J . Phys. Chem. 1991,95, 10561-10563
La&,:
10561
A Soluble Dimetallofullerene Marcos M. Alvarez, Edward G.Gillan, Kiroly Holczer? Richard B. Kaner,* Kyu S. Min, and Robert L. Whetten* Department of Chemistry and Biochemistry and Solid State Science Center, University of California, Los Angeles, California 90024- 1569 (Received: November 19, 1991)
Application of the Kratschmer-Huffman method to a lanthana-graphite mixture yields an extractable molecule of formula La&,, in addition to the recently reported Lacs2. Its solubility properties suggest that it is similar to the well-characterized higher fullerenes c76, c78,and C84, and its estimated yield, based on laser-desorption mass spectra of (*)-charged molecules, is comparable to these. The finding of a series of particularly extractable molecules n = 0, 1,2) could indicate either a special type of structure or an isoelectronic series. We are attempting to isolafe this molecule by chromatography.
Introduction Since the discovery by Kratschmer, Huffman, and co-workers (KH)'qz of a method for preparing macroscopic quantities of Cm, and the confirmation of the hollow cagelike structures ('fullerenes") for c 6 0 1 and for higher fullerenes (C70 and bey ~ n d ) , +it~has been apparent that the objective of trapping various atoms within a carbon network, long suggested to be feasible by molecular beam experiment^,^ might lie within reach. Recent additional support has come from bombardment experiments in which He and Ne are trapped in preformed c60 and C70.839 However, only very recently have some initial successes been reported on preparing macroscopic quantities of fullerene-encapsulated atoms: Chai et a1.I0 described a hybrid method (laser ablation of lanthana-graphite mixtures at elevated temperatures) for making Lacs2, an air-stable, extractablesoluble metallofullerene, in addition to a number of nonextractable, sublimable species including Lacso. They proposed that this species is describable as endohedral h2+.c822-, an unusual oxidation state for lanthanum. Subsequently, Johnson et al." employed the straight KH procedure to also produce soluble kc82 and reported an EPR spectrum attributable to it and interpretable as La3+C8z3-. Motivated by these developments, we have followed the second approach" (see below), in an attempt to prepare, separate, and characterize metallofullerenes such as Laca, following the route taken with the larger fullerene~.~ But our initial attempts uncovered, surprisingly, that La2Csowas the major extractable species. Experimental Methods Graphite sticks (Poco Graphite, Inc.), 1.5 in. long by 0.125 in. diameter, are drilled out (to 1.25 in.) by a 0.056 in. bit, and filled with a mixture of La203 (Fisher, 99%) and high-strength graphite cement (HSGC-Dylon Industries). Roughly, the filled rods are 20% La203by weight, corresponding to approximately 1.8 La per 100 carbon atoms. They are cured at 200 OC overnight and then heated to 900 OC in vacuum for at least 12 h. Each rod is used as a lower electrode in a gravity-driven resistive-heating setup described previously,2 and the other conditions are identical. From the total collected lanthanum/carbon material, several kinds of samples are prepared, including the raw material, or as extracted in various solvents (CS2, toluene, hexane), or the residual 'insolubles". These were handled in air without special precautions. The laser-desorption mass spectrometry (LDMS) procedure has been described previously.12 Samples as dissolved in solvents or as slumes are coated onto quartz or metal substrate rods, briefly heated to drive off liquid, and then mounted in vacuum, either (i) in the ion-extraction region of the reflection-typetime-of-flight instrument or (ii) in a gas-nozzle source. Radiation pulses s) from an excimer laser at 6.4 eV photon energy (ArF band) are *Towhom correspondence should be addressed. 'Present address: Laboratoire de Physique des Solides, Bitiment 510, 91405, Orsay Cedex, France. 0022-3654/91/2095-10561$02.50/0
used, at a fluence set for desorption conditions by attenuating, spatially filtering, and weakly focusing the radiation onto the sample. Charged desorption products of either sign are detected by pulsing the extraction voltage to f 2 kV, which acceleratesthem into the reflection mass spectrometer in high vacuum. Digitized time-of-flight waveforms are accumulated over 100-5000 pulses and are analyzed on a calibrated time-to-mass scale.
