Facile and systematic access to the least-coordinating WCA [(R<sup>F</sup>O)<sub>3</sub>Al–F–Al(OR<sup>F</sup>)<sub>3</sub>]<sup>−</sup> and its more Lewis-basic brother [F–Al(OR<sup>F</sup>)<sub>3</sub>]<sup>−</sup> (R<sup>F</sup> = C(CF<sub>3</sub>)<sub>3</sub>)
By reaction of the Lewis acid Me 3 Si-F-Al(OR F ) 3 with a series of [PF 6 ] À salts, gaseous PF 5 and Me 3 Si-F are liberated and salts of the anion [F-Al(OR F ) 3 ] À ([f-al] À ; R F ¼ C(CF 3 ) 3 ) can be obtained. By addition of another equivalent of Me 3 Si-F-Al(OR F ) 3 to [f-al] À , gaseous Me 3 Si-F is released and salts of the least coordinating anion [(R F O) 3 Al-F-Al(OR F ) 3 ] À ([al-f-al] À ) are formed. Both procedures work for a series of synthetically useful cations including Ag + , [NO] + , [Ph 3 C] + and in very clean reactions with 5 g batch sizes giving excellent yields typically exceeding 90%. In addition, the synthesis of Me 3 Si-F-Al(OR F ) 3 has been optimized and scaled up to 85 g batches in an one-pot procedure. These anions could previously only be obtained by difficult to control decomposition reactions of [Al(OR F ) 4 ] À or by halide abstraction reactions with Me 3 Si-F-Al(OR F ) 3 , generating relatively large countercations that are unsuited for further use as universal starting materials. Especially [al-f-al] À is of interest for the stabilization of reactive cations, since it is even weaker coordinating than [Al(OR F ) 4 ] À and more stable against strong electrophiles. This bridged anion can be seen as an adduct of [f-al] À and Al(OR F ) 3 . Thus, it is similarly Lewis acidic as BF 3 and eventually reacts with nucleophiles (Nu) from the reaction environment to yield Nu-Al(OR F ) 3 and [f-al] À . This prevents working with [al-f-al] À salts in ethereal or other donor solvents. By contrast, the [f-al] À anion is no longer Lewis acidic and may therefore be used for reactions involving stronger nucleophiles than the [al-f-al] À anion can withstand. Subsequently it may be transformed into the [al-f-al] À salt by simple addition of one equivalent of Me 3 Si-F-Al(OR F ) 3 .
Introduction<!>Results and discussion<!>Synthesis of [f-al]<!>Spectroscopic differentiation of the anions<!>Molecular structures of [f-al] À and [al-f-al] À salts<!>Some aspects for single crystal X-ray diffraction renements<!>Improved WCA performance of [al-f-al] À<!>Thermodynamic stability of [al-f-al] À<!>The Lewis acidity of [al-f-al] À : implications for solvent compatibility<!>Taking advantage of [f-al] À<!>Limits of [f-al] À<!>Conclusion<!>Conflicts of interest
<p>For many years our group has been interested in the stabilization of reactive cations with the weakly coordinating anion [Al(OR F ) 4 ] À (WCA, R F ¼ C(CF 3 ) 3 ). With this anion shown in Scheme 1, a large variety of reactive cations being electrophilic like [CX 3 ] + , RZn + (R ¼ Me, Et), [Sn(Cp)] + , [P 2 X 5 ] + (X ¼ Cl, Br, I) and [P 9 ] + , oxidizing like [NO] + , [NO 2 ] + or [Ag(X 2 ) n ] + -complexes, or very weakly bound to a central atom like the huge number of [M(L) n ] + complexes with M ¼ Cu, Ag, Au and L ¼ CO, C 2 H 2 , C 2 H 4 , P 4 , S 8 , S 12 , Se 6 , Se 12 or Se 19 could be stabilized. [1][2][3][4][5] One of the biggest advantages of this [Al(OR F ) 4 ] À anion is its facile and fast synthesis in high yields. 6 Other commonly used WCAs are [B(C 6 F 5 ) 4 ] À , perhalogenated carborane anions [R-CB 11 X 12 ] À (X ¼ F, Cl, Br) and the teates [E(OTeF 5 ) x ] À (E ¼ B, Al, Sb, As, Bi, Nb; x ¼ 4, 6). 7,8 However, the synthesis of these anions either require handling of explosive LiC 6 F 5 , 9 elaborate syntheses in small scales, 10 or handling of the toxic and hydrolysis sensitive HOTeF 5 . 8,11 Yet, especially the carborane anions are chemically more stable against erce electrophiles than [Al(OR F ) 4 ] À , e.g. against small silylium ions. 12 As shown in eqn (1a), [Al(OR F ) 4 ] À decomposes in the presence of small silylium ions [R 3 Si] + above Scheme 1 Overview to the anions presented in this paper and their abbreviations. Cite this: Chem. Sci., 2018, 9, 7058</p><p>All publication charges for this article have been paid for by the Royal Society of Chemistry À30 C to give R 3 Si-F-Al(OR F ) 3 . 