J. Phys. Chem. C 2008, 112, 6087-6092
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Direct Metalation of a Phthalocyanine Monolayer on Ag(111) with Coadsorbed Iron Atoms Yun Bai,1 Florian Buchner,1 Matthew T. Wendahl, Ina Kellner, Andreas Bayer, Hans-Peter Steinru1 ck, Hubertus Marbach,* and J. Michael Gottfried* UniVersita¨t Erlangen-Nu¨rnberg, Lehrstuhl fu¨r Physikalische Chemie II and Interdisciplinary Center for Molecular Materials (ICMM), Egerlandstr. 3, 91058 Erlangen, Germany ReceiVed: NoVember 22, 2007; In Final Form: February 7, 2008
An ordered monolayer of phthalocyanine on a silver(111) surface reacts with the stoichiometric amount of coadsorbed Fe atoms to form iron(II)-phthalocyanine. This surface-confined redox reaction was studied with X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). The formation of iron(II)-phthalocyanine was confirmed by comparison with the directly deposited complex. No side reactions such as the coordination of Fe atoms on the peripheral nitrogen atoms of the phthalocyanine molecules were observed.
1. Introduction Coordination compounds on solid supports are promising candidates for novel, regularly nanostructured catalysts. The coordinated metal ions represent well-defined and uniform active sites, whereas the solid support ensures the simple separation of the catalyst and products. Thus, these catalysts combine the best of the two worlds of homogeneous and heterogeneous catalysis. The most promising molecular precursors are phthalocyanines and porphyrins because they can coordinate a large variety of metal ions and the resulting metal complexes are usually very stable and readily available.2 In addition, their planar geometries facilitate the formation of well-ordered arrays of strongly adsorbed molecules.3-19 Several examples of this class of catalysts have been studied;20-24 however, the major developments in this field certainly lie in the future. From a fundamental point of view, it is important to understand how the interaction of the metal complex with the surface of the support influences the electronic structure25 and the reactivity of the active site and how this active site responds to the adsorption of potential reactants.26 Little is known about these aspects and about the general reactivity of adsorbed metal complexes. Recently, it has been demonstrated that adsorbed metalloporphyrins can be synthesized directly on a surface by metalation of the adsorbed porphyrins with vapor-deposited metal atoms, for example Fe, Co, and Zn.12,27-30 There are strong indications that the order of deposition is not relevant; the porphyrins also react with metal atoms that have been deposited prior to the adsorption of the porphyrin. This has been shown for the reaction between tetraphenylporphyrin (2HTPP) and Zn.31 The metalation reaction starts with the coordination of the neutral metal atom by the nitrogen atoms of the intact porphyrin. Thereafter, the pyrrolic hydrogen atoms migrate to the metal center, where they complete the reaction by desorbing as H2. * Authors to whom correspondence should be addressed: Dr. J. Michael Gottfried. Address: Universita¨t Erlangen-Nu¨rnberg, Lehrstuhl fu¨r Physikalische Chemie II, Egerlandstr. 3, 91058 Erlangen, Germany. Phone: +49 913185-27314.Fax: +49913185-28867.E-Mail:
[email protected]. Dr. Hubertus Marbach. Address: Universita¨t ErlangenNu¨rnberg, Lehrstuhl fu¨r Physikalische Chemie II, Egerlandstr. 3, 91058 Erlangen, Germany. Phone: +49 9131 85-27316. Fax: +49 9131 85-28867. E-Mail:
[email protected].
