CO2 coordination by inorganic polyoxoanion in water

Article abstract of DOI:10.1021/ja801560t

Two inorganic polyoxoanions react with carbon dioxide in aqueous solution to yield two polymeric chainlike complexes, where an activated CO₂ molecule covalently bonds to polyoxoanions in a linear μ-η¹,η¹-OCO style. For the first time, the CO₂ coordination is realized by an economic and environmentally friendly approach. Copyright

Full text of DOI:10.1021/ja801560t

Published on Web 07/26/2008
CO2 Coordination by Inorganic Polyoxoanion in Water
Guanggang Gao, Fengyan Li, Lin Xu,* Xizheng Liu, and Yanyan Yang
Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry,
Northeast Normal UniVersity, Changchun, Jilin 130024, P. R. China
Received March 1, 2008; E-mail: linxu@nenu.edu.cn
Carbon dioxide has been well-recognized as one of the green-
house gases causing global warming.1 Although people currently
are more focused toward understanding the storage of CO2,2,3
searching for economic methods to fix CO2 remains a hot issue.
The chemical transformation using carbon dioxide as renewable
carbon resources has attracted much attention in the past decades,
because of its application in the synthesis of fine chemicals and
clean fuels.4-6 However, highly thermodynamic stability and kinetic
inertness of CO2 hamper its activation in the chemical transforma-
tion. A promising route to CO2 fixation is to bind CO2 by a simple
complex, so that the resulting complex becomes either a useful
chemical or CO2-activated reagent. So far, the number of structurally
characterized CO2-containing compounds is still limited.7 These
reported complexes represented four coordination modes of η1-
CO2, η2-COO, η1-OCO, and µ-η2, η2-CO2.8,9 The reactions of these
organometallic complexes strongly depend on toxic organic solvents
such as benzene and tetrahydrofuran, and the remaining toxic
solvents become again a new environmental problem. In addition,
the expensive anhydrous solvents and raw materials make these
reactions impractical for an industrially sustainable process. The
development of environmentally friendly and economic methods
for CO2 fixation therefore remains a long-standing challenge. Our
special attention thus focused on the chemical reaction in water to
avoid the use of toxic or carcinogenic organic solvents as well as
reduce solvent cost.
Figure 1. Thermal ellipsoid plots of polyoxoanions located in 1 (top left;
Mo 50% and Co 50% for E1; symmetry code i ) -x, 1 - y, z) and 2 (top
right; Mo 60% and Co 40% for E1; Mo 90% and Co 10% for E2; symmetry
code i ) 2 - x, 1 - y, z). Ball-stick representation of polymeric chain
structure of complex 1 (bottom). All imidazole and water molecules are
omitted.
considered to be a mono-CoII-substituted Keggin-type [PMo12O40]3-
or [SiMo12O40]3- anion. The chain skeleton is bridged by the CO2
ligand in an axial direction, showing a linear geometry. But the
CO2 groups show slightly bent arrangements with O-C-O angles
of 158.7(14)° for 1 and 166.8(10)° for 2. Two O atoms of CO2
link MoVI and CoII from two neighboring polyoxoanions, respec-
tively. Atoms Mo(1) and Co of 1 and Mo(1), Co(1), Mo(3), and
Co(2) of 2 were refined as mixed atoms (Mo(1) 50% and Co (50%)
of 1; Mo(1) (64%), Co(1) (36%), Mo(3) (86%), Co(2) (14%) of
2). The polyanionic chains arrange each other along the a axis in
the two complexes (Figure S1). The data of the elemental analysis
and X-ray photoelectron spectra (Figure S2) well identify these
atoms. The binding energies of Mo 3d5/2 (232.6 eV) and Co 2p3/2
(781.0 eV) for both 1 and 2 are in agreement with the reported
data of Mo6+ and Co2+ ions.18,19 Although the C-O distances
obtained from single crystal data are not exact values due to the
disorder of Mo and Co atoms in the crystal structure, the values
