Reversible single-crystal-to-single-crystal transformation and highly selective adsorption property of three-dimensional cobalt(II) frameworks

Article abstract of DOI:10.1021/ic101618n

A three-dimensional (3D) coordination polymer, [Co₃(L) ₂(BTEC)(H₂O)₂] · 2H₂O [1, HL = 3,5-di(imidazol-1-yl)benzoic acid, H₄BTEC = 1,2,4,5- benzenetetracarboxylic acid], with tfz-d topology has been hydrothermally synthesized. The framework of 1 has high thermal stability and exhibits single-crystal-to-single-crystal (SCSC) transformations upon removing and rebinding the noncoordinated and coordinated water molecules. X-ray crystallographic analyses revealed that the coordination geometry of Co(II) changes from octahedral to square pyramid upon dehydration, accompanying the appearance of one-dimensional (1D) open channels with dimensions of 2.0 × 2.8 A. The dehydrated form [Co₃(L)₂(BTEC)] (2) exhibits highly selective adsorption of water molecules over N₂,CH ₃OH, and CH₃Ch₂OH, which could be used as sensors for water molecules. Furthermore, the magnetic properties of 1 and 2 were investigated, showing the existence of ferromagnetic interaction between the Co(II) atoms within the trinuclear subunit.

Full text of DOI:10.1021/ic101618n

Inorg. Chem. 2011, 50, 985–991 985
DOI: 10.1021/ic101618n
Reversible Single-Crystal-to-Single-Crystal Transformation and Highly Selective
Adsorption Property of Three-Dimensional Cobalt(II) Frameworks
Zhi Su,† Min Chen,† Taka-aki Okamura,‡ Man-Sheng Chen,† Shui-Sheng Chen,† and Wei-Yin Sun*,†
†Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, School of Chemistry and
Chemical Engineering, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093,
‡
China, and Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka,
Osaka 560-0043, Japan
Received August 9, 2010
A three-dimensional (3D) coordination polymer, [Co3(L)2(BTEC)(H2O)2] 2H2O [1, HL = 3,5-di(imidazol-1-yl)benzoic
3
acid, H4BTEC = 1,2,4,5-benzenetetracarboxylic acid], with tfz-d topology has been hydrothermally synthesized. The
framework of 1 has high thermal stability and exhibits single-crystal-to-single-crystal (SCSC) transformations upon
removing and rebinding the noncoordinated and coordinated water molecules. X-ray crystallographic analyses re-
vealed that the coordination geometry of Co(II) changes from octahedral to square pyramid upon dehydration,
˚
accompanying the appearance of one-dimensional (1D) open channels with dimensions of 2.0 ꢀ 2.8 A. The dehydrated
form [Co3(L)2(BTEC)] (2) exhibits highly selective adsorption of water molecules over N2, CH3OH, and CH3CH2OH,
which could be used as sensors for water molecules. Furthermore, the magnetic properties of 1 and 2 were in-
vestigated, showing the existence of ferromagnetic interaction between the Co(II) atoms within the trinuclear subunit.
Introduction
(SCSC) or single-crystal-to-amorphous transformations,3
while the others with switchable guest molecules are unstable
after the removal of the guest, which leads to the collapse of
the frameworks.4 It has been demonstrated that the SCSC
transformation is highly desirable and opens routes to the
systematic study of gas storage, separation, and solid state
reaction, since it allows direct visualization of how the crystal
structure is changing during the transformation process and
of the location and orientation of guest molecules in the voids,
where single crystallographic analysis, as the most direct
evidence of this transformation, can supply enough structural
information.5 However, the exploration via SCSC transfor-
mations is uncommon so far.
Recently, the design and construction of porous coordina-
tion polymers (PCPs) with large surface area have been
of great interest due to their potential applications in gas
storage, separation, ion exchange, catalysis, sensors, and
magnetism.1 Multicarboxylate ligands were widely used as
bridging linkers in the construction of PCPs due to the high
thermal and chemical stabilities of polycarboxylate-metal
fragments.2 Particularly, some reported flexible porous frame-
works display unique dynamic behaviors in response to ex-
ternal stimuli with reversible changes of their structures and
properties which are called single-crystal-to-single-crystal
*To whom correspondence should be addressed. Fax: þ86-25-83314502.
E-mail: sunwy@nju.edu.cn.
(1) (a) Llewellyn, P. L.; Horcajada, P.; Maurin, G.; Devic, T.; Rosenbach,
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J. Am. Chem. Soc. 2010, 132, 3605. (b) Zeng, M. H.; Wang, Q. X.; Tan, Y. X.;
Hu, S.; Zhao, H. X.; Long, L. S.; Kurmoo, M. J. Am. Chem. Soc. 2010, 132,
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r
2011 American Chemical Society
Published on Web 01/10/2011
pubs.acs.org/IC

986 Inorganic Chemistry, Vol. 50, No. 3, 2011
Su et al.
