Facile CO2 Cycloaddition to Epoxides by Using a Tetracarbonyl Metal Selenotungstate Derivate [{Mn(CO)3}4(Se2W11O43)]8-

Article abstract of DOI:10.1021/acs.inorgchem.8b02321

A novel polyoxometalate cluster, i.e., Na_1.5H_4.5[(CH₃)₄N]₂{[Mn(CO)₃]₄(Se₂W₁₁O₄₃)}·9H₂O (1), has been successfully synthesized under moderately acidic conditions. Compound 1 contains four electron-donating {Mn(CO)₃}⁺ organometallic entities, which are grafted over an unprecedented heteropolytungstate electron-acceptor group. Compound 1 was further structurally characterized by various physicochemical techniques like elemental analyses, inductively coupled plasma (ICP) analyses, IR and UV-vis spectroscopy, electrochemistry, and single-crystal X-ray diffraction, and so on. The polyoxoanion of 1 comprises a novel {Se₂W₁₁} fragment, which is obtained from molecular assembly of rare {SeW₃} and {SeW₈} species. Evaluation of the data from solution-state IR spectrum showed excellent agreement with the solid state IR spectrum, indicating the intact clusters in the CH₃CN/Na₂SO₄ solvent. Also, negative electrospray ionization mass spectrometry (ESI-MS) was an alternative tool to verify the stability of 1 in the mixed solvent. Additionally, the resulting hybrid can act as a catalyst for cyclic carbonate formation from the reactants epoxides and CO₂ under modest reaction conditions in conjunction with a 1-ethyl-1-methylpyrrolidinium bromide (2). The good activity can be substantiated due to the cooperative influence of polyoxoanion and Br^- ions. Complex 1 can also be easily recycled and reused three times without obvious decrease of catalytic activity.

Full text of DOI:10.1021/acs.inorgchem.8b02321

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Facile CO2 Cycloaddition to Epoxides by Using a Tetracarbonyl
Metal Selenotungstate Derivate [{Mn(CO)3}4(Se2W11O43)]8−
Jingkun Lu, Xinyi Ma, Vikram Singh, Yujiao Zhang, Ping Wang, Junwei Feng, Pengtao Ma,
Jingyang Niu,* and Jingping Wang*
Henan Key Laboratory of Polyoxometalate, Institute of Molecular and Crystal Engineering, College of Chemistry and Chemical
Engineering, Henan University, Kaifeng, 475004 Henan, PR China
S
* Supporting Information
ABSTRACT: A novel polyoxometalate cluster, i.e.,
Na1.5H4.5[(CH3)4N]2{[Mn(CO)3]4(Se2W11O43)}·9H2O (1),
has been successfully synthesized under moderately acidic
conditions. Compound 1 contains four electron-donating
{Mn(CO)3}+ organometallic entities, which are grafted over
an unprecedented heteropolytungstate electron-acceptor group.
Compound 1 was further structurally characterized by various
physicochemical techniques like elemental analyses, inductively
coupled plasma (ICP) analyses, IR and UV−vis spectroscopy,
electrochemistry, and single-crystal X-ray diffraction, and so on.
The polyoxoanion of 1 comprises a novel {Se2W11} fragment,
which is obtained from molecular assembly of rare {SeW3} and
{SeW8} species. Evaluation of the data from solution-state IR
spectrum showed excellent agreement with the solid state IR
spectrum, indicating the intact clusters in the CH3CN/Na2SO4 solvent. Also, negative electrospray ionization mass
spectrometry (ESI-MS) was an alternative tool to verify the stability of 1 in the mixed solvent. Additionally, the resulting hybrid
can act as a catalyst for cyclic carbonate formation from the reactants epoxides and CO2 under modest reaction conditions in
conjunction with a 1-ethyl-1-methylpyrrolidinium bromide (2). The good activity can be substantiated due to the cooperative
influence of polyoxoanion and Br− ions. Complex 1 can also be easily recycled and reused three times without obvious decrease
of catalytic activity.
economic necessity, there is an increasing demand for cyclic
carbonates.
