Reactivity and Regioselectivity of Noble Gas Endohedral Fullerenes Ng@C60 and Ng2@C60 (Ng=He–Xe)

FULL PAPER DOI: 10.1002/chem.200901224 Reactivity and Regioselectivity of Noble Gas Endohedral Fullerenes Ng@C60 and Ng2@C60 (Ng=He–Xe) Slvia Osuna,[a] Marcel Swart,*[a, b] and Miquel Sol*[a] ACHTUNGRE À1 Abstract: Recently, it was shown that equivalent bonds in free C60, Ng@C60, less than 2 kcalmol ) for bonds closer genuine NgÀNg chemical bonds are and Ng2@C60 (Ng= He, Ne, Ar, Kr, and to the C5 symmetry axis is found. This present in the endohedral fullerenes Xe). Our BP86/TZP calculations reveal picture changes dramatically for the Ng2@C60 in the case of Ng= Xe, while that introduction of single noble gas endohedral compounds with heavier it is more debatable whether a chemi- atoms in Ng@C60 and noble gas dimers noble gas dimers. Encapsulation of cal bond exist for Ng= Ar and Kr. The He2 and Ne2 in Ng2@C60 has almost no these noble gas dimers clearly enhan- lighter homologues with helium and effect on the exohedral reactivity com- ces the reaction, both under thermody- neon are weakly bonded van der Waals pared to free C60, in agreement with namic and kinetic control. Moreover, complexes. The presence of a noble gas experimental results. In all these cases in the case of Xe2@C60, addition to dimer inside the cage is expected to cycloaddition is clearly favored at the [6,6] and [5,6] bonds becomes equally modify the exohedral reactivity of the [6,6] bonds in the fullerene cage. For viable. These reactivity changes in en- C60 cage with respect to that of free the endohedral compounds He2@C60 dohedral fullerenes are attributed to C60. To investigate the impact of encap- and Ne2@C60 a slight preference (by stabilization of the LUMO, increased sulated diatomic noble gas molecules fullerene strain energy, and greater on the chemical reactivity of C ,we compression of the encapsulated Ng 60 Keywords: cycloaddition · density 2 analyzed the thermodynamics and the unit along the He to Xe series. functional calculations · fullerenes · kinetics of [4+2] Diels–Alder cycload- noble gases · reaction mechanisms dition of 1,3-cis-butadiene at all non- [2] Introduction noble gas cations with neutral C60. In 1993, Saunders and co-workers showed that, in fullerene preparation by the The possibility of encapsulating atoms or molecules inside standard method (graphite arc in low-pressure helium), the fullerenes was considered after the first fullerene was dis- endohedral compound He@C60 was obtained for about one covered in 1985,[1] when Heath et al. synthesized the first en- in a million free fullerene molecules.[3] Moreover, heating dohedral compound La@C60. Six years later, endohedral fullerenes to high temperatures under several atmospheres compounds with encapsulated noble gases were synthesized of 3He or Ne produced the respective endohedral com- + by collision of helium, neon, or argon atoms with C60 or of pounds. The presence of potassium cyanide in the reaction mixture enhances the rate of the reaction.[4] Using the same procedure, Saunders et al. discovered that all noble gas endohedral compounds could be produced under the same [a] S. Osuna, Dr. M. Swart, Prof. Dr. M. Sol conditions.[3,5] The proposed formation mechanism is reversi- Institut de Qumica Computacional and Departament de Qumica Universitat de Girona, Campus Montilivi, 17071 Girona (Spain) ble breaking of a carbon–carbon bond of the fullerene sur- [b] Dr. M. Swart face, which opens a “window” large enough to allow noble [6] Instituci Catalana de Recerca i Estudis AvanÅats (ICREA) gas encapsulation. Jimnez-Vzquez et al. used classical Pg. Llus Companys 23, 08010 Barcelona (Spain) statistical mechanics to estimate the equilibrium constants [7] Supporting information for this article is available on the WWW for encapsulation of noble gas atoms in C60. Since then, under http://dx.doi.org/10.1002/chem.200901224. It contains Cartesian several studies on purification and enrichment of these com- coordinates of all species, including imaginary frequencies of the TSs, pounds have been carried out.[8] A significant advance in the a figure showing the orientation of the noble gas dimer in the different products for Ng= He, Ne, Ar, and Xe, and HOMOs and LUMOs insertion of atoms and molecules into the C60 cage and pro- of the different Ng2@C60 species. duction of macroscopic quantities of endohedral fullerenes Chem. Eur. J. 2009, 15, 13111 – 13123 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 13111 was made by Komatsu et al.