Synthesis and physical properties of tris(dialkylamino)cyclopropenium dicyanamide ionic liquids

The synthesis and properties of 16 tris(dialkylamino)cyclopropenium (TDAC) cations with the dicyanamide (DCA) anion, [N(CN)2]−, are described. D3h- and C3h-symmetric cations ([C3(NR2)3]DCA (R = Me, Et, Pr, Bu, Pent, Hex, Dec) and [C3(NRMe)3]DCA (R = Bu, St), respectively) were synthesised by reaction of C3Cl5H with the corresponding amine. Reaction of the alkoxydiaminocyclopropenium salt [C3(NEt2)2(OMe)]+ with amines led to a series of C2v-symmetric salts [C3(NEt2)2(NR2)]DCA (R = Me, Bu, Hex) and two Cs-symmetric salts and [C3(NEt2)2(NRMe)]DCA (R = Me, Bu). Similarly, [C3(NMe2)2(OMe)]+, was used to prepare the Cs-symmetric salts [C3(NMe2)2(NRMe)]DCA (R = Pr, Bu). In addition to characterisation by NMR, mass spectrometry and microanalysis, the salts were characterised by DSC, TGA, density, viscosity, conductivity and miscibility/solubility studies. Comparisons have been made with similar series of bistriflimide (NTf2−) salts that have been previously reported to see whether the same trends are observed with a different anion.


