Microwave-assisted synthesis of dimolybdenum tetracarboxylates and a decanuclear osmium cluster

Article abstract of DOI:10.1016/j.jorganchem.2007.11.036

An improved method for the synthesis of dimolybdenum tetracarboxylates has been developed. The standard procedure for the preparation of these compounds involves extended reflux (up to 20 h) of a mixture of Mo(CO)₆ and the appropriate carboxylic acid, along with its anhydride, under an inert atmosphere. Using a microwave reactor and a closed vessel, Mo₂(acetate)₄, Mo₂(propionate)₄ and Mo₂(benzoate)₄ have been prepared in superior yields in less than 1 h. Furthermore, this new method does not require the use of the acid anhydride or the use of an inert gas. An improved method for the preparation of [N(PPh₃)₂]₂[Os₁₀C(CO)₂₄] directly from Os₃(CO)₁₂ has also been developed. The published synthesis of this osmium cluster calls for the pyrolysis of Os₃(CO)₁₁(py) for 64 h. Microwave irradiation of a mixture of Os₃(CO)₁₂ and diglyme in a closed vessel yields the desired product in just over 1 h.

Full text of DOI:10.1016/j.jorganchem.2007.11.036

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Journal of Organometallic Chemistry 693 (2008) 1712–1715
www.elsevier.com/locate/jorganchem
Note
Microwave-assisted synthesis of dimolybdenum tetracarboxylates
and a decanuclear osmium cluster
Kara D. Johnson, Gregory L. Powell *
Department of Chemistry and Biochemistry, Abilene Christian University, ACU Box 28132, Abilene, TX 79699, USA
Received 29 September 2007; received in revised form 21 November 2007; accepted 21 November 2007
Available online 31 December 2007
Abstract
An improved method for the synthesis of dimolybdenum tetracarboxylates has been developed. The standard procedure for the
preparation of these compounds involves extended reflux (up to 20 h) of a mixture of Mo(CO)6 and the appropriate carboxylic acid,
along with its anhydride, under an inert atmosphere. Using a microwave reactor and a closed vessel, Mo2(acetate)4, Mo2(propionate)4
and Mo2(benzoate)4 have been prepared in superior yields in less than 1 h. Furthermore, this new method does not require the use of the
acid anhydride or the use of an inert gas. An improved method for the preparation of [N(PPh3)2]2[Os10C(CO)24] directly from Os3(CO)12
has also been developed. The published synthesis of this osmium cluster calls for the pyrolysis of Os3(CO)11(py) for 64 h. Microwave
irradiation of a mixture of Os3(CO)12 and diglyme in a closed vessel yields the desired product in just over 1 h.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Microwave synthesis; Molybdenum acetate dimer; Dimolybdenum tetracarboxylate; Quadruple bond; Osmium carbonyl cluster; Carbide
cluster
1. Introduction
eliminate all traces of oxygen. For example, the standard
procedure for the preparation of Mo2(O2CCH3)4 involves
refluxing acetic acid, acetic anhydride and Mo(CO)6 under
N2 gas for 20 h to obtain a 37% yield [3]. The method is
improved with the use of selected solvents with higher
boiling points. As Cotton noted [1], ‘‘Superior yields
(80%) are obtained only when a solvent such as diglyme
or 1,2-dichlorobenzene is used.” However, a 20–24 h reflux
period is still required. A significant hazard associated with
this reaction is the ease with which Mo(CO)6 sublimes from
the reaction mixture and deposits in the reflux condenser.
The reaction setup has been known to explode as the con-
denser becomes completely blocked with small white crys-
tals of Mo(CO)6.
We report herein the results of our attempts to improve
upon the syntheses of dimolybdenum tetracarboxylates by
using a microwave energy source and a closed reaction
vessel. The use of microwave irradiation in chemical syn-
thesis has grown increasingly more important and wide-
spread [4]. Advantages include acceleration of reaction
rates, improved yields and simplified procedures. Using
Tetra-l-acetatodimolybdenum(II), Mo2(O2CCH3)4, is
by far the most important synthon for preparing complexes
with Mo–Mo quadruple bonds. It is the starting material of
choice for many of the thousands of known quadruply
bonded dimolybdenum compounds [1]. It has also found
use as a chromophore in the determination of the absolute
configuration of UV–Vis transparent amino acids, hydroxy
acids, amino alcohols and 1,2- and 1,3-diols [2]. In such
chiroptical studies, Mo2(O2CCH3)4 is mixed with the chiral
molecule in DMSO and the circular dichroism spectrum of
the resulting solution is recorded and analyzed. Dimolyb-
denum tetracarboxylates are typically synthesized by
refluxing the carboxylic acid (along with its anhydride when
available) with hexacarbonylmolybdenum(0), Mo(CO)6,
under an inert atmosphere [1]. This often takes many hours
and results in a low yield even when measures are taken to
*
Corresponding author. Tel.: +1 325 674 2171; fax: +1 325 674 6988.
E-mail address: powellg@acu.edu (G.L. Powell).
0022-328X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.11.036

