Four alkali-induced 3d strontium(II) coordination polymers constructed from imidazole-4,5-dicarboxylate: Syntheses, crystal structures, and properties

Article abstract of DOI:10.1002/ejic.201200796

Four alkali-induced 3D coordination polymers, namely [Sr(HIDC)(H ₃IDC)(H₂O)]_n·nH₂O (1), [Sr(HIDC)]_n (2), [Sr₃(HIDC)₂(H ₂IDC)₄]_n (3), and [Sr₂(HIDC) ₂]_n (4), were constructed from strontium(II) and imidazole-4,5-dicarboxylic acid (H₃IDC) and were characterized by elemental analysis, IR, TG, PL, powder and single-crystal X-ray diffraction. Complex 1 is an eca net with the Schlaefli symbol of (3⁹.4 ¹².5⁷) by considering the Sr^II cation as an 8-connected node. Complex 2 presents a rare 5-connected network with Schaefli symbol of (4⁵.6⁵) when the Sr^II cation and HIDC^2- dianion are viewed as 5-connected nodes. Complex 3 is a complicated 3D network, which can be regarded as a H₂IDC ^- monoanion sustained porous framework filled by another HIDC ^2- dianion and H₂IDC^- monoanion. Complex 4 exhibits a 3D porous framework with 1D trigonal hydrophobic channels. The ligand H₃IDC in the four complexes exists as the H₃IDC molecule, the H₂IDC^- monoanion, and the HIDC^2- dianion. In addition, the μ₂ (κ¹N:κ¹O: κ¹O) mode of the H₃IDC molecule in complex 1, the μ₃ (κ¹O:κ¹O: κ¹O:κ¹O) and μ₃ (κ¹N:κ¹O:κ¹O: κ¹O) modes of the H₂IDC^- monoanion in complex 3, as well as the μ₅ (κ¹N: κ²O:κ¹O:κ²O: κ¹O) and μ₆ (κ¹N: κ²O:κ²O:κ²O: κ²O) modes of the HIDC^2- dianion in complexes 2 and 4 are reported for the first time. Diverse topological architectures of the four complexes depend mainly on the coordination spheres of the Sr^II cation, the coordination modes of the anions, the different alkali, and the ligand to alkali ratio. TG analysis indicates that complex 2 presents high thermal stability, while the TG curve and temperature-dependent PXRD spectra of complex 4 indicate that its framework is stable up to 350 °C. Moreover, the solid-state luminescent properties demonstrate that complexes 1 to 4 exhibit strong blue emission at room temperature. Copyright

Full text of DOI:10.1002/ejic.201200796

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DOI: 10.1002/ejic.201200796
1
Four Alkali-Induced 3D Strontium(II) Coordination Polymers Constructed
from Imidazole-4,5-dicarboxylate: Syntheses, Crystal Structures, and
Properties
Xian-Fa Zhang,[a] Zhao-Peng Deng,[a] Li-Hua Huo,*[a] Qing-Mao Feng,[a] and Shan Gao*[a]
Keywords: Strontium / Coordination modes / Carboxylate ligands / Thermal stability / Luminescence
6
Four alkali-induced 3D coordination polymers, namely
[Sr(HIDC)(H3IDC)(H2O)]n·nH2O (1), [Sr(HIDC)]n (2),
cule, the H2IDC– monoanion, and the HIDC2– dianion. In ad-
dition, the μ2 (κ1N:κ1O:κ1O) mode of the H3IDC molecule in
complex 1, the μ3 (κ1O:κ1O:κ1O:κ1O) and μ3 (κ1N:κ1O:
κ1O:κ1O) modes of the H2IDC– monoanion in complex 3, as
well as the μ5 (κ1N:κ2O:κ1O:κ2O:κ1O) and μ6 (κ1N:κ2O:κ2O:
κ2O:κ2O) modes of the HIDC2– dianion in complexes 2 and 4
are reported for the first time. Diverse topological architec-
tures of the four complexes depend mainly on the coordina-
tion spheres of the SrII cation, the coordination modes of the
anions, the different alkali, and the ligand to alkali ratio. TG
analysis indicates that complex 2 presents high thermal sta-
bility, while the TG curve and temperature-dependent PXRD
spectra of complex 4 indicate that its framework is stable up
to 350 °C. Moreover, the solid-state luminescent properties
demonstrate that complexes 1 to 4 exhibit strong blue emis-
[Sr3(HIDC)2(H2IDC)4]n (3), and [Sr2(HIDC)2]n (4), were con-
structed from strontium(II) and imidazole-4,5-dicarboxylic
acid (H3IDC) and were characterized by elemental analysis,
IR, TG, PL, powder and single-crystal X-ray diffraction. Com-
plex 1 is an eca net with the Schläfli symbol of (39.412.57) by
considering the SrII cation as an 8-connected node. Complex
2 presents a rare 5-connected network with Schäfli symbol
of (45.65) when the SrII cation and HIDC2– dianion are viewed
as 5-connected nodes. Complex 3 is a complicated 3D net-
work, which can be regarded as a H2IDC– monoanion sus-
tained porous framework filled by another HIDC2– dianion
and H2IDC– monoanion. Complex 4 exhibits a 3D porous
framework with 1D trigonal hydrophobic channels. The li-
26
31
36
11
16
21
gand H3IDC in the four complexes exists as the H3IDC mole- sion at room temperature.
