Unusual thermal C-H bond activation by a tungsten allene complex

Article abstract of DOI:10.1021/ja026951p

Gentle thermolysis of the 18e alkyl-allyl complex, Cp*W(NO)(CH₂CMe₃)(η³-3,3-Me₂C₃H₃) (1), generates a reactive 16e allene intermediate, Cp*W(NO)(η²-CH₂=C=CMe₂) (A), with the concomitant evolution of neopentane via hydrogen abstraction from the dimethylallyl ligand. A has been structurally characterized as its PMe₃ adduct and is capable of effecting single and multiple C-H bond activations of hydrocarbon solvents. For example, the thermal reaction of 1 with cyclohexane leads to the formation of the 18e cyclohexenyl hydrido complex, Cp*W(NO)(η³-C₆H₉)(H) (5), as a result of three successive C-H activations of the alkane solvent. Copyright

Full text of DOI:10.1021/ja026951p

Published on Web 07/23/2002
Unusual Thermal C-H Bond Activation by a Tungsten Allene Complex1
Stephen H. K. Ng, Craig S. Adams, and Peter Legzdins*
Department of Chemistry, The UniVersity of British Columbia, VancouVer, British Columbia, Canada V6T 1Z1
Received May 17, 2002
The search for complexes that exhibit selective C-H bond
activation is the focus of considerable research in organometallic
chemistry at the present time, primarily due to the potential for
functionalization of readily available hydrocarbons into more
desirable products.2,3 Our previous contributions to this area have
included the development of 16e alkylidene4,5 and acetylene6-9
nitrosyl complexes of tungsten that activate aliphatic and aromatic
C-H bonds selectively under mild conditions. We now wish to
report the generation of a reactive 16e allene complex, Cp*W(NO)-
(η2-H2CdCdCMe2) (A), that effects single and triple C-H bond
activations of hydrocarbon substrates under thermal conditions.10
The chemistry we have discovered is summarized in Scheme 1.
Scheme 1
Figure 1. Solid-state molecular structure of 2 with 50% probability thermal
ellipsoids shown. Selected interatomic distances (Å) and angles (deg):
W(1)-N(1) ) 1.760(5), W(1)-C(16) ) 2.217(6), W(1)-C(11) ) 2.182(6),
W(1)-C(12) ) 2.433(6), N(1)-O(1) ) 1.237(7), C(11)-C(12) ) 1.46(1),
C(12)-C(13) ) 1.348(9), W(1)-N(1)-O(1) ) 170.7(5), N(1)-W(1)-
C(16) ) 96.7(2), W(1)-C(11)-C(12) ) 81.2(4), C(11)-C(12)-C(13) )
124.9(7), C(12)-C(13)-C(14) ) 125.6(7), C(12)-C(13)-C(15) ) 120.1(7).
69% yield by recrystallization from ether/hexanes at -30 °C.
Consistently, a similar thermal reaction of 1 in C6D6 leads to the
formation of the corresponding deuterated complex, Cp*W(NO)-
(C6D5)(η3-3,3-Me2-allyl-d1) (3-d6). Interestingly, 3-d6 exists as a
mixture of three isotopomers, with deuterium being incorporated
Thermolysis of the 18e complex Cp*W(NO)(CH2CMe3)(η3-3,3-
at the two methyl groups and the central allyl carbon, whose exact
Me2C3H3) (1)11 in Me4Si at 50 °C for 6 h results in the evolution
of neopentane and the quantitative formation (as determined by
1H NMR spectroscopy) of the 18e alkyl-allyl complex, Cp*W-
(NO)(CH2SiMe3)(η3-3,3-Me2C3H3) (2). Complex 2 is thermally
stable in Me4Si, and the known12 bis(alkyl) complex, Cp*W(NO)-
(CH2SiMe3)2, is not formed on prolonged reaction times. 2 is an
orange air-stable solid that can be isolated in 60% yield by
crystallization from the final reaction mixture at -30 °C. Its solid-
state molecular structure13 (Figure 1) exhibits metrical parameters
that are consistent with solution NMR data14 and reflect σ-π
distortion15 of the dimethylallyl ligand (i.e., C(11)-C(12) ) 1.46-
(1) Å and C(12)-C(13) ) 1.348(9) Å). Furthermore, the asym-
metric allyl ligand is rotated away from the idealized exo or endo
orientations to maximize the π-interaction between the nonbonding
orbital of the allyl ligand and the metal center.16
modes of formation remain to be ascertained. The thermolysis
reaction in C6D6 exhibits first-order kinetics for the loss of 1 at 50
°C (kobs ) 2.2(1) × 10-4 s-1), a feature consistent with the rate-
determining step being the intramolecular generation of interme-
diate A.
