Aldiminium Cations as Countercations to Discrete Main Group Fluoroanions

The reactions of group 14 tetrafluorides (SiF4, GeF4, and SnF4) and group 15 pentafluorides (PF5, AsF5, and SbF5) with the CAAC-based trifluoride reagent [MeCAACH][F(HF)2] led to the isolation of salts containing discrete 5- or 6-coordinated fluoroanions. The syntheses of [MeCAACH][SiF5], [MeCAACH][GeF5], [MeCAACH][(THF)SnF5], and the structurally related [MeCAACH][(dioxane)SnF5], [MeCAACH][PF6], [MeCAACH][AsF6], and [MeCAACH][SbF6] are effective, selective and in high yield. All compounds were characterized by X-ray single-crystal structure analysis, NMR and Raman spectroscopy. It is worth noting that the synthesized [MeCAACH][GeF5] is a rare example of a structurally characterized compound with discrete [GeF5]− anion, while [MeCAACH][(THF)SnF5] and [MeCAACH][(dioxane)SnF5] represent the first compounds with discrete octahedrally coordinated tin fluoride anions with incorporated solvent molecules. Finally, the aldiminium-based cation [MeCAACH]+ proved to be suitable for the stabilization of rare discrete main group fluoride anions.


Introduction
Cyclic (alkyl)(amino)carbenes (CAACs) are a class of compounds that have been extensively studied since their discovery in 2005 [1].They have been shown to be among the most nucleophilic and electrophilic stable carbenes known to date [2].Their electronic and steric properties make them suitable ligands for the stabilization of highly reactive and unusual main group and transition metal species in their low or high oxidation states [2][3][4].CAACs can be prepared from the corresponding aldiminium-based salts by deprotonation.Nowadays, several methods for the preparation of CAAC precursors are known [2].It all started with the discovery of aldiminium-based triflate salts by Bertrand's group [1] and was extended with the synthesis of aldiminium-based chloride salts [5].Both salts can be effectively deprotonated to form CAACs. Interestingly, CAAC complexes could also be prepared starting from CAAC(H)OH in aqueous solution [6].Later, aldiminium-based tetrafluoroborate salts also found their use as CAAC precursors [7].They are usually prepared by ion exchange from the corresponding chloride derivatives using KBF 4 [8].Recently, [ Me CAACH][BF 4 ] was prepared by Jana's group from CAAC: carbene and NO [BF 4 ] [9].The Me CAAC: carbene, which acted as a one-electron reducing agent, was chemically oxidized with NO[BF 4 ] to form a transient radical cation, which was subsequently converted to [ Me CAACH][BF 4 ] by hydrogen abstraction [9].The same method led to the formation of [ Me CAACH][SbF 6 ] when NO[SbF 6 ] was used [9].
Aldiminium-based triflate and chloride salts can be used not only as CAAC precursors but also as unusual group transfer reagents in the chemistry of aluminum hydrides [10].Recently, we have prepared a series of aldiminium-based fluoride and poly(hydrogen fluoride) compounds that can be used as nucleophilic fluorination reagents [11].In particular, the aldiminium-based trifluoride salt [ Me CAACH][F(HF) 2 ] proved to be very effective in organic transformations due to its ability to convert benzyl bromides, 1-and 2-alkyl bromides, sulfonyls, and silanes to the target fluorides [11].
particular, the aldiminium-based trifluoride salt [ Me CAACH][F(HF)2] proved to be very effective in organic transformations due to its ability to convert benzyl bromides, 1-and 2-alkyl bromides, sulfonyls, and silanes to the target fluorides [11].
In this work, we tested the reactivity of the recently developed [ Me CAACH][F(HF)2] with inorganic fluoride compounds, in particular with group 14 and 15 elements.Our main goal was to extend the knowledge of aldiminium-based compounds and to prepare a series of compounds that could potentially exhibit group transfer properties analogous to the corresponding triflate and chloride salts.It is well known that cations have a strong influence on the form of the anions.For example, it has been shown that the steric bulkiness of the [IPrH] + cation is suitable for the stabilization of discrete [SiF5] − , and [GeF5] − anions, while less sterically demanding cations preferentially form octahedrally coordinated [SiF6] 2− species [12].With this in mind, we were particularly interested in whether the aldiminium-based reagent would allow the stabilization of discrete group 14 fluoroanions.