During our efforts toward the rationaldesign and synthesis
of PCPs with mixed imidazoles containing ligands and car-
boxylic acid,6 we have prepared a new Co(II) complex, [Co3-
Table 1. Crystal Data and Structure Refinements for Complexes 1 and 2
1
2
empirical formula
fw
temp/K
C36H28Co3N8O16
1005.45
293
C36H20Co3N8O12
933.39
433
(L)2(BTEC)(H2O)2] 2H2O [1, HL=3,5-di(imidazol-1-yl)-
3
benzoic acid, H4BTEC=1,2,4,5-benzenetetracarboxylic acid],
through the hydrothermal method: the complex has a three-
dimensional (3D) host framework with the inclusion of water
molecules as the guests. Remarkably, the reversible de/
rehydrated SCSC transformation was found in the heating
and after standing in the air. Interestingly, [Co3(L)2(BTEC)]
(2) exhibits highly selective adsorption of the water molecules
over N2, CH3OH, and CH3CH2OH, which can be used as
sensors for water molecules. Furthermore, the magnetic pro-
perties of 1 and 2 were investigated.
cryst syst
space group
triclinic
P1
triclinic
P1
˚
a/A
8.550(4)
10.077(3)
11.255(4)
77.672(13)
73.974(17)
88.701(17)
909.8(6)
1
8.683(6)
9.849(6)
10.927(7)
71.60(2)
73.81(3)
86.83(3)
851.0(10)
1
˚
b/A
˚
c/A
R/deg
β/deg
γ/deg
3
˚
V (A )
Z
Dc (g cm-3
F(000)
)
1.835
509
1.821
469
Experimental Section
θ range/deg
3.1 - 27.5
8656
4138
1.082
0.0312
0.0813
3.1 - 27.5
8033
3875
1.102
0.0382
0.1015
reflns collected
independent reflns
goodness of fit
R1a (I > 2σ (I))
wR2b (I > 2σ (I))
All commercially available chemicals are of reagent grade
and were used as received without further purification. The
ligandHLwaspreparedaccordingtotheproceduresreported
previously.7 Elemental analyses of C, H, and N were per-
formed on a Perkin-Elmer 240C elemental analyzer at the
analysis center of Nanjing University. Infrared spectra (IR)
were recorded on a Bruker Vector22 FT-IR spectrophoto-
meter using KBr pellets. Thermogravimetric analyses
(TGA) were performed on a simultaneous SDT 2960 thermal
P
P
P
P
a R1 = ||Fo| - |Fc||/ |Fo|. b wR2 = | Σw(|Fo|2 - |Fc|2)|/ |w(Fo)2|1/2
,
where w = 1/[σ2(Fo ) þ (aP)2 þ bP]. P = (Fo þ 2Fc2)/3.
2
2
1234(w), 1073(s), 935(w), 908(w), 843(m), 787(w), 738(m), 656(m),
522(w).
analyzer under nitrogen at a heating rate of 10 ꢀC min-1
VariabletemperaturepowderX-raydiffraction(PXRD) ana-
lyses were performed on a Shimadzu XRD-6000 X-ray dif-
.
Rehydration of 2. Standing the crystal sample of 2 in the air for
several hours gave rise to the rehydrated complex 10.
Single Crystal X-Ray Structural Determinations. The crystal-
lographic data for 1 and 2 were collected on a Rigaku RAXIS-
RAPID imaging plate diffractometer at 293 and 433 K, respec-
tively, with graphite-monochromated Mo KR radiation (λ =
˚
fractometer with Cu KR (λ = 1.5418 A) radiation. Solid state
UV-visible spectra were measured in diffuse reflectance
mode on a Shimadzu UV3600 spectrophotometer coupled
to a MPC-3100 unit equipped with an integrating sphere
coated with BaSO4. The magnetic measurements in the
temperaturerangeof1.8to300KwerecarriedoutonaQuan-
tum Design MPMS7 SQUID magnetometer in a field of 2000
Oe. Diamagnetic corrections were made with Pascal’s con-
stants for all samples. Nitrogen (N2), methanol (CH3OH),
ethanol (CH3CH2OH), and H2O sorption experiments were
carried out on a Belsorp-max volumetric gas sorption instru-
ment. The sample was dried by using the “outgas” function
of the surface area analyzer for 10 h at 160 ꢀC to remove the
H2O molecules.
