Iron(II/III) complexes of non-porphyrin macrocyclic [N 4] 2− ligands: structure and axial reactivity

Abstract

The orientation of axial ligand planes in fifteen octahedral homo- and (in one case) hetero-ligand diadducts of the porphyrin-like macrocyclic iron(II)/(III) complexes Fe 1a, Fe 1b, Fe 2 and Fe 3 have been characterised by X-ray structure analysis. In contrast to iron porphyrins, the equatorial Fe–N distances are shorter than those of the axial Fe–N bonds. The bond lengths within the first coordination sphere are nearly independent of the oxidation step of the central atom. The relative orientation of the axial ligand planes is discussed with respect to the oxidation step of iron, the symmetry of the ring system and—especially for derivatives of the tetraaza[14]annulene Fe 3—to the saddle-shaped distortion of the macrocyclic plane. The reactivity of the axial coordination sites has been evaluated by equilibrium studies of the consecutive ligand substitution in the octahedral diadducts. The quotient K 1/ K 2 for the individual steps of ligand exchange, which represents a quantitative measure of ‘push–pull’ effects of different axial ligands in mixed adducts, is particularly high for the combination of P(OEt) 3 with pyridine bases as trans ligands.