Unconventional cage structures of endohedral metallofullerenes1

Journal of Molecular Structure ŽTheochem. 461᎐462 Ž1999. 97᎐104

Unconventional cage structures of endohedral metallofullerenes 夽 Shigeru Nagase a,U , Kaoru Kobayashi a , Takeshi Akasakab a

Department of Chemistry, Graduate School of Science, Tokyo Metropolitan Uni¨ ersity, Hachioji, Tokyo 192-0397, Japan b Graduate School of Science and Technology, Niigata Uni¨ ersity, Niigata 950-2181, Japan Received 29 May 1998; accepted 2 July 1998

Abstract New structures are reported for endohedral metallofullerenes. Through theoretical calculations of Ca@C 72 , Ca@C 74 , and Sc 2 @C 74 , it is found that unconventional cage structures are significantly stabilized by endohedral metal-doping, which violate the isolated pentagon rule or contain heptagonal rings. It is expected that these unconventional structures will extend and enrich the research area of endohedral metallofullerenes. Also reported is a new endohedral metallofullerene La 2 @C 79 N with a heteroatom incorporated in the cage. 䊚 1999 Elsevier Science B.V. All rights reserved. Keywords: Endohedral metallofullerene; Violation of the isolated pentagon rule; Heptagon-containing structure; Metal-doped heterofullerene

1. Introduction Since the first success in the extraction of La@C 82 in 1991 w1x, endohedral metallofullerenes Žendohedrally metal-doped fullerenes . have attracted special interest as new promising molecules for material and catalytic applications. Recent important progress has been marked by the isolation and purification in macroscopic quantities; the interesting electronic properties 夽 Dedicated to Professor Keiji Morokuma in celebration of his 65th birthday. U Corresponding author. Tel.: q81 426 772554; fax: q81 426 772525; e-mail: [email protected]

and reactivities have been extensively investigated from both theoretical and experimental points of view, as shown in a recent review w2x. However, the structures are still elusive and almost unknown. This determination is currently of primary interest since it is an important clue in disclosing the formation mechanism and developing new routes to bulk production. For empty fullerenes, only the isomers with pentagons and hexagons, which satisfy the isolated pentagon rule ŽIPR. w3,4x, have been isolated and experimentally characterized w5x. Therefore, it has been often assumed that metals are encapsulated inside the most stable or abundant fullerene isomers. Very recently, the structures and symmetry of

0166-1280r99r$ - see front matter 䊚 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 1 2 8 0 Ž 9 8 . 0 0 4 6 5 - 5

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representative metallofullerenes such as Ca@C 82 , Sc@C 82 , Sc 2 @C 84 , and La 2 @C 80 have been disclosed through a close interplay between theoretical prediction and experiment, as summarized in reviews w6,7x. In all these cases, the cage structures satisfy IPR and contain only pentagons and hexagons, as in the case of empty fullerenes. However, the cage structures do not necessarily coincide with those of the most stable fullerene isomers of C n . For example, in Sc@C 82 the Sc atom is encapsulated inside the highly unstable C 2v isomer of C 82 w8,9x while in La 2 @C 80 two La atoms are encapsulated inside the most unstable I h isomer of C 80 w10x. This is because electron transfer from metals to C n changes drastically the relative stability of the isomers. It is an important question whether metals can stabilize unconventional cage structures during growth and annealing processes. We here report possible violation of IPR and the appearance of heptagon-containing structures through theoretical calculations of Ca@C 72 , Ca@C 74 , and Sc 2 @C 74 . We also report an unconventional metallofullerene La 2 @C 79 N in which a cage carbon is substituted with a heteroatom.

3. Results and discussion 3.1. Ca@C7 2 For the C 72 fullerene, there is only one isomer of D6d symmetry which satisfies IPR w20x. The endohedral structure Ža. of Ca@C 72 optimized by placing a Ca atom inside this IPR-satisfying D6d isomer is shown in Fig. 1, which has C s symmetry. For the present purpose, a total of 431 240 closed cage structures composed of pentagons, hexagons, and one heptagon was generated for C 72 by using the GSW program w21x; among these, 11 190 structures correspond to conventional fullerenes consisting of only pentagons and hexagons. By searching a huge number of generated structures, we have succeeded in finding that there are much more stable endohedral structures w22x. As shown in Fig. 1, structures b ŽC 2 . and c ŽC 2v . contain a pair of adjacent pentagons. Neverthe-

