Nonchelated d0 zirconium-alkoxide-alkene complexes

Article abstract of DOI:10.1021/ja0575225

The reaction of Cp′₂Zr(O^tBu)Me (Cp′ = C₅H₄Me) and [Ph₃C][B(C₆F ₅)₄] yields the base-free complex [Cp′ ₂Zr(O^tBu)][B(C₆F₅)₄] (6), which exists as Cp′₂Zr(O^tBu)(CIR)⁺ halocarbon adducts in CD₂Cl₂ or C₆D ₅Cl solution. Addition of alkenes to 6 in CD₂Cl ₂ solution at low temperature gives equilibrium mixtures of Cp′_2- Zr(O^tBu)(alkene)⁺ (12a-l), 6, and free alkene. The NMR data for 12a-l are consistent with unsymmetrical alkene bonding and polarization of the alkene C=C bond with positive charge buildup at C_int and negative charge buildup at C_term. These features arise due to the lack of d-π* back-bonding. Equilibrium constants for alkene coordination to 6 in CD₂Cl₂ at -89 °C, K _eq = [12][6]^-1[alkene]^-1, vary in the order: vinylferrocene (4800 M^-1) ? ethylene (7.0) ≈ α-olefins > cis-2-butene (2.2) > trans-2-butene (<0.1). Alkene coordination is inhibited by sterically bulky substituents on the alkene but is greatly enhanced by electron-donating groups and the β-Si effect. Compounds 12a-l undergo two dynamic processes: reversible alkene decomplexation via associative substitution of a CD₂Cl₂ molecule, and rapid rotation of the alkene around the metal-(alkene centroid) axis.

Full text of DOI:10.1021/ja0575225

Published on Web 06/06/2006
Nonchelated d0 Zirconium-Alkoxide-Alkene Complexes
Edward J. Stoebenau, III and Richard F. Jordan*
Contribution from the Department of Chemistry, The UniVersity of Chicago,
5735 South Ellis AVenue, Chicago, Illinois 60637
Received November 3, 2005; Revised Manuscript Received April 26, 2006; E-mail: rfjordan@uchicago.edu
Abstract: The reaction of Cp′2Zr(OtBu)Me (Cp′ ) C5H4Me) and [Ph3C][B(C6F5)4] yields the base-free complex
[Cp′2Zr(OtBu)][B(C6F5)4] (6), which exists as Cp′2Zr(OtBu)(ClR)+ halocarbon adducts in CD2Cl2 or C6D5Cl
solution. Addition of alkenes to 6 in CD2Cl2 solution at low temperature gives equilibrium mixtures of Cp′2-
Zr(OtBu)(alkene)+ (12a-l), 6, and free alkene. The NMR data for 12a-l are consistent with unsymmetrical
alkene bonding and polarization of the alkene CdC bond with positive charge buildup at Cint and negative
charge buildup at Cterm. These features arise due to the lack of d-π* back-bonding. Equilibrium constants
for alkene coordination to 6 in CD2Cl2 at -89 °C, Keq ) [12][6]-1[alkene]-1, vary in the order: vinylferrocene
(4800 M-1) . ethylene (7.0) ≈ R-olefins > cis-2-butene (2.2) > trans-2-butene (<0.1). Alkene coordination
is inhibited by sterically bulky substituents on the alkene but is greatly enhanced by electron-donating
groups and the â-Si effect. Compounds 12a-l undergo two dynamic processes: reversible alkene
decomplexation via associative substitution of a CD2Cl2 molecule, and rapid rotation of the alkene around
the metal-(alkene centroid) axis.
Introduction
Zirconocene-catalyzed polymerization of alkenes is an in-
dustrially important reaction that enables the efficient synthesis
of polyolefins with exquisite and predictable control of polymer
structure.1 Chain growth in this process occurs by coordination
of an alkene to a d0 Cp2ZrR+ metal-alkyl species, followed
by insertion into the metal-alkyl bond,2 as shown in Scheme
1. The intermediate Cp2Zr(R)(alkene)+ species have not been
detected, and computational studies predict that the alkene
insertion step has a low barrier or is near barrierless.3,4
Analogous d0 metal-hydride-alkene species are likely inter-
mediates in â-H elimination, an important chain transfer process
(Scheme 1). Studies of model d0 metal-alkene complexes may
provide insight into many features of polymerization processes,
including catalyst structure/performance relationships, insertion
regiochemistry, tacticity control, and monomer reactivity trends
in copolymerization.
Figure 1. Dewar-Chatt model of metal-alkene bonding in dn metal
complexes.
Scheme 1
The Dewar-Chatt model of dn metal-alkene bonding (n g
2) comprises two components: σ-donation from the alkene π
orbital to a metal-acceptor orbital, and π-back-bonding from a
metal d orbital to the alkene π* orbital (Figure 1).5 The d-π*
back-bonding strengthens the metal-alkene interaction and
results in symmetrical alkene coordination, i.e., nearly identical
M-Cvinyl distances, even for unsymmetrical alkenes.6 For d0
metals, d-π* back-bonding is not possible,7 and therefore
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9
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J. AM. CHEM. SOC. 2006, 128, 8162-8175
10.1021/ja0575225 CCC: $33.50 © 2006 American Chemical Society

Nonchelated d0 Zr
−Alkoxide−Alkene Complexes
A R T I C L E S
Chart 1
Chart 2
insertion. Casey and Bercaw described chelated d0 zirconium-
alkyl-alkene complexes that incorporate SiMe2 units in the
chelate arm to enhance coordination and prevent â-elimination
reactions (G), and have studied the effects of alkene and Cp
ligand substitution on the dynamics of these compounds.11 Royo
reported d0 zirconium and hafnium-alkyl-alkene complexes
in which a â-Si-substituted alkene is tethered to a Cp or indenyl
ligand (H).12 The NMR properties of E-H are similar to those
of the structurally characterized examples A and B,8 which
suggests that unsymmetrical alkene coordination and concomi-
tant polarization of the CdC bond may be a general feature of
d0 metal-alkene complexes.13
As chelation may influence the structure, dynamics, and
reactivity of d0 metal-alkene complexes, it is desirable to study
nonchelated systems to probe the fundamental features of the
d0 metal-alkene interaction. Hessen14 and Green15 reported
cationic vanadium- and niobium-imido-alkene complexes (I
and J, Chart 2). The NMR coordination shifts of the propylene
adducts of VV and NbV are similar to those of A and B, again
suggesting similar unsymmetrical alkene coordination. Kress
observed several related tungsten-alkylidene-cycloalkene com-
plexes (K) at low temperature.16
metal-alkene binding will be weak, and the complexes will be
susceptible to ligand substitution reactions. The rapid insertion
reactions and low metal-alkene bond strengths of d0 metal-
alkene complexes make these species challenging targets for
study.
Jordan and co-workers reported detailed studies of chelated
zirconium and titanium-alkene complexes LnM(OCMe2CH2-
CH2CHdCH2)+ (LnM ) Cp2Zr (A); LnM ) rac-(EBI)Zr (B);
EBI ) 1,2-(1-indenyl)2ethane; LnM ) (C5H4SiMe2NtBu)Ti (C);
LnM ) (C5Me4SiMe2NtBu)Ti (D)) which are shown in Chart
1.8 An X-ray crystallographic analysis of B showed that the
alkene is coordinated in a highly unsymmetrical fashion that is
essentially an η1-alkene binding mode. For this species, d(Zr-
Cterm) ) 2.634(5) Å, d(Zr-Cint) ) 2.819(4) Å, and ∆d ) d(Zr-
Cint) - d(Zr-Cterm) ) 0.185(6) Å. The structure of A is similar.
Key NMR parameters for A-D include large downfield
coordination shifts for Hint (∆δ ) δcoord - δfree ) 0.7-1.6)
and Cint (∆δ ) 18-25), and a large upfield coordination shift
for Cterm (∆δ ) -11 - -22) with respect to the corresponding
LnM(OCMe2CH2CH2CHdCH2)(THF)+ complexes in which the
alkene is not coordinated. These results imply that the alkene
is polarized with positive charge buildup on Cint and negative
charge buildup on Cterm as a result of the unsymmetrical binding.
Nonchelated d0 metal-alkyl-alkene species are very rare.
Casey studied the coordination of propylene, 1-butene, and
(10) (a) Casey, C. P.; Klein, J. F.; Fagan, M. A. J. Am. Chem. Soc. 2000, 122,
4320. (b) Casey, C. P. Fagan, M. A.; Hallenbeck, S. L. Organometallics
1998, 17, 287. (c) Casey, C. P.; Fisher, J. J. Inorg. Chim. Acta 1998, 270,
5. (d) Casey, C. P.; Hallenbeck, S. L.; Wright, J. M.; Landis, C. R. J. Am.
Chem. Soc. 1997, 119, 9680. (e) Casey, C. P.; Hallenbeck, S. L.; Pollock,
D. W.; Landis, C R. J. Am. Chem. Soc. 1995, 117, 9770.
(11) (a) Brandow, C. G.; Mendiratta, A.; Bercaw, J. E. Organometallics 2001,
20, 4253. (b) Casey, C. P.; Carpenetti, D. W., II; Sakurai, H. Organome-
tallics 2001, 20, 4262.
(12) (a) Mart´ınez, G.; Royo, P. Organometallics 2005, 24, 4782. (b) Cano, J.;
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345, 15. (c) Galakhov, M. V.; Heinz, G.; Royo, P. Chem. Commun. 1998,
17.
1
The JCH coupling constants of the vinyl unit are essentially
unchanged by coordination, which indicates that the alkene
structure is not significantly perturbed by coordination.
Casey studied chelated neutral d0 yttrium-alkyl-alkene
complexes (E)9 and cationic zirconium-alkyl-alkene com-
plexes (F),10 in which the chelate ring size prevents alkene
(13) Other chelated d0 metal-alkene complexes: (a) Horton, A. D.; Orpen, A.
G. Organometallics 1992, 11, 8. (b) Erker, G. Acc. Chem. Res. 2001, 34,
309. (c) Dahlmann, M.; Erker, G.; Bergander, K. J. Am. Chem. Soc. 2000,
122, 7986. (d) Karl, J.; Dahlmann, M.; Erker, G.; Bergander, K. J. Am.
Chem. Soc. 1998, 120, 5643. (e) Karl, J.; Erker, G. J. Mol. Catal. A: Chem.
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J.; Huffman, J. C.; Mindiola, D. J. Organometallics 2005, 24, 1886. (g)
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80. (h) Yoshida, M.; Jordan, R. F. Organometallics 1997, 16, 4508. (i)
Evans, W. J.; Kozimor, S. A.; Brady, J. C.; Davis, J. C.; Nyce, G. W.;
Seibel, C. A.; Ziller, C. A.; Doedens, R. J. Organometallics 2005, 24, 2269.
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Soc. 1997, 119, 10561.
(7) (a) Back-bonding from a metal-ligand bonding orbital is possible. For
this effect in d0 metal-carbonyl complexes, see: (b) Guo, Z.; Swenson,
D. C.; Guram, A. S.; Jordan, R. F. Organometallics 1994, 13, 766. (c)
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5867. (b) Carpentier, J. F.; Wu, Z.; Lee, C. W.; Stro¨mberg, S.; Christopher,
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J.-F.; Maryin, V. P.; Luci, J.; Jordan, R. F. J. Am. Chem. Soc. 2001, 123,
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9
J. AM. CHEM. SOC. VOL. 128, NO. 25, 2006 8163

