Two three-dimensional metal-organic frameworks constructed from alkaline earth metal cations (Sr and Ba) and 5-nitroisophthalicacid - Synthesis, charaterization, and thermochemistry

Article abstract of DOI:10.1002/zaac.200800047

Two MOFs of [Sr^II(5-NO₂-BDC)(H₂O) ₆] (1) and [Ba^II(5-NO₂-BDC)(H ₂O)₆] (2) have been synthesized in water using alkaline earth metal salts and the rigid organic ligand

Full text of DOI:10.1002/zaac.200800047

DOI: 10.1002/zaac.200800047
Two Three-Dimensional Metal-Organic Frameworks Constructed from Alkaline
Earth Metal Cations (Sr and Ba) and 5-Nitroisophthalicacid ؊ Synthesis,
Charaterization, and Thermochemistry
Sanping Chen, Qi Shuai, and Shengli Gao*
Xi’an / P. R. China, Northwest University, Department of Chemistry, Shaanxi Key Laboratory of Physico-Inorganic Chemistry
Received December 17th, 2007; accepted February 8th, 2008.
Abstract. Two MOFs of [SrII(5-NO2-BDC)(H2O)6] (1) and
[BaII(5-NO2-BDC)(H2O)6] (2) have been synthesized in water using
alkaline earth metal salts and the rigid organic ligand 5-NO2-
H2BDC. The compounds were characterized by elemental analysis,
infrared spectrum, thermal analysis, and X-ray crystallography.
Crystal structure analyses have shown that the two complexes are
isostructural as evidenced by IR spectra and TG-DTA. Both com-
pounds present three-dimensional frameworks built up from infi-
nite chains of edge-sharing twelve-membered rings through
OϪH···O hydrogen bonds. The specific heat capacities of the title
complexes have been determined by an improved RD496-III micro-
calorimeter with the values of (109.29 0.693) J molϪ1 KϪ1 and
(81.162 0.858) J molϪ1 KϪ1 at 298.15 K, and the molar enthalpy
changes of the formation reactions of complexes at 298.15 K were
calculated as (4.897
0.008) kJ molϪ1 and (2.617
0.009) kJ
molϪ1, respectively.
Keywords: Alkaline earth metals; Metal-organic framework;
5-Nitroisophthalic acid; Crystal structures
1 Introduction
favored to the reaction of hard bases which have stronger
ability to present electrons like oxygen of carboxylate group
based on the soft and hard acid and base theory.
In our previous work, the relationship of thermal prop-
erty with the structure for the alkaline earth metal MOF
based on 5-hydroxyisophtalic acid (5-OH-H2BDC) has
been studied [32]. In the extend investigation, we con-
structed the alkaline earth metal MOF with 5-NO2-H2BDC
and investigated their thermal properties comprehensively.
MOF structures are composed of two main components:
the organic linkers and the metal ions [1]. The linkers play
a role of “struct” that bridge metal atoms, which in turn
act as “joints” in the resulting MOF structure. The class of
materials has received widespred attention due to the diver-
sity of linkers and metal ions and the products of their as-
sembly could be crystallized. Particular interest is associ-
ated with MOF constructed from carboxylateϪcontaining
organic linkers due to the diversity of the binding modes
[2Ϫ10]. As a electron withdrowing group, the nitryl group
could take on special spatial effects in the formation of net-
works and effect on the electron density of the whole li-
gands and proneness to crystallization in aqua solution [11].
In this account, we select 5-NO2-H2BDC as a ligand with
C2v symmetry bearing two carboxylate groups and a nitryl
group. Previous studies about 5-NO2-H2BDC are focused
mainly on those of transition [12Ϫ26] or lanthanide ele-
ments [11, 27Ϫ30]. To the best of our knowledge, coordi-
nation polymers assembled by alkaline earth metal salts
have seldom been reported possibly due to the bigger elec-
tropositivity of alkaline earth, which results in the difficulty
to accept a pair of electrons from ligands. In fact, alkaline
earth metal irons with high coordination numbers [31] are
2 Experimental Section
2.1 Materials and Analytical methods
Reagents were purchased commercially and used without further
purification. Elemental analyses (C, H and N) were carried out on
a Vario EL III CHNOS instrument made in German. The TG-
DTA experiments for the complexes were performed using a model
WRT-2P under a static air atmosphere with the heating rate of
10 °C minϪ1 from ambient temperature to 900 °C. The IR analysis
was recorded as KBr pellets using a BEQ, UZNDX-550 spec-
trometer.