2. Computational methods Geometries were fully optimized at the Hartree᎐Fock ŽHF. level. The effective core potentials and basis sets of Hay and Wadt w11x were used for Ca, Sc, and La. The contraction schemes employed for the basis sets were Ž5s5p.rw4s4px for Ca, Ž5s5p5d.rw4s4p3dx for Sc, and Ž5s5p3d.r w4s4p3dx for La in standard notation; the split-valence 3-21G basis set w12x was used for C and N. Energies were improved by single-point non-local hybrid density functional calculations w13x at the B3P w14,15x and B3LYP w14,16x levels with a large 6-31G basis set for C w17x. For the electron density analysis, all-electron calculations were carried out at the B3LYP level by using the Ž4333r43.rw43321r421x basis set augmented by p-type polarization function Žexponent 0.059. for Ca w18x and 6-31G for C w17x. All calculations were performed with the Gaussian94 program w19x.

Fig. 1. The optimized structures of Ca@C 72 with only pentagons and hexagons. Structure a ŽC s . satisfies IPR while b ŽC 2 . and c ŽC 2v . contain a pair of adjacent pentagons. The distances between Ca and the nearest neighbor carbon are ˚ 2.777 Ža., 2.840 Žb., and 2.783 Žc. A.

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less, b is 38.2 ŽB3P. and 37.4 ŽB3LYP. kcalrmol more stable than a, while c is 36.6 ŽB3P. and 36.2 ŽB3LYP. kcalrmol more stable than a. These are remarkable as the first examples invalidating IPR established in fullerene chemistry. Upon optimization without Ca, the carbon cage of b becomes 10.6 ŽB3P. and 10.2 ŽB3LYP. kcalrmol less stable than the IPR-satisfying C 72 ŽD6d . cage, because it has a pair of adjacent pentagons. Interestingly, the non-IPR carbon cage of c is 8.7 ŽB3P. and 8.4 ŽB3LYP. kcalrmol more stable than C 72 ŽD6d . even after optimization without Ca. This IPR-violation for the empty case is noteworthy since the structure and stability of C 72 have long been controversial w20,23x. As Fig. 2 shows, the carbon cage of c is formally obtainable by adding C 2 onto a hexagonal ring in the belt of the IPR-satisfying C 70 ŽD5h .. Other important findings are structures d ŽC s .

Fig. 2. Formal addition of one or two C 2 units on C 70 ŽD5h . leads to the cage structures of c, e and j.

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and e ŽC s . shown in Fig. 3. The carbon cage of d contains one heptagonal ring Žsurrounded by two groups consisting of two and three adjacent pentagons., thereby not obeying the traditional definition of fullerenes. However, d was calculated to be 19.0 ŽB3P. and 18.4 ŽB3LYP. kcalrmol more stable than a. This is the first example of a heptagon-containing structure that is more stable than the conventional fullerene structure. A stable structure with one heptagonal ring has been recently calculated for C 62 w24x; however, it should be noted that C 62 has no IPR-satisfying structure. Upon optimization without Ca, the heptagon-containing cage of d becomes 26.2 ŽB3P and B3LYP. kcalrmol less stable than C 72 ŽD6d .. On the other hand, structure e possesses two heptagonal rings. Nevertheless, e is 1.6 ŽB3P. and 2.1 ŽB3LYP. kcalrmol more stable than a. In the absence of Ca, the two-heptagon containing cage of e is 24.7 ŽB3P. and 24.2 ŽB3LYP. kcalrmol less stable than C 72 ŽD6d .; it is formally obtainable by adding C 2 on C 70 ŽD5h . Žsee Fig. 2. and 1.5 ŽB3P. and 2.0 ŽB3LYP. kcalrmol more stable than the oneheptagon-containing cage of d. As is obvious from these results, heptagon-containing structures as well as non-IPR structures can be highly stabilized by endohedral metal-doping. The binding energies Žrelative to Ca q C 72 . are 36, 16, 33, and 15 kcalrmol for b, c, d, and e at the B3LYP level, respectively. These confirm that a Ca atom can be endohedrally strongly bound to the non-IPR and heptagon-containing C 72 cages. In contrast, encapsulation of Ca inside the IPR-

Fig. 3. The heptagon-containing structures optimized for Ca@C 72 . Structures d ŽC s . and e ŽC s . have one and two heptagonal rings, respectively. The distances between Ca and ˚ the nearest neighbor carbon are 2.819 Žd. and 2.689 Že. A.

