Lanthanum nitride endohedral fullerenes La3N@C2n (43 <or= n <or= 55): preferential formation of La3N@C96

While the trimetallic nitrides of Sc, Y and the lanthanides between Gd and Lu preferentially template C(80) cages, M(3)N@C(80), and while those of Ce, Pr and Nd preferentially template the C(88) cage, M(3)N@C(88), we show herein that the largest metallic nitride cluster, La(3)N, preferentially leads to the formation of La(3)N@C(96) and to a lesser extent the La(3)N@C(88). This is the first time that La(3)N is successfully encapsulated inside fullerene cages. La(3)N@C(2n) metallofullerenes were synthesized by arcing packed graphite rods in a modified Kr?tschmer-Huffman arc reactor, extracted from the collected soot and identified by mass spectroscopy. They were isolated and purified by high performance liquid chromatography (HPLC). Different arcing conditions were studied to maximize fullerene production, and results showed that yields have a high La(2)O(3)/C dependence. Relatively high yields were obtained when a 1:5 ratio was used. Three main fractions, La(3)N@C(88), La(3)N@C(92), and La(3)N@C(96), were characterized by UV/Vis-NIR and cyclic voltammetry. Unlike other trimetallic nitride metallofullerenes of the same carbon cage size, La(3)N@C(88) exhibits a higher HOMO-LUMO gap and irreversible reduction and oxidation steps.     

Electronic properties and 13C NMR structural study of Y3N@C88

In this paper, we report the synthesis, purification, (13)C NMR, and other characterization studies of Y(3)N@C(88). The (13)C NMR, UV-vis, and chromatographic data suggest an Y(3)N@C(88) having an IPR-allowed cage with D(2)(35)-C(88) symmetry. In earlier density functional theory (DFT) computational and X-ray crystallographic studies, it was reported that lanthanide (A(3)N)(6+) clusters are stabilized in D(2)(35)-C(88) symmetry cages and have reduced HOMO-LUMO gaps relative to other trimetallic nitride endohedral metallofullerene cage systems, for example, A(3)N@C(80). In this paper, we report that the nonlanthanide (Y(3)N)(6+) cluster in the D(2)(35)-C(88) cage exhibits a HOMO-LUMO gap consistent with other lanthanide A(3)N@C(88) molecules based on electrochemical measurements and DFT computational studies. These results suggest that the reduced HOMO-LUMO gap of A(3)N@C(88) systems is a property dominated by the D(2)(35)-C(88) carbon cage and not f-orbital lanthanide electronic metal cluster (A(3)N)(6+) orbital participation.     

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.     

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.     

The electronic and vibrational structure of endohedral Tm3N@C80 (I) fullerene--proof of an encaged Tm3+

The electronic and vibrational structure of the nitride clusterfullerene Tm3N@C80 (I) was investigated by cyclic voltammetry, FTIR, Raman, and X-ray photoemission spectroscopy. The electrochemical energy gap of Tm3N@C80 (I) is 1.99 V, which is 0.13 V larger than that of Sc3N@C80 (I). FTIR spectroscopy showed that the C80:7 (I(h)) cages in Tm3N@C80 (I), Er3N@C80 (I), Ho3N@C80 (I), Tb3N@C80 (I), Gd3N@C80 (I), and Y3N@C80 (I) have the same bond order. The analysis of low-energy Raman spectra points to two uniform force constants which can be used to describe the interaction between the encaged nitride cluster and the C80:7 (I(h)) cage in M3N@C80 (I) (M = Tm, Er, Ho, Tb, Gd, and Y). Because the M3N-C80 bond strength is strongly dependent on the charge of the metal ions, this is a direct hint for a 3+ formal valence state of the metal ions in these nitride clusterfullerene series, including Tm3N@C80 (I). Photoemission spectra of the Tm 4d core level and the Tm 4f valence electrons provided a direct proof for a (4f)12 electronic configuration of the encapsulated thulium. In conclusion, thulium in Tm3N@C80 (I) has a formal electronic ground state of +3, in contrast to the +2 state found in Tm@C82. It is demonstrated that the valence state of metal atoms encaged in fullerenes can be controlled by the chemical composition of the endohedral fullerene.     

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.     

Charge-induced reversible rearrangement of endohedral fullerenes: electrochemistry of tridysprosium nitride clusterfullerenes Dy3N@C2n (2n=78, 80)

The electrochemistry of three new clusterfullerenes Dy3N@C2n (2n=78, 80), namely two isomers of Dy3N@C80 (I and II) as well as Dy3N@C78 (II), have been studied systematically including their redox-reaction mechanism. The cyclic voltammogram of Dy3N@C80 (I) (Ih) exhibits two electrochemically irreversible but chemically reversible reduction steps and one reversible oxidation step. Such a redox pattern is quite different from that of Sc3N@C80 (I), and this can be understood by considering the difference in the charge transfer from the encaged cluster to the cage. A double-square reaction scheme is proposed to explain the observed redox-reaction behavior, which involves the charge-induced reversible rearrangement of the Dy3N@C80 (I) monoanion. The first oxidation potential of Dy3N@C80 (II) (D5h) has a negative shift of 290 mV relative to that of Dy3N@C80 (I) (Ih), indicating that lowering the molecular symmetry of the clusterfullerene cage results in a prominent increase in the electron-donating property. The first and second reduction potentials of Dy3N@C78 (II) are negatively shifted relative to those of Dy3N@C80 (I, II), pointing to the former's lowered electron-accepting ability. The significant difference in the electrochemical energy gaps of Dy3N@C80 (I), Dy3N@C80 (II), and Dy3N@C78 (II) is consistent with the difference in their optical energy gaps.     

