Full exploration of the Diels-Alder cycloaddition on metallofullerenes M3N@C80 (M = Sc, Lu, Gd): the D(5h) versus I(h) isomer and the influence of the metal cluster

In this work a detailed investigation of the exohedral reactivity of the most important and abundant endohedral metallofullerene (EMF) is provided, that is, Sc(3)N@I(h)-C(80) and its D(5h) counterpart Sc(3)N@D(5h)-C(80) , and the (bio)chemically relevant lutetium- and gadolinium-based M(3)N@I(h)/D(5h)-C(80) EMFs (M = Sc, Lu, Gd). In particular, we analyze the thermodynamics and kinetics of the Diels-Alder cycloaddition of s-cis-1,3-butadiene on all the different bonds of the I(h)-C(80) and D(5h)-C(80) cages and their endohedral derivatives. First, we discuss the thermodynamic and kinetic aspects of the cycloaddition reaction on the hollow fullerenes and the two isomers of Sc(3)N@C(80). Afterwards, the effect of the nature of the metal nitride is analyzed in detail. In general, our BP86/TZP//BP86/DZP calculations indicate that [5,6] bonds are more reactive than [6,6] bonds for the two isomers. The [5,6] bond D(5h)-b, which is the most similar to the unique [5,6] bond type in the icosahedral cage, I(h)-a, is the most reactive bond in M(3)N@D(5h)-C(80) regardless of M. Sc(3)N@C(80) and Lu(3)N@C(80) give similar results; the regioselectivity is, however, significantly reduced for the larger and more electropositive M = Gd, as previously found in similar metallofullerenes. Calculations also show that the D(5h) isomer is more reactive from the kinetic point of view than the I(h) one in all cases which is in good agreement with experiments.     

Trimetallic nitride endohedral metallofullerenes: reactivity dictated by the encapsulated metal cluster

The first derivatives of Y(3)N@C(80) have been synthesized and fully characterized. 1,3-Dipolar cycloaddition of N-ethylazomethine ylide yielded mainly the pyrrolidine monoadduct of the icosahedral (I(h)()) symmetry cage exclusively at a [6,6] double bond. The same regioselectivity on a [6,6] double bond was observed when the endohedral compound was cyclopropanated with diethyl bromomalonate. These results are in pronounced contrast to those observed for icosahedral symmetry Sc(3)N@C(80), for which all reported derivatives add completely regioselectively to [5,6] double bonds. (1)H NMR, (13)C NMR, and HMQC spectroscopy revealed that the addition pattern on Y(3)N@C(80) resulted in a pyrrolidinofullerene derivative with unsymmetric pyrrolidine carbons and symmetric geminal protons. The cyclopropanated monoadduct exhibited symmetric ethyl groups on the malonate, consistent with regioselective addition at a [6,6] double bond. Attempts to perform the same cyclopropanation reaction on (I(h)()) Sc(3)N@C(80) failed to yield any identifiable products. These observations clearly indicate that the reactivity of trimetallic nitride endohedral metallofullerenes toward exohedral chemical functionalization is profoundly affected and effectively controlled by the nature of the endohedral metal cluster.     

Manganese(III)-catalyzed free radical reactions on trimetallic nitride endohedral metallofullerenes

The first reactions of trimetallic nitride templated endohedral metallofullerenes (TNT EMFs) with carbon radicals generated from diethyl malonate catalyzed by manganese(III) acetate are reported. Two methano monoadducts, Sc3N@C80-A and Sc3N@C80-B, were isolated and characterized. Sc3N@C80-A contains two ester moieties, whereas Sc3N@C80-B contains only one ester group and a hydrogen atom on the central carbon of the addend. NMR spectroscopy of the two monoadducts suggests that the addition occurs regioselectively at a 6,6-ring juncture on the surface of the icosahedrally (Ih) symmetric Sc3N@C80, forming the first 6,6-ring-bridged methano Ih Sc3N@C80 derivatives. The measured 1J(C,H) = 147 Hz for the methano carbon with its hydrogen in monoadduct Sc3N@C80-B nearly perfectly matches the data for pi-homoaromatic systems, indicating an open [6,6]-methano structure. Geometry optimization also found that the "closed" [6,6]-methano structures were energetically unstable and always led to the open forms. Thus, an "open" [6,6]-methanofulleride structure is proposed, which was induced by the norcaradiene rearrangement, resulting in the cleavage of the cyclopropane ring and the formation of energetically stable open cage fullerene derivatives. These are the first examples of thermodynamically stable adducts of the "open" type at the 6,6-ring juncture of Ih Sc3N@C80, differing greatly from the "closed" 5,6-ring juncture adducts reported previously. In addition, bis-, tri-, and up to octaadducts of Sc3N@C80 were detected by matrix-assisted laser desorption ionization time-of-flight mass spectrometry; this synthetic method was also applied to Lu3N@C80, producing adducts with up to 10 substituents on the carbon cage. These are the highest levels of substitution of TNT metallofullerenes reported so far.     

