Anionic ansa-zirconocenes with pentafluorophenyl-substituted borato bridges

Article abstract of DOI:10.1021/om010028z

The ansa-borane complex (Me₂S)(C₆H₅)B (C₅H₄)₂ZrCl₂ (1) reacts selectively with 2 equiv of LiC₆F₅ to give (Me₂S)(C₆H₅)B (C₅H₄)₂Zr(C₆ F₅)₂ (2) and with 3 equiv LiC₆F₅ to form the borato-bridged complex [Li(Et₂O)₃][(C₆F₅)(C₆ H₅)B(C₅H₄) ₂Zr(C₆F₅)₂] (3a). The C₆F₅ groups are exchanged for Me by reaction with AlMe₃ to form [Li(Et₂O)_x][(C₆F₅)(C₆ H₅) B(C₅H₄)₂ZrMe₂] (4). Treatment of BCl₃ with (SiMe₃)(SnMe₃)C₅H₄ affords BCl(C₅H₄SiMe₃)₂ (5), which undergoes a dehalosilylation reaction with ZrCl₄(Me₂S)₂ to give Cl(Me₂S)B(C₅H₄)ZrCl₂ (6). The reaction of 6 with LiC₆F₅ proceeds in a fashion similar to that of 2, leading first to (Et₂O)ClB(C₅H₄)₂Zr(C₆ F₅)₂ (7) and with 5 equiv of LiC₆F₅ to [Li(Et₂O)₄][(C₆F₅)₂ B(C₅H₄)₂Zr (C₆F₅)₂] (9a) in 28% yield. The yield is improved through the use of C₆F₅MgBr at 60°C. C₆F₅ groups are readily exchanged for Me through reaction with LiMe to give [Li(Et₂O)₄] [(C₆F₅)₂B (C₅H₄)₂ZrMe₂] (10a). Complexes 3, 9, and 10 are catalysts for ethene polymerization when activated with MAO or AlBu₃ⁱ/[CPh₃] [B(C₆F₅)₄].

Full text of DOI:10.1021/om010028z

Organometallics 2001, 20, 2093-2101
2093
An ion ic a n sa -Zir con ocen es w ith
P en ta flu or op h en yl-Su bstitu ted Bor a to Br id ges
Simon J . Lancaster* and Manfred Bochmann
Wolfson Materials and Catalysis Centre, School of Chemical Sciences, University of East
Anglia, Norwich NR4 7TJ , U.K.
Received J anuary 16, 2001
The ansa-borane complex (Me2S)(C6H5)B(C5H4)2ZrCl2 (1) reacts selectively with 2 equiv
of LiC6F5 to give (Me2S)(C6H5)B(C5H4)2Zr(C6F5)2 (2) and with 3 equiv LiC6F5 to form the
borato-bridged complex [Li(Et2O)3][(C6F5)(C6H5)B(C5H4)2Zr(C6F5)2] (3a ). The C6F5 groups are
exchanged for Me by reaction with AlMe3 to form [Li(Et2O)x][(C6F5)(C6H5)B(C5H4)2ZrMe2]
(4). Treatment of BCl3 with (SiMe3)(SnMe3)C5H4 affords BCl(C5H4SiMe3)2 (5), which
undergoes a dehalosilylation reaction with ZrCl4(Me2S)2 to give Cl(Me2S)B(C5H4)ZrCl2 (6).
The reaction of 6 with LiC6F5 proceeds in a fashion similar to that of 2, leading first to
(Et2O)ClB(C5H4)2Zr(C6F5)2 (7) and with 5 equiv of LiC6F5 to [Li(Et2O)4][(C6F5)2B(C5H4)2Zr-
(C6F5)2] (9a ) in 28% yield. The yield is improved through the use of C6F5MgBr at 60 °C. The
C6F5 groups are readily exchanged for Me through reaction with LiMe to give [Li(Et2O)4]-
[(C6F5)2B(C5H4)2ZrMe2] (10a ). Complexes 3, 9, and 10 are catalysts for ethene polymerization
when activated with MAO or AlBui /[CPh3][B(C6F5)4].
3
In tr od u ction
by us and others in modifying the structure of the active
species (or its immediate precursor complex) by explor-
ing electrically neutral analogues and zwitterionic
structures where the anionic component is covalently
attached to the cationic species. Examples are complexes
with borato-substituted cyclopentadienyl ligands, as in
II7 and III,8 “self-activating” complexes containing
Lewis-acidic Cp substituents as in IV,9 Erker’s allylic
zwitterion V,10 half-sandwich complexes of type VI,11
and complexes with dianionic ligands such as boroles
(VII)11d,12 and trimethylenemethane complexes (VIII).13
A long-term aim of our research has been the synthesis
of zwitterionic ansa-metallocenes where the weakly
As is now well established,1 the active species in
metallocene-based olefin polymerization catalysts are
electron-deficient cationic metallocene alkyl complexes
of the type [Cp2MR]+ (Chart 1, structure I), which are
stabilized by weakly coordinating anions such as
[MeMAO]- or [B(C6F5)4]-.2-6 This has sparked interest
(1) For reviews see: (a) Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt,
R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995,
34, 1143. (b) Bochmann, M. J . Chem. Soc., Dalton Trans. 1996, 255.
(c) Chen, E. Y. X.; Marks, T. J . Chem. Rev. 2000, 100, 1391.
