Aluminum alkoxides as synthons for methylalumoxane (MAO): Product-catalyzed thermal decomposition of [Me2Al(μ-OCPh3)]2

Article abstract of DOI:10.1021/om0106128

The thermal decomposition of [Me₂Al(μ-OCPh₃)]₂, to yield Ph₃CMe and methylalumoxane ([MeAlO]_n, MAO), is initially catalyzed by the addition of AlMe₃; however, the reaction is also catalyzed by the MAO product. The overall reaction rate takes the form: rate = k_TMA[{Me₂Al(μ-OCPh₃)}₂][AlMe ₃] + k_MAO[{Me₂Al(μ-OCPh₃)}₂][MAO], where k_MAO ? k_TMA. The ΔH? for the AlMe₃- and MAO-catalyzed reactions have been determined as 175 ± 8 and 190 ± 15 kJ·mol^-1, respectively. Both reactions show a large positive value of ΔS? (41 ± 8 eu and <53 eu, respectively), indicative of a dissociative reaction. The thermal decomposition of [Me₂Al(μ-OCPh₃)]₂ is also catalyzed by Lewis acids, including AlCl₃, AlCl₂Me, and AlClMe₂. On the basis of the relative rates of the AlMe₃-catalyzed thermal decomposition of [Me₂Al-(μ-OCPh₃)]₂, [Me₂Al(9-Ph-fluoroxy)]₂ (1), and [Me₂Al(9-Me-fluoroxy)]₂ (2) and the MAO-catalyzed C-methylation of [Me₂Al(μ-OR)]₂ [R = CMePh₂ (3), CMe₂Ph (4), CH₂Ph, C₆H₁₁, C₆H₄-4-^tBu (5)], it is proposed that the rate-determining step for C-methylation involves heterolytic cleavage of the O-C bond and the formation of a carbonium ion. The more stable the carbonium ion, the faster the reaction. Additionally, it is proposed that Lewis acid catalysis is due to the formation of an asymmetrically bridged hemi-alkoxide, whose formation is an equilibrium process such that the observed rate of the reaction will be dependent on the equilibrium for the reaction of [Me₂Al(μ-OCPh₃)]₂ with the Lewis acid.

Full text of DOI:10.1021/om0106128

5162
Organometallics 2001, 20, 5162-5170
Alu m in u m Alk oxid es a s Syn th on s for Meth yla lu m oxa n e
(MAO): P r od u ct-Ca ta lyzed Th er m a l Decom p osition of
[Me2Al(µ-OCP h 3)]2
Stephen J . Obrey,1a Simon G. Bott,1b and Andrew R. Barron*,1a
Department of Chemistry and Center for Nanoscale Science and Technology, Rice University,
Houston, Texas 77005, and Department of Chemistry, University of Houston,
Houston, Texas 77204
Received J uly 9, 2001
The thermal decomposition of [Me2Al(µ-OCPh3)]2, to yield Ph3CMe and methylalumoxane
([MeAlO]n, MAO), is initially catalyzed by the addition of AlMe3; however, the reaction is
also catalyzed by the MAO product. The overall reaction rate takes the form: rate )
kTMA[{Me2Al(µ-OCPh3)}2][AlMe3] + kMAO[{Me2Al(µ-OCPh3)}2][MAO], where kMAO . kTMA. The
∆Hq for the AlMe3- and MAO-catalyzed reactions have been determined as 175 ( 8 and 190
( 15 kJ ‚mol-1, respectively. Both reactions show a large positive value of ∆Sq (41 ( 8 eu
and <53 eu, respectively), indicative of a dissociative reaction. The thermal decomposition
of [Me2Al(µ-OCPh3)]2 is also catalyzed by Lewis acids, including AlCl3, AlCl2Me, and AlClMe2.
On the basis of the relative rates of the AlMe3-catalyzed thermal decomposition of [Me2Al-
(µ-OCPh3)]2, [Me2Al(9-Ph-fluoroxy)]2 (1), and [Me2Al(9-Me-fluoroxy)]2 (2) and the MAO-
catalyzed C-methylation of [Me2Al(µ-OR)]2 [R ) CMePh2 (3), CMe2Ph (4), CH2Ph, C6H11,
C6H4-4-tBu (5)], it is proposed that the rate-determining step for C-methylation involves
heterolytic cleavage of the O-C bond and the formation of a carbonium ion. The more stable
the carbonium ion, the faster the reaction. Additionally, it is proposed that Lewis acid
catalysis is due to the formation of an asymmetrically bridged hemi-alkoxide, whose formation
is an equilibrium process such that the observed rate of the reaction will be dependent on
the equilibrium for the reaction of [Me2Al(µ-OCPh3)]2 with the Lewis acid.
In tr od u ction
alcohols, where one of the substituents was an aryl
group; however, as the number of aryl substituents was
reduced, the reaction was found to require more heat
and longer reaction times.
In 1974, Mole and co-workers published a series of
papers describing the AlMe3 C-methylation of tertiary
alcohols, ketones, and carboxylic acids.2-4 Ordinarily,
the reaction of AlMe3 with tertiary alcohols, R3COH,
would be expected to yield the alkoxide, i.e., eq 1.5
Based on the isolation of similar products from the
thermolysis of a mixture of R3COH and AlMe3 as well
as the previous isolation of [Me2Al(µ-OCR3)]2, it was
proposed that the C-methylation reaction occurs via the
aluminum alkoxide compound.2 The thermolysis of the
alkoxide compounds was found to be slow unless per-
formed in the presence of an excess of AlMe3. It was
noted by Mole and co-workers that the role of the AlMe3
in increasing the rate of the reaction, and reducing the
occurrence of side products, was due to the possible
formation of the hemi-alkoxide, Me2Al(µ-OCR3)(µ-Me)-
AlMe2. Despite this, the mechanism of C-methylation
was proposed to involve protonation of the alkoxide
oxygen since water was observed to further catalyze the
C-methylation.2,6 Although the specific role of the ad-
dition of the Lewis acid was unclear, it was known at
that time that addition of water to AlMe3 yields the
formation of methylalumoxane ([MeAlO]n, MAO).7
1
2
AlMe3 + R3COH f [Me2Al(µ-OCR3)]2 + CH4 (1)
When AlMe3 was reacted with a tertiary alcohol in
toluene solution at room temperature, then heated in a
sealed ampule at temperatures ranging from 80 to 300
°C for up to 36 h, the C-methylation products were
isolated, eq 2.2
AlMe3
R3COH
8 R3CMe
(2)
This reaction was found to be generally applicable for
tertiary alcohols as well as secondary and primary
* To whom correspondence should be addressed (url: www.rice.edu/
barron).
(1) (a) Rice University. (b) University of Houston.
(2) Harney, D. W.; Meisters, A.; Mole, T. Aust. J . Chem. 1974, 27,
1639.
(3) Meisters, A.; Mole, T. Aust. J . Chem. 1974, 27, 1655.
(4) Meisters, A.; Mole, T. Aust. J . Chem. 1974, 27, 1665.
(5) (a) Mole, T. Aust. J . Chem. 1966, 19, 373. (b) Rogers, J . H.;
Apblett, A. W.; Cleaver, W. M.; Tyler, A. N.; Barron, A. R. J . Chem.
Soc., Dalton Trans. 1992, 3179.
(6) The effect of water in increasing the rate of MAO formation has
been confirmed, as well as the effect of solid MAO; see: Sangokoya, S.
