Parallel transformations of cyclohexene mediated by the Cp*W(NO) fragment

Article abstract of DOI:10.1021/ja0426692

Hydrogenolysis of Cp*W(NO)(CH₂CMe₃)₂ at room temperature in cyclohexene results in the formation of the intermediate 16e organometallic complex, [Cp*W(NO)(η²-cyclohexene)]. This intermediate leads to three parallel transformations of cyclohexene, namely (a) C-H activation of cyclohexene to form an η³-cyclohexenyl hydrido complex, (b) combination of cyclohexene and H₂ to form a cyclohexyl hydrido complex, and (c) coupling of two molecules of cyclohexene with concomitant loss of two hydrogen atoms to form a complex containing a novel η¹,η³-(cyclohexyl)cyclohexenyl ligand. Single-crystal X-ray crystallographic analyses of products resulting from transformations (b) and (c) have been effected. Copyright

Full text of DOI:10.1021/ja0426692

Published on Web 04/22/2005
Parallel Transformations of Cyclohexene Mediated by the Cp*W(NO)
Fragment
Xing Jin, Peter Legzdins,* and Miriam S. A. Buschhaus
Department of Chemistry, The UniVersity of British Columbia, VancouVer, British Columbia, Canada V6T 1Z1
Received December 6, 2004; E-mail: legzdins@chem.ubc.ca
One of the principal uses of transition-metal organometallic
complexes is as reagents and catalysts in organic syntheses.1 Most
of these applications exploit the fact that the chemistry of an organic
molecule bound to a transition-metal center is markedly different
from that exhibited by the molecule in its free state. In recent years,
considerable attention has focused on simple hydrocarbons as the
organic molecules to be transformed, primarily due to the potential
for functionalization of these readily available materials into more
desirable, higher-value products.2 Our previous contributions to this
area have included the development of 16-electron organometallic
complexes of molybdenum and tungsten based on the Cp*M(NO)
(Cp* ) η5-C5Me5; M ) Mo, W) fragments for the sequential
activation of hydrocarbon C-H bonds.3 We now wish to report
that the Cp*W(NO) fragment can also be used to mediate the three
transformations of cyclohexene summarized in Scheme 1.
The three transformations are initiated by coordination of
cyclohexene to the tungsten center and formally involve (a)
intramolecular C-H activation of cyclohexene to form an η3-
Figure 1. Solid-state molecular structure of 3 with 50% probability thermal
ellipsoids shown. Selected interatomic distances (Å) and angles (deg):
cyclohexenyl hydrido complex,4 (b) combination of cyclohexene
and H2 to form an η1-cyclohexyl hydrido complex,5 and (c) coupling
of two molecules of cyclohexene with concomitant loss of two
hydrogen atoms to form a novel η1,η3-(cyclohexyl)cyclohexenyl
ligand.6 The requisite 16e [Cp*W(NO)(η2-cyclohexene)] intermedi-
ate can be generated by hydrogenolysis of an appropriate Cp*W-
(NO)(hydrocarbyl)2 precursor in cyclohexene. The specific bis-
(neopentyl) system that we have examined in detail is illustrated
in Scheme 2 along with our rationale that accounts for the formation
of the isolated products 1, 2, and 3. Thus, treatment of wine-red
Cp*W(NO)(CH2CMe3)2 with H2 (1 atm) in neat cyclohexene in a
1:30:100 molar ratio under ambient conditions for 3 h results in
the formation of a red-brown solution that contains 1, 2, and 3 as
the main organometallic products in an approximate 6:1:2 ratio,
W(1)-N(1) ) 1.780(2), N(1)-O(1) ) 1.225(3), W(1)-C(1) ) 2.273(2),
W(1)-C(8) ) 2.284(2), W(1)-C(9) ) 2.268(2), W(1)-C(10) ) 2.434(2),
C(8)-C(9) ) 1.413(3), C(9)-C(10) ) 1.401(3), W(1)-N(1)-O(1) )
174.8(2), C(1)-W(1)-C(8) ) 70.62(9), C(8)-W(1)-C(10) ) 61.68(8),
C(8)-C(9)-C(10) ) 118.8(2).
Scheme 1
1
respectively (as estimated by H NMR spectroscopy). Extraction
of the dried reaction residue with cold Et2O and crystallization of
the extracted material from THF at -30 °C affords red diamond-
shaped crystals of 3. The remaining material was purified by
chromatography on silica with hexanes/Et2O as eluant to eventually
obtain 2‚Et2O as dark red crystals and 1 as a yellow powder.7
The identity of complex 1 as an η3-cyclohexenyl hydrido
complex has been established by its characteristic spectroscopic
properties, most notably its 1H NMR spectrum (C6D6) that contains
a distinctive hydride resonance at δ -0.57 (1JWH ) 131.7 Hz) and
is indicative of a WdW double bond that enables each tung-
sten center to attain an 18-valence-electron configuration. As
shown in Scheme 2, we believe that 2 arises via adduct formation
between the transiently formed 16-electron hydrido complexes,
Cp*W(NO)(H)(C6H11) and Cp*W(NO)(H)(CH2CMe3).
Finally, monometallic complex 3 has been characterized by
conventional spectroscopic methods, and its solid-state molecular
structure (Figure 1) has been established by an X-ray crystal-
lographic analysis.11 Its most interesting structural feature is the
η1,η3-(cyclohexyl)cyclohexenyl ligand, which again results in the
metal center attaining an 18-electron configuration. The unsym-
metrical η1,η3 (as opposed to an η2,η2) mode of attachment of this
coupled ligand to the metal center is probably reflective of electronic
factors. We have previously seen manifestations of similar factors
