J. Phys. Chem. B 2004, 108, 14025-14031
14025
Photoinduced Reactions of a Carbene Precursor with Chemisorbed CO on a Metal Surface: Diphenyldiazomethane on CO-Covered Ru(0001) Markus Gunia,† Peter Jakob,‡ Wolfram Sander,§ and Christof Wo1 ll*,† Lehrstuhl fu¨r Physikalische Chemie I, Ruhr-UniVersita¨t Bochum, D-44780 Bochum, Germany, Fachbereich Physik, Philipps-UniVersita¨t Marburg, D-35032 Marburg, Germany, and Lehrstuhl fu¨r Organische Chemie II, Ruhr-UniVersita¨t Bochum, D-44780 Bochum, Germany ReceiVed: February 27, 2004; In Final Form: May 11, 2004
The adsorption of the carbene precursor diphenyldiazomethane (DPDMA) on clean and CO-precovered Ru(0001) has been investigated using infrared reflection absorption spectroscopy (IRAS). Deposition of DPDMA in a control experiment onto the clean metal surface leads to a cleavage of the diazo group, yielding an upright standing diphenylmethylene species. To inhibit this instant dissociation and to control the state of the carbene precursor, adsorption was studied on a CO-spacer layer on the Ru-surface. The reaction of the photochemically or thermally formed diphenylcarbene with the underlying CO molecules yields a new carbonylcontaining species, which is proposed to be a surface bound diphenylketene.
Introduction Carbenes are important reactive intermediates in organic chemistry and have been investigated extensively in the past with spectroscopic and theoretical methods.1-6 In surface chemistry, metal carbene complexes have been proposed to play a key role in several reactions important to heterogeneous catalysis, i.e., the Fischer-Tropsch synthesis, where a carbene intermediate is supposed to insert into the metal-carbon bond of chemisorbed CO to yield a ketene.7,8,31 While reactions of diazo compounds with transition metal complexes are well-known, there are only few studies of reactions of diazo compounds on metal surfaces. Since it is rather difficult to introduce a reactive carbene directly into a UHV system, the use of a precursor molecule such as a diazo compound is required. A large number of studies of carbenes generated from diazo compounds, e.g., in noble gas matrixes, have been reported.9-15 The main advantage of using diazo compounds as precursors is that the carbene is easily generated by removing N2 either thermally or photochemically. The N2 molecule interacts only weakly with the freshly formed carbene and is easily removed from the system. Here we describe experiments where diphenyldiazomethane (DPDMA), a precursor of diphenylcarbene, has been deposited on a CO-saturated ruthenium surface. This diazo compound is one of the most extensively investigated carbene precursors in organic chemistry.1,2,4,16,17 An adlayer of CO on ruthenium is a relevant model system, which has been characterized in considerable detail in a number of previous investigations.18-20 Unfortunately, the large sensitivity of the DPDMA molecule toward X-ray photons (see Figure 1S, Supporting Information) prohibits the application of X-ray photoelectron spectroscopy (XPS), a standard method for characterizing molecular adsorbates. The main technique used here to characterize the molecule adlayers is infrared reflection absorption spectroscopy (IRAS). * To whom correspondence should be addressed. E-mail:
[email protected]. † Lehrstuhl fu ¨ r Physikalische Chemie I, Ruhr-Universita¨t Bochum. ‡ Philipps-Universita ¨ t Marburg. § Lehrstuhl fu ¨ r Organische Chemie II, Ruhr-Universita¨t Bochum.
Figure 1. Comparison of IR-data: multilayer of DPDMA (360 Langmuir) adsorbed at 80 K (dashed line) vs DPDMA in an argon matrix at 20 K (solid line).
