Ionothermal syntheses of six three-dimensional zinc metal-organic frameworks with 1-alkyl-3-methylimidazolium bromide ionic liquids as solvents

Article abstract of DOI:10.1021/ic701393w

We have performed ionothermal reactions between Zn(NO₃) ₂ and H₃BTC in 1-alkyl-3-methylimidazolium bromide ionic liquids with the alkyl group varying from ethyl to amyl. Six 3-D metal-organic frameworks (MOFs), including two isomeric compounds [Zn₃(BTC) ₂(H₂O)₂]·2H₂O (1 and 2) (H₃BTC = 1,3,5-benzenetricarboxylate acid), [EMI]-[Zn(BTC)] (3) (E = ethyl, MI = 3-methylimidazolium), [PMI][Zn(BTC)](4) (P = propyl), [BMI] ₂[Zn₄(BTC)₃(OH)-(H₂O)₃] (5) (B = butyl), and [AMI][Zn₂(BTC)(OH)Br] (6) (A = amyl), have been synthesized and structurally characterized. Compounds 1 and 2 are isomeric compounds, in which the coordination modes of Zn atoms and the BTC^3- ligands are considerably different. Compounds 3-6 crystallize with the corresponding ionic liquid cations incorporated in the frameworks. Their crystal structures show various features including various coordination geometries of Zn²⁺ and various bridging modes of the BTC^3- ligands. The incorporated cations appear to have strong interactions with the frameworks.

Full text of DOI:10.1021/ic701393w

Inorg. Chem. 2007, 46, 10670−10680
Ionothermal Syntheses of Six Three-Dimensional Zinc Metal−Organic
Frameworks with 1-Alkyl-3-methylimidazolium Bromide Ionic Liquids as
Solvents
Ling Xu,† Eun-Young Choi,‡ and Young-Uk Kwon*,†,‡,§
Department of Chemistry, BK-21 School of Chemical Materials Sciences, Sungkyunkwan
UniVersity, Suwon, 440-176 Korea, Institute of Basic Science, Sungkyunkwan UniVersity,
Suwon, 440-176 Korea, and SKKU AdVanced Institute of Nanotechnology, Sungkyunkwan
UniVersity, Suwon, 440-176 Korea
Received July 13, 2007
We have performed ionothermal reactions between Zn(NO3)2 and H3BTC in 1-alkyl-3-methylimidazolium bromide
ionic liquids with the alkyl group varying from ethyl to amyl. Six 3-D metal organic frameworks (MOFs), including
two isomeric compounds [Zn3(BTC)2(H2O)2] 2H2O (1 and 2) (H3BTC 1,3,5-benzenetricarboxylate acid), [EMI]-
[Zn(BTC)] (3) (E ethyl, MI 3-methylimidazolium), [PMI][Zn(BTC)](4) (P propyl), [BMI]2[Zn4(BTC)3(OH)-
(H2O)3] (5) (B butyl), and [AMI][Zn2(BTC)(OH)Br] (6) (A amyl), have been synthesized and structurally
−
‚
)
)
)
)
)
)
characterized. Compounds 1 and 2 are isomeric compounds, in which the coordination modes of Zn atoms and the
-
BTC3 ligands are considerably different. Compounds 3
−6 crystallize with the corresponding ionic liquid cations
incorporated in the frameworks. Their crystal structures show various features including various coordination
+
-
geometries of Zn2 and various bridging modes of the BTC3 ligands. The incorporated cations appear to have
strong interactions with the frameworks.
Introduction
and organic ligands. With these attributes, ionothermal
synthesis is considered to be more environment-friendly and
safer than the traditional hydro/solvothermal reactions, which
are frequently used in the exploratory syntheses of inorganic
and inorganic-organic hybrid materials. In addition, iono-
thermal solvent systems can provide different reaction
conditions from those of hydro/solvothermal systems, by
which novel structure types such as zeolite and zeo-type
frameworks can be obtained.2,3a,b,5
Ionothermal synthesis is a novel material synthesis
method,1-3 in which ionic liquids (ILs) function as reaction
media, templates, or charge-compensating groups. ILs have
many intriguing characteristic physicochemical properties
such as high ionic conductivity, nonflammability, and
negligible vapor pressure.4 The coexistence of ionic and
organic groups and the large temperature windows of ILs
make them ideal media for the reactions between metal ions
There are a number of examples in which ILs have been
successfully applied to the syntheses of novel metal-organic
frameworks (MOFs).6 Since the physical properties of ILs
can be tailored, it follows naturally to explore ionothermal
MOF synthesis systems by employing ILs that are systemati-
cally varied in structure. One such variation is the length of
the alkyl group on the imidazolium cation. With the increase
of the alkyl chain length, the viscosity of the IL increases
and the melting point decreases, although such a trend is
reverted when there are more than seven carbons in the alkyl
chain.7,8
* Corresponding author. E-mail: ywkwon@skku.edu.
