Communication
ꢀ
The crystal structures of phase II and phase I, listed in the
Supporting Information and CIF files, were resolved from neu-
tron diffraction patterns recorded at room temperature (RT)
and 9 K, respectively. Due to the high quality of the data even
the hydrogen atoms could be located during the Rietveld re-
finement. Phase II is stable at RT and crystallizes in the non-
centrosymmetric monoclinic space group P21 (No. 4) [a=
well as the cation and anion orientation. Each [FeCl ] tetrahe-
4
dron is stacked with six ClꢀCl contacts (<3.9 ); two in a one-
dimensional zigzag manner along the b axis (with slightly
longer distances, 3.87(3) ) and four in two linear chains
propagated along the bisectors of the ac plane (3.63(4) (2),
3.81(4) (2)). No p–d interactions of the aromatic rings with
ꢀ
the [FeCl ] metal complexes are observed in either phase, be-
4
6
.542(3), b=14.037(3), c=6.536(3) , b=90.01(3)8, V=
cause the distances between them (near 4 ) are considerably
larger than the sum of the van der Waals (vdw) radii of C(H)
3
ꢀ3
6
00.2(4) , Z=2, 1 =1.631 gcm , T=300 K]. Phase I crys-
calcd
[18]
tallizes in the non-centrosymmetric orthorhombic space group
and Cl atoms (3.57 ). The Dimim cation displays six and
[19]
P2 2 2 (No. 19) [a=9.092(1), b=13.731(2), c=8.974(1) , V=
twelve weak (long) hydrogen bonds for phases II and I, re-
1
1 1
3
ꢀ3
[20]
1120.2(3) , Z=4, 1calcd =1.748 gcm , T=9 K]. Both structures
spectively, according to the IUPAC rule
(Figure S7 and
can be described as layers of cations and anions stacked along
Table S5 in the Supporting Information). It should be men-
tioned that the typical most acidic hydrogen atom of the imi-
the b axis, with the closest FeꢀFe distances being more than
[21]
6
inside a layer. The presence of a 2 screw axis in both
dazolium ring, C2ꢀH position, is not involved in the hydro-
gen bond network of phase II (nomenclature depicted in Fig-
ure S7 in the Supporting Information). Although this issue dif-
fers from the data reported for the Emim [MCl ] (M=Co, Ni
1
ꢀ
phases yields a change in the orientation of the [FeCl ] metal
4
complex anions from layer to layer (see inset of Figure 1 and
2
4
[
17]
and Fe)
and DimimCl (Dimim=1,3-dimethylimidazolium)
[22]
ionic liquids, in which the strongest hydrogen bond was lo-
cated in this position; similar behaviour was observed in the
[23]
[16a]
Emim[BF4] and Emim[FeCl4]
MILs. In phase I, significantly
stronger (shorter) hydrogen bonds are found, which is consis-
[24]
tent with the Raman spectra. At 4 K, the Raman bands of
the CꢀH stretching modes of the imidazolium ring, located be-
ꢀ
1
tween 3100 and 3200 cm , increase in intensity, which is char-
acteristic for strong hydrogen bonds at the C2 and C4/5 posi-
[19]
tions. In addition, in the low-frequency range, a splitting of
ꢀ
the Raman modes of the [FeCl4] complex, associated to the
crystallographic phase transition, was detected (Figure S8 of
the Supporting Information).
Measurements of magnetic susceptibility as a function of
temperature show a sharp maximum at approximately 5.6 K
(
Figure 1), indicating the existence of a 3D antiferromagnetic
ordering. This result indicates that a smaller chain length in
Figure 1. Temperature dependence of c
m m
(blue circles) and c T (black circles)
+
the imidazolium cation [from Bmin that shows no 3D order-
for Dimim[FeCl ] measured under 1 kOe. The solid red and green lines are
4
+
+
the fit according to Equation (2) in the Supporting Information. The inset
shows the crystal structure view along the [001] direction of phase I at 9 K.
Brown (iron), green (chloride), black (carbon), blue (nitrogen) and light pink
ing through Emim (4 K) to Dimim (5.6 K)] stabilizes the mag-
netic ordering. Above 10 K, a paramagnetic behaviour is dis-
ꢀ1
ꢀ1
played, with a c T value of 4.25 emuKmol Oe (5.83 m ) at
m
B
(hydrogen). The dotted square represents the unit cell.
room temperature. At temperatures higher than 10 K, the mag-
netic susceptibility data follow the Curie–Weiss law with Weiss
Figure S6 in the Supporting Information). This tetrahedrally co-
ordinated anion is fairly regular (mean values of Cl-Fe-Cl bond
angles are 109(2)8 for I and 110(3)8 for II) with slightly different
bond lengths for phase I and II (respective mean values 2.16(2)
and 2.19(3) ). The Dimim cations lie antiparallel to each other
along the b direction, and are stacked nearly identically one
above the other in the a and c directions, although slightly dis-
torted in I. The dihedral angles of the methyl groups to the
imidazolium ring [a(C-N-C-N)] in both phases range from 177
to 1798. The refined values for the CꢀC and CꢀN bond lengths
lie in the expected range and are comparable to those found
in other imidazolium compounds, for example, in Bmim [XCl ]
temperatures, q , close to ꢀ7.0 K with a Curie constant of C=
p
1
ꢀ
ꢀ1
4.34 emuKmol Oe [see Eq. (1) in Supporting Information].
This value corresponds to an effective paramagnetic moment
(m ) of 5.88 m per Fe ion; this agrees with the expected value
eff
B
ꢀ
1
ꢀ1
3+
of 4.375 emuKmol Oe (5.92 m ) for an Fe ion with a mag-
B
[25]
netic spin S=5/2.
Due to the anisotropy of the crystal,
a combination of a modified expression of the one-dimension-
[26]
al Bonner and Fisher model, for the interplane interactions,
[27]
and a two-dimensional Rushbrook and Wood model, for the
intraplane interactions, was used to extract the magnetic ex-
3
+
change couplings between the Fe ions (see magnetic fitting
calculations in the Supporting Information). The best fit leads
2
17]
4
[
(
Bmim=1-butyl-3-methylimidazoliumX=Fe, Ni and Co) and
to the parameters J =ꢀ0.511 K for the interplane exchange
?
[
16a]
Emim[FeCl4].
interaction and J =ꢀ0.359 K value for the intraplane interac-
j j
The topologies of the unit cell in both phases are quite simi-
lar, showing virtually the same cation–anion arrangement as
tions with g=2.03. These small values point to a very weak an-
tiferromagnetic interactions, in accordance with the large dis-
Chem. Eur. J. 2014, 20, 72 – 76
73
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Garcia-Saiz, Abel
