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.     

Computational Studies on Non‐covalent Interactions of Carbon and Boron Fullerenes with Graphene

First‐principles DFT calculations are carried out to study the changes in structures and electronic properties of two‐dimensional single‐layer graphene in the presence of non‐covalent interactions induced by carbon and boron fullerenes (C₆₀, C₇₀, C₈₀ and B₈₀). Our study shows that larger carbon fullerene interacts more strongly than the smaller fullerene, and boron fullerene interacts more strongly than that of its carbon analogue with the same nuclearity. We find that van der Waals interactions play a major role in governing non‐covalent interactions between the adsorbed fullerenes and graphene. Moreover, a greater extent of van der Waals interactions found for the larger fullerenes, C₈₀ and B₈₀, relative to smaller C₆₀, and consequently, results in higher stabilisation. We find a small amount of electron transfer from graphene to fullerene, which gives rise to a hole‐doped material. We also find changes in the graphene electronic band structures in the presence of these surface‐decorated fullerenes. The Dirac cone picture, such as that found in pristine graphene, is significantly modified due to the re‐hybridisation of graphene carbon orbitals with fullerenes orbitals near the Fermi energy. However, all of the composites exhibit perfect conducting behaviour. The simulated absorption spectra for all of the graphene–fullerene hybrids do not exhibit a significant change in the absorption peak positions with respect to the pristine graphene absorption spectrum. Additionally, we find that the hole‐transfer integral between graphene and C₆₀ is larger than the electron‐transfer integrals and the extent of these transfer integrals can be significantly tuned by graphene edge functionalisation with carboxylic acid groups. Our understanding of the non‐covalent functionalisation of graphene with various fullerenes would promote experimentalists to explore these systems, for their possible applications in electronic and opto‐electronic devices.     

Structure and Electronic Properties of Fullerenes C52q+: Is C522+ an Exception to the Pentagon Adjacency Penalty Rule?

The structure, vibrational spectra and electronic properties of the neutral, singly and doubly charged C52 fullerenes were studied by means of the Hartree-Fock method and density functional theory. Different isomers were considered, in particular those with the lowest possible number (five or six) of adjacent pentagons, and an isomer with a four-atom ring. For neutral and singly charged species, the most stable isomer is that with the lowest number of adjacent pentagons, namely five. However, for C(52)2+, the most stable structure has six adjacent pentagons. This finding, which contradicts the pentagon adjacency penalty rule, is a consequence of complete filling of the HOMO pi shell and the near-perfect sphericity of the most stable isomer. The simulated vibrational spectra show important differences in the positions and intensities of the vibrations for the different isomers.     

Tunable Electronic Properties of a Proton‐Responsive N,N‐Dimethylaminoazobenzene Fullerene (C60) Dyad

A basic N,N‐dimethylaminoazobenzene–fullerene (C₆₀) dyad molecular skeleton is modelled and synthesized. In spite of the myriad use of azobenzene as a photo‐ and electrochromic moiety, the idea presented herein is to adopt a conceptually different path by using it as a bridge in a donor–bridge–acceptor single‐molecular skeleton, connecting the electron acceptor N‐methylfulleropyrrolidine with an electron donor N,N‐dimethylaniline. Addition of trifluoroacetic acid (TFA) results in a drastic colour change of the dyad from yellow to pink in dichloromethane (DCM). The structure of the protonated species are established from electronic spectroscopy and time‐dependent density functional theory (TD‐DFT) calculations. UV/Vis spectroscopic investigations reveal the disappearance of the 409 nm ¹(π→π*) transition with appearance of new features at 520 and 540 nm, attributed to protonated β and α nitrogens, respectively, along with a finite weight of the C₆₀ pyrrolidinic nitrogen. Calculations reveal intermixing of n_(N?N)→π*_(N?N) and charge transfer (CT) transitions in the neutral dyad, whereas, the n_(N?N)→π*_(N?N) transition in the protonated dyad is buried under the dominant ¹(π →π*) feature and is red‐shifted upon Gaussian deconvolution. The experimental binding constants involved in the protonation of N,N‐dimethylanilineazobenzene and the dyad imply an almost equal probability of existence of both α‐ and β‐protonated forms. Larger binding constants for the protonated dyads imply more stable dyad complexes than for the donor counterparts. One of the most significant findings upon protonation resulted in frontier molecular orbital (FMO) switching with the dyad LUMO located on the donor part, evidenced from electrochemical investigations. The appearance of a new peak, prior to the first reduction potential of N‐methylfulleropyrrolidine, clearly indicates location of the first incoming electron on the donor‐centred LUMO of the dyad, corroborated by unrestricted DFT calculations performed on the monoanions of the protonated dyad. The protonation of the basic azo nitrogens thus enables a rational control over the energetics and location of the FMOs, indispensable for electron transport across molecular junctions in realizing futuristic current switching devices.     

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.     

In Silico Olefin Metathesis with Ru‐Based Catalysts Containing N‐Heterocyclic Carbenes Bearing C60 Fullerenes

Density functional theory calculations have been used to explore the potential of Ru‐based complexes with 1,3‐bis(2,4,6‐trimethylphenyl)imidazolin‐2‐ylidene (SIMes) ligand backbone ( A ) being modified in silico by the insertion of a C₆₀ molecule ( B and C ), as olefin metathesis catalysts. To this end, we investigated the olefin metathesis reaction catalyzed by complexes A , B , and C using ethylene as the substrate, focusing mainly on the thermodynamic stability of all possible reaction intermediates. Our results suggest that complex B bearing an electron‐withdrawing N‐heterocyclic carbene improves the performance of unannulated complex A . The efficiency of complex B is only surpassed by complex A when the backbone of the N‐heterocyclic carbene of complex A is substituted by two amino groups. The particular performance of complexes B and C has to be attributed to electronic factors, that is, the electronic‐donating capacity of modified SIMes ligand rather than steric effects, because the latter are predicted to be almost identical for complexes B and C when compared to those of A . Overall, this study indicates that such Ru‐based complexes B and C might have the potential to be effective olefin metathesis catalysts.     