† Department of Chemistry.
‡ Institute of Basic Science.
§ SKKU Advanced Institute of Nanotechnology.
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10670 Inorganic Chemistry, Vol. 46, No. 25, 2007
10.1021/ic701393w CCC: $37.00
© 2007 American Chemical Society
Published on Web 11/15/2007

Three-Dimensional Zinc Metal-Organic Frameworks
Table 1. Main IR Characteristic Absorption Peaks for Compounds 1-6
compd
υas(COO-), cm-1
υs(COO-), cm-1
δ(C-H)(MI), cm-1
δ(C-N)(MI), cm-1
δ(C-H), cm-1
1
2
3
4
5
6
1630, 1565
1626, 1575
1630, 1565
1612, 1560
1633, 1536
1625, 1567
1437, 1362
1446, 1379
1438, 1350
1430, 1365
1455, 1385
1446, 1379
2980, 2924
2973, 2923
2973, 2930
2972, 2890
2974, 2890
2974, 2893
3150, 3066
3147, 3097
3117, 3041
3156, 3076
1160
1164
1156
1162
In the present study, we synthesized and used four sorts
of 1-alkyl-3-methylimidazolium bromide ILs with different
alkyl chain lengths from two to five carbons, (1-ethyl-3-
methylimidazolium bromide ([EMI]Br), 1-propyl-3-meth-
ylimidazolium bromide ([PMI]Br), 1-butyl-3-methylimida-
zolium bromide ([BMI]Br), and 1-amyl-3-methylimidazolium
bromide ([AMI]Br), as solvents. In order to isolate the effects
of the variation on the cation part, we have fixed all the other
variables such as the anion of ILs, the types of reagent metal
ion, and ligand. In the present paper, the anion is Br- and
metal ion is Zn2+. An aromatic rigid polycarboxylic acid,
1,3,5-benzenetricarboxylic acid (H3BTC), a versatile and
powerful ligand with its three carboxylate groups as short
bridges and a benzene ring as a long bridge between metal
centers leading to structural varieties, is chosen as the ligand.
In fact, this Zn-BTC system is one of the most extensively
explored systems in the endeavor for synthesizing novel
MOFs9,10 While many different solvent systems and reaction
techniques have been used, there is not a single case of Zn-
BTC system under ionothermal conditions reported in the
literature. Surprisingly, our results reveal that the ionothermal
reaction technique is extremely versatile in producing many
new MOFs with some new features. Herein, the syntheses
and crystal structures of six novel 3-D Zn-BTC MOFs, [Zn3-
(BTC)2(H2O)2]‚2H2O (1 and 2), [EMI][Zn(BTC)] (3), [PMI]-
[Zn(BTC)](4), [BMI]2[Zn4(BTC)3(OH)(H2O)3] (5), and [AMI]-
[Zn2(BTC)(OH)Br] (6), are presented.
Experiment Section
Syntheses of the Compounds. All chemicals were commercially
purchased and used without further purification. The ILs were
prepared according to the literature method.11,5a Under inert nitrogen
atmosphere conditions, degassed alkyl bromide was refluxed with
the distilled 1-methylimidazole to give the corresponding ILs. The
four sorts of ILs were washed with ethyl acetate and then dried
under a vacuum at least for 10 h: [EMI]Br (white solid, mp )
80-82 °C, yield: 92%), [PMI]Br (pale yellow oil, yield: 80%),
[BMI]Br (pale yellow oil, yield: 78%), [AMI]Br (pale brown oil,
yield: 68%).
[Zn3(BTC)2(H2O)2]‚2H2O (1 and 2). Zn(NO3)2‚6H2O (1.5 mmol,
0.446 g for 1; 2.0 mmol, 0.595 g for 2) and H3BTC (0.5 mmol,
0.105 g) were placed in a 25 mL Teflon-lined stainless-steel
autoclave mixed with [EMI]Br (1.0 g). The mixtures were kept in
a furnace at 160 °C for 5 days and then naturally cooled to room
temperature to get colorless crystals of 1 and 2 suitable for X-ray
diffraction. Yield for 1, 69%, for 2, 72% (based on H3BTC).
Elemental analysis found (calcd) for 1: C, 31.59(31.68); H, 2.13-
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(m), 1437(m), 1362(s), 1101(m), 731(w), 615(w), 574(w), 460-
(w); for 2: 3419(vs), 2973(w), 2923(m), 1626(vs), 1575(s),
1485(w), 1446(s), 1379(s), 1051(w), 719(w), 649(w), 568(w), 463-
(w).
