Mechanistic studies of the formation of zirconium alkylidene complexes [η5-C5H3-1,3-(SiMe2CH2PPr(i)2)2]Zr=CHR(Cl) (R = Ph, SiMe3)

Article abstract of DOI:10.1021/ja982969h

The reaction of [P₂Cp]ZrC1₃ (1) with 2 equiv of KCH₂Ph generates an equilibrium mixture of alkyl complexes consisting of [P₂Cp]ZrCl₂(CH₂Ph) (2), [P₂Cp]ZrCl(CH₂Ph)₂ (3), and [P₂Cp]Zr(CH₂Ph)₃ (4). Thermolysis of this mixture yields the alkylidene complex [P₂Cp]Zr=CHPh(Cl) (5) in 85% overall yield. Kinetic studies reveal a composite mechanism that incorporates the above preequilibrium, followed by an intramolecular α-abstraction reaction of dibenzyl 3 which follows a first-order rate, with the rate parameters ΔH((+)) = 19(1) kcal mol^-1 and ΔS((+)) = -22(5) cal^-1 mol K^-1. A kinetic isotope effect of 3.0(5) was measured at 70°C for the perdeuterated analogue [P₂Cp]ZrCl(CD₂C₆D₅)₂. The reaction of 1 with 2 equiv of LiCH₂EMe₃ (E = C, Si) produces a similar equilibrium mixture as observed for the benzyl analogues, consisting of [P₂Cp]ZrCl₂(CH₂EMe₃) (7), [P₂Cp]Zr(CH₂EMe₃)₃ (8), and [P₂Cp]ZrCl(CH₂EMe₃)₂ (9). Thermolysis of this mixture yields [P₂Cp]Zr=CHEMe₃(Cl) (6). A kinetic analysis conducted on 9 (E = Si) indicated a first-order reaction from which the activation parameters ΔH((+)) = 6(1) kcal mol^-1 and ΔS((+)) = -62(5) cal mol^-1 K^-1 were obtained. The results indicate that reaction rates follow the order CH₂Ph > CH₂SiMe₃ > CH₂CMe₃, an exact reversal of the trend for the homoleptic Ta systems Ta(CH₂R)₅. The role of phosphine coordination is discussed to account for this trend. A crystal structure determination obtained for 6b reveals an α-agostic interaction and a structure analogous to that of 5.

Full text of DOI:10.1021/ja982969h

2478
J. Am. Chem. Soc. 1999, 121, 2478-2487
Mechanistic Studies of the Formation of Zirconium Alkylidene
Complexes [η5-C5H3-1,3-(SiMe2CH2PPri2)2]ZrdCHR(Cl) (R ) Ph,
SiMe3)
Michael D. Fryzuk,*,† Paul B. Duval,† Shane S. S. H. Mao,† Michael J. Zaworotko,‡ and
Leonard R. MacGillivray‡
Contribution from the Department of Chemistry, UniVersity of British Columbia, 2036 Main Mall,
VancouVer, B.C., Canada V6T 1Z1, and Department of Chemistry, St. Mary’s UniVersity,
Halifax, N.S., Canada B3H 3C3
ReceiVed August 18, 1998
Abstract: The reaction of [P2Cp]ZrCl3 (1) with 2 equiv of KCH2Ph generates an equilibrium mixture of alkyl
complexes consisting of [P2Cp]ZrCl2(CH2Ph) (2), [P2Cp]ZrCl(CH2Ph)2 (3), and [P2Cp]Zr(CH2Ph)3 (4).
Thermolysis of this mixture yields the alkylidene complex [P2Cp]ZrdCHPh(Cl) (5) in 85% overall yield.
Kinetic studies reveal a composite mechanism that incorporates the above preequilibrium, followed by an
intramolecular R-abstraction reaction of dibenzyl 3 which follows a first-order rate, with the rate parameters
∆H‡ ) 19(1) kcal mol-1 and ∆S‡ ) -22(5) cal-1 mol K-1. A kinetic isotope effect of 3.0(5) was measured
at 70 °C for the perdeuterated analogue [P2Cp]ZrCl(CD2C6D5)2. The reaction of 1 with 2 equiv of LiCH2-
EMe3 (E ) C, Si) produces a similar equilibrium mixture as observed for the benzyl analogues, consisting of
[P2Cp]ZrCl2(CH2EMe3) (7), [P2Cp]Zr(CH2EMe3)3 (8), and [P2Cp]ZrCl(CH2EMe3)2 (9). Thermolysis of this
mixture yields [P2Cp]ZrdCHEMe3(Cl) (6). A kinetic analysis conducted on 9 (E ) Si) indicated a first-order
reaction from which the activation parameters ∆H‡ ) 6(1) kcal mol-1 and ∆S‡ ) -62(5) cal mol-1 K-1 were
obtained. The results indicate that reaction rates follow the order CH2Ph > CH2SiMe3 > CH2CMe3, an exact
reversal of the trend for the homoleptic Ta systems Ta(CH2R)5. The role of phosphine coordination is discussed
to account for this trend. A crystal structure determination obtained for 6b reveals an R-agostic interaction and
a structure analogous to that of 5.
Introduction
Scheme 1
Metal alkylidene1 complexes continue to generate interest as
intermediates in a number of important catalytic reactions
involving olefins, including alkene metathesis2,3 and related
transformations such as ring-opening metathesis polymerization
(ROMP)4-7 and ring-closing metathesis (RCM)8,9 catalysis.
Many examples of alkylidene complexes are known for the
groups 5 and 6 metals,10-12 which is in stark contrast to the
paucity of group 4 derivatives.13,14 The reason for the discrep-
ancy between the above-mentioned groups has been attributed
† University of British Columbia.
‡ St. Mary’s University.
to the mechanism by which these complexes are formed; as
shown in Scheme 1, a sterically congested polyalkyl precursor
is necessary since this promotes R-abstraction between two cis-
disposed alkyl ligands.1
For groups 5 and 6 precursors this arrangement can be met
either with bulky homoleptic alkyl complexes such as Ta(CH2-
CMe3)515 or similar derivatives such as Cp*TaCl2(CH2CMe3)2.16
However, the analogous group 4 complexes contain at least one
less alkyl ligand and hence are at a considerable disadvantage
to achieve a similarly crowded coordination sphere. Therefore,
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10.1021/ja982969h CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/03/1999

Formation of [η5-C5H3-1,3-(SiMe2CH2PPri2)2]ZrdCHR(Cl)
J. Am. Chem. Soc., Vol. 121, No. 11, 1999 2479
most group 4 alkylidene complexes reported thus far have
required the addition of either inter- or intramolecular phosphine
donors,17,18 and of these examples, only a few Ti derivatives
have been isolated as thermally stable compounds.19-23
In the slow limit of this process, one phosphine coordinates to
the metal while the other is dissociated. By comparison, in 4
the three bulky benzyl ligands crowd the metal center and
prevent either phosphine from coordinating, leaving both
sidearms dangling at all temperatures.
