Intermolecular activation of hydrocarbon C-H bonds under ambient conditions by 16-electron neopentylidene and benzyne complexes of molybdenum

Article abstract of DOI:10.1021/ja0349094

Cp*Mo(NO)(CH₂CMe₃)₂ (1), a complex with α-agostic C-H...Mo interactions, evolves neopentane in neat hydrocarbon solutions at room temperature and forms the transient 16-electron alkylidene complex, Cp*Mo(NO)(=CHCMe₃), which subsequently activates solvent C-H bonds. Thus, it reacts with tetramethylsilane or mesitylene to form Cp*Mo(NO)(CH₂CMe₃)(CH₂ SiMe₃) (2) or Cp*Mo(NO)(CH₂CMe₃)(η²- CH₂C₆H₃-3,5-Me₂) (3), respectively, in nearly quantitative yields. Under identical conditions, 1 in p-xylene generates a mixture of sp² and sp³ C-H bond activation products, namely Cp*Mo(NO)(CH₂CMe₃)(C₆H₃-2, 5-Me₂) (4, 73%) and Cp*Mo(NO)(CH₂CMe₃)(η² -CH₂C₆H₄-4-Me) (5, 27%). In benzene at room temperature, 1 transforms to a mixture of Cp*Mo(NO)(CH₂CMe₃)(C₆H₅) (6) and Cp*Mo(NO)(C₆H₅)₂ (7) in a sequential manner. Most interestingly, the thermal activation of 6 at ambient temperatures gives rise to two parallel modes of reactivity involving either the elimination of benzene and formation of Cp*Mo(NO)(=CHCMe₃) or the elimination of neopentane and formation of the benzyne complex, Cp*Mo(NO)(η²-C₆H₄). In pyridine, these intermediates are trapped as the isolable 18-electron adducts, Cp*Mo(NO)(=CHCMe₃)(NC₅H₅) (8) and Cp*Mo(NO)(η²-C₆H₄) (NC₅H₅) (9), and, in hydrocarbon solvents, they effect the intermolecular activation of aliphatic C-H bonds at room temperature to generate mixtures of neopentyl- and phenyl-containing derivatives. However, the distribution of products resulting from the hydrocarbon activations is dependent on the nature of the solvent, probably due to solvation effects and the presence of σ- or π-hydrocarbon complexes on the reaction coordinates of the alkylidene and the benzyne intermediates. The results of DFT calculations on these processes in the gas phase support the existence of such hydrocarbon complexes and indicate that better agreement with experimental observations is obtained when the actual neopentyl ligand rather than the simpler methyl ligand is used in the model complexes.

Full text of DOI:10.1021/ja0349094

Published on Web 05/14/2003
Intermolecular Activation of Hydrocarbon C-H Bonds under
Ambient Conditions by 16-Electron Neopentylidene and
Benzyne Complexes of Molybdenum
Kenji Wada,†,§ Craig B. Pamplin,† Peter Legzdins,*,† Brian O. Patrick,†
Irina Tsyba,‡ and Robert Bau‡
Contribution from the Departments of Chemistry, UniVersity of British Columbia,
VancouVer, British Columbia, Canada V6T 1Z1 and UniVersity of Southern California,
Los Angeles, California 90089
Received February 27, 2003; E-mail: legzdins@chem.ubc.ca
Abstract: Cp*Mo(NO)(CH2CMe3)2 (1), a complex with R-agostic CsH‚‚‚Mo interactions, evolves neopentane
in neat hydrocarbon solutions at room temperature and forms the transient 16-electron alkylidene complex,
Cp*Mo(NO)(dCHCMe3), which subsequently activates solvent CsH bonds. Thus, it reacts with tetra-
methylsilane or mesitylene to form Cp*Mo(NO)(CH2CMe3)(CH2SiMe3) (2) or Cp*Mo(NO)(CH2CMe3)(η2-
CH2C6H3-3,5-Me2) (3), respectively, in nearly quantitative yields. Under identical conditions, 1 in p-xylene
generates a mixture of sp2 and sp3 CsH bond activation products, namely Cp*Mo(NO)(CH2CMe3)(C6H3-
2,5-Me2) (4, 73%) and Cp*Mo(NO)(CH2CMe3)(η2-CH2C6H4-4-Me) (5, 27%). In benzene at room temperature,
1 transforms to a mixture of Cp*Mo(NO)(CH2CMe3)(C6H5) (6) and Cp*Mo(NO)(C6H5)2 (7) in a sequential
manner. Most interestingly, the thermal activation of 6 at ambient temperatures gives rise to two parallel
modes of reactivity involving either the elimination of benzene and formation of Cp*Mo(NO)(dCHCMe3) or
the elimination of neopentane and formation of the benzyne complex, Cp*Mo(NO)(η2-C6H4). In pyridine,
these intermediates are trapped as the isolable 18-electron adducts, Cp*Mo(NO)(dCHCMe3)(NC5H5) (8)
and Cp*Mo(NO)(η2-C6H4)(NC5H5) (9), and, in hydrocarbon solvents, they effect the intermolecular activation
of aliphatic CsH bonds at room temperature to generate mixtures of neopentyl- and phenyl-containing
derivatives. However, the distribution of products resulting from the hydrocarbon activations is dependent
on the nature of the solvent, probably due to solvation effects and the presence of σ- or π-hydrocarbon
complexes on the reaction coordinates of the alkylidene and the benzyne intermediates. The results of
DFT calculations on these processes in the gas phase support the existence of such hydrocarbon complexes
and indicate that better agreement with experimental observations is obtained when the actual neopentyl
ligand rather than the simpler methyl ligand is used in the model complexes.
Introduction
Our contribution to this area of chemistry derives from our
abiding interest in the family of Cp′M(NO)R2 complexes (Cp′
) Cp (η5-C5H5) or Cp* (η5-C5Me5); M ) Mo or W; R ) alkyl
or aryl) due to their unique thermal behavior in hydrocarbon
solutions. Previous reports from our laboratories have described
the thermal conversion of Cp*W(NO)(hydrocarbyl)2 complexes
into transient tungsten alkylidene,2,3 acetylene,4 or allene5 species
that subsequently activate the CsH bonds of hydrocarbon
solvents, often in an unprecedented manner. The reactive
Considerable effort is currently being devoted to the study
of selective, intermolecular C-H bond-activation processes with
a view to developing efficient and catalytic methods for the
direct conversion of hydrocarbon feedstocks into functionalized
organic compounds.1 To date, advances in this field have
included the discoveries of various types of metal-containing
complexes that effect selective C-H bond activations. Notable
examples are late-transition-metal complexes that oxidatively
add C-H bonds to the metal center, transition-metal, lanthanide,
and actinide complexes that facilitate C-H activation via M-C
σ-bond metathesis and early- to mid-transition-metal complexes
that add substrate C-H bonds across MdNR or MdCHR
linkages.1
(2) (a) Tran, E.; Legzdins, P. J. Am. Chem. Soc. 1997, 119, 5071-5072. (b)
Adams, C. S.; Legzdins, P.; Tran, E. J. Am. Chem. Soc. 2001, 123, 612-
624.
(3) For three other reported examples of intermolecular C-H activations by
alkylidene complexes, see: (a) Coles, M. P.; Gibson, V. C.; Clegg, W.;
Elsegood, M. R. J.; Porelli, P. A. J. Chem. Soc., Chem. Commun. 1996,
1963-1964. (b) van der Heijden, H.; Hessen, B. J. Chem. Soc., Chem.
Commun. 1995, 145-146. (c) Cheon, J.; Rogers, D. M.; Girolami, G. S. J.
Am. Chem. Soc. 1997, 119, 6804-6813.
(4) (a) Debad, J. D.; Legzdins, P.; Lumb, S. A.; Batchelor, R. J.; Einstein, F.
W. B. J. Am. Chem. Soc. 1995, 117, 3288-3289. (b) Legzdins, P.; Lumb,
S. A. Organometallics 1997, 16, 1825-1827. (c) Debad, D.; Legzdins, P.;
Lumb, S. A.; Rettig, S. J.; Batchelor, R. J.; Einstein, F. W. B. Organo-
metallics 1999, 18, 3414-3428.
(5) Ng, S. H. K.; Adams, C. S.; Legzdins, P. J. Am. Chem. Soc. 2002, 124,
9380-9381.
† University of British Columbia.
‡ University of Southern California.
§ Current address: Department of Energy & Hydrocarbon Chemistry,
Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-
8501, Japan.
(1) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507-514 and references
therein.
9
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J. AM. CHEM. SOC. 2003, 125, 7035-7048
7035

