Olefin Metathesis Activity of Double Bond Contacts to a Conducting Solid

Jun 22, 2009 - We report a surface spectroscopy investigation of metathesis reactions among propene, ethene, 1-butene, 1,3-butadiene, norbornene, and ...
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J. Phys. Chem. C 2009, 113, 12331–12339

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Olefin Metathesis Activity of Double Bond Contacts to a Conducting Solid: Alkylidenes on β-Mo2C Mohamed Siaj, Nathalie Dubuc, and Peter H. McBreen* De´partement de chimie, UniVersite´ LaVal, Que´bec, Que´bec, Canada, G1K 7P4 ReceiVed: January 25, 2009; ReVised Manuscript ReceiVed: May 8, 2009

We report a surface spectroscopy investigation of metathesis reactions among propene, ethene, 1-butene, 1,3-butadiene, norbornene, and cyclopentylidene on a planar polycrystalline β-Mo2C surface. Surface cyclopentylidene was prepared by exposure of the clean metal carbide to cyclopentanone. Propagator species, surface methylidene, ethylidene, propylidene, propylidiene, and ring-opened norbornene were detected using reflectance absorbance infrared spectroscopy. High-resolution X-ray photoelectron data were obtained for the interaction between propene and surface cyclopentylidene. Vibrational and thermal desorption measurements as a function of the number of exposure cycles were carried out for the reaction with propene. Turnover to yield butene was detected. The carbonyl bond dissociative adsorption of propanal and acrolein was used to prepare propylidene and propylidiene to further identify the metathesis propagator species. Given that molybdenum carbide displays metallic conductance, olefin metathesis at the chemisorption bond may find use in the design of devices in which good electrical contact between an electrode and an organic phase is desired. More generally, it offers the opportunity to use routine surface science techniques to study olefin metathesis on extended surfaces. Introduction Terminal alkylidenes may be prepared and isolated on the surfaceofpolycrystallineβ-Mo2C.1-3 Thealkylidenemetal-carbon double bond is also the chemisorption bond, and the carbide is metallic with an electrical conductivity of ∼1.4 × 106 S m-1 at room temperature.4 Alkylidene layers on such conducting materials may combine a number of useful properties for the design of molecular devices and metal-organic hybrid materials. First, the adsorption geometry is unusually well-defined; by definition, terminal alkylidenes form only atop adsorption geometries. Second, Ning et al.5 show that in the case of a 2,4,6octatriene chain connected via double bonds between two ruthenium slabs, (Ru)d(C8H8)d(Ru), the conjugation of the chain extends into the electrodes. This π-continuity leads to significantly higher conductance as compared to that calculated for the case that CH bonds are added at each end of the chain to form two single chemisorption bonds, (Ru)s(C8H10)s(Ru). The beneficial role of metal-carbon double bond contacts in electron transport is also supported by computational studies by Tulevski et al.6 and by Lang and Kagan.7 Furthermore, Chen et al. have recently reported experimental evidence for Ru nanoparticle8a and Ru film-mediated8b intervalence charge transfer between ferrocene moieties attached to the metal using conjugated alkylidene contacts. Third, surface alkylidene systems display high thermal stability,6,9-11 an important quality for forming robust metal-molecule interfaces. Fourth, we have demonstrated that the alkylidene functionalized β-Mo2C surface9,12 is active for olefin metathesis reactions. Similarly, Tulevski et al.6,13 have demonstrated olefin metathesis activity for alkylidene-functionalized Ru thin films and nanoparticles. The latter studies suggest that olefin metathesis at the chemisorption bond might find use in the design of materials for which excellent * To whom correspondence should be addressed. E-mail: peter.mcbreen@ chm.ulaval.ca.

electronic contact between an organic phase and a metallic phase is a targeted property. In this paper, we extend our previous surface spectroscopy study of olefin metathesis on cyclopentylidene-functionalized β-Mo2C.12 Data obtained using reflectance absorbance infrared spectroscopy (RAIRS), high-resolution X-ray photoelectron spectroscopy (XPS), and thermal temperature programmed desorption (TPD) are presented. Results for ethene, propene, 1-butene, butadiene, norbornene, acrolein, and propanal are combined to explore the olefin metathesis reactivity of the metal carbide surface. There are relatively few reports of surface science investigations of olefin metathesis at alkylidene chemisorption sites on extended metallic surfaces. Olefin metathesis has been reported for β-Mo2C9,12 and a molybdenum-aluminum alloy14 under ultrahigh vacuum conditions and for Ru thin films6,8b and nanoparticles in solution.13,15 The active ruthenium films were prepared by exposure of the freshly prepared surface to either 4-bromophenyldiazomethane or trimethylsilyldiazomethane. The active Ru nanoparticles were also prepared using diazo compounds. The active β-Mo2C samples were prepared by exposure of the clean surface to cyclobutanone2,9 or cyclopentanone3 to yield surface cycloalkylidene groups. The latter reaction, which was also reported for acetaldehyde1 and cyclohexanone,10,11 is analogous to the formation of homogeneous alkylidene-W-oxo complexes.16 The metathesis-active MoAl thin films were prepared by exposure to CH2I2.14 Surface science investigations of chemical reactivity on metals are often motivated by a desire to learn more about heterogeneous catalytic processes occurring on highly dispersed supported metal systems. However, it is not yet evident how to compare olefin metathesis on extended metal or metal carbide surfaces with that on conventional catalysts prepared by impregnating an oxide support with a metal precursor. Experimental studies of the latter systems are hindered by the fact that the number of active sites typically corresponds to much less than 1% of the total number of metal atoms.17 Hence,

