Copper(I) and silver(I) 2-methylimidazolates: Extended isomerism, isomerization, and host-guest properties

Article abstract of DOI:10.1021/ic300083k

Syntheses, structures, and properties of univalent coinage metal 2-methylimidazolate supramolecular isomers [M(mim)] (1, M = Cu; 2, M = Ag) were investigated in detail. In addition to the known isomers, namely, zigzag chains [Cu(mim)] (1a) and [Ag(mim)] (

Full text of DOI:10.1021/ic300083k

Article
pubs.acs.org/IC
Copper(I) and Silver(I) 2-Methylimidazolates: Extended Isomerism,
Isomerization, and Host−Guest Properties
Yu Wang, Chun-Ting He, Yi-Jiang Liu, Tian-Qi Zhao, Xiao-Min Lu, Wei-Xiong Zhang, Jie-Peng Zhang,*
and Xiao-Ming Chen
MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, School of
Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China
S
* Supporting Information
ABSTRACT: Syntheses, structures, and properties of uni-
valent coinage metal 2-methylimidazolate supramolecular
isomers [M(mim)] (1, M = Cu; 2, M = Ag) were investigated
in detail. In addition to the known isomers, namely, zigzag
chains [Cu(mim)] (1a) and [Ag(mim)] (2a), molecular
o c t a g o n [ C u 8 ( m i m ) 8 ] · C 6 H 6 ( 1 b ) , d e c a g o n
[Cu10(mim)10]·C8H10 (1c), helical chain [Ag4(mim)4]·C6H6
(2b), and S-shaped chain [Ag4(mim)4]·C8H10 (2c), two new
structures including a polyrotaxane [Cu10(mim)10]·[Cu(mim)] (1d, C2/m, a = 14.452(4) Å, b = 27.712(7) Å, c = 11.427(3) Å, β
= 125.899(4)°, V = 3707(2) Å3) and a new octagon [Ag8(mim)8]·Me2CO (2d, C2/c, a = 21.852(3) Å, b = 12.101(2) Å, c =
20.907(3) Å, β = 90.875(2)°, V = 5528(2) Å3) were discovered. The potential porous properties of guest-containing [M(mim)]
isomers were studied by thermogravimetry, X-ray powder diffraction, vacuum thermal desorption, and CO2 sorption
experiments. The isomers show distinctly different guest removal behaviors depending on their pore structures. By heating, the
guest-containing isomers, 1b−1c and 2b−2d, undergo irreversible, two-step, crystal-to-crystal structural transformations to form
the guest-free isomers 1a or 2a, respectively. Except 1b, other guest-containing isomers can retain their porous structures after
removal of the template molecules, which were confirmed by CO2 sorption experiments.
helical chains, large polygons, and other unusual super-
structures. Among a variety of imidazolate derivatives, 2-
methylimidazolate showed the most abundant structure
diversity and only one case of template-induced isomerism,
which may be due to the fact that methyl group has a suitable
size to interact with guest and avoid too much steric hindrance.
