(W6I8)Cl4– A Basic Model Compound for Photophysically Active [(W6I8)L6]2–Clusters?

Article abstract of DOI:10.1002/zaac.201600316

The heteroleptic cluster compound (W₆I₈)Cl₄was prepared by thermal conversion of the homoleptic clusters W₆I₁₂and W₆Cl₁₂at 700 °C to yield a bright yellow powder. The presence of the smaller chlorido ligands in apical positions of [(W₆I₈)Cl₆]^2–creates nearly spherically clusters showing thermal and chemical inertness. Photoluminescence studies revealed a strong red phosphorescence from excited spin-triplet states.

Full text of DOI:10.1002/zaac.201600316

Journal of Inorganic and General Chemistry
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ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
DOI: 10.1002/zaac.201600316
(W6I8)Cl4 – A Basic Model Compound for Photophysically Active
[(W6I8)L6]2– Clusters?
Markus Ströbele,[a] David Enseling,[b] and Thomas Jüstel,[b]H. -Jürgen Meyer*[a]
Keywords: Tungsten cluster; Synthesis; Luminescence
Abstract. The heteroleptic cluster compound (W6I8)Cl4 was prepared spherically clusters showing thermal and chemical inertness. Photo-
by thermal conversion of the homoleptic clusters W6I12 and W6Cl12 at
700 °C to yield a bright yellow powder. The presence of the smaller
chlorido ligands in apical positions of [(W6I8)Cl6]2– creates nearly
luminescence studies revealed a strong red phosphorescence from ex-
cited spin-triplet states.
a crucial role regarding properties. This prompted us to de-
velop the basic heteroleptic cluster compound (W6I8)Cl4 as a
prototype compound for [M6I8L6]2– photosensitizers, which is
reported herein.
Introduction
M6X12 compounds with M = Mo, W and X = Cl, Br, I are
well known.[6–10] Their structures are based on the octahedral
i
[M6X8 ] cluster core containing eight inner (μ3) and six outer
halide ligands, of which four are shared between adjacent clus-
Results and Discussion
ters (i = innen or inner, a = außen or outer) according to nota-
i
a
tion [(M6X8 )X2 X4/2a–a].
Synthesis and Structure
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 for
a manifold of exchange reactions with various inorganic or
organic ligands.[1]
Syntheses of homoleptic M6X12 (M = Mo, W; X = Cl, Br, I)
cluster compounds have been described based on several dif-
ferent procedures given in the literature.[5] For the preparation
of the new heteroleptic cluster compound (W6I8)Cl4 we use an
unconventional but simple solid-state reaction between W6I12
and W6Cl12. This reaction takes advantage of the higher ther-
mal stability of W6I12 compared to W6Cl12. W6I12 can be
treated at 700 °C in a sealed quartz tube without decomposi-
tion, whereas W6Cl12 has been shown to decompose into the
elements near 550 °C when treated under similar conditions.
When W6I12 and W6Cl12 were reacted, the formation of some
elemental tungsten as side-phase was inevitable, which intro-
duced a light greying to the observed (W6I8)Cl4 powder.
(W6I8)Cl4 was obtained as an orange-yellow crystalline
powder containing plate-like single-crystals, which were used
for structure determination and refinement by means of X-ray
diffraction. The substitution of the apical iodido ligands in
W6I12 by chlorido ligands in the heteroleptic (W6I8)Cl4 in-
volves a change of the color from brown to bright yellow (Fig-
ure 1), which is related to the canary yellow color of W6Cl12.
The crystal structure of (W6I8)Cl4 refined with the space
group Cmca (Table 1 and Table 2), is closely related to the
structure of W6X12 (X = Cl, Br, I). The cluster connectivity of
Photophysical studies on homoleptic and heteroleptic
i
a 2–
[W6X8 X6 ] cluster compounds (in which Xi and Xa are Cl,
Br, I) have revealed that the all iodide cluster shows the highest
luminescence intensity (φem = 0.39) and the most blue shifted
i
a
emission band (at 698 nm for (TBA)2[W6X8 X6 ]) in aceto-
nitrile.[2]
Compounds with the octahedral molybdenum halide cluster
[Mo6I8L6]2–, in which L is an inorganic or organic ligand have
received great attention due to their remarkable photophysical
and photochemical properties.[3] These compounds exhibit
photoluminescence properties with typical emissions in the red
to infra-red region, and excellent potential in the generation of
singlet oxygen, in solid state and in solution.