A. US Patent, 6,013,820 2000.
(7) It should be noted that at this time it was known that addition
of water to AlMe3 yields the formation of methylalumoxane (MAO);
see for example: Manyik, R. M.; Walker, W. E.; Wilson, T. P. U.S.
Patent 3,242,099, 1966.
10.1021/om0106128 CCC: $20.00 © 2001 American Chemical Society
Publication on Web 10/26/2001

Thermal Decomposition of [Me2Al(µ-OCPh3)]2
Organometallics, Vol. 20, No. 24, 2001 5163
At the simplest mechanistic level, it is possible to
envision that the formation of the C-methylated prod-
ucts would result from a SN2-type methylation of the
quaternary carbon, i.e., a hydroxide/methyl exchange
reaction (eq 3) related to the reaction of Ph3EOH (E )
Sn, Pb) and [(tBu)2Ga(µ-OH)]3 with AlMe3.8
R3C-OH + Me2Al-Me f R3C-Me + Me2Al-OH
(3)
Since tertiary alcohols generally do not undergo hy-
droxide/methyl exchange reactions, this mechanism is
open to question.9 We and others have reported that the
reaction of Ph3COH with AlMe3 at room temperature
gave [Me2Al(µ-OCPh3)]2 in quantitative yield.2,8
Given the unusual nature of the C-methylation reac-
tion and the similarity to the synthesis of catalytically
active MAO through the hydroxide/methyl exchange
between Ph3EOH (E ) Sn, Pb) with AlMe3,8 we have
undertaken a study of the thermal decomposition of
[Me2Al(µ-OCPh3)]2 and the role of Lewis acids in cata-
lyzing its thermal decomposition.
F igu r e 1. Representative 1H NMR spectra, at 60, 290, and
300 min, for the thermolysis of [Me2Al(µ-OCPh3)]2 with 1
equiv of AlMe3 in toluene-d8 at 90 °C.
OCPh3)]2 with 1/2, 1, 2, and 3 equiv of AlMe3 at 90 °C
reaches final completion after 350, 302, 204, and 141
min, respectively. The catalytic nature of the AlMe3 is
such that the reaction proceeds, albeit slowly, with as
little as 10-5 molar equiv of AlMe3. It is thus difficult
to measure the uncatalyzed reaction time; however,
extrapolation of the total reaction time as a function of
the AlMe3/[Me2Al(µ-OCPh3)]2 mole ratio provides an
estimate that the presence of trace quantities of AlMe3
is sufficient to promote C-methylation. This result also
demonstrates the importance of using material that has
been repeatedly recystallized for the kinetic measure-
ments, described below, to ensure the removal of traces
of AlMe3. As would be expected, as the concentration of
AlMe3 gets very large, the total reaction time becomes
infinitely small. This relationship is observed experi-
mentally as a large excess of AlMe3 in the reaction
mixture allows the decomposition of [Me2Al(µ-OCPh3)]2
even at room temperature.
Resu lts a n d Discu ssion
Thermolysis of [Me2Al(µ-OCPh3)]2 in toluene-d8 for 24
h at 100 °C results in no change in the 1H NMR
spectrum or precipitation from solution. Subsequent
addition of 1 molar equiv of AlMe3 (per alkoxide dimer)
shows no reaction at room temperature for 8 h. Upon
heating the reaction mixture to 80 °C, conversion of
[Me2Al(µ-OCPh3)]2 to Ph3CMe was complete in 16 h. A
comparable result was reported by Mole and co-
workers.2
1
The aliphatic region of the H NMR spectrum of the
reaction mixture shows, in addition to the methyl peak
of Ph3CMe (δ ) 2.01 ppm) and the toluene-d7 septet (δ
) 2.09 ppm), a broad two-component feature between
0 and -0.5 ppm. The sharper resonance (δ ) -0.4 ppm)
is due to AlMe3,10 and the broad resonance is generally
indicative of methylalumoxane ([MeAlO]n, MAO).11 The
identity of the products was confirmed by NMR and
catalytic activity (see Experimental Section) to be
Ph3CMe and MAO, eq 4.12
Mole and co-workers2 noted that the C-methylation
reactions were all initially slow, but showed an in-
creased reaction rate with reaction time. They proposed
that the reaction was “autocatalytic”. To better under-
stand the formation of MAO, the product formation was
AlMe3
catalyst8
1
2
[Me2Al(µ-OCPh3)]2
1
followed by H NMR as a function of reaction time.
[MeAlO]n + Ph3CMe (4)
A solution of [Me2Al(µ-OCPh3)]2 in toluene-d8 with 1
1
equiv of AlMe3 was heated to 90 °C and the H NMR
Thus, AlMe3 appears to catalyze the thermal decompo-
sition of [Me2Al(µ-OCPh3)]2. This is confirmed by the
observation that the initial rate increases with increased
AlMe3 concentration; see below. Furthermore, the over-
all reaction time decreases with increased AlMe3 con-
centration. For example, the reaction of [Me2Al(µ-
spectrum collected every 10 min. Typical 1H NMR
spectra are shown in Figure 1. During the first 200 min
1
the H NMR remains almost unchanged, with only the
slow growth in the CH3 peak due to Ph3CMe, and the
concomitant decrease in the Al-CH3 peak due to [Me2Al(µ-
OCPh3)]2 (δ -0.81 ppm).13 The extent of the reaction
as a function of time was measured by the relative
conversion of [Me2Al(µ-OCPh3)]2 to Ph3CMe; see Figure
2. The formation of Ph3CMe does not follow simple first-
order kinetics over the entire reaction lifetime. Instead,
(8) Obrey, S. J .; Barron, A. R. J . Chem. Soc., Dalton Trans. 2001,
2456.
(9) Eisch, J . J . In Comprehensive Organometallic Chemistry; Wilkin-
son, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford, 1986;
Vol. 1, Chapter 6.
(10) See: Apblett, A. W.; Barron, A. R. Organometallics 1990, 9,
2137, and references therein.
(11) See for example: Resconi, L.; Bossi, S.; Abis, L. Macromolecules
1990, 23, 4489.
(12) It should be noted that MAO is known to be mixture of species,
ordinarily obtained by adding water to AlMe3, in which the Al:Me ratio
is variable; however, for the present study a general formula of
[MeAlO]n will be employed for convenience.
(13) During the course of the reaction small peaks are observed at
-0.52, -0.72, and -1.27 ppm. These resonances are not present in
the [Me2Al(µ-OCPh3)]2 or AlMe3 starting materials, and their relative
concentrations remain constant until the end of the reaction when they
are no longer observed. The chemical shift of these peaks is typical of
an Al-CH3 group, and they may be indicative of an intermediate
species.

5164 Organometallics, Vol. 20, No. 24, 2001
Obrey et al.
F igu r e 2. Plot of the % formation of Ph3CMe as a function
of time (min) for the thermolysis of [Me2Al(µ-OCPh3)]2 with
1 equiv of AlMe3 in toluene-d8 at 90 °C.
F igu r e 3. Plot of kobs versus [AlMe3] for the decomposition
at 90 °C of [Me2Al(µ-OCPh3)]2 catalyzed by AlMe3 (kTMA
)
3.120 × 10-5 mol-1‚dm3‚s-1, R ) 0.993).
the rate of Ph3CMe formation is initially slow with only
6% conversion after 200 min, at which point a rapid
increase in rate is observed, such that 80% of the
reaction is complete in the final 10 min. The form of
Figure 2 suggests that the reaction is catalyzed not only
by the AlMe3 (eq 4) but also by the MAO product, i.e.,
eq 5.
