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Are Aluminoxanes Nanotubular Structural Evidence from a Quantum Chemical Study.

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Angewandte
Chemie
structures over other proposed alternatives was demonstrated
by quantum chemical calculations on methyl-substituted[6, 7]
and unsubstituted[8] aluminoxanes. Herein, we report a HFand B3LYP-level quantum chemical study of the structures of
alkylaluminoxanes.
The structural determination of MAO is further complicated by association of trimethylaluminum[1] into the
{AlO(Me)} core. This led us to an unexpected observation
while studying the subsequent reactions between the unsubstituted [AlO(H)]n cages and AlH3. The products of the
reactions for cages with n = 6 and 8, which are the unsubstituted counterparts of two cages characterized crystallographically by Barron et al., are illustrated in Figure 1. The
Polymerization Catalysts
DOI: 10.1002/ange.200600197
Are Aluminoxanes Nanotubular? Structural
Evidence from a Quantum Chemical Study
Mikko Linnolahti,* John R. Severn, and
Tapani A. Pakkanen
Single-site a-olefin polymerization catalysts hold promise for
a rational tailoring of the polymer microstructure and hence
its properties. One of the critical components in these systems
is the co-catalyst, which can profoundly influence the activity,
stereoselectivity, and molecular-weight capability of the
catalytic system.[1] The archetypal co-catalyst is a solution of
methylaluminoxanes (MAO), the structural characterization
of which has remained challenging and elusive. As a
consequence, the understanding and control of the polymerization process, together with optimization of the co-catalyst,
has been handicapped by the inability to determine the
structure of the active component.
Due to the lack of precise crystallographic and spectroscopic characterization, several structural models have been
proposed for MAO. The first proposals included chains and
rings with three-coordinate, highly Lewis acidic Al centers.[2]
The preference for four-coordinate Al and three-coordinate
O atoms was demonstrated in 1983 by Atwood et al. by a
crystal structure analysis of [Al7O6Me16] .[3] Further progress
towards interpretation of polyhedral cages as the most
relevant structural alternative was made in the mid-90s by
Sinn[4] and Barron et al.[5] Following the synthesis of [AlO(tBu)]n cages (n = 6–9 and 12),[5] the preference for cage
[*] Dr. M. Linnolahti, Prof. T. A. Pakkanen
Department of Chemistry
University of Joensuu
P.O. Box 111, 80101 Joensuu (Finland)
Fax: (+ 358) 13-251-3390
E-mail: mikko.linnolahti@joensuu.fi
Dr. J. R. Severn
Borealis Polymers Oy
P.O. Box 330, 06101 Porvoo (Finland)
Supporting Information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. 2006, 118, 3409 –3412
Figure 1. The formation of aluminoxane nanotubes by association of
four {AlH3} groups into [AlO(H)]6 and [AlO(H)]8 cages.
AlH3 units attach to the [AlO(H)]n core by breaking the Al
O bond between {Al2O2} squares; the mechanism of this
“latent Lewis acidicity”[9] has been described by Zurek and
Ziegler.[10] For both cages, the reaction is exothermic for
addition of up to four AlH3 groups,[11] after which all {Al2O2}
squares are opened to form six-membered rings exclusively.
The resulting molecular structures are striking, with sections
of armchair (2,2) nanotubes capped with two AlH3 groups at
each end. These species do not undergo further reaction with
AlH3. Changing the hydrogen atoms to methyl groups leads a
similar, although somewhat more pronounced, result: in the
case of the smaller cage the reaction energy increases from
102.7 kJ mol 1 to 157.2 kJ mol 1.
Next, we investigated the cage dimer of [AlO(H)]6, whose
methylated counterpart has been proposed as a possible
component of MAO.[12] It turns out that this cage dimer is
unstable and undergoes a structural deformation. Depending
on the orientations of the two cages, the dimer has three
isomers; the energetically most favored one, by a margin of
over 40 kJ mol 1, is shown in Figure 2. This S4-symmetric
isomer, which is favored over two non-interacting cages by
340 kJ mol 1, takes the form of a capped (2,2) nanotube. As
the dimer is capped with three adjacent {Al2O2} squares,
oligomerization of the cages probably proceeds beyond the
dimer. Alternatively, incorporation of trialkylaluminum
would terminate the tube growth.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 2. The formation of a capped (2,2) armchair aluminoxane
nanotube by dimerization of face-bridged [AlO(H)]6 cages.
