(Phenylboratabenzene)zirconium complexes: Tuning the reactivity of an olefin polymerization catalyst

Article abstract of DOI:10.1021/om970103z

The reaction of 2 equiv of lithium 1-phenylboratabenzene with ZrCl₄ in ether affords bis(1-phenylboratabenzene)zirconium dichloride (2), while [4-tert-C₄H₉C₅H₄BPh] ₂ZrCl₂ (3), obtained in a similar manner, was subjected to a single-crystal X-ray diffraction study. The reaction of a solution of 2 and methylaluminoxane (MAO) with C₂H₄ (1 atm, 25°C) affords ethylene oligomers (dimers of 1-alkenes as the major product, together with minor amounts of 1- and 2-alkenes).

Full text of DOI:10.1021/om970103z

2492
Organometallics 1997, 16, 2492-2494
(P h en ylbor a ta ben zen e)zir con iu m Com p lexes: Tu n in g
th e Rea ctivity of a n Olefin P olym er iza tion Ca ta lyst
Guillermo C. Bazan* and George Rodriguez
Department of Chemistry, University of Rochester, Rochester, New York 14627-0216
Arthur J . Ashe III,* Saleem Al-Ahmad, and J eff W. Kampf
Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055
Received February 11, 1997X
Summary: The reaction of 2 equiv of lithium 1-phenyl-
boratabenzene with ZrCl4 in ether affords bis(1-phen-
ylboratabenzene)zirconium dichloride (2), while [4-tert-
C4H9C5H4BPh]2ZrCl2 (3), obtained in a similar manner,
was subjected to a single-crystal X-ray diffraction study.
The reaction of a solution of 2 and methylaluminoxane
(MAO) with C2H4 (1 atm, 25 °C) affords ethylene
oligomers (dimers of 1-alkenes as the major product,
together with minor amounts of 1- and 2-alkenes).
The development of new metallocene-based homoge-
neous catalysts for olefin polymerization is of immense
practical importance1 and continues to attract intense
research.2 Bis(boratabenzene)metal complexes,3 e.g.,
1-3, result from the replacement of the metallocene
cyclopentadienyl groups by the aromatic heterocycle
boratabenzene.4 Recently, catalysts prepared from
[C5H5BN(i-Pr)2]2ZrCl2 (1) have been found to have high
activity.5 Since the boron atoms of 1 interact strongly
with their exocyclic substituents, we have proposed that
boratabenzene-metal catalysts may be electronically
tunable by changing the pendant group. In this com-
munication, we report on the synthesis and character-
ization of bis(1-phenylboratabenzene)zirconium dichlo-
rides (2 and 3) and show that olefin polymerization by
2 involves increased rates of â-hydrogen elimination.
is approximately equidistant from each of the 10 ring
carbon atoms (average Zr-CR ) 2.62(3) Å, average Zr-
(7) Bis(1-phenylboratabenzene)zirconium dichloride (2). A solution
of LDA (6.5 mmol) in 10 mL of ether was added dropwise to a solution
of 1-phenyl-1-boracyclohexa-2,5-diene (1.0 g, 6.5 mmol) in 20 mL of
ether at -78 °C. The color changed to yellow when the reaction mixture
was allowed to warm to room temperature. After this solution was
recooled to -78 °C, it was added with stirring to a suspension of ZrCl4
(0.75 g, 3.25 mmol) in 10 mL of ether at -78 °C. The reaction mixture
was allowed to warm to room temperature with stirring over 4 h. The
solvent was removed under reduced pressure, and the residue was
extracted with toluene. After filtration, the filtrate was concentrated
under reduced pressure and treated with pentane. Upon standing in
the freezer for 24 h, a yellow crystalline product was obtained. The
product was collected by filtration, washed with pentane, and dried
in vacuum affording 1.30 g (80%) of product, mp 165 °C. HRMS(EI):
calcd for C22H2011B235Cl290Zr m/e, 466.0175; found m/e, 466.0164. 1H
NMR (C6D6): δ 6.17 (t, J ) 6.5 Hz, H(4)), 6.57 (d, J ) 11.2 Hz, H(2,6)),
7.22 (dd, J ) 11.2, 6.8 Hz, H(3,5)), 7.32 (t, J ) 7.2 Hz, p-Ph), 7.41 (t,
J ) 7.2 Hz, m-Ph), 7.94 (d, J ) 7.2 Hz, o-Ph). 13C NMR (C6D6): δ 112.7
(C(4)), 126.4 (C(2,6)), 146.8 (C(3,5)), 128.9 (m-Ph), 130.7 (p-Ph), 133.9
(o-Ph), ipso-Ph not observed. 11B NMR (C6D6): δ 39.6. Anal. Calcd for
C22H20B2Cl2Zr: C, 56.40; H, 4.31. Found: C, 56.31; H, 4.24.
