Efficient fixation of atmospheric CO2 as carbonate by lanthanide-based complex via synergistic effect of zinc ion

Article abstract of DOI:10.1039/c3dt50230a

Reactions of [Zn(L)(H₂O)] precursor and EuCl₃ with the aid of CO₃^2- ions derived from atmospheric CO ₂ affording an unusual heteropolynuclear cluster [Zn ₄L₄Eu₄(CO₃)₆] ·EtOH. In the core of the cluster, four hetero-binuclear [ZnLEu] units linked by six CO₃^2- anions forms a well defined [Zn ₄Eu₄] skeleton with good planarity, which plays an important role in stabilizing the cluster.

Full text of DOI:10.1039/c3dt50230a

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Efficient fixation of atmospheric CO as carbonate by
2
lanthanide-based complex via synergistic effect of zinc
ion†
Cite this: DOI: 10.1039/c3dt50230a
Received 23rd January 2013,
Accepted 17th April 2013
DOI: 10.1039/c3dt50230a
Bin Zhang, Xiaodu Zheng, Hao Su, Yu Zhu, Chenxia Du* and Maoping Song*
www.rsc.org/dalton
2
Reactions of [Zn(L)(H
2
O)] precursor and EuCl
3
with the aid of CO
3
Despite the advantages of these kinds of clusters, the syn-
−
ions derived from atmospheric CO
Eu (CO
2
affording an unusual hetero- thesis of multinuclear lanthanide(III) complexes that efficiently
8
polynuclear cluster [Zn
4
L
4
4
3
)
6
]·EtOH. In the core of the capture aerial carbon dioxide still remains a great challenge.
2
−
cluster, four hetero-binuclear [ZnLEu] units linked by six CO
3
This is particularly due to the ligand controlled hydrolysis pro-
anions forms a well defined [Zn Eu ] skeleton with good planar- cesses of lanthanide(III)-based compounds are hard to control,
4
4
ity, which plays an important role in stabilizing the cluster.
which are especially favorable for the formation of lanthanide
9
carbonates. To the best of our knowledge, only two carbonato-
As is well-known, carbon dioxide has become a major contri-
butor to climate change as the fossil-fuel-dependent develop-
2−
3+
bridged lanthanide(III) clusters with the CO
3
–Ln
ratio
higher than 1 : 1 was obtained from fixation of atmospheric
ment results in the sharp increase of CO
2
concentration in the
7d,10
CO2.
carbonic anhydrase plays a critical function in respiration,
transporting CO between metabolizing tissues and the lungs,
which has inspired numerous studies about zinc(II) hydroxide
complexes as potential candidates for CO capture in the past
Considering the fact that zinc-containing enzymes
atmosphere over recent times. Therefore, reducing the CO
concentration in the atmosphere is currently a top priority.
2
1
2
Among different methods developed for capturing aerial
carbon dioxide from the viewpoint of better utilization of
carbon resources and the increasing concern on our environ-
2
11
two decades. Thus, we sought to develop Zn(II)–Ln(III) hetero-
nuclear complexes and to investigate whether it is possible for
zinc ions to play a synergistic effect in improving the ability of
lanthanide(III) clusters to capture carbon dioxide in air.
2
ment, porous materials consisting of metal–organic frame-
works (MOFs) have shown great potential in CO adsorption
2
2
due to their high specific surface areas (SSAs) (up to 5000 m
−
1
3
−1 3
g
) and pore size (up to 5 cm g ). However, most MOFs uti-
It is well known that the hexadentate Schiff-bases with
alkoxy groups at the 3-position can improve metal-bridging
capability and are useful for obtaining the 3d–4f heterometal-
lized for this application are very expensive and moisture sen-
4
sitive. An alternative method for efficient atmospheric CO2
fixation is the absorption of CO
2
with an in situ generated car-
12
lic complexes. And a variety of binuclear [MLn] complexes
5
bonato-bridged complexes. In this regard, more recently, poly-
nuclear lanthanide complex seems more promising in view of
the strong ability of lanthanide(III) hydroxides to interact with
carbon dioxide in air and the diversified coordination mode of
have been successfully obtained employing compartmental
ligands and demonstrated practical applications arising from
13
different metal ions. We herein decide to use hexadentate
Schiff-base as ligand to investigate the synergistic effect of zinc
ion in improving the ability of lanthanide-based complex for
6
center ions. This kind of carbonato-bridged polynuclear
lanthanide complexes, although still rare, have proven to
exhibit not merely appealing structures and unique magnetic
fixing CO . In this communication, the reaction of europium
2
trichloride and [ZnL(H
2
O)] precursor (H
2
L
=
N,N̲′-bis-
properties, but also potential applications in fixing atmos-
(
3-methoxy-salicylidene)-1,2-propanediamine) with the aid of
7
pheric CO
2
under ambient conditions.
