Alkyl substituent effects on reductive elimination reactions in zirconocene alkyl hydride complexes. Manipulation of the alkyl steric environment allows the synthesis of a zirconocene dinitrogen complex

Article abstract of DOI:10.1021/om0302239

The rate of reductive elimination as a function of alkyl ligand has been measured for a series of zirconocene alkyl hydride complexes, Cp*Cp″Zr(R)H (Cp* = η⁵-C₅Me₅, Cp″ = η⁵-C₅H₃-1,3-(SiMe₃)₂, R = CH₃, CH₂(CH₂)₂CH₃, CH₂(CH₂)₆CH₃, CH₂^cC₆H₁₁, CH₂CHMe₂, CH₂-CMe₃), where the steric disposition of the alkyl ligand has been systematically varied. The rate of reductive elimination increases modestly as the steric bulk of the alkyl ligand is increased. This trend is attributed to ground state destabilization arising from unfavorable steric interactions between the alkyl ligand and the cyclopentadienyl substituents. The effect is magnified when more voluminous cyclopentadienyl ligands are incorporated into the metallocene framework. Thus, the Cp*Cp?Zr(R)H (Cp? = η⁵-C₅H₂-1,2,4- (SiMe₃)₃, R = CH₃, CH₂ (CH₂)₂CH₃, CH₂(CH₂)₆CH₃) series of alkyl hydride complexes lose alkane more readily than the corresponding Cp*Cp″Zr(R)H complexes. In addition, the rate of reductive elimination has also been examined for Cp*Cp″Zr(Ph)(H) and Cp*Cp″Zr(CH₂Ph)H (Ph = C₆H₅) and is slower than the alkyl hydride series. The sluggish rates of reductive elimination are a result of ground state stabilization imparted by a strong zirconium-phenyl bond and by η² coordination of the benzyl ligand, respectively. This interaction, along with the solid state structure of Cp*Cp″Zr(CH₃)H, has been characterized by X-ray diffraction. The kinetic data led to the synthesis of Cp*₂Zr(CH₂CMe₃)H, which undergoes reductive elimination of alkane and coordination of dinitrogen.

Full text of DOI:10.1021/om0302239

Organometallics 2003, 22, 2797-2805
2797
Alk yl Su bstitu en t Effects on Red u ctive Elim in a tion
Rea ction s in Zir con ocen e Alk yl Hyd r id e Com p lexes.
Ma n ip u la tion of th e Alk yl Ster ic En vir on m en t Allow s th e
Syn th esis of a Zir con ocen e Din itr ogen Com p lex
J aime A. Pool, Emil Lobkovsky, and Paul J . Chirik*
Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853
Received March 24, 2003
The rate of reductive elimination as a function of alkyl ligand has been measured for a
series of zirconocene alkyl hydride complexes, Cp*Cp′′Zr(R)H (Cp* ) η5-C5Me5, Cp′′ ) η5-
c
C5H3-1,3-(SiMe3)2, R ) CH3, CH2(CH2)2CH3, CH2(CH2)6CH3, CH2 C6H11, CH2CHMe2, CH2-
CMe3), where the steric disposition of the alkyl ligand has been systematically varied. The
rate of reductive elimination increases modestly as the steric bulk of the alkyl ligand is
increased. This trend is attributed to ground state destabilization arising from unfavorable
steric interactions between the alkyl ligand and the cyclopentadienyl substituents. The effect
is magnified when more voluminous cyclopentadienyl ligands are incorporated into the
metallocene framework. Thus, the Cp*Cp′′′Zr(R)H (Cp′′′ ) η5-C5H2-1,2,4-(SiMe3)3, R ) CH3,
CH2(CH2)2CH3, CH2(CH2)6CH3) series of alkyl hydride complexes lose alkane more readily
than the corresponding Cp*Cp′′Zr(R)H complexes. In addition, the rate of reductive
elimination has also been examined for Cp*Cp′′Zr(Ph)(H) and Cp*Cp′′Zr(CH2Ph)H (Ph )
C6H5) and is slower than the alkyl hydride series. The sluggish rates of reductive elimination
are a result of ground state stabilization imparted by a strong zirconium-phenyl bond and
by η2 coordination of the benzyl ligand, respectively. This interaction, along with the solid
state structure of Cp*Cp′′Zr(CH3)H, has been characterized by X-ray diffraction. The kinetic
data led to the synthesis of Cp*2Zr(CH2CMe3)H, which undergoes reductive elimination of
alkane and coordination of dinitrogen.
In tr od u ction
studied less thoroughly. Schwartz and co-workers de-
scribed ligand-induced reductive elimination, whereby
addition of a phosphine ligand to Cp2Zr(R)H (Cp ) η5-
C5H5; R ) CH2C6H11) triggered loss of alkane and
formation of Cp2Zr(PR3)2.6 More recently, Ashe observed
a similar phenomenon upon addition of PMe3 to bo-
ratabenzene-ligated zirconium(IV) dialkyl complexes.7
Nitrogen donors such as (dimethylamino)pyridine
(DMAP)8 and acetylenes9 have also been shown to
induce reductive elimination of alkane and dihydrogen,
respectively, in zirconocene complexes. Legzdins has
also reported ligand-induced reductive elimination in
piano-stool tungsten aryl hydride complexes upon ad-
dition of isocyanides and organic carbonyls.10 Ligand-
induced reductive elimination has also been implicated
in the stoichiometric preparation of cyclic â-phosphino
imines with zirconium11 as well as in the titanium-
Reductive elimination of alkane from a transition
metal alkyl hydride is one of the fundamental trans-
formations in organotransition metal chemistry and
constitutes a key bond-forming step in numerous cata-
lytic cycles (eq 1).1 The microscopic reverse, oxidative
addition of a carbon-hydrogen bond, has attracted
considerable interest for its utility in alkane activation.2
Alkane reductive elimination reactions with late transi-
tion metal compounds are quite common3 and have been
the subject of numerous mechanistic4 and computational
studies.5
Early transition metal reductive elimination reac-
tions, although known for quite some time, have been
(6) (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.
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Chem., Int. Ed. Engl. 1997, 36, 2014.
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and Applications of Organotransition Metal Chemistry; University
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(2) For a recent review see: Labinger, J . A.; Bercaw, J . E. Nature
2002, 417, 507.
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Chem. Soc. J pn. 1981, 54, 1857.
(4) (a) Williams, B. S.; Goldberg, K. I. J . Am. Chem. Soc. 2001, 123,
2576. (b) Crumpton, D. M.; Goldberg, K. I. J . Am. Chem. Soc. 2000,
122, 962.
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1997, 119, 1139.
(11) Cadierno, V.; Zablocka, M.; Donnadieu, B.; Igau, A.; Majoral,
J .-P.; Skowronska, A. J . Am. Chem. Soc. 1999, 121, 11086.
10.1021/om0302239 CCC: $25.00 © 2003 American Chemical Society
Publication on Web 05/30/2003

2798 Organometallics, Vol. 22, No. 13, 2003
Pool et al.
catalyzed ene reaction12 and in the reductive cyclization
of enones.13
information could be used to design new zirconocene
alkyl hydrides that undergo facile reductive elimination
to form cyclometalated zirconocene hydrides that are
reactive toward dinitrogen and other small molecules.
