The elusive halides VCl5, MoCl6, and ReCl6

Article abstract of DOI:10.1002/anie.201207552

Vanadium pentachloride and molybdenum and rhenium hexachloride are all thermally unstable but can be prepared by metathesis from the corresponding fluorides with BCl₃ at low temperatures. MoCl₆ is structurally related to β-WCl₆, and ReCl₆ to α-WCl₆. VCl₅ is a dimer in the solid state (see structure; V red, Cl green). Copyright

Full text of DOI:10.1002/anie.201207552

Angewandte
Chemie
DOI: 10.1002/anie.201207552
Highest Oxidation States
The Elusive Halides VCl , MoCl , and ReCl **
5
6
6
Farhad Tamadon and Konrad Seppelt*
It is common knowledge that fluorine can stabilize high
oxidation states. Well-known examples include ReF7 and
PtF . Even more striking is the recent generation of HgF in
6
4
[
1]
a neon matrix at 4 K. Compounds such as AuF , PdF , and
6
5
PdF could also exist under matrix-isolation conditions. In
6
contrast, the realm of high-valent metal chlorides is not only
smaller than that of fluorides, it is fraught with reports that are
either incorrect or highly questionable. This is certainly the
situation with the three chlorides addressed in this work,
VCl , MoCl , and ReCl .
5
6
6
VF is a well-known compound. It is a strong oxidant,
5
which suggests that VCl might be unstable with respect to Cl2
5
and the well-characterized liquid VCl , the highest known
4
chloride of vanadium. Note that the homologous compounds
NbCl and TaCl were prepared decades ago and are well-
Figure 1. Structure of VCl in the solid state, ORTEP representation,
ellipsoids set at 50% probability. Bond lengths are given in pm.
5
5
5
characterized. In 1969 and 1970, the reaction of VOCl with
3
PCl was reported to produce “VCl ” with an unbelievably
amounts of MoCl could be prepared by refluxing H MoO in
5
5
6
2
4
[
2]
[9]
high melting point of 260–2658C. It was quickly shown by
SOCl or by chlorinating molybdenum metal. It was also
2
+
À
others that the product was actually a mixture of PCl VCl5
reported that MoCl could be sublimed. However, the only
4
6
+
À
[3]
and PCl4 VOCl4
.
characterization reported was X-ray powder diffraction, and
[
10]
[11]
We attempted to prepare VCl by two different routes.
it is known that b-WCl6 and b-MoCl4 crystallize in the
same space group type and have very similar lattice constants.
In view of the facts that we could not reproduce these results,
5
The first was irradiation of a VCl /Cl2 solution at low
4
temperature, similar to the procedure reported for the
[
4]
synthesis of metastable AsCl5. However, upon irradiation,
the orange color of the VCl /Cl solution became black, and
and that genuine MoCl prepared in this work (see below)
6
cannot be sublimed, it is likely that the microcrystalline
4
2
VCl could not be detected spectroscopically: The Raman
product prepared in the 1967 work was b-MoCl . It is certainly
5
4
spectrum of the product exhibited only an extremely intense
fluorescence background and the V NMR spectrum showed
no signals other than a trace amount of VOCl3.
possible that SOCl could have acted as a reducing agent.
2
5
1
We prepared MoCl by reacting MoF and BCl . The
6
6
3
reaction is rapid at room temperature and very slow at
The second approach was the reaction of VF with BCl at
À788C. At À258C shiny black hexagonal plates crystallized.
5
3
À788C. The reaction mixture became dark violet and
The structure of MoCl , determined from diffraction data
6
precipitated black crystals. The structure of VCl is shown in
collected at À1408C is shown in Figure 2. It is isostructural
5
Figure 1. It is a chloride-bridged dimer, similar to the
with b-WCl and UCl , and like these two structures it is made
6
6
[
5]
[6]
[7]
[8]
structures of Re Cl , Nb Cl , Ta Cl , W Cl , and
up of rigorously octahedral MoCl molecules. The unit cell
2
10
2
10
2
10
2
10
6
[
4]
Sb Cl . A Raman spectra could not be obtained, again
contains two independent, nearly identical molecules. The
2
10
because of an intense fluorescence background. However,
5
1
a
V NMR spectrum recorded at low temperature exhibited
a new resonance at d = 984.6 ppm (d(VOCl ) = 0). The new
3
compound vanadium pentachloride melts at À108C with
decomposition. It is even less stable in solution and decom-
poses above À408C.
WCl and UCl are the only known binary hexachlorides.
6
6
They are very stable, and melt without decomposition at 275
and 177.58C, respectively. In 1967, it was reported that small
[
*] Dr. F. Tamadon, Prof. Dr. K. Seppelt
Institut fꢀr Anorganische und Analytische Chemie
Freie Universitꢁt Berlin
Fabeckstrasse 34–36, 14195 Berlin (Germany)
E-mail: seppelt@zedat.fu-berlin.de
[**] We thank the Deutsche Forschungsgemeinschaft for the support of
Figure 2. Structure of a-MoCl , viewed along 001. ORTEP representa-
6
this work.
tion, ellipsoids set at 50% probability.
Angew. Chem. Int. Ed. 2013, 52, 767 –769
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
767

