Synthesis and Application of Strong Brønsted Acids Generated from

Article pubs.acs.org/Organometallics

Synthesis and Application of Strong Brønsted Acids Generated from the Lewis Acid Al(ORF)3 and an Alcohol Anne Kraft,† Jennifer Beck,† Gunther Steinfeld,‡ Harald Scherer,† Daniel Himmel,† and Ingo Krossing*,† †

Institut für Anorganische und Analytische Chemie, Freiburger Materialforschungszentrum (FMF) and Freiburg Institute for Advanced Studies (FRIAS), Universität Freiburg, Albertstraße 19, 79104 Freiburg, Germany ‡ Zürcher Hochschule für Angewandte Wissenschaften, Grüental, 8820 Wädenswil, Switzerland S Supporting Information *

ABSTRACT: The strong neutral Brønsted acids [(R)OH→Al(OC(CF3)3)3] (R = −C(CF3)3 (1), −C6F5 (2), (−)-menthyl (3)) were synthesized by complexation of perfluoro tert-butyl alcohol, pentafluorophenol, and (−)-menthol with the Lewis superacid Al(OC(CF3)3)3. The 1:1 composition of the compounds was proven by NMR (except for compound 2), IR, and partially Raman spectroscopy and X-ray crystallography. Of the structures, 2 crystallized with a coordinated toluene molecule, which might be seen as a frozen intermediate or prestep to form the classical Wheland complex of protonated toluene. This interaction was calculated to be exothermic by 32 (no dispersion) or 88 kJ mol−1 (Grimmes D3 dispersion correction included @ RI-BP86/SV(P)). 1 proved suitable to protonate mesitylene and Et2O, giving the acidic cationic Brønsted acids [H(C6H3(CH3)3)]+ (4b) and [H(OEt2)2]+ (5) with the respective weakly coordinating anion [Al(OC(CF3)3)4]−. In dichloromethane solution 4b decomposes at room temperature, leaving the room-temperature-stable salt [H(C6H3(CH3)3)]+[((CF3)3CO)3Al−F−Al(OC(CF3)3)3]− (4a; XRD). The acidities reached with 4 and 5 are discussed in terms of our recently introduced absolute Brønsted acidity scale. The absolute chemical potentials of a 0.001 M solution of protonated mesitylene and Et2O amount to −944 and −1015 kJ mol−1 orin terms of absolute pHabs valuesto 165 and 178 and thus are at the threshold of superacidity of −975 kJ mol−1 or pHabs of 171.

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INTRODUCTION Combining highly acidic systems with weakly coordinating anions (WCAs) seems to be one of the promising strategies to create highly reactive protonated cations.1,2 In recent years superacids and their use to protonate arenes and fullerenes were in the focus of experimental and theoretical work.1,3−8 While classical superacids such as HF/SbF5, HSO3F/SbF5 (Magic acid), and HBr/AlBr3 allow the synthesis of highly reactiveoften oxidizedcations, their resulting anions are often too nucleophilic, too basic, or too corrosive so that decomposition reactions result.9,10 Therefore, a nonoxidizing although extremely Brønsted acidic system with a conjugate base as poorly nucleophilic as possible would be desirable. Promising examples for this type of Brønsted acids are Reed'sunfortunately still isolable only on a small scale carborane acids H[CB11H6X6] (X = Cl, Br), which at a 1 equiv level can protonate weak bases such as benzene11,12 and fullerenes10 and were used for Et2Cl+ formation13 and protonation-induced rearrangement reactions.14 Recent additions to these type of acids include H[RCB11F11] (R = H, C2H5), C60(C2F5)5H, H[HCB11Cl11], and H2[B12X12] (X = Cl, Br).4,15,16 To the best of our knowledge, no pure conjugate acids of completely perfluorinated anions are currently known. For example, due to acidic cleavage of the B−C bond, H[B(C6F5)4] is not suitable for superacid chemistry and the © 2012 American Chemical Society

