Description
High-purity [5,6]-Fullerene-C70 (carbon 70 fullerenes, or C70) for use in thermal evaporation systems, where increased optical absorption is important.
Purification Methods
It was purified from the soluble toluene extract (400mg) of the soot (Fullerite) formed from resistive heating of graphite by adsorption on neutral alumina (100g, Brockmann I, 60 x 8cm). Elution with toluene/hexane (5:95 v/v) gives ca 250mg of quite pure C6 0. It has characteristic spectral properties (see below). Further elution with toluene/hexane (20:80 v/v, i.e. increased polarity of solvent) provides 50mg of "pure" C7 0 [Allemand et al. J Am Chem Soc 113 1050 1991]. Chromatography on alumina can be improved by using conditions which favour adsorption rather than crystallisation. Thus the residue from toluene extraction (1g) in CS2 (ca 300mL) is adsorbed on alumina (375g, standard grade, neutral ca 150 mesh, Brockmann I) and loaded as a slurry in toluene/hexanes (5:95 v/v) to a 50 x 8cm column of alumina (1.5Kg) in the same solvent. To avoid crystallisation of the fullerenes, 10% of toluene in hexane is added quickly followed by 5% of toluene in hexane after the fullerenes had left the loading fraction (2-3hours). With a flow rate of 15mL/minute the purple C6 0 fraction is eluted during a 3-4hour period. Evaporation of the eluates gives 550-630mg of product which, after recrystallisation from CS2/cyclohexane yields 520-600mg of C60 which contains adsorbed solvent. On drying at 275o/10-3mm for 48hours a 2% weight loss is observed although the C60 still contains traces of solvent. Further elution of the column with 20% of toluene in hexane provides 130mg of C7 0 containing 10-14% of C60 (by NMR). This was rechromatographed as above using a half scale column and adsorbing the 130mg in CS2 (20mL) on alumina (24g) and gave 105mg of recrystallised C7 0 (containing 2% of C6 0). The purity of C6 0 can be improved further by washing the crystalline product with Et2O and Me2CO followed by recrystallisation from *C6H6 and vacuum drying at high temperatures. Carbon soot from resistive heating of a carbon rod in a partial helium atmosphere (0.3bar) under specified conditions is extracted with boiling *C6H6 or toluene, filtered and the red-brown solution is evaporated to give crystalline material in 14% yield which is mainly a mixture of fullerenes C6 0 and C7 0. Chromatographic filtration of the 'crude' mixture with *C6H6 allows no separation of components, but some separation was observed on silica gel TLC with n-hexane or n-pentane, but not cyclohexane as eluents. Analytical HPLC with hexanes (5_m Econosphere silica) gave satisfactory separation of C6 0 and C7 0 (retention times of 6.64 and 6.93minutes respectively) at a flow rate of 0.5mL/minute and using a detector at 256nm. HPLC indicated the presence of minor (<1.5% of total mass) unidentified Cn species with retention times of 5.86 and 8.31minutes. Column chromatography on flash silica gel with hexane gives a few fractions of C6 0 with 95% purity, but later fractions contain mixtures of C6 0 and C7 0. These can be obtained in 99.85 and >99% purity, respectively, by column chromatography on neutral alumina. [Ajie et al. J Phys Chem 94 8630 1990.] Separation of C60 and C7 0 can be achieved by HPLC on a dinitroanilinopropyl (DNAP) silica (5_m pore size, 300. pore diameter) column with a gradient from n-hexane to 50% CH2Cl2 using a diode array detector at wavelengths 330nm (for C6 0) and 384nm (for C7 0). [Cox et al. J Am Chem Soc 113, 2940, 1991.] Soxhlet extraction of the "soot" is a good preliminary procedure, or if material of only ca 98% purity is required. Soxhlet extraction with toluene is run (20 minutes per cycle) until colourless solvent filled the upper part of the Soxhlet equipment (10 hours). One-third of the toluene remained in the pot. After cooling, the solution was filtered through a glass frit. This solid (purple in toluene) was ca 98% C6 0. This powder was again extracted in a Soxhlet using identical conditions as before, and the C60 was recrystallised from toluene to give 99.5% pure C60. C70 has greater affinity than C6 0 for toluene. [Coustel et al. J Chem Soc, Chem Commun 1402 1992.]
Purification of C60 from a C60/C70 mixture was also achieved by dissolving it in an aqueous solution of (but not ) cyclodextrin (0.02M) upon refluxing. The rate of dissolution (as can be followed by the UV spectra) is quite slow and constant up to 10-5M of C60. The highest concentration of C60 in H2O obtained was 8 x 10-5M and a 2 cyclodextrin:1 C60 clathrate is obtained. C60 is extracted from this aqueous solution by toluene and C60 of >99 purity is obtained by evaporation. With excess of cyclodextrin more C6 0 dissolves and the complex precipitates. The precipitate is insoluble in cold H2O but soluble in boiling H2O to give a yellow solution. [Andersson et al. J Chem Soc, Chem Commun 604 1992.]
C60 and C70 can also be readily purified by inclusion complexes with p-tert-butylcalix[6] and [8]arenes. Fresh carbon-arc soot (7.5g) is stirred with toluene (250mL) for 1hour and filtered. To the filtrate is added p-tertbutylcalix[ 8]arene, refluxed for 10minutes and filtered. The filtrate is seeded and set aside overnight at 20o. The C60 complex separates as yellow-brown plates and is recrystallised twice from toluene (1g from 80mL) to give a 90% yield. Addition of CHCl3 (5mL) to the complex (0.85g) gave C60 (0,28g, 92% from recrystallised complex).
p-tert-Butylcalix[6]arene-(C60)2 complex is prepared by adding p-tert-butylcalix[6]arene (4.4mg) to a refluxing solution of C60 (5mg) in toluene (5mL). The hot solution is filtered rapidly and cooled overnight to give prisms (5.5mg, 77% yield). Pure C60 is obtained by decomposing the complex with CHCl3 as above. The p-tert-butylcalix[6]arene-(C70)2 complex is obtained by adding p-tert-butylcalix[6]arene (5.8mg) to a refluxing solution of C70 (5mg) in toluene (2mL), filtering hot and slowly cooling to give red-brown needles (2.5mg, 31% yield) of the complex. Pure C7 0 is then recovered by decomposing the complex with CHCl3. Decomposition of these complexes can also be achieved by boiling a toluene solution over KOH pellets for ca 10minutes. The calixarenes form Na salts which do not complex with the fullerenes. These appear to be the most satisfactory means at present for preparing large quantities of relatively pure fullerene C60 and C70 and is considerably cheaper than previous methods. [Atwood et al. Nature 368 229 1994.]
Repeated chromatography on neutral alumina yields minor quantities of solid samples of C76, C84, C90 and C94 believed to be higher fullerenes. A stable oxide C70 has been identified. These have been separated by repeated flash chromatography on alumina with gradient elution using hexane/toluene mixtures (starting from 95:5 and increasing proportions of toluene until the ratio of 50:50 was attained) [Diederich et al. Science 2 5 2 548 1991].