Few studies, particularly in early transition metal
chemistry, have focused on the effect of alkyl ligand on
the rate of reductive elimination. Seminal studies by
Halpern on square-planar bis-phosphine platinum alkyl
hydride complexes demonstrated accelerated rates of
alkane loss with increased size of the platinum alkyl.22
Similar observations were made with related bis-neo-
pentyl platinum phosphine complexes23 and have also
been noted in palladium,24 rhodium,25 and gold com-
pounds.26 In this report we describe a systematic study
of the effect of alkyl ligand on the rate of reductive
elimination in zirconocene alkyl hydride complexes.
Application of our data has allowed the synthesis of a
zirconocene dinitrogen complex by reductive elimina-
tion.
Examples of alkane reductive elimination reactions
that proceed thermally, without addition of exogeneous
ligands, have been described for metallocenes of groups
4, 5, and 6. For example, Cp*2Zr(CH2CHMe2)H14 (Cp*
) η5-C5Me5) and (CpR5)2W(CH3)H15 (Rn ) Me or H) lose
alkane upon thermolysis. More recently, Parkin has
observed thermal reductive elimination of dihydrogen
in an ansa-tantalocene trihydride complex16 and con-
ducted a detailed mechanistic investigation into the
nature of this reaction in related ansa-molybdenocene
and ansa-tungstocene dihydrides and alkyl hydrides.17
Choukroun18 and Teuben19 have also observed bimetal-
lic reductive eliminations for the group 4 metals result-
ing in cyclopentadienyl ligand and dinitrogen activation,
respectively.
Recent work from our laboratory has focused on the
application of early transition metal alkane and dihy-
drogen reductive elimination reactions to the activation
of small molecules such as elemental phosphorus20 and
dinitrogen.21 For group 4 metallocene complexes, we
discovered that sterically demanding cyclopentadienyl
substituents such as [SiMe3] groups are effective in
promoting alkane reductive elimination under mild
conditions.21 Mechanistic studies on the zirconocene
isobutyl hydride complex Cp′′2Zr(CH2CHMe2)H (Cp′′ )
η5-C5H2-(1,3-SiMe3)2) revealed that dinitrogen activation
occurred from a zirconocene cyclometalated hydride
rather than from direct reductive elimination of alkane
(eq 2).
Resu lts a n d Discu ssion
P r ep a r a tion a n d Ch a r a cter iza tion of Zir con o-
cen e Alk yl Hyd r id e Com p lexes. Synthesis of the
desired zirconocene alkyl hydride complexes proceeded
in a straightforward manner by addition of the ap-
propriate alkyllithium to the zirconocene hydrido chlo-
ride complex Cp*Cp′′Zr(H)Cl (1).27 Initial studies fo-
cused on the preparation of a series of alkyl hydride
complexes where the steric disposition of the alkyl
ligand could be systematically varied. Thus, Cp*Cp′′Zr-
(CH3)H (2), Cp*Cp′′Zr(CH2(CH2)2CH3)H (3), Cp*Cp′′Zr-
c
(CH2(CH2)6CH3)H (4), Cp*Cp′′Zr(CH2 C6H11)H (5), and
Cp*Cp′′Zr(CH2CMe3)H (7) were synthesized and iso-
lated in approximately 70% yield (eq 3). One exception
With an understanding of how cyclopentadienyl sub-
stituents influence the rate of reductive elimination, we
became interested in learning how the alkyl ligand could
be manipulated to control the rate of alkane loss. Such
(12) Sturla, S. J .; Kablaoui, N. M.; Buchwald, S. L. J . Am. Chem.
Soc. 1999, 121, 1976.
(13) Kablaoui, N. M.; Buchwald, S. L. J . Am. Chem. Soc. 1995, 117,
6785.
(14) McAlister, D. R.; Erwin, D. K.; Bercaw, J . E. J . Am. Chem. Soc.
1978, 100, 5966.
(15) (a) Bullock, M. R.; Headford, C. E. L.; Kegley, S. E.; Norton, J .
R. J . Am. Chem. Soc. 1985, 107, 727. (b) Cooper, N. J .; Green, M. L.
H.; Mahtab, R. J . J . Chem. Soc., Dalton Trans. 1979, 1557. (c) Parkin,
G.; Bercaw, J . E. Organometallics 1989, 8, 1172.
(16) Shin, J . H.; Parkin, G. Chem. Commun. 1999, 887.
(17) (a) Churchill, D. G.; J anak, K. E.; Wittenberg, J . S.; Parkin, G.
J . Am. Chem. Soc. 2003, 125, 1403. (b) J anak, K. E.; Churchill, D. G.;
Parkin, G. Chem. Commun. 2003, 22.
(18) Choukroun, R.; Raoult, Y.; Gervais, D. J . Organomet. Chem.
1990, 391, 189.
(19) de Wolf, J . M.; Blaauw, R.; Meetsma, A.; Teuben, J . H.; Gyepes,
R.; Varga, V.; Mach, K.; Veldma, N.; Spek, A. L. Organometallics 1996,
15, 4977.
(20) Chirik, P. J .; Pool, J . A.; Lobkovsky, E. Angew. Chem., Int. Ed.
2002, 41, 3463.
(21) Pool, J . A.; Lobkovsky, E.; Chirik, P. J . J . Am. Chem. Soc. 2003,
125, 2241.
is the preparation of 2. Monitoring the reaction in situ
by 1H NMR spectroscopy revealed a relatively clean
alkylation reaction upon addition of LiCH3. However a
small (∼5-10%) amount of a blue, paramagnetic im-
purity was also observed. Unfortunately, this small
amount of impurity could only be removed through
(22) Halpern, J . Acc. Chem. Res. 1982, 15, 332.
(23) Foley, P.; DiCosmo, R.; Whitesides, G. M. J . Am. Chem. Soc.
1980, 102, 6713.
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41, 4289.
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1997, 16, 434. (b) Northcutt, T. O.; Wick, D. D.; Vetter, A. J .; J ones,
W. D. J . Am. Chem. Soc. 2001, 123, 7257.
(26) Kramer, A. V.; Osborn, J . A. J . Am. Chem. Soc. 1974, 96, 7823.
(27) Pool, J . A.; Bradley, C. A.; Chirik, P. J . Organometallics 2002,
21, 1271.

Zirconocene Dinitrogen Complex
Organometallics, Vol. 22, No. 13, 2003 2799
successive recrystallizations, resulting in an overall 15%
isolated yield for pure 2.
The zirconocene isobutyl hydride complex Cp*Cp′′Zr-
(CH2CHMe2)H (6) was also prepared by reaction of the
zirconocene dichloride with 2 equiv of tert-butyllithium
as described previously (eq 4).27
The zirconocene alkyl hydrides containing â-hydro-
gens (3-6) may also be prepared by olefin insertion into
the zirconocene dihydride Cp*Cp′′ZrH2. Although this
route provided the desired compounds, the alkyl hydride
products are invariably contaminated with the zir-
conocene cyclometalated hydride (eq 5). For this reason
the salt metathesis route is preferred. Attempts to
prepare zirconocene alkyl hydrides with secondary alkyl
ligands by reaction of 1 with cyclopentyl or cyclohexyl-
lithium were unsuccessful. Likewise, attempts to insert
cyclopentene or cyclohexene into Cp*Cp′′ZrH2 did not
yield the desired alkyl hydride product.
