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8/10/2019 AlCp2](+)Structure, Properties and Isobutene Polymerization
Abstract. Reaction of the protonic ether compound [H(OEt2)2]-
[Al(ORF)4] (RF C(CF3)3) with AlCp3 leads to the formation of
the [AlCp2]-cation, which is stabilized by the weakly coordinating
[Al(ORF)4] ion. Besides [AlCp2], the ion [AlCp2 · 2Et2O], which
is stabilized by two ether molecules, is formed in an equilibrium
reaction. The so far unknown molecular structure of [AlCp2] and
Introduction
About 15 years ago, we described the structure, bonding
and spectroscopic properties of [AlCp2*][Cp*AlCl3] (1) [1].
Later, this MgCp*2 analogue, a sandwich compound, was
obtained by Shapiro et al. [2] and Jutzi et al. [3] on different
routes. The high stability of 1 and its poor ability to initiate
the cationic polymerization of isobutene is in contrast to
the performance of [AlCp2][MeB(C6F5)3] (2), observed by
Bochmann [4]. Compound 2 decomposes in CH2Cl2 above
20 °C. Below this temperature, 2 is a highly active poly-
merization initiator. Compound 2 has been obtained byreaction of Cp2AlMe and B(C6F5)3 and, because of its low
stability, has so far only been characterized in solution at
low temperatures. In order to explore the possibility of a
fine tuning of the stability and activity of the AlR2 cation,
Shapiro et al. [2] varied the cyclopentadienyl rings to
Cp C5Me4H and determined the structure of
[Cp2Al][B(C6F5)4] (3). Furthermore, Shapiro et al. con-
vincingly demonstrated that 3 is a better initiator for the
* Prof. Dr. H. SchnoeckelFax: 49-721-6084854E-Mail: [email protected]
[a] Institute for Inorganic ChemistryUniversity of KarlsruheEngesserstrasse 1576131 Karlsruhe, Germany
[b] Freiburg Materials Research CenterAlbert-Ludwigs-Universität FreiburgStefan-Meier-Straße 2179104 Freiburg i. Br., Germany
[c] Institute for Inorganic and Analytical ChemistryAlbert-Ludwigs-Universität FreiburgAlbertstr. 2179104 Freiburg i. Br., GermanySupporting information for this article is available on theWWW under www.zaac.wiley-vch.de or from the author.
M. Huber, A. Kurek, I. Krossing, R. Mülhaupt, H. SchnöckelARTICLE
Calculations of the Equilibrium between 2a and 2b
Including the Crystalline Compounds
These considerations were made to obtain some infor-
mation about the concentration of the [AlCp2] ion, which
is necessary for the estimation of its initiating activity for
the polymerization of isobutene. Only with this background
we were able to compare our results of the polymerization
experiments (see below) with those obtained by Bochmann
et al [4]. Therefore, some difficulties were expected because
of the presence of diethyl ether in the reaction mixture,
since, in regard to the above mentioned considerations,
compound 2b should be favored. However, its activity as an
initiator should be negligible compared to that of the ether
free aluminocenium cation in 2a, because of the lower
Lewis acidity of 2b (see below). Nevertheless, in order to
force the equilibrium [Equation (2)] on the side of 2a, the
solvents of the reaction mixture (dichloromethane and es-
pecially diethyl ether with its high volatility) were com-
pletely removed within half an hour after the reagents were
combined. Therefore, the residue used for the polymeriz-
ation should mainly contain 2a as a result of the equilib-rium [Equation (2)]. The complete removal of ether was
controlled by 1H-NMR measurements. The solid obtained
finally, i.e. the catalyst, was stored at 78 °C prior to use
in order to avoid the expected decomposition of [AlCp2]
in CH2Cl2 [4].
In order to understand the influence of the crystalline
compounds 2a and 2b on the position of the equilibrium
[Equation (2)], we also worked out a BornFajansHaber
cycle for the heterogeneous equilibrium between 2a and 2b
in the gas phase [24] and in solid state (Figure 3). With the
(3)
Figure 3. Born Fajans Haber cycle for the formation of 2a and 2b as well as in the gas phase as a model for the solution, and in thesolid state. All energies are Gibbs energies at 298 K in kJ · mol1. The Gibbs energies for the gas phase (∆rG 0(gas)) were calculated using
the program package Turbomole (BP86/SV(P)). To calculate ∆G 0(solv) experimental data were used [31].
