© Copyright 2002 American Chemical Society
MARCH 5, 2002 VOLUME 18, NUMBER 5
Letters Identification of Surface Allenyl and Its Transformation into Propargyl with C3H3Br Adsorption by RAIRS on Ag(111) Yu-Jui Wu, Wei-Hua Wang, and Chao-Ming Chiang* Department of Chemistry, National Sun Yat-Sen University, Kaohsiung, Taiwan 80424 Received October 10, 2001 Chemisorbed allenyl/propargyl (two possible C3H3 isomers) intermediates on Ag(111) were isolated and identified by reflection-absorption infrared spectroscopy (RAIRS). Following the adsorption of propargyl bromide at 110 K, surface heating resulted in the cleavage of the CsBr bond at 200 K (confirmed by X-ray photoelectron spectroscopy) and surface hydrocarbon species bearing the characteristic CdCdC stretching vibration in RAIRS, indicative of the surface-bound allenyl. This assignment was supported by the same experiment using bromopropadiene (allenyl bromide) as the precursor, in which the identical spectral change was observed. A distinct molecular transformation occurred around 300 K featuring the appearance of new IR bands, particularly the CtC stretch mode, suggesting that surface propargyl was formed. This propargylic intermediate remained thermally stable to 450 K before further decomposition and reactions on the surface.
Introduction Metal complexes bearing unsaturated C3H3 fragments have come under increased scrutiny in recent years because of their structural diversity, modes of bonding, and rich reactivity.1-3 The C3H3 group allows a structure corresponding either to the propargyl or the allenyl form (Scheme 1). The metal-allenyl can be viewed as tautomeric with the propargyl species via a 1,3-hydrogen shift. However, studies show that the isomerization actually favors a pathway involving a 1,3-metal sigmatropic rearrangement,4,5 in which the metal migrates over the hydrocarbon framework to cause a change in the locus of * To whom correspondence should be addressed. Phone: 8867-525-3939. Fax: 886-7-525-3908. E-mail:
[email protected]. (1) Tsutsumi, K.; Ogoshi, S.; Quyoum, R.; Nishiguchi, S.; Kurosawa, H. J. Am. Chem. Soc. 1998, 120, 1938. (2) Doherty, S.; Corrigan, J. F.; Carty, A. J.; Sappa, E. Adv. Organomet. Chem. 1995, 37, 39. (3) Wojcicki, A. New. J. Chem. 1994, 18, 61. (4) Pu, J.; Peng, T.-S.; Arif, A. M.; Gladysz, J. A. Organometallics 1992, 11, 3232. (5) Keng, R.-S.; Lin, Y.-C. Organometallics 1990, 9, 289.
Scheme 1
metal-ligand bonding (Scheme 1). Generally, a reversible and facile interconversion is implied. Propargyl/allenyl-metal compounds possess utility as carbon transfer agents in organic synthesis. For example, they can afford either allenic or acetylenic derivatives when treated with electrophiles, depending on whether the reaction takes place at the propargylic or the allenic site.6-9 Since both forms often exist as an equilibrium (6) Creary, X. J. Am. Chem. Soc. 1977, 99, 7632. (7) Westmijze, H.; Kleijn, H.; Bos, H. J. T.; Vermeer, P. J. Organomet. Chem. 1980, 199, 293. (8) Yamamoto, H. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Heathcock, C. H., Eds.; Pergamon: Oxford, 1991; Vol. 2, pp 81.
