Rapid synthesis of Group VI carbonyl complexes by coupling borohydride catalysis and microwave heating

Article abstract of DOI:10.1016/j.inoche.2012.09.030

Several Group VI tetracarbonyl phosphine and tertiary amine complexes [M(CO)₄ L₂, M = Cr, Mo, W, L₂ = 2PPh ₃, dppm, dppe, dppp, dppb, bpy, phen, dppf] were synthesized in minutes in the microwave at moderate temperature, atmospheric pressure, and utilizing NaBH₄ as a catalyst. The reactions were optimized by careful solvent selection. The octahedral complexes were isolated in percent yields ranging from 17 to 95. The lower temperatures, shorter reaction times, benign solvents, and lower pressures as compared to the traditional thermal syntheses provide a rapid, eco-friendly synthetic route to these common Group VI complexes.

Full text of DOI:10.1016/j.inoche.2012.09.030

Inorganic Chemistry Communications 26 (2012) 69–71
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Rapid synthesis of Group VI carbonyl complexes by coupling borohydride catalysis
and microwave heating
⁎
Kurt R. Birdwhistell , Brian E. Schulz, Paula M. Dizon
Chemistry Dept. Loyola University New Orleans, 6363 St. Charles Ave, New Orleans, LA, USA, 70118
a r t i c l e i n f o
a b s t r a c t
Article history:
Several Group VI tetracarbonyl phosphine and tertiary amine complexes [M(CO)4 L2, M=Cr, Mo, W, L2=
2PPh3, dppm, dppe, dppp, dppb, bpy, phen, dppf] were synthesized in minutes in the microwave at moderate
temperature, atmospheric pressure, and utilizing NaBH4 as a catalyst. The reactions were optimized by care-
ful solvent selection. The octahedral complexes were isolated in percent yields ranging from 17 to 95. The
lower temperatures, shorter reaction times, benign solvents, and lower pressures as compared to the tradi-
tional thermal syntheses provide a rapid, eco-friendly synthetic route to these common Group VI complexes.
© 2012 Elsevier B.V. All rights reserved.
Received 22 May 2012
Accepted 29 September 2012
Available online 7 October 2012
Keywords:
Group VI tetracarbonyl complexes
Rapid synthesis
Microwave
Catalysis
Sodium borohydride
cis-Selectivity
Introduction
microwave [18]. Leadbeater synthesized several ruthenium carbonyl
cluster complexes in a microwave reactor at high pressures in minutes
The Group VI carbonyl complexes have been of interest for many
years. These complexes are used in a number of different roles. Molybde-
num carbonyl complexes have been used as catalysts [1–3]. Metal car-
bonyls have been used for biological assays using the intense IR signal
of the coordinated carbonyl ligand [4–6]. Recently, metal carbonyl com-
plexes have been studied as therapeutic CO-release molecules [7–9]. In
many cases the traditional thermal syntheses of these complexes require
high temperatures and long reaction times (4–60 h) [10,11]. With tem-
perature sensitive ligands these stringent synthetic conditions lead to
ligand and complex degradation. An alternate low temperature rapid
synthesis of these types of complexes would be desirable especially for
complexes with temperature sensitive biologically relevant ligands.
Various strategies have been used to reduce these stringent reaction
conditions. Catalysts such as sodium borohydride [12,13] and sodium
hydroxide [14,15] have been employed which significantly reduce the
time and temperature required to produce these Group VI carbonyl
complexes.
Recently, several groups have used microwave heating to significant-
ly decrease the time to synthesize metal carbonyl complexes. For exam-
ple, Green used high temperatures and high pressures to synthesize
several Group VI carbonyl complexes in minutes [16]. Hogarth employed
high temperatures at atmospheric pressure to rapidly synthesize several
Group VI carbonyl complexes [17]. Several arenecarbonylchromium
complexes were prepared at high temperatures and pressures in a
[19].
In this report we have combined two of the above strategies for the
rapid synthesis of several common Group VI carbonyl complexes. We
use sodium borohydride in alcohol solvents to rapidly synthesize sever-
al Group VI carbonyl complexes at relatively low temperatures and
atmospheric pressure in a microwave. The lower temperature, shorter
reaction times, and lower pressure as compared to the traditional syn-
theses provide a relatively rapid synthetic procedure under milder
conditions.
Results and discussion
Chatt was the first to use borohydride as a catalyst for the synthesis
of Group VI carbonyl complexes [12]. We modified Chatt's procedure by
heating the reaction mixture containing NaBH4 in a microwave and
using various alcohol solvents which allowed us to extend his proce-
dure to a greater range of Group VI complexes and make the procedure
more efficient. The combination of alcohol solvents and borohydride
salts provides an ideal set of reaction conditions for the application of
microwave heating. The alcohol hydroxyl group absorbs microwaves
strongly via the dipolar absorption mechanism and the borohydride
salts absorb through the ion conduction mechanism, resulting in a
rapid temperature increase of the reaction mixture [20].
The ligand substitution reactions using a Group VI carbonyl and var-
ious ligands were carried out under nitrogen in alcohol solvents at atmo-
spheric pressure in the microwave leading to octahedral disubstituted
carbonyl complexes. These reactions are shown in Fig. 1. The ligands
and abbreviations are shown in Fig. 2. The general procedure used a
⁎
Corresponding author. Tel.: +1 504 865 3272; fax: +1 504 865 3269.
E-mail address: birdwhis@loyno.edu (K.R. Birdwhistell).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2012.09.030

