A beneficial kinetic effect of an η5-C5Me 4H ligand

Article abstract of DOI:10.1021/ja107834g

C-H activation of benzene at 26 °C by (η⁵-C ₅Me₅)W(NO)(CH₂CMe₃) (η³-CH₂CHCHMe) results after 4 h in the production of five new organometallic complexes, only two of which are isomers of the desired (η⁵-C₅Me₅)W(NO)(C₆H ₅)(η³-CH₂CHCHMe) compound. In contrast, the identical reaction involving the η⁵-C₅Me₄H analogue affords only the phenyl complexes during the first 24 h, thereby facilitating their isolation in good yields. This striking difference in reactivity can be attributed to the lesser steric demands of the η⁵-C₅Me₄H ligand that result in its complexes reacting at a significantly slower rate.

Full text of DOI:10.1021/ja107834g

Published on Web 10/06/2010
A Beneficial Kinetic Effect of an η5-C5Me4H Ligand
Rhett A. Baillie, Tommy Tran, Michelle E. Thibault, and Peter Legzdins*
Department of Chemistry, The UniVersity of British Columbia, VancouVer, British Columbia, Canada V6T 1Z1
Received August 31, 2010; E-mail: legzdins@chem.ubc.ca
form the corresponding phenyl derivatives as the two isomers 2a-
Abstract: C-H activation of benzene at 26 °C by (η5-
d6 and 2b-d6. However, 2a-d6 and 2b-d6 ultimately convert to the
C5Me5)W(NO)(CH2CMe3)(η3-CH2CHCHMe) results after 4 h in the
hydrido complexes 3-d6 and 4-d6, respectively, by intramolecular
production of five new organometallic complexes, only two of
which are isomers of the desired (η5-C5Me5)W(NO)(C6H5)(η3-
CH2CHCHMe) compound. In contrast, the identical reaction
involving the η5-C5Me4H analogue affords only the phenyl com-
exchange of the newly formed phenyl ligand with a terminal
hydrogen atom on the allyl ligand. To the best of our knowledge,
this type of isomerization, which is probably mediated by the metal
center, is without precedent in the chemical literature.
plexes during the first 24 h, thereby facilitating their isolation in
good yields. This striking difference in reactivity can be attributed
to the lesser steric demands of the η5-C5Me4H ligand that result
in its complexes reacting at a significantly slower rate.
Scheme 1
Pentahapto-cyclopentadienyl groups such as η5-C5H5 (Cp) and
η5-C5Me5 (Cp*) are ubiquitous in transition-metal organometallic
chemistry and have been utilized as nonreactive, stabilizing ligands
in a wide variety of complexes that undergo all the fundamental
organometallic reactions.1 On occasion, however, the particular
cyclopentadienyl ligand employed can control the characteristic
chemical properties of a particular compound. A striking illustration
of this fact is provided by the observation by Chirik and co-workers
that replacement of Cp* ligands in zirconocenes by the less sterically
demanding η5-C5Me4H (Cp′) groups alters how N2 coordinates to
the metal centers and how it subsequently reacts with H2 and CO.2
In a similar vein Evans and co-workers recently utilized Cp′ instead
of Cp* ligands to prepare the first dinitrogen complex of scandium,
[Cp′2Sc]2(µ-η2:η2-N2).3 We now wish to report that a similar
replacement in one of our C-H activating “piano-stool” nitrosyl
complexes results in cleaner chemical transformations that afford
the desired products in significantly better isolated yields. This
useful outcome is a manifestation of the less crowded Cp′ complex
undergoing reactions that are primarily dissociative in nature at a