In the course of the two hydrogen transfer steps, the metal is successively oxidized to the corresponding dication.30 We note that similar reactions have been used for the synthesis of surfaceconfined metal-organic networks32 and, recently, for the mechanical switching of adsorbed [2]-catenane molecules by complexation with copper atoms.33 An important application of the direct metalation is the in situ synthesis of reactive metalloporphyrins such as iron(II)tetraphenylporphyrin (FeTPP), which has a high affinity toward oxygen.34 Monolayers of this complex can in principle be prepared by synthesis in solution, isolation, and subsequent vapor deposition. However, the high reactivity can lead to the formation of byproducts as for example superoxo complexes with axial O2 ligands, resulting in chemically inhomogeneous, contaminated monolayers. In contrast, the in situ metalation of porphyrin monolayers under ultrahigh vacuum conditions provides clean and uniform iron(II)-porphyrin monolayers with very high degrees of metalation.12 Potentially, this route can also be employed to synthesize extremely reactive metalloporphyrins, which can only exist in the presence of a surface. For example, unfavorable oxidation states of the metal ion may be stabilized by the interaction with the underlying surface, which can act as an electron donor.25 For practical applications, phthalocyanines are more suitable than porphyrins because of their higher stability and lower price. Thus, the question arises whether the direct metalation reaction can also be employed for the in situ synthesis of metallophthalocyanine monolayers. Naively, one may assume that the reactivity of phthalocyanine toward metal atoms is similar to the reactivity of porphyrins because both provide the same coordination environment. However, the phthalocyanine molecule also contains four peripheral iminic nitrogen atoms (Figure 1), which could coordinate coadsorbed metal atoms and thus give rise to an unwanted side reaction. The fact that such a side reaction is theoretically possible has been demonstrated in a recent study of the interaction of tetrapyridylporphyrin (2HTPyP) with Fe on Cu(111). In this study, it was found that the iminic nitrogen atoms of the pyridyl groups strongly attract the coadsorbed Fe atoms.35 Phthalocyanine may react in an analogous way because the peripheral meso-bridging nitrogen atoms
10.1021/jp711122w CCC: $40.75 © 2008 American Chemical Society Published on Web 03/27/2008
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Bai et al. 2. Experimental Section
Figure 1. Constant-current STM image (a) of a complete monolayer of 2HPc and (b) after the deposition of iron (θFe ) 0.012) onto the 2HPc monolayer. In part b some molecules exhibit a central bright spot, suggesting the formation of FePc. (c) Height profile extracted along the green line in b. Space-filling models of 2HPc and FePc are shown above and below the height profile, respectively. The arrows indicate the positions of the molecules in the images and the profile. The tunneling parameters of the STM micrographs were (a) -0.67 V, 39 pA and (b) -1.45 V, 33 pA.
should show a reactivity similar to that of the nitrogen atoms of the pyridyl groups of 2HTPyP. Another difficulty may arise from the fact that the metal atoms, which are vapor-deposited on the complete monolayer of the ligand molecules, need to diffuse to find a vacant coordination site. In the case of a monolayer of tetraphenylporphyrin (2HTPP), it was found that Fe and Co atoms are sufficiently mobile on the densely packed 2HTPP monolayer for an almost complete metalation at room temperature. However, the phthalocyanine monolayer may allow less mobility of the metal atoms because their tetrapyrrole macrocyles are in direct contact to the surface. In contrast, the peripheral phenyl groups of 2HTPP (which are rotated out of the porphyrin plane) act as spacers and create a gap between the porphyrin macrocycle and the surface.25 It appears likely that the coadsorbed metal atoms are able to diffuse in this gap with lower activation energy than between a phthalocyanine molecule and the surface. Alternatively, diffusion of the metal atoms on the molecular layer or between the molecules may be possible, but both mechanisms seem energetically unfavorable. In the first case, the bond between the metal atom and the surface must be broken, whereas in the second case the molecules in the densely packed layer must be laterally displaced. For these reasons, metalation of a phthalocyanine monolayer may be slow or require an excess of the metal. Despite these potential complications, we found that the metalation of well-ordered monolayers of phthalocyanine on an Ag(111) surface proceeds rapidly at room temperature and leads to almost complete metalation. In addition, the process appears to be highly selective; that is, no indications of side reactions were found. The progress of the reaction was monitored with X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM).
