Flexible porous coordination polymers constructed from 1,2-bis(4-pyridyl) hydrazine via solvothermal in situ reduction of 4,4′-azopyridine

Article abstract of DOI:10.1039/c1dt10682a

Solvothermal reactions of Zn(NO₃)₂, 1,4-benzenedicarboxylic acid (H₂bdc), and 4,4′-azopyridine (azpy) in different conditions yielded [Zn(bdc)(bphy)]·DMF·H ₂O (1a, bphy = 1,2-bis(4-pyridyl)hydrazine, DMF = N,N-dimethylformamide) and [Zn(bdc)(bphy)]·EtOH·H₂O (1b) with two-fold interpenetrated dmp topology and [Zn₂(bdc) ₂(bphy)]·1.5EtOH·H₂O (2a) and [Zn ₂(bdc)₂(bphy)]·DMA·1.5H₂O (2b, DMA = N,N-dimethylacetamide) with two-fold interpenetrated pcu topology. The in situ reduction of azpy to bphy was confirmed by single-crystal structures and LC-MS analyses of the acid-digested crystalline samples, as well as controlled solvothermal experiments. Removal of the guest molecules in 1a/1b and 2a/2b converts the materials to guest-free phases [Zn(bdc)(bphy)] (1) and [Zn ₂(bdc)₂(bphy)] (2), respectively, which were identified by PXRD. CO₂ sorption experiments performed at 195 and 298 K showed low porosity for 1 and gated sorption behavior for 2. At 298 K, 2 exhibits high selectivity for adsorbing CO₂ over CH₄. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1dt10682a

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PAPER
Flexible porous coordination polymers constructed from
1,2-bis(4-pyridyl)hydrazine via solvothermal in situ reduction of
4,4¢-azopyridine†
Xiao-Min Liu, Lin-Hua Xie, Jian-Bin Lin, Rui-Biao Lin, Jie-Peng Zhang* and Xiao-Ming Chen
Received 16th April 2011, Accepted 8th June 2011
DOI: 10.1039/c1dt10682a
Solvothermal reactions of Zn(NO3)2, 1,4-benzenedicarboxylic acid (H2bdc), and 4,4¢-azopyridine
(azpy) in different conditions yielded [Zn(bdc)(bphy)]·DMF·H2O (1a, bphy =
1,2-bis(4-pyridyl)hydrazine, DMF = N,N-dimethylformamide) and [Zn(bdc)(bphy)]·EtOH·H2O (1b)
with two-fold interpenetrated dmp topology and [Zn2(bdc)2(bphy)]·1.5EtOH·H2O (2a) and
[Zn2(bdc)2(bphy)]·DMA·1.5H2O (2b, DMA = N,N-dimethylacetamide) with two-fold interpenetrated
pcu topology. The in situ reduction of azpy to bphy was confirmed by single-crystal structures and
LC-MS analyses of the acid-digested crystalline samples, as well as controlled solvothermal
experiments. Removal of the guest molecules in 1a/1b and 2a/2b converts the materials to guest-free
phases [Zn(bdc)(bphy)] (1) and [Zn2(bdc)2(bphy)] (2), respectively, which were identified by PXRD. CO2
sorption experiments performed at 195 and 298 K showed low porosity for 1 and gated sorption
behavior for 2. At 298 K, 2 exhibits high selectivity for adsorbing CO2 over CH4.
On the other hand, novel coordination structures with in situ
Introduction
generated ligands (especially those that are inaccessible by direct
preparation) have attracted considerable attention not only in
coordination chemistry but also in organic chemistry for dis-
covery of new organic reactions.20–22 To date, many metal/ligand
reactions have been found and used to generate a large number
of coordination polymers.23–27 For example, we have discovered
and elucidated several types of copper-mediated, oxidative ligand
conversions under solvothermal conditions, such as hydroxylation
of 2,2¢-bipyridyl-like ligands28 and one-pot syntheses of the 3,5-
disubstituted-1,2,4-triazole from organonitriles and ammonia.29
We have also utilized these reactions to synthesize a series of novel
supramolecular architectures with interesting photoluminescent
and porous properties.22
Herein, we report flexible PCPs constructed by reactions of
4,4¢-azopyridine and 1,4-bezenedicarboxlic acid with Zn(NO3)2.
