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2007, 111, 5821-5824 Published on Web 04/03/2007
Surface-Confined Two-Step Synthesis of the Complex (Ammine)(meso-tetraphenylporphyrinato)-zinc(II) on Ag(111) Ken Flechtner, Andreas Kretschmann, Liam R. Bradshaw, Marie-Madeleine Walz, Hans-Peter Steinru1 ck, and J. Michael Gottfried* Lehrstuhl fu¨r Physikalische Chemie II, UniVersita¨t Erlangen-Nu¨rnberg, Egerlandstr. 3, 91058 Erlangen, Germany ReceiVed: February 23, 2007; In Final Form: March 20, 2007
We report the first example of a surface-confined two-step synthesis of axially coordinated metalloporphyrin complexes in an ultrahigh vacuum environment. Specifically, a monolayer of tetraphenylporphyrin on an Ag(111) surface was metalated with coadsorbed Zn atoms, and thereafter, NH3 ligands were attached to the metal centers. The surface reactions were monitored with X-ray photoelectron spectroscopy. The approach outlined in this work can be employed to produce and study adsorbates of various axially coordinated porphyrin complexes, including biologically relevant systems.
Introduction Surface-confined coordination chemistry aims at the functionalization of surfaces on the nanoscale and has seen a rapid development in recent years. Important examples are the selfassembly of metal-organic networks,1,2 the fabrication of surface-mounted nanoscopic devices,3,4 and, very recently, the in situ synthesis of adsorbed planar metal complexes.5-8 Most previous studies, however, have focused mainly on structural aspects, using scanning tunneling microscopy (STM), while the chemical properties of the adsorbed coordination compounds have largely been ignored. For example, little is known about how the electronic structure of a coordinated metal ion changes as a result of the interaction with the underlying surface9 and how this interaction influences the chemical reactivity or catalytic activity of the metal complex. This chemical information, however, is very important in view of potential applications of these surface-confined coordination compounds, for example as catalysts or sensors. Especially interesting with respect to a chemical functionalization of surfaces are planar metal complexes such as metalloporphyrins and metallophthalocyanines10,11 (see ref 9 for a full list of references), in which the metal ion possesses two unsaturated axial coordination sites. In the adsorbed state, one of the axial sites can interact with the surface,9 whereas the other one can coordinate (and potentially activate)12 an additional ligand.13 The interaction with the surface can be used to modify the electronic structure and, thereby, the chemical reactivity or catalytic activity of the metal center.9 (A similar mechanism is found in metalloporphyrin-based enzymatic systems as for example heme-thiolate proteins,12,14 in which the axial thiolate ligand acts as an electron donor.) A particular advantage of these complexes is that the interaction between surface and metal center can be tuned with spacer substituents on the periphery of the porphyrin ligand.9 * To whom correspondence should be addressed. Phone: +49 9131 8527314. Fax: +49 9131 85-28867. E-mail: michael.gottfried@ chemie.uni-erlangen.de.
