Synthesis and characterization of [(C5Me4H) 2Al]+, an initiator for the polymerization of isobutene. X-ray crystal structures of [(C5Me4H)2Al(μ- Cl)]2 and [(C5</s

Article abstract of DOI:10.1021/om0606439

The octamethylaluminocenium cation [Cp′₂Al]⁺ (Cp′ = C₅Me₄H) was formed both by halide abstraction in the reaction between [Cp′₂AlCl]₂ and AlCl ₃ and by cyclopentadienide abstract

Full text of DOI:10.1021/om0606439

5582
Organometallics 2006, 25, 5582-5586
Synthesis and Characterization of [(C5Me4H)2Al]+, an Initiator for
the Polymerization of Isobutene. X-ray Crystal Structures of
[(C5Me4H)2Al(µ-Cl)]2 and [(C5Me4H)2Al][B(C6F5)4]
Suh-Jane Lee, Pamela J. Shapiro,* and Brendan Twamley
Department of Chemistry, UniVersity of Idaho, Moscow, Idaho 83844-2343
ReceiVed July 17, 2006
The octamethylaluminocenium cation [Cp′2Al]+ (Cp′ ) C5Me4H) was formed both by halide abstraction
in the reaction between [Cp′2AlCl]2 and AlCl3 and by cyclopentadienide abstraction in the reaction between
Cp′3Al and [CPh3][B(C6F5)]4. [Cp′2Al]+ is less thermally stable than [Cp*2Al]+ (Cp* ) C5Me5) but is
more active as an initiator for isobutene polymerization. The X-ray crystal structures of [Cp′2AlCl]2 and
[Cp′2Al][B(C6F5)4] are reported.
Equation 1 illustrates how the aluminocenium cation is
presumed to initiate the polymerization.2,8 Coordination of an
isobutene molecule to the aluminum generates a carbenium ion,
from which the polymer is propagated. The striking effect of
cyclopentadienyl ligand size on the stability and activity of the
aluminocenium cation prompted us to explore the possibility
of fine-tuning the stability and activity of the cation by varying
the number of methyl substituents on its cyclopentadienyl rings.
For this purpose, we pursued the synthesis of the octamethy-
laluminocenium cation [Cp′2Al]+ (Cp′ ) C5Me4H). As described
herein, methanide ion abstraction from Cp′2AlMe by B(C6F5)3
in the manner demonstrated for Cp2AlMe and Cp*AlMe was
not a viable route to the cation. Therefore, an alternate synthetic
route to the octamethylaluminocenium cation was sought.
Toward this goal, we developed the new compound [Cp′2AlCl]2
(1). We also demonstrated, for the first time, the abstraction of
a cyclopentadienyl anion from a metal by the trityl cation.
Introduction
Organoaluminum cations are of interest as initiators for ring-
opening polymerization of alkene oxides1 and carbocationic
alkene polymerization2 and as potential catalysts for the
coordination/insertion polymerization of alkenes.3 Stable, two-
coordinate organoaluminum cations are rare since their elec-
tronic and coordinative unsaturation, the very properties that
make them attractive as potential catalysts, also make them
especially vulnerable to decomposition. Those that have been
isolated and structurally characterized have been stabilized either
by the presence of bulky, sterically protecting hydrocarbyl
ligands on the aluminum or by inert, weakly coordinating
counteranions.4 Among this group is the decamethylaluminoce-
nium cation [Cp*2Al]+ (Cp* ) C5Me5), first isolated by
Schno¨ckel and co-workers from the reaction between [Cp*Al]4
and AlCl3.5 The counteranion in this case was [Cp*AlCl3]-.
