Reactions of Os3(CO)12 with carboxylic acids in a microwave reactor; Synthesis of Os2(benzoate)2(CO) 6, a dinuclear osmium(I) compound with aromatic carboxylate ligands

Article abstract of DOI:10.1016/j.jorganchem.2012.09.014

Microwave heating is used to synthesize compounds of the type Os ₂(μ-O₂CR)₂(CO)₆, including the first example with aromatic carboxylate ligands. The one-step preparation of these complexes is straightforward and does not require an air-free reaction environment. The compounds for which R = Me (1) or Et (2) are produced in higher yield than previously reported by the microwave irradiation of Os ₃(CO)₁₂ for less than 15 min in acetic or propionic acid, respectively. This method may also be used to prepare the new compound Os ₂(μ-O₂CH)₂(CO)₆ (3) by reaction with formic acid, but a higher yield is obtained when 1,2-dichlorobenzene is used as a co-solvent. Irradiating a mixture of Os₃(CO)₁₂ and excess benzoic acid in 1,2-dichlorobenzene gives another new complex Os ₂(μ-O₂CC₆H₅)₂(CO) ₆ (4). X-ray crystallographic analyses of 3 and 4 reveal molecular structures similar to that of 1 with approximate C_2v symmetry. When the molar ratio of benzoic acid to Os₃(CO)₁₂ is 1:1, then the major product is the trinuclear cluster Os₃(μ-H)(μ-O ₂CC₆H₅)(CO)₁₀ (5) instead of a dinuclear compound. This represents the first instance in which a cluster of this type has been produced directly from Os₃(CO)₁₂ instead of through a multi-step procedure.

Full text of DOI:10.1016/j.jorganchem.2012.09.014

Journal of Organometallic Chemistry 723 (2013) 103e107
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Note
Reactions of Os3(CO)12 with carboxylic acids in a microwave reactor; synthesis
of Os2(benzoate)2(CO)6, a dinuclear osmium(I) compound with aromatic
carboxylate ligands
,
Kayla J. Pyper a, Jade Y. Jung a, Brittney S. Newton a, Vladimir N. Nesterov b, Gregory L. Powell a
*
a Department of Chemistry & Biochemistry, Abilene Christian University, ACU Box 28132, Abilene, TX 79699, USA
b Department of Chemistry, University of North Texas, Denton, TX 76203, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Microwave heating is used to synthesize compounds of the type Os2(m-O2CR)2(CO)6, including the first
Received 25 August 2012
Received in revised form
19 September 2012
example with aromatic carboxylate ligands. The one-step preparation of these complexes is straight-
forward and does not require an air-free reaction environment. The compounds for which R ¼ Me (1) or
Et (2) are produced in higher yield than previously reported by the microwave irradiation of Os3(CO)12
for less than 15 min in acetic or propionic acid, respectively. This method may also be used to prepare the
Accepted 20 September 2012
new compound Os2(
1,2-dichlorobenzene is used as a co-solvent. Irradiating a mixture of Os3(CO)12 and excess benzoic acid in
1,2-dichlorobenzene gives another new complex Os2( -O2CC6H5)2(CO)6 (4). X-ray crystallographic
analyses of 3 and 4 reveal molecular structures similar to that of 1 with approximate C2v symmetry.
When the molar ratio of benzoic acid to Os3(CO)12 is 1:1, then the major product is the trinuclear cluster
m-O2CH)2(CO)6 (3) by reaction with formic acid, but a higher yield is obtained when
Keywords:
Microwave synthesis
Osmium carbonyl cluster
Diosmium(I) compound
Oxidative addition of carboxylic acids
Bridging carboxylate ligands
m
Os3(m-H)(m-O2CC6H5)(CO)10 (5) instead of a dinuclear compound. This represents the first instance in
which a cluster of this type has been produced directly from Os3(CO)12 instead of through a multi-step
procedure.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
Cotton in 1971 [4]. The molecular geometry was shown to have
approximate C2v symmetry with cis bridging carboxylate ligands as
Dinuclear osmium(I) carbonyl complexes with carboxylate
ligands are relatively rare. Lewis, Johnson and coworkers discov-
ered in 1969 that the diosmium(I) compounds Os2(m-O2CR)2(CO)6,
illustrated by structure A in Chart 1. Spectroscopic evidence
suggests that the other Os2( -O2CR)2(CO)6 compounds have
a similar structural arrangement.