10.1021/jp900740w CCC: $40.75  2009 American Chemical Society Published on Web 06/22/2009

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although olefin metathesis on supported molybdena was first reported more than 40 years ago,18 there is still a great lack of definitive information on the active catalyst.19,20 In contrast, Basset and co-workers pioneered the design of well-defined, single-site, heterogenized olefin metathesis catalysts using the surface organometallic chemistry method.21 These catalysts are prepared by reacting well-defined metal complexes with isolated siloxy groups on a silica support. The surface complexes are amenable to study by NMR, and the detailed level to which they may be characterized is exemplified by a recent report of the isolation of alkylidene propagator and metallacyclobutane intermediate species for a silica-supported single site [(≡SiO)W(≡NAr)(dCHtBu)(2,5-Me2NC4H2)] olefin metathesis catalyst.22 Surface-initiated ring-opening olefin metathesis polymerization at homogeneous catalysts attached to linkers on silicon, silica, or gold, for example, may be used to grow polymer layers, localized surface structures, and hybrid inorganic-organic materials.23 The study of well-defined olefin metathesis catalysts supported on polymers is also a very active area of research.24 The approach, described in this paper, of performing olefin metathesis reactions directly at alkylidene chemisorption sites on a conducting solid is complementary to the above methods. It is expected that the polycrystalline molybdenum carbide foil used in the present study presents a range of distinct chemisorption sites. However, since the chemical composition of the surface region is Mo2C, as measured by XPS, the extended metal carbide is a better-defined system than a conventional dispersed metal oxide, molybdena, metathesis catalyst. In addition to olefin metathesis activity, molybdenum carbide displays very interesting surface reaction chemistry. The chemisorption properties of metal carbides have been extensively explored by Chen and co-workers.25 Their studies provide ample support for the idea first proposed by Levy and Boudart that carburization renders the catalytic activity of tungsten close to that of platinum.26 Both molybdenum carbide and molybdenum oxycarbide are excellent heterogeneous catalysts.27 For example, zeolite-supported Mo2C is active for the aromatization of methane.28 Hence, the sample used in the present study may be considered as a catalytically active material for hydrocarbon transformation reactions. The results presented herein show that the alkylidene-functionalized carbide surface is selectively active for olefin metathesis reactions. Experimental Section Oyama and Ramanthan29 prepared the sample using a temperature-programmed reaction method to carburize a pure Mo foil in a CH4/H2 mixture. XRD analysis of the carbided foil confirmed that it was essentially pure bulk β-Mo2C. The polycrystalline sample was cleaned by repeated cycles of Ar+ sputtering at 500 K to remove sulfur impurities, followed by annealing at 1385 K. An optical pyrometer was used to countercheck the thermocouple measurements, since the thermocouple attached to the tantalum support rather than to the sample. Residual oxygen in the surface region was minimized by depositing carbon on the surface through ethylene or propylene decomposition at 500-600 K, followed by heating to 1385 K. The sample was annealed to 1300 K and then cooled down to the desired starting temperature prior to the first measurement in each experiment. The surface cleanliness was verified using XPS measurements. The stoichiometry of the surface region, as measured using the Mo(3d5/2)/C(1s) ratio, was 2:1. All of the RAIRS data were recorded at 100 K. Most of the experiments were performed in an in-house, ultrahigh-vacuum system equipped for XPS, TPD, and RAIRS.

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Figure 1. RAIRS spectra recorded for the interaction of propene with cyclopentylidene functionalized planar β-Mo2C. (a) Spectrum of the functionalized surface prior to exposure to propene. (b) Spectrum recorded at 100 K following exposure to 2 × 10-8 Torr during a 1 K/s temperature ramp from 100 to 400 K. (c) Spectrum recorded at 100 K following a subsequent temperature ramp to 470 K. (d) Spectrum at 100 K following the interaction of 2 × 10-8 Torr propene with clean β-Mo2C during a 100-470 K temperature ramp.