Other synthetic approaches to [Cp*2Al]+ were later reported
by us6 and by Jutzi and co-workers.7 The importance of steric
stabilization by the bulky Cp* ligand is underscored by the poor
stability of [Cp2Al][MeB(C6F5)3], formed by Bochmann and co-
workers from the reaction between Cp2AlMe and B(C6F5)3.2
This species decomposes in CH2Cl2 solution at temperatures
above -20 °C. Below this temperature it is a highly active
initiator for the carbocationic polymerization of isobutene and
isoprene, even at -78 °C. By contrast, [Cp*2Al][MeB(C6F5)3]
is a poor initiator for isobutene polymerization, exhibiting no
activity below -20 °C.6
Results and Discussion
We initially explored methanide anion abstraction from Cp′2-
AlMe by B(C6F5)3 as a potential route to [Cp′2Al][MeB(C6F5)]
since this approach was successful for preparing aluminocenium
cations from Cp2AlMe2 and Cp*2AlMe.6 Unlike Cp2AlMe and
Cp*2AlMe, Cp′2AlMe is not isolable because it undergoes ligand
(1) (a) Munoz-Hernandez, M. A.; Mckee, M. L.; Keizer, T. S.; Yearwood,
B. C.; Atwood, D. A. J. Chem. Soc., Dalton Trans. 2002, 410-414. (b)
Jegier, J. A.; Munoz-Hernandez, M. A.; Atwood, D. A. J. Chem. Soc.,
Dalton Trans. 1999, 2583-2588. (c) Munoz-Hernandez, M. A.; Sannigrahi,
B.; Atwood, D. A. J. Am. Chem. Soc. 1999, 121, 6747-6748.
(2) Bochmann, M.; Dawason, D. M. Angew. Chem., Int. Ed. Engl. 1996,
35, 2226-2227.
1
redistribution in toluene to form Cp′3Al (identified by H and
27Al NMR spectroscopy) and possibly Cp′AlMe2.
(3) (a) Radzewich, C. E.; Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc.
1998, 120 (36), 9384-9385. (b) Coles, M. P.; Jordan, R. F. J. Am. Chem.
Soc. 1997, 119 (34), 8125-8126. (c) Ihara, E.; Young, V. G., Jr.; Jordan,
R. F. J. Am. Chem. Soc. 1998, 120 (32), 8277-8278.
An equivalent of B(C6F5)3 was added to the product mixture
from the 2:1 reaction between Cp′Na and MeAlCl2 to determine
if the equilibria might be shifted in favor of selective formation
of [Cp′2Al][MeB(C6F5)3]; however, an ill-defined mixture of
products was obtained instead. Therefore, we pursued a dicy-
clopentadienylaluminum halide precursor as a potential precursor
to a [Cp′2Al]+ salt.
(4) (a) Young, J. D.; Khan, M. A.; Wehmschulte, R. J. Organometallics
2004, 23, 1965-1967. (b) Kim, K.-C.; Reed, C. A.; Long, G. S.; Sen, A.
J. Am. Chem. Soc. 2002, 124, 7662-7663.
(5) Dohmeier, C.; Schno¨ckel, H.; Robl, C.; Schneider, U.; Ahlrichs, R.
Angew. Chem., Int. Ed. Engl. 1993, 32, 1655-1657.
(6) Burns, C. T.; Shapiro, P. J.; Budzelaar, P. H. M.; Willett, R.; Vij, A.
Organometallics 2000, 19, 3361-3367.
(7) Hotmann, U.; Jutzi, P.; Kuehler, T.; Neumann, B.; Stammler, H.-G.
Organometallics 1999, 18, 5531-5538.
(8) Grattan, D. W.; Plesch, P. H. Makromol. Chem. 1980, 181, 751-
775.
10.1021/om0606439 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 10/11/2006

Synthesis and Characterization of [(C5Me4H)2Al]+
Organometallics, Vol. 25, No. 23, 2006 5583
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1
Al(1)-C(1)
Al(1)-C(10)
Al(1)-C(11)
Al(1)-C(14)
C(1)-C(2)
C(1)-C(5)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
1.997(2)
2.046(2)
2.355(2)
2.346(2)
1.485(3)
1.495(3)
1.351(3)
1.461(3)
1.349(3)
C(4)-C(5)
1.349(3)
1.438(3)
1.440(3)
1.401(3)
1.417(3)
1.499(3)
2.3511(11)
2.3409(9)
C(10)-C(11)
C(10)-C(14)
C(11)-C(12)
C(12)-C(13)
C(13)-C(14)
Al(1)-Cl(1)
Al1-Cl(1A)
Cl(1A)-Al(1)-Cl(1)
Al(1A)-Cl(1)-Al(1)
84.86(3)
95.14(3)
aSymmetry generation ) -x + 1, -y + 1, -z + 1.
