Langmuir 1994,10, 505-509
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Adsorbed Phenyl Groups as Traps for Radical Intermediates in Reactions on Copper Surfaces Ming Xi and Brian E. Bent*'+ Department of Chemistry, Columbia University, New York,New York 10027 Received May 24,1993.In Final Form: November 2 3 , 1 9 9 P Identification of radical intermediates in surface reactions is difficult, particularly if the radicals are not evolved from the surface. In this paper, we report that adsorbed phenyl groups formed on Cu(ll1) by thermal or electron-induceddissociationof iodobenzenecan function as efficient traps for alkyl radicals generated in the dissociativeadsorptdon of alkyl iodides. Results from temperature-programmedreaction studies show that phenyl groups couple with methyl, ethyl, propyl, butyl, and neopentyl iodides at temperatures below 160 K to form the corresponding alkylbenzene. The upper limit to the coupling temperature (160-175 K)is quite close to the temperature range for C-I bond scission in the iodoalkanes on this surface. Since the presence of alkyl radicals upon the dissociation of C-I bonds in alkyl halides on Cu(ll1) has been independently verified (Lin, J.-L.; Bent, B. E. J. Am. Chem. SOC.1993,115,2849), we propose that these cross coupling reactions with adsorbed phenylgroupsinvolve transient alkyl radicals. The observation of croaa couplingproducts in the reactionwith sterically-hinderedneopentyliodide indicates that S Nor~ other concerted mechanisms are unlikely. 1. Introduction Direct evidence for radical intermediates in surface reactions is rare although free radicals are proposed to play a role in a number of heterogeneous processes. For example, it has been recognized and generally agreed that the formation of Grignard reagents from the reaction of CH3Br and magnesium surfaces involves methyl radical intermediates.lV2 In this case, as well as for other radicalgenerating reactions in solution, the evidence for radicals is both chemical (radical rearrangements, radical-radical reactions? and radical trapping') and spectroscopic (electron spin resonances and chemically-induced dynamic nuclear polari~ations).~ In gas/surface reactions, radical identification is more difficult, particularly if the radicals are not evolved from the surface where mass spectrometry or photon spectroscopies can be used to detect them. It is therefore desirable to identify adsorbates that can function as radical traps for establishing the presence of surface-retained radical intermediates. In this paper we report that adsorbed phenyl groups can function as efficient traps for alkyl radicals generated in the dissociativeadsorption of alkyl iodides on a Cu(ll1) surface. The presence of radicals in the thermal dissociation of alkyl iodides on this surface has been independently established by studies reported elsewhere.8e9 In particular, it was shown that when the C-I bond in iodomethane dissociates at -140 K on Cu(lll), some of + Presidential Young Inveatigator,A. P. Sloan Fellow,and Camille and Henry Dreyfua Teacher Scholar. AbetractpublishedinAduaceACSAbstracts,February 1,1994. (1) (a) Walboreky, H. M. Acc. C h m . Res. lS90,!23,28. (b) Garst, J. F. Acc. Chem. Rea. 1991,24,96. (c) Walling,C. Acc. Chem. Res. 1991,
24,265. (2) Kochi, J. K. OrganometallicMechoniemsandCataly8is;Academic Press: New York, 1978. (3) (a) Krusic, P. J.;Fagan,P. J.; Filippo, J. S., Jr. J. Am. Chem. Soc. 1977,99,260. (b) Griller,D.;Ingold,K. U. Acc. Chem.Res.1980,13,317. (4) Lappert, M. F.; Lednor, P. W. J. Chem. SOC.,Chem. Commun. 1973,948. (6)Filippo, J. S.;Silbermann,J.; Fagan, P. J. J. Am. Chem. SOC.1978, loo, 4834. (6) Kramer, A. V.; Oaborn, J. A. J. Am. Chem. Soc. 1974,96, 7832. (7) (a)Pryor, W. A. FreeRadicals;McGraw-HillBookCompany: New York, 1966. (b)Lazar, M.; Rychly, J.; Klimo, V.; Pelikan,P.; Valko, L. Free Radicals in Chemistry and Biology; -. CRC Prm: Boca Raton, FL, 1989. (8) Lin, J.-L.;Bent,B. E. J. Am. Chem. SOC.1993,115,284R J. Phys. Chem. 1993,97, 9713. (9) Lin, J.-L.; Bent, B. E. J. Am. Chem. SOC.,in press.
the methyl groups are ejected into the gas phase as radicals and some remain adsorbed on the surface as strongly bound methyl groups ( E C , , ~ 30 kcal/mol).8 For longer chain alkyl iodides, radical evolution into the gas phase has not been detected, but there is indirect evidence from coupling and disproportionation products that radicals are formed during C-I bond scission for these compounds as well.9 Here, we show that these surface-retained alkyl radicals can be trapped using adsorbed phenyl groups. The product alkylbenzenecan be desorbed from the surfacemolecularly intact for identification by mass spectrometry. The adsorbed phenyl groups in this study are generated on a Cu(ll1) surface at low temperatures (