A R T I C L E S
Wada et al.
Table 1. X-ray Crystallographic Data for Complexes 1, 3, 6, 8, and 9·C6H6
1
3
6
8
9‚C H
6 6
empirical formula
formula weight
crystal size (mm3)
crystal system
space group
volume (Å3)
a (Å)
C20H37MoNO
403.45
0.37 × 0.24 × 0.015
monoclinic
P21/n
2097.4(7)
8.9121(17)
21.702(4)
11.260(2)
90
105.610(3)
90
4
1.278
0.710 73
130(2)
C24H37MoNO
451.49
0.20 × 0.20 × 0.20
monoclinic
P21/n
2305.4(3)
13.1900(11)
11.8175(7)
15.1108(13)
90
101.828(4)
90
4
1.301
0.710 73
173(2)
C21H31MoNO
409.41
0.30 × 0.20 × 0.20
monoclinic
P21/c
2061.86(17)
10.2911(5)
14.6191(6)
14.4274(8)
90
108.209(2)
90
4
1.319
0.710 69
173(2)
C20H30MoN2O
410.40
0.57 × 0.22 × 0.18
triclinic
P1h
1015.4(9)
8.372(4)
8.944(5)
14.795(8)
81.399(9)
79.463(8)
69.442(8)
2
C27H30MoN2O
494.47
0.20 × 0.05 × 0.05
orthorhombic
P212121
2300.8(2)
7.8177(4)
14.3141(9)
20.5603(10)
90
90
90
4
1.428
0.710 73
173(2)
5.91
1024
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
Z
density (g cm-3
wavelength (Å)
temp (K)
)
1.342
0.710 73
160(2)
6.54
428
µ (cm-1
F000
)
6.30
856
5.81
952
6.43
856
R1
0.0444
0.0413
0.0285
0.0473
0.0327
wR2
0.1038
0.1018
0.0780
0.1230
0.0662
tungsten species are typically formed by thermolysis of ap-
propriate organometallic precursors at temperatures ranging from
50 to 80 °C. However, under these conditions some of the Cs
H activation processes lead to cascade-type or decomposition
processes, and the operative chemistry is often obscured by the
thermal instability of the organometallic complexes involved.
Consequently, we decided to extend our studies to encompass
the congeneric molybdenum precursors in hopes of overcoming
this difficulty. We began our investigations in this regard with
Cp*Mo(NO)(CH2CMe3)2 (1), since we knew from our previous
work that the related CpMo(NO)(CH2CMe3)2 complex in CH2-
Cl2 eliminates neopentane below room temperature and forms
the bimetallic complex [CpMo(NO)](µ-η1:η2-NO)(µ-CHCMe3)-
[CpModCHCMe3] via a transient alkylidene species (eq 1).6
Figure 1. ORTEP plot of the solid-state molecular structure of Cp*Mo-
(NO)(CH2CMe3)2 (1) with 50% probability ellipsoids. Selected bond lengths
(Å) and angles (deg): Mo(1)-N(1) ) 1.757(3), N(1)-O(1) ) 1.216(4),
Mo(1)-C(11) ) 2.163(3), Mo(1)-C(16) ) 2.098(4), Mo(1)-H(4) ) 2.18-
(6), Mo(1)-N(1)-O(1) ) 168.5(3), N(1)-Mo(1)-C(11) ) 98.73(13),
N(1)-Mo(1)-C(16) ) 98.55(15), and C(11)-Mo(1)-C(16) ) 108.77-
(15).
In this report, we present the results of our investigations of
the thermal chemistry of 1 in various solvents, a survey that
has revealed both similarities and differences with that estab-
lished for its well-studied tungsten analogue. We also describe
the results of DFT calculations on some of these processes in
the gas phase that provide additional insight as to the mecha-
nistic pathways being followed during these metal-mediated
transformations. A portion of this work has been communicated
previously.7
Cp*M(NO)(CH2CMe3)2 (M ) Mo, W) molecular complexes
are isomorphous and crystallize in the P21/n space group with
essentially the same unit-cell dimensions (Table 1). A single-
crystal X-ray diffraction analysis of 1 at 130(2) K has confirmed
its monomeric nature and has provided the ORTEP diagram
and intramolecular metrical parameters presented in Figure 1.
The Cp*-Mo and Mo-NO intramolecular dimensions are
similar to those found in related complexes.9 There is some
rotational disorder in the methyl groups of one neopentyl ligand;
however, this disorder has no effect on the positions of the atoms
of its methylene group. One of the methylene C-H bonds,
C(16)-H(4), is strongly distorted toward the Mo atom in an
orientation consistent with interaction between this C-H bond
and the vacant metal-based d orbital. The Mo(1)-C(16)-H(4)
angle associated with this bond (81(3)°) is much lower than
the normal value (109.5°) expected for an sp3-hybridized C
atom, and the Mo(1)‚‚‚H(4) distance of 2.18(6) Å is in the range
Results and Discussion
Solid-State and Solution Molecular Structures of Cp*Mo-
(NO)(CH2CMe3)2 (1). As might have been expected, the
molecular structures of 1 in both the solid state and solutions
are virtually identical to those of its tungsten congener.8 Both
(6) (a) Legzdins, P.; Rettig, S. J.; Veltheer, J. E. J. Am. Chem. Soc. 1992, 114,
6922-6923. (b) Legzdins, P.; Rettig, S. J.; Veltheer, J. E.; Batchelor, R.
J.; Einstein, F. W. B. Organometallics 1993, 12, 3575-3585.
(7) Wada, K.; Pamplin, C. B.; Legzdins, P. J. Am. Chem. Soc. 2002, 124, 9680-
9681.
(8) Bau, R.; Mason, S. A.; Patrick, B. O.; Adams, C. S.; Sharp, W. B.; Legzdins,
P. Organometallics 2001, 20, 4492-4501.
(9) Legzdins, P.; Veltheer, J. E. Acc. Chem. Res. 1993, 26, 41-48.
9
7036 J. AM. CHEM. SOC. VOL. 125, NO. 23, 2003

C−H Activation by Neopentylidene and Benzyne Complexes
A R T I C L E S
anticipated for an agostic interaction. In addition to this agostic
C-H‚‚‚Mo interaction, another methylene hydrogen, H(2),
appears to be interacting very weakly with the Mo atom, but
this interaction is much more subtle [C(11)-H(2) ) 0.97(4)
Å; Mo(1)-C(11)-H(2) ) 99(2)°]. Nevertheless, the structural
deformations ascribed to agostic interactions in 1 are very similar
to those revealed in its tungsten congener by a low-temperature
X-ray analysis. This “unequal double R-agostic” motif has been
more definitively established for the tungsten complex by a
neutron diffraction analysis at 120 K.8
spectra that indicate that the bulky 3,5-dimethylbenzyl ligand
is attached to the metal in an η2 fashion on average in solution.11
The existence of this coordination mode is indicated by a broad
1H NMR resonance at δ 5.94 that can be attributed to the ortho
H atoms of this ligand and the occurrence of a high-field ipso
carbon resonance at δ 118.4 in the 13C{1H} spectrum. Our
inference of this mode of attachment is further supported by
the solid-state molecular structure of 3 (Figure 2) that clearly
shows an η2-3,5-dimethylbenzyl ligand having a bonding contact
between the ipso carbon and the metal center (Mo(1)-C(12)
) 2.496(3) Å) and an acute Mo-C-Cipso bond angle of 84.0-
(2)°. The other metrical parameters associated with the η2-3,5-
dimethylbenzyl ligand of 3 are similar to those extant in
Cp*Mo(NO)(CH2SiMe3)(η2-CH2C6H5) and related benzyl com-
plexes.12 The propensity of benzyl ligands to exhibit η2
coordination modes in such complexes has been attributed to
their ability to provide in this manner the necessary extra
electron density to impart an 18e configuration to the metal
center.
The 1H and gated-decoupled 13C NMR spectra of 1 in toluene-
d8 at 0 °C indicate that the R-agostic interactions evident in the
solid-state structure persist in solutions. First, there is a large
chemical-shift difference (∆δ ) 4.02) between the methylene
proton signals that appear as doublets in the 1H NMR spectrum
with one doublet appearing at δ ) -1.06 ppm. Second, there
1
are unequal JCH values (∆1JCH ) 24 Hz) evident for the
R-carbon signals in the gated-decoupled 13C NMR spectrum
with one being significantly reduced (i.e., 1JCH ) 101 Hz) when
compared to normal values. Indeed, both NMR spectra of 1
closely resemble those exhibited by Cp*W(NO)(CH2CMe3)2
(e.g., for the W compound, ∆1JCH ) 23 Hz).8 In summary, there
is essentially no difference between the molecular structures of
the Mo and W complexes either in the solid state or in solutions.
Both isostructural complexes are appropriately configured to
undergo cleavage of an R-H bond by a metal-assisted abstraction
process, and hence, there is no apparent structural basis for the
experimentally observed differences in thermal sensitivity of
the Mo and W compounds (vide infra).
C. Activation of p-Xylene. The reaction of 1 in p-xylene at
room temperature generates products derived from the activation
of solvent sp2 and sp3 C-H bonds. Specifically, the aryl
complex Cp*Mo(NO)(CH2CMe3)(C6H3-2,5-Me2) (4, 73%) and
the known12b 4-methylbenzyl complex Cp*Mo(NO)(CH2CMe3)-
(η2-CH2C6H4-4-Me) (5, 27%) are formed (eq 4). Although
Thermal C-H Bond-Activation Chemistry Initiated by
Cp*Mo(NO)(CH2CMe3)2 (1). A. Activation of Tetramethyl-
silane. Stirred solutions of 1 in tetramethylsilane at room
temperature evolve neopentane over 28 h as the known10 mixed
alkyl complex, Cp*Mo(NO)(CH2CMe3)(CH2SiMe3) (2), is
formed in quantitative yield (eq 2). Complex 2 is formed from
isomeric complexes 4 and 5 have not been isolated in pure form
from the final reaction mixture, they have been identified on
the basis of NMR spectroscopic, mass spectral, and elemental
analysis data. The NMR spectral features of 4 are consistent
with the formation of a single isomer with the structure shown
in eq 4, unlike the related tungsten complex Cp*W(NO)(CH2-
CMe3)(C6H3-2,5-Me2), which exists as two rapidly interchanging
isomers in solution at room temperature.13 The solution 1H NMR
spectrum of 5, like that of the related complex 3, reveals the
presence of an η2-4-methylbenzyl ligand and exhibits four sets
of resonances due to the diastereotopic methylene H atoms.
D. Activation of Benzene-d6 and Mechanistic Consider-
ations. Complex 1 in C6D6 is converted to Cp*Mo(NO)-
(CHDCMe3)(C6D5) (6-d6) over the course of 28 h at room
temperature (eq 5). The 1,2-CsD bond addition of C6D6 across
1 via the activation of an sp3 C-H bond of a solvent molecule,
and it does not react further with tetramethylsilane under these
mild reaction conditions. In contrast, the analogous tungsten
complex reacts with tetramethylsilane under the requisite
thermolysis conditions (70 °C, 40 h) to generate a mixture of
Cp*W(NO)(CH2CMe3)(CH2SiMe3) (97%) and Cp*W(NO)(CH2-
SiMe3)2 (3%).2 The second product presumably results from
the elimination of neopentane from Cp*W(NO)(CH2CMe3)(CH2-
SiMe3) and formation of transient Cp*W(NO)(dCHSiMe3) that
subsequently activates a second solvent molecule.
B. Activation of Mesitylene. The thermal reaction of 1 in
neat mesitylene affords Cp*Mo(NO)(CH2CMe3)(η2-CH2C6H3-
3,5-Me2) (3) in ca. 95% yield after 28 h at room temperature
(eq 3). No signals attributable to any other C-H bond-activation
(11) Legzdins, P.; Jones, R. H.; Phillips, E. C.; Yee, V. C.; Trotter, J.; Einstein,
F. W. B. Organometallics 1991, 10, 986-1002.
(12) (a) Dryden, N. H.; Legzdins, P.; Phillips, E. C.; Trotter, J.; Yee, V. C.
Organometallics 1990, 9, 882-884. (b) Dryden, N. H.; Legzdins, P.;
Trotter, J.; Yee, V. C. Organometallics 1991, 10, 2857-2870.
(13) Adams, C. S.; Legzdins, P.; Tran, E. Organometallics 2002, 21, 1474-
1486.
1
products are evident in the H NMR spectrum of the final
1
reaction mixture. Complex 3 exhibits H and 13C{1H} NMR
(10) Debad, J. D.; Legzdins, P.; Rettig, S. J.; Veltheer, J. E. Organometallics
1993, 12, 2714-2725.
9
J. AM. CHEM. SOC. VOL. 125, NO. 23, 2003 7037

