Introduction

In view of their diverse and easily tailored structures, along with their various potential applications in catalysis and photocatalysis [1, 2], separation [3], gas storage [4], carbon dioxide capture [5] and so on [6, 7], functional inorganic–organic hybrid porous materials constructed from coordination complexes have gained continuing attention [6, 8]. Most recently, such coordination complexes have been found to have potential as heterogeneous photocatalysts due to the presence of catalytically active metals and/or functional organic linkers as well as easily tailorable physical and chemical functions and catalytic properties [9]. In recent years, much attention has been given to the use of such complexes for the degradation of organic pollutants [10–12], CO2 reduction [13, 14] and water splitting [15, 16].

The construction of such coordination complexes is influenced by many factors including but not limited to the types of metal atoms, organic ligands, solvents, and counter-ions [10, 17]. In particular, the alteration of the organic ligands can lead to very different structures [18, 19]. As bridging ligands, carboxylates, especially multi-carboxylates, are of immense interest in the construction of coordination architectures owing to the fact that the resulting complexes have a wide range of structural diversities and potential applications as porous materials [17, 20, 21]. In recent years, rigid 5-R-isophthalic acids (5-R-H2ipa, R = –OH, –CH3, –Br, –OCH3, –NO2) have been used for the preparation of various coordination compounds [22–26]. In order to investigate the influence of functional groups attached to the ipa ligand on the structure of the resulting coordination complexes, in this paper, three different multicarboxylic acid ligands, namely 5-bromoisophthalic acid (5-Br-ipa), 5-nitroisophthalic acid (5-NO2-ipa), and 3,4-dicarboxyl-(3′,4′-dicarboxylazophenyl) benzene (3,4-H4dczpb) (Scheme 1) along with 1,10-phenanthroline (phen) as a terminal chelating ligand were utilized to build three novel manganese(II) coordination complexes with different structures, formulated as Mn(phen)(5-Br-ipa) (1), Mn(phen)(5-NO2-ipa) (2), and [Mn(phen)2(3,4-H2dczpb)]·H2O (3). The crystal structures of the complexes, as well as their optical gaps and photocatalytic activities toward the degradation of methylene blue (MB), have been investigated.

Scheme 1
scheme 1

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

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Experimental

Materials and methods

All chemicals were commercially available reagent grade and used without further purification. CNH elemental analyses were obtained using an Elementar Vario EL-III instrument. The FTIR spectra, in the region (400–4000 cm−1), were recorded on a Nicolet 6700 Fourier Transform infrared spectrophotometer. UV–Vis diffuse reflection spectroscopy (DRS) of solid samples were measured from 200 to 1200 nm with a PerkinElmer Lamda 650S spectrophotometer using BaSO4 was used as the standard with 100 % reflectance.

Synthesis of Mn(phen)(5-Br-ipa) (1)

A mixture of MnCl2·4H2O (0.0594 g, 0.3 mmol), 5-Br-H2ipa (0.0735 g, 0.3 mmol) and 1,10-phen (0.1189 g, 0.6 mmol) was sealed in a 25 mL Teflon-lined stainless steel Parr bomb containing deionized H2O (20 mL), heated at 160 °C for 72 h, and then cooled down to room temperature. Small yellow rod-like crystals of complex 1 (yield 90 % based on MnCl2·4H2O) were isolated and washed with deionized water and ethanol. Anal. Calcd. for 1, C20H11BrMnN2O4: C, 50.2; N, 5.9; H, 2.3. Found: C, 50.3; N, 5.9; H, 2.4 %. IR (KBr)/cm−1: 3417m, 3089m, 1619s, 1567m, 1530m, 1518m, 1451m, 1426s, 1344m, 1145w, 1102w, 920m, 865m, 852m, 790m, 730m, 639m, 537w, 421w.

Synthesis of Mn(phen)(5-NO2-ipa) (2)

Yellow block-like crystals of 2 (yield 61 % based on MnCl2·4H2O) were synthesized from a mixture of MnCl2·4H2O (0.0594 g, 0.3 mmol), 5-NO2-H2ipa (0.0663 g, 0.3 mmol) and 1,10-phen (0.1189 g, 0.6 mmol) under the same conditions as complex 1. Anal. Calcd. for 2, C20H11MnN3O6: C, 54.0; N, 6.3; H, 2.5. Found: C, 54.2; N, 6.2; H, 2.7 %. IR (KBr)/cm−1: 3417m, 3091m, 1607s, 1573m, 1532m, 1425s, 1375m, 1351m, 1140w, 1102w, 1078m, 922m, 843m, 789m, 731m, 640m, 545w, 421w.

