated temperatures in CD2Cl2.
Figure 4 summarizes the results for the ionic hydro-
genation of Et2CdO for the series of C2-bridged (2) and
C3-bridged (3) phosphine catalyst precursors. Of the C2-
bridged catalysts 2Cy forms the most active catalyst,
followed by the 2tBu and 2Ph systems. This reactivity
trend is similar to those of the unbridged system, where
the Mo-PCy3 catalyst was superior to Mo-PPh3.23,24 The
C3-bridged-PPh2 system is more reactive than its C2-
bridged-PPh2 analogue under these reaction conditions.
The unbridged Mo-PCy3 catalyst precursor HMo-
(CO)2Cp(PCy3) gave a more active catalyst than the
bridged 2Cy catalyst. While 2Cy produced 2.8 turnovers
in 6 h, HMo(CO)2Cp(PCy3) led to about 8.6 turnovers
in 6 h.23,24 This comparison is based on related catalysts
with cyclohexyl groups on the phosphine, but note that
the steric bulk of the untethered catalyst is larger, since
it has a PCy3 ligand, while the tethered complex 2Cy
has a C2H4 bridge linking the PCy2 moiety to the
cyclopentadienyl. Decomposition of the unbridged
Mo-PCy3 catalyst occurs much more quickly than for the
new bridged catalysts. Thus, even though the bridged
catalysts give slower initial rates, they are longer lived,
and thus give higher TON numbers.
In all cases, complex 1H NMR spectra of the C2-
bridged and C3-bridged catalytic solutions were ob-
served. Generally one or two major species were ob-
served, along with minor species. We were unable to
characterize all the species in solution or to make a
definitive assignment of the resting state. It appears
that both ketone and alcohol complexes are present,
especially in the C2-PPh2 and C3-PPh2 systems, but
attempted isolation of pure ketone complexes was not
successful. For the previously studied unbridged sys-
tems, cis and trans isomers of the ketone complexes,
As was found from the experiments in CD2Cl2, the
counterion can have a large effect on the rate of
-
catalysis. The BAr′4 anion provides more turnovers
-
than those obtained by using BF4 or PF6-. The effect
of H2 pressure was examined for reactions using the
Mo-C2-PCy2 system. An increase in turnover frequency
was found for reactions carried out under 56 atm (820
psi initial H2 pressure at 23 °C, before heating) com-
pared to reactions conducted at 4 atm. Of particular
interest are the results shown in entry 10, where use
of 0.35 mol % 2Cy (50 °C; 56 atm H2) produced 94%
hydrogenation of Et2CdO after 3 days, corresponding
[M(CO) Cp(PR3)(Et2CdO)] BAr′4-, were the major spe-
+
2

6226 Organometallics, Vol. 24, No. 25, 2005
Table 3. Hydrogenation of Neat 3-Pentanone by Mo Catalysts at 23, 50, or 75 °C
Kimmich et al.
P(H2) at
RT (atm)
TONtotal at
TONtotal at
line
catalyst precursor
ligand
anion
T, °C
mol % cat.
ca. 10 daysa,b
ca. 30 daysb
-
1
2Ph
C2-PPh2
C2-PPh2
C2-PPh2
C3-PPh2
C3-PPh2
C3-PPh2
C2-PCy2
C2-PCy2
C2-PCy2
C2-PCy2
C2-PCy2
C2-PCy2
C2-PCy2
C2-PCy2
C2-PCy2
C2-PCy2
C2-PCy2
C2-PCy2
C2-PtBu2
C2-PtBu2
C2-PtBu2
C2-PtBu2
PCy3
BAr′4-
BAr′4-
BAr′4-
BAr′4-
BAr′4-
BAr′4-
BAr′4-
BAr′4-
BAr′4-
BAr′4
23
50
75
23
50
75
23
50
50
50
50
50
50
75
75
75
75
75
23
50
75
75
23
50
50
50
50
75
75
75
0.35
0.35
0.17
0.35
0.35
0.17
0.35
0.35
0.12
0.35
0.35
0.35
0.35
0.17
0.17
0.17
0.086
0.086
0.35
0.35
0.17
0.17
0.35
0.35
0.35
0.35
0.17
0.17
0.17
0.17
4
4
4
4
4
4
4
4
4
56
4
4
4
4
4
4
4
54
4
4
4
4
4
4
4
4
4
4
4
4
6
62
87
20
2
2Ph
3
2Ph
87
15
105
89
59
4
3
5
3
6
3
88
23
7
2Cy
8
2Cy
120
122
283 (5 days)
99
9
2Cy
177
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
2Cy
-
2Cy
BF4-
137
124
212
206
102
482
621
482
49
2Cy
PF6
OTf
78
4Cy
120
141
74
-
2Cy
BAr′4
-
2Cy
BF4
4Cy
OTf
OTf
OTf
388
462
481
46
4Cy
4Cy
