Crystalline and liquid crystalline organic-inorganic hybrid salts with cation-sensitized hexanuclear molybdenum cluster complex anion luminescence

Article abstract of DOI:10.1002/ejic.201100365

The salts [C_nmim]₂[Mo₆Cl₁₄] (mim = methylimidazolium; n = 4, 6, 12, 16, 18) have been obtained by reaction of C_nmimCl with MoCl₂. Thermal analysis shows the melting point decreases with increasing alkyl chain length of the cation. The imidazolium chloromolybdates(II) with n = 6-18 decompose above 340 °C; [C₁₈mim]₂[Mo₆Cl₁₄] is thermally stable up to 390 °C. All compounds are insensititve to the constituents of the atmosphere. Of the higher melting salts [C_nmim] ₂[Mo₆Cl₁₄] (n = 4, 6), high-quality single crystals could be obtained. Single-crystal X-ray structural analyses clearly show that the cluster complex anion [Mo₆Cl₁₄]^2- has an electron-precise octahedral {Mo₆} cluster. For [C ₄mim]₂[Mo₆Cl₁₄], two polymorphs differing in the cation alkyl-side-chain conformation were obtained. The less thermodynamically stable modification with a gauche conformation undergoes monotropic phase transition to the more stable form with the all-trans conformation. Long alkyl-side-chain salts, [C_nmim] ₂[Mo₆Cl₁₄] with n = 16 and 18, exhibit thermotropic liquid-crystalline behaviour. All compounds show bright red luminescence centred at about 730 nm with a lifetime of about 0.15 ms, originating from the cluster complex anion, which can be sensitized by the imidazolium cations.

Full text of DOI:10.1002/ejic.201100365

FULL PAPER
DOI: 10.1002/ejic.201100365
Crystalline and Liquid Crystalline Organic–Inorganic Hybrid Salts with
Cation-Sensitized Hexanuclear Molybdenum Cluster Complex Anion
Luminescence
[a]
[b]
[a]
[a]
[b]
Joanna Bäcker, Svenja Mihm, Bert Mallick, Mei Yang, Gerd Meyer,* and
Anja-Verena Mudring*[
a]
Dedicated to Professor John D. Corbett on the occasion of his 85th birthday
Keywords: Cluster compounds / Ionic liquids / Liquid crystals / Luminescence / Synthesis design
The salts [C
n
mim]
2
[Mo
6
Cl14] (mim = methylimidazolium; n =
mimCl
2
. Thermal analysis shows the melting point de-
6 4 2 6
precise octahedral {Mo } cluster. For [C mim] [Mo Cl14], two
4, 6, 12, 16, 18) have been obtained by reaction of C
n
polymorphs differing in the cation alkyl-side-chain confor-
mation were obtained. The less thermodynamically stable
modification with a gauche conformation undergoes mono-
tropic phase transition to the more stable form with the all-
with MoCl
creases with increasing alkyl chain length of the cation. The
imidazolium chloromolybdates(II) with n = 6–18 decompose
above 340 °C; [C18mim]
90 °C. All compounds are insensititve to the constituents of
the atmosphere. Of the higher melting salts [C mim]
Mo Cl14] (n = 4, 6), high-quality single crystals could be ob-
tained. Single-crystal X-ray structural analyses clearly show
2
[Mo
6
Cl14] is thermally stable up to
trans conformation. Long alkyl-side-chain salts, [C
n 2
mim] -
3
[Mo Cl14] with n = 16 and 18, exhibit thermotropic liquid-
6
n
2
-
crystalline behaviour. All compounds show bright red lumi-
nescence centred at about 730 nm with a lifetime of about
0.15 ms, originating from the cluster complex anion, which
can be sensitized by the imidazolium cations.
[
6
2
–
that the cluster complex anion [Mo
6
Cl14
]
has an electron-
cluster bonding;[4] in a molecular orbital (MO) view these
occupy 12 bonding orbitals, leaving a rather wide gap to
Introduction
Halides of heavier Group 5 and 6 metals in lower oxi- the anti-bonding LUMO (see below for luminescence). The
dation states show an extensive tendency towards metal– occupation of 12 bonding, cluster-based orbitals clearly ac-
[
1]
metal bonding. It was a big surprise when structure deter- counts for a very stable cluster and, thus, a very stable an-
2–
mination of the light yellow salt of MoCl exhibited octahe- ionic cluster complex, [{Mo }Cl ] , because {Mo }Cl
2
6
14
6
12
dra of molybdenum atoms, {Mo }, surrounded by eight may be dissolved in hydrochloric acid and salts may be crys-
6
face-capping [μ , or i for “innere Liganden” (inner ligands)] tallized from such a solution. Hg[{Mo }Cl ] was perhaps
3
6
14
and six terminal [over the vertices, μ , or a for “äußere Li- the first cluster complex salt to be structurally characterized
1
ganden” (outer ligands)] chloride ligands.[
2]
The by single-crystal X-ray diffraction. Many characteriza-
{Mo }Cl Cl ] cluster complexes need to share four a- tions have followed, either homo- or heteroleptic salts of
[5]
i
a 2–
[
6
8
6
i
a
2–
[6]
type ligands to achieve the composition MoCl2
=
the [{Mo }L L ] cluster complex anion (Figure 1).
