Mechanism of reversible alkyne coupling at zirconocene: Ancillary ligand effects

Article abstract of DOI:10.1021/ja800025u

The mechanism of reversible alkyne coupling at zirconium was investigated by examination of the kinetics of zirconacyclopentadiene cleavage to produce free alkynes. The zirconacyclopentadiene rings studied possess trimethylsilyl substituents in the α-positions, and the ancillary Cp₂, Me ₂C(η⁵-C₅H₄)₂, and CpCp* (Cp* = η⁵-C₅Me₅) bis(cyclopentadienyl) ligand sets were employed. Fragmentation of the zirconacyclopentadiene ring in Cp₂Zr[2,5-(Me₃Si) ₂-3,4-Ph₂C₄] with PMe₃, to produce Cp₂Zr(η²-PhC≡CSiMe₃)-(PMe ₃) and free PhC≡CSiMe₃, is first-order in initial zirconacycle concentration and zero-order in incoming phosphine (k_obs = 1-4(2) × 10^-5 s^-1 at 22°C), and the activation parameters determined by an Eyring analysis (ΔH ^? = 28(2) kcal mol^-1 and ΔS ^? = 14(4) eu) are consistent with a dissociative mechanism. The analogous reaction of the ansa-bridged complex Me₂C(η ⁵-C₅H₄)₂Zr[2,5-(Me ₃Si)₂-3,4-Ph₂C₄] is 100 times faster than that for the corresponding Cp₂ complex, while the corresponding CpCp* complex reacts 20 times slower than the Cp₂ derivative. These rates appear to be largely influenced by the steric properties of the ancillary ligands.

Full text of DOI:10.1021/ja800025u

Published on Web 03/13/2008
Mechanism of Reversible Alkyne Coupling at Zirconocene:
Ancillary Ligand Effects
Adam D. Miller, Jennifer L. McBee, and T. Don Tilley*
Department of Chemistry, UniVersity of California, Berkeley, Berkeley, California 94720-1460
Received January 2, 2008; E-mail: tdtilley@berkeley.edu
Abstract: The mechanism of reversible alkyne coupling at zirconium was investigated by examination of
the kinetics of zirconacyclopentadiene cleavage to produce free alkynes. The zirconacyclopentadiene rings
studied possess trimethylsilyl substituents in the R-positions, and the ancillary Cp2, Me2C(η5-C5H4)2, and
CpCp* (Cp* ) η5-C5Me5) bis(cyclopentadienyl) ligand sets were employed. Fragmentation of the
zirconacyclopentadiene ring in Cp2Zr[2,5-(Me3Si)2-3,4-Ph2C4] with PMe3, to produce Cp2Zr(η2-PhCtCSiMe3)-
(PMe3) and free PhCtCSiMe3, is first-order in initial zirconacycle concentration and zero-order in incoming
phosphine (kobs ) 1.4(2) × 10-5 s-1 at 22 °C), and the activation parameters determined by an Eyring
analysis (∆Hq ) 28(2) kcal mol-1 and ∆Sq ) 14(4) eu) are consistent with a dissociative mechanism. The
analogous reaction of the ansa-bridged complex Me2C(η5-C5H4)2Zr[2,5-(Me3Si)2-3,4-Ph2C4] is 100 times
faster than that for the corresponding Cp2 complex, while the corresponding CpCp* complex reacts 20
times slower than the Cp2 derivative. These rates appear to be largely influenced by the steric properties
of the ancillary ligands.
Introduction
via the coupling of diynes of the type RCtC-R′-CtCR
containing rigid spacer groups (R′). These investigations have
The reductive coupling of alkynes by zirconocene is an
important carbon-carbon bond-forming reaction.1-3 The result-
ing zirconacyclopentadienes are useful precursors to a wide
range of organic molecules including (for example) dienes,4
arenes,5-10 cyclopentadienes,11-13 thiophenes,14,15 phospholes,16
thiophene oxides,17 and bicyclic compounds.18 We have em-
ployed this reaction for the development of efficient synthetic
routes to polymers,19-21 oligomers,17,22 and macrocycles,23-30
shown that the nature of the alkyne substituents is critical in
determining the structure of the resulting coupled products. One
critical factor relates to the propensity of a substituent to adopt
the R- (-2 and -5) or â- (-3 and -4) positions of a
zirconacyclopentadiene ring. For example, a â-directing alkyne
substituent (Râ), which forces the carbon it is bound to into the
â-position of the zirconacyclopentadiene ring, will incorporate
the spacer of a diyne (R′) into the backbone of an oligomer or
polymer (eq 1). The regioselectivity associated with alkyne
coupling also plays a role in the formation of macrocycles, but
a more important requirement in this case is that the alkyne
substituents promote reversibility for zirconacyclopentadiene
formation.30
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J. AM. CHEM. SOC. 2008, 130, 4992-4999
10.1021/ja800025u CCC: $40.75 © 2008 American Chemical Society

Reversible Alkyne Coupling at Zirconocene
A R T I C L E S
Scheme 1
Scheme 2
It is well known that certain sterically demanding alkyne
substituents, such as trimethylsilyl,31 tert-butyl,18,32 and diphe-
nylphosphino33 groups, direct alkyne couplings such that they
adopt the R-positions of the zirconacyclopentadiene ring. These
groups also promote reversibility for alkyne couplings and give
labile metallacycles.34,35 Thus, zirconocene couplings of diynes
of the type Me3SiCtC-R′-CtCSiMe3 are particularly useful
in the synthesis of macrocycles.24 On the other hand, electron-
withdrawing alkyne substituents can preferentially adopt the
â-positions of a zirconacyclopentadiene ring, and this has been
explained in terms of charge distributions in the transition state
for alkyne coupling. This transition state has a highly unsym-
metrical structure according to DFT calculations and is best
described as an alkyne adduct of a zirconacyclopropene.36
Although the synthetic utility of the reductive coupling of
alkynes by zirconocene is firmly established, there has been
little investigation into the mechanism of such couplings. In
addition, the possible effects of alternative ancillary ligand sets,
on (for example) regioselectivity, reversibility, and coupling
rates, have yet to be determined. Characterization of the
mechanism is necessary to gain a better understanding of the
factors that influence the course of alkyne couplings at
zirconium. Kinetic and mechanistic investigations of zir-
conocene-mediated alkyne couplings are inherently difficult,
given the high rates of these reactions, the uncertainty in the
exact nature of the zirconocene species that initiate the coupling,
and the fact that intermediate species (i.e., zirconacyclopropenes)
are fleeting and difficult to observe. To the best of our
knowledge, zirconacyclopropenes containing the Cp2Zr moiety
have been isolated only as base adducts;34,37-39 however, related
complexes with substituted cyclopentadienyl ligands of the type
C5H5-nMen (n ) 2-5) are known.40,41
It has been assumed that alkyne coupling occurs in a stepwise
manner, and evidence for this exists in the fact that some
zirconacyclopentadienes react with a Lewis base to eliminate 1
equiv of alkyne to form a base-stabilized zirconacyclopropene
(Scheme 1).34 Thus, it seemed possible to probe the mechanism
of alkyne coupling by examining the reverse process, and this
approach was taken in the research described herein. These
mechanistic investigations were used to probe ancillary ligand
effects on this chemistry, and for this purpose the Cp2, Me2C-
(η5-C5H4)2, and CpCp* (Cp* ) η5-C5Me5) bis(cyclopentadienyl)
ligand sets were employed. For each zirconocene system, kinetic
and mechanistic investigations targeted the fragmentation of a
zirconacyclopentadiene, the microscopic reverse of alkyne
coupling (Scheme 1, L ) PMe3). X-ray structure determinations
have also contributed to the mechanistic picture that emerges
from these studies.
