Towards direct correlations between spin-crossover and structural features in iron(II) complexes

Acta Cryst. (2003). B59, 479–486

Smoothness of the spin transition extracted from the magnetic curve versus the S–C distance of the S⋅⋅⋅H(C) intermolecular hydrogen interaction from the crystal structure.

The spin crossover [Fe(PM-BiA)₂(NCS)₂] complex, where PM is N-(2-pyridylmethylene) and BiA 4-aminobiphenyl, can crystallize in two polymorphs. The orthorhombic phase undergoes a very abrupt spin transition at T_1/2 = 167 K while the monoclinic phase undergoes a gradual spin transition at T_1/2 = 205 K. The polymorphism of this compound has been used to underline the structural properties responsible for such a difference in the magnetic behavior. New structural parameters are defined, and directly linked to the magnetic properties, in particular the cooperativity of the spin transition. The results are extended to all the complexes belonging to the same family of general formula [Fe(PML)₂(NCS)₂]. In this family, the spin-transition features are driven by weak intermolecular interactions such as S⋅⋅⋅H(C) hydrogen-like contacts.