6 8 6
i
a
a–a
[1a,3]
{
Mo }Cl
= {Mo }Cl 8/1Cl 2/1Cl 4/2.
The {Mo6}
2–
6
12
6
The anion [{Mo }Cl ] is a large anion with a diameter
6 14
cluster is understood to be an electron-precise 24-electron
cluster in the sense that 24 electrons are available for intra-
of about 1 nm. Crystallization from solution can be ac-
complished with a large variety of cations, not only simple
+
2+
mono- or divalent cations, such as Cs or Hg , but – even
a] Fakultät für Chemie, Anorganische Chemie I – Festkörperchemie more advantageous for better crystallization – large cations,
und Materialien, Ruhr-Universität Bochum,
[
2+
+
such as [Ca(dmso) ] or [Rb(12c4)2] (12c4 = 12-crown-
). A large variety of solvents and solvent mixtures have
6
4
4780 Bochum, Germany
[
6c]
4
Fax: +49-234-32-14951
E-mail: anja.mudring@rub.de
been tested for crystallization, not only aqueous solutions,
but also polar organic solvents, such as alcohols, dimethyl
sulfoxide or acetonitrile, may be used.
Molten salts, especially imidazolium-based tetrachlo-
roaluminates have also been used as solvents, even at room
[
b] Department für Chemie, Lehrstuhl für Anorganische
Festkörper- und Koordinationschemie, Universität zu Köln,
Greinstraße 6, 50939 Köln, Germany
Fax: +49-221-470-5083
E-mail: gerd.meyer@uni-koeln.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100365.
[
7]
temperature. At present, this molten salt chemistry is ex-
Eur. J. Inorg. Chem. 2011, 4089–4095
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4089

G. Meyer, A.-V. Mudring et al.
FULL PAPER
i
a
2–
i
a
[
{Mo }Cl Cl ] . Mo–Mo, Mo–Cl , and Mo–Cl distances
6 8 6
correspond very well with those statistically found for a
2–
[6c]
large number of salts with the [{Mo }Cl ] anion.
6
14
Table 1. Crystallographic data for [C
n
mimCl]
2
[Mo
6
Cl14] with n = 4
(
1a, 1b) and n = 6 (2); the space group is P2
1
/c (no. 14) in all cases.
1a
1b
2
a [pm]
b [pm]
c [pm]
β [°]
V [Å ]
T [K]
R1 [%]
wR2 [%]
Goof
991.1(5)
1684.3(8)
1186.4(5)
111.010(9)
1849(2)
153(2)
5.58
14.5
1.14
982.6(1)
1826.6(2)
1147.8(1)
109.59(13)
1940.4(4)
293(2)
3.18
1176.8(2)
1012.1(2)
1908.9(4)
107.64(1)
2166.0(8)
293(2)
2.75
Figure 1. Ball-and-stick model of the anionic cluster complex
3
i
a
2–
i
a
6 8 6
[{Mo }L L ] ; ligands L (light green) and L (dark green) may
or may not be identical.
4.70
0.722
5.95
0.952
periencing a revival through applications in ionic-liquid
[
8]
(
IL) chemistry, with a large variety of cations and anions.
Distances and angles in the n-alkyl-methylimidazolium
cation of 1a, 1b and 2 (Table 2) are the same as those com-
monly observed.
With the incorporation of complex f-metal anions in ionic
liquids, a new generation of ILs with interesting properties,
such as luminescence or magnetism, have become access-
[
11]
However, there are distinct differences
[
9]
in the conformation of the cations. For compounds 1a and
1b, the main difference is the conformation of the butyl side
chain of the imidazolium cation (Figure 2). In 1a a gauche
conformation is found along the C5–C6 bond, whereas 1b
features only anti conformations. Furthermore, the orienta-
tion of the alkyl chain with respect to the imidazolium ring
plane is different. In 1a the alkyl chain is oriented back-
wards with an (N2–C5–C6) angle of –111.85(2)°, in 1b it is
forward with an angle of 113(1)°. The hexyl side chain in 2
also features only anti-conformations and bends backwards
with an angle of –111.67(1)°.
ible. Likewise, d-element ions in ILs give rise to auspicious
[
10]
properties.
