A lewis base promoted alkyl/alkoxide ligand redistribution: Reaction of [Me2Al(μ-OCPh3)]2 with THF

Article abstract of DOI:10.1021/om010394i

The reaction of [Me₂Al(μ-OEPh₃)]₂ with pyridine yields the expected acid-base complexes AlMe₂(OEPh₃)(py) [E = C (1) and Si (2)]. In contrast, the reaction with THF yields AlMe-(OEPh₃)₂(THF) [E = C (5) and Si (6)], although the dimethyl compounds, AlMe₂(OEPh₃)-(THF) [E = C (3) and Si (4)], are observed in THF-d₈ solution. The reaction of [Me₂Al(μOCPh₃)]₂ with THF was followed by ¹H NMR and found to occur by a two-step process. First, the Al₂O₂ core of [Me₂Al(μ-OEPh₃)]₂ is cleaved by THF to form compound 3. Second, two molecules of AlMe₂(OCPh₃)(THF) react with each other, with prior dissociation of THF from at least one complex, resulting in the ligand redistribution and the formation of 5 and AlMe₃-(THF). The conversion of [Me₂Al(μ-OCPh₃)]₂ into compound 3 is exothermic, and the subsequent formation of 5 and AlMe₃(THF) is endothermic. The rate equations for the formation of 3 and its conversion to 5 have been determined. The observation of both alkoxide cleavage and alkyl/alkoxide exchange requires a fine balance between a Lewis base that is of sufficient strength to cleave the dimeric alkoxide, [R₂Al(μ-OR′)]₂, while being sufficiently weak to allow dissociation from the monomeric complex, AlR₂(OR′)(L).

Full text of DOI:10.1021/om010394i

Organometallics 2001, 20, 5119-5124
5119
A Lew is Ba se P r om oted Alk yl/Alk oxid e Liga n d
Red istr ibu tion : Rea ction of [Me2Al(µ-OCP h 3)]2 w ith THF
Stephen J . Obrey,1a Simon G. Bott,1b and Andrew R. Barron*,1a
Department of Chemistry, Rice University, Houston, Texas 77005, and Department of
Chemistry, University of Houston, Houston, Texas 77204
Received May 14, 2001
The reaction of [Me2Al(µ-OEPh3)]2 with pyridine yields the expected acid-base complexes
AlMe2(OEPh3)(py) [E ) C (1) and Si (2)]. In contrast, the reaction with THF yields AlMe-
(OEPh3)2(THF) [E ) C (5) and Si (6)], although the dimethyl compounds, AlMe2(OEPh3)-
(THF) [E ) C (3) and Si (4)], are observed in THF-d8 solution. The reaction of [Me2Al(µ-
OCPh3)]2 with THF was followed by 1H NMR and found to occur by a two-step process. First,
the Al2O2 core of [Me2Al(µ-OEPh3)]2 is cleaved by THF to form compound 3. Second, two
molecules of AlMe2(OCPh3)(THF) react with each other, with prior dissociation of THF from
at least one complex, resulting in the ligand redistribution and the formation of 5 and AlMe3-
(THF). The conversion of [Me2Al(µ-OCPh3)]2 into compound 3 is exothermic, and the
subsequent formation of 5 and AlMe3(THF) is endothermic. The rate equations for the
formation of 3 and its conversion to 5 have been determined. The observation of both alkoxide
cleavage and alkyl/alkoxide exchange requires a fine balance between a Lewis base that is
of sufficient strength to cleave the dimeric alkoxide, [R2Al(µ-OR′)]2, while being sufficiently
weak to allow dissociation from the monomeric complex, AlR2(OR′)(L).
In tr od u ction
ation reactions.6 Ligand disproportionation reactions
between AlR3 and [R′2Al(µ-OR′′)]2 have also been studied
in detail,7 and the disproportionation of [Me2Al(µ-OMe)]3
in THF or pyridine has been reported.8 During our
investigation of the thermal decomposition of [Me2Al-
(µ-OCPh3)]2,9 we noted the formation of AlMe(OCPh3)2-
(THF) when the former was exposed to traces of THF.
This observation prompted an investigation of the
reaction of [Me2Al(µ-OCPh3)]2 with Lewis bases.