commonly written HBF4 or HSbF6 acids only exist in the form H(H2O)n+[BF4]− and H(HF)n+[SbF6]−.15 So far the hypothetical pure H2[B12F12] superacid has only been isolated as [(H3O)2]2+[B12F12]2−·6H2O.17 In a broader sense, the almost perfluorinated neutral radical [HCB11(CF3)nF11−n]• (n = 5, 6; only stable at −60 °C) can be mentioned here, which was synthesized by oxidation of the respective anion.18 In analogy to the classical Lewis/Brønsted acid mixtures HF/ SbF5 and HSO3F/SbF5 we increased the Brønsted acidity of perfluorinated or chiral alcohols by complexation with a nonoxidizing Lewis superacid.5 Thus, stoichiometric reactions of PhF→Al(ORF)3 or in situ prepared donor-free Al(ORF)3 (RF = C(CF3)3) and the alcohols HO−R (R = −C(CF3)3 (1), −C6F5 (2), (−)-menthyl (3)) form an alcohol−alane adduct, which represents the protonated weakly coordinating anion H[Al(OR)(ORF)3]. Neutral alcohol−water aluminum complexes are only scarcely described in the literature, due to their high tendency to hydrolyze with classical aluminum-based Lewis acids such as trihaloalanes.19 In a wider perspective, also the water and methanol complexes of E(C6F5)3 (E = B, Al) are cited here.20 The properties of the employed alcohols clearly influence the overall acidity of the prepared system: the pKa Received: August 10, 2012 Published: October 3, 2012 7485

dx.doi.org/10.1021/om300776a | Organometallics 2012, 31, 7485−7491

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recommended to use 1 in the nonpolar solvent perfluorohexane, in which the dissociation tendency is minimized. 3 is stable in dichloromethane, while 2 is only scarcely soluble in common solvents such as dichloromethane, SO2, and 1,2-F2C6H4. Overall the calculated complexation enthalpies ΔrH°gas (eq 1, (RI-)BP86/def-TZVP, 298 K) increase from HOC(CF3)3 over HOC6F5 to menthol. This leveling is in agreement with the enhanced stability and decreased acidity of the compounds in solution as well as the calculated GAs. The NMR and IR spectroscopic data as well as the X-ray crystal structure data agree with 1:1 compositions of the compounds. Spectroscopic Characterization of the Acids. The IR spectrum of 1 shows the O−H stretch at 3413 cm−1 as a significantly (by 220 cm−1) red-shifted and broadened band in comparison to that of free HOC(CF3)3, indicating the formation of hydrogen bonds. This finding is in agreement with the proton being localized at an oxygen atom and involved in a hydrogen bond (to a (C−)F or (C−)O atom). The DFToptimized structure (see the Supporting Information) suggests internal hydrogen bonding from a protonated oxygen atom to a fluorine or oxygen atom of a OC(CF3)3 group. In the IR spectra of 2 and 3 only weak O−H bands could be detected, but all other bands are as expected and are in good agreement with the calculations (see the Supporting Information). NMR experiments further suggest the stoichiometric composition of the adducts 1 and 3. Unfortunately, 2 is insoluble in all our commonly used solvents; therefore, no solution NMR spectra could be recorded. In 3 the chiral alcohol is coordinated to the Lewis acid Al(ORF)3 also in dichloromethane solution. In the 27Al spectrum a broad signal at 39 ppm (Δν1/2 = ∼600 Hz) was detected, which according to our experience is only possible if the Lewis base is strongly bound to Al(ORF)3. Apart from a very small signal of the free alcohol, only one 19F signal in the typical chemical shift range of Al(ORF)3 compounds at −75.6 ppm was detected.29 A combination of 1H,13C HMBC, 1H,13C HSQC, and 1H,19F HOESY experiments achieved the assignment of 1H and 13C resonances. In agreement with the coordination to a very strong Lewis acid, the 1H resonances of the hydroxyl and the OC−H group are shifted downfield by 3 and 1.2 ppm, in comparison to free (−)-menthol. Except for the signal of the proton of the isopropyl group, all other proton resonances have higher frequencies. Of the carbon resonances, the signal of the OC−H group shows the most significant downfield shift and appears at 87.6 ppm versus 71.2 ppm for free (−)-menthol. All other carbon resonances are shifted to high field, as expected when the hydroxyl group bears an electron-withdrawing group (cf. the Supporting Information).30 Finally, in the 1H,19F HOESY NMR spectrum the cross peaks between the 19F signal and the 1 H signals of the isopropyl and hydroxyl groups of (−)-menthol show the small distance between menthol and Al(ORF)3. Taken together, this proves that 3 does not dissociate in dichloromethane solution. 19 F NMR spectra of 1 in SO2 and 1,2-F2C6H4 show two signals in the characteristic range for the chemically equivalent CF3 groups of perfluoro tert-butoxy moieties (Table 2). The ratio of the integrals of these two signals is 1:3. Whereas in SO2 solution the line width of the two signals is small, they are significantly broadened in 1,2-F2C6H4. When the 1,2-F2C6H4 solution of 1 was cooled to 0 and −35 °C, the signals became considerably sharper (see the Supporting Information). Obviously, there is a chemical exchange between the two different perfluoro tert-butoxy groups in the 1,2-F2C6H4

