© Copyright 1998 by the American Chemical Society
VOLUME 102, NUMBER 37, SEPTEMBER 10, 1998
LETTERS Distinct Demonstration of Methylene Insertion into the Metal-Carbon Bond on Ag(111) Hsiao-Jung Wu and Chao-Ming Chiang* Department of Chemistry, National Sun Yat-Sen UniVersity, Kaohsiung, Taiwan 80424 ReceiVed: June 8, 1998
We provide a clear demonstration of the migratory insertion of methylene into the metal-carbon bond on a silver single-crystal surface. The insertion reaction is characterized by the interaction between methylene and perfluoromethyl groups coadsorbed on Ag(111). The CH2 insertion into the Ag-CF3 bond followed by β-fluoride elimination produces CH2dCF2, which is our experimental evidence for the insertion mechanism. Our observations confirm the step concerning the propagation of the carbon chain in the Fischer-Tropsch processes.
There is general agreement that the Fischer-Tropsch reaction proceeds as a stepwise polymerization of methylene groups on certain transition-metal surfaces.1 The propagation of the carbon chain is thought to be achieved by sequential insertion of methylenes into metal-carbon bonds. However, the identification of such a key mechanistic step utilizing modern surface techniques under ultrahigh vacuum conditions on atomically clean single-crystal surfaces has been scarce, except for one study on copper surfaces.2 In this paper, we present evidence for the insertion reaction of methylene on the Ag(111) surface, which is based on the interaction between methylene and trifluoromethyl groups coadsorbed on this surface. Methylene was generated on the surface by thermal disruption of the C-I bonds in adsorbed methylene iodide (CH2I2) at or below 200 K. CH2I2 has been demonstrated as a source of adsorbed methylene on a variety of metal surfaces by several research groups.3-8 Similarly, as reported by White et al.,9-11 the surface-bound trifluoromethyl was prepared by the cleavage of the C-I bond in the chemisorbed trifluoromethyl iodide (CF3I) well below 200 K. The experiments were performed in an ultrahigh-vacuum chamber equipped with an ion sputtering gun and a retardingfield analyzer for both Auger electron spectroscopy and lowenergy electron diffraction. In addition, a quadrupole mass spectrometer was shielded in a differentially pumped housing with a 2 mm diameter aperture to conduct temperature-pro-
grammed-reaction (TPR) studies. Following the adsorption of 0.2L (1L ) 10-6 Torr s) CF3I at 180 K, we found CF3 radical ejection (monitored as CF2+, m/e ) 50, which is the most abundant fragment of the CF3 radical in the mass spectrometer8) near 320 K, as indicated in the TPR spectra of Figure 1A. In agreement with White,9-11 our observations confirm that CF3I thermally dissociates at a submonolayer coverage on Ag(111) to form CF3(ad) and I(ad). The decomposition channel is limited to C-I bond scission, and there is no evidence for C-C bond formation on this surface. As far as methylene iodide is concerned, analogous to its chemistry on copper surfaces,4,5 TPR of a submonolayer coverage of CD2I2 (0.2L) shows that the only gas-phase product is ethylene-d4 (C2D4+, m/e ) 32) with a peak temperature at 250 K, Figure 1C. This result implicates that CD2I2 initially dissociates on Ag(111) below 200 K to produce adsorbed deuterated methylene and iodine. TPR of CD2(ad) subsequently leads to self-coupling and ethylene evolution. It should be noted that the effects of the coadsorbed iodine are known to be restricted to site blocking. The surface reaction pathways for alkyls in the presence and absence of halogens are unchanged. The reaction peak temperatures in TPR experiments are always perturbed by less than 10 K, corresponding to a change in the activation energy by 1 kcal/mol if the firstorder preexponential factor is 1013 s-1.12 We aim to use the coadsorption of CD2I2 and CF3I on a silver surface to investigate the possibility of cross-coupling between
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7076 J. Phys. Chem. B, Vol. 102, No. 37, 1998
Letters
Figure 1. TPR spectra for selected mass fragments after the adsorption of (A) 0.2L CF3I, (B) 0.2L CF3I + 0.2L CD2I2, and (C) 0.2L CD2I2 on Ag(111) at 180 K. The representative ions are m/e ) 32, 66, and 50 monitored at 70 eV mass spectrometer ionization energy. The heating rate is 1 K/s.
