Triple-stranded ferric helices: a pi - pi interaction-driven structural hierarchy of Fe sub(5), Fe sub(7), and Fe sub(17) clusters

Three polypyridine ligand-supported multinuclear iron complexes, [Fe sub(5)], [Fe sub(7)] and [Fe sub(17)], 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 pi - pi stacking. The pentanuclear iron complex consisted of two Fe super(II) and three Fe super(III) ions, supported by three ligands, while the heptanuclear complex comprised four Fe super(II) centres, three Fe super(III) ions, and six ligands, and the heptadecanuclear complex contained seventeen Fe super(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.