Combination effects of cation and anion of ionic liquids on the cadmium metal-organic frameworks in ionothermal systems

Article abstract of DOI:10.1021/ic800071g

The effects of cation and/or anion of two groups of ionic liquids ([EMI]X and [PMI]X, where EMI = 1-ethyl-3-methylimidazolium; PMI = 1-propyl-3- methylimidazolium; X = Cl, Br, and I) on the ionothermal reactions between Cd(NO₃)₂·4H₂O and 1,3,5- benzenetricarboxylic acid (H₃BTC) were studied. Three different Cd-BTC metal-organic frameworks, [EMI][Cd₂(BTC)Cl₂](1), [EMI][Cd(BTC)](2), and [PMI][Cd(BTC)](3), were formed into crystalline phases. 1 was obtained from reactions in [EMI]Cl, while the same reactions with Cl replaced by Br or I produced a known compound 2. The replacement of EMI ⁺ by PMI⁺ produced 3, irrespective of the nature of X.

Full text of DOI:10.1021/ic800071g

Inorg. Chem. 2008, 47, 1907-1909
Combination Effects of Cation and Anion of Ionic Liquids on the
Cadmium Metal-Organic Frameworks in Ionothermal Systems
Ling Xu,† Eun-Young Choi,‡ and Young-Uk Kwon*,†,‡,§
Department of Chemistry, BK-21 School of Chemical Materials Science, Institute of Basic Science,
and SKKU AdVanced Institute of Nanotechnology, Sungkyunkwan UniVersity, Suwon, 440-176
Korea
Received January 15, 2008
The effects of cation and/or anion of two groups of ionic liquids
([EMI]X and [PMI]X, where EMI ) 1-ethyl-3-methylimidazolium;
PMI ) 1-propyl-3-methylimidazolium; X ) Cl, Br, and I) on the
ionothermal reactions between Cd(NO3)2 ·4H2O and 1,3,5-ben-
zenetricarboxylic acid (H3BTC) were studied. Three different Cd-
BTC metal-organic frameworks, [EMI][Cd2(BTC)Cl2](1),
[EMI][Cd(BTC)](2), and [PMI][Cd(BTC)](3), were formed into
crystalline phases. 1 was obtained from reactions in [EMI]Cl, while
the same reactions with Cl replaced by Br or I produced a known
compound 2. The replacement of EMI+ by PMI+ produced 3,
irrespective of the nature of X.
organic cations and inorganic or organic anions. The variation
of the cation and/or anion can provide a means to tune the
solvent properties, which could be a means to design the
structures of MOFs.1,3 For example, Morris’ group showed
that the hydrophilicity/hydrophobicity of the anion part gives
rise to significant changes in the MOF structures.1h Further-
more, the ionic species of ILs can participate into MOFs,
making the nature of the cation/anion an important variable
for the design of MOFs. Many of the MOFs from ionother-
mal syntheses show that the cations of ILs can function as
structure-directing agents. Our recent paper also showed that
the tuning of the MOF structures by the variation of the
cation of ILs could be extensive.3b
However, such efforts toward understanding the influence
of the cation and anion were limited until now. Here, we
present our results that show more subtle variation by the
chemical identity of the halide anion, coupled with the
influence of the cation part. Halide anions of ILs are classified
as strongly hydrophilic, implying that their behaviors are
similar to one another. However, our results demonstrate that
there is finer subtlety with the variation of halide ions.
We employed two groups of ILs as solvents for ionother-
mal reactions between Cd(NO3)2 · 4H2O and 1,3,5-benzen-
etricarboxylic acid (H3BTC). The ILs we used are [EMI]X
(EMI ) 1-ethyl-3-methylimidazolium) and [PMI]X (PMI )
1-propyl-3-methylimidazolium) with X ) Cl, Br, and I for
each group. The ILs were synthesized according to the
literature methods.3a,4
The ionothermal reaction technique for the syntheses of
metal-organic frameworks (MOFs) is a rapidly growing
research field in recent years.1 Ionic liquids (ILs), the solvents
for ionothermal reactions, possess interesting and useful
physicochemical properties such as high thermal stability and
negligible vapor pressures,2 whose features may be advanta-
geous over the conventional solvents for MOF syntheses with
improved safety and reduced environmental problems.
