Parallel modes of C-H bond activation initiated by Cp*Mo(No)(Ch2CMe3)(C6H5) at ambient temperatures

Article abstract of DOI:10.1021/ja027080m

The molybdenum nitrosyl complex Cp*Mo(NO)(CH₂CMe₃)(C₆H₅) reacts at room temperature via elimination of neopentane or benzene to form the transient species Cp*Mo(NO)(=CHCMe₃) and Cp*Mo(NO)(η²

Full text of DOI:10.1021/ja027080m

Published on Web 07/24/2002
Parallel Modes of C-H Bond Activation Initiated by
Cp*Mo(NO)(CH2CMe3)(C6H5) at Ambient Temperatures1
Kenji Wada,† Craig B. Pamplin, and Peter Legzdins*
Department of Chemistry, The UniVersity of British Columbia, VancouVer, British Columbia, Canada V6T 1Z1
Received May 28, 2002
Scheme 1
The functionalization of hydrocarbon C-H bonds by soluble
transition-metal complexes is of increasing practical interest.2,3
Previously, reports from this group have described the thermal
conversion of Cp*W(NO)(hydrocarbyl)2 complexes into transient
tungsten alkylidene,4,5 acetylene,6 or allene7 species that can
subsequently activate the C-H bonds of hydrocarbon solvents in
a variety of ways. The reactive tungsten species are typically formed
by thermolysis of suitable organometallic precursors at temperatures
ranging from 50 to 80 °C. However, under these conditions some
of the C-H activation processes are not clean, thereby leading us
Scheme 2
to surmise that some of the operative chemistry might be obscured
by the thermal instability of the organometallic complexes involved.
Consequently, we decided to extend our studies to encompass the
congeneric molybdenum precursors. We began our investigations
with Cp*Mo(NO)(CH2CMe3)2 (1)8 since we knew from previous
work that the related CpMo(NO)(CH2CMe3)2 complex forms a
dimeric molybenum alkylidene complex in CH2Cl2 below room
temperature.9 We now report the preliminary results of our
investigations of the thermal chemistry of this molybdenum complex
which reveal parallel C-H bond activation chemistry mediated not
only by transient alkylidene complexes but also by transient benzyne
complexes under unusually mild conditions.
At first glance, the thermal reactivity of 1 resembles that reported
previously for its isostructural10 tungsten analogue. For example,
the reactions of 1 with tetramethylsilane or mesitylene for 30 h at
ambient temperatures result in the clean formation of free neopen-
tane and Cp*Mo(NO)(CH2CMe3)(CH2SiMe3) (2) or Cp*Mo(NO)-
complex, Cp*W(NO)(CH2CMe3)2,4b and they are consistent with
rate-determining R-hydrogen abstraction from a neopentyl meth-
ylene group and formation of the reactive alkylidene species,
Cp*Mo(NO)(dCHCMe3), probably as its hydrocarbon solvate.11
Remarkably, the reaction of 1 with C6H6 at room temperature
leads to the sequential activation of two molecules of benzene. Thus,
a C6H6 solution of 1 reacts at ambient temperatures to yield Cp*Mo-
(NO)(CH2CMe3)(C6H5) (4) along with the diphenyl complex,
Cp*Mo(NO)(C6H5)2 (5) (Scheme 1). As shown, the reaction of 4
with C6H6 at room temperature also cleanly generates 5. Of note,
the analogous reaction of 4 with C6D6 affords primarily 4-d6 (27%)
and 5-d6 (68%), along with traces of Cp*Mo(NO)(C6D5)2 (5-d10).
In addition, solutions of 5 in C6D6 are converted to 5-d6 at room
temperature, and 5-d10 is fully formed after 48 h at 40 °C. The
intermediacy of a transient benzyne complex in this chemistry (see
Scheme 2) is suggested by the following observations. First, when
(CH2CMe3)(η2-CH2C6H3-3,5-Me2) (3), respectively. Neither product
undergoes further reactivity under these mild reaction conditions.
Similarly, the reaction of 1 with C6D6 generates predominantly
Cp*Mo(NO)(CHDCMe3)(C6D5) (4-d6) and neopentane, with ste-
reospecific deuterium incorporation at the synclinal methylene
position of the neopentyl ligand. A kinetic analysis of this reaction
in the temperature range 26-40 °C (monitored by 1H NMR
spectroscopy) reveals a first-order loss of 1, and a linear Eyring
plot affords ∆Hq ) 99(1) kJ mol-1 and ∆Sq ) -11(4) J mol-1
K-1. These parameters are very similar to those determined
previously at higher temperatures for the analogous tungsten
2
the conversion of 4-d6 to 5-d10 in C6D6 is monitored by H{1H}
NMR spectroscopy, no incorporation of deuterium into the Cp*
methyl substituents is observed. Mechanisms involving intra-
molecular oxidative addition of a Cp* C-H bond to a metal center
and formation of intermediate “tucked-in” complexes are known
to result in such deuterium scrambling.12 Second, the thermolysis
of 4 in pyridine-d5 at room temperature for 30 h affords a mixture
* To whom correspondence should be addressed. E-mail: legzdins@chem.ubc.ca.
† Current address: Department of Energy & Hydrocarbon Chemistry, Graduate
School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan.
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10.1021/ja027080m CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
the benzyne intermediates.11,20 More detailed mechanistic studies
and DFT calculations of these facile and parallel C-H bond
activation systems are currently in progress, with a view to
developing new avenues of hydrocarbon activation chemistry. The
results of these investigations will be reported in due course.
