Spin Crossover in Iron(III) Complexes with Pentadentate Schiff Base Ligands and Pseudohalido Coligands

Iron(III) mononuclear complexes that involve pentadentate Schiff base ligands and chlorido, azido, cyanido, cyanato, thiocyanato, or selenocyanato coligands were synthesized, structurally characterized, and subjected to a magnetochemical investigation. The Schiff bases were derived either from 5-chlorosalicylaldehyde or the 2-hydroxyacetonaphthone analogues by using an asymmetric 1,6-diamino-4-azahexane. A polymorphism that originated from different pentadentate ligand conformations on the iron center or different arrangements of noncovalent contacts was detected for the thiocyanato complexes. The central iron(III) atoms are mostly in the high-spin states, except for that with the coordinated cyanido ligand. Four complexes that contain the thiocyanato or selenocyanato ligand exhibit spin crossover, centered at the critical temperature (Tc) of 42, 114, 282, and 293 K, respectively. The magnetic data of all compounds were analyzed using the spin Hamiltonian formalism including the zero-field splitting (ZFS) term, and in the case of the spin-crossover compounds, the Ising-like model with vibrations was applied.