Introduction
Ionic liquids (ILs) are an increasingly important class of so materials that have undergone rapid development in recent years; many of these materials are now commercially available. 1 These materials are frequently considered as green alternatives to classical organic solvents due to their general properties of almost zero vapour pressure, low ammability, tunability, excellent solvent properties and potential for efficient recycling. 2 Additionally, their conductivity and other useful properties have led to many applications. 3 Traditionally, there are four major cation-based classes of ILs: imidazolium, pyridinium, phosphonium and ammonium. Although cyclopropenium salts have been known since the rst report of [C 3 Ph 3 ] + in 1957 (ref. 4) and the triaminocyclopropenium (TAC) salts have been known for more than 40 years, 5 we did not report on the IL properties of TAC salts until 2011. 6 TAC salts are remarkably stable due to a combination of the aromaticity of the 2p-electron ring system as well as the strong p donation from the three amino groups. TAC salts more generally have been recently reviewed by Komatsu and Kitagawa as well as Bandar and Lambert. 7,8 TAC cations have greater charge delocalisation than ammonium, phosphonium, and guanidinium cations, while they have reduced hydrogen bond donor capabilities, compared to the imidazolium, pyridinium and triazonium cations, due to a lack of aromatic C-H groups. The high-lying non-bonding HOMO of TAC cations gives particularly weak cation-anion interactions 9 and their salts consequently have unusual properties: in some early work, Weiss and co-workers prepared iodide-iodoacetylene and iodide-iodoarene adducts. 10 More recently, we isolated an interesting discrete dichloride hexahydrate cube, [Cl 2 (H 2 O) 6 ] 2À , in which the solid state structure is very similar to the calculated structure. 11 Even more surprising is that, upon removal of the solvate, the cations form dicationic pairs with remarkably short p-p stacking distances. 12 In 2015, we reported on a series of tris(dialkylamino)cyclopropenium (TDAC) NTf 2 À ILs in which the effects of cation size and symmetry were investigated. Cation symmetry classes included D 3h , C 3h , C 2v and C s while the cation size varied from the hexamethyl cation [C 3 (NMe 2 ) 3 ] + to the hexadecyl cation [C 3 (N(C 10 H 21 ) 2 ) 3 ] + . 13 Variation of the alkyl chain length in IL cations is generally used to ne tune the properties of an IL. In contrast, exchanging the anion frequently leads to much larger step changes in properties such as the viscosity, conductivity and solubility/miscibility proles. However, the properties of an ionic liquid are not just the sum of the cations and the anions, the properties are also dependent on how the anions and cations interact: for example, their relative sizes, shapes and charge distributions, as well as any hydrogen bonding, impact on the intermolecular forces and thus their physical properties. Therefore, we sought to investigate how exchanging from one anion to another impacts on trends in the physical properties of TAC ILs. Following our detailed study on bistriimide salts, we now report on the corresponding dicyanamide ([N(CN) 2 ] À , DCA) TDAC salts, an anion which would be expected to produce lower viscosities, higher conductivities and greater hydrophilicity. Some of this work has been communicated. 6,14 2. Results and discussion 2.1 Synthesis TDAC salts were rst prepared by reaction of tetrachlorocyclopropene with secondary amines, 5 and more recently by reaction with pentachlorocyclopropane. 6,14,15 These routes (Scheme 1) provide the D 3h -and C 3h -symmetric cation chloride salts [C 3 (NR 2 ) 3 ]Cl (1) and [C 3 (NRR 0 ) 3 ]Cl (2), respectively. Treatment of these salts with aqueous NaDCA readily provides the corresponding dicyanamide salts (3 and 4, respectively) which can be extracted into an organic solvent such as chloroform or dichloromethane. Earlier, we communicated the syntheses and some properties of the [C 3 (NR 2 ) 3 ]DCA salts for R ¼ Et (3b) 14 and Bu (3d). 6 Here we additionally include the related syntheses for R ¼ Me (3a), Pr (3c), Pent (3e), Hex (3f) and Dec (3g) as well as If the secondary amine is bulky, such as HN i Pr 2 or HN(C 6 H 11 ) 2 , then its reaction with C 3 Cl 5 H or C 3 Cl 4 gives the corresponding diaminochlorocyclopropenium cation [C 3 (NR 2 ) 2 Cl] + which can then be treated with a smaller secondary amine to provide a limited range of cations with C 2v and C s symmetry: [C 3 (NR 2 ) 2 (NR 0 2 )] + and [C 3 (NR 2 ) 2 (NR 0 R 00 )] + , respectively, in which R is bulky and NR 0 R 00 is reasonably small. 16,17 Here we have used this route (Scheme 2) to prepare [C 3 (N i Pr 2 ) 2 (NBu 2 )]DCA (5). Due to the limited versatility of this route, we developed a route via reaction of secondary amines with the alkoxydiaminocyclopropenium cations [C 3 (NMe 2 ) 2 (-OMe)] + (6) and [C 3 (NEt 2 ) 2 (OMe)] + (7) (Scheme 3). As with the chloride salts, addition of aqueous NaDCA followed by extraction with an organic solvent allows one to isolate the DCA salts. Scheme 3 illustrates the three series of ILs that were prepared in this way: two C s -symmetric series, 8 and 9, via 6 and 7, respectively; and a C 2v -symmetric series, 10, prepared via 7. Earlier, we communicated the syntheses and some properties of the salts [C 3 (NEt 2 ) 2 (NRMe)]DCA (R ¼ Bu (9a), Hex (9b)) and [C 3 (NEt 2 ) 2 (NR 2 )]DCA (R ¼ Me (10a), Bu (10b), Hex (10c)). 14 When looking at trends within these series, note that some highersymmetry species may also belong to these series, i.e., 3a can be included in series 8; 3b in series 10; and 10a in series 9. Similarly, 3a can be considered part of the C 3h 4 series of cations.
TDAC cations can also be prepared by alkylation of a cyclopropenimine aer deprotonation of the corresponding protic TAC cation. In the case of 8a, it was more convenient (and cheaper) to use this route via the protic TAC cation [C 3 -(NMe 2 ) 2 (NPrH)] + (11) (Scheme 4).
All new compounds were characterised by 1 H-and 13 C{ 1 H}-NMR spectroscopy as well as ES-MS and microanalysis. Chloride and water contents were determined for ILs prior to measurement of their physical properties; namely, DSC, TGA, viscosity, conductivity and density. These impurities in particular are known to impact on the physical properties of ILs.
With respect to the NMR spectra, it should rstly be borne in mind that rotation about the exocyclic C-N bonds is fast on the NMR timescale; thus, a C s -symmetric cation such as 8 exhibits C 2v symmetry on the NMR timescale, and there is only one 1 H and 13 C-NMR signal for the NMe 2 groups. Typical 1 H-and 13 C-NMR ranges were tabulated previously. 13 2.2 DSC data DSC data were collected at 10 C min À1 and the results are given in Table 1. Not surprisingly, the only salt that is not an IL by denition (arbitrarily dened as a melting point less than 100 C) is that with the smallest and most symmetric (D 3h ) cation, 3a, with a melting point (T m ) of 117 C. A large number of Scheme 1 Synthesis of D 3h -and C 3h -symmetric TDAC salts. factors are known to inuence T m : the various intermolecular forces (coulombic, van der Waals, hydrogen-bonding etc.), conformational exibility and the shape or symmetry of the species. Generally, it is found that ILs have high melting points for small ions in which coulombic attractions dominate. They also have high melting points for large ions in which van der Scheme 3 Synthesis of C 2v -and C s -symmetric TDAC salts.