values of HOC(CF3)3 and HOC6F5 are very similar (5.4/5.5 in H2O, 20.55/20.11 in CH3CN, gas-phase acidities (GAs) 1357/ 1343 kJ mol−1).21 Thus, both should qualify to form with Al(ORF)3 the first completely perfluorinated, neutral, but still isolable Brønsted acids 1 and 2. In support, we have calculated the GAs of compounds 1−3 at the BP86/def-TZVP level as 1041, 1052, and 1169 kJ mol−1, respectively, ranking them between the classical Brønsted acid H2SO4 and the very strong neutral isolable carborane acid H[CB11H6Cl6] (Table 1).3 Table 1. Calculated Gas-Phase Acidities ((RI-)BP86/defTZVP) of 1−3 Compared to a Representative Selection of Classical Brønsted Acidsa Brønsted acid

GA (kJ mol−1)

HBr H2SO4 HSO3F HN(SO2CF3)2 (NC)2C(CNH) menthol−Al(ORF)3 (3) H[Al(OC6F5)(ORF)3] (2) H[CB11H6Cl6] H[Al(ORF)4] (1) H[HCB11Cl11] H[CB11F12] H2B12F12

1341, [1333], 133222 1272, [1261], 126523,24 1233, 125523 1220, [1218], 120025 1209 1169 1052 1044, [1014] 1041 997, [965] 921, [891] 914

a Values in brackets are taken from the literature;3 ORF = OC(CF3)3. Values in italics are experimental data taken from the literature.

With the current work we present the easily available in large scale Brønsted acid 1, which protonates weak bases such as 1,3,5-mesitylene and Et2O as their weakly coordinating anion salts. Apart from [H−C6Me6]+[Al(ORF)4]−, which was mentioned as a byproduct,26 no protonated arenes with the [Al(ORF)4]− counteranion are currently known. By using the chiral alcohol (−)-menthol we introduce chirality to the system, which may find applications similar to those for classical chiral anions and Brønsted acids:27 e.g., chiral solid catalysts based on menthol were used for asymmetric Diels−Alder reactions.28 Further applications for 1−3 are in the focus of ongoing work.

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RESULTS AND DISCUSSION Synthesis of the Neutral Acids. The simple route to synthesize the neutral Brønsted acids 1−3 is shown in eq 1.

While 1 needs to be synthesized from freshly prepared donorfree Al(ORF)3, 2 and 3 can be generated by substitution of the PhF ligand in PhF→Al(ORF)3 with HOC6F5 or menthol. After the compounds are crystallized at −20 or −40 °C, they can be isolated as colorless powders in yields between 44 (2) and 88% (1). 1 is best stored at −40 °C, whereas 2 and 3 can be handled at room temperature. 1 is highly soluble in PhF, 1,2-F2C6H, CH2Cl2, and SO2 but tends to dissociate in solution (see below). In donor solvents such as SO2, Et2O, and MeCN adduct formation is almost impossible to suppress, as will be discussed later. In THF polymerization occurs. Thus, it is 7486