bears one η1 and one η3 Cp′ ring. The Al-C(1) σ-bond distance
is slightly shorter, by 0.04-0.05 Å, than the distance between
Al and C(10), the shortest bond with the η3 ring. C(11) and
C(14) of the η3 ring are 0.3 Å more distant from the aluminum
than C(10). C(12) and C(13) are 0.3-0.4 Å farther still. Whereas
the η1 ring exhibits discrete single and double C-C bond
lengths, with single-bond lengths of 1.461(3)-1.495(3) Å and
double-bond lengths of 1.349(3) and 1.351(3) Å, the C-C bond
lengths in the η3 ring, ranging from 1.401(3)-1.499(3) Å, are
delocalized.
Only a small number of chloride-bridged cyclopentadienyl-
aluminum compounds have been structurally characterized:
[(η5-C5Me4Et)(Ph)Al-µ-Cl]2,9 [(η5-C5Me4Et)(Cl)Al-µ-Cl]2,9 [(η3-
C5Me5)(Cl)Al-µ-Cl]2,10 [(η3-C5Me5)(CH3)Al-µ-Cl]2,11 and [(η3-
C5Me5)(i-Bu)Al-µ-Cl]2.11 The hapticities of the cyclopentadienyl
ring in these compounds vary between η3 and η5 in the solid
state. There are no striking structural differences between 1 and
these other compounds.
Figure 1. Thermal ellipsoids diagram (30%) of [η1,η3-Cp′2Al-µ-
Cl]2 (1). Hydrogen atoms are omitted for clarity.
Table 1. Crystallographic Data for Compounds 1 and 3
1
3
formula
mol wt
cryst syst
space group
a (Å)
C36H52Al2Cl2
609.64
triclinic
C48H31AlBBrF20
1105.43
monoclinic
C2/c
22.644(5)
23.988(5)
17.049(3)
90
107.02(3)
90
P1h
8.622(17)
10.595(2)
10.614(2)
108.88(3)
111.74(3)
92.98(3)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
835.7(4)
1
8855(3)
8
Compound 1 is stable indefinitely in the solid state below
-20 °C under nitrogen. At room temperature the solid turns
purple over the course of a week. 1H and 27Al NMR spectra of
solutions of the compound in CDCl3 and CD2Cl2 exhibit ligand
redistribution and ionization products from 1. Four distinct
resonances at δ ) 103, 70, -37, and -113 were observed in
the 27Al NMR spectrum. The resonance at 103 ppm is consistent
with the reported chemical shift of [AlCl4]-.12 We attribute the
-113 ppm resonance to [Cp′2Al]+ due to its extreme upfield
shift,13 similar to that of [Cp*2Al]+ (δ -115)5 and [Cp2Al] (δ
-128).2
Synthesis and Structural Characterization of [Cp′2Al]-
[B(C6F5)4]. Since AlCl3 itself can serve as a halide-abstracting
agent, we also examined the reaction between Cp′3Al and AlCl3
in a 1:2 ratio in CH2Cl2, instead of the 2:1 ratio used to generate
1. This reaction afforded a brown solid that exhibited resonances
for [Cp′2Al]+, [AlCl4]-, and 1 in the 1H and 27Al NMR spectra
of a sample in CDCl3. Efforts to crystallize the aluminocenium
salt from this mixture afforded only crystals of 1.
T (K)
λ (Å)
203(2)
0.71073
1.211
0.270
0.25 × 0.15 × 0.146
2.07 to 25.25
-10 e h e 10,
-12 e k e 12,
-12 e l e 12
8693
84(2)
0.71073
1.658
1.069
0.18 × 0.08 × 0.08
1.57 to 27.50
-29 e h e 29,
-31 e k e 31,
-22 e l e 21
58 723
F
calc (Mg/m3)
µ (mm-1
)
cryst size (mm3)
θ range (deg)
index ranges
no. reflns collected
no. indep reflns
3011 [R(int) )
0.0248]
3011/0/189
10 166 [R(int) )
0.1092]
10 166/78/562
no. data/restraints/
params
GOF
1.054
0.0385
0.0995
0.308, -0.235
1.018
0.0687
0.1454
0.709, -0.788
R1 [I > 2σ(I)]a
wR2 [I > 2σ(I)]b
largest diff peak,
hole (e‚Å-3
)
a R1 ) ∑|Fo| - |Fc|/∑|Fo|. wR2 ) {∑[w(Fo - Fc )2]/∑[w(Fo )2]}1/2
.
