The Journal of
Physical Chemistry
0 Copyright 1993 by the American Chemical Society
VOLUME 97, NUMBER 24, JUNE 17,1993
LETTERS Preparation of Hydrofullerenes by Hydrogen Radical Induced Hydrogenation Moetaz I. AttaUa,'*t Anthony M. Vassallo$ BNW N. Tattam,*and John V. Hnnnat CSIRO Division of Coal and Energy Technology, PO Box 136, North Ryde NSW, 2113 Australia, and Department of Pharmacy, Sydney University, NS W, 2006 Australia Received: November 30, 1992; In Final Form: March 30, I993
Fullerite (a mix of crude c60 and c70) has been hydrogenated using hydrogen radical species (H') a t elevated temperatures (400 "C) and hydrogen pressures (6.9 MPa (cold)). Iodoethane was employed as the hydrogen radical promoter. Analysis of the reduced fullerite by fast atom bombardment mass spectrometry revealed a mixture of hydrofullerenes comprising mainly C60H36 and C70H36. No spectral evidence for the ethylation or iodination of the fullerite was found. Prepared under these conditions, the product is only sparingly soluble in common solvents. Solid-state I3Cnuclear magnetic resonance spectroscopy and infrared spectroscopy were used to characterize the reduced fullerite. The structure of the C60H36 produced under these conditions is of lower symmetry than Th and most likely D3d or C3i symmetry.
Introduction The spherical all-carbon molecule Cao has been the subject of intense research since its isolation in macroscopic amounts by Kratschmer et al. in 1990.' One aspect of this research has been the preparation of fullerene based molecules by the addition of reagents such as chlorine,2s3bromine: f l u ~ r i n e ,hydrogen,' ~?~ and ethylenediamine8 and the alkylation of C60e9These molecules are of interest not only for their intrinsic properties but also because they may reveal how the cage structure distorts when some of the carbons have higher hybridization. It has been reported that a hydrocarbon, CaH36, was prepared by Smalley and co-workers using Birch reduction.' They suggested that hydrogen added to the molecule so that 12 isolated double bonds remained, one in each pentagon, and that the reaction was fully reversible. However, no subsequent characterization has been published. Theoreticalstudies have indicated that additionof more hydrogen would be energetically more favorable if these hydrogens were to be incorporated within the cage,lo resulting in hydrogens both outside and inside the ball. In view ofthe relatively mild conditions of the Birch reduction, we report the hydrogen radical induced hydrogenation as a method of adding hydrogen to Cm. Theoretical t CSIRO Division of t Sydney University.
Coal and Energy Technology.
0022-3654/93/2097-6329$04.00/0
calculations and symmetry constraints indicate that the most stable isomers in the CaH1Zn series would be CboH36 and CaHsa."JZ
Experimental Section Fullerite (approximately 85% Ca, remainder C,Oand higher fullerenes) was prepared using the contact arc method of Haufler et a1.' The hydrogenation method used in this investigation is based on a technique using alkyl halides developed at this laboratory for the hydrogenation of coal and simple aromatic c o m p o ~ n d s . ' ~ -In~ ~a typical reaction, fullerite (150 mg) was placed in a glass vessel inside a Parr autoclave (250 an3) with 2 g of iodoethane and pressurized with hydrogen to 6.9 MPa (1000psig cold charge). The hydrogenation was carried out at 400 "C for 1 h. After cooling, the product was removed and washed with aqueous Na2S203 (25% w/w, 75 cm3) to remove residual iodine and then with excess water. A light brown solid (122 mg, 81% w/w) was recovered. Similar experiments in the absence of iodoethane resulted in no hydrogenation and total recovery of the unaltered fullerite. Reactions at higher temperature (450 "C) and pressure (10.4 MPa) resulted in a product with very little hydrogen and a broad IR spectrum. Iodine alone can also be used as the hydrogen radical promoter. 0 1993 American Chemical Society
Letters
6330 The Journal of Physical Chemistry, Vol. 97, No. 24, 1993 Solid-state 13C NMR spectroscopy was carried out at 22.5 MHz with a Bruker CXP-90 spectrometer, using conventional magic angle sample spinning and cross-pplarization techniques. Spectra were acquired with a recycledelay of 3 8, a contact period of 2 ms, and a IH r / 2 pulse length of 4 ps. The sample was spun at 4 kHz in a Doty 7-mm single air bearing electromagnet probe, and chemical shifts were referenced externally to TMS. Infrared spectroscopy was carried out using the alkali halide pellet technique on a Digilab FTS-80 spectrometer at 2-cm-' resolution. Mass spectra wereobtained using a Finnigan/Mat TSQ-46 triplestage quadrupole mass spectrometer, operated in the single scanning mode using the first quadrupole (Ql). A 2-9 scan time was used to cover the mass interval between 400 and 1000 Da. An Ion Tech Ltd. fast atom gun was used to produce fast argon atoms (8 keV). Sample preparation consisted of dissolving the hydrofullerene in nitrobenzene (-2 pL) and mixing with 2 pL of dithioerthrito1:dithiothreitol:glycerol matrix (5:1:1, w/w). Approximately 1 pL of the final mixture was placed onto the copper sample stage. The sample was introduced into the mass spectrometer via the solid probe interface with the sample at 90° to the fast atom beam. Calculations at the semiempirical level, using the AM 1 Hamiltonian,I6 were carried out to determine the molecular geometry of a number of CmH36 isomers. From this optimized geometry, force constants were evaluated and the vibrational frequenciesof each isomer determined. Thesecomputationswere carried out using the UniChem package (Cray Research, Inc.) on a Cray Y-MP4E computer.
