Encapsulation of atmospheric CO2 by a self-assembled decanuclear cadmium complex via unfamiliar perchlorato and carbonato bridges

Article abstract of DOI:10.1039/c2cc35171d

A decanuclear Cd complex has been found as a carbonate-containing capsule. The structure strongly resembles a ten-blade waterwheel with a central carbonate ligand surrounded by two superimposed Cd₅O₅ crowns with a pentagonal antiprism-like disposition. The capsule is doubly capped by two pentadentate perchlorate anions. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc35171d

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2012, 48, 9915–9917
www.rsc.org/chemcomm
COMMUNICATION
Encapsulation of atmospheric CO2 by a self-assembled decanuclear
cadmium complex via unfamiliar perchlorato and carbonato bridgesw
Ana M. Garcı
´
Jesus Sanmartı
a-Deibe,a Cristina Portela-Garcı
´
´
a,a Matilde Fondo,a Antonio J. Motab and
´
n-Matalobos*a
Received 18th July 2012, Accepted 17th August 2012
DOI: 10.1039/c2cc35171d
A decanuclear Cd complex has been found as a carbonate-
containing capsule. The structure strongly resembles a ten-blade
waterwheel with a central carbonate ligand surrounded by two
superimposed Cd5O5 crowns with a pentagonal antiprism-like
disposition. The capsule is doubly capped by two pentadentate
perchlorate anions.
The chemical reactivity of carbon dioxide at metal centres has
drawn longstanding scientific interest both in terms of funda-
mental understanding and applicability.1 A series of zinc,
cadmium, copper, silver, cobalt, nickel and lanthanide com-
plexes have been successfully used to afford carbonato com-
plexes by reaction with atmospheric CO2.2–6 Pariya et al.6a
reported the first example of a dinuclear cadmium complex
with a carbonato bridge by spontaneous reaction, under
atmospheric conditions, of the starting mononuclear complex
with CO2. Dinuclear cadmium hydroxo complexes have also
been shown to react with atmospheric CO2 to form carbonato-
bridged species.6c–e
Studies on dinuclear zinc, nickel and cobalt complexes4a
have shown that the formation of carbonato complexes in a
basic reaction medium occurs through an attack of a hydroxo
nucleophilic species to the electrophilic carbon atom of the
CO2 molecule. This hydroxo complex seems to derive from an
aqua-bridged complex [M2L(m-OH2)]+, where the aqua ligand
is activated by the two Lewis-acidic metal centres, and this
should favour its deprotonation to give a hydroxo-bridged
complex of the type [M2L(m-OH)]+.
In the course of our investigations to pursue systems with high
nuclearity involving CdII coordination, our synthetic attempts to
introduce both the linear H2L [H2L = (E)-2-((1H-imidazol-
2-yl)methyleneamino)phenol), Fig. 1 (middle)] and a non-planar
molecule (perchlorate) as bridging ligands led to the isolation of
Fig. 1 Schematic representation of the reactions that give rise to the
cadmium complexes presented in this work. See ESIw for details.
the cage structure [Cd10(L)4(HL)6(ClO4)2(CO3)]2+ (1), shown in
Fig. 1 (top left), which resembles a ten-blade waterwheel based
on a decorated Cd10O10 double crown (Fig. S1 in the ESIw file).
The study was initiated with the reaction between imidazole-
2-carboxaldehyde (1 equiv.), 2-aminophenol (1 equiv.) and
[Cd(H2O)6](ClO4)2 (0.5 equiv.). Under these conditions, only
Cd(HL)2 (2, Fig. S3 in ESIw) is formed in bulk form (Fig. 1,
right bottom). 2 was also obtained from Cd(AcO)2Á2H2O
(0.5 equiv.) and isolated H2L (1 equiv.) as starting reagents
(Fig. S4 in ESIw). However, when both Cd(AcO)2Á2H2O and
[Cd(H2O)6](ClO4)2 (1 and 0.5 equiv., respectively) were used,
the main reaction product was Cd2(L)(HL)(HO) (3, see ESIw).