˚
0.71075 A). The structures were solved by direct methods with
SIR92 and expanded using Fourier techniques.8,9 All non-
hydrogen atoms were refined anisotropically using the full-
matrix least-squares method on F2. Hydrogen atoms of the li-
gands in 1 and 2 were generated geometrically, and those of
water molecules (O7, O8) in 1 were located directly. All calcula-
tions were carried out on an SGI workstation using the teXsan
crystallographic software package of Molecular Structure
Corporation.10 The crystallographic data collections for 10 were
carried out on a Bruker Smart Apex CCD area-detector dif-
fractometer with graphite-monochromated Mo KR radiation
Preparation of [Co3(L)2(BTEC)(H2O)2] 2H2O (1). A mixture
3
˚
of CoCl2 6H2O (57.4 mg, 0.2 mmol), HL (25.4 mg, 0.1 mmol),
(λ = 0.71073 A) at 293 K using the ω-scan technique. The
3
diffraction data were integrated using the SAINT program,11
which was also used for the intensity corrections for the Lorentz
and polarization effects. Semiempirical absorption correction
was applied using the SADABS program.12 The structure was
solved by direct methods, and all non-hydrogen atoms were
refined anisotropically on F2 with the full-matrix least-squares
technique using the SHELXL-97 crystallographic software
package.13 All of the hydrogen atoms of ligands were generated
geometrically, those of water molecule (O8) were located
directly, and those of O7 were not found. All calculations
were performed on a personal computer with the SHELXL-97
H4BTEC (25.4 mg, 0.1 mmol), NaOH (20.0 mg, 0.5 mmol), and
H2O (10 mL) was sealed in a 16 mL Teflon lined stainless steel
container and heated at 210 ꢀC for 3 d. Complex 1 was isolated in
red block crystalline form by filtration and washed by water and
ethanol several times with a yield of 63%. Anal. Calcd for
C36H28Co3N8O16 (%): C, 43.00; H, 2.81; N, 11.14. Found: C,
43.11; H, 2.75; N, 11.10. IR (KBr, cm-1): 3448(s, br), 1651(s),
1616(m), 1582(s), 1514(s), 1478(m), 1418(s), 1376(m), 1323(s),
1234(w), 1073(s), 935(w), 905(w), 843(m), 787(w), 738(m),
656(m), 522(w).
Preparation of [Co3(L)2(BTEC)] (2). Complex 2 was obtained
by heating the crystal sample of 1 at 160 ꢀC for 2 h. Anal.
Calcd for C36H20Co3N8O12 (%): C, 46.32; H, 2.16; N; 12.00.
Found: C, 46.30; H, 2.13; N; 12.08. IR (KBr, cm-1): 1651(s),
1617(m), 1581(s), 1513(s), 1478(m), 1417(s), 1375(m), 1324(s),
(8) SIR92: Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.;
Giacovazzo, C.; Guagliardi, A.; Polidori, G. J. Appl. Crystallogr. 1994, 27,
435.
(9) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.;
Gelder, R. de; Israel, R. Smits, J. M. M. DIRDIF94: The DIRDIF-94
Program System, Technical Report of the Crystallography Laboratory,
University of Nijmegen, The Netherlands, 1994.
(6) (a) Su, Z.; Cai, K.; Fan, J.; Chen, S. S.; Chen, M. S.; Sun, W. Y.
CrystEngComm 2010, 12, 100. (b) Su, Z.; Bai, Z. S.; Xu, J.; Okamura, T.-a.; Liu,
G. X.; Chu, Q.; Wang, X. F.; Sun, W. Y.; Ueyama, N. CrystEngComm 2009, 11,
873. (c) Su, Z.; Fan, J.; Okamura, T.-a.; Chen, M. S.; Chen, S. S.; Sun, W. Y.;
Ueyama, N. Cryst. Growth Des. 2010, 10, 1911. (d) Su, Z.; Bai, Z. S.; Fan, J.; Xu,
J.; Sun, W. Y. Cryst. Growth Des. 2009, 9, 5190.
(10) teXsan; Molecular Structure Corporation: The Woodlands, TX, 1999.
(11) SAINT, version 6.2; Bruker AXS, Inc.: Madison, WI, 2001.
€
€
(12) Sheldrick, G. M. SADABS; University of Gottingen: Gottingen,
Germany.
(7) Fan, J.; Gan, L.; Kawaguchi, H.; Sun, W. Y.; Yu, K. B.; Tang, W. X.
Chem.;Eur. J. 2003, 9, 3965.
(13) Sheldrick, G. M. SHELXTL, version 6.10; Bruker Analytical X-ray
Systems: Madison, WI, 2001.

