Cationic Dialkyl Metal Compounds of Group 13 Elements (E = Al, Ga

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Cationic Dialkyl Metal Compounds of Group 13 Elements (E = Al, Ga, In) Stabilized by the Weakly Coordinating Dianion [B12Cl12]2
Mathias Kessler, Carsten Knapp,* and Ardiana Zogaj Institut f€ur Anorganische und Analytische Chemie, Albert-Ludwigs-Universit€at Freiburg, Albertstrasse 21, 79104 Freiburg i. Br., Germany

bS Supporting Information ABSTRACT: The reactions of the trityl salt of the weakly coordinating dianion [B12Cl12]2
([Ph3C]2[B12Cl12]) with Et3Al, Et3Ga 3 (OEt2) and Et3In in 1,2-difluorobenzene yielded (Et2Al)2B12Cl12, [Et2Ga(OEt2)2]2[B12Cl12], and (Et2In)2B12Cl12. The products were characterized by NMR (1H, 11B, 13C), IR, and Raman spectroscopy. Investigation of the symmetric carbon
metal stretching vibration (Raman) of the [Et2E]+ unit in (Et2E)2B12Cl12 (E = Al, In) compounds indicated a linear structure for E = In and a bent structure for E = Al. The latter was confirmed by a crystal structure determination of (Et2Al)2B12Cl12. While the reaction of the triethyl compounds Et3E (E = Al, In) and Et3Ga 3 (OEt2) proceeded via βhydride abstraction and release of ethylene, Me3Al reacts with [Ph3C]2[B12Cl12] under methide transfer. The gaseous byproduct ethene was identified by IR spectroscopy, and solid byproducts (Ph3CH or Ph3CMe) were observed by NMR spectroscopy in solution. The formation of (Me2Al)2B12Cl12 was proven by X-ray diffraction and NMR spectroscopy. In the crystal structures of (Me2Al)2B12Cl12 and (Et2Al)2B12Cl12 the aluminum atoms are bound to two chlorine atoms, resulting in a distorted tetrahedral environment around aluminum. The aluminum
chlorine contacts are longer than a typical Al
Cl single bond but significantly shorter than the sum of the van der Waals radii. The bonding in both compounds can be described as ion-like. The underlying thermodynamics for β-hydride abstraction and methide transfer were investigated in the gas phase by DFT calculations, in 1,2difluorobenzene solution by applying the COSMO solvation model, and in the solid state by Born
Haber
Fajans cycles using a volume-based approach to estimate lattice enthalpies. These estimations show that the reactions are unfavorable in the gas phase but become favorable when solvation and lattice energies are taken into account.

1. INTRODUCTION Only few examples of cationic dialkyl metal compounds of group 13 elements (E = Al, Ga, In) are known without a stabilizing donor ligand.1,2 Donor-free, very electrophilic [R2E]+ cations would have unique properties, and their bonding situations are of fundamental interest. Cationic compounds that approximate these properties in the solid state or solution are important as catalysts or co-catalysts for polymerization and have been applied in C(sp3)
F bond activation and alkylative defluorination.3 Trityl ([CPh3]+, triphenylcarbenium) salts of weakly coordinating anions are known to be good starting materials for either the β-hydride elimination or the alkide abstraction from organometallic molecules, and at the same time they introduce a weakly coordinating anion, which is able to stabilize the electrophilic cation formed.4 For instance, cationic trialkyl compounds of silicon and germanium were generated using this methodology.5,6 However, in these compounds the cation is associated with the weakly coordinating anion. Thus, the term ionic is rather misleading, and therefore the term ion-like was suggested to describe this type of weakly associated ions.6
8 r 2011 American Chemical Society

Consequently, reactions of these trityl salts with trialkyl compounds of group 13 elements should give the desired dialkyl metal moieties. Two-coordinate thallium and indium cations are well known, while the gallium and aluminum compounds were described only recently.1 The persistence of the cations strongly depends on the stability of the counteranion against electrophilic attack by the generated very reactive cation. Bochmann et al. showed that using tetrakis(pentafluorophenyl)borate [B(C6F5)4]
as an anion in the reaction with AlR3 led to only a transient dialkyl aluminum cation followed by degradation of the anion due to abstraction of C6F5
.9 In 2002 Reed et al. prepared (Et2Al)(HCB11H5X6) (X = Cl, Br) using a robust, partly halogenated carborate anion and reported the first structural characterization of a donor-free ion-like dialkyl aluminum compound.8,10 Previously, cationic dialkyl aluminum compounds were stabilized by mono- or bidentate neutral ligands resulting in a tetrahedral geometry around the metal center and Received: April 19, 2011 Published: June 22, 2011 3786

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Organometallics

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Table 1. 1H and 13C Chemical Shifts and Element
Carbon Stretching Frequencies in (R2E)2B12Cl12 (R = Me, Et; E = Al, In), [Et2Ga(OEt2)2]2[B12Cl12], and ER3

a

δ(1H) NMR (rt, C6D6)

δ (13C) NMR

Raman νs

IR νas

CH2(q)/CH3(t)

(rt, C6D6) CH2/CH3

(E
C) [cm
1]

(E
C) [cm
1]

(Et2In)2B12Cl12a Et3In

1.01/1.30 0.57/1.44

14.3/9.9 6.3/11.8 b

[Et2Ga(OEt2)2]2[B12Cl12]a

0.67/1.03

4.6/7.1

527

584

Et3Ga 3 (OEt2)

0.55/1.34

4.7/10.4

492

535c

(Et2Al)2B12Cl12

0.27/0.82

4.8/7.1

535

678

Et3Al

0.30/1.10

0.0/8.5

430/560d,29

64930

53030

68930

(Me2Al)2B12Cl12


0.44 (s)


4.5

Me3Al


0.35 (s)


7.4

448 44728

687

NMR (C6D6/1,2-F2C6H4). b 13C NMR (d8-THF). c Raman Et3Ga 3 (OEt2). d Al2Et6.