Interestingly, 4,4¢-azopyridine was found to be in situ reduced
to a flexible ligand 1,2-bis(4-pyridyl)hydrazine, giving two PCPs
crystallizing in four different crystal structures, which showed
interesting framework flexibility and gas sorption properties.
During the last decade, porous coordination polymers (PCPs)
rapidly emerged as a new type of porous materials and have
attracted considerable attention from scientists in many research
fields.1–3 These kinds of materials possess some advantages such
as high crystallinity, tunability of pore character and structural
flexibility, when compared with the conventional porous mate-
rials, i.e. zeolites and activated carbons.4–9 Flexible PCPs are of
special interest because the host frameworks can show struc-
tural rearrangements during sorption processes. Such structural
transformations may be guest-dependent, and consequently the
adsorbents are able to distinguish different adsorbates with high
selectivity. Therefore, flexible PCPs may find applications in chem-
ical sensors, switches, gas separation, etc.10 Generally, framework
flexibility is imparted by weak bonding interactions,11–13 sliding
of multiple networks,14–16 changing coordination geometries,17
and/or rotation around single coordination/covalent bonds.18,19
Presently, rational design and synthesis of flexible PCPs are
basically based on choosing suitable ligands.
Experimental section
MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, State
Key Laboratory of Optoelectronic Materials and Technologies, School of
Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou,
510275, P. R. China. E-mail: zhangjp7@mail.sysu.edu.cn; Fax: +86 20
84112245; Tel: +86 20 84113986
† Electronic supplementary information (ESI) available: PXRD patterns,
TGA curve, additional plots, selected bond lengths and angles, and
crystallographic data. CCDC reference numbers 822607–822610. For ESI
and crystallographic data in CIF or other electronic format, see DOI:
10.1039/c1dt10682a
Materials and general methods
Commercially available reagents were used as received without
further purification. 4,4¢-Azopyridine (azpy) was prepared ac-
cording to a reported method (Fig. S1 and Fig. S2†).30 ESI-
MS spectra were measured on a SHIMADZU LCMS-2010A
apparatus using an electrospray ionization source with MeOH as
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1
the mobile phase. H-NMR spectra were recorded on a Varian
0.25 mmol), and azpy (0.023 g, 0.125 mmol) was sealed in a 15 mL
Teflon-lined reactor and heated at 160 ◦C for 84 h, then cooled
to room temperature. The product was washed with DMF and
collected by filtration (0.1060 g, yield 76%). Anal. calcd. (%) for
C29H29N4O10.5Zn2: C, 47.56; H, 3.99; N, 7.65. found: C, 47.12; H,
3.54; N, 7.94. IR: 3390(m), 3022(w), 2480(w), 1634(s), 1566(m),
1389(s), 1201(w), 1016(w), 910(w), 826(m), 748(s), 552(m), 498(w).
Mercury-Plus (300 MHz) spectrometer. Infrared spectra were
recorded with a Bruker TENSOR 27 Fourier transform FT-IR
spectrophotometer on KBr pellets in the range of 4000–400 cm-1.
Elemental (C, H, and N) analyses were performed on a Perkin-
Elmer 240 elemental analyzer. Thermogravimetry (TG) analyses
were performed on a Netzsch TG 209 instrument in flowing N2
◦
with a heating rate of 10 C min-1. Powder X-ray diffraction
[Zn2(bdc)2(bphy)]·DMA·1.5H2O (2b). A procedure similar
to that for 2a was utilized to prepare 2b by using N,N-
dimethylacetamide (DMA) in place of ethanol (0.137 g, yield
92%). Anal. calcd. (%) for C30H30N5O10.5Zn2: C, 47.45; H, 3.98;
N, 9.22. found: C, 47.82; H, 3.26; N, 9.65. IR: 3301(m), 3200(w),
3104(w), 2939(w), 2877(w), 2014(m), 1698(s), 1514(m), 1447(s),
1262(s), 1142(m), 1041(w), 860(s), 546(s), 478(s), 415(m).