b
2
2
2
Synthesis and Structural Characterization of [Cp′2Al(µ-
Cl)]2. [Cp′2Al(µ-Cl)]2, 1, was formed via ligand redistribution
between Cp′3Al and AlCl3, which were combined in a 2:1 ratio
in dichloromethane. Efforts to form 1 by reacting AlCl3 with
Cp′2Mg were unsuccessful, possibly due to the tendency of the
aluminum compound to form a matrix with the MgCl2 byprod-
When Cp′3Al and AlCl3 were combined in a 1:2 ratio in
bromobenzene and the solution was layered with hexane, a pink,
crystalline product was obtained in 55% yield. We tentatively
formulate this compound as [Cp′2Al][AlCl4] (2) (eq 2) on the
(9) Schulz, S.; Roesky, H. W.; Noltemeyer, M.; Schmidt, H. G. J.
Organomet. Chem. 1995, 493, 69-75.
1
uct. Although the H NMR spectra of 1 in C6D6 and CDCl3
indicated the presence of multiple species due to ligand
redistribution, single crystals of 1 crystallized from a chloroform
solution of the compound in 42% yield. The molecular structure
of the compound is shown in Figure 1. Crystallographic data
and selected bond distances and angles are listed in Tables 1
and 2, respectively.
(10) Koch, H. J.; Schulz, S.; Roesky, H. W.; Noltemeyer, M.; Schmidt,
H. G.; Heine, A.; Herbst-Irmer, R.; Stalke, D.; Sheldrick, G. M. Chem.
Ber. 1992, 125, 1107-1109.
(11) Schonberg, P. R.; Paine, R. T.; Campana, C. F.; Duesler, E. N.
Organometallics 1982, 1, 799-807.
(12) (a) Atwood, J. L.; Cowley, A. H.; Hunter, W. E.; Mehrotra, S. K.
Inorg. Chem. 1982, 21 (4), 1354-1356. (b) Cowley, A. H.; Kemp, R. A.;
Wilburn, J. C. J. Chem Soc., Chem Commun. 1982, 5, 319-320.
(13) (a) Gauss, J.; Schneider, U.; Ahlrichs, R.; Dohmeier, C.; Schno¨ckel,
H. J. Am. Chem. Soc. 1993, 115, 2402-2408.
The molecule sits on a center of inversion in the crystal, so
each half of the dimer is related by symmetry. The aluminum

5584 Organometallics, Vol. 25, No. 23, 2006
Lee et al.
Cp′3Al + [Ph3C]+[B(C6F5)4]- C H Br8
Table 3. Selected Bond Lengths (Å) for 3
6
5
Al1-C25
Al1-C26
Al1-C27
Al1-C28
Al1-C29
C25-C29
C25-C26
C26-C27
C27-C28
2.105(4)
2.142(4)
2.168(4)
2.167(4)
2.131(4)
1.423(6)
1.426(7)
1.427(6)
1.430(6)
C28-C29
Al2-C34
Al2-C35
Al2-C36
Al2-C37
Al2-C38
1.437(6)
[Cp′2Al]+[B(C6F5)4]-
2.154(16)
2.093(12)
2.031(12)
2.060(15)
2.141(17)
(3)
(3)
The asymmetric unit of the crystal contains two symmetry
unique half aluminocenium cations, a [B(C6F5)4]- counteranion,
and a 50% disordered bromobenzene molecule from the solvent
of crystallization. Thermal ellipsoid drawings of the two
aluminocenium ions are shown in Figure 2. One aluminocenium
cation is linear with staggered η5 Cp′ rings. The centroid-Al
bond distance is 1.765 Å (vs 1.78 Å for [Cp*2Al]), and a 180
°C centroid-Al-centroid angle is imposed by the symmetry
of the crystal. The second aluminocenium cation is disordered.