separated cations and anions in the solid state.1 The reaction of [CPh3][1-Me-CB11F11] with R3Al (R = Me, Et) led to similar results. A crystal structure shows the aluminum atom to be connected to two fluorine atoms of two different anions, leading to a dimeric structure in the solid state.11 Only very recently Wehmschulte et al. reported bulky cationic arylethylaluminum compounds, which were stabilized by the [HCB11H5Cl6]
and [HCB11H5I6]
carborate anions.12 They were synthesized by either β-hydride abstraction by the trityl cation and elimination of ethene or ethide transfer to trialkyl silylium compounds. Almost linear diaryl metal cations of gallium and aluminum were reported using different weakly coordinating anions and very bulky substituents that protect the metal center.13
15 The perchlorinated closo-dodecaborate [B12Cl12]2
was shown to be a chemically very robust weakly coordinating dianion,16
22 for which improved syntheses have been reported recently.16,23 The successful syntheses of trialkyl silylium compounds (R3Si)2B12Cl12 and of the methylating agent Me2B12Cl12 illustrate its potential to stabilize electrophilic cations.18,19 Thus, the perchlorinated closo-dodecaborate [B12Cl12]2
anion is a promising candidate for the stabilization of group 13 dialkyl metal ion-like compounds. The reactions of [CPh3]2[B12Cl12]16 with Et3M (M = In, Al), Et3Ga 3 (OEt2), and Me3Al will be described, and the crystal structures of (R2Al)2B12Cl12 (R = Et, Me) are reported.

2. RESULTS AND DISCUSSION 2.1. Synthesis of (Et2E)2B12Cl12 (E = Al, In), [Et2Ga (OEt2)2]2[B12Cl12], and (Me2Al)2B12Cl12. In previous work we

showed that the reaction of the trityl salt [Ph3C]2[B12Cl12] with trialkylsilanes R3SiH gave the corresponding silylium compounds (R3Si)2B12Cl12 (R = Me, Et, iPr).18 Transfer of this reaction from group 14 to group 13 compounds should give similar results for reactions of trialkyl metal compounds of group 13. Therefore in an analogous manner [Ph3C]2[B12Cl12] was treated with an excess of Et3M (M = Al, In), Et3Ga 3 (OEt2), and Me3Al in 1,2-difluorobenzene (eqs 1
3). The excess was used to obtain complete conversion in tolerable reaction times. Similar reaction conditions were used to prepare the corresponding silylium compounds.18 1;2
difluorobenzene, rt

2Et3 E + ½CPh3 2 ½B12 Cl12  s f ðEt2 EÞ2 B12 Cl12 E ¼ Al, In

+ 2HCPh3 + 2C2 H4

ð1Þ

1;2
difluorobenzene, rt

4Et3 Ga 3 ðOEt2 Þ + ½CPh3 2 ½B12 Cl12 s f½Et2 GaðOEt2 Þ2 2 ½B12 Cl12 

+ 2HCPh3 + 2C2 H4 + 2Et3 Ga

ð2Þ

1;2
difluorobenzene, 80 °C

2Me3 Al + ½CPh3 2 ½B12 Cl12 s fðMe2 AlÞ2 B12 Cl12

+ 2MeCPh3 1

ð3Þ

13

Table 1 summarizes the H and C chemical shifts and the element
carbon stretching frequencies of the reaction products (R2E)2B12Cl12 (R = Me, Et; E = Al, In) and [Et2Ga(OEt2)2]2[B12Cl12] and the corresponding R3E starting materials. The IR spectra of all products show the expected bands for the C
H and C
C vibrations as well as the characteristic signals for the [B12Cl12]2
dianion.16 The symmetric E
C stretching frequencies (νs) were identified in the Raman spectra, and the asymmetric E
C stretching vibrations (νas) were observed mostly by IR spectroscopy. The symmetric and the asymmetric stretching vibrations in a three-atomic unit are coupled, and thus the difference between νs and νas (Δν in cm
1) for a C
E
C framework depends on the C
E
C angle. The coupling is zero for an orthogonal unit (Δν = 0) and increases with increasing C
E
C angle (νs < νas). The asymmetric E
C stretching vibration is shifted to higher and the symmetric E
C stretching vibration to lower wave numbers, respectively.24 The dependency of Δν on the C
In
C angle was previously investigated by Hausen et al.25 The reaction of Et3In with [Ph3C]2[B12Cl12] leads to (Et2In)2B12Cl12 (see eq 1), which shows proton NMR signals for the ethyl groups of the [Et2In]+ cation at 1.01 (q) and 1.30 (t) ppm (see Figure S10). An 115In NMR signal could not be observed due to the fast quadrupolar relaxation of the indium nucleus (I(115In) = 9/2) in these molecules. In contrast to the lighter homologues, dialkyl indium cations can exist in a basefree, nearly linear form.2,26 Already three decades ago the crystal structure of [Me2In]+Cl
containing an almost linear [Me2In]+ cation (C
In
C = 167.3°) was determined. The symmetric In
C stretching vibration in (Et2In)2B12Cl12 (νs(In
C) = 448 cm
1 (Raman), see Figure S12) appears at a lower frequency than in [Me2In]+Cl
(νs(In
C) = 500 cm
1 (Raman))27 and is very close to the calculated stretching frequency for linear [Et2In]+ in the gas phase (νs(In
C) = 450 cm
1, νas(In
C) = 529 cm
1). These results indicate the presence of a linear [Et2In]+ cation in (Et2In)2B12Cl12. The reaction of [Ph3C]2[B12Cl12] with Et3Ga 3 (OEt2) leads to [Et2Ga(OEt2)2]2[B12Cl12] (see eq 2). As evident from NMR spectroscopy, two ether molecules are bound to the gallium 3787