(PXRD) measurements were performed on a Bruker D8 AD-
VANCE X-ray diffractometer with Cu-Ka radiation. For variable-
temperature PXRD measurements, the diffraction patterns at
different temperatures were recorded after the sample had stayed
at the respective temperature for 30 min in a N2 atmosphere. CO2
and CH4 sorption measurements were performed with Belsorp-
Max and Belsorp-HP automatic volumetric adsorption apparatus.
Crystallography
Synthesis
All the single-crystal XRD data were collected on a Bruker
Apex CCD area-detector diffractometer with Mo-Ka radiation
at 293(2) K. The structures were solved by the direct or Patterson
methods and refined by the full-matrix least-squares method on F2
using SHELXTL. All hydrogen atoms were placed geometrically
and anisotropic thermal parameters were used to refine all
non-hydrogen atoms of the frameworks. Most disordered guest
molecules in compounds 1a–2b could not be modeled and their
electron density peaks were removed by the SQUEEZE route in
PLATON, and the amount of them were determined by TG results
and elemental analyses. The crystal data and structure refinement
results for these compounds are listed in Table 1.
[Zn(bdc)(bphy)]·DMF·H2O
(1a). An
N,N-dimethyl-
formamide (DMF) solution (9 mL) of Zn(NO3)2·6H2O (0.074
g, 0.25 mmol), 1,4-benzenedicarboxylic acid (H2bdc, 0.042 g,
0.25 mmol), and azpy (0.046 g, 0.25 mm◦ol) was sealed in a 15 mL
Teflon-lined reactor and heated at 160 C for 84 h, then cooled
to room temperature. The products were washed with DMF and
collected by filtration (0.0602 g, yield 37%). Anal. calcd. (%) for
C21H23N5O6Zn: C, 49.76; H, 4.57; N, 13.82; found: C, 49.82; H,
4.52; N, 13.45. IR: 3239(m), 2510(w), 2481(w), 1621(w), 1618(s),
1501(w), 1357(s), 1215(s), 1063(w), 1057(m), 1020(m), 828(m),
750(m).
[Zn(bdc)(bphy)]·EtOH·H2O (1b). A similar synthesis proce-
dure for 1a was adapted to prepare 1b, except that the DMF
was replaced by EtOH (0.145 g, yield 90%). Anal. calcd. (%) for
C20H22N4O6Zn: C, 50.06; H, 4.62; N, 11.68. found: C, 51.12; H,
4.36; N, 12.25. IR: 3245(m), 3141(w), 3040(w), 2998(w), 2791(w),
1616(s), 1501(w), 1359(s), 1216(s), 1061(m), 1020(s), 828(m),
750(m), 526(w).
Results and discussion
Synthesis
Solvothermal reactions of Zn(NO3)2, 1,4-benzenedicarboxylic
acid (H2bdc) and 4,4¢-azopyridine (azpy) in different condi-
tions yielded four crystalline products, [Zn(bdc)(bphy)]·DMF·
H2O (1a), [Zn(bdc)(bphy)]·EtOH·H2O (1b), [Zn2(bdc)2(bphy)]·
1.5EtOH·H2O (2a), and [Zn2(bdc)2(bphy)]·DMA·1.5H2O (2b).
[Zn2(bdc)2(bphy)]·1.5EtOH·H2O (2a). An ethanol solution (9
mL) of Zn(NO3)2·6H2O (0.0743 g, 0.25 mmol), H2bdc (0.042 g,
Table 1 Crystallographic data and refinement parameters for 1a, 1b, 2a and 2b
1a
1b
2a
2b
Formula
F.W.
