Mass Spectral Evidence of Alkali Metal Insertion into C60

Mass Spectral Evidence of Alkali Metal Insertion into C60-Cyclooctatetraene Complexes: M+@C60-C8H8•3Cheryl D. Stevenson,* James R. Noyes, and Richard C. Reiter Department of Chemistry, Illinois State University, Normal, Illinois 61790-4160 [email protected]. Received July 5, 2002

C60 can be reduced to its trianion anion radical in hexamethylphosphoramide with potassium or cesium metal. The addition of water to these solutions, followed by toluene extraction, yields materials that exhibit the expected mass spectral peaks for the Birch reduction products of C60•3(C60Hn). However, when cyclooctatetraene (COT) is present in the solution, the mass spectral signature for the Birch reduction products of M+@C60-COT•3- and C60-COT•3- are also found. The trianion radical of C60 reacts with COT in HMPA to yield a [2 + 2] cycloaddition product, and subsequent ring opening provides a passageway for the Cs+ or K+ counterion to the interior of the fullerene. Analogous results are not observed when the smaller metals (Na and Li) are used as the reducing agents. Only the larger alkali metal cations form tight ion pairs with the trianion of C60COT. The tight ion association is necessary to bring the cation into a sufficiently close proximity to the trianion for the cation to proceed to the interior. Introduction In 1985 Smalley et al. used mass spectrometry to analyze C60 samples that were prepared from the laser ionization of LaCl3-impregnated graphite.1 The most exciting result was the appearance of a peak corresponding to [email protected] The result, however, proved to be controversial,2 as it was later claimed that the peak corresponding to the mass of La + C60 was actually due to an ion cluster (gas phase ion pair) rather than to a truly endohedral species.3 Later photophysical and chemical studies carried out in the magnetic trap of the FTICR mass spectrometer resolved the issue and clearly indicated the formation of [email protected] Since these early studies a number of inert gases and lanthanides have been encapsulated in the C60 spheroidal cage via the vaporization of impregnated carbon.4 More recently Rubin and co-workers have performed a “molecular surgery” upon the C60 moiety, opening a passageway large enough to allow the introduction of bimolecular hydrogen and helium into the neutral C60 cage, and the endohedral H2 was observable by NMR.5 The size of the opening had to be quite large to allow the unencumbered passage of the molecular hydrogen into (1) Heath J. R.; O’Brien, S. C.; Zhang, Q.; Liu, Y.; Curl, R. F.; Smalley, R. E. J. Am. Chem. Soc. 1985, 107, 7779. (2) Weiss, F. D.; Elkind, J. L.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. J. Am. Chem. Soc. 1988, 110, 4464. (3) (a) Cox, D. M.; Trevor, D. J.; Reichmann, K. C.; Kaldor, A. J. Am. Chem. Soc. 1986, 108, 2457. (b) Cox, D. M.; Reichmann, K. C.; Kaldor, A. J. Chem. Phys. 1988, 88, 1588. (4) (a) Bubnov, V. P.; Laukhina, E. E.; Kareev, I. E.; Koltover, V. K.; Prokhorova, T. G.; Yagubskii, E. B.; and Kozmin, Y. P. Chem. Mater. 2002, 14, 1004. (b) References in (a). (c) Syamala, M. S.; Cross, R. J.; Saunders, M. J. Am. Chem. Soc. 2002, 124, 6216. (d) Giblin, D. E.; Gross, M. L.; Saunders, M.; Jimenez-Vazquez, H.; Cross, R. J. J. Am. Chem. Soc. 1997, 119, 9883. (5) Rubin, Y.; Jarrosson, T.; Wang, G.; Bartberger, M. B.; Houk, K. N.; Schick, G.; Saunders, M.; Cross, R. J. Angew. Chem., Int. Ed. 2001, 40, 1543.

