Spin crossover complexes with N4O2 coordination sphere—The influence of covalent linkers on cooperative interactions

About 10 years ago Jäger and co-workers demonstrated the possibility of preparing an iron(II) spin crossover complex of a Schiff-base like N2O2 coordinating ligand in combination with imidazole or pyridine as axial ligands. Starting at this point, we have now prepared over 50 octahedral iron(II) complexes of this ligand type. Thirty of these new compounds are spin crossover complexes that can be subdivided into monomer (9), polymer (12) and dimer (9) systems. This review gives an overview of this complex class. Special consideration was given to the influence of covalent linkers on cooperative interactions during the spin transition, as the preparation of highly cooperative spin crossover complexes with wide thermal hysteresis loops around room temperature is still one of the major goals of a preparative chemist working in this area of research. In order to more closely evaluate the influence of the covalent linkers on the spin transition properties the complexes were not only investigated in the solid state but also in solution. For this purpose the proton NMR spectra of the complexes were analyzed in dependence on temperature and we will introduce the reader to the benefits of this method.