C21H23N5O6Zn
506.81
Fdd2
28.942(3)
58.653(5)
21.5267(19)
90
90
90
36 542(6)
64
1.474
1.122
1.000
0.0664
0.1868
0.461(9)
C20H22N4O6Zn
479.79
Pnna
29.50(2)
14.724(13)
10.274(9)
90
90
90
4463(7)
8
1.428
C29H29N4O10.5Zn2
732.30
P1
C30H30N5O10.5Zn2
759.33
P1
¯
¯
Space group
˚
a/A
10.877(7)
17.619(12)
20.499(14)
110.788(12)
93.326(13)
90.408(12)
3665(4)
4
1.327
1.363
1.004
0.0544
10.9028(13)
10.9453(13)
15.8533(18)
88.159(2)
80.542(2)
85.314(2)
1859.6(4)
2
1.356
1.347
1.009
0.0412
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Z
Dc/g cm-3
m/mm-1
GOF
1.143
1.005
0.0600
0.1677
a
R1 (I > 2s)
b
wR2 (all data)
0.1469
0.1105
Flack parameter
ꢀ
ꢀ
ꢀ
ꢀ
2
a R1 = ꢀFo| - |Fcꢀ/ |Fo|. b wR2 = [ w(Fo - Fc2)2/ w(Fo2)2]1/2
.
8550 | Dalton Trans., 2011, 40, 8549–8554
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1a/1b and 2a/2b were synthesized by feeding ratios
Zn(NO3)2 : H2bdc : azpy = 1 : 1 : 1 and 1 : 1 : 0.5, respectively. Dif-
ferent solvents resulted in inclusion of different guests. Though
their synthetic conditions were only slightly different, all phases
were obtained in high purity, which were confirmed by PXRD
patterns (Fig. S8†). Evidence of the in situ reduction of azpy to
bphy will be discussed below.
Structures
X-Ray crystallographic analyses revealed that 1a and 1b crystallize
in the orthorhombic space groups Fdd2 and Pnna, respectively,
with very different cell parameters. There are four independent
Zn2+ ions, four bdc2- ligands and four bphy ligands in the
asymmetric unit of 1a, but 1b contains only two independent Zn2+
ions (both lie on the twofold axes with half occupancy), one bdc2-
and one bphy (in general positions with no imposed symmetry).
The coordination modes between the Zn2+ ions, bdc2- and bphy
ligands in the two structures are basically similar (Fig. 1a). Each
Zn2+ ion in 1a/1b is four-coordinated with two carboxylate O
2-
˚
atoms [Zn–O 1.925(3)–2.187(3) A] from two bdc ligands and
˚
two N atoms [Zn–N 2.004(3)–2.073(3) A] from two bphy ligands
in distorted tetrahedral geometries. The other two carboxylate O
atoms form either weak coordination or non-bonding with the
˚
Zn ion [Zn–O 2.299(4)–2.915(3) A] (Fig. S4 and Table S1†). Each
bdc2- ligand bridges two Zn2+ ions by its two carboxylate ends,
and each bphy ligand links two Zn2+ ions by its two pyridine
nitrogen atoms. Based on the following facts, the ligand in 1a/1b
should be bphy rather than the starting agent azpy. The N–N bond
˚
lengths [1.365(5)–1.440(5) A] in 1a/1b are in good accordance with
˚
reported values of those in free bphy [1.3934(14) A] and other
compounds containing N–N single bonds [1.368(15)–1.434(17)
Fig. 1 Perspective views of a typical coordination environment of the
zinc atoms, the simplified topology networks (dmp and pcu) and the 3D
packing structures with two-fold interpenetration for 1a/1b (a, b, c) and
2a/2b (d, e, f). (cyan sphere, red stick, and blue stick represent Zn, bdc2-,
and bphy, respectively).