Results and Analysis Composition measurements have been made on variously treated samples obtained from two batches collected from vaporizing a total of eight La20,/graphite rods. Parts a and b of Figure 1 show positive- and negative-ion LDMS spectra obtained from the toluene extract (at 25 "C). Similarly, extracted samples of vaporized graphite containing no lanthana show only Car C70, and traces of the higher fullerenes c76, c78, and C84s2 However, now one finds also three higher mass peaks, corresponding to Lacsz, LaC&(H), and LazCBo. The fmt of these was reported,Io as h @ c 8 2 , to be an %uniquely stable" metal-carbon molecule, soluble in toluene and air-stable, originating from laser-vaporization of a lanthana-graphite composite rod under flowing argon gas at 1200 OC. It was later (1) (a) Kritschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990,347,354. (b) Kritschmer, W.; Fostiropoulos, K.; Huffman, D. R. Chem. Phys. Lett. 1990, 170, 167. (c) See also: Bethune, D. S.; Meijer, G.;Tang, W. C.; Rosen, H. J. Chem. Phys. Lett. 1990, 174, 219. (2) For descriptions of variations on this method, see: Ajie, H. et al. J . Phys. Chem. 1990,94, 8630. Haufler et al. Ibid. 1990,94, 8634. Whetten, R. L., et al. Muter. Res. Soc. Proc. 1991, 206, 639-650. Haufler, R. E., et al. Ibid. 1991, 206, 627-638. (3) Taylor, R.; Hare, H. P.; Abdul-Sada, A. K.; Kroto, H. J. Chem. Soc., Chem. Commun. 1990. 20. 1423. (4) Johnson, R. D.; Meijer, G.; Salem, J. R.; Bethune, D. S. J . Am. Chem. Soc. 1991, 113,3619. (5) Diederich, F.; Ettl, R.; Rubin, Y.; Whetten. R. L.: Beck. R. D.: Alvarez, M. M.; Anz, S. J.; Sensharma, D.; Wudl, F.; Khemani, K. C.; Koch, A. Science 1991, 252, 548. (6) Ettl, F.; Chao, I.; Diederich, F.; Whetten, R. L. Nuture 1991,352, 149. (7) For reviews of fullerene history and nomenclature, see: Smalley, R. E.; Curl, R. F. Science 1988,242, 1017. Kroto, H. Ibid. 1988,242, 1139. For early discussions of encapsulation, see: Heath, J. R.; OBrien, S. C.; Zhang, Q.;Liu, Y.; Curl, R. F.; Kroto, H. W.; Tittel, F. K. Smalley, R. F. J. Am. Chem. Soc. 1985, 107, 7779, and ref 14 below. (8) Ross, M. M.; Callahan, J. H. J. Phys. Chem. 1991,95,5720. Caldwell, K. A.; Giblin, D. E.; Hsu, C. S.; Cox, D. M.; Gross, M. L. J. Am. Chem. Soc., in press. (9) Wieske, T.; Bijhme, D. K.; Hrusik, J.; Kritschmer, W.; Schwarz, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 884; J . Phys. Chem., submitted for publication. (IO) Chai, Y.; Guo, T.; Jin, C.; Haufler, R.E.; Chibante, L. P. F.; Fure, J.; Wang, L.; Alford, J. M.; Smalley, R. E. J . Phys. Chem. 1991, 95, 7564. (1 1) Johnson, R. D.;deVries, M. S.; Salem, J.; Bethune, D. S.; Yannoni, C. S. Nuture; Electron Spin Resonance of La@& submitted for publication. (12) Beck, R. D.; St. John, P.; Alvarez, M. M.; Diederich, F.; Whetten, R. L. J . Phys. Chem. 1991, 95,8402. See also ref 2. Schriver, K. E. Ph.D. Thesis, University of California, 1990. Diederich, F.; Rubin, Y.; Knobler, C. B.; Whetten, R.L.; Schriver, K. E.; Houk, K. N.; Li, Y. Science 1989, 245, 1088.
(13) Cox, D. M.,et al., Isolation and characterization of Cm, submitted for publication.