considered.[12] Thus, synthesis of AE metal coordination
Introduction
polymers with attractive structures and properties through
the rational selection and adjustment of N-heterocyclic
carboxylates with the versatile coordination modes induced
by different types and amounts of alkali is still significant
work.
56
61
66
71
76
Considerable attention has been devoted to the rational
design and synthesis of alkaline earth (AE) metal coordina-
tion polymers due to their fascinating architectures along
with potential applications in the fields of catalysis, material
science, and biochemistry.[1–4] Similar to the most reported
transition metal coordination polymers, the assembly of AE
metal coordination polymers is affected by many intricate
influence factors,[5–9] among which the judicious selection
of multifunctional organic ligands is an effective approach
to construct metal–organic complexes with unique architec-
tures and functions.[10] It is well known that aromatic poly-
carboxylates are excellent candidates for constructing new
architectures because of their versatile coordination modes
and variable valences induced by different amounts of al-
kali.[11] However, the AE coordination polymers con-
structed from the N-heterocyclic carboxylates are rarely
41
46
51
In this context, imidazole-4,5-dicarboxylic acid (H3IDC)
is an excellent candidate for assembling new polymeric net-
works owing to the arrangement of the two N atoms with
a N–X–N angle of about 144°. In particular, H3IDC can be
partially or fully deprotonated at different pH values, which
is controlled by diverse alkali and the ligand to alkali ratio.
However, it is surprising that research on AE imidazole-4,5-
dicarboxylates is rarely observed, which may be caused by
their unpredictable coordination numbers and geometries
as no ligand field stabilization effects govern their coordina-
tive bonding. The AE metals present high affinity for oxy-
gen donors, particularly water, to form solvated metal cen-
ters that inhibit the metal–ligand cross-linking required to
form an extended structure. Up to now, only a few imid-
azole-4,5-dicarboxylate complexes that incorporate AE
metals have been reported.[13] In 2008, Gurunatha et al. pre-
sented three metal–organic hybrid complexes of MgII,
which exhibited structural evolutions from 1D, 2D to 3D by
controlling the stoichiometry and temperature.[14] Further
[a] Key Laboratory of Functional Inorganic Material Chemistry,
Ministry of Education, Heilongjiang University,
Harbin 150080, People’s Republic of China
Fax: +86-0451-86608040
E-mail: lhhuo68@yahoo.com
shangao67@yahoo.com
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201200796.
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investigation of the CSD research [Cambridge Structure reaction was limited within the range of 5 to 6 by using
Database search, CSD Version 5.27 (November 2005) with different inorganic and organic alkali and by controlling the
19 updates (January 2006-Nov 2011)] revealed that there ratio of the ligand to inorganic alkali. Subsequently, four
is no report on imidazole-4,5-dicarboxylate complexes that new 3D coordination polymers were obtained as illustrated 116
contain a SrII cation with a large cation radius and diverse in Scheme 2.
coordination numbers. Hence, we report the synthesis,
structure, and properties of four new 3D coordination poly-
mers based on strontium and imidazole-4,5-dicarboxylic
acid (H3IDC), namely [Sr(HIDC)(H3IDC) (H2O)]n·nH2O
(1), [Sr(HIDC)]n (2), [Sr3(HIDC)2(H2IDC)4]n (3), and
81
86
91
96
[Sr2(HIDC)2]n (4). The four complexes exhibit diverse topo-
logical structures that are mainly dependent on the coordi-
nation spheres of the SrII cation, coordination modes of the
anions, the different alkali, and the ligand to alkali ratio. In
addition, the μ2 (κ1N:κ1O:κ1O) mode of the H3IDC mole-
cule in complex 1, the μ3 (κ1O:κ1O:κ1O:κ1O) and μ3
(κ1N:κ1O:κ1O:κ1O) modes of the H2IDC– monoanion in
complex 3, as well as the μ5 (κ1N:κ2O:κ1O:κ2O:κ1O) and μ6
(κ1N:κ2O:κ2O:κ2O:κ2O) modes of the HIDC2– dianion in
complexes 2 and 4 are reported for the first time
(Scheme 1).[13,14]
Scheme 2. Synthetic process for the four complexes.
