Noncovalent Interactions in Halogenated Pyridinium Salts of the Weakly Coordinating Anion [Al(OTeF5)4]−

Abstract The synthesis and the first structural characterization of the halogenated pyridinium salts [C5F5NH]+, [C5F4ClNH]+, [(C5F5N)2H]+, [(C5Cl5N)2H]+ of the weakly coordinating anion (WCA) [Al(OTeF5)4]−, showing noncovalent interactions in the solid state, are presented. The salts were characterized by the multinuclear NMR and IR spectroscopy as well as X‐ray diffraction. Hirshfeld surface analysis and solid state structures reveal various intermolecular anion‐π and σ‐hole interactions between the corresponding halogenated pyridinium cations and the anion [Al(OTeF5)4]−.


Introduction
In contrast to pyridine, the perfluorinated pyridine C 5 F 5 N, exhibits an highly reduced basicity. While non fluorinated pyridine can be protonated by HCl or HBr, pentafluoropyridine can only be protonated by Brønsted superacids such as HF/AsF 5 and HF/SbF 5 . [1] The strong Brønsted superacid [o-C 6 H 4 F 2 -H][Al-(OTeF 5 ) 4 ] is known for the protonation of weak bases like benzene and white phosphorus, resulting in [C 6 H 7 ] + and [P 4 H] + , respectively. [2] This superacid can be potentially used for protonation of halogenated pyridines. Moreover, the perhalogenated pyridinium cations are strong acids themself and should give rise to a wide variety of binding types. Namely the NÀ H bond, which is a strong hydrogen bond donor and the halogen-carbon bond that serve as halogen bond donor. Additionally, perhalogenated pyridines exhibit positive quadrupolar moments like in C 6 F 6 , which can result in anion-π interactions. [3] In contrary to the well-known cation-π interactions, anion-π interactions are extremely rare and far less investigated. [4] Anion-π interactions occur between electrondeficient arenes with a positive quadrupole moment and negatively charged or electron-rich sites. [3] They are directional and located above the aromatic ring periphery as a short contact and are predominantly investigated in the solid state. [5] Previous studies showed anion-π interactions between non-fluorinated nitrogen-containing hetrerocycles and small weakly coordinating anions (WCAs) like [BF 4 ] À and [PF 6 ] À [6] or between perfluoroarenes, like C 5 F 5 N, C 6 F 6 and C 10 F 8 with different halide anions. [7] In this work, we investigated the protonation of C 5 F 5 N, C 5

Results and Discussion
In order to study the basicity of some easily available perhalogenated pyridines, we calculated their proton affinities. As expected, the basicity trend follows the row C 5 F 5 N < C 5 F 4 ClN < C 5 Cl 5 N ( Figure 1). Their proton affinities (PAs) are higher than for ortho-difluorobenzene (743.0 kJ/mol) and therefore all of the pyridine derivatives can be in principle protonated by the superacid 1 a.
The computed plots of the electrostatic potentials (ESP) of the pyridinium cations illustrate that in all cases, their strongest interaction with an anion will be observed via the NÀ H moiety (Figure 2). Beside the hydrogen bond site, there is the region of depleted electron density above and below the aromatic pyridinium ring. This site is also known from the literature as π-hole and can lead to anion-π interactions. [8] While the cations 2 and 3 show a strong depletion of the π-electron density above the π-bonds in a plane perpendicular to the pyridinium ring, for 4 only a weak depletion is observed. The chlorinated pyridinium cations 3 and 4 show additionally σ-holes -an area of enhanced positive electrostatic potential, located on the carbon-chlorine axis. [9] For the cation 4 differently sized σ-holes on the chlorine atoms are observed, see Figure 2. The biggest σ-holes are found for the 2,6-position. In salts with cation 3, it might be possible to study all above discussed interactions simultaneously.
In order to obtain the corresponding pyridinium salts, we treated C 5 F 5 N, C 5 F 4 ClN or C 5 Cl 5 N with the Brønsted superacid 1 a in ortho-difluorobenzene (oDFB) (Scheme 1).
The salt 2 a crystallizes in the triclinic space group P � 1 ( Figure 3) by slowly cooling down the reaction mixture to À 24°C. The short N···F distance of 270.82 (25) pm, is in agreement with a strong hydrogen bond between the NÀ H moiety and a fluorine atom of the WCA. [10] Figure 4 shows the co-planar arrangement of two neighboring moieties of 2 with a C3-F3-F4-C4 dihedral angle of 180(2)°. These moieties are symmetry related by an inversion center. As short fluorine/fluorine (F4···F3) contacts of 276.8 (21) pm are observed between the two cations we further inspected the fluorine-specific interaction in an isolated [C 5 F 5 NH] + -dimer by means of atoms in molecules (AIM) analysis. At the bond critical points (CP) between F3 and F4 the electron density is very low (ρ CP = 0.045 e A À 3 ) and the Laplacian is small and positive (! 2 ρ CP = 0.035 e A À 5 ). Together with a value of the electron localization function (ELF) close to zero (0.01) and a ratio of the absolute potential (j V j) and the kinetic energy density (G) below 1 (0.74), this indicates a non-shared interaction such as a van-der-Waals interaction. An ionic interaction appears to be unlikely for this dimer of two cations.
From the reaction of 1 a with an excess of C 5 F 5 N the salt [(C 5 F 5 N) 2 H][Al(OTeF 5 ) 4 ] (5 a) was obtained. It crystallizes in the monoclinic space group Cc ( Figure 6). In the solid state, a pentafluoropyridinum cation interacts with its NÀ H moiety with the nitrogen atom of another pentafluoropyridine forming a pyridinium-pyridine dimer, as it has been described for the nonfluorinated analogue before. [11] The NÀ N distance in 5 a is measured to be 275.46(83) pm which is about 35 pm shorter than the sum of the van der Waals radii of 310 pm. [10] The pentafluoropyridine rings are  arranged co-planar, which is rather unusual. Single molecule calculations at the B3LYP-D3/def2-TZVPP level of theory for the dimer predict a structure with a dihedral angle of about 55.5°t o be lower in enegy by 16.4 kJ/mol and find the coplanar arrangement to be a transition state. This can be explained by solid state anion-π interactions, in which the fluorine atoms of the OTeF 5 moiety above and below the plane of the penta-fluoropyridines are stabilizing the planar arrangement (Figure 7).
The Hirshfeld surface plot depicts short contacts from F27, F21, F17 and F29 of the WCA to the rings of cation 5 (see Supporting Information Figure SI 3.1). For 4-halotetrafluoropyridines, the halogen bond donor strength weakens in the order I, Br, Cl. [12] The halogen bond donor strength can be increased considerably by N-methylation, like it was shown in case of 4iodo-2,3,5,6-tetrafluoropyridine and resulted in the formation of a halogen bond with the WCA [Al(OTeF 5 ) 4 ] À in the solid state. [13] Analogously, an increase of the halogen bond donor strength in halogenated pyridines should be achieved by their protonation. In order to study this behaviour, we reacted one equivalent of 4-chloro-2,3,5,6-tetrafluoropyridine with 1 a (Scheme 1).
[ The NÀ H moiety is interacting with two fluorine atoms, F12 and F12', via close fluorine-specific hydrogen bond interactions. A similar arrangement is found for the chlorine atom Cl4 and

