A tungsten complex with a bidentate, hemilabile N-heterocyclic carbene ligand, facile displacement of the weakly bound W-(C=C) bond, and the vulnerability of the NHC ligand toward catalyst deactivation during ketone hydrogenation

Article abstract of DOI:10.1021/om700694e

The initial reaction observed between the N-heterocyclic carbene IMes (IMes = l,3-bis(2,4,6trimethylphenyl)imidazol-2-ylidene) and molybdenum and tungsten hydride complexes CpM(CO)₂(PPh₃)H (M = Mo, W) is deprotonation of the metal hydride by IMes, giving [(IMeS)H]⁺[CpM(CO) ₂(PPh₃)]. At longer reaction times and higher temperatures, the reaction of IMes with CpM(CO)₂(PR₃)H (M = Mo, W; R = Me, Ph) produces CpM(CO)₂(IMeS)H. Hydride transfer from CpW(CO)₂(EvIeS)H to Ph₃C⁺B(C₆F ₅)₄^- gives CpW(CO)₂(IMeS) ⁺B(C₆F₅)₄^-, which was crystallographically characterized using X-ray radiation from a synchrotron. The IMes is bonded as a bidentate ligand, through the carbon of the carbene as well as forming a weak bond from the metal to a C=C bond of one mesityl ring. The weakly bound C=C ligand is hemilabile, being readily displaced by H₂, THF, ketones, or alcohols. Reaction of CpW(CO)_2- (IMes)⁺ with H₂ gives the dihydride complex [CpW(CO)₂(IMes)(H) ₂]⁺. Addition of Et₂CH-OH to CpW(CO) ₂(IMeS)⁺B(C₆F₅)₄ ^- gives the alcohol complex [CpW(CO)₂(IMes)(Et ₂CH-OH)]⁺[B(C₆F₅)₄] ^-, which was characterized by crystallography and exhibits no evidence for hydrogen bonding of the bound OH group. Addition of H₂ to the ketone complex [CpW(CO)₂(IMes)(Et₂C=O)] ⁺[B(C₆F₅)₄]- produces an equilibrium with the dihydride [CpW(CO)₂(IMeS)(H)₂]⁺ (K_eq = 1.1 × 10³ at 25° C). The tungsten ketone complex [CpW(CO)₂(IMeS)(Et₂C=O)I+[B(C₆F ₅)₄]^- serves as a modest catalyst for hydrogenation of Et₂C=O to Et₂CH-OH in neat ketone solvent. Decomposition of the catalyst produces [H(IMes)]⁺B(C ₆F₅)₄^-, indicating that these catalysts with N-heterocyclic carbene ligands are vulnerable to decomposition by a reaction that produces a protonated imidazolium cation.

Full text of DOI:10.1021/om700694e

Organometallics 2007, 26, 5079-5090
5079
A Tungsten Complex with a Bidentate, Hemilabile N-Heterocyclic
Carbene Ligand, Facile Displacement of the Weakly Bound
W-(CdC) Bond, and the Vulnerability of the NHC Ligand toward
Catalyst Deactivation during Ketone Hydrogenation
Fan Wu,† Vladimir K. Dioumaev,† David J. Szalda,†,‡ Jonathan Hanson,† and
R. Morris Bullock*,†,§
Chemistry Department, BrookhaVen National Laboratory, Upton, New York 11973-5000, and
Chemical Sciences DiVision, Pacific Northwest National Laboratory, P.O. Box 999, K2-57,
Richland, Washington 99352
ReceiVed July 12, 2007
The initial reaction observed between the N-heterocyclic carbene IMes (IMes ) 1,3-bis(2,4,6-
trimethylphenyl)imidazol-2-ylidene) and molybdenum and tungsten hydride complexes CpM(CO)2(PPh3)H
(M ) Mo, W) is deprotonation of the metal hydride by IMes, giving [(IMes)H]+[CpM(CO)2(PPh3)]-. At
longer reaction times and higher temperatures, the reaction of IMes with CpM(CO)2(PR3)H (M ) Mo,
W; R ) Me, Ph) produces CpM(CO)2(IMes)H. Hydride transfer from CpW(CO)2(IMes)H to Ph3C+B(C6F5)4-
-
gives CpW(CO)2(IMes)+B(C6F5)4 , which was crystallographically characterized using X-ray radiation
from a synchrotron. The IMes is bonded as a bidentate ligand, through the carbon of the carbene as well
as forming a weak bond from the metal to a CdC bond of one mesityl ring. The weakly bound CdC
ligand is hemilabile, being readily displaced by H2, THF, ketones, or alcohols. Reaction of CpW(CO)2-
(IMes)+ with H2 gives the dihydride complex [CpW(CO)2(IMes)(H)2]+. Addition of Et2CH-OH to
-
CpW(CO)2(IMes)+B(C6F5)4 gives the alcohol complex [CpW(CO)2(IMes)(Et2CH-OH)]+[B(C6F5)4]-,
which was characterized by crystallography and exhibits no evidence for hydrogen bonding of the bound
OH group. Addition of H2 to the ketone complex [CpW(CO)2(IMes)(Et2CdO)]+[B(C6F5)4]- produces
an equilibrium with the dihydride [CpW(CO)2(IMes)(H)2]+ (Keq ) 1.1 × 103 at 25 °C). The tungsten
ketone complex [CpW(CO)2(IMes)(Et2CdO)]+[B(C6F5)4]- serves as a modest catalyst for hydrogenation
of Et2CdO to Et2CH-OH in neat ketone solvent. Decomposition of the catalyst produces
[H(IMes)]+B(C6F5)4 , indicating that these catalysts with N-heterocyclic carbene ligands are vulnerable
-
to decomposition by a reaction that produces a protonated imidazolium cation.
metal dihydride to a ketone is followed by hydride transfer from
a neutral metal hydride. The lifetime of the catalysts was limited
by decomposition pathways that appear to be initiated by
dissociation of a phosphine. A second generation of catalysts
was designed to suppress phosphine dissociation by connecting
the cyclopentadienyl and phosphine ligands through a two-
carbon bridge,4 and improved lifetimes and increased thermal
stability were obtained.
Introduction
Traditional homogeneous catalysts for ketone hydrogenation
use Rh or Ru as the metal and proceed by mechanisms involving
insertion of a ketone into a metal hydride bond.1 Removing the
mechanistic requirement for an insertion reaction opens the
possibility of alternative mechanisms that may be feasible using
a wider range of metals, including inexpensive alternatives to
precious metals.2 We found that phosphine-containing Mo and
N-Heterocyclic carbene (NHC) ligands5 can offer significant
advantages in comparison to phosphine ligands. Experimental
studies6,7 as well as computations7,8 have shown that NHC
W complexes [Cp(CO)2(PR3)M(OdCEt2)]+BAr′4 [R ) Me,
-
Ph, Cy; Ar′ ) 3,5-bis(trifluoromethyl)phenyl] serve as catalyst
precursors for the ionic hydrogenation of ketones.3 Mechanistic
studies show that these reactions proceed by an ionic hydro-
genation mechanism,2 in which proton transfer from a cationic
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* Address correspondence to this author at PNNL. E-mail:
morris.bullock@pnl.gov.
† Brookhaven National Laboratory.
‡ Research Collaborator at Brookhaven National Laboratory. Permanent
address: Department of Natural Sciences, Baruch College, New York, NY.
§ Pacific Northwest National Laboratory.
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10.1021/om700694e CCC: $37.00 © 2007 American Chemical Society
Publication on Web 08/28/2007

5080 Organometallics, Vol. 26, No. 20, 2007
Wu et al.
ligands generally form stronger bonds to the metal than
phosphines, thus reducing the dissociation of the ligand from
the metal. NHC ligands, like phosphines, are normally intended
to function as innocent spectator two-electron donors. NHC
ligands can become bidentate ligands by undergoing ortho-
metalation or cyclometalation9-13 or dehydrogenation reac-
tions.14 Insertion reactions15 and C-N cleavage13,16 have also
been observed in complexes with NHC ligands. These types of
reactions alter the backbone of the NHC ligand by rupturing
strong C-H or C-C bonds. In other cases, reversible weak
coordination modes, such as agostic bonding or hydrogen
bonding, have been observed for NHC ligands bearing aromatic
ligands that are not protected by steric bulk.10,17 We report
synthetic, structural, and reactivity studies on a series of
molybdenum and tungsten complexes with the widely used NHC
ligand IMes (IMes ) 1,3-bis(2,4,6-trimethylphenyl)imidazol-
2-ylidene) and an evaluation of their use in the catalytic ionic
hydrogenation of ketones.18 We describe examples of cationic
complexes where the IMes ligand functions as an overall four-
electron donor, through the expected interaction of a bond to
the carbene carbon and additionally through a weak interaction
of the metal with a CdC bond of the mesityl ring. The NHC
ligand is thus a bidentate hemilabile19 ligand. Hemilabile ligands
can be advantageous in catalysis, since the flexibility of multiple
binding modes enables the ligand to temporarily protect a vacant
coordination site. Our studies also resulted in the observation
of cleavage of the NHC ligand during catalysis, resulting in
protonated imidazolium cations. This exposes a vulnerability
of these catalysts containing NHC ligands.
Results and Discussion
Synthesis and Characterization of CpM(CO)2(IMes)H.
Neutral molybdenum and tungsten complexes CpM(CO)2-
(IMes)H were synthesized from CpM(CO)2(PR3)H (R ) Me,
Ph) by displacement of the phosphine ligand by the N-
heterocyclic carbene ligand IMes. The reaction of IMes with
CpMo(CO)2(PPh3)H at room temperature in toluene results in
the formation of a new complex that exhibits IR bands from
CO ligands at 1778 and 1700 cm-1, indicating a metal carbonyl
anion. For comparison, the CO bands of Li+[CpMo(CO)2(PMe3)]-
appear at 1786 and 1646 cm-1 in THF.20 In addition to
resonances for the Cp and PPh3 ligands, the 1H NMR spectrum
displays a singlet at δ 10.82 in C6D6 (δ 11.35 in CD2Cl2)
assigned to the CH proton of the imidazolium cation. For
comparison, the 1H NMR spectrum of [H(IMes)]+Cl- in
CD2Cl2 exhibits a resonance at δ 11.27. These spectral features
indicate that the complex is [H(IMes)]+[CpMo(CO)2(PPh3)]-,
resulting from deprotonation of the metal hydride by IMes.
Recrystallization of this complex gave an analytically pure
sample. Analogous ionic complexes were observed spectro-
scopically in the reaction of IMes with CpW(CO)2(PPh3)H and
CpMo(CO)2(PMe3)H, but were not isolated in pure form.
The deprotonation of CpMo(CO)2(PPh3)H and related metal
hydrides by the NHC can be understood by considering the
acidity of the metal hydride and the basicity of the NHC. Norton
and co-workers determined the pKa of CpW(CO)2(PMe3)H to
be 26.6 in CH3CN.21,22 The acidity of the Mo analogue was
not determined, but Mo hydrides are more acidic than those of
W; the pKa of CpMo(CO)3H in CH3CN is 13.9, versus 16.1 for
CpW(CO)3H.21 A computational study predicted the pKa values
for a series of protonated NHC ligands in CH3CN and DMSO
solvents.23 The pKa found for an analogue of protonated IMes
(with xylyl groups in the computational study rather than mesityl
groups in IMes) was about 28.2 in CH3CN and 16.8 in DMSO.
