Self‐Assembly of Aminocyclopropenium Salts: En Route to Deltic Ionic Liquid Crystals

Abstract Aminocyclopropenium ions have raised much attention as organocatalysts and redox active polymers. However, the self‐assembly of amphiphilic aminocyclopropenium ions remains challenging. The first deltic ionic liquid crystals based on aminocyclopropenium ions have been developed. Differential scanning calorimetry, polarizing optical microscopy and X‐ray diffraction provided insight into the unique self‐assembly and nanosegregation of these liquid crystals. While the combination of small headgroups with linear p‐alkoxyphenyl units led to bilayer‐type smectic mesophases, wedge‐shaped units resulted in columnar mesophases. Upon increasing the size and polyphilicity of the aminocyclopropenium headgroup, a lamellar phase was formed.

Herein we report, for the first time,t hat ILCs can be obtained from aminocyclopropenium derivatives,w hose phase geometry and temperature range is controlled by the headgroup.

Synthesis of aminocyclopropenium salts and related compounds
At the outset of our study,wechose a4-alkoxyphenyl and wedge-shaped 4-[(3,4,5-trialkoxybenzoyl)oxy]phenyl moieties as different mesogenic cores for attachment to the aminocyclopropenium headgroups because these core units are known to promote mesophase formation in guanidinium ILCs. [55,56] Thes ynthesis of the cyclopropenium transfer reagents 3a-c is shown in Scheme 1.
To access cyclopropenium ILCs 7,4 -alkoxyanilines 6a-c [55a] were treated with cyclopropenium chloride 3a in the presence of LiCl and Hünigsb ase in CH 2 Cl 2 for four days.After chromatographic purification, the resulting product was submitted to salt metathesis with NaBF 4 in EtOH to yield the cyclopropenium tetrafluoroborates 7a-c in 31-41 % yield (Scheme 2). Under similar conditions,a mmonium trifluoroacetate 9 derived from the corresponding Bocprotected precursor [55b] was treated with reagents 3a or 3b to provide the wedge-shaped tetrafluoroborates 10 a or 10 b in 14 %a nd 16 %y ields,r espectively (Scheme 2). Presumably, the acylated phenol of 9 reduces the nucleophilicity of the nitrogen atom relative to compounds 6-8,t hus leading to diminished yields of 10.I ts hould also be emphasized that efforts undertaken to optimize the yields were not extensive.
To assess the influence of the unique aminocyclopropenium headgroup on the liquid crystalline self-assembly process,s tructurally related guanidinium salts 12 a and 12 b, and known compound 13 a and 13 b [55b] with asimilar core unit and side chain lengths as well as the same counterion, were chosen as reference compounds (Scheme 2). As described earlier for related ILCs, [55a] guanidinium tetrafluoroborates 12 a and 12 b were prepared from the corresponding guanidinium chlorides through salt metathesis.
Furthermore,t he trimethylammonium salt 14 was prepared to compare this ILC,c arrying as mall spherical headgroup,w ith derivatives carrying much larger planar delocalized guanidinium or aminocyclopropenium headgroups.Compound 14 was obtained from 15 in 68 %byacidic removal of the N-Boc group and methylation with an excess of methyl iodide.
Ther eaction of trifluoroacetate 9 with cyclopropenium salt 3c afforded the desired aminocyclopropenium chloride 10 c in 8% yield. Thed isubstituted derivative 11 was also isolated in 4%yield;its formation was unexpected because of the bulky substituents,although similar bis(cyclopropenium)substituted amines carrying iPr groups have been reported in the literature. [58,59] Fortunately,single crystals of 8awith aC 12 side chain were obtained, which were suitable for X-ray crystal structure analysis ( Figure 2). Derivative 8a is oriented in al inear extended all-trans conformation in the solid state.T he distance N1H1···F3 between the NH as H-donor and F3 of the tetrafluoroborate anion as H-acceptor is 2.05 .Aweak interaction between C21H21, C17H17, and C6H6 as Hdonors,a nd the Fa toms of BF 4 as H-acceptor with H···F distances ranging between 2.36 and 2.55 ,w ere found. Interdigitation of the alkyl chains is visible in the cell plot. A hydrophobic interaction between the chains,w hich are stacking perpendicular to the ac diagonal, was not observed (Supporting Information). [60] Mesomorphic properties of cyclopropenium salts and related compounds Them esomorphic properties of the aminocyclopropenium salts 7, 8, 10,and ammonium salt 9 were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD  (Figures 3a,b), suggesting the presence of smectic A( SmA) mesophases.
