Introduction

Considering their diverse and easily tailored structures, along with their various potential applications [1] in catalysis [2–5], separation [6], gas storage [7], carbon dioxide capture [8] and so on [9], coordination compounds as examples of functional organic–inorganic hybrid porous materials have gained increasing attention [9–11]. Recently, coordination complexes and their derivatives have been found to have potential applications as heterogeneous photocatalysts, due to the presence of catalytically active metal sites and/or functional organic linkers as well as easily tailorable physical and chemical functionality of the catalytic moieties [12]. Hence, much attention has been paid to the catalytic degradation of organic pollutants [13–15], CO2 reduction [15, 16], Cr(VI) reduction [15, 17, 18] and water splitting [19–21] using coordination compounds as photocatalysts.

The structures of such coordination compounds are primarily influenced by factors including the choice of metal centers, organic linkers, solvents and counterions [9, 14, 22]; variation of the organic linker can lead to very different structures [23, 24]. As typical ligands, multi-carboxylate linkers are of immense interest in the construction of versatile coordination architectures, due to the fact that they can link metal centers together and/or compensate the positive charge of the metal ions [24–26]. In order to investigate the influence of different organic linkers on the structures and properties of coordination compounds, three different multi-carboxylic acid ligands, namely 5-nitroisophthalic acid (5-NO2-H2ipa); 3,4-dicarboxyl-(3′,4′-dicarboxylazophenyl) benzene (3,4-H4dczpb); and 6,12-dihydroxy-1,2,6,12-tetrahydroindazolo[2,1-a]indazole-1,7-dicarboxylic acid (L), as shown in Scheme 1, along with 1,10-phenanthroline (phen) as chelating ligand, were utilized to build four coordination compounds with different structures, i.e., [CuCl(phen)2](5-NO2-Hipa)·2H2O (BUC-10), [Cu(phen)(3,4-H2dczpb)(H2O)]·H2O (BUC-11), [Fe(phen)(L)(H2O)]2 (BUC-12), and [Fe(phen)3](3,4-H3dczpb)2·0.6H2O (BUC-13). The crystal structures of the complexes, as well as their optical gaps and photocatalytic activities toward degradation of methylene blue (MB), have been investigated. Factors influencing the activity and stability of BUC-12 in photocatalytic reactions were also studied.

Scheme 1
scheme 1

Structural formulae of 5-NO2-ipa (a), 3,4-H4dczpb (b) and L (c)

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Experimental

Materials and methods

All chemicals were commercially available reagent grade and used without further purification. C, N, H elemental analyses were obtained using an Elementar Vario EL-III instrument. The Fourier transform infrared (FTIR) spectra were recorded on a Nicolet-6700 spectrophotometer in the region (4000–400 cm−1). UV–Vis diffuse reflection spectra of solid samples were measured from 200 to 800 nm on a PerkinElmer Lamda 650S spectrophotometer, in which BaSO4 was used as the reference with 100% reflectance.

Synthesis of BUC-10

A mixture of CuCl2·2H2O (0.3 mmol, 0.0511 g), 5-NO2-H2ipa (0.3 mmol, 0.0663 g) and 1,10-phen (0.6 mmol, 0.1189 g) in a mole ratio of 1:1:2 was sealed in a 25-mL Teflon-lined stainless steel Parr bomb containing deionized H2O (20 mL). The mixture was heated at 160 °C for 72 h and then cooled to room temperature. Green rod-like crystals of [CuCl(phen)2](5-NO2-Hipa)·2H2O (BUC-10) (yield 92% based on CuCl2·2H2O) were isolated by filtration and washed with deionized water and ethanol. Anal. Calcd. for BUC-10, C32H24ClCuN5O8: C, 54.4; N, 9.9; H, 3.4. Found: C, 54.8; N, 10.0; H, 3.4%. IR (KBr)/cm−1: 3578, 3108 m, 1953s, 1599, 1529, 1517, 1453 m, 1425 s, 1370, 1342 m, 1202, 1223, 1182, 1143, 1091, 1075w, 859, 782, 725, 646 m, 507, 428w.

