D10-Metal coordination polymers based on analogue di(pyridyl)imidazole derivatives and 4,4′-oxydibenzoic acid: Influence of flexible and angular characters of neutral ligands on structural diversity

Article abstract of DOI:10.1039/b809336a

A series of mixed-ligand coordination complexes, namely [Zn(L ¹)(oba)] (1), [Cd(L¹)(oba)] (2), [Zn₂(L ²)(oba)₂]·8H₂O (3), [Cd ₂(L²)(oba)₂]·2H₂O (4), [Zn₃(L³)(oba)₃] (5), [Cd₂(L ³)(oba)₂]·(L³) (6), [Cd(L ⁴)(oba)]·H₂O (7) and [Cd(L⁵)(oba)] ·3H₂O (8), where L¹ = 2-(2-pyridyl)imidazole, L² = 1,4-bis[2-(2-pyridyl)imidazol-1-yl]butane, L³ = 1,4-bis[2-(2-pyridyl)imidazol-1-ylmethyl]benzene, L⁴ = 1,3-bis[2-(2-pyridyl)imidazol-1-ylmethyl]benzene, L⁵ = 1,2-bis[2-(2-pyridyl)imidazol-1-ylmethyl]benzene and H₂oba = 4,4′-oxydibenzoic acid, have been synthesized under hydrothermal conditions. Their structures have been determined by single crystal X-ray diffraction analyses and further characterized by elemental analyses, IR spectra, and thermogravimetric (TG) analyses. In compounds 1 and 2, oba ^2-, L¹ ligand and Zn^II or Cd^II ions assemble to form the parallel chains or parallel sheets which are linked by the weak hydrogen bonding and π...π stacking interactions to give the 2D supramolecular sheet or 3D supramolecular net, respectively. For 3, L ² ligands connect [Zn(oba)] chains to generate a unusual (10,3)-b topological structure which is the first example for eight-fold interpenetrating framework based on the (10,3)-b net. In 4, L² ligands link [Cd(oba)] double-chains to give a 2D sheet which is assembled by π...π stacking interactions to obtain a 3D supramolecular net. In 5, L³ ligands link Zn^II ions from α-Po net formed by Zn^II ions and oba^2- anions to show a novel 3D 8-connected self-penetrating framework with the unreported (4¹⁶·6¹¹·8) topological structure. In 6, the double-chains constructed by Cd^II and oba^2- anions are linked by one kind of L³ ligand to form a layer-like structure which is assembled by π...π stacking interactions to show a 3D supramolecular structure. In 7, oba^2- anions coordinate to Cd^II cations to form chains which are connected by L⁴ to form a four-fold interpenetrating diamond network. In 8, the weak hydrogen bonding and π...π stacking interactions connect the [Cd(L⁵)(oba)] chains to give a 2D supramolecular sheet. By careful inspections of the structures of 1-8, we believe that the different flexible and angular neutral ligands, coordination geometries of metal centers and weak interactions (hydrogen bonds and π...π stacking interactions) are crucial factors for the formation of the different structures. The photoluminescent properties of 1-8 have been studied in the solid state at room temperature.

Full text of DOI:10.1039/b809336a

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PAPER
www.rsc.org/dalton | Dalton Transactions
d10-Metal coordination polymers based on analogue di(pyridyl)imidazole
derivatives and 4,4¢-oxydibenzoic acid: influence of flexible and angular
characters of neutral ligands on structural diversity†
Ya-Qian Lan, Shun-Li Li, Yao-Mei Fu, Yan-Hong Xu, Lu Li, Zhong-Min Su* and Qiang Fu*
Received 3rd June 2008, Accepted 28th July 2008
First published as an Advance Article on the web 22nd October 2008
DOI: 10.1039/b809336a
A series of mixed-ligand coordination complexes, namely [Zn(L1)(oba)] (1), [Cd(L1)(oba)] (2),
[Zn2(L2)(oba)2]·8H2O (3), [Cd2(L2)(oba)2]·2H2O (4), [Zn3(L3)(oba)3] (5), [Cd2(L3)(oba)2]·(L3) (6),
[Cd(L4)(oba)]·H2O (7) and [Cd(L5)(oba)]·3H2O (8), where L1 = 2-(2-pyridyl)imidazole,
L2 = 1,4-bis[2-(2-pyridyl)imidazol-1-yl]butane, L3 = 1,4-bis[2-(2-pyridyl)imidazol-1-ylmethyl]benzene,
L4 = 1,3-bis[2-(2-pyridyl)imidazol-1-ylmethyl]benzene, L5 = 1,2-bis[2-(2-pyridyl)imidazol-1-
ylmethyl]benzene and H2oba = 4,4¢-oxydibenzoic acid, have been synthesized under hydrothermal
conditions. Their structures have been determined by single crystal X-ray diffraction analyses and
further characterized by elemental analyses, IR spectra, and thermogravimetric (TG) analyses. In
compounds 1 and 2, oba2-, L1 ligand and ZnII or CdII ions assemble to form the parallel chains or
parallel sheets which are linked by the weak hydrogen bonding and p ◊ ◊ ◊ p stacking interactions to give
the 2D supramolecular sheet or 3D supramolecular net, respectively. For 3, L2 ligands connect
[Zn(oba)] chains to generate a unusual (10,3)-b topological structure which is the first example for
eight-fold interpenetrating framework based on the (10,3)-b net. In 4, L2 ligands link [Cd(oba)]
double-chains to give a 2D sheet which is assembled by p ◊ ◊ ◊ p stacking interactions to obtain a 3D
supramolecular net. In 5, L3 ligands link ZnII ions from a-Po net formed by ZnII ions and oba2- anions
to show a novel 3D 8-connected self-penetrating framework with the unreported (416·611·8) topological
structure. In 6, the double-chains constructed by CdII and oba2- anions are linked by one kind of L3
ligand to form a layer-like structure which is assembled by p ◊ ◊ ◊ p stacking interactions to show a 3D
supramolecular structure. In 7, oba2- anions coordinate to CdII cations to form chains which are
connected by L4 to form a four-fold interpenetrating diamond network. In 8, the weak hydrogen
bonding and p ◊ ◊ ◊ p stacking interactions connect the [Cd(L5)(oba)] chains to give a 2D supramolecular
sheet. By careful inspections of the structures of 1–8, we believe that the different flexible and angular
neutral ligands, coordination geometries of metal centers and weak interactions (hydrogen bonds and
p ◊ ◊ ◊ p stacking interactions) are crucial factors for the formation of the different structures. The
photoluminescent properties of 1–8 have been studied in the solid state at room temperature.
organic anionic ligands not only due to their various coordination
Introduction
modes to metal ions, resulting from completely or partially
deprotonated sites allowing for the large diversity of topologies,4
but also because of their abilities to act as H-bond acceptors and
donors to assemble various supramolecular structures.5
Metal–organic frameworks (MOFs) are rapidly increasing not
only because of their tremendous potential applications in func-
tional materials, nanotechnology and biological recognition,1 but
also because of their intriguing variety of architectures and
topologies.2 One of the obvious challenges for construction of
novel coordination polymers is the rational and controllable prepa-
ration of metal–organic frameworks, which is greatly affected by
the ligand nature, anions, template, metal ions and other factors.3
Among them, multi-carboxylate ligands act as multifunctional
Among aromatic multi-carboxylates, V-shaped aromatic dicar-
boxylate H2oba (4,4¢-oxydibenzoic acid)6 is introduced because
it has irregular orientations when it coordinates to metals, which
may produce various structural topologies, large pores or beautiful
helical structures, owing to its bidentate carboxylate arms and
its steric bulk. The use of H2oba as a bridging ligand in con-
structing high-dimensional structures is relatively mature. There-
fore, the resulting topological structures can be predicted and
designed.