We have isolated a series of supramolecular isomers of Cu(I)/
Ag(I) 2-methylimidazolates (Figure 1). For example, close-
packing zigzag chains of [Cu(mim)] (1a) and [Ag(mim)] (2a)
were obtained without template. Molecular octagon
[Cu8(mim)8]·C6H6 (1b) and 81 helix [Ag4(mim)4]·C6H6
(2b) were isolated using benzene as template, while larger
t e m p l a t e p - x y l e n e g a v e m o l e c u l a r d e c a g o n
[Cu1 0 (mim)1 0 ]·C8 H1 0 (1c) and S-shaped chain
[Ag4(mim)4]·C8H10 (2c).8 It is worth noting that, although
Cu(I) and Ag(I) possess the same coordination mode, the
structures of [Cu(mim)] and [Ag(mim)] polymers are very
different. For instance, a Ag(I)-based molecular polygon has
not been observed previously, which has been attributed to the
differences of ionic radius and metallophilicity that change the
sizes and packing fashions of the polygons.5a Certainly, such a
reasonable, hypothetic isomer may be realized by more
extensive synthetic studies. Also, considering that current
studies on porous coordination polymers (PCPs) are mostly
INTRODUCTION
■
Coordination polymers have attracted a great deal of academic
and commercial interest due to their fascinating structures and
potential applications.1 Controlling the structures and proper-
ties of coordination polymers is a long-standing challenge.2
Supramolecular isomerism,3 as an interesting phenomenon in
supramolecular and coordination chemistry,4 plays an im-
portant role in understanding the self-assembly and structure−
property relationship of coordination polymers.5 Guest-induced
supramolecular isomers have been well documented, which can
be obtained by rational introduction of different template
molecules. Strictly speaking, isomeric frameworks depending on
inclusion of different guest molecules are not genuine
supramolecular isomers, because the crystals have different
chemical compositions. However, if the template molecules can
be removed, these crystals become genuine supramolecular
isomers with different porous structures.5a Nevertheless, the
potential porosity of guest-induced supramolecular isomers has
received little attention.6
Metal azolate frameworks (MAFs)7 have been demonstrated
as a simple and effective metal−ligand system for the
construction of supramolecular isomers.5a Univalent coinage
metal imidazolates are straightforward for simple chain-like
polymeric structures, because the exobidentate ligand enforces
univalent coinage metal to be linear two-coordinated. Further,
the moderate bridging angle of imidazolate (ca.135−140°)
renders not only ordinary zigzag chains but also highly curved
Received: January 13, 2012
Published: April 2, 2012
© 2012 American Chemical Society
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[Cu10(mim)10]·2C8H10 (1c). p-Xylene (2 mL) was used as the
template (yield ca. 45%). EA calcd (%) for C56H70Cu10N20: C 40.55, H
4.25, N 16.89; found: C 40.95, H 4.39, N 16.50. A few crystals of
[Cu10(mim)10]·[Cu(mim)] (1d), which did not crack during heating
and vacuum treatments, could be found occasionally.
Phase-pure microcrystalline powder can be also prepared by
pouring a solution of [Cu(NH3)2]OH (1.0 mmol) in aqueous
ammonia/methanol (10/10 mL) into a solution of Hmim (1.0 mmol,
0.081 g) in methanol (20 mL) or methanal/template (10/10 mL) with
rapid stirring constantly, followed by filtration, wash, and drying. The
whole process must be carried out in N2 atmosphere, but the final
products easily turn green in air.
Ag(I) 2-Methylimidazolate Isomers. A solution of Ag2O (1 mmol,
0.120 g) in aqueous ammonia/ethanol (5/5 mL) was poured into a
solution of Hmim (1 mmol, 0.081 g) in methanol (10 mL) or
methanal/template (5/5 mL) with rapid stirring constantly. White
precipitate appeared quickly, and the slurry was stirred for 15 min.
After then, the white powder was filtered, washed by ethanol, and
dried in air. The phase purity was verified by PXRD.
[Ag(mim)] (2a). No template was used (yield ca. 72%). EA calcd
(%) for C4H5AgN2: C 25.42, H 2.67, N 14.82; found: C 25.59, H 3.03,
N 14.78.
[Ag4(mim)4]·C6H6 (2b). Benzene (5 mL) was used as the template
(yield 74%). EA calcd (%) for [Ag4 (mim)4 ]·0.6C6 H6
(C19.6H23.6Ag4N8): C 29.33, H 2.96, N 13.96; found: C 29.25, H
3.35, N 13.59. The chemical formula was also supported by TG results,
which indicated that the bulk sample was not fully saturated by
benzene, or the guest molecules were removed partially after the
washing and drying procedures.
[Ag4(mim)4]·C8H10 (2c). p-Xylene (5 mL) was used as the template
(yield 77%). EA calcd (%) for C24H30Ag4N8: C 33.44, H 3.51, N
13.00; found: C 33.09, H 3.68, N 13.34.