the interior of the cage. It occurred to us that if the C60 cage were highly negatively charged, a much smaller opening might allow the passage of positive alkali metal ions.6 The predicted superconductivity of endohedral alkali metal fullerenes and the observed superconductivity of K3C60,7,8 coupled with their other unique magnetic and electronic properties, have prompted tremendous interest in such materials.9,10 The high solution electron affinities of the fullerenes11 along with the low ionization potentials of the alkali metals would result in an electron from the alkali metal residing on the carbon cage (a cation inside a trianionic paramagnetic cage).7,9 As a result of the very large dipole-dipole interaction involving the cation and carbons with the unpaired electron, this trait would make the M+@C60•n- systems NMR “invisible” but still detectable by mass spectroscopy. Despite intense investigation, there are only a couple of reports of mass spectral observations suggesting the existence of endohedral alkali metal-C60 complexes.12 This is reportedly due to their extreme lability.9,10 (6) Stevenson, C. D.; Noyes, J. R.; Reiter, R. C. J. Am. Chem. Soc. 2000, 122, 12905. (7) Hebard, A. F.; Rosseinsky, M. J.; Haddon, R. C.; Murphy, D. W.; Glarum, S. H.; Palstra, T. T. M.; Ramirez, A. P.; Kortan, A. R. Nature 1991, 350, 600. (8) Wang, P.; Maruyama, Y.; Metzger, R. M. Langmuir 1996, 12, 3932. (9) Smalley, R. E. In Fullerenes: Synthesis, Properties, and Chemistry of Large Carbon Clusters; Hammond, G. S., Kuck, V. J., Eds.; American Chemical Society: Washington, DC, 1992; pp 141-159. (10) Ogawa, T.; Sugai, T.; Shinohara, H. J. Am. Chem. Soc. 2000, 122, 3538. (11) Candida, M.; Shohoji, B. L.; Luisa, M.; Franco, M. B.; Celina, M.; Lazana, R. L. R.; Nakazawa, S.; Sato, K.; Shiomi, D.; Takui, T. J. Am. Chem. Soc. 2000, 122, 2962. (12) (a) Tellgmann, R.; Krawez, N.; Lin, S.-H.; Hertel, I. V.; Campbell, E. E. B. Nature 1996, 382, 407. (b) Chai, Y.; Guo, T.; Jin, C.; Haufler, R. E.; Chibante, L. T. F.; Fure, J.; Wang, L.; Alford, J. M.; Smalley, R. E. J. Phys. Chem. 1991, 95, 7564. 10.1021/jo0261451 CCC: $22.00 © 2002 American Chemical Society

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Published on Web 11/05/2002

C60-Cyclooctatetraene Complexes SCHEME 1

SCHEME 2

Patchkovskii and Thiel suggest that the [2 + 2] cyclodimerization of C60 followed by ring opening can provide a window (Scheme 1) sufficiently large to allow the passage of noble gas atoms into C60.13 In an analogous reaction, the anion radical of cyclooctatetraene (COT), in hexamethylphosphoramide (HMPA), spontaneously dimerizes at ambient temperatures.14-16 The [2 + 2] dimer subsequently undergoes ring opening to form the dianion and anion radical of [16]annulene, reaction 1. Since C60 also readily undergoes [2 + 2]

cycloaddition,17,18 it seemed reasonable that the trianion radical of C60 would react with COT in HMPA to also yield a [2 + 2] cycloaddition product, and subsequent ring opening could provide a passageway for the alkali metal counterion to the interior of the fullerene (Scheme 2). Results and Discussion The reduction (with a freshly distilled alkali metal mirror in a sealed glass assembly under high vacuum) (13) Patchkovskii, S.; Thiel, W. J. Am. Chem. Soc. 1998, 120, 556. (14) Stevenson, C. D.; Nebgen, M. A. J. Am. Chem. Soc. 1985, 107, 5501. (15) Stevenson, C. D.; Reiter, R. C.; Sedwick, J. B. J. Am. Chem. Soc. 1983, 105, 6521. (16) Stevenson, C. D.; Burton, R. D., Peters, S. J.; Reiter, R. C. J. Org. Chem. 1993, 58, 5838. (17) Fujitsuka, M.; Luo, C.; Ito, O.; Murata, Y.; Komatsu, K. J. Phys. Chem. A 1999, 103, 7155. (18) Vassilikogiannakis, G.; Orfanopoulos, M. J. Org. Chem. 1999, 64, 3392.

