A hydro­stable and twofold inter­penetrating three-dimensional zinc–organic framework with rob topology based on 4,4′-oxydibenzoate and 3,3′-dimethyl-4,4′-bi­pyridine ligands

Metal–organic frameworks (MOFs) are a new class of porous materials that have received widespread attention due to their potential applications in gas storage and/or separation, catalysis, luminescence, and so on. The title com­pound, poly[[(μ₂-3,3′-dimethyl-4,4′-bi­pyridine-κ²N:N′)bis­(μ₄-4,4′-oxydibenzoato-κ⁴O:O′:O′′:O′′′)dizinc] tetra­hydrate], {[Zn₂(C₁₄H₈O₅)₂(C₁₂H₁₂N₂)]·4H₂O}_n, has been prepared by the solvothermal assembly of Zn(NO₃)₂·6H₂O, 4,4′-oxydi(benzoic acid) and 3,3′-dimethyl-4,4′-bi­pyridine. The two Zn^II atoms adopt the same five-coordinated distorted square-pyramidal geometry (i.e. ZnO₄N), bonding to four O atoms from four different 4,4′-oxydibenzoate (oba) ligands and one N atom from a 3,3′-dimethyl-4,4′-bi­pyridine (dmbpy) ligand. The supra­molecular secondary building unit (SBU) is a paddle-wheel [Zn₂(COO)₄] unit and these units are linked by oba ligands within the layer to form a two-dimensional net parallel to the b axis, with the dmbpy ligands pointing alternately up and down, which is further extended by dmbpy ligands to form a three-dimensional framework with rob topology. The single net leaves voids that are filled by mutual inter­penetration of an independent equivalent framework in a twofold inter­penetrating architecture. The title compound shows thermal stability up to 673 K and is stable in aqueous solutions in the pH range 5–9. Excitation and luminescence data observed at room temperature show that it emits a bright-blue fluorescence.

S1. Introduction

Metal–organic frameworks (MOFs), as a relatively new type of inorganic–organic hybrid materials, have received widespread attention over the past decade due to their structural diversity, fascinating topologies, chemical tailorability and tenability, as well as their excellent properties with promising applications, such as gas storage and separation (Li et al., 2015; Waller et al., 2015; Alezi et al., 2015), nonlinear optics (He et al., 2015), catalysis (Chen et al., 2016; Kitagawa et al., 2004), magnetism (Aulakh et al., 2015), luminescence (Lin et al., 2015), drug delivery (Ma, Li, Xiao et al., 2015), sensing (Weiss et al., 2016) and detection (Xie et al., 2015). During the attainment of MOFs, many factors can influence the construction progress, e.g. metal ions, organic ligands, solvents, pH values, reaction temperatures, and so on (Rao et al., 2009; Liu et al., 2010). Among many on-going efforts to develop high-performance MOF materials, the selection and utilization of different organic ligands are considered to be the most significant factors that affect the structures and properties of the final products (Wan et al., 2015; Liu et al., 2011).

Moreover, a number of reports have shown that many carboxyl­ate-based MOFs involving transition metal ions (e.g. Zn²⁺, Cd²⁺ and Co²⁺) with or without nitro­gen-containing auxiliary ligands are unstable and lose their structural integrity rapidly when exposed to air due to the relatively weak metal–oxygen bonds within the frameworks are easy to be attacked by water molecules (Ma et al., 2011; Canivet et al., 2014; Zhang et al., 2014). This drawback has been recognized as an imperative issue for their practical applications. Four main strategies are used to prepare the hydro­stable carboxyl­ate-based MOFs: (i) using high oxidation state metals (e.g. Zr⁴⁺, Cr³⁺ etc.) (Kandiah et al., 2010); (ii) introducing hydro­phobic groups (e.g. methyl and ethyl) to the organic ligands (Ma et al., 2011); (iii) doping hybrid composites (e.g. carbon nanotubes or hetero-metals) into the frameworks to prepare the complex (Li et al., 2012); (iv) using inter­penetration or catenation of the frameworks to narrow the pore size (Jasuja & Walton, 2013).

3,3'-Di­methyl-4,4'-bi­pyridine (dmbpy) may act in bidentate bridging mode (Ma, Li, Wu et al., 2015), whereas the 4,4'-oxydibenzoate (oda) ligand has versatile binding and coordination modes, and can also be used to construct multinuclear structures (Yang et al., 2012). To our knowledge, very few examples of MOFs based on carboxyl­ate and dmbpy ligands have been reported. On the basis of these considerations, we chose to react Zn(NO₃)₂·6H₂O with 4,4'-oxydi(benzoic acid) (H₂oba) and dmbpy, and obtained the title compound, {[Zn₂(oba)₂(dmbpy)]·4H₂O)}_n, (I).

S2. Experimental

Reagent-grade 4,4'-oxydi(benzoic acid) (H₂oba) and Zn(NO₃)₂·6H₂O metal salts were obtained from Aladdin and used as received. 3,3'-Di­methyl-4,4'-bi­pyridine (dmbpy) was isolated based on the procedure of Ma et al. (2011). Elemental analyses for C, H, and N were carried out using a Vario EL III Elemental Analyzer. IR spectra were recorded (4000–400 cm^-1) as KBr disks on a Shimadzu IR-440 spectrometer. Thermogravimetric analyses (TGA) were performed on an automatic simultaneous thermal analyzer (DTG-60, Shimadzu) under a flow of N₂ at a heating rate of 10 K min^-1 between ambient temperature and 1073 K. Powder XRD investigations were carried out on a Bruker AXS D8-Advanced diffractometer at 40 kV and 40 mA with Cu Kα (λ = 1.5406 Å) radiation. Luminescence spectra for crystalline samples were recorded at room temperature on an Edinburgh FLS920 phospho­rimeter.