2−
CO3 ions derived from atmospheric CO under proper con-
2
ditions results in a new heteropolynuclear cluster [Zn
4
L
4
Eu
4
-
2
−
(
3 6 3
CO ) ]·EtOH, which contains a metal–CO
core with high
2
−
2+
3+
CO3 –metal (Zn and Eu ) ratio of 3 : 4. To the best of our
knowledge, this is the first carbonato-bridged 3d–4f heteropo-
The College of Chemistry and Molecular Engineering, Zhengzhou University,
Zhengzhou, 450052, P. R. China. E-mail: dcx@zzu.edu.cn, mpsong@zzu.edu.cn;
Tel: +86 0371 67767896
2
lynuclear cluster generated by fixation of atmospheric CO .
The mononuclear precursor, [Zn(L)(H O)], was prepared in
2
†
Electronic supplementary information (ESI) available: Synthetic part, Fig. S1,
excellent yield (92%) by a similar method according to the lit-
erature. Air and moisture stable cluster was then synthesized
Tables S1 and S2. CCDC 910926 for [Zn Eu (CO ]·EtOH. For ESI and crystal-
4
4
L
4
3 6
)
1
4
lographic data in CIF or other electronic format see DOI: 10.1039/c3dt50230a
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Fig. 1 Molecular structure of [Zn
4
Eu
4
L
4
(CO
3
)
6
]·EtOH. Solvent molecules and
Fig. 2 The cluster core bridged by carbonate ion and the pseudo-octahedron
hydrogen atoms are omitted for clarity.
4 3 2
formed by [Eu (CO ) ].
by the reaction of EuCl ·6H O with [Zn(L)(H O)] in the molar (1.952(4)–1.970(3) Å) from O atoms of carbonate groups are sig-
3
2
2
ratio of 1 : 1 in ethanol with the presence of triethylamine (for nificantly shorter than those from the phenoxo oxygen atoms.
2
+
detailed synthesis see ESI†). Prismatic yellow crystals of The strong interactions between Zn ions and carbonate
Zn Eu L (CO ) ]·EtOH suitable for X-ray diffraction were anions imply that zinc ions may play a pivotal role in the
[
4
4
4
3 6
obtained in 32% yield by slow evaporation of the solution efficient adsorption process of CO
under aerobic conditions over two weeks. The sample was con- The four Eu(III) ions are situated in very similar coordi-
firmed by elemental analysis and IR spectroscopy (ESI†). The nation environments. Each Eu ion is located in the outer
2
.
3
+
−1
strong absorption bands at around 1536 and 1505 cm can be
2 2
O O site of the [ZnL] component, and is surrounded by nine
assigned to the stretching vibrations ν(C–O) of the two kinds oxygen atoms with a distorted tri-capped trigonal prismatic
2
−
of CO3 ions. The out-of-plane bending vibration of π(C–O) geometry. Four oxygen atoms are arising from the phenolic
appears at 849 cm− as a relatively weak absorption band. groups and the methoxy side arms of the Schiff-base ligand,
More carefully identification of different C–O vibration modes and the remaining five oxygen atoms are provided by four car-
seems impractical owing to the disturbance of Schiff-base bonate anions with both chelating and bridging modes. The
1
absorptions.