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J . A.; Elder, M. J . Eur. Patent Appl. 0426 637, 1990. (c) Ewen, J . A.;
Elder, M. J . Eur. Patent Appl. 0427 697, 1991. (d) Yang, X.; Stern, C.
L. Marks, T. J . J . Am. Chem. Soc. 1991, 113, 3623. (e) Ewen, J . A.;
Elder, M. J . Makromol. Chem., Macromol. Symp. 1993, 66, 179. (f)
Ewen, J . A. Stud. Surf. Sci. Catal. 1994, 89, 405.
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T. J . J . Am. Chem. Soc. 1997, 119, 2582. (f) Chen, Y. X.; Metz, M. V.;
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Chem. 1998, 76, 1249. (b) Ko¨hler, K.; Piers, W. E.; J arvis, A. P.; Xin,
S.; Feng, Y.; Brasakis, A. M.; Collins, S.; Clegg, W.; Yap, G. P. A.;
Marder, T. B. Organometallics 1998, 17, 3557. (c) Williams, V. C.; Piers,
W. E.; Clegg, W.; Elsegood, M. R. J .; Collins, S.; Marder, T. B. J . Am.
Chem. Soc. 1999, 121, 3244. (d) Williams, V. C.; Dai, C.; Li, Z.; Collins,
S.; Piers, W. E.; Clegg, W.; Elsegood, M. R. J .; Marder, T. B. Angew.
Chem., Int. Ed. 1999, 38, 3695. (e) Williams, V. C.; Irvine, G. J .; Piers,
W. E.; Li, Z.; Collins, S.; Clegg, W.; Elsegood, M. R. J .; Marder, T. B.
Organometallics 2000, 19, 1619.
(5) Delocalized diborate anions have been described: (a) Lancaster,
S. J .; Walker, D. A.; Thornton-Pett, M.; Bochmann, M. Chem. Commun.
1999, 1533. (b) LaPointe, R. E. WO 99/42467, 1999. (c) LaPointe, R.
E.; Roof, G. R.; Abboud, K. A.; Klosin, J . J . Am. Chem. Soc. 2000, 122,
9560. (d) Zhou, J .; Lancaster, S. J .; Walker, D. A.; Beck, S.; Thornton-
Pett, M.; Bochmann, M. J . Am. Chem. Soc. 2001, 123, 223.
(7) (a) Bochmann, M.; Lancaster, S. J .; Robinson, O. B. J . Chem.
Soc., Chem. Commun. 1995, 2081. (b) Lancaster, S. J .; Thornton-Pett,
M.; Dawson, D. M.; Bochmann, M. Organometallics 1998, 17, 3829.
(c) Ruwwe, J .; Erker, G.; Fro¨hlich, R. Angew. Chem., Int. Ed. Engl.
1996, 35, 80. (d) Burlitch, J . M.; Burk, J . H.; Leonowicz, M. E.; Hughes,
R. E. Inorg. Chem. 1979, 18, 1702. (e) Braunschweig, H.; Wagner, T.
Chem. Ber. 1994, 127, 1613. (f) Bohra, R.; Hitchcock, P. B.; Lappert,
M. F.; Au-Yeung, S. F.; Leung, W. P. J . Chem. Soc., Dalton Trans.
1995, 2999.
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J . Organomet. Chem. 1997, 545-546, 597.
(9) (a) Reetz, M. T.; Bru¨mmer, H.; Kessler, M.; Kuhnigk, J . Chimia
1995, 49, 501. (b) Duchateau, R.; Lancaster, S. J .; Thornton-Pett, M.;
Bochmann, M. Organometallics 1997, 16, 4995. (c) Lancaster, S. J .;
Al-Benna, S.; Thornton-Pett, M.; Bochmann, M. Organometallics 2000,
19, 1599.
(10) (a) Temme, B.; Erker, G.; Karl, J .; Luftmann, R.; Fro¨hlich, R.;
Kotila, S. Angew. Chem., Int. Ed. Engl. 1995, 34, 1755. (b) Karl, J .;
Erker, G.; Fro¨hlich, R. J . Am. Chem. Soc. 1997, 119, 11165. Karl, J .;
Erker, G. J . Mol. Catal. A: Chem. 1998, 128, 85. (c) Karl, J .; Dahlmann,
M.; Erker, G.; Bergander, K. J . Am. Chem. Soc. 1998, 120, 5643.
(11) (a) J ime´nez Pindado, G.; Thornton-Pett, M.; Bouwkamp, M.;
Meetsma, A.; Hessen, B.; Bochmann, M. Angew. Chem., Int. Ed. Engl.
1997, 36, 2358. (b) J ime´nez Pindado, G.; Thornton-Pett, M.; Bochmann,
M. J . Chem. Soc., Dalton Trans. 1997, 3115. (c) J ime´nez Pindado, G.;
Thornton-Pett, M.; Hursthouse, M. B.; Coles, S. J .; Bochmann, M. J .
Chem. Soc., Dalton Trans. 1999, 1663. (d) J ime´nez Pindado, G.;
Lancaster, S. J .; Thornton-Pett, M.; Bochmann, M. J . Am. Chem. Soc.
1998, 120, 6816. (e) Corradi, M. M.; J ime´nez Pindado, G.; Sarsfield,
M. J .; Thornton-Pett, M.; Bochmann, M. Organometallics 2000, 19,
1150.
