The Encapsulation of Trimetallic NitrideClusters in Fullerene Cages

H. C. Dorn,1 S. Stevenson,' J. Craft,1 F. Cromer,1 J. Duchamp,1 G. Rice,1T. Glass,' K. Harich,1 P. W. Fowler,2 T. Heine,3 E. Hajdu,4 R. Bible,4M. M. Olmstead,5 K. Maitra,5 A. J. Fisher5 and A. L. Balch5

I) Department of Chemistry, Virginia Tech, Blacksburg, VA 240612) 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, Italy4) 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 endohedralmetallorullerenes in low yields with a wide array of different products, but the introduction ofnitrogen 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 atrimetallic nitride template (TNT) process in relatively high yields. The archetypal representative ofthis new class is the stable endohedral metallofullerene, Sc3N@C80 containing a triscandium nitridecluster encapsulated in an icosahedron (Ih), C80 cage. The Sc3N@C80 is formed in yields evenexceeding 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 characterizationby various spectroscopic approaches. In this paper we focus on the characterization and properties ofthis fascinating new class of materials.

1. Introduction

For the empty C80-Ih cage, computational results1"3 suggest significant stabilizationupon donation of 6 electrons, (C80)6' and experimental evidence supports anicosahedral cage for the La2@C80 endohedral.3 Recently, we reported the firstexamples of a new family of stable metal (A,B) endohedral metallofullerenes, A3_nBnN@C80 (n=0-3) that are stabilized by donation of 6 electrons to the C80-Ih cage.4The endohedral nature of Sc3N@C80 was confirmed via a single crystal X-raydiffraction study of (Sc3N@C80)« Con(OEP> 1.5 chloroform-0.5 benzene.4 TheSc3N@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-Scdistance 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.

CP544, Electronic Properties of Novel Materials—Molecular Nanostructures, edited by H. Kuzmany, et al.© 2000 American Institute of Physics 1-56396-973-4/007$ 17.00

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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). Thefacile formation of scandium and yttrium TNT members, allows competitiveformation 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% atomicratio), powdered graphite mixture, and cobalt oxide. As previously reported fornanotube production, low levels of cobalt (and nickel) enhance nanotube formation.5'6The rods were subsequently vaporized in a Kratschmer-Huffman generator (He/N2mixture).7 We have observed similar enhancements of both the TNT and non-TNTendohedral metallofullerenes by factors of 3-6 relative to empty-cage fullerenes withthe inclusion of low levels (100-180 mg) of cobalt oxide. The soot obtained from thegenerator was extracted with carbon disulfide and the soluble fraction (fullerenes andendohedral metallofullerenes) was analyzed by negative-ion mass spectrometry.

Figure 1

The NI-DCI mass spectra for "mixed" Group III and scandium, A3.nScnN@C80 TNTmembers are shown in Fig. 2. This data clearly confirms the higher yield advantage forA3N cluster formation of Group III (relative to the non-TNT members and empty-cages) by the TNT process. As illustrated in Fig. 2, the yield enhancement forSc3N@C80 is at least an order of magnitude greater than the usually prominent non-TNT Sc2@C84 formed under non-TNT conditions (absence of N2). The high yieldssuggest favorable spatial and/or bonding interactions for the YSc2N and Y2ScNclusters inside the cage. Although all members of the AnSc3.nN@C80, (n=0-3) GroupIII family are observable in the soluble fraction, La3N@C80 is formed at a very lowlevel (-10% of Sc2@C84). The mixed lanthanum TNT members LaSc2N@C80 andLa2ScN@C80 are also formed in relatively lower quantities. The lower yields for theLanSc3.nN@C80 family members are consistent with the significant increase in the ionicradii for La (0.1045 nm) relative to Y (0.0900 nm) and Sc (0.0745 nm). For all GroupIII 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 bondingto the central nitrogen atom and two electrons for cage stabilization, (Cm)6~vide supra.

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a) Sc3N@C80

b)

C84C96

Sc3N@C80

JlJwtU

Y2ScN@C80

Y3N@C80

Sc3N@C80s,

.t.|iJl»̂ .iV'lt̂ |11l-

LaSc2N@C8Q

La2ScN@C8Q

m/z

Figure 2NI-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) formationfor 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 encapsulatedmetal ions. The high stability of these new TNT members will allow isolation ofnumerous 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, andendohedral metallofullerenes has been the isolated-pentagon rule (IPR).1'8"9 Althoughexceptions have been predicted,10"12 there are no well characterized violations of thisrule. Most endohedral metallofullerenes isolated and characterized to date13"15 havecarbon 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 carboncages with less than seventy atoms, only the well recognized C60-Ih can satisfies theisolated 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 theseries by the trimetallic nitride template (TNT) process in a dynamic mixture ofnitrogen and helium gas (20/300 ml/min) into the Kratschmer-Huffinan apparatus.7Besides Sc3N@C80, the other homologues, Sc3N@C68 and Sc3N@C78 are produced inslightly higher abundance than the most prominent non-TNT Sc2@C84, the endohedralmetallofullerene usually formed in larger amounts in scandium soots under non-TNTconditions. After the usual chromatographic isolation protocol, the purity ofSc3N@C68 and Sc3N@C78 was established by negative-ion mass spectrometry.

