Synthesis, structures, and luminescence of two 2-D microporous metal-organic frameworks in the zinc (cadmium)-dicarboxylate-imidazolate system

Article abstract of DOI:10.1080/00958972.2016.1172070

Two 2-D microporous metal–organic frameworks, [Zn(BDC)(MbIm)]·2DMF (1) and [Cd₃(BDC)₃(MbIm)₂(DMF)₂]·2DMF (2), have been synthesized by solvothermal reaction of 1,4-benzenecarboxylic acid (H₂BDC) and 2-methylbenzimidazole (MbIm) with zinc/cadmium nitrate. Single-crystal X-ray diffraction analysis indicates that 1 consists of the well-known zinc paddle-wheel motif which is linked by bridging dicarboxylates to form 2-D square grids. The 2-D layers stack offset due to the effect of the spatial structure of MbIm ligand and hydrogen-bonding interaction between MbIm and guest molecules. Similarly, 2 is constructed by six-connected Cd₃(μ-O₂CR)₆(MbIm)₂ units and bridging carboxylates, resulting in a 2-D layer structure with triangular grids. Topology analysis reveals that 1 exhibits a 2-D tetragonal plane network with {4⁴·6²} topology symbol, while 2 possesses a six-connected {3⁶·4⁶·5³} topological network. Analysis of the luminescence spectra demonstrates that the complexes have good luminescent intensities with greater red-shift (82 nm for 1 and 69 nm for 2) corresponding to free MbIm. Elemental analyses, infrared spectra, powder X-ray diffraction, and thermogravimetric analyses of 1 and 2 have been investigated.

Full text of DOI:10.1080/00958972.2016.1172070

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Synthesis, structures and luminescence of two 2-D
microporous metal-organic frameworks in the zinc
(cadmium)-dicarboxylate-imidazolate system
Xiaoqing Guo, Miao Wang, Xuefang Gu, Jinli Zhu, Yanfeng Tang, Jiang
Guoqing & Junfeng Bai
To cite this article: Xiaoqing Guo, Miao Wang, Xuefang Gu, Jinli Zhu, Yanfeng Tang, Jiang
Guoqing & Junfeng Bai (2016): Synthesis, structures and luminescence of two 2-D microporous
metal-organic frameworks in the zinc (cadmium)-dicarboxylate-imidazolate system, Journal of
Coordination Chemistry, DOI: 10.1080/00958972.2016.1172070
To link to this article: http://dx.doi.org/10.1080/00958972.2016.1172070
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Date: 06 April 2016, At: 21:45

Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://dx.doi.org/10.1080/00958972.2016.1172070
Synthesis, structures and luminescence of two 2-D microporous metal-organic
frameworks in the zinc (cadmium)-dicarboxylate-imidazolate system
XIAOQING GUO†, MIAO WANG†, XUEFANG GU†, JINLI ZHU†, YANFENG TANG†, GUOQING JIANG*†
and JUNFENG BAI*‡
†School of Chemistry and Chemical Engineering, Nantong University, Nantong, 226019, P. R. China
‡The State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering,
Nanjing University, 22 Hankou Road, Nanjing, 210093 (P.R. China)
Two 2-D microporous metal-organic frameworks, [Zn(BDC)(MbIm)]·2DMF (1) and
[Cd3(BDC)3(MbIm)2(DMF)2]·2DMF (2), have been synthesized by solvothermal reaction of 1,4-
benzenecarboxylic acid (H2BDC) and 2-methylbenzimidazole (MbIm) with zinc/cadmium
nitrate. Single crystal X-ray diffraction analysis indicates that 1 consists of the well-known zinc
paddle-wheel motif which is linked by bridging dicarboxylates to form 2-D square grids. The
2-D layers stack offset due to the effect of the spatial structure of MbIm ligand and hydrogen-
bonding interaction between MbIm and guest molecules. Similarly, 2 is constructed by six-
connected Cd3(μ-O2CR)6(MbIm)2 units and bridging carboxylates, resulting in a 2-D layer
structure with triangular grids. Topology analysis reveals that 1 exhibits a 2-D tetragonal plane
network with {44•62} topology symbol, while 2 possesses a 6-connected {36•46•53} topological
network. Analysis of the luminescence spectra demonstrates that the complexes have good
luminescent intensities with greater red-shift (82 nm for 1 and 69 nm for 2) corresponding to free
MbIm. Elemental analyses, infrared spectra (IR), powder X-ray diffraction (PXRD), and
thermogravimetric analyses (TGA) of 1 and 2 have been investigated.
Keywords: Metal-organic frameworks; Paddle-wheel motif; Solvothermal synthesis;
Luminescence; Microporous framework
*Corresponding authors. Email: jgq3518@163.com (G. Jiang); bjunfeng@nju.edu.cn (J. Bai)
1

