Crystal structure of 1,3-dimethylimidazolium bis(fluorosulfonyl)amide: Unexpectedly high melting point arising from polydentate hydrogen bonding

Article abstract of DOI:10.1246/cl.130982

1,3-Dimethylimidazolium bis(fluorosulfonyl)amide, [C₁mim]FSA, is an ionic liquid with a lower viscosity than the bis(trifluoromethanesulfonyl) amide salt, [C₁mim]NTf₂, but with a higher melting point. The single-crystal X-ray structure analysis reveals that this is because the [C₁mim]FSA crystal has low coordination ion number and three types of highly symmetric polydentate CH.O hydrogen bonds.

Full text of DOI:10.1246/cl.130982

Received: October 22, 2013 | Accepted: November 19, 2013 | Web Released: November 23, 2013
CL-130982
Crystal Structure of 1,3-Dimethylimidazolium Bis(fluorosulfonyl)amide:
Unexpectedly High Melting Point Arising from Polydentate Hydrogen Bonding
Kozo Fujii, Tomohiro Mukai, and Keiko Nishikawa*
Graduate School of Advanced Integration Science, Chiba University,
1-33 Yayoi-cho, Inage-ku, Chiba 263-8522
(E-mail: k.nishikawa@faculty.chiba-u.jp)
1,3-Dimethylimidazolium
bis(fluorosulfonyl)amide,
using a laboratory-made calorimeter.12 The crystal-liquid phase
[C1mim]FSA, is an ionic liquid with a lower viscosity than
the bis(trifluoromethanesulfonyl)amide salt, [C1mim]NTf2, but
with a higher melting point. The single-crystal X-ray structure
analysis reveals that this is because the [C1mim]FSA crystal
has low coordination ion number and three types of highly
symmetric polydentate C-H£O hydrogen bonds.
transition for each salt was visually determined using a Linkam
TMS94 temperature-controlled stage, and the melting point was
taken to be the endothermic peak maximum. The viscosities
of the two salts were measured using an Anton Paar AMVn
viscometer at 333 K because the liquid [C1mim]FSA crystallized
immediately below its melting point (331 K) in the glass
capillary cell. [C1mim]FSA single-crystals were obtained by
recrystallization from methanol. X-ray diffraction data were
collected at 173 K.
Room-temperature ionic liquids (RTILs) are a new class of
liquids that have attracted a lot of attention because of their
interesting and characteristic properties.1,2 It is crucial that
general correlation between the structure and physicochemical
properties is clarified so that this can be achieved. For example,
it has been demonstrated that the variety of conformation of the
molecular ion plays an important role to obtain RTILs with low
melting point.3 Many studies have been focused on the way how
varieties and flexibilities of alkyl chain conformations in cations
relate to the physicochemical properties, and it has been shown
that the flexibility of the RTIL cation strongly affects the phase-
transition behavior, thermal properties, structure, and dynam-
ics.3,4 In this way, there have been many studies focused on
RTIL cations.
Melting points and viscosities of [C1mim]FSA and
[C1mim]NTf2 are listed in Table 1, and those of the correspond-
¹
ing [C2mim]+ salts are included for comparison.13 Each [FSA]
¹
salt was less viscous than the corresponding [NTf2] salt, which
could probably be attributed to the lower molecular weight and
¹
weaker nucleophilicity of bis(sulfonyl)amide group in [FSA]
¹
than that in [NTf2] , because the fluoro group is more
electronegative than the CF3 group.14 Melting points of both
of the [C2mim] salts were almost the same, but the [C1mim]FSA
melting point was markedly higher than that of [C1mim]NTf2,
which is difficult to explain considering molecular weights,
electrostatic interactions, and conformational variations. A
characteristic cation-anion interaction geometry might be
speculated to exist in the [C1mim]FSA; therefore, we attempted
to investigate this possibility by performing single-crystal X-ray
diffraction analysis.