The importance of [Mo6I8L6]2– photosensitizers inspired us
to create corresponding [(W6I8)L6]2– clusters, which are cur-
rently developed.[4] In these compounds, terminal ligands play
* Prof. Dr. H. -J. Meyer
E-Mail: juergen.meyer@uni-tuebingen.de
[a] Section of Solid State and Theoretical Inorganic Chemistry
Institute of Inorganic Chemistry
Eberhard Karls University Tübingen
Auf der Morgenstelle 18
i
a
[(W6I8 )Cl2 Cl4/2a–a] is represented by four inter-cluster-bridg-
ing chlorido ligands (Cla–a) forming the characteristic two-di-
mensional layer structure of W6X12 compounds, as displayed
in Figure 2. The octahedral [W6] cluster core contains W–W
distances within the range 265.04(7)–265.23(7) pm, with an
average value of 265.11(6) pm. W–I contacts with (inner) face-
72076 Tübingen, Germany
[b] Department of Chemical Engineering
University of Applied Sciences Münster
Stegerwaldstrasse 39
48565 Steinfurt, Germany
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Table 2. Atomic coordinates and equivalent isotropic displacement pa-
rameters (pm2 ϫ10–1) for (W6I8)Cl4. Ueq is defined as one third of the
trace of the orthogonalized Uij tensor.
Atom
x
y
z
Ueq
W(1)
W(2)
W(3)
I(1)
I(2)
Cl(1)
Cl(2)
0.1201(1)
0
0
0.1253(1)
0.1259(1)
0.2778(2)
0
0
½
13(1)
13(1)
13(1)
25(1)
25(1)
26(1)
33(1)
0.0851(1)
0.1296(1)
0.0472(1)
0.2267(1)
0
0.6295(1)
0.4151(1)
0.2733(1)
0.5473(1)
½
0.3122(3)
0.3121(3)
Figure 1. Optical photographs of crystalline powders (top row) of
W6I12 starting material (left), (W6I8)Cl4 under daylight (middle), and
(W6I8)Cl4 under 366 nm excitation (right). A crystal of (W6I8)Cl4 is
shown below under daylight and under 366 nm excitation (bottom
row).
capping iodido ligands range within 278.62(6)–280.34(6) pm,
with an average value of 279.37(6) pm. Six chlorido ligands
are situated in (outer) apical positions of the cluster. Four of
them generate a bridging connectivity with W–Cl distances of
253.6(3) pm (Figure 2). Another two apical chlorido ligands
appear as terminal ligands with W–Cl distances at
246.2(3) pm. All in all, the crystal structure of (W6I8)Cl4 corre-
sponds to the structure of its homoleptic W6X12 counterparts,
i
a
best described by the [(W6I8 )Cl2 Cl4/2a–a] connectivity.
Table 1. Crystal data and structure refinement data for (W6I8)Cl4.
(W6I8)Cl4
Figure 2. Two-dimensional projection of a section of the crystal struc-
Formula weight
Temperature /K
Wavelength /pm
Crystal system
Space group
2260.10
293(2)
71.073
i
a
ture of [(W6I8 )Cl4/2a–aCl2 ] (color code: W blue, I violet, and Cl
green).
orthorhombic
Cmca (no. 64)
Unit cell dimensions
a /pm
b /pm
compounds, which are summarized in Table 3. A major differ-
ence to corresponding compounds of this type is the enlarge-
ment of W–Cl–W angles (148.8°), due to the smaller chlorido
ligands in (W6I8)Cl4. The smaller chlorido ligands in apical
positions of the [(W6I8)Cl6]2– fragment in Figure 3 make up
for a nearly spherical cluster moiety, having a diameter in the
order of 100 nm.
1560.6(2)
1209.5(2)
1210.5(2)
2.2848(6)
4
6.570
41.343
3744
0.12ϫ0.1ϫ0.007
c /pm
Volume /nm3
Z
Density (calculated) /g·cm–3
Absorption coefficient /mm–1
F(000)
Outer ligands of [(M6Xi8)Xa ]2– clusters (M = Mo, W) are
Crystal size /mm3
6
Theta range for data collection /° 3.366 to 25.327
usually more loosely bound to the cluster core, which may be
due also to the repulsion with inner X ligands (matrix effect),
provoking that outer ligands have relatively long M–X dis-
tances, and can be more easily removed or substituted. With
the presence of the smaller chlorido ligands in apical positions
of the [(W6I8)Cl6]2– cluster this repulsion may be reduced, as
we note significantly short M–X distances for W–Cla and
W–Cla–a in (W6I8)Cl4 (Table 3). (W6I8)Cl4 appears as a ther-
mally and chemically stable material. According to a thermal
treatment in an evacuated silica tube (W6I8)Cl4 is thermally
stable up to 750 °C. In air it behaves stable up to 250 °C ac-
cording to DTA (differential thermal analysis) measurements.