In addition, a careful selection of the properties of the
second ligands, such as shape, functionality, flexibility, angle
and symmetry, is a key step for the rational design of struc-
tures and specific chemical and physical properties.7 In previous
studies,8 we focused our attention on the construction of metal
Institute of Functional Material Chemistry; Key Lab of Polyoxometa-
late Science of Ministry of Education, Faculty of Chemistry, Northeast
Normal University, Changchun, 130024, China. E-mail: zmsu@nenu.edu.
cn, fuq836@nenu.edu.cn; Tel: +86 431 85099108
† Electronic supplementary information (ESI) available: Additional fig-
ures, TGA of 1–8 and luminescent spectrum of L1 and L2. CCDC reference
numbers 687548–687555 for 1–8. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/b809336a
6796 | Dalton Trans., 2008, 6796–6807
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©

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Chart 1 Structures of the ligands L1–L5.
organic frameworks with metal salts and flexible bis(imidazol) lig-
ands, for example, 1,4-bis(imidazol-1-yl)butane, 1,2-bis(imidazol-
1-ylmethyl)benzene, 1,3-bis(imidazol-1-ylmethyl)benzene and so
on. In comparison with the above-mentioned ligands, 2-(2-
pyridyl)imidazole (L1) may tune the frameworks of the coordina-
tion polymers through non-covalent interactions such as p ◊ ◊ ◊ p
stacking and hydrogen bonding interactions. In addition, four
different flexible and angular exo-bis(bidentate) ligands (L2–L5;
Chart 1) with various conformations and coordination modes are
engaged in this system.
Well-known d10 metal ions, such as ZnII and CdII, have
attracted considerable attention not only because of their ap-
pealing structures but also because of their good luminescent
properties.9 Here, we will describe a series of luminescent
Zn(II) and Cd(II) coordination complexes with diverse network
structures. Eight mixed-ligand coordination complexes, namely
[Zn(L1)(oba)] (1), [Cd(L1)(oba)] (2), [Zn2(L2)(oba)2]·8H2O (3),
[Cd2(L2)(oba)2]·2H2O (4), [Zn3(L3)(oba)3] (5), [Cd2(L3)(oba)2]·(L3)
(6), [Cd(L4)(oba)]·H2O (7) and [Cd(L5)(oba)]·3H2O (8), have been
synthesized under hydrothermal conditions. Their structures have
been determined by single crystal X-ray diffraction analyses
and further characterized by elemental analyses, IR spectra,
and thermogravimetric analyses. The effects of neutral ligands,
coordination geometries of metal centers and weak interactions
(hydrogen bonds and p ◊ ◊ ◊ p stacking interactions) for the for-
mation of coordination complexes were elucidated. Thermal
and photoluminescent properties for the compounds were also
investigated.
steel container. The container was heated to 150 ◦C and held◦at that
temperature for 72 h, then cooled to 100 ◦C at a rate of 5 C h-1,
and held for 8 h, followed by further cooling to 30 ◦C at a rate of
3 ◦C h-1. Colorless crystals of 1 were collected in 80% yield based
on Zn(OAc)2·2H2O. Anal. Calcd for C22H15N3O5Zn (466.74): C,
56.61; H, 3.24; N, 9.00. Found: C, 56.49; H, 3.34; N, 9.01%. IR
(cm-1): 3122 (w), 1600 (s), 1546 (s), 1499 (m), 1474 (m), 1409 (s),
1354 (s), 1296 (w), 1235 (s), 1156 (m), 875 (m), 846 (w), 777 (m),
695 (w).
Synthesis of [Cd(L1)(oba)] (2). A mixture of 4,4¢-oxydibenzoic
acid (0.16 g, 0.50 mmol), L1 (0.07 g, 0.50 mmol), Cd(OAc)2·2H2O
(0.13 g, 0.50 mmol), NaOH (0.04 g, 1.00 mmol) and H2O (10 mL)
was stirred for 1 h and then sealed in a 25 mL Teflon-lined stainless
steel container. The container was heated to 150 ◦C and held◦at that
temperature for 72 h, then cooled to 100 ◦C at a rate of 5 C h-1,
and held for 8 h, followed by further cooling to 30 ◦C at a rate of
3 ◦C·h-1. Colorless crystals of 2 were collected in 81% yield based
on Cd(OAc)2·2H2O. Anal. Calcd for C22H15CdN3O5 (513.77): C,
51.43; H, 2.94; N, 8.18. Found: C, 51.51; H, 3.00; N, 8.20%. IR
(cm-1): 3070 (w), 1683 (s), 1594 (s), 1500 (m), 1422 (s), 1293 (m),
1245 (s), 1158 (s), 1104 (m), 1009 (w), 936 (w), 859 (m), 770 (m),
695 (w), 648 (w).
Synthesis of [Zn2(L2)(oba)2]·8H2O (3). A mixture of 4,4¢-
oxydibenzoic acid (0.16 g, 0.50 mmol), L2 (0.17 g, 0.50 mmol),
Zn(OAc)2·2H2O (0.11 g, 0.50 mmol), NaOH (0.04 g, 1.0 mmol)
and H2O (10 mL) was stirred for 1 h and then sealed in a 25 mL
Teflon-lined stainless steel container. The container was heated
◦
to 150 C and held at that temperature for 72 h, then cooled to
100 ◦C at a rate of 5 ◦C h-1, and held for 8 h, followed by further
cooling to 30 ◦C at a rate of 3 ◦C h-1. Colorless crystals of 3 were
collected in 70% yield based on Zn(OAc)2·2H2O. Anal. Calcd for
C48H52N6O18Zn2 (1131.70): C, 50.94; H, 4.63; N, 7.42. Found: C,
51.01; H, 4.68; N, 7.39%. IR (cm-1): 3394 (m), 1680 (s), 1597 (s),
1548 (m), 1499 (m), 1414 (m), 1295 (m), 1241 (s), 1159 (s), 1103
(w), 878 (w), 774 (m), 694 (w), 658 (w).