In 1958 Bradley summarized the structural trends in
the chemistry of main group and transition metal
alkoxides as “alkoxide derivatives adopt the smallest
structural unit consistent with all atoms attaining a
higher coordination sphere”.2 The archetypal example
of this effect is the structural reorganization exhibited
by Al(OiPr)3. When freshly prepared, Al(OiPr)3 is tri-
meric in which each aluminum has a coordination
number of four. Under ambient conditions, the trimeric
structure converts to a tetrameric structure in which
three aluminums are four-coordinate and one has a six-
coordinate geometry.3 Reoligomerization reactions are
a common feature of alkylaluminum alkoxides, but they
do not necessarily result in higher coordination num-
bers. For example, in the absence of overwhelming steric
bulk, dimethylaluminum alkoxides, [Me2Al(µ-OR)]n, are
trimeric when synthesized at low temperatures, but
reoligomerize to dimers at elevated temperatures.4
Increased steric bulk, of either the alkoxide or the
aluminum alkyl groups, results in dimeric structures
for [R2Al(µ-OR′)]n.4 It is only with sufficiently exagger-
ated steric bulky ligands, such as the 2,6-di-tert-butyl-
4-methylphenol ligand, that monomeric structures are
formed.5 Unlike the dimer/trimer equilibria, potential
monomer/dimer systems have not been studied, due to
the presence of additional complicating disproportion-
Resu lts a n d Discu ssion
Reaction of [Me2Al(µ-OEPh3)]2 (E ) C, Si) with an
excess of pyridine gives the expected monomeric Lewis
acid-base complex, AlMe2(OEPh3)(py), E ) C (1), Si
(2).10 The cleavage of the alkoxides’ Al2O2 core by the
pyridine agrees with our previous studies.11 Dissolution
of [Me2Al(µ-OEPh3)]2 in THF-d8 also results in the
formation of the Lewis acid-base complexes, AlMe2-
(OEPh3)(THF-d8), E ) C (3), Si (4). Isolation of either
compound followed by redissolution in C6D6 gives a
(5) (a) Healy, M. D.; Wierda, D. A.; Barron, A. R. Organometallics
1988, 7, 2543. (b) Petrie, M. A.; Olmstead, M. M.; Power, P. P. J . Am.
Chem. Soc. 1991, 113, 8704. (c) Swowron˜ska-Ptasin˜ska, M.; Starow-
ieyski, K. B.; Pasynkiewicz, S.; Carewska, M. J . Organomet. Chem.
1978, 160, 403.
(6) Shreve, A. P.; Mulhaupt, R.; Fultz, W.; Calabrese, J .; Robbins,
W.; Ittel, S. D. Organometallics 1988, 7, 409.
(7) (a) Mole, T. Aust. J . Chem. 1966, 19, 381. (b) J effery, E. A.; Mole,
T. Aust. J . Chem. 1970, 23, 715.
(8) The formation of AlMe3L was observed by 1H NMR spectroscopy;
see: Mole, T. Aust. J . Chem. 1966, 19, 373.
* To whom correspondence should be addressed (url://www.rice.
edu/barron).
(1) (a) Rice University. (b) University of Houston.
(2) (a) Bradley, D. C. Nature 1958, 182, 1211. (b) Bradley, D. C.
Adv. Chem. Ser. 1959, 23, 10.
(9) Obrey, S. J .; Bott, S. G.; Barron, A. R. Organometallics 2001, in
press.
(10) Obrey, S. J .; Barron, A. R., J . Chem. Soc., Dalton Trans. 2001,
in press.
(3) Mehrotra, R. C. J . Indian Chem. Soc. 1953, 30, 585.
(4) Rogers, J . H.; Apblett, A. W.; Cleaver, W. M.; Tyler, A. N.; Barron,
A. R. J . Chem. Soc., Dalton Trans. 1992, 3179.
(11) (a) Healy, M. D.; Ziller, J . W.; Barron, A. R. J . Am. Chem. Soc.
1990, 112, 2949. (b) Healy, M. D.; Power, M. B.; Barron, A. R. J . Coord.
Chem. 1990, 21, 363.
10.1021/om010394i CCC: $20.00 © 2001 American Chemical Society
Publication on Web 11/02/2001

5120 Organometallics, Vol. 20, No. 24, 2001
Obrey et al.
F igu r e 3. Plot of Al-O (0) and Si-O (9) bond distance
(Å) as a function of Al-O-Si bond angle (deg) for tri-
phenylsiloxide compounds of aluminum.