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 987
˚
Table 2. Selected Bond Lengths [A] and Bond Angles [deg] for Complexes 1 and 2
which (O7) is from a terminal water molecule, and two
nitrogen atoms (N12, N32C) from two different L- li-
gands as shown in Figure 1a. The average Co2-O bond
1a
˚
length is 2.11 A, and the Co2-N bond distances are
Co1-O1
2.0612(13) Co1-O5#1
2.1111(14)
2.1535(18) Co2-O6#1
2.1208(17) Co2-O1#2
2.1138(17) Co2-N32#3
95.42(5)
87.58(6)
180.00
88.29(6)
84.58(5)
92.02(6)
89.45(6)
81.53(5)
177.84(5)
91.13(6)
166.31(7)
95.42(7)
2.0538(14)
˚
2.1208(17) and 2.1164(17) A (Table 2). It is interesting
Co1-O3#2
Co2-O3
2.0697(14)
2.0959(14)
2.1164(17)
to find that the Co1 links two Co2 atoms by sharing two
edges of a coordination octahedron to form a Co3(OCO)6
linear trinuclear secondary building unit (SBU) with a
Co2-N12
Co2-O7
O1-Co1-O3
O1-Co1-O5#1
O1-Co1-O1#2
O3-Co1-O5#1
O1#2-Co1-O3
O3-Co2-N12
O3-Co2-O6#1
O1#2-Co2-O3
O3-Co2-N32#3
O6#1-Co2-O7
O1#2-Co2-O7
O7-Co2-N32#3
O3-Co1-O3#2
180.00
˚
Co1 Co2 distance of 3.08 A (Scheme 1).
O5#1-Co1-O5#4 180.00
3 3 3
On the other hand, each L- ligand links three SBUs
using its two imidazole groups and a carboxylate in the
μ2-η1:η1-bidentate bridging mode to form an infinite two-
dimensional (2D) network (Figure S1, Supporting In-
formation). The 2D layer can be simplified to a (3,6)-
connected kgd net with SBU-SBU distances of 12.11,
O1#2-Co1-O5#1
O3#2-Co1-O5#1
92.42(6)
91.71(6)
O6#1-Co2-N12
O1#2-Co2-N12
N12-Co2-N32#3
O1#2-Co2-O6#1
O3-Co2-O7
176.11(6)
89.46(6)
89.73(7)
94.32(5)
85.98(7)
85.38(7)
-
˚
13.36, and 16.64 A, respectively, if the SBU and L ligand
O7-Co2-N12
are considered a six-connected node and three-connector,
respectively (Scheme S1 and Figure S1, Supporting In-
formation).14 Each BTEC4- ligand connects two SBUs
using its four μ2-η2:η0-bridging carboxylate groups to
form an infinite one-dimensional (1D) chain with a SBU-
O6#1-Co2-N32#3 88.88(7)
O1#2-Co2-N32#3 97.24(6)
2a
Co1-O1
2.0401(18) Co1-O5#1
2.0710(19)
2.1535(18) Co2-O1#2
2.1220(19)
˚
SBU distance of 8.55 A (Scheme S1, Figure S2, Support-
Co1-O3
ing Information). The 2D networks are further connected
by BTEC4- ligands to result in the formation of a three-
dimensional (3D) framework of 1 (Figure 1b). Therefore,
each SBU is eight-connected by six L-’s in the 2D net-
work and two SBUs via the BTEC4- ligands. The 3D
framework of 1 can be viewed as a binodal (3,8)-connected
Co2-O3
2.076(2)
2.127(2)
Co2-O6#1
1.982(2)
2.043(2)
96.56(7)
89.64(8)
180.00
83.44(7)
93.67(8)
89.90(9)
92.46(9)
80.59(7)
174.72(7)
159.41(9)
Co2-N32#3
Co2-N12
O1-Co1-O3
O1-Co1-O5#1
O1-Co1-O1#2
O1-Co1-O3#2
O3#2-Co1-O5#1
O3-Co2-N12
O3-Co2-O6#1
O1#2-Co2-O3
O3-Co2-N32#3
O6#1-Co2-N12
O3-Co1-O5#1
O3-Co1-O3#2
O5#1-Co1-O5#4
O1#2-Co1-O5#1
86.33(8)
180.00
180.00
90.36(8)
€
net, referred to as the tfz-d net with a Point (Schlafli)
O1#2-Co2-N12
N32#3-Co2-N12
O1#2-Co2-O6#1
O6#1-Co2-N32#3 91.98(9)
O1#2-Co2-N32#3 95.74(8)
99.22(8)
86.94(10)
101.35(8)
symbol of (43)2(46 618 84) (Figure 1c).15
3
3
The most striking feature of complex 1 is the existence
of open channels running along the c axis with effective
˚
dimensions of ca. 2.4 ꢀ 3.4 A. Four water molecules (two
a Symmetric transformations used to generate equivalent atoms for
1-2. #1: 1 þ x, y, -1 þ z. #2: 2 - x, -y, -z. #3: 2 - x, 1 - y, 1 - z. #4:
1 - x, -y, 1 - z.
coordinated and two noncoordinated) per formula unit
are located within the channels (Figure 1d) and are
involved in extensive hydrogen-bonding interactions
with the framework (Table S3, Supporting Information).