Experimental section
General procedures
Chemicals were purchased from commercial sources and used
without further purification. Ligands 2-(2-pyridyl)imidazole
1,4-bis[2-(2-pyridyl)imidazol-1-yl]butane, 1,4-bis[2-(2-pyridyl)-
imidazol-1-ylmethyl]benzene, 1,3-bis[2-(2-pyridyl)imidazol-1-yl-
methyl]benzene, 1,2-bis[2-(2-pyridyl)imidazol-1-ylmethyl]benzene
were prepared according to the literature.10
Synthesis of [Cd2(L2)(oba)2]·2H2O (4). A mixture of 4,4¢-
oxydibenzoic acid (0.16 g, 0.50 mmol), L2 (0.17 g, 0.50 mmol),
Cd(OAc)2·2H2O (0.13 g, 0.50 mmol), NaOH (0.04 g, 1.0 mmol)
and H2O (10 mL) was stirred for 1 h and then sealed in a 25 mL
Teflon-lined stainless steel container. The container was heated
Synthesis of [Zn(L1)(oba)] (1). A mixture of 4,4¢-oxydibenzoic
acid (0.16 g, 0.50 mmol), L1 (0.07 g, 0.50 mmol), Zn(OAc)2·2H2O
(0.11 g, 0.50 mmol), NaOH (0.04 g, 1.00 mmol) and H2O (10 mL)
was stirred for 1 h and then sealed in a 25 mL Teflon-lined stainless
◦
to 150 C and held at that temperature for 72 h, then cooled to
100 ◦C at a rate of 5 ◦C h-1, and held for 8 h, followed by further
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cooling to 30 ◦C at a rate of 3 ◦C h-1. Colorless crystals of 4 were
collected in 58% yield based on Cd(OAc)2·2H2O. Anal. Calcd for
C48H40Cd2N6O12 (1117.66): C, 51.58; H, 3.61; N, 7.52. Found: C,
51.61; H, 3.69; N, 7.48%. IR (cm-1): 3050 (w), 1682 (m), 1596 (s),
1547 (s), 1497 (s), 1468 (m), 1372 (s), 1228 (s), 1160 (m), 876 (w),
779 (m), 690 (w), 656 (w).
(m), 1496 (m), 1427 (s), 1310 (s), 1249 (s), 1157 (s), 1102 (w), 932
(m), 858 (w), 765 (s), 700 (m), 544 (w).
Physical measurements. The C, H, and N elemental analysis
was conducted on a Perkin–Elmer 240C elemental analyzer. The
FT-IR spectra were recorded from KBr pellets in the range 4000–
400 cm-1 on a Mattson Alpha-Centauri spectrometer. TGA was
performed on a Perkin–Elmer TG-7 analyzer heated from 35
to 800◦C under nitrogen. The emission/excitation spectra were
recorded on a Varian Cary Eclipse spectrometer.
Synthesis of [Zn3(L3)(oba)3] (5).
A
mixture of 4,4¢-
oxydibenzoic acid (0.16 g, 0.50 mmol), L3 (0.20 g, 0.50 mmol),
Zn(OAc)2·2H2O (0.11 g, 0.50 mmol), NaOH (0.04 g, 1.0 mmol)
and H2O (10 mL) was stirred for 1 h and then sealed in a 25 mL
Teflon-lined stainless steel container. The container was heated
X-Ray crystallography
◦
to 150 C and held at that temperature for 72 h, then cooled to
100 ◦C at a rate of 5 ◦C h-1, and held for 8 h, followed by further
cooling to 30 ◦C at a rate of 3 ◦C h-1. Colorless crystals of 5 were
collected in 56% yield based on Zn(OAc)2·2H2O. Anal. Calcd for
C66H44N6O15Zn3 (1357.18): C, 58.40; H, 3.27; N, 6.19. Found: C,
58.40; H, 3.29; N, 6.22%. IR (cm-1): 3046 (w), 1683 (s), 1592 (s),
1501 (m), 1423 (m), 1292 (s), 1249 (s), 1158 (s), 1104 (m), 935 (w),
858 (m), 768 (w), 544 (w).
Single-crystal X-ray diffraction data for compounds 1–8 were
recorded on a Bruker Apex CCD diffractometer with graphite-
˚
monochromated MoKa radiation (l = 0.71073 A) at 293K. Ab-
sorption corrections were applied using multi-scan technique.
All the structures were solved by Direct Method of SHELXS-
9711 and refined by full-matrix least-squares techniques using
the SHELXL-97 program12 within WINGX.13 Non-hydrogen
atoms were refined with anisotropic temperature parameters. The
hydrogen atoms of the organic ligands were refined as rigid groups.
The hydrogen atoms of the O3w molecule in 8 could not be
located, but have been included in the formula. And other H
atoms of water molecules were located from difference Fourier
maps. The disordered 2-(2-pyridyl)imidazole group in compound
1 was refined using C and N atoms split over two equivalent sites,
with a total occupancy of 1. The detailed crystallographic data
and structure refinement parameters for 1–8 are summarized in
Table 1.
Synthesis of [Cd2(L3)(oba)2]·(L3) (6). A mixture of 4,4¢-
oxydibenzoic acid (0.16 g, 0.50 mmol), L3 (0.20 g, 0.50 mmol),
Cd(OAc)2·2H2O (0.13 g, 0.50 mmol), NaOH (0.04 g, 1.0 mmol)
and H2O (10 mL) was stirred for 1 h and then sealed in a 25 mL
Teflon-lined stainless steel container. The container was heated
◦
to 150 C and held at that temperature for 72 h, then cooled to
100 ◦C at a rate of 5 ◦C h-1, and held for 8 h, followed by further
cooling to 30 ◦C at a rate of 3 ◦C h-1. Colorless crystals of 6 were
collected in 52% yield based on Cd(OAc)2·2H2O. Anal. Calcd for
C76H56Cd2N12O10 (1522.13): C, 59.67; H, 3.71; N, 11.04. Found: C,
59.69; H, 3.68; N, 11.00%. IR (cm-1): 3067 (w), 1682 (w), 1596 (s),
1546 (s), 1479 (m), 1401 (s), 1291 (m), 1247 (s), 1156 (s), 1096 (m),
1004 (w), 934 (w), 876 (w), 782 (m), 737 (m), 692 (m), 655 (m).
Results
Structures
Synthesis of [Cd(L4)(oba)]·H2O (7). A mixture of 4,4¢-
oxydibenzoic acid (0.16 g, 0.50 mmol), L4 (0.20 g, 0.50 mmol),
Cd(OAc)2·2H2O (0.13 g, 0.50 mmol), NaOH (0.04 g, 1.0 mmol)
and H2O (10 mL) was stirred for 1 h and then sealed in a 25 mL
Teflon-lined stainless steel container. The container was heated
Structure description of 1. The asymmetric unit consists of one
kind of ZnII cation, oba2- anion and L1 ligand (Fig. 1a). Each ZnII is
in a distorted octahedral coordination environment, in which four
oxygen atoms (Zn(1)–O(1) = 2.078(3), Zn(1)–O(2) = 2.299(4),
˚
◦
Zn(1)–O(4)#1 = 2.103(3) and Zn(1)–O(5)#1 = 2.216(4) A) from
to 150 C and held at that temperature for 72 h, then cooled to
two oba2- anions and two nitrogen atoms (Zn(1)–N(2)= 2.119(4)
100 ◦C at a rate of 5 ◦C h-1, and held for 8 h, followed by further
cooling to 30 ◦C at a rate of 3 ◦C h-1. Colorless crystals of 7 were
collected in 43% yield based on Cd(OAc)2·2H2O. Anal. Calcd for
C38H30CdN6O6 (779.08): C, 58.58; H, 3.88; N, 10.79. Found: C,
58.61; H, 3.84; N, 10.82%. IR (cm-1): 3057 (w), 1683 (s), 1594 (s),
1499 (m), 1424 (s), 1292 (s), 1249 (s), 1157 (m), 1103 (m), 935 (w),
859 (m), 768 (m), 692 (w), 543 (w).