S2.1. Synthesis and crystallization

A mixture of Zn(NO₃)₂·6H₂O (0.089 g, 0.3 mmol), H₂oba (0.077 g, 0.3 mmol) and dmbpy (0.028 g, 0.15 mmol) was dissolved in N,N-di­methyl­acetamide (DMA; 10 ml) and stirred for 30 min. Tthe solution was then heated to 403 K for 3 d in a 23 ml Teflon-lined stainless steel autoclave, followed by cooling to room temperature to yield colourless single crystals (yield 40%, based on H₂oba). Analysis calculated for C₂₀H₁₈NO₇Zn: C 53.37, H 4.00, N 3.11%; found: C 53.59, H 4.16, N 3.02%. IR (KBr, cm^-1): 3444 (vs), 2977 (w), 2927 (w), 1609 (vs), 1373 (s), 1228 (s), 1155 (s), 1083 (m), 866 (m), 775 (s), 662 (s), 593 (w), 507 (w), 440 (w).

S2.2. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. All H atoms were positioned geometrically and constrained using the riding-model approximation, with C—H = 0.93 (aromatic) or 0.96 Å (methyl) and U_iso (H) = 1.2 U_eq (C). The routine SQUEEZE routine (Spek, 2015) in PLATON (Spek, 2009) was applied to remove diffuse electron density caused by badly disordered water molecules. The formula unit was arrived at through combination of elemental analyses, IR spectra and thermogravimetric characterization. The dmbpy ligand was disordered strongly. We have used the PART instruction to deal with this problem; however, in the PART 2 section, the –CH₃ group still vibrated in two positions, namely C29B and C29C. This complicated disorder led to difficulty in adding the 0.25 H atoms at C21B and C19B, so the final formula is lacking 0.5 H atoms.

S2.3. IR analysis

The FT–IR spectrum of (I) was recorded as KBr pellets (Fig. S1 in the Supporting information). In the IR spectrum, strong broad bands at 3444 cm^-1 can be assigned to the ν(O—H) stretching vibrations of the water molecules. Features at 2977 and 2927 cm^-1 are associated with the methyl groups of dmbpy ligands. The features at 1609 and 1373 cm^-1 are associated with the asymmetric (C—O—C) and symmetric (C—O—C) stretching vibrations.

S3. Results and discussion

S3.1. Structure analysis

The asymmetric unit of (I) includes one Zn²⁺ ion, one oba^2- dianionic ligand, half a dmbpy ligand and two lattice water molecules. Each Zn^II centre is five-coordinated by four carboxyl­ate O atoms from four different oba^2- ligands and one N atom from a dmbpy ligand (Fig. 1), adopting a distorted square-pyramidal geometry (i.e. ZnO₄N), with Zn—O bond lengths and O—Zn—O angles in the ranges 2.023 (3)–2.047 (3) Å and 88.63 (17)–160.50 (11)°, respectively, all of which are within the reasonable range of those reported for other five-coordinated Zn^II complexes with O- and N-donating ligands (Ma et al., 2011). In the polymeric structure of (I), the oba^2- ligands adopt a µ₄-bridging mode connecting four Zn^II ions, whereas the dmbpy ligand is coordinated in a trans-bidentate bridging mode linking two Zn^II ions from dinuclear zinc cores separated by 13.913 (2) Å. The four oba^2- ligands link two Zn^II ions to construct a dinuclear zinc building block, [Zn₂(COO)₄], which can be regarded as a supra­molecular secondary building unit (SBU) or cluster node. The connectivity of the oba^2- ligand to Zn²⁺ ions leads to the formation of a two-dimensional net, where the dinuclear zinc cores are separated by 14.165 (3) Å (Fig. 2). The rigid µ₂-dmbpy struts connect the layer to other ones alternately up and down, resulting in a three-dimensional structure (Fig. 3). The free volume of voids created by the formal removal of the lattice waters is 872.0 ų, which is about 20.5% of the unit-cell volume. The single net leaves voids that are filled by mutual inter­penetration of an independent equivalent framework in a twofold inter­penetrating architecture (Fig. 4).

A better insight into this framework can be achieved by topology analysis (Blatov et al., 2014). In this structure, each SBU can act as a 6-connected node, with oba^2- and dmbpy ligands as linkers, and the title compound can be represented as a rob net with a point symbol of (4⁸.6⁶.8) (Fig. 5).

Besides (I), there are only five three-dimensional frameworks with rob topology involving the oba^2- and Zn^II/Mg^II ions with or without N-containing auxiliary ligands, viz. [Zn₂(oba)₂(bpy)]·DMA (DMA is N,N-di­methyl­acetamide and bpy is -4,4'-bypyridine; Pramanik et al., 2011), (II), and [Zn₂(oba)₂(bpy)]·DMF (DMF is di­methyl­formamide; Tan et al., 2012), (III), [Zn₂(oba)₂(bpe)]·2DMF (bpe is ???; Kondo et al., 2007), [Zn₂(oba)₂(3-BPT)]·2.5DMF (3-BPT is ???; Xie & Wu, 2014) and [Mg₃(oba)₃(H₂O)(DMA)₂]·2DMA (Li et al., 2011). From these, (II) and (III) show the same twofold inter­penetrating framework with different inclusions (Pramanik et al., 2011; Tan et al., 2012). Indeed, the participation of angular ligands (like oba^2-) with Zn^II is an essential condition in the design of new inter­penetrating networks with rob topology. Thus, similar to compounds (II) and (III), which are built from oba^2- and 4,4'-bi­pyridine ligands, the title compound was constructed from oba^2- and methyl-functionalized bi­pyridine (dmbpy). However, this functionalization does not affect the inter­penetration, since structures (I)–(III) demonstrate the same twofold inter­penetration pattern, which differs from the inter­penetration mode of the other three compounds. Moreover, in comparison to (I)–(III), the two twofold inter­penetrating three-dimensional zinc-based MOFs involving dmbpy ligands and linear benzene-1,4-di­carboxyl­ate or naphthalene-1,4-di­carboxyl­ate ligands (Ma et al., 2011; Ma, Li, Xiao et al., 2015) exhibit pcu topology.