Two orientations of the Cp′ were identified in this cation and
modeled as rigid groups using the coordinates of the ordered
aluminocenium cation. Both octamethylcyclopentadienyl ligands
are η5 bonded to aluminum, but the centroid-Al-centroid angle
deviates from linearity and is 174.7° for both disordered models.
The centroid-Al-centroid bond lengths are inequivalent. One
is 0.06 Å shorter and the other is 0.07 Å shorter than the Al-
(1)-centroid bond length in the ordered, linear molecule. The
disorder suggests that the Cp′ rings may be wagging back and
forth as a result of their inadequate steric shielding of the low-
coordinate, electrophilic aluminum center.
Al-(centroid 1)
Al-(centroid 2)
1.765
1.708/1.836
basis of its spectroscopic data. 1H and 27Al NMR spectra of the
compound in bromobenze-d5 revealed signals for only [Cp′2-
1
Al]+ (δ -113) and [AlCl4]- (δ 103). The H NMR spectrum
showed Cp′ methyl resonances at 1.65 and 1.73 ppm and a Cp′
allylic proton resonance at 5.85 ppm. Efforts to elucidate the
molecular structure of 2 from X-ray diffraction data were
unsuccessful.
C6H5Br
3[Cp′ Al]+[AlCl ]-
2Cp′3Al + 4AlCl3
8
(2)
2
4
(2)
Ligand redistribution occurred immediately when 2 was
dissolved in CDCl3. Two sets of Cp′ methyl signals (δ 1.91,
2.02; 2.14, 2.23) and two Cp′ allylic proton peaks (δ 5.77, 6.46)
1
were observed in the H NMR spectrum. There was a slight
splitting in the δ 6.46 resonance that is reminiscent of the
splitting observed in the Cp* methyl resonance in the 1H NMR
spectra of [Cp*2Al]+ salts.6,7 We believe that this splitting is
associated with ion pairing since it is solvent and temperature
dependent.
The stability of compound 3 in both bromobenzene-d5 and
CDCl3 solvents was monitored by 1H and 27Al NMR spectros-
copy. The compound was stable in bromobenzene-d5 at room
temperature for over a week. It was stable in CDCl3 at -20 °C
for more than 48 h and gradually decomposed as the temperature
was warmed to 20 °C. The cation [Cp′2Al]+ exhibits an 27Al
NMR chemical shift of δ -113 in both CDCl3 and bromoben-
zene-d5. A splitting of the allylic peak was observed in the room-
Compound 2 initiates the polymerization of isobutene in CH2-
Cl2; however, due to ligand redistribution, we could not rule
out the involvement of [AlCl2]+ 8 or [Cp′AlCl]+ as initiators of
the polymerization. With the knowledge that a [Cp′2Al]+ salt
was isolable, we sought an alternate route to an aluminocenium
salt with an inert counteranion. Although the trityl cation is best
known for its ability to abstract hydride and methyl anions from
metals, it proved to also be competent at abstracting a [Cp′]-
group from Cp′3Al. Clear, light pink crystals of [Cp′2Al]-
[B(C6F5)4], 3, were isolated in 43% yield from the reaction
between Cp′3Al and [CPh3][B(C6F5)4] in a bromobenzene/
hexane solvent mixture (eq 3). The pink coloration of the crystals
could be due to the presence of trace impurities. An X-ray crystal
structure determination was performed on the compound.
Crystallographic data and selected bond distances and angles
are listed in Tables 1 and 3, respectively.
1
temperature H NMR spectra of the compound in CDCl3 that
was not seen in brombenzene-d5.
Isobutene Polymerization. The isobutene polymerization
activities of compound 3 and [Cp*2Al][MeB(C6F5)3] were
compared under similar conditions. Whereas 3 activated the
polymerization of isobutene at -20 °C in CH2Cl2, [Cp*2Al]-
[MeB(C6F5)3] was completely inactive at this temperature. This
was confirmed in both NMR scale and preparative experiments.
