J. Phys. Chem. 1992,96,9609-9612
9609
Anaeroblc Sampllng and Characterization of Lanthanofullerenes: Extraction of Lac,, and Other Lac,, Shunji Bandow, Hiroshi Kitagawa, Tadaoki Mitani, Hiroo Inokuchi, Institute for Molecular Science, Myodaiji Okazaki, 444 Japan
Yahachi Saito, Department of Electric and Electronic Engineering, Mi'e University, Tsu 51 4, Japan
Hiroki Yamaguchi, Naoyuki Hayashi, Hiroyasu Sato, and Hisanori Shinobara* Department of Chemistry for Materials, Mi'e University, Tsu 514, Japan (Received: September 21, 1992)
Several new types of lanthofullerenes including have been produced and extracted by a newly developed fullerene arc generator equipped with an anaerobic (air-free) sampling and extraction mechanism of metallofullerenes. Other than the reporterd Lacs2,at least three different lanthanum metallofullerenes have been extracted by toluene and observed by ESR and mass spectrometry under anaerobic sampling conditions. The newly extracted lanthanofullerenesexhibit equally-spaced octet ESR lines of equal signal intensities with different hyperfine coupling constants (0.44,3.15,2.12G) and g values (2.0043, 2.0042, 2.0010). Furthermore, these lanthanofullerenes are found to be air-sensitive, and in particular, the ESR signal intensity for one of the newly found metallofullerenes has decreased drastically to less than 5% of the original intensity when the sample and Lacs4. was exposed to air for 5 days. The probable candidates for these fullerenes are presented, including
Introduction Since the first extraction of a lanthanum fullerene, La@Cs2, from the laboratory-produced soot by Chai et al.,I several endohedral metallofullerenes, including La@C82,24 Y @ C8z,3,5,6 and Sc@C82,3~7,8 have kcen produced, extracted, and characterized by ESR and mass spectrometry. One of the most intriguing and important properties of these novel forms of molecules is that a substantial charge transfer from encaged metal atoms to the fullerene cages is occurring and that these species can formally be described as La3+@Cs23-,2 Y3+@C823-:and
[email protected] Furthermore, even a metal trimer has been found to be encapsulated by the fullerene c82, Le., Sc3@Cs2,as revealed by ESR and mass spectr~metry.~,~ Because of the scarcity of the metallofullerenesof the (M),Csz type present in laboratory-produced soot generated by the metal/graphite composite arc method, ultrasensitive and speciesspecific detection techniques such as electron spin resonance and mass spectrometry have been used for characterizing the metallofullerenes. So far, the reported solvent-extractable metallofullerenes are of the (M),Csz ( n = 1, 2, 3) typel-10with a few exceptions such as L2@CsO?Y2@Cs4,596 and
[email protected] the present stage, the metallofullerenesencaged by Cm have not been extracted and characterized by electron spin resonance. The probable reasons for this are (i) the M@C60 fullerenes are airsensitive and are subjected to reactions with oxygen or water, so that the normal "aerobic" sampling and extraction are not suited (ii) the for the detection and ESR characterization of M@Cmfullerenes are insoluble to various solvents (in contrast to M @ Cs2);and (iii) the M 8Csofullerenes are simply not formed, at least, by the metal/graphite composite arc method. Under these circumstances, if reason i is responsible for the inability to extract M @ C60 by various solvents, "anaerobic" (air-free) sampling, extraction, and characterization should be employed for investigating the presence or nonpresence of M@Cmand other M@Ch types of metallofullerenes. and In the present Letter, we report the extraction of other Lach types of lanthanofullerenes under "anaerobic" sampling conditions by using a newly developed fullerene generator" which is equipped with an anaerobic sampling and solvent-extraction mechanism. The so-prepared metallofullerenes are then characterized by ESR and laser-desorption mass spectrometry also under anaerobic conditions. The ESR measurements of these To whom correspondence should be addressed.
