Evans–Showell-type polyoxometalate-based metal–organic complexes with novel 3D structures constructed from flexible bis-pyrazine–bis-amide ligands and copper metals: syntheses, structures, and fluorescence and catalytic properties

Abstract

Two new Evans–Showell-type polyoxometalate (POM)-based metal–organic complexes, namely {Cu₃(L₁)_1.5(H₂O)₅[Co₂Mo₁₀H₄O₃₈]}·5H₂O (1), {[Cu(L₂)_0.5(H₂O)₂]₂[Co₂Mo₁₀H₄O₃₈]}·6H₂O (2) (L₁ = N,N′-bis(2-pyrazinecarboxamide)-1,4-butane, L₂ = N,N′-bis(2-pyrazinecarboxamide)-1,6-hexane), were successfully synthesized and structurally characterized by single-crystal X-ray diffraction, elemental analysis, IR spectroscopy, powder X-ray diffraction (PXRD) and thermogravimetric analyses (TGA). In complex 1, the adjacent [Co₂Mo₁₀H₄O₃₈]⁶⁻ polyoxoanions are linked by Cu^II ions to form a 1D Cu–[Co₂Mo₁₀H₄O₃₈]⁶⁻ inorganic chain, which is further linked by ligand L₁ and [Co₂Mo₁₀H₄O₃₈]⁶⁻ polyoxoanions, forming a 3D metal–organic framework. In complex 2, the adjacent [Co₂Mo₁₀H₄O₃₈]⁶⁻ polyoxoanions link the Cu^II ions to generate a 2D Cu–[Co₂Mo₁₀H₄O₃₈]⁶⁻ inorganic layer, which is further connected with bidentate ligands L₂ to obtain a 3D metal–organic framework. The structural diversities of compounds 1 and 2 showed that the spacer lengths of the flexible bis-pyrazine–bis-amide ligands play important roles in tuning the structures of the title complexes. Compounds 1 and 2 represent the first examples of 3D frameworks based on the Evans–Showell-type polyoxoanions and Cu-bis-pyrazine–bis-amide coordination complexes. Moreover, the ligand L₁ was first successfully introduced into the POM system. The electrochemical and fluorescence properties of compounds 1 and 2 were discussed. As heterogeneous catalysts, compounds 1 and 2 have good catalytic activity for the oxidation of benzyl alcohol. Moreover, compound 2 has higher catalytic performance with 100% conversion and 98.0% selectivity for benzoic acid at 10 h. The difference in their catalytic performance may be mainly due to the difference of their structures. The catalysts can be recovered and reused without displaying any significant loss of activity.