CD2(ad) and CF3(ad) or, in more explicit terms, the migratory insertion of methylene into the Ag-CF3 bond. Figure 1B illustrates the TPR spectra of 0.2L CD2I2 and 0.2L CF3I coadsorbed on Ag(111) at 180 K. The TPR spectra for this mixture, showing m/e ) 32, 66, 50 ions, evidently differ from those for the molecules adsorbed individually, shown in Figure 1A and 1C. In addition to the clear reduction in intensity for both m/e ) 32 and 50, the more intriguing change is the emergence of new desorption peaks at 258 K of m/e ) 50 (CF2+) and 66 (C2D2F2+) that we attribute to CD2dCF2. What type of interaction involving both CD2(ad) and CF3(ad) can result in a product such as CD2dCF2? We propose the following reaction mechanism to account for its formation: In Scheme 1, CD2(ad) is inserted into the Ag-CF3 bond to form an adsorbed fluoroethyl group CF3CD2(ad), which then undergoes β-fluoride elimination to produce liberated fluoroethylene (CD2dCF2) and adsorbed fluorine. Although direct spectroscopic confirmation of the intermediate fluoroethyl species on the surface would be desirable, the proposed mechanism is still strongly inferred by the fact that TPR results of CH2(ad) + CF3(ad) were found to be the same as those obtained from CF3CH2(ad) generated exclusively on the surface by thermal rupturing of the C-I bond in the adsorbed CF3CH2I. Specifically, Figure 2A shows the TPR spectra of m/e ) 64 and 83 ions after adsorption of 0.2L CF3I and 1.0L CH2I2 on Ag(111). The heaViest ion detected at 258 K is C2H2F2+, m/e ) 64, which is also the molecular weight of CF2d CH2 (fluoroethylene). Extensive masses were searched, and we did not observe any C3-C6 species in the desorption spectra at this temperature. Again, the formation of CF2dCH2 implies the insertion of methylene into the Ag-CF3 bond. Furthermore, the lack of intensity of m/e ) 83 (C2H2F3+) at 258 K suggests that the evolution of CF3CH2 radicals as the fate of the methylene insertion can be ruled out. In fact, once the CF3CH2(ad) is formed, β-fluoride elimination in conjunction with CF2dCH2 desorption is the only subsequent reaction pathway. Figure 2B shows the TPR spectra of CF3CH2(ad) generated by thermal dissociation of the C-I bond in the adsorbed CF3CH2I at 180 K. In good agreement with the work of Gellman et al.,13,14 CF3CH2(ad) on Ag(111) does not self-couple or desorb as a radi-
Figure 2. TPR spectra for selected mass fragments after the (A) coadsorption of 0.2L CF3I and 1.0L CH2I2 and (B) adsorption of 0.2L CF3CH2I on Ag(111) at 180 K. The representative ions are m/e ) 64 (CF2CH2+) and 83 (CF3CH2+).