m
R ¼ Me (1) or Et (2) could be synthesized by heating a mixture of
Os3(CO)12 and neat acetic acid or neat propionic acid in a sealed
Carius tube at 185 ꢀC [1]. Compound 1 was produced in 76% yield
after 5 h of heating, and 2 was prepared in 70% yield after 14 h of
heating. In 1987, Deeming et al. employed the same method to
To our knowledge, there have been no reports of direct reactions
of aromatic carboxylic acids with Os3(CO)12. Because of the diffi-
culty of removing the first two CO ligands from Os3(CO)12, the
standard practice is to initially convert this cluster into derivatives
with more labile ligands. Such activated clusters react with either
aliphatic or aromatic carboxylic acids to generate triosmium
synthesize Os2(m-O2CCHMe2)2(CO)6 in 71% yield after 6 h of heating
in isobutyric acid [2]. In 1988, Deeming’s group showed that
products of the type Os3(m-H)(m-O2CR)(CO)10, which have the
molecular structure represented by B in Chart 1. Thus the starting
heating a mixture of Os3(CO)12 and trifluoroacetic acid at 120 ꢀC for
only 2 h produced Os2(
m
-O2CCF3)2(CO)6 in 50% yield, while heating
materials Os3(CO)10(MeCN)2, Os3(CO)10(C6H8), Os3(CO)10(C8H14)2,
at higher temperatures led to the formation of mononuclear
products [3]. To date, only these four aliphatic carboxylic acids have
been used to prepare Os22þ compounds. An X-ray crystallographic
Os3(m-H)(m-OH)(CO)10, and the silica-anchored species Os3(m-H)(m-
OeSieOe)(CO)10 have been used to synthesize dozens of such
compounds [5e19].
analysis of Os2(
m
-O2CMe)2(CO)6 was carried out by Bullitt and
Microwave heating is becoming more widespread in the prep-
aration of coordination and organometallic complexes due to
reaction rate enhancements, energy savings, higher product yields,
automated control of reaction parameters, and expanded ranges of
reaction temperature and pressure [20]. Several recent publications
* Corresponding author. Tel.: þ1 325 674 2171; fax: þ1 325 674 6988.
E-mail address: powellg@acu.edu (G.L. Powell).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.09.014

104
K.J. Pyper et al. / Journal of Organometallic Chemistry 723 (2013) 103e107
R
R
O
O
O
O
O
R
O
C
O
O
C
C
Os
O
O
O
C
C
C
O
O C
Os
Os
CO
O
O
C
C
Os
Os
C
C
O
O
H
C
C
C
O
C
O
O
O
A
B
Chart 1. Principal products of the reactions of Os3 clusters with carboxylic acids.
have focused on microwave-promoted reactions of Os3(CO)12
,
,
mixture. Yield: 161.2 mg (94.5%) of Os2(
nCO, CHCl3): 2100 (m), 2066 (vs), 2014 (s) and 1998 (vs) cmꢁ1. Anal.
Calc. for C10H6O10Os2: C, 18.02; H, 0.91%. Found: C, 18.06; H, 0.97%.
1H NMR (CDCl3):
2.07 (s, CH3) ppm.
m-O2CCH3)2(CO)6 (1). IR
2ꢁ
including its conversion to Os3(m-H)2(CO)10
,
[Os10C(CO)24
]
(
Os3(CO)11(NCMe), Os3( -H)(
m
m-OH)(CO)10, and Os6(CO)18 [21e24].
We chose to use a microwave reactor to carry out reactions
between Os3(CO)12 and various carboxylic acids with the intent of
d
improving the yield of known Os2(m-O2CR)2(CO)6 compounds as
2.3. Synthesis of Os2(m-propionate)2(CO)6 (2)
well as synthesizing new complexes including one with bridging
aromatic carboxylate ligands. This note presents the results of our
investigations with formic, acetic, propionic and benzoic acids.