in Cp′M(NO)(η4-trans-diene) complexes (Cp′ ) Cp (η5-C5H5) or
Cp*) of both molybdenum12 and tungsten.6
its IR spectrum as a KBr pellet that exhibits a νWH at 1898 cm-1
.
We have previously isolated 1 in 32% yield from the gentle
thermolysis of Cp*W(NO)(CH2CMe3)(η3-H2CCHCMe2) at 50 °C
in neat cyclohexane.8
Bimetallic complex 2‚Et2O is quite thermally sensitive, and its
identity has been confirmed by a single-crystal X-ray diffraction
analysis.9 Its intramolecular metrical parameters resemble those
previously established for related complexes containing W2(µ-H)2
central cores.10 Specifically, its W-W bond length of 3.0315(2) Å
9
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10.1021/ja0426692 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
Supporting Information Available: Experimental procedures and
complete characterization data for complexes 1, 2, and 3 (PDF), and
full details of the crystal structure analyses of 2 and 3 including
associated tables (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
Consistent with the reaction sequences contained in Scheme 2
are the following facts. Dihydrogen is an essential reactant since
Cp*W(NO)(CH2CMe3)2 simply does not react with cyclohexene
alone under ambient conditions. Furthermore, the use of D2 in
place of H2 does not result in any incorporation of deuterium
into 1 or 3. Finally, 1 does not convert to 3 when exposed to
cyclohexene under the reaction conditions employed. Interestingly,
when effected in a 1:10 mixture of cyclohexene/hexanes, the
reaction of Cp*W(NO)(CH2CMe3)2 with H2 affords only bimetallic
2, which can be isolated by crystallization from the final reaction
mixture at -30 °C. The preliminary results of DFT calculations
on these systems are generally consistent with the transformations
depicted in Scheme 2 being operative in neat cyclohexene.
However, they also suggest that 16-electron Cp*W(NO)H2 plays a
pivotal role as a reactive intermediate in a 1:10 cyclohexene/hexanes
mixture.13
In summary, we have succeeded in isolating and characterizing
three different types of organometallic products resulting from the
transformations of cyclohexene effected at the Cp*W(NO) fragment.
Current studies with this system are directed at establishing which
other unsaturated substrates will undergo these types of conversions
and at delineating the characteristic chemistry of the product
complexes. The results of these investigations will be reported in
due course.
References
(1) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals,
3rd ed.; Wiley & Sons: New York, 2001; Chapter 14.
(2) ActiVation and Functionalization of C-H Bonds; Goldberg, K. I.,
Goldman, A. S., Eds.; ACS Symposium Series 885; American Chemical
Society: Washington, DC, 2004.
(3) Pamplin, C. B.; Legzdins, P. Acc. Chem. Res. 2003, 36, 223.
(4) Evidence for the occurrence of acyclic alkene to π-allyl hydride rear-
rangements during the thermal reactions of W(CO)4(alkene)2 complexes
has been presented previously. See: Szyman´ska-Buzar, T.; Jaroszewski,
M.; Wilgocki, M.; Zio´łkowski, J. J. J. Mol. Catal. A: Chem. 1996, 112,
203.
(5) The insertion of coordinated alkenes into M-H bonds is a fundamental
transformation in transition-metal organometallic chemistry; see ref 1.
(6) For comparison, the dimerization of 1,3-cyclooctadiene mediated by the
Cp*M(NO) [M ) Mo, W] fragments affords a complex in which the
coupled ligand is attached to the metal centers in a bis-η2 fashion. See:
Debad, J. D.; Legzdins, P.; Young, M. A.; Batchelor, R. J.; Einstein, F.
W. B. J. Am. Chem. Soc. 1993, 115, 2051.
(7) Complete experimental details are provided in the Supporting Information.
(8) Ng, S. H. K.; Adams, C. S.; Hayton, T. W.; Legzdins, P.; Patrick, B. O.
J. Am. Chem. Soc. 2003, 125, 15210.
(9) (9) Crystal data for 2‚Et2O: triclinic, space group P1h, a ) 9.3217(1) Å,
b ) 11.5165(2) Å, c ) 17.5796(3) Å, R ) 85.575(1)°, â ) 89.308(1)°,
γ ) 89.733(1)°, V ) 1863.29(5) Å3, Z ) 2, R1 ) 0.0210, wR2 ) 0.0461,
and GOF(F2) ) 1.138 for 8657 reflections and 402 variables.
(10) Legzdins, P.; Martin, J. T.; Einstein, F. W. B.; Willis, A. C. J. Am. Chem.
Soc. 1986, 108, 7971 and references therein.
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 a postgraduate scholarship to M.S.A.B. We
also thank Dr. K. Wada of Kyoto University for providing us with
the preliminary results of his DFT calculations on these systems
and Dr. B. O. Patrick and Ms. A. Lam of this department for
assistance during the X-ray crystallographic analyses of 2 and 3.
P.L. gratefully acknowledges The Canada Council for the Arts for
the award of a Killam Research Fellowship during 2002-2004.
(11) Crystal data for 3: monoclinic, space group C2/c, a ) 22.1618(3) Å, b
) 13.0316(2) Å, c ) 17.9373(3) Å, â ) 131.682(2)°, V ) 3868.93(10)
Å3, Z ) 8, R1 ) 0.0168, wR2 ) 0.0379, and GOF(F2) ) 1.068 for 4813
reflections and 231 variables.
(12) (a) Hunter, A. D.; Legzdins, P.; Einstein, F. W. B.; Willis, A. C.; Bursten,
B. E.; Gatter, M. G. J. Am. Chem. Soc. 1986, 108, 3843. (b) Christensen,
N. J.; Hunter, A. D.; Legzdins, P. Organometallics 1989, 8, 930.
(13) Wada, K. Kyoto University, Kyoto, Japan. Personal communication, 2005.
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