Typically, the complex metal anion is ob-
tained by dissolving the respective binary metal salt in an
IL containing the same anion. In this context, salts such as
MoCl , which contain a stable metal cluster complex (see
2
above), drew our attention. We have, therefore, undertaken
a more systematic study of the dissolution of MoCl in alk-
2
ylimidazolium chloride ILs [C mim]Cl (mim = methylimid-
n
azolium; n = 4, 6, 12, 16, 18) with the aim of evaluating
whether it is possible to obtain ILs or low melting salts with
2
–
the cluster complex anion [{Mo }Cl ] .
6
14
Table 2. Selected interatomic distances for 1a, 1b and 2.
1
a
d [Å]
1b
d [Å]
2
d [Å]
Results and Discussion
Mo1–Cl1a
Mo1–Cl4
Mo1–Cl7
2.432(6) Mo1–Cl1a 2.418(4) Mo1–Cl1a 2.43(1)
2.471(6) Mo1–Cl4
2.473(8) Mo1–Cl5
2.473(7) Mo1–Cl7
2.476(8) Mo1–Cl6
2.427(2) Mo2–Cl2
2.47(1)
2.49(1)
2.479(2) Mo2–Cl7
2.483(2) Mo2–Cl6
2.42(2)
2.467(3) Mo3–Cl7
2.472(2) Mo3–Cl5
2.474(6) Mo3–Cl6
2.477(5) Mo3–Cl4
2.607(5) Mo1–Mo3 2.599(1) Mo1–Mo2 2.60(2)
2.610(2) Mo1–Mo2 2.602(9) Mo1–Mo2 2.60(4)
2.610(2) Mo1–Mo2 2.604(9) Mo1–Mo2 2.60(1)
i
i
i
Syntheses
2.47(2)
2.471(2) Mo1–Cl7
2.474(2) Mo1–Cl4
2.48(2)
2.42(2)
2.46(1)
2.467(2) Mo2–Cl7
2.468(2) Mo2–Cl5
2.48(1)
2.436(6) Mo3–Cl3
2.463(9) Mo3–Cl7
2.47(1)
2.474(9) Mo3–Cl4
2.478(8) Mo3–Cl6
Mo1–Cl6
2.47(1)
2.47(3)
2.47(1)
2.48(2)
2.43(4)
2.43(4)
2.47(1)
2.47(2)
2.47(2)
2.42(3)
2.47(1)
2.47(4)
2.48(1)
2.48(4)
i
i
i
i
i
i
The reactions of five alkylimidazolium chlorides [C mim]- Mo1–Cl6
n
i
i
i
Cl (n = 4, 6, 12, 16, 18) with MoCl were carried out in Mo1–Cl5
Mo1–Cl5
Mo2–Cl2
Mo2–Cl4
2
a
a
a
Mo2–Cl2
Mo2–Cl7
Mo2–Cl4
Mo2–Cl5
Schlenk tubes under vacuum at 100–110 °C for 3 d. Yellow
to orange salts of [C mim] [Mo Cl ] solidified upon cool-
i
i
i
Mo2–Cl5
Mo2–Cl4
n
2
6
14
i
i
i
ing to ambient temperature. Compounds with alkyl chains
i
i
i
i
i
i
with n = 12, 16, 18 (compounds 3, 4, and 5, respectively) Mo2–Cl6
Mo2–Cl6
a
a
a
Mo3–Cl3
Mo3–Cl4
Mo3–Cl5
Mo3–Cl7
Mo3–Cl3
were wax-like and their wax-like behaviour increased with
increasing alkyl chain length. High-quality single crystals
of compounds with shorter alkyl chains, [C mim] [Mo Cl ]
i
i
i
i
i
i
Mo3–Cl5
4
2
6
14
i
i
i
i
i
i
(
1a and 1b) and [C mim] [Mo Cl ] (2), were obtained and Mo3–Cl6
6
2
6
14
allowed X-ray structure analyses. Compounds 1a and 1b Mo1–Mo2
Mo1–Mo3
were synthesized and crystallized independently in pure
Mo1–Mo3
phases by using slightly different synthetic procedures.