A R T I C L E S
Wada et al.
Scheme 1
of two molecules of benzene and the formation of the two
complexes Cp*Mo(NO)(CH2CMe3)(C6H5) (6) and Cp*Mo(NO)-
(C6H5)2 (7) in the final reaction mixture (Scheme 1). As shown
in Scheme 1, the reaction of 6 with C6H6 at room temperature
also cleanly generates 7. The yield of 6 can therefore be
maximized by workup of the crude reaction mixture upon
complete consumption of 1, that is, after ca. 28 h at room
temperature. Complex 6 can be purified by extraction from the
crude residue into cold pentane followed by column chroma-
tography on alumina, and it is isolable as a red crystalline solid
(27% yield).
Figure 2. ORTEP plot of the solid-state molecular structure of Cp*Mo-
(NO)(CH2CMe3)(η2-CH2C6H3-3,5-Me2) (3) with 50% probability ellipsoids.
Selected bond lengths (Å) and angles (deg): Mo(1)-N(1) ) 1.761(2),
N(1)-O(1) ) 1.226(3), Mo(1)-C(11) ) 2.189(3), Mo(1)-C(12) ) 2.496-
(3), Mo(1)-C(20) ) 2.226(3), C(11)-C(12) ) 1.450(4), Mo(1)-N(1)-
O(1) ) 168.1(2), Mo(1)-C(11)-C(12) ) 84.00(19), and C(11)-Mo(1)-
C(12) ) 35.28(11).
the ModC bond of the intermediate alkylidene complex is
stereospecific, with incorporation of deuterium at the synclinal
methylene position being indicated by the absence of a
1
corresponding resonance in the H NMR spectrum of 6-d6.
Furthermore, the anticlinal methylene-proton signal appears as
a broad singlet due to an unresolved coupling to the geminal
deuterium nucleus. These features are similar to the spectral
characteristics noted previously for the related tungsten complex,
which is formed in a similar manner but at higher temperatures.2b
Figure 3. ORTEP plot of the solid-state molecular structure of Cp*Mo-
(NO)(CH2CMe3)(C6H5) (6) with 50% probability ellipsoids. Selected bond
lengths (Å) and angles (deg): Mo(1)-N(1) ) 1.769(2), N(1)-O(1) ) 1.214-
(2), Mo(1)-C(11) ) 2.148(2), Mo(1)-C(17) ) 2.091(2), Mo(1)-H(17a)
) 2.13(3), Mo(1)-H(17b) ) 2.57(3), C(17)-C(18) ) 1.530(3), Mo(1)-
N(1)-O(1) ) 167.5(2), Mo(1)-C(17)-H(17a) ) 78(2), Mo(1)-C(17)-
H(17b) ) 110(2), N(1)-Mo(1)-C(11) ) 94.13(8), N(1)-Mo(1)-C(17)
) 99.4(1), and C(11)-Mo(1)-C(17) ) 113.98(8).
Kinetic studies of R-hydrogen elimination from 1 have been
carried out in C6D6 solution by monitoring the decay of the
Cp* resonance of 1 using 1H NMR spectroscopy in the
temperature range 26-40 °C. The onset of neopentane elimina-
tion from 1 is indicated by a growth in the resonances due to
6-d6 and neopentane (δ 0.90). The activation parameters ∆H‡
) 99(1) kJ mol-1 and ∆S‡ ) -11(4) J mol-1 K-1 for this
transformation can be determined using regression methods from
a linear plot of ln(k/T) versus 1/T, and they are consistent with
the rate-determining step being the elimination of neopentane
from 1. In addition, the thermolysis of 1 in a 1:1 mixture of
benzene/benzene-d6 at 23 °C yields an intermolecular KIE of
1.04(4), a value similar to that exhibited by its tungsten congener
at 70 °C. In other words, all current indications are that the
Cp*M(NO)(CH2CMe3)2 (M ) Mo, W) complexes follow very
similar mechanistic pathways during their thermal activations
of hydrocarbon C-H bonds.
The solid-state molecular structure of 6 has been established
by a single-crystal X-ray crystallographic analysis. The resulting
ORTEP diagram of the three-legged piano-stool structure is
shown in Figure 3, and pertinent crystal and refinement data
are collected in Table 1. The most notable feature of the structure
is the presence of one very strong methylene C-H‚‚‚Mo agostic
interaction (Mo(1)-H(17a) ) 2.13(3) Å, Mo(1)-C(17)-H(17a)
) 78(2)°) that persists in solutions of the complex. The other
intramolecular metrical parameters of this molecule resemble
those reported for related complexes. In general, aryl complexes
of this type, that is, Cp′M(NO)(aryl)(R) (Cp′ ) Cp, Cp*; M )
Mo, W; R ) alkyl, aryl), are known to be thermally
sensitive.2b,14-16
(14) Dryden, N. H.; Legzdins, P.; Rettig, S. J.; Veltheer, J. E. Organometallics
1992, 11, 2583-2590.
(15) Sharp, W. B., Ph.D. Dissertation, The University of British Columbia, 2001.
(16) Debad, J. D., Ph.D. Dissertation, The University of British Columbia, 1994.
E. Activation of Benzene. Remarkably, the reaction of 1 with
C6H6 at room temperature results in the sequential activation
9
7038 J. AM. CHEM. SOC. VOL. 125, NO. 23, 2003

C−H Activation by Neopentylidene and Benzyne Complexes
A R T I C L E S
The pentane-insoluble residue formed from solutions of 1 in
benzene consists primarily of the diphenyl complex 7, which
can be isolated in moderate yield (30%) as violet needles by
recrystallization from ether. Alternatively, 7 can be prepared in
higher yields simply by stirring benzene solutions of 1 (or 6) at
room temperature for extended periods of time. Thus, it is
formed virtually quantitatively in a benzene solution of 1 after
ca. 1 week at ambient temperatures. Complex 7 in C6D6
solutions is converted to 7-d6 at room temperature, and 7-d10 is
1
the only identifiable organometallic complex detected by H
NMR spectroscopy after 48 h at 40 °C, although higher reaction
temperatures also result in the formation of several unidentified
decomposition products. The intermediacy of a transient benzyne
complex in this chemistry is indicated by the fact that monitoring
of the conversion of 6-d6 to 7-d10 in C6D6 by H{1H} NMR
2
Figure 4. ORTEP plot of the solid-state molecular structure of Cp*Mo-
(NO)(dCHCMe3)(NC5H5) (8) with 50% probability ellipsoids. Selected
bond lengths (Å) and angles (deg): Mo(1)sC(11) ) 1.951(3), C(11)s
H(11) ) 0.97(4), Mo(1)sN(1) ) 1.764(3), N(1)sO(1) ) 1.231(4), Mo-
(1)sN(1)sO(1) ) 172.4(3), Mo(1)sC(11)sC(12) ) 134.9(2), and C(11)s
Mo(1)sN(2) ) 101.0(1).
spectroscopy shows no incorporation of deuterium into the Cp*
methyl substituents. Hence, mechanisms involving the intramo-
lecular oxidative addition of a Cp* C-H bond to a metal center
and formation of intermediate “tuck-in” complexes that are
known to result in such deuterium scrambling17-21 can be ruled
out for this chemistry. Another possible mechanism for the C-H
bond activation of benzene by 6 involves intermolecular
oxidative addition to form a high-valent species such as Cp*Mo-
(NO)(H)(C6H5)2(CH2CMe3). However, since there is no inter-
molecular kinetic isotope effect evident during the initial rates
of the reaction of 6 with benzene/benzene-d6, this pathway is
deemed to be unlikely.
The sequential activation of two molecules of benzene by 1
differs from the otherwise similar chemistry observed previously
for the related tungsten complex Cp*W(NO)(CH2CMe3)2, which
is converted cleanly to the phenyl complex, Cp*W(NO)(CH2-
CMe3)(C6H5), upon thermolysis in benzene at 70 °C. Cp*W-
(NO)(C6H5)2 is not present in the final reaction mixture. In this
connection, it may be noted that Cp*W(NO)(CH2CMe3)(C6H5)
converts at 110 °C in benzene solutions containing PMe3 to
Cp*W(NO)(C6H5)2(PMe3), presumably via a transient benzyne
complex.16 Another difference between the molybdenum and
tungsten systems is that Cp*W(NO)(C6H5)2 may be synthesized
by treating Cp*W(NO)Cl2 with 1 equiv of (C6H5)2Mg‚
x(dioxane), but analytically pure samples of Cp*Mo(NO)(C6H5)2
(7) are only accessible via the C-H activation of benzene.
Previous attempts to prepare 7 by metathetical means provided
samples contaminated with MgCl2, possibly due to the formation
of adducts in which the Lewis-basic nitrosyl ligand of 7 acts as
a donor to the Lewis-acidic Mg atom of the magnesium chloride
byproduct.22
CMe3)4. The γ-hydrogen activation pathway is about 25 times
slower than the R-hydrogen abstraction process. Interestingly,
the Ti(CH2CMe3)3(C6H5) product reacts further via elimination
of a second equivalent of neopentane to generate an ill-defined
benzyne intermediate prior to decomposition.
F. Trapping of the Intermediates. Given our previous
success in trapping and identifying transient intermediates by
carrying out the thermolyses of the precursor bis(alkyl) com-
plexes in Lewis bases that are able to stabilize coordinatively
unsaturated tungsten2 and molybdenum23,24 complexes, we
effected the thermolysis of 1 in neat pyridine under ambient
conditions. The intermediacy of a neopentylidene complex in
the thermal CsH bond activation reactions of 1 is confirmed
by the isolation of the base-stabilized complex Cp*Mo(NO)-
(dCHCMe3)(NC5H5) (8) in moderate yield (55%) from this
reaction (eq 6). Complex 8 crystallizes as orange prisms from
hexanes, and its solid-state molecular structure has been
established by a single-crystal X-ray crystallographic analysis.
The resulting ORTEP plot is shown in Figure 4. The notable
metrical parameters of 8 include a short Mo(1)-C(11) bond
distance of 1.951(3) Å and an obtuse Mo-C(11)-C(12) bond
angle of 134.9(2)°. The neopentylidene ligand is oriented
anticlinal to the Cp* group, thereby minimizing steric interac-
tions and optimizing the synergic π-bonding between the
π-donor alkylidene and π-acceptor NO ligands.25-27 In C6D6
solution, 8 (which is formed as a single isomer) exhibits typical
To put these observations in context, it may be noted that
Girolami and co-workers have shown that thermolysis of the
titanium tetrakis(neopentyl) complex, Ti(CH2CMe3)4, in benzene
results in the formation of Ti(CH2CMe3)3(C6H5).3c Deuterium-
labeling studies in C6D6 indicate that this reaction proceeds via
distinct alkylidene and titanacyclobutane intermediates formed
via R- or γ-hydrogen elimination of neopentane from Ti(CH2-
(17) Bercaw, J. E. AdV. Chem. Ser. 1978, 167, 136-148.
(18) Chirik, P. J.; Day, M. W.; Bercaw, J. E. Organometallics 1999, 18, 1873-
1881.
(23) Legzdins, P.; Rettig, S. J.; Veltheer, J. E.; Batchelor, R. J.; Einstein, F. W.
B. Organometallics 1993, 12, 3575-3585.
(19) Takao, T.; Amako, M.; Suzuki, H. Organometallics 2001, 20, 3406-3422.
(20) Kraft, B. M.; Lachicotte, R. J.; Jones, W. D. J. Am. Chem. Soc. 2001, 123,
10973-10979.
(24) Legzdins, P.; Veltheer, J. E.; Young, M. A.; Batchelor, R. J.; Einstein, F.
W. B. Organometallics 1995, 14, 407-417.
(21) Polse, J. L.; Andersen, R. A.; Bergman, R. G. J. Am. Chem. Soc. 1998,
120, 13405-13414.
(25) Schilling, B. E. R.; Hoffman, R.; Faller, J. W. J. Am. Chem. Soc. 1979,
101, 592-598.
(22) Legzdins, P.; Rettig, S. J.; Sa´nchez, L. Organometallics 1988, 7, 2394-
2403.
(26) Gibson, V. C. J. Chem. Soc., Dalton Trans. 1994, 1607-1618.
(27) Smith, K. M.; Poli, R.; Legzdins, P. Chem.sEur. J. 1999, 5, 1598-1608.
9
J. AM. CHEM. SOC. VOL. 125, NO. 23, 2003 7039

