A new high-spin iron(III) complex with a pentadentate macrocyclic amidopyridine ligand: A change from slow single-ion paramagnetic relaxation to long-range antiferromagnetic order in a hydrogen-bonded network

A new, stable iron(III) complex with a pentadentate amide-containing macrocyclic ligand was prepared and fully characterized. The complex adopted a pentagonal-bipyramidal geometry, where an equatorial plane is occupied by the pyridine nitrogen, two deprotonated amide nitrogens, and two secondary amines from the macrocycle, and two axial positions are available for monodentate ligand (chloride anion or solvent molecule) coordination. The rigid, planar iron-amide building blocks are linked in a three-dimensional network via a system of hydrogen bonds, with the shortest Fe-Fe separation of 8.02 Å. The coordination of strongly electron-donating, negatively charged deprotonated amide groups resulted in expected stabilization of a high oxidation state of iron (the redox potential of the Fe^IIL/Fe^IIL couple, -0.57 V vs SCE). In contrast to the majority of the iron complexes with polydentate amide ligands, the pentagonal-bipyramidal geometry of the macrocyclic complex described in this work affords a high-spin configuration of the central metal ion (room-temperature magnetic moment is 5.84 μ_β). Variable-temperature iron-57 Mössbauer spectroscopy and ac and dc magnetization studies indicate slow paramagnetic relaxation and a crossover to long-range antiferromagnetic order at T < ∼3.2 K.