Molecular Oxygen Modulated Luminescence of an Octahedro-hexamolybdenum Iodide Cluster having Six Apical Thiocyanate Ligands

Article abstract of DOI:10.1002/zaac.201600021

The synthesis, characterization, and luminescence properties of (Bu₄N)₂[Mo₆I₈(NCS)₆] are reported. Orange crystals of (Bu₄N)₂[Mo₆I₈(NCS)₆] were c

Full text of DOI:10.1002/zaac.201600021

Journal of Inorganic and General Chemistry
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DOI: 10.1002/zaac.201600021
Molecular Oxygen Modulated Luminescence of an Octahedro-
hexamolybdenum Iodide Cluster having Six Apical Thiocyanate Ligands
Lara Riehl,[a] Markus Ströbele,[a] David Enseling,[b] Thomas Jüstel,[b] and
H.-Jürgen Meyer*[a]
Keywords: Molybdenum cluster; Structure; Luminescence; Thiocyanate
Abstract. The synthesis, characterization, and luminescence properties cence studies on the crystalline solid revealed a broad excitation in
of (Bu4N)2[Mo6I8(NCS)6] are reported. Orange crystals of (Bu4N)2- the UV/Vis region and an emission around 690 nm. The red emission
[Mo6I8(NCS)6] were crystallized from acetone solution and used for intensity decreases with increasing partial pressure of atmospheric mo-
X-ray single-crystal structure determination and refinement, yielding a lecular O2.
structure with a centrosymmetric [Mo6I8(NCS)6]2– ion. Photolumines-
addressed to the apical ligands considered as antenna effect.
Introduction
The influence of apical ligands (L) in [Mo6I8L6]2– clusters is
Octahedral molybdenum halide cluster compounds based on
not fully understood and may be related to the significant or-
the [Mo6X14]2– ion exhibit remarkable photophysical and pho-
bital mixing of apical ligand MOs with the HOMO of the
tochemical properties because they show photoluminescence
[Mo6X8] cluster core.[17]
properties in solid state and in solution with typical emissions
Another important functionality of these clusters is the lumi-
in the red to infra-red region with high quantum yields, plus
nescence quenching in the presence of molecular oxygen, due
excellent potential in the generation of singlet oxygen.[1–8]
to the formation of singlet oxygen. This can be explained by
i
a
Syntheses of (nBu4N)2[Mo6I8 L6 ] compounds (i = innen or
inner, a = außen or outer) (L = F, Cl, Br, I) have been reported
some years ago, also substitutes with outer ligands (L) such as
an intersystem crossing from excited singlet states into triplet
states of the cluster, which are responsible for the excitation
of triplet oxygen. In this context, the formation of singlet oxy-
gen has been shown by direct measurement of the phosphores-
cence of singlet oxygen.[18]
i
a
trifluoroacetate in (Bu4N)2[Mo6Cl8 (CF3COO)6 ] or alkoxide
i
a
in Na2[Mo6Cl8 (OR)6 ].[9–13] Many clusters of this type behave
chemically stable under various conditions and it is clear that
the apical (outer) ligands are more loosely bound to the cluster
core, thus allowing a manifold of exchange reactions with vari-
ous inorganic or organic ligands.
The combination of two interfering functionalities, namely
the luminescence and singlet oxygen production, make these
materials unique and raises potential for applications in molec-
ular oxygen sensing, singlet oxygen generation, and so on.
Similar properties are evident not only for halide compounds
such as [Mo6X8] and [W6X8], but also in [Re6Q8] cluster cores
(Q = S, Se, Te).[19,20] However, studies on [W6I8] compounds
are systematically missing because during the past, because
there has been no useful tungsten iodide source available.