[Ag8(mim)8]·0.5Me2CO·0.5H2O (2d). Acetone (5 mL) was used as
the template (yield 79%). EA calcd (%) for C33.5H44Ag8N16O: C 25.96,
H 2.86, N 14.46; for guest-free [Ag8(mim)8] (C32H40Ag8N16): C
25.42, H 2.67, N 14.82; found: C 25.92, H 3.05, N 14.73. Single
crystals of 2d were obtained by layering a solution of Hmim (0.1
mmol, 0.008 g) in acetone (2 mL) on a solution of Ag2O (0.1 mmol,
0.012 g) in aqueous ammonia (25%, 2.0 mL).
X-ray Crystallography. Single-crystal X-ray diffraction data for 1d
and 2d were collected on a Bruker Apex CCD area-detector
diffractometer with Mo Kα monochromatic radiation. The structures
were solved by the direct method and refined by the full-matrix least-
squares method on F2 using the SHELXTL software package. All
hydrogen atoms were placed geometrically, and anisotropic thermal
parameters were used to refine all nonhydrogen atoms. The crystal
data and structure refinement results are listed in Table 1.
Density Functional Theory (DFT) Studies. Full geometry
optimization was carried out for the model molecules based on the
crystal data with the Perdew, Burke, and Ernzerhof (PBEPBE)
functional for both exchange and correlation,10 which was usually
thought to give good results for weak intermolecular interactions.
Considering both the calculation cost and the accuracy, we used the
effective core potential (ECP) and Stuttgart-Dresden ECP plus DZ
(SDD) basis sets for transition metal elements (Ag or Cu) and 6-31G
(d) for the rest atoms in the geometry optimization. Considering that
the local spin density approximation (LSDA) usually overestimates the
binding energy, while the generalized gradient approximation (GGA)
significantly underestimates, both the LSDA and PBEPBE (one of the
GGA functional) method were investigated. Simultaneously, the 6-
311G (d, p) basis set was used for C, N, O, and H, and the SDD basis
set with ECP was used for transition metal elements with correcting
basis set superposition error effects (BSSE). The binding energy ΔE
was calculated by:
Figure 1. Known supramolecular isomers of 1 and 2.
focused on three-dimensional (3D) or two-dimensional (2D)
coordination frameworks, these template-induced isomers,
[M(mim)]·guest, may serve as unique 0D and 1D porous
frameworks.9
As an extension of our previous crystal engineering study on
univalent coinage metal 2-methylimidazolates, here,we report a
detailed study on the syntheses, structures, and properties of
this unique coordination polymer system. Besides the
optimized synthetic method and discovery of new supra-
molecular isomers, thermal stabilities, structural dynamic
behaviors, and porous properties of the guest containing
isomers were also investigated.
EXPERIMENTAL SECTION
■
Materials and General Methods. All solvents and starting
materials were purchased commercially and used as received.
Elemental analyses (EA) were performed by a Perkin-Elmer 240
elemental analyzer (C, H, N). Thermogravimetric (TGA) analyses
were performed at a rate of 3 °C/min under N2 using a NETZSCH
TG 209 system. Vacuum thermal desorption (VTD) was performed at
a rate of 3 °C/min using the degas port of a Micromeritics ASAP 2020
M instrument equipped with a turbo molecular pump and a μmHg
pressure gauge. Powder X-ray diffraction (PXRD) data were recorded
on a Bruker D8 ADVANCE X-ray powder diffractometer (Cu Kα). In
variable-temperature powder X-ray diffraction (VTPXRD) measure-
ments, the diffraction patterns for different temperatures were
recorded after the sample had stayed at the respective temperature
for 30 min. Differential scanning calorimetry (DSC) was performed
under N2 using a Netzsch DSC 204 system under N2. CO2 sorption
measurements were performed using a Belsorp-Max automatic
volumetric adsorption apparatus.
Syntheses. Cu(I) 2-Methylimidazolate Isomers. A mixture of
Cu(NO3)2·3H2O (0.242 g, 1.0 mmol), Hmim (0.081 g,1.0
mmol), and aqueous ammonia (25%, 5 mL), as well as an
appropriate template if necessary, was stirred for 15 min in air,
then transferred and sealed in a 10 mL Teflon-lined vessel,
which was heated in an oven to 160 °C for 80 h, and then
cooled to room temperature at a rate of 5 °C h−1. The resulting
crystals were filtered, washed by ethanol, and dried under N2.
