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J. Phys. Chem. C 2008, 112, 3794-3799
Formation of Methyl Isocyanide from Dimethylamine on Pt(111) Kumudu Mudiyanselage and Michael Trenary* Department of Chemistry, UniVersity of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607-7061
Randall J. Meyer Department of Chemical Engineering, UniVersity of Illinois at Chicago, 810 South Clinton, Chicago, Illinois 60607-7000 ReceiVed: October 1, 2007; In Final Form: December 4, 2007
The conversion of dimethylamine ((CH3)2NH) to methyl isocyanide (CNMe) through a methylaminocarbyne (CNHMe) intermediate on Pt(111) has been studied experimentally with reflection absorption infrared spectroscopy (RAIRS) and theoretically with density functional theory (DFT). (CH3)2NH adsorbs molecularly at 85 K through the nitrogen lone pair and dehydrogenates to produce CNHMe at 350 K. Both experimental observations and the results of DFT calculations show that CNHMe is more stable on the surface than either (CH3)2NH or CNMe. However, surface CNHMe can be converted to the less stable CNMe via two different pathways. A small amount of CNMe is formed at temperatures above 350 K due to the dehydrogenation of CNHMe accompanied by desorption of H2, which drives the reaction forward. The second pathway involves the reaction at 85 K with water adsorbed from the background, which leads to a complete conversion of CNHMe to form both on-top and bent-bridge-bonded CNMe. It is assumed that the dehydrogenation of the CNHMe molecules produces hydrated hydronium ions of the general formula ((H2O)nH3O)+, although direct spectroscopic evidence for such species was not obtained.
Introduction A central theme of surface chemistry research has been the formation and characterization of thermodynamically stable molecular species on surfaces. A more challenging yet equally important problem is to uncover ways to isolate and characterize less stable surface species. As a case in point, dimethylamine ((CH3)2NH) has been observed to spontaneously convert to the methylaminocarbyne (CNHMe) species on the Pt(111) surface.1 Similarly, methyl isocyanide (CNMe) readily reacts with surface hydrogen to also form methylaminocarbyne.2 These observations suggest that, of the three molecules, methylaminocarbyne is the most stable, a conclusion supported by the density functional theory (DFT) calculations reported here. This suggests that if conversion of (CH3)2NH to CNMe on the Pt(111) surface can be achieved at all, it will require subtle manipulations of the surface chemistry involving these species. To achieve this goal, organometallic complexes can serve as a guide to possible reactions on metal surfaces because of the many similarities between the bonding and reactions of ligands in such complexes and the analogous chemistry on metal surfaces.3-5 There have been many studies of aminocarbyne complexes derived from electrophilic attack of isocyanides at the N atom in organometallic systems,6,7 and this class of reactions also occurs on metal surfaces as shown by the formation of CNHMe from the hydrogenation of CNMe adsorbed on Pt(111).2 The reverse reaction, deprotonation of an aminocarbyne to form an isocyanide, is also well-known in organometallic systems. Because of the acidic character of aminocarbyne ligands, they can be deprotonated by a base such as NEt3 or NBu4OH to regenerate the parent isocyanide ligands.7,8 The simplest aminocarbyne species, CNH2, bonded to a single metal site in [ReCl(CNH2)(dppe)2][BF4] (dppe ) Ph2PCH2CH2PPh2) con-
verts to [ReCl(CNH)(dppe)2] by reactions with NEt3 or NBu4OH9 or under reductive conditions.10 The protonation of the methyl isocyanide complex trans-[Mo(CNMe)2(dppe)2] leads to the formation of the monoaminocarbyne complex trans-[Mo(CNHMe)(CNMe)(dppe)2]+, which undergoes intramolecular hydrogen migration from the aminocarbyne to the metal forming the hydride complex [MoH(CNMe)2(dppe)2]+ with methyl isocyanide ligands.8 The detailed pathway for this intramolecular H-migration process was not established, but the involvement of an η1 f η2 rearrangement of the aminocarbyne ligand was predicted.11 Even though reactions in which an aminocarbyne is deprotonated to an isocyanide ligand are well-known in organometallic systems, analogous reactions on metal surfaces have not been reported. One important difference concerns the assignment of charges in the two types of systems. The protonation reaction of a neutral isocyanide complex produces an aminocarbyne complex with a charge of +1. On the Pt surface, charge can flow to and from the metal atoms so that it is not necessary to formally balance charges