Synthesis of [Mn(phen)2(3,4-H2dczpb)]·H2O (3)

Yellow block-like crystals of 3 (yield 40 % based on MnCl2·4H2O) were synthesized from a mixture of MnCl2·4H2O (0.0594 g, 0.3 mmol), 3,4-H2dczpb (0.1074 g, 0.3 mmol) and 1,10-phen (0.1189 g, 0.6 mmol) in 1:1:2 M ratio under the same conditions as complex 1. Anal. Calcd. for 3, C40H26MnN6O9: C, 60.8; N, 10.6; H, 3.3. Found: C, 60.9; N, 10.5; H, 3.5 %. IR (KBr)/cm−1: 3449m, 3047m, 1938m, 1694m, 1589m, 1517s, 1427s, 1375m, 1261m, 1101w, 1049m, 865m, 845m, 789m, 776m, 727m, 655m, 638m, 515w, 420w.

X-ray crystallography

X-ray single-crystal data collection for the complexes 1–3 was performed with 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 [27] was used for data collection and SAINT software [27] for data extraction. Empirical absorption corrections were performed with the SADABS program [28]. The structures were solved by direct methods (SHELXS-97) [29] and refined by full-matrix-least squares techniques on F 2 with anisotropic thermal parameters for all of the non-hydrogen atoms (SHELXL-97) [29]. 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 [29]. Crystallographic data and structural refinements for the complexes 1–3 are summarized in Table 1. Selected bond lengths and angles for both compounds are listed in Table 2.

Table 1 Details of X-ray data collection and refinement for compounds 1–3
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Table 2 Selected bond lengths (Å) and angles (°) for compound 1–3
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Photocatalytic degradation of MB

The activities of complexes 1–3 as photocatalysts were evaluated via degradation of MB at room temperature under 500 W Hg lamp irradiation in a photocatalytic assessment system (Beijing Aulight Co. Ltd.). The distance between the light source and the beaker containing reaction mixture was fixed at 5 cm. The solid photocatalyst (50 mg) was added to 200 mL of MB (10 mg/L) aqueous solution in a 300-mL beaker. Prior to irradiation, the suspension was magnetically stirred in the dark for 120 min to ensure the establishment of an adsorption/desorption equilibrium. During the photocatalytic degradation reaction, stirring was maintained to keep the mixture in complete suspension. Aliquots of volume 1 ml were extracted at regular intervals using a 0.45-μm syringe filter (Shanghai Troody) for analysis. A Laspec Alpha-1860 spectrometer was used to monitor the changes in dye absorbance in the range of 400–800 nm in a spectrometric quartz cell with 1 cm path length. The MB concentration was determined from its maximum absorbance at 664 nm.

Results and discussion

Structures of the complexes

All these three complexes are stable in water and common organic solvents like methanol, alcohol, ether, N,N-dimethylformamide, and so on.

Mn(phen)(5-Br-ipa) (1) was synthesized under hydrothermal conditions. The crystal structure analysis reveals that complex 1 is built up of 2D neutral Mn(phen)(5-Br-ipa) sheets, as illustrated in Fig. 1b. The nearly identical Mn1 and Mn2 centers, in an octahedral geometry, are each six-coordinated by two nitrogen atoms from a phen ligand, two oxygen atoms from two different monodentate 5-Br-ipa2− ligands, and two oxygen atoms from a chelating 5-Br-ipa2− ligand. Taking Mn1 as an example, N1 and O5 occupy the axial positions, and the remaining atoms N2, O1, O3 and O4 atoms lie in the four sites of the equatorial plane of Mn1, as depicted in Fig. 1a. In the equatorial plane, the N2–Mn1–O3#1, O1–Mn1–O4#1, O3#1–Mn1–O4#1, and O1–Mn1–N2 bond angles are 150.37(13), 92.95(13), 57.82(13), and 114.35(13)°, respectively, and the N1–Mn1–O5 bond angle is 161.22(14)°, implying that the Mn-centered coordination octahedron is seriously distorted. The Mn–O and Mn–N bond distances compare with the normal values for these bonds as found in similar CPs [17].