Synthesis and Structural Characterization
In our work, we tested the reactivity of group 14 tetrafluorides (SiF4, GeF4, and SnF4) and group 15 pentafluorides (PF5, AsF5, and SbF5) using the CAAC-based trifluoride reagent [ Me CAACH][F(HF)2].Since the properties of the starting compounds differ significantly even within the same group, slightly different synthesis procedures were required to prepare salts with 5-or 6-coordinated fluoroanions.The synthesis and characterization of all salts are systematically presented in the following chapters.Crystallographic data for all newly characterized compounds are compiled in the Supporting Information.] − anions, large cations must be used [12,17], and the sterically demanding cation [CAACH] + was found to be suitable for this purpose.
bond lengths and angles are not compared with the previously reported structures due to the heavy disorder of the anion.The structural features of our [GeF5] − anion in [ Me CAACH][GeF5] are also consistent with the previously described [IPrH][GeF5] [12].It is worth noting that [IPrH][GeF5] represents the first example in which the discrete [GeF5] − anion is present, while [ Me CAACH][GeF5] is only the second example.To stabilize discrete [SiF5] − and [GeF5] − anions, large cations must be used [12,17], and the sterically demanding cation [CAACH] + was found to be suitable for this purpose.Removal of the volatiles resulted in the formation of a brown solid that could not be crystallized despite several attempts.Several attempts have been made in the past to crystallize the monomeric [SnF 5 ] − anion, but none of them was successful [18].Since tin preferentially forms octahedrally coordinated units in the presence of fluoride, [SnF 5 ] − units usually form oligomeric or polymeric structures [19].Octahedrally coordinated discrete tin fluoride anions, such as [SnF 6 ] 2− , are more common and better studied [20][21][22].However, only a limited number of structurally characterized [SnF 6 ] 2− anions with organic cations are known.To date, there are only 12 discrete structures in the CCDC crystal structure database [23].was previously prepared and structurally characterized by Jana's group [9].Our results agree well with the reported data and provide a simple alternative approach for the formation of [ Me CAACH][SbF6] in high yields.In addition to the tetrafluoroborate salts, the CAAC-based chloride and triflate salts are still used as precursors for CAAC: carbenes.In the past, we have demonstrated their usefulness as group transfer reagents in aluminum hydride chemistry [10].During our

NMR Spectroscopy
The 1 H, 13 C, 19 F, and other heteronuclear NMR spectra are given in the Supporting Information for all synthesized salts.All spectra are consistent with crystal structures determined by single-crystal X-ray diffraction.In the 1 H and 13 C NMR spectra, the characteristic signals for the [ Me CAACH] + cation are visible.In addition, the 1 H and 13 C NMR spectra of [ Me CAACH][(THF)SnF5] contain signals for THF and residual hexane.
Measured 19 F and other heteronuclear NMR peaks ( 11 B, 29 Si, 119 Sn, 31 P, 75 As, and 121 Sb) are listed in Table 1.The 74 Ge NMR spectra could not be measured due to the characteristics of the broadband probe.The frequency of 74 Ge was outside the range of the broadband probe used.The data collected for the discrete fluoroanions of the main group elements agree well with the data reported for other related compounds.

