Carbon-hydrogen bond activation with a cyclometalated zirconocene hydride: Mechanistic differences between arene and alkane reductive elimination

Article abstract of DOI:10.1021/om0508379

Thermolysis of the cyclometalated zirconocene hydride (η⁵- C₅H₃-1,3-(SiMe₃)₂) (η⁵-C₅H₃-3-SiMe3-1-η¹- SiMe₂CH₂)ZrH, at 45°C in benzene-d₆ or toluene-d₈, resulted in H/D exchange arising from reversible C-H activation of sp²-hybridized bonds. More careful inspection of the isotopic exchange reactions by NMR spectroscopy revealed formation of zirconocene aryl hydride complexes in equilibrium with the cyclometalated hydride compound. In contrast, activation of (dimethylamino)pyridine (DMAP) was complete at 23°C and afforded the ortho-metalated (dimethylamino)pyridyl hydride. Performing the thermolysis of the cyclometalated zirconocene hydride in alkane solvents resulted in activation of a second [SiMe3] group on the opposite cyclopentadienyl ring, forming the C₂-symmetric double-cyclometalated complex (η⁵-C₅H ₃-1,3-(SiMe₃)₂)(η⁵-C ₅H₃-3-SiMe3-1-η¹-SiMe₂CH ₂)₂Zr. Isotopic labeling studies demonstrated that neopentane reductive elimination to yield the cyclometalated hydride proceeds through a cyclometalated assisted pathway, while the analogous extrusion of benzene occurs by direct C-H bond reductive coupling, possibly a consequence of the lower orbital reorganization energy associated with the sp2-hybridized carbon. Activation of DMAP follows a different course, where the nucleophilicity of the heterocycle induces reductive elimination and subsequent ortho metalation. In contrast, conversion of the cyclometalated zirconocene hydride to the double-cyclometalated derivative most likely occurs by a-bond metathesis, highlighting the various pathways available for carbon-hydrogen bond activation.

Full text of DOI:10.1021/om0508379

1092
Organometallics 2006, 25, 1092-1100
Carbon-Hydrogen Bond Activation with a Cyclometalated
Zirconocene Hydride: Mechanistic Differences between Arene and
Alkane Reductive Elimination
Wesley H. Bernskoetter, Jaime A. Pool, Emil Lobkovsky, and Paul J. Chirik*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell UniVersity,
Ithaca, New York 14853
ReceiVed September 29, 2005
Thermolysis of the cyclometalated zirconocene hydride (η5-C5H3-1,3-(SiMe3)2)(η5-C5H3-3-SiMe3-1-
η1-SiMe2CH2)ZrH, at 45 °C in benzene-d6 or toluene-d8, resulted in H/D exchange arising from reversible
C-H activation of sp2-hybridized bonds. More careful inspection of the isotopic exchange reactions by
NMR spectroscopy revealed formation of zirconocene aryl hydride complexes in equilibrium with the
cyclometalated hydride compound. In contrast, activation of (dimethylamino)pyridine (DMAP) was
complete at 23 °C and afforded the ortho-metalated (dimethylamino)pyridyl hydride. Performing the
thermolysis of the cyclometalated zirconocene hydride in alkane solvents resulted in activation of a second
[SiMe3] group on the opposite cyclopentadienyl ring, forming the C2-symmetric double-cyclometalated
complex (η5-C5H3-3-SiMe3-1-η1-SiMe2CH2)2Zr. Isotopic labeling studies demonstrated that neopentane
reductive elimination to yield the cyclometalated hydride proceeds through a cyclometalated assisted
pathway, while the analogous extrusion of benzene occurs by direct C-H bond reductive coupling, possibly
a consequence of the lower orbital reorganization energy associated with the sp2-hybridized carbon.
Activation of DMAP follows a different course, where the nucleophilicity of the heterocycle induces
reductive elimination and subsequent ortho metalation. In contrast, conversion of the cyclometalated
zirconocene hydride to the double-cyclometalated derivative most likely occurs by σ-bond metathesis,
highlighting the various pathways available for carbon-hydrogen bond activation.
and the isolated cyclometalated ion-pair complex [(η5-C5H2-
1,2,4-(SiMe3)3)(η5-C5H2-2,4-(SiMe3)2-1-η1-SiMe2CH2)Zr][MeB-
(C6F5)3] has been shown to be unreactive toward ethylene.9
Introduction
Cyclometalated zirconocene hydride complexes are of inter-
est, given their role in H/D exchange reactions involving
saturated C-H bonds of the cyclopentadienyl substituents1-3
and as potential conduits to low-valent zirconium species that
activate small molecules such as dinitrogen.4,5 Examples of
isolable or even observable cyclometalated zirconocene hydrides
are relatively rare and are typically limited to complexes con-
taining sterically demanding silyl substituents. Mach and co-
workers have reported and crystallographically characterized (η5-
C5Me4SiMe3)(η5-C5Me4-η1-SiMe2CH2)ZrH, isolated from mag-
nesium reduction of the corresponding zirconocene dichloride
precursor.6 Irreversible C-H activation of cyclopentadienyl
substituents in zirconocene-catalyzed olefin polymerization is
also believed to be an important catalyst deactivation pathway,7,8
Our laboratory has been studying the chemistry of cyclo-
metalated zirconocene hydride complexes with the goal of
using these molecules for the activation of dinitrogen, obviating
the need for alkali-metal reduction procedures and offering the
potential for catalytic N2 functionalization.10,11 Several ex-
amples of “mixed ring” cyclometalated hydrides have been
prepared from reductive elimination of alkane (Figure 1).4,12
Unfortunately, these complexes have shown no propensity to
coordinate N2, most likely a result of relatively electron do-
nating cyclopentadienyl substituents that raise the activation
barrier for C-H reductive elimination, prohibiting access to the
desired reduced zirconium species. Introduction of purely
silylated cyclopentadienyl ligands into the zirconium coor-
dination sphere produced more facile alkane reductive elimina-
tion reactions, as extrusion of isobutane from (η5-C5H3-1,3-
(SiMe3)2)2Zr(CH2CHMe2)H occurs over the course of minutes
at ambient temperature to furnish the side-on-bound dinitrogen
complex [(η5-C5H3-1,3-(SiMe3)2)2Zr]2(µ2,η2:η2-N2) (Figure 1).4
Only one diastereomer of the cyclometalated zirconocene
hydride (η5-C5H3-1,3-(SiMe3)2)(η5-C5H3-3-SiMe3-1-η1-SiMe2-
CH2)ZrH (1) was identified during the alkane reductive elimina-
tion reaction.
* To whom correspondence should be addressed. E-mail: pc92@
cornell.edu.
(1) McAlister, D. R.; Erwin, D. K.; Bercaw, J. E. J. Am. Chem. Soc.
1978, 100, 5966.
(2) Chirik, P. J.; Day, M. W.; Bercaw, J. E. Organometallics 1999, 18,
1873.
(3) Chirik, P. J.; Henling, L. M.; Bercaw, J. E. Organometallics 2001,
20, 534.