13,14 However, it is compatible with the bulky [Si(C 6 Me 5 ) 3 ] + silylium ion. 15 More general, very small and very potent electrophiles, like "[PCl 2 ] + " or "[SiCl 3 ] + " induce [Al(OR F ) 4 ] À decomposition, very oen under formation of [(R F O) 3 Al-F-Al(OR F ) 3 ] À ([al-f-al] À , eqn (1b)), which is even less coordinating than [Al(OR F ) 4 ] À . 16 For more than a decade, access to a starting material with this anion was only accessible by decomposition of Ag[Al(OR F ) 4 ]. However, this synthesis proved to be delicate to reproduce, requires the two-step synthesis of Ag[Al(OR F ) 4 ], and is combined with loss of Ag + and OR F moieties. The bridging motif of [al-f-al] À may be seen in analogy to related bridged anions known to the literature, 17 e.g. as in [(C 6 F 5 ) 3 B-CN-B(C 6 F 5 ) 3 ] À , [(C 6 F 5 ) 3 B-NH 2 -B(C 6 F 5 ) 3 ] À and [(C 6 F 5 ) 3 E-F-E(C 6 F 5 ) 3 ] À (E ¼ B, Al, Ga). 18 In some cases, where decomposition of [Al(OR F ) 4 ] À occurred, formation of [al-f-al] À could not be observed. Instead the formal [F-Al(OR F ) 3 ] À anion ([f-al] À ) was obtained, oen as part of a neutral compound, like in Me 3 Si-F-Al(OR F ) 3 or Cp*Be-F-Al(OR F ) 3 . 13,19 Therefore, both were rationalized as being ion-like compounds that behave intermediate between being covalently bound or separated ions. Nevertheless also compounds, where [f-al] À and the cation are separated, were isolated. 20 Me 3 Si-F-Al(OR F ) 3 has already shown its usefulness for silylation reactions, 13,21 the polymerization of isobutene, 13 halide abstraction reactions, 21 and abstraction of [Cp] À . 3 It is available in an one pot synthesis that was optimized here to 85 g scale. Upon investigation of its reactivity, formation of [al-f-al] À and [f-al] À could oen be observed and proved to be easily controllable. 13,21 Therefore we investigated here, if it is possible to use Me 3 Si-F-Al(OR F ) 3 to synthesize both anions with cations that would make them good starting materials for further chemistry.</p><!><p>Large-scale optimized synthesis of Me 3 Si-F-Al(OR F ) 3</p><p>In order to establish optimized routes to the anions [al-f-al] À and [f-al] À , we needed access to larger amounts of Me 3 Si-F-Al(OR F ) 3 . Therefore, we optimized this synthesis and performed an upscaling from 40 g (ref. In order to maximize the yield and purity of Me 3 Si-F-Al(OR F ) 3 it is highly important to dry all glassware with a gas burner in vacuo until the ame turns orange. When opening the reaction vessel containing the AlEt 3 , the argon ow must be carefully controlled. If the argon ow is too high, the AlEt 3 may start to smoke due to swirling of ambient air. Additionally, the HOR F has to be dried to a water content of less than 1 ppm, which can be achieved by stirring with P 4 O 10 for two weeks. It is also highly advisable to use as large of a stir bar as possible, since vigorous stirring is crucial for this synthesis and during the reaction, the viscosity of the reaction solution increases.</p><p>The Me 3 Si-F-Al(OR F ) 3 used for all syntheses in this paper was prepared according to the following optimized procedure: AlEt 3 (15.0 mL, 109 mmol, 93% purity) was dissolved in heptane (120 mL) and cooled to À40 C. Under vigorous (!) stirring, the rst two equivalents of HOR F (30.0 mL, 215 mmol, 2.0 eq.) were added dropwise to the reaction mixture, while keeping the cooling bath between À40 and À20 C. During the addition of HOR F , gas evolution (C 2 H 6 ) was observed, the viscosity of the solution increased and small amounts of precipitate formed. Aer complete addition of the rst HOR F portion (typically within 1 hour), Me 3 SiF (12.5 g, 136 mmol, 1.2 eq.) was condensed onto the reaction mixture at À40 C. The solution was stirred for 10 min, and then the third equivalent of HOR F (20.0 mL, 143 mmol, 1.3 eq.) was