INTRODUCTION
■
The depletion of fossil carbon sources and the concomitant
dependence of the chemical industry on their cracking
products have aroused great research attention.1 In addition
to biomass, CO2 is under the spotlight as C1 building block
and has been viewed as an inexhaustible, cheap, nontoxic,
nonflammable green source of carbon.2 Concerning about
global warming, together with the incoming necessity to find
alternative feedstock to fossil fuels, CO2 as chemical feedstock
may help to mitigate CO2 emissions thus benefiting in
sustainable development of society.3 One of the most admired
routes in this direction is the cycloaddition reaction of CO2
and epoxides to produce five-membered cyclic carbonates. The
cyclic carbonates production on an industrial scale represents a
much greener alternative to the use of phosgene.4 In particular,
cyclic carbonates have gained a lot of attention as precursors
for obtaining polycarbonates, polymers, aprotic polar solvents,
electrolytes in rechargeable batteries, and intermediates in the
production of pharmaceuticals and fine chemicals.5 In an
increasing global market for polycarbonates, cyclic carbonates
also gain importance as raw materials for (biodegradable)
plastics.6 Thus, in view of their broad scope along with their
Recently, a large variety of homogeneous and heterogeneous
catalysts including quaternary ammonium or phosphonium
salts,7 alkali halides (e.g., KI),8 main group and transition metal
complexes,9 zeolite,10 metal oxides,11 and metal oxide
frameworks12 have been developed for the synthesis of cyclic
carbonates. Of particular interest are that polyoxometalate
(POM) systems have been reported as active and selective
catalysts for such catalytic transformation. In 2004, Man-
ikandan et al. were the first to study a zinc-substituted
sandwich-type POM, Na12[WZn3(H2O)(ZnW9O34)2]·
46H2O,9c in conjunction with a Lewis base (dimethyl amino
pyridine) which comes out as an efficient catalyst in the
cycloaddition reaction. In 2005, Sakakura et al. evaluated a
series of transition-metal-substituted POMs, such as [(n-
C7H15)4N]6[α-SiW11O39Co] and [(n-C7H15)4N]6[α-Si-
W11O39Mn], as the catalysts in the synthesis of cyclic
carbonate under elevated CO2 pressure.13 In particular,
carbonyl complexes of ReI and MnI have proven effective for
Received: August 16, 2018
© XXXX American Chemical Society
A
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the transformation of CO2. ReI and MnI complexes have been
found to be active electrocatalysts for the electro- and
photochemical reduction of CO2.14 Surprisingly, their
applications in the coupling of CO2 with epoxides remain
rare. In 2006, a proposed mechanism for the catalytic CO2/
expoxide coupling reaction catalyzed by manganese(I)
complexes was reported by Lin et al.15 In 2007, a novel
tricarbonyl rhenium(I) complex using as a recyclable catalyst
for chemical fixation of carbon dioxide in ionic liquid was
reported by Wong et al.16
Some previous reports have suggested that parallel Lewis
base activation of CO2 and Lewis acid activation of epoxides is
important in CO2 cycloaddition reaction.17 Various binary
catalyst systems containing Lewis acid and base centers
exhibited high catalytic activity in the synthesis of cyclic
carbonates from CO2 and epoxides under mild reaction
conditions.16,18 Hill et al. have investigated that ReI/MnI atom
is Lewis acid center when it is in its excited state, since the
POM moiety acts as a strong electron withdrawing group.19
Furthermore, room-temperature ionic liquids (ILs) can not
only offer an excellent reaction medium for the transformation
of CO2 (because of their high CO2 solubility, high
conductivity, wide electrochemical window, and nonvolatility),
but also make a nucleophilic attack (the halide ions in ILs) on
the sterically hindered β-carbon atom of the favorable epoxide
and strongly lower the activation energy barrier of the reaction.
The use of ILs as reaction media also allows the catalysts to be
recycled efficiently.20 Hence, it still remains a tremendous
challenge in the synthesis of novel POM-supported man-
ganese/rhenium carbonyl derivatives as catalysts with
enhanced properties for CO2 conversion.
mass spectrometer. All samples were injected directly at a flow rate of
5 μL min−1 using a syringe pump. All spectra were collected in
negative mode and analyzed using the PeakView software. X-ray
powder diffraction (XPRD) measurements were performed on a
Bruker D8 Advance instrument with Cu Kα radiation (λ = 1.54056 Å)
in the angular range 2θ = 5−40° at 293 K. UV−vis absorption
spectrum was obtained with a U-4100 spectrometer at room
temperature. Elemental analyses (C, H, and N) were performed on
a PerkinElmer 2400-II CHNS/O analyzer. ICP-OES analyses were
performed with a Perkin-Elemer Optima 2100DV spectrometer.
Thermogravimetry (TG) analyses were conducted on a Mettler−
Toledo TGA/SDTA851e analyzer under the nitrogen gas atmosphere
with a heating rate of 10 °C min−1 from 25 to 800 °C. X-ray
photoelectron spectra (XPS) were recorded with an Axis Ultra X-ray
photoelectron spectrometer. Magnetic susceptibility measurements
were carried out with a Quantum Design SQUID (MPMS3 and
PPMS-9) magnetometer in the temperature range of 1.8−300 K with
an applied field of 1000 Oe. GC analyses were performed on Bruker
450-GC with a flame ionization detector equipped with a 30 m
column (GsBP-5, 0.25 mm internal diameter and 0.25 μm film
thickness) with nitrogen as carrier gas. All electrochemical measure-
ments were performed at room temperature in a standard three-
electrode cell connected to a LK98 microcomputer-based electro-
chemical system. A freshly cleaned glassy carbon disk electrode (3
mm diameter) was used as the working electrode, a platinum wire
served as the counter electrode, and Ag/AgCl was the reference
electrode.
Synthesis of Compound 1. Na2WO4·2H2O (0.198 g, 0.600
mmol) and SeO2 (0.011 g, 0.099 mmol) were dissolved in a solution
consisting of 0.2 mL of acetic acid (1 M) and 10 mL of distilled water.
After vigorous stirring for about 20 min, the pH of this mixture was
adjusted to 5.04 by using acetic acid (1 M). Then adding 3 mL of
Mn(CO)5Br (0.055 g, 0.200 mmol) solution, which refluxed in 3 mL
of CH3CN at 70 °C in the dark for 0.5 h. After then, the solution
mixture was heated at 70 °C for 1 h, and a translucent, orange
solution was observed. Additional tetramethylammonium chloride
(0.1 g, 1.0 mmol) was added, then heated at 70 °C for 10 min, then
cooled, filtered, and left to evaporate slowly in the dark. Red block
crystals of 1 were obtained at room temperature after about 2 weeks.
