La2@Cs(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 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 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.     

New Isolated‐Pentagon‐Rule and Skeletally Transformed Isomers of C100 Fullerene Identified by Structure Elucidation of their Chloro Derivatives

High‐temperature chlorination of C₁₀₀ fullerene followed by X‐ray structure determination of the chloro derivatives enabled the identification of three isomers of C₁₀₀ from the fullerene soot, specifically numbers 18, 425, and 417, which obey the isolated pentagon rule (IPR). Among them, isomers C₁‐C₁₀₀(425) and C₂‐C₁₀₀(18) afforded C₁‐C₁₀₀(425)Cl₂₂ and C₂‐C₁₀₀(18)Cl_28/30 compounds, respectively, which retain their IPR cage connectivities. In contrast, isomer C_2v‐C₁₀₀(417) gives C_s‐C₁₀₀(417)Cl₂₈ which undergoes a skeletal transformation by the loss of a C₂ fragment, resulting in the formation of a nonclassical (NC) C₁‐C₉₈(NC)Cl₂₆ with a heptagon in the carbon cage. Most probably, two nonclassical C₁‐C₁₀₀(NC)Cl_18/22 chloro derivatives originate from the IPR isomer C₁‐C₁₀₀(382), although both C₁‐C₁₀₀(344) and even nonclassical C₁‐C₁₀₀(NC) can be also considered as the starting isomers.     

Regioselective Cage Opening of La2@D2(10611)‐C72 with 5,6‐Diphenyl‐3‐(2‐pyridyl)‐1,2,4‐triazine

The thermal reaction of the endohedral metallofullerene La₂@D₂(10611)‐C₇₂, which contains two pentalene units at opposite ends of the cage, with 5,6‐diphenyl‐3‐(2‐pyridyl)‐1,2,4‐triazine proceeded selectively to afford only two bisfulleroid isomers. The molecular structure of one isomer was determined using single‐crystal X‐ray crystallography. The results suggest that the [4+2] cycloaddition was initiated in a highly regioselective manner at the C? C bond connecting two pentagon rings of C₇₂. Subsequent intramolecular electrocyclization followed by cycloreversion resulted in the formation of an open‐cage derivative having three seven‐membered ring orifices on the cage and a significantly elongated cage geometry. The reduction potentials of the open‐cage derivatives were similar to those of La₂@D₂‐C₇₂ whereas the oxidation potentials were shifted more negative than those of La₂@D₂‐C₇₂. These results point out that further oxidation could occur easily in the derivatives.     

New Insights into the Hexaphenylethane Riddle: Formation of an α,o‐Dimer

Upon reduction of a 1H‐cyclobuta[de]naphthalene‐4,5‐diylbis(diarylmethylium) species, a new C? C bond is formed between the C_α and C_ortho atoms of the two chromophores, which presents an unprecedented coupling pattern for the dimerization of two trityl units. By attaching an annulated cyclobutane ring at the opposite peri position of the naphthalene core, the distance between the C_α carbon atoms was elongated beyond the limit of σ‐bond formation through “scissor effects”. The suppression of C_α? C_α bond formation, which would lead to hexaphenylethane‐type compounds, is key to the first successful isolation of the α,o‐adducts. The 5‐diarylmethylene‐6‐triarylmethyl‐1,3‐cyclohexadiene unit in the α,o‐adducts is stable, and isomerization of the cyclohexadiene unit into an aromatic system was not observed. The newly formed C_α? C_ortho bond was cleaved upon two‐electron oxidation to regenerate the dicationic dye.     

Stable Non‐IPR C60 and C70 Fullerenes Containing a Uniform Distribution of Pyrenes and Adjacent Pentagons

The most abundant fullerenes, C₆₀ and C₇₀, and all the pure carbon fullerenes larger than C₇₀, follow the isolated‐pentagon rule (IPR). Non‐IPR fullerenes containing adjacent pentagons (APs) have been stabilized experimentally in cases where, according to Euler’s theorem, it is topologically impossible to isolate all the pentagons from each other. Surprisingly, recent experiments have shown that a few endohedral fullerenes, for which IPR structures are possible, are stabilized in non‐IPR cages. We show that, apart from strain, the physical property that governs the relative stabilities of fullerenes is the charge distribution in the cage. This charge distribution is controlled by the number and location of APs and pyrene motifs. We show that, when these motifs are uniformly distributed in the cage and well‐separated from one other, stabilization of non‐IPR endohedral and exohedral derivatives, as well as pure carbon fullerene anions and cations, is the rule, rather than the exception. This suggests that non‐IPR derivatives might be even more common than IPR ones.     

Synthesis and Structure of LaSc2N@Cs(hept)‐C80 with One Heptagon and Thirteen Pentagons

The synthesis and single‐crystal X‐ray structural characterization of the first endohedral metallofullerene to contain a heptagon in the carbon cage are reported. The carbon framework surrounding the planar LaSc₂N unit in LaSc₂N@C_s(hept)‐C₈₀ consists of one heptagon, 13 pentagons, and 28 hexagons. This cage is related to the most abundant I_h‐C₈₀ isomer by one Stone–Wales‐like, heptagon/pentagon to hexagon/hexagon realignment. DFT computations predict that LaSc₂N@C_s(hept)‐C₈₀ is more stable than LaSc₂N@D_5h‐C₈₀, and suggests that the low yield of the heptagon‐containing endohedral fullerene may be caused by kinetic factors.     

Facile Separation and Crystal Structure Determination of C2‐C82(3) Fullerene

A report of the facile separation of C₂‐C₈₂(3) fullerene is given, which has resulted in the isolation of isomerically pure C₈₂(3) on a preparative scale, avoiding time consuming recycling procedures. By single‐crystal X‐ray analysis of C₈₂(3)·Ni(OEP) (Nickel octaethylporphyrine) the first crystal structure containing pristine C₈₂(3) has been revealed.     

Synthesis and Investigation of 1,3‐Bis(5‐nitraminotetrazol‐1‐yl)propan‐2‐ol and its Salts

1,3‐Bis(5‐nitraminotetrazol‐1‐yl)propan‐2‐ol ( 5 ) was prepared by the reaction of 5‐aminotetrazole and 1,3‐dichloroisopropanol under basic conditions. Obtained 1,3‐bis(5‐aminotetrazol‐1‐yl)propan‐2‐ol ( 3 ) was nitrated with 100 % nitric acid. In this context in situ hydrolysis of the nitrate ester was studied. Metal and nitrogen‐rich salts of the neutral compound 5 were prepared and analyzed. Crystal structures of three salts and the sensitivities toward impact, friction and electrostatic discharge were determined as well. The performance values of the compounds were calculated using the EXPLO5 program. A detailed comparison of the different salts is also enclosed.     

Supramolecular [60]Fullerene Liquid Crystals Formed By Self‐Organized Two‐Dimensional Crystals

Fullerene‐based liquid crystalline materials have both the excellent optical and electrical properties of fullerene and the self‐organization and external‐field‐responsive properties of liquid crystals (LCs). Herein, we demonstrate a new family of thermotropic [60]fullerene supramolecular LCs with hierarchical structures. The [60]fullerene dyads undergo self‐organization driven by π–π interactions to form triple‐layer two‐dimensional (2D) fullerene crystals sandwiched between layers of alkyl chains. The lamellar packing of 2D crystals gives rise to the formation of supramolecular LCs. This design strategy should be applicable to other molecules and lead to an enlarged family of 2D crystals and supramolecular liquid crystals.