Results and Discussion
Since trimethylsilyl substituents are known to enhance
reversibility for zirconocene-mediated alkyne couplings, inves-
tigations targeted 2,5-bis(trimethylsilyl)-3,4-diphenylzircona-
cyclopentadiene derivatives. To explore the influence of ancil-
lary ligands on zirconocene coupling, various bis(cyclopenta-
dienyl) substituents were also examined. Zirconacycles contain-
ing the ancillary ligands Me2C(η5-C5H4)2 and CpCp* are of
particular interest since most reported zirconacyclopentadienes
feature the Cp2 or Cp*2 ligand sets.42-46
(31) Erker, G.; Zwettler, R.; Kruger, C.; Hylakryspin, I.; Gleiter, R. Organo-
metallics 1990, 9, 524-530.
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1003-1004.
(33) Miquel, Y.; Igau, A.; Donnadieu, B.; Majoral, J. P.; Dupuis, L.; Pirio, N.;
Meunier, P. Chem. Commun. 1997, 279-280.
Synthesis of Zirconacyclopentadiene and Zirconacyclo-
propene Complexes. The starting compounds used in this study
(34) Erker, G.; Zwettler, R. J. Organomet. Chem. 1991, 409, 179-188.
(35) Takahashi, T.; Kotora, M.; Hara, R.; Xi, Z. F. Bull. Chem. Soc. Jpn. 1999,
72, 2591-2602.
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H.; Tilley, T. D. J. Am. Chem. Soc. 2003, 125, 4199-4211.
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V. V.; Shur, V. B. Z. Anorg. Allg. Chem. 1995, 621, 77-83.
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P.; Mach, K. Organometallics 1996, 15, 3752-3759.
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1998, 270, 399-404.
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J. Am. Chem. Soc. 2004, 126, 16937-16950.
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594, 147-153.
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Gautheron, B.; Dousse, G. Organometallics 1995, 14, 162-169.
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Acta 2002, 334, 17-24.
9
J. AM. CHEM. SOC. VOL. 130, NO. 14, 2008 4993

A R T I C L E S
Scheme 3
Miller et al.
a The reducing agent was n-BuLi rather than Mg/HgCl2.
Scheme 4
Scheme 5
a This intermediate was observed only for zirconacycle 1.
were synthesized by reduction of the appropriate zirconocene
dichloride in the presence of an alkyne, with either butyl lithium
in THF (Negishi protocol)47 or magnesium amalgam48 as the
reductant. Zirconacyclopentadienes 1-4 were prepared by
generation of the zirconocene synthon via the method of Negishi
in 28-68% yields (Scheme 2). A similar method was used to
obtain the zirconacyclopropene complex CpCp*Zr(pyr)(η2-Me3-
SiCtCSiMe3) (5), which was expected to serve as a convenient
synthon for the zirconocene CpCp*Zr. This compound is related
to Rosenthal’s Cp2Zr(pyr)(η2-Me3SiCtCSiMe3),39 which is a
mild and efficient synthetic source of zirconocene.26 Synthesis
of zirconacyclopropenes 6-8 typically required a different
reduction procedure (Scheme 3). These complexes were ob-
tained in 29-51% yields via treatment of the zirconocene
dichloride with magnesium powder and mercury chloride in
THF at -78 °C, followed by addition of the alkyne and then
the appropriate Lewis base (pyridine or PMe3). All new
compounds were obtained as analytically pure solids that were
equiv of PhCtCPh (tolan) in benzene-d6 occurred over 24 h at
75 °C, to give zirconacyclopentadiene 3 in quantitative yield
1
(by H NMR spectroscopy; Scheme 4). During the course of
1
this reaction, two intermediates were observed by H NMR
spectroscopy with sets of CpCp* resonances at 5.73, 1.65 ppm
and 5.58, 1.55 ppm. These spectra also contained new reso-
nances for coordinated pyridine and bound Me3SiCtCSiMe3,
implying that these intermediates are CpCp*Zr(η2-PhCtCPh)-
(pyr) and CpCp*Zr[2,3-(Me3Si)2-4,5-Ph2C4], respectively. In
contrast, ansa complex 6 was converted cleanly to zirconacycle
4 within only 30 min at ambient temperature under analogous
reaction conditions.
