Highly-fluorinated Triaminocyclopropenium Ionic Liquids.

A series of highly-fluorinated triaminocyclopropenium salts, with up to six fluorous groups, were prepared and their properties as ionic liquids investigated. Reaction of pentachlorocyclopropane or tetrachlorocyclopropene with bis(2,2,2-trifluoroethyl)amine, HN(CH2 CF3 )2 , occurs in the presence of a trialkylamine, NR3 , to give cations with two fluorinated amino groups, [C3 (N(CH2 CF3 )2 )2 (NR2 )]+ (R=Et, Pr, Bu, Hex), with traces of [C3 (N(CH2 CF3 )2 )3 ]+ . Use of appropriate reagent ratios and reaction times and subsequent addition of a dialkylamine, HNR'R", gives cations with one fluorinated amino group, [C3 (N(CH2 CF3 )2 )(NR2 )(NR'R")]+ ((NR2 )(NR'R")=(NBu2 )2 , (NEt2 )(NPr2 ), (NBu2 )(NBuMe)). These cations were isolated as chloride salts and some of these were converted to bistriflamide, dicyanamide and triflate salts to provide ionic liquids. These salts were characterised by thermal (DSC and TGA) measurements and miscibility/solubility properties (determined in a range of solvents). Ionic liquids (ILs) were also characterised by density, viscosity and conductivity measurements where possible. X-ray diffraction studies of chloride salts showed the formation of fluorous regions and more hydrophilic ionic regions in the solid state.

Introduction

Research on ionic liquids (ILs) and their applications is now an intense area of activity. The reasons are well‐known: in particular, their effectively zero vapor pressure, low flammability, easy tunability and potential for recycling. Inclusion of fluorine atoms in an ionic liquid can cause a significant change to their properties. A wide range of IL properties such as melting point, viscosity, density, conductivity, solubility, liquid range, thermal stability and electrochemical stability can be altered by modifying non‐fluorinated ILs into fluorinated ILs.

The fluorinated ionic liquids (FILs) reported so far most commonly contain fluorinated anions (FA‐ILs), for which there is a large variety; such as BF4 −, PF6 −, triflate (OTf−), bistriflamide (NTf2 −), CF3(CF2)nCO2 −, CF3(CF2)nSO3 − and [PF3(C2F5)3]−.[ , ] In contrast, there is a much smaller number of ILs with fluorinated cations (FC‐ILs).[ , , , , , ] Many of the FC‐ILs in fact have both ions fluorinated (FAC‐ILs). The first FC‐ILs, with 3‐methyl‐1‐(2,2,2‐trifluoroethyl)imidazolium, were reported by Bonhôte and co‐workers in 1996 and were characterized as hydrophobic, highly conductive, ambient‐temperature molten materials. However, the vast majority of fluorinated cations contain a single fluorinated alkyl chain with an ethylene spacer, ‐CH2CH2(CF2) F to the cationic moiety (Scheme 1, class II). The value of n varies greatly, but six is most common. Although the cationic moiety is typically imidazolium,[ , , , , , , , ] examples with a cation such as pyridinium,[ , , ] ammonium,[ , ] phosphonium[ , ] or triazolium are also known.[ , ] There is also a number of examples with longer spacer groups.[ , , , , , ] Related to this class is the benzyl‐functionalised imidazolium (IM) IL [BuIMCH2C6F5]NTf2 which contains a perfluorophenyl group. Methylene spacers are much less common (class I), and are mostly associated with a single 2,2,2‐trifluoroethyl group on imidazolium or pyridinium.[ , , ] These FC‐ILs are fundamentally different from other FC‐ILs due to the close proximity of both the cationic charge and the electronegative F atoms to the methylene protons, which can therefore make them significant hydrogen‐bond donor groups. FC‐ILs with no spacer (class 0) are also very rare. These have been attached to imidazolium and triazolium cations.[ , ]

Scheme 1

Classes of known FC‐ILs based on the number of fluorinated groups and the spacing to the cationic moiety.

FC‐ILs with more than one fluorinated alkyl group are only known for classes II[ , , , ] (imidazolium, triazolium, pyridinium, ammonium and phosphonium) and II (phosphonium). Alpers et  al. also reported some dicationic diammonium and diphosphonium ILs with two fluorinated alkyl groups.

Fluorine‐containing ILs are interesting materials as they can be used in applications such as gas absorption and for the formation of hydrophobic materials. Vanhoutte et  al. have studied the solubility of oxygen in piperidinium and pyrrolidinium FAC‐ILs and concluded that the concentration of dissolved oxygen in fluorinated ILs is higher than that of commercial ILs without a fluorinated alkyl chain. Tindale and Ragogna have synthesized a series of highly‐fluorinated phosphonium‐based ILs and investigated them as media for the generation of superhydrophobic coatings.