200 180 160 140 120 100 80
80 40 20
0
chemical shift (6 ppm)
Figure 1. Solid-state 13C NMR spectrum of hydrogenated C,&&.
Results and Discussion The light brown solid recovered was found to be insoluble in many common solvents, but slightly soluble in nitrobenzene. This contrasts with the reported solubility of the product prepared by Birch reduction' of Cm in a G H I ~ / C solvent S ~ mixture. In any event, it is probable that under these reaction conditions the product formed differs from those reported by Smalley et al.7 Element analysis of the material was found to be C 95.1%; H 5.1% (calculated value based on 85% CmH36 and 15% C7OH36 is C 95.31% H 4.69%). Attempts to obtain the mass spectrum using normal electron impact (70 eV) and laser desorption FTICR were unsucCc88fu1, largely due to thermal breakdown of the product to lesser hydrogenated products (e&, C60H18, CmHs, etc.). Thermogravimetric analysis and infrared emission spectroscopy reveal that the hydrogenated product begins todecompose at -350 OC, and full details of these studies will be reported elsewhere. However, the FAB MS techniqueprovided a viable mass spectrum with strong signals at m / z 757 and 877 which correspond to the H+ adduct of CmH3,5and C70H36, respectively. The detection of C70H36 implies a mechanism whereby only pentagons, and not hexagons, are hydrogenated under these conditions. The solid-state 13CNMR spectrum of the product is shown in Figure 1. The spectrum reveals the presence of at least four types of sp3 hybridized carbon, exhibiting chemical shifts of 31, 38,44, and 49 ppm, and at least three types of spZ hybridized carbon,withchemicalshiftsat128,134,and 138ppm. Integration of these resonances reveals a ratio of 22:38 for spZ to sp3carbon types. As mentioned above, the 36 H isomer is the most probable, and although the NMR integrates to 38 protons (assuming all s$ carbons are bonded to hydrogen), this discrepancyis considered to be within the experimental error of the technique given the nature of the mixture and the differing IH TI, lH Tip, and TCH relaxation times of the species within the product. According to theoretical studies," the distribution of hydrogens in CmH36 with Th symmetryshould correspond to one set of 12equivalent protons and one set of 24 equivalent protons, with the former being secondnearest neighbors and the latter nearest neighbors to sp2bonded carbon, respectively. This proton distribution should result in 2
Figure 2. Three possible etructure of MU: (a) Th,looking down one 9 axis, (b) Dw view perpendicular to 4 axis, and (c) Cj, view perpendicular to C3 axis.
13C NMR lines (or groups of lines) in a ratio of 2:l for the tetrahedral carbons. As can be seen in Figure 1, this raw pattern is present but is composed of at least four lines. This splitting may arise from nonequivalence of molecules in the unit cell or a lower symmetry arrangement of the double bonds; however, the former is unlikely as the chemical shift difference in these splittings is quite large (4-5 ppm). Lower symmetry structures will give rise to extra NMR resonances, and two structures of lower symmetry are proposed in Figure 2. The D~structurehas one aromatic ring at each pole and a belt of double bonds around and parallel with the equator, while the C3, structure has the doublebondsaround but normal to theequator. The&dstructure should have at least six NMR distinguishable carbons in a ratio
Letters
The Journal of Physical Chemistry, Vol. 97, No. 24, 1993 6331 not consistent with a structure of Th symmetry. Calculations at the semiempiricallevel of theory predict only one IR-active C-€4 vibration for the Th structure but at least five for the D3dstructure and at least 17 for the Cji structure. The calculated infrared spectrum of these three structures is also shown in Figure 3. On the basis of these predictions, we feel that the D3d theoretical spectrummost closely resembles the measured spectrum;however, effects such as band splitting due to crystal packing interactions and Fermi resonance, etc., have not been taken into account. A more detailed description of the infrared analysis and molecular modeling is in preparation, and the results of an electron and X-ray diffraction study will be reported elsewhere.'* Finally, although it is conceivable that ethylation and/or iodination could occur; both the NMR and MS results show no evidence that either reaction has taken place. In contrast to studies of coal,15 where the alkyl group may become incorporated into the product, it appears that in reaction with fullerite the alkyl group does not react with the molecule. Acknowledgment. Dr. K. Fisher of the University of NSW carried out the FTICR laser ablation mass spectrometry and Dr. D. Jardieneof Maquarie University obtained the E1 mass spectra. Weacknowledge the helpof Mr. S.P.Chatfieldin the preparation of model structures O f CmH36 and Dr. Robert Bell and Dr. Mareck Michalewicz from the CSIRO Division of Information Technology for assistance in the use of the UniChem package. References and Notes (1) Kratschmer, W.; Fostiropoulous, K.; Huffman, D. R. Chem. Phys. Lett. 1990,170, 167.