This pKa value in CH3CN shows that deprotonation of metal
hydrides such as CpMo(CO)2(PPh3)H by NHCs should readily
occur. NHC ligands with alkyl rather than aryl substituents were
more basic, with computed pKa values of their conjugate acids
around 32-34 in CH3CN and 21-24 in DMSO, in reasonable
agreement with experimental values determined in DMSO24 or
aqueous solution.25 Thus while some caution is needed in
comparing relative acidities measured in different solvents (or
extrapolated from one solvent to another), the specific values
as well as the general trends support our experimental observa-
tions of initial deprotonation of metal hydrides by NHC ligands.
Displacement of a phosphine by an N-heterocyclic carbene
ligand is commonly used as a synthetic procedure, but is
generally applied to reactions where the other ligands on the
metal are unreactive with the free NHC. Previous examples of
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Tungsten Complex with an N-Heterocyclic Carbene Ligand
Organometallics, Vol. 26, No. 20, 2007 5081
Scheme 1
displacement of a phosphine by an NHC ligand on Ru11,26 or
Rh27 hydride complexes involved metal hydrides that had two
or more phosphines. Presumably the much lower acidity of such
hydride complexes makes an initial deprotonation of the metal
hydride by the NHC less favorable.
Figure 1. ORTEP diagram of complex CpW(CO)2(IMes)H (30%
probability ellipsoids). H atoms are omitted except for the WH.
Selected bond lengths (Å) and angles (deg): W(1)-C(11) 1.936-
(6), W(1)-C(12) 1.927(5), W(1)-C(111) 2.183(5), C(11)-W(1)-
C(12) 77.7(2), C(11)-W(1)-C(111) 80.3(2), C(12)-W(1)-C(111)
109.68(19), N(112)-C(111)-W(1) 128.8(3), N(115)-C(111)-
W(1) 129.3(3). Closest W‚‚‚Cmesityl nonbonding distances (Å):
W(1)‚‚‚C(121) 3.637(5), W(1)‚‚‚C(151) 3.663(5).
The rate of conversion of ionic [H(IMes)]+[CpM(CO)2(PR3)]-
to the neutral product CpM(CO)2(IMes)H (Scheme 1) is highly
solvent-dependent. On a preparative scale, CpMo(CO)2(IMes)H
was isolated in 86% yield after heating CpMo(CO)2(PPh3)H and
IMes at 95 °C in toluene for 3 h. In more polar solvents,
however, the ionic complex is stabilized, and conversion of
[H(IMes)]+[CpMo(CO)2(PPh3)]- to CpMo(CO)2(IMes)H is
much slower. In THF-d8, 60% conversion was observed after 1
day at 95 °C. When [H(IMes)]+[CpMo(CO)2(PPh3)]- was
heated in CD3CN for 7 days at 95 °C, only 5% conversion to
CpMo(CO)2(IMes)H was detected. This remarkable solvent
effect is attributed to the stabilization of the ionic species
[H(IMes)]+[CpM(CO)2(PR3)]- by polar solvents.
While the hydride complexes CpM(CO)2(IMes)H (for both
M ) Mo and W) can be purified by recrystallization to separate
them from the PPh3 released from CpM(CO)2(PPh3)H, use of
CpM(CO)2(PMe3)H as the starting material allows the volatile
PMe3 to be readily removed under vacuum. The entire reaction
can be conveniently carried out under dynamic vacuum in the
molten phase of neat reagents, CpM(CO)2(PMe3)H and IMes,
at 95-120 °C.
We suggest that the observed proton transfer reaction is an
unproductive equilibrium and that CpM(CO)2(IMes)H forms as
the product despite the intervention of this equilibrium. An
alternate possibility is that [H(IMes)]+[CpM(CO)2(PR3)]- is an
intermediate on the pathway to formation of CpM(CO)2(IMes)H.
This mechanism would involve oxidative addition of the C-H
bond of the imidazolium cation to the tungsten, presumably
following dissociation of the PPh3 ligand to generate a vacant
site. Oxidative addition of imidazolium cations to metals has
been observed in reactions of Ni, Pd, and Pt complexes,28,29 as
well as in studies of Rh and Ir complexes,30 so there is
substantial precedent for such reactivity of late metal complexes.
On the other hand, oxidative addition pathways are less
commonly observed for tungsten complexes of the type studied
here (compared to Pd, Ir, etc.). We cannot rule out the direct
oxidative addition pathway shown by the dotted arrow in
Scheme 1.
1
The H and 13C chemical shifts of CpW(CO)2(IMes)H are
indicative of the normal bonding mode of IMes through the
NCN carbon, rather than through C-4 or C-5 of the imidazolium
ring, denoted as “abnormal” by Crabtree.31 In the 13C{1H} NMR
spectrum at 27 °C, the CO ligands of CpW(CO)2(IMes)H appear
as a slightly broadened singlet at δ 238, but at -100 °C the
two CO ligands are inequivalent, appearing at δ 247.4 and 232.3.
The four ortho-methyl groups of the IMes ligand appear as a
singlet (12 H) at 27 °C in the 1H NMR spectrum, but at -100
°C two separate singlets (6H each) are observed; a similar
situation is found in the 13C{1H} NMR spectrum. An estimate
of the OC-W-CO angle can be made from the relative areas
of the two CO bands using the equation tan2 θ ) Iasym/Isym
,
where 2θ is the OC-M-CO angle and Iasym and Isym are the
relative integrated intensities of the asymmetric and symmetric
CO bands.32 The IR spectrum of CpW(CO)2(IMes)H in
CD2Cl2 exhibits νsym(CO) at 1906 cm-1 and νasym(CO) at 1810
cm-1. The experimentally measured value of Iasym/Isym ) 0.95
leads to a predicted OC-M-CO angle of 89°, indicating that
the two CO ligands are cis to each other. This configuration
renders the metal a chiral center, in agreement with the low-
temperature NMR data.
Molecular Structures of CpW(CO)2(IMes)H and CpMo-
(CO)2(IMes)H by Single-Crystal X-ray Diffraction. The
structures of CpW(CO)2(IMes)H and CpMo(CO)2(IMes)H were
determined by X-ray diffraction and are unremarkable four-
legged piano stools (Figures 1 and 2). The carbonyls are cis, as
surmised by the NMR and IR data. The IMes ligand is
coordinated symmetrically, as indicated by very similar N-C-W
angles.
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W.; Melnikoff, A.; Stahl, R. Angew. Chem., Int. Ed. Engl. 1967, 4, 692-
693.

5082 Organometallics, Vol. 26, No. 20, 2007
Wu et al.
Figure 3. ORTEP diagram of the cation of CpW(CO)2(IMes)+-
B(C6F5)4- (W) (30% probability ellipsoids). H atoms are omitted.
Selected bond lengths (Å) and angles (deg): W(1)-C(6) 1.928-
(15), W(1)-C(7) 1.934(19), W(1)-C(11) 2.188(12), W(1)-C(121)
2.901(13), W(1)-C(122) 3.072(13), C(6)-W(1)-C(7) 77.9(6),
C(6)-W(1)-C(11) 129.5(5), C(7)-W(1)-C(11) 84.1(5), N(12)-
C(11)-W(1) 113.9(8), N(15)-C(11)-W(1) 146.6(8).
Figure 2. ORTEP diagram of complex CpMo(CO)2(IMes)H (30%
probability ellipsoids). H atoms are omitted except for the MoH.
Selected bond lengths (Å) and angles (deg): Mo1-C161 1.948-
(11), Mo1-C171 1.955(12), Mo1-C11 2.187(8), Mo2-C211
1.917(10), Mo2-C271 1.935(11), Mo2-C21 2.171(8), C161-
Mo1-C171 78.7(4), C161-Mo1-C11 104.9(3), C171-Mo1-C11
83.2(4), N12-C11-Mo1 128.6(6), N15-C11-Mo1 128.7(6),
C211-Mo2-C271 80.0(4), C211-Mo2-C21 104.9(4), C271-
Mo2-C21 82.5(4), N22-C21-Mo2 129.1(6), N25-C21-Mo2
128.5(6). Closest Mo‚‚‚Cmesityl nonbonding distances (Å): Mo1-
C121 3.638(10), Mo1-C151 3.699(10).
1
which allows for H NMR but not 13C NMR characterization.
Adducts are cleanly formed with coordinating solvents such as
THF or ketones (Vide infra).
Molecular Structure of CpW(CO)2(IMes)+. The lack of
suitable solvents frustrated attempts to obtain single crystals of
W suitable for conventional X-ray diffraction. However, use
of high-intensity X-ray radiation at the National Synchrotron
Light Source enabled a structural determination using the very
small crystals (0.010 × 0.050 × 0.100 mm for W) obtained by
slow diffusion of reactants in toluene. While the formula, CpW-
(CO)2(IMes)+, appears to indicate a 16-electron configuration
at W, the crystal structure (Figure 3) reveals that a CdC bond
of one of the mesityls forms a weak but distinct interaction to
the metal: W(1)-C(121) 2.901(13) Å and W(1)-C(122) 3.072-
(13) Å.
Hydride Transfer from CpM(CO)2(IMes)H to Ph3C+B-
(C6F5)4- to Give CpM(CO)2(IMes)+B(C6F5)4-. Hydride trans-
fer from CpM(CO)2(IMes)H to Ph3C+B(C6F5)4- gives cationic
complexes CpM(CO)2(IMes)+B(C6F5)4- (abbreviated as Mo or
W) (eq 1). The reaction is complete within minutes at 25 °C,
These bonds are much longer than those found in complexes
containing an η2-arene ligand that is not chelated; M-(CdC)
distances in η2-arene complexes are typically 2.1-2.2 Å.33 For
complexes containing η2-arenes constrained to the metal through
chelation (as is the case in W) M-C lengths are often
significantly longer, with distances as long as 2.5-2.8 Å being
found in Mo and W complexes.34
The p-orbitals of the CdC point directly at the metal in W,
as expected for this type of bonding interaction. (See Figure
S1 of the Supporting Information.) In CpW(CO)2(IMes)H, where
there is no interaction of the CdC bonds to W, the N-C-W
angles are nearly equivalent (128.8(3)° and 129(3)°; see Figure
S2a of the Supporting Information). In contrast, distortions of
the IMes ligand in W (Figure S2b of the Supporting Informa-
(33) (a) Winemiller, M. D.; Kelsch, B. A.; Sabat, M.; Harman, W. D.
Organometallics 1997, 16, 3672-3678. (b) Chin, R. M.; Dong, L.; Duckett,
S. B.; Partridge, M. G.; Jones, W. D.; Perutz, R. N. J. Am. Chem. Soc.
1993, 115, 7685-7695. (c) Tagge, C. D.; Bergman, R. G. J. Am. Chem.
Soc. 1996, 118, 6908-6915. (d) Reinartz, S.; White, P. S.; Brookhart, M.;
Templeton, J. L. J. Am. Chem. Soc. 2001, 123, 12724-12725.
(34) (a) Cobbledick, R. E.; Dowdell, L. R. J.; Einstein, F. W. B.; Hoyano,
J. K.; Peterson, L. K. Can. J. Chem. 1979, 57, 2285-2291. (b) Jeffery, J.
C.; Moore, I.; Razay, H.; Stone, F. G. A. J. Chem. Soc., Chem. Commun.