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Inorganic Chemistry, Vol. 46, No. 25, 2007 10671

Xu et al.
Table 2. Factors Affecting the Reaction System and Resulting MOFs
connectivity modes
stoichiometry coordination geometries
3-
compd
ionic liquid coordination modes of BTC3- of BTC3- in Scheme 1 of nZn:nBTC
of Zn
[Zn3(BTC)2(H2O)2]·2H2O (1)
[EMI]Br
[EMI]Br
[EMI]Br
bridging monodentate
and bis-didentate
chelating/bridging bidentate
and bis-bidentate
bidentate and
A
B
F
1.5:0.5
2.0:0.5
3.0:0.5
6, 4
[Zn3(BTC)2(H2O)2]·2H2O (2)
[EMI][Zn(BTC)] (3)
6, 5
4
bis-monodentate
tri-monodentate
tri-monodentate
and tri-bidentate
monodentate and
bis-bidentate
[PMI][Zn(BTC)] (4)
[BMI]2[Zn4(BTC)3(OH)(H2O)3] (5)
[PMI]Br
[BMI]Br
H
C, H
1.5:0.5
2.0:0.5
3
4, 4, 6, 4
[AMI][Zn2(BTC)(OH)Br] (6)
[AMI]Br
D
3.5:0.5
5, 4
[EMI][Zn(BTC)] (3). Zn(NO3)2‚6H2O (3.0 mmol, 0.893 g) and
H3BTC (0.5 mmol, 0.105 g), were mixed with 1.0 g [EMI]Br in a
25 mL Teflon-lined stainless-steel autoclave. The mixture was
heated at 160 °C for 5 days in a furnace to obtain colorless crystals
of 3. Yield: 70% (based on H3BTC). Elemental analysis found
(calcd): C, 46.91(46.96); H, 3.36(3.68); N, 7.16(7.30). IR data (in
KBr, cm-1): 3420(w), 3150(m), 3066(m), 2973(m), 2930(m), 1630-
(s), 1565(s), 1438(s), 1350(s), 1160(s), 834(w), 764(s), 725(s), 538-
(m), 446(m).
crystallographic software12 and refined by the full-matrix least-
squares technique on F2. All non-hydrogen atoms were refined
anisotropically, and the hydrogen atoms were included in the final
stages of the refinements on calculated positions bonded to their
carrier atoms.
X-ray powder diffraction (XRPD) data were recorded on a
Rigaku D/max-RC diffractometer with Cu KR radiation (λ ) 1.5406
Å, 30 kV, and 40 mA) with a scan speed of 2° min-1 and a step
size of 0.02° in 2θ. The experimental XRPD patterns for compounds
1 to 6 match well with those simulated from single-crystal structure
data (Supporting Information; Figure S2), indicating that all six
compounds were isolated as single phases.
Elemental analysis (C, H, and N) was performed on an EA1110
elemental analyzer, and the data compared experiment with
calculation, confirming the single phases of six compounds.
FT-IR spectra were obtained on a Bruker Tensor27 FT-IR
spectrometer using KBr pellets dispersed with sample powders in
the range of 4000-400 cm-1. The characteristic absorption peaks
of the main functional groups for 1-6 are listed in Table 1. The
asymmetric stretching vibrations νas(COO-) were observed in the
rang 1536-1633 cm-1 and symmetric stretching vibrations νs(COO-)
in 1350-1455 cm-1. The above stretching was shifted to lower
values, compared to the carbonyl frequencies of free H3BTC ligand.
[PMI][Zn(BTC)](4). A mixture of Zn(NO3)2‚6H2O (1.5 mmol,
0.446 g), H3BTC (0.5 mmol, 0.105 g), and [PMI]Br (1.0 mL)
in a 25 mL Teflon-lined stainless-steel autoclave was heated to
160 °C for 5 days in a furnace. The resulting mixture was naturally
cooled until colorless crystals of 4 obtained. Yield: 60% (based
on H3BTC). Elemental analysis found (calcd): C, 48.45(48.32);
H, 3.97(4.06); N, 6.95(7.04). IR data (in KBr, cm-1): 3425(s),
3147(w), 3097(w), 2972(w), 2890(w), 1612(s), 1560(s), 1430(w),
1384(s), 1365(s), 1164(w), 1051(s), 879(w), 725(w), 536(w), 463-
(w).
[BMI]2[Zn4(BTC)3(OH)(H2O)3] (5). Zn(NO3)2‚6H2O (2.0 mmol,
0.595 g) and H3BTC (0.5 mmol, 0.105 g) were mixed with [BMI]-
Br (1.0 mL) in a 25 mL Teflon-lined stainless-steel autoclave and
then heated to 160 °C for 5 days. The mixture was naturally cooled
to get colorless crystals of 5. Yield: 70% (based on H3BTC).