NMR Spectroscopy
The 1 H, 13 C, 19 F, and other heteronuclear NMR spectra are given in the Supporting Information for all synthesized salts.All spectra are consistent with crystal structures determined by single-crystal X-ray diffraction.In the 1 H and 13  Measured 19 F and other heteronuclear NMR peaks ( 11 B, 29 Si, 119 Sn, 31 P, 75 As, and 121 Sb) are listed in Table 1.The 74 Ge NMR spectra could not be measured due to the characteristics of the broadband probe.The frequency of 74 Ge was outside the range of the broadband probe used.The data collected for the discrete fluoroanions of the main group elements agree well with the data reported for other related compounds.[13], −138.4 ppm [14], and −137.08 ppm [12]) and [GeF 5 ] -anions ( 19 F: −136.60 ppm [12]).The 29 Si NMR signal of [SiF 5 ] -in [ Me CAACH][SiF 5 ] ( 29 Si: −149.86 ppm) is also consistent with the previously reported data for [SiF 5 ] -anions ( 29 Si: −147.5 ppm [14]).
Since the [(THF)SnF 5 ] -anion is not symmetrical, two distinct signals corresponding to the anion are seen in the 19 F NMR spectra.A doublet is observed at −160.37 ppm belonging to the 4 cis-fluorine atoms and a quintet at −170.20 ppm belonging to the trans-fluorine atom.The same NMR pattern was observed by Röschenthaler's group for [RSnF 5 ] − anions (R = (Me 2 N) 2 C and C 5 H 10 N 2 ) [24].The 119 Sn NMR spectrum for the [(THF)SnF 5 ] -anion contains a multiplet at −789.93 ppm.Our results are in a similar range to those reported for [RSnF 5 ] − anions ( 119 Sn: -749.75 ppm) [24].The 119 Sn NMR spectra of [RSnF 5 ] − anions give a doublet of quintets with 1 J SnF(cis) = 2160 Hz and 1 J SnF(trans) = 1490 Hz [24].The 119 Sn NMR spectrum of our [(THF)SnF 5 ] -anion has a similar shape with 1 J 119SnF = 1880 Hz.However, not all peaks are visible in the NMR spectrum due to the high background noise.
The 19 F NMR signal of the [SbF 6 ] -anion in [ Me CAACH][SbF 6 ] consists of two sets of peaks.The first set of peaks with slightly higher intensity ( 19 F: −123.96ppm with J = 1936.3Hz) is split into six lines due to scalar coupling with 121 Sb (I = 5/2, natural abundance 57.25%), while the second set of peaks with slightly lower intensity ( 19 F: −123.96ppm with J = 1050.4Hz) is split into eight lines due to scalar coupling with 123 Sb (I = 7/2, natural abundance 42.75%).The chemical shift of the 19 F NMR spectra agrees well with the previously reported results for [SbF 6 ] -anions ( 19 F: −113.69 ppm [31]).The difference of about 10 ppm can be attributed to the use of a different NMR solvent.published in the literature [33].Assigning the As-F vibration in [ Me CAACH][AsF 6 ] was difficult because the more intense peak at 681 cm −1 , which is in better agreement with the literature data, is likely a signal from the [ Me CAACH] + cation since it is present in the Raman spectra of all other aldiminium-based salts.

Caution
F 2 and aHF are extremely corrosive and highly dangerous gases.They should be handled with care by an experienced experimenter in a well-ventilated hood.F 2 is also a strong oxidizer that should be handled in nickel or copper equipment.All equipment used for reactions with F 2 should be thoroughly cleaned, degreased, and fluorinated before use.The gases BF 3 , SiF 4 , GeF 4 , PF 5 , and AsF 5 are also toxic.They readily hydrolyze in the presence of water to form HF. Take care to avoid inhalation and contact with skin.Always wear protective clothing, gloves, and a face mask when handling corrosive gases.

General
All syntheses were carried out under anhydrous conditions.Nonvolatile compounds were stored and handled in a glovebox (M.Braun) maintained below 0.1 ppm O 2 and H 2 O, while gaseous and volatile compounds were handled via a nickel and polytetrafluoroethylene (PTFE) vacuum line.All reactions were performed in tetrafluoroethylenehexafluoropropylene (FEP) reaction vessels equipped with PTFE valves.Before use, the vessels were passivated with F 2 at 1 bar for 2 h and then evacuated.Crystallization of all newly prepared compounds proceeded by vapor diffusion.Approximately 20 mg of each salt was dissolved in 0.5 mL of solvent (usually DCM) in a small vial.The small vial was placed in a larger wide-neck vial containing 2.5 mL of antisolvent (usually cyclopentane) and capped with a screw cap.After a few days, small crystals formed in the small vial.