Scheme 4
Alternative synthesis of C s -symmetric 8a. Table 1 DSC (10 C min À1 ) and TGA data for DCA salts Waals interactions dominate. Reducing the symmetry or increasing the conformational exibility of the side chains tends to reduce T m since they reduce the factors that favour efficient packing in the solid state. Fig. 1 shows a plot of T m versus cation molecular weight (MW) for the D 3h class of cations for both the NTf 2 À salts as well as the DCA salts. This shows a rapid drop in T m from 3a as both size and conformational exibility rapidly increase. Generally the DCA salts might be expected to have higher T m due to closer electrostatic attractions to the cation, however, the hexaethyl and hexadecyl salts have lower T m and, more generally, the TDAC NTf 2 À salts do not show a clear trend towards lower T m ; presumably the greater size difference in the hexadecyl case explains that one. Unfortunately, T m transitions were not identied for the hexapentyl and hexahexyl salts. Remarkably, the hexapropyl salts have a higher T m than the hexaethyl and hexabutyl analogues for both the NTf 2 À and DCA salts. This may be a result of chain exibility issues. There is insufficient data to discern clear trends for the other series. Notably, 4b, with very long C 18 chains, has a signicantly higher T m than the similarlysized hexadecyl salt 3g. That is also the case for the NTf 2 À analogues. Salt 5 also has a signicantly higher T m (47 C) than similarly-sized salts, and this can be attributed to the lack of exibility of the isopropyl groups.

Thermal decomposition
Thermal decomposition data were collected at both 1 C min À1 and 10 C min À1 and are given in Table 1. We found with the NTf 2 À salts that the major factor in determining the thermal decomposition onset temperature, T d , is the number of methyl groups, and especially the number of dimethylamino groups. When none of the six alkyl groups are methyl groups, T d is relatively invariant: at 1 C min À1 , T d ranges 275-305 C for the DCA salts versus 343-364 C for NTf 2 À salts, while at 10 C min À1 , it ranges from 315-350 C for the DCA salts versus 393-409 C for the NTf 2 À salts. Typically then, the DCA salts have signicantly lower T d than the NTf 2 À salts. Fig. 2 illustrates the trends for the 10 C data only. One to three methyl groups doesn't have a signicant impact except for 4b for which the onset temperature is signicantly lower at 279 C. This is odd since 4a has a T d of 331 C.

Density
Density data were determined from 20-90 C where possible; the results for the 20 C and 50 C data are given in Table 2 and molar volumes are provided in the ESI. † Unlike melting points, viscosities and conductivities, vide infra, densities for TDAC DCA salts are effectively independent of cation shape and depend only on the cation size (or mass). Fig. 3 shows a plot of densities at 20 C versus the MW of the cations for the DCA salts. At long chain lengths, densities must approach that of very low density polyethylene (VLDPE), 0.88-0.915 g mL À1 , as the cation core and anions are increasingly diluted. The equation on Fig. 3 describes the tted curve. This is made up of a volume for the permethyl salt 3a of 360Å 3 and a factor of 27.5Å 3 per CH 2 group added (n) to this core. This CH 2 volume agrees with that obtained by Ye and Shreeve for ILs of 28Å 3 and what we found for the NTf 2 À salts of 27.7Å 3 . 13, 18 We estimated a volume of 280Å 3 for the [C 3 (NMe 2 ) 3 ] + cation in TDAC NTf 2 À ILs which gives us a volume for DCA of 80Å 3 . This is somewhat smaller than the 86 A 3 deduced by Ye and Shreeve. 18 At 20 C and n ¼ N, a density of 0.847 g mL À1 is calculated for TDAC DCA salts compared to 0.842 g mL À1 for the NTf 2 À salts, effectively this is "liquid polyethylene".
The densities of ILs have a linear dependency on temperature and can be well-tted by the equation r ¼ a À bT. Parameter a represents a theoretical density at 0 K. Pleasingly, a plot of parameter a versus cation MW can also be t to a similar equation as the density data obtained at 20 C (ESI, Fig. 1S †). In this case, we calculate a volume for each CH 2 group at 0 K of 23 A 3 while the volume of 3a at 0 K was found to be 309Å 3 . This gives a "free volume" of 51Å 3 for 3a at 20 C and 4.5Å 3 per CH 2 .  Parameter a for "liquid polyethylene" (density at 0 K and n ¼ N) is 1.015 g mL À1 .
Density parameter b represents the temperature dependency of the density. This parameter is rarely commented on, however, this parameter can also t, although not as well, by an equation of the type used for the density at 20 C and parameter a (ESI, Fig. 2S †). This illustrates a decrease in temperature dependence with MW. The combination of the equations for a and b allow us to derive a temperature-dependent equation for all TDAC DCA salts (eqn (1)).
The thermal expansion coefficient a p can be obtained from the slope of a plot of ln(r) versus T, i.e. À[v ln (r)/vT] P ¼ Àc. Although a p can vary with temperature, 19 and ln(r) versus T can be t with quadratic or cubic functions, we have not done so due to the limited temperature range. The values of a p range from 0.605 Â 10 À3 K À1 to 0.685 Â 10 À3 K À1 which is noticeably lower than the typical values for NTf 2 À salts of 0.680-0.705 Â 10 À3 K À1 . Fig. 4

