A nona-vacant Keggin-type tricarbonyl rhenium derivative {[PMo3O16][Re(CO)3]4}5- and its catalytic performance for CO2 cycloaddition reactions

Article abstract of DOI:10.1039/c5ra14201f

A nona-vacant Keggin-type tricarbonyl rhenium derivative [(NH₄)₅]{[PMo₃O₁₆][Re(CO)₃]₄}·1.5H₂O was obtained and characterized. Its frontier orbitals were computed by density functional theory (DFT) calculations. Furthermore, it could act as a Lewis acid catalyst and promote the conversion of CO₂ to cyclic carbonate under mild reaction conditions with pyrrolidinium bromide as a co-catalyst.

Full text of DOI:10.1039/c5ra14201f

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A nona-vacant Keggin-type tricarbonyl rhenium
5
ꢀ
derivative {[PMo O ][Re(CO) ] } and its catalytic
3
16
3 4
Cite this: RSC Adv., 2015, 5, 69006
performance for CO cycloaddition reactions†
2
Received 24th July 2015
Accepted 31st July 2015
Zhiyuan Huo, Jipeng Guo, Jingkun Lu, Qiaofei Xu, Pengtao Ma, Juan Zhao,
Dongdi Zhang, Jingyang Niu* and Jingping Wang*
DOI: 10.1039/c5ra14201f
www.rsc.org/advances
A nona-vacant Keggin-type tricarbonyl rhenium derivative [(NH
[PMo 16][Re(CO) }$1.5H O was obtained and characterized. Its liquids. Recently, our group reported a POM-based trirhenium
frontier orbitals were computed by density functional theory (DFT) carbonyl derivative, [(AsW O ){Re(CO) } (m -OH)(m -OH)
4 5
) ]
the cycloaddition of CO to epoxides effectively in ionic
2
10
{
3
O
]
3 4
2
6
ꢀ
,
1
1
39
3 3
3
2
calculations. Furthermore, it could act as a Lewis acid catalyst and which could catalyze the cycloaddition reaction under harsh
11
promote the conversion of CO to cyclic carbonate under mild conditions.
2
reaction conditions with pyrrolidinium bromide as a co-catalyst.
Hence, the need to develop the design of new POM-
supported rhenium carbonyl derivatives still remains, as the
derivatives may have enhanced catalytic properties toward
effective CO2 conversion. In this paper, we have obtained a
nona-vacant Keggin-type tricarbonyl rhenium derivative
Polyoxometalate (POM)-supported carbonyl metal derivatives,
1
as a category of interesting organometallic oxides, have
attracted growing attention owing to their rather unique struc-
[
(NH ) ]{[PMo O ][Re(CO) ] }$1.5H O (1), which has been
4 5 3 16 3 4 2
2
tures and range of fascinating properties. A series of these
structurally characterized and computed by density functional
theory (DFT). We also investigated the catalytic properties of the
derivative in the cycloaddition of CO to epoxides under mild
2
compounds with their synthetic processes, special structures,
3
–7
and catalytic properties has been reported previously.
However, most of these derivatives are dominated by Lindqvist-
reaction conditions in pyrrolidinium bromide.
3–5
type POMs, in contrast, Keggin-type POM-supported carbonyl
metal derivatives have rarely been reported mainly because
Keggin-type polyoxoanions have smaller charge densities to
combine with metal carbonyl groups. Recently, an increasing
amount of attention has been directed towards rhenium con-
taining POMs due to the attractive catalytic properties of the
rhenium atom. Re(I) complexes have been demonstrated to be
4 6
Compound 1 was prepared by the reaction of (NH ) -
Mo O $4H O, Na HPO $12H O, Mn(AC) $4H O and Re(CO) Cl
7
24
2
2
4
2
2
2
5
in a CH OH–H O mixed solvent (Section S1, ESI†). Single crystal
3
2
X-ray diffraction analysis reveals that compound 1 crystallizes in
the monoclinic space group C
2
/m. Compound 1 consists of a
5ꢀ
+
{
[PMo
3
O
16][Re(CO)
3
]
4
}
(1a) unit, ve NH
4
cations, and one
9ꢀ
3 16
and a half crystal water molecules. Notably, the [PMo O ]
effective for the transformation of CO and have usually been
2
fragment presents the highest vacant Keggin-polyoxometalate
derivative. It can be considered as a unique fragment by
removal of three corner-shared Mo O triads from a saturated
8
used as efficient photocatalysts for the reduction of CO
However, their application in the coupling of CO
to produce cyclic carbonates remains very rare. Hua et al. rst
described that [Re(CO) Br] could catalyze the synthesis of cyclic
2
.
