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Chemists have produced millions of organic and organometallic compounds, but only recently have they produced one of the most aesthetically pleasing theoretical molecules after decades of trying - hexaferrocenylbenzene. Peter Vollhardt of the University of California at Berkeley and colleagues have finally succeeded in synthesising this super-crowded organometallic complex. NMR spectroscopy revealed that despite the crowded nature of the molecule this dendritic structure has considerable flexibility and average symmetry at room temperature. The team's success means this previously elusive structure can be investigated for its electronic, magnetic, optical and catalytic properties. The first reported attempts on the hexaferrocenylbenzene structure were made in 1967. The molecule consists of a central benzene ring in which each of its six hydrogen atoms has been swapped for a ferrocenyl ligand, this group is composed of a central iron ion sandwiched between two cyclopentadienyl molecules. With such a crowded structure packed with aromatic rings and iron ions, it was always going to be a difficult synthesis. Now, Vollhardt and colleagues at the University of Southern Denmark, in Odense and New Mexico Highlands University, in Las Vegas, have used a ferrocenylation reaction (Negishi type) six times on hexabromo- or hexaiodo-benzene to produce this compound. They were building on an earlier five-fold synthesis that was reluctant to succumb to the sixth substitution. The final yield is a mere 4%, with 56% side product in the form of the penta version of the molecule. Nevertheless, after forty years of chemists trying, 4% is better than zero! In addition to its potentially interesting electromagnetic and optical properties, Vollhardt and his colleagues are keen to learn more about the nanoscopic properties of this supercrowded arene. Indeed, there are hints from theoretical chemists that it might function as a molecular gear, or be used as the starting material for constructing even more unwieldy molecules such as a cyclic hexa-decker ?Ferris-wheel? ferrocene. It might also be used as a precursor in the synthesis of another elusive molecule hexacyclopentacoronene, C36H12. X-ray diffraction confirmed the precise structure and physical testing showed it to be remarkably stable yet sensitive to air, especially in solution. Vollhardt and his team are now endeavouring to optimise the synthesis with a view to boosting that 4% yield. Related links: |
 Vollhardt solves chemical crowding  Hexaferrocenylbenzene - the chemical incrowd |