2tBu
BAr′4-
-
2tBu
BAr′4-
BAr′4
OTf
82
112
81
58
2tBu
65
4tBu
45
-
HMo(CO)2Cp(PCy3)
HMo(CO)2Cp(PCy3)
HMo(CO)2Cp(PCy3)
HMo(CO)2Cp(PCy3)
TfOMo(CO)2Cp(PCy3)
HMo(CO)2Cp(PCy3)
TfOMo(CO)2Cp(PCy3)
TfOMo(CO)2Cp(PPh3)
BAr′4-
37
PCy3
BAr′4
72
111
19
22
13
57
22
13
-
PCy3
PF6-
13
PCy3
BF4
OTf
19
PCy3
8
-
PCy3
BAr′4
OTf
OTf
55
PCy3
18
PPh3
6
a TONtotal includes the formation of the (Et2CH)2O, which typically comprises 5-10% of the concentration of the predominant product
alcohol. See Table S1 in the Supporting Information for further details. b Actual time of sampling for 10 days ranged from 9.6 to 10.7
days; actual time of sampling for the 30 day time ranged from 28.9 to 32 days. See Table S1 in the Supporting Information for further
details.
to about 90 turnovers/day. At t ) 5 days, essentially
complete (99.5%) hydrogenation of neat ketone to the
alcohol had occurred. In contrast to the marginal per-
experiments demonstrated that catalysts loadings as
low as 0.1 mol % can be used successfully.
The last nine entries in Table 3 provide a comparison
of the unbridged vs bridged catalysts. Since our previous
investigations used these complexes in CD2Cl2,23,24 it
was worthwhile to assess how these catalysts might
perform under the solvent-free conditions examined
here for the bridged catalysts. The improved lifetime
found in neat Et2CdO does suggest that some of the
decomposition previously found for these catalysts was
likely due to decomposition caused by reactions with the
CD2Cl2 solvent. Despite this observation, however, the
unbridged catalysts still give lower turnovers compared
to the bridged catalyst precursors reported here.
Conclusions. A series of new molybdenum carbonyl
hydride complexes with a C2 bridge linking the Cp and
phosphine ligands was prepared and fully characterized.
Abstraction of hydride from these catalyst precursors
-
formance obtained with counterions such as BF4 or
PF6- in most cases, the activity of the Mo-OTf complex
with the Mo-C2-PCy2 system is comparable to that using
BAr′4-. When the triflate complex 4Cy is dissolved in
CD3CN, the nitrile binds to the metal, giving {Mo(CO)2-
[η5:η1-C5H4(CH2)2PCy2(NCCD3)]}+OTf-; this reactivity
is encouraging since ionization of the bound triflate to
form a triflate counterion is needed for these catalyst
precursors to operate by an ionic mechanism. Compari-
son of lines 8 and 13 shows comparable performance
for BAr′4- vs OTf, and the experiments at 75 °C for the
Mo-C2-PCy2 system give better results for OTf (cf. line
14 vs line 16). An appealing aspect of using complexes
with triflate counterions (or triflate ligands) is their cost-
effectiveness, since triflates are less expensive than
borate anions such as BAr′4-. Comparison of lines 17
and 18 surprisingly shows no significant increase in
activity when the Mo-OTf system is carried out under
higher pressures of H2, in contrast to the results using
with Ph3C+BAr′4 in the presence of Et2CdO leads to
-
ketone hydrogenation. These catalysts slowly hydroge-
nate Et2CdO at 23 °C and 4 atm H2 pressure, but are
more active at higher temperatures (50 or 75 °C) and
higher pressures. The maximum turnover frequency
obtained for 2Cy is about 90 turnovers/day (over a 3
day period) at 50 °C at 56 atm H2 pressure. The new
catalysts are used in the solvent-free hydrogenation of
ketones, and they can be used at low loading of catalyst
-
the BAr′4 counterion for the Mo-C2-PCy2 system. Use
of the Mo-C2-PCy2 system with OTf is sufficiently
reactive and long-lived to result in total TONs in the
range of 500, which is a marked improvement in lifetime
over the previously reported unbridged systems.