As noted by Pereiro and coworkers, the physical characterisations of most of the FC‐ILs reported to date has been very limited and would typically only include the synthesis and thermal (DSC and TGA) properties. Coupled with the limited number of FC‐ILs, it is therefore often difficult to distinguish clear trends in their properties. However, the viscosity of fluorinated ILs is generally quite high, and conductivity is rather low.

In the work reported here we will describe the synthesis and properties of FC‐ILs based on triaminocyclopropenium (TAC) cations which allow us to introduce up to six fluorinated alkyl groups. TAC salts have been investigated for almost 50 years, but interest in applications of TAC salts has significantly increased lately due to the favourable properties that these cations exhibit. They have a high‐lying HOMO, which results in particularly weak interactions with anions,[ , , , ] and a reversible oxidation to the radical dication, which makes them useful for electrochemical processes. TAC salts also have excellent thermal stability, despite the ring strain, due to their aromaticity, charge delocalization, and high‐energy HOMO. Other areas of interest include anion switches, polyurethanes, ionic liquid crystals, polyelectrolytes, phase‐transfer catalysis,[ , ] organocatalysis, hypergolic fluids, and lipase activation. We initially reported the IL properties of TAC‐based salts in 2011, and this was followed by studies on tris(dialkylamino)cyclopropenium (TDAC) NTf2 − ILs and dicyanamide (DCA) ILs, as well as protic and amino acid‐functionalised TAC ILs.[ , ] We now report our use of trifluoroethyl groups to produce fluorinated TAC cations of the new classes I, I and I (cations with 2, 4 and 6 CH2CF3 groups, respectively) along with an investigation of their properties.

Results and Discussion

Synthesis

Addition of secondary amines to either pentachlorocyclopropane or tetrachlorocyclopropene is well known for the synthesis of triaminocyclopropenium (TAC) salts.[ , ] However, this does not work well with weakly‐nucleophilic amines such as bis(2,2,2‐trifluoroethyl)amine, NH(CH2CF3)2. We found however, that the addition of a trialkylamine NR3 (R=Et, Pr, Bu, Hex) to a solution with C3Cl4 and NH(CH2CF3)2 would readily give a TAC salt with two fluorinated amino groups and one dialkylamino group derived from the trialkylamine, [C3(N(CH2CF3)2)2(NR2)]Cl. A proposed mechanism is shown in Scheme 2 in which C3Cl4 initially undergoes nucleophilic substitution by an amine, with loss of Cl−, to give intermediate A which can then lose a chloroalkane to form the dichloroaminocyclopropenium intermediate B (we isolated PrCl from a reaction using NPr3). This cationic salt is now more readily attacked by NH(CH2CF3)2 to form the resultant TAC chloride salt ([1  a–d]Cl). Interestingly, a small amount (ca. 8%) of [C3(N(CH2CF3)2)3]Cl ([2]Cl) is also formed; this does not form in the absence of NR3. We believe that this is due to a slow reaction of NH(CH2CF3)2 with the cationic intermediate A (which would be faster than a reaction with neutral C3Cl4).

Scheme 2

Synthesis of TAC salts with two or three fluorinated amino groups.

Optimization of reagent ratios is critical to maximizing the yield of the desired product. With an excess of NEt3, for example, the major product is instead [Et3NCClCHCONEt2]Cl (3) with [1  a]Cl and [C3(N(CH2CF3)2)(NEt2)2]Cl ([4  a]Cl) as minor products (Scheme 3). It's not clear where the O atom in 3 came from, but we presume it came from traces of water in the undried amine. ES‐MS of the crude product mixtures was found to be useful by giving near quantitative information on the product ratios (see Supporting Information). It was found that C3Cl5H could also be used instead of C3Cl4, but the reactions were not as clean. For the synthesis of [1  a]Cl, a C3Cl5H/NH(CH2CF3)2/NEt3 ratio of 1 : 5 : 2 was found to be best.

Scheme 3

Reaction of C3Cl4 with NH(CH2CF3)2 and an excess of NEt3 (starting from intermediate B).

Salt 3 was also characterized crystallographically as the monohydrate and found to contain a rare discrete 1D linear {Cl(H2O)−}∞ chain. Details are provided in the Supporting Information.

TAC cations with one fluorinated amine could be prepared in two steps by the later addition of a secondary dialkylamine NHRR’ which reacts faster than the remaining NH(CH2CF3)2. Thus, if addition of NH(CH2CF3)2/NBu3 is followed by addition of HNBu2 (with further tertiary amine) then the C 2v‐symmetric cation [4  c]Cl is formed (Scheme 4). In this manner, it is also possible to form C s‐symmetric cations with three different amino groups, and we isolated [C3(N(CH2CF3)2)(NEt2)(NPr2)]Cl ([5  a]Cl) and [C3(N(CH2CF3)2)(NBu2)(NBuMe]Cl ([5  b]Cl) via this route.