S3.2. Thermal and chemical stability

To characterize the thermal stability of (I), we performed the thermogravimetric analysis (TGA). The relevant experiments for the crystalline sample of (I) were performed under an N₂ atmosphere wherein the sample was heated to 1073 K at a rate of 10 K min^-1. The TG–DTA curves are shown in Fig. 6 and the results reveal that (I) demonstrate a good thermal stability. The weight loss in the temperature range 373–573 K corresponds to the removal of two lattice water molecules (calculated 8.01%, observed 8.22%). Upon further heating, the framework was stable up to 643 K and then a sharp weight loss was observed above 643 K due to the degradation of the framework.

The pH-dependent stabilities of (I) in aqueous solutions were investigated by XRPD (Fig. 7). For these tests, 50 mg of as-synthesized crystals were soaked in aqueous solutions with different pH values and stirred for 3 d at room temperature. According to these results, the title compound is stable at pH range from 5 to 9. Stirring under more basic (pH = 13–14) or acidic (pH = 2–3) conditions, show complete decomposition of their framework. The stabilities of title compound are similar to several aluminum-isophthalate-based MOFs (CAU-10-X, where X = H, CH₃, OCH₃, NO₂, NH₂ or OH) (Reinsch et al., 2013), but lower than the series of carboxyl­ate-based MOFs involving zirconium ions (Schaate et al., 2011). However, the results is very remarkable, especially when compared with other MOFs constructed from aromatic carboxyl­ate N-containing auxiliary ligands and zinc/cadmium ions (Tan et al., 2015; Chen et al., 2006).

S3.3. Luminescence properties

Luminescence properties of coordination polymers with d¹⁰ metal centres have attracted intense inter­est because of their potential applications as sensing luminescent materials (An et al., 2011; Gole et al., 2011). Herein, we examined the luminescence properties of (I) in the solid state at room temperature. The photoluminescence spectrum of H₂oba shows an emission maximum at 432 nm (Hu et al., 2012). The emission band of the free ligand is probably attributable to the π*–π transition. Upon complexation of the ligand with Zn^II ions, the emission peak occur at 426 nm (λ_ex = 310 nm) (Fig. 8). The small blue-shift indicates that the luminescence of (I) probably origins from the oba^2- ligands and the role of the Zn^II centre is believed to stabilize the ligands and increase the conformational rigidity of the ligands by coordination.

S4. Conclusions

The assembly of a zinc(II) salt with H₂oba and methyl-functionalized dmbpy ligands under solvothermal conditions results in the formation of the title twofold inter­penetrating three-dimensional MOF, (I), with rob underlying topology. It have been shown that (I) is stable in aqueous solutions in the pH range 5–9, and it emits a bright blue fluorescence (λ_em = 426 nm). The compound represents a new example of a twofold inter­penetrating three-dimensional framework in an isoreticular series of structures with rob topology based on an angular ligand (oba^2-) in combination with a linear N-containing auxiliary ligand (dmbpy).

Structure description

Metal–organic frameworks (MOFs), as a relatively new type of inorganic–organic hybrid materials, have received widespread attention over the past decade due to their structural diversity, fascinating topologies, chemical tailorability and tenability, as well as their excellent properties with promising applications, such as gas storage and separation (Li et al., 2015; Waller et al., 2015; Alezi et al., 2015), nonlinear optics (He et al., 2015), catalysis (Chen et al., 2016; Kitagawa et al., 2004), magnetism (Aulakh et al., 2015), luminescence (Lin et al., 2015), drug delivery (Ma, Li, Xiao et al., 2015), sensing (Weiss et al., 2016) and detection (Xie et al., 2015). During the attainment of MOFs, many factors can influence the construction progress, e.g. metal ions, organic ligands, solvents, pH values, reaction temperatures, and so on (Rao et al., 2009; Liu et al., 2010). Among many on-going efforts to develop high-performance MOF materials, the selection and utilization of different organic ligands are considered to be the most significant factors that affect the structures and properties of the final products (Wan et al., 2015; Liu et al., 2011).

Moreover, a number of reports have shown that many carboxyl­ate-based MOFs involving transition metal ions (e.g. Zn²⁺, Cd²⁺ and Co²⁺) with or without nitro­gen-containing auxiliary ligands are unstable and lose their structural integrity rapidly when exposed to air due to the relatively weak metal–oxygen bonds within the frameworks are easy to be attacked by water molecules (Ma et al., 2011; Canivet et al., 2014; Zhang et al., 2014). This drawback has been recognized as an imperative issue for their practical applications. Four main strategies are used to prepare the hydro­stable carboxyl­ate-based MOFs: (i) using high oxidation state metals (e.g. Zr⁴⁺, Cr³⁺ etc.) (Kandiah et al., 2010); (ii) introducing hydro­phobic groups (e.g. methyl and ethyl) to the organic ligands (Ma et al., 2011); (iii) doping hybrid composites (e.g. carbon nanotubes or hetero-metals) into the frameworks to prepare the complex (Li et al., 2012); (iv) using inter­penetration or catenation of the frameworks to narrow the pore size (Jasuja & Walton, 2013).

3,3'-Di­methyl-4,4'-bi­pyridine (dmbpy) may act in bidentate bridging mode (Ma, Li, Wu et al., 2015), whereas the 4,4'-oxydibenzoate (oda) ligand has versatile binding and coordination modes, and can also be used to construct multinuclear structures (Yang et al., 2012). To our knowledge, very few examples of MOFs based on carboxyl­ate and dmbpy ligands have been reported. On the basis of these considerations, we chose to react Zn(NO₃)₂·6H₂O with 4,4'-oxydi(benzoic acid) (H₂oba) and dmbpy, and obtained the title compound, {[Zn₂(oba)₂(dmbpy)]·4H₂O)}_n, (I).