A R T I C L E S
Stoebenau and Jordan
-
1-hexene to Cp*2YR (Cp* ) C5Me5).17 Low-temperature NMR
spectra of mixtures of Cp*2YR and alkenes contain one set of
exchange-averaged alkene signals that shift in same directions
as observed for chelated yttrium-alkene compounds.10 These
results imply that Cp*2YR(alkene) adducts (L) are formed and
undergo fast intermolecular exchange with free alkene on the
NMR time scale. Resconi characterized an [(indenyl)2ZrMe]-
[indole‚B(C6F5)3] ion pair (M), which NMR evidence implies
is held together by η2-CdC indole coordination.18
Scheme 2. Anion ) B(C6F5)4
In addition to these d0 transition metal-alkene complexes,
other alkene complexes in which π-back-bonding is absent have
been reported, including chelated metal-alkene adducts of alkali
metals,19 alkaline earth metals,20 SmIII,21 and Al, Ga, and In,22
as well as norbornyl-type cations in which an alkene is
coordinated to Si, Ge, Sn, or Pb.23 The ion pair [Bu3Sn][H2Cd
CHCH2B(C6F5)3], which is generated from B(C6F5)3 and Bu3-
SnCH2CHdCH2, may be viewed as an alkene complex of
Bu3Sn+ and exhibits NMR properties that are similar to those
of d0 transition metal-alkene complexes.24 Metal-alkene
complexes in which back-bonding is very weak, such as AgI
systems,25 are also known.26
This paper describes the generation and properties of non-
chelated Cp′2Zr(OtBu)(alkene)+ (Cp′ ) C5H4Me) and Cp2Zr-
(OtBu)(alkene)+ complexes. These species are models for the
(C5R5)2Zr(polymeryl)(alkene)+ intermediates in Zr-catalyzed
alkene polymerization reactions.27
species and characterization of exchange processes. The B(C6F5)4-
anion was chosen for its weak coordinating ability29 and low
reactivity with zirconocene cations.30
Results
Targets. The objective of this work was to study nonchelated
alkene complexes of the d0-metal species (C5H4R)2Zr(OtBu)+
(R ) H, Me), to probe the ZrIV-alkene interaction. The tert-
butoxide ligand was used to provide stable, electrophilic
zirconocene species that cannot undergo alkene insertion, due
to the strong Zr-O bond,28 and to enable direct comparisons
with the chelated systems A and B.8 The Cp′ ligand provides a
convenient symmetry probe to facilitate identification of reactive
(C5H4R)2Zr(OtBu)Me Complexes. The reaction of Cp′2-
ZrMe2 (1) with excess tBuOH in benzene produces Cp′2Zr(Ot-
Bu)Me (2) within 15 min at 22 °C (Scheme 2).31 No further
reaction of 2 with tBuOH to give Cp′2Zr(OtBu)2 (3) occurs after
3 d under these conditions. However, treatment of 2 with
tBuOH-d9 resulted in an equilibrium mixture of 2, Cp′2Zr(OC-
(CD3)3)Me (2-d9), tBuOH, and tBuOH-d9 within 1 h (K1 ) 0.99-
(4), C6D6, 22 °C).32 This result shows that protonolysis of the
Zr-OtBu bond of 2 is favored over protonolysis of the Zr-
CH3 bond, probably due to the availability of the lone pairs on
the alkoxide ligand. Similar kinetic basicity effects have been
observed in reactions of zirconium alkoxides and zirconium
alkyls with hydroxylated Al and Si surfaces, and in the reaction
of MeAl(O-2,6-tBu-4-Me-Ph)2 with H2O.33 The analogous
compound Cp2Zr(OtBu)Me (4) was generated from Cp2ZrMe2
(5) on an NMR scale by the procedure used for 2. Compounds
2 and 4 are Cs symmetric in solution, and exhibit OCMe3 13C
NMR signals at δ 76.9 and 77.1, respectively.
(17) (a) Casey, C. P.; Tunge, J. A.; Lee, T.-Y.; Fagan, M. A. J. Am. Chem. Soc.
2003, 125, 2641. (b) Casey, C. P.; Lee, T.-Y.; Tunge, J. A.; Carpenetti, D.
W., II. J. Am. Chem. Soc. 2001, 123, 10762.
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Macromol. Chem. Phys. 2004, 205, 319.
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G. W. Chem. Commun. 2001, 1858.
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2004, 43, 6208.
(21) Evans, W. J.; Perotti, J. W.; Brady, J. C.; Ziller, J. W. J. Am. Chem. Soc.
2003, 125, 5204.
(22) (a) Hata, G. Chem. Commun. 1968, 7. (b) Dolzine, T. W.; Oliver, J. P. J.
Am. Chem. Soc. 1974, 96, 1737. (c) Pietryga, J. M.; Jones, J. N.; Mullins,
L. A.; Wiacek, R. J.; Cowley, A. H. Chem. Commun. 2003, 2072.
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Schleyer, P. Angew. Chem., Int. Ed. Engl. 1997, 36, 626. (b) Mu¨ller, T.;
Bauch, C.; Bolte, M.; Auner, N. Chem. Eur. J. 2003, 9, 1746. (c) Mu¨ller,
T.; Bauch, C.; Ostermeier, M.; Bolte, M.; Auner, N. J. Am. Chem. Soc.
2003, 125, 2158.
(C5H4R)2Zr(OtBu)+ Species. Compounds 2 and 4 react with
[Ph3C][B(C6F5)4]30b in C6D5Cl or C6H6 within 15 min at 22 °C
to yield cationic Zr-alkoxide species [(C5H4R)2Zr(OtBu)]-
[B(C6F5)4] (6: R ) Me; 7: R ) H), as shown in Scheme 2.34
(24) (a) Blackwell, J. M.; Piers, W. E.; McDonald, R. J. Am. Chem. Soc. 2002,
124, 1295. (b) Blackwell, J. M.; Piers, W. E.; Parvez, M. Org. Lett. 2000,
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Dias, H. V. R.; Wang, Z.; Jin, W. Inorg. Chem. 1997, 36, 6205. (c) Dias,
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(27) For preliminary communications, see: (a) Stoebenau, E. J., III; Jordan, R.
F. J. Am. Chem. Soc. 2003, 125, 3222. (b) Stoebenau, E. J., III; Jordan, R.
F. J. Am. Chem. Soc. 2004, 126, 11170.
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Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 1, 245.
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(32) K1 ) [2-d9][tBuOH][2]-1[tBuOH-d9]-1
.
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9
8164 J. AM. CHEM. SOC. VOL. 128, NO. 25, 2006

Nonchelated d0 Zr
−Alkoxide−Alkene Complexes
A R T I C L E S
Scheme 3
Compound 6 was isolated as a pale-yellow, base-free solid by
washing with benzene to remove Ph3CMe, followed by washing
with hexanes and vacuum drying. Complex 7 was generated
and used in situ. The 13C NMR OCMe3 resonances in C6D5Cl
of 6 (δ 84.1) and 7 (δ 84.6) are shifted ca. 7 ppm downfield
from the corresponding signals of 2 and 4.
Figure 2. ORTEP view (50% probability ellipsoids) of the structure of
the (Cp2Zr)2(µ-OH)22+ dication of 8. Hydrogen atoms on the Cp rings are
omitted.
Compound 6 is a weaker Lewis acid than the related alkyl
cation Cp′2ZrMe+. Competition experiments show that THF
preferentially binds to Cp′2ZrMe+ over 6.35 In addition, the
dinuclear intermediate {Cp′2Zr(OtBu)}2(µ-Me)+ is not observed
when reactions of 2 and Ph3C+ are monitored by NMR
spectroscopy,36 while {Cp′2ZrMe}2(µ-Me)+ is observed in the
reactions of 1 with Ph3C+.37
Compound 6 is very stable in C6D5Cl solution at 22 °C (no
decomposition after 7 d) and in the solid state at -35 °C (no
decomposition after 1 year). Compound 6 is also stable for at
least 8 h at -89 °C in CD2Cl2, but decomposes completely
within 17 h at 22 °C in this solvent to unknown products.
Compound 7 is less stable than 6 and decomposes completely
within 5 d at 22 °C in C6D5Cl solution to poly(isobutene) and
the dinuclear dicationic complex [{Cp2Zr}2(µ-OH)2][B(C6F5)4]2
(8), which separates as yellow crystals. Compound 8 forms by
net isobutene elimination from 7, as shown in Scheme 3. The
isobutene is cationically polymerized under the reaction condi-
tions, and Cp2Zr(OH)+ dimerizes to 8. A similar decomposition
reaction was observed for Cp2Zr(StBu)(THF)+.38
Molecular Structure of {Cp2Zr}2(µ-OH)22+. The structure
of 8 was determined by X-ray crystallography. The dication
consists of two normal bent zirconocene units linked by two
bridging hydroxide groups (Figure 2).39 The Zr2O2 core is planar.
The Zr-O bond lengths (2.161(3) and 2.158(2) Å) are similar
those in other complexes with Zr2(µ-OH)2 cores, such as {Cp2-
Zr(OCOCF3)(µ-OH)}2 (9), (Cp2Zr{imidazole‚B(C6F5)3})2(µ-
OH)2 (10), and [{Cp2Zr(NCnPr)}2(µ-OH)2][BPh4]2 (11).40 The
O-Zr-O angle in 8 (72.0(1)°) is 6-7° larger, and the Zr-
Figure 3. Possible solution structures for 6.
O-Zr angle (108.0(1)°) is 6-7° smaller than the corresponding
angles in 9-11, where an additional ligand is present.40a-c There
is a close contact (2.24 Å) between the hydroxide proton and a
m-fluorine of the anion in 8, which is less than the sum of the
hydrogen and fluorine van der Waals radii (2.67 Å).41
Solution Structure of (C5H4R)2Zr(OtBu)+ Cations. The 1H
and 13C NMR spectra of 6 in CD2Cl2 each contain two Cp′ CH
signals, even at -89 °C, indicative of C2V symmetry. This result
suggests four possible solution structures for 6. First, 6 may
contain a three-coordinate Cp′2Zr(OtBu)+ cation in which the
alkoxide ligand occupies the central coordination site in the
zirconocene wedge (N, Figure 3), similar to Cp*2Sm(O-2,3,5,6-
Me4-Ph),42 or exchanges rapidly between the sides of the
zirconocene wedge. Second, 6 may be a contact ion pair (O),
which undergoes fast site epimerization at Zr (i.e. fast exchange
of the tert-butoxide ligand and anion between the sides of the
zirconocene wedge). Coordination of B(C6F5)4- to electrophilic
Zr, Th, Al, and Sc cations is known.43 Third, 6 may be a solvent
adduct (P), which undergoes rapid site epimerization at Zr.
Several d0 metal-haloarene complexes have been characterized,
including Cp2Zr(CH2Ph)(ClPh)+, Cp*2ZrCl(ClPh)+, Cp*2Sc-
(34) For related (C5R5)2Zr(OR)(L)+ species, see: (a) Collins, S.; Koene, B. E.;
Ramachandran, R.; Taylor, N. J. Organometallics 1991, 10, 2092. (b) Hong,
Y.; Kuntz, B. A.; Collins, S. Organometallics 1993, 12, 964. (c) Hong,
Y.; Norris, D. J.; Collins, S. J. Org. Chem. 1993, 58, 3591. (d) Jordan, R.
F.; Dasher, W. D.; Echols, S. F. J. Am. Chem. Soc. 1986, 108, 1718.
(35) For the 6 + Cp′2ZrMe(THF)+ / Cp′2Zr(OtBu)(THF)+ + Cp′2ZrMe+
equilibrium, K2 ) [ZrOR(THF)][ZrMe][6]-1[ZrMe(THF)]-1 ) 0.34(6) in
C6D5Cl at 22 °C.
(40) (a) Klima, S.; Thewalt, U. J. Organomet. Chem. 1988, 354, 77. (b) Ro¨ttger,
D.; Erker, G.; Fro¨hlich, R.; Kotila, S. J. Organomet. Chem. 1996, 518, 17.
(c) Aslan, H.; Eggers, S. H.; Fischer, R. D. Inorg. Chim. Acta 1989, 159,
55. (d) Martin, A.; Uhrhammer, R.; Gardner, T. G.; Jordan, R. F.; Rogers,
R. D. Organometallics 1998, 17, 382.
(41) Porterfield, W. W. Inorganic Chemistry: A Unified Approach, 2nd ed.;
Academic Press: San Diego, 1993; p 214.
(42) Evans, W. J.; Hanusa, T. P.; Levan, K. R. Inorg. Chim. Acta 1985, 110,
191.
(43) (a) Goodman, J. T.; Schrock, R. R. Organometallics 2001, 20, 5205. (b)
Yang, X.; Stern, C. L.; Marks, T. J. Organometallics 1991, 10, 840. (c)
Korolev, A. V.; Delpech, F.; Dagorne, S.; Guzei, I. A.; Jordan, R. F.
Organometallics 2001, 20, 3367. (d) Bouwkamp, M. W.; Budzelaar, P. H.
M.; Gercama, J.; Del Hierro Morales, I.; de Wolf, J.; Meetsma, A.;
Troyanov, S. I.; Teuben, J. H.; Hessen, B. J. Am. Chem. Soc. 2005, 127,
14310.
(36) {Cp′2Zr(OtBu)}2(µ-Me)+ can be generated from a 2:1 mixture of 2 and
[Ph3C][B(C6F5)4] in C6D5Cl.
(37) (a) Bochmann, M.; Lancaster, S. J. Angew. Chem., Int. Ed. Engl. 1994,
33, 1634. (b) Wu, F.; Dash, A. K.; Jordan, R. F. J. Am. Chem. Soc. 2004,
126, 15360.
(38) Piers, W. E.; Koch, L.; Ridge, D. S.; MacGillivray, L. R.; Zaworotko, M.
Organometallics 1992, 11, 3148.
(39) Cardin, D. J.; Lappert, M. F.; Raston, C. L. Chemistry of Organo-Zirconium
and -Hafnium Compounds; Ellis Horwood Ltd: Chichester, U.K., 1986;
pp 68-75.
9
J. AM. CHEM. SOC. VOL. 128, NO. 25, 2006 8165