.
Angewandte
Communications
structures of all three hexachlorides are best described as
hexagonal close-packed (HCP) lattices of Cl atoms with the
metal atoms in one-third of the Oh holes. In this low-
single-crystal structural determinations, and may, in any case,
[18]
be dynamic. This is indeed the case for crystalline ReF6.
The synthesis of ReCl6 has been controversial. The first
[
12]
temperature phase of MoCl , there is always a 25% disorder
report, the reaction of rhenium metal with Cl at 6008C,
2
6
[
13–16]
of the molybdenum atoms within the hexagonal chlorine
lattice. For comparison we have obtained the single-crystal
was later challenged by different groups.
More recently, it
was reported that ReCl can be electrochemically oxidized
À
6
[17]
structure of a- and b-WCl , as the previous structure
to ReCl6.
6
determinations were carried out on powder data. Both
modifications show no disorder.
The MoCl6 crystallization is spontaneous during its
formation, and there is no possibility for recrystallization
owing to its insolubility at temperatures below 08C and its
thermal instability at elevated temperatures.
The F/Cl exchange method which we used to prepare
MoCl6 in this work has been used by others to prepare
[
12,13,15]
ReCl6,
but apparently only impure samples of the
hexachloride were obtained. In this work, we reacted ReF6
with BCl3 very slowly at À258C, which resulted in the
formation of shiny black hexagonal crystals. The structure of
ReCl is shown in Figure 2. It is isostructural with a-WCl . As
When the single crystal used to collect the diffraction data
at À1408C was warmed to À108C, a single-crystal-to-single-
crystal phase change occurred. When diffraction data of this
new phase were collected again at À1408C, the HCP lattice of
6
6
in the case of crystalline MoCl , crystals of ReCl were always
6
6
twinned. The structure can be solved when the overlapping
reflections were excluded from the refinement.
Cl atoms is virtually unchanged. In this b-MoCl , the
The crystallinity of ReCl , like with MoCl , cannot be
6
6
6
molybdenum atoms are now disordered over two positions
improved by recrystallization, which is again due to its
insolubility at temperatures below 08C and its thermal
instability above this temperature.
at z = 0 and z = 0.5 in a 1:1 manner, while in a-MoCl there is
6
a 1:2 distribution at z = 0 and z = 0.5 (or 5:7, if the 75:25%
disorder is taken into account). The interchange of the a- into
the b-modification can be envisioned as a shift of the MoCl6
columns along the z axis until complete disorder is achieved.
The b-modification has very weak and diffuse reflection
which can be fitted into the lattice if a and b are increased by
a factor of four each. This superstructure could not be solved.
Only when this problem is solved a reliable description for the
interconversion between a-MoCl6 and b-MoCl6 might be
possible.
As expected, molecules of ReCl are octahedral to within
6
the uncertainties. Distances are listed in Table 1. Like MoCl6,
samples of ReCl decomposed to ReCl during several days at
6
5
room temperature with evolution of Cl2.
Experimental Section
VCl : Boron trichloride (3 g, 25.6 mol) was condensed into a PFA
5
tube (PFA = poly(perfluorovinyl ether-co-tetrafluoroethylene) at
Differential scanning calorimetry of samples of MoCl6
revealed a transition at 1008C that is not observed for samples
of MoCl . Above 1008C the DSC curves for MoCl and MoCl