Reagent-grade 4,4'-oxydi(benzoic acid) (H₂oba) and Zn(NO₃)₂·6H₂O metal salts were obtained from Aladdin and used as received. 3,3'-Di­methyl-4,4'-bi­pyridine (dmbpy) was isolated based on the procedure of Ma et al. (2011). Elemental analyses for C, H, and N were carried out using a Vario EL III Elemental Analyzer. IR spectra were recorded (4000–400 cm^-1) as KBr disks on a Shimadzu IR-440 spectrometer. Thermogravimetric analyses (TGA) were performed on an automatic simultaneous thermal analyzer (DTG-60, Shimadzu) under a flow of N₂ at a heating rate of 10 K min^-1 between ambient temperature and 1073 K. Powder XRD investigations were carried out on a Bruker AXS D8-Advanced diffractometer at 40 kV and 40 mA with Cu Kα (λ = 1.5406 Å) radiation. Luminescence spectra for crystalline samples were recorded at room temperature on an Edinburgh FLS920 phospho­rimeter.

The FT–IR spectrum of (I) was recorded as KBr pellets (Fig. S1 in the Supporting information). In the IR spectrum, strong broad bands at 3444 cm^-1 can be assigned to the ν(O—H) stretching vibrations of the water molecules. Features at 2977 and 2927 cm^-1 are associated with the methyl groups of dmbpy ligands. The features at 1609 and 1373 cm^-1 are associated with the asymmetric (C—O—C) and symmetric (C—O—C) stretching vibrations.

The asymmetric unit of (I) includes one Zn²⁺ ion, one oba^2- dianionic ligand, half a dmbpy ligand and two lattice water molecules. Each Zn^II centre is five-coordinated by four carboxyl­ate O atoms from four different oba^2- ligands and one N atom from a dmbpy ligand (Fig. 1), adopting a distorted square-pyramidal geometry (i.e. ZnO₄N), with Zn—O bond lengths and O—Zn—O angles in the ranges 2.023 (3)–2.047 (3) Å and 88.63 (17)–160.50 (11)°, respectively, all of which are within the reasonable range of those reported for other five-coordinated Zn^II complexes with O- and N-donating ligands (Ma et al., 2011). In the polymeric structure of (I), the oba^2- ligands adopt a µ₄-bridging mode connecting four Zn^II ions, whereas the dmbpy ligand is coordinated in a trans-bidentate bridging mode linking two Zn^II ions from dinuclear zinc cores separated by 13.913 (2) Å. The four oba^2- ligands link two Zn^II ions to construct a dinuclear zinc building block, [Zn₂(COO)₄], which can be regarded as a supra­molecular secondary building unit (SBU) or cluster node. The connectivity of the oba^2- ligand to Zn²⁺ ions leads to the formation of a two-dimensional net, where the dinuclear zinc cores are separated by 14.165 (3) Å (Fig. 2). The rigid µ₂-dmbpy struts connect the layer to other ones alternately up and down, resulting in a three-dimensional structure (Fig. 3). The free volume of voids created by the formal removal of the lattice waters is 872.0 ų, which is about 20.5% of the unit-cell volume. The single net leaves voids that are filled by mutual inter­penetration of an independent equivalent framework in a twofold inter­penetrating architecture (Fig. 4).

A better insight into this framework can be achieved by topology analysis (Blatov et al., 2014). In this structure, each SBU can act as a 6-connected node, with oba^2- and dmbpy ligands as linkers, and the title compound can be represented as a rob net with a point symbol of (4⁸.6⁶.8) (Fig. 5).

Besides (I), there are only five three-dimensional frameworks with rob topology involving the oba^2- and Zn^II/Mg^II ions with or without N-containing auxiliary ligands, viz. [Zn₂(oba)₂(bpy)]·DMA (DMA is N,N-di­methyl­acetamide and bpy is -4,4'-bypyridine; Pramanik et al., 2011), (II), and [Zn₂(oba)₂(bpy)]·DMF (DMF is di­methyl­formamide; Tan et al., 2012), (III), [Zn₂(oba)₂(bpe)]·2DMF (bpe is ???; Kondo et al., 2007), [Zn₂(oba)₂(3-BPT)]·2.5DMF (3-BPT is ???; Xie & Wu, 2014) and [Mg₃(oba)₃(H₂O)(DMA)₂]·2DMA (Li et al., 2011). From these, (II) and (III) show the same twofold inter­penetrating framework with different inclusions (Pramanik et al., 2011; Tan et al., 2012). Indeed, the participation of angular ligands (like oba^2-) with Zn^II is an essential condition in the design of new inter­penetrating networks with rob topology. Thus, similar to compounds (II) and (III), which are built from oba^2- and 4,4'-bi­pyridine ligands, the title compound was constructed from oba^2- and methyl-functionalized bi­pyridine (dmbpy). However, this functionalization does not affect the inter­penetration, since structures (I)–(III) demonstrate the same twofold inter­penetration pattern, which differs from the inter­penetration mode of the other three compounds. Moreover, in comparison to (I)–(III), the two twofold inter­penetrating three-dimensional zinc-based MOFs involving dmbpy ligands and linear benzene-1,4-di­carboxyl­ate or naphthalene-1,4-di­carboxyl­ate ligands (Ma et al., 2011; Ma, Li, Xiao et al., 2015) exhibit pcu topology.

To characterize the thermal stability of (I), we performed the thermogravimetric analysis (TGA). The relevant experiments for the crystalline sample of (I) were performed under an N₂ atmosphere wherein the sample was heated to 1073 K at a rate of 10 K min^-1. The TG–DTA curves are shown in Fig. 6 and the results reveal that (I) demonstrate a good thermal stability. The weight loss in the temperature range 373–573 K corresponds to the removal of two lattice water molecules (calculated 8.01%, observed 8.22%). Upon further heating, the framework was stable up to 643 K and then a sharp weight loss was observed above 643 K due to the degradation of the framework.