Reaction of 10 mL of isobutene with 58 µg of 3 in 5 mL of
CH2Cl2 for 3 h at at -20 °C, followed by quenching with
methanol, yielded 3.2 g of polyisobutene as a viscous oil. A
similar scale reaction using [Cp*2Al][MeB(C6F5)3] afforded 2.1
Figure 2. Thermal ellipsoid drawing (30%) of [η5,η5-Cp′2Al]+[B(C6F5)4]- (3). Hydrogen atoms are omitted and only one of the disordered
models for Al2 is shown for clarity.

Synthesis and Characterization of [(C5Me4H)2Al]+
Organometallics, Vol. 25, No. 23, 2006 5585
1.95/1.97 (2s, 6H, (C5(CH3)4)); 2.00/2.03 (2s, 6H, (C5(CH3)4)); 3.28/
4.45(2s, 1H, (Me4C5H)). 13C{1H} NMR (75 MHz): δ 11.67, 12.10,
13.91 (C5(CH3)4); 123.51 (Me4C5H). Anal. Found (calcd): C, 68.49
(69.94); H, 7.93 (7.34). Due to the thermal instability of the
compound, the carbon value was consistently low upon reanalysis.
[Cp′2Al]+[AlCl4]- (2). Aluminum trichloride (0.268 g, 2.01
mmol) was added from a sidearm solid dispenser to a solution of
Cp′3Al (0.39 g, 1.0 mmol) in 20 mL of bromobenzene at room
temperature. The pinkish reaction mixture was stirred for 30 min.
Slow diffusion of hexane into the mixture at room temperature
yielded a pink, clear, crystalline solid after 24 h. The pink solid
was isolated by decanting off the bromobenzene solvent and rinsed
with petroleum ether, then dried under vacuum (yield 0.37 g, 56%).
1H NMR(300 MHz, 297 K, bromobenzene-d5): δ 1.65 (s, 6H, (C5-
(CH3)4)); 1.73 (s, 6H, (C5(CH3)4)); 5.85 (s, 1H, (Me4C5H)). 13C-
{1H} NMR (75 MHz): δ 27.21 (C5(CH3)4); 103.70, 118.79
(Me4C5H). 27Al NMR (78 MHz): δ -113 [Cp′2Al+]; 103 [AlCl4-].
Anal. Found (calcd) for C18H26Al2Cl4: C, 46.04 (49.3); H, 5.74
(5.93). The analysis is most consistent with the formulation given
for 2. The presence of some cocrystallized bromobenzene in the
crystals could account for the low carbon value.
g of polyisobutene after being run overnight at room temper-
ature. These results combined with the results from Bochmann’s
group indicate the following relative initiator activities: [Cp2-
Al]+ . [Cp′2Al]+ > [Cp*2Al]+. They also offer strong evidence
that the aluminocenium cation is indeed the initiator for the
polymerization, as opposed to protons generated by trace
amounts of water. Furthermore, these results show that activity
of the aluminocenium initiator is highly sensitive to the number
of methyl substituents on its cyclopentadienyl rings.
Summary and Conclusions
In summary, the inaccessibility of Cp′2AlMe necessitated the
development of a alternate route to [Cp′2Al]+ besides methyl
anion abstraction by B(C6F5)3. Toward this goal, Cp2′AlCl (1)
was prepared as a potential precursor to the [Cp′2Al]+ cation
via a ligand redistribution reaction between Cp′3Al and AlCl3.
Chloride ligand abstraction from 1 by AlCl3 appears to afford
[Cp′2Al][AlCl4]. Neither 1 nor [Cp′2Al][AlCl4] maintains their
integrity in solution, however, equilibrating to mixtures of 1,
[Cp′2Al][AlCl4], and Cp′3Al. An alternate route to [Cp′2Al]+
was found involving an unprecedented abstraction of a cyclo-
pentadienyl anion from Cp′3Al by [Ph3C][B(C6F5)4]. [Cp′2Al]-
[B(C6F5)4] (3) is less stable thermally than [Cp*2Al][MeB-
(C6F5)3] but is more active as an initiator of the carbocationic
polymerization of isobutene. We attribute the relative activities
of the aluminocenium cations ([Cp2Al]+ . [Cp2Al]+ . [Cp*2-
Al]+) as initiators primarily to the steric influence of their
cyclopentadienyl rings. The bulkier substituted rings should
make access to the aluminum center by substrates such as
isobutene more difficult and should also interfere with inter-
molecular decomposition pathways, which is reflected in the
relative thermal stabilities of these cations.
