The Encapsulation of Trimetallic Nitride Clusters in Fullerene Cages

The Encapsulation of Trimetallic Nitride Clusters in Fullerene Cages H. C. Dorn,1 S. Stevenson,' J. Craft,1 F. Cromer,1 J. Duchamp,1 G. Rice,1 T. Glass,' K. Harich,1 P. W. Fowler,2 T. Heine,3 E. Hajdu,4 R. Bible,4 M. M. Olmstead,5 K. Maitra,5 A. J. Fisher5 and A. L. Balch5 I) Department of Chemistry, Virginia Tech, Blacksburg, VA 24061 2) School of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4 QD UK 3) Dipartimento di Chemica 'G. Ciamician', Universita di Bologna, via Selmi 2, Bologna 1-40126, Italy 4) Searle, 4901 Searle Parkway, Skokie, II 60077 5) Department of Chemistry, University of California, Davis, California 95616, USA

Abstract: The Kratschmer-Huffman electric-arc generator typically produces endohedral metallorullerenes in low yields with a wide array of different products, but the introduction of nitrogen leads to a new family of encapsulates. A family of endohedral metallorullerenes AnB3. nN@C2n (n=0-3, x=34, 39, and 40) where A and B are Group III and rare-earth metals is formed by a trimetallic nitride template (TNT) process in relatively high yields. The archetypal representative of this new class is the stable endohedral metallofullerene, Sc3N@C80 containing a triscandium nitride cluster encapsulated in an icosahedron (Ih), C80 cage. The Sc3N@C80 is formed in yields even exceeding empty-cage C84. Other prominent scandium TNT members are Sc3N@C68 and Sc3N@C78. The former Sc3N@C68 molecule represents an exception to the well known isolated pentagon rule (IPR). These new molecules were purified by chromatography with corresponding characterization by various spectroscopic approaches. In this paper we focus on the characterization and properties of this fascinating new class of materials.

1. Introduction For the empty C80-Ih cage, computational results1"3 suggest significant stabilization upon donation of 6 electrons, (C80)6' and experimental evidence supports an icosahedral cage for the La2@C80 endohedral.3 Recently, we reported the first examples of a new family of stable metal (A,B) endohedral metallofullerenes, A3_ 4 nBnN@C80 (n=0-3) that are stabilized by donation of 6 electrons to the C80-Ih cage. The endohedral nature of Sc3N@C80 was confirmed via a single crystal X-ray diffraction study of (Sc3N@C80)« Con(OEP> 1.5 chloroform-0.5 benzene.4 The Sc3N@C80 molecule is in close proximity but not covalently bound to the Con(OEP) molecule, which makes face-to-face contact with another Con(OEP) moiety. The N-Sc distance and the closest Sc-C bond distance are 0.198±0.002 and, 0.220±0.002 nm, respectively. In the solid, the scandium ions face three pentagons within the C80 cage.

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2. A3.nScnN@C80 (n=0-3) Group III Endohedral Metallofullerenes The A3.nScn@C80 family members were prepared utilizing the TNT approach (presence of nitrogen) with a typical example illustrated, Y3N@C80 (Fig. la). The facile formation of scandium and yttrium TNT members, allows competitive formation conditions for comparisons with other Group III and rare-earth elements. Cored graphite rods were packed with A2O3 and Sc2O3 (constant A/Sc, 3/2% atomic ratio), powdered graphite mixture, and cobalt oxide. As previously reported for nanotube production, low levels of cobalt (and nickel) enhance nanotube formation.5'6 The rods were subsequently vaporized in a Kratschmer-Huffman generator (He/N2 mixture).7 We have observed similar enhancements of both the TNT and non-TNT endohedral metallofullerenes by factors of 3-6 relative to empty-cage fullerenes with the inclusion of low levels (100-180 mg) of cobalt oxide. The soot obtained from the generator was extracted with carbon disulfide and the soluble fraction (fullerenes and endohedral metallofullerenes) was analyzed by negative-ion mass spectrometry.

