Insights in Reaction Mechanistics: Isotopic Exchange during the

Article pubs.acs.org/JPCC

Insights in Reaction Mechanistics: Isotopic Exchange during the Metalation of Deuterated Tetraphenyl-21,23D‑porphyrin on Cu(111) Michael Röckert,† Matthias Franke,† Quratulain Tariq,† Dominik Lungerich,‡ Norbert Jux,‡ Michael Stark,† Andre Kaftan,† Stefanie Ditze,† Hubertus Marbach,† Mathias Laurin,† Jörg Libuda,† Hans-Peter Steinrück,† and Ole Lytken*,† †

Lehrstuhl für Physikalische Chemie II, Universität Erlangen-Nürnberg, Egerlandstraße 3, D-91058 Erlangen, Germany Lehrstuhl für Organische Chemie II, Universität Erlangen-Nürnberg, Henkestraße 42, D-91054 Erlangen, Germany

‡

S Supporting Information *

ABSTRACT: Through the use of temperature-programmed desorption (TPD), the self-metalation and dehydrogenation of deuterated 5,10,15,20tetraphenyl-21,23D-porphyrin on Cu(111) have been studied, resulting in new insight into the metalation of porphyrins on surfaces. The metalation is found to proceed through the transfer of the central aminic hydrogen atoms to the Cu(111) surface and not, as suggested by gas phase calculations, through the combination of the hydrogen atoms to molecular hydrogen above the partially inserted metal center. This finding suggests that the metalation reaction could be significantly influenced by the stability of hydrogen on the substrate surface. The metalation reaction and the subsequent hydrogenation and dehydrogenation of the periphery of the porphyrin molecule leading to hydrogen−deuterium exchange are modeled with a simple microkinetic reaction model. The model is able to describe the main features of the TPD spectra.

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INTRODUCTION Planar transition metal complexes adsorbed on solid supports are promising for heterogeneous catalysis and sensor applications.1−3 An important feature of such compounds, compared to supported metal clusters, is that the active sites, that is, the coordinated metal centers with their vacant axial coordination sites, are well-defined and uniform. From the application point of view, this promises high selectivity, and from the experimental point of view, easier interpretation of data. Metalloporphyrins are particularly suitable, because they combine a structure-forming element, the porphyrin framework, with an active site, the coordinated metal ion. Its planar coordination environment leaves two axial coordination sites available for additional ligands. A very similar situation is obtained for other porphyrinoid-derived metal complexes, such as metallophthalocyanines. Many metalloporphyrins can be deposited on surfaces by evaporation. Alternatively, it is possible to deposit the parent free-base porphyrin and metalate the porphyrin molecule in an ultrahigh vacuum (UHV) via preor postadsorption of metal atoms, as has been shown for cobalt, iron, zinc, copper, nickel, and cerium.4−23 Mediated by an STM tip, even metalation with Ag is possible.24 For some metalloporphyrins, notably Fe(II) porphyrins, only the metalation of deposited free-base porphyrins with iron guarantees well-defined and clean adsorbate layers.4−7,13,18 Despite the fact that the surface-mediated metalation of porphyrinoids is well-established from the experimental point © 2014 American Chemical Society

of view, the mechanistic understanding is only at its infancy. The available literature on the adsorption and metalation of porphyrins on well-defined metal surfaces is mostly limited to electron spectroscopic 5,6,8,9,11,13−19,21,23 and scanning probe5−7,10−13,15,20,21,23 techniques. From a theoretical point of view, due to the size of the porphyrin molecules, the existing understanding of the metalation reaction has been limited to density functional theory (DFT) calculations of the gas-phase reaction, that is, without including the surface.25−27 One experimental technique that can provide significant additional insight is temperature-programmed desorption (TPD), where the amount of hydrogen released during the metalation reaction and also during further dehydrogenation steps can be analyzed. Although this technique has been very successfully used to study the dehydrogenation of smaller organic molecules,28,29 only a very few studies were performed to study metalation or dehydrogenation of porphyrins on surfaces.27,30,31 This is mostly due to the fact that standard setups are not suited to detect the small desorption signal stemming from the single hydrogen molecule released per porphyrin molecule during the metalation reaction. In the present study, we have used TPD to follow the self-metalation and dehydrogenation of deuterated 5,10,15,20-tetraphenylReceived: July 22, 2014 Revised: October 15, 2014 Published: October 15, 2014 26729