The XPS and STM experiments were performed in two separate UHV systems with background pressures below 2 × 10-10 mbar. The XPS experiments were performed with a Scienta ESCA-200 spectrometer equipped with an Al KR X-ray source (1486.6 eV), an X-ray monochromator, and a hemispherical energy analyzer (SES-200). The overall energy resolution amounts to 0.3 eV. All binding energies were referenced to the Fermi edge of the clean Ag surface (Eb ≡ 0). The photoelectrons were detected at an angle of 70° to the surface normal for increased surface sensitivity. The STM experiments were carried out using an RHK UHV VT STM 300 with an RHK SPM 100 electronics. All given voltages refer to the sample, and the images were recorded in constant-current mode. Moderate low pass filtering was applied for noise reduction. Cut Pt/Ir tips were used as STM probes. The STM data were processed with the software WSxM, (http://www.nanotec.es); a description of the program can be found in ref 36. The substrate was an Ag single crystal (purity >99.999%, purchased from Surface Preparation Laboratory, DE Zaandam, The Netherlands) with a polished (111) surface, which was aligned to 99% and >90%, respectively, SigmaAldrich) were outgassed in vacuo by heating to 410 K for 24 h and, later, to 670 K for 3 h. Well-ordered monolayers of 2HPc and FePc were prepared by vapor deposition of multilayers onto the Ag(111) substrate held at 300 K and subsequent annealing at 530 K (2HPc) or 560 K (FePc). During deposition, the temperature of the Knudsen cell was 670 K for 2HPc and 680 K for FePc, which led to a flux of approximately 0.016 monolayers per minute. The deposition of Fe atoms was carried out with an Omicron EFM 3 electron-beam evaporator. The evaporants were iron rods with diameters of 2 mm and purities of 99.99% purchased from Mateck. The coverage θ of the phthalocyanine monolayers on the Ag substrate, defined as the number of adsorbed molecules per surface atom, was experimentally determined with STM and corresponded to θ ) 0.037. 3. Results and Discussion 3.1. Scanning Tunneling Microscopy. Ordered monolayers of phthalocyanine (2HPc) on Ag(111) were prepared by vapor deposition of multilayers and subsequent annealing as described in the experimental section. This procedure takes advantage of the different desorption temperatures of multilayers and monolayers and has been applied successfully for the preparation of well-defined monolayers of various metallophthalocyanines37 and porphyrins.11,12,25,27,30 The phthalocyanine molecules in the monolayer have a thermal stability sufficient to survive the annealing step, as has been confirmed by X-ray photoelectron spectroscopy (XPS). In Figure 1a, a high-resolution STM constant current image of the 2HPc monolayer is reproduced. The molecules lie flat on the surface and show symmetric cross-like features. The dark center of each molecule represents the inner cavity, which is surrounded by the four central nitrogen atoms.13 The unit cell has a square geometry with lattice vectors of 1.4 ( 0.05 nm. This type of arrangement has also been found for monolayers of phthalocyanine (2HPc) and various metallophthalocyanines (CuPc, CoPc, NiPc, FePc, SnPc, PdPc) on Cu(100),15 Ag(111),17 Au(111),3,4,19 MoS216, and graphite (HOPG).13,16,19 The submolecular resolution shows the orientation of the molecules relative
Metalation of Phthalocyanine with Fe on Ag(111)
J. Phys. Chem. C, Vol. 112, No. 15, 2008 6089
Figure 2. (a) Constant-current STM image of a monolayer of 2HPc exhibiting two topographically different 2HPc species, denoted as 2HPc_mod1 and 2HPc_mod2. (b-d) Clockwise arranged sequence of STM images after the incremental deposition of Fe (θFe ) 0.012 in each step). The bar graphs in the right-hand part of the figure show the percentages of the two 2HPc species with different apparent heights (blue, green) and of phthalocyanines with coordinated Fe atoms () FePc) (red). The tunneling parameters of the STM micrographs were (a) -0.26 V, 22 pA, (b) -1.44 V, 34 pA, (c) -1.42 V, 22 pA, and (d) -0.99 V, 0.33 pA.
to the adsorbate lattice, giving an azimuthal angle between one vector of the unit cell and the molecular axis of δ ) 60 ( 3° as indicated in Figure 1a. By calculating the ratio of substrate and adsorbate unit cell areas, a coverage of θ ) 0.037 was determined. This number equals the stoichiometric coverage of vapor-deposited Fe atoms necessary for the complete metalation of a 2HPc monolayer. Figure 1b shows an STM image of a monolayer of 2HPc after the deposition of iron, θFe ) 0.012 ( 0.003, which is approximately 30% of the stoichiometric amount. Following Fe deposition, some of the molecules appear with a bright central spot, while the other molecules remain unchanged. Figure 1c shows a profile across one modified and two unmodified molecules, acquired at the position of the green dotted line in Figure 1b. It clearly demonstrates an increased apparent height in the former and central cavities in the latter, indicating that the vapor-deposited Fe atoms occupy the central positions of the 2HPc molecules. This result is interpreted as the coordination of the Fe atoms by the phthalocyanine molecules. No Fe atoms can be seen at sites between the 2HPc molecules, which proves that the coordination