Non-classical (NC), heptagon-containing fullerenes obtained via chlorination-promoted cage transformations: C76(NC2a)Cl24 and C76(NC2b)Cl28.

High-temperature chlorination of C76 fullerene with SbCl5 proceeds via five Stone-Wales rearrangements, resulting in non-classical (NC) C76(NC1a)Cl24 with two heptagons and 14 pentagons partically fused in pairs and triples. C76(NC2b)Cl28 with isomeric carbon cage was obtained by chlorination-promoted cage shrinkage of C80via two C2 losses. The pathways of skeletal cage trasformations, the chlorination patterns, and formation energies are discussed in detail.

Chlorination-Promoted Skeletal Transformations in Isolated-Pentagon Rule (IPR) Isomers of Fullerene C86 to Non-IPR Chloro- and Trifluoromethyl Derivatives.

Fullerene C86 contains two isomers obeying the Isolated-Pentagon Rule (IPR), CS-C86(16) and C2-C86(17). Both isomers undergo unprecedented skeletal transformations at high-temperature (400 °C) chlorination with SbCl5. One-step Stone-Wales rearrangement (SWR) in C86(17) results in the pentagon-fused #63614C86 cage found in the structure of #63614C86Cl24. CF3 derivatives with the same cage, two isomers of #63614C86(CF3)18 and #63614C86(CF3)18O2, were obtained by high-temperature trifluoromethylation of the chlorination products with CF3I, followed by HPLC separation. The skeletal transformation of C86(16) proceeds via two SWRs under the formation of a #63624C86 cage with one fused-pentagon pair found in the structure of #63624C86(CF3)18. The addition patterns in skeletally transformed molecules are discussed in detail, disclosing the influence of the pentagon fusions, isolated C=C bonds, and benzenoid rings on the stability of the molecules with non-IPR C86 cages. The chlorination-promoted SWRs in C86 isomers have been observed for the first time, which contribute a lot to the understanding of skeletal transformations in fullerenes.

Dimeric and 1D polymeric low-chlorinated C60 fullerenes, (C60Cl5)2 and (C60Cl4)∞.

Low-chlorinated fullerenes, dimeric (C60Cl5)2 and one-dimensional, polymeric (C60Cl4)∞, were obtained by high-temperature (270 °C) chlorination of C60 with a SbCl5/SbCl3 mixture, as revealed by X-ray crystallography. The compounds were characterized by IR and Raman spectroscopy and theoretical calculations. This is the first observation of a fullerene polymer with single C-C bonding and neutral building blocks.

Structural Chemistry of Pentagon-Fused C82 Fullerene Derivatives #39173C82(CF3)14,16,18 and #39173C82Cl28.

High-temperature chlorination of the most stable Isolated-Pentagon-Rule (IPR) isomer of fullerene C82, C2-C82(3), invariably produces non-IPR #39173C82Cl28, containing one pentagon-pentagon fusion in the carbon cage. High-temperature trifluoromethylation of #39173C82Cl28 followed by HPLC separation resulted in the isolation and structure elucidation of eight #39173C82(CF3)n (n = 14, 16, 18) compounds. Structural chemistry of #39173C82(CF3)14,16,18 and #39173C82Cl28 is characterized by the variation of the addition patterns in the region of a pentagon-pentagon fusion. The regiochemistry of CF3 addition in the remaining cage region is similar to that of the known IPR C82(3)(CF3)n compounds. Theoretical calculations revealed that #39173C82(CF3)n possess lower thermodynamic stability than isomeric IPR derivatives.

Fused-Pentagon Carbon Cages in Chloro and Trifluoromethyl Derivatives of C60: Non-IPR 1809C60Cl8, 1806C60(CF3)14, and Nonclassical C60(NC)Cl14.