10.1021/la011534w CCC: $22.00 © 2002 American Chemical Society Published on Web 02/01/2002
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mixture, regiochemical ambiguity inevitably arises. The preference for adoption of the propargyl or allenyl form is also dependent upon the metal. To achieve high regioselectivity in the formation of either the allenic or the alkynic products, better knowledge of the thermal stability and the isomerization mechanisms in the propargyl/allenyl-metal interaction appears to be especially valuable. Ligands of interest to the inorganic chemist are good candidates for studies on surfaces. Concepts learned from the chemisorption system can contribute to the understanding of organometallic and coordination chemistry. Hence, we attempt to isolate the C3H3 species on a surface at cryogenic temperatures, where propargyl and/ or allenyl moieties might have long lifetimes, to permit direct spectroscopic recognition. Experimental Section The reflection-absorption infrared spectroscopy (RAIRS) experiments were performed in an ultrahigh vacuum (UHV) chamber evacuated with a turbomolecular pump and an ion pump to a base pressure of 1 × 10-10 Torr or below. RAIRS was performed by taking the infrared beam from a Nicolet Magna 560 FTIR spectrometer and focusing it at grazing incidence (85°) through a polarizer and a KBr window onto the Ag(111) in the UHV chamber. The reflected beam was then passed through a second KBr window and refocused on a liquid N2 cooled HgCdTe detector. All spectra correspond to the average of 1500 scans at 4 cm-1 resolution and are ratioed against the background spectra from the clean metal surface. The X-ray photoemission spectra (XPS) were recorded in an UHV chamber connected to a wide-range (10-1500 eV) spherical grating monochromator beamline at the Synchrotron Radiation Research Center in Hsin-Chu, Taiwan. The sample preparation and gas handling all followed standard procedures, and were described in detail elsewhere.10
Results and Discussion The use of hydrocarbon halides as precursors has proven useful for forming adsorbed intermediates of interest.11,12 Here this methodology was applied to the C3H3 system by adsorbing propargyl bromide (CHtCCH2Br) and/or bromopropadiene (CH2dCdCHBr) dissociatively on Ag(111) under UHV conditions. With the aid of surface-sensitive RAIRS, we were able to determine in situ that the allenyl form is favored below 240 K, and then it is thermally transformed into the propargyl form at higher temperatures on the surface. By exposing Ag(111) to 2 Langmuirs (L, 1 L ) 10-6 Torr‚s) of propargyl bromide (commercially available from TCI, >97% purity) at 110 K, resulting in a fraction of a monolayer coverage,13 a series of temperature-dependent Br 3d XPS spectra is shown in Figure 1. The broad curve (a) at 110 K can be systematically decomposed into two sets of 3d3/2/3d5/2 spin-orbit-split peaks, one at the binding energies (BE) 70.6/69.8 eV and the other at 69.2/68.2 eV. The former peaks are assigned to the C-Br binding energies, and the latter are assigned to Br bound directly to Ag. In agreement with previous work regarding CH3Br photodissociation on Ag(111) by Zhou and White,14 we take the higher BE features as adsorbed molecules with the C-Br bond intact and the lower BE state as a species (9) Tsuji, J.; Mandai, T. Angew. Chem., Int. Ed. Engl. 1995, 34, 2589. (10) Wu, H.-J.; Hsu, H.-K.; Chiang, C.-M. J. Am. Chem. Soc. 1999, 121, 4433. (11) Zaera, F. Acc. Chem. Res. 1992, 25, 260. (12) Bent, B. E. Chem. Rev. 1996, 96, 1361. (13) Surface coverage of adsorbates was built up by background dosing. A monolayer is defined in terms of the maximum exposure that does not give any multilayer desorption peak (135 K) intensity in the temperature programmed desorption. (14) Zhou, X.-L.; White, J. M. Surf. Sci. 1991, 241, 259.
Figure 1. (Left) X-ray photoelectron spectra of Br 3d acquired from Ag(111) exposed to 2 L of propargyl bromide at 110 K following annealing at the specified temperatures. The dots represent the collected data, and the solid lines correspond to the best fit obtained with Gaussian-broadened Lorentzian spinorbit doublets after Shirley background subtraction. (Right) TPR spectra showing one of the surface reaction products, HBr, for 2 L of propargyl bromide adsorbed on Ag(111).
with the C-Br bond cleaved. Therefore, propargyl bromide must adsorb both molecularly and dissociatively at this temperature. Annealing to 200 K, curve b, leads to only one set of peaks at 69.2/68.2 eV, indicating complete C-Br bond cleavage and generation of C3H3 fragments on the surface. Further annealing to 240 K and higher temperatures, curves c-f, produces no core-level shift, except that the peak areas start to decrease above 600 K. There is no detectable Br XP signal after heating to 900 K. As a reference, the condensed multilayer RAIR spectrum is shown in Figure 2(a, left) after dosing 20 L of propargyl bromide on Ag(111) at 110 K. All features agree well with the major infrared bands of the gas-phase compound15 (see assignments in Table 1). The presence of terminal acetylenic C-H and methylene CH2 groups is evidenced by the stretching modes at 3280 cm-1, 3012 cm-1 (asymmetric), and 2964 cm-1 (symmetric), respectively. The skeletal modes ν(CtC) and ν(CsC) appear at 2123 and 956 cm-1, respectively. The remaining modes at 1422, 1212, and 1148 cm-1 are attributed to γ(CH2, scissoring), ω(CH2, wagging), and τ(CH2, twisting) vibrations. In contrast, Figure 2(b, left) displays a RAIR spectrum after Ag(111) is exposed to 2 L of propargyl bromide at 110 K, where the low intensities (as low as 10-4 absorbance units) of the spectrum reasonably manifest IR signals from a submonolayer coverage of adsorbates. Aside from peaks (3012, 2123, 1422, 1212, 1148, and 956 cm-1) attributable to the intact molecules, two additional features at 1812 and 892 cm-1 do not correspond to any modes in the (15) Shimanouchi, T. J. Phys. Chem. Ref. Data 1977, 6, 993-1102.