Moreover, the decanuclear cadmium cluster 1 was also
obtained in small amount (B2%), by extended exposure of
the mother liquour (MeOH) to an air atmosphere, while being
slowly evaporated (top-left in Fig. 1). This suggests that,
a
´ ´ ´
Departamento de Quımica Inorganica, Facultad de Quımica,
Universidad de Santiago de Compostela,
15782-Santiago de Compostela, Spain.
E-mail: jesus.sanmartin@usc.es; Fax: +34 981 528073
b Departamento de Quı´mica Inorga´nica, Facultad de Ciencias,
Universidad de Granada, 18002-Granada, Spain
w Electronic supplementary information (ESI) available: Syntheses
and characterization of the ligand and its complexes, X-ray diffraction
data of 1, 2, and H2L, additional views of 1, and computational details
of 4. CCDC 892600–892602 (1, 2, and H2L). For crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2cc35171d
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 9915–9917 9915

View Article Online
similarly to that reported for other dinuclear metal complexes,4a
the carbonato complex 1 could be formed by an attack of the
hydroxo species 3 to the carbon atom of the CO2 from the
atmosphere. Both, water present in starting metal salts and
Lewis acidity of Cd2+ could have assisted in the formation of 3.
With the aim of checking if the hydroxo dinuclear complex 3
could give rise to the carbonate encapsulation by reaction with
CO2, we have passed a stream of this gas through a methanol
solution of 3, after adding cadmium perchlorate. Powder
X-ray diffraction studies on the bulk solid isolated upon
filtration matched well with the diffraction pattern obtained
from the single-crystals of 1, which indicates the participation
of 3 in the formation of 1. Complex 1 gave satisfactory
elemental analysis, and its IR spectrum shows not only bands
associated with the phenol-based ligand and the perchlorate
group, but also the presence of two strong bands at 1463 and
1440 cmÀ1, consistent with C–O stretching frequencies observed
for carbonato bridges.4d,h,5b,6b
Two of the faces of the final structure are formed by the two
perchlorato ligands that, since they bridge more than two
metal atoms, can be considered as 2D ligands.9 Fig. S6 (top),
in the ESIw file, shows the coordination modes (m2, m3, m4 and
m12) described to date when a perchlorate anion acts as a
bridging ligand in polynuclear compounds.10
Finally, the cage contains a single carbonato ligand presenting
two new coordination modes (m8 and m9) in the two cations
of 1 found in the asymmetric unit of the crystal (Fig. 3). The
coordination modes described to date for carbonate anions
acting as bridging ligands in polynuclear compounds11 (m2, m3,
m4 and m6) are summarised, in the ESIw file, in Fig. S6
(bottom). The structure of the capsule shows evidence that
carbonate becomes permanently trapped within it and cannot
be released without breaking one or more chemical bonds.
The structure of Cd2(L)(HL)(ClO4)(MeOH)2 (4), which did
not crystallise in our hands, was elucidated on the basis of IR,
NMR and mass spectroscopies. The mass spectrum of this
complex shows a peak at 696.8 m/z with an isotopic profile
which matches well with that calculated for the cationic species
[Cd2(L)(HL)(ClO4) + H]+ (Fig. S7, ESIw). The IR spectrum
of 4 contains triply split band maxima at about 1145, 1120 and
1087 cmÀ1 and two peaks at about 636 and 625 cmÀ1 that were
attributed to a perchlorato ligand with local C2v symmetry,
supporting its coordination in a chelating bidentate fashion.12
A strong complex absorption with numerous submaxima
originating from non-fundamental n(NH) modes and n(CH)
vibrations is present between 3150 and 2500 cmÀ1 in the IR
spectrum of H2L, although the NH stretching would be
expected13 at around 3090 cmÀ1. Upon complexation, the
relative intensities of the NH absorptions decrease and, as a
With the aim of establishing the significance of the presence
of the acetate species as a source of hydroxide in the enhance-
ment of the reactivity of the cadmium complexes towards
atmospheric CO2, we also performed another reaction in the
absence of Cd(AcO)2Á2H2O, i.e. we only used [Cd(H2O)6](ClO4)2
(1 equiv.) and H2L (1 equiv.) as starting reagents. As a result, a
compound with Cd2(L)(HL)(ClO4)(MeOH)2 stoichiometry (4)
(main), and Cd(HL)2 (2) (by-product) were obtained (top-right in
Fig. 1), showing that the presence of acetate is critical to form the
carbonato complex 1.
