Surface-Confined Coordination Chemistry with Porphyrins and Phthalocyanines: Aspects of Formation,...

Z. Phys. Chem. 223 (2009) 53–74 . DOI 10.1524.zpch.2009.6024© by Oldenbourg Wissenschaftsverlag, München

Surface-Confined Coordination Chemistry with

Porphyrins and Phthalocyanines: Aspects of

Formation, Electronic Structure, and Reactivity

By J. Michael Gottfried* and Hubertus Marbach

Department Chemie und Pharmazie, Lehrstuhl für Physikalische Chemie II, UniversitätErlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany

Dedicated to Prof. Dr. Klaus Christmann on the occasion of his 65th birthday

(Received September 25, 2008; accepted October 2, 2008)

Porphyrin . Phthalocyanine . Coordination Chemistry .

Molecular Self-Assembly . Photoelectron Spectroscopy .

Scanning Tunneling Microscopy

Recent years have seen rapid progress in the field of surface-confined coordination chemistry.Adsorbed metal complexes of tetrapyrroles (porphyrins, phthalocyanines, corroles) are espe-cially interesting in this context, since they combine a planar structure-determining elementwith an active site. While earlier studies of adsorbed metallo-tetrapyrroles mainly addressedaspects of molecular self-assembly, the focus of interest has shifted gradually to electronicstructure and chemical reactivity. This article gives an overview of recent advances in the fieldof surface chemistry with tetrapyrroles. In particular, the following aspects will be discussed:intramolecular conformation and supramolecular ordering, electronic interaction with the sub-strate, surface-confined synthesis, and ligand-related effects such as the surface trans effect.

1. Introduction

Functionalization of surfaces on the nanoscale is the key to designing novelcatalysts, sensors, and other devices that are based on the interaction of an activesurface with the surrounding medium. Metalloporphyrins and similar planarmetal complexes are especially suitable for this task, because they combine astructure forming element (the porphyrin framework) with an active site, usuallythe coordinated metal center. In the free complex, this metal center is oftencoordinated by only the tetradentate, planar ligand (porphyrin, phthalocyanine,

* Corresponding author. E-mail: [email protected]

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54 J. M. Gottfried and H. Marbach

or corrole, in the following referred to as tetrapyrroles), and, thus, coordinativelyunsaturated. This unsaturated character, resulting in two vacant axial coordina-tion sites, is a central reason for the outstanding importance of this class ofmolecules in biological systems, in which they represent the active centers ofmany enzymes. For example, the ubiquitous heme-thiolate proteins (with thecytochrome P450 type enzymes as an important class of representatives) containan iron-porphyrin as the active center [1, 2]. The axial thiolate ligand attachedto the Fe center plays an important role in controlling the reactivity of the activesite, especially with respect to its ability to reduce oxygen. Other importantexamples are magnesium porphyrins in chlorophyll [3], a cobalt corrin in cobala-min (vitamin B12) [4], or an iron porphyrin in heme, which is essential for theoxygen transport in the blood of mammals [1].

Despite the fact that the metal center is undercoordinated, metallo-tetrapyr-roles are often remarkably stable, presumably because of the tetradentate natureof the ligand. This unique combination of global stability and local reactivity isprobably the reason why porphyrins were so successful in evolution as the build-ing blocks of enzymes.

If metallo-tetrapyrroles are adsorbed on metal surfaces, they usually assumea geometry with the porphyrin plane parallel to the surface – both on the solid.

vacuum [5–12] and the solid.liquid interface [13, 14]. In this geometry, one ofthe two axial coordination sites is occupied by the underlying surface, which canact as an unconventional axial ligand and can influence the electronic structure(and, thus, the chemical reactivity) of the metal center [15, 16]. The remainingaxial site is now exposed to the surrounding medium (liquid or gas) and repre-sents a reactive center with potential catalytic [17–19], or sensor functionality[20–26].

For example, cobalt(II) tetraphenylporphyrin (CoTPP) supported on TiO2 pow-der proved to be an efficient de-NOx catalyst, i.e., it catalyzes the reduction of NOand NO2 with CO or H2 [17, 27–29]. Since neither the unsupported CoTPP norTiO2 are active alone, there must be some synergistic interplay between CoTPP andTiO2, causing the catalytic activity. Based on EPR and UV-VIS data obtained frompowder samples, it was proposed that modification of the electronic structure of theCo ion, caused by electron transfer from the TiO2 support, is responsible for thecatalytic activity [17]. Although rather speculative at its time, this approach – tofocus on the local interaction between metal center and support to understand theelectronic structure and reactivity of adsorbed coordination compounds – provedto be visionary and has since been supported by a number of studies on more ele-mentary, better defined systems [11, 15, 16]. It also illustrates one central point ofthe work summarized in this article: the control of the reactivity of the metal centerby the interaction with the support. In principle, the strength of this interaction canbe influenced by changing the chemical nature of support or metal ion and by ad-justing the distance between the two [15].