2.2 Syntheses of the complexes
2.2.1 [Sr(II)(5-NO2-BDC)(H2O)6] (1)
A mixture of 5-NO2-H2BDC (0.2111 g, 1 mmol), NaOH (0.08 g,
2 mmol) and distillated water (20 mL) was stirring until pH ϭ 5.
To which 20 mL 0.5 mol LϪ1 SrCl2 ·6H2O (0.26662 g, 1 mmol)
aqueous solution was added under the condition of stirring for two
hours at room temperature, then filtrated. After one week, color-
less, triangular single crystals of 1 suitable for XϪray diffraction
* Shengli Gao
Department of Chemistry
Shaanxi Key Laboratory of Physico-Inorganic Chemistry
Northwest University
Xi’an 710069 / P. R. China
E-mail: gaoshli@nwu.edu.cn
Z. Anorg. Allg. Chem. 2008, 634, 1591Ϫ1596
© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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S. Chen, Q. Shuai, S. Gao
SrCl2 ·6H2O (0.26662 g, 1 mmol) aqueous solution, to which 5 mL
DMF was added under the condition of stirring. After two week,
two types of colorless, single crystals of 2 were obtained in the
filtrate. One are rectangular, the other are triangular which are tar-
geted single crystals. Anal. Calc. for 2: C, 21.14; H, 3.33; N, 3.08,
Ba, 30.21. Found: C, 21.05; H, 3.35; N, 3.07, Ba, 30.28 %.
Table 1 Crystallographic data and structure refinement parame-
ters for 1 and 2
Complex
1
2
Empirical formula
Formula weight
Temperature /K
Crystal system
Space group
C8 H15NO12Sr
404.83
C8 H15NO12Ba
454.55
273(2)
monoclinic
Cc
273(2)
monoclinic
Cc
2.3 X-ray crystallography
Flack parameter
0.003(19)
11.7031(11)
18.4855(16)
7.3184(7)
118.9330(10)
1385.6(2)
4
1.941
3.958
816
0.003(19)
11.884(3)
18.718(5)
7.5815(15)
19.971(3)
1460.9(6)
4
2.067
2.782
888
˚
a /A
˚
The data were collected on a Bruker SMART CCD area-detector
b /A
˚
c /A
diffractometer with graphite monochromated Mo-Kα radiation
β /°
˚
(λϭ0.71073 A) at 293(2) K, and the data reduction was performed
3
˚
Volume /A
using Bruker SAINT [20]. The structures were solved with direct
methods and refined by fullmatrix least-squares refinement on F2
with anisotropic displacement parameters for non-H atoms using
SHELXTL [33]. All non-hydrogen atoms were refined aniso-
tropically. The hydrogen atoms except for those of water molecules
were generated geometrically. Hydrogen atoms of water were found
in the difference Fourier map directly. Structural plots were gener-
ated with SHELXTL and OLEX [34]. A summary of the crystallo-
graphic data for the complexes is given in Table 1. Selected bond
lengths and angles are tabulated in Table 2.
Z
Dcalc /(mg/m3)
Absorption coefficient /mmϪ1
F (000)
Crystal size /mm
Range hkl collected
0.37 x 0.34 x 0.07 0.51 x 0.39 x 0.30
Ϫ14 Յ h Յ 14
Ϫ23 Յ kՅ22
Ϫ4 Յ l Յ 9
3771 / 1915
0.0277
98.9
1915 / 2 / 200
1.061
R1 ϭ 0.0392
wR2 ϭ 0.1044
R1 ϭ 0.0402
wR2 ϭ 0.1055
Ϫ11 Յ h Յ 14
Ϫ22 Յ k Յ 22
Ϫ9 Յ l Յ 7
3613 / 1805
0.0274
99.8
1805 / 2 / 195
1.071
R1 ϭ 0.0195
wR2 ϭ 0.0483
R1 ϭ 0.0199
wR2 ϭ 0.0483
Reflections collected / unique
R (int)
Completeness /%
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I>2sigma(I)]
3 Results and Discussion
R indices (all data)
3.1 Structural description
Ϫ3
˚
Largest diff. peak and hole /eA
0.750 and Ϫ0.846 0.454 and Ϫ0.410
The formulas for the polymeric networks are identified as
[SrII(5-NO2-BDC)(H2O)6] for 1 and [BaII(5-NO2-BDC)-
(H2O)6] for 2. Single crystal X-ray analyses reveal that both
of the structures are isostructural, in which the 5-NO2-
H2BDC ligands are completely deprotonated.