Ta ble 1. Selected Kin etic Da ta for th e
Decom p osition of [Me2Al(µ-OCP h 3)]2 Ca ta lyzed by
AlMe3 (kTMA) a n d MAO (kMAO
)
temperature
kTMA
kMAO
(°C)
(mol-1‚dm3‚s-1
)
(mol-1‚dm3‚s-1
)
80
90
100
8.218 × 10-6
3.120 × 10-5
2.101 × 10-4
9.991 × 10-3
1.402 × 10-2
1.860 × 10-1
[Me2Al(µ-OCPh3)]2 MAO catalyst8
1
2
indicates a dissociative reaction,14 while the value for
∆Hq suggests of significant bond breaking in reaching
the transition state.
[MeAlO]n + Ph3CMe (5)
It would be desirable to directly measure the MAO-
catalyzed rate directly by the addition of MAO to
[Me2Al(µ-OCPh3)]2; however, such a comparison is dif-
ficult for three reasons: (a) the presence of significant
quantities of AlMe3 (and other proprietary additives) in
commercial samples,15 (b) uncertainties as to the nature
of MAO (oligomerization and structure distribution),
and (c) uncertainties as to whether the MAO formed
from [Me2Al(µ-OCPh3)]2 has the same composition as
that formed from the hydrolysis of AlMe3. We have,
therefore, used the rate data from the final stage of the
reaction.
Product catalysis was confirmed by the addition of 10
mol % of commercial MAO solution to a solution of
[Me2Al(µ-OCPh3)]2; see Experimental Section. With the
addition of MAO complete conversion of [Me2Al(µ-
OCPh3)]2 to Ph3CMe and MAO occurred in 30 min at
only 80 °C. This should be compared to >200 min for
the reaction with an equivalent amount of AlMe3.
AlMe3- ver su s MAO-Ca t a lyzed Decom p osit ion .
To gain a better understanding of the overall decompo-
sition reaction, as well as the relative effects of AlMe3
and the MAO product, we have undertaken a kinetic
1
study using H NMR spectroscopy. For simplicity, the
During the final (rapid) stage of the thermal decom-
position of [Me2Al(µ-OCPh3)]2 the rate shows first-order
dependence with respect to the concentration of remain-
ing [Me2Al(µ-OCPh3)]2. A plot of kobs versus the concen-
tration of aluminum that is present as MAO (i.e.,
[AlMAO]) also shows a first-order dependence. Thus, the
MAO-catalyzed rate equation takes the form shown in
eq 7 (where kMAO is the rate constant associated with
eq 5).
process will be considered as two separate reactions, in
which AlMe3 catalysis (eq 4) is operable during the
initial reaction and MAO catalysis is dominant after
sufficient product (i.e., MAO) has been produced. While
the former reaction is relatively straightforward to
study, the latter requires several assumptions; see
below.
The thermal decomposition of [Me2Al(µ-OCPh3)]2
shows a first-order dependence during the early stages
of the reaction. A plot of kobs versus [AlMe3] also shows
a first-order dependence; see Figure 3. Thus, the initial
reaction takes the form of eq 6 (where kTMA is the rate
constant associated with eq 4). A summary of values
for kTMA is given in Table 1.
-d[{Me2Al(µ-OCPh3)}2]/dt )
k
MAO[{Me2Al(µ-OCPh3)}2][AlMAO] (7)
The values for kMAO are approximately 2 orders of
magnitude larger than those for kTMA under the same
reaction conditions (Table 1).
-d[{Me2Al(µ-OCPh3)}2]/dt )
Determination of the temperature dependence for
kTMA[{Me2Al(µ-OCPh3)}2][AlMe3] (6)
kMAO gives a value for ∆Hq of 190 ( 15 kJ ‚mol-1. Due
Determination of the temperature dependence for kTMA
(14) Atwood, J . D. Inorganic and Organometallic Reaction Mecha-
nisms; VCH: New York, 1997; p 14.
(15) Barron, A. R. Organometallics 1995, 14, 3581.
allows for the determination of ∆Hq (175 ( 8 kJ ‚mol-1
)
and ∆Sq (41 ( 8 eu). The large positive value of ∆Sq

Thermal Decomposition of [Me2Al(µ-OCPh3)]2
Organometallics, Vol. 20, No. 24, 2001 5165
Ta ble 2. Rela tive Ca ta lytic Activity of Lew is Acid s
for th e Decom p osition of [Me2Al(µ-OCP h 3)]2
time for complete reaction
Lewis acid
25 °C
50 °C
80 °C
MAO
AlCl3
AlCl2Me
AlClMe2
AlMe3
<1 h
<1 h
<24 h
no reaction
no reaction
<24 h
slow reaction
Figu r e 4. Relative stability of [9-phenylfluorene]+, [Ph3C]+,
<24 h
and [9-methylfluorene]+ carbonium ions.
to the uncertainties in the MAO speciation, only an
estimation of ∆Sq can be obtained (<53 eu).16 It is
interesting that the activation parameters for the MAO-
catalyzed reaction (eq 5) are essentially the same as
those for the AlMe3-catalyzed reaction (eq 4). The
similarity between the ∆Hq for the MAO- and AlMe3-
catalyzed reactions is unexpected given an empirical
observation of the relative rates. The mechanistic
implication of this observation is discussed below. On
the basis of the above the overall rate for the thermal
decomposition of [Me2Al(µ-OCPh3)]2 is given by eq 8.
-d[{Me2Al(µ-OCPh3)}2]/dt ) (kTMA[AlMe3] +
k
MAO[AlMAO])[{Me2Al(µ-OCPh3)}2] (8)
Rea ction of [Me2Al(µ-OCP h 3)]2 w ith Lew is Acid s.
To determine the generality of the Lewis acid-catalyzed
decomposition of [Me2Al(µ-OCPh3)]2, the effects of the
Lewis acids AlCl3, MeAlCl2, and Me2AlCl were com-
pared to that of AlMe3 and MAO.
Reaction of an equimolar solution of [Me2Al(µ-
OCPh3)]2 and the appropriate Lewis acid resulted in the
formation of MAO and Ph3CMe; see Experimental
Section. The relative rate of the reaction was found to
increase with the accepted increased Lewis acidity of
these compounds, i.e., AlCl3 > AlCl2Me > AlClMe2 >
AlMe3. The reaction with AlCl3 was found to be com-
parable to that with MAO, both reactions being com-
plete in less than 1 h at 25 °C (Table 2). It should be
noted that during reaction of AlCl3 with [Me2Al(µ-
OCPh3)]2, a bright yellow color appeared which persisted
for 15 min; the presence and significance of this color
change is discussed below.
The ability of any Lewis acid to catalyze the thermal
decomposition of [Me2Al(µ-OCPh3)]2 and the proportion-
ality of the reaction rate to the Lewis acid strength
suggest that the C-methylation reaction is not catalyzed
by protonation of the alkoxide as previously suggested,2
but by Lewis acid activation of the alkoxide.
Affect of Ca r bon iu m Ion Sta bility. During the
investigation of the reaction of [Me2Al(µ-OCPh3)]2 with
Lewis acids, we have noticed the presence of a transient
bright yellow color. In the case of the reaction with
AlCl3, this color persists for 15 min during the initial
reaction sequence even at elevated temperatures. In
contrast, the reaction with AlMe3 is colorless under the
conditions required for reaction to proceed (g80 °C).
