Isomerization and Reactivity of Allyl Groups Adsorbed on Coinage

Using allyl halides (C3H5X, X = Cl, Br) to generate adsorbed C3H5 fragments and halogen atoms, we have studied the isomerization and reactivity of σ-...
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J. Phys. Chem. B 2001, 105, 3908-3916

Isomerization and Reactivity of Allyl Groups Adsorbed on Coinage Metals† H. Celio and J. M. White* Center for Materials Chemistry and Texas Materials Institute, UniVersity of Texas at Austin, Austin, Texas 78712-1167 ReceiVed: September 23, 2000; In Final Form: January 12, 2001

Using allyl halides (C3H5X, X ) Cl, Br) to generate adsorbed C3H5 fragments and halogen atoms, we have studied the isomerization and reactivity of σ-bonded (η1) and π-bonded (η3) C3H5 on two metal substrates, Cu(100) and Ag(111), using vibrational and thermal desorption spectroscopies. On Cu(100), two forms of η3-C3H5, denoted endo-η3-C3H5 T exo-η3-C3H5, are identified that undergo a reversible temperature-dependent isomerization. When dissociative adsorption of C3H5X saturates on Cu(100), only exo-η3-C3H5 is found between 77 and 300 K. On Ag(111), η1-C3H5 and η3-C3H5 (both exo and endo) form at 110 K, depending on the dose. The η1-C3H5 and exo-η3-C3H5 irreversibly isomerize to endo-η3-C3H5 upon annealing to 175 K. When either metal substrate, held at a selected temperature between 77 and 150 K, is dosed with C3H5Br, there is a reaction with η3-C3H5 to form 1,5-hexadiene, C6H10. This reaction does not occur upon dosing with C3H5Cl. When η3-C3H5 is heated above 200 K, different results obtain for the two metals: on Cu(100), in agreement with previous work, C3H5 radical desorption at 450 K dominates, while dimerization and desorption as C6H10 at ∼250 K occurs on Ag(111).

I. Introduction

SCHEME 1: Allyl Isomerization

In recent papers,1,2 we described 1,5-hexadiene (C6H10) synthesis from chemisorbed η3-allyl and precursor 3-bromopropene (C3H5Br) on Cu(100) at temperatures as low as 77 K. In the language used to discuss homogeneous reactions of organometallics, this process can be described as an organometallic nucleophile attacking an organic halide electrophile. This precursor-assisted reaction occurs for physisorbed C3H5Br, but not 3-chloropropene (C3H5Cl), and contrasts with typical Langmuir-Hinshelwood carbon-carbon bond-forming reactions between two chemisorbed alkenyl and alkyl groups on Cu, Ag, and other metals.3,4 Vibrational and thermal desorption spectra, the central diagnostics used in this work, have previously been used to characterize adsorbed allyl, C3H5, on Ag(110).5 Adsorbed C3H5 dimerized on Ag(110) to form adsorbed C6H10 at 310 K, a Langmuir-Hinshelwood process limited, in agreement with calculations,6 by surface diffusion of C3H5 groups bound to Ag. On Cu(100), near-edge X-ray absorption fine structure (NEXAFS) showed that for saturation coverage, the C3 plane of C3H5 was parallel within 10° to the Cu surface.7 During temperature-programmed desorption (TPD), over 80% of the allyl groups on Cu(100) were ejected as radicals between 350 and 450 K. Dosing C3H5Br on Al(100) at 300 K yielded propenyl groups, which hydrogenated and desorbed as propylene at ∼500 K.8 On Pt(111), submonolayer coverages of C3H5Br yielded η3-allyl (π-bonded) at 185 K, which rearranged to η1allyl (σ-bonded) groups at saturation.9 Annealing to 320 K induced a reaction between η1-allyl and surface hydrogen to produce propylene. We recently published the first reflection absorption infrared spectroscopy (RAIRS) study of allyl on Cu(100),1 finding evidence for η3-C3H5 consistent with high†

Part of the special issue “John T. Yates, Jr. Festschrift”. * Corresponding author: e-mail [email protected]; fax 512-4719495; phone 512-471-3704.

resolution electron energy loss spectroscopy (HREELS) work on Ag(110).5 In related catalysis work (e.g., Fischer-Tropsch synthesis10) involving small metal particles, various isomeric forms of allyl on metal clusters have been postulated as key intermediates. A reversible hydrogen transfer (tautomerism) between η1-allyl and η1-propenyl has been proposed. Further, in a recent study of the isomerization of η3-C3H5 (bonded to a Ti metal center), a critical step was the formation, via β-hydrogen transfer, of titanium hydride coordinated with π-bonded allene (C3H4) and subsequent rearrangement to η1-propenyl.11 In the work reported here, we found no evidence in RAIRS for η1-propenyl, i.e., no evidence for CH3 groups. Turning to organometallic molecules, metal-catalyzed C3H5 isomerization is often fluxional, the most common pathway linking η3- and η1-forms.12 Taking account of the coadsorbed halogen that is present in our system, we outline in Scheme 1 a plausible set of energetically accessible and fluxionally interconnected isomers. The fluxionality can be described as η3 to η1 rearrangement followed by rotation around a carboncarbon bond to re-form the η3-isomer. In Scheme 1, the rotation is around the bond between the 2 and 3 carbons, and the syn/ anti exchange is indicated in the upper panel. In the presence

