AlCp2](+)Structure, Properties and Isobutene Polymerization

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ARTICLE

DOI: 10.1002/zaac.200801378

[AlCp2]: Structure, Properties and Isobutene Polymerization

Michael Huber,[a] Alexander Kurek,[b] Ingo Krossing,[c] Rolf Mülhaupt,[b] andHansgeorg Schnöckel[a]*

Dedicated to Professor Martin Jansen on the Occasion of His 65th Birthday

Keywords: Sandwich complexes; Aluminium; Cyclopentadienyl ligands; Polymerizations

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.

Z. Anorg. Allg. Chem. 2009, 635, 17871793 © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1787

the molecular structure of [AlCp2 · 2Et2O] are presented in this

work. To get insight in the formation and the equilibrium between

these two cations, quantum mechanical calculations were per-

formed. Moreover, the influence of counterions like [Al(ORF)4]

and [MeB(C6F5)3] on the activity of [AlCp2] to act as a polymer-

ization initiating agent for isobutene was investigated.

isobutene polymerization than [AlCp*2] but worse than

[AlCp2] in 2. Within this context, we want to answer two

remaining questions in this contribution:

1) The so far unknown structure of the [AlCp2] ion sta-

bilized with the large WCA (weakly c oordinating anion)

[Al(ORF)4] (RF C(CF3)3) (2a) [5, 6] will be presented in

the crystalline state.

2) Furthermore, it will be shown that the [Al(ORF)4]

ion has weaker contacts to [AlCp2] in 2a than

[MeB(C6F5)3] in 2, inducing a higher activity of [AlCp2]

with respect to the polymerization of 2a in solution relativeto 2, since the [MeB(C6F5)3] anion is better suited to form

a Cp2AlMeB(C6F5)3 ion pair than [Al(ORF)4] (similar

complexes were observed earlier, e.g. Cp2Y-Me-B(C6F5)3(Cp C5H4SiMe3) [7]. The high activity of 2a is finally

supported by quantum chemical calculations (Fluoride Ion

Affinity, FIA) [8, 9].

In order to understand the formation of

[AlCp2 · 2Et2O], which was also prominent in the solution

and was isolated and structurally characterized as

[AlCp2 · 2Et2O][Al(ORF)4] (2b), additional quantum chemi-

cal calculations were performed.

Results and DiscussionX-ray Structure Determination

In 2004, P. Jutzi et al. succeeded in synthesizing the

SiCp*-ion an isoelectronic species to monomeric AlCp*

[10, 11] by reaction of SiCp*2 and [C5Me5H2], both in

solution as in the solid state with [B(C6F5)4] as counterion

[12] and later also by using [H(OEt2)2][Al(ORF)4] (RF

C(CF3)3) as a proton source [13]. For the synthesis of the

AlCp2-cation we similarly used the proton transfer reagent

[H(OEt2)2][Al(ORF)4]. The compounds 2a and 2b were



M. Huber, A. Kurek, I. Krossing, R. Mülhaupt, H. SchnöckelARTICLE

(1)

obtained by reaction of one equivalent of AlCp3 and

[H(OEt2)2][Al(ORF)4] at 30 °C in dichloromethane

[Equation (1)]. At 28 °C colorless crystals of 2a and 2b,

suitable for crystal structure analysis, were obtained from

the same reaction mixture. As found by analysis of severalbatches, either 2a or 2b seem to be slightly preferred de-

pending on the reaction conditions (temperature, concen-

tration of the ether molecules, concentration of the ions).

Compound 2a crystallizes in the monoclinic space group

C 2/c with one ion pair in the asymmetric unit (Figure 1,

Table 1). It forms a distorted CsCl packing with an average

cationcation distance of 10.9 A; the average distance be-

tween the anions is 9.78 A. The structure of the cationic

AlCp2-ion is given in Figure 2. In 2a the Cp rings are η5-

bonded (stagger angle β ¯ 30.55°) [14] with an average

XAl distance of 178.9 pm (X is the distance between the

center of the Cp-rings and the aluminum atom) and an

XAlX angle of 179.5°. The average AlX distances in[AlCp*2] (1) and [AlCp2] (3) are 178 pm and 176.5 pm

respectively [1, 2]. Interestingly, the less demanding Cp-li-

gand induces a longer AlX distance (AlC distances)

than the bulkier Cp and Cp* ligands: the AlX distance

Figure 1. Section of the crystal structures of the compounds[AlCp2][Al(ORF)4] (2a) and [AlCp2.2Et2O][Al(ORF)4] (2b).