F igu r e 1. Molecular structure of AlMe(OCPh3)2(THF) (5).
Thermal ellipsoids are shown at the 20% level, and
hydrogen atoms attached to carbon are omitted for clarity.
Selected bond lengths (Å) and angles (deg): Al(1)-O(2) )
1.710(4), Al(1)-O(1) ) 1.853(8), Al(1)-O(2)-C(10) )
141.2(4).
5 and 6 is dominated by the sterically bulky alkoxide
and siloxide ligands. The Al-O and Si-O bond dis-
tances are essentially independent of the Al-O-Si
angle. This is expected when considered with regard to
the structures of other Al-OSiPh3 compounds; see
Figure 3.15
In vestiga tion of THF -P r om oted Alk yl/Alk oxid e
Exch a n ge Rea ction . Since dimeric dimethylaluminum
alkoxides do not usually disproportionate into methyl-
aluminum dialkoxide species in the presence of a Lewis
base,8 we deemed this reaction suitable for further
investigation. In this regard, the reaction of [Me2Al-
(OCPh3)]2 with 2 equiv of THF in C6D6 was followed by
1H NMR over a period of one week under ambient
conditions.
Upon initial analysis of the mixture, a single Al-Me
resonance (δ ) 0.81 ppm) due to [Me2Al(µ-OCPh3)]2 is
the only species observed. After 2 h, the integration
(relative to the signal due to the residual protons in the
solvent) of [Me2Al(µ-OCPh3)]2 has decreased, and a
resonance is observed at 0.66 ppm that may be assigned
to AlMe2(OCPh3)(THF) (3), i.e., eq 1.16
F igu r e 2. Molecular structure of AlMe(OSiPh3)2(THF) (6).
Thermal ellipsoids are shown at the 20% level, and
hydrogen atoms attached to carbon are omitted for clarity.
Selected bond lengths (Å) and angles (deg): Al(1)-O(1) )
1.715(4), Al(1)-O(2) ) 1.699(3), Al(1)-O(1S) ) 1.869(5),
Si(1)-O(1) ) 1.596(4), Si(2)-O(2) ) 1.596(4), Al(1)-O(1)-
Si(1) ) 149.8(2), Al(1)-O(2)-Si(2) ) 165.5(3).
K1
[Me2Al(µ-OCPh3)]2 + 2 THF y\z
2 AlMe2(OCPh3)(THF)
(1)
3
1
complex H NMR spectrum. The use of only a trace of
With increased reaction time the relative concentrations
of [Me2Al(µ-OCPh3)]2 and AlMe2(OCPh3)(THF) continue
to change; however, additional resonances due to AlMe3-
(THF)17 and AlMe(OCPh3)2(THF) (5) are also observed.
The relative ratio of AlMe3(THF) and AlMe(OCPh3)2-
(THF) is approximately 1:1, suggesting that they are
formed by the ligand disproportionation of AlMe2-
THF results in the isolation of the dialkoxide complex,
AlMe(OCPh3)2(THF) (5).12 A similar reaction occurs for
the reaction of [Me2Al(µ-OSiPh3)]2 to give AlMe(OSiPh3)2-
(THF) (6).
Compounds 1, 2, 5, and 6 have been characterized
by NMR spectroscopy. The 27Al NMR spectra are
consistent with analogous AlR2(OR)(L) and AlR(OR)2-
(L) environments.13 The solid state structures of com-
pounds 5 and 6 have been confirmed by X-ray diffraction
and are shown in Figures1 and 2. As we have previously
observed,11,14 the coordination geometry in compounds
(14) Francis, J . A.; McMahon, C. N.; Bott, S. G.; Barron, A. R.
Organometallics 1999, 18, 4399.
(15) (a) Apblett, A. W.; Warren, A. C.; Barron, A. R. Can. J . Chem.
1992, 70, 771. (b) Apblett, A. W.; Barron, A. R. J . Crystallogr. Spectrosc.
Res. 1993, 23, 529. (c) Wengrovius, J . H.; Garbauskas, M. F.; Williams,
E. A.; Going, R. C.; Donahue, P. E.; Smith, J . F. J . Am. Chem. Soc.
1986, 108, 982. (d) Atwood, D. A.; Hill, M. S.; J egier, J . A.; Rutherford,
D. Organometallics 1997, 16, 2659. (e) Veith, M.; J arczyk, M.; Huch,
V. Angew. Chem., Int. Ed. Engl. 1997, 36, 117.
(16) Assignments were based upon the integration of new peaks and
in comparison with the formation of AlMe2(OCPh3)(THF) in THF-d8.