0 1991 American Chemical Society
10562 The Journal of Physical Chemistry, Vol. 95, No. 26, 1991
Letters
90.0
80.0
.-c
E
A
70.0
Q, c
-C 80.0 50.0
40.0
9Cn Figure 2. LDMS for the insoluble fraction of the lanthana-graphite soot in the positive-ion mode.
toluene, or CS2. These experiments indicate that the solubility of La2Csois similar to fidlerenes containing about the same number of carbon atoms, lower than those containing much fewer, and higher than those containing much more (beyond Cg4,cf. ref 5): C60rC70
Figure 1. (a) LDMS in positive-ion mode, showing La2Cso+as only major high-mass peak. (b) Same as (a), but in the negative-ion mode, La2C8cis unusually abundant. (c) Same as (a), but extracted in boiling
toluene to obtain fullerenes larger than c84; see text for discussion. observed in the LDMS of Johnson et al.,” who also reported an EPR spectrum ascribed to it. The second is evidently derived from the fmt-we refer to it as an oxide (or hydroxide) impurity-and was not reported in refs 10 and 11. It is not clear whether the oxygen comes from the lanthana, from O2 impurity in the chamber, or from reactions of preformed Lacs2. (Similar oxide peaks, Ca0 and C7oO have been reported and correspond to stable, separable c o m p o ~ n d s . ~ The J ~ ) third, and by far the strongest, of these peaks occurs at 1238 f 2 amu, corresponding to La2Cm. It has no satellite above 2%of its intensity. The measurements were done under extreme low-fluence conditions, as evidenced by an absence of the characteristic fullerene from fragmentation peaks c 5 8 ’ and c68’. Figure I C shows a positive-ion spectrum obtained similarly, except that the extraction was performed in boiling toluene, in order to better dissolve the higher fullerenes. (The negative-ion spectrum for this sample is very similar.) Accordingly, one can now see, in addition to the above-mentioned peaks, the entire higher fullerene series from c76 to c96, including well-known maxima a t 76, 78, 84, 90, and 96.234 The Lacs2 peak lies very near (-5 amu) to C94rbut the La2Csopeak is clearly observed and is seen to have an intensity at least equal to c 7 6 even in the negative-ion spectrum. This is important, because earlier positive-ion mass spectra of fullerene/metal-fullerene mixtures were shown to be strongly biased by the lower ionization energies of the metal-containing species.14 The high abundance of La2CBo suggests that it can be separately obtained in macroscopic quantities using the same methods that yielded pure c 7 6 and two isomers of C78.691s The solubility properties have been explored by examining mixtures extracted by various means, using hexane, cold or boiling (14) Cox,D. M.;Trevor, D. J.; Reichmann, K. C.;Kaldor, A. J . Am. Chem. Soc. 1986, 208,2457. Cox,D. M.;Reichmann, K. C.; Kaldor, A. J . Chem. Phys. 1988,88, 1588. (15) Diederich, F.; Whetten, R. L.;Thilgen, C.; Ettl, R.; Chao, I.;.Alvarez, in press. M. M. Science, Fullerene Isomerism: Two Structures for CY8,
’C76rC78,C84,LaC82,La2C80’
c86-96
>> c98-200
This solubility sequence remains to be quantified by further experiments. We have also analyzed the insoluble fraction of the lanthanagraphite samples, in order to determine the extent of lanthanum-containing material in the overall sample. A typical result is shown in Figure 2. Normally, the spectrum for such insoluble material requires somewhat higher laser fluence to obtain and contains residual C60rC70, and so on as in the pattern already described (Figure IC), in addition to higher fullerenes from c98 up to the limits of the spectrometer sensitivity (see ref IC). However, in this spectrum all the strongest peaks beyond C70 correspond to La-containing fullerenes, intermingled with the C, abundance pattern described in the preceding paragraph. The following patterns are notable: 1. For the most part, the low-mass region agrees with the ‘sublimed” spectrum of Chai et a1.,I0 with Laca, and Lacs2 particularly abundant. 2. Beyond c84, the fullerene peaks (C,) are all much weaker than the adjacent (high-mass side) metallofullerene peak (LaCfF12). 3. The dimetallofullerene series La2C, becomes evident at n = 74, has a maximum at La2CBo,and achieves an equal intensity to the Lac, series a t La2Cloo. 4. The trimetallofullerene series has an onset at La3CIo2and rises sharply to dominate the pattern starting with La3ClM. 5. The relative decline of this series, to become a minority component at La3Cls0,could indicate the onset of dominance of an La& series a t La4Cj18. We have made no speaal effort to determine whether the metal atoms can be removed by photofragmentation (shrink-wrapping experiment16),although it is significant that our fragment patterns i.e., loss of cz,c4,c6. are the same for both ca and Furthermore, the metal loadings implied by these formulas are all smaller than the minima-La2C~ and La3C88-found by Chai et al.1° through laser vaporization of lanthana-graphite composites in an R-ICR spectrometer, for which such tests were made. [One should not confuse laser desorption of resistively vaporized composites under helium (KH procedure), as used here, with the laser vaporization mass spectrometric measurements made under much more severe conditions on unreacted material.] It might also be noted that the statistical weighting of La:C across the high-mass end of the spectrum is in line with what would be expected from (16) Weiss, F. D.; OBrien, S. C.; Elkind, J. L.;Curl, R. F.; Smalley, R. E. J . Am. Chem. SOC.1988, 110.4464.