30–36
˚
A],
and are obviously longer than the typical N N bond
36
˚
length [1.2513(15) A] in azpy. Furthermore, the absolute values
of C–N–N–C torsion angles [89.3(5)–100.8(5)◦] in 1a/1b are
distinctly deviated from the planar configuration of C–N N–
C moieties in azpy (180◦), but similar to the reported value
for the free bphy [84.37(3)◦] (Table 2). As depicted in Fig. 1b,
bdc2- and bphy ligands can be regarded as 2-connected edges,
while Zn2+ ions can be treated as 4-connected vertexes, forming
a 3D uninodal 4-connected dmp net (vertex symbol 6.6.6.6.62.83).
This kind of net is rare and has been only observed in several
coordination polymers.37 Two such dmp nets interpenetrate in
1a and 1b, inducing the solvent accessible voids to 30.3% and
28.5%, respectively (Fig. 1c).38 The void was occupied by one
DMF and one water molecules per formula unit in 1a, and
one EtOH and one water molecules per formula unit in 1b,
which were confirmed by TG and elemental analyses for crystal
Table 2 Comparison of the N–N/N N bond lengths and torsion angles in hydrazine/azo derivatives
C–N–N–C torsion angle [◦]
Ref.
˚
Compounds
N–N bond length [A]
azobenzene
1.253(3)
1.2513(15)
1.3934(14)
1.394(5)
180
180
84.37(3)
75.59(7)
31, 33, 34
4,4¢-azopyridine
1,2-bis(4-pyridyl)hydrazine
1,2-diphenylhydrazine
1,2-bis(4-pyridyl)hydrazine
1a
30, 31, 34, 36
30, 31, 34, 36
31, 33, 34
this work
this work
1.387(1)
136.5(2)
N2–N3 1.431(5)
N6–N7 1.365(5)
N10–N11 1.440(5)
N14–N15 1.424(6)
N2–N3 1.386(8)
N2–N3 1.371(6)
N6–N7 1.375(8)
N3A–N4A 1.419(16)
N3B–N4B 1.364(14)
100.8(5), -89.6(5), -89.3(5), -96.2(6)
1b
2a
-98.7(7)
-142.9(7), 149.4(6)
this work
this work
2b
-142.0(13), 144.5(14)
this work
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samples. Though 1a and 1b have the same topology, there are
some slight differences for the detailed coordination environments
of the Zn2+ ions and conformations of the bphy ligands. The
coordination configuration of each Zn2+ ion is different from each
other, especially in the Zn ◊ ◊ ◊ O distances of the weak coordination
and/or non-bonding interactions between the metal and the
carboxylate O atoms. The conformations of bphy ligands are
also similar, but deviations from the free bphy ligand are evident.
According to ab initio calculations, the stable C–N–N–C torsion
angle of free bphy is 136.5(2)◦, but the values are -89.3(5), -89.6(5),
-96.2(6), and 100.8(5)◦ in 1a and -98.7(7)◦ in 1b (Fig. S5 and
Table S2†). Accordingly, it is probably the size of the bigger
guests which results in the more serious deformation of the bphy
ligands (DMF in 1a and EtOH in 1b). Combination of slightly
different coordination geometries and flexible ligands endows the
framework with various conformations, resulting in the different
crystallographic symmetries of 1a and 1b.
hydrazopyridinium.41 Recently, Beckhaus et al. isolated bphy as a
byproduct using azpy as starting material.36 Whereas, the reason
for the reduction of azpy was unclear in the latter two examples.
Although X-ray crystallographic analysis strongly suggested the
presence of bphy ligands in the four obtained crystal structures,
we performed LC-MS analysis on the acid digested sample and
control experiments to further confirm the in situ reduction of azpy
(Scheme 1). Acetic acid was used to decompose the crystalline
compounds, and then the resultant solution was analyzed by LC-
MS, showing clearly one peak of bphy instead of azpy (Fig. S3†).
Meanwhile, we also treated a sample of the starting material azpy
with the above-mentioned conditions for LC-MS measurement,
and the results showed azpy remained intact. Therefore it was
confirmed that the ligands in 1a, 1b, 2a and 2b are bphy rather
than azpy, and azpy was reduced to bphy during the reaction
processes.