among the adsorbates themselves. Thus, we use the terms protonation/deprotonation and hydrogenation/dehydrogenation interchangeably since there is no way of knowing if an H or H+ is the species transferred in the reaction. Aside from the issue of charge distribution, there are several motivations for exploring this chemistry on surfaces. For example, isocyanides are attractive for applications in molecular electronics because of the strong metal-CNR bond.12-16 In this context, it was recently shown17 that the N-H bond of a single CNHMe molecule could be selectively broken with an electron pulse from the tip of a scanning tunneling microscope to form CNMe on Pt(111) at 4.7 K. This was notable as it was a demonstration of a chemical cycle at the single
10.1021/jp709581j CCC: $40.75 © 2008 American Chemical Society Published on Web 02/16/2008
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molecule level, an idea that is relevant to the development of molecular switches. It would thus be of great interest to achieve the conversion of an aminocarbyne to an isocyanide on a surface by a purely chemical route. At a practical level the fact that isocyanides are foul smelling and unpleasant to work with and are often unavailable commercially makes the prospect of preparing them in situ on surfaces from more benign precursors such as amines quite attractive. Here, we provide a detailed description of the formation of methyl isocyanide on Pt(111) through the reaction of methylaminocarbyne and background water at 85 K, and through the thermal dehydrogenation of methylaminocarbyne at temperatures of 350-400 K. Experimental Section The experiments were performed in two separate ultrahigh vacuum (UHV) chambers using two different Pt(111) crystals. The data shown in Figure 6 were obtained in a chamber (chamber 1) with a base pressure of ∼2 × 10-10 Torr. The system has been described in detail elsewhere.18 In brief, it is equipped for low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) with a quadrupole mass spectrometer (QMS), and reflection absorption infrared spectroscopy (RAIRS) with a Fourier transform infrared (FTIR) spectrometer, a Mattson RS-10000. Except for the results of Figure 6, all of the RAIRS experiments were performed in a second UHV chamber (chamber 2) with a base pressure of ∼1 × 10-10 Torr. A detailed description of this system can be found elsewhere.19 In brief, it consists of a stainless steel chamber equipped for LEED, Auger electron spectroscopy (AES), and TPD experiments with a QMS. The chamber is coupled to a commercial FTIR spectrometer, a Bruker IFS 66 v/S. For both RAIRS systems, the IR beam enters and exits the UHV chamber through differentially pumped O-ring sealed KBr windows and passes through a polarizer before reaching the infrared detector. The spectra were obtained at 4 cm-1 resolution with an MCT (HgCdTe) detector. In cases where the sample was annealed to a temperature above 85 K, the sample was then cooled back to 85 K before the spectrum was acquired, unless otherwise noted. Although reproducible RAIRS results were obtained in each chamber, RAIRS results from chamber 2 with the Bruker IFS 66 v/S spectrometer show better signal-to-noise ratios and therefore those spectra are presented here. Dimethylamine (99.5% pure) was purchased from Matheson Gas Products Inc. and was used without further purification. The Pt(111) surface was cleaned by a procedure described earlier.20 Briefly, the crystal was heated in 1 × 10-7 Torr of O2 at ∼825 K for 1 h after Ar+ bombardment. This procedure is effective at removing surface carbon as shown by the subsequent exposure to 2 Langmuirs (L; 1 L ) 1 × 10-6 Torr s) of O2 at 85 K and performing a TPD experiment from 85 to 1000 K in which the desorption of O2 without CO2 is observed above 300 K. The absence of impurities other than carbon was verified (in chamber 2) with Auger electron spectroscopy. Computational Methods Density functional theory calculations were performed using the program VASP21,22 (Vienna Ab initio Simulation Package), a DFT code for periodic systems. The calculations utilized a plane wave basis set and ultrasoft pseudopotentials. Exchange and correlation energies were calculated using the PerdewWang ’91 form23 of the generalized gradient approximation. Convergence tests were made with respect to the number of k-points and basis set size. A plane wave cutoff energy of 400 eV was used in the calculations. Geometries were judged to be
Figure 1. RAIR spectra following exposure to 0.4 L of (CH3)2NH at 85 K, annealing to the indicated temperatures, and cooling back to 85 K where the spectra were acquired.