F igu r e 1. Molecular structure of one enantiomer of 2 with
30% probability ellipsoids. Hydrogen atoms, except the
metal hydride, are omitted for clarity.
with those previously reported for zirconocene hy-
drides27,28 and alkyls,29 respectively.
A similar protocol produced X-ray quality crystals of
the zirconocene benzyl hydride, 10, and the molecular
structure is shown in Figure 2. As with 2, the X-ray data
were of sufficient quality to locate each of the hydrogens
in the molecule, providing a zirconium hydride distance
of 1.81(2) Å. The most notable feature of the structure
is the η2 coordination of the benzyl ligand with a short
Zr-C(12) distance of 2.7805(18) Å. This value is in
agreement with the average distance of 2.618 Å found
for zirconium η2 benzyl complexes.30
In addition to studies with the [Cp*Cp′′Zr] series, we
also investigated the effect of alkyl substituents on the
rate of reductive elimination with the more sterically
hindered [Cp*Cp′′′Zr] (Cp′′′ ) η5-C5H2-1,2,4-(SiMe3)3)
motif. Preparation of the requisite hydrido chloride
complex, Cp*Cp′′′Zr(H)Cl (11), was accomplished by
addition of 1 equiv of LiCMe3 to a toluene solution of
the zirconocene dichloride (eq 6).
Using the procedures described above, Cp*Cp′′Zr(CH2-
SiMe3)H (8), Cp*Cp′′Zr(Ph)H (9), and Cp*Cp′′Zr(CH2-
Ph)H (10) were also prepared and isolated as air- and
moisture-sensitive solids in moderate yields (∼75%) by
recrystallization from pentane (eq 3).
Each of the zirconocene alkyl hydride complexes,
2-10, was readily identified by 1H and 13C NMR
spectroscopy. Typical spectra display inequivalent [SiMe3]
groups, one Cp* resonance, and three cyclopentadienyl
resonances on the Cp′′ ring, consistent with C1 molec-
ular symmetry. Diagnostic Zr-H resonances are typically
observed around 5 ppm in the 1H NMR spectrum in
benzene-d6, although for 3 and 4, relatively upfield
values of 3.84 and 3.86 ppm are observed, respectively.
The alkyl hydride complexes also display diastereotopic
Zr-CH2 hydrogens that are upfield of SiMe4, consistent
with a methylene unit coordinated to a chiral zirconium
center.
Cooling a concentrated pentane solution of 2 to -35
°C produced clear, colorless crystals suitable for X-ray
diffraction. The asymmetric unit contains two enantio-
meric molecules, one of which is shown in Figure 1. The
solid state structure reveals the expected coordination
environment for a monomeric, bent metallocene. The
X-ray data were of sufficient quality such that all of the
hydrogen atoms, including the zirconium hydride, could
be located and refined. The Zr-H distance of 1.85(3) Å
and the Zr-C(1) distance of 2.282(2) Å are in accord
Alkylation of 11 with LiCH3 resulted in elimination
of lithium chloride accompanied by clean formation of
the zirconocene methyl hydride complex Cp*Cp′′′Zr-
(CH3)H (12) (eq 7). Unlike 2, pure 12 has been obtained
(28) (a) Highcock, W. J .; Mills, R. M.; Spencer, J . L.; Woodward, P.
J . Chem. Soc., Dalton Trans. 1986, 821. (b) Stones, S. B.; Peterson, J .
L. Inorg. Chem. 1981, 20, 2889. (c) Kraft, B. M.; Lachicotte, R. J .; J ones,
W. D. J . Am. Chem. Soc. 2001, 123, 10973.
(29) (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.;
Meyer, T. Y.; J ordan, R. F. Organometallics 1993, 12, 2897. (c) J ordan,
G. T.; Liu, F.-C.; Shore, S. G. Inorg. Chem. 1997, 36, 5597. (d) Liu,
F.-C.; Liu, J .; Meyers, E. A.; Shore, S. G. Inorg. Chem. 1998, 37, 3293.

2800 Organometallics, Vol. 22, No. 13, 2003
Pool et al.
F igu r e 2. Molecular structure of 10 with 30% probability ellipsoids. Hydrogen atoms, except the metal hydride, are omitted
for clarity.
Ta ble 1. Ra te Con sta n ts for Alk a n e Red u ctive
directly from the reaction mixture without the need for
Elim in a tion for 2-7 a t 60 °C
successive recrystallizations. This methodology has also
compound
R
kobs (s-1
)
krel
been extended to include the butyl hydride derivative
Cp*Cp′′′Zr(CH2(CH2)2CH3)H (13) and the n-octyl hy-
dride Cp*Cp′′′Zr(CH2(CH2)6CH3)H (14), both of which
may be isolated as thick yellow oils in high yield (eq 7).
2
3
4
5
6
7
CH3
5.1(3) × 10-6
4.9(3) × 10-4
4.4(3) × 10-4
4.4(1) × 10-4
1.7(4) × 10-3
2.0(1) × 10-3
1
96
86
86
333
392
CH2CH2CH2CH3
CH2(CH2)6CH3
CH2cC6H11
CH2CHMe2
CH2CMe3
Ta ble 2. Ra tes of Red u ctive Elim in a tion for 8-10
a t 60 °C
compound
R
k )
obs (s-1
8
9
10
CH2SiMe3
Ph
CH2Ph
3.3(1) × 10-6
5.5(2) × 10-6
4.7(1) × 10-5
elimination31 were conveniently measured by H NMR
spectroscopy at 60 °C and were determined by integrat-
ing the resonances of the zirconocene alkyl hydride
versus 15 as a function of time. The observed rate
constants for reductive elimination for the zirconocene
complexes 2-7 where the alkyl ligand was systemati-
cally varied are contained in Table 1.
In general, the rate of alkane formation increases as
the steric disposition of the alkyl increases. This trend
is in accord with data previously reported for late
transition metal reductive elimination reactions.22-24
The thermolysis of the zirconocene alkyl hydrides, 8-10,
was also studied. The measured first-order rate con-
stants for reductive elimination for these complexes are
contained in Table 2.
1
Kin etics of Red u ctive Elim in a tion . Thermolysis
of the zirconocene alkyl hydrides Cp*Cp′′Zr(R)H results
in smooth, first-order elimination of alkane and cyclo-
metalation of one of the cyclopentadienyl trimethylsilyl
groups. As reported previously,21 only one diastereomer
of the cyclometalated zirconocene hydride, 15, is ob-
served (eq 8). These reactions are best viewed as a
The rates of reductive elimination were also measured
for the Cp*Cp′′′Zr(R)H complexes at 60 °C (eq 9). As
with the Cp*Cp′′Zr(R)H reductive eliminations, one
diastereomer of the cyclometaled hydride 16 is
formed.21,32 In this case, rate constants for the least
sterically hindered complexes were measured because,
at this temperature, the other reactions were too fast
composite of alkane reductive elimination followed by
oxidative addition of a C-H bond of a [SiMe3] substitu-
ent. The first-order rate constants for overall reductive
(31) We use the term overall reductive elimination to signify the
conversion of the zirconocene alkyl hydride to the corresponding
cyclometalated hydride with loss of alkane. The observed rate con-
stants, which we refer to as “rate constants for reductive elimination”,
are actually the products of several elementary steps (see Scheme 1).