Elemental analysis found (calcd): C, 41.09(40.92); H, 3.68(3.43);
N, 4.44(4.66). IR data (in KBr, cm-1): 3423(vs), 3117(w), 3041-
(w), 2974(s), 2890(w), 1633(s), 1536(w), 1455(s), 1385(s), 1325-
(w), 1269(w), 1156(w), 1051(s), 881(w), 600(w), 436(w).
The difference ∆(νas(COO-) - νs(COO-)) were around 200 cm-1
,
characteristic for coordinated carboxylate groups.13 The peaks for
1-6 from 2890 to 2980 cm-1 are from C-H stretching vibrations.14
The peaks for 3-6 around 1160 cm-1 can be assigned as Nalkyl-C
stretching of the imidazolium rings.15
[AMI][Zn2(BTC)(OH)Br] (6). Zn(NO3)2‚6H2O (3.5 mmol,
1.041 g), H3BTC (0.5 mmol, 0.105 g), and [AMI]Br (1.0 mL)
were heated at 160 °C for 5 days in a 25 mL Teflon-lined stainless-
steel autoclave in a furnace. Colorless crystals of 6 resulted from
the natural cooling. Yield: 75% (based on H3BTC). Elemental
analysis found (calcd): C, 36.71(36.83); H, 3.68(3.43); N, 4.71-
(4.77). IR data (in KBr, cm-1): 3419(s), 3156(s), 3124(s), 3096-
(s), 3076(s), 2974(s), 2893(w), 1625(s), 1567(s), 1446(s), 1379(s),
1162(s), 1051(s), 935(s), 719(s), 624(w), 583(w), 551(w), 487(s),
463(w).
Results and Discussion
Each of the compounds 1-6 was obtained as a single
phase product under the conditions given in the Experimental
Section. Compounds 1-3 were obtained from the reactions
between Zn2+ and BTC3- in a [EMI]Br medium. The only
difference among their synthesis conditions is the different
Zn/H3BTC ratios (Table 2). Remarkably, the crystal structural
data reveal that the variation of starting composition is
Characterizations. Thermogravimetric (TG) analyses were
carried on a TA4000/SDT 2960 instrument in flowing N2 with a
heating rate of 10 °C‚min-1. TG data of the six compounds show
that they have high thermal stabilities. The weight losses in
compounds 1, 3, 4, and 6 occur in single steps, while 2 and 5 show
two-step weight losses with the first ones corresponding to water
losses (Supporting Information; Figure S1).
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Crystallographic data were collected at -100 °C for 1 and
25 °C for the others on Bruker CCD diffractiometers in different
locations including Korea Basic Science Institute. The structures
were solved by the direct methods using SHELXTL package of
10672 Inorganic Chemistry, Vol. 46, No. 25, 2007

Three-Dimensional Zinc Metal-Organic Frameworks
Scheme 1. Connectivity Modes of BTC Ligands with Zn Found in the Literatures on Zn-BTC MOFs and the Crystal Structures of the Present Paper
significant enough to result in totally different compounds.
As will be discussed below, 1 and 2 are free of template,
while 3 is formed with EMI+ as a template. The other
compounds 4-6 were obtained from the reactions between
Zn2+ and H3BTC in IL media with longer alkyl chains and
were constituted with the corresponding organic cations
incorporated. All of the crystal structures of compounds 1-6
can be described as 3-D frameworks constructed by con-
necting Zn2+ atoms and BTC3- ligands in various modes
and various ratios.
Zn, and there are µ2-oxo bridges and µ2-carboxylic bridges
between two Zn ions.
According to Scheme 1, the connectivity modes of BTCs
found in 1-6 are A (in 1), B (2), C (5), D (6), F (3), and H
(4 and 5). It is noteworthy that BTCs in 1-6 tend to be
bonded to large numbers of Zn atoms of from four to six
(Scheme 1). While 1 and 2 have unique connectivity modes,
3, 4, 5, and 6 show popular ones each of which appeared
five through nine times from the 41 Zn-BTC MOFs in the
literature.