A R T I C L E S
Stoebenau and Jordan
(FPh)2+, and Cp*2Sc(κ2-o-F2C6H4)+.37b,43d,44 Finally, 6 may be
a dinuclear dication with two bridging tert-butoxide ligands (Q)
analogous to 8. Dinuclear yttrium and chromium complexes with
µ-OtBu ligands are known,45 and smaller alkoxides can bridge
between Zr centers.40d
temperature. Addition of isobutene or styrene to 6 results in
rapid cationic polymerization of the alkene at -78 °C, and the
target Zr-isobutene or Zr-styrene adducts were not detected.46
In the case of tert-butyl vinyl ether, cationic polymerization
competes with π-complex formation, but the π-complex 12f
was observed. Finally, trace oxidation of vinylferrocene occurs
during the formation of 12h, probably due to the presence of
trace Ph3C+ in 6.47
NMR Properties and Solution Structures of Cp′2Zr(OtBu)-
(alkene)+ Complexes. (A) General Features. The 1H and 13C-
{1H} NMR spectra of 12a,j,k, which contain symmetrical
alkenes, each contain four Cp′ CH resonances, one Cp′ ipso-C
resonance, and one Cp′Me resonance, consistent with Cs-
symmetric structures. The 1H and 13C spectra of 12a contain a
single coordinated-ethylene resonance, even at -89 °C. This
result implies that the bound ethylene rotates around the Zr-
(ethylene centroid) axis rapidly on the NMR time scale. The
NMR spectra of 12j,k contain one set of dCHR resonances
(i.e. the ends of the alkene are equivalent) which, along with
the Cs symmetry, is consistent with fast rotation around the Zr-
(alkene centroid) axis or a static structure in which the alkene
CdC bond is perpendicular to the O-Zr-(alkene centroid)
plane, which is unlikely on steric grounds.48
The reaction of a 1:1 mixture of 2 and 4 with [Ph3C]-
[B(C6F5)4] (1 equiv based on total Zr) gives only 6 and 7. No
new resonances or line broadening effects are observed in NMR
spectra of the product mixture that can be ascribed to a mixed
Cp2Zr(µ-OtBu)2ZrCp′22+ complex, which would be expected to
form if 6 and 7 contained dinuclear dications. This result implies
that 6 and 7 do not contain dinuclear dications of type Q.
Differentiation between structures N-P is more difficult.
Addition of PhCl or C6D5Cl to CD2Cl2 solutions of 6 at -89
1
°C leads to selective shifts in the H NMR Cp′-CH and OtBu
resonances of 6. Moreover, the coordination of ethylene to 6 in
CD2Cl2 solution is perturbed by addition of PhCl but is not
affected by addition of [Ph3C][B(C6F5)4] as an anion source
(vide infra). Finally, the 19F NMR spectrum of 6 shows no
evidence of anion coordination to Zr down to -89 °C, although
the effects of weak, labile anion coordination on the 19F NMR
spectrum may be subtle. Collectively, these results are most
consistent with 6 and 7 existing as halocarbon adducts of type
P in halocarbon solution.
Generation of Cp′2Zr(OtBu)(alkene)+ Complexes. The
addition of excess alkene to a CD2Cl2 solution of 6 at low
temperature produces an equilibrium mixture of [Cp′2Zr(OtBu)-
(alkene)][B(C6F5)4] (12a-l), 6, and free alkene, as shown in
eq 1. In a typical experiment, the alkene was added by vacuum
transfer to a frozen solution of 6 in CD2Cl2 in an NMR tube.
The tube was thawed, stored at -78 °C, and transferred to a
precooled NMR probe with minimal transfer time (<1 min) to
minimize side reactions. Alkene complexes 12a-l were char-
acterized by NMR spectroscopy. These species are thermally
sensitive, which precluded isolation or ESI-MS analysis.
The NMR spectra of 12b-h, which contain monosubstituted
alkenes, each contain eight Cp′ CH resonances, two Cp′ ipso-C
resonances, and two Cp′Me49 resonances at -89 °C, charac-
teristic of C1 symmetry. The Cp′ rings are diastereotopic due
to the chiral center at Cint of the bound alkene. The NMR spectra
of 12b-h contain a single set of alkene resonances, which is
consistent with the presence of a single rotamer or, more likely,
fast metal-(alkene centroid) rotation.48
(B) Monosubstituted Alkene Adducts. The alkene 13C NMR
resonances of monosubstituted alkene complexes 12b-h display
characteristic coordination shifts that are listed in Table 1. The
Cint resonances shift downfield by 15-30 ppm, while the Cterm
resonances shift upfield by 10-25 ppm, compared to the free
alkene positions. The vinyl region of the 13C{1H} NMR
spectrum of propylene adduct 12b is shown in Figure 4 and
illustrates the divergent coordination shifts of the two vinyl
resonances.
1
The H NMR resonances for the vinyl units of 12b-h are
also strongly perturbed by coordination, as summarized in Table
1. The Hint signals shift far downfield (∆δ ) 1.5-1.8) upon
coordination, while the Htrans and Hcis resonances display smaller
1
coordination shifts. The JCH coupling constants of the vinyl
units of 12b-h are only slightly perturbed by coordination
(Table 2), and the vinyl nJHH coupling constants are essentially
unchanged the from free alkene values.
The NMR properties of 12b-h are very similar to those of
structurally characterized chelated ZrIV-alkoxide-alkene com-
plexes A and B,8 and thus imply that the alkene is bound in a
similar unsymmetrical fashion (i.e. d(Zr-Cint) > d(Zr-Cterm))
and is polarized with positive charge buildup on Cint and
negative charge buildup on Cterm, as shown in Figure 5. The
Side reactions complicated studies of the binding of certain
alkenes to 6. Ethylene polymerization occurs above -40 °C in
the presence of 6, most likely due to the presence of trace Zr-
alkyl species, though adduct 12a could be studied up to this
(46) η2-Isobutene and η2-styrene adducts of VV are known. Witte, P. T.
Vanadium Complexes Containing Amido Functionalized Cyclopentadienyl
Ligands. Ph.D. Thesis, University of Groningen, Groningen, The Nether-
lands, 2000.
(47) Trace oxidation of ferrocene also occurs in the presence of 6 under similar
conditions (CD2Cl2, -91 °C). It is probable that trace Ph3C+ is the oxidant.
(48) To date, NOESY spectra have not enabled clarification of this issue.
(49) The 1H NMR Cp′Me resonances of 12b-h often overlap.
(44) Bochmann, M.; Jaggar, A. J.; Nicholls, J. C. Angew. Chem., Int. Ed. Engl.
1990, 29, 780.
(45) (a) Evans, W. J.; Boyle, T. J.; Ziller, J. W. Organometallics 1993, 12,
3998. (b) Chisholm, M. H.; Cotton, F. A.; Extine, M. W.; Rideout, D. C.
Inorg. Chem. 1979, 18, 120.
9
8166 J. AM. CHEM. SOC. VOL. 128, NO. 25, 2006

Nonchelated d0 Zr
−Alkoxide−Alkene Complexes
A R T I C L E S
Figure 4. Partial 13C{1H} NMR spectrum of an equilibrium mixture of 12b, 6, and free propylene (CD2Cl2, -89 °C). Propylene coordination shifts are
shown by arrows. Assignments: δ 154 (12b Cint), 147 (d, anion), 137 (d, anion), 136 (d, anion), 134 (C3H6 Cint), 129 (6 Cipso), 127 (2 s, 12b Cipso), 123 (br,
anion), 117 (4 s, 12b Cp′ CH), 117 (large s, 6 Cp′ CH), 115 (C3H6 Cterm), 114 (6 Cp′ CH), 114-111 (4 s, 12b Cp′ CH), 103 (12b Cterm).
Table 1. NMR Coordination Shifts (∆δ) for Monosubstituted
Alkenes Coordinated to 6a
cpd
alkene
Cint
Cterm
Callylic
Hint
Htrans
Hcis
12b propylene
12c 1-hexene
19.6 -12.4 3.0 1.55
17.5 -11.9 2.4 1.53
16.7 -11.3 2.2 1.46
0.37
0.41
0.38
0.38
0.14
0.12
xc
Figure 6. Resonance stabilization of Cp′2Zr(OtBu)(η2-H2CdCHOtBu)+
(12f).
12d 1,5-hexadieneb
12e 4,4-Me2-1-pentene 18.8 -11.7 1.5 1.46
12f t-butyl vinyl ether 32.5 -23.8
0.24
s
1.58 -0.26 -0.21
0.00 0.00
1.77 -0.56 -0.52
12g allyltrimethylsilane 31.6 -19.7 9.0 1.80
12h vinylferrocene
30.3 -27.8
s
a
In CD2Cl2, -89 °C; ∆δ ) δcoord - δfree.
b Values for the bound vinyl
unit are shown; values for the pendant vinyl unit are essentially unchanged
from free substrate values. c Resonance not detected due to overlap with
free alkene resonance.
1
Table 2. JCH Coupling Constants (Hz) for Monosubstituted
Alkenes Coordinated to 6a
cpd
alkene
C
int coord
free
Cterm coord
free
12b
12c
12d
12e
12f
propylene
1-hexene
156
156
155
150
173
150
156
153
151
150
150
174
150
155
156
156
157
150
154
153
153
155
155
155
156, 154
161, 155
157, 153
159, 155
1,5-hexadieneb
4,4-Me2-1-pentene
tert-butyl vinyl ether
allyltrimethylsilane
vinylferrocene
Figure 7. Resonance stabilization of Cp′2Zr(OtBu)(H2CdCHC5H4FeCp)+
(12h).
12g
12h
a Values from 13C{gated-1H} NMR spectra; in CD2Cl2, -89 °C.
b Values for the bound vinyl unit are shown; values for the pendant vinyl
unit are essentially unchanged from free substrate values.
Figure 8. â-Si stabilization of Cp′2Zr(OtBu)(H2CdCHCH2SiMe3)+ (12g).
coordination shifts. Enhanced polarization of the alkene CdC
bond in 12f-h due to resonance effects (Figures 6 and 7)51 or
â-Si hyperconjugation (Figure 8),52 which stabilizes the partial
positive charge on Cint, explains these coordination shifts. These
results support the proposed unsymmetrical binding and alkene
polarization model.
Figure 5. Polarization of the CdC bond of the alkene in Cp′2Zr(OtBu)-
(alkene)+ complexes.
CdC polarization explains the downfield coordination shifts of
the Cint, Hint, and Callylic resonances and the upfield coordination
1
shifts of the Cterm resonances. The JCH values imply that the
(C) Symmetrical Alkene Adducts. The alkene NMR coor-
dination shifts of 12a,j,k are listed in Table 3. The Cvinyl and
vinyl units of 12b-h are not significantly structurally distorted
(i.e. the hybridization at Cint and Cterm is not significantly
perturbed) by coordination.50
As summarized in Table 1, the coordination shifts for the
Cint, Cterm, and Hint of the tert-butyl vinyl ether (12f), vinylfer-
rocene (12h), and allyltrimethylsilane (12g) adducts are larger
than those for simple alkene complexes 12b-e. Additionally,
the Htrans and Hcis resonances of 12f-h are shifted less downfield
than those in 12b-e, and in some cases exhibit upfield
Callylic resonances exhibit quite small coordination shifts, while
the Hvinyl and Hallylic resonances show moderate downfield
1
coordination shifts.53 The vinyl JCH coupling constants of
12a,j,k are unperturbed from the free alkene values.
(51) For stabilization of positive charge by a ferrocenyl substituent, see: (a)
Ceccon, A.; Giacometti, G.; Venzo, A.; Paolucci, D.; Benozzi, D. J.
Organomet. Chem. 1980, 185, 231. (b) Allenmark, S. Tetrahedron Lett.
1974, 15, 371. (c) Behrens, U. J. Organomet. Chem. 1979, 182, 89. (d)
Prakash, G. K. S.; Buchholz, H.; Reddy, V. P.; de Meijere, A.; Olah, G.
A. J. Am. Chem. Soc. 1992, 114, 1097. (e) Koch, E.-W.; Siehl, H.-U.;
Hanack, M. Tetrahedron Lett. 1985, 26, 1493. (f) Siehl, H.-U.; Kaufmann,
F.-P.; Apeloig, Y.; Braude, V.; Danovich, D.; Berndt, A.; Stamatis, N.
Angew. Chem., Int. Ed. Engl. 1991, 30, 1479. (g) Kreindlin, A. Z.;
Dolgushin, F. M.; Yanovsky, A. I.; Kerzina, Z. A.; Petrovskii, P. V.;
Rybinskaya, M. I. J. Organomet. Chem. 2000, 616, 106. (h) Koridze, A.
A.; Astakhova, N. M.; Petrovskii, P. V. J. Organomet. Chem. 1983, 254,
345.
(50) An alternative formulation of 12a-l as insertion products with chelated
ether ligands, i.e., Cp′2Zr(κ2-C,O-CH2CHROtBu)+ alkoxymetallocyclobu-
tane complexes, can be safely rejected. Related neutral azametallocyclobu-
tane complexes formed by alkene addition to Zr-imido species exhibit
very large upfield coordination shifts for the “alkene” resonances, and much
higher “metal-alkene” rotation barriers. Walsh, P. J.; Hollander, F. J.;
Bergman, R. G. Organometallics 1993, 12, 3705.
9
J. AM. CHEM. SOC. VOL. 128, NO. 25, 2006 8167