¯
2a and 2b both crystallize in the triclinic space group P1, but
the unit-cell volume of 2b is almost half that of 2a. There are
four Zn2+ ions, four bdc2- and two bphy ligands in the asymmetric
unit of 2a, but 2b contains only half that of 2a (Fig. 1d). The N–
˚
N bond lengths [1.364(14)–1.419(16) A] and the absolute values
of C–N–N–C torsion angles [142.0(13)–149.4(6)◦] in 2a/2b are
also consistent with those for bphy (Fig. S7 and Table S4†). The
coordination modes between each independent Zn2+ ions, bdc2-
ligands, and bphy ligands in 2a/2b are also basically similar. Each
Scheme 1 Reduction of azpy into bphy.
Zn2+ ion in 2a/2b is five-coordinated with four carboxylate O
In view of the lack of reduction ability for zinc nitrate and
terephthalic acid, we speculated that the employed solvents
were critical for the in situ reducing of azpy into bphy under
solvothermal conditions. Different solvents including pure water
were reacted with azpy under solvothermal condition without the
addition of zinc ions or H2bdc ligands. LC-MS analysis indicated
that the reduction of azpy to bphy can be furnished at 160 ◦C in
water.
2-
˚
atoms [Zn–O 2.007(4)–2.061(4) A] from four bdc ligands and
˚
one N atom [Zn–N 1.993(4)–2.018(3) A] from one bphy ligand in a
square-pyramidal geometry (Fig. S6 and Table S3†). Two Zn2+ ions
are bridged by four carboxylate groups from four bdc2- ligands,
leading to a paddle-wheel dinuclear unit. Each bdc2- ligand bridges
two dinuclear units by its two carboxylate ends, extending to a 2D
sql net, which are pillared by bphy to form a 3D elongated primitive
pcu net (Fig. 1e).39,40 The overall 3D structure of 2a/2b is composed
of a pair of such pcu nets interpenetrated with each other, with
voids of 40.3% in 2a and 37.3% in 2b, which are occupied by 1.5
EtOH and 1 H2O molecules per formula in 2a and 1 DMA and
1.5 H2O molecules per formula in 2b (Fig. 1f). The reason for
the structural discrepancies between 2a and 2b is similar to that
mentioned for 1a/1b. Though there is no obvious difference in the
Zn2+ coordination environment, the C–N–N–C torsion angles in
2a [142.9(7)–149.4(6)◦] are larger than that [136.5(2)◦] in the free
bphy and those in 2b [142.0(13)–144.5(14)◦].
Framework stability
The TG curves show that 1a releases one H2O molecule per
formula unit (observed 3.8%, calc. 3.6%) below 145 ◦C and
subsequently one DMF (observed 17.0%, calc. 16.7%) before
210 ◦C. 1b shows a 9.3% of weight loss at about 90 ◦C and
another 3.7% weight loss before 135 ◦C, which are respectively
corresponding to an EtOH molecule (calc. 9.1%) and a water
molecule (calc. 3.5%) per formula unit. 1a and 1b decompose
similarly above 320–330 ◦C. (Fig. S9†). 2a loses all its guest
molecules below 130 ◦C, corresponding to 1.5 EtOH and 1 water
molecules (15.0%, calc. 14.6%) per formula unit. A step in weight
loss before 190 ◦C is observed for 2b, which is ascribed to the
release of 1 DMA and 1.5 water molecules per formula unit (14.0%,
calc. 13.7%). 2a and 2b also decompose similarly around 340–350
◦C (Scheme 2).
