The influence of cage size on the reactivity of trimetallic nitride metallofullerenes: a mono- and bis-methanoadduct of Gd3N@C80 and a monoadduct of Gd3N@C84

A reactivity study of the higher TNT EMFs of gadolinium is reported here showing that the reactivity substantially decreases when the fullerene cage gets larger.     

Structural and magnetic properties of Gd3N@C80

Using relativistic and on-site correlation-corrected density functional theory, we have investigated the structural and magnetic properties of recently synthesized Gd3N@C80. The most stable structure of Gd3N@C80 has the three magnetic Gd ions pointing to the centers of hexagons in C80. The magnetic ground state of this structure has the three coplanar spins (S = 7/2) offset by 120 degrees angles. At the same time, the state with the highest multiplicity, where all the spins are parallel aligned, is found only about 4.5 meV higher in energy. Therefore, at room temperature, we expect Gd3N@C80 to be paramagnetic with the spin fluctuating between different multiplicities. As a result, Gd3N@C80 may exhibit greater proton relaxivity than Gd@C60 and Gd@C82 and serve as a possible candidate for the next generation of commercially available magnetic resonance imaging contrast agents.     

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.     

Expanding the number of stable isomeric structures of the C80 cage: a new fullerene Dy3N@C80

The production, isolation, and spectroscopic characterization of a new Dy3N@C80 cluster fullerene that exhibits three isomers (1-3) is reported for the first time. In addition, the third isomer (3) forms a completely new C80 cage structure that has not been reported in any endohedral fullerenes so far. The isomeric structures of the Dy3N@C80 cluster fullerene were analyzed by studying HPLC retention behavior, laser desorption time-of-flight (LD-TOF) mass spectrometry, and UV-Vis-NIR and FTIR spectroscopy. The three isomers of Dy3N@C80 were all large band-gap (1.51, 1.33, and 1.31 eV for 1-3, respectively) materials, and could be classified as very stable fullerenes. According to results of FTIR spectroscopy, the Dy3N@C80 (I) (1) was assigned to the fullerene cage C80:7 (I(h)), whereas Dy3N@C80 (II) (2) had the cage structure of C80:6 (D(5h)). The most probable cage structure of Dy3N@C80 (III) (3) was proposed to be C80:1 (D(5d)). The significant differences between Dy3N@C80 and other reported M3N@C80 (M = Sc, Y, Gd, Tb, Ho, Er, Tm) cluster fullerenes are discussed in detail, and the strong influence of the metal on the nitride cluster fullerene formation is concluded.     

Synthesis and Characterization of Lu3N@C80O

Oxygenated hollow cage fullerenes have been intensively studied due to their applications in biomedicine in recent years. Clusterfullerenes have become a focus of endohedral fullerene researches for their exceptionally high yield and thermal stabilities. However, oxide derivatives of clusterfullerene remain unexplored to date. Herein, we present the photochemical synthesis of an oxide derivative of clusterfullerene, Lu₃N @C₈₀O , for the first time. The compound was characterized by matrix‐assisted laser desorption ionization time‐of‐flight mass spectrometry, UV –vis‐NIR , cyclic voltammetry, and FTIR spectroscopy. The results suggest that one oxygen atom bridges with the fullerene cage after the oxidation of Lu₃N @C₈₀ . Moreover, the oxidation has a major impact on the electrochemical behavior of Lu₃N @C₈₀ .     

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.     

Chemically adjusting plasma temperature, energy, and reactivity (CAPTEAR) method using NOx and combustion for selective synthesis of Sc3N@C80 metallic nitride fullerenes

Goals are (1) to selectively synthesize metallic nitride fullerenes (MNFs) in lieu of empty-cage fullerenes (e.g., C60, C70) without compromising MNF yield and (2) to test our hypothesis that MNFs possess a different set of optimal formation parameters than empty-cage fullerenes. In this work, we introduce a novel approach for the selective synthesis of metallic nitride fullerenes. This new method is "Chemically Adjusting Plasma Temperature, Energy, and Reactivity" (CAPTEAR). The CAPTEAR approach with copper nitrate hydrate uses NOx vapor from NOx generating solid reagents, air, and combustion to "tune" the temperature, energy, and reactivity of the plasma environment. The extent of temperature, energy, and reactive environment is stoichiometrically varied until optimal conditions for selective MNF synthesis are achieved. Analysis of soot extracts indicate that percentages of C60 and Sc3N@C80 are inversely related, whereas the percentages of C70 and higher empty-cage C2n fullerenes are largely unaffected. Hence, there may be a "competitive link" in the formation and mechanism of C60 and Sc3N@C80. Using this CAPTEAR method, purified MNFs (96% Sc3N@C80, 12 mg) have been obtained in soot extracts without a significant penalty in milligram yield when compared to control soot extracts (4% Sc3N@C80, 13 mg of Sc3N@C80). The CAPTEAR process with Cu(NO3)2.2.5H2O uses an exothermic nitrate moiety to suppress empty-cage fullerene formation, whereas Cu functions as a catalyst additive to offset the reactive plasma environment and boost the Sc3N@C80 MNF production.     