However, if the reaction mixture is cooled (25 °C) during
the reaction, a bright yellow color is observed, but goes
away again as the reaction is reheated to 80 °C. The
source of this color may be identified by UV-visible
spectroscopy (λmax ) 430 nm) as being due to the
F igu r e 5. Molecular structure of [Me2Al(9-Ph-fluoroxy)]2
(1). Thermal ellipsoids are shown at the 20% level, and
hydrogen atoms are omitted for clarity.
formation of the [Ph3C]+ carbonium.17 In addition,
quenching the yellow reaction mixture with EtOH
results in the formation of Ph3COEt.
The transient observation of the carbonium ion,
[Ph3C]+, suggests that its formation is important in the
catalyzed decomposition of [Me2Al(µ-OCPh3)]2 to give
MAO. If this is true, then the rate of AlMe3-catalyzed
decomposition of [Me2Al(µ-OR)]2 will be dependent on
the stability of the carbonium ion [R]+. With this in
mind, the relative decomposition rates of three structur-
ally related compounds have been investigated on the
basis of carbonium ions with differing stabilities: 9-phen-
ylfluorene, Ph3C, and 9-methylfluorene (Figure 4). The
choice of the fluorene derivatives was dictated by the
desire to provide carbonium ions that were compara-
tively more and less stable than [Ph3C]+.
The syntheses of [Me2Al(9-Ph-fluoroxy)]2 (1) and
[Me2Al(9-Me-fluoroxy)]2 (2) were accomplished by the
reaction of AlMe3 with 9-phenylflourenol and 9-fluo-
renone, respectively (see Experimental Section). Com-
1
pounds 1 and 2 were characterized by H, 13C, and 27Al
NMR spectroscopy and mass spectrometry. Their dimer-
ic nature in solution and solid state was confirmed by
molecular weight measurements and X-ray crystal-
lography. The molecular structures of [Me2Al(9-Ph-
fluoroxy)]2 (1) and [Me2Al(9-Me-fluoroxy)]2 (2) are shown
in Figures 5 and 6; selected bond lengths and angles
are given in Tables 3 and 4, respectively. The steric bulk
(16) The extent of MAO oligomerization will have no effect on the
slope of -ln(k5/T) versus 1/T, but will affect the intercept, i.e., the value
for ∆Sq.
(17) Katz, T. J .; Gold, E. H. J . Am. Chem. Soc. 1964, 86, 1600.

5166 Organometallics, Vol. 20, No. 24, 2001
Obrey et al.
F igu r e 7. Plot of the reaction of [Me2Al(9-Ph-fluoroxy)]2
(b), [Me2Al(µ-OCPh3)]2 (0), and [Me2Al(9-Me-fluoroxy)]2 (9)
with 1 equiv of AlMe3 at 60, 90, and 130 °C, respectively.
than those in compound 2 [1.454(3) Å] due to the greater
steric bulk of the 9-Ph-fluoroxy ligand in the former.
Each of [Me2Al(9-Ph-fluoroxy)]2 (1), [Me2Al(9-Me-
fluoroxy)]2 (2), and [Me2Al(OCPh3)]2 were reacted with
1 equiv of AlMe3. Initial investigations into the relative
reactivities showed that the [Me2Al(9-Ph-fluoroxy)]2
reacted readily on gentle warming of the reaction
mixture, while [Me2Al(9-Me-fluoroxy)]2 required heating
at 110 °C for 2 days for complete conversion. In each
case the appropriate alkylation product was isolated and
the resulting reaction mixture was found to contain
catalytically active MAO; see Experimental Section.
To obtain an indication of the relative reactivities of
these three compounds, the reactions were followed by
1H NMR. The reactions of both [Me2Al(9-Ph-fluoroxy)]2
and [Me2Al(OCPh3)]2 were carried out in toluene-d8
solution at 60 and 80 °C, respectively. In contrast, to
obtain a sufficiently fast reaction to be followed by NMR
spectroscopy, the reaction of [Me2Al(9-Me-fluoroxy)]2
was performed in o-xylene-d10 at 130 °C. All samples
were equimolar with 1 equiv of AlMe3. From the shape
of the plots of % conversion versus time (Figure 7) it is
clear that despite the differences in the temperature
required for reaction, each compound follows a similar
pathway; initial slow AlMe3-catalyzed reaction, followed
by rapid product (MAO)-catalyzed decomposition.
The time for the complete reaction follows the order
[Me2Al(9-Ph-fluoroxy)]2 (175 min @ 60 °C) > [Me2Al(µ-
OCPh3)]2 (350 min @ 90 °C) > Me2Al(9-Me-fluoroxy)]2
(480 min @ 130 °C). The first-order rate constants for
these reactions were determined: Me2Al(9-Me-fluor-
oxy)]2, 4.06 × 10-5 mol-1‚dm3‚s-1 (@ 130 °C); [Me2Al-
(µ-OCPh3)]2, 5.45 × 10-5 mol-1‚dm3‚s-1 (@ 80 °C);
[Me2Al(9-Ph-fluoroxy)]2, 8.69 × 10-5 mol-1‚dm3‚s-1
(@ 60 °C). The relative rate of C-methylation for
[Me2Al(µ-OR)]2 is clearly dependent on the relative
stability of the carbonium ion, [R]+.
F igu r e 6. Molecular structure of [Me2Al(9-Me-fluoroxy)]2
(2). Thermal ellipsoids are shown at the 30% level, and
hydrogen atoms are omitted for clarity.
Ta ble 3. Selected Bon d Len gth s (Å) a n d An gles
(d eg) in [Me2Al(9-P h -flu or oxy)]2 (1)
Al(1)-O(1)
Al(1)-C(1)
Al(2)-O(1)
Al(2)-C(3)
O(1)-C(11)
1.864(2)
1.937(4)
1.858(2)
1.943(3)
1.461(3)
Al(1)-O(2)
Al(1)-C(2)
Al(2)-O(2)
Al(2)-C(4)
O(2)-C(21)
1.851(2)
1.942(4)
1.861(2)
1.947(3)
1.470(3)
O(1)-Al(1)-O(2)
O(1)-Al(1)-C(2)
O(2)-Al(1)-C(2)
O(1)-Al(2)-O(2)
O(1)-Al(2)-C(4)
O(2)-Al(2)-C(4)
Al(1)-O(1)-Al(2)
Al(2)-O(1)-C(11)
Al(1)-O(2)-C(21)
79.39(9)
O(1)-Al(1)-C(1)
O(2)-Al(1)-C(1)
C(1)-Al(1)-C(2)
O(1)-Al(2)-C(3)
O(2)-Al(2)-C(3)
C(3)-Al(2)-C(4)
Al(1)-O(1)-C(11)
Al(1)-O(2)-Al(2)
Al(2)-O(2)-C(21)
112.3(1)
110.5(1)
122.8(2)
110.6(1)
113.2(1)
122.1(2)
130.2(2)
99.3(1)
110.7(1)
112.8(2)
79.29(9)
113.0(1)
110.7(1)
99.0(1)
128.8(2)
129.2(2)
129.8(2)
Ta ble 4. Selected Bon d Len gth s (Å) a n d An gles
(d eg) in [Me2Al(9-Me-flu or oxy)]2 (2)
Al(1)-O(1)
Al(1)-C(22)
1.853(2)
1.944(3)
Al(1)-C(21)
O(1)-C(1)
1.945(3)
1.454(3)
O(1)-Al(1)-O(1′)
80.17(7) O(1)-Al(1)-C(21)
113.9(1)
O(1)-Al(1)-C(22) 112.5(1)
Al(1)-O(1)-Al(1′)
C(21)-Al(1)-C(22) 117.8(2)
99.83(7) Al(1)-O(1)-C(1)
131.6(1)
of the 9-alkylfluoroxy ligands appears to control the
orientation of the alkoxide ligand and consequently the
geometry of the Al2O2 unit. The 9-Me-fluoroxy ligands
in compound 2 are in an anti conformation with a
centrosymmetric structure to the dimer and as expected
a planar Al2O2 core.18 In contrast, compound 1 crystal-
lizes as a dimer with the 9-Ph-fluoroxy in an eclipsed
conformation, resulting in a noncrystallographic mirror
plane coplanar with the AlMe2 moieties. The asymmetry
with respect to the Al2O2 core causes a slight buckling
of the core [the fold angle along the Al(1)‚‚‚Al(2) vector
is 160°]. Despite this slight distortion all the bond
lengths and angles are within previously reported
ranges.19 It is worth noting that the O-C distances in
compound 1 [1.461(3) and 1.470(3) Å] are slightly longer
Rea ction of [Me2Al(µ-OR)]2 w ith MAO. As was
noted earlier, Mole and co-workers investigated the
reaction of a range of tertiary alcohols with a slight
(19) See for example: (a) Francis, J . A.; McMahon, C. N.; Bott, S.
