General synthesis of racemic Me2Si-bridged bis(indenyl) zirconocene complexes [12]

Full text of DOI:10.1021/ja001005z

J. Am. Chem. Soc. 2000, 122, 8093-8094
8093
General Synthesis of Racemic Me2Si-Bridged
Bis(indenyl) Zirconocene Complexes
Xingwang Zhang,† Qingming Zhu,† Ilia A. Guzei,‡ and
Richard F. Jordan*,§
Department of Chemistry
The UniVersity of Iowa, Iowa City, Iowa 52242
Department of Chemistry
Iowa State UniVersity, Ames, Iowa, 50011
Department of Chemistry, The UniVersity of Chicago
5735 South Ellis AVenue, Chicago, Illinois 60637
Figure 1. Molecular structure of Zr{PhN(CH2)3NPh}Cl2(THF)2 (1). Bond
distances (Å): Zr-Cl(1) 2.4785(5), Zr-Cl(2) 2.4565(5), Zr-O(1)
2.321(1), Zr-O(2) 2.302(2). Bond angles (deg): Cl(1)-Zr-Cl(2)
164.00(2), O(1)-Zr-O(2) 79.32(5). Torsion angles (deg): N(1)-Zr-
N(2)-C(10) -133.8(2), N(2)-Zr-N(1)-C(4) -127.8(2).
ReceiVed March 21, 2000
Chiral racemic ansa-zirconocene complexes can be activated
by MAO or other cocatalysts to generate excellent catalysts for
isotactic R-olefin polymerization and other stereoselective reac-
tions.1 Racemic SiMe2-bridged bis(indenyl) zirconocenes that con-
tain methyl and aryl substituents at the indenyl 2 and 4 positions,
respectively, are among the best metallocene catalysts for the pro-
duction of high molecular weight, isotactic poly(R-olefins).2 ansa-
Zirconocenes are normally synthesized by salt-elimination reac-
tions between ansa-bis(indenyl) dianion reagents and ZrX4 or Zr-
X4L2 compounds. However, the factors that control chemoselec-
tivity (i.e. metallocene vs dinuclear products) and diastereose-
lectivity (i.e. rac/meso selectivity) in these reactions are not well
understood, and extensive screening studies of reagents, counter-
ions, solvents, use of added ligands, and reaction conditions are
required for each case to optimize yields.3 Amine elimination
reactions of ansa-bis(indenes) and Zr(NR2)4 compounds provide
efficient routes to simple ansa-zirconocenes, but this approach
is not successful for sterically crowded cases.4 Here we report a
general, high-yield synthesis of rac-SiMe2-bridged bis(indenyl)
zirconocenes that exploits the conformational properties of a
simple chelating diamide ligand to control diastereoselectivity.
The chelated propylene-diamide complex Zr{PhN(CH2)3NPh}-
Cl2(THF)2 (1) can be prepared by two methods as shown in
Scheme 1. The reaction of ZrCl4 and 2 equiv of Li2[PhN(CH2)3-
NPh] in toluene affords Zr{PhN(CH2)3NPh}2 as a yellow solid
in 73% isolated yield. The reaction of ZrCl4 and Zr{PhN(CH2)3-
NPh}2 in THF/Et2O (1:1 by volume) yields 1 as a yellow solid
quantitatively.5 Alternatively, 1 can be prepared directly by the
reaction of ZrCl4 with 1 equiv of Li2[PhN(CH2)3NPh] in THF/
Et2O in 81% isolated yield.
Scheme 1
1 is monomeric and has approximate C2 symmetry with the C2
axis lying along the Zr- - -C(2) vector and bisecting the O(1)-
Zr-O(2) angle. The geometry at Zr is distorted octahedral and
the weak donor THF ligands are trans to the strong donor amide
groups. The Zr-N bond distances (2.082(2), 2.080(2) Å) are
normal and the N(1)-Zr-N(2) angle (91.63(6)°) is close to the
ideal octahedral value. The six-membered C(1)-N(1)-Zr-N(2)-
C(3)-C(2) chelate ring adopts a twist conformation.7 The N(1),
Zr, N(2), and C(2) atoms are coplanar to within 0.02 Å, and C(1)
and C(3) lie 0.79 Å above and below the N(1)-Zr-N(2)-C(2)
plane, respectively. This conformation places the two phenyl rings
on opposite sides of the N(1)-Zr-N(2)-C(2) plane; the C(4)-
A view of the molecular structure of 1 which highlights the
conformation of the chelate ring is shown Figure 1.6 Compound
† The University of Iowa.