1981, 1255-1258. (c) Curtis, M. D.; Messerle, L.; D’Errico, J. J.; Solis,
H. E.; Barcelo, I. D.; Butler, W. M. J. Am. Chem. Soc. 1987, 109, 3603-
3616. (d) Shiu, K.-B.; Chou, C.-C.; Wang, S.-L.; Wei, S.-C. Organometallics
1990, 9, 286-288. (e) Howard, J. A. K.; Jeffery, J. C.; Li, S.; Stone, F. G.
A. J. Chem. Soc., Dalton Trans. 1992, 627-634. (f) Shiu, K.-B.; Yeh, L.-
Y.; Peng, S.-M.; Cheng, M.-C. J. Organomet. Chem. 1993, 460, 203-211.
(g) Cheng, T.-Y.; Szalda, D. J.; Bullock, R. M. J. Chem. Soc., Chem.
Commun. 1999, 1629-1630.
and the analytically pure product precipitates from toluene in
high yield. IR bands for the CO ligands appear at much higher
frequencies than those for the neutral hydrides. Bands for Mo
(Nujol) appear at 1999 and 1905 cm-1, while those for W appear
at 1980 and 1890 cm-1. Spectroscopic characterization of W is
complicated by its reactivity and solubility characteristics, as it
is insoluble in hexane or toluene, and reacts with more polar
solvents. Prompt decomposition to unidentified products occurs
when W is dissolved in CD2Cl2. Solutions of W in CF3Ph are
stable for longer times (roughly 10% decomposition in 22 h),

Tungsten Complex with an N-Heterocyclic Carbene Ligand
Organometallics, Vol. 26, No. 20, 2007 5083
tion) provide further indications of the bonding interaction: the
N(12)-C(11)-W(1) angle is compressed to 113.9(8)°, and the
N(15)-C(11)-W(1) angle is expanded to 146.6(8)°.
resonances (ν1/2 ≈ 230 Hz) are observed. Further sharpening
of the inequivalent hydride resonances occur as the temperature
is lowered, and at -100 °C two distinct resonances appear at δ
1.19 and -2.97. In the 13C NMR spectrum at -100 °C, the
two CO ligands are inequivalent (δ 205.2 and 203.1), as are
the ortho-methyl groups on the mesityl fragment. These features
indicate a lack of symmetry in the dihydride. The activation
parameters of the fluxional process were determined by line
shape analysis of the hydride resonances (∆Hq ) 9.3 ( 0.3
kcal mol-1 and ∆Sq ) -2 ( 1 cal mol-1 K-1, measured over
the range -100 to 27 °C). The low entropy of activation is
indicative of an intramolecular mechanism, but these data are
insufficient to deduce the exact nature of the process. Thus,
the enthalpy barrier value is consistent with several plausible
dynamic processes, such as a hindered rotation of the bulky
IMes ligand, a turnstile rotation of the W(CO)(H)2 or W(CO)2H
tripods, or even more elaborate mechanisms involving transient
formation of dihydrogen complexes.37 A cis-dicarbonyl con-
figuration of WH2 is assigned on the basis of the relative
intensity of the two carbonyl IR bands (Iasym/Isym ) 1.1;
predicted OC-W-CO angle 93°) The structure suggested in
eq 2 is consistent with the data that require inequivalent hydrides
and cis carbonyls. This geometry, with one hydride “trans” to
the Cp ligand, is analogous to that determined in the crystal
structure of [CpW(CO)2(PMe3)(H)2]+[OTf]-,38 but other ge-
ometries cannot be ruled out on the basis of the spectral data.
Formation of 18e- Adducts of [CpM(CO)2(IMes)]+-
[B(C6F5)4]- with THF, Et2CdO, and Et2CH-OH. The
weakly bound CdC ligand in [CpM(CO)2(IMes)]+ is readily
displaced by oxygen donor ligands (THF, ketones, alcohols) to
form the adducts [CpM(CO)2(IMes)(THF)]+[B(C6F5)4]- (Mo-
(THF) or W(THF)), [CpM(CO)2(IMes)(Et2CdO)]+[B(C6F5)4]-
(Mo(Et2CdO) or W(Et2CdO), and ([CpW(CO)2(IMes)(Et2-
CH-OH)]+[B(C6F5)4]- (W(Et2CHOH)) (eq 3). The metal
carbonyl IR ν(CO) bands indicate adduct formation: 1977 and
1882 cm-1 for [CpM(CO)2(IMes)(THF-d8)]+[B(C6F5)4]-, Mo-
(THF-d8), and 1962 and 1859 cm-1 for W(THF-d8), indicating
more electron density on the metal than in Mo or W, but less
than in CpM(CO)2(IMes)H (1918 and 1843 cm-1 for CpMo-
(CO)2(IMes)H in THF-d8; 1913 and 1822 cm-1 for CpW(CO)2-
(IMes)H). The relative intensities of the CO bands are consistent
with cis orientation of the carbonyls.
The electronic unsaturation in W results in distortion of the
IMes ligand and bending of the mesityl substituent toward the
metal for coordination. In contrast, other reported examples of
electronic unsaturation in metal complexes with IMes ligands
exhibit tilt of the mesityl groups away from the metal. Thus
computations35 on ruthenium complexes involved in the olefin
metathesis catalysts developed by Grubbs and co-workers36
indicate that the 14-electron species (NHC)Cl2RudC(H)Ph
(formed by loss of PCy3 from (NHC)(PCy3)Cl2RudC(H)Ph)
has a characteristic tilt of mesityl groups away from the metal
and alkylidene ligand to release steric pressure.
The methyl group of the interacting CdC fragment in W is
bent out of the plane of the mesityl ring by 13.4° and away
from the metal, in accord with the change in hybridization. As
previously reported,18 DFT computations also provide support
for the bonding of the CdC bond of the mesityl ring to the
metal for both Mo and W. The methyl substituent bonded to
the CdC is displaced out of plane of the mesityl ring in the
computed structures (8.2° for W and 7.2° for Mo), providing
convincing evidence of bonding.
The solid-state structure of Mo was also determined from
synchrotron data. It appears to be similar to that of W, but with
shorter distances from the metal to the coordinated CdC (2.77-
(3) and 3.02(3) Å). Unfortunately, the quality of the structural
determination for Mo is marginal, so detailed structural
comparisons are not warranted. Further information on the
structure determination is provided in the Supporting Informa-
tion.
Synthesis and Characterization of the Dihydride Complex
[CpW(CO)2(IMes)(H)2]+[B(C6F5)4]-. Reaction of H2 with a
suspension of W in toluene produces the dihydride complex
[CpW(CO)2(IMes)(H)2]+[B(C6F5)4]- (WH2) (eq 2). The car-
bonyl IR bands of WH2 appear at 2065 and 2006 cm-1 in C6D6
solution, with the higher frequency compared to W reflecting
the higher oxidation state of WH2. At 27 °C, a singlet is
The ketone complex [CpW(CO)2(IMes)(Et2CdO)]+[B(C6F5)4]-
(W(Et2CdO)) is reasonably stable in solution but leaches
ketone upon attempted recrystallization or washing with hy-
drocarbon solvents. Broadened resonances are observed in the
1H NMR spectrum in C6D6 for the CH2 and CH3 groups of the
ketone in [CpW(CO)2(IMes)(Et2CdO)]+[B(C6F5)4]-, indicative
of exchange of the ketone on and off the metal.
When Et2CHOH is added to a suspension of W in benzene,
a purple solution is formed. After 2 days, purple crystals of
alcohol complex W(Et2CHOH) are deposited (eq 3). In the IR
spectrum (KBr), the ν(O-H) stretch is observed as a weak,
broad band at 3420 cm-1. The crystals were too small (0.050
× 0.050 × 0.005 mm) for a structure determination by
conventional X-ray crystallography, so the structure was
determined by single-crystal X-ray diffraction using the high-
intensity X-ray radiation at the National Synchrotron Light
observed for the two hydrides at δ -0.72 in THF-d8 solvent.
This resonance broadens at lower temperatures, with ν1/2 ≈ 138
Hz at -18 °C. At -39 °C the resonance is nearly coalesced
into the baseline (ν1/2 ≈ 900 Hz), and at -60 °C separate broad
(36) (a) Sanford, M. S.; Ulman, M.; Grubbs, R. H. J. Am. Chem. Soc.
2001, 123, 749-750. (b) Sanford, M. S.; Love, J. A.; Grubbs, R. H. J. Am.
Chem. Soc. 2001, 123, 6543-6554.
(37) Ryan, O. B.; Tilset, M.; Parker, V. D. J. Am. Chem. Soc. 1990,
112, 2618-2626.
(38) Bullock, R. M.; Song, J.-S.; Szalda, D. J. Organometallics 1996,
15, 2504-2516.
(35) Cavallo, L. J. Am. Chem. Soc. 2002, 124, 8965-8973.

5084 Organometallics, Vol. 26, No. 20, 2007
Wu et al.
Source. As shown in Figure 4, the alcohol ligand is cis to the
IMes ligand. The W-O bond length of 2.250(7) Å is longer
than the W-O bond length of 2.176(8) Å found in [Cp(CO)3W-
provided strong evidence of hydrogen bonding of the methanol
ligandtotheClO4- anionintherheniumcomplex[ReH(CO)(NO)-
(PPh3)2(MeOH)]+ClO4 reported by Grundy and Robertson.41
-
(HOiPr)]+OTf-.39 In the H NMR spectrum of W(Et2CHOH)
Bochmann and co-workers showed42 from a crystal structure
that Et2O forms a hydrogen bond to the bound isopropyl alcohol
ligand in [Cp2Zr(OiPr)(HOiPr)]+‚OEt2. In contrast, Gladysz and
co-workers concluded that their spectroscopic data did not
1
in C6D6, the resonance for the OH of the bound alcohol appears
at δ 0.62. The OH resonance for the free alcohol (0.037 M in
C6D6) appears at δ 0.67, indicating that coordination of the
alcohol to the metal results in a very small perturbation of its
chemical shift. We interpret this to indicate that there is little
or no hydrogen bonding of the bound alcohol to any proton
acceptor sites. We find no evidence for significant hydrogen
bonding of the OH of the bound alcohol in the solid state of
W(Et2CHOH). The shortest O‚‚‚F separation was 4.355 Å
[from O(3) to F(85)], which is much too long of a distance to
be considered for a hydrogen-bonding interaction. In contrast,
most previously reported alcohol ligands engage in hydrogen
bonding, as indicted by structural studies, as well as significant
provide strong evidence for hydrogen bonding in the rhenium
- 43
alcohol complexes [CpRe(NO)(PPh3)(ROH)]+BF4
.
The adduct complexes readily convert into each other when
two oxygen donors are present simultaneously. Thus, 1H NMR
signals of the THF complex W(THF) and ketone complex W-
(Et2CdO) are broad in C6D6 but are sharp in neat THF or
ketone. This behavior is consistent with a facile dissociation of
the oxygen donor ligand and suppression of such dissociation
by a large excess of ligand.