A substitution of all apical iodide ligands in [MoI8I6]2–, i.e.
with trifluoroacetate ligands, induces several changes of op-
tical properties such as a blue shift of the emission band and
a significant increase of the luminescence quantum yield.[18]
Experiments have shown that the optical properties of
[Mo6X14]2– compounds are largely influenced by the nature of
ligands (X). Halide ligands or their substitutes play an impor-
tant role for introducing a charge and thereby influence the
absorption in the visible and near-ultraviolet region. Thus, cor-
responding electronic transitions appear halide dependent
causing a red shift when going from chloride to bromide, and
from bromide to iodide.[1] This had led to the conclusion that
the absorption bands are influenced by ligand-to-metal charge
transfer (LMCT) transitions.[14–16] A significant influence is
i
a 2–
Among the relatively small number of [Mo6I8 Y6 ] clusters
that have been studied to date, the trifluoroacetate compound
* Prof. Dr. H.-J. Meyer
E-Mail: juergen.meyer@uni-tuebingen.de
[a] Section for Solid State and Theoretical Inorganic Chemistry
Institute of Inorganic Chemistry
University of Tübingen
i
a
(nBu4N)2[Mo6I8 (CF3COO)6 ] is reported to have the highest
quantum yield (nearly 100% in acetonitrile solution) and alter-
natively a high rate of singlet-oxygen formation (85%).[18]
More examples, in which the apical ligands are represented by
(n-C3F7COO)–,[21] (pyrene–COO)– or (anthracene–COO)–,[22]
(SC6F4H)–,[23] [CϵCC(O)OMe]–,[24] and (NO3)–,[25,26] have
been reported, also showing good results. Still more examples
Auf der Morgenstelle 18
72076 Tübingen, Germany
[b] Department of Chemical Engineering
Münster University of Applied Sciences
Stegerwaldstraée 39
48565 Steinfurt, Germany
Z. Anorg. Allg. Chem. 2016, 642, (5), 403–408
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of compounds are needed to get more insight into the relation- Table 1. Crystal data and structure refinement for (TBA)2[Mo6I8(NCS)6].
ship between the nature of ligands and the fine tuning of prop-
erties.
(TBA)2[Mo6I8(NCS)6]
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
a /pm
b /pm
C38H72I8Mo6N8S6
2424.23
293(2) K
71.073 pm
triclinic
¯
P1
1161.5(1)
1259.6(1)
1331.1(1)
116.118(7)
97.064(7)
90.841(7)
1.7301(3)
1
2.327
4.836
1128
0.16ϫ0.16ϫ0.08
[Mo6X8(NCS)6]2– clusters contain 24 electrons in Mo–Mo
bonding states and are expected like other clusters of this type
to exhibit a singlet ground state resulting in diamagnetic be-
havior. Electronic structure calculations on [Mo6X8(NCS)6]2–
compounds (X = Cl, Br, and I) predict luminescence properties
and evidence for ligand-to-metal charge transfer (LMCT).[27]
The electronic features imply that the HOMO is largely iso-
thiocyanate based and that the lowest excited states are cen-
tered on the [Mo6X8] core. Experimental data on these com-
c /pm
α /°
β /°
γ /°
i
a
Volume /nm3
pounds are missing although (nBu4N)2[Mo6I8 (NCS)6 ]·
Me2CO has been reported in the literature[28,29] and the synthe-
Z
Density (calculated) /g·cm–3
Absorption coefficient /mm–1
F(000)
i
a
sis and the crystal structure of (nBu4N)2[Mo6Cl8 (NCS)6 ]
have been reported.[30]
Crystal size /mm3
Herein we report the all synthetic steps of the synthesis of
a ligand substituted cluster compound having a new crystal Theta range for data collection /° 2.286 to 25.027
i
a
Index ranges
–13 Յ h Յ 13
–14 Յ k Յ 14
–15 Յ l Յ 15
16970
structure. Solvent free (nBu4N)2[Mo6I8 (NCS)6 ] was formed
via several steps of solid state and solution reactions. The crys-
tal structure was solved and refined on basis of single-crystal
data and photoluminescence properties are reported.