The phase purity of each sample was verified by PXRD.
[Cu(mim)] (1a). No template was used (yield ca. 10%). EA calcd
(%) for C4H5CuN2: C 33.22, H 3.48, N 19.37; found: C 33.14, H 3.68,
N 19.04.
ΔE = E
octagon·ntemplate
− E
octagon
− nE
+ E
BSSE
template
[Cu8(mim)8]·C6H6 (1b). Benzene (2 mL) was used as the template
(yield ca. 25%). EA calcd (%) for C38H46Cu8N16: C 36.95, H 3.75, N
18.14; found: C 36.61, H 3.96, N 17.93.
Where n is the number of template molecules, Eoctagon•ntemplate, Eoctagon,
Etemplate, and EBSSE are the energies of the complex consisting of an
octagon and n template molecules, the single molecular octagon, the
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single crystals of the new phase, i.e., the octagon
[Ag8(mim)8]·Me2CO (2d), were latterly obtained by the liquid
diffusion reaction, which enable structure determination in spite
of poor reproducibility and low yield. It is worth noting that the
mixing speed of metal ions and ligands has great impact on the
product purity. Only quickly pouring the aqueous ammonia of
Ag2O into the methanol solution of Hmim can generate the
guest-containing phases 2b, 2c, and 2d as pure phases.
Otherwise, the products always contain 2a as an impurity,
indicating that the zigzag chain structure is the thermodynami-
cally favored phase, and the guest-containing structures are
kinetic favored ones (Figure S1, Supporting Information).
Structures. As expected, the univalent coinage metal
imidazolate frameworks are all constructed by linear coordi-
nated metal ions and exobidentate mim− ligands. Depending on
the related orientation of the adjacent ligands, these structurally
simple coordination polymers can form different super-
tructures, including zigzag chains, helical chains, and polygons.
Among various reported imidazolate derivatives, mim− is the
most special one because its superstructure can be controlled
by appropriate templates. Without adding guest molecules,
adjacent mim− ligands adopt approximately trans-configuration,
giving rise to close-packing zigzag chains 1a and 2a.
Table 1. Crystallographic Data and Structure Refinement
Details
a
1
2
formula
FW
C44H63Cu11N22O4
1663.1
C36.5H51Ag8N16O2.5
1616.89
123(2)
T/K
113(2)
space group
a/Å
C2/m
C2/c
14.452(4)
27.712(7)
11.427(3)
125.899(4)
3707(2)
2
21.852(3)
12.101(2)
20.907(3)
90.875(2)
5528(2)
4
b/Å
c/Å
β/deg
V/Å3
Z
Dc/g cm−3
μ/mm−1
R1 (I > 2σ)
wR2 (all data)
GOF
1.490
1.943
3.134
2.817
0.0776
0.0614
0.2192
0.1712
1.087
1.071
a
2
2
R1 = ∑||Fo| − |Fc||/∑|Fo|, wR2 = [∑w(Fo − Fc2)2/∑w(Fo )2]1/2
.
single template molecule, and the BSSE correction, respectively. All
calculations were carried out using Gaussian 03 package.
Using aromatic templates with different sizes, Cu(I) 2-
methylimidazolate crystallize as molecular octagons 1b and
decagons 1c. If the guests were removed from 1b and 1c, the
void would be 20.5% and 39.7%, respectively,12 suggesting
potential porosity. However, because of the staggered stacking
of octagons, the cavities inside the octagons (6.5 × 9.5 × 7.5
Å3) are not interconnected (aperture diameter 3.4 Å) to each
other in 1b. Although the decagons stack in a similar fashion,
the larger inner cavities (8.6 × 10.3 × 11.5 Å3) are connected
large apertures (6.1 × 8.6 Å2) to form 1D channels in 1c
(Figure 2).