In contrast to these results, treating the starting trichloride
complex 1 with 2 equiv of KCH2Ph in THF does not give the
dibenzyl complex [P2Cp]ZrCl(CH2Ph)2 (3) as the sole product,
but instead yields a thermally and photochemically labile
compound isolated as a greenish-orange oil consisting of a
mixture of 2, 3, and 4, in the ratio of 5:4:5, respectively, at
room temperature (eq 1):
We are currently exploring the organometallic chemistry of
early transition-metals and f-block elements stabilized by the
tridentate ligand [η5-C5H3-1,3-(SiMe2CH2PPri2)2] ([P2Cp]). This
system has allowed for the isolation and solid-state structure
determination of the first stable zirconium alkylidene complex
[P2Cp]ZrdCHPh(Cl) (5).24 The high yield and overall stoichi-
ometry of this product suggested that the reaction proceeds via
a thermally induced R-abstraction of the dialkyl precursor [P2-
Cp]ZrCl(CH2Ph)2 (3). However, this species was not initially
detected and therefore was assumed to be an unobserved
intermediate in this reaction.24 Further investigation has now
provided for the detection of 3 and permitted us to study the
mechanism of its transformation to 5. Although the mechanistic
(1)
2, 3 and 4 are all observed together as products of this reaction
as long as the stoichiometry of KCH2Ph used is greater than 1
equiv and less than 3 equiv, the relative proportion of tribenzyl
4 present in the mixture increasing with the amount of benzyl
reagent used. Attempts to separate individual species from this
mixture have repeatedly proved unsuccessful. The subsequent
thermal or photochemical reaction of this ensemble to produce
the benzylidene complex [P2Cp]ZrdCHPh(Cl) (5) will be
discussed later.
details of alkylidene formation have received considerable
attention,25 the kinetic study presented here provides the first
insight into this reaction from the perspective of group 4
derivatives.
The generality of this reaction has been extended to the
synthesis of the alkylidene analogues, [P2Cp]ZrdCHEMe3(Cl)
(6a: E ) C, 6b: E ) Si). A kinetic study of the formation of
6b is included in this report as a comparison to the mechanism
determined for 5, along with the crystal structure determination
for 6b.
The identification of 3 within this mixture was nontrivial and
is based on the assignment of resonances in the 1H and 31P{1H}
NMR spectra by comparison to those resonances attributed to
known 2 and 4. The coexistence of all three species in solution
1
Results and Discussion
generates a complicated H NMR spectrum with some peaks
overlapping even at 500 MHz, thereby rendering some reso-
nances unassignable. However, the integration of those reso-
nances attributable to 3 supports the stoichiometry of the
compound as a dibenzyl complex (two CH2Ph ligands per
[P2Cp] moiety).
NMR Spectroscopic Identification of [P2Cp]ZrCl(CH2Ph)2
(3). The starting complex [P2Cp]ZrCl3 (1) reacts rapidly with 1
equiv of KCH2Ph in THF to produce the monobenzyl complex
[P2Cp]ZrCl2(CH2Ph) (2), which can be isolated with difficulty
as red crystals from a cold (-40 °C), concentrated, hexameth-
yldisiloxane solution. Similarly, 1 reacts readily with 3 equiv
of KCH2Ph in THF to generate cleanly the tribenzyl derivative
[P2Cp]Zr(CH2Ph)3 (4) as a hydrocarbon-soluble orange oil. The
solution behavior of these species as monitored by variable-
temperature NMR spectroscopy has been described previously.26
At ambient temperature the pendant phosphines of 2 are
fluxional, undergoing rapid exchange via an associative pathway.
The ambient temperature 31P{1H} NMR spectrum of the
reaction mixture reveals a broad singlet for 3 at 0.5 ppm, which
sharpens as the temperature is elevated to 95 °C. As the
temperature is lowered below room temperature, this peak
gradually broadens until it disappears into the baseline near -70
°C, beyond which a reappearance of new peaks is not observed
even down to -95 °C. These observations are consistent with
fluxional coordination of the sidearm phosphines to the metal
center, as previously noted with other [P2Cp]ZrRxCl3-x com-
plexes.26
(17) Schwartz, J.; Gell, K. I. J. Organomet. Chem. 1980, 184, C1.
(18) Hartner, F. W.; Schwartz, J.; Clift, S. M. J. Am. Chem. Soc. 1983,
105, 640.
(19) van der Heijden, H.; Hessen, B. J. Chem. Soc., Chem. Commun.
1995, 145.
(20) van Doorn, J. A.; van der Heijden, H.; Orpen, A. G. Organometallics
1995, 14, 1278.
The resonances due to the benzylic (CH2Ph) protons of 3
are temperature dependent, a feature that is not observed in the
1
corresponding spectra of either 2 or 4. This portion of the H
NMR spectrum of the mixture is relatively unobscured, which
permitted a variable-temperature study. At -40 °C the benzylic
protons of 3 give rise to a doublet of multiplets centered near
2.45 ppm; the pattern resembles an AB multiplet with additional
coupling to phosphorus-31. As the temperature is raised to -10
°C this doublet of multiplets evolves into a doublet of doublets,
which gradually converges to two broad singlets at 30 °C.
Eventually these peaks coalesce as the temperature approaches
40 °C, and a further temperature increase gradually sharpens
(21) Binger, P.; Muller, P.; Benn, R.; Mynott, R. Angew. Chem., Int.
Ed. Engl. 1989, 28, 610.
(22) Sinnema, P.-J.; Veen, L.; Spek, A. L.; Veldman, N.; Teuben, J. H.
Organometallics 1997, 16, 4245.
(23) Kahlert, S.; Go¨rls, H.; Scholz, J. Angew. Chem., Int. Ed. Engl. 1998,
37, 1857.
(24) Fryzuk, M. D.; Mao, S. S. H.; Zaworotko, M. J.; MacGillivray, L.
R. J. Am. Chem. Soc. 1993, 115, 5336.
(25) Li, L.; Hung, M.; Xue, Z. J. Am. Chem. Soc. 1995, 117, 12746.
(26) Fryzuk, M. D.; Mao, S. S. H.; Duval, P. B.; Rettig, S. J. Polyhedron
1995, 14, 11.

2480 J. Am. Chem. Soc., Vol. 121, No. 11, 1999
Fryzuk et al.
Table 1. Equilibrium Concentrations of 2, 3, and 4 from Mixed
Solutions at 25 °C and the Resulting Equilibrium Constant (K)
Calculated According to Eq 2
Scheme 2
[2] (M)
[3] (M)
[4] (M)
K
0.55
0.23
0.15
0.04
0.12
0.10
0.11
0.07
0.04
0.07
0.17
0.17
0.7
0.5
0.7
0.7
this singlet further. To account for these observations, in the
low-temperature limit, it is proposed that the benzyl ligands of
3 are chemically equivalent but the methylene protons are
inequivalent; a structure based on a pseudo trigonal bipyramid
with one phosphine arm coordinated and the other dangling is
shown below. As the temperature is raised the rate of phosphine
donor exchange gradually increases until it coincides with the
NMR time scale at coalescence, and eventually in the fast
exchange limit of this process a singlet is observed as an average
of two chemical environments. Thus on average, two equivalent
benzyl groups are present and only one pendant phosphine donor
is coordinated.
Equilibrium between [P2Cp]ZrCl2(CH2Ph) (2), [P2Cp]-
ZrCl(CH2Ph)2 (3), and [P2Cp]Zr(CH2Ph)3 (4). Monobenzyl
2 and tribenzyl 4 are each isolable in high yield from reactions
conducted under the appropriate stoichiometric conditions, yet
as already mentioned, attempts to prepare dibenzyl 3 always
result in mixtures. A possible explanation for the coexistence
of 2, 3, and 4 from the reaction in eq 1 is that there is an
equilibrium between dibenzyl 3, monobenzyl 2, and tribenzyl
4 (eq 2):
fluxional behavior of various species and by the temperature
dependence of the chemical shifts of some resonances, precise
integration measurements of the spectra obtained proved dif-
ficult. As a result, the van’t Hoff plots obtained were subject to
considerable uncertainty in the linear fits, and reproducible
values for the thermodynamic parameters ∆H°, ∆S°, and ∆G°
could not be calculated. However, the measurements were
sufficiently reliable to be able to discern a small increase in the
concentration of 3 relative to 2 and 4 as the temperature was
increased over a wide (-65 to 60 °C) range. Therefore, a
qualitative assessment of the thermodynamics in this reaction
could be made. The minimal temperature dependence of the
equilibrium constant apparent from the results suggests that the
magnitude of ∆H° is fairly small, a positive value being inferred
from the observed increase in K with temperature. Also, as there
are an equal number of species on each side of eq 2 and all are
approximately similar in structure, one could argue that ∆S°
should also be relatively small. Overall, this would give a low
value for ∆G° in this reaction, although admittedly there is
considerable uncertainty associated with the assumptions made
in this analysis.