K.D. Johnson, G.L. Powell / Journal of Organometallic Chemistry 693 (2008) 1712–1715
1713
an open vessel in a modified conventional microwave oven,
2.2. Preparation of Mo2(O2CCH3)4 in a closed vessel
Hogarth and coworkers have prepared Mo2(O2CCH3)4 in
48% yield [5]. In the absence of an inert atmosphere, how-
ever, this experiment failed. We have found that a closed-
vessel microwave method allows for the preparation of
Mo2(O2CCH3)4 in yields as high as 89% without the use
of a co-solvent or an inert gas and without concern for
the buildup of Mo(CO)6 deposits. Several other dimolybde-
num tetracarboxylates can also be synthesized more effi-
ciently via this method, and our success in preparing
these Mo2(O2CR)4 compounds in the microwave reactor
led us to attempt the synthesis of a larger metal cluster
complex. Our initial target was the decanuclear osmium
cluster [N(PPh3)2]2[Os10C(CO)24]. The standard method
for its synthesis involves pyrolysis for 64 h [6]. We have
found that microwave irradiation can be used to make this
cluster in just over 1 h.
Mo(CO)6 (0.397 g, 1.50 mmol) was added to acetic acid
(7 mL, 122 mmol) in a 35-mL reaction vessel equipped with
a stir bar. The vessel was capped, placed in the microwave
reactor and irradiated at 300 W with the stirring rate set to
high. After 7 min the power automatically dropped because
the pressure was nearing the limit of 300 psi, but the power
remained between 270 and 290 W for 3 more minutes. The
maximum temperature reached was 185 °C and the maxi-
mum pressure reached was 287 psi. The reaction mixture
was cooled to 54 °C and briefly vented before being irradi-
ated for an additional 6 min at 300 W. The reaction mix-
ture was cooled to room temperature, and then filtered
through a glass frit. The yellow product was rinsed with
ethanol, hexane, and ether, then placed in a vacuum desic-
cator for 12 h. Yield: 0.284 g, 88.5%. IR (solid, ATR): 1507
s, 1491 s, 1439 vs, 1407 s, 1352 m, 1045 m, 1033 m, 960 m,
937 m, 899 s, 854 m, 674 vs cmÀ1. Anal. Calc. for
C8H12O8Mo2: C, 22.45; H, 2.83. Found: C, 22.39; H,
2.76%.
2. Experimental
All reactions were carried out in a Discover-S micro-
wave reactor (2455 MHz, CEM Corp., Matthews, NC)
that allows for a maximum temperature of 300 °C and a
maximum pressure of 300 psi. Thick-walled 35-mL
reaction vessels with Teflon-lined caps were provided by
CEM. Caution must be exercised due to the toxic nature
of CO and Mo(CO)6. All manipulations must be carried
out in a highly efficient fume hood. Extra care must be
taken when the reactions are under pressure; the
microwave reactor must be placed in the fume hood
and the hood sash must be left down until a few minutes
after the pressure has been released. Caution is also
advised when using concentrated acids. A new cap should
be used to seal the reaction vessel for each reaction
described below. All chemicals were purchased from
Aldrich and used as received. Infrared spectra were
recorded on a Nicolet Avatar 320 FTIR spectrophotome-
ter. The IR spectra of the dimolybdenum compounds
were obtained using solid samples in attentuated
total reflectance mode, hereafter denoted: solid, ATR.
Elemental analyses were performed by QTI of White-
house, NJ.
2.3. Preparation of tetra-l-propionatodimolybdenum(II),
Mo2(O2CCH2CH3)4
Mo(CO)6 (0.365 g, 1.38 mmol) was added to propionic
acid (8 mL, 107 mmol) and hexane (2 mL) in a 35-mL reac-
tion vessel equipped with a stir bar. The vessel was capped,
placed in the microwave reactor and irradiated for 22 min
at 300 W with the stirring rate set to high. The maximum
temperature reached was 190 °C and the maximum pres-
sure reached was 287 psi. The reaction mixture was cooled
to 30 °C and briefly vented before being irradiated for an
additional 10 min at 300 W. The mixture was cooled to
54 °C, removed from the microwave reactor, and placed
in a freezer at À10 °C for 5 h. The yellow product was fil-
tered off and washed with ethanol and diethyl ether. Yield:
0.238 g, 71.0%. IR (solid, ATR): 1506 vs, 1466 s, 1447 m,
1427 vs, 1382 m, 1296 s, 1082 m, 1074 mm 1005 m, 957
m, 888 vs, 859 s, 806 s, 735 m, 675 vs cmÀ1. Anal. Calc.
for C12H20O8Mo2: C, 29.77; H, 4.16. Found: C, 30.08; H,
3.84%.
2.4. Preparation of tetra-l-benzoatodimolybdenum(II),
Mo2(O2CC6H5)4
2.1. Preparation of tetra-l-acetatodimolybdenum(II),
Mo2(O2CCH3)4, in an open vessel
Mo(CO)6 (0.317 g, 1.20 mmol) and benzoic acid
(1.710 g, 13.99 mmol) were added to diglyme (2 mL) and
hexane (2 mL) in a 35-mL reaction vessel equipped with
a stir bar. The vessel was capped, placed in the microwave
reactor and irradiated for 10 min at 300 W with the stirring
rate set to high. The maximum temperature reached was
200 °C and the maximum pressure reached was 232 psi.
The reaction mixture was cooled to 54 °C and briefly
vented before being irradiated for an additional 5 min at
300 W. The mixture was cooled to 54 °C, removed from
the microwave reactor, and allowed to cool to room tem-
Mo(CO)6 (1.15 g, 4.36 mmol) was added to 25 mL of
acetic acid (437 mmol) and 10 mL of diglyme in a conven-
tional 100-mL round-bottom glass flask equipped with a
stir bar and connected to a large reflux condenser. The
flask was placed in the microwave reactor cavity in open-
vessel mode with the maximum temperature set to
135 °C. The mixture was irradiated at 150 W for 1 h, and
then allowed to cool to room temperature. The bright
yellow product was filtered off and washed with ethanol,
hexane and diethyl ether. Yield: 0.603 g, 64.7%.