Experimental Section The UHV system used in this investigation has been described earlier.21 Briefly, it consists of a highly stable setup for infrared absorption measurements in reflection geometry (incident angle 84° from the surface normal). An MCT-detector is used to detect the reflected p-polarized IR-light. The standard settings of data collection include coaddition of 2000 scans at 2 cm-1 resolution which takes about 15 min. The sample temperature during all IR-measurements was 80 K. A clean Ru(0001) surface was prepared by flashing the sample to 1500 K for at least two times. The absence of significant amounts of carbon contaminations on the surface is demostrated by recording thermal desorption spectra (TDS) after adsorption of oxygen on the surface. As has been demostrated in previous work,22 residual carbon contaminations cause a peak in the CO-TDS data above 500 K. Since the current experimental setup did not allow deposition of DPDMA and the recording of IR-spectra at the same sample
10.1021/jp0401877 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/24/2004
14026 J. Phys. Chem. B, Vol. 108, No. 37, 2004 position, the movement of the manipulator had to be controlled very accurately. Due to the high reproducibility of the sample position (0.01 mm along each axis and 0.5° in rotation), the IR data for the different preparation steps can be directly compared without any further normalization procedure. Furthermore, no baseline correction has been applied to the IR data in order to avoid the loss of any spectral information. The reference spectra needed for the IR-normalization were recorded at the end of each set of experiments after annealing the hydrocarbon layers to 500 K. The base pressure of the UHV-system amounts to 5 × 10-11 mbar. For the deposition of DPDMA layers a special type of evaporator was built. The evaporator consists of a water-cooled metal tube. This setup allows for an accurate temperature stabilization and in particular ensures that the maximum temperature is not exceeded while the tube is heated by a thermocoax wire. A thermocouple tightly clamped to the end of the metal tube allows for accurate temperature measurements. Outside of the vacuum chamber the container for the diazo compound is connected to the evaporation tube via a valve and a screw cap connection. Since contact with metal surfaces causes decomposition of DPDMA, all inner surfaces of the metal parts, including the tube, the valve, and the container itself are coated with a thin ceramic layer (Silcosteel, Restek Corporation). Inside the UHV-system the evaporation tube ends with a Teflon cap with an orifice of 1 mm diameter. During bake-out of the UHVsystem, the flask containing the DPDMA was cooled with icewater in order to minimize thermal decomposition of the organic compound. This evaporator design meets the requirements to keep the background pressure during deposition of the diazo compound in UHV as small as possible to avoid contamination and to produce a molecular beam that hits the whole sample surface. The DPDMA was synthesized using a previously described method,23 subsequently purified by HPLC, and then stored under a nitrogen atmosphere at 250 K. After the flask was connected to the evaporator, several pump and freeze cycles were applied to further reduce any contamination. Since DPDMA easily decomposes at elevated temperatures, a series of measurements was carried out to find a temperature where the vapor pressure is sufficiently high for a vapor-phase deposition but which on the other hand causes a minimum of thermal decomposition. For the deposition of monolayers a temperature of 275 K resulted in the preparation of DPDMA layers with only a very little amount of contamination at a vapor pressure suitable to prepare layers in the monolayer regime. Benzophenone (BZP) is always present as an impurity in DPDMA, with concentrations as large as 5%. Because of the larger vapor pressure, evaporation of a BZP/DPDMA mixture at 275 K onto a ruthenium crystal results in a layer containing a majority of BZP, which effectively separates the DPDMA molecules from each other, as will be discussed later. For the experiments with pure layers of DPDMA (multilayer, clean Rusurface), a charge of DPDMA which was additionally purified by HPLC with a content of BZP below the detection limit was used. As a result of this cleaning procedure, in these experiments the IR data recorded for deposited DPDMA adlayers do not show the typical absorption bands of BZP (see Table 1S, Supporting Information). In a first set of experiments, X-ray photoelectron spectroscopy (XPS) was used to monitor the deposition of the diphenyldiazomethane molecule on a chemically inert SiO2 surface. Unfortunately, this molecule is so sensitive toward X-ray photons that this spectroscopic technique could not be used (see
Gunia et al. TABLE 1: IR Spectroscopic Data of a Multilayer of DPDMA at 80 K and in an Argon Matrix at 20 K wavenumbers (cm-1) multilayer (360 Langmuir) at 80 K
argon matrix at 20 K
651 694
651 692
760
756
DPDMA on Ru(0001) at 80 K 660 703 777
901 934 1027 1032
936
928
1034 1076
1076 1444 1454 1491
1076 1447 1457 1497
1496 1578 1594 1600 2052
1502 1582
1493/1496
3072 a
1598 2046 3070
2816 2992 3069
assignmenta C-C deform. ip arom. C-H deform. op arom. C-C deform. op arom. C-H deform. op arom. C-C deform. op arom. C-H deform. op arom. C-H str. (17b) ring (12) C-C deform. ip arom. C-H deform. ip (19a) C-C deform. ip (18b) C-C deform. ip (18b) C-C deform. ip (18a) C-H deform. C-C deform. ip (18a) C-C deform. ip (9b) C-C deform. ip (9a) C-C deform. ip (9a) NdN-str. C-H-str. (CH2) C-H-str. (CH) C-H-str. arom.
