Unusual Crystal Packing in a Family of [Fe{2,6-bis(pyrazol-3-yl)pyridine}2]2+ Compounds and the Effect on the Occurrence of Spin Crossover and Its Cooperative Character

The crucial relationship between the spin-crossover (SCO) phenomenon and the crystal lattice can be investigated by tuning the intermolecular interactions of a complex with little change to the electronic environment of the metal. With this aim, the synthesis of the new ligand 2,6-bis[5-(naphth-2-yl)-pyrazol-3-yl] pyridine (H2L) and four different forms (1-4) of the corresponding [Fe(H2L)(2)](2+) complex cation are presented. H2L exhibits an increased number of aromatic rings with respect to previous analogues to augment the density of pi center dot center dot center dot pi contacts within the system. The four compounds group into three different structural arrangements that coincide with two different SCO behaviour types and a locked high-spin state. The intermolecular interactions that embody the various crystallographic organisations have been analysed through Hirshfeld plots, and the SCO properties of 1-4 are also discussed in light of the local structural analysis around the Fe-II centres.