10.1021/jp071531d CCC: $37.00
An example for the activation of a metalloporphyrin by an active support is tetraphenylporphyrinato-cobalt(II) (CoTPP) adsorbed on TiO2, which catalyzes the reduction of NO. The oxide donates electron density to the metalloporphyrin and, in this way, influences its reactivity.15 Other technological applications of adsorbed metalloporphyrins include usage as photosensitizers in dye solar cells16 and as colorimetric gas sensors.17 In all these examples, axial coordination and/or interaction with the underlying surface is decisive for the specific functionalities of the systems. In this work, we report on the controlled two-step synthesis of monolayers of (ammine)(meso-tetraphenylporphyrinato)zinc(II) ((NH3)ZnTPP) on an Ag(111) surface as a simple model system for the described planar metal complexes with additional axial ligand. For monitoring the surface reactions, we employed X-ray photoelectron spectroscopy (XPS), which provides chemical information and distinguishes our work from most previous related studies, which focus on the geometric structure of adsorbed complexes. Moreover, to our knowledge, this is the first study in which a well-defined, adsorbed metal complex is produced in a multistep synthesis in an ultrahigh vacuum environment. Experimental Section All experiments were performed with a commercial X-ray photoelectron spectrometer (Scienta ESCA-200) equipped with an Al KR X-ray source (1486.6 eV) with monochromator and a hemispherical energy analyzer (SES-200). The overall energy resolution amounts to 0.3 eV. The reported binding energies are referenced to the Fermi edge of the clean Ag surface (EB ≡ 0). The base pressure in our ultrahigh-vacuum (UHV) system is below 1 × 10-10 mbar. Well-ordered monolayers of H2TPP and ZnTPP were prepared by evaporation deposition of multilayers at 300 K and subsequent annealing at 550 K as described in ref 9. The formation of square domains with long range order and a lattice © 2007 American Chemical Society
5822 J. Phys. Chem. C, Vol. 111, No. 16, 2007
Letters
monolayer19
Figure 1. N 1s XP spectra of (A) a of H2TPP on Ag(111) and (B) after deposition of a monolayer of H2TPP and the stoichiometric amount of Zn (θZn ) 0.037) at 300 K and subsequent heating to 550 K. (C) N 1s XP spectrum of a monolayer of directly deposited ZnTPP on Ag(111) for comparison. (D) (NH3)ZnTPP on Ag(111) at 140 K, NH3 background pressure 1 × 10-8 mbar. (E) (NH3)ZnTPP produced with directly deposited ZnTPP for comparison, conditions as in (D). Line colors: red, H2TPP; orange, ZnTPP; green, NH3.
constant of 1.4 nm was confirmed by LEED. The porphyrins (purity >98%, Porphyrin Systems GbR) were degassed in vacuo for 24 h at 420 K prior to use. The sample was an Ag single crystal with a polished (111) surface. The NH3 gas (Linde) was of the purity >99.98%. Results and Discussion The synthesis starts from the reactants meso-tetraphenylporphyrin (H2TPP), Zn, and NH3 and comprises the following steps: (1) the metalation of adsorbed H2TPP with coadsorbed Zn atoms
H2TPP(ad) + Zn(ad) f ZnTPP(ad) + H2v
(1)
and (2) the coordination of NH3 as an axial ligand
ZnTPP(ad) + NH3(gas) f H3NsZnTPP(ad)
(2)
As previously shown, the metalation reaction (1) is very efficient, and no side reaction with Ag atoms occurs.6 First, we prepared an ordered monolayer of H2TPP on an Ag(111) surface as described in the experimental section. The N 1s signal in the XP spectrum of this monolayer, displayed in Figure 1A, shows two peaks at 398.2 eV (iminic N) and 400.1 eV (pyrrolic N).5 (We found no evidence for a coordination of the iminic nitrogen atoms to the Ag(111) surface, which is at variance to recent observations by Katsonis et al.18 for mesotetradodecylporphyrin on Au(111). The cause for this discrepancy is probably given by the different peripheral substituents: the rotationally restricted phenyl groups in H2TPP lead to a larger distance between the nitrogen atoms and the Ag surface, compared to the alkyl-substituted porphyrin.)
Figure 2. Zn 2p3/2 XP spectra. (A) Zn/Ag(111), θZn ) 0.037,19 (B) after successive deposition of a monolayer of H2TPP, the stoichiometric amount of Zn (θZn ) 0.037), and heating to 550 K. (C) Zn 2p3/2 XP spectrum of a monolayer of directly deposited ZnTPP on Ag(111) for comparison. (D) (NH3)ZnTPP on Ag(111) at 140 K, NH3 background pressure 1 × 10-8 mbar. (E) (NH3)ZnTPP produced with directly deposited ZnTPP, conditions as in (D).
