Insights on the Lewis Superacid Al(OTeF5)3: Solvent Adducts, Characterization and Properties

Abstract Preparation and characterization of the dimeric Lewis superacid [Al(OTeF5)3]2 and various solvent adducts is presented. The latter range from thermally stable adducts to highly reactive, weakly bound species. DFT calculations on the ligand affinity of these Lewis acids were performed in order to rank their remaining Lewis acidity. An experimental proof of the Lewis acidity is provided by the reaction of solvent‐adducts of Al(OTeF5)3 with [PPh4][SbF6] and OPEt3, respectively. Furthermore, their reactivity towards chloride and pentafluoroorthotellurate salts as well as (CH3)3SiCl and (CH3)3SiF is shown. This includes the formation of the dianion [Al(OTeF5)5]2−.

In 2017 we reported first attempts of the synthesis of the Lewis acid Al(OTeF 5 ) 3 . [14] With a FIA of 591 kJ mol À 1 for its molecular unit it can be counted as one of the strongest known isolable Lewis acids. The compound was analyzed by IR and Raman spectroscopy, revealing the dimeric form [Al(OTeF 5 ) 3 ] 2 in the solid state. Still, its temperature sensitivity made the handling of the compound tedious as it rapidly decomposed at temperatures above 0°C, which might be accounted to impurities (see below).
Herein we report on an improved synthesis of [Al(OTeF 5 ) 3 ] 2 , which can be prepared on a multigram scale. The Lewis superacid is room-temperature stable for several hours and isolable as an adduct-free, amorphous powder. With this neat Lewis acid in hand, we investigated its complexation with a broad range of different solvents, ranging from thermally stable, strongly bound adducts to weakly bound, reactive species. We then further elaborate on the reactivity of these so formed solvent-adducts.

Results and Discussion
In our previous study we firstly reported on the formation of the Lewis acid [Al(OTeF 5 ) 3 ] 2 by the reaction of triethylaluminium, AlEt 3 , with teflic acid, HOTeF 5 , in n-pentane in a stoichiometric ratio of 1 : 3. [14] This reaction yields a colorless powder which is unstable at room temperature. Analyzing a solution of this product in SO 2 ClF by low-temperature NMR spectroscopy revealed the presence of residual alkyl moieties that presumably led to a decreased thermal stability (decomposition above 0°C) of this compound. Therefore, an improved synthesis of the dimeric [Al(OTeF 5 ) 3 ] 2 by employing different reactants and conditions was needed.
In our new approach AlMe 3 was used as a starting material (cf. Scheme 1). Analogue to the reported synthesis with AlEt 3 , treatment in n-pentane with 3 equivalents of HOTeF 5 and warming of the mixture from À 196°C to À 40°C results in the formation of a colorless precipitate. After removing the solvent, a yet again temperature-sensitive powder remains. Low-temperature NMR measurements in SO 2 ClF show the presence of a methyl group at À 0.01 ppm in the 1 H NMR spectrum, indicating an incomplete substitution of the methyl groups by À OTeF 5 (teflate) groups at the aluminum center. In the 19 F NMR spectrum, two sets of signals for magnetically inequivalent À OTeF 5 groups are observed, while the 27 Al NMR shows a broad signal at 48 ppm, typical for a tetrahedrally coordinated Al center. Single crystals suitable for low-temperature single crystal X-ray diffraction grew in a cooled ortho-difluorobenzene (o-DFB) solution and the molecular structure of the neutral dimer [Al(OTeF 5 ) 2 Me] 2 was obtained (cf. Figure 2). The dimer bridged by two À OTeF 5 groups crystallizes in the monoclinic space group P2 1 /n.
