Spin-State Variations of Iron(III) Complexes with Tetracarbene Macrocycles

Starting from their six-coordinate iron(II) precursor complexes [L^8R Fe(MeCN)]²⁺ , a series of iron(III) complexes of the known macrocyclic tetracarbene ligand L^8H and its new octamethylated derivative L^8Me , both providing four imidazol-2-yliden donors, were synthesized. Several five- and six-coordinate iron(III) complexes with different axial ligands (Cl^- , OTf^- , MeCN) were structurally characterized by X-ray diffraction and analyzed in detail with respect to their spin state variations, using a bouquet of spectroscopic methods (NMR, UV/Vis, EPR, and ⁵⁷ Fe Mößbauer). Depending on the axial ligands, either low-spin (S=1/2) or intermediate-spin (S=3/2) states were observed, whereas high-spin (S=5/2) states were inaccessible because of the extremely strong in-plane σ-donor character of the macrocyclic tetracarbene ligands. These findings are reminiscent of the spin state patterns of topologically related ferric porphyrin complexes. The ring conformations and dynamics of the macrocyclic tetracarbene ligands in their iron(II), iron(III) and μ-oxo diiron(III) complexes were also studied.