Formation and unexpected catalytic reactivity of organoaluminum boryloxides

Article abstract of DOI:10.1021/ic000895g

The synthesis and structural characterization of mono- and tris-boryloxide complexes of aluminum are described. The mono-boryloxide aluminum species are found to catalyze the conversion of borinic acids to cyclic boroxines (RBO)₃ with eliminati

Full text of DOI:10.1021/ic000895g

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
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Communications
Inorganic Chemistry, Vol. 40, No. 5, 2001 827
An obvious potential alternative route to bis(boryloxide) and
tris(boryloxide) species is via treatment of Me3Al with 2 equiv
of (Mes)2BOH and excess (Mes)2BOH, respectively. However,
these reactions, rather than affording the higher aluminum
boroxide species, gave copious quantities of the white crystalline
cyclic boroxine, (MesBO)3, along with concomitant formation of
mesitylene (by NMR). Analysis of the residue showed that, in
each case, only one methyl of Me3Al is substituted to give 1 in
addition to the boroxine ring. A series of control experiments
were performed: under identical experimental conditions but in
the absence of Me3Al or (1), (Mes)2BOH does not afford
(MesBO)3. Even after prolonged (24 h) reflux in toluene,
(Mes)2BOH may be recovered unchanged. Contrastingly, upon
addition of a catalytic amount of Me3Al (5 mol %) to a solution
of (Mes)2BOH, quantitative conversion to (MesBO)3 occurs within
12 h at 60 °C, confirming the catalytic role of organoaluminum
species in the formation of the boroxine. Historically, boroxines
have been synthesized by a number of different routes, the most
common being dehydration of boronic acids of the type RB(OH)2
at 110 °C in the solid state10,11 or in refluxing toluene.12 To our
knowledge, boroxines have not previously been accessed via
borinic acid precursors.
Figure 2. The molecular structure of 4.
To probe the catalytic role of the organoaluminum species, a
series of NMR experiments were carried out in which the
conversion of (Mes)2BOH to (MesBO)3 was monitored at room
temperature in the presence of 1. The 1H NMR spectra recorded
over 180 min are shown in Figure 3. At t ) 0 (spectrum a),
resonances attributable to (Mes)2BOH and 1 only are observable.
Spectra b-d show the progressive loss of (Mes)2BOH and the
appearance of signals assignable to mesitylene and (MesBO)3.
Significantly, no methane is formed. The intensities of the
resonances attributable to 1 remain constant throughout the course
of the reaction, which is first order in borinic acid (rate constant
10 mol-1 s-1 (293 K)). The reaction of (Mes)2BOH with 2
afforded (MesBO)3 exclusively; no mixed mesityl/2,6-dimeth-
ylphenyl boroxines were observed, and there was no exchange
of the boryloxide ligands of 2. Monitoring this reaction over the
temperature range 293-333 K afforded the activation param-
eters: ∆H‡ 89.1 kJ mol-1; ∆S‡ 145 J mol-1 K-1; ∆G‡ (293 K)
46.7 kJ mol-1. The turnover frequency (TOF) at 298 K is
dependent upon the aluminum and boryl substituents, the chlo-
roaluminum catalyst 3 (TOF ) 8.0 h-1) giving a more active
catalyst than 1 (TOF ) 1.8 h-1) or the 2,6-dimethylphenylboron
catalyst 2 (TOF ) 1.3 h-1).
The detailed mechanism by which 1-3 catalyzes the conversion
of borinic acids R2BOH into boroxines (RBO)3 remains to be
established, and we refrain here from speculation. The elimination
of mesitylene solely from the borinic acid highlights a pathway
that contrasts with that observed in other systems where the
elimination usually involves a methyl from the aluminum center
to generate methane. The catalyzed formation of boroxines
(RBO)3 by boryloxide-aluminum species highlights the possibil-
ity that the formation of species such as [RAlO]n by hydrolysis
of alkyl precursors may be autocatalyzed by intermediates formed
during the hydrolytic procedure.
Figure 3. 1H NMR spectra (t ) 0-180 min) showing the conversion of
(mes)2BOH to [mesBO]3 catalyzed by 1 (* resonance due to the residual
protio impurity in C6D6).
geometry at aluminum is distorted tetrahedral with three coordina-
tion sites being occupied by boryloxide ligands and the fourth
occupied by a strongly bound MeCN ligand [Al-N(1) 1.954(2)
Å]. All three O-Al-N angles are noticeably contracted [ranging
between 102.2(1) and 103.8(1)°] indicating a tendency toward
trigonal pyramidal geometry with the acetonitrile ligand occupying
the apical site. The Al-O distances range between 1.693(2) and
1.708(2) Å and the associated O-B bond lengths between 1.322-
(3) and 1.350(3) Å. Two Al-O-B units have essentially the same
geometry, but the third, which has significantly longer Al-O and
shorter O-B distances, also has an Al-O-B angle that shows
the largest departure from linearity [144.2(2)°, cf. 163.9(2)° and
158.8(2)°].
Acknowledgment. BP Chemicals Ltd is thanked for financial
support.
Note Added in Proof. Subsequent to acceptance of this work
for publication, a related study by Serwatowski and co-workers
has been published: Inorg. Chem. 2000, 39, 5763.
Supporting Information Available: Synthetic procedures for 1-4
and kinetic data. Three X-ray crystallographic files in CIF format. This
material is available free of charge via the Internet at http://pubs.acs.org.
IC000895G
(11) Green, M. L.; Wagner, M. J. Chem. Soc., Dalton Trans. 1996, 2467.
(12) Sporzynski, A.; Szatylowicz, H. J. Organomet. Chem. 1994, 470, 31.