Structure of [Sr(HIDC)(H3IDC)(H2O)]n·nH2O (1)
As shown in Figure 1 (a), the asymmetric unit of com-
plex 1 contains one SrII cation, one HIDC2– dianion, one
H3IDC molecule, one coordinated water molecule, and one 121
lattice water molecule. Each SrII cation is eight-coordinate
and presents a distorted tetragonal prism geometry (Fig-
ure 1, b). The C–O bond lengths of C4–O1, C4–O2, C5–
O3, and C5–O4 (Table S2) indicate the existence of H atoms
attached to the two carboxyl groups. In comparison, the C– 126
O bond lengths of C9–O5, C9–O6, C10–O7, and C10–O8
(Table S2) indicate that the H atoms attached to the two
carboxyl groups are deprotonated. The HIDC2– dianion
and the H3IDC molecule play different roles in constructing
the final topological architecture. For the HIDC2– dianion, 131
Scheme 1. New coordination modes of H3IDC, H2IDC–, and
HIDC2– in complexes 1 to 4.
the two carboxylates adopt the same μ2 (bis-monodentate)
coordination mode and bridge adjacent SrII cations to form
two separate 1D linear (O7–C10–O8) and zig-zag (O5–C9–
O6) motifs along different directions (Figure 1, c). The
Sr···Sr separations in the two types of chains are 6.893(2) Å 136
(linear chain) and 6.917(2) Å (zig-zag chain). Combination
of the two types of chains affords the formation of the
101 Results and Discussion
Syntheses
As stated above, imidazole-4,5-dicarboxylic acid wave-like 2D 44 grid with diagonal Sr···Sr separations of
(H3IDC) can be successively deprotonated to generate the 9.757(2) and 9.775(2) Å (Figure 1, c). The H3IDC mole-
H2IDC– monoanion, the HIDC2– dianion, and the IDC3– cules link the above mentioned 44 grids in a new μ2 141
106 trianion by changing the type or amount of the alkali and (κ1N:κ1O:κ1O) coordination mode to generate a 3D net-
hence may result in diverse architectures through reaction work (see Scheme 1, Figure S1 in the Supporting Infor-
with metal cations that possess large cation radii and high mation). In addition, strong π–π stacking interactions along
coordination numbers. With this concept in mind, the c axis between the imidazole rings of H3IDC molecules
Sr(NO3)2 was employed to react with the proper amount of with a centroid-to-centroid distance of 3.783(4) Å are de- 146
111 the ligand H3IDC in a typical hydrothermal synthesis. To tected, which further strengthen the stability of the whole
avoid the hydrolyzation of the SrII cation, the pH of the crystal structure.
2
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Four Alkali-Induced 3D Strontium(II) Coordination Polymers
node and connection nets. As observed in Figure S1, each
eight-coordinate SrII cation links adjacent eight SrII cations
through three HIDC2– dianions and two H3IDC molecules.
Thus, each SrII cation can be viewed as 8-connected nodes.
The whole structure can thus be represented by an eca net. 156
An analysis by TOPOS4[16] reveals that the network of com-
plex 1 is an 8-connected net with the Schläfli symbol of
(39.412.57) topology (Figure 1, d).
Structure of [Sr(HIDC)]n (2)
Another 3D network, namely complex 2, is formed by 161
changing the ligand to alkali (NaOH) ratio to 1:2. As
shown in Figure 2 (a), the asymmetric unit of complex 2
contains one SrII cation and one HIDC2– dianion. As op-
posed to complex 1, each SrII cation is seven-coordinate
and the coordination sphere can be best described as a dis- 166
Figure 1. (a) Perspective view of the asymmetric unit of 1. (b) Anti-
tetragonal prism geometry of SrII cation. (c) 2D wave-like 44 grid
containing 1D linear and 1D zig-zag chain motifs. (d) The eca net
of 1.
Figure 2. (a) Perspective view of the asymmetric unit of 2.
(b) Monocapped trigonal prism geometry of SrII cation. (c) 3D
network (right) formed by 1D ribbon chain (left and up) and 2D
layer (left and down) with the 1D channels along the a axis. (d)
The (45.65) topology of 2 by considering HIDC2– dianion (up) and
A better insight into the nature of this intricate network
can be achieved by the application of the topological ap-
151 proach, reducing multidimensional structures to simple SrII cation (down) as 5-connected nodes.