1. Introduction
The design and construction of metal-organic frameworks (MOFs) is of interest, due to their
intriguing structures and potential application as crystalline functional solid-state materials, in
gas adsorption and separation, ion conductivity and transport, luminescence, catalysis, analytical
chemistry, etc. [1-7]. The combination of flexibility of linkers (organic ligands) with diverse
geometry, size and functionality and richness of connectors (metal ions/clusters) has led to
various MOFs with different structures and properties [8, 9]. For example, a series of MOFs
based on Zn(BDC)(H2O)·(DMF) (MOF-2, one of the first materials constructed from a paddle-
wheel structure) [10] are synthesized by replacing connectors (eg. Cu2+, Fe2+) and linkers
(eg. Cl2-BDCH2, dichlorobenzene-1,4-dicarboxylic acid) [11-14]. Replacing connectors has been
investigated extensively, while the strategy of replacing terminal ligands is rarely reported.
Generally, the combination of ditopic organic carboxylates and metal ions/clusters through
coordination interactions can lead to 2-D layers with various grids and further linking into a 3-D
framework by hydrogen-bond interaction. For instance, Chen and co-workers reported a
microporous framework, Cu(BDC-OH)(H2O)·0.5DEF (where BDC-OH = 2-hydroxybenzene-
1,4-dicarboxylic acid), which exhibits approximately square grids in 2-D microporous layers and
diamond pores in 3-D perspective [11]. Consequently, the effect of packing mode of layers on
crystal structure and porosity is very important in 2-D networks. The packing mode is commonly
influenced by terminal ligands and solvent molecules because terminal ligands have a certain
spatial structure and tend to form hydrogen-bond interactions with adjacent layers or solvent
molecules (bridging two layers), further influencing the interlayer distance and offset.
Rational selection of terminal ligands, to some extent, is a factor in determining the
structures and properties of 2-D frameworks. Benzimidazole has been extensively studied in the
preparation of zeolitic imidazolate frameworks (ZIFs) [15-17], but the application of
2-substituent benzimidazole (especially MbIm) in metal-dicarboxylate-imidazolate system is
relatively rare. Several reasons contribute to the employment of MbIm in this research: (i) The
introduction of MbIm may increase the complexity of pore composition and framework,
resulting in multifunctionality of the pores [18-20]; (ii) Benzimidazole derivatives with a greater
conjugative effect usually exhibit strong luminescence and can be a suitable candidate for
luminescent MOFs [21-23]; (iii) As an interesting N-containing ligand, MbIm possesses two
2

kinds of coordination modes — monodentate or bidentate, which may lead to diversity of crystal
structures [24-26]. Transition metal ions (such as Zn2+ and Cd2+) often show strong coordination
for imidazolates and carboxylates, which may effectively restrain the tendency for phase
separation when multicomponent ligands are used in the synthesis of MOFs [27, 28].
Herein, two new zinc(cadmium)-dicarboxylate-imidazolate frameworks,
[Zn(BDC)(MbIm)]·2DMF (1) and [Cd3(BDC)3(MbIm)2(DMF)2]·2DMF (2), have been
synthesized under the same conditions, exhibiting 2-D microporous layers arising from metal
clusters and bridging dicarboxylates and MbIm as terminal ligand bridging the adjacent layers
through hydrogen-bonding interactions (scheme 1). Their synthesis, crystal structures,
luminescent properties and other properties are discussed.
Scheme 1. Synthesis of 1 and 2.
2. Experimental
2.1. Materials and measurements
2-Methylbenzimidazole (MbIm) was prepared according to the literature procedure [29, 30]. All
other materials were obtained from commercial sources and used without purification. Elemental
analyses of C, H and N were performed on a Perkin-Elmer 240C analytical instrument. IR
spectra were recorded on an AVATAR360 FT-IR spectrophotometer using discs from 4000-
400 cm-1. Thermogravimetric analyses (TGA) were performed on a NETZSCH STA 449F3
analyzer under N2. Powder X-ray diffraction (PXRD) patterns were measured by a Bruker D8
Advance diffractometer (Cu/Kα, λ = 1.5418 Å). The solid state luminescent spectra were
recorded at room temperature on a Hitachi-F-7000 instrument.
3

