Rigid versus semi-rigid bis(imidazole) ligands in the assembly of two Co(ii) coordination polymers: structural variability, electrochemical properties and photocatalytic behavior

Abstract

The hydrothermal reactions of 1,2,4,5-cyclohexanetetracarboxylic acid (H₄L) with CoCl₂·2H₂O and rigid or semi-rigid bis(imidazole) ligands were able to generate two Co(II) coordination polymers (CPs), {[Co₅(L)₂(1,4-bimb)(μ₃-OH)₂(H₂O)₈]·2H₂O}_n (1), {[Co(L)_0.5(1,4-bib)]·H₂O}_n (2) (1,4-bimb = 1,4-bis(imidazol-1-ylmethyl)benzene, 1,4-bib = 1,4-bis(1H-imidazol-1-yl)benzene). CPs 1 and 2 were structurally characterized by elemental analysis, IR spectroscopy, X-ray powder diffraction and single crystal X-ray diffraction. CP 1 features a 3D 3,3,4,4,5-connected framework with an unprecedented {4²·8·10²·12}{4³·6·8⁶}₂{4³}₂{4⁶}₂{6²·8}₂ topology, which represents the first example of CPs with such a topology. CP 2 possesses a three-fold interpenetration 3D framework with mog topology. The distinct structures of the two CPs may result from diverse coordination modes of the (L)⁴⁻ ligands and different structural characteristics of rigid or semi-rigid N-donor ligands. The thermal stabilities, photoluminescence properties and electrochemical behavior in the solid state for CPs 1 and 2 have been investigated. The photophysical studies indicated that CPs 1 and 2 are potential semiconductive materials. Moreover, both CPs 1 and 2 show high photocatalytic efficiency for the degradation of methylene blue (MB) under UV light irradiation and exhibit good stability and recyclability. A possible photocatalytic mechanism is speculated by introducing t-butyl alcohol (TBA) as a widely used ˙OH scavenger.