G.; Barron, A. R. Organometallics 1999, 18, 4399. (b) Francis, J . A.;
Bott, S. G.; Barron, A. R. J . Organomet. Chem. 2000, 597, 29. (c)
Cetinkaya, B.; Hitchcock, P. B.; J asim, H. A.; Lappert, M. F. Polyhedron
1990, 9, 239. (d) Kumar, R.; Sierra, M.; de Mel, V. S. J .; Oliver, J . P.
Organometallics 1990, 9, 484.
(18) In the absence of steric effects, dimeric aluminum alkoxide
forms centrosymmetric dimers in which the Al2O2 core is planar; see:
Oliver, J . P.; Kumar, R. Polyhedron 1990, 9, 409.

Thermal Decomposition of [Me2Al(µ-OCPh3)]2
Organometallics, Vol. 20, No. 24, 2001 5167
Ta ble 5. Alk yla tion P r od u cts a n d Rela tive Ra tes
for th e Com p lete Rea ction of [Me2Al(µ-OR)]2 w ith
MAO a t 80 °C a n d th e Com p a r itive Ra te of th e
Rea ction Ca ta lyzed by AlMe3
comparison
reaction
time
to AlMe3
catalyzed
R
product
Ph3C-Me
Ph3C-
Ph2MeC-
PhMe2C-
PhCH2-
9-Me-fluorone-
9-Ph-fluorone-
<30 min 6 h @90 °C
Ph2MeCMe
PhMe2C-Me
PhCH2-Me
9,9-Me2-fluorone
9-Me-9-Ph-fluorone <30 min 4 h @60 °C
<12 h
<12 h
<12 h
<12 h
20 h @85 °C
142 h @90 °C
no reaction
8 h @ 130 °C
of the bridging group (i.e., µ-Cl versus µ-Me). Similar
ligand exchange reactions are well known in the litera-
ture.9
C6H11
tert-butyl-
4-tert-butylphenol- no reaction
-
C6H11-Me
Me3C-Me
<3 day
<12 h
n/a
no reaction
42 h @ 120 °C
n/a
Keq
[Me2Al(µ-OCPh3)]2 + Al2Me6 y z
molar excess of AlMe3.2 Upon the basis of the foregoing
it is reasonable to propose that these reactions occurred
by the in-situ formation of the alkoxide compounds
whose decomposition was catalyzed initially by the
slight excess of AlMe3, and subsequently by the forma-
tion of MAO as the aluminum-containing product. Even
under the conditions employed, these reactions were
slow and required elevated temperatures. Given the
greater catalytic activity of MAO, it should be possible
to decrease the reaction times of these reactions and
catalyze the decomposition of previously stable alkox-
ides.
2[Me2Al(µ-OCPh3)(µ-Me)AlMe2] (9)
We have previously demonstrated that the catalyti-
cally active structures of alkylalumoxanes are caged
compounds,20 and we have proposed their activity
derives from their latent Lewis acidity of the opening
of the cage structure.21 We have recently reported that
AlMe3 reacts with the tert-butylalumoxane [(tBu)Al(µ3-
O)]6 to give cage-opened structures (e.g., II).22 It is
reasonable to propose that a similar structure would be
formed with dimethylaluminum alkoxides, i.e., III.23
A series of dimethylaluminum alkoxides were syn-
thesized by the reaction of AlMe3 with the corresponding
alcohol; see Experimental Section. Addition of a 10%
molar equiv of commercial MAO was used as the
catalyst, and the time for formation of the alkane
1
product was monitored by H NMR spectroscopy. The
results of these studies are presented in Table 5. As
expected, these reactions are all significantly faster than
the equivalent AlMe3-catalyzed reactions. However,
more important is that the secondary alkoxide, [Me2Al-
(µ-OC6H11)]2, undergoes C-methylation. In Mole’s origi-
nal investigations, secondary alcohols did not undergo
C-methylation.2 Thus, MAO may be used as a catalyst
for the C-methylation of secondary alcohols. We note
that the 4-tert-butyl phenoxide does not show any
reaction, even after 7 days. On the basis of our proposed
reaction dependence on the stability of the carbonium
ion, however, the unreactivity of phenoxide derivatives
is not surprising.
Mech a n ism of C-Meth yla tion . Although the C-
methylation reaction was extensively studied by Mole
and co-workers,2 there were a number of questions that
remained unanswered. What is the role of the Lewis
acid in the facilitation of the reaction, and why are rates
of the AlMe3- and MAO-catalyzed reactions vastly
different despite the similarity in the ∆Hq for the
reactions?
Although the formation of a complex between the
Lewis acid and [Me2Al(µ-OCPh3)]2 is reasonable in view
of the measured rate equations, it does not explain the
activation parameters. The large value for ∆Sq is
suggestive of a dissociative reaction, while the value for
Mole and co-workers2 postulated that the increased
reaction rate in the presence of excess AlMe3 was due
to the formation of an asymmetrically bridged dimeric
alkoxide, i.e., [Me2Al(µ-OCPh3)(µ-Me)AlMe2] (I, X ) X′
) X′′ ) Me). Our observation of the reaction rate being
first order with respect to both [Me2Al(µ-OCPh3)]2 and
AlMe3 agrees with this pathway. This proposal would
also explain the enhancement of the rate with alumi-
num chlorides. Since the formation of the mixed dimer,
I, would be an equilibrium process (eq 9). The position
of the equilibrium would be dependent on the stability
(20) (a) Mason, M. R.; Smith, J . M.; Bott, S. G.; Barron, A. R. J .
Am. Chem. Soc. 1993, 115, 4971. (b) Harlan, C. J .; Mason, M. R.;
Barron, A. R. Organometallics 1994, 13, 2957.
(21) Harlan, C. J .; Bott, S. G.; Barron, A. R. J . Am. Chem. Soc. 1995,
117, 6465.
(22) Watanabe, M.; McMahon, C. N.; Harlan, C. J .; Barron, A. R.
Organometallics 2001, 20, 460.
(23) It should be noted that while a bridging alkoxide ligand in III
would be preferential on electronic grounds, steric factors may favor a
terminal alkoxide.

5168 Organometallics, Vol. 20, No. 24, 2001
Obrey et al.