10.1021/jp003446a CCC: $20.00 © 2001 American Chemical Society Published on Web 03/08/2001

Allyl Groups Adsorbed on Coinage Metals

J. Phys. Chem. B, Vol. 105, No. 18, 2001 3909

of coadsorbed halogens (X ) Cl or Br), two η3 conformations, endo and exo, with respect to the halogen are plausible, as indicated in Scheme 1. While there are examples of endo and exo isomers coordinated to metal atoms,13 we are unaware of evidence for them on extended metal surfaces. Using RAIRS and TPD, we report here on the structure and reactivity of C3H5 isomers adsorbed on Cu(100) and Ag(111). We find RAIRS evidence for σ-bonded (η1-C3H5) and for the exo and endo conformations of π-bonded (η3-C3H5) allyl groups. The latter evidences a previously unexplored role for the coadsorbed halogen. The involvement of these isomeric structures in C6H10 formation is assessed. II. Experimental Section The experiments were done in a two-level ultrahigh vacuum (UHV) chamber, the upper level of which is equipped for Fourier transform infrared measurements.2 By use of a narrowband HgCdTe (MCT) detector, spectra (peak-to-peak noise ∼0.004% ∆R/R units) spectra were recorded by coadding 1500 scans at 4 cm-1 resolution. For RAIRS of surfaces covered with both hydrocarbon and halogen species, an appropriate reference spectrum, either Br- or Cl-covered metal, was chosen to match the changes in reflectivity (e.g., electronic absorption) produced by halogen atoms. The lower chamber is equipped with a singlepass cylindrical mirror analyzer for Auger electron spectroscopy (AES) and a differentially pumped quadrupole mass spectrometer for TPD measurements. The adsorbate purification and dosing procedures, the sample holder, and the Cu(100) cleaning procedures are described elsewhere.2 The Ag(111) crystal was cleaned by repeated cycles of Ar+ ion grazing-incidence sputtering at 300 K (3.0 kV, 6 µA, 15 min) and annealing at 775 K (10 min). This procedure was repeated until impurities were below AES detection limits. Exposures are given in langmuirs and are based on Ar pressure rises that occur in the UHV chamber when a fixed and reproducible pressure is placed behind a preset leak valve.2 On Cu(100), adsorbed Cl was prepared by dosing C3H5Cl at low temperatures, annealing at 550 K to desorb C3H5, and recooling.7 For a 0.8 langmuir dose of C3H5Cl, this procedure led to a Cl/Cu AES ratio of 1.11. Under these conditions, the absolute coVerage of Cl, calibrated by CO titration,2 was 9 Cl/ 100 Cu [i.e., 0.09 monolayer (ML)]. From the stoichiometry of C3H5Cl, we infer an equal C3H5 coverage formed and desorbed. Three repetitions of the dosing/annealing cycle saturated the AES ratio at 2.47, from which we deduce a Cl coverage of 0.20 ( 0.01 ML. Br coverages were prepared by dosing and annealing C3H5Br. The Br AES signals were weak and precluded verification of saturation, but three cycles of adsorption and desorption, followed by CO titration, indicated a Br coverage also equal to 0.20 ( 0.01 ML. On clean Ag(111), adsorbed Cl was produced in the same way but the titration method fails because CO does not adsorb.14 Instead, with calibrated XPS data taken from a different sample and instrument, the absolute coverage was 1 Cl/8 surface Ag atoms, i.e., θCl ) 0.12,14 a value in agreement with a rough AES determination that used a reported Cl/Ag AES ratio for an absolute coverage of 0.5 Cl/surface Ag.15 In a separate publication, we show that this Cl coverage is nonuniform after annealing, with some regions containing as many as 1 Cl/3 Ag surface atoms.14 III. Results This section is organized in two major subsections describing and discussing RAIRS and TPD results, first for Cu(100) and

Figure 1. RAIRS spectra for indicated doses of C3H5Br on Cu(100) at 275 K. Each spectrum is labeled with the total exposure (in langmuirs) of C3H5Br and the estimated coverage of exo-η3-C3H5(c).