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Organometallics atom, likely forming an overall tetrahedral geometry. Proton NMR signals are observed at 0.67 (q) and 1.03 (t) originating from the ethyl groups bound to gallium and at 1.14 (t, Et2O) and 3.75 (q, Et2O) ppm for the coordinated ether molecules. The intensity ratio between the signals of 2:3:6:4 is in accord with two ethyl groups and two ether molecules bound to the gallium atom. The additional ether molecules originate from the excess of Et3Ga 3 (OEt2) used for this reaction and fill the free coordination sites on the electron-deficient cationic gallium center. A 71Ga NMR signal (I(71Ga) = 3/2) could not be observed owing to the aforementioned reasons. The Raman spectrum shows the symmetric Ga
C stretching vibration at 527 cm
1 (see Figure S9), and the asymmetric Ga
C stretching vibration is observed at 584 cm
1 (IR, see Figure S14). The stretching frequencies are shifted to higher wave numbers, but Δν remains unchanged compared to the starting material Et3Ga 3 (OEt2), in accord with a similar tetrahedral coordination sphere around Ga. The reaction of [Ph3C]2[B12Cl12] with Et3Al yields (Et2Al)2B12Cl12 according to eq 1. The 1H NMR spectrum (see Figure S1) shows signals for the ethyl groups at 0.27 (q) and 0.82 (t) ppm, in agreement with those observed for the ethyl groups in (Et2Al)(HCB11H5Br6) (0.27 (q), 0.87 (t), d6benzene).8 Furthermore the proton NMR spectrum of (Me2Al)2B12Cl12 (see eq 3) shows a signal at
0.44 (s) ppm, and the 13C NMR spectrum a chemical shift of
4.5 ppm for the methyl groups (see Figure S17). These results are significantly altered for the reactant Me3Al (δ(1H) =
0.35/δ(13C) =
7.4, d6-benzene). An 27Al NMR signal (I(27Al) = 5/2) could not be observed for both products, although it is present in samples of the starting materials Et3Al and Me3Al. This might be a result of significantly lower concentrations of (R2Al)2B12Cl12 due to low solubility compared to the samples of pure R3Al in C6D6. The symmetric Al
C stretching frequency for (Et2Al)2B12Cl12 appears at 535 cm
1 in the Raman spectrum (see Figure S4, calcd [Et2Al]+ νs(Al
C) = 477 cm
1) and the asymmetric Al
C stretching vibration at 678 cm
1 in the IR spectrum. These results are in accord with a bent C
Al
C moiety, which has been confirmed by X-ray diffraction (see Section 2.3). The reaction mechanism of the reaction of [Ph3C]+ with Et3Al and Me3Al, respectively, is completely different. The triethyl compounds Et3E (E = Al, In) and Et3Ga 3 (OEt2) react with the trityl cation under β-hydride abstraction and formation of triphenylmethane and ethene as byproducts, which were identified by gas-phase IR spectroscopy and NMR spectroscopy (see Figures S2 and S6 in the Supporting Information). These results are in analogy with the findings of Reed et al.8 In contrast, trimethylaluminum does not possess β-hydrogen atoms, and a methide transfer to the carbon atom of [Ph3C]+ is occurring instead (see eq 3). The kinetic barrier for this reaction is presumably higher, because heating is required to get the reaction to proceed. The reaction progress on heating is evident from a decoloration of the yellow (due to the presence of the trityl cation) solution. The byproduct MeCPh3 was identified by NMR spectroscopy (see Figure S16). All synthesized products show low solubility in either C6D6 or a mixture of C6D6 and 1,2-F2C6H4. Even coordination of two ether molecules to the cation in the case of [Et2Ga(OEt2)2]2[B12Cl12] does not significantly increase the solubility. The low solubility of these compounds is presumably caused by the double negative charge of the dianion and thus the resulting higher lattice enthalpy of the products.

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Table 2. Calculated (PBE0/def2-TZVPP) Reaction Enthalpies and Free Reaction Energies (298.15 K) in the Gas Phase and in 1,2-Difluorobenzene (εr = 13.38)32 Solution According to eq 4 and Estimated Reaction Enthalpies in the Solid State ΔHR0 (gas phase)

ΔGR0 (gas phase)

ΔGR0 (1,2-difluorobenzene)

ΔHR (solid state)

Et3E

[kJ mol
1]

[kJ mol
1]

[kJ mol
1]

[kJ mol
1]

Et3Al

153

110

49


111

Et3Ga

104

61


35


192

Et3In

33


11


110


235

Table 3. Calculated (PBE0/def2-TZVPP) Reaction Enthalpies and Free Reaction Energies (298.15 K) in the Gas Phase and in 1,2-Difluorobenzene (εr = 13.38)32 Solution According to eq 5 and Estimated Reaction Enthalpies in the Solid State ΔHR0

ΔGR0

ΔGR0

ΔHR

R3E

(gas phase) [kJ mol
1]

(gas phase) [kJ mol
1]

(1,2-difluorobenzene) [kJ mol
1]

(solid state) [kJ mol
1]

Me3Al

106

104

33

Et3Al

84

95

34


180

Et3Ga

35

45


50


261

Et3In


36


26


126


304


229

2.2. Calculated Reaction Enthalpies. To gain some insight into the thermodynamics of the two competing reaction pathways (β-hydride abstraction and release of ethene (eq 4 and Table 2) or alkide transfer (eq 5 and Table 3)), reaction enthalpies (ΔHR) were calculated by DFT methods (PBE0/ def2-TZVPP). Solvation effects were taken into account by applying the COSMO model and using the experimental dielectric constant of 1,2-difluorobenzene.