NMR Spectroscopy
Samples were prepared under inert atmosphere in a glovebox (M.Braun).NMR spectra were recorded in 5 mm glass NMR tubes with FEP inlays.Measurements were performed at the Slovenian NMR Centre (National Institute of Chemistry) using a Bruker AVANCE NEO 600 MHz NMR spectrometer (Bruker Corporation, Billerica, MA, USA).Chemical shifts of 1 H and 13 C were referenced to residual MeCN-d 3 signals and reported relative to TMS (tetramethylsilane).The chemical shifts of 19

Crystal Structure Determination
Crystal data for all compounds were collected with a Gemini A diffractometer (Agilent Technologies, Santa Clara, CA, USA) equipped with an Atlas CCD detector using graphite-monochromated Cu Kα radiation at 150 K.The data were processed using the CrysAlisPro software package [37].An analytical absorption correction was applied to all data sets [38].Structures were solved using the SHELXT program [39].Structure refinement was performed using the SHELXL software [40] implemented in the Olex2 program package [41].The figures were created using Diamond [42].

Raman Spectroscopy
Samples were filled into 0.3 mm quartz capillaries under an inert atmosphere in a glovebox (M.Braun).Raman spectra were recorded using a Horiba Jobin Yvon Labram-HR spectrometer (HORIBA, Ltd., 2 Miyanohigashi, Kisshoin, Minami-ku Kyoto, 601-8510 Japan) coupled with an Olympus BXFM-ILHS microscope (Olympus Corporation, Shinjuku, Tokyo, Japan) at room temperature.Samples were excited with the 633 nm emission line of a He-Ne laser.

Conclusions
Aldiminium-based salts can be used as CAAC precursors or as unusual group transfer reagents.Since not many aldiminium-based compounds are yet known, we have carried out a systematic study on the synthesis of aldiminium-based salts of fluoroanions of groups 14 and 15.To achieve this goal, the reactivity of group 14 tetrafluorides (SiF 4 , GeF 4 , and SnF 4 ) and group 15 pentafluorides (PF 5 , AsF 5 , and SbF

