non-IPR Fullerenes Research Articles

Electroactive Polymerization Behaviors of Fused-Pentagon Chlorofullerenes: #1809C60Cl8 and #271C50Cl10

The fullerenes that violate isolated pentagon rule (IPR) have unusual electronic properties resulting from their fused-pentagon structures. Numerous non-IPR fullerenes have now been captured by chlorination, affording opportunity to go insight into the properties involved in non-IPR fullerenes in the forms of chlorofullerenes (CFs). Here cyclic voltammetry (CV) is employed to probe the electrochemical properties of non-IPR #1809C60Cl8 in comparison with those of #271C50Cl10. Differing from IPR-satisfying CFs such as C60Cl8 and C60Cl10 (referring to I h-symmetric C60), the two non-IPR CFs exhibit divergent electroactive polymerization characters. In addition, the electrocatalytic effect of ferrocene that is otherwise employed as internal reference has been shown in the CV process of CFs.

Systematic investigation of the stability of all dihydro isomers of the more stable C38fullerene isomer with C2 symmetry

The local aromaticity of fullerene C[Formula: see text] ([Formula: see text] was studied using the bond resonance energy (BRE) method. The global aromaticity of all 349 possible isomers of C[Formula: see text]H2based on [Formula: see text] symmetry was investigated using the topological resonance energy (TRE) method. The TRE results show that most of the C[Formula: see text]H2isomers have greater stability than C[Formula: see text] ([Formula: see text]. Based on the BRE results, the preferred addition sites of hydrogen atoms are discussed. The relationship between the addition sites and BREs is analyzed and discussed. We found that the addition sites of hydrogen atoms are strongly dependent on the magnitude of the parent cage BRE values. The most stable isomers of C[Formula: see text]H2are often produced by diminishing of the [Formula: see text]-bonds from those sites in the parent cage at which are located the two carbon atoms with the lowest BRE values. Based on this rule, the preferred addition patterns for non-IPR fullerene cages can be easily predicted.

(Invited) New Vistas on Chemistry of Endohedral Metallofullerenes

The isolated pentagon rule (IPR) states that stable fullerenes have each of their 12 pentagons surrounded by five hexagons. On the contrary, in 2000, that idea was altered by the discovery of the first non-IPR fullerenes, Sc2@C66 and Sc3N@C68. Themolecular structure of Sc2@C66 was assigned to possess a C2v(4348)-C66 cage that contained two sets of double fused pentagons by using 13C NMR spectroscopy and powder X-ray diffraction data analysis. However, that structure has remained a long-standing controversy, since it is thermodynamically unfavorable. Here, we demonstrate definitively that Sc2@C66 does not have the structure that was long believed to be but a brand new type. 2D-NMR spectroscopic and single-crystal X-ray analysis disclose that Sc2@C66 has a C2v(4059)-C66 cage containing two sets of unsaturated linear triquinanes (ULTs), in which three pentagons abut one another and two scandium ions are located within the folds of each of the ULT units. Our identication of Sc2@C66 as Sc2@C2v(4059)-C66 shows that new motifs in carbon cage geometry continue to be discovered and calls attention to the ULT unit, whose aromaticity and coordination chemistry deserve further study.

Functionalization of pentagon–pentagon edges of fullerenes by cyclic polysulfides: A DFT study

We have performed a computational study to investigate the cyclosulfurization of the pentagon–pentagon (p–p) junctions in the non-IPR fullerenes C60(D3) and C70(C2v), and also Stone-Wales defective C60 fullerene. Our results indicate the exothermic character of cyclosulfurization processes which can be related to the increase of pyramidalization angle (spherical excesses) and p characters of natural hybrid orbitals of C atoms at the p–p junctions. In fact these lead to the structural strain relief and stability of the cyclosulfurization derivatives of the non-IPR fullerenes. Moreover, the cyclosulfurization reaction of p–p bonds on the C70(C2v) is more energetically favorable than that of C60(D3), due to the higher curvature of carbon sites and the larger values of the p characters of natural hybrid orbitals in the C70(C2v). On the other hand, localization of the excess electrons on the C atoms at the p–p junctions leads to the low tendency of the charged non-IPR fullerenes to cyclosulfurization process. The desulfurization pathway of the exohedral derivatives of C70(C2v) indicates that it is energetically unfavorable for the functionalized fullerenes to break into individual fullerene and sulfur molecules. HOMO–LUMO gaps almost are independent of the number of pentathiepin rings while sensitive to the type of parent fullerene.