2.2. Crystallographic studies
X-ray diffraction for 1 and 2 were performed on a Bruker SMART APEX II diffractometer at
293 K with graphite monochromated Mo/Kα radiation (λ = 0.71073 Å). The structure,
established with direct methods, was refined by weighted full-matrix least-square refinements on
F2 using SHELXS-97 [31]. All the non-hydrogen atoms were refined with anisotropic
displacement parameters and hydrogens of the organic ligands were placed in geometrically
idealized positions and refined using a riding model with 1.2 × Ueq of the attached atoms. A
summary of the parameters for data collection and refinement of 1 and 2 are listed in table 1.
Selected bond lengths (Å) and angles (°) for 1 and 2 are given in table S1.
2.3. Preparation of 2-methylbenzimidazole (MbIm)
A mixture of acetic acid (1.44 g, 24 mmol) and o-phenylenediamine (2.16 g, 20 mmol) was
refluxed for 12 h in hydrochloric acid (4.5 M, 16 mL). The resulting gray solution was poured
into ice water (50 mL) with vigorous stirring and then neutralized by saturated sodium carbonate
solution. The white solid product was collected and washed with distilled water (Yield: 65%). IR
(KBr, cm-1): 3059(m), 2991(m), 2917(m), 2787(m), 2678(m), 1449(s), 1418(s), 1386(m),
1271(s), 1026(m), 736(s).
2.4. Synthesis of [Zn(MbIm)(BDC)] 2DMF (1)
A solid mixture of zinc(II) nitrate hexahydrate Zn(NO3)2·6H2O (17.6 mg, 0.05 mmol),
terephthalic acid (16.6 mg, 0.1 mmol) and 2-methylbenzoimidazole (13.3 mg, 0.1 mmol) was
dissolved in DMF (3 mL) in a 20 mL vial to obtain a colorless solution. The vial was tightly
capped and heated at 5 °C/min to 100 °C in a programmable oven for 96h, and then cooled to
room temperature at a rate of 10 °C h-1 to obtain colorless block-shaped crystals of 1 (Yield: 61%
based on Zn). Elemental analysis (%) for C22H26N4O6Zn Calcd: C, 51.98; H, 5.12; N, 11.03.
Found: C, 51.86; H, 5.21; N, 11.12. IR (KBr, cm-1): 3127(m), 2929(m), 1666(s), 1501(m),
1458(m), 1386(s), 1283(m), 1254(w), 1089(w), 822(m), 746(s).
2.5. Synthesis of [Cd3(MbIm)2(BDC)3(DMF)2]•2DMF (2)
This complex was prepared in a similar procedure as for 1 by using Cd(NO3)2·4H2O (17.1 mg,
0.055 mmol) in place of Zn(NO3)2·6H2O. The resulting colorless crystalline product of 2 was
4

