La₂@C_s(17 490)-C₇₆: A new non-IPR dimetallic metallofullerene featuring unexpectedly weak metal-pentalene interactions

Although all the pure-carbon fullerene isomers above C₆₀ reported to date comply with the isolated pentagon rule (IPR), non-IPR structures, which are expected to have different properties from those of IPR species, are obtainable either by exohedral modification or by endohedral atom doping. This report describes the isolation and characterization of a new endohedral metallofullerene (EMF), La₂@C₇₆, which has a non-IPR fullerene cage. The X-ray crystallographic result for the La ₂@C₇₆/[Ni^II(OEP)] (OEP=octaethylporphyrin) cocrystal unambiguously elucidated the C_s(17 490)-C₇₆ cage structure, which contains two adjacent pentagon pairs. Surprisingly, multiple metal sites were distinguished from the X-ray data, which implies dynamic behavior for the two La³⁺ cations inside the cage. This dynamic behavior was also corroborated by variable-temperature ¹³⁹ La NMR spectroscopy. This phenomenon conflicts with the widely accepted idea that the metal cations in non-IPR EMFs invariably coordinate strongly with the negatively charged fused-pentagon carbons, thereby providing new insights into modern coordination chemistry. Furthermore, our electrochemical and computational studies reveal that La₂@C_s(17 490)-C₇₆ has a larger HOMO-LUMO gap than other dilanthanum-EMFs with IPR cage structures, such as La₂@D_3h(5)-C₇₈ and La₂@I _h(7)-C₈₀, which implies that IPR is no longer a strict rule for EMFs. Breaking the dogma again! Single-crystal X-ray structure analysis of a newly isolated metallofullerene, La₂@C_s(17 490)-C₇₆, revealed a cage structure that violates the isolated-pentagon rule (IPR; see figure). More surprisingly, dynamic behavior of the two La³⁺ ions is observed inside the cage, which conflicts with the common belief that metal cations invariably coordinate strongly with nearby negatively charged fused-pentagon carbons in non-IPR EMFs.