(W6I8)Cl4 is chemically inert in the presence of concentrated
Index ranges
–18 Յ h Յ 18
–14 Յ k Յ 14
–14 Յ l Յ 14
10388
1083
0.0173
Reflections collected
Independent reflections
R(int)
Completeness to θ = 25.242°
Refinement method
98.9%
Full-matrix least-squares on F2
1083 / 0 / 47
1.159
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I Ͼ 2σ(I)]
R indices (all data)
R1 = 0.0212, wR2 = 0.0552
R1 = 0.0217, wR2 = 0.0554
Largest diff. peak and hole /e·Å–3 3.132 and –1.335
Interatomic distances in (W6I8)Cl4 can be well related with acids (HCl, HF, HNO3, or H2SO4) and stable in strong alkaline
corresponding distances obtained in similar W6X12 cluster solutions.
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Table 3. Comparison of distances /pm, selected angles /°, and unit cell volumes V /Å3 in M6X12 compounds with M = Mo, W.
[6]
[7]
[8]
[9]
[10]
Mo6Cl12
Mo6Br12
Mo6I12
W6Br12
(W6I8)Cl4
W6I12
M–M
M–Xi
M–Xa–a
M–Xa
M–I–M
V
260.63(7)
247.0(1)
249.3(2)
238.0(2)
131.18(6)
1785.2(8)
263.1(1)
259.7(1)
265.3(2)
255.1(2)
131.76(7)
2048.8(3)
267.2(4)
2.773(3)
289.1(5)
2.805(5)
133.5(2)
2494(1)
263(2)
262(4)
265(5)
250(3)
132.0(2)
2061.5(3)
265.11(6)
279.37(6)
253.6(3)
246.2(3)
148.76(1)
2284.8(6)
266.5(3)
279.5(6)
288.9(4)
279.7(5)
134.3(2)
2508(1)
Figure 3. Space filling structure drawing of an isolated [(W6I8)Cl6]2–
fragment (left) contained in the (W6I8)Cl4 structure and a correspond-
ing drawing, in which atoms are shown as contracted spheres, intercon-
nected with lines that characterize interatomic bonds (right).
Photoluminescence
First photophysical studies on Mo6Cl12 were reported sev-
eral years ago, showing that molybdenum halide clusters are
strongly photoluminescent.[12] M6Cl12 compounds containing
the [(M6Cl8)Cl6]2– ion (M = Mo, W) show red luminescence
in solid state under UV/Vis photoexcitation.[15] When dis-
solved in organic solvents, the photoluminescence of
[(Mo6Cl8)Cl6]2– cluster ions is increasingly quenched with the
amount of molecular oxygen in solution.[11] Alongside with a
rather long luminescence emission lifetime and other experi-
mental evidence, the excited state properties were interpreted
as phosphorescence emission from excited spin-triplet states.
The temperature dependence of the emission and of the decay
time of [(Mo6Cl8)Cl6]2– cluster anions were explained by the
emission from the three excited triplet-state sublevels.[12]
Solid (W6I8)Cl4 shows a broad absorption band in the range
between 250 and 550 nm, displayed in Figure 4. Upon exci-
tation at 450 nm there appears a very broad emission band
peaking at 635 nm (1.95 eV) with a FWHM of 110 nm
(0.33 eV), with a decay time of about 57 μs.
Figure 4. Excitation spectrum of (W6I8)Cl4 monitored at 650 nm.
Temperature dependent emission spectra are obtained upon
450 nm excitation. The emission intensity of (W6I8)Cl4 is low
at room temperature, as can be estimated from the emission
spectra in Figure 5, which may be caused be concentration
quenching of adjacent clusters in crystal grains. Expectedly,
Figure 5. Emission spectra of (W6I8)Cl4 between 100 and 500 K upon
450 nm excitation.