1
˚
and Zn(1)–N(3) = 2.114(3) A) from one L ligand are located at
the six vertexes. The Zn–O and Zn–N bond lengths are all within
the normal ranges.14 Each oba2- anion coordinates to two ZnII
cations by a bis(bidentate) coordination mode (Chart S1a) to give
1
˚
a sinusoidal chain with a period of 25.228 A (Fig. 1b). The L
chelates to each ZnII cation as a terminal ligand with the dihedral
angle of 1.6◦ between pyridine and imidazole rings in the same
ligand (Fig. S1).
Synthesis of [Cd(L5)(oba)]·3H2O (8). A mixture of 4,4¢-
oxydibenzoic acid (0.16 g, 0.50 mmol), L5 (0.20 g, 0.50 mmol),
Cd(OAc)2·2H2O (0.13 g, 0.50 mmol), NaOH (0.04 g, 1.0 mmol)
and H2O (10 mL) was stirred for 1 h and then sealed in a 25 mL
Teflon-lined stainless steel container. The container was heated
Interestingly, there are weak p ◊ ◊ ◊ p and hydrogen bonding
interactions in the structure. The p ◊ ◊ ◊ p stacking interactions15
show between pyridine and imidazole rings from adjacent layers,
with the plane-to-plane distance of 3.46 A and the centroid–
centroid distance of 4.00 A. And each L donates one hy-
˚
◦
1
˚
to 150 C and held at that temperature for 72 h, then cooled to
100 ◦C at a rate of 5 ◦C h-1, and held for 8 h, followed by further
cooling to 30 ◦C at a rate of 3 ◦C h-1. Colorless crystals of 8 were
collected in 73% yield based on Cd(OAc)2·2H2O. Anal. Calcd for
C38H34CdN6O8 (815.11): C, 55.99; H, 4.20; N, 10.31. Found: C,
56.01; H, 4.16; N, 10.31%. IR (cm-1): 3042 (w), 1682 (m), 1595
drogen bond to the carboxylic oxygen atom with the N–
˚
H ◊ ◊ ◊ O distance of 3.116(7) A (Table S1). So an interesting
2D supramolecular layer is finally formed by linking the sinu-
soidal chains through p ◊ ◊ ◊ p and hydrogen bonding interactions
(Fig. 1c).
6798 | Dalton Trans., 2008, 6796–6807
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Table 1 Crystal data and structure refinements for compounds 1–8
1
2
3
4
5
6
7
8
Formula
Fw
Crystal system Monoclinic
C22H15N3O5Zn C22H15CdN3O5 C48H52N6O18Zn2 C48H40Cd2N6O12 C66H44N6O15Zn3 C38H28CdN6O5 C38H30CdN6O6 C38H34CdN6O8
466.74
513.77
Monoclinic
P21/c
11.7140(4)
16.0590(4)
11.2640(5)
90
96.482(1)
90
1131.70
Monoclinic
P21/n
17.309(3)
16.441(2)
18.128(3)
90
92.439(3)
90
5154.1(13)
4
1117.66
Triclinic
P1
1357.18
Monoclinic
C2/c
14.3040(4)
17.4700(4)
25.636(1)
90
94.069(1)
90
6390.1(3)
4
761.06
Triclinic
¯
P1
779.08
Monoclinic
P21/n
12.4090(3)
20.5790(4)
14.6440(6)
90
114.637(1)
90
815.11
Triclinic
¯
P1
¯
Space group
P21/n
7.6100(5)
18.888(1)
14.7840(13)
90
98.806(1)
90
2100.0(3)
4
˚
a/A
10.0580(4)
10.6750(4)
12.0480(5)
85.841(1)
68.774(1)
66.171(1)
1098.90(8)
1
9.6950(11)
12.4600(13)
15.1400(16)
111.061(2)
96.603(2)
101.133(2)
1640.3(3)
1
11.4920(4)
11.7810(4)
14.6520(5)
71.945(1)
73.436(1)
80.084(1)
1800.08(11)
2
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
2105.38(13)
4
3399.14(17)
4
Z
m/mm-1
1.208
1.476
1.076
1.621
1.009
1.458
1.041
1.689
1.188
1.411
0.721
1.541
0.700
1.522
0.668
1.504
D
calcd/g cm-3
F(000)
952
1024
12 844/5054
2344
27 392/10240
562
6902/5038
2768
19 563/7664
772
10 241/7502
1584
20 887/8188
832
11 223/8218
Reflns collected/ 12 692/5007
unique
R(int)
0.0227
1.016
0.0202
1.045
0.0250
0.0603
0.0639
0.0454
1.017
0.0584
0.1421
0.1572
0.0161
1.039
0.0345
0.0728
0.0777
0.449/-0.409
0.0288
1.042
0.0399
0.1202
0.1272
1.517/-0.630
0.0436
0.957
0.0606
0.1214
0.1335
0.0192
1.023
0.0308
0.0695
0.0755
0.0137
1.030
0.0341
0.0855
0.0907
GOF on F2
a
R1 [I > 2s(I)] 0.0420
b
wR2
wR2 for all data 0.1316
Largest
0.1141
0.391/-0.588 0.377/-0.426 1.019/-0.482
0.864/-0.841 0.591/-0.539 0.963/-0.555
-3
˚
residuals/e A
ꢀ
ꢀ
ꢀ
ꢀ
a R1 = ꢀFo| - |Fcꢀ/ |Fo|. b wR2 = | w(|Fo|2 - |Fc|2)|/ |w(Fo2)2|1/2
.
ric unit (Fig. 2a). CdII ion shows a seven-coordinated pentagonal
bi-pyramidal coordination geometry which is finished by five
carboxylic oxygen atoms (Cd(1)–O(5) = 2.223(4), Cd(1)–O(4) =
2.655(3), Cd(1)–O(1)#1 = 2.255(3) Cd(1)–O(2)#1 = 2.704(3) and
2-
˚
Cd(1)–O(1)#2 = 2.462(4) A) from three oba anions and two
nitrogen atoms (Cd(1)–N(1) = 2.2892(17) and Cd(1)–N(3) =
1
˚
2.3973(18) A) from one L ligand. The Cd–O and Cd–N bond
lengths are all within the normal ranges.16 Each oba2- anion
coordinates three CdII ions (Chart S1b) to give a 2D sheet
(Fig. 2b) which contains a [Cd2(CO2)4] unit with the Cd ◊ ◊ ◊ Cd
1
˚
distance of 3.72 A (Fig. S2a). L coordinates chelately to each
CdII cation as a terminal ligand with the dihedral angle of
4.8◦ between pyridine and imidazole rings in the same ligand
(Fig. S2b).
