Syntheses, Structures And Properties Of Ni(â…¡) Coordination Polymers Based On Pyridine-3,4-dicarboxylate

Metal-organic coordination polymers have attracted extensive attention in the past decade due to their fascinating structures and potential applications in magnetism, adsorption, separation, catalysis, sensor and optics. The exploration of assembly principles and the construction of coordination polymers with intrteresting structures and excellent properties are one of hot topics. The organic ligands are well known to play an crucial role in the formation of coordination polymers, as a result, it is the key for acquiring the desirable coordination polymers through the deliberate selection of organic ligands and the adjustment of influencing factors in self-assmebly process.In this contribution, we have selected the mixed ligands of unsymmetric pyridine-3,4-dicarboxylic acid (3,4-H2pydc) and bipyridyl or bis(imidazolyl) ligands with different spacer between coordination groups, such as 1,2-di-(4-pyridyl)ethane (dpe), 1,2-di(4-pyridyl)ethylene (dpy) and 1,3-di(4-pyridyl)propane (dpp), 1,2-bis(imidazol-1’-yl)ethane (bime), 1,3-bis(imidazol-1’-yl)propane (bimp), 1,4-bis(imidazol-1’-yl)butane (bimb) and 2,5-bis(1H-imidazol-1’-yl)thiophene (bimt). The hydrothermal reactions of these mixed ligands and Ni(NO3)2·6H2O or Ni(CH3COO)2·4H2O gave rise to eight Ni(Ⅱ) coordination polymers, [Ni2(3,4-Pydc)2(dpe)(H2O)2]n·(H2O)n (1), [Ni2(3,4-pydc)2(dpy)(H2O)2]n·(H2O)n (2), [Ni2(3,4-pydc)2(H2O)3]n(H2O)4n (3), [Ni(3,4-pydc)(bime)(H2O)]n (4), [Ni(3,4-pydc)(bimp)(H2O)]n (5), [Ni(3,4-pydc)(bimb)0.5(H2O)2]n·(H2O)n (6), [Ni(3,4-pydc)(bimt)(H2O)]n·(H2O)0.5n (7) and [Ni2(3,4-pydc)2(bimt)2]n·(H2O)5n (8), respectivley. The crystal structures of complexes 1-8 were defined by single crystal X-ray diffraction analysis. Ni(Ⅱ) ions in 1-8 adopt a distorted octahedral geometry.3,4-pydc in 1-3 serves as aμ4-bridge, while 3,4-pydc in 4-7 adopts a μ3-bridging mode.1,2.7 and 8 are three-dimension (3-D) non-interpentrating pillared-layer structures.3 does not contain dpp and is a 3-D network.4 and 5 are 2-D network, while 6 is a novel bilayer structure. The effects of the coordination groups and the spacers bewteen coordination groups on the structures and properties have been systematically investigated. All of the complexes were also characterized by X-ray powder diffraction, IR spectra, thermogravimetric analysis and elemental analysis. The magnetism for 1,2,3 and 8 was also investigated. Although 1,2,3 and 8 consist of dinuclear Ni(Ⅱ)-carboxylate units, but the difference of carboxylate bridging modes results in antiferromagnetic coupling in 1,2 and 3, and a ferromagnetic interaction in 8.