Ionic hydrogenation of ketones with molybdenum pentabenzylcyclopentadienyl hydride catalysts

Article abstract of DOI:10.1021/om800379z

A study of the reactivity of [MoCp^Bz(CO)₃H] (Cp ^Bz = pentabenzylcyclopentadienyl) as a catalyst precursor for the ionic hydrogenation of 3-pentanone is presented. The complexes [MCp ^Bz(CO)₃OTf] (M = Mo (3), W (4)), [MoCp^Bz(CO) ₃(O=CEt₂)]BF₄ (9), and [MoCp ^Bz(CO)₃(O=CMe₂)]BF₄ (12) are described. 9⁺ and [MoCp^Bz(CO)₃(OHCHEt ₂)]⁺ (8⁺) were identified by NMR under catalytic conditions. The reaction of [WCp^Bz(CO)₃H] with triflic acid led to [WCp^Bz(CO)₃(H)₂] ⁺ (7⁺), which was identified by NMR as a dihydride complex. The influence of steric and electronic effects on the catalyst activity is discussed on the basis of experimental results and a mechanism proposal resulting from DFT calculations. The proposed mechanism includes facile protonation of the acetone O atom promoted by a dihydride Mo(IV) complex, followed by slower hydride transfer from the metal center to the carbonyl C atom. The results indicate strong coordination of both the product (isopropyl alcohol) and the reactant (acetone), leading to some blocking of the catalyst active site. Regeneration of the catalyst active species is the rate-limiting step of the catalytic cycle. The steric bulk of the Cp^Bz ligand increases the catalyst activity.

Full text of DOI:10.1021/om800379z

Organometallics 2008, 27, 4589–4599
4589
Ionic Hydrogenation of Ketones with Molybdenum
Pentabenzylcyclopentadienyl Hydride Catalysts†
So´nia Namorado, Maria Augusta Antunes, Luis F. Veiros, Jose´ R. Ascenso,
M. Teresa Duarte, and Ana M. Martins*
Centro de Qu´ımica Estrutural, Instituto Superior Te´cnico, AV. RoVisco Pais, 1049-001 Lisboa, Portugal
ReceiVed April 30, 2008
A study of the reactivity of [MoCpBz(CO)3H] (CpBz ) pentabenzylcyclopentadienyl) as a catalyst
precursor for the ionic hydrogenation of 3-pentanone is presented. The complexes [MCpBz(CO)3OTf]
(M ) Mo (3), W (4)), [MoCpBz(CO)3(OdCEt2)]BF4 (9), and [MoCpBz(CO)3(OdCMe2)]BF4 (12) are
described. 9+ and [MoCpBz(CO)3(OHCHEt2)]+ (8+) were identified by NMR under catalytic conditions.
The reaction of [WCpBz(CO)3H] with triflic acid led to [WCpBz(CO)3(H)2]+ (7+), which was identified
by NMR as a dihydride complex. The influence of steric and electronic effects on the catalyst activity is
discussed on the basis of experimental results and a mechanism proposal resulting from DFT calculations.
The proposed mechanism includes facile protonation of the acetone O atom promoted by a dihydride
Mo(IV) complex, followed by slower hydride transfer from the metal center to the carbonyl C atom. The
results indicate strong coordination of both the product (isopropyl alcohol) and the reactant (acetone),
leading to some blocking of the catalyst active site. Regeneration of the catalyst active species is the
rate-limiting step of the catalytic cycle. The steric bulk of the CpBz ligand increases the catalyst activity.
cis/trans isomerization may be the limiting step of hydride
transfer reactions.11,12
Introduction
The reduction of ketones to alcohols by an ionic mechanism
is an important reaction that has attracted much interest.1-7 In
the presence of strong acids the oxygen atom of ketones is
protonated and the subsequent hydride transfer from a transition-
metal hydride complex completes the reduction of the CdO
group. Studies on the protonation of [WCp(CO)2(PMe3)H] have
shown that the extent of the reaction depends on the solvents
and the formation of acid/base homoconjugate pairs. Although
a nonclassical η2-H2 ligand has not been directly observed, the
kinetic site of protonation is the hydride ligand.8 The cationic
dihydrides [WCp(CO)2(PR3)(H)2]+ have been observed at low
temperatures by NMR9,10 and by X-ray diffraction (R ) Me).10
The hydricity of complexes [MCp′(CO)3-n(PR3)nH] (Cp′ ) Cp,
Cp*, Ind; R ) Me, Ph, Cy; n ) 0, 1; M ) Cr, Mo, W) shows
that (i) phosphine-containing compounds are better hydride
sources, (ii) hydride transfer is faster for trans isomers, and (iii)
Recently Bullock and co-workers have turned the ionic
reduction of ketones into a true hydrogenation catalytic reac-
tion.13 Using as catalyst precursors [MoCp(CO)2(PR3)H] and
the trityl cation, the complexes catalyze the reduction of
3-pentanone in the presence of hydrogen. The influence of the
phosphine ligands on the catalyst activity revealed a dependence
on the phosphine cone angles, and the dissociation of PR3 was
identified as a decomposition pathway of the catalysts.14,15
Replacement of phosphine by an N-heterocyclic carbene led to
inefficient ketone hydrogenation catalysts, due to the vulner-
ability of carbene ligands to protonation, producing an imida-
zolium cation.16
Our interest in molybdenum and tungsten pentabenzylcyclo-
pentadienyl complexes17-19 led us to assess the properties of
[MCpBz(CO)3H] in the catalytic hydrogenation of ketones. The
lack of ligands susceptible to participate in the catalyst
decomposition and the influence of a very bulky cyclopenta-
dienide ligand in the ionic hydrogenation of ketones catalyzed
* To whom correspondence should be addressed. E-mail:
ana.martins@ist.utl.pt.
† This paper is dedicated to the memory of Professor Alberto R. Dias.
(1) Gaus, P. L.; Kao, S. C.; Youngdahl, K.; Darensbourg, M. Y. J. Am.
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Soc. 1998, 120, 13121–13137.
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(15) Bullock, R. M. Chem. Eur. J. 2004, 10, 2366–2374.
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10.1021/om800379z CCC: $40.75
2008 American Chemical Society
Publication on Web 08/26/2008

4590 Organometallics, Vol. 27, No. 18, 2008
Namorado et al.
Scheme 1
Table 1. IR Carbonyl Frequencies (cm-1
)
by molybdenum hydrides are discussed and related to a DFT20
study of the reaction mechanism.
compd
ν(Ct O)
3
2058
2041
2011
2008
2073
1981
1975
1934
1928
1995
1956
1956
1919
1822
1962
[MoCpBz(CO)3Cl]
[MoCpBz(CO)3Me]
[MoCpBz(CO)3H]
[MoCp(CO)3OTf]
Results and Discussion
Chemical Studies. Syntheses and Characterization. All
new compounds are represented in Scheme 1. Addition of
dichloromethane solutions of triflic acid to equimolar amounts
of the metal hydrides [MCpBz(CO)3H] (M ) Mo (1), W (2);
CpBz ) pentabenzylcyclopentadienyl)19 dissolved in the same
solvent resulted in immediate darkening of the light yellow
solutions. The reactions led to the formation of the complexes
[MCpBz(CO)3OTf] (M ) Mo (3), W (4)), which were isolated
in high yields as red crystals (M ) Mo, 94% yield) and a
microcrystalline orange solid (M ) W, 92% yield). Complexes
3 and 4 may alternatively be obtained by reactions of
[MCpBz(CO)3Me] (M ) Mo (5), W (6)) with HOTf in similar
yields.