High-temperature chlorination of conventional IPR C60 can produce chloro derivatives of non-IPR C60 by skeletal transformations via Stone-Wales rearrangements (SWRs) of the carbon cage. We report the synthesis and structure elucidation of non-IPR 1809C60Cl8 and nonclassical C60(NC)Cl14. The present isolation of 1809C60Cl8 hints at the possibility that the same product in the previously reported chlorine-doped arc-discharge synthesis could have, likewise, resulted from the initially formed IPR C60. C60(NC)Cl14 is the first chloride containing a nonclassical carbon cage with one heptagon and 13 pentagons known previously only in a CF3 derivative. Additionally, trifluoromethylation of non-IPR chlorides revealed the formation of 1806C60(CF3)14 with a new non-IPR carbon cage and unusual trifluoromethylation pattern. Thereby, the number of different, structurally confirmed non-IPR carbon cages of C60 now reaches eight.

Crippling the C70 fullerene: non-classical C68Cl26(OH)2 and C68Cl25(OH)3 with three heptagons and only fused pentagons via chlorination-promoted skeletal transformations.

High-temperature (440 °C) chlorination of C70 with SbCl5 promotes Stone-Wales transformations and loss of the C2 fragment, which results in a non-classical C68Cl28 partially hydrolyzed to C68Cl26(OH)2 and C68Cl25(OH)3. X-ray diffraction reveals an unprecedented C68 cage with three heptagons and 15 pentagons arranged in fused pairs and triples. The shortest possible transformation pathways include one C2 loss step and four Stone-Wales transformation steps.

Two Isolated-Pentagon-Rule Isomers C98(110) and C98(111) Isolated as Trifluoromethylfullerenes C98(CF3)22.

The fullerene C98 has 259 topologically possible isomers that obey the isolated-pentagon rule (IPR). In this work, a family of experimentally confirmed IPR isomers of C98 fullerene is extended by the high-performance liquid chromatography isolation and X-ray structural characterization of two trifluoromethyl derivatives, C1-C98(110)(CF3)22 and C1-C98(111)(CF3)22. The carbon cages of isomers Cs-C98(110) and Cs-C98(111) differ by a Stone-Wales rotation of only one C-C bond, which results in very similar addition patterns of 22 CF3 groups in the C98(CF3)22 molecules. The stabilizing substructures in both C98(CF3)22 molecules include six benzenoid rings and four isolated C═C bonds. Both Cs-C98(110) and Cs-C98(111) belong to the isomers of moderate relative stability among altogether seven IPR isomers of C98 fullerene with experimentally confirmed cage structures.

Trifluoromethyl derivatives of pentagon-fused C60: 1809C60(CF3)n (n = 10, 12, 14, 16).

The carbon cage of buckminsterfullerene Ih-C60, obeying the Isolated-Pentagon Rule (IPR), can be transformed to the non-IPR C2v-1809C60 cage by a single Stone-Wales rearrangement (SWR) in the course of high-temperature chlorination of C60 with SbCl5. The following high-temperature trifluoromethylation of the chlorination products with CF3I afforded non-IPR CF3 derivatives, 1809C60(CF3)n. X-ray diffraction studies of 1809C60(CF3)n (n = 10, 12, 14, 16) revealed that the sites of pentagon-pentagon fusions on the carbon cage are preferentially occupied by CF3 groups. The addition patterns of 1809C60(CF3)n and related 1809C60Cln are compared, demonstrating a prevailing role of pentagon-pentagon fusions in the stability and structural chemistry of these compounds. Further SWR skeletal transformations of 1809C60 are discussed and compared with the experimental data available.

Chloro- and Trifluoromethyl Derivatives of a Pentagon-Fused C60: 1810C60Cl24, 1810C60Cl20, and 1810C60(CF3)14.

The carbon cage of Ih-C60, obeying the isolated-pentagon rule (IPR), can be transformed to the non-IPR D2h-1810C60 cage via two successive Stone-Wales rearrangements in the course of high-temperature chlorination of C60 with SbCl5. Two chloro derivatives, C2v-1810C60Cl24 and C2v-1810C60Cl20, have been isolated by high-performance liquid chromatography (HPLC). High-temperature trifluoromethylation of the chlorination products with CF3I, followed by HPLC separation, afforded a non-IPR CF3 derivative, Cs-1810C60(CF3)14. Structural elucidation of the isolated compounds revealed that all eight sites of pentagon-pentagon fusions on the carbon cage are preferentially occupied by Cl atoms or CF3 groups. According to density functional theory calculations, chloro and CF3 derivatives of 1810C60 are more stable than the isomeric derivatives of 1809C60 or IPR 1812C60, possessing respectively four or no sites of pentagon fusion in their carbon cages.