In addition to the higher turnover numbers obtained
with the new bridged catalysts reported here, a further
improvement is that lower catalyst loadings can be
successfully employed. All of the experiments in Table
3 use less than 0.4 mol % catalyst loading, and some
-
(0.1-0.4 mol %). The BAr′4 provided much longer
lifetimes and turnover frequencies compared to catalysts
with either BF4- or PF6- anions. A molybdenum triflate
complex, (TfO)Mo(CO)2[η5:η1-C5H4(CH2)2PCy2], was found
to provide catalytic activity similar to that found using

Molybdenum Carbonyl Complexes
Organometallics, Vol. 24, No. 25, 2005 6227
the BAr′4- anion, providing an advantage in the use of
tions, the crystals contain up to 1 equiv of toluene. The sample
sent for elemental analysis was shown by 1H NMR to contain
0.2 molar equiv of toluene. Anal. Calcd for C21H19O2PMo‚
0.2toluene: C, 59.80; H, 4.57. Found: C, 59.93; H, 4.56.
General Methods for Crystal Structure Determina-
tions. X-ray diffraction data were collected using Mo KR
radiation on a Bruker SMART 1K CCD diffractometer using
a standard focus tube. The structures were solved by direct
methods using teXsan(SIR-92) and were refined using Z
program suite (Calabrese). Refinement was by full-matrix least
squares on F. All non-hydrogen atoms were refined anisotro-
pically. Scattering factors, including anomalous terms for Mo
and P, were from International Table for X-ray Crystal-
lography, Vol. IV. Further details on the crystallography are
provided in the Supporting Information.
-
less expensive triflate counterions compared to BAr′4
.
The Mo catalysts represent a substantial improvement
over previously prepared catalysts, which had no bridge
between the cyclopentadienyl and phosphine ligands.
We seek further improvements in an attempt to move
this type of complex toward practical utility.
Experimental Section
General Methods. All manipulations were carried out
under an argon atmosphere using standard Schlenk or vacuum
line techniques or in a drybox. THF, Et2O, toluene, hexane,
and pentane were distilled from Na/benzopheneone, and
CH2Cl2 was distilled from P2O5. Deuterated NMR solvents
were similarly dried and vacuum transferred. HOTf and HOAc
were purified by distillation. 3-Pentanone was dried over
molecular sieves and vacuum transferred. Reaction solutions
Preparation of HMo(CO)2[η5:η1-C5H4(CH2)2PCy2] (2Cy).
A solution of LiC5H4(CH2)2PCy2 (733.8 mg, 2.48 mmol) in THF
(10 mL) was added to a solution of Mo(CO)3(diglyme) (777.6
mg, 2.48 mmol, 1.00 equiv) in THF (15 mL). The yellow
solution was stirred for 30 min at room temperature. IR
spectra [ν(CO) in THF: 1902 (s), 1890 (sh), 1805(s), 1779 (m),
1714 (s) cm-1] and 31P{1H} NMR spectra [(THF) δ -1.8] were
consistent with the formation of Li{Mo(CO)3[η5-C5H4(CH2)2-
PCy2}. HOAc (180 µL, 3.14 mmol, 1.27 equiv) was added, the
color of the solution lightened to a pale yellow, and salt
formation was observed. The solvent was evaporated, and the
residue was extracted with toluene (25 mL). The reaction
mixture was filtered, and the toluene was evaporated. Recrys-
tallization from a mixture of toluene, THF, and pentane at
-30 °C produced HMo(CO)2[η5:η1-C5H4(CH2)2PCy2] as a pale
yellow solid (557.0 mg, 1.26 mmol, 51% yield). The NMR
assignments below were determined with the help of 1H,
1H{31P}, 31P{1H}, 13C{1H}, 1H-13C HMQC, and 1H-1H COSY.
were stored in the absence of light for all long-term reactions.