Chlorination of IPR C100 Fullerene Affords Unconventional C96Cl20 with a Nonclassical Cage Containing Three Heptagons

Chlorination of C₁₀₀ fullerene with a mixture of VCl₄ and SbCl₅ afforded C₉₆Cl₂₀ with a strongly unconventional structure. In contrast to the classical fullerenes containing only hexagonal and pentagonal rings, the C₉₆ cage contains three heptagonal rings and, therefore, should be classified as a fullerene with a nonclassical cage (NCC). There are several types of pentagon fusions in the C₉₆ cage including pentagon pairs and pentagon triples. The three‐step pathway from isolated‐pentagon‐rule (IPR) C₁₀₀ to C₉₆(NCC‐3hp) includes two C₂ losses, which create two cage heptagons, and one Stone–Wales rotation under formation of the third heptagon. Structural reconstruction established C₁₀₀ isomer no. 18 from 450 topologically possible IPR isomers as the starting C₁₀₀ fullerene. Until now, no pristine C₁₀₀ isomers have been confirmed based on the experimental results.     

Antiferromagnetic Ground State of a C60‐Covered Si(001) Surface

Modeling magnetism: The antiferromagnetic ground state of the C₆₀/Si(001)‐c(4×4) surface is predicted by means of density functional theory calculations. Two adjacent dangling bonds (DBs) generated by the adsorption of C₆₀ are antiferromagnetically coupled with each other. This study demonstrates that magnetic Si surfaces can be prepared by engineering single Si DBs with unpaired electrons.

     

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.     

Orienting Effect of the Cage Addends: The Case of Nucleophilic Cyclopropanation of C2‐C70(CF3)8

C₂‐C₇₀(CF₃)₈ was found to be a very promising substrate in the Bingel and the Bingel–Hirsch reactions combining perfect regioselectivity with much higher reactivity compared to its analogs. The reactions with diethyl malonate yield a single isomer of the monoadduct C₇₀(CF₃)₈[C(CO₂Et)₂] and a single C₂‐symmetrical bisadduct C₇₀(CF₃)₈[C(CO₂Et)₂]₂. The Bingel–Hirsch variation is particularly interesting in that it additionally affords, in a similar regioselective manner, the unexpected alkylated derivatives C₇₀(CF₃)₈[CH(CO₂Et)₂]H and C₇₀(CF₃)₈[C(CO₂Et)₂][CH(CO₂Et)₂]H. The novel compounds have been isolated and structurally characterized by means of ¹H and ¹⁹F NMR spectroscopy as well as single‐crystal X‐ray diffraction. The mechanistic and regiochemical aspects of the reaction are explained with the aid of DFT calculations.     

[6,6]‐Open and [6,6]‐Closed Isomers of C70(CF2): Synthesis,Electrochemical and Quantum Chemical Investigation

Novel difluoromethylenated [70]fullerene derivatives, C₇₀(CF₂)_n (n=1–3), were obtained by the reaction of C₇₀ with sodium difluorochloroacetate. Two major products, isomeric C₇₀(CF₂) mono‐adducts with [6,6]‐open and [6,6]‐closed configurations, were isolated and their homofullerene and methanofullerene structures were reliably determined by a variety of methods that included X‐ray analysis and high‐level spectroscopic techniques. The [6,6]‐open isomer of C₇₀(CF₂) constitutes the first homofullerene example of a non‐hetero [70]fullerene derivative in which functionalisation involves the most reactive bond in the polar region of the cage. Voltammetric estimation of the electron affinity of the C₇₀(CF₂) isomers showed that it is substantially higher for the [6,6]‐open isomer (the 70‐electron π‐conjugated system is retained) than the [6,6]‐closed form, the latter being similar to the electron affinity of pristine C₇₀. In situ ESR spectroelectrochemical investigation of the C₇₀(CF₂) radical anions and DFT calculations of the hyperfine coupling constants provide evidence for the first example of an inter‐conversion between the [6,6]‐closed and [6,6]‐open forms of a cage‐modified fullerene driven by an electrochemical one‐electron transfer. Thus, [6,6]‐closed C₇₀(CF₂) constitutes an interesting example of a redox‐switchable fullerene derivative.     

Quantum chemical calculations of N@Cn endofullerenes (n ≤ 60)

The geometric parameters and energy characteristics of small endofullerenes N@C_n (n = 20, 24, 30, 32, 40, 50) and N@C₆₀ in the quartet ground state were calculated by the B3LYP/6-31G* method. The N atom is located at the center of the carbon cage in all molecules except N@C₃₀, where it is bound to the cage wall. Encapsulation of nitrogen atom has little effect on the fullerene cage geometry for n = 40, 50, and 60. No significant charge transfer from the N endo-atom to the cage was revealed for all the N@C_n endofullerenes studied. The calculated spin density on the nitrogen endo-atom increases as the size (n) of the carbon cage increases. The relative stabilities of C_n fullerenes and corresponding endofullerenes N@C_n are discussed. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 15–20, January, 2006.