Porous coordination polymer with flexibility imparted by coordinatively changeable lithium ions on the pore surface

Article abstract of DOI:10.1021/ic902077j

Solvothermal reactions of equimolar zinc acetate, lithium acetate, and 1, 3, 5-benzenetricarboxylic acid (H₃btc) in different mixed solvents yielded isostructural three-dimensional frameworks [LiZn(btc)(cG)] ? |G [cG and |G denote coordinated and lattice guests, respectively; cG=(nmp) _0,5(H₂O)_0.55, IG=(EtOH)_0.5 (1a); CG=H₂O, IG = EtOH (1b); nmp=N-methyl-2pyrrolidone] with one-dimensional channels occupied by guest molecules and solvent-coordinated, extrusive Li⁺ ions. Thermogravimetry analyses and powder X-ray diffraction measurements revealed that both 1a and 1b can lose all lattice and coordinated guests to form a desolvated phase [LiZn(btc)] (MCF-27, 1) and almost retains the original framework structure. Gas adsorption measurements on 1 confirmed its permanent porosity but suggested a structural transformation from 1a/1b to 1. It is noteworthy that only 1a can undergo a single-crystal to single-crystal (SCSC) transformation into 1 upon desolvation. The crystal structure of 1 revealed that the Li⁺ ions were retracted into the channel walls via complementary coordination to the carboxylate oxygen atoms in the framework rather than being exposed on the pore surface. Single-crystal X-ray diffraction analyses were also performed for N_2- and CO _2-loaded samples of 1, revealing that the framework remained unchanged when the gases were adsorbed. Although the gas molecules could not be modeled, the residue electrons inside the channels demonstrated that the retracted Li⁺ ions still behave as the primary interacting site for CO₂ molecules. Nevertheless, solvent molecules such as H²O can readily compete with the framework oxygen atom to retrieve the extrusive Li⁺ ions, accompanying the reverse structural transformation, i.e., from 1 to 1a/1b.

Full text of DOI:10.1021/ic902077j

1158 Inorg. Chem. 2010, 49, 1158–1165
DOI: 10.1021/ic902077j
Porous Coordination Polymer with Flexibility Imparted by Coordinatively
Changeable Lithium Ions on the Pore Surface
Lin-Hua Xie, Jian-Bin Lin, Xiao-Min Liu, Yu Wang, Wei-Xiong Zhang, Jie-Peng Zhang,* and Xiao-Ming Chen*
MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic
Materials, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China
Received October 21, 2009
Solvothermal reactions of equimolar zinc acetate, lithium acetate, and 1,3,5-benzenetricarboxylic acid (H3btc) in different
mixed solvents yielded isostructural three-dimensional frameworks [LiZn(btc)(cG)] lG [cG and lG denote coordinated and
3
lattice guests, respectively; cG= (nmp)0.5(H2O)0.5, lG=(EtOH)0.5 (1a); cG=H2O, lG= EtOH (1b); nmp=N-methyl-2-
pyrrolidone] with one-dimensional channels occupied by guest molecules and solvent-coordinated, extrusive Liþ ions.
Thermogravimetry analyses and powder X-ray diffraction measurements revealed that both 1a and 1b can lose all lattice
and coordinated guests to form a desolvated phase [LiZn(btc)] (MCF-27, 1) and almost retains the original framework
structure. Gas adsorption measurements on 1 confirmed its permanent porosity but suggested a structural transformation
from 1a/1b to 1. It is noteworthy that only 1a can undergo a single-crystal to single-crystal (SCSC) transformation into 1
upon desolvation. The crystal structure of 1 revealed that the Liþ ions were retracted into the channel walls via
complementary coordination to the carboxylate oxygen atoms in the framework rather than being exposed on the pore
surface. Single-crystal X-ray diffraction analyses were also performed for N2- and CO2-loaded samples of 1, revealing that
the framework remained unchanged when the gases were adsorbed. Although the gas molecules could not be modeled,
the residue electrons inside the channels demonstrated that the retracted Liþ ions still behave as the primary interacting
site for CO2 molecules. Nevertheless, solvent molecules such as H2O can readily compete with the framework oxygen
atom to retrieve the extrusive Liþ ions, accompanying the reverse structural transformation, i.e., from 1 to 1a/1b.
Introduction
seldom been found in traditional inorganic and carbon-based
porous materials, namely, flexibility, which refers to the fact that
some frameworks are capable of changing their internal struc-
tures as responses to external stimuli, such as temperature, light,
pressure, and guests. It is believed that these kinds of PCPs
would be a new class of advanced materials for selectivity
recognition, increased storage, facile delivery, sensor, and
switch. So far, however, reported dynamic PCPs are still rare,
and the rational design and synthesis of these kinds of environ-
mentally responsive materials remain a challenge.3-12
In the past decade, porous coordination polymers (PCPs)
have been extensively investigated because of their potential in
gas storage, catalysis, separation, and many other fields.1 Vari-
ous combinations of a large number of metal ions and available
organic ligands led to a large family of PCPs. As a new type of
porous material, high internal surface area and stability are two
important factors for PCPs. Up to now, many stable PCPs with
permanent porosity have been successfully isolated, and some
of them exhibit ultrahigh surface areas over 4000 m2/g.2 On the
other hand, there is an important feature for PCPs that has
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mail.sysu.edu.cn (J.-P.Z.), cxm@mail.sysu.edu.cn (X.-M.C.).
