J.-Y. He et al. / Polyhedron 155 (2018) 42–49
43
much higher than previously reported values for such systems
[36].
The single crystals suitable for X-ray analysis for 1 and 2 were
achieved via a diffusion process, namely, the powdered sample of
1 or 2 were dissolved in the minimum amount of DMF, and abso-
lute ethanol diffuses into the DMF solution of 1 or 2 to give the cor-
responding crystals.
With respect to [M(mnt)2]ꢀ (M = Ni or Pt) salts widely studied,
the [M(mnt)2]2ꢀ salts have been rare investigated. Recently, we
have synthesized and characterized a series of salts of [Ni(mnt)2]2ꢀ
with pyridinium derivatives, and found that the molecule structure
and charge of the counter cation strongly influence on the
crystal structure and dielectric properties and ionic liquid crystal
nature of [Ni(mnt)2]2ꢀ salts [39,40]. The dielectric phase transition
is one of the most attractive features of material since the conver-
sion between the distinct high and low dielectric states has suc-
ceeded in photoelectronic fields. As one of the most promising
strategies to assemble such a functional material, usually, to design
and synthesize molecule compounds with structural transforma-
tions effectively acquires the typically temperature-dependent
dielectric states. To continuously explore this context, herein, we
present our study of the crystal structures and dielectric properties
of two [Ni(mnt)2]2ꢀ salts containing noncentrosymmetric organic
cations 1,4-dimethyl-DABCO2+ and TMSF+. The two cations maybe
show order–disorder structural transformation in their crystals
with the temperature changes. Two new salts show similar packing
structure and novel dielectric properties.
2.3. Physical measurements
Elemental analyses (C, H and N) were carried out on a Perkin
Elmer 240C analytical instrument. IR spectra were recorded in
KBr pellets with a Nicolet 170 SXFT-IR spectrophotometer in the
4000–400 cmꢀ1 region. The UV–vis-NIR absorption spectra in solid
state were taken using a Shimadzu UV-3100 spectrometer. Powder
X-ray diffraction (PXRD) data were collected on a Riga-Ku/max-
2550 diffractometer with Cu K
a radiation (k = 1.5418 Å). Thermo-
gravimetric (TG) experiments were performed with
a
TA2000/2960 thermogravimetric analyzer from 293 K to 1063 K
at a heating rate of 20 K minꢀ1 under a nitrogen atmosphere, and
the polycrystalline samples were placed in an aluminum crucible.
Differential scanning calorimetry (DSC) was carried out for 1 and
2 on a Pyris 1 power-compensation differential scanning calorime-
ter with a warming rate of 10 K minꢀ1 during the heating process.
Temperature- and frequency-dependent dielectric permittivity and
AC impedance measurements were carried out on a Concept 80
system (Novocontrol, Germany) in 153–403 K and 153–373 K for
1 and 2, respectively. The powdered disc, with a thickness of ca.
1.68 mm (for 1) and 1.36 mm (for 2) as well as a diameter of
7.0 mm, was coated by gold films on the opposite surfaces and
sandwiched by platinum electrodes and the AC frequencies span
from 1 to 107 Hz.
2. Experimental
2.1. Chemicals and materials
All reagents and chemicals, including 1,4-diazoniabicyclo[2.2.2]
octane (abbr. DABCO), trimethylsulfonium bromide (abbr. [TMSF]
Br) and iodomethane, were purchased from commercial sources
and used without further purification.
Disodium maleonitriledithiolate (Na2mnt) was synthesized fol-
lowing the procedure published [41].
2.4. X-ray single crystallography
The typical process is described for the preparation of (1,4-
The single crystal X-ray diffraction data were collected for 1 and
dimethyl-DABCO)I2.
1,4-diazabicyclo[2.2.2]octane
(112 mg,
2 with graphite monochromated Mo K
a (k = 0.71073 Å) on a CCD
1 mmol) was solved in 20 mL acetone, and iodomethane (300 mg,
2.1 mml) was added into the above solution drop by drop and stir-
ring at room temperature. The white precipitate formed gradually,
and the mixture was further stirred for 30 min, and the precipitate
was separated by suction and washing by absolute ethanol for
three times, and dried at 60 °C overnight. Yield ca. 85% (based on
DABCO). The final product was characterized by 1H NMR (D2O,
400 MHz; ref. Fig. S1) with d: 4.8 (s, D2O-D), 4.13 (S, 12H, H-
CH2), 3.43 (S, 6H, H-CH3).
area detector (Bruker-SMART). Data reductions and absorption cor-
rections were performed with the SAINT [42] and SADABS [43] soft-
ware packages, respectively. Structures were solved by direct
methods using the SHELXL-97 software package [44]. The non-H
atoms were anisotropically refined using the full-matrix least
squares method on F2. All H atoms were placed at calculated posi-
tions and refined riding on the parent atoms. The crystallographic
details about data collection and structure refinement are summa-
rized in Table 1.
2.2. Preparation of 1 and 2
3. Results and discussion
2.2.1. [1,4-Dimethyl-DABCO][Ni(mnt)2] (1)
3.1. Crystal structure analysis
A H2O solution of Na2mnt (186 mg, 1 mmol) was added to a H2O
solution of NiCl2ꢂ6H2O (120 mg, 0.5 mmol), the mixture was stirred
at room temperature for 10 min, and then filtered. To the filtrate
was added dropwise a H2O solution of (1,4-dimethyl-DABCO)I2
(200 mg, 0.5 mmol), the immediately formed dark red precipitate
was filtered out, washed with H2O up to the filtrate being colorless.
The precipitate was collected and dried at 60 °C in vacuum for 2 h.
Yield ca. 58% (calculation based on NiCl26H2O). Calc. for C16H18N6-
NiS4: C, 39.93; H, 3.770; N, 17.46%; Found: C, 40.00; H, 3.86; N,
17.85%.
Salt 1 crystallizes in monoclinic space group C2/c. As shown in
Fig. 1a, an asymmetric unit of 1 contains one half of [Ni(mnt)2]2ꢀ
dianion and one half of divalent 1,4-Dimethyl-1,4-diazoniabicy-
clo[2.2.2]octane (abbr. Me2-DABCO2+) cation. The Ni2+ ion locates
at an inversion center, which lead to the NiS4 core being cofacial
in [Ni(mnt)2]2ꢀ. The [Ni(mnt)2]2ꢀ has Ci point group symmetry,
and the planar mnt2ꢀ ligand makes a dihedral angle of 7.8° with
the plane of NiS4 core. Two crystallographically inequivalent Ni–S
bond lengths are 2.1753(6) and 2.1780(6) Å, respectively; and the
S–Ni–S bite angle is 92.59(2)°. These bond parameters in [Ni
(mnt)2]2ꢀ are comparable to that in other [Ni(mnt)2]2ꢀ salts
reported [39,40]. The Me2-DABCO2+ shows the C2 point group sym-
metry, and the twofold rotation axis is parallel to the b-axis and
passes through the midpoint of a segment connecting N3 and
N3#1 with the symmetric code #1 = 1 ꢀ x, y, 0.5 ꢀ z.
2.2.2. (TMSF)2[Ni(mnt)2] (2)
This salt was prepared following a similar process for prepara-
tion of 1, just replaced (1,4-dimethyl-DABCO)I2 by [TMSF]Br. Yield
ca. 77%. Calc. for C14H18N4NiS6: C, 34.08; H, 3.677; N, 11.36%;
Found: C, 33.33; H, 3.73; N, 11.33%.