RESULTS AND DISCUSSION
■
Syntheses. Having very low solubility, metal imidazolate
frameworks easily precipitate as microcrystalline powders. The
growth of single crystals for X-ray structure analysis often
requires using a solvothermal or liquid diffusion method.5a,7 In
our preliminary studies, single crystals of the [Cu(mim)] (1a),
[Cu8(mim)8]·C6H6 (1b), and [Cu10(mim)10]·C8H10 (1c) were
discovered by solvothermal reactions,8a while [Ag(mim)] (2a),
[Ag4(mim)4]·C6H6 (2b), and [Ag4(mim)4]·C8H10 (2c) were
furnished by liquid diffusion reactions.8b
In order to optimize the synthesis of these compounds and
search for new isomers, we tried different reaction methods and
conditions. Rapid mixing an aqueous ammonia solution of
metal oxide/hydroxide with a solution of ligands has been
demonstrated to be efficient for producing a pure microcrystal-
line sample for several MAF systems.11 This method can be
applied for not only rapid syntheses of known isomers but also
discovery of new isomers of Cu(I) and Ag(I) 2-methylimida-
zolates. Nevertheless, the rapid solution mixing method for
Cu(I) complexes requires inert atmosphere protection, and the
powder of Cu(I) 2-methylimidazolates obtained by this method
tends to be oxidized in air, probably due to their small particle
sizes. Therefore, the more convenient solvothermal method,
despite its long reaction time and relatively low yield, was used
for preparation of the Cu(I) 2-methylimidazolate isomers, for
which the large single crystals can be handled in the air.
Although liquid diffusion can produce high-quality single
crystals of Ag(I) 2-methylimidazolate isomers, it also has many
drawbacks such as long reaction time, low yield, low purity,
and/or low repeatability. The crystals of guest-containing
isomers generally grow at the template buffering layer, while the
guest-free isomer 1a always appears at other places
simultaneously. Ag(I) ions are relatively stable in air but can
be easily reduced at high temperatures so that the solvothermal
method is not suitable for Ag(I) 2-methylimidazolates.
Therefore, the rapid solution mixing method is suitable for
synthesizing the Ag(I) 2-methylimidazolate isomers. More
importantly, using acetone as a solvent, a new structure was
obtained, as evident by PXRD characterization. Fortunately,
Figure 2. Perspective views of the framework and pore (shown as
Connolly surfaces with probe diameter 2.5 Å) structures of (a) 1b and
(b) 1c.
While single crystals of 1c tend to crack during thermal and/
or vacuum treatments, a few specimens remain intact, which
were identified as a polyrotaxane-like structure
[Cu10(mim)10]·[Cu(mim)] (1d) by X-ray single-crystal
diffraction study. Having very similar cell parameters and the
same space group with 1c, 1d also contains the [Cu10(mim)10]
decagons, but its cavities are occupied by Cu(I) ions and mim−
ligands rather than the xylene molecules. Interestingly, the extra
Cu(I) ions and mim− ligands connect each other into simple
zigzag chains penetrating through multiple decagons, which
form a rare polyrotaxane-like structure (Figure 3). Masciocchi
et al. have reported a cocrystal structure of chain and ring
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Figure 3. Perspective view of the polyrotaxane-like structure of 1d.
isomers for [Cu(pymo)] (Hpymo = 2-hydroxypyrimidine), in
which the small [Cu6(pymo)6] hexagons are not penetrated by
the steric hindered helical chains.13
Being different with Cu(I) 2-methylimidazolate, Ag(I) 2-
methylimidazolate containing aromatic templates are 81 helix
chains and S-shaped chains instead of polygons. The potential
voids in 2b and 2c would be 22.5 and 25.6%, respectively, when
the template molecules are omitted. Packing of the highly
curved chains leads to S-shaped channels, with widest and
narrowest cross section sizes of ca. 7.1 × 4.3 Å2 and 7.6 × 3.8
Å2 for 2b, and 6.7 × 3.9 Å2 and 7.7 × 3.2 Å2 for 2c, respectively
(Figure 4).
Figure 5. Perspective views of the octagonal molecular structure in (a)
ball-and-stick (thermal ellipsoid drawn at 50% probability. Symmetry
code: a = 1 − x, −y, 1 − z) and (b) space-filling mode, and (c) packing
of octagons and (d) pore structure (shown as Connolly surfaces with
probe diameter 2.5 Å) of 2d.