Table 2. Selected bond lengths /pm for the compounds 1, 2a, 2b and 3.

1 av. [min./max] 3 av. [min./max] 2a av. [min./max] 2b av. [min./max]

d (AlX) 177.7(6) 176.5(4) 179.5

d (AlCCp) 215.5(4) [213.7(5)/217(6)] 214.3(4) [210.5(5)/216.1(4)] 215.8(4) [212.8(4)/216.3(4)] 199.1(3) [199.6(3)/198.6(3)]d (CpcCpc) 142.9(4) [141(4)/144(4)] 142.8(4) [142.3(3)/142.6(5)] 141.8(4) [140.6(3)/143.1(5)] 140.5(3) [134.9(2)/146(2)]

X denotes the center of the Cp-rings.

www.zaac.wiley-vch.de © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Z. Anorg. Allg. Chem. 2009, 178717931788

Table 1. Selected crystallographic data of 2a und 2b.

2a 2b

Molecular formula C26H10Al2F36O4 C34H40Al2F36O6

T /K 150(2) 150(2)M r /g · mol1 1124.26 1272.54Crystal color colorless colorlessCrystal system monoclinic monoclinicSpace group C 2/c P21/na /A 19.172(4) 29.964(4)b /A 19.540(4) 9.5526(19)c /A 19.405(4) 24.831(5) /° 90 90

β /° 98.02(3) 91.05(3)γ /° 90 90V /A 3 7401(3) 4734.7(16)Z 8 4

ρcalcd. /g · cm3 2.023 1.785 µ /mm1 0.299 0.246F (000) 4396 2528hkl -area 24 h 23, 22 h 22,

23 k 23, 10 k 10,0 l 23 27 l 27

2θ limits /° 1.7325.89 4.0823.25Measured refl. 13079 24105Independent refl. 7120 6755Rint 0.0581 0.0529Observed refl. [F > 4σ(F )] 4688 4796No. Parameter / Restraints 663/36 1034/211Scan method Φ ΦGooF 1.045 1.030R1 / wR2 [F > 4σ(F )] 0.0475/0.151 0.0992/0.2377R1 / wR2 (all data) 0.0839/0.1276 0.1291/0.2587Max/min peaks /e · A3 0.279/0.386 0.646/0.469

in 2a is 0.9 pm (2.4 pm) longer than those in 1 (3). Thismay be explained by a slight movement of the CH3-groups

of the Cp (Cp*)-rings in 1 (3) of about 1.8° (2.0°) out of

the Cp-plane [1, 2]. It is in agreement with the more electron

rich nature of the latter two ligands that are better electron

donors, which leads to a closer contact between aluminum

and the substituted Cp rings. Accordingly, the average

AlCcp distance in AlCp2 is with 215.8 pm in the same

range as those observed in [AlCp*2] (215.5 pm) and

[AlCp2] (214.3 pm). The CC distance in the Cp-rings is

141.8 pm ([AlCp*2]: 142.9 pm, [AlCp2]: 142.8 pm).

Selected bond lengths of 1, 2a, 2b and 3 are given in

Table 2.

In 2b the Cp rings are no longer η5- bonded. Because of the presence of two diethyl ether molecules in solution

[from the proton source H(OEt2)2], two of them are coor-




dinated to the aluminocenium cation, the Cp ligands are

now σ-bonded (η1). Compound 2b crystallizes in the mono-

clinic space group P21/n. A representation of the cation is

given in Figure 2. The central aluminum atom is tetra-

hedrally surrounded by two Cp and two diethyl ether mol-

ecules. The average AlCCp distance is 199.1 pm and the

distances to the oxygen atoms of the ether molecules are

186.2 pm on average.

Figure 2. Representation of the AlCp2-cation: a) Sandwich com-

plex with η5 bonded Cp-rings (2a). b) staggered conformation of the Cp-ligands (view along the XAlX axis) in 2a. c) Projectionof the AlCp2

ion with two coordinating molecules of diethylether (2b).