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solution is not suitable for discussion (see the Supporting Information). However, the solution of the single-crystal data is also in agreement with the assignment of 1 as H[Al(OC(CF3)3)4]. The aluminum atoms in 2·tol and 3 have distortedtetrahedral coordination, and the sums of the FRO−Al−ORF (ORF = OC(CF3)3) bond angles are 347° in 2·tol and 341° in 3. The average Al−O distance (∼169.4 pm, OC(CF3)3) in both structures is consistent with the values for Al(ORF)3 complexes already described in the literature.29,32 In tetracoordinate Al compounds using C6F5O and menthoxy as anionic ligands, dAl−O is significantly shorter than in the dative Al−O bonds to the coordinated alcohols (188.5 pm in 2·tol vs 178.7 pm in AlMe2(OC6F5)[N(C2H4)3CH]33/184.2 pm in Al2(OC6F5)6,34 average 183.0 pm in 3 vs 177.4 pm in [Li(thf)2(μ-O(−)Ment)2Al(H)2]35 (Ment = menthyl) but similar to that in a bridging situation (184.1 pm in [Me2Al(μ-O(−)-Ment)]236). Due to the high Lewis acidity of Al(ORF)3, the C−O distances of the coordinated alcohols are strongly elongated in comparison to those in the free alcohols (140.6 vs 136.0 pm37 in 2·tol, 151.1 vs 142.8 pm38 in 3). In 3 the position of one of the hydrogen atoms of the two independent moieties could not be found on the difference Fourier map and needed to be restrained. Nevertheless, their position at the oxygen atom of menthol was already proven by NMR spectroscopy and is in agreement with 3 being the weakest acid of all three compounds (cf. also the high aqueous pKa value of menthol of 19).39 The hydrogen atom on the oxygen atom in 2·tol was located, and the position was refined isotropically in the final model. A hydrogen bond between the O−H donor and the π electron cloud of the toluene ring was observed: cf. the related B(C6F5)3·H2O·C6H6.41 The oxygen atom is positioned over three carbon atoms of the toluene ring with an O···toluene centroid distance of 318 pm. This distance lies between those of [(C2B10H10Hg)3·H2O]2·C6H642 (299(1) pm) and B(C6F5)3·H2O·C6H641 (323.1(3) pm). The short contact distance is nicely visualized with the Hirshfeld surface of toluene (Figure 2b). Furthermore, the influence of the proton on the electrostatic potential of toluene is significant (see the Supporting Information), although the structural data of toluene remain almost unchanged. This type of NH−/OH−π intermolecular interaction was in the focus of several gas-phase experimental and theoretical studies.43 They presume an increase of the hydrogen bond strength with toluene in comparison to benzene due to the stronger π-electron density caused by the additional electron-donating methyl group.41 Thus, the calculated interaction energy (ΔrH°gas(298 K)) between 2 and toluene is significant at 32 kJ mol−1 but also includes π stacking, as indicated by the frontier orbitals as well as from the mentioned electrostatic interaction (see the Supporting Information).44 Using Grimme's D3-dispersion correction (2010), the calculated interaction energy increases to 88 kJ mol−1 (with D2-dispersion correction 86, with D3dispersion correction 88 kJ mol−1, (RI-)BP86/SV(P)).45 Overall the constitution of the interaction of 2 with toluene might be seen as a frozen intermediate or prestep to form a classical Wheland complex, in this case protonated toluene. Successful isolation of a protonated arene as an intermediate of electrophilic aromatic substitution was only realized when using the strongest acid 1 (vide infra).

Table 2. Experimental NMR Data (ppm) of 1, 3, and 4a (ORF = OC(CF3)3) compound

solventb

1

SO2

reference 1

SO2 1,2-F2C6H4

3

CD2Cl2

reference 4b

CD2Cl2 SO2

1

H

19

F

27

Al

4.96 4.96

−75.4 −74.3 −74.4 −75.5

4.26 4.40a

−74.6 −75.6

39

−75.7

36.3

1.39 2.77 2.92 4.57 7.67

35.3

assignment SO2→Al(ORF)3 HORF free HORF 1,2-F2C6H4→ Al(ORF)3 HORF Ment−OH→ Al(ORF)3 free menthol [H(C6H3(CH3)3)] [Al(ORF)4]

a

Only the H−O signal is given. For all other 1H chemical shifts see the Supporting Information. bSpectra measured in SO2 were calibrated using a reference sample of HOC(CF3)3 in SO2.

solution. This was proven by a 19F,19F EXSY experiment (see the Supporting Information). Most likely the mechanism for this chemical exchange is a replacement of an alcohol molecule bound to Al(ORF)3 by a solvent molecule. This leads to an equilibrium between the free alcohol HOC(CF3)3 and the ligands of 1,2-F2C6H4→Al(ORF)3 through the formation of H[Al(ORF)4]. In SO2 this chemical exchange is much slower if present at allbecause in SO2→Al(ORF)3 the donor is more strongly bound by about 39 kJ mol−1 than in 1,2-F2C6H4→ Al(ORF)3 (ΔrH°(dissociation) = +49 (SO2) vs +10 kJ mol−1 (1,2-F2C6H4), (RI-)BP86/def-TZVP, see also ref 29). This suggestion was confirmed by the following observations. The proton chemical shift of the OH group of HOC(CF3)3 in the SO2 solution of 1 is identical with the chemical shift of the pure alcohol in the same solvent. Furthermore, and in contrast to the 1,2-F2C6H4 solution, a broad aluminum signal similar to that for the known SO2→Al(ORF)3 was detected in the SO2 solution.29 Crystal Structures of the Acids. The molecular structures of 2·tol (tol = toluene) and 3 were determined by X-ray diffraction studies (Figures 1 and 2).31 We also collected multiple crystal structure data of independent batches of 1, but due to the strong disorder of the compound, the structure

Figure 1. Molecular structure of 3. Displacement ellipsoids are drawn at the 50% probability level. The asymmetric unit contains two independent moieties, only one of which is shown. Selected distances in pm: Al1−O4 = 183.5(5), CMent−O4 = 151.4(1.0). 7487


