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US researchers are using ultraviolet spectroscopy and other techniques to answer the question - how does the reactivity of metallofullerenes correlate with the size of the cage? Their work could lead to the further development of these materials as more effective contrast agents for magnetic resonance imaging (MRI). Buckyballs, or to give them their more formal name, fullerenes, sparked a huge amount of interest in their chemistry during the 1990s. They promised some wildly unrealistic applications and even a mention in the UK government's House of Lords, but technologies have been slow to emerge. Since then, their elongated cousins, the carbon nanotubes stole some of their limelight and are on the verge of producing real applications in molecular electronics and materials science. However, chemists have not given up on the soccerball-shaped molecules and have continued to investigate their chemistry, add functional groups and slip metals into these carbon cages. Most recently, Luis Echegoyen and colleagues at Clemson University in South Carolina and Luna Innovations Inc in Danville, Virginia, have uncovered the reactivity of a specific and technologically relevant class of fullerene, the higher trimetallic nitride endohedral metallofullerene cages (TNT EMF). Endohedral metallofullerenes, have metal atoms within the carbon cage as opposed to chemical cousins in which metal atoms are bonded to the outer surface. The key to reactivity of these endohedral materials, the US scientists have found, apparently lies in the narrowing of the gap between the material's molecular orbitals for bigger fullerene cages. For a larger cage, the gap between the highest occupied molecular orbital (the HOMO) and the lowest unoccupied molecular orbital (the LUMO) is smaller than it is for a smaller cage. A smaller HOMO-LUMO gap would normally mean less energy being required to shuffle bonding electrons and so would imply an increase in reactivity. However, this is not the case with the EMFs. The researchers explain this contradictory phenomenon by looking at the formation of pyramidalised bonds in the fullerene shells. The greater the degree of pyramidalisation, among the reacting carbon atoms, the more reactive they become. Reactivity of these materials is key to adding useful functionality to their surface. Fullerenes containing more than 80 carbon atoms can trap three gadolinium atoms, together with an attendant nitrogen atom, within their cage to form Gd3N@C80, Gd3N@C84 and Gd3N@C88, to make gadolinium nitride cluster EMFs. The @ sign highlights that the metal nitride cluster is within the fullerene cage rather than bonded to the surface. Interest in these materials emerged because they have great potential as strong contrast agents for MRI, that improve on conventional gadolinium complexes. As such, researchers are investigating what functional chemical groups might be added to give them particular properties. The present team used high-performance liquid chromatography (HPLC), mass spectrometry, UV-Vis-NIR spectroscopy, and cyclic voltammetry in order to isolate and characterise the resulting functionalised materials. The highly paramagnetic nature of the cluster within the fullerenes, of course, precluding investigation by NMR spectroscopy. The addition of particular functional groups might alter physical properties, such as water solubility, which is important for a medical agent as it could allow it to be taken by mouth, rather than having to be administered through intravenous injection. However, understanding their reactivity as it relates to cage size, whether C80, C84, or C88 could speed up the development of such agents by allowing chemists to target individual cages that will respond best to a given reagent rather than focusing on unreactive systems. Echegoyen and colleagues have taken the cyclopropanation reaction with bromomalonate, the so-called Bingel reaction, as a prototypical process and compared how well each fullerene cage fares in this reaction. "Up to now, there have been no reports of the reactivity of the higher TNT EMF cages (larger than C80)", says Echegoyen. The team found that Gd3N@C80 gives rise to a mono- and a bis-malonate adduct with apparent regioselectivity. Gd3N@C84 produces only a mono-malonate adduct, while Gd3N@C88 which has the smallest HOMO-LUMO gap, is wholly unreactive. The US team is now working on adding different functional groups, not just the diethyl malonates, on to the surface of fullerene cages to endow the compounds with water solubility. Related links:
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