(17) 1H NMR: δ 3.34 (4H, m, OCH2), 1.00 (4H, m, OCH2CH2), -0.41
(9H, s, Al-CH3).
(12) The initial observation of this effect was due to a cross
contamination of our hexane still by THF through the nitrogen lines.
However, subsequent experiments have demonstrated that the syn-
thesis of 5 and 6 may be accomplished in a rational manner by using
ca. 2 equiv of THF in toluene solution.
(13) (a) Barron, A. R. Polyhedron 1995, 14, 3197. (b) Healy, M. D.;
Power, M. B.; Barron, A. R. Coord. Chem. Rev. 1994, 130, 63.

Reaction of [Me2Al(µ-OCPh3)]2 with THF
Organometallics, Vol. 20, No. 24, 2001 5121
Ta ble 1. Su m m a r y of Th er m od yn a m ic a n d Kin etic
Da ta
a
Values assuming no dissociation of AlMe(OCPh3)2(THF).
b
Values assuming complete dissociation of AlMe(OCPh3)2(THF)
to AlMe(OCPh3)2 and THF. c Values assuming complete dissocia-
tion of AlMe(OCPh3)2(THF) and taking into account potential
equilibrium with excess THF.
such a process with aluminum. In fact, the ∆S calcu-
lated for the reaction in eq 2 appears to be similar to
values obtained for the dissociation of dimeric aluminum
alkyls (123-142 J ‚K-1 mol-1).19 Thus, the entropy of
reaction appears to suggest a dissociative process. It is
well known, however, that Lewis acid-base complexes
of aluminum dissociate in solution.18 Furthermore, the
extent of dissociation is increased the greater steric bulk
of the ligands. Thus, AlMe(OCPh3)2(THF) would be
expected to be significantly dissociated in solution (i.e.,
eq 5), in which case K2 could be represented by eq 6.
F igu r e 4. Plot of the formation of the relative concentra-
tion of [Me2Al(µ-OCPh3)]2 (0), AlMe2(OCPh3)(THF) (9), and
AlMe(OCPh3)2(THF) (b) as a function of reaction time for
the reaction of [Me2Al(µ-OCPh3)]2 with 2 equiv of THF at
50 °C.
(OCPh3)(THF), eq 2. The mole fractions of each species
as a function of reaction time is shown in Figure 4. Both
reactions reach equilibria over a wide temperature
range; for example, at 50 °C equilibrium is reached after
13 h.
K2
K3
2 AlMe2(OCPh3)(THF) y\z
AlMe(OCPh3)2(THF) y\z AlMe(OCPh3)2 + THF (5)
AlMe(OCPh ) (THF)
+ AlMe3(THF) (2)
3 2
[AlMe(OCPh3)2][THF][AlMe3(THF)]
5
K2 )
(6)
K3[AlMe2(OCPh3)(THF)]2
The equilibrium constant for the formation of AlMe2-
(OCPh3)(THF) (eq 1) as defined by eq 3 may be deter-
mined as 30.2 mol-1 dm3 at 40 °C.
Like other similar reactions, the reaction shown in eq
5 would be rapid on the NMR time scale, and the
traditional method of determining K3 is complicated due
to the multiple reactions.18 Values for this modified K2
can be estimated if complete dissociation occurs. On the
basis of these assumptions the ∆H values do not vary
significantly (see Table 1), and the ∆S [83(3) J ‚K-1
mol-1] is closer to that expected for similar reactions;
see above. We propose, therefore, that the dissociation
of THF from AlMe(OCPh3)2(THF) is part of the driving
force for the formation of AlMe(OCPh3)2(THF) and
AlMe3(THF).
The conversion of [Me2Al(µ-OCPh3)]2 into AlMe2-
(OCPh3)(THF) shows a first-order dependence on the
concentration of [Me2Al(µ-OCPh3)]2. A plot of kobs versus
[THF] also shows a first-order dependence. Thus, the
rate of the initial reaction takes the form of eq 7.
