The selective preparation of an aluminum oxide and its isomeric C-H-activated hydroxide

Article abstract of DOI:10.1021/ja052400y

An aluminum oxide [LAlO]₂ (1) has been prepared by the oxidative addition of aluminum(I) monomer LAl (L = HC[(CMe)(NAr)]₂, Ar = 2,6-iPr₂C₆H₃) with molecular oxygen. The short Al-O bonds in Al₂(μ-O)₂ result in short Al...Al contacts and subsequent steric crowding of the Ar substituents from the two oriented L. 1 hydrolyzes to form [LAl(OH)]₂(μ-O) (2). A C-H-activated aluminum hydroxide 4, an isomer of 1, however, is obtained by hydrolysis of the bulky aluminum amide 3 rather than by a conversion by high temperature treatment of 1. This indicates selective preparation of isomers 1 and 4. Copyright

Full text of DOI:10.1021/ja052400y

Published on Web 06/30/2005
The Selective Preparation of an Aluminum Oxide and Its Isomeric
C-H-Activated Hydroxide
Hongping Zhu,† Jianfang Chai,† Vojtech Jancik,† Herbert W. Roesky,*,† William A. Merrill,‡ and
Philip P. Power‡
Institut fu¨r Anorganische Chemie der UniVersita¨t Go¨ttingen, Tammannstrasse 4, D-37077 Go¨ttingen, Germany, and
Department of Chemistry, UniVersity of California, DaVis, One Shields AVenue, DaVis, California 95616.
Received April 13, 2005; E-mail: roesky@gwdg.de
Alumoxane can be used as the active catalyst in the polymeri-
zation of epoxides,1a-c aldehydes,1d,e and olefins1f and, therefore,
attracts much attention to chemists. In 1980, methylalumoxane
(MAO, [MeAlO]n) was found to be a highly active cocatalyst for
group 4 metallocenes, catalyzing ethylene and propylene poly-
merization.2 This has been of remarkable industrial importance. In
general, alumoxanes of formula [RAlO]n or [R2AlOAlR2]n can be
synthesized by the controlled hydrolysis of organoaluminum
compounds using water, or water contained in hydrated metal salts,3
or alternatively by their reaction with oxygen-containing species.4
This is well documented with the structural characterization of such
alumoxanes prepared in recent years, although the related reaction
process is complex.3,4e,f Meanwhile, it has been shown that the
hydrolysis of those organoaluminum compounds often leads to the
generation of Al-OH-containing species.3
It is known that alkylaluminum(III) compounds reacting with
O2 normally undergo insertion reaction into the Al-alkyl bond and
result in aluminum alkoxides5 or alkylperoxides.6 In contrast, the
reaction of low-valent aluminum compounds with O2, which could
Figure 1. Molecular structure of 1 (50% probability thermal ellipsoids).
directly yield alumoxanes, has not been investigated so far. Herein,
Selected bond lengths [Å] and angles [°]: Al(1)-O(1) 1.760(1), Al(1)-
we report on the reaction of aluminum(I) monomer LAl (L )
O(1A) 1.763(1), Al(1)‚‚‚Al(1A) 2.472(1), C(15)H(15)(56%)‚‚‚XPh(1A) 2.71,
HC[(CMe)(NAr)]2, Ar ) 2,6-iPr2C6H3)7 with molecular oxygen
C(15C)H(15C)(44%)‚‚‚XPh(1) 3.02, O(1)-Al(1)-O(1A) 90.86(5), Al(1)-
O(1)-Al(1A) 89.14(5).
and the isolation of an aluminum oxide [LAlO]2 (1). Further
hydrolysis of 1 to an aluminum oxide hydroxide 2 and the reaction
of LAl with N3Ar′′ in the presence of a small amount of water to
a C-H-activated aluminum hydroxide 4, an isomer of 1, are also
described (Scheme 1).
Scheme 1
A toluene solution of LAl in an atmosphere of O2 under stirring
in the temperature range of -78 °C to room temperature changed
slowly the color from red to orange, to yellow, and finally to almost
colorless. At ca. -15 °C, 1 separated from the solution as a
crystalline solid. The formation of 1 involves the O2 oxidation of
two central Al atoms. This might proceed through a LAl(η2-O2)
intermediate, which is not stable and reacts further with another
molecule of LAl. Comparable examples are reported for the reaction
8
of copper(I) with O2 and the matrix experiment of AlX and O2.9
Recently, the reaction of LAl with a large bulky azide N3Ar′ has
shown two types of intramolecular addition to an AldN multiple
bond species.10 The reproducible reaction of LAl with N3Ar′′
containing a small amount of H2O leads to the respective isolation
of 3, 4, and H2NAr′′ (5) and indicates the partial hydrolysis of a
C-H-activated product (3 to 4) at low temperature. The Al-N bond