A R T I C L E S
Wada et al.
Figure 5. 1H NMR spectrum (300 MHz, 25 °C, pyridine-d5) of the solution formed by thermolysis of Cp*Mo(NO)(CH2CMe3)(C6H5) (6) in pyridine-d5.
Resonances due to the complexes Cp*Mo(NO)(dCHCMe3)(NC5D5) (8-d5) and Cp*Mo(NO)(η2-C6H4)(NC5D5) (9-d5) are indicated. Residual solvent protons
are labeled with an asterisk, and d5 labels are omitted for clarity.
alkylidene NMR resonances at δ 12.43 and 298.4 for the
R-proton and the R-carbon of the neopentylidene ligand,
respectively.28 Interestingly, Cp*Mo(NO)(CH2CMe3)(CH2-
SiMe3) (2) reacts in neat pyridine-d5 solution in a similar manner
to form primarily 8-d5 with concomitant elimination of tetra-
methylsilane. A small amount of neopentane is also evolved
during this conversion, but no organometallic coproducts have
yet been definitively identified.
The room-temperature thermolysis of 6 in neat PMe3 for 24
h results in the formation of a brown solution containing
Cp*Mo(NO)(dCHCMe3)(PMe3) that can be identified by its
1
characteristic H NMR spectrum. Interestingly, a similar ther-
molysis of 6 in pyridine affords an orange mixture of the
alkylidene complex 8 and the benzyne complex Cp*Mo(NO)-
Figure 6. ORTEP plot of the solid-state molecular structure of Cp*Mo-
(η2-C6H4)(NC5H5) (9) (eq 7). Monitoring of this reaction by
(NO)(η2-C6H4)(NC5H5) (9) with 50% probability ellipsoids. Selected bond
lengths (Å) and angles (deg): Mo(1)-C(11) ) 2.124(4), Mo(1)-C(12) )
2.153(4), C(11)-C(12) ) 1.356(6), Mo(1)-N(1) ) 1.774(4), N(1)-O(1)
) 1.228(5), Mo(1)-N(1)-O(1) ) 169.2(4), and C(11)-Mo(1)-C(12) )
36.96(16).
The benzyne complex 9 can be readily isolated from the final
reaction mixture by fractional crystallization from ether, and it
is obtained as a canary yellow powder in 17% yield based on
1H NMR spectroscopy in pyridine-d5 reveals that 8-d5 is formed
preferentially (in an approximate 3:1 ratio), and signals due to
liberated neopentane and benzene are also detectable in the 1H
NMR spectrum of the crude reaction mixture (Figure 5). The
aromatic region of the 1H NMR spectrum of 9-d5 exhibits
inequivalent benzyne signals with overlapping multiplets at δ
7.48 (2H), 7.78 (1H), and 7.89 (1H). The 13C{1H} NMR
spectrum shows two low-field signals at δ 158.4 and 162.2
which correspond to the ipso carbon atoms of the benzyne ligand
as well as four other benzyne carbon signals. The line shapes
of these signals are invariant from 20 to 85 °C, thereby
indicating that 9 is stereochemically rigid in pyridine-d5 and
that the Mo-η2-C6H4 linkage is static. Also, pyridine ligand
exchange apparently does not occur, since the NMR spectra of
9 in pyridine-d5 at 85 °C are invariant with time.
6 (i.e., a theoretical maximum yield of 25%). Recrystallization
of 9 from benzene provides yellow needles suitable for analysis
by single-crystal X-ray diffraction, and the resulting ORTEP
diagram of the solid-state molecular structure of 9 is shown in
Figure 6. As anticipated, the dihedral angle between the planar
benzyne ligand and the Mo(1)-C(11)-C(12) plane is only 1.2-
(2)°. The length of the benzyne C-C bond coordinated to the
molybdenum center is 1.356(6) Å, while the other C-C bond
distances in this ligand range from 1.374(6) to 1.394(6) Å with
an average value of 1.383 Å. The two Mo-C(benzyne) distances
of 2.124(4) and 2.153(4) Å are unequal and larger than those
(2.011 and 2.056 Å) extant in the only previously reported
molybdenum aryne complex, namely Mo(η2-2-MeC6H3)(2-
MeC6H4)2(PMe2Ph)2.29 The infrared spectrum of 9 in KBr
(29) Koschmieder, S. U.; McGilligan, B. S.; McDermott, G.; Arnold, J.;
Wilkinson, G.; Hussain-Bates, B.; Hursthouse, M. B. J. Chem. Soc., Dalton
Trans. 1990, 3427-3433.
(28) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds; Wiley: New
York, 1988; Chapter 4.
9
7040 J. AM. CHEM. SOC. VOL. 125, NO. 23, 2003

C−H Activation by Neopentylidene and Benzyne Complexes
A R T I C L E S
Scheme 2
Figure 7. DFT optimized geometries of CpMo(NO)(C6H5)(CH3) (G-G′′′)
and CpMo(NO)(C6H5)(CH2CMe3) (GG).
Table 2. DFT Optimized Structural Parameters for
CpMo(NO)(C6H5)(CH3) (G-G′′′) and CpMo(NO)(C6H5)(CH2CMe3)
(GG) (see Figure 7)
exhibits strong absorptions at 1537 and 1583 cm-1, which can
be assigned to νNO and νC≡C stretching frequencies, respectively.
C-H Bond-Activation Chemistry Initiated by 6. Solutions
of 6 in C6D6 are converted to primarily 6-d6 (27%) and 7-d6
(68%), along with traces of Cp*Mo(NO)(C6D5)2 (7-d10) (eq 8).
Thermolysis of 6 in tetramethylsilane at room temperature for
48 h results in the clean formation of 2 (41%) and Cp*Mo-
(NO)(CH2SiMe3)(C6H5) (10, 59%) as measured after 70%
conversion (Scheme 2).
parameter
G
G′
G′′
G′′′
GG
Mo-C1
2.143
2.117
2.594
1.106
1.773
1.240
113.8
101.1
173.8
2.147
2.121
2.597
1.106
1.768
1.243
112.5
101.1
173.7
2.142
2.124
2.553
1.107
1.763
1.204
114.4
98.5
2.160
2.133
2.593
1.102
1.780
1.193
114.9
100.2
172.7
2.131
2.130
2.379
1.121
1.772
1.242
111.3
88.4
Mo-C2
Mo-H*
C1-H*
Mo-N
N-O
C1-Mo-C2
Mo-C1-H*
Mo-N-O
172.4
172.2
Theoretical Investigations into C-H Activations by Cp-
Mo(NO) Complexes. A. Optimal Structures of CpMo(NO)-
(C6H5)(alkyl) Complexes. The molecular structure of CpMo-
(NO)(C6H5)(CH3) optimized by DFT calculations with LanL2DZ
(G), SDD (G′), LanL2DZ-II (G′′), or LanL2DZ-III (G′′′) basis
sets and that of CpMo(NO)(C6H5)(CH2CMe3) optimized by
using the LanL2DZ basis set (GG) are shown in Figure 7.
Intramolecular geometrical parameters of these structures are
summarized in Table 2. The Cp-Mo, Mo-NO, and Mo-C6H5
intramolecular dimensions of structure G are generally com-
parable to those extant in the solid-state molecular structure of
Cp*Mo(NO)(CH2CMe3)(C6H5) (6) shown in Figure 3. One of
the methyl C-H bonds in G, namely C1-H*, is distorted
toward the Mo atom in an orientation consistent with there being
an interaction between this C-H bond and the vacant metal-
based d orbitals.35 Thus, the Mo-C1-H* angle (101.1°) is
smaller than the normal value (109.5°) expected for an sp3-
hybridized C atom, and the Mo-H* distance (2.594 Å) is
distinctly shorter than the others. Nevertheless, the extent of
this calculated R-agostic interaction is significantly lower than
that evident in the X-ray structure of 6, in which the Mo-C1-
H* angle and the Mo-H* distance are 78(2)° and 2.13(3) Å,
respectively. The use of larger basis sets such as LanL2DZ-II
or LanL2DZ-III instead of the normal LanL2DZ basis set only
alters the extent of the R-agostic interaction slightly. Thus, the
Mo-C1-H* angles extant in structures G′′ (98.5°) and G′′′
(100.2°) are slightly more acute than that in G, while, in the G′
The distribution of products resulting from the reactions of
hydrocarbons with 6 thus appears to be dependent on the nature
of the hydrocarbon substrate, possibly due to the presence of
σ- or π-hydrocarbon complexes on the reaction coordinates of
the alkylidene and benzyne intermediates. [This possibility is
specifically addressed during the theoretical investigations
summarized in the next section.] Although the insertion of
unsaturated small molecules into a metal-carbon bond of a
benzyne complex is a well-established phenomenon, only a
relatively few benzyne complexes (either isolated or invoked)
have been reported to undergo intermolecular C-H bond
activation processes and then usually at temperatures in excess
of 100 °C.16,30-32 In addition, Jones has implicated various
benzyne species during the C-F bond activation of fluorinated
arenes,20,33 and Hughes recently prepared the first tetrafluo-
robenzyne complexes via ortho-fluoride abstraction from pen-
tafluorophenyl ligands.34
(33) (a) Edelbach, B. L.; Kraft, B. M.; Jones, W. D. J. Am. Chem. Soc 1999,
121, 10327-10331. (b) Kraft, B. M.; Lachicotte, R. J.; Jones, W. D.
Organometallics 2002, 21, 727-731.
(30) Erker, G. J. Organomet. Chem. 1977, 134, 189-202.
(31) Fagan, P. J.; Manriquez, J. M.; Maatta, E. A.; Seyam, A. M.; Marks, T. J.
J. Am. Chem. Soc. 1981, 103, 6650-6667.
(32) (a) Hartwig, J. F.; Andersen, R. A.; Bergman, R. G. J. Am. Chem. Soc.
1989, 111, 2717-2719. (b) Hartwig, J. F.; Bergman, R. G.; Andersen, R.
A. J. Am. Chem. Soc. 1991, 113, 3404-3418.
(34) (a) Hughes, R. P.; Williamson, A.; Sommer, R. D.; Rheingold, A. L. J.
Am. Chem. Soc. 2001, 123, 7443-7444. (b) Hughes, R. P.; Laritchev, R.
B.; Williamson, A.; Incarvito, C. D.; Zakharov, L. N.; Rheingold, A. L.
Organometallics 2002, 21, 4873-4885.
(35) Poli, R.; Smith, K. M. Organometallics 2000, 19, 2858-2867.
9
J. AM. CHEM. SOC. VOL. 125, NO. 23, 2003 7041