Metalloporphyrin monolayers or thin films have also been employed as sen-sors [20–26]. Recently, the development of an electric sensor was reported in



Surface-Confined Coordination Chemistry with Porphyrins … 55

which a monolayer of a substituted zinc porphyrin, covering the gate of a transis-tor (SOI-MOSFET), represents the active element for the detection of nitrogenbases such as pyridine [26]. The coordination of the N bases to the metal centersleads to changes in the drain current. Suitable for repeated use due to the reversi-ble coordination of the N bases, this sensor detects amounts below one femto-mole and can be rapidly reset by exposure to light, possibly because of photo-induced cleavage of the coordination bond.

In the following, we will discuss recent advances on the field of surface-confined coordination chemistry with metallo-tetrapyrroles. Although focusingon the solid.vacuum (particularly metal.vacuum) interface, we will also draw,when appropriate, connections to related work at the solid.liquid interface,which has been reviewed recently [13]. In addition, we will discuss the relevanceof these fundamental studies for potential applications in catalysis and sensortechnology.

2. Structure: Intramolecular conformation andsupramolecular ordering aspects of tetrapyrroles onsurfaces

The structural characterization of metallo-tetrapyrroles on various surfaces is afast-growing field due to their potential applications in functional devices basedon self-assembled monolayers. The scanning tunneling microscope (STM) is byfar the most prominent tool to study the microscopic properties in terms oftopography and local electronic structure of large molecules and extended supra-molecular structures on flat substrates [5, 6, 30]. If not stated differently, in allcases discussed below the tetrapyrroles are lying flat on the substrates, i.e., theplane of the macrocycle is oriented parallel to the surface plane. Two mainfeatures are in the focus of STM investigations of tetrapyrroles: a) the intramo-lecular conformation.electronic structure of individual molecules and b) the longrange order of self-assembled islands or monolayers.

In a low temperature (LT) STM under ultra-high vacuum conditions, thedirect investigation of isolated adsorbed tetrapyrrole molecules is possible [31–36]. For zinc(II)-etioporphyrin I (ZnEP), adsorbed on NiAl(110) at 13 K, Qiu etal. found that this molecule exists in two different conformations and can bereversibly switched between the two states by applying voltage pulses (1.8 Vand –1.8 V for 100 ms) via the STM tip [34]. The same group investigatedZnEP on a thin oxide film (Al2O3.NiAl(110)), where the molecule exhibited sixdistinguishable conformations [31]. Here, the STM tip was used to characterizethe local electronic structure by acquiring dI.dV spectra and to locally excitephotons; interestingly, the corresponding conformations are associated with dif-ferent electronic fingerprints and only two out of the six molecular conformationsluminesced, thus demonstrating that the electronic properties can strongly dependon the actual molecular conformation [31]. In another set of single molecule




experiments performed by Moresco et al., the STM tip was used at ~15 K tolaterally manipulate the comparably bulky copper(II)-5,10,15,20-tetrakis-(3,5-di-tert-butyl)-phenylporphyrin (CuTTBPP) on Cu(211) and Cu(100) [32, 33]. It wasfound that the dihedral angle between the di-tert-butyl-phenyl (DBP) ligands(with the attached tert-butyl groups) and the porphyrin plane changes upon lateralmanipulation, demonstrating the ability of the large molecule to conformationallyadapt to the surface topography.

A similar effect had already been described in 1997 by Jung et al., whoreported that the overall conformation of CuTTBPP can be described by anantisymmetric rotation of two opposite DBP substituents. The authors observedthat the corresponding deformations of the molecule in STM depend on theactual substrate (Cu(100), Au(110), Au(110), and Ag(110)) [8]. The resultingdihedral angle of the DBPs is then interpreted as a result of the delicate balanceof an attractive molecule-substrate interaction and the steric repulsion betweenthe ortho-substituents. It is important to note that the investigations were per-formed on self-assembled arrays of CuTTBPP, which exhibited different molecu-lar arrangements depending on the actual substrate and, thus, the intramolecularconformations. Recently, Buchner et al. found four different phases (three coex-isting phases at room temperature and one extremely stable and well orderedherringbone arrangement formed upon thermal activation, see Fig. 1d–f) for thesimilar CoTTBPP molecule on Ag(111) [12]. To achieve a consistent picture ofthe deformations of the individual molecules in STM, an additional tilt angle ofthe DBP substituents had to be taken into account. Regarding the significantlydifferent intramolecular conformations of the CoTTBPP molecule on the samesubstrate, also intermolecular interactions within the long-range ordered phasesmust contribute to the deformations of the individual molecules. The diversityof monolayer structures found for CuTTBPP and CoTTBPP suggests that theinteractions that finally lead to the formation of self-assembled arrays of tetrapyr-roles might be very complex and depend on various parameters. The role of theside groups can also be illustrated by comparing results for tetraphenylporphyrins(TPP) and tetrapyridylporphyrins (TPyP), which differ only in that one carbonatom in each side-group is replaced by a nitrogen atom. TPP molecules generallytend to arrange in a square order with a lattice constant of ~1.4 nm (for example,different TPPs on Cu(111) [37], Au(111) [9, 38], and Ag(111) [39–43], see alsoFig. 1a–c), whereas TPyP molecules arrange in a herringbone structure onAg(111) [37, 42]. From these observations, at least two conclusions can bedrawn: i) the ordering of TPP is independent of the substrates and the centralmetal atoms tested in the works cited above, ii) the differences in the moleculararrangements of TPP and TPyP on the same silver substrate must be solely dueto the slight variation in the corresponding side group. For the generation oftailored supramolecular tetrapyrrole networks, the variation of side groups ap-pears to be a suitable tool. An example for this approach is given by Veld et al.,who reported the formation of covalently linked tetraarylporphyrins (TAP) onCu(110) after a thermal treatment [44]. A similar reaction was studied by Grill