[Cp′2Al]+[B(C6F5)4]- (3). Ph3CB(C6F5)4 (0.786 g, 0.85 mmol)
was added through a solid dispenser to a solution of Cp′3Al (0.34
g, 0.87 mmol) in 20 mL of bromobenzene at room temperature.
The mixture appeared reddish-purple and then changed to a
brownish-yellow color and was stirred for 30 min. Slow diffusion
of hexane into the mixture at room temperature afforded a pink,
clear, crystalline solid after 36 h. The pink solid was isolated by
decanting bromobenzene solvent and rinsed with petroleum ether
1
(yield 0.35 g, 43%). H NMR (300 MHz, 297 K, CDCl3): δ 2.19
(s, 6H, (C5(CH3)4)); 2.25 (s, 6H, (C5(CH3)4)); 6.25(d, 1H, (Me4C5H)).
13C{1H} NMR(75 MHz): δ 10.72, 12.52 (C5(CH3)4) 107.46
(Me4C5H); 127.74, 130.90, 132.40(B(C6F5). 27Al NMR(78 MHz):
-113. 19F NMR (282 MHz): -132, -163, -166 (B(C6F5). 11B
NMR (54 MHz): δ -13.6. Anal. Found (calcd) for C40H26-
AlBF20‚C6H5Br: C, 49.26 (48.90); H, 2.98 (2.69).
Experimental Section
Isobutene Polymerizations by 3 and by [Cp*2Al][MeB-
(C6F5)3]. Compound 3 (83 mg, 0.058 mmole) was dissolved in 5
mL of CH2Cl2 in a 200 mL two-neck round-bottom flask. Isobutene
(10 mL) was condensed into the solution of 3 cooled to -78 °C.
The reaction mixture was warmed to -20 °C, stirred for 3 h at
that temperature, and then quenched with 0.5 mL of MeOH. A
viscous polymer was observed after additional MeOH was added
to the reaction. The reaction was warmed to room temperature,
and the solvent was removed under reduced pressure, leaving a
red-brown viscous oil. This material was redissolved in CH2Cl2,
and the red-brown organic layer was washed twice with distilled
water, which did not remove the color caused by impurities from
the catayst. After removal of the CH2Cl2, the oil was further dried
in a 60 °C oven, yielding 3.25 g of polyisobutene, which was
identified by its H NMR spectrum. H NMR (300 MHz, 297 K,
CDCl3): δ.1.056 (CH3), δ.1.35 (CH2)).15
The reaction between 47.8 mg (0.058 mmol) of [Cp*2Al][MeB-
(C6F5)3] and isobutene (10 mL) under identical experimental
conditions did not yield any polymer.
X-ray Crystal Structure Determinations. For both compounds
1 and 3, crystals were removed from the flask and covered with a
layer of hydrocarbon oil. A suitable crystal was selected, attached
to a glass fiber, and placed in the low-temperature nitrogen stream.16
Data for 1 and 3 were collected at 203(2) and 84(2) K, respectively,
using a Bruker/Siemens SMART APEX instrument (Mo KR
radiation, λ ) 0.71073 Å) equipped with a LT-2A low-temperature
device (203(2) K) or a Cryocool NeverIce low-temperature device
(84(2) K). Data for each crystal were measured using omega scans
General Considerations. All experiments were performed under
inert atmosphere conditions through a combination of glovebox,
high-vacuum, and Schlenk line techniques. All solvents were dried
over alumina columns and stored in line-pots over sodium/
benzophenone or CaH2 for CH2Cl2 and CHCl3 solvents. Argon was
purified by passage over an oxytower BASF catalyst (Aldrich) and
4A molecular sieves. NMR spectra were recorded on IBM NR-
1
300 (300 MHz H; 75.4 MHz, 13C; 78 MHz, 27Al) and Bruker
1
Avance 500 (500 MHz, H; 125 MHz, 13C; 470 MHz, 19F; 130
MHz, 27Al) spectrometers. All chemical shifts are reported in ppm
1
and referenced to the solvent (13C, H) or Al(OH)3 (27Al, external
reference, 0 ppm). Deuterated solvents were dried over activated
4A molecular sieves. Elemental analyses were determined by
Desert Analyses (Tucson, AZ).