A similar behavior of emission bands was studied in much
the intensity increases with decreasing temperature. Usually, detail, where spectral fitting procedures have resulted in three
the temperature dependency of the position of the emission triplet states for [Mo6Cl14]2– and four triplet states for
maximum is a result of the redistribution of excited triplet [Re6S8X6]4– (X = Cl–, Br–, I–).[13]
spin-states. For (W6I8)Cl4 we monitored a peculiar temperature
Different oxygen partial pressures showed no influence on
dependency, shown in Figure 5. The emission maximum shows the luminescence intensity of solid (W6I8)Cl4. This give raise
a red-shift between 100 to 200 K and then a blue-shift from to the assumption that (W6I8)Cl4 is not efficient in the genera-
200 to 500 K. The red-shift behavior of the emission maximum tion of singlet oxygen. The same was demonstrated for solid
is due to emission bands originating from several excited spin- (TBA)2[W6I14].[4] Oxygen quenching is only apparent once
triplet state sublevels, whereby the occupation of these energy [W6I14]2– clusters are dissolved into solution (e.g. in aceto-
levels varies with temperature.
nitrile, acetone, alcohol), and a strongly enhanced photophysi-
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W6Cl12 Ǟ 6W + 6Cl2 (side reaction)
(2)
cal activity with respect to luminescence intensity or singlet
oxygen production is obtained for compounds containing
[M6X8 L6 ] ions (M = Mo, W) mentioned in the introduction
[15]
i
a 2–
A mixture of 1:2 molar ratio of W6Cl12
(0.033 mmol, 50 mg) and
W6I12[10,16] (0.066 mmol, 172 mg) was ground in a mortar and charged
into a silica tube. The ampoule was flame-sealed under vacuum. The
mixture was heated for 12 h at 600 °C with a heating and cooling rate
of 2 K·min–1. (W6I8)Cl4 was obtained as a yellow-orange powder in
60% yield [Equation (1)] with elementary tungsten as a side-phase
[Equation (2)].
part.
i
a
Moreover, (TBA)2[M6X8 L6 ] (M = Mo, W) exhibits strong
luminescence quenching in the presence of molecular oxygen
in solid state,[3,4] in contrast to what is obtained for (W6I8)Cl4.
Conclusions
The new compound (W6I8)Cl4 appears as a crystalline yel-
low to orange powder. The crystal structure was characterized
by single-crystal XRD techniques and the photoluminescence
behavior was studied. (W6I8)Cl4 reveals a broad excitation in
the UV/Vis range and shows a red emission at low tempera-
tures. Compared to ligand substituted [Mo6I8L6]2– clusters
there is no significant luminescence quenching of (W6I8)Cl4
by molecular oxygen in solid state.
References
[1] W. Preetz, D. Bublitz, H. G. von Schnering, J. Saßmannshausen,
Z. Anorg. Allg. Chem. 1994, 620, 234–246; D. H. Johnston, D. C.
Gaswick, M. C. Lonergan, C. L. Stern, D. F. Shriver, Inorg.
Chem. 1992, 31, 1869–1873.
[2] T. C. Zietlow, D. G. Nocera, H. B. Gray, Inorg. Chem. 1986, 25,
1351–1353.
[3] M. N. Sokolov, M. A. Mihailov, E. V. Peresypkina, K. A. Brylev,
N. Kitamura, V. P. Fedin, Dalton Trans. 2011, 40, 6375–6377; K.
Kaplan, P. Kubát, M. Dusek, K. Fejfarvová, V. Sícha, J. Mosinger,
K. Lang, Eur. J. Inorg. Chem. 2012, 3107–3111.
Experimental Section
X-ray Crystallographic Study of (W6I8)Cl4: An orange single-crys-
tal of (W6I8)Cl4 was measured with a single-crystal X-ray dif-
fractometer (STOE-IPDS) at room temperature (T = 293 K) using Mo-
Kα radiation (λ = 0.71073 Å). The crystal structure refinement and
solution was performed with direct methods (SHELXS) and least
square refinements on F2 (SHELXL14[14]).
[4] L. Riehl, A. Seyboldt, M. Ströbele, D. Enseling, T. Jüstel, M.
Westberg, P. R. Ogilby, H.-J. Meyer, Dalton Trans. 2016, 45,
15500–15506.
[5] M. A. Riche, Ann. Chim. Phys. 1857, 3, 5–80; E. Defacqz, C. R.
Acad. Sci. 1898, 176, 962–964; E. Defacqz, Ann. Chim. Phys.
1901, 22, 238–288; R. D. Hogue, R. E. McCarley, Inorg. Chem.
1970, 9, 1354–1360; H. Schäfer, H. G. Schulz, Z. Anorg. Allg.
Chem. 1984, 516, 196–200.
[6] H. Schäfer, H.-G. Schnering, J. Tillack, F. Kuhnen, F. Wöhrle, H.