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Figure 2. (a) Section of the molecular structure of 2·tol. Displacement ellipsoids are drawn at the 50% probability level. Selected distances in pm: Al−O4 = 188.49(12), O4−C13 = 140.56(19), av Al−O = 169.4, O4−H = 0.80(2), O4−Ctol = 335, 307, and 318 (C19, C20, C21); distances within toluene C19−C20 = 138.9(3), C20−C21 = 138.9(2), C21−C22 = 138.5(3), C22−C23 = 138.4(3), C23−C24 = 139.8(2), C24−C25 = 150.6(3), C19−C24 = 139.0(2). The distance between the toluene plane and the oxygen atom of HOC6F5 is 302 pm, and the angle between the planes is 13°. (b) Hirshfeld surface40 of the coordinated toluene molecule, showing the strong OH−π intermolecular interaction.

First Applications of 1 to Protonate Ether and Mesitylene. Synthesis. The protonated 1,3,5-mesitylene salt [H(C6H3(CH3)3)]+[Al(ORF)4]− (4b)an isolated Wheland complexwas synthesized by mixing 1 (prepared in situ) with mesitylene in perfluorohexane at −20 °C (eq 2). The yellow

reaction mixture is highly acidic and reactive; thus, in situ preparation is recommended. 4b is highly soluble in SO2 but needs to be handled at −20 °C. In a similar reaction carried out in dichloromethane single crystals of the well-ordered salt [H(C6H3(CH3)3)]+[(FRO)3Al−F−Al(ORF)3]− (4a) were isolated. From earlier work it is known that the counterion [(FRO)3Al−F−Al(ORF)3]− is a frequently observed decomposition product of [Al(ORF)4]− that results from the action of the very strong Lewis acid Al(ORF)3 on the [Al(ORF)4]− counterion.46 The 1H NMR spectrum of 4b in SO2 shows the typical signals of protonated mesitylene (Table 2).12 Since an excess of mesitylene and free alcohol was used in this reaction, the resonances of these compounds were also detected. However, also smaller signals of several side products were observed (for a discussion see the Supporting Information).47 In the 19F and 27 Al spectra the characteristic signals for [Al(ORF)4]− were detected.48,49 1 can also be used to protonate Et2O to [H(OEt2)2]+[Al(ORF)4]− (5) just by mixing pure 1 with Et2O, giving the known signals in the NMR spectra.49 However, due to the high tendency of 1 to dissociate in solution, the formation of Et2O→ Al(ORF)3 was also observed with 24 mol % next to 76 mol % 5 (see the Supporting Information).34 Crystal Structure. Structural data of 4a are typical for protonated mesitylene as a classical σ complex (the H−C−H angle at C72 is around 97°) and the μ-F bridged anion, already described in the literature (Figure 3).11,12,46 Therefore, a detailed structural discussion is not given here (see the Supporting Information).

Figure 3. Molecular structure of 4a. Displacement ellipsoids are drawn at the 50% probability level. The disorder of one of the C(CF3)3 groups is removed for clarity. Selected distances in pm: Al1−F1 = 176.4(3), Al2−F1 = 177.0(3), av Al−O = 169.7, C71−C72 = 144.3(9), C72−C73 = 144.9(9), C73−C74 = 136.1(8), C74−C75 = 139.7(1.0), C75−C76 = 139.4(1.0), C76−C71 = 135.6(9). Al1−F1− Al2 = 176.0(2)°. The hydrogen atoms at C72 were located and the positions were refined isotropically in the final model.

It is noteworthy that just a weak interaction between the anion and the cation was observed. The shortest distance between the CH2 group of the cation and a fluorine atom of the anion is about 299 pm; thus, the distance is at the sum of the van der Waals radii considering weak hydrogen bonds. However, the structural data of the cation slightly differ from those reported with the carborane anions, and the cyclohexadienyl character is more pronounced in 4a.11 Comparison of experimental and calculated data is best for the cation of 4a (Table 3). This reveals that the pseudo-gas-phase conditions are strongly introduced with the μ-F bridged anion, indicating the aluminates as very weakly coordinating.2 Analysis of the Absolute Acidities. Furthermore, we evaluated the strengths of the Brønsted acidity of solutions of the cationic acids protonated mesitylene and diethyl ether in the respective solvents SO2 and Et2O. In a Born−Haber− Fajans cycle (see the Supporting Information) using the experimental gas-phase basicity of mesitylene and calculated Gibbs solvation energies (COSMO and rCCC), we calculated 7488


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1−3 protonated weakly coordinating anions, which are promising starting materials to stabilize new protonated species. As a first test, 1 was used to protonate mesitylene (4a,b) and Et2O (5), underlining the potential of these acids. However, choosing the ideal combination of nonbasic solvent and weak but non-σ-basic base is necessary to reach the high possible Brønsted acidity and to suppress the stable σ-adduct formation. Here Et2O already presents problems, as shown by the roughly 24% Et2O→Al(ORF)3 byproduct observed during the protonation of ether. Further applications of 1−3 are in the focus of current experiments. An evaluation of the absolute acidities of solutions of 4 and 5 revealed that the weakly coordinating anion [Al(ORF)4]− tolerates remarkable acidities up to a level similar to that of neutral liquid H2SO4 with a pHabs of 171.