The exohedral Diels-Alder reactivity of the titanium carbide endohedral metallofullerene Ti2C2@D(3h)-C78: comparison with D(3h)-C78 and M3N@D(3h)-C78 (M=Sc and Y) reactivity

The chemical functionalization of endohedral (metallo)fullerenes has become a main focus of research in the last few years. It has been found that the reactivity of endohedral (metallo)fullerenes may be quite different from that of the empty fullerenes. Encapsulated species have an enormous influence on the thermodynamics, kinetics, and regiochemistry of the exohedral addition reactions undergone by these species. A detailed understanding of the changes in chemical reactivity due to incarceration of atoms or clusters of atoms is essential to assist the synthesis of new functionalized endohedral fullerenes with specific properties. Herein, we report the study of the Diels-Alder cycloaddition between 1,3-butadiene and all nonequivalent bonds of the Ti(2)C(2)@D(3h)-C(78) metallic carbide endohedral metallofullerene (EMF) at the BP86/TZP//BP86/DZP level of theory. The results obtained are compared with those found by some of us at the same level of theory for the D(3h)-C(78) free cage and the M(3)N@D(3h)-C(78) (M=Sc and Y) metallic nitride EMFs. It is found that the free cage is more reactive than the Ti(2)C(2)@D(3h)-C(78) EMF and this, in turn, has a higher reactivity than M(3)N@D(3h)-C(78). The results indicate that, for Ti(2)C(2)@D(3h)-C(78), the corannulene-type [5,6] bonds c and f, and the type B [6,6] bond 3 are those thermodynamically and kinetically preferred. In contrast, the D(3h)-C(78) free cage has a preference for addition to the [6,6] 1 and 6 bonds and the [5,6] b bond, whereas M(3)N@D(3h)-C(78) favors additions to the [6,6] 6 (M=Sc) and [5,6] d (M=Y) bonds. The reasons for the regioselectivity found in Ti(2)C(2)@D(3h)-C(78) are discussed.     

Chemical reactivity of D3h C78 (metallo)fullerene: regioselectivity changes induced by Sc3N encapsulation

We report here for the first time a full comparison of the exohedral reactivity of a given fullerene and its parent trinitride template endohedral metallofullerene. In particular, we study the thermodynamics and kinetics for the Diels-Alder [4 + 2] cycloaddition between 1,3-butadiene and free D3h'-C78 fullerene and between butadiene and the corresponding endohedral D3h-Sc3N@C78 derivative. The reaction is studied for all nonequivalent bonds, in both the free and the endohedral fullerenes, at the BP86/TZP//BP86/DZP level. The change in exohedral reactivity and regioselectivity when a metal cluster is encapsulated inside the cage is profound. Consequently, the Diels-Alder reaction over the free fullerene and the endohedral derivative leads to totally different cycloadducts. This is caused by the metal nitride situated inside the fullerene cage that reduces the reactivity of the free fullerene and favors the reaction over different bonds.     