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satisfying C 72 ŽD6d . cage leading to structure a was calculated to be 12 kcalrmol less stable at the B3LYP level than Ca q C 72 . This reflects the leapfrog structure of C 72 ŽD6d . with a large HOMO᎐LUMO gap w20x. Two electrons are transferred in Ca@C 72 from Ca to C 72 . However, a high energy cost is required in the case of C 72 ŽD6d . because of the high-lying LUMO level at y0.73 eV. It seems unlikely that a Ca atom is wrapped in the IPR-satisfying C 72 cage during annealing rearrangements. Very recently, Ca@C 72 has been isolated and purified w25x, for which a 13 C NMR study is in progress w26x. We predict that Ca@C 72 takes unconventional structures such as b and c. We could suggest also similar cage structures for La 2 @C 72 isolated recently w27x, though a conventional fullerene structure satisfying IPR is illustrated in the experimental paper. Since only trace mass spectral evidence is available for C 72 itself, the extraction and isolation of Ca@C 72 and La 2 @C 72 are apparently owing to metal-mediated stabilization. The HOMO levels of b, c, d, and e are y7.0, y6.4, y7.4, and y6.6 eV at the HF level, respectively. These are 0.8᎐1.8 eV lower than that of y5.6 eV for a, suggesting that unconventional structures b᎐e are less reactive especially toward oxygen than a. The local strain inherent in closed cage structures also plays an important role in determining the reactivities. In this context, the pyramidalization angles from the ␲ orbital axis vector ŽPOAV. analysis, ␪p s Ž ␪␴␲ y 90.⬚ Žfor ␪␴␲ , see chart., provide a useful index of the local strain at each carbon atom: an increase in ␪p leads to a high local strain Ža high curvature on the cage. w28x. The pyramidalization angles calculated with the Table 1 ␪pma x Ž⬚. in C 72 and Ca@C 72

a b c d e

C72

Ca@C72

12.3 16.0 18.3 16.1 16.2

14.2 16.6 15.1 16.4 15.1

POAV3 program are given in Table 1. The maximum value Ž ␪pmax . for the IPR-satisfying C 72 cage is 12.3⬚ and highly increased to 14.2⬚ upon endohedral Ca-doping. As Table 1 shows, the ␪pmax values for the non-IPR and heptagon-containing C 72 cages are larger because of the defects and become rather smaller Žor change little. upon endohedral Ca-doping. However, the resultant values of 15.1᎐16.6⬚ are larger than those of 11.0᎐12.1⬚ calculated for four stable isomers of Ca@C 82 w29x. This is consistent with the fact that Ca@C 72 is more reactive and less abundantly produced than Ca@C 82 w30x. To provide insight into the nature of the bonding in endohedral metallofullerenes, a topological analysis of electron density ␳ Ž r . and its Laplacian ⵜ 2␳ Ž r . was carried out for structure c as a representative example, according to the ‘atoms in molecules’ theory developed by Bader w31x. For this purpose, we have located the relevant bond critical points Ž r b . between atoms, at which ⵜ␳ Ž r b . s 0 and electron density is minimum along the bond and maximum in the other two directions. The values of the Laplacian ⵜ 2␳ Ž r . identify whether the electron density is locally concentrated Žnegative values. or depleted Žpositive values.. Thus for covalent Žor polar. bonds arising from electron sharing, the values of ⵜ 2␳ Ž r b . become highly negative as a result of electron concentration, and the values of ␳ Ž r b . provide a measure of the strength of covalent bonding. For ionic bonds, ⵜ 2␳ Ž r b . is positive and ␳ Ž r b . is relatively low in value because of the electron contraction toward the interacting atoms, reflecting closed-shell interactions. Bond paths of maximum electron density through a bond critical point Ž r b .