‡ Iowa State University.
§ The University of Chicago.
(1) (a) Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.; Waymouth,
R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143. (b) Hoveyda, A. H.;
Morken, J. P. Angew. Chem., Int. Ed. Engl. 1996, 35, 1262.
(2) (a) Spaleck, W.; Ku¨ber, F.; Winter, A.; Rohrmann, J.; Bachmann, B.;
Antberg, M.; Dolle, V.; Paulus, E. F. Organometallics 1994, 13, 954. (b)
Stehling, U.; Diebold, J.; Kirsten, R.; Ro¨ll, W.; Brintzinger, H. H.; Ju¨ngling,
S.; Mu¨lhaupt, R.; Langhauser, F. Organometallics 1994, 13, 964. (c) Spaleck,
W.; Antberg, M.; Rohrmann, J.; Winter, A.; Bachmann, B.; Kiprof, P.; Behm,
J.; Herrmann, W. A. Angew. Chem., Int. Ed. Engl. 1992, 31, 1347. (d) Ju¨ngling,
S.; Mu¨lhaupt, R.; Stehling, U.; Brintzinger, H. H.; Fischer, D.; Langhauser,
F. J. Polym. Sci. A: Polym. Chem. 1995, 33, 1305. See also: (e) Resconi, L.;
Balboni, D.; Baruzzi, G.; Fiori, C.; Guidotti, S.; Mercandelli, P.; Sironi, A.
Organometallics 2000, 19, 420.
1
N(1)- -N(2)-C(10) torsion angle is 145.2°.8 However, the H
NMR spectrum of 1 contains two methylene resonances for the
diamide ligand in a 2:1 intensity ratio down to -105 °C (THF-
d8), which implies that ring inversion is fast on the NMR scale
in solution.
(3) The best reported syntheses of rac-Me2Si-bridged zirconocenes that
we are aware of are given in ref 2 and the following: (a) Strickler, J. R.;
Power, J. M. U.S. Patent 5,847,175, 1998; Chem. Abstr. 1998, 130, 52581.
(b) Rohrmann, J.; Ku¨ber, F. U.S. Patent 5,616,747, 1997. (c) Fischer, D.;
Schweier, G.; Brintzinger, H. H.; Damrau, H. R. H. European Patent
Application 0 745 606 A2, 1996; Chem. Abstr. 1996, 126, 75069.
(4) (a) Christopher, J. N.; Diamond, G. M.; Jordan, R. F. Organometallics
1996, 15, 4038. (b) Diamond, G. M.; Jordan, R. F.; Petersen, J. L. J. Am.
Chem. Soc. 1996, 118, 8024.
(6) X-ray data for 1: tetragonal, P42/n, a ) b ) 24.977(1) Å, c ) 7.8479(4)
Å, V ) 4895.8(4) Å3, Z ) 8, T ) 173(2) K, Dcalc ) 1.440 g/cm3; R1 )
0.0248, wR2 ) 0.0708 for 5002 independent reflections with I > 2σ(I); GOF
on F2 ) 0.997.
(7) March, J. AdVanced Organic Chemistry; Wiley: New York, 1985; p 124.
(8) In contrast, the chelate rings in 4- and 5-coordinate Zr(IV) and Ti(IV)
propylene-diamide complexes containing bulky aryl or silyl N-substituents
adopt boat conformations. (a) Scollard, J. D.; McConville, D. H.; Vittal, J. J.
Organometallics 1995, 14, 5478. (b) Scollard, J. D.; McConville, D. H.; Payne,
N. C.; Vittal, J. J. Macromolecules 1996, 29, 5241. (c) Lee, C. H.; La, Y.;
Park, J. W. Organometallics 2000, 19, 344.
(5) This procedure is based on the reported synthesis of Zr(NMe2)2Cl2-
(THF)2. Brenner, S.; Kempe, R.; Arndt, P. Z. Anorg. Allg. Chem. 1995,
621, 2021.