Kinetic Stability of Non-IPR Fullerene Molecular Ions.

Many fullerenes that violate the isolated pentagon rule (IPR) form stable metallofullerenes. In general, a fullerene cage is kinetically stabilized by acquiring a given number of electrons. Kinetic stability of negatively charged non-IPR fullerenes, including the recently isolated endohedral metallofullerene with a heptagonal face, was rationalized in terms of bond resonance energy (BRE). Interestingly, molecular anions of conventional fullerenes found in most isolated metallofullerenes are kinetically stable with large positive BREs for all CC bonds. As we pointed out in 1993, the IPR does not apply to charged fullerenes because π-bonds shared by two five-membered rings are aromatized to varying extents.

Origin of kinetic instability of fullerenes that violate the isolated pentagon rule.

The isolated pentagon rule (IPR) holds without exceptions for neutral fullerene molecules. Unlike those in non-IPR fullerenes, 5/5 bonds (i.e., π-bonds shared by two pentagons) in many planar polycyclic π-systems are kinetically rather stable with large positive bond resonance energies (BREs), where BRE is a graph-theoretically defined index of kinetic stability. Geometric conditions were explored for designing planar polycyclic π-systems with unstable 5/5 bonds. We then found that the kinetic instability of non-IPR fullerenes stems from the coexistence of pentalene substructures and nearby disjoint pentagons. Proper arrangements of fused pentagons and disjoint pentagons make the 5/5 bonds highly reactive with large negative BREs.

Sm@C2v(19138)-C76: A Non-IPR Cage Stabilized by a Divalent Metal Ion.

Although a non-IPR fullerene cage is common for endohedral cluster fullerenes, it is very rare for conventional endofullerenes M@C2n, probably because of the minimum geometry fit effect of the endohedral single metal ion. In this work, we report on a new non-IPR endofullerene Sm@C2v(19138)-C76, including its structural and electrochemical features. A combined study of single-crystal X-ray diffraction and DFT calculations not only elucidates the non-IPR cage structure of C2v(19138)-C76 but also suggests that the endohedral Sm(2+) ion prefers to reside along the C2 cage axis and close to the fused pentagon unit in the cage framework, indicative of a significant metal-cage interaction, which alone can stabilize the non-IPR cage. Furthermore, electrochemical studies reveal the fully reversible redox behaviors and small electrochemical gap of Sm@C2v(19138)-C76, which are comparable to those of IPR species Sm@D3h-C74.

Sc2@C66 revisited: an endohedral fullerene with scandium ions nestled within two unsaturated linear triquinanes.

The geometries of fullerenes are governed by the isolated pentagon rule (IPR), which states that stable fullerenes have each of their 12 pentagons surrounded by five hexagons. At the dawn of fullerene science, it was widely believed that the IPR would also be applicable for endohedral fullerenes. In 2000, that idea was altered by the discovery of the first non-IPR fullerenes, Sc2@C66 and Sc3N@C68. The structural data for Sc2@C66 were interpreted to indicate the presence of a pair of doubly fused pentagons. However, that structure has remained a long-standing mystery, since it is thermodynamically unfavorable. Here, we demonstrate definitively that Sc2@C66 does not have the structure that was long believed to be but a brand new type. 2D NMR spectroscopic and single-crystal X-ray analyses disclose that Sc2@C66 has a C2v(4059)-C66 cage containing two sets of unsaturated linear triquinanes (ULTs), in which three pentagons abut one another and two scandium ions are located within the folds of each of the ULT units.

Maximum Aromaticity As a Guiding Principle for the Most Suitable Hosting Cages in Endohedral Metallofullerenes