The IR spectra of 3 and 4 show three absorption bands for
the carbonyl ligands at values slightly higher than those reported
for other [MCpBz(CO)3X] complexes17,21 but shifted to lower
values when compared to those reported for [MCp(CO)3OTf]
(M ) Mo, W)22 (Table 1). This shows that metal-carbonyl
back-bonding is more extensive in 3 and 4 than in analogous
Cp derivatives, and thus, the five benzyl substituents on the Cp
ring make the pentabenzylcyclopentadienide ligand a better
donor than cyclopentadienide. The IR spectra also display a
4
2050
2032
2009
2008
2067
1968
1967
1926
1901
1982
1947
1942
1893
[WCpBz(CO)3Cl]
[WCpBz(CO)3Me]
[WCpBz(CO)3H]
[WCp(CO)3OTf]
1945
series of peaks centered at ca. 3000 cm-1 and below 1600 cm-1
characteristic of the CpBz ligand and three bands assigned to
the νas(SO3) and νs(SO3) bands of the coordinated triflate group.
1
The H NMR spectra of complexes 3 and 4 show one singlet
corresponding to the methylenic protons (δ (CD2Cl2) 3.73 ppm
for 3 and 3.80 ppm for 4) and two multiplets in the aromatic
region, indicating that there are no constraints to the free rotation
of the five benzyl groups in solution. Accordingly the 13C NMR
spectra present one resonance for the methylenic carbons, one
resonance for the Cp ring carbons, four benzyl aromatic carbon
resonances, and two resonances for the carbonyl carbons that
are integrated 2:1, as expected for four-legged piano-stool metal
coordination. The 19F NMR spectra exhibit one singlet at δ
-77.4 ppm for 3 and δ -76.8 ppm for 4, assigned to the fluorine
atoms of the triflate group.
Dark red needles of 3 and orange crystals of 4 suitable for
single-crystal X-ray diffraction determination were obtained
from diethyl ether solutions at -20 °C. The compound molec-
ular structures depicted in Figures 3 and 4 are discussed later.
Reactions of 1 and 2 with HOTf were carried out in sealed
NMR tubes and studied by 1H and 19F NMR. Upon addition of
(20) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and
Molecules; Oxford University Press: New York, 1989.
(21) Song, L. C.; Zhang, L. Y.; Hu, Q. M.; Huang, X. Y. Inorg. Chim.
Acta 1995, 230, 127–131.
(22) Appel, M.; Schloter, K.; Heidrich, J.; Beck, W. J. Organomet.
Chem. 1987, 322, 77–88.

Ketone Hydrogenation with MoCpBz(CO)3H
Organometallics, Vol. 27, No. 18, 2008 4591
Figure 1. Catalytic hydrogenation of 3-pentanone with [MoCp′(CO)3-n(PMe3)nH] (Cp′ ) Cp, CpBz, n ) 0, 1) catalysts in CD2Cl2 (catalyst:
ketone ) 1:10, P(H2) ) 4 atm).
Figure 2. Free energy profile calculated (PBE1PBE) for the hydrogenation of acetone catalyzed by [MoCp(CO)3]+. The minima and transition
states were optimized, and the structures obtained are present. Free energies (kcal/mol) are referred to the dihydrogen complex
[MoCp(CO)3(H)2]+ (A), and the values in italics represent energy barriers.
1 equiv of HOTf to a frozen solution of 2 in CD2Cl2, the tube
was sealed under vacuum and the temperature of the mixture
was raised to -80 °C. The formation of a new species, identified
as the dihydride [WCpBz(CO)3(H)2]OTf (7), was detected by
the appearance of a new resonance at δ -1.96 ppm. The
assignment of this resonance was based on the spin-lattice
relaxation time (T1) criterion described by Crabtree, which
allows one to distinguish between dihydride and dihydrogen
ligands.23-25 According to this, short T1 values (4-100 ms)
are observed for dihydrogen ligands, whereas classical dihydride

4592 Organometallics, Vol. 27, No. 18, 2008
Namorado et al.
The NMR data described are consistent with the establishment
of equilibrium between 2 and 7 as indicated in eq 1. The
exchange process is slow at -80 °C, and at -10 °C the rate of
the exchange is such that the two hydride resonances collapse.
The rate exchange for the process was determined as 0.48 s-1
at -70 °C and 0.19 s-1 at -80 °C. Above 10 °C decomposition
of 7 into 4 starts to take place (eq 2). This reaction led to the
formation of H2, observable when the solution attained room
temperature.
Figure 3. Molecular structure of 3 showing thermal ellipsoids at
the 50% probability level. Hydrogen atoms and disordered solvent
molecules are omitted for clarity.
The reactivity pattern observed for complex 2 is similar to
that reported by Bullock and co-workers for [WCp(CO)3H] and
[WCp*(CO)3H].10 A noticeable difference between the three
hydrides refers to the stability of the corresponding dihydrides
[WCp′(CO)3(H)2]+ (Cp′ ) CpBz, Cp*, Cp), which follows the
1
trend Cp > Cp* > CpBz. The H NMR spectrum run at -80
°C immediately after addition of 1.2 equiv of HOTf to a CD2Cl2
solution of 2 at room temperature revealed the formation of 4
in 49% yield and a total conversion of 2 in 66% yield (2:4:7 )
34:49:17). In equivalent conditions, conversion of analogous
Cp and Cp* hydrides are 16% and 50%, respectively, thus
meaning that the basicity of [WCp′(CO)3H] increases in the
sequence Cp < Cp* < CpBz.
Addition of HOTf to a CD2Cl2 solution of [MoCpBz(CO)3H]
(1) at -80 °C, as described for the tungsten analogue, led to
the immediate formation of [MoCp(CO)3OTf] (3), revealing that
the stability of [MoCpBz(CO)3(H)2]+ hampers its detection
and the transformation represented in eq 2 prevails despite the
temperature.26 Nevertheless, treatment of a dichloromethane-
d2 solution of 1 with triflic acid in the presence of 1 equiv of
3-pentanone revealed the formation of 3 and a new species
that was identified by NMR as the alcohol derivative
[MoCpBz(CO)3(Et2CHOH)]OTf (8). Characteristic NMR data
for 8 at -10 °C are observed at δ 3.79 (CH2Ph), 3.02 (m, CH),
1.67 (multiplet, br., CH2), and 0.96 ppm (t, CH3) in the proton
spectrum and at δ 90.6 (CH), 31.9 (CH2Ph), 27.6 (CH2), and
9.6 ppm (CH3) in the carbon spectrum. During this time, as the
temperature is raised from -80 to 0 °C, 1 is completely
converted into 3 and 8 in a 1:4 ratio. Further heating to room
temperature revealed that ketone conversion attained 73% and
that 3 was now the major complex in solution (3:8 ) 5:1). This
result shows that the competition toward metal coordination
between the alcohol and triflate favors the anion coordination
and is also consistent with the lack of reactivity between 3 and
3-pentanone. However, in the presence of a more poorly
coordinating anion such as BF4-, hydride elimination from 1
in the presence of 3-pentanone led to the synthesis of
[MoCpBz(CO)3(OdCEt2)]BF4 (9), which was isolated as a pink
Figure 4. Molecular structure of 4 showing thermal ellipsoids at
the 50% probability level. Hydrogen atoms and disordered solvent
molecules are omitted for clarity.
species cause significantly longer T1 values, reflecting longer
H · · · H distances. A value of 472 ms, which is consistent with
a dihydride formulation, was determined for 7 at -70 °C, when
the T1 vs temperature curve reaches the minimum. At this
temperature the integration of the new signal at -1.96 ppm
versus the hydride resonance in 2 (-6.72 ppm) revealed 50%
conversion of 2 into 7. As the temperature was raised,
broadening of all hydride resonances was observed. Simulta-
neously, the two sets of resonances assigned to the benzylic
protons of the CpBz ligands in 2 and 7 overlapped, and when
the temperature reached -10 °C, the hydride resonances of both
species collapsed into the baseline. At 10 °C a new resonance
due to the methylenic protons of the triflate derivative 4 appeared
at δ 3.76 ppm. Cooling the sample to -80 °C caused the
sharpening of the peaks associated with the hydride protons of
2 and 7. A mixture of 2, 7, and 4 in the ratio 2:1:7 was observed.
