A Comparative Study of a Triphenylene Tricarbonyl Chromium

Mar 23, 2009 - ... Universität Bonn, 53121 Bonn, Germany, Lehrstuhl für Physikalische Chemie II, Universität Erlangen-Nürnberg, 91058 Erlangen, Ge...
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J. Phys. Chem. C 2009, 113, 6014–6021

A Comparative Study of a Triphenylene Tricarbonyl Chromium Complex and Its Uncoordinated Arene Ligand on the Ag(111) Surface: Influence of the Complexation on the Adsorption Christoph H. Schmitz,*,† Carola Rang,‡ Yun Bai,§ Iordan Kossev,† Julian Ikonomov,† Yang Su,† Konstantinos Kotsis,† Serguei Soubatch,|,⊥ Olga Neucheva,|,⊥ F. Stefan Tautz,|,⊥ Frank Neese,† Hans-Peter Steinru¨ck,§ J. Michael Gottfried,§ Karl Heinz Do¨tz,‡ and Moritz Sokolowski† Institut fu¨r Physikalische und Theoretische Chemie, UniVersita¨t Bonn, 53115 Bonn, Germany, Kekule´ Institut fu¨r Organische Chemie and Biochemie, UniVersita¨t Bonn, 53121 Bonn, Germany, Lehrstuhl fu¨r Physikalische Chemie II, UniVersita¨t Erlangen-Nu¨rnberg, 91058 Erlangen, Germany, Institut fu¨r Bio- und Nanosysteme (IBN-3), Forschungszentrum Ju¨lich, 52425 Ju¨lich, Germany, JARA, Fundamentals of Future Information Technology; 52425 Ju¨lich, Germany ReceiVed: December 8, 2008; ReVised Manuscript ReceiVed: January 26, 2009

An aromatic triphenylene molecule (2,3-diethyl-1,4-dimethoxytriphenylene; TPH) and its regioselectively Cr(CO)3-labeled complex (η6-(1,2,3,4,4a,12b)-tricarbonyl(2,3-diethyl-1,4-dimethoxytriphenylene)chromium(0); R-TPHC) were deposited on a Ag(111) surface by vapor deposition. The monolayers were investigated by X-ray photoelectron spectroscopy and scanning tunneling microscopy. Both substances adsorb with the extended π-system parallel to the surface and form long-range ordered monolayers. The Cr(CO)3 group of the R-TPHC complex is oriented toward the vacuum. Although the footprints of both substances are similar, the additional Cr(CO)3 group on the R-TPHC leads to a lateral order (unit cell) that is significantly different from that of TPH. SCHEME 1

1. Introduction Organometallic complexes have already assumed an important role in homogeneous catalysis, with applications ranging from the bench scale to industrial size.1 One major challenge will be to implement these complexes also into the field of heterogeneous catalysis by their immobilization on solid surfaces to exploit the advantages of heterogeneous catalysts. In this context, surface science aspects of these materials are highly relevant. In principle, well-defined model catalysts can be studied by adsorption of organometallic complexes on single crystal surfaces. Evidently, the activity of these catalysts will depend strongly on the structural arrangement of the adsorbate. Hence, the adsorption geometry plays a key role, and it is therefore necessary to understand the forces that lead to a specific arrangement. Recent studies of organometallic complexes on surfaces have focused mainly on metalloporphyrins,2-6 metallophthalocyanines,7-9 and metallocenes.10,11 These substances have one major characteristic in common: in all of these complexes, the metal atom is coordinated centrosymmetrically by an extended π-system. Therefore, it does not influence the orientation of the dipole moment in the molecular plane that is important for the lateral packing of the molecules on a surface. A recent example7 are phthalocyanines, for which the metal * Corresponding author address: Institut fu¨r Physikalische and Theoretische Chemie, Universita¨t Bonn, Wegelerstraβe 12, 53115 Bonn, Germany. Phone: +49 228 73-2520. Fax: +49 228 73-2551. E-mail: christoph.schmitz@ pc.uni-bonn.de. † Institut fu¨r Physikalische und Theoretische Chemie, Universita¨t Bonn. ‡ Kekule´ Institut fu¨r Organische Chemie und Biochemie, Universita¨t Bonn. § Universita¨t Erlangen-Nu¨rnberg. | Forschungszentrum Ju¨lich. ⊥ JARA.