Probably, the more exciting aspect of ILs is the possibility
of designing the structures of MOFs. ILs are composed of
* To whom correspondence should be addressed. E-mail:
ywkwon@skku.edu.
†
Department of Chemistry, BK-21 School of Chemical Materials
Science.
‡
Institute of Basic Science.
SKKU Advanced Institute of Nanotechnology.
§
The nature of X of the [EMI]X solvent system turned out
to be a crucial factor in determining the forms of MOFs.
(1) (a) Lin, Z.; Wragg, D. S.; Morris, R. E. Chem. Commun. 2006, 2021–
2023. (b) Liao, J. H.; Wu, P. C.; Huang, W. C. Cryst. Growth Des.
2006, 6, 1062–1063. (c) Sheu, C. Y.; Lee, S. F.; Lii, K. H. Inorg. Chem.
2006, 45, 1891–1893. (d) Tsao, C. P.; Sheu, C. Y.; Nguyen, N.; Lii,
K. H. Inorg. Chem. 2006, 45, 6361–6364. (e) Liao, J. H.; Huang, W. C.
Inorg. Chem. Commun. 2006, 9, 1227–1231. (f) Lin, Z.; Slawin,
A. M. Z.; Morris, R. E. J. Am. Chem. Soc. 2007, 129, 4880–4881. (g)
Parnham, E. R.; Morris, R. E. Acc. Chem. Res. 2007, 40, 1005–1013.
(h) Lin, Z; Wragg, J. E.; Morris, R. E. J. Am. Chem. Soc. 2007, 129,
10334–10335.
When
X
)
Cl, we obtained
a
new MOF,
[EMI][Cd2(BTC)Cl2] (1), with Cl taking a part of the MOF
structure, while the X ) Br and I reactions produced
[EMI][Cd(BTC)] (2) without any X in the structure. 2 was
previously reported by another group.1b Quite contrarily,
(2) (a) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; Wiley-
VCH: New York, 2002; Chapter 3. (b) Gutowski, K. E.; Broker, G. A.;
Willauer, H. D.; Huddleston, J. G.; Swatloski, R. P.; Holbrey, J. D.;
Rogers, R. D. J. Am. Chem. Soc. 2003, 125, 6632–6633. (c) Copper,
E. R.; Andrew, C. D.; Wheatley, P. S.; Webb, P. B.; Wormald, P.;
Morris, R. E. Nature 2004, 430, 1012–1016.
(3) (a) Parnham, E. R.; Morris, R. E. Chem. Mater. 2006, 18, 4882–4887.
(b) Xu, L.; Choi, E.-Y.; Kwon, Y.-U. Inorg. Chem. 2007, 46, 10670–
10680.
(4) Bonhote, P.; Dias, A. P.; Papageorgiou, N.; Kalyanasundaram, K.;
Cratzel, M. Inorg. Chem. 1996, 35, 1168–1178.
10.1021/ic800071g CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 6, 2008 1907
Published on Web 02/19/2008

COMMUNICATION
Figure 2. Structure of 3: (a) asymmetric unit; (b) 3D view with the PMI
(yellow) lying in the channels.
Figure 1. Structure of 1: (a) asymmetric unit; (b) 3D view along the b
direction with EMI+ (orange) located in the channels.
atoms are linked by O12, O15, and µ2-O16 atoms of BTC3-
ligands. These neighboring antiparallel chains are paired to
form double chains by Cd2-O11 bonds connecting the
[Cd2Cl4] units (Supporting Information, Figure S4a). Con-
nected by the Cd1-O13 bridges, the double chains are fused
into a battlement-like 2D layer along the ac plane, stacking
in an -ABAB- fashion (Supporting Information, Figure
S4b). A 3D framework is generated by the 2D layers linked
by Cd1-O14 bonds, in which the channels are formed along
the b direction with 349.6 Å3 (37.8% of the total volume)
extraframework volume calculated by PLATON.7 The EMI+
cations anchor in the channels (Figure 1b) and are stabilized
by weak d-π interactions with the framework in addition
to the electrostatic interaction.