1
After 24 h at room temperature the H NMR spectrum, run at
(24) Crabtree, R. H.; Lavin, M. J. Chem. Soc., Chem. Commun. 1985,
1661–1662.
(25) Desrosiers, P. J.; Cai, L. H.; Lin, Z. R.; Richards, R.; Halpern, J.
J. Am. Chem. Soc. 1991, 113, 4173–4184.
-80 °C, revealed that 4 was the only species in solution.
(23) Crabtree, R. H.; Lavin, M.; Bonneviot, L. J. Am. Chem. Soc. 1986,
108, 4032–4037.
(26) Quadrelli, E. A.; Kraatz, H. B.; Poli, R. Inorg. Chem. 1996, 35,
5154–5162.

Ketone Hydrogenation with MoCpBz(CO)3H
Organometallics, Vol. 27, No. 18, 2008 4593
solid in 80% yield. The NMR spectra of 9 in CD2Cl2 show that
the ketone is labile and its dissociation gives rise to the formation
of two new species identified as the dichloromethane adduct
[MoCpBz(CO)3(CH2Cl2)]BF4 (10) and [MoCpBz(CO)3(FBF3)]
(11), which exist in solution in a 1:3 ratio. At -80 °C the proton
NMR spectrum displays, in addition to characteristic resonances
due to the CpBz ligand, a broad resonance at δ 5.37 ppm that
presents NOE coupling with the aromatic ortho benzyl protons
and is due to metal-bonded dichloromethane in 10. The 19F
NMR spectrum at room temperature consists of two resonances
at δ -149.92 (br, free BF4-) and -156.10 ppm (s, MoBF4)
that upon cooling at -80 °C splits into a broad quartet
(δ -338.24 ppm, MoFBF3) and two doublets integrated in the
ratio 1:4 at δ -153.04 and -152.97 ppm (2JFF ) 93.8 Hz),
assigned to the terminal fluorines MoFBF3 of the 10B and 11B
isotopomers.27
Treatment of an acetone solution of 1 with 1 equiv of
Ph3CBF4 in CH2Cl2 led to [MoCpBz(CO)3(OdCMe2)]BF4 (12).
The spectroscopic features of 12 obtained by IR and NMR attest
to its similarity to 9. As described for 9, 12 also releases ketone
and is converted into a mixture of 10 and 11 in CH2Cl2 solution.
Despite this behavior, it was possible to grow crystals of 12
from a freshly prepared solution in dichloromethane layered with
index (WI)28,29 of 0.604. These values can be compared with
the corresponding ones in free H2 (d ) 0.744 Å, WI ) 1.000),
providing a clear indication of a complex with coordinated
dihydrogen, rather than a dihydride species.
The first step in the mechanism corresponds to the breaking
of the H-H bond in A, yielding a dihydride complex (B), as
indicated by a large H-H separation (1.864 Å) as well as by
the small Wiberg index associated with that interaction (WI )
0.060), precluding the existence of an H-H bond. This is an
oxidative addition process, going from Mo(II) in A to Mo(IV)
in B. The corresponding transition state (TSAB) has an inter-
mediate structure, between A and B. In TSAB, there is a weak
H-H interaction (d ) 1.278 Å, WI ) 0.237), indicating that
the process of bond breaking is halfway through when the
transition state is reached. The two isomers, A and B, are equally
stable, in practical terms (∆G ) 0.1 kcal/mol), and the low
activation energy (∆Gq ) 3.2 kcal/mol) calculated for the
oxidative addition shows a facile process. It should be noted
that a mechanism starting directly from the dihydrogen complex
(A) and not involving B was thoroughly investigated. In all
cases, the first step corresponds to the formation of dihydride
species (B), as indicated in Figure 2.
Acetone addition to B originates C, a complex with a
hydrogen interaction between the hydride opposite to the Cp in
the metallic fragment, and the O atom of acetone (dO-H ) 1.765
Å). The metal hydride that interacts with acetone is determined
by electronic factors. In fact, the two hydride ligands in B are
not equivalent. The H atom opposite to the Cp ligand is
considerably electron poorer than the hydride closer to Cp, as
shown by the corresponding atomic charges derived from a
natural population analysis (NPA):30-37 0.19 and 0.11, respec-
tively. Thus, the O atom of an incoming acetone molecule
approaches the more positive hydride in B: that is, the more
acidic MoH bond. Formation of C from free acetone and the
dihydride complex, B, is favorable (∆G ) -3.3 kcal/mol), from
a thermodynamic point of view.
hexane at -20 °C.
Ketone Hydrogenation. The reactions were performed in
CD2Cl2 and monitored by NMR. Typically, a mixture of
[MoCp′(CO)3-n(PMe3)nH] (Cp′ ) Cp, n ) 0; Cp′ ) CpBz, n )
0, 1), 3-pentanone (10 equiv) and Ph3CBAr′4 (Ar′ ) C6H3(CF3)2;
Mo:Ph3C+ ) 1:1) in CD2Cl2 was frozen and evacuated at liquid
nitrogen temperature and the tube was filled with H2 at 1 atm.
The mixture was warmed to room temperature and the reaction
1
progress checked by H NMR. Slow hydrogenation of 3-pen-
tanone proceeds along the time as shown in Figure 1, which
also displays the results obtained using [MoCp(CO)2(PMe3)H]
as the catalyst precursor reported by Bullock and co-workers
for comparison.14 The catalytic activity of [MoCpBz(CO)3H] (1)
is higher than that displayed by all other complexes, and
[MoCp(CO)3H] is the less active compound. The complexes
[MoCp′(CO)2(PMe3)H] (Cp′ ) Cp, CpBz) show similar activi-
ties, and moreover, Figure 1 shows that they give rise to catalytic
The following step, from C to D, corresponds to H transfer
from Mo to the acetone O atom, or, in other words, to
protonation of acetone. In the product, D, there are two
molecular units connected together by a Mo-HO interaction
(dMo-H ) 2.604 Å): one can formally be viewed as a neutral
monohydride complex and the other is protonated acetone,
[MoCp(CO)3H]-(CH3)2COH+. Thus, this process corresponds
to a reductive elimination step, bringing the metal back to
Mo(II). The corresponding transition sate (TSCD) is a rather early
one, with the Mo-H bond still far from broken, presenting
values of bond length and Wiberg index (dMo-H ) 1.834 Å,
systems which deactivate slowly than 1.
DFT Calculations. The mechanism of the catalytic hydro-
genation of acetone to isopropyl alcohol was investigated by
means of DFT calculations.20 In the model used for this study,
the catalyst active species, [MoCp(CO)3]+, corresponds to the
metal fragment with the simplest π ligand, i.e., plain cyclopen-
tadienide, for computational expediency. The catalyst with CpBz,
[Mo(CpBz)(CO)3]+, is used for a comparative study of relevant
parts of the calculated mechanism (see below). The free energy
profile calculated for the reaction is represented in Figure 2.
WI ) 0.400) not far from those of intermediate C (dMo-H
)
1.768 Å, WI ) 0.454). On the other hand, formation of the
Coordination of H2 to the metallic fragment [MoCp(CO)3]+
results in a 22.0 kcal/mol stabilization of the system, reflecting
the electron-deficient character of the metal in the initial
complex, a 16-electron species. This is a barrierless process,
since no transition state could be located for the addition of
hydrogen, at the theory level employed. In the resulting product,
[MoCp(CO)3(H2)]+ (A), formally an 18-electron species, there
is an “upright” arrangement of the H2 fragment, with the MoHH
plane perpendicular to the plane of the Cp ring. In A, the H-H
bond is short (0.848 Å) and strong, as demonstrated by a Wiberg
(28) Wiberg, K. B. Tetrahedron 1968, 24, 1083–1096.