Wilson notation for vibrations of the phenyl rings in parentheses.
Supporting Information Figure 1S). The data, which were recorded for a multilayer of DPDMA deposited on a SiO2 surface, yield a binding energy for the two nitrogen atoms of the diazo compound clearly distinguishable and in excellent agreement with data in the literature.24 However, after 1 h of irradiation (12 kV, 20 mA emission current) the spectra reveal significant radiation damage. Because of this pronounced sensitivity to X-ray photons, XPS is not the method of choice for the investigation of this particular molecule. We have therefore used infrared reflection absorption spectroscopy, IRAS, to study the deposited layers: there is no indication that infrared photons cause any damage to the diazo compounds. Results In Figure 1 we display IR-spectra recorded for DPDMA multilayers prepared by vapor deposition using an evaporator temperature of 318 K. Good agreement is observed with IRspectra recorded for DPDMA in an Ar-matrix also shown in Figure 1. For assignment of the absorption bands, see Table 1. The thickness of the DPDMA layers in the monolayer regime on the ruthenium surface was determined using the following procedure. First, a clean NO/O (2 × 1) layer25 was prepared on the Ru(0001) surface, to which a small amount of 13CO had been coadsorbed. Deposition of diphenyldiazomethane was then followed by monitoring the spectral features such as the shape and the position of the CO-stretch absorption band. As shown in Figure 2, initially the 13CO-band shows a small shift and some changes in shape until the first DPDMA monolayer is completed. Adsorption of additional DPDMA does not lead to further changes but only to an increase of the νNdN-band of DPDMA. This procedure allows estimation of the completeness of the DPDMA monolayer from the evaporation time. The deposition time required to yield a full monolayer determined in this calibration experiment as well as the other parameters (temperatures of the Ru(0001) surface and the DPDMA, sample position) were also applied in the following experiments.
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J. Phys. Chem. B, Vol. 108, No. 37, 2004 14027
Figure 2. Calibration of monolayer preparation: dependence of reflectivity of the diphenyldiazomethane.