The same method used to prepare 1 was employed with 92.0 mg
(0.101 mmol) of Os3(CO)12 and 8 mL of propionic acid. The yield of 2
was 85.3 mg (80.7%). IR (nCO, CHCl3): 2099 (m), 2066 (vs), 2014 (s)
and 1997 (vs). Anal. Calc. for C12H10O10Os2: C, 20.75; H, 1.45%.
2. Experimental
Found: C, 21.39; H, 1.35%. 1H NMR (CDCl3):
(t, 3H, CH3) ppm.
d 2.34 (q, 2H, CH2), 1.10
2.1. Methods and materials
All syntheses were performed using a Discover-SP microwave
reactor (2455 MHz, CEM Corp., Matthews, NC) with thick-walled
35-mL glass vessels equipped with a magnetic stir bar and
a PFTE-lined cap. In each case, the reactor was set to a maximum
pressure of 300 psi, and the highest stir rate was used. Caution was
exercised due to the toxic nature of CO and metal carbonyl
compounds. All manipulations were carried out in a highly efficient
fume hood. Extra precautions were taken when the reactions were
under pressure; the microwave reactor was placed in a fume hood
and the hood sash was left down until a few minutes after the
pressure had been released. The Os3(CO)12 was purchased from
Strem; all other reagents and solvents were obtained from Aldrich.
All chemicals were used as received. Infrared spectra were obtained
using a Nicolet Avatar 320 FTIR with a CaF2 solution cell. 1H NMR
spectra were recorded on an EM360 NMR spectrometer. Prepara-
tive thin-layer chromatography (TLC) was carried out on Analtech
silica gel 60 (0.50 mm) plates. Columbia Analytical Services carried
out the elemental analyses.
2.4. Synthesis of Os2(m-formate)2(CO)6 (3)
A 91.3 mg (0.101 mmol) sample of Os3(CO)12 was added to 8 mL
of formic acid, and the mixture was irradiated at an initial power of
300 W. After 3 min of heating, the temperature reached 177 ꢀC and
the pressure reached 300 psi. At this point the microwave source
was turned off, the vessel was cooled to 60 ꢀC, and the pressure was
released. Microwave heating was then resumed, and the temper-
ature reached 177 ꢀC after 2.2 min. This temperature was main-
tained for 9 additional minutes, and then the reaction mixture was
evaporated under a stream of nitrogen. The residue was dissolved
in dichloromethane and filtered through silica gel. The crude
product formed upon slow evaporation. It was recrystallized from
a 1:3 dichloromethane/hexanes mixture. The yield of 3 was 60.6 mg
(62.8%).
In order to investigate the source of the pressure buildup
mentioned above, we conducted a control experiment in which
8 mL of formic acid was subjected to microwave heating in the
absence of Os3(CO)12. A pressure of 300 psi was reached after
4.4 min of irradiation at 180 ꢀC. After cooling to 25 ꢀC, the pressure
was still 190 psi. We assume that these high pressures are due to
the well-known thermal decomposition of formic acid to produce
hydrogen and carbon dioxide [25].
In an effort to minimize the decomposition of formic acid and
the resulting pressure buildup, we decided to attempt the prepa-
ration of 3 with 1,2-dichlorobenzene as a solvent. A 96.8 mg
(0.107 mmol) sample of Os3(CO)12 was added to 7 mL of 1,2-
dichlorobenzene and 2.5 mL of formic acid. After 4 min of micro-
wave heating, the temperature reached 177 ꢀC and the pressure
reached 155 psi. This temperature was maintained for 10 additional
minutes. The pressure reached a maximum of 162 psi. The reaction
mixture was evaporated under a stream of nitrogen, and the
2.2. Synthesis of Os2(m-acetate)2(CO)6 (1)
A 154.6 mg (0.1705 mmol) sample of Os3(CO)12 was added to
7 mL of glacial acetic acid, and the mixture was irradiated at an
initial power of 300 W. After 2 min, the temperature reached 180 ꢀC
and this temperature was maintained as the sample was irradiated
for an additional 10 min. Distilled water (70 mL) was added to the
resulting colorless solution, and the product precipitated as a white
solid. A portion of CH2Cl2 (55 mL) was then added to re-dissolve the
crude product, and the mixture was stirred for 10 min before being
placed in a separatory funnel. The CH2Cl2 layer was collected, dried
with anhydrous MgSO4, and then allowed to evaporate. The crude
product was recrystallized from a 1:3 dichloromethane/hexanes

K.J. Pyper et al. / Journal of Organometallic Chemistry 723 (2013) 103e107
105
product was isolated as described above. The yield of 3 was 81.1 mg
3. Results and discussion
(79.4%). IR (nCO, CHCl3): 2104 (m), 2071 (vs), 2021 (s) and 2004 (vs).