(6) Zwitterionic alternatives to cationic active species have been
discussed, see: (a) Piers, W. E. Chem. Eur. J . 1998, 4, 1. (b) Piers, W.
E.; Sun, Y.; Lee, L. W. M. Top. Catal. 1999, 7, 133.
10.1021/om010028z CCC: $20.00 © 2001 American Chemical Society
Publication on Web 04/19/2001

2094 Organometallics, Vol. 20, No. 10, 2001
Lancaster and Bochmann
Ch a r t 1
Sch em e 1
coordinated anion is incorporated into the bridge, to give
compounds of type IX. Early attempts to access such
structures using preformed bis(cyclopentadienyl)borates
[(C6F5)2B(C5H5)2]- had not been successful since these
borato ligands proved elusive. We report here an
alternative route to complexes of type IX.
A number of groups have reported the synthesis of
boron-bridged ansa-metallocene complexes. The com-
plexes can be broadly divided into two classes: (i)
zirconocenes bridged by aminoboryl moieties where the
electrophilicity of boron is attenuated by partial π-bond-
ing to nitrogen;14 and (ii) zirconocenes bridged with
Lewis-acidic boryl linkages.15,16 These offer the prospect
of coordinating Lewis-basic ligands to boron. However,
in the absence of strongly coordinating donors such as
PMe3 these complexes proved unstable toward alkyl-
ation.15
An alternative strategy to first synthesizing a borato-
bis(cyclopentadienyl) ligand is the preparation of ansa-
borane complexes of the type that Shapiro and others
have made, followed by their subsequent conversion to
ansa-borates (structure IX). We believed that the ideal
bridging group would be [-B(C6F5)2-], which, from our
experience of the tetrakis(pentafluorophenyl)borate an-
ion chemistry, should have greater stability toward
electron and ligand transfer than the known -BPh(Cl)-
and -BPh(Me)- bridged examples.
We report here the synthesis of bis(pentafluorophe-
nyl)borato-bridged ansa-zirconocene complexes and their
alkylation products.
Resu lts a n d Discu ssion
The ansa-borane complex (Me2S)(C6H5)B(C5H4)2ZrCl2
(1) was prepared following the literature procedure.15
Bright yellow 1 crystallizes very readily from hot
toluene solution. The green color reported15 was only
observed on exposure of solutions of 1 to air.
Very recently, while this work was in progress,
Shapiro has reported the first synthesis of borato-
bridged ansa-metallocenes by the reaction of (Me2S)-
(C6H5)B(C5H4)2ZrCl2 (1) with PPNCl or Cp*2AlMe to
give [PPN][(Cl)(C6H5)B(C5H4)2ZrCl2] and [Cp*2Al][(Me)-
(C6H5)B(C5H4)2ZrCl2], respectively (Scheme 1).17
The reaction of 1 equiv of LiC6F5 with complex 1 was
expected to either lead to the borato complex Li[(C6F5)-
(C6H5)B(C5H4)2ZrCl2] or result in alkylation to give
(Me2S)(C6H5)B(C5H4)2Zr(C6F5)Cl. In practice we ob-
tained a mixture of unreacted 1 and a new complex, 2.
The difference in solubility between 1 and 2 made
separation straightforward. Elemental analysis indi-
cated that 2 had lost both chloride ligands; this coupled
with the presence of a Me2S donor, a very similar 11B
NMR chemical shift to 1, and 19F o-C6F5 resonances
consistent with bonding to zirconium led to the identi-
fication of the product as (Me2S)(C6H5)B(C5H4)2Zr(C6F5)2
(2) (Scheme 2). Spectroscopic data of 1 and all new
(12) (a) Quan, R. W.; Bazan, G. C.; Kiely, A. F.; Schaefer, W. P.;
Bercaw, J . E. J . Am. Chem. Soc. 1994, 116, 4489. (b) Pastor, A.; Kiely,
A. F.; Henling, L. M.; Day, M. W.; Bercaw, J . E. J . Organomet. Chem.
1997, 528, 65. (c) Kiely, A. F.; Nelson, C. M.; Pastor, A.; Henling, L.
M.; Day, M. W.; Bercaw, J . E. Organometallics 1998, 17, 1324.
(13) Rodriguez, G.; Bazan, G. C. J . Am. Chem. Soc. 1997, 119, 343.
(14) (a) Braunschweig, H.; Dirk, R.; Mu¨ller, M.; Nguyen, P.; Re-
sendes, R.; Gates, D. P.; Manners, I. Angew. Chem., Int. Ed. Engl. 1997,
36, 2338. (b) Braunschweig, H.; von Koblinski, C.; Wang, R. Eur. J .
Inorg. Chem. 1999, 69. (c) Ashe, A. J .; Fang, X.; Kampf, J . W.
Organometallics 1999, 18, 2288.
(15) Stelck, D. S.; Shapiro, P. J .; Basickes, N.; Rheingold, A. L.
Organometallics 1997, 16, 4546.