dx.doi.org/10.1021/jp507303h | J. Phys. Chem. C 2014, 118, 26729−26736

The Journal of Physical Chemistry C

Article

DCl in D2O (35 wt %, Sigma-Aldrich). After stirring overnight, the layers were separated and the organic layer was washed several times with D2O until the green color of the dicationic species was replaced by the typical purple color of porphyrin solutions. A miniscule amount of dry triethylamine was added, and a final washing with D2O afforded a deep purple solution, which was taken to dryness. The compound was dried in vacuo overnight. Gas-phase mass spectra of 2DTPP and 2HTPP were acquired by evaporation from a Knudsen cell held at 600−620 K, directly into the ionization source of the mass spectrometer. Deposition on the Cu(111) surface was done from an identical Knudsen cell. Prior to each experiment, the Cu(111) single crystal was cleaned by 1 kV Ar+-ion sputtering and annealing at 850 K. XPS was used to confirm the cleanliness of the sample and to measure the 2DTPP coverage, as described previously.23 The XPS measurements were performed with a Scienta ESCA 200 spectrometer with a monochromatized Al Kα X-ray source (1486.6 eV) and a hemispherical energy analyzer (SES 200). To characterize the degree of deuteration of the synthesized 2DTPP, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements were performed in a Bruker Vertex 80v IR spectrometer equipped with the Praying Mantis Diffuse Reflection Accessory from Harrick (10 mm sampling cup, 0.25 mL volume). The samples were ground thoroughly in a mortar and then mixed with KBr. Traces of water were removed by purging the sample compartment of the spectrometer with dry air at least 30 min after introduction of the sample. IR spectra were recorded with a spectral resolution of 2 cm−1, 256 scans, and a scanning speed of 40 kHz. The reference to the spectra is pure KBr acquired under the same conditions.

21,23D-porphyrin molecules on Cu(111). Using a deuterated molecule allowed us to follow the reaction in unprecedented detail and, thus, yielded unique new insights into the reaction pathway of the metalation process and into the specific influence of the substrate on the reaction.

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EXPERIMENTAL SECTION TPD and high-resolution X-ray photoelectron spectroscopy (XPS) measurements were conducted in an ultrahigh-vacuum system with a base pressure of 2 × 10−10 mbar. Gas-phase mass spectra and TPD measurements were performed with a Pfeiffer HiQuad QMA 400 mass spectrometer with QMH 410-2 and QMG 700 electronics. To improve the sensitivity and signal-tonoise ratio of the TPD setup, the mass spectrometer was mounted inside a copper Feulner cup, the opening of which was positioned to within 1 mm of the sample surface.32 The substrate was a Cu(111) single crystal with a purity >99.999%, aligned to within 0.1° of the nominal orientation. Deuterated 5,10,15,20-tetraphenyl-21,23D-porphyrin (2DTPP; see Figure 1) was synthesized by dissolving 100 mg

Figure 1. 5,10,15,20-Tetraphenyl-21,23D-porphyrin (2DTPP).

of 5,10,15,20-tetraphenylporphyrin (2HTPP) in 50 mL of CDCl3 and treating this solution with commercially available

Figure 2. Hydrogen desorption from the reactions of tetraphenylporphyrin (2HTPP) with Cu(111) as a function of coverage and temperature.31 For the completed checkerboard structure (0.56 molecules/nm2), metalation (450 K), partial dehydrogenation (545 K), and complete dehydrogenation (600−900 K) are visible as three distinct hydrogen desorption peaks in the TPD spectra. At lower coverage, metalation and partial dehydrogenation merge into one peak (514 K). 26730