(29) Wiberg indices are electronic parameters related to the electron
density between atoms. They can be obtained from a natural population
analysis and provide an indication of the bond strength.
(30) Carpenter, J. E.; Weinhold, F. J. Mol. Struct. (THEOCHEM) 1988,
46, 41–62.
(31) Carpenter, J. E. Ph.D. Thesis, University of Wisconsin, Madison,
WI, 1987.
(32) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980, 102, 7211–
7218.
(33) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1983, 78, 4066–4073.
(34) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1985, 83, 1736–1740.
(35) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985,
83, 735–746.
(36) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88,
(27) Namorado, S.; Antunes, M. A.; Azevedo, C. G.; Duarte, M. T.;
Ascenso, J. R.; Lemos, M. A.; Martins, A. M. Manuscript (containing full
details for complexes 10 and 11) in preparation. See also ref 12.
899–926.
(37) Weinhold, F.; Carpenter, J. E. The Structure of Small Molecules
and Ions; Plenum: New York, 1988; p 227

4594 Organometallics, Vol. 27, No. 18, 2008
Namorado et al.
Scheme 2. Comparative Reactivity of the Isopropyl Alcohol
Complex with Cp and CpBz as Coligands36
new O-H bond is only incipient in TSCD, with a long distance
(1.592 Å), denoting a weak interaction (WI ) 0.174), still far
from what is observed in D, where formation of the O-H bond
is completed (dO-H ) 1.018 Å, WI ) 0.558). A clear positive
charge (0.278) calculated for the transferring H atom, in TSCD
,
confirms this as a protonation step. Acetone protonation, from
C to D, is a favorable process (∆G ) -6.6 kcal/mol) with a
negligible activation free energy of 1.3 kcal/mol.
From D, reorientation of the protonated acetone with respect
to the metallic fragment brings the OH group to the proximity
of one CO ligand in the complex, establishing a classical H
bond, in E, with a O-HO separation of 1.506 Å. This process
causes a stabilization of 3.1 kcal/mol, and thus, the system
continues to go downhill in terms of free energy. The low value
of the activation free energy associated (2.8 kcal/mol) indicates
a facile rearrangement.
The following step, from E to F, represents H transfer from
Mo to the C atom of the acetone carbonyl group. In this step
the reaction goes from an H-bonded complex between two
molecular units, protonated acetone and the monohydride
complex [MoCp(CO)3H], in E, to a single molecule in F. This
intermediate (F) corresponds to a Mo complex with a molecule
of isopropyl alcohol coordinated by the C-H bond of the
secondary C atom, [MoCp(CO)3{(CH3)2CHOH}]+. In the
corresponding transition state (TSEF), formation of the new
C-H bond is only incipient (dC-H ) 1.842 Å, WI ) 0.170),
while Mo-H bond breaking is just starting, as shown by a
separation of 1.764 Å and a Wiberg index of 0.457, values not
far from those of intermediate E (dMo-H ) 1.704 Å, WI )
0.536). The H-transfer process from E to F corresponds, in fact,
to hydride transfer from the metal to the C atom of the carbonyl
group in protonated acetone, as demonstrated by a negative
charge calculated for the transferring H atom in TSEF (-0.026).
This step is slightly unfavorable (∆G ) 0.2 kcal/mol), and the
corresponding activation free energy (7.4 kcal/mol), although
reachable, presents the highest value for the reaction, from the
aggregate between acetone and the dihydride complex (C) to
the complex with O-coordinated isopropyl alcohol (G).
The final step of the mechanism, from F to G, is a
rearrangement of isopropyl alcohol in the coordination sphere
of the metal. The system goes from a coordination established
by the C-H bond of the central C atom, in F, to O coordination
in the product, G. In the transition state, TSFG, the structure is
intermediate between the two minima involved, F and G. In
TSFG, the Mo-H bond is practically broken (dMo-H ) 2.218
Å, WI ) 0.058), while formation of the new Mo-O bond is
only beginning with values of bond distance (2.955 Å) and
Wiberg index (0.040) far from the ones found in the product,
G (dMo-O ) 2.242 Å, WI ) 0.299). This step is very favorable,
from a thermodynamic point of view (∆G ) -17.3 kcal/mol),
and the activation free energy calculated (4.4 kcal/mol) is
accessible.
The energy profile calculated for the reaction (Figure 2) shows
that, overall, the system goes downhill, in free energy terms,
from the initial aggregate between acetone and the dihydride
complex (C) until G, the complex with isopropyl alcohol
coordinated through the O atom. In fact, an energy variation of
∆G ) -26.8 kcal/mol is associated with that process, from C
to G, corresponding to an exergonic reaction and indicating the
stability of the product, with respect to the reagent. In addition,
coordination of one molecule of acetone (one of the reagents)
to the metal center is a competitive process, since the reaction
of ligand exchange from isopropyl alcohol, in G, to acetone, in
H, is exergonic (∆G ) -3.7 kcal/mol; see Figure 2). The
stabilities of the complexes with coordinated isopropyl alcohol
(G) and with coordinated acetone (H) are most probably the
major drawbacks of this system with respect to catalysis, since
an overstabilization of those species means that both the reaction
product and the reagent are, at least partially, blocking the metal
coordination position and, thus, the catalytic site. This effect is
reflected in the closure of the catalytic cycle, that is, regeneration
of the dihydrogen complex (A) from H, releasing one molecule
of acetone and coordinating one molecule of H2. The energy
variation associated with that process, from H back to A
(∆G ) 12.0 kcal/mol; see Figure 2), is higher than any energy
barrier calculated for the elementary steps of the mechanism,
thus corresponding to the rate-limiting step of the entire process
and explaining the poor performance of the Cp complex as a
catalyst for the reaction, as experimentally observed.
Key features of the catalytic reaction with the pentabenzyl-
cyclopentadienyl (CpBz) complex as catalyst were also inves-
tigated computationally, for comparison purposes. Thus, regen-
eration of the dihydrogen complex [MoCpBz(CO)3(H2)]+ (ABz)
from the acetone complex [MoCpBz(CO)3(OdCMe2)]+ (HBz)
is less unfavorable than the equivalent process for the Cp system,
with an energy variation 4.2 kcal/mol lower (see Scheme 2).38
(38) Energy values obtained for the gas phase at the PBE1PBE/b1 level
(see Computational Details).

Ketone Hydrogenation with MoCpBz(CO)3H
Organometallics, Vol. 27, No. 18, 2008 4595
This corresponds to the rate-limiting step of the catalytic cycle
(see above), and thus, a lower energy cost associated with the
CpBz catalyst suggests that this should be a better catalyst than
the Cp system, in good accord with the experimental observa-
tions. An increased steric bulk of CpBz, when compared with
Cp, destabilizes the coordination of acetone in HBz. Accordingly,
acetone coordination is weaker in the CpBz complex than in
the Cp analogue, as shown by a longer bond length (2.234 and
2.202 Å in HBz and H, respectively) and the associated Wiberg
indices (0.307 in HBz and 0.333 in H). Interestingly, the ligand
exchange process, switching of isopropyl alcohol and acetone
(from G to H and from GBz to HBz), has similar energy
variations in both systems (within 0.5 kcal/mol;38 see Scheme
2) since the two ligands have equivalent steric requirements.
The highest energy barrier calculated for the consecutive steps
of the hydrogenation reaction (from C to G) corresponds to
hydride transfer from the metal center to protonated acetone,
from E to F in the path calculated for the Cp catalyst,
represented in Figure 2. This step was also calculated for the
CpBz catalyst (see Figure S1 in the Supporting Information).
The general features of the process are equivalent for both
catalysts and, thus, a detailed discussion is unnecessary, but
some aspects deserve further notice. The first is the negative
charge (-0.023) calculated for the transferring H atom in the
transition state (TSEFBz), indicative of a hydride nature, similarly
to what happens in TSEF with the Cp catalyst, discussed above.