A number of high-resolution and high surface sensitivity experiments were performed at the ELETTRA Super-ESCA beamline, Trieste. The following photon energies were used to obtain high surface sensitivity: 400 eV, C(1s) and Mo(3d); 650 eV, O(1s). The same sample preparation procedures were used in both experimental systems. The sample was mechanically mounted in the in-house system on a tantalum support held on Ta wires between liquid-nitrogen-cooled copper feedthroughs. A K-type thermocouple was spot-welded to the tantalum support. The general procedure for preparing cyclopentylidene groups on β-Mo2C was as follows: The clean surface, at 100 K, was exposed to cyclopentanone. Annealing to 180 K eliminated the multilayer, and further annealing to 300-500 K caused carbonyl bond scission, resulting in the formation of surface alkylidenes characterized by νas(CH2) and νs(CH2) RAIRS bands3 at 2960 and 2872 cm-1, respectively (Figure 1, spectrum a). The integrated absorbance of these bands reaches a maximum at ∼500 K.10 Carbonyl bond scission is accompanied by the appearance of a band at ∼981 cm-1, characteristic of an ν(ModO) stretching vibration. By reference to previously reported data for oxo species formed from cyclobutanone,2 acetaldehyde,1 and dissociated NO and O2 on β-Mo2C30 and to the oxidative addition of aldehydes and ketones to WCl2(PMePh2)4 reported by Bryan and Mayer,16 it is assumed that cyclopentonone carbonyl bond scission yields a surface alkylidene-Mo-oxo complex. The surface coverage of cyclopentylidene groups at 500 K was estimated by comparison to O(1s) data for saturation coverages of atomic oxygen, NO, and CO on β-Mo2C at 100 K. These probe molecules were used to titrate the number of chemisorption sites on the surface. Oxygen chemisorption on the β-Mo2C sample at 100 K saturates at an exposure of ∼2 L (1 L ) 1 × 10-6 Torr · s).31 The O(1s) signal for the molecular adsorption of NO and CO at 100 K saturate at 0.89 and 0.66, respectively, of the O(1s) signal for the oxygen monolayer. The ratio of O2 to CO uptake on the β-Mo2C surface (1.5:1) is lower

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SCHEME 1: Chauvin Mechanism for Olefin Metathesis Reactions between Propene and Cyclopentylidene on β-Mo2C

than the values of ∼3.5:1 and ∼6:1 reported for R-Mo2C (0001) at 150 K32 and for supported molybdenum carbide catalysts at 307 K,33 respectively. This suggests that the clean β-Mo2C surface is more reactive toward CO chemisorption than the latter two systems, possibly due to the presence of grain boundary sites in the polycrystalline foil or simply due to the presence of more Mo-terminated surface sites. Comparison to sensitivity factor corrected C(1s) data for 5 L cyclopentanone exposure at 100 K followed by an anneal to 500 K yields a cyclopentylidene surface coverage equivalent to approximately 0.3 of an oxygen monolayer. In this comparison, all the C(1s) signal except that due to the metal carbide (282.8 eV) and extensively dehydrogenated surface carbon (283.4 eV)10,11,30 is attributed to surface cyclopentylidene. This is a very rough estimate, given that some ring-opened molecular fragments11 could also contribute to the signal that is attributed to cyclopentylidene. Nevertheless, the very strong RAIRS signal for cyclopentylidene on β-Mo2C (Figure 1a) clearly indicates a relatively high surface coverage. The important practical point is that carbonyl bond dissociation of aldehydes and ketones on β-Mo2C yields surface coverages of alkylidenes that are high enough to make their study by surface science techniques readily possible. Cross-metathesis experiments were performed by exposing the alkylidene-functionalized surface to the olefin while heating the sample at 1 K/s. Olefin exposures were carried out in the 10-7-10-8 Torr range using uncorrected gauge readings. Spectroscopic experiments were performed by interrupting the heating ramp at a selected temperature, turning off the olefin doser, cooling down to 100 K and verifying the molecular composition of the surface using RAIRS. This procedure was repeated by exposing the carbide to the olefin again while heating from 100 K to the next selected, higher temperature. As detailed below, RAIRS evidence for metathesis reactions becomes clear only following heating to the 400-500 K range. It should be kept in mind that the alkylidene functionalized surface is prepared using a carbonyl bond scission reaction. Hence, atomic oxygen is deposited on the surface during the preparation procedure. As discussed elsewhere,30 atomic oxygen on molybdenum carbide can be distributed into oxo, highcoordination, and subsurface states. Furthermore, some dissolved oxygen is always present in the sample. The present experiments do not permit us to establish the role, if any, of oxygen in the surface metathesis processes. The desorption of oxygenated products, other than that of the reformed parent ketone,9-11 was not observed. Results and Discussion The Chauvin mechanism34 for the transalkylidenation reaction between propene and surface cyclopentylidene, 1, is illustrated in Scheme 1. Reaction pathways A and B involve, respectively, a molybdacyclobutane with the methyl substituent at positions (1, 2) and (1, 3). For pathway A, decomposition of the moly-

