Facile reduction of early transition metal halides with nonconventional, mild reductants (see abstract)

Article abstract of DOI:10.1016/j.ica.2002.12.001

Reduction of MoCl₅ with Bi in a sealed borosilicate ampule at 350°C, followed by sublimation of by-product BiCl₃ and addition of aqueous hydrochloric acid, yielded chloromolybdic acid, (H₃O) ₂Mo₆(μ₃-Cl)₈Cl ₆·6H₂O, in 80% yield (unoptimized) after double recrystallization. Chloromolybdic acid was thermolyzed to Mo₆Cl ₁₂ in 95% yield.

Full text of DOI:10.1016/j.ica.2002.12.001

Inorganica Chimica Acta 357 (2004) 644–648
www.elsevier.com/locate/ica
Facile reduction of early transition metal halides with
nonconventional, mild reductants. 6. A new, lower-temperature,
solid-state synthesis of the cluster hexamolybdenum dodecachloride
Mo6Cl12 from MoCl5, via chloromolybdic acid,
(H3O)2[Mo6(l3-Cl)8Cl6] ꢀ 6H2O
Daniel N.T. Hay, Juan A. Adams, Jason Carpenter, Stephanie L. DeVries,
John Domyancich, Bruce Dumser, Shawn Goldsmith, Melissa A. Kruse, Angela Leone,
Farid Moussavi-Harami, Jennifer A. OÕBrien, Jennifer R. Pfaffly, Michael Sylves,
*
Parisa Taravati, Jacob L. Thomas, Breck Tiernan, Louis Messerle
Department of Chemistry, The University of Iowa, 461 Chemistry Building, Iowa City, IA 52242-1294, USA
Received 2 November 2002; accepted 5 December 2002
Abstract
Reduction of MoCl5 with Bi in a sealed borosilicate ampule at 350 °C, followed by sublimation of by-product BiCl3 and addition
of aqueous hydrochloric acid, yielded chloromolybdic acid, (H3O)2Mo6(l3-Cl)8Cl6 ꢀ 6H2O, in 80% yield (unoptimized) after double
recrystallization. Chloromolybdic acid was thermolyzed to Mo6Cl12 in 95% yield.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Molybdenum dichloride; Hexamolybdenum dodecachloride; Molybdenum pentachloride reduction; Bismuth; Chloromolybdic acid
1. Introduction
compounds to be synthesized (in 1859) [3], although the
actual structure was determined far later [4]. A large
body of octahedral Mo6 cluster coordination chemistry
has been reported [5–17] over the past four decades.
Mo6Cl12, discrete cluster derivatives, and related mate-
rials have attracted considerable photochemical interest
[18–32] because of their phosphorescence, luminescence,
and electrogenerated chemiluminescence. The structures
and interesting photochemistry have led to considerable
theoretical effort [33–41] in order to understand cluster
bonding and excited states. The chemistry of chalco-
genide derivatives has been explored [42–46], in part as a
possible molecular precursor route to Chevrel phases.
Mo6Cl12 and its derivatives have been used in catalysis
[47–49], intercalation chemistry [50], radiochemistry
[51], and electrochemical, sensor, and conductor re-
search [52–61].
There are two principal structure types for octahedral,
hexanuclear cluster halides of mid-valent second- and
third-row early transition metals. Group 5 hexanuclear
clusters possess terminal and octahedral edge–bridging
halides, whereas Group 6 hexanuclear clusters have ter-
minal and octahedral face–bridging halides. An alternate
description of the Group 6 cluster core is a cubic array of
l3-halides with metal atoms at the centers of each face.
Both cluster types are found as discrete molecular clus-
ters or as clusters linked in extended arrays by sharing of
halides [1,2].
Molybdenum dichloride (hexamolybdenum dodeca-
chloride, Mo6Cl12) was one of the first metal cluster
* Corresponding author. Tel.: +1-319-335-1372; fax: +1-319-335-
1270.
E-mail address: lou-messerle@uiowa.edu (L. Messerle).
Many synthetic routes to Mo6Cl12 and chloromolyb-
dic acid ((H3O)2[Mo6(l3-Cl)8Cl6] ꢀ 6H2O, the discrete,
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2002.12.001