Et3 E + ½Ph3 C+ f ½Et2 E+ + Ph3 CH + C2 H4 ðE ¼ Al, Ga, InÞ ð4Þ R 3 E + ½Ph3 C+ f ½R 2 E+ + Ph3 CR ðE ¼ Ga, In, R ¼ Et; E ¼ Al, R ¼ Me, EtÞ ð5Þ All reactions become less endothermic declining in the group from Al to In following the trend of the decreasing E
C bond energy. The free reaction energies in the gas phase for both reaction pathways differ by only 15 kJ mol
1. Only for indium are the gas-phase-calculated free energies exergonic. Taking solvation into account all reactions benefit by about 60
100 kJ mol
1, which makes the reactions according to eqs 4 and 5 thermodynamically favorable for gallium and indium but not for aluminum. However, it has been experimentally proven that the reactions proceed in the desired direction for all three group 13 elements. The evolution of the gaseous byproduct ethene certainly drives reaction 4 to the right side, but the methide transfer from Me3Al remains endergonic according to the calculations even if solvation energies are taken into account. Therefore the actual phase of each compound has to be taken into account. Reaction enthalpies accounting for the phase of each compound were estimated by Born
Haber
Fajans cycles (BHC, see S3). For this purpose the ion-like dialkyl metal compounds of group 13 were considered to be purely ionic salts [R2E]2[B12Cl12], and their lattice enthalpies were estimated using the 3788

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volume-based approach.31 The lattice enthalpies of the [R2E]+ salts exceed that of the trityl salt [Ph3C]2[B12Cl12] because of their smaller cation sizes. Therefore the reaction enthalpies are significantly altered to more exothermic values. Even the reactions leading to (Et2Al)2B12Cl12 and (Me2Al)2B12Cl12 are calculated to be exothermic by
111 and
229 kJ mol
1, respectively. Exemplarily, the Born
Haber
Fajans cycle for the reaction leading to [Et2Al]2[B12Cl12] is shown in Figure 1. 2.3. Crystal Structures of (Et2Al)2B12Cl12 and (Me2Al)2B12Cl12. Colorless crystals suitable for X-ray diffraction were obtained from 1,2-difluorobenzene solution by slow removal of the solvent by applying a temperature gradient by cooling the other leg of the vessel. The crystal structure of (Et2Al)2B12Cl12 is depicted in Figure 2, showing an ion-like structure with an aluminum atom bound to two chlorine atoms with aluminum
chlorine distances of 247.9(3) and 249.0(3) pm, respectively. These values are considerably smaller than the sum of the van der Waals radii (∑(Al,Cl) = 359 pm)33 but longer than the bond distance in gaseous monomeric AlCl3 (205 pm).34 They are comparable to the Al
Cl bond distances in (Et2Al)(HCB11H5Cl6) (244 and 243 pm).8 The aluminum atom is bridging two chlorine atoms (Cl1 and Cl2), resulting in a distorted tetrahedral structure. The C
Al
C angle of 124.3(12)° is in between that for an ideal tetrahedral (109.5°) and a linear structure (180°) and smaller than the C
Al
C angle found in (Et2Al)(HCB11H5Cl6) (136.6°).8 The second aluminum atom is bound in the same manner on the reverse side (to Cl9 and Cl12; for the numbering scheme see Figure S19). The B
Cl bonds of chlorine

atoms bound to the aluminum (180.1(4) and 182.4(6) pm) are elongated (av B
Cl 177.6 pm). Altogether the bonding situation is related to that in (Et3Si)2B12Cl1218 and Me2B12Cl1219 with the distinction of two chlorine interactions to the electrophilic aluminum atom to complete its tetrahedral coordination sphere. In comparison with (Et2Al)2B12Cl12 the structural characteristics of (Me2Al)2B12Cl12 are almost identical. Its crystal structure is shown in Figure 3, and unlike (Me2Al)(1-Me-CB11F11), it does not form a dimeric11 but a monomeric structure. The Al
Cl bond distances (245.74(5) and 239.61(5) pm) are shorter than those in the diethylaluminum moiety, and the C
Al
C angle of 133.84(7)° is widened. The Al
Cl distances are significantly elongated in comparison with dimeric (Me2AlCl)2 (230.3 pm), and the Al
C distances (191.67(15) and 191.97(14) pm) are slightly shorter than in (Me2AlCl)2 (193.5 pm).35 The B
Cl distances of the chlorine atoms bound to aluminum (181.74(14) and 182.05(13) pm) are also elongated (av B
Cl 177.9 pm). Averaged Brown bond valences for the Al
Cl bonds of 0.30 and 0.35 v.u. for (Et2Al)2B12Cl12 and (Me2Al)2B12Cl12, respectively, indicate only weak Al
Cl contacts and thus a separation of the [R2E]+ cations from the dianion [B12Cl12]2
. Therefore the Al
Cl interaction should be described as ion-like. This is reminiscent of a bonding analysis for (Et2Al)(HCB11H5Cl6).10 Selected bond valences and bond lengths are listed in Table 4 in comparison with (Et2Al)(HCB11H5Cl6),8 (Me2Al)(1-MeCB11F11),11 [Dipp*AlEt][HCB11H5Cl6] (Dipp* = 2,6-(2,6-iPr2C6H3)2C6H3
)),12 [DcpAlEt][HCB11H5Cl6] (Dcp = 2,6-(2,6-Cl2C6H3)2C6H3
)),12 and (Me2AlCl)2.35

Figure 1. Born
Haber
Fajans cycle for the formation [Et2Al]2[B12Cl12] in the condensed phase (295.15 K).

3. CONCLUSION AND OUTLOOK In conclusion, we have shown that cationic dialkyl metal compounds of group 13 elements (E = Al, Ga, In) can be stabilized by the weakly coordinating perchlorinated dianion [B12Cl12]2
. [Ph3C]2[B12Cl12] reacts with Et3E (E = Al, In), Et3Ga 3 (OEt2), or Me3Al under β-hydride abstraction or methide transfer, respectively, to the corresponding (R2E)2B12Cl12 (E = Al, In; R = Me, Et) compounds and to [Et2Ga(OEt2)2]2[B12Cl12]. The crystal structures of (Me2Al)2B12Cl12 and (Et2Al)2B12Cl12 show very similar features regarding the cation
anion interaction compared to those previously reported for the related halogenated

of

Figure 2. Part of the crystal structure of (Et2Al)2B12Cl12. Selected bond lengths [pm] and angles [deg]: Al1
Cl1 247.9(3), Al1
Cl2 249.0(3), Al1
C1 192(2), Al1
C3 191.5(11), C1
C2 156.2(15), C3
C4 154.5(14), B1
Cl1 180.1(4), B2
Cl2 182.4(6), B3
Cl3 177.7(4), B4
Cl4 177.5(6), Cl1
Al1
Cl2 91.23(10), C1
Al1
C3 124.3(12). Thermal ellipsoids are shown at the 50% probability level, and the hydrogen atoms are shown as spheres of arbitrary size. Selected bond lengths [pm] and Brown bond valences (italics) are given within the picture. 3789

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Figure 3. Part of the crystal structure of (Me2Al)2B12Cl12. Selected bond lengths [pm] and angles [deg]: Al1
Cl1 245.74(5), Al1
Cl2 239.61(5), Al1
C1 191.67(15), Al1
C2 191.97(14), B1
Cl1 181.74(14), B2
Cl2 182.05(13), B3
Cl3 177.73(13), B4
Cl4 177.80(14), B5
Cl5 178.59(12), B6
Cl6 177.46(14), Cl1
Al1
Cl2 92.802(17), C1
Al1
C2 133.84(7). Thermal ellipsoids are shown at the 50% probability level, and the hydrogen atoms are shown as spheres of arbitrary size. Selected bond lengths [pm] and Brown bond valences (italics) are given within the picture.

