Synthesis, crystal structures and fluorescence properties of two 1D Zn(II) homologous coordination polymers

2.1 Materials and physical methods

All chemicals were commercially available and used as received without further purification. The elemental analyses (C, H, and N) were performed on a Perkin-Elmer 240C apparatus. FT-IR spectra were recorded in the range of 4000–450 cm⁻¹ on a Perkin Elmer Frontier spectrometer. Thermogravimetric analyses (TG) were performed under nitrogen with a heating rate of 10 K min⁻¹ using a Perkin Elmer Thermogravimetric Analyzer TGA4000. Photoluminescence spectra were measured on a Varian Cary Eclipse fluorescence spectrophotometer with a xenon arc lamp as the light source. In the measurements of emission and excitation spectra, the slit width was 5 nm.

2.2 Synthesis of [Zn(Hdba)₂(bib)] _n (1)

A mixture of Zn(CH₃COO)₂·2H₂O (0.05 mmol), H₂dba (0.1 mmol), bib (0.05 mmol), NaOH (0.1 mmol) and aqueous ethanol (10 mL/5 mL) was added to a 25 mL Teflon-lined stainless steel reactor and heated at 140 °C for 4 days, and then slowly cooled to room temperature. Colorless block-shaped single crystals suitable for X-ray data collection were obtained by filtration. Yield: 65% (based on Zn). Anal. for C₂₆H₂₀N₄O₆Zn (549.83): calcd. C 56.74, H 3.64, N 10.18; found C 56.76, H 3.61, N 10.17. IR (KBr): ν = 3349(w), 1610(s), 1525(m), 1328(s), 1270(s), 1064(m), 910(m), 851(m), 771(m), 742(w), 638(w), 531(w), 459(w) cm⁻¹.

2.3 Synthesis of [Zn(Hdba)₂(bmib)] _n (2)

The synthesis of 2 was similar to that for 1 except for bmib (0.05 mmol) being substituted for bib (0.05 mmol). Colorless block-shaped single crystals suitable for X-ray data collection were obtained by filtration. Yield: 45% (based on Zn). Anal. for C₂₈H₂₄N₄O₆Zn (577.88): calcd. C 58.14, H 4.15, N 9.69; found C 58.15, H 4.13, N 9.73. IR (KBr): ν = 3341(w), 1615(s), 1569(m), 1455(s), 1375(m), 1243(s), 1175(m), 1130(m), 909(w), 859(m), 777(m), 665(w), 620(w), 526(w), 497(w), 457(w) cm⁻¹.

2.4 X-ray crystallographic studies

Suitable single crystals were selected and investigated on a XtaLAB Mini (ROW) diffractometer with graphite-monochromatized MoKα radiation (λ = 0.71073 Å). Each crystal was kept at T = 298.15 K during data collection. Using Olex2 [47], the structure was solved with the Shelxt [48] structure solution program using Intrinsic Phasing and refined with the Shelxl [49] refinement package using least-squares minimization. All non-hydrogen atoms were refined with anisotropic displacement parameters. Carbon-bound hydrogen atoms were placed in calculated positions (d = 0.93 Å for CH and d = 0.96 Å for CH₃) and were included in the refinement in the riding (CH) or rotating (CH₃) model approximation, with U_iso(H) set to 1.2U_eq(C) for -CH and 1.5U_eq(C) for –CH₃. The H atoms of phenolic hydroxyl groups in 1 and 2 were refined as rotating groups, with d_O–H = 0.82 Å and U_iso(H) = 1.5U_eq(O). The structures were examined using the ADDSYM subroutine of Platon [50] to ensure that no additional symmetry could be applied to the models. Pertinent crystallographic data collection and refinement parameters are collated in Table 1. Selected bond lengths and angles in 1 are given in Table 2. Hydrogen bond geometry data are presented in Table 3.

CCDC 2219961 (1) and 2219962 (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data request/cif.