Each of the aluminum centers is coordinated by a methyl group, a terminal teflate group and two bridging teflate moieties, leading to a heavily distorted tetrahedral coordination sphere with bond angles between 79.2(2)°and 118. 5 4(2) pm). [14] This difference in bond distances is analogue to the difference in bond distances of bridging and terminal perfluoroalcoholates in the compound Et 2 Al(μ-OR f ) 2 Al(Et)(OR f ) with OR f = OC(CF 3 ) 3 ) reported by Krossing et al. [15] The AlÀ C bond distances (d(Al1-C1) = 192.3(8) pm) are shortened when compared to the molecular structure of dimeric [AlMe 3 ] 2 , underlining the increased Lewis acidity of the Al centers in [Al(OTeF 5 ) 2 Me] 2 . [16] Recently, we reported on a comparable molecular structure of the higher homologue gallium Et 2 Ga(μ-
To obtain the fully teflate-substituted Lewis acid [Al-(OTeF 5 ) 3 ] 2, either starting from AlMe 3 and HOTeF 5 or [Al-(OTeF 5 ) 2 Me] 2 , a slight excess of teflic acid and further heating to room-temperature is needed. Removing the solvent at reduced pressure again leads to the isolation of a colorless powder. The comparison of recorded IR and Raman spectra of [Al(OTeF 5 ) 3 ] 2 to [Al(OTeF 5 ) 2 Me] 2 shows the absence of residual methyl groups (cf. spectra in Supporting Information). Furthermore, this product is stable for several hours at room temperature and can be stored at À 20°C under an argon atmosphere for months without any decomposition.

Solvent adducts
The reactivity and acidity of the Lewis acid [Al(OTeF 5 ) 3 ] 2 in further reactions is clearly dependent on the solvent that is used. In the following section the interaction of the dimer [Al(OTeF 5 ) 3 ] 2 with different solvents is described. While either no solubility at low temperatures or decomposition was observed with non-polar solvents such as alkanes and methylene chloride, a number of different solvent adducts are obtained with stronger donors (cf. Figure 3).
In our previous work, we briefly discussed the synthesis of the solvent-adduct [Al(OTeF 5 ) 3 (MeCN)] by the reaction of HOTeF 5 with AlEt 3 in n-pentane and the subsequent addition of equimolar amounts of MeCN. [14] For this work we extended the range of nitriles and used acetonitrile as well as benzonitrile (PhCN) as solvent (cf. Scheme 2) and further analyzed the formed compounds by NMR and, in the case of the benzonitrile adduct, also by SC-XRD. In both cases the removal of all volatiles yields a room-temperature stable colorless powder in almost quantitative yields, which can be stored under an argonatmosphere at room temperature for several months without any sign of decomposition.
In the case of the benzonitrile adduct colorless single crystals suitable for SC-XRD could be obtained. The compound [Al(OTeF 5 ) 3 (PhCN) 3 ] crystallizes in the monoclinic space group P2 1 /c with three molecules per asymmetric unit (cf. Figure 4). . [20] The AlÀ N bond distances range from 200.3(3) to 203.7(3) pm. All of the OÀ AlÀ O bond angles are with an average of 95.8°l arger than the NÀ AlÀ N bond angles with an average of 86.7°, leading to a slight distortion of the coordination sphere.
To enable a structural characterization of the autoionized species, bipyridine (bipy) is added to a solution of [Al-(OTeF 5 ) 3 (PhCN) 3 ] in CH 2 Cl 2 . A similar approach was recently reported by Gerken et al. for the autoionization of SbF 5 . [21] The anticipated salt [Al(OTeF 5 ) 2 (bipy) 2 ][Al(OTeF 5 ) 4 (bipy)] crystallizes in the triclinic space group P � 1 (cf. Figure 5). The molecular structure shows two octahedrally coordinated aluminum complexes. The anionic fragment shows AlÀ O bond distances between 183.9(2) and 188.0(2) pm which are comparable to the reported AlÀ O bond distances in [Al(OTeF 5 ) 4 (thf) 2 ] À . [20] The AlÀ O bond distances of the cation are in a similar range and not shortened as one might expect. The same holds true for the AlÀ N bond distances in anion and cation.
Compared to the neat Lewis acid [Al(OTeF 5 ) 3 ] 2 , the nitrile adducts form room-temperature stable compounds and can be easily handled. The main drawback is the quenched Lewis acidity due to the coordination of the nitrile molecules. In order to preserve a high reactivity of the underlying Lewis acid we aimed to prepare adducts with weaker donor solvents. Therefore, the solid dimer [Al(OTeF 5 ) 3 ] 2 was dissolved in fluorobenzene and SO 2 ClF, respectively (cf. Scheme 4).