reaction is fast on the time scale of our experiment (several hours) and that the Fe atoms prefer to bind to the centers of the molecules rather than to the meso-bridging nitrogen atoms at the periphery or to other sites at the Ag surface. At this point, it is not clear whether this coordination leads to the formation of Fe(II)-phthalocyanine (FePc) through a reaction that includes oxidation of the Fe atom and the release of the pyrrolic hydrogen atoms as H2 or whether the Fe atom is coordinated by a more or less intact 2HPc molecule. The definite proof for the formation of FePc will be given below by X-ray photoelectron spectroscopy (XPS) and by a direct comparison of the STM features of in situ prepared and directly deposited commercial FePc. For the time being, this conclusion is also
supported by comparison with literature STM data for iron(II)phthalocyanine on Au(111)4,14 and graphite (HOPG).13 In these references, the FePc molecules show a central protrusion, which has been attributed to an orbital-mediated tunneling contribution through the Fe(II) d6 system.4,14 However, it is noteworthy to mention that these FePc monolayers were prepared by direct vapor deposition of FePc, contrary to our approach. FePc is sensitive to oxidation and difficult to obtain as a pure substance. Therefore, the adsorbed FePc that was prepared by in situ metalation in UHV is most likely of higher purity than that obtained by direct deposition of FePc. This difference may slightly limit the significance of this comparison with literature data. The long-range order of an extended area of the 2HPc monolayer is displayed in Figure 2a. The other STM images (Figure 2b-d) were acquired after the successive deposition of equal amounts of Fe (θFe ) 0.012 in each step) onto this monolayer. Apparently, the number of protrusions (FePc molecules) increases roughly proportional to the amount of deposited Fe. The corresponding numbers of the fraction of protrusions from Figure 2 are (b) 37%, (c) 71%, and (d) 95%. For comparison, the numbers expected from a stoichiometric reaction are (b) 32%, (c) 65%, and (d) 97%. The small deviations between the experimental and ideal degrees of metalation are within the margins of error (25% of the amount of deposited Fe). Figure 2a also shows that the ordered monolayer of 2HPc contains two species with different apparent heights; 83% of the molecules appear brighter and 17% dimmer. We note that the contrast of Figure 2a was enhanced to make the small difference in the apparent heights more obvious. In Figure 2bd, an increasing fraction of the molecules appear as protrusions, representing the already described Fe-coordinated phthalocyanine species. As a result of the reduced contrast in comparison
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Figure 3. Constant-current STM images of (a) a mixed monolayer of 2HPc (95%) and FePc (5%), generated by direct deposition of a 2HPc/ FePc mixture; (b) after vapor deposition of Fe (θFe ) 0.012 ( 0.003) on this mixed monolayer; (c) a larger area of the layer of image 3b, showing 32% FePc. The following tunneling parameters were applied: (a) -0.88 V, 25 pA; (b and c) -1.22 V, 34 pA.
to Figure 2a, the two different 2HPc species show similar brightness in the latter three images. The ratio of the three different species (two species of 2HPc and FePc) is constant over large areas of the surface, as verified by extended STM images (size: 50 × 50 nm2, ∼1300 molecules) recorded after each deposition step (not shown). The origin of the two different apparent heights of the 2HPc molecules cannot be addressed adequately at this point. The existence of two different molecular conformations would be a possible explanation, but seems unlikely, considering the planarity of the molecules and the stiffness of the aromatic systems. As another possibility, some of the molecules may have a certain lateral position relative to the substrate atoms, which enables them to weakly coordinate Ag atoms with their central nitrogen atoms. A similar effect has been observed for tetradodecylporphyrin on Au(111).9 However, in contrast to this study, our XPS data do not show an additional signal that could be related to this interaction. Furthermore, the images in Figure 2 prove that both 2HPc species have a high affinity to coadsorbed Fe atoms, although the dimmer molecules are metalated more rapidly. This is illustrated by the bar diagrams in Figure 2, which represent the relative fractions of the different species as derived from large images with ∼1300 molecules. The reason for the different reactivities is subject to ongoing investigations and is not focus of this publication. After metalation, the formerly distinguishable 2HPc species are indistinguishable in the STM images. For a direct comparison of in situ prepared and directly deposited FePc, a mixture of 2HPc and commercial FePc was vapor deposited to create a mixed monolayer with 5% FePc (Figure 3a). Subsequent vapor deposition of Fe atoms (θFe ) 0.012 ( 0.003) leads to the formation of additional FePc (Figure 3b), which is indistinguishable from the directly deposited FePc molecules. Figure 3c shows a larger area of this layer, proving that in situ and ex situ prepared FePc lead to identical features in STM.