Encapsulation of metal clusters in endohedral metallofullerenes takes place in cages that in most cases are far from being the most stable isomer in the corresponding hollow fullerenes. In large metal clusters and small fullerene cages, the election is mainly driven by the need to reduce strain. For the not so large metal clusters, it is usually said that the cage is chosen to minimize the electrostatic repulsion between charged five-membered rings.1 We have proved that maximizing the aromaticity is the main driving force that determines the most suitable hosting carbon cage for a given metallic cluster.2 This simple rule, the so-called Maximum ARomaticity Criterion (MARC), allow us to directly compare IPR (isolated pentagon rule) and non-IPR anionic isomers regardless the number of adjacent pentagon pairs (APP) they have. We have tested the MARC for the majority of endohedral metallofullerenes from C66 to C104 reported to date and the results show that MARC successfully predicts the most suitable C2n cage for hosting a given metal cluster among the manifold existing C2n isomers. In addition, using the Maximum ARomaticity Criterion, one can justify why the IPR rule is not always fulfilled when EMFs are considered.[Figure 1 here]Figure 1. Classification in terms of the ALA index of the anionic IPR and non-IPR fullerene isomers with 1-3 Adjacent Pentagon Pairs (APPs) for the most common C2 n (2n = 66–104) EMFs reported to date. Isomers experimentally observed are marked using black dots, and their corresponding isomer numbers are indicated. The ALA prediction ordering for the experimentally observed isomers are given in parentheses. A. Rodríguez-Fortea, N. Alegret, A.L. Balch, J.M. Poblet, Nat. Chem. 2010, 2, 955.M. Garcia-Borràs, S. Osuna, M. Swart, J.M. Luis, and M. Solà, Angew. Chem. Int. Ed. 2013, 52, 9275.

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 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.

Bingel–Hirsch Addition on Non-Isolated-Pentagon-Rule Gd3N@C2n (2n = 82 and 84) Metallofullerenes: Products under Kinetic Control

Bingel-Hirsch reactions on fullerenes take place under kinetic control. We here predict, by means of DFT methodology, the products of the Bingel-Hirsch addition on non-isolated-pentagon-rule (non-IPR) metallofullerenes Gd3N@C2n (2n = 82, 84), as modeled by closed-shell Y3N@C2n systems. Adducts on [6,6] B-type bonds placed near the pentalene unit are predicted for the two cages, as found for other non-IPR endohedral fullerenes such as Sc3N@C68.

Electronic structure of Sc3N@C68

Ultraviolet photoelectron spectra (UPS) and X-ray photoelectron spectra (XPS) of non-IPR endohedral fullerene Sc3N@C68 are measured. A simulated spectrum generated from a geometry optimized structure calculated by the density functional theory reproduces the UPS very well. XPS chemical shift of Sc2p3/2 level and electron population analysis of Sc3N@C68 suggest the electronic configuration of (Sc3N)6+@C686-. The Sc2p3/2 and N1s levels of Sc3N@C68, Sc3N@C78 and Sc3N@C80 move toward lower binding energy in accordance with cage size contraction.

Fullerene C72Cl4: The Exception that Proves the Rule?

In a stable condition: The first example of a non-IPR hollow fullerene that is more stable than the IPR isomer has been reported for the C2v-C72 fullerene (see picture). This molecule has been trapped by the exohedral chlorination reaction that affords the #11188C72Cl4 molecule, the structure of which has been unambiguously determined by X-ray analyses.

Thermal Properties of Non-IPR Fullerene Solids and their Reactivity with C2

not Available.

Non-IPR endohedral fullerene Yb@C76: density functional theory characterization

Download options Please wait... Supplementary files Supplementary information PDF (73K) Article information DOI https://doi.org/10.1039/C1JM12230D Article type Communication Submitted 19 May 2011 Accepted 23 Jun 2011 First published 11 Jul 2011 Download Citation J. Mater. Chem., 2011,21, 12206-12209 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Non-IPR endohedral fullerene Yb@C76: density functional theory characterization T. Yang, X. Zhao, Q. Xu, C. Zhou, L. He and S. Nagase, J. Mater. Chem., 2011, 21, 12206 DOI: 10.1039/C1JM12230D To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Search articles by author Tao Yang Xiang Zhao Qian Xu Caihua Zhou Ling He Shigeru Nagase

C72Cl4: A Pristine Fullerene with Favorable Pentagon-Adjacent Structure

A long-sought empty non-IPR fullerene, (#11188)C72, which is more stable than the sole IPR isomer in the fullerene[72] family, has been retrieved and crystallographically characterized as (#11188)C72Cl4. Mass spectrometric data support the facile dechlorination of (#11188)C72Cl4 and, in turn, the possible stability of pristine (#11188)C72.