Scheme 4

Synthesis of TAC salts with one fluorinated amino group.

Interestingly, we were able to prepare a C s‐symmetric cation with two fluorinated amino groups by using a shorter time for stirring with more NH(CH2CF3)2 and NBu3 followed by addition of HNBuMe to give [C3(N(CH2CF3)2)2(NBuMe)]Cl ([1  e]Cl). This suggests the presence of fluorinated analogues of intermediate A in Scheme 2 and that the route taken and the products formed depends on a number of factors such as the concentration, relative proportions of the reagents, time allowed between steps, and the nature of the tertiary amine.

Attempts to prepare [2]Cl in greater yield were unsuccessful. We also found that it was always isolated with a significant impurity in which one F atom has been replaced by an H atom to give [C3(N(CH2CF3)2)2(N(CH2CF3)(CH2CHF2))]Cl ([6]Cl), as confirmed by 1H‐NMR and ES‐MS (Supporting Information). However, the mechanism for the formation of this compound is unclear.

In order to investigate the potential usefulness of these cations in forming ionic liquids, a selection of the chloride salts was converted to salts with bistriflamide, dicyanamide and triflate anions, as listed in Table 1, using conventional metathesis routes. We then investigated their physical properties (DSC, TGA, viscosity, density and conductivity) where possible. Unfortunately, we were unable to prepare ionic liquids with 2 + due to the low yields and very low solubility of the chloride salt in organic solvents.

Table 1

DSC and TGA data for fluorinated TAC salts.

Salt	Yield/%	T g/°C	T m/°C	T d at 1 °C min−1/°C	T d at 10 °C min−1/°C
[C3(N(CH2CF3)2)2(NEt2)]Cl ([1  a]Cl)	41	–	206	243	273
[C3(N(CH2CF3)2)2(NPr2)]Cl ([1  b]Cl)	39	–	191	238	271
[C3(N(CH2CF3)2)2(NBu2)]Cl ([1  c]Cl)	41	–	174	238	273
[C3(N(CH2CF3)2)2(NHex2)]Cl ([1  d]Cl)	10	–	120	238	268
[C3(N(CH2CF3)2)2(NBuMe)]Cl ([1  e]Cl)	15	–	191	236	267
[C3(N(CH2CF3)2)3]Cl ([2]Cl)	6	–	236	231	260
[C3(N(CH2CF3)2)(NBu2)2]Cl ([4  c]Cl)	35	–	81.3	246	280
[C3(N(CH2CF3)2)(NEt2)(NPr2)]Cl ([5  a]Cl)	16	–	63.7	250	281
[C3(N(CH2CF3)2)(NBu2)(NBuMe)]Cl ([5  b]Cl)	43	−39.4	RTIL	242	275
[C3(N(CH2CF3)2)2(NEt2)]NTf2 ([1  a]NTf2)	71	–	88.1	273	328
[C3(N(CH2CF3)2)2(NBu2)]NTf2 ([1  c]NTf2)	90	−37.0	56.6	277	331
[C3(N(CH2CF3)2)2(NHex2)]NTf2 ([1  d]NTf2)	89	−38.2	36.4	283	330
[C3(N(CH2CF3)2)2(NBuMe)]NTf2 ([1  e]NTf2)	90	–	88.8	278	336
[C3(N(CH2CF3)2)(NBu2)2]NTf2 ([4  c]NTf2)	88	−59.1	RTIL	305	388
[C3(N(CH2CF3)2)(NBu2)(NBuMe)]NTf2 ([5  b]NTf2)	73	−55.3	RTIL	311	368
[C3(N(CH2CF3)2)2(NBu2)]DCA ([1  c]DCA)[a]	64	−21.4	68.7	199	235
[C3(N(CH2CF3)2)2(NHex2)]DCA ([1  d]DCA)[a]	80	−29.8	RTIL	186	232
[C3(N(CH2CF3)2)(NBu2)2]DCA ([4  c]DCA)[a]	80	−47.1	RTIL	206	252
[C3(N(CH2CF3)2)(NBu2)(NBuMe)]DCA ([5  b]DCA)[a]	64	−45.4	RTIL	218	254
[C3(N(CH2CF3)2)2(NBu2)]OTf ([1  c]OTf)	76	−28.2	RTIL	253	342
[C3(N(CH2CF3)2)2(NHex2)]OTf ([1  d]OTf)	91	−28.8	RTIL	275	343
[C3(N(CH2CF3)2)(NBu2)2]OTf ([4  c]OTf)	91	−45.1	RTIL	306	357
[C3(N(CH2CF3)2)(NBu2)(NBuMe)]OTf ([5  b]OTf)	74	−43.5	RTIL	294	366

[a] DCA = dicyanamide ([N(CN)2]−).