Figure 2. RAIRS data for the interaction of ethene with cyclopentylidene functionalized β-Mo2C. (a) Reference spectrum recorded prior to exposure to ethene. Spectra recorded at 100 K following exposure to 2 × 10-8 Torr ethene during sequential 1 K/s ramps from 100 to (b) 300 and (c) 470 K.

bdacyclobutane gives ethylidene cyclopentane, 1c, in the gas phase and surface methylidene, 1a, as the propagator alkylidene. The second pathway (B) forms gas-phase methylene cyclopentane, 1d, and surface ethylidene, 1b. Figure 1 shows IR spectra recorded following exposure of the cyclopentylidene functionalized carbide to 2 × 10-8 Torr propene as the sample temperature is raised from 100 at 1 K/s. No significant change (spectrum b) is observed before heating above 400 K. That is, spectra a and b are characteristic of surface cyclopentylidene alone. In contrast, heating to 470 K causes the appearance of five new bands in the CH stretching region (3075, 2976, 2943, 2922, and 2855 cm-1). The observed frequencies of 2976, 2922, and 2855 cm-1 are in good agreement with those found for 1b formed through acetaldehyde dissociation on β-Mo2C,1 as well as with calculated1,35 and experimental data for ModCHCH3 species.1,36 The features at 3075 and 2943 cm-1 can be attributed to 1a, by reference to literature data for matrix-isolated H2CdRe(O)2OH37 and metathesis-active Od ModCH2 prepared by exposing photoreduced Mo/SiO2 to cyclopropane.36 The latter two systems display a band at 3080 cm-1 as well as a second band at 2986 and 2945 cm-1, respectively. The observation of intense νas(CH2) and νs(CH2) bands shows that the methylene group is tilted away from the normal to the carbide surface. Control experiments (spectrum 1d) show that the interaction of propene with the clean β-Mo2C surface does not lead to the formation of surface alkylidenes. This observation demonstrates that 1a and 1b formed by interaction of propene with 1 result from a cross-metathesis reaction and not from some intrinsic surface reaction of propene. The identification of 1a was further confirmed (Figure 2) by showing that ethene reacts with 1 at 400-470 K to form 1a, as characterized by intense bands at 3077 and 2955 cm-1. As shown in Figure 3, the transalkylidenation reactions were also monitored by performing temperatureprogrammed reaction experiments. Figure 3b shows desorption data for the interaction of propene with 1. The m/e ) 67 signal is the most abundant peak of both 1c and 1d. The onset for desorption of the two primary transalkylidenation products is ∼370 K, and the temperature-programmed reaction peak extends to ∼520 K. The temperature range is in the upper range of that reported for olefin metathesis reactions on supported molybdena catalysts.19b As shown in Figure 3a, the desorption spectrum measured without exposure to propene does not display any

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Figure 4. (Left panel) Mass spectrometry data showing a decreasing rate of methylidene cyclopentane and ethylidene cyclopentane formation as a function of the number of exposures to propene. Each cycle was performed by exposure to 2 × 10-8 Torr propene while ramping the temperature of the sample from 100 to 870 K. The areas of the combined desorption peak are plotted in the inset. (Right panel) RAIRS data showing a decrease in the relative intensity of the cyclopentylidene band at ∼2960 cm-1 during three cycles of exposure to propene. Cycles were performed by exposure to 2 × 10-8 Torr propene while ramping the sample from 100 to 470 K. The area of the cyclopentylidene band at 2960 cm-1 is plotted in the inset. Figure 3. Thermal desorption data for the interaction of propene with cyclopentylidene-functionalized β-Mo2C. (a) Reference spectrum showing the featureless desorption trace of cyclopentanone from the cyclopentylidene-functionalized surface. (b) From bottom to top: data (m/e ) 67) for the interaction of CD2CDCD3, CH2CHCD3, CD2CHCH3, and CH2CHCH3 with the cyclopentylidene-functionalized surface. (c) Data (m/e ) 67) for the interaction of ethene with the cyclopentylidenefunctionalized surface. The reactions were performed by exposing the surface to 2 × 10-8 Torr olefin while raising the sample temperature from 100 at 1 K/s.