D.N.T. Hay et al. / Inorganica Chimica Acta 357 (2004) 644–648
645
molecular cluster chloro acid derivative) have been re-
ported [62–72], and one has been developed for under-
graduate laboratory instruction in high temperature,
solid-state inorganic synthesis [73]. The most commonly
used methods, usually refinements of earlier methods
are: (1) conproportionation of MoCl5 and Mo metal at
650 °C to MoCl3, followed by disproportionation at
650 °C to Mo6Cl12 and MoCl4 and recycling/conpro-
portionation of MoCl4 with Mo metal, for an overall
yield of 85–91% [68], (2) reduction, with concomitant
mechanical shaking, of MoCl5 with Al in a chloroalu-
minate melt at 200 °C for 6 h and then at 450 °C for 48 h,
for a 98% yield of chloromolybdic acid [71], and (3)
high temperature (720 °C) conproportionation of MoCl5
and Mo in the presence of NaCl for a Mo6Cl12 yield
of 70–80% [72]. Two of these methods require sub-
stantial labor or quartz tubing because of the elevated
temperatures.
with a vortex mixer prior to introduction into the am-
puleÕs end receiver chamber using a long stem funnel in
order to minimize contamination of the constriction
surfaces. The ground-jointed end was closed with a gas
inlet adapter, the ampule evacuated using a Schlenk line,
and the ampule flame-sealed under vacuum.
MoCl5 (Cerac Inc., Milwaukee, WI), Bi (325 mesh,
Cerac), hydrochloric acid (12 M, Fisher), 8-hydroxy-
quinoline (Fisher), NaOH (pellets, EM Science), acetic
acid (glacial, EM Science), and ethanol (95%, Phar-
maco) were used as received. Powder X-ray diffraction
was performed on a Siemens D5000 diffractometer, with
moisture-sensitive samples protected by a 5 lm film of
polyethylene.
2.1. Preparation of Mo6Cl12 via reduction of MoCl5 with
bismuth
Chloromolybdic acid [63,68,71,74,75] is easily pre-
pared from Mo6Cl12 via dissolution in and recrystalli-
zation from concentrated hydrochloric acid, and can be
thermolyzed in vacuo back to Mo6Cl12. The chloro acid
is often used as an intermediate in the purification of
Mo6Cl12.
We recently reported several convenient, lower tem-
perature, and high yield solid-state syntheses of the
hexatungsten cluster compounds W6Cl12 and (H3O)2-
[W6(l3-Cl)8Cl6](H2O)x by reduction of WCl6 with the
nonconventional reductants bismuth, antimony, and
mercury [76]. This simple approach has been imple-
mented in a local undergraduate inorganic synthesis lab
course [77]. We have extended this versatile, lower-
temperature methodology to the preparation of the
known lower tungsten chloride (WCl4)x [78], the new
tungsten chloride clusters W3Cl10 and Na3W3Cl13 [79],
and the known hexatantalum clusters Ta6X14 (X ¼ Cl,
Br) [80,81]. This success led us to examine the applica-
bility of this approach [77] to the synthesis of Mo6Cl12.
An ampule with MoCl5 (5.00 g, 18.3 mmol) and Bi
(3.825 g, 18.3 mmol) in the end reaction chamber was
placed in the center of a horizontal tube furnace and the
temperature slowly raised to 230 °C over 2 h and then to
350 °C over 2 h. The ampule was repositioned with part
of the receiver chamber out of the furnace in order to
remove BiCl3 by sublimation. Heating was continued at
350 °C for 24 h. After cooling, the ampule was opened in
the glovebox. The crystalline, inhomogeneous nonvola-
tiles, dark brown with areas of yellow, weighed 4.065 g.
This weight was consistent with appreciable bismuth
content since the theoretical yield of Mo6Cl12 was
3.053 g.
A portion of the product (2.587 g) was treated with 25
ml of 12 M HCl with agitation, resulting in a slight exo-
therm. The mixture was dissolved with heating and re-
crystallized as described below, resulting in 1.025 g of
orange-yellow needles of (H3O)2[Mo6(l3-Cl)8Cl6](OH2)x
(58% yield, for x ¼ 6, based on MoCl5).
In a somewhat more labor-intensive procedure re-
sulting in improved yield, MoCl5 (5.00 g, 18.3 mmol)
and Bi (3.825 g, 18.3 mmol) were sealed in the receiver
chamber of an ampule. The ampule was placed in the
center of a horizontal tube furnace, the temperature
slowly raised to 230 °C over 2 h and then to 350 °C over
2 h. The ampule was heated at 350 °C for 2.5 days. After
cooling to 100 °C, the end of the receiver chamber was
moved out of the furnace. The furnace was heated to
350 °C over 2 h and then at 350 °C for 12 h. The ampule
was allowed to cool and then removed from the furnace.
The yellow-brown nonvolatile material was homoge-
nized by shaking, the ampule returned to the center of
the furnace, and the ampule heated to 350 °C over 3 h.
The end of the receiver chamber was moved out of the
furnace, the furnace reoriented to a slight angle (10°–
15°) from the horizontal, and the ampule heated at
350 °C for 24 h. The ampule was allowed to cool and
opened in the glovebox. The homogeneous, crystalline,
2. Experimental
Moisture-sensitive precursors and products were
manipulated in a Vacuum Atmosphere glovebox under a
N2/He atmosphere. Thermolyne Model 21100 (61200
°C) and Marshall Model 1046 single-zone (62000 °C)
tube furnaces equipped with positionable thermocouples
were used in solid-state syntheses. All syntheses em-
ployed dual-chamber borosilicate glass ampules of 25
mm OD and 30–60 ml total chamber volume, with a 14/
20 or 19/22 ground glass joint at one end and constric-
tions between the end reaction chamber and receiver
chamber and between the receiver chamber and joint.
Ampules were oven dried at 130 °C overnight and then
cooled under evacuation in the glove box antechamber.
Reactants were homogenized in a 20 ml scintillation vial