Letters combinatorics (binomial distribution), given the rod‘s atomic composition of 1.8:lOO.
Discussion and Conclusions First, we are concerned with the differences between our results and those reported in refs 10 and 11. Second, we discuss the abundance and solubility of this molecule in relation to the possibility of obtaining separated samples as in the case of the larger fullerenes. Third, we present some speculation about the structure in light of the observation of a particularly stable/soluble series La,Cs,,-2n, n = 0, 1, 2. In our composition analysis by laser-desorption mass spectrometry, we find that the LazCs0abundance appears to be several times stronger than the previously observed Lacx2peak, although the former was not reported in refs 10 and 11. Two obvious differences between our procedures and theirs might account for this: 1. In our samples a relatively high La:C ratio is assured by the geometry of our loading-2W0 La203by mass (corresponding to 1.8:lOO atom ratio), as compared to 10% by the IBM group” and 7% by the Rice group.1° However, this “statistical factor” of two or three seems insufficient to account for the observed change. 2. The photoionization process is different, despite use of the same (6.4 eV) light sources, in that we ionize (or attach electrons) in the condensed phase as part of the desorption process, as contrasted with gas-phase photoionization, where the ionization potential (IP) is a critical parameter. It seems possible that, whereas the C, have IPS well above 6.4 eV and Lac, have IP < 6.4 eV, as was clearly demonstrated much earlier by Cox et al.,14 the La2Csomight also have IP > 6.4 eV and therefore be strongly discriminated. At present, we are inclined toward this second possibility. The high abundance of La2Csoin extracted samples is indicated by both the positive and negative ion mass spectra. Based on past experience in analyzing mixtures and pure samples of higher fullerenes, it appears to correspond to one percent or more of the total soluble materia1,29p6J2J5 prior to any attempt to optimize the production conditions. By all indications, it has solubility properties similar to the known fullerenes C76,C78(2), and Cs4, which have been purified in larger quantities (order of 0.01 g) by ~hromatography.6.~~ Given the recent trend in scaling up such separation^,'^ we believe it will be profitable to attempt such a separation for La2Csoand have begun collaborative work in this direction. However, it is important to realize that the mass spectral analysis of the undissolved fraction indicates that many other metallofullerenes are present and thus are “air-stable” in at least this sense;1° this includes Lacm, which may have a higher abundance than Lacsz or La2CXoand therefore be a more attractive target for separation once a suitable procedure is found. At the same time, our results add to the previously reported indications (on sublimed samples)lo that metallofullerenes do have preferential forms-magic numbers 82 and 80-that differ from the favored fullerene numbers-76,78, and 84 (with 80 absent and little or no 82).235 Finally, we turn to speculation on the structure of La2CS0,the basis for its preferential formation and extraction, and its possible relation to Lacsz and CS4. Assuming that both atoms are encapsulated within a fullerene cage, one can rely on the undisputed success of the principle of minimizing adjacent pentagonsI8-using its corollary, the isolated-pentagon rule (1PR)-in rationalizing fullerene magic numbers to eliminate all structures except the seven IPR-satisfying structures for fullereneCXo.l9Among these, it seems reasonable to consider elongated (prolate) structures, such as the D5aCso structure2’ (homologue to the realized zh-cm and (17) Achiba, Y. et al., submitted for publication. (18) See reviews cited in ref 7, and references therein. (19) Manolopoulos, D. E.; Fowler, P. W. Chem. Phys. Lert., in press.