The pH-dependent stabilities of (I) in aqueous solutions were investigated by XRPD (Fig. 7). For these tests, 50 mg of as-synthesized crystals were soaked in aqueous solutions with different pH values and stirred for 3 d at room temperature. According to these results, the title compound is stable at pH range from 5 to 9. Stirring under more basic (pH = 13–14) or acidic (pH = 2–3) conditions, show complete decomposition of their framework. The stabilities of title compound are similar to several aluminum-isophthalate-based MOFs (CAU-10-X, where X = H, CH₃, OCH₃, NO₂, NH₂ or OH) (Reinsch et al., 2013), but lower than the series of carboxyl­ate-based MOFs involving zirconium ions (Schaate et al., 2011). However, the results is very remarkable, especially when compared with other MOFs constructed from aromatic carboxyl­ate N-containing auxiliary ligands and zinc/cadmium ions (Tan et al., 2015; Chen et al., 2006).

Luminescence properties of coordination polymers with d¹⁰ metal centres have attracted intense inter­est because of their potential applications as sensing luminescent materials (An et al., 2011; Gole et al., 2011). Herein, we examined the luminescence properties of (I) in the solid state at room temperature. The photoluminescence spectrum of H₂oba shows an emission maximum at 432 nm (Hu et al., 2012). The emission band of the free ligand is probably attributable to the π*–π transition. Upon complexation of the ligand with Zn^II ions, the emission peak occur at 426 nm (λ_ex = 310 nm) (Fig. 8). The small blue-shift indicates that the luminescence of (I) probably origins from the oba^2- ligands and the role of the Zn^II centre is believed to stabilize the ligands and increase the conformational rigidity of the ligands by coordination.

The assembly of a zinc(II) salt with H₂oba and methyl-functionalized dmbpy ligands under solvothermal conditions results in the formation of the title twofold inter­penetrating three-dimensional MOF, (I), with rob underlying topology. It have been shown that (I) is stable in aqueous solutions in the pH range 5–9, and it emits a bright blue fluorescence (λ_em = 426 nm). The compound represents a new example of a twofold inter­penetrating three-dimensional framework in an isoreticular series of structures with rob topology based on an angular ligand (oba^2-) in combination with a linear N-containing auxiliary ligand (dmbpy).

Synthesis and crystallization

A mixture of Zn(NO₃)₂·6H₂O (0.089 g, 0.3 mmol), H₂oba (0.077 g, 0.3 mmol) and dmbpy (0.028 g, 0.15 mmol) was dissolved in N,N-di­methyl­acetamide (DMA; 10 ml) and stirred for 30 min. Tthe solution was then heated to 403 K for 3 d in a 23 ml Teflon-lined stainless steel autoclave, followed by cooling to room temperature to yield colourless single crystals (yield 40%, based on H₂oba). Analysis calculated for C₂₀H₁₈NO₇Zn: C 53.37, H 4.00, N 3.11%; found: C 53.59, H 4.16, N 3.02%. IR (KBr, cm^-1): 3444 (vs), 2977 (w), 2927 (w), 1609 (vs), 1373 (s), 1228 (s), 1155 (s), 1083 (m), 866 (m), 775 (s), 662 (s), 593 (w), 507 (w), 440 (w).

Refinement details

Crystal data, data collection and structure refinement details are summarized in Table 1. All H atoms were positioned geometrically and constrained using the riding-model approximation, with C—H = 0.93 (aromatic) or 0.96 Å (methyl) and U_iso (H) = 1.2 U_eq (C). The routine SQUEEZE routine (Spek, 2015) in PLATON (Spek, 2009) was applied to remove diffuse electron density caused by badly disordered water molecules. The formula unit was arrived at through combination of elemental analyses, IR spectra and thermogravimetric characterization. The dmbpy ligand was disordered strongly. We have used the PART instruction to deal with this problem; however, in the PART 2 section, the –CH₃ group still vibrated in two positions, namely C29B and C29C. This complicated disorder led to difficulty in adding the 0.25 H atoms at C21B and C19B, so the final formula is lacking 0.5 H atoms.

Computing details

Data collection: SMART (Bruker, 2008); cell refinement: SMART (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2005; software used to prepare material for publication: SHELXTL (Bruker, 2008).

Figures

	Fig. 1. A view of the coordination environment of the Zn^II ion in (I). All H atoms have been omitted for clarity. [Symmetry code: (i) -x, -y+1, -z+1.] [Please provide a fully labelled plot]
	Fig. 2. A view of the two-dimensional network of (I) along the b axis.
	Fig. 3. A view of the three-dimensional framework of (I) along the c axis.
	Fig. 4. A view of the twofold interpenetration in (I) parallel to the c axis.
	Fig. 5. A view of the rob topological network of (I).
	Fig. 6. TGA–DTA (thermogravimetric analysis–differential thermal analysis) curves of complex (I).
	Fig. 7. Powder X-ray diffraction (PXRD) patterns of (I).
	Fig. 8. Solid-state excitation and emission spectra of (I) at room temperature.

Poly[[(µ₂-3,3'-dimethyl-4,4'-bipyridine-κ²N:N')bis(µ₄-4,4'-oxydibenzoato-κ⁴O:O':O'':O''')dizinc] tetrahydrate]

Crystal data

[Zn2(C14H8O5)2(C12H12N2)]·4H2O	F(000) = 1848
Mr = 899.5	Dx = 1.41 Mg m^−3
Orthorhombic, Pcca	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P2a2ac	Cell parameters from 8103 reflections
a = 16.864 (4) Å	θ = 2.6–25.2°
b = 11.067 (2) Å	µ = 1.20 mm^−1
c = 22.764 (4) Å	T = 296 K
V = 4248.6 (14) Å^3	Block, colorless
Z = 4	0.22 × 0.20 × 0.18 mm

Data collection

Bruker SMART APEX DUO CCD diffractometer	3849 independent reflections
Radiation source: fine-focus sealed tube	3314 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.025
phi and ω scans	θmax = 25.3°, θmin = 1.8°
Absorption correction: multi-scan (SADABS; Bruker, 2008)	h = −20→15
Tmin = 0.781, Tmax = 0.815	k = −13→13
26264 measured reflections	l = −26→27

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.045	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.128	H-atom parameters constrained
S = 1.06	w = 1/[σ^2(Fo^2) + (0.0634P)^2 + 4.8367P] where P = (Fo^2 + 2Fc^2)/3
3849 reflections	(Δ/σ)max = 0.016
266 parameters	Δρmax = 0.53 e Å^−3
246 restraints	Δρmin = −0.50 e Å^−3

