Fullerenes: Breaking the rules

Two C₆₀ isomers that do not obey the isolated pentagon rule have been isolated.

Buckminsterfullerene – a highly symmetric C₆₀ isomer with a soccer-ball-like structure – was previously thought to be the only experimentally isolable C₆₀ isomer. This fact is all the more surprising given that theoretically there are a total of 1812 possible isomers. Importantly, the structure of buckminsterfullerene is the only isomer of C₆₀ that obeys the widely accepted isolated pentagon rule (IPR). With the various isomers built up of 5- and 6-membered rings of carbon, this rule states that no stable isomer will have fused 5-membered rings.

Now, Su-Yuan Xie, Lan-Sun Zheng and co-workers¹ from Xiamen University in China have shown that chlorination allows the isolation of two isomers of C₆₀ that do not obey the IPR. “We first observed mass spectrometric signals of one of these chlorinated compounds as early as 2001,” says Xie. “At that time however, we naturally assumed that they were due to chlorinated fullerenes that obeyed the IPR.”

Fused pentagons were observed twice in one compound, and three times in another. Both the new structures resembled a slightly-flattened soccer ball (Fig. 1). One (C₆₀Cl₈) retained the mirror symmetry exhibited by buckminsterfullerene itself while the other structure (C₆₀Cl₁₂) no longer had mirror symmetry and was chiral.

Fig. 1: Buckminsterfullerene (top) and two polychlorinated C₆₀ isomers that break the isolated pentagon rule C₆₀Cl₈ (left) and C₆₀Cl₁₂ (right). The fused 5-membered rings are highlighted in yellow.

Xie and colleagues went on to investigate the reactivity of these two new structures, and have shown that they undergo site-selective Friedel-Crafts reactions—each molecule of C₆₀Cl₈ reacts with no more than four molecules of benzene in the presence of iron(III) chloride —which could lead to multifunctional materials built from heterosubstituted fullerenes.

Non-IPR fullerenes may form in the gas phase synthesis of fullerenes, but that they quickly isomerize forming, in the case of C₆₀, solely buckminsterfullerene. Ultimately, it comes down to a question of thermodynamic stability. In the case of the chlorinated fullerenes, C₆₀Cl₈ and C₆₀Cl₁₂ the non-IPR structures are thermodynamically more favorable. By studying the high temperature dechlorination of these new materials in an inert atmosphere, this group have shown that non-IPR C₆₀ structures do exist in the gas phase. “We now hope to go on and investigate the electronic, mechanistic and magnetic properties of these non –IPR fullerenes,” says Xie.