1714
K.D. Johnson, G.L. Powell / Journal of Organometallic Chemistry 693 (2008) 1712–1715
perature. The orange-yellow product was filtered off and
washed with diethyl ether. Yield: 0.366 g, 90.0%. IR (solid,
ATR): 1490 s, 1478 m, 1438 m, 1400 vs, 1070 m, 1027 m,
959 m, 929 m, 896 m, 854 m, 842 s, 721 vs, 709 vs, 681
vs cmÀ1. Anal. Calc. for C28H20O8Mo2: C, 49.72; H,
2.98. Found: C, 49.69; H, 2.81%.
nitrogen in an open vessel for 1 h produced a 65% yield.
Although these open-vessel methods drastically reduce
the reaction time, they still require a large amount of sol-
vent and an inert atmosphere.
On the other hand, the closed-vessel microwave
method offers multiple advantages over the previously
published procedures. No longer is nitrogen or argon
gas required nor is anhydride or an extra solvent needed.
There is no need to monitor the buildup of sublimed
Mo(CO)6 in a condenser. The reaction time is short,
yet the desired product is formed in 89% yield. This
new method requires less energy and uses fewer chemi-
cals. The acetic acid-to-Mo(CO)6 mole ratio in the stan-
dard method is 230:1 while this ratio is 81:1 in our
closed-vessel microwave approach. Venting the reaction
vessel after the initial irradiation period is necessary in
order to release some of the CO gas generated by the
reaction. If the second period of irradiation is not per-
formed, then a small amount of unreacted Mo(CO)6
remains deposited near the top of the reaction vessel.
Alternatively, we have found that the reaction may be
carried out in a single step if it is done on a smaller scale
and for a longer period of time (less than 0.3 g of
Mo(CO)6 and over 30 min of irradiation).
We were unable to find any mention in the scientific lit-
erature of a yield associated with the preparation of
Mo2(propionate)4, although it was also first reported in
1964 [7]. Unlike the acetate, which is insoluble in acetic
acid and readily precipitates, the propionate is somewhat
soluble in propionic acid. When microwave reactions were
performed with Mo(CO)6 in propionic acid alone, the
highest yield obtained was 61%. We chose to include a
small amount of hexane to accelerate the dissolution of
Mo(CO)6 and to expedite the precipitation of the desired
product. The addition of hexane allowed us to achieve a
71% yield.
The standard synthesis of Mo2(benzoate)4 is fairly simi-
lar to that of Mo2(acetate)4 and Mo2(propionate)4 with the
exception of the use of diglyme as a solvent and a much
shorter reflux period of 4 h. The published procedure stres-
ses that the diglyme must be dried and that there must be
no presence of oxygen whatsoever [3]. Yields up to 65%
may be obtained by this method, but the closed-vessel
microwave technique offers several advantages. In our
experiments no measures were taken to exclude all mois-
ture from the diglyme nor was nitrogen or argon used to
exclude O2, yet we obtained a better yield of the same
orange-yellow solid in only 20 min. This new method also
reduces the amount of diglyme used per gram of Mo(CO)6
by 83%, although it does require the addition of a small
amount of hexane.
2.5. Preparation of [N(PPh3)2]2[Os10C(CO)24]
Os3(CO)12 (0.250 g, 0.276 mmol) was added to 8 mL of
diglyme in a 35-mL reaction vessel equipped with a stir
bar. The vessel was capped, placed in the microwave
reactor and irradiated at 300 W with the stirring rate set
to high. After 38.5 min, the temperature had reached
231 °C and the pressure had reached 287 psi, so the
reactor automatically reduced the power to 100 W over
the next 30 s. The power remained between 90 and
110 W for 9 additional minutes of irradiation. The
reaction vessel was cooled to 50 °C and briefly vented
before being irradiated at 300 W for 4 more min. At that
time, the power began to drop steadily over the next
1 min until it reached 140 W. The vessel was then cooled
to 54 °C and briefly vented again before being irradiated
at 300 W for an additional 4 min. The mixture was cooled
to 54 °C, poured into a watch glass over a steam bath,
and evaporated to a volume of less than 0.5 mL. The res-
idue was washed with warm hexane, dissolved in a mix-
ture of 12 mL of acetone and 3 mL of methanol, and
then filtered through a glass frit. A solution of 0.105 g
(0.164 mmol) of [N(PPh3)2]2Cl in 3 mL methanol was
added to the filtrate. Slow evaporation of the resulting
solution led to the formation of small crystals of dark
red-brown [N(PPh3)2]2[Os10C(CO)24], which was then
recrystallized from an acetone/methanol solution. Finally,
the product was filtered off and washed with cold 1-pro-
panol. Yield: 0.174 g, 57.4%. IR (CH2Cl2) t(CO): 2034
s, 1986 s cmÀ1. Anal. Calc. for C97H60N2O24P4Os10: C,
31.80; H, 1.65; N, 0.76. Found: C, 31.92; H, 1.54; N,
0.81%.
3. Results and discussion
3.1. Preparation of Mo2(carboxylate)4
The synthesis of Mo2(O2CCH3)4 has been known since
1964 [7] and its structure since 1965 [8]. As is the case in
the other dimolybdenum tetracarboxylates prepared
herein, the pair of quadruply bonded Mo atoms is bridged
by the four carboxylate ligands in an eclipsed conformation
so that the oxygen atoms for each ligand are directly oppo-
site one another. In 2004, Hogarth and coworkers reported
obtaining a 48% yield by heating a mixture of acetic acid,
diglyme and Mo(CO)6 under argon in an open vessel for
45 min in a modified conventional microwave oven [5].
We decided to perform a similar experiment with the com-
puter-controlled microwave reactor. Irradiation at 150 W
of a mixture of acetic acid, diglyme and Mo(CO)6 under
3.2. Preparation of [N(PPh3)2]2[Os10C(CO)24]
The [Os10C(CO)24]2À anion of this compound has
virtual Td symmetry and consists of a carbide atom sur-
rounded by a tetracapped octahedral osmium framework

