The influence of ClO_4~- and PF_6~- anions and the steric hindrance of flexible bis-imidazole ligands on the formation and structures of iron(II) coordination polymers

摘要

Treatments of the flexible 1,4-bis(imidazolyl)butane (bib) ligand with Fe(II) salts in a mixture of ethanol and water (1:1) as a solvent at room temperature resulted in the formation of two novel coordination polymers, {[Fe(bib)_3](ClO_4)_2}_n (1) and {[Fe(bib)_3](PF_6)_2}_n (2), with two distinctly different network structures due to the variation of the ClO_4~- and PF_6~- moieties in these compounds. Their infrared spectra, X-ray powder diffraction (XRPD), elemental and thermal analyses were also investigated in detail. The results of the X-ray analyses demonstrate that in these structures, the iron(II) ion lies on an inversion center and adopts an FeN_6 octahedral geometry. Each μ_2-bib serves as a bridging ligand connecting two adjacent iron(II) ions. In the structure of 1, six μ_2-bib ligands are simultaneously coordinated to one central iron(II) ion to form an open 3D 2-fold interpenetrating framework with an a-polonium(pcu) topology, while 2 has an open 3D network with an acs [point symbol (4~9.6~6)] structure. The structure of [bib~(Me)H](PF_6) [bib~(Me) = 1,4-bis(2-methylimidazolyl)butane] (3) has also been studied by X-ray crystallography as an unexpected product in an attempt to synthesize the {[Fe(bib~(Me))_3]~(2+)}_n cation with the PF_6~- anion.