Mo1–Mo2
2.613(2) Mo1–Mo3 2.61(1)
Mo1–Mo3 2.609(9)
Views of the crystal structures of 1a, 1b and 2 along the
Single-Crystal X-ray Structures
two shorter crystallographic axes (Figure 3) show that the
2–
Selected crystallographic data for 1a, 1b and 2 are sum- packing of imidazolium cations and [{Mo }Cl ] anions,
6
14
marized in Table 1. Comparison of measured X-ray powder in a 2:1 ratio, is very similar for these compounds. However,
+
patterns and those calculated with crystal structure data of the relative orientation of the [C mim] cations and the
4
2–
1
a, 1b and 2 confirm the correctness of the structure solu- [{Mo }Cl ] anions are slightly different for 1a and 1b.
6 14
tions and the phase purity of the compounds. Compounds The layers are farther apart in 1b than those in 1a which is
a, 1b and 2 all feature the complex cluster anion attested by the longer b axis, 1684.3(8) versus 1826.6(2) pm
1
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 4089–4095

Crystalline and Liquid Crystalline Hybrid Salts
(1a vs. 1b). Quite clearly, the all-anti butyl chains in 1b are
more voluminous. Indeed, the cell volume of 1b is larger
Ϫ3
than that of 1a, hence the density of 1a (2.426 gcm ) is
Ϫ3
larger than that of 1b (2.311 gcm ). The effect on the
structure of extending the alkyl chain by two carbon atoms
becomes evident when comparing the structures of 1a/1b
and 2 (Figure 3). In all three cases, cationic and anionic
layers alternate along [100] and they become more sepa-
rated with the length of the alkyl chains.
IR Spectroscopy
IR spectra for all [C mim] [Mo Cl ] compounds investi-
n
2
6
14
gated in this work show the characteristic vibrations of the
cations (see the Supporting Information). Between 2870
Figure 2. Conformations of the n-alkyl-methylimidazolium cations
in the crystal structures of 1a, 1b and 2.
–1
3
2
and 3165 cm , the stretching modes for sp - and sp -C–H
vibrations are observed. The C=N and C=C stretching
–1
modes are found at around 1572 cm . Furthermore, a C–
–1
H deformation mode can be observed at about 1466 cm .
A comparison of the IR spectra of 1a and 1b shows ad-
ditional modes for 1b in the region where C–C modes are
–1
active at 1258 and 1030 cm . This may be attributed to the
mixed anti-gauche conformation of the alkyl chain of the
methylimidazolium cation in 1b, compared with the all-
trans conformation in 1a. These vibrations are absent in the
IR spectrum of [C mim] [Mo Cl ] (3), and indeed, the so-
6
2
6
14
lid-state structure shows the cation to be in the all-trans
conformation. However, they start to appear again in the
compounds with longer alkyl chains; an indication of mixed
conformations of the longer alkyl chains.
Thermal Behaviour
All [C mim] [Mo Cl ] compounds investigated herein
n
2
6
14
show surprisingly high decomposition points (Tables 3 and
; for a graphical display of the TG curves see the Support-
ing Information). It seems that the low Lewis basicity of the
4
2–
[
Mo Cl ] cluster anion is responsible for the high thermal
6 14
stability.
Table 3. Thermal data for [C
n
mim]
2
[Mo
6
Cl14] (n = 4, 6, 12).
1a, b
2
3
T
°C]
ΔH
T
ΔH
T
ΔH
–1
–1
–1
[
[kJmol ] [°C] [kJmol ] [°C] [kJmol ]
Heating
ss[a] 126.3 (1a)
6.0
m
c
d
226.7
200.5
354
43.05
–37.58
226.2 50.17
139.4 –23.1
354
167.2 15.02
123.5 –15.04
344
Cooling
Heating
[
a] s = monotropic solid–solid phase transition, m = melting, c =
crystallization, d = decomposition.
With respect to their thermal behaviour, below the de-
composition point the compounds can be grouped in two
classes. Compounds 1–3 show distinct melting and crystalli-
zation [graphical representations of the differential scanning
calorimetry (DSC) traces can be found in the Supporting
Information]. Compounds 1 and 2 both melt at around
Figure 3. Projections of the crystal structures of [C
Mo Cl14] with n = 4 (1a, 1b) and n = 6 (2) showing the packing
of cations and anions in two different directions.
n 2
mim] -
[
6
Eur. J. Inorg. Chem. 2011, 4089–4095
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4091

G. Meyer, A.-V. Mudring et al.
FULL PAPER
Table 4. Thermal data for [C
n
mim]
2
[Mo
6
Cl14] (n = 16, 18).
4
T
5
T
[°C]
ΔH
[kJmol ]
ΔH
[kJmol ]
–1
–1
[
°C]
–0.1
3.4
132.6
159.8
144.2
140.7
–5.6
Heating (2nd)
G–M[a]
7.0
–25.51
25.61
14.80
–17.41
–0.73
–5.82
–31.0
62.9
8
–41.53
28.35
6.57
–8.12
–9.37
M–M
M–L
L–M
M–M
M–G
d
133.0
147.8
123.1
101.7
–37.9
390
Cooling (1st)
Heating
351
[
a] G–M = glass–mesophase transition, M–M = mesophase–meso-
phase transition, M–L = mesophase–liquid transition, d = decom-
position.