complexation does not induce a structural phase transition, since the centrosymmetry of the electronic structure is preserved. However, systematic studies that analyze the influence of the metal center on the molecular arrangement are missing, in particular, studies in which non-centrosymmetric organometallic complexes are compared to their metal-free analogues with the distinct aim to elucidate the role of the metal center. The substances that we have chosen for our studies are the chromium complex η6-(1,2,3,4,4a,12b)-tricarbonyl(2,3-diethyl1,4-dimethoxytriphenylene)chromium(0) (R-TPHC; see Scheme 1 and Figure 1) and the corresponding free ligand 2,3-diethyl1,4-dimethoxytriphenylene (TPH, Scheme 1). R-TPHC belongs to the class of the η6-arene tricarbonyl chromium complexes,

10.1021/jp810788n CCC: $40.75  2009 American Chemical Society Published on Web 03/23/2009

Adsorption of an arene(CO)3Cr complex on Ag(111)

J. Phys. Chem. C, Vol. 113, No. 15, 2009 6015 2. Experimental

Figure 1. Ball-stick and hard-sphere models of R-TPHC. The dimensions include the van der Waals radii of the atoms. The contour of the hard-sphere model is used in Figure 5b for illustrating the twodimensional molecular arrangement of the adsorbates. Color assignment: carbon, blue; hydrogen, white; oxygen, red; chromium, purple. For details of the calculation methods, see text.

which are also called “half-sandwich” compounds. The chromium atom is octahedrally coordinated by the persubstituted hydroquinoid ring of the aromatic ligand on the one and by three carbon monoxide molecules on the other facial plane. The aromatic ligand occupies three coordination sites of the Cr and is hexahapto-bonded via its aromatic π-orbitals. The bonding of the carbonyl ligands is understood in terms of a concerted σ-type electron donation from the carbon lone pair into a vacant Cr d orbital and a π backbonding from a filled Cr d orbital into the π* orbital of the carbonyl ligand. The chromium is zerovalent, forming a stable 18-electron complex. However, considering the extended aromatic system of the triphenylene ligand, the persubstituted ring is not the only possible coordination site for the Cr(CO)3 moiety. Alternative coordination sites are the two unsubstituted terminal rings and the central ring. The exclusive coordination at the persubstituted ring is ensured by the choice of the synthesis route involving a chromium-templated benzannulation of a Fischer carbene complex by an alkene.12-14 In fact, complexes with the Cr(CO)3 moiety coordinated to one of the two unsubstituted terminal rings (β-TPHC, Scheme 1) are the thermodynamically favored regioisomers. The presence of these two alternative coordination sites makes the arene tricarbonyl chromium complexes interesting also for a possible application as a “molecular switch”.15,16 The activation energy required for shifting the Cr(CO)3 moiety from its original coordination site to the thermodynamically favored coordination site is typically in the range of 20-30 kcal/mol.17,18 This process is called “haptotropic migration”, as introduced by Hoffmann and co-workers.19 The haptotropic migration has been subject to a large number of recent experimental16,20,21 as well as theoretical18,22 studies and has also been demonstrated for the system R-TPHC/β-TPHC. The migration experiments are presently carried out exclusively in solution. In the case of TPHC, the metal shift occurs at temperatures between 363 and 373 K. For possible applications, it would be advantageous that the migration (switching) occurs for locally fixed complexes; for example, for complexes adsorbed on a surface. In this paper, we will show that it is possible to prepare highly ordered layers of TPH molecules and R-TPHC complexes on the Ag(111) surface by vapor deposition. The remarkable influence of the Cr(CO)3 moiety on the 2D ordered structures will be discussed.

The scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) experiments were carried out in two separate UHV systems. Both were equipped with standard methods for sample cleaning and preparation and were held at a base pressure of 2 × 10-10 mbar. STM data were acquired using a commercial RHK variable temperature (VT-) STM with an operating temperature range from 20 K to room temperature. All images were taken in constant-current mode using mechanically cut Pt/Ir tips. Given bias voltages refer to the sample. Noise reduction of the images was done by moderate low-pass filtering or smoothing. Lateral and vertical distance calibration was performed using atomically resolved images of the clean Ag(111) sample surface. XPS data were acquired using a Scienta ESCA-200 spectrometer equipped with an Al KR X-ray source, an X-ray monochromator, and a hemispherical energy analyzer of the type SES-200. All binding energies were referenced to the Fermi edge of the clean Ag(111) sample. To increase the surface sensitivity, the angle between the surface normal and the analyzer was set to 70°. The C 1s and O 1s spectra were measured at a pass energy of 150 eV and the Cr 2s spectra at a pass energy of 300 eV. Degradation due to beam damage was not observed. The substrate, an Ag(111) single crystal, was prepared by repeated cycles of sputtering with Ar+ ions with a kinetic energy of 800 eV and subsequent annealing to 770 K. The key steps of the R-TPHC synthesis are the formation of a Fischer carbene complex prepared from commercially available 9-bromophenanthrene followed by a chromium-templated benzannulation by 3-hexyne closing the additional persubstituted hydroquinoid ring that is coordinated by the retaining Cr(CO)3 moiety.12 R-TPHC forms orange crystals. R-TPHC was purified by gradient sublimation at 354 K and a pressure of 10-5 mbar. Decomposition and haptotropic migration of the complex during the sublimation were excluded by NMR analysis of the sublimate. Because of the low evaporation temperature and the limited long-term stability of the arene complex in the presence of ambient air and humidity, special precautions had to be taken. We used an evaporation cell combined with a load lock that could be filled under an inert gas atmosphere, pumped and baked out at low temperatures separately, and subsequently inserted into the UHV chamber by a linear manipulator without breaking the vacuum. TPH was prepared via decomplexation of R-TPHC in solution under air, sunlight and heating. Details of the synthesis of both TPH and R-TPHC will be published elsewhere. After standard work up, TPH was also purified by means of gradient sublimation at 373 K and 10-5 mbar. Ordered monolayers of TPH were prepared by vapor deposition at a cell temperature of 373 K and a crystal temperature of 300 K. R-TPHC was evaporated at a cell temperature of 354 K. One monolayer (ML) is defined as the amount of TPH or R-TPHC forming a closed layer with the structure described below. We have deposited the complexes on Ag(111) crystals at temperatures from 80 to 300 K, followed by subsequent annealing at 340 K. The best results in the sense of large, defectfree domains were achieved by deposition of slightly more than one monolayer at a sample temperature of 300 K and subsequent annealing for 2 min. The deposition times for one monolayer differed from below 1 min to 15 min due to very different distances between evaporation cell and sample in the STM and XPS systems.

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Schmitz et al.

TABLE 1: Stoichiometries of TPH and r-TPHC as Derived from the C 1s XP Spectra and Comparison with Theoretically Calculated Valuesa stoichiometry (%) R-TPHC

TPH Caromatic (C-1) Cethyl (C-2) Cipso (C-3) CCO (C-5) Cmethoxy (C-4)

theory

exptl

theory

exptl

66.7 16.7 8.3

66.3 17.0 8.5

8.3

8.2

59.3 14.8 7.4 11.1 7.4

59.5 15.2 7.1 10.6 7.6

Accuracy: (1%. The very broad satellite (C-6, cf. Figure 2) was not included in the calculation of the stoichiometries. a

TABLE 2: Comparison of Experimentally Determined XPS Binding Energies for TPH, r-TPHC, and Comparable Arene(tricarbonyl)chromium Complexes from the Literature binding energy (eV monolayer

powder

TPH TPHC BTCa BTCb ATCc ATCd CTCe C 1s Caromatic (C-1) Cethyl (C-2) Cipso (C-3) CCO (C-5) Cmethoxy (C-4) Car. π-π* (C-6) O 1s OCO (O-2) Omethoxy (O-1)

285.0 285.0 285.5 286.0 284.6f 286.1f 285.7 285.5 285.5 286.2 286.0 286.5 286.8 287.2 286.6 287.7 287.0 287.2 287.0 289.1 290.2 533.4 533.1 533.3 531.9 533.3 532.9 533.6 533.6

a BTC ) benzene(tricarbonyl)chromium; see ref 43. b See ref 41. ATC ) anisole(tricarbonyl)chromium; see ref 42. d See ref 41. e CTC ) chlorobenzene(tricarbonyl)chromium; see ref 41. f No separation into aromatic and methoxy carbon was performed. c

The ORCA electronic structure program package23 was used for all quantum chemical calculations reported here. Minimum energy structures were optimized with the BP86 functional24,25 (DFT) in combination with the Ahlrichs’ TZVP basis set for the Cr(CO)3 template and the smaller SV(P) basis set for the arene system. The resolution-of-the-identity RI approximation26-28 was used in the split-RI-J variant29 with the appropriate coulomb fitting sets. The minimum structures were verified through frequency calculations that were performed by two-sided differentiation of analytic gradients. Plots of all frontier orbitals are provided in the Supporting Information. 3. Results and Discussion a. X-ray Photoelectron Spectroscopy. The fact that we are able to sublime an intact complex does not necessarily mean that the adsorption of the complexes on Ag(111) takes place without decomposition. The interaction between the surface and the aromatic system of the complex competes with the bonding between the aromatic system and the metal fragment. This may lead to a cleavage of the Cr(CO)3 moiety by weakening of its coordinative bond. To obtain information about the chemical state of the adsorbed complex, we acquired X-ray photoelectron spectra of an R-TPHC monolayer and a TPH monolayer on Ag(111). The complete results of the quantitative analysis of the photoelectron spectra are summarized in the Tables 1 and 2. The analysis was performed by fitting pseudo-Voigt profiles to the experimental spectra using the binding energies, the fwhm and the Lorentzian-to-Gaussian ratios as fit parameters. For the determination of the stoichiometries, the peak areas were