(32) For other examples of [SiMe3] group participation in low-valent
zirconium chemistry see: (a) Horacek M.; Stepnicka P.; Kubista J .;
Fejfarova K.; Gyepes R.; Mach K. Organometallics 2003, 22, 861. (b)
Larsonneur, A.-M.; Choukroun, R.; J aud, J . Organometallics 1993, 12,
3216.
(30) Cambridge Structural Database version 5.23. For representa-
tive examples of zirconium η2-benzyl complexes: (a) Ziniuk, Z.;
Goldberg, I.; Kol, M. Inorg. Chem. Commun. 1999, 2, 549. (b) Bei, X.;
Swenson, D. C.; J ordan, R. F. Organometallics 1997, 16, 3282. (c) Male,
N. A. H.; Thorton-Pett, M.; Bochmann, M. J . Chem. Soc., Dalton Trans.
1997, 2487. (d) Cloke, F. G. N.; Geldbach, T. J .; Hitchcock, P. B.; Love,
J . B. J . Organomet. Chem. 1996, 506, 343.

Zirconocene Dinitrogen Complex
Organometallics, Vol. 22, No. 13, 2003 2801
Sch em e 1
Ta ble 3. Rela tive Ra tes of Alk a n e Red u ctive
reductively eliminate by the pathway shown in Scheme
1.
Elim in a tion for 12-14 a t 60 °C
compound
R
kobs (s-1
)
krel
12
13
14
CH3
1.0(2) × 10-5
5.5(2) × 10-3
1.0(5) × 10-2
1
550
1000
CH2CH2CH2CH3
CH2(CH2)6CH3
1
to be accurately determined by H NMR spectroscopy.
The rate constants for reductive elimination for Cp*-
Cp′′′Zr(R)H complexes 12, 13, and 14 are contained in
Table 3.
One open question regarding the mechanism proposed
in Scheme 1 is the possibility of a reversible reductive
elimination (step iii) and the intermediacy of an alkane
σ-complex. Both J ones33 and Parkin17 have noted that
what is commonly referred to as a “reductive elimination
reaction” is actually a composite of two reactions,
namely, reductive coupling of the alkyl and hydride
ligands, followed by dissociation (eq 11). An intermedi-
ate alkane σ-complex or reassociation of the alkane with
the zirconocene alkyl intermediate D (Scheme 1) would
lead to multiple H/D exchange processes with an ap-
propriately isotopically labeled alkyl hydride complex.
Isotop ic La belin g Stu d ies. Previous studies from
our laboratory have focused on the mechanism of alkane
reductive elimination from zirconocene isobutyl hydride
complexes.21 Isotopic labeling and kinetic isotope effects
are consistent with a mechanism involving rapid migra-
tion of the zirconium hydride to the Cp* ring followed
by rate-determining oxidative addition of a [SiMe3]
group. Reductive elimination of the alkane and subse-
quent hydrogen migration produce the observed zir-
conocene cyclometalated hydride. Isotopic labeling stud-
ies were conducted with isotopomers of 2 in order to
determine whether this proposed mechanistic pathway
was operable for alkyls other than isobutyl.
To probe this issue, Cp*Cp′′Zr(CD3)H (2-d 3) was
prepared by addition of LiCD3 to 1. Thermolysis of 2-d 3
at 60 °C resulted in smooth formation of the zirconocene
cyclometalated hydride 15 along with CD3H as the sole
methane isotopomer (eq 12). The absence of CD2H2 and
Preparation of Cp*Cp′′Zr(D)Cl (1-d 1) was accom-
plished by addition of 1 equiv of LiC(CD3)3 to Cp*Cp′′-
ZrCl2 as described previously.27 Alkylation of 1-d 1 with
LiCH3 afforded Cp*Cp′′Zr(CH3)D (2-d 1). Thermolysis of
2-d 1 at 60 °C resulted in formation of CH4 and the
cyclometalated zirconocene deuteride 15-d 1 (eq 10).
Analysis of the reaction mixture by a combination of 1H
2
and H{1H} NMR spectroscopy demonstrated that CH4
was the only methane isotopomer produced and the
deuterium atom remained in the organometallic prod-
uct. Therefore, alkyls other than isobutyl seem to
(33) J ones, W. D. Acc. Chem. Res. 2003, 36, 140.

2802 Organometallics, Vol. 22, No. 13, 2003
Pool et al.
Sch em e 2
other methane isotopomers demonstrates the irrevers-
ibility of the reductive elimination step; oxidative ad-
dition of free alkane is not competitive with hydrogen
migration from the cyclopentadienyl ring to the metal
center. Sole formation of CD3H also demonstrates that
the lifetimes of methane σ-complexes are not significant
on the reductive elimination pathway.
constants in Table 1, the rate of reductive elimination
of alkane increases modestly with the increased steric
disposition of the alkyl ligand. For example, the neo-
pentyl hydride complex 7 undergoes reductive elimina-
tion almost 400 times faster than the corresponding
methyl hydride 2. Provided that the mechanism pro-
posed in Scheme 1 is operative, the observed alkyl
effects may be rationalized on the basis of ground state
effects. We previously have shown that competitive
reductive elimination between Cp*Cp′′Zr(CH2CHMe2)H
and Cp*(η5-C5H3-1,3-(Si(CD3)3)2Zr(CH2CHMe2)H yields
a primary kinetic isotope effect of 3.6(3) at 60 °C.21 This
result was interpreted as [SiMe3] group oxidative ad-
dition as the rate-determining step during the alkane
reductive elimination reaction.
On the basis of the proposed mechanism in Scheme
1, dominant transition state effects would yield slower
rates of alkane loss as the size of the alkyl ligand
increases. The larger alkyls would serve to hinder
oxidative addition of the [SiMe3] carbon-hydrogen bond
and raise the overall activation barrier for reductive
elimination. If ground state effects were dominant, an
increased rate of reductive elimination with increased
alkyl sterics would be anticipated. Unfavorable steric
interactions between the bulky alkyl ligands and the
cyclopentadienyl substituents would destabilize the
ground state for the more hindered alkyl complexes and
would result in a lower activation barrier for reductive
elimination. The observed kinetic data clearly support
ground state destabilization as the dominant effect for
the increased rates of reductive elimination.
P ossibility of σ-Bon d Meta th esis. The kinetic and
mechanistic data presented herein and in our previous
report21 do not definitively exclude σ-bond metathesis
pathways (Scheme 2). One possibility is a concerted
addition of a C-H bond from a [SiMe3] group to the
zirconium alkyl group, liberating alkane and forming
the cyclometalated zirconocene hydride (path 1, Scheme
2). Although our isotopic labeling studies are consistent
with this pathway, the preference for C-H bond activa-
tion of the alkyl group over the hydride is inconsistent
with previous observations in related scandocene alkyls.34
Another possibility is concerted addition of the [SiMe3]
C-H bond to the zirconium hydride, liberating free
dihydrogen and forming an unobserved cyclometalated
zirconocene alkyl. To account for the observed products,
the alkyl group must undergo selective hydrogenolysis
to yield the cyclometalated zirconocene hydride (path
2, Scheme 2). The possibility of this pathway has been
definitively excluded by hydrogenolysis of the cyclo-
metalated zirconocene hexyl complex. Addition of dihy-
drogen results in initial formation of the zirconocene
hexyl hydride rather than 15 and free hexane (eq 13).