Equilibrium between [AlCp2 ] (2a) and [AlCp2 ·2Et 2O]

(2b) in Solution

Although the compounds 2a and 2b could be charac-

terized by crystal structure analysis, it was difficult to ob-

tain NMR spectra of the different moieties in solution (1H,13C, 27Al). Thus, only a signal at δ 35 ppm for the central

aluminum atom of the anion [Al(ORF)4] could be detected

in the 27Al-NMR spectrum of the reaction mixture at differ-

ent temperatures. Although other authors described signals

for [AlCp2] [4], we could not observe any signals for the

cationic aluminum atoms in 2a and 2b. In the 1H-NMR

spectrum of 2b, signals for the protons of the two coordin-

ating ether molecules [ δ 1.45(t), 4.13 (q)] and the C5H5-

rings [ δ 6,17(s)] could be detected. However, we did not

find any signal for the protons of the η5-C5H5-rings in 2a,

which was expected at δ 7.05 [4]. Along these signals, we

observed some weak signals in the area of coordinated ethermolecules [ δ 4.28(q), 4.33(q) and δ 1.50(t) 1.51(t)] and

at δ 6.36, 6.38, 6.40 and 6.41, which were assigned to the

protons of the Cp-rings. To complement the NMR spectro-

scopic results, we investigated the equilibrium between 2a

and 2b in the gas phase [Equation (2)] with the help of

quantum chemical calculations.

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Z. Anorg. Allg. Chem. 2009, 17871793 © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.zaac.wiley-vch.de 1789

Quantum Chemical Calculations on the Relative Stability of

[AlCp2 ] (2a) and [AlCp2 ·2Et 2O] (2b)

The crystalline compounds 2a and 2b were obtained un-

der similar conditions; moreover, the NMR spectra indi-

cated the presence of intermediate ether solvates. Therefore,

we assumed that equilibrium exists in solution with com-

parable concentrations of 2a and 2b. To assure this con-clusion, quantum chemical calculations were performed

[1519]. These calculations result in a ∆rH 0 gain of

97.2 kJ · mol1 (0 K) by the coordination of two diethyl

ether molecules. The change of the Gibbs energy ∆rG 0(gas)at 248 and 298 K was additionally calculated to estimate

the position of the equilibrium (Table 2). Accordingly the

position of equilibrium [Equation (2)] is slightly on the side

of 2b (∆rG 0(gas) 10.7 kJ · mol1) at 248 K and on the

side of 2a at 298 K (∆rG 0(gas) 3.8 kJ · mol1). The calcu-

lated values of the Gibbs energy and the estimated equilib-

rium position suggested that signals of the protons of 2a

should be visible in the 1H-NMR spectra. However, the

measured NMR spectra showed that the cations in 2a and

2b are only the extremes of an equilibrium that contains

further, unknown entries. Thus, the short lifetime of all

species in this dynamic equilibrium and probably the fluctu-

ation of their structure should play an important role

[2022]. Such processes are serious in 27Al-NMR spec-

troscopy, because the usually broad signals in most cases

could be further broadened until disappearing [23]. The

chemical environment of the central aluminum atoms in the

considered equilibrium changes very fast at the time scale

of NMR spectroscopic measurements. Because of the nu-

merous dynamic processes, which are present in the ex-

tended equilibrium, an extreme broadening of the signals

in the 27Al-NMR spectra results, so that they cannot be

detected anymore. In one borderline case there is a η5 coor-

dination of the Cp ligands (2a), in the other case their hap-

ticity is one and the central aluminum atom is additionally

coordinated by two diethyl ether molecules (2b). The

change from one state to the other should proceed through

intermediates, in which only one or two ether molecules are

coordinated, one of which is more coordinated than the

other. The hapticity in these fluctuating intermediates prob-

ably varies between 5 and 1. These considerations are in

line with the observed signals in the 1H-NMR spectra, in

which no signal is detected for the protons of the undis-

turbed [AlCp2] cation ( δ 7.05). Instead, in addition to

the signals of [AlCp2 · 2Et2O] [ δ 1.45(t), 4.13 (q), 6,17(s)],

we could observe some intermediate signals in the range of Cp ligands [ δ 6.36(s), 6.38(s),6.40(s), 6.41(s)] and in the

range that is typical for coordinating ether molecules [ δ

4.28(q), 4.33(q) and δ 1.50(t) 1.51(t)]. The last mentioned

signals are broadening while rising the temperature. In sum-

mary, these observations suggest dynamic processes in a dy-

namic, temperature dependent equilibrium [Equation (2)].




