Structural characterization of Al10O6 iBu16(μ-H)2, a high aluminum content cluster: Further studies of methylaluminoxane (MAO) and related aluminum complexes

Article abstract of DOI:10.1021/ic060291y

The first structurally characterized isobutyl-containing aluminoxane compound is presented. The Al₁₀O₆ⁱBu ₁₆(μ-H)₂ (I) cluster is produced from neat octakis-isobutyltetraluminoxane (Al₄O₂ⁱBu ₈) at 80°C in 6-8 h followed by slow crystallization. The crystal is triclinic (space group P1) with the molecule lying on an inversion center. This aluminoxane contains both nearly linear, 154(2)°, aluminum-bridging hydrides and three-coordinate aluminum sites. Solid-state ²⁷Al magic-angle spinning (MAS) NMR experiments were done at 19.6 and 40 T (833 MHz and 1.703 GHz, ¹H) and at 30-35 kHz spinning speeds, leading to the determination of the C_q and η values for the two four-coordinate Al sites and a lower limit of C_q for the three-coordinate Al site. Geometry-optimized restricted Hartree-Fock calculations at the double-ζ level of an idealized structure (methyl substituted, D_2h geometry) yielded C_q and η in close agreement with experiment; C _q agrees within 3 MHz.

Full text of DOI:10.1021/ic060291y

Inorg. Chem. 2007, 46, 44−47
i
Structural Characterization of Al10O6 Bu16(
µ-H)2, a High Aluminum
Content Cluster: Further Studies of Methylaluminoxane (MAO) and
Related Aluminum Complexes
Feng-Jung Wu,*,† Larry S. Simeral,*,† Anthony A. Mrse,‡ Jan L. Eilertsen,‡ Lacramioara Negureanu,‡
Zhehong Gan,§ Frank R. Fronczek,‡ Randall W. Hall,*,‡ and Leslie G. Butler*,‡
Albemarle Corporation, Process DeVelopment Center, P.O. Box 341,
Baton Rouge, Louisiana 70821, Department of Chemistry, Louisiana State UniVersity,
Baton Rouge, Louisiana 70803, and National High Magnetic Field Laboratory,
1800 East Paul Dirac DriVe, Tallahassee, Florida 32310
Received February 20, 2006
i
The first structurally characterized isobutyl-containing aluminoxane compound is presented. The Al10O6 Bu16
(
µ
-H)2
i
(I) cluster is produced from neat octakis-isobutyltetraluminoxane (Al4O2 Bu8) at 80
°
C in 6
−8 h followed by slow
crystallization. The crystal is triclinic (space group P1
contains both nearly linear, 154(2) , aluminum-bridging hydrides and three-coordinate aluminum sites. Solid-state
27Al magic-angle spinning (MAS) NMR experiments were done at 19.6 and 40 T (833 MHz and 1.703 GHz, 1H)
and at 30 35 kHz spinning speeds, leading to the determination of the Cq and values for the two four-coordinate
Al sites and a lower limit of Cq for the three-coordinate Al site. Geometry-optimized restricted Hartree Fock calculations
at the double- level of an idealized structure (methyl substituted, D2h geometry) yielded Cq and in close agreement
with experiment; Cq agrees within 3 MHz.
h) with the molecule lying on an inversion center. This aluminoxane
°
−
η
−
ú
η
Introduction
to isobutyl analogues which are less bulky (thereby more
MAO-like) and can be synthesized from commercially
available, inexpensive triisobutylaluminum. Herein, we report
the crystal structure and NMR spectra, both calculated and
experimental, for a novel isobutylaluminoxane cage contain-
ing both bridging hydride sites and three-coordinate alumi-
num sites.
The controlled reaction of water with aluminum alkyls
leading to numerous complex structures has a long history.1,3,4
For trimethylaluminum, hydrolysis yields MAO, a complex
system with, as yet, unknown structure(s).1,2 Controlled
hydrolysis of tri-tert-butylaluminum with water or hydrated
metal salts leads to cage-type structures with four-coordinate
aluminum sites bound to oxygen and one or more alkyl
groups.5,6 Cages consisting of six, seven, eight, and nine
aluminums have been structurally characterized. Aluminox-
Methylaluminoxane (MAO) has been the activator of
choice for metallocene related catalysts, a new generation
of single site catalysts revolutionizing polyolefin synthesis.1,2
Interestingly, the molecular structure(s) of MAO is unknown.