Yield: 0.074g (14.71%) for 1 based on Mn(CO)5Br. Anal. Calcd: C,
6.40; H, 1.19; N, 0.75; W, 53.85; Se, 4.21; Mn, 5.85. Found: C, 6.68;
H, 1.01; N, 0.85; W, 52.75; Se, 4.31; Mn, 5.39. Selected IR (KBr,
cm−1): 2030 (vs), 1920 (vs), 1630 (m), 1485 (m), 952 (s), 838 (s),
800 (w), 653 (w).
General Procedure for Cycloaddition Reaction of CO2 to
Epoxide. The cycloaddition of CO2 to glycidyl phenyl ether was
carried out in a stainless-steel autoclave (20 cm3 inner volume). A
typical reaction was carried out as follows. A stainless-steel high-
pressure reactor was charged with 5.0 mmol of substrate, 80 mg of
ionic liquid 2, 0.15 mol % catalyst 1, and a stirring bar. After cautious
flushing with CO2, the autoclave was pressurized to 1.5 bar of CO2 at
room temperature. Then, the charged autoclave was heated under
stirring to the desired temperature. After reaction completion, the
reactor was cooled to room temperature in cold water, and the
unreacted carbon dioxide pressure was slowly released. The reaction
mixture remaining in the autoclave was extracted with hot ethyl
acetate and further analyzed by GC to quantify conversion and
selectivity. After removal of the ethyl acetate under vacuum, this
reside of catalyst 1 and ionic liquid 2 (catalyst system) in the bottom
of autoclave was recycled. The results show that the catalyst system
could be recycled at least three times without noticeable loss of
catalytic activity because the charge-transfer polyoxometalate may be
produced by 1 and 2 via electrostatic attraction.
However, the development of POM-based metal carbonyl
derivatives (PMCDs) still remains very slow. Most of the
reported compounds are dominated by Lindqvist-type
derivatives (Table S1),21 although they possess the dual
advantages of metal carbonyls and POMs. Particularly, d6
metal carbonyls (manganese/rhenium) and POMs are
important and remain very challenging in the POM-based
metal carbonyl derivatives. Our group recently reported several
PMCDs such as [(CH3)4N]5H23[(PW11O39){Re(CO)3}3(μ3-
O)(μ2-OH)]4·24H2O,22 [(NH4)5][{PMo3O16}{Re(CO)3}4]·
2 3
1 . 5 H O ,
a n d
[ N a ( H O ) ] -
2
2 5
(NH4)7[P2W15O56Co3(H2O)3(OH)3Mn(CO)3]·19H2O,24
Pursuant to our own efforts toward the development of novel
POM-supported manganese/rhenium carbonyl derivatives for
coupling CO2 with epoxides under mild conditions, herein, an
unprecedented selenotungstate with decorated manganese
c a r b o n y l u n i t s , N a 1 . 5 H 4 . 5 [ ( C H 3 ) 4 N ] 2 { [ M n -
(CO)3]4(Se2W11O43)}·9H2O (1), was obtained by a conven-
tional mixed-solvent solution method. To the best of our
knowledge, compound 1 has the largest number of metal
carbonyl groups. In addition, 1 was used as an efficient and
recyclable catalyst for the formation of cyclic carbonate with 1-
ethyl-1-methylpyrrolidinium bromide (2).
EXPERIMENTAL SECTION
■
Materials and Physical Measurements. All chemicals were
purchased from commercial sources and used without further
purification. Ionic liquid (1-ethyl-1-methylpyrrolidinium bromide;
CAS: 69227−51−6) is also commercially available. IR spectra
(including solid- and solution-state samples) were measured on a
Bruker-Vertex 70 FT-IR spectrometer. Electrospray Ionization Mass
Spectrometry (ESI-MS) was performed using a Triple TOF 4600−1
X-ray Crystallography. Crystallographic and experimental details
of the crystal structure determination are given in Table 1. A red block
crystal with dimensions of 0.18 × 0.16 × 0.12 mm3 for 1 was sealed in
a capillary tube for indexing and intensity data collection. The
collection of the intensity data were collected using an APEX-II CCD
detector with the use of graphite-monochromatized Mo Kα radiation
(λ = 0.71073 Å). Using Olex2,25 the structure was solved with the
B
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Table 1. Crystal Data and Structure Refinement of
Scheme 1. Schematic Synthetic Process of Compound 1
Compound 1
formula
Na1.5H4.5[(CH3)4N]2{[Mn(CO3)]4(Se2W11O43)}
9H2O
empirical formula
C20H24Mn4N2Na1.5O60Se2W11
fw (g mol−1
temp. (K)
cryst syst
space group
a (Å)
)
3686.93
296.15
monoclinic
C2/c
36.612(4)
b (Å)
23.476(3)
c (Å)
24.428(3)
β (deg)
V (Å3)
102.867(2)
20469(4)
Z
8
Dcalcd (g cm−1
)
2.393
13.574
μ (mm−1
)
F000
13092.0
limiting indices
−43 ≤ h ≤ 43, −26 ≤ k ≤ 28, −18 ≤ l ≤ 29
reflns collected
52671
data/restraints/
parameters
18151/38/826
Rint
GOF on F2
0.1240
0.968
R1, wR2 [I > 2σ(I)]
R1, wR2 [all data]
R1 = 0.0625, wR2 = 0.1396
R1 = 0.1764, wR2 = 0.1931
ShelXT structure solution program using Direct Methods and refined
with the ShelXL refinement package using Least Squares minimiza-
tion.26,27 Routine Lorentz and polarization corrections were applied,
and an absorption correction was performed using the SADABS
program. [(CH3)4N]+ and lattice H2O were determined by using
elemental analysis and thermogravimetric analysis. For 1, no hydrogen
atoms associated with the water molecules were located from the
difference Fourier map and all the non-hydrogen atoms were refined
anisotropically. CCDC 1843010 contains the supplementary data for
this paper. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
date_request/cif.