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torted monocapped trigonal prism geometry (Figure 2, b).
The imidazole N2 atom retains the hydrogen atom while
the two carboxylates are deprotonated and their corre-
sponding C–O bond lengths are similar (Table S2), which
171 indicates that the two carboxylates are delocalized. It
should be noted that this HIDC2– dianion exhibits a new
μ5 (κ1N:κ2O:κ1O:κ2O:κ1O) coordination mode as shown in
Scheme 1. Although the two carboxylates of the HIDC2–
dianion display the same μ3 coordination mode, they play
176 different roles in constructing the final topological architec-
ture (Figure 2, c). In the carboxylate that contains O1–C4–
O2, two carboxylate O1 atoms bridge two adjacent SrII cat-
ions to form a four-membered Sr2O2 ring with a Sr···Sr sep-
aration of 3.796(2) Å. The adjacent Sr2O2 rings are further
181 linked by the carboxylate O2 atoms to generate a 2D layer
motif parallel to the bc plane (Figure 2, c, left and down).
This interconnection leads to the formation of another six-
teen-membered Sr4C4O8 ring. In the carboxylate that con-
tains O3–C5–O4, the four-membered Sr2O2 rings that are
186 bridged by O3 atoms [Sr···Sr separation of 4.079(1) Å] are
linked by four adjacent carboxylate O4 atoms to afford a
1D ribbon chain along the a axis (part c of Figure 2, left
and top). Thus, the intercrossing of the aforementioned
layer and ribbon gives rise to the formation of the final 3D
191 network (Figure 2, c, right). In addition, the π–π stacking
interactions between the imidazole rings with a centroid-to-
centroid distance of 3.925(4) Å are detected, which further
strengthen the stability of the whole crystal structure.
One can conclude from the 3D network that each SrII
196 cation is coordinated to five surrounding HIDC2– dianions
and each HIDC2– dianion bridges five neighboring SrII cat-
ions. Thus, a better insight into the nature of this intricate
network can be achieved by the application of the topologi-
cal approach, reducing multidimensional structures to sim-
201 ple node and connection nets. As discussed above, each
seven-coordinate SrII cation and HIDC2– dianion can be
viewed as 5-connected nodes (Figure 2, d, left). Therefore,
the whole structure can be defined as a rare 5-connected
network with Schäfli symbol of (45.65) topology (Figure 2,
206 d, right).[16] This type of topology, to the best of our knowl-
Figure 3. (a) Perspective view of the asymmetric unit of 3. (b) Anti-
tetragonal prism geometry of Sr1 cation and tetragonal prism ge-
ometry of Sr2 cation. (c) Topological representation of the 3D
framework with an 8·8·102 net. (d) 3D network with the channels
filled by the HIDC2– dianion (green) and the N5-containing
H2IDC– monoanion (yellow).
edge, has rarely been documented in any other coordination in Figure S2, excluding the N5-containing H2IDC–
complexes.
monoanion, the HIDC2– dianion, and the Sr2 cation, the
residual N3-containing H2IDC– monoanions bridge adja-
cent SrII cations in a new μ3 (κ1N:κ1O:κ1O:κ1O) coordina-
tion mode to form a 3D framework. The topological analy- 226
sis of this network reveals that it is a rare topological net,
Structure of [Sr3(HIDC)2(H2IDC)4]n (3)
Another 3D network, namely complex 3, was obtained if the Sr1 cations and the N3-containing H2IDC–
211 by changing the inorganic alkali (NaOH) to triethylamine. monoanions are viewed as 3-connected nodes, and the Ver-
Single-crystal X-ray diffraction analysis reveals that the tex Symbol of this net is 8·8·102 (Figure 3, c).[16] The chan-
asymmetric unit of complex 3 contains two SrII cations, one nels of the above network are filled by the HIDC2– dianion 231
HIDC2– dianion, and two H2IDC– monoanions, in which and the N5-containing H2IDC– monoanion is in a different
the Sr2 cation is located in a special position (Figure 3, a). μ3 (κ1O:κ1O:κ1O:κ1O) coordination mode (Figure 3, d). It
216 Both of the two eight-coordinate SrII cations exhibit dis- should be noted that if the N3-containing H2IDC–
torted tetragonal prism geometry (Figure 3, b).
monoanions are omitted, the 3D network cannot be re-
Despite the HIDC2– dianion adopting the same coordi- tained by the other two anions without the assistance of 236
nation mode as observed in complex 2, the new coordina- Sr2 cations. Viewed along the c axis, the eight-coordinate
tion mode of the H2IDC– monoanion makes complex 3 Sr2 cations are embedded in the channels formed by the
221 construct a completely different 3D network. As illustrated three types of anions and Sr1 cations (Figure S3).