The 13C{1H} NMR spectrum of W(THF-d8) in THF-d8 at
-40 °C indicates inequivalent CO ligands (δ 247.1, 246.2). In
its cis configuration the metal is chiral and the two CO ligands
are expected to be inequivalent. Inequivalent resonances for the
CHdCH vinyl carbons (δ 128.4, 126.6) are observed in the
13C{1H} NMR. In addition, the two sides of the IMes ligand
are inequivalent, exhibiting four separate resonances for the
1
downfield shifts in the H NMR spectrum of the bound OH
peak. The crystal structure of [Cp(CO)3W(HOiPr)]+OTf- 39
shows that the OH ligand is strongly hydrogen bonded to an
oxygen of the triflate anion in the solid state, as evidenced
by a short O‚‚‚O distance of 2.63(1) Å. Evidence that the
O-H‚‚‚O hydrogen bonding persists in solution comes from
thechemicalshiftofδ7.34oftheOHof[Cp(CO)3W(HOiPr)]+OTf-
1
ortho-CH3 groups on the mesityl rings in both the H NMR
1
in the H NMR spectrum in CD2Cl2 solution, with the large
spectrum at -30 °C and the 13C{1H} NMR spectrum at -40
°C. These spectroscopic data indicate restricted rotation about
the W-C bond of the NHC ligand, as well as restricted rotation
about the N-C(ipso mesityl) bond. While in many cases NHC
ligands rotate freely about the M-C bond, several previous
examples of restricted rotation for NHC ligands were observed,
with steric hindrance often being cited as the cause.12,44
Equilibria for Displacement of the Ketone or THF Ligand
by H2. Addition of Et2CdO to the tungsten cation W led to
the immediate formation of the ketone complex [CpW(CO)2-
(IMes)(Et2CdO)]+[B(C6F5)4]- (W(Et2CdO); 0.047 M). When
shift downfield compared to the free alcohol being indicative
of hydrogen bonding. Beck and co-workers reported the crystal
-
structure of [Cp(CO)(PPh3)Ru(EtOH)]+BF4
,
which had
-
O-H‚‚‚F hydrogen bonding of the alcohol to the BF4 ligand
and an O‚‚‚F distance of 2.66 Å.40 Spectroscopic evidence
1
H2 (4 atm) was added to this NMR tube, a H NMR spectrum
recorded at 0 °C showed that 58% of the tungsten complex had
been converted to the dihydride complex WH2 (eq 4). The
concentration of H2 dissolved in solution was 9 mM; the
integration of H2 was corrected for the fact that 25% of the H2
is para-H2, which is NMR-silent. The equilibrium constant
determined under these conditions was Keq ) 1.4 × 103. The
NMR tube was warmed to 25 °C, and the Keq determined at
(39) (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.
(40) Milke, J.; Missling, C.; Su¨nkel, K.; Beck, W. J. Organomet. Chem.
1993, 445, 219-227.
Figure 4. ORTEP of [CpW(CO)2(IMes)(Et2CH-OH)]+ (30%
probability ellipsoids). H atoms are omitted except for the OH.
Selected bond lengths (Å) and angles (deg): W(1)-C(6) 1.945-
(14), W(1)-C(7) 1.966(12), W(1)-O(3) 2.250(7), C(6)-W(1)-
C(7) 73.3(5), C(6)-W(1)-C(11) 111.9(4), C(7)-W(1)-C(11)
77.3(4).
(41) Grundy, K. R.; Robertson, K. N. Inorg. Chem. 1985, 24, 3898-
3903.
(42) Farrow, E.; Sarazin, Y.; Hughes, D. L.; Bochmann, M. J. Orga-
nomet. Chem. 2004, 689, 4624-4629.
(43) Agbossou, S. K.; Smith, W. W.; Gladysz, J. A. Chem. Ber. 1990,
123, 1293-1299.

Tungsten Complex with an N-Heterocyclic Carbene Ligand
Organometallics, Vol. 26, No. 20, 2007 5085
Scheme 2
this temperature was 1.1 × 103. The Keq measured after 1 h at
each of these temperatures was the same within experimental
uncertainty. Even though the magnitude of the Keq indicates a
strong thermodynamic preference for formation of the dihydride
from the ketone complex, both species are present under these
conditions, due to the much lower concentration of H2 (9 mM)
compared to Et2CdO solvent (9.4 M).
Table 1. Hydrogenation of Neat Et2CdO with 0.34 mol %
of Catalyst
catalyst precursor
T, °C
pressure, atm
time, h
TONa
W
W
W
W
W
W
W
W
Mo
Mo
Mo
Mo
23
23
50
50
23
23
50
50
23
23
50
50
4
4
4
24
238
23
164
24
240
24
168
24
2.1 (0)
10.0 (0)
15.1 (0.4)
29.9 (0.7)
7.8 (0.2)
86.0 (6.0)
15.9 (3.8)
60.9 (12.6)
0.9 (0)
4
54.4
54.4
54.4
54.4
4
4
4
4
The dihydride WH2 is also favored over W(THF); Keq
)
[WH2][THF-d8]/[W(THF-d8)][H2] ≈ 3 × 103 in THF-d8 at 298
K. Solutions of W(THF-d8) eventually become viscous and turn
into a gel, likely due to ring opening and oligomerization of
the THF-d8 solvent.
240
24
240
0.9 (0)
0.8 (0)
1.0 (0)
Addition of Et2CdO to a solution of [CpW(CO)2(IMes)(Et2-
CH-OH)]+[B(C6F5)4]- in C6D6 results in displacement of the
alcohol by the ketone and formation of [CpW(CO)2(IMes)-
(Et2CdO)]+[B(C6F5)4]-. A lower limit of Keq > 100 was
estimated for the equilibrium Keq ) [W(Et2CdO)+][Et2CH-
OH]/[W(Et2CH-OH)+][Et2CdO], based on an estimated NMR
detection limit of 3%. These experiments establish the relative
binding strength to [CpW(CO)2(IMes)]+ as (H)2 > Et2CdO >
Et2CH-OH.
Catalytic Hydrogenation of Et2CdO. Modest activity for
the hydrogenation of Et2CdO (eq 5) is obtained using com-
plexes W as the catalyst precursor, which is readily converted
into the ketone adduct W(Et2CdO) under these conditions.
a Turnover number (TON) is the total number of moles of 3-pentanone
hydrogenated per mole of catalyst. It includes TON for the formation of
the direct hydrogenation product, 3-pentanol, and the secondary condensation
product, (Et2CH)2O. Each equivalent of the ether is counted as two turnovers
of the catalyst, since it is formed from 2 equiv of the alcohol. The number
in parentheses is a TON for the ether alone.
10% decomposition is observed in 1 day at 23 °C and 30% in
10 days. At 50 °C, 30% decomposition occurs in 1 day and
>97% of the catalyst has decomposed after 7 days. The catalytic
activity of Mo (∼1 turnover in 1 day at 23 °C, 4 atm H2) is
lower than that of W, in contrast to the trend observed for
phosphine-containing catalysts CpM(CO)2(PR3)(Et2CdO)+, where
the Mo catalysts were notably more active than the W
analogues.3,4 Complex Mo is less stable under hydrogenation
conditions. Since only one turnover was obtained, strictly
speaking, the Mo complex is not a catalyst.
The proposed mechanism of ionic hydrogenation of ketones
by Mo or W catalysts (Scheme 2) involves displacement of a
bound ketone by H2 as the first step (eq 4, discussed above),
followed by proton and hydride transfer reactions from metal
hydride complexes, as previously outlined3,4 for Mo and W
catalysts reported earlier.
Catalytic hydrogenation of neat Et2CdO by W (0.34 mol %)
at 23 °C gives 2.1 turnovers in 1 day at 4 atm H2 (see Table 1).
At higher pressure (54.4 atm H2), 7.8 turnovers are found after
1 day, and a maximum of 86 turnovers are detected after 10
days. Higher activity is observed at 50 °C (15.1 turnovers in 1
day at 4 atm H2), but the lifetime of the catalyst decreases. Thus,
Catalyst Deactivation and Formation of [H(IMes)]+.
Vulnerability of Metal-NHC Catalysts. Decomposition of
the molybdenum and tungsten catalysts was found to produce
multiple metal species, but only one species containing IMes,
as judged by the 1H NMR spectrum. The single product
containing the IMes entity was isolated and identified as
[H(IMes)]+B(C6F5)4- (cf. Scheme 1). It was probed for activity
under ionic hydrogenation conditions, but proved to be inactive.
The formation of the protonated form of IMes highlights a
susceptibility of these catalysts to decomposition. Our experi-
ments do not distinguish a reductive elimination process that
(44) (a) Doyle, M. J.; Lappert, M. F. J. Chem. Soc., Chem. Commun.
1974, 679-680. (b) Herrmann, W. A.; Goossen, L. J.; Spiegler, M. J.
Organomet. Chem. 1997, 547, 357-366. (c) Weskamp, T.; Schattenmann,
W. C.; Spiegler, M.; Herrmann, W. A. Angew. Chem., Int. Ed. 1998, 37,
2490-2493. (d) Enders, D.; Gielen, H. J. Organomet. Chem. 2001, 617-
618, 70-80. (e) Chianese, A. R.; Li, X.; Janzen, M. C.; Faller, J. W.;
Crabtree, R. H. Organometallics 2003, 22, 1663-1667. (f) Dinger, M. B.;
Nieczypor, P.; Mol, J. C. Organometallics 2003, 22, 5291-5296. (g) Silva,
L. C.; Gomes, P. T.; Veiros, L. F.; Pascu, S. I.; Duarte, M. T.; Namorado,
S.; Ascenso, J. R.; Dias, A. R. Organometallics 2006, 25, 4391-4403.

5086 Organometallics, Vol. 26, No. 20, 2007
Wu et al.
forms the imidazolium cation from other possible mechanisms.
Mechanistic studies would be complicated by the low concen-
tration of the metal complexes under catalytic conditions,
together with the formation of more than one metal-containing
decomposition product.
pentamethylimidazolium and Pd(0) products.49 Computational
studies have explored the geometrical influences on the reductive
elimination50 and the effect of electronic and steric factors on
the barrier for elimination.51 Attempted catalytic olefin hydro-
formylation using Co2(CO)6(IMes)2 led to a decomposition
product thought to be [H(IMes)]+[Co(CO)4]-.52
In our earlier studies on catalytic reactions using [Cp(CO)2-
-
(PR3)M(OdCEt2)]+BAr′4 as catalyst precursors for the ionic
Conclusions. The NHC-substituted Mo and W hydrides
CpM(CO)2(IMes)H are synthesized in good yield by a displace-
ment of the phosphine in CpM(CO)2(PR3)H by IMes. In most
cases the initially observed reactivity leads to ionic complexes,
[H(IMes)]+[CpM(CO)2(PR3)]-, arising from deprotonation of
the metal hydride by the highly basic free NHC. Hydride
abstraction from CpW(CO)2(IMes)H gives CpW(CO)2(IMes)+-
B(C6F5)4-, in which the IMes ligand is a hemilabile bidentate
ligand, with one CdC bond of the arene weakly bonding to the
metal. Ketones or alcohols readily displace the weak CdC bond,
producing [CpW(CO)2(IMes)(Et2CdO)]+ or [CpM(CO)2(IMes)-
(Et2CH-OH)]+. Displacement of the ketone or alcohol by H2
leads to the dihydride complex [CpW(CO)2(IMes)(H)2]+. These
W complexes are catalyst precursors for the hydrogenation of
neat Et2CdO, but activities are modest, and decomposition of
the catalyst occurs. The product of the catalyst decomposition
is [H(IMes)]+, resulting from protonation of the NHC ligand,
exposing a vulnerability of the NHC ligand in this catalytic
reaction.
hydrogenation of ketones3 decomposition to phosphonium
cations, HPR3+, was identified as a catalyst deactivation
pathway. The higher binding strength of NHC ligands versus
phosphines was a primary motivation for comparing the
reactivity of these new NHC complexes to those with phosphine
ligands. More research will be needed to tune the stability
properties of these catalysts for hydrogenations, to decrease the
susceptibility to proton-mediated decomposition. In contrast to
these hydrogenations, hydrosilylation of ketones catalyzed by
W(Et2CdO) gives much longer catalyst lifetimes.45 In the case
of hydrosilylation of aliphatic ketones, a readily recyclable
catalyst is obtained that produces over 1000 total turnovers.