Reflections collected
Independent reflections
Completeness to θ = 25.027°
Refinement method
6068 [Rint = 0.0180]
99.3%
Full-matrix least-squares on F2
6068 / 0 / 298
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I Ͼ 2σ(I)]
R indices (all data)
1.176
R1 = 0.0310, wR2 = 0.0553
R1 = 0.0422, wR2 = 0.0587
Results and Discussion
Largest diff. peak and hole /e·Å–3 0.423 and –0.454
Synthesis and Structure
Syntheses of [Mo6I8Y6]2– cluster compounds have been de-
scribed based on several procedures given in the literature. A
comprehensive preparation procedure for these clusters, de-
parting from molybdenum and iodine was never given. Espe-
cially the first step of this synthesis is critical and the develop-
ment of a new route for the preparation of Mo6I12 would be
desirable. Our preparation begins with the classical solid-state
synthesis of Mo6I12 from elemental molybdenum and iodine,
which can be tricky with respect to the employed Mo powder
(see Experimental Section). From Mo6I12, the preparation of
Cs2[Mo6I14] proceeds almost quantitatively, which indeed is
an excellent precursor for the following reaction steps by
means of solution chemistry.[31] The exchange of cesium by
TBA [(nBu4N)+] ions largely increases the solubility of the
cluster compound, and the introduction of labile nitrato ligands
in apical positions of (TBA)2[Mo6I8(NO3)6] yields a good pre-
cursor for ligand exchange with thiocyanato ligands.[25]
Beautiful orange crystals of (TBA)2[Mo6I8(NCS)6] are finally
crystallized from acetone solution. Crystallographic data are
summarized in Table 1. The crystal structure of (TBA)2-
[Mo6I8(NCS)6] contains an arrangement of [Mo6I8(NCS)6]2–
Figure 1. Crystal structure of (TBA)2[Mo6I8(NCS)6] projected on the
bc plane.
clusters and tetrabutylammonium ions (TBA) crystallizing
2–
¯
with the space group P1. [Mo6I8(NCS)6] clusters are ar-
ranged to form a slightly distorted hexagonal primitive packing
arrangement, which is stuffed with (TBA)+ ions, as shown in
Figure 1.
TBA ions are responsible for the low symmetry of the crys-
tal structure with the centrosymmetric [Mo6I8(NCS)6]2– cluster
(see Figure 2).
Z. Anorg. Allg. Chem. 2016, 403–408
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time it has been shown that the HOMO is clearly influenced
by contributions of apical ligands, which is at least part of the
reason for the appearance of the stronger luminescence of
some heteroleptic [Mo6I8Y6]2– cluster compounds compared to
their homoleptic counterparts [Mo6X14]2–.
The identity of (TBA)2[Mo6I8(NCS)6] was confirmed by
ESI mass spectroscopy in methanol solution (Bruker Esquire
3000 plus) in negative mode, showing a mass peak (m/z) at
2182.5 [M–TBA]–. Orange crystals of (TBA)2[Mo6I8(NCS)6]
show surface effects after storage in air for several days. The
mass spectrum still showed the same mass peak, however, with
the evolution of minor peaks.
Photoluminescence (PL), Reflection Spectra, and Decay
Curves
(TBA)2[Mo6I8(NCS)6] displays strong red photolumines-
cence peaking at 690 nm as shown in Figure 3. Its excitation
spectrum shows a very broad band between 250 and 600 nm
as presented in Figure 4. Due to the large full width at half
maximum (ca. 0.28 eV) and the nature of the compound, we
assume that the intense photoluminescence originates from a
ligand-to-metal charge-transfer transition between the ligands
and the divalent molybdenum.
Figure 2. Structure of the centrosymmetric [Mo6I8(NCS)6]2– cluster.
The [Mo6I8] cluster core exhibits bonding Mo–Mo distances
within 265.92(6)–267.34(7) pm with an average value of
266.68(7) pm for the octahedral arrangement of Mo atoms.
Mo–I contacts range within 277.79(6)–279.33(6) pm for the
face-capping (μ3) iodide ligands with an average value of
278.42(6) pm. Six thiocyanato ions, in which the nitrogen
atoms are connected to the molybdenum cluster in apical posi-
tions complete the centrosymmetric [Mo6I8(NCS)6]2– ion.
Average Mo–N distances are at 212.3(5) pm; average distances
of thiocyanato ions are at 115.0(6) pm for N–C, and at
160.9(6) pm for C–S contacts.
