. Introduction Coordination compounds which exhibit a high surface area and large pore volume are of great current interest for their specific structural features [1–7] and potential applications, such as gas storage [8–10], separation [11,12], carbon dioxide capture [13] and catalysis [14]. Just recently, an increasing number of reports on coordination compounds as photocatalysts have appeared to indicate that they provide a unique opportunity for integrating different molecular functional blocks to obtain good performance of organic pollutant degradation [15–21], CO2 reduction [22–24] and Cr(VI) reduction [25,26]. Compared to traditional semiconductor photocatalytic systems, photoactive coordination compounds have some advantages to act as efficient photocatalysts: (i) the well-defined crystalline structures of coordination compounds are beneficial in the characterization and study of the structure– property relationship of these solid photocatalysts; (ii) the modular nature of the synthesis of coordination compounds allows the rational design and fine tuning of these catalysts at the molecular level, allowing the electronic structure of the coordination compounds catalysts to be easily tailored; (iii) the structural features of tunable active sites (i.e., metal-oxo clusters and organic linkers) in coordination compounds lead to more efficiency in solar harnessing; (iv) the intrinsic porosity and high surface area can facilitate fast transport guest molecules through the open channels, which is very essential for a high photocatalytic reaction efficiency. As is well known, the construction of coordination compounds is mainly dependent on the combination of several factors, such as the organic ligands, solvents, metal atoms and counter-ions [1–3,5,17,18,27]. Polycarboxylate ligands, as good candidates for the construction of coordination compounds, have aroused a good deal of interest from chemists. 2,20 ,3,30 -Oxydiphthalic acid (2,20 ,3,30 -odpt) had been used as a flexible and semi-flexible exo-multidentate ligand for the design and construction of novel coordination compounds due to its versatile coordination modes [28,29]. In this paper, in addition to 2,20 ,3,30 -odpt, two special exo-multidentate ligands, 3,5-di(3,4-dicarboxylphenyloxy) benzoic acid (dcpb) and 1,3-(30 ,40 -carboxylphenoxy) benzene (cpb) were introduced to construct four transition metal-based compounds, namely [Co2(phen)4(2,20 ,3,30 -H2odpt)2]14H2O (1), [Co2(phen)4(H4dcpb)2]14H2O (2), [Cu(phen)(H3dcpb)] (3) and [Mn(phen)2(H2cpb)] (4), with the aid of 1,10-phenanthroline (phen). The optical gaps, photocatalytic activities and possible degradation mechanism towards methylene blue (MB) and methyl orange (MO) in aqueous solution were studied.

Photocatalytic degradation of MB and MO Methylene blue (MB) with the molecular formula C16H18N3SCl (FW 319.85 g/mol) and methyl orange (MO) with the molecular formula C14H14N3NaO3S (FW 327.33 g/mol), as illustrated in Scheme 1, are difficult to degrade in wastewater and were used as model organic dye pollutants to evaluate the photocatalytic activities of compounds 1–4 at room temperature and 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 the reaction mixture was fixed at 5 cm. 50 mg of powder for each compound, with a particle size less than 147 lm and obtained by grinding the as-prepared single crystals, were put into 200 ml of MB (10 mg/L) or MO (30 mg/L) aqueous solution in a 300 ml flask. 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 photodegradation reaction, stirring was maintained to keep the mixture in suspension. One milliliter sample was extracted at regular intervals using a 0.45 lm syringe filter (Shanghai Troody) for analysis. A Laspec Alpha-1860 spectrometer was used to monitor the changes of the dye absorbance in the range 400–800 nm in a 1 cm path length spectrometric quartz cell. The MB and MO concentrations were estimated by the absorbance at 664 and 463 nm, respectively.

Results and discussion

Both compounds 1 and 2 crystallize in the triclinic space group P-1 and possess similar crystal structures, except for the organic carboxylic acid (2,20 ,3,30 -H4odpt in 1 and H5dcpb in 2) and the number of the lattice water molecules (14 lattice water molecules and 6 lattice water molecules respectively). Hence, only the structure of 1 is described in detail. The crystal structure analysis reveals that the compound 1 consists of the discrete neutral [Co2(phen)4(2,20 ,3,30 -H2odpt)2] complex and fourteen lattice water molecules. The Co(II) ion, in an octahedral geometry, is six-coordinated by four nitrogen atoms from two different phen ligands and two oxygen atoms from two different 2,20 ,3,30 -H2odpt2 ligands, in which one nitrogen and oxygen atom (N1 and O3) occupy the axial positions, and the remaining three nitrogen atoms (N2, N3 and N4) and one oxygen atom (O4#1) lie in the four sites of the equatorial plane, as shown in Fig. 1(a). The Co–O and Co–N bond distances compare with the normal values for these bonds [7,34]. In the equatorial plane, the O4#1–Co1–N2, O4#1–Co1–N3, N2–Co1–N4 and N3–Co1–N4 bond angles are 96.86(14), 91.10(14), 94.2(2) and 78.1(2), respectively, and the N1–Co1–O3 bond angle is 170.77(15), implying the Co-centered coordination octahedron is slightly distorted. The partly deprotonated 2,20 ,3,30 -H2odpt2 ligands act as both bis-monodentate ligands to link two Co(II) ions and counter-ions to compensate the charge of [Co(phen)2] 2+, which is very different from the coordination modes reported previously [28,29]. The dihedral angles between the phenyl rings of the 2,20 ,3,30 -H2odpt2 ligands are 88.3(2), exhibiting the typical V-shaped mode with the presence of an O atom. The neighboring [Co2(phen)4(2,20 ,3,30 -H2odpt)2] units are further linked into a 3D framework by rich hydrogen bonding interactions, as illustrated in Fig. 1(b) and (c) and Table 3.