SCHEME 2: Chauvin Mechanism for the Olefin Metathesis Reaction between Ethene and Cyclopentylidene on β-Mo2C

signal due to 1c and 1d, ruling out the possibility that the m/e ) 67 peaks in Figure 3b arise from some intrinsic reaction of surface cyclopentylidene. TPD results for the cross-metathesis reaction of ethene and cyclopentylidene to form the expected (Scheme 2) gas-phase product 1d are shown in Figure 3c. In summary, the combined RAIRS and TPD data are fully consistent with the metathesis reactions illustrated in Schemes 1 and 2. The observed ν(CH2) frequencies for methylidene, 1a, do not match those reported for CH2 groups on a variety of metal surfaces. The difference arises because CH2 forms a double bond to a single metal atom on β-Mo2C, whereas it typically bridgebonds to extended metal surfaces to form a structure close to tetrahedral coordination.38 Terminal carbenes are also rarely found in dinuclear organometallic complexes.39 Data for CH2

groups on Fe(110), Ru(001), Rh(111), Mo(110), and Pd/SiO2 show ν(CH2) bands in the 2870-2984 cm-1 range.40 This is consistent with data for µ2-CH2 organometallic complexes showing νas(CH2) and νs(CH2) bands at 2958-2984 and 2918-2933 cm-1, respectively.41 In contrast, 1a displays a ν(CH2) spectrum that closely matches both experimental and calculated data for terminal methylidene species. Matrix isolated MdCH2 species, where M is Fe, Cu, Co, or Cr, display νs(CH2) bands in the 2907-2961 cm-1 range.42 Matrix-isolated (C2H4)FedCH242a and CudCH2 species42b display νas(CH2) bands at 3012 and 3046 cm-1, respectively. Calculated νas(CH2) and νs(CH2) frequencies for OdModCH2, where the Mo atom is also bonded to two adjacent deprotonated silanol groups on a silica cluster, are 3051-3076 and 2917-2939 cm-1, respectively.35 Hence, the observation of a band at approximately 3080 cm-1 provides unequivocal evidence for the formation of ModCH2 surface groups. The formation of metal-carbon double bonds, rather than bridged alkylidenes, on β-Mo2C implies that the molybdenum atoms at the carbide surface behave as if they are isolated singleatom chemisorption sites.3 The selectivity of β-Mo2C toward terminal alkylidene formation may be rationalized by noting that carbide formation leads to increased Mo-Mo distances,43 to metal-carbon covalent bonding,44 and to charge transfer from Mo to C.43 We propose that the combined effect of these three factors serves to increase the electronic corrugation of the carbide surface relative to that of the pure transition metal and that this increased corrugation favors single-site adsorption states. Figure 4 shows TPD (left panel) and RAIRS (right panel) data for additional experiments performed by cycling the cyclopentylidene-functionalized surface to propene.. The TPD data were obtained for three consecutive exposures of 2 × 10-8 Torr propene while the sample was ramped at 1 K/s from 100 to 870 K. The combined desorption signal (m/e ) 67) due to

Olefin Metathesis Activity 1c and 1d, labeled y in the inset, decreases as the number of cycles is increased. The RAIRS data were obtained at 100 K betwen cycles of heating from 100 to 870 K under 2 × 10-8 Torr propene. The variation of the integrated absorbance of the cyclopentylidene band at ∼2960 cm-1 is plotted in the inset. We make the rough assumption that the RAIRS signal is proportional to the surface coverage of 1. Experiments performed over several cycles show that although the cyclopentylidene signal decreases, it is not completely removed. This indicates that metathesis occurs at a faster rate at the propagator sites and that the amount of unreacted initiator, 1, decays slowly with further exposure to the olefin. As discussed below, this analysis assumes that there is negligible decomposition of cyclopentylidene species during the temperature ramps. The thermal desorption and RAIRS data in Figure 4 illustrate a very important and useful property of the carbide: alkylidenes on β-Mo2C display very high thermal stability.9-11 Heating the sample to temperatures in the 470-870 K range does not eliminate metathesis activity. This is consistent with spectroscopic data showing the presence of alkylidenes on β-Mo2C at 900 K.10 Thermally stable carbenes have also been reported for supported catalysts. Vikulov et al.36 showed that alkylidenes on a metathesis catalyst formed by photoreduction of Mo6+/ SiO2 were stable under vacuum to 700 K. An explanation for the anomalous thermal stability of alkylidenes on molybdenum carbide is presented elsewhere.9-11 Briefly, extensive decomposition of a fraction of the ketone or aldehyde precursor layer deposits excess carbon on the surface, thereby rendering it highly inert toward decomposition of any surviving alkylidene groups. This interpretation implies that the metathesis-active surface consists of active alkylidene sites isolated on an otherwise inert surface. It is assumed that surface metathesis primarily takes place through adsorption of the olefin reactant unto the inert component of the surface, followed by diffusion to the active alkylidene sites. This description is analogous to that used by Henry et al. for adsorption onto oxide-supported metal particles.45 The unambiguous detection by RAIRS of various alkylidene species was used as a basis for molecularly specific XPS studies (Figure 5). High-resolution C(1s) spectra were recorded separately at 470 K for cyclopentylidene 1 (spectrum 5c), ethylidene 1b (spectrum 5a), and cyclopentylidene reacted with propene (spectrum 5b). Acetaldehyde dissociation1 on β-Mo2C was used to prepare 1b (spectrum 5a). The broad, low, binding energy feature in each of the spectra is due to a combination of signal for carbide carbon and excess surface carbon.30 This is shown by the deconvoluted peaks at 282.8 and 283.4 eV in spectrum 5a. The C(1s) binding energy for the clean carbide is 282.8 eV; however, the latter signal is largely hidden due to the high surface sensitivity of the synchrotron XPS spectra. The term excess carbon is used to describe extensively hydrogenated carbon deposited on the surface through decomposition of the precursor ketone or aldehyde molecule or though decomposition of the olefin reactant10,11,30 The C(1s) signal due to surface alkylidenes appears at higher binding energies. In particular, ethylidene (spectrum 5a) and cyclopentylidene (spectrum 5c) display peaks at 284.2 and 284.6 eV, respectively. The C(1s) spectrum obtained after the metathesis reaction between propene and 1 at 470 K (spectrum 5b) displays a new maximum at 284.2 eV. By comparison to spectrum 5a, the peak at 284.2 eV is attributed in part to 1b formed as in Scheme 1. Obviously, as shown by the RAIRS data in Figures 1 and 4, the transalkylidenation reaction also leads to the formation of surface methylidene 1a, and some cyclopentylidene 1 may remain