3.1 Description of the structure of [Zn(Hdba)₂(bib)] _n (1)

Single crystal X-ray diffraction data have revealed that 1 and 2 are a pair of 1D homologous coordination polymers with similar molecular structures but crystallize in different crystal systems and space groups, and with distinctly different cell parameters (Table 1). Only the structure of 1 is discussed in detail here to show key structural features of 1 and 2. The asymmetric unit of 1 consists of a half-molecule of the formula [Zn(Hdba)₂(bib)] with a half of Zn²⁺, half of a neutral auxiliary bib ligand with crystallographic inversion symmetry, and one Hdba^– ligand (Figure 1). Each Zn atom is tetra-coordinated with two O atoms (O1 and O1ⁱ, symmetry code i seen in Table 2) and two N atoms (N1 and N1ⁱ) from two symmetry-related Hdba^– and bib ligands, respectively, resulting in a slightly distorted tetrahedral {ZnN₂O₂} configuration. The Zn–O bond lengths are 1.950(2) Å, and the Zn–N distances are 2.002(3) Å long, within the normal range (Table 2). The N(O)–Zn–O(N) angles fall in the range 99.59(14)–124.44(10)°.

Figure 1:

View of the zigzag chain in 1 (symmetry codes: (i) 1/2 – x, y, 1 – z; (ii) 1 – x, 2 – y, 1 – z; (iii) −1/2 + x, 2 – y, z; (iv) –x, 2 – y, 1 – z).

The detailed coordination modes of the Hdba⁻ and bib ligands are shown in Figure 1. It is interesting to note that the deprotonated carboxylates of the Hdba^– anion coordinate Zn(II) in a monodentate bridging mode (η¹: η⁰) and the bib ligands feature a trans-conformation for the bridging mode (μ₂) with a dihedral angle between the two imidazole rings of 0° as imposed by symmetry. Hence all atoms of the bib ligand are in one plane. Two symmetry-related Hdba^– ligands are bound to each Zn²⁺ cation in an upside-down fashion through their deprotonated carboxylate oxygen atom, resulting in substructural blocks {[Zn(Hdba)]₂}. These neighboring {[Zn(Hdba)]₂} units are further interconnected through the μ₂-bib bridge to generate infinite zigzag chains with Zn···Zn separations of 13.55(9) Å.

In the crystal, the polymeric chains of structure 1 are further associated by a pair of symmetry-related hydrogen bonding interactions, involving the uncoordinated phenolic hydroxyl OH and the uncoordinated carboxylate oxygen atom. Two intermolecular O3–H3···O2^v bonds enclose the Hdba^– ligands to a dimer [Hdba]₂ with a hydrogen bonded R 2 14 ring (Figure 2a and Table 3) [51]. These symmetry-related inversion dimers [Hdba]₂ are further connected with Zn²⁺, forming an interesting hydrogen bonded infinite chain along the c axis with a Zn···Zn distance of 11.06(9) Å. The coordination-based chains and the hydrogen bonding-based chains are interlocked in a reversely alternating parallel arrangement, assembling 1 into a 3D supramolecular structure. In addition, an intermolecular non-classical hydrogen bond C–H···O also plays a role in assembling the substructural unit [Zn(Hdba)₂(bib)] into a 3D supramolecular network with the phenolic hydroxyl O atom (O3 as acceptors), i.e. C10–H10···O3^viii (Table 3).

Figure 2:

(a) View of a Zn²⁺ center connected with two hydrogen bonded dimers [Hdba^–] and two bib ligands in 1 (symmetry codes: (i) 1/2 – x, y, 1 – z; (ii) 1 – x, 2 – y, 1 – z; (iii) −1/2 + x, 2 – y, z; (iv) –x, 2 – y, 1 – z; (v) 1/2 – x, 1/2 – y, 1/2 – z; (vi) x, 1/2 – y, 1/2 + z; (vii) 1/2 – x, 1/2 – y, 3/2 – z); (b) the simplified 3D O–H···O hydrogen bonded framework with a 4-connected {6⁶} topology in 1; (c) the simplified 2D π-stacking interactions between bib with a 3-connected {4⁴·6²} topology in 1.