of a heterogeneous (C60 is insoluble in HMPA) 1:1 mixture of COT or COT-d8 and C60 in HMPA with potassium or cesium metal immediately gives rise to a solution yielding the strong EPR signal for C60•-. The signal consists of a single resonance with a peak-to-peak line width (∆wpp) of about 0.9 G. Continued exposure of the mixture to the alkali metal surface results in drastic changes in the EPR pattern. After more than 3 mol of metal per mol of C60 are added, the spectrum evolves into a well-resolved hyperfine pattern resulting from a metal nucleus (a133Cs ) 0.725, or a39K < 0.2 G).19 This pattern is further split by a single 31P nucleus with a31P ) 0.93 G.6 The broad line width (0.55 G) is partially attributed to the unresolved splittings from the carbon-13 nuclei. This doublet species is invariant whether “off-the-shelf” or freshly sublimed C60 is used. The species giving rise to this EPR spectrum is C603-(HMPA)•,M+ where the fullerene contains three extra (added) electrons and the resulting anionic species has condensed with one HMPA unit. Further reduction of this species with alkali metal results in the loss of the EPR signal. This is probably due to the formation of the diamagnetic tetra-anion of C60. The addition of water to the HMPA solutions containing the C603-(HMPA)•,M+ (M ) K, Cs) complex yields a brown intractable material. The solids were collected via filtration and were extracted with toluene for 24 h in a Soxhlet. Matrix-assisted laser desorption/ionization (MALDI) mass spectroscopy of the extracted solids reveals peaks at each mass to charge unit from 720 to 727. These represent the parent (P) and P + 1 peaks of the Birch reduction products of the anions of C60. No heavier materials were found, and in no case was there evidence of a mass-to-charge ratio consistent with one or more HMPA molecules attached to a C60. Evidently workup and isolation results in the loss of the tenuously attached HMPA. When this same procedure is repeated in the presence of COT (1 mol per mol of C60), the EPR results are unaltered. However, the Soxhlet extracts reveal mass spectral patterns containing signals consistent with the presence of COT and the metal used to carry out the reduction. When potassium is the reducing agent, metal peaks corresponding to the Birch reduction product ions of COT-C60n- are also observed at 824-831 mass to charge units. In some cases there are also clear signals in the 863-870 region due to one COT unit plus one C60 unit plus one potassium (see Figure 1).6 The COT has apparently undergone a [2 + 2] cycloaddition to the anionic C60, and the potassium metal is intimately associated with the COT-C60 complex. When cesium metal is used in place of the potassium, the signals in the 863-870 region are replaced with signals in the 957-964 region, which correspond to the Birch reduced COT-C60-Cs anionic complex, Figure 2. Both potassium and cesium reductions have resulted in mass spectral signatures of the COT-C60-metal complex and of the COT-C60 complex upon both positive and negative ion detection. However, in the absence of COT no mass spectral peaks for ions heavier than those from Birch reduced C60 are found. Clearly the presence of the COT on the C60 allows the formation of (19) The low gyromagnetic ratio of 39K accounts for the unobservable hyperfine from this nucleus.

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Stevenson et al.

FIGURE 2. The MALDI mass spectrum (negative ion detecFIGURE 1. The MALDI mass spectrum (negative ion detection) of the toluene extract obtained from the reduction of C60 and COT with potassium metal followed by the addition of water. The inset represents a separate experiment.

metal complexes that are sufficiently robust to withstand a water workup and either positive or negative ion mass spectral detection. As mentioned above peaks are observed for Birch reduced alkali metal@C60 with no COT attached. Presumably the metal cation enters the system while the COT-C60 attachment is intact. The COT later comes off of the C60 unit. It is well-known that radical ions (both positive and negative) of 2 + 2 dimers spontaneously undergo cleavage, which is especially important in biochemical systems.20 Hence, the reverse of the first reaction in Scheme 2 is not surprising. It may even take place in the mass spectrometer during analysis. We contemplated the possibility that a complex is formed where the metal simply resides in the concave area between the alkene and the C60 ball as an ion pair, structure I. However, such ion paired complexes could not remain intact in the gas phase (mass spectral conditions) under both positive and negative ion detection. Attempts were made to carry out mechanical and semiempirical calculations upon the system depicted in (20) For example, see: (a) Jacobsen, J. R.; Cochran, A. G.; Stephans, J. C.; King, D. S.; Schultz, P. G. J. Am. Chem. Soc. 1995, 117, 5453. (b) Austin, R.; McMordie, S.; Altmann, E.; Begley, T. P. J. Am. Chem. Soc. 1993, 115, 10370. (c) Hartman, R. F.; Van Camp, J. R.; Rose, S. D. J. Org. Chem. 1987, 52, 2684.

