Solvent-modulated zincII networks with different stacking arrangements

Two ZnII complexes, [Zn2(L)2(DMF)(H2O)2]·DMF (1) and [Zn(L)(DEF)]·DEF (2), were synthesized by solvothermal reactions using 4,4′-(2,3,5,6-tetramethylbenzene1,4-diyl)dibenzoic acid (H2L) and Zn(NO3)2·6H2O in different solvents of DMF, ethanol, and water for 1 and N,N-diethylformamide (DEF) for 2. The L2− with different coordination modes connect [Zn2(COO)2] secondary building units (SBUs) to generate a wavy 2-D (4,4) network of 1 while in 2 there are paddlewheel [Zn2(COO)4] SBUs which are connected by L2− to form a planar 2-D (4,4) network. The 2-D layered structures show different stacking arrangements and are further linked by hydrogen bonding or C–H⋯π interactions to give 3-D architectures. The different structures and stacking arrangements of 1 and 2 result from different reaction solvents. Photoluminescence properties of the complexes were investigated. Graphical Abstract


Introduction
The synthesis of metal-organic frameworks (MOFs) with diverse structures and specific properties remains an intensive research area [1,2]. Many MOFs with 1-D, 2-D, and 3-D structures have been reported using pre-designed organic ligands to link metal ions or secondary building units (SBUs) [3,4]. It is helpful to recognize the SBUs which determine the final structures while fixing the organic ligands. The geometry of the SBUs is dependent on the bridging groups of the ligands, metal ions, reaction solvent, etc. [5]. Solvent molecules with different sizes, polarities, and coordination abilities usually play important roles in the construction of MOFs from both thermodynamic and kinetic aspects: (i) solvent molecules as ligands coordinate with metal ions; (ii) solvent molecules as guests exist in the voids of frameworks. In fact, the reaction solvent certainly influences the crystal growth and the final structure of MOFs [6].

Materials and methods
All commercially available chemicals and solvents are of reagent grade and were used as received. H 2 L ligand was prepared according to the previously reported method [7a]. Elemental analyses for C, H, and N were performed on a Perkin-Elmer 240C elemental analyzer. The thermogravimetric analyses (TGA) were performed on a simultaneous SDT 2960 thermal analyzer at a heating rate of 10°C min −1 under nitrogen. FT-IR spectra were recorded from 400 to 4000 cm −1 on a Bruker Vector22 FT-IR spectrophotometer using KBr pellets. PXRD patterns were obtained on a Bruker D8 Advance X-ray diffractometer with Cu Kα (λ = 1.5418 Å) radiation at room temperature. Photoluminescence spectra for solidstate samples at room temperature were measured on a Perkin Elmer LS55.

X-ray crystallography
Crystallographic data of 1 and 2 were collected on a Bruker Smart Apex CCD with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 293(2) K using the ω-scan technique. The data were integrated using the SAINT program [11(a)], which was also used for intensity corrections for Lorentz and polarization effects. Semiempirical absorption correction was applied using SADABS [11(b)]. The structures were solved by direct methods and all non-hydrogen atoms were refined anisotropically on F 2 by full-matrix least-squares using the SHELXL crystallographic software package [11(c)]. The hydrogens of coordinated water molecules in 1 were found directly and all hydrogens of organic ligands were generated geometrically and refined isotropically using the riding model. The details of the crystal parameters, data collection, and refinements of the complexes are summarized in table 1, and selected bond lengths and angles are listed in table 2.

Results and discussion
3.1. Structure description of [Zn 2 (L) 2 (DMF)(H 2 O) 2 ]·DMF (1) The asymmetric unit of 1 consists of two Zn II ions, two L 2− , two coordinated water molecules, and one coordinated and one free DMF. As shown in figure 1(a), Zn1 is surrounded by four oxygens from three different L 2− ligands and one water molecule to give tetrahedral coordination geometry, while Zn2 is surrounded by six oxygens from three distinct L 2− ligands, one water molecule, and one DMF to form distorted octahedral coordination geometry. In 1, two L 2ligands present the different coordination modes as illustrated in scheme 1(a) and (b): (κ 2 )-(κ 1 )-μ 2 -L 2− and (κ 1 -κ 1 )-(κ 1 -κ 1 )-μ 4 -L 2− . Two Zn II (one Zn1 and one Zn2) ions are linked by two carboxylate groups each with (κ 1 -κ 1 )-μ 2 coordination to give a  figure  1(e). The hydrogen bonding data are summarized in table S1.