(8) Bis(4-tert-butyl-1-phenylboratabenzene)zirconium dichloride
(3): A solution of 4-tert-butyl-1,1-dibutyl-1-stannacyclohexa-2,5-diene
(2.0 g, 5.6 mmol) in 10 mL of pentane was added with stirring to a
solution of phenylboron dichloride (0.94 g, 5.9 mmol) in 20 mL of
pentane at -78 °C. The reaction mixture was allowed to stir at -78
°C for 1 h and then allowed to warm to room temperature. The solvent
and dibutyltin dichloride were removed by vacuum distillation, leaving
a residue of 0.5 g (43%) of 4-tert-butyl-1-phenyl-1-boracyclohexa-2,5-
diene. 1H NMR (C6D6): δ 0.85 (s, 9H, CMe3), 2.67 (m, 1H, CH(4)), 7.36
(m, 7H), 8.21 (m, 2H, o-Ph). This crude product was used without
further purification. A solution of LDA (2.3 mmol) in 10 mL of ether
was added dropwise to a solution of 4-tert-butyl-1-phenyl-1-boracyclo-
hexa-2,5-diene (0.5 g, 2.3 mmol) in 20 mL of ether at -78 °C. The color
changed to yellow on warming to room temperature. After this solution
was recooled to -78 °C, it was added dropwise with stirring to a
suspension of ZrCl4 (0.28 g, 1.2 mmol) in 10 mL of ether at -78 °C.
The reaction mixture was allowed to warm to room temperature with
stirring over 4 h. The solvent was removed under reduced pressure,
leaving a yellow residue which was extracted with hexane (4 × 20 mL).
The hexane extracts were cooled in the freezer for 12 h, which afforded
0.25 g (36%) of yellow crystals, mp 118-120 °C. HRMS(EI): calcd for
C30H3611B235Cl290Zr m/e, 578.1427; found m/e, 578.1438. 1H NMR
(C6D6): δ 1.19 (s, CMe3), 6.94 (d, J ) 11.4 Hz, H(2,6)), 7.47 (d, J )
11.4 Hz, H(3,5)), 7.35 (t, J ) 7.4 Hz, p-Ph), 7.48 (t, J ) 7.4 Hz, m-Ph),
8.19 (d, J ) 7.4 Hz, o-Ph). 13C NMR (C6D6): δ 31.4 (CMe3), 37.0 (CMe3),
127.7 (C(2,6)), 137.6 (C(4)), 143.9 (C(3,5)), 128.9 (m-Ph), 130.9 (p-Ph),
133.7 (o-Ph), ipso-Ph not observed. 11B NMR (C6D6): δ 36.9. Anal. Calcd
for C30H36B2ZrCl2: C, 62.08; H, 6.26. Found: C, 62.33; H, 6.29. For
the synthesis of 1,1-dibutyl-4-tert-butyl-1-stannacyclohexa-2,5-diene,
see: ref 6 and Ashe, A. J ., III; Diephouse, T. R.; El-Sheikh, M. Y. J .
Am. Chem. Soc. 1982, 104, 5693.
The reaction of ZrCl4 with 2 equiv of Li[C5H5BPh]6
in ether produces [C5H5BPh]2ZrCl2 (2) in 80% yield (eq
1).7 Crystals suitable for X-ray diffraction could only be
8
obtained for the derivative [4-tert-C4H9C5H4BPh]2ZrCl2
(3), for which the molecular structure is illustrated in
Figure 1.9 The sterically demanding tert-butyl groups
of 3 are positioned to the sides of the open metallocene
wedge, which brings the less sterically demanding
B-Ph groups near the more crowded closed face of the
wedge. The Zr atom is η6-coordinated to both rings and
X Abstract published in Advance ACS Abstracts, May 1, 1997.