- 42
Mo(CO)3(diglyme),29 Mo(CO)3(NCMe)3,41 Ph3C+BAr′4
,
Ph3COTf,43 spiro[2.4]hepta-4,6-diene,44 and LiC5H4(CH2)nPR2
(n ) 2, R ) Ph and Bu; n ) 3, R ) Ph)31 were prepared
t
following literature methods. The related ligand with R ) Cy,
LiC5H4(CH2)2PCy2, was prepared by an analogous procedure;
-
details are in the Supporting Information. Ph3C+BF4 and
-
Ph3C+PF6 were purchased from Aldrich and purified by
recrystallization from CH2Cl2/Et2O. Most NMR tube experi-
ments were carried out in NMR tubes equipped with a Teflon
J. Young valve. Reactions carried out under hydrogen gas were
filled with 1.0-1.1 atm H2 at 77 K (liquid N2), and the valve
was closed. This corresponds to a pressure of about 4 atm at
room temperature (298/77 ) 3.9). NMR measurements were
recorded on Bruker AM-300 NMR or Bruker Avance 400
instruments. 1H NMR spectra were referenced to the residual
proton peaks of the deuterated solvents, and 31P NMR spectra
were referenced to 85% H3PO4. NMR probe temperatures were
determined using the standard MeOH test.45 IR measurements
were recorded on a Mattson Polaris FT-IR spectrometer.
Elemental analyses were carried out by Schwarzkopf Mi-
croanalytical Laboratory (Woodside, NY).
Preparation of HMo(CO)2[η5:η1-C5H4(CH2)2PPh2] (2Ph).
Mo(CO)3(diglyme) (557 mg, 1.77 mmol) and LiC5H4(CH2)2PPh2
(504 mg, 1.77 mmol, 1.00 equiv) were placed in a Schlenk flask,
and THF (20 mL) was added. The solution was stirred for 2 h
at room temperature, at which time the reaction was ∼80%
complete. The solution was refluxed for an additional 2 h. IR
spectra [ν(CO) in THF: 1903 (s), 1906 (sh), 1804 (s), 1784 (m),
1715 (s) cm-1] and 31P{1H} NMR spectra [(THF) δ -13.2] were
consistent with the formation of Li{Mo(CO)3[η5-C5H4(CH2)2-
PPh2]}. HOAc (100 µL, 1.92 mmol, 1.08 equiv) was added to
the yellow solution, which lightened, and salt formation was
observed. The solution was stirred for 3 h at room temperature,
and the solvent was evaporated. The residue was extracted
with toluene (25 mL) and filtered. The resulting pale red air-
sensitive solution was diluted with pentane (30 mL) and stored
in a -30 °C freezer. HMo(CO)2[η5:η1-C5H4(CH2)2PPh2] crystal-
lized as a yellow solid (230 mg, 0.530 mmol, 30% yield). 1H
NMR (300 MHz, CD2Cl2): δ -5.25 (d, 2JPH ) 27.6, 1H, MoH),
2.34 (dt, 2JPH ) 28.1 Hz, 3JHH ) 6.8 Hz, 2H, PCH2), 3.00 (“q”,
3JPH ) 3JHH ) 7.4 Hz, 2H, C5H4CH2), 5.10 (br, 2H, C5H4), 5.30
(br, 2H, C5H4), 7.42-7.55 (m, 10H, Ph). 31P{1H} NMR (121.9
MHz, CD2Cl2): δ 74.4 (br, w1/2 ) 32 Hz). IR (THF): ν(CO) )
1939 (s), 1864 (vs) cm-1. Depending on crystallization condi-
2
1H NMR (400 MHz, CD2Cl2): δ -5.81 (d, JPH ) 26.8, 1H,
MoH), 1.24-2.00 (m, 22H, Cy), 2.38 (m, 2H, PCH2), 2.47 (m,
2H, C5H4CH2), 4.96 (br, 2H, R-C5H4), 5.09 (m, 2H, â-C5H4).
31P{1H} NMR (162.0 MHz, CD2Cl2): δ 80.6 (br, w1/2 ) 200 Hz).
13C{1H} NMR (100.6 MHz, CD2Cl2): δ 25.8 (br, PCH2), 26.7
(s, m- or p-Cy), 27.6 (d, 2JCP ) 13 Hz, o-Cy), 29.0 (s, m- or p-Cy),
2
37.0 (d, JCP ) 23 Hz, ipso-Cy), 37.3 (v br, C5H4CH2), 81.8 (s,
3
o-C5H4), 87.4 (s, m-C5H4), 125.1 (d, JCP ) 6 Hz, ipso-C5H4),
234 (v br, CO). IR (THF): ν(CO) ) 1927 (s), 1848 (vs) cm-1
.