K.D. Johnson, G.L. Powell / Journal of Organometallic Chemistry 693 (2008) 1712–1715
1715
with two terminal carbonyl ligands per osmium in the
Acknowledgements
central Os6 octahedron and three terminal CO ligands
per capping osmium atom. The published procedure for
the synthesis of this decanuclear osmium cluster involves
converting Os3(CO)12 to Os3(CO)11(C5H5N), sealing it
under vacuum in a Carius tube and heating it at 250 °C
for 64 h to obtain a 65% yield [6]. This represents a 58%
yield based on Os3(CO)12 since Os3(CO)11(py) is prepared
from Os3(CO)12 in 90% yield [6]. Previous attempts to pre-
pare the Os10 cluster by the pyrolysis of Os3(CO)12 resulted
in only a 5% yield, while the pyrolysis of Os3(CO)11(py) for
15 h instead of 64 h gave a 15% yield [6]. The microwave-
assisted technique proved superior once again since a
57% yield was obtained directly from Os3(CO)12 in just
over 1 h.
We gratefully acknowledge the financial support of The
Welch Foundation (Grant R-0021). This paper is dedicated
to the memory of F.A. Cotton in recognition of his many
important contributions to inorganic and organometallic
chemistry. GLP remains indebted to Prof. Cotton for intel-
lectual support and scientific inspiration for over 25 years.
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enhanced reaction rates, improved yields, and the use of
fewer reagents. The increased temperature and pressure
associated with a closed vessel allow for a more efficient
reaction compared to an open-vessel approach, although
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byproducts. All of the dimolybdenum tetracarboxylates
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quickly and in very high yield for a single step process.
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