The influence of ClO4− and PF6− anions and the steric hindrance of flexible bis-imidazole ligands on the formation and structures of iron(II) coordination polymers

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](ClO4)2}n (1) and {[Fe(bib)3](PF6)2}n (2), with two distinctly different network structures due to the variation of the ClO4− and PF6− 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 FeN6 octahedral geometry. Each μ2-bib serves as a bridging ligand connecting two adjacent iron(II) ions. In the structure of 1, six μ2-bib ligands are simultaneously coordinated to one central iron(II) ion to form an open 3D 2-fold interpenetrating framework with an α-polonium (pcu) topology, while 2 has an open 3D network with an acs [point symbol (49.66)] structure. The structure of [bibMeH](PF6) [bibMe = 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(bibMe)3]2+}n cation with the PF6− anion.

Two Fe(II) coordination polymers with the imidazole based ligand bib were obtained with different structures. In each of these complexes, the metal centre adopts an almost regular FeN6 octahedral environment. The polymer {[Fe(bib)3](ClO4)2}n has an open 3D 2-fold interpenetrating framework with an α-polonium topology, while {[Fe(bib)3](PF6)2}n exhibits an open 3D network with the rare acs structure.Figure optionsDownload as PowerPoint slide