It seems appropriate to briefly overview the structures of
Zn-BTC MOFs in the literatures before discussing on the
details of our compounds.9,10 There are 41 different Zn-
BTC MOFs reported until now. These structures can be
classified into 11 different types of connectivity modes
around the BTC ligand. Each of compounds 1 and 2 shows
yet a new mode to make total of 13 types of connectivity
modes of BTC with Zn, which are shown in Scheme 1. In
this scheme, the modes are arranges in the order of decreasing
number of Zn atoms bonded to a BTC from six to one. Mode
L with two Zn atoms is the most frequently found with ten
examples, followed by mode H (with three Zn) and mode F
(with four Zn) each with seven examples. Modes C (with
six Zn) and D (with five Zn) are also found in five cases
each (Supporting Information; Table S1). The 13 modes span
all of the possible features of bonds between a carboxylic
group and a Zn atom (or Zn atoms). For example, there are
protonated and deprotonated carboxylic groups, there are
monodentate and bidentate carboxylic groups bonded to a
It needs to be emphasized that the connectivity modes in
compounds 1 and 2 have not been reported despite that Zn-
BTC systems have been studied extensively. This probably
has something to do with the solvent systems used for the
syntheses. To the best of our knowledge, this is the first time
to present Zn-BTC coordination polymers synthesized in
IL systems. Probably the unique properties of ILs as solvents,
i.e., the large dielectric constant and the high viscosity, as
well as the possibility to function as templates, affect the
reactions and reinforce new structures. Therefore, even if
ionic liquid is not incorporated in the final frameworks for
compounds 1 and 2 (unlike 3, 4, 5, and 6 which contain the
cations of the ILs) these compounds show some new features
such as the new connectivity modes of BTC.
Description of Crystal Structures 1-6. The structure of
1 features a 3-D framework in which the BTC3- ligand
exhibits a µ6 coordination mode: three carboxylate groups
adopt bridging monodentate and bis-bidentate coordination
fashion (mode A in Scheme 1). As shown in Figure 1a, Zn-
Inorganic Chemistry, Vol. 46, No. 25, 2007 10673

Xu et al.
Figure 1. Structure of 1: (a) Coordination environment of Zn. (b) The 2-D layer formed by linking the [(Zn2)4(BTC)2] units with the water molecules
being omitted for clarity. (c) The topological drawing of 2-D layer. (d) 3-D framework.
(1) atom is in an octahedral geometry constructed by four
carboxylic oxygen atoms (dZn(1)-O ) 2.160(2)-2.310(2) Å)
in the equatorial positions and two symmetry-related water
molecules in the axial positions (dZn(1)-O(1W) ) 2.467(2) Å).
Zn(2) center is coordinated by four carboxylic oxygen atoms
from four BTC3- ligands to form a distorted tetrahedral
geometry (dZn(2)-O ) 2.191(1)-2.342(2) Å). BTC3- ligands
are paired with the phenyl rings parallel to each other, and
they are connected to four Zn(2) atoms to form two kinds
of 16-membered rings [Zn2(BTC)2]; the expansion of such
units produces a ladder-like chain along the b-direction. The
ladders so generated are further connected to the next ones
through the linkage between O(3) and Zn(2) into a 2-D layer
along the bc plane (Figure 1b) with a distance of 9.879 Å
between the ladders. The 2-D layer is connected to the next
ones through Zn(1)O4 tetrahedral bridges to form a 3-D
framework structure (Figure 1d). Water molecules O(2W)
are stabilized in the framework by hydrogen bonds with
carboxylic oxygen atoms (O(2W)‚‚‚O(3) ) 2.984(3) Å;
O(2W)‚‚‚O(5) ) 2.933(3) Å). The centroid-to-centroid
distance of 3.808 Å between the paired phenyl rings of
BTC3- ligands indicate the existence of a weak π···π stacking
interaction.
2 has the same composition as 1 and is also a 3-D
framework structure. However, as mentioned earlier, they
show different connectivity modes, which, in turn, make the
two structures different in many aspects. The BTC3- ligand
in 2 shows the connectivity mode B in Scheme 1 and is
bonded to six Zn atoms in a chelating/bridging bidentate and
bis-bidentate fashion. The coordination environment of Zn-
(1) atom is an octahedron (dZn(1)-O ) 2.025(1)-2.144(2) Å),
similar to that of Zn(1) in 1. However, Zn(2) is surrounded
10674 Inorganic Chemistry, Vol. 46, No. 25, 2007

Three-Dimensional Zinc Metal-Organic Frameworks
Figure 2. Structure of 2: (a) Coordination environment of Zn. (b) The 2-D layer formed by linking the [(Zn2)4(BTC)2] units with the water molecules
being omitted for clarity. (c) The topological drawing of 2-D layer. (d) 3-D framework.