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D.N.T. Hay et al. / Inorganica Chimica Acta 357 (2004) 644–648
olive green/brown nonvolatiles weighed 4.066 g (theory
for Mo6Cl12, 3.053 g).
A portion of this solid (2.000 g) was treated with 12
M HCl as described below and recrystallized to yield
20 min. To this suspension 30% H2O2 was added
dropwise (8–12 drops) until evolution of gas was ob-
served. The mixture was allowed to stand for ꢁ1 h,
during which all solids dissolved to yield a yellow so-
lution that bleached while standing. The nearly color-
less solution was transferred to a 125 ml Erlenmeyer
flask and the scintillation vial repeatedly rinsed with
water to give a final volume of 40–50 ml. The Erlen-
meyer flask was placed in a boiling water bath for 30
min in order to complete oxidation of molybdenum
species and to destroy excess H2O2; during this time the
solution became colorless. The flask was then removed
from the bath and the pH adjusted to ꢁ5 with acetic
acid. The flask was returned to the water bath and
heated for 30 min. To the heated solution 5 ml of a 0.25
M ethanolic 8-hydroxyquinoline solution was slowly
added, resulting in a yellow precipitate. The mixture
was stirred and returned to the water bath for 15 min.
8-Hydroxyquinoline solution was added dropwise to
check for completeness of precipitation. The precipitate
was filtered hot through a tared medium porosity frit-
ted funnel, rinsed with 100–150 ml of hot water, dried
overnight at ꢁ130 °C, and weighed.
1.456
g
of orange-yellow chloromolybdic acid,
(H3O)2[Mo6(l3-Cl)8Cl6](OH2)x (80% yield, for x ¼ 6,
based on MoCl5).
X-ray diffraction patterns for chloromolybdic acid
from both procedures were empirically identical.
A recrystallized sample of chloromolybdic acid
(0.8095 g) was thermolyzed in vacuo using the method of
Michel and McCarley [43] by slowly (2.5 h) raising the
temperature to 350 °C and heating for 24 h. The Mo
analysis of the resulting 0.632 g (95.0% recovery based on
chloromolybdic acid) orange-yellow material was con-
sistent with approximately anhydrous Mo6Cl12 (Anal.
Calc. for Cl2Mo: Mo, 57.50. Found (average of two
trials): Mo, 57.64%). The anhydrous Mo6Cl12 changed to
a paler color upon short exposure to air (10–15 min),
characteristic of water absorption by the sample. Mo
analysis of an air-exposed sample was consistent with the
neutral dihydrate Mo6(l3-Cl)8Cl4(OH2)2 (Anal. Calc.:
Mo 55.50. Found (average of three trials): 55.28%).
2.2. Procedure for isolation and purification of crude
Mo6Cl12 via recrystallization and thermolysis of
(H3O)2[Mo6(l3-Cl)8Cl6](OH2)x
3. Results and discussion
The reduction of MoCl5 with Bi (Eq. (1)) is a straight-
forward, low-temperature route to Mo6Cl12 via the inter-
mediate chloromolybdic acid. With a 28 h reaction time
Concentrated aqueous HCl (12 M, 25 ml per 2.0–4.0 g
of impure chloromolybdic acid) was added to the impure
product (after BiCl3 sublimation) in a 250 ml Erlenmeyer
flask (attached to a water bubbler for trapping HCl vapor)
and the mixture heated with intensive agitation to dissolve
the material. An initial hot filtration through a medium
