J. Phys. Chem. 1980, 84, 423-427
423
Electron Spin Resonance Study of the Generation of Stable Arene Radical Cations in Molten Antimony Trichloride A. C. Buchanan, 111," R. Livingston, A. S. Dworkln, and G. P. Smith Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 (Received July 30, 1979) Publication costs assisted by Oak Ridge National Laboratory
We have found that pure molten antimony trichloride behaves as an oxidizing agent toward polycyclic arenes, producing stable solutions of radical cations. We have also shown that the oxidizing power of the Sb3+/Sbo couple can be considerably increased or decreased by the addition, respectively, of a few mole percent of a chloride ion acceptor (AlC13)or donor (Me4NC1). ESR spectra and coupling constants are reported for several arene radical cations in this medium, including those for the previously unreported acenaphthene radical cation.
Introduction Our interest in the basic chemistry of polycyclic aromatic hydrocarbons in antimony trichloride stems from the fact that molten SbC13is an effective hydrocracking catalyst for coal,l analogous to the more widely known molten salt catalyst ZnClz.z Procedures for the generation of stable radical cations of polycyclic aromatic hydrocarbons (which are model compounds for some of the structural units of coal) in solution have generally involved strong oxidizing agents such as the protonic acid HzS04 or the powerful Lewis acid SbC15.3i4However, there is at present considerable disagreement as to whether the mild Lewis acid antimony trichloride can similarly behave as an oxidizing agent toward arenes. Bauer and c o - ~ o r k e r measured s~~~ the reversible halfwave potentials for the oxidation of several arenes in molten SbClJ (mp 73 "C), from which they concluded that some of the more easily oxidizable arenes (e.g., perylene) should be spontaneously oxidized to radical cations in SbC13with the concomitant reduction of the SbC13to Sb'. However, the only spectroscopic evidence reported for the presence of an arene radical cation was a broad unresolved ESR line (assigned to the perylene radical cation) for a solution of perylene in solid SbC13.'j Their investigation of the change in the redox potential of the Sb3+/Sbocouple as a function of chloride ion concentration also suggested the possibility that the oxidizing power of the SbC13melt could be altered in a controlled manner by changes in the Lewis acidity of the solvent (via added chloride ion acceptors or donors). In contrast, Baughan and co-workers stated that molten SbC13could not be an oxidizing agent because the SbC13would have to be reduced to antimony metal8 They reported that the production of arene radical cations in SbC13melts required the presence of oxygen or SbC16 i m p ~ r i t i e s . ~ - ~ During the course of our studies of the reactions of aromatic hydrocarbons in molten SbC13,we have developed evidence which suggests that arene radical cations (produced from Oxidation by the SbC13 solvent) are key intermediates.l0-lZ These data included the isolation of stoichiometric quantities of antimony metal from the reaction mixtures.12 Moreover, we found that these reactions were strongly influenced by the presence of added chloride ion acceptors or donors.l1J2 The intent of this work is to examine whether polycyclic arenes can be oxidized to radical cations in pure SbC13 without other oxidizing agents and to determine whether the oxidizing power of the SbC13 solvent could be controlled by changes in the Lewis acidity of the solution. In 0022-3654/80/2084-0423$01.00/0
this study, we have examined by electron spin resonance (ESR) spectroscopy the nature of molten SbC13solutions of the following polycyclic arenes which are listed in order of increasing oxidizability in SbCl/ naphthalene, acenaphthene, pyrene, anthracene, 9,lO-diphenylanthracene (DPA), 9,lO-dimethylanthracene (DMA), perylene, and naphthacene.
Experimental Section Materials. The organic compounds studied were commercial reagents of the highest purity (99+ to 99.9+%). DPA and DMA were used without further purification. Naphthalene and acenaphthene were recrystallized from EtOH, whereas anthracene was recrystallized, first from acetic acid and then from toluene, and sublimed. Pyrene, perylene, and naphthacene were sublimed before use. The purity of these organic reagents was confirmed by W, GC, or high-performance LC. SbC13 was further purified from anhydrous material obtained from three different vendors-Apache (99.9999% ), Atomergic Chemetals Co., Gallard-Schlesinger (99.999%), and Ventron-Alfa (99.99%). The material was first refluxed over antimony metal (Apache (99.999%)) in an argon atmosphere to reduce any SbC15 present. Sublimation under vacuum followed by distillation under argon in a sealed apparatus gave a white solid which melted at 73 OC to give a clear, colorless liquid. Spark source mass spectrometry did not detect any inorganic oxidizing impurities above 2 ppm. A1C13 was prepared from zone refined 99.9999% aluminum metal and semiconductor grade HC1, yielding a colorless solid. This procedure has been fully described in the 1iterat~re.l~ Commercial (CH3)4NC1was purified by precipitation from a hot methyl alcohol-acetone solution upon addition of acetone. The precipitate was filtered and the operation repeated in Schlenk-ware in a dry argon atmosphere. The product was then dried under vacuum at room temperature. Sample Preparation. The sample tubes used in these studies were constructed from 4-mm 0.d. 3-mm i.d. fused silica. Molten SbC13is a lossy solvent in the microwave field, and thus it was necessary for the lower 3 cm of the tube (the part in the microwave cavity) to be flattened with an inside thickness of about 0.5 mm. All material transfers were performed in a controlled atmosphere drybox whose argon atmosphere was constantly circulated through a purification system and continuously monitored for moisture and oxygen content, which was kept at a level 0 1980 American Chemical Society
424
The Journal of Physical Chemistty, Vol. 84, No. 4, 1980
of