4
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Four Alkali-Induced 3D Strontium(II) Coordination Polymers
Structure of [Sr2(HIDC)2]n (4)
241
By replacing triethylamine with ethylenediamine, com-
plex 4 is obtained, which exhibits a 3D porous framework
with 1D hydrophobic channels. Single-crystal X-ray diffrac-
tion analysis reveals that complex 4 crystallizes in the space
group R3c. As shown in Figure 4 (a), the asymmetric unit
246 of complex 4 contains two SrII cations and two HIDC2–
dianions. The two SrII cations also exhibit different coordi-
nation spheres (Figure 4, b). The Sr1 cation is seven-coordi-
nated and displays monocapped trigonal prism geometry.
The Sr2 cation is nine-coordinate and the coordination
251 sphere can be best described as distorted tetrakaidecahed-
ron geometry.
In comparison with the HIDC2– dianion in the previous
three complexes, the two HIDC2– dianions in the present
complex exhibit two different types of coordination modes.
256 Part a of Figure 4 shows that the N1-containing HIDC2–
dianion adopts a new μ6 (κ1N:κ2O:κ2O:κ2O:κ2O) coordina-
tion mode, while the N3-containing HIDC2– dianion
adopts μ4 (κ1N:κ2O:κ2O:κ1O:κ1O) coordination mode.
With such complicated coordination modes, adjacent SrII
261 cations are bridged to form a 3D porous framework with
1D hydrophobic channels as illustrated in part c of Fig-
ure 4. It is interesting to note that no guest molecules are
encapsulated in the channels, owing to the fact that this
complex is obtained from a mixed water/ethanol solution,
266 and the rigidity of the H3IDC ligand is responsible for the
stability of the large void. PLATON[17] calculations indi-
cated that the resulting effective free volume is 12.7% of
the crystal volume (812.6 Å3 out of the 6407.3 Å3 unit cell
volume).
271
A close inspection reveals that this hydrophobic channel
possesses a trigonal nanoporous A with maximum window
dimensions (the distances between the vertex of Sr2) of
13.9ϫ 13.9ϫ 13.9 Å3 (Figure 4, d, Figure S2†). The chan-
nel walls are composed of HIDC2– dianions linked Sr1 tape.
276 The thickness of 3.398(5) Å between the opposite imidazole
rings indicates the existence of strong π–π stacking interac-
tions that further strengthen the porous structure. More-
over, the assembly of the hydrophobic channels leads to the
formation of another small trigonal channel B with maxi-
281 mum window dimensions (the distances between the vertex
of N1) of 5.3ϫ5.3ϫ5.3 Å3 (Figure S4).
Alkali-Induced Structure Diversities
Since complexes 1 to 4 were obtained from the same re-
actants, namely Sr(NO3)2 and H3IDC, under similar hydro-
286 thermal conditions (reaction temperature and time) but
with a change of alkali and ligand to alkali ratio
(Scheme 2), undoubtedly, the types and amounts of the al-
kali play an important role in controlling the structure of
Figure 4. (a) Perspective view of the asymmetric unit of 4. (b) Mon-
ocapped trigonal prism geometry of Sr1 cation and tetrakaide-
cahedron geometry of Sr2 cation. (c) 3D porous framework with
1D hydrophobic channels. (d) Side view (left) and front view (right)
of the trigonal channels.