obtained in 50% yield (based on Cd). Elemental analysis (%) for C52H56N8O16Cd3 Calcd: C,
45.01; H, 4.04; N, 8.08. Found: C, 45.08; H, 4.12; N, 8.01. IR (KBr, cm-1): 3434(m), 3130(m),
2930(m), 1655(s), 1602(s), 1574(s), 1505(m), 1456(m), 1385(vs), 1281(m), 843(m), 750(s),
523(m).
3. Results and discussion
3.1. Crystal structure of 1
Single-crystal structure analysis reveals that the paddle-wheel structure of 1 is structurally
similar to the well-known MOF-2 [10]. This complex crystallizes in monoclinic space group
C2/c. In the asymmetrical unit of the unit cell, each Zn(1) exhibits a distorted square pyramidal
geometry with four oxygens from four independent carboxylate groups and one nitrogen of
MbIm in the apical position as shown in figure 1a. Both the Zn-O bond lengths (from
2.0303(18) Å to 2.0532(16) Å) and the Zn-N bond length (2.0053(19) Å) fall in the normal range
[32, 33], and Zn(1) lies 0.40 Å above the basal O1-O3a-O2-O4a plane. The Zn···Zn distance of
3.0236(3) Å in 1 is longer than the value in MOF-2 (2.940(3) Å), which may be caused by the
coordinated geometry. Similar to MOF-2, two Zn ions and four carboxylates consist of a
Zn2(O2CR)4 SBU, where each carboxylate is bonded to two adjacent Zn ions in a di-monodentate
mode. The remaining carboxylate groups of BDC ligands extend from the Zn2(O2CR)4 SBU at
approximately 90°, resulting in 2-D microporous layers with quadrangular pores (see figure 1b).
The first type of diamond pore has maximum diameter of 7.4 Å with consideration of the van der
Waals radii, while the methyl groups of MbIm stretch into the pore to form the second type of
hourglass-shaped pore (figure 1c). The layers are packed along an axis by intermolecular
hydrogen-bonding interactions between O of DMF and methyl C of MbIm ligand, resulting in
two types of 1-D pores (figure 2). Complex 1 exhibits total solvent-accessible volume of 52.9%
of the unit cell volume, as calculated using PLATON [34]. Topology analysis reveals that if the
repeat unit Zn2(O2CR)4(MbIm)2 can be considered as a four-connected node, and the bdc2- can
be viewed as linear linkers, then 1 can be simplified into a tetragonal plane framework with the
point symbol of {44•62} (figure 1d).
Compared with those previously reported for Zn(BDC)(DMF)(H2O) (MOF-2) and
Zn(BDC)(DMF) [35], the crystal structure of 1 reveals a similar 2-D layer with square grids.
However, the packing mode of the 2-D layers and the complexity of pores are completely
5

different, mainly influenced by displacement of the terminal ligands coordinating to the
peripheral Zn(II) ions. As shown in table 2, the perpendicular separation between layers of
MOF-2 and Zn(BDC)(DMF) is 4.21 Å and 6.10 Å, respectively. In 1, the offset of adjacent
layers is much larger, 9.54 Å. Because 1 needs to stack with a large offset to accommodate the
MbIm molecules.
3.2. Crystal structure of 2
This compound was prepared in a similar procedure as 1 using Cd(NO3)2·4H2O instead of
Zn(NO3)2·6H2O. Single-crystal X-ray analysis reveals that 2 crystallized in monoclinic space
group P21/c and exhibits two types of coordinated Cd(II) ions in the asymmetrical unit of the
unit cell (figure 3a). All the Cd-O bond lengths (from 2.210(6) Å to 2.455(5) Å) and the Cd-N
bond length (2.252(5) Å) fall in the normal range [32, 36]. In each Cd3(μ-O2CR)6(MbIm)2 SBU,
the two symmetry-related Cd(1) ions with a distorted [CdO5N] octahedral geometry are six
coordinate with one nitrogen from MbIm, one oxygen of DMF and four oxygens from three
individual carboxylates. The three carboxyl groups bridge to central Cd(2), resulting in an
approximately octahedral coordination environment. Each bridging carboxylate radiates from the
Cd3(μ-O2CR)6(MbIm)2 at approximately 60° angles to form a 2-D layer with triangular grids in
the bc plane (figure 3b). The pores of as-synthesized 2 are occupied by DMF molecules, with
solvent-accessible void volume of 26.7% by PLATON [34]. The six-connected SBUs and the 2-
D networks of 2 are similar to those of linear trinuclear M3(COO)n complexes —
[NH2ET2]2[Zn3(μ-BDC)4]·2.5DEF [37] and [Zn3(NH2BDC)3(H2O)2]·5DMF [38]. In both
complexes, the 2-D layers are linked into a 3D structure with bridging BDC2- ligands. However,
in 2, the MbIm ligand coordinates to peripheral Cd(II) as a terminal ligand, preventing the 2-D
extending to a 3-D framework. From a topological viewpoint, two MbIm ligands and three
Cd(II) ions constitute one repeat unit Cd3(MbIm)2, and each unit as a node links six BDC2- ions,
therefore 2 can be viewed as a 6-connected plane with {36•46•53} topology (figure 3c).
3.3. Powder X-ray diffraction patterns and thermal analysis
The X-ray powder diffraction (XRD) was conducted to check the phase purity of 1 and 2 (see
figure S3). All the peaks in experimental patterns almost match those in the calculated patterns
6