(16) (a) Rufanov, K. A.; Kotov, V. V.; Kazennova, N. B.; Lemenovskii,
D. A.; Avtomonov, E. V.; Lorberth, J . J . Organomet. Chem. 1996, 525,
287. (b) Rufanov, K. A.; Avtomonov, E. V.; Kazennova, N. B.; Kotov,
V. V.; Khvorost, A.; Lemenovskii, D. A.; Lorberth, J . J . Organomet.
Chem. 1997, 536-537, 361. (c) Reetz, M. T.; Wiluhn, M.; Psiorz, C.;
Goddard, R. J . Chem. Soc., Chem. Commun. 1999, 1105.
(17) Burns, C. T.; Stelck, D. S.; Shapiro, P. J .; Ashwani, V.; Kunz,
K.; Kehr, G.; Concolino, T.; Rheingold, A. L. Organometallics 1999,
18, 5432.

Borato Bridged Zirconocenes
Organometallics, Vol. 20, No. 10, 2001 2095
Sch em e 2
complexes are collected in Table 1. 2 can be obtained in
70% yield by employing 2 equiv of LiC6F5. Complex 2
was repeatedly observed as a byproduct in crude reac-
tion mixtures with excess LiC6F5. The observation that
the first two equivalents of LiC6F5 will selectively
alkylate at zirconium is in contrast with the literature
report stating that clean alkylation first requires ex-
change of the Me2S for a more strongly bound PMe3
donor.17
The use of a third equivalent of LiC6F5 led to
displacement of dimethyl sulfide and formation of an
ansa-borate complex, [Li(Et2O)3][(C6F5)(C6H5)B(C5H4)2-
Zr(C6F5)2] (3a ). Borate formation is indicated by the low
solubility of 3a in aromatic solvents, the 11B NMR
chemical shift (δ -11.81), and the presence of reso-
nances for both Zr-C6F5 and B-C6F5 moieties in the
19F NMR spectrum. The presence of two different phenyl
substituents on the boron results in four resonances for
the cyclopentadienyl protons and two inequivalent Zr-
C6F5 ligands.
into a Zr-C6F5 bond is likely to be facile. Preliminary
NMR reactions indicated that the exchange reaction
with AlMe3 did proceed but was slow and led to the
formation of side-products. The smoothest reaction,
leading to the best characterized zirconium methyl
species, was that between the lithium salt 3 and 10
equiv of AlMe3 carried out over 36 h in dichloromethane.
Although the product was not pure, the spectroscopic
characterization of the major product was consistent
with structure 4 (Scheme 2). The 11B NMR chemical
shift (δ -10.9) and observation of a B-C6F5 group
indicate that the borate bridge is intact. As expected,
1
two resonances are observed with H (δ 0.14 and 0.10)
and 13C NMR chemical shifts (δ 28.92 and 28.59) that
are consistent with Zr-Me groups.
A further indication of the ability of aluminum alkyls
to exchange with the Zr-C6F5 groups and produce
reactive species is given by ethene polymerization
results obtained in the 3b/MAO system, which show
good activities after short induction times (Table 2).
Following the formation of stable ansa-borate com-
plexes with one C6F5 and one C6H5 substituent, we
wished to prepare the analogue with two C6F5 groups.
This compound should have improved stability due to
the greater electron-withdrawing capability and steric
bulk of the C6F5.
The yield of complex 3a is improved if 4 equiv of
LiC6F5 are used, presumably because reaction at boron
is in competition with the decomposition of the lithium
reagent.18
The cation of complex 3a can be exchanged readily
for [NEt4]+ or [PPh4]+ to give complexes 3b and 3c,
respectively (Scheme 2). This leads to small changes in
Boron trichloride reacts with 2 equiv of (SiMe3)-
(SnMe3)C5H4 with selective dehalostannylation to yield
BCl(C5H4SiMe3)2 (5) (Scheme 3). Addition of ZrCl4-
(Me2S)2 at 70 °C gave Cl(Me2S)B(C5H4)ZrCl2 (6), which
was purified by recrystallization from dichloromethane.
The elemental analysis was consistent with three
chlorine atoms per molecule, and the characteristic four
resonances were observed for the inequivalent cyclo-
pentadienyl protons. The 11B NMR chemical shift (δ 5.0)
indicates a neutral four-coordinate boron atom similar
to that observed for the neutral boron bridges with Me2S
1
the H NMR spectra of the anion.
Having demonstrated the formation of ansa-borate
complexes with a B-C6F5 substituent, we wished to
selectively exchange the Zr-C6F5 ligands for more
reactive methyl groups. Efficient zwitterion formation
and alkene insertion chemistry would require alkyl
exchange since neither C6F5 abstraction nor insertion
(18) LiC6F5 decomposes above ca. -20 °C in a potentially explosive
reaction to give LiF and benzyne: Chernega, A. N.; Graham, A. J .;
Green, M. L. H.; Haggitt, J .; Lloyd, J .; Mehnert, C. P.; Metzler, N.;
Souter, J . J . Chem. Soc., Dalton Trans. 1997, 2293. See also ref 3f.