Figure 4. Defect textures of the LC states of 4 (top) and 5 (bottom)
as observed by POM when cooling the isotropic melt at the respec-
tive temperatures.
It seems that none of the compounds form a complete
crystalline state upon cooling. The cracks in the material,
which the POM image shows for 4 at –49 °C, evidently re-
sult from mechanical stress. However, the LC texture is still
present, which illustrates that the material freezes into a
semi-ordered state. Upon heating, when enough thermal en-
ergy becomes available, partial ordering sets in at some
point, as observed by the exothermic events present in both
heating curves of 4 and 5.
2
26–227 °C. However, compound 1 crystallizes more read-
ily, whereas 2 shows a substantial supercooling of –90 °C
before it crystallizes. Compound 3 shows a significantly
lower melting point than 1 and 2 and the degree of super-
cooling (≈ 45 °C) is less than that of 2. Thus, it appears
that longer alkyl chains kinetically hamper crystallization.
Compound 1a shows a monotropic phase transition into 1b
at 126.3 °C. After this temperature, the thermal behaviour
of the sample is identical to that of 1b. Modification 1b is,
therefore, the thermodynamically more stable modification,
although with a rather small enthalpy of transformation of
Optical Spectroscopy
Ϫ1
only 6 kJmol . This behaviour can be attributed to the
conformation of the butyl chain, which is all-anti in 1b.
The optical properties of compounds 1–5 were studied
The phase behaviour of the long-alkyl-chain compounds by UV/Vis absorption and luminescence spectroscopy. The
4
and 5 is far more complex. Compounds 4 and 5 melt to distinct absorption maxima determined by UV/Vis absorp-
form isotropic liquids at 159.8 and 147.8 °C, respectively. tion spectroscopy (see the Supporting Information) agree
Thus, the tendency, already observed for 1–3, for the melt- very well with the observed yellow–orange colour of the
ing point to decrease with the length of the alkyl chain on compounds, as commonly found for compounds with the
2–
[12]
the imidazolium cation is followed further. However, upon [{Mo }Cl ] cluster complex anion.
The absorption
6
14
cooling from the isotropic melt, neither 4 nor 5 crystallizes. maxima appear at the same value as the first maximum ob-
Rather, both form liquid-crystalline (LC) phases. Figure 4 served in the excitation spectrum and seem to be indepen-
displays the defect textures observed by polarizing optical dent of the countercation. The corresponding electronic
microscopy (POM) for the two compounds at the respective transitions can be attributed to intra-cluster transitions.
temperatures. Subtle changes of the defect texture can be However, a second transition is found in the excitation spec-
seen, which are mirrored by the small enthalpy changes in trum at lower energies, which depends on the cation. In this
the DSC. For both compounds, a mesophase–mesophase area, inter-cluster transitions also take placeϪas observed
1
1
transition is observed above 100 °C.
for MoCl itselfϪas well as the allowed π– π* and forbid-
2
1
3
[13]
Small-angle X-ray scattering (SAXS) experiments show, den π– π* transitions within the imidazolium cation (see
for the high-temperature mesophase, repeating units of the Supporting Information). All transitions have a rather
about 27.4 and 26.6 Å for [C mim] [Mo Cl ] and 36.5 and short lifetime and cannot be well separated by time-resolved
1
6
2
6
14
3
3.3 Å for [C mim] [Mo Cl ], which shift with tempera- measurements. With increasing alkyl chain length at the cat-
18 2 6 14
ture, according to thermal expansion and contraction (for ion a blueshift from 489 nm for 1 to 458 nm for 5 is ob-
diffraction patterns taken at various temperatures, see the served (for further details see the Supporting Information).
Supporting Information). Below the mesophase–mesophase
In the emission spectrum no cation emission can be ob-
phase transition, only one repeating distance can be de- served, indicating efficient energy transfer between the
2–
tected in the SAXS area: at about 29.4 Å for [C mim] - imidazolium cation and the [{Mo }Cl ] cluster complex
1
6
2
6
14
[
Mo Cl ] and 32.2 Å for [C mim] [Mo Cl ]. The latter anion. Upon excitation at both 360 (excitation into the
6 14 18 2 6 14
distances roughly amount to the diameter of one complex complex cluster anion levels) and 475 nm (excitation into
anion plus the length of the respective cation. The POM the imidazolium head group), all compounds show a bright
and SAXS observations are in agreement with the forma- red emission centred at around 730 nm (Figure 5). This
tion of a highly ordered hexagonal LC phase, which con- emission is typical for electronic transitions within the
2–
verts to a rectangular one at higher temperatures.