porosity fritted glass funnel was necessary in order to
remove gray-black (presumably Bi-containing) insoluble
impurities. The solid was washed with hot concentrated
aqueous HCl in order to recover any product that may
have crystallized during filtration. The yellow filtrate was
allowed to cool, resulting in orange-yellow crystals. The
crystalline product was returned to the Erlenmeyer flask
with aqueous HCl, redissolved by heating, and recovered
by filtration after slow cooling. One to three recrystalli-
zations were sufficient to obtain orange-yellow needles of
(H3O)2[Mo6(l3-Cl)8Cl6](OH2)x.
6MoCl5 þ 6Bi ! Mo6Cl12 þ 6BiCl3
ð1Þ
the chloromolybdic acid yield was 58%. The yield im-
proved to 80% with extra heating time (total of ꢁ4.5
days) and homogenization steps, and decreased mark-
edly with shorter heating times. The homogenization
step can be performed by either (1) opening the ampule
in a glove box, grinding the contents with a mortar and
pestle, and resealing the powder in a new ampule or (2)
temporarily removing the BiCl3 by sublimation, which
results in a free-flowing nonvolatile mixture that is easily
shaken and rehomogenized. The second approach re-
quires more time but less labor.
Bismuth is a useful reductant because it (1) is inex-
pensive, (2) is nontoxic, (3) is not impeded by a surface
oxide coating, (4) generates a volatile by-product that
does not exert significant pressures within an ampule
under reaction conditions, for improved safety, and (5)
does not readily overreduce Mo6Cl12. The dissolution of
bismuth in molten BiCl3 may also assist in the reduction.
The yields are unoptimized and, presumably, may be
improved by elevated temperature, increased reaction
time, or additional homogenization of reactants during
the procedure. The only disadvantage of this new route
is the extended time for BiCl3 sublimation; however,
sublimation can be performed unattended.
2.3. Analytical procedure for molybdenum
The molybdenum content was determined gravi-
metrically as MoO2(C9H6ON)2 (C9H6ON ¼ 8-hydrox-
yquinolinate) [82]. All samples were handled under
inert atmosphere in order to prevent exposure to
moisture until sample decomposition in the procedure.
In a 20 ml scintillation vial an accurately weighed
sample (ca. 50 mg) was decomposed with 7 ml of
aqueous 1 M NaOH, forming a brown suspension after

D.N.T. Hay et al. / Inorganica Chimica Acta 357 (2004) 644–648
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Acknowledgements
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This research was supported in part with funds
from the National Science Foundation (CHE-
0078701), the National Cancer Institute (1 R21
RR14062-01), the Roy J. Carver Charitable Trust
(Grant 01-93), and the Department of EnergyÕs En-
ergy-Related Laboratory Equipment Program (tube
furnace). The Siemens D5000 automated powder dif-
fractometer was purchased with support from the Roy
J. Carver Charitable Trust. L.M. acknowledges the
support services for manuscript preparation provided
by the Obermann Center for Advanced Studies at The
University of Iowa.
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