The second result is a slightly lower activation free energy
calculated for hydride transfer with the CpBz catalyst, in
comparison with that obtained for the Cp system. This indicates
that hydride transfer is favored for the CpBz system, resulting
from the presence of a better electron donor (CpBz vs Cp).
Finally, a difference of less than 2 kcal/mol was obtained38 for
the energy barriers of hydride transfer calculated in the gas
phase, on comparing the two catalysts (with CpBz and with Cp),
suggesting that the model used for the mechanistic investiga-
tions, [MoCp(CO)3]+, provides a good description of the system.
Figure 5. Molecular structure of 12 showing thermal ellipsoids at
the 50% probability level. Hydrogen atoms are omitted for clarity.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 3, 4, and
12
[MoCpBz(CO)3
[MoCpBz(CO)3OTf] [WCpBz(CO)3OTf] (OCMe2)]BF4
(3)
(4)
(12)
M-CpBz(CT)
M-C(6)
2.0004(4)
2.006(5)
1.994(4)
2.057(5)
1.131(6)
1.135(5)
1.139(6)
2.187(3)
1.828(5)
1.9984(4)
2.074(12)
1.981(10)
2.011(12)
1.120(12)
1.148(11)
1.135(13)
2.174(6)
2.0245(8)
2.032(9)
1.973(9)
2.037(9)
1.125(9)
1.149(9)
1.119(9)
2.156(5)
1.169(11)
M-C(7)
M-C(8)
C(6)-O(1)
C(7)-O(2)
C(8)-O(3)
M-O(4)
C(9)-S(1)/O(4)
1.799(12)
M-C(6)-O(1)
M-C(7)-O(2)
M-C(8)-O(3)
M-O(4)-S(1)/
C(9)
173.1(5)
179.6(4)
177.0(4)
140.5(2)
176.5(9)
178.9(8)
172.7(9)
138.3(5)
176.0(7)
178.1(7)
172.9(7)
144.8(7)
Crystallographic Studies. Crystals of 3, 4, and 12 suitable
for X-ray diffraction structure determinations have been obtained
from diethyl ether (3 and 4) and CH2Cl2/hexane (12) at -20
°C. Complexes 3 and 4 crystallize in the monoclinic space group
j
P21/c, and 12 crystallizes in the triclinic space group P1 with
as the M-C-O angles are typical for terminal carbonyl ligands.
The M-O(4) distances in 3 and 4 are close to the value of
2.212(2) Å reported for [MoCp(CO)3OTf]39 and slightly longer
than the M-O(4) distance in 12, reflecting the usual difference
between neutral and anionic metal-oxygen bond lengths.40 The
carbonyl carbon of the coordinated ketone in 12 is planar,
consistent with sp2 character. The methyl groups of the ketone
ligand are almost perpendicular to the cyclopentadienyl ring
plane (86.56(5)°) and coplanar with the CO ligand in a trans
position. Interestingly, the structure optimized for 12 by means
of DFT calculations (corresponding to HBz), is in excellent
agreement with the X-ray structure, reproducing all conforma-
tional arrangements discussed above: namely, the “upright”
conformation of coordinate acetone, perpendicular to CpBz, and
the spacial arrangement of the substituents in the cyclopenta-
dienyl ring. In addition, the calculated bond distances (Mo-CCO
) 2.00-2.01 Å, Mo-Oacet ) 2.23 Å, and Mo-CpBz(CT) )
2.01 Å) compare well with the experimental values (see Table
2). These results show that the theoretical method used in the
two independent molecules in the unit cell. The molecular
structures are shown in Figures 3- and 5, and selected bond
distances and angles are shown in Table 2 (only one set of data
are shown for 12).
In all complexes the metal coordination geometry can be best
described as a distorted four-legged piano stool with CpBz-
(CT)-M-O(4) angles of 109.79(9), 109.45(18), and 104.84(13)°
for 3, 4, and 12, respectively, and CpBz(CT)-M-C(i) (i ) 6,
7, 8) angles in the ranges 108.4-128.9° for 3 and 4 and
107.7-124.0° for 12 (CpBz(CT) ) centroid of the cyclopenta-
dienyl ring). The spatial arrangements of the cyclopentadienyl
substituents in all compounds exhibit four benzyl aromatic
groups opposite to the metal and one bent down toward it,
although, as expected for 18-electron complexes, sufficiently
far apart to prevent any bonding interaction. The relative
conformation of the phenyl rings may be characterized by the
torsion angles defined by M-C(CpBz)-C(CH2)-Cipso (see the
Supporting Information). The arrangement observed in 3, 4, and
12 is analogous to that observed for other MCpBz (M ) Mo(II),
W(II)) complexes previously reported.17,19 The M-CpBz(CT)
and the M-C(i) (i ) 1-5) bond distances are comparable to
the values determined for other pentabenzylcyclopentadienyl
complexes,17-19 and the M-C and C-O bond distances as well
(39) Lindner, E.; Henes, M.; Wielandt, W.; Eichele, K.; Steimann, M.;
Luinstra, G. A.; Gortz, H.-H. J. Organomet. Chem. 2003, 681, 12–13.
(40) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson,
D. G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1–S83.

4596 Organometallics, Vol. 27, No. 18, 2008
Namorado et al.
41
Scheme 3. Catalytic Cycle for Ketone Hydrogenation
Me],17 [WCpBz(CO)3Me],17 and Ph3CBAr′4 were prepared ac-
cording to methods described in the literature. Triflic acid and
Ph3CBF4 were used as received from Aldrich. NMR spectra were
recorded on Bruker AvanceII 300 and Bruker AvanceII 400
spectrometers. Chemical shifts for 1H were referenced to resonances
of the residual protonated solvents relative to tetramethylsilane, and
13C spectra were referenced to the solvent carbon resonance. 19F
spectra were referenced to external CF3COOH (δ -76.55 ppm).
11B spectra were referenced to external BF3 · Et2O (δ 0 ppm). IR
spectra were recorded as KBr pellets on a Jasco FI/IR-4100
spectrometer. Elemental analyses were performed at the Laborato´rio
de Ana´lises do IST, Lisbon, Portugal.
[MoCpBz(CO)3OTf] (3). A solution of triflic acid (45 µL, 0.50
mmol) in dichloromethane (5 mL) was added dropwise, at room
temperature, to a solution of [MoCpBz(CO)3H] (0.340 g, 0.50 mmol)
(15 mL) in the same solvent. The color of the mixture changed
immediately from pale yellow to dark red. The solution was stirred
mechanistic studies provides a good description of the system,
from a structural point of view. Short intermolecular contacts
were found in 3, 4, and 12. In 12 each anion interacts with two
cations in the asymmetric unit, displaying hydrogen bonds of
the type F · · · H-C, ranging from 2.401(9) to 2.769(9) Å (see
Figure S2 in the Supporting Information).