Chemistry-A European Journal
Research Article doi.org/10.1002/chem.202202749 the fluorine atoms F4 and F4', respectively. The rather strong halogen-halogen bond exhibts a chlorine-fluorine distance of 302.0(17) pm which is 20 pm below the sum of the van der Waals radii. Multiple short F Anion -C Ar contacts below the sum of the van der Waals radii of 317 pm were found. [10] For the anionπ interaction in heteroaromatic systems, the α-carbon next to the heteroatom is usually found to form the strongest interactions. [6] This is also the case for 3 a in the solid state. Hirshfeld surface analysis reveals interactions between the fluorine atoms F8 and F6 of the WCA and the π-system of cation 3 ( Figure S3.2). The reaction of two equivalents of pentachloropyridine with 1 a yielded single crystals of [(C 5 Cl 5 N) 2 H][Al(OTeF 5 ) 4 ]·C 6 H 4 F 2 (6 a·C 6 H 4 F 2 ). The compound crystallizes in the monoclinic space group P2 1 /c ( Figure 9).
As for the fluorine analogue 5 a the pyridine rings form a pyridinium-pyridine dimer. The NÀ N distances in dimers 5 a and 6 a are indistinguishable within the experimental error. Moreover, the two pentachloropyridine rings in 6 are staggered with an angle between the planes of 95.8(1)°, in contrast to the fluorinated analogue 5 which is arranged coplanar (Figure 4). Single molecule calculations on B3LYP-D3/def2-TZVPP level of theory of the perchlorinated dimer 6 find the coplanar arrangement to be a 2 nd order saddle point that is 68.2 kJ/mol higher in energy than the staggered conformation with a dihedral angle of 90.2°representing the global minium. This structural difference in the solid state between the cations 5 and 6 can be attributed to the weaker anion-π interactions (see Figure 2) as well as the bigger repulsion between the chlorine atoms in 2,6position. One of the perchloropyridine rings shows a πinteraction with a cocrystalized oDFB molecule. They exhibit a face-to-face arrangement with a centroid-centroid distance of 355.8(1) pm which indicates a rather strong interaction. [14] Several anion-π interactions are observed between the fluorine atoms F1, F9, F16, and F19 of the WCA and 6 (see Supporting Information Figure S3 In the IR spectra of 2 a and 3 a several broad bands for the NÀ H stretch are observed. The NÀ H band for 2 a exhibits two maxima at 3104 cm À 1 and 2953 cm À 1 , respectively. The NÀ H band for 3 a shows three maxima at 3301 cm À 1 , 3204 cm À 1 and 2953 cm À 1 , respectively. In contrast to the spectra of the monomeric cations the spectra of the pyridinum-pyridine dimers show very broad bands in the region from 3000 to 1750 cm À 1 for 5 a and from 3500 to 2000 cm À 1 for 6 a attributed to the NÀ H stretching vibration. This is well combarable to the reported spectrum of the nonhalogenated pyridinum-pyridine dimer. [15] Conclusion H] + were structurally characterized for the first time. In the solid state rather strong noncovalent interactions between these electron deficient aromatic cations and the weakly coordinating anion [Al(OTeF 5 ) 4 ] À are observed. Due to the protonation of the halogenated pyridines, their ability to form halogen bonds as well as anion-π interactions is increased compared to the neutral species.