Comparison of the catalyst lifetimes for hydrogenation versus
hydrosilylation is complicated by the different conditions being
used, as the hydrosilylation reactions were all conducted at room
temperature, whereas many of the hydrogenations were carried
out at 50 °C. Some of the enhanced stability for hydrosilylation
may be due to more catalyst decomposition occurring at the
higher temperatures employed in the hydrogenation reactions.
The longer lifetime of the hydrosilylation catalysts may also
be related to the reactivity of the silyl hydride complex [CpW-
(CO)2(IMes)(SiEt3)H]+ compared to the dihydride complex
WH2. Further studies on the chemical properties and acidities
of these complexes would be needed to better understand the
differences in catalyst stability.
Experimental Section
All manipulations were performed in Schlenk-type glassware on
a dual-manifold Schlenk line or in an argon-filled Vacuum
Atmospheres glovebox. NMR spectra were obtained on a Bruker
1
Avance 400 FT NMR spectrometer (400 MHz for H). All NMR
spectra were recorded at 25 °C unless stated otherwise. Chemical
1
shifts for H and 13C NMR spectra were referenced using internal
Previously reported metal-NHC complexes demonstrated a
wide range of reactivity (or lack thereof) with acids. For
example, Haynes and co-workers found that the Rh-C bond
of Rh(I)(NHC)(CO) was cleaved by HCl.46 In contrast, Crabtree,
Faller, and co-workers found that HOAc did not induce any
Pd-C bond cleavage of a Pd(NHC) complex, even after 16 h
at 55 °C.29 Herrmann and co-workers reported Pd(II) catalysts
for the activation of methane. Their catalytic reaction was carried
out at 80-100 °C in CF3CO2H, using a Pd complex that had a
chelating bis-(NHC)carbene ligand that demonstrates remarkable
stability toward acid.47
solvent resonances and are reported relative to tetramethylsilane.
External standards of trifluorotoluene (set as δ ) -63.73) and 85%
H3PO4 (set as δ ) 0) were used for referencing 19F and 31P NMR
spectra. 13C{1H} and 31P{1H} NMR spectra were recorded with
broadband 1H decoupling unless stated otherwise. For quantitative
1H NMR measurements the relaxation delay was set at 30 s. NMR
measurements performed in non-deuterated solvents (e.g., Et2Cd
O) contained sealed capillaries of deuterated solvents (CD2Cl2), the
spectrometer was locked on the deuterated solvent, and the chemical
shift reference used the deuterated solvent. The chemical shifts
referenced against standards in sealed capillaries can be substantially
different (ca. 1 ppm) from the values obtained by conventional
referencing against standards dissolved in the bulk of the sample.
Large solvent peaks for the resonances of the solvent obscure some
parts of the spectrum from 1 to 3 ppm, but pertinent regions of the
spectrum (e.g., Cp, vinyl, and aromatic resonances) can be reliably
observed and integrated. GC-MS spectra were recorded on an
Agilent Technologies 5973 mass selective detector connected to
an Agilent Technologies 6890N gas chromatograph equipped with
an HP-5ms column (5% phenyldimethylpolysiloxane). Infrared
spectra were recorded on a Mattson Polaris or ThermoNicolet FTIR
spectrometer. Elemental analyses were performed by Schwarzkopf
Microanalytical Laboratory, Inc. (Woodside, NY) or Atlantic
Microlab, Inc. (Norcross, GA).
Recent studies have provided an improved understanding of
the ways in which metal complexes with NHC ligands decom-
pose.48 Examples in which alkyl groups on NHC ligands react
(often by C-H activation pathways) were mentioned in the
Introduction. Such pathways alter the structure of the NHC
ligand, but generally leave the M-C bond of the NHC intact.
The decomposition pathway we found is less common, resulting
in loss of the NHC ligand from the metal. Related reactions
that produce imidazolium cations have been reported. Cavell
and co-workers have reported kinetic and computational studies
on the reactivity of Pd NHC complexes. They reported that
-
[Pd(CH3)(tmiy)(PR3)2]+BF4 complexes (tmiy ) 1,3,4,5-tet-
Hydrocarbon solvents were dried over Na/K-benzophenone.
Benzene-d6 was dried over Na/K. 3-Pentanone was dried over CaH2.
ramethylimidazole-2-ylidene) decompose by a reductive elimi-
nation reaction that follows first-order kinetics to produce
(49) McGuinness, D. S.; Saendig, N.; Yates, B. F.; Cavell, K. J. J. Am.
Chem. Soc. 2001, 123, 4029-4040.
(45) Dioumaev, V. K.; Bullock, R. M. Nature 2003, 424, 530-532.
(46) Martin, H. C.; James, N. H.; Aitken, J.; Gaunt, J. A.; Adams, H.;
Haynes, A. Organometallics 2003, 22, 4451-4458.
(47) Muehlhofer, M.; Strassner, T.; Herrmann, W. A. Angew. Chem.,
Int. Ed. 2002, 41, 1745-1747.
(50) Graham, D. C.; Cavell, K. J.; Yates, B. F. Dalton Trans. 2005,
1093-1100.
(51) Graham, D. C.; Cavell, K. J.; Yates, B. F. Dalton Trans. 2006,
1768-1775.
(52) van Rensburg, H.; Tooze, R. P.; Foster, D. F.; Slawin, A. M. Z.
Inorg. Chem. 2004, 43, 2468-2470.
(48) Crudden, C. M.; Allen, D. P. Coord. Chem. ReV. 2004, 248, 2247-
2273.

Tungsten Complex with an N-Heterocyclic Carbene Ligand
Organometallics, Vol. 26, No. 20, 2007 5087
CF3Ph was vacuum transferred from LiAlH4. H2 was used as
received. 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes),53
CpMo(CO)2(PPh3)H,54 CpMo(CO)2(PMe3)H,55 CpW(CO)2(PPh3)H,54
and CpW(CO)2(PMe3)H55 were synthesized according to the
2.38 (s, 6H, p-Me-Mes), 2.19 (s, 12H, o-Me-Mes). 31P NMR (THF-
d8): δ 87.9 (s, PPh3). 1H NMR (CD3CN): δ 8.73 (br s, 1H, HCN2),
7.71 (s, 2H, dCH), 7.51 (m, 6H, PPh), 7.22 (m, 9H, PPh), 7.17
(s, 4H, m-H-Mes), 4.67 (s, 5H, Cp), 2.38 (s, 6H, p-Me-Mes), 2.13
(s, 12H, o-Me-Mes). 31P NMR (THF-d8): δ 89.0 (s, PPh3). IR
literature procedures. A sample of Ph3C+B(C6F5)4 was donated
-
data: IR (C6D6): νsym(CO) 1778 (vs), νasym(CO) 1700 (vs) cm-1
,
by Albemarle Corporation.
I
ν
asym/Isym ) 1.20 (predicted OC-W-CO angle 95°). IR (THF-d8):
Simulations of the dynamic NMR spectra were carried out using
the gNMR software package (v3.6.5 for Macintosh, Cherwell
Scientific Publishing Limited). The rates of exchange as a function
of temperature were determined from visual comparison of the
experimental and simulated spectra. The errors in the rate constants
of ca. 10% were estimated on the basis of subjective judgments of
the sensitivity of the fits to changes in the rate constants. The
temperature of the NMR probe was calibrated using methanol.56
The activation parameters and their uncertainties were calculated
using KINPAR, a Macintosh computer program provided by Prof.
Jack Norton (Columbia University).
sym(CO) 1785 (vs), νasym(CO) 1707 (vs) cm-1, Iasym/Isym ) 1.18
(predicted OC-W-CO angle 94°).
Observation of CpMo(CO)2(PMe3)-[H(IMes)]+ in the Reac-
tion of CpMo(CO)2(PMe3)H with IMes. CpMo(CO)2(PMe3)H (2.9
mg, 0.01 mmol), IMes (3.1 mg, 0.01 mmol), and 0.6 mL of C6D6
were placed in an NMR tube equipped with a Teflon valve. The
light yellow solids dissolved to produce a dark purple solution. 1H
NMR spectrum acquired after 5 min at room temperature showed
ca. 40% of the starting CpMo(CO)2(PMe3)H, ca. 20% of CpMo-
(CO)2(IMes)H + free PMe3, and ca. 40% of a new product,
CpMo(CO)2(PMe3)-[H(IMes)]+. H NMR (C6D6): δ 11.53 (br s,
1H, HCN2), 6.76 (s, 4H, m-H-Mes), 6.29 (br s, 2H, dCH), 4.92
(br s, 5H, Cp), 2.12 (s, 6H, p-Me-Mes), 2.11 (s, 12H, o-Me-Mes),
1.38 (d, 2JHP ) 7 Hz, 9H, PMe). 31P NMR (C6D6): δ 29.4 (s, PMe3).
1
Synthesis of cis-CpMo(CO)2(IMes)H from CpMo(CO)2-
(PPh3)H. In a glovebox CpMo(CO)2(PPh3)H (480.0 mg, 1.000
mmol), IMes (306.0 mg, 1.000 mmol), and 10 mL of toluene were
placed in a glass tube equipped with a Teflon valve. The light
yellow solids dissolved to produce a dark purple solution, and a
new light yellow precipitate formed almost immediately. The tube
was heated at 95 °C for 3 h. The product was recrystallized from
toluene-hexanes (1:3) to yield 449 mg (86%) of pure CpMo(CO)2-
Synthesis of CpW(CO)2(IMes)H from CpW(CO)2(PMe3)H.
CpW(CO)2(PMe3)H (346.0 mg, 0.900 mmol), IMes (275.0 mg,
0.900 mmol), and toluene (1 mL) were placed in a glass tube
equipped with a Teflon valve. The light yellow solids dissolved to
produce a dark purple solution, then a new light yellow precipitate
formed almost immediately. The volatiles were removed under
vacuum, and the residue was heated under vacuum for 10 min at
120 °C to remove free phosphine and reestablish the equilibrium
in favor of CpW(CO)2(IMes)H. The product was recrystallized from
toluene-hexanes (1:1) to yield pure CpW(CO)2(IMes)H (416 mg,
76%) as light yellow crystals with 0.5 equiv of crystallization
1
(IMes)H as light yellow crystals. H NMR (THF-d8): δ 7.16 (s,
2H, dCH), 7.02 (s, 4H, m-H-Mes), 4.62 (s, 5H, Cp), 2.34 (s, 6H,
p-Me-Mes), 2.09 (s, 12H, o-Me-Mes), -4.73 (s, 1H, MoH). 13C
NMR (THF-d8; some assignments were aided by recording 13C APT
2
(attached proton test) experiments): δ 243.3 (d, JCH ) 11 Hz,
2
Mo-CO), 200.2 (d, JCH ) 12 Hz, NCN), 139.5 (m, i-Mes), 139.2
2
2
(q, JCH ) 6 Hz, p-Mes), 136.9 (q, JCH ) 6 Hz, o-Mes), 130.0
1
1
2
(dm, JCH ) 156 Hz, m-Mes), 124.3 (dd, JCH ) 196 and JCH
)
1
solvent (C6H5CH3) per W detected by NMR. H NMR (C6D6): δ
1
12 Hz, )CH), 89.0 (d of quintets, JCH ) 174 and JCH ) 6 Hz,
6.80 (s, 4H, m-H-Mes), 6.19 (s, 2H, dCH), 4.60 (s, 5H, Cp), 2.12
Cp), 21.2 (qt, 1JCH ) 126 and 3JCH ) 4 Hz, p-Me-Mes), 18.8 (qm,
(s, 6H, p-Me-Mes), 2.10 (s, 12H, o-Me-Mes), -5.93 (s, 1JWH ) 45
1JCH ) 128 Hz, o-Me-Mes). IR (THF-d8): νsym(CO) 1918 (vs), νasym
-
1
Hz, 1H, WH). H NMR (THF-d8, -100 °C): δ 7.40 (s, 2H, d
(CO) 1843 (vs) cm-1, Iasym/Isym ) 1.0 (predicted OC-Mo-CO
angle 90°). IR (hexanes): νsym(CO) 1930 (vs), νasym(CO) 1858 (vs)
cm-1, Iasym/Isym ) 0.93 (predicted OC-Mo-CO angle 88°). Anal.