Theoretical Study. To further understand the template
effect involved in the syntheses of these compounds, we used
density functional theory (DFT) to calculate the binding
energy (ΔE) of the [Cu8(mim)8]/[Ag8(mim)8] octagons with
different guest molecules (observed in the crystal structures or
hypothetic) (Figure S2 and Tables S1−S8, Supporting
Information). As shown in Table 2, both benzene and toluene
Figure 4. Perspective views of the framework and pore (shown as
Connolly surfaces with probe diameter 2.5 Å) structures of (a) 2b and
(b) 2c.
Table 2. Calculated Binding Energies of [M8(mim)8]
Octagons for Different Guests
Crystal structure analysis showed that 2d contains four
independent linear two-coordinated Ag(I) ions and four
exobidentate mim− ligands (Ag−N 2.056(9)−2.099(9) Å,
N−Ag−N 170.6(4)−177.0(4)o), as well as disordered acetone
and water guest molecules. The adjacent mim− ligands are
oriented in a cis-fashion (the dihedral angle, DA, between
adjacent im rings is 8.6−29.3°), giving rise to a semicircular-
shaped Ag4(mim)4 fragment. Two of such fragments join (DA
24.1°) to generate a centrosymmetric, neutral, flattened
elliptical octagon (Figure 5a). The inner cavity of 2d has a
size of (7.0 × 10.0 Å2), which is slightly larger than that of the
Cu(I) analog 1b (6.5 × 9.5 Å2). Comparison of the cavity sizes
with the molecular sizes of benzene (6.6 × 7.3 Å2), cyclohexane
(6.6 × 7.2 Å3), and p-xylene (6.6 × 9.1 Å3) indicates that the
guests are suitable for 1b but smaller for 2d as template (Figure
5b). Similar to other univalent coinage metal imidazolates,
[Ag8(mim)8] also stack by metallophilic interactions
(Ag1···Ag3 2.9996(12) Å, Ag1···Ag4 2.9870(13) Å, Ag2···Ag4
3.0530(12) Å) to form high-dimensional structure (Figure 5c).
There are 1D curved channels (void = 26.7%) with the
narrowest and widest diameter ca. 4.3 Å and 10.0 Å,
respectively, running along the c-axis. (Figure 5d).
host + guest
PBEPBE (kJ/mol)
LSDA (kJ/mol)
[Ag8(mim)8] + benzene
[Cu8(mim)8] + benzene
[Ag8(mim)8] + toluene
[Cu8(mim)8] + toluene
[Ag8(mim)8] + 2acetone
[Cu8(mim)8] + 2acetone
−1.78
−7.28
−2.40
−8.49
−15.06
−31.32
−8.57
−18.43
−13.28
−23.33
−25.48
−67.44
can be inserted into the [Cu8(mim)8] and [Ag8(mim)8]
octagons with attractive interactions (binding energy ΔE <
0). However, the binding energy of [Cu8(mim)8] with benzene
and toluene are 6−10 kJ·mol−1 larger than those for
[Ag8(mim)8], indicating that the stabilization effects of benzene
and toluene for [Cu8(mim)8] are stronger than for
[Ag8(mim)8]. The difference can be explained by the relatively
large inner cavity of [Ag8(mim)8], which has weak confinement
effect for benzene and toluene. On the other hand, when a pair
of acetone molecules were inserted into the [Ag8(mim)8]
octagon, the binding energy was much lower than for those
containing aromatics, which was consistent with our exper-
imental results. It can be explained that the size of a pair of
acetone molecules are suitable for the cavity size of
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[Ag8(mim)8] octagon. Unexpectedly, the binding energy of
[Cu8(mim)8] octagon with a pair of acetone molecules was the
lowest among all calculation results, indicating that a pair of
acetone molecules could be also inserted into a smaller pore,
illustrating the flexible nature of the supramolecular acetone
dimer. This result also suggests that acetone may also promote
the formation of [Cu8(mim)8] octagons. As predicted, the
microcrystalline product obtained by rapid solution mixing
reaction of Cu(I) 2-methylimidazolate using acetone as the
template displays a PXRD pattern (Figure S1g, Supporting
Information) very similar to that of 1b.
