Triple-stranded ferric helices: a π–π interaction-driven structural hierarchy of Fe5, Fe7, and Fe17 clusters

Abstract

Three polypyridine ligand-supported multinuclear iron complexes, [Fe₅], [Fe₇] and [Fe₁₇], were synthesized and their physical properties were investigated. The complexes had triple-stranded helical structures with pseudo threefold symmetry, and were stabilized by varying degrees of intramolecular π–π stacking. The pentanuclear iron complex consisted of two Fe^II and three Fe^III ions, supported by three ligands, while the heptanuclear complex comprised four Fe^II centres, three Fe^III ions, and six ligands, and the heptadecanuclear complex contained seventeen Fe^III ions and nine ligands. Electrochemical studies revealed that the pentanuclear and heptanuclear iron complexes showed pseudo-reversible three- and five-step redox behaviours, respectively. Magnetic measurements conducted on the pentanuclear and heptanuclear complexes revealed that antiferromagnetic interactions were operative between neighbouring iron ions through the oxo- and pyrazole-bridges.