optimized when the forces were within a convergence tolerance of 0.02 eV/Å. Approximately four layers of vacuum separated the four-layer slabs in the z-direction. Results Methylaminocarbyne forms from either the N-protonation of CNMe2 or the partial dehydrogenation of (CH3)2NH.1 Figure 1 shows the formation of CNMe starting from (CH3)2NH through a CNHMe intermediate. The (CH3)2NH adsorbs molecularly at 85 K as shown in the topmost spectrum of Figure 1. Peak assignments for adsorbed (CH3)2NH are based on our previous study of (CH3)2NH on Pt(111).1 The peak at 888 cm-1 is due to the NH bending mode, and a symmetric CH3 deformation is observed at 1451 cm-1. The C-H stretch region at 2895 cm-1 displays a complex set of peaks, possibly including overtone and/or combination bands interacting by Fermi resonance with the C-H stretch fundamentals. An asymmetric CN stretch is observed at 1025 cm-1. A 300 K anneal mainly leads to a sharpening of the dimethylamine peaks seen at 85 K, thus demonstrating that (CH3)2NH is stable on the surface up to 300 K. Essentially the same changes with a 300 K anneal were reported earlier,1 where they were attributed to better twodimensional order in the annealed (CH3)2NH layer. In addition, a small amount of the new species produced by the 350 K anneal is present as indicated by weak peaks at 1403 and 1476 cm-1 in the 300 K spectrum. No new peaks are seen in the C-H stretch region, but separate peaks due to adsorbed (CH3)2NH are resolvable at 2944, 2906, 2882, 2816, and 2767 cm-1 after the 300 K anneal, which are within a few wavenumbers (cm-1) of what was seen in our earlier (CH3)2NH spectra1 under the same conditions. A completely different spectrum is seen after a 350 K anneal, and the peaks are assigned to CNHMe as previously reported.1 Thus, the peaks at 1476 and 3406 cm-1 are assigned to the CN and NH stretches, respectively, of
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Figure 3. Plot of the peak intensities of the CN and NH stretches of CNMe and CNHMe, respectively, expressed as percentages of their maximum values, as a function of time at 85 K. The corresponding spectra were obtained after exposing the surface to 0.4 L of (CH3)2NH at 85 K, annealing to 350 K to form CNHMe, and then cooling back to 85 K where the peak intensities were monitored as a function of time.
Figure 2. RAIR spectra obtained after annealing CNMe formed at 85 K as in Figure 1 to the indicated temperatures, followed by cooling back to 85 K where the spectra were acquired.