Baumann, Z. Anorg. Allg. Chem. 1967, 353, 281–310.
[7] Y.-Q. Zheng, Y. Grin, H.-G. Schnering, Z. Kristallogr. (NCS)
1998, 213, 497.
[8] Z. G. Aliev, L. A. Klinkova, I. V. Dubrovin, L. O. Atovmyan, Zh.
Neorg. Khim. 1981, 26, 1964–1967.
Some results and final R values are shown in Table 1, atom positions
are given in Table 2, and some selected distances are collected in
Table 3.
Further details of the crystal structure investigations may be obtained
from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-
Leopoldshafen, Germany (Fax: +49-7247-808-666; E-Mail:
crysdata@fiz-karlsruhe.de, http://www.fiz-karlsruhe.de/request for de-
posited data.html) on quoting the depository number CSD-429579.
[9] Y.-Q. Zheng, K. Peters, W. Hoenle, Y. Grin, H.-G. Schnering, Z.
Kristallogr. 1997, 212, 453–457.
[10] J. D. Franolic, J. R. Long, R. H. Holm, J. Am. Chem. Soc. 1995,
117, 8139–8153.
[11] A. W. Maverick, H. B. Gray, J. Am. Chem. Soc. 1981, 103, 1298–
1300; J. A. Jackson, C. Turro, M. D. Newsham, D. G. Nocera, J.
Phys. Chem. 1990, 94, 4500–4507; J. A. Jackson, M. D. New-
sham, C. Worsham, D. G. Nocera, Chem. Mater. 1996, 8, 558–
564; R. N. Ghosh, G. L. Baker, C. Ruud, D. G. Nocera, Appl.
Phys. Lett. 1999, 75, 2885–2887; D. J. Osborn, G. L. Baker, J.
Sol-Gel Sci. Technol. 2005, 36, 5–10; R. N. Ghosh, P. A. Aske-
land, S. Kramer, R. Loloee, Appl. Phys. Lett. 2011, 98, 221103;
Reaction products were inspected by powder X-ray diffraction with a
Stadi-P (STOE, Darmstadt) diffractometer with germanium monochro-
mated Cu-Kα1 radiation.
Photoluminescence Measurements: Excitation and emission spectra
as well as decay curves were collected with a fluorescence spectrome-
ter FLS920 (Edinburgh Instruments) equipped with a 450 W ozone
free xenon arc lamp (OSRAM) and a sample chamber installed with a
mirror optic for powder samples. For detection, a R2658P single-pho-
ton counting photomultiplier tube (Hamamatsu) was used. All lumines-
ˇ
K. Kirakci, P. Kubát, M. Kucˇeráková, V. Sícha, H. Gbelcová, P.
Lovecká, P. Grznárová, T. Ruml, K. Lang, Inorg. Chim. Acta
2016, 441, 42–49.
cence spectra were recorded with a spectral resolution of 1 nm, a dwell [12] Y. Saito, H. K. Tanaka, Y. Sasaki, T. Azumi, J. Phys. Chem. 1985,
89, 4413–4415; T. Azumi, Y. Saito, J. Phys. Chem. 1988, 92,
1715–1721; H. Miki, T. Ikeyama, Y. Sasaki, T. Azumi, J. Phys.
Chem. 1992, 96, 3236–3239.
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.
[13] N. Kitamura, Y. Ueda, S. Ishizaka, K. Yamada, M. Aniya, Y. Sa-
saki, Inorg. Chem. 2005, 44, 6308–6313.
[14] G. M. Sheldrick, Acta Crystallogr., Sect. A 2008, 64, 112–122.
[15] V. Kolesnichenko, L. Messerle, Inorg. Chem. 1998, 37, 3660; M.
Ströbele, T. Jüstel, H. Bettentrup, H.-J. Meyer, Z. Anorg. Allg.
Chem. 2009, 635, 822–827.
[16] M. Ströbele, C. Castro, R. F. Fink, H.-J. Meyer, Angew. Chem.
Int. Ed. 2016, 55, 4814–4816.
Synthesis of (W6I8)Cl4
Received: September 5, 2016
W6Cl12 + 2W6I12 Ǟ 3[W6I8]Cl4
(1)
Published Online: ᭿
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M. Ströbele, D. Enseling, T. Jüstel, H. -J. Meyer* .......... 1–5
(W6I8)Cl4 – A Basic Model Compound for Photophysically
Active [(W6I8)L6]2– Clusters?
Z. Anorg. Allg. Chem. 0000, 0–0
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