Understanding spin crossover

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This work focuses on synthesizing and characterizing new one-dimensional iron(II) spin-crossover (SCO) coordination polymers featuring an octahedral N4O2 coordination sphere around the iron centers. It involves the development of new tetradentate equatorial ligands based on a Schiff base-like system, alongside bidentate bipyridine and bisimidazole derivatives that facilitate the formation of infinite 1D chains. By varying functional groups on the equatorial ligands, such as hydroxy and phenyl groups, the interactions between the chains were optimized through intermolecular interactions like hydrogen bonds and π-stacking. The free equatorial ligand can exist in two tautomeric forms: enol-imine and keto-enamine. For the ligand H2L1c, studies using 1H NMR spectroscopy and X-ray structure analysis demonstrated a preference for the keto-enamine form, contrasting with traditional Schiff base ligands. Coordination to a metal center influences the bond lengths of the conjugated π-system, indicating a balance between the tautomeric structures. A diverse ligand pool enabled the synthesis of various SCO compounds exhibiting different spin transition behaviors, from gradual to abrupt transitions with thermal hysteresis. Detailed investigations using magnetic measurements, X-ray analysis, Mössbauer spectroscopy, DSC, and paramagnetic NMR spectroscopy led to new models explaining the formation of stepwise spin transitions and wide thermal h