A R T I C L E S
Stoebenau and Jordan
Table 3. NMR Coordination Shifts (∆δ) for Symmetrical Alkenes
Coordinated to 6a
The coordination of ethylene to 6 to give 12a was probed in
detail. The equilibrium constant for ethylene coordination is Keq
) 7.0(6) M-1 in CD2Cl2 at -89 °C. Under these conditions, at
[6]initial ) 0.040 M-1 and [C2H4]initial ) 0.20 M-1, ca. 55% of
the total Cp′2Zr(OtBu)+ exists as 12a. This equilibrium constant
is unchanged over the concentration ranges [6]initial ) 0.038-
0.080 M-1 and [C2H4]initial ) 0.13-0.29 M-1, consistent with
the equilibrium shown in eq 1. Addition of [Ph3C][B(C6F5)4]
(2.4 equiv per total Zr) as an anion source does not affect Keq
at -89 °C. If 6 existed as the ion-pair structure O (Figure 3),
cpd
alkene
Cvinyl
Callylic
Hvinyl
Hallylic
12a
12j
12k
ethylene
cis-2-butene
cyclopentene
-3.8
0.2
1.8
-
3.7
1.7
0.56
0.99
0.78
-
0.40
0.34
a In CD2Cl2 at -89 °C; ∆δ ) δcoord - δfree
.
Table 4. Equilibrium Constants for Alkene Coordination to 6a
1
cpd
alkene
K
eq (M-
)
-
12h
12f
12g
12a
12i
12b
12c
12d
12j
12k
12e
12l
-
H2CdCHFcb
4800(1100)
largec
1700(400)
7.0(6)
6.7(3)
5.4(2)
4.8(8)
3.7(1.1)
2.2(1)
2.1(1)
1.9(1)
e0.1d
xe
addition of B(C6F5)4 would shift the equilibrium in eq 1 to
H2CdCHOtBu
H2CdCHCH2SiMe3
H2CdCH2
- 55
the left, since 12a is unlikely to bind B(C6F5)4
. In contrast,
when PhCl (8 equiv per total Zr) is added to mixtures of 6 and
ethylene at -89 °C in CD2Cl2, Keq is decreased to 5.0(3) M-1
,
H2CdCdCH2
H2CdCHMe
consistent with stronger binding of PhCl vs CD2Cl2 to 6. This
result supports the proposal that 6 exists as a solvent adduct in
RCl solution (P, Figure 3) and that the equilibrium in eq 1
involves substitution of CD2Cl2 by alkene.
H2CdCHnBu
η2-1,5-hexadiene
cis-MeHCdCHMe
cyclopentene
t
H2CdCHCH2 Bu
H2CdCHCl
The data in Table 4 show how steric and electronic effects
influence the binding of alkenes to 6. Alkene binding strength
varies in the order ethylene ≈ R-olefins > cis-alkenes > trans-
alkenes (no coordination observed). There is only a small
difference in the Keq values for ethylene, propylene, and
1-hexene. R-Olefins are better σ-donors but are more sterically
crowded than ethylene, and these effects compensate each other.
However, more dramatic increases in the steric bulk of the
alkene do inhibit binding; e.g. 4,4-dimethyl-1-pentene adduct
12e is significantly less stable than ethylene adduct 12a, and
3,3-dimethyl-1-butene (tert-butylethylene) and vinyltrimethyl-
silane do not bind to 6 at -89 °C. cis-Alkenes show decreased
Keq values due to increased steric crowding near the double
bond, and trans-2-butene does not bind to 6. The reason for
the stronger binding of cis-2-butene vs that of trans-2-butene
to 6 is unknown. The relative binding abilities of cis- and trans-
alkenes are highly system-dependent.11a,56
H2CdCHtBu
H2CdCHSiMe3
trans-MeHCdCHMe
H2CdCHCHdCH2
C6H6
-
xe
-
xe
-
xe
-
xe
-
H2CdCMe2
H2CdCHPh
polyf
-
polyf
a In CD2Cl2 solution, -89 °C; Keq ) [Zr-L][6]-1[L]-1; L ) alkene. b Fc
) ferrocenyl. c Complete coordination observed. See text for details.
d Adduct formation detected by line broadening of vinyl chloride 13C NMR
signals. e No complex formed; Keq e 0.1 M-1 f Cationic polymerization
.
rapidly occurs.
These data can be rationalized by the unsymmetrical coor-
dination/alkene polarization model proposed above for mono-
substituted alkene complexes, coupled with fast exchange of
the two ends of the alkene, e.g. by rotation around the metal-
(alkene centroid) axis. Under slow alkene rotation conditions,
one alkene carbon resonance of an unsymmetrically bound
symmetrical alkene would shift upfield, while the other would
shift downfield, as observed for 12b-h. Exchange of the two
ends of the CdC bond would average these resonances, resulting
in the small, net coordination shifts observed for 12a,j,k.
However, structurally characterized AgI-ethylene adducts,
where back-bonding is minimal, contain symmetrically bound
ethylene ligands.26a-c
Electron-donating groups on the alkene significantly enhance
alkene coordination to 6. tert-Butyl vinyl ether binds to 6 very
strongly to give 12f, in which the partial positive charge on
Cint is stabilized by the OtBu group (Figure 6). In fact, 12f is
stable without excess tert-butyl vinyl ether in solution between
-89 and -69 °C in CD2Cl2. The Keq value for tert-butyl ether
coordination to 6 could not be determined but must be at least
600 times larger than that for the sterically similar alkene 4,4-
dimethyl-1-pentene. However, the bound alkene in 12f is
displaced by PMe3 to give [Cp′2Zr(OtBu)(PMe3)][B(C6F5)4] (13)
and free tert-butyl vinyl ether.
Thermodynamics of Alkene Binding to 6. Equilibrium
constants for alkene binding to 6 in eq 1, Keq ) [12][6]-1
-
[alkene]-1, were determined by 1H NMR and are listed in Table
4.54 The equilibria were studied directly for 12a-e,i-k. Alkene
complexes 12g,h are formed quantitatively when 1 equiv of the
substrate is added to 6, and therefore Keq values in these cases
were determined by competition experiments with propyne and
allyltrimethylsilane, respectively.
Vinylferrocene also binds strongly to 6 to form 12h, despite
the presence of the large ferrocenyl substituent close to the
double bond. In fact, vinylferrocene binds almost 700 times
more strongly to 6 than ethylene does. The partial positive
charge at Cint in 12h is stabilized by the ferrocenyl group (Figure
7).51 Allyltrimethylsilane also binds strongly to 6 to form 12g.
The equilibrium constant for the allyltrimethylsilane coordina-
tion is 900 times larger than that for the sterically similar alkene
4,4-dimethyl-1-pentene. The partial positive charge on Cint in
12g is stabilized by the â-Si group (Figure 8).52
(52) Lambert, J. B. Tetrahedron 1990, 46, 2677.
(53) Cyclopentene adduct 12k exhibits only one Hallylic resonance, whereas two
resonances are expected if rotation around the metal-(alkene centroid) bond
is fast. Most likely the two Hallylic resonances overlap. However, it is also
possible that the Hallylic hydrogens are exchanged by nondissociative alkene
face exchange. This process has been invoked to explain trans-1,3-
enchainment in Zr-catalyzed cyclopentene polymerization. Kelly, W. M.;
Wang, S.; Collins, S. Macromolecules 1997, 30, 3151.
(54) If the solvent term is included, the equilibrium expression for eq 1 is Keq′
) Keq[CD2Cl2] where Keq is defined as in the text. If the solvent
concentration is assumed to be independent of temperature, the value of
∆H° is not affected, but the entropy term becomes ∆S°′ ) ∆S° + R ln-
[CD2Cl2], where R ln[CD2Cl2] ≈ 5.5 eu.
(55) Control experiments confirmed that Ph3C+ does not interact with C2H4 or
6 under these conditions.
(56) (a) Liu, W.; Brookhart, M. Organometallics 2004, 23, 6099. (b) Muhs, M.
A.; Weiss, F. T. J. Am. Chem. Soc. 1962, 84, 4697.
9
8168 J. AM. CHEM. SOC. VOL. 128, NO. 25, 2006