1
donors in 1 and 2. The H NMR chemical shift of Me2S

2096 Organometallics, Vol. 20, No. 10, 2001
Ta ble 1. 1H, 11B, 13C, a n d 19F NMR Da ta for Com p lexes 1-10
Lancaster and Bochmann

Borato Bridged Zirconocenes
Organometallics, Vol. 20, No. 10, 2001 2097
Ta ble 2. Eth en e P olym er iza tion P r od u ctivities (1 ba r )
complex
(µmol)
AlBui
(µmol)
temp
(°C)
time
(s)
polymer yield
(g)
3
activator (µmol)
productivitya
0.28
3b
(10)
3b
MAO
20
60
20
60
60
20
60
20
60
20
60
20
60
600
240
1200
600
600
180
180
300
300
300
300
300
300
0.466
0.537
0.332
0.560
0.913
0.226
0.533
0.095
0.286
0.120
0.153
0.255
0.162
(5000)
MAO
0.80
0.19
0.67
1.10
0.90
2.13
0.23
0.69
0.29
0.37
0.61
0.39
(10)
9a b
(5)
(5000)
MAO
(5000)
9a
MAO
(5)
(5000)
9a c
(10)
9a
MAO
(6000)
[CPh3][B(C6F5)4]
(10)
500
500
(5)
9a
[CPh3][B(C6F5)4]
(10)
MAO
(10000)
MAO
(10000)
[CPh3][B(C6F5)4]
(5)
[CPh3][B(C6F5)4]
(5)
[CPh3][B(C6F5)4]
(10)
[CPh3][B(C6F5)4]
(10)
(5)
10a
(5)
10a
(5)
10a
(5)
10a
(5)
10b
(5)
500
500
500
500
10b
(5)
a
In 106 g PE/(mol M)‚h‚bar. b Ethene uptake rate increases markedly after first 5 min. c MAO and 9a premixed 1 h before polymerization.
Sch em e 3
adduct, possibly due to reduced steric crowding in the
chloro- versus the phenylborane. The identity of 7 was
confirmed by elemental analysis, indicating the presence
of one chloride ligand; the 11B NMR chemical shift (δ
14.0) and a set of four different cyclopentadienyl proton
1
signals implied two different ligands on boron. The H
NMR signals for the coordinated Et2O show a large
downfield shift (δ 4.82 and 1.70). There is no evidence
for exchange with free diethyl ether in solution at 20
°C, indicating very strong coordination of the ligand. The
19F NMR spectrum shows two different Zr-C6F5 ligands
but no B-C6F5 groups. Crude product mixtures from
reactions between 6 and up to 6 equiv of LiC6F5
invariably contained both 7 and a second new com-
pound, 9a . The physical properties of 9a are consistent
with formulation as an ionic compound; 9a is poorly
soluble in toluene and can be best purified by precipi-
tating as an oil from a toluene solution by cooling to
-30 °C. This oil retains some toluene even when dried
under vacuum to an amorphous foam. Unfortunately it
proved impossible to crystallize either 9a or the [NEt4]+
salt, 9b. Examination of the spectroscopic data (Table
1) provides strong evidence for the formulation of the
anion of 9 as [(C6F5)2B(C5H4)2Zr(C6F5)2]-. The 11B NMR
chemical shift at δ -12.2 ppm indicates a four-
coordinate anionic borate atom similar to that seen for
3. Unlike 3, compound 9 has C2v symmetry, and only
two resonances are observed for the cyclopentadienyl
protons. The 19F NMR spectrum (Figure 1) indicates just
two types of C6F5 group of equal intensity, one with a
very broad o-F resonance at δ -118 (Zr-C6F5) and a
sharper signal for o-F at δ -132 (B-C6F5). The o-F
signals for the Zr-C6F5 group are consistently observed
to be broad at room temperature, presumably due to
hindered rotation about the Zr-C bond.
(δ 2.61) is 0.45 ppm downfield of that found for 1, which
indicates greater Lewis acidity at the boron center. Like
1, complex 6 is soluble in warm toluene and dichloro-
methane but poorly soluble in light petroleum or diethyl
ether.
Using a method similar to that employed for the
preparation of 3, we followed the reaction between 6 and
4 equiv of LiC6F5. Again the more reactive site in the
molecule was not at boron but at zirconium, and the
first step in the reaction sequence was alkylation to form
(L)ClB(C5H4)2Zr(C6F5)2 (7) (Scheme 4). This compound
could be isolated pure by crystallization from diethyl
ether. Unlike complex 2, the alkylation product was the
diethyl ether adduct rather than the dimethyl sulfide
The yield of 9a obtained through reaction of 6 with
LiC6F5 was found to be highly variable. The use of larger
excesses of LiC6F5 and longer reaction times did not

2098 Organometallics, Vol. 20, No. 10, 2001
Lancaster and Bochmann
F igu r e 1. 19F NMR spectrum of 9a , [Li(Et2O)4][(C6F5)2B(C5H4)2Zr(C6F5)2] (282.4 MHz, CDCl3, 20 °C).
Sch em e 4
significantly improve the yield. The instability of the
LiC6F5 reagent and the low yields for complete conver-
sion to 9a suggest that LiC6F5 decomposition is in
competition with arylation at the boron center. Unlike
the lithium reagent, C6F5MgBr is thermally stable and
can be warmed above room temperature without rapid
decomposition. Reactions between 4 equiv of C6F5MgBr
and 6 (Scheme 4) at room temperature again only lead
to compound 7. However, in small-scale reactions warm-
ing to 60 °C leads rapidly to conversion to the anion 9.
On a larger scale reaction times could be as long as 18
h, and it was necessary to monitor progress by 1H NMR.

Borato Bridged Zirconocenes
Organometallics, Vol. 20, No. 10, 2001 2099
F igu r e 2. 1H NMR spectrum of complex 10b, [NEt4][(C6F5)2B(C5H4)2ZrMe2] (300.13 MHz, CDCl3, 20 °C).