The labile complex Cp2Zr[2,5-(Me3Si)2-3,4-Ph2C4] (9) readily
undergoes substitution by alkynes less sterically encumbered
than Me3SiCtCPh (e.g., tolan) to form zirconacyclopentadienes
derived from the newly introduced alkyne, with liberation of 2
equiv of Me3SiCtCPh.32,35 To compare the reactivities of 1,
2, and 9 in this type of alkyne-substitution process, slightly more
than 2 equiv of tolan in benzene-d6 were added to each complex,
and the reactions were monitored by NMR spectroscopy. The
ansa complex 2 readily underwent alkyne ligand exchange with
tolan (2.1 equiv) at ambient temperature, and complete conver-
sion to 4 was observed after 1 h (Scheme 5). Compound 9
smoothly reacted with 2.1 equiv of tolan at 80 °C to yield the
tetraphenylzirconacyclopentadiene (10) in less than 1 h. By
comparison, only 50% of the CpCp* complex 1 had reacted
1
fully characterized by H, 13C, and/or 31P NMR spectroscopy.
Reactions of Zirconocene Complexes with Alkynes. To
evaluate the utility of compounds 5 and 6, the analogues to
Rosenthal’s Cp2Zr(pyr)(η2-Me3SiCtCSiMe3), as precursors to
zirconacyclopentadiene complexes, their reactions with alkynes
were examined. The reaction of the CpCp* compound 5 with 2
(47) Negishi, E.; Cederbaum, F. E.; Takahashi, T. Tetrahedron Lett. 1986, 27,
2829-2832.
(48) Thanedar, S.; Farona, M. F. J. Organomet. Chem. 1982, 235, 65-68.
9
4994 J. AM. CHEM. SOC. VOL. 130, NO. 14, 2008

Reversible Alkyne Coupling at Zirconocene
A R T I C L E S
Cpcent; 115.6°), which results in a more exposed, less sterically
congested zirconium center. The constraint imposed by the ansa-
Me2C bridge should result in less orbital overlap between the
cyclopentadienyl ring systems and zirconium, such that this
ligand set is a relatively poor electron donor.50 The presence of
a Cp* ligand might be expected to increase the Cpcent-Zr-
Cpcent, but the steric influence of the -SiMe3 groups in the
R-positions of the metallacycle acts to counter this effect, such
that this angle in 1 (121.8°) is less than that in the Cp2 analogue
9 (124.4°). Thus, compound 1 may be viewed as possessing a
considerably more crowded metal center and the most electron-
donating ligand set. The relief of steric pressure around the metal
center associated with the ansa bridge of 2 results in the shortest
distance from the metal to the R-carbon of the cyclopentadiene
ring (Zr-C1, 2.243(4) Å). For comparison, the corresponding
distances in 1 and 9 are 2.266(3) and 2.264(2) Å, respectively.
Kinetic Studies of Alkyne Substitution. To quantify dif-
ferences in rates of alkyne substitution (Vide supra), a kinetic
study was initiated. It was previously reported that Cp2Zr[2,5-
(Me3Si)2-3,4-Ph2C4] readily fragments to form a zirconacyclo-
propene, with liberation of 1 equiv of trimethylsilylphenylacet-
ylene in the presence of a donor ligand such as PMe3.34
Similarly, treatment of the zirconacycles 1, 2, and 9 with 10
equiv of PMe3 at ambient temperature in benzene-d6 resulted
in quantitative formation of zirconacyclopropenes 7, 8, and 11,
respectively, with release of 1 equiv of free Me3SiCtCPh (by
1H NMR spectroscopy). Under these pseudo-first-order condi-
tions, data were collected for up to five half-lives and the
products were identified by comparisons to the independently
synthesized compounds described above, or to those previously
reported in the literature.34 The observed, pseudo-first-order rate
constants are summarized in Scheme 6. The rate constant
observed for the fragmentation of 9 with excess PMe3 (10 equiv)
at 50 °C was found to be kobs ) 1.08(5) × 10-3 s-1. This value
is similar to that for the rate of substitution of 1 equiv of Me3-
SiCtCPh in 9 in the presence of an excess of tolan (10 equiv)
at 50 °C (kobs ) 0.88(3) × 10-3 s-1). The substitution of Me3-
SiCtCPh is expected to occur in a sequential process to first
form the unsymmetrical Cp2Zr[2-(Me3Si)-3,4,5-Ph3C4], which
then undergoes a second substitution to form the tetraphenylzir-
conacyclopentadiene (10). The formation of Cp2Zr[2-(Me3Si)-
Figure 1. ORTEP depiction of the solid-state molecular structure of 1.
Hydrogen atoms are omitted for clarity. Ellipsoids are drawn with 50%
probabilities. Compound 1 was crystallized in the P1h space group with the
following refinement parameters: R1 ) 0.0334, wR2 ) 0.0930, and GOF
) 1.086. Selected bond lengths (Å), bond angles (°): Zr-C1, 2.266(3);
C1-C2, 1.360(4); C2-C3, 1.523(4); C2-C1-Si1, 121.8(2); Cpcent-Zr-
Cpcent, 135.4.
Figure 2. ORTEP depiction of the solid-state molecular structure of 2.
Hydrogen atoms are omitted for clarity. Ellipsoids are drawn with 50%
probabilities. Compound 2 was crystallized in the P1h space group with the
following refinement parameters: R1 ) 0.0402, wR2 ) 0.0825, and GOF
) 0.821. Selected bond lengths (Å), bond angles (°): Zr-C1, 2.243(4);
C1-C2, 1.348(5); C2-C3, 1.538(6); C2-C1-Si1, 125.5(3); Cpcent-Zr-
Cpcent, 115.6.
1
3,4,5-Ph3C4] is supported by H NMR spectra which contain
resonances for an intermediate species with -SiMe3 (-0.14
ppm) and Cp (6.14 ppm) groups. The similarity in the
substitution rates of Me3SiCtCPh with either PMe3 or tolan
suggests that the two substitution reactions proceed by related
mechanisms (Scheme 1).
It is believed that the initial reaction steps of zirconocene-
promoted alkyne coupling involve the formation of a transient
16-electron monoalkyne complex.3,36,51,52 The intermediate
zirconacyclopropene then rapidly adds a second equivalent of
alkyne to give the zirconacylopentadiene. Given that alkyne
substitution by PMe3 in zirconacyclopentadienes represents a
process very similar to the reverse of alkyne coupling, it seemed
that this reaction might occur via a reversible ring fragmentation
to generate a zirconacyclopropene followed by trapping with
1
under the same conditions, and by H NMR spectroscopy the
main product appears to be the unsymmetrical zirconacyclopenta-
diene formed by a single alkyne substitution. This product ex-
hibits resonances at -0.04 (R-SiMe3), 1.76 (Cp*), and 6.37 (Cp)
ppm. Increasing the temperature to 100 °C resulted in formation
of the expected zirconacycle 3 over 14 h. Thus, the influence
of ancillary ligands on the rates of these alkyne substitution
reactions follows the pattern Me2C(C5H4)2 > Cp2 > CpCp*.