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > 2sigma(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
C1	0.0546 (2)	0.3973 (3)	0.40649 (16)	0.0589 (9)
C2	0.0883 (2)	0.3310 (4)	0.35506 (15)	0.0614 (9)
C3	0.0575 (2)	0.2214 (4)	0.33640 (18)	0.0680 (11)
H3	0.0147	0.1879	0.3563	0.082*
C4	0.0892 (2)	0.1613 (4)	0.28896 (18)	0.0734 (11)
H4	0.0681	0.0874	0.2773	0.088*
C5	0.1520 (2)	0.2106 (4)	0.25892 (18)	0.0697 (10)
C6	0.1844 (3)	0.3200 (4)	0.27632 (19)	0.0781 (12)
H6	0.2269	0.3534	0.2560	0.094*
C7	0.1525 (3)	0.3787 (4)	0.32435 (18)	0.0730 (11)
H7	0.1744	0.4517	0.3365	0.088*
C8	0.23239 (13)	0.1993 (2)	0.17420 (11)	0.0698 (11)
C9	0.31206 (15)	0.1671 (2)	0.17514 (11)	0.0838 (14)
H9	0.3297	0.1076	0.2008	0.101*
C10	0.36539 (11)	0.2240 (3)	0.13770 (11)	0.0773 (12)
H10	0.4187	0.2025	0.1383	0.093*
C11	0.33907 (12)	0.3131 (2)	0.09932 (9)	0.0522 (8)
C12	0.25940 (13)	0.3453 (2)	0.09838 (11)	0.0658 (10)
H12	0.2418	0.4049	0.0727	0.079*
C13	0.20606 (10)	0.2884 (2)	0.13582 (13)	0.0773 (12)
H13	0.1528	0.3099	0.1352	0.093*
C14	0.3975 (2)	0.3798 (3)	0.06168 (14)	0.0539 (8)
N1A	0.13457 (17)	0.7306 (2)	0.47790 (12)	0.0619 (9)	0.564 (6)
C18A	0.2118 (2)	0.7201 (3)	0.4572 (2)	0.0697 (18)	0.564 (6)
H18A	0.2312	0.6451	0.4456	0.084*	0.564 (6)
C19A	0.2600 (2)	0.8217 (4)	0.4537 (2)	0.0743 (18)	0.564 (6)
H19A	0.3117	0.8147	0.4398	0.089*	0.564 (6)
C20A	0.2310 (2)	0.9338 (3)	0.47094 (15)	0.0701 (12)	0.564 (6)
C21A	0.15383 (19)	0.9443 (2)	0.4917 (2)	0.0785 (18)	0.564 (6)
C22A	0.10559 (13)	0.8427 (3)	0.4952 (2)	0.0719 (17)	0.564 (6)
H22A	0.0539	0.8497	0.5090	0.086*	0.564 (6)
N1B	0.12462 (15)	0.7505 (2)	0.48373 (12)	0.0619 (9)	0.436 (6)
C18B	0.1182 (2)	0.8396 (3)	0.52649 (17)	0.074 (2)	0.436 (6)
H18B	0.0844	0.8286	0.5583	0.088*	0.436 (6)
C19B	0.1625 (2)	0.9451 (3)	0.52171 (19)	0.076 (2)	0.436 (6)
C20B	0.2130 (2)	0.9616 (2)	0.47418 (16)	0.0701 (12)	0.436 (6)
C21B	0.2194 (3)	0.8724 (4)	0.43142 (12)	0.079 (2)	0.436 (6)
C22B	0.1752 (3)	0.7669 (3)	0.43620 (11)	0.073 (2)	0.436 (6)
H22B	0.1795	0.7072	0.4076	0.088*	0.436 (6)
C23A	0.1193 (7)	1.0573 (8)	0.5101 (4)	0.120 (4)	0.50
H23A	0.1591	1.1061	0.5287	0.179*	0.50
H23B	0.0771	1.0418	0.5375	0.179*	0.50
H23C	0.0986	1.0992	0.4765	0.179*	0.50
C23B	0.2713 (7)	0.8897 (13)	0.3822 (5)	0.111 (6)	0.25
H23D	0.2562	0.9616	0.3614	0.166*	0.25
H23E	0.2676	0.8214	0.3564	0.166*	0.25
H23F	0.3249	0.8978	0.3959	0.166*	0.25
C23C	0.1536 (11)	1.0333 (15)	0.5664 (7)	0.116 (6)	0.25
H23G	0.1191	1.0964	0.5527	0.174*	0.25
H23H	0.2045	1.0668	0.5759	0.174*	0.25
H23I	0.1311	0.9964	0.6007	0.174*	0.25
O1	−0.00252 (16)	0.3506 (2)	0.43328 (11)	0.0686 (7)
O2	0.08591 (19)	0.4959 (3)	0.41976 (11)	0.0804 (9)
O3	0.1801 (2)	0.1448 (3)	0.21242 (14)	0.0899 (10)