RESULTS AND DISCUSSION
■
Synthesis and Crystal Structure. Compound 1 was
obtained from the reaction of Na2WO4, SeO2, and Mn-
(CO)5Br with a pH-controlled assembly (pH 4.8−5.6 for 1) in
a mixed CH3CN/H2O solvent. The acidification by acetic acid
was necessary in vew of its extensive utilization for acidifying
Se-based polyoxotungstates.28 The procedure followed for the
synthesis of 1 is summarized in Scheme 1. Single-crystal X-ray
analysis reveals that 1 crystallizes in the monoclinic system
with space group C2/c. Compound 1 consists of a discrete
cluster anion [{Mn(CO)3}4(Se2W11O43)]8−, 2 [(CH3)4N]+
counter cations, 9 lattice water molecules, 1.5 Na+ ions, and
4.5 protons for the charge balance (Figure 1a). This new
polyoxoanion can be considered as grafting four [Mn(CO)3]+
pendants onto the previous unreported selenotungstate
fragment [Se2W11O43]8− via five μ3-O atoms (O4, O17, O26,
O35, O39) and six μ2-O atoms (O9, O24, O27, O30, O32,
O37) (Figure S1). All the Mn−C and C−O bond lengths are
in the ranges of 1.753−1.903 Å and 1.093−1.193 Å,
respectively, and the Mn−C/C−O bonds lengths are equal
within experimental errors.29
Figure 1. (a) Ball-and-stick and polyhedral representations of the
polyanion. (b) and (c) Bond connectivity modes of four [Mn(CO)3]+
groups with representative bond distances (Å).
pounds.30 As previously researched, the [M(CO)3]+ (M = Mn,
Re, and Rh) moiety is generally attached to a trigonal adjacent
O atoms on the surface of POMs, forming kinetically stable d6
low-spin octahedral 18-electron moiety fac-[(OC)3ML3]+ (L =
ligand).30b,31 The bonding of the [M(CO)3]+ fragment to a
triangle of bridging oxygen atoms is usually the rule within
PMCDs.21e,31 In this structure, only Mn1 is bonded to a POM
unit via three μ3-O atoms (O4, O17, and O26) in a similar way
to the reported compounds. Mn3 and Mn4 centers are
coordinated to {Se2W11} unit by two μ2-O atoms (O9, O32;
Interestingly, this grafting fashion of a [Mn(CO)3]+ pendant
(Mn2, Mn3, Mn4) connecting to a POM unit via bridging
oxygen atoms is different from the most reported com-
C
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Figure 2. Ball-and-stick representation of the polyoxoanion (a), basic building blocks (b) {SeW3} and {SeW8} species, and the (c) {Se2W11}
fragment. Color code: W, cyan; Se, rose; Mn, green; O, gray in {Se2W11} fragment; C, black; O atoms are red in [Mn(CO)3]+ groups.
O24, O37) and one μ3-O atom (O35; O39), respectively
(Figure S1). Mn2 atom is connected to the POM unit by two
double-bridging oxygen atoms (O27, O30). Furthermore, the
four Mn+ cations in polyoxoanion are all 6-coordinate
octahedral configurations but they reside in different
coordination environments (Figure 1b,c). As shown in Figure
S1, Mn1 is bonded to O4, O17, and O26 and forms six Mn−
O−W bonds [Mn1−O, 2.021−2.125 Å; W−O, 1.894−2.112
Å] to the POM moiety. It is likely that Mn2 was linked to
[Se2W11O43]8− through two Mn−O−W bridged linkages
[Mn2−O, 1.985−2.004 Å; W−O, 1.773−1.807 Å]. Mn3 and
Mn4 are similar and bonded to O9, O32, and O35 and O24,
O37, and O39, respectively, forming six Mn−O−W bonds
[Mn3−O, 2.059−2.063 Å; W−O, 1.735−2.022 Å; Mn4−O,
2.024−2.480 Å; W−O, 1.750−2.335 Å] and two Mn−O−Se
bonds to the [Se2W11O43]8− [Se2−O, 1.729 Å]. As a new
building block, such {Se2W11} unit has not been discovered in
POMs before. Interestingly, this structure is completely
Figure 3. Polyhedral and ball-and-stick representation of {Se2W11}
different from the reported {Se2W11} fragment addressed by
Chen in 2016.32 As reported, the {Se2W11} fragment is
composed of the unreported {SeW4} and {SeW7} species,
which are linked into a Wells−Dawson-like assembly (Figure
S2). In our structure, {Se2W11} fragment is comprised of the
novel {SeW3} moiety and {SeW8} unit via two μ2-O atoms
(O13, O21) (Figure 2). As shown in Figure 3, the
selenium(IV) center in {SeW3} unit is coordinated to three
terminal atoms (O35, O39, O42) of three edge-shared {WO6}
octahedra. Besides, the {SeW8} cluster can be considered as a
{SeW6} fragment connecting to two edge-sharing WO6
octahedra via O4, O10, O17, and O23 (Figure S3). The
{SeW6} fragment can be derived from the well-known trivacant
Keggin {α-SeW9O33} by omitting three vertex-sharing WO6
octahedra. Additionally, the remaining two edge-sharing WO6
octahedra were hinged onto the {SeW6} cluster via two μ2-O
atoms (O10, O23) and two μ3-O atoms (O4, O17), forming
the {SeW8} cluster.
fragment and its basic building blocks. Color code: W, cyan; Se, rose;
O, red.
discussed based on BVS calculations (Table S3). Complete
single-crystal X-ray data for 1 is available, and selected
interatomic distances and angles are presented in Tables S4
and S5. The XPS spectra of 1 were also recorded to further
consolidate the chemical valences of Mn, Se and W atoms
(Figure 4). The Mn 2p3/2 binding energy is 641.2 eV for 1,
consistent with the Mn 2p3/2 binding energy of 641.9 eV for
[Mn(CO)5]Br,34 which indicates that the oxidation state of the
Mn ion in 1 is +1 (Figure 4b). The result can also be further
confirmed by magnetic susceptibility spectrum (Figure S7).