(2)
In support of this proposal, when pure 2 and 4 are mixed in
solution, one immediately observes the formation of 3, irrespec-
tive of the order of addition, and ultimately a mixture of all
three compounds. The ratio of species observed in the final
reaction mixture depends on the relative amounts of monobenzyl
2 and tribenzyl 4 initially added. Table 1 shows the equilibrium
concentrations obtained from separately prepared solutions in
which the initial proportions of 2 and 4 added together were
varied. From the data, it is evident that these species are indeed
in equilibrium (eq 2) with an equilibrium constant (K ) [3]2/
[2][4]) equal to 0.7(1) at 25 °C. The estimated uncertainty in K
is a function of the imprecision in the integration measurements
The mechanism that converts monobenzyl 2 and tribenzyl 4
to 2 equiv of dibenzyl 3 involves a mutual exchange of benzyl
and chloro ligands. An intermediate is not observed in this
reaction, but a plausible candidate could be the dinuclear adduct
A shown in Scheme 2. There are two options for dissociation
of the bridging ligands in complex A: path a regenerates 2 and
4, while path b leads to the formation of 2 equiv of 3; both
pathways are consistent with the reaction in eq 2.
1
of the H NMR spectra, as evaluated from repetitive experi-
ments.
It should be stated at this point that both mononuclear 3 and
the dinuclear adduct A possess an identical stoichiometry of
two benzyl ligands for each [P2Cp] group. The identification
of the species observed in equilibrium with monobenzyl 2 and
tribenzyl 3 as mononuclear 3 was ascertained from the equi-
Variable-temperature NMR experiments were then conducted
on sample mixtures of 2, 3, and 4 to obtain thermodynamic
information from the temperature dependence of the equilibrium
constant depicted in eq 2. Because of the aforementioned

Formation of [η5-C5H3-1,3-(SiMe2CH2PPri2)2]ZrdCHR(Cl)
J. Am. Chem. Soc., Vol. 121, No. 11, 1999 2481
Scheme 3
Figure 1. Nonlinear fit to the first-order decomposition of 3 (at 353
K).
Table 2. Estimated Rate Constants for the First-Order
R-Abstraction of 3
T (K)
104k2 (s-1
)
343(1)
353(1)
358(1)
368(1)
0.7(2)
1.6(2)
2.5(2)
5.4(3)
A kinetic analysis of the thermolysis of this reaction mixture
was conducted by monitoring the 31P{1H} NMR spectra for
separate reactions performed at 70, 80, 85, and 95 °C,
respectively. A similar study was also conducted at 70 °C on
the perdeuterated analogue [P2Cp]ZrCl(CD2C6D5)2 (3-d7) to
examine a possible kinetic isotope effect. In all instances the
decline in [3] followed a pattern as shown in Figure 1. The
nonlinear fit of the plot of ln[3] versus time illustrates a
departure from first-order kinetics; the preequilibrium in this
composite mechanism is likely responsible for these complicated
results. The steep initial decrease in the concentration of 3
corresponds more closely to the actual first-order R-abstraction
reaction rate, but curvature in the plot in Figure 1 arises due to
a steady decrease in the observed rate (kobs) as the continued
lowering in the concentrations of the equilibrium species makes
it more difficult to replenish 3 from bimolecular collisions of 2
and 4.
To evaluate the complicating factor of the preequilibrium,
the results were simulated (Chemical Kinetics Simulator)27 from
input of the initial concentrations and the kinetic parameters
k1, k-1, and k2. Knowledge of the approximate equilibrium
constant K gives the ratio k1/k-1, with limits for these values
set (1 > k > 10-3 s-1) by the lack of exchange broadening
observed in the line widths of the equilibrium species in the
NMR spectra, and by the observation of k2 as the slow step in
this composite reaction mechanism. A more precise estimate
of these parameters, as well as the rate constant k2 for the
irreversible R-abstraction step, could be made by iteration of
the simulation results to give the best agreement with the
experimentally determined changes in [3], [4], and [5] as a
function of time; only a very narrow range of values of k2 (given
in Table 2) produces the observed values of [3], [4], and [5]
for a given temperature. The inclusion of the preequilibrium in
the simulation analysis reproduces the characteristic decay of
dibenzyl 3 depicted in Figure 1.
librium concentration measurements. As shown in Table 1, a
process involving dinuclear A in direct equilibrium with
monobenzyl 2 and tribenzyl 4 (i.e., in which path b in Scheme
2 is omitted) is not in accord with the data. An equilibrium
constant calculated assuming a dinuclear complex would require
that K ) [A]/[2][4]; this in turn requires that the values for [A]
determined by integration of the NMR spectra would equal one-
half the values listed for 3 in Table 1, and therefore shows poor
correlation between the separate sample mixtures. Furthermore,
if this equilibrium were in effect then dilution of the reaction
mixture should induce further disproportionation of A to restore
the equilibrium, a feature that is not observed.
Mechanistic Studies on the Formation of [P2Cp]Zrd
CHPh(Cl) (5). As previously reported,24 thermolysis of a
toluene solution of the reaction mixture consisting of 2, 3, and
4 at 65 °C for 24 h produces the alkylidene complex [P2Cp]-
ZrdCHPh(Cl) (5). The same result is obtained when the reaction
mixture is exposed to visible light for a period of 12 h. 5 is
thermally stable and can be isolated as moderately air- and
moisture-sensitive orange crystals from cold pentane in high
yield (85%).
The unusual transformation of the mixture of benzyl com-
plexes 2, 3, and 4 to produce alkylidene 5 prompted us to
investigate the mechanism employed. Since it was found that
each of 2 and 4 is thermally and photochemically stable under
the conditions that generate alkylidene 5 from the mixture, it
seemed likely that the dibenzyl complex 3 is the precursor to
the alkylidene 5 via R-abstraction; this is summarized in Scheme
3. Accordingly, the continued depletion of 3 in forming 5 would
perturb the preequilibrium in Scheme 3, thus prompting the
conversion of additional monobenzyl 2 and tribenzyl 4 to
replenish 3, eventually resulting in the observed high yield
production of 5. Added support that 3 is the reactive species is
provided by heating reaction mixtures consisting of unequal
proportions of 2 and 4. In each case the formation of 5 ceases
when the precursor 3 can no longer be replenished, which occurs
once the limiting component (either 2 or 4) as a source for 3
has been consumed.
The rate constants (k2) obtained for each temperature from
the simulated data are shown in Table 2; a kinetic isotope effect
of 3.0(0.5) was observed at 70 °C for the perdeuterated species,
in support of rate-determining C-H bond activation16,25 in a
concerted, four-centered transition state as shown in Scheme

2482 J. Am. Chem. Soc., Vol. 121, No. 11, 1999
Fryzuk et al.
Figure 3. ORTEP view of 6b; 50% probability thermal ellipsoids are
shown for the non-hydrogen atoms and H(19).
Scheme 4
Figure 2. Eyring plot of the first-order decomposition of 3 (arbitrary
error bars of (0.1 are indicated).