integrated. We compared only the relative intensities of the components of the C 1s signal and, separately, of the O 1s signal, but did not compare the C 1s with the O 1s intensities. Hence, cross sections and instrumental factors did not have to be taken into account. The C 1s spectrum of TPH is discussed first and is shown in Figure 2a. It can be fitted with four components. The main component (C-1) at a binding energy of 285.0 eV stems from the carbon atoms of the aromatic backbone of the ligand (with the exception of the two ipso-carbons carrying the methoxy substituents). The binding energy is consistent with the values reported for adsorbates of other weakly bonded aromatic systems on Ag, such as naphthalene adsorbed on Ag(100) with a binding energy of 285.0 eV.30 Interestingly, the oxygen-containing methoxy groups do not play a role for the position of the C 1s signal related to the aromatic backbone. This is in contrast to the situation seen for 3,4,9,10-perylenetetracarboxylic acid dianhydride (PTCDA) on Ag(111), in which the oxygencontaining anhydride groups have a strong influence on the C 1s binding energy of the aromatic backbone and which induce a stronger (chemisorptive) bonding of the molecule.31 There are two additional signal components (C-2 and C-3) that contribute to the asymmetric peak shape of the main signal. First, the two ethyl groups cause a component at a binding energy of 285.5 eV (C-2). The difference of 0.5 eV between the aromatic and the aliphatic carbons is slightly higher than the difference of approximately 0.3 eV often observed for polymers; for example, between the aliphatic chain and the phenyl groups in polystyrene.32 This may be explained by a more effective screening effect of the aromatic system due to stronger substrate-adsorbate interaction, which causes a stronger decrease in the binding energy compared to the ethyl groups. Such a larger difference in the binding energy between aliphatic and aromatic C in the monolayer regime has also been reported for the adsorption of the amino acid 3-(3,4-dihydroxyphenyl)L-alanine (L-DOPA) on Au(110), where the C 1s binding energies for the methylene and the aromatic carbon differ by 0.6 eV.33 The other contribution (C-3) originates from the two aromatic ipso-carbons (carbon atoms 1 and 4 in Scheme 1) at a binding energy of 286.2 eV. Solomon et al. observed an asymmetric peak shape of the C 1s spectrum of phenol adsorbed on Ag(110), as well. They were able to separate the signal into the main peak and a contribution of the ipso-C with a difference in binding energy of 1.6 eV.34 This finding is in very good agreement with our measurements, especially because we expect a decrease in the partial charge of the ipso-C by charge transfer within the extended π-system, resulting in a smaller difference in binding energy with respect to the main peak. The fourth component (C-4) at 287.2 eV is assigned to the two methoxy groups. The binding energy is comparable to the binding energy of the methoxy carbon in poly(methyl methacrylate) (PMMA) at 286.8 eV35 and the methoxy groups of a multilayer trimethoxybenzene on Cu(111).36 For the latter system, Wegner et al. describe a component in the C 1s spectrum at 286.9 eV that is a superposition of the three methoxy as well as the three ipso-carbons.36 Finally, we note that the integrated intensity ratio of the peaks in the TPH C 1s spectrum matches the stoichiometry of the molecule very well, within 1% of the values determined (cf. Table 1). The O 1s spectrum of TPH (Figure 2c) shows, as expected, only one peak (O-1). The binding energy of 533.6 eV agrees with the O 1s binding energy of (partly) aromatic ether groups in polymers, which is in the range of 533.0-533.5 eV,37 and

Adsorption of an arene(CO)3Cr complex on Ag(111)

J. Phys. Chem. C, Vol. 113, No. 15, 2009 6017

Figure 2. C 1s and O 1s XP spectra of the ligand TPH (a, c) and R-TPHC (b, d) on Ag(111) for coverages of 1 ML. The spectra of R-TPHC show one additional peak (red, C-5 and O-2), attributed to the three carbonyl groups of the complex. The fit model matches the stoichiometries of the complex and the ligand. Excitation: Al KR. For additional details, see the text and Table 1.