We are now interested in the relationship between the
physicochemical properties and the anion structures. Bis(fluoro-
sulfonyl)amide and bis(trifluoromethanesulfonyl)amide (abbre-
¹
¹
viated to [FSA] and [NTf2] , respectively), which are typical
RTIL anions, contain two stable rotamers with a small energy
difference.5,6 A number of crystal structure analyses7 and
theoretical and spectroscopic studies8 have been performed on
the imidazolium-based NTf2 salts, whereas the number of
structure studies on the FSA salts is limited.9,10 Comparing the
structures and physicochemical properties of the two anions will
provide useful information. We synthesized 1,3-dimethylimida-
zolium bis(fluorosulfonyl)amide ([C1mim]FSA) (Figure 1) and
measured its viscosity and melting point, preformed single-
crystal X-ray analysis, and compared its structural characteristics
with those of [C1mim]NTf2.7b
The [C1mim]FSA salt crystallized in the orthorhombic
Pbcm space group. The detailed crystal structure data are
presented in Table S1 in the Supporting Information.11 As
shown in Figure 2, both the cation and anion form a completely
symmetric structure, with the C1 and H1 of the cation and N2 of
the anion located on the crystallographic mirror plane. The
¹
[FSA] conformation was observed to be perfectly cisoid with
F-S-N-S torsion angles of «74.1(1)°. On the other hand, if
¹
[NTf2] would form a completely symmetric cisoid conformer,
it would be unstable because of steric effects involving the two
bulky CF3 groups. To the best of our knowledge, a perfectly
cisoid conformation of a bis(sulfonyl)amide-based anion has
never been found in other salts.
The characteristic structure unit shown in Figure 2 indicates
that the crystal has a bilaterally symmetric cation-anion
The salts [C1mim]FSA and [C1mim]NTf2 were synthesized
following a previously established procedure,2 as described in
the Supporting Information.11 Calorimetric measurements were
¹1
conducted from 213 to 353 K at a scanning rate of 5 mK s
Table 1. Melting point and viscosity of various ionic liquids
[C1mim]+
Tm/K
©a/mPa s
331.0
298.5
[C2mim]+
Anion
Tm/K
©b/mPa s
¹
[FSA]
8.2
13.2
260.1c
256.8c
24.5c
45.9c
¹
[NTf2]
Figure 1. Structure of 1,3-dimethylimidazolium bis(fluorosul-
fonyl)amide ([C1mim]FSA).
b
c
aAt 333 K. At 298 K. ref 13.
Chem. Lett. 2014, 43, 405–407 | doi:10.1246/cl.130982
© 2014 The Chemical Society of Japan | 405

(a)
(b)
Figure 3. (a) In-plane hydrogen bonding and (b) out-of-plane
hydrogen bonding in a [C1mim]FSA crystal. Grey: carbon, blue:
nitrogen, yellow: fluorine, orange: sulfur, red: oxygen, and
white: hydrogen. Symmetry operations: (iv) 2 ¹ x, 1/2 + y,
1/2 ¹ z; (v) 1 ¹ x, 1/2 + y, 1/2 ¹ z.
Figure 2. Asymmetric unit with symmetry equivalent atoms
(symmetry operation: (i) x, y, 1/2 ¹ z) of [C1mim]FSA with 50%
ellipsoidal probability. The solid line indicates the crystallo-
graphic mirror plane.
Table 2. Hydrogen bonding in the crystal of [C1mim]FSAa
dH£O dH£O ¹ vdWb
dC£O
/¡
¾
C-H£O
C-H£O
/¡
/¡
/°
C1-H1£O1
C3-H3A£O1
C2-H2£O2ii
2.43
2.45
2.55
¹0.29
¹0.27
¹0.17
¹0.09
3.242(2)
3.362(2)
3.491(2)
3.442(2)
143
154
169
141
C3-H3C£O2iii 2.63
aSymmetry operations: (ii) 2 ¹ x, 1/2 + y, z; (iii) 1 ¹ x,
1/2 + y, z. vdW: sum of the van der Waals radii.
Figure 4. Hydrogen-bonded undulating chain structure of
[C1mim]FSA projected along the c axis (z = 0-1/2). Blue:
cation and red: anion.
b
interaction geometry. We used the Bondi van der Waals radii as
atomic radii,15 and studied the interactions between the cations
and anions in the crystal.
formed the relatively weak bidentate hydrogen bonds H3C£O2iii
and H3Ci£O2v (Figure 3b).