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pubs.acs.org/IC
Published on Web 01/05/2010
2010 American Chemical Society

Article
Additionally, many recent works have been devoted to PCP
Inorganic Chemistry, Vol. 49, No. 3, 2010 1159
Scheme 1. Schematic Representation of Structural Transformations
for 1a
systems containing coordinatively unsaturated metal centers
(UMCs) to enhance the interaction between the host frame-
works and adsorbates, especially hydrogen.13 However, it is
difficult to realize and characterize UMCs in these systems
because removal of the coordinated solvent frequently leads to
corruption of the framework or transformation of the metal
coordination geometry that even the framework retains.14 It is
meaningful to determine the crystal structure of the desolvated
phase to confirm the final coordination status of the metal ion.
With consideration of their gravimetric gas storage capacity,
some theoretical studies had been performed on models of
PCPs doped by the lightest metal lithium atoms exposed on
their internal surface, revealing a significant improvement of
the H2 uptake capacity.15 Experimentally, many investigations
have also been carried out to immobilize coordinatively
unsaturated Liþ ions on the pore surface of PCPs.16-20
Eddaoudi and co-workers reported that the extra-framework
cations (Kþ or dimethylammonium) of zeolite-like PCPs were
exchangeable with Liþ as aqua complexes. Nevertheless, it was
found that the removal of aqua ligands of Liþ would result in
degradation of the frameworks.16 Hupp and co-workers
reported that interwoven PCPs doped with Liþ exhibited
enhanced H2 uptake, but detailed investigations revealed that
the enhanced H2 uptake resulted from displacement of inter-
woven frameworks rather than exposed Liþ because more
extensive Liþ doping decreased the H2 loading.17 Hartmann
and co-workers reported that a MIL-53(Al) structural analo-
gue with a ligand containing a pendant hydroxyl group could
be lithium(þ) alkoxide modified. Both the H2 uptake and
isosteric heat of H2 adsorption increased after such a modi-
fication. However, because the framework undergoes a slow
a Sky-blue and yellow spheres represent Liþ ions on the pore surface
of the framework and coordinated guests, respectively.
temperature-induced transformation from the high-tempera-
ture phase to the low-temperature phase (decreased pore
volume) upon cooling to perform the adsorption experiment,
it was believed that the observed increase of H2 uptake was
not caused exclusively by the Liþ doping, and separation of
the pure effect of Liþ doping from the effect of the phase
transformation remained to be studied.18 Champness’s and
€
Schroder’s groups showed that dimethylammonium cations in
an indium-based PCP could be exchanged with Liþ ions. After
exchange, the H2 uptake was enhanced. However, it was
believed that Liþ ions in the PCP were not accessible to H2
molecules because a lower isosteric heat of H2 adsorption was
found for the exchanged material, even though complete
desolvation of the Liþ ions was confirmed.19a Recently, they
reported that piperazinium dications in an indium-based PCP
could be exchanged with Liþ ions. After activation of the
exchanged phase, an enhanced H2 adsorption capacity
coupled to an increased isosteric heat of H2 adsorption was
observed. According to these findings, the Liþ ions were
supposed to be partially exposed after removal of the coordi-
nated guest, although the precise structure of the desolvated
phase has not been determined.19b All of these reports reveal
that the coordination status of the Liþ ions in the activated
framework is difficult to identify but significant for elucidation
of the relationship between their structures and sorption
performances. Actually, there is no report dealing with the
crystal structural characterization of coordinatively unsatu-
rated Liþ ions in PCPs.
(10) Kondo, A.; Noguchi, H.; Ohnishi, S.; Kajiro, H.; Tohdoh, A.;
Hattori, Y.; Xu, W. C.; Tanaka, H.; Kanoh, H.; Kaneko, K. Nano Lett.
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H. C. J. Am. Chem. Soc. 2006, 128, 11734–11735. (c) Forster, P. M.; Eckert, J.;
Heiken, B. D.; Parise, J. B.; Yoon, J. W.; Jhung, S. H.; Chang, J. S.; Cheetham, A.
K. J. Am. Chem. Soc. 2006, 128, 16846–16850. (d) Dinca, M.; Dailly, A.; Liu, Y.;
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Chem. Soc. 2009, 131, 4995–5000.
In this contribution, we present a new, porous, heterome-
tallic carboxylate framework [LiZn(btc)] (MCF-27, 1; H3-
btc=1,3,5-benzenetricarboxylic acid), which can be obtained
by thermal activation of its as-synthesized, solvated forms
(14) (a) Chen, C. L.; Goforth, A. M.; Smith, M. D.; Su, C. Y.; zur Loye,
H. C. Angew. Chem., Int. Ed. 2005, 44, 6673–6677. (b) Suh, M. P.; Cheon, Y. E.;
Lee, E. Y. Chem.;Eur. J. 2007, 13, 4208–4215. (c) Sakamoto, H.; Matsuda, R.;
Bureekaew, S.; Tanaka, D.; Kitagawa, S. Chem.;Eur. J. 2009, 15, 4985–4989.