(4) Pool, J. A.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2003,
125, 2241.
(5) Pool, J. A.; Lobkovsky, E.; Chirik, P. J. Organometallics 2003, 23,
2797.
(6) Horacek, M.; Stepnicka, P.; Kubista, J.; Fehfarova, K.; Geypes, R.;
Mach, K. Organometallics 2003, 22, 861.
(7) (a) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994,
116, 10015. (b) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc.
1991, 113, 3623.
(8) Bertuleit, A.; Fritze, C.; Erker, G.; Fro¨lich, R. Organometallics 1997,
16, 2891.
(9) Choukroun, R.; Wolff, F.; Lorber, C.; Donnadieu, B. Organometallics
2003, 22, 2245.
(10) Pool, J. A.; Chirik, P. J. Can. J. Chem. 2005, 83, 286.
(11) Mori, M. J. Organomet. Chem. 2004, 689, 4210.
(12) Pool, J. A.; Bradley, C. A.; Chirik, P. J. Organometallics 2002, 21,
1271.
10.1021/om0508379 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 02/03/2006

C-H Bond ActiVation with a Zirconocene Hydride
Organometallics, Vol. 25, No. 5, 2006 1093
Figure 1. Cyclopentadienyl substituent effects on the reactivity of cyclometalated zirconocene hydride complexes.
On the basis of this enhanced reactivity, we became interested
in developing a reliable, preparative-scale synthetic route to 1
and exploring its reactivity with traditionally robust substrates
such as N2 and alkanes. In this contribution, we report our
progress toward this objective with the isolation and crystal-
lographic characterization of 1, along with a series of isotopic
exchange and C-H activation reactions with deuterated arenes.
During the course of these investigations, we have discovered
that arene and alkane reductive elimination proceeds by two
distinct mechanistic pathways and that the cyclometalated
zirconocene hydride activates C-H bonds by a variety of
different mechanisms, depending on the substrate.
Figure 2. (a) Molecular structure of 1 shown with 30% probability
ellipsoids. (b) Top view of the molecule. Hydrogen atoms, except
for the zirconium hydride, are omitted for clarity.
Results and Discussion
Synthesis and Characterization of (η5-C5H3-1,3-(SiMe3)2)-
(η5-C5H3-3-SiMe3-1-η1-SiMe2CH2)ZrH (1). Previously we
have reported that treatment of the zirconocene dichloride (η5-
C5H3-1,3-(SiMe3)2)2ZrCl2 with 2 equiv of tert-butyllithium
furnished the corresponding isobutyl hydride complex (η5-C5H3-
1,3-(SiMe3)2)2Zr(CH2CHMe2)H, which undergoes alkane reduc-
tive elimination to afford 1.4 Since our initial report, we have
found an improved route to 1 by reductive elimination of
neopentane from (η5-C5H3-1,3-(SiMe3)2)2Zr(CH2CMe3)H (eq 1).
on the carbon bound to the zirconium. The zirconium hydride
resonance appears at 5.62 ppm and has been confirmed by
isotopic labeling (vide infra). We were previously unable to
assign the stereochemistry of 1, due to its propensity to undergo
further chemistry (vide infra). Subsequently, we have discovered
that cyclohexane-d12 solutions of 1 prepared under argon or
vacuum are stable for extended periods.
Definitive assignment of the preferred diastereomer of 1 was
accomplished by single-crystal X-ray diffraction. The compound
crystallizes with two independent, enantiomeric molecules in
the unit cell, one of which is presented in Figure 2. In the solid
state, the two cyclopentadienyl rings are oriented in a near-
gauche arrangement to avoid transannular interactions of the
sterically demanding [SiMe3] substituents. The intact [SiMe3]
group on the cyclopentadienyl ring containing the cyclometa-
lated substituent is oriented in a lateral position with respect to
the metallocene wedge, a consequence of [SiMe2CH2] coordina-
tion in the equatorial plane. This arrangement gears the [SiMe3]
on the opposite ring, producing the gauche conformation. The
Zr(1′)-C(11′) bond length of 2.271(5) Å is in the range typically
observed for zirconocene alkyls.13 The observed diastereomer
most likely arises from C-H activation of a preferred rotamer
that minimizes transannular steric interactions. Previous studies
on related mixed-ring derivatives allowed observation of the
less favored diastereomer upon thermolysis, suggesting that
reversible C-H activation proceeds with a lower barrier than
rotamer interconversion.4 While a similar energetic profile is
The zirconocene alkyl hydride was readily prepared in a
straightforward manner by alkylation of (η5-C5H3-1,3-(SiMe3)2)2-
Zr(H)Cl12 with LiCH2CMe3. This synthetic route is preferred,
as the larger neopentyl group undergoes more facile reductive
elimination than does the corresponding isobutyl complex,
owing to ground-state destabilization imparted by the larger alkyl
ligand.5 The isobutyl hydride route is also complicated by the
need for carefully titrated tert-butyllithium solutions, as excess
alkyllithium formed unidentified side products. Use of solid,
recrystallized LiCH2CMe3 obviates these complications and,
following alkane reductive elimination, reproducibly yielded
pure 1.
As we have described previously,4 1H and 13C NMR spectra
of benzene-d6 solutions of 1 exhibit the number of peaks
consistent with formation of a single diastereomeric cyclom-
etalated zirconocene hydride. Diagnostic doublets were observed
at -4.44 and 0.62 ppm, assigned to the methylene hydrogens
(13) For representative examples see: (a) Harlan, C. J.; Bott, S. G.;
Barron, A. R. J. Chem. Soc., Dalton Trans. 1997, 637. (b) Crowther, D. J.;
Borkowsky, S. L.; Swenson, D.; Mayer, T. Y.; Jordan, R. F. Organometallics
1993, 12, 2897. (c) Jordan, G. T.; Liu, F. C.; Shore, S. G. Inorg. Chem.
1997, 36, 5597. (d) Liu, F. C.; Liu, J.; Meyes, E. A.; Shore, S. G. Inorg.
Chem. 1998, 37, 3293.

1094 Organometallics, Vol. 25, No. 5, 2006
Bernskoetter et al.
likely in the present case, the inability to observe other
diastereomers does not exclude a Curtin-Hammett argument
as the origin of the selectivity.
hydride. On the basis of 1H NMR spectroscopy and degradation
experiments, the three new organometallic products have been
identified as isomers of the zirconocene tolyl-d7 deuteride
complex (η5-C5H3-1,3-(SiMe3)2)2Zr(C7D7)D (3-d8).