(1) (a) Rotman, D. Chem. Week 1996, 158 (36), 37. (b) Thayer, A.
M. Chem. Eng. News 1995, 73 (37), 15.
(2) For recent 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) Ziegler Catalysts; Fink, G., Mu¨lhaupt, R.,
Brintzinger, H. H., Eds.; Springer-Verlag: Berlin, Germany, 1995.
(3) For older work see: (a) Herberich, G. E.; Greiss, G.; Heil, H. F.
Angew. Chem. Int., Ed. Engl. 1970, 9, 805. (b) Herberich, G. E.; Ohst,
H. Adv. Organomet. Chem. 1986, 25, 199. (c) Ashe, A. J ., III; Meyer,
E.; Shu, P.; VonLehmann, T.; Bastide, J . J . Am. Chem. Soc. 1975, 97,
6865.
(4) For recent work, see: (a) Herberich, G. E.; Englert, U.; Schmidt,
M. U. Organometallics 1996, 15, 2707. (b) Ashe, A. J ., III; Kampf, J .
W.; Mu¨ller, C.; Schneider, M. Organometallics 1996, 15, 387. (c) Hoic,
D. A.; Davis, W. M.; Fu, G. C. J . Am. Chem. Soc. 1996, 118, 8176.
(5) Bazan, G. C.; Rodriguez, G.; Ashe, A. J ., III; Al-Ahmad, S.;
Mu¨ller, C. J . Am. Chem. Soc. 1996, 118, 2291.
(6) Ashe, A. J ., III; Shu, P. J . Am. Chem. Soc. 1971, 93, 1804.
S0276-7333(97)00103-9 CCC: $14.00 © 1997 American Chemical Society

Communications
Organometallics, Vol. 16, No. 12, 1997 2493
F igu r e 2. Partial GC/MS trace of the product mixture
obtained from reaction of 2/MAO and 1 atm of C2H4.
min (T ) 25 °C, 1 atm of C2H4). Each peak differs in
mass by one C2H4 unit while their “fine structure” is
due to different isomers (i.e., different m + n combina-
tions). Also produced at 1 atm of C2H4 are 1-alkenes
and 2-alkenes in 11% and 9% yields, respectively, as
determined by NMR spectroscopy.13 The 2/MAO cata-
lyst mixture forms polyethylene at higher temperatures,
F igu r e 1. ORTEP view of 3, showing the atom-numbering
scheme. Hydrogen atoms were omitted for clarity. Selected
bond distances (Å): Zr-B(1), 2.826(2); Zr-B(2), 2.773(3);
Zr-C(1), 2.619(2); Zr-C(2), 2.608(2); Zr-C(3), 2.660(2); Zr-
C(4), 2.599(2); Zr-C(5), 2.606(2); Zr-C(16), 2.577(2); Zr-
C(17), 2.584(2); Zr-C(18), 2.668(2); Zr-C(19), 2.659(2); Zr-
C(20), 2.663(2).
and there is a nearly linear dependence of Mw (Mw
weight average molecular weight) on ethylene pressure
(at 60 °C: Mw ) 10K (30 psi); Mw ) 20K (75 psi); Mw
)
)
110K (300 psi)). Monomer consumption increases with
pressure and temperature (i.e., a rate of 1180 kg of PE/
(h [Zr] mol) is observed at 60 °C and 300 psi).
Câ ) 2.61(2) Å, average Zr-Cγ ) 2.66(1) Å, while the
Zr-B distance (ave 2.80(3) Å) is somewhat longer.10 The
geometry of the ligand and its juxtaposition to the metal
are virtually identical to those found in late transition
metal complexes,3b,11 implying an invariant electronic
character for phenylboratabenzene. Since phenylbo-
ratabenzene is a poorer donor than Cp,3b,c the Zr atoms
of 2 and 3 should be more electrophilic than those of
standard metallocenes. In marked contrast, the Zr atom
of 1 shows a large slip-distortion away from B (Zr-B )
2.98 Å) and toward Cγ (Zr-Cγ ) 2.483(5) Å).5 The
strong π donation to B by the pendant N(i-Pr)2 group of
1 saturates the electropositive boron, making the Zr of
1 less electrophilic than that of 2.