Liao et al. reported that the same reaction as for 1 in
[EMI]Br, instead of [EMI]Cl, produced 2 without Br in the
framework.1b We confirmed their result and found that
[EMI]I also produced 2 (Supporting Information, Figure S3).
It seems that the sizes of X and the Cd-X bond strengths
are the factors that determine the structures. To the contrary,
such a differentiation by X does not appear in the [PMI]X
solvent system.
The asymmetric unit of 3 consists of one Cd atom, one
BTC3- ligand, and one PMI+. The asymmetric Cd1 atom is
in a slightly distorted five-coordinated square pyramid CdO5
geometry with τ ) 0.0156, whose equatorial plane is
composed of four O atoms from three BTC3- ligands with
distances of Cd-O from 2.038(2) to 2.600(1) Å and whose
axial position is occupied by a carboxylic O atom (O15;
Figure 2a). Four Cd1 atoms are connected by three separate
BTC3- ligands with chelating and monodentate mode to form
a 29-membered ring [Cd4(BTC)3]. The growth of the 29-
membered rings results in a stairlike chain along the b
direction, and the neighboring antiparallel stairs further
extend to form a 2D layer in the bc plane (Supporting
Information, Figure S5). The neighboring parallel layers are
further connected via Cd1-O15 bonds to generate a 3D
framework. The channels are formed with 1599.0 Å3 (48.5%)
extraframework volume,7 which is fully occupied by the
PMI+ cations (Figure 2b). There are π-π stacking and
electrostatic interactions between the guest and the frame-
work.
reactions using [PMI]X as solvents turned out to be insensi-
tive to the nature of X, all producing a new MOF,
[PMI][Cd(BTC)] (3). Interestingly, the frameworks of 2 and
3 show the same topology, although their structures are
different in some details.
The crystal structures of 1 and 3 were characterized by
single-crystal X-ray diffraction analyses,5 and the phase
identity of 2 was confirmed by comparing the powder X-ray
diffraction (PXRD) pattern with the calculated one from the
crystal data in the literature.1b
There already exist 28 different Cd-BTC MOFs in the
literature. These MOFs can be classified into 17 different
types depending on the coordination mode of the BTC ligand.
The 17 modes span most of the possible combinations of
the bonding modes around BTC ligands. These are sum-
marized in the Supporting Information (Scheme S1 and Table
S1) along with their references. Compound 1 shows the
highest coordination number of 6 for BTC3-; compounds 2
and 3 also show a relatively high coordination number of 4.
These and our previous analysis on more extensively
investigated Zn-BTC MOFs3b suggest that MOFs from
ionothermal reactions tend to have higher degrees of
condensation between the metal and ligand, probably because
of the higher polarity of ILs than other types of solvents.
The asymmetric unit of 1 has two Cd atoms, one BTC3-
ligand, one EMI+, and two Cl- ligands (Figure 1a). The
BTC3- ligand is bonded to six Cd atoms in a chelating/
bridging bidentate and bis-bidentate fashion (mode Q in
Scheme S1). The Cd1 atom is in a distorted six-coordinated
CdO5Cl octahedron, and Cd2 is in a distorted five-
coordinated CdO2Cl3 trigonal bipyramid with τ ) 0.652.6
Cd1 is coordinated equatorially by four carboxylic O atoms
from three separate BTC3- ligands and axially by one
carboxylic O atom (O13) and one Cl-. Cd2 is surrounded
by one carboxylic O atom (O11) and two Cl- in the
equatorial plane, one carboxylic O atom (O16), and one Cl-
in the axial position. The Cd-O average distance is 2.30 Å.
There are two kinds of µ2-bridging Cl- atoms: Cl1 asym-
metrically bridges Cd1 and Cd2, with Cd-Cl distances being
2.759(6) and 2.479(5) Å, respectively, and Cl2 links two
symmetry-related Cd2 atoms.
The description of the structure of 1 starts from a -Cd-
BTC- chain along the a direction in which neighboring Cd1
Thermogravimetric analysis (TGA) data of 1 and 3 show
that they have high thermal stability and start to decompose
in single steps at 379 °C for 1 and 383 °C for 3 (Supporting
Information, Figure S1). 2 is also reported to have high
decomposition temperature.1b It seems that the strong
(5) Sheldrick, G. M. SHELXTL Programs, version 5.1; Bruker AXS GmbH:
Karlsruhe, Germany, 1998.