10.1021/ja001005z CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/04/2000

8094 J. Am. Chem. Soc., Vol. 122, No. 33, 2000
Communications to the Editor
Figure 2. Molecular structure of rac-(MBSBI)Zr{PhN(CH2)3NPh} (3c).
Bond distances (Å): Zr-N(1) 2.073(2), Zr-N(2) 2.122(2). Torsion angles
(deg): N(1)-Zr-N(2)-C(10) -141.9(3), N(2)-Zr-N(1)-C(6) -131.2(3).
The reaction of 1 with 1 equiv of the lithium ansa-bis(indenyl)
reagents Li2[SBI′](Et2O) 2a-d (2a, SBI′ ) (1-indenyl)2SiMe2
(SBI); 2b, SBI′ ) (2-methyl-1-indenyl)2SiMe2 (MSBI); 2c, SBI′
) (2-methyl-4,5-benz-1-indenyl)2SiMe2 (MBSBI); 2d, SBI′ ) (2-
methyl-4-phenyl-1-indenyl)2SiMe2, (MPSBI)) in Et2O affords the
corresponding rac-(SBI′)Zr{PhN(CH2)3NPh} zirconocenes 3a-d
in >90% NMR yield (Scheme 1). The 1H NMR spectra of 3a-d
each contain one SiMe2 resonance, one 2-H (for 3a) or 2-Me
(for 3b-d) resonance, three NCH2CH2CH2N methylene reso-
nances, and appropriate indenyl resonances consistent with C2
symmetry. The meso isomers of 3a-d were not detected.
Compounds 3c and 3d were isolated in pure form as red solids
in 90% and 87% yield, respectively.
The molecular structure of 3c was determined by X-ray crys-
tallography (Figure 2).9 The Zr-diamide unit in 3c is structurally
similar to that in 1. The twist conformation of the chelate ring,
the large C(6)-N(1)- -N(2)-C(10) torsion angle (142.8°), and
the N(1)-Zr-N(2) angle (86.77(9)°) are very similar to the
corresponding features in 1. The Zr-centroid distances (2.342,
2.291 Å) and centroid-Zr-centroid angle (123.1°) are compa-
rable to those in rac-(MBSBI)ZrCl2 (2.247 Å, 127.9°).2b
A reasonable explanation for the high selectivity for rac-
metallocene products in Scheme 1 is that the twist conformation
of the Zr propylene-bisamide chelate ring constrains the two
N-Ph groups to lie above and below the N-Zr-N plane, which
accommodates the rac-metallocene structure but sterically dis-
favors the meso structure (and the transition state and η5,η1-bis-
(indenyl) intermediate leading thereto).10 For comparison, the
reaction of 2c with Zr(NMePh)2Cl2(THF)2, the nonchelated
analogue of 1, yields a 1/1 mixture of rac- and meso-(MBSBI)-
Zr(NMePh)2 along with 20% of the dinuclear species (MBSBI)-
{Zr(NMePh)2Cl}2.11 X-ray crystallographic analyses show
that the Zr(NMePh)2 units in Zr(NMePh)2Cl2(THF)2 and rac-
(MBSBI)Zr(NMePh)2 (Figure 3a) are structurally similar to the
Zr{PhN(CH2)3NPh} units in 1 and 3c, with approximate C2
symmetry, large C(Ph)-N- -N-C(Ph) torsion angles (155.5° and
152.8°, respectively), and placement of the two N-phenyl groups
on opposite sides of the N-Zr-N plane. However, meso-
(MBSBI)Zr(NMePh)2 can form because the nonchelated Zr-
NMePh ligands can rotate to relieve steric crowding between
the N-phenyl and indenyl groups on the crowded side of the
metallocene, as illustrated in Figure 3b. Similarly, the reaction
Figure 3. Schematic drawings of the molecular structures of rac- and
meso-(MBSBI)Zr(NMePh)2 (a, b) and rac- and meso-(MBSBI)Zr(PhCH2-
CH2NPh) (c, d) based on X-ray crystallographic analyses (R4 and R5 )
benzo).
of 2c with the 0.5 equiv of the ethylene-diamide complex
{Zr(PhNCH2CH2NPh)Cl(THF)}2(µ-Cl)2 yields a 2/1 mixture of
rac- and meso-(MBSBI)Zr(PhNCH2CH2NPh).11 X-ray crystal-
lographic studies show that the ZrNCH2CH2N chelate rings in
rac- and meso-(MBSBI)Zr(PhNCH2CH2NPh) (Figure 3c,d) adopt
envelope conformations in which one N-Ph group lies in the
N-Zr-N plane, which allows the meso isomer to form.