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RESULTS AND DISCUSSION

In a very recent TPD study of nondeuterated tetraphenylporphyrin (2HTPP) on Cu(111) we identified the main reactions of 2HTPP as a function of coverage and temperature.31 The results are summarized in Figure 2. At low coverage, the 2HTPP molecules are adsorbed as isolated molecules on the Cu(111) surface, but as the coverage is increased beyond 0.37 molecules/nm2, the molecules start forming a well-ordered checkerboard-like structure, with every second molecule slightly elevated above the surface.23,33 At a coverage of 0.56 molecules/nm2, the checkerboard structure completely covers the surface, and additional molecules begin to form multilayers. For the completed checkerboard structure (0.56 molecules/ nm2), three distinct features are observed in the TPD spectra in Figure 2; they are attributed to metalation of 2HTPP with copper substrate atoms to CuTPP at 450 K, (2) partial dehydrogenation of CuTPP at 545 K (the fusing of pyrrole and phenyl rings), and, finally (3) complete dehydrogenation between 600 and 950 K.31 The fusing of pyrrole and phenyl rings has been observed previously on both Cu(111)21 and Ag(111)34 surfaces and, for brominated and iodated porphyrins, also in liquid medium.35,36 At coverages below 0.37 molecules/nm2, the first two reaction steps occur simultaneously, leading to a single desorption peak at 514 ± 4 K in Figure 2, which is more or less independent of coverage. As the coverage is increased beyond 0.37 molecules/nm2, where the checkerboard structure starts forming,23,33 the metalation becomes faster and the partial dehydrogenation slower, and the two peaks at 450 and 545 K (0.56 molecules/nm2) evolve out of the single peak at ∼514 K. This can be explained by a weaker interaction of the iminic nitrogen atoms with the copper surface in the checkerboard structure, leading to faster metalation,23 and the stabilizing effect of T-type interactions in the CuTPP islands formed at high coverage after metalation, leading to slower dehydrogenation.31 To learn more about the mechanism of the metalation reaction, we performed TPD measurements of deuterated 2DTPP. To clearly distinguish the metalation step from the first dehydrogenation step, a coverage of 0.54 ± 0.03 molecules/ nm2 was chosen, which is roughly equal to the coverage of the completed checkerboard structure. The resulting desorption of H2 (mass 2), HD (mass 3), and D2 (mass 4) is shown in Figure 3. The intensities of H2, HD, and D2 have been divided by the square roots of the masses, to correct for the pumping speed of the small opening of the Feulner cup. However, the ratios between the masses may still be slightly skewed by different transmission probabilities in the mass spectrometer. The combined area under the three desorption spectra is normalized such that it corresponds to 15 molecules, that is, 30 atoms. The dashed lines indicate zero-order fits to the leading edges of the peaks and are used to separate the areas of the different peaks. The high-temperature dashed line for H2 indicates degassing from the sample holder. Summing the D2, HD, and H2 signals in the peaks at 450 and 545 K and between 600 and 950 K yields a ratio of 0.71:4.17:10.12, which is in good agreement with the nominal value of 1:4:10, according to the scheme in Figure 2. From the TPD spectra and the corresponding integrated intensities in Figure 3 (denoted as numbers next to the three peaks), it is immediately apparent that the amount of deuterium desorbing during metalation, that is, at 450 K, is much less (0.02) than the one deuterium molecule expected for

Figure 3. Desorption of H2 (mass 2), HD (mass 3), and D2 (mass 4) from 0.54 2DTPP molecules/nm2 on Cu(111) at a heating rate of 5 K/s. For clarity, the HD and D2 intensities have been magnified by 10 and 50, respectively. Three distinct reactions can be indentified: metalation (450 K), partial dehydrogenation (545 K), and complete dehydrogenation (600−950 K). The numbers indicate the integrated intensities of the shaded areas, normalized to make the sum of all desorption peaks equal to the expected 15 molecules per porphyrin molecule.31 The dashed lines indicate how the peaks are separated for integration; the high-temperature dashed line for hydrogen is desorption from the sample holder as it heats.

