Bromination of C60 in benzene the product. C60Br6 with an average of one occluded bromine molecule was synthesized and crystallographically determined.
Fullerenes Sciences & Technology, 6 (1998) 689-694
CATALYTIC PREPARATION AND CHARACTERIZATION OF C60Br24 1 A. Djordjevic1*, M. Vojinovic-Miloradov , N. Petranovic2, 2 A. Devecerski ,
D. Lazar3 and B. Ribar3 1
Faculty of Sciences, Department of Chemistry, University of Novi Sad, Yugoslavia, 2Faculty of Physical Chemistry, University of Belgrade, Yugoslavia, 3 Faculty of Sciences, Department of Physics, University of Novi Sad, Yugoslavia ABSTARCT In this paper the procedure for catalytical bromination of C60 with elementary bromine with FeBr3 as a catalyst is described. In this procedures only one reaction product - C60 Br24 is obtained. The twenty four bromine atoms are symmetrically distributed over the C60 sphere, which was confirmed by thermogravimetric analysis. The yield of bromine derivative in this reaction is 98%. INTRODUCTION The C60 molecule has attributes of a ″big atom″ (approximately 1 nm), since it has a close spherical electronic shell and possesses a unique icosahedral symmetry processes is in most cases not yet clear. Derivatizations with halogens are very important reactions in the synthesis of significat intermediates in various properties and the fifth order symmetry axis. There are 60 equivalent C-atoms, 20 hexagonal faces and 12 additional pentagonal faces to form carbon cluster cage. The caracteristic reactions are therefore addition. The mechanism of the addition nucleophilic reactions. In the first attempt to halogenate C60, it was fluorinated
*
Faculty of Sciences, Department of Chemistry, University of Novi Sad, Trg Dositeja Obradovi}a 3, 21000 Novi Sad, E-mail dva@eunet,yu
[2,3], chlorinated and brominated [4,5] under non-controlled reaction conditions, and the products were not defined or exactly explained, containing 24 to 28 chlorine and bromine atoms per C60 molecule. By elementary microanalysis it was determined that molecule C60 binds 28 bromine atoms, of which 24 are covalently bound, while 4 bromine atoms (two bromine molecules) are occluded on the surface of the sphere [6,7]. During thermal degradation of bromine adduct on 150°C for 24 hours, all bromine atoms are eventually lost [4] in cascade, which was observed by IR spectroscopy. C60Br24 (1, 3, 6, 8, 11, 13, 16, 18, 21, 23, 26, 28, 31,33,36,38,41,44,46,49,51,54,57,60-tetra-icosa-bromofulleren) is an orange compound, insoluble in meny organic solvents. C60Br24 has a simple IR spectrum, which indicates the symmetrical distribution of addends on the sphere. Bromination of C60 in CS2 or chloroform yields C60Br8 in 80% and 58% yield, respectively. C60Br8 (1,3,6,11,13,18,28,31-octabromo-fullerene) is insoluble in organic solvents and very soluble in elementary bromine. X-ray analysis of a monocrystal has shown that the substance crystallizes with bromine molecules which are chaotically distributed. IR spectrum of C60Br8 is much more complicated, which is in accordance with its symmetry. Bromination of C60 in benzene the product C60Br6 with an average of one occluded bromine molecule was synthesized and crystallographically determined [6].
C60Br6
moderately
(1,6,9,12,15,18-hexabromo-fullerene) soluble
in
organic
solvents.
It
is
is less
symmetrical compared to the previous two derivatives [6,7].
During
thermical
degradation
in
CCl4,
this
molecule is disproportioned in to C60Br8 and C60. The instability of C60Br6 and C60Br8 can be explained by the presence of the butadiene fragment in the central pentagon, as well as eclipse 1,2-bromine steric interactions in C60Br6. The high stability of C60Br24 can be explained by the reduction double and reorganisation single bonds, which require the lowest energy of the addition derivative of C60. This structure was justified by the X-ray analysis of a monocrystal [8]. MATERIALS AND METHODS Bromine derivative C60Br24 was obtained in the reaction between C60 and elementary bromine in the presence of the catalytic quantities of FeBr3 according to the following procedure: 300 mg (99.8%) C60 and 2 ml of elementary bromine in the presence of the catalytic quantities of FeBr3 was stirred for 40 minutes at room temperature. After completing the reaction the excess of unreacted bromine was evaporated and the catalyst was separated by dissolving in mixture ethanol-H2O 1:2. In the reaction vessel 1077 mg of an orange-brown powder was left, which represents the bromine derivative of C60. The product is insoluble in organic solvents, and dissolves very weakly in dimethylsulfoxide. IR spectra were taken in KBr on the Perkin Elmer 457 apparatus (Faculty of Sciences, Department of Chemistry, University of
Novi Sad), and
thermogravimetric analysis was completed at DuPont 1090 TA, TGA 951 (Faculty of Physical Chemistry University
of
Beograd). X-ray diffractograms were taken at Phillips powder
diffractograph, PW 1020, V=30 kV, A=30mA (Faculty of Sciences, Department of Physics University of
Novi Sad).