Figure 1 The NI-DCI mass spectra for "mixed" Group III and scandium, A3.nScnN@C80 TNT members are shown in Fig. 2. This data clearly confirms the higher yield advantage for A3N cluster formation of Group III (relative to the non-TNT members and emptycages) by the TNT process. As illustrated in Fig. 2, the yield enhancement for

Sc3N@C80 is at least an order of magnitude greater than the usually prominent nonTNT Sc2@C84 formed under non-TNT conditions (absence of N2). The high yields suggest favorable spatial and/or bonding interactions for the YSc2N and Y2ScN clusters inside the cage. Although all members of the AnSc3.nN@C80, (n=0-3) Group III family are observable in the soluble fraction, La3N@C80 is formed at a very low level (-10% of Sc2@C84). The mixed lanthanum TNT members LaSc2N@C80 and La2ScN@C80 are also formed in relatively lower quantities. The lower yields for the LanSc3.nN@C80 family members are consistent with the significant increase in the ionic radii for La (0.1045 nm) relative to Y (0.0900 nm) and Sc (0.0745 nm). For all Group III examples, we have assumed a trivalent state for each encapsulated metal atom (La, Y, and Sc). This suggests a contribution of one electron (per metal atom) for bonding to the central nitrogen atom and two electrons for cage stabilization, (Cm)6~vide supra.

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a)

Sc3N@C80

b) C

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LaSc2N@C8Q

Sc3N@C80 s ,

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11

.t.|iJl»^.iV'lt^| l-

m/z Figure 2 NI-DCI mass spectra for soluble extract: a) graphite rods packed with Sc2O3; b) graphite rods packed with 3/2% Y2O3/Sc2O3; c) graphite rods packed with 3/2% In summary, Group III and rare-earth trimetallic nitride template (TNT) formation for the A3N cluster in the icosahedral C80 cage is consistent with two chief factors: 1) an optimum A3N cluster size, and 2) a requisite trivalent character of the encapsulated metal ions. The high stability of these new TNT members will allow isolation of numerous purified samples in the near future.

3. Sc3N@C68: A Violation of the Isolated Pentagon Rule One of the sacrosanct rules in the evolving field of fullerenes, nanotubes, and endohedral metallofullerenes has been the isolated-pentagon rule (IPR).1'8"9 Although exceptions have been predicted,10"12 there are no well characterized violations of this rule. Most endohedral metallofullerenes isolated and characterized to date13"15 have carbon cages of seventy or more carbon atoms (e.g., La2@C72, Sc2@C74, Er2@C82, Sc2@C84, Sc3@C82, Sc3N@C80) with IPR allowed structures. Of all possible carbon cages with less than seventy atoms, only the well recognized C60-Ih can satisfies the isolated pentagon rule. Scandium encapsulates, Sc3N@C68, Sc3N@C78, and Sc3N@C80

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are formed in a Kratschmer-Huffinan generator by the trimetallic nitride template (TNT) process (in the presence of nitrogen) in soluble extract yields of- 1%, 1%, and 10%, respectively. We observe enhanced production of three members of the series by the trimetallic nitride template (TNT) process in a dynamic mixture of nitrogen and helium gas (20/300 ml/min) into the Kratschmer-Huffinan apparatus.7 Besides Sc3N@C80, the other homologues, Sc3N@C68 and Sc3N@C78 are produced in slightly higher abundance than the most prominent non-TNT Sc2@C84, the endohedral metallofullerene usually formed in larger amounts in scandium soots under non-TNT conditions. After the usual chromatographic isolation protocol, the purity of Sc3N@C68 and Sc3N@C78 was established by negative-ion mass spectrometry. The 45Sc NMR spectrum for Sc3N@C68 exhibits a single symmetric peak in carbon disulfide at 296 K with a 45Sc NMR linewidth of-5600 Hz. This linewidth is slightly greater than the previously reported value for Sc3N@C80 and the chemical shift corresponds to considerably greater shielding (92.5 ppm relative to external ScCl3).4 On the 45Sc NMR timescale, the results for Sc3N@C68 suggests that the three Sc atoms are equivalent at this temperature, which is consistent with three-fold symmetry. The Sc3N@C68 species represents a clear departure from previous endohedral metallofullerenes, and encapsulation of a four-atom molecular cluster in a relatively small carbon cage of only sixty-eight carbon atoms gives rise to a clear violation of the isolated-pentagon rule (IPR). For fullerenes smaller than C70only C^has a cage with a classic IPR allowed icosahedral cage. For a C68 cage, the spiral algorithm1 finds 6332 distinct fullerenes. In neutral fullerene cages it is well established that each fused pentagon pair carries an energy penalty of 70-90 kJ/mole.16 The qualitative preference for low-Np isomers (Np=number of fused pentagons) is confirmed by model DFTB calculations17 that treat the cage as an empty fullerene capable of accepting electrons from a central reservoir. A highly symmetric structure (D3, Np=3) is proposed for Sc3N@C68 that is consistent with this computational approach.