Figure 4c also shows the high-resolution XPS spectra for Se 3d
of compound 1, in which a weak peak at 59.6 eV was observed,
indicating the presence of Se4+.35 Besides, the two apparent
signals appearing at 37.7 and 35.5 eV in the energy regions of
W 4f5/2 and W 4f7/2 are confirmed to be W6+ centers (Figure
4d).36
Analyses of BVS, XPS, IR, and XPRD. Bond Valence
Sum Calculations and XPS Spectra. The bond valence sum
(BVS) calculations of 1 manifest that the oxidation states of all
Se and W atoms are assigned to +4 and +6, respectively (Table
S2).33 All the oxygen atoms on the POM fragments are also
FT-IR Spectroscopy and XPRD Pattern. The phase purity of
1 was confirmed by comparison of the experimental X-ray
powder pattern (XPRD) with the simulated XPRD from the
crystal data (Figure S8). The diffraction peaks of both
D
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Figure 4. XPS spectra of 1 for (a) C 1s; (b) Mn 2p3/2; (c) Se 3d; (d) W 4f5/2 and W 4f7/2
.
simulated and experimental patterns match in the key
positions, indicating the phase purity of the compound. The
difference in reflection intensities is probably due to
preferential orientations in the powder sample of compound
1. The solid FT-IR spectra were recorded using KBr pellets in
the range of 4000−450 cm−1. In the IR spectrum of 1 (Figure
S9), the terminal W−Ot stretching vibrations are seen as
strong bands in the range of 955−938 cm−1. The characteristic
peaks corresponding to ν(W−Ob) and ν(W−Oc) are located at
838 and 653 cm−1, respectively.37 Moreover, a pair of
absorption peaks are observed at 2030 and 1920 cm−1,
respectively, which arise from the antisymmetric and
symmetric stretching vibration of the C−O bond. Similar
bands are observed for some C3v metal carbonyl complexes.21a
Compared with that of Mn(CO)5 (Figure S10), the peaks in 1
display obviously red shift as a result of the combination of the
metal carbonyl and POMs fragments. In summary, the results
of IR spectroscopy are in good agreement with the single-
crystal X-ray structural analysis.
Electrospray Ionization Mass Spectroscopy (ESI-MS)
and Solution-State IR Studies. ESI-MS study of 1 dissolved
in the mixed solvent CH3CN−H2O (1:3, volume ratio)
confirmed that the compound retained its integrity in solution
through all the m/z values of the main peaks. We also carried
out ESI-MS experiments to see what other species remain
following crystallization in the solution (Figure S11). The
formula for the anions observed were confirmed by the
experimental values of m/z as well as the isotopic profile. All
the results are presented in Figure 5 and Table 2. In the
spectrum of complex 1, there were some major peaks located
at m/z 1179.329, 1185.516, 1193.043, 1199.005, 1210.081,
and 1231.69 which belong to {[(CH3)4N]H4[Mn-
(CO)3]4(Se2W11O43)(H2O)2}3− (calcd 1179.641), {H5[Mn-
(CO)3]4(Se2W11O43)(H2O)7}3− (calcd 1185.295),
{[(CH3)4N]NaH3[Mn(CO)3]4(Se2W11O43)(H2O)3}3− (calcd
1192.971), {[(CH3)4N]NaH3[Mn(CO)3]4(Se2W11O43)-
(H2O)4}3− (calcd 1198.975), {[(CH3)4N]2H3[Mn-
(CO)3]4(Se2W11O43)(H2O)3}3− (calcd 1210.007),
{[(CH3)4N]Na3H[Mn(CO)3]4(Se2W11O43)(H2O)7}3− (calcd
1231.640), respectively. Furthermore, two peaks at m/z
876.255 and 889.033 assigned to {NaH3 [Mn-
(CO)3]4(Se2W11O43)(H2O)3}4− (calcd 876.204) and {H4[Mn-
(CO)3]4(Se2W11O43)(H2O)7}4− (calcd 888.720) were also
observed (Figure S12 and Table S6). IR spectrum of a queous
solution of 1 could be obtained in the range of 4000−500 cm−1
(Figure S13).The spectrum is compared with that recorded in
the solid state, indicating the cluster is intact in the mixed
solvent CH3CN−H2O (1:3, volume ratio). Infrared vibration
band in the vicinity of 3300 cm−1 belongs to the stretching
vibration absorption peak for H−O in H2O. Bending vibration
band of H2O appears at about 1630 cm−1. In addition, the
spectrum of solution-state IR show similar characteristics for
the C−O bond and tungsten-oxo framework, respectively.