fully deuterated 2DTPP. Furthermore, it is evident that significant amounts of deuterium desorb as HD and D2 during dehydrogenation of the periphery of the molecule in the peaks at 545 and 600−950 K. The interpretation of the TPD results in Figure 3 depends critically on the degree of deuteration of the 2DTPP molecules and on where the deuteration has taken place. A significant degree of deuteration at the periphery of the molecules would drastically change the conclusions. To answer these questions, we characterized the synthesized 2DTPP molecules with gasphase mass spectrometry and DRIFTS measurements (see Figure 4). In the mass spectra in Figure 4a, the deuterated fraction of the synthesized 2DTPP was determined by the following procedure: The measured spectrum (top, red data) was fitted by a superposition of five copies of the nondeuterated 2HTPP spectrum, which are shifted to each other by 1 amu, to represent the expected mass spectra for tetraphenylporphyrin containing zero, one, two, three, and four deuterium atoms. The three dominant masses for 2HTPP (614, 615, and 616 amu) visible in Figure 4a are a result of the natural abundance of carbon-13 (1.1%).37 The linear combination of the five spectra, best describing the measured 2DTPP spectra shown in Figure 4a, yielded fractions of 9% nondeuterated, 45% with one deuterium atom, 45% with two deuterium atoms, and no porphyrin molecules with three or more deuterium atoms, corresponding to an average of 1.35 deuterium atoms per porphyrin molecule. 26731



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Figure 4. Characterization of the synthesized 5,10,15,20-tetraphenyl-21,23D-porphyrin (2DTPP) used in this study: (a) gas-phase mass spectra showing the deuterated fractions; (b) Kubelka−Munk-corrected DRIFTS spectra, comparing 2HTPP with 2DTPP, normalized to the integrated area of the fingerprint region from 1650 to 600 cm−1.

vibration at 3312 cm−1 decreases by 62% and a new band appears at 2482 cm−1, corresponding to the ND stretching vibration of the deuterated molecule.38 Sixty-two percent is in good agreement with the 68% measured by gas-phase mass spectrometry. The absence of any CD vibrations at 2300−2200 cm−139−41 also corroborates the mass spectrometry finding that the deuteration has taken place only at the central two aminic hydrogen atoms and not at the periphery of the molecule. The conclusions from the DRIFTS and gas-phase mass spectrometry measurements are clear: The deuteration is limited almost exclusively to the two central aminic hydrogen atoms, and the degree of deuteration is 62−68%. Thus, these questions remain: Why do only 3% (0.02 of 0.71, see Figure 3) of deuterium atoms per porphyrin molecule desorb as D2 during metalation and not 45% as expected for the above determined degree of deuteration (see Table 1), and why do the majority of D atoms desorb as HD during the dehydrogenation of the periphery of the molecule, that is, between 500 and 950 K? The only valid explanation is that an exchange of aminic deuterium with hydrogen atoms in the periphery of the molecule must take place during the metalation reaction. Furthermore, when the desorption from the partial dehydrogenation (i.e., the peak at 545 K) is subtracted from the TPD spectra, and the resulting metalation peaks are normalized to have the same heights, as shown in Figure 5, a clear shift between the desorption of H2, HD, and D2 is evident, with D2 desorbing earlier and H2 later. None of these observations is in agreement with a reaction model similar to that calculated for the gas-phase reaction.25−27 There, the aminic hydrogen atoms are transferred sequentially to the partially inserted metal center, where they combine to molecular hydrogen. This reaction pathway is not consistent with an exchange of aminic deuterium with hydrogen atoms in the periphery of the molecule or a shift in desorption temperature between H2, HD, and D2 during metalation. Although isotope effects, due to the different ground-state vibrational energies (zero-point motion), have been observed for dehydrogenation of CH and CD bonds42−44 and desorption of H2 and D2,45 those effects would shift desorption of D2 in the opposite direction, that is, to higher and not lower temperatures.

From these fractions we can calculate the deuteration when limited to two positions in the molecules. Assuming an equal probability p for deuteration at each of the N sites in the molecule, the fraction f of porphyrin molecules with D deuterium atoms can be calculated as f = p D × (1 − p)N − D × (N! − (N − D) ! )/D!