Encouraged by these observations, we studied the tubular
section of the unsubstituted aluminoxanes by means of
periodic ab initio calculations for the open-ended infinite
tubes. In analogy with carbon nanotubes,[13] zigzag, armchair,
and chiral nanotubes can be derived. The relative stabilities of
the tubes are listed in Table 1 using the preferred [AlO(H)]n
cage (T-symmetric Al28O28H28[8]) as a reference. The favored
structures for each family are illustrated in Figure 3.
Table 1: Diameters [nm] and stabilities [kJ mol 1] relative to the preferred
cage (Al28O28H28[8]) for [AlO(H)]n nanotubes of infinite length.
[AlO(H)]n nanotube
Diameter
(2,2)
(3,3)
(4,4)
(3,0)
(4,0)
(5,0)
(6,0)
(2,1)
(3,1)
(3,2)
(4,1)
(4,2)
0.69
0.87
1.03
0.56
0.75
0.81
0.95
0.55
0.71
0.74
0.79
0.88
DE (nHF)
1.4
2.3
3.3
3.8
10.6
10.6
7.2
15.7
5.0
4.4
8.8
3.7
DE (nB3LYP)
3.3
4.1
1.0
1.6
11.5
11.4
8.2
13.3
6.5
6.1
9.9
5.3
The aluminoxane nanotubes are favored over the cage
whatever the method applied. The tubes reach their energy
minimum at diameters somewhat below 1 nm: armchair in
(3,3), zigzag in (4,0), and chiral in (4,1). The preference for
zigzag tubes is due to the repulsion between the hydrogen
substituents. The zigzag arrangement allows the largest
separation between the neighboring substituents (3.76 E in
the case of the preferred (4,0), compared to 2.98 E for
armchair (3,3) and 3.27 E for chiral (4,1)). The preference for
relatively thin tubes, on the other hand, is due to the optimal
curvature for sp3-hybridized Al. While the Al28O28H28 cage is
a representative of optimal curvature, the cages are handicapped by the {Al2O2} squares necessary for cage closure. In
this regard, nanotubular shapes appear more reasonable
owing to their larger relative proportion of favorable sixmembered rings.
To verify that the preference for the tubular shape is not
characteristic for unsubstituted aluminoxanes alone, we
repeated the cage dimerization study (Figure 2) for methylsubstituted cages. The substitution pattern does not affect our
conclusions, the dimerization energy in the case of methyl
substituents being 330 kJ mol 1 (compared to 340 kJ mol 1
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Figure 3. Optimized structures of the favored [AlO(H)]n nanotubes of
infinite length for each family: armchair (3,3) (top), zigzag (4,0)
(middle), and chiral (4,1) (bottom).
for the unsubstituted case). We then optimized the methyland tert-butyl-substituted zigzag (4,0) and (3,0) nanotubes,
respectively, and compared their stabilities with those of the
correspondingly substituted cages. The calculations of Ziegler
et al. suggest that Al12O12Me12 is the favored [AlO(Me)]n
cage,[6] therefore we selected this as the reference structure.
No data for the relative stabilities of tert-butyl-substituted
cages are available, therefore a cage synthesized by Barron
et al., namely [AlO(tBu)]6, was selected as a reference.[5] The
optimized structures of the substituted cages and nanotubes,
together with their relative stabilities, are given in Figure 4.
The methylaluminoxane nanotube, in line with the unsubstituted ones, is clearly favored over the cage, the difference in
relative energy being almost 20 kJ mol 1 per {AlO(Me)} unit.
The opposite is observed for tert-butyl substituents, however,
as the marked overcrowding due to the vicinity of the bulky
substituents results in significant destabilization—the [AlO(tBu)]6 cage is favored by more than 70 kJ mol 1 per {AlO(tBu)} unit. One should note that the energy difference is due
to destabilization of the tube rather than stabilization of the
cage owing to the presence of tert-butyl substituents. Preliminary studies on the (4,0) nanotube suggest that the tubes
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2006, 118, 3409 –3412
Angewandte
Chemie
Figure 4. Structures and relative stabilities of selected methyl- and tertbutyl-substituted aluminoxane cages and zigzag nanotubes.
become increasingly overcrowded as a function of the tube
diameter.