Synthesis of BUC-11

Small black rod-like crystals of [Cu(phen)(3,4-H2dczpb)(H2O)]·H2O (BUC-11) (yield 68% based on CuCl2·2H2O) were synthesized from a mixture of CuCl2·2H2O (0.3 mmol, 0.0511 g), H4dczpb (0.3 mmol, 0.1074 g) and 1,10-phen (0.6 mmol, 0.1189 g) in 1:1:2 M ratio under the same conditions as for BUC-10. Anal. Calcd. for BUC-11, C28H20CuN4O10: C, 52.8; N, 8.8; H, 3.1. Found: C, 52.9; N, 8.8; H, 3.2%. IR (KBr)/cm−1: 3362, 3063, 2015, 1733, 1670, 1614, 1592, 1520, 1487 m, 1386 s, 1371, 1333, 1280 m, 1060w, 854, 802, 723 m, 673w.

Synthesis of BUC-12

Black rod-like crystals of [Fe(phen)(L)(H2O)]2 (BUC-12) (yield 92% based on FeSO4·7H2O) were synthesized from a mixture of FeSO4·7H2O (0.3 mmol, 0.0834 g), L (0.3 mmol, 0.1074 g) and 1,10-phen (0.6 mmol, 0.1189 g) in 1:1:2 M ratio under the same conditions as for BUC-10. Anal. Calcd. for BUC-12, C56H32Fe2N8O14: C, 58.3; N, 9.7; H, 2.8. Found: C, 58.4; N, 9.7; H, 2.9%. IR (KBr)/cm−1: 2925, 1717, 1676, 1601 m, 1426 s, 1387, 1360, 1300 m, 1151, 1099w, 920 m, 880, 843w, 817, 771, 747 m, 657, 596w.

Synthesis of BUC-13

Red block-like crystals of [Fe(phen)3](3,4-H3dczpb)2·0.6H2O (BUC-13) (yield 90% based on FeSO4·7H2O) were synthesized from a mixture of FeSO4·7H2O (0.3 mmol, 0.0834 g), 3,4-H4dczpb (0.3 mmol, 0.1074 g) and 1,10-phen (0.6 mmol, 0.1189 g) in 1:1:2 M ratio under the same conditions as for BUC-10. Anal. Calcd. for BUC-13, C68H43.20FeN10O16.60: C, 62.3; N, 10.7; H, 3.2. Found: C, 62.8; N, 10.8; H, 3.4%. IR (KBr)/cm−1: 3417, 3061, 1579, 1517, 1426 s, 1355, 1263 m, 1204, 1061, 847w, 775, 724 m, 656, 531w.

X-ray crystallography

X-ray single-crystal data collections for the complexes were performed with a Bruker Smart 1000 CCD area detector diffractometer with graphite-monochromatized MoKα radiation (λ = 0.71073 Å) using φ − ω mode at 293(2) K. The SMART software package [27] was used for data collection and SAINT software [28] for data extraction. Empirical absorption corrections were performed with the SADABS program [29]. The structures were solved by direct methods (SHELXS-97) [30] and refined by full-matrix-least squares techniques on F 2 with anisotropic thermal parameters for all non-hydrogen atoms (SHELXL-97) [30]. The hydrogen atoms of the organic ligands were added according to theoretical models, and those of water molecules were found by difference Fourier maps. All structural calculations were carried out using the SHELX-97 program package [30]. Crystallographic data and structural refinements for BUC 10–13 are summarized in Table 1. Selected bond lengths and angles are listed in Table 2.

Table 1 Details of X-ray data collection and refinement for BUC 10–13
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Table 2 Selected bond lengths and angles for BUC 10-13 [Å and °]
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Photocatalytic degradation of MB

The activities of BUC 10–13 as photocatalysts were evaluated using MB as a model organic pollutant at room temperature and under UV light irradiation in a photocatalytic assessment system (Beijing Aulight Co. Ltd.). The distance between the light source and the beaker containing the reaction mixture was fixed at 5 cm. The powdered photocatalyst, with a particle size less than 147 μm, was added to 200 mL of MB (10 mg/L) aqueous solution in a 300-mL beaker. Prior to UV irradiation, the suspension was magnetically stirred in the dark for 120 min to ensure the adsorption/desorption equilibrium. During the photocatalytic degradation, stirring was maintained to keep the mixture fully in suspension. One milliliter aliquots were extracted at regular time intervals using a 0.45-μm syringe filter (Shanghai Troody) for analysis. The solution was analyzed on a Laspec Alpha-1860 spectrometer over the range of 400–800 nm in a spectrometric quartz cell with 1 cm path length. The remaining MB concentration was determined at 664 nm.