Carbon-Hydrogen Bond Activation with 1. Gently warm-
ing a benzene-d6 solution of 1 to 45 °C over the course of 2
To identify each isomer formed and determine their relative
amounts, the protio isotopologue (η5-C5H3-1,3-(SiMe3)2)2Zr-
(C7H7)H (3) was prepared from thermolysis of 1 in toluene and
the products treated with D2 gas. This procedure liberated free
isoptopically labeled arene and formed the zirconocene dideu-
teride [(η5-C5H3-1,3-(SiMe3)2)2ZrD2]n (6-d2) (vide infra). Analy-
1
days and monitoring the reaction by H NMR spectroscopy
resulted in gradual disappearance of the resonances for the
zirconium hydride, the cyclometalated methylene protons, and
the [SiMe3] groups. Removing the deuterated solvent and
2
analyzing the resulting zirconocene by H NMR spectroscopy
1
in benzene confirmed deuterium incorporation into these posi-
tions, arising from H/D exchange with the NMR solvent. Careful
monitoring of the isotopic exchange reaction as a function of
time at 45 °C allowed observation of a new Cs-symmetric
organometallic product, identified as the zirconocene phenyl-
d5 deuteride complex (η5-C5H3-1,3-(SiMe3)2)2Zr(C6D5)D (2-d6)
(eq 2). Continued thermolysis of 1 in benzene-d6 at 45 °C for
sis of the toluene-d isotopomers by a combination of H and
2H NMR spectroscopy established isotopic incorporation into
only the meta and para positions of the free arene. Control
experiments involving addition of D2 gas to 6-d2 in C7H8 did
not affect H-D exchange over the time scale of the deuteriolysis
experiment. A similar degradation procedure was performed
with DCl and yielded the zirconocene dichloride (η5-C5H3-1,3-
(SiMe3)2)2ZrCl2 and the same isotopomers of toluene-d. In both
degradation experiments, no evidence for benzylic or ortho
activation of the arene was obtained. The lack of ortho activation
is most likely a consequence of the steric inaccessibility of the
C-H bond adjacent to the methyl group.15 Correlating the
amounts of free toluene-d with the ratios of the isomers of 3
observed by 1H NMR spectroscopy allows tentative assignment
of the major isomer as the para-activated product formed in a
7:5 ratio to the exo-meta- and endo-meta-activated products (eq
4). A 3:1 ratio of the meta products was observed; however,
5 days resulted in a constant 2:1 ratio of 2-d6 to 1-dn, indicating
the two species are in equilibrium and C-H bond activation is
reversible. Synthesis of a zirconocene phenyl hydride com-
plex by C-H activation of benzene has been described
previously, as thermolysis of the ansa-zirconocene dimethyl
complex Me2Si(η5-C5Me4)2ZrMe2 under an atmosphere of H2
at 80 °C affords Me2Si(η5-C5Me4)2Zr(C6H5)H.14
Because conversion to 2-d6 was incomplete from the ther-
molysis of 1 and did not exhibit its full complement of
resonances due to partial deuteration, an independent synthesis
of the protio isotopologue 2 was performed. Addition of 1 equiv
of phenylithium to (η5-C5H3-1,3-(SiMe3)2)2Zr(H)Cl in pentane
followed by filtration and solvent removal furnished a yellow
oil identified as (η5-C5H3-1,3-(SiMe3)2)2Zr(C6H5)H (2) on the
basis of 1H and 13C NMR spectroscopy (eq 3). Notable features
overlapping resonances did not allow confident assignment of
the exo and endo isomers. Observation of distinct exo- and endo-
meta isomers indicates restricted rotation about the zirconium-
aryl bond.
Carbon-hydrogen activation was also explored with hetero-
cycles. Addition of 1 equiv of (dimethylamino)pyridine (DMAP)
to 1 resulted in ortho metalation over the course of 8 h at 23
°C to afford a single isomer of 4 (eq 5). Identification of the
of the 1H NMR spectrum of 2 are the observation of two
[SiMe3] environments, three cyclopentadienyl hydrogens, and
a Zr-H resonance centered at 5.49 ppm. Importantly, the
chemical shifts of the cyclopentadienyl and [SiMe3] resonances
confirm the formation of 2-d6 from thermolysis of 1 in ben-
zene-d6.
Similar isotopic exchange experiments were conducted with
1 and toluene-d8. Thermolysis of 1 at 45 °C resulted in levels
of deuterium incorporation similar to those observed with
benzene-d6 after 3 days. Examination of the 1H NMR spectrum
of the thermolysis reaction indicated the formation of three new
zirconocene products accounting for approximately 45% of the
organometallic species in solution. The remaining zirconium
was 1-dn, again establishing an equilibrium between the
cyclometalated zirconocene hydride and the corresponding aryl
regiochemistry of the C-H-activated product was accomplished
by treatment of 4 with deuterium chloride, yielding (η5-C5H3-
1,3-(SiMe3)2)2ZrCl2 and DMAP-d with deuterium exclusively
in the ortho position of the arene. Unfortunately, definitive
assignment of the stereochemistry, namely exo versus endo
selectivity, was not accomplished, due to poor resolution of the
coordinated DMAP resonances that are key for structural
elucidation by two-dimensional NMR experiments. For simplic-
ity, the exo isomer (relative to the pyridine nitrogen) is depicted
(14) Lee, H.; Desrosiers, P. J.; Guzei, I.; Rheingold, A. L.; Parkin, G. J.
Am. Chem. Soc. 1998, 120, 3255.
(15) Jones, W. D. Acc. Chem. Res. 2003, 36, 140.

C-H Bond ActiVation with a Zirconocene Hydride
Organometallics, Vol. 25, No. 5, 2006 1095
Synthesis and Characterization of [(η5-C5H3-1,3-
(SiMe3)2)2ZrH2]n. During the process of identifying the product
ratio arising from toluene activation, D2 gas was added to the
mixture of 1 and the isomers of 3, liberating toluene-d.
Concomitant with this process is the formation of a white
powder identified as the zirconocene dideuteride [(η5-C5H3-1,3-
(SiMe3)2)2ZrD2]n (6-d2). This compound is sparingly soluble in
aromatic hydrocarbons and is essentially insoluble in pentane.
Moreover, the protio isotopologue 6 can be prepared from H2
1
addition to pure 1, 2, or 3 (eq 7). The H NMR spectrum of 6
Figure 3. (a) Molecular structure of 5 shown with 30% probability
ellipsoids. (b) Top view of the molecule. Hydrogen atoms are
omitted for clarity.
in eq 5 on the basis of literature precedent with cationic zir-
conocene complexes16 and a related neutral bis(indenyl)zir-
conium derivative, which has been structurally characterized.17
Thermolysis in Alkane Solvents. The facile isotopic ex-
change observed with 1 and deuterated arenes prompted further
exploration of the scope of the C-H bond activation reaction.
Of particular interest was the reactivity of 1 with saturated
alkanes. Thermolysis of 1 in cyclohexane-d12 at 85 °C for several
days did not result in isotopic exchange into the zirconium
hydride, the methylene hydrogens, or the [SiMe3] substituents,
suggesting that productive, reversible oxidative addition of the
sp3-hybridized C-D bonds of the solvent was not operative.