(15) (a) Han, S. S.; Goddard, W. A. J. Am. Chem. Soc. 2007, 129, 8422–
8423. (b) Blomqvist, A.; Araujo, C. M.; Srepusharawoot, P.; Ahuja, R. Proc. Natl.
Acad. Sci. U.S.A. 2007, 104, 20173–20176. (c) Klontzas, E.; Mavrandonakis, A.;
Tylianakis, E.; Froudakis, G. E. Nano Lett. 2008, 8, 1572–1576.
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J. F.; Eddaoudi, M. J. Am. Chem. Soc. 2008, 130, 3768–3770. (b) Nouar, F.;
Eckert, J.; Eubank, J. F.; Forster, P.; Eddaoudi, M. J. Am. Chem. Soc. 2009, 131,
2864–2870.
(17) (a) Mulfort, K. L.; Hupp, J. T. J. Am. Chem. Soc. 2007, 129, 9604–
9605. (b) Mulfort, K. L.; Hupp, J. T. Inorg. Chem. 2008, 47, 7936–7938. (c)
Mulfort, K. L.; Wilson, T. M.; Wasielewski, M. R.; Hupp, J. T. Langmuir 2009,
25, 503–508.
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(19) (a) Yang, S. H.; Lin, X.; Blake, A. J.; Thomas, K. M.; Hubberstey, P.;
[LiZn(btc)(cG)] lG [cG and lG denote coordinated guests
3
and lattice guests, respectively; cG = (nmp)0.5(H2O)0.5
,
lG=(EtOH)0.5 (1a); cG=H2O, lG=EtOH (1b); nmp=N-
methyl-2-pyrrolidone] with solvent-bound Liþ ions extru-
sive into the pores as potentially useful UMCs. In addition to
thermogravimetry (TG) analyses, powder X-ray diffraction
(PXRD) measurements, and gas sorption measurements,
single-crystal structure determinations for guest-free and gas-
loaded 1 were also conducted to investigate the coordination
status of the Liþ ions and flexibility of the framework. It is
revealed that dramatic environmental changes of the Liþ ion
take place when all of the guests are removed, and after the
transformation, the Liþ ions distinctively respond to the
stimulations of different guests (Scheme 1).
€
Champness, N. R.; Schroder, M. Chem. Commun. 2008, 6108–6110. (b) Yang,
Experimental Section
S. H.; Lin, X.; Blake, A. J.; Walker, G. S.; Hubberstey, P.; Champness, N. R.;
Materials and General Methods. Solvents and reagents
were obtained from commercial sources and used as received.
€
Schroder, M. Nat. Chem. 2009, 1, 487–493.
(20) Dinca, M.; Long, J. R. J. Am. Chem. Soc. 2007, 129, 11172–11176.

1160 Inorganic Chemistry, Vol. 49, No. 3, 2010
Xie et al.
Table 1. Crystal Data and Structure Refinements for 1a, 1b, 1, and N2- and CO2-Loaded 1
1a
1b
1
N2-loaded 1
CO2-loaded 1
empirical formula
M
LiZnC12.5H11.5N0.5O7.5
361.03
103
tetragonal
P41212
16.592(2)
10.856(3)
2988.5(10)
8
1.605
1.678
LiZnC11H11O8
343.51
293
tetragonal
P41212
16.6332(9)
10.7833(11)
2983.3(4)
8
1.530
1.677
LiZnC9H3O6
279.42
293
tetragonal
P41212
16.3463(10)
11.2519(13)
3006.5(4)
8
1.235
1.640
LiZnC9H3O6
279.42
103
tetragonal
P41212
16.3514(6)
11.2764(8)
3015.0(3)
8
1.235
1.640
LiZnC9H3O6
279.42
195
tetragonal
P41212
16.3136(1)
11.3049(2)
3008.61(6)
8
1.235
1.640
temperature (K)
cryst syst
space group
˚
a (A)
˚
c (A)
3
V (A )
˚
Z
Dcalc (g/cm3)
μ (mm-1
)
reflns collected
indep reflns
14 311
3053
9855
3057
13 561
3074
12 084
3038
11 190
2381
θ range (deg)
GOF
2.24-26.37
1.069
0.0483
0.1359
0.03(2)
1.73-26.37
0.994
0.0470
0.1074
0.05(2)
2.49-26.37
1.093
0.0366
0.0971
-0.01(2)
0.667/-0.270
1.76-26.37
1.060
0.0337
0.0805
-0.002(16)
0.385/-0.418
3.83-62.45
1.338
0.0624
0.2129
0.05(8)
1.264/-1.512
R1 [I > 2σ(I)]a
wR2 (all data)b
Flack parameter
3
˚
ΔFmax/min (e/A )
0.736/-0.871
0.549/-0.320
P
P
P
P
a R1 = ||Fo| - |Fc||/ |Fo|. b wR2 = [ w(Fo2 - Fc2)2/ w(Fo2)2]1/2
.