added within 5 min. The cooling bath was removed and the reaction mixture was allowed to reach rt, which led to dissolution of the precipitate. Aer 30 min, more HOR F (5.0 mL, 36 mmol, 0.3 eq.) was added and the solution was stirred overnight. From the clear solution, slowly a white powder crystallized. For product isolation, simply the solvent was removed in vacuo yielding a white crystalline powder (85.4 g, 103 mmol, 94%). Note: one needs the excess of the uorinated alcohol, as this is very volatile (b.p. ¼ +45 C) and the continuous stream of evolving ethane removes the excess alcohol through the bubbler.</p><p>The quality of the Me 3 Si-F-Al(OR F ) 3 can be veried by different methods: if the obtained powder seems to be moist, but cannot be completely dried, there are probably residual Al-Et groups present in the product. In this case, more HOR F should be added. Another qualitative test for the purity of the product is by dissolving it in 1,2-diuorobenzene (o-DFB). A solution of pure Me 3 Si-F-Al(OR F ) 3 is nearly colorless, while impurities (mainly residual Al-Cl groups stemming from the AlEt 3 ) induce a light yellow color. These impurities usually enrich at the top of the crystalized product and stick to the glass of the reaction vessel. These crystals are yellow colored and isolable. Therefore they should not be scratched out of the reaction vessel. Purication of the white powder can be achieved either by sublimation at 60 C in dynamic vacuum (10 À3 mbar) or by washing with warm CH 2 Cl 2 , let it cool and then lter it. Additionally, NMR spectra of the product should be taken in both, o-DFB and CH 2 Cl 2 . Me 3 Si-F-Al(OR F ) 3 is only poorly soluble in CH 2 Cl 2 , but not its impurities. Therefore, if a NMR spectrum in CH 2 Cl 2 shows only negligible amounts of impurities, the product can be considered pure. In o-DFB Me 3 Si-F-Al(OR F ) 3 is highly soluble and here the signal intensity is increased compared to the impurities. Therefore, NMR spectra in CH 2 Cl 2 may show a purity of only 60%, while the real purity is >90%, as can be seen from NMR spectra in o-DFB. Pure Me 3 Si-F-Al(OR F ) 3 can be stored in a glove box for at least a year without decomposition. The calculated rather exergonic reaction energies are included with eqn (2) and are given in kJ mol À1 (BP86-D3(BJ)/def-SV(P)).</p><!><p>The preferred reaction conditions for all investigated compounds are listed in Table 1. The formation of the respective anion is controlled by the stoichiometry of Me 3 Si-F-Al(OR F ) 3 used (1 : 1 or 2 : 1)the very Pearson hard Li + case presents the only exception and stops at the [f-al] À stage. The reactions performed at rt are usually nished within a few minutes and are accompanied by a vigorous evolution of PF 5 aer addition of the solvent. In the following, we briey describe the reactions for each cation separately.</p><p>Lithium salts. Due to the poor solubility of Li[PF 6 ] in o-DFB even at 60 C, a donor has to be added. We chose Cl 3 CCN as a weak donor in order to obtain a lithium cation with a Lewis acidity as high as possible. From this reaction the compound [Li(NCCCl 3 )][f-al] could be obtained. As the only exception from eqn (2), the [al-f-al] À anion could not be obtained in combination with a lithium cation. This is probably due to strong coordination of Li + to [f-al] À (see molecular structures  3e and f). This suggests that the third o-DFB molecule is only weakly bound.</p><p>The reaction in SO 2 has to be performed at À20 C in order to prevent loss of the solvent, while at the same time the evolving</p><p>can only be obtained with signicant amounts of impurities, therefore no yield is given.