In addition, each L1 from one sheet donates one hydrogen
bond to the adjacent carboxylic oxygen atom with the N–H ◊ ◊ ◊ O
˚
distance of 2.767(2) A. Weak p ◊ ◊ ◊ p stacking interactions are
exhibited between pyridine and imidazole rings from adjacent
˚
layers, with the plane to plane distance of 3.46 A and the centroid–
˚
centroid distance of 3.87 A. So a 3D supramolecular framework
different from 1 is finally formed through p ◊ ◊ ◊ p and hydrogen
bonding interactions (Fig. 2c).
Fig. 1 (a) Coordination environment of ZnII atom in 1 with the ellipsoids
drawn at the 30% probability level, all hydrogen atoms were omitted for
clarity. (b) The infinite chain-like structure of compound 1 (symmetry
transformations used to generate equivalent atoms: #1 x - 3/2, -y +
1/2, z - 1/2). (c) View of the 2D supramolecular layer constructed by
p ◊ ◊ ◊ p stacking (green dashed lines) and H-bonds (yellow dashed lines)
interactions connecting the adjacent chains.
Comparing the structures of 1 and 2, they show different 1D
and 2D structures, when using ZnII and CdII ion. The reasons may
be that ZnII and CdII ions have closed d10 electronic shell, and
there are no crystal field stabilization energies for them when their
complexes are formed. The radius of CdII cation is larger than that
of ZnII cation, and the coordination number of CdII cation in 2 is
larger than that of ZnII cation in 1. So compounds 1 and 2 show
different structural types.
Structure description of 2. Compound 2 is a 3D supramolec-
ular structure using CdII ions instead of ZnII ions. There is one
kind of CdII ion, oba2- anion and L1 ligand in the asymmet-
Structure description of 3. To evaluate the effects of differ-
ent bridging spacers within the framework formation of their
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Fig. 2 (a) Coordination environment of CdII atom in 2 with the ellipsoids
drawn at the 30% probability level, all hydrogen atoms were omitted for
clarity (symmetry transformations used to generate equivalent atoms: #1
x + 1, -y + 1/2, z - 1/2; #2 -x, y + 1/2, -z + 1/2). (b) Ball-and-stick repre-
sentation of the layer-like structure of 2. (c) Ball-and-stick representation
of the 3D supramolecular structure of 2.
complexes, four structurally related ligands L2–L5 which derive
from L1 have been utilized to synthesize six different frameworks.
When L1 has been replaced by L2 and reacted with H2oba and
Zn cations, a particularly fascinating 3D structure of 3 has been
obtained. As shown in Fig. 3a, the structure of 3 contains two
kinds of unique Zn(II) atoms, two kinds of unique oba2- anions,
and one kind of L2 ligand. Zn1 and Zn2 show six-coordinated
octahedral geometries which are surrounded by four oxygen
atoms (Zn(1)–O(3)#1 = 2.056(3), Zn(1)–O(1) = 2.061(3), Zn(1)–
O(4)#1 = 2.350(4), Zn(1)–O(2) = 2.375(4), Zn(2)–O(7) = 2.087(3),
Zn(2)–O(10)#3 = 2.144(3), Zn(2)–O(9)#3 = 2.181(3) and Zn(2)–
Fig. 3 (a) Coordination environment of ZnII atoms in 3 with the ellipsoids
drawn at the 30% probability level, all hydrogen atoms and water molecules
were omitted for clarity (symmetry transformations used to generate
equivalent atoms: #1 x - 1/2, -y + 3/2, z - 1/2; #3 x - 1/2, -y - 1/2,
z + 1/2). (b) Ball-and-stick and space filling representations two kinds of
chains formed by oba2-, Zn1 and Zn2, respectively. (c) and (d) Ball-stick
and polyhedral and schematic representations of the 3D structure of 3.
(e) Schematic view of eight-fold interpenetrating topological framework.
(f) The water clusters in the structure (symmetry transformations used to
generate equivalent atoms: #2 x, y - 1, z - 1; #9 1/2 - x, -1/2 + y, 3/2 - z;
#10 1 - x, -y, 1 - z; #11 1/2 + x, 1/2 - y, 1/2 + z; #12 x - 1/2, 1/2 - y,
z - 1/2; #13 x + 1/2, -y - 1/2, z + 1/2; #14 1 + x, y, z).
O(6) = 2.343(3) A) from two oba2- anions and two nitrogen atoms
˚
(Zn(1)–N(1) = 2.036(3), Zn(1)–N(2) = 2.114(3), Zn(2)–N(4) =
2
˚
2.050(3) and Zn(2)–N(5) = 2.094(3) A) from the same L ligand,
respectively. Two oba2- anions coordinate to two kinds of ZnII
cations by bis(bidentate) coordination modes (Chart S1a) to give
two kinds of [Zn(oba)] chains, respectively. And the two kinds of
chains are arranged by vertical modes (Fig. 3b). L2 ligand exhibits
a TTT (T = trans) conformation (Fig. S3a) and coordinates to Zn1
and Zn2 atoms from different chains to generate a 3D framework
(Fig. 3c). From the topological view, if each Zn atom is considered
as a three-connected node (Fig. S3b) and oba2- and L2 ligands as
linkages, the structure can be classified as an unusual (10,3)-b (or
ThSi2) net (Fig. 3d).
Because of the spacious nature of the single network, the po-
tential voids are filled via mutual interpenetration of identical 3D
frameworks, generating an eight-fold interpenetrating architecture
(Fig. 3e). In a previous report, a few (10,3) nets have already
been defined by Wells, Koch and Fischer.17 To our knowledge,
only one eight-fold interpenetrating framework based on (10,3)
6800 | Dalton Trans., 2008, 6796–6807
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net has been reported18, in which there are four right- and four
left-handed (10,3)-a nets. Of the (10,3)-b type, there are currently
only a few known examples.3e,19 Some of them show two-fold or
three-fold interpenetrating frameworks. The framework of 3 is
the first example for eight-fold interpenetrating framework based
on the (10,3)-b net which is clearly different from other reported
network topologies.
Water clusters have been widely studied both theoretically
and experimentally.20 A variety of water clusters, [(H2O)n, n =
2–18], found in a number of crystal hosts, have been characterized
and display different configurations. There exist (H2O)16 polymers
which are formed by two crystallographically-related (H2O)8
octamers in the structure of 3, and the geometrical parameters
of the water polymers and its association with oba2- ligands are
provided in Table S1, whereas these distances in regular ice, liquid
20c
˚
water, and water vapor are 2.74, 2.85, and 2.98 A, respectively.
As shown in Fig. 3f, the sixteen lattice water molecules in 3 are
assembled into a centrosymmetric polymer by two eight octamers.
In each octamer, five water molecules (O3W, O7W#9, O6W#10,
O2W#10 and O5W#2) assemble through hydrogen bonds in
an approximate pentagon with five water molecules at each
corner (O3W ◊ ◊ ◊ O7W#9 = 2.769(6), O3W ◊ ◊ ◊ O5W#2 = 2.854(6),
O5W#2 ◊ ◊ ◊ O2W#10 = 2.882(6), O2W#10 ◊ ◊ ◊ O6W#10 = 2.878(6)
˚
and O6W#10 ◊ ◊ ◊ O7W#9 = 2.722(6) A). And O3W donates a
Fig. 4 (a) Coordination environment of CdII atom in 4 with the ellipsoids
drawn at the 30% probability level, all hydrogen atoms and water molecules
were omitted for clarity (symmetry transformations used to generate
equivalent atoms: #1 x - 1, y, z + 1; #2 -x + 2, -y + 1, -z + 1).
