Yan-Qiu Zhang1 • Chong-Chen Wang1,2 • Xin-Xing Guo1 • Peng Wang1 • Shi-Jie Gao1

Abstract 

Two 1D coordination polymers based on transition metals, namely [Co(4,40 -bpy)2(H2bptc)] (1) and [Ni(4,40 -bpy)(H2O)4](H2bptc)2H2O (2) (4,40 -bpy = 4,40 - bipyridine; H4bptc = 3,30 ,4,40 -biphenyltetracarboxylic acid), had been synthesized under hydrothermal conditions and characterized by single-crystal X-ray diffraction, FTIR spectroscopy, elemental analysis, powder X-ray diffraction, and UV–Vis diffuse reflection spectroscopy. The photocatalytic degradation of methylene blue, methyl orange, and rhodamine B by these complexes under UV light irradiation was investigated and shows that both complexes can catalyze each of these reactions. A possible catalytic mechanism is proposed in terms of the HOMO– LUMO gap and the associated light-induced transitions in these complexes. This was further supported by  OH radicals trapping experiments using isopropanol as radical scavenger. Complex 1 showed no obvious decay of photocatalytic efficiency even after recycling five times, implying excellent stability and recyclability.

Photocatalytic degradation experiments 

The potential of these complexes as photocatalysts was evaluated via degradation of MB, MO, and RhB dyes 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.

Results and discussion 

Structures of complex 1 and complex 2 

The complex [Co(4,40 -bpy)2(H2bptc)] (1) was synthesized under hydrothermal conditions, and is insoluble in common solvents including but not limited to water, methanol, ethanol, and ether. The crystal structure analysis reveals that complex 1 is built up of 1D neutral [Co(4,40 -bpy)2 (H2bptc)] chains, as illustrated in Fig. 1d. The Co(II) center, in an octahedral geometry, is six-coordinated by two nitrogen atoms from two different 4,40 -bpy ligands, two oxygen atoms from two different monodentate H2 bptc2- ligands, and two oxygen atoms from one H2bptc2- ligand in a chelating mode, such that two nitrogen atoms (N1 and N3) occupy the axial positions and the remaining four carboxylic oxygen atoms (O3, O4, O7, O8) lie in the four sites of the equatorial plane, as depicted in Fig. 1a. The Co–O and Co–N bond distances are comparable with the normal values for these bonds found in similar CPs [18, 24]. In the equatorial plane, the O4#1–Co1–O8, O3–Co1– O4#1, O3–Co1–O7, and O7–Co1–O8 bond angles are 88.05(15), 106.40(17), 104.73(16), and 59.67(14), respectively, while the N1–Co1–N3 bond angle is 167.32(18), revealing that the co-centered coordination octahedron is slightly distorted. The H4bptc ligand has often been adopted as a component for building CPs with or without the auxiliary ligands. This polydentate ligand which may act as a linker with different geometric effects to connect metal centers into multidimensional structures via various coordination modes is shown in Scheme 1a–h [19, 25, 26]. In complex 1, the partly deprotonated H2bptc2- acts as a tridentate ligand, joining the Co(II) centers via both chelating and monodentate modes to form one-dimensional [Co(H2bptc)] chain, as illustrated in Scheme 1a, Fig. 1b, c. Again, in complex 1, only one nitrogen atom of the 4,40 -bpy ligands is coordinated to the metal, with the other nitrogen being terminal without coordinating to any metal ions, hence the 4,40 -bpy acted as a monodentate ligand to complete the Co centers’ coordination environment, much different from its role as typical bidentate linker in other reported CPs [27– 29]. The complex [Ni(4,40 -bpy)(H2O)4](H2bptc)2H2O (2) was synthesized under identical conditions to those used for complex 1, and is also insoluble in common solvents