then for Ag(111). Most comparisons of Cu(100) and Ag(111) are postponed to section IV. Overall, for Cu(100) dosed with C3H5Br at both 275 and 77 K, RAIRS shows formation of coverage-dependent distributions of allyl isomers. There is an interesting thermally reversible allyl isomerization process attributed to exo- and endo-η3-C3H5. The influence of preadsorbed Cl on the structure and reactivity of allyl groups is examined by both RAIRS and TPD. For Ag(111) dosed with C3H5Br or C3H5Cl at 110 K, TPD provides a reactivity comparison with Cu(100) and among the halides. Below 150 K, a precursor-assisted reaction, as on Cu(100), is observed between η3-C3H5 and C3H5Br. Unlike Cu(100), radicals do not desorb above 200 K; rather, 1,5-hexadiene forms by linking η3-C3H5. Distinguished by RAIRS, dissociative adsorption of low doses of C3H5Br and C3H5Cl on Ag(111) leads to different isomers of C3H5, including η1-C3H5. 1. Results on Cu(100): Dosing C3H5Br on Cu(100) at 275 K. When Cu(100) held at 275 K is exposed to relatively large doses (6-18 langmuirs) of C3H5Br, RAIRS (Figure 1) reflects dissociative adsorption and the growth and ordering of the allyl isomer, η3-C3H5. Consider the following. First, the spectra are consistent with those reported for η3-allyl groups in metal complexes and on metal surfaces.5,9,16 Second, and as expected, there is no evidence for the accumulation of C3H5Br, since 275 K is higher than the temperature required for C-Br bond breaking (225 K) and is above the multilayer C3H5Br desorption (130 K).1 Regarding ordering, the 884 cm-1 band narrows as its intensity rises, which we take to indicate increased local ordering of the adsorbates (more detail below). On the basis of a recent calculation of η3allyl-Cr complexes,16 the 884, 964, and 992 cm-1 bands are assigned, contrary to our previous report,1 to wagging modes, Fω(CH2)sym and Fω(CH), and to the skeletal mode, ν(CCC)sym, respectively. The proposed η3-allyl structure, Scheme 1, has Cs symmetry with delocalized π-type C-C-C bonding. According to NEXAFS,7 the C-C-C plane is tilted 80-85° away from

3910 J. Phys. Chem. B, Vol. 105, No. 18, 2001 the surface normal. As for π-bonded ethylene,17 the CH2 wagging mode is the most intense. In Scheme 1, we also make a distinction between endo- and exo-η3-allyl. In a separate study, we found that the CH2 wagging mode of π-bonded adsorbates (e.g., C2H4 and propene) is exceptionally sensitive to the presence of Cl and is blue-shifted compared to adsorption on clean Ag(111).14 If the endo and exo conformations are compared, the CH2 wagging mode will be perturbed less in the latter (i.e., the two CH2 groups are further from the halogen). Since other data (below) exhibit stronger shifts and are, on that basis, assigned to endo, we assign the 884, 964, and 992 cm-1 bands to exo-η3-C3H5 and postpone assignment of the 937 cm-1 band. The integrated area of the Fω(CH2)sym mode was used as a measure of the exo-η3-allyl coverage. Absolute coverages, indicated on the left-hand side of Figure 1, were calculated by a calibration based on the Fω(CH2)sym intensity for a known coverage, 0.09 ML, prepared by dissociating C3H5Cl as indicated in the Experimental Section. For this coverage, the integrated intensity, I(CH2wag) = (∆R/R) dν, is (9.5 ( 0.3) × 10-3 cm-1.2 Strikingly, the largest coverage of exo-η3-allyl (0.074 ( 0.004 ML) occurs for an intermediate, not the highest, dose. Consistent with other work,17 we conclude that, for low doses, exo-η3allyl accumulates at 275 K but that it subsequently reacts with C3H5Br in a precursor state to form 1,5-hexadiene, C6H10. The reaction probability per incident C3H5Br, 1.2 × 10-3,15 is low but not negligible. The reaction can be written