Although at room temperature compound 7 is the only
intermediate observed, in reactions that were warmed
to 60 °C for prolonged periods, a second intermediate
elusive. Repeated washing, attempted recrystallization,
or precipitation steps led only to the growth of new
impurity signals, presumably hydrolysis products with
very similar solubility properties.
1
could be identified by H NMR (Scheme 4). Compound
8 could not be isolated, and apart from a new set of four
cyclopentadienyl resonances indicating two inequivalent
groups bonded to boron and a broad 11B resonance at δ
-1 ppm, no other data are available. Compound 8 is
tentatively assigned as (L)(C6F5)B(C5H4)2Zr(C6F5)2, where
L is presumably Et2O.
Attempts to prepare the dimethyl anion [(C6F5)2-
B(C5H4)2ZrMe2]- in analogous fashion to 4 were unsat-
isfactory. The reaction between 9 and AlMe3 appeared
to be very slow indeed, and quantitative conversion to
a new complex was not achieved. The reaction between
9 and AlMe3 will establish equilibrium, and large
quantities of AlMe3 may be required to achieve high
conversions.19
By contrast, the reaction of 9a with LiMe is exother-
mic. Spectroscopic investigation of the crude reaction
mixture indicated the formation of a new compound,
10a . Where reactions were attempted on a larger scale,
it was thought prudent to add the LiMe at 0 °C and
warm slowly to room temperature. When the initial
reaction exotherm was avoided, the reaction proceeded
much more slowly and a reaction time of 12 h was
required for complete alkyl exchange to form 10a . The
[NEt4]+ salt 10b was prepared similarly from 9b.
Compounds 10a and 10b were obtained >90% pure by
1H NMR. However, samples of either 10a or 10b pure
enough to provide good elemental analysis proved
Anion 10 is assigned the structure [(C6F5)2B(C5H4)2-
ZrMe2]- on the basis of 1H, 11B, 13C, 2D H-C HETCOR,
1
and 19F NMR spectra. The H NMR spectrum of 10b is
shown in Figure 2. Apart from the cation [NEt4]+ and a
signal for [Li(Et2O)x]+, the NMR spectrum is very
simple, consisting of two pseudo-triplets for the AA′BB′
pattern of a substituted cyclopentadienyl ligand, broad-
ened by proximity to the quadrupolar boron nucleus,
and a sharp CH3 singlet at δ -0.6. The identity of this
singlet as the Zr-Me resonance was confirmed by the
observation of coupling to a 13C NMR signal at δ 23.56
ppm. The 11B NMR chemical shift is the same as that
observed for 9. The 19F NMR indicates just one type of
C6F5 group, as expected, but although sharp, the o-F
resonance for the B-C6F5 group is observed to be
downfield shifted at δ -114.85 ppm; the reason for this
is not clear.
Eth en e P olym er iza tion s. The ionic complexes 3b,
9a , 10a , and 10b were employed as precatalysts for
ethene polymerization, and the results are presented
in Table 2. The productivity observed for 3b is similar
to that observed for Shapiro’s ansa-borates [PPN][(Cl)-
(C6H5)B(C5H4)2ZrCl2] (0.57 × 106 g PE mol-1 h-1 bar-1
)
and [Cp*2Al][(Me)(C6H5)B(C5H4)2ZrCl2] (0.9 × 106 g PE
mol-1 h-1 bar-1).17 The productivity for 9a activated
with MAO is comparable. Somewhat higher productivi-
ties are obtained if the catalyst is premixed with MAO,
which is consistent with slow generation of the active
species. For simple zirconocenes productivity is highly
(19) Bochmann, M.; Sarsfield, M. J . Organometallics 1998, 17, 5908.

2100 Organometallics, Vol. 20, No. 10, 2001
Lancaster and Bochmann
dependent on the mode of activation.5d The activation
P r ep a r a tion of [Et4N][(C6F 5)(C6H5)B(C5H4)2Zr (C6F 5)2]
(3b). To 4 mmol of crude 3a was added 0.84 g (4 mmol) of
[Et4N]Br, followed by 30 mL of dichloromethane. The reaction
mixture was then stirred for 1 h. The solids were allowed to
settle and separated by filtration. Removal of the solvent
yielded a yellow-brown foam, 3.2 g (3.4 mmol, 85%). Repeated
of 9a with AlBui /[CPh3][B(C6F5)4] gives a productivity
3
of 2.13 × 106 g PE mol-1 h-1 bar-1 at 60 °C. However,
the use of the dimethyl complex 10a does not lead to
an increase in productivity over 9a under these polym-
erization conditions. Polymerizations with 10b indicate
that in the presence of an excess of aluminum reagent
the nature of the precatalyst countercation and any
associated diethyl ether does not have a dramatic
impact on productivity.
attempts to recrystallize from
a range of solvents were
unsuccessful. Anal. Calcd: C, 54.36; H, 3.58; N, 1.51. Found:
C, 54.05; H, 3.6; N, 1.35.
P r ep a r a tion of [P P h 4][(C6F 5)(C6H5)B(C5H4)2Zr (C6F 5)2]
(3c). Following a procedure similar to that for 3b, 1.40 g (1.45
mmol) of 3a and 0.78 g (1.87 mmol) of [PPh4]Br were reacted
in 20 mL of dichloromethane. Removal of excess PPh4Br and
solvent left a dark yellow foam, yield 1.5 g (1.30 mmol, 90%).