Dissolution of [Al(OTeF 5 ) 3 ] 2 in fluorobenzene at 0°C results in a light green, clear solution. Warming the mixture to roomtemperature leads to visible decomposition of the compound. Low-temperature NMR spectroscopic measurements revealed the presence of a single aluminum species in the 27 Al NMR spectrum. The broad signal at À 46.1 ppm is in the typical range for tetrahedrally coordinated Al centers and in agreement with the chemical shift of the literature-known [Al{OC(CF 3 ) 3 } 3 (PhF)], thus pointing to the formation of [Al(OTeF 5 ) 3 (PhF)]. [5] The 19 F NMR spectrum shows signals corresponding to one AB 4 spin system with F a at À 40.7 ppm and F b at À 46.1 ppm with a coupling constant of 2 J FF = 191 Hz. A signal for the coordinated PhF could not be observed in the 19 F NMR spectrum since the spectra were recorded in fluorobenzene. Therefore, an exchange of the solvent molecules bound to the Al center can be expected. Attempts to isolate the compound as a neat substance did not yield in any success.  Concentration of a solution of [Al(OTeF 5 ) 3 ] 2 in PhF and further cooling to À 40°C resulted in yellow-green single crystals which were examined by single crystal X-ray diffraction. Instead of a tetrahedral aluminum complex, the five-fold coordinated complex [Al(OTeF 5 ) 3 (PhF) 2 ] was found (cf. Figure 6). The com-pound crystallizes in the monoclinic space group P2 1 /n. In this structure the aluminum center has a trigonal-bipyramidal coordination sphere with three À OTeF 5  (2) pm). [5] Analogous to the findings of Krossing et al., [5] the CÀ F bond lengths of the bound fluorobenzene molecules are elongated by about 7 pm compared to neat fluorobenzene. [22] Changing the solvent and treating solid [Al(OTeF 5 ) 3 ] 2 with an excess of SO 2 ClF results in a clear colorless solution. In contrast to the experiments with fluorobenzene, this mixture is sufficiently stable at room temperature. Interestingly, the adduct formation with SO 2 ClF yields the trigonal-bipyramidal complex [Al(OTeF 5 ) 3 (SO 2 ClF) 2 ] in solution and in the solid state.
The 27 Al NMR spectrum shows a very broad signal at 34.0 ppm (FWHM = 2200 Hz), which lies between the typical regions of four-fold and six-fold coordinated Al centers. Similar to the experiments with PhF, the 19 F NMR spectrum shows only one AB 4 spin system belonging to the three magnetically equivalent À OTeF 5 groups.
A colorless powder is isolated by removing all volatiles at reduced pressure. This powder is stable for several hours at room temperature and was analyzed by IR and Raman spectroscopy. Besides the typical bands of Al(OTeF 5 ) 3 in the IR spectrum, additional bands at 1436 (Raman: 1428) and 1188 (Raman: 1182) cm À 1 for the SO 2 stretching vibrations of the  coordinated SO 2 ClF molecules are observed. Compared to free SO 2 ClF (ν as (SO 2 ) = 1455 cm À 1 , ν s (SO 2 ) = 1224 cm À 1 ) these bands are slightly red-shifted, which indicates a coordination of the SO 2 ClF molecules via the oxygen atom. [23] After concentrating the solution of [Al(OTeF 5 ) 3 (SO 2 ClF) 2 ] and slowly cooling it to À 80°C single crystals suitable for single crystal X-ray diffraction were obtained. The compound [Al-(OTeF 5 ) 3 (SO 2 ClF) 2 ] crystallizes in the triclinic space group P � 1 (cf. Figure 7). Analogous to the molecular structure of [Al-(OTeF 5 ) 3 (PhF) 2 ], the À OTeF 5 groups build the equatorial plane and the SO 2 ClF molecules are bound in axial position to the central aluminum atom, resulting in a trigonal bipyramidal coordination sphere. For the AlÀ O bonds between aluminum and the teflate groups an average bond distance of 174.3 pm with average bond angles of 119.9°is found, which is comparable to the distances and angles in [Al(OTeF 5 ) 3 (PhF) 2 ]. As already discovered by vibrational spectroscopy, the SO 2 ClF molecules are bound by their oxygen atoms with AlÀ O bond lengths of 210.4(6) and 198.9(6) pm. To the best of our knowledge, only one other example of a molecular structure with an oxygen-bound SO 2 ClF molecule has been reported so far, which is [Xe(OTeF 5 )(SO 2 ClF)][Sb(OTeF 5 ) 6 ]. Here, the SO 2 ClF molecule is coordinated to the [Xe(OTeF 5 )] + cation. [24] A remarkable example for a weakly coordinated aluminum complex was shown by Cowley, Jones, and coworkers, when they first synthesized the arene complexes [Al(C 6 F 5 ) 3 (η 1 -C 6 H 6 )] and [Al(C 6 F 5 ) 3 (η 1 -C 7 H 8 )] starting from AlMe 3 and B(C 6 F 5 ) 3 in benzene or toluene. [25] Own attempts to form an arene adduct in an analoguous route with B(OTeF 5 ) 3 as À OTeF 5 group transfer reagent were unsuccessful. Also dissolving the solid [Al-(OTeF 5 ) 3 ] 2 in toluene led to the decomposition of the Lewis acid. The failed reaction can be explained by the low binding energy of a toluene complex compared to the dimeric species and will be discussed in a later section.