According to the refined crystal structure it is clear that the
ambident thiocyanato ion is linked to the cluster by the harder
nitrogen atom. Considering the HSAB principle, the Mo2+ ion
could be considered as a hard acid, so it will form a more
stable core with the hardest base. A similar behavior has been
described for halides in apical cluster positions, where increas-
ing stability follows the order F– Ͼ Cl– Ͼ Br– Ͼ I–.[32,33]
As can be expected, the thiocyanato ligands are linear with
an average N–C–S angle at 179.1°. However, Mo–N–C angles
are bent with an average angle of 163.0°. This deviation from
linearity could be explained with the matrix effect, resulting
from the arrangement with TBA ions in the structure. At the
Figure 3. Emission spectra of (TBA)2[Mo6I8(NCS)6] in different at-
mospheres under 400 nm excitation.
Most striking is that the photoluminescence emission inten-
same time there is a difference in bonding behavior, because sity of the μ-scale powder is dependent on the respective atmo-
besides the σ donation, there appears some π back donation sphere. As depicted in Figure 3 and Figure 4, the photolumi-
between the nitrogen atom of the thiocyanato ligands and the nescence intensity is strongest in pure nitrogen. With an in-
molybdenum cluster. A bending effect was not considered in crease of the oxygen partial pressure, the emission intensity
the calculation of the electronic structure of [Mo6I8(NCS)6]2–, continuously decreases, while the shape of the excitation and
which displayed a π antibonding character in the frontier orbit- emission spectrum is not changed at all.
als region of (NCS)– ions introducing a non-stabilizing orbital
The reflection spectrum (Figure 5) does not change with
interaction.[27] A nonbonding or slightly antibonding interac- amending the atmosphere. This indicates that the absorption
tion is not unexpected for this type of cluster and gives raise edge and strength is not a matter of the atmosphere, but that
to the assumption that these clusters may undergo facile elec- the luminescence process is quenched by the presence of oxy-
trochemical one-electron reversible oxidation. At the same gen.
Z. Anorg. Allg. Chem. 2016, 403–408
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Figure 4. Excitation spectra of (TBA)2[Mo6I8(NCS)6] in different at-
mospheres monitored for 690 nm emission.
Figure 6. Decay curves of (TBA)2[Mo6I8(NCS)6] in different atmo-
spheres for 400 nm excitation and monitored at 670 nm emission.
photophysical properties as shown herein for the crystalline
solid. Solid (TBA)2[Mo6I8(NCS)6] reveals a broad excitation
in the UV/Vis range and shows a strong red emission in the
absence of oxygen. The luminescence quenching behavior in
the presence of molecular oxygen can be explained the same
way as reported for other compounds containing [Mo6I8(L)6]2–
ions, namely with the excitation of triplet oxygen from excited
triplet states of the cluster. (TBA)2[Mo6I8(NCS)6] is a perspec-
tive material for surface grafting on Au or Ag surfaces.
After finishing this work, we found a publication on in vitro
studies of water soluble Na2[Mo6I8(NCS)6].[35]
Experimental Section
Materials and Methods: Unless otherwise noted, all reactions were
carried out in an argon atmosphere in dried and degassed solvents
using Schlenk techniques. All solvents were purchased from Sigma-
Aldrich, dried and degassed with an MBraun SPS-800 solvent purifica-
tion system. Chemicals used were obtained from commercial suppliers
and were used without further purification.
Figure 5. Reflection spectrum of (TBA)2[Mo6I8(NCS)6] relative to
BaSO4 as a white standard.
Syntheses: The preparation of the title compound departs from solid
state reactions of elements, yielding Mo6I12, which is used to prepare
Cs2[Mo6I14].[31] The following reaction stages involve the preparation
of the TBA (nBu4N)+ salt (TBA)2[Mo6I14] in aqueous solution and the
preparation of the nitrate salt (TBA)2[Mo6I8(NO3)6], before the labile
nitrate ligands are exchanged with thiocyanate.
The presence of photoluminescence quenching can also be
proven by recording the decay curves. From these curves, as
depicted in Figure 6, the following decay times were derived:
(i) in pure nitrogen τe = 82 μs
(ii) in air (21% O2) τe = 70 μs
(iii) in pure oxygen τe = 55 μs
These findings are in line with photoluminescence quench-
ing as mentioned before. The ratio of the decay constants can
be used in order to estimate the extent of photoluminescence
quenching, i.e. the application of oxygen reduces the photolu-
minescence intensity by 33%.