In addition, the crystal has two types of π···π stacking interactions: between the imidazole and benzene rings (5-membered ring, Cg1: N1/C8/N2/C10/C9 and 6-membered ring, Cg2: C11/C12/C13ⁱⁱ/C11^ii/C12ⁱⁱ/C13) related to the bib ligands, and between the Hdba^– phenyl rings (6-membered ring, Cg3: C2/C3/C4/C5/C6/C7). The dihedral angles between the Cg1/Cg2^ix (Cg1/Cg2^x, Cg2/Cg1^xi, and Cg2/Cg1^x) or Cg3/Cg3^xii (symmetry codes: ix x, −1 + y, z; x 1 – x, 1 – y, 1 – z; xi x, 1 + y, z; xii 1/2 – x, −1/2 – y, 1/2 – z) are 2.73(18)° and 0°, with centroid-to-centroid distances of 3.6493(19) and 3.981(3) Å, and perpendicular distances of 3.2952(14) and 3.5680(19) Å, respectively. Furthermore, a C–H···π interaction exists between the flanking imidazole ring of bib and benzene C–H of Hdba^– with H3A···Cg1^xiii or C3···Cg1^xiii distances of 2.76(3) Å and 3.643(5) Å, respectively, and the C3–H3A···Cg1^xiii angle of 158° (symmetry code: xiii 1/2 – x, −1 + y, 1 – z). Thus, through hydrogen bonds and π···π as well as with C–H···π interactions, the one-dimensional network is further expanded into a three-dimensional supramolecular architecture.

Interestingly, from the topological point of view, each Zn core unit acts as a 4-connected node, linked by two hydrogen bonded dimers [Hdba^–]₂ and two bib ligands, and each [Hdba^–]₂ dimer or bib unit bridges two Zn atoms serving as a linker (Figure 2a). The 3D hydrogen bonded framework of 1 can be simplified as a 4-c unimodal dia Diamond topology with a Schläfli symbol {6⁶} and a 4/6/c1 or sqc6 topos & RCSR.ttd as analyzed with the program Topos 4.0 [52] (Figure 2b). Also, each Zn core unit acts as a 4-connected node, linked by two Hdba^– ligands and two Cg1/Cg2^ix, and each Hdba^– ligand or Cg1/Cg2^ix bridges two Zn atoms acting as a linker. The 2D π-stacking structure of 1 can be simplified as a 4-c unimodal sql topology with a Schläfli symbol {4⁴·6²} and a Shubnikov tetragonal plane net (Figure 2c).

3.2 Description of the structure of [Zn(Hdba)₂(bmib)] _n (2) and structural comparison of 1 and 2

As discussed above, using the same metal and carboxylic acid, and different neutral auxiliary ligands, the same synthesis strategy will likely afford homologous structures. The largest similarity of 1 and 2 is that they both have almost the same coordination {ZnO₂N₂} unit and the same zigzag coordination chain (Figures 1 and 3). However, crystals of 1 and 2 belong to different crystal systems and space groups (Table 1). Small differences between the bond lengths and angles of the two molecules can probably be accounted for by differences in the molecular packings. The additional methyl substituents at the imidazole rings in the bmib ligand in 2 as compared to 1 cause crucial differences in the dihedral angles between these rings. The dihedral angles between the two imidazole rings in bib are 0° in 1, as imposed by symmetry, and 49.86(5)° in bmib in 2 thus preventing co-planarity of the rings and weakening π conjugation in 2. It should be noted that the bmib ligands in 2 have crystallographic C₂ (2) symmetry.

Figure 3:

(a) The coordination environment for Zn²⁺ in 2 (symmetry codes: (i) 1 – x, y, 1/2 – z; (ii) 1 – x, y, 3/2 – z; (iii) x, y, −1 + z); (b) view of the zigzag chain in 2 (symmetry codes: (i) 1 – x, y, 1/2 – z; (ii) 1 – x, y, 3/2 – z; (iii) x, y, −1 + z; (iv) x, y, 1 + z).