(b) Ball-and-stick representation of the double-chain structure formed
by oba2- and Cd1. (c) Ball-stick and polyhedral representation of the
2D structure of 4. (d) and (e) Ball-and-stick representation of the 3D
supramolecular structure of 4.
˚
hydrogen bond to O4W#9 (O3W ◊ ◊ ◊ O4W#9 = 2.895(7) A) and
O2W donates a hydrogen bond to O1W#11 (O2W ◊ ◊ ◊ O1W#11 =
˚
2.787(5) A) which gives hydrogen bonds to one carboxylic
oxygen atom (O7#13) and O8W#14 (O1W#11 ◊ ◊ ◊ O7W#13 =
˚
2.854(5) and O1W#11 ◊ ◊ ◊ O8W#14 = 2.770(8) A). As is well
known, water molecules often show a three-coordinate mode
through hydrogen bond interactions (O1W#11, O2W#9, O4W#9,
O5W#2, O6W#10, O7W#9 and O8W#11);21 however, O3W of
this water polymer shows a four-coordinate mode.
structural type from others has been obtained. Single crystal X-ray
analysis reveals that compound 5 is constructed by two kinds of
ZnII cations, two kinds of oba2- anions (oba1 and oba2) and one L3
ligand (Fig. 5a). Zn1 atom is six-coordinated and exhibits a twisted
octahedral geometry which is surrounded by four carboxylic
oxygen atoms (Zn(1)–O(6) = 2.001(2), Zn(1)–O(3) = 2.015(2),
Structure description of 4. When using CdII cations instead
of ZnII cations, a 3D supramolecular compound of 4 has been
isolated. As shown in Fig. 4a, the structure of 4 contains of
one kind of CdII cation, L2 ligand and oba2- anion. Cd1 atom is
six-coordinated and shows an octahedral coordination geometry,
which is completed by four oxygen atoms (Cd(1)–O(3) = 2.216(2),
Cd(1)–O(1)#1 = 2.341(2), Cd(1)–O(1)#2 = 2.3600(19) and Cd(1)–
˚
Zn(1)–O(1)#3 = 2.125(2) and Zn(1)–O(2)#3 = 2.328(2) A) from
three oba2- ligands and two nitrogen atoms (Zn(1)–N(1) = 2.335(3)
3
˚
and Zn(1)–N(2) = 2.068(3) A) from the same L ligand, while Zn2
2-
˚
O(2)#2 = 2.544(2) A) from three oba anions and two nitrogen
lies on an inversion center and is coordinated by six carboxylic
˚
atoms (Cd(1)–N(1) = 2.270(2) and Cd(1)–N(3) = 2.369(2) A) from
oxygen atoms (Zn(2)–O(4) = 2.195(2), Zn(2)–O(5) = 2.280(2), and
the same L2 ligand. The oba2- anion shows a different coordination
mode (Chart S1c) from 1–3. Two oba2- anions link two Cd1 cations
to give a double-chain structure (Fig. 4b) in which there is a 26-
membered ring (I). And the L2 ligand with a TTT conformation
(Fig. S4a) coordinates to two Cd cations from different double-
chains to form a 2D sheet (Fig. 4c). Each water molecule is
stabilized in the structure by donating one hydrogen bond to one
2-
˚
Zn(2)–O(2)#2 = 2.320(2) A) from six oba ligands and shows
a distorted octahedral geometry. The oba1 ligands (Chart S1d)
link Zn1 atoms to form a chain which is connected by Zn(2)–
O(5) and Zn(2)–O(2)#2 bonds to generate a sheet (Fig. 5b).
And then the sheets are pillared by oba2 (Chart S1e) ligands
to give a 3D framework (Fig. 5c). In the 3D structure, the Zn2
octahedron is a crystallographic inversion center that connects
two crystallographically equivalent Zn1 octahedra in a vertex-
sharing mode to form a [Zn3(CO2)6] polyhedral cluster by twelve
carboxylic groups from four oba1 and two oba2 ligands with the
˚
carboxylate oxygen atom (O(1W) ◊ ◊ ◊ O(3) = 2.825(5) A).
In addition, the strong p ◊ ◊ ◊ p stacking interactions between two
phenyl rings of two oba2- anions from adjacent layers (Fig. 4d),
˚
with the plane to plane distance of 3.40 A and the centroid–
centroid distance of 3.59 A which are in the normal ranges (3.3–
3.8 A), link above-mentioned 2D sheets to give a interesting 3D
supramolecular framework (Fig. 4e and S4b).
˚
nonbonding Zn1 ◊ ◊ ◊ Zn2 distance of 3.66 A. If each [Zn3(CO2)6]
˚
polyhedral cluster is considered as a six-connected node and each
oba2- anion as a linkage, the structure formed by oba2- anions and
Zn cations shows a a-Po (pcu) net (Fig. 5b).
In addition, each [Zn3(CO2)6] cluster is further linked by
two L3 ligands with an ‘S’-shaped conformation (Fig. S5a) to
22
˚
Structure description of 5. When using the semi-flexible L3
ligand instead of the flexible L2 ligand, compound 5 with a different
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membered cycle catenates the three shortest four-membered cycles.
The structure of 5 shows a new type of 3D eight-connected self-
penetrating framework, which could help us deeply understand
the nature of coordination polymer frameworks and better design
functional materials.
Structure description of 6. Compound 6 shows a different
structure from 5, by using CdII cation instead of ZnII cation in
the same reaction conditions. Single crystal X-ray analysis reveals
that compound 6 contains one kind of CdII cation and oba2- anion,
and two kinds of L3 ligands (Fig. 6a). The CdII ion is coordinated
by four oxygen atoms (Cd(1)–O(4)#1 = 2.264(3), Cd(1)–O(2) =
˚
2.343(4), Cd(1)–O(1) = 2.382(4) and Cd(1)–O(4)#2 = 2.455(3) A)
from three distinct oba2- ligands and two nitrogen atoms
Fig. 5 (a) Coordination environment of ZnII atoms in 5 with the ellipsoids
drawn at the 30% probability level (symmetry transformations used to
generate equivalent atoms: #1 -x + 1/2, -y + 1/2, -z + 1; #2 -x + 1/2,
y - 1/2, -z + 1/2; #3 x, -y + 1, z + 1/2). (b) Ball-and stick representation
of the layer-like structure formed by one kind of oba2- anion and ZnII
cations. (c) and (d) Ball-and stick and schematic representations of the
3D framework by the other kind of oba2- anion pillaring the layers.
(e) Schematic representation of the 3D self-penetrating framework of 5.
form a complicated 3D framework. It is interesting that each L3
ligand penetrates two 2D sheets formed by the Zn and oba1.