2. 1 Scheme 1 .
Scheme 1. Synthesis of [ Me CAACH][SiF5] and [ Me CAACH][GeF5].Single crystals of [ Me CAACH][SiF5] and [ Me CAACH][GeF5] were prepared by vapor diffusion crystallization using DCM as solvent and cyclopentane as antisolvent.The crystal structures of [ Me CAACH][SiF5] and [ Me CAACH][GeF5] are shown in Figure 1.Both salts crystallize in the monoclinic space group P21/n and their asymmetric units consist of a heterocyclic cation [ Me CAACH] + and a discrete pentacoordinated anion.The [SiF5] − and [GeF5] − anions have a trigonal bipyramidal geometry with disordered positions of the fluorine atoms.Two preferred orientations were modeled and refined, with domain A and B occupancies in [SiF5] − of 75% and 25%, respectively, and domain A and B occupancies in [GeF5] − of 67% and 33%, respectively.The crystal structures with disordered anions are shown along with separate images of domains A and B in Figures S31 and S32 in the Supporting Information.The crystal structures of [SiF5] − anions are often disordered and rarely well determined [13,14].The structural features of our [SiF5] − anion in [ Me CAACH][SiF5] are consistent with the previously reported structures [15-17], but their Scheme 1. Synthesis of [ Me CAACH][SiF 5 ] and [ Me CAACH][GeF 5 ].Single crystals of [ Me CAACH][SiF 5 ] and [ Me CAACH][GeF 5 ] were prepared by vapor diffusion crystallization using DCM as solvent and cyclopentane as antisolvent.The crystal structures of [ Me CAACH][SiF 5 ] and [ Me CAACH][GeF 5 ] are shown in Figure 1.Both salts crystallize in the monoclinic space group P2 1 /n and their asymmetric units consist of a heterocyclic cation [ Me CAACH] + and a discrete pentacoordinated anion.The [SiF 5 ] − and [GeF 5 ] − anions have a trigonal bipyramidal geometry with disordered positions of the fluorine atoms.Two preferred orientations were modeled and refined, with domain A and B occupancies in [SiF 5 ] − of 75% and 25%, respectively, and domain A and B occupancies in [GeF 5 ] − of 67% and 33%, respectively.The crystal structures with disordered anions are shown along with separate images of domains A and B in Figures S31 and S32 in the Supporting Information.The crystal structures of [SiF 5 ] − anions are often disordered and rarely well determined [13,14].The structural features of our [SiF 5 ] − anion in [ Me CAACH][SiF 5 ] are consistent with the previously reported structures [15-17], but their bond lengths and angles are not compared with the previously reported structures due to the heavy disorder of the anion.The structural features of our [GeF 5 ] − anion in [ Me CAACH][GeF 5 ] are also consistent with the previously described [IPrH][GeF 5 ] [12].It is worth noting that [IPrH][GeF 5 ] represents the first example in which the discrete [GeF 5 ] − anion is present, while [ Me CAACH][GeF 5 ] is only the second example.To stabilize discrete [SiF 5 ] − and

Figure 1 .
Figure 1.Structure of the asymmetric unit of (a) [ Me CAACH][SiF5] and (b) [ Me CAACH][GeF5].The ellipsoids are drawn at 50% probability.The positions of the fluorine atoms in both anions are disordered.For clarity, only the fluorine atoms in domain A are shown and all hydrogen atoms except those on the aldiminium ring are omitted.

Figure 1 .
Figure 1.Structure of the asymmetric unit of (a) [ Me CAACH][SiF 5 ] and (b) [ Me CAACH][GeF 5 ].The ellipsoids are drawn at 50% probability.The positions of the fluorine atoms in both anions are disordered.For clarity, only the fluorine atoms in domain A are shown and all hydrogen atoms except those on the aldiminium ring are omitted.The SnF 4 reacted with [ Me CAACH][F(HF) 2 ] in anhydrous HF with quantitative formation of [ Me CAACH][SnF 5 ], as shown in Scheme 2. Removal of the volatiles resulted in the formation of a brown solid that could not be crystallized despite several attempts.Several attempts have been made in the past to crystallize the monomeric [SnF 5 ] − anion, but none of them was successful [18].Since tin preferentially forms octahedrally coordinated units in the presence of fluoride, [SnF 5 ] − units usually form oligomeric or polymeric structures [19].Octahedrally coordinated discrete tin fluoride anions, such as [SnF 6 ] 2− , are more common and better studied [20-22].However, only a limited number of structurally characterized [SnF 6 ] 2− anions with organic cations are known.To date, there are only 12 discrete structures in the CCDC crystal structure database [23].

Figure 1 .
Figure 1.Structure of the asymmetric unit of (a) [ Me CAACH][SiF5] and (b) [ Me CAACH][GeF5].The ellipsoids are drawn at 50% probability.The positions of the fluorine atoms in both anions are disordered.For clarity, only the fluorine atoms in domain A are shown and all hydrogen atoms except those on the aldiminium ring are omitted.The SnF4 reacted with [ Me CAACH][F(HF)2] in anhydrous HF with quantitative formation of [ Me CAACH][SnF5], as shown in Scheme 2. Removal of the volatiles resulted in the formation of a brown solid that could not be crystallized despite several attempts.Several attempts have been made in the past to crystallize the monomeric [SnF5] − anion, but none of them was successful [18].Since tin preferentially forms octahedrally coordinated units in the presence of fluoride, [SnF5] − units usually form oligomeric or polymeric structures [19].Octahedrally coordinated discrete tin fluoride anions, such as [SnF6] 2− , are more common and better studied [20-22].However, only a limited number of structurally characterized [SnF6] 2− anions with organic cations are known.To date, there are only 12 discrete structures in the CCDC crystal structure database [23].