Zn1	0.05787 (2)	0.59574 (4)	0.490891 (16)	0.05170 (17)
O4	0.37543 (16)	0.4771 (2)	0.03891 (11)	0.0665 (7)
O5	0.46489 (17)	0.3341 (3)	0.05522 (13)	0.0755 (8)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.063 (2)	0.072 (2)	0.0422 (18)	−0.0232 (18)	0.0015 (15)	−0.0008 (16)
C2	0.059 (2)	0.077 (2)	0.0478 (19)	−0.0234 (19)	0.0049 (16)	−0.0076 (17)
C3	0.060 (2)	0.080 (3)	0.064 (2)	−0.029 (2)	0.0082 (17)	−0.013 (2)
C4	0.066 (2)	0.081 (3)	0.073 (3)	−0.021 (2)	0.005 (2)	−0.023 (2)
C5	0.064 (2)	0.082 (3)	0.064 (2)	−0.012 (2)	0.0092 (18)	−0.017 (2)
C6	0.071 (3)	0.092 (3)	0.071 (3)	−0.025 (2)	0.021 (2)	−0.012 (2)
C7	0.076 (3)	0.082 (3)	0.062 (2)	−0.032 (2)	0.013 (2)	−0.015 (2)
C8	0.070 (3)	0.078 (3)	0.061 (2)	0.000 (2)	0.0121 (19)	−0.017 (2)
C9	0.071 (3)	0.119 (4)	0.062 (2)	0.017 (3)	−0.002 (2)	0.027 (3)
C10	0.054 (2)	0.111 (3)	0.067 (2)	0.026 (2)	0.0013 (18)	0.027 (2)
C11	0.0530 (19)	0.062 (2)	0.0419 (16)	0.0199 (16)	−0.0027 (14)	−0.0049 (15)
C12	0.057 (2)	0.058 (2)	0.082 (3)	0.0184 (18)	−0.0099 (19)	0.000 (2)
C13	0.052 (2)	0.066 (2)	0.114 (4)	0.0128 (19)	0.004 (2)	−0.010 (2)
C14	0.060 (2)	0.059 (2)	0.0428 (17)	0.0258 (17)	0.0010 (15)	−0.0055 (15)
N1A	0.0648 (18)	0.0636 (18)	0.0571 (17)	−0.0475 (16)	0.0012 (14)	−0.0045 (14)
C18A	0.070 (3)	0.064 (3)	0.075 (4)	−0.040 (3)	0.014 (3)	−0.009 (3)
C19A	0.066 (4)	0.067 (3)	0.090 (4)	−0.039 (3)	0.018 (3)	−0.015 (3)
C20A	0.073 (3)	0.064 (2)	0.074 (2)	−0.049 (2)	0.011 (2)	−0.0075 (19)
C21A	0.084 (3)	0.061 (3)	0.090 (4)	−0.039 (3)	0.022 (3)	−0.014 (3)
C22A	0.069 (3)	0.069 (3)	0.077 (4)	−0.046 (3)	0.016 (3)	0.000 (3)
N1B	0.0648 (18)	0.0636 (18)	0.0571 (17)	−0.0475 (16)	0.0012 (14)	−0.0045 (14)
C18B	0.082 (4)	0.068 (3)	0.071 (4)	−0.043 (3)	0.002 (4)	0.004 (3)
C19B	0.086 (4)	0.068 (3)	0.075 (4)	−0.046 (3)	0.015 (4)	−0.003 (3)
C20B	0.073 (3)	0.064 (2)	0.074 (2)	−0.049 (2)	0.011 (2)	−0.0075 (19)
C21B	0.087 (4)	0.074 (4)	0.074 (4)	−0.054 (4)	0.018 (4)	−0.008 (3)
C22B	0.082 (4)	0.068 (4)	0.070 (4)	−0.053 (3)	0.006 (3)	−0.007 (3)
C23A	0.102 (7)	0.077 (5)	0.180 (9)	−0.030 (5)	0.049 (7)	−0.034 (7)
C23B	0.142 (13)	0.077 (10)	0.113 (12)	−0.066 (10)	0.083 (10)	−0.048 (9)
C23C	0.120 (12)	0.084 (10)	0.143 (12)	−0.020 (10)	0.031 (11)	−0.029 (9)
O1	0.0738 (17)	0.0752 (17)	0.0568 (15)	−0.0265 (14)	0.0152 (13)	−0.0091 (13)
O2	0.100 (2)	0.0868 (19)	0.0540 (15)	−0.0488 (17)	0.0197 (14)	−0.0175 (14)
O3	0.096 (2)	0.087 (2)	0.087 (2)	−0.0156 (18)	0.0344 (18)	−0.0249 (17)
Zn1	0.0558 (3)	0.0568 (3)	0.0425 (2)	−0.03991 (19)	0.00089 (16)	0.00041 (16)
O4	0.0709 (16)	0.0594 (15)	0.0693 (16)	0.0290 (13)	0.0059 (13)	0.0073 (13)
O5	0.0667 (16)	0.0844 (19)	0.0753 (17)	0.0388 (15)	0.0175 (14)	0.0181 (15)