C3H5Br(p) + η3-C3H5(c)+ Br(c) f C6H10(p) + 2Br(c) (1) where (p) and (c) denote physisorbed and chemisorbed states, respectively. Note that chemisorbed η3-C3H5 is replaced by chemisorbed Br and that physisorbed C3H5Br is replaced by physisorbed C6H10. There is one other interesting aspect of Figure 1. As dosing and reaction proceed, the 884 cm-1 bandwidth reproducibly drops from a full width at half-maximum (fwhm) of 13.1 (9.0 langmuir dose) to 10.8 cm-1 (18 langmuir dose). This is taken to indicate increasing homogeneity of the adsorbate-substrate structure as the reactions proceed; i.e., exo-η3-C3H5(c) and Br(c) become more highly ordered with respect to each other and the Cu(100) substrate. Dosing C3H5Br on Cu(100) at 77 K. The situation is very different when dosing occurs at 77 K rather than 275 K. Since the rate of reaction (eq 1) becomes too slow to detect below 95 K,2 large doses (Figure 2) reflect both dissociative adsorption of C3H5Br (886 cm-1) and accumulation of molecular C3H5Br (863, 933, 988, 1194, and 1213 cm-1). Since the C3H5Br bands are red-shifted no more than 3-6 cm-1 from those for solid C3H5Br,18 we suppose that C3H5Br is not directly interacting with Cu but is physisorbed to previously formed η3-C3H5(c) and Br(c). The 886 cm-1 band is assigned, as above, to exo-η3-C3H5, and the intensity corresponds to a coverage of 0.08 MLsabout the same as for a 9 langmuir dose at 275 K (Figure 1). By inference, there is an equal Br(c) coverage. This is the first direct evidence for dissociation of C3H5Br at 77 K to form η3-C3H5. C3H5Cl dosed at 77 K also dissociates to form exo-η3-C3H5 (not shown). Interestingly, the results for 0.2 and 0.4 langmuir doses differ from the others; while there are four bands, at 902, 938, 984, and 1012 cm-1, we see no evidence for either C3H5Br or exoη3-C3H5(c). It is striking that the band positions change abruptly for doses exceeding 0.4 langmuir; the 902 cm-1 intensity is strong for 0.2 and 0.4 langmuir doses but is replaced by strong

Celio and White

Figure 2. RAIRS spectra for indicated doses of C3H5Br on Cu(100) at 77 K. As marked, bands are attributed to endo-η3-C3H5(c), exo-η3C3H5(c), and C3H5Br(p).

intensity at 886 cm-1 in the 0.8 langmuir spectrum. We propose that whatever forms at low doses transforms into exo-η3-C3H5(c) as the coverage increases. Noting the persistent 20 cm-1 blue shift compared to exo-η3-C3H5, we assign the 902, 984, and 1012 cm-1 bands to endo-η3-C3H5. As in Figure 1, the band at 938 cm-1 appears only for doses near those required to saturate dissociative adsorption where halogen coverages are highest. It is assigned, speculatively, to a third form of η3-allyl perturbed by multiple halogen atoms. Thermal Isomerization of Allyl. The cryogenic formation of endo-η3-allyl followed by its isothermal coverage-induced isomerization to exo-η3-allyl at 77 K suggests that the thermal activation barrier between these two isomers is very small ( 40 K), symmetric shape, and peak position that decreases (by 15-20 K) with increasing η3-C3H5 coverage. This is consistent with previous descriptions of C3H5(c) on Ag(110).5,6 Interestingly, the high-temperature C6H10 intensity for 1.2 langmuir doses is 3-fold greater for C3H5Cl than for C3H5Br, an observation related to reactions at lower temperature that we now discuss. On the basis of the 67 amu signal (C5H7+), specific for C6H10, that accompanies the 41 amu signal at 146 K (not shown), the 1,5-hexadiene desorption rate maximizes no more than 2 or 3 K before the C3H5Br. For a 1.2 langmuir dose, the 41 amu peak comprises 37% C6H10 and 63% C3H5Br. For doses less than 1.2 langmuirs, the low-temperature C6H10 contribution drops and becomes negligible for a dose of 0.4 langmuir . The only other desorbing species are the allyl halides that exhibit three coverage-dependent peaks, denoted R, β, and γ. The γ peaks are unsaturable and attributed to desorption of C3H5X that has no contact with either Ag or the dissociation products, C3H5 and Cl or Br, i.e., adsorbed on C3H5X. The more interesting R and β peaks are assigned to molecular C3H5X interacting with species other than C3H5X. Because the C3H5Br case is complicated by 1,5-hexadiene desorption, we first consider C3H5Cl. At the lowest dose (0.12 langmuir), β-C3H5Cl is evident but R-C3H5Cl is not. The β intensity grows monotonically with dose, while the R state exhibits an onset (it is barely evident for 0.6 langmuir and is saturated at 1.2 langmuirs). The β form is ascribed to weakly chemisorbed C3H5Cl involving coupling to Ag through its -CH2Cl functionality. The downward shift of the peak temperature reflects weakening ensemble average bonding as the accompanying coverages of C3H5 and Cl increase with dose. We estimate, on the basis of mass spectrometer signals, that for C3H5Cl doses between 0.1 and 1.0 langmuir, a third desorbs as C3H5Cl and two-thirds dissociates to form C6H10. The R-C3H5Cl feature appears only for doses near saturation of the β-desorption at Cl-modified Ag sites. The stabilization, compared to β-C3H5Cl, is ascribed to π-bonding between the CdC bond of the vinylic (-HCdCH2) functionality and electron-deficient Ag atoms.14 Formation and Isomerization of Allyls from C3H5Cl. RAIRS of a 0.6 langmuir dose of C3H5Cl on Ag(111) at 110