from the single-crystal X-ray diffraction data, indicating the purity of the as-synthesized
complexes.
According to TGA data measured under nitrogen from 30 °C to 800 °C, both 1 and 2
show high thermal stability and decompose about 370 °C, as shown in figure S4. With increase
of temperature, 1 exhibits weight loss of 14.67% from 110 °C to 220 °C, corresponding to one
guest DMF (Calcd: 14.39%). Then evaporation of another DMF is divided into two stages: a
mild reduced phase (220-310 °C) and a sharp reduced phase (310-360 °C). The total weight loss
of 28.62% in the temperature range of 110-360 °C is consistent with the loss of two DMF
molecules (Calcd: 28.78%). This phenomenon can be caused by differences of intensity of
hydrogen-bonding interaction between the guest DMF molecules and crystal framework [39].
2 loses 11.21% weight at 100-220 °C, corresponding to release of two DMF molecules (Calcd:
10.54%), and begins to collapse at 370 °C.
3.4. Luminescent properties of 1 and 2
To systematically study the luminescent properties of 1 and 2, the solid-state emission spectra of
MbIm, H2BDC, 1 and 2 have been recorded at room temperature. Similar to previously reported
zinc (cadmium) coordination polymers [40, 41], figure 4 indicates that 1 and 2 exhibit ligand-
centered strong luminescent emission at 441 nm (λex = 372 nm) and 428 nm (λex = 330 nm),
respectively. Free MbIm and H2BDC have also been investigated under the same conditions and
exhibit luminescent emission band at 359 nm (λex = 305 nm) and 390 nm (λex = 350 nm),
respectively, which may be attributed to the лӿ л transitions [42, 43]. MbIm plays a main role in
luminescence of 1 and 2. Complex 1 displays red-shifts of 82 nm and 51 nm from the ligand
emission bands of free MbIm and H2BDC, while the red-shift of 2 is 69 nm (relative to MbIm)
and 38 nm (relative to H2BDC), respectively. In order to systematically evaluate the luminescent
properties of 1 and 2 with similar materials, a detailed comparison is listed in table 3. The red-
shifts of 1 and 2 are far larger than the values of other compounds, which makes them potential
luminescent materials. The enhanced luminescence and larger red-shifts of 1 and 2 could be
caused by charge transfer from the respective ligands to the d10 metal ions, Zn(II) and Cd(II),
ligand-to-metal charge transition (LMCT), which contributes to increase in the rigidity of the
ligand and reduces the loss of energy [34-46]. In addition, the red-shift and luminescent emission
intensity of 1 is much higher than for 2, which may be primarily assigned to the high molar ratio
7

(MbIm:H2BDC = 1:1) of MbIm in 1 (1:3 in 2) and the different structures caused by the
heterolinks in this work.
4. Conclusion
We have synthesized two 2-D microporous metal-organic frameworks based on zinc(cadmium)-
dicarboxylate-imidazolate system under solvothermal conditions. After the terminally
coordinated water molecules of MOF-2 are substituted by MbIm ligands, the crystal structure
and properties of 1 are quite different. Complex 1 needs greater interlayer distance and offset to
accommodate MbIm. Similar to 1, MbIm is a terminal ligand coordinating to the peripheral
Cd(II) ions in 2, and further linking 2-D layers by intermolecular hydrogen-bonding interactions.
Topology analysis reveals that 1 exhibits a 2-D layer network with a tetragonal plane framework
with the point symbol of {44•62}, while 2 possesses a 6-connected plane with {36•46•53}
topology. In comparison with other zinc and cadmium coordination polymers based on BDC and
benzimidazole derivatives [40, 43], both 1 and 2 show strong luminescence along with red-shift
(82 nm for 1 and 69 nm for 2), which makes them possess potential application in luminescent
materials. On the basis of the current work, our group is committed to prepare a new class of
coordination polymers with better luminescent properties through increasing the rigidity of
ligands and functionalizing the organic ligands for higher electronic delocalization.
Supplementary material
Details of the refinement procedures for the crystal structures are supplied. The deposition
numbers of 1 and 2 are CCDC 1417969 and 1417970, respectively. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing data-
request@ccdc.cam.ac.uk, or by contacting the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033.
Acknowledgements
This work was supported by the National Science Foundation of China (Grant Nos. 21173122,
21376124, 21476117 and 21501100).
8

