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

Abstract 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.


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][2][3][4][5][6][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)(H 2 O)·(DMF) (MOF-2, one of the first materials constructed from a paddle-wheel structure) [10] are synthesized by replacing connectors (e.g. Cu 2+ , Fe 2+ ) and linkers (e.g. Cl 2 -BDCH 2 , dichlorobenzene-1,4-dicarboxylic acid) [11][12][13][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)(H 2 O)·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 [15][16][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][19][20]; (ii) Benzimidazole derivatives with a greater conjugative effect usually exhibit strong luminescence and can be a suitable candidate for luminescent MOFs [21][22][23]; (iii) As an interesting N-containing ligand, MbIm possesses two kinds of coordination modes -monodentate or bidentate, which may lead to diversity of crystal structures [24][25][26]. Transition metal ions (such as Zn 2+ and Cd 2+ ) 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 [Cd 3 (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.  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 disks from 4000 to 400 cm −1 . Thermogravimetric analyses (TGA) were performed on a NETZSCH STA 449F3 analyzer under N 2 . Powder X-ray diffraction 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.

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 F 2 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 × u eq 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.

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 1(a) 1(b)). 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 1(c)). 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 Zn 2 (O 2 CR) 4 (MbIm) 2 can be considered as a four-connected node, and the bdc 2− can be viewed as linear linkers, then 1 can be simplified into a tetragonal plane framework with the point symbol of {4 4 ·6 2 } ( figure 1(d)). Compared with those previously reported for Zn(BDC)(DMF)(H 2 O) (MOF-2) and Zn(BDC)(DMF) [35], the crystal structure of 1 reveals a similar 2-D layer to that of square grids. However, the packing mode of the 2-D layers and the complexity of pores are completely 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 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.

Crystal structure of 2
This compound was prepared in a similar procedure as 1 using Cd(NO 3 ) 2 ·4H 2 O instead of Zn(NO 3 ) 2 ·6H 2 O. 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 3(a)). All the Cd-O bond lengths (from 2.210(6) to 2.455(5) Å) and the Cd-N bond length (2.252(5) Å) fall in the normal range [32,36]. In each Cd 3 (μ-O2CR) 6 (MbIm) 2 SBu, the two symmetry-related Cd(1) ions with a distorted [CdO 5 N] 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 Cd 3 (μ-O 2 CR) 6 (MbIm) 2 at approximately 60° angles to form a 2-D layer with triangular grids in the bc plane ( figure 3(b)). 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 M 3 (COO) n complexes -[NH 2 ET 2 ] 2 [Zn 3 (μ-BDC) 4 ]·2.5DEF [37] and [Zn 3 (NH 2 BDC) 3 (H 2 O) 2 ]·5DMF [38]. In both complexes, the 2-D layers are linked to a 3-D structure with bridging BDC 2− 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 Cd 3 (MbIm) 2 , and each unit as a node links six BDC 2− ions, therefore 2 can be viewed as a 6-connected plane with {3 6 ·4 6 ·5 3 } topology ( figure 3(c)).

Powder X-ray diffraction patterns and thermal analysis
The X-ray powder diffraction 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 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 to 800 °C, both 1 and 2 show high thermal stability and decompose about 370 °C, as shown in figure S4. With increase in temperature, 1 exhibits weight loss of 14.67% from 110 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]. Complex 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.

Luminescent properties of 1 and 2
To systematically study the luminescent properties of 1 and 2, the solid-state emission spectra of MbIm, H 2 BDC, 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  , respectively, which may be attributed to the π * -π transitions [42,43]. MbIm plays a main role in luminescence of 1 and 2. Complex 1 displays red-shifts of 82 and 51 nm from the ligand emission bands of free MbIm and H 2 BDC, while the red-shift of 2 is 69 nm (relative to MbIm) and 38 nm (relative to H 2 BDC), 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 d 10 metal ions, Zn(II) and Cd(II), ligand-to-metal charge transition, which contributes to increase in the rigidity of the ligand and reduces the loss of energy [34][35][36][37][38][39][40][41][42][43][44][45][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 (MbIm:H 2 BDC = 1 : 1) of MbIm in 1 (1 : 3 in 2) and the different structures caused by the heterolinks in this work.

Conclusion
We have synthesized two 2-D microporous MOFs 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 {4 4 ·6 2 }, while 2 possesses a six-connected plane with {3 6 ·4 6 ·5 3 } 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 suitable for 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 Center, 12 union Road, Cambridge CB2 1EZ, uK; Fax: +44 1223 336033.