Nevertheless, it was possible to obtain an arene adduct by a detour (cf. Scheme 5). In the first step, a solution of AlEt 3 in toluene is treated with 4 equivalents of HOTeF 5 , which leads to the protonation of toluene, thereby forming the strong Brønsted acid [H-C 7 H 8 ][Al(OTeF 5 ) 4 ]. Similar procedures are already described in the literature. [14,26] In the next step, a slight excess of triethylsilane, Et 3 SiH, is added to the mixture, followed by a gas formation accompanied by the decolorization of the bright orange solution. Upon addition of Et 3 SiH, presumably the cationic silylium species [SiEt 3 ] + is formed alongside the evolution of gaseous H 2 , which is a sufficiently strong electrophile to abstract an À OTeF 5 group from the anion [Al(OTeF 5 ) 4 ] À . Thereby, Et 3 SiOTeF 5 and the Lewis acid Al(OTeF 5 ) 3 are formed, the latter of which is stabilized by the present toluene. Warming the reaction mixture above À 40°C results in the visible decomposition of the sample. Therefore, the reaction solution was analyzed by low-temperature NMR spectroscopy. The 19 F NMR spectrum reveals two magnetically inequivalent À OTeF 5 groups, assigned to the formed Et 3 SiOTeF 5 and the solventadduct [Al(OTeF 5 ) 3 (η 1 -C 7 H 8 )]. In the 27 Al NMR spectrum a broad signal at 48 ppm for [Al(OTeF 5 ) 3 (η 1 -C 7 H 8 )] is observed. The 1 H and 29 Si NMR spectra confirm the presence of Et 3 SiOTeF 5 and residual Et 3 SiH.
Further cooling of the reaction mixture to À 80°C led to colorless crystals suitable for single crystal X-ray diffraction. The compound [Al(OTeF 5 ) 3 (η 1 -C 7 H 8 )]·C 7 H 8 crystallizes in the monoclinic space group P2 1 /c (cf. Figure 8). The aluminum center is distorted tetrahedrally coordinated by three À OTeF 5 ligands and a toluene molecule via its para-carbon atom. The AlÀ O bond lengths are in the same range as the aforementioned solvent   4 ], whereby the highly Lewis acidic silylium ion is η 1 -coordinated by toluene (d(Si-C toluene ) = 218 pm). [27] The geometry at the C4 atom indicates a sp 2 hybridization (ff(C3-C4-C5) = 119.4(10)°) and the donor-acceptor bond between Al and C is therefore best described as a π-arene complex (ff(Al1-C4-C3) = 95.8(6)°and ff(Al1-C4-C5) = 91.7(6)°). This is further supported by the maintained planarity of the toluene molecule (largest torsion angle: 3.9°) and similar aromatic CÀ C bond lengths (ranging between 137.6(14) and 141.2(14) pm). The contrary case of a Wheland-type σ-complex would require a sp 3 hybridized carbon atom C4 with bond angles close to 109°, alternating CÀ C bond lengths and a loss of planarity of the aromatic ring.