Elemental analyses were performed on a Perkin-Elmer 240
elemental analyzer (C, H, and N). IR spectra were recorded in
the range of 400-4000 cm-1 on a Bruker TENSOR 27 Fourier
transform IR (FT-IR) spectrophotometer using KBr pellets. TG
analyses were performed on a Netzsch TG 209 instrument in
flowing N2 with a heating rate of 10 °C/min. PXRD measure-
ments were performed on a Bruker D8 ADVANCE X-ray
diffractometer with Cu KR radiation. For variable-temperature
PXRD (VTPXRD) measurements, the diffraction patterns for
different temperatures were recorded after the sample had
stayed at the respective temperature for 30 min in a N2 atmo-
sphere. Ar, N2, and H2 sorption measurements were performed
using a Micromeritics ASAP 2020 instrument. CO2 sorption
measurement was performed with a Belsorp-Max automatic
volumetric adsorption apparatus.
solved by direct methods and refined by a full-matrix least-
squares method on F2 using the SHELXTL crystallographic
software package. All hydrogen atoms were placed geometri-
cally, and anisotropic thermal parameters were used to refine all
non-hydrogen atoms. For N2- and CO2-loaded 1, the gas
molecules cannot be modeled; only the frameworks were re-
fined. For 1a and 1b, disordered guest molecules cannot be
modeled and were treated by the SQUEEZE routine,21 and their
amounts were determined by the TG results and elemental
analyses. The crystal data and structure refinement results for
these compounds are listed in Table 1.
Results and Discussion
Syntheses and Crystal Structures. 1a and 1b were iso-
lated by the solvothermal reaction of equimolar zinc
acetate, lithium acetate, and a H3btc ligand in different
solvents. Mixed solvents were necessary for the syntheses
of both compounds. Before solvothermal reactions, stir-
ring of the heterogeneous solutions for a period of time
was helpful for a high purity of both compounds, which
was confirmed by their PXRD patterns (Figure S2 in the
Supporting Information). It ought to be mentioned that
when the sample of 1b was exposed to the air for hours,
several additional peaks appeared in its PXRD pattern
compared with that of its as-synthesized sample (Figure
S2e in the Supporting Information). Commonly, this kind
of phenomenon can result from the loss of guests. How-
ever, the original PXRD pattern of 1b cannot be recov-
ered by immersion of the exposed sample in the mother
liquor (Figure S2f in the Supporting Information). Thus,
the structural transformation should not be caused by
desolvation. Instead, the new phase possibly resulted
from the interaction between the sample and moisture
in the air.22
Syntheses of [LiZn(btc)(nmp)0.5(H2O)0.5] 0.5EtOH (1a) and
3
[LiZn(btc)(H2O)] EtOH (1b). Zn(CH3COO)2 2H2O (0.132 g,
0.6 mmol), Li(CH3COO) 2H2O (0.062 g, 0.6 mmol), and H3btc
3
3
3
(0.147 g, 0.6 mmol) were added to a mixed solvent of nmp (6 mL)
and anhydrous ethanol (18 mL) in a 40-mL Teflon-lined reactor.
The resultant suspension was stirred for about 1 h, then sealed,
and heated at 160 °C for 4 days. Block-shaped colorless crystals
of 1a were obtained in a yield of 0.176 g (81.2%). Anal. Calcd
(%) for LiZnC12.5H11.5N0.5O7.5: C, 41.58; H, 3.21; N, 1.94.
Found: C, 40.27; H, 3.24; N, 2.10. FT-IR (KBr pellet, cm-1):
3421(m), 3072(w), 2968(w), 2939(w), 2877(w), 2501(w), 1627(s),
1566(s), 1544(s), 1438(s), 1367(s), 1110(m), 1041(w), 941(w),
883(w), 775(m), 725(m), 663(w), 580(w), 474(w).
Compound 1b was synthesized in under conditions similar to
those of 1a except that the solvents were replaced by anhydrous
ethanol (12 mL) and anhydrous methanol (12 mL), and the
reaction temperature was 90 °C. Yield: 0.172 g, 83.5%. Anal.
Calcd (%) for LiZnC11H11O8: C, 38.46; H, 3.23. Found: C,
38.71; H, 3.34. FT-IR (KBr pellet, cm-1): 3409(m), 1620(s),
1566(s), 1440(s), 1371(s), 1107(w), 1110(w), 941(w), 883(w),
819(w), 767(m), 725(m), 547(w), 466(w).
X-ray Crystallography. Single-crystal XRD data were col-
lected on a Bruker Apex CCD area-detector diffractometer with
Mo KR monochromatic radiation or an Oxford Gemini S Ultra
X-ray single-crystal diffractometer with Cu KR monochromatic
radiation. The single crystal of 1 was obtained by heating a
single crystal of 1a in a capillary at 280 °C under high vacuum for
about 3 h and then sealed by fire (Figure S1 in the Supporting
Information). The N2- and CO2-loaded single crystals of 1 were
obtained from 1 in a capillary backfilled with N2 or CO2 and
finally sealed by fire under a liquid-N2 bath. The structures were
Single-crystal XRD reveals one Zn2þ, one Liþ, one
btc3- ligand, half of a water molecule, and half of a nmp
molecule in the asymmetric unit of 1a. The Zn2þ ion is
coordinated by four carboxylate oxygen atoms (O1, O2,
O3, and O6) from four btc3- ligands in a distorted
(21) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13.