[9] with chemical opening, inser- The finding of two noble gas atoms close to each other tion, and chemical closing of the cage in what has been was quite an achievement, since the attractive force between called fullerene surgery. This process allowed the synthesis two noble gas atoms is small. Therefore, a free dimer of [10,11] 3 of He@C60 and other endohedral fullerenes. He@C60 such atoms is only stable at extremely low temperatures or species are chemically interesting because 3He NMR spec- for a very short time in a collision. When these atoms are in- troscopy can be used to study reactions of fullerenes.[12, 13] corporated into the fullerene structure, they are forced into 3 3 NMR studies on He@C60 and He@C70 indicate significant contact by the cage and vibrate and rotate within it like dia- [14] diamagnetic rings in C60 and very large ones in C70. Al- tomic molecules. Moreover, the NgÀNg distances in though Ng@C60 (Ng=He–Xe) compounds have been theo- Ng2@C60 are much shorter than those in the free Ng2, which retically studied in several works,[15,16] none of them ana- are weakly bonded van der Waals complexes.[25] lyzed their reactivity. The viability of dimer formation inside fullerenes has at- Interestingly, endohedral fullerenes with a single noble tracted interest. Scuseria and co-workers performed theoret- gas atom were detected in deposits associated with the ical calculations on Ng@C60 (Ng=He, Ne, Ar, Kr, and Xe) [16] impact of a large bolide (asteroid or comet) with the Earth and Ngn@C60 (Ng= He, n=2–4; Ng=Ne, n=2). In all [17] (i.e., the Sudbury Impact Crater). The Sudbury fullerenes cases studied they concluded that C60 was hardly deformed À1 (C60 and C70) were found to contain anomalous ratios of after encapsulation (less than 0.5 kcalmol for a single 3He/4He indicating that the fullerene component is extrater- noble gas atom, and 1 kcalmolÀ1 for multiple complexes at restrial in origin. Other endohedral fullerenes containing the B3LYP/6-31G**//LDA/3-21G level). The interaction en- noble gases such as 40Ar/36Ar in nonterrestrial ratios were ergies for the systems considered were predicted to be re- À1 also found. Endohedral compounds with single noble gas pulsive: around 1 kcalmol for He@C60 and Ne@C60, 7 and À1 À1 atoms are stable, although the rate of decomposition to the 14 kcalmol for Ar@C60 and Kr@C60, and 7–30 kcalmol free fullerene and noble gas is faster if traces of trapped sol- for the He2@C60 and He4@C60 complexes. Note, however, vent are not removed.[18] that the level of theory used in their study is inadequate for Frunzi, Cross, and Saunders explored experimentally the weak interactions.[26] This was shown more explicitly by Lee exohedral reactivity of these single-Ng compounds. They and McKee for the interaction energy of one hydrogen mol- performed the Diels–Alder reaction between 9,10-dimethy- ecule in a C60 fullerene (H2@C60), which was calculated as 129 3 À1 À1 lanthracene (DMA) and Xe@C60 or He@C60 to investi- +1.7 kcalmol with B3LYP and À6.5 kcalmol at the gate the effect of the trapped noble gas.[19] They observed more appropriate M05-2X level.[27] that the thermodynamics of the Diels–Alder reaction was Recently, Krapp and Frenking studied the chemical bond- 3 favored at low temperatures in the case of He@C60, while ing of the noble gas atoms in the endohedral fullerenes [25] at high temperatures the equilibrium constant of the reac- Ng2@C60 (Ng=He, Ne, Ar, Kr and Xe). The MP2/ 129 tion was larger for Xe@C60. Moreover, they obtained 85% TZVPP//BP86/TZVPP calculations showed that equilibrium of bis-adducts and no measurable amount of unreacted geometries for the considered compounds have D3d symme- 3 À1 He@C60 in the case of helium. When xenon is encapsulated try with the exception of Xe2@C60, which was 2.8 kcalmol 129 inside, some unreacted Xe@C60 and large amounts of more stable in the D5d symmetry. Moreover, they observed mono-adduct were detected. According to Frunzi, Cross, that precession of the helium dimer in He2@C60 has practi- [19] 129 and Saunders , the decreased reactivity of Xe@C60 to- cally no barrier. The charge distribution and orbital analysis 3 wards addition of DMA compared to He@C60 at low tem- showed that the partial charge of the Ng2 dimer is close to peratures must be attributed to the fact that the p-electron zero, except for xenon. The sum of the atomic partial charg- cloud of the C60 cage is pushed outward by the xenon atom. es of the Xe2 moiety is +1.06 and +1.52 for the lowest lying The same reason was given by Komatsu et al. to justify the singlet and triplet states, respectively. Therefore between 1 [11] decreased reactivity of (H2)2@C70 compared to H2@C70. and 2 electrons are transferred from the noble gas dimer to q+ qÀ Recently, Diels–Alder and retro-Diels–Alder reaction be- the fullerene structure, that is, Xe2 @C60 (1<q<2).

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