In order to estimate the ligand affinity of the Lewis acid Al(OTeF 5 ) 3 towards the different solvents and judge the remaining Lewis acidity of the solvent adducts, the fluoride ion affinities and the complexation energies of the adducts have been calculated on the BP86/def-SV(P) and B3LYP/def2-TZVPP level of theory. The results are summarized in Table 1. For the discussion of the calculated FIA values, the BP86/def-SV(P) level of theory and the isodesmic reactions with trimethylsilane as anchor are used to allow a comparison with a previously reported FIA calculation. [2,28] In general, by expanding the ligand coordination sphere of the aluminum center starting from a tetrahedral over a trigonal bipyramidal to an octahedral coordination, a decrease of the Lewis acidity is observed. For the tetrahedrally coordinated complexes, the Lewis acidity of the Al center decreases in the order toluene > SO 2 ClF > PhF > MeCN > PhCN > Et 2 O. This is also reflected in the experiment, whereby the complex stability increases in the same order. This trend is also in agreement with experimental donor numbers of the respective solvents.   In some cases, it was not possible to successfully form a solvent-adduct by just adding an excess of solvent to the neat Lewis acid [Al(OTeF 5 ) 3 ] 2 . This problem could be circumvented by starting from weakly bound adducts. As an example, we added a small excess of diethyl ether to [Al(OTeF 5 ) 3 (SO 2 ClF) 2 ] in SO 2 ClF, resulting in the formation of the diethyl ether adduct. By slowly cooling the reaction mixture to À 80°C it was possible to obtain colorless crystals of the product. The compound [Al-(OTeF 5 ) 3 (Et 2 O) 2 ] crystallizes in the triclinic space group P � 1 (cf. Figure 9). Similar to the adducts with PhF and SO 2 ClF, the complex possesses a trigonal bipyramidal coordination sphere at the Al center. The bond distances between the aluminum and the oxygen atom of Et 2 O (d(Al1-O4) = 195.7(2) and d(Al1-O5) = 197.1(3) pm) are comparable to the analogue bond lengths in literature-known [Al(OC 5 F 4 N) 3 (Et 2 O) 2 ]. [11] This reflects well on the similar Lewis acidity of the two compounds.
An established method for experimentally gauging the acidity of a Lewis acid is the Gutmann-Beckett method, in which the 31 P NMR chemical shift of triethylphosphine oxide, OPEt 3 , in a Lewis acid complex is analyzed in respect to free OPEt 3 . [30] By reacting an equimolar amount of OPEt 3 with [Al(OTeF 5 ) 3 ] 2 in SO 2 ClF it was possible to obtain the tetrahedral complex [Al(OTeF 5 ) 3 OPEt 3 ]. Note that any excess of the phosphine will lead to multiple coordination to the Al center and therefore result in ambiguous signals in the corresponding NMR spectra. The 31 P NMR of this compound gave a signal at 83.9 ppm. Compared to free OPEt 3 (δ in CD 2 Cl 2 : 50 ppm) this resonance is shifted by 33.9 ppm and clearly surpasses other aluminum based Lewis superacids such as Al(C 6 F 5 ) 3 (Δδ: 26.0 ppm) [31] and Al[OC(C 6 F 5 ) 3 ] 3 (Δδ: 23.9 ppm). [12] This high value is also in line with the calculated FIA of Al(OTeF 5 ) 3 . Therefore, this compound combines both, a high global (according to FIA) and effective (according to GB method) Lewis acidity. [32] By cooling a concentrated solution of [Al(OTeF 5 ) 3 OPEt 3 ] in SO 2 ClF colorless crystals were obtained. The compound [Al-(OTeF 5 ) 3 OPEt 3 ] crystallizes in the orthorhombic space group  Pbca (cf. Figure 10) 5 ] crystallizes in the monoclinic spacegroup P2 1 /c (cf. Figure 11) 0,1,2,3). The 27 Al NMR spectrum of this reaction shows three sharp signals at 80.5, 65.1, and 47.6 ppm, corresponding to the [Al(OTeF 5 ) 2 Cl 2 ] À , [Al(OTeF 5 ) 3 Cl] À , and [Al(OTeF 5 ) 4 ] À ions. [14] These resonances are flanked by 125 Te satellites of the corresponding isotopologues. In the 19 F NMR spectrum three AB 4 patterns are observed, which are assigned to the chloroaluminates by their integral ratio.