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[44] Y.B. Dong, P. Wang, R.Q. Huang, M.D. Smith. Inorg. Chem., 43, 4727 (2004).
[45] G. Wu, X.F. Wang, T. Okamura, W.Y. Sun, N. Ueyama. Inorg. Chem., 45, 8523 (2006).
[46] D.M. Ciurtin, N.G. Pschirer, M.D. Smith, U.H.F. Bunz, H.C. zur Loye. Chem. Mater.,
13, 2743 (2001).
11

Table 1. Crystal data and structure refinement for 1 and 2.
Complex
1
2
Formula
C22H26N4O6Zn
507.86
C52H56N8O16Cd3
1386.28
296
FW
T (K)
293
Crystal system
Monoclinic
C2/c
Monoclinic
P21/c
Space group
a (Å)
22.4814(4)
10.9583(2)
21.6165(4)
90
16.9678(5)
9.6195(3)
18.8606(6)
90
b (Å)
c (Å)
α (°)
β (°)
110.667(1)
90
109.133(1)
90
γ (°)
V (Å3)
4982.69(16)
8
2908.41(16)
2
Z
DC (g cm-3)
1.354
1.583
F(000)
μ (mm-1)
2112
1392
1.028
1.159
Theta Min-Max [°]
Data collected/uniq. (Rint)
R1, wR2 [I > 2 sigma(I2)]
R1, wR2 (all data)
1.9, 27.5
21595/5763(0.018)
0.0503, 0.1727
0.0758, 0.2195
1.08
2.4, 27.6
23545/6744(0.018)
0.0557, 0.1705
0.0646, 0.1952
1.07
2
Goodness-of-fit on F
Residues (e Å-3)
-2.39/1.26
-2.82/1.98
12

Table 2. Comparison of the effect of different terminal ligands on the packing modes of the
layers.
Terminal
ligand
Interlayer
distance (Å)
Interlayer
offset (Å)
Compound
Zn(BDC)(MbIm)·(DMF)
MbIm
H2O
10.51
5.29
6.55
5.11
9.54
4.21
3.27
6.10
Zn(BDC)(H2O)·(DMF) [10]
Zn2(CB-BDC)2(H2O)2·(H2O)3(DMF)1.8 [33]
Zn(BDC)(DMF) [32]
H2O
DMF
13

Table 3. Comparison of luminescent properties with zinc (cadmium) coordination polymers based
on (bimb, imb, PPDA).
Ligand
Complex
Red-shift
(nm)
Complexes
emission bonds emission bonds
(λex)/nm
(λex)
Complex 1
359(305)
359(305)
428/464(350)
305(271)
305(271)
305(285)
441(372)
428(330)
432/465(350)
313(271)
336(271)
336(300)
323(283)
435(328)
419(300)
82
69
4/1
6
Complex 2
[Zn(bimb)(bdc)]·H2O [28]
{[Zn(imb)(SO4)]·H2O}n [23]
{[Cd2(imb)(SO4)2H2O]·CH3OH}n [23]
{[Cd(imb)(bdc)(H2O)]·CH3OH}n [41]
31
31
18
35
19
{[Cd(imb)(Hbtc)(CH3OH)]·2H2O٠CH3OH}n [41] 305(285)
{[Cd(PPDA)(H2O)]·H2O}n [36]
400(270)
{Zn(HPPDA)2(H2O)4}n [36]
400(270)
14

Figure captions
Figure 1. (a) Paddle-wheel coordination environment of Zn2+ in 1. Color code: Zn, blue; C, gray;
N, green; O, red. Hydrogens have been omitted for clarity. (b) One layer viewing along the a
axis. (c) View of alternating diamond and hourglass-shaped pores. (d) 2-D 44·62 topological layer
in the bc plane.
Figure 2. View of two kinds of C-H···O hydrogen bonds linking adjacent layers of 1 [symmetry
codes: (a) –1/2+x, -3/2+y, -1+z; (b) -x, -y, -z; (c) -1/2+x, -1/2+y, -1+z; (d) -x, -1-y, -z].
Figure 3. (a) The coordination environment of Cd(II) in 2; (b) Space-filling representation of 2-D
triangular channels in the bc plane; (c) 2-D 36·46·53 topological layer, with the six-connected
SBUs as the node and BDC as the stick.
Figure 4. Solid-state emission and excitation spectra of MbIm, H2BDC, 1 and 2 at room
temperature.
15

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Graphical abstract
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