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DSC and TGA data

DSC data was collected at 10 °C min−1 and the results are summarized in Table 1. For the chloride salts (Figure 1), melting point is primarily dependent on the number of fluorinated amino groups with [2]Cl (F18) having the highest m.p. (236 °C) and only the mono‐fluorinated amino‐group (F6) salts being ILs; secondarily on symmetry, with the m.p. decreasing from [4  c]Cl to [5  a]Cl to [5  b]Cl (81.3 °C, 63.7 °C and RTIL, respectively); and finally on alkyl chain length with decreasing m.p. for the difluorinated amino‐group (F12) salts [1  a–d]Cl (206 °C, 191 °C, 174 °C and 120 °C, respectively). Similar trends are found for the NTf2 − and DCA salts, however, all of these classify as ILs, with the F6 salts being RTILs. In the case of the triflate salts, all of these are RTILs.

Figure 1

Melting points of fluorinated TAC chloride salts. Green circles (F6 salts), blue diamonds (F12 salts), orange triangle (F18 salt [2]Cl).

Thermal decomposition temperatures (T d) were determined at both 1 °C min−1 and 10 °C min−1 and are given in Table 1. The main factor that determines T d in these salts is the anion, with T d decreasing NTf2 −∼OTf−>Cl−>DCA. For comparison, TDAC NTf2 − salts (without any Me groups) have T d values around 400 °C, which is about 70 °C higher than the F12 NTf2 − salts. The lower stability of the DCA salts compared to the Cl− salts is interesting, especially in light of their much lower m.p.s. F12 DCA salts have T d values about 100 °C lower than TDAC DCA salts. On the other hand, F12 Cl− salts have T d values about only 30 °C higher than TDAC Cl− salts.[ , ] The nature of the alkyl group has very little effect on T d whereas an additional fluorinated amino group decreases T d by 10–20 °C for the Cl−, OTf− and DCA salts and 30–50 °C for the NTf2 − salts.

Density

Density data were determined from 20–90 °C for the relatively low viscosity IL [4  c]NTf2 (Tables 1S and 2S). At 20 °C, the density is 1.298202 g/cm3 with a molar volume of 579.82 cm3/mol. Parameters a and b, for the linear fit of the data using the equation ρ=a–bT, were found to be 1.317240 g/cm3 and 0.000955 g cm−3 K−1, respectively.

The densities of the other, more viscous, ILs at 20 °C was estimated using the group additivity method for determining molecular volume (Tables 2 and 3S). The molecular volume of [4  c]NTf2 is 962.5 Å3. We used the following volumes for the individual groups as determined by various authors: 280 Å3 for the [C3(NMe2)3]+ core, 27.7 Å3 (CH2), 5 Å3 (H), 80 Å3 (DCA), 248 Å3 (NTf2 −),125 Å3 (OTf−) and 27 Å3 (Cl−).[ , , ] The volume for a CF3 group was thus found to be 56.2 Å3.

Table 2

MW and selected density, viscosity, and conductivity data forfluorinated ILs.

Ionic liquid	MW	Density [g cm−3]		Viscosity [cP]		Conductivity [mS cm−1]		ΔW at 20 °C
	[g/mol]	20 °C	50 °C	20 °C	50 °C	20 °C	50 °C
[C3(N(CH2CF3)2)(NBu2)(NBuMe)]Cl ([5  b]Cl)	465.95	1.175[a]	–	–	2510	0.0033	0.0481
[C3(N(CH2CF3)2)(NBu2)2]NTf2 ([4  c]NTf2)	752.72	1.298202	1.269463	979	142	0.0858	0.389	0.31
[C3(N(CH2CF3)2)(NBu2)(NBuMe)]NTf2 ([5  b]NTf2)	710.64	1.342[a]	–	492	83	0.1082	0.500	0.56[b]
[C3(N(CH2CF3)2)2(NHex2)]DCA ([1  d]DCA)	646.56	1.276[a]	–	–	2818	0.0021	0.0396
[C3(N(CH2CF3)2)(NBu2)2]DCA ([4  c]DCA)	538.62	1.125[a]	–	3906	321	0.0371	0.320	0.16[b]
[C3(N(CH2CF3)2)(NBu2)(NBuMe)]DCA ([5  b]DCA)	496.54	1.159[a]	–	6770	411	0.0452	0.352	1.02[b]
[C3(N(CH2CF3)2)2(NBu2)]OTf ([1  c]OTf)	673.48	1.442[a]	–	–	5425	0.0007	0.0215
[C3(N(CH2CF3)2)2(NHex2)]OTf ([1  d]OTf)	729.59	1.367[a]	–	–	2878	0.0003	0.0083
[C3(N(CH2CF3)2)(NBu2)2]OTf ([4  c]OTf)	621.64	1.229[a]	–	6919	480	0.0710	0.325	−0.40[b]
[C3(N(CH2CF3)2)(NBu2)(NBuMe)]OTf ([5  b]OTf)	579.56	1.272[a]	–	9766	555	0.0158	0.1465	0.15[b]

[a] Calculated density; [b] Estimated using the calculated density at 20 °C.