with the binding energy of a multilayer trimethoxybenzene on Cu(111) at 533.7 eV.36 The C 1s spectrum of R-TPHC is shown in Figure 2b. Although the overall appearance of the spectrum is similar to that of TPH, a closer look shows that the dip found in the TPH spectrum at 286.8 eV between the main peak and the peak of the methoxy groups is absent here. Subtraction of the spectra for R-TPHC and TPH reveals one additional peak at 286.5 eV. We have used the fitting results from the TPH spectrum as starting parameters for the analysis of the C 1s spectrum of R-TPHC. The peak assigned to the aromatic carbons (C-1) and the ethyl groups (C-2) are found again at the same binding energies of 285.0 and 285.5 eV, respectively. The signals of the ipso-carbons (C-3) and methoxy groups (C-4) are slightly shifted to smaller binding energies by 0.2 to 286.0 and 287.0 eV, respectively. This is, however, within the accuracy of the fit results. The substrate-adsorbate interaction is not decisively different for R-TPHC with respect to TPH and may, hence, be considered to be weak, too. The additional component (C-5) at 286.5 eV originates from the three carbonyl groups of the Cr(CO)3 fragment. The integrated intensity ratio of the peaks reflects the stoichiometry of the complex R-TPHC within an accuracy of 1% (cf. Table 1); this difference is much smaller than the margins of error of our measurements and data analysis. Especially, the presence of an additional peak caused by the three carbonyl groups with the correct intensity provides strong evidence for the adsorption of undecomposed R-TPHC on the Ag(111) surface. However, it does not yet exclude the possibility that the Cr(CO)3 group adsorbs separately from the triphenylene ligand on the Ag surface. Evidence against such a partial decomposition is provided by the STM data presented below, which show that R-TPHC is more densely packed on the surface than the triphenylene ligand alone (see below). If the Cr(CO)3 group and the triphenylene ligand adsorbed separately, one would expect the opposite. The C 1s binding energy for CO in the gas phase has been measured as 296-297 eV.38,39 As proposed by Siegbahn et al.,40

one would expect a binding energy of ∼291 eV for CO in the solid state. The significantly smaller binding energy in the complex (286.5 eV) originates from the strong π backbonding; that is, the chromium atom donates electron density back into the carbonyl carbon, and therefore, the binding energies of the core electrons are lowered. The here-reported C 1s binding energy for the CO ligands of R-TPHC is in good agreement with literature values of simple arene(tricarbonyl)chromium complexes derived from powder XP spectra that reveal a binding energy of 286.6-287.7 eV.41-43 In the case of the reference values of anisole(tricarbonyl)chromium, the authors only distinguished between the carbon atoms of the aromatic system and the carbonyl groups.41,42 Our data suggest that there is a partial overlap of the C 1s signal components for the methoxy group and the carbonyl. As a consequence, the true C 1s binding energy of the carbonyl group may be slightly smaller than the value reported in this reference, which would improve the agreement with the values measured here even further. All C 1s binding energies are summarized in Table 2. In most cases reported in the literature, sample charging effects made it necessary to assume fixed values for the binding energy of the aromatic carbons. Therefore, one should compare only the differences between the C 1s binding energies of the ligand and the carbonyl groups. These are in the range of 1.2-2.0 eV. Our value of 1.5 eV obtained for R-TPHC fits perfectly into this range. In a series of spectra for increasing coverage, we could observe that the relative distances between signal components remained unchanged, whereas the whole signal shifted slightly to higher binding energies; for example, by 0.2 eV for a coverage of approximately 10 ML. The shift is presumably caused by a more efficient core hole screening in the monolayer regime. Alternatively, the shift could result from a small coverage-driven work function change. The fact that the appearance of the monolayer and the multilayer spectra remains unchanged (i.e., that there are no differential chemical shifts) again shows the weakness of the substrate-adsorbate interac-

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Figure 3. Cr 2s XP spectrum of R-TPHC on Ag(111) at a coverage of 1 ML. The background was fitted using a fourth grade polynomial function. (a) Raw spectrum and (b) after subtraction of the background.