Table 3. Comparison of the Experimental and Calculated Bond Lengths (pm) of Mesitylenium Cations bond

4a

pbe0/def2TZVPP

[H(C6H3(CH3)3)] [CB11H6Br6]11

C71−C72 C71−C76 C72−C73 C73−C74 C74−C75 C75−C76

144.3(9) 135.6(9) 144.9(9) 136.1(8) 139.7(1.0) 139.4(1.0)

147.7 136.2 147.7 136.2 141.0 141.0

141.4(1.2) 138.7(1.1) 142.5(1.2) 138.4(1.1) 137.0(1.0) 138.0(1.0)

the pKa value of protonated mesitylene in SO2 to be 13. According to our recently reported acidity scale,7,8 we calculated the absolute chemical potentials of a 0.001 M solution of protonated mesitylene and Et2O as −944 and −1015 kJ mol−1 orin terms of absolute pHabs valuesto 165 and 178 (using the rCCC model with the anchor points ΔsolvG° = −898 kJ mol−1 for H+ in SO2 and −998 kJ mol−1 for H+ in Et2O).7 Since we established the border of superacidity (neutral sulfuric acid) at an absolute chemical potential of −975 kJ mol−1 or an absolute pHabs value of 171, we conclude that the weakly coordinating anion [Al(ORF)4]− tolerates remarkable acidities up to a superacidic environment, as for protonated mesitylene in SO2 solution. It appears that the counterion [(FRO)3Al−F−Al(ORF)3]− is slightly more tolerant against protons and the salt [H(C6H3(CH3)3)]+[(FRO)3Al−F−Al(ORF)3]− (4a) is therefore stable at room temperature. This is in agreement with earlier experiences with this anion. Strengths of the Do→Al(ORF)3 Adduct Formation as a Stability Governing Factor. Summarizing all hitherto available experimental and calculated data of L→Al(ORF)3 (L = menthol, Et2O, C6F5OH, HOC(CF3)3, SO2, PhF, 1,2F2C6H4), it is obvious that in the presence of σ donors such as Et2O and SO2 or in this case menthol the reasonably polar, but nonbasic and accordingly more weakly bound ligands PhF, 1,2F2C6H4, and HOC(CF3)3 are easily substituted (Figure 4).