Tetramethylcyclopentadiene was prepared from 3-pentanone and
acetaldehyde as described in the literature.14 Aluminum trichloride
was purified by sublimation before use.
1
1
[Cp′2AlCl]2 (1). Aluminum trichloride (0.135 g, 1.01 mmol) was
added with a sidearm solid dispenser to a solution of Cp′3Al (0.78
g, 2.0 mmol) in 30 mL of methylene chloride at -78 °C. The
mixture was warmed to room temperature slowly and stirred for 3
h. The solvent was removed from the resulting clear yellow solution
under reduced pressure, leaving behind a white solid. The white
solid product was washed with 20 mL of petroleum ether and dried
under vacuum (yield 0.38 g, 42%). Two major species were
observed in the 1H NMR spectrum of 1 as a result of ligand
1
redistribution in CDCl3. H NMR (300 MHz, 297 K, CDCl3): δ
(14) (a) Fendrick, C. M.; Schertz, L. D.; Day, V. W.; Marks, T. J.
Organometallics 1988, 7, 1828-1838. (b) Kohl, F. X.; Jutzi, P. J.
Organomet. Chem. 1983, 243, 31-34.
(15) Mallwitz, F.; Grasmu¨ller, M.; Ismeier, J. R.; Eckelt, R.; Nuyken,
O.; Goedel,W. A. Macromol. Chem. Phys. 1999, 200 (5), 1014-1022.
(16) Hope, H. Prog. Inorg. Chem. 1995, 41, 1.

5586 Organometallics, Vol. 25, No. 23, 2006
Lee et al.
of 0.3° per frame for 30 s, and a full sphere of data was collected.
A total of 2132 frames were collected with a final resolution of
0.84 Å for 1 and 0.77 Å for 3. The first 50 frames were re-collected
at the end of data collection to monitor for decay. Cell parameters
were retrieved using SMART17 software and refined using SAINT-
Plus18 on all observed reflections. Data reduction and correction
for Lp and decay were performed using the SAINTPlus software.
Absorption corrections were applied using SADABS.19 The struc-
tures were solved by direct methods and refined by least-squares
method on F2 using the SHELXTL program package.20 The
structures for 1 and 3 were solved in the space groups P1h (# 2)
and C2/c (# 15), respectively, by analysis of systematic absences.
In the structure determination for 3, the bromobenzene solvent was
disordered and modeled as a rigid body using coordinates obtained
from the Cambridge Structure Database with each orientation at
46% and 54% occupancy. The second aluminocinium cation is also
disordered. Two orientations of the Cp′ ligand were identified and
modeled as rigid groups using the coordinates of the ordered
aliminocenium cation. The occupancies of both orientations were
refined at 50%. Soft restraints were applied to the disordered carbon
atom thermal displacement parameters, and all atoms were refined
aniostropically. No decomposition was observed during data
collection for either crystal.
Acknowledgment. We are grateful to the donors of the
Petroleum Research Fund, administered by the American
Chemical Society and the Idaho NSF EPSCoR program (EPS-
0132626) for supporting this research. The establishment of a
Single-Crystal X-ray Diffraction Laboratory and the purchase
of a 500 MHz NMR spectrometer were supported by the M. J.
Murdock Charitable Trust of Vancouver, WA, National Science
Foundation, and the NSF Idaho EPSCoR Program.
(17) SMART: V. 5.626, Bruker Molecular Analysis Research Tool; Bruker
AXS: Madison, WI, 2002.
(18) SAINTPlus: V. 6.36a, Data Reduction and Correction Program;
Supporting Information Available: CIF files the X-ray crystal
structures of compounds 1 and 3. This material is available free of
charge via the Internet at http://pubs.acs.org.
Bruker AXS: Madison, WI, 2001.
(19) SADABS: V. 2.01, an empirical absorption correction program;
Bruker AXS Inc.: Madison, WI, 2001.
(20) Sheldrick, G. M. SHELXTL: V. 6.10, Structure Determination
Software Suite; Bruker AXS Inc.: Madison, WI, 2001.
OM0606439