[AlMe2(OCPh3)(THF)]2
[{Me2Al(OCPh3)}2][THF]2
K1 )
(3)
Similarly, the equilibrium constant associated with the
ligand exchange reaction (eq 2) may be defined by eq 4
and is 6.85 × 10-2 at 40 °C.
[AlMe(OCPh3)2(THF)][AlMe3(THF)]
K2 )
(4)
[AlMe2(OCPh3)(THF)]2
The temperature dependence of K1 and K2 was
measured, from which ∆H and ∆S have been deter-
mined. A summary of the thermodynamic data for eqs
1 and 2 is given in Table 1. Interestingly, the conversion
of [Me2Al(µ-OCPh3)]2 into AlMe2(OCPh3)(THF) is exo-
thermic, while the subsequent formation of AlMe-
(OCPh3)2(THF) and AlMe3(THF) is endothermic. Thus,
the former reaction is enthalpically driven, while the
latter is entropically driven.
We note that the value for ∆S determined for the
reaction shown in eq 2 [116(1) J ‚K-1 mol-1] is larger
than would be expected for a reaction in which there is
no change in molecularity, i.e., no change in the number
of species. Based upon previous studies,18 a value of
between 30 and 70 J ‚K-1 mol-1 would be expected for
-d[{Me2Al(µ-OCPh3)}2]/dt )
k1[{Me2Al(µ-OCPh3)}2][THF] (7)
Measurement of the temperature dependence for k1
allows for the determination of ∆Hq and ∆Sq; see Table
1. The positive value of ∆Sq indicates a dissociative
reaction.20
At high temperatures, K1 is reached rapidly enough
that it is possible to determine the rate for the conver-
sion of AlMe2(OCPh3)(THF) to AlMe(OCPh3)2(THF) and
(19) (a) Smith, M. B. J . Organomet. Chem. 1974, 70, 13. (b) Smith,
M. B. J . Phys. Chem. 1967, 71, 354.
(20) Atwood, J . D. Inorganic and Organometallic Reaction Mecha-
nisms; VCH: New York, 1997; p 14.
(18) (a) Power, M. B.; Nash, J . R.; Healy, M. D.; Barron, A. R.
Organometallics 1992, 11, 1830. (b) Stanford, T. S.; Henold, K. L. Inorg.
Chem. 1975, 14, 2426.

5122 Organometallics, Vol. 20, No. 24, 2001
Obrey et al.
F igu r e 5. Reaction coordinate diagram for the reaction of [Me2Al(µ-OCPh3)]2 with THF.
AlMe3(THF). As may be expected of a ligand exchange
reaction, the disappearance of the H NMR signal due
to the Al-CH3 groups of the monomeric dimethylalumi-
num alkoxide species occurs in a second-order fashion,
terms in the denominator. Unfortunately, we were
unable to find conditions under which a zero-order rate
dependence is observed, since conditions of low THF
concentration do not initiate the first reaction, eq 1.
Thus, k2 cannot be determined. Since KTHF is a constant,
∆Hq may be determined from kobs for a given concentra-
tion of THF and an upper value for ∆Sq may be
estimated; see Table 1.
1
i.e., eq 8. Second-order observed rate constants, kobs
,
were calculated from the corresponding plot of 1/[AlMe2]
versus time.
-d[AlMe2]/dt ) kobs[AlMe2]2
(8)
The reaction coordinate diagram for the reaction of
[Me2Al(µ-OCPh3)]2 with THF is given in Figure 5.
A plot of kobs versus [THF] shows an inverse depen-
dence. As is typical for Lewis acid-base complexes of
group 13 metals, AlMe2(OCPh3)(THF) undoubtedly ex-
ists in rapid equilibrium in solution (eq 9).
Con clu sion s
The Lewis base (L) cleavage of an alkoxide dimer,
[R2Al(µ-OR′)]2 (eq 1), is enthalpically driven. In contrast,
the ligand redistribution reaction (eq 2) is endothermic
and entropically driven. Based on the inverse depen-
dence on the concentration of the Lewis base, it appears
that a prior dissociation of the Lewis base is required
(eq 9) in order for the ligand redistribution reaction to
continue. If the Lewis base is a sufficiently strong donor
ligand, dissociation will be negligible, and ligand redis-
tribution will not occur. In order for ligand redistribu-
tion to be facile, the Lewis base must be a poor donor
ligand (resulting in a weak Al-L bond). If the Lewis
base is too weak, the cleavage of the alkoxide dimer will
not proceed.