Over time, hydrogenation of the alkyl hydride is ob-
27
served, affording the zirconocene dihydride Cp*Cp′′ZrH2
and hexane. On the basis of this result and observations
contrary to those reported previously,34 we do not favor
σ-bond metathesis over the mechanism proposed in
Scheme 1.
To further substantiate our hypothesis, similar stud-
ies were also conducted with a more substituted cyclo-
pentadienyl ligand. A more voluminous ligand array
would be expected to magnify the observed ground state
effects and increase the overall rate of reductive elimi-
nation as well as provide greater kinetic discrimination
between similar alkyl ligands (e.g., CH2(CH2)2CH3
versus CH2(CH2)6CH3). Comparison of the rate con-
stants in Table 2 for the Cp*Cp′′Zr(R)H series to those
in Table 3 for the Cp*Cp′′′Zr(R)H complexes supports
this hypothesis. Comparing the methyl derivatives 2
and 12 reveals a doubling in rate constant for reductive
elimination upon addition of a [SiMe3] to one cyclopen-
tadienyl ring. The difference in rate constants increases
with the size of the alkyl ligand. For example, the
n-butyl derivatives 3 and 13 display an 11-fold rate
enhancement upon addition of a [SiMe3] group, whereas
the more bulky n-octyl complexes 4 and 14 offer a 22-
fold rate enhancement.
Exp la n a tion of th e Obser ved Alk yl Effects on
Red u ctive Elim in a tion . As demonstrated by the rate
(34) 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.
The more sterically hindered cyclopentadienyl ligand
environment also magnifies subtle differences between

Zirconocene Dinitrogen Complex
Organometallics, Vol. 22, No. 13, 2003 2803
similar alkyl ligands. For the [Cp*Cp′′Zr] ligand motif,
the rate constants for reductive elimination between the
n-butyl derivative 3 and the n-octyl complex 8 are
indistinguishable within experimental error. However,
for the [Cp*Cp′′′Zr] series, the n-octyl derivative 14
reductively eliminates alkane approximately twice as
fast as the n-butyl complex 13. More substantial effects
are observed in changing the alkyl from methyl to a long
chain hydrocarbon. In the [Cp*Cp′′Zr] case, the methyl
reductively eliminates approximately 100 times slower
than the n-butyl, whereas addition of a cyclopentadienyl
[SiMe3] group results in a 550-fold rate enhancement.
All of the observations are consistent with ground state
destabilization being the most important factor govern-
ing the rate of alkane reductive elimination.
increased strength of the titanium-phenyl bond relative
to other alkyl substituents.
Ap p lica tion of Alk yl Su bstitu en t Effects to Din i-
tr ogen Activa tion . We have previously demonstrated
that manipulation of cyclopentadienyl ring substituents
may be used to increase the rate of alkane reductive
elimination to form cyclometalated zirconocene hydrides
that are reactive toward atmospheric nitrogen.21 It is
important to note that it is the stability of the cyclo-
metalated hydride, not the alkyl hydride, that dictates
reactivity toward dinitrogen. We therefore hypothesized
that alkyl effects may be used to increase the rate of
reductive elimination in a zirconocene complex that
forms a N2-reactive cyclometalated hydride.
As a demonstration of principle, we focused on the
reductive elimination of Cp*2Zr(R)H complexes. This
ligand set was chosen because the dinitrogen complex
[Cp*2Zr(η1-N2)]2(µ2-N2) (18) has been previously syn-
thesized by sodium amalgam reduction of the corre-
sponding dichloride complex.42 Presumably alkali metal
reduction affords a zirconocene cyclometalated hydride
which upon exposure to N2 forms 18. It should also be
noted that Cp*2Zr(CH2CHMe2)H has been prepared and
its thermal reductive elimination studied.14 However,
the temperatures required to initiate isobutane reduc-
tive elimination from the isobutyl hydride exceed the
thermal stability of 18, thus precluding isolation of the
N2 complex by reductive elimination. Preparation of
sterically demanding alkyl hydride complexes may take
advantage of the ground state destabilization phenom-
enon and hence lower the activation barrier for alkane
loss and provide a facile route to 18 under the relatively
mild and reproducible conditions of reductive elimina-
tion.
The ground state effect is also manifested, perhaps
more subtly, with other types of alkyl ligands. For
example, the trimethylsilylmethyl hydride 8 undergoes
reductive elimination slower than the corresponding
neopentyl hydride 7. At first glance this observation
may seem inconsistent with the proposed ground state
destabilization model. However, the longer carbon-
silicon bond length in ZrCH2-SiMe3 as compared to
ZrCH2-CMe3 removes unfavorable steric interactions
with the cyclopentadienyl substituents. This assertion
is supported by the average carbon-silicon bond length
of 2.187(95) Å versus carbon-carbon bond lengths of
1.534(16) Å reported for group 4 transition metal alkyls
of this type.35 Similar explanations have been proposed
for the difference in the rate of R-elimination in the
decomposition of the homoleptic tantalum alkyls, Ta-
(CH2ESiMe3)5 (E ) Si > C),36 as well as in thermal
alkane eliminations from silica-supported chromium
alkyls.37
Synthesis of Cp*2Zr(H)Cl (16) has been achieved by
addition of 1 equiv of LiCMe3 to the corresponding
dichloride complex as described previously.21 Alkylation
with 1 equiv of LiCH2CMe3 in toluene affords Cp*2Zr-
(CH2CMe3)H (17) as a yellow crystalline solid in 69%
Ground state effects may also be used to explain the
relatively slow rates of reductive elimination of the
phenyl and benzyl hydrides 9 and 10. For the latter, η2
coordination of the benzyl ligand provides electronic
saturation at the metal center and hence stabilizes the
ground state and slows the rate of reductive elimination.
The phenyl hydride 9 reductively eliminates alkane at
a rate comparable to the methyl hydride 2. Although
the planarity of the phenyl ring may be in part respon-
sible for the slow loss of benzene, the relatively high
metal-carbon bond strength is the more likely explana-
tion. Bergman has rationalized the slower reductive
elimination of benzene as compared to cyclohexane in
Cp*Ir(PMe3)(R)H complexes on the basis of the in-
creased iridium bond enthalpy for the phenyl complex.38
J ones and Feher also estimated a 13 kcal/mol increase
in the bond enthalpy for rhodium-phenyl versus rhod-
ium-methyl complexes.39 A similar rationale was also
used to explain relatively slow rates of benzene reduc-
tive elimination in tantalocene phenyl dihydride com-
plexes.40 Studies by Wolczanski41 have also noted the
1
yield (eq 14). The H NMR spectrum of 17 in benzene-
d6 displays a Cp* resonance at 1.94 ppm, alkyl reso-
nances at 0.06 (CH2) and 1.22 (CMe3), and a zirconium
hydride resonance at 6.59 ppm in accord with previously
reported alkyl hydride complexes.14,21,27
Stirring a toluene solution of 17 at 22 °C under 1 atm
of pure dinitrogen over the course of 2 weeks resulted
in slow reductive elimination of neopentane and forma-
tion of the dinitrogen complex 18 (eq 15) in 75% yield.
In addition to 18, small, variable amounts (10-20%) of
the dihydride complex Cp*2ZrH2 were also observed. We
believe the dihydride complex arises from stirring 18
for extended periods, resulting in decomposition by
cyclometalation with liberation of free H2.43 Attempts
(35) Cambridge Structural Database version 5.23.