Table 4. Brown Bond Valences,36 Selected Bond Lengths, and C
Al
C Angles of (Et2Al)2B12Cl12 (1), (Me2Al)2B12Cl12 (2), (Et2Al)(HCB11H5Cl6) (3),8 (Me2Al)(1-Me-CB11F11) (4),11 [Dipp*AlEt][HCB11H5Cl6] (Dipp* = 2,6-(2,6-i-Pr2C6H3)C6H3
)) (5),12 [DcpAlEt][HCB11H5Cl6] (Dcp = 2,6-(2,6-Cl2C6H3)C6H3
)) (6),12 and (Me2AlCl)2 (7)35 1

411

512

7a 35

612

[pm]

v.u.

[pm]

v.u.

[pm]

v.u.

[pm]

v.u.

[pm]

v.u.

[pm]

v.u.

[pm]

v.u.

247.9

0.30

245.7

0.32

244.0

0.33

196.4

0.32

239.5

0.38

249.6

0.29

230.3

0.48

249.0

0.29

239.6

0.37

243.0

0.34

196.9

0.32

241.5

0.36

239.7

0.37

180.1

0.87

181.7

0.83

183.1

0.80

142.6

0.68

183.2

0.80

182.3

0.82

182.4

0.83

182.1

0.83

182.8

0.81

142.0

0.69

182.9

0.81

183.4

0.79

Al
C1

192.0

0.97

191.7

0.95

193.4

0.91

192.2

0.94

194.8

0.88

194.6

0.88

193.5

0.90

Al
C2/C3 C1
C2

191.5 156.2

0.96 0.99

192.0

0.95

192.1 150.2

0.94 1.17

192.8

0.93 153.9

1.06

149.9

1.18

C3
C4

154.5

1.03

146.0

1.31

C
Al
C [deg]

124.3

b

Al
X1/X2

B
X1/X2b

a

38

2

133.8

136.6

147.6

132.9

139.3

126.9

Gas-phase electron diffraction data. b X = Cl, F.

carborates.8,11 Vibrational data for (Et2In)2B12Cl12 suggest the presence of a linear [Et2In]+ cation, while the aluminum moiety should be bent, which was confirmed by crystal structure determinations. Very recently, silylium compounds of the [B12Cl12]2
anion have been shown to be active catalysts for C
F activation.23 The weakly coordinating perchlorinated dianion [B12Cl12]2
is easily accessible and possesses similar weakly coordinating properties to the halogenated carborates.16,18,20 Preliminary investigations showed (Et2Al)2B12Cl12 to be a very active catalyst for cationic isobutene polymerization.37 Thus, it can be anticipated that the compounds reported herein are also active catalysts for C
F activation or other polymerization reactions, which may be proven in the future.

4. EXPERIMENTAL SECTION General Remarks. Air- and moisture-sensitive solid reagents were manipulated using standard vacuum and Schlenk techniques or in a glovebox with an atmosphere of dry argon (H2O and O2 < 1 ppm). The

reactions were carried out in H-shaped glass vessels with J. Young Teflon in glass valves and an incorporated fine frit. The solvents 1,2-difluorobenzene (Fluorochem, 98%) and benzene-d6 (Deutero) were dried over CaH2 (Merck) and distilled prior to use. n-Pentane was purified using a Grubbs solvent purification system. Me3Al (2.0 M in hexane, Aldrich) and Et3Al (ABCR, 93%) are commercially available and were used as received. [Ph3C]2[B12Cl12],16 Et3In,38 and Et3Ga 3 (OEt)39 were prepared by published procedures. IR spectra were recorded on a Nicolet Magna 760 IR spectrometer equipped with a diamond or ZnSe ATR attachment. Measurements using the ATR attachments did not exclude moisture and air completely due to technical reasons. Gas-phase IR spectra were measured using a 10 cm IR gas cell equipped with KBr windows. FT-Raman spectra were recorded in flame-sealed capillaries on a Bruker Vertex 70 IR spectrometer equipped with a Bruker RAM II Raman module using a highly sensitive Ge detector and a Nd:YAG laser (1064 nm). 1H, 11B, and 13C spectra were measured on a Bruker Avance II WB 400 MHz spectrometer in 5 mm NMR tubes at room temperature with a 5 mm BBFO probehead. Chemical shifts are given with respect to Me4Si (1H, 13C) and BF3 3 OEt2 (11B). Two-dimensional HSQC and 3790