KTHF
AlMe2(OCPh3)(THF) y z AlMe2(OCPh3) + THF
(9)
The inverse dependence on the THF concentration
suggests that prior dissociation of THF is required for
the rate-controlling reaction of an “AlMe2(OCPh3)”
moiety with a second AlMe2(OCPh3)(THF) molecule (eq
10).21 The AlMe(OCPh3)2 is in equilibrium with its THF
complex, eq 5.
AlMe2(OCPh3) + AlMe2(OCPh3)(THF) h
AlMe(OCPh3)2 + AlMe3(THF) (10)
Assuming that (1) the ligand redistribution occurs by
the reactions given above, (2) the ligand exchange is the
rate-determining step, and (3) the concentration of
reactants, as determined from the 1H NMR spectra,
[AlMe2], can be expressed as eq 11, then the rate of
ligand exchange can be expressed by eq 12.
The observation of both alkoxide cleavage and alkyl/
alkoxide exchange requires a fine balance between a
Lewis base that is of sufficient strength to cleave the
dimeric alkoxide, [R2Al(µ-OR′)]2, while being sufficiently
weak to allow dissociation from the monomeric complex,
AlR2(OR′)(L). Such a balance in reactivity may be
thought of as fitting a Goldilocks model: “not too hot,
not too cold, just right”. While it is clearly fortuitous12
that THF has a near optimum Lewis basicity to allow
for both cleavage and ligand redistribution reactions to
occur with [Me2Al(µ-OCPh3)]2 and [Me2Al(µ-OSiPh3)]2,
we are continuing to study these reactions to develop a
[AlMe2] ) [AlMe2(OCPh3)(THF)] + [AlMe2(OCPh3)]
(11)
k2KTHF[AlMe2]2
rate )
(12)
[THF] + KTHF
For this rate law, the dependence on THF varies from
(21) Presumably, two “AlMe2(OCPh3)” groups are not involved;
0 to -1, depending on the relative magnitude of the two
otherwise the re-formation of [Me2Al(µ-OCPh3)]2 would be favored.

Reaction of [Me2Al(µ-OCPh3)]2 with THF
Organometallics, Vol. 20, No. 24, 2001 5123
Ta ble 2. Su m m a r y of X-r a y Diffr a ction Da ta
predictive model of the factors controlling group 13
alkoxide oligomerization and ligand distribution.
AlMe(OCPh3)2(THF) AlMe(OSiPh3)2(THF)
(5)
(6)
Exp er im en ta l Section
emp form
cryst size, mm
cryst syst
space group
a, Å
C
43H41AlO3
C41H41AlO3Si2
0.6 × 0.6 × 0.1
monoclinic
P21/c
16.425(3)
8.740(2)
27.048(4)
105.06(3)
3749(1)
4
1.178
0.15
2.5 to 46.7
9558
4458
0.5 × 0.4 × 0.2
monoclinic
P21/m
8.759(2)
24.128(5)
9.213(2)
114.33(3)
774.1(6)
2
1.184
0.095
5 to 47
6921
2646
Mass spectra were obtained on a Finnigan MAT 95 mass
spectrometer operating with an electron beam 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 unless
otherwise stated. NMR spectra were obtained on Bruker
Avance 200 and 400 MHz spectrometers using (unless other-
wise stated) C6D6 solutions. Chemical shifts are reported
relative to internal solvent resonances (1H and 13C) and
external [Al(H2O)6]3+(27Al). [Me2Al(µ-OCPh3)]2 and [Me2Al(µ-
OSiPh3)]2 were prepared according to previously described
procedures.10 Microanalyses were performed by Oneida Re-
search Services, Inc., Whitesboro, NY. All other chemicals were
obtained from Aldrich and used without further purification
(unless otherwise noted).
AlMe2(OCP h 3)(p y) (1). [Me2Al(µ-OCPh3)]2 (0.100 g, 0.158
mmol) was dissolved in toluene (10 mL), and a solution of
pyridine (25 mg, 0.158 mmol) in toluene (5 mL) was added
slowly at room temperature. The reaction mixture was stirred
for 6 h, after which the solvent was removed under vacuum,
yielding a white powder. Yield: 95%. 1H NMR: δ 8.38 [2H, d,
J (H-H) ) 6.2 Hz, o-CH, py], 7.61 [6H, d, J (H-H) ) 7.1 Hz
o-CH], 7.11 (3H, m, p-CH), 7.04 (6H, m, m-CH), 6.91 (1H, m,
p-CH, py), 6.56 (2H, m, m-CH, py), -0.51 (6H, s, Al-CH3). 27Al
NMR: δ 130 (W1/2 ) 7250 Hz).