(36) Li, L.; Hung, M.; Xue, Z. J . Am. Chem. Soc. 1995, 117, 12746.
(37) Ajou, J . A. N.; Rice, G. L.; Scott, S. L. J . Am. Chem. Soc. 1998,
120, 13436.
(38) Buchanan, J . M.; Stryker, J . M.; Bergman, R. G. J . Am. Chem.
Soc. 1986, 108, 1537.
(39) J ones, W. D.; Feher, F. J . J . Am. Chem. Soc. 1984, 106, 1650.
(40) Bregel, D. C.; Oldham, S. M.; Eisenberg, R. J . Am. Chem. Soc.
2002, 124, 13827.
(41) Bennett, J . L.; Vaid, T. P.; Wolczanski, P. T. Inorg. Chim. Acta
1998, 270, 414.
(42) Manriquez, J . M.; Bercaw, J . E. J . Am. Chem. Soc. 1974, 96,
6229.
(43) Kraft, B. M.; Lachicotte, R. J .; J ones, W. D. J . Am. Chem. Soc.
2001, 123, 10973.

2804 Organometallics, Vol. 22, No. 13, 2003
Pool et al.
0.79 (SiMe3), 12.52 (C5Me5), 44.17 (Zr-CH3), 115.58, 116.94,
123.58, 125.58, 126.57 (Cp), 117.93 (C5Me5).
to increase the rate of reductive elimination and hence
dinitrogen complex formation by incorporation of bulky
methyladamantyl ligands produced only modest rate
enhancements.
P r ep a r a tion of Cp *Cp ′′Zr (CH2(CH2)2CH3)H (3). A scin-
tillation vial was charged with 55 mg (0.12 mmol) of Cp*Cp′′Zr-
(H)Cl and approximately 10 mL of pentane. The solution was
n
chilled in the cold well, and 73 µL (0.12 mmol) of 1.6 M BuLi
in hexane was added. The resulting reaction mixture was
warmed to room temperature and stirred for 15 min, after
which time the white precipitate was removed by filtration
through Celite. The solvent was then removed in vacuo,
leaving 45 mg (76%) of a yellow oil identified as 3. 1H NMR
(benzene-d6): δ -0.39 (m, 2H, CH2CH2CH2CH3), 0.27 (s, 9H,
SiMe3), 0.41 (s, 9H, SiMe3), 1.09 (t, 3H, CH2CH2CH2CH3), 1.30
(m, 4H, CH2CH2CH2CH3), 1.82 (s, 15H, C5Me5), 3.84 (s, 1H,
Zr-H), 5.16 (m, 1H, Cp), 5.29 (m, 1H, Cp), 5.51 (m, 1H, Cp).
13C NMR (benzene-d6): δ 0.89 (SiMe3), 0.93 (SiMe3), 12.09,
14.81, 28.50, 37.43 (CH2CH2CH2CH3), 12.40 (C5Me5), 112.81,
113.99, 114.70, 121.01, 123.04, 125.64 (Cp), 119.49 (C5Me5).
P r ep a r a tion of Cp *Cp ′′Zr (CH2(CH2)6CH3)H (4). This
compound was prepared in a manner identical to 3 employing
84 mg (0.15 mmol) of 1 and 19 mg (0.15 mmol) of LiCH2(CH2)6-
Exp er im en ta l Section
1
CH3, yielding 77 mg (74%) of a yellow oil identified as 4. H
Gen er a l Con sid er a tion s. All air- and moisture-sensitive
manipulations 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 manipulations were dried and
deoxygenated using literature procedures.44 In addition, tolu-
ene and pentane were further dried by storage over ti-
tanocene45 and were vacuum transferred immediately before
use. Deuterated solvents for NMR spectroscopy were distilled
from sodium metal under an atmosphere of argon and stored
over titanocene. Argon and hydrogen gas were purchased from
Airgas Incorporated and passed through a column containing
Ridox oxygen scavenger and 4 Å molecular sieves before
admission to the high-vacuum line. Preparations of 1,27 7,27
Cp*Cp′′′ZrCl2,21 15,21 and 1621 were accomplished using pro-
cedures described previously.
Single crystals suitable for X-ray diffraction were coated
with polyisobutylene oil in a drybox and were quickly trans-
ferred to the goniometer head of a Siemens SMART CCD area
detector system equipped with a molybdenum X-ray tube
(λ ) 0.71073 Å). Preliminary data revealed the crystal system.
A hemisphere routine was used for data collection and deter-
mination 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 .
P r ep a r a tion of Cp *Cp ′′Zr (CH3)H (2). A scintillation vial
was charged with 211 mg (0.447 mmol) of 1 and approximately
10 mL of toluene. The solution was chilled in the cold well,
and 279 µL of 1.6 M MeLi in diethyl ether was added. The
resulting reaction mixture was warmed to room temperature
and stirred for 10 h, after which time the white precipitate
was removed by filtration through Celite. The solvent was then
removed in vacuo, leaving a powder blue oil. Repeated recrys-
tallization from pentane afforded 32 mg (16%) of analytically
pure colorless crystals. Anal. Calcd for C22H40Si2Zr: C, 58.47;
H, 8.92. Found: C, 58.25; H, 9.23. 1H NMR (benzene-d6): δ
-0.34 (s, 3H, Zr-CH3), 0.21 (s, 9H, SiMe3), 0.37 (s, 9H, SiMe3),
1.89 (s, 15H, C5Me5), 5.15 (m, 1H, Cp), 5.37 (m, 1H, Cp), 6.75
(m, 1H, Cp), 5.75 (s, 1H, Zr-H). 13C NMR (benzene-d6): δ 0.72,
NMR (benzene-d6): δ -0.002 (m, 2H, CH2(CH2)6CH3), 0.29 (s,
9H, SiMe3) 0.43 (s, 9H, SiMe3), 0.92 (t, 3H, CH2(CH2)6CH3),
1.24-1.40 (m, 12H, CH2(CH2)6CH3), 1.84 (s, 15H, C5Me5), 3.86
(s, 1H, Zr-H), 5.17 (m, 1H, Cp), 5.30 (m, 1H, Cp), 5.56 (m, 1H,
Cp). 13C NMR (benzene-d6): δ 0.92, 0.96 (SiMe3), 12.42 (C5Me5),
29.85, 30.58, 32.32, 32.38, 35.56, 37.76, 37.99 (CH2(CH2)6CH3),
65.89 (CH2(CH2)6CH3)112.79, 113.99, 116.76, 119.51, 123.05
(Cp), 114.69 (C5Me5).
P r ep a r a tion of Cp *Cp ′′Zr (CH2cC6H11)H (5). This com-
pound was prepared in a manner identical to 3 employing 96
mg (0.20 mmol) of 1 and 21 mg (0.20 mmol) of LiCH2(CH2)6-
1
CH3, yielding 96 mg (86%) of a yellow oil identified as 5. H
c
NMR (benzene-d6): δ -0.36 (m, 1H, CH2 C6H11), -0.34 (m, 1H,
CH2cC6H11), 0.24 (s, 9H, SiMe3), 0.29 (s, 9H, SiMe3), 0.87, 1.19,
1.44, 1.72 (m, H, CH2cC6H11), 1.89 (s, 15H, C5Me5), 5.07 (m,
1H, Cp), 5.36 (m, 1H, Cp), 5.75 (s, 1H, Zr-H), 6.79 (m, 1H, Cp).