Single-crystal X-ray diffraction analysis reveals that in 1,
perchlorato, carbonato and phenoxo bridges, i.e. metal–ligand
interactions, are responsible for the self-assembly event through
reaction with atmospheric CO2. This decanuclear cadmium
complex shows several remarkable features such as a high
nuclearity that results in the formation of a 28 vertex capsule
(Cd10Cl2O16) (Fig. 2, left), in which the metal ions are either
penta-, hexa-, or heptacoordinated (Fig. 2, right). Taking only
the cadmium centres into consideration, the cage strongly
resembles the geometrical structure of a Cd10O10 pentagonal
antiprism, giving the appearance of a wheel or a double crown
(Fig. S1, ESIw).
Furthermore, this cage is formed from two previously
unreported pentacoordinate perchlorato ligands (m5), and ten
pentacoordinate phenol-based ligands displaying the very rare
m3 bridging mode.7 The binding sites of the phenol-based
ligands form ten of the edges of the capsule in a similar manner
to that reported in scaffold architectures involving 1D ligands.8
result, the more prominent band (which is around 3060 cmÀ1
)
could be used as a sign of the presence of the NH group in 4
(Fig. S8, ESIw). Compared with the 1H-NMR spectrum
of H2L, that of 4 shows a typical pattern of a coordinated
Schiff-base ligand with imine and aromatic protons showing
only small shifts.14 The signals at about 3.16 (3H) and
4.12 (1H) ppm reveal the presence of coordinated methanol
(Fig. S9, ESIw).
Computational studies15 were undertaken in order to gain
insight into the structure and type of perchlorato coordination
mode involved in 4. Theoretical calculations (DFT/B3LYP,
see ESIw for further details) performed in gas phase predict
Fig. 2 Two views of the Cd10Cl2O16 capsule. Left: the cage containing
a carbonato ligand as an encapsulated guest. Right: the pseudo-
pentagonal antiprism of the decanuclear cluster showing the coordination
polyhedra of cadmium ions with both capping perchlorato ligands.
Fig. 3 Cd10O10 double crowns with their carbonate guests, showing
two new coordination modes m8 (left) and m9 (right), found in the
asymmetric unit of 1. Non-coordination Cd–O distances (A) are
indicated (see ESIw).
c
9916 Chem. Commun., 2012, 48, 9915–9917
This journal is The Royal Society of Chemistry 2012

View Article Online
Inorg. Chem., 2008, 47, 7784; (d) L.-Y. Kong, Z.-H. Zhang,
H.-F. Zhu, H. Kawaguchi, T. Okamura, M. Doi, Q. Chu,
W.-Y. Sun and N. Ueyama, Angew. Chem., Int. Ed., 2005,
44, 4352; (e) B. Sarkar, B.-J. Liaw, C.-S. Fang and C. W. Liu,
Inorg. Chem., 2008, 47, 2777; (f) A. Graham, S. Meier, S. Parsons
and E. P. Winpenny, Chem. Commun., 2000, 811; (g) J.-M. Chen,
W. Wei, X.-L. Feng and T.-B. Lu, Chem.–Asian J., 2007, 2,
710–719; (h) X.-L. Tang, W.-H. Wang, W. Dou, J. Jiang,
W.-S. Liu, W.-W. Qin, G.-L. Zhang, H.-R. Zhang, K.-B. Yu and
L.-M. Zheng, Angew. Chem., Int. Ed., 2009, 48, 3499.
5 (a) S.-D. Bian, J.-H. Jia and Q.-M. Wang, J. Am. Chem. Soc., 2009,
131, 3422; (b) D. Sun, G.-G. Luo, N. Zhang, R.-B. Huang and
L.-S. Zhenga, Chem. Commun., 2011, 47, 1461.
6 (a) C. Pariya, V. G. Puranik and N. R. Chaudhuri, Chem.