Fig. 1
figure 1

a Asymmetric unit of Mn(phen)(5-Br-ipa) (1) and coordination environments around the Mn(II) atoms. b The 2D sheet of 1 viewed from a-axis. c 1D chains of 1 from b-axis. d The highlights of Mn(II) environment coordinated by phen and 5-Br-ipa ligand in 1 (H atoms are omitted for clarity)

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The 5-Br-H2ipa ligand has often been adopted as a component for building coordination compounds with or without auxiliary ligands, as it is a kind of polydentate ligand which may act as a linker with different geometric effects to connect metal centers into multidimensional structures via various coordination modes [30–35]. In complex 1, the completely deprotonated 5-Br-ipa2− acts as a tridentate ligand, joining Mn(II) centers via both chelating and bis-mono modes to form two-dimensional Mn(5-Br-ipa) sheets modified by phen ligands, as illustrated in Fig. 1b–d and Scheme 2a. The 2D Mn(phen)(5-Br-ipa) sheets are further linked into a three-dimensional structure via π-π stacking interactions, as listed in Table 4.

Scheme 2
scheme 2

Coordination environments of the Mn(II) centers in complexes 1–3

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Mn(phen)(5-NO2-ipa) (2) was synthesized under identical conditions as 1. Again, the Mn1 and Mn2 centers in 2, are almost identical. For example, Mn1 is octahedrally coordinated by N3 and N4 from a phen ligand, O1 and O7 atoms from two different monodentate 5-NO2-ipa2− ligands, and O9 and O10 from one chelating 5-NO2-ipa2− ligand, as shown in Fig. 2a, b. The coordination environment of Mn(II) and coordination mode of the 5-NO2-ipa2− ligand in complex 2 are thus nearly identical with their counterparts in complex 1. However, complexes 1 and 2 differ in possessing 1D chains as illustrated in Fig. 2c, d and Scheme 2b and 2D sheets as depicted in Fig. 1b, respectively, which can be explained by the slight difference of O–Mn–O bond angles as listed in Table 2 and Scheme 2a, b. Finally, in complex 2, a three-dimensional supramolecular structure is built up of one-dimensional Mn(phen)(5-NO2-ipa) chains via π–π stacking interactions as listed in Table 4.

Fig. 2
figure 2

a Asymmetric unit of Mn(phen)(5-NO2-ipa) (2) and coordination environments around the Mn(II) atoms. b Highlight of the coordination environment of the Mn(II) centers. c Packing view of 1D chains from the a-axis. d 1D chain of compound 2

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The 3D supramolecular structure of [Mn(phen)2(3,4-H2dczpb)]·H2O (3) is constructed from discrete zero-dimensional [Mn2(phen)4(3,4-H2dczpb)] units and lattice water molecules via intermolecular hydrogen bonding interactions [O2-O5(x-1, y, z-1): 2.544 Å; O7-O6: 2.381 Å] (Fig. 3b, c; Table 3) and π–π stacking interactions as listed in Table 4. In the structure of complex 3, Mn(II) is six-coordinated in a distorted octahedral geometry connected by four nitrogen atoms (N1, N2, N3 and N4) from two different phen ligands, plus two oxygen atoms (O3 and O4) from a bis-monodentate 3,4-H2dczpb2− ligand, as illustrated in Fig. 3a. The 3,4-H2dczpb2− anions acts as both bis-monodentate ligands to join two different Mn2+ centers and as counter-ions. The coordination mode of 3,4-H2dczpb2− in complex 3 is similar to that of the previously reported [Co2(phen)4(H2dczpb)2]·5H2O [36], while being quite different from the [Mn2(phen)4(3,4-H2dczpb)](3,4-H4dczpb)2·2H2O [12]. The terminal COOH substituents on the other benzene ring of the 3,4-H2dczpb2− ligand and four coordination sites of Mn2+ being occupied by phen resulted in the low-dimensional structure of complex 3.

