Fang-Hua Zhao

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]·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.

Metal–ligand ratio-controlled assembly of two pairs of Co(II) complexes: syntheses, structures and magnetic properties

Reactions of Co(II) salts with rigid 5-amino-isophthalic acid (AIP) and flexible bis(imidazole) ligands by controlling the metal–ligand ratio gave rise to two pairs of new complexes: [Co(AIP)(bimb)] (1) and [Co(AIP)(bimb)0.5] (2), [Co(AIP)(bix)]·H2O (3) and [Co2(AIP)2(bix)1.5] (4) (bimb = 1,4-bis (imidazol-1-yl)-butane, bix = 1,4-bis(imidazol-1-yl-methylene)-benzene). Comparative study revealed that the different metal–ligand ratios affected the coordination modes of the ligand and thus resulted in four different structures. All complexes display 2D structures and extend into 3D supramolecular frameworks via intermolecular hydrogen bonds. Complex 1 features a (4,5)-connected hydrogen-bond network. Complex 2 displays a 2D bilayered structure with a rare (3,4)-connected (63)(65 ·8) topology. Complex 3 is an 2D (4,4) network showing 2-fold parallel interpenetrating 2D → 2D structure, which features the 2-fold interpenetrating cds hydrogen-bond net. Complex 4 shows a trinodal (4,4,5)-connected hydrogen-bond net. Magnetic studies of 1 reveal ferromagnetic interaction at low temperature and 4 displays antiferromagnetic behavior.

Two Acentric Mononuclear Molecular Complexes with Unusual Magnetic and Ferroelectric Properties

Two acentric, i.e., noncentrosymmetric, mononuclear complexes, Co(5-ATZ)(4)Cl(2), 1, and Cu(5-ATZ)(4)Cl(2), 2, where 5-ATZ is the monodentate 5-amino-1-H-tetrazole ligand, have been prepared and characterized. Both complexes crystallize in the tetragonal system with the P4nc space group, a member of the polar noncentrosymmetric 4mm class, and thus both 1 and 2 can exhibit ferroelectric and nonlinear optical properties. Magnetic studies indicate that 1 is a paramagnetic high-spin cobalt(II) complex with a rather extensive spin-orbit coupling, modeled as a zero-field splitting parameter, D, of ±91(3) cm(-1) and with very weak long-range antiferromagnetic exchange interactions. Direct current (dc) and ac magnetic studies indicate that 2 is a paramagnetic copper(II) complex that exhibits weak ferromagnetic exchange interactions below 15 K. Both 1 and 2 exhibit ferroelectric hysteresis loops at room temperature with remanent polarizations of 0.015 μC/cm(2) and coercive electric fields of 5.5 and 5.7 kV/cm, respectively.

Three 3d–4f heterometallic complexes constructed from oxalic acid and benzimidazole-5,6-dicarboxylic acid

Three 3d–4f heterometallic complexes, namely, [Co3Ln2(L)4(C2O4)2(H2O)2·6H2O]n (Ln = Pr (1), Nd (2), Sm (3); H2L = benzimidazole-5,6-dicarboxylic acid, H2C2O4 = oxalic acid), have been synthesized hydrothermally and characterized by single crystal X-ray diffraction study, IR, TGA and elemental analysis. The three complexes are isostructural; in their structures, the Co(II) centers are connected into a linear Co3 cluster by the C2O42− anions, the Ln(III) centers are connected into one-dimensional (1D) {Ln–O–C}n rod-like chains by the carboxyl groups of L2− anions, the linear Co3 clusters and {Ln–O–C}n rod-like chains are further connected by the L2− anions, and finally construct three 3D frameworks, which contain 1D channels along the b direction occupied by (H2O)8 clusters. In addition, magnetic studies of the three complexes all reveal antiferromagnetic behaviors.

A 2D→3D net with coexistence of polycatenation and polythreading constructed by long rigid and flexible ligands

A new nickel complex, [Ni(pim)(bibp)1.5(H2O)]·1.25H2O (1, pim = pimelic acid, bibp = 4,4′-bis(1-imidazolyl)biphenyl), was obtained via hydrothermal synthesis. The complex shows an unprecedented 2D→3D net with the coexistence of polycatenation and polythreading based on the T-shaped bilayer motifs.