Structure and enhanced reactivity rates of the D5h Sc3N@C80 and Lu3N@C80 metallofullerene isomers: the importance of the pyracylene motif

In this paper we report enhanced reactivity of the D(5h) isomers in comparison with the more common I(h) isomers of Sc(3)N@C(80) and Lu(3)N@C(80) toward Diels-Alder and 1,3-dipolar tritylazomethine ylide cycloaddition reactions. Also, the structure of the D(5h) isomer of Sc(3)N@C(80) has been determined through single-crystal X-ray diffraction on D(5h)-Sc(3)N@C(80).Ni(OEP).2benzene (OEP = octaethylporphyrin). The Sc(3)N portion of D(5h)-Sc(3)N@C(80) is strictly planar, but the plane of these four atoms is tipped out of the noncrystallographic, horizontal mirror plane of the fullerene by 30 degrees . The combination of short bond length and high degree of pyramidization for the central carbon atoms of the pyracylene sites situated along a belt that is perpendicular to the C(5) axis suggests that these are the sites of greatest reactivity in the D(5h) isomer of Sc(3)N@C(80). Consistent with the observation of higher reactivity observed for the D(5h) isomers, cyclic voltammetry and molecular orbital (MO) calculations demonstrate that the D(5h) isomers have slightly smaller energy gaps than those of the I(h) isomers. The first mono- and bis-adducts of D(5h) Sc(3)N@C(80) have been synthesized via 1,3-dipolar cycloaddition of tritylazomethine ylide. The NMR spectrum for the monoadduct 2b is consistent with reaction at the 6,6-ring juncture in the pyracylene unit of the D(5h) Sc(3)N@C(80) cage and is the thermodynamically stable isomer. On the other hand, monoadduct 2a undergoes thermal conversion to other isomeric monoadducts, and three possible structures are proposed.     

Unexpected chemical and electrochemical properties of M3N@C80 (M = Sc, Y, Er)

The unexpected isomerization of N-ethyl [6,6]-pyrrolidino-Y3N@C80 to the [5,6] regioisomer is reported, as well as the synthesis, characterization, and electrochemical analysis of Er3N@C80 derivatives. A complete electrochemical study of the M3N@C80 species (M = Sc, Y, Er) and their derivatives is presented. We introduce electrochemistry as a new tool in the characterization of the [5,6] and [6,6] regioisomers of trimetallic nitride endohedral metallofullerenes.     

Tunable charge-transport properties of I(h)-C80 endohedral metallofullerenes: investigation of La2@C80, Sc3N@C80, and Sc3C2@C80

Fullerene crystals or films have drawn much interest because they are good candidates for use in the construction of electronic devices. The results of theoretical calculations revealed that the conductivity properties of I(h)-C(80) endohedral metallofullerenes (EMFs) vary depending on the encapsulated metal species. We experimentally investigated the solid-state structures and charge-carrier mobilities of I(h)-C(80) EMFs La(2)@C(80), Sc(3)N@C(80), and Sc(3)C(2)@C(80). The thin film of Sc(3)C(2)@C(80) exhibits a high electron mobility μ = 0.13 cm(2) V(-1) s(-1) under normal temperature and atmospheric pressure, as determined using flash-photolysis time-resolved microwave conductivity measurements. This electron mobility is 2 orders of magnitude higher than the mobility of La(2)@C(80) or Sc(3)N@C(80).     

Electrosynthesis of a Sc3N@I(h)-C80 methano derivative from trianionic Sc3N@I(h)-C80

The electrosynthetic method has been used for the selective synthesis of fullerene derivatives that are otherwise not accessible by other procedures. Recent attempts to electrosynthesize Sc(3)N@I(h)-C(80) derivatives using the Sc(3)N@I(h)-C(80) dianion were unsuccessful because of its low nucleophilicity. Those results prompted us to prepare the Sc(3)N@C(80) trianion, which should be more nucleophilic and reactive with electrophilic reagents. The reaction between Sc(3)N@C(80) trianions and benzal bromide (PhCHBr(2)) was successful and yielded a methano derivative, Sc(3)N@I(h)-C(80)(CHPh) (1), in which the >CHPh addend is selectively attached to a [6,6] ring junction, as characterized by MALDI-TOF mass spectrometry and NMR and UV-vis-NIR spectroscopy. The electrochemistry of 1 was studied using cyclic voltammetry, which showed that 1 exhibits the typical irreversible cathodic behavior of pristine Sc(3)N@I(h)-C(80), resembling the behavior of other methano adducts of Sc(3)N@I(h)-C(80). The successful synthesis of endohedral metallofullerene derivatives using trianionic Sc(3)N@I(h)-C(80) and dianionic Lu(3)N@I(h)-C(80), but not dianionic Sc(3)N@I(h)-C(80), prompted us to probe the causes using theoretical calculations. The Sc(3)N@I(h)-C(80) trianion has a singly occupied molecular orbital with high spin density localized on the fullerene cage, in contrast to the highest occupied molecular orbital of the Sc(3)N@I(h)-C(80) dianion, which is mainly localized on the inside cluster. The calculations provide a clear explanation for the different reactivities observed for the dianions and trianions of these endohedral fullerenes.     