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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 and the complete set of data is provided in the Supporting Information (Table 4S). The discussion here will refer to the 50 °C data, which is more comprehensive. Firstly, additional fluorinated amino groups increase the viscosities significantly, for example, 480 cP for [4  c]OTf versus 5425 cP for [1  c]OTf. Similarly, the F6 ILs [4  c]NTf2 and [5  b]NTf2 (979 and 492 cP, respectively) have much higher viscosities than TDAC NTf2 − ILs of similar MW, which have viscosities of about 200 cP. Likewise, TDAC DCA ILs have viscosities of about 250 cP versus 3906 and 6770 cP for [4  c]DCA and [5  b]DCA, respectively. A similar trend was reported for fluorinated versus non‐fluorinated imidazolium ILs.[ , , ] In terms of anion effects, the ability to hydrogen bond and a small size appear to be key factors, with Cl−≫OTf−>DCA≫NTf2 −. The much lower viscosities of the NTf2 − ILs compared to the DCA ILs is in complete contrast to the TDAC ILs in which the NTf2 − ILs have slightly higher viscosities.[ , ] Presumably, steric effects hinder the NTf2 − ions from hydrogen bonding with the CH2CF3 groups.

The viscosity data was fit to both the Arrhenius and Vogel‐Fulcher‐Tammann equations; these parameters are given in the Supporting Information (Table 5S). The values of D, a measure of the deviation from Arrhenius behavior, lie in the range 5–10 which is typical for “fragile” liquids. A “fragility plot” of log(viscosity) versus T g/T (Figure 1S) also confirms that these are fragile ILs.

Conductivity

Conductivity data was collected from 20–90 °C where possible. Results at 20 °C and 50 °C are given in Table 2 and the complete set of data, along with the fitting data (to both the Arrhenius and Vogel‐Fulcher‐Tammann equations), is provided in the Supporting Information (Tables 6S and 7S). Conductivity generally decreases with an increase in the viscosity of the IL, but ion‐pairing or clustering will also have a significant impact on the conductivity. Thus, conductivity is found to be lowest for the F12 ILs as well as the chloride IL [5  b]Cl, as these all have high viscosities. Of these, [1  d]OTf has an especially low conductivity relative to its viscosity. On the other hand, of the F6 ILs, [4  c]OTf has a relatively high conductivity for its viscosity.

Ionicity

A Walden plot, log(Λ) versus log(1/η), is frequently used to investigate the ionicity of an IL (Figure 2). The deviation from the diagonal line on the Walden plot (ΔW) is given in Table 2. Low ionicity is generally attributed to the formation of ion‐pairs, ion‐triplets or cluster aggregates, however, correlated and anti‐correlated motions of ions, for example, provide alternative explanations for decreased conductivity. Methods to determine ionicity have been discussed recently by Nordness and Brennecke.

Figure 2

Walden plot for fluorinated TAC ILs. The solid diagonal line represents 1 M KCl(aq). Blue 4  c +; red 5  b +; diamonds NTf2 −; circles DCA; triangles OTf−.

A Walden plot requires knowledge of the fluid's density to determine the molar conductivity. Unfortunately, we were only able to measure one set of density data, so the density for the other five ILs that are liquid at 20 °C was estimated as described above. Thus, ΔW for these ILs are estimates. From Figure 2 we can see that the NTf2 − salts are “good” ILs, the triflate salts are “very good” to “superionic” ILs and the DCA salts are highly variable.

Miscibility and solubility

The miscibility and solubility properties were investigated at 25 °C (Table 3). As would be expected, the chloride salts are soluble or miscible in the amphiprotic solvents water and ethanol and insoluble/immiscible in toluene and hexane, although the low‐symmetry and conformationally‐flexible salt [5  b]Cl is miscible with toluene (and diethylether) and has some miscibility in hexane. Proton acceptor ability seems to be quite important as these salts are soluble/miscible in ethylacetate (and acetonitrile) and most have some solubility in diethylether. Conversely, a number of them have low solubility in the polar solvent CH2Cl2, contrary to typical ILs. This is likely because CH2Cl2 is a very weak proton‐acceptor solvent.

