How does the reactivity of metallofullerenes correlate with the size of the cage? 

Luis Echegoyen and co-workers at Clemson University, US and Luna Innovations Inc., Danville, Virginia have uncovered the reactivity of the higher trimetallic nitride endohedral metallofullerene cages (TNT EMF). 

Endohedral metallo fullerenes (EMF) are fullerenes that have metal atoms enclosed within their inner spheres. The US scientists explain that as the fullerene size gets larger, the HOMO-LUMO gap decreases and the reactivity is expected to increase, but instead it was observed that the reactivity decreases. This is most likely due to less strain in the bigger fullerene systems. 

In order to demonstrate how the size of fullerene cages affect the reactivity of the cyclopropanation reaction with bromomalonate also known as the Bingel reaction, Echegoyen compared the reactivity differences of Gd3N@C80, Gd3N@C84 and Gd3N@C88. They found that Gd3N@C80 gives rise to a mono- and bis-malonate adduct with apparent regioselectivity. Gd3N@C84 gives solely a mono-malonate adduct, while Gd3N@C88 which has the smallest HOMO-LUMO gap, is completely unreactive. 'Up to now, there have been no reports of the reactivity of the higher TNT EMF cages (larger than C80)', says Echegoyen. 

Echegoyen and colleagues believe that new types of MRI contrast agents based on these TNT EMFs will soon be available. The US team is currently working on placing different functional groups (beyond the diethyl malonates described in this work) on the fullerene cages which will impart the desired water solubility required for MRI contrast agents. 

'Challenges in this area include the lack of characterisation techniques available due to the paramagnetic nature of the enclosed Gd3N cluster,' explains Echegoyen. For this current work, characterisation was done via MALDI-TOF MS, UV-Vis spectroscopy and electrochemistry. 

Kathleen Too