The Journal of Physical Chemistry, Vol. 95, No. 26, 1991 10563 D5h’C70) or the twisted DZ-Cwstructure (homologue to the realized D2-C76and Dz-C84)6321 instead of oblate and quasi-spherical structures, like Z~-C80.zo Beyond such structural considerations, Chai et a1.I’ have pointed out an intriguing numerical coincidence that appears to link the most stable/extractable of the higher fullerenes, CX4and (276, to the high abundances of Lacsz and Lac7,. It was proposed that there is a high stability associated with 84 and 76 p?r electrons on the fullerene, an arrangement that would involve an unusual La2+oxidation state. By extension, LaZCs0would be explained similarly as (Laz+)2.Cso4-,preserving the 84-electron p?r count. The high stability/extractability series is thus written concisely as La,Csek. In practice, the small lanthanum hyperfine coupling observed for Lacs2, along with its g value, strongly suggests that all three La electrons are transferred to orbitals derived from carbon pr orbitals, i.e. La3+.Cs2*. Such a conclusion would largely destroy the reasoning underlying the proposal of an isoelectronic series. There remains a second possibility that has not been adequately ruled out, namely, the metal atoms in these extractable compounds could be part of the cage’s bonding network, and somehow each takes the place of two carbon atoms (in contrast to boron substitutionZ2),naturally explaining the series La,Csc2,. Then the high stability of the extracted species would be accounted for by a special 84-fullerene structure, e.g., DZ-Cs4,rather than electron count. In their discussion of the EPR spectra of Lacs2, Johnson et al.” considered just the two extreme cases-extemal attachment vs encapsulation-and ruled out the former, based partly on the moderate strength of the anisotropic broadening appearing in the solid-state spectrum. However, it is possible that even this moderate broadening is too large to be explained by an encapsulated atom, preserving the quasi-spherical nature of the fullerene, whereas a site bonded directly to a few carbon atomsz3breaks the symmetry and could thus explain the sensitivity of the La hyperfine component line widths to the solid state. We therefore know of no experimental result, on condensed-phasesamples or molecular beams, that rules out this possibility. After this work was substantially complete, we learned that Weaver et al.24have completed a series of experiments on yttrium-fullerenes, Y,C,, in which a soluble compound Y2C8z has been observed, as “the first multimetal endohedral complex fullerene to be observed as a stable species in solution, YZ@c82=. Second, in subsequent work, Yeretzian et aLz5have scattered La2&,- with silicon surfaces at energies to 190 eV without fragmentation (on the lo4 s time scale), leading to a estimated lower bound for the fragmentation of 3.9 eV (90 kcal/mol), in support of the notion of tightly bound or encapsulated La atoms. Acknowledgment. We thank Richard E.Smalley and Robert D. Johnson for many helpful and stimulating discussions on preparation methods and for providing preprints of refs 1,2, and 24, W. J. Evans for advice on lanthanum chemistry, and Franpis Diederich for encouragement and for discussions on metallofullerenes and higher fullerenes. This work has been supported by an NSF grant (to R.L.W.) and by Packard Foundation Fellowships (to R.B.K. and R.L.W.). (21) Labastie, P.; Whetten, R. L.; Cheng, H.-P.; Holczer, K., submitted to Chem. Phys. Lett. Fowler, P. W. J. Chem. Soc. Faraday Trans. 1991,87, 1945. (22) Smalley, R. E., et al., ACS Symp. Ser., in press. One possibility is analagous to lanthanum-phthalocyanine, in which the metal is coordinated in a square-planar arrangement to the N atoms of pyridine-like rings; in a fullerene, that La would replace a C2unit and lie at the intersection of four five-membered rings. (23) A moderate-strength interaction is suggested by bis-$-arene complexes Anderson, D. M.; Cloke, F.G.N.; Cox, P. A.; Edelstein, N.; Green, J. C.; Pang, T.; Sameh, A. A.; Shalimoff, G. J. Chem. Soc., Chem. Commun. 1x19,53, 1989. (24) Weaver, J. H.; Chai, Y.; Kroll, G. H.; Jin, C.; Ohno, T. R.; Haufler, R. E.; Guo, T.; Alford, J. M.; Conceicao, J.; Chibante, L. P. F.; Jain, A.; Palmer, G.; Smalley, R. E. Nature, submitted for publication. (25) Yeretzian, C., et al., to be published.