1
for /2 h. The solvent was evaporated under vacuum, and the red
residue obtained was extracted in diethyl ether and the extract
filtered. Concentration and cooling to -20 °C yielded red crystals
(0.398 g, 0.47 mmol, yield 94%). 1H NMR (CD2Cl2): δ 7.15-7.08
(m, 15 H, p-C6H5, m-C6H5), 6.84-6.81 (m, 10 H, o-C6H5), 3.73
(s, 10 H, CH2Ph). 13C NMR (CD2Cl2): δ 243.9 (CO trans OTf),
225.5 (CO), 138.0 (i-C6H5), 129.0 (o-C6H5), 128.9 (m-C6H5), 127.4
(p-C6H5), 117.4 (C5Bz5), 32.4 (CH2Ph). 19F NMR (CD2Cl2): δ
-77.4 (s, CF3SO3). 1H NMR (toluene-d8): δ 6.86-6.84 (m, 15 H,
p-C6H5, m-C6H5), 6.60-6.57 (m, 10 H, o-C6H5), 3.54 (s, 10 H,
CH2Ph). 13C NMR (toluene-d8): δ 244.9 (CO trans OTf), 225.5
(CO), 137.5 (i-C6H5), 128.9 (o-C6H5), 128.7 (m-C6H5), 127.2 (p-
C6H5), 117.1 (C5Bz5), 32.2 (CH2Ph). 19F NMR (toluene-d8): δ
Conclusion
Scheme 3 shows a catalytic cycle for the hydrogenation of
ketones with [MoCp(CO)3(H)2]+. According to the calculated
mechanism, the catalytic reaction depends on a balance between
the acidity of the Mo(IV) dihydride, [MoCp(CO)3(H)2]+, the
hydricity of Mo(II) hydride species, [MoCp(CO)3H], and the
stability of [MoCp(CO)3L]+ cations (L ) acetone, isopropyl
alcohol). The higher activity displayed by [MoCpBz(CO)3H] in
comparison with that of the Cp analogue may be envisaged as
a consequence of the higher steric constraint created by the CpBz
ligand compared to Cp and the consequent destabilization of
the species [MoCp′(CO)3L]+ (L ) ketone, alcohol) with the
steric bulk of the Cp′ ligand. As a consequence, blocking of
hydrogen activation by the ketone or the alcohol (iv and v in
Scheme 3) is less effective in the case of [MoCpBz(CO)3]+,
making this complex a better catalyst. The lability of the alcohol
and ketone ligands revealed by the NMR results of 8, 9, and
12 is consistent with this assumption. Moreover, the preferred
orientation of the ketone ligand in 12 is a further argument to
justify the lower stability of the [MoCpBz(CO)3(ketone)]+ cation
in comparison with its Cp analogue. The much lower activity
of [MoCp(CO)3H] most probably reflects the stability of the
species [MoCp(CO)3L]+ that constitute deactivation pathways
of the catalytic system if dissociation of L does not take place.
This reasoning is also consistent with the results reported for
the hydrogenation activity of [MoCp(CO)2(PR3)H] (R ) Me,
Ph, Cy), which increases with the phosphine cone angle.14 The
better catalytic performance of [MoCpBz(CO)3H] in comparison
with Cp or CpBz dicarbonyl phosphine analogues must also be
a reflection of the higher stability of that catalyst, which is not
prone to ligand protonation.
-73.4 (s, CF3SO3). IR (KBr pellet): νCt O 2058, 1981, 1956 cm-1
;
νas(SO3) 1336, 1196 cm-1; νs(SO3) 1014 cm-1. Anal. Calcd for
C44H35O6F3MoS: C, 62.40; H, 4.17. Found: C, 62.61; H, 4.04.
[WCpBz(CO)3OTf] (4). Method 1. A solution of triflic acid
(45 µL, 0.5 mmol) in dichloromethane (10 mL) was added
dropwise, at room temperature, to a solution of [WCpBz(CO)3H]
(0.392 g, 0.50 mmol) in dichloromethane (20 mL). A color change
from pale yellow to dark orange was observed. The mixture was
1
stirred for /2 h. The solvent was evaporated under vacuum, and
the orange residue obtained was extracted in diethyl ether and the
extract filtered. Concentration of the solution and cooling to -20
°C yielded a microcrystalline orange solid (0.429 g, 0.46 mmol,
yield 92%).
Method 2. A solution of triflic acid (29 µL, 0.33 mmol) in
dichloromethane was added dropwise, at -50 °C, to a solution of
[WCpBz(CO)3Me] (0.263 g, 0.33 mmol) in dichloromethane. The
mixture was slowly warmed to room temperature, and a color
change from yellow to dark orange was observed. The mixture was
stirred for 1.5 h at room temperature. Removal of the solvent under
vacuum resulted in the formation of an orange solid that was
extracted in diethyl ether; the extract was filtered, concentrated,
and cooled to -20 °C to afford orange crystals (0.285 g, 0.31 mmol,
1
yield 92%). H NMR (CD2Cl2): δ 7.15-7.07 (m, 15 H, p-C6H5,
m-C6H5), 6.84-6.81 (m, 10 H, o-C6H5), 3.80 (s, 10 H, CH2Ph).
13C NMR (CD2Cl2): δ 234.0 (CO trans OTf), 220.1 (CO), 138.1
(i-C6H5), 129.0 (o-C6H5), 128.9 (m-C6H5), 127.4 (p-C6H5), 114.9
(C5Bz5), 32.6 (CH2Ph). 19F NMR (CD2Cl2): δ -76.8 (s, CF3SO3).
IR (KBr pellet): νCt O 2050, 1968, 1947 cm-1; νas(SO3) 1346, 1195
cm-1; νs(SO3) 1009 cm-1. Anal. Calcd for C44H35O6F3WS: C,
56.64; H, 3.78; S, 3.44. Found: C, 57.38; H, 4.09; S, 3.44.
Protonation of [WCpBz(CO)3H] To Give [WCpBz(CO)3-
(H)2]OTf (7). In an NMR tube containing a frozen solution of triflic
acid (5 µL, 0.05 mmol) in CD2Cl2 a CD2Cl2 solution of
[WCpBz(CO)3H] (0,024 mg, 0.03 mmol) was added, and the tube
was sealed. The tube was placed in a bath at -80 °C and the mixture
stirred. The NMR spectra run at -80 °C showed the formation of
Experimental Section
General Procedures. All operations were performed under a
dry nitrogen atmosphere using standard Schlenk and glovebox
techniques. Solvents were previously dried with 4 Å molecular
sieves and distilled under a dry nitrogen atmosphere over CaH2
(dichloromethane and hexane) or sodium/benzophenone (diethyl
ether and toluene). Dichloromethane-d2 and toluene-d8 were dried
with 4 Å molecular sieves, degassed, and stored under dry nitrogen.
HCpBz 21
,
[MoCpBz(CO)3H],19 [WCpBz(CO)3H]],19 [MoCpBz(CO)3-
(41) Bahr, S. R.; Boudjouk, P. J. Org. Chem. 1992, 57, 5545–5547.