Carbon Origami via an Alumina-Assisted Cyclodehydrofluorination Strategy.

The synthesis of pristine non-planar nanographenes (NGs) via a cyclodehydrofluorination strategy is reported and the creation of highly strained systems via alumina-assisted C-F bond activation is shown. Steric hindrance could execute an alternative coupling program leading to rare octagon formation offering access to elusive non-classical NGs. The combination of two alternative ways of folding could lead to the formation of various 3D NG objects, resembling the Japanese art of origami. The power of the presented "origami" approach is proved by the assembly of 12 challenging nanographenes that are π-isoelectronic to planar hexabenzocoronene but forced out of planarity.

Structural Studies of Giant Empty and Endohedral Fullerenes.

Structure elucidations of giant fullerenes composed of 100 or more carbon atoms are severely hampered by their extremely low yield, poor solubility and huge numbers of possible cage isomers. High-temperature exohedral chlorination followed by X-ray single crystal diffraction studies of the chloro derivatives offers a practical solution for structure elucidations of giant fullerenes. Various isomers of giant fullerenes have been determined by this method, specially, non-classical giant fullerenes containing heptagons generated by the skeletal transformations of carbon cages. Alternatively, giant fullerenes can be also stabilized by encapsulating metal atoms or clusters through intramolecular electron transfer from the encapsulated species to the outer fullerene cage. In this review, we present a comprehensive overview on synthesis, separation and structural elucidation of giant fullerenes. The isomer structures, chlorination patterns of a series of giant fullerenes C2n (2n = 100-108) and heptagon-containing non-classical fullerenes derived from giant fullerenes are summarized. On the other hand, giant endohedral fullerenes bearing different endohedral species are also discussed. At the end, we propose an outlook on the future development of giant fullerenes.

Three Isolated-Pentagon-Rule Isomers of C96 Fullerene Isolated as Trifluoromethyl Derivatives.

The family of experimentally confirmed isolated-pentagon-rule (IPR) isomers of C96 fullerene is extended by trifluoromethylation of a C96 fraction of the fullerene soot, high-performance liquid chromatography separation of CF3 derivatives, and a single-crystal X-ray diffraction study of C96(CF3)n compounds with the use of synchrotron radiation. New cage isomers were revealed in C96(94)(CF3)18/20 and C96(182)(CF3)18 compounds, whereas isomer C96(181), previously known in the adduct with nickel porphyrinate, was confirmed in C96(181)(CF3)18/20 derivatives. Common and special features of the addition patterns of CF3 groups on C96 carbon cages are discussed in more detail. The investigated isomers belong to the most stable C2-C96(181) and slightly less stable C1-C96(94) and C2-C96(182) among the altogether 15 experimentally confirmed IPR isomers of C96 fullerene.

Fused-Pentagon C70Cl6 and C70Cl8 Obtained via Chlorination-Promoted Skeletal Transformation of IPR C70.

The isolated-pentagon-rule (IPR) D5h-C70 fullerene is least susceptible to skeletal transformations in comparison with higher fullerenes and even C60. A cage transformation in IPR C70 via a one-step Stone-Wales rearrangement was accomplished by high-temperature (440 °C) ampule chlorination with SbCl5. Subsequent dechlorination at 450 °C, followed by high-performance liquid chromatography separation, allowed the isolation of non-IPR C70Cl6 and C70Cl8. X-ray diffraction study revealed the presence of an unprecedented C70 carbon cage, possessing two pairs of fused pentagons and the chlorination patterns located on one cage hemisphere. A high energetic and thermal stability of both non-IPR chlorides was also confirmed by theoretical calculations of formation energies. Pathways of skeletal transformations of IPR C70 in comparison with those in C60 are discussed.

