Understanding Spin Crossover: A Contribution

This work deals with the synthesis and characterisation of new one-dimensional iron(II) SCO coordination polymers which exhibit an octahedral N4O2-coordination sphere around the iron centres. On the one side, new tetradentate equatorial ligands were synthesised, which base on the Schiff base-like ligand system. On the other side, new bidentate bipyridine and bisimidazole derivatives were synthesised which, offer the ability to link the complexes at the axial position to infinite 1D chains. Through specific variation of functional groups at the equatorial ligands (e.g. hydroxy groups, phenyl groups), it was possible to optimise the interactions between the chains (inter-molecular interactions, e.g. hydrogen bonds, p-stacking). These functional groups also influence the ligand field at the central atom, but this effect plays a minor role for the system presented here.

The free equatorial ligand offers two possible tautomeric structures with enol-imine functional groups or keto-enamine, respectively. Using the example of the newly developed ligand H2L1c, it could be proven by 1H NMR-spectroscopy and X-ray structure analysis that the equilibrium is shifted to the keto-enamine structure. This is in contrast to classic Schiff base ligands like the salen ligand, for example. Upon coordination to a metal centre, the bond lengths of the conjugated p-system of the deprotonated ligand indicate a balance between both contributing structures and a delocalisation of the negative charge over the six-membered chelate ring.

Manifold combinations from a ligand pool, containing 9 axial and 9 equatorial partial newly developed ligands, allowed the synthesis of numerous SCO compounds with versatile spin transition behaviours: from gradual or abrupt, to stepwise and with thermal hysteresis. Through detailed investigation of the outcomes of magnetic measurements, X-ray structure analysis, Mössbauer spectroscopy, DSC and paramagnetic NMR spectroscopy in solution, new models were developed in this work to explain the formation of stepwise spin transitions and wide thermal hysteresis loops, as observed for some of the compounds presented. On the way to the purposeful synthesis of SCO materials and a better understanding and predictability of the SCO phenomenon, this is highly important.