Nonchelated d0 Zr
−Alkoxide−Alkene Complexes
A R T I C L E S
Table 5. Thermodynamic Data for Equilibria in eq 1 and
Activation Parameters for Decomplexation (k-1) for Ethylene and
Propylenea
q
q
q
∆
H
°
∆
S
(eu)
°
∆
H-
∆
S-
(eu)
∆G-
1
1
1
adduct
alkene
(kcal/mol)
(kcal/mol)
(kcal/mol)b
12a
12b
H2CdCH2
H2CdCHMe -3.8(2) -17(1)
-3.6(1) -16(4)
7.7(5)
8.1(9)
-15(2)
-12(4)
11.2(1)
10.9(1)
a In CD2Cl2 solution. b At -39 °C.
Figure 10. Eyring plot for ethylene (circles, solid line) and propylene
(triangles, dashed line) decomplexation (k-1) from 12a,b in CD2Cl2 solution.
The two rightmost data points for each line overlap.
of coordinated ethylene by free ethylene (eq 2) does not occur
to a significant extent. Instead, the data imply that ethylene is
lost from 12a to give 6, and ethylene coordinates to 6 to give
12a (eq 1). An Eyring plot for ethylene decomplexation from
12a is shown in Figure 10, and activation parameters for
ethylene decomplexation are listed in Table 5.
Figure 9. van’t Hoff plot for ethylene (circles, solid line) and propylene
(triangles, dashed line) coordination in 12a,b in CD2Cl2 solution.
The presence of an electron-withdrawing group on the alkene
inhibits alkene binding to 6. When vinyl chloride is added to a
solution of 6 in CD2Cl2 at -89 °C, the only evidence for
coordination is selective line broadening of the vinyl chloride
13C NMR signals, suggestive of minor formation of 12l (Keq e
0.1 M-1) and fast intermolecular exchange of free and bound
vinyl chloride. An alternative formulation of 12l as a Cl-bound
halocarbon adduct cannot be ruled out. Vinyl chloride binds
much more weakly than the sterically similar substrate pro-
pylene.
Enthalpy and Entropy of Ethylene and Propylene Coor-
dination to 6. Keq values for ethylene and propylene coordina-
tion to 6 were determined over a wide temperature range,
enabling determination of binding enthalpies and entropies.
These values are listed in Table 5. As the temperature is raised,
formation of the alkene complexes 12a,b become less favored
and the equilibria shift to free alkene and 6. Representative van’t
Hoff plots are given in Figure 9. The ∆H° and ∆S° values are
both very similar for the two alkenes. The ∆S° values are
modestly negative and would be less negative if a solvent term
were included in the Keq expression.54
Similarly, when an equilibrium mixture of 12b, 6, and free
propylene in CD2Cl2 is warmed above -89 °C, the NMR signals
of 12b broaden and eventually coalesce with those of 6 and
free propylene. First-order rate constants were found by line
shape analysis of the Hint resonance of 12b using gNMR.57
Propylene decomplexation rates are independent of the con-
centration of free propylene over the range [C3H6] ) 0.070-
0.22 M. Therefore, as for ethylene, free propylene does not
directly displace coordinated propylene. The activation param-
eters for propylene decomplexation from 12b (Figure 10 and
Table 5) are very similar to those for ethylene decomplexation
from 12a.
q
The negative ∆S-1 values for ethylene and propylene
decomplexation suggest that these reactions proceed by associa-
tive mechanisms in which the coordinated alkene is displaced
by a CD2Cl2 molecule.
A free energy diagram that compares propylene and ethylene
coordination to 6 at -39 °C is shown in Figure 11. It is clear
that ethylene and propylene interact with the Cp′2Zr(OtBu)+
cation in a nearly identical manner. The reason for this is that
the steric and electronic differences between these two alkenes
are small and exert compensating effects.
Alkene Decomplexation. The NMR resonances of 12a, 6,
and free ethylene in an equilibrium mixture in CD2Cl2 solution
broaden as the temperature is raised above -89 °C. The Cp′
1
CH H NMR signals of 12a coalesce with those of 6 between
-68 and -48 °C, and the coordinated ethylene signal coalesces
with the free ethylene signal at ca. -30 °C. Study of this system
in the fast exchange regime was difficult due to the lopsided
equilibrium above -38 °C. These dynamic effects are consistent
with the exchange of 12a with 6 and of coordinated and free
ethylene.
Electronic effects strongly influence the rate of alkene
decomplexation and parallel the electronic effects on Keq.
Decomplexation of allyltrimethylsilane from 12f is very slow,
with k-1 ) 0.046(3) s-1 at -89 °C in CD2Cl2, as determined
1
from a H EXSY spectrum. In contrast, the rate constants for
First-order rate constants for ethylene decomplexation from
12a (k-1 in eq 1) were determined from the excess line
ethylene and propylene decomplexation under the same condi-
tions are k-1 ) 1.5(1) and 2.3(1) s-1 respectively, as calculated
from the corresponding activation parameters. The tert-butyl
vinyl ether adduct 12f also has a high decomplexation barrier.
1
broadening of the ethylene H NMR signal of 12a under slow
exchange conditions. Ethylene decomplexation rates from 12a
are independent of the free ethylene concentration over the range
[C2H4] ) 0.045-0.23 M-1. Therefore, associative displacement
(57) gNMR, v. 4.1.2; Adept Scientific: Letchworth, U.K., 2000.
9
J. AM. CHEM. SOC. VOL. 128, NO. 25, 2006 8169

A R T I C L E S
Stoebenau and Jordan
Table 6. NMR Coordination Shifts (∆δ) for Vinyl Ethers and
Sulfidesa
cpd
substrate
Cint
Cterm
Hint
Htrans
Hcis
14
15
H2CdCHOEt
H2CdCHSMe
-5.4
-7.1
14.3
15.7
-0.54
<0b
0.77
0.86
0.97
>0b
a In CD2Cl2 at -89 °C; ∆δ ) δcoord - δfree b The resonances for the
.
coordinated substrate are obscured by Cp′ CH resonances, but the signs of
the coordination shifts can be established because the Hint and Hcis resonances
of free methyl vinyl sulfide are downfield and upfield of the entire Cp′ CH
region, respectively.
exchange, i.e. exchange of the Cp′2Zr(OtBu)+ unit between the
two enantiofaces of the alkene without decomplexation.
Alkene enantioface exchange in 12f probably proceeds by
slippage of the alkene to an O-coordination mode (12f′), rotation
around the O-Cint bond, and slippage back to the π-complex
isomer (eq 3). This process is a functional-group-assisted olefin
face exchange.58 As noted below, ethyl vinyl ether, which is
smaller than tert-butyl vinyl ether, forms a stable O-adduct with
6.
Figure 11. Free energy diagram (-39 °C) comparing ethylene and
propylene coordination to 6 in CD2Cl2 solution.
An alternative possible mechanism for alkene face exchange
in 12f is rotation around the CdC double bond, which may be
weakened due to the Cp′2Zr(OtBu)(H2CC+(H)OtBu) carbo-
cation-like resonance form (Figure 6). This kind of mechanism
has been established for CpFe(CO)2(H2CdCHNR2)+ vinylamine
complexes.59 In addition to alkene face exchange, this mecha-
nism results in exchange of Hcis with Htrans of the bound alkene.
However, for 12f, the Htrans and Hcis resonances are sharp
between -89 and -69 °C, ruling out the CdC double bond
rotation mechanism.
Figure 12. Variable-temperature 1H NMR spectra of a mixture of 12f and
6 in CD2Cl2 solution. The Cp′ CH region is shown. The sets of exchanging
resonances of 12f are labeled a-d and were established by EXSY
spectroscopy. The large resonances at δ 6.36 labeled 6 are from 6. The
resonance at δ 6.18 labeled x is an unknown impurity.
Vinyl Ether and Vinyl Sulfide Adducts. The reaction of 6
with ethyl vinyl ether or methyl vinyl sulfide yields the
corresponding [Cp′2Zr(OtBu)(η1-RECHdCH2)][B(C6F5)4] ad-
ducts 14 (RE ) EtO) and 15 (RE ) MeS), as shown in eq 4.
The NMR properties of 14 and 15 are very different from those
of 12f or the other monosubstituted alkene complexes 12b-
e,g,h. In particular, 14 and 15 display upfield coordination shifts
for Hint and Cint, and downfield coordination shifts for Cterm
(Table 6), which are opposite in sign to those for 12b-h. In
addition, 14 and 15 display Cs symmetry at -89 °C instead of
the C1 symmetry observed for 12b-h. These results show that
the alkenes in 14 and 15 are coordinated by the heteroatom
and not by the CdC bond. The putative O-bound tert-butyl vinyl
ether adduct 12f′ in eq 4 is probably electronically favored over
the CdC π-complex 12f but is destabilized by steric crowding.
Line broadening of the tert-butyl vinyl ether NMR resonances
of 12f is not observed up to -69 °C, and exchange between
12f and 6 is not observed by EXSY spectroscopy at -89 °C.
Moreover, no loss of 12f is observed over 6 h at -89 °C in
CD2Cl2. If decomplexation of tert-butyl vinyl ether from 12f
occurred, the free alkene would likely be cationically polym-
erized, resulting in net conversion of 12f to 6.
Dynamics of Cp′2Zr(OtBu)(H2CdCHOtBu)+. The 1H NMR
Cp′ CH resonances of 12f exhibit selective broadening between
-89 and -69 °C and start to coalesce in a pairwise manner at
1
-69 °C (Figure 12). A H EXSY spectrum of 12f (-89 °C)
contains cross-peaks for pairwise exchange of the Cp′ CH
resonances but does not show cross-peaks for exchange of 12f
with 6. In addition, the 13C NMR ipso-Cp′ and Cp′Me
resonances of 12f display line broadening at -79 °C. The Zr-
OtBu and other alkene resonances of 12f remain sharp between
-89 and -69 °C. The resonances of 6 also remain sharp in
this temperature range.
These results show that the two Cp′ rings of 12f exchange
with each other without decomplexation of the tert-butyl vinyl
ether ligand and without exchange of the two sides of a given
Cp′ ligand (site epimerization). The simplest exchange process
that explains these results is nondissociative alkene face
Diene Adducts. It might be expected that 1,3-butadiene
would bind strongly to 6 in an η2-fashion, as the positive charge
9
8170 J. AM. CHEM. SOC. VOL. 128, NO. 25, 2006