Performing the thermolysis on a preparative scale for 1 week
in heptane solution afforded a yellow solid identified as one
isomer of the double-cyclometalated zirconocene (η5-C5H3-3-
SiMe3-1-η1-SiMe2CH2)2Zr (5) (eq 6).
recorded in benzene-d6 exhibits the number of peaks for two
compounds. The major resonances correspond to a molecule
with a Cs-symmetric ligand environment that also exhibits
distinct peaks for bridging and terminal zirconium hydrides
centered at -3.57 and 3.41 ppm, respectively.2 Additional
signals for a minor C2V-symmetric compound were also ob-
served, tentatively assigned as the ziroconocene dihydride
monomer. Quantitation of the amount of monomer versus dimer
in solution could not be reliably accomplished, due to the
partial solubility of 6 in benzene-d6.
Formation of relatively robust hydride bridges in 6 are in
contrast with the monomeric, tert-butyl-substituted congener (η5-
C5H3-1,3-(CMe3)2)2ZrH2.2 The difference in position of the
monomer-dimer equilibrium between the two zirconocene
dihydrides is most likely a result of cooperative steric and
electronic effects. The metal center in 6 is less hindered
compared to that in (η5-C5H3-1,3-(CMe3)2)2ZrH2, owing to
longer C-Si bonds relative to C-C bonds with the purely alkyl
substituents. Likewise, studies from our laboratory4 and others18
have shown the silyl-substituted cyclopentadienyl ligands impart
more electrophilic metal centers compared to their alkylated
counterparts. Taken together, these effects support the formation
of significant quantities of coordinatively saturated zirconocene
dihydride dimers.
The 1H NMR spectrum of 5 in cyclohexane-d12 or benzene-
d6 exhibits the number of resonances consistent with a C2-
symmetric compound, indicating that the second C-H activation
occurs on the cyclopentadienyl ring opposite the first. Two
diagnostic doublets with coupling constants of 14 Hz were
observed upfield of SiMe4 at -2.37 and -1.40 ppm for the
diastereotopic methylene hydrogens on the carbon bound to
zirconium. A single zirconium methylene resonance was also
observed at 16.9 ppm by 13C NMR spectroscopy. Confirmation
of the assignment of the observed isomer was provided by X-ray
diffraction. The solid-state structure of 5 (Figure 3) contains
nearly idealized C2 molecular symmetry and confirms cyclo-
metalation of the [SiMe3] substituents from the opposite
cyclopentadienyl ring. The Zr(1)-C(16) and Zr(1)-C(17) bond
distances are statistically invariant at 2.320(2) and 2.316(2) Å
and are slightly elongated from the lengths typically encountered
for zirconocene alkyls.13 Similar observations have been made
by Mach and co-workers, where thermolysis of a cyclometalated
zirconocene hydride yielded a double-cyclometalated zir-
conocene where activation of the second silyl group occurred
on the ring opposite of the first.6
Both 1 and 6 are extremely sensitive molecules. The presence
of even trace amounts of water with either compound cleanly
afforded the zirconocene µ-hydride complex [(η5-C5H3-1,3-
(SiMe3)2)2ZrH]2(µ2-O) (7) (eq 8). Colorless blocks suitable for
X-ray diffraction were isolated from cooling a concentrated
pentane solution to -35 °C. The solid-state structure is presented
in Figure 4, and the data were of sufficient quality such that all
of the hydrogens in the molecule were located and refined. Two
data sets were collected on two independently prepared samples
and produced identical results. An idealized C2 molecular
(16) Jordan, R. F.; Taylor, D. F.; Baenziger, N. C. Organometallics 1990,
9, 1546.
(17) Bradley, C. A.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2003,
125, 8110.
(18) Zachmanoglou, C. E.; Docrat, A.; Bridgewater, B. M.; Parkin, G.
E.; Brandow, C. G.; Bercaw, J. E.; Jardine, C. N.; Lyall, M.; Green, J. C.;
Kiester, J. B. J. Am. Chem. Soc. 2002, 124, 9525.

1096 Organometallics, Vol. 25, No. 5, 2006
Bernskoetter et al.
temperature, and deuterium incorporation was observed in the
bridging and terminal deuteride positions as well as in the
[SiMe3] groups of the cyclopentadienyl ligands (eq 9). These
results do not distinguish between direct deuteriolysis of the
Zr-CH2 linkage or reductive elimination of the cyclometalated
zirconocene hydride to form a transient sandwich compound
that undergoes D2 oxidative addition followed by reversible
cyclometalation. By way of comparison, the double-cyclom-
etalated zirconocene 5 is resistant toward hydrogenation (deu-
teration), requiring heating to 65 °C for 2 days for conversion
to 6.
Isolation of the zirconocene phenyl hydride complex 2
allowed the study of isotopic exchange in aryl hydride com-
plexes. Thermolysis of 2 at 45 °C over the course of 8 h in
benzene-d6 resulted in disappearance of the phenyl resonances
between 6.98 and 7.19 ppm along with the zirconium hydride
centered at 5.49 ppm. A combination of 1H and 2H NMR
spectroscopy revealed clean conversion to 2-d6 before any 1
could be detected (eq 10). Significantly, the disappearance of
Figure 4. Molecular structure of 7 shown with 30% probability
ellipsoids. Hydrogen atoms, except for the zirconium hydride, are
omitted for clarity. The oxygen atom and the zirconium hydrides
are disordered over two positions; only one is shown.
geometry with a transoid orientation of the zirconium hydrides
is observed. The structure possesses half occupancy of both the
oxygen and hydride atoms. The Zr-H and Zr-O distances of
1.88(5) and 1.931(3) Å, respectively, are in good agreement
with those previously reported for zirconium hydride and oxo
compounds.19 The bent Zr-O-Zr bond (bond angle 153.05-
(17)°) is unusual, as typically linear linkages of this type are
observed. Examples of bent Zr-O-Zr bonds have been reported
in several different µ-oxo zirconocenes.20
Another notable feature of the solid-state structure is the
coplanarity of the two zirconocene fragments. An orthogonal
orientation of the two metallocene cores would be expected if
Zr-O π-bonding was dominant. However, previous work from
our laboratory with zirconocenes4 and from Evans with lan-
thanides21 has established that this specific (bis)cyclopentadienyl
prefers coplanarity in the solid state of dimeric structures. The
solution NMR data collected on the same batch of crystals used
for the diffraction experiment were also consistent with a dimeric
structure with coplanar zirconocene fragments, as Cs rather than
C1 symmetry was observed.
the phenyl and zirconium hydride resonances occurred at the
same rate and only trace amounts of deuterium incorporation
into the [SiMe3] groups were observed during the transforma-
tion. At longer reaction times, the equilibrium between isoto-
pologues of 2-dn and 1-dn was established and additional isotopic
exchange into the [SiMe3] groups was observed.
Formation of the isotopologue of 2-d6 with an equal propor-
tion of Zr-D and Zr-Ph-d5 was unexpected, on the basis of
the previously proposed mechanism for alkane reductive
elimination from zirconocene alkyl hydride complexes.1,4,5 An
example of this pathway is presented in Figure 5 and proceeds
through initial metal-to-ring hydrogen transfer followed by
oxidative addition of a C-H bond. This could either be
intramolecular, arising from a cyclopentadienyl substituent, or
intermolecular, derived from addition of an external C-H bond
from arene solvent. Reductive elimination of alkane then occurs
followed by hydrogen migration to yield the observed product.