Treating the adduct [Al(OTeF 5 ) 3 (SO 2 ClF) 2 ] with trityl chloride CPh 3 Cl in SO 2 ClF immediately yields an intense yellow solution, already indicating the formation of the carbocation [CPh 3 ] + . Analysis by NMR spectroscopy shows beside the formation of the desired cation again a mixture of anions [Al(OTeF 5 ) 4-n Cl n ] À (n = 0,1,2,3) as mentioned above (more details in Supporting Information). A similar ligand scrambling of the anion was reported by Riddlestone et al. when they treated the Lewis acid Al(OC 5 F 4 N) 3 with trityl chloride. [11] In an attempt to obtain halogen-bridged adducts of the form [Al(OTeF 5 ) 3

(Me 3 SiF)] and [Al(OTeF 5 ) 3 (Me 3 SiCl)] [33] in analogy to the literature-known [Al{OC(CF 3 ) 3 } 3 (Me 3 SiF)] and [Al{OC-(CF 3 } 3 ) 3 (Me 3 SiCl)]
, [15] we treated [Al(OTeF 5 ) 3 (SO 2 ClF) 2 ] with the respective trimethylsilyl halides. Instead of the desired reaction,  the formation of the species Me 3 SiOTeF 5 is observed in both cases by NMR spectroscopy. [34] Moreover, in the reaction of [Al(OTeF 5 ) 3 (SO 2 ClF) 2 ] with Me 3 SiCl the formation of AlCl 3 is observed in the 27 Al NMR spectrum, while in the case of Me 3 SiF a colorless, insoluble solid precipitates, which is likely insoluble AlF 3 . Subsequently, a substitution of the À OTeF 5 groups by the halogen atom of the trimethylsilyl halides takes place. This is due to two reasons: The high Lewis acidity and the steric accessibility of the Al atom in [Al(OTeF 5 ) 3 (SO 2 ClF) 2 ] allow a dynamic ligand exchange. Further, the formation of Me 3 SiOTeF 5 and AlF 3 are thermodynamically favored and therefore drive the reaction.

Conclusion
In this work we report the improved synthesis of the Lewis superacid Al(OTeF 5 ) 3 in its neat dimeric form in gram-scale, as well as the synthesis and characterization of a variety of solvent adducts. These range from octahedral complexes with strong donor molecules to extremely weakly bound tetrahedral complexes. Theoretical calculations on the complexation energies and fluoride ion affinities of these adducts show that, depending on the solvent, the reactivity of the Lewis acid can be controlled, while a very high acidity is retained. Experimental validation of its Lewis acidity by the Gutmann Beckett method and fluoride abstraction from a hexafluoroantimonate salt confirm the Lewis superacidity of Al(OTeF 5 ) 3 . As expected, fluoride and chloride abstractions can be easily realized with this species, but the accessibility of the aluminum center can also lead to ligand scrambling. This allows Al(OTeF 5 ) 3 and its solvent-adducts to be used in the future whenever an extreme high fluoride ion affinity is needed, a recent example being the successful synthesis of the perfluorinated trityl cation. [35]

Experimental Section
All preparative work was carried out using standard Schlenk techniques. Glassware was greased with Triboflon III. The pentafluoroorthotelluric acid HOTeF 5 was prepared as described elsewhere. [36] All solid materials were handled inside a glove box with an atmosphere of dry argon (O2 < 0.5 ppm, H2O < 0.5 ppm). All solvents were freshly dried with CaH 2 before use and stored on molecular sieve. HSiEt 3 , FSiMe 3 and ClSiMe 3 were degassed prior to use and CPh 3 Cl was dried in dynamic vacuum overnight and stored in a dry argon box. Raman spectra were recorded on a Bruker MultiRAM II equipped with a low-temperature Ge detector (1064 nm, 50-100 mW, resolution 4 cm À 1 ). IR spectra were measured on a Bruker ALPHA FTIR spectrometer equipped with a diamond ATR attachment in a glove box filled with argon (resolution 4 cm À 1 ). NMR spectra were recorded on a JEOL 400 MHz ECS or ECZ spectrometer. All reported chemical shifts were referenced to the Ξ values given in IUPAC recommendations of 2008 using the 2 H signal of the deuterated solvent as internal reference. [37] Chemical shifts and coupling constants of 19 F NMR spectra are given as simulated by gNMR. [38] Crystal data were collected with MoK α radiation on a Bruker D8 Venture diffractometer with a CMOS area detector. Single crystals were picked at À 40°C under nitrogen atmosphere and mounted on a 0.15 mm Micromount using perfluoroether oil. The structures were solved with the ShelXT [39] structure solution program using intrinsic phasing and refined with the ShelXL [40] refinement package using least squares on weighted F2 values for all reflections using OLEX2. [41]  )]) contain(s) the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service. The Turbomole program [42] was used to perform calculations at the unrestricted Kohn-Sham DFT level, using the BP86 or B3LYP hybrid functional [43] (with RI [44] ) in conjunction with basis sets def-SV(P) and def2-TZVPP. [45] Minima on potential energy surfaces were characterized by normal mode analysis. Thermochemical data is provided without counterpoise correction but including zero-point energy correction as obtained from harmonic vibrational frequencies.