Axial Perturbation of Intermediate Spin (S = 3/2) Iron(III) Complexes with Square Pyramidal N4X Coordination: Solid State Structures and Electronic Properties

Abstract

The combination of planar‐directing, strong‐field [N4] ligands with a weak axial field component is known to stabilize five‐coordinate iron(III) complexes in otherwise uncommon spin states. In this work, a series of axially perturbed iron(III) complexes of an [N4] macrocyclic ligand of the Jäger type, [Fe(L)X] (with X: Cl, Br, I, NCS) gave intermediate‐spin ground states with axially steered admixture of the high‐spin state, S = 3/2; 5/2. The nature of the electronic ground‐state as deduced from SQUID magnetometry, 57Fe Mössbauer spectroscopy in the solid, and 1H NMR and EPR spectroscopies in (frozen) solutions was discussed in the light of the solid‐state structures, which were obtained from single‐crystal X‐ray diffraction. A DFT‐based protocol was established to quantify the axial field effects and, by including relevant [N4] systems, predict the differential equatorial ligand field components.