Divalent ansa-Zirconocenes: Stereoselective Synthesis and High Activity for Propylene Polymerization

Full text of DOI:10.1021/ja0381611

Published on Web 12/09/2003
Divalent ansa-Zirconocenes: Stereoselective Synthesis and High Activity for
Propylene Polymerization
Eugene Y.-X. Chen,*,† Richard E. Campbell, Jr.,‡ David D. Devore,*,‡ D. Patrick Green,‡
Bernie Link,‡ Jorge Soto,‡ David R. Wilson,‡ and Khalil A. Abboud§
The Dow Chemical Company, Midland, Michigan 48674, Department of Chemistry, Colorado State UniVersity,
Fort Collins, Colorado 80523, and Department of Chemistry, UniVersity of Florida, GainesVille, Florida 32611
Received August 27, 2003; E-mail: eychen@lamar.colostate.edu; devoredd@dow.com
Scheme 1
The development of new metalation methodologies/reagents for
the stereoselective synthesis of group 4 ansa-metallocenes in high
racemic purity has been a subject of intense research for the past
eight years.1 This significant interest arises from the critical need
to substantially suppress or completely eliminate the achiral meso-
isomeric coproduct often formed up to 50% during the synthesis
of the chiral racemic ansa-metallocene catalyst, which is essential
for stereoselective olefin polymerization. A success in this regard
will require little or no purification of the product and achieve high
to quantitative conversion of the expensive ansa-ligands to the
desired chiral racemic catalyst.
Jordan’s seminal amine elimination approach that utilizes the
reaction of neutral ligands with Zr(NR2)4 works well for simple
ansa-bis(indenyl) ligands. However, this approach is not successful
for sterically crowded ligands such as 2,4-substituted and related
indenes;1b-c,e-f the incorporation of such a ligand framework into
reduction of ZrCl4 using sodium amalgam (Na/Hg) in the presence
a propylene-polymerization-catalyst structure is necessary for
of PMe3. The current approach produces the Zr(II) diene complexes
achieving high polymer isotacticity and molecular weight.2 The use
in high isolated yields (90-93%) under mild reaction conditions
1b
of Zr{PhN(CH2)3NPh}Cl2(THF)2 solved this problem, but the
with the avoidance of Hg. As is demonstrated below, the reaction
needed conversion of the zirconocene diamide product to the
sequence consists of a rate-limiting step for the reduction of ZrCl4-
dichloride precatalyst seems not as straightforward as with the
(PR3)2 by Li to the chloride-bridged Zr(III) dimer [ZrCl3(PR3)2]2
unsubstituted indenyl derivative.1b The Zr biphenolate complexes
(2)7 and a fast diene-driven disproportionation of the Zr(III) dimer
Zr(OAr-ArO)Cl2(THF)21a are also found to undergo stereoselective
to the Zr(II) 1 and ZrIVCl4(PR3)2. The one-half of 1 equiv of ZrCl4-
reaction with dianionic salts of various ansa-bis(indenyl) ligands;
(PR3)2 generated in the fast disproportionation step re-enters the
reduction cycle, and the reaction proceeds until all of the diene is
consumed (Scheme 1).
with highly substituted indenyl ligands, the reaction gives an initially
kinetically controlled mixture of rac- and meso-zirconocene bi-
phenolates, which requires subsequent heating to yield the pre-
dominant rac-product.
The use of a suitable solvent, reductant, and phosphine ligand is
critical for achieving high isolated yields of 1. In hydrocarbon
solvents, Li metal only reduces ZrCl4(PR3)2 but not the diene;
however, in ethereal solvents Li metal reacts rapidly with the diene
to form the dianionic 1,4-diphenyl-2-butene-1,4-diyl species. The
reaction of such diyl species with ZrCl4(PR3)2 produced 1 in low
yield and a significant amount of free diene. Among the reducing
agents (Na, Na/naphthalenide, Li, Mg, Rieke Mg, K, KC8, Na/K,
lithium alkyls, and Grignard reagents) investigated, low-sodium Li
powder gave the best result. Because of the poor solubility of ZrCl4-
(PMe3)2 in toluene, the reaction does not proceed to completion
with this phosphine, but with PEt3 and P(nPr)3 the reaction proceeds
cleanly to produce green 2 in quantitative yield in the absence of
1,4-diphenyl-1,3-butadiene, or purple 1 in the presence of a
stoichiometric amount of the diene. The independent reaction of
the Zr(III) dimer 2b with 1,4-diphenyl-1,3-butadiene is fast, leading
to a 1:1 mixture of the Zr(II) 1b and the Zr(IV) ZrCl4(PnPr3)2.5
The structure of 1a (Figure 1) reveals a square pyramidal
coordination sphere about Zr, with the π-bound diene ligand
occupying the apex of the pyramid and adopting an s-cis orientation.
The ∆d8 parameter for the diene ligand in 1a is -0.027 Å, which
is consistent with the diene ligand being bound in a π-fashion.9
We have been interested in the stereoselective synthesis of chiral
diValent group 4 metallocenes and recognized that, in addition to
having a high degree of control over chemo- and diastereoselec-
tivity, it is also important that the synthesis lead directly to the
final precatalyst. To this end, we have recently developed a highly
efficient, stereoselective synthesis of racemic, divalent ansa-
zirconocene catalysts using the Zr(II) diene synthon.3 Attractive
attributes of this novel synthesis include the formation of rac-only
Zr(II) complexes in high yield for substituted ligands such as 2-Me-
4-Ph-indene4 and a one-pot reaction for the generation of the final
precatalyst which can be readily activated to the highly active
catalyst for propylene polymerization with common activators.