Structural Studies. The molecular structures of zirconacy-
clopentadienes 1 and 2 were determined by single-crystal X-ray
crystallography, and ORTEP diagrams49 of each are shown in
Figures 1 and 2. The structure of compound 9 was previously
reported by Erker and co-workers.31 The ansa-bridged zircona-
cyclopentadiene 2 exhibits the smallest angle associated with
binding of the bis(cyclopentadienyl) ligand set (Cpcent-Zr-
(50) Lee, H.; Desrosiers, P. J.; Guzei, I.; Rheingold, A. L.; Parkin, G. J. Am.
Chem. Soc. 1998, 120, 3255-3256.
(51) Buchwald, S. L.; Nielsen, R. B. J. Am. Chem. Soc. 1989, 111, 2870-
2874.
(52) Imabayashi, T.; Fujiwara, Y.; Nakao, Y.; Sato, H.; Sakaki, S. Organome-
tallics 2005, 24, 2129-2140.
(49) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
9
J. AM. CHEM. SOC. VOL. 130, NO. 14, 2008 4995

A R T I C L E S
Scheme 6
Miller et al.
Scheme 7
PMe3 (Scheme 7). The first step of this mechanism is consistent
with the observed first-order dependence on the concentration
of the starting zirconacyclopentadiene, exhibited by the reactions
of 1, 2, and 9 with excess PMe3 (10 equiv). Attempts to
determine the rate dependence on the concentration of PMe3
involved monitoring the kinetics of reactions of 1, 2, and 9 with
varying PMe3 concentrations (see the Supporting Information
for details). For these reactions, second-order kinetics were not
observed and the reaction rates were found to be insensitive to
the phosphine concentration. Additional mechanistic insight was
gained by measuring the activation parameters for the alkyne
substitution in zirconacycle 9 by PMe3. An Eyring plot of the
rate data for the temperature range of 22-72 °C (Figure 3)
provided the activation parameters ∆Hq ) 28(2) kcal mol-1 and
∆Sq ) 14(4) eu. The significantly positive entropy of activation
and the observed rate law strongly indicate the operation of a
dissociative mechanism for the alkyne/PMe3 exchange.53
Attempts to measure k1 (Scheme 7) independently, as a
verification of the results obtained under pseudo-first-order
conditions, involved 2-D EXSY NMR experiments designed
to measure the exchange rate of free Me3SiCtCPh with the
bound alkyne of zirconacyclopentadiene A. However, this
exchange could not be observed below the decomposition
temperature of 9 (ca. 75 °C).
the observation that the rate constant does not vary with changes
in the concentration of PMe3, suggests that the rate constant
for trapping with PMe3 (k2) must be extremely large.
The above mechanistic studies have established that the rates
of alkyne substitution by PMe3 in a zirconacyclopentadiene are
first-order in zirconacycle concentration and have no observed
dependence on the concentration of PMe3. The generality of
this dissociative mechanism, in which zirconacyclopentadiene
fragmentation is the rate-determining step, is supported by the
close similarity in rates for alkyne substitutions with PMe3 or
tolan, suggesting that both reactions occur through the same
zirconacyclopropene intermediate. These results have implica-
tions for the reductive coupling of alkynes by the zirconocene
The observed first-order rate law, kobs[zirconocene], is
consistent with the two-step mechanism of Scheme 7, with
alkyne dissociation as the rate-limiting step. For a dissociative
mechanism, it might be expected that added alkyne (PhCt
CSiMe3, C) would suppress the rate of substitution. However,
with C/PMe3 ratios of 3, 9, 42, and 55, substitution reactions
of zirconacycle 9 proceeded at similar rates. This, along with
Figure 3. Eyring plot for the rate of disappearance of 9 over the temperature
range from 22 to 72 °C.
(53) Atwood, J. D. Inorganic and Organometallic Reaction Mechanisms; Brooks/
Cole Pub. Co.: Monterey, CA, 1985.
9
4996 J. AM. CHEM. SOC. VOL. 130, NO. 14, 2008

Reversible Alkyne Coupling at Zirconocene
A R T I C L E S
fragment, which presumably goes through the same zircona-
cyclopropene intermediate.
of Cp*2Zr(CO)2 (ν(CO)avg ) 1899.5 vs 1932.0 cm-1 for Cp2-
Zr(CO)2), which indicate greater back-donation in these com-
pounds.54 The Me2C(η5-C5H4)2Zr fragment appears to have a
slightly destabilized HOMO compared to that of the Cp2Zr
fragment, despite the fact that the carbonyl stretching frequencies
in Me2C(η5-C5H4)2Zr(CO)2 are slightly higher in energy (ν-
(CO)avg ) 1935.0 cm-1).54 Thus, on the basis of the theoretical
results cited above, the influence of ancillary ligand sets on the
rates of alkyne coupling should be ordered such that CpCp* >
Cp2 > Me2C(η5-C5H4)2. The experimental results presented
above, however, reflect the opposite trend, with the fastest rate
for alkyne coupling being associated with the Me2C(η5-C5H4)2-
Zr fragment. This indicates that steric factors override electronic
influences in these coupling reactions.