3. From the Eyring plot in Figure 2 the activation parameters
∆H‡ ) 19(1) kcal mol-1 and ∆S‡ ) -22(5) cal mol-1 K-1
were obtained. The value for ∆H‡ is in accord with kinetic
studies conducted on other systems involving alkylidene forma-
tion and related C-H activation processes.28-30 In those studies
∆H‡ ranges between 20 and 30 kcal mol-1. However, the
experimentally determined value of ∆S‡ for 3 is rather large in
comparison to these other systems (typically between 0 and -10
cal mol-1 K-1), although the decomposition of Ti(CH2CMe3)4
via an alkylidene intermediate has a similar large negative ∆S‡
NMR spectrum and the triplet at 209 ppm in the 13C{1H} NMR
spectrum, for the alkylidene proton (ZrdCHCMe3) and carbon,
respectively. In contrast, the silyl analogue 6b can be isolated
as thermally stable, air- and moisture-sensitive yellow crystals
in very good yield. Spectroscopic data for 6b reveal a singlet
in the 1H NMR spectrum at 8.99 ppm for the alkylidene proton
(ZrdCHSiMe3) and a triplet at 225 ppm in the 13C{1H} NMR
spectrum for the alkylidene carbon (ZrdCHSiMe3); the triplet
pattern in the 13C{1H} NMR spectrum arises from 2JPC coupling
to two equivalent phosphines.
An X-ray structure determination of 6b was undertaken; the
molecular structure is shown in Figure 3, with selected bond
lengths and bond angles given in Table 3. The very short Zr-C
bond distance of 2.015(9) Å is similar to that of 2.024(4) Å for
Zr benzylidene 5.24 As also observed for 5, the alkylidene
fragment in 6b is oriented syn to the unique cyclopentadienyl
carbon, with the R-hydrogen atom directed below the metal
center to form the same type of agostic C-H interaction. This
hydrogen was located and refined, and the Zr‚‚‚H distance of
2.28(8) Å, together with the distorted Zr-C-Si angle of
164.3(6)°, is characteristic of an R-agostic C-H bond.32 All
other bond lengths and angles are similar to those in the
corresponding alkylidene 5.24
) -16 cal mol-1 K-1 31
An in-depth analysis of the kinetic
.
results for 3 will be given in conjunction with the analogous
results for the R-abstraction of [P2Cp]ZrCl(CH2SiMe3)2 (9b)
to give alkylidene [P2Cp]ZrdCHSiMe3(Cl) (6b).
Other Zirconium Alkylidene Complexes. In view of the
success in the synthesis of the benzylidene complex 5, we were
interested in testing both the generality of this reaction mech-
anism and the versatility of the [P2Cp] ligand in stabilizing other
Zr alkylidene complexes. Two examples of alkyl groups that
have been commonly chosen for the synthesis of alkylidene
complexes of the early transition-metals are the neopentyl
(CH2CMe3)10 and neosilyl (CH2SiMe3) ligands.25 Treatment of
1 with 2 equiv of LiCH2EMe3 (E ) C, Si) in toluene at -78
°C yields a bright yellow oil after workup. Thermolysis of this
material in toluene at 95 °C generates the corresponding
alkylidene complex [P2Cp]ZrdCHEMe3(Cl) (6a: E ) C, 6b:
E ) Si) (Scheme 4). The formation of 6a requires 6 days at 95
°C, while the analogous reaction to produce 6b is complete
within 3 days under the same conditions.
The neopentylidene (6a) was obtained as a green oil,
contaminated with inseparable side-products (approximately
20% by NMR spectroscopy), presumably a result of the
prolonged duration of the thermolysis. Diagnostic of the
alkylidene moiety is the singlet observed at 8.56 ppm in the 1H
To examine the formation of 6a and 6b in comparison to the
mechanism determined for 5, reaction mixtures derived accord-
ing to Scheme 4 were studied by NMR spectroscopy prior to
the thermolysis. The spectra obtained exhibit a complexity that
has been similarly noted from the products of the analogous
reaction in eq 1, suggesting the occurrence of an equilibrium
mixture of the corresponding mono-, di-, and trialkyl complexes.
To help assign these species, the alkyl complexes [P2Cp]-
(27) Hinsberg, W.; Houle, F., Almaden Research Center, IBM, 1996,
San Jose, CA.
(28) Thorn, M. G.; Hill, J. E.; Waratuke, S. A.; Johnson, E. S.; Fanwick,
P. E.; Rothwell, I. P. J. Am. Chem. Soc. 1997, 119, 8630.
(29) Vaughan, W. M.; Abboud, K. A.; Boncella, J. M. J. Am. Chem.
Soc. 1995, 117, 11015.
(30) McDade, C.; Green, J. C.; Bercaw, J. E. Organometallics 1982, 1,
1629.
(31) Cheon, J.; Rogers, D. M.; Girolami, G. S. J. Am. Chem. Soc. 1997,
119, 6804.
(32) Brookhart, M.; Green, M. L. H.; Wong, L. L. Prog. Inorg. Chem.
1988, 36, 1.

Formation of [η5-C5H3-1,3-(SiMe2CH2PPri2)2]ZrdCHR(Cl)
J. Am. Chem. Soc., Vol. 121, No. 11, 1999 2483
Table 3. Selected Bond Lengths (Å) and Bond Angles (deg) for
6b
Table 4. Rate Constants for the First-Order Portion of the
Decomposition of 9b
Zr(1)-Cl(1)
Zr(1)-P(2)
Zr(1)-CP(2)
Zr(1)-CP(4)
Zr(1)-Cent
P(1)-Cl(1)
P(1)-C(10)
P(2)-C(13)
Si(1)-CP(1)
Si(1)-CP(4)
Cp(1)-CP(5)
CP(3)-C(4)
Si(3)-C(20)
Zr(1)-H(19)
2.508(3)
2.820(3)
2.549(8)
2.555(8)
2.577(8)
1.827(9)
1.845(12)
1.881(10)
1.878(9)
1.855(9)
1.398(13)
1.412(16)
1.841(12)
2.28(8)
Zr(1)-P(1)
Zr(1)-CP(1)
Zr(1)-CP(3)
Zr(1)-CP(5)
Zr(1)-C(19)
P(1)-C(7)
2.838(3)
2.569(8)
2.528(8)
2.555(8)
2.015(9)
1.855(11)
1.810(9)
1.839(11)
1.794(11)
1.437(12)
1.405(12)
1.848(9)
1.10(8)
T (K)
105k2 (s-1
)
353(1)
368(1)
383(1)
398(1)
3.7(3)
4.8(3)
7.7(4)
9.7(5)
P(2)-C(6)
P(2)-C(16)
Si(1)-Cl(1)
Cp(1)-CP(2)
Cp(2)-CP(3)
Si(3)-C(19)
C(19)-H(19)
A reexamination of the products obtained from the reaction
of 1 with 2 equiv of LiCH2EMe3 reveals the presence of 7 and
8, and another species which is observed as a singlet in the 31
P
1
NMR spectrum and identified from the H NMR spectrum as
the dialkyl complex [P2Cp]ZrCl(CH2EMe3)2 (9a: E ) C, 9b:
E ) Si). Again, it seems that a similar equilibrium (eq 5) is
established between 7, 8, and 9 for the neopentyl and neosilyl
derivatives as has been observed for the benzyl derivative, since
combining separate solutions of 7 and 8 produces the same
mixture of all three species.
Cl(1)-Zr(1)-P(1)
Cl(1)-Zr(1)-C(19) 108.0(3)
76.48(8) Cl(1)-Zr(1)-P(2)
78.44(9)
Zr(1)-C(19)-Si(3) 164.3(6)
P(1)-Zr(1)-P(2)
P(1)-Zr(1)-C(19)
Zr(1)-P(1)-Cl(1)
Zr(1)-P(2)-C(6)
Cp(4)-Si(2)-C(4)
154.91(8) Si(3)-C(19)-H(19) 106.5
95.9(3)
111.8(3)
111.7(4)
110.6(4)
P(2)-Zr(1)-C(19)
Zr(1)-P(1)-C(7)
92.5(3)
116.7(4)
Cp1(1)-Si(1)-C(1) 106.8(4)
Si(1)-CP(1)-CP(2) 123.2(7)
Si(1)-CP(1)-CP(5) 131.0(7)
P(1)-C(1)-Si(1)
Cl(1)-Zr(1)-Cent
P(2)-Zr(1)-Cent
Zr(1)-C(19)-H(19)
119.1(6)
141.23(8)
98.62(7)
89.5(5)
P(2)-C(6)-Si(2)
P(1)-Zr(1)-Cent
C(19)-Zr(1)-Cent
113.6(5)
100.42
110.7(3)
(5)
ZrCl2(CH2EMe3) (7a: E ) C, 7b: E ) Si) and [P2Cp]Zr(CH2-
EMe3)3 (8a: E ) C, 8b: E ) Si) were independently prepared
from reactions of 1 with the appropriate stoichiometry of LiCH2-
EMe3 in toluene (eq 3, 4):
Detailed analyses of these equilibria were precluded by the
overlap of peaks in the 1H and 31P{1H} NMR spectra for various
species at separate temperatures. However, it could be ascer-
tained that below the temperature at which one observes
alkylidene formation, the equilibrium increasingly favors the
disproportionation of the dialkyl species 9 as the temperature
is raised (i.e., the value of K decreases with an increase in
temperature), in contrast to the observations for 3 in the
equilibrium of eq 2.