A R T I C L E S
Wada et al.
Figure 8. DFT optimized geometries of complexes in the CpMo(NO)(C6H5)(CH3) (G) system.
structure optimized by the use of the SDD basis set, it is very
close to that in G (Table 2).
theoretical studies of CpW(NO)R2 systems.36 The transition state
from the methyl phenyl complex G to E via a one-step, σ-bond
metathesis pathway is a late one (F-TS), with the hydrogen atom
being transferred being more than halfway to the H-acceptor
carbon.
On the other hand, the structure of the phenyl neopentyl
complex GG optimized by the B3LYP/LanL2DZ calculations
involves one strongly agostic methylene hydrogen. Indeed, the
computed acute Mo-C1-H* angle (88.4°) and the short Mo-
H* distance (2.379 Å) indicate that GG is a better model for
the actual complex 6, probably for both steric and electronic
reasons. For instance, the inductive effect of a tBu group would
increase the basicity of the methylene C-H linkages and
enhance the interaction between the R-hydrogen and the strongly
Lewis-acidic9 molybdenum center. The present study clearly
shows that the use of the simplified model G results in an
underestimation of R-agostic interactions which often play very
important roles during the cleavage of R-C-H bonds by metal-
assisted abstraction pathways.8
Based on these initial findings, we next examined the
intermolecular C-H bond activation pathways initiated by the
neopentyl phenyl complex 6 by the use of both models, G and
GG, to assess what effects the simplification of the alkyl ligands
has on these pathways. The LanL2DZ basis set was used during
the following calculations in order to minimize computational
expense. At this point, it may also be noted that, in general, the
HOMOs and LUMOs of the various structures optimized during
this work resemble those described by Poli and Smith during
their theoretical investigations of related systems.35
B. CsH Activations Initiated by CpMo(NO)(C6H5)(CH3).
Optimal geometries have been determined for various species
included in the activation processes, and the results of these
computations are presented in Figure 8 and Table S1 of the
Supporting Information. A π-benzene adduct of the carbene
fragment CpMo(NO)(dCH2) has been successfully located as
a distinct intermediate (E) for the process involving eventual
benzene expulsion.36 Another stable intermediate (E′) with the
π-benzene ligand positioned away from the Cp ring has also
been found, and its calculated energy is almost the same as that
of E. Similar stable intermediates have been found during the
On the other hand, for the pathway involving methane loss
from the methyl phenyl reactant, a stable σ-methane benzyne
intermediate I has been found. The methane ligand is coordi-
nated in an η2 fashion to the metal center with MosH and Mos
C bond distances of 2.211 Å and 2.877 Å, respectively. Both
distances are slightly longer than those previously computed
for the corresponding methane methylene intermediate, CpMo-
(NO)(σ-CH4)(dCH2) (C).35 The transition state H-TS from G
to I is a very late one in which the formation of the benzyne
and methane ligands is almost complete. Thus, the distance
between the transferring hydrogen, H*, and the methyl acceptor
is only 1.389 Å, whereas that between H* and C3 is 1.554 Å.
In a related theoretical study of plausible mechanisms for
generating tungsten carbene species from Cp′W(NO)(CH3)2 (Cp′
) Cp or Cp*), Fan and Hall37 found the stable intermediates
Cp′W(NO)(H)(dCH2)(CH3) that result from R-H oxidative
addition. Nevertheless, they concluded that the one-step, σ-bond
metathesis-like mechanism is the preferred one. We therefore
attempted to locate CpMo(NO)(H)(η2-C6H4)(CH3), a stable
intermediate resulting from the oxidative addition of a â-hy-
drogen of the phenyl ring, during the course of the transforma-
tion from G to I. However, such a species was not found at the
present calculation level even after extensive searching with
various initial reactant geometries. Finally, the transition state
L-TS has been located during the course of the reaction from
the π-benzene benzyne intermediate K to the diphenyl complex
M. The molecular structure of L-TS is very similar to that of
the benzene intermediate K. Remarkably, the structure of the
pyridine adduct of the benzyne intermediate, N, optimized by
the B3LYP/LanL2DZ calculation is almost the same as the
X-ray structure of 9. Thus, the computed C2sC3 bond length
of the benzyne ligand is 1.364 Å, only slightly longer than the
actual value (1.356(6) Å). The other C-C bond distances in
(36) Adams, C. S.; Legzdins, P.; McNeil, W. S. Organometallics 2001, 20,
4939-4955.
(37) Fan, Y.; Hall, M. B. J. Chem. Soc., Dalton Trans. 2002, 713-718.
9
7042 J. AM. CHEM. SOC. VOL. 125, NO. 23, 2003

C−H Activation by Neopentylidene and Benzyne Complexes
A R T I C L E S
Table 3. Relative Energies of Complexes in the
CpMo(NO)(C6H5)(CH3) Model System (see Figure 9)
longer (∼0.01 Å) than those found in the phenyl methyl system.
In FF-TS the MosH distance is much shorter (by more than
0.02 Å) than that in F-TS. Remarkably, the employment of a
model including a neopentyl group greatly reduces the calculated
barrier for the formation of a reactive alkylidene intermediate.
Thus, the relative energy of the transition state FF-TS is 5.58
kcal mol-1 lower than that calculated for the simpler methyl
system (F-TS). On the other hand, the presence of a bulky
neopentyl ligand does not result in any significant changes of
the activation barrier for the formation of the benzyne inter-
mediate. Hence, the relative energy of the transition state HH-
TS is 34.70 kcal mol-1, only slightly higher than that of H-TS
(33.62 kcal mol-1). As a result, the barrier for the generation
of the active benzyne intermediate is 10.83 kcal mol-1 higher
than that for the neopentylidene species, again indicating that
in the neopentyl phenyl complex R-hydrogen abstraction to form
the ModCHCMe3 linkage is more facile than the â-hydrogen
elimination to form the benzyne species. It may also be noted
that the optimized structure of the pyridine adduct of the
alkylidene intermediate OO (a known compound)6b compares
well with the solid-state molecular structure of the corresponding
complex 8 (Figure 4). Thus, the MosC1 bond length (1.951
Å), the MosC1sC(tBu) angle (138.4°), and the coordination
geometry of the pyridine ligand (i.e., MosN2 ) 2.205 Å)
compare very well with those found in the experimentally
determined structure. Finally, as for N, the computed energy
level of OO is relatively low (-13.52 kcal mol-1), again
indicating its relatively high thermal stability.
∆E
∆G
(kcal mol-1
)
complex
(kcal mol-1
)
D
E
E′
F-TS
G
29.53
16.73
16.44
29.45
0.00
18.86
16.18
16.82
31.02
0.00
34.96
23.84
17.50
19.04
28.90
-3.69
-5.54
H-TS
I
J
K
L-TS
M
33.62
25.11
27.85
19.02
26.52
-4.18
-6.83
N
Figure 9. Reaction profile showing the calculated energies of complexes
derived from CpMo(NO)(C6H5)(CH3) (G) (see Table 3).
the benzyne ligand (1.399-1.420 Å), the two Mo-C(benzyne)
distances (2.115 and 2.150 Å), and the coordination geometry
of the pyridine ligand (e.g., Mo-N2 ) 2.227 Å) also compare
very well with those found in the X-ray structure (Figure 6).
D. CsH Activations Initiated by CpMo(NO)(CH2CMe3)2.
In addition to investigating the parallel generation of the active
intermediates from the alkyl phenyl complexes, we have also
studied mechanisms for generating molybdenum alkylidene
species from CpMo(NO)(CH2CMe3)2 (AA) by neopentane
elimination. Apart from the methyl groups on the Cp ring, the
structural parameters of AA closely resemble those in the solid-
state molecular structure of 1 (Figure 1) except for the
geometries of the R-hydrogens. The optimized structure of
CpMo(NO)(CH2CMe3)2 involves two “moderately agostic”
R-hydrogens (Mo to R-H ) 2.484 Å and 2.478 Å). Nevertheless,
the extent of R-agostic interaction in AA is significantly larger
than those found in previous studies employing model dimethyl
complexes.35,37 As summarized in Table 5, the activation energy
for the one-step transformation of AA into the stable σ-neo-
pentane neopentylidene intermediate CC is calculated to be
27.34 kcal mol-1 (27.58 kcal mol-1 based on ∆Eo calculated
from the sum of electronic and zero-point energies) According
to the previous report,35 the computed activation barrier from
CpMo(NO)(CH3)2 (A) to CpMo(NO)(dCH2)(σ-CH4) (C) is
32.62 kcal mol-1 based on ∆Eo. A comparison of these data
clearly indicates that the use of “unsimplified” models greatly
lowers the barrier for the R-hydrogen elimination pathway.
Hence, it is worth noting at this point that while the insights
provided by computations can be very useful, the results
obtained using simplified models such as Cp′M(NO)(CH3)2 in
place of the actual and more complicated bis(alkyl) compounds
should be interpreted with caution. In fact, in this particular
system the dimethyl complexes do not even exhibit R-H
elimination chemistry, spontaneously converting instead to their
The data summarized in Table 3 and presented pictorially in
Figure 9 show the complete energy profile accompanying these
C-H activation processes originating at G. The calculated
activation barrier for the generation of the active benzyne
intermediate from G (33.62 kcal mol-1) is slightly higher than
that for the methylene intermediate (29.45 kcal mol-1), and it
is consistent with the results obtained in competition experiments
(vide supra). The benzyne intermediate stabilized by π-coor-
dinated benzene (K) is 19.02 kcal mol-1 less stable than the
reactant G, and the energy of the transition state L-TS is only
7.50 kcal mol-1 higher than that of K. This result indicates that
once the reactive benzyne intermediate is formed in benzene
solution, the reaction spontaneously proceeds to form the
thermodynamically stable diphenyl complex M. Conversely, the
diphenyl complex needs to overcome a relatively large barrier
(30.70 kcal mol-1) in order to generate an active benzyne
intermediate. Finally, the low energy level of the pyridine adduct
N (-6.83 kcal mol-1) indicates the high thermal stability of
such Lewis-base adducts, a feature discussed previously by other
investigators.35
C. CsH Activations Initiated by CpMo(NO)(C6H5)-
(CH2CMe3). All the optimized structures are shown in Figure
10, and their key structural parameters are summarized in Table
4 and Table S2 in the Supporting Information. The relative
energies of the various species are collected in Table 5 and are
presented pictorially in Figure 11. The MosC1 bond lengths
in the optimal structures of DD, EE, and FF-TS are slightly
9
J. AM. CHEM. SOC. VOL. 125, NO. 23, 2003 7043

A R T I C L E S
Wada et al.
Figure 10. DFT optimized geometries of complexes in the CpMo(NO)(C6H5)(CH2CMe3) (GG) system.
Table 4. DFT Optimized Structural Parameters for Neopentane
on the substrates involved, thereby indicating that the rate of
traversal of the two competitive C-H bond-activation pathways
depends on the nature of the hydrocarbon substrates. The
theoretical part of this work clearly indicates the presence of
σ- or π-hydrocarbon complexes on the reaction coordinates of
the alkylidene and the benzyne intermediates.13,36 Consistent
with the calculations is the observation that trapping of the
intermediates with Lewis bases such as pyridine results in more
of the trapped neopentylidene complex, since it is more readily
formed from 6. However, the neopentylidene intermediate is
computed to effect C-H bond activations more slowly, and thus,
with substrates such as benzene and tetramethylsilane, the
products resulting from the benzyne intermediate should pre-
dominate, as is indeed observed experimentally. In other words,
the less abundant intermediate is responsible for the majority
of the C-H activated products. This qualitative agreement of
theory and experiment is gratifying, especially since the
theoretical calculations were effected on the model Cp com-
plexes in the gas phase rather than the actual Cp* complexes
in solutions for which solvent effects would also be expected
to play a role.
σ-Complexes HH-TS and II (see Figure 10)
parameter
HH-TS
II
Mo-C1
2.407
2.100
2.228
1.861
1.460
1.534
1.784
1.237
116.9
80.1
2.905
2.107
2.138
2.156
1.123
2.492
1.786
1.238
127.3
90.1
Mo-C2
Mo-C3
Mo-H*
C1-H*
C3-H*
Mo-N1
N-O
C1-Mo-C2
C1-Mo-C3
Mo-N-O
172.6
173.2
Table 5. Relative Energies of Complexes in the
CpMo(NO)(C6H5)(CH2CMe3) Model System (see Figure 11)
∆E
∆G
(kcal mol-1
)
complex
(kcal mol-1
)
AA
BB-TS
CC
DD
EE
FF-TS
GG
HH-TS
II
J
K
4.58
31.92
14.79
24.60
12.38
23.87
0.00
34.70
22.09
29.42
20.59
28.09
-2.61
-13.52
4.48
33.41
13.39
12.51
11.00
24.44
0.00
36.30
19.74
17.39
18.92
28.78
-3.81
-14.18
Epilogue
Our initial investigations of the thermal reactivity of 1 and 6
have revealed several unique characteristics of CsH bond
activation by the reactive molybdenum alkylidene and benzyne
complexes, Cp*Mo(NO)(dCHCMe3) and Cp*Mo(NO)(η2-
C6H4). First, the activation of CsH bonds occurs at room
temperature via the 1,2-CsH addition of the substrate to the
ModC double bond of Cp*Mo(NO)(dCHCMe3) with a slight
kinetic preference for the strongest CsH bond. Second, parallel
modes of CsH bond activation can be initiated from 6 by
elimination of either alkane or arene, resulting in the simulta-
neous formation of the Cp*Mo(NO)(dCHCMe3) and Cp*Mo-
(NO)(η2-C6H4) intermediates, both of which can activate CsH
bonds. It is thus not unreasonable to expect that thermally
induced R-H elimination of various hydrocarbons from other
mixed-hydrocarbyl congeners of the large family of available
L-TS
M
OO
oxo imido isomers, Cp′M[N(CH3)](O)(CH3), via nitrosyl Ns
O bond cleavage under the experimental conditions employed.38
In summary, thermolysis of the neopentyl phenyl complex 6
in hydrocarbon solutions affords products via both the neopen-
tylidene and benzyne intermediates at ambient temperatures.
However, the products are formed in different ratios depending
(38) Sharp, W. B.; Daff, P. J.; McNeil, W. S.; Legzdins, P. J. Am. Chem. Soc.
2001, 123, 6272-6282.
9
7044 J. AM. CHEM. SOC. VOL. 125, NO. 23, 2003