the complexes. Based on the structures of the four com- of 1:1 and 1:2, two 3D networks were obtained with dif-
291 plexes, the influence of the alkali on the structure is its ef- ferent topology of eca (complex 1) and (45.65) (complex 2), 296
fects on the degree of deprotonation of H3IDC, which then in which complex 1 contains H3IDC molecules and HIDC2–
generates different anions and hence results in diverse com- dianions whereas complex 2 only contains HIDC2– di-
plexes with new architectures. By using NaOH in the ratio anions. By changing the inorganic alkali to an organic base,
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such as triethylamine and ethylenediamine, another two H3IDC. In comparison with the free H3IDC, the absorption
301 new 3D complexes were obtained. Complex 3 contains bands at 1695 cm–1 in 1 and 1698 cm–1 in 2 can be assigned 356
HIDC2– dianions and H2IDC– monoanions while complex to the C=O stretching vibrations, which indicates that some
4 only contains HIDC2– dianions. Complex 3 is a compli- of the carboxyl groups have not been completely deproton-
cated 3D network that can be regarded as a H2IDC– ated. The absorption bands at 1624, 1589, and 1378 cm–1
monoanion sustained porous network filled by another for 1, 1590 and 1374 cm–1 for 2, 1625, 1591, and 1376 cm–1
306 HIDC2– dianion and H2IDC– monoanion. Interestingly, for 3, 1592 and 1384 cm–1 for 4 correspond to asymmetric 361
complex 4 exhibits a 3D porous framework with 1D hydro- stretching and symmetric stretching of carboxyl groups,
phobic channels. From the above description, in compari- respectively. The sharp bands at 3177 cm–1 in 1, 3156 cm–1
son with the inorganic alkali, the organic alkali not only in 2, 3179 cm–1 in 3, and 3129 cm–1 in 4 are attributed to
results in the deprotonation of the carboxylic groups but the N–H stretching vibrations, which demonstrates that the
311 also acts as a template in the reaction process to induce hydrogen atoms of the imidazole ring are not deprotonated 366
porous structures. In summary, different alkali and ligand under the present pH value. The broad bands centered at
to alkali ratios play important roles in the structural diver- 3346 cm–1 were assigned to the O–H stretching vibrations
sity and evolution of these complexes.
of water molecules and hydroxy groups, as well as the for-
mation of extensive hydrogen bonding interactions.
Coordination Modes of HxIDC(3–x)– (x = 1–3) and
316 Coordination Spheres of SrII Cation
Thermal Stability
371
As illustrated in the introduction, H3IDC has been
widely used to construct metal–organic complexes. Owing
to the employment of different types and amounts of alkali,
H3IDC can be partially or fully deprotonated to generate
321 H2IDC–, HIDC2–, and IDC3–, thus providing versatile co-
ordination modes. Besides the reported μ3:η4 and μ4:η7 in
complexes 1, 3, and 4, it should be noted that five new
coordination modes for the H3IDC molecule, H2IDC–
monoanion, and HIDC2– dianion are reported for the first
326 time as described above (Scheme 1).[13,14] As a result of
these new and intricate coordination modes, four new 3D
SrII-based coordination polymers were obtained. Another
reason to form high-dimensional architectures is the large
radius and high coordination numbers of the SrII cation,
331 which increases the metal–ligand cross-linking required to
form an extended structure. The SrII cations in these four
complexes are all coordinated by five independent
HxIDC(3–x)– (x = 1–3) species and present high coordina-
tion numbers (7–9) and versatile coordination spheres. In
336 complexes 1 and 2, the SrII cations exhibit single coordina-
tion spheres of anti-tetragonal prism and monocapped tri-
gonal prism geometry with coordination numbers of 8 and
7, respectively. In contrast, the SrII cations in complexes 3
and 4 exhibit two types of coordination spheres. The two
341 SrII cations in complex 3 are both eight-coordinate and
present anti-tetragonal and tetragonal prism geometry,
whereas the two SrII cations in complex 4 are seven- and
nine-coordinate and present monocapped trigonal prism
and tetrakaidecahedron geometry. These different coordi-
346 nation spheres result in different connections, thus generat-
ing various architectures. Based on the afore-mentioned two
points, it is reasonable to obtain these four new metal–or-
ganic networks through the rational choice of SrII cation
and the adjustment of the HxIDC(3–x)– (x = 1–3) species
351 with different types and amounts of alkali.
The thermal stability of the four complexes was analyzed
on crystalline samples by thermogravimetric analyses
(TGA) from 30 to 800 °C at a rate of 10 °C min–1 under a
N2 atmosphere. As shown in Figure 5, the TGA curve indi-
cates that complex 1 loses the lattice and coordinated water 376
molecules between 116 and 229 °C with an observed weight
loss of 8.35% (calcd. 8.30%). The next weight loss in the
temperature range of 229 to 585 °C is due to the decompo-
sition of organic components. The residue has a composi-
tion of SrCO3 (obsd. 34.13%, calcd. 34.03%). Complex 2 381
loses the organic components from 420 to 577 °C with a
sharp weight loss step. The remaining weight of 61.48%
corresponds to the percentage (calcd. 61.08%) of SrCO3.
Complex 3 also loses the organic components with a nearly
sharp weight loss from 310 to 564 °C. The residue has a 386
composition of SrCO3 (obsd. 37.45%, calcd. 37.17%).
Complex 4 exhibits a similar weight loss to complex 2 where
the loss of organic components occurs from 353 to 610 °C,
and the remaining weight of 60.95% corresponds to the
percentage (calcd. 61.08%) of SrCO3. Moreover, in order 391
to study the stability of the porous structure, temperature-
dependent PXRD spectra of complex 4 were measured to
IR Spectra
The infrared spectra of the four complexes were recorded
to confirm the deprotonation of the carboxyl groups of Figure 5. TG curves of the four complexes.