(3)
The monoalkyl complexes 7a and 7b are each obtained as
It is evident that the mechanistic details for the formation of
the neopentyl and neosilyl alkylidene complexes 6 are similar
to those previously outlined for benzylidene 5. For example,
heating either the monoalkyl 7 or trialkyl 8 derivatives
independently does not generate an alkylidene complex, and
the production of alkylidene from the thermolysis of the
equilibrium mixture in eq 5 ceases when the supply of dialkyl
precursor 9 has been exhausted. Once again this points to a
mechanism analogous to that in Scheme 3, where the dialkyl
complex 9 is the thermally labile species that leads directly to
the alkylidene product in a first-order mechanism.
Experimental complications associated with the long reaction
time and the formation of interfering impurities precluded a
kinetic study of the neopentyl system. However, an analysis of
the reaction kinetics for the thermal rearrangement of neosilyl
9b was conducted by 31P NMR spectroscopy, for reactions
performed at 80, 95, 110, and 125 °C. In all instances the same
pattern for the decrease in [9b] over time was observed as has
been noted for benzyl 3 in Figure 1. Therefore, the results for
9b were simulated by the same iterative procedure followed
for 3, from which the first-order rate constant (k2) was obtained
for each temperature (Table 4). The corresponding Eyring plot
is shown in Figure 4, from which the activation parameters ∆H‡
) 6(1) kcal mol-1 and ∆S‡ ) -62(5) cal mol-1 K-1 were
(4)
yellow oils that could not be isolated free of impurities. Some
of the impurities arise from the slow disproportionation of 7 to
give 1 and other unidentified species. Both 7a and 7b exhibit
fluxional behavior similar to that observed for monobenzyl 2,26
although the processes could not be satisfactorily studied by
variable-temperature NMR spectroscopy due to the inherent
impurities.
The trialkyl complexes 8a and 8b can each be isolated cleanly
as hydrocarbon-soluble oils. Singlets are observed in the 31P
NMR spectrum at -5.5 and -5.9 ppm for 8a and 8b,
respectively. These resonances remain unchanged at lower
temperatures and are indicative of dangling phosphines, as is
found for tribenzyl 4. Apparently, the crowded geometry
imposed by three relatively bulky alkyl ligands prevents the
phosphines from coordinating to the metal center.

2484 J. Am. Chem. Soc., Vol. 121, No. 11, 1999
Fryzuk et al.
Scheme 5
Figure 4. Eyring plot of the first-order decomposition of 9b (arbitrary
error bars of (0.1 are indicated).
negative ∆S‡ term. The first is the coordination of both bulky
sidearm phosphines, considered to help lower the activation
energy by increasing steric saturation at the metal center to
induce R-abstraction as a means of relieving the steric pressure.
The second feature is the necessity for the two alkyl ligands to
attain a cis configuration to permit the concerted abstraction
step to proceed via the four-centered transition state. The steric
difficulties that must be surmounted to reach the transition state
are even more pronounced for the bulkier dialkyl species 9b.
For example, the singlet in the 31P{1H} NMR spectrum for 9b
at -4.5 ppm is consistent with uncoordinated phosphine donors
as shown in Scheme 5. In this case, the difference between the
ground state, in which both arms are unbound, and the highly
ordered transition state leads to the very large negative ∆S‡.
Since the ∆H‡ terms for these processes do not differ much
from Ta(CH2R)5 systems discussed above, it is the large negative
value of ∆S‡ for the bulkier alkyl derivatives that results in the
observed reversal of the reactivity trend; in other words, entropic
factors are important in the formation of these group 4 alkylidene
complexes. These factors also help to explain the otherwise
contradictory observation that the bulky trialkyl derivatives are
thermally unreactive, as these complexes are too sterically
hindered in comparison to the dialkyl analogues to allow the
bulky phosphines to coordinate in the transition state.
obtained; these values are significantly different from the
parameters obtained for 3.
The trend reported in the literature for the relative rates of
alkylidene complex formation follows the order neopentyl >
neosilyl > benzyl, which reflects faster rate-determining
R-abstraction for complexes bound by alkyl ligands that can
provide greater steric congestion at the metal center. For
example, for the first-order decomposition of Ta(CH2R)5 (R )
Ph, SiMe3, CMe3), the rate constants follow the order k ) 4.3
× 10-5 (at 313 K) < 3.5 × 10-4 (311 K) < too fast to monitor,
for R ) Ph,33 SiMe3,25 and CMe3,25 respectively. Our results
indicate a reVersal of this trend for the complexes studied here.
The neopentyl system that produces alkylidene 6a was found
to be too slow to study, and a comparison of the first-order rate
constants in Table 2 and Table 4 for the dialkyl species studied
reveals the slower thermal reactivity of neosilyl 9b versus benzyl
3. This surprising observation may be partially related to the
unusual preequilibrium step associated with the postulated
mechanism (Scheme 3), where disproportionation of the dialkyl
species is favored at higher temperatures for the bulkier
derivatives, in competition with the irreversible step leading to
alkylidene formation. However, the effect of the preequilibrium
is evidently secondary in comparison to the activation param-
eters, particularly ∆S‡, associated with the rate-determining
R-abstraction step. As mentioned above, both near-zero and
negative values (0 to -10 cal mol-1 K-1) have been typically
observed for the majority of C-H bond abstraction processes
that have been studied. The results from this work show
considerable departure from these values, especially with the
large negative ∆S‡ of -62(5) cal mol-1 K-1 for 9b, which has
been rarely seen for a first-order process. To account for this
remarkable difference it is useful to examine the proposed
transition state structure in this reaction as shown in Scheme 5.
For the dibenzyl complex 3, the 31P{1H} NMR data indicate
that one phosphine binds in the ground state; to access the
transition state, there are two features that must be attained,
both of which lead to a significantly more ordered structure in
the transition state in comparison to the ground state structure
of the dialkyl derivatives, which could generate a relatively large
Conclusions
The ancillary ligand [P2Cp] has been utilized in the synthesis
of the first reported examples of stable zirconium alkylidene
complexes, 5 and 6b. The structural parameters are similar for
each and the solid-state structure is maintained in solution. Both
complexes are monomeric with Cs symmetry, and the alkylidene
units are oriented in a similar fashion with respect to the
ancillary ligand. An agostic interaction between the alkylidene
R-C-H moiety and the metal center is also present in both
compounds, a common feature for unsaturated alkylidene
complexes.
The alkylidene complexes 5 and 6 are formed via the thermal
decomposition of the corresponding dialkyl precursors 3 and
9, respectively. However, this is complicated by a preequilibrium
step in which 3 and 9 also undergo reversible ligand redistribu-
tion, presumably via a bridging dinuclear adduct, to give the
(33) Malatesta, V.; Ingold, K. U.; Schrock, R. R. J. Organomet. Chem.
1978, 152, C53.

