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 $^{1}_{\infty}\rm\{[Fe^{III}(NO_{3})_{2}({\rm \mu}-4,4^{\prime}-bipy)(H_{2}O)_{2}]OH\cdot H_{2}O\}$ ( 1 ) and $^{1}_{\infty}\rm\{[Fe^{II}(H_{2}PO_{4})_{2}({\rm \mu}-4,4^{\prime}-bipy)(H_{2}O)]\cdot H_{2}O\cdot (4,4^{\prime}-bipy)\}$ ( 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. 57 Fe 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 H 2 PO 4 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).