Given that uncertainty, a strong need exists for well-
characterized aluminoxanes as either structural models for
MAO, spectroscopic references, or replacements for MAO.
Although a number of well-characterized aluminoxanes with
sterically encumbered alkyl groups have already been
reported, it is highly desirable to extend the modeling study
* To whom correspondence should be addressed. E-mail: lbutler@lsu.edu
(L.G.B.), robert_wu@albemarle.com (F.-J.W.), larry_simeral@albemarle.com
(L.S.S.), rhall@lsu.edu (R.W.H.), gan@magnet.fsu.edu (Z.G.), jlasseei@
online.no (J.L.E.), lnegur1@lsu.edu (L.N.). Telephone: 703-292-4955
(L.G.B.), 225-768-5696 (F.-J.W.), 703-768-5995 (L.S.S.), 225-578-3472
(R.W.H.), 850-644-4662 (Z.G.). Fax: 703-292-9047 (L.G.B.), 225-768-
5689 (F.-J.W.), 703-768-5990 (L.S.S.), 225-578-8248 (R.W.H.), 850-644-
1366 (Z.G.).
(3) Storr, A.; Jones, K.; Laubengayer, A. W. J. Am. Chem. Soc. 1968, 90
(12), 3173-7.
† Albemarle Corporation.
‡ Louisiana State University.
(4) Giannini, U.; Albizzati, E.; Zucchini, U. Inorg. Chim. Acta 1985, 98
(3), 191-4.
§ National High Magnetic Field Laboratory.
(1) Kaminsky, W.; Sinn, H. Proc. IUPAC, IUPAC, Macromol. Symp. 1982,
28, 247.
(5) Mason, M. R.; Smith, J. M.; Bott, S. G.; Barron, A. R. J. Am. Chem.
Soc. 1993, 115 (12), 4971-84.
(2) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. (Washington, DC, U.S.) 2000,
(6) Harlan, C. J.; Mason, M. R.; Barron, A. R. Organometallics 1994, 13
(8), 2957-69.
100 (4), 1391-1434.
44 Inorganic Chemistry, Vol. 46, No. 1, 2007
10.1021/ic060291y CCC: $37.00
© 2007 American Chemical Society
Published on Web 12/13/2006

i
Structural Characterization of Al10O6 Bu16(µ-H)2
i
i
Table 1. Calculated and Experimental 27Al NMR Parameters
calculated experimental
charge/e- Cq/MHz δ/ppma Cq/MHz δ/ppma
Scheme 1. Al10O6 Bu16(µ-H)2, R ) Bu
site
η
η
Al1, AlCO3
Al3, AlC2O
Al4,
0.446
0.572
0.336
-17.3 0.319 113
37.2 0.731 211
-20.9 0.663 149
17.5
0.41 117
22.9
0.67 158
AlC2O(µ-H)
a Experimental chemical shifts are referenced to the Al(NO3)3 solution
(0 ppm); calculated chemical shifts are referenced to the trimethylaluminum
dimer with 5-coordinate bridging carbons (154 ppm).
fields were calibrated with R-Al2O3 (-8.8 ppm).9 Magic-angle
spinning (MAS) rotors, 2.0 mm ZrO2 rotors with tight-fitting Kel-F
caps, were loaded in an argon-filled glovebox and transported to
the NMR instrument in capped vials and spun with air to 35.7 kHz
for up to 40 min. Under these conditions, the more air-sensitive
MAO samples gave spectra without noticeable decomposition
products and three different samples of I gave very similar spectra.
Despite the field calibration with R-Al2O3, the 40 T field was
observed to drift between sample changes; hence, the chemical shifts
reported here are based on the 19.6 T spectra. Alternative data
acquisition strategies, such as an internal chemical shift standard,
were considered, but none were found; AlB2 was considered as it
has a chemical shift well away from I, but the AlB2 resonance has
a broad intrinsic line width.10 Other experimental details for 40 T
NMR spectroscopy are reported elsewhere.11
anes with ring and ladder structures have been synthesized
starting from (Mes*AlH2)2, where Mes* is 2,4,6-tri-tert-
butylbenzene.7 Controlled thermolysis of anionic methyl-
aluminoxanes has yielded a large, open anionic cluster.8
During studies of hydrolyzed isobutylaluminoxane, as the
MAO replacement, we serendipitously obtained nice single
i
crystals of Al10O6 Bu16(µ-H)2 (I, Scheme 1). This material
is extremely useful for NMR characterization and molecular
orbital calculation assessment as a model for methyl-
aluminoxanes.