Synthesis, isolation, and addition patterns of trifluoromethylated D5h and I(h) isomers of Sc3N@C80: Sc3N@D5h-C80(CF3)18 and Sc3N@I(h)-C80(CF3)14

Sc(3)N@D(5h)-C(80) and Sc(3)N@I(h)-C(80) were trifluoromethylated with CF(3)I at 400 °C, affording mixtures of CF(3) derivatives. After separation with HPLC, the first multi-CF(3) derivative of Sc(3)N@D(5h)-C(80), Sc(3)N@D(5h)-C(80)(CF(3))(18), and three new isomers of Sc(3)N@I(h)-C(80)(CF(3))(14) were investigated by X-ray crystallography. The Sc(3)N@D(5h)-C(80)(CF(3))(18) molecule is characterized by a large number of double C-C bonds and benzenoid rings within the D(5h)-C(80) cage and a fully different position of the Sc(3)N unit compared to that in the pristine Sc(3)N@D(5h)-C(80). A detailed comparison of five Sc(3)N@I(h)-C(80)(CF(3))(14) isomers reveals a strong influence of the exohedral additions on the behavior of the Sc(3)N cluster inside the I(h)-C(80) cage.     

New M(3)N@C(2n) endohedral metallofullerene families (M=Nd, Pr, Ce; n=40-53): expanding the preferential templating of the C(88) cage and approaching the C(96) cage

Three new families of trimetallic nitride template endohedral metallofullerenes (TNT EMFs), based on cerium, praseodymium, and neodymium clusters, were synthesized by vaporizing packed graphite rods in a conventional Kr?tschmer-Huffman arc reactor. Each of these families of metallofullerenes was identified and characterized by mass spectroscopy, HPLC, UV/Vis-NIR spectroscopy, and cyclic voltammetry. The mass spectra and HPLC chromatograms show that these larger metallic clusters are preferentially encapsulated by a C(88) cage. When the size of the cluster is increased, the C(96) cage is progressively favored over the predominant C(88) cage. It is also observed that the smaller cages (C(80)-C(86)) almost disappear on going from neodymium to cerium endohedral metallofullerenes. The UV/Vis-NIR spectra and cyclic voltammograms confirm the low HOMO-LUMO gap and reversible electrochemistry of these M(3)N@C(88) metallofullerenes.     

Vibrational structure of endohedral fullerene Sc3N@C78 (D3h'): evidence for a strong coupling between the Sc3N cluster and C78 cage.

The vibrational structure of the endohedral cluster fullerene Sc(3)N@C(78) is studied by FTIR spectroscopy, Raman spectroscopy and DFT-based quantum chemical calculations. Remarkably good agreement between experimental and calculated spectra is achieved and a full assignment of the Sc(3)N-based vibrational modes is given. Significant differences in the vibrational structure of the endohedral cluster fullerene Sc(3)N@C(78) and the empty, charged C(78) (6-): 5 (D(3h)') are rationalized by the strong coupling between the Sc(3)N cluster and the fullerene cage. This coupling has its origin in a significant overlap of the Sc(3)N and C(78) molecular orbitals, and causes atomic-charge and bond-length redistributions compared to the neutral C(78) and the C(78) (6-) anion. An ionic model is not sufficient to describe the electronic, geometric and vibrational structure of the Sc(3)N@C(78) nitride cluster fullerene.     