PLATON analysis suggests that the channels occupy
crystallographic software package. In addition, the crystallo-
graphic data for 1, 2, and 10 can be collected from the same single
crystal, showing reversible dehydration and rehydration cycles.
Details of the crystal parameters, data collection, and refine-
ments are summarized in Table 1 for 1 and 2 and Table S1 (Sup-
porting Information) for 10, and selected bond lengths and
angles for 1, 2, and 10 are listed in Tables 2 and S2 (Support-
ing Information), respectively. Further details are provided in
the Supporting Information.
3
3 16
˚
˚
6.5% of the total volume [59.2 A /909.8(6) A ].
Thermal Stability and Powder X-ray diffraction (PXRD).
To examine the thermal stabilities of complexes 1 and 2,
thermogravimetric analyses (TGA) and PXRD were
carried out (Figures S3 and S4, Supporting Information).
A two-step weight loss of complex 1 was observed in the
temperature range of 40-200 ꢀC, namely, 3.41% between
40 and 135 ꢀC and 3.73% between 135 and 200 ꢀC, which
is ascribed respectively to the loss of noncoordinated and
coordinated water molecules (total calcd, 7.16%), to give
the dehydrated form 2. The further weight loss in the
range of 330-600 ꢀC was attributed to the decomposition
of the framework to form CoO as a final product (ob-
served, 22.96%; calcd, 21.50%). No obvious weight loss
Results and Discussion
Crystal Structure of Complex 1. Single crystal X-ray
diffraction analysis revealed that complex 1 crystallizes in
the triclinic P1 space group (Table 1). In the asymmetric
unit of 1, there are two different Co(II) atoms (Co1, Co2),
one of which (Co1) is sitting on an inversion center; one
L- and half a molecule of BTEC4-; and one coordinated
and one free water molecule. Each Co1 with slightly
distorted octahedral coordination geometry is six-coor-
dinated by four oxygen atoms (O1, O3, O1A, O3A) from
two distinct BTEC4- ligands and other two (O5B, O5D)
from two different L- ligands (Figure 1a). The average
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Eur. J. Inorg. Chem. 2006, 3041. (c) Xu, G. J.; Zhao, Y. H.; Shao, K. Z.; Lan,
Y. Q.; Wang, X. L.; Su, Z. M.; Yan, L. K. CrystEngComm 2009, 11, 1842. (d)
Garibay, S. J.; Stork, J. R.; Wang, Z. Q.; Cohen, S. M.; Telfer, S. G. Chem.
Commun. 2007, 4881. (e) Feng, Y.; Yang, E. C.; Fu, M.; Zhao, X. J. Z. Anorg.
Allg. Chem. 2010, 636, 253.
˚
bond length of Co1-O is 2.07 A. Each Co2 also with
distorted octahedral coordination geometry is six-coor-
dinated by four oxygen atoms, three of which (O1A, O3,
O6B) are from three distinct BTEC4- ligands and one of
(16) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7.

988 Inorganic Chemistry, Vol. 50, No. 3, 2011
Su et al.
Figure 1. (a) The coordination environment of Co(II) atoms in 1 with the ellipsoids drawn at the 30% probability level. The hydrogen atoms of the ligands
were omitted for clarity. (b) The 3D framework of complex 1. (c) The (3,8)-connected tfz-d net of 1. (d) Space-filling view of the 3D framework of 1 along the
c axis, where the water molecules are represented by the red balls.
Scheme 1. View of the Trinuclear SBU in 1
of 2 was observed at the starting temperature, and the
framework decomposition occurred in the temperature
range of 330-620 ꢀC to give the final CoO product (ob-
served, 23.86%; calcd, 23.15%).
Figure S4 (Supporting Information) shows the PXRD
patterns of the as-synthesized 1 in the range of 25-350 ꢀC.
It is clear that the diffraction profiles below 330 ꢀC are
almost the same, indicating that the framework is stable
under this temperature and the crystal lattice remains
intact after removal of the water molecules. However,
the sample became amorphous when the temperature
was increased to 350 ꢀC, as shown by the absence of

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 989
Figure 3. Reversible single-crystal-to-single-crystal transformations of
dehydration and rehydration between 1 and 2.
Figure 2. (a) The coordination environment of Co(II) atoms in 2 with
the ellipsoids drawn at the 30% probability level. The hydrogen atoms of
ligands were omitted for clarity. (b) Space-filling view of the 3D frame-
work of 2 along c axis.
Figure 4. Solvent vapor adsorption and desorption isotherms of 2 at
298 K, which shows highly selective adsorption of H2O vapor over other
organic solvents.
diffraction peaks in the PXRD pattern. It can be con-
cluded that 1 has excellent thermal stability.
Reversible Single-Crystal-to-Single-Crystal Transfor-
mations upon Dehydration and Rehydration (1-2-10).