(1)

. If the deuteration of 1.35 deuterium atoms per molecule is limited to only the two aminic hydrogen atoms in the center of the molecule (position 21,23), the degree of deuteration, and thereby the probability p for deuteration, is 1.35/2 = 0.68 (68%). Using eq 1, the expected fractions of deuterated molecules are then calculated to be 10%, nondeuterated, 44% with one D atom, 46% with two D atoms, and no molecules with three or more D atoms, in perfect agreement with the measured fractions (see Table 1, 2 sites). Table 1. Measured and Calculated Fractions of Deuterated Molecules for Deuteration at the Two Aminic Hydrogen Atoms versus Deuteration at the 28 Hydrogen Atoms at the Periphery of the Molecule, Showing Clear Evidence for Deuteration at the Two Aminic Hydrogen Atoms D atoms

measured (%)

2 sites (%)

28 sites (%)

0 1 2 3 4

9 45 45 0 0

10 44 46 0 0

25 36 24 11 3

If the deuteration would instead have occurred at the periphery of the molecule (28 sites), the degree of deuteration becomes 1.35/28 = 0.048 (4.8%). Again, using eq 1, the expected fractions of deuterated molecules are calculated yielding 25% nondeuterated, 36% with one D atom, 24% with two D atoms, 11% with three D atoms, and 3% with four D atoms, in complete disagreement with the measured fractions. The deuteration must therefore be limited almost exclusively to the two central aminic hydrogen atoms. This conclusion is also supported by the DRIFTS measurements in Figure 4b. After deuteration, the NH stretching 26732



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molecule begin to dehydrogenate, more and more hydrogen is produced, increasing the amounts of HD and later H 2 desorbing. In principle, the addition of a single deuterium atom (i.e., the formation of one CHD unit) at the periphery would be enough to cause the scrambling. However, we cannot rule out that the addition of a second one (i.e., the formation of two CHD units) is required, yielding a more stable intermediate, with a fully saturated double bond and thus a longer lifetime. This second step is indicated in parentheses in Figure 6. The long lifetime is essential to explain the delayed desorption of H2 as compared to D2. Copper-based catalysts are mostly used for methanol synthesis and low-temperature water gas shift46 and, unlike more reactive surfaces such as Pt(111),28,29 most alkanes desorb intact from Cu(111) without dehydrogenating.47 However, although other metals are usually preferred, copper can also be used to catalyze hydrogenation of olefins48 and dehydrogenation of parafins.49 It is, therefore, reasonable that copper could catalyze the hydrogenation and subsequent dehydrogenation of the periphery of the porphyrin molecule when hydrogen is present on the surface. To verify that the reaction model is indeed suitable to describe the observed desorption of H2, HD, and D2, we simulated the reaction and fitted the kinetic parameters of the simulation to the measured desorption rates. The microkinetic model, the fitted kinetic parameters, and the simulated and measured desorption rates for the metalation reaction are denoted in Figure 7. The model is able to describe not only the absolute rates of desorption of H2, HD, and D2 but also the temperature shift between them. The microkinetic model consists of four principal reactions (see Figure 7), with corresponding rate constants:

Figure 5. Normalized TPD spectra of H2 (mass 2), HD (mass 3), and D2 (mass 4) released during the metalation of 0.54 2DTPP molecules/nm2 on Cu(111) at a heating rate of 5 K/s. For clarity, the zero-order fits to the leading edges of the partial dehydrogenation peaks (see Figure 3) have been subtracted. A shift in temperature between H2, HD, and D2 is clearly observed, with D2 desorbing earlier and H2 later.