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Figure 5. C(1s) data obtained using synchrotron radiation. (a) Spectrum of ethylidene formed through the dissociative chemisorption of acetaldehyde on clean β-Mo2C. (b) Spectrum recorded following the interaction of propene with cyclopentylidene functionalized β-Mo2C at 470 K. (c) Spectrum of cyclopentylidene formed by exposing clean β-Mo2C to cyclopentanone. All of the spectra were recorded at 470 K.

unreacted. We do not attempt to ascertain the relative concentrations of these species through a full deconvolution of the spectra; however, the shift in the peak maximum from 284.6 to 284.2 eV clearly shows that the initiator species 1 is largely replaced by propagator species. The primary propagator alkylidenes 1a and 1b can enter back into the productive catalytic cycle in propene metathesis to yield gas-phase butene and ethene (Scheme 3). The inset to Figure 6 shows butene (m/e ) 56), 1c, and 1d desorption signals recorded as a function of temperature during the continuous exposure of cyclopentylidene-functionalized β-Mo2C to 2 × 10-8 Torr propene. The detection of butene shows that catalytic turnover can be achieved under uhv conditions. All measurements show the simultaneous onset of butene, 1c, and 1d formation. However, in all experiments, the rate of butane formation was observed to increase as the rate of 1c and 1d formation decreases. The main part of Figure 6 shows data that were obtained by exposing the functionalized sample to 2 × 10-8 Torr propene while ramping the temperature by 1 K/s from 100 to 720 K and then holding the temperature constant. The butene signal grows as the temperature is increased above ∼340 K and remains constant as the sample temperature is held at 720 K. Measurements were performed using 1-butene and 1,3butadiene to explore the metathesis activity of 1 toward progressively more complex olefins. Figure 7 displays spectra for an experiment in which the cyclopentylidene-functionalized carbide was exposed to a background of 3 × 10-8 Torr of 1-butene as a function of increasing temperature. As shown in Scheme 4, the Chauvin mechanism predicts the formation of methylidene, 1a, and propylidene, 2b, propagators. The RAIRS spectrum recorded on heating to 400 K displays only RAIRS bands characteristic of 1. However, on increasing the temperature to 470 and 550 K, the spectrum exhibits new bands at ∼3071, 2977, 2955, ∼2911, and 2845 cm-1. These bands may be assigned to 1a (3071 and 2955 cm-1), 2b (2977, 2911, 2877, and 2845 cm-1), and unreacted 1 (2966 and 2877 cm-1). The identification of 2b is made by reference to vibrational data for liquid46 and chemisorbed propanal47 and by reference to the

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SCHEME 3: Chauvin Mechanism Illustrating the Formation of Gas Phase Ethene and Butene in a Second Catalytic Cycle

Figure 6. Inset: Mass spectrometry data showing the formation of methylidene cyclopentane and ethylidene cyclopentane (m/e ) 67) and butene (m/e ) 56) during exposure of cyclopentylidene-functionalized β-Mo2C to 2 × 10-8 Torr propene during a 1 K/s temperature ramp starting at 100 K. The main part of the Figure shows the butene signal for an experiment performed by ramping from 100 to 720 K in the presence of 2 × 10-8 Torr propene and then holding the sample temperature constant at 720 K. The signal for background propene is also shown.