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

T 248.15 K 88.4 77.7 10.7T 298.15 K 88.6 91.4 3.8

Solid State ∆G latt,1 /kJ · mol1 ∆G latt,2 /kJ · mol1 ∆G solv /kJ · mol1 ∆G (s) /kJ · mol1

T 248.15 K 200.8 181.8 4.5 3.8T 298.15 K 200.8 181.8 9 13.8


experimental X-ray crystal diffraction data, it is possible to

estimate the lattice enthalpy and entropy of the solids at

298 K (see Experimental Section) [2530]. The calculated

Gibbs energies, which were gained by crystallization of 2a

and 2b, are given in Table 3. Thus, the crystallization of 2a

is about 13.8 kJ · mol1 exergonic (∆rG 0(s)) at 298 K com-

pared to 2b (Figure 3) (The equilibrium in the gas phase is

only slightly on the side of 2a at that temperature). There-fore, its crystallization should be preferred. On the other

hand because of the entropy, the crystalline state of 2b is

favored at low temperatures, e.g. 248 K.

Overall, the calculations are in line with the experiments:

crystalline 2a, which is necessary for best polymerization

activity, is favored at higher temperatures and by lower

ether concentration. However, these conditions, especially

the higher temperatures, limit the crystallization of 2a

through its decomposition above 0 °C.

Orientating Investigations for the Isobutene Polymerization

In 1996, Bochmann et al. [4] performed the polymeriz-ation of isobutene (IB) [Equation (3)] with [AlCp2] (coun-

terion: [MeB(C6F5)3]) and detected its high initiating

activity. Compared to [MeB(C6F5)3], the homoleptic

borate [B(C6F5)4] is known to be a better anion and to be

similar in coordination quality to [Al(ORF)4] [7]. Thus, the

interactions between dissolved AlCp2 and the counterion

used here, should be weaker than those employing

[MeB(C6F5)3]. Consequently, it is expected that the alumi-

nocenium cation with [Al(ORF)4] as anion has a higher

activity relative to [AlCp2][MeB(C6F5)3] [4]. Therefore,




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].

Lewis Acid FIA /kJ · mol1 pF

[AlCp2] 773 18.34[Al(Cp)2] 687 16.40[AlCp*2] 686 16.38[AlCp2 · 2Et2O] 311 7.42

expected to be virtually inactive. On the other hand it would

be possible that traces of protonated ether [H(OEt2)2],

used for the synthesis of 2a, induce a too high activity of

the initiator 2a. This scenario is very unlikely, because al-

ready very low concentrations of H(OEt2)2[Al(ORF)4]

are very potent IB polymerization initiators. Thus, 5 mg of

H(OEt2)2[Al(ORF)4] are sufficient to polymerize 12 g

(20 mL) of IB at 60 °C in one second in quantitative yield

[36, 37]. In our case, 2a did not initiate polymerization at

70 and 60 °C. If traces of H(OEt2)2[Al(ORF)4] in 2a

would be the reason for the initiation of IB polymerization,

the polymerization should have also started at these lowertemperatures. Thus, our IB polymerization is due to the ac-

tions of the aluminocenium cation and not induced by

traces of proton sources like H(OEt2)2[Al(ORF)4].

Summary

The reaction of AlCp3 with [H(OEt2)2][Al(ORF)4] re-

sults in Cp protonation and formation of the aluminocen-

ium cation [AlCp2], which is stabilized both in solution

and in the solid state by the weakly coordinating anion

[Al(ORF)4] (RF C(CF3)3). We crystallized two com-

pounds [AlCp2][Al(ORF)4] (2a) and as the main product

the ether stabilized species [AlCp2 · 2Et2O)]

[Al(ORF

)4]

(2b). With the help of quantum chemical calculations we

were able to explain the dynamic and temperature depen-

dent equilibrium in solution between 2a and 2b, in which

intermediates exhibiting fluctuating structures should play

an essential role. Therefore, we could detect only signals of

2b and other intermediates but not those of 2a by NMR

spectroscopy in solution. In order to explain the preferred

crystallization of 2b, BornFajansHaber cycle consider-

ations showed that 2b is slightly favored at low tempera-

tures. Furthermore, these calculations confirmed the exper-

imentally observed difficulties during the crystallization of

2a: in the heterogeneous equilibrium the donor free

[AlCp2]

2a is preferred near room temperature. However,2a decomposes far below this temperature; under the con-

ditions described here as well as under those in the experi-

ments of Bochmann et al [4]. Therefore, it is not surprising

that the crystallization of 2a requires “green fingers” be-

cause its formation is restricted to very sharp experimental

conditions.