In order to investigate the effect of pH on the photocatalytic MB degradation, the initial pH values of the 10 mg/L MB solution were adjusted to 2.0, 4.0, 6.0, 8.0 and 10.0, using HCl and NaOH solutions of suitable concentrations (50 mg of BUC-12 was added to 200 mL of MB solution). In order to study the effect of salts, on the photocatalytic performance of BUC-12, tap water was used as a solvent containing sodium (18.42 mg/L), calcium (70.05 mg/L), magnesium (30.59 mg/L) and chloride (0.1 mg/L). The total dissolved solids content of the tap water was 452 mg/L.

Results and discussion

All of these coordination compounds were stable under air and insoluble in water and common organic solvents, including ethanol, methylbenzene, chloroform, ether, DMSO and DMF.

Structure of BUC-10

[CuCl(phen)2](5-NO2-Hipa)·2H2O (BUC-10) was synthesized under hydrothermal conditions. Although the crystal structure of BUC-10 has been previously reported by Xiao and coworkers [32], the current refinement was of better quality; the values of R gt(F) and ωR ref(F 2) of BUC-10 in the present work were 0.0572 and 0.1213, respectively, compared to those from the previously study (R gt(F) = 0.103, ωR ref(F 2) = 0.204 [32]). The crystal structure analysis reveals that it is built up of discrete [CuCl(phen)2] units, partly deprotonated 5-NO2-Hipa− anions and two lattice water molecules. The Cu(II) center, in a distorted trigonal pyramidal geometry, is five-coordinated by four nitrogen atoms from two phen ligands and one chloride, such that N1, N2, N3 and N4 occupy the equatorial plane, and chloride occupies the vertex, as depicted in Fig. 1. The Cu–N bond distances range from 1.988(3) to 2.173(4) Å, and the Cu–Cl bond length is 2.2810(15) Å; these values are comparable with those typically found in similar coordination compounds [31]. In the equatorial plane, the N1–Cu1–N3 and N2–Cu1–N4 bond angles are 174.57(17)° and 108.80(15)°, respectively, showing that the copper’s coordination sphere is severely distorted. In the crystal, a 3D supramolecular framework is assembled via abundant hydrogen-bonding interactions, as documented in Fig. S1(ESI) and Table 3.

Fig. 1
figure 1

ORTEP drawing of BUC-10 showing the Cu(II) coordination environment with thermal ellipsoids at 30% probability (all hydrogen atoms attached on carbon atoms, oxygen atoms and the corresponding hydrogen atoms of lattice water molecules are omitted for clarity)

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Table 3 Hydrogen bonds for BUC 10–13 [Å and o]
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Structure of BUC-11

In [Cu(phen)(3,4-H2dczpb)(H2O)]·H2O (BUC-11), the Cu(II) center has a square-pyramidal geometry, being coordinated by N1 and N2 from a phen ligand, O4 and O5 from two different 3,4-H2dczpb2− ligands, plus atom O9 from a water ligand, as shown in Fig. 2a. The partly deprotonated 3,4-H2dczpb2− ligands adopt a bis-monodentate mode to link two [Cu(phen)(H2O)]2+ units into an approximately rectangular molecular box, as illustrated in Fig. 2a. These [Cu2(phen)2(3,4-H2dczpb)2(H2O)2] molecular boxes are further joined into a two-dimensional supramolecular layer structure in the ab-plane with the help of hydrogen-bonding interactions, as detailed in Fig. 2b and Table 3.