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between atoms are images of chemical bonds, the existence being a necessary condition for the existence of chemical bonds Žor interactions .. In Fig. 1, a total of 108 bond lines are drawn between carbon atoms for structure c. Accordingly, 108 bond paths Žand bond critical points. were located between the carbon atoms. The bond path lengths were calculated to be somewhat longer than the corresponding interatomic lengths, as a result of the fact that the C᎐C bonds are slightly curved toward the outside of the C 72 cage. The high ␳ Ž r b . values of 0.24᎐0.32 a.u. and the negative ⵜ 2␳ Ž r b . values of y0.44 to y0.81 a.u. confirm that there are strong covalent interactions between carbon atoms. The bond ellipticities at r b w31x Žthe derivation of electron distribution from cylindrical symmetry or a measure of ␲-type interactions . in the range of 0.10᎐0.17 are comparable to that of 0.14 calculated at the same level for benzene ŽC 6 H 6 ., suggesting that the C᎐C cage bonds preserve p-bond character even after receiving two electrons from Ca. We turn to the bonding features between Ca and C 72 . Bond paths Žand bond critical points. were located only between Ca and each C atom in adjacent pentagons. In other words, Ca interacts only with adjacent pentagons. The magnitudes of ␳ Ž r b . and ⵜ 2␳ Ž r b . are 0.02 and 0.09 a.u., respectively. This very low ␳ Ž r b . value and the positive ⵜ 2␳ Ž r b . value verify that the bonds between Ca and C are highly ionic, as anticipated from the electronic structure described formally 2y Ž as Ca2q C 72 because of two-electron transfer from Ca to C 72 .. This delocalized electron interaction is reflected in high bond ellipticities of 0.79᎐3.28 due to electron anisotropy. These confirm that the binding energies of endohedral metallofullerenes are mainly dominated by electrostatic interactions w2,6,7x. The same trends were also calculated for other structures. 3.2. Ca@C74 and Sc 2 @C74 In an attempt to generalize the appearance of unconventional structures, endohedral Ca-doping was investigated for C 74 . The C 74 fullerene has only one IPR-satisfying isomer of D 3h symmetry w20x. The endohedral structure Žf. of Ca@C 74

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Fig. 4. The optimized structures of Ca@C 74 . Structure f ŽC 2v . satisfies IPR while g ŽC 1 . and h ŽC 1 . contain a pair of adjacent pentagons. The distances between Ca and the nearest neigh˚ bor carbon are 2.802 Žf., 2.761 Žg., and 2.746 Žh. A.

optimized with this D 3h isomer is shown in Fig. 4, which has C 2v symmetry. Although C 74 itself has not yet been successfully isolated up to now and remained as a ‘missing’ fullerene between C 60 and C 94 w5x, Ca@C 74 has been recently isolated and purified w25x. A total of 615 576 closed cage structures were generated for C 74 which are composed of pentagons, hexagons, and one heptagon. Two electrons are transferred in Ca@C 74 from Ca to C 74 . 2y Therefore, the relative energies of the C 74 isomers are of great help in predicting correctly the most favorable carbon cages, as demonstrated in several examples w2,6,7x. From the 615 576 2y structures, two cage isomers stable for C 74 were searched for, which contain a pair of adjacent pentagons. The endohedral structures Žg and h. of Ca@C 74 optimized with these isomers are shown in Fig. 4, which have C 1 symmetry. However, both g and h were calculated to be less stable than f by 15.8 and 18.2 kcalrmol at the B3P level and 15.5 and 18.1 kcalrmol at the B3LYP level, respectively. This suggests that the

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IPR-satisfying f is the most probable structure for Ca@C 74 . Therefore, encapsulation of two Sc atoms was next tested. The optimized structure Ži. of Sc 2 @C 74 with the IPR-satisfying C 74 cage is shown in Fig. 5, which has C 2v symmetry w32x. Since a total of four electrons are transferred to 4y was seC 74 in Sc 2 @C 74 , a stable cage for C 74 lected from the 615 576 structures of C 74 , which contains two pairs of adjacent pentagons. Fig. 5 shows the endohedral structure Žj. of Sc 2 @C 74 optimized with this defect cage. It is remarkable that j is only 1.5 kcalrmol less stable at both B3P and B3LYP levels than the IPR-satisfying i. Interestingly, the carbon cage of j is also formally obtainable by adding two C 2 units onto two different hexagonal rings in C 70 ŽD5h ., as shown in Fig. 2. The HOMO and LUMO levels of y6.2 and y1.5 eV calculated at the HF level for j are 0.4 eV lower and 0.6 higher than those of i, respectively. This clearly indicates that j is much less reactive than i. We propose that Sc 2 @C 74 is an interesting target for the experimental study of unconventional structures. With the availability of a purified sample of Sc 2 @C 74 , Dorn and co-workers have very recently succeeded in observing the 13 C NMR spectrum consisting of a total of 39 lines from 130 to 160 ppm w33,34x. Generally, the carbon atoms in IPR-satisfying fullerene cages exhibit the 13 C chemical shifts in the range of 130᎐151 ppm. Therefore, the two highly deshielded lines at