dx.doi.org/10.1021/om2003333 |Organometallics 2011, 30, 3786–3792

Organometallics HMBC experiments were carried out to obtain 13C NMR chemical shifts and JC
H coupling constants. All actual spectra are included with the Supporting Information. Safety Note. Trialkyl metal compounds of group 13 are pyrophoric and have to be handled under exclusion of air and moisture. They react very violently with water, which should be avoided in any case. (Me2Al)2B12Cl12. Me3Al (0.8 mL, 2 M in hexane, 10 equiv) was added to [Ph3C]2[B12Cl12] (0.16 g, 0.16 mmol) dissolved in 10 mL of 1,2F2C6H4. The reaction mixture was stirred for 24 h at rt and heated to 80 °C for another 16 h. The color of the reaction mixture changed from yellow to colorless during heating. After addition of 10 mL of pentane a white precipitate formed, which was separated by filtration, washed several times with pentane (3  5 mL), and dried under vacuum. (Me2Al)2B12Cl12 remained as a white solid (0.08 g, 0.10 mmol, 68%). 1 H NMR (400.17 MHz, d6-benzene, 298 K): δ
0.44 (s, 12H, CH3). 11 B NMR (128.39 MHz, d6-benzene, 298 K): δ
11.6. 13C NMR (100.63 MHz, d6-benzene, 298 K): δ
4.5 (CH3, 1JC
H = 120 Hz). IR (diamond, cm
1): ν 3470 (br, H2O), 1620 (br, H2O), 1506 (w), 1389 (w), 1197 (m), 1030(vs) [B12Cl12]2
, 742 (s), 687 (s) (νas(Al
C)), 583 (m), 531 (vs) [B12Cl12]2
. Raman (cm
1): ν 303 (100) [B12Cl12]2
, 126 (50) [B12Cl12]2
. (Et2Al)2B12Cl12. Et3Al (0.16 g, 1.40 mmol, 10 equiv) was added to [Ph3C]2[B12Cl12] (0.15 g, 0.14 mmol) dissolved in 10 mL of 1,2F2C6H4. The reaction mixture was stirred for 1.5 h at rt. The color of the reaction mixture changed from yellow to colorless. An IR spectrum of the gaseous byproduct confirmed the formation of C2H4. After addition of 10 mL of pentane a white precipitate formed, which was separated by filtration, washed several times with pentane (3  5 mL), and dried under vacuum. (Et2Al)2B12Cl12 remained as a white solid (0.05 g, 0.08 mmol, 56%). 1H NMR (400.17 MHz, d6-benzene, 298 K): δ 0.27 (q, 8H, CH2), 0.82 (t, 12H, CH3). 11B NMR (128.39 MHz, d6-benzene, 298 K): δ
11.6. 13C NMR (100.63 MHz, d6-benzene, 298 K): δ 4.8 (CH2, 1JC
H = 127 Hz), 7.1 (CH3, 1JC
H = 120 Hz). IR (ZnSe, cm
1): ν 2954 (m), 2873 (m), 1452 (w), 1400 (w), 1032 (vs) [B12Cl12]2
, 977 (sh), 678 (s) (νas(Al
C)). Raman (cm
1): ν 2955 (10), 2936 (20), 2919 (10), 2892 (10), 2874 (20), 2862 (30), 2854 (5), 2745 (5), 1455 (10), 1401 (5), 1199 (10), 680 (5), 535 (15) (νs(Al
C)), 301 (100) [B12Cl12]2
, 225 (10), 124 (40) [B12Cl12]2
. [Et2Ga(OEt2)2]2[B12Cl12]. An excess of Et3Ga 3 (OEt) (3.77 mmol, 23.5 equiv) was added to [Ph3C]2[B12Cl12] (0.17 g, 0.16 mmol) dissolved in 10 mL of 1,2-F2C6H4. The reaction mixture was stirred for 10 h at rt. The color of the reaction mixture changed from yellow to light yellow. An IR spectrum of the gaseous byproduct confirmed the formation of C2H4. After addition of 10 mL of pentane a white precipitate formed, which was separated by filtration, washed several times with pentane (3  5 mL), and dried under vacuum. [Et2Ga(OEt2)2]2[B12Cl12] remained as a white solid (0.09 g, 0.08 mmol, 50%). 1 H NMR (400.17 MHz, d6-benzene/1,2-F2C6H4, 298 K): δ 0.67 (q, 8H, CH2), 1.03 (t, 12H, CH3), 1.14 (t, 24H, CH3 (Et2O)), 3.75 (q, 16H, CH2 (Et2O)). 11B NMR (128.39 MHz, d6-benzene/1,2-F2C6H4, 298 K): δ
11.7. 13C (100.63 MHz, d6-benzene/1,2-F2C6H4, 298 K): δ 4.6 (CH2), 7.1 (CH3), 13.3 ((CH3), Et2O), 67.6 ((CH2), Et2O). IR (diamond, cm
1): ν 2956 (br), 2873 (sh), 1468 (w), 1445 (w), 1389 (w), 1288 (vw), 1188 (w) 1147(w), 1023(vs) [B12Cl12]2
, 991 (sh), 891 (s), 830 (w), 762 (m), 672 (m), 584 (w) (νas(Ga
C)), 531 (vs) [B12Cl12]2
, 452 (vw). Raman (cm
1): ν 3062 (5), 2980 (25), 2938 (80), 2875 (15), 2744 (10), 1454 (30), 1392 (5), 1321 (5), 1211 (5), 1089 (10), 1003 (10), 893 (5), 832 (5), 773 (5), 527 (30) (νs(Ga
C)), 300 (100) [B12Cl12]2
, 128 (80) [B12Cl12]2
. (Et2In)2B12Cl12. Et3In (0.34 g, 1.68 mmol, 9 equiv) was added to [Ph3C]2[B12Cl12] (0.19 g, 0.18 mmol) dissolved in 10 mL of 1,2F2C6H4. The reaction mixture was stirred at rt for 10 h. The color of the reaction mixture changed from yellow to colorless. An IR spectrum of the gaseous byproduct confirmed the formation of C2H4. After addition

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Table 5. Crystallographic Data for (Et2Al)2B12Cl12 and (Me2Al)2B12Cl12 (Et2Al)2B12Cl12

a

(Me2Al)2B12Cl12

empirical formula

C8H20Al2B12Cl12

C4H12Al2B12Cl12

M/g mol
1 cryst syst

725.32 monoclinic

669.22 monoclinic

space group

C2/m

P21/c

a/pm

1172.1(2)

890.67(3)

b/pm

1385.0(3)

1044.67(4)

c/pm

904.89(18)

1403.31(5)

β/deg

94.75(3)

95.590(2)

V/nm3

1.4639(5)

1.29951(8)

Z δcalcd./g 3 cm
3

2 1.645

2 1.710

θ range/deg

3.11
26.00

2.30
33.28

T/K

160(2)