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.     

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.     

Titanium/yttrium mixed metal nitride clusterfullerene TiY2N@C80: synthesis, isolation, and effect of the group-III metal

Titanium/yttrium mixed metal nitride clusterfullerene (MMNCF) TiY(2)N@C(80) has been successfully synthesized, representing the first Ti-containing non-scandium MMNCF. TiY(2)N@C(80) has been isolated by multistep HPLC and characterized by various spectroscopies in combination with DFT computations. The electronic absorption property of TiY(2)N@C(80) was characterized by UV-vis-NIR spectroscopy, indicating the resemblance to that of TiSc(2)N@C(80) with broad shoulder absorptions. The optical band gap of TiY(2)N@C(80) (1.39 eV) is very close to that of TiSc(2)N@C(80) (1.43 eV) but much smaller than that of Y(3)N@C(80)(I(h), 1.58 eV). Such a resemblance of the overall absorption feature of TiY(2)N@C(80) to TiSc(2)N@C(80) suggests that TiY(2)N@C(80) has a similar electronic configuration to that of TiSc(2)N@C(80), that is, (TiY(2)N)(6+)@C(80)(6-). FTIR spectroscopic study and DFT calculations accomplish the assignment of the C(80):I(h) isomer to the cage structure of TiY(2)N@C(80), with the C(1) conformer being the lowest energy structure, which is different from the C(s) conformer assigned to TiSc(2)N@C(80). The electrochemical properties of TiY(2)N@C(80) were investigated by cyclic voltammetry, revealing the reversible first oxidation and first reduction step with E(1/2) at 0.00 and -1.13 V, respectively, both of which are more negative than those of TiSc(2)N@C(80), while the electrochemical energy gap of TiY(2)N@C(80) (1.11 V) is almost the same as that of TiSc(2)N@C(80) (1.10 V). Contrary to the reversible first reduction step, the second and third reduction steps of TiY(2)N@C(80) are irreversible, and this redox behavior is dramatically different from that of TiSc(2)N@C(80), which shows three reversible reduction steps, indicating the strong influence of the encaged group-III metal (Y or Sc) on the electronic properties of TiM(2)N@C(80) (M = Y, Sc).     

Preparation and crystallographic characterization of a new endohedral,Lu3N@C80.5 (o-xylene), and comparison with Sc3N@C80.5 (o-xylene)

The trimetallic nitride template (TNT) approach has been successfully utilized to prepare the new endohedral Lu(3)N@C(80). Well-ordered crystals of Lu(3)N@C(80).5 (o-xylene) and Sc(3)N@C(80).5 (o-xylene) form upon cooling of o-xylene solutions of these endohedrals and they are isomorphous. Although the positions of the fullerene cage (which is fully ordered and located at a crystallographic center of symmetry) and the o-xylene molecules are nearly identical in these two structures, the positioning of the metal ions in the two crystals differ in significant ways. However, the expected difference in sizes of lutetium and scandium does not affect the dimensions of the C(80) cage. Nevertheless, the positions of the metal atoms do produce a slight outward dislocation of the immediately adjacent carbon atoms.     

New egg-shaped fullerenes: non-isolated pentagon structures of Tm3N@C(s)(51 365)-C84 and Gd3N@C(s)(51 365)-C84

Although there are 51 568 non-IPR and 24 IPR structures for C84, the egg-shaped endohedral fullerenes Tm3N@C(s)(51 365)-C84 and Gd3N@C(s)(51 365)-C84 utilize the same non-IPR cage structure as found initially for Tb3N@C(s)(51 365)-C84.     

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.     

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.     

Isolation and structural characterization of a family of endohedral fullerenes including the large, chiral cage fullerenes Tb(3)N@C(88) and Tb(3)N@C(86) as well as the I(h) and D(5)(h) isomers of Tb(3)N@C(80)

The recent finding that isomer 2 of Tb(3)N@C(84) uses one of the 51,568 possible nonisolated pentagon rule (non-IPR) structures for the C(84) cage rather than one of the 24 cage isomers that do obey the IPR suggests that further experimental work on the structure of larger endohedrals is needed to observe the utility of the IPR rule in this uncharted territory. The structures of the newly synthesized endohedral fullerenes--Tb(3)N@C(88), Tb(3)N@C(86), and the Ih and D(5)(h) isomers of Tb(3)N@C(80)--have been determined by single-crystal X-ray diffraction on samples cocrystallized with NiII(octaethylporphyrin). In contrast to the situation for isomer 2 of Tb(3)N@C(84), the structures of Tb(3)N@C(88) and Tb(3)N@C(86) do conform to the IPR. Both Tb(3)N@C(88) and Tb(3)N@C(86) have chiral structures with D(2) symmetry for Tb(3)N@C(880 and D(3) symmetry for Tb(3)N@C(86). Within this group of endohedrals, the size of the carbon cage affects the Tb-N and Tb-C distances, the orientations of the carbon cage with respect to the porphyrin plane, the locations of the metal ions and their orientations relative to the porphyrin plane, and the degree of pyramidalization of the Tb(3)N unit.     