Anal. Calc. for C8H2O10Os2: C, 15.05; H, 0.32%. Found: C, 15.46; H,
3.1. Improved preparation of previously known diosmium(I)
compounds
0.41%. 1H NMR (CDCl3):
d 8.44 (s, CH). Single crystals for X-ray
analysis were grown by slow evaporation of a hexane/CH2Cl2
solution.
Compounds 1 and 2 were prepared much more efficiently with
microwave heating than with conventional heating. The acetate
compound was produced in 95% yield in only 12 min of microwave
irradiation, while the conventional method produced a 76% yield
with 5 h of heating. For the propionate compound, the new method
gave an 81% yield in 14 min compared to a 70% yield in 14 h for the
old method. Not only are the yields higher and the rates faster for
the microwave method, it is also simpler and more convenient to
perform. There is no need to carefully place the reactants in a Carius
tube, seal it with a torch under nitrogen or vacuum, and then cut it
open to retrieve the products. Instead reactants are easily loaded
into a wide-mouthed glass vessel exposed to room atmosphere and
a cap is snapped onto the top.
2.5. Synthesis of Os2(m-benzoate)2(CO)6 (4)
Benzoic acid (66.4 mg, 0.544 mmol) and Os3(CO)12 (70.4 mg,
0.0776 mmol) were added to 6 mL of 1,2-dichlorobenzene and the
mixture was irradiated at an initial power of 300 W. After 3.5 min
the temperature reached 188 ꢀC, and this temperature was main-
tained as the sample was irradiated for an additional 15 min. A light
yellow solution was produced. The solvent was removed by evap-
oration under a stream of N2 gas. The residue was dissolved in
CH2Cl2 and subjected to TLC with an eluent of 3.5:1 n-hexane/
CH2Cl2. Two bands were collected. The top band (Rf ¼ 0.69) con-
tained 2.9 mg of an unidentified light yellow product. IR (nCO
,
CHCl3): 2075 (m), 2065 (s), 2048 (m), 2027 (vs), 2017 (s, sh), 2003
3.2. Synthesis of new diosmium(I) compounds
(w),1982 (w). The second band (Rf ¼ 0.54) contained 61.4 mg (66.7%
yield) of colorless Os2(m-O2CC6H5)2(CO)6 (4). IR (nCO, CHCl3): 2099
The use of a microwave reactor allowed for the rapid, single-step
(m), 2067 (vs), 2015 (s), 2002 (vs), 1994 (w). Anal. Calc. for
preparation of the new compounds 3 and 4 directly from Os3(CO)12
.
C20H10O10Os2: C, 30.38; H, 1.27%. Found: C, 30.42; H, 1.30%. 1H NMR
The formate compound 3 may be synthesized by the same method
used to prepare 1 and 2 (using neat carboxylic acid), but the use of
1,2-dichorobenzene as a solvent eliminated several problems and
produced a higher yield. Our attempts to prepare 4 without
a solvent were unsuccessful, while the use of 1,2-dichorobenzene
as a solvent worked well. This benzoate complex is the first
(CDCl3):
d 8.07 (m, 2H, C6H5), 7.95 (m, 1H, C6H5), 7.44 (m, 2H,
C6H5) ppm. Crystals were grown by slow evaporation of a hexane/
CH2Cl2 solution.