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EXPERIMENTAL SECTION

General Considerations. All reactions were performed under an argon atmosphere using standard vacuum and Schlenk techniques or a glovebox. PhF→Al(OC(CF3)3)3 was synthesized according to ref 32. AlEt3 (>93% purity) and HOC6F5 were used as provided. PhF, perfluorohexane, SO2, Et2O, toluene, 1,2-F2C6H4, and 1,3,5-mesitylene were dried over CaH2. HOC(CF3)3 was dried over P4O10. Solvents were distilled prior to use or directly added to the reaction mixture using condensation techniques. All reactions were performed in special double J. Young flasks sealed with grease-free Teflon or glass valves or special J. Young NMR tubes to exclude air and moisture. NMR spectra were recordedif not otherwise mentionedat room temperature in CD2Cl2, SO2, or 1,2-F2C6H4 on a Bruker Biospin Avance II+ 400 MHz WB or a Bruker DPX 200 spectrometer with the software Topspin 2.1. 19 F NMR spectra in 1,2-F2C6H4, which were recorded without deuterium lock, were calibrated to the 19F solvent signal at −139 ppm. Spectra measured in SO2 were calibrated using a reference sample of HOC(CF3)3 in SO2. For single-crystal measurements, the temperature-sensitive crystals were mounted in perfluoroether oil on MicroMounts at −20 to −30 °C using a low-temperature mounting device. X-ray crystallographic data were collected on a Bruker Quazar APEX2 CCD area detector diffractometer or a Rigaku R-Axis SPIDER diffractometer with Mo Kα X-ray (0.710 73 Å) sources. Structures were solved by direct methods in the SHELXS/XL51 program or with OLEX2.52 Disorder in the anions (OC(CF3)3 groups) was restricted with SADI and SAME instructions. The hydrogen atoms, which were not found on the difference Fourier map, were added with HFIX instructions. ORTEP was used for the graphics of the crystal structures.53 Crystal structure data of 2·tol: a = 10.7030(2) Å, b = 10.8399(2) Å, c = 15.5105(3) Å, α = 72.0560(10)°, β = 80.9760(10)°, γ = 77.2220(10)°, P1̅, Z = 2, R1 = 0.0375, wR2 = 0.0842. Crystal structure data of 3: a = 11.4149(3) Å, b = 17.9673(6) Å, c = 15.8175(4) Å, α = 90°, β = 96.312(2)°, γ = 90°, P21, Z = 4, R1 = 0.0685, wR2 = 0.1680. Crystal structure data of 4a: a = 10.447(2) Å, b = 33.489(7) Å, c = 15.380(3) Å, α = 90°, β = 105.82(3)°, γ = 90°, P21/c, Z = 4, R1 = 0.0701, wR2 = 0.1820. CCDC 862693 (2·tol), 865147 (3), and 865081 (4a) contain supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc. cam.ac.uk/data_request/cif. For reflective IR measurements a Nicolet Magna-IR 760 Fouriertransform spectrometer with a Diamond cell was used (OMNIC software, usually 32 scans). The data were ATR corrected using the default values (Diamond ATR correction: angle of reflection 45°, number of reflections 1, index of refraction 1.50). The samples were prepared in a glovebox, squeezed in the Diamond cell, and measured immediately under dry air. Raman spectra were recorded on a Bruker RAM II Fourier-transform Raman module for the VERTEX 70 spectrometer (Nd:YAG laser, excitation wavelength 1064 nm, software package OPUS). The samples were prepared in a glovebox and enclosed in flame-sealed glass tubes to exclude air and moisture. Details on the quantum chemical calculations are included with the Supporting Information.

Figure 4. Ordering of the donor strength of several solvents and σ donors in the shown equilibrium according to their calculated complexation enthalpies (Do = σ donor, L = ligand at the (RI)BP86/def-TZVP level50).

Thus, it is recommended to use 1 in perfluorohexane, since otherwise stable σ-adduct formation is difficult to suppress, as already discussed with the NMR experiments. To take full advantage of the high acidity of 1, the choice of the right solvent (1,2-F2C6H4 or, even better, perfluorohexane) and a weak but non-σ-basic base such as mesitylene is the best combination for successful protonations.

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CONCLUSION The high stability of the corresponding anion is the key to high protic acidity of neutral Brønsted acids. It can be realized with a large anion sizeto delocalize the negative chargeand shielding induced by inert substituents such as halides.1 Starting from the Lewis superacid Al(ORF)3 and bulky alcohols such as HOC(CF3)3, HOC6F5, and menthol, we created with 7489