Scheme 2 .
Scheme 2. Synthesis of [ Me CAACH][(THF)SnF5].In our work, we wanted to demonstrate the presence of [SnF5] − units in a discrete form.Fortunately, [ Me CAACH][SnF5] dissolved in THF and reacted with it to form a new compound.Immediately, a white solid precipitated from the THF solution, which was later characterized as a salt containing octahedrally coordinated [(THF)SnF5] − anions.Single crystals of [ Me CAACH][(THF)SnF5] suitable for X-ray analysis formed by vapor diffusion crystallization using THF as solvent and hexane as antisolvent.When [ Me CAACH][SnF5] was suspended in dioxane, it reacted in the same way as with THF to form [ Me CAACH][(dioxane)SnF5].The latter compound was crystallized by vapor

Scheme 2 .
Scheme 2. Synthesis of [ Me CAACH][(THF)SnF 5 ].In our work, we wanted to demonstrate the presence of [SnF 5 ] − units in a discrete form.Fortunately, [ Me CAACH][SnF 5 ] dissolved in THF and reacted with it to form a new compound.Immediately, a white solid precipitated from the THF solution, which was later characterized as a salt containing octahedrally coordinated [(THF)SnF 5 ] − anions.Single crystals of [ Me CAACH][(THF)SnF 5 ] suitable for X-ray analysis formed by vapor diffusion crystallization using THF as solvent and hexane as antisolvent.When [ Me CAACH][SnF 5 ] was suspended in dioxane, it reacted in the same way as with THF to form [ Me CAACH][(dioxane)SnF 5 ].The latter compound was crystallized by vapor diffusion crystallization, using dioxane as solvent and hexane as antisolvent and formed single crystals of [ Me CAACH][(dioxane)SnF 5 ]•dioxane.The crystal structures of the two compounds are shown in Figure 2.
diffusion crystallization, using dioxane as solvent and hexane as antisolvent and formed single crystals of [ Me CAACH][(dioxane)SnF5]•dioxane.The crystal structures of the two compounds are shown in Figure2.

2. 1 . 2 .
Group 15    The one pot syntheses of [Me CAACH][PF6] and [ Me CAACH][AsF6] are shown in Scheme 3. First, [ Me CAACH][F(HF)2] was generated in situ by the reaction of [ Me CAACH][Cl(HCl)0.5] in aHF.MF5 gases (M = P, As) were then added in excess to the reaction mixture, which was stirred overnight.After removal of volatiles, [ Me CAACH][MF6] (M = P, As) was formed in quantitative yield.We were also able to prepare [ Me CAACH][SbF6] using a similar procedure (Scheme 4).In this case, the SbF5 was first freshly prepared in situ from SbF3 and F2 in aHF.Then, [ Me CAACH][F(HF)2] was added to the reaction mixture.To avoid the presence of chlorine atoms in the reaction

Figure 3 .Scheme 3 .Scheme 4 .
Figure 3. Structure of the asymmetric units of (a) [ Me CAACH][PF6] and (b) [ Me CAACH][AsF6].The ellipsoids are drawn at 50% probability.For clarity, all hydrogen atoms except those on the heterocyclic ring are omitted.The two compounds are isostructural, with the hexafluoroarsenate compound having only a slightly larger unit cell.The asymmetric units of both compounds contain a heterocyclic cation [ Me CAACH] + and a discrete octahedrally coordinated anion [MF6] -(M = P, As).The structural features of the anions agree well with the structures of the previously described species.The average P-F distance of 1.591 Å of [ Me CAACH][PF6] is consistent with the average P-F distances of 1.570 Å [28], 1.588 Å [29], or 1.600 Å [29] reported elsewhere.Similarly, the slightly longer average As-F distance of 1.710 Å of

