Iron cluster assumes snakelike structure

Taking advantage of a little-used O,N,N,O chelating ligand, George Christou and coworkers at the University of Florida have created new iron cluster compounds, including a record-length Fe₁₈ molecular chain with an unusual structure that resembles a double-headed serpent with open jaws (Chem. Commun. 2007, 3359). Transition-metal clusters???in particular those with an iron-oxygen core???are of interest for their magnetic properties and as models for biomolecules such as the iron-storage protein ferritin. Alkoxide-based ligands such as tripodal alcohols typically have been used to connect iron atoms to make these clusters. Christou's group decided to try creating new cluster types by using an ethylenediamine ligand with hydroxyethyl arms, HO(CH₂)₂NH(CH₂)₂NH(CH₂)₂OH. One result is the Fe₁₈ cluster in which the oxygen atoms of the ligands bridge different pairs of iron atoms (Fe-O core shown) and the nitrogen atoms bind to a single iron atom. The molecule surpasses a Cr₁₂Ni₃ compound as the longest chainlike metal cluster yet discovered.