Five novel cobalt coordination polymers: effect of metal–ligand ratio and structure characteristics of flexible bis(imidazole) ligands

Abstract

Five novel metal–organic frameworks (MOFs) have been hydrothermally synthesized, namely, [Co(bimb)(2,4′-bpdc)(H₂O)₂]·H₂O (1), [Co₂(bimb)₃(2,4′-bpdc)₂(H₂O)₂]·H₂O (2), [Co(bix)(2,4′-bpdc)] (3), [Co(bix)_0.5(2,4′-bpdc)(H₂O)_0.25] (4) and [Co(bimh)(2,4′-bpdc)] (5), based on mixed ligands, 2,4′-biphenyldicarboxylic acid (2,4′-H₂bpdc) as organic linkers and different bis(imidazole) ligands as co-ligands: (1,4-bis(imidazol-1-yl)-butane (bimb), 1,4-bis(imidazol-1-yl-methylene)-benzene (bix) and 1,6-bis(imidazol-1-yl)-hexane (bimh)). The coordination polymers were characterized by single-crystal X-ray diffraction, thermogravimetric analysis (TGA) and element analysis (EA). Reactions between CoCl₂·6H₂O, 2,4′-H₂bpdc and neutral bimb ligand afforded two crystallographically different coordination polymers (1 and 2) through tuning the ligand/metal ratio meticulously. Compound 1 crystallizes in a noncentrosymmetric Pna2₁ space group with 6⁵·8 CdSO₄-type topology, while compound 2 is isolated with P2₁/c space group and displays a 4-connected 6⁶ network. By substituting bix for bimb ligand, alteration of the metal/ligand ratio in the initial reaction system induced a transition from 2D layer (compound 3) to 3D architecture (compound 4). Notably, the structure of 4 is constructed from [Co(2,4′-bpdc)] layers, which are further pillared by bix ligands into a 3D 3,4-connected (6³)·(6³·10³) network. In 5, the left- and right-handed helical chains constructed by Co(II) and bimh ligands are bridged by 2,4′-bpdc²⁻ anions to form a layer-like structure.