À1968, traces of Cl
2
and HCl were pumped off at À788C. VF
5
(
300 mg, 2 mol) was condensed in at À1968C. Warming to À608C
affords a dark violet solution, and slow cooling to À788C gave black
5
6
5
crystals, while much of the VCl remained in solution. X-ray data at
5
are identical. We conclude that solid MoCl6 decomposes
À1408C: a = 594.2(2), b = 644.6(2), 879.2(2) pm, a = 108.94(1), ß =
rapidly to MoCl at about 1008C. At room temperature, the
6
3
¯
90.95(1), g = 116.08(1)8, V= 279.0(2) ꢀ 10 pm . Z = 2, triclinic, P1,
5
5
1
decomposition of MoCl , with evolution of Cl , required
6
2
R = 0.0309, wF = 0.0983. V-NMR (VOCl ), d = 984.6 ppm; cf. VF :
1
2
3
5
several days.
d = À802 ppm.
1
[11]
The d molecule ReCl , like ReF ,
a Jahn–Teller distortion. Calculated structural parameters
for molecular ReCl6 (Table 1) show deviations from Oh
symmetry as in ReF , such as a compression along a fourfold
should exhibit
MoCl
,
6
ReCl
6
:
BCl
3
(3 g, 25.6 mmol) and MoF
6
(300 mg,
6
6
1.43 mmol) or ReF6 (300 mg, 1 mmol) were reacted as described
above. Upon warming to À208C, the solution turned dark red in both
cases. Within 2–3 hours, hexagonal black crystals of MoCl6 were
formed that after few more hours turned into black needles. The
ReCl6 crystallization requires 12–14 hours, giving hexagonal black
6
axis, that are so small that they are unlikely to be observed in
plates. a-MoCl : X-ray data at À1408C: a = 1034.1(3), c = 554.0(2)
6
6
3
¯
pm, V= 513 ꢀ 10 pm , Z = 3, trigonal, P3m1, R = 0.0383, wR =
1
2
[
19]
[20]
Table 1: Experimental and calculated bond lengths in MoCl and
6
0.1209. b-MoCl
:
X-ray data at À1408C: a = 596.4(1), c =
6
ReCl , compared to WCl .
6 3
6
6
553.9(1) pm, V= 170.63 ꢀ 10 pm , Z = 1, hexagonal, P6 mcm, R =
3
1
À1
0
.0193, wR = 0.0413. Raman (solid): n˜ = 403 (10, A ), 175 cm (4,
MÀCl
2
1
g
T ). ReCl : X-ray data: a = 597.5(2), c = 1645.3(9) pm, V= 508.7 ꢀ
2
g
6
a-MoCl , X-ray
230.4(9) molecule 1
28.4(9), 231.8(9) molecule 2
231.1(1)
232.3
6
3
¯
6
1
0 pm , rhombohedral, R3, R = 0.0282, wR = 0.0489. Raman (solid):
1
2
2
À1
n˜ = 404(10), 169(3) cm
.
b-MoCl , X-ray
MoCl , MP2, calcd
6
6
Received: September 18, 2012
Revised: October 19, 2012
Published online: November 22, 2012
ReCl , X-ray
226.3(6)
230.6(2ꢂ)
6
ReCl , MP2, calcd
6
2
31.5(4ꢂ)
a-WCl , X-ray
227.5(3)
6
b-WCl , X-ray
228.5(1) molecule 1
6
2
28.5(1), 228.6(1) molecule 2
Keywords: molybdenum hexachloride · rhenium hexachloride ·
vanadium pentachloride
WCl , MP2, calcd
231.8
.
6
768
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Angew. Chem. Int. Ed. 2013, 52, 767 –769

Angewandte
Chemie
9
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1
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e-mail: crysdata@fiz-karlsruhe.de), on quoting the depository
numbers CSD 425143 (a-MoCl ), 425144 (b-MoCl ), 425145
[
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[
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[
[
[
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6
6
[
9] M. Mercer, Chem. Commun. 1967, 119.
6 5 6 6
(ReCl ), 425146 (VCl ), 425147 (a-WCl ), and 425148 (b-WCl ).
[
[
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core potential for 28 electrons, Re: 8s7p6d[6s5p3d], core
potential for 60 electrons, Cl, V: 6-311 + g(d,p).
1
Angew. Chem. Int. Ed. 2013, 52, 767 –769
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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769