After preparation of the H2TPP monolayer, the stoichiometric amount of Zn (θZn ) 0.037)19 was vapor deposited on the surface and the sample was heated to 550 K to complete the metalation reaction, which is activated.6 Comparison of the N 1s XP signal of the resulting metal complex (Figure 1B) with that of directly deposited ZnTPP (Figure 1C) shows that ZnTPP was formed with high yield. This result is confirmed by the Zn 2p3/2 spectra in Figure 2. The second step of the synthesis, the coordination of the axial NH3 ligand, requires low temperatures. Figure 1D shows the N 1s spectrum taken after dosing an excess of NH3 (1 × 10-8 mbar, 20 min) at 140 K. (The measurements were performed in the presence of an NH3 background pressure of 1 × 10-8 mbar as will be explained below.) The additional peak at 401.5 eV is attributed to NH3 that is coordinated to a Zn ion. (Further evidence for this assignment will be provided below.) The peak intensity cannot be increased by raising the NH3 background pressure to 1 × 10-7 mbar, which indicates that the respective coordination site is saturated. For reference purposes, we also studied the coordination of NH3 to ZnTPP that was prepared ex situ and directly deposited on the surface as described in the experimental section (coverage one monolayer). The respective N 1s spectrum (Figure 1E) is almost identical to Figure 1D, except for some additional intensity around 400 eV in Figure 1D, which probably stems from NH3 coordinated to some residual unmetalated H2TPP. This explanation is in agreement with our observation that NH3 adsorption on H2TPP layers at 140 K increases the signal at 400 eV (Supporting Information, Figure S1). Note that coordination of NH3 on residual H2TPP is certainly not related to the signal at 401.5 eV, because this signal is even more intense in the case of coordination on the reference ZnTPP sample with the slightly higher degree of metalation (Figure 1E). The N 1s signal of the four porphyrin nitrogens shows no significant changes upon adsorption of NH3 (Figure 1B-E), in contrast to the Zn 2p3/2 signal, which shifts by 0.2 eV toward
Letters
Figure 3. Mass spectrometer signal (m/z ) 17) recorded during the thermal decomposition of the complex (NH3)ZnTPP on Ag(111). Heating rate 1.0 K/s. For preparing the complex, NH3 was dosed for 100 s at 110 K at a pressure of 3 × 10-8 Torr. Inset: Leading edge analysis,20 plot of ln(rate) vs 1/T.
lower binding energy (Figure 2B-E). This shift of the Zn 2p3/2 signal provides further evidence that NH3 coordinates to the Zn ion. At 120 K, the complex (NH3)ZnTPP is stable without NH3 background pressure (pNH3 < 1 × 10-10 mbar) over at least several hours. However, we have chosen to use a higher temperature of 140 K combined with a small NH3 pressure in order to prevent adsorption of small amounts of water from the residual gas during the extended periods of data acquisition (∼2 h, see Figure S2, Supporting Information). The strength of the bond between NH3 and the Zn ion was estimated from a thermal decomposition experiment, in which the sample temperature was steadily increased with a rate of 1.0 K/s. Simultaneously, the released NH3 was detected with a mass spectrometer. The resulting signal (Figure 3) shows a maximum at 145 K. (Note that no NH3 desorption from the clean surface was observed under otherwise identical conditions.) An Arrhenius analysis (or leading edge analysis)20 of the