Calcd for C28H30N2O2Mo: C, 64.37; H, 5.79; N, 5.36. Found: C,
64.13; H, 6.05; N, 5.34.
CH), 7.06 (s, 4H, m-H-Mes), 4.71 (s, 5H, Cp), 2.34 (s, 6H, p-Me-
Mes), 2.12 (br s, 6H, o-Me-Mes), 2.01 (br s, 6H, o-Me-Mes), -6.43
(s, 1JWH ) 45 Hz, 1H, WH). 13C NMR (C6D6) δ 238.1 (br, W-CO),
184.1 (d, 2JCH ) 14.8 Hz, NCN), 139.1 (m, i-Mes), 138.9 (q, 2JCH
2
1
) 6 Hz, p-Mes), 136.6 (q, JCH ) 6 Hz, o-Mes), 129.9 (dm, JCH
CpMo(CO)2(PPh3)-(H+IMes). CpMo(CO)2(PPh3)H (306 mg,
0.64 mmol) and IMes (194 mg, 0.64 mmol) were charged into a
100 mL flask, and toluene (20 mL) was added, giving an orange
solution. After stirring at 23 °C for 10 min, hexane (40 mL) was
added, and an orange precipitate formed immediately. The pre-
cipitate was washed with hexane (3 × 10 mL) and dried under
vacuum to give an orange solid (409 mg, 82%). Anal. Calcd for
C46H45N2O2PMo: C, 70.40; H, 5.78; N, 3.57. Found: C, 70.13;
H, 6.01; N, 3.08. 1H NMR (C6D6, 23 °C): δ 10.82 (br s, 1H, HCN2),
7.89 (t m, JHH ) 8 Hz, 6H, PPh), 6.98 (m, 9H, PPh), 6.70 (s, 4H,
m-H-Mes), 5.97 (s, 2H, dCH), 4.71 (s, 5H, Cp), 2.09 (s, 6H, p-Me-
Mes), 1.98 (s, 12H, o-Me-Mes). 31P NMR (C6D6, 23 °C): δ 89.7
(s, PPh3). 13C{1H} NMR (C6D6, 23 °C): δ 243.0 (Mo-CO), 242.9
(Mo-CO), 159.6 (NCN), 145.7 (d, i-PPh3, J ) 30), 141.3 (i-Mes),
135.3 (p-PPh3), 134.2 (d, o-PPh3, J ) 13), 132.0 (p-Mes), 130.2
(o-Mes), 127.5 (d, m-PPh3, J ) 8), 127.4 (m-Mes), 124.1 ()CH),
1
2
) 157 Hz, m-Mes), 122.9 (dd, JCH ) 195 and JCH ) 12 Hz, d
1
CH), 87.4 (d quintet, JCH ) 177 and JCH ) 7 Hz, Cp), 21.4 (qt,
1JCH ) 126 and JCH ) 5 Hz, p-Me-Mes), 19.1 (qm, JCH ) 127
Hz, o-Me-Mes). 13C{1H} NMR (THF-d8, -100 °C): δ 247.4 (br
s, W-CO), 232.3 (br s, W-CO), 181.5 (s, NCN), 139.4 (s, p-Mes or
i-Mes), 138.9 (s, p-Mes or i-Mes), 137.1 (br s, o-Mes), 136.6 (br s,
o-Mes), 129.8 (br s, m-Mes), 124.1 (br s, dCH), 88.0 (s, Cp), 21.3
(br s, p-Me-Mes), 19.4 (br s, o-Me-Mes), 18.9 (br s, o-Me-Mes).
3
1
IR (toluene): νsym(CO) 1915 (vs), νasym(CO) 1824 (vs) cm-1, Iasym
/
-
Isym ) 0.93 (predicted OC-W-CO angle 88°). IR (THF-d8): νsym
(CO) 1913 (vs), νasym(CO) 1822 (vs) cm-1, Iasym/Isym ) 0.90
(predicted OC-W-CO angle 87°). IR (CD2Cl2): νsym(CO) 1906
(vs), νasym(CO) 1810 (vs) cm-1, Iasym/Isym ) 0.95 (predicted OC-
W-CO angle 89°). Anal. Calcd for C31.5H34N2O2W (formula
includes 0.5 equiv of crystallization solvent, C6H5CH3, per W): C,
57.63; H, 5.22; N, 4.27. Found: C, 57.52; H, 5.07; N, 4.14.
1
86.3 (Cp), 21.5 (p-Me-Mes), 17.8 (o-Me-Mes). H NMR (THF-
Synthesis of cis-CpW(CO)2(IMes)H from CpW(CO)2(PPh3)H.
CpW(CO)2(PPh3)H (608 mg, 1.07 mmol), IMes (333 mg, 1.09
mmol), and toluene (3 mL) were placed in a glass tube equipped
with a Teflon valve. The yellow solids dissolved to produce a
brown-red solution, and a light yellow precipitate formed within
10-20 min. The color faded slowly to yellow-gray, indicating
completion of the reaction after 2 days at 23 °C. The product was
washed with hexane (2 × 7 mL) and recrystallized from toluene-
hexane (1:1) to yield pure CpW(CO)2(IMes)H (568 mg, 87%) as
light yellow crystals. The product was identified by comparison to
d8): δ 10.44 (br s, 1H, HCN2), 7.85 (s, 2H, dCH), 7.43 (m, 6H,
PPh), 7.12 (s, 4H, m-H-Mes), 7.01 (m, 9H, PPh), 4.20 (s, 5H, Cp),
(53) Arduengo, A. J., III; Dias, H. V. R.; Harlow, R. L.; Kline, M. J.
Am. Chem. Soc. 1992, 114, 5530-5534.
(54) (a) Bainbridge, A.; Craig, P. J.; Green, M. J. Chem. Soc. (A) 1968,
2715-2718. (b) Beck, W.; Schloter, K.; Su¨nkel, K.; Urban, G. Inorg. Synth.
1989, 26, 96-106.
(55) Kalck, P.; Pince, R.; Poilblanc, R.; Roussel, J. J. Organomet. Chem.
1970, 24, 445-452.
(56) Van Geet, A. L. Anal. Chem. 1970, 42, 679-680.

5088 Organometallics, Vol. 26, No. 20, 2007
Wu et al.
an authentic sample of CpW(CO)2(IMes)H, which was synthesized
by an independent route.
cis-[CpW(CO)2(IMes)(THF-d8)]+[B(C6F5)4]-. 1H NMR (THF-
d8, -30 °C): δ 7.99 and 7.87 (d, JHH ) 2 Hz, 1H, dCH), 7.26,
1
Observation of CpW(CO)2(PPh3)-[H(IMes)]+ Intermediate
in the Reaction of CpW(CO)2(PPh3)H with IMes. CpW(CO)2-
(PPh3)H (5.7 mg, 0.01 mmol), IMes (3.1 mg, 0.01 mmol), and C6D6
(0.6 mL) were placed in an NMR tube equipped with a Teflon
valve. The light yellow solids dissolved to produce a brown-red
solution. An 1H NMR spectrum acquired after 5 min at room
temperature showed ca. 40% of the starting CpW(CO)2(PPh3)H and
ca. 60% of a new product, CpW(CO)2(PPh3)-[H(IMes)]+. 1H NMR
(C6D6): δ 10.95 (br s, 1H, HCN2), 7.88 (t, JHH ) 8 Hz, 6H, PPh),
6.98 (m, 9H, PPh), 6.74 (s, 4H, m-H-Mes), 6.23 (s, 2H, dCH),
4.61 (s, 5H, Cp), 2.12 (s, 6H, p-Me-Mes), 2.05 (s, 12H, o-Me-
Mes).
7.19, 7.16, and 7.03 (s, 1H, m-H-Mes), 5.36 (s, 5H, Cp), 2.41, 2.31,
2.30, 2.23, 2.14, and 2.02 (s, 3H, p-Me-Mes and o-Me-Mes). 13C-
{1H} NMR (THF-d8, -40 °C): δ 247.1 and 246.2 (s, W-CO), 179.6
(s, NCN), 149.0 (br d, 1JCF ) 240 Hz, o-C6F5), 141.3 and 140.0 (s,
1
p-Mes or i-Mes), 139.1 (dm, JCF ) 242 Hz, p-C6F5), 137.9 (s,
p-Mes or i-Mes), 137.0 (dm, 1JCF ) 244 Hz, m-C6F5), 137.5, 136.7,
136.5, and 135.8 (s, o-Mes), 130.7, 130.3, 130.2, and 129.4 (s,
m-Mes), 128.4 and 126.6 (br s, dCH), 125 (br m, i-C6F5), 95.4 (s,
Cp), 21.1 and 21.0 (s, p-Me-Mes), 19.7, 18.9, 18.7, and 18.6 (s,
o-Me-Mes). 19F NMR (THF-d8, -30 °C): δ -133.5 (d, 8F, 3JFF
)
3
11 Hz, o-C6F5), -164.9 (t, 4F, JFF ) 21 Hz, p-C6F5), -168.5 (t,
8F, 3JFF ) 18 Hz, m-C6F5). IR (THF-d8): νsym(CO) 1962 (vs), νasym
-
(CO) 1859 (vs) cm-1, Iasym/Isym ) 1.06 (predicted OC-W-CO
angle 92°).
Synthesis of [CpMo(CO)2(IMes)]+[B(C6F5)4]- (Mo). In a
glovebox CpMo(CO)2(IMes)H (52.4 mg, 0.100 mmol) was added
[CpW(CO)2(IMes)(H)2]+[B(C6F5)4]- (WH2). [CpW(CO)2-
(IMes)]+[B(C6F5)4]-‚CH3Ph (30 mg, 0.022 mmol) was suspended
in toluene (5 mL) and placed in a tube equipped with a Teflon
valve. The tube was filled with about 1.1 atm H2 at -196 °C, sealed,
and warmed to room temperature with vigorous stirring. The color
of the suspension changed almost immediately from brown to
yellow. The supernatant was removed, and the precipitate was dried
under vacuum to yield pure [CpW(CO)2(IMes)(H)2]+[B(C6F5)4]-
(25 mg, 91%) as a yellow powder. The sample suspended in C6D6
was found to contain [CpW(CO)2(IMes)(H)2]+[B(C6F5)4]- and 1
equiv of toluene. Anal. Calcd for C59H39BF20N2O2W (formula
includes 1 equiv of C6H5CH3, per W): C, 51.25; H, 2.84; N, 2.03.