imposed by the bulky Ar′′ in 3 might preferentially react with H2O
to form 4, an isomer of 1. 3 and 5 are characterized by spectral
analysis (Supporting Information), and 3 is further confirmed by
X-ray measurement (Supporting Information).
(m/z 920.4, 30%) are assigned to the molecular ion [M+]. 1 is
soluble in hot aromatic solvents (toluene and benzene), whereas
its solubility is poor at room temperature. Compound 4 was isolated
as colorless crystals, which melted in the range of 333-335 °C. 1
and 4 both have been characterized by spectroscopy, as well as by
X-ray crystallography.
The structural analysis of 1 (Supporting Information) reveals a
dimer with crystallographic centro-symmetry (Figure 1). The cen-
tral Al2(µ-O)2 core is formed in a nearly perfect square
(Al-O: 1.760(1), 1.763(1) Å; O-Al-O: 90.89(1)°; Al-O-Al:
89.11(1)°). The Al-O bond lengths are a little longer than those
observed in compounds with four-coordinate µ-oxo-aluminum
Compound 1 was obtained in high yield (80%). It melts at 314-
315 °C, and the isotope-distributed peaks in the EI MS spectrum
† Universita¨t Go¨ttingen.
‡ University of California, Davis.
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10.1021/ja052400y CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
activity, the notable structural character of 1 prompted us to test
its cocatalytic property for the dimethylzirconocene polymerization
of ethylene in toluene solution at either 25 or 80 °C. However, no
activity was observed. Moreover, no reaction occurred between 1
and Cp2ZrMe2.
In summary, we have prepared an aluminum oxide [LAlO]2 (1)
by reaction of LAl with O2. 1 has a stronger steric crowding than
that of its isomer 4 obtained from the hydrolysis of the bulky
aluminum amide 3. An attempt to convert 1 to 4 by high
temperature treatment, whether in toluene solution or in its solid
state, was not successful. The preparation of isomers 1 and 4 is
possible using selective routes.
Acknowledgment. This work was supported by the Deutsche
Forschungsgemeinschaft and the Go¨ttinger Akademie der Wissen-
schaften.
Figure 2. Molecular structure of 4 (30% probability thermal ellipsoids).
Selected bond lengths (Å) and angles (°): Al(1)-O(1) 1.910(1), Al(1)-
O(2) 1.850(1), Al(1)-C(13) 1.986(2), C(13)-C(12) 1.549(2), Al(2)-O(1)
1.852(1), Al(2)-O(2) 1.905(1), Al(2)-C(42) 1.989(2), C(42)-C(41)
1.554(3), Al(1)‚‚‚Al(2) 2.950(1), O(1)-Al(1)-O(2) 73.84(6), Al(1)-O(2)-
Al(2) 103.54(6), O(2)-Al(2)-O(1) 73.92(6), Al(2)-O(1)-Al(1) 103.25(6).
Supporting Information Available: The Experimental Section
including the detailed synthetic procedures, analytical, spectral, and
crystallographic characterization data (PDF). The molecular structure
of 4. CIF data for 1, 3 (0.12 toluene, 0.05 n-hexane), and 4. This
material is available free of charge via the Internet at http://pubs.acs.org.
(1.659-1.753 Å).11 However, they are much shorter than the
predicted one (1.96 Å).12 This may be largely due to ionic
contribution to the Al-O bond.13 The short Al-O bonds in
Al2(µ-O)2 result in short Al‚‚‚Al contacts (2.472(1) Å) and
subsequent steric crowding of the Ar substituents from the two
oriented L. Similar structural features are discussed for compounds
L2M (M ) Mg, Ca, Sr, Ba)15a and suggested for [LGaO]2.15b This
structural character is further recognized in solution by NMR
spectroscopic analysis. The iPr groups at Ar of L give rise to four
separated septets and eight doublets, indicating different steric
environments of two Ar groups of the LAl moiety. Moreover, one
septet and one doublet of those appear at higher field (δ 2.63 and
0.22 ppm) in comparison to the resonance range (δ 4.14-3.06,
1.58-0.72 ppm) of similar compounds.11e,14 This suggests that the
corresponding methine (CH) and methyl (CH3) protons of iPr are
shielded due to the ring current effect within the aryl groups caused
by steric crowding.16 The 13C{1H} NMR data also exhibit the
methine carbon resonance at high field (δ 71.4 ppm). This property
may result in a not energetically favorable approach to 1 by using
[LAlCl(µ-O)]2 as a precursor (Supporting Information).
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