Bai et al. The observation that all vapor-deposited Fe atoms find a coordination site confirms that the reaction is a surface-mediated process. Clearly, the adsorbed Fe atoms must diffuse across the Ag surface before they are captured by a 2HPc molecule. However, it is debatable whether the Fe atoms diffuse between the molecular layer and the Ag surface, on top of the molecules, or between the 2HPc molecules while the contact to the Ag surface persists. All three processes seem energetically unfavorable because they imply the cleavage of bonds to the surface or the lateral displacement of molecules. As mentioned above, 2HPc and 2HTPP have notably different distances between the plane of their tetrapyrrole macrocycles and the underlying surface because 2HTPP rests on its four peripheral phenyl substituents, which the planar phthalocyanine lacks. As a result, the distance to the surface of the porphyrin cycle in 2HTPP is larger by approximately 0.3 nm compared to the 2HPc molecule, as has been derived from the van-derWaals dimensions.25 For this reason, one may suspect that Fe atoms diffusing within a 2HTPP monolayer have a higher mobility than Fe atoms within an 2HPc monolayer, at least if these atoms diffuse between the Ag surface and the molecular layer. However, if this is the case, then it has little influence on the efficiency of the metalation reaction. High degrees of metalation have been found for both molecules, with an even higher value for 2HPc (95% as compared to 89% for 2HTPP12). 3.2. X-ray Photoelectron Spectroscopy (XPS). The STM images in Figures 1 and 2 clearly show that the Fe atoms coordinate to the centers of the phthalocyanine molecules. However, the clarification of the chemical states of the coordinated Fe atoms and of the phthalocyanine ligands requires complementary investigation with photoelectron spectroscopy. The phthalocyanine molecule contains two chemically different types of nitrogen atoms, two pyrrolic (-NH-) and six iminic nitrogen atoms (dN-) (Figure 1). Four of the iminic nitrogen atoms occupy the bridging meso-positions at the periphery of the molecule, while the other two are in the center along with the two pyrrolic nitrogen atoms. In the N 1s region of the X-ray photoelectron spectrum, the respective signals appear at 400.2 eV (-NH-) and 398.5 eV (dN-) (Figure 4a), in line with previous XPS data.38 The ratio of the peak intensities after deconvolution is 1:3.05, in good agreement with the stoichiometry of the molecule. (The difference in the chemical shifts between the iminic nitrogen atoms in the peripheral mesobridging positions and those in the center amounts to only 0.4 eV38 and is neglected in this discussion.) No additional signals were observed that could be related to the two 2HPc species with different apparent heights in the STM image (Figure 2a), indicating that these species are in chemically similar states. Deposition of a sub-stoichiometric amount of Fe atoms (θFe ) 0.027) on the 2HPc monolayer at room temperature leads to significant changes in the N 1s signal. The two components of adsorbed 2HPc lose intensity, while another signal appears at 398.7 eV (Figure 4b). With a slight excess of Fe atoms (θFe ) 0.044), the two signals from 2HPc vanish completely and give way to a single peak at 398.7 eV (Figure 4c). Apparently, the four central nitrogen atoms are now in a chemically identical (or very similar) state. To clarify the question of whether the coordination of the Fe atoms results in the formation of iron(II)-phthalocyanine (FePc), a monolayer of this commercially available complex was prepared as described in the experimental section. The corresponding XP spectrum, displayed in Figure 4d, is virtually identical to the spectrum in Figure 4c for the metalated 2HPc. The agreement between these two spectra provides strong evidence that the reaction of adsorbed 2HPc
Metalation of Phthalocyanine with Fe on Ag(111)
Figure 4. N 1s XP spectra of a monolayer of 2HPc (A), a monolayer of 2HPc after the deposition of increasing amounts of iron, θFe ) 0.027 (B) and θFe ) 0.044 (C), and a monolayer of directly deposited commercial FePc as a reference (D). The fit neglects the small binding energy difference of 0.4 eV38 between the iminic nitrogen atoms in the peripheral meso-positions and in the center, for both 2HPc with 6 and FePc with 8 iminic nitrogen atoms.