</p><p>PF 5 is allowed to evaporate from the solution. The turbid reaction solution is then stirred at À20 C until the gas evolution has completely stopped. Subsequently, the reaction solution is allowed to reach 0 C, followed by removal of the solvent. The solvent free product is obtained as an off-white powder. Nitrosonium salts. Due to the poor solubility of [NO][PF 6 ] in CH 2 Cl 2 at rt, SO 2 has to be used as solvent. Here, the reaction is also carried out at À20 to À30 C to allow the use of a bubbler for removal of the evolving PF 5 . The reaction is then allowed to slowly warm up to 0 C over the course of 45 min to ensure complete conversion, followed by removal of all volatiles in vacuo. Due to the incompatibility of [NO] + with n-pentane, a quantitative crystallization in analogy to the silver salts is not possible.</p><p>Trityl salts. When a few drops of o-DFB or CH 2 Cl 2 are added to a mixture of [Ph 3 C][PF 6 ] and Me 3 Si-F-Al(OR F ) 3 , immediately a erce evolution of PF 5 takes place and is nished in less than a minute. Nevertheless, the reaction solution should be stirred for another 30 min in order to allow for a complete conversion. Aer removal of the solvent in vacuo, the product is obtained as a yellow powder.</p><!><p>Both anions can be differentiated by standard spectroscopic methods (IR, Raman, NMR). However, in all cases spectra of [fal] À and [al-f-al] À are very similar and therefore we highlight key differences in the following.</p><p>Differentiation by NMR-spectroscopy. Fig. 1 shows the characteristic NMR signals of [al-f-al] À and [f-al] À in direct comparison. For [al-f-al] À the 19 F NMR spectrum shows a broad signal for the bridging uorine atom at À184.5 ppm and a resonance for the C(CF 3 ) 3 groups at À75.5 ppm with a small doublet splitting of 0.5 Hz due to the 5 J(F-F) coupling to the bridging uorine atom. In the 27 Al NMR spectrum, only a very broad resonance at about 37 ppm (Dn 1/2 ¼ 1200 Hz) is obtained. This broadness of the signal is caused by a fast relaxation of the Al atoms. As a result, the 1 J Al-F coupling is not resolved neither in the 27 Al nor in the 19 F NMR spectra. The presence of this coupling can be deduced from the broadening of the signal of the bridging uorine atom in the 19 F NMR spectrum. In the 19 F { 27 Al} NMR spectrum this signal is sharp, which unambiguously proves that the broadening of this signal results from the 1 J Al-F coupling (see ESI † for detailsinvestigations on the Al(OR F ) 3 exchange reactions).</p><p>By contrast, for [f-al] À the Al-F signal in the 19 F NMR spectrum generates a sextet, due to coupling to the aluminum atom. This signal is usually found between À185 and À200 ppm, but may also be found at À145 ppm in case of the [NO] + and [SeCl 3 ] + salts. The signal of the alkoxy groups is visible at À75.6 ppm with 5 J(F-F) ¼ 1.5-2 Hz due to coupling to the Al-bound uorine atom. In the 27  The main differences between these anions can be found in the region between 900 and 500 cm À1 (Fig. 2a). In the IR spectra additional bands at 636 (n as (Al-F-Al)) and 862 cm À1 are visible for [al-f-al] À , and at 762 (n(Al-F)) and 808 cm À1 for [f-al] À , when compared to [Al(OR F ) 4 ] À . The Raman spectra of these anions also show new bands at 636 and 723 cm À1 , respectively. One of the most characteristic differences between the three discussed anions in the Raman spectra is the frequency of the vibrational bands at $750 and $800 cm À1 and their intensities. These bands are approximately equally intense for [Al(OR F ) 4 ] À . When looking at [al-f-al] À and [f-al] À , the vibrational band at $750 cm À1 is about twice as intense as the band at $800 cm À1 .</p><p>Especially for [f-al] À , the coordination of the cation to the (Al-)F atom may have an impact on the vibrational bands in the vibrational spectra. Therefore, we compared the IR and Raman spectra of Ag[f-al], K[f-al] and [NO][f-al] (Fig. 2b). Among these cations, [NO] + is considered to be the least and Ag + (donor-free) to be the strongest coordinating. Most of the vibrational bands in these compounds are identical. Only the molecular vibrations containing partial Al-F contributions ($810, $760 and $730 cm À1 ) are inuenced by the coordination to the cation. These bands appear at higher wave numbers, the weaker coordinating the cation is.  ) x ] + salts would be interesting, it proved to be rather complicated, since the vibrational bands of these cations overlap with the ones of the anion.</p><!><p>Although our commonly used [Al(OR F ) 4 ] À anion usually does not show signicant coordination, very small cations, like Li + and [Ag(L)] + are able to be coordinated by its oxygen atoms. 4,5,23 As can be seen from Fig. 3, [f 2). A similar coordination of the oxygen atoms of [al-f-al] À to a cation is highly unlikely for sterical reasons and was not observed in the compounds presented in this work. However, the [ZnEt] + cation is small enough to coordinate to these alkoxy groups.</p><!><p>For [Al(OR F ) 4 ] À compounds, the structure determination by single crystal X-ray diffraction frequently tends to be problematic. Due to the very weak interactions of the peruorinated residues with the cation, but also with each other, these residues are able to rotate around the C-O and the Al-O bonds. As a result, the OC(CF 3 ) 3 moieties in this anion are oen heavily disordered and/or feature rather large anisotropic displacement ellipsoids. This is one of the reasons, why we introduced the DSR program code, nowadays included with OLEX2 (ref. 24) and ShelXLe, 25 to facilitate the description of these oen very complicated disorders. 26 These problems are typically less pronounced in salts containing [al-f-al] À . This is due to the further shortened Al-O bonds (169.7 pm vs. 172.5 pm on average) 6,16 that force the six OC(CF 3 ) 3 groups to be distributed over the rather small Al-F-Al core and increase the steric strain. For these reasons, the OC(CF 3 ) 3 groups are not able to rotate as freely as in [Al(OR F   This suggests a ligand scrambling reaction and formation of [AlF x (OR F ) 4Àx ] À (x ¼ 1-3) and their F-bridged analogues. From the 11 B NMR spectrum only formation of BF(OR F ) 2 was visible. Since this solution contained lots of precipitate, another sample was prepared in C 6 H 5 F/acetone-d 6 in order to dissolve this precipitate. Here NMR spectra showed mainly the formation of unbridged [AlF x (OR F ) 4Àx ] À and several boron containing species. The formation of [AlF x (OR F ) 4Àx ] À occurs due to the Lewis basicity of acetone and will be discussed later. These NMR spectra suggested a complex mixture of different boron and aluminum containing species. Inter alia, [Ag(PhF)] [F 2 B(OR F ) 2 ] (see ESI † for details) crystallized from one of the mixtures. Interestingly, in this structure the Ag + is coordinated to the oxygen atoms of the anion, since the more Lewis basic (B-)F atoms form good hydrogen bonds with the coordinated solvent molecules.</p><p>Due to the lower Lewis acidity of the evolving BF 3 compared to PF 5 , the ligand scrambling reaction would rather be expected when using [PF 6 ] À salts for these reactions (FIA ¼ 346 vs. 380 kJ mol À1 ; the CIA, HIA and MIA are also higher for PF 5 ). 27 The same trend holds, when comparing the calculated [OR F ] À ion affinities of BF 3 (208 kJ mol À1 ) and PF 5 (239 kJ mol À1 ) at BP86-D3(BJ)/def-SV(P) level. Therefore, a kinetic reason for this ligand scrambling has to be considered. The according reaction intermediate very likely involves Al-O(R F )-B/P and Al-F-B/P bridging. In case of PF 5 , this transition state is energetically less accessible due to its increased size and decreased tendency to undergo bridging.</p><p>Additionally, the stabilities of the theoretical reaction products, i.e. BF 2 (OR F ) and PF 4 (OR F ), also should be considered, but this is rather a secondary effect. BF 2 (OR F ) features a tricoordinated, planar boron atom, which is able to undergo p back bonding with the lone pairs of the alkoxy moiety. This back bonding is stronger than that of uoride and leads to a considerable stabilization of this molecule. The BF 2 (OR F ) is able to undergo a second ligand scrambling to form the observed BF(OR F ) 2 . In PF 4 (OR F ) this p back bonding is not present, presumably resulting in less favorable thermodynamics in matters of the discussed ligand scrambling reaction. Therefore, the isolation of [f-al] À and [al-f-al] À is best possible when using [PF 6 ] À salts, although the Lewis acidities of PF 5 and BF 3 would suggest otherwise.