Regardless of whether the C-H activation event is intra- or
intermolecular, the starting zirconium hydride (the deuteride is
illustrated in Figure 5 for clarity) remains in the complex and
is not transferred to the alkane. Thus, if this mechanism were
operative for the isotopic exchange with 2 and benzene-d6, the
expected product is not 2-d6 but rather its isotopologue, (η5-
C5H3-1,3-(SiMe3)2)2Zr(C6D5)H (2-d5).
Isotopic Labeling Studies and Mechanistic Considerations.
A series of isotopic labeling experiments were conducted in
order to gain insight into the mechanism of H/D exchange and
arene C-H bond activation. Because it is well-established that
zirconocene dihydrides promote H/D exchange,1,2 isotopic
substitution during the preparation of 6 from 1 was initially
studied. One atmosphere of D2 gas was added to a thawing
2
benzene solution of 1 and the H NMR spectrum immediately
recorded. Formation of 6-dn was instantaneous at ambient
(19) Crystallographically characterized hydrides: (a) Highcock, W. J.;
Mills, R. M.; Spencer, J. L.; Woodward, P. J. Chem. Soc., Dalton Trans.
1986, 821. (b) Stones, S. B.; Petersen, J. L. Inorg. Chem. 1981, 20, 2889.
(c) Kraft, B. M.; Lachicotte, R. J.; Jones, W. D. J. Am. Chem. Soc. 2001,
123, 10973. Zirconium oxo: (d) Reid, A. F.; Shannon, J. S.; Swan, J. M.;
Wailes, P. C. Aust. J. Chem. 1965, 18, 173. (e) Clarke, J. F.; Drew, M. G.
B. Acta Crystallogr., Sect. B 1974, B30, 2267. (f) Petersen, J. L. J.
Organomet. Chem. 1979, 166, 179.
(20) For representative examples see: (a) Clarke, J. F.; Drew, M. G. B.
Acta Crystallogr., Sect. B 1974, 30, 2267. (b) Petersen, J. L. J. Organomet.
Chem. 1979, 166, 179. (c) Koyama, M.; Kimura, K.; Nakano, M.; Yamazaki,
H. Chem. Lett. 1998, 1139. (d) Green, J. C.; Green, M. L. H.; Taylor, G.
C.; Saunders: J. Dalton Trans. 2000, 317.
To further probe if cyclometalated-assisted reductive elimina-
tion was operative with this specific zirconocene aryl hydride
complex, the zirconocene phenyl deuteride (η5-C5H3-1,3-
(SiMe3)2)2Zr(C6H5)D (2-d) was prepared from (η5-C5H3-1,3-
(SiMe3)2)2Zr(D)Cl and phenyllithium. Thermolysis of a pentane
solution of 2-d at 45 °C for 10 h and monitoring of the reaction
(21) Evans, W. J.; Allen, N. T.; Ziller, J. W. Angew. Chem., Int. Ed.
2002, 41, 359.

C-H Bond ActiVation with a Zirconocene Hydride
Organometallics, Vol. 25, No. 5, 2006 1097
Figure 5. Cyclometalated-assisted pathway for alkane reductive elimination from zirconocene alkyl hydride complexes.
2
1
by H and H NMR spectroscopy revealed formation of ben-
bond reductive coupling,15,23 the more directional orbital raises
the barrier for direct reductive elimination and the cyclometa-
lated-assisted pathway becomes competitive.
zene-d along with 1 (eq 11). Importantly, no deuterium incor-
While direct C-H bond formation was observed for arene
reductive coupling, this mechanism does not account for the
experimentally observed isotopic exchange with 1. A competing
pathway must account for deuterium incorporation into the
zirconium hydride, methylene, and [SiMe3] groups. Two alter-
native mechanisms seem plausible. The first (Figure 6, path A)
is a σ-bond metathesis pathway,24 involving exchange of an
arene C-D bond with the zirconium methylene. Subsequent
benzene-d5 reductive elimination followed by reversible oxida-
tive addition and reductive elimination sequences involving
isotopomers and isotopologs of 1-dn would account for the
experimental observation of deuterium in the Zr-H, Zr-CH2,
and [SiMe3] positions. The major limitation of this mechanism
is that it contradicts established σ-bond metathesis preferences,
whereby metal hydrides are known to undergo metathesis more
rapidly than more directional sp3-hybridized metal-alkyl bonds.24
An alternative σ-bond metathesis mechanism with the zirconium
hydride is also possible, liberating HD gas, which then hydrog-
enates the zirconium-aryl or -alkyl bond (Figure 6). Only
productive pathways to account for deuterium incorporation into
the cyclopentadienyl ligand are illustrated in Figure 6.
poration was observed in 1, clearly demonstrating direct benzene
reductive elimination from 2-d.
Because the results of benzene reductive elimination from
2-d contrast with the cyclometalated-assisted pathway pre-
viously reported for isobutane reductive elimination from both
(η5-C5Me5)2Zr(CH2CHMe2)H1 and (η5-C5Me5)(η5-C5H3-1,3-
(SiMe3)2)Zr(CH2CHMe2)H,4 the mechanism of neopentane
extrusion from (η5-C5H3-1,3-(SiMe3)2)2Zr(CH2CMe3)D was
examined. Alkylation of (η5-C5H3-1,3-(SiMe3)2)2Zr(D)Cl with
LiCH2CMe3 at 23 °C furnished (η5-C5H3-1,3-(SiMe3)2)2Zr(CH2-
CMe3)D. Allowing the zirconocene neopentyl deuteride to stand
at ambient temperature for 3 h resulted in clean conversion to
1-d with loss of neopentane (eq 12). Examination of the products
The second possibility is a reversible oxidative addition-
reductive elimination sequence (Figure 6, path B). For observ-
able H-D exchange to occur, this pathway must occur by a
cyclometalated-assisted mechanism, allowing deuterium from
the free arene to incorporate into the zirconocene product. While
direct C-H reductive coupling for 2-d and 2 occurs with a lower
barrier than the cyclometalated-assisted alternative, the observa-
tion of deuterium scrambling after isotopic exchange of 2 with
benzene-d6 suggests that a higher barrier cyclometalated-assisted
pathway may indeed be accessible at longer reaction times.
The conversion of 1 to the double-cyclometalated zirconocene
5 provided some insight into the barrier for σ-bond metathesis.
Recall that this reaction takes place over the course of 1 week
at 85 °C, in comparison to the isotopic exchange reactions,
by 2H and 1H NMR spectroscopy revealed formation of
isotopomers of 1-d1 with deuterium in the zirconium hydride,
zirconium methylene, and the [SiMe3] substituents. The poor
2
resolution of H NMR spectroscopy did not allow definitive
assignment of the [SiMe3] groups that were deuterated and did
not show if other diastereomers of 1 were accessed during the
reductive elimination reaction. Significantly, no deuterium
incorporation into the free alkane was observed, clearly sup-
porting cyclometalated-assisted reductive elimination for neo-
pentane formation.