CNHMe. A CH3 deformation appears at 1403 cm-1, and the 1294 cm-1 peak is due to a mode with mixed CH3 rocking/NH bending character. The peaks observed at 2065 and 1805 cm-1 are due to on-top and bridge-bonded CO, respectively,24,25 that is adsorbed from the background. A weak peak at 2233 cm-1 is assigned to the CN stretch of the on-top form of methyl isocyanide as previously reported.2,26,27 This assignment is unambiguous as the bridge-bonded form of CNMe has a distinctly lower frequency, and peaks due to dimethylamine and methylaminocarbyne do not occur at all in the ∼1600-2700 cm-1 region.2,26,27 The bottom three spectra of Figure 1 were taken with the crystal held at 85 K for increasing periods of time. These spectra show a decrease of the NH stretch peak of CNHMe at 3406 cm-1 as a function of time with a concomitant increase of the CN stretch peak of CNMe. After 90 min at 85 K, the NH stretch peak of CNHMe has completely disappeared and the observed peaks, except for the one at 1515 cm-1, can be assigned to two forms of adsorbed CNMe. Specifically, the peaks at 2236 and 1704 cm-1 are assigned to CN stretches of on-top and bent-bridge-bonded CNMe, respectively. The CH3 deformation and rocking modes of bent-bridged CNMe appear at 1387 and 1130 cm-1, respectively. Figure 2 shows the formation of CNHMe from the hydrogenation of CNMe, which was derived from the dehydrogenation of CNHMe at 85 K. The top spectrum of Figure 2 is the same as the bottom spectrum of Figure 1. There are only some very minor changes in the spectrum upon annealing to 300 K, which include the appearance of a weak peak at 3406 cm-1, a sharpening of the peak at 1800 cm-1, and some redistribution of intensity in the weak features between 1449 and 1704 cm-1. A 350 K anneal leads to a dramatic change with the development of peaks due to CNHMe at 3406, 2995, 2922, 1478, 1403, and 1295 cm-1. The peaks at 1804 and 2063 cm-1 are assigned to bridge-bonded and on-top CO adsorbed from the background.24,25 The conversion of the on-top site CNMe to bridgebonded CNHMe evidently makes on-top sites available for CO,
which accounts for the peak at 2063 cm-1, whereas the small shift from 1800 to 1804 cm-1 of the bridge-bonded CO is due to a small coverage change in this form of adsorbed CO. With the 400 K anneal, there is decomposition of CNHMe accompanied by desorption of H2 making more sites available for CO adsorption. This leads to a higher coverage of CO and hence a much higher intensity of the on-top CO peak at 2078 cm-1. The fact that there are fewer coadsorbed species results in CO frequencies that are much closer to their values on the clean surface of ∼1850 cm-1 for the bridge-bonded CO and ∼2100 cm-1 for the on-top CO.24,25 The hydrogen needed to hydrogenate CNMe to CNHMe presumably comes either from adsorption of hydrogen due to exposure of the sample to background gases (which contain a large fraction of H2) for a long period of time at 85 K or from the original dehydrogenation of CNHMe at 85 K, or from a combination of both. Further annealing leads to the decomposition of CNHMe to one or more surface species. After annealing to 400 K, the NH stretch peak at 3406 cm-1 has completely disappeared. The CN stretch peak of CNMe is shifted from 2233 to 2239 cm-1, and its increase in intensity indicates the formation of additional CNMe. The IR peaks at 1253, 1411, and 1527 cm-1 observed after annealing to 450 K are due to an unidentified common intermediate formed from several precursors, including dimethylamine, methyl isocyanide, and trimethylamine at 450 K.1 Figure 3 shows the time dependence of the percentage of the CN and NH stretch peak intensities relative to their values at the maximum coverages of CNMe and CNHMe, respectively, at 85 K. Except for the last data points for each species, the increase in CN stretch intensity and the accompanying decrease in intensity of the NH stretch peak are seen to change approximately linearly with time, with slopes of similar magnitude (within a factor of ∼1.5) but opposite signs. This dehydrogenation process to form methyl isocyanide does not occur at 150 K as shown by the RAIR spectra in Figure 4. The topmost spectrum was obtained after a 0.4 L (CH3)2NH exposure at 85 K. The next two spectra were obtained after heating to 300 and 350 K, followed by cooling to 150 K. The last two spectra were obtained following the 350 K anneal after leaving the sample at 150 K for the indicated times. CNHMe forms at 350 K as discussed in connection with Figure 1. After 60 min
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Figure 4. RAIR spectra following a 0.4 L (CH3)2NH exposure at 85 K and annealing to the indicated temperatures. All the spectra were acquired at 150 K except the top spectrum, which was acquired at 85 K.
