Octamer of iridium complex cations

Many of you are familiar with “sandwich” compounds in which a metal atom is complexed between two five- to eight-membered aromatic rings. Early examples are ferrocene [Fe(cp)₂; cp is cyclopentadienyl] and bis(benzene)chromium.

As many other sandwich compounds were being synthesized, chemists began to make “half-sandwich” or “piano-stool” compounds. The latter term arose because the molecular structure resembles a stool in which an arene is the seat and the opposite ligands are the legs. One useful compound in this category is methylcyclopentadienylmanganese tricarbonyl, which has been used as a replacement for tetraethyllead to increase the octane rating of gasoline.

In the past 20 years or so, researchers have been synthesizing piano-stool complexes in which the “legs” are amino acids. In addition to having properties similar to those of other half-sandwiches, the amino acid ligands in these species have a propensity to bridge to other complexes and oligomerize into polynuclear rings. In particular, a piano-stool complex that consists of a metal with cyclopentadienide, amino acid, and chloride ligands, when treated with a poorly coordinating anion such as tetrafluoroborate (BF₄^–) to displace the chloride, forms a trinuclear compound.

At least, that’s what chemists thought until Joseph S. Merola and colleague David Morris at Virginia Tech (Blacksburg) began to experiment with different metals, amino acids, and anions.

When the researchers chose iridium as the metal, pentamethylcyclopentadienide (Cp*) as the arene, and glycine as the amino acid [formula Cp*Ir(glycinato)Cl] and displaced chloride with BF₄^–, the product was the normal three-unit complex. But when they switched BF₄^– to hexafluorophosphate (PF₆^–) as the displacing anion, an unprecedented octametallic structure formed (see figures). The authors explain that the way that PF₆^– is encapsulated within the nascent complex is responsible for their unexpected finding.

Image credits: 3-D structure, Joseph S. Merola; octopus drawing, Christine DuChane.