We previously reported that rac-(SBI)Zr(NMe2)2 and rac-(EBI)-
Zr(NMe2)2 (EBI ) 1,2-ethylenebis(indenyl)) are quantitatively
converted to rac-(SBI)ZrCl2 (4a) and rac-(EBI)ZrCl2 by reaction
with Me3SiCl.4 Similarly, 3a is cleanly converted to 4a (100%
NMR) by reaction with Me3SiCl in CD2Cl2 at 60 °C (sealed tube,
Scheme 1). In contrast, no reaction is observed between 3d and
Me3SiCl in CD2Cl2 (100 °C, 30 h, sealed tube). However, 3b-d
react cleanly with HCl in Et2O or toluene at -78 °C to afford
the corresponding rac zirconocene dichlorides 4b-d in high yield
(Scheme 1). rac-(MBSBI)ZrCl2 (4c) was isolated in 70% yield
(vs 1) by initial generation of 3c from 1 and 2c, filtration to
remove the LiCl coproduct, treatment with HCl at -78 °C,
removal of the solvent, and washing with benzene to remove the
PhNH(CH2)3NHPh coproduct. Dichlorides 4b and 4d were
isolated in 76% and 51% yield, respectively (vs ZrCl4), by in
situ generation of 1 from ZrCl4 and Li2[PhN(CH2)3NPh], treatment
with the appropriate Li2[SBI′](Et2O) reagent, filtration to remove
the LiCl coproduct, treatment with HCl at -78 °C, and filtration.
The lower isolated yield for 4d is due to its high solubility.
These results show that reaction of the easily accessible
propylene-diamide complex Zr{PhN(CH2)3NPh}Cl2(THF)2 (1)
with Li2[SBI′](Et2O) reagents provides a general, high-yield,
stereoselective route to rac-(SBI′)Zr{PhN(CH2)3NPh} complexes,
including those with 2-Me substituents. These zirconocene-
diamide complexes, which can be isolated or used in situ, can be
converted to the corresponding rac-(SBI′)ZrCl2 complexes by
reaction with Me3SiCl (for 3a) or HCl (for 3b-d). The confor-
mational properties of the PhN(CH2)3NPh2- ligand are the key
to the stereoselectivity in these reactions and this method should
be applicable to a wide variety of ansa-metallocenes. Work is in
progress to extend this approach to the enantioselective synthesis
of ansa-metallocenes using chiral diamide ligands.
(9) X-ray data for 3c: triclinic, P1h, a ) 10.3355(6) Å, b ) 12.1037(7) Å,
c ) 14.8777(8) Å, R ) 93.503(1)°, â ) 97.219(1)°, γ ) 107.125(1)°,
V ) 1755.1(2) Å3, Z ) 2, T ) 173(2) K, Dcalc ) 1.382 g/cm3; R1 ) 0.0453,
wR2 ) 0.0751 for 7121 independent reflections with I > 2σ(I); GOF on
F2 ) 1.010.
(10) Wiesenfeldt, H.; Reinmuth, A.; Barsties, E.; Evertz, K.; Brintzinger,
H.-H. J. Organomet. Chem. 1989, 369, 359.
(11) (a) Zhang, X. W.; Jordan, R. F. Manuscript in preparation. (b) X-ray
crystallographic analyses of Zr(NMePh)2Cl2(THF)2, rac- and meso-(MBSBI)-
Zr(NMePh)2, {Zr(PhNCH2CH2NPh)Cl(THF)}2(µ-Cl)2, and rac- and meso-
(MBSBI)Zr(PhNCH2CH2NPh) will be included in a full report.
Acknowledgment. This work was supported by the Albemarle
Corporation.
Supporting Information Available: Synthetic procedures, charac-
terization data for new compounds, and details of the X-ray structure
determinations of 1 and 3c (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
JA001005Z