826
Inorg. Chem. 2001, 40, 826-827
Formation and Unexpected Catalytic Reactivity of Organoaluminum Boryloxides
Vernon C. Gibson,* Sergio Mastroianni, Andrew J. P. White, and David J. Williams
Department of Chemistry, Imperial College, South Kensington, London, SW7 2AY, U.K.
ReceiVed August 8, 2000
Aluminoxanes [RAlO]n have been shown to play a special role
in the activation of R-olefin polymerization precatalysts.1 Within
the general family of aluminoxanes, the most commonly utilized
derivative is methylaluminoxane (MAO, A) which is often
described as a mixture of linear and cyclic oligomers, and cage
structures.2 Studies by Barron and co-workers have revealed that
sterically hindered alkylaluminoxanes, such as (ButAlO)n, exist
as hexameric cages (B) and that their Lewis acidity arises by
cleavage of one of the framework aluminum-oxygen bonds.3
More recently, these researchers have also shown that the [But-
Al] units of (ButAlO)6 may be replaced by [MeAl] groups upon
treatment with Me3Al to give hybrid tert-butyl-methylaluminox-
anes and even hexameric (MeAlO)6.4
Scheme 1
num counterparts due to the higher electronegativity of boron
[2.04] relative to aluminum [1.62] (Pauling scale). Known
complexes of aluminum containing (-OBR2) ligands are limited
to a few examples.4-8 Here, we describe reactions of Me2AlX
(X ) Me, Cl) with the borinic acids HOBR2 (where R ) mesityl
or 2,6-dimethylphenyl) to give novel organoboryloxide aluminum
products. An unexpected outcome of these studies was the finding
that Al-OBR2 species are capable of catalyzing the formation
of boroxine (RBO)3, a trimeric boron relative of the hexanuclear
aluminoxanes which may be viewed as being composed of two
stacked (RAlO)3 rings.
Treatment of (mes)2BOH with excess Me3Al (5 equiv) in
refluxing toluene for 12 h, followed by recrystallization from
acetonitrile, afforded a white crystalline compound, whose
characterizing data are consistent with the mono-boryloxide
aluminum species 1 (Scheme 1). The 2,6-dimethylphenyl deriva-
tive 2 can be prepared by an analogous procedure. Crystals of 1
suitable for an X-ray structure determination were grown from a
saturated acetonitrile solution at room temperature;9 the structure
is shown in Figure 1. The molecule has crystallographic inversion
symmetry about the center of the Al2O2 ring which is planar and
has transannular O‚‚‚O and Al‚‚‚Al separations of 2.44 and 2.86
Å, respectively, and Al-O-Al angles of 98.98(10)°. The Al-
O-Al bridge is symmetric with Al-O distances of 1.876(2) [Al-
O] and 1.879(2) Å [Al-O′], values that lie within the range
normally observed for oxygen-bridged ring systems.
With a view to obtaining a greater understanding of MAO, we
became interested in modeling the incipient highly Lewis acidic
sites that give rise to their exceptional reactivity. In their simplest
form, these sites may be viewed as consisting of an aluminum
methyl moiety bonded to two highly electron withdrawing
(OAlX2) units (C) which, in coordination chemistry terms, can
be regarded as quasi-alkoxide ligands. It is also likely that species
of this type form during the early stages of MAO formation by
the hydrolytic route. For example, Roesky and co-workers have
shown that hydrolysis of the sterically hindered model compound
mes3Al (mes ) 2,4,6-trimethylphenyl) in thf affords [mes2AlOH]2
as a thf adduct.5 However, due to the lack of controlled routes to
HOAlX2 species, and thereby to -OAlX2 ligands, we decided to
model these groups using boryloxide units of the type (-OBX2)
(D) whose metal derivatives may be readily synthesized via the
more amenable borinic acids (HOBR2). The R substituents may
be utilized to modulate the steric and electronic environment
around the aluminum center as well as the solubility of the
resultant boryloxide products. These boron-containing ligands are
also expected to be more electron-withdrawing than their alumi-
The reaction of (mes)2BOH with excess Me2AlCl under
analogous conditions affords the closely related dimeric mono-
boryloxide product 3 (Scheme 1). Upon reflux in acetonitrile, 3
reacts to give the tris(boryloxide)aluminum product (R2BO)3Al-
(NCMe) (4). The X-ray structure of 49 is shown in Figure 2. The
(1) See: Kaminsky, W. J. Chem. Soc., Dalton Trans. 1998, 1413 and
references therein.
(2) See: (a) Pasynkiewicz, S. Macromol. Symp. 1995, 97, 1. (b) Barron, A.
R. Macromol. Symp. 1995, 97, 15.
(3) (a) Mason, M. R.; Smith, J. M.; Bott, S. G.; Barron, A. R. J. Am. Chem.
Soc. 1993, 115, 4971. (b) Koide, Y.; Bott, S. G.; Barron, A. R.
Organometallics 1996, 15, 5514.
(4) Barron, A. R. Abstracts of Papers, 218th National Meeting of the
American Chemical Society, New Orleans, August 1999; American
Chemical Society: Washington, DC, 1999; INOR 293.
(5) Storre, J.; Klemp, A.; Roesky, H. W.; Schmidt, H.-G.; Noltemeyer, M.;
Fleischer, R.; Stalke, D. J. Am. Chem. Soc. 1996, 118, 1380.
(6) Ko¨ster, R.; Tsay, H.-Y.; Kru¨ger, C.; Serwatowski, J. Chem. Ber. 1986,
119, 1174.
(7) Anulewicz-Ostrowska, R.; Lulinski, S.; Serwatowski, J. Inorg. Chem.
1999, 38, 3796.
(8) Ko¨ster, R.; Tsay, H.-Y.; Kru¨ger, C.; Serwatowski, J. Chem. Ber. 1986,
119, 1301.
(9) See Supporting Information for crystal data.
(10) Washburn, R. M.; Levens, E.; Albright, C. F.; Billig, F. A. Org. Synth.
1959, 39, 3.
Figure 1. The molecular structure of 1.
10.1021/ic000895g CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/23/2001