Anion effect on melting points of salts with organic cations

Prevalence of bis(perfluoroalkanesulfonyl)imide anions in low-melting salts

Contribution from Axel's Collection of Notes and Communications
by Axel Drefahl • www.axeleratio.com

Comparison of various anions shows that bis(perfluoroalkanesulfonyl)imide anions such as bis(trifluoromethanesulfonyl)imide, [(CF3SO2)2N] –, and bis(pentafluoroethanesulfonyl)imide, [(C2F5SO2)2N] –, typically result in the lowest melting salt when paired with an alkyl-substituted imidazolium cation.
Here, we present salts with their melting points from published sources in such a way that for a given organic cation the anions are ordered by increasing values of the melting point for the salts they build with that cation.

H. L. Ngo et al. found for a large set of imidazolium salts that those containing the [(CF3SO2)2N] – anion had lower melting points compared to the other investigated anions [1]:

Imidazolium cation Anion order with melting points in °C
[1Et3MeIM] + [(CF3SO2)2N] – (-15) < [(C2F5SO2)2N] – (-1) < [BF4] – (11) < [(CF3SO2)3C] – (39)
< [AsF6] – (53) < [PF6] – (62) < Br – (79), I – (79) < Cl – (89)
[1Me2Me3PrIM] + [(CF3SO2)2N] – (15) < [(C2F5SO2)2N] – (34) < [PF6] – (78) < Cl – (138)
[1Me3[iPr]IM] + [(CF3SO2)2N] – (16) < [PF6] – (102) < I – (114)
[Me5IM] + [(CF3SO2)2N] – (118) < [PF6] – (166) < I – (213)
The cations have been ordered (top to bottom) with increasing melting points of their [(CF3SO2)2N] – salts. As noted by H. L. Ngo et al., the melting points decrease with the larger, more asymmetric imidazolium cations.

Similarly, but with different anions, we order imidazolium salts by their melting points taken from another comprehensive study [2]:

Imidazolium cation Anion order with melting points
in °C
[1Et3Et5MeIM] + [(CF3SO2)2N] – (-22) < [CF3SO3] – (35)
[1Et3MeIM] + [CF3CO2] – (-14) < [CF3SO3] – (-9) < [(CF3SO2)2N] – (-3) < [CF3(CF2)3SO3] – (28)
[1Bu3MeIM] + [(CF3SO2)2N] – (-4) < [CF3SO3] – (16) < [CF3(CF2)3SO3] – (20)
[1Et3Me5MeIM] + [(CF3SO2)2N] – (-3) < [CF3SO3] – (6)
[1Et3EtIM] + [(CF3SO2)2N] – (14) < [CF3SO3] – (23)
[1Et2Me3MeIM] + [(CF3SO2)2N] – (20) < [CF3CO2] – (59) < [CF3SO3] – (109)
[1Me3MeIM] + [(CF3SO2)2N] – (22) < [CF3SO3] – (39) < [CF3CO2] – (5)
[1Et2Et3MeIM] + [(CF3SO2)2N] – (28) < [CF3SO3] – (113)
With one exception (see [1Et3MeIM] +), alkyl-substituted imidazolium bis(trifluoromethanesulfonyl)imide salts have the lowest melting points. However, the other anions (trifluoromethanesulfonate, [(CF3SO2)2N] –, nonafluorobutanesulfonate, [CF3(CF2)3SO3] –, and trifluoroacetate, [CF3CO2] –) form low-melting salts with many imidazolium cations.

For 1-alkyl-2-methylpyrrolinium salts, the difference of the melting temperature between the bis(trifluoromethanesulfonyl)imide and iodide exceeds 60 degrees [3]:

Pyrrolinium cation Anion order with melting points
in °C
[1Pr2MePN] + [(CF3SO2)2N] – (19) < I – (85)
[1Bu2MePN] + [(CF3SO2)2N] – (36) < I – (111)

Since salts with bis(perfluoroalkanesulfonyl)imide anions have been reported of good chemical stability in air, salts with these anions will often be excellent ionic liquid candidates for many applications.

Cation dictionary:

1,2-Diethy-3-methylimidazolium [1Et2Et3MeIM] +
1,2-Dimethy-3-propylimidazolium [1Me2Me3PrIM] +
1,3-Diethylimidazolium [1Et3Et5MeIM] +
1,3-Diethyl-5-methylimidazolium [1Et3Et5MeIM] +
1,3-Dimethylimidazolium [1Me3Me5IM] +
1-Butyl-2-methylpyrrolinium [1Bu2MePN] +
1-Butyl-3-methylimidazolium [1Bu3MeIM] +
1-Ethyl-2,3-dimethylimidazolium [1Et2Me3MeIM] +
1-Ethyl-3-methyl-5-methylimidazolium [1Et3Me5MeIM] +
1-Ethyl-3-methylimidazolium [1Et3MeIM] +
1-Methyl-3-isopropylimidazolium [1Me3[iPr]IM] +
1-Propyl-2-methylpyrrolinium [1Pr2MePN] +
Pentamethylimidazolium [Me5IM] +

References:

[1] Ngo, H. L.; LeCompte K.; Hargens L.; McEwen, A. B. “Thermal properties of imidazolium ionic liquids” Thermochim. Acta 2000, 357-358, 97-102.
[2] Bonhôte, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram, K.; Grätzel, M. “Hydrophobic, Highly Conductive Ambient-Temperature Molten Salts” Inorg. Chem. 1996, 35, 1168-1178.
[3] Sun, J.; MacFarlane, D. R.; Forsythe, M. “A new family of ionic liquids based on the 1-alkyl-2-methyl pyrrolinium cation” Electrochim. Acta 2003, 48, 1707-1711. DOI: 10.1016/S0013-4686(03)00141-5 .


Last update was made on June 17 in 2008 by Axel Drefahl
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