J. Phys. Chem. C, Vol. 112, No. 15, 2008 6091 According to this model, which was first suggested by Gunnarson and Scho¨nhammer,39 the main peak at 707.2 eV corresponds to most efficiently screened core holes, while the satellites at higher binding energies result from less efficiently screened core holes. Which of these two explanations is correct cannot be determined on the basis of our experimental data. Deposition of a slight excess of Fe (θFe ) 0.044, Figure 5b) causes only minor changes in the spectrum compared to the spectrum for stoichiometric Fe deficiency (Figure 5a). The main signal at 707.2 eV grows relative to the satellites because its position coincides with the position for uncoordinated Fe(0). The reasons for this deviation from the typical peak position for Fe(II) have been established for similar systems and will be discussed below. For comparison, the Fe 2p3/2 signal of a directly deposited monolayer of commercial FePc is displayed in Figure 5c, which is almost identical to Figure 5b and confirms the formation of FePc by direct metalation. Figure 5d shows the Fe 2p3/2 region of a multilayer (∼10 monolayers) of commercial FePc. This spectrum is in good agreement with XPS data for FePc multilayers published previously.40 The broad and asymmetric shape of this signal has been attributed to the open-shell structure of the coordinated Fe ion, which leads to a coupling between the spin of the core hole and the spins in the valence shell.40 Compared to the monolayer spectrum, the maximum of the multilayer signal is shifted to higher binding energy, 709 eV. This is a typical value for Fe in the oxidation state +2, whereas the main signal of the monolayer spectrum at 707.2 eV is rather typical for Fe(0). A very similar difference in the peak positions of multilayer and monolayer has been described recently for the Co 2p3/2 signal of cobalt-tetraphenylporphyrin (CoTPP) on Ag(111).25,26 A detailed investigation with X-ray and UV photoelectron spectroscopy revealed that the Co ions in the CoTPP monolayer interact strongly with the Ag surface, which transfers electron density to the Co ion. Thus, the observed peak shift results to a large extent from a partial reduction of the Co ion.25 It is likely that a similar effect causes the different positions of the Fe 2p3/2 signal for multilayers and monolayers of FePc. Because of the planar geometry of the molecule, the distance between the Fe ion and the Ag surface is probably shorter than the Co-Ag distance of adsorbed CoTPP, making the Fe-Ag interaction even more effective. 4. Conclusions
Figure 5. Fe 2p3/2 XP spectra of a monolayer of 2HPc after the deposition of increasing amounts of iron, θFe ) 0.027 (A) and θFe ) 0.044 (B); a monolayer of directly deposited commercial FePc as a reference (C), and FePc multilayers (∼10 monolayers) (D).
with coadsorbed Fe atoms leads to the formation of iron(II)phthalocyanine. This conclusion is further supported by XP signals of the coordinated metal. Figure 5a shows the Fe 2p3/2 signal after deposition of a substoichiometric amount of Fe (θFe ) 0.027) on the 2HPc monolayer. This spectrum corresponds to the same state as the N 1s spectrum in Figure 4b. The Fe 2p3/2 signal shows a main peak at 707.2 eV, which is accompanied by a satellite structure between 708 and 712 eV. A similar structure was obtained for the Co 2p3/2 signal of cobalt(II)-tetraphenylporphyrin (CoTPP) on Ag(111).25 In a detailed photoemission study of this system, the satellite structure was explained with the open-shell character of the metal ion, which results in final states of different spins and, thus, different energies.25 As an alternative model, a distribution of different efficiencies in the screening of the final core hole by the underlying metal surface can be assumed.