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Figure 6. TPD spectra taken after indicated exposures of C3H5Cl (left panel) and C3H5Br (right panel) on Ag(111) at 110 K. In both panels, the broad peaks above 220 K are due to C6H10 from coupling of C3H5(c) groups. Below 200 K, the left panel shows three distinct desorption states of C3H5Cl: multilayer (γ at 126 K), and two other peaks, β and R, ascribed to C3H5Cl interacting with Ag and with previously formed decomposition products. The right panel shows similar states of C3H5Br below 200 K; however, the 146 K peak is composed of C3H5Br (63%) and 1,5-hexadiene (37%). The latter is from a precursor-assisted reaction between chemisorbed η3-allyl and physisorbed C3H5Br.

Figure 7. RAIRS spectra obtained at 110 K after incremental exposures of C3H5Cl on Ag(111) at 110 K (A-C), followed by a 175 K annealing and cooling back to 110 K (D). Each spectrum is labeled with the total exposure of C3H5Cl. Note the 2× scale factor for spectrum A.

K, before and after desorbing C3H5Cl by annealing to175 K, underscores major thermal effects (Figure 7C,D). After annealing, three bands appear at 870, 972, and 1009 cm-1, consistent with η3-C3H5(c) found in coordination metal complexes,16 on Cu(100) (Figure 1) and on Ag(110).5 Under the assumption that the local symmetry is Cs, the 870, 972, and 1009 cm-1 peaks are assigned to the Fω(CH2)sym, Fω(CH), and ν(CCC)sym vibrational modes of endo-η3-C3H5(c). The endo assignment is

made on the basis of the Fω(CH2)sym mode at 870 cm-1; the exo form is discussed below. Strikingly, without annealing (spectra A, B, and C) there is no evidence for η3-C3H5(c). Instead, we identify three species: η1-C3H5(c), β-C3H5Cl, and γ-C3H5Clsas marked. To establish these assignments, first consider the two lowest doses, 0.04 and 0.4 langmuir at 110 K. These spectra exhibit six identifiable vibrational bands, three of whichs927, 981, and

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Celio and White

Figure 8. RAIRS spectra (all taken at 110 K) obtained after incremental exposures of C3H5Br on Ag(111) at 110 K (A-D), followed by a 130 K anneal (E). The last clearly shows the formation of 1,5-hexadiene (914 cm-1).

Figure 9. RAIRS spectra after incremental exposures of C3H5Br on Ag(111) at 135 K (A, B), followed by annealing to 175 K (C). All spectra were taken at 135 K.

1251 cm-1sare assigned to β-C3H5Cl.19 The red shift of at least 5 cm-1 compared to solid C3H5Cl, particularly the CH2 wagging mode at 1251 cm-1, is taken to reflect substrate interactions sufficient to form a weak chemisorption bond between Ag and -CH2Cl. The other three bandss912, 975, and 1169 cm-1sare assigned to η1-C3H5(c) on the basis of arguments presented below. For the 0.6 langmuir dose, a third sets894, 936, 994, and 1255 cm-1 emerges; all are within 2 cm-1 of the bands for solid C3H5Cl18 and are assigned to γ-C3H5Cl identified in TPD (Figure 6), i.e., C3H5Cl adsorbed on C3H5Cl. Formation and Isomerization of Allyls from C3H5Br. As expected on the basis of the TPD results, RAIRS of dosed C3H5Br differs from that of C3H5Cl. Two substrate temperatures were chosen for dosing: 110 K (Figure 8) to correspond with conditions used for C3H5Cl, and 135 K to minimize retention of C3H5Br (Figure 9). After a dosing of 0.2 langmuir at 110 K, bands reflecting dissociative adsorption and formation of exoη3-C3H5 are evident at 855, 972, and 1004 cm-1. Unlike the case for comparable doses of C3H5Cl, there is no evidence for η1-C3H5. Dosing more C3H5Br (0.4-1.2 langmuir doses) increases the intensity of exo-η3-C3H5 and C3H5Br bands, several of which overlap. Annealing to 130 K leads to interesting results. As expected, the band at 1004 cm-1 (exo) blue-shifts to 1015 cm-1 (endo). More importantly, a band emerges at 914 cm-1, assigned to physisorbed 1,5-hexadiene. This band, evidenced by a weak but distinct shoulder even before annealing to 130 K, is identified unambiguously as the CH2 wagging mode of the 1,5-hexadiene group on the basis of RAIRS of C6H10 directly dosed on bromine-covered Cu(100).1 Thus, on Ag(111) some C6H10 forms at cryogenic temperatures when C3H5Br(p) interacts with η3-C3H5, as reflected in Scheme 1 and eq 1. It is not clear from these data whether the exo or endo forms of η3-C3H5, or both, react with C3H5Br(p) to form C6H10. Spectra after dosing of 0.2 and 2.0 langmuirs at 135 K, and after annealing of the latter to 175 K, contain three bands in the 800-1200 cm-1 region (Figure 9). For 0.2 langmuir these lie at 856, 967, and 1004 cm-1, while for 2.0 langmuirs and for