Nonchelated d0 Zr
−Alkoxide−Alkene Complexes
A R T I C L E S
Reactions of Cp′2Zr(OtBu)+ with Other Potential Ligands.
Neither N2 (1 atm) nor H2 (1 atm) displaces CD2Cl2 from 6 at
-89 °C in CD2Cl2. N2 and H2 complexes of d0 metals are as
yet unknown.64,65 Hydrogenolysis of Zr-alkyl bonds is thought
to involve transient H2 complexes.66 Neither benzene nor in situ
generated Ph3CMe coordinate to 6 at -89 °C in CD2Cl2.
Unfavorable steric interactions between the arenes and the tert-
butoxide and Cp′ ligands may prevent coordination in this
system. Arene complexes of d0 metals are known in more
sterically open systems.67
Figure 13. Unobserved 1,5-hexadiene coordination modes.
on Cint could be stabilized as an allyl-type cation.60 However,
1,3-butadiene does not coordinate to 6. The lack of 1,3-butadiene
coordination may be due to conjugation. The conjugative
stabilization of 1,3-butadiene is 3-4 kcal/mol,61 while ∆H° of
coordination for nonconjugated alkenes is ca. -3.7 kcal/mol
(Table 5). Thus, if 1,3-butadiene were to lose conjugation upon
η2-coordination, perhaps due to rotation around the C2-C3 bond
to minimize steric crowding, there likely would be no enthalpic
advantage to coordination. Therefore, as ∆S° is expected to be
negative, Keq for 1,3-butadiene coordination would be extremely
small. On the other hand, chelated η2- and η4-cyclopentadiene
ZrIV complexes are known.13h,37b
Discussion
NMR Properties and Structures of Cp′2Zr(OtBu)(alkene)+
Species. The close similarity of the NMR data for nonchelated
12b-h and structurally characterized chelates A and B8 pro-
vides strong evidence that these species have similar unsym-
metrical alkene binding modes. The lack of d-π* back-bonding
and the shape of the alkene π-orbital allow the coordinated
alkene to slide from a symmetrical to an unsymmetrical
coordination mode to delocalize the positive charge from the
metal to the alkene. For monosubstituted-alkene complexes
12b-h, the slippage occurs such that the charge is delocalized
The NMR spectra of the 1,5-hexadiene adduct 12d contain
one set of strongly perturbed vinyl resonances and one set of
only slightly shifted resonances, indicative of coordination of
only one alkene unit. There is no evidence for formation of
chelated bis-alkene complex R or dinuclear complex S (Figure
13).60 The inability of 1,5-hexadiene to coordinate to two Zr
centers to form S reflects the weakness of R-olefin coordination
to 6. Assuming that the Keq value for formation of S from 6
and 12d is the same as that for formation of 12d from 6 and
1,5-hexadiene, under typical NMR conditions the concentration
of S would be less than ca. 0.15 mM (<0.05[12d]), and
therefore, this species would not be detectable by NMR. There
is no evidence that the two ends of the 1,5-hexadiene ligand of
12d undergo an associative intramolecular exchange process
(i.e. with R as a transition state or intermediate).62
primarily to Cint, which can stabilize the charge better than Cterm
,
as secondary carbocations are more stable than primary
carbocations. Incorporation of â-Si, R-OR, or R-metal-
locene functionalities in the alkene results in additional charge
delocalization.
The NMR data for symmetrical-alkene complexes 12a,j,k can
be accommodated by the unsymmetrical binding model, with
fast rotation around the metal-(alkene centroid) axis on the
NMR time scale. However, the observed coordination shifts for
symmetrical alkene complexes are small (Table 3) and are also
consistent with symmetrical alkene binding, as has been
established for AgI-ethylene adducts by X-ray crystallographic
analysis.26a-c
The proposed unsymmetrical alkene binding in 12b-h is
supported by computational results. DFT calculations predict
highly unsymmetrical propylene coordination (∆d(Zr-C) )
0.54 Å) in {H2Si(C5H4)2}Zr(CH3)(H2CdCHMe)+, with the Zr-
Cp2Zr(OtBu)(H2CdCHMe)+. The addition of excess pro-
pylene to 7 at -89 °C produces an equilibrium mixture of [Cp2-
Zr(OtBu)(H2CdCHMe)][B(C6F5)4] (16), 7, and free propylene,
analogous to the formation of the Cp′2Zr analogue 12b in eq 1.
The 1H and 13C NMR spectra of 16 each contain two Cp
resonances, consistent with the expected C1 symmetry. The
NMR data for the bound propylene of 16 are very similar to
the data for 12b. The equilibrium constant for formation of 16,
Keq ) [16][7]-1[H2CdCHMe]-1, is 14.7(7) M-1 (CD2Cl2, -89
°C), almost 3 times greater than that for formation of 12b under
the same conditions (eq 1). This difference is due to the greater
Lewis acidity of 7 vs 6, which results from the weaker electron-
donating ability of Cp- compared to that of Cp′-.63 Steric factors
may also make a minor contribution to the difference in
propylene binding strength in 16 vs 12b.
C
term distance (2.57 Å) being significantly shorter than the Zr-
Cint distance (3.11 Å).68 Similar unsymmetrical propylene
binding was predicted for the several possible propylene
complexes in the rac-Me2C{1-(3-tBu-indenyl)2}Zr(CH2CHMeEt)-
(H2CdCHMe)+ system (∆d(Zr-C) ) 0.27-0.73 Å).69
Interestingly, DFT calculations also predict unsymmetrical
ethylene binding in Cp2Zr(CH3)(C2H4)+ (∆d(Zr-C) ) 0.22 Å)
and {H2Si(C5H4)2}Zr(CH3)(C2H4)+ ∆d(Zr-C) ) 0.24 Å),4a in
which the calculated Zr-Cvinyl lengths are similar to those in
the calculated propylene adducts. Ab initio calculations predict
unsymmetrical ethylene binding in (C5H4SiH2NtBu)Ti(CH3)-
(64) MacKay, B. A.; Fryzuk, M. D. Chem. ReV. 2004, 104, 385.
(65) (a) Fryzuk, M. D.; Love, J. B.; Rettig, S. J.; Young, V. G. Science 1997,
275, 1445. (b) Basch, H. et al. J. Am. Chem. Soc. 1999, 121, 523.
(66) Gell, K. I.; Posin, B.; Schwartz, J.; Williams, G. M. J. Am. Chem. Soc.
1982, 104, 1846.
(58) The styrene complex (C5H4CH2CH2NiPr)V(dNtBu)(H2CdCHPh)+ under-
goes a similar enantioface exchange by slippage to η-phenyl coordinated
transient intermediates.46
(59) (a) Matchett, S. A.; Zhang, G.; Frattarelli, D. Organometallics 2004, 23,
5440. (b) Chang, T. C. T.; Foxman, B. M.; Rosenblum, M.; Stockman, C.
J. Am. Chem. Soc. 1981, 103, 7361.
(67) (a) Hayes, P. G.; Piers, W. E.; Parvez, M. J. Am. Chem. Soc. 2003, 125,
5622. (b) Green, M. L. H.; Sassmannshausen, J. Chem. Commun. 1999,
115. (c) Sassmannshausen, J. Organometallics 2000, 19, 482. (d) Gillis,
D. J.; Quyoum, R.; Tudoret, M.-J.; Wang, Q.; Jeremic, D.; Roszak, A. W.;
Baird, M. C. Organometallics 1996, 15, 3600. (e) Lancaster, S. J.; Robinson,
O. B.; Bochmann, M.; Coles, S. J.; Hursthouse, M. B. Organometallics
1995, 14, 2456. (f) Jia, L.; Yang, X.; Stern, C. L.; Marks, T. J.
Organometallics 1997, 16, 842.
(60) The formation of Cp′2Zr(OtBu)(η4-H2CdCHCHdCH2)+ or Cp′2Zr(OtBu)-
+
(alkene)2 species are probably precluded by O-Zr π-bonding.
(61) Wiberg, K. B.; Rosenberg, R. E.; Rablen, P. R. J. Am. Chem. Soc. 1991,
113, 2890.
(62) The pendant vinyl group of the 1,5-hexadiene may not be able to wrap
around correctly to displace the bound vinyl group.
(63) Wieser, U.; Babushkin, D.; Brintzinger, H.-H. Organometallics 2002, 21,
920.
(68) Schaper, F.; Geyer, M.; Brintzinger, H. H. Organometallics 2002, 21, 473.
(69) Moscardi, G.; Resconi, L.; Cavallo, L. Organometallics 2001, 20, 1918.
9
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A R T I C L E S
Stoebenau and Jordan
(C2H4)+ (∆d(Ti-C) ) 0.29 Å).70 However, DFT computations
predict more symmetrical ethylene binding in certain [Cp2ZrMe-
(C2H4)][MAO] ion pairs (∆d(Zr-C) ) 0.05-0.27 Å).71 Further
studies will be required to fully understand the coordination
mode of symmetrical alkenes to Cp2Zr(X)+ species.
and solvent adducts are observed the for nonchelated systems,
while A and B exist only as the alkene adduct even with the
less weakly coordinating MeB(C6F5)3- anion.8a,b This difference
is expected due to the more favorable entropy of coordination
in the chelated systems compared to that in nonchelated systems.
Alkene Exchange. Alkene adducts 12a,b undergo reversible
displacement of the alkene by CD2Cl2 solvent. Neither direct
displacement of the coordinated alkene by free alkene nor
The NMR properties and proposed unsymmetrical alkene
binding of d0 complexes 12a-k are significantly different from
those of related d2 metal-alkene complexes. For example, the
ZrII-butene complexes Cp2Zr(H2CdCHEt)(THF-d8) (T) and
Cp2Zr(H2CdCHEt)(PMe3) (U) exhibit large upfield NMR
coordination shifts for both alkene carbons (∆δ ) ca. -75)
and for the three vinyl protons ∆δ ) ca. -5.0), in sharp contrast
to the coordination shifts of 12a-k listed in Table 1.72 The 1JCH
values for the alkene unit of U (137-145 Hz) are significantly
decreased from the values of free alkenes and 12a-k (Table
2).73 An X-ray crystallographic analysis shows that the butene
ligand in U is coordinated symmetrically, with ∆d(Zr-C) )
0.01(1) Å.74 The chelated ZrII-phosphine-alkene complexes
Cp′′2Zr(PPh2CH2CH2CHdCH2) (Cp′′ ) 1,2-C5H3Me2) and Cp2-
Zr(PPh2CH2CH2CH2CHdCH2) also exhibit symmetrical alkene
binding (∆d(Zr-C) ) 0.034(6), 0.058(4) Å).75 The symmetrical
coordination in these ZrII systems provides good evidence that
the unsymmetrical coordination proposed for 12b-h and
observed in A and B does not result from steric crowding or
from chelation.
The ZrII-ethylene complexes, Cp2Zr(H2CdCH2)(THF) and
Cp2Zr(H2CdCH2)(pyridine), exhibit large upfield alkene coor-
dination shifts, low 1JCH values (140-145 Hz), and symmetrical
ethylene binding (∆d(Zr-C) ) 0.04(1) Å) in the solid state,
similar to what is observed for ZrII(H2CdCHR) adducts.76 The
upfield coordination shifts and symmetrical alkene binding in
ZrII-alkene complexes are caused by d-π* back-bonding in
these d2 complexes, and these species can be considered to be
metallocyclopropanes. The alkene CdC distances in these
complexes are lengthened by coordination to ca. 1.44 Å,
consistent with metallocyclopropane formulation. In contrast,
the absence of back-bonding in d0 species results in divergent
alkene NMR coordination shifts consistent with unsymmetrical
alkene coordination.
-
displacement of the bound alkene by the B(C6F5)4 anion is
observed in 12a,b. However, these processes may occur in less
coordinating solvents.
Comparison of 12a-l with d0 Metal-Alkene Complexes
of Group 3, 5, and 6 Metals. The enthalpies of propylene
binding in 12b and Cp*2Y(R)(propylene) species L are similar.17
However, 6 is stabilized by solvent coordination, Cp*2YR
compounds are stabilized by agostic CH interactions,17a and the
Cp′2Zr(OtBu)+ unit is less crowded than the Cp*2YR units.
Therefore, the alkene binding enthalpies of 12b and L may not
accurately reflect the relative strengths of ZrIV-alkene and YIII-
alkene interactions. The Keq for ethylene binding for ZrIV (12a)
in chlorobenzene is much smaller than that for ethylene binding
to NbV (J; 220(10) M-1 at -30 °C in C6D5Cl), as 12a does not
form in this solvent.15 Alkene binding in 12a,b is also
substantially weaker than alkene binding in VV complex I, where
adduct formation is even favorable at ambient temperature.14,46
Additionally, cyclopentene binding in 12k is much weaker than
cycloheptene binding to WVI in K.16 A reasonable explanation
for the stronger alkene binding in the groups 5 and 6 systems
is that the VV, NbV, and WVI centers in I, J, and K are more
electrophilic than the ZrIV center in 12a-k. Weak nonclassical
back-bonding from MdNR, M-NR2, or MdCR2 bonding or
nonbonding orbitals to the alkene π* orbital may also enhance
alkene binding in the groups 5 and 6 systems.
Alkene decomplexation from L and related YIII systems is
much faster than for 12a-k, which may reflect the fact that
the displacing group is an incipient agostic C-H bond on the
alkyl ligand in L.17 Alkene decomplexation barriers for I, J,
and K have not been reported. However, qualitative observations
strongly suggest that decomplexation is slower than for 12a-
l,14-16 which is consistent with stronger alkene coordination in
the groups 5 and 6 systems.
NMR line broadening indicative of hindered metal-(alkene
centroid) rotation is observed in the NbV and WVI compounds
J and K,15,16 whereas in contrast, alkene rotation is fast on the
NMR time scale for 12a-k even at -89 °C. This difference
may be due to the presence of nonclassical back-bonding in J
and K, especially given the similar structures of the Cp2Nb(d
NtBu)+ and Cp′2Zr(OtBu)+ cations. However, differences in
metal-alkene distances may influence relative alkene rotation
rates.
Thermodynamics of Alkene Coordination to 6. Zirconium-
alkene complexes 12a-l exhibit weak metal-alkene binding.
The ∆H° values show that the Zr-ethylene and Zr-propylene
bonds are only ca. 3.7 kcal/mol stronger than the Zr-ClCD2Cl
bond, which underscores the weakness of alkene coordination
in these d0 metal complexes. The weak binding is due to the
lack of d-π* back-bonding. Consistent with a pure σ-donation
binding mode, coordination is enhanced by the introduction of
electron-donating groups in the alkene and by incorporation of
functional groups to stabilize the positive charge buildup on
Cint.
Comparison of 12a-l with Cp2ZrR+ Species and Implica-
tions for Polymerization. A Cp2Zr(OR)+ cation is a weaker
Lewis acid than an analogous Cp2ZrR+ cation, due to O-Zr
π-donation.34d,77,78 This difference is reflected in the preferential
binding of THF to Cp′2ZrMe+ vs 6 in competition experiments.35
Therefore, it is expected that alkene coordination in 12a-l and
16 is weaker than what would be expected in similar Cp2Zr-
(R)(alkene)+ species. However, the effects of alkene structure
The metal-alkene interaction in 12a-l is weaker than that
in the chelates A and B.8 Equilibrium mixtures of alkene adducts
(70) Lanza, G.; Fragala`, I. L.; Marks, T. J. Organometallics 2002, 21, 5594.
(71) Zurek, E.; Ziegler, T. Faraday Discuss. 2003, 124, 93.
(72) (a) Dioumaev, V. K.; Harrod, J. F. Organometallics 1997, 16, 1452. (b)
Negishi, E.-i.; Holmes, S. J.; Tour, J. M.; Miller, J. A.; Cederbaum, F. E.;
Swanson, D. R.; Takahashi, T. J. Am. Chem. Soc. 1989, 111, 3336.
(73) Binger, P.; Mu¨ller, P.; Benn, R.; Rufin˜ska, A.; Gabor, B.; Kru¨ger, C.; Betz,
P. Chem. Ber. 1989, 122, 1035.
(74) Goddard, R.; Binger, P.; Hall, S. R.; Mu¨ller, P. Acta Crystallogr., Sect. C
1990, 46, 998.
(75) Yamazaki, A.; Nishihara, Y.; Nakajima, K.; Hara, R.; Takahashi, T.
Organometallics 1999, 18, 3105.
(76) Fischer, R.; Walther, D.; Gebhardt, P.; Go¨rls, H. Organometallics 2000,
19, 2532.
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Nonchelated d0 Zr
−Alkoxide−Alkene Complexes
A R T I C L E S
on alkene binding strength should be parallel for both Cp2ZrR+
and Cp2Zr(OR)+ cations.
propyne, cis-2-butene, trans-2-butene, hydrogen, and vinyl chloride
were used as received. Cyclopentene, tert-butyl vinyl ether, ethyl vinyl
ether, vinyltrimethylsilane, 1-hexene, 4,4-dimethyl-1-pentene, and
allyltrimethylsilane were dried over CaH2 before use. Styrene was
degassed prior to use. PMe3, 1,5-hexadiene, and 3,3-dimethyl-1-butene
were dried over 3 Å molecular sieves prior to use.
Early transition metal catalysts are usually more active for
ethylene polymerization than propylene polymerization (i.e.
ethylene is consumed faster).79a Furthermore, ethylene/R-olefin
copolymerization reactivity ratios show that ethylene is pref-
erentially inserted vs the R-olefin into a Cp2Zr(copolymeryl)+
species, regardless of which monomer was previously in-
serted.79b,80 Finally, ethylene inserts much faster into Y-H and
Y-C bonds than propylene does.17a The present work shows
that ethylene and propylene bind to 6 with approximately the
same strength and exchange on and off with very similar rates
in CD2Cl2 solution. These results suggest that the general lower
activity for propylene polymerization compared to that for
ethylene polymerization is not due to differential substrate
binding but rather arises because propylene insertion (from the
metal-alkene adduct) has a higher barrier than ethylene
insertion. The larger size of propylene would lead to a more
crowded insertion transition state and may explain this differ-
ence.
(C5R5)2ZrR+ polymerization catalysts typically have a strong
preference for 1,2-insertion of R-olefins, i.e., the migrating Zr-R
group migrates to Cint).1b The proposed unsymmetrical binding
in 12b-h and the resulting partial positive charge on Cint provide
a simple explanation for this trend. The polarization should make
Cint more susceptible to nucleophilic attack than Cterm. Also,
unfavorable steric interactions between the alkyl group of the
R-olefin and the cyclopentadienyl ligands in Cp2Zr(R)(H2Cd
CHR′)+ species may favor the rotamer in which Cint is oriented
toward the metal-alkyl bond, which may contribute to the
preference for 1,2 insertion.
Elemental analyses were performed by Midwest Microlab (India-
napolis, IN). ESI-MS experiments were performed with an HP Series
1100MSD instrument using direct injection via a syringe pump (ca.
10-3 M solutions). Good agreement between observed and calculated
isotope patters was observed, and the listed m/z value corresponds to
the most intense peak in the isotope pattern. NMR spectra were recorded
on Bruker DRX 500 or 400 spectrometers in Teflon-valved NMR tubes
at ambient probe temperature unless otherwise noted. 1H and 13C
chemical shifts are reported relative to SiMe4 and were referenced to
the residual solvent signals. 19F NMR spectra are reported and
referenced relative to external CFCl3. 11B NMR spectra are referenced
to external BF3‚OEt2. 31P NMR spectra are reported to external 85%
H3PO4 and are referenced to free PMe3 (δ -61.0). NMR probe
temperatures were calibrated by a MeOH thermometer.82 Coupling
constants are reported in Hz. Where 13C-{gated-1H} NMR spectra are
reported, standard 13C{1H} NMR spectra were also recorded to assist
in interpretation and assignment. For H2CdCHX substrates, Hcis is the
H that is cis to Hint, and Htrans is the H that is trans to Hint.
-
NMR spectra of ionic compounds contain B(C6F5)4 resonances at
the free anion positions. 19F NMR spectra were obtained for all
compounds that contain this anion. Resonances for Ph3CMe are present
when cationic compounds are generated with Ph3C+ and used in situ.
-
The data for B(C6F5)4 and Ph3CMe are given in the Supporting
Information.
Generation of Cp2Zr(OtBu)Me (4). An NMR tube was charged
with Cp2ZrMe2 (13.4 mg, 0.0533 mmol), and C6D6 (0.69 mL) and tert-
butyl alcohol (0.0770 mmol, 1.445 equiv) were added separately by
vacuum transfer at -196 °C. The tube was warmed to 22 °C, shaken,
and maintained at 22 °C for 30 min to afford a colorless solution. A
1H NMR spectrum was obtained that established that 4 had formed
quantitatively. The volatiles were removed under vacuum, C6D6 (0.65
mL) was added by vacuum transfer at -196 °C, the tube was warmed
to 22 °C, and NMR spectra were recorded. This species was used in
Experimental Section
General Procedures. All reactions were performed using glovebox
or Schlenk techniques under a purified N2 atmosphere, or on a high
vacuum line. N2 was purified by passage through columns of activated
molecular sieves and Q-5 oxygen scavenger. CD2Cl2, C6D5Cl, and C6H5-
Cl were distilled from P2O5. C6D6 was distilled from Na/benzophenone.
Benzene and hexanes were purified by passage though columns of
activated alumina and BASF R3-11 oxygen removal catalyst. Cp′2-
ZrMe2 (1), Cp2ZrMe2 (5), and methyl vinyl sulfide were synthesized
by literature procedures.81 Other reagents were received from standard
commercial sources. tert-Butyl alcohol was dried over K2CO3 and stored
under vacuum or as a stock solution in benzene. [Ph3C][B(C6F5)4],
vinylferrocene, isobutene, ethylene, propylene, allene, 1,3-butadiene,
1
situ to prepare 7. H NMR (C6D6): δ 5.77 (s, 10H, Cp), 1.02 (s, 9H,
OtBu), 0.32 (s, 3H, ZrMe). 13C{1H} NMR (C6D6): δ 110.2 (Cp), 77.1
(OCMe3), 31.8 (OCMe3), 17.6 (ZrMe).
Synthesis of [Cp′2Zr(OtBu)][B(C6F5)4] (6). A flask was charged
with Cp′2ZrMe2 (1, 1.1826 g, 4.231 mmol) and benzene (40 mL). The
resulting solution was stirred at 22 °C, and tert-butyl alcohol (3.6 mL
of a 2.1 M solution in benzene, 7.6 mmol, 1.8 equiv) was added by
syringe. Gas evolution occurred. The colorless solution was stirred for
1.25 h, the volatiles were removed under vacuum, and the product was
dried under vacuum for 22 h, giving a clear, colorless oil. A 1H NMR
(77) For discussion of M-OR π-donation for early transition metals and
lanthanides, see: (a) Steffey, B. D.; Fanwick, P. E.; Rothwell, I. P.
Polyhedron 1990, 9, 963. (b) Coffindaffer, T. W.; Steffy, B. D.; Rothwell,
I. P.; Folting, K.; Huffman, J. C.; Streib, W. E. J. Am. Chem. Soc. 1989,
111, 4742. (c) Kerschner, J. L.; Fanwick, P. E.; Rothwell, I. P.; Huffman,
J. C. Inorg. Chem. 1989, 28, 780. (d) Caulton, K. G. New. J. Chem. 1994,
18, 25. (e) Howard, W. A.; Trnka, T. M.; Parkin, G. Inorg. Chem. 1995,
34, 5900. (f) Hillier, A. C.; Liu, S.-Y.; Sella, A.; Elsegood, M. R. J. Inorg.
Chem. 2000, 39, 2635. (g) Petrie, M. A.; Olmstead, M. M.; Power, P. P.
J. Am. Chem. Soc. 1991, 113, 8704. (h) Russo, M. R.; Kaltsoyannis, N.;
Sella, A. Chem. Commun. 2002, 2458. (i) Manz, T. A.; Fenwick, A. E.;
Phomphari, K.; Rothwell, I. P.; Thomson, K. T. J. Chem. Soc., Dalton
Trans. 2005, 668.
(78) (a) Marsella, J. A.; Moloy, K. G.; Caulton, K. G. J. Organomet. Chem.
1980, 201, 389. (b) Britovsek, G. J. P.; Ugolotti, J.; White, A. J. P.
Organometallics 2005, 24, 1685.
(79) (a) Mo¨hring, P. C.; Coville, N. J. J. Organomet. Chem. 1994, 479, 1. (b)
Kaminsky, W.; Arndt, M. AdV. Polym. Sci. 1997, 127, 143.
(80) For a recent example of preferential R-olefin incorporation vs ethylene,
see: Irwin, L. J.; Reibenspies, J. H.; Miller, S. A. J. Am. Chem. Soc. 2004,
126, 16716.
1
spectrum confirmed that the oil was pure 2.31 Data for 2: H NMR
(C6D6): δ 5.69 (m, 2H, Cp′ CH), 5.65 (m, 4H, Cp′ CH), 5.51 (m, 2H,
Cp′ CH), 2.01 (s, 6H, Cp′Me), 1.06 (s, 9H, OtBu), 0.25 (s, 3H, ZrMe).
1H NMR (C6D5Cl): δ 5.78-5.73 (m, 4H, Cp′ CH), 5.71 (m, 2H, Cp′
CH), 5.58 (m, 2H, Cp′ CH), 2.05 (s, 6H, Cp′Me), 1.07 (s, 9H, OtBu),
0.13 (s, 3H, ZrMe). 13C{1H} NMR (C6D6): δ 121.7 (Cp′ ipso), 111.9
(Cp′ CH), 111.8 (Cp′ CH), 108.2 (Cp′ CH), 107.8 (Cp′ CH), 76.9
(OCMe3), 32.1 (OCMe3), 19.6 (ZrMe), 15.1 (Cp′Me). Compound 2 so
obtained was dissolved in benzene (20 mL). A separate flask was
charged with solid [Ph3C][B(C6F5)4] (3.8030 g, 4.123 mmol, 0.98 equiv)
and benzene (70 mL), giving a suspension of a yellow solid in a gold
solution. The solution of 2 was added to the flask containing the [Ph3C]-
[B(C6F5)4] suspension. The mixture was stirred for 2.5 h at 30 °C to
give a mixture of a dark oil and yellow supernatant. The supernatant
was removed by cannula. The oil was washed with benzene (3 × 60
(81) (a) Couturier, S.; Tainturier, G.; Gautheron, B. J. Organomet. Chem. 1980,
195, 291. (b) Samuel, E.; Rausch, M. D. J. Am. Chem. Soc. 1973, 95, 6263.
(c) Doering, W. v. E.; Schreiber, K. C. J. Am. Chem. Soc. 1955, 77, 514.
(82) Van Geet, A. L. Anal. Chem. 1970, 42, 679.
9
J. AM. CHEM. SOC. VOL. 128, NO. 25, 2006 8173