-
slowly to a stirred solution of Ph3C+B(C6F5)4 (96.6 mg, 0.105
mmol) in toluene (5 mL). A dark purple precipitate formed, and
the stirring was continued for 40 min. The bright yellow mother
liquor was removed, and the precipitate was washed with toluene
until the washings were colorless (5 × 3 mL). The product was
washed with hexanes (3 × 3 mL) and dried in Vacuo to yield dark
purple crystals of pure CpMo(CO)2(IMes)+B(C6F5)4 (112 mg,
-
87%) with 0.5 equiv of crystallization solvent (C6H5CH3) per Mo.
The product was insoluble in common noncoordinating NMR
solvents. For spectra in THF-d8 see [CpMo(CO)2(IMes)(THF-d8)]+-
[B(C6F5)4]-. IR (Nujol): νsym(CO) 1999 (vs), νasym(CO) 1905 (vs)
cm-1, Iasym/Isym ) 1.05 (predicted OC-Mo-CO angle 91°). Anal.
Calcd for C55.5H33BF20N2O2Mo (with 0.5 equiv of crystallization
solvent, C6H5CH3, per Mo): C, 53.47; H, 2.67; N, 2.25. Found:
C, 53.18; H, 2.77; N, 2.43.
1
Found: C, 51.03; H, 2.68; N, 2.17. H NMR (C6D6, 23 °C): δ
6.71 (s, 4H, m-H-Mes), 5.94 (s, 2H, dCH), 4.09 (s, 5H, Cp), 2.09
(s, 6H, p-Me-Mes), 1.58 (s, 12H, o-Me-Mes), -1.15 (br s, ν1/2
)
14 Hz 2H, WH). The low solubility in C6D6 precluded reliable 13
C
cis-[CpMo(CO)2(IMes)(THF-d8)]+[B(C6F5)4]-, Mo(THF-d8).
1H NMR (THF-d8): δ 7.83 (s, 2H, dCH), 7.13 (s, 4H, m-H-Mes),
5.14 (s, 5H, Cp), 2.36 (s, 6H, p-Me-Mes), 2.11 (s, 12H, o-Me-
Mes). 13C{1H} NMR (THF-d8): δ 251 (br, Mo-CO), 187.3 (s,
NMR measurements. 1H NMR (THF-d8, -40 °C): δ 7.82 (s, 2H,
dCH), 7.17 (s, 4H, m-H-Mes), 5.46 (s, 5H, Cp), 2.37 (s, 6H, p-Me-
Mes), 2.06 (s, 12H, o-Me-Mes), -0.7 (br s, ν1/2 ) 1400 Hz, 2H,
1
WH). H NMR (THF-d8, -100 °C): δ 7.95 (s, 2H, dCH), 7.19
1
NCN), 149.3 (dm, JCF ) 246 Hz, o-C6F5), 141.0 (br s, p-Mes or
(s, 4H, m-H-Mes), 5.59 (s, 5H, Cp), 2.38 (s, 6H, p-Me-Mes), 2.07
(s, 12H, o-Me-Mes), 1.19 (br s, ν1/2 ) 13 Hz, 1H, WH), -2.97
1
i-Mes), 139.2 (dm, JCF ) 243 Hz, p-C6F5), 137.4 (br s, p-Mes or
1
i-Mes), 137.2 (dm, JCF ) 244 Hz, m-C6F5), 136.5 (br s, o-Mes),
2
1
(∼br d, ν1/2 ) 12 Hz, JHH ) 3 Hz, JHW ) 34 Hz, 1H, WH).
13C{1H} NMR (THF-d8, -100 °C): δ 205.2 and 203.1 (s, W-CO),
130.3 (br s, m-Mes), 127.6 (br s, dCH), 125 (br m, i-C6F5), 96.9
(s, Cp), 21.0 (s, p-Me-Mes), 18.7 (br s, o-Me-Mes). 19F NMR (THF-
1
160.7 (s, NCN), 148.8 (br d, JCF ) 242 Hz, o-C6F5), 141.0 (br s,
d8) δ -132.9 (d, 8F, 3JFF ) 10 Hz, o-C6F5), -165.1 (t, 4F, 3JFF
)
1
p-Mes or i-Mes), 139.0 (dm, JCF ) 242 Hz, p-C6F5), 138.5 (s,
21 Hz, p-C6F5), -168.6 (t, 8F, 3JFF ) 18 Hz, m-C6F5). IR (THF):
sym(CO) 1977 (vs), νasym(CO) 1882 (vs) cm-1, Iasym/Isym ) 1.15
p-Mes or i-Mes), 137.0 (dm, 1JCF ) 247 Hz, m-C6F5), 136.4 (br s,
o-Mes), 130.6 and 130.5 (s, m-Mes), 127.9 (br s, dCH), 124.5 (br
m, i-C6F5), 88.6 (s, Cp), 21.2 (s, p-Me-Mes), 18.7 and 18.3 (s, o-Me-
ν
(predicted OC-Mo-CO angle 94°).
Synthesis of [CpW(CO)2(IMes)]+[B(C6F5)4]- (W). In a glove-
Mes). 19F NMR (THF-d8, -40 °C): δ -133.5 (d, 8F, JFF ) 11
3
box CpW(CO)2(IMes)H (244.0 mg, 0.400 mmol) was added slowly
Hz, o-C6F5), -164.9 (t, 4F, 3JFF ) 21 Hz, p-C6F5), -168.5 (t, 8F,
3JFF ) 18 Hz, m-C6F5). [CpW(CO)2(IMes)(H)2]+[B(C6F5)4]- is not
stable in THF-d8 at 23 °C; [CpW(CO)2(IMes)(THF-d8)]+[B(C6F5)4]-
started to form. Additionally, [CpW(CO)2(IMes)(THF-d8)]+-
[B(C6F5)4]- does not react with H2 in THF-d8 at 23 °C. IR
(Nujol): νsym(CO) 2073 (vs), νasym(CO) 2018 (vs) cm-1, Iasym/Isym
) 1.18 (predicted OC-W-CO angle 95°). IR (C6D6): νsym(CO)
-
to a stirred solution of Ph3C+B(C6F5)4 (387.0 mg, 0.420 mmol)
in 10 mL of toluene, and a dark purple precipitate formed. The
stirring was continued for 30 min. The bright yellow mother liquor
was discarded, and the precipitate was washed with toluene until
the washings were colorless (5 × 3 mL). The product was washed
with hexanes (3 × 3 mL) and dried under vacuum to yield dark
purple crystals of pure [CpW(CO)2(IMes)]+[B(C6F5)4]- (490 mg,
91%) with 1 equiv of crystallization solvent (C6H5CH3) per W.
The product was insoluble in common noncoordinating NMR
solvents. For spectra in THF-d8 see [CpW(CO)2(IMes)(THF-d8)]+-
2065 (w), νasym(CO) 2006 (w) cm-1
.
[CpW(CO)2(IMes)(Et2CdO)]+[B(C6F5)4]-, W(Et2CdO). [CpW-
(CO)2(IMes)]+[B(C6F5)4]-‚CH3Ph (53 mg, 0.038 mmol) and Et2Cd
O (300 µL, 2.83 mmol) were mixed to produce a dark purple
solution and placed in an NMR tube equipped with a Teflon valve.
The volatiles were removed in Vacuo, giving [CpW(CO)2-
(IMes)(Et2CdO)]+[B(C6F5)4]- as a purple product. 1H NMR
(C6D6): δ 6.6 (br s, 4H, m-H-Mes), 6.10 (s, 2H, dCH), 4.49 (s,
5H, Cp), 2.08 (s, 6H, p-Me-Mes), 1.9 (br s, 4H, CH3CH2), 1.70
(br s, 12H, o-Me-Mes), 0.72 (br s, 6H, CH3CH2). 13C{1H} NMR
(Et2CdO and a sealed capillary of CD2Cl2 for lock, -30 °C): δ
248.1 and 246.4 (s, W-CO), 241.1 (s, Et2CdO), 177.4 (s, NCN),
1
[B(C6F5)4]-. H NMR (CF3Ph and a sealed capillary of C6D6 for
lock): δ 6.76 (br s, 4H, m-H-Mes), 6.66 (br s, 2H, dCH), 5.13 (br
s, 5H, Cp), 2.09 (s, 6H, p-Me-Mes), 2.06 (s, 12H, o-Me-Mes). IR
(Nujol): νsym(CO) 1980 (vs), νasym(CO) 1890 (vs) cm-1. IR (CF3-
Ph): νsym(CO) 1983 (vs), νasym(CO) 1900 (vs) cm-1. Anal. Calcd
for C59H37BF20N2O2W (formula includes 1 equiv of crystallization
solvent, C6H5CH3, per W): C, 51.33; H, 2.70; N, 2.03. Found: C,
51.24; H, 3.35; N, 2.02. Single crystals of CpW(CO)2(IMes)+B-
-
1
(C6F5)4 were grown directly from the reaction mixture by slow
148.7 (dm, JCF ) 244 Hz, oC6F5), 140.8 (br s, p-Mes or i-Mes),
1
diffusion of reagents.
138.7 (dm, JCF ) 247 Hz, p-C6F5), 136.9 (s, p-Mes or i-Mes),

Tungsten Complex with an N-Heterocyclic Carbene Ligand
Organometallics, Vol. 26, No. 20, 2007 5089
1
3
136.7 (dm, JCF ) 247 Hz, m-C6F5), 136.7 (s, o-Mes), 130.2 (s,
3.14 (m, 1H, CH-OH), 1.24 (m, 4H, CH3CH2), 0.82 (t, JHH ) 8
Hz, 6H, CH3CH2), 0.67 (d, 1H, JHH ) 5 Hz, CH-OH).
3
m-Mes), 128 and 126 (v b, dCH tentative assignment), 124.6 (br
m, i-C6F5), 96.0 (s, Cp), 37.8 (s, CH3CH2), 21.0 (s, p-Me-Mes),
18.8, 18.6, and 17.9 (s, o-Me-Mes), 8.9 (s, CH3CH2). 19F NMR δ
(Et2CdO and a sealed capillary of CD2Cl2 for lock, -30 °C):
Isolation of [H(IMes)]+B(C6F5)4]-. The Mo-catalyzed hydro-
genation mixture described above (300 mM Et2CdO in C6D6) was
heated for 30 min at 100 °C to complete catalyst decomposition.
