A relationship between the coordination octahedron parameters and ligand conformation during spin transition in the cationic complex [N, N′-3,6-diazaoctane-1,8-diylbis(salicylidenaldiminato)]iron(iii) [FeIII(Sal2trien)]+

Magnetostructural relationships between the spin state of the FeIII ion, conformations of the ethylene bridges (-CH2-CH2-) in the five-membered chelates of the ligand, and Fe-N/O bonds were constructed due to an analysis of the literature data for 29 structures based on the [N,N′-3,6-diazaoctane-1,8-diylbis(salicylidenaldiminato)]iron(iii) [FeIII(Sal2trien)]+ (H2Sal2trien is the hexadentate ((Nam)2(Nim)2O2) Schiffbase, condensation product of triethylenetetramine and salicylaldehyde). According to the analysis results for the dependences of the Fe-N bond lengths on the fraction of the high-spin state (γHS, S = 5/2), the Fe-Nam and Fe-Nim bonds depend most strongly on the change in γHS, since the donor N atoms are characterized by high σ-overlapping with the d-orbitals of the metal ion. The conformational switch of the ethylene bridges of the ligand occurs in the range 40% < γHS < 85% at the relative elongations of the Fe-Nam and Fe-Nim bonds equal to 4.0–8.5% and 3.4–8.6%, respectively. Relationships were revealed between the γHS values and ratios of high- and low-spin conformers of the complex cation in the unit cell. The scatter of bond length values was found in the polyhedron FeN4O2 to 4% at γHS = 0% and to 5% γHS = 100%, which is caused by the effect of the crystalline environment of the complex cation. The determined features of the structural rearrangement of the hexadentate macrocyclic ligand of the Saltrien type can be used as a criterion for the estimation of γHS and development of molecular design of FeIII compounds with controlled spin transition.