A R T I C L E S
Stoebenau and Jordan
Table 7. Summary of X-ray Diffraction Data for 8‚C6D5Cl
mL); at each wash the supernatant was removed by cannula, leaving a
yellow oil. The oil was then washed with hexanes (3 × 50 mL; 1 × 70
mL; 1 × 20 mL) yielding 6 (3.30 g, 80%) as a yellow solid.83 The
product was stored at -35 °C. Data for 6: 1H NMR (C6D5Cl): δ 5.86
(m, 4H, Cp′ CH), 5.83 (m, 4H, Cp′ CH), 1.86 (s, 6H, Cp′Me), 1.02 (s,
9H, OtBu). 1H NMR (C6D5Cl, -35 °C): δ 5.83 (m, 8H, Cp′ CH), 1.86
formula
fw
C68H22B2O2F40Zr2‚C6D5Cl
1952.51
cryst syst
space group
a (Å)
b (Å)
c (Å)
triclinic
P1h
9.900(2)
12.250(3)
14.111(3)
89.956(4)
85.384(4)
87.814(4)
1704.5(7)
2
(s, 6H, Cp′Me), 1.02 (s, 9H, OtBu). H NMR (CD2Cl2, -89 °C): δ
1
6.35 (br s, 4H, Cp′ CH), 6.33 (br s, 4H, Cp′ CH) 2.18 (s, 6H, Cp′Me),
1.22 (s, 9H, OtBu). 13C{1H} NMR (C6D5Cl): δ 129.2 (Cp′ ipso), 117.7
(Cp′ CH), 114.9 (Cp′ CH), 84.4 (OCMe3), 31.3 (OCMe3), 14.5 (Cp′Me).
13C{1H} NMR (C6D5Cl, -35 °C): δ 117.4 (Cp′ CH), 114.5 (Cp′ CH),
84.0 (OCMe3), 31.0 (OCMe3), 14.4 (Cp′Me). The ipso Cp′ peak is
obscured by solvent. 13C{1H} NMR (CD2Cl2, -89 °C): δ 129.4 (ipso
Cp′), 116.7 (Cp′ CH), 114.2 (Cp′ CH), 84.1 (OCMe3), 30.9 (OCMe3),
14.8 (Cp′Me). Anal. Calcd for C40H23BOF20Zr: C, 47.97; H, 2.31.
Found: C, 48.08; H, 2.51. ESI-MS (C6D5Cl/PhMe solution): Major
cation observed: [Cp′2Zr(OtBu)+ - H] calcd m/z 320.1, found 320.0.
Generation of [Cp2Zr(OtBu)][B(C6F5)4] (7). A solution of Cp2Zr-
(OtBu)Me (4, 0.0533 mmol) was prepared as described above, and the
volatiles were removed under vacuum. Solid [Ph3C][B(C6F5)4] (49.3
mg, 0.0534 mmol) was added, and C6D5Cl (0.48 mL) was added by
vacuum transfer at -196 °C. The tube was warmed to 22 °C and
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
T (K)
100
cryst color, habit
GOF on F2
R indices (I > 2σ(I))a
pale yellow, rod
1.048
R1 ) 0.0433
wR2 ) 0.1004
R1 ) 0.0533
wR2 ) 0.1041
R indices (all data)a
2
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) [∑[w(Fo - Fc )2]/∑[w(Fo )2]]1/2
,
where w) q /[σ2(Fo ) + (aP)2 + bP].
2
6H, Cp′Me), 1.22 (s, 9H, OtBu, overlaps with signal for 6). 13C{1H}
1
shaken, giving a deep yellow-orange solution. A H NMR spectrum
1
NMR (CD2Cl2, -89 °C): δ 127.1 (Cp′ ipso), 119.0 (t, JCH ) 160,
was obtained and showed that 7 had formed in 96% yield versus Ph3-
CMe. 1H NMR (C6D5Cl): δ 5.98 (s, 10H, Cp), 0.98 (s, 9H, OtBu). 1H
NMR (CD2Cl2, -89 °C): δ 6.50 (s, 10H, Cp), 1.19 (s, 9H, OtBu).
13C{1H} NMR (C6D5Cl): δ 116.0 (Cp), 85.0 (OCMe3), 31.1 (OCMe3).
13C{gated-1H} NMR (CD2Cl2, -89 °C): δ 115.5 (d, 1JCH ) 177, Cp),
84.6 (s, OCMe3), 30.5 (q, JCH ) 126, OCMe3).
Conversion of 7 to [{Cp2Zr}2(µ-OH)2][B(C6F5)4]2 (8). The above
NMR tube was maintained at 22 °C for 5 d, during which time yellow
C2H4), 116.6 (Cp′ CH), 116.2 (Cp′ CH), 114.3 (Cp′ CH), 110.5 (Cp′
CH), 84.8 (OCMe3), 30.7 (OCMe3), 14.7 (Cp′Me).
X-ray Crystallographic Analysis of 8. Single crystals of 8 were
obtained by the decomposition of 7 in C6D5Cl solution after 5 d.
Crystallographic data are summarized in Table 7. Data were collected
on a Bruker Smart Apex diffractometer using Mo KR radiation (0.71073
Å). Repeated difference Fourier maps allowed recognition of all
expected non-hydrogen atoms as well as a disordered C6H5Cl solvent
molecule. The bridging atoms between Zr atoms were determined to
be oxygen on the basis of bond lengths and thermal parameters. A
weak peak about 0.8 Å from this oxygen was assigned to H to give an
overall neutral charge. Final refinement was anisotropic for non-
hydrogen atoms and isotropic for H atoms. The occupancy of the Cl
atom of the solvent was refined to a value of 0.51 consistent with a
disordered C6H5Cl solvent molecule. No anomalous bond lengths or
thermal parameters were noted.
Determination of Thermodynamic Parameters and Exchange
Rates. (A) General Variable-Temperature NMR Procedure. NMR
samples were prepared as described for 12a above. The NMR tube
was stored at -78 °C and placed in a precooled NMR probe. The probe
was maintained at a given temperature for 20-30 min to allow the
sample to reach thermal equilibrium, and then NMR experiments were
conducted. When necessary, the temperature was raised 5 or 10 °C
and the procedure repeated until the desired temperature range had been
studied.
(B) Equilibrium Constant Measurements. Equilibrium constants,
Keq ) [12][6]-1[alkene]-1, were determined by multiple (typically 2-4)
1H NMR experiments in which [6]initial and [alkene]initial were varied.
For 12a-e,i-k, Keq values were determined by direct study of the
equilibria in eq 1. For the equilibrium involving 12a, relative amounts
of 12a, 6, and free ethylene were determined from the integrations of
the coordinated C2H4 resonance of 12a, the Cp′Me resonance of 6 after
the contribution from 12a was subtracted, and the free C2H4 resonance,
respectively. The number of moles of 12a and 6 were determined by
finding their respective mole fractions and multiplying by the original
amount of 6 used. The number of moles of free ethylene was determined
from the ratio of the integrations of free ethylene to 12a. Concentrations
were then determined by dividing the number of moles by the solution
volume. Keq values were found between -89 °C and -38 °C. The Keq
values for formation of 12b-e,i-k were determined similarly; details
are given in the Supporting Information.
1
1
crystals separated from solution. A H NMR spectrum revealed the
complete disappearance of 7 and the formation of poly(isobutene) (54%
based on Ph3CMe) and unidentified metallocene products (total of 13%
based on Ph3CMe). An X-ray crystallographic analysis revealed the
yellow crystals to be [{Cp2Zr(µ-OH)}2][B(C6F5)4]2.
General Procedure for Generation of [Cp′2Zr(OtBu)(alkene)]-
[B(C6F5)4] (12a-l). An NMR tube was charged with 6 (25-50 mg),
and CD2Cl2 (0.5-0.7 mL) was added by vacuum transfer at -78 °C.
The tube was shaken at this temperature, giving a yellow solution. The
tube was cooled to -196 °C, and the alkene (excess) was added by
vacuum transfer. The tube was warmed to -78 °C and shaken, yielding
a yellow solution. The tube was placed in an NMR probe that had
been precooled to -89 °C. NMR spectra were obtained and showed
that a mixture of 12a-l, 6, and free alkene was present. The detailed
procedure and data for ethylene adduct 12a are given below. Data for
12b-k, and 13-16 are given in the Supporting Information.
Generation of [Cp′2Zr(OtBu)(H2CdCH2)][B(C6F5)4] (12a). An
NMR tube was charged with 6 (26.8 mg, 0.0267 mmol), and CD2Cl2
(0.67 mL) was added by vacuum transfer at -78 °C. The tube was
shaken at this temperature, giving a yellow solution. The tube was
cooled to -196 °C, and ethylene (0.183 mmol) was added by vacuum
transfer. The tube was warmed to -78 °C and shaken, yielding a yellow
solution. The tube was placed in an NMR probe that had been precooled
to -89 °C. NMR spectra were obtained and showed that a mixture of
12a (0.022 M), 6 (0.018 M), and free ethylene (0.17 M) was present.
Raising the temperature from -89 °C has the effect of decreasing the
amount of 12a in solution while increasing the amounts of 6 and free
ethylene, and broadening the signals for 12a and to a lesser extent those
of 6 and free ethylene. The signals for 12a eventually coalesce with
1
those of 6 and free ethylene. Data for 12a: H NMR (CD2Cl2, -89
°C): δ 6.35 (br m, 2H, Cp′ CH, overlaps with signal for 6), 6.23 (br
m, 4H, Cp′ CH), 6.11 (m, 2H, Cp′ CH), 5.92 (s, 4H, C2H4), 2.17 (s,
(83) One crop of 6 was contaminated with ca. 2% [Ph3C][B(C6F5)4]. Control
experiments showed that this contaminant does not influence alkene
coordination to 6.
The Keq value for allyltrimethylsilane coordination in 12g was
determined by a competition experiment with free propyne and the
9
8174 J. AM. CHEM. SOC. VOL. 128, NO. 25, 2006