One question raised by these results is as follows: why does
the mechanism of reductive elimination change depending on
whether alkyl or aryl substituents are coordinated in the
zirconocene wedge? The lower orbital reorganizational energy
associated with C-H bond formation from an sp2 aryl ligand
most likely makes the direct reductive elimination pathway
energetically accessible.22 In the case of sp3 alkyl group C-H
(22) (a) Brown, M. P.; Puddephatt, R. J.; Upton, C. E. E.; Lavington, S.
W. J. Chem. Soc., Dalton Trans. 1974, 1613. (b) Bullock, R. M.; Headford,
C. E. L.; Kegley, S. E.; Norton, J. R. J. Am. Chem. Soc. 1985, 107, 727.
(c) Low, J. J.; Goddard, W. A., III. J. Am. Chem. Soc. 1984, 106, 6928. (d)
Low, J. J.; Goddard, W. A., III. J. Am. Chem. Soc. 1984, 106, 8321. (e)
Obara, S.; Kitaura, K.; Morokuma, K. J. Am. Chem. Soc. 1984, 106, 7482.
(23) (a) Churchill, D. G.; Janak, K. E.; Wittenberg, J. S.; Parkin, G. J.
Am. Chem. Soc. 2003, 125, 1403. (b) Janak K. E.; Churchill, D. G.; Parkin,
G. Chem. Commun. 2003, 22.
(24) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan,
M. C.; Santarsiero, B. D.; Schaefer, W. D.; Bercaw, J. E. J. Am. Chem.
Soc. 1987, 109, 203.

1098 Organometallics, Vol. 25, No. 5, 2006
Bernskoetter et al.
Figure 6. Possible mechanisms for isotopic exchange with 1 and benzene-d6.
which occur in hours at 45 °C. In addition, hydrogenation of 5
to yield 1 and ultimately 6 is slow, requiring 2 days at 65 °C.
To further probe the possibility of σ-bond metathesis, the
zirconocene phenyl hydride complex 2 was exposed to deute-
rium gas at 23 °C. The deuteration to 6-dn was slow, requiring
2 days to reach completion. Significantly, no deteurium was
incorporated into to either the Zr-H or Zr-Ph positions prior
to conversion to the zirconocene deuteride product. While not
conclusive evidence, taken together, these results suggest that
σ-bond metathesis pathways with these molecules occur with
relatively high barriers, supporting cyclometalated-assisted
reductive elimination-oxidative addition sequences as the
mechanism for H-D exchange.
plausible is a ligand-induced reductive elimination pathway,
initially observed in zirconocene chemistry by Schwartz and
co-workers,26 whereby the DMAP nitrogen coordinates to 1 and
induces direct C-H bond reductive coupling and dissociation.27
Concluding Remarks
A reliable synthetic route to a crystallographically character-
ized, yet reactive, cyclometalated zirconocene hydride has been
devised. Isotopic exchange with arene solvents such as benzene-
d6 and toluene-d8 occurred over the course of hours at 45 °C
and proceeds through observable zirconocene aryl hydride
(deuteride) complexes. More careful examination of the mech-
anism of reductive elimination revealed that reductive coupling
with zirconocene aryl substituents occurred through a direct
C-H bond-forming process, while the analogous reaction with
zirconocene alkyls proceeds through a cyclometalated-assisted
pathway. The lower orbital reorganization energy in the former
case is believed to account for the observed mechanistic
preferences. However, it is postulated that a cyclometalated-
assisted pathway competes with the direct process and is
responsible for isotopic exchange. Carbon-hydrogen bond
activation of more nucleophilic heterocyclic substrates such as
(dimethylamino)pyridine occurs through a ligand-induced re-
ductive elimination pathway. Conversion of 1 to 5 most likely
occurs by σ-bond metathesis, highlighting the variety of
mechanistic pathways available to the cyclometalated zir-
conocene hydride for C-H bond activation.
The mechanism of ortho C-H activation of DMAP was also
explored. Addition of 1 equiv of 4-(dimethylamino)pyridine-
2,6-d2 (DMAP-d2)25 to 1 furnished 4-d2 with deuterium located
only in the ortho pyridyl position and the zirconium deuteride
(eq 13). As with the isotopic exchange between 2 and benzene-
d6, these results are inconsistent with a cyclometalated-assisted
pathway. In addition, no deuterium incorporation was observed
in the [SiMe3] substituents, ruling out a σ-bond metathesis path-
way involving the arene and zirconium methylene. While the
alternate σ-bond metathesis mechanism involving reaction of
Zr-H with the heterocycle liberating H-D cannot be defini-
tively excluded, we believe this process is unlikely, as a com-
pletely isotopically selective capture of the intermediate zir-
conocene aryl cyclometalate with H-D must be invoked. More
(25) Rubottom, G. M.; Evain, E. J. Tetrahedron 1990, 46, 5055.
(26) (a) Gell, K. I.; Schwartz, J. J. Am. Chem. Soc. 1978, 100, 3246. (b)
Gell, K. I.; Schwartz, J. J. Organomet. Chem. 1978, 162, C11. (c) Yoshifuji,
M.; Gell, K. I.; Schwartz, J. J. Organomet. Chem. 1978, 153, C15.
(27) We are unable to distinguish between this pathway and one involving
reversible reductive elimination and oxidative addition of the cyclometalated
hydride, forming a transient bis(cyclopentadienyl)zirconium sandwich which
then oxidatively adds the sp2-hybridized ortho C-H bond.

C-H Bond ActiVation with a Zirconocene Hydride
Organometallics, Vol. 25, No. 5, 2006 1099
temperature and stirred overnight under vacuum. Following filtra-
tion, the solvent was removed in vacuo to yield 0.255 g (89%) of
a yellow oil identified as 1. Analytically pure material can be
obtained in lower yields by recrystallization of concentrated pentane
solutions at -35 °C. Anal. Calcd for C22H42ZrSi4: C, 51.80; H,
Experimental Section
General Considerations. All air- and moisture-sensitive ma-
nipulations were carried out using standard high-vacuum-line,
Schlenk, or cannula techniques or in an M. Braun inert-atmosphere
drybox containing an atmosphere of purified nitrogen. The M. Braun
drybox was equipped with a cold well designed for freezing samples
in liquid nitrogen. Solvents for air- and moisture-sensitive manipu-
lations were dried and deoxygenated using literature procedures.28
Toluene, benzene, pentane, and heptane were further dried by
distillation from “titanocene”.29 Deuterated solvents for NMR
spectroscopy were distilled from sodium metal under an atmosphere
of argon and stored over 4 Å molecular sieves. Benzene-d6 and
toluene-d8 were further dried by storage and distillation from
“titanocene” immediately before use. Argon and hydrogen gas were
purchased from Airgas Inc. and passed through a column containing
manganese oxide on vermiculite and 4 Å molecular sieves before
admission to the high-vacuum line. 1H and 13C NMR spectra were
recorded on a Varian Inova 400 spectrometer operating at 399.860
MHz (1H) and 100.511 MHz (13C). All chemical shifts are reported
1
8.30. Found: C, 51.66; H, 7.94. H NMR (benzene-d6): δ -4.44
(d, 13 Hz, 1H, CH2SiMe3), 0.62 (d, 13 Hz, 1H, CH2SiMe3), 0.18
(s, 9H, SiMe3), 0.22 (s, 9H, SiMe3), 0.32 (s, 9H, SiMe3), 0.37 (s,
3H, SiMe2), 0.44 (s, 3H, SiMe2), 5.34 (m, 1H, Cp), 5.60 (m, 1H,
Cp), 6.16 (m, 1H, Cp), 6.21 (m, 1H, Cp), 6.98 (m, 1H, Cp), 7.49
(m, 1H, Cp), 5.62 (s, 1H, Zr-H). 13C{1H} NMR (cyclohexane-
d12): δ -0.067, 0.37, 0.40, 0.83, 1.53 (SiMe), 36.11 (ZrCH2SiMe2),
112.74, 113.39, 114.68, 115.99, 117.02, 122.03, 122.13, 126.02,
126.87, 130.21 (Cp).