by five carboxylate oxygen atoms (dZn(2)-O ) 1.972(2)-
2.239(2) Å) from four BTC3- ligands to display a distorted
trigonal bipyramidal coordination environment with τ )
0.664 [τ ) (â-R)/60, where R and â are the two biggest
bond angles around Zn center; τ ) 0 for ideal square
pyramid, and τ ) 1 for ideal trigonal bipyramid],16 which is
different from the tetrahedron of Zn(2) in 1 (Figure 2a). Two
BTC3- ligands link four Zn(2) atoms to form two kinds of
8- and 16-membered rings, whose linkage results in a ladder-
like chain as that in 1. However, the neighboring chains in
2 are antiparallel to each other, which is different from the
parallel ones in 1 (Figure 1c and Figure 2c); these are further
connected by Zn(2)-O(1) and Zn(2)-O(2) bonds into a 2-D
layer (Figure 2b). These layers are linked through Zn(1)O6
octahedra into a 3-D framework (Figure 2d). Water mol-
ecules O(2W) are located between the 2-D layers and form
hydrogen bonds of O(2W)‚‚‚O(1W) (3.079(1) Å). The
neighboring phenyl rings in BTC3- ligands show both
π‚‚‚π interaction with the centroid-to-centroid distance 3.754
Å and d‚‚‚π interaction between C(14)-H and the phenyl ring
with C(14)-to-centroid distance 3.387 Å.
Compound 3 was obtained from a Zn2+-richest condition
among the reactions performed in [EMI]Br (compounds
1-3). Different from 1 and 2, its crystal structure shows
that EMI+ cation functions as a template. Zn(1) atom is
tetrahedrally coordinated by four carboxylic oxygen atoms
(dZn(1)-O ) 1.814(2)-2.233(2) Å) from four BTC3- ligands
(Figure 3a). The crystal structure can be described as stacking
of puckered layers that are composed of eight-membered
rings formed by two Zn atoms and two bridging bidentate
BTC3- ligands and 32-membered rings formed by four Zn
atoms and four BTC3- ligands between them in monodentate
and bidentate modes. The 32-membered rings share common
edges to form a 2-D layer and the eight-membered rings
between them (Figure 3b). The layers are further bridged
via Zn(1)-O(2) bonds with the BTC3- ligand displaying
bidentate and bis-monodentate coordination modes to
(16) Addision, A. W.; Rao, T. N.; Reedijk, J.; Riju, J. V.; Verschoor, G.
C. J. Chem. Soc., Dalton Trans. 1984, 1349-1356.
Inorganic Chemistry, Vol. 46, No. 25, 2007 10675

Xu et al.
Figure 3. Structure of 3: (a) Coordination environment of Zn. (b) The 2-D layer. (c) 3-D framework with EMI located in the channels (EMI are represented
by yellow and orange colors).
form a 3-D framework. There are channels formed along
the b-direction where the EMI+ cations are located (Figure
3c).
appears to be significant contribution from hydrogen bond
of C(4)-H‚‚‚O(11) (2.684(6) Å, 121.20°) between them.
There is also a weak d‚‚‚π interaction between C(3)-H and
the phenyl ring of BTC3- (dC(3)-H···centroid ) 3.594 Å, -C(3)-
H-centroid ) 118.17°).
The asymmetric unit of 4 contains one Zn atom, one
BTC3- ligand, and one PMI+. Zn(1) atom is coordinated with
six oxygen atoms with three short (dZn(1)-O ) 2.015(2)-
2.284(3) Å) and three long bonds (dZn(1)-O ) 2.568-2.763
Å) (Figure 4a). In fact, the latter three Zn-O bond distances
are too long to be considered as normal coordination bonds.
The valence sums of these three bonds total smaller than
0.1, almost negligible. Probably, the 3-D network structure
of compound 4 exerts a matrix effect that keeps these oxygen
atoms from coming closer, and the Zn(1) position is the
energy minimum position under the restriction. The structure
of 4 can be described as an interpenetration structure of two
identical 3-D frameworks each constructed by enantiomeric
pairs of two types of helices formed by [Zn(BTC)]- repeating
units (Figure 4b and 4c). The two types of helices appear as
a large 32-membered ring that surrounds PMI+ ions and a
small 16-membered ring in the view along the a-axis. The
pitch height of these helices are l ) 16.005 Å. The two types
of helices are fused by sharing common edges composed of
Zn and BTC3- into a 3-D framework structure. The
interpenetration makes the pore rather small which is filled
by PMI+ ions (Figure 4d). The interaction between PMI+
and the framework is electrostatic principally, but there also
There are four symmetry-independent Zn atoms and
BTC3- ligands in 5 in the asymmetric unit (Figure 5a). Zn-
(1), Zn(2), and Zn(4) centers are in tetrahedra, and Zn(3)
center is in an octahedron. O(1) acts as a µ3-bridge to connect
Zn(1), Zn(2), and Zn(3) with Zn-O bond distances of 1.986-
(1), 1.994(1), and 2.148(1) Å, whose bond-valence sum is
calculated to be 1.23 indicating that O(1) is OH-; this
assignment is in accord with the charge balance requirement.