RESULTS AND DISCUSSION C60Br24 is an orange powder insoluble in organic solvents. Dissolving C60Br24 in ethyl-methyl-ketone eliminated bromine atoms from the C60 sphere, which was manifested by the color change and the appearance of the black precipitate of C60. The IR (KBr) spectrum and 13C NMR (CDCl3, C6D6; 1:10) spectrum of the black precipitate contain bands characteristic for fullerene C60: 527,576,1183,1430 cm-1 and δ=143.0 ppm respectively [9,10]. IR spectrum of C60Br24 (KBr) contains characteristic bands at: 1250,1180,1140,1080,1050,960,930,850,780,760,730, 600, 560, 550 cm-1. IR spectrum of the catalytically synthesized C60Br24 (Fig. 1) is in accordance with the literature data [6]. The thermogravimetric curve contains only one sharp peak at the temperature of 165.2°C and the loss of mass is 72.59%. The sharp peak in the TGA curve (Fig.2) shows that in the process of thermal transformation all bromine atoms are lost, which is the characteristic of the completely symmetrical distribution of bromine over the C60 molecule. Experimental loss of mass of 72.59% is in accordance with the theoretical loss of 24 bromine atoms of 72.70%. Microanalysis conforming the existens of 24 bromine atoms on C60 molecule. In this reaction of the catalytic bromination only one reaction product is obtained in yield of 98 %, without occluded bromine atoms and without presence of organic solvents. The IR, 13C and X-ray spectrum was taken on the remains of thermogravimetric analysis, and contains bands characteristic for C60 molecule, which points to the one-step loss of all bromine atoms from the product. Experimental evidence of the symmetry of C60Br24, which is obtained by the thermogravimetric analysis, is completely in accordance
with the theoretical predictions of the stability of this molecule. The results of Xray analysis of C60Br24 powder are as follows d values: 4.13,3.97, 3.91,3.68,3.56,3.43,3.34,3.21,3.15,3.05,3.02,2.93,2.82,2.74,2.62,2.55,2.45,1.83, 1.81,1.77,1.70 (Fig.3). Energetically lowest, i.e. most stable structure is the one in which 30 double bonds are located at the junctions of hexagons (6,6-double bonds), i.e. in exo-position to the pentagons. Minimization of 5,6-double bonds is one of the basic, dominant factors, which directs the regiochemistry of the addition reactions [1] . In these reactions the first step is always addition in 1,2position at 6,6-double bond, without the introduction of 5,6-double bonds. In the case of voluminous addends, the rearranging to 1,4-position because of the eclipse interactions between them follows 1,2-addition. The distribution of 24 added bromine atoms over the C60 molecule is represented in the Schlegel diagram in Fig.4. CONCLUSION The catalytic bromination of C60 in the presence of FeBr3 in short time results in one reaction product - C60Br24, without occluded bromine atoms and solvent molecules in the 98.0% yield. The separation of the reaction mixture is a simple evaporation of the unreacted bromine and the dissolving of FeBr3.
The sharp
peak in the curve obtained by thermogravimetric analysis shows that in the process of thermal transformation all bromine atoms are lost in one step, which is the characteristic of the completely symmetrical distribution of bromine over the C60 molecule. Experimental loss of mass of 72.59% is in accordance with the theoretical loss of 24 bromine atoms of 72.70%. The structure of C60Br24, is in accordance with the theoretical predictions of the reactions of this type. The defined structure of the bromine derivative enable procedures of the further derivatization.
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Fig1. IR specrum C60Br24
Fig. 2. TG curve C60Br24
Fig.3 Diffractogram of C60Br24
Fig 4 Schlegel diagram of C60Br24
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