Anal. Calcd for C21H31O2PMo: C, 57.02; H, 7.06. Found: C,
56.92; H, 6.99.
Preparation of HMo(CO)2[η5:η1-C5H4(CH2)2PtBu2] (2tBu).
A solution of Mo(CO)3(diglyme) (793.2 mg, 2.52 mmol) in THF
(10 mL) was added to a solution of LiC5H4(CH2)2PtBu2 (616.8
mg, 2.52 mmol, 1.00 equiv) in THF (10 mL). The solution was
stirred for 1.5 h at room temperature. IR spectra [ν(CO) in
THF: 1902 (s), 1890 (sh), 1807 (s), 1780 (m), 1715 (s) cm-1
]
and 31P{1H} NMR spectra [(THF) δ 31.8] were consistent with
the formation of Li{Mo(CO)3[η5-C5H4(CH2)2PtBu2}. HOAc (170
µL, 2.97 mmol, 1.18 equiv) was added. The yellow solution
darkened to orange, and salt formation was observed. After
30 min at room temperature, solvent was evaporated. The
residue was extracted with toluene (25 mL) and filtered, and
the solvent was evaporated. Recrystallization from toluene/
hexane at -30 °C produced HMo(CO)2[η5:η1-C5H4(CH2)2PtBu2]
as a yellow solid (710 mg, 1.82 mmol, 72% yield). 1H NMR
2
(300 MHz, CD2Cl2): δ -5.74 (d, JPH ) 24.6, 1H, MoH), 1.34
(d, 3JPH ) 12.9 Hz, 18 H, tBu), 2.34-2.51 (m, 4H, CH2’s), 4.96
(br, 2H, C5H4), 5.07 (br, 2H, C5H4). 31P{1H} NMR (121.9 MHz,
CD2Cl2): δ 103.8 (br, w1/2 ) 34 Hz). IR (THF): ν(CO) ) 1922
(s), 1845 (vs) cm-1. Anal. Calcd for C17H27O2PMo: C, 52.31;
H, 6.97. Found: C, 52.36; H, 7.17.
Preparation of HMo(CO)2[η5:η1-C5H4(CH2)3PPh2] (3).
LiC5H4(CH2)3PPh2 (1.543 g, 5.17 mmol) was dissolved in THF
(30 mL), giving an orange solution. This solution was added
to a solution of Mo(CO)3(diglyme) (1.625 g, 5.17 mmol, 1.00
equiv) in THF (30 mL). The resulting intense yellow-brown
solution was stirred for 1 h at room temperature. IR spectra
(41) Tate, D. P.; Knipple, W. R.; Augl, J. M. Inorg. Chem. 1962, 1,
433-434.
(42) Bahr, S. R.; Boudjouk, P. J. Org. Chem. 1992, 57, 5545-5547.
(43) Straus, D. A.; Zhang, C.; Tilley, T. D. J. Organomet. Chem.
1989, 369, C13-C17.
(44) Bartsch, R. A.; Lee, J. G. J. Org. Chem. 1991, 56, 2579-2582.
(45) Van Geet, A. L. Anal. Chem. 1970, 42, 679-680.

6228 Organometallics, Vol. 24, No. 25, 2005
Kimmich et al.
[ν(CO) in THF: 1902(s), 1892 (sh), 1804(s), 1779 (m), 1717 (s)
cm-1] and 31P{1H} NMR spectra [(THF) δ -14.4] were consis-
tent with the formation of Li[Mo(CO)3[η5-C5H4(CH2)3PPh2] as
the predominant product. HOAc (100 µL, 1.92 mmol, 1.08
equiv) was added, and salt formation was observed. After
stirring at room temperature for 1.5 h, the solvent was
evaporated and the residue was extracted with toluene (25
mL). The reaction mixture was filtered, the solvent volume
was reduced to 15 mL, and pentane (15 mL) was added. A
yellow solid formed at -30 °C, along with an orange oil. While
the actual yield of the reaction was approximately 80% by
NMR, only a small amount of solid could be isolated in
analytically pure form (190 mg, 0.43 mmol, 8% yield). The
NMR assignments below were determined with the help of 1H,
1H{31P}, 31P{1H}, 13C{1H}, 1H-13C HMQC, and 1H-1H COSY.