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., and we thank Dr. B. O. Patrick of
this Department for collecting X-ray diffraction data. P. L. is a
Canada Council Killam Research Fellow.
Supporting Information Available: Experimental procedures and
characterization data for the new compounds described (PDF) and X-ray
crystallographic data for 7•C6H6 (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
Figure 1. ORTEP diagram of Cp*Mo(NO)(η2-C6H4)(NC5H5) (7) 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), Mo(1)-N(2) 2.223(3); Mo(1)-N(1)-O(1) 169.2(4).
References
(1) Presented in part at the 223rd National Meeting of the American Chemical
Society, Orlando, FL, April 2002, Abstract INOR 0112.
(2) (a) ActiVation and Functionalization of Alkanes; Hill, C. L., Ed.; Wiley-
Interscience: New York, 1989. (b) Shilov, A. E.; Shul’pin, G. B.
ActiVation and Catalytic Reactions of Saturated Hydrocarbons in the
Presence of Metal Complexes; Kluwer Academic: Dordrecht, The
Netherlands, 2000.
(3) For leading reviews, see: (a) Crabtree, R. H. Chem. ReV. 1985, 85, 245-
269. (b) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H.
Acc. Chem. Res. 1995, 28, 154-162. (c) Crabtree, R. H. Chem. ReV. 1995,
95, 987-1007. (d) Shilov, A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97,
2879-2932. (e) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem.,
Int. Ed. 1998, 37, 2180-2192. (f) Crabtree, R. H. J. Chem. Soc., Dalton
Trans. 2001, 2437-2450.
(4) (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.
(5) For other examples of intermolecular C-H bond activation by metal
alkylidene complexes, see: (a) Coles, M. P.; Gibson, V. C.; Clegg, W.;
Elsegood, M. R. J.; Porrelli, 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.
(6) (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, J. D.; Legzdins,
P.; Lumb, S, A.; Rettig, S. J.; Batchelor, R. J.; Einstein, F. W. B.
Organometallics 1999, 18, 3414-3428.
of the trapped alkylidene complex, Cp*Mo(NO)(dCHCMe3)-
(NC5D5) (6-d5, 75%) and the trapped η2-benzyne complex, Cp*Mo-
(NO)(η2-C6H4)(NC5D5) (7-d5, 25%). The benzyne proton resonances
of 7-d5 are inequivalent, occurring as overlapping multiplets at δ
7.48 (2H), 7.78 (1H), and 7.89 (1H). The line shapes of these signals
are invariant from 20 to 85 °C in pyridine-d5, thereby indicating
that 7 is stereochemically rigid in this solution and that the Mo-
η2-C6H4 linkage is static.
Complexes 6 and 7, prepared by thermolysis of 4 in pyridine,
can be readily separated by fractional crystallization. Compound 7
as its benzene solvate has been subjected to a single-crystal X-ray
crystallographic analysis, and the resulting ORTEP diagram of 7
is shown in Figure 1.13 The length of the benzyne C-C bond
coordinated to the molybdenum center is 1.356(6) Å, and the other
C-C 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 extant in the molybdenum toluyne complex, Mo(η2-2-
MeC6H3)(2-MeC6H4)2(PMe2Ph)2 (2.011 and 2.056 Å).14 The in-
frared spectrum of 7 in KBr exhibits strong absorptions at 1537
and 1583 cm-1, which are assigned to νNO and νCdC stretching
frequencies, respectively.
The neopentylidene and benzyne intermediates derived from 4
react readily with the aliphatic C-H bonds of various substrates
to generate mixtures of products. For example, thermolysis of 4 in
tetramethylsilane at room temperature for 48 h results in the
formation of 2 (29%) and Cp*Mo(NO)(CH2SiMe3)(C6H5) (8,
41%).15 Similarly, 4 reacts with mesitylene to yield 3 and Cp*Mo-
(NO)(η2-CH2C6H3-3,5-Me2)(C6H5) (9). 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.16-19 The distribution
of products resulting from the reactions of hydrocarbons with 4
(Scheme 2) appears to be dependent on the nature of the
hydrocarbon substrate, possibly due to the presence of σ-hydro-
carbon complexes on the reaction coordinates of the alkylidene and
(7) Ng, S. H. K.; Adams, C. S.; Legzdins, P. J. J. Am. Chem. Soc., in press.
(8) (a) Debad, J. D.; Legzdins, P.; Rettig, S. J.; Veltheer, J. E. Organometallics
1993, 12, 2714-2725. (b) Legzdins, P.; Young, M. A.; Batchelor, R. J.;
Einstein, F. W. B. J. Am. Chem. Soc. 1995, 117, 8798-8806.
(9) (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.
(10) Bau, R.; Mason, S. A.; Patrick, B. O.; Adams, C. S.; Sharp, W. B.;
Legzdins, P. Organometallics 2001, 20, 4492-4501.
(11) Adams, C. S.; Legzdins, P.; McNeil, W. S. Organometallics 2001, 20,
4939-4955.
(12) Schock, L. E.; Brock, C. P.; Marks, T. J. Organometallics 1987, 6, 232-
241.
(13) Crystal data for 7‚C6H6: orthorhomic, space group P212121, a ) 7.8177-
(4) Å, b ) 14.3141(9) Å, c ) 20.5603(10) Å, V ) 2300.8(2) Å3, Z ) 4,
R1 ) 0.052, wR2 ) 0.090.
(14) 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.
(15) Product distributions were determined by 1H NMR spectroscopy in C6D6.
(16) Debad, J. D. Ph.D. Dissertation, University of British Columbia, 1989.
(17) Erker, G. J. Organomet. Chem. 1977, 134, 189-202.
(18) Fagan, P. J.; Manriques, J. M.; Maatta, E. A.; Seyam, A. M.; Marks, T.
J. J. Am. Chem. Soc. 1981, 103, 6650-6667.
(19) (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.
(20) Adams, C. S.; Legzdins, P.; Tran, E. Organometallics 2002, 21,
1474-1486.
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