The colorless precipitate was separated, dried under vacuum, and
3
3
-133.3 (dm, 8F, JFF ) 11 Hz, o-C6F5), -164.3 (tm, 4F, JFF
)
21 Hz, p-C6F5), -168.2 (tm, 8F, 3JFF ) 17 Hz, m-C6F5). IR (Et2Cd
O): νsym(CO) 1960 (vs), νasym(CO) 1859 (vs) cm-1, W(Et2CdO)
carbonyl band is obscured by the solvent, Iasym/Isym ) 1.22 (predicted
1
1
redissolved in THF-d8. H NMR (THF-d8): δ 9.24 (t, JHH ) 2
1
Hz, 1H, HCN2), 8.06 (d, JHH ) 2 Hz, 2H, dCH), 7.19 (s, 4H,
m-H-Mes), 2.37 (s, 6H, p-Me-Mes), 2.17 (s, 12H, o-Me-Mes). 13C-
{1H} NMR (THF-d8): δ 142.8 (s, p-Mes or i-Mes), 138.2 (s, p-Mes
or i-Mes), 135.3 (s, o-Mes), 132.0 (s, NCN), 130.7 (s, m-Mes), 126.3
(s, dCH), 21.1 (s, p-Me-Mes), 17.3 (s, o-Me-Mes). Due to the low
concentration of the sample, the 13C NMR signals of C6F5 were
not reliably observed or assigned. 19F NMR (THF-d8): δ -132.9
(d, 8F, 3JFF ) 11 Hz, o-C6F5), -165.3 (t, 4F, 3JFF ) 21 Hz, p-C6F5),
OC-W-CO angle 96°). IR (C6D6): νsym(CO) 1959 (vs), νasym
-
(CO) 1857 (vs) cm-1, ν(CdO) ) 1717 (w) cm-1, low solubility
precludes reliable determination of relative intensities. UV(tolu-
ene): λmax ) 498 nm (ꢀ ) 1 × 103 L mol-1 cm-1).
[CpW(CO)2(IMes)(Et2CH-OH)]+[B(C6F5)4]-, W(Et2CHOH).
[CpW(CO)2(IMes)]+[B(C6F5)4]-‚CH3Ph (85 mg, 0.062 mmol) was
suspended in C6H6 (5 mL), and Et2CH-OH (50 µL, 0.46 mmol)
was added. All the solid dissolved to yield a purple solution, which
was stirred for another hour. This solution was then maintained at
23 °C without stirring. After 2 days, a purple crystalline solid
formed at the bottom of the flask. The supernatant was carefully
decanted, and the solid was washed with hexanes (2 × 5 mL) and
C6H6 (1 mL) and dried under vacuum for 30 min. [CpW(CO)2-
(IMes)(Et2CH-OH)]+[B(C6F5)4]- (55 mg, 65%) was collected as
a purple crystalline solid. X-ray-quality crystals were obtained, and
the structure was determined by high-intensity X-ray radiation at
3
-168.8 (t, 8F, JFF ) 17 Hz, m-C6F5).
Comparative Substrate (Et2CdO, Et2CH-OH, and H2)
Coordination. The relative binding strength of various substrates
(Et2CdO, Et2CH-OH, and H2) is important in the catalytic ionic
hydrogenation reactivity and was compared using the CpW(CO)2-
(IMes)+ system.
The measurement is illustrated by the following example of Et2-
CH-OH and Et2CdO at 23 °C: [CpW(CO)2(IMes)(Et2CH-OH)]+-
[B(C6F5)4]- (8.0 mg, 0.006 mmol) was prepared in C6D6 (0.6 mL)
according to the above procedure, with Et2CH-OH (13 µL, 0.12
1
the National Synchrotron Light Source. H NMR (C6D6): δ 6.53
1
mmol, 20 equiv) present. The H NMR spectrum contained one
(br s, 4H, m-H-Mes), 5.89 (br s, 2H, dCH), 4.66 (br s, 5H, Cp),
3.12 (br s, 1H, CH-OH), 1.98 (br s, 6H, p-Me-Mes), 1.83 (br s,
12H, o-Me-Mes), 1.22 (br s, 4H, CH3CH2), 0.79 (br s, 6H, CH3-
CH2), 0.62 (br s, 1H, CH-OH). 19F NMR (C6D6): δ -131.7 (d,
set of broad resonances for Et2CH-OH, indicative of a fast
intermolecular exchange of free and coordinated alcohol on the
NMR time scale. Upon addition of Et2CdO (4.4 µL, 0.042 mmol,
1
7 equiv), the H NMR spectrum showed that [CpW(CO)2(IMes)-
3
3
(Et2CdO)]+[B(C6F5)4]- had formed quantitatively. One set of
broad, averaged resonances for free and coordinated Et2CdO and
sharp resonances for free Et2CH-OH were observed. This evidence
is consistent with the complete displacement of Et2CH-OH from
the W center upon addition of Et2CdO, indicating that Et2CdO
binding is favored over Et2CH-OH coordination. If 3% of total
W is used as the NMR detection limit for the remaining W(Et2-
8F, JFF ) 11 Hz, o-C6F5), -162.3 (t, 4F, JFF ) 21 Hz, p-C6F5),
-166.2 (t, 8F, 3JFF ) 18 Hz, m-C6F5). The low solubility in C6D6
precluded reliable 13C NMR measurements. IR (C6D6): νsym(CO)
1973 (vs), νasym(CO) 1891 (vs) cm-1, Iasym/Isym ) 0.79 (predicted
OC-W-CO angle 83°).
1
For comparison, H NMR of free Et2CH-OH in C6D6 ([Et2-
CH-OH] ) 0.037 M) was obtained at 23 °C. 1H NMR (C6D6): δ
Table 2. Crystallographic Collection and Refinement Data
CpMo(CO)2(IMes)H‚
0.5 (C7H8)
C31.5H34MoN2O2
[CpW(CO)2(IMes)]+[B(C6F5)4]-‚ [CpW(CO)2(IMes)(Et2CH-OH)]+-
CpW(CO)2(IMes)H
C28H30WN2O2
0.5(C7H8)
[B(C6F5)4]-
formula
fw
C55.5H33BWF20O2
1334.50
C57H41BWF20O3N2
1376.58
610.39
568.55
temp
293(2) K
triclinic
P1h (No. 2)
8.9340(18)
11.258(2)
13.631(3)
80.97(3)
72.96(3)
70.79(3)
1235.0(4)
2
4.704
0.71073
1.641
.33 × 0.33 × 0.27
2.40 to 27.47
5652
293(2) K
triclinic
150(2) K
150(2) K
cryst syst
space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/c (No. 14)
18.262(4)
17.725(4)
17.657(4)
orthorhombic
Pbca (No. 61)
17.636(4)
17.126(3)
35.532(7)
P1h (No. 2)
8.5080(17)
16.515(3)
21.351(4)
106.97(3)
95.45(3)
92.94(3)
2846.6(10)
4
0.490
0.71073
1.327
0.60 × 0.40 × 0.25
2.01 to 26.00
10 795
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
113.87(3)
5226.6(18)
4
2.327
0.90350
5440
8
2.27
0.92200
µ (mm-1
λ (Å)
)
F
calc (g cm-3
)
1.696
1.704
cryst size (mm)
θ range (deg)
.100 × 0.050 × 0.010
3.02 to 28.25
4800
.050 × 0.050 × 0.005
1.49 to 31.16
22 257
total no. of reflns
no. of indep reflns,
I g 3.0σ(I)
5652
10 795
4800
6833 [R(int) ) 0.0693]
no. of params
final R indices
[I > 2σ(I)]
299
660
749
758
R1 ) 0.0284, wR2 ) 0.0656 R1 ) 0.0559, wR2 ) 0.1206
R1 ) 0.0573, wR2 ) 0.1528
R1 ) 0.0607, wR2 ) 0.1539
R indices (all data)
R1 ) 0.0681, wR2 ) 0.0724 R1 ) 0.2577, wR2 ) 0.1596
R1 ) 0.0585, wR2 ) 0.1540
R1 ) 0.0952, wR2 ) 0.1733
1.047
goodness-of-fit on F2 1.056
O.979
1.022
none
none
extinction coeff
absorp corr
none
ψ scans
none
none
Fourier(XABS2)a
Fourier(XABS2)b
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) {∑[w(|Fo | - |Fc |)2]/∑[w|Fo | ]}1/2
.
b Parkin, S.; Moezzi, B.; Hope, H. J. Appl. Crystallogr. 1995, 28, 53-56.
2
2
2 2

5090 Organometallics, Vol. 26, No. 20, 2007
Wu et al.
CH-OH)+ adduct, the lower limit of Keq is estimated as [W(Et2Cd
O)+][Et2CH-OH]/[W(Et2CH-OH)+][Et2CdO] > 100.
was carried out in a constant-temperature bath. Aliquots were
removed by cooling the tube to -196 °C, evacuating the H2,
refilling the tube with Ar, and taking it into the glovebox. After
removal of an aliquot (ca. 60 µL), the tube was again freeze-
pump-thawed, filled with H2 at -196 °C, and resealed.
X-ray Structure Determinations. X-ray crystallographic data
sets for CpW(CO)2(IMes)H and CpMo(CO)2(IMes)H were collected
on an Enraf Nonius CAD-4 diffractometer. The structures of W
and W(Et2CHOH) were determined from data collected at beamline
X7B at the X-ray radiation at the National Synchrotron Light
Source. Crystal data and information about the data collection are
provided in Table 2.
Similarly, H2 completely displaces coordinated Et2CdO from
the CpW(CO)2(IMes)(Et2CdO)+ adduct to yield the CpW(CO)2-
+
+
(IMes)(H)2 complex in C6D6. Moreover, CpW(CO)2(IMes)(H)2
remains intact upon addition of excess Et2CdO in C6D6. Addition-
ally, introduction of H2 to CpW(CO)2(IMes)(Et2CH-OH)+ affords
+
free Et2CH-OH and Cp2W(CO)2(IMes)(H)2 in C6D6, indicative
of a stronger binding strength of H2 versus Et2CH-OH.
Example of Catalytic Hydrogenation at 54.4 atm (800 psi).
CpW(CO)2(IMes)+B(C6F5)4 (53.0 mg, 0.040 mmol) and 3-pen-
-
tanone (1.20 mL, 11.3 mmol) were placed in a stainless steel high-
pressure autoclave. H2 (54.4 atm) was added, and the reaction was
carried out in a constant-temperature bath. Prior to removal of each
sample for analysis, the bottom of the autoclave was cooled at -196
°C, and the pressure was slowly vented. The autoclave was taken
into a glovebox, and the sample for NMR analysis was removed.
Then the autoclave was resealed and H2 was again added.
Acknowledgment. Research at Brookhaven National Labo-
ratory was carried out under contract DE-AC02-98CH10886
with the U.S. Department of Energy and was supported by its
Division of Chemical Sciences, Office of Basic Energy Sciences.
Research at Pacific Northwest National Laboratory was funded
by the Division of Chemical Sciences, Office of Basic Energy
Sciences, U.S. Department of Energy, and by an LDRD grant.
Pacific Northwest National Laboratory is operated by Battelle
for the U.S. Department of Energy.
Example of Catalytic Hydrogenation at 4 atm. CpW(CO)2-
-
(IMes)+B(C6F5)4 (26.5 mg, 0.019 mmol) and 3-pentanone (600
µL, 5.65 mmol) were placed in a glass tube (125 mL capacity)
equipped with a Teflon valve. The solution was freeze-pump-
thawed, then frozen again, and the entire tube was submersed in
liquid nitrogen. The tube was then filled with about 1 atm of H2 at
-196 °C, sealed, and warmed to room temperature. As a result,
the tube contained 20 mmol of H2 (ca. 4 atm at room temperature;
P1/T1 ) P2/T2, so when P1 ) 1 atm, T1 ) 77 K, T2 ) 295 K, the
H2 pressure in the sealed tube at 23 °C was ca. 4 atm). The reaction
Supporting Information Available: Additional ORTEPs of W
and crystallographic information for Mo. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM700694E