Figure 3. IR spectrum of a monolayer of DPDMA evaporated on Ru(0001) at 80 K. The thermal development of this layer up to 177 K is shown. The broad absorption band at 1900 cm-1 is caused by a small amount CO on the surface (approximately 4-5%).42
In a first set of experiments DPDMA was deposited on the clean Ru(0001) surface at 80 K. The corresponding IR-spectra are shown in Figure 3. The spectra do not reveal the characteristic νNdN-band of DPDMA but show deformation modes at 760-780, 700 (out-of-plane), and 1076 cm-1 (in-plane) characteristic of phenyl groups. Upon heating to 177 K the in-planemode increases in intensity, while the out-of-plane-modes become weaker. This finding is consistent with a more upright orientation of the molecule, since the transition dipole moment for the out-of-plane-modes is oriented normal to the phenyl ring and thus should, because of the surface selection rule, become weaker when the ring-plane is orientated normal to the surface of the metal substrate. The opposite is true for the in-planeband, which has its transition dipole moment oriented within the ring-plane.26 This temperature dependent behavior points to an orientation effect resulting in a diphenylmethylene species standing upright on the ruthenium surface. These data also reveal that, as expected, the contact of DPDMA with the rather reactive Ru surface leads to decomposition of the molecule even at this low temperature. The assignment of the bands is summarized in Table 1. In a second set of experiments we investigated the deposition of DPDMA on the less reactive CO passivated surface. The
13CO
absorption band versus evaporation time of
preparation of the CO-adlayers (for both 13CO and 12CO) on the Ru(0001) surface was carried out by dosing 5 Langmuir of carbon monoxide at 80 K followed by annealing the layers to 250 K. This preparation method has been studied in detail previously and yields a well-defined (5x3 x5x3)R30° CO adlayer with a coverage of θ ) 2/3.18,20 The corresponding IR data of the resulting CO monolayers are shown in the Supporting Information (Figure 2S). The isotopic shift amounts to 2.1% (referred to the higher frequency). On this CO adlayer an additional layer of DPDMA was deposited as described above. To avoid unwanted interaction between the single diphenylcarbene units in the next step by irradiation with light, for this set of experiments a charge of DPDMA was used where the concentration of the BZP was particularly high. Since the vapor pressure of BZP is slightly larger than that of DPDMA, deposition at a container temperature of 275 K results in a layer containing a majority of BZP. The IR data recorded for the mixed benzophenone/diphenyldiazomethane layers are shown in the next section in context with the experiments on CO-adlayers. The different BZP bands in the IR-data presented here are in good agreement with previously published data for BZP condensed in an Ar-matrix1 (see Table 1S, Supporting Information). In previous works, experiments have been carried out where the diphenylcarbene has been embedded in organic glasses of BZP, which demonstrates that BZP in fact is completely unreactive toward diphenylcarbene13-15,27,28 and effectively avoids the dimerization of the carbenes. To avoid the overlap of the νNdN-band of DPDMA and the stretch vibration of CO adsorbed on Ru, the isotopomer 13CO was used first in this experiment. Irradiation Experiments. Diphenyldiazomethane/13CO/ Ru(0001). A saturated layer of 13CO on the Ru-surface covered by approximately half a monolayer of DPDMA diluted in BZP was prepared as described above. The corresponding IR-spectra are shown in Figure 4 and clearly reveal the different bands expected for such a mixed adlayer. To achieve cleavage of the diazo group, the DPDMA adlayer was irradiated with light from a Hg-lamp (full arc, λ > 515 nm) for 15 min at 80 K to produce the diphenylcarbene in its triplet ground state.29 Illumination was carried through a glass window absorbing at wavenumbers below 400 nm from outside of the UHV-chamber. As expected, after irradiation the char-
14028 J. Phys. Chem. B, Vol. 108, No. 37, 2004
Figure 4. IR data of half a monolayer of DPDMA diluted in BZP (a). After irradiation of this layer, the characteristic νNdN-band of DPDMA disappeared (b). Annealing up to 250 K leads to a new absorption band at 1776 cm-1 (c).
Gunia et al.
Figure 6. IR data of a monolayer of DPDMA diluted in BZP at 80 K on Ru(0001) (a). After this layer was annealed to 165 K, a new absorption band at 1780 cm-1 appears simultaneously to the attenuation of the characteristic νNdN-band of diphenyldiazomethane (b). Further annealing up to 200 K (c) and 240 K (d) enhances the absorption band.