Eu–O bond lengths, involving the bridging/chelating deproto-
The molecular structure of the complex, determined by nated phenol oxygen atoms in the range of 2.402(3) Å to 2.437(3)
single-crystal X-ray diffraction, is depicted in Fig. 1. The crystal Å, are significantly shorter than those associated with the
parameters are shown in Table S1,† and selected bond lengths chelating methoxy oxygen atoms (2.768(4)–2.794(3) Å, average
1
6
and angles are given in Table S2 ESI.† Structural analysis 2.781 Å), which are similar to those reported in the literature.
2
−
shows that the complex is crystallized in the triclinic space The Eu–O bond lengths from oxygen atoms of CO3 anions
group P ˉ1 . The asymmetric unit comprises only half of the are with an average of 2.433 Å, which are in good agreement
6
molecules, with the other half being generated by a centre of with those previously reported in the literature.
inversion. And the molecule contains four nine-coordinated
One of the most notable structural features of the cluster is
Eu(III) and four five-coordinated Zn(II) cations, four ligands that four hetero-binuclear [ZnLEu] units linked by four carbon-
2
−
(
H L) and six CO
anions.
ate groups forms two well defined parallelogram skeletons
2
3
As shown in Fig. 1, the outer O O moieties of the [ZnL] (Zn , Eu ) with good planarity. In addition, they are all located
2
2
4
4
3
+
components coordinate to Eu ions, resulting in the for- in the same plane. The Eu⋯Eu distances range from 4.1475(5)
mation of hetero-binuclear [ZnLEu] units with the Zn⋯Eu sep- Å to 4.1528(4) Å and the angles vary from 88.950(9)° to 91.050(9)°,
aration of 3.4158(7)–3.4328(8) Å. The dinuclear units interact while the Zn⋯Zn distances are from 8.1905(7) Å to 8.3003(7) Å
with six carbonate ions through both chelating and bridging and the angles are from 89.929(9)° to 90.071(9)°, respectively
modes
resulting
in
a
heteropolynuclear
cluster (Fig. S1†). The four carbonate anions serve as tridentate
[
Zn Eu L (CO ) ]·EtOH.
Each Zn ion has a usual five-coordinate environment and a μ
bridges, which link the Zn(II) ion to the adjacent Eu(III) ions in
4
4
4
3 6
2
+
1
1
2
2−
3
–η :η :η fashion. The other two CO
3
ions lying above
adopts a distorted square pyramidal geometry, composed of and below the main plane are located on the inversion centre.
2
−
1
1
2
the inner N O core from the Schiff-base ligand (L ) as the Each of them bridges four Eu ions in a μ –η :η :η fashion,
2
2
4
basal plane and one coordinated O atom from the coordinated resulting in a pseudo-octahedron core formed by [Eu
CO3
4
(CO
occupying the apical position. The zinc atoms lie as shown in Fig. 2. This coordination mode is rarely observed
slightly above the N O planes with atom-to-plane distances of in carbonato-bridged polynuclear lanthanide complexes,
3 2
) ]
2
−
2
2
0
.5959 Å and 0.5772 Å for Zn1 and Zn2 respectively (Fig. 2). which may play an important role in stabilizing the cluster. In
Moreover, in the asymmetric unit, two N O basal planes are other words, the overall structure of the complex can be well
2
2
almost perpendicular to one another with a dihedral angle of described as a pseudo-octahedron [Eu (CO ) ] core surrounded
4
3 2
8
7.863°. The average bond lengths for Zn–O (phenolic) and stabilized by four [ZnN
2 3
O ] square pyramids.
(
2.028 Å) and Zn–N (2.040 Å) are comparable to those found in The above results illustrated that the presence of ZnL as a
1
5
the literature. Notably, the Zn–O bond lengths depend on seed may be critical for the formation of carbonato-bridged
the nature of the oxygen atoms, in which the bond lengths 3d–4f heteropolynuclear cluster generated by fixation of
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atmospheric CO
anism, we design a simple experiment to validate whether [Zn-
L)(H O)] plays a synergistic effect in fixing CO . The CO is
bubbled through ethanol solution of the precursor [Zn(L)-
H O)] containing NEt as a base at room temperature for
2
. As an attempt to lend credence to the mech-
(
2
2
2
(
2
3
about three hours, the resultant precipitate is obtained by eva-
porating the solvent under reduced pressure. A comparison of
the IR spectra of the precipitate with that of [Zn(L)(H O)] pro-
2
−
vides evidence for the formation of [LZn–OCO
2
H] species.