Regioselective Mono- and Dialkylation of [6,6]-open C60 (CF2 ): Synthetic and Kinetic Aspects.

Alkylation of homofullerene [6,6]-C60 (CF2 )2- dianion with the set of alkyl halides, RX, was established to demonstrate an effect of RX nature on the conversion, product composition, and regioselectivity. The respective C60 (CF2 )RH, C60 (CF2 )R2 and C60 (CF2 )R'R'' compounds were obtained in the reaction with sterically unhindered RX, isolated by HPLC and unequivocally characterized. The kinetic studies evidenced SN 2 mechanism for both alkylation steps, yielding mono- and dialkylated C60 (CF2 ), respectively, and indicated the negative charge localization at the bridgehead carbon atoms as well as a steric hindrance of the CF2 moiety likely to be a key factors for the SN 2 reaction mechanism and observed regioselectivity. The significant difference in the rate constants of the first and the second steps is attributed to the different activation barriers predicted by DFT calculations which makes possible to develop synthetic methods for the regioselective preparation of monoalkylated C60 (CF2 )RH and heterodialkylated C60 (CF2 )R'R'' derivatives.

Fused-Pentagon Isomers of C60 Fullerene Isolated as Chloro and Trifluoromethyl Derivatives.

The carbon cage of buckminsterfullerene Ih -C60 , which obeys the Isolated-Pentagon Rule (IPR), can be transformed to non-IPR cages in the course of high-temperature chlorination of C60 or C60 Cl30 with SbCl5 . The non-IPR chloro derivatives were isolated chromatographically (HPLC) and characterized crystallographically as 1809 C60 Cl16 , 1810 C60 Cl24 , and 1805 C60 Cl24 , which contain, respectively two, four, and four pairs of fused pentagons in the carbon cage. High-temperature trifluoromethylation of the chlorination products with CF3 I afforded a non-IPR CF3 derivative, 1807 C60 (CF3 )12 , which contains four pairs of fused pentagons in the carbon cage. Addition patterns of non-IPR chloro and CF3 derivatives were compared and discussed in terms of the formation of stabilizing local substructures on fullerene cages. A detailed scheme of the experimentally confirmed non-IPR C60 isomers obtained by Stone-Wales cage transformations is presented.

Trifluoromethyl Derivatives of Elusive Fullerene C98.

Data concerning the isomeric composition of C98 and the chemistry of C98 derivatives are scarce due to very low abundance of C98 in the fullerene soot. Trifluoromethylation of C98 -containing mixtures followed by HPLC separation of CF3 derivatives and single crystal X-ray diffraction study resulted in structural characterization of four compounds C98 (248)(CF3 )18/20 , C98 (116)(CF3 )18 , and C98 (120)(CF3 )20 . To date, these compounds represent the largest fullerenes isolated as CF3 derivatives with experimentally determined molecular structures. The addition patterns of C98 (CF3 )18/20 are discussed in detail revealing the stabilizing factors, such as isolated double C=C bonds and benzenoid rings on C98 fullerene cages. A detailed comparison with the addition patterns of the known C98 Cln allowed us to contribute to the better understanding the chemistry of elusive C98 fullerene.

Fused-pentagon C70Cl26 obtained via chlorination-promoted Stone-Wales cage transformations of C70.

High-temperature (360 °C) chlorination of C70 with VCl4 or SbCl5 yields only IPR C70Cl26/28. Chlorination with SbCl5 at 440 °C resulted in a skeletal transformation via a two-step Stone-Wales rearrangement and the formation of non-IPR 8005C70Cl26 with two fused pentagon pairs in the carbon cage which was established by single crystal X-ray diffraction.

Structures of Gd3N@C80 Prato Bis-Adducts: Crystal Structure, Thermal Isomerization, and Computational Study.