So the [Zn3(CO2)6N4] is a 8-connected node (Fig. S5b), and the
whole structure of 5 is a 3D eight-connected self-penetrating
framework with the (4166118) topological structure (Fig. 5e). To
our best knowledge, only a few examples of eight-connected 3-
D self-penetrating MOFs are reported until now,6b,23 and the
structure of 5 is completely different from that of the well-
known eight-connected self-penetrating frameworks with (424563),
(42068), (42464) and (4125667728) topologies.6b,23 In the reported
3D eight-connected self-penetrating frameworks, the two shortest
four-membered cycles are catenated. And in this case, one four-
Fig. 6 (a) Coordination environment of CdII atom in 6 with the ellipsoids
drawn at the 30% probability level, all hydrogen atoms were omitted for
clarity (symmetry transformations used to generate equivalent atoms:
#1 x, y, z + 1; #2 -x + 1, -y, -z). (b) Ball-and-stick representation
of the double-chain structure formed by oba2- and Cd1. (c) Ball-stick
and polyhedral representation of the 2D structure of 6. (d) and (e)
Ball-and-stick and space filling representations of the 3D supramolecular
structure of 6.
6802 | Dalton Trans., 2008, 6796–6807
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˚
(Cd(1)–N(1) = 2.387(4) and Cd(1)–N(2) = 2.323(4) A) from
one L3 ligand, showing distorted octahedral coordination geome-
try (Fig. 6a). Two oba2- anions link two CdII cations (Chart S1f)
to give a double-chain structure (Fig. 6b) in which there is a 26-
membered ring (I). And one kind of L3 ligand with an ‘S’-shaped
conformation coordinates two Cd cations from different double-
chains to form a 2D sheet (Fig. 6c). And the other kind of L3
ligand with the same conformation co-crystallizes in the structure
(Fig. S6).
In addition, there are p ◊ ◊ ◊ p stacking interactions in the
structure (Fig. 6d). The weak p ◊ ◊ ◊ p stacking interactions between
two pyridine rings of two L3 ligands coordinating to metal centers
˚
from adjacent layers, with the plane to plane distance of 3.42 A and
˚
the centroid–centroid distance of 3.93 A, connect sheets to give a
3D supramolecular structure. And the p ◊ ◊ ◊ p stacking interactions
between pyridine and imidazole rings of two kinds of L3 ligands,
˚
with the plane to plane distance of 3.47 A and the centroid–
3
˚
centroid distance of 3.82 A, stabilize the isolated L ligands in
the structure (Fig. 6e).
Structure description of 7. When using the different angular
character of L4 instead of L3, compound 7 with four-fold inter-
penetrating diamond topological framework has been obtained.
The structure of 7 contains one kind of CdII ion, oba2- anion
and L4 ligand (Fig. 7a). Each CdII ion is seven-coordinated by
four carboxylic oxygen atoms (Cd(1)–O(2) = 2.3332(16), Cd(1)–
O(5)#2 = 2.3507(16), Cd(1)–O(4)#2 = 2.4158(15) and Cd(1)–
2-
˚
O(1) = 2.6436(18) A) from two oba anions and three nitrogen
atoms (Cd(1)–N(2) = 2.2723(17), Cd(1)–N(5)#1 = 2.2869(18)
4
˚
and Cd(1)–N(3) = 2.6357(19) A) from two L ligands, showing a
pentagonal bi-pyramidal coordination geometry. Each of the oba2-
anion with a bis(bidentate) coordination mode (Chart S1a) links
two CdII ions to give a chain (Fig. 7b). Unexpectedly, L4 ligand
coordinates to two CdII ions (Fig. S7a) from different chains with
a mono(bidentate) coordination mode to form a 3D framework
(Fig. 7c).
Fig. 7 (a) Coordination environment of CdII atom in 7 with the ellipsoids
drawn at the 30% probability level, all hydrogen atoms and water molecule
were omitted for clarity (symmetry transformations used to generate
equivalent atoms: #1 x - 1/2, -y + 1/2, z - 1/2; #2 x - 1/2, -y + 3/2,
z + 1/2). (b) Ball-and-stick representation of the chain structure formed
by oba2- and Cd1. (c) Ball-stick and polyhedral representation of the 3D
structure of 7. (d) and (e) Schematic view of four-fold interpenetrating
topological framework.
From the topological view, if each CdII ion is considered as
a 4-connected node (Fig. S7b), and each oba2- anion and L4
ligand as linkages, the structure of 7 is a 3D diamond topological
net (Fig. 7d). Because of the spacious nature of the single
network, the potential voids are filled via mutual interpenetration
of identical 3D frameworks, generating a four-fold interpenetrat-
ing architecture (Fig. 7e). This kind of four-fold interpenetrat-
ing structure has been described in previous reports.24 Lattice
water molecule occupies the residual space and is hydrogen-
bonded to the carboxylate groups (O(1W) ◊ ◊ ◊ O(1) = 2.859(3) and
5
˚
the Cd ◊ ◊ ◊ Cd distance of 3.85 A. It is unexpected that L acts as
a chelate and terminal ligand and coordinates to a Cd ion with a
‘V’-shaped conformation (Fig. S8).
In addition, there are weak p ◊ ◊ ◊ p interactions between two
pyridine rings of L5 ligands from adjacent chains with the plane
˚
to plane distance of 3.51 A and the centroid–centroid distance
˚
of 3.99 A. At the same time, O1W donates two hydrogen bonds
to an N atom of L5 ligand and an O atom (O(1W) ◊ ◊ ◊ N(5)#4 =
˚
O(1W) ◊ ◊ ◊ O(5)#5 = 2.920(3) A).
2.950(5) and O(1W) ◊ ◊ ◊ O(4)#1 = 2.870(3) A) of oba2- anion from
˚
Structure description of 8. When the smallest angular L5 ligand
was selected to react with CdII ion and H2oba, we obtained
compound 8. There is one kind CdII ion, oba2- anion and L5
ligand (Fig. 8a). The CdII ion exhibits a pentagonal bi-pyramidal
coordination geometry which is completed by five carboxylic oxy-
gen atoms (Cd(1)–O(2) = 2.2935(18), Cd(1)–O(4)#1 = 2.4048(19),
Cd(1)–O(5)#2 = 2.4049(18), Cd(1)–O(1) = 2.4492(18) and Cd(1)–
adjacent chains. And O2W donates two hydrogen bonds to O1W
and O3W (O(2W) ◊ ◊ ◊ O(1 W) = 2.804(5) and O(2W) ◊ ◊ ◊ O(3 W) =
˚
2.958(9) A). So the non-covalent interactions play important roles
for linking the chains to generate a 2D supramolecular sheet
(Fig. 8c).
Effect of the anion and neutral ligand on the structures
O(5)#1 = 2.4594(19) A) from three oba2- anions and two nitrogen
˚
˚
atoms (Cd(1)–N(2) = 2.262(2) and Cd(1)–N(3) = 2.401(2) A) from
From the above structural descriptions, it can be seen that the
anion and neutral ligands have great influences on the frameworks
of the complexes.
one L5 ligand. The oba2- anions (Chart S1b) link Cd ions to give a
double-chain structure (Fig. 8b) which contains a [Cd2] unit with
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˚
ions coordinated by the same neutral ligands) of 14.06 and 14.93 A.