</p><!><p>When considering the calculated electrostatic potentials of [Al(OR F ) 4 ] À , [al-f-al] À and [f-al] À (Fig. 4) it can be concluded that [al-f-al] À should be the least coordinating of these anions and about as coordinating as [(C 6 F 5 ) 3 Al-F-Al(C 6 F 5 ) 3 ] À . This is mainly due to the delocalization of the negative charge over 55 uorine atoms, but also because the negative charge is partially localized at the bridging uorine atom with an NPA charge of À0.67 vs. À0.34 for the terminal F atoms, similar to [f-al] À (NPA charge ¼ À0.69 vs. À0.35; BP86/def-SV(P)). This makes [al-fal] À more stable towards F À and [R F O] À abstraction than [Al(OR F ) 4 ] À (see later). 16,28,29 The reactions of Ag[Al(OR F ) 4 ] with halosilanes R 3 SiX (R ¼ Me, tBu, Ph; X ¼ Cl, Br, I) in CH 2 Cl 2 were already described. 13,14 At rt these reactions yield R 3 Si-F-Al(OR F ) 3 , while at À50 C the   Although the formation of this cation is in agreement with the ndings of Reed et al. that silylium ions are able to dissolve AgBr, 30 at the same time it is contradictory to the reactions of Ag [Al(OR F ) 4 ], where halide abstraction from halosilanes was observed. 13,14 This leads to the conclusion that [Al(OR F ) 4 ] À cannot be innocent in these reactions as similar halide abstractions are observed with Ag[ClO 4 ]. Only when Me 3 SiI was added to Ag[al-f-al] in toluene/1,2,3-C 6 H 3 F 3 in a 1 : 1 stoichiometry, the formation of AgI could be observed. NMR spectra of this solution predominantly showed decomposition of the anion and formation of Me 3 Si-F-Al(OR F ) 3 next to other unidentied products.</p><p>Judged from these reactions and their solution NMR spectra, we propose that upon reaction of Ag[WCA] and halosilanes, R 3 SiX, always the according silver complexes [Ag(X-SiR 3 ) x (solv) y ] + are formed (eqn (3a)). These complexes contain an activated silicon and are strong silylating reagents. They react with nucleophiles (Nu) under formation of Nu-SiR 3 and AgX. It appears, that for anions like [ClO 4 ] À or even [Al(OR F ) 4 ] À this nucleophile can also be the anion (!), which subsequently may decompose (eqn (3b)). However, apparently the nucleophilicity of the [al-f-al] À WCA is too low, to induce AgX separation. In addition, the silylation strengths of the [Ag(X-SiR 3 ) x (solv) y ] + cation increases, when going from X ¼ Cl to I, mainly due to weakening of the Si-X bond. Overall, the lacking formation of AgX by reaction of Ag[al-f-al] with R 3 SiX (X ¼ Cl, Br) can be seen as rst experimental proof for the less coordinating nature of [al-f-al] À with respect to [Al(OR F ) 4 ] À or other WCAs that react with halosilanes under separation of AgX.</p><p>Another evidence for the weaker coordination and increased stability of [al-f-al] À compared to [Al(OR F ) 4 ] À is the intermediate synthesis of silylium ions by reaction of [Ph 3 C][WCA] with Me 3 SiH in CH 2 Cl 2 . In case of [WCA] À ¼ [Al(OR F ) 4 ] À only Me 3 Si-F-Al(OR F ) 3 is formed at rt within 30 min. In contrast to this, in case of [WCA] À ¼ [al-f-al] À the solvent is attacked under formation of Me 3 SiCl. In the NMR spectra of the latter reaction, the anion is mostly intact but decomposes within two days at room temperature. We were not able to identify the according cation, but judged by the formation of Me 3 SiCl, it might be a chlorinated carbocation resulting from the solvent CH 2 Cl 2 . Exchanging the CH 2 Cl 2 for o-DFB, we were not able to isolate silylium ions at rt by this route when using Me 3 SiH or iPr 3 SiH. Again, we mainly observed formation of R 3 Si-F-Al(OR F ) 3 and other decomposition products, similar to the reaction of Ag[alf-al] with Me 3 SiI.</p><!