In the presence of MAO, 1 produces high molecular
weight polyethylene under 1 atm of C2H4. The analo-
gous reaction with 2 proceeds differently.12 At pressures
under 2 atm, the major product is a mixture of 2-alkyl-
1-alkenes of various chain lengths. Figure 2 shows part
of the GC/MS trace for the product obtained after 30
In analogy to well-established metallocene chemistry,2
we propose that, at low ethylene pressure, 2/MAO/C2H4
react according to Scheme 1. Multiple C2H4 insertions
into hydride A14 result in the propagating alkyl B. Fast
â-H elimination from B limits chain growth generating
1-alkenes and A. At longer reaction times, the concen-
tration of 1-alkenes increases and their insertion into
B takes place to give the â-disubstituted alkyl C. The
2-alkyl-1-alkenes (D) arise from â-H elimination at C,
which also regenerates A. The 2-alkenes are probably
the result of 2,1-insertion, followed by â-hydrogen
elimination.15
For a given unit of reaction time, the total mass of
organic product reflects the rate of C2H4 uptake. At 1
atm and [MAO]/[2] ) 480, the activity obtained is 84
Kg of 2-alkyl-1-alkenes/(h [Zr] mol). For 1, under
similar conditions ([MAO]/[1] ) 450), one obtains 69 kg
of PE/(h [Zr] mol). Therefore, under the experimental
conditions described above, changing the boron sub-
stituent from diisopropylamine to phenyl increases the
rate of â-H elimination from B more greatly (kâ in
Scheme 1) than the rate of ethylene insertion into A
(kins).
(9) Crystal data for C30H36B2Cl2Zr: space group P21/c with a )
10.0660(8) Å, b ) 12.2679(10) Å, c ) 22.807(2) Å, â ) 91.472(5)°, Z )
4, F ) 1.369 g/cm3. A total of 7454 reflections (h,k,l) were collected in
the range 2.62° < 2θ < 26.00°, with 5519 being used in the structure
refinement by full-matrix least-squares techniques (462 variables).
Final R1 ) 0.0294, wR2 ) 0.0596.
(10) The steric effects of the ring substituents on the hapticity of
the boratabenzene ligands of 3 are probably minor since the electroni-
cally similar but sterically divergent 1-methylboratabenzene is η6-
coordinated in (C5H5BMe)2ZrCl2. Herberich, G. E. Pure Appl. Chem.,
in press.
(11) Huttner, G.; Gartzke, W. Chem. Ber. 1974, 107, 3786.
(12) A typical procedure follows. A 2 mL toluene solution of 2 (10
mg, 0.021mmol) was added dropwise to a 20 mL toluene solution of
MAO (0.600 g, 10.3 mmol). After the reaction mixture was stirred for
10 min, the reaction vessel was evacuated and 1 atm of ethylene
introduced with vigorous stirring. After 40 min, the reaction was
quenched with 30 mL of a 10% solution of HCl in methanol. The organic
layer was separated from the aqueous fraction and placed under
vacuum to remove the solvent. This fraction gave 1.199 g of an oily
organic material. The product was characterized by GC/MS and 1H
and 13C NMR as a mixture of 2-alkyl-1-alkenes (∼80%), 1-alkenes
(∼10%), and 2-alkenes (∼10%).
The following observations are consistent with Scheme
1. Quenching after 5 min at 1 atm of C2H4 gives a
product of 60% 1-alkenes with 40% 2-alkyl-1-alkenes.