TABLE 2: Vibrational Data of Some Metal Complexes Including a Formal Bonded Ketene Group
Figure 5. IR data of half a monolayer of DPDMA diluted in BZP at 80 K (a). After irradiation of this layer, no significant changes in the IR data can be seen (b). Annealing up to 180 K leads to the appearance of a new absorption band at 1814 cm-1 (c); further annealing to 240 K enhances the new absorption band (d).
acteristic νNdN-band of DPDMA has disappeared, pointing to the formation of diphenylcarbene.1,2 Unfortunately, due to the rather small concentration of DPDMA in this diluted phase, the typical bands of the carbene as known from the matrix isolation studies1,2 cannot be seen in this spectrum. After the irradiated DPDMA adlayer was annealed to 250 K, a new absorption band at 1776 cm-1 was recognized, indicating the formation of a carbonyl containing species. Note, that this new absorption band does not appear directly after irradiation but only after heating to 250 K. To aid the assignment of the new absorption band at 1776 cm-1, the same experiment was carried out with the 12CO isotopomer. The overall scenario is virtually identical to that described above, the only difference being that after heating, the new band is shifted to 1814 cm-1 (see Figure 5). These results strongly indicate that upon heating, the CO adsorbed on the Ru-surface undergoes a reaction with the carbene, yielding a carbonyl-containing species. The isotope shift also clearly demonstrates that the carbonyl group of the BZP cannot be involved in this reaction, as expected from the experiments with the carbenes embedded in organic glasses (see the discussion above).
substance
conditions
ν12CO (cm-1)
ν13CO (cm-1)
spectroscopic method
143 244 3a44 3b44 444 531
C6H6, RT CH2CL2, -52 °C KBr, RT KBr, RT CH2Cl2, NaCl-cell, RT Pt(111), 100 K
1777 1752 1632 1656 1728 1717
1719 1610 1619 1693
IR IR IR IR IR HREELS
From experiments carried out in noble gas matrixes, it is wellknown that diphenylcarbene reacts with CO to form diphenylketene.29 On the basis of this fact, we assign the new species observed after heating to diphenylketene. For free diphenylketene molecules embedded in a matrix the carbonyl-band is located at significantly higher frequencies, e.g., at 2109 cm-1 in an Ar matrix at 20 K.30 The large shift with regard to the frequency observed for the adsorbed species (1814 cm-1) is attributed to the interaction of the molecule with the ruthenium surface. In previous works on ketenes adsorbed on different metal surfaces, similar carbonyl stretch vibration shifts toward lower wavenumbers have been observed31,32 (see Table 2). By comparison with these data, the region of the observed new absorption band gives support for a ketene species bonded to the metal surface via the carbon-carbon double bond. Binding via the double bond of the carbonyl-part of the ketene group would give rise to an absorption band at significantly lower wavenumbers;31,33,34 see also Table 2. Annealing Experiments. Diphenyldiazomethane/13CO/ Ru(0001). In a different set of experiments an almost complete monolayer of DPDMA diluted in BZP was adsorbed on a saturated layer of 13CO. In this case the concentration of DPDMA was somewhat higher as in the former experiments, as can be seen from the higher intensity of the DPDMA νNdN absorption band. To investigate the effect of surface temperature, this adlayer was annealed to different temperatures without any irradiation (Figure 6). The IR data presented in Figure 6 reveal a strong attenuation of the νNdN-band of DPDMA after heating to 165 K, indicating cleavage of the diazo group. At the same time a new absorption band appears at about 1780 cm-1. Since the temperature of 165 K is significantly lower than the temperature where a decom-
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J. Phys. Chem. B, Vol. 108, No. 37, 2004 14029
CHART 1: Structures of Different Model Compounds with a Formal Bonded Ketene Group
SCHEME 1: Reaction Scheme for DPDMA (bottom) and Diazomethane (top) on the Clean Ru-Surface