signal yields an activation energy of Ea,des ) 40 kJ/mol and a pre-exponential factor of 3 × 1013 s-1. If the formation of the Zn-NH3 bond is nonactivated, then Ea,des equals the bond dissociation energy Ebond (otherwise Ebond ) Ea,des - Ea,ads with the adsorption activation energy Ea,ads). The intensity ratio of the two N 1s peaks is 1.36:4 in Figure 1D and 1.39:4 in Figure 1E, compared to the expected ratio of 1:4 for the case that NH3 adsorbs exclusively on the Zn ions. The apparent excess of NH3 is an artifact and due to the detection geometry, which results in a higher sensitivity to NH3 as compared to the porphyrin nitrogens. The photoelectrons were detected at 80° relative to the surface normal, which increases the surface sensitivity by a factor of 1/cos 80° ∼ 6 compared to detection at 0°.21 At 80°, electrons emitted from the porphyrin nitrogens must permeate parts of the porphyrin layer, which reduces the signal intensity (Figure S3, Supporting Information). In contrast, the NH3 molecules on-top of the Zn atoms are sufficiently exposed and the respective photoelectrons are much less affected. Therefore, the N 1s signal of the NH3 molecule is more intense than that of a nitrogen atom in the porphyrin ring. For detection in normal emission (0°), we expect to find a ratio of 1:4. However, the reduced surface sensitivity at this angle resulted in a much larger noise level, which made the error in the intensity ratio too large for a meaningful comparison. We also note that no NH3 adsorption was observed with XPS on the clean Ag surface under these conditions. This and the
J. Phys. Chem. C, Vol. 111, No. 16, 2007 5823 fact that the signal at 401.5 eV shows saturation behavior exclude adsorption at the surface or at other parts of the ZnTPP molecules as a possible reason for the oversized NH3 signal. What is the character of the NH3sZn bond? The N 1s binding energy of the coordinated NH3, 401.5 eV, is higher than in typical ammine complexes, where energies of ∼400 eV have been found.22-24 Comparison with N 1s binding energies of typical ammonium compounds, ∼402 eV,25 suggests that the coordinated NH3 in (NH3)ZnTPP has partial ammoniumcharacter, caused by a transfer of electron density from NH3 to the Zn ion. Further evidence for this electron transfer is provided by the Zn 2p3/2 signal, which shifts to lower binding energy upon NH3 coordination. These findings suggest that the NH3s Zn bond is a weak dative bond (or coordinate covalent bond) with partial ionic character. Alternatively, the high N 1s binding energy may be caused by a final state effect: the coordinated NH3 has a rather large distance to the metal surface and is not embedded in a matrix of neighboring molecules. This may result in a poor screening of the final hole state and, thus, lead to a high apparent N 1s binding energy. Conclusions We have demonstrated that well-defined monolayers of a metalloporphyrin complex with an axial ligand can be assembled in two reaction steps on an Ag(111) surface. We first deposited the tetraphenylporphyrin ligand, which establishes the longrange order of the adsorbate layer, then the central metal of the complex (Zn), and finally the axial ligand, NH3. In vacuo, the complex (NH3)ZnTPP decomposes at 145 K to gaseous NH3 and adsorbed ZnTPP. This approach can be used for the controlled synthesis of a large variety of adsorbed, axially coordinated porphyrin complexes, including biologically relevant systems such as (O2)FeTPP or (NO)CoTPP. Acknowledgment. This work was financially supported by the Deutsche Forschungsgemeinschaft (DFG) through SFB 583. The authors thank Norbert Jux, Hubertus Marbach, and Florian Maier for insightful discussions. Supporting Information Available: Additional XP spectra: Adsorption of ammonia on tetraphenylporphyrin layers and adsorption of water on ZnTPP layers at low temperatures; detection geometry. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Barth, J. V.; Costantini, G.; Kern, K. Nature 2005, 437, 671. (2) Lin, N.; Stepanow, S.; Vidal, F.; Kern, K.; Alam, M. S.; Stro¨msdo¨rfer, S.; Dremov, V.; Mu¨ller, P.; Landa, A.; Ruben, M. Dalton Trans. 2006, 2794. (3) Vaughan, O. P. H.; Williams, F. J.; Bampos, N.; Lambert, R. M. Angew. Chem.-Int. Ed. 2006, 45, 3779. (4) Zheng, X. L.; Mulcahy, M. E.; Horinek, D.; Galeotti, F.; Magnera, T. F.; Michl, J. J. Am. Chem. Soc. 2004, 126, 4540. (5) Gottfried, J. M.; Flechtner, K.; Kretschmann, A.; Lukasczyk, T.; Steinru¨ck, H.-P. J. Am. Chem. Soc. 2006, 128, 5644. (6) Kretschmann, A.; Walz, M.-M.; Flechtner, K.; Steinru¨ck, H.-P.; Gottfried, J. M. Chem. Commun. 2007, 568. (7) Buchner, F.; Schwald, V.; Comanici, K.; Steinru¨ck, H.-P.; Marbach, H. ChemPhysChem 2007, 8, 241. (8) Auwa¨rter, W.; Weber-Bargioni, A.; Brink, S.; Riemann, A.; Schiffrin, A.; Ruben, M.; Barth, J. V. ChemPhysChem 2007, 8, 250. (9) Lukasczyk, T.; Flechtner, K.; Merte, L. R.; Jux, N.; Maier, F.; Gottfried, J. M.; Steinru¨ck, H.-P. J. Phys. Chem. C 2007, 111, 3090. (10) Barlow, D. E.; Scudiero, L.; Hipps, K. W. Langmuir 2004, 20, 4413.
5824 J. Phys. Chem. C, Vol. 111, No. 16, 2007 (11) Scudiero, L.; Barlow, D. E.; Mazur, U.; Hipps, K. W. J. Am. Chem. Soc. 2001, 123, 4073. (12) Macyk, W.; Franke, A.; Stochel, G. Coord. Chem. ReV. 2005, 249, 2437. (13) Williams, F. J.; Vaughan, O. P. H.; Knox, K. J.; Bampos, N.; Lambert, R. M. Chem. Commun. 2004, 1688. (14) Mansuy, D.; Battioni, P. Diversity of reactions catalyzed by hemethiolate proteins. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 4, p 1. (15) Mochida, I.; Suetsugu, K.; Fujitsu, H.; Takeshita, K. J. Catal. 1982, 77, 519. (16) Schmidt-Mende, L.; Campbell, W. M.; Wang, Q.; Jolley, K. W.; Officer, D. L.; Nazeeruddin, M. K.; Gra¨tzel, M. ChemPhysChem 2005, 6, 1253. (17) Rakow, N. A.; Suslick, K. S. Nature 2000, 406, 710. (18) Katsonis, N.; Vicario, J.; Kudernac, T.; Visser, J.; Pollard, M. M.; Feringa, B. L. J. Am. Chem. Soc. 2006, 128, 15537.
Letters (19) The coverage θ is defined as the number of adsorbed molecules or atoms divided by the number of substrate atoms, while “monolayer” denotes a closed adlayer of molecules with direct contact to the substrate surface. For example, the monolayers of ZnTPP and H2TPP correspond to θ ) 0.037. (20) Habenschaden, E.; Ku¨ppers, J. Surf. Sci. 1984, 138, L147. (21) Hu¨fner, S. Photoelectron Spectroscopy. Principles and Applications, 3rd ed.; Springer: Berlin, 2002. (22) Hendrickson, D. N.; Hollander, J. M.; Jolly, W. L. Inorg. Chem. 1969, 8, 2642. (23) Yatsimirskii, K. B.; Nemoskalenko, V. V.; Aleshin, V. G.; Bratushko, Y. I.; Moiseenko, E. P. Chem. Phys. Lett. 1977, 52, 481. (24) Maier, F.; Gottfried, J. M.; Rossa, J.; Gerhard, D.; Schulz, P. S.; Schwieger, W.; Wasserscheid, P.; Steinru¨ck, H.-P. Angew. Chem.-Int. Ed. 2006, 45, 7778. (25) Aduru, S.; Contarini, S.; Rabalais, J. W. J. Phys. Chem. 1986, 90, 1683.