Fig. 2
figure 2

a ORTEP drawing of BUC-11 showing the Cu(II) coordination environment with thermal ellipsoids at 30% probability (all hydrogen atoms attached on carbon atoms, oxygen atoms and the corresponding hydrogen atoms of lattice water molecules are omitted for clarity). Symmetry code: a −x + 1, −y + 2, −z + 1 and b Two-dimensional supramolecular layer structure of BUC-11 formed by hydrogen-bonding interactions

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Structure of BUC-12

Similar to the structure of BUC-11, [Fe(phen)(L)(H2O)]2 (BUC-12) is based on a [Fe(phen)(L)(H2O)]2 molecular box, in which Fe(II) is six-coordinated in a distorted octahedral geometry provided by N3 and N4 from a phen ligand, O1 and O3 oxygen atoms from carboxylate and carbonyl groups from the same L ligand, O5A oxygen atom from another L ligand and O7 from a water ligand, as illustrated in Fig. 3a. In BUC-12, the completely deprotonated L ligand coordinates via a single O atom from its COO− group plus chelating O atoms from both COO− and OH− groups to link Fe(II) centers. The [Fe(phen)(L)(H2O)]2 molecular boxes are linked into a chain-like structure via hydrogen-bonding interactions, as detailed in Fig. 3b and Table 3.

Fig. 3
figure 3

a ORTEP drawing of BUC-12 showing the Fe(II) coordination environment with thermal ellipsoids at 30% probability (all hydrogen atoms attached on carbon atoms, oxygen atoms and the corresponding hydrogen atoms of lattice water molecules are omitted for clarity). Symmetry code: a −x + 1, −y + 1, −z + 1 and b The packing structure of BUC-12 viewed along the c axis

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Structure of BUC-13

The structure of BUC-13 ([Fe(phen)3](3,4-H3dczpb)2·0.6H2O) consists of discrete [Fe(phen)3]2+ units, partly deprotonated 3,4-H3dczpb− anions and lattice water molecules. The Fe(II) center is in a distorted octahedral geometry provided by six nitrogen atoms from three phen ligands, as illustrated in Fig. 4a. The cationic charge of [Fe(phen)3]2+ is balanced by two 3,4-H3dczpb− anions. These partly deprotonated 3,4-H3dczpb− anions are assembled into sheets via hydrogen-bonding interactions (as listed in Fig. 4b; Table 3) and further connected by cationic [Fe(phen)3]2+ moieties into a 3D supramolecular structure via electrostatic and weak intermolecular interactions.

Fig. 4
figure 4

a ORTEP drawing of BUC-13 showing the Fe(II) coordination environment with thermal ellipsoids at 30% probability (all hydrogen atoms attached on carbon atoms, oxygen atoms and the corresponding hydrogen atoms of lattice water molecules are omitted for clarity). Symmetry code: a −x + 1, y, −z + 3/2. b 3D framework of BUC-13 viewed along the a axis

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Optical and catalytic properties

In order to investigate the optical properties of BUC 10–13, their UV–Vis adsorption spectra for powdered samples were recorded at room temperature [33]. As shown in Fig. 5, these coordination polymers showed an adsorption peak in the range 250–400 nm, indicative of their selective absorptions in the ultraviolet region.