Both 1 and 2 contain nine-coordinated alkaline earth me-
tal(II) cations (Fig. 1) in distorted tricapped trigonal prism
(Fig. 2) where O1W, O2W and O5W form the top plane of
were obtained in the filtrate. Anal. Calc. for 1: C, 23.74; H, 3.73;
N, 3.46, Sr, 21.64. Found: C, 23.79; H, 3.70; N, 3.47; Sr, 21.64 %.
2.2.2 [Ba(II)(5-NO2-BDC)(H2O)6] (2)
2 was obtained by similar procedures to 1 using 20 mL 0.5 mol
LϪ1 BaCl2 ·2H2O (0.2443 g, 1 mmol) aqueous solution instead of
˚
Table 2 Selected bond lengths/A and angles/° for 1 and 2
1
Sr(1)-O(2W)
Sr(1)-O(5W)
Sr(1)-O(6W)
2.500(5)
2.570(5)
2.713(5)
Sr(1)-O(2)
Sr(1)-O(4W)
Sr(1)-O(3)#1
2.518(4)
2.579(5)
3.002(5)
Sr(1)-O(1W)
Sr(1)-O(3W)
Sr(1)-O(4)#2
2.561(5)
2.650(5)
3.053(5)
O(2W)-Sr(1)-O(2)
78.57(15)
82.73(17)
151.79(16)
93.10(16)
135.21(15)
127.63(16)
136.55(14)
O(2W)-Sr(1)-O(1W)
O(2)-Sr(1)-O(5W)
O(2)-Sr(1)-O(4W)
O(2W)-Sr(1)-O(3W)
O(5W)-Sr(1)-O(3W)
O(2)-Sr(1)-O(6W)
O(4W)-Sr(1)-O(6W)
74.74(17)
72.07(16)
73.63(16)
91.15(16)
146.62(14)
136.59(16)
73.11(16)
O(2)-Sr(1)-O(1W)
O(1W)-Sr(1)-O(5W)
O(1W)-Sr(1)-O(4W)
O(2)-Sr(1)-O(3W)
O(4W)-Sr(1)-O(3W)
O(1W)-Sr(1)-O(6W)
O(3W)-Sr(1)-O(6W)
139.40(18)
74.66(15)
131.12(16)
74.55(16)
77.05(15)
83.96(15)
71.44(14)
O(2W)-Sr(1)-O(5W)
O(2W)-Sr(1)-O(4W)
O(5W)-Sr(1)-O(4W)
O(1W)-Sr(1)-O(3W)
O(2W)-Sr(1)-O(6W)
O(5W)-Sr(1)-O(6W)
2
Ba(1)-O(2)
Ba(1)-O(1W)
Ba(1)-O(6W)
2.649(3)
2.764(4)
2.850(4)
Ba(1)-O(2W)
Ba(1)-O(5W)
Ba(1)-O(3)#1
2.700(4)
2.782(4)
3.016(4)
Ba(1)-O(4W)
Ba(1)-O(3W)
Ba(1)-O(4)#2
2.722(4)
2.815(4)
3.101(4)
O(2)-Ba(1)-O(2W)
O(2)-Ba(1)-O(1W)
O(2W)-Ba(1)-O(5W)
O(2)-Ba(1)-O(3W)
O(1W)-Ba(1)-O(3W)
O(2W)-Ba(1)-O(6W)
O(5W)-Ba(1)-O(6W)
77.44(12)
135.45(14)
78.60(13)
74.02(12)
138.46(12)
128.10(13)
141.05(12)
O(2)-Ba(1)-O(4W)
72.58(12)
134.01(13)
96.81(13)
91.33(12)
145.03(11)
73.34(12)
O(2W)-Ba(1)-O(4W)
O(2)-Ba(1)-O(5W)
O(1W)-Ba(1)-O(5W)
O(4W)-Ba(1)-O(3W)
O(2)-Ba(1)-O(6W)
O(1W)-Ba(1)-O(6W)
O(3W)-Ba(1)-O(6W)
149.45(12)
71.12(12)
70.65(13)
75.11(11)
135.43(12)
89.10(13)
70.10(11)
O(4W)-Ba(1)-O(1W)
O(4W)-Ba(1)-O(5W)
O(2W)-Ba(1)-O(3W)
O(5W)-Ba(1)-O(3W)
O(4W)-Ba(1)-O(6W)
Symmetry transformations used to generate equivalent atoms:
1 #1 xϩ1/2, Ϫyϩ1/2, zϩ1/2; #2 xϩ1, y, zϩ1; #3 xϪ1/2, Ϫyϩ1/2, zϪ1/2; #4 xϪ1, y, zϪ1
2 #1 xϪ1/2, Ϫyϩ1/2, zϪ1/2; #2 xϪ1, y, zϪ1; #3 xϩ1/2, Ϫyϩ1/2, zϩ1/2; #4 xϩ1, y, zϩ1
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Z. Anorg. Allg. Chem. 2008, 1591Ϫ1596

Two Three-Dimensional Metal-Organic Frameworks
Fig. 2 Coordination polyhedron for metal(II) ions in 1 and 2. The
hydrogen and carbon atoms are omitted for clarity.
Fig. 1 Coordination environment of alkaline earth metal(II)
cations in 1 and 2. The hydrogen atoms are omitted for clarity