XRD experiments with 7b showed adistinct reflection in the small-angle region at 2q = 2.28 8/38.7 ,which was assigned as the (001) layer reflex, and abroad halo around 2q = 15-268 8, which is typical for the molten alkyl chains of the LC selfassembly (Figures 4a,b).
Te mperature-dependent XRD measurements provided the smectic layer distance of 7b and 7c as af unction of temperature (Figure 4c). Ther atio of the experimentally determined smectic layer distance l 001 of 39.6 (at 105 8 8C), with respect to the calculated molecular length of 27 for 7b using the Avogadro program, [61] suggested aSmA bilayer-type arrangement with considerable interdigitation.
Increasing the steric bulkiness of the aminocyclopropenium head group resulted in complete loss of mesomorphism, as observed for phenoxy bis(diisopropylamino)cyclopropenium tetrafluoroborates 8 (Table 1). These derivatives were non-mesomorphic with melting points ranging from 113-114 8 8C. In the solid state asimilar ionic sublayer was found for derivative 8a,a sc ompared to the mesophase packing of 7band 7cwith less bulky head groups.A lthough direct correlations between solidstate structures and mesophase structures have to be made with great care,c rystallographic data might provide some useful insight into mesomorphic selfassembly or rationale for the absence of mesomorphism. [62,63] Despite the hydrogen bond NH À F3 between the aminocyclopropenium NÀHu nit and the fluoro group of the counterion, which should promote liquid crystallinity,i nterdigitation of the alkyl chains,and thus,van der Waals interactions are less pronounced in compound 8a.M oreover, p-p stacking was absent in the solid-state structure,i n contrast with the corresponding mesogenic guanidinium salts. [64] These counterbalanced effects,i n particular the interdigitation present only in small compartments (Supporting Information, Figure S15), together with the steric bulkiness of the cationic headgroup,s eem to disfavor mesomorphic self-assembly. Forc omparison, the mesomorphic properties of the guanidinium tetrafluoroborates 12 a and 12 b with the same chain lengths and counterion as the corresponding aminocyclopropenium tetrafluoroborates 7a and 7b were studied. POM investigations of the guanidinium derivatives showed Maltese crosses ( Supporting Information, Figures S9d,e). XRD studies revealed as harp (001) reflex and ab road halo (Supporting Information, Figures S13 and S14). Thel ayer distance calculated from the (001) reflex decreased with  increasing temperature.T ogether,this indicates aSmA phase for these derivatives.
Comparing the results for 7 and 12,both are forming SmA phases,s howing that the bis(dimethylamino)cyclopropenium cation behaved as an extended deltic guanidinium cation with as lightly decreased mesophase stability and temperature range.N otably,t he smectic layer arrangement is favored despite the increase of the cross-sectional area of the head group of 7,a sc ompared to the guanidinium derivatives 12.
Thes mall-angle X-ray scattering (SAXS) profile of 10 a displayed three sharp reflections with aratio of 1:1/ p 3:1/ p 4, which were indexed as (10), (11) and (20) reflections of ah exagonal columnar lattice with p6mm symmetry,a nd alattice parameter of 49.0 was calculated. In the wide-angle region ab road halo around 4.5 was observed (Table 2; Supporting Information, Figure S11). Taking the value of 4.5 for the average chain distance into the calculation of the number of molecules per columnar repeat (Z), Z = 5w as obtained. [66] In contrast, the increased bulkiness of the 1,2-bis(diisopropylamino)cyclopropenium headgroup in derivative 10 b strongly disfavored mesomorphic self-assembly.