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tion) of the toluene extract obtained from the reduction of C60 and COT with cesium metal followed by the addition of water. The peaks for Cs@HnC60- are presumably from a loss of COT from Cs@HnC60-COT-. The inset represents the results from this same experiment but with positive ion detection.

structure I. However, when the metal is placed near the concave surface (as a starting point), even a simple mechanical energy minimization results in widening the “hole” in the C60, and consequent insertion of the metal cation, followed by a renarrowing of the orifice. The very strong Coulombic attraction between the positive metal cation and the tri-anionic cage represents a force suf-

C60-Cyclooctatetraene Complexes

FIGURE 3. A 300-MHz 1H NMR spectrum of the vinyl region of a HMPA-d18 solution exhibiting the EPR signal for C603--

HMPA•,Cs+. The large NMR signal for free COT is shown off scale. The multiplet from 5.8 to 6.4 ppm is due to COT attached to C60. The resonances are broadened as a result of the presence of paramagnetic material. A small simulation (using gNMR) of the NMR spectrum is shown in the inset. It was generated using the parameters shown on the structure. A broad signal from an “impurity” is observed under the real spectrum in the region from 6.1 to 6.3 ppm.

ficient to overcome the apparent steric restriction that is revealed by space-filling models of the COT-C60 [2 + 2] complex. Solutions containing the C603-(HMPA)•,Cs+ complex were generated in HMPA-d18, and 1H NMR spectra were recorded. The solutions contain paramagnetic material, but broadened spectra of the COT-C60 [2 + 2] complex were found. The complex pattern has the appropriate chemical shifts for COT protons where the COT moiety is connected to a large aromatic substituent. The pattern (Figure 3) originates from four groups of protons (two in each group) with considerable second-order effects. It can, however, be simulated with couplings and chemical shifts that are consistent with the structure shown in structure I without the metal present. The experimental and theoretical evidence reported here are most consistent with the formation of endohedral alkali metal complexes, [K@C60-COT]•2- and [Cs@C60COT]•2-, which can be formally written as K+@C60COT•3-and Cs+@C60-COT•3- (reaction 2).

Experimental Section All reductions were carried out in sealed Pyrex glass apparatuses, charged with known amounts of COT and/or C60, under high vacuum. The HMPA was then distilled directly into an evacuated apparatus from a potassium/HMPA solution. The alkali metals were distilled into an apparatus to form metal mirrors. After the apparatuses were sealed from the high vacuum line, the HMPA mixtures were exposed for various amounts of time to the freshly distilled metal mirrors by inverting the apparatus. Samples were reduced to the point where the EPR spectrum for C603-(HMPA)•,M+ was observed, as shown in Figure 4. In the case of the NMR analysis, a NMR tube attached to the apparatus was sealed and removed. The tube containing the

FIGURE 4. (A) A 10-G EPR spectrum of C603--HMPA• obtained after the addition of ca. 3 equiv of Cs metal to the same solution. (B) A computer simulation generated using a single 133Cs splitting of 0.725 G, a 31P splitting of 0.93 G, and a line width of 0.55 G. A small contribution (4%, slightly shifted downfield) from residual paramagnetic C60n- species is included. In contrast, the analogous EPR spectrum of the potassium system shows only a single broad line. The gyromagnetic ratio of potassium is too small to allow resolution of the 39K hyperfine. The unresolved hyperfine simply broadens the resonance and resolution. excess metal was then removed, and the samples were quenched with DI water. In some cases the water solutions were titrated with standardized HCl solutions. This procedure was used to find the number of cesium atoms per C60 molecule in the original HMPA solutions. After the addition of water, the resulting brown intractable materials were collected via filtration and were extracted with toluene for 24 h in a Soxhlet. They were then submitted to the mass spectrometry at the University of Illinois where they were analyzed on a matrixassisted laser desorption time-of-flight mass spectrometer (MALDI-TOF MS).

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Stevenson et al. were carried out using EWSim, v. 4.0, from Scientific Software Services, Normal, IL. 1H NMR spectra were recorded on a 300 MHz spectrometer. Neither Li+ nor Na+ form tight ion pairs with C60-COT3-(HMPA)•. Consequently, no endohedral materials were observed in any of our lithium studies. Sodium cannot be studied using the mass spectral protocol described above. Apparently the better solvation of the smaller cations by the HMPA21 prevents ion association. Rubidium studies were hampered by the presence of extraneous mass spectral peaks in the 907913 critical region, which were present in the analysis of samples from the other metals. Full geometry optimization calculations on the C60 complex were carried out using the PM3 protocol and the TITAN (Jaguar 3.5, Schro¨dinger, Inc., Portland, Oregon, 1998).

Acknowledgment. We are grateful to the National Science Foundation (CHE-9617066) for support of this work. X-band EPR data were obtained using a spectrometer operating at 9.77 GHz with a standard rectangular cavity. Modulation frequency was 100 kHz; modulation amplitude was 0.1 G. Computer simulations of the experimental EPR spectra

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JO0261451 (21) Stevenson, C. D.; Valentine, J.; Williams, E.; Caldwell, G.; Alegria, A. E. J. Am. Chem. Soc. 1979, 101, 515.