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

Five novel metal-organic frameworks (MOFs) have been hydrothermally synthesized, namely, [Co(bimb)(2,4'-bpdc)(H2O)(2)]center dot H2O (1), [Co-2(bimb)(3)(2,4'-bpdc)(2)(H2O)(2)]center dot 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'-H(2)bpdc) 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 center dot 6H(2)O, 2,4'-H(2)bpdc 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 Pna2(1) space group with 6(5).8 CdSO4-type topology, while compound 2 is isolated with P2(1)/c space group and displays a 4-connected 6(6) 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 (6(3)).(6(3).10(3)) network. In 5, the left- and right-handed helical chains constructed by Co(II) and bimh ligands are bridged by 2,4'-bpdc(2-) anions to form a layer-like structure.