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 Fe-III 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) [Fe-III(Sal(2)trien)](+) (H(2)Sal(2)trien is the hexadentate ((N-am)(2)(N-im)(2)O-2) 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 (gamma(HS), S = 5/2), the Fe-N-am and Fe-N-im bonds depend most strongly on the change in gamma(HS), since the donor N atoms are characterized by high sigma-overlapping with the d-orbitals of the metal ion. The conformational switch of the ethylene bridges of the ligand occurs in the range 40% < gamma(HS) < 85% at the relative elongations of the Fe-N-am and Fe-N-im bonds equal to 4.0-8.5% and 3.4-8.6%, respectively. Relationships were revealed between the gamma(HS) 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 gamma(HS) = 0% and to 5% gamma(HS) = 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 gamma(HS) and development of molecular design of Fe-III compounds with controlled spin transition.

Automated summary

In this study, the relationship between the spin state of the Fe-III ion, conformations of the ethylene bridges in the ligand, and Fe-N/O bonds was analyzed based on literature data for 29 structures. It was found that the Fe-N-am and Fe-N-im bonds are most strongly influenced by the change in spin state, and the conformational switch of the ethylene bridges occurs within a specific range of gamma(HS).