The thermal reaction of 1 in a solution of excess trimethylphos-
phine (10 equiv) in cyclohexane cleanly affords the base-stabilized
form of A, Cp*W(NO)(η2-H2CdCdCMe2)(PMe3) (4), which has
been isolated in 24% yield. The solution molecular structure of 4
is readily inferred from its 13C {1H} and 1H NMR spectra (C6D6),
which display a central allene carbon resonance at δ 165.67 and
CH3 (δ ) 2.44, 1.78), CH2 (δ ) 0.67, 1.97), and PMe3 (δ ) 1.27)
proton resonances which also exhibit the expected NOE enhance-
ments (as determined by selective NOE spectroscopy).17,18 A single-
crystal X-ray crystallographic analysis of 4 has been conducted,
and the resulting ORTEP of its solid-state molecular structure is
shown in Figure 2.19 The nonlinear nature of the dimethylallene
carbon backbone (C(11)-C(12)-C(13) ) 134.0(5)°) and the
relatively long C(11)-C(12) bond length of 1.436(7) Å are
In a similar manner, Cp*W(NO)(C6H5)(η3-3,3-Me2C3H3) (3) is
produced quantitatively via thermolysis of 1 under the same
conditions in benzene solution. 3 is isolable as orange blocks in
* To whom correspondence should be addressed. E-mail: legzdins@chem.ubc.ca.
9
9380
J. AM. CHEM. SOC. 2002, 124, 9380-9381
10.1021/ja026951p CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Acknowledgment. We are grateful to the Natural Sciences and
Engineering Council of Canada for support of this work in the form
of grants to P.L. and postgraduate scholarships to C.S.A. We also
thank Dr. B. O. Patrick of this Department for assistance during
the X-ray crystallographic analyses of 2 and 4. P.L. gratefully
acknowledges The Canada Council for the Arts for the award to
him of a Killam Research Fellowship.
Supporting Information Available: Experimental procedures and
complete characterization data for complexes 1-5 and full details of
the crystal structure analysis including associated tables for 2 and 4
(PDF). This material is available free of charge via the Internet at http://
pubs.acs.org.
References
(1) Presented in part at the 223rd National Meeting of the American Chemical
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Figure 2. Solid-state molecular structure of 4 with 50% probability thermal
ellipsoids shown. Selected interatomic distances (Å) and angles (deg):
W(1)-N(1) ) 1.795(4), W(1)-C(11) ) 2.205(4), W(1)-C(12) ) 2.148(5),
W(1)-P(1) ) 2.4660(12), N(1)-O(1) ) 1.224(5), C(11)-C(12) ) 1.436(7),
C(12)-C(13) ) 1.324(7), W(1)-N(1)-O(1) ) 175.0(4), N(1)-W(1)-
C(11) ) 91.00(18), C(12)-W(1)-C(13) ) 38.49(18), C(11)-C(12)-
C(13) ) 134.0(5), C(12)-C(13)-C(14) ) 121.8(5), C(12)-C(13)-C(15)
) 124.3(5).
(4) Adams, C. S.; Legzdins, P.; Tran, E. J. Am. Chem. Soc. 2001, 123, 612.
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manifestations of the considerable back-donation of electron density
from the metal to the alkene π* orbitals.20
(10) Thermolyses of alkyl-allyl complexes of iridium at 120 °C are believed
to form similar allene intermediates. However, these intermediates have
not been isolated or characterized, and they are only capable of effecting
single C-H bond activations; see ref 22.