We instead propose a reaction model where the aminic hydrogen atoms do not recombine into molecular hydrogen above the partial inserted metal center as in the gas phase, but are transferred to the Cu(111) surface as atomic hydrogen instead. Once on the surface, hydrogen is consumed by two competing reactions: simple recombination and desorption as molecular hydrogen or hydrogenation of the periphery of the porphyrin molecule yielding temporary CHD units. In the latter case, subsequent dehydrogenation of these CHD units at the periphery of the molecule will release either the deuterium or the hydrogen. This explains the scrambling of deuterium on the surface, which eventually leads to HD and H2 desorption during metalation of 2DTPP; this process is schematically shown in Figure 6. If the lifetime of the hydrogenated molecule on the surface is sufficient to act as a sink, the hydrogen released by dehydrogenation will be delayed compared to the hydrogen released by metalation. Hence, initially, as 2DTPP begins to metalate, mostly deuterium is produced on the surface, but, as the hydrogenated parts of the porphyrin

metalation (NH dehydrogenation) r1 = k1 × θNH

k1 = ν1 × exp( −E1/RT )

hydrogenation of periphery r2 = k 2 × θH × θCH

k 2 = ν2 × exp( −E2 /RT )

dehydrogenation of periphery r3 = k 3 × θCHH

k 3 = ν3 × exp( −E3/RT )

dihydrogen desorption r4 = k4 × θH 2

k4 = ν4 × exp( −E4 /RT )

These four principal equations (rates and coverages) have to be considered for all possible combinations of the H/D isotopes, that is, H2, HD, and D2, as well as CH, CH, CHH, CHD, and CDD; the complete set of rate equations is given in the Supporting Information. The initial reaction rates are calculated on the basis of the initial coverages and the four rate constants. On the basis of these reaction rates, the coverages at a small time step, Δt, later are calculated. These new coverages are then used to calculate new reaction rates and the process is repeated. If the time steps are kept small relative to the reaction rates, the simulation is accurate. To accurately describe the faster reaction rates at higher temperatures, 500 000 time steps were used, getting progressively smaller from 100 ms at 350 K to 0.02 ms at 490 K. The kinetic parameters denoted in Figure 7 are those that gave the best fit between the simulated and measured desorptions of H2, HD, and D2. The kinetic parameters for desorption were fixed to the values measured for clean Cu(111) by Anger et al.: 1010 nm2/molecules·s and 75

Figure 6. Proposed hydrogenation and subsequent dehydrogenation mechanism on Cu(111) responsible for the scrambling of deuterium, produced during the metalation of 2DTPP, with hydrogen in the periphery of the porphyrin molecule. The periphery is here illustrated as a phenyl ring, but we expect the same exchange mechanism to occur at the pyrrole rings (for further details, see the text). 26733



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Figure 7. Measured desorption rates (solid lines) of H2 (mass 2), HD (mass 3), and D2 (mass 4) during the metalation of 0.54 2DTPP molecules/ nm2 on Cu(111) at a heating rate of 5 K/s, together with the simulated desorption rates (dashed lines) for 65% deuterated 2DTPP based on the reaction model and kinetic parameters shown to the right.

kJ/mol.50 To further simplify the fit, the prefactors for the other three reactions were fixed at 1015 s−1 for metalation (the value measured by Ditze et al. at low coverage10), 1013 nm2/ molecules·s for hydrogenation, and 1013 s−1 for dehydrogenation. The degree of deuteration was assumed to be 65% (an average of the IR and mass spectrometry characterizations). Considering the slight uncertainty on the sensitivity of the mass spec to the different masses, the agreement between the measured and simulated data in Figure 7 is quite good and, importantly, the experimentally observed shift to lower temperatures for HD and D2 desorption (H2, 456 K; HD, 452 K; D2, 446 K) is reproduced by the simulation (H2, 459 K; HD, 454 K; D2, 450 K). Finally, we analyze the broad dehydrogenation peak between 600 and 950 K in Figures 3 and 8, which show that desorption of HD and D2 is shifted toward higher temperatures compared to H2. This behavior is attributed to the isotope effect mentioned earlier. The difference in ground-state vibrational energy increases the activation energy barrier for dehydrogenation of a CD bond by 5 kJ/mol compared to a CH bond.42−44 During the TPD measurement, CH bonds are continuously dehydrogenating slightly more rapidly than the CD bonds, causing the ratio of CD to CH remaining on the surface and, thereby, the ratio of HD to H2 desorbing to continuously increase. Toward the end of the desorption peak, even D2 can be seen desorbing, as can be seen in Figure 3. To illustrate this effect we have simulated two simple firstorder reactions with a 5 kJ/mol difference in activation energy and identical prefactors, representative of CH and CD bond breaking. The absolute values of the activation energies and prefactors have been chosen to produce broad peaks covering roughly the same temperature range as the experimentally observed for the complete dehydrogenation peak (600−950 K) (see Figure 8). The ratio of CD to CH was chosen to be 0.029, identical to the experimentally observed ratio. The ratio is easily calculated from Figure 3 by dividing the total amount of deuterium (HD = 0.57) by the total amount of hydrogen (HD + 2 × H2 = 0.57 + 2 × 9.54 = 19.65). However, because each deuterium atom produced results in one HD molecule desorbing, whereas each hydrogen atom produced only results in 1/2 H2 molecule, the simulated CD to CH reaction ratio has to be multiplied by 2 before comparison to the measured desorption ratio of HD to H2. The good agreement between the observed behavior and that expected from a kinetic isotope effect allows us to conclude that the