In the following the hydrogen bonding and π-conjugation stacking of crystalline 2 are discussed. The bmib ligand has two methyl groups more than bib, which induces distinctly different hydrogen bonding modes for 1 and 2. The Hdba^– ligands form hydrogen bonded chains with a R 2 14 hydrogen bonding ring (–O–C–C–C–C–O–H–)₂ of two O3–H3···O2^v units in 1 (Figure 2a), but the Hdba^– ligands are forming hydrogen bonded double chains with a R 2 22 hydrogen bonding ring (–O–C–O–Zn–O–C–C–C–C–O–H–)₂ of two O3–H3···O2^v (Table 3) units in 2. Not surprisingly, given the additional methyl substituents, the π-stacking patterns in 1 and 2 are totally different. In 1, two imidazole and two benzene rings of bib or Hdba^– ligands, participate in the formation of π···π and C–H···π stacking interactions, while each bimb or Hdba^– is not obviously involved in any form of π-stacking interactions in 2. In summary, these factors ultimately cause 1 and 2 to assemble in closely similar coordination polymer chains which are accommodated in noticeably different three-dimensional supramolecular structures.

3.3 Infrared spectra of 1 and 2

Comparing infrared spectra of the free ligands to those of their complexes provides information about the coordination of the ligand. CPs 1 and 2 show similar infrared bands in the range 4000–450 cm⁻¹, which are different from those of the free ligand. The broad band centered at 3349 cm⁻¹ for 1 and at 3341 cm⁻¹ for 2 reveal the O–H characteristic stretching vibration of the uncoordinated phenolic hydroxy group. Bands assigned to ν_as(COO⁻) and ν_s(COO⁻), which are observed for the free H₂dba ligand at 1725 cm⁻¹ and 1405 cm⁻¹, respectively, are shifted to 1525 cm⁻¹ for 1, 1569 cm⁻¹ for 2 and 1328 for 1, 1375 cm⁻¹ for 2, indicating that deprotonation of the carboxylate group occurring upon coordination. The difference between the asymmetric and symmetric stretches, Δ(ν_as(COO⁻) − ν_s(COO⁻)) is slightly smaller than 200 cm⁻¹ (197 and 194 cm⁻¹, respectively), indicating a monodentate carboxylate [26, 53, 54]. The low intensity bands in the region of 600–400 cm⁻¹ are attributed to Zn–N and Zn–O vibrations [26]. All these spectroscopic features of H₂dba and 1–2 are consistent with the result of the crystal structure determinations.

3.4 PXRD and thermal analysis

In order to confirm the bulk purity of 1 and 2, the PXRD patterns were checked at room temperature. The experimental patterns are almost identical with the simulated patterns based on the single-crystal data, confirming that 1 and 2 have been obtained as a pure crystalline phase (Figure 4). In order to explore the thermal stability of 1 and 2, TG studies have been performed in a nitrogen atmosphere at a heating rate of 10 K min⁻¹ between T = 50 and 800 °C. 1 and 2 have similar TG curves. Up to 300 °C, almost no weight loss was observed, which indicates the absence of occluded solvent and guest molecules in the samples of 1 and 2. According to TG data, then a continuous decomposition of the framework occurs above 310 °C for 1 and 2 (Figure 5).

Figure 4:

The simulated and experimental PXRD patterns of 1 and 2.

Figure 5:

The TG curves for 1 and 2.

3.5 Luminescent properties

Luminescent coordination polymers (CPs) or metal-organic frameworks (MOFs) or hydrogen-bonded organic frameworks (HOFs) have received much attention owing their potential applications in photocatalysis, biomedical imaging, and fluorescent sensors [23, 24, 41, 42]. The solid state fluorescence properties of 1 and 2 have been investigated at room temperature (Figure 6). The maximum sharp emission peaks are centered at 410 nm (λ_ex = 380 nm) with the intensity of 866 a.u. for 1, which may be attributed to π*→n or π*→π transitions. The maximum broad emission peaks are centered at 328 nm (λ_ex = 290 nm) with the intensity of 633 a.u. for 2. The emission intensity of 2 is much lower than that of 1 by 233 a.u., which may be assigned to different hydrogen-bonded organic frameworks.

Figure 6:

The solid state fluorescence spectra of 1 and 2.