Synthesis of MoI2:
Mo + I2 ǞMoI2
(1)
Mo powder (Strem, 99.95%, 100 mesh) was treated in a flow of
NOXAL (7% H2 in Ar) at 700 °C for 8 h prior use. Afterwards, a 1:5
molar mixture of elemental molybdenum (0.031 mmol, 0.3 g) and iod-
ine (0.014 mmol, 3.5 g) were placed into a silica tube and fused under
vacuum. The mixture was slowly heated in a muffle furnace to 700 °C
and remained there for 4 d following Equation (1). The ampoule was
placed in the furnace so that the vaporized iodine could condense and
flow back into the reaction mixture. The resulting black powder was
Conclusions
The new compound (TBA)2[Mo6I8(NCS)6] appears as a red-
orange crystal powder. The crystal structure reveals the centro-
symmetric [Mo6I8(NCS)6]2– ion, which provides interesting obtained in 97% yield, as estimated from XRD patterns.
Z. Anorg. Allg. Chem. 2016, 403–408
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Attention: The silica tube may explode, or produce low yields of
Luminescence Measurements: Excitation and emission spectra as
MoI2, if the Mo powder was not sufficiently reduced (activated) with well as decay curves were collected with a fluorescence spectrometer
NOXAL.
FLS920 (Edinburgh Instruments) equipped with a 450 W ozone-free
xenon arc lamp (OSRAM) and a sample chamber installed with a mir-
ror optic for powder samples. For detection, a R2658P single-photon
counting photomultiplier tube (Hamamatsu) was used. All lumines-
cence spectra were recorded with a spectral resolution of 1 nm, a dwell
time of 0.4 s in 1 nm steps, and three repeats. Reflection spectra were
monitored by placing the sample into an integrating sphere coated with
spectralon and using a synchronous scan, i.e., the excitation and emis-
sion monochromator were adjusted to the same wavelength and tuned
synchronously. These reflection spectra were recorded with an Edin-
burgh Instruments FS900 spectrometer equipped with a 450 W Xe arc
lamp and cooled single-photon counting photomultiplier (Hamamatsu
R928). BaSO4 (99%, Sigma-Aldrich) was used as a reflectance stan-
dard.
Synthesis of Cs2[Mo6I14]:
6MoI2 + 2CsI ǞCs2[Mo6I14]
(2)
A mixture of 3:1 molar ratio of MoI2 (3.4 mmol, 1.2 g) and CsI
(1.1 mmol, 297.14 mg) was ground and placed into a silica tube. The
ampoule was flame-sealed under vacuum. The mixture was heated for
3 d at 700 °C. Cs2[Mo6I14] was obtained as a dark red powder in 89%
yield and behaves stable in air [Equation (2)].
Synthesis of (TBA)2[Mo6I14]:
Cs2[Mo6I14] + 2(TBA)I Ǟ (TBA)2[Mo6I14] + 2CsI
(3)
Solid Cs2[Mo6I14] (0.93 mmol, 2.44 g) was added to a solution of
(TBA)I (2.7 mmol, 1.03 g, Aldrich Ն 99.95%) in CH2Cl2/H2O
(300 mL, 1:1) under magnetic stirring for 2 d at ambient temperature
following Equation (3). The organic phase was separated and filtered.
After evaporation of the solvent the resulting red powder was washed
three times with distilled water. The product was re-dissolved in
CH2Cl2 and filtered. A red powder was obtained in 90% yield after
evaporation of the solvent.
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Reaction products were inspected by powder X-ray diffraction (XRD)
using a Stadi-P (STOE, Darmstadt) diffractometer with germanium
monochromated Cu-Kα1 radiation. The powder patterns of products
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and purity by comparison with corresponding patterns from the ICSD.
Z. Anorg. Allg. Chem. 2016, 403–408
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Zeitschrift für anorganische und allgemeine Chemie
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Received: January 14, 2016
Published Online: March 9, 2016
Z. Anorg. Allg. Chem. 2016, 403–408
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