AlMe2(OSiP h 3)(p y) (2). [Me2Al(µ-OSiPh3)]2 (100 mg, 0.150
mmol) was dissolved in toluene (10 mL), whereupon a solution
of pyridine (24 mg, 0.303 mmol) in toluene (5 mL) was added
slowly at room temperature. The reaction mixture was stirred
for 6 h, after which the solvent was removed under vacuum,
yielding a white powder. Yield: 95%. 1H NMR: δ 8.57 [2H, d,
J (H-H) ) 5.7 Hz, o-CH, py], 7.69 [6H, d, J (H-H) ) 7.1 Hz,
o-CH], 7.11 (6H, m, m-CH), 7.04 (3H, m, p-CH), 6.56 (2H, m,
m-CH, py), 6.31 (1H, m, p-CH, py), -0.51 (6H, s, Al-CH3). 27Al
NMR: δ 130 (W1/2 ) 6700 Hz).
b, Å
c, Å
â, deg
V, Å3
Z
D
calc, g‚cm-3
µ
calc, mm-1
2θ range, deg
no. of reflns colld
no. of ind reflns
no of reflns obsd
weighting scheme
SHELXTL params
R
1300
0.10, 0.0
1564
0.10, 0.0
0.114
0.270
0.40
0.0485
0.113
0.15
Rw
largest diff peak,
e Å-3
mL). All samples were heated to the appropriate temperature
within the NMR spectrometer, and a series of 1H NMR spectra
were collected at equal increments. The temperature of the
NMR spectrometer probe was calibrated using the chemical
shifts of ethylene glycol.22 The relative integration of the
aluminum methyl protons was used to determine the rate of
the reactions at seven different temperatures (313-373 K)
until equilibrium had been reached. The first-order observed
rate constants were determined from a plot of -ln[{Me2Al(µ-
OCPh3)}2] versus time. The second-order observed rate con-
stant was determined from a plot of 1/[AlMe2] versus time
using preequilibrium kinetics.
The rate dependence on the concentration of THF for k1 was
determined using a series of four samples of [Me2Al(µ-OCPh3)]
2 with 1, 2, 5, and 10 equiv of THF prepared from two stock
solutions. The first solution was prepared by dissolving [Me2-
Al(µ-OCPh3)]2 (90 mg) in toluene-d8 (2.91 g). Samples of this
solution (0.500 g) were accurately weighted into a series of 5
mm NMR tubes. A second solution was prepared by dissolving
THF (114 mg) in toluene-d8 (1.896 g). This solution was
accurately weighed into the four NMR tubes (250, 125, 50, and
25 mg) followed by addition of toluene-d8. All samples were
heated to 353 K within the NMR spectrometer, and a series
AlMe2(OCP h 3)(THF ) (3). [Me2Al(µ-OCPh3)]2 (10 mg, 0.016
mmol) was dissolved in THF-d8 (0.75 mL) and allowed to stand
for 1 h at room temperature. NMR analysis showed the
1
formation of a single product. H NMR (THF-d8): δ 7.38 [6H,
d, J (H-H) ) 7.9 Hz, o-CH], 7.18 (6H, m, m-CH), 7.08 (3H, m,
p-CH), -1.09 (6H, s, Al-CH3).
AlMe2(OSiP h 3)(THF ) (4). [Me2Al(µ-OSiPh3)]2 (10 mg, 0.015
mmol) was dissolved in THF-d8 (0.75 mL) and allowed to stand
for 1 h at room temperature. NMR analysis showed the
1
of H NMR spectra were collected at equal increments.
The rate dependence on the concentration of THF for k2 was
determined using a series of four samples of [Me2Al(µ-OCPh3)]2
with 5, 10, 20, and 50 equiv of THF, prepared by accurately
weighting 20 mg of [Me2Al(µ-OCPh3)]2 into 5 mm NMR tubes
followed by addition of toluene-d8 (0.500 g). A solution of THF
(455 mg) was diluted with toluene-d8 (545 mg). This solution
was prepared and accurately weighed into the NMR tubes (25,
50, 125, and 250 mg) followed by addition of toluene-d8. All
samples were heated to 353 K within the NMR spectrometer,
1
formation of a single product. H NMR (THF-d8): δ 7.55 (6H,
m, o-CH), 7.06-7.22 (9H, m, p-, m-CH), -0.94 (6H, s, Al-CH3).