Figure 8. (Top) Measured complete dehydrogenation region (600− 950 K) from Figure 3, compared with (middle) the simulated firstorder CH and CD dehydrogenation reactions assuming a 5 kJ/mol difference in activation energies caused by the difference in groundstate vibrational energies. Also plotted (bottom) is the measured HD to H2 desorption ratio and the simulated CD to CH dehydrogenation ratio, scaled to compensate for the desorption of one HD molecule per D produced, but only half a H2 molecule per H produced.

observed enrichment of deuterium in the desorbing molecules with increasing temperature is simply a consequence of the difference in ground-state vibrational energy of CH versus CD.

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CONCLUSIONS Performing TPD experiments with deuterated 2DTPP on Cu(111) provided detailed information on the mechanistics of the self-metalation reaction to CuTPP and the subsequent dehydrogenation. During metalation of tetraphenylporphyrin with Cu atoms from the Cu(111) substrate, the aminic deuterium atoms of the porphyrin molecule are found to be abstracted to the Cu(111) surface, producing adsorbed deuterium atoms. This is in contrast to the gas-phase reaction, 26734



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which proceeds through the recombination of the aminic deuterium atoms to molecular deuterium above the partially inserted metal center.25−27 Once produced on the Cu(111) surface, adsorbed deuterium partially exchanges with hydrogen atoms in the periphery of the porphyrin molecule, yielding isotopic scrambling during the metalation of 5,10,15,20-tetraphenyl-21,23D-porphyrin. The exchange is suggested to occur through the hydrogenation and subsequent dehydrogenation of the periphery of the molecule and is clearly visible as desorption of H2, HD, and D2 in temperature-programmed desorption. Using simple assumptions, a microkinetic model is able to describe the main features of the reactions. As a consequence of the suggested reaction pathway, the metalation reaction might depend strongly not just on the metal atom inserted but also on the surface on which the metalation takes place.

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ASSOCIATED CONTENT

S Supporting Information *

The full set of rate equations used to simulate the data in Figure 7. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(O.L.) Phone: +49 9131 85-27320. Fax: +49 9131 85-28867. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the German Science Foundation (DFG) for financial support through FOR 1878 (f unCOS) and SFB 953 “Synthetic Carbon Allotropes”; infrastructural contributions by the Erlangen Cluster of Excellence “Engineering of Advanced Materials” are also gratefully acknowledged.

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ABBREVIATIONS TPD: temperature-programmed desorption XPS: X-ray photoelectron spectroscopy DRIFTS: diffuse reflectance infrared Fourier transform spectroscopy UHV: ultrahigh vacuum 2HTPP: 5,10,15,20-tetraphenyl-21,23H-porphyrin 2DTPP: 5,10,15,20-tetraphenyl-21,23D-porphyrin CuTPP: copper 5,10,15,20-tetraphenylporphyrin

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REFERENCES

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Insights in Reaction Mechanistics: Isotopic Exchange during the

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