The structures of two bis-ethylpyrrolidinoadducts of Gd3N@Ih-C80, obtained by regioselective 1,3-dipolar cycloadditions, were elucidated by single crystal X-ray, visible-near infrared (vis-NIR) spectra, studies on their thermal isomerization, and theoretical calculations. The structure of the minor-bis-adduct reveals a C2-symmetric carbon cage with [6,6][6,6]-addition sites and with an endohedral Gd3N cluster that is completely flattened. This is the first example of a crystal structure of Gd3N@Ih-C80 derivatives. The structure of the major-bis-adduct was inferred by the vis-NIR spectrum being corresponded to the structure of a previously reported major-bis-adduct of Y3N@Ih-C80 known to have an asymmetric [6,6][6,6]-structure. Based on experimental results showing that the minor-bis-adduct of Gd3N@Ih-C80 isomerized to the major-adduct, a possible second addition site was elucidated with support from density functional theory calculations.

Diversion of the Arbuzov reaction: alkylation of C-Cl instead of phosphonic ester formation on the fullerene cage.

We report an "inversed" Arbuzov reaction of the fullerene derivatives C60Ar5Cl with trialkyl phosphites P(OR)3 producing alkylated fullerene derivatives C60Ar5R (R = Me, Et, iPr, nBu) with almost quantitative yields. This reaction provides a convenient synthetic route for the preparation of a large variety of functionalized fullerene derivatives with tailored properties, e.g. water-soluble compounds demonstrating promising antiviral activities against HCMV, HSV1, HIV and several influenza virus strains.

Chlorination-Promoted Skeletal Transformations of Fullerenes.

Classical fullerenes are built of pentagonal and hexagonal rings, and the conventional syntheses produce only those isomers that obey the isolated-pentagon rule (IPR), where all pentagonal rings are separated from each other by hexagonal rings. Upon exohedral derivatization, the IPR fullerene cages normally retain their connectivity pattern. However, it has been discovered that high-temperature chlorination of fullerenes with SbCl5 or VCl4 can induce skeletal transformations that alter the carbon cage topology, as directly evidenced by single crystal X-ray diffraction studies of the chlorinated products of a series of fullerenes in the broad range of C60 to C102. Two general types of transformations have been identified: (i) the Stone-Wales rearrangement (SWR) that consists of a rotation of a C-C bond by 90°, and (ii) the removal of a C-C bond, i.e., C2 loss (C2L). Single- or multistep SWR and/or C2L transformations afford either classical non-IPR fullerenes bearing fused pentagons (highlighted in red in the TOC picture) or nonclassical (NCx) fullerenes with x = 1-3 heptagonal rings (highlighted in blue in the TOC picture) often flanked by fused pentagons. Several subtypes of the SWR and C2L processes can be further discerned depending on the local topology of the transformed region of the cage. Under the chlorination conditions, the non-IPR and NC carbon cages that would be energetically unfavorable and mostly labile in their pristine state are instantaneously stabilized by chlorination of the pentagon-pentagon junctions and by delimitation of the original spherical π-system into smaller favorable aromatic fragments. The significance of the chlorination-promoted skeletal transformations within the realm of fullerene chemistry is demonstrated by the growing body of examples. To date, these include single- and multistep SWRs in the buckminsterfullerene C60 and in the higher fullerenes C76(1), C78(2), C82(3), and C102(19), single and multistep C2Ls (i.e., cage shrinkage) in C86(16), C88(33), C90(28), C92(50), C96(80), C96(114), and C102(19), and multistep combinations of SWRs and C2Ls in C88(3), C88(33), and C100(18), (IPR isomer numbering in parentheses is according to the spiral algorithm). Remarkably, an IPR precursor can give rise to versatile transformed chlorinated fullerene cages formed via branched pathways. The products can be recovered either in their initial chlorinated form or as more soluble CF3/F derivatives obtained by an additional trifluoromethylation workup. Reconstruction of the skeletal transformation pathways is often complicated due to the lack of the isolable intermediate products in the multistep cases. Therefore, it is usually based on the principle of selecting the shortest pathways between the starting and the final cage. The quantum-chemical calculations illustrate the detailed mechanisms of the SWR and C2L transformations and the thermodynamic driving forces behind them. A particularly important aspect is the interplay between the chlorination patterns and the regiochemistry of the skeletal transformations.