tion. Furthermore, spectra of R-TPHC powder were acquired (not shown). These spectra can be interpreted and analyzed with the model developed for the monolayer coverage; however, charging effects lead to substantial peak broadening, which makes comparison to the monolayer data more difficult. The O 1s spectrum of R-TPHC shows one peak with a fwhm that is slightly larger than the fwhm of the respective peak for the TPH monolayer. Apparently, the peak is a superposition of two contributions, which are related to the two methoxy and the three carbonyl groups, respectively. We have used the binding energy obtained for the methoxy group of TPH as a starting value for the fit and a fixed stoichiometry of 3:2. The resulting O 1s binding energies are again 533.6 eV, as for TPH, for the methoxy (O-1) and 533.4 eV for the carbonyl groups (O-2). Both the C 1s and the O 1s spectra clearly prove the undecomposed adsorption of R-TPHC on the Ag(111) surface. The C 1s as well as the O 1s spectrum show one additional peak for the three additional carbonyl groups of the complex, as is expected. A direct proof for the undecomposed adsorption would also be the detection of chromium. Unfortunately, the commonly used peak for chromium analysis, the Cr 2p doublet, is superimposed by the Ag 3d substrate signal. Hence, we used the Cr 2s signal that, however, possesses only a small cross section and is neighbored again by a substrate signal originating from the Ag 3s photoelectrons. We were able to resolve the weak signal on the strong background (Figure 3). After subtraction of the empirically fitted background, a peak at 695.7 eV can be discerned. The ratio of the integrated peak areas of the C 1s and the Cr 2s signals, taking into account the different photoelectron sensitivity factors44 and the different pass energies, points to an atomic ratio of 27: 0.74 (ideal: 27: 1). The deviation is possibly caused by (i) the nonconsideration of the spectrometer response function, (ii) the choice of the background, and (iii) a partial damping of the photoelectrons emitted from the

Schmitz et al. chromium atom by interaction with the surrounding carbonyl ligands. Due to this limitation of the accuracy, the clear presence of the Cr 2s peak should be considered as a qualitative proof for the adsorption of undecomposed R-TPHC on Ag(111). b. Scanning Tunneling Microscopy. Figure 4a shows an STM constant-current image of a TPH monolayer on Ag(111). The molecules adsorb flat lying on the surface in a nearly hexagonal close-packed arrangement. We were able to resolve both the adsorbate and the substrate within a single STM image (not shown). This allowed us to exactly derive the dimension and orientation of the unit cell with respect to the substrate. The lengths of the cell vectors are a ) 1.20 ( 0.05 nm and b ) 1.12 ( 0.05 nm; they enclose an angle of γ ) 116.6 ( 0.5°. Vector a is exactly parallel to the [101j] direction of the substrate. Within the margin of error, the length of the vector corresponds to 4 times the distance between next-nearest neighbor atoms on the Ag(111) surface, pointing to a commensurate structure along one crystal axis. The unit cell illustrated in Figure 4c has an area of 1.2 nm2 and contains one molecule. Hence, TPH exhibits an adsorption behavior that is similar to many planar closed-ring molecules without functional groups, such as perylene or coronene.45,46 These extended aromatic molecules also adsorb in a (pseudo-) hexagonal close-packed arrangement, which is typical for a weak substrate-adsorbate interaction. The additional functional groups, ethyl and methoxy, of TPH apparently do not have an important influence on the adsorption geometry. This is in contrast to what is known for strong electron-withdrawing groups. For example, the anhydride groups of PTCDA induce a strong quadrupole moment that dominates the lateral structural order.47,48 The orientation of the molecules in the unit cell (cf. Figure 4) has been constructed from their geometric shape by minimizing the lateral overlap, because we have no definite submolecular contrast in the STM images that can be clearly assigned to the ethyl or methoxy groups and would allow us to fix the azimuthal orientation. Assuming that the aromatic system is oriented parallel to the surface, the (tricarbonyl)chromium complex R-TPHC exhibits the same footprint on the surface as the uncoordinated ligand TPH. However, a large area STM constant current image in Figure 5a already shows a completely different structure, and the pseudohexagonal structure of TPH cannot be found here. We made a statistical analysis of a large number of STM images to completely rule out drift and creep effects and derived a unit cell size with a ) 2.25 ( 0.05 nm, b ) 1.65 ( 0.03 nm, and an enclosed angle of γ ) 90.2 ( 0.6° (cf. Figure 4). The unit cell occupies an area of 3.7 nm2 and contains four complexes. A summary of all structural parameters of TPH and R-TPHC is given in Table 3. The arrangement of the complexes within the unit cell can be nicely seen in Figure 5b. The size and the orientation of the complexes are illustrated with the contour of a hard-sphere model, including the van der Waals radii. In this STM image, the triangular shape of the triphenylene ligand is visible, containing two brighter protrusions on one ring, which stem from the Cr(CO)3 moiety. This is marked with the indicated carbonyl groups within the outer contours of the model. The observation of two bright protrusions instead of three, which would correspond to the topography of the three CO groups, can be explained by the electronic structure of the Cr(CO)3 moiety. DFT calculations show that the electron density is highest at the persubstituted ring carrying the Cr(CO)3 group. The calculated orbitals show either one or no nodal plane along the long axis of the complex in the relevant area and, hence, explain the symmetry of the two bright protrusions that was observed in Figure 5b.

