Channel-containing criss-crossed and hydrogen-bonded one-dimensional coordination polymers of iron(III) and (II) with 4,4′-bipyridine: Hydrothermal syntheses, structures, absorption and mössbauer studies showing spin crossover for the iron(III) polymer

The iron-4,4′-bipyridine coordination polymers _∞¹{[Fe^III(NO₃) ₂(μ-4,4′-bipy)(H₂O)₂]OH·H ₂O} (1) and _∞¹{[Fe^II(H ₂PO₄)²-(μ-4,4′-bipy)(H ₂O)]·H₂O·(4,4′-bipy)} (2) have been synthesized by hydrothermal methods. The structure of 1 is a channel-containing open framework constructed through the hydrogen-bond supported criss-cross arrangement of the 1D Fe-bipy chains. ⁵⁷Fe Mössbauer spectroscopy reveals a gradual spin crossover behavior for 1 with a magnetic ordering at low temperature. The structure of 2 is constructed from parallel oriented, hydrogen-bonded 1D Fe-bipy chains. The 4,4′-bipyridine solvate in 2 is held by hydrogen bonding between the H₂PO₄ groups on adjacent iron centers along the chain and is situated in a π-π stacking contact to the coordinated 4,4′-bipy ligand. The chains in 2 are interconnected through hydrogen bonding to a three-dimensional supramolecular network. Crystals of 2 appear dark red or orange depending on the orientation (pleochroism, dichroism).