Geometric parameters (Å, º)

C1—O1	1.251 (4)	C20A—C21A	1.3900
C1—O2	1.249 (4)	C20A—C20A^i	1.599 (4)
C1—C2	1.494 (5)	C21A—C22A	1.3900
C2—C3	1.387 (5)	C21A—C23A	1.443 (8)
C2—C7	1.392 (5)	C22A—H22A	0.9300
C3—C4	1.376 (6)	N1B—C22B	1.3900
C3—H3	0.9300	N1B—C18B	1.3900
C4—C5	1.374 (5)	N1B—Zn1	2.0557 (17)
C4—H4	0.9300	C18B—C19B	1.3899
C5—O3	1.369 (5)	C18B—H18B	0.9300
C5—C6	1.386 (6)	C19B—C20B	1.3900
C6—C7	1.381 (6)	C19B—C23C	1.416 (9)
C6—H6	0.9300	C20B—C21B	1.3899
C7—H7	0.9300	C20B—C20B^i	1.509 (4)
C8—O3	1.378 (3)	C21B—C22B	1.3901
C8—C9	1.3900	C21B—C23B	1.435 (9)
C8—C13	1.3900	C22B—H22B	0.9300
C9—C10	1.3900	C23A—H23A	0.9600
C9—H9	0.9300	C23A—H23B	0.9600
C10—C11	1.3900	C23A—H23C	0.9600
C10—H10	0.9300	C23B—H23D	0.9600
C11—C12	1.3900	C23B—H23E	0.9600
C11—C14	1.500 (4)	C23B—H23F	0.9600
C12—C13	1.3900	C23C—H23G	0.9600
C12—H12	0.9300	C23C—H23H	0.9600
C13—H13	0.9300	C23C—H23I	0.9600
C14—O4	1.251 (4)	O1—Zn1^ii	2.050 (3)
C14—O5	1.253 (4)	Zn1—O2	2.017 (3)
N1A—C18A	1.3900	Zn1—O5^iii	2.040 (3)
N1A—C22A	1.3900	Zn1—O4^iv	2.046 (3)
Zn1—N1A	1.9971 (17)	Zn1—O1^ii	2.050 (3)
C18A—C19A	1.3900	Zn1—Zn1^ii	2.9107 (7)
C18A—H18A	0.9300	O4—Zn1^v	2.046 (3)
C19A—C20A	1.3900	O5—Zn1^vi	2.040 (3)
C19A—H19A	0.9300
O1—C1—O2	124.6 (4)	C21A—C22A—H22A	120.0
O1—C1—C2	118.2 (3)	N1A—C22A—H22A	120.0
O2—C1—C2	117.2 (3)	C22B—N1B—C18B	120.0
C3—C2—C7	118.0 (3)	C22B—N1B—Zn1	120.44 (15)
C3—C2—C1	121.8 (3)	C18B—N1B—Zn1	119.56 (15)
C7—C2—C1	120.3 (3)	C19B—C18B—N1B	120.0
C4—C3—C2	121.2 (4)	C19B—C18B—H18B	120.0
C4—C3—H3	119.4	N1B—C18B—H18B	120.0
C2—C3—H3	119.4	C18B—C19B—C20B	120.0
C3—C4—C5	119.8 (4)	C18B—C19B—C23C	117.8 (10)
C3—C4—H4	120.1	C20B—C19B—C23C	122.2 (10)
C5—C4—H4	120.1	C21B—C20B—C19B	120.0
O3—C5—C4	116.1 (4)	C21B—C20B—C20B^i	109.6 (3)
O3—C5—C6	123.3 (4)	C19B—C20B—C20B^i	125.49 (13)
C4—C5—C6	120.6 (4)	C20B—C21B—C22B	120.0
C7—C6—C5	118.9 (4)	C20B—C21B—C23B	120.0 (7)
C7—C6—H6	120.5	C22B—C21B—C23B	120.0 (6)
C5—C6—H6	120.5	N1B—C22B—C21B	120.0
C6—C7—C2	121.5 (4)	N1B—C22B—H22B	120.0
C6—C7—H7	119.3	C21B—C22B—H22B	120.0
C2—C7—H7	119.3	C21B—C23B—H23D	109.5
O3—C8—C9	119.8 (2)	C21B—C23B—H23E	109.5
O3—C8—C13	120.2 (2)	H23D—C23B—H23E	109.5
C9—C8—C13	120.0	C21B—C23B—H23F	109.5
C10—C9—C8	120.0	H23D—C23B—H23F	109.5
C10—C9—H9	120.0	H23E—C23B—H23F	109.5
C8—C9—H9	120.0	C19B—C23C—H23G	109.5
C9—C10—C11	120.0	C19B—C23C—H23H	109.5
C9—C10—H10	120.0	H23G—C23C—H23H	109.5
C11—C10—H10	120.0	C19B—C23C—H23I	109.5
C10—C11—C12	120.0	H23G—C23C—H23I	109.5
C10—C11—C14	119.90 (19)	H23H—C23C—H23I	109.5
C12—C11—C14	119.99 (19)	C1—O1—Zn1^ii	129.9 (2)
C13—C12—C11	120.0	C1—O2—Zn1	125.0 (2)
C13—C12—H12	120.0	C5—O3—C8	118.5 (3)
C11—C12—H12	120.0	N1A—Zn1—O2	97.99 (11)
C12—C13—C8	120.0	N1A—Zn1—O5^iii	97.88 (12)
C12—C13—H13	120.0	O2—Zn1—O5^iii	88.61 (14)
C8—C13—H13	120.0	N1A—Zn1—O4^iv	101.70 (12)
O4—C14—O5	124.6 (4)	O2—Zn1—O4^iv	87.04 (13)
O4—C14—C11	117.7 (3)	O5^iii—Zn1—O4^iv	160.35 (10)
O5—C14—C11	117.7 (3)	N1A—Zn1—O1^ii	101.71 (11)
C18A—N1A—C22A	120.0	O2—Zn1—O1^ii	160.30 (10)
C18A—N1A—Zn1	126.48 (16)	O5^iii—Zn1—O1^ii	88.45 (12)
C22A—N1A—Zn1	113.42 (16)	O4^iv—Zn1—O1^ii	89.22 (11)
C19A—C18A—N1A	120.0	N1A—Zn1—N1B	8.5
C19A—C18A—H18A	120.0	O2—Zn1—N1B	105.34 (11)
N1A—C18A—H18A	120.0	O5^iii—Zn1—N1B	93.62 (11)
C20A—C19A—C18A	120.0	O4^iv—Zn1—N1B	106.02 (11)
C20A—C19A—H19A	120.0	O1^ii—Zn1—N1B	94.28 (11)
C18A—C19A—H19A	120.0	N1A—Zn1—Zn1^ii	178.25 (11)
C19A—C20A—C21A	120.0	O2—Zn1—Zn1^ii	82.68 (7)
C19A—C20A—C20A^i	132.6 (4)	O5^iii—Zn1—Zn1^ii	80.51 (7)
C21A—C20A—C20A^i	107.3 (4)	O4^iv—Zn1—Zn1^ii	79.93 (7)
C20A—C21A—C22A	120.0	O1^ii—Zn1—Zn1^ii	77.63 (7)
C20A—C21A—C23A	123.4 (5)	N1B—Zn1—Zn1^ii	170.04 (10)
C22A—C21A—C23A	116.6 (5)	C14—O4—Zn1^v	127.6 (2)
C21A—C22A—N1A	120.0	C14—O5—Zn1^vi	127.2 (2)

Symmetry codes: (i) −x+1/2, −y+2, z; (ii) −x, −y+1, −z+1; (iii) x−1/2, −y+1, −z+1/2; (iv) −x+1/2, y, z+1/2; (v) −x+1/2, y, z−1/2; (vi) x+1/2, −y+1, −z+1/2.