Adsorption of an arene(CO)3Cr complex on Ag(111)

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Figure 4. STM images (size: 8 × 8 nm2) of TPH (a) and R-TPHC (b) with inserted unit cells and substrate orientation plus corresponding adsorption model (c, d). (c) Unit cell of TPH, cell dimensions: a ) 1.20 ( 0.05 nm; b ) 1.12 ( 0.05 nm; γ ) 116.6 ( 1°. The unit cell vector a is parallel to the close-packed [101j] direction of the substrate (see text). Within the margins of error, its length corresponds to 4 times the Ag-Ag distance, pointing to a commensurate adlayer along a. The adsorption site of the molecule within the cell was obtained from the size and the shape of the molecule by minimization of the overlap. (d) Unit cell of R-TPHC, cell dimensions: a ) 2.25 ( 0.05 nm; b ) 1.65 ( 0.03 nm; γ ) 90.2 ( 0.6°. The symmetry operations shown here correspond to a rectangular unit cell, which then would have the plane symmetry group p2gg. The cell contains four molecules, arranged in two nested zigzag chains with opposite directions. The chain directions are indicated by black arrows (b and d) which correspond to the electrostatic dipole moment of the molecule. Tunneling parameters are -1.21 V, 126 pA (a) and -0.49 V, 370 pA (b).

We further note that the very high resolution in Figure 5b was achieved after a sudden change in contrast during scanning. Because this resolution was present only for a limited time and we were not able to reproduce it deliberately in later images by choosing identical tunneling parameters, we assume that it was caused by a sudden change of the tip due to adsorption of a complex or a fragment of the complex. Contrast changes are known for adsorbate-covered STM tips; for example, if the tip apex is covered with a single CO molecule in a controlled way.49 In the structural model in Figure 5b, there is a small overlap of the van der Waals radii. However, this overlap is likely artificial, since the contour was taken from the geometry optimization in gas phase. The overlap is caused by the ethyl and methoxy side groups, which are able to rotate to the side or even out of plane, thus avoiding the overlap on the surface.

From the analysis of the STM images, we conclude that the R-TPHC complexes adsorb in a flat-lying geometry, with the aromatic system oriented parallel to the surface and the metal fragment on top pointing toward the vacuum. They arrange in two nested zigzag chains with - identifying the persubstituted ring with the arrowhead - opposite directions. This is illustrated in the adsorption model (Figure 4d) with black arrows. The complexes are twisted by approximately 35° out of the b direction. Assuming a rectangular unit cell, which is within the statistical error, the cell has the 2D symmetry group p2gg. The symmetry operations are indicated in the hardsphere model of the unit cell in Figure 4d. They include four glide reflections, two parallel to the a- and to the b-vector, respectively, and 2-fold rotation axes at the corners, the medians, and in the center of the unit cell. The unit cell contains four complexes. The

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Figure 5. (a) Large area STM image (65 × 65 nm2) of a monolayer R-TPHC on Ag(111). Substrate directions are valid for both images. (b) Magnification (3.2 × 2.6 nm2) of the unit cell with submolecular contrast. The two neighboring protrusions are assigned to the Cr(CO)3 moiety of the complex, and the position is indicated with the contour of a hard-sphere model including the van der Waals radii. The complexes arrange in two nested zigzag chains with opposite directions (in these two images, they are oriented horizontally). Tunneling parameters are 1.50 V, 386 pA (a) and -1.30 V, 17.2 pA (b).

TABLE 3: Unit Cell Parameters of the Observed Structures unit cell parameters a (nm) b (nm) γ (°) A (nm2) n per unit cell Aeff per molecule (nm2) symmetry group