precipitate from the solution. The mixture was stored at -30
°C overnight to complete the crystallization. (TfO)Mo(CO)2-
[η5:η1-C5H4(CH2)2PtBu2] (4tBu) was isolated as an orange-red
solid (177.5 mg, 0.33 mmol, 81% yield, >95% pure). The
isolated product is a 9:1 mixture of cis:trans isomers. 1H NMR
2
(400 MHz, CD2Cl2) of cis isomer: δ 1.29 (d, JHP ) 13.2 Hz,
t
2
t
CH3 of Bu), 1.43 (d, JHP ) 13.0 Hz, CH3 of Bu), 2.56-2.88
(m, CH2’s), 4.53 (m, 1H, C5H4), 4.66 (m, 1H, C5H4), 5.97 (m,
1H, C5H4), 6.11 (m, 1H, C5H4). 1H NMR (400 MHz, CD2Cl2) of
t
trans isomer: δ 1.45 (overlapping with cis, CH3 of Bu), 1.95
(“q”, 3JPH ) 3JHH ) 7.6 Hz, 2H, C5H4CH2), 2.26 (dt, 2JPH ) 23.8
Hz, JHH ) 7.2 Hz, 2H, PCH2) 5.71 (m, 4H, C5H4). 31P NMR
3
(162.0 MHz, CD2Cl2): δ 114.1 (trans), 94.4 (cis). IR (THF):
ν(CO) ) 1992 (s), 1904 (w), 1862 (s) cm-1
.
Hydrogenations of 3-Pentanone in CD2Cl2. A series of
hydrogenations of 3-pentanone were performed in CD2Cl2 in
5 mm NMR tubes equipped with a J. Young valve, following
a procedure described previously.24 Experiments were prepared
by placing HMo(CO)2[η5:η1-C5H4(CH2)nPR2] (2.1 × 10-5 mol, n
2
1H NMR (400 MHz, CD2Cl2): δ -5.95 (br d, JPH ) 33.9 Hz,
1H, MoH), 1.60 (t m, 2JPH ) 24.5 Hz, 2H, PCH2), 2.20 (m, CH2),
2.49 (m, CH2), 5.04 (br, 2H, m-C5H4), 5.27 (br, 2H, o-C5H4),
7.37-7.40 (m, 6H, m- and p-Ph), 7.57 (m, 4H, o-Ph). 31P{1H}
NMR (162.0 MHz, CD2Cl2): δ 54.9 (br, w1/2 ) 200 Hz).
13C{1H} NMR (100.6 MHz, CD2Cl2): δ 22.0 (br s, PCH2), 27.7
(s, PCH2CH2), 28.2 (v br, C5H4CH2), 86.9 (br, o-C5H4), 88.4 (s,
m-C5H4), 104.2 (vbr, ipso-C5H4), 130.2 (s, p-Ph), 128.8 (s, m-Ph),
132.6 (d, 2JPC ) 10 Hz, o-Ph), 139.1 (br d, 1JPC ) 43.2 Hz, ipso-
Ph), 234 (v br, CO). IR (THF): ν(CO) ) 1936 (s), 1858
(vs) cm-1. The sample sent for elemental analysis had 0.5
equiv of toluene, as determined by 1H NMR. Anal. Calcd
C22H21O2PMo‚0.50toluene: C, 62.46; H, 5.14. Found: C, 62.14;
H, 5.26.
t
) 2, R ) Ph, Cy, Bu and n ) 3, R ) Ph) and 1 equiv (2.1 ×
10-5 mol) of Ph3C+A- (A- ) BAr′4-, BF4-, or PF6-) in an NMR
tube, followed by 700 µL of a CD2Cl2 solution that was 300
mM in 3-pentanone. Typical concentrations were 300 mM in
3-pentanone, 30 mM in catalyst, and 30 mM in bibenzyl, an
internal standard for 1H NMR integrations. The resulting
solutions ranged from dark orange to purple in color; the tubes
were wrapped in foil to avoid possible photochemical reactions.
The NMR tubes were filled with H2 (∼1.1 atm) while frozen
in liquid nitrogen, giving about 4 atm H2 when warmed to room
temperature. The available headspace of the NMR tube
allowed for about 1.5 mL of H2 to be added. Even at 4 atm,
this meant that at most 11 molar equiv of H2 were present
during the reaction. Additional H2 was therefore added during
the reactions as needed. In many cases, additional 3-pentanone
(21.2 µL, 10.0 equiv) was added to the NMR tubes after most
of the substrate had been hydrogenated. The additional
substrate was added in the drybox under argon, then H2 was
added again.