100(2)

μ/mm
1

1.199

1.343

reflns collected

34 643

18 721

indep reflns

1495 [Rint = 0.0304]

4933 [Rint = 0.0271]

R1, wR2 (I > 2σ(I))a

0.0468, 0.1102

0.0263, 0.0577

R1, wR2 (all data) largest diff peak and hole

0.0475, 0.1104 0.546,
0.609

0.0367, 0.0620 0.372,
0.373

R1 = ∑||Fo|
|Fc||/∑|Fo |, wR2 = (∑[w(Fo2
Fc2)2]/∑[wFo4])1/2.

of 10 mL of pentane a white precipitate formed, which was separated by filtration, washed several times with pentane (3  5 mL), and dried under vacuum. (Et2In)2B12Cl12 remained as a white solid (0.11 g, 0.08 mmol, 65%). 1H NMR (400.17 MHz, d6-benzene/1,2-F2C6H4, 298 K): δ 1.01 (q, 8H, CH2), 1.30 (t, 12H, CH3). 11B NMR (128.39 MHz, d6benzene/1,2-F2C6H4, 298 K): δ
11.6. 13C NMR (100.63 MHz, d6benzene/1,2-F2C6H4, 298 K): δ 9.9 (CH3), 14.3 (CH2). IR (ZnSe, cm
1): ν 3456 (br, H2O) 2964 (m), 2924 (m), 2853 (w), 1652 (br, H2O), 1456 (w), 1412 (w), 1264 (s), 1102 (sh), 1029 (vs) [B12Cl12]2
, 864 (sh), 809 (s), 664 (m). Raman (cm
1): ν 2965 (10), 2928 (50), 2879 (15), 2758 (5), 1460 (10), 1179 (35), 448 (100) (νs(In
C)), 305 (80) [B12Cl12]2
, 233 (20), 128 (70) [B12Cl12]2
. X-ray Crystallography. Single crystals suitable for X-ray diffraction were obtained from 1,2-difluorobenzene solution by slow removal of the solvent by applying a temperature gradient by cooling the other leg of the vessel. Single-crystal X-ray structure determinations were carried out on a Rigaku R-AXIS Spider image plate diffractometer ((Et2Al)2B12Cl12) or on a Bruker Smart Apex-II CCD system ((Me2Al)2B12Cl12) using Mo KR (0.71073 Å) radiation. The crystals were mounted onto a kryo loop using fluorinated oil and frozen in the cold nitrogen stream of the goniometer. Details of the crystallographic data collection and refinement parameters are given in Table 5. The structures were solved by direct methods (SHELXS).40 Subsequent leastsquares refinement on F2 (SHELXL) located the positions of the remaining atoms in the electron density maps.40 All atoms were refined anisotropically. Hydrogen atoms were placed in calculated positions using a riding model and were refined isotropically in blocks. The data were corrected for absorption (semiempirical from equivalents). The crystal of (Et2Al)2B12Cl12 was presumably twinned. The disorder was modeled over two sites. Several attempts to obtain better crystals failed. Graphical representations of the structures were prepared with the program DIAMOND.41 Calculations of the Brown bond valences were performed with the program VALENCE.42 Quantum Chemical Calculations. DFT calculations were performed using the PBE0 functional with the def2-TZVPP basis set as implemented in the program TURBOMOLE.43,44 Frequencies were 3791

dx.doi.org/10.1021/om2003333 |Organometallics 2011, 30, 3786–3792

Organometallics calculated analytically with the AOFORCE module at the same level for all species.45 All calculated species are true minima on the energy hypersurface, as shown by the absence of imaginary frequencies. Raman intensities were calculated with the RAMAN tool; IR intensities, with the AOFORCE module of TURBOMOLE. Calculated coordinates and total energies are included in the Supporting Information. Calculations of the Gibbs solvation energies were performed applying the COSMO solvation model.46 The solvent 1,2-difluorobenzene was described by the experimental static dielectric constant (ε = 13.38).32 As recommended by the developers, the optimized atomic radii for the COSMO calculations were used.47,48 The calculations approximate Gibbs free energies at 298 K in the gas phase or in solution, respectively. Thermodynamic data used for the construction of the Born
Haber
Fajans cycles are given in section S3 of the Supporting Information.

’ ASSOCIATED CONTENT

bS

Supporting Information. Crystallographic data (CIF) for (Et2Al)2B12Cl12 and (Me2Al)2B12Cl12. Actual NMR and vibrational spectra. Calculated absolute energies, symmetries, and coordinates for all calculated species. Born
Haber
Fajans cycles (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: +49-761-2036001. Tel: +49-761-203-6150.

’ ACKNOWLEDGMENT This work was supported by the Albert-Ludwigs-Universit€at Freiburg and the Deutsche Forschungsgemeinschaft. We are grateful to Dr. Harald Scherer for helpful discussions, Hannes B€ohrer for the synthesis of Et3In, and Sebastian Spieler for help with the experimental work. We thank Prof. Dr. Steven H. Strauss, Colorado State University, for making the crystal structure data of (Me2Al)(1-Me-CB11F11) available to us. ’ REFERENCES (1) Dagorne, S.; Atwood, D. A. Chem. Rev. 2008, 108, 4037–4071. (2) Atwood, D. A. Coord. Chem. Rev. 1998, 176, 407–430. (3) Gu, W.; Haneline, M. R.; Douvris, C.; Ozerov, O. V. J. Am. Chem. Soc. 2009, 131, 11203–11212. (4) Krossing, I.; Raabe, I. Angew. Chem. 2004, 116, 2116–2142. Angew. Chem., Int. Ed. 2004, 43, 2066–2090. (5) Reed, C. A. Acc. Chem. Res. 2010, 43, 121–128. (6) Wright, J. H.; Mueck, G. W.; Tham, F. S.; Reed, C. A. Organometallics 2010, 29, 4066–4070. (7) Reed, C. A. Acc. Chem. Res. 1998, 31, 325–332. (8) Kim, K. C.; Reed, C. A.; Long, G. S.; Sen, A. J. Am. Chem. Soc. 2002, 124, 7662–7663. (9) Bochmann, M.; Sarsfield, M. J. Organometallics 1998, 17, 5908–5912. (10) Pandey, K. K. Inorg. Chem. 2003, 42, 6764–6767. (11) Strauss, S. H.; Ivanov, S. V. US 6645903 B2, Nov. 11, 2003. (12) Klis, T.; Powell, D. R.; Wojtas, L.; Wehmschulte, R. J. Organometallics 2011, 30, 2563–2570. (13) Wehmschulte, R. J.; Steele, J. M.; Young, J. D.; Khan, M. A. J. Am. Chem. Soc. 2003, 125, 1470–1471. (14) Young, J. D.; Khan, M. A.; Wehmschulte, R. J. Organometallics 2004, 23, 1965–1967.