Fig. 5
figure 5

UV–Vis diffuse reflectance spectra of BUC 10–13

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The photocatalytic performances of BUC 10–13 for the decomposition of MB were assessed under UV irradiation. Control experiments on MB degradation without any catalyst under identical conditions were also carried out. In order to confirm the powder purity of BUC-12 used in this experiment, powder X-ray diffraction was conducted. The powder X-ray pattern matched well with that simulated from the X-ray single-crystal diffraction data, as shown in Fig. 8, confirming the phase purity of bulk samples of BUC-12. The slight differences in intensities may be assigned to the preferred orientation of the crystalline powder samples [34]. The photocatalytic performances were monitored by measuring the maximum absorbance intensity at 664 nm, in order to determine the residual MB concentration. The degradation efficiencies of MB in the presence of BUC 10–13 after prior adsorption equilibrium in the dark are shown in Fig. 6. All quantitative data for degradation efficiencies are average values from three parallel experiments. It can be seen that the degradation efficiency of MB increased from 14.4% (control experiment) to 57.8, 46.1, 92.1 and 29.3%, respectively, with BUC-10, BUC-11, BUC-12 and BUC-13 as photocatalysts for a runtime of 120 min. All of these photocatalytic degradation reactions followed a pseudo-first-order kinetic model. Thus, plots of −ln(C/C 0) versus reaction time were linear, giving R 2 values of 0.989, 0.698, 0.946 and 0.976, respectively. The pseudo-first-order rate constants (k) for the photocatalytic degradation of MB with BUC-10, BUC-11, BUC-12 and BUC-13 as photocatalysts were 0.0067, 0.0038, 0.0218 and 0.0031 min−1, respectively. The noticeable differences in their photocatalytic performances might arise from the structures of these four coordination compounds, especially the role of the polycarboxylic acids. The 5-NO2-Hipa− and 3,4-H3dczpb− in BUC-10 and BUC-13 are discrete anions and uncoordinated to the metal, whereas the 3,4-H2dczpb2− ligand in BUC-11 is partly coordinated to the metal, with two COOH groups being undeprotonated. However, the L ligand in BUC-12 is completely coordinated to the Fe2+ center. With this in mind, we propose that the uncoordinated COOH groups might capture ·OH radicals, affecting the catalysis. Further research will be required in order to clarify the relationship between the photocatalytic performances of these complexes and their structures.

Fig. 6
figure 6

Plots of concentration versus irradiation time for MB under irradiation with Hg lamp light using BUC 10–13 as photocatalysts, and the control experiment without any photocatalyst

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The pH of the solution is another important parameter, which can influence photocatalytic degradation reactions [35]. Thus, the effect of initial pH on the degradation efficiency of MB using BUC-12 as catalyst was tested. The results revealed that BUC-12 was an effective catalyst over a wide pH range from 2.0 to 10.0, as illustrated in Fig. 7a. In order to investigate the activities of the organic ligands as photocatalysts, free L and phen were utilized to conduct MB degradation. Free L showed no photocatalytic activity, while phen was slightly soluble in water, and reacted stoichiometrically with MB under the experimental conditions, rather than acting as a photocatalyst. Also, tap water was used to prepare a MB solution in order to study the influence of dissolved salts on the photocatalytic MB decomposition. The photocatalytic degradation efficiency was inhibited by the total salts in water, as illustrated in Fig. 7b. The effects of individual cations and anions will be discussed in detail in our future work.

Fig. 7
figure 7

a Effect of initial pH on the degradation of MB under UV irradiation using BUC-12 as photocatalyst. b The effect of salts and ligands on the degradation of MB

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In order to test the practicability of these complexes as photocatalysts, BUC-12 was selected to test the recyclability and stability by repeated runs in the photocatalytic degradation of MB under the usual reaction conditions. The results in Table 4 demonstrated that the photocatalytic performance remained almost unchanged after five runs, implying that BUC-12 is stable under the catalytic conditions. Furthermore, the PXRD diffraction patterns of the BUC-12 after five runs again matched well with the simulated pattern generated from the single-crystal diffraction data, as shown in Fig. 8. Hence, BUC-12 shows good reusability for the catalytic degradation of organic pollutants.

Table 4 Effect of recycling BUC-12 in the degradation of MB (10 mg/L)
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Fig. 8
figure 8

PXRD patterns of BUC-12 before and after photocatalytic reaction and the simulated XRD pattern from the single-crystal structure

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Conclusions

Four coordination compounds were synthesized via the hydrothermal method and characterized using single-crystal X-ray diffraction analysis, FTIR, UV–Vis DSR and CHN elemental analysis. The crystal structure analyses revealed that the supramolecular frameworks of BUC-10, BUC-11, BUC-12 and BUC-13 are constructed from discrete zero-dimensional units and lattice molecules via intermolecular hydrogen-bonding interactions. All of the complexes exhibited photocatalytic activities to decompose MB under UV light irradiation, implying their potential application as solid photocatalytic materials.

Supplementary materials

CCDC 1042968, 1042981, 1042975 and 1042979 contain the supplementary crystallographic data for compounds BUC-10, BUC-11, BUC-12 and BUC-13. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.