Simple Combustion Production and Characterization of Octahydro[60]fullerene with a Non-IPR C60 Cage

For the first time an easier, operable combustion method is employed for the synthesis of non-IPR fullerene, and an octahydro[60]fullerene with a non-IPR C(60) cage (C(60) isomer (#1809)C(60)) produced by combustion is isolated and characterized by MS, UV-vis, IR, and NMR spectroscopies in combination with DFT calculations. This finding shows that, in addition to chlorine, hydrogen can be an ample cataloreactant for the production of non-IPR fullerene derivatives under such conditions as arc-burning and diffusion combustion.

Chlorofullerenes featuring triple sequentially fused pentagons

The triple sequentially fused pentagons (TSFP) motif is one of the basic subunits that could be used for constructing fullerenes, but it violates the isolated pentagon rule (IPR) and has not been found in carbon cages to date. The properties of TSFP-incorporating fullerenes are thus poorly explored both theoretically and experimentally. Reported herein are four chlorinated derivatives of three different fullerene cages, all with the TSFP motif. X-ray crystallographic analyses indicate that the molecular strain inherent to the pentagon adjacency of a TSFP is significantly relieved upon exohedral chlorination, leaving one of the four pentagon fusion sites unsaturated and rendering the present derivatives chiral. This unique reactivity, in stark contrast to that of previously reported non-IPR fullerenes containing double fused pentagons or triple directly fused pentagons, can be rationalized by density functional theory calculations, and are expected to stimulate further studies of these new members of the fullerene family, both theoretically and experimentally.

Synthesis of [formula omitted] using a low-pressure benzene/oxygen diffusion flame: Another pathway toward non-IPR fullerenes

In addition to the traditional Krätschmer–Huffman graphite arc-discharge method, a combustion method has been developed to synthesize fullerenes that defy the isolated pentagon rule (IPR). The C 3 v -symmetric # 1911 C 64 H 4 , as well as other non-IPR fullerenes such as the possible non-IPR C 60 isomer, have been synthesized as fullerene hydrides in a low-pressure benzene/oxygen diffusion flame. The non-IPR structure of the chromatographically isolated C 64 species was unambiguously identified by its HPLC retention time, UV absorption and mass spectrum by comparison with the reference material C 3 v - # 1911 C 64 H 4 . The synthesis, isolation and identification of C 3 v - # 1911 C 64 H 4 , as described in this work, show that this combustion method is successful, is energy efficient and is a promising candidate for the macroscopic synthesis of non-IPR fullerenes.

Properties of non-IPR fullerene films versus size of the building blocks

This perspective focuses on the cage size dependent properties of novel solid fullerene nanofilms grown by soft-landing of mass-selected C(n)(+) (48, 50, 52, 54, 56, 58, 62, 64, 66 and 68) onto room temperature graphite surfaces under ultra-high vacuum conditions. Such non-isolated-pentagon-ring (non-IPR) fullerene materials are not accessible to standard fullerene preparation methods. The component molecular building blocks of non-IPR films were generated by electron impact induced ionization/fragmentation of sublimed IPR-C(70)(D(5h)) (-->C(n) (n = 68, 66, 64, 62)) or IPR-C(60)(I(h)) (-->C(n) (n = 58, 56, 54, 52, 50)). Non-IPR fullerene films on graphite grow via formation of dendritic C(n) aggregates, whereas deposition of IPR fullerenes under analogous conditions (via deposition of unfragmented C(60)(+) and C(70)(+)) leads to compact islands. The latter are governed by weak van der Waals cage-cage interactions. In contrast, the former are stabilized by covalent intercage bonds as mediated by the non-IPR sites (primarily adjacent pentagon pairs, AP). A significant fraction of the deposited non-IPR C(n) cages can be intactly (re)sublimed by heating. The corresponding mean desorption activation energies, E(des), increase from 2.1 eV for C(68) up to 2.6 eV for C(50). The densities of states in the valence band regions (DOS), surface ionization potentials (sIP) and HOMO-LUMO gaps (Delta) of semiconducting non-IPR films were measured and found to vary strongly with cage size. Overall, the n-dependencies of these properties can be interpreted in terms of covalently interconnected oligomeric structures comprising the most stable (neutral) C(n) isomers-as determined from density functional theory (DFT) calculations. Non-IPR fullerene films are the first known examples of elemental cluster materials in which the cluster building blocks are covalently but reversibly interconnected.