Influence of Ancillary Ligands on Alkyne Coupling. The
observed first-order rate constants for the fragmentations of
zirconacyclopentadienes with PMe3 depend very much on the
nature of the ancillary bis(cyclopentadienyl) ligand sets, with
the ratio of relative rates being 0.046:1:100 (CpCp*/Cp2/Me2C-
(η5-C5H4)2). Thus, the rate is greater with less sterically
demanding and less electron-donating ancillary ligands. The
influence of these ligands on the dissociative substitutions of
alkynes can be rationalized in terms of the mechanism in
Scheme 7, which is consistent with the investigations described
above, and with computational studies.36,52,54 The transition state
for alkyne coupling at zirconocene has been calculated for Cp2-
Zr[2,5-Ph2-3,4-(C6F5)2]36 (eq 2). These results indicate that the
transition state is highly unsymmetrical, with one alkyne strongly
bonded to zirconium to give a zirconacyclopropene structure
(C-C bond length: 1.296 Å) while the other alkyne is very
weakly bonded with little perturbation of its geometry (C-C
bond length: 1.236 Å). As the coupling reaction proceeds, the
more activated alkyne moves into the wedge formed by the two
Cp ligands, and repulsive steric interactions between the Cp
groups and the phenyl ring of this alkyne ligand are further
intensified by a “pivoting” of this alkyne as the zirconacyclo-
pentadiene ring forms. The geometric changes associated with
this coupling result in a constriction of the Cpcent-Zr-Cpcent
angle, as indicated by the relatively small value calculated for
the transition state (128.7°) vs that observed for Cp2Zr[2,5-Ph2-
3,4-(C6F5)2] (136.6°). Thus, bis(cyclopentadienyl) ligand sets
that enforce a smaller Cpcent-Zr-Cpcent angle, such as Me2C-
(η5-C5H4)2, should lower the transition-state energy. On the other
hand, the sterically hindered Cp* ligand prevents a significant
reduction in the Cpcent-Zr-Cpcent angle and leads to a higher
transition-state energy.
Conclusions
Given the synthetic utility of reversible alkyne couplings
involving zirconacyclopentadienes,23-30,32,34 it is important to
develop a detailed understanding of the mechanism of this
process and the manner in which steric and electronic factors
influence reactivity. The results reported here, which represent
the first kinetic and mechanistic studies of this reaction, provide
insights into how these couplings occur. Although the kinetic
studies focused on the fragmentation of zirconacyclopentadienes
(the reverse of alkyne coupling), a number of conclusions
regarding the mechanism of alkyne coupling can be made.
Consistent with previous computational studies, the experimental
results point to stepwise activations of the two alkynes. Thus,
the dissociative nature of the zirconacycle fragmentation is
consistent with alkyne couplings that occur via reaction of the
second alkyne with a zirconacyclopropene complex.
The kinetic analysis described here offers an opportunity to
probe steric and electronic effects on alkyne couplings at
zirconium. Kinetic and mechanistic information is very useful
for the development of an understanding of factors that control
regiochemistry and the supramolecular chemistry associated with
reversible couplings.19-35 Steric effects dominate the rate of
alkyne substitution in zirconacyclopentadienes, but not in the
manner that might be expected. Thus, sterically crowded
zirconacyclopentadienes do not eliminate alkyne more rapidly.
The least sterically demanding ancillary ligand set, Me2C(η5-
C5H4)2, leads to the fastest rate for alkyne substitution, and the
more sterically demanding CpCp* ligands decrease the rate for
alkyne substitution (relative to Cp2). Combined with structural31
and computational36,52 studies on these coupling reactions, the
results indicate that facile alkyne couplings at a zirconocene
center require considerable space in the plane bisecting the
cyclopentadienyl ligands to accommodate necessary pivoting
motions for the two alkyne ligands. This information regarding
the influence of ancillary ligands will be useful for future
synthetic efforts that employ new alkyne substituents in regi-
oselective and/or reversible couplings to small molecules,
oligomers, polymers, macrocycles, and cages.
The electronic properties of the bis(cyclopentadienyl) ligand
set may also play an important role in influencing the rate of
alkyne coupling. Theoretical calculations suggest that electron-
donating ancillary ligands stabilize the transition state for
alkyne-alkyne coupling at zirconium.52 Therefore, more electron-
donating bis(cyclopentadienyl) ligand sets might be expected
to increase the rate of alkyne coupling. The Cp* ligand is
relatively electron-donating, and calculations on the Cp*2Zr
fragment indicate that the Cp* ligands raise the HOMO energy
of this fragment compared to that of Cp2Zr.54 This has also been
experimentally confirmed by the lower energy carbonyl stretches
Experimental Section
General Procedures. All manipulations were performed under a
dry, nitrogen atmosphere using standard Schlenk techniques or a
nitrogen-filled glovebox. Dry, oxygen-free solvents were employed.
Olefin impurities were removed from pentane by treatment with
concentrated H2SO4, 0.5 N KMnO4 in 3 M H2SO4, and saturated
NaHCO3. Pentane was then dried over MgSO4, stored over activated 4
(54) Zachmanoglou, C. E.; Docrat, A.; Bridgewater, B. M.; Parkin, G.; Brandow,
C. G.; Bercaw, J. E.; Jardine, C. N.; Lyall, M.; Green, J. C.; Keister, J. B.
J. Am. Chem. Soc. 2002, 124, 9525-9546.
9
J. AM. CHEM. SOC. VOL. 130, NO. 14, 2008 4997

A R T I C L E S
Miller et al.
Å molecular sieves, and distilled from potassium benzophenone ketyl
under a nitrogen atmosphere. Toluene was purified by stirring with
concentrated H2SO4, washing with saturated NaHCO3, then drying over
CaCl2 before distillation from sodium under a nitrogen atmosphere.
Spectrophotometric grade diethyl ether and reagent grade THF were
distilled from sodium benzophenone ketyl under a nitrogen atmosphere.
Acetonitrile was refluxed over P2O5 and distilled under a nitrogen
atmosphere. Benzene-d6 was vacuum distilled over sodium/potassium
alloy and benzophenone.
