Syntheses, structural diversity, magnetic properties and dye absorption of various Co(ii) MOFs based on a semi-flexible 4-(3,5-dicarboxylatobenzyloxy)benzoic acid

Abstract

Five novel Co(II) metal–organic frameworks (MOFs) constructed from semi-flexible 4-(3,5-dicarboxylatobenzyloxy)benzoic acid (H₃L), namely {[Co_1.5(HL)(4,4′-bidpe)₂(H₂O)]·3H₂O}_n (1), {[Co₃(L)₂(4,4′-bibp)₃(μ₂-O)₂]·2H₂O}_n (2), {[Co(HL)(1,3-bitl)]·(1,4-Diox)}_n (3), [Co₂(HL)₂(3,5-bipd)₂]_n (4), and {[Co(HL)(tib)]·0.5H₂O·NMP}_n (5) (4,4′-bidpe = 4,4′-bis(imidazolyl)diphenyl ether, 4,4′-bibp = 4,4′-bis(imidazol-1-yl)biphenyl, 1,3-bitl = 1,3-bis(1-imidazoly)toluene, 3,5-bipd = 3,5-bis(1-imidazoly)pyridine, and tib = 1,3,5-tris(1-imidazolyl)benzene), were synthesized under solvothermal conditions and further characterized by elemental analysis, IR spectra, powder X-ray diffraction (PXRD), thermogravimetric (TG) analysis and single-crystal X-ray diffraction. Different architectural topologies have been generated by adjusting the N-donor ligands. Single-crystal X-ray diffraction analysis reveals that complex 1 shows a rare 1D → 2D polyrotaxane network. Complex 2 possesses an unprecedented 2-nodal (3,10)-connected 3D framework with a Schläfli symbol of (4³)²(4⁶·6³²·8³)(4³)². When the 2-connected points (H₃L and 1,3-bitl ligands) are not calculated, complex 3 shows a hcb uninodal 3-connected 2D network with the Schläfli symbol (6³), which further constructs a 3D supramolecular structure through O–H⋯O hydrogen bonds; while the 2-connected points are taken into account, complex 3 exhibits an unprecedented 3-nodal (2,2,4)-connected network. Complex 4 presents an unprecedented 2-nodal (3,5)-connected 3D framework with a (4·6²)(4·6⁶·8³) topology, while complex 5 exhibits another unprecedented 2-nodal (3,5)-connected 2D framework with a (4²·6⁷·8)(4²·6) Schläfli symbol and shows 2D → 3D supramolecular structure through O–H⋯O hydrogen bonds. Meanwhile, the magnetic properties of complexes 2 and 4 are discussed. Moreover, the dye adsorption and mechanism studies indicate that the pore size, the uncoordinated O atoms in carboxyl groups and the uncoordinated carboxyl groups of the MOFs have significant effects on the dye adsorption capacity.