SCHEME 1
cal but, instead, decomposes via β-fluoride elimination, yielding CF2dCH2 (m/e ) 64) at ∼250 K as the sole gas-phase product. The uniqueness of this study lies in the employment of a transition-metal surface known for its selectivity toward carboncarbon bond formation reactions and fluorine-substituted methyl, CF3(ad), as the target for methylene insertion. Ag is too inert to activate C-F and C-H bonds; therefore, the adsorbed intermediates of interest, such as CF3(ad) and CH2(ad), can remain intact until their interaction occurs. On the basis of the fact that adsorbed alkyl groups tend to dimerize on the Ag(111) surface to yield alkanes of twice the chain length, established by White and co-workers,15-18 the migratory insertion of CH2(ad) into the Ag-CF3 bond (a C-C bond formation reaction) should be plausible. As the Ag-CH2CF3 species is formed after the insertion reaction, β-fluoride elimination, the characteristic and dominant reaction pathway when the adsorbed alkyl group is fluorinated at the β-carbon position on Ag(111), terminates the propagation of the carbon chain. The final product CH2dCF2, dimerized and fluorine-labeled, provides a simple and definite proof of the methylene insertion. Altogether, CH2(ad) + CF3(ad) on the Ag(111) surface appears to be the system of choice for the purpose of manifesting the insertion mechanism.
Letters In conclusion, the TPR spectra taken after the coadsorption of methylene and trifluoromethyl groups on Ag(111) give distinct evidence for the migratory insertion of methylene into the metal-carbon bond. The identical results between CH2(ad) + CF3(ad) and CF3CH2(ad) corroborate the mechanism proposed in Scheme 1. To our knowledge, this is the first example of using a mixture of methylene and perfluoroalkyl groups to demonstrate the methylene insertion reaction on a single-crystal surface under ultrahigh-vacuum conditions. Acknowledgment. This research was supported by the National Science Council of the Republic of China under Contract No. 87-2113-M-110-008 and Startup Fund from National Sun Yat-Sen University. C.-M.C, dedicates this to his mentor, the late Prof. Brian E. Bent. References and Notes (1) Brady, R. C., III.; Pettit, R. J. Am. Chem. Soc. 1980, 102, 6181. (2) Chiang, C.-M.; Wentzlaff, T. H.; Bent, B. E. J. Phys. Chem. 1992, 96, 1836.
J. Phys. Chem. B, Vol. 102, No. 37, 1998 7077 (3) Domen, K.; Chuang, T. J. J. Am. Chem. Soc. 1987, 109, 5288. (4) Chiang, C.-M.; Wentzlaff, T. H.; Jenks, C. J.; Bent, B. E. J. Vac. Sci. Technol. A 1992, 10, 2185. (5) Imre, K.; Solymosi, F. J. Phys. Chem. B 1997, 101, 5397. (6) Weldon, M. K.; Friend, C. M. Surf. Sci. 1994, 321, L202. (7) Wu, G.; Bartlett, B. F.; Tysoe, W. T. Surf. Sci. 1997, 373, 129. (8) Tjandra, S.; Zaera, F. J. Catal. 1993, 144, 361. (9) Castro, M. E.; Pressley, L. A.; Kiss, J.; Pylant, E. D.; Jo, S. K.; Zhou, X.-L.; White, J. M. J. Phys. Chem. 1993, 97, 8476. (10) Szabo, A.; Converse, S. E.; Whaley, S. R.; White, J. M. Surf. Sci. 1996, 364, 345. (11) Junker, K. H.; Sun, Z.-J.; Scoggins, T. B.; White, J. M. J. Chem. Phys. 1996, 104, 3788. (12) Chiang, C.-M.; Bent, B. E. Surf. Sci. 1992, 279, 79. (13) Paul, A.; Gellman, A. J. J. Am. Chem. Soc. 1995, 117, 9056. (14) Paul, A.; Gellman, A. J. Langmuir 1995, 11, 4433. (15) Zhou, X.-L.; White, J. M. Catal. Lett. 1989, 2, 375. (16) Zhou, X.-L.; Solymosi, F.; Blass, P. M.; Cannon, K. C.; White, J. M. Surf. Sci. 1989, 219, 294. (17) Zhou, X.-L.; White, J. M. J. Phys. Chem. 1991, 95, 5575. (18) Zhou, X.-L.; Blass, P. M.; Koel, B. E.; White J. M. Surf. Sci. 1992, 271, 427.