The guest-free phases of 1a and 1b show new and similar PXRD
patterns, confirming they have the same flexible frameworks, i.e., 1
(Fig. S10†). When 1 adsorbs DMF or EtOH, 1a or 1b is recovered,
respectively, whatever sample 1 was obtained. Analogously, 2a and
2b could be also transformed to the same activated phase of 2
after complete removal of their guests, but the crystallinity of 2
obtained from 2b was poor (Fig. S11†). 2a can be recovered from
2 by adsorption of EtOH, but 2b can be hardly recovered even
Reduction of azpy to bphy
The reductions of aromatic azo compounds have been studied ex-
tensively and have focused on their electrochemical behavior by po-
larography, cyclic voltammetry, controlled-potential coulometry,
electron spin resonance spectroscopy, and ultraviolet spectroscopy
during the 1970s,31–35 while few studies involving the reduction of
4,4¢-azopyridine have been reported via other methods.36 Launay’s
group reported the pH induced metal-to-ligand charge-transfer
intramolecular redox reaction in an azpy/bphy system.30 They
also synthesized bphy by reducing azpy with granulated metallic
Zn in ethanol and concentrated ammonia at 80 ◦C under argon.
Williams et al. introduced azpy to synthesize hydrazopyridinium-
containing [2]catenanes, and observed the reduction of azpy to
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Considering the gating sorption behavior of 2 may be utilized
to selectively separate different gases, CO2 and CH4 sorption
isotherms were also measured at 298 K with pressures up to 40
bar (Fig. 3). The CO2 isotherm is similar to that measured at 195
K and an uptake of 160 cm3 g-1 is reached at 40 bar. In contrast,
almost no CH4 can be adsorbed up to 40 bar. Consequently, the
CO2/CH4 adsorption ratio is extremely large, which results from
the fact that the flexible structure can be selectively opened by
only CO2. Common porous materials based on adsorption affinity
only have relatively low selectivities. Finally, PXRD patterns of the
samples after CO2/CH4 sorption experiments matched well with
2, indicating the reversibility of structure transformation during
adsorption and desorption (Fig. S12†).
Scheme 2 Relationships between 1a/1b, 1, 2a/2b and 2.
by exposing 2 to DMA vapor for several days. The reason may
be that the DMA guests in 2b were difficult to remove because
of their large molecular size and high boiling points, making the
framework partially collapse during activation.
Sorption studies
CO2 sorption isotherms of 1 and 2 were measured at 195 K (Fig.
2). The CO2 uptake of 1 increased with increasing pressure and
reach 25.6 cm3 g-1 at P/P0 = 1. The low porosity indicated the
coordination frameworks of 1a/1b were severely deformed into
a relatively condensed phase after guest removal, which could be
hardly opened by CO2. In contrast, 2 exhibited an interesting
stepwise adsorption at 195 K. The adsorption isotherm gradually
reached an uptake of 45.7 cm3 g-1 at P/P0 = 0.1, indicating low
porosity of 2, but being higher than 1. Then, it sharply increased
to reach saturation at about P/P0 = 0.3, indicating that the abrupt
expansion of the framework largely extended the porosity. From
the saturation uptake of 193 cm3 g-1, the pore volume of 2 was
calculated to be 0.34 cm3 g-1, being similar to the calculated result
of 0.35 cm3 g-1 from the single-crystal structure of 2a. The isotherm
of 2 showed a clear hysteresis, where desorption occurred at P/P0
< 0.05, being similar to reported sorption behavior of flexible
frameworks.42
Fig. 3 Adsorption isotherms of CO2 and CH4 for 2 at 298 K and 0–40
bar.
Conclusions
In situ formation of 1,2-bis(4-pyridyl)hydrazine from reduction of
4,4¢-azopyridine under solvothermal conditions has been found
in the syntheses of two new porous coordination polymers crys-
tallized as four crystalline phases. Inclusion of slightly different
solvents distinctly changes the total symmetry of the framework,
indicating flexibility in the frameworks and ligands. Depending
on the topologies, the two types of frameworks exhibited very
different sorption behaviors, one of which showed interesting
gating adsorption of CO2, which can be utilized to efficiently
separate CO2 from CH4.
Acknowledgements
This work was supported by NSFC (20821001 & 90922031),
the “973 Project” (2007CB815302), the Chinese Ministry of
Education (109125 & ROCS), and the Open Fund of the State
Key Laboratory of Optoelectronic Materials and Technolo-
gies.
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