NMR spectra were recorded in benzene-d6 solutions on a Bruker
AVB-400 instrument operating at 400.1 MHz (1H), 100.7 MHz (13C),
and 162 MHz (31P), with a Bruker AV-300 instrument operating at
300.1 MHz (1H), or with a Bruker DRX-500 instrument operating at
500.1 MHz (1H). Chemical shifts are reported in ppm downfield from
SiMe4 and were referenced internally to residual solvent resonances
(1H, 13C) or externally to 85% H3PO4 (31P). Elemental analyses were
performed by the Microanalytical Laboratory in the College of
Chemistry at the University of California, Berkeley.
syringe at -78 °C. After stirring for 1 h at -78 °C, tolan (0.200 g, 1.1
mmol) in THF (5 mL) was added. The reaction mixture was allowed
to warm to ambient temperature over 2-3 h and then stirred for an
additional 48 h. The THF was then removed under vacuum, and then
toluene (20 mL) was added. The solution was cannula-filtered and
concentrated to 10 mL, and then pentane (8 mL) was added. The
solution was cooled to -80 °C for 96 h to give the product, isolated
by filtration, as orange crystals in 60% yield (0.216 g, 0.34 mmol). 1H
NMR: δ 7.00 (m, 4 H), 6.95 (m, 4 H), 6.86 (t, 4 H, 3J ) 7.8 Hz), 6.79
(m, 4 H), 6.70 (m, 4 H), 6.09 (s, 5 H), 1.79 (s, 15 H). 13C{1H} NMR:
δ 197.5 (-ZrCPhdCPh), 148.2 (-ZrCPhdCPh), 144.0 (Ph), 142.9
(Ph), 131.8 (Ph), 129.1 (Ph), 127.3 (Ph), 127.1 (Ph), 124.9 (Ph), 123.4
(Ph), 121.2 (η5-C5Me5), 112.5 (η5-C5H5), 12.3 (η5-C5Me5). Anal. Calcd
for C43H40Zr: C, 79.70; H, 6.22. Found: C, 79.35; H, 6.07.
Me2C(η5-C5H4)2Zr[2,3,4,5-Ph4C4] (4). See the preparation of zir-
conacyclopentadiene 1 for the synthetic protocol. The amounts of
reagents used were Me2C(η5-C5H4)2ZrCl2 (0.250 g, 0.75 mmol) in THF
(10 mL), n-BuLi (0.94 mL, 1.5 mmol), and tolan (0.402 g, 2.3 mmol)
in THF (5 mL). The solid residue was purified by washing with pentane
(2 × 20 mL), followed by addition of toluene (12 mL). The solution
was cannula-filtered, concentrated, cooled to -30 °C for 18 h, and
then cooled to -80 °C for 20 h to give the product, isolated by filtration,
All reagents were purchased from Aldrich and used as received
unless stated otherwise. The 1-phenyl-2-(trimethylsilyl)acetylene was
vacuum distilled and stored over 3 Å molecular sieves. Pyridine was
refluxed over calcium hydride and distilled under a nitrogen atmosphere.
n-Butyl lithium was used as a 1.6 M solution in hexanes. The
1
as orange crystals in 32% yield (0.150 g, 0.27 mmol). H NMR: δ
zirconocene dichlorides, CpCp*ZrCl2 and Me2C(η5-C5H4)2ZrCl2,56
55
7.11 (t, 4 H, 3J ) 7.6 Hz), 7.01 (m, 4 H), 6.88 - 6.79 (m, 14 H), 6.70
3
(m, 2 H), 5.06 (t, 4 H, J ) 2.8 Hz), 1.09 (s, 6 H). 13C{1H} NMR: δ
were prepared according to literature procedures. Compound 10 was
prepared according to the method of Negishi et al.47 Compound 11
was prepared in situ, and the spectroscopic characterization was
consistent with the literature.34
194.3 (-ZrCPhdCPh), 149.3 (-ZrCPhdCPh), 141.2 (Ph), 139.9 (Ph),
131.1 (Ph), 128.1 (Ph), 127.1 (Ph), 126.9 (Ph), 125.2 (Ph), 123.3 (Ph),
118.0 (Me2C(η5-C5H4)), 115.5 (Me2C(η5-C5H4)), 109.6 (Me2C(η5-
C5H4)), 37.5 (Me2C(η5-C5H4)), 23.7 (Me2C(η5-C5H4)). Anal. Calcd for
C41H34Zr: C, 79.74; H, 5.55. Found: C, 79.42; H, 5.50.
CpCp*Zr[2,5-(Me3Si)2-3,4-Ph2C4] (1). To CpCp*ZrCl2 (0.208 g,
0.57 mmol) in THF (15 mL), n-BuLi (0.7 mL, 1.1 mmol) was added
dropwise via syringe at -78 °C. After stirring for 1 h at -78 °C,
1-phenyl-2-(trimethylsilyl)acetylene (0.200 g, 1.1 mmol) in THF (5 mL)
was added. The reaction mixture was allowed to warm to ambient
temperature over 2-3 h and was then stirred for an additional 16 h.
The THF was removed under vacuum, and then pentane (20 mL) was
added. The solution was cannula-filtered and pumped to dryness, and
then diethyl ether (10 mL) was added until all the solid had dissolved.
To this solution was slowly added acetonitrile until solid persisted in
solution. The solution was cooled to -30 °C for 16 h to give the
product, isolated by filtration, as yellow-orange crystals in 28% yield
(0.101 g, 0.16 mmol). 1H NMR: δ 6.91 (m, 6 H), 6.77 (m, 2 H), 6.71
(m, 2 H), 6.20 (s, 5 H), 1.90 (s, 15 H), -0.06 (s, 18 H). 13C{1H}
NMR: δ 207.2 (-ZrC(SiMe3)dCPh), 152.7 (-ZrC(SiMe3)dCPh),
146.2 (Ph), 131.1 (Ph), 130.7 (Ph), 126.9 (Ph), 126.5 (Ph), 125.1 (Ph),
120.2 (η5-C5Me5), 111.3 (η5-C5H5), 12.7 (η5-C5Me5), 4.6 (-SiMe3).
Anal. Calcd for C37H48Si2Zr: C, 69.42; H, 7.56. Found: C, 69.08; H,
7.50.
CpCp*Zr(η2-Me3SiCtCSiMe3)(pyr) (5). To CpCp*ZrCl2 (0.50 g,
1.4 mmol) in THF (20 mL) was added bis(trimethylsilyl)acetylene (0.31
mL, 1.4 mmol) via syringe. This solution was cooled to -78 °C, and
n-BuLi (1.7 mL, 2.8 mmol) was added dropwise via syringe. The
mixture was stirred for 10 min at -78 °C, the cold bath was removed,
and the reaction mixture was stirred for 16 h at ambient temperature.