2.6. Synthesis of Os3(m-H)(m-benzoate)(CO)10 (5)
example of
carboxylate ligands.
a Os2(m-O2CR)2(CO)6 compound with aromatic
Benzoic acid (9.9 mg, 0.081 mmol) and Os3(CO)12 (71.2 mg,
0.0785 mmol) were added to 8 mL of 1,2-dichlorobenzene in a 35-
mL reaction vessel equipped with a magnetic stir bar. The vessel
was sealed and irradiated at an initial power of 300 W. After 3 min,
the temperature reached 189 ꢀC. A temperature range of 188e
190 ꢀC was maintained as the sample was irradiated for an addi-
tional 12 min. A yellow-brown solution was produced. The solvent
was removed by evaporation under a stream of N2 gas. The residue
was dissolved in CH2Cl2 and subjected to TLC with an eluent of 3:1
n-hexane/CH2Cl2. Two bands were collected. The top band
3.3. Crystallographic characterization of 3 and 4
X-ray crystallographic analyses of 3 and 4 were carried out, and
the data collection and refinement parameters are given in Table 1.
The molecular structure of 3 is illustrated in Fig. 1. For 4, there are
two independent molecules in the unit cell, and the structure of
Table 1
(Rf ¼ 0.81) contained 38.4 mg (50.3% yield) of Os3(
m-H)(m-
X-ray crystallographic data and processing parameters for clusters 3 and 4.
O2CC6H5)(CO)10 (5). IR (nCO, CHCl3): 2113 (m), 2099 (w), 2074 (vs),
2064 (vs), 2024 (vs), 2017 (vs), 1986 (m, sh). Anal. Calc. for
C17H6O12Os3: C, 20.99; H, 0.62%. Found: C, 21.03; H, 0.65%. 1H NMR
Compound
3
4
CCDC entry no.
Cryst syst
897819
Triclinic
P ꢁ1
896037
Monoclinic
P2(1)/n
10.1543(6)
11.6656(6)
35.796(2)
90
92.845(1)
90
4235.0(4)
C20H10O10Os2
790.68
(CDCl3):
d
ꢁ12.19 (s, 1H, OsH), 7.27 (m, 2H, C6H5), 7.07 (m, 1H, C6H5),
Space group
6.52 (m, 2H, C6H5) ppm. The second band (Rf ¼ 0.59) contained
2.6 mg of an unidentified yellow product. IR (nCO, CHCl3): 2125 (m),
2082 (m), 2036 (s), 2024 (vs), 2004 (m), 1980 (s).
ꢀ
a, A
7.3402(6)
7.6918(6)
11.665(1)
75.890(1)
84.795(1)
79.924(1)
628.08(9)
C8H2O10Os2
638.50
ꢀ
b, A
ꢀ
c, A
a
, deg
, deg
b
2.7. X-ray crystallography
g
, deg
3
ꢀ
V, A
The crystal structure determination of compounds 3 and 4 were
carried out using a Bruker APEX2 CCD-based diffractometer
equipped with a low-temperature device and an Mo-target X-ray
tube. The X-ray data were collected at 100(2) K. Data collection,
indexing, and initial cell refinements were carried out using APEX2
[26], with the frame integrations and final cell refinements carried
out using SAINT [27]. An absorption correction was applied using
SADABS [28], and all non-hydrogen atoms were refined anisotrop-
ically. All hydrogen atoms were placed in idealized positions and
were refined using a riding model. The structures were examined
using the Addsym subroutine of PLATON to ensure that no addi-
tional symmetry could be applied to the models [29]. The structure
was solved and refined using SHELXTL [30]. Refinement details and
structural parameters for the compounds are summarized inTable 1.
Mol formula
FW
Formula units per cell (Z)
2
8
Dcalcd (Mg/m3)
3.376
2.480
ꢀ
l
(Mo K
a
), A
0.71073
20.258
0.71073
12.045
Absorption coeff (mmꢁ1
Abs corr factor
)
0.6209/0.1159
7607
0.5842/0.1725
49,875
Total reflections
Independent reflections
Data/res/parameters
2756
9392
2756/0/182
0.0180
9392/0/577
0.0258
R1a [I ꢂ 2
s(I)]
wR2b
0.0478
0.0664
GOF on F2
1.033
1.252/ꢁ1.862
1.017
1.122/ꢁ1.380
3
ꢀ
Dr(max), Dr(min) (e/A )
a
R1 ¼ Sj jFoj ꢁ jFcj j/SjFoj.
b
R2 ¼ {S[w(F2o ꢁ F2c)2]/S[w(Fo2)2]}1/2
.