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(15) Young, J. D.; Khan, M. A.; Powell, D. R.; Wehmschulte, R. J. Eur. J. Inorg. Chem. 2007, 1671–1681. (16) Knapp, C.; Guttsche, K.; Geis, V.; Scherer, H.; Uzun, R. Dalton Trans. 2009, 2687–2694. (17) Knapp, C.; Schulz, C. Chem. Commun. 2009, 4991–4993. (18) Kessler, M.; Knapp, C.; Sagawe, V.; Scherer, H.; Uzun, R. Inorg. Chem. 2010, 49, 5223–5230. (19) Bolli, C.; Derendorf, J.; Kessler, M.; Knapp, C.; Scherer, H.; Schulz, C.; Warneke, J. Angew. Chem. 2010, 122, 3616–3619. Angew. Chem., Int. Ed. 2010, 49, 3536–3538. (20) Derendorf, J.; Kessler, M.; Knapp, C.; R€uhle, M.; Schulz, C. Dalton Trans. 2010, 39, 8671–8678. (21) Boere, R. T.; Kacprzak, S.; Kessler, M.; Knapp, C.; Riebau, R.; Riedel, S.; Roemmele, T. L.; R€uhle, M.; Scherer, H.; Weber, S. Angew. Chem. 2011, 123, 572–575. Angew. Chem., Int. Ed. 2011, 50, 549–552. (22) Warneke, J.; D€ulcks, T.; Knapp, C.; Gabel, D. Phys. Chem. Chem. Phys. 2011, 13, 5712–5721. (23) Gu, W.; Ozerov, O. V. Inorg. Chem. 2011, 50, 2726–2728. (24) Weidlein, J.; M€uller, U.; Dehnicke, K. Schwingungsspektroskopie; Thieme: Stuttgart, 1988. (25) Hausen, H. D.; Mertz, K.; Weidlein, J.; Schwarz, W. J. Organomet. Chem. 1975, 93, 291–296. (26) Neum€uller, B.; Gahlmann, F. J. Organomet. Chem. 1991, 414, 271–283. (27) Hausen, H. D.; Mertz, K.; Veigel, E.; Weidlein, J. Z. Anorg. Allg. Chem. 1974, 410, 156–164. (28) Huang, Z. S.; Park, C.; Anderson, T. J. J. Organomet. Chem. 1993, 449, 77–84. (29) Yamamoto, O. Bull. Chem. Soc. Jpn. 1961, 35, 619–622. (30) Kvisle, S.; Rytter, E. Spectrochim. Acta, Part A 1984, 40, 939–951. (31) Glasser, L.; Jenkins, H. D. B. Chem. Soc. Rev. 2005, 34, 866–874. (32) Lide, D. R. Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Danvers, 2003. (33) Mantina, M.; Chamberlin, A. C.; Valero, R.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. A 2009, 113, 5806–5812. (34) Aarset, K.; Shen, Q.; Thomassen, H.; Richardson, A. D.; Hedberg, K. J. Phys. Chem. A 1999, 103, 1644–1652. (35) Brendhaugen, K.; Haaland, A.; Novak, D. P. Acta Chem. Scand. A 1974, 28, 45–47. (36) Brown, I. D. Acta Crystallogr., Sect. B 1992, 48, 553–572. (37) Kessler, M.; Knapp, C.; Krossing, I.; Lichtenthaler, M., to be published. (38) Runge, F.; Zimmermann, W.; Pfeiffer, H.; Pfeiffer, I. Z. Anorg. Allg. Chem. 1952, 267, 39–48. (39) Tang, H.; Fabicon, R.; Richey, H. G. Organometallics 1998, 17, 139–145. (40) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112–122. (41) Diamond-Visual Crystal Structure Information System, Version 3.2e; Crystal Impact - K. Brandenburg & H. Putz GbR: Bonn, 2011. (42) Brown, I. D. J. Appl. Crystallogr. 1996, 29, 479–480. The contacts (in valence units [v.u.]) are defined as s = exp[(R0
R)/B], where R is the experimental distance, and R0 and B are constants. The used values are [Al(+3)
Cl(
1) R0 = 2.032, Al(+3)
C(
4) R0 = 1.899, B(+3)
Cl(
1) R0 = 1.750, C(
4)
C(
4) R0 = 1.560, Al(+3)
F(
1) R0 = 1.545, B(+3)
F(
1) R0 = 1.281; B = 0.37]. (43) TURBOMOLE Version 6.10, a development of University of Karlsruhe and Forschungszentrum Karlsruhe GmbH, 1989
2009; TURBOMOLE GmbH: Karlsruhe, 2009. Available from http://www. turbomole.com. (44) Ahlrichs, R.; B€ar, M.; H€aser, M.; Horn, H.; K€olmel, C. Chem. Phys. Lett. 1989, 162, 165–169. (45) Deglmann, P.; Furche, F.; Ahlrichs, R. Chem. Phys. Lett. 2002, 362, 511–518. (46) Klamt, A.; Sch€u€urmann, G. J. Chem. Soc., Perkin Trans. 2 1993, 799–805. (47) Klamt, A.; Eckert, F. Fluid Phase Equilib. 2000, 172, 43–72. (48) Eckert, F.; Klamt, A. AIChE J. 2002, 48, 369–385. 3792

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