the trigonal prism, and the bottom plane is completed by
O6W, O3W and O4W, while O2, O3A and O4B capped
quadrilateral face formed by O2W, O3W, O4W, O5W; O1W,
O6W, O4W, O5W and O1W, O6W, O3W, O2W, respectively.
Among all the nine coordinated oxygen atoms, three (O2,
O3A and O4B) from three different carboxyl groups, six
(O1W, O2W, O3W, O4W, O5W and O6W) from coordi-
nated water molecules. The Sr (1)-O bond distances range
Fig. 3 A fragment of the one-dimensional chain structure in 1
and 2. The hydrogen atoms are omitted for clarity.
˚
from 2.506(5) to 3.055(5) A (Table 2) with the mean of
˚
2.679 (6) A, which are similar to those observed in other
strontium carboxylate complexes with the mean of 2.696
˚
˚
(4) A [35], 2.636 (7) A [36]. The bond length of Ba-Ocarboxyl-
˚
bonds range from 2.649(3) to 3.102(4) A and Ba-OW
bonds from 2.706(4) to 2.851(4) A, which compares well
ate
˚
with the average values determined from the CSD [2.798
˚
˚
(7) A for Ba-Ocarboxylate bond and 2.847 (7) A for Ba-OW
bond].
In 1 and 2 the nitro group from 5-NO2-BDC does not
coordinate with M2ϩ ions when two carboxyl groups are
employed in bridging and monodentate fashions to link
three M2ϩ ions. In this case two M2ϩ ions are linked to-
gether through two 5-NO2-BDC ligands to form twelve-
membered rings and neighboring twelve-membered rings
share common C-O-M edges, extending to one-dimensional
chain (Fig. 3). Just like the previous works, the coordination
modes of two carboxyl groups in the 5-NO2-BDC ligands
exhibit multiplicate coordination modes, such as monodent-
ate mode [12], bidentate chelating mode [16, 29], μ3-bridge
mode and μ2-bridge mode [19, 27, 31]. Both 1 and 2 present
three-dimensional layer structures constructed from one-
dimensional chains through OϪH···O hydrogen bonds
(Fig. 4).
Fig. 4 View of three-dimensional layer structure along c axis in
1 and 2. MII ions and H atoms are omitted for clarity.
Z. Anorg. Allg. Chem. 2008, 1591Ϫ1596
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1593

S. Chen, Q. Shuai, S. Gao
3.2 IR spectroscopy
26.70 %) for 1 and 19.34 % (calculated 23.79 %) for 2. The
process is accompanied by one exothermic peak centered at
101.16 °C for 1 and 86.58 °C for 2, respectively. The pyro-
lysis of the remaining M(II)(5-NO2-BDC), yielding the
compound MIICO3, occurs in two consecutive stages at
382-570 °C for 1 and 400-596 °C for 2, giving the mass
losses of 38.01 % (calculated 36.83 %) for 1 and 31.26 %
(calculated 32.80 %) for 2. During this process, DTA curve
show two exothermic peaks at 449 °C and 522 °C for 1,
together with 444 °C and 497 °C for 2.