C−H Activation by Neopentylidene and Benzyne Complexes
A R T I C L E S
Figure 11. Reaction profile showing the calculated energies of complexes derived from CpMo(NO)(C6H5)(CH2CMe3) (GG) (see Table 5).
(Me3CCH2)2Mg‚x(dioxane) (0.486 g, 1.85 mmol) at -196 °C was added
THF (40 mL) by vacuum transfer. The resulting brown suspension was
warmed to 0 °C and stirred for 20 min, whereupon a purple solution
formed. The volatiles were then removed from the reaction mixture
under reduced pressure. The flask containing the residue was charged
with a second portion of (Me3CCH2)2Mg‚x(dioxane) (0.462 g, 1.76
mmol), and Et2O (40 mL) was added by vacuum transfer at -196 °C.
The resulting mixture was allowed to warm to room temperature, and
the red suspension was stirred for 30 min. The solvent was removed
from the final reaction mixture in vacuo, and the residue was extracted
with 4:1 hexanes/Et2O (3 × 10 mL). The red extracts were filtered
through a column of neutral alumina (3 × 1 cm) supported on a
medium-porosity frit, and the column was washed with 4:1 hexanes/
Et2O until the filtrate was colorless. The combined filtrates were
concentrated and stored at -30 °C for 2 days to obtain burgundy needles
of 1 (0.797 g, 56% in three crops).
Cp*Mo(NO)(R)(R′) complexes will provide a wide variety of
coordinatively unsaturated complexes suitable for the activation
of hydrocarbons in unprecedented ways. Our studies in this
regard with these and related complexes are currently in
progress, and the results of these investigations will be reported
in due course.
Experimental Section
General Methods. All reactions and subsequent manipulations were
performed under rigorously anaerobic and anhydrous conditions utiliz-
ing either high vacuum or an atmosphere of prepurified argon or
dinitrogen with conventional gloveboxes and vacuum-line Schlenk
techniques. The gloveboxes used were Innovative Technologies Lab-
Master 100 and MS-130 BG dual-station models equipped with freezers
maintained at -30 °C. Thermolysis reactions were performed in thick-
walled glass bombs equipped with Kontes greaseless stopcocks, and
NMR spectroscopic analyses were conducted in J. Young NMR tubes.
All solvents were dried and distilled prior to use or vacuum
transferred directly from the appropriate drying agent. Hydrocarbons
and diethyl ether were distilled from sodium benzophenone ketyl,
tetrahydrofuran was distilled from molten potassium, and dichlo-
romethane and chloroform were distilled from calcium hydride.
Deuterated solvents were purchased from Cambridge Isotope Labora-
tories and then degassed and vacuum-transferred from K (C6D6), Na
(toluene-d8), or activated 4 Å molecular sieves (CD2Cl2). Trimeth-
ylphosphine (Aldrich) was dried and stored over potassium. Pyridine
was dried over KOH and distilled onto activated 4 Å molecular sieves.
Celite and neutral alumina I (60-325 mesh) were oven-dried and cooled
under vacuum prior to use.
Anal. Calcd for C20H37NOMo: C, 59.54; H, 9.24, N, 3.47. Found:
C, 59.33; H, 9.51; N, 3.43. IR (Nujol): ν(NO) 1599 (s) cm-1. 1H NMR
2
(toluene-d8, 300 MHz, 0 °C) δ -1.06 (d, JHH ) 11.7 Hz, 2H, CHsyn
-
2
HCMe3), 1.37 (s, 18H, CMe3), 1.50 (s, 15H, C5Me5), 2.96 (d, JHH
11.7 Hz, 2H, CHantiHCMe3). Gated 13C NMR (toluene-d8, 75.5 MHz,
0 °C): δ 9.5 (q, JCH ) 127.2 Hz, C5Me5), 33.5 (q, JCH ) 124.2 Hz,
CMe3), 38.7 (s, CMe3), 97.4 (dd, 1JCH ) 101 Hz, 1JCH ) 125 Hz, CH2),
109.3 (s, C5Me5). MS (EI, 100 °C): m/z 405 [P+].
)
1
1
Preparation of Cp*Mo(NO)(CH2CMe3)(CH2SiMe3) (2). A red
solution of 1 (0.096 g, 0.24 mmol) in Me4Si (8 mL) was stirred at
room temperature for 30 h. The solution was then evaporated to dryness
in vacuo, and the red solid was extracted with hexanes (5 mL). The
extract was filtered through a column of neutral alumina, and the filtrate
was stored at -30 °C for several days to induce the deposition of 2 as
a red crystalline mass (0.032 g, 32%). The IR, NMR, and mass spectral
data of these crystals were identical to those reported previously for
2.10
Preparation of Cp*Mo(NO)(CH2CMe3)(η2-CH2C6H3-3,5-Me2) (3).
A red solution of 1 (0.100 g, 0.249 mmol) in mesitylene (10 mL) was
stirred at room temperature for 30 h. The final solution was taken to
dryness in vacuo, and the solid orange residue was extracted with 10:1
hexanes/Et2O (3 × 10 mL). The combined extracts were filtered through
a column of neutral alumina (0.5 × 5 cm), and the filtrate was stored
at -30 °C overnight to induce the deposition of 3 as yellow needles
(0.030 g, 27%).
Infrared spectra were recorded on an ATI Mattson Genesis Series
FT-IR spectrometer. NMR spectra were recorded at room temperature,
unless otherwise noted, on Bruker AV300 or AMX 500 spectrometers.
1H and 13C chemical shifts are recorded in ppm relative to the residual
proton or natural-abundance carbon signal(s) of the solvents employed;
coupling constants are reported in Hz. 2H{1H} NMR signals were
1
referenced to C6H5D (δ 7.15). Correlation and assignment of H and
13C NMR resonances were aided by 2D HMQC and HMBC experi-
ments when necessary. Low-resolution mass spectra (EI, 70 eV) were
recorded by the staff of the UBC mass spectrometry facility using a
Kratos MS-50 spectrometer. Elemental analyses were performed by
Canadian Microanalytical Service Ltd., Delta, B.C., Canada.
Anal. Calcd for C24H37MoN2O: C, 63.84; H, 8.26; N, 3.10. Found:
Preparation of Cp*Mo(NO)(CH2CMe3)2 (1). This complex was
prepared by a modification of the previously reported method.10,39 To
a Schlenk tube containing [Cp*Mo(NO)Cl2]2 (1.156 g, 1.74 mmol) and
C, 63.85; H, 8.23; N, 3.05. IR (Nujol): ν(NO) 1601 (s) cm-1. 1H NMR
2
(C6D6, 300 MHz, 25 °C) δ -2.48 (d, JHH ) 12.4 Hz, 1H, CHsyn
-
2
HCMe3), 1.12 (d, JHH ) 12.4 Hz, 1H, CHantiHCMe3), 1.16 (s, CMe3,
9H), 1.61 (s, C5Me5, 15H), 1.96 (s, C6H3Me2, 6H), 2.01 (d, 2JHH ) 4.7
(39) Legzdins, P.; Young, M. A.; Batchelor, R. J.; Einstein, F. W. B. J. Am.
Chem. Soc. 1995, 117, 8798-8806.
2
Hz, 1H, CHsynHC6H3Me2), 3.43 (d, JHH ) 4.7 Hz, 1H, CHantiHC6H3-
9
J. AM. CHEM. SOC. VOL. 125, NO. 23, 2003 7045