∆Hq is larger than we have determined for the base
cleavage of the [Me2Al(µ-OCPh3)]2 [73(3) kJ ‚mol-1].24
Therefore, the rate-determining step in the reaction
shown in eq 4 is not associated with cleavage of the
dimer; that is, eq 9 is not the rate-determining step but
a preequilibrium reaction.
generality of the effect on a coordinated ligand by a
group 13 Lewis acid as being the increase of positive
charge on the â-substituent, for example, the Lewis
acidic group 13 halide activation of (a) alkyl halides for
Friedel-Craft alkylation of aromatic hydrocarbons (IV),26
(b) organic carbonyls toward alkylation and reduction
(V),27,28 and (c) weak protic acids (water and alcohols)
toward alkane elimination (VI).29 Recently, we have
reported examples in which a group 13 Lewis acid will
activate a second metal through a similar effect (VII).30
The similarities of the ∆Hq for the AlMe3- and MAO-
catalyzed reactions suggest a commonalty of rate-
determining steps. Given that the formation of I and
III would be expected to energetically different, it is
unlikely that the formation of these species is the rate-
determining step. We have demonstrated that the
stability of the carbonium ion is of primary importance
in determining the rate of C-methylation reaction. In
addition, we have observed the formation of [CPh3]+
spectroscopically. This suggests that the heterolytic
cleavage of the O-C bond is important and the meas-
ured values for ∆Hq (175-190 kJ ‚mol-1) would be
appropriate for the cleavage of an O-C bond.25 We
propose, therefore, that the formation of the carbonium
ion from the mixed dimer, I, is the rate-determining step
(eq 10). The formation of MeCPh3 will undoubtedly occur
via abstraction of Me- from the anionic alumoxane (cf.,
eq 10).
kO-C
[Me2Al(µ-OCPh3)(µ-Me)AlMe2]
8
[Me2Al(µ-O)(µ-Me)AlMe2]- + [CPh3]+ (10)
The formation of [Me2Al(µ-OCPh3)(µ-Me)AlMe2] may
be considered as a Lewis acid-base complex between
AlMe3 and the oxygen atom of the aluminum alkoxide.
In this regard, it is expected that an increased positive
charge would be placed on the alkoxide’s quaternary
carbon (VIII), which in turn will promote the heterolytic
cleavage of the O-C bond. A similar effect would be
expected for MAO and the alkylaluminum chlorides.
If the cleavage of an O-C bond is the rate-determin-
ing step, then the rate will follow that of a classical
preequilibrium system in which the mixed intermediate,
e.g., [Me2Al(µ-OCPh3)(µ-Me)AlMe2], is in equilibrium
with the reactants. The rate law will have the form of
second-order reaction (eq 11) with a composite rate
constant dependent on the equilibrium constant for the
formation of the adduct (Keq, cf. eq 9) with the Lewis
acid and the rate of O-C bond cleavage (kO-C), eq 12.
Con clu sion s
-d[{Me2Al(µ-OCPh3)}2]/dt )
We propose the following general pathway for the
Lewis acid-catalyzed C-methylation of aluminum alkox-
ides resulting in the formation of methylalumoxane
(MAO). The dimethylaluminum alkoxide reacts with a
Lewis acid to form a mixed intermediate (I). This
equilibrium reaction (cf. eq 9) is dependent on the
identity and concentration of the Lewis acid. The Lewis
acid activation of the alkoxide’s quaternary carbon
results in the heterolytic cleavage of the O-C bond and
the formation of an ion pair.31 Although we have no
k[{Me2Al(µ-OCPh3)}2][Lewis acid] (11)
k ) kO-CKeq
(12)
Thus, the activation energy will be solely dependent on
the cleavage of the O-C bond, which in turn is a
function of the stability of the resulting carbonium ion.
However, the observed rate of the reaction will be de-
pendent on the equilibrium for the reaction of [Me2Al(µ-
OCPh3)]2 with the Lewis acid. On the basis of the
stability of the bridging ligands, the Keq, and hence the
reaction rate, would be expected to follow the order AlCl3
> AlCl2Me > AlClMe2 > AlMe3. This is indeed observed.
Furthermore, the relative rate of the MAO-catalyzed
reaction suggests that the formation of a complex with
[Me2Al(µ-OCPh3)]2 is favored in comparison with the
more traditional Lewis acids.
(26) (a) Whitmore, F. C. J . Am. Chem. Soc. 1932, 54, 3274. (b) Olah,
G. A.; Kuhn, S. J .; Flood, S. H. J . Am. Chem. Soc. 1962, 84, 1688. (c)
Olah, G. A.; Kobayashi, S.; Tashiro, M. J . Am. Chem. Soc. 1972, 94,
7448.
(27) See: Power, M. B.; Nash, J . R.; Healy, M. D.; Barron, A. R.
Organometallics 1992, 11, 1830, and references therein.
(28) See for example: (a) Power, M. B.; Bott, S. G.; Atwood, J . L.;
Barron, A. R. J . Am. Chem. Soc. 1990, 112, 3446. (b) Power, M. B.;
Apblett, A. W.; Bott, S. G.; Atwood, J . L.; Barron, A. R. Organometallics
1990, 9, 2529. (c) Power, M. B.; Bott, S. G.; Clark, D. L.; Atwood, J . L.;
Barron, A. R. Organometallics 1990, 9, 3086.
(29) McMahon, C. N.; Bott, S. G.; Barron, A. R. J . Chem. Soc., Dalton
Trans. 1997, 3129.
(30) (a) Borovik, A. S.; Bott, S. G.; Barron, A. R. Angew. Chem., Int.
Ed. 2000, 39, 4117. (b) Borovik, A. S.; Bott, S. G.; Barron, A. R. J . Am.
Chem. Soc. 2001, in press.
Given the formation of a Lewis acid complex with
[Me2Al(µ-OCPh3)]2, why should this activate the O-C
bond toward cleavage? We have previously discussed the
(24) Obrey, S. J .; Bott, S. G.; Barron, A. R. Organometallics 2001,
in press.
(31) We note that the formation of an anionic alumoxane of the
formula [Me2Al(µ-O)AlMe3]- has been previously reported and struc-
turally characterized; see: Atwood, J . L.; Zaworotko, M. J . J . Chem.
Soc., Chem. Commun. 1983, 302.
(25) Typical values for an alcohol O-C bond strength are 380 kJ ‚
mol-1
: Lowery, T. H.; Richardson, K. S. Mechanism and Theory in
Organic Chemistry; Harper Collins: New York, 1987; pp 161-162.

Thermal Decomposition of [Me2Al(µ-OCPh3)]2
Organometallics, Vol. 20, No. 24, 2001 5169
yielding a white solid, confirmed to be polyethylene by 13C
NMR spectroscopy35 and TG/DTA.36
direct evidence, it is expected that this is the rate-
determining step. The ion pair will react further to
alkylate the carbonium ion and form a neutral alumox-
ane.32 The relative catalytic activity of a Lewis acid is
dependent on its ability to form a mixed intermediate
(I).