106
K.J. Pyper et al. / Journal of Organometallic Chemistry 723 (2013) 103e107
Fig. 1. Thermal ellipsoid plot of the molecular structure of 3 at the 50% probability level.
one of these is illustrated in Fig. 2. Both of these new complexes
have a molecular geometry similar to that of 1 [4]. The carboxylate
ligands adopt a cis arrangement and two of the CO ligands are
axially coordinated while the other four are bound in equatorial
sites. The axial carbonyl ligands are bent toward the carboxylate
ligands and are more weakly bound to the metal atoms than the
equatorial CO ligands. Selected bond distances and angles for 1, 3,
and 4 are compared in Table 2. The OseOs bond distances get
Fig. 2. Thermal ellipsoid plot at the 50% probability level of the molecular structure of one of the two independent molecules of 4.

K.J. Pyper et al. / Journal of Organometallic Chemistry 723 (2013) 103e107
107
Table 2
Os3(CO)12 with other reagents such as di-, tri-, and tetracarboxylic
acids.
Selected average bond distances (Å) and angles (ꢀ) for Os2(
m
-O2CR)2(CO)6 complexes.
R ¼ H (3)
R ¼ C6H5 (4)
R ¼ CH3 (1)
Acknowledgments
Bond distances
OseOs
OseO
OseCax
OseCeq
2.7545(2)
2.124(6)
1.983(4)
1.877(4)
1.257(5)
1.124(5)
1.147(5)
2.748(1)
2.119(3)
1.989(8)
1.873(4)
1.265(5)
1.124(5)
1.150(5)
2.731(2)
2.076(6)
1.96(2)
1.79(1)
1.29(1)
1.10(2)
1.16(1)
We gratefully acknowledge the financial support of The Welch
Foundation (Grant R-0021).
CeOcarboxylate
C
C
axeOax
eqeOeq
Appendix A. Supplementary material
CCDC 897819 and 896037 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Center via
http://www.ccdc.cam.ac.uk/data_request/cif.
Bond angles
OseOseCax
OseOseCeq
OseOseO
168.0(6)
93(1)
83.0(3)
90.1(1)
83.4(7)
126.8(3)
169.8(4)
94.5(5)
83.0(3)
90.1(5)
84.1(3)
125.2(2)
170.5(6)
93.4(3)
83.5(2)
89.7(7)
83.5(4)
124(1)
C
eqeOseCeq
OeOseO
OeCeO
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3.4. Preparation of Os3(m-H)(m-benzoate)(CO)10 (5)
Using the same reaction conditions that were successful for the
synthesis of 4, but changing the molar ratio of benzoic acid to
Os3(CO)12 from 7:1 to 1:1 produces the trinuclear cluster Os3(
m-
H)( -O2CC6H5)(CO)10 (5) in 50% yield with no trace of the dinuclear
m
compound. As noted above, it has previously been necessary to
utilize multi-step procedures involving the initial conversion of
Os3(CO)12 to a more reactive species in order to prepare compounds
of the type Os3(m-H)(m-O2CR)(CO)10. A three-step method was used
to prepare 5 with an overall yield of 39% based on Os3(CO)12 [1].
Thus a microwave reactor allows for a higher yield preparation in
only one step.
[16] A.J. Amoroso, B.F.G. Johnson, J. Lewis, C.-K. Li, M.C. Ramirez de Arellano,
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4. Conclusion
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Microwave heating is shown to be effective for the high-yield,
single-step synthesis of dinuclear osmium(I) compounds of the
a Tool for
type Os2(
H or Ph. Using a one-step microwave method, the trinuclear cluster
Os3( -H)( -benzoate)(CO)10 is also easily prepared in higher yield
m-O2CR)2(CO)6, including the first examples for which R is
m
m
than previously reported. This is accomplished without the need to
replace two CO groups with more labile ligands, and is the first
instance in which a cluster of this type has been synthesized
directly from Os3(CO)12. The use of a microwave reactor has many
advantages over traditional preparative methods including very
rapid reaction rates, automated control of temperature and pres-
sure, and simpler procedures with no need for inert gases. We
intend to continue exploring the microwave-promoted reactions of
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