13C NMR (benzene-d6): δ 1.11 (SiMe3), 12.86 (C5Me5), 27.07,
27.36, 27.72, 27.88, 34.04, 40.34, 41.41 (CH2cC6H11), 68.40
(CH2cC6H11), 115.86, 117.78, 117.97, 123.57, 129.52 (Cp),
123.96 (C5Me5).
P r ep a r a tion of Cp *Cp ′′Zr (CH2CMe3)H (7). A scintillation
vial was charged with 88 mg (0.19 mmol) of Cp*Cp′′Zr(H)Cl
and about 10 mL of pentane. The solution was chilled in the
cold well, and 15 mg (0.19 mmol) of LiCH2Me3 was dissolved
in pentane and added to the solution. The resulting reaction
mixture was warmed to room temperature and stirred for 1
h, after which time the white precipitate was removed by
filtration through Celite. The solvent was then removed in
1
vacuo, leaving 63 mg (67%) of a yellow oil identified as 7. H
NMR (benzene-d6): δ 0.24 (s, 9H, SiMe3), 0.37 (s, 9H, SiMe3),
1.19 (s, CH2CMe3), 1.89 (s, 15H, C5Me5), 4.96 (m, 1H, Cp), 5.46
(m, 1H, Cp), 7.30 (m, 1H, Cp), 6.15 (s, 1H, Zr-H). 13C NMR
(benzene-d6): δ 0.88 (SiMe3), 1.18 (SiMe3), 12.88 (C5Me5), 37.03
(CH2CMe3), 39.17 (CH2CMe3), 69.40 (CH2CMe3), 116.28, 117.43,
123.11, 123.27, 129.68 (Cp), 118.48 (C5Me5).
P r ep a r a tion of Cp *Cp ′′Zr (CH2SiMe3)H (8). This com-
pound was prepared in a manner identical to 3 employing 57
mg (0.12 mmol) of 1 and 11 mg (0.12 mmol) of LiCH2SiMe3
and diethyl ether as a solvent. This procedure yielded 47 mg
1
(75%) of a yellow oil identified as 8. H NMR (benzene-d6): δ
-0.57 (d, 1H, CH2SiMe3), -0.22 (d, 1H, CH2SiMe3), 0.21 (s,
9H, SiMe3), 0.26 (s, 9H, SiMe3), 0.36, (s, 9H, SiMe3), 1.88 (s,
15H, C5Me5), 5.01 (m, 1H, Cp), 5.48 (m, 1H, Cp), 6.96 (m, 1H,
Cp), 5.87 (s, 1H, Zr-H). 13C NMR (benzene-d6): δ 0.71 (SiMe3),
1.28 (SiMe3), 5.50 (SiMe3), 12.80 (C5Me5), 49.38 (CH2SiMe3),
115.95, 117.37, 122.25, 123.24, 129.33 (Cp), 118.14 (C5Me5).
P r ep a r a tion of Cp *Cp ′′Zr (P h )H (9). This compound was
prepared in a manner identical to 3 employing 91 mg (0.21
mmol) of 1 and 420 µL of 0.5 M (0.21 mmol) PhLi, yielding 79
(44) Pangborn, A.; Giardello, M.; Grubbs, R. H.; Rosen, R.; Timmers,
F. Organometallics 1996, 15, 1518.
(45) Marvich, R. H.; Brintzinger, H. H. J . Am. Chem. Soc. 1971,
93, 203.

Zirconocene Dinitrogen Complex
Organometallics, Vol. 22, No. 13, 2003 2805
1
mg (73%) of a white solid identified as 9. H NMR (benzene-
d6): δ 0.28, 0.45 (s, 18H, SiMe3), 1.76 (s, 15H, C5Me5), 5.24
(m, 1H, Cp), 5.36 (m, 1H, Cp), 5.51 (m, 1H, Cp), 5.31 (s, 1H,
Zr-H), 7.02 (d, 2H, o-C6H5), 7.09 (t, 2H, p-C6H5), 7.29 (d, 2H,
m-C6H5). 13C NMR (benzene-d6): δ 0.25 (SiMe3), 1.00 (SiMe3),
12.06 (C5Me5), 115.94, 121.78, 122.95, 124.11, 126.62, 127.01,
136.76 (Cp, Ph), 117.02 (C5Me5).
1H, Cp). 13C NMR (benzene-d6): δ 1.47, 2.42, 2.53 (SiMe3),
13.05 (C5Me5), 29.98, 30.12, 32.45, 32.33, 32.10, 37.51 (CH2(CH)6-
CH3), 71.91 (CH2(CH)6CH3), 119.51, 119.44, 121.45, 125.45,
132.97 (Cp), 118.13 (C5Me5).
P r ep a r a tion of Cp *2Zr Np H (17). A scintillation vial was
charged with 186 mg (0.47 mmol) of 16 and approximately 10
mL of toluene. The solution was chilled in the cold well, and
36 mg (0.47 mmol) of LiCH2CMe3 was dissolved in pentane
and added to the solution. The resulting reaction mixture was
warmed to room temperature and stirred for 10 h, after which
time, the white precipitate was removed by filtration through
Celite. The solvent was then removed in vacuo, leaving 139
mg of a yellow solid (69%) identified as 17. Anal. Calcd for
P r ep a r a tion of Cp *Cp ′′Zr (CH2P h )H (10). This compound
was prepared in a manner identical to 3 employing 0.105 g
(0.223 mmol) of 1 and 29 mg (0.223 mmol) of KCH2Ph, yielding
96 mg (82%) of an orange solid identified as 10. Anal. Calcd
for C28H42Si2Zr: C, 63.69; H, 8.02. Found: C, 63.78; H, 8.42.
1H NMR (benzene-d6): δ 0.21 (s, 9H, SiMe3), 0.24 (s, 9H,
SiMe3), 1.24 (d, 1H, CH2Ph), 1.77 (d, 1H, CH2Ph), 1.86 (s, 15H,
C5Me5), 5.00 (m, 1H, Cp), 5.35 (m, 1H, Cp), 6.17 (m, 1H, Cp),
5.55 (s, 1H, Zr-H), 6.78 (d, 2H, C6H5), 6.92 (t, 1H, p-C6H5), 7.20
(d, 2H, C6H5). 13C NMR (benzene-d6): δ 0.86 (SiMe3), 1.27
(SiMe3), 12.64 (C5Me5), 61.72 (CH2Ph), 115.94, 121.56, 123.99,
124.68, 126.19, 126.24, 126.72, 128.44, 129.27 (Cp, CH2Ph),
117.77 (C5Me5).
C
25H42Zr1: C, 69.21; H, 9.76. Found: C, 69.10; H, 9.43. 1H
NMR (benzene-d6): δ 0.060 (s, 2H, CH2CMe3), 1.23 (s, 9H,
CH2CMe3), 1.94 (s, 15H, C5Me5), 6.59 (s, 1H, Zr-H). 13C NMR
(benzene-d6): δ 12.57 (C5Me5), 37.39 (CH2CMe3), 38.47 (CH2-
CMe3), 68.32 (CH2CMe3), 118.40 (C5Me5).