significantly slower rate than its Cp* analogue.
The conversions depicted in Scheme 1 have been monitored by
1H NMR spectroscopy, and they have been effected on a preparative
scale with C6H6 as the substrate. All nondeuterated new complexes
have been isolated and characterized by conventional spectroscopic
methods, including single-crystal X-ray crystallographic analyses
of representative compounds.6 Crystallization of 2′ from pentane
affords crystals that contain both 2a′ and 2b′ in the lattice (Figure
1), but similar treatment of 2* produces crystals that contain only
2a* whose intramolecular dimensions are essentially identical to
those of 2a′.6 As in Cp*W(NO)(alkyl)(η3-allyl) complexes,7 the
allyl ligands of complexes 2 are in endo orientations with the meso
protons pointing away from the cyclopentadienyl rings. Further-
more, there is a σ-π distortion of the allyl ligands brought about
by the electronic asymmetry at the metal centers.8 The spectroscopic
properties of complexes 2 indicate that they retain their “piano-
stool” molecular structures in solutions. Interestingly, dissolution
of crystals of 2a* in C6D6 immediately results in a solution whose
1H NMR spectrum exhibits signals attributable to both 2a* and
2b* thereby indicating that the two isomers are in rapid equilibrium
with each other.
We have established previously that thermolysis of Cp*W(NO)-
(CH2CMe3)(η3-CH2CHCHMe) (1*)4 at ambient temperatures results
in the loss of neopentane and the formation of the 16e η2-diene
intermediate complex, Cp*W(NO)(η2-CH2dCHsCHdCH2) (A*).5
In the presence of linear alkanes, A* effects single CsH activations
of the hydrocarbons exclusively at their terminal carbons and cleanly
forms 18e Cp*W(NO)(n-alkyl)(η3-CH2CHCHMe) complexes.5 In
contrast, treatment of 1* with benzene at room temperature affords
“a complex mixture of organometallic compounds”.5 Fortunately,
replacement of the Cp* group in 1* [νNO (Nujol) ) 1594 cm-1] by
the Cp′ ligand to form 1′ [νNO (Nujol) ) 1595 cm-1] has enabled
us to establish that the initial activation of benzene does indeed
produce the expected phenyl derivatives for both complexes
(Scheme 1 with C6D6 as the reactive substrate). However, these
phenyl complexes are thermodynamically unstable and eventually
isomerize to a mixture of η3-phenylallyl hydrido complexes, the
isomerizations occurring more rapidly in the Cp* system.
The synthesis of isomeric complexes 3* (minor) and 4* (major)
is facilitated by warming of the reaction mixture to 45 °C for 24 h.
Their molecular structures have been deduced from their diagnostic
1H and 13C-APT NMR spectra, and the structure of 4* has been
confirmed by an X-ray crystallographic analysis (Figure 2). The
signals due to the hydride ligands in 3* and 4* occur at δ -0.63
ppm (1JWH ) 124 Hz) and -0.05 ppm (1JWH ) 127 Hz),
respectively, and the intramolecular metrical parameters of 4* are
similar to those of the other new complexes isolated during this
work. Interestingly, the allyl ligand in 4* is in an exo orientation,
As shown in Scheme 1, the η2-diene intermediate complexes A
(i.e., either A* or A′)4 activate C6D6 in the anticipated manner to
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15160 J. AM. CHEM. SOC. 2010, 132, 15160–15161
10.1021/ja107834g  2010 American Chemical Society