[Me2Al(9-P h -flu or oxy)]2 (1). To a solution of AlMe3 (0.500
g, 6.95 mmol) in toluene (50 mL) was added dropwise a
solution of 9-phenylfluorenol (1.790 g, 6.94 mmol) in toluene
(50 mL) over a period of an hour. The reaction mixture was
stirred for 6 h, whereupon the solvent was removed under
vacuum. The resulting white powder was redissolved in CH2Cl2
(50 mL) and filtered. The solution was concentrated and cooled
to -33 °C. The resulting clear colorless crystals were collected
The most common method for formation of MAO is
the reaction of AlMe3 with water. Unfortunately, this
is not an easily controlled reaction.33 Gaining insight
into the mechanism of formation for MAO is an impor-
tant step in the development of synthetic strategies to
yield MAO with high and (perhaps what is more
important) consistent catalytic activity. The synthesis
of MAO from readily prepared dimethylaluminum alkox-
ides with much lower sensitivity to air and moisture
offers an interesting entry into the controlled synthesis
of MAO. Furthermore, while we have demonstrated that
the MAO prepared herein has a catalytic activity
comparable to that of commercial samples, our future
studies will concentrate on the reproducible nature of
the reaction as well as the relative reactivities of various
MAO samples. Furthermore, we note that the MAO
prepared by the methods described herein is soluble in
aliphatic hydrocarbon solvents. This is in contrast to
MAO prepared from the hydrolysis of AlMe3 that is
soluble in aromatic solvents. Finally, we note that MAO
appears to be suitable as an active agent for the
C-methylation of alcohols that do not undergo reaction
with AlMe3.
1
by filtration. Yield: 89%. Mp: 170 °C (dec). H NMR: δ 7.55
[4H, d, J (H-H) ) 14.3 Hz, o-CH], 7.32 [4H, d, J (H-H) ) 14.3
Hz, m-CH], 6.9-7.12 (14H, m, CH), -0.85 (12H, s, AlCH3).
13C NMR: δ 148.7 (IX, A), 142.8 (OCC, Ph), 141.0 (IX, F), 130.4
(IX, B), 128.8 (o-Ph), 128.0 (p-Ph), 127.3 (m-Ph), 126.9 (IX,
C), 123.2 (IX, D), 121.1 (IX, E), 88.1 (OC), -5.76 (Al-CH3). 27Al
NMR: δ 140 (W1/2 ) 7650 Hz).
[Me2Al(9-Me-flu or oxy)]2 (2). To a solution of AlMe3 (1.00
g, 13.89 mmol) in toluene (50 mL) was added dropwise a
solution of 9-fluorenone (2.25 g, 12.5 mmol) in toluene (50 mL)
over a period of an hour. The reaction mixture was stirred for
6 h, whereupon the solvent was removed under vacuum. The
resulting white powder was redissolved in CH2Cl2 (50 mL) and
filtered. The solution was concentrated and cooled to -33 °C.
The resulting colorless crystals were collected by filtration.
Exp er im en ta l Section
The syntheses of [Me2Al(µ-OR)]2 were performed according
to the literature methods.2,5b,7,23 Ethylene (Matheson polymer
grade) was used as received. MAO (30 wt % in toluene) and
AlMe3 were generously provided by Albemarle Corporation.
Unless otherwise noted all procedures were performed under
purified nitrogen or argon. All solvents were distilled and
degassed before use. Microanalyses were performed by Oneida
Research Services, Inc., Whitesboro, NY. Mass spectra were
obtained on a Finnigan MAT 95 mass spectrometer operating
with an electron beam energy of 70 eV for EI mass spectra.
IR spectra (4000-400 cm-1) were obtained using a Nicolet 760
FT-IR infrared spectrometer. IR samples were prepared as
Nujol mulls between KBr plates. NMR spectra were obtained
on Bruker AM-250 and Avance 200 and 400 spectrometers
using (unless otherwise stated) benzene-d6 solutions. Chemical
shifts are reported relative to internal solvent resonances (1H
and 13C) and external [Al(H2O)6]3+ (27Al).
1
Yield: 94%. Mp: 240-248 °C. H NMR (CDCl2): δ 7.59 [4H,
s, J (H-H) ) 7.0 Hz, CH], 7.46 [4H, d, J (H-H) ) 7.0 Hz, CH],
7.36 [4 H, td, J (H-H) ) 7.4 Hz, J (H-H) ) 1.2 Hz, CH], 7.29
[4H, td, J (H-H) ) 7.4 Hz, J (H-H) ) 1.2 Hz, CH], 1.85 (6H,
s, CH3), -1.28 (12H, s, Al-CH3). 13C NMR: δ 147.5 (IX, A),
138.9 (IX, F), 129.6 (IX, B), 128.2 (IX, C), 124.7 (IX, D), 120.2
(IX, E), 82.4 (OC), 26.9 (CH3), -7.61 (Al-CH3). 27Al NMR: 147
(W1/2 ) 7350 Hz).
Th er m a l Decom p osit ion of [Me2Al(9-P h -flu or oxy)]2.
[Me2Al(9-Ph-fluoroxy)]2 (1) (500 mg, 0.79 mmol) was dissolved
in toluene (100 mL) containing AlMe3 (57 mg, 0.79 mmol). The
reaction was heated for 24 h at 60 °C. The reaction mixture
was divided into two parts. The first part was hydrolyzed (5
mL) followed by extraction with Et2O (3 × 50 mL). The organic
layer was washed with NaHCO3 and brine and dried over
MgSO3. Removal of the volatiles under vacuum gave a white
solid, which was determined to be 9-methyl-9-phenylfluorene
by 1H and 13C NMR spectroscopy.37 To the second fraction was
added Cp2ZrCl2 (231 µg, 0.79 µmol). Ethylene was bubbled
through the reaction mixture, yielding a white solid confirmed
to be polyethylene.33,34
Th er m a l Decom p osit ion of [Me2Al(9-Me-flu or oxy)]2.
[Me2Al(9-Me-fluoroxy)]2 (2) (400 mg, 0.79 mmol) was dissolved
in o-xylene (100 mL) containing AlMe3 (57 mg, 0.79 mmol).
The reaction was heated for 24 h at 130 °C. The reaction
mixture was divided into two parts. The first part was
hydrolyzed (5 mL) followed by extraction with Et2O (3 × 50
mL). The organic layer was washed with NaHCO3 and brine
and dried over MgSO3. Removal of the volatiles under vacuum
Th er m a l Decom p osition of [Me2Al(µ-OCP h 3)]2. [Me2Al-
(µ-OCPh3)]2 (500 mg, 0.79 mmol) was dissolved in toluene (100
mL) containing AlMe3 (57 mg, 0.79 mmol). The reaction was
heated for 16 h at 80 °C. The reaction mixture was divided
into two parts. The first part was hydrolyzed (5 mL) followed
by extraction with Et2O (3 × 50 mL). The organic layer was
washed with NaHCO3 and brine and dried over MgSO3.
Removal of the volatiles under vacuum gave a white solid that
1
was determined to be Ph3CMe by H and 13C NMR spectros-
copy.34 To the second fraction was added Cp2ZrCl2 (231 µg, 0.79
µmol). Ethylene was bubbled through the reaction mixture,
(32) The disproportionation of [Me2Al]2O to MAO and AlMe3 has
been previously discussed, see: Pasynkiewicz, S. Polyhedron 1990, 9,
429.
(33) For a recent review see: Barron, A. R. In Metallocene-Based
Polyolefins; Scheirs, J ., Kaminsky, W., Eds.; Wiley: Chichester, 2000;
Chapter 2.
(35) Vander Hart, D. L.; Pe´rez, E. Macromolecules 1986, 19, 1902.
(36) Breslow, D. S.; Newburg, N. R. J . Am. Chem. Soc. 1959, 81,
81.
(37) Pouchert, C. J .; Cambell, J . R. The Aldrich Library of NMR
Spectra; Aldrich Chemical Co.: Milwaukee, WI, 1974; Vol. 2, p 47c.
(34) 1H NMR (C6D6): δ 7.00-7.14 (15H, m, C6H5), 2.03 (3H, s,
C-CH3). 13C NMR (C6D6): δ 144.7 (MeCC), 131.1 (o-CH), 128.8 (m-
CH), 128.47 (p-CH), 89.90 (CPh3), 26.9 (CH3).