Fig. 3
figure 3

a Ortep view of [Mn(phen)2(3,4-H2dczpb)]·H2O (3). Lattice water molecules and H atoms are omitted for clarity. b 3D framework of 3 viewed along c-axis. c Hydrogen bonding interactions in the complex 3

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Table 3 Hydrogen bonds for compound 3 [Å and °]
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Table 4 Defined ring and relative parameters of π-π interactions in compound 1–3
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Optical energy gap

To explore the conductivities of the complexes 1–3, UV–Vis DRS was used on powder samples to obtain their corresponding band gap E g [37, 38], determined as the intersection point between the energy axis and the line extrapolated from the linear portion of the absorption edge in a plot of Kubelka–Munk function F against energy E. The Kubelka–Munk function, F = (1 − R)2/2R, was calculated from the recorded UV–Vis DRS data, where R is the reflectance of an infinitely thick layer at a given wavelength. The plots of F versus E for these complexes are shown in Fig. 4, where steep absorption edges are displayed. The E g values were obtained as 2.9, 3.0 and 3.2 eV for complexes 1, 2 and 3, respectively, implying that all three complexes show selective absorption in the visible and ultraviolet spectrum region [39, 40].

Fig. 4
figure 4

Kubelka-Munk transformed diffuse reflectance spectra for complexes 1–3

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Photocatalytic studies

The photocatalytic performances of these complexes for the decomposition of MB were studied under UV irradiation. Control experiments on degradation of MB in the absence of catalysts were also performed. The photocatalytic performances were monitored by measuring the maximum absorbance intensity at λ = 664 nm to determine the residual concentration of MB. The degradation efficiencies of MB in the presence of complexes 1–3 after adsorption equilibrium in the dark are depicted in Fig. 5. All data are the average values of three parallel experiments. The degradation efficiency of MB increased from 27.5 % (control experiment without any photocatalyst) to 66.1, 74.2 and 91.9 % with 1, 2 and 3, respectively, after 120 min. These photocatalytic degradation reactions each followed a pseudo-first-order kinetic model, with R 2 values of 0.949, 0.865 and 0.926 for 1, 2 and 3, respectively, evidenced by the linear plots of −ln(C/C 0) versus reaction time t, such that the pseudo-first-order rate constants (k) were 0.0090, 0.0095 and 0.018 min−1, respectively.

Fig. 5
figure 5

Plots of concentration of MB versus irradiation time under irradiation with Hg lamp light with complexes 1–3 as photocatalysts, plus a control experiment without any photocatalyst

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In the presence of UV light, charge transfer presumably takes place from the HOMO of O and/or N atoms, to the LUMO, which is likely to be centered on the Mn atom [11, 41–43]. Excited state electrons in the LUMO are usually very easily lost, while the HOMO can readily accept an electron to return to its stable ground state [11]. Therefore, electrons are captured from water molecules to produce ·OH active species, which can destroy organic dyes such as MB efficiently in the photocatalytic process [43, 44].

In order to confirm the proposed mechanism, radical trapping experiments were carried out to detect the main oxidative species in these reactions [45]. The results revealed that the addition of 1 mM isopropanol (IPA) as a radical scavenger significantly inhibited the degradation efficiencies of MB (decreased from 91.9 to 40.9 %) under UV irradiation for 120 min, with k being 0.0046 min−1 (compared to 0.018 min−1 without IPA as radical trapping agent), as illustrated in Fig. 5. This is consistent with the involvement of ·OH radicals in the photodegradation reactions.

Conclusions

Three novel coordination compounds were synthesized by hydrothermal methods and characterized by X-ray diffraction analysis, as well as physico-chemical and spectroscopic analysis. Complex 1 consisted of 2D neutral sheets, while complex 2 had a 1D structure. The different dimensionalities can be attributed to the different O–Mn–O angles in these complexes, although the Mn(II) centers and the organic ligands adopted identical coordination modes. Complex 3 was a supramolecular 0D coordination complex. All three complexes exhibited photocatalytic activities to decompose MB under UV irradiation, believed to be mediated by ·OH radicals. Compared to those complexes already described in the literatures [11], complex 3 possesses average catalytic activities to degrade MB. Further research should be carried out to investigate their photocatalytic activities with respect to other organic pollutants.

Supplementary material

CCDC 1042969, 1042971 and 1042976 contain the supplementary crystallographic data for compounds 1, 2 and 3. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.