[{Mo }Cl ] cluster anion. It agrees well with observa-
6
14
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 4089–4095

Crystalline and Liquid Crystalline Hybrid Salts
2
–
[C
n
mim]Cl: Following a common literature procedure,[8] 1-methyl-
imidazole (1 equiv.) and the respective chloroalkane (1 equiv.) were
heated in acetonitrile at reflux for 3 d. After cooling, the mixture
was slowly added to cold toluene (–30 °C). The white precipitate
tions for other compounds containing the [{Mo }Cl ]
cluster anion, such as the starting material MoCl2,
with theoretical calculations.
6
14
[
14]
and
6
was filtered off quickly and dried under vacuum. [C mim]Cl was
obtained as a viscous and colourless liquid.
1
[C
4
mim]Cl: H NMR [298 K, 250 MHz, CDCl
3
(99.8%, Deutero
GmbH)]: δ = 0.9 (s, H), 1.3 (m, H), 1.8 (m, H), 4.0 (s, H), 4.2 (t,
3
3
3
J
H,H = Hz, H), 7.2 (d, JH,H = Hz, H), 7.2 (d, JH,H = Hz, H),
10.9 (s, H) ppm.
1
[
=
C
6
mim]Cl: H NMR [300 MHz, D
2
O (99.9%, Deutero GmbH)]: δ
3
3
0.78 (t, JH,H = Hz, 3 H); 1.22 (m, 6 H); 1.80 (t, JH,H = Hz, 2
H); 3.83 (s, 3 H); 4.13 (t, JH,H = Hz, 2 H); 7.40 (d, JH,H = Hz,
3
3
2
H); 8.67 (s, 1 H) ppm.
Figure 5. Comparison of the excitation spectra, monitored at
12mim]Cl: 1H NMR [298 K, 250 MHz, D
[
C
2
O (99.9%, Deutero
7
3
30 nm (left) and emission spectra, obtained upon excitation at
60 nm (right) of 1 (ᮀ), 2 (᭺), 3 (Δ), 4 (ٌ), and 5 (᭛).
3
GmbH)]: δ = 0.74 (t, JH,H = Hz, 3 H); 1.12 (s, 20 H); 3.83 (s, 3
H); 4.1 (t, 3JH,H = Hz, 2 H), 7.44 (d, 3JH,H = Hz, 1 H); 7.48 (d,
3
J
H,H = Hz, 1 H) ppm.
Lifetimes of the excited states for 1–5 of 0.154 to
.132 ms at room temperature correspond to typical values [C16mim]Cl: 1H NMR [298 K, 250 MHz, D
O (99.9%, Deutero
0
2
[15]
3
for the cluster anion.
To a first approximation, the life- GmbH)]: δ = 0.74 (t, JH,H = Hz, 3 H), 1.21 (s, 28 H); 3.84 (s, 3
H); 4.16 (t, JH,H = Hz, 2 H), 7.44 (d, 3JH,H = Hz, 1 H), 7.48 (d,
3
time of the excited state becomes shorter with increasing
alkyl chain length due to increased vibronic quenching.
3
J
H,H = Hz, 1 H) ppm.
[
C
18mim]Cl: 1H NMR [298 K, 250 MHz, D
2
O (99.9%, Deutero
3
GmbH)]: δ = 0.75 (t, JH,H = Hz, 3 H); 1.16 (s, 32 H); 3.84 (s, 3
H); 4.16 (t, JH,H = Hz, 2 H), 7.44 (d, 3JH,H = Hz, 1 H), 7.48 (d,
3
Conclusions
3
J
H,H = Hz, 1 H) ppm.
Alkyl-methylimidazolium chlorides, [C mim]Cl with n =
n
MoCl
of MoCl
2
: The synthesis succeeded by a conproportionation reaction
4, 6, 12, 16, 18, react at 100–110 °C with molybdenum di-
[
16]
5
and Mo at 850 °C in a sealed silica ampoule.