Figure 6. Plot of the CN and NH stretch peak intensities of CNMe and CNHMe respectively, as a function of time. The crystal was exposed to 0.4 L of (CH3)2NH at 85 K, and the spectra were acquired at the indicated temperatures.
Figure 5. RAIR spectra following a 1 L (CH3)2NH exposure at 300 K, annealing to the indicated temperatures, and cooling back to 300 K where the spectra were acquired.
at 150 K, no significant change occurs in the spectrum, although the NH stretch peak intensity is somewhat lower. Most
importantly, the CN stretch peak of CNMe does not appear even after 60 min at 150 K. A small amount of CNMe also forms from CNHMe at higher temperatures as shown by the spectra in Figure 5, all of which were obtained with the crystal at 300 K. The topmost spectrum is of molecularly adsorbed (CH3)2NH that was obtained after exposure at 300 K. As implied by the annealing results following (CH3)2NH exposure at 85 K in Figure 1, (CH3)2NH is stable up to 300 K. The spectrum obtained after simply leaving the crystal at 300 K for 15 min leads to the development of peaks at 3405, 1462, 1403, and 1291 cm-1 due to CNHMe. The subsequent spectra were obtained by heating to increasingly higher temperatures followed by cooling back to 300 K. The appearance of the CN stretch peak at 2206 cm-1 after annealing to 380 K indicates the formation of CNMe. The intensity of this peak increases slightly more after annealing to 390 K, but higher temperatures lead to the dissociation of CNMe into different species, and the 420 K spectrum shows a diminished CN stretch of CNMe at 2228 cm-1. The results shown in Figure 6 characterize the hightemperature reaction by monitoring the increase of the CN stretch of CNMe and the decrease of the NH stretch of CNHMe as a function of time from spectra obtained at the indicated temperatures. Unlike the procedure used for Figure 5, (CH3)2NH was initially adsorbed at 85 K for the results of Figure 6. (CH3)2NH adsorbed at 85 K slowly dehydrogenates to produce CNHMe at 300 K as shown in Figure 6a. Even though the NH stretch peak intensity starts to decrease after about 1 h at 300 K, the CN stretch peak of CNMe does not appear. Figure
3798 J. Phys. Chem. C, Vol. 112, No. 10, 2008 TABLE 1: Total Adsorption Energies Relative to the Energy of the (CH3)2NH(gas) + Pt(111) Slab species
total energy (eV)
(CH3)2NH(gas) + Pt(111) (CH3)2NH(ads) CNHMe(ads) + 3H(ads) CNMe(ads) + 4H(ads) CHNMe(ads) + 3H(ads)
0.0 -0.69 -1.14 -0.85 -0.16
6b shows that the NH stretch peak of CNHMe is completely eliminated after about 40 min at 350 K, at which point the CN stretch peak of CNMe starts to appear. This suggests that the breaking of the N-H bond of CNHMe does not immediately produce CNMe but that an intermediate species is formed first. The appearance of NH and CN stretch peaks together in Figure 6c demonstrates the presence of both CNHMe and CNMe upon annealing to 370 K of the (CH3)2NH adsorbed at 85 K. However, after about 10 min at 370 K, the NH stretch peak has completely disappeared and the intensity of the CN stretch peak of CNMe gradually increases with time. Only the CN stretch peak of CNMe is present even for the shortest times at 390 K, as shown in Figure 6d. At first, the intensity of this peak gradually increases with time and then it starts to decrease as the CNMe dissociates into different species on the surface. Discussion The objective of the experiments described here was to determine if dimethylamine could be used as a precursor to form methyl isocyanide on the