position of the free molecule is observed, the cleavage of the N2-group must therefore result from an interaction of the DPDMA with the Ru-surface. We propose that at temperatures of 160 K the mobility of the CO layer on the Ru(0001) is already sufficiently high19 to allow single DPDMA molecules to get closer to the metal surface and, as a result, to decompose in analogy to the behavior seen for the deposition of the molecule on the clean surface. As demonstrated in the first set of experiments (see above), the reactivity of the Ru surface is sufficiently high to decompose DPDMA even at temperatures as low as 80 K. The wavenumber of the new band formed after decomposition of the DPDMA is in the same range as observed in the irradiation experiments and suggests the same product resulting from a reaction of the carbene with an adsorbed CO molecule. A comparison of our results with complexes of ketene compounds known in organometallic chemistry and experiments with methylketene (CH2CO) on Pt(111)31 nicely corroborates our assignment of the spectral features. In this interesting work the thermal evolution of methylketene coadsorbed with hydrogen or oxygen on the platinum surface was followed by HREELS. All of the organometallic compounds used for comparison contain a central metal atom (rhodium, iridium). An organic compound (a formal diphenylketene or phenylketene) is bonded to this metal atom, and the vibration of the carbonyl band is observed by IR. The structures of these model compounds are shown in Chart 1. Discussion The IR-spectra for the DPDMA multilayer (Figure 1) demonstrate that multilayers of this rather sensitive molecule could be deposited under UHV conditions without a significant amount of decomposition. The absence of the NdN vibration mode in the spectra recorded in a control experiment for a
monolayer of DPDMA deposited on the clean Ru surface at a temperature of 80 K clearly demonstrates that the molecule decomposes when contacting the clean Ru substrate. The IR deformation modes at 760-780, 700 (out-of-plane), and 1076 cm-1 (in-plane) characteristic of phenyl groups significantly change in intensity upon annealing: the intensity of the outof-plane modes decrease while the in-plane band increases in intensity. This is in accord with a reorientation to a more uprightstanding surface-bonded diphenylmethylene species during the annealing. The small red shift of approximately 4 cm-1 of the out-of-plane bands during annealing also points to a decrease of interaction between the phenyl rings and the ruthenium surface. A higher degree of decomposition during preparation of such a layer, for example the formation of phenyl rings directly bonded at the metal surface, should result in at least two in-plane bands detectable by IRAS. The broad absorption band in the proximity of 1090 cm-1 leads to a well-defined absorption at 1076 cm-1 (fwhm 7 cm-1) during annealing, which suggests that there is only one species present on the surface. Further decomposition of the phenyl rings is also not supported by the IR data, since the decomposition products of the phenyl rings (e.g., ethene, propene, or 1,3-butadiene) should exhibit characteristic absorption bands which are not observed.35-39 However, the absence of the νNdN-band characteristic of DPDMA clearly shows the capability of the Ru(0001) surface to cleave the diazo group from our carbene precursor even at temperatures as low as 80 K (Scheme 1). This finding is in excellent agreement with the results of a previous work where the reaction of diazomethane with a Ru(001)-surface was studied.40 In this case the formation of a surface-bonded methylcarbene was reported when diazomethane was deposited on a clean surface at 80 K (see also Scheme 1). This behavior of the smallest carbene methylene has also been observed on a Ag(111)-surface.41
14030 J. Phys. Chem. B, Vol. 108, No. 37, 2004 In the main set of experiments a direct contact between DPDMA and the Ru-surface was excluded by introducing a CO spacer layer. In the corresponding IR-spectra the presence of the NdN-band clearly indicates that a monolayer of DPDMA (diluted in BZP) was successfully deposited on top of the CO adlayer. Irradiation leads - as expected - to the abstraction of the diazo group as indicated by the disappearance of the Nd N-band after irradiation. Surprisingly, at this point no reaction product could be identified. This is unexpected, since under the conditions of matrix isolation, diphenylcarbene rapidly reacts with CO to form diphenylketene.30 We explain this unexpected observation by the fact that the CO molecules are highly oriented at the ruthenium surface due to their bonding to the surface via the carbon atom. As a result, and in contrast to the corresponding matrix experiments, the CO carbon atom is well protected from any interaction with the carbene. Other reactions of the carbene, e.g., with other carbene