RAIRS spectrum for propanal annealed to 400 K on β-Mo2C (Figure 7, top spectrum). By analogy to earlier work on acetaldehyde,1 propanal is assumed to produce 2b through carbonyl bond scission on the carbide surface. RAIRS spectra recorded for the interaction between 1,3butadiene and 1, performed as for 1-butene, are shown in Figure 8. The Chauvin mechanism, illustrated in Scheme 5, predicts the formation of 1a and propylidiene, 3b, propagators. The transalkylidenation process proceeds at approximately the same temperature range as for propene and ethene. Below ∼410 K, neither new bands nor the attenuation of the cyclopentylidene bands was observed. In contrast, at 480 K, new peaks at ∼3087, ∼3061, 2951 and ∼2894 cm-1 grow in. By reference to HREELS data for adsorbed acrolein47,48 and by reference to the RAIRS spectrum of acrolein annealed on β-Mo2C to 400 K (spectrum 8c), the bands at ∼3087 and ∼3061 cm-1 may be assigned to a mixture of 1a and 3b. Scission of the acrolein

Figure 7. RAIRS spectra for the interaction of 1-butene with cyclopentylidene-functionalized β-Mo2C. Spectra were recorded at 100 K following exposures to 3 × 10-8 Torr 1-butene during consecutive temperature ramps from 100 to (a) 400, (b) 470, and (c) 550 K. The uppermost spectrum was obtained by exposing clean β-Mo2C to 50 L propanal at 105 K, followed by an anneal to 400 K.

SCHEME 4: Chauvin Mechanism for Olefin Metathesis Reactions between 1-Butene and Cyclopentylidene on β-Mo2C

carbonyl bond on the carbide surface is assumed to yield 3b in the same way that the dissociative adsorption of acetaldehyde yields 1b.1 The formation of polymer layers on substrates is an important method for modifying surfaces and for incorporating organic

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J. Phys. Chem. C, Vol. 113, No. 28, 2009 12337 SCHEME 6: Illustration of Ring-Opening Metathesis Polymerization (ROMP) of Norbornene at a Cyclopentylidene Surface Site

Figure 8. RAIRS spectra for the interaction of 1,3-butadiene with cyclopentylidene-functionalized β-Mo2C. Spectra recorded at 100 K following exposure to 2 × 10-8 Torr 1,3-butadiene during consecutive 1 K/s temperature ramps to (a) 400 and (b) 480 K. Spectrum c was obtained by exposing clean β-Mo2C to 15 L acrolein at 100 K, followed by an anneal to 400 K.

occurs at approximately 170 K. Exposure at 400 K leads to a slight broadening of the intense cyclopentylidene peak, ∼2960 cm-1. Exposure at 500 K shows that this broadening is due to the growth of a new band at 2953 cm-1. In addition, new bands appear at ∼3058, ∼2914, and 2877 cm-1. The interaction of norbornene with the cyclopentylidenefunctionalized surface differs from that observed by Hostetler et al.49 for the adsorption of norbornene on Pt(111) and on H2exposed Pt(111). The latter authors found desorption from the

SCHEME 5: Chauvin Mechanism for Olefin Metathesis Reactions between 1,3-Butadiene and Cyclopentylidene on β-Mo2C

components into hybrid materials.23,24a For example, tethered Grubbs catalysts have been used to carry out surface-initiated ring-opening metathesis (ROMP) polymerization on a variety of substrates.23 The metathesis activity of molybdenum carbide offers the possibility of growing polymers from its surface. The formation and spectroscopic isolation of propylidiene, 3b, through cross-metathesis with 1,3-butadiene or through the adsorption of acrolein is significant in that it represents an example of the synthesis of a short-chain conjugated unit directly attached to a conducting solid. As described next, experiments performed using norbornene show that the surface is active for ROMP insertion, giving rise to the possibility of forming longer chains from the surface. Results for test ROMP reactions (Scheme 6) carried out using norbornene as the monomer and surface cyclopentylidene, 1, as the initiator are shown in Figure 9a. Exposure of cyclopentylidene-functionalized β-Mo2C at 100 K to norbornene yields vibrational bands at 3063, 2976, 2950, 2920, and 2872 cm-1 characteristic of condensed molecular norbornene.49,50 The latter bands are not present in spectra taken on exposure to norbornene at 300 K; rather, the spectrum is simply characteristic of cyclopentylidene (Figure 9b), indicating that norbornene desorbs rather than reacts at 300 K. Desorption from the multilayer