the annealed surface they are blue-shifted to 875, 973, and 1017 cm-1. As in Figure 7, the blue-shifted bands are assigned to endo-η3-C3H5(c) (Figure 9B,C), while the bands in Figure 9A are assigned to exo-η3-C3H5(c). The 2.0 langmuir spectrum contains two additional bands and a shoulders868, 887, and 932 cm-1sascribed to C3H5Br (Figure 2). After annealing to 175 K, loss of C3H5Br is indicated by the absence of bands from C3H5Br. In addition, increased ordering after annealing is reflected in sharper peaks, particularly at 1017 cm-1, where the fwhm decreases from 23 to 6.6 cm-1. IV. Discussion In this discussion, the focus is on similarities and differences in the structure and reactivity of C3H5(c) on Cu(100) and Ag(111). Dissociation at Low Temperatures. One difference between Cu(100) and Ag(111) is the low-temperature (110 K) dissociation of C3H5Cl. The results concur with general observations; copper is typically somewhat more effective than silver in catalyzing bond breaking. Our experiments show that whereas dissociation mixed with molecular adsorption occurs readily when C3H5Cl is dosed on Ag(111), dissociation dominates on Cu(100). As noted below, these differences impact the structures and reactions of adsorbed allyl groups because the local surroundings differ. In the case of C3H5Br, dissociation dominates for low doses at ∼100 K on both metals. Structure of η3-Allyl (Orientation of CCC Plane). From RAIRS we conclude that η3-C3H5 forms on both Ag(111) and Cu(100) (Figures 1 and 9). However, relative intensity differences indicate structural differences of η3-C3H5(c) on the two metals. For example, as is true for ethylene π-bonded on transition metals,17 the CH2 wagging mode is the most intense on Cu(100). This is not the case for Ag(111), where the CCC stretching mode is strongest. We propose that the difference is ascribable to different orientations of the C-C-C plane of η3C3H5(c). From RAIRS, the intensity ratio of the wagging and twisting modes, Fω(CH2)sym/Fω(CH), is much higher for Cu(100) than for Ag(111) (4.1 versus 0.84). On Cu(100) the

Allyl Groups Adsorbed on Coinage Metals C-C-C plane lies no more than 10° away from the surface,7 whereas on the basis of a calculation invoking the infrared surface selection rule, it lies 70° away on Ag(111). We are unaware of other determinations of this orientation angle on extended Ag surfaces or allyl complexes bonded to a Ag center. Another interesting structural difference is the preference for the exo- and endo-η3-C3H5 isomers. On Cu(100) the preference on heating is to form exo-η3-C3H5 (red-shifted), but on Ag(111) the reverse occurs (blue-shifted). Furthermore, another structure, identified as η1-allyl, forms upon dosing of Ag(111) with C3H5Cl at 110 K; on Cu(100), neither C3H5Br or C3H5Cl dissociation leads to η1-allyl. Finally, although only irreversible transformations among isomers of C3H5 were identified on Ag(111), we saw a reversible isomerization, identified as exo-η3C3H5 T endo-η3-C3H5, on Cu(100) between 77 and 225 K, with the endo form dominant at 77 K and exo at 225 K. By analogy with organometallic complexes (e.g., an η3-allyl-palladium complex12,13) we propose a syn/anti exchange between endoand exo-η3-C3H5 isomers (Scheme 1). Role of Halogens. The significant role played by halogens is evident for both Ag(111) and Cu(100). The electronwithdrawing character of Cl and Br influences the form taken by chemisorbed C3H5. For example, η3-C3H5 formation on Cu(100) at 77 K requires dosing C3H5Br (or C3H5Cl) until the dissociative process is nearly saturated. Similarly, electron withdrawal from nearby Ag atoms rationalizes why η1-C3H5 accumulates at 110 K when C3H5Cl is dosed on Ag(111) but not Cu(100). We propose that on Ag(111) the dissociative and nondissociative adsorption rates are competitive and their concentrations accumulate together, whereas on Cu(100) dissociative adsorption dominates at low doses. We propose that the presence of C3H5Cl restricts the formation of η3-C3H5, but not η1-C3H5, and attribute this inhibition to C3H5Cl accumulating at electron-deficient Ag that surrounds Cl(c), limiting C3H5 to η1-C3H5. Consistent with this proposal, heating η1-C3H5 mixed with C3H5Cl to 175 K (Figure 7) desorbs C3H5Cl and forms endo-η3-C3H5. It is noteworthy that Shustorovich6 calculated that η3-C3H5 was 8.2 kJ mol-1 more stable than η1-C3H5 on Ag(110). Reactivity of η3-C3H5(c) above 200 K. We have already noted the dissociation differences between Cu(100) and Ag(111). There are other reactivity differences; C3H5 species desorb as radicals from Cu(100),7 whereas they recombine and desorb as C6H10 from Ag(111) and Ag(110).5 The temperature regimes are also quite different: C3H5 desorbs from Cu(100) between 350 and 450 K, while C6H10 desorbs from Ag(111) between 200 and 300 K. Clearly, C3H5 is either much less mobile on Cu(100) than on Ag(111) or the activation energy for recombining, once two C3H5 are in contact, is much higher. Previous interpretations have focused on charge-transfer attributes of activation energy for recombination, but the role of diffusion cannot be dismissed. One other issue deserving further investigation is the production of small amounts of C3H6 (propylene) and of C3H4 (presumably allene) on Cu(100).7 The absolute C6H10 yield attributed to recombination of C3H5 (peak between 250 and 265 K) was estimated on the basis of directly dosing C6H10 and saturating a weakly chemisorbed TPD peak at 181 K (multilayer desorption peaks 56 K lower). We assume that a single 2D layer is packed like liquid C6H10 (density ) 0.710 g cm-3). This comprises 3 × 1014 molecules cm-2, compared to one layer of Ag(111) that comprises 1.38 × 1015 atoms cm-2 and yields an estimated C6H10 saturation coverage of 0.22 ML. Since C6H10 does not dissociate and is weakly held on Ag(111), this estimate is considered