Both Zn(1) and Zn(2) atoms are surrounded by three
carboxylic oxygen atoms from three BTC3- ligands (dZn(1)-O
) 1.940(1)-1.983(2) Å and dZn(2)-O ) 1.953(1)-1.982(1)
Å) and O(1) from OH-. On the other hand, all of the four
oxygen atoms bonded to Zn(4) atom are from four different
BTC3- ligands (dZn(4)-O ) 1.929(1)-1.967(1) Å). The
octahedron around Zn(3) is formed by two carboxylic oxygen
atoms (dZn(3)-O ) 2.127(1)-2.136(1) Å) from two BTC3-
ligands located on axial positions, three (dZn(3)-O(1W) ) 2.048-
(1)-2.086(1) Å) and OH- lying in the equatorial plane. The
three water molecules O(1W), O(2W), and O(3W) have
bond-valence sum of 0.40, 0.39, and 0.36, suggesting that
they are all neutral coordination water molecules.
10676 Inorganic Chemistry, Vol. 46, No. 25, 2007

Three-Dimensional Zinc Metal-Organic Frameworks
Figure 4. Structure of 4: (a) Coordination environment of Zn. (b) A 3-D along c direction showing doubly interpenetrated frameworks (violet and pale
violet color show each fragment). (c) One fragment of them along c direction. (d) Overall 3D framework generated with the PMI (orange and yellow) lied
in the channels.
The structural description of 5 starts from a Zn(3)-BTC
chain along the a-direction in which neighboring Zn(3) atoms
are bridged by O(32) and O(33) atoms of BTC3-. These
chains are paired to form double chains through bridging
Zn(4) atoms (Figure 5b), and the double chains are fused
into a 2-D layer along the ab plane by bridging Zn(2) atoms
through Zn(2)-O(14), Zn(2)-O(34), and Zn4-O11 bonds
(Figure 5c). A rigid 3-D framework is constructed by several
Zn-O bonds (Zn1-O1, O16, and O31; Zn3-O22; Zn4-
O24) those link the 2-D layers (Figure 5d). The BMI+ cation
is located in the channels (Figure 5e), which interacts with
the negatively charged framework. There are also two kinds
of hydrogen bonds formed, the one between BMI+ and O(25)
(C(45)-H‚‚‚O25 ) 2.959(5) Å, 163.1°) Å and the other
between water molecules and carboxylic oxygen atoms
(O(1W)‚‚‚O(21) ) 2.650(2), O(2W)‚‚‚O(12) ) 2.697(2),
O(2W)‚‚‚O(23) ) 2.747(2), O(2W)‚‚‚O(3) ) 3.064(2),
O(3W)‚‚‚O(13) ) 2.814(2), O(3W)‚‚‚O(26) ) 2.780(2) Å).
Compound 6 distinguishes itself from the previous MOFs
in that Br- constitutes the structure with Zn-Br bonds. As
shown in Figure 6a, the asymmetric unit consists of two
independent Zn atoms: Zn(1) is five-coordinated square
pyramid ZnO5 (τ ) 0.375) and Zn(2) is four-coordinated
distorted tetrahedron ZnO3Br. There is a µ3-bridge oxygen
O(1) that is bonded to Zn(1), Zn(1A) (A ) -x, 1 - y, 1 -
z) and Zn(2) with Zn-O bond distances of 2.024(9), 2.095
(9), and 1.944(9) Å. The calculated valence-sum for O(1) is
1.29, indicating that O(1) should be OH-, which is in
agreement with the charge balance consideration. Zn(1) is
coordinated equatorially by two carboxylic oxygen atoms
from two BTC ligands (dZn(1)-O(11) ) 1.994(9), dZn(1)-O(16)
)
2.074(1) Å) and two O(1) atoms of the µ3-OH- groups
(dZn(1)-O(1) ) 2.024(9) and 2.095(9) Å) and axially by a
carboxylic oxygen (dZn(1)-O(14) ) 2.017(1) Å). Zn(2) is
coordinated by two carboxylic oxygen atoms (dZn(2)-O(12)
)
1.964(1), dZn(2)-O(15) ) 1.975(1) Å), O(1) and Br(1) (dZn(2)-Br(1)
) 2.321(3) Å). A µ3-OH- and its symmetry-related atom
bridge four Zn atoms to form a subunit [Zn4(OH)2Br2]4- with
a crystallographic inversion center at the midpoint of Zn-
(1)‚‚‚Zn(1A) (Figure 6b). By coordinating with Zn(1) and
Zn(2A) by bidentate and with Zn(1A) by monodentate
coordination modes, BTC3- ligands bridge the [Zn4(OH)2Br2]4-
subunits to form a 36-membered ring, whose expansion
results in a 2-D layer. With the connection of Zn(1)-O(11)
and Zn(2)-O(12), the 2-D layer is further grown into a 3-D
framework with channels along the a-direction. AMI+ cations
are located between the layers (Figure 6c). The interaction
between AMI+ and the framework is solely electrostatic.