5170 Organometallics, Vol. 20, No. 24, 2001
Obrey et al.
Ta ble 6. Su m m a r y of X-r a y Diffr a ction Da ta
gave a white solid, which was determined to be 9,9-dimethyl-
1
fluorene38 from its melting point and H NMR spectroscopy.39
[Me2Al(9-Ph-
fluoroxy)]2 (1)
[Me2Al(9-Me-
fluoroxy)]2 (2)
To the second fraction was added Cp2ZrCl2 (231 µg, 0.79 µmol).
Ethylene was bubbled through the reaction mixture, yielding
a white solid confirmed to be polyethylene.33,34
emp form
Mw
cryst syst
space group
a, Å
C
42H38Al2O2
C32H34Al2O2
504.55
orthorhombic
Pbca
7.962(2)
14.984(3)
24.378(5)
628.29
monoclinic
P21/c
15.407(3)
10.704(2)
22.137(4)
105.14(3)
3524(1)
4
[Me2Al(µ-OCMeP h 2)]2 (3). To a solution of AlMe3 (1.00 g,
13.89 mmol) in toluene (50 mL) was added dropwise a solution
of Ph2MeCOH (2.480 g, 12.51 mmol) in toluene (50 mL) over
a period of an hour. The reaction mixture was stirred for 16
h, whereupon the solvent was removed under vacuum. Yield:
63%. Mp: 133-135 °C. 1H NMR: δ 7.15-7.25 (20H, m, C6H5),
2.16 (6H, s, CH3), -0.60 (12H, s, Al-CH3). 13C NMR: δ 145.9
(OCC, Ph), 128.8 (o-CH), 128.5 (p-CH), 127.2 (m-CH), 31.7
(CH3), -5.26 (Al-CH3). 27Al NMR: δ 145 (W1/2 ) 8230 Hz).
[Me2Al(µ-OCMe2P h )]2 (4). To a solution of AlMe3 (1.00 g,
13.89 mmol) in toluene (50 mL) was added dropwise a solution
of Ph2MeCOH (1.680 g, 12.3 mmol) in toluene (50 mL) over a
period of an hour. The reaction mixture was stirred for 6 h,
whereupon the solvent was removed under vacuum. Yield:
b, Å
c, Å
â, deg
V, Å3
2907(1)
4
1.153
0.126
10 317
Z
D
µ
calc, g‚m-3
1.185
0.117
12 297
calc, mm-1
no. of reflns
collected
no. of ind reflns
no. of reflns obsd
weighting scheme
SHELXTL params
R
5082
3040
0.0927, 0
2097
1635
0.10, 0
1
68%. Mp: 100-104 °C. H NMR: δ 7.33-7.38 (4H, m, C6H5),
0.0503
0.1336
0.28
0.0535
0.1424
0.27
7.09-7.15 (4H, m, C6H5), 7.01-7.07 (2H, m, C6H5), 1.53 (12H,
s, CH3), -0.56 (12H, s, Al-CH3). 13C NMR: δ 145.4 (OCC),
128.9 (o-CH), 128.5 (p-CH), 126.6 (m-CH), 31.3 (CH3), -5.57
(Al-CH3). 27Al NMR: δ 137 (W1/2 ) 8230 Hz).
Rw
largest diff peak, e Å-3
mine the rate of the reactions at three different temperatures
until the reaction had reached completion. All calculations
were performed using standard methods.14
[Me2Al(µ-OC6H4-4-tBu )]2 (5). To a solution of AlMe3 (1.00
g, 13.89 mmol) in toluene (50 mL) was added dropwise a
solution of 4-tBuPhOH (1.878 g, 12.5 mmol) in toluene (50 mL)
over a period of an hour. The reaction mixture was stirred for
6 h, whereupon the solvent was removed under vacuum.
Yield: 88%. 1H NMR: δ 7.08 [4H, d, J (H-H) ) 7.0 Hz, o-CH],
7.01 [4H, d, J (H-H) ) 7.0 Hz, m-CH], 1.15 [18H, s, C(CH3)3],
-0.20 (12H, s, Al-CH3). 13C NMR: δ 149.9 (OC), 128.7 (o-CH),
127.8 (p-CH), 118.7 (m-CH), 34.6 (CH3), 31.8 [C(CH3)3], -10.3
(Al-CH3).
X-r a y Cr ysta llogr a p h y. Crystals of all compounds were
sealed in glass capillaries under argon. Data for compounds 1
and 2 were collected on a Bruker CCD SMART system,
equipped with graphite-monochromated Mo KR radiation (λ
) 0.71073 Å) and corrected for Lorentz and polarization effects.
The structures were solved using the direct methods program
XS41 and difference Fourier maps and refined by using full
matrix least-squares method.42 All non-hydrogen atoms were
refined with anisotropic thermal parameters. Hydrogen atoms
involved in hydrogen bonding were found, but not refined.
Remaining hydrogen atoms were placed in calculated positions
[Uiso ) 0.08; d(C-H) ) 0.96 Å] for refinement. Refinement of
positional and anisotropic thermal parameters led to conver-
gence (see Table 6).
P olym er iza tion Test. To a sample of MAO solution formed
from the Lewis acid-catalyzed decomposition of [Me2Al(µ-
OCPh3)]2 was added Cp2ZrCl2 (ca. 0.1 mol %) and the resulting
mixture stirred for 30 min. Ethylene was then bubbled through
the reaction mixture for 10 min, at room temperature. Quench-
ing with methanol and filtration resulted in the isolation of
polyethylene as characterized by 13C CPMAS NMR and TGA
analysis.34,35
Ack n ow led gm en t. Financial support for this work
is provided by the Welch Foundation. The Bruker
Avance 200 and 500 NMR spectrometer was purchased
with funds from ONR Grant N00014-96-1-1146 and
NSF Grant CHE-9708978, respectively.
Kin etic Mea su r em en ts. Series samples were prepared in
5 mm NMR tubes from standard solutions (0.75 mL) of
[Me2Al(µ-OCPh3)]2 (0.0596 M) and AlMe3 [0.0299 M (1/2 equiv),
0.0596M (1 equiv), 0.119 M (2 equiv), 0.178 M (3 equiv)] in
toluene-d8. All samples were heated to the appropriate tem-
perature within the NMR spectrometer, and a series of 1H
NMR spectra were collected at equal time increments. The
temperature of the NMR spectrometer probe was calibrated
using the chemical shifts of ethylene glycol.40 The relative
integration of the triphenylethane protons was used to deter-
Su p p or tin g In for m a tion Ava ila ble: Full listings of bond
length and angles, anisotropic thermal parameters, and
hydrogen atom parameters; elemental analysis, MS, IR spec-
tra, and selected 1H NMR and Eyring plots. This material is
available free of charge via the Internet at http://pubs.acs.org.
(38) Harvey, R. G.; Fu, P. P.; Rabideau, P. W. J . Org. Chem. 1976,
41, 2706.
OM0106128
(39) Mp: 95-96 °C. 1H NMR (CDCl3): δ 7.67-7.05 (8H, m, fluorene),
1.45 (6H, s, CH3).
(41) Sheldrick, G. M. Acta Crystallogr. Sect. A 1990, A46, 467.
(42) Sheldrick, G. M. SHELXTL; Bruker AXS, Inc.: Madison, WI,
1997.
(40) (a) van Geet, A. L. Anal. Chem. 1968, 40, 2227. (b) Gordon, H.
J .; Ford, R. A. The Chemists Companion; Wiley: New York, 1972.