J. Phys. Chem. B, Vol. 105, No. 18, 2001 3915 reliable to within 15%. Based on this estimate, the maximum C6H10 yield is 0.12 ML, observed after dosing of 1.2 langmuirs of C3H5Cl on Ag(111) at 110 K. At the lowest exposure (0.12 langmuir of C3H5Cl), the C6H10 yield is, like the C3H5Cl dose, an order of magnitude lower (0.016). Reactivity of η3-C3H5 below 200 K. On both metals, C6H10 forms below 200 K, through a mechanism that is not controlled by diffusion-limited coupling of η3-C3H5. Rather, as reported recently for Cu(100),2 it is best described in terms of an extrinsic precursor-assisted reaction between C3H5Br and chemisorbed η3-C3H5. C6H10 forms for C3H5Br dosed on η3-C3H5 but not for C3H5Cl, presumably because the activation energy for the precursor-assisted reaction, Er, is higher than the activation energy for C3H5Cl desorption, Ed; i.e., (Ed - Er) < 0.0. For C3H5Br, the reverse holds and (Ed - Er) ) 12 kJ mol-1.2 The C6H10 at 146 K on Ag(111) is consistent with our previously reported work on Cu(100).1,2 However, the yield under comparable of C3H5 coverages is 1.66 times higher on Cu(100) than on Ag(111). This estimate is based on the ratio of integrated intensities of the 914 cm-1 mode of C6H10: (1.8 ( 0.03) × 10-2 cm-1 for Cu(100) and (5.6 ( 0.3) × 10-3 for Ag(111). These reactivity differences may be related to the different orientations of the C-C-C plane of η3-C3H5(c), approximately 70° away from the surface normal on Ag(111) and only 10° away on Cu(100).9 Extrinsic Precursor-Mediated Reaction. The extrinsic precursor-mediated formation of C6H10 on Cu(100) and Ag(111) contrasts with typical carbon-carbon formation reactions of adsorbed alkenyl and alkyl groups on coinage metals.3,4 With respect to the incoming C3H5Br, the η3-C3H5(c)/M (M ) Ag, Cu) substrate can be described as an organometallic nucleophile that attacks electrophiles such as C3H5Br, yielding C6H10. We propose that the precursor-assisted surface reaction reported here for Ag(111) and elsewhere for Cu(100)1,2 involves electron transfer from η3-C3H5(c) to the C-Br bond of C3H5Br(p) that results in the transient formation of a temporary anion, (C3H5Br)-. The anion reacts, with significant probability at ∼130 K, with the surface η3-C3H5(c) to form 1,5-hexadiene, C6H10(p), and Br(c). This reaction is analogous, for example, to the reaction of organic halides (RX) with (η3-allyl)-Ni halides in a so-called nucleophilic radical chain reaction, SRN1, mechanism that involves the short-lived (∼1 µs) radical anion, (RX)-.20 Within this framework, electron transfer to C3H5Cl(p) is energetically more demanding and does not occur. V. Summary On both Cu(100) and Ag(111), allyl halides (C3H5X, X ) Cl, Br) dissociate to C3H5 and halogen atoms at temperatures as low as 77 K. The formation, isomerization and reactivity of σ-bonded (η1-C3H5) allyl and π-bonded (η3-C3H5) allyl groups on Ag(111) and Cu(100) were characterized with RAIRS and TPD. A reversible exo-η3-C3H5 to endo-η3-C3H5 isomerization of C3H5(c) was observed on Cu(100); these differ in the orientation of the CH2 groups with respect to the halogen. As expected for such energetically similar isomers, the enthalpy change is small (∆H ) -4.3 kJ/mol), favoring the endo-η3form on Cu(100). Conversion to exo-η3-C3H5 occurs when either the temperature is raised or halogen atoms are preadsorbed. At 110 K on Ag(111), η1-C3H5 forms during low doses of C3H5Cl, while η3-C3H5 forms during doses of C3H5Br. Independent of coverage, η1-C3H5 and η3-C3H5 isomerize irreversibly to endo-η3-C3H5 when the adsorbate-Ag(111) system is annealed to 175 K. Because the reactivity of η3-C3H5 varies with its adsorption structure, results differ on Cu(100) and Ag(111). Above 200