The three-coordinate site was not included in the fit because the
calculated Cq (37.2 MHz; Table 1) is much too large to yield a
detectable resonance at 19.6 or 40 T and at a 35 kHz spin rate.9
The experimental 27Al Cq, η, and isotropic chemical shift values
are given in Table 1. The four-coordinate aluminum sites have, as
expected, similar chemical shifts, while the three-coordinate site is
believed to be shifted as much as 100 ppm downfield.12 In an effort
to extract the maximum information from the 19.6 and 40 T spectra,
a spectral fitting procedure was designed to simultaneously fit all
spectra, using a Nelder-Mead Simplex algorithm13 provided in
Matlab. I has quite high symmetry as it lies on an inversion center;
hence, the 10 aluminum sites in the molecule reduce to three NMR
distinct sites in the MAS spectra. The four sites labeled “AlCO3”
(see Scheme 1) contribute to one 27Al resonance, and the four sites
labeled “AlC2O(µ-H)” contribute to another; the two sites labeled
“AlC2O” have Cq values too large to be observed, even at 40 T.9
(The tert-butyl analogue of the dimer (R2AlOAlR2)2 has been
structurally characterized by single-crystal X-ray determination. In
contrast, the isobutyl analogue is a liquid at room temperature. See
refs 5, 6, and 24.)
Experimental Section
i
Single crystals of Al10O6 Bu16(µ-H)2 (I, Scheme 1) were obtained
from long-term storage (>6 months at ambient temperature) of neat
(iBu2AlOAliBu2)2 obtained by hydrolyzing triisobutylaluminum with
0.5 equiv of water; several attempts to make compound I starting
i
from Bu2AlH failed. The formation of the crystals from neat
(iBu2AlOAliBu2)2 is a very slow process at room temperature,
and the yield is low (<0.1% after one year). Subsequently, we
found that the reaction can be accelerated by heating neat
(iBu2AlOAliBu2)2 at ∼80 °C for 6-8 h, during which a dispro-
portionation reaction occurs generating AliBu3, I, and a number of
hydride-containing species:
(R2AlOAlR2)2 thermolysis8 AlR3 + Al10O6R16(µ-H)2 + others
The reaction can be conveniently monitored by 1H NMR for the
formation of AliBu3 and the hydride. The thermolysis, if done at
60 °C or >110 °C, results in either too slow a reaction or the
formation of a black Al metal, respectively. After removal of AliBu3
from the mother liquor (80 °C/0.3 mmHg), the yield of I can reach
up to ∼10%.
Results and Discussion
The 1H NMR spectrum of the crystals, having an empirical
i
The crystal of Al10O6 Bu16(µ-H)2 is triclinic (space group P1h)
i
formula of Al10O6 Bu16H2 (I), exhibits characteristics very
and contains both near-linear bridging hydrides ( Al4-H-Al5 )
154(2)°, d(Al-H) ) 1.72(3) Å) and nearly planar three-coordinate
aluminum sites ( C9-Al3-C13 ) 134.6(1)°; Al3 is 0.0304(6) Å
out of the C9-C13-O3 plane).
different from those of (R2AlOAlR2)2, namely, four different
alkyl environments (in a 1:1:1:1 ratio), very sharp resonances
(indicative of slow or no alkyl exchange), and the presence
NMR studies of this air-sensitive compound were done at the
National High Magnetic Field Laboratory on a narrow-bore Magnex
19.6 T (833 MHz, H) magnet fitted with a Bruker console and a
Samoson probe. NMR spectra were also acquired on the hybrid 40
T magnet with a Tecmag console and the Samoson probe. Magnetic
(9) Bryant, P. L.; Harwell, C. R.; Mrse, A. A.; Emery, E. F.; Gan, Z.;
Caldwell, T.; Reyes, A. P.; Kuhns, P.; Hoyt, D. W.; Simeral, L. S.;
Hall, R. W.; Butler, L. G. J. Am. Chem. Soc. 2001, 123 (48), 12009-
12017.
1
(10) Eastman, M. A. J. Magn. Reson. 1999, 139 (1), 98-108.
(11) Gan, Z. H.; Gor’kov, P.; Cross, T. A.; Samoson, A.; Massiot, D. J.
Am. Chem. Soc. 2002, 124 (20), 5634-5635.
(7) Wehmschulte, R. J.; Power, P. P. J. Am. Chem. Soc. 1997, 119 (35),
8387-8388.