A R T I C L E S
Wada et al.
Me2), 5.94 (s, benzyl Hortho, 2H), 7.06 (s, benzyl Hpara, 1H). 13C{1H}
NMR (C6D6, 75 MHz, 25 °C) δ 10.6 (C5Me5), 21.1 (C6H3Me2), 34.8
(CMe3), 37.1 (CMe3), 46.0 (CH2C6H3Me2), 57.8 (CH2CMe3), 108.4 (C5-
Me5), 118.4 (benzyl Cipso), 131.1 (benzyl Cortho), 132.2 (benzyl Cpara),
138.5 (benzyl Cmeta). MS (EI, 100 °C): m/z 453 [P+].
Thermal Reaction of 1 in p-Xylene. A red solution of 1 (0.101 g,
0.251 mmol) in p-xylene (10 mL) was stirred at room temperature for
44 h. The final red solution was then taken to dryness in vacuo, and
the solid residue was dissolved in C6D6 for analysis by 1H NMR
spectroscopy. This analysis indicated the presence of Cp*Mo(NO)-
(CH2CMe3)(C6H3-2,5-Me2) (4, 73%) and Cp*Mo(NO)(CH2CMe3)(η2-
CH2C6H4-4-Me) (5, 27%). Attempts to separate complexes 4 and 5 by
column chromatography were unsuccessful, but the mixture was
sufficiently pure to afford an acceptable elemental analysis. Anal. Calcd
for C23H35MoNO: C, 63.15; H, 8.06; N, 3.20. Found: C, 63.13; H,
7.62; N, 3.59. MS (EI, 120 °C): m/z 439 [P+].
vacuo, the dark red residue was extracted with hexanes (2 × 5 mL),
and the extracts were filtered through a column of neutral alumina (0.5
× 5 cm) supported on glass wool. The filtrate was stored at -30 °C
overnight to induce the deposition of red blocks of 6-d6 (0.064 g, 62%).
Anal. Calcd for C21H25D6MoNO: C, 60.71; N, 3.37. Found: C,
60.56; N, 3.38. IR (KBr): ν(NO) 1611 (s) cm-1. 1H NMR (C6D6, 300
MHz, 25 °C) δ 1.24 (s, 9H, CMe3), 1.51 (s, 15H, C5Me5), 4.66 (s, 1H,
CDHanti). 2H{1H} NMR (C6D6, 77 MHz, 25 °C) δ -1.76 (br s, CDsynH),
7.15 (s, C6D6), 7.10 (br m, overlapping Dmeta and Dpara), 7.60 (br m,
D
ortho). 13C{1H} NMR (C6D6, 75 MHz, 25 °C) δ 10.0 (C5Me5), 33.4
1
(CMe3), 41.1 (CMe3), 111.1 (C5Me5), 112.6 (m, CHD), 126.5 (t, JCD
) 14.6 Hz, phenyl Cpara), 134.7 (t, 1JCD ) 24.1 Hz, phenyl Cortho), 180.1
(phenyl Cipso). Additional phenyl signals (Cmeta) are obscured by the
benzene-d6 peak. MS (EI, 150 °C): m/z 417 [P+].
The thermolysis of 1 in a 1:1 mixture of benzene/benzene-d6 at 23
°C yields an intermolecular kinetic isotope effect of 1.04(4), a value
similar to that exhibited by its tungsten congener at 70 °C.36
Preparation of Cp*Mo(NO)(dCHCMe3)(NC5H5) (8). A red solu-
tion of 1 (0.102 g, 0.253 mmol) in pyridine (8 mL) was stirred at room
temperature for 30 h. The final orange-yellow solution was taken to
dryness in vacuo, the remaining solid was dissolved in hexanes (10
mL), and the solution was filtered through a column of Celite (0.5 ×
2 cm) supported on glass wool. The filtrate was kept at -30 °C
overnight to induce the deposition of orange crystals which were
collected by filtration, washed with pentane (2 × 1 mL), and dried in
vacuo (0.058 g, 55% in two crops).
Characterization data for 4: 1H NMR (C6D6, 300 MHz, 25 °C) δ
1.24 (s, 9H, CMe3), 1.39 (s, 3H, aryl CH3), 1.53 (s, 15H, C5Me5), 1.55
(d, 1H, CHsynH), 2.14 (d, 1H, CHantiH), 2.16 (s, 3H, aryl CH3), 6.87
3
3
(br d, JHH ) 7.9 Hz, 1H, phenyl H), 7.09 (br d, JHH ) 7.9 Hz, 2H,
phenyl H). The methylene hydrogen doublets occurring at δ 1.55 (syn)
and 2.14 (anti) are partially obscured by the aryl CH3 and Cp*
2
resonances, respectively, and JHH could not be determined.
Characterization data for 5: 1H NMR (C6D6, 300 MHz, 25 °C) δ
-2.83 (d, JHH ) 12.4 Hz, 1H, CHsynHCMe3), 0.97 (d, JHH ) 12.4
Hz, 1H, CHantiHCMe3), 1.19 (s, 9H, CMe3), 1.57 (s, 15H, C5Me5), 2.00
2
2
2
(d, JHH ) 4.9 Hz, 1H, CHsynHAr), 2.13 (s, 3H, aryl CH3), 3.36 (d,
Anal. Calcd for C20H30MoN2O: C, 58.53; H, 7.37; N, 6.83. Found:
2JHH ) 4.9 Hz, 1H, CHantiHAr), 6.19 (d, JHH ) 7.9 Hz, 2H, phenyl
3
1
C, 58.39; H, 7.20; N, 6.87. H NMR (C6D6, 300 MHz) δ 1.60 (s, 9H,
3
3
H), 6.59 (d, JHH ) 7.9 Hz, 2H, phenyl H). These data are consistent
CMe3), 1.79 (s, 15H, C5Me5), 6.18 (t, JHH ) 6.9 Hz, 2H, pyridine
with those previously reported for this complex in CDCl3.12b
Thermal Reaction of 1 in Benzene. A red solution of 1 (0.205 g,
0.509 mmol) in C6H6 (10 mL) was stirred at room temperature for 30
h. The final red-purple solution was taken to dryness in vacuo, the
violet residue was extracted with pentane (5 × 5 mL), and the combined
extracts were filtered through a column of alumina (1 × 2 cm) supported
on a medium-porosity glass frit. The filtrate was concentrated under
reduced pressure to the point of incipient crystallization and was then
stored at -30 °C. After 12 h, the solution had deposited wine red
crystals of analytically pure Cp*Mo(NO)(CH2CMe3)(C6H5) (6) (0.056
g, 27%).
The solid remaining after pentane extraction was dissolved in Et2O
(5 mL), and the solution was filtered through a column of Celite (0.5
× 2 cm) supported on glass wool. The column was washed with Et2O
(3 × 5 mL), and the combined filtrates were concentrated under reduced
pressure to the point of incipient crystallization. Violet needles (0.064
g, 30%) of Cp*Mo(NO)(C6H5)2 (7) were deposited after the solution
had been stored at -30 °C for several hours
H
meta), 6.58 (t, 3JHH ) 7.6 Hz, 1H, pyridine Hpara), 8.15 (d, 3JHH ) 4.9
Hz, 2H, pyridine Hortho), 12.43 (s, 1H, ModCH). 13C{1H} NMR (C6D6,
75 MHz) δ 10.4 (C5Me5), 32.7 (CMe3), 49.5 (CMe3), 108.0 (C5Me5),
124.1 (pyridine Cmeta), 136.6 (pyridine Cpara), 154.8 (pyridine Cortho),
155.0 (pyridine Cortho), 298.4 (ModCH). MS (EI, 120 °C): m/z 412
[P+]. The NMR spectroscopic data for Cp*Mo(NO)(dCHCMe3)-
(NC5D5) (8-d5) recorded in pyridine-d5 are contained in Figure 6 and
are provided below.
1H NMR (pyridine-d5, 500 MHz) δ 1.47 (s, 9H, CMe3), 1.83 (s,
15H, C5Me5), 12.38 (s, 1H, ModCH). 13C{1H} NMR (pyridine-d5, 125
MHz) δ 10.4 (C5Me5), 32.6 (CMe3), 49.5 (CMe3), 108.4 (C5Me5), 124.6
(t, 1JCD ) 24.5 Hz, pyridine Cmeta), 137.4 (t, 1JCD ) 25.1 Hz, pyridine
1
C
para), 154.9 (t, JCD ) 28.3 Hz, pyridine Cortho), 298.1 (ModCH).
Preparation of Cp*Mo(NO)(η2-C6H4)(NC5H5) (9). A red solution
of 6 (0.056 g, 0.136 mmol) in pyridine (5 mL) was stirred at room
temperature for 24 h. The final solution was taken to dryness in vacuo,
the remaining orange solid was dissolved in Et2O (10 mL), and the
solution was filtered through a column of Celite (0.5 × 2 cm) supported
on glass wool. The filtrate was stored at -30 °C for 4 h to induce the
deposition of 9 as a yellow solid. The product was isolated by decanting
the orange supernatant solution (from which complex 8 can be
recovered), and the yellow precipitate was washed with cold Et2O (2
× 1 mL) and pentane (2 × 1 mL) and then dried in vacuo (0.010 g,
17%). Recrystallization of the yellow solid from benzene provided
yellow needles of 9‚C6H6 suitable for analysis by X-ray diffraction.
The benzyne complexes Cp*Mo(NO)(η2-C6H4)(NC5H5) (9) and Cp*Mo-
(NO)(η2-C6H4)(NC5D5) (9-d5) are not sufficiently soluble in C6D6 to
characterize by 13C{1H} NMR spectroscopy. Consequently their
diagnostic NMR spectra were obtained with pyridine-d5 solutions.
Anal. Calcd for C21H24MoN2O: C, 60.58; H, 5.81; N, 6.73. Found:
C, 60.02; H, 6.23; N, 6.36. IR (KBr): ν(NO) 1537 (s) cm-1, ν(C≡C)
Characterization data for 6. Anal. Calcd for C21H31MoNO: C, 61.61;
H, 7.63; N, 3.42. Found: C, 61.51; H, 7.91; N, 3.38. IR (KBr): ν(NO)
1612 (s) cm-1. 1H NMR (C6D6, 300 MHz, 25 °C) δ -1.75 (d, 2JHH
)
9.7 Hz, 1H, CHsynH), 1.25 (s, 9H, CMe3), 1.51 (s, 15H, C5Me5), 4.77
2
(d, JHH ) 9.7 Hz, 1H, CHantiH), 7.07 (m, 1H, phenyl Hpara), 7.16 (m,
2H, phenyl Hmeta), 7.57 (d, 3JHH ) 6.7 Hz, 2H, phenyl Hortho). 13C{1H}
NMR (C6D6, 75 MHz, 25 °C) δ 10.0 (C5Me5), 33.2 (CMe3), 41.3
(CMe3), 111.1 (C5Me5), 114.5 (CH2), 127.3 (phenyl Cpara), 127.6 (phenyl
C
meta), 135.4 (phenyl Cortho), 180.1 (phenyl Cipso). MS (EI, 120 °C):
m/z 411 [P+], 381 [P+ - NO].
The NMR and IR spectroscopic data for 7 matched those previously
reported for the compound isolated from the reaction of [Cp*Mo(NO)-
Cl(µ-Cl)]2 with (C6H5)2Mg‚x(dioxane).14 However, satisfactory elemen-
tal analyses for this compound are reported here for the first time. Anal.
Calcd for C22H25MoNO: C, 63.61; H, 6.07; N, 3.37. Found: C, 63.40;
H, 5.94; N, 3.34.
1
1583 cm-1. H NMR (pyridine-d5, 500 MHz, 25 °C) δ 1.67 (s, 15H,
3
C5Me5), 7.37 (t, JHH ) 6.4 Hz, 2H, pyridine Hmeta), 7.48 (m, 2H,
3
overlapping benzyne HmetaA and HmetaB), 7.76 (t, JHH ) 7.4 Hz, 1H,
Preparation of Cp*Mo(NO)(CHDCMe3)(C6D5) (6-d6). A red
solution of 1 (0.100 g, 0.248 mmol) in C6D6 (8 mL) was stirred at
room temperature for 16 h. The final solution was taken to dryness in
pyridine Hpara), 7.78 (m, 1H, benzyne HorthoΑ), 7.89 (m, 1H, benzyne
H
(pyridine-d5, 125 MHz, 25 °C) δ 10.1 (C5Me5), 110.2 (C5Me5), 125.8
3
orthoB), 9.02 (d, JHH ) 5.1 Hz, 2H, pyridine Hortho). 13C{1H} NMR
9
7046 J. AM. CHEM. SOC. VOL. 125, NO. 23, 2003