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Complex 1. A mixture of AlEt3 (0.075 g, 0.66 mmol) in perfluorohexane (3−5 mL) was cooled to 253 K in a special J. Young flask sealed with greaseless glass valves. HOC(CF3)3 (0.5 mL, 3.4 mmol, 5.2 equiv) was added with strong stirring over 30 min. The reaction mixture was warmed to 273 K for 5 min. Afterward H[Al(OC(CF3)3)4] was crystallized at 233 K. After the solvent was removed at 253 K, the compound could be isolated as a colorless powder (0.56 g, 88%). 1H NMR (400.17 MHz, SO2, 298 K): δ 4.96 (s, 1H, free HOC(CF3)3). 19F NMR (376.53 MHz, SO2, 298 K): δ −74.3 (s, 9F, HOC(CF3)3), −75.4 (s, 27F, 3 × C(CF3)3), in a 1:3 ratio. 27Al NMR (104.27 MHz, SO2, 298 K): δ 35.3 (bs, SO2→Al(OC(CF3)3)3).54 IR (Diamond ATR, corrected): ν̃ 464 (w), 539 (w), 581 (w), 728 (m), 890 (w), 938 (w), 979 (s), 1104 (m), 1149 (w), 1222 (m), 1257 (vs), 1301 (m), 1359 (w), 3413 (w, O−H). Complex 2. In one neck of a special double-necked J. Young flask sealed with greaseless glass valves PhF→Al(OC(CF3)3)3 (0.135 g, 0.16 mmol) was dissolved in 1,2-F2C6H4 (2 mL) at 253 K; in the other neck C6F5OH (0.030 g, 0.16 mmol, 1 equiv) was dissolved in toluene (2 mL). The 1,2-F2C6H4 phase was slowly covered with the toluene phase. The title compound crystallized at 253 K between the two layers and could be isolated as a colorless powder. The yield was determined in a reaction made in 1,2-F2C6H4 (0.066 g, 44%). 2 is insoluble in SO2, 1,2-F2C6H4, dichloromethane, and toluene; therefore, no NMR spectra could be recorded. IR (Diamond ATR, corrected): ν̃ 458 (w), 494 (w), 539 (w), 582 (w), 635 (w), 675 (w), 728 (m), 915 (vw), 976 (s), 1002 (s), 1020 (m), 1152 (w), 1191 (m), 1221 (m), 1256 (vs), 1303 (m), 1362 (w), 1490 (vw), 1526 (m), 1662 (vw), 3648 (vbroad, vw). Complex 3. (−)-Menthol (0.02 g, 0.13 mmol) and PhF→ Al(OC(CF3)3)3 (0.105 g, 0.13 mmol, 1 equiv) were dissolved in perfluorohexane (3 mL) at 253 K in a special J. Young flask sealed with greaseless glass valves. The reaction mixture was cooled to 253 K to crystallize. After removal of the solvent by cannula and under vacuum the title compound could be isolated as a colorless powder (0.075 g, 66%). 19F NMR (376.53 MHz, CD2Cl2, 298 K): δ −75.6 (s, 27F, 3 × C(CF3)3). 27Al NMR (104.27 MHz, CD2Cl2, 298 K): δ 39 (bs, Al(OC(CF3)3)3). For 1H and 13C NMR data see the Supporting Information. IR (Diamond ATR, corrected): ν̃ 537 (w), 569 (w), 630 (vw), 667 (w), 727 (m), 812 (w), 843 (vw), 862 (w), 884 (w), 906 (w), 975 (s), 1012 (w), 1041 (vw), 1095 (vw), 1180 (m), 1219 (m), 1249 (vs), 1266 (vs), 1300 (m), 1355 (w), 1392 (vw), 1459 (w), 2875 (m), 2930 (m), 2959 (m), 3566 (m), 3650 (vbroad). FT Raman: ν̃ 538 (m), 571 (w), 751 (s), 774 (m), 812 (m), 884 (w), 982 (w), 1041 (w), 1083 (w), 1180 (w), 1279 (w), 1355 (w), 1463 (m), 1503 (m), 2758 (m), 2864 (s), 2882 (m), 2939 (s), 2969 (s). Complex 4b. A mixture of AlEt3 (0.196 g, 1.7 mmol) in perfluorohexane (5 mL) and 1,3,5-mesitylene (0.5 mL, 3.2 mmol, 1.9 equiv.) was cooled to 253 K in a special J. Young flask sealed with greaseless glass valves. HOC(CF3)3 (1.6 mL, 11.2 mmol, 6.6 equiv) was added with strong stirring over 30 min with complete EtH evolution. Strong shaking of the reaction mixture suddenly changed the color to yellow. Isolation of the compound is not possible due to the high reactivity; therefore, in situ preparation is recommended. The compound is soluble in liquid SO2. The reaction temperature should always be below 253 K. In a reaction carried out in dichloromethane we isolated crystals of [C9H13][((CF3)3CO)3Al−F−Al(OC(CF3)3)3] (4a). No further characterization of the crystals 4a was performed. Data for 4b are as follows. 1H NMR (400.17 MHz, SO2, 233 K): δ 2.77 (s, 6H, o-CH3), 2.92 (t, J = 3.7 Hz, 3H, p-CH3), 4.57 (bs, 2H, CH2), 7.67 (s, 2H, m-CH). 19F NMR (376.53 MHz, SO2, 233 K): δ −75.7 (s, 36F, 4 × C(CF3)3). 27Al NMR (104.27 MHz, SO2, 233 K): δ 36.3 (s, [Al(OC(CF3)3)4]−). 13C NMR (100.63 MHz, SO2, 233 K): δ 26.6 (s, 2C, o-CH3), 29.0 (s, 1C, p-CH3), 53.6 (s, 2C, m-CH), 135.3 (s, 1C, CH2). The 13C resonances are taken from an HSQC spectrum. For a discussion see the Supporting Information.

Article

ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Fonds der Chemischen Industrie, the AlbertLudwigs-Universität Freiburg, the ERC with the UniChem Project, and the DFG for support.

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REFERENCES

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NOTE ADDED AFTER ASAP PUBLICATION In the version of this paper published on Oct 3, 2012, incorrect data were given in a few places in the text as well as in the Supporting Information. The data that now appear as of Oct 11, 2012, are correct.

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Synthesis and Application of Strong Brønsted Acids Generated from

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