(22) Zhang, J.-P.; Ghosh, S. K.; Lin, J.-B.; Kitagawa, S. Inorg. Chem.
2009, 48, 7970–7976.

Article
Inorganic Chemistry, Vol. 49, No. 3, 2010 1161
Figure 1. Coordination environment of 1a. For clarity, all hydrogen
atoms are omitted and only the coordinated oxygen atom (yellow) is
shown for the coordinated solvent molecule. Symmetry codes: a, -0.5 þ
y, 0.5 - x, 0.75 þ z; b, 1 - y, 1 - x, 1.5 - z; c, -0.5 þ y, 0.5 - x, -0.25 þ z;
d, 0.5 - y, 0.5 þ x, -0.75 þ z; e, 1 - y, 1 - x, 0.5 - z; f, 0.5 - y, 0.5 þ x,
0.25 þ z; g, 1 - y, 1 - x, 1.5 - z.
tetrahedral geometry with the Zn-O bond length in the
Figure 2. Perspective view of the coordination framework of 1a along
the c axis. Simplification and color codes follow those of Figure 1.
˚
range of 1.942(3)-2.055(3) A and also weakly coordi-
nated by another carboxylate oxygen atom [Zn1-O4=
the Supporting Information), which should result from
the small distortion of the framework when the desol-
vated phase 1 was produced. Note that the peak inten-
sities of the PXRD patterns became stronger when the
temperature was gradually elevated. In the TG curve of
1b, there is an abrupt weight loss of 13.8%, corresponding
to the release of the ethanol guest molecules (calcd 13.4%)
before 100 °C, followed by a gradual weight loss of 5.1%
until 350 °C, corresponding to the removal of coordinated
water molecules (calcd 5.2%; Figure S6 in the Supporting
Information). The VTPXRD patterns show that there is
an apparent structural rearrangement when the ethanol
guest molecules were removed at 60 °C. However, all of
the PXRD patterns above this temperature appear to be
identical with that of 1 only with slight differences in the
intensities of the peaks (Figure S7 in the Supporting
Information). This fact implies that the cell parameters
of the sample were basically maintained when the coor-
dinated water molecules were gradually removed at high-
er temperatures because the TG results indicated the loss
of coordinated water molecules at relatively high tem-
perature above 100 °C.
Sorption Studies. To evaluate the porosity of 1, gas
sorption studies were conducted. The sample of guest-free
1 was obtained by heating 1a at 280 °C for 8 h under a high
vacuum. Ar (87 K), N2 (77 K), and CO2 (195 K) sorption
measurements for 1 gave type I isotherms typical for
microporous materials, and the uptakes around 1 atm
are 271, 211, and 174 cm3/g, respectively (Figure 3). The
apparent Brunauer-Emmett-Teller and Langmuir sur-
face areas of 1 calculated from the Ar adsorption iso-
therm are 725 and 962 m2/g, respectively. Assuming that
the gases are micropore filling in the channels of desol-
vated 1a as the liquid (densities for liquid Ar, N2, and
CO2: 1.40, 0.808, and 1.101 g/cm3), the theoretically
saturated uptakes of the three types of gases can be
calculated as 255, 210, and 182 cm3/g, respectively. Com-
monly, the measured uptakes should be less than the
calculated ones, especially for guest molecules with large
size and polarity (dipole, quadrupole, etc.) because of
the preferred orientation of the guest molecules and
space incommensurateness. In other words, Ar sorption
þ
˚
2.424(4) A; Figure 1]. The Li ion is coordinated by three
carboxylate oxygen atoms (O1, O4, and O5) from three
btc3- ligands and one oxygen atom (O7) from the aqua or
˚
nmp ligand [Li-O=1.864(12)-1.990(10) A], which are
statistically disordered at the same position with half-
occupancy. The coordination geometry of Liþ is a slightly
distorted tetrahedron [O-Li-O = 101.3(5)-115.2(6)°].
The btc3- ligand bridges four Zn2þ ions and three Liþ
ions by its six carboxylate oxygen atoms. Along the c axis,
an alternate connection of Zn2þ and Liþ ions by btc3-
carboxylates results in a 41 helical chain (Figure S3 in
the Supporting Information). Such chains are intercon-
nected with four neighboring equivalent chains via btc3-
ligands, giving rise to a three-dimensional (3D) structure
(Figure 2). There are helical one-dimensional (1D) chan-
˚
nels with diameters ranging from 5.8 to 6.3 A running
along the c axis, which exhibit 40.3% volume of the
crystal.21 These channels are occupied by the aqua and
nmp solvents as well as disordered guest ethanol mole-
cules. It is noteworthy that the Liþ ions are extrusive into
the channels, which suggests that they can potentially
serve as UMCs after removal of the guests and coordi-
nated molecules. 1b is isostructural to 1a except that all of
the coordinated solvents in 1b are water molecules. Note
that both 1a and 1b are spontaneously resolved racemic
conglomerates and crystallize in the chiral space group of
P41212 (No. 92) or P43212 (No. 96).