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Figure 2. (Left) RAIR spectra of (a) 20 L of propargyl bromide on Ag(111) at 110 K (This exposure was sufficient to populate the multilayer.) and (b) 2 L of propargyl bromide adsorbed at 110 K. Spectra c-g were measured at 110 K following momentary annealing of sample b to the temperatures indicated. (Right) RAIR spectra acquired at 110 K (a) following 10 L (multilayer) of bromopropadiene adsorption, and annealing to the specified temperatures (b-f). The absorbance scale is marked as a vertical line. Table 1. Vibrational Assignments (in cm-1) of Gaseous Propargyl Bromide (HCtCsCH2Br) and Bromopropadiene (H2CdCdCHBr), and Comparison with the Frequencies Observed in the RAIR Spectra after Propargyl Bromide or Bromopropadiene Adsorption on Ag(111) normal modea
gas-phase CHCCH2Brb
propargyl bromide, 20 L dosed at 110 K
ν(tCsH) νa(CH2) νs(CH2) ν (CtC)
3335 3006 2976 2138
3280 3012 2964 2123
γ(CH2) ω(CH2)
1431 1218
1422 1212
τ(CH2)
1152
1148
ν(CsC)
961
normal modea
gas-phase CH2CCHBrb
bromopropadiene, 10 L dosed at 110 K
ν(CH) νa(CH2) νs(CH2)
3080 3080 3005
3063
νa(CdCdC) γ(CH2)
1961 1432
1956 1421
δ(CH)
1217
1210
νs(CdCdC) F(CH2)
1078 1000
1087
862
878
propargyl bromide or bromopropadiene annealed at 300 K 2913 (tC‚‚‚H+δ‚‚‚Br-δ) 2843 2026
1812
1120
956
942 ω(CH2)
a
propargyl bromide or bromopropadiene annealed at 200 K
892
Notation: ν ) stretching, γ ) scissoring, ω ) wagging, τ ) twisting, δ ) bending, F ) rocking. b Reference 15.
precursor. We believe that these two peaks are characteristic of the adsorbed C3H3 fragments. After the surface is annealed to 200 K (above the molecular desorption temperature), only these two peaks remain in the spectrum (Figure 2(c, left)). The 1812 and 892 cm-1 frequencies are reminiscent of the fingerprint modes for allenic species: the asymmetric CdCdC stretching and the terminal d CH2 wagging modes. For example, in the gas-phase bromopropadiene (see Table 1) these two bands appear at 1961 and 862 cm-1. In the metal-allenyl complexes, CpRe(NO)(PPh3)(η1-CHdCdCH2)4 and CpW(CO)3(η1-CHdCd CH2),5 as well as the allenic compound (Me3Sn)2CdCd C(Me3Sn)2,16 the νa(CdCdC) band was observed at 1926, 1900, and 1850 cm-1, respectively. Yet our band at 1812 cm-1 differs from these frequencies in the homogeneous systems by about 40-110 cm-1. Strong interactions between the linear CdCdC unit and surface atoms must (16) West, R.; Jones, P. C. J. Am. Chem. Soc. 1969, 91, 6156.
be invoked to account for such a large frequency reduction, a situation similar to those in the rare dinuclear µ-η3allenyl palladium complexes reported by Kurosawa,17 in which charge back-donation from the metal to the empty π* MO of the allenyl ligand is implicated. However, the surface dipole selection rule in RAIRS allows excitations only of dynamic dipoles with components normal to the surface. Since the ν(CdCdC) mode is not invisible in this spectrum, we believe that the CdCdC axis cannot be entirely horizontal or, rather, it is slightly tilted with respect to the Ag(111) surface (not exactly an η3-bonding). However, could the 1812 cm-1 peak be assigned to the CtC stretches (2123 cm-1 for propargyl bromide) redshifted by interactions between the triple bond and the surface? To support the argument for CdCdC not CtC, we adsorbed bromopropadiene on Ag(111) as a direct route (17) Ogoshi, S.; Tsutsumi, K.; Ooi, M.; Kurosawa, H. J. Am. Chem. Soc. 1995, 117, 10415.