Formation of [η5-C5H3-1,3-(SiMe2CH2PPri2)2]ZrdCHR(Cl)
J. Am. Chem. Soc., Vol. 121, No. 11, 1999 2485
The assignment of resonances for 3 is based on the known resonances
for 2 and 4. Note that some peaks for 3 are obscured by overlapping
resonances arising from 2 and 4.
corresponding mono and tris(alkyl) species (Scheme 2). Kinetic
studies show that the decomposition of 3 and 9 follows first-
order kinetics, with a deviation in the first-order plot associated
with the presence of the preequilibrium. The activation param-
eters obtained from the reaction are consistent with an intramo-
lecular R-abstraction process, as is the kinetic isotope effect of
3.0 (0.5) obtained for the benzyl derivative. However, in the
systems studied here the reaction rates follow the order benzyl
> neosilyl > neopentyl, exactly opposite to that found previ-
ously in other systems. These results reflect a subtle balance
between the steric crowding imposed by the alkyl ligands and
the sidearm phosphines of the ancillary ligand. The bulkier
neopentyl ligands crowd the metal center to a greater extent
than do the benzyl groups, thus providing less opportunity for
the relatively large Pri2P donors to coordinate. As alkylidene
formation can be promoted by phosphine coordination,34 this
may account for the faster rates observed for the less bulky
benzyl derivative, in addition to the otherwise surprising lack
of thermal reactivity of the trialkyl species, where phosphine
coordination remains absent under all conditions.
3: 1H NMR (20 °C, C6D6) δ 0.33 and 0.44 (s, 6H, Si(CH3)2), 0.89
(m, 4H, SiCH2P), 1.23 (m, 24H, CH(CH3)2), 2.34 (m, 4H, CH2Ph),
4
4
6.49 (d, 2H, JHH ) 1 Hz, Cp-H), 6.65 (d, 4H, JHH ) 7 Hz, o-C6H5),
4
3
6.87 (t, 1H, JHH ) 1 Hz, Cp-H), 7.03 (t, 4H, JHH ) 7 Hz, m-C6H5),
7.22 (t, 2H, 3JHH ) 7 Hz, p-C6H5). 31P{1H} NMR (20 °C, C6D6) δ 0.1
(s).
[P2Cp]ZrCl2(CH2CMe3) (7a). A solution of LiCH2CMe3 (19 mg,
0.24 mmol) in 30 mL of toluene was added dropwise over a period of
20 min to a cooled (-78 °C) stirring solution of 1 (156 mg, 0.24 mmol)
dissolved in 60 mL of toluene. The color of the solution gradually
changed from pale to bright yellow. The solution was stirred at -78
to 0 °C for another 30 min, and then allowed to warm slowly to room
temperature. The solvent was then removed in vacuo, and the oily
residue extracted with hexanes and filtered. The filtrate was reduced
to yield 0.87 g of a slightly impure (by 1H and 31P NMR spectroscopy)
yellow oil consisting of 6a as the main (>90%) product. 1H NMR (20
2
°C, C6D6) δ 0.26 and 0.36 (s, 6H, Si(CH3)2), 0.70 (dd, 4H, JHH ) 6
2
Hz, JPH ) 7 Hz, SiCH2P), 1.00 (m, 24H, CH(CH3)2), 1.47 (s, 9H,
CH2C(CH3)3), 1.52 (m, 4H, CH(CH3)2), 1.68 (s, 2H, CH2C(CH3)3), 6.54
4
4
(t, 1H, JHH ) 1 Hz, Cp-H), 6.98 (d, 2H, JHH ) 1 Hz, Cp-H). 31P
Experimental Section
NMR (20 °C, C6D6) δ 0.7 (br s).
[P2Cp]ZrCl2(CH2SiMe3) (7b). The procedure followed was similar
to that for 6a, using LiCH2SiMe3 (27 mg, 0.29 mmol) and 1 (185 mg,
General Considerations. All manipulations were performed under
an atmosphere of prepurified nitrogen in a Vacuum Atmospheres HE-
553-2 glovebox equipped with a MO-40-2H purification system or in
0.29 mmol), to yield an impure yellow oil (>85% pure by 1H and 31
P
1
NMR spectroscopy). H NMR (20 °C, C6D6) δ 0.31 (s, 9H, CH2Si-
(CH3)3), 0.38 and 0.50 (s, 6H, Si(CH3)2), 0.69 (s, 2H, CH2Si(CH3)3),
0.71 (dd, 4H, 2JHH ) 6 Hz, 2JPH ) 7 Hz, SiCH2P), 1.05 (m, 24H, CH-
1
standard Schlenk-type glassware on a dual vacuum/nitrogen line. H
NMR spectra (referenced to nondeuterated impurity in the solvent) were
performed on one of the following instruments depending on the
complexity of the particular spectrum: Bruker WH-200, Varian XL-
300, or a Bruker AM-500. 13C NMR spectra (referenced to solvent
peaks) were run at 75.429 MHz on the XL-300 instrument, and 31P
NMR spectra (referenced to external P(OMe)3 at 141.0 ppm) were run
at 121.421 and 202.33 MHz on the XL-300 and Bruker AM-500
instruments, respectively. All chemical shifts are reported in ppm and
all coupling constants are reported in Hz. Elemental analyses were
performed by Mr. P. Borda of this department.
Reagents. Hexanes and tetrahydrofuran (THF) were predried over
CaH2 followed by distillation from sodium-benzophenone ketyl under
argon. Toluene was dried over sodium under argon, and hexamethyl-
disiloxane was distilled from sodium-benzophenone ketyl under
nitrogen. The deuterated solvents C6D6 and C6D5CD3 were dried over
molten sodium, vacuum transferred to a bomb, and degassed by freeze-
pump-thaw technique before use. KCH2Ph (and the perdeuterated
analogue KCD2C6D5), LiCH2CMe3, and LiCH2SiMe3 were prepared
according to literature methods. (η5-C5H5)2Fe (ferrocene) was sublimed
before use, and trimethylphosphine was vacuum distilled by vacuum
transfer. Samples of ferrocene and PMe3 were dissolved in benzene-d6
and prepared in sealed melting point tubes for 1H NMR external
reference. The procedures for the preparation of [P2Cp]ZrCl3 (1),26
[P2Cp]ZrCl2(CH2Ph) (2),26 [P2Cp]Zr(CH2Ph)3 (4),26 and [P2Cp]Zrd
CHPh(Cl) (5)24 have been described elsewhere.
[P2Cp]ZrCl(CH2Ph)2 (3). (a) From [P2Cp]ZrCl3 (1) and KCH2Ph.
A solution of KCH2Ph (0.100 mg, 0.43 mmol) in 10 mL of THF was
added with stirring to a cooled (-78 °C) solution of 1 (65 mg, 0.23
mmol) dissolved in 30 mL of THF. The dark red color of the KCH2Ph
solution was discharged upon addition and the color of the reaction
mixture became greenish-orange. The mixture was allowed to slowly
warm to room temperature, whereupon all the volatiles were then
removed under vacuum and the residue extracted with hexanes and
filtered. The filtrate was reduced under vacuum to give a thermally
and photochemically labile sensitive orange oil. NMR spectra of this
oil indicate the presence of 3, in addition to both 2 and 4.
4
(CH3)2), 1.61 (m, 4H, CH(CH3)2), 6.50 (t, 1H, JHH ) 1 Hz, Cp-H),
7.05 (d, 2H, 4JHH ) 1 Hz, Cp-H). 31P NMR (20 °C, C6D6) δ 1.5 (br s).
[P2Cp]ZrCl(CH2CMe3)2 (9a). A solution of LiCH2CMe3 (1.09 mg,
0.24 mmol) in 60 mL of toluene was added dropwise to a cooled (-78
°C) stirring solution of 1 (350 mg, 0.58 mmol) dissolved in 60 mL of
toluene. The color of the solution immediately changed in intensity
from pale to bright yellow. The solution was allowed to warm slowly
to room temperature and stirred for another 2 h. The solvent was then
removed in vacuo, and the residue extracted with hexanes and filtered.