Figure 9. RAIRS data for the reaction of norbornene with cyclopentylidene-functionalized β-Mo2C. (a) The spectra labeled 140 and 170 K were recorded after exposure of clean β-Mo2C at 100 K to 11 L norbornene followed by annealing to the indicated temperatures. The spectrum labeled 300 K was recorded following a subsequent 1 K/s anneal of sample from 100 to 300 K in the presence of 2 × 10-8 Torr. The sample was then exposed to 3 × 10-8 Torr during a temperature ramp from 100 to 400 K. The spectrum labeled 500 K was then recorded after two 1 K/s ramps from 100 to 500 K in the presence of 7 × 10-8 Torr. (b) A reference spectrum of cyclopentylidene-functionalized β-Mo2C prior to exposure to norbornene.

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multilayer at ∼178 K, CH scission above 220 K to form a norborneyl intermediate, and decomposition to yield benzene and surface CxHy at 470-520 K. Their RAIRS spectra of norbornene on Pt(111) and on H2/Pt(111) at 300 K display intense ν(CH) bands at approximately 2864, 2900, and 2940 cm-1. These bands are completely absent at 300 K (Figure 9) on the cyclopentylidene/β-Mo2C surface, where only the bands due to cyclopentylidene are observed. However, exposure to norbornene at 500 K leads to a spectrum consistent with ringopening metathesis. The new bands observed at 500 K are very similar to those (2946, 2910, and 2866 cm-1) reported by Kim et al.23x for the surface-initiated ring-opening of norbornene from a Grubbs catalyst tethered to Si/SiO2, showing that norbornene inserts into 1 through ring-opening. The band at 3063 cm-1 due to the alkene CH vibration is close in frequency to the value of 3055 cm-1 reported for polynorbornene grafted to gold particles.23y These results are consistent with weak adsorption of norbornene on the inert component, the carbon passivated regions of the surface, and norbornene insertion through ROMP at active alkylidene sites. Conclusions This paper expands on a previous report of the activity of molybdenum carbide for olefin metathesis reactions.12 The detailed information gained using surface science techniques offers the prospect of moving toward the rational use of heterogeneous metathesis catalysis to construct materials where excellent electronic contact between the organic and metal surfaces is desired.51 In this context, it is already well-established that metal carbides,52 including Mo2C,52d,e may be used as ohmic contacts to pure carbon materials such as nanotubes and diamond. The results suggest that the Chauvin mechanism34 for homogeneous olefin metathesis also applies to olefin metathesis on metal surfaces. That is, the active site for each catalytic cycle contains a surface alkylidene group. The fact that alkylidenes display anomalous thermal stability on the carbide surface ensures a high coverage of active sites under reaction conditions. The cyclopentylidene-functionalized β-Mo2C system is a relatively well-defined metathesis catalyst in that its average surface composition is characterized by XPS and in that it is possible to obtain vibrational spectra of the initiator and propagator alkylidenes. Acknowledgment. Financial support from the Natural Sciences and Engineering Research Council of Canada is gratefully noted. We acknowledge the contribution of C. Maltais and I. Temprano to some of the experimental measurements. References and Notes (1) Siaj, M.; Reed, C.; Oyama, S. T.; Scott, S. L.; McBreen, P. H. J. Am. Chem. Soc. 2004, 126, 9514. (2) Siaj, M.; Oudghiri-Hassani, H.; Zahidi, E.; McBreen, P. H. Surf. Sci. 2005, 579, 1. (3) Siaj, M.; Temprano, I.; Dubuc, N.; McBreen, P. H. J. Organomet. Chem. 2006, 5497, 2425. (4) Tsakalakos, L.; Rahmane, M.; Larsen, M.; Gao, Y.; Denault, L.; Wilson, P.; Balch, J. J. Appl. Phys. 2005, 98, 044317. (5) Ning, J.; Qian, Z. K.; Li, R.; Hou, S. M.; Rocha, A. R.; Sanvito, S. J. Chem. Phys. 2007, 126, 174706. (6) Tulevski, G. S.; Myers, M. B.; Hybertsen, M. S.; Steigerwald, M. L.; Nuckolls, C. Science 2005, 309, 591. (7) Lang, N. D.; Kagan, C. R. Nano Lett. 2006, 6, 2955. (8) (a) Chen, W.; Chen, S.; Ding, F.; Wang, H.; Brown, L. E.; Konopelski, J. P. J. Am. Chem. Soc. 2008, 130, 12156. (b) Chen, W.; Brown, L. E.; Konopelski, J. P.; Chen, S. Chem. Phys. Lett. 2009, 471, 283. (9) Zahidi, E.; Oudghiri Hassani, H.; McBreen, P. H. Nature 2001, 409, 1023.

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