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to the allenyl form. Bromopropadiene was prepared in our laboratory on the basis of the method reported by Jacobs and Brill.18,19 Figure 2(a, right) shows the multilayer RAIR spectrum after dosing 10 L of bromopropadiene on Ag(111) at 110 K, which correlates well with the gasphase assignments (see Table 1). In particular, the characteristic CH stretching, CH bending, CdCdC astretching, CdCdC s-stretching, and CH2 wagging modes are recognizable. The spectrum changes significantly after the momentary annealing of the surface at 200 K (Figure 2(b, right)), in which only 1812 and 892 cm-1 frequencies are observable; and this spectrum is essentially identical with Figure 2(c, left). Although only two peaks are observed in the spectra (due to the very limited signal-to-noise ratio of RAIRS in this chemisorption system), fortunately the 1812 cm-1 peak is fairly diagnostic of allenyl. The simple spectra shown in both Figure 2(c, left) and 2(b, right) can be reconciled with the allenyl form of a chemisorbed C3H3 intermediate. The spectra start to display drastic changes around 240 K, at which new bands emerge (see Figure 2(d, left) and 2(c, right)). Warming to 300 K eventually leads to the development of RAIR spectra 2(e, left) and 2(d, right), indicative of a dramatic molecular transformation. Most notably, the intense band at 2026 cm-1 is attributable to the ν(CtC), despite a more than 50 cm-1 deviation from those in the gas-phase propargyl bromide and metalpropargyl compounds.4,5,20 The substantial reduction of the CC stretch frequency again suggests strong interactions between the triple bond π orbitals and the surface. When the evidence is complemented with the 942 cm-1 peak, assigned to ν(CsC), a surface alkynic species containing the CsCtC unit becomes evident. However, this carbon framework cannot lie flat on the surface; otherwise, these vibration modes should be IR-inactive. Intriguingly, the terminal tCsH stretching mode (3280 cm-1 in propargyl bromide) seems to be missing. One possible interpretation is that the acetylenic ν(tCsH) vibration is not absent but, instead, associated with the pronounced 2913 cm-1 frequency in Figure 2(e, left) and 2(d, right) because the “band splitting” feature (labeled *) resembles the peak around 3280 cm-1 in spectrum 2(a, left) of multilayer propargyl bromide. If it is true, such remarkable C-H mode softening (3283 cm-1 - 2913 cm-1 ) 370 cm-1) can be rationalized by the following argument: (1) A hydrogen bonded to a sp-hybridized carbon atom is much more acidic than other hydrocarbons. As a (18) Bromopropadiene was synthesized by stirring and heating propargyl bromide with dry cuprous bromide (CuBr). The product of the rearrangement can be obtained by distillation using the difference of their boiling points (72.8 °C bromopropadiene vs 90 °C propargyl bromide). 1H NMR indicates ∼90% purity in the liquid phase. See ref 19 for details. (19) Jacobs, T.; Brill, W. F. J. Am. Chem. Soc. 1953, 75, 1314. (20) Jolly, P. W.; Pettit, R. J. Organomet. Chem. 1968, 12, 491.
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result, removal of a proton from a terminal alkyne can be accomplished with relative ease.21 (2) A neighboring coadsorbed Br atom may act as the base which is responsible for weakening the tCsH bond to a great extent via a tC-δ‚‚‚H+δ‚‚‚Br-δ type of interaction. In addition, a temperature-programmed reaction (TPR) experiment detected the evolution of HBr above 550 K (Figure 1, right panel), somewhat supporting our speculation. However, this argument remains speculative until further evidence is provided. On the basis of the spectral analysis, it is reasonable to state that the surface species isolated at 300 K is the propargyl form of C3H3 and that an irreversible process, converting allenyl into propargyl, is directly demonstrated by RAIRS. Figure 2(f, left) does not change much compared to 2(e, left); therefore, the propargyl persists on the surface up to 450 K. Further heating to 550 K causes most of the absorption bands to disappear in Figure 2(g, left) and 2(f, right), which coincides with the propyne (C3H4) desorption above 450 K found in TPR measurements (data not shown here). The origin of the residual vibration feature at 1138 cm-1 is unknown but is tentatively attributed to the ν(C-C) mode from surface hydrocarbon fragments.22 Finally, heating the surface to 900 K removes all the adsorbateinduced IR signals. In summary, we have successfully isolated the reactive C3H3 intermediates on Ag(111) by adsorption and thermal dissociation of propargyl bromide and bromopropadiene. The preference for adoption of the allenyl or propargyl form on this surface is temperature dependent, which might have implications for the regiochemical control in synthetic applications. RAIRS has unequivocally shown that the allenyl is preferred below 240 K, and observed the complete rearrangement of allenyl to propargyl occurring at 300 K. The propargyl form is thermally stable up to 450 K. Despite numerous investigations on propargyl/allenyl-metal complexes, this report presents the first complementary surface study executed on a clean metallic single crystal under UHV conditions. Further research on bonding and reactivity to discern the analogies and differences between adsorbate/surface and ligand/ metal systems is in progress. Acknowledgment. This research was supported by the National Science Council of the Republic of China under Contract No. 89-2113-M-110-030. We thank Dr. Y. W. Yang and Dr. L. J. Fan for their assistance at the Synchrotron Radiation Research Center. LA011534W (21) Wade, L. G., Jr. Organic Chemistry, 4th ed.; Prentice Hall International: London, U.K., 1999; pp 388-389. (22) XPS discovered that about 50% of the adsorbed C3H3 decomposes irreversibly to surface carbon.