Preparation of (η5-C5H3-1,3-(SiMe3)2)2Zr(C6H5)H (2). A 20
mL scintillation vial was charged with 0.120 g (0.220 mmol) of
(η5-C5H3-1,3-(SiMe3)2)2Zr(H)Cl, 122 µL of 1.8 M PhLi (0.220
mmol) in cyclohexane/diethyl ether, and approximately 10 mL of
pentane. The resulting solution was stirred for 16 h, turning pale
yellow over time. The mixture was filtered through Celite and the
solvent removed in vacuo to yield 0.105 g (81%) of a yellow oil
1
relative to SiMe4 using H (residual) or 13C NMR chemical shifts
of the solvent as a secondary standard. (η5-C5H3-1,3-(SiMe3)2)2-
Zr(H)Cl,4 LiCH2CMe330 and DMAP-d225 were prepared as described
previously. The zirconocene deuterio chloride (η5-C5H3-1,3-
(SiMe3)2)2Zr(D)Cl was prepared in a manner analogous to that for
the hydride isotopomer using KBEt3D.31
1
identified as 2. H NMR (benzene-d6): δ 0.09 (s, 18H, SiMe3),
0.23 (s, 18H, SiMe3), 5.49 (s, 1H, Zr-H), 6.00 (m, 2H, Cp), 6.41
(m, 1H, Cp), 6.98-7.19 (Ph). 13C{1H} NMR (benzene-d6): δ 0.35,
0.61 (SiMe3), 113.20, 114.12, 122.82, 123.28, 125.25, 125.99,
127.16, 127.68, 128.20 (Cp/Ph).
Single crystals suitable for X-ray diffraction were coated with
polyisobutylene oil in a drybox and were quickly transferred to
the goniometer head of a Siemens SMART CCD area detector
system equipped with a molybdenum X-ray tube (λ ) 0.710 73
Å). Preliminary data revealed the crystal system. A hemisphere
routine was used for data collection and determination of lattice
constants. The space group was identified, and the data were
processed using the Bruker SAINT program and corrected for
absorption using SADABS. The structures were solved using direct
methods (SHELXS) completed by subsequent Fourier synthesis and
refined by full-matrix least-squares procedures. Elemental analyses
were performed at Robertson Microlit Laboratories, Inc., in
Madison, NJ.
Spectroscopic Identification of (η5-C5H3-1,3-(SiMe3)2)2Zr(η1-
C6H5Me)H (3). A flame-dried 50 mL round-bottom flask was
charged with 0.347 g (0.636 mmol) of (η5-C5H3-1,3-(SiMe3)2)2Zr-
(H)Cl, 0.050 g (0.640 mmol) of LiCH2CMe3, and 25 mL of toluene,
and a 180° needle valve was attached. The resulting reaction mixture
was stirred at 45 °C for 4 days. Aliquots of the solution were taken,
and the progress of the reaction was monitored periodically by 1H
NMR spectroscopy. Analysis showed a mixture of 1 and three
isomers of (η5-C5H3-1,3-(SiMe3)2)2Zr(η1-C6H5Me)H. 1H NMR
(benzene-d6): major isomer (3-para), 0.08 (s, 18H, SiMe3), 0.26
(s, 18H, SiMe3), 2.26 (s, 3H, C6H5Me), 5.28 (s, 1H, Zr-H), 6.02
(m, 2H, Cp), 6.42 (m, 1H, Cp), 6.82-7.12 (C6H5Me); minor isomers
(3-meta), 0.09 (s, 18H, SiMe3), 0.14 (s, 18H, SiMe3), 0.23 (s, 18H,
SiMe3), 0.27 (s, 18H, SiMe3), 2.12 (s, 3H, C6H5Me), 2.20 (s, 3H,
C6H5Me), 5.25 (s, 1H, Zr-H), 5.28 (s, 1H, Zr-H), 5.50 (m, 1H,
Cp), 6.00 (m, 1H, Cp), 6.04 (m, 1H, Cp), 6.12 (m, 1H, Cp), 6.45
(m, 1H, Cp), 6.51 (m, 1H, Cp), 6.82-7.12 (C6H5Me).
Spectroscopic Identification of (η5-C5H3-1,3-(SiMe3)2)2Zr-
(CH2CMe3)H. A flame-dried 100 mL round-bottom flask was
charged with 0.963 g (1.77 mmol) of (η5-C5H3-1,3-(SiMe3)2)2Zr-
(H)Cl and 0.137 g (1.77 mmol) of neopentyllithium. The flask was
attached to a swivel frit apparatus and removed from the drybox.
On the vacuum line, pentane was added by vacuum transfer at -78
°C. The resulting reaction mixture was stirred under argon for 30
min, after which time the white precipitate was removed by filtration
and washed with cold pentane. The solvent was removed in vacuo,
yielding a yellow oil identified as (η5-C5H3-1,3-(SiMe3)2)2Zr(CH2-
Preparation of (η5-C5H3-1,3-(SiMe3)2)2Zr(2-NC5H3-4-NMe2)H
(4). A 20 mL scintillation vial was charged with 0.111 g (0.218
mmol) of 1, 0.031 g (0.250 mmol) of 4-(dimethylamino)pyridine,
and 15 mL of diethyl ether. The reaction mixture was stirred for
16 h, turning pale yellow. The solvent was removed in vacuo and
the residue extracted with pentane. Chilling the concentrated
solution to -35 °C overnight yielded 0.069 g (51%) of faint brown
crystals identified as 4. Anal. Calcd for C29H52ZrSi4N2: C, 55.09;
1
CMe3)H. H NMR (benzene-d6): δ 0.18 (s, 2H, CH2CMe3), 0.31
(s, 18H, SiMe3), 0.33 (s, 18H, SiMe3), 1.12 (s, 9H, CH2CMe3), 5.47
(m, 1H, Cp), 6.20 (m, 1H, Cp), 6.67 (m, 1H, Cp), 5.77 (s, 1H,
Zr-H). 13C{1H} NMR (cyclohexane-d12): δ 1.65, 1.71 (SiMe3),
26.64 (CMe3), 114.13, 115.84, 117.74, 119.86, 120.11 (Cp).