(6) τ ) (ꢀ-R)/60, where R and ꢀ are the two biggest bond angles around
the Cd center, τ ) 0 for an ideal square pyramid, and τ ) 1 for an
ideal trigonal bipyramid. Addision, A. W.; Rao, T. N.; Reedijk, J.; Riju,
J. V.; Verschoor, G. C. J. Chem. Soc., Dalton Trans. 1984, 1349–1353.
(7) Spek, A. L. Acta Crystallogr. 1990, A46, C43.
1908 Inorganic Chemistry, Vol. 47, No. 6, 2008

COMMUNICATION
features of the MOFs to be formed in the present systems.
In this regard, 1 is a deviation from the general behavior,
which may be attributed to the smaller size of Cl than Br or
I and the stronger Cd-Cl bond than Cd-Br or Cd-I. The
absence of such a deviation in the products from the [PMI]X
reactions suggests that the host–guest compatibility of the
structure of 1 with EMI+ may not be possible with the larger
PMI+ guests.
In summary, we have explored the ionothermal reaction
systems between Cd(NO3)2 and H3BTC in two groups of IL
media, in which the ILs are [EMI]X and [PMI]X (X ) Cl,
Br, and I). The nature of the halide ion of the [EMI]X solvent
system turned out to be a crucial factor in determining the
forms of MOFs: a new MOF, [EMI][Cd2(BTC)Cl2], is
obtained from [EMI]Cl solvent, while [EMI]X (X ) Br and
I) produced [EMI][Cd(BTC)] without any X in the structure.
However, reactions using [PMI]X turned out to be insensitive
to the nature of halide, all producing a new MOF,
[PMI][Cd(BTC)]. The resulting compounds feature 3D
structures with IL cations anchoring in the channels, in which
the ILs act as structure-directing templates and charge-
compensating groups, endowing the MOFs with high thermal
stability. Our results show that there is finer subtlety with
the variation of halide ions, coupled with the influence of
the cation part in the IL.
Figure 3. (a and b) Environments of channels occupied with EMI+ and
PMI+ in 2 and 3, respectively. Note that one Cd-O bond (red) is broken
on moving from 2 to 3. (c and d) Topologies of the [Cd(BTC)]- networks
in 2 and 3, in which red and gray circles represent Cd2 dimers and BTC3-
ligands, respectively. (e and f) Views along [001] directions in compounds
2 and 3.
interaction between the guest cations and the frameworks
increases the thermal stability of this class of compounds.
It is noteworthy that compounds 2 and 3 have some
common features. Both have the same general chemical
formula [cation][Cd(BTC)], and their structures have the
same topology with channels in the [001] directions with
EMI+ or PMI+ cations in the channels. It is interesting that
the additional methylene group of the cation on moving from
2 to 3 makes several slight changes in the structure while
maintaining the same topology (see Figure 3): (a) the [001]
channel of compound 3 becomes broadened compared to that
of compound 2 (6.3 × 7.5 Å2 for 2 and 7.8 × 6.3 Å2 for 3);
(b) the extraframework volume of 3 is also slightly larger
with 1599.0 Å3 than that of 2 with 1563.1 Å3;7 (c) the
coordination mode of one carboxylic group in the BTC3-
ligand is changed from chelating for 2 (mode L in Scheme
S1 in the Supporting Information) to a monodentate mode
for 3 (mode I in Scheme S1 in the Supporting Information)
to accommodate the larger PMI cation. These observations
suggest that the topology and the general chemical formula
[cation][Cd(BTC)] of 2 and 3 are some of the most favorable
Acknowledgment. This work was supported by the Korea
Research Foundation grant funded by the Korean Govern-
ment (MOEHRD; Grant KRF-2005-005-J11903). We thank
CNNC for partial support and CCRF and KBSI for the
crystallographic data.
Supporting Information Available: Experiment section, physi-
cal measurements, TGA graphs, IR data, XRPD patterns, X-ray
crystallographic files for compounds 1 and 3 (CCDC: 659182 and
659183), and Cd-BTC compounds and coordination modes in the
literature. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC800071G
Inorganic Chemistry, Vol. 47, No. 6, 2008 1909