6
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Four Alkali-Induced 3D Strontium(II) Coordination Polymers
verify the stability of the framework. As shown in Figure 6,
complex 4 retains its framework up to 350 °C. When the
396 temperature rises to 400 °C, most of the diffraction peaks
disappear except for the peaks at the 2θ of 6.36, 11.04,
12.75, 13.48, 22.63, 29.46, and 41.33°. Meanwhile, the ap-
pearance of characteristic diffraction peaks of (111), (112),
(132), and (113) indicates that SrCO3 is formed. Therefore,
401 it can be concluded that the porous framework is partially
collapsed at the temperature of 400 °C.
Figure 7. Emission spectra of the four complexes and ligand
H3IDC in the solid-state at room temperature.
net with the Schläfli symbol of (39.412.57) while complex 2 426
presents a rare 5-connected network with Schäfli symbol of
(45.65). Complex 3 is a dense 3D network and complex 4
exhibits a 3D porous framework with 1D trigonal hydro-
phobic channels. The structural evolutions depend mainly
on the coordination spheres of the SrII cation, the coordina- 431
tion modes of the anions, the different alkali, and the ligand
to alkali ratio. In addition, five new coordination modes
for the H3IDC molecule, H2IDC– monoanion, and HIDC2–
dianion were reported for the first time. The TG curve and
temperature-dependent PXRD spectra of complex 4 indi- 436
cate that its framework is stable up to 350 °C. Moreover,
complexes 1 to 4 exhibit strong blue emission at room tem-
perature. This study presents a good example that the new
topological architectures can be constructed from metal cat-
ions with large radii and N-heterocyclic carboxylates with 441
versatile coordination modes and variable valences that are
modulated by the different types and amounts of alkali.
Further studies on the synthesis, structure, and luminescent
properties of alkaline earth metal complexes that contain
other N-heterocyclic molecules are underway in our labora- 446
tory.
Figure 6. Temperature-dependent PXRD patterns of complex 4 un-
der N2 atmosphere.
Luminescent Property
The luminescent properties of complexes 1 to 4 and the
free ligand in the solid-state at room temperature were in-
406 vestigated. As shown in Figure 7, upon excitation at
391 nm, free ligand H3IDC exhibits emission maximum at
467 nm, which can probably be attributed to the π*–π tran-
sitions. In contrast to the emission of the free ligand
H3IDC, the strong blue fluorescent emission band at
411 460 nm for complex 1, 458 nm for complex 2, 468 nm for
complex 3, and 467 nm for complex 4 can probably be as-
signed to the intraligand (IL) π–π* transitions because of
the resemblance of the emission spectra to that of the free
ligands (Figure 7).[18] The high-dimensional polymeric
416 structures of the four complexes leads to significant en-
hancement of fluorescence intensity compared to that of
the free ligand.[19] Moreover, the emission intensity of com-
plex 1 is evidently stronger than those of complexes 2 to 4,
which can be attributed to the existence of water molecules
421 and the formation of the hydrogen bonding interactions.
Experimental Section
Materials and Methods: All of the chemicals and solvents were AR
grade and were used without further purification in the syntheses.
Elemental analyses were carried out with a Vario MICRO from
Elementar Analysensysteme GmbH and the infrared spectra (IR)
were recorded from KBr pellets in the range of 4000 to 400 cm–1
with a Bruker Equinox 55 FTIR spectrometer. Powder X-ray dif-
fraction (PXRD) patterns for the three complexes were measured
at 293 K with a Bruker D8 diffractometer (Cu-Kα, λ = 1.54059 Å).
Thermogravimetric analysis (TGA) was performed with a Perkin–
Elmer TG/DTA 6300 thermal analyzer under flowing N2 with a
heating rate of 10 °C min–1. Luminescence spectra were measured
with a Perkin–Elmer LS 55 luminance spectrometer.