C−H Activation by Neopentylidene and Benzyne Complexes
A R T I C L E S
(pyridine Cmeta), 127.6 (benzyne CorthoB), 128.8 (benzyne CorthoA), 129.1
(benzyne CmetaA), 129.5 (benzyne CmetaB), 138.6 (pyridine Cpara), 156.3
(pyridine Cortho), 158.4 (benzyne Cipso), 162.2 (benzyne Cipso). MS (EI,
120 °C): m/z 418 [P+]. The spectra of 9-d5 in pyridine-d5 (e.g., Figure
6) are identical except that no pyridine signals are evident.
(NO)(dCHCMe3)(NC5H5) (8), basically the same strategy was fol-
lowed. The position of the unique methine hydrogen was located from
a difference Fourier map while all other hydrogens were placed in
calculated positions. In the subsequent least-squares refinement, all non-
hydrogen atoms were refined anisotropically, and all hydrogens were
refined isotropically in a riding manner along with their attached carbons
except the unique hydrogen of the dCH group which was allowed to
refine independently. The final position of that H atom corresponds to
a normal sp2-hybridized carbon atom with very little hint of an Mo‚‚
‚H agostic interaction.
Thermal Reaction of 6-d6 in C6D6. A glass bomb equipped with a
Kontes valve was charged with a solution of 6-d6 (0.012 g, 0.029 mmol)
in C6D6 (4 mL) and maintained at 35 °C for 48 h. A 1H NMR spectrum
of the final solution indicated the incomplete formation of 7-d10 (δ
1.51) and neopentane-d2 (br, δ 0.86). The proton-decoupled deuterium
spectrum of the final solution was also recorded. 2H{1H} NMR (C6D6,
76.8 MHz, 25 °C) δ -1.75 (s, CDsynH of unreacted 6-d6), 0.86 (s,
neopentane-d2), 7.15 (C6D6), 7.57 (s, phenyl), 7.70 (s, phenyl). No
deuterium signal was evident in the Cp* methyl region (ca. δ 1.5).
Thermal Reaction of 6 in Tetramethylsilane. Thermolysis of 6
(0.008 g, 0.020 mmol) in Me4Si (5 mL) at room temperature for 48 h
results in the formation of a violet solution. Evaporation of the volatile
components from the final reaction mixture provided a deep red solid
that was dissolved in C6D6. Analysis of this solution by NMR
spectroscopy revealed a mixture of Cp*Mo(NO)(CH2CMe3)(CH2SiMe3)
(2, 41%) and Cp*Mo(NO)(CH2SiMe3)(C6H5) (10, 59%) (product ratio
determined after ca. 70% conversion). Product ratios were determined
by integration of the distinctive methylene proton resonances which
appear between 2.5 and 5.0 ppm. Complex 10 was not isolated but
Data collection and structure solution for Cp*Mo(NO)(CH2CMe3)-
(η2-CH2C6H3-3,5-Me2) (3), Cp*Mo(NO)(CH2CMe3)(C6H5) (6), and
Cp*Mo(NO)(η2-C6H4)(NC5H5)‚C6H6 (9‚C6H6) were conducted at the
University of British Columbia. All measurements were recorded at
-100(1) °C on a Rigaku/ADSC CCD area detector using graphite-
monochromated Mo KR radiation. For 3, the data were collected to a
maximum 2θ value of 55.8° in 0.50° oscillations with 12.0 s exposures.
A sweep of data was done using φ oscillations from 0.0 to 190.0° at ø
) -90.0°, and a second sweep was performed using ω oscillations
between -17.0° and 23.0° at ø ) -90°. The crystal-to-detector distance
was 38.19 mm, and the detector swing angle was -5.58°. For 6, the
data were collected to a maximum 2θ value of 55.8° in 0.50° oscillations
with 20.0 s exposures. A sweep of data was done using φ oscillations
from 0.0 to 190.0° at ø ) -90.0°, and a second sweep was performed
using ω oscillations between -17.0° and 23.0° at ø ) -90°. The
crystal-to-detector distance was 38.14 mm, and the detector swing angle
was -5.60°. For 9‚C6H6, the data were collected to a maximum 2θ
value of 55.7° in 0.50° oscillations with 57.0 s exposures. A sweep of
data was done using φ oscillations from 0.0° to 190.0° at ø ) -90.0°,
and a second sweep was performed using ω oscillations between -17.0°
and 23.0° at ø ) -90°. The crystal-to-detector distance was 37.99 mm,
and the detector swing angle was -5.60°. All three data sets were
corrected for Lorentz and polarization effects. The solid-state structures
were solved by direct methods43 and expanded using Fourier tech-
niques.44 The non-hydrogen atoms were refined anisotropically. For 3,
only the methylene hydrogens on C(11) and C(20) were refined
isotropically. For 6, the phenyl hydrogens and the methylene hydrogens
on C(17) were refined isotropically. All other hydrogen atoms in 3, 6,
1
1
was characterized in situ by H NMR spectroscopy. H NMR (C6D6,
2
300 MHz, 25 °C) δ -0.43 (d, JHH ) 10.7 Hz, 1H, CHsynH), 0.23 (s,
9H, CMe3), 1.49 (s, 15H, C5Me5), 2.79 (d, 2JHH ) 10.7 Hz, 1H, CHantiH),
7.09 (m, 3H, phenyl Hmeta and Hpara), 7.46 (d, 3JHH ) 6.6 Hz, 2H, phenyl
Hortho).
Thermal Reaction of 6 in PMe3. Thermolysis of 6 (0.049 g, 0.122
mmol) in PMe3 (1 mL) at room temperature for 24 h resulted in the
formation of a brown solution containing Cp*Mo(NO)(dCHCMe3)-
1
(PMe3). This complex was not isolated but was characterized by a H
NMR spectrum of the dried reaction residue in C6D6. 1H NMR (C6D6,
2
300 MHz, 25 °C) δ 0.95 (d, JHP ) 8.5 Hz, 9H, PMe3), 1.40 (s, 9H,
CMe3), 1.84 (s, 15H, C5Me5), 12.38 (d, 3JHP ) 4.4 Hz, 1H, ModCH).
X-ray Crystallographic Analyses. Data collection and structure
solution for Cp*Mo(NO)(CH2CMe3)2 (1) and Cp*Mo(NO)(dCHCMe3)-
(NC5H5) (8) were conducted at the University of Southern California.
X-ray data were collected at low temperature (130(2) K for 1 and 160-
(5) K for 8) on a Bruker SMART APEX CCD diffractometer with
graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å). The cell
parameters for each compound were obtained from the least-squares
refinement of the spots (from 60 collected frames) using the SMART
program.40 In each case, a hemisphere of data was collected up to a
resolution of 0.75 Å, and the intensity data were processed using the
SAINT PLUS program.41 All calculations for the structural analyses
were carried out using the SHELXTL package.42 Initial positions of
the Mo atoms were located by direct methods, and the rest of the atoms
were found by using conventional heavy-atom techniques. The struc-
tures were ultimately refined by least-squares methods using data in
the range of 2θ ) 3.5-55.0°.
and 9‚C6H6 were placed in calculated positions. The final cycles of
2
full-matrix least-squares refinements (function minimized, Σw(Fo
-
Fc2)2) were based on 5058 observed reflections and 270 variable
parameters for 3, 4610 observed reflections and 253 variable parameters
for 6, and 5081 observed reflections and 285 variable parameters for
9‚C6H6. All calculations were performed using the teXsan crystal-
lographic software package of Molecular Structure Corporation45 and
SHELXL-97.42 Selected crystallographic data for the X-ray structures
of 1, 3, 6, 8, and 9‚C6H6 are presented in Table 1. Full details of all
crystallographic analyses are provided as Supporting Information.
Kinetic Studies. Kinetic studies of the reaction of 1 in C6D6 were
performed using a J. Young NMR tube containing a solution of 1 (12
mg, 0.03 mmol) in 1 mL of C6D6 with a fused capillary tube containing
hexamethyldisilane as an internal standard. The reaction was conducted
in the NMR probe at constant temperature, and 1H NMR spectra were
recorded at intervals. The rate constants were calculated using linear
regression methods from first-order plots of the loss of starting material
versus time.
For the complex Cp*Mo(NO)(CH2CMe3)2 (1), all hydrogen atoms
were placed in calculated positions except the four H atoms of the two
CH2 groups. Those were readily located from difference Fourier maps
and were subsequently refined isotropically. The structure of 1 is
remarkably similar to that of the analogous tungsten complex, Cp*W-
(NO)(CH2CMe3)2.8 As in the case of the tungsten analogue, one of the
neopentyl groups in 1 has disordered methyl groups, but fortunately,
the methylene portion of this CH2CMe3 ligand was ordered, and its
two H atoms could be located unambiguously. For the complex Cp*Mo-
Theoretical DFT Calculations. To compare the single-point ener-
gies of model complexes in a consistent manner, all calculations were
carried out using geometries optimized by density functional theory
(43) For SIR97, see: Altomare, A.; Burla, M. C.; Cammalli, G.; Cascarano,
M.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.;
Spagna, A. J. Appl. Crystallogr. 1999, 32, 115-119.
(44) DIRDIF-94 program system; Technical Report of the Crystallography
Laboratory; University of Nijmegen: The Netherlands, 1994.
(45) teXsan, Crystal Structure Analysis Package; Molecular Structure Corpora-
tion: 3200 Research Forest Drive, The Woodlands, TX, 77381, 1985 and
1992.
(40) SMART APEX program system, version 5.0; Bruker Analytical X-ray
System, Inc.: Madison, WI, 2001.
(41) SAINT PLUS program system, version 5.0; Bruker Analytical X-ray System,
Inc.: Madison, WI, 2001.
(42) Sheldrick, G. M. SHELXTL, version 5.1; Bruker Analytical X-ray System,
Inc.: Madison, WI, 1997.
9
J. AM. CHEM. SOC. VOL. 125, NO. 23, 2003 7047

A R T I C L E S
Wada et al.
(DFT). Calculations were performed using the Gaussian 98 RevA.11.146
implementation of B3LYP (Becke three-parameter exchange functional
(B3)47 and the Lee-Yang-Parr correlation functional (LYP)48) in the
parallel execution environment using the Scientific Computing As-
sociates’ Linda program suite on the Fujitsu VPP-800 supercomputer
at the Academic Center for Computing and Media Studies, Kyoto
University. A part of the computations was performed on Intel PII
computers at the University of British Columbia. Four basis sets were
considered. Most of the calculations utilized the LanL2DZ basis set
that included D95 basis functions for H, C, N, and O49 and the
relativistic electron core potential (ECP) sets of Hay and Wadt for Mo.50
In this connection, it should be noted that the alkylidene fragment
CpMo(NO)(dCH2) D, the σ-methane complex CpMo(NO)(dCH2)(σ-
CH4) C, the dimethyl complex CpMo(NO)(CH3)2 A, and the transition-
state species CpMo(NO)(dCH2‚‚‚H‚‚‚CH3) B-TS have previously been
optimized by Poli and Smith.35 The second basis set, designated SDD,
was the combination of the Huzinage-Dunning double-ú basis set on
lighter elements with the Stuttgart-Dresden basis set-RECP combina-
tion.51 The third basis set, LanL2DZ-II, combined the D95* basis
functions on the lighter elements with LanL2DZ on the molybdenum
atom, supplemented by f-type polarization functions taken from the
work of Frenking and co-workers.52 In the fourth basis set, LanL2DZ-
III, the modified ECP in which the two outermost p functions were
replaced by a (41) split of the optimized 5p function53 was employed
for the Mo atoms together with the 6-31G* basis set for all C, N, and
O atoms, the 6-31G** basis set for all H atoms on CH3 and C6H5
groups, and the 6-31G basis set for all cyclopentadienyl H atoms. No
constraints were imposed on any of the systems. Frequency calculations
on optimized species established that all the transition states possessed
only one imaginary frequency and that the products and intermediates
possessed no imaginary frequencies. Energies with thermal corrections
(E) and Gibbs free energies (G) were computed for species in the gas
phase at standard temperature (298.15 K) and pressure (1 atm). Spatial
plots of the optimized geometries and frontier orbitals were obtained
from Gaussian 98 output using Cambridge Soft Corporation’s Chem
3D Pro v4.0.
Acknowledgment. We are grateful to the Natural Sciences
and Engineering Research Council of Canada for support of
this work in the form of grants to P.L. We also thank Professor
W. Stephen McNeil for his advice and assistance with the DFT
calculations and Professor Take-aki Mitsudo for enabling our
use of the supercomputer at Kyoto University. P.L. is a Canada
Council Killam Research Fellow.
(46) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.11.1; Gaussian,
Inc.: Pittsburgh, PA, 1998.
Supporting Information Available: Full details of crystal-
lographic (CIF) and theoretical (PDF) analyses. This material
is available free of charge via the Internet at http://pubs.acs.org.
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(48) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. 1988, B37, 785-789.
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