The difference of the distance is from the different flexibilities of L2
and L3 ligands, so in 6 the semi-flexible L3 ligands can link the
double-chains to give a bigger pore which can accommodate the
other isolated L3 ligand. While the L4 ligand with a smaller angle
II
˚
can coordinate to two Cd ions with the distance of 13.48 A, which
is benefit to form a different structure for 4 and 6. So the structural
characters and coordination modes of five neutral ligands have
significant influences on the ultimate structures.
Effect of the weak interactions on the structures
Interestingly, the p ◊ ◊ ◊ p stacking interactions in 1, 2, 4, 6 and
8 can link lower polymeric units (1D or 2D) to from higher
supramolecular structures (2D or 3D) (Scheme 1). In addition, L1
donates one hydrogen bond to the carboxylic oxygen atom in 1 and
2, so interesting 2D and 3D supramolecular structures are finally
formed by combining p ◊ ◊ ◊ p and hydrogen bonding interactions.
In 8, the hydrogen bonding interactions play an important role
on generating higher dimensional structure. And the hydrogen
bonds in 8 are formed by water clusters which are different from
1 and 2. Although there is a hydrogen bond in 4, it does not assist
to form the higher supramolecular structure. So in 4 and 6, the
higher dimensional supramolecular structures are constructed by
p ◊ ◊ ◊ p stacking interactions. And those are formed by p ◊ ◊ ◊ p and
hydrogen bonding interactions in 1, 2 and 8.
Fig. 8 (a) Coordination environment of CdII atom in 8 with the
ellipsoids drawn at the 30% probability level, all hydrogen atoms and
water molecule were omitted for clarity (symmetry transformations used
to generate equivalent atoms: #1 x, y, z - 1; #2 -x, -y, -z + 2). (b)
Ball-and-stick representation of the chain structure of 8. (c) View of the
2D supramolecular layer constructed by p ◊ ◊ ◊ p (black dashed lines) and
H-bonds (green dashed lines) interactions connecting the adjacent chains.
Each of these anions may act as a versatile ligand to construct
various MII–oba metal–organic units. In this case, oba2- anions
display a variety of coordination modes (Chart S1a–S1f) and
coordinate to 2–4 metal centers to give different MII–oba metal–
organic units. In 1, 3 and 7, oba2- anions link ZnII or CdII ions
to generate 1D chain-like structures, and they show 1D double
chain-like structures in 4 and 6. While in 2 and 5, oba2- anions
assemble ZnII or CdII ions to form 2D and 3D nets, respectively.
The various MII–oba metal–organic units can be decorated five
neutral ligands to form the ultimate compounds. And a variety of
coordination modes for oba2- anions may be expected.
Due to the structural features of five neutral ligands, L1 ligand
tends to act as a terminal ligand coordinating to one metal
center with two nitrogen atoms, and L2–L5 maybe prefer to
adopt bis(bidentate)-cheating bridging modes with four aromatic
N atoms coordinating to two metal centers. Because the differ-
ent coordination behaviors of L1–L5, it is anticipated that the
structures of 1–8 are various. For 1 and 2, L1 ligands decorate
with metal centers from 1D and 2D MII–oba units as terminal
ligands to show the ultimate 1D and 2D structures, respectively.
For 3, 4, 6 and 7, L2–L4 ligands link various 1D MII–oba units by
coordinating to two metal centers as bridges to give different 2D
or 3D frameworks, respectively. In 5, L3 ligands link the 3D MII–
oba unit to exhibit a 3D self-penetrating topological framework.
Unexpectedly, L5 as terminal ligands connect one metal center to
show a 1D chain in 8.
Comparing the structures of 1 and 3, the result indicates that
the neutral bridging ligand (L2) can link similar MII–oba units
(1D MII–oba unit in 1 and 3) to form higher dimensional structures
(1D in 1 and 3D in 3) than neutral terminal ligand (L1). A similar
change of dimensionalities also occurs in compounds 4, 6, and 7.
In 4 and 6, the similar MII–oba double-chains are pillared by L2
and L3 to generate a sheet with the two CdII ions distances (CdII
Scheme 1 Schematic view of eight compounds in this work.
Based on the above discussions, many factors, such as the
various coordination modes of oba2- anions, the coordination
modes and conformations of different neutral ligands, the radii of
metal cations, the versatilities of the metal coordination geometries
and p ◊ ◊ ◊ p stacking and hydrogen bonding interactions play
fundamental roles in the formation of the final products. These
factors work together to affect the structures.
Luminescent properties
Luminescent compounds are of great current interest because of
their various applications in chemical sensors, photochemistry
and electroluminescent display.25 The photoluminescent spectra
of compounds 1–8 and free neutral ligands L1–L5 were measured
at room temperature (Fig. 9), and the wavelengths of the emission
maximums and excitation are listed in Table 2. In comparison with
the free ligands, the emission maximums of compounds 1–8 have
changed. This may be caused by the charge-transfer transition
between ligands and metal centers,10c,26 and ligand coordinating
6804 | Dalton Trans., 2008, 6796–6807
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Table 2 The wavelengths of the emission maximums and excitation (nm)
7 and 8, the weight loss attributed to the gradual release of water
molecules is observed in the range 20–103 ◦C for 7 and 18–146 ◦C
for 8, and the decomposition of the residual composition occurs
from 194 ◦C to 492 ◦C for 7 and 248 ◦C to 502 ◦C for 8, leading
to the formation of CdO as the residue (obsd 16.8%, calcd 16.5%)
for 7 and (obsd 16.2%, calcd 15.8%) for 8.
Ligand
L1
L2
L3
L4
L5
lem
lex
504 532 360, 506 547 356, 500
370 370 320 480 320
Compound
1
2
3
4
5
6
7
8
Conclusion
lem
lex
379 411 375
320 320 290
398 388
310 310
364 382 376
300 290 330
In conclusion, we have been designed and synthesized eight
coordination polymers based on analogue bis(pyridyl)imidazole
derivatives and 4,4¢-oxydibenzoic acid. By careful inspec-
tions of the structures of 1–8, we believe that various
bis(pyridyl)imidazole derivatives with different conformation and
coordination modes, metal centers with different coordination
numbers, 4,4¢-oxydibenzoic acid showing various coordination
modes and weak interactions (hydrogen bonds and p ◊ ◊ ◊ p stacking
interactions) have influences on the structures. It is a feasible
method to introduce ancillary ligands with different characters to
construct coordination polymers with different structural types.
It is expected that the integration of novel neutral ligands and
multi-carboxylate anion may offer new opportunities to construct
new types of MOFs. Investigations in this direction are currently
under way.
Fig. 9 Solid-state photoluminescent spectra of 1–8 at room temperature.
to the metal center which effectively increases the rigidity of the
ligand and reduces the loss of energy by radiation-less decay27
In all, the emissions of compounds 1–8 may be attributed to
a joint contribution of the intra-ligand transitions or charge-
transfer transitions between the coordinated ligands and the
metal center. These observations indicate that these condensed
polymeric materials may be excellent candidates for potential
photoactive materials, since they are thermally stable and insoluble
in common polar and nonpolar solvents.
Acknowledgements
We are thankful for financial support from the Program for
Changjiang Scholars and Innovative Research Team in University,
the National Natural Science Foundation of China (No. 20573016
and 20703008), and the Science Foundation for Young Teachers
of Northeast Normal University (No. 20070309).
Thermal Analysis
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