Synthesis and reactivity of N@C60O

The endohedral fullerene epoxide N@C60O was synthesised, isolated by High Performance Liquid Chromatography (HPLC), and characterised by Electron Spin Resonance (ESR). This nitrogen radical displays predominantly axial symmetry characteristics as expected for a monoadduct, evidenced by a zero-field splitting D parameter of 6.6 MHz and an E parameter of 0.5 MHz in powder at 77 K. Photo- and thermally-activated silencing of the nitrogen radical were observed, the latter showing the evolution of a new spin signal during heating at 100 degrees C. We suggest that loss of nitrogen spin is due to coupling with a radical formed by opening of the epoxide ring. This implies that the reaction of C60O with C60 in the solid state proceeds via a radical, rather than ionic, intermediate.     

Complexation of Sc3N@C80 endohedral fullerene with cyclic Zn-bisporphyrins: solid state and solution studies

We report the synthesis of two cyclic β-pyrrole unsubstituted meso-tetraphenyl bisporphyrins in which the porphyrin units are connected by two 2,3-hexadiynyl-1,6-dioxo or two hexyl-1,6-dioxo spacers, respectively. Both cyclic porphyrin dimers exist in solution as mixtures of two conformational isomers. In the solid state, the receptor with diynyl spacers forms a 1:1 complex with the icosahedral (I(h)) isomer of the trimetallic nitride endohedral fullerene Sc(3)N@C(80). In this complex the receptor adopts a scoop-shaped conformation having a dihedral angle of 87.25° between the two porphyrin planes. The hexyl spaced analogue, however, adopts a similar conformation upon encapsulation of one molecule of Sc(3)N@C(80) in a self-assembled dimeric capsule. The capsular complexes pack in columns and render the fullerene units completely isolated. In toluene solution, (1)H NMR experiments indicate that the endohedral fullerene Sc(3)N@C(80) is exclusively bound by the expanded isomer of both dimers. UV-vis and fluorescence titration experiments confirmed the existence of strong π-π interactions between the fullerene Sc(3)N@C(80) and the flexible bisporphyrin dimer with hexyl spacers. At micromolar concentration, the flexible receptor forms only a 1:1 complex with the endohedral fullerene with stability constant value of K(a) = 2.6 ± 0.3 × 10(5) M(-1).     

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.     

Gadolinium-based mixed-metal nitride clusterfullerenes Gd(x)Sc(3-x)N@C80 (x=1, 2).

The first gadolinium-based mixed-metal nitride clusterfullerenes Gd(x)Sc(3-x)N@C(80) (I) (1, x=2; 2, x=1) have been successfully synthesized by the reactive gas atmosphere method and isolated facilely by recycling high-performance liquid chromatography (HPLC). The sum yield of 1 and 2 is 30-40 times higher than that of Gd(3)N@C(80) (I). Moreover, an enhanced relative yield of 2 over the Sc(3)N@C(80) (I) is achieved under the optimized synthesis conditions. According to the UV/Vis/NIR spectroscopic characterization, 1 and 2 are both stable fullerenes with large optical band-gaps while 1 has higher similarity to Gd(3)N@C(80) (I) and 2 resembles Sc(3)N@C(80) (I). The vibrational structures of 1 and 2 are studied by Fourier-transform infrared (FTIR) spectroscopy as well as density functional theory (DFT) computations. In particular, the structures of the encaged Gd(x)Sc(3-x)N clusters within 1 and 2 are analyzed.     

The role of an asymmetric nitride cluster on a fullerene cage: the non-IPR endohedral DySc2N@C76

The first non-IPR C(76) cage based on a mixed metal nitride cluster, DySc(2)N@C(76), was successfully synthesized and isolated. DySc(2)N@C(76) is a stable fullerene with a small band gap of 0.96 eV. According to the FTIR spectroscopic study in combination with extensive DFT calculations, the cage structure of DySc(2)N@C(76) has been assigned to the non-IPR C(s): 17490-I isomer having two pairs of the adjacent pentagons. DySc(2)N@C(76)provides the first example of stabilization of the non-IPR C(76) cage by encapsulation of an asymmetric DySc(2)N mixed cluster, revealing the role of the cluster structure on the stability of the fullerene cage. As the asymmetric DySc(2)N cluster has a more suitable geometry for the inner space of the C(76) cage compared to that of the homogeneous clusters like Sc(3)N or Dy(3)N, the highest yield for C(76)-based cluster fullerenes with the Dy(x)Sc(3)-(x)N mixed nitride cluster is achieved for the DySc(2)N@C(76)     

Influence of cluster size on the structures and stability of trimetallic nitride fullerenes M3N@C80.