Preparation of (TfO)Mo(CO)2[η5:η1-C5H4(CH2)2PPh2]
(4Ph). HMo(CO)2[η5:η1-C5H4(CH2)2PPh2] (29.9 mg, 6.95 × 10-5
mol) was dissolved in toluene (2 mL). HOTf (7.0 µL, 7.9 × 10-5
mol, 1.1 equiv) was added to the solution. The pale yellow
solution rapidly darkened to deep purple. The solution was
stirred for 30 min at room temperature. Hexane (2 mL) was
added to the toluene solution, and (TfO)Mo(CO)2[η5:η1-
C5H4(CH2)2PCy2] (4Ph) crystallized at -30 °C as a purple-
red solid (165.0 mg, 0.28 mmol, 62% yield; 1.5:1 mixture of
trans:cis isomers). The trans:cis ratio did not significantly
The hydrogenations were monitored by 1H and 31P NMR.
The TON (turnover number) was determined on the basis of
the methine CH integrations of the pentanol and its ether in
the 1H NMR. In cases where the CH2 peak (δ 2.93) of the
internal standard (bibenzyl) overlapped with other resonances,
the ortho-phenyl resonance of the BAr′4- anion (δ 7.74, broad)
was used as an integration standard.
Hydrogenations of Neat 3-Pentanone (30 mM catalyst,
>100 mL of H2 at 4 atm). A series of hydrogenations of neat
3-pentanone were performed in glass tubes under H2 at 23,
50, and 75 °C. The reactions were not stirred, since stirring
appeared to have little, if any, effect on the rate of hydrogena-
1
change in CD2Cl2 after 18 h at room temperature. H NMR
(400 MHz, CD2Cl2): δ 2.09 (dt, 2H, 2JHP ) 27.6 Hz, 3JHH ) 7.0
Hz, trans-PCH2), 2.50-2.56 (m, 1H, cis-CH2), 2.72 (m, 2H,
trans-C5H4CH2), 2.91-3.03 (m, 1H, cis-CH2), 3.36-3.45 (m, 2H,
cis-CH2), 4.71 (m, 1H, cis-C5H4), 4.85 (m, 1H, cis-C5H4), 5.78
(m, 2H, trans-C5H4), 5.95 (m, 2H, trans-C5H4), 6.28 (m, 1H,
cis-C5H4), 6.31 (m, 1H, cis-C5H4). 31P NMR (162.0 MHz,
CD2Cl2): δ 63.3 (cis, 39%), 74.9 (trans, 61%).
Preparation of (TfO)Mo(CO)2[η5:η1-C5H4(CH2)2PCy2]
(4Cy). HMo(CO)2[η5:η1-C5H4(CH2)2PCy2] (201.0 mg, 0.454
mmol) was dissolved in CH2Cl2 (6 mL). HOTf (40 µL, 67 mg,
0.45 mmol) was added to the solution. The pale yellow solution
rapidly darkened to deep red. The solution was stirred for 30
min at room temperature. The solvent was evaporated, and
the product was recrystallized from toluene at -30 °C. 4Cy
crystallized as a red solid (165.0 mg, 0.28 mmol, 62% yield).
The isolated product is a 6:1 mixture of cis:trans isomers. The
product ratio did not significantly change in CD2Cl2 after 2
days at room temperature. 1H NMR (300 MHz, CD2Cl2): δ
1.20-2.52 (m, cis- and trans-CH2 and Cy), 2.69-2.85 (m, cis-
and trans-CH2), 4.58 (m, 1H, cis-C5H4), 4.63 (m, 1H, cis-C5H4),
5.65 (m, 2H, trans-C5H4), 5.70 (m, 2H, trans-C5H4), 6.13 (m,
1H, cis-C5H4), 6.20 (m, 1H, cis-C5H4). 31P NMR (121.9 MHz,
CD2Cl2): δ 66.5 (cis), 79.1 (trans). IR (THF): ν(CO) ) 1980
(s), 1869 (s) cm-1. Anal. Calcd for C22H30O5F3PSMo: C, 44.75,
H, 5.12. Found: C, 44.59; H, 5.04.