Viscosity
Viscosity data were collected from 20-90 C where possible; the results for the 20 C and 50 C data are given in Table 2. As is to be expected, viscosity generally increases with MW, as shown in     5 for viscosity at 20 C. The data is colour-coded by symmetry class, however, it seems that symmetry has little effect except for a couple of cases: the propyl salt 8a appears to be an outlier in Fig. 5 and 6 with a much higher viscosity than the butyl derivative 8b. The hexabutyl salt 3d also seems higher than the other D 3h salts. Fig. 6 compares the viscosity of the low MW salts to those of other cations at 20 C. Generally, the TDAC salts have lower viscosities relative to their MW and this might be attributed to relatively weak cation-anion interactions. A similar effect was observed for the NTf 2 À salts. 13 The viscosity data was t to both the Arrhenius (h ¼ A exp (E a / RT)) and Vogel-Fulcher-Tammann (VFT, eqn (2)) equations (also, D ¼ B/T 0 ); these parameters are given in the ESI. † There are not many obvious trends: E a tends to increase with MW; the values of D, a measure of the deviation from Arrhenius behaviour, lie in the range 4-10 which is typical for "fragile" liquids. 22 A "fragility plot" of log(viscosity) versus T g /T (ESI, Fig. 4S †) similarly shows that these materials are typical of fragile ILs.

Conductivity
Conductivity data was also collected from 20-90 C where possible; results at 20 C and 50 C are given in Table 2. Fig. 7 illustrates the data at 60 C for which a strong trend of decreasing conductivity with MW is observed. Although, as expected from the high viscosity of 8a, compared to 8b, it has     (3b-g)).
a signicantly lower conductivity than 8b. Interestingly, although the imidazolium DCA ILs generally have a higher conductivity, this is due to their lower MWs: Fig. 7 shows that the trendline for the imidazolium ILs lies well below that of the TDAC ILs. A plot of log(conductivity) versus MW gives a reasonably straight line (ESI, Fig. 5S †).

Ionicity
Conductivity and viscosity are linked through Walden's rule: Lh ¼ k (in which k is a temperature-dependent constant, the Walden product). Walden plots, log(L) versus log(1/h), were used to investigate the ionicity of the ILs, with deviations from the ideal diagonal (1 M KCl(aq) line) being ascribed to the formation of ion pairs or aggregates ( Fig. 8 and 9). The majority of ILs fall in a narrow band with only a small deviation from ideal, as measured by the average deviation DW. These would thus be considered as "good ILs". The dihexyl salt 10c lies further from the ideal (DW ¼ 0.28) than do the other non-D 3h -symmetric cations (DW ¼ 0.10-0.20), but as it did for the corresponding NTf 2 À salt. 13 However, it does not show the strongly non-linear plot that the NTf 2 À salt does. The D 3h -symmetric ILs (Fig. 9) show increasing deviation from ideal as size increases from 3d to 3g (DW ¼ 0.04, 0.27, 0.50 and 0.84 for 3d-g, respectively). The same effect was observed for the corresponding NTf 2 À salts.
Curiously, the hexapropyl salt 3c (DW ¼ 0.46) has a greater deviation than 3d and 3e, and the hexabutyl salt 3d has a remarkably small deviation of 0.04.

Miscibility and solubility
The miscibility and solubility properties of the salts were investigated at 25 C. The results are given in Table 3 in order of increasing MW. DCA salts are more hydrophilic than NTf 2 À salts. However, the DCA salts are miscible with water for only small MW (<ca. 315 g mol À1 ); they are immiscible for MW > ca. 400 g mol À1 and partially miscible for intermediate MWs. All of the salts were found to be soluble or miscible in MeOH, EtOH, CH 2 Cl 2 and EtOAc. The notable exception being the insolubility in everything of 4b which has three C 18 chains. That contrasts signicantly with the similarly-sized 3g with six C 10 chains that is soluble in everything except water. Most compounds are at least partially miscible in toluene, the small and symmetric 3a and 10a salts, as well as 4b, being the exceptions. As expected, larger alkyl chains favour miscibility in the non-polar solvents diethylether and hexane. Partial miscibility starts at about 320 g mol À1 in diethylether and 430 g mol À1 in hexane. As was found with the NTf 2 À salts, greater symmetry decreases solubility/ miscibility whereas increased exibility increases solubility/ miscibility. Thus 5 (with inexible isopropyl groups) and D 3hsymmetric 3c are insoluble in diethylether and hexane, and C ssymmetric 9b (with a exible hexyl chain) is soluble in hexane despite a relatively small MW.   3 ]DCA (4b) 9 5 0 I I