Nonchelated d0 Zr
−Alkoxide−Alkene Complexes
A R T I C L E S
propyne adduct, [Cp′2Zr(OtBu)(HCtCMe)][B(C6F5)4] (17).27a The
equilibrium constant for the reaction:
width in absence of exchange for the coordinated ethylene resonance
at each temperature. First-order rate constants were determined by the
formula k-1 ) π(ω - ω0), where ω is the line width at half-height of
the coordinated ethylene peak and ω0 is the line width at half-height
of the benzene peak. All measurements were made in the slow-exchange
region over the temperature range -89 to -39 °C.
17 + H2CdCHCH2SiMe3 / 12g + HCtCMe
was defined to be K1 ) [12g][HCtCMe][17]-1[H2CdCHCH2SiMe3]-1
.
The relative amounts of 12g, 17, free allyltrimethylsilane, and free
propyne were found from the SiMe3 resonance of 12g, the Hterm
resonance of 17, the SiMe3 resonance of free allyltrimethylsilane, and
the methyl resonance of free propyne, respectively. The equili-
First-order rate constants for propylene decomplexation from 12b
were determined by visually fitting simulated spectra to experimental
spectra. Simulations were performed using gNMR.57 The average line
width of the trityl cation resonances was used as the line width in the
absence of exchange of the coordinated propylene resonances at each
temperature. The Hint resonance of coordinated propylene was simulated.
Coupling constants of 12b were determined directly, or when not
possible, by using the coupling constants of free propylene. Coupling
constants were assumed to be temperature independent. The gNMR
program yields rates, which were converted to first-order rate constants
by the formula: rate ) k-1[12b], where k-1 is the rate constant for
propylene decomplexation from 12b. All measurements were made in
the slow-exchange region over a temperature range of -89 to -39 °C.
For 12a and 12b, first-order decomplexation rate constants k-1 were
determined in at least three experiments with different initial concentra-
tions of 6 and alkene. The k-1 values were found to be independent of
the concentrations of 6, of free alkene, and of 12, consistent with the
rate law rate ) k-1[12]. Activation parameters for decomplexation were
determined from Eyring plots of ln(k-1/T) versus 1/T. The reported
values are the weighted averages of results from at least three runs.84
brium constant for allyltrimethylsilane coordination to 6 is Keq,12g
[12g][6]-1[H2CdCHCH2SiMe3]-1 ) K1‚Kpro where Kpro ) [17][6]-1
[HCtCMe]-1
)
-
.
The Keq value for vinylferrocene coordination in 12h was determined
by a competition experiment with allyltrimethylsilane. The equilibrium
constant for the reaction:
12g + H2CdCHFc / 12h + H2CdCHCH2SiMe3
was defined to be K2
) -
[12h][H2CdCHCH2SiMe3][12g]-1
[H2CdCHFc]-1. The relative amounts of 12h, 12g, free vinylferrocene,
and free allyltrimethylsilane were found from the Hint resonance of 12h,
the Hint resonance of 12g, the Htrans resonance of free vinylferrocene,
and the Hallylic resonance of free allyltrimethylsilane, respectively. The
equilibrium constant for allyltrimethylsilane coordination to 6 is Keq,12h
) [12h][6]-1[H2CdCHFc]-1 ) K2‚Keq,12g where Keq,12g is defined above.
Thermodynamic parameters (∆H° and ∆S°) were determined from
van’t Hoff plots of ln(Keq) versus 1/T. Reported values for each case
are the weighted average of results from at least three runs in which
the initial concentrations of 6 and alkene were varied.84
Acknowledgment. We thank the NSF (CHE-0212210) for
financial support, the University of Chicago for a William
Rainey Harper fellowship (E.J.S.), Dr. Ian Steele for determi-
nation of the crystal structure of 8, and Drs. Orson Sydora and
Chang-Jin Qin for the ESI-MS of 6.
(C) Exchange Rate Measurements. Variable-temperature 1H NMR
spectroscopy was used to determine the kinetics of ligand exchange
for 12a and 12b. First-order rate constants for ethylene decomplexation
from 12a (k-1) were determined by measuring the line width (in Hz)
at half-height of the coordinated ethylene peak. The line width of the
benzene peak (added as a line width standard) was used as the line
Supporting Information Available: Full citation for ref 65b;
NMR data for free alkenes, 12b-k, and 13-16; additional
experimental procedures; dynamics of allene complex 12i,
details of the crystal structure of 8 (pdf, cif). This material is
available free of charge via the Internet at http://pubs.acs.org.
(84) The weighted average xj is defined as (∑wixi)/(∑wi) and the weighted
standard deviation σ is defined by σ2 ) {∑wi(xi - xj)2}/{∑wi} where the
weighing factors are wi ) 1/σi2.
JA0575225
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J. AM. CHEM. SOC. VOL. 128, NO. 25, 2006 8175