The openness of the channels makes it possible to remove
the uncoordinated and coordinated water molecules from
1. When crystals of 1 were heated at 160 ꢀC for 2 h to re-
move the water molecules, the dehydrated product,
[Co3(L)2(BTEC)] (2), was obtained with retention of the
single crystallinity, accompanying a color change from
red to dark-violet (solid state UV-visible spectra of 1 and
2 shown in Figure S5, Supporting Information). Single
crystal X-ray diffraction of 2 confirms that the frame-
work structure and packing mode of 1 are retained and
the space previously occupied by water molecules be-
comes devoid of any appreciable electron density (Figure
2a and b). Although the crystal system and space group
remain unchanged (triclinic, P1), the coordination geo-
metry of Co2 changed from octahedral to square pyra-
mid, where the bond angle of O6#1-Co2-N12 [sym-
metric code: 1þx, y, -1 þ z] decreased from 176.11(6)
to 159.41(9)ꢀ and the bond lengths of Co2-O6#1 and
Co2-N12 decreased from 2.0697(14) and 2.1208(17) to
Furthermore, the dehydrated process can be reversed
by exposing the dehydrated sample to the air or water
vapor or through the immersion of 2 into water at room
temperature for several hours. The crystallographic in-
vestigation confirmed that the framework structure of the
rehydrated sample 10 is the same as that of 1, accompany-
ing a return of the original color (Figure 3). Thus, the Co2
atom provides a coordination site for the water molecule
and returns to the octahedral coordination geometry. In
conclusion, the SCSC transformations between 1-2-10
involve dynamic motions altering the coordination geo-
metry of Co(II) from/to an octahedron to/from a square
pyramid as well as the shrinkage/expansion of pore
deformation.
Sorption Properties. On the basis of the well-defined
porous structure of the dehydrated product 2, the adsorp-
tion isotherms for different solvents (H2O, CH3OH,
CH3CH2OH) at 298 K as well as N2 at 77 K were mea-
sured. The amount of water vapor uptaken increases ab-
ruptly at the beginning and reaches approximately 4.59
H2O molecules per formula unit at 1.0 atm, which pre-
sents a typical type-I curve, indicating there is diffusion
of water molecules into the channels (Figure 4).17 The
observed hysteresis between the adsorption-desorption
˚
1.982(2) and 2.043(2) A, respectively (Figure 2a, Table 2).
The void volume occupied 3.8% of the total crystal vol-
˚
3
ume for 2 [32.7/851.0(10) A ] with dimensions of 2.0 ꢀ
˚
2.8 A for the open channels along the c axis, which is ob-
˚
viously smaller than that in 1 (2.4 ꢀ 3.4 A). Moreover,
(17) (a) Zhong, D. C.; Lin, J. B.; Lu, W. G.; Jiang, L.; Lu, T. B. Inorg.
Chem. 2009, 48, 8656. (b) Choi, H. S.; Suh, M. P. Angew. Chem., Int. Ed. 2009,
48, 6865. (c) Zhu, A. X.; Lin, J. B.; Zhang, J. P.; Chen, X. M. Inorg. Chem. 2009,
48, 3882. (d) Chen, M. S.; Bai, Z. S.; Okumura, T.-a.; Su, Z.; Chen, S. S.; Sun,
W. Y.; Ueyama, N. CrystEngComm 2010, 12, 1935.
the unit-cell volume is decreased from 909.8(6) for 1 to
3
851.0(10) A for 2, decreasing the volume by about
˚
6.5%. In addition, the distance between Co1 and Co2
˚
within the SBU decreased from 3.08 to 3.04 A.

990 Inorganic Chemistry, Vol. 50, No. 3, 2011
Su et al.
curves may be attributed to the intercrystalline voids.18
It should be noticed that only surface adsorption has
occurred in the CH3OH and CH3CH2OH vapor adsorp-
tion measurements, and the isotherms are characteristic
type-II curves. Namely, complex 2 has a highly selective
adsorption of H2O over CH3OH and CH3CH2OH, which
should be ascribed to the different kinetic diameters of
these molecules.19
Obviously, the N2 gas is different from the solvent
vapor, which has no ability to bend or rotate molecular
linkages. The resulting N2 adsorption isotherm is shown
in Figure S6 (Supporting Information), which is a char-
acteristic type-H3 sorption behavior.20 Furthermore,
hysteresis was also observed between the adsorption-
desorption curves. The result implies that only surface
adsorption has occurred, indicating that nitrogen mole-
cules cannot diffuse into the channels at this temperature.