Furthermore, we investigated the IB polymerization ac-

tivity of [AlCp2]. The results were compared qualitatively

with those made by Bochmann et al. The higher activity of

“our” aluminocenium cation is most likely caused by the




less coordinating anion [Al(ORF)4] relative to the earlier

used [Me(B(C6F5)3]. Hence, in our experiments the ac-

tivity of the [AlCp2] cation in solution is enhanced due

to the weaker interactions with the counterion. The Lewis

acidities and therefore the activity of 1, 2a, 2b and 4,

with respect to IB polymerization were quantitatively com-

pared for the first time by calculating their FIA values

([AlCp2]

>> [AlCp2]

[AlCp*2]

>>> [AlCp2 · 2Et2O]

).

Experimental Section

All manipulations were carried out under nitrogen with solvents

that were pre-dried by using standard procedures. The reagents

AlCp3 and H(OEt2)2[Al(ORF)4] were synthesized as described in

literature [21, 38].

The NMR spectra were recorded with a Bruker AVANCE 400 in

CD2Cl2 at 30 °C and at room temperature. The chemical shifts

are given in ppm. They refer to C6D5H (δ 7.16, 1H), C6D6 and

the external standards 0.5 [Al(H2O)6]3 (δ 0, 27Al) and Cl3CF

(δ 0, 19F).

Preparation of [AlCp2 ][Al(ORF )4 ] 2a and

[AlCp2 · 2Et 2O][Al(ORF )4 ] (2b)

[H(OEt2)2][Al(ORF)4] (2.14 g, 2.0 mmol) was dissolved in CH2Cl2(10 mL) and afterwards combined at 78 °C with AlCp3 (445 mg,

2.0 mmol) dissolved in CH2Cl2 (5 mL). Thereby the color of the

reaction mixture changed from colorless to slight yellow. After

magnetic stirring for one hour at 78 °C, the reaction mixture was

warmed to 30 °C and it was kept for two hours at this tempera-

ture. Afterwards, the reaction mixture was concentrated and stored

at 28 °C. After several days, colorless crystals of 2a and 2b were

obtained, which were suitable for crystal structure analysis. Crystals

of 2a are preferred in more concentrated solution after removal of

the solvents in vacuo. In the 1H-NMR spectra, only compound 2b

is observed (see Supporting information). Yield (sum of 2a and 2b):1.249 g, 0.99 mmol, 41 %. 1H NMR (400 MHz, 30 °C), CD2Cl2:

δ 1.467 (t, 3J (H,H) 7,1 Hz, 12 H), 4.134 (q, 3J H,H 7,1 Hz,

8 H), 6,17 (s, 10 H). 13C NMR (100 MHz, 30 °C), CD2Cl2: δ

13.92 (s), 70.54 (s), 114.3 (s), 121,54 (q, 1J C,F 291.5 Hz). 19F

NMR (376 MHz, 30 °C), CD2Cl2: δ 75.47 (s). 27Al NMR

(104 MHz, 30 °C), CD2Cl2: δ 35 (s) ppm.

Preparation of the Catalyst (2a)

[H(OEt2)2][Al(ORF)4] (3.21 g, 2.5 mmol) was dissolved in CH2Cl2(15 mL) and afterwards combined at 78 °C with AlCp3 (668 mg,

2.5 mmol) dissolved in CH2Cl2 (10 mL). Thereby the color of the

reaction mixture changed from colorless to slight yellow. After

magnetic stirring for five minutes at

78 °

C, the reaction mixturewas warmed to 30 °C. Afterwards, the solvent was completely

removed within 30 minutes and a colorless powder of 2a was ob-

tained. The catalyst was stored at 78 °C prior to the polymeriz-

ation procedure. 13C NMR (100 MHz, 30 °C), CD2Cl2: δ 13.9

(s), 70.5 (s), 114.3 (s), 121,5 (q, 1J C,F 291.5 Hz). 27Al NMR

(104 MHz, 30 °C), CD2Cl2: δ 35 (s).