0022-365419212096-9609$03.00/0
samples reveal the presence of, at least, three new octet lines with different hyperfine coupling constants and g values, which are different from the reported octet absorption due to
[email protected] octet signals are either absent or only very weakly observed under aerobic sampling/characterization condition^.^
Experimental Section Details of the production of carbon raw soot containing metallofullerenes by the arc method, which is equipped with an "anaerobic" (air-free) sampling and extraction mechanism of the soot, will be published elsewhere." The present apparatus is specially designed for the production, sampling, and solvent extraction of metallofullerenesunder complete anaerobic conditions. The main idea of the present soot generator is based on the so-called "gas flow-cold trap" method.I2 The carbon soot containing various lanthanofdlerenes was produced in the atmosphere of helium gas flow. Under the constant flow of helium gas (100 Torr, 43 Torr-L/s), an arc between a graphite rod (negative electrode) and a lanthnum-graphite composite rod (positive electrode) was sparked at a current density of 2.3-2.5 A/mmz in the dc mode. The vapor of extraction solvents such as toluene (which was distilled in an Ar atmosphere from CaH2 to remove even a trace amount of water and was completely degassed by the freeze and thawing method in vacuo) was introduced in the helium gas stream and then trapped on the cold wall cooled with liquid nitrogen. After the collection of the soot with the solvent, the trap was filled with Nzgas at 1 atm for a further anaerobic handling of the sample. The lanthanum-graphite composite rods (1 3 mm in diameter; 8 wt %) were purchased from Toyo Tanso Co. Ltd. The extracts were analyzed by laser-desorption time-of-flight mass spectrometry (LD-TOF-MS).6p7 Samples as dissolved in various solvents were coated onto a quartz plate, quickly heated to drive off liquid, and then mounted in vacuum in the ion-extraction region of a reflectron type TOF mass spectrometer. All of these handlings of the sample were performed under dry nitrogen flow conditions to achieve an anaerobic sample preparation. Since the fragmentation of ions complicates the mass spectral identification of parent (intact) ions, the desorption (and ionization) laser (Nd:YAG third harmonic, Spectra-PhysicsGCR-11) was kept at very low fluence below 100 pJ/l-mm-diameter spot on the sample. The accelerationvoltage was typically in the range 1500-1700 V. The time-of-flight signals were accumulated over 200-600 shots and were analyzed by a transient digitizer (Lecroy 0 1992 American Chemical Society
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9610 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992
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H/G Figure 1. Typical ESR spectrum of “anaerobically” sampled and extracted lanthanofullerenw in toluene. The spectrum was recorded at the microwave frequency of 9.4299 GHz with 0.2-mW power at 200 K. Besides the main octet (a), four other octets (b-e) are observed.
TABLE I: Observed ESR Spin Panmeters of Anaerobically Sampled Laatbanofullerenes obsd octet’ A. G g value M.G re1 intensit9 a (La@C,,) 1.20 2.0008 0.049 100 2.0002 0.052 66 0.83 b 2.0043 0.061 27 c (Lac,,)‘ 0.44 0.067 7 3.15 2.0042 d 2.0010 0.037 2 e 2.12
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F i w e 2. ESR spectra of (A) the normal aerobically_prGa .. I lanthanohllerenes in tdluene at 220 K and (B) the column chromatographed portion of the sample (A) at 220 K. The column chromatography was performed by using an ethanol-deactivatedsilica gel column with a 100% toluene eluent. The deactivation was done by using ethanol-hexane (1:l) mixed solution. Note that the lanthanofullerene giving octet b does not come out from the present column chromatography.
~~~
a For the assignment of octets a-e, see Figure 1. Relative intensity is scaled to the integrated intensity of La@Cs2(100). cTentative assignment (see text).
9400 A). The conversion from time to mass scale and the analysis of the TOF spectra were performed on a personal computer (NEC PC9801-VX). Electron spin resonance spectra were recorded by an ESR spectrometer (Bruker ESP 300 E) in a X-band frequency at 9.42 GHz with typically 0.2-mW power. The anaerobically extracted and filtered solution was scaled in a thin-wall quartz tube. Special efforts were made to obtain better spectral resolution. We have found that ESR line widths of the metallofullerenes attain the minimum values at 220 K, so that the ESR spectra presented in this Letter were recorded at this temperature.