Fig. 5. The optimized structures of Sc 2 @C 74 . Structure i ŽC 2v . satisfies IPR while j ŽC 2v . contains two pairs of adjacent ˚ pentagons. The Sc᎐Sc distances are 3.390 Ži. and 3.630 Žj. A. The distances between Sc and the nearest neighbor carbon ˚ are 2.337 Ži. and 2.397 Žj. A.

161.11 and 160.66 ppm imply the presence of adjacent pentagons. According to our calculations, however, the non-IPR C s structure proposed by Dorn and co-workers does not correspond to an energy minimum, which collapses to a more stable C 2v structure. It is an interesting subject to find an unconventional structure which is consistent with the observed 13 C NMR spectrum. 3.3. La2 @C7 9 N It is of growing interest to prepare a new class of fullerene molecules whose properties are significantly enhanced by substitution of one or more cage carbons with heteroatoms. Most interesting is the recent preparation of azafullerene ions such as C 59 Nq and C 69 Nq w35᎐37x. However, the resultant C 59 N and C 69 N radicals are highly reactive. For this reason, only the dimers or derivatives have been isolated and their properties have been characterized w38᎐44x. In the meantime, we have found that the La-doped azafullerene ion La 2 @C 79 Nq is efficiently formed by fast atom bombardment mass ŽFABMS. fragmentation of the adducts of La 2 @C 80 and benzyl azide w45x. La 2 @C 79 Nq is isoelectronic with La 2 @C 80 . Calculations show that almost all three valence electrons are transferred in La 2 @C 79 Nq from each La Ž5d1 6s 2 . to the LUMO, LUMOq 1, and LUMO q 2 of the azafullerene cage, as in La 2 @C 80 w46,47x. Consequently, La 2 @C 79 Nq has a closed shell electronic structure. Acceptance of an electron leads to the La 2 @C 79 N radical. An important finding is that the electron is not distributed on the cage but localized on each La with a spin density of 0.5. Thus, the cage atoms of La 2 @C 79 N have no significant radical character, since two La atoms can serve as a spin-absorbent. This is in sharp contrast with the high spin localization onto the carbons near nitrogen in C 59 N and C 69 N which leads to facile dimerization and abstraction w35,38᎐44x. The most stable optimized structure of La 2 @C 79 N is shown in Fig. 6, which ˚ has C s symmetry. The La᎐La distance of 3.585 A ˚ for is shorter than those of 3.655 and 3.622 A La 2 @C 80 w46,47x and La 2 @C 79 Nq, respectively, as a result of the acceptance of an electron into

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References

Fig. 6. Two views of the optimized structure ŽC s . of La 2 @C 79 N.

the La᎐La bonding orbital and the decreased positive charge on La. In La 2 @C 79 N, the nitro˚ gen atom occupies the position which is 4.518 A away from each La. On the basis of the unique stability and properties, we are currently working on the isolation of La 2 @C 79 N in macroscopic amounts and the experimental confirmation of the structure. 4. Conclusions Through theoretical calculations, we have proposed new cage structures which violate the isolated pentagon rule and contain heptagonal rings or a heteroatom. It is expected that these unconventional cage structures will extend and enrich the research area of endohedral metallofullerenes. A further study is in progress. Acknowledgements We thank Dr. M. Yoshida and Prof. E. Osawa for making the GSW program available for the present study, Prof. H. Shinohara, Dr. T.J.S. Dennis, and Prof. M. Takata for private communication on the experimental results, and Prof. R.C. Haddon for providing us with a copy of the POAV3 program. We are also grateful to Prof. H.C. Dorn for making the 13 C NMR spectrum of Sc 2 @C 74 available before publication. This work was supported in part by a grant from the Ministry of Education, Science, Sports, and Culture of Japan.

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