molecules, are suppressed by the high dilution of the carbene in BZP. Benzophenone itself is unreactive toward diphenylcarbene in organic glasses.27,28 On the basis of these considerations, we propose that the irradiation leads to diphenylcarbene physisorbed on top of the CO-adlayer. Although the small concentration of the carbene and the fairly small excitation probability of the IR modes in the carbene make a direct identification via IR-spectroscopy impossible, the results of the heating experiments suggest that a reactive species must be present on the surface. Upon heating the surface to 250 K we observe the formation of a new carbonyl band at 1770-1780 cm-1 (13CO adlayer) and at 1814 cm-1 (12CO adlayer), respectively. The most reasonable assignment of this band is a carbonyl band in surface-bonded diphenylketene formed by the reaction of the diphenylcarbene with CO molecules adsorbed at the surface. As a result of the associated fairly large transition dipole moment, the CdO str. absorption band of the diphenylketene is five times stronger than the strongest band in the carbene. Thus, we are only able to directly observe the ketene but not the carbene. Experiments carried out with 12CO fully support this reaction scheme, since the band at 1774 shifts to 1814 cm-1. This reaction only takes place at elevated temperatures and was not observed directly after photolysis due to the larger thermal-induced motion of the CO molecules at higher temperatures. We propose that the population of librations at higher temperatures results in a reduction of the distance between the carbonyl C atom and the carbene to a point where the reaction becomes possible. The importance of thermally induced mobility and reorientation of the CO layer is demonstrated in a third set of experiments. Here a complete monolayer of diphenyldiazomethane diluted in BZP was prepared, as described above. After annealing to temperatures above 165 K the NdN-band at 2047 cm-1 strongly decreases and simultaneously the band at 1784 cm-1, already observed in the previous set of experiments, appears. Again, this band is assigned to the carbonyl str. vibration in surface-bound diphenylketene formed by reaction of diphenylcarbene with the CO adlayer. In this set of experiments, however, the carbene is not formed photochemically as in the previous irradiation experiments but must result from a removal of the diazo group when the DPDMA molecule gets in the vicinity of the Ru surface (Scheme 2). Conclusions In this paper we describe the outcome of several experiments investigating the reactivity of DPDMA and the corresponding diphenylcarbene on clean and adsorbate-precovered Ru-surfaces. When DPDMA comes into contact with the clean Ru-surface
Gunia et al. SCHEME 2: Reaction Scheme Proposed for DPDMA Adsorbed on a CO/Ru(0001) Surface
the IR-data of this control experiment presented here demonstrate the formation of a surface-bound diphenylmethylene species, fully consistent with previous experiments on the interaction of diazomethane with transition metal surfaces. DPDMA molecules could be stabilized on the ruthenium surface using a CO spacer layer. The photon-induced abstraction of the diazo group in DPDMA layers deposited on such a spacer layer leads to the unexpected observation that the carbene formed in this process does not immediately react with the underlying COmolecules. Only after heating the surface to temperatures of about 180 K is a new carbonyl band at 1774 cm-1 (13CO) observed, which is assigned to a diphenylketene formed by reaction of diphenylcarbene with the surface-bound 13COmolecules. Heating of the DPDMA on the CO spacer layer without prior irradiation leads to the same reaction product, indicating that the catalytic activity of Ru is sufficiently large to bring about an abstraction of the diazo group without photochemistry. In summary our experiments demonstrate that it is possible to stabilize a carbene precursor at a precovered solid surface and to study its reactions in detail. Acknowledgment. We gratefully acknowledge the work of K. Gomann for the synthesis of the DPDMA. Supporting Information Available: The XPS-spectra of DPDMA, the IR-data of the prepared CO-monolayers on the Ru-surface, and the IR spectroscopic data of the BZP are available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Sander, W. J. Org. Chem. 1989, 54, 333. (2) Sander, W. Angew. Chem. 1986, 98, 255.
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