The reduction of ZrCl4(PR3)2 in toluene or hexanes with Li
powder and in the presence of 1 equiv of trans-1,4-diphenyl-1,3-
butadiene proceeds cleanly at ambient temperature to yield the
Zr(II) diene complexes Cl2Zr(1,4-Ph2C4H4)(PR3)2 (R ) Et, 1a; R
n
) Pr, 1b).5 Green et al. first reported6 the phosphine-stabilized
Zr(II) diene complexes prepared in low yield (18%) from the
† Colorado State University.
‡ The Dow Chemical Co.
§ University of Florida.
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J. AM. CHEM. SOC. 2004, 126, 42-43
10.1021/ja0381611 CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
coordination in 3. Diene coordination isomerization has previously
been reported for bis-Cp zirconium diene complexes, where heat
and light interconverted a σ-bound s-cis and a π-bound s-trans
butadiene or alkyl substituted diene ligand.10 It seems unlikely in
the current case that such an interconversion is occurring because
an aryl substituted diene ligand is used, which previously have been
shown not to interconvert, and 3 has been subjected to temperatures
as high as refluxing octane with no change in diene coordination.
The most likely scenario is a coordination isomerization during the
stepwise metalation reaction. After one-half of the ansa-bis(indenyl)
ligand coordinates to the zirconium center, the diene ligand
isomerizes to the π-bound s-trans coordination mode and then
directs the second half of the ansa-bis(indenyl) ligand to coordinate
to Zr in exclusively the rac-orientation.
Figure 1. Molecular structure of 1a. Selected bond lengths (Å): Zr-C1
) 2.438(3), Zr-C2 ) 2.488(5), Zr-C3 ) 2.424(5), Zr-C4 ) 2.421(3),
C1-C2 ) 1.263(3), C2-C3 ) 1.406(5), C3-C4 ) 1.267(3).
The divalent ansa-zirconocene 3 can be readily activated with
common activators such as MAO and perfluorophenyl derivatives
of neutral or ionic B and Al complexes, generating a highly active
propylene polymerization catalyst. For example, upon activation
with B(C6F5)3/iBu2Al(BHT), zirconocene 3 produces isotactic
polypropylene having Tm ) 157 °C, Mw ) 1.92 × 105, PDI )
1.79 at a 70 °C polymerization temperature and with an extremely
high efficiency of 1.17 × 108 g PP/g Zr.3,5
In conclusion, this work introduces a novel, highly efficient
synthesis of racemic ansa-zirconocene(II) catalysts. The proposed
configurational interplay between the diene and ansa-bis(indenyl)
ligands accounts for the high stereoselectivity in this synthesis.
Supporting Information Available: Experimental details (PDF)
and complete X-ray crystallographic data for complexes 1a and 3 (CIF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
References
Figure 2. Molecular structure of 3. Selected bond lengths (Å): Zr-C35
) 2.523(2), Zr-C36 ) 2.394(2), Zr-C37 ) 2.387(2), Zr-C38 )
2.515(2), C35-C36 ) 1.423(3), C36-C37 ) 1.412(2), C37-C38 )
1.417(3).
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The two phosphine ligands are arranged in a trans orientation, as
are the two chloride ligands.
The reaction of 1 with Li2{Me2Si(2-Me-4-Ph-Ind)2} (Scheme 1)
in toluene proceeds cleanly to form the desired racemic ansa-
zirconocene(II) diene complex 3 in quantitative yield (for an
isolation of 8.6 g of 3, the yield was 98%).5 At no time during the
1
reaction is any meso-isomer detected by H NMR spectroscopy.
The crystal structure of 3 (Figure 2) features the s-trans, π-bound
diene ligand, consistent with the Zr center being in the formal +2
oxidation state. The average Zr-C bond distance to the diene
termini is 0.128 Å longer than those to the internal carbon atoms,
which supports the Zr(II) assignment. The indenyl ligands are
arranged in the desired rac-orientation, and the “lock and key” fit
between the 1,4-diphenyl-1,3-butadiene ligand and the two substi-
tuted indenyl ligands is readily apparent from the structure, where
the two diene phenyl groups are placed into the coordination sphere
voids of the rac-structure. This “lock and key” arrangement clearly
minimizes the steric interactions of the diene phenyl rings with the
methyl and phenyl indenyl substituents. A meso-indenyl arrange-
ment would dispose at least two phenyl rings (one diene and one
indenyl) to very close proximity that clearly would be destabilizing;
this is probably why the meso-isomer is not observed.
(3) Chen, E. Y.; Campbell, R. E.; Devore, D. D.; Green, D. P.; Patton, J. T.;
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99/46270, 1999.
(4) For reactions involving other ligands, see ref 3.
(5) See the Supporting Information for synthesis and characterization details.
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Schrock, R. R.; Day, C. S. Inorg. Chem. 1981, 20, 1844-1849. (b) For
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The question arises as to how the diene-bonding mode transforms
from a π-bound, s-cis coordination in 1 to a π-bound s-trans
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