Figure 3 .
Figure 3. Structure of the asymmetric units of (a) [ Me CAACH][PF6] and (b) [ Me CAACH][AsF6].The ellipsoids are drawn at 50% probability.For clarity, all hydrogen atoms except those on the heterocyclic ring are omitted.The two compounds are isostructural, with the hexafluoroarsenate compound having only a slightly larger unit cell.The asymmetric units of both compounds contain a heterocyclic cation [ Me CAACH] + and a discrete octahedrally coordinated anion [MF6] -(M = P, As).The structural features of the anions agree well with the structures of the previously described species.The average P-F distance of 1.591 Å of [ Me CAACH][PF6] is consistent with the average P-F distances of 1.570 Å [28], 1.588 Å [29], or 1.600 Å [29] reported elsewhere.Similarly, the slightly longer average As-F distance of 1.710 Å of mixture, [ Me CAACH][F(HF)2] was used.Similar procedures have been used in the past for the synthesis of compounds containing [AsF6] − , and [SbF6] − anions [27].[ Me CAACH][SbF6]

Figure 3 .
Figure 3. Structure of the asymmetric units of (a) [ Me CAACH][PF6] and (b) [ Me CAACH][AsF6].The ellipsoids are drawn at 50% probability.For clarity, all hydrogen atoms except those on the heterocyclic ring are omitted.

Figure 3 .
Figure 3. Structure of the asymmetric units of (a) [ Me CAACH][PF 6 ] and (b) [ Me CAACH][AsF 6 ].The ellipsoids are drawn at 50% probability.For clarity, all hydrogen atoms except those on the heterocyclic ring are omitted.The two compounds are isostructural, with the hexafluoroarsenate compound having only a slightly larger unit cell.The asymmetric units of both compounds contain a heterocyclic cation [ Me CAACH] + and a discrete octahedrally coordinated anion [MF 6 ] - (M = P, As).The structural features of the anions agree well with the structures of the previously described species.The average P-F distance of 1.591 Å of [ Me CAACH][PF 6 ] is consistent with the average P-F distances of 1.570 Å [28], 1.588 Å [29], or 1.600 Å [29] reported elsewhere.Similarly, the slightly longer average As-F distance of 1.710 Å of [ Me CAACH][AsF 6 ] is comparable to the average As-F distances of 1.715 Å [30] and the range of 1.691(2)-1.757(2)Å reported for other [AsF 6 ] -anions [27].

2. 1
.3.CAAC Precursors Finally, we prepared the [ Me CAACH][BF 4 ] salt by the same procedure described for [ Me CAACH][PF 6 ] and [ Me CAACH][AsF 6 ].The in situ prepared [ Me CAACH][F(HF) 2 ] reacted with BF 3 gas and quantitatively formed [ Me CAACH][BF 4 ], as shown in Scheme 5.The same compound was recently prepared by Jana's group from CAAC: carbene and NO[BF 4 ] [9].Our approach provides a simple and straightforward alternative for the preparation of CAAC-based tetrafluoroborate compounds in high yield.

2. 1 Scheme 5 .
Scheme 5. Synthesis of [ Me CAACH][BF4].Single crystals of [ Me CAACH][BF4] suitable for X-ray diffraction were prepared by vapor diffusion crystallization using DCM as solvent and cyclopentane as antisolvent.[ Me CAACH][BF4] crystallizes in the monoclinic space group P21/n and its crystal structure is shown in Figure 4.The asymmetric units contain a heterocyclic cation [ Me CAACH] + and a discrete, tetrahedrally coordinated [BF4] -anion.The positions of the fluorine atoms in the anion are disordered.Two preferred orientations were modeled and refined, with domains A and B occupied by 73% and 27%, respectively.The crystal structures with disordered anions are shown along with separate images of domains A and B in Figure S34 in the Supporting Information.The structural features of our [BF4] − anion in [ Me CAACH][BF4]are consistent with the previously described structures[8], however, the bond lengths and angles are not compared to the previously reported structures due to the heavy disorder of the [BF4] − anion.