Pyridine (0.11 mL, 1.4 mmol) was then added via syringe, which caused
the reaction mixture to immediately turn opaque brown. The solvent
was then removed under vacuum to near dryness, pentane (30 mL)
was added, and the solution was stirred for 30 min. The solution was
cannula-filtered, concentrated to 10 mL, and then cooled to -80 °C
for 6 days. The solvent was removed by cannula filtration, and the
resulting solid was recrystallized from a concentrated pentane solution
at -30 °C for 16 h to give the product, isolated by filtration, as dark
1
purple crystals in 16% yield (0.122 g, 0.23 mmol). H NMR: δ 8.34
(m, 2 H), 6.87 (m, 1 H), 6.55 (m, 2 H), 5.70 (s, 5 H), 1.58 (s, 15 H),
0.33 (s, 18 H). 13C{1H} NMR: δ 222.8 (-ZrC(SiMe3)dC(SiMe3),
detected by gHMBC experiment), 153.4 (pyr), 136.1 (pyr), 123.1 (pyr),
115.0 (η5-C5Me5), 108.5 (η5-C5H5), 11.9 (η5-C5Me5), 3.4 (-SiMe3).
Anal. Calcd for C28H43NSi2Zr: C, 62.16; H, 8.01; N, 2.59. Found: C,
62.37; H, 8.10; N, 2.72.
Me2C(η5-C5H4)2Zr[2,5-(Me3Si)2-3,4-Ph2C4] (2). See the preparation
of zirconacyclopentadiene 1 for the synthetic protocol. The amounts
of reagents used were Me2C(η5-C5H4)2ZrCl2 (0.200 g, 0.60 mmol) in
THF (10 mL), n-BuLi (0.75 mL, 1.2 mmol), and 1-phenyl-2-
(trimethylsilyl)acetylene (0.210 g, 1.2 mmol) in THF (5 mL). Crystal-
lization from pentane resulted in yellow crystals in 35% yield (0.130
Me2C(η5-C5H4)2Zr(η2-Me3SiCtCSiMe3)(pyr) (6). A Schlenk tube
was loaded with Me2C(η5-C5H4)2ZrCl2 (0.250 g, 0.75 mmol), magne-
sium powder (0.091 g, 3.75 mmol), and mercury chloride (0.204 g,
0.75 mmol) and was then cooled to -78 °C. A solution of bis-
(trimethylsilyl)acetylene (0.17 mL, 0.75 mmol) in THF (10 mL) was
added, and the mixture was then stirred for 1.5 h while warming to
-20 °C. The flask was then placed in an ice bath for 15 min, and
pyridine (0.06 mL, 0.75 mmol) was added via syringe. The reaction
mixture was allowed to warm to ambient temperature and was stirred
for 16 h. The solvent was removed under vacuum, then pentane (20
mL) was added, and the solution was stirred for 15 min. The solution
was cannula-filtered, concentrated to 5 mL, and cooled to -30 °C for
18 h to give the product, isolated by filtration, as dark purple crystals
1
g, 0.21 mmol). H NMR: δ 6.87 (m, 4 H), 6.80 (m, 2 H), 6.66 (m, 4
H), 6.58 (t, 4 H, 3J ) 2.8 Hz), 5.81 (t, 4 H, 3J ) 2.8 Hz), 1.60 (s, 6 H),
-0.07 (s, 18 H). 13C{1H} NMR: δ 198.3 (-ZrC(SiMe3)dCPh), 145.2
(-ZrC(SiMe3)dCPh), 143.3 (Ph), 129.5 (Ph), 126.6 (Ph), 124.8 (Ph),
115.4 (Me2C(η5-C5H4)), 112.5 (Me2C(η5-C5H4)), 107.4 (Me2C(η5-
C5H4)), 36.9 (Me2C(η5-C5H4)), 23.8 (Me2C(η5-C5H4)), 2.3 (-SiMe3).
Anal. Calcd for C35H42Si2Zr: C, 68.90; H, 6.94. Found: C, 68.59; H,
6.90.
CpCp*Zr[2,3,4,5-Ph4C4] (3). To CpCp*ZrCl2 (0.203 g, 0.56 mmol)
in THF (15 mL) was added n-BuLi (0.7 mL, 1.1 mmol) dropwise via
(55) Wolczanski, P. T.; Bercaw, J. E. Organometallics 1982, 1, 793-799.
(56) Quijada, R.; Dupont, J.; Silveira, D. C.; Miranda, M. S. L.; Scipioni, R. B.
Macromol. Rapid Commun. 1995, 16, 357-362.
1
in 46% yield (0.176 g, 0.34 mmol). H NMR: δ 8.64 (m, 2 H), 6.80
(m, 1 H), 6.38 (m, 2 H), 6.13 (m, 2 H), 5.34 (m, 4 H), 4.93 (m, 2 H),
9
4998 J. AM. CHEM. SOC. VOL. 130, NO. 14, 2008

Reversible Alkyne Coupling at Zirconocene
A R T I C L E S
1.55 (s, 3 H), 1.42 (s, 3 H), 0.55 (s, 9 H), 0.05 (s, 9 H). 13C{1H} NMR:
δ 217.6 (-ZrC(SiMe3)dC(SiMe3)), 195.7 (-ZrC(SiMe3)dC(SiMe3)),
152.8 (pyr), 136.5 (pyr), 123.0 (pyr), 122.9 (Me2C(η5-C5H4)), 113.9
(Me2C(η5-C5H4)), 108.9 (Me2C(η5-C5H4)), 94.9 (Me2C(η5-C5H4)), 92.1
(Me2C(η5-C5H4)), 35.7 (Me2C(η5-C5H4)), 24.7 (Me2C(η5-C5H4)), 24.3
(Me2C(η5-C5H4)), 2.5 (-SiMe3), 2.3 (-SiMe3). Anal. Calcd for C26H37-
NSi2Zr: C, 61.11; H, 7.30; N, 2.74. Found: C, 60.88; H, 7.35; N,
2.59.