Framework Stability. Similar to other univalent coinage
metal azolates, Cu(I) and Ag(I) 2-methylimidazolates also have
good chemical stability. Crystal samples of isomers 1a−1c can
be handled in air for short time, and its color changes from
yellow to light green after 2 days, reflecting the oxidation of
Cu(I) ions in air and moisture. However, after 1 month of
exposure, the PXRD pattern of the light-green crystals still
resembled that of isomers 1a−1c, respectively. For samples of
isomers 2a−2d, no color change was evident when powder
samples were exposed to ambient light for weeks.
Thermal stability and template removing property of 1b, 1c,
2b, 2c, and 2d were estimated by thermogravimetry (TG) and
variable-temperature PXRD (VTPXRD) measurements. TG
curve of 1b showed a weight loss 6.36% (cal. 6.32%) between
165 and 240 °C, followed by a plateau to 350 °C. The initial
guest removal temperature is much higher than the boiling
point of benzene, corresponding well to the unconnected
cavities. In contrast, 1c containing 1D S-shape channel began to
lose weight at 60 °C, and all p-xylene can be removed below
170 °C (Figure 6).
Figure 7. TG curves of 2b (black), 2c (blue), and 2d (red).
phase above 260 °C, corresponding well with the plateau in the
TG curve (Figure 8a). Similarly, 1c also transformed to two
new phases at 120 and 250 °C, respectively (Figure 8b).
Interestingly, the diffraction patterns of the first and second
new phases of 1b are very similar to those of the first and
second ones of 1c, respectively. Moreover, the diffraction
patterns of the second new phases were very similar to that of
1a, indicating that both 1b and 1c finally transformed to the
Figure 6. TG curves of 1b (black) and 1c (blue).
TG curves of 2b and 2c are similar, which lose weight from
room temperature to 155 °C, and decompose above 220 °C. It
is interesting that the TG curve of 2d did not show the guest
removal process, although its decomposed temperature was
similar to those of 2b and 2c (Figure 7). This phenomenon can
be explained by the large pores of 2d, which can hardly retain
the guest molecules with low boiling point and small size after
routine filtration, washing, and drying procedures.
VTPXRD patterns of 1b showed that the as-synthesized
phase can be stable up to 160 °C. New diffraction peaks
appeared at 180 °C, which became stronger and stronger below
240 °C. This phenomenon is consistent with the guest removal
temperature range in the TG curve. The as-synthesized phase
disappeared above 240 °C. At 250 °C, some other new peaks
appeared, and the sample completely transformed to another
Figure 8. VTPXRD patterns of (a) 1b and (b) 1c.
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thermally stable, nonporous zigzag chain structure, which was
confirmed by Rietveld refinement of the PXRD pattern (Table
S9 and Figures S3 and S4, Supporting Information). The Ag(I)
2-methylimidazolate isomers 2b, 2c, and 2d also show similar
two-step phase transitions. The original structures are stable
below 130 °C. The intermediate phase appeared above 140 °C,
and the final stable phase (2a) appeared above 190 °C (for 2b,
2c) and 160 °C (for 2d), respectively (Figure S5, Supporting
Information). Differential scanning calorimetry (DSC) was
used to reveal the phase transitions for guest-free 2d (to avoid
the energetic effect of guest removal). The DSC curve of 2d
exhibits a sharp exothermic peak (−43.92 kJ g−1) centered at
153 °C, followed by another broad exothermic one (−20 kJ
g−1) centered at 182 °C (Figure S6, Supporting Information),
corresponding well to the two-step phase transition. We cannot
solve the crystal structures for the intermediate phases, because
they always admix with 1b/1c and 1a or 2b/2c/2d and 2a. The
structural transformations from the porous isomers to the
nonporous ones for 1 and 2 may be explained by a local
melting mechanism, as there is no obvious structural relation-
ship among the superstructures (e.g., packing, position, and
short contacts of rings and chains) of these isomers, except the
local coordination modes and 0D/1D topologies.