Most interestingly, the thermolysis of 1 in cyclohexane produces
principally the cyclohexenyl hydrido complex, Cp*W(NO)(η3-
C6H9)(H) (5), which can be isolated in 32% yield. 5 formally results
from three C-H bond activations of the hydrocarbon solvent.21 Its
1H NMR spectrum (C6D6) contains a distinctive hydride resonance
at δ -0.57 (1JWH ) 131.7 Hz), and its IR spectrum as a KBr pellet
exhibits a νWH stretch at 1898 cm-1.4 GC-MS studies also reveal
the presence of the coupled organic product, 1,1-dimethylpropyl-
cyclohexane, in the final reaction mixture. This transformation
constitutes a novel mode of multiple C-H activations of cyclo-
hexane, a relatively inert solvent that has frequently been used to
study the C-H activations of other hydrocarbons.22-24 For com-
parison, the well-studied bis(alkyl) species such as Cp*W(NO)-
(CH2CMe3)2 reacts with cyclohexane in a completely different
manner. Their thermolyses in the presence of an excess of
trimethylphosphine in cyclohexane at 70 °C for 40 h result in the
formation of two base-stabilized complexes, the alkylidene and the
cyclohexene adducts. However, in the absence of a suitable Lewis
base, only decomposition occurs to afford intractable products.4
In summary, we have succeeded in generating a reactive 16e
allene complex by the gentle thermolysis of an alkyl-allyl
precursor. We have isolated and fully characterized this allene
complex as its 18e PMe3 adduct. In addition to effecting single
C-H bond activations which are the reverse of the reaction used
to generate it, the 16e allene complex also effects multiple C-H
bond activations of a substrate that is normally difficult to activate.
These reactivity differences suggest that new avenues of alkane
activation chemistry may well be accessible through alkyl-allyl
complexes such as 1. Studies in this regard are ongoing, and the
results of these investigations will be reported in due course.
(11) 1 is prepared by the metathesis reaction of the chloro precursor with the
appropriate allyl Grignard reagent.
(12) Bau, R.; Mason, S. A.; Patrick, B. O.; Adams, C. S.; Sharp, W. B.;
Legzdins, P. Organometallics 2001, 20, 4492.
(13) Crystal data for 2: monoclinic, space group C2/c, a ) 15.3944(6) Å,
b ) 8.4977(3) Å, c ) 32.326(1) Å, â ) 90.753(3)°, V ) 4228.4(3) Å3,
Z ) 8, R1 ) 0.053, wR2 ) 0.0107, and GOF(F 2) ) 1.60 for 4410
reflections and 221 variables.
(14) For instance, the 13C{1H} NMR spectrum (C6D6) of 2 exhibits a resonance
at 101.93 ppm (allyl-CH) characteristic of an sp2-like carbon and a signal
at 39.31 ppm (allyl-CH2) indicative of an sp3-like terminal carbon. In
addition, the orientation of the Me2C allyl terminus trans to NO (as shown
in Scheme 1 and Figure 1) is further supported by evidence gathered from
selective NOE spectroscopy experiments.
(15) Greenhough, T. J.; Legzdins, P.; Martin, D. T.; Trotter, J. Inorg. Chem.
1979, 18, 3268.
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(19) Crystal data for 3: monoclinic, space group P21/c, a ) 9.1458(5) Å, b )
13.6500(7) Å, c ) 15.9576(9) Å, â ) 98.724(3)°, V ) 1969.10(18) Å3,
Z ) 4, R1 ) 0.0358, wR2 ) 0.0667, and GOF(F 2) ) 0.946 for 4135
reflections and 217 variables.
(20) The exceptionally strong π-donor ability of the related Cp*W(NO)(PPh3)
fragment has been documented, see: Burkey, D. J.; Debad, J. D.; Legzdins,
P. J. Am. Chem. Soc. 1997, 119, 1139.
(21) Preliminary single-crystal X-ray crystallographic analysis of 5 has
confirmed the atomic connectivity shown in Scheme 1. However, the
quality of the diffraction data collected precludes meaningful discussion
of its metrical parameters.
(22) McGhee, W. D.; Bergman, R. G. J. Am. Chem. Soc. 1988, 110, 4246.
(23) Cummins, C. C.; Baxter, S. M.; Wolczanski, P. T. J. Am. Chem. Soc.
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