Repeated attempts at recrystallization were unsuccessful.
P r ep a r a tion of [Li(Et2O)x][(C6F 5)(C6H5)B(C5H4)2Zr Me2]
(4). To a solution of 10 mL of AlMe3 (1 M, 10 mmol) in
dichloromethane was added 0.75 g of 3a (0.84 mmol) and the
reaction mixture stirred. There was an initial vigorous reac-
tion. The reaction mixture was allowed to stand for 36 h. Light
petroleum was added to precipitate the product, and the
solvents were filtered off before drying in vacuo to give crude
4 as a tan-colored foam.
P r ep a r a tion of ClB(C5H4SiMe3)2 (5). To a solution of 15
mmol of BCl3 dissolved in 150 mL of toluene was added 9.03
g (30 mmol) of C5H4(SiMe3)(SnMe3). The reaction mixture was
heated to reflux for 5 h, during which time the solution
darkened slightly in color. Removal of the volatiles at 0.1
mmHg, 50 °C, yielded a yellow-orange oil, which crystallized
very slowly when stored at 5 °C. The 1H NMR spectrum
indicated that the yield was essentially quantitative. The
compound was used without further purification.
Con clu sion s
The chloro borane bridged ansa-zironocene Cl(Me2S)B-
(C5H4)2ZrCl2 is accessible through the dehalosilylation
reaction of BCl(C5H4SiMe3)2 with ZrCl4(Me2S)2. The
reaction of this complex and the phenyl analogue
((Me2S)(C6H5)B(C5H4)2ZrCl2) with 2 equiv of LiC6F5
leads to selective arylation to form neutral bis(penta-
fluorophenyl)zirconium compounds. The use of 5 equiv
of LiC6F5 leads to attack at boron, affording anionic
borates. The Zr-C6F5 groups are exchanged for Zr-Me
through reaction with AlMe3 or LiMe to give [Li(Et2O)x]-
[(C6F5)(C6H5)B(C5H4)2ZrMe2] and [Li(Et2O)4][(C6F5)2B-
(C5H4)2ZrMe2], respectively. These anionic ansa-zircono-
cenes can be activated with MAO or AlBui /[CPh3]-
3
[B(C6F5)4] to give ethene polymerization catalysts.
Exp er im en ta l Section
P r ep a r a tion of (Me2S)ClB(C5H4)2Zr Cl2 (6). 5 (15 mmol)
was dissolved in 100 mL of toluene, and 5.4 g (15 mmol) of
ZrCl4(Me2S)2 was added. The reaction mixture was warmed
to 70 °C and stirred overnight. The solution gradually dark-
ened to orange-brown, and a small amount of dark brown solid
precipitated. The toluene was removed under vacuum and the
product extracted with dichloromethane. The solution was
concentrated until crystals persisted and cooled to -25 °C
overnight, giving a mass of pale yellow crystals. Yield (two
fractions): 4 g (10 mmol, 66%). 1H NMR spectra of samples of
6 indicate the presence of between 0.1 and 0.2 equiv of toluene,
which is only very slowly lost under vacuum. Anal. Calcd for
Gen er a l Con sid er a tion s. All manipulations were per-
formed under a dinitrogen atmosphere using Schlenk tech-
niques. Solvents were distilled under N2 over sodium (toluene),
Na/K alloy (diethyl ether, light petroleum (bp 40-60 °C)), or
CaH2 (dichloromethane). NMR solvents were dried over 4 Å
molecular sieves (C6D6, CD2Cl2, CDCl3). NMR spectra were
recorded on a Bruker DPX300 spectrometer. Chemical shifts
are reported in ppm and referenced to residual solvent
resonances (1H, 13C NMR) or external BF3‚OEt2 (11B) and
CFCl3 (19F). Ethene polymerizations (1 bar) were performed
as described.5d {(Me2S)(C6H5)B(C5H4)2}ZrCl2 was prepared
according to a literature procedure.15
C
12H14BCl3SZr‚0.15(C7H8): C, 38.00; H, 3.71; Cl, 25.78.
P r ep a r a tion of {(Me2S)(C6H5)B(C5H4)2}Zr (C6F 5)2 (2). To
a solution of 0.94 g (3.8 mmol) of C6F5Br in light petroleum,
held at -78 °C, was added slowly 2.4 mL of a 1.6 M solution
of n-BuLi (3.8 mmol). The reaction mixture was stirred for 1
h at -78 °C before 1.87 g (1.9 mmol) of solid 1 was added. The
reaction mixture was stirred for 2 h before warming to room
temperature. A yellow solution formed above a pale solid. The
solid was allowed to settle and the solution filtered off. The
solvent was taken off under reduced pressure, leaving a yellow
foam. Extraction with cold (-30 °C) toluene and removal of
Found: C, 38.00; H, 3.47; Cl, 26.60.
P r ep a r a tion of [(C6F 5)2B(C5H4)2Zr (C6F 5)2][Li(Et2O)4]
(9a ). To a solution of 27.2 mmol of LiC6F5 in 100 mL of Et2O
at -78 °C was added 6 (2.17 g, 5.44 mmol). The reaction
mixture was allowed to warm very slowly to room temperature
over a period of 6 h. Removal of the volatiles in vacuo,
extracting with 60 mL of toluene, and cooling to -25 °C
precipitated 9a as brown oil, yield 1.84 g (1.53 mmol, 28%).