UV Spectra and Electrochemistry. To investigate the
properties of 1 in the mixed solvent CH3CN−H2O (1:3,
volume ratio), in situ UV−vis spectra were monitored. As
shown in Figure S14a, a cuspidal absorption band at about 206
nm can be attributed due to the Ot → W charge transfer
transitions,38 whereas, the broad absorption band at ca. 389
nm could be assigned to the Mn (π) → CO (π*) transitions,
indicating the presence of manganese carbonyl groups.39 The
stability of 1 was assessed by monitoring its in situ UV−vis
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Figure 5. (a) Negative mode ESI-MS spectrum of 1 in mixed solvent of CH3CN/H2O. (b) Expansions of the {[(CH3)4N]H4[Mn-
(CO)3]4(Se2W11O43)(H2O)2}3− signals; (c) {H5[Mn(CO)3]4(Se2W11O43)(H2O)7}3− signals; (d) {[(CH3)4N]NaH3[Mn(CO)3]4(Se2W11O43)-
(H2O)3}3− signals; (e) {[(CH3)4N]NaH3[Mn(CO)3]4(Se2W11O43)(H2O)4}3− signals; (f) {[(CH3)4N]2H3[Mn(CO)3]4(Se2W11O43)(H2O)3}3−
signals; and (g) {[(CH3)4N]Na3H[Mn(CO)3]4(Se2W11O43)(H2O)7}3− signals, with experimental results in blue, and simulated spectra in black,
respectively.
spectra in intervals for up to 16 h (Figure S14b). A
complementary cross-check of this stability and the redox
properties of 1 were studied through cyclic voltammetry (CV)
in 0.4 M CH3CN/Na2SO4 (1:3, volume ratio) electrolyte
solution. Figure 6a shows the CV pattern of 1 (1 × 10−3 M) in
the range of potential values from −1.0 to +1.0 V vs Ag/AgCl
at a scan rate of 100 mV s−1. It can be clearly seen that the
redox peaks of WVI centers fragments appear at a more
negative potential region than that assigned to the MnI centers,
as expected. The electrochemical behaviors exhibited one pair
of closely spaced reversible peaks, two irreversible cathodic
peaks and two anodic peaks, which results from the
cooperation of the redox processes of the WVI atoms and
[Mn(CO)3]+ pendants.37b As shown in Figure 6a, at the
positive potential region of 1, two anodic peaks and one
cathodic peak might be assigned to the oxidation of MnI
F
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and the corresponding anodic peak potentials shift toward the
positive direction with increasing scan rates. The average peak
potentials did not change on the whole, although the peak-to-
peak separation between the corresponding anodic and
cathodic peaks increased. In contrast, different scan rates
(from 20−200 mV s−1) experiments reveal that surface-
controlled is dominative during the redox process in view of
the peak currents (Figure 6b), which are linearly proportional
to the square root of the scan rate.
Catalysis. Considering the coexistence of Lewis acidic
POM sites (catalyst 1) and basic ionic liquids (2), their
catalytic activities were studied in CO2 cycloaddition reactions.
In our initial tests on the chemical fixation of CO2 with
epoxides, glycidyl phenyl ether (2a) was used as a template
substrate to produce 3-phenoxy-1,2-propylene carbonate (3a).
On the basis of previous reports,41 a proposed mechanism for
the catalytic CO2/expoxide coupling reaction using a binary
catalyst system is shown in Scheme 2. The nucleophilic attack
Table 2. Summary of the Observed ESI-MS (Negative
Mode) Peaks of 1
identification
cal. (m/z) exp. (m/z)
{[(CH3)4N]H4[Mn(CO)3]4(Se2W11O43)
1179.641
1179.329
(H2O)2}3−
{H5[Mn(CO)3]4(Se2W11O43)(H2O)7}3−
1185.295
1192.971
1185.516
1193.043
{[(CH3)4N]NaH3[Mn(CO)3]4(Se2W11O43)
(H2O)3}3−
{[(CH3)4N]NaH3[Mn(CO)3]4(Se2W11O43)
(H2O)4}3−
1198.975
1210.007
1231.640
1199.005
1210.081
1231.691
{[(CH3)4N]2H3[Mn(CO)3]4(Se2W11O43)
(H2O)3}3−
{[(CH3)4N]Na3H[Mn(CO)3]4(Se2W11O43)
(H2O)7}3−
Scheme 2. Proposed Reaction Mechanism for the
Cycloaddition of CO2 to Epoxides
of bromide ion on the less sterically hindered β-carbon anion
atom of the epoxide is the first step. The Lewis acidic site of
the POM is also of benefit to reduce the energy barrier for the
ring-opening step through a polarization of the epoxide carbon
atoms. Subsequently, the nucleophilic alkoxide intermediate
generated from ring-opening then attacks CO2 to give a CO2-
adduct, which leads to the formation of the cyclic carbonate
and regeneration of the catalyst. Table 3 summarizes the effects
of various reaction parameters on the catalytic performance. As
shown in Table 3, entry 1, ionic liquid 2 itself could convert
the epoxide into 3a through a Lewis acid−Lewis base
mechanism with Br− as a nucleophile. However, 2a displays
a slower conversion at 70 °C in absence of catalyst 1, leading
to only a 39.3% yield, whereas on a small addition of catalyst 1
(0.15 mol %) over a short reaction time (1.5 h), 3a was
produced in excellent 97.2% yield with a turnover frequency
(TOF) of 432 h−1 (Table 3, entry 2). The result shows that the
cooperative action of an anion Br− (nucleophile) and a metal
center M (Lewis acid) of a catalyst promotes the ring-opening
of the epoxide, CO2 attack, and subsequent ring-closing
reaction. An analogous result addressed by Kleij et al. has
shown that the ring-opening procedure initiated by direct
Figure 6. Cyclic voltammograms of 1 in aqueous 0.4 M CH3CN/
Na2SO4 (1:3, volume ratio) electrolyte solution. POM concentration
= 1 × 10−3 M. The scan rate was 100 mV s−1 (a). CVs of 1 at different
scan rates: from inside to outside: 20, 40, 60, 80, 100, 120, 140, 160,
180, and 200 mV s−1 (b). Inset: Representation of the current as a
function of the square root of the scan rate. The working electrode
was glassy carbon, and the reference electrode was Ag/AgCl.