The filtrate was reduced to give a bright yellow oil consisting of 7a, in
addition to 6a and 8a. The assignment of resonances for 7a is based
on the known resonances for 6a and 8a obtained by independent
1
syntheses. H NMR (20 °C, C6D6) δ 0.45 and 0.47 (s, 6H, Si(CH3)2),
0.63 (d, 4H, 2JHH ) 6 Hz, SiCH2P), 0.89 (m, 24H, CH(CH3)2), 1.23 (s,
18H, CH2C(CH3)3), 1.31 and 1.78 (d, 2H, 2JHH ) 8 Hz, CH2C(CH3)3),
4
1.55 (m, 4H, CH(CH3)2), 6.82 (d, 2H, JHH ) 1 Hz, Cp-H), 7.03 (t,
4
1H, JHH ) 1 Hz, Cp-H). 31P NMR (20 °C, C6D6) δ -5.9 (s).
[P2Cp]ZrCl(CH2SiMe3)2 (9b). The procedure followed is analogous
to that for 9a above, using LiCH2SiMe3 (117 mg, 1.24 mmol) and 1
(398 mg, 0.62 mmol). The resulting greenish oil consists of 7b, 8b,
and 9b. Again, the assignment for 9b is based on integration of the 1H
NMR spectrum. 1H NMR (20 °C, C6D6) δ 0.32 (s, 18H, CH2Si(CH3)3),
2
0.41 and 0.43 (s, 6H, Si(CH3)2), 0.63 (d, 4H, JHH ) 6 Hz, SiCH2P),
2
0.70 and 1.14 (d, 2H, JHH ) 12 Hz, CH2Si(CH3)3), 0.99 (m, 24H,
CH(CH3)2), 1.57 (m, 4H, CH(CH3)2), 6.81 (t, 1H, 4JHH ) 1 Hz, Cp-H),
6.86 (d, 2H, 4JHH ) 1 Hz, Cp-H). 31P NMR (20 °C, C6D6) δ -4.5 (s).
[P2Cp]Zr(CH2CMe3)3 (8a). A solution of LiCH2CMe3 (56 mg, 0.72
mmol) in 30 mL of toluene was added dropwise to a cooled (-78 °C)
stirring solution of 1 (154 mg, 0.24 mmol) dissolved in 60 mL of
toluene. The color of the solution immediately turned bright yellow
upon addition. The mixture was warmed to room temperature, and
stirring was continued for 30 min. The solvent was then removed in
vacuo, and the oily residue extracted with hexanes and filtered. The
filtrate was reduced under vacuum to give 8a as a bright yellow oil
that remained soluble in pentane even at -40 °C, precluding the
isolation of crystalline material. Yield 130 mg, 72%. 1H NMR (20 °C,
(b) From [P2Cp]ZrCl2(CH2Ph) (2) and [P2Cp]Zr(CH2Ph)3 (4).
Mixing together stoichiometric benzene-d6 solutions of 2 and 4 produced
a mixed solution with the same spectroscopic results as observed for
procedure (a) above.
2
C6D6) δ 0.48 and 0.49 (s, 6H, Si(CH3)2), 0.70 (dd, 4H, JHH ) 6 Hz,
2JPH ) 4 Hz, SiCH2P), 0.99 (m, 24H, CH(CH3)2), 1.21 (s, 6H, CH2C-
(CH3)3), 1.22 (s, 27H, CH2C(CH3)3), 1.55 (m, 4H, CH(CH3)2), 6.68 (d,
4
4
2H, JHH ) 1 Hz, Cp-H), 6.94 (t, 1 Hz, JHH ) 1H, Cp-H). 31P NMR
(34) Rupprecht, G. A.; Messerle, L. W.; Fellmann, J. D.; Schrock, R. R.
J. Am. Chem. Soc. 1980, 102, 6236.
(20 °C, C6D6) δ -5.5 (s).

2486 J. Am. Chem. Soc., Vol. 121, No. 11, 1999
Table 5. Crystallographic Dataa
Fryzuk et al.
compound
formula
fw
crystal system
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
6ba
transmission factors
scan type
data collected
2θmax, deg
total reflcns
unique reflcns
Rmerge
no. with I g 3σ(I)
no. of variables
R(F) (I g 3σ(I))
Rw(F) (I g 3σ(I))
R(F2) (all data)
Rw(F2) (all data)
gof
0.823-0.999
ω-2θ
(h,+k,+l
45
4817
4640
0.040
3028
311
0.056
0.055
0.056
0.055
3.16
C27H57ClP2Si2Zr
654.62
monoclinic
P21/n (No. 14)
12.982(5)
16.679(6)
16.710(5)
76.50(3)
90
90
3573(22)
4
Z
D
calc, g/cm3
1.22
F(000)
1392
5.1
0.30 × 0.40 × 0.50
max ∆/σ (final cycle)
0.000
µ, cm-1
residual density e/Å3
-0.610 to 0.880 (near Zr)
crystal size, mm
a Temperature 294 K, Nonius CAD-4 diffractometer, Mo KR radiation (λ ) 0.70930 Å), graphite monochromator, takeoff angle 6.0°, aperture
6.0 × 6.0 mm at a distance of 285 mm from the crystal, stationary background counts at each end of the scan (scan/background time ratio 2:1),
σ2(F2) ) [S2(C + 4B)]/Lp2 (S ) scan rate, C ) scan count, B ) normalized background count), function minimized ∑w(|Fo| - |Fc|)2 where w )
4Fo /σ2(Fo ), R(F) ) ∑||Fo| - |Fc||/∑|Fo|, Rw(F) ) (∑w(|Fo| - |Fc|)2/∑w|Fo| )1/2, and gof (F) ) [∑w(|Fo| - |Fc|)2/(m - n)]1/2
.
2
2
2
[P2Cp]Zr(CH2SiMe3)3 (8b). The procedure followed was similar
to that for 8a, using LiCH2SiMe3 (82 mg, 0.87 mmol) and 1 (185 mg,
0.29 mmol), to yield a pale yellow oil. Yield 176 mg, 76%. H NMR
(CH3)3), 35.4 and 35.5 (CH(CH3)2), 119.0 (Cp-CH2), 121.9 (CpSi),
129.5 (Cp-CH), 213.8 (t, 2JCP ) 12 Hz, CHSi(CH3)3). Anal. Calcd for
C27H57ClP2Si3Zr: C, 49.54; H, 8.78; Cl, 5.42. Found: C, 49.58; H,
8.71; Cl, 5.65.
NMR Study of the Equilibrium between 2, 3, and 4. Separate
stock solutions of 2 and 4 were prepared in benzene-d6, the concentra-
tions of which were determined by comparing the integrated intensity
of the CH2Ph signals relative to external ferrocene. Incrementally varied
proportions of the initial solutions of 2 and 4 were then combined
together to generate mixed solutions consisting of 2, 3, and 4. 1H NMR
spectra of these solutions were obtained, and the concentrations of 2,
3, and 4 were calculated from the integration of the CH2Ph signals
with respect to that of the ferrocene standard.
1
(20 °C, C6D6) δ 0.28 (s, 27H, CH2Si(CH3)3), 0.29 and 0.45 (s, 6H,
2
Si(CH3)2), 0.32 and 0.63 (d, 2H, JHH ) 6 Hz, SiCH2P), 0.68 (s, 6H,
CH2Si(CH3)3), 1.00 (m, 24H, CH(CH3)2), 1.59 (m, 4H, CH(CH3)2), 6.74
(d, 2H, 4JHH ) 2 Hz, Cp-H), 6.91 (t, 1H, 4JHH ) 2 Hz, CpH). 31P NMR
(20 °C, C6D6) δ -5.9 (s).