To provide insight into the influence of encaged clusters on the structures and stability of trimetallic nitride fullerenes (TNFs), extensive density functional theory calculations were performed on Sc3N@C80, Y3N@C80, and La3N@C80 as well as their encaged clusters. The calculated results demonstrated that both Sc3N and Y3N units are planar, whereas La3N units are pyramidal inside C80-I(h), and that both of the Y3N@C80 and La3N@C80 cages deform considerably in the planes of Y3 and La3. The calculated results suggest that M-cage attraction/repulsion and M-M repulsion interactions determine the geometries of these three complex molecules and the dynamics of the corresponding encaged clusters. These calculated findings distinctly reveal the influence of the size of the encaged clusters on the structures and stability of TNFs and may rationalize their significant differences in yields and chemical reactivity.     

Reactivity of metallic nitride endohedral metallofullerene anions: electrochemical synthesis of a Lu3N@Ih-C80 derivative

Electrochemically generated Lu(3)N@I(h)-C(80) dianions react with the electrophile, PhCHBr(2), to produce a methano derivative of Lu(3)N@I(h)-C(80)(CHC(6)H(5)) (1) with high regioselectivity. The compound was characterized by MALDI-TOF, NMR, and UV-vis-NIR absorption spectroscopy. Electrochemical characterization of this Lu(3)N@I(h)-C(80)(CHC(6)H(5)) derivative showed the typical irreversible reductive behavior of the pristine Lu(3)N@I(h)-C(80), similar to those observed for Bingel adducts of Lu(3)N@I(h)-C(80). Using the same conditions, the reaction between Sc(3)N@I(h)-C(80) dianions and PhCHBr(2) was conducted for comparison. Unexpectedly, no nucleophilic reaction was observed, indicating that Sc(3)N@I(h)-C(80) dianions are not reactive toward the electrophile. Theoretical studies for both dianionic Lu(3)N@I(h)-C(80) and Sc(3)N@I(h)-C(80) showed that the HOMO is more highly localized on the fullerene cage for [Lu(3)N@I(h)-C(80)](2-) and more localized on the inside cluster for [Sc(3)N@I(h)-C(80)](2-), providing an explanation for the drastically different reactivities observed.     

The change of the state of an endohedral fullerene by encapsulation into SWCNT: a Raman spectroelectrochemical study of Dy3N@C80 peapods

Raman and in situ Raman spectroelectrochemical studies of Dy3N@C80@SWCNT peapods have been carried out for the first time. The formation of peapods by the encapsulation of gaseous Dy3N@C80 has been confirmed by HR-TEM microscopy and by the successful transformation of Dy3N@C80@SWCNT into double-walled carbon nanotubes. The Raman spectra of the endohedral fullerene cluster changed dramatically in the interior of the single-walled carbon nanotube (SWCNT). The electrochemical charging of the peapod indicates a slight reversible attenuation of the Raman intensities of fullerene features during anodic doping. The results support the assignment of Raman bands to the Dy3N@C80 moiety inside a SWCNT.     

Characterization of the bis-silylated endofullerene Sc3N@C80

The photochemical reaction of Sc(3)N@C(80) with 1,1,2,2-tetramesityl-1,2-disilirane affords the adduct as a bis-silylated product. The adduct was characterized by NMR spectroscopy and single-crystal X-ray structure analysis. The dynamic behavior of the disilirane moiety and the encapsulated Sc(3)N cluster were also investigated. The unique redox property of the adduct is reported by means of CV and DPV. Experimental results were confirmed by density functional calculations.     

Gd3N@C2n (n = 40, 42, and 44): remarkably low HOMO-LUMO gap and unusual electrochemical reversibility of Gd3N@C88

High-performance liquid chromatography was used to isolate two new trimetallic nitride endohedral fullerenes, Gd3N@C2n (n = 42 and 44), and they were characterized by MALDI-TOF mass spectrometry, UV-vis-NIR, and cyclic voltammetry. It was found that their electronic HOMO-LUMO gaps depend pronouncedly on the size of the cage, from a large band gap for Gd3N@C80 (2.02 V) to a small band gap for Gd3N@C88 (1.49 V). The electrochemical properties also change dramatically with the size of the cage, going from irreversible for the C80 cage to reversible for Gd3N@C88. The latter is the largest trimetallic cluster inside C88 isolated and characterized to date. Gd3N@C88 has one of the lowest electrochemical energy gaps for a nonderivatized metallofullerene.