The 45Sc NMR spectrum for Sc3N@C68 exhibits a single symmetric peak in carbondisulfide at 296 K with a 45Sc NMR linewidth of-5600 Hz. This linewidth is slightlygreater than the previously reported value for Sc3N@C80 and the chemical shiftcorresponds to considerably greater shielding (92.5 ppm relative to external ScCl3).4On the 45Sc NMR timescale, the results for Sc3N@C68 suggests that the three Sc atomsare equivalent at this temperature, which is consistent with three-fold symmetry. TheSc3N@C68 species represents a clear departure from previous endohedralmetallofullerenes, and encapsulation of a four-atom molecular cluster in a relativelysmall carbon cage of only sixty-eight carbon atoms gives rise to a clear violation of theisolated-pentagon rule (IPR). For fullerenes smaller than C70only C^has a cage with aclassic IPR allowed icosahedral cage. For a C68 cage, the spiral algorithm1 finds 6332distinct fullerenes. In neutral fullerene cages it is well established that each fusedpentagon pair carries an energy penalty of 70-90 kJ/mole.16 The qualitative preferencefor low-Np isomers (Np=number of fused pentagons) is confirmed by model DFTBcalculations17 that treat the cage as an empty fullerene capable of accepting electronsfrom a central reservoir. A highly symmetric structure (D3, Np=3) is proposed forSc3N@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 carbondisulfide solutions of Sc3N@C68, Sc3N@C78, and Sc3N@C80, respectively, illustratechanges 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 isomersof Sc3N@C68, Sc3N@C78, and Sc3N@C80, but the chromatographic behavior of thisTNT family still provides information regarding the charge distribution and polarity ofthese three species. It has been previously established that less polar chromatographicstationary phases, such as PBB (pentabromobenzyl) generally exhibit weakerintermolecular interactions and give chromatographic retention times proportional tothe 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 nelectron population to the cage surface, although possibly attenuated by the presenceof the central electronegative nitrogen atom in the encapsulated cluster.

0.3-

0.2-

0.1-

-0.2 :

-0.3-

-0.4-

-0.5

Sc3N@C8oSc3N@C78

Sc3N@C68

60 70 80 90 100

carbon cage number (C2n)

Figure 3Chromatographic retention data for TNT members and empty-cage

fullerenes PBB/CS2, solvent (capacity factor, k'=tr-t0/t0) versuscarbon cage number.

5. Air Oxidation Study of Sc3N@C80

A black film of Sc3N@C80 sample was evaporated from a carbon disulfide solutiondirectly onto a Au pad. The corresponding photoelectron spectroscopy (XPS) spectrumis shown in Fig. 4b. The observed spectrum suggests an absorption centered at ~400.9eV for the 2p3/2 core level and a second peak due to spin-orbital coupling 4.8 eV higherenergy than the 2p3/2 peak. Although a much smaller nitrogen peak is observed at396.4 eV, the scandium and nitrogen XPS signal areas (corrected for relativesensitivities) 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 favorablywith the value reported for scandium nitride,19 396.2 eV, as well as the 2p3/2 core levelSc peak, 400.7 eV. The binding energy for the nitrogen in Sc3N@C80 (396.4 eV) alsodisfavors a structure of an encapsulated Sc3N cluster with a rapidly inverting electronlone pair located on a sp3 hybridized nitrogen atom. Takahashi and coworkers20 havereported XPS results for Sc2@C84 with a 2p3/2 peak at -401 eV that we have alsorepeated for reference purposes (Fig. 4a) illustrating a typical scandium endohedralmetallofullerene (without encapsulated nitrogen).

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The black Sc3N@C80 film prepared for the XPS experiments described in Fig. 4 wasslowly heated in air. At a temperature of 663-673 K, the black film was converted to awhite crystalline film and the subsequent XPS spectrum (Fig. 4c) for the scandium2p3/2 core level shifted to 402.9 eV with the disappearance of the nitrogen peak. Inaddition, the carbon Is peak centered at 285.2 eV was greatly attenuated relative to thecorresponding carbon peak before sample heating. These results suggest nearlycomplete conversion of the Sc3N@C80 film into scandium oxide, Sc2O3. A SEM imageof the scandium oxide film (Fig. 4d) indicates crystals with dimensions in the range of0.1 - 0.6 jum. Elemental analysis of the crystals by energy dispersive X-rayspectroscopy indicates only the presence of scandium and oxygen with significantlylower levels of carbon.

6. Conclusions

The results of this study illustrate a wide range of new TNT endohedralmetallofullerenes can be prepared in relatively high yields and purity . The limitedphysical and chemical properties suggest a wide range of applications in the emergingnano-science field. The unique structural, chemical, and reactivity features for thesenew encapsulates will clearly provide new directions in host-guest chemistry.

7. Acknowledgments

AIB thanks the National Science Foundation and HCD thanks LUNA Innovationsfor supporting phases of this study. HCD also acknowledge technical support from P.Phillips. PWF and TH acknowledge support from contract 'FMRX CT96 0126USEFULL' under the European Union TMR Network scheme.

a) Sc3N@Cgo

Sc2@C84 (D2d) 401.0 407-4 / \ (after heat

Figure 4XPS 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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