X-ray crystallography
The salts 3a and 3c were investigated by X-ray diffraction. Salt 3a was crystallised in air and consequently found to have one equivalent of water solvate. It packs in the C2/c space group with one independent cation, anion and water solvate in each unit cell. The DCA and water molecules form hydrogen-bonded chains that are parallel to chains of cations in the same layer. The layers above and below have the chains of cations above and below the chains of DCA and water. Fig. 10 illustrates the asymmetric unit with the atomic labelling scheme. The C-C distances range 1.3801 (13) Each cation has a DCA anion above and below with the shortest distances being C1/N4 at 3.305(1)Å and C3/N4 at 3.310(1)Å as well as a number of other inter-ion distances of less than 3.5Å.
Salt 3c packs in the C2/c space group with one independent cation and anion in the unit cell. Fig. 11 shows the two ions with the atomic labelling scheme. Each cation is sandwiched between a propyl group from another cation and a DCA anion that is parallel to the cyclopropenium plane (Fig. 12). The closest cyclopropenium ring to DCA distance is C3-N4 at 3.332 A which is very similar to a distance of 3.351(2)Å for the distance between two positively-charged cyclopropenium cations in [C 3 (NEt 2 ) 3 ]I. 12 One propyl group is disordered such that the attached N atom is non-planar. It is not clear why this would be the case, although it should be noted that the trisdiisopropyl isomer has one pyramidal N atom due to steric strain and a low rotational barrier of the amino groups in TAC cations. 25 However, in that case, the C 3 N 3 core is signicantly asymmetric due to the incomplete delocalisation, whereas in 3c the C 3 N 3 core is still close to D 3h -symmetric.

Conclusions
We have prepared a series of TDAC DCA salts for comparison with a similar series of NTf 2 À salts. Like those salts, the melting points can be rationalised by consideration of MW, symmetry and conformational exibility. TGA studies found that dimethylamino groups generally decrease thermal stability. Density was similarly found to be strongly dependent on MW and temperature, but not symmetry. Density tting parameters a and b are strongly correlated with MW, as is the thermal expansion coefficient a P (which was not correlated in the NTf 2 À salts). Viscosity appears to be less dependent on symmetry than in the NTf 2 À salts and, similarly, conductivity is essentially MW dependent only. A Walden plot revealed that most of the ILs can be classied as "good ILs" with only the larger ILs showing deviations from ideality. The DCA salts are more hydrophilic than the NTf 2 À salts, but even so, only the smaller cations are soluble/miscible in water. Solubility/miscibility in non-polar   solvents was, like the NTf 2 À salts, found to be largely dependent on MW, but also subtly dependent on symmetry and conformational exibility. Only the largest cation, 3g, is fully miscible with hexane. The extensive characterisation of a second comparative anion series of TDAC salts will make it easier to predict properties of TDAC salts with other anions and thus ease their tunability.
Solubility and miscibility studies were carried out by taking 0.5 mL of sample and adding step-wise 10 Â 0.05 mL of solvent followed by 9 Â 0.5 mL of solvent. Aer each addition of solvent the sample was mixed and allowed to equilibrate at 25 C to determine whether the sample was miscible or immiscible. In the case of solid samples, a 0.1 g sample was taken and 2.5 mL of solvent was added and the sample was equilibrated at 25 C. In some cases, the solid sample was observed to form two immiscible liquid layers. Water contents of water-saturated ILs were measured by Karl-Fischer titration aer equilibration at 25 C for 24 hours followed by centrifugation.
X-ray crystallography: single crystals of 3a$H 2 O and 3c formed in the neat liquid. A suitable crystal of each was selected and mounted on a SuperNova, Dual, Cu at zero, Atlas diffractometer. Using Olex2, 26 the structures were solved with the XS structure solution program 27 using Direct Methods and rened with the XL renement package 27 using Least Squares minimisation. Crystal data and structure renement and structural details are given in the ESI † along with the atom numbering schemes.