This highly selective adsorption of H2O over others is
attributed to the presence of the narrow pore size (2.0 ꢀ
˚
2.8 A) of 2, which is very close to the kinetic diameter
˚
of H2O (2.75 A) and significantly smaller than that of
˚
˚
˚
CH3OH (3.8 A), CH3CH2OH (4.3 A), and N2 (3.64 A)
and does not permit these molecules access to the
channels.21 It should be noted that 2 exhibits a highly
selective adsorption of water molecules over N2, CH3OH,
and CH3CH2OH, which could be used as a sensor for
water molecules with a clear color change.
Magnetic Properties of 1 and 2. The temperature de-
pendencies of magnetic susceptibilities of 1 and 2 were
investigated from 300 to 1.8 K with an applied magnetic
field of 2000 Oe. The χM and χMT vs T curves for 1 and
2 are shown in Figure 5. The center to center distances
Figure 5. Temperature dependencies of magnetic susceptibility χM (O)
and χMT (9) for 1 (a) and 2 (b). The solid lines represent the fitted curves.
between the two trinuclear SBUs are long, for example,
-
˚
˚
9.51, 9.05, and 5.66 A separated by L and 8.55 A sepa-
rated by BTEC4- in 1. Therefore, the magnetic interac-
tions between the adjacent trinuclear SBUs can be ignored,
and the interactions can be considered to occur between the
two adjacent metal ions bridged by carboxylate groups
within the SBU. As is known, the magnetic analysis for
the Co(II) complexes is rather complicated because of its
spin-orbital coupling, and some approximate methods
are applied to analyze the magnetic interactions between
the Co(II) ions.22 For 1, the χMT value at 300 K is 8.76
emu K mol-1, which is much higher than the sum of three
isolated spin-only Co(II) ions of 5.63 emu K mol-1 with
g = 2.0 and S = 3/2, ascribing to the contribution of
Co(II) ions (Figure 5a). Similar to the magnetic behavior
of other reported Co(II) complexes, the χMT value of
1 slowly decreases upon cooling to 33 K, which is a typical
manner of spin-orbit coupling and is mainly due to the
single-ion behavior of Co(II).23 However, the χMT value
has an upturn below 33 K, and χMT reaches a maximum
of 10.47 emu K mol-1 at 3 K, indicating that the ferro-
magnetic coupling between Co(II) ions occurs in this system
and is strong enough to compensate for the single-ion
behavior resulting from spin-orbital coupling. The suc-
cedent decrease in the χMT vs T curve of 1 should be
derived from the inter-Co(II) ions’ antiferromagnetic
interactions and/or zero field splitting (ZFS). Factually,
the overall tendencies in the plot of χMT vs T exhibit the
mutual competition between the ferromagnetic coupling
and single-ion behavior of Co(II) ions.
(18) (a) Vishnyakov, A.; Ravikovitch, P. I.; Neimark, A. V.; Bulow, M.;
Wang, O. M. Nano Lett. 2003, 3, 713. (b) Lee, J. Y.; Jagiello, J. J. Solid State
Chem. 2005, 178, 2527.
(19) (a) Zeng, M. H.; Hu, S.; Chen, Q.; Xie, G.; Shuai, Q.; Gao, S. L.;
An attempt was made, through an expression for the
S = 3/2 system with eqs 1-4, for 1 to fit the magnetic data
above 33 K in order to evaluate dominant zero field
splitting effects, D, and the magnetic coupling (zJ) be-
tween the neighboring Co(II) centers linked by oxygen
atoms, and the interactions between the two terminal
Co(II) centers within the SBU were omitted:
Tang, L. Y. Inorg. Chem. 2009, 48, 7070. (b) Jiang, J. J.; Li, L.; Lan, M. H.; Pan,
€
M.; Eichhofer, A.; Fenske, D.; Su, C. Y. Chem.;Eur. J. 2010, 16, 1841.
(20) (a) Sing, K. S. W. Pure Appl. Chem. 1982, 54, 2201. (b) Rowcell,
J. L. C.; Eckert, J.; Yaghi, O. M. J. Am. Chem. Soc. 2005, 127, 14904. (c) Xu, J.;
Bai, Z. S.; Chen, M. S.; Su, Z.; Chen, S. S.; Sun, W. Y. CrystEngComm 2009, 11,
2728. (d) Liu, B.; Shiyama, H.; Akita, T.; Xu, Q. J. Am. Chem. Soc. 2008, 130,
5390.
(21) (a) Coriani, S.; Halkier, A.; Rizzo, A.; Ruud, K. Chem. Phys. Lett.
2000, 326, 269. (b) Cheon, Y. E.; Suh, M. P. Chem.;Eur. J. 2008, 14, 3961. (c)
Hazra, A.; Kanoo, P.; Mohapatra, S.; Mostafa, G.; Maji, T. K. CrystEngComm
2010, DOI: 10.1039h/b924511a.
(22) (a) Zhang, S. H.; Song, Y.; Liang, H.; Zeng, M. H. CrystEngComm
2009, 11, 865. (b) Zeng, M. H.; Yao, M. X.; Liang, H.; Zhang, W. X.; Chen, X. M.