Thermal Behaviors. To evaluate the thermal behaviors
of 1a and 1b, TG analyses and VTPXRD measurements
were conducted. For 1a, the TG curve shows two steps
of weight loss below 350 °C, followed by a plateau until
430 °C when the compound became to decompose (Figure
S4 in the Supporting Information). The first weight loss
between room temperature and 120 °C was 6.5%, corre-
sponding to the departure of ethanol guests (calcd 6.4%).
The 16.7% weight loss between 120 and 350 °C corre-
sponds to the release of the coordinated water and nmp
molecules (calcd 16.2%). The VTPXRD patterns for 1a
were collected from 30 to 380 °C under a N2 atmosphere.
Some peaks of its diffraction pattern were slightly shifted
when the sample was heated at 140-220 °C (Figure S5 in

1162 Inorganic Chemistry, Vol. 49, No. 3, 2010
Xie et al.
Information). This value of the isosteric heat of adsorp-
tion is comparable to that of MOF-5 (5.2 kJ/mol) but
apparently lower than that of some PCPs containing
UMCs.23
Single-Crystal to Single-Crystal (SCSC) Transforma-
tion. To interpret the results of adsorption experiments, it
is necessary to obtain the precise structure of 1. Inspired
by its VTPXRD patterns, we attempted to perform SCSC
transformation experiments for single crystals of 1a. It
was found that the single crystallinity of the crystals of 1a
was retained after being heated at 280 °C for 3 h under a
high vacuum to remove the uncoordinated and coordi-
nated solvents. In contrast, the SCSC transformation
experiment from 1b to 1 was unsuccessful. Single crystals
of 1b easily crack after heating. Its structural rearrange-
ment induced by the loss of ethanol guests may be
responsible for the friable character, whereas the frame-
work of 1a is unchanged when its ethanol guests are
removed, as demonstrated by the VTPXRD patterns.
Single-crystal XRD of 1 revealed that the crystal retains
the tetragonal crystal system and the original space
group, with the unit cell parameters being slightly chan-
ged. The a and b axes are shortened from 16.592(2) to
Figure 3. Adsorption isothermsofAr (black), N2 (red), and CO2 (green)
for 1 measured at 87, 77, and 195 K, respectively.
˚
16.3463(10) A, while the c axis is lengthened from
10.856(3) to 11.2519(13) A. There are one Zn2þ, one
˚
Liþ, and one btc3- ligand in the asymmetric unit of 1.
The Zn2þ ion is also coordinated by four carboxylate
oxygen atoms [O1, O2, O3, and O5; Zn-O = 1.901-
˚
(2)-2.040(2) A] in a distorted tetrahedral geometry and
weakly coordinated bþy another oxygen O6 atom [2.669(3)
˚
A]. However, the Li ion is now coordinated by four
carboxylate oxygen atoms [O1, O4, O5, and O6; 1.855-
˚
3-
(6)-2.025(7) A] from four btc ligands in a more dis-
torted tetrahedral geometry [O-Li-O = 83.9(2)-
137.8(4)°; Figure S8 in the Supporting Information].
Compared with the structure of 1a, it is not difficult to
rationalize the structural transformation process. When
the coordinated guest (O7) is removed, Li1 and O6
approach each other to form a new coordination bond.
Meanwhile, O4, O5, and O6 either approach or depart
from Zn1, resulting in the formation or cleavage of
coordination bonds. O1, O2, and O3 are also slightly
shifted to adapt the new coordination geometry of the
metal atoms (Figure 5). In short, the Liþ ion is shifted into
a position of lower potential when the coordinated sol-
vent is removed rather than being exposed on the pore
surface (Figure 6), although the framework seems to be
almost unchanged during the transformation according
to the PXRD patterns. This rationalization of the struc-
tural change in the SCSC transformation from 1a to 1 is in
accordance with our previous observation of a “carboxy-
late shift” around metal centers in the SCSC transforma-
tion of dehydration of a 3D chiral cadmium(II) dicarboxy-
late coordination polymer.24 These results reveal that it is
more appropriate to regard the Liþ ion as coordinatively
changeable rather than coordinatively unsaturated. To
our best knowledge, this is the first time that the removal
of a coordinated solvent on a Liþ ion is structurally
characterized, which is fundamental for the investigations
on PCPs with Li doping strategies.
Figure 4. H2 adsorption isotherms for 1 recorded at 77 K (blue) and 87
K (red).
measurement should generally give the most reliable pore
characteristic among the selected gases used in this study.
One may notice that the measured uptakes are very close
to the predicted ones. Especially, the measured Ar uptake
is obviously higher than the predicted value. The exces-
sive adsorption of 1 compared to the predicted value
implies that the coordination framework of 1a is slightly
expanded after the removal of all coordinated solvents
and guests.
H2 adsorption experiments were also conducted for 1 at
77 and 87 K, which show 163 and 120 cm3/g uptakes
around 1 atm, respectively (Figure 4). By fitting of the
Langmuir-Freundlich equation to the isotherms, the
saturated H2 uptake for 1 is estimated to be 235 cm3/g.
Virial analysis23 of the H2 adsorption isotherms revealed
that the isosteric heat of adsorption of 1 at zero surface
coverage is 5.9 kJ/mol and slightly decreases as the
H2 uptake increases (Figure S12 in the Supporting
(23) Chen, B.; Zhao, X.; Putkham, A.; Hong, K.; Lobkovsky, E. B.;
Hurtado, E. J.; Fletcher, A. J.; Thomas, K. M. J. Am. Chem. Soc. 2008, 130,
6411-6423 and references cited therein.