C O M M U N I C A T I O N S
probably because of the two substituents on C(13) (Figure 2).9 The
1
final H NMR spectrum of the mixture of 3* and 4* in C6D6 also
exhibits resonances that may be assigned to the minor coordination
isomer of 3* in which the phenyl substituent on the allyl ligand is
situated at the opposite end adjacent to the nitrosyl group.
Figure 3. Distribution of organometallic complexes during the C-H
activation of benzene by 1′ at 26 °C.
Figure 1. Solid-state molecular structures of 2a′ (right) and 2b′ (left) as
they occur in the asymmetric unit with 50% probability thermal ellipsoids
shown. Selected interatomic distances (Å) and angles (deg): 2a′: W(2)-C(29)
) 2.360(3), W(2)-C(30) ) 2.331(3), W(2)-C(31) ) 2.291(3), W(1)-
C(33) ) 2.214(3), W(2)-N(2) ) 1.775(3), N(2)-O(2) ) 1.223(4),
C(29)-C(30) ) 1.378(5), C(30)-C(31) ) 1.423(5), C(31)-C(32) )
1.512(5), C(29)-C(30)-C(31) ) 118.2(3), W(2)-N(2)-O(2) ) 172.1(3).
2b′: W(1)-C(10) ) 2.232(3), W(1)-C(11) ) 2.357(3), W(1)-C(12) )
2.506(3), W(1)-C(14) ) 2.210(3), W(1)-N(1) ) 1.773(3), N(1)-O(1) )
1.225(4), C(10)-C(11) ) 1.424(5), C(11)-C(12) ) 1.367(5), C(12)-C(13)
) 1.501(5), C(10)-C(11)-C(12) ) 119.2(3), W(1)-N(1)-O(1) )
169.0(3).
4 h under identical conditions contains at least six such compounds,
i.e. “a complex mixture”.5 Clearly, the isolation of the desired
products resulting from the C-H activation of benzene is easiest
for the Cp′ system.
It has been previously reported that Cp* complexes can undergo
rapid intramolecular rearrangements.10 This work has demonstrated
that the less sterically demanding Cp′ group becomes the ligand of
choice when the desired initial products undergo such subsequent
rearrangements. Studies are currently in progress to establish the
mechanisms of the rearrangements that result in the hydrido
compounds 3 and 4 and to determine why the alkyl ligands in the
related Cp*W(NO)(n-alkyl)(η3-CH2CHCHMe) complexes evidently
do not undergo such rearrangements.
Kinetic analyses of the benzene-activation reactions yield pseudo-
first-order rate constants (s-1), Arrhenius activation energies (kJ
mol-1) of (8.5 ( 0.2) × 10-5 and 79.1 ( 1.9 and (3.6 ( 0.1) ×
10-5 and 93.2 ( 6.6 for 1* and 1′, respectively. The corresponding
Eyring parameters ∆H‡ (kJ mol-1) and ∆S‡ (J K-1 mol-1) are 90.6
( 6.6 and -27.4 ( 3.4 and 76.5 ( 1.9 and -66.6 ( 3.0,
respectively. The subsequent isomerizations of 2a and 2b to the
hydrido complexes 3 and 4 also occur more rapidly for the Cp*
complexes. Thus, signals due to 3* and 4* begin to appear in the
1H NMR spectrum of the benzene reaction mixture after 4 h at 26
°C when only 75% of 1* has been consumed. In contrast, hydride
resonances attributable to 3′ and 4′ only begin to appear after 24 h
at 26 °C when 96% of 1′ has reacted. In other words, for the first
24 h the Cp′ reaction mixture contains only three organometallic
complexes (cf. Figure 3) whereas the Cp* reaction mixture after
Acknowledgment. We are grateful to The Dow Chemical
Company for continuing support of this work, and we thank Drs.
David Graf and Bill Tenn for assistance and helpful discussions.
Supporting Information Available: A PDF file providing full
details of experimental procedures and characterization data for all new
complexes and a CIF file containing details of the crystallographic
analyses of complexes 2a′ and 2b′, 2a*, and 4*. This material is
available free of change via the Internet at http://pubs.acs.org.
References
(1) Hartwig, J. F. Organotransition Metal Chemistry: From Bonding to
Catalysis; University Science Books: Sausalito, CA, 2010; Section 3.6 and
references cited therein.
(2) Knobloch, D. J.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2010,
132, 10553, and references cited therein.
(3) Demir, S.; Lorenz, S. E.; Fang, M.; Furche, F.; Meyer, G.; Ziller, J. W.;
Evans, W. J. J. Am. Chem. Soc. 2010, 132, 11151.
(4) Throughout this report compounds are designated by bold numbers or letters.
The superscripts * and ′ after the numbers or letters indicate the Cp*- and
Cp′-containing complexes, respectively. If no superscript is shown, then
the designation encompasses both the Cp* and the Cp′ compounds.
(5) Tsang, J. Y. K.; Buschhaus, M. S. A.; Graham, P. M.; Semiao, C. J.;
Semproni, S. P.; Kim, S. J.; Legzdins, P. J. Am. Chem. Soc. 2008, 130,
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(6) See Supporting Information.
(7) Tsang, J. Y. K.; Buschhaus, M. S. A.; Fujita-Takayama, C.; Patrick, B. O.;
Legzdins, P. Organometallics 2008, 27, 1634.
(8) Semproni, S. P.; McNeil, W. S.; Baillie, R. A.; Patrick, B. O.; Campana,
C. F.; Legzdins, P. Organometallics 2010, 29, 867.
Figure 2. Solid-state molecular structure of 4* with 50% probability
thermal ellipsoids shown. Selected interatomic distances (Å) and angles
(deg): W(1)-C(11) ) 2.279(3), W(1)-C(12) ) 2.279(3), W(1)-C(13) )
2.461(3), W(1)-H(1) ) 1.68(4), W(1)-N(1) ) 1.774(2), N(1)-O(1) )
1.222(3), C(15)-C(13) ) 1.499(4), C(13)-C(12) ) 1.400(4), C(12)-C(11)
) 1.417(4), C(15)-C(13)-C(12) ) 119.4(2), C(13)-C(12)-C(11) )
124.2(3), W(1)-N(1)-O(1) ) 175.4(2).
(9) Related η3-H2CCHCMe2 complexes also exhibit exo orientations of their
allyl ligands; see: Ng, S. H. K.; Adams, C. S.; Hayton, T. W.; Legzdins,
P.; Patrick, B. O. J. Am. Chem. Soc. 2003, 125, 15210.
(10) Besora, M.; Vyboishchikov, S. F.; Lledo´s, A.; Maseras, F.; Carmona, E.;
Poveda, M. L. Organometallics 2010, 29, 2040.
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