2
with epoxides
3
10
Keggin [PMo O ] unit. The POM moiety has twelve terminal
1
2 40
5
oxygen atoms and three m -O atoms, providing a large coordi-
2
9
carbonates under harsh conditions. Subsequently, Wong et al.
nation ability for bonding the Re centers of the rhenium
carbonyl clusters, and forming the stable structure of
reported that a tricarbonyl rhenium(I) complex could catalyze
5
ꢀ
{
[PMo
3
O
16][Re(CO)
3
]
4
}
(Fig. 1a). The tricarbonyl rhenium
units here can be divided into two categories, on one hand, the
Re1 atom coordinated with three carbonyl ligands is attached to
the Mo O trimer via three m -O atoms, meanwhile, the other
Henan Key Laboratory of Polyoxometalate Chemistry, Institute of Molecular and
Crystal Engineering, College of Chemistry and Chemical Engineering, Henan
University, Kaifeng 475004, Henan, China. E-mail: jyniu@henu.edu.cn; jpwang@
henu.edu.cn; Fax: +86 371 23886876
3
16
2
two Re2 atoms and the Re3 atom is attached to the terminal
oxygen atoms of the Mo O trimer and the PO tetrahedron.
†
Electronic supplementary information (ESI) available: Experimental section,
3
16
4
supplementary crystal structure gures, elemental analyses, IR spectrum, XRPD Notably, the cubane subunit of {ReMo
3
O
4
} was formed by the
patterns, thermogravimetric analysis, and computational methods. CCDC Re1 atom and three Mo atoms (Fig. 1b). The bond valence sum
1
057225. For ESI and crystallographic data in CIF or other electronic format see
(BVS) calculations show that all Mo and P atoms of 1a are in
DOI: 10.1039/c5ra14201f
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the +6 and +5 oxidation states, respectively (Table S4, ESI†).
Furthermore, the Re–C bond lengths in 1a range from 1.81 to
˚
˚
1
.938 A, while the C–O bond lengths range from 1.101 to 1.29 A
(
Table S2, ESI†). These bond lengths are consistent with that
6
c,7
observed for other Re(I) carbonyl complexes.
As seen in Fig. 2, the DFT calculations prove that the HOMO
Fig. 2 Several highest occupied (H ¼ HOMO) and lowest unoccupied
orbitals are localized on the tricarbonyl rhenium fragment of (L ¼ LUMO) orbitals of 1a and their orbital energies in eV.
5
ꢀ
{
[PMo
3
O
16][Re(CO)
3
]
4
}
, in contrast, the LUMO orbitals wholly
9
ꢀ
existing in the [PMo
3
O
16
]
moiety are principally W–O anti-
bonding orbitals. Electrons could transfer easily from the Re
(
entry 2). The yield of 3a was slightly affected by the CO pressure
2
9
ꢀ
atoms to the [PMo O ] moiety when the Re atoms are in their
3
16
in the low pressure range (entries 4 and 5). However, the yield
excited states, which makes the Re atoms more electropositive.
was strongly dependent on the reaction temperature (entries 6–
˚
ꢁ
ꢁ
Coincidentally, the bond length (Re2–C5: 1.94 A; Fig. 1a) is
8). The increase of the reaction temperature from 60 C to 65 C
˚
longer than others (z1.86 A), indicating that the Re2–C5 bond
resulted in a rapid increase in the yield from 36.8 to 82.7%
is highly activated and more prone to breaking. Thus, the Re2
atom is proven to be the Lewis acid center when it is in its
(
7
entries 6 and 7). On increasing the reaction temperature to
5 C, 97.8% yield was achieved (entry 8). When compound 1 was
ꢁ
excited state, since the POM moiety acts as a strong electron- replaced by [(NH ) ]{[Mo O H ][Re(CO) ] }$14H O (two Re
4
4
8
30
6
3 2
2
withdrawing group. Exhilaratingly, as reported in the litera-
ture recently, there are many binary catalyst systems containing
Lewis acid and Lewis base centers that provide good yields for
centers) as the catalyst at the same loading, 80.6% yield was
obtained. By contrast, using a [(CH ) N] [(OC) Re(H AsMo O )]$
3
4
3
3
3
9 32
2
11H O catalyst (just one Re center) at the same loading, only
12
the cycloaddition of CO
have good catalytic properties toward the CO
2
to epoxides. Hence, compound 1 may
cycloaddition
7
2.5% yield was achieved. However, no yield was obtained
ꢁ
2
without co-catalyst 2 at 70 C over 30 minutes (Table 1, entry 13).
DFT calculations and the study on the catalytic properties of
these POM-based rhenium carbonyl derivatives indicated that
reaction. Meanwhile, room-temperature ionic liquids used as
co-catalysts are especially benecial for the transformation of
CO
negligible vapor pressure and excellent selectivity.
more, the halide ions in ionic liquids usually act as Lewis base
2
because of their high CO solubility, high ion conductivity, the Re centers were regarded as active sites.