3916 J. Phys. Chem. B, Vol. 105, No. 18, 2001 K, η3-C3H5 couples to form 1,5-hexadiene at ∼250 K on Ag(111), whereas on Cu(100), desorption as allyl radicals occurs at ∼450 K. At temperatures below 150 K, extrinsic precursorassisted synthesis of C6H10 occurs when the activation energy for reaction between η3-C3H5 and a physisorbed electrophile, e.g., C3H5Br(p), is low enough to compete with alternatives, e.g, C3H5X(p) desorption. This process occurs with significantly higher probability on Cu(100) than on Ag(111), perhaps because the η3-C3H5 structures differ. Finally, electronic structure changes of Ag(111) and Cu(100) induced by coadsorbed halogen atoms are correlated with the isomeric form taken by allyl groups. Acknowledgment. This work is supported in part by the U.S. Department of Energy, Office of Basic Energy Sciences, and by the Robert A. Welch Foundation. References and Notes (1) Celio, H.; Smith, K. C.; White, J. M. J. Am. Chem. Soc. 1999, 121, 10422. (2) Celio, H.; Smith, K. C.; White, J. M. J. Am. Chem. Soc. 2001 (submitted for publication). (3) Bent, B. E. Chem. ReV. 1996, 96, 1361. (4) Zaera, F. Chem. ReV. 1995, 95, 2651.

Celio and White (5) Carter, N. R.; Anton, A. B.; Apai, G. J. Am. Chem. Soc. 1992, 114, 4410. (6) Shustorovich, E. Surf. Sci. 1992, 279, 355. (7) Gurevich, A. B.; Teplyakov, A. V.; Yang, M. X.; Bent, B. E.; Holbrook, M. T.; Bare, S. R. Langmuir 1998, 14, 1419. (8) Bent, B. E.; Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1991, 113, 1143. (9) Scoggins, T. B.; White, J. M. J. Phys. Chem. 1997, 101, 7958. (10) Mann, B. E.; Turner, M. L.; Quyoum, R.; Marsih, N.; Maitlis, P. M. J. Am. Chem. Soc. 1999, 121, 6497. (11) Brinkman, P. H. P.; Luinstra, G. A.; Saenz, A. J. Am. Chem. Soc. 1998, 120, 2854. (12) Breutel, C.; Pregosin, P. S.; Salzmann, R.; Togni, A. J. Am. Chem. Soc. 1994, 116, 4067. (13) Trost, B. M.; Van Vranken, D. L. Chem. ReV. 1996, 96, 395. (14) Kershen, K.; Celio, H.; Lee, I.; White, J. M. Langmuir 2001 (in press). (15) Using the reaction probability equation, Sr ) R/{1 + (kd/kr) exp[-(Ed - Ea)/RT]} where R ∼ 1 (trapping probability), (kd/kr) ) 105, (Ed - Ea) ) 12 kJ/mol, T ) 275 K, and R is the gas constant, we calculated Sr ) 1.2 × 10-3. Additional details are given in ref 2. (16) Swang, O.; Blom, R. J. Organomet. Chem. 1998, 561, 29. (17) Sheppard, N.; De La Cruz, C. AdV. Catal. 1996, 41, 1. (18) Durig, J. R.; Jalilian, M. R. J. Phys. Chem. 1980, 84, 3543. (19) Barnes, A. J.; Holroyd, S.; George, W. O.; Goodfield, J. E.; Maddams, W. F. Spectrochim. Acta 1982, 38A, 1245. (20) Hegedus, L. S.; Thompson, D. H. P. J. Am. Chem. Soc. 1985, 107, 5663.