(8) Atwood, J. L.; Hrncir, D. C.; Priester, R. D.; Rogers, R. D.
Organometallics 1983, 2 (8), 985-9.
(12) Benn, R.; Janssen, E.; Lehmkuhl, H.; Rufinska, A. J. Organomet.
Chem. 1987, 333 (2), 155-168.
(13) Lagarias, J. C.; Reeds, J. A.; Wright, M. H.; Wright, P. E. SIAM J.
Optim. 1998, 9 (1), 112-147.
Inorganic Chemistry, Vol. 46, No. 1, 2007 45

Wu et al.
Figure 1. 1H NMR spectrum of I.
i
Figure 2. ORTEP of the core of the cluster, Al10O6 Bu16(µ-H)2, I. For
each isobutyl group, only one carbon is shown. With a molecular symmetry
of C2h, the unique sites are as follows: Al1, a four-coordinate AlCO3 site;
Al3, a three-coordinate AlC2O site; and Al4, a four-coordinate AlC2O(µ-
H) site.
of a broad hydride resonance at 3.80 ppm (Figure 1). The
presence of a hydride (8:1 isobutyl/hydride ratio) is unex-
pected, but not surprising considering the ease with which
an isobutyl group can undergo â-H elimination.
Figure 3. (a) 27Al MAS NMR spectrum at B0 ) 19.6 T, νr ) 35.7 kHz
(two runs) and B0 ) 40 T, νr ) 35 kHz. (b) Best Simplex fits of the central
transition of the 27Al MAS NMR spectrum.
This complex (Figure 2) is the first aluminoxane found to
contain a bridging hydride. Among related alkyl systems,
trimeric di-tert-butylaluminum hydride has a slightly bent
bridging hydride with d(Al-H) ) 1.726(5) Å and Al-
H-Al ) 151(1)°.14 The anionic alane, Na[(CH3)3Al-H-
Al(CH3)3] has a linear bridging hydride with d(Al-H) )
1.665(1) Å.15 A bridging hydride, d(Al-H) ) 1.803(7) and
1.829(8) Å, is found in the benzoate complex, R2Al(µ-H)-
(µ-O2C-C6H5)AlR2 (R ) CH(SiMe3)2).16 A similar bond
length, 1.727(1) Å, is found in a doubly bridging hydride,
bis(bis(trimethylsilyl)amino)alane dihydride, {[(Me3Si)2N]2Al-
(µ-H)}2.17
which yield extremely broad resonances, even at high
magnetic fields and fast MAS rates.9 In advance of the NMR
studies of I (Figure 3), ab initio molecular orbital calculations
were performed for an idealized structure; isobutyl groups
were replaced by methyl groups and the molecular symmetry
was increased from C2h to D2h. Previous calculations have
demonstrated that the Hartree-Fock-level calculations are
sufficient to obtain Cq and η values within 10% of experi-
mental solid-state NMR parameters.9 The ab initio restricted
Hartree-Fock calculations, with a double-ú basis set (cc-
pVDZ), yielded an optimized geometry for which Lo¨wdin
charges,18 27Al quadrupolar NMR parameters, Cq, and η were
calculated (Table 1).
Aluminoxanes have been difficult to study with 27Al MAS
NMR because of very large 27Al quadrupolar interactions
In the MAO/metallocene polyolefin catalysis, three-
coordinate aluminum sites in MAO were originally thought
necessary for catalytic activation of the metallocenes. It was
proposed that the Lewis acidity of a three-coordinate
aluminum site would enable the extraction of a ligand from
(14) Uhl, W. Z. Anorg. Allg. Chem. 1989, 570, 37-53.
(15) Atwood, J. L.; Hrncir, D. C.; Rogers, R. D.; Howard, J. A. K. J. Am.
Chem. Soc. 1981, 103 (22), 6787-8.
(16) Uhl, W.; Graupner, R.; Pohl, S.; Saak, W.; Hiller, W.; Neumayer, M.
Z. Anorg. Allg. Chem. 1997, 623 (5), 883-891.
(17) Janik, J. F.; Wells, R. L.; Rheingold, A. L.; Guzei, I. A. Polyhedron
1998, 17 (23-24), 4101-4108.
(18) Lo¨wdin, P.-O. Phys. ReV. 1955, 97, 1474-89.