Expanding the world of endohedral fullerenes--the Tm3N@C2n (39< or =n< or =43) clusterfullerene family

A new family of endohedral fullerenes, based on an encaged trithulium nitride (Tm(3)N) cluster, was synthesised, isolated and characterised by HPLC, mass spectrometry, and visible-NIR and FTIR spectroscopy. Tm(3)N clusterfullerenes with cages as small as C(76) and as large as C(88) were prepared and six of them were isolated. Tm(3)N@C(78) is a small clusterfullerene. The two isomers of Tm(3)N@C(80) (I and II) were the most abundant structures in the fullerene soot. Tm(3)N@C(82), Tm(3)N@C(84), and Tm(3)N@C(86) represent a new series of higher clusterfullerenes. All six isolated Tm(3)N clusterfullerenes were classified as large energy-gap structures with optical energy gaps between approximately 1.2 and approximately 1.75 eV. Tm(3)N@C(80) (I) and Tm(3)N@C(80) (II) were assigned to the C(80) cages C(80):7 (I(h)) and C(80):6 (D(5h)). For Tm(3)N@C(78), the analysis pointed to an elliptical carbon cage with C(78):1 (D(3)) or C(78):4 (D(3h)) being the probable structures.     

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.     

Poly(perfluoroalkylation) of metallic nitride fullerenes reveals addition-pattern guidelines: synthesis and characterization of a family of Sc3N@C80(CF3)n (n = 2-16) and their radical anions

A family of highly stable (poly)perfluoroalkylated metallic nitride cluster fullerenes was prepared in high-temperature reactions and characterized by spectroscopic (MS, (19)F NMR, UV-vis/NIR, ESR), structural and electrochemical methods. For two new compounds, Sc(3)N@C(80)(CF(3))(10) and Sc(3)N@C(80)(CF(3))(12,) single crystal X-ray structures are determined. Addition pattern guidelines for endohedral fullerene derivatives with bulky functional groups are formulated as a result of experimental ((19)F NMR spectroscopy and single crystal X-ray diffraction) studies and exhaustive quantum chemical calculations of the structures of Sc(3)N@C(80)(CF(3))(n) (n = 2-16). Electrochemical studies revealed that Sc(3)N@C(80)(CF(3))(n) derivatives are easier to reduce than Sc(3)N@C(80), the shift of E(1/2) potentials ranging from +0.11 V (n = 2) to +0.42 V (n = 10). Stable radical anions of Sc(3)N@C(80)(CF(3))(n) were generated in solution and characterized by ESR spectroscopy, revealing their (45)Sc hyperfine structure. Facile further functionalizations via cycloadditions or radical additions were achieved for trifluoromethylated Sc(3)N@C(80) making them attractive versatile platforms for the design of molecular and supramolecular materials of fundamental and practical importance.     

A large family of dysprosium-based trimetallic nitride endohedral fullerenes: Dy3N@C2n (39 </= n </= 44)

Dysprosium-based trimetallic nitride endohedral fullerenes (clusterfullerenes)-the Dy(3)N@C(2)(n) (38 相似文献   

Preparation and structure of CeSc2N@C80: an icosahedral carbon cage enclosing an acentric CeSc2N unit with buried f electron spin

Herein, we report the preparation, purification, and characterization of a mixed trimetallic nitride endohedral metallofullerene, CeSc(2)N@C(80). Single-crystal X-ray diffraction shows that CeSc(2)N@C(80) consists of a four-atom asymmetric top (CeSc(2)N) inside a C(80) (I(h)()) carbon cage. Unlike the situation in most endohedrals of the M(3)N@C(2)(n)() type, the nitride ion is not located at the center of the carbon cage but is offset by 0.36 A in order to accommodate the large Ce(III) ion. The cage carbon atoms near the endohedral Ce and Sc atoms exhibit significantly larger pyramidal angles than the other carbon atoms on the C(80) cage. Surprisingly, at ambient temperature, the (13)C NMR spectrum exhibits isotropic motional averaging yielding only two signals (3 to 1 intensity ratio) for the icosahedral C(80) cage carbons. At the same temperature, the (45)Sc NMR exhibits a relatively narrow, symmetric signal (2700 Hz) with a small temperature-dependent Curie shift. A rotation energy barrier (E(a) = 79 meV) was derived from the (45)Sc NMR line-width analysis. Finally, the XPS spectrum for CeSc(2)N@C(80) confirms a +3 oxidation state for cerium, Ce(3+)(4f(1)5d(0)).This oxidation state and the Curie shift are consistent with a weakly paramagnetic system with a single buried f electron spin.