(24) Xue, D.-X.; Zhang, W.-X.; Chen, X.-M.; Wang, H.-Z. Chem.
Commun. 2008, 1551–1553.

Article
Inorganic Chemistry, Vol. 49, No. 3, 2010 1163
174 cm3/g (84%). These data imply that the adsorbed
N2 and CO2 molecules are not ideally in their liquid status
like the adsorbed inert Ar molecules are. The small Ar
molecules are spherical and without a quadrupole, thus
more effectively occupying the pores. In contrast, the
adsorbed N2 and CO2 molecules may interact with the
pore walls and occupy the pores with the preferred
orientations, resulting in some wasted space. The calcu-
lated value of the micropore filling of liquid H2 (density:
0.0708 g/cm3) for 1 is 291 cm3/g, also higher than the
calculated saturated amount of 235 cm3/g (81%) ob-
tained by fitting of the isotherms with the Langmuir-
Freundlich equation. Additionally, according to the
structure of 1, the Liþ ions are four-coordinated with
carboxylate oxygen atoms rather than exposed, providing
the reason why the isosteric heat of adsorption for its H2
uptake is relatively low.
Host-Guest Studies. Structural determination of the
gas-loaded PCPs would be pivotal for the understanding
of the adsorption mechanism by providing structural
details of the guest inclusion phases. Such information
at least illustrates whether the host framework is trans-
formed or not after gas adsorption. Therefore, we also
determined the structures of N2- and CO2-loaded single
crystals of 1 at 103 and 195 K, respectively. Though the
adsorbed gases cannot be modeled, it is unambiguous
that the framework remained unchanged. The refinement
for the diffraction data of N2-loaded 1 revealed no
significant peaks of electron density in the channels.
However, there is an apparent decrease of the R factors
for structural refinement after the SQUEEZE treatment
(Table S1 in the Supporting Information). This fact
suggests that the adsorbed N2 molecules are highly dis-
ordered because of their very weak interaction with the
framework. For CO2-loaded 1, there are many pro-
nounced electron density peaks presented in the channels.
The 3D electron density map (Fo - Fc) shows that the
electron contribution of the adsorbed CO2 molecules is
primarily localized along the channel walls (Figure 7).
The maxþimum of the electron density was in the vicinity
Figure 5. Coordination environments of the metal atoms in 1a (a) and 1
(b). Simplification and color codes follow those in Figure 1. Symmetry
codes: a, -0.5 þ y, 0.5 - x, 0.75 þ z; b, 1 - y, 1 - x, 1.5- z; c, x, y, 1 þ z; d,
0.5 - y, 0.5 þ x, 0.25 þ z; e, x, y, 1 þ z.
˚
of the Li ion with a distance of 3.41 A (Figure 8). This
fact implies that the Liþ ions still behave as the primary
interacting sites for the adsorbed CO2 molecules,
although they only adopt distorted tetrahedral coordina-
tion geometries and were almost unexposed in 1. This
result is reminiscent of the situation of MOF-5 containing
Zn2þ ions in similar coordination environments, in which
the faces of ZnO4 tetrahedra behave as the primary
adsorption sites.25
Figure 6. Perspective view of the framework of 1 along the c axis.
Simplification and color codes follow those of Figure 1.
According to the SCSC studies, the structure of 1 may
be regarded as the accurate structure of the activated
sample of 1a for the gas adsorption experiments. The
channels of 1 along the c axis are close to being linear in
spite of their helical nature (Figure S8 in the Supporting
Information). The diameters of the channel sections are in
Although the interactions between the Liþ ions and the
adsorbed gases are weak in 1, the Liþ ions may strongly
interact with solvent molecules capable of coordination,
such as water molecules. This deduction is supported by
the fact that single crystallinity of 1 is immediately lost
upon exposure to a moist atmosphere. The PXRD pat-
tern of hydrated 1 reveals a slight structural rearrange-
ment after hydration (Figure 9). The PXRD pattern of
hydrated 1 matches well with that of the exposed sample
of 1b (Figure 9), which suggests that the framework of 1
was changed to that of 1b, concomitant with a small
˚
the range of 7.1-7.3 A; the volume for the channels
corresponds to 45.3% of the structure;21 the calculated
density for the porous framework is 1.235 g/cm3. With
these results, calculated saturated gas adsorption
amounts of 1 can be obtained. For Ar, this amount is
288 cm3/g, close to the obtained value 271 cm3/g (94%).
The calculated values for N2 and CO2 are 237 and
206 cm3/g, respectively, slightly higher than the experi-
mentally obtained values of 211 cm3/g (89%) and
(25) Rowsell, J. L. C.; Spencer, E. C.; Eckert, J.; Howard, J. A. K.; Yaghi,
O. M. Science 2005, 309, 1350–1354.

1164 Inorganic Chemistry, Vol. 49, No. 3, 2010
Xie et al.
Figure 9. PXRD patterns for (a) an as-prepared sample of 1, (b) a
sample of 1 after exposure to air for minutes, and (c) a sample of 1b after
exposure to air for hours.