451
456
Conclusions
[Sr(HIDC)(H3IDC)(H2O)]n·nH2O (1): A mixture of Sr(NO3)2·
4H2O (211 mg, 1 mmol), H3IDC (156 mg, 1 mmol), NaOH (40 mg,
1 mmol), and deionized water (10 mL) was sealed in a 25 mL Tef-
461
In summary, four new 3D metal coordination polymers
constructed from SrII and H3IDC were synthesized and ex-
hibit diverse topological architectures. Complex 1 is an eca lon-lined stainless steel vessel (pH ≈ 5) and heated at 140 °C for
Eur. J. Inorg. Chem. 0000, 0–0
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X.-F. Zhang, Z.-P. Deng, L.-H. Huo, Q.-M. Feng, S. Gao
FULL PAPER
3 days. After the solution was slowly cooled to room temperature,
colorless crystals of complex 1 that were suitable for X-ray diffrac-
tion were obtained; yield 78% (based on Sr). C10H10N4O10Sr
(433.83): calcd. C 27.69, H 2.32, N 12.91; found C 27.64, H 2.36,
refinement parameters of complexes 1–4, as well as the selected
bond distances for complexes 1–4.
526
466
471
Acknowledgments
N 12.87. IR (KBr): ν = 3346 (m), 3177 (m), 1695 (m), 1624 (s),
˜
1589 (s), 1551 (m), 1482 (s), 1454 (m), 1378 (s), 1252 (m), 1072 (m),
This work was financially supported by the Key Project of the Nat-
ural Science Foundation of Heilongjiang Province (grant number
ZD200903), the Education Bureau of Heilongjiang Province (grant
numbers 12511z023 and 2011CJHB006), the Innovation Team of
the Education Bureau of Heilongjiang Province (grant number
2010td03), the Ministry of Education (Program for New Century
Excellent Talents in University, grant number NCET-06-0349), and
the National Scientific Foundation of China (NSFC, grant number
61271126). We thank the University of Heilongjiang (grant number
Hdtd2010-04) for supporting this study.
958 (m), 929 (m), 870 (m), 786 (m), 631 (m), 520 (m) cm–1.
[Sr(HIDC)]n (2): A similar procedure to that used for complex 1
was employed to prepare complex 2 by changing the amount of
NaOH to 2 mmol (pH ≈ 6). Colorless crystals of complex 2 that
were suitable for X-ray diffraction were isolated from the solution
after it was slowly cooled to room temperature; yield 82% (based
on Sr). C5H2N2O4Sr (241.70): calcd. C 24.85, H 0.83, N 11.59;
531
536
476
found C 24.89, H 0.78, N 11.62. IR (KBr): ν = 3156 (m), 1590 (s),
˜
1562 (s), 1490 (s), 1452 (s), 1374 (s), 1228 (m), 1081 (m), 954 (m),
875 (m), 787 (m), 622 (m), 526 (m) cm–1.
481
486
491
[Sr3(HIDC)2(H2IDC)4]n (3): A mixture of Sr(NO3)2·4H2O (211 mg,
1 mmol), H3IDC (156 mg, 1 mmol), triethylamine (0.1 mL), EtOH
(4 mL), and deionized water (8 mL) was sealed in a 25 mL Teflon-
lined stainless steel vessel (pH ≈ 5) and heated at 140 °C for 3 days.
After the solution was slowly cooled to room temperature, colorless
crystals of complex 3 that were suitable for X-ray diffraction were
obtained; yield 76% (based on Sr). C30H16N12O24Sr3 (1191.38):
calcd. C 30.24, H 1.35, N 14.11; found C 30.20, H 1.41, N 14.15.
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X-ray Crystallographic Measurements
Table S1 provides a summary of the crystal data, data collection,
and refinement parameters for the complexes 1 to 4. All of the
diffraction data were collected at 295 K with a RIGAKU RAXIS-
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gen atoms attached to carbon were placed in calculated positions
with C–H = 0.93 Å and U(H) = 1.2Ueq (C) in the riding model
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506
511
516
CCDC-861988 (for 1), -808003 (for 2), -861989 (for 3), and -861990
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Supporting Information (see footnote on the first page of this arti-
cle): Additional figures, PXRD patterns, crystal data and structure
8
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Four Alkali-Induced 3D Strontium(II) Coordination Polymers
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Published Online: ᭿
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X.-F. Zhang, Z.-P. Deng, L.-H. Huo, Q.-M. Feng, S. Gao
FULL PAPER
Strontium Coordination Polymer
631
636
Self-assembly of strontium and imidazole-
4,5-dicarboxylic acid leads to the formation
of four new 3D coordination polymers that
exhibit high thermal stability and blue em-
issions in the solid state at room tempera-
ture.
X.-F. Zhang, Z.-P. Deng, L.-H. Huo,*
Q.-M. Feng, S. Gao* ...................... 1–10
Four Alkali-Induced 3D Strontium(II)
Coordination Polymers Constructed from
Imidazole-4,5-dicarboxylate:
Syntheses,
Crystal Structures, and Properties
Keywords: Strontium
/
Coordination
641
modes / Carboxylate ligands / Thermal
stability / Luminescence
10
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