title: La2@C(s)(17,490)-C76: a new non-IPR dimetallic metallofullerene featuring unexpectedly weak metal-pentalene interactions.
abstract: 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 La2@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 La(3+) 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.
authors.affiliation: Foundation for Advancement of International Science (FAIS), Tsukuba, Ibaraki 305-0821 (Japan); Department of Chemistry, Tokyo Gakugei University, Koganei, Tokyo 184-8501 (Japan); Life Science Center of Tsukuba Advanced Research Alliance (TARA), University of Tsukuba, Tsukuba, Ibaraki 305-8577 (Japan).
authors.forename: Mitsuaki
authors.initials: M
authors.lastname: Suzuki
authors.forename: Naomi
authors.initials: N
authors.lastname: Mizorogi
authors.forename: Tao
authors.initials: T
authors.lastname: Yang
authors.forename: Filip
authors.initials: F
authors.lastname: Uhlik
authors.forename: Zdenek
authors.initials: Z
authors.lastname: Slanina
authors.forename: Xiang
authors.initials: X
authors.lastname: Zhao
authors.forename: Michio
authors.initials: M
authors.lastname: Yamada
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Maeda
authors.forename: Tadashi
authors.initials: T
authors.lastname: Hasegawa
authors.forename: Shigeru
authors.initials: S
authors.lastname: Nagase
authors.forename: Xing
authors.initials: X
authors.lastname: Lu
authors.forename: Takeshi
authors.initials: T
authors.lastname: Akasaka
keywords: X-ray diffraction; cocrystallization; density functional calculations; isolated pentagon rule; metallofullerenes