2
13–15
Further-
2
centers that could greatly lower the activation energy barrier of Table 1 Effect of the reaction parameters on the coupling of CO and
chloromethyloxirane catalyzed by catalyst 1 using ionic liquid 2 as the
co-catalyst
the cycloaddition reaction. In this paper, we use 1-ethyl-1-
methylpyrrolidinium bromide (2) as the co-catalyst.
a
The ability of compound 1 to catalyze the synthesis of cyclic
carbonates from CO and epoxides with the co-catalyst of ionic
2
liquid 2 was researched. Furthermore, the effects of several
reaction parameters on the cycloaddition reaction were studied
in detail by using the coupling of chloromethyloxirane (2a) with
CO2 to produce 4-chloromethyl-1,3-dioxolan-2-one (3a) as a
model reaction. In a blank experiment (Table 1, entry 1), ionic
liquid 2 itself could convert 2a into 3a through a Lewis acid–
Lewis base mechanism, though the yield was much lower.
However, the yield was remarkably increased to 93.6% by the
addition of 0.3 mol% (relative to epoxide) of 1 for 30 minutes
b
ꢁ
c
Entry
Catalyst
P (MPa)
T ( C)
Time (min)
Yield (%)
16
1
2
3
4
5
6
7
8
—
1
1
1
1
1
1
1
1
1
4
5
1
1.0
1.0
1.0
0.5
1.5
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
70
70
70
70
70
60
65
75
70
70
70
70
70
30
30
15
30
30
30
30
30
30
30
30
30
30
37.2
93.6
46.1
85.9
96.8
36.8
82.7
97.8
91.5
88.3
80.6
72.5
—
d
9
e
1
1
1
1
0
1
2
3
f
g
h
a
Reaction conditions: catalyst 1 (0.3 mol%), chloromethyloxirane 2a (5
b
mmol, 390 mL), co-catalyst 2 (7 mol%, 70 mg). Initial pressure at room
temperature. Determined by GC using dimethyl phthalate as an
internal standard, the selectivity was over 99% in all cases. Yield of
the 2nd run in the recycling studies. Yield of the 3rd run in the
c
d
e
f
recycling studies. Using an [(NH
4
)
4
]{[Mo
8
O
30
H
6
][Re(CO)
3
]
2
}$14H
2
O
a
g
catalyst (catalyst 4) at the same loading.
(CH N] [(OCh) Re(H AsMo 32)]$11H
3 4
O }. same loading. Without ionic liquid 2.
Using
Fig. 1 (a) Polyhedral and ball-and-stick representation of polyanion
[
3
)
4
3
3
3
9
O
2
O catalyst (catalyst 5) at the
1
a. (b) Ball-and-stick representation of cubane subunit {ReMo
PO , light orange; WO , light blue.
4
6
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The recyclability of the catalyst system was examined in supported rhenium carbonyl derivatives with diverse structures
subsequent reactions. The results showed that it could be and potentially powerful catalytic properties for the cycloaddi-
recycled at least 3 times with a general decline in yield to 88.3% tion reaction.
for the 3rd run (Table 1, entry 10). Furthermore, the IR spectra
of the recycled catalyst system were highly similar to that of the
fresh one (Fig. S7, ESI†), indicating that the catalyst system was
Acknowledgements
stable.
We gratefully acknowledge support from the NSFC (grants
We then focused on further application of the catalyst 91222102 and 21371048) and Dr Fu Qiang Zhang (Shangqiu
system’s promising catalytic potential. As summarized in Table Normal College) for his dedication to the DFT computations
2, the catalytic activity of the system depends extremely on the and constructive suggestions.
structure of the employed epoxides. Both glycidyl methacrylate
(
2b) and glycidyl phenyl ether (2c) are highly polar. Glycidyl
Notes and references
ꢁ
methacrylate (2b) exhibited an attractive yield of 92.7% at 70 C
for 1 h (Table 2, entry 1) and glycidyl phenyl ether (2c) displayed
a good yield of 90.6% (Table 2, entry 2). Styrene oxide (2d) was
conrmed as a less-reactive epoxide, with only 63.2% yield
achieved in 2 h followed by a slow increase in yield to 87.5%
over a further 3 h (Table 2, entry 3 and 4). This may result from
the steric hindrance of the phenyl group. However, the aliphatic
terminal epoxide 1,2-epoxyhexane (2e), which is relatively non-
polar, showed very sluggish reactivity (Table 2, entry 5 and 6).
For low boiling propylene oxide (2f), only 10.8% yield was
obtained aer 1 h, which is because propylene oxide exists as a
gas at 1.0 MPa and 70 C, most of which then stays in the
headspace region of the reaction vessel and hardly participates
in the reaction.
In summary, we have reported a nona-vacant Keggin-type
tricarbonyl rhenium derivative in this paper. Furthermore,
compound 1 shows good catalytic activity for the cycloaddition
reaction with co-catalyst ionic liquid 2. The experimental results
and DFT calculations greatly promote the design of more POM-
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ꢁ
a
b
Entry
1
Substrate
Time (h)
1
Product
Yield (%)
2b
2c
92.7
90.6
7
2
1
3
4
2d
2d
2
3
63.2
87.5
5
6
2e
2e
4
9
43.1
91.3
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a
Reaction conditions: catalyst 1 (0.3 mol%), epoxide (5 mmol), ionic
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b
liquid 2 (7 mol%, 70 mg). Determined by GC using dimethyl
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2
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