Table 3

Miscibility and solubility properties of fluorinated TAC salts at 25 °C.[a]

Compound	Water	EtOH	CH2Cl2	EtOAc	Toluene	Et2O	Hexane
[C3(N(CH2CF3)2)2(NEt2)]Cl (1  a)	S	S	I	S	I	I	I
[C3(N(CH2CF3)2)2(NPr2)]Cl (1  b)	S	S	P	S	I	P	I
[C3(N(CH2CF3)2)2(NBu2)]Cl (1  c)	S	S	S	S	I	P	I
[C3(N(CH2CF3)2)2(NHex2)]Cl (1  d)	S	S	S	S	I	P	I
[C3(N(CH2CF3)2)2(NBuMe)]Cl (1  e)	S	S	P	S	I	I	I
[C3(N(CH2CF3)2)3]Cl (2)	S	S	I	S	I	I	I
[C3(N(CH2CF3)2)(NBu2)2]Cl (4  c)	S	S	S	S	I	P	I
[C3(N(CH2CF3)2)(NBu2)(NBuMe)]Cl (5  b)	M	M	M	M	M	M	≥53% IL
[C3(N(CH2CF3)2)2(NEt2)]NTf2 (1  a)	I	S	P	S	I	P	I
[C3(N(CH2CF3)2)2(NBu2)]NTf2 (1  c)	I	S	S	S	I	P	I
[C3(N(CH2CF3)2)2(NHex2)]NTf2 (1  d)	I	S	S	S	N	S	I
[C3(N(CH2CF3)2)2(NBuMe)]NTf2 (1  e)	I	S	P	S	I	P	I
[C3(N(CH2CF3)2)(NBu2)2]NTf2 (4  c)	N	M	M	M	≥50% IL	M	≥56% IL
[C3(N(CH2CF3)2)(NBu2)(NBuMe)]NTf2 (5  b)	N	M	M	M	≥53% IL	M	≥53% IL
[C3(N(CH2CF3)2)2(NBu2)]DCA (1  c)	I	S	S	S	I	P	I
[C3(N(CH2CF3)2)2(NHex2)]DCA (1  d)	N	M	M	M	≥67% IL	≥53% IL	≥53% IL
[C3(N(CH2CF3)2)(NBu2)2]DCA (4  c)	N	M	M	M	≥53% IL	≥53% IL	≥59% IL
[C3(N(CH2CF3)2)(NBu2)(NBuMe)]DCA (5  b)	N	M	M	M	≥50% IL	≥53% IL	≥53% IL
[C3(N(CH2CF3)2)2(NBu2)]OTf (1  c)	N	M	M	M	≥59% IL	≥53% IL	≥53% IL
[C3(N(CH2CF3)2)2(NHex2)]OTf (1  d)	N	M	M	M	≥59% IL	M	≥67% IL
[C3(N(CH2CF3)2)(NBu2)2]OTf (4  c)	N	M	M	M	M	M	≥71% IL
[C3(N(CH2CF3)2)(NBu2)(NBuMe)]OTf (5  b)	N	M	M	M	M	M	≥53% IL

[a] S=soluble; I=insoluble; M=miscible; N=immiscible liquid; P=partial solubility/miscibility.

The NTf2 −, DCA and OTf− salts tend to have similar solubility properties: they are all insoluble/immiscible in water but soluble/miscible in EtOH, CH2Cl2 and EtOAc. Differences appear when using non‐polar solvents. Notably, the NTf2 − salts tend to be less miscible/soluble in these solvents, which is not the case for TDAC salts.[ , ] They are also more soluble/miscible in diethyl ether than toluene or hexane, presumably due to hydrogen bonding between the cation CH2 groups and the ether O atom. Interestingly, the DCA salts have very similar miscibilities in toluene, diethylether and hexane, being mostly partially miscible. In this case, the methylene groups are presumably hydrogen bonding with DCA rather than ether. The triflate salts, however, are mostly fully miscible in diethylether, suggesting weaker hydrogen bonding between the cation and anion.

Solid state structures

We have previously reported the solid‐state structures and infrared spectra of some of the chloride salts as discrete chloride hydrates. In particular, [1  c]Cl.H2O forms a dichloride dihydrate square [Cl2(H2O)2]2−, [1  d]Cl.0.5H2O forms a dichloride hydrate [Cl2(H2O)]2−, [1  e]Cl.H2O forms a 1D chain {Cl(H2O)−}∞, [4  c]Cl.H2O forms a dichloride dihydrate square [Cl2(H2O)2]2−, and [5  a]Cl.H2O forms a tetrachloride tetrahydrate [Cl4(H2O)4]4−. We now report the solid‐state structures of three additional chloride salts, the anhydrous salt [2]Cl, the monohydrate [1  b]Cl.H2O which contains a [Cl2(H2O)2]2− square, and the hydrate [1  a]Cl.2.17H2O which contains a discrete 1D ribbon of the chloride hydrate {[Cl6(H2O)13]6−}∞. We also report the structure of the bistriflamide salt [1  a]NTf2.