1.287(9) Å for 1 and 1.304(13) Å for 2 are still acceptable as
comparedwiththevaluesobservedinotherCO2-ligatedcompounds.8,9
The coordination binding of the two O atoms from CO2 with
electropositive MoVI and CoII ions should reduce the electron
density of O atoms in 1 and 2, resulting in the increased C-O
bond lengths.
Polyoxometalates (POMs), as a class of large cluster polyoxo-
anions consisting of both metal ion (Mo, W, and V elements) and
oxo ligand, have been widely used in diverse areas such as
catalysis,10 materials science,11 and medicine.12 The previous
researches have proved that the polyoxoanions could covalently
link a carboxylic or carbonato ligand in aqueous solution.13,14
Inspired by these results, we consider that CO2 may combine with
polyoxoanions. When attempting to synthesize an imidazole-grafted
polyoxoanion in aqueous solution together with in situ generated
CO2, fortuitously, we obtained an unusual CO2-ligated compound,
(C3H5N2)3(C3H4N2)[PMo11CoO38(CO2)]·4H2O (1). Compound 1
could feasibly be synthesized by an aqueous solution containing
[H2PMo11CoO40]5- polyoxoanion15 with bubbled CO2. Also, the
analogic compound (C3H5N2)4[SiMo11CoO38(CO2)]·4H2O (2) was
isolated by the same process.16 These two compounds, for the first
time, represent the CO2 coordination by inorganic polyoxoanions
in aqueous solution. Noteworthy, the µ-η1,η1-OCO linear coordina-
tion mode is first shown among the known CO2 complexes. The
reactions depend on clean water solvent and inexpensive raw
materials, and only mechanical stirring expends the electric power.
Such an economic and environmentally friendly approach should
be more suitable for the current industrial process of CO2 fixation.
The single-crystal X-ray diffraction data17 revealed the molecular
structures of 1 and 2. The skeletons of 1 and 2 are composed of
[PMo11CoO38(CO2)]n and [SiMo11CoO38(CO2)]n polymeric chains,
respectively (Figure 1). The polyoxoanion in the chain could be
The infrared spectra of the crystalline samples exhibit the
absorption bands at 2169 cm-1 for 1 and 2165 cm-1 for 2 (Figure
S3), indicative of the coordinated CO2 ligands. These absorption
bands clearly show the red shift in comparison with Vasym ) 2348
cm-1 of free CO2.8 This is in accordance with the structural features
that CO2 ligands appear in longer C-O bond distances for 1 and
2. When 1 and 2 are prepared from the isotopically labeled 13CO2
gas, these absorption bands show a stronger red shift (at 2108 cm-1
9
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2008 American Chemical Society

C O M M U N I C A T I O N S
511.8), [(CO2)(SiMo11O38)(CO2)(SiMo11O39)H6]4- (calcd m/z )
902.6), and [(CO2)SiMo11CoO38(CO2)H2]2- (calcd m/z ) 919.8)
for 2. These results also suggest the occurrence of a C-O bond
break in the polymeric chains.23
The successful syntheses of 1 and 2 provide a promising route
to capture CO2 by simple inorganic polyoxoanions in aqueous
medium. The linear pattern of the CO2 ligand in 1 and 2 should
become a significant structural and functional model for catalytic
conversion of CO2 into useful chemicals. In particular, most of the
current CO2 reactions depend on organic solvent.24 Thus, 1 and 2
represent a green chemical approach to CO2 coordination chemistry.
Acknowledgment. The authors are thankful for the financial
supports from the National Natural Science Foundation of China
(Grant No. 20671017; 20731002) and the Specialized Research
Fund for the Doctoral Program of Higher Education.
Supporting Information Available: Experimental details, structural
figures, IR, UV-vis, NMR, XPS, ESI-MS spectra, and crystallographic
data. This material is available free of charge via the Internet at http://
pubs.acs.org.
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