70
K.R. Birdwhistell et al. / Inorganic Chemistry Communications 26 (2012) 69–71
Fig. 1. Synthesis of Group VI metal carbonyl complexes.
Fig. 2. Ligand structures and abbreviations.
Table 1
10% excess of the ligand combined with 1.0 mmol of the metal carbonyl
and 3 equivalents of NaBH4 in 20 mL of alcohol solvent. The microwave
was set to operate at 400 W with a temperature ramp of 1.5 min to the
reflux temperature. Once reflux temperature was reached the micro-
wave power was significantly reduced. The reaction mixture was held
at reflux temperature for 5–20 min. The resulting complexes are disub-
stituted octahedral metal carbonyl complexes. The conditions and yields
for these reactions are compiled in Table 1. The products were isolated by
addition of water to the reaction mixture. Addition of water promotes
precipitation of the products while destroying any remaining sodium
borohydride. The complexes were characterized by comparison to their
literature spectra [12,17,21,22]. Details of the spectra are compiled in
the supplementary material. A detailed general procedure is found as a
footnote [23].
Results of the microwave-assisted borohydride catalyzed synthesis of Group VI metal
carbonyl complexes.
Entry Complex Time(min)/ Solvent1
#
% Yield
Thermal
[12] (BH4)
% yield/time %yield/
(min) time(min)
Microwave
[16]
temp (°C)
1
1a
20/100
1-Butanol
30, trans 45/300,
trans
54
90
85
2
3
4
1b
1c
1d
20/105
20/110
20/125
1-Butanol
THFA
THFA/
5%THF
THFA/
5%THF
–
–
–
55/5
86/.5
–
5
1e
20/125
69
–
–
6
7
8
9
1f
20/90
10/90
10/90
20/85
1-Propanol 76
1-Propanol 17
–
–
1g
1h
2a
–
68/5
–
1-Propanol
0
–
Results of microwave assisted NaBH4 catalyzed syntheses of M(CO)4L2
complexes
(1:1)3
73(cis)
90/300,
trans
10
112
12
13
14
15
16
17
18
2b
20/90
1-Propanol 80
1-Propanol b5
1-Propanol 95
1-Propanol 79
1-Propanol 75
1-Propanol 71
1-Propanol 82
1-Propanol 50
58/5
The yields for these reactions range from 17 to 95%. Heating times
varied from 40 to 3 min depending on metal and ligand. The pyridyl
amine ligands reacted very rapidly and decomposed rapidly leading
to lower overall yields. Entry 8 shows no Cr(phen)(CO)4 ,1g, was iso-
lated. Entry 11 in Table 1 is a control reaction run without catalyst
that produced 5% product as determined by 31PNMR. The low yield
demonstrates the essential need for borohydride in these syntheses.
In the sixth column in Table 1 are the yields and times from Chatt's
borohydride synthesis [12] and the seventh column gives yields and
times for Green's microwave synthesis [16] for comparison. Com-
pounds were identified by comparison to their NMR and IR literature
spectra [11,12,17,21,22].
Selectivity of the borohydride microwave synthesis. Product 2a
is a good example of the selectivity that can be achieved by combining
microwave heating and borohydride catalysis versus using solely micro-
wave heating or longer conventional thermal synthetic pathways. In this
case (entry 9) we were able to isolate pure cis-(CO)4(PPh3)2Mo in 73%
2c,Cntrl 20/90
–
2c
2d
2e
2f
2g
2h
3a
20/90
20/90
20/90
15/90
3/90
80/300
80/.5
–
–
–
–
–
–
–
–
99/.5
–
4/90
20/110
THFA
48(mix)4 70/300,
trans
19
20
21
22
23
24
25
3b
3c
3d
3e
3f
40/120
20/105
40/120
40/120
20/105
20/105
20/90
THFA
1-Butanol
THFA
THFA
THFA
91
60
94
75
79
80
80/300
50/300
40/300
–
62/5
64/2
–
–
–
–
3g
3h
Butanol
80/300
50/300
75/10
–
1-Propanol 55
1
THF=tetrahydrofuran, THFA=tetrahydrofurfuryl alcohol.
Control reaction without catalyst.
1:1 Mixture of 1-propanol:ethanol.
2
3
4
Mixture of cis and trans isomers.