Results and Discussion Observation of Five Different Octet ESR Absorption Dw to Lac2,.Figure 1 shows a typical ESR spectrum of lanthanofullerenes diluted in toluene, which is sampled, extracted, and measured under completely uanaerobicn (air-free) conditions. Besides the strongest octet lines (series a) of La@Cs2,four different octet lines (series b-e) are observed with Werent line widths (AH), hyperfine coupling constants (A), and g values. The observed AH, A, and g values are summarized in Table I along with the relative abundance of the five octets. The temperature dependence of the line width indicates that AH attains the minimum value at 220 K. All of the ESR measurements reported here are made at this temperature to obtain better spectral resolution unless otherwise described. Octet b has been assigned by Suzuki et ale3as one of the La@Cs2isomers which have different Cg2cages. Structural isomers of the ‘hollow” Ca2fullerene have been analyzed by ”C NMR measurements by Kikuchi et al.I3 Very recently, Hoinkis et ai? have also reported the presence of octet b. On the basis of mass spectral evidence that the only lanthanofullereneobserved for the extracts was La@c82, they have ascribed octet b to three probable species: (i) the La atom occupies a different position from that of octet a in the cs2 cage, (ii) one of the La@Cs2 isomers, or (iii) adducts of h @ c g 2 (i.e., octet a) with solvent or hollow fullerenes. The reported relative intensities, I(octet b)/
Z(octet a), are 27% and 33% by Suzuki et aL3and Hoinkis et a1.,4 respectively. We have also observed octet b not only in aerobic but also in anaerobic sampling conditions. However, in anaerobic sampling and characterization, we have found three distinct features which, apparently, are not observed by the previous studies3v4under aerobic conditions. Firstly, the relative intensity, I(octet b)/Z(cctet a), was much larger than the above aerobic values (27% and 33%):~~ typically in the range 5046%. Secondly, we have found that the intensity of octet b decreased when an anaerobically prepared sample was exposed to air: the ESR signal intensity, for example, decreased to 70% of the original intensity upon exposing it to air for 4 days, whereas that of Octet a (due. to La@Ca2)remained almost the same within an experimental uncertainty. Finally and most importantly, octet b can be separated from octet a by column chromatography. Figure 2 shows ESR spectra of (A) conventionally prepared lanthanofullerenes in toluene under the aerobic sampling and of (B) a column chromatographed sample of (A) by using an ethanol-deactivated silica gel column with toluene eluent. The second and third results indicate that octet b possesses a much higher degree of air sensitivity and a much greater ability of adsorption to the silica column than those of octet a (due to La@&). In order to further investigate the origin of octet b in Figure 1, we have measured laser-desorption (LD) time-of-flight (TOF) mass spectra of the anaerobically-preparedlanthanofullerene sample. Figure 3 exhibits a typical LD-TOF mass spectrum at 355 nm of the anaerobically-prepared sample. As in the previous rep o r t ~ ,beaides ~ ~ *the ~ ~hollow ~ ~ fullerenes (Ca,c70, C76,c78,Cs4) the most intense peak is due to La@Cs2. In addition, the mass spectrum reveals two other distinct peaks due to lanthanofullerenes: and Lacw We have tentatively assigned octet c in Figure 1 to on the basis of air-sensitivity experiments (cf. section 3.2). Unfortunately, at present it is not possible to unambWusly identify the origin of octet b. However, in addition to the reported assignment^,^,^ the mass spectra and air-sensitivity experiments (cf, section 3.2) seem to suggest that this multiplet is associated with another type of the lanthanofullerene such as Laca,. ~ ~ h ~ p r e p u e d u r Conditioos. Figure 1 exhibits at least three other octet ESR signals: octets c-e. The observed g values and the hyperfine coupling constants of octets c and d are the same as Hoinkis et al.4 within experimental errors. Octet e is newly found in this experiment. The line widths are much narrower in the present measurement than those of Hoinkis et al. because the observation
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The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 9611
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Time-of-Flight /,us Figure 3. Laser-desorption (355 nm) TOF mass spectrum of an anaerobically prepared toluene extract involving lanthanofullerenes. The anaerobically prepared sample was handled and set in the ionization region of the TOF mass spectrometer under anaerobic (nitrogen flow) is particularly conditions. Note that the peak corresponding to enhanced in addition to La@(&. 9 = 2.0042
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Figure 4. High-resolution ESR spectra of the octet c region measured at 220 K in toluene: (A) an anaerobically prepared sample as that of Figure 1; (B) the normal aerobically prepared sample, showing no trace of octet c. The peaks marked by circle and triangle belong to octets d and e, respectively. The prominent peaks (sealed out) in (A) and (B) are due to octet a (cf. Figure 1). The hyperfine structure appearing in both high and low magnetic field of the prominent peak in (B) is due to hyperfine coupling to ')C in natural abundance on the fullerene cages.
