Peripheral Ligand Modification to Tune Spin Crossover Behavior of Fe(III) Complexes

Two major factors governing the spin crossover (SCO) behavior are the electronic and steric aspects of the ligand. The electronic effects directly alter the energy gap between the frontier molecular (t2g and eg) orbitals, whereas the steric effects can lead to a distorted structure, which then can change the electronic properties of the complex, leading to a change in the spin state. Here, we investigated how the steric modulation of the ligand by its peripheral modification affects the spin states of the corresponding Fe(III) complexes. We chose three different amines, N-ethylethylenediamine, N-phenylethylenediamine, and N-benzylethylenediamine, and condensed them with salicylaldehyde and 3-methoxysalicylaldehyde to afford the corresponding Schiff base ligands, HL1, HL3, and HL2, respectively. Interaction of these ligands with Fe(III) salts afforded [Fe(L1)2]NCS (1), [Fe(L2)2NCS]·H2O (2·H2O), and [Fe(L3)2]NCS (3), respectively. 3-Methoxysalicylaldehyde with N-phenylethylenediamine afforded ligand HL2 and complex 2·H2O. 1 and 3 exhibit SCO behavior, while complex 2·H2O remains in the high spin state. These results were rationalized by a combination of experimental and theoretical studies.