All of the cation-anion interactions in [C1mim]FSA were
found to involve C-H£O hydrogen bonds, and are summarized
in Table 2. The fluoro group in the [FSA] did not interact with
Differently from the [C1mim]NTf2 crystal, all the cation-
anion interactions in the [C1mim]FSA crystal could be charac-
terized by polydentate hydrogen bonding (Figure 3). This is
the cause for the unexpectedly high melting point of the
[C1mim]FSA crystal, because the highly symmetric polydentate
interactions will strongly inhibit rotation around the S-N-S
bonds and decrease the flexibility of the anion in the crystal.
A packing diagram for [C1mim]FSA is shown in Figure 4.
Successive in-plane interactions along the b axis result in an
undulating chain structure. Concomitantly, out-of-plane hydro-
gen bonding between the H3C and O2iii contributes to a stack of
chains along the a axis. There are no cation-anion interactions
along c axis (Figure S4 in the Supporting Information).11
In conclusion, in spite of the fact that the [C1mim]FSA
liquid has a low viscosity compared with the [C1mim]NTf2
liquid, the former salt has an unexpectedly high melting point.
[C1mim]FSA crystal structure revealed a low coordination ion
number and a perfectly symmetric polydentate hydrogen bond-
ing. The rigid packing structure contributed to its higher melting
point than the related salts. In the future, we intend to study
the flexibility and dynamics of salts containing the amide-anion
to elucidate the relationship between conformation and proper-
ties.
¹
the cation. It is well-known that strong interactions can
be formed by the hydrogen atom at the 2-position of the
imidazolium ring.7,16 In the studied crystal, the H1£O1 and
H3A£O1 interatomic distances were significantly shorter than
the sum of van der Waals radii, indicating strong interactions.
H1 is the hydrogen atom at the 2-position of the imidazolium
ring. The mirror symmetry operation of the asymmetric unit
revealed the presence of characteristic W-shaped tridentate
hydrogen bonding (Figure 3a), which is typically observed in
1,3-dialkylimidazolium-based NTf2 salts.7b,7d,7e
In-plane hydrogen bonding was also observed around the 4-
and 5-positions of the imidazolium ring, forming bidentate
hydrogen bonding (Table 2 and Figure 3a). In-plane interactions
¹
were formed by a single [C1mim]+ and two [FSA] , and vice
versa. However, bidentate hydrogen bonding at the 4- and 5-
positions was not found in [C1mim]NTf2, and each H atom
¹
interacted with a different [NTf2] . The in-plane hydrogen
¹
bonding showed that three [NTf2] surrounded [C1mim]+ 7b
(Figure S3 in the Supporting Information).11 We believe that
the lower coordination ion number in [C1mim]FSA than in the
[C1mim]NTf2 leads to stronger cation-anion interactions and
plays an important role in [C1mim]FSA having a higher melting
point.
This study was supported by the Ministry of Education,
Culture, Sports, Science and Technology of Japan [for K. N.,
No. 21245003 Grant-in-Aid for Scientific Research (A)]. The
authors thank Dr. T. Mandai for his experimental support.
The anion above the imidazolium plane (at the symmetry
position (iii) 1 ¹ x, 1/2 + y, z and (v) 1 ¹ x, 1/2 + y, 1/2 ¹ z)
406 | Chem. Lett. 2014, 43, 405–407 | doi:10.1246/cl.130982
© 2014 The Chemical Society of Japan

References and Notes
Umebayashi, S.-i. Ishiguro, J. Phys. Chem. B 2006, 110,
8179. b) J. C. Lassègues, J. Grondin, R. Holomb, P.
Johansson, J. Raman Spectrosc. 2007, 38, 551. c) W. A.
Henderson, P. C. Trulove, H. C. De Long, V. G. Young, Jr.,
ECS Trans. 2007, 3, 83.
K. Fujii, S. Seki, S. Fukuda, R. Kanzaki, T. Takamuku, Y.
Umebayashi, S.-i. Ishiguro, J. Phys. Chem. B 2007, 111,
12829.
1
2
3
a) T. Welton, Chem. Rev. 1999, 99, 2071. b) P. Wasserscheid,
W. Keim, Angew. Chem., Int. Ed. 2000, 39, 3772.
P. Bonhôte, A.-P. Dias, N. Papageorgiou, K.
Kalyanasundaram, M. Grätzel, Inorg. Chem. 1996, 35, 1168.
a) R. Ozawa, S. Hayashi, S. Saha, A. Kobayashi, H.-o
Hamaguchi, Chem. Lett. 2003, 32, 948. b) J. D. Holbrey,
W. M. Reichert, M. Nieuwenhuyzen, S. Johnston, K. R.
Seddon, R. D. Rogers, Chem. Commun. 2003, 1636.
a) M. Imanari, K.-i. Uchida, K. Miyano, H. Seki, K.