46 Inorganic Chemistry, Vol. 46, No. 1, 2007

i
Structural Characterization of Al10O6 Bu16(µ-H)2
the metallocene, thus opening a coordination site for an
olefin.1,2,19 However, that concept has evolved into a more
sophisticated mechanism of “latent Lewis acidity”19 in which
four-coordinate aluminum sites have, because of strain within
cage complexes, the potential to rupture and thus generate
reactive three-coordinate sites. Another scheme by which
three-coordinate aluminum sites might be generated is the
cleavage of a bridging methyl bond. Recently, bridging
methyls have been observed in MAO by Fourier transform
infrared (FTIR) spectroscopy20 and it was reported that the
tert-butylaluminoxane hexamer reacts with trimethylalumi-
num to form methyl bridges, and this product is catalytically
active with metallocenes.21 Well-defined three-coordinate
aluminum sites are rare, with the sterically hindered, non-
catalytically active site of Al(OR)3 (R ) 2,6-di-tert-butyl-
4-methylphenyl) available as one example.22 Preliminary
testing shows that I has little or no activating power for a
A search for three-coordinate sites in MAO was made with
field-swept NMR with the assumption that 27Al Cq > 30
MHz, but no such sites were found.9 In the present work,
the calculated Cq ) 37.2 MHz and, based on the accuracy
for the two other sites, we estimate an accuracy of (3 MHz.
With such a large Cq and after reconsidering the earlier field-
swept spectra, the three-coordinate sites may not have been
detected if they were present in less than 10% abundance.
The charge distribution among the aluminum sites shows
that the three-coordinate sites have the highest positive
charge, consistent with their proposed role as Lewis acids.
The hydride site creates the potential for a facile change in
the aluminum charge upon rupture of an Al-H-Al bond
for one of the aluminum sites, from least positive to most
positive. Because of the crystalline form of this aluminoxane,
it has unique value in the calculations and NMR spectroscopy
of MAO, especially with the validation of the calculated
quadrupolar interactions and chemical shifts.
1
number of metallocenes tested. Typically, the H NMR
spectrum of Cp2ZrMe2 in C6D6 remains unchanged after
mixing with 1 equiv of I. This lack of reactivity is apparently
due to the bulky isobutyl groups protecting the three-
coordinate aluminum (Al3).
Acknowledgment. The support of the National Science
Foundation, CHE-9977124, and the Research Council of
Norway (J.L.E.) is gratefully acknowledged, as is the funding
for NMR and computer cluster equipment from the Louisiana
Board of Regents. The National High Magnetic Field
Laboratory is sponsored by the National Science Foundation
through Cooperative Agreement DMR-9527035 and by the
State of Florida. Calculations were performed on computer
resources at the LSU Computer Center.
Despite the lack of catalytic activity, the title compound
is highly important to MAO structural research for several
reasons: a well-characterized three-coordinate aluminum site
and a bridging hydride site, a large aluminum cluster size, a
total of 10 aluminum sites, and an empirical formula
(Al1.0O0.6R1.6) which is similar to that determined for MAO
(Al1O0.8-0.75R1.4-1.5).23 The lack of catalytic activity is at-
tributed to the bulk isobutyl groups which prevent the three-
coordinate aluminum sites from extracting a methyl from
dimethyl zirconocene and adopting a tetrahedral geometry.
Supporting Information Available: Crystallographic data,
including the coordinates of the idealized structure used for the
calculations (9 pages). This material is available free of charge via
the Internet at http://pubs.acs.org
(19) Harlan, C. J.; Bott, S. G.; Barron, A. R. J. Am. Chem. Soc. 1995, 117
(24), 6465-74.
IC060291Y
(20) Ystenes, M.; Eilertsen, J. L.; Liu, J.; Ott, M.; Rytter, E.; Støvneng, J.
A. J. Polym. Sci., Part A: Polym. Chem. 2000, 38 (17), 3106-27.
(21) Watanabe, M.; McMahon, C. N.; Harlan, C. J.; Barron, A. R.
Organometallics 2001, 20 (3), 460-467.
(23) Imhoff, D. W.; Simeral, L. S.; Blevins, D. R.; Beard, W. R. ACS Symp.
Ser. 2000, 749, 177-191.
(24) Boleslawski, M.; Serwatowski, J. J. Organomet. Chem. 1983, 254 (2),
159-66.
(22) Healy, M. D.; Barron, A. R. Angew. Chem., Int. Ed. Engl. 1992, 31
(7), 921-2.
Inorganic Chemistry, Vol. 46, No. 1, 2007 47