4000
3000
2000
1000
wavenumber Figure 3. Infrared transmission spectrum of hydrogenated CSO/C~O anc calculated IR spectra of three possible C&36 structures.
of 2:2:1:1:2:2, arranged from the two sp2hybridized carbons on the polar hexagon to the two sp2 carbons on the equatorial pentagon. The C3i structure should have at least 10 NMR inequivalent carbons, with the sp2carbons in a ratio of 2: 1:1. On this basis, the D3d structure is more consistent with the observed solid-state NMR spectrum;however, a more definitiveassignment must wait until a suitable solvent is found and a solution NMR spectrum obtained. The small broad resonance at 127 ppm can be assigned to the sp2carbons in hydrogenated c70, based on the integrated NMR intensity and the m / z 877 signal (assigned to C70H36) in the mass spectrum. The infrared spectrum (KBr disc, Figure 3) has three strong C-H stretching absorptions at 2912, 2847, and 2827 cm-' and a weaker band at 1490 cm-l in addition to a number of still weaker bands. (Absorptions at -3400 and 1640 cm-l are due to traces of water in the product.) Residual c 6 0 / c 7 0 is not evident in the IR spectra, although a number of bands in the hydrogenated material are of similar frequency to CSOor c 7 0 . For example, the 526-cm-1 band of c 6 0 (and C70) is present, but the 576-, 1183-, and 1430-cm-1bands are missing, as is the intense 577-cm-l band of C70.17The presence of bands at 470,526,667,691,1174, and 1448 cm-I would indicate the spherical fullerene structure is still present but considerablydistorted. The infrared spectrum is also
(2) Tebbe, F. N.; Becker, J. Y.;Chase, D. B.; Firment, L. E.; Holler, E. R.; Malone, B. S.; Krusic, P. J.; Wasserman, E. J . Am. Chem.Soc. 1991,113, 9900. (3) Olah, G. A.; Bucsi, 1.; Lambert, C.; Aniszfeld, R.; Trivuii, N. J.; Sensharma, D. K.;Prakash, G. K. S . J. Am. Chem. Soc. 1991,113,9385. (4) Birkett, P. R.; Hitchwck, P. B.; Kroto, H. W.; Taylor, R.; Walton, R. M. Nature 1992,357,479. ( 5 ) Selig, H.; Lifshitz, C.; Peres, T.; Fischer, J. E.; McGhie, A. R.; Romanow, W. J.; McCauley, Jr. J. P.; Smith, 111, A. B. J . Am. Chem. Soc. 1991, 113,5475. (6) Holloway, J. H.; Hope, E. G.; Taylor, R.; Langley, J.; Avent, A. G.; Dennis, T. J.; Hare, J. P.; Kroto, H. W.; Walton, D. R. M. J. Chem. Soc., Chem. Commun. 1991, 966. (7) Haufler, R. E.;Conceicao, J.; Chibante, P. F.; Chai, Y.;Byme, N. E.; Flanagan, S.; Haley, M. M.; O'Brien, S. C.; Pan, C.; Xiao, 2.;Billup, W. E.; Ciufolini, M. A.; Hauge, R. H.; Margrave, J. L.; Wilson, L. J.; Curl, R. F.;Smalley, R. E. J. Phys. Chem. 1990,94, 8634. (8) Seshadri, R.; Govindaraj, A.; Nagarajan, R.; Radeep, T.; Rao, C. N. R. Teirahedron Lett. 1992,33,2069. (9) Bausch, J. W.;Prakash, G. K. S.; Olah, G. A. J. Am. Chem. Soc. 1991, 113,3205. 110) Saunders. M. Science 1991. 253. 330. (Jlj Dunlap, B. I.; Brenner, D. W.; Mintmire, J. W.; Mowrey, R. C.; White, C. T. J. Am. Chem. Soc. 1991,95,5763. (12) Guo, T.; Scuseria, G. E. Chem. Phys. Letr. 1992. 191, 527. (13) Vassallo, A. M.; Wilson, M. A.; Aitalla, M. I. Energy Fuels 1988, 2, 539. (14) Attalla,M.I.;Wilson,M.A.;Quezada,R.A.;Vassallo,A.M.Energy Fuels 1989,3, 59. (1 5) Attalla, M. I. Ph.D. Thesis, MaquarieUniversity,Sydney, Australia, 1992. (16) Dewar, M. J. S.;Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P.J . Am. Chem. Soc. 1985,107,3902. (17) Chase, B.; Herron, N.; Holler, E. J. Phys. Chem. 1992, 96, 4262. (18) Hall, L. G.; McKenzie, D. R.; Attalla, M. I.; Vassallo, A. M.; Davis, L.;Dunlop, J.; Cockayne, D. J. H. J. Phys. Chem., in press.
