Two I_h-symmetry-breaking C₆₀ isomers stabilized by chlorination

One abiding surprise in fullerene science is that I_h-symmetric buckminsterfullerene C₆₀ (ref. 1) (I_h-C₆₀ or ^#1,812C₆₀, the nomenclature specified by symmetry or by Fowler's spiral algorithm²) remains the sole C₆₀ species experimentally available. Setting it apart from the other 1,811 topological isomers (isobuckminsterfullerenes) is its exclusive conformity with the isolated-pentagon rule³, which states that stable fullerenes have isolated pentagons. Although gas-phase existence of isobuckminsterfullerenes has long been suspected^4-7, synthetic efforts have yet to yield successful results. Here, we report the realization of two isobuckminsterfullerenes by means of chlorination of the respective C _2v- and C_s-symmetric C₆₀ cages. These chlorinated species, ^#1,809C₆₀Cl₈(1) and ^#1,804C₆₀Cl₁₂(2), were isolated in experimentally useful yields. Structural characterization by crystallography unambiguously established the unique pentagon-pentagon ring fusions. These distinct structural features are directly responsible for the regioselectivity observed in subsequent substitution of chlorines, and also render these unprecedented derivatives of C₆₀ isomers important for resolving the long-standing puzzle of fullerene formation by the Stone-Wales transformation scheme^8-11.