Ketone Hydrogenation with MoCpBz(CO)3H
Organometallics, Vol. 27, No. 18, 2008 4597
Table 3. Crystallographic Data for 3, 4, and 12
[MoCpBz(CO)3OTf] (3) [WCpBz(CO)3OTf] (4)
[MoCpBz(CO)3(OCMe2)]BF4 (12)
formula
C44H35F3O6SMo · 1/4Et2O
C44H35F3O6SW · 1/4Et2O
C46H41BF4O4Mo
840.54
150(2)
0.710 69
triclinic
j
P1
formula wt
temp (K)
wavelength (Å)
cryst syst
space group
unit cell dimens
a (Å)
854.01
941.91
150(2)
0.710 73
monoclinic
P21/c
150(2)
0.710 73
monoclinic
P21/c
10.0960(19)
17.894(4)
22.843(5)
10.0710(7)
17.8840(12)
22.7960(16)
12.661(5)
15.214(5)
21.710(7)
b (Å)
c (Å)
R (deg)
83.015(18)
89.04(2)
89.57(2)
4150(2)
ꢀ (deg)
98.102(6)
97.901(4)
γ (deg)
V (Å3)
4085.6(14)
4066.8(5)
Z
4
4
2
calcd density (Mg/m3)
1.373
0.432
1728
1.523
2.952
1856
1.345
0.376
1728
abd coeff (mm-1
F(000)
)
cryst size (mm)
θ range for data collection (deg)
limiting indices
0.40 × 0.15 × 0.15
0.30 × 0.10 × 0.02
0.24 × 0.18 × 0.04
1.45-27.99
1.45-26.40
0.94-26.51
-13 e h e 13
-23 e k e 23
-30 e l e 30
85 319/9803 (Rint ) 0.0706)
99.5 (θ ) 27.99°)
multiscan
-12 e h e 12
-21 e k e 22
-28 e l e 28
74 326/8317 (Rint ) 0.1398)
99.5 (θ ) 26.40°)
multiscan
-15 e h e 15
-19 e k e 18
-27 e l e 27
45 230/16 918 (Rint ) 0.0997)
98.3 (θ ) 26.51°)
multiscan
no. of collected/unique rflns
completeness to θ (%)
abs cor
refinement method
full-matrix least squares on F2
9803/2/520
full-matrix least squares on F2
8317/2/505
full-matrix least squares on F2
16 918/0/1009
0.940
no. of data/restraints/params
goodness of fit on F2
final R indices (I > 2σ(I))
R indices (all data)
1.093
1.081
R1 ) 0.0563, wR2 ) 0.1744
R1 ) 0.0802, wR2 ) 0.2068
1.791, -1.295
R1 ) 0.0523, wR2 ) 0.1296
R1 ) 0.1126, wR2 ) 0.1737
1.979, -1.893
R1 ) 0.0787, wR2 ) 0.1825
R1 ) 0.1707, wR2 ) 0.2114
0.589, -0.851
largest diff peak, hole (e Å-3
)
1
7 in 50% yield. H NMR (CD2Cl2, -70 °C): δ 7.17-7.06 (m, 15
H, p-C6H5, m-C6H5), 6.80-6.76 (m, 10 H, o-C6H5), 3.89 (s, 10 H,
CH2Ph), -1.96 (s, 2 H, W(H)2). 13C NMR (CD2Cl2, -70 °C): δ
215.9 (CO), 193.2 (CO cis WH2), 136.0 (i-C6H5), 128.7 (o-C6H5),
127.7 (m-C6H5), 127.4 (p-C6H5), 120.1-115.9 (CF3), 110.2 (C5Bz5),
30.6 (CH2Ph). 19F NMR (CD2Cl2, -70 °C): δ -0.48 (s, CF3SO3).
T1 and kD values (300 MHz, CD2Cl2): -80 °C, T1 ) 498 ms, kD )
31.9 (CH2Ph), 27.6 ((CH3CH2)2CHOH), 9.6 ((CH3CH2)2CHOH).
19F NMR (CD2Cl2, -50 °C): δ -76.8 (s, CF3SO3).
-
Reaction of [MoCpBz(CO)3H] (1) with Ph3CBF4 in the
Presence of 3-Pentanone. Synthesis of [MoCpBz(CO)3-
(OdCEt2)]BF4 (9). A dichloromethane (5 mL) solution of
-
Ph3C+BF4 (0.144 g, 0.44 mmol) was added to a solution of
[MoCpBz(CO)3H] (0.304 g, 0.44 mmol) and 3-pentanone (1 mL,
14 mmol) in the same solvent (5 mL). The solution immediately
turned red. After the mixture was stirred for 15 min, hexane (20
mL) was added and a precipitate formed. The solid was allowed to
settle, and the solution was filtered. The pink microcrystalline solid
obtained was dried under vacuum (0.352 g, 0.4 mmol, yield 80%).
1H NMR (CD2Cl2, 25 °C): for 9, δ 7.20-7.00 (m, 15 H, p-C6H5,
m-C6H5), 6.82-6.68 (m, 10 H, o-C6H5), 3.82 (s, 10 H, CH2Ph),
2.74-2.66 (q, 4 H, (CH3CH2)2CdO), 1.32-1.27 (t, 6 H,
(CH3CH2)2CdO); for 10, δ 7.20-7.00 (m, 15 H, p-C6H5, m-C6H5),
6.82-6.68 (m, 10 H, o-C6H5), 5.37 (br, MoClCH2Cl), 3.76 (s, 10
H, CH2Ph); for 11, δ 7.20-7.00 (m, 15 H, p-C6H5, m-C6H5),
6.82-6.68 (m, 10 H, o-C6H5), 3.71 (s, 10 H, CH2Ph). 13C NMR
(CD2Cl2, 25 °C): δ 240.0, 227.4, 226.5, 225.6 (CO), 138.0, 137.8,
137.6 (i-C6H5), 129.2, 129.0, 128.9 (o-C6H5), 128.9, 128.7, 128.6
(m-C6H5), 127.5, 127.3, 127.2 (p-C6H5), 118.0, 117.7, 117.0
(C5Bz5), 54.0 (MoClCH2Cl, 10), 38.9 ((CH3CH2)2CdO, 9), 32.3,
32.2, 32.1 (CH2Ph), 9.9 ((CH3CH2)2CdO, 9). 19F NMR (CD2Cl2,
0.19 s-1; -70 °C, T1 ) 472 ms, kD ) 0.48 s-1
.
Reaction of [MoCpBz(CO)3H] (1) with Triflic Acid in the
Presence of 3-Pentanone. In an NMR tube, a solution of 1 (25.3
mg, 0.04 mmol) and 3-pentanone (4 µL, 0.04 mmol) in dichlo-
romethane-d2 was frozen and treated with a solution of triflic acid
(3.5 µL, 0.04 mmol) in the same solvent. The tube was sealed, and
the reaction was followed by NMR from -50 °C to room
temperature. At -50 °C the 1H and 13C spectra revealed the
presence of three different molybdenum species that could be
identified as compounds 1, 3, and [MoCpBz(CO)3(Et2CHOH)]OTf
(8). The spectra also showed the resonances due to free 3-pentanone
(1H NMR (CD2Cl2, -50 °C) δ 2.64-2.57 (q, 4 H, (CH3CH2)2CO),
1.06-1.01 (t, 6 H, (CH3CH2)2CO); 13C NMR (CD2Cl2, -50 °C) δ
35.5 ((CH3CH2)2CO), 7.5 ((CH3CH2)2CO)). 1H NMR (CD2Cl2, -50
°C): for 1, δ 7.11-7.06 (m, 15 H, p-C6H5, m-C6H5), 6.85-6.67
(m, 10 H, o-C6H5), 3.82 (s, 10 H, CH2Ph), -5.19 (s, 1 H, Mo-H);
for 3, δ 7.11-7.06 (m, 15 H, p-C6H5, m-C6H5), 6.85-6.67 (m, 10
H, o-C6H5), 3.70 (s, 10 H, CH2Ph); for 8 (CD2Cl2, -10 °C), δ
7.08 (m, 15 H, p-C6H5, m-C6H5), 6.73 (m, 10 H, o-C6H5), 3.79 (s,
10 H, CH2Ph), 3.02 (m, 1 H, (CH3CH2)2CHOH), 1.67 (m, 4 H,
(CH3CH2)2CHOH), 0.96 (t, 6 H, (CH3CH2)2CHOH), 0.10 (s, 1 H,
(CH3CH2)2CHOH). 13C NMR (CD2Cl2, -50 °C): δ 244.1, 241.2,
224.8, 222.1 (CO), 139.9, 138.0, 137.3 (i-C6H5), 128.5, 128.3, 128.2,
128.0 (o,m-C6H5), 126.8, 126.7, 126.0 (p-C6H5), 121.4-117.1
(q, CF3), 116.4, 116.1, 110.9 (C5Bz5), 89.7 ((CH3CH2)2CHOH, 8),
31.7, 31.5, 31.4 (CH2Ph), 27.0 ((CH3CH2)2CHOH, 8), 9.1
((CH3CH2)2CHOH, 8). 13C NMR data for 8 (CD2Cl2, -10 °C): δ
241.7 (CO), 228.0 (CO), 137.4 (i-C6H5), 128.7 (o-C6H5), 128.3 (m-
C6H5), 127.1 (p-C6H5), 116.6 (C5Bz5), 90.6 ((CH3CH2)2CHOH),
2
-80 °C): δ -150.28 (br, BF4-), -152.97 (d, JFF ) 93.7 Hz, (µ-
2
F)10BF3, 11), -153.04 (d, JFF ) 93.8 Hz, (µ-F)11BF3, 11),
2
-338.24 (q, JFF ) 92.1 Hz, (µ-F)BF3, 11). 11B NMR (CD2Cl2,
25 °C): δ 0.06 ((µ-F)BF3, 11), -0.26 (BF4-). IR (KBr pellet): νCt
O
2055, 1975, 1954 cm-1; νOdCEt 1633 cm-1; νBF 1084, 1058, 1029
2
4
cm-1. Anal. Calcd for C48H45O4F4MoB: C, 66.37; H, 5.22. Found:
C, 66.38; H, 4.82.