CpCp*Zr(η2-Me3SiCtCPh)(PMe3) (7). See the preparation of
zirconacyclopropene 6 for synthetic protocol. The amounts of reagents
used were Cp*CpZrCl2 (0.40 g, 1.1 mmol), magnesium powder (0.13
g, 5.5 mmol), mercury chloride (0.30 g, 1.1 mmol), 1-phenyl-2-
(trimethylsilyl)acetylene (0.19 g, 1.1 mmol), and PMe3 (0.11 mL, 1.1
mmol) in THF (15 mL). Crystallization from pentane resulted in yellow
crystals in 51% yield (0.305 g, 0.56 mmol). 1H NMR: δ 7.20 (t, 2 H,
3J ) 8.5 Hz), 6.94 (m, 1 H), 6.80 (m, 2 H), 5.40 (s, 5 H), 1.74 (s, 15
H), 0.83 (d, 9 H, 2JH-P ) 5 Hz), 0.37 (s, 9 H). 31P{1H} NMR: δ -10.2.
Anal. Calcd for C29H43PSiZr: C, 64.27; H, 8.00. Found: C, 64.27; H,
8.20.
ferrocene (ca. 5 mg, 27 µmol) were loaded into a Teflon-sealed NMR
tube and then dissolved in benzene-d6 (ca. 500 mg). The tube was then
heated at 50 °C directly in the spectrometer for 1 h.
1
Data points were gathered by H NMR spectroscopy, and the rate
of disappearance of 9 was monitored by integrating the -SiMe3 peak
relative to that of Cp2Fe. Rate constants were calculated from first-
order plots using data from the first three to five half-lives, and the
observed rate constant (kobs ) 0.88(3) × 10-3 s-1) is an average of
three runs.
Kinetic Study of the Zirconacylopentadiene Fragmentation with
PMe3. Zirconacyclopentadienes 1, 2, or 9 (ca. 9.7 µmol) and ferrocene
(ca. 2 mg, 10.7 µmol) were dissolved in benzene-d6 (ca. 500 mg) and
then loaded into a Teflon-sealed NMR tube. To this solution was added
PMe3 (10 µL, 97 µmol), and the samples containing 1 were quickly
frozen to prevent fragmentation before analysis.
1
Data points were gathered by H NMR spectroscopy, and the rate
of disappearance of 1, 2, or 9 was monitored by integrating the -SiMe3
peak relative to that of Cp2Fe. Rate constants were calculated from
first-order plots using data from the first three to five half-lives. All
high-temperature analyses were performed directly in the spectrometer,
and the temperature was calibrated with an external ethylene glycol
standard.58
Me2C(η5-C5H4)2Zr(η2-Me3SiCtCPh)(PMe3) (8). See the prepara-
tion of zirconacyclopropene 6 for synthetic protocol. A change to the
above procedure was that the reaction mixture was stirred for only 2 h
at ambient temperature before removal of THF. The amounts of reagents
used were Me2C(η5-C5H4)2ZrCl2 (0.322 g, 0.97 mmol), magnesium
powder (0.118 g, 4.85 mmol), mercury chloride (0.263 g, 0.97 mmol),
1-phenyl-2-(trimethylsilyl)acetylene (0.19 mL, 0.97 mmol), and PMe3
(0.1 mL, 1 mmol) in THF (10 mL). Crystallization from pentane
resulted in yellow crystals in 29% yield (0.145 g, 0.28 mmol). 1H
NMR: δ 7.20 (t, 2 H, 3J ) 8 Hz), 6.96 (m, 1 H), 6.72 (m, 2 H), 6.43
(m, 2 H), 5.09 (m, 2 H), 4.97 (m, 2 H), 4.92 (m, 2 H), 1.64 (s, 3 H),
Influence of [PMe3] on the Rate of Zirconacyclopentadiene
Fragmentation. Zirconacyclopentadienes 1, 2, or 9 (ca. 6 µmol) and
ferrocene (ca. 2 mg, 10.7 µmol) were dissolved in benzene-d6 (ca. 500
mg) and then loaded into a Teflon-sealed NMR tube. To this solution
was added PMe3 in varying concentrations (three different concentra-
tions for each), and the samples containing 1 were quickly frozen to
prevent fragmentation before analysis. Experiments for zirconacyclo-
pentadiene 1 were performed at 20 °C, those for 2 were done at 75 °C,
and those for 9 were done at 50 °C.
2
1.38 (s, 3 H), 0.81 (d, 9 H, JH-P ) 6 Hz), 0.35 (s, 9 H). 31P{1H}
NMR: δ -7.1. Anal. Calcd for C27H37SiPZr: C, 63.35; H, 7.29.
Found: C, 63.38; H, 7.29.
1
Data points were gathered by H NMR spectroscopy, and the rate
of disappearance of 1, 2, or 9 was monitored by integrating the -SiMe3
peak relative to that of Cp2Fe. Rate constants were calculated from
first-order plots using data from the first three to five half-lives. See
the Supporting Information for further details.
Cp2Zr[2,5-(Me3Si)2-3,4-Ph2C4] (9). See the preparation of zircona-
cyclopentadiene 1 for the synthetic protocol. The amounts of reagents
used were Cp2ZrCl2 (0.20 g, 0.68 mmol) in THF (15 mL), n-BuLi (0.85
mL, 1.4 mmol), and 1-phenyl-2-(trimethylsilyl)acetylene (0.24 g, 1.4
mmol) in THF (5 mL). Crystallization from pentane resulted in yellow
crystals in 26% yield (0.100 g, 0.18 mmol). 1H NMR: δ 6.88 (t, 4 H,
Supporting Information Available: Tables of bond lengths
and angles, details of the kinetic experiments, and the X-ray
crystallographic data for compounds 1 and 2 (CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
3
3
3J ) 7.3 Hz), 6.77 (t, 2 H, J ) 7.5 Hz), 6.68 (d, 4 H, J ) 7.3 Hz),
6.16 (s, 10 H), -0.16 (s, 18 H). This is consistent with the reported 1H
NMR spectrum.57
Reaction of Cp2Zr[2,5-(Me3Si)2-3,4-Ph2C4] (9) with Tolan. Zir-
conacycle 9 (ca. 6.5 mg, 11 µmol), tolan (ca. 21 mg, 118 µmol), and
JA800025U
(57) Erker, G.; Zwettler, R.; Kruger, C.; Schlund, R.; Hylakryspin, I.; Gleiter,
(58) Ammann, C.; Meier, P.; Merbach, A. E. J. Magn. Reson. 1982, 46, 319-
R. J. Organomet. Chem. 1988, 346, C15-C18.
321.
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