At 15 min, the olefin ratio changes to 29% 1-alkenes to
71% 2-alkyl-1-alkenes. Doubling the ethylene pressure
(13) 1H NMR (CDCl3, 300 MHz): δ 4.58 (s, 2-alkyl-1-alkenes, 2H),
4.81 (dm, J ) 10.5 Hz, 1-alkenes, 1H), 4.89 (dm, J ) 17 Hz, 1-alkenes,
1H), 5.3 (m, 2-alkenes, 2H), 5.7 (m, 1-alkenes, 1H). 13C NMR (75.3 MHz,
CDCl3): δ 107.5 (2-alkyl-1-alkenes), 108.6 (2-alkyl-1-alkenes), 114.3
(1-alkenes), 123.7 (2-alkenes), 131.1 (2-alkenes), 139.3 (1-alkenes),
150.5 (2-alkyl-1-alkenes), 151.9 (2-alkyl-1-alkenes).
(14) We assume that MAO serves its standard role; alkylation
followed by ligand extraction. The hydride is generated from insertion
of C2H4 followed by â-hydrogen elimination.
(15) For a discussion of the regiochemistry of 1-alkene insertion in
Ziegler-Natta catalysts see: Busico, V.; Cipullo, R.; Chadwick, J . C.;
Modder, J . F.; Sudmeijer, O. Macromolecules 1994, 27, 7538.

2494 Organometallics, Vol. 16, No. 12, 1997
Communications
Sch em e 1
from 1 to 2 atm increases the (m + n) average from
15(1) to 30(1). Direct addition of 1-alkenes yields the
expected products. For example, 1-octene is dimerized
to 2-hexyl-1-decene by 2/MAO in less than 5 min (eq
2).16 In addition, the reaction of 1,5-hexadiene with
boratabenzene)zirconium catalysts increase the rate of
â-hydrogen eliminations so that 1-alkenes and their
dimers are obtained. In the broader context, we have
demonstrated that boratabenzenes are electronically
tunable surrogates for cyclopentadienyl in an important
class of organometallic compounds. Since the cyclopen-
tadienyl ligand is ubiquitous in organometallic chem-
istry, it should be possible to effect similar modification
of other important reactions.17
Ack n ow led gm en t. Work at the University of Roch-
ester was supported by Exxon Chemical Co. and the
Petroleum Research Fund, administered by the Ameri-
can Chemical Society, while work at the University of
Michigan was supported by the National Science Foun-
dation, Grant No. CHE-9224907, and the Petroleum
Research Fund. We are also grateful to Dr. R. Fisher
for the high-temperature polymerizations and GPC
data.
2/MAO results in quantitative production of methyl-
enecyclopentane (eq 2).
In summary, we have shown that a change of the
boron substituent of a boratabenzene-zirconium cata-
lyst has a profound effect on its chemical reactions.
While ((diisopropylamino)boratabenzene)zirconium cata-
lysts form high molecular weight polyethylene at low
monomer pressure, the more electrophilic (phenyl-
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
data and structure refinement, atomic coordinates, bond
lengths and angles, anisotropic displacement prarameters, and
hydrogen coordinates and ORTEP diagram for 3 (9 pages).
Ordering information is given on any current masthead page.
(16) Dimerization of 1-octene: To a 10 mL toluene solution of 2 (10
mg, 0.21 mmol) and MAO (600 mg, 0.01 mol) was added 1-octene (0.5
g, 4.5 mmol). The reaction was quenched after 2 h with 20 mL of a
10% solution of HCl in methanol. The organic layer was separated and
the solvent removed to give 453 mg of a clear colorless liquid. 1H NMR
(CDCl3, 400 MHz): δ 4.85 (s, 2H, H2CdC), 2.05 (m, 4H, H2CdC(CH2)2),
1.45 (m, 4H, CH2CH3), 1.32 (b, 16H, CH2), 0.91 (m, 6H, CH3). 13C NMR
(CDCl3): δ 150.1, 109.1, 36.5, 32.3, 32.1, 30.6, 30.2, 29.9, 29.5, 28.2,
28.1, 23.05, 23.03, 14.3 (two carbon resonances overlap). GC/MS shows
a single component with MW ) 224. A similar procedure was carried
out for the cyclization of 1,5-hexadiene to methylenecyclopentane.
OM970103Z
(17) For an earlier perceptive example, see: Bo¨nnemann, H. Angew.
Chem., Int. Ed., Engl. 1985, 24, 248.