temperature is different (room temperature vs 220 K). The most important point of the present observation is that the relative intensities Z(octet c)/Z(octet a) and Z(octet d)/Z(octet a) are 27% and 71,respectively. The values are much larger than those of Hoinkis et al. (less than where the lanthanofullerenes were prepared and characterized under aerobic conditions. Octet c is expanded in Figure 4A to show further details of the multiplet. The comsponding ESR spectrum of a lanthanofullerene sample, which lacks octet c, is also presented in Figure 4B for comparison. Besides the main octet, further hyperfine structures are seen. These structures appeared in both high and low magnetic field sides of one of the main (octet a) absorption and have been assigned as hyperfine coupling to 13C in natural abundance on the carbon cages.) In Figure 4A, there are still other hyperfine structures. The peaks marked by a circle and triangle can be assigned as one and two of octets d and e, respectively (cf. Figure 1). In octet c, the peaks due to hyperfine coupling to IT, similar to those of octet a, are difficult to trace at the present ESR spectral resolution. Hoinkis et aim4 have suggested that this multiplet is possibly associated with Lac7+ AS described above' the present anaerobic experiment can produce a fairly good amount of Octet c (27% relative to octet a) so that some quantitative descriptions for the multiplet become
possible: First, octet c has showed a much stronger air sensitivity than that of octet b. For instance, upon exposing to air for 4 days in toluene solution, the relative ESR signal intensity, I(octet c)/I(octet a), decreased from 27% to 1096, and a further exposure finally led to the value of the normal aerobic sampling, 1-21,as reported by Hoinkis et al.4 Furthermore, it was found that the decrease of the signals was much accerelated when the sample was stored and exposed to air in the solid (extract powder) form. Second, LD-TOF mass spectra (cf. Figure 3) show a relatively strong peak due to h c 7 6 . The mass spectral intensity of also decreased in, qualitatively, the same fashion as observed in the corresponding ESR signals. On the other hand, Lac7( has been barely detectable in the LD-TOF mass spectrum of the normal (aerobically prepared) lanthanofullerene sample. Lastly, octet c has not been detected (i.e., less than 1% relative to octet a) whenever the aerobic samples (in the form of the original soot or extracts) are exposed to air for more than a day before the ESR measurement. On the basis of these observations, at present we think that octet c stems most probably from which is unstable in the ambient atmosphere. However, further studies (such as the separation of from LacB2)are necessary to unambiguously identify the origin of octet c. On account of the observed instability of toward air, it is highly probable that hc76 has a tendency to form nonparamagnetic complexes with oxygen or to form the adducts with solvent molecules or hollow fullerenes.4J0 The origin of the special reactivity of as compared with that of LacB2,is not clear at the moment. This undoubtedly is related to the structure of Lac7,, which involves the crucial problem of the endo- and exohedral nature of this metallofullerene. Other Solvent-Extractable Lanthaaofullerenes Lact, under Anaerobic Conditions. Figure 1 also reveals other octets labeled as (d) and (e). Obviously, these equally-spaced octets of equal intensity are due to other types of lanthanofullerenes, Lach. Because of signal-to-noise limitations, unambiguous correlations between ESR and mass spectroscopicobservations are not possible for these multiplets. However, the air-sensitivity experiment has revealed that these octets are extremely air-sensitive, to such an extent that the relative intensities, Z(0ctet d)/Z(octet a) and Z(0ctet e)/Z(octet a), decreased from 7% and 2% to the aerobic sampling values of 1% and