The
chloride, MoCl , yielding light yellow organic–inorganic hy-
2
reaction product was checked for identity and phase purity by X-
ray powder diffraction. The recorded powder pattern compared
well with data from the PDF database and can be found in the
brid salts, [C mim] [{Mo }Cl ]. Crystal structures were
n
2
6
14
solved for the first two members of the series. For 1, two
modifications were secured, of which 1a was the least ther- Supporting Information.
modynamically stable, although with the highest density,
due to the partly gauche conformation of the butyl side
chain. Salts with longer alkyl chains solidified into wax-like
[
C
n
mim]
and MoCl
10 °C in an oil bath for 3 d. After allowing the reaction mixtures
2
[Mo
6 n
Cl14], n = 4, 6, 12, 16 or 18: An excess of [C mim]Cl
2
were reacted in a Schlenk tube under vacuum at 100–
1
compounds and had rather complicated thermal behaviour, to cool to room temperature in the oil bath, compounds 1a and 2
resulting in thermotropic LC behaviour for [C mim] - were obtained as orange block-shaped crystals and (due to the
n
2
[
{Mo }Cl ] (n = 16 and 18). All five salts showed a bright smaller particle size) a lighter, yellow powder, respectively. Com-
6 14
pounds 3, 4 and 5 formed as waxy, sticky orange solids. For further
red luminescence, centred at around 730 nm, upon exci-
tation either into the imidazolium head group (475 nm) or
into cluster based levels at 360 nm. Lifetimes decreased
from 0.154 to 0.132 ms and the length of the alkyl side
chain of the cation was subject to enhanced vibronic
quenching.
purification, the solids were washed with water and filtered. The
solid residue was washed off the filter with CH Cl or acetone. The
2 2
respective solvent was then removed under reduced pressure and
the remaining solids were dried for several hours at 100 °C.
[C
4
mim]
2 6 6 4
[Mo Cl14] (1): C16H30Cl14Mo N (1350.42): calcd. C
14.23, N 4.15, H 2.24; found C 14.26, N 4.16, H 2.34.
[
C
6
mim] [Mo Cl14] (2): C20 (1406.53): calcd. C
17.08, N 3.98, H 2.72; found C 17.38, N 3.93, H 2.73.
12mim] [Mo Cl14] (3): C32 (1574.85): calcd. C
6.64, N 3.45, H 3.85; found C 26.25, N 3.84, H 4.28.
[C mim] [Mo Cl ] (4): C H Cl Mo N (1687.07): calcd. C
2
6
6 4
H38Cl14Mo N
Experimental Section
[C
2
6
6 4
H62Cl14Mo N
General: All reaction products were insensitive to the constituents
of air. Handling for further analyses was undertaken in air, unless
mentioned otherwise.
2
1
6
2
6
14
40 78 14
6
4
2
8.55, N 3.33, H 4.43; found C 29.19, N 3.33, H 5.44.
Materials: 1-Methylimidazole (Ͼ 99%, Sigma Aldrich), 1-chlo-
robutane (99+%, Acros Organics), 1-chlorohexane (95%, Acros
Organics), 1-chlorododecane (99%, Acros Organics), 1-chlorohex-
adecane (Ն 97%, Fluka), 1-chlorooctadecane (Ն 97%, Fluka),
CH Cl (99.99%, Fisher Scientific) and acetone (p.a., Fisher Scien-
2 2
tific) were used as received. Acetonitrile (99.5%, J. T. Baker) and
toluene (99.5%, Prolabo) were dried prior to use.
[C mim] [Mo Cl ] (5): C H Cl Mo N (1743.17): calcd. C
1
8
2
6
14
44 86 14
6
4
30.32, N 3.21, H 4.97; found C 31.86, N 3.26, H 5.83.
mim] [Mo Cl14]: A second modification of 1b was obtained by
reacting MoCl in a 0.5 mol-% excess of [C mim]Cl in a sealed
ampoule for 24 h at 130 °C in an oven. After cooling at 2 °Ch
orange crystals were obtained. Estimated yield: 75%.
(1350.42): calcd. C 14.23, N 4.15, H 2.24; found
C 16.43, N 4.75, H 2.86.
[C
4
2
6
2
4
Ϫ1
,
Syntheses: Syntheses were carried by using standard Schlenk and
argon glovebox techniques.
16 6 4
C H30Cl14Mo N
Eur. J. Inorg. Chem. 2011, 4089–4095
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4093

G. Meyer, A.-V. Mudring et al.
FULL PAPER
Powder X-ray Diffraction: Powder X-ray diffraction data were ob-
trometer (Varian, Palo Alto, USA). For measurements, small
tained by using an image plate Guinier diffractometer (Huber G
n 2 6
amounts of [C mim] [Mo Cl14] (n = 4, 6, 12, 16, 18) were mixed
6
70, Rimsting; Mo-Kα1). Temperature-dependent SAXS experi-
carefully with KCl (analytical grade, Riedel-de Haën) and pressed
(Perkin–Elmer hydraulic press) to give thin pellets. The samples
were placed on a solid sample holder 5 mm in diameter and fixed
with silicone paste (Baysilone Paste, Bayer, Leverkusen).
ments were carried out at the A2 Beamline of DORIS III, Hasylab,
DESY, Hamburg, Germany, at a fixed wavelength of 1.5 Å. The
data were collected with a MarCCD detector. The detector was
calibrated with silver behenate. The sample–detector position was
fixed at 635.5 mm. For measurements, the samples were placed in
a copper sample holder between aluminium foil. The sample tem-
perature was controlled by a JUMO IMAGO 500 multi-channel
process and program controller. Data reduction and analysis, cor-
rection or background scattering and transmission were carried out
by using a2tool (Hasylab).