Figure 7. Perspective view of the 3D electron density map (Fo - Fc) in
the unit cell of CO2-loaded 1 along the c axis. Color codes for the contours
metal ion can be categorized as a kind of complementary
coordination action, which has been scarcely observed in
the literature.14c However, the occurrence of this type of
behavior is dependent on the nature of the guests. As
mentioned above, adsorption of gas did not lead to any
significant change of the coordination state of the Liþ ion.
Therefore, it may be concluded that there is a competition
between the guest and framework carboxylate oxygen
atoms in coordination to the Liþ ions.
3
(e/A ): blue, 0.6; yellow, 0.9; red, 1.2. Color codes for the atoms follow
˚
those of Figure 1.
Conclusion
We have described a porous 3D heterometallic carboxylate
framework [LiZn(btc)], which represents a unique prototype
for the study of the framework flexibility, coordinatively
changeable Liþ ions, and sorption property. At first glance,
the material appears as a new PCP incorporated with
coordinatively unsaturated Liþ ions. The as-synthesized,
solvated material [LiZn(btc)(cG)] lG contains solvent-coor-
3
dinated, extrusive Liþ ions on the pore surface, and the
framework retention after removal of the guest and coordi-
nated solvent molecules was also confirmed by the TG and
PXRD studies. However, such a rigid structure elucidation,
which was commonly used in other similar cases, cannot
satisfactorily explain the gas sorption data of the activated
material [LiZn(btc)]. The measured H2 adsorption enthalpy
for the activated material [LiZn(btc)] is also lower than
expected.
Figure 8. Section ofthe 3Delectron densitymap (Fo - Fc) in the unit cell
of CO2-loaded 1 passing through a Liþ ion. The contours are plotted with
3
an interval of 0.1 e/A .
˚
The single-crystal structures of the activated materials in
guest-free and gas-loaded forms show that the Liþ ions on the
pore surface are actually retracted into the channel walls by
complementary coordination to the framework carboxylate
oxygen atoms rather than being exposed on the pore surface.
The framework is not altered after N2 or CO2 adsorption.
Although the binding strength is weak, the unexposed Liþ
ions still behave as the primary interacting sites of the porous
framework, which can be revealed by the electron density
distribution of the CO2-adsorbed single crystal. For much
strongly coordinative guests such as H2O, the retracted Liþ
ions can readily extrude back to coordinate with the guest.
The direct structural information for the desolvated Liþ
ions would provide useful implication for the understanding
amount of an unknown phase after exposure to a moist
atmosphere. This finding also justifies our previous spec-
ulation that the structural transformation of 1b after
exposure to air resulted from the interaction between
the sample and moisture. In other words, when stimu-
lated by moisture, the Liþ ions of 1 extrude back to
coordinate water molecules. When coordinated solvents
are removed, the Liþ ions of 1a are coordinated to the
carboxylate oxygen atoms of the framework to compen-
sate for their unstable coordination state; when being
stimulated by moisture, the Liþ ions of 1 are coordinated
to water molecules and recover its previous coordination
state (Figure 5). This type of coordination behavior for a

Article
Inorganic Chemistry, Vol. 49, No. 3, 2010 1165
of Li-doping PCPs, where the coordination environments of
the doped Liþ ions on the pore surface are fundamental to the
sorption mechanism. It is well-known that coordinatively
unsaturated Liþ ions are very energetically unstable as
compared to other transition-metal ions such as Cu2þ. In
contrast to zeolites, PCPs are usually not rigid enough to
sustain the very unstable, coordinatively unsaturated Liþ
ions. Structural transformations must occur to lower the
system energies after the introduction of coordinatively
unsaturated Liþ ions. Frequently, structural transformations
result in the collapse of the PCPs. Even occasionally when
PCPs are able to maintain their crystallinity after structural
transformations, the Liþ ions would very likely be trans-
formed to certain unexposed environments. The dramatic
changes in the coordination environments of the Liþ ions
may not cause a large structural variation of the crystal lattice
and, hence, do not lead to critical changes in the PXRD
patterns. Thus, PXRD patterns cannot serve as sufficient
methods to justify the generation of UMCs, especially for Liþ
ions, whereas the unexposed Liþ ions still behave as the
primary interaction sites for guests compared to other places
like organic ligands of the PCPs. This fact is possibly because
of the relatively strong local electric fields around the inor-
ganic moieties, which would have an influence on their gas
(especially for polar molecules) sorption performances.
In summary, this work illustrates the critical importance of
single-crystal structure determination for elucidation of the
sorption properties of flexible PCPs, especially those showing
coordinatively changeable Liþ ions and weak interactions
between the guests and host framework.
Acknowledgment. This work was supported by NSFC
(Grants 20821001 and 20531070), the “973 Project”
(Grant 2007CB815302), and the Chinese Ministry of
Education (Grant 109125).
Supporting Information Available: PXRD, TG curves, addi-
tional structural figures, photograph of a single crystal of 1
sealed in a capillary, detail about the analysis of H2 adsorption
isotherms, and X-ray crystallographic files in CIF format. This
material is available free of charge via the Internet at http://
pubs.acs.org.