[C3(N(CH2CF3)2)3]Cl ([2]Cl) crystals were obtained by slow evaporation of an acetonitrile/DCM solution at room temperature. It crystallizes in the triclinic space group P‐1 and the asymmetric unit has one cation and chloride anion (Table 8S). Each chloride ion is surrounded by three cations and has six CH−Cl hydrogen‐bond interactions, with the closest α‐hydrogen atoms as shown in Figure 3. The cation adopts a conformation in which fluoroalkyl chains in two of the amino groups are alternating either side of the C3 plane and in the other amino group they are on the same side of the C3 plane. Every chloride is “chelated” by three cations and every cation chelates three chlorides to form a 3D network of CH−Cl− hydrogen bonds (Table 14S). This extensive network accounts for its high melting point of 236 °C.

Figure 3

Plot of [2]Cl illustrating the environment around the chloride ion.

Despite the electronegativity of the trifluoromethyl groups, the ring C−C and exocyclic C−N bond distances (average 1.381 Å and 1.335 Å, respectively) are typical for peralkylated TAC salts. Two of the amino groups are relatively planar and in the plane of the C3 ring, however, one is a distorted trigonal plane (sum of angles=355.3°) with one substituent rotated out of the C3 plane (C3‐C2‐N2‐C23=−46.4(4)°). However, these distortions are not especially unusual.

Suitable crystals of [1  b]Cl for X‐ray diffraction were grown by slow evaporation of an undried CH2Cl2/MeOH solution. The salt crystallizes in the monoclinic space group C2/c (Table 9S) in which the asymmetric unit consists of one cation, a chloride ion and two half‐water solvates (Figure 4a). The chloride hydrate then forms a [Cl2(H2O)2]2− square with crystallographic C 2v symmetry (Figure 4b). The cation adopts a conformation in which three trifluoroethyl groups and one propyl group are in one side of the C3N3 plane while one trifluoroethyl group and one propyl group are in the opposite side of the plane. The dichloride dihydrate cluster is sandwiched between two cations with four additional cations which also have hydrogen‐bonding interactions with the cluster (Figure 6S and Table 17S).

Figure 4

(a) Asymmetric unit of [1  b]Cl.H2O; (b) The [Cl2(H2O)2]2− cluster and its environment within [1  b]Cl.H2O.

Due to the different π‐donor abilities of the amino groups, the exocyclic C−N bond to the NEt2 group is shorter than to the N(CH2CF3)2 groups (1.3101(15) Å versus 1.3388 Å average, respectively) and the cyclic C−C bond opposite the NEt2 group is also shorter (1.3690(14) Å versus 1.3884 Å average, respectively). However, the average C−C and C−N distances are essentially the same as [2]Cl (1.382 Å and 1.329 Å versus 1.381 Å and 1.335 Å, respectively). Also like 2 +, two of the amino groups are relatively planar and in the plane of the C3 ring, while one is a distorted trigonal plane (sum of angles=355.9°) with one substituent rotated out of the C3 plane (C1‐C2‐N2‐C21=20.5(3)°). Regarding the chloride hydrate cluster, there is only one other reported C 2v‐symmetric [Cl2(H2O)2]2− cluster, in the closely‐related [1  c]Cl.H2O salt. The cluster in [1  b]Cl.H2O has longer Cl−O distances (3.262(5) and 3.31(5) Å versus 3.2051(15) and 3.2975(15) Å) and, consequently, the infrared stretching bands (Figure 7S) are at higher energy and the bending bands are at lower energy (Table 18S). Curnow and Crittenden have recently reviewed the structures and infrared spectra of halide hydrates in the solid state.

Crystals of [C3(N(CH2CF3)2)2(NEt2)]Cl.2.17H2O ([1  a]Cl.2.17H2O) were obtained by slow evaporation of a CH2Cl2/EtOH solution at room temperature. It crystallizes in the triclinic space group P‐1 (Table 9S). The asymmetric unit contains six cations and anions along with 12 fully‐occupied water molecules and two half‐occupied water molecules, thus the stoichiometry is 13 waters per six cations and anions (Figure 5). Two of the cations have disordered CH2CF3 groups. In all of the cations, the CH3 groups are alternating either side of the C3N3 plane. On the other hand, a variety of conformations relative to the C3N3 plane are observed for the CH2CF3 groups: in four of the cations, the CF3 groups are on the same side of the plane; while in the other two cations, two CF3 groups are on same side of the plane and the other two are alternating either side of the plane (Figure 20S).