4. Chromatographic Retention Behavior of Sc3N@C68, Sc3N@C78, and Sc3N@C80 The characteristic colors bluish-purple, green, and reddish-orange for carbon disulfide solutions of Sc3N@C68, Sc3N@C78, and Sc3N@C80, respectively, illustrate changes in the electronic structure as a function of carbon cage differences (C68, C78, and C80). To date, we have found no chromatographic evidence for additional isomers of Sc3N@C68, Sc3N@C78, and Sc3N@C80, but the chromatographic behavior of this TNT family still provides information regarding the charge distribution and polarity of these three species. It has been previously established that less polar chromatographic stationary phases, such as PBB (pentabromobenzyl) generally exhibit weaker intermolecular interactions and give chromatographic retention times proportional to the polarizability/ 7t-electron count of the fullerene cage.4'18 We find that Sc3N@C68, Sc3N@C78, and Sc3N@C80 when injected onto a PBB chromatographic column (carbon disulfide solvent) have elution times corresponding to empty cages C74-C75,

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C82-C83, and C84-C85, respectively, (Fig. 3). This suggests significant transfer of n electron population to the cage surface, although possibly attenuated by the presence of the central electronegative nitrogen atom in the encapsulated cluster. 0.30.20.1-

Sc3N@C8o Sc3N@C78

Sc3N@C68

-0.2: -0.3-0.4-0.5

60

70

80

90

100

carbon cage number (C2n) Figure 3 Chromatographic retention data for TNT members and empty-cage fullerenes PBB/CS2, solvent (capacity factor, k'=tr-t0/t0) versus carbon cage number.

5. Air Oxidation Study of Sc3N@C80 A black film of Sc3N@C80 sample was evaporated from a carbon disulfide solution directly onto a Au pad. The corresponding photoelectron spectroscopy (XPS) spectrum is shown in Fig. 4b. The observed spectrum suggests an absorption centered at ~400.9 eV for the 2p3/2 core level and a second peak due to spin-orbital coupling 4.8 eV higher energy than the 2p3/2 peak. Although a much smaller nitrogen peak is observed at 396.4 eV, the scandium and nitrogen XPS signal areas (corrected for relative sensitivities) provide good agreement for a 3 to 1 ratio of atoms for the Sc3N cluster. In addition, the binding energy for the nitrogen peak (396.4 eV) compares favorably with the value reported for scandium nitride,19 396.2 eV, as well as the 2p3/2 core level Sc peak, 400.7 eV. The binding energy for the nitrogen in Sc3N@C80 (396.4 eV) also disfavors a structure of an encapsulated Sc3N cluster with a rapidly inverting electron lone pair located on a sp3 hybridized nitrogen atom. Takahashi and coworkers20 have reported XPS results for Sc2@C84 with a 2p3/2 peak at -401 eV that we have also repeated for reference purposes (Fig. 4a) illustrating a typical scandium endohedral metallofullerene (without encapsulated nitrogen).

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The black Sc3N@C80 film prepared for the XPS experiments described in Fig. 4 was slowly heated in air. At a temperature of 663-673 K, the black film was converted to a white crystalline film and the subsequent XPS spectrum (Fig. 4c) for the scandium 2p3/2 core level shifted to 402.9 eV with the disappearance of the nitrogen peak. In addition, the carbon Is peak centered at 285.2 eV was greatly attenuated relative to the corresponding carbon peak before sample heating. These results suggest nearly complete conversion of the Sc3N@C80 film into scandium oxide, Sc2O3. A SEM image of the scandium oxide film (Fig. 4d) indicates crystals with dimensions in the range of 0.1 - 0.6 jum. Elemental analysis of the crystals by energy dispersive X-ray spectroscopy indicates only the presence of scandium and oxygen with significantly lower levels of carbon. 6. Conclusions The results of this study illustrate a wide range of new TNT endohedral metallofullerenes can be prepared in relatively high yields and purity . The limited physical and chemical properties suggest a wide range of applications in the emerging nano-science field. The unique structural, chemical, and reactivity features for these new encapsulates will clearly provide new directions in host-guest chemistry. 7. Acknowledgments AIB thanks the National Science Foundation and HCD thanks LUNA Innovations for supporting phases of this study. HCD also acknowledge technical support from P. Phillips. PWF and TH acknowledge support from contract 'FMRX CT96 0126 USEFULL' under the European Union TMR Network scheme. a)

Sc3N@Cgo

Sc2@C84 (D2d)

407 4

401.0

-

/

\

(after heat

Figure 4 XPS of: a) Sc2@C84 (D2d) film; b) Sc3N@C80 film; c) Sc3N@C80 film after heat treatment 673K; and d) SEM of Sc3N@C80 after heat treatment.

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