Recently, we reported that Ag(I) 2-isopropylimidazolate
[Ag(ipim)] could undergo a similar temperature-induced two-
step crystal-to-crystal structural transformation from a chicken-
wire isomer to a quintuple helix one.14 In contrast with 1 and 2,
the stable phase of [Ag(ipim)] at high temperature is not the
simple chain (although the simple chain isomer can be
synthesized), and there are very similar structural features
between the chicken-wire and quintuple helix isomers in favor
of a topochemical transformation mechanism. Moreover, there
is no observable intermediate phase in the PXRD pattern, and
there are a pair of DSC peaks, an endothermic one followed by
an exothermic one, during the phase transition.
Figure 9. Comparison of DTG and VTD curves of (a) 1b and (b) 1c.
Similarly, VTD curves showed that 2b and 2c can be readily
activated at 60 °C under high vacuum for 24 h, which are also
confirmed by TG and PXRD measurements of the activated
samples (Figures S8−S10, Supporting Information).
Sorption Properties. In order to confirm the permanent
porosity of 1c, 2b, 2c, and 2d, CO2 sorption isotherms were
measured at 195 K. As shown in Figure 10, all isotherms have
Sample Activation. According to the structural analysis,
except 1b, other guest-containing isomers might be activated to
serve as porous materials. However, from the TG and PXRD
results, only 2d could easily lose the guest molecules and
remain in the open structure. When the guest molecules are
removed, other isomers undergo phase transitions and very
likely transform to nonporous structures. To investigate
whether these isomers can be activated at lower temperatures
to avoid phase transitions, we performed vacuum thermal
desorption (VTD) experiments by recording the guest
desorption rate of samples with slow elevation of temperature
under high vacuum. The VTD experiment has the same
physical meaning as the differential TG curves (DTG) but
provides information under high vacuum rather than ambient
pressure.
For 1b, the DTG and VTD curves show sharp peaks at the
same temperature (Figure 9a), meaning that the reducing
pressure cannot induce an observable effect on the desorption
temperature. This fact indicates that the benzene molecules are
held very tightly in the crystal of 1b, because the cavities are
isolated. For 1c, the peak in the VTD curve appears at a
temperature 50 °C lower than that in the DTG curve (Figure
9b). Therefore, 1c could be activated by heating at 80 °C under
high vacuum (the same as VTD experiment) for 24 h. TG and
PXRD measurements showed that the guest molecules have
been completely removed, and the activated sample retains the
as-synthesized structure, though the diffraction peaks are
weakened and broadened (Figure S7, Supporting Information).
Figure 10. Adsorption (filled) and desorption (open) isotherms of
CO2 for 1c (black), 2b (red), 2c (blue), and 2d (green) measured at
195 K, respectively.
type-I characteristics, except the small steps and hystereses for
1c and 2b, which indicate certain framework flexibility. Fitting
the CO2 adsorption isotherms give large BET (Brunauer−
Emmett−Teller) surface areas of 114−286 m2 g−1 and
Langmuir surface areas of 181−388 m2 g−1 (Table S10,
Supporting Information), indicating that the sorption occurs
inside the crystals. The pore volumes of the samples were also
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calculated from the saturated uptakes for judging the sample
quality. The measured pore volume of 1c was much less than
the theoretical one, indicating partial collapse of the porous
framework after guest removal. In contrast, the measured pore
volumes of 2b, 2c, and 2d were close to the theoretical ones,
confirming that the porous structures were well retained.
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■
Depending on the chemical nature of targeted coordination
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ASSOCIATED CONTENT
■
S
* Supporting Information
PXRD patterns, TG curves, DSC curves, additional structural
plots, DFT calculation data, and X-ray crystallographic files in
CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
(12) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7.
(13) Masciocchi, N.; Ardizzoia, G. A.; LaMonica, G.; Maspero, A.;
Corresponding Author
Sironi, A. Angew. Chem., Int. Ed. 1998, 37, 3366.
(14) Zhang, J. P.; Qi, X. L.; He, C. T.; Wang, Y.; Chen, X. M. Chem.
Commun. 2011, 47, 4156.
*E-mail: zhangjp7@mail.sysu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the “973 Project”
(2012CB821706), NSFC (21121061 and 21001120), and
Chinese Ministry of Education (NCET-10-0863 and ROCS).
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