Figure 5

Asymmetric unit of [1  a]Cl.2.17H2O (the waters are not shown for clarity).

The chloride hydrate forms a 1D ribbon of {[Cl6(H2O)13]6−}∞ within relatively hydrophilic (CH2) channels (Figure 6) to form ionic regions within the crystal. The chloride anions have a number of weak hydrogen‐bonding interactions with the cation; there are 20 CH−Cl hydrogen bonding interactions with H−Cl distances within 2.61–3.00 Å (Figure 19S and Table 21S). Of those 20 hydrogen bonds, only one is associated with an ethyl group; this illustrates the better hydrogen‐bond donor ability of trifluorinated ethyl groups. Figure 6b clearly illustrates the formation of ionic regions surrounded by fluorous regions.

Figure 6

(a) The {[Cl6(H2O)13]6−}∞ ribbon in [1  a]Cl.2.17H2O (the likely hydrogen‐bonding arrangement is indicated; the two half‐occupied waters are in essentially the same position‐on top of each other in this view); (b) chloride hydrate ribbons within ionic channels surrounded by fluorous regions.

The structure of the bistriflamide salt of [1  a]+ was also determined; crystals of [1  a]NTf2 were obtained by slow evaporation of a methanol solution at room temperature. It crystallizes in the monoclinic space group C2/c (Table 9S) and the asymmetric unit contains one and a half independent cations and one and a half anions (Figure 7). In both cations, the CF3 and CH3 groups are alternating either side of the C3N3 plane. All of the nitrogen atoms are trigonal planer with the sum of the C−N−C angle around each of the nitrogen atom close to 360° (N1=359.9°; N2=358.8°; N3=358.7°). One of the anions is disordered. The anions are each surrounded by seven cations via weak CH−F, CH−O and CH−N hydrogen bonds as detailed in the Supporting Information.

Figure 7

Thermal ellipsoid plot of the independent cations and anions in [C3(N(CH2CF3)2)2(NEt2)]NTf2 ([1  a]NTf2).

Conclusion

We have prepared a series of highly‐fluorinated TAC salts. Trialkylamines NR3 were found to facilitate the reaction of the amine HN(CH2CF3)2 with C3Cl4 or C3Cl5H. This allows the preparation of TAC cations with one, two or three fluorinated amino groups, including cations with three different amino groups. The use of trialkylamines to promote the addition of a weakly‐nucleophilic amine suggests that a general route for the addition of similar weakly‐nucleophilic amines is now available. Curiously, the cation with three fluorinated amino groups, 2 +, was found to readily form a cation with just one F atom replaced by an H atom. Unfortunately, these salts were not available in high yields.

Melting points were found to increase significantly with fluorinated amino groups, decrease with alkyl chain length and lower symmetry, and decrease with the anion in the order Cl−>NTf2 −>DCA∼OTf−. T d values were found to significantly decrease with the addition of fluorinated amino groups, especially for the DCA anion, but not so much for chloride.

Viscosities for the ILs are generally much higher (and conductivities lower) than TDAC salts, and this can be attributed to hydrogen bonding with the methylenes on the fluorinated groups. Interestingly, the NTf2 − salts have lower viscosities than the DCA salts, which is probably also related to the hydrogen bonding ability.

Miscibilities and solubilities appear to be highly dependent on the hydrogen‐bonding ability of the solvent. Thus, CH2Cl2 is sometimes a poor solvent and diethylether is generally a better solvent than both toluene and hexane.

The solid state structure of [1  a]Cl.2.17H2O displays the clear formation of ion‐rich regions (including the CH2 groups of the cations) surrounded by fluorous regions.

The synthesis of novel highly‐fluorinated cations by use of TAC cations, with up to six fluorous groups, is a significant step towards our goal to synthesize cations that are either completely fluorinated or are fully encased within a sphere fluorous groups. These species promise to provide significant differences in their physical properties compared to the partially‐fluorinated cations known to date.

Supporting Information Summary

Synthesis and characterization details; density data (with fit parameters) for [4  c]NTf2, calculated molar volumes and densities at 20 °C; viscosity, viscosity fit parameters, a fragility plot, conductivity, conductivity fit parameters; IR spectrum and assignments for [1  b]Cl.H2O; and crystallographic data, bond lengths and angles for 3, [2]Cl, [1  b]Cl.H2O, [1  a]Cl.2.17H2O and [1  a]NTf2; ES‐MS spectra of crude product mixtures. Crystallographic data (CDCC 2141678‐2141682) is also available free of charge from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge, CB2 1EZ, UK; E‐mail: deposit@ccdc.cam.ac.uk.

Conflict of interest

The authors declare no conflict of interest.

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Supporting Information

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