centers. In the negative region of potentials, the CVs are
restricted to the one chemically reversible W-reduction waves
peaking at E1/2 = −0.832 V (IV/IV′) [E1/2 = (Epa + Epc)/2],
and one distinct reduction wave appeared at −0.738 V (V). It
is worth noting that the ΔEp [ΔEp = Epa − Epc] value of 69 mV
(IV/IV′) for the oxidation process suggests a quasi-reversible
one-electron process (the theoretical value of ΔEp for a
reversible electron transfer is about 59 mV).40 Moreover, the
cathodic peak potentials shift toward the negative direction
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Table 3. Optimization of the Reaction Conditions of Cycloaddition of CO2 to Glycidyl Phenyl Ether (2a) Catalyzed by
a
Catalyst 1 with Ionic Liquid 2
b
c
d
entry
cat. 1 (mol %)
2 (mol %)
T (°C)
press. (Mpa)
time (h)
yield (%)
TOF (h−1
)
1
2
3
4
5
6
7
8
8
8
8
8
8
8
8
8
8
8
4
6
70
70
65
60
70
70
70
70
70
70
70
70
75
70
70
70
70
70
1.5
1.5
1.5
1.5
1.0
0.5
1.5
1.5
1.5
1.5
0.15
0.15
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.0
2.0
0.15
0.15
1.5
1.5
1.5
1.5
1.5
1.5
39.3
97.2
80.6
49.1
87.4
73.5
89.2
75.3
85.1
98.2
52.6
79.3
174.7
432
0.15
0.15
0.15
0.15
0.15
0.12
0.10
0.15
0.15
0.15
0.15
0.15
0.15
0.15
358.2
218.2
388.4
326.7
396.4
334.7
378.2
436.4
233.8
352.4
9
10
11
12
13
14
15
16
17
18
e
8
8
8
8
8
92.5
88.2
45.2
40.1
58.7
411.1
392
200.9
178.2
260.9
f
g
h
i
a
b
Reaction conditions: catalyst 1 (0.15 mol %), epoxide (5 mmol, 680 μmol), ionic liquid 2 (8 mol %, 80 mg). CO2 was charged into autoclave
c
under an initial pressure of 1.5 MPa at room temperature. Determined by GC analyses using dimethyl phthalate as an internal standard; The
d
e
f
g
selectivity for cyclic carbonates were over 99% in all examined cases. In [molpropylene‑carbonate molcatalyst−1 h−1]. Second run. Third run. Na2WO4·
h
i
2H2O (0.15 mol %) was used as a catalyst. SeO2 (0.15 mol %) was used as a catalyst. Mn(CO)5Br (0.15 mol %) was used as a catalyst.
insertion of CO2 need much harsher reaction conditions (i.e.,
higher temperatures and pressures).42 Interestingly, the TOF
of 432 h−1 was superior to that of [Na(H2O)5]-
(NH4)7[P2W15O56Co3(H2O)3(OH)3Mn(CO)3]·19H2O re-
ported by Niu et al. in 2017 (∼264 h−1).24 To further
optimize reaction conditions, the yield of 3a get generally
increased with the subsequent increase in temperature (Table
3, entries 2−4), and it reached its maximum at 70 °C. In order
to seek optimal conditions, several factors were investigated.
We found that the yield was strongly affected by the reaction
temperature. By increasing the temperature from 60 to 70 °C,
the yield of 3a enhanced from 97.2 to 49.1%. The obvious
elevated activity of the catalytic system verifies that the energy
barrier of the cycloaddition of CO2 has been reduced by a
higher temperature. In addition, the yield of 3a was slightly
affected by the CO2 pressure (Table 3, entries 2, 5, and 6). An
increase in the CO2 pressure from 0.5 to 1.5 MPa results in
improved yield from a moderate 73.5 to 97.2%. A moderate
yield obtained in the low CO2 pressure shows the amount of
reagent is not enough to coordinate to the Lewis acid sites.
The influence of the catalyst dosage was also investigated.
Lowering the catalyst loading to 0.12 mol % and 0.1 mol % led
to decreased yields of 89.2% and 75.3%, respectively (Table 3,
entries 7 and 8). Subsequently, the reaction duration was also
studied, and the catalyst amount, temperature and CO2
pressure were fixed at the previously determined optimum
conditions. When the time was prolonged from 1 to 1.5 h , the
yield enhanced to 12.1% (Table 3, entry 9). Extending the
reaction to 2 h did not significantly affect 3a yield, which
confirms that the optimum reaction time was 1.5 h (Table 3,
entry 10). Besides, the raw materials, like Na2WO4·2H2O,
SeO2, and Mn(CO)5Br, showed somewhat low catalytic
activity compared with that of catalyst 1 (Table 3, entries
16−18). When SeO2 was used as a catalyst for the
cycloaddition reaction, only a 40.1% yield of 3a was observed.