[P2Cp]ZrdCHCMe3(Cl) (6a). A solution of LiCH2CMe3 (51 mg,
0.65 mmol) in 20 mL of toluene was added dropwise with stirring to
a cooled (-78 °C) solution of 1 (210 mg, 0.33 mmol) dissolved in 60
mL of toluene. The color of the solution immediately changed to bright
yellow. The solution was stirred at -78 to 0 °C for another 30 min,
and then for another 3 h at room temperature. The solvent was then
removed in vacuo, and the residue extracted with hexanes and filtered.
The filtrate was reduced under vacuum to give a green oil that was
redissolved in 30 mL of toluene. This solution was placed in a 95 °C
oil bath for 6 days, after which no further reaction could be detected
by NMR spectroscopy. The solvent was removed, and the oil extracted
with pentane to give a green oil, containing 6a and other unidentified
products. Repeated attempts to obtain crystals from cold pentane were
unsuccessful. Estimated yield 65% by 31P NMR spectroscopy. 1H NMR
Kinetic Studies of the Formation of Alkylidene 5 and 6b. A
sample of the equilibrium mixture consisting of monobenzyl 2, dibenzyl
3, and tribenzyl 4 was dissolved in toluene-d8 and used as a stock
solution for kinetic study. The initial concentrations in this solution
were calculated from the integration of the 31P NMR signals of a
representative sample with respect to that of the internal standard PPh3.
The disappearance of 3 and 4 (and the concomitant appearance of 5)
as recorded by 31P{1H} NMR spectroscopy was monitored for a period
of at least 3 half-lives; samples with an internal PPh3 reference were
employed to calibrate the measurements. Separate kinetic runs were
conducted on the Varian 300-XL instrument at temperatures set to 70,
80, 85, and 90 °C. The uncertainty in the temperature settings was
estimated to be (1 °C. Similar experiments with the CD2C6D5 labeled
complexes were performed at 70 °C to examine kinetic isotope effects
(KIE) associated with the proposed mechanism. Each experiment was
repeated at least once to indicate reproducibility. For the duration of
the kinetic experiments, all sample tubes were protected from light
sources where possible by using metal foil wrap to minimize the
competitive photochemical transformation of 3 to 5.
A similar procedure was followed for the study into the formation
of the silyl alkylidene 6b, except the 31P{1H} NMR spectra were
conducted at 20 °C, as the resonances due to dialkyl 9b and trialkyl
8b merge together at higher temperatures. For a typical run, a 0.5 mL
sample of the toluene solution was placed in a 5 mm NMR tube, the
solution frozen and degassed, and the tube then flame sealed and placed
in a constant temperature oil bath (uncertainty in the temperature
measurements estimated to be 1 °C). The tube was removed from the
bath after a period of 1.5 h and immersed in an ice bath to quench the
reaction. A 31P{1H} NMR spectrum was recorded at 20 °C on the
Bruker-AMX 500 MHz instrument, and the tube returned to the oil
bath. This procedure was repeated with further 1.5 h heating intervals,
and the disappearance of 9b and 8b (and the concomitant appearance
of 6b) as recorded by 31P{1H} NMR spectroscopy was monitored for
a period of at least 3 half-lives. Kinetic experiments were conducted
at four different temperatures (80, 95, 110, and 125 °C) to obtain
separate reaction rates.
(20 °C, C6D6) δ 0.38 and 0.40 (s, 6H, Si(CH3)2), 0.80 (dd, 4H, 2JHH
)
6 Hz, 2JPH ) 7 Hz, SiCH2P), 1.10 and 1.30 (m, 12H, CH(CH3)2), 1.50
4
(s, 9H, CHC(CH3)3), 2.10 (m, 4H, CH(CH3)2), 6.60 (d, 2H, JHH ) 1
4
Hz, C5H), 7.41 (t, 1H, JHH ) 1 Hz, C5H), 8.20 (s, 1H, CHC(CH3)3).
31P NMR (20 °C, C6D6) δ 14.0 (s).
[P2Cp]ZrdCHSiMe3(Cl) (6b). A solution of LiCH2SiMe3 (155 mg,
1.64 mmol) in 30 mL of toluene was added dropwise with stirring to
a cooled (-78 °C) solution of 1 (525 mg, 0.82 mmol) dissolved in 90
mL of toluene. The color of the solution immediately changed from
pale yellow to bright yellowish green. The solution was stirred at -78
to 0 °C for another 30 min, and then for another 2 h at room
temperature. The solvent was then removed, and the residue extracted
with hexanes and filtered. The filtrate was reduced under vacuum to
give a greenish yellow oil that was redissolved in 60 mL of toluene.
This solution was placed in a 95 °C oil bath for 3 days, after which the
volume was reduced under vacuum to 1 mL and 5 mL of hexanes added.
Bright yellow crystals were slowly obtained from this concentrated
solution at -40 °C. The supernatant solution was filtered away, and
the crystals washed twice with cold hexanes and dried under vacuum.
More crystalline material could be obtained from the mother liquor.
1
Yield 440 mg, 82%. H NMR (20 °C, C6D6) δ 0.21 and 0.53 (s, 6H,
Si(CH3)2), 0.32 (s, 9H, CHSi(CH3)3), 0.68 and 0.72 (dd, 4H, 2JHH ) 6
2
Hz, JPH ) 7 Hz, SiCH2P), 1.04 and 1.48 (m, 12H, CH(CH3)2), 2.11
and 2.29 (m, 2H, CH(CH3)2), 6.07 (d, 2H, 4JHH ) 1 Hz, C5H), 7.30 (t,
1H, 4JHH ) 1 Hz, C5H), 8.99 (s, CHSi(CH3)3). 31P NMR (20 °C, C6D6)
δ 16.2 (s). 13C NMR (20 °C, C6D6) δ 5.9 and 11.1 (Si(CH3)2), 7.9
(CH(CH3)2), 16.5 (SiCH2P), 25.6 and 30.9 (CH(CH3)2), 26.6 (CHSi-

Formation of [η5-C5H3-1,3-(SiMe2CH2PPri2)2]ZrdCHR(Cl)
J. Am. Chem. Soc., Vol. 121, No. 11, 1999 2487
X-ray Data Collection and Reduction. X-ray quality crystals of
6b were obtained from the preparation as described above. A
representative crystal was mounted and sealed within a capillary to
maintain a dry, O2-free environment for the sample during the
crystallographic analysis. Diffraction experiments were performed at
23 °C on a Nonius CAD-4 diffractometer equipped with graphite-
monochromatized Mo KR (0.70930 Å) radiation. The final unit-cell
parameters were obtained by least-squares for 24 reflections with 2θ
) 30.00-43.00 degrees. The data were processed and corrected for
Lorentz and polarization effects, and absorption.
The structure was solved by conventional heavy atom methods, the
coordinates of the Zr, P, and Si atoms being determined from the
Patterson function and those of the remaining atoms from subsequent
difference Fourier syntheses. After anisotropic refinement of all non-
hydrogen atoms, methylene and ring sp2 hydrogen atoms were fixed
in calculated positions (dC-H ) 1.08 Å) with temperature factors based
upon the carbon to which they are bonded. Methyl hydrogen atoms
were located and fixed via inspection of a difference Fourier map,
temperature factors based upon the carbon to which they are bonded.
H19 was located via difference Fourier map inspection and refined.
All crystallographic calculations were conducted with the PC version
of the NRCVAX program package locally implemented on an IBM-
compatible 80486 computer.
Crystallographic data appear in Table 5.
Acknowledgment. Financial support for this research was
provided by NSERC of Canada in the form of a Research Grant
to M.D.F. and a postgraduate scholarship to P.B.D.
Supporting Information Available: Additional tables in-
clude crystallographic data and atomic positional and thermal
parameters for 6b (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
JA982969H