The structural differences between MAO and tert-butylsubstituted aluminoxanes can now be discussed in a new light.
The [AlO(tBu)]n cages could actually be considered to be very
short tubes for which the tube growth is prematurely
interrupted after two or three repetitive cycles as a consequence of repulsion between the bulky substituents. In the
case of MAO, no interruption of the tube growth occurs as the
repulsion between the methyl groups is insignificant. One
should note that the current interpretation of the structure of
MAO being in a dynamic equilibrium of variably sized cages[7]
is basically based on BarronHs tert-butyl-substituted polyhedral cages.[5] Taking into account the significantly different
structural preferences of methyl- and tert-butyl-substituted
aluminoxanes, the structure of MAO might involve a dynamic
equilibrium mostly between nanotubes of variable lengths
and thicknesses, perhaps containing fractions of proposed
cages and other structural fragments.
In order to understand the function of nanotubular MAO
as a co-catalyst in polymerization catalysis, further studies are
required to determine its average molar mass, tube growth
mechanism, capping of the tube ends, and association of
trimethylaluminum into the AlO(Me) core. As far as the
molar mass is concerned, various experimental values have
been reported.[1] Recently, the size of the [Me MAO] anion
was determined from a pulsed field-gradient NMR study by
Babushkin and Brintzinger.[14] The observed mean effective
hydrodynamic radius of 12.2–12.5 E, assuming a spherical
structure, corresponds to about 150–200 Al atoms in each
MAO molecule. Possibly due to its size, catalytic activity
declines strongly at MAO concentrations below an Al/Zr
ratio of 200–300:1.[15] The sizes of the preferred MAO cages
reported previously[7] are about an order of magnitude lower,
hence they are unlikely to account for the structure of MAO.
Instead, the presence of hundreds of Al atoms in nanotubular
shapes could be a plausible explanation. In the case of the
zigzag (4,0) tube, 200 Al atoms would correspond to 50
Angew. Chem. 2006, 118, 3409 –3412
repetitive {Al4O4Me4} rings. As the tubular section of MAO is
inert towards reactions with trimethylaluminum, it is likely
that trimethylaluminum becomes associated into the tube
ends (see Figure 1). Building on this theory, one could reason
that the functional sites of MAO as a co-catalyst are
exclusively at the tube ends, hence a MAO molecule
containing hundreds of Al atoms arranged into a nanotubular
shape would possess, at most, two active sites, namely both
ends of the tube. The relatively few active sites of large MAO
molecules given the tasks of the co-catalyst, that is, activation
of the catalyst precursor and scavenging of impurities, would
explain why thousandfold Al/Zr ratios are generally required
in catalytic polymerizations.
In summary, we have provided new evidence concerning
the structures of aluminoxanes, among which methylaluminoxane is particularly important due to its application as a
co-catalyst in single-site homogeneous polymerization catalysis. The data presented suggest that MAO is nanotubular in
shape. The aluminoxanes are capable of adopting armchair,
zigzag, and chiral analogues of the well-known carbon
nanotubes and prefer diameters of about 1 nm. Trimethylaluminum is likely to become associated at the tube ends,
which act as functional sites in MAO.
Methods
All structures, including the periodic ones, were fully optimized.
Aluminoxane cages were constrained to the symmetries in question,
namely T for [AlO(H)]28, T for [AlO(Me)]12, and C3v for [AlO(tBu)]6,
and were characterized as true minima by frequency calculations.
Periodic calculations of unsubstituted aluminoxanes were performed
at the HF and B3LYP levels of theory; in all other examples the HF
method was applied. Periodic calculations generally require optimized basis sets. The optimized 8-5-11G* and 8-411G* basis sets were
used for aluminum and oxygen,[16] respectively, together with the
standard 6-31G** basis set for hydrogen, as reported in a previous
study.[8] For carbon, a 6-21G* basis set with a modified outer sp
exponent was adopted.[17] Identical basis sets were applied for the
clusters and the periodic tubes. The calculations were performed with
Gaussian 03 software.[18]
Received: January 17, 2006
Published online: April 7, 2006
.
Keywords: ab initio calculations · aluminoxanes ·
nanostructures · polymerization · structure elucidation
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2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Zuschriften
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The energy of AlMe3 was taken as being that of its dimer,
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2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2006, 118, 3409 –3412
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