The yield was slightly increased to 45.2% when the catalyst was
changed to Na2WO4·2H2O, and an obvious enhanced 58.7%
yield was generated by Mn(CO)5Br.
If ionic liquid 2 was added incrementally from 4 to 6 mol %,
then the yield increased from 52.6 to 79.3% (Table 3, entries
11−12). In the absence of 2, no conversion of 2a occurred
under the applied reaction conditions, indicating that ionic
liquid 2 was necessary for cycloaddition reaction (Table 3,
entry 13). The result without ionic liquid also indicates that
the high energies in the ring-opening process. In addition to
the 2, the search of other low-cost additives is another
alternative choice. In Table S7, we first tried to use
tetrabutylammonium chloride containing Cl− anion as an
additive, but only 15.6% yield was obtained. Subsequently,
tetrabutylammonium bromide and tetrabutylammonium io-
dide were also used as additives, yields of 36.2 and 13.4% were
obtained under identical reaction conditions, respectively. The
activity order of quaternary ammonium halides was found to
be tetrabutylammonium bromide > tetrabutylammonium
chloride > tetrabutylammonium iodide. This order is a
combined result of considering the nucleophilicity (Br−
>
Cl− > I−) and leaving ability (Br− > Cl− > I−) of halide
anions.43 Consequently, the above results suggested that
cooperation of Lewis acid and nucleophilic agent was necessary
for obtaining high activity for cycloaddition reaction.44
Experiments were also carried out to examine the recyclability
of the catalyst system using 2a as the substrate. The results
showed that it could be reused at least 3 times with a general
decline in yield to 88.2% for the third run (Table 3, entry 15).
H
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Table 4. Cycloaddition of Various Epoxides with CO2 in the Presence of 1 with Ionic Liquid 2a
a
b
Reaction conditions: catalyst 1 (0.15 mol %), epoxide (5 mmol, 680 μmol), ionic liquid 2 (8 mol %, 80 mg), 70 °C, 1.5 MPa. Determined by GC
c
−1
analyses using dimethyl phthalate as an internal standard; the selectivities were over 99% in all examined cases. In [molpropylene‑carbonate molcatalyst
h−1].
After reaction, the catalyst system was washed three times with
ethyl acetate. The IR spectra and XPRD patterns of the
recovered catalyst system are displayed in Figures S17 and S18.
We also find that the POM maintained its structural integrity
after three cycles but that the structure cannot remain
crystalline.
representing a new member in inorganic family of POMs.
Meanwhile, the successful preparation of 1 provides us an
enlightening synthetic method for covalent grafting of metal
carbonyl groups onto POM cluster and further guides the
design of high nuclear POM-supported metal carbonyl
derivatives, though it is quite difficult to obtain them due to
the steric hindrance of the organic groups. Notably, it showed
good activity in catalyzing cyclic carbonate synthesis from
epoxides and CO2 under mild reaction conditions with ionic
liquid 2. Various epoxides can be catalyzed by the
heterogeneous catalytic system, and it can be easily separated
and reused for three successive cycles without significantly loss
in its catalytic activity.
Using catalyst 1 and 2, cycloadditions of CO2 with other
substrates were also examined at 1.5 MPa and 70 °C, and the
results are depicted in Table 4. Epoxides bearing chloromethyl
(2b) electron-withdrawing substituent were converted to
corresponding cyclic carbonates in 94.7% yield in 1 h (Table
4, entry 1). In the case of the boiling propylene oxide (2c),
only 12.9% yield was obtained (Table 4, entry 2), mainly due
to most of it staying in the headspace region of the reaction
vessel and hardly participating in the reaction. Electron-
donating substituents like 1,2-epoxyhexane (2d) and 1,2-
epoxyoctane (2e) can be converted to the respective
carbonates in good to very good yields with prolonging
reaction time (Table 4, entries 3−5). Glycidyl methacrylate
(2f) was also examined (Table 4, entry 6) and displayed an
excellent yield of 90.5% under the optimal reaction conditions.
Styrene oxide (2g), a sterically more challenging substrate
(entry 7) was confirmed as a less-reactive epoxide, with only
71.8% yield over a further 3 h. This probably owed to the steric
hindrance of the phenyl group.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02321.
BVS, XPRD, IR, ESI-MS, UV−vis, TGA, and catalytic
properties of 1 and the structures of 1 (PDF)
Accession Codes
CCDC 1843010 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
CONCLUSIONS
■
In summary, we have successfully prepared for the first time
the unprecedented tungstoselenite, 1, stabilized by the
attachement of four {Mn(CO)3} organometallic groups. The
polyoxoanion contains novel {SeW3} and {SeW8} species,
I
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Article
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from Carbon Dioxide. Chem. Commun. 2009, 0, 1312−1330.
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of CO2 and Epoxides to Provide Polycarbonates and Cyclic
Carbonates. Chem. Soc. Rev. 2012, 41, 1462−1484.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: jyniu@henu.edu.cn (J.N.).
*E-mail: jpwang@henu.edu.cn (J.W.).
(8) Huang, J.-W.; Shi, M. Chemical Fixation of Carbon Dioxide by
NaI/PPh3/PhOH. J. Org. Chem. 2003, 68, 6705−6709.
ORCID
Jingkun Lu: 0000-0002-1962-8223
Jingyang Niu: 0000-0001-6526-7767
Jingping Wang: 0000-0001-8079-5939
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the National
Key R&D Program of China (2018YFB0605802) and the
Natural Science Foundation of China (Grants 20771034,
21401042).
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