Experimental Section
For detailed experimental data, see electronic Supporting Information. All reactions were carried out under inert conditions using standard Schlenk techniques. Glass vessels were greased with Triboflon III. All solid materials and triethylaluminium (Al(C 2 H 5 ) 3 , 93 %) were handled inside a glove box with an atmosphere of dry argon (O 2 < 0.5 ppm, H 2 O < 0.5 ppm). The pentafluoroorthotelluric acid was synthesized as reported in the literature. [16] All solvents were dried either with CaH 2 or with Sicapent® before use. IR spectra were collected on a Bruker ALPHA FTIR spectrometer equipped with a diamond ATR attachment in an argon-filled glove box. NMR spectra were recorded on either a JEOL 400 MHz ECS or ECZ-R spectrometer. Reported chemical shifts are referenced to the Ξ values given in IUPAC recommendations of 2008 using the 2 H signal of the deuterated solvent as internal reference. For external locking [D 6 ]-acetone was flame sealed in a glass capillary and the lock oscillator frequency was adjusted to give δ( 1 H) = 7.26 ppm for a CHCl 3 sample locked on the capillary. Chemical shifts and coupling constants of strongly coupled spin systems are given as simulated in gNMR. [17] Crystal structures were obtained on a Bruker D 8 Venture diffractometer with a PHOTON 100 CMOS area detector using MoÀ Kα radiation. Single crystals were coated with a perfluoroether oil at À 25°C and selected under nitrogen atmosphere. Using Olex2, [18] the structures were solved with the ShelXT [19] structure solution program by intrinsic phasing and refined with the ShelXL [20] refinement package using least-squares minimization. All calculations were performed using a general-purpose High-Performance Computer at ZEDAT (CURTA), [21] provided by Freie Universität Berlin. For density functional calculations the Gaussian 16 [22] software was used with its implementation of B3LYP, and Grimme-D3 [23] together with the basis set def2-TZVPP. [24] Calculated structures, as well as crystal structures, were visualized with Diamond. [25] Electrostatic potentials were calculated using the Turbomole software and visualized with a VMD editor. Molecular Hirshfeld surface contours were performed using the Crystal Explorer. In this study, all the Hirshfeld surfaces were generated using a high (standard) surface resolution. For 5 a, the 3-D dnorm surfaces were mapped using a fixed color scale of À 0.4727 (red) to 0.5303 (blue), using an isovalue of 0.5. For 2 a, 3 a and 6 a·o-C 6 H 4 F 2 , the 3-D dnorm surfaces were mapped using a fixed color scale of À 0.0964 (red) to 0.9305 (blue), using an isovalue of 0.5. Red contours indicated a contact less than the sum of the van der Waals radii of the respective elements. Blue and white contours indicate that the nearest external atom is at a distance greater or equal to the sum of the van der Waals radii respectively from atomic coordinates. Atoms in Molecules (AIM) analysis of an isolated [C 5 F 5 NH] + -dimer based on calculations at B3LYP-D3/cc-pVTZ [26] level were performed with the MultiWFN program. [27] The dimer structure was extracted from the experimental crystal structure and not further optimized.

Synthesis of [C 5 F 5 NH][Al(OTeF 5 ) 4 ] (2 a):
A sample of Al-(C 2 H 5 ) 3 (78 mg, 0.68 mmol, 1 equiv.) was dissolved in 3 mL of ortho-difluorobenzene (oDFB). The solution was degassed and HOTeF 5 (652 mg, 2.72 mmol, 4 equiv.) was condensed onto it at À 196°C. The reaction mixture was warmed up to À 30°C. Afterwards a gas bubbler was added and the reaction mixture was stirred for 30 min. A yellow solution was obtained indicating the formation of the corresponding Brønsted superacid. C 5 F 5 N (115 mg, 0.68 mmol, 1 equiv.) was condensed into a Schlenk tube with a PTFE cap and dissolved in 2 mL of oDFB. This solution was added to reaction vessel via syringe under an argon stream. The gas bubbler was exchanged with a stopper and the mixture was stirred for another 30 min at À 30°C. The pressure in the reaction vessel was reduced and the flask was placed in a À 24°C fridge for crystallization. [

Synthesis of [C 5 F 4 ClNH][Al(OTeF 5 ) 4 ] (3 a):
The synthesis was analogous to the one of [C 5 F 5 NH][Al(OTeF 5 ) 4 ] using C 5 F 4 ClN (130 mg, 0.7 mmol, 1 equiv.) that was previously dissolved in 2 mL of oDFB. After the reaction was complete, all volatile parts were removed in vacuo. The flask with the remaining solution was put into a À 24°C fridge for crystallization. [