Commun., 1997, 1307; (b) D. E. Janzen, M. E. Botros,
D. G. VanDerveer and G. J. Grant, Dalton Trans., 2007, 5316;
(c) R. A. Allred, S. A. Huefner, K. Rudzka, A. M. Arif and
L. M. Berreau, Dalton Trans., 2007, 351; (d) R. A. Allred,
A. M. Arif and L. M. Berreau, J. Chem. Soc., Dalton Trans.,
2002, 300; (e) R. A. Allred, L. H. Alexander, A. M. Arif and
L. M. Berreau, Inorg. Chem., 2002, 41, 6790.
7 (a) A. Ray, G. M. Rosair, R. Rajeev, R. B. Sunoj, E. Rentschler
and S. Mitra, Dalton Trans., 2009, 9510; (b) S. Biswas, S. Naiya,
M. G. B. Drew, C. Estarellas, A. Frontera and A. Ghosh, Inorg.
Chim. Acta, 2011, 366, 219.
8 R. Vilar, D. M. P. Mingos, A. J. P. White and D. J. Williams,
Angew. Chem., Int. Ed. Engl., 1998, 37, 1258.
9 J. W. Steed, D. R. Turner and K. J. Wallace, Core Concepts in
Supramolecular Chemistry and Nanochemistry, John Wiley & Sons
Ltd., Chichester, England, 2007.
Fig. 4 DFT/B3LYP calculated structure for complex 4, from the
collected spectroscopic data.
that the perchlorate bridged isomer (m2:Z2) is less stable than
the mono-coordinated one by 6.0 kcal molÀ1 (DH = +4.3,
DG = +4.2), although several species (Fig. S5, ESIw) could
participate in a complex equilibrium before delivering the final
product. The structure of the chelating bidentate isomer of the
dinuclear cadmium complex is shown in Fig. 4. Interestingly,
the LÀ form is much more stable than the methoxo form by
13.6 kcal molÀ1 (DH = +12.8, DG = +10.7), allowing us to
correctly balance the negative charge, indeed.
In conclusion, we reported the synthesis of a novel closed
structure by metal-directed self-assembly, using both one-
dimensional and two-dimensional ligands. The capsule shows
evidence that a CO32À ion, which is derived from atmospheric
CO2 through chemical conversion in the presence of a weak
Brønsted base, becomes permanently trapped within it. In the
core of the cage, the carbonato ligand adopts new bridging
modes (m8 or m9) to link the Cd2+ ions. In the shell of the cage,
each one of the perchlorato ligands adopts an unprecedented
m5 bridging mode. Moreover, the phenolato ligands close the
cage via a very rare m3 bridging mode.
10 A. R. Paital, V. Bertolasi, G. Aromi, J. Rivas-Arino and D. Ray,
Dalton Trans., 2008, 861.
11 (a) N. W. Davies, A. S. P. Frey, M. G. Gardiner and J. Wang, Chem.
Commun., 2006, 4853; (b) Y. Jiang, X. Wang, X. Ying, F. Zhong, J. Cai
and K. He, Inorg. Chem. Commun., 2006, 9, 1063; (c) N. Kitajima,
T. Koda, S. Hashimoto, T. Kitagawa and Y. Moro-oka, J. Am. Chem.
Soc., 1991, 113, 5664; (d) S. C. Rawle, C. J. Harding, P. Moore and
N. W. Alcock, J. Chem. Soc., Chem. Commun., 1992, 1701;
(e) G. Kolks, S. J. Lippard and J. Waszczak, J. Am. Chem. Soc.,
1980, 102, 4832; (f) X.-M. Chen, Q.-Y. Deng and G. Wang, Poly-
hedron, 1994, 13, 3085; (g) C. Bazzaicalupi, A. Bencini, A. Bencini,
A. Bianchi, F. Corana, V. Fusi, C. Giorgi, P. Paoli, P. Paoletti,
B. Valtancoli and C. Zanchini, Inorg. Chem., 1996, 35, 5540;
(h) N. N. Murthy and K. Karlin, J. Chem. Soc., Chem. Commun.,
1993, 1236; (i) X. Liang, J. A. Parkinson, S. Parsons, M. Weishaupl
and P. J. Sadler, Inorg. Chem., 2002, 41, 4539; (j) L.-Y. Kong,
H.-F. Zhu, Y.-Q. Huang, T.-A. Okamura, X.-H. Lu, Y. Song,
G.-X. Liu, W.-Y. Sun and N. Ueyama, Inorg. Chem., 2006, 45, 8098.