Isola tion of (Et2O)ClB(C5H4)2Zr (C6F 5)2 (7). Otherwise
following the procedure described for 9a , the reaction mixture
was warmed to room temperature, and the solids were
separated by filtration. The diethyl ether solution was cooled
to -25 °C overnight, and the resulting pale yellow crystals of
7 were isolated and dried under vacuum. Anal. Calcd for
C26H18BClF10OZr‚0.66C4H10O: C, 47.59; H, 3.43; Cl, 4.90.
Found: C, 47.74; H, 3.43; Cl, 5.72.
P r ep a r a tion of [Et4N][(C6F 5)2B(C5H4)2Zr (C6F 5)2] (9b). A
solution of 80 mmol of C6F5MgBr in 200 mL of Et2O was added
via a cannula to a cold (0 °C) solution of 6.4 g (16 mmol) of 6
dissolved in 200 mL of toluene. The brown reaction mixture
was allowed to warm to room temperature, and ca. 150 mL of
Et2O was removed under reduced pressure. The solution was
then warmed to 60 °C and the reaction monitored by 1H NMR.
After 18 h the toluene was stripped off under vacuum. Solid
[Et4N]Br, 3.36 g (16 mmol), was added, followed by 200 mL of
CH2Cl2. The mixture was stirred for 1 h before 100 mL of
1
the toluene under vacuum gave 2 as a yellow solid. H NMR
indicated that 2 retained 0.5 equiv of toluene. The yield of 2
was 1.0 g (1.3 mmol, 70%). Anal. Calcd for C30H19BF10SZr‚
0.5C7H8: C, 53.68; H, 3.09. Found: C, 53.85; H, 3.08.
P r ep a r a t ion of [Li(E t 2O)x][(C6F 5)(C6H 5)B(C5H 4)2Zr -
(C6F 5)2] (3a ). To 6 mmol of LiC6F5 in 60 mL of diethyl ether
at -78 °C was added 1.25 g (2 mmol) of 1 and the mixture
stirred for 1 h and allowed to warm very slowly. At ca. -60
°C all the solid 1 had dissolved, leaving only a very small
amount of white solid. By ca. -50 °C a thick precipitate had
formed. As the suspension was warmed further, the solid
dissolved, leaving only a small amount of insoluble material
after stirring overnight. The solids were separated by filtration,
and the solvent was taken off under reduced pressure to give
a yellow foam. Crude NMR analysis indicated a mixture of 2
and a new complex, 3a . Washing with toluene to remove 2
gave 3a , yield 1.3 g (1.35 mmol, 67%).

Borato Bridged Zirconocenes
Organometallics, Vol. 20, No. 10, 2001 2101
petroleum ether was added to assist with solid settling and
filtration. The solvents were removed to give a sticky brown
solid which still contained some CDCl3-insoluble material as
well as a quantity of (C6F5)B(C5H4)2Zr(C6F5)2‚OEt2 (8). The
solid was washed with petroleum ether to remove 8 and re-
extracted with toluene. Removal of the toluene gave compound
9b as a pale brown foam, yield 6.58 g (6.4 mmol, 40%). The
NMR data are very similar to those of 9a .
P r ep a r a tion of [Li(Et2O)4][(C6F 5)2B(C5H4)2Zr Me2] (10a ).
To a solution of 3.2 g (2.66 mmol) of 9a in 60 mL of diethyl
ether was added 3.4 mL of 1.6 M LiMe (5.4 mmol). The solution
began to reflux. After stirring for a further 2 h, the mixture
was filtered to remove a dark insoluble precipitate and taken
to dryness. The 1H NMR spectrum of the crude product was
consistent with the formation of 10a . Extraction with toluene
and precipitation with light petroleum produced an oil, which
was thoroughly dried under vacuum to give a sticky yellow
solid, yield 1.45 g (1.62 mmol, 30%).
appeared to be complete. The solution was filtered and
separated from a dark insoluble material and the ether
removed under reduced pressure. The product was then
separated from metal halides by extraction with dichloro-
methane. Removal of the dichloromethane and characteriza-
1
tion by H NMR indicated the presence of impurities, particu-
larly the presence of diethyl ether. A second extraction with
toluene produced a pale yellow solution, which was filtered.
Removal of the toluene and thorough drying under vacuum
yielded 10b. NMR signals due to Et2O persisted even after
prolonged drying, suggesting the presence of a small amount
of [Li(Et2O)n]+ cation.
Ack n ow led gm en t. This work was supported by the
Engineering and Physical Sciences Research Council.
Su p p or tin g In for m a tion Ava ila ble: For compounds for
which analyses are not reported, copies of their NMR spectra
are available. This material can be obtained free of charge via
the Internet at http://pubs.acs.org.
P r ep a r a tion of [Et4N][(C6F 5)2B(C5H4)2Zr Me2] (10b). To
a suspension of 5.3 g (5.1 mmol) of 9b in 100 mL of diethyl
ether was added 6.4 mL of 1.6 M LiMe. The reaction was
monitored by 1H NMR and after 4 h at room temperature
OM010028Z