K.R. Birdwhistell et al. / Inorganic Chemistry Communications 26 (2012) 69–71
71
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in the trans isomer or a mixture of cis and trans. Chatt isolated the
trans-(CO)4(PPh3)2Mo after 5 h of heating [12]. We optimized the
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ture of 1:1 ethanol:1-propanol. This solvent mixture reduces the tem-
perature of the reaction and promotes product precipitation upon
formation. Most literature preparations of the cis-Mo(CO)4(PPh3)2 pro-
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Mo(CO)4(nbd) and replacing those ligands with PPh3 in a second step
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Conclusions
In conclusion, the combined use of microwave energy and borohy-
dride catalysis provides a low temperature, low pressure, and rapid
synthesis of Group VI octahedral carbonyl complexes in good yields.
The mild rapid reaction conditions allows one to selectively isolate
the cis-(Ph3P)2Mo(CO)4 complex directly from Mo(CO)6. The pyridyl
amine complexes are generally produced in lower yields than the
phosphine complexes using this method.
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Acknowledgements
We would like to acknowledge partial funding of this work through
the National Science Foundation grant number 0535957 and Motorola
Corporation. I would like to thank TLB for comments on the manuscript.
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magnetic triple resonance in complex spin systems. III. Symmetrical ditertiary
Appendix A. Supplementary material
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783–790.
Supplementary data on the compounds isolated can be found in
the online version at http://dx.doi.org/10.1016/j.inoche.2012.09.030.
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parameters: cyclic bisphosphine complexes of molybdenum, tungsten, and platinum.
Bond angle dependence of metal shieldings, metal-phosphorus coupling constants,
and the phosphorus-31 chemical shift anisotropy in the solid state, Organometallics
11 (1992) 1033–1043.
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MARS microwave fitted with a fiber optic temperature probe and a port on top
for a reflux condenser.1 mmol of Mo(CO)6 and dppe (0.425 g, 1.1 mmol) were
combined with 20 mL of 1-propanol in a two-neck 100 mL RB flask. To this mixture
was added NaBH4 (0.128 g, 3.3 mmol). The flask was placed in the microwave and a
reflux condenser attached through a hole in the top of the microwave. The mixture
was sparged with nitrogen. The mixture was heated under nitrogen at 400 W for
1.5 min to reach reflux temperature. Once the reflux temperature was reached the
microwave power was reduced and the temperature maintained for 18 min. The
mixture was cooled to room temperature and 2–4 mL of water was added to the
reaction to dissolve excess NaBH4 and promote product precipitation. The reaction
was cooled −10 °C for several hours. The light yellow complex was filtered in air
and washed with 2×5 mL of petroleum ether/diethyl ether(1:1) mixture resulting
in 580 mg of Mo(CO)4dppe after drying, a 95% yield.
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