><p>[al-f-al] À is stable against very strong silylating agents, such as [Ag(X-SiR 3 ) x (solv) y ] + (X ¼ Cl, Br), but not against small silylium ions. Therefore, we were interested in evaluating its general stability towards uoride abstraction. This was done by calculating the decomposition reaction energies of [al-f-al] À and [Al(OR F ) 4 ] À using to the FIA procedure (Table 3). 31 Although this was already done in the literature, 28 we repeated these calculations with inclusion of the D3(BJ) dispersion correction (BP86-D3(BJ)/def-SV(P)). This seemed reasonable, since the strength of the Al-F-Al bond in [al-f-al] À is underestimated by DG ðgÞ ¼ 66 kJ mol À1 at BP86/def-SV(P) level. This also holds for the weak interaction between the evolving epoxide C 4 F 8 O and the corresponding Lewis acids. Yet, these calculations show only minor discrepancies to the literature.</p><p>Again, these calculations support the previous experiments and show that [al-f-al] À is more stable against uoride abstraction than [Al(OR F ) 4 ] À . Additionally, the decomposition of [al-f-al] À by [Me 3 Si] + was calculated to be endergonic in CH 2 Cl 2 by DG ¼ 10 kJ mol À1 and to be less favored than the decomposition of CH 2 Cl 2 . This is also in accordance to our reactions of [Ph 3 C][WCA] with Me 3 SiH in CH 2 Cl 2 . It should be noted, that the calculated reactions are only the rst step in the decomposition of CH 2 Cl 2 , [Al(OR F ) 4 ] À and [al-f-al] À and are not the nal reaction products. As a result, [al-f-al] À is not stable against small silylium ions at rt, although these calculations suggest so.</p><!><p>The increased stability of [al-f-al] À against electrophiles compared to [Al(OR F ) 4 ] À seems to make it the anion of choice for the synthesis of reactive cations. 28 However, this increased stability comes for a price: it's clearly noticeable Lewis acidity. Thus, when investigating the NMR spectra of K + [al-f-al] À we learned that as soon as we added Et Using the Fluoride Ion Affinity (FIA) as a measure, the Lewis acidity of [al-f-al] À is with a FIA-value of 329 kJ mol À1 in the region of BF 3 (346 kJ mol À1 ). This limits the choice of solvents: in basic solvents such as Et 2 O and MeCN the equilibrium shown in Scheme 2 is on the side of LB-Al(OR F ) 3 and [f-al] À . In SO 2 , PhF, o-DFB, CH 2 Cl 2 and CHCl 2 F the uoride bridged anion [alf-al] À was found to be completely intact by NMR spectroscopy. However, since SO 2 -Al(OR F ) 3 and Ph-F-Al(OR F ) 3 are known to be relatively stable, 33 an equilibrium between [al-f-al] À and LB-Al(OR F ) 3 /[f-al] À (here LB ¼ SO 2 , PhF) may be possible. For SO 2 this equilibrium was calculated to be on the side of [al-f-al] À by DG solv ¼ À66 kJ mol À1 using the COSMO model (3 ¼ 16.3) at BP86-D3(BJ)/def-SV(P) level. This is in agreement with our experimental data, since we were able to synthesize [al-f-al] À salts free of [f-al] À in SO 2 . Theoretically, the [f-al] À anion may also act as Lewis base here, i.e. when the [al-f-al] À salt is contaminated with [f-al] À . In this case, an exchange of Al(OR F ) 3 moieties between [f-al] À and [al-f-al] À is expected. However, the presence of the 1 J Al-F and 5 J F-F coupling for both anions in this mixture denitely excludes this exchange. We did also not observe such an exchange in the 19 F, 19 F-EXSY NMR spectrum or by line shape analysis. In contrast to this, the analogous chloride bridged anion [al-cl-al] À dissociates in PhF. 34 Therefore, we suggest using CHCl 2 F, CH 2 Cl 2 , o-DFB, PhF and SO 2 as solvents of choice, as they are polar and feature low Lewis basicity.</p><!><p>As already discussed, [al-f-al] À decomposes in presence of nucleophiles under formation of its more coordinating brother [f-al] À . This anion is less suited for the stabilization of reactive cations, since the anion is very likely to undergo ion pairing due to the strongly coordinating Al-bound uorine atom. However, the characterization of reactive cations in combination with WCAs can be very demanding, e.g. because of bad crystallization behavior due to the very low Coulomb interactions between anion and cation. In these cases the use of [f-al] À as a coordinating anion may be very helpful. Due to the stronger interaction between the anion and the cation, the crystallization of the reaction product may be facilitated. In a different project to be published with all details separately, 35  ) with SeCl 4 and to crystallize all three compounds (Fig. 6).