Structural and magnetic studies of atr-based 1-D、2-D、3-D frameworks

Departmaent of Chemistry, National Taiwan University* National Synchrotron Radiation Research Center,Hsinchu, Taiwan**
○Wei-Lun Ho* Gene-Hsiang Lee* Hwo-Shuenn Sheu** Yu Wang*

New iron(II) coordination polymers, 1-D {[Fe(atr)2(μ-atr)(H2O)2](BF4)2.4H2O}n (atr = trans-4,4'-azo-1,2,4-triazole), 2-D {Fe(μ-admtr) (μ-4,4'-bipyridine) (NCS)2}n (admtr= trans-4,4'-azo-3,5-dimethyl-1,2,4-triazole) and 3-D {[Fe(μ-atr)3](BF4)2.2H2O}n have been synthesized and characterized. The crystal structure of 1-D and 3-D compounds have similar formula ,but contain two types of coordination: In 1-D compound the coordination of Fe contains two water, two terminal atr ligands ,and one bridging atr ligand, while in 3-D compound the metal coordination contains three bridging atr ligands between iron centers, resulting in a interpenetrated structure. Powder X-ray Diffraction of dehydrated 1-D compounds shows that it actually turns into 3-D structure, which is confirmed by the corresponding magnetic behavior being changed from paramagnetic to a spin crossover at 290K. We can also presume that the path of structural transformation from the 1-D and 3-D crystal structure.
In another 2-D structure, the Fe coordination contains one methyl-substituted atr bridging ligand, one 4-4'-bipyridine bridging ligand ,and two NCS anions. The 2-D compound shows a structural phase transition at 105K. At room temperature, the iron is at the center of inversion (Z=1, in P-1), while at 120K, it is not (Z=2, in P-1) with c-axis doubled. The SQUID measurement indicates that there is a weak ferromagnetic interaction below 50K.