12 (a) D. A. House, P. J. Steel and A. A. Watson, J. Chem. Soc.,
Chem. Commun., 1987, 1575; (b) J. L. Pascal and F. Favier, Coord.
Chem. Rev., 1998, 178, 865 (Coord. Chem. Rev., 2000, 209, 453)
erratum.
Notes and references
1 (a) W. Leitner, Coord. Chem. Rev., 1996, 153, 257; (b) X. L. Yin
and J. R. Moss, Coord. Chem. Rev., 1999, 181, 27;
(c) D. B. Dell’Amico, F. Calderazzo, L. Labella, F. Marchetti
and G. Pampaloni, Chem. Rev., 2003, 103, 3857; (d) G. A. Olah,
A. Goeppert and G. K. S. Prakash, J. Org. Chem., 2009, 74, 487;
(e) K. Caldeira, A. K. Jain and M. I. Hoffert, Science, 2003,
299, 2052; (f) D. S. Jenkinson, D. E. Adams and A. Wild, Nature,
1991, 351, 304; (g) R. Quinn, J. B. Appleby and G. P. Pez, J. Am.
Chem. Soc., 1995, 117, 329; (h) K. Yamaguchi, T. S. Calderwood
and D. T. Sawyer, Inorg. Chem., 1986, 25, 1289.
2 (a) L. Y. Kong, Z. H. Zhang, H. F. Zhu, H. Kawaguchi,
T. Okamura, M. Doi, Q. Chu, W. Y. Sun and N. Ueyama, Angew.
Chem., 2005, 117, 4426 (Angew. Chem., Int. Ed., 2005, 44, 4352);
(b) L. Y. Kong, H. F. Zhu, Y. Q. Huang, H. Kawaguchi, X. H. Lu,
Y. Song, G. X. Liu, W. Y. Sun and N. Ueyama, Inorg. Chem.,
2006, 45, 8098; (c) B. Verdejo, J. Aguilar, E. G. Espana, P. Gavina,
J. Latorre, C. Soriano, J. M. Llinares and A. Domenech, Inorg.
Chem., 2006, 45, 3803; (d) A. Escuer, F. A. Mautner, E. Penalba
and R. Vicente, Inorg. Chem., 1998, 37, 4190; (e) N. Kitajima,
S. Hikichi, M. Tanaka and Y. Moro-oka, J. Am. Chem. Soc., 1993,
115, 5496.
13 (a) P. Naumov, G. Jovanovski and A. Todorovska, J. Mol. Struct.,
2001, 563–564, 341; (b) P. Naumov, M. Ristova, B. Soptrajanov
and M. Zugik, J. Mol. Struct., 2001, 598, 235.
14 J. M. Lazaro Martınez, P. N. Romasanta, A. K. Chattah and
G. Y. Buldain, J. Org. Chem., 2010, 75, 3208.
15 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li,
H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta,
F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin,
V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari,
A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi,
N. Rega, N. J. Millam, M. Klene, J. E. Knox, J. B. Cross,
V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma,
V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg,
S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz,
J. Cioslowski and D. J. Fox, Gaussian 09, Revision A. 2, Gaussian,
Inc., Wallingford CT, 2009.
3 (a) B. Kersting, Angew. Chem., 2001, 113, 4109 (Angew. Chem.,
Int. Ed., 2001, 40, 3987); (b) M. Kato and T. Ito, Inorg. Chem.,
1985, 24, 504; (c) M. Kato and T. Ito, Inorg. Chem., 1985, 24, 509;
(d) M. Kato and T. Ito, Bull. Chem. Soc. Jpn., 1986, 59, 285.
4 (a) B. Kersting, Angew. Chem., Int. Ed., 2001, 40, 3987;
(b) E. Garcia-Espana, P. Gavina, J. Latorre, C. Soriano and
B. Verdejo, J. Am. Chem. Soc., 2004, 126, 5082;
(c) P. Mukherjee, M. G. B. Drew, M. Estrader and A. Ghosh,
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 9915–9917 9917