Angew Chem. Int. Ed. 2007, 46, 1832. (c) Jia, H. P.; Li, W.; Ju, Z. F.; Zhang, J.
Dalton Trans. 2007, 3699.
Ng2μ2B 1 þ 9 e-2D=k T
B
χjj
¼
ð1Þ
4ð1 þ e-2D=k T
Þ
B
kBT
(23) Sun, H. L.; Wang, Z. M.; Gao, S. Inorg. Chem. 2005, 44, 2169.

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 991
Ng2μB2 4 þ ð3kBT=DÞð1 - e-2D=k T
Þ
B
were used to fit the data between 22 and 300 K for com-
,
plex 2 and obtained values of g=2.45, D=65.38 cm-1
and zJ=0.75 cm-1 [R=1.31ꢀ10-3].
χ^
¼
ð2Þ
4ð1 þ e-2D=k T
Þ
B
kBT
Comparing the fitting results of 1 and 2, the values of
D and zJ decreased and increased upon dehydration,
respectively, which is consistent with the structural
changes between 1 and 2. The distances of Co1-Co2 are
χjj þ 2χ^
χ0 ¼
ð3Þ
ð4Þ
3
˚
˚
3.08 A in 1 and 3.04 A in 2. The shortened distance be-
tween Co1 and Co2 in dehydrated 2 leads to the magnetic
coupling interaction (zJ) being stronger and the zero-field
splitting effect (D) being weaker.
χ0
!
χ ¼
2zJ
Ng2μ2B
1 -
χ0
Conclusions
where N is Avogadro’s number, μB is the Bohr magneton,
kB is Boltzmann’s constant, and g is the Lande g value.
The best fit in the range of 33-300 K was obtained with
The 3D tfz-d topological coordination polymer based on
Co(II), 3,5-di(imidazol-1-yl)benzoic acid (HL), and 1,2,4,5-
benzenetetracarboxylic acid (H4BTEC) shows interesting
SCSC transformations and reversible dehydration and rehy-
dration phenomena. The dehydrated complex has highly selec-
tive adsorption of water molecules over N2 and other solvent
vapors accompanying a color change, which may be used as
a sensor for water molecules. The results of the present study
provide a nice example of PCPs not only with high stability
and SCSC transformation but also with interesting pro-
perties of highly selective sorption and sensory for specific
molecules.
values of g=2.50, D=80.15 cm-1, and zJ=0.42 cm-1 24
.
P
The agreement factor R, defined as
[(χMT)obsd -
P
(χMT)calcd]2/ (χMT)2, is equal to 8.50 ꢀ 10-4
.
Furthermore, the field dependence of the magnetiza-
tions of 1 at 1.8 K shows a nearly linear increase below
10 kOe and reaches a value of 8.21 Nβ at 70 kOe (Figure
S7, Supporting Information). This value is less than the
expected saturation value of 9 Nβ for the CoII cluster
3
with S = 3/2 and g = 2, which indicates the possible
existence of S=1/2 components in the Kramers doublets
formed by zero-field splitting.25
Similarly, the magnetic measurements for the dehydrated
product 2 show that complex 2 has similar magnetic
property as 1 (Figure 5b and Figure S8, Supporting Infor-
Acknowledgment. This work was financially supported
by the National Natural Science Foundation of China
(Grant nos. 20731004 and 21021062) and the National
Basic Research Program of China (Grant nos. 2007CB-
925103 and 2010CB923303).
mation). The χMT value at 300 K is 8.42 emu K mol-1
.
The upturn was found at 22 K and then reached a maxi-
mum of 8.61 emu K mol-1. The same eqs 1-4 used for 1
Supporting Information Available: X-ray crystallographic file
in CIF format, crystal structure (Scheme S1, Figures S1 and S2),
TGA (Figure S3), PXRD (Figure S4), solid state UV-visible
spectra (Figure S5), N2 adsorption isotherm (Figure S6), mag-
netic data (Figures S7 and S8). This information is available free
of charge via the Internet at http://pubs.acs.org.
(24) (a) Marshall, S. R.; Rheingold, A. L.; Dawe, L. N.; Shum, W. W.;
Kitamura, C.; Miller, J. S. Inorg. Chem. 2002, 41, 3599. (b) Duran, N.; Clegg,
W.; Cucurull-Sanchez, L.; Coxall, R. A.; Jimenez, H. R.; Moratal, J. M.; Lloret, F.;
Gonzalez-Duarte, P. Inorg. Chem. 2000, 39, 4821. (c) Nelson, D.; Haar, L. W. T.
Inorg. Chem. 1993, 32, 182.
(25) Kahn, O. Molecular Magnetism; VCH: Weinheim, Germany, 1993.