Luminescence Spectroscopy: Absorption and emission spectra were
recorded on a FluoroLog-3 (Horriba Jobin Yvon) spectrometer by
using a 450-W xenon lamp as the excitation source and a photo-
multiplier as the detector. Samples of [C
2, 16, 18) were prepared in silica tubes sealed with wax or Parafilm
Pechiney Plastic Packing).
n 2 6
mim] [Mo Cl14] (n = 4, 6,
1
(
Infrared Spectroscopy: Measurements were performed on an Alpha
FT-IR spectrometer (Bruker, Karlsruhe) in the attenuated total re-
flection (ATR ) mode. The ATR unit was equipped with a diamond
crystal, which was coated with the neat sample.
Crystal Structure Determinations: A few crystals of 1a were selected
under perfluorinated polyether (viscosity 1600 cSt, ABCR GmbH,
Karlsruhe) with the aid of an optical microscope. The best speci-
men was chosen and adhered to a glass capillary with oil. A com-
plete intensity data set was collected by using a Bruker smart 1000
Polarizing Optical Microscopy (POM): The samples for POM mea-
single-crystal X-ray diffractometer (Bruker, Karlsruhe) with graph- surements were prepared under ambient conditions. Small amounts
ite-monochromated Mo-K
α
radiation (λ = 0.71073 Å) at –60 °C.
n 2 6 14
of [C mim] [Mo Cl ] (n = 16, 18) were placed between thin glass
Data reduction was carried out with the program package
slides under a microscope equipped with crossed polarizers and a
video camera (Axio Imager A1, Zeiss). For heating and cooling of
the samples, a THMS 600 (Linkam Scientific Instruments, GB)
[
17]
SAINT and a numerical absorption correction with the program
[
18]
SADABS.
For data collection for 1b, single crystals were sealed in Lindemann
glass capillaries of 0.2 mm o.d. and, for 2, crystals were selected
again under perfluorinated polyether. Data collection was per-
formed on a IPDS-I single-crystal diffractometer (Stoe, Darm-
stadt). Data reduction was carried out with the program package
heating and cooling stage was used. The measurements were car-
Ϫ1
ried out at a thermal ramp of 5 Kmin
.
Supporting Information (see footnote on the first page of this arti-
cle): X-ray powder diffraction patterns, IR spectra, absorption
spectra and thermoanalytical data (TG, DSC) for all compounds.
[
19]
X-AREA
and a numerical absorption correction with the pro-
[
20]
gram X-RED.
Crystal structure solutions by direct methods using SIR 92[21]
yielded heavy-atom positions. Subsequent difference Fourier analy-
Acknowledgments
ses and least-squares refinement with SHELXL-97[22] allowed the
This work has been supported by the State of Nordrhein-Westfalen
through the Universities of Cologne and Bochum, especially within
the framework of the Collaborative Research Program 608 (Com-
plex transition metal compounds with spin and charge degrees of
freedom and disorder) and the Priority Program SPP 1191 (Ionic
Liquids) supported by the Deutsche Forschungsgemeinschaft
location of the remaining atom positions. In the final step of the
crystal structure refinement, hydrogen atoms of idealized –CH,
–
CH
atom mode; their isotropic displacement factor was chosen to be 1.2
–CH, –CH ) and 1.5 (–CH ) times the preceding carbon atom.
2 3
and –CH groups were added and treated with the riding
(
2
3
(DFG), Bonn. A. V. M. is grateful for a Dozentenstipendium
For further information see the Supporting Information.
awarded by the Fonds der Chemischen Industrie, Frankfurt/Main.
CCDC-12345 (for 1a), -12345 (for 1b) and -12345 (2) contain sup-
plementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif. For draw-
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Differential Scanning Calorimetry (DSC): Melting point and phase-
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2
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6
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Received: April 5, 2011
Published Online: August 10, 2011
Miki, T. Ikeyama, Y. Sasaki, T. Azumi, J. Phys. Chem. 1992,
Eur. J. Inorg. Chem. 2011, 4089–4095
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4095