AlMe(OCP h 3)2(THF ) (5). A solution of THF (114 mg, 1.58
mmol) in toluene (1.0 mL) was added to a solution of [Me2Al-
(µ-OCPh3)]2 (0.500 g, 0.791 mmol) in toluene (10 mL) and
allowed to stand at room temperature for 2 weeks, after which
clear, colorless crystals precipitated out of solution. Yield: 55%.
1
Mp: 199-201 °C. H NMR: δ 7.61 [12H, d, J (H-H) ) 8.0 Hz
o-CH], 7.14 (12H, m, m-CH), 7.06 (6H, m, p-CH), 3.23 (4H, m,
OCH2, THF), 0.78 (4H, m, OCH2CH2, THF), -0.99 (3H, s, Al-
CH3). 27Al NMR: δ 80 (W1/2 ) 7090 Hz).
1
and a series of H NMR spectra were collected at equal time
increments.
X-r a y Cr ysta llogr a p h ic Stu d ies. Crystals of 5 and 6 were
sealed in glass capillary tubes under argon. Crystal and data
collection details are given in Table 2. Standard procedures
in our laboratory have been described previously.23 Data 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
AlMe(OSiP h 3)2(THF ) (6). A solution of THF (108 mg, 1.506
mmol) in toluene (1.0 mL) was added to a solution of [Me2Al-
(µ-OSiPh3)]2 (0.500 g, 0.753 mmol) in toluene (10 mL) and
allowed stand at room temperature for 3 weeks, after which
clear, colorless crystals precipitated out of solution. Yield: 95%.
1H NMR: δ 7.45-7.70 (12H, m, o-CH), 7.05-7.20 (18H, m, m
and p-CH), 3.12 (4H, m, O-CH2), 0.72 (4H, m, OCH2CH2),
-1.07 (3H, s, Al-CH3). 27Al NMR: δ 85 (W1/2 ) 6440 Hz).
Kin etic Mea su r em en ts. A series of samples were prepared
in 5 mm NMR tubes from standard solutions of [Me2Al(µ-
OCPh3)]2 (0.02766 M) and THF (0.2766 M) in toluene-d8 (0.75
(22) (a) van Geet, A. L. Anal. Chem. 1968, 40, 2227. (b) Gordon, H.
J .; Ford, R. A. The Chemists Companion; Wiley: New York, 1972.
(23) Mason, M. R.; Smith, J . M.; Bott, S. G.; Barron, A. R. J . Am.
Chem. Soc. 1993, 115, 4971.

5124 Organometallics, Vol. 20, No. 24, 2001
Obrey et al.
were solved using direct methods program XS24 and difference
Fourier maps and refined using full matrix least-squares
methods.25 The THF ligand in compound 5 resides on a
crystallographic mirror plane and is, therefore, constrained to
be planar. Attempts to resolve a disorderd model were unsuc-
cessful. All atoms were refined with anisotropic thermal
parameters. All the hydrogen atoms were placed in calculated
positions [Uiso ) 0.08; d(C-H) ) 0.96 Å] for refinement of
positional and anisotropic parameters, leading to convergence.
Avance 200 NMR spectrometer was purchased with
funds from ONR Grant N00014-96-1-1146. AlMe3 was
generously provided by Albemarle Corp. The authors
acknowledge the questions of one of the referees for
clarifying our thoughts on the thermodynamic aspects
of this paper.
Su p p or tin g In for m a tion Ava ila ble: Full listings of bond
lengths and angles, anisotropic thermal parameters, and
hydrogen atom parameters; 13C NMR, MS, IR, and analytical
data; selected equilibrium and kinetic plots. This material is
available free of charge via the Internet at http://pubs.acs.org.
Ack n ow led gm en t. Financial support for this work
was provided by the Welch Foundation. The Bruker
(24) Sheldrick, G. M. Acta Crystallogr. Sect. A 1990, A46, 467.
(25) Sheldrick, G. M. SHELXTL; Bruker AXS, Inc.: Madison, WI,
1997.
OM010394I