When cyclopropenium chloride 10 c was heated in the DSC,a ne ndothermal Cr-Cr transition at 54 8 8Ca nd an endothermal melting transition at 103 8 8Cw as measured. No clearing transition could be detected until decomposition occurred, but POM revealed aclearing temperature of 112 8 8C.   were observed that were assigned as (001) and (002) smectic layer reflexes (Figure 5b). Thew ide-angle X-ray scattering (WAXS) profile showed ab road halo around 4.2 (Fig-ure 5a). As ac onsequence of increasing decomposition of the sample with prolonged exposure time, temperature-dependent layer spacings could not be determined. Presumably,t he dimethoxybenzyl (DMB) protecting groups were cleaved under these conditions. [67] Structurally related guanidinium derivatives were considered for comparison. Compounds 13 a and 13 b are already known [27] and demonstrated ac olumnar hexagonal (Col h )m esophase.F or derivative 13 a with triflate anion, an enantiotropic mesophase was observed between 51 8 8Ca nd 146 8 8C. Derivative 13 b,with tetrafluoroborate anion, had ab roader mesophase range because of al ower melting point at 33 8 8Ca nd am uch higher clearing temperature at 228 8 8C, leading to decomposition upon clearing.D erivative 14,w ith atrimethyl ammonium head group, was also investigated. DSC analysis revealed am esophase between 31 8 8Ca nd 139 8 8C. XRD studies, together with POM textures (Supporting Information, Figures S9c and S12) revealed the presence of aC ol h mesophase.
Initially,wesurmised that the bulky 2,4-dimethoxybenzyl protecting group might deteriorate any liquid crystalline selfassembly,i np articular considering the size misfit of headgroup and core unit. However,t he results demonstrate that nanosegregation is favored because of polyphilic interactions. Presumably,asmectic bilayer-type organization is realized by a p-p stacked electron-rich aryl layer, followed by acharged layer, where the cyclopropenium cations are counterbalanced by the tetrafluoroborate anions,f ollowed by an aryl layer of the gallic acid phenyl esters and ahydrophobic layer. Notably, derivative 11 carrying two cyclopropenium headgroups was non-mesomorphic.T hus,the combination of steric hindrance and Coulomb repulsion of two cations in ac lose vicinity seems to disfavor liquid crystallinity.

Conclusion
We demonstrate,f or the first time,t hat aminocyclopropenium salts self-assemble into liquid crystalline mesophases. Nanosegregation of immiscible parts,e lectrostatic interactions,a nd volume requirements of both the head group and hydrophobic parts play am ajor role in controlling the mesophase stability.T he geometry of the mesophase was determined by the effective volume of headgroup versus hydrophobic part, in agreement with Israelachvilisp acking model for lyotropic liquid crystals. [68] Thus,a minocyclopropenium salts with small N,N-dimethylamino substituents and  as ingle alkoxy chain attached to the aryl unit (7b and 7c) form lamellar geometries (Figures 6and 7), which is similar to the corresponding guanidinium salts 12 a and 12 b.W ith increasing steric demand of the headgroup mesomorphism is lost (for example, N,N-diisopropylamino 8a-c). Precursor 9 with ap olar ammonium headgroup was capable of forming aC ol r mesophase with p2mm symmetry.O nt he other hand, wedge-shaped aminocyclopropenium salts with N,N-dimethylamino groups self-assemble into micellar-like columnar geometries (for example, 10 a;F igure 7), which is similar to trimethylammonium (14)and guanidinium salts 13 a and 13 b. Again, mesomorphism was lost with more sterically demanding headgroups (diisopropylamino). However,w hen the headgroup surpasses acertain size requiring asimilar volume as the hydrophobic part and provides additional polyphilic interactions,l amellar mesophases were found again (for example, 10 c). These results are in good agreement with molecular dynamics (MD) simulations on pyridinium ILCs, which revealed that only those compounds with ar elatively large volume ratio of cation to anion form stable SmA phases. [69] In conclusion, aminocyclopropenium ILCs serve as welldefined model compounds to study self-assembly and nanosegregation, which are important in polyelectrolytes used for battery materials.T hese ILCs bridge the gap between low molecular weight organocatalysts and polymeric electrolytes, and thus,c ontribute to the general utility of 3-ring aromatic compounds.  . Comparisono fmesophases of alkoxyphenyla nd phenylalkoxybenzoate-based ILCs depending on the headgroup ammonium (9,14), guanidinium (12,13), and aminocyclopropenium (7, 10;* denotes decomposition). The values of 13 a and 13 b were taken from ref. [27].