Pt(111) surface. Isocyanides are not customarily synthesized from secondary amines, so the surface reaction reported here represents novel and unexpected chemistry. The only apparent connection between the surface chemistry of (CH3)2NH and that of CNMe is the fact that both molecules form the common intermediate, methylaminocarbyne, which suggested the possibility that a pathway connecting (CH3)2NH to CNMe might be found. Previous experimental work2 had shown that exposure of H2 at 280 K to a Pt(111) surface with a submonolayer coverage of CNMe would completely convert this adsorbate to CNHMe, suggesting that the reverse reaction would not occur spontaneously. Furthermore, DFT calculations confirmed that CNMe + H was less thermodynamically stable than CNHMe, implying that the reaction should not occur under equilibrium conditions starting with only CNHMe on the surface. To assess the possibility that dehydro-
Mudiyanselage et al. genation of methylaminocarbyne might occur by an isomerization reaction in which an H atom is transferred from the N atom to the R-carbon atom, calculations were also performed on the formimidine species, CHNMe. The adsorption energies relative to gas-phase (CH3)2NH are given in Table 1 for (CH3)2NH, CNMe + 4H, CNHMe + 3H, and CHNMe + 3H. The given values are for each species at its lowest energy adsorption site; namely (CH3)2NH, CHNMe, and CNMe are at on-top sites, CNHMe is at a twofold bridge site, and H is at fcc threefold hollow sites. A fortuitous observation was that leaving the CNHMe covered surface at 85 K for an extended period led to its conversion to CNMe, proving that the reaction was feasible. The increase of the CN stretch peak intensity of CNMe with time is accompanied by a decrease in intensity of the NH stretch peak of CNHMe as shown in Figure 1, indicating a conversion of CNHMe to CNMe. This observation implicated adsorption of a background gas, despite the relatively good UHV conditions present during the experiments. The three most abundant background gases in our chambers are H2, CO, and H2O. Hydrogen was ruled out since it promotes the formation of CNHMe. It is conceivable that CO could abstract a proton from CNHMe to form the formyl (HCO) species.3 However, CO desorbs from the clean surface at temperatures above 300 K, so the fact that temperatures below 150 K are needed argues against the reaction involving CO. This conclusion was confirmed by spectra (not shown) obtained in an experiment in which CO was deliberately dosed onto the CNHMe-covered surface. The implication then that adsorption of background water was responsible is consistent with the low desorption temperature of 140-180 K of water from the clean surface.28 The role of water was confirmed by spectra (not shown) obtained after the CNHMe-covered surface was deliberately exposed to D2O at 85 K, which converted the CNHMe to CNMe in only a few minutes. In the deprotonation of CNHMe to CNMe, H2O acts as a base that would then be converted to a protonated form of water, the simplest of which is H3O+. There is precedent for H2O protonation in surface reactions, although it appears that the actual species is not H3O+ itself, but a hydrated hydronium ion. Wagner and Moylan have reported the formation of H3O+ from the coadsorption of H2O and HF at 100 K and after annealing of H2O and H2 coadsorbed at 90 to 150 K on Pt(111).29,30 However, Chen et al., studied the thermodynamics of the formation
Figure 7. Proposed reaction scheme for the formation of CNMe from (CH3)2NH on Pt(111).