Preparation of (η5-C5H3-1,3-(SiMe3)2)(η5-C5H3-3-SiMe3-1-η1-
SiMe2CH2)ZrH (1). A flame-dried 25 mL round-bottom flask was
charged with 0.301 g (0.551 mmol) of (η5-C5H3-1,3-(SiMe3)2)2Zr-
(H)Cl and 0.043 g (0.551 mmol) of neopentyllithium and attached
to a 180° needle valve. The assembly was removed from the drybox,
and approximately 10 mL of pentane was added by vacuum transfer
at -78 °C. The resulting reaction mixture was warmed to room
1
H, 8.29; N, 4.43. Found: C, 54.90; H, 8.20; N, 4.05. H NMR
(benzene-d6): δ 0.22 (s, 18H, SiMe3), 0.40 (s, 18H, SiMe3), 2.37
(s, 6H, NMe2), 3.78 (s, 1H, Zr-H), 5.79 (m, 1H, Cp), 5.99 (m,
2H, Cp), 5.97 (m, 1H, py), 7.20 (m, 1H, py), 7.89 (m, 1H, py).
13C{1H} NMR (benzene-d6): δ 1.09, 1.38 (SiMe3), 39.34 (NMe2),
108.5, 111.6, 115.9, 117.4, 119.5 (Cp), 109.3, 114.3, 147.8, 154.5,
193.8 (py).
Preparation of (η5-C5H3-3-SiMe3-1-η1-SiMe2CH2)2Zr (5). A
thick-walled flame-dried reaction vessel was charged with 0.038 g
(0.075 mmol) of 1. On a high-vacuum line, the contents of the
vessel were degassed and approximately 5 mL of heptane was added
by vacuum transfer at -78 °C. The resulting reaction mixture was
then heated to 85 °C for 1 week. After this time, the solvent was
removed in vacuo to give an oily solid. This yielded yellow blocks
of 5 following recrystallization from pentane at -35 °C. Anal. Calcd
for C22H40ZrSi4: C, 52.00; H, 7.93. Found: C, 52.19; H, 8.26. 1H
(28) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
(29) Marvich, R. H.; Brintzinger, H. H. J. Am. Chem. Soc. 1971, 93,
203.
(30) Schrock, R. R.; Fellmann, J. D. J. Am. Chem. Soc. 1978, 100, 3359.
(31) Klusener, P. A. A.; Brandsma, L.; Verkruijsse, H. D.; Schleyer, P.
v. R.; Friedl, T.; Pi, R. Angew. Chem., Int. Ed. Engl. 1986, 25, 465.

1100 Organometallics, Vol. 25, No. 5, 2006
Bernskoetter et al.
NMR (cyclohexane-d12): δ -2.37 (d, 2H, 14 Hz, CH2SiMe3),
-1.40 (d, 2H, 14 Hz, CH2SiMe3), 0.19 (s, 18H, SiMe3), 0.25 (s,
6H, SiMe2), 0.29 (s, 6H, SiMe2), 5.73 (m, 2H, Cp), 5.93 (m, 2H,
Cp), 6.93 (m, 2H, Cp). 13C{1H} NMR (cyclohexane-d12): δ -0.85,
0.30, 0.41, 1.73 (SiMe), 16.9 (ZrCH2), 110.8, 116.5, 118.1, 118.8,
127.4 (Cp).
needle valve. Against an Ar counterflow, 6 µL of distilled water
was added to the yellow solution via microsyringe. The resulting
reaction mixture was stirred at ambient temperature for 30 min.
The solvent was removed in vacuo and the residue washed with
cold pentane, yielding 0.087 g (57%) of an off-white powder
identified as 7. Anal. Calcd for C44H86Zr2Si8: C, 50.90; H, 8.35.
Found: C, 50.22; H, 7.97. 1H NMR (benzene-d6): δ 0.47 (s, 36H,
SiMe3), 0.48 (s, 36H, SiMe3), 5.18 (s, 2H, Zr-H), 5.58 (m, 4H,
Cp), 5.89 (m, 4H, Cp), 6.53 (m, 4H, Cp). 13C{1H} NMR (benzene-
d6): δ 2.04, 2.11 (SiMe3), 118.16, 120.27, 120.51, 120.88, 125.28
(Cp).
Preparation of [(η5-C5H3-1,3-(SiMe3)2)2ZrH2]2 (6). A thick-
walled reaction vessel was charged with 0.151 g (0.297 mmol) of
1 and approximately 10 mL of pentane. On a high-vacuum line,
the vessel was submerged in liquid nitrogen and evacuated and 4
atm of dihydrogen was admitted. The contents of the vessel were
warmed to ambient temperature, and the resulting reation mixture
was stirred for 16 h. The excess H2 was removed in vacuo and the
precipitate collected by filtration, yielding 0.077 g (51%) of a white
powder identified as 6. Anal. Calcd for C44H88Zr2Si8: C, 51.59;
H, 8.66. Found: C, 51.57; H, 8.27. 1H NMR (benzene-d6): δ -3.57
(br, 2H, Zr-H), 0.23 (s, 36H, SiMe3), 0.51 (s, 36H, SiMe3), 3.41
(br, 2H, Zr-H), 5.05 (br, 4H, Cp), 6.08 (br, 4H, Cp), 6.59 (br, 4H,
Cp). 13C{1H} NMR (benzene-d6): δ -0.33, 1.69 (SiMe3), 114.05,
117.75, 120.14 (Cp). (η5-C5H3-1,3-(SiMe3)2)2ZrH2: 1H NMR
(benzene-d6) δ 0.58 (s, 36H, SiMe3), 5.91 (m, 4H, Cp), 7.80 (m,
2H, Cp), Zr-H not located; 13C{1H} NMR (benzene-d6) δ 2.22
(SiMe3), 119.80, 122.56 (Cp). Quaternary carbons were not located
in either case.
Acknowledgment. We thank the National Science Founda-
tion (CAREER Award) and the Director, Office of Basic Energy
Sciences, Chemical Sciences Division, of the U.S. Department
of Energy (Contract No. DE-FG02-05ER15659) for financial
support. P.J.C. is a Cottrell Scholar supported by the Research
Corp. and is a science and engineering fellow of the David and
Lucile Packard Foundation.
Supporting Information Available: A figure giving a repre-
sentative 1H NMR spectrum of 2 in benzene-d6 and CIF files giving
crystallographic data for 1, 5, and 7. This material is available free
of charge via the Internet at http://pubs.acs.org.
Preparation of [(η5-C5H3-1,3-(SiMe3)2)2ZrH]2(µ2-O) (7). A 25
mL round-bottom flask was charged with 0.150 g (0.295 mmol) of
1 and approximately 10 mL of diethyl ether and attached to a 180°
OM0508379