Figure 4 . 5 .
Figure 4. Structure of the asymmetric unit of [ Me CAACH][BF4].The ellipsoids are drawn at 50% probability.The positions of the fluorine atoms in the anion are disordered.For clarity, only the fluorine atoms in domain A are shown and all hydrogen atoms except those on the heterocyclic ring are omitted.In addition to the tetrafluoroborate salts, the CAAC-based chloride and triflate salts are still used as precursors for CAAC: carbenes.In the past, we have demonstrated their usefulness as group transfer reagents in aluminum hydride chemistry[10].During our

2. 1 Scheme 5 .
Scheme 5. Synthesis of [ Me CAACH][BF4].Single crystals of [ Me CAACH][BF4] suitable for X-ray diffraction were prepared by vapor diffusion crystallization using DCM as solvent and cyclopentane as antisolvent.[ Me CAACH][BF4] crystallizes in the monoclinic space group P21/n and its crystal structure is shown in Figure 4.The asymmetric units contain a heterocyclic cation [ Me CAACH] + and a discrete, tetrahedrally coordinated [BF4] -anion.The positions of the fluorine atoms in the anion are disordered.Two preferred orientations were modeled and refined, with domains A and B occupied by 73% and 27%, respectively.The crystal structures with disordered anions are shown along with separate images of domains A and B in Figure S34 in the Supporting Information.The structural features of our [BF4] − anion in [ Me CAACH][BF4]are consistent with the previously described structures[8], however, the bond lengths and angles are not compared to the previously reported structures due to the heavy disorder of the [BF4] − anion.

Figure 4 .
Figure 4. Structure of the asymmetric unit of [ Me CAACH][BF4].The ellipsoids are drawn at 50% probability.The positions of the fluorine atoms in the anion are disordered.For clarity, only the fluorine atoms in domain A are shown and all hydrogen atoms except those on the heterocyclic ring are omitted.

Figure 4 .Figure 5 .
Figure 4. Structure of the asymmetric unit of [ Me CAACH][BF 4 ].The ellipsoids are drawn at 50% probability.The positions of the fluorine atoms in the anion are disordered.For clarity, only the fluorine atoms in domain A are shown and all hydrogen atoms except those on the heterocyclic ring are omitted.In addition to the tetrafluoroborate salts, the CAAC-based chloride and triflate salts are still used as precursors for CAAC: carbenes.In the past, we have demonstrated their usefulness as group transfer reagents in aluminum hydride chemistry [10].During our work, we succeeded in preparing [ Me CAACH][Cl] and [ Me CAACH][OTf] according to known procedures from the literature [1,10], crystallizing them and structurally characterizing them.The crystal structures of [ Me CAACH][Cl] and [ Me CAACH][OTf] are shown in Figure 5. [ Me CAACH][Cl] crystallizes in the monoclinic space group P2 1 /c, while
C NMR spectra, the characteristic signals for the [ Me CAACH] + cation are visible.In addition, the 1 H and 13 C NMR spectra of [ Me CAACH][(THF)SnF 5 ] contain signals for THF and residual hexane.

Molecules 2023, 28
[12,17] PEER REVIEW 3 of 16 bond lengths and angles are not compared with the previously reported structures due to the heavy disorder of the anion.The structural features of our [GeF5] − anion in [ Me CAACH][GeF5] are also consistent with the previously described [IPrH][GeF5] [12].It is worth noting that [IPrH][GeF5] represents the first example in which the discrete [GeF5] − anion is present, while [ Me CAACH][GeF5] is only the second example.To stabilize discrete [SiF5] − and [GeF5] − anions, large cations must be used[12,17], and the sterically demanding cation [CAACH] + was found to be suitable for this purpose.