Bingel-Hirsch reactions on non-IPR Gd3N@C2n (2n = 82 and 84)

The Bingel-Hirsch reactions on non-isolated pentagon rule (non-IPR) Gd(3)N@C(2n) (2n = 82, 84) are studied. Computational results show that the two metallofullerenes display similar reactivity according to their related topologies. Long C-C bonds with large pyramidalization angles lead to the most stable adducts, the [5,6] bonds in the adjacent pentagon pair being especially favored. The lesser regioselectivity observed for Gd(3)N@C(82) is probably due to the activation of some C-C bonds by means of the metal cluster.     

Comparative investigation on non-IPR C68 and IPR C78 fullerenes encaging Sc3N molecules

A computational study on the experimentally detected Sc(3)N@C(68) cluster is reported, involving quantum chemical analysis at the B3LYP/6-31G level. Extensive computations were carried out on the pure C(68) cage which does not conform with the isolated pentagon rule (IPR). The two maximally stable C(68) isomers were selected as initial Sc(3)N@C(68) cage structures. Full geometry optimization leads to a confirmation of an earlier assessment of the Sc(3)N@C(68) equilibrium geometry (Nature 2000, 408, 427), namely an eclipsed arrangement of Sc(3)N in the C(68) 6140 frame, where each Sc atom interacts with one pentagon pair. From a variety of theoretical procedures, a D(3h) structure is proposed for the free Sc(3)N molecule. Encapsulated into the C(68) enclosure, this unit is strongly stabilized with respect to rotation within the cage. The complexation energy of Sc(3)N@C(68) cage is found to be in the order of that determined for Sc(3)N@C(80) and exceeding the complexation energy of Sc(3)N@C(78). The cage-core interaction is investigated in terms of electron transfer from the encapsulated trimetallic cluster to the fullerene as well as hybridization between these two subsystems. The stabilization mechanism of Sc(3)N@C(68) is seen to be analogous to that operative in Sc(3)N@C(78). For both cages, C(68) and C(78), inclusion of Sc(3)N induces aromaticity of the cluster as a whole.     

Purification of endohedral trimetallic nitride fullerenes in a single, facile step

A major hurdle hampering the development of fullerenes, endohedral metallofullerenes, and nanotubes has been the difficulty of obtaining high purity samples. Soots prepared in the usual manner via a Kr?tschmer-Huffman electric-arc generator consist of mixtures of insoluble carbonaceous materials and soluble fullerenes: C60, C70, C76, C78, C84, etc. When metals are introduced as endohedral species the complexity of the resultant soot is even greater because of the presence of multiple isomers of both the empty fullerenes and the endohedral metallofullerenes. Here, for the first time, we report that lanthanide trimetallic nitride endohedral metallofullerenes, A3N@C80 (A = lanthanide atom, e.g., Er, Gd, Ho, Lu, Sc, Tb, Tm, Y), can be obtained in pure form directly from as-prepared soots in a single facile step by taking advantage of their extraordinary kinetic chemical stability with respect to the other fullerenes in Diels-Alder reactions with a cyclopentadiene-functionalized resin. We show that careful control of conditions (stoichiometry, time, temperature) allows separation of fullerenes with different cage sizes, as well as isomeric species. Furthermore, the Diels-Alder reaction is thermally reversible, and we demonstrated that the bound empty-cage fullerenes and classical endohedral metallofullerenes can be recovered by displacement with maleic anhydride.     

Pyramidalization of Gd3N inside a C80 cage. The synthesis and structure of Gd3N@C80

The X-ray crystal structure of Gd(3)N@C(80).Ni(II)(OEP).1.5(benzene) shows that the Gd(3)N unit within the I(h) C(80) cage is pyramidal, whereas Sc(3)N@C(80), Sc(3)N@C(78), Sc(3)N@C(68), Lu(3)N@C(80) and Sc(2)ErN@C(80) have planar M(3)N units.     

A symmetric derivative of the trimetallic nitride endohedral metallofullerene, Sc3N@C80.

The reaction of Sc3N@C80 with 6,7-dimethoxyisochroman-3-one (13C labeled) provides the first functionalized derivative of the trimetallic nitride template (TNT) endohedral metallofullerene family. The reaction mixture is dominated by a single 13C labeled monoadduct product that was purified by HPLC. The 13C labeled monoadduct was characterized by 1H NMR, 13C NMR, and MALDI-TOF mass spectrometry. The proposed structure for this novel symmetric monoadduct is consistent with derivatization at the [5,6] ring juncture on the Sc3N@C80 cage.     