Polymerization Procedure

Isobutene was dried with triisobutylaluminum and recondensed

prior to use. All polymerizations were carried out in standard


Schlenk tubes. For the 10 minutes polymerization procedure, isobu-

tene (10 mL) was condensed into the reaction tubes and allowed to

equilibrate at the appropriate temperature. Compound 2a

(50 µmol) was dissolved in CH2Cl2 (5 mL) at 20 °C. After cooling

to the appropriate temperature, this solution was added to the rig-

orously stirred isobutene. The reaction was quenched after 10 mi-

nutes with methanol (0.5 mL). Finally, the solvents were evaporated

in an oil-pump vacuum and the remaining polymer was dried in

vacuo until constant weight. By using a polymerization time of 120minutes, 2a (58 µmol) was dissolved in CH2Cl2 (5 mL) at 20 °C

in a reaction tube. Isobutene (10 mL) was condensed into this solu-

tion and cooled to 78 °C. The reaction mixture was warmed to

the appropriate temperature, stirred for 2 hours at this temperature,

and then quenched with methanol (0.5 mL). The obtained polymer

was dried as described above.

Polymer Characterization

Number- (M n) and weight-average (M w) molecular weights and po-

lydispersities (M w/M n) were determined by gel permeation chroma-

tography (GPC) vs. polystyrene standards. The GPC measurements

were carried out at 30 °C in chloroform with PSS-SDV columns

(8.0 mm 30 mm, 5 µm particles, 103, 104, 105, 106 A pore size).For detection a refractive index detector was used.

Crystallographic Analysis for 2a and 2b

Crystals of 2a and 2b were mounted on a glass fiber in silicone oil

at 123 °C. The data were collected with a STOE IPDS two-circle

diffractometer using graphite-monochromated Mo-K α radiation

and a STOE IPDS area detector. The data were corrected for ab-

sorption by the STOE IPDS software. Lorentz polarization and

absorption corrections were applied. The structural solution (calcu-

lated by direct methods) and refinement (on F 2, hydrogen atoms

calculated) were carried out using the SHELX97 program suite.

Crystallographic data (excluding structure factors) for the struc-tures reported in this paper have been deposited with the Cam-

bridge Crystallographic Data Center as supplementary publication

no. CCDC-637292 (2a) and CCDC-688755 (2b). Copies of the data

can be obtained free of charge on application to CCDC, 12 Union

Road, Cambridge CB2 1EZ, UK [Fax: -44-1223-336-033; E-Mail:

[email protected]].

Quantum Chemical Calculations

All DFT calculations were carried out with the TURBOMOLE

program package [1519]. For all compounds the BP86 functional

with SV(P) basis sets were used. Vibrational frequencies were calcu-

lated with AOFORCE at the BP86/SV(P) level and used to verify

the nature of the obtained minima. On the basis of the calculatedfrequencies, thermal corrections to the enthalpy and Gibbs free en-

ergy have been calculated with the FREEH module implemented in

TURBOMOLE. Calculation of solvation energies (solvent CH2Cl2with ε 11.16 at T 30 °C) were performed as single-point

DFT calculations (BP86/SV(P)) with the COSMO [39] module.

The Gibbs lattice energies for the BornHaberFajans cycle have

been calculated using a molecular volume based modified Kapus-

tinskii equation as introduced by Jenkins and Glasser [25, 2731].

Similarly, the solid state entropy was calculated according to

Jenkins and Glasser [26].




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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In this case shape lines with a small full width half-maximumare detected.

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[29] H. D. B. Jenkins, J. F. Liebman, Inorg. Chem. 2005, 44, 6359.[30] H. D. B. Jenkins, J. Chem. Edc. 2005, 82, 950.[31] The standard formation enthalpies and standard entropies

were taken from the website of the National Institute forStandards and Technology http://www.webbook.nist.gov/chemistry/.

[32] At 30° the reaction could only be carried out when the reac-tion mixture was slowly warmed from 78 °C to the reactiontemperature.

[33] Results of the polymerizations experiments at 70 °C: t

10 min: yield /g: 0.05 (0.30), Mw /104 g · mol1: 17 (18), PDI:1.8 (2.8), t 120 min: yield /g: 0.47, Mw /104 g · mol1: 16,PDI: 1.9.

[34] With these calculations we wanted to show that only [AlCp 2]

and not [AlCp2 · 2Et2O] reacts as an initiator for the isobutenepolymerization, as described in this paper.

[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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Part 1, 1993, 799; b) A. Schäfer, A. Klamt, D. Sattel, J. C. W.Lohrenz, F. Eckert, Phys. Chem. Chem. Phys. 2000, 2, 2187.

Received: December 11, 2008Published Online: February 13, 2009

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AlCp2](+)Structure, Properties and Isobutene Polymerization