Formation of CNMe from (CH3)2NH on Pt(111) of H3O+ from coadsorbed H2 and H2O on (2 × 1)-Pt(110) and reported that the species formed is probably not H3O+, but is most likely a mixture of hydrated complexes such as H5O2+, H7O3+, or H9O4+.31 We assume that such hydrated complexes are also formed in the reaction between CNHMe and H2O. However, we do not have clear spectroscopic evidence for the formation of these complexes. The weak RAIRS peak at 1515 cm-1 in Figure 1 has not been assigned and could be due to a ((H2O)nH3O)+ species. The same species might also be responsible for the peak at 1704 cm-1, although it is more likely due to a bent-bridged form of CNMe, which can have CdN stretch values from 1600 to 1770 cm-1 on Pt(111).26 DFT calculations show that even though H2O + CNHMe is more energetically favorable than H3O+ + CNMe by 0.71 eV, the combination of the hydrated hydronium ion H9O4+ with CNMe is more favorable than H8O4 + CNHMe by 0.29 eV on the Pt(111) surface. Once the CNMe forms from the reaction with background water, it is not readily converted back to CNHMe. This is demonstrated by the results of Figure 2, where heating the CNMe + protonated water layer to 300 K leaves the CNMe spectrum largely unchanged. Presumably, heating to 300 K desorbs the water leaving H and CNMe on the surface, which is thermodynamically unstable with respect to formation of CNHMe. The activation barrier for the CNHMe formation reaction must be fairly high as it does not occur until the surface is heated to 350 K. This is somewhat higher than the temperature of 280 K that was found necessary to hydrogenate CNMe in our earlier study under somewhat different conditions.2 Figures 5 and 6 demonstrate that the dissociation of methylaminocarbyne upon annealing to 350 K and above forms different species, including a small amount of CNMe having a broad CN stretch peak at 2206-2228 cm-1. This hightemperature conversion is probably driven by H2 desorption via the reaction
2CNHMe(ads) f 2CNMe(ads) + H2(g) which removes hydrogen from the system and hence shifts the equilibrium in favor of CNMe. However, the results in Figure 6b show that the NH stretch peak of CNHMe is completely eliminated after about 40 min at 350 K and only then does the CN stretch peak of CNMe start to appear. This lag indicates that the conversion may occur by way of an intermediate species, with the most likely one being formimidine (CHNMe). Formimidine has been observed in organometallic systems32 with a range of CdN stretch values depending on the complex. It occurs at 1617 cm-1 for the (η5-C5Me5)2Zr(H)(CHdNCH3) complex.33 The fact that formimidine exists in complexes suggests that it might also form on metal surfaces. Even though the DFT results in Table 1 show that CHNMe is not stable on the surface relative to the other species, it may persist long enough to account for the lag observed in Figure 6b. This higher temperature path forms only a small amount of CNMe because the dissociation of CNMe occurs simultaneously at the higher temperatures needed to desorb hydrogen. A simple reaction scheme summarizing our results for the formation of methyl isocyanide from dimethylamine on Pt(111) is shown in Figure 7. Conclusions The methylaminocarbyne species, CNHMe, provides a link between the surface chemistry of dimethylamine and methyl isocyanide on Pt(111). Molecularly adsorbed (CH3)2NH readily dehydrogenates to produce CNHMe at 350 K. Upon cooling to 85 K, the CNHMe-covered surface slowly adsorbs background