Structure, stability, and cluster-cage interactions in nitride clusterfullerenes M3N@C2n (M = Sc, Y; 2n = 68-98): a density functional theory study

Extensive semiempirical calculations of the hexaanions of IPR (isolated pentagon rule) and non-IPR isomers of C(68)-C(88) and IPR isomers of C(90)-C(98) followed by DFT calculations of the lowest energy structures were performed to find the carbon cages that can provide the most stable isomers of M(3)N@C(2n) clusterfullerenes (M = Sc, Y) with Y as a model for rare earth ions. DFT calculations of isomers of M(3)N@C(2n) (M = Sc, Y; 2n = 68-98) based on the most stable C(2n)(6-) cages were also performed. The lowest energy isomers found by this methodology for Sc(3)N@C(68), Sc(3)N@C(78), Sc(3)N@C(80), Y(3)N@C(78), Y(3)N@C(80), Y(3)N@C(84), Y(3)N@C(86), and Y(3)N@C(88) are those that have been shown to exist by single-crystal X-ray studies as Sc(3)N@C(2n) (2n = 68, 78, 80), Dy(3)N@C(80), and Tb(3)N@C(2n) (2n = 80, 84, 86, 88) clusterfullerenes. Reassignment of the carbon cage of Sc(2)@C(76) to the non-IPR Cs: 17490 isomer is also proposed. The stability of nitride clusterfullerenes was found to correlate well with the stability of the empty 6-fold charged cages. However, the dimensions of the cage in terms of its ability to encapsulate M(3)N clusters were also found to be an important factor, especially for the medium size cages and the large Y(3)N cluster. In some cases the most stable structures are based on the different cage isomers for Sc(3)N and Y(3)N clusters. Up to the cage size of C(84), non-IPR isomers of C(2n)(6-) and M(3)N@C(2n) were found to compete with or to be even more stable than IPR isomers. However, the number of adjacent pentagon pairs in the most stable non-IPR isomers decreases as cage size increases: the most stable M(3)N@C(2n) isomers have three such pairs for 2n = 68-72, two pairs for n = 74-80, and only one pair for n = 82, 84. For C(86) and C(88) the lowest energy IPR isomers are much more stable than any non-IPR isomer. The trends in the stability of the fullerene isomers and the cluster-cage binding energies are discussed, and general rules for stability of clusterfullerenes are established. Finally, the high yield of M(3)N@C(80) (Ih) clusterfullerenes for any metal is explained by the exceptional stability of the C(80)(6-) (Ih: 31924) cage, rationalized by the optimum distribution of the pentagons leading to the minimization of the steric strain, and structural similarities of C(80) (Ih: 31924) with the lowest energy non-IPR isomers of C(760(6-), C(78)(6-), C(82)(6-), and C(84)(6-) pointed out.     

Crystallographic characterization and structural analysis of the first organic functionalization product of the endohedral fullerene Sc(3)N@C(80)

The structure of Sc3N@C80-C10H12O2, a Diels-Alder cycloadduct of Sc3N@C80, has been determined. The crystallographic data shows that cycloaddition occurs at a C-C bond of 6:5 ring junction, and that the fullerene C1-C2 bond is elongated and pulled out from the fullerene. The Sc3N unit is well-ordered within the C80 cage and positioned away from the site of addition. The proximity of the Sc atoms to the cage carbon atoms causes those carbon atoms to protrude slightly from the surface of the fullerene cage.     

Sc3N@C78: encapsulated cluster regiocontrol of adduct docking on an ellipsoidal metallofullerene sphere

The first N-tritylpyrrolidino derivatives of D(3h) (78:5) Sc(3)N@C(78) were successfully synthesized and isolated. The addition sites for the two nearly equivalent kinetic monoadducts 1a and 1b are across two different 6,6 junction sites on the Sc(3)N@C(78) cage that are offset from the horizontal plane defined by the Sc(3)N cluster. The adducts were characterized by NMR experiments, DFT calculations and X-ray crystallographic analysis of Sc(3)N@C(78) derivative 1a. A unique finding of this study is the regiocontrol of adduct docking by the internal Sc(3)N cluster.