Omitting the guest IL cations, the void volumes of the
MOFs were calculated to be 45.8% (3), 50.2% (4), 42.3%
Inorganic Chemistry, Vol. 46, No. 25, 2007 10677

Xu et al.
Figure 5. Structure of 5: (a) Coordination environment of Zn. (b) The double chains formed from “Zn3-BTC chain” through bridging Zn4 atoms. (c) The
extended 2-D layer formed by bridging Zn-O bonds. (d) 3-D framework omitted ILs [BMI]+. (e) A view along a direction. EMI molecules as charge-
compensating groups and template located in the channels and are represented by ball and stick model (yellow).
(5), and 54.0% (6) by PLATON.17 One may notice that the
trend of the void volume does not correlate with the size
of the cations. In order to see the possibility of obtaining
porous MOFs, we have attempted to exchange the
organic cations with smaller ones such as Na+ and K+.
However, in most of the cases we failed to find the right
solvent in which the MOFs were stable or the ion-exchange
reactions did not occur. The TG analyses of the six
compounds indicate the skeletons of the frameworks and the
ionic liquids decompose nearly synchronously, showing that
the cations interact strongly with the frameworks, which
appears to be the reason for the difficulties in our attempted
ion-exchange reactions. Further studies on such possibilities
are in progress.
In Table 2, we summarize some factors affecting the
reaction system and resulting MOFs. Though we used the
same reagents and reaction conditions, the six compounds
show diverse structures. Even the coordination modes of
BTC3- ligands are different. Although the systematic varia-
tion of the alkyl chain lengths of 1-alkyl-3-methylimidazo-
lium bromide ILs may bring in systematic variations in the
physical properties such as the melting point and viscosity,
there are many other factors, such as solvent-solute and Zn-
BTC interactions, which prohibit generalization. However,
(17) Spek, A. L. Acta Crystallogr., Sect. A: Found. Crystallogr. 1990,
A46, C43.
10678 Inorganic Chemistry, Vol. 46, No. 25, 2007

Three-Dimensional Zinc Metal-Organic Frameworks
Figure 6. Structure of 6: (a) The coordination spheres of Zn atoms. (b) The negative subunit [Zn4(OH)2Br2]4-. (c) 3-D framework generated with AMI
(yellow) filled in the layers and the subunits [Zn4(OH)2Br2]4- represented by polyhedron.
one may agree on that the imidazolium cations with longer
chains have the tendency to be incorporated into the
frameworks to result in through increased van der Waals
interactions and that the EMI lies on the boundary between
the templated and template-free MOFs.
systemization of the resultant structures in relation with the
ILs used, our results demonstrate the richness of the structural
diversity of the ionothermal reaction systems in obtaining
novel MOF compounds. Further investigation on such
systems may yield many new MOF compounds with
interesting features and useful properties. The tendency of
high degree of condensation between Zn2+ and BTC3- in
1-6 appears to be the feature relevant to the nature of the
ionothermal reactions. Certainly, a large volume of experi-
mental data needs to be accumulated for deeper understand-
ing on the nature of ionothermal systems. Our present data
and the few of works of the pioneering groups are just the
beginning of such a paramount task.
Conclusions
In this study, we have explored the ionothermal reaction
systems between Zn(NO3)2 and H3BTC in a series of IL
media. For this, we have employed four different 1-alkyl-
3-methylimidazolium bromide ILs with the alkyl chain length
varying from ethyl to amyl. Under the same reaction
conditions except for the type of ILs and Zn:H3BTC ratios,
we obtained six 3-D zinc MOFs with diverse features,
including various coordination number of Zn2+, the bonding
modes of the BTC ligand from µ3 to µ6, and the role of the
imidazolium cation in the final structures. Although the
complex nature of the reaction systems prohibited any
Acknowledgment. This work was supported by the Korea
Research Foundation Grant funded by the Korean Govern-
ment (MOEHRD) (KRF-2005-005-J11903). We also thank
CNNC and KOSEF for financial support.
Inorganic Chemistry, Vol. 46, No. 25, 2007 10679

Xu et al.
Supporting Information Available: Crystallographic data,
bond distances and angles, TG graphs, and observed and calculated
XRPD patterns for compounds 1-6, Zn-BTC compounds and
coordination modes in the literatures. This material is available free
of charge via the Internet at http://pubs.acs.org. Crystallographic
data for 1-6 have been deposited at the Cambridge Crystallographic
Data Center as supplementary publications (CCDC: 653389-
653394). These data can be obtained free of charge at www.ccd-
c.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK; fax: +441223336033; e-mail:
deposit@ccdc.cam.ac.uk).
IC701393W
10680 Inorganic Chemistry, Vol. 46, No. 25, 2007