Ordered monolayers of iron(II)-phthalocyanine on an Ag(111) surface were obtained by direct metalation of phthalocyanine monolayers with the stoichiometric amount of vapordeposited Fe atoms. The reaction proceeds rapidly at room temperature and leads to high degrees of metalation of the phthalocyanine molecules (95%). The STM investigations reveal that the Fe atoms are exclusively coordinated by the central nitrogen atoms of the phthalocyanine molecules; no competing coordination on the peripheral meso-bridging nitrogen atoms was observed. In addition, the fact that all deposited Fe atoms up to the stoichiometric amount find a coordination site indicates that the Fe atoms are sufficiently mobile on the surface even in the presence of the densely packed phthalocyanine monolayer. The N 1s and Fe 2p XPS data prove that the adsorbed iron(II)-phthalocyanine (FePc) prepared in situ is chemically identical to directly deposited FePc. In general, the ultrahigh vacuum environment with its very low concentration of contaminants provides excellent conditions for the in situ preparation of such reactive metal complexes. Therefore, the procedure described here may also be applied successfully for the synthesis of monolayers of M(II)-phthalocyanines with metal ions that are
6092 J. Phys. Chem. C, Vol. 112, No. 15, 2008 usually not stable in the +2 oxidation state. The presence of the substrate surface may have an additional stabilizing influence on such unusual oxidation states. Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft through Sonderforschungsbereich 583 is gratefully acknowledged. MTW thanks the Deutscher Akademischer Austauschdienst (DAAD) for a RISE Fellowship. We thank Robert Staehle, who participated in some of the XPS measurements. References and Notes (1) These authors contributed equally to this manuscript and should share first authorship. (2) The Porphyrin Handbook; Kadish, K. M., Guilard, R., Smith, K. M., Eds.; Academic Press: New York, 2000; Vols. 1-4. (3) Lu, X.; Hipps, K. W.; Wang, X. D.; Mazur, U. J. Am. Chem. Soc. 1996, 118, 7197. (4) Lu, X.; Hipps, K. W. J. Phys. Chem. B 1997, 101, 5391. (5) Scudiero, L.; Barlow, D. E.; Mazur, U.; Hipps, K. W. J. Am. Chem. Soc. 2001, 123, 4073. (6) Hipps, K. W.; Scudiero, L.; Barlow, D. E.; Cooke, M. P. J. Am. Chem. Soc. 2002, 124, 2126. (7) Barlow, D. E.; Scudiero, L.; Hipps, K. W. Langmuir 2004, 20, 4413. (8) Sekiguchi, T.; Wakayama, Y.; Yokoyama, S.; Kamikado, T.; Mashiko, S. Thin Solid Films 2004, 464-65, 393. (9) Katsonis, N.; Vicario, J.; Kudernac, T.; Visser, J.; Pollard, M. M.; Feringa, B. L. J. Am. Chem. Soc. 2006, 128, 15537. (10) Auwa¨rter, W.; Weber-Bargioni, A.; Riemann, A.; Schiffrin, A.; Gro¨ning, O.; Fasel, R.; Barth, J. V. J. Chem. Phys. 2006, 124, 194708. (11) Buchner, F.; Comanici, K.; Jux, N.; Steinru¨ck, H.-P.; Marbach, H. J. Phys. Chem. C 2007, 111, 13531. (12) Buchner, F.; Schwald, V.; Comanici, K.; Steinru¨ck, H.-P.; Marbach, H. ChemPhysChem 2007, 8, 241. (13) (a) Nilson, K.; Åhlund, J.; Brena, B.; Go¨thelid, E.; Schiessling, J.; Mårtensson, N.; Puglia, C. J. Chem. Phys. 2007, 127, 114702. (b) Åhlund, J.; Schnadt, J.; Nilson, K.; Go¨thelid, E.; Schiessling, J.; Besenbacher, F.; Mårtensson, N.; Puglia, C. Surf. Sci. 2007, 601, 3661. (14) Cheng, Z. H.; Gao, L.; Deng, Z. T.; Jiang, N.; Liu, Q.; Shi, D. X.; Du, S. X.; Guo, H. M.; Gao, H.-J. J. Phys. Chem. C 2007, 111, 9240. (15) Lippel, P. H.; Wilson, R. J.; Miller, M. D.; Wo¨ll, C.; Chiang, S. Phys. ReV. Lett. 1989, 62, 171. (16) Ludwig, C.; Strohmaier, R.; Petersen, J.; Gompf, B.; Eisenmenger, W. J. Vac. Sci. Technol., B 1994, 12, 1963. (17) Grand, J.-Y.; Kunstmann, T.; Hoffmann, D.; Haas, A.; Dietsche, M.; Seifritz, J.; Mo¨ller, R. Surf. Sci. 1996, 366, 403. (18) Hipps, K. W.; Lu, X.; Wang, X. D.; Mazur, U. J. Phys. Chem. 1996, 100, 11207. (19) (a) Walzer, K.; Hietschold, M. Surf. Sci. 2001, 471, 1. (b) Gopakumar, T. G.; Lackinger, M.; Hackert, M.; Mu¨ller, F.; Hietschold, M. J. Phys. Chem. B 2004, 108, 7839. (c) Gopakumar, T. G.; Lackinger, M.; Hietschold, M. Jpn. J. Appl. Phys. 2006, 45, 2268. (20) Hulsken, B.; Van, Hameren, R.; Gerritsen, J. W.; Khoury, T.; Thordarson, P.; Crossley, M. J.; Rowan, A. E.; Nolte, R. J. M.; Elemans,
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