The IR spectra of the two complexes show the character-
istic bands of the dicarboxylate groups of 5-OH-BDC at
1621 cmϪ1 for 1 and 1624 cmϪ1 for 2 for the asymmetric
vibration and at 1452 cmϪ1 for 1 and 1457 cmϪ1 for 2 for
the symmetric vibration. The absence of the expected ab-
sorption at 1680Ϫ1765 cmϪ1 for the protonated dicar-
boxylate groups illustrates the complete deprotonation of
the dicarboxylate groups in the complexes. The broad bands
at 3244Ϫ3414 cmϪ1 of 1 and 3443Ϫ3417 cmϪ1 of 2 are at-
tributed to the characteristic vibrations of the water.
3.4 Specific heat capacity
3.3 Thermogravimetric analysis
The calorimetric methods and experimental conditions are
identical with that of reference [32]. The specific heat ca-
pacities of the empty cell and the standard substances were
determined according to the above method and listed in
Table 3. The measurement precisions are in the range of 5 ϫ
10Ϫ4. All the measurement accuracies of the two standard
As shown in Figure 5, the two as-prepared compounds ex-
perience two similar thermal processes in TG-DTA runs un-
der the static air which show three weight loss steps. Be-
cause the TG curves showed overlapped decomposition
stages that precluded the isolation and further analysis of
the intermediates, only the final residues were identified by
X-ray powder diffraction techniques. Observed from the
TG-DTA curves, the complexes are thermally stable up to
60 °C. Afterwards, six coordination water molecules from
the complexes are eliminated in the first step up to 400 °C,
resulting in the weight losses of 26.70 % (calculated
samples are 8 ϫ 10Ϫ4
.
The mean heats of disequilibria for the complexes (1:
m1 ϭ 4.11643 g; 2: m2 ϭ 2.99262 g) are (8668.012 3.276)
mJ and (8334.025 2.131) mJ, respectively, which are meas-
ured based on 6 times measurements for each sample. The
specific heat capacities calculated from the data are in
Fig. 5 TGϪDTA curves of 1 and 2.
Table 3 Data of the heat and heat capability of the empty cell and the standard substances at 298.15 K
Heats of disequilibrium
The empty cell
Standard Ͱ-Al2O3
Sublimed benzoic acid
and Specific heat capacity
q(1)/mJ
q(2)/mJ
7940.761
7953.287
8347.927
8349.710
8620.310
8633.710
q(3)/mJ
q(4)/mJ
7958.855
7933.804
8372.466
8361.101
8603.060
8633.198
q(5)/mJ
q(6)/mJ
7930.113
7936.176
8350.931
8369.453
8629.164
8614.664
q (Mean SD) /mJ
The relative standard deviation
Specific heat capacity
7942.166 4.674
8358.598 4.351
5.206ϫ10Ϫ4
79.098 2.364 (79.03
8622.351 4.928
5.718ϫ10Ϫ4
145.202 1.627(145.327
5.884ϫ10Ϫ4
[37]
[38]
)
)
c/(J molϪ1 KϪ1
)
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Z. Anorg. Allg. Chem. 2008, 1591Ϫ1596

Two Three-Dimensional Metal-Organic Frameworks
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5-NO2-H2BDC ϩ 2NaOH ϩ MIICl2 ·6H2O
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4 Conclusions
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were determined as (109.29
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Supplemental material. Supplementary data are available from
the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: ϩ44-1233-336033; e-mail:
deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk) on request,
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Acknowledgements. We gratefully acknowledge the financial sup-
port from the National Natural Science Foundation of China
(Grant Nos. 20471047 and 20771089), the National Natural Sci-
ence Fundation of Shaanxi Province(No. 2007B02). and Shaanxi
Physico-chemical Key Laboratory and Shaanxi key Laboratory of
Chemical Reaction Engineering.
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© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2008, 1591Ϫ1596