-
Reaction of [MoCpBz(CO)3H] (1) with Ph3CBF4 in the
Presence of Acetone. Synthesis of [MoCpBz(CO)3(OdCMe2)]-
BF4 (12). A dichloromethane (5 mL) solution of Ph3CBF4 (0.170
g, 0.5 mmol) was added to a solution of [MoCpBz(CO)3H] (0.349
g, 0.5 mmol) and acetone (0.2 mL, 2.7 mmol) in the same solvent

4598 Organometallics, Vol. 27, No. 18, 2008
Namorado et al.
(5 mL). The solution immediately turned red. After the mixture
was stirred for 15 min, hexane (20 mL) was added and a precipitate
formed. The solid was allowed to settle, and the solution was filtered
off. The compound was recrystallized at -20 °C from diffusion of
hexane into a dichloromethane solution, yielding purple crystals
of the solvent molecule, while in complex 4 the solvent molecule
was refined isotropically. During the refinement of the structure of
compound 12 a significant solvent-accessible void was found, and
as it was not possible to determine the solvent position, SQUEEZE47
(PLATON48) was used. Data for complexes 3, 4, and 12 were
deposited with the CCDC under the deposit numbers CCDC
684304, 684305, and 684306, respectively, and can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.uk/data_request/cif.
Computational Details. All calculations reported in the text were
performed using the Gaussian 03 software package49 and the
PBE1PBE functional, without symmetry constraints. That functional
uses a hybrid generalized gradient approximation (GGA), including
25% mixture of Hartree-Fock50 exchange with DFT20 exchange
correlation, given by the Perdew, Burke, and Ernzerhof functional
(PBE).51 The optimized geometries were obtained with the LanL2DZ
basis set52-55 augmented with an f-polarization function56 for Mo
and standard 6-31G(d,p) basis sets57-61 for the remaining elements
(basis b1). Transition state optimizations were performed with the
synchronous transit-guided quasi-Newton method (STQN) devel-
oped by Schlegel et al.62,63 Frequency calculations were performed
to confirm the nature of the stationary points, yielding one imaginary
frequency for the transition states and none for the minima. Each
transition state was further confirmed by following its vibrational
mode downhill on both sides and obtaining the minima presented
on the energy profiles. A natural population analysis (NPA)30-37
and the resulting Wiberg indices28,29 were used to study the
electronic structure and bonding of the optimized species.
1
(0.397 g, 0.45 mmol, yield 90%). H NMR (CD2Cl2, 25 °C): for
12, δ 7.12-7.04 (m, 15 H, p-C6H5, m-C6H5), 6.82-6.77 (m, 10 H,
o-C6H5), 3.80 (s, 10 H, CH2Ph), 2.53 (s, 6 H, (CH3)2CdO); for 10,
δ 7.12-7.04 (m, 15 H, p-C6H5, m-C6H5), 6.82-6.77 (m, 10 H,
o-C6H5), 5.30 (br, MoClCH2Cl), 3.76 (s, 10 H, CH2Ph); for 11, δ
7.12-7.04 (m, 15 H, p-C6H5, m-C6H5), 6.82-6.77 (m, 10 H,
o-C6H5), 3.71 (s, 10 H, CH2Ph). 13C NMR (CD2Cl2, 25 °C): δ
242.3, 240.5, 234.4, 233.7 (CO), 226.6 (CO, OCMe2, 12), 226.3,
225.6 (CO), 138.0, 137.7, 137.4 (i-C6H5), 129.2, 129.0, 128.9 (o-
C6H5), 128.9, 128.8, 128.7 (m-C6H5), 127.4, 127.1 (p-C6H5), 118.0,
117.8, 117.0 (C5Bz5), 54.0 (MoClCH2Cl, 10), 34.2 ((CH3)2CdO,
12), 32.3, 32.0 (CH2Ph). 19F NMR (CD2Cl2, -80 °C): δ -149.82
(br, BF4-), -152.96 (d, 2JFF ) 93.7 Hz, (µ-F)10BF3, 11), -153.02
2
(d, 2JFF ) 93.8 Hz, (µ-F)11BF3, 11), -338.24 (q, JFF ) 93.6 Hz,
(µ-F)BF3, 11). 11B NMR (CD2Cl2, 25 °C): δ 0.49 ((µ-F)BF3, 11),
-0.23 (BF4-). IR (KBr pellet): νCt O 2057, 2036, 1978, 1958 cm-1
;
νOdCMe 1657 cm-1; νBF 1056 cm-1. Anal. Calcd for C46H41-
2
4
O4F4MoB · 0.5CH2Cl2: C, 63.11; H, 4.79. Found: C, 63.20; H, 4.49.
Catalytic Hydrogenations. [MCpBz(CO)3H] (0.0125 mmol) and
-
1 equiv of Ph3CBAr′4 (0.0125 mmol) were placed in an NMR
tube equipped with a J. Young valve, and a solution of 3-pentanone
(0.123 mmol) in CD2Cl2 (400 µL) was added. Under these
conditions the concentration of catalyst is 30 mM and the ketone
concentration is 300 mM. The NMR tube was immersed in liquid
nitrogen, evacuated, and filled with H2 at 1 atm, leading to an actual
pressure of 4 atm at room temperature. The hydrogenation reaction
The energy values presented in Figure 2 result from single-point
energy calculations using a VTZP basis set (basis b2) and the
1
proceeded at room temperature and was monitored by H NMR
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of catalyst present.
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Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
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General Procedures for X-ray Crystallography. Pertinent
details for the individual compounds can be found in Table 3.
Crystallographic data for compounds 3, 4, and 12 were collected
using graphite-monochromated Mo KR radiation (R ) 0.710 73
Å) on a Bruker AXS-KAPPA APEX II area detector diffractometer
equipped with an Oxford Cryosystem open-flow nitrogen cryostat,
and data were collected at 150 K. Cell parameters were retrieved
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on all observed reflections. Absorption corrections were applied
using SADABS.42 The structures were solved by direct methods
using either SHELXS-9743 or SIR 9744 and refined using full-matrix
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programs are included in the package of programs WINGX-version
1.64.05.45 All non-hydrogen atoms were refined anisotropically,
and all hydrogen atoms were placed in idealized positions and
refined riding on the parent carbon atom. The molecular structures
were drawn with ORTEP3 for Windows.46 3 and 4 data presented
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positions within the unit cell, which were ascribed to residual
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Ketone Hydrogenation with MoCpBz(CO)3H
Organometallics, Vol. 27, No. 18, 2008 4599
geometries optimized at the PBE1PBE/b1 level. Basis b2 consisted
of the Stuttgart/Dresden ECP with valence triple- (SDD)62-64 and
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was based on the united atom topological model applied on UAHF
radii, optimized for the HF/6-31G(d) level.
Acknowledgment. We thank the Fundac¸a˜o para a Cieˆncia
e a Tecnologia, Lisbon, Portugal, for funding (POCTI/QUI/
55744/2004, SFRH/BD/13374/2003, SFRH/BPD/26745/
2006).
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Supporting Information Available: Tables giving atomic
coordinates of all optimized species, CIF files giving X-ray
crystallographic data for compounds 3, 4, and 12 and Figure S1,
giving a free energy profile. This material is available free of charge
via the Internet at http://pubs.acs.org.
OM800379Z
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