J. Phys. Chem. C, Vol. 112, No. 10, 2008 3799 water, which dehydrogenates the CNHMe to produce CNMe on the surface. This reaction would not be expected to occur in the absence of coadsorbed water, as CNHMe is thermodynamically stable relative to CNMe + H as shown by DFT calculations. In the deprotonation reaction, water acts as a base to abstract the acidic H from the NH group of CNHMe to form CNMe and, presumably, a hydrated form of protonated water. Dissociation of CNHMe upon annealing to 350 K and above forms a small amount of CNMe as hydrogen is removed from the system by desorption. This thermal dehydrogenation reaction most likely occurs through a CHNMe intermediate. Direct spectroscopic evidence for the protonated forms of water and for the CHNMe species was not obtained. Acknowledgment. This work was supported by the National Science Foundation under Grant CHE-0714562. References and Notes (1) Kang, D.-H.; Trenary, M. Surf. Sci. 2002, 519, 40. (2) Kang, D.-H.; Trenary, M. J. Phys. Chem. B 2002, 106, 5710. (3) Albert, M. R.; Yates, J. T., Jr. The Surface Scientist’s guide to organometallic chemistry; American Chemical Society: Washington, DC, 1987. (4) Canning, N.; Madix, R. J. J. Phys. Chem. 1984, 88, 2437. (5) Muetterties, E. L.; Rhodin, T. N.; Band, E.; Brucker, C. F.; Pretzer, W. R. Chem. ReV. 1979, 79, 91. (6) Pombeiro, A. J. L.; Carvalho, M. F. N. N.; Hitchcock, P. B.; Richards, R. L. J. Chem. Soc., Dalton Trans. 1981, 1629. (7) Pombeiro, A. J. L.; Guedes da Silva, M. F. C. J. Organomet. Chem. 2001, 65, 617-618. (8) Pombeiro, A. J. L.; Guedes da Silva, M. F. C.; Michelin, R. A. Coord. Chem. ReV. 2001, 218, 43. (9) Guedes da Silva, M. F. C.; Lemos, M. A. N. D. A.; Frausto da Silva, J. J. R.; Pombeiro, A. J. L.; Pellinghelli, M. A.; Tiripicchio, A. J. Chem. Soc., Dalton Trans. 2000, 373. (10) Lemos, M. A. N. D. A.; Guedes da Silva, M. F. C.; Pombeiro, A. J. L. Inorg. Chim. Acta 1994, 226, 9. (11) Henderson, R. A.; Pombeiro, A. J. L.; Richards, R. L.; Frausto da Silva, J. J. R.; Wang, Y. J. Chem. Soc., Dalton Trans. 1995, 1193. (12) Chen, J.; Calvet, L. C.; Reed, M. A.; Carr, D. W.; Grubisha, D. S.; Bennett, D. W. Chem. Phys. Lett. 1999, 313, 741. (13) Hong, S.; Reifenberger, R.; Tian, W.; Datta, S.; Henderson, J.; Kubiak, C. P. Superlattices Microstruct. 2000, 28, 289. (14) Lee, Y.; Morales, G. M.; Yu, L. Angew. Chem., Int. Ed. 2005, 44, 4228. (15) Stapleton, J. J.; Daniel, T. A.; Uppili, S.; Cabarcos, O. M.; Naciri, J.; Shashidhar, R.; Allara, D. L. Langmuir 2005, 21, 11061. (16) Swanson, S. A.; McClain, R.; Lovejoy, K. S.; Alamdari, N. B.; Hamilton, J. S.; Scott, J. C. Langmuir 2005, 21, 5034. (17) Katano, S.; Kim, Y.; Hori, M.; Trenary, M.; Kawai, M. Science (Washington, DC, U.S.) 2007, 316, 1883. (18) Kang, D. H.; Trenary, M. Surf. Sci. 2000, 470, L13. (19) Brubaker, M. E.; Trenary, M. J. Chem. Phys. 1986, 85, 6100. (20) Jentz, D.; Celio, H.; Mills, P.; Trenary, M. Surf. Sci. 1995, 341, 1. (21) Kresse, G.; Furthmu¨ller, J. Phys. ReV. B 1996, 54, 11169. (22) Kresse, G.; Furthmu¨ller, J. Comput. Mater. Sci. 1996, 6, 15. (23) Perdew, J. P.; Wang, Y. Phys. ReV. B 1992, 45, 13244. (24) Schweizer, E.; Persson, B. N. J.; Tu¨shaus, M.; Hoge, D.; Bradshaw, A. M. Surf. Sci. 1989, 213, 49. (25) Tu¨shaus, M.; Schweizer, E.; Hollins, P.; Bradshaw, A. M. J. Electron Spectrosc. Relat. Phenom. 1987, 44, 305. (26) Avery, N. R.; Matheson, T. W. Surf. Sci. 1984, 143, 110. (27) Avery, N. R.; Matheson, T. W.; Sexton, B. A. Appl. Surf. Sci. 1985, 384, 22-23. (28) Haq, S.; Harnett, J.; Hodgson, A. Surf. Sci. 2002, 505, 171. (29) Wagner, F. T.; Moylan, T. E. Surf. Sci. 1987, 182, 125. (30) Wagner, F. T.; Moylan, T. E. Surf. Sci. 1988, 206, 187. (31) Chen, N.; Blowers, P.; Masel, R. I. Surf. Sci. 1999, 419, 150. (32) Singleton, E.; Oosthuizen, H. E. AdV. Organomet. Chem. 1983, 22, 209. (33) Wolczanski, P. T.; Bercaw, J. E. J. Am. Chem. Soc. 1979, 101, 6450.