P r ep a r a tion of [Cp *2Zr N2]2(µ-N2) (18) by Red u ctive
Elim in a tion . A 25 mL round-bottom flask was charged with
0.068 g (0.157 mmol) of 17 and approximately 10 mL of
pentane. The resulting reaction mixture was stirred for 2
weeks, after which time the solvent was removed in vacuo,
leaving a purple solid. Recrystallization from pentane afforded
0.052 g (75%) of 18 by comparison to previously reported
data.42
Rea ction of 15 w ith 1-Hexen e. A J . Young NMR tube was
charged with 51 mg (0.12 mmol) of 15, and approximately 0.5
mL of benzene-d6 was added. On the vacuum line, 1 equiv of
1-hexene was added via calibrated gas bulb. The tube was
shaken thoroughly. 1H NMR (benzene-d6): δ -3.00, -0.21 (d,
2H, ZrCH2SiMe2), -0.01 (dt, 2H, CH2CH2CH2CH2CH2CH3),
0.27 (s, 9H, SiMe3), 0.64 (s, 3H, SiMe2), 0.95 (t, 3H, CH2CH2-
CH2CH2CH2CH3), 1.39 (m, 8H, 1.67 (s, 15H, CpMe5), 5.08, 5.20,
6.20 (m, 3H, Cp), 8.96 (s, 1H, HN3SiMe3) 1 SiMe2 peak not
located. 13C NMR (benzene-d6): δ -0.80, 2.97 (SiMe2), 1.57
(SiMe3), 12.71 (C5Me5), 13.51, 15.28, 24.05, 32.95, 36.47, 38.49
(hexyl) 55.79 (ZrCH2SiMe2), 114.30, 114.90, 116.45, 119.75,
125.12 (Cp), 117.72 (C5Me5).
Gen er a l P r oced u r e for Kin etic Deter m in a tion s. In the
drybox, 0.50 mL of a 0.070 M stock solution of the desired
zirconocene alkyl hydride in benzene-d6 was charged into a J .
Young NMR tube and sealed with a Teflon valve. The NMR
probe was calibrated to the desired temperature using an
ethylene glycol standard, and the sample was inserted into
the probe. Approximately 10-15 spectra were recorded at
regular intervals over the duration of 2-3 half-lives. Peak
intensities of the reactant and product were measured. The
observed rate constants were obtained from slopes of plots of
ln[Cp*(CpRn)Zr(R′)H)] versus time.
P r ep a r a tion of Cp *Cp ′′′Zr (H)Cl (11). In the drybox, a 50
mL round-bottom flask was charged with 290 mg (0.502 mmol)
of Cp*Cp′′′ZrCl2, which was then dissolved in approximately
20 mL of toluene. The solution was placed in the cold well until
t
frozen. While thawing, 334 µL of 1.5 M BuLi was added by
microsyringe. The resulting reaction mixture was warmed to
room temperature and stirred. After 2 h, a white precipitate
formed and was removed by filtration through a pad of Celite.
The solvent was removed in vacuo from the resulting solution,
leaving a brown oil. Recrystallization from pentane afforded
172 mg (63%) of a light brown powder identified as 11. Anal.
Calcd for C24H45Cl1Si3Zr1: C, 52.94; H, 8.33. Found: C, 52.45;
1
H, 7.96. H NMR (benzene-d6): δ 0.26 (s, 9H, SiMe3), 0.32 (s,
9H, SiMe3), 0.51 (s, 9H, SiMe3), 2.01 (s, 15H, C5Me5), 6.00, 6.91
(m, 2H, Cp), 6.89 (s, 1H, Zr-H). 13C NMR (benzene-d6): δ 1.28
(SiMe3), 2.19 (SiMe3), 2.40 (SiMe3), 13.34 (C5Me5), 123.29,
124.73, 125.26, 131.26, 133.45, 136.99 (Cp), 121.17 (C5Me5).
P r ep a r a tion of Cp *Cp ′′′Zr (CH3)H (12). This compound
was prepared in a manner identical to 3 employing 54 mg
(0.099 mmol) of 11 and 62 µL (0.099 mmol) of 1.6 M MeLi,
1
yielding 43 mg (83%) of a yellow oil identified as 12. H NMR
(benzene-d6): δ -0.30 (s, 3H, Zr-CH3), 0.19 (s, 9H, SiMe3), 0.32
(s, 9H, SiMe3), 0.47 (s, 9H, SiMe3), 2.00 (s, 15H, C5Me5), 5.69
(m, 1H, Cp), 7.18 (m, 1H, Cp), 6.12 (s, 1H, Zr-H). 13C NMR
(benzene-d6): δ 1.22 (SiMe3), 2.35 (SiMe3), 2.40 (SiMe3), 13.07
(C5Me5), 44.42 (CH3), 114.93, 123.39, 125.47, 129.81, 135.61
(Cp), 118.84 (C5Me5).
P r ep a r a tion of Cp *Cp ′′′Zr (CH2CH2CH2CH3)H (13). This
compound was prepared in a manner identical to 3 employing
80 mg (0.150 mmol) of 11 and 92 µL (0.15 mmol) of 1.6 M
n
-
BuLi, yielding 53 mg (61%) of a yellow oil identified as 13. 1H
NMR (benzene-d6): δ -1.84 (m, 1H, CH2CH2CH2CH3), 0.25
(s, 9H, SiMe3), 0.36 (s, 9H, SiMe3), 0.49 (s, 9H, SiMe3), 1.04 (t,
3H, CH2CH2CH2CH3), 1.42 (m, 3H, CH2CH2CH2CH3), 1.99 (s,
15H, C5Me5), 5.54 (m, 1H, Cp), 7.24 (m, 1H, Cp), 5.62 (s, 1H,
Zr-H). 13C NMR (benzene-d6): δ 1.47 (SiMe3), 2.39 (SiMe3), 2.52
(SiMe3), 13.02 (C5Me5), 14.01, 25.15, 30.19, 34.49 (CH2CH2-
CH2CH3), 114.93, 119.06, 119.45, 125.44, 132.92 (Cp), 118.14
(C5Me5).
Ack n ow led gm en t. For financial support, we would
like to thank Cornell University, the donors of the
Petroleum Research Fund administered by the Ameri-
can Chemical Society, and the National Science Foun-
dation for a CAREER award. J .A.P. also thanks the
National Institutes of Health for support through the
Chemistry and Biology Interface Training Grant at
Cornell.
P r ep a r a tion of Cp *Cp ′′′Zr (H)n Octyl (14). This compound
was prepared in a manner identical to 3 employing 75 mg
(0.140 mmol) of 11 and 19 mg (0.16 mmol) of LiCH2(CH2)6-
Su p p or tin g In for m a tion Ava ila ble: Crystallographic
data for 2 and 10 including full atom labeling schemes and
bond distances and angles. Representative NMR spectra and
kinetic data. This material is available free of charge via the
Internet at http://pubs.acs.org.
1
CH3, yielding 42 mg (53%) of a yellow oil identified as 13. H
NMR (benzene-d6): δ -1.79 (m, 2H, CH2(CH2)6CH3), 0.26 (s,
9H, SiMe3) 0.36 (s, 9H, SiMe3), 0.50 (s, 9H, SiMe3), 0.92 (t,
3H, CH2(CH2)6CH3), 1.23-1.52 (m, 12H, CH2(CH2)6CH3), 2.00
(s, 15H, C5Me5), 5.55 (m, 1H, Cp), 5.63 (s, 1H, Zr-H), 7.26 (m,
OM0302239