Structure description of [Zn(L)(DEF)]·DEF (2)
When the reaction solvent was changed from the mixed solvents of DMF, EtOH, and H 2 O to DEF, 2 was isolated; the asymmetric unit of 2 consists of one Zn II , one L 2− , one coordinated DEF and one free DEF. As shown in figure 2(a), each Zn II is coordinated by  (2) Zn (2) and (CH 3 CH 2 ) 2 NH, generated in situ by the decomposition of DEF and DMF, respectively [13], which leads to the deprotonation of H 2 L to give L 2− . The complete deprotonation of H 2 L was further confirmed by IR spectral data of 1 and 2 since no vibration bands were observed between 1680 and 1760 cm −1 [figure S (2)]. In addition, DMF and DEF serve as both ligands and guests in these two complexes, which may hinder the formation of higher dimensional frameworks [14]. In 1, there are coordinated H 2 O molecules and dinuclear [Zn 2 (COO) 2 ] SBUs linked by ligands to form a wavy network, while in 2 there are wellknown paddlewheel [Zn 2 (COO) 4 ] SBUs which are joined together by ligands to generate a planar layer structure.

Powder X-ray diffraction and thermal analysis
The pure phases of 1 and 2 are confirmed by PXRD measurements. As shown in figure 3, the PXRD pattern of the as-synthesized sample is in accord with the simulated one.  TGA was performed in nitrogen for 1 and 2 and the TG curves are shown in figure S3. The TG curve of 1 reveals a weight loss of 17.39% (Calcd 17.21%) between 30 and 280°C, which corresponds to the loss of two coordinated water molecules, one free DMF, and one coordinated DMF. The TG curve of 2 shows a weight loss of 31.14% (Calcd 31.56%) from 30 to 300°C, which corresponds to the release of one free DEF and one coordinated DEF.

Photoluminescence properties
Solid-state emission spectra of 1 and 2 at room temperature are shown in figure 4. Emission bands are observed at 354 nm (λ ex = 303 nm) for H 2 L, 365 nm (λ ex = 308 nm) for 1, and 338 nm (λ ex = 282 nm) for 2. The emissions of the complexes are neither metal-to-ligand charge transfer nor ligand-to-metal charge transfer due to d 10 electronic configuration of Zn II which is difficult to oxidize or reduce [15]. Therefore, the photoluminescence emissions of the complexes can be assigned to ligand transitions because of their similarity with the free H 2 L ligand. Such ligand-based emission is ascribed to the π* → n or π* → π electronic transitions. The dissimilarity of the emissions of the complexes and the ligands may originate from the coordination of the ligands [16]. Complex 2 has an obvious blue-shift compared with 1. It may be ascribed to different interactions between the layers, the hydrogen bonds in 1 and the C-H⋯π interactions in 2 [16(b), and (d)].

Conclusion
We synthesized and characterized two Zn II -L 2− complexes with different [Zn 2 (COO) 2 ] and [Zn 2 (COO) 4 ] SBUs from different solvents. Both are 2-D (4,4) networks but with different stacking arrangements of the layers, an ABAB stacking mode of the wavy layers in 1 and an ABCD stacking mode of the planar layers in 2. The layers are further connected into 3-D frameworks through weak supramolecular interactions. Complexes 1 and 2 exhibit solid-state emissions at 365 and 338 nm upon excitation at 308 and 282 nm, respectively. The result implies that the reaction solvent plays an important role in determining the structure of the complexes.

Supplementary material
Crystallographic data for the structures reported in this article have been deposited with the Cambridge Crystallographic Data Center as Supplementary Publication No. CCDC-1011710 (1) and 1011711 (2). Copies of the data can be obtained at http://www.ccdc.cam. ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: + 44 1223336033; E-mail: deposit@ccdc.cam.ac.uk).