</p><p>While single crystal X-ray diffraction measurements on these crystals yielded a superstructure at 100 K for [SeCl 3 ][Al(OR F ) 4 ], a simpler solution was obtained, when using [f-al] À and [al-fal] À as anions at 100 K. Especially the crystal structure of Cl 3 Se-[f-al], which crystallized as a contact ion pair, showed no disorder at all and only contains one molecule in the asymmetric unit. The bond valence 36 (bv) of only 0.16 for the Se-F contact suggests a weak interaction between the cation and the</p><p>a Thermodynamic values for F À were calculated using the Sackur-Tetrode equation. Only the solvation free energy was calculated at DFT level. Additionally to crystallization, [f-al] À salts can be used as key intermediates towards [al-f-al] À salts. Since [f-al] À is compatible with nucleophiles, it can be used for the synthesis of cations, which require the presence of nucleophiles or coordinating solvents. The resulting product can then be transformed into the [al-f-al] À salt by addition of Me 3 Si-F-Al(OR F ) 3 . The only prerequisite for this transformation is that the cation and [f-al] À have to be separated in solution to some extent, i.e. as solvent separated ion pair providing access to small amounts of free [f-al] À for the reaction with Me 3 Si-F-Al(OR F ) 3 .</p><!><p>To this point [f-al] À seemed to be rather stable against highly Lewis acidic cations, like [SeCl 3 ] + and [Me 3 Si] + (FIA ¼ 815 and 952 kJ mol À1 , respectively; BP86/def-SV(P)), due to formation of stable adducts. A comparison of the Al-F bond valences of 0.68 (Cl 3 Se-[f-al]) and 0.50 (Me 3 Si-F-Al(OR F ) 3 ) 13 already suggests, that extremely electrophilic cations might be able to abstract the Al-bound uorine atom. Yet, the relatively high bv in Me 3 Si-F-Al(OR F ) 3 is astonishing, considering the FIA of [Me 3 Si] + is higher by 415 kJ mol À1 compared to Al(OR F ) 3 (FIA ¼ 537 kJ mol À1 ). 32  The ratio of the products also suggests that the halide abstraction from PF x Cl 3Àx is preferred over PCl 3 . Interestingly again, the Ag[al-f-al] formed did not participate in further halide abstraction reactions (see our comments above).</p><p>As a conclusion, [f-al] À may be well suited for the stabilization of reactive cations as neutral ion-like Cat-F-Al(OR F ) 3 compounds. However, the presence of another [f-al] À anion may also lead to formation of [al-f-al] À and release of Cat-F. In these cases the best way to obtain a desired Cat-F-Al(OR F ) 3 may be the reaction of Cat-F with Al(OR F ) 3 or its equivalent Me 3 Si-F-Al(OR F ) 3 .</p><!><p>By reaction of Me 3 Si-F-Al(OR F ) 3 with different [PF 6 ] À salts we were able to selectively synthesize the anions [(R F O) 3 Al-F-Al(OR F ) 3 ] À ([al-f-al] À ) and [F-Al(OR F ) 3 ] À ([f-al] À ). In the presence of nucleophiles or coordinating solvents, however, [al-fal] À was shown to react under formation of [f-al] À and Nu-Al(OR F ) 3 . By intermediate generation of small silylium ions and isolation of [Ag(tBu 3 SiBr) 2 (CH 2 Cl 2 ) 2 ] + we were able to support the role of [al-f-al] À as "least-coordinating-anion": 16 thus, the elimination of AgX from the formed solvated [Ag(XSiR 3 ) x ] + cation requires the back-side attack of a nucleophile (solvent,  Due to the facile synthesis of Me 3 Si-F-Al(OR F ) 3 and the anions [f-al] À and [al-f-al] À in large scales and high yields, we believe that these anions will nd wide-spread use for the generation and stabilization of reactive cations, but also suggest they may be suitable for catalytic processes. Here, the [f-al] À anion may even be helpful to stabilize in a hemilabile coordination scheme the resting state of the catalysis process.</p><!><p>There are no conicts to declare.</p>