Triple-stranded ferric helices: a π-π interaction-driven structural hierarchy of Fe5, Fe7, and Fe17 clustersElectronic supplementary information (ESI) available: Mössbauer data of complex 2. CCDC 942566-942568. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3dt51480c

Three polypyridine ligand-supported multinuclear iron complexes, [Fe 5 ], [Fe 7 ] and [Fe 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 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. Polypyridyl ligands were used to support triple-stranded helical [Fe 5 ], [Fe 7 ] and [Fe 17 ] complexes. The redox-active complexes were stabilized by increasing degrees of π-π stacking between the ligand moieties.