−
1
The signals corresponding to ν(C–O) at 1697 and 1295 cm
are in good agreement with those of [ZnOCO H] in other
−
2
1
7
−1
systems. The peaks at 696 and 835 cm can be assigned to
the rocking vibration of ρ (OCO) and out-of-plane bending
vibration of π(C–O) respectively.
r
1
8
Moreover, the broad
−1
vibration at 3456 cm in the resultant precipitate compared Fig. 4 The suggested reaction pathway for the formation of [Zn
2
O)] at 3418 cm is considered as evi-
dence that there exists an –OH group of the [LZn–OCO H] .
4 4 4 3 6
L Eu (CO ) ]
−
1
cluster. The labeling of atoms is illustrated on the individual structures.
with that of [Zn(L)(H
−
2
2
−
Considering that the conversion of CO
2
to CO
3
may be
reaction, (iv) assembly and rearrangement of the intermediate
C resulting in the formation of the final product (D). The
suggested reaction pathway is depicted in Fig. 4. However, due
to the diversified coordination modes of europium ion, the
presence of other intermediates in the reaction process can
not be completely excluded. Further work is needed to fully
understand the reaction mechanism.
In conclusion, we have described the preparation and struc-
tural characterization of a new carbonato-bridged 3d–4f hetero-
polynuclear cluster, and the result shows that this complex
similar to the mechanism of carbonic anhydrase demonstrated
in a series of zinc(II) complexes. It is reasonable to assume
that the initial deprotonation of zinc-bound water molecule
Zn(L)(OH)] plays a fundamental role in the formation of zinc
bicarbonate derivative [LZn–OCO H] , which involves the
nucleophilic attack of hydroxo species to the electrophilic C
1
9
−
[
−
2
atom of CO
Fig. 3.
2
. The suggested reaction scheme is illustrated in
The above experiment results illustrate that the ZnL species
bound to the Eu ions may act as nucleophiles in the hydration
of CO and play a synergistic effect in fixing CO , which should
be critical to the formation of the cluster. Therefore, the
overall reaction pathways for the formation of the final cluster
can be recommended as the following steps: (i) instant for-
could efficiently capture atmospheric CO under ambient con-
2
2
2
ditions. To the best of our knowledge, such a lanthanide-
2−
3+
based polynuclear complex with the high CO
3
–Eu ratio of
2−
10b
3
: 2 (CO3 –metal of 3 : 4) is very scarce in the literature.
This characteristic may provide important evidence for the
synergistic effect of the Zn ions in improving the ability of
Eu ions to capture carbon dioxide in air. Remarkably, this
synthetic approach may represent a promising new route
towards the design of new lanthanide clusters for enhance-
3
+
mation of [Zn(L)(H
EuCl into the ethanol solution of [Zn(L)(H
ence of triethylamine, (ii) deprotonation of the zinc-bound
water molecule with Et N to give the active zinc hydroxide
derivative [Zn(L)(OH)Eu(H
2
O)Eu(H
2
O)
4
]
hydrate (A) upon addition of
2+
O)] with the pres-
3+
3
2
1
2
3
2
+
2
4
O) ] , followed by nucleophilic
ment of CO capture as well as novel functional materials with
2
attack of the zinc bound hydroxide at the carbon dioxide
substrate to give a hetero-binuclear bicarbonate intermediate
(B), (iii) reaction with transformation
of species B into species C through further CO2 uptake
intrinsic properties arising from different meal ions. Studies
focused on magnetic properties of the Gd(III) and Dy(III) ana-
logues and a spectroscopic study of Nd(III) clusters are in
progress.
2+
2 2 4
[Zn(L)(OCO H)Eu(H O) ]
This work was supported by a grant from the National
Natural Science Foundation of China [Grant No. 20771093].
Notes and references
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structures.
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