TPH

R-TPHC

1.20 ( 0.05 1.12 ( 0.05 116.6 ( 1.0 1.2 1 1.2 p1

2.25 ( 0.05 1.65 ( 0.03 90.2 ( 0.6 3.7 4 0.93 p2gg (γ ) 90°) p1 (γ * 90°)

symmetry operations are valid for the adsorbate only due to the incommensurability. A convolution of the adsorbate and the substrate breaks the symmetry; complexes with different orientation within the unit cell are adsorbed on different sites of the substrate. Thus, the appearance of single complexes in the STM images changes with orientation, as can be seen in Figure 5b. Although the overall appearance of the complexes with different orientations is unchanged, especially the apparent height of the two protrusions varies with respect to each other within the structure. As mentioned above, the footprints of TPH and R-TPHC are equal, but the effective area, Aeff, per molecule or complex inside the unit cell is different. R-TPHC adsorbs in a more dense packing with an area occupancy of 0.93 nm2 per complex compared to 1.2 nm2 per molecule for TPH. The higher density of the lateral packing and the more complicated adsorption geometry presumably reflect a stronger attractive adsorbateadsorbate interaction of R-TPHC compared to TPH. The (tricarbonyl)chromium fragment withdraws electron density from the aromatic system. The direction of the electrostatic dipole moment is mainly perpendicular to the π system, but because the chromium fragment is located on the persubstituted terminal ring, it strongly influences the lateral charge distribution in the aromatic system and, thus, induces a dipole moment parallel to the surface that plays a role in the lateral adsorption geometry of the complex. These dipoles are schematically illustrated in Figure 4d. On the basis of this “dipole model” of the structure, the mainly attractive interactions within the structure can be understood. Along the zigzag chains, we have attractive head-to-tail dipole interactions. Between adjacent zigzag chains, the dipole moments of two next-nearest neighbor

Schmitz et al. complexesareantiparallel,thusalsocausingattractivedipole-dipole interactions. This motif alternates between two neighboring chains in the direction perpendicular to the chains (cf. Figure 4d). The second complex of the middle chain (counted in the direction of the dipoles) is antiparallel aligned to the third complex of the lowermost chain, the third complex of the middle chain is antiparallel aligned to the second complex of the topmost chain, and so on. The only repulsive head-to-head interaction can be found between two complexes at a larger distance from each other in adjacent chains; for example, the second complex in the middle chain and the first complex in the lowermost chain (Figure 4d). However, these complexes are facing each other with their ethyl groups, which should additionally weaken the repulsive interactions. A further refinement of the unit cell by low energy electron diffraction, however, failed because the complex decomposes under electron bombardment. This is known for simple complexes such as the deuterated benzene(tricarbonyl)chromium and has been interpreted as a decomplexation of the complex, mainly by loss of the aromatic ligand.50 4. Conclusions Ordered monolayers of TPH and the chromium complex R-TPHC were prepared on Ag(111). XPS analysis shows an adsorption of undecomposed R-TPHC. Both R-TPHC and TPH are weakly bonded (possibly physisorbed) to the Ag(111) surface. STM investigations reveal a planar adsorption geometry of TPH and R-TPHC with a flat-lying aromatic π system on the surface. In the case of R-TPHC, the Cr(CO)3 fragment is on top of the adsorbed aromatic ligand; hence, pointing toward the vacuum. Thus, both substances have the same footprint on the surface. Nevertheless, they show different ordered structures on Ag(111). The free ligand TPH is comparable to other aromatic adsorbates without strong electron-withdrawing groups. It forms a nearly hexagonal close-packed arrangement with weak intermolecular interactions. By contrast, R-TPHC adsorbs in a more complicated structure with a higher packing density. This is likely caused by the electron-withdrawing effect of the Cr(CO)3 group that also influences the lateral charge distribution within the aromatic system, leading to different lateral interactions between the complexes. Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft through SFB 624 “From the Design of Chemical Templates towards Reaction Control” and SFB 583 “Redox-Active Metal Complexes: Control of Reactivity via Molecular Architecture” is gratefully acknowledged. Supporting Information Available: Plots of all calculated frontier orbitals of R-TPHC. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Cornils, B.; Herrmann, W. A. Applied Homogeneous Catalysis with Organometallic Compounds; VCH: Weinheim, 1996. (2) Zotti, L. A.; Teobaldi, G.; Hofer, W. A.; Auwa¨rter, W.; WeberBargioni, A.; Barth, J. V. Surf. Sci. 2007, 601, 2409. (3) Jung, T. A.; Schlittler, R. R.; Gimzewski, J. K. Nature 1997, 386, 696. (4) Scudiero, L.; Barlow, D. E.; Hipps, K. W. J. Phys. Chem. B 2000, 104, 11899. (5) Comanici, K.; Buchner, F.; Flechtner, K.; Lukasczyk, T.; Gottfried, J. M.; Steinru¨ck, H.-P.; Marbach, H. Langmuir 2008, 24, 1897. (6) Flechtner, K.; Kretschmann, A.; Steinru¨ck, H.-P.; Gottfried, J. M. J. Am. Chem. Soc. 2007, 129, 12110.

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