tion. The catalysts were formed in situ by placing 2 or 3 (2.1
× 10-5 mol, 1 equiv) and Ph3C+A- (A ) BAr′4-, BF4-, or PF6
;
-
2.1 × 10-5 mol; 1 equiv) in a small vial and adding 3-pentanone
(630 µL, 537 mg, 6.24 mmol, 283 equiv). After all the solids
had dissolved, the reaction solution was transferred to a 125
mL glass bulb with a Teflon valve. The solution was removed
from the drybox, connected to a vacuum line, and freeze-
pump-thawed two times. The sample was frozen a third time,
and the entire tube was submersed in liquid N2. The tube was
then filled with 1.1 atm H2, sealed, and warmed to room
temperature. The resulting tube contained about 4.1 atm H2
(125 mL at 4.1 atm, 20.1 mmol, 1000 equiv) at room temper-
ature. The tube was warmed to the appropriate temperature.
Samples were removed for analysis at 10, 20, and 30 days.
Reactions using isolated catalyst precursors such as 4, (TfO)-
Mo(CO)2Cp(PCy3), or (TfO)Mo(CO)2Cp(PPh3) were prepared by
dissolving the isolated complexes in 3-pentanone; all other
conditions were identical.
Preparation of (TfO)Mo(CO)2[η5:η1-C5H4(CH2)2PtBu2]
(4tBu). HMo(CO)2[η5:η1-C5H4(CH2)2PtBu2] (163.4 mg, 0.419
mmol) was dissolved in toluene (4 mL). HOTf (36 µL, 61 mg,
0.41 mmol) was added to the solution. The pale yellow solution
initially turned dark purple, then blood red. The solution was
stirred for 30 min at room temperature. The product began to
The samples were removed by cooling the solution to 77 K
(liquid N2) and flowing H2 over the solution. The tube was then
warmed to room temperature, and a 60 µL aliquot was

Molybdenum Carbonyl Complexes
Organometallics, Vol. 24, No. 25, 2005 6229
removed. All manipulations took place under 1 atm hydrogen.
After sampling, the tube was once again cooled with liquid
nitrogen and evacuated. The tube was then filled with 1.1 atm
H2 at 77 K, sealed, and placed back in the constant-temper-
ature bath. The aliquot (60 µL) was dissolved in 500 µL of
CD2Cl2 (in the air) for NMR analysis.
accurate, as the CH3 resonances are barely baseline separated
in the H NMR spectrum.
1
Acknowledgment. We thank the U.S. Department
of Energy, Office of Science, Laboratory Technology
Research Program, and the Division of Chemical Sci-
ences, Office of Basic Energy Sciences, for support. This
research was carried out at Brookhaven National Labo-
ratory under contract DE-AC02-98CH10886 with the
U.S. Department of Energy. We thank DuPont for
additional support of this work through a CRADA grant.
High-pressure reactions were performed in a clean stainless
steel Parr microreactor (100 mL). A typical reaction involved
placing 2Cy (27.9 mg, 6.30 × 10-5 mol) and Ph3C+BAr′4- (69.6
mg, 6.30 × 10-5 mol, 1.00 equiv) in the reactor, in the glovebox.
3-Pentanone (1.9 mL, 1.62 g, 17.9 mmol, 285 equiv) was added
by syringe to give a red solution. The reactor was sealed under
argon and removed from the glovebox. The reactor was then
placed under 820 psi H2. The reactor was warmed to 50 °C,
and the reaction was run for about 10 days (the pressure
increased to 860 psi at 50 °C). The reactor was then cooled to
room temperature, and the reactor was slowly depressurized.
The reactor was then opened, and a small aliquot of the
solution was removed. The sample was dissolved in CD2Cl2,
and the 1H NMR was taken and analyzed as before. The TON
values were most accurately determined by comparing the CH2
resonances of the ketone with the product alcohol and ether
resonances. Integration of the CH3 resonances usually agreed
within 5% (and often within 1%), but this tends to be less
Supporting Information Available: Spectral data on
{M(CO)2[η5:η1-C5H4(CH2)nPR2](NCCD3)}+ complexes, addi-
tional NMR characterization and assignments for 2Cy and 3,
characterization of HW(CO)2[η5:η1-C5H4(CH2)2PtBu2], an ex-
panded version of Table 3, details on the preparation of
LiC5H4(CH2)2PCy2, further crystallographic details, and ad-
ditional information and plots of catalytic experiments in
CD2Cl2. This material is available free of charge via the
Internet at http://pubs.acs.org.
OM050564H