Nishikawa, Phys. Chem. Chem. Phys. 2010, 12, 2959. b)
M. Imanari, K. Fujii, T. Endo, H. Seki, K.-i. Tozaki, K.
Nishikawa, J. Phys. Chem. B 2012, 116, 3991. c) T. Endo, T.
Kato, K.-i. Tozaki, K. Nishikawa, J. Phys. Chem. B 2010,
114, 407.
9
10 a) J. F. Sawyer, G. J. Schrobilgen, S. J. Sutherland, Inorg.
Chem. 1982, 21, 4064. b) W. Isenberg, M. Noltemeyer,
G. M. Sheldrick, Acta Crystallogr., Sect. B: Struct. Crys-
tallogr. Cryst. Chem. 1982, 38, 2887. c) A. Haas, C. Klare,
P. Betz, J. Bruckmann, C. Krüger, Y.-H. Tsat, F. Aubke,
Inorg. Chem. 1996, 35, 1918. d) M. Beran, J. Příhoda, Z.
Žák, M. Černík, Polyhedron 2006, 25, 1292.
4
5
J. N. C. Lopes, K. Shimizu, A. A. H. Pádua, Y. Umebayashi,
S. Fukuda, K. Fujii, S.-i. Ishiguro, J. Phys. Chem. B 2008,
112, 9449.
L. Xue, C. W. Padgett, D. D. DesMarteau, W. T. Pennington,
Solid State Sci. 2002, 4, 1535.
a) J. J. Golding, D. R. MacFarlane, L. Spiccia, M. Forsyth,
B. W. Skelton, A. H. White, Chem. Commun. 1998, 1593. b)
J. D. Holbrey, W. M. Reichert, R. D. Rogers, Dalton Trans.
2004, 2267. c) W. A. Henderson, M. Herstedt, V. G. Young,
Jr., S. Passerini, H. C. De Long, P. C. Trulove, Inorg. Chem.
2006, 45, 1412. d) A. R. Choudhury, N. Winterton, A.
Steiner, A. I. Cooper, K. A. Johnson, CrystEngComm 2006,
8, 742. e) Y. U. Paulechka, G. J. Kabo, A. V. Blokhin, A. S.
Shaplov, E. I. Lozinskaya, D. G. Golovanov, K. A.
Lyssenko, A. A. Korlyukov, Y. S. Vygodskii, J. Phys. Chem.
B 2009, 113, 9538. f) T. Mukai, K. Nishikawa, RSC Adv.
2013, 3, 19952.
11 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html. Experimental details and Crystallographic data
in CIF(CCDC 963540).
12 a) K.-i. Tozaki, H. Inaba, H. Hayashi, C. Quan, N. Nemoto,
T. Kimura, Thermochim. Acta 2003, 397, 155. b) K.
Nishikawa, S. Wang, H. Katayanagi, S. Hayashi, H.-o
Hamaguchi, Y. Koga, K.-i. Tozaki, J. Phys. Chem. B 2007,
111, 4894. c) T. Endo, K.-i. Tozaki, T. Masaki, K.
Nishikawa, Jpn. J. Appl. Phys. 2008, 47, 1775.
13 M. Ishikawa, T. Sugimoto, M. Kikuta, E. Ishiko, M. Kono,
J. Power Sources 2006, 162, 658.
14 J. Mullay, J. Am. Chem. Soc. 1985, 107, 7271.
15 A. Bondi, J. Phys. Chem. 1964, 68, 441.
16 D. G. Golovanov, K. A. Lyssenko, Y. S. Vygodskii, E. I.
Lozinskaya, A. S. Shaplov, M. Y. Antipin, Russ. Chem. Bull.
2006, 55, 1989.
6
7
8
a) K. Fujii, T. Fujimori, T. Takamuku, R. Kanzaki, Y.
Chem. Lett. 2014, 43, 405–407 | doi:10.1246/cl.130982
© 2014 The Chemical Society of Japan | 407