Table 3. Calculated Gibbs energies for the Born Fajans Haber cycle in Figure 3.
Gas Phase ∆rH (g) /kJ · mol1 T ∆rS (g) /kJ · mol1 ∆rG (g) /kJ · mol1
[AlCp2]: Structure, Properties and Isobutene Polymerization
Table 4. Results of the isobutene polymerization compared withthe experimental results (in parentheses) obtained by other authors[4]. The experiments at 70 °C cannot be compared, because of the poor solubility of 2a at this temperature [33].
t 10 min 50 °C 30 °C
Yield /g 0.58 (0.18) 1.56 (0.08)M w /104 g · mol1 6.1 (62) 1.9 (32)PDI 2.0 (2.0) 1.7 (1.8)
t 120 min 50 °C* 30 °C
Yield /g 6.51 5.74M w /104 g · mol1 6.1 2.1PDI 6.0 3.7
* At 50 °C the polymerization had to be interrupted because of the high viscosity of the reaction mixture.
we performed isobutene polymerizations under similar con-
ditions as those run by Bochmann et al [4].
Comparing the polymerization results (Table 4), our
experiments show a lower weight-average molecular weight
M w and a polydispersity of the polymers, which is in the
same range at the same temperatures. The higher yield of polyisobutene in our experiments is remarkable. When the
experiments were conducted over a period of two hours at
50 and 30 °C, they had to be interrupted because of the
high viscosity of the reaction mixture [32]. The here pre-
sented polymerization experiments suggest a higher activity
of [AlCp2], which is a result of the weaker interaction be-
tween [AlCp2] and [Al(ORF)4] relative to the experiments
performed by Bochmann et al [4].
Comparison of the Isobutene-Polymerization of [AlCp*2 ]
(1), [AlCp2 ] (4) [AlCp2 ]
(2a) and [AlCp2 · 2Et 2O]
(2b)
Furthermore, we attempted to scale the different activi-
ties of the cations [AlCp*2] (1), [AlCp2] (4) [AlCp2]
(2a) and [AlCp2 · 2Et2O] (2b) [34]. Shapiro et al. already
concluded that the relative activity of the cations rises from
1 to 2a, and related this increase to the lower steric demand
of the Cp rings and the hence resulting better access of IB
to the positively charged aluminum atoms [2].
Generally, the activity should not only, but strongly de-
pend on the Lewis acidity of the cationic species. Therefore,
one should expect that the higher the Lewis acidity, the
higher is the activity according to IB polymerization. A
quantitative measure for the Lewis acidity is the fluoride
ion affinity (FIA) [7, 8, 35]. The calculated values for 1, 2a,2b and 3 are given in Table 5. Comparing these values
it is possible to list the Lewis acidities and the activity for
the polymerization of IB in the following sequence:
[AlCp2] >> [AlCp2] [AlCp*2] >>> [AlCp2 · 2Et2O].
Compound 2a, which was used for the polymerization
experiments, contains only traces of 2b. The large difference
in the FIA between 2a and 2b and also the fact, that the
FIA of 2b is lower than that of 1 (the polymerization of
isobutene does not start with 1 as initiator even at 20 °C)
are strong hints, that any 2b still present in samples of 2a is
Table 5. Calculated FIA of the compounds 1, 2a, 2b and 3. Thereported pF values are only to order the cations in a quantitativescale for Lewis acidities given by Christe and Dixon [8].
[AlCp2]: Structure, Properties and Isobutene Polymerization
Supporting Information (see footnote on the first page of this arti-
cle): The presence of 2b in solution is confirmed by the 1H NMR
spectrum.
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[35] The FIA of 1, 2a and 3 was calculated according to [Equation(4)]. For 2b [Equation (5)] was used. Reference is theexperimental value for the FIA of OCF2, which is209 /kJ · mol1.
(4)
(5)
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Received: December 11, 2008Published Online: February 13, 2009