Fig. 1. Constant current STM images of self-assembled monolayers of CoTPP (left, a–c, ISet =56 pA, UGap = –1.2 V) and CoTTBPP (right, d–f, ISet = 300 pA, UGap = +1.2 V) acquired atroom temperature with the corresponding detail enlargements and molecular models. The differ-ences in the long-range order are obvious: the CoTPP layer exhibits a square order with alattice constant of ~1.4 nm, whereas the CoTTBPP molecules arrange in a highly interwovenherringbone structure [12]. The appearance of the CoTPP is dominated by a saddle shapedeformation of the macrocycle (brighter atoms in c). The four bright spots in the CoTTBPPimage in e can be attributed to the upper tert-butyl groups (marked black in f), and the fourslightly dimmer spots can be identified with the lower tert-butyl groups. The visibility of bothgroups is in line with the DBP plane rotated only 20° out of the porphyrin plane.

et al., who observed the formation of covalently linked porphyrin arrays onAu(111) upon loss of Br atoms from bromophenyl porphyrins [45]. A zinc-porphyrin with two attached DBPs and two cyano-phenyl (CP) groups at oppo-site sides of the macrocycle self-arranges in a porous network [46–48]. The porescan either serve as a host system for guest molecules such as C60 [47] or can actas bearings for multiposition rotary devices for the latter porphyrin derivates[46]. This concept can be expanded by linking two zinc-porphyrins (triply fuseddi-porphyrins), again with alternatingly attached DBP and CP groups, resultingin altered pore distances [48]. There are other examples from the infinite possibil-ities to combine the different building blocks, all yielding individual structuralthemes, such as the assembly of meso-tetramesitylporphyrin (TMP) on Cu(100)[49] or the formation of copper phthalocyanine (Pc) or free-base porphyrin deri-vates with attached alkane chains on a graphite substrate [50]. Another way to




generate distinct assemblies is to mix different tetrapyrrolic molecules such asF16CoPc and NiTPP on Au(111), which leads to the formation of well definedassemblies with alternating rows of the two species.

Other objects of STM investigations are dynamic processes such as the nucle-ation and supramolecular assembly of PtTTBPP on Cu(100) at 80 K, where thenucleation process could be described with a 2D gas-solid phase transition model[51], or the observation of rotating tetrapyrrole derivatives [46, 52].

The aforementioned mobility of PtTTBPP on Cu(100) at 80 K [51] is exem-plary for most of the systems discussed above. A strategy to immobilize themolecules is the anchoring of thiol-functionalized tetrapyrrols on gold substrates[18, 53]. Within this approach, completely different adsorption geometries havebeen reported; for example, for a thiol-functionalized cobalt porphyrin anchoredon Au(111), the orientation of the macrocycle is almost perpendicular to thesurface [18].

Even though STM is clearly a powerful tool to study the supramolecularassembly and intramolecular conformation of tetrapyrroles on various surfaces,it is of limited accuracy in the determination of the distances between the atomsin the molecule and the substrate surface. Such distances have been successfullymeasured with normal-incidence X-ray standing waves (NI-XSW) [54–56].Tin(II)-phthalocyanine (SnPc) on Ag(111) was independently studied by Stadleret al. [54] and Wooley et al. [55]; this complex has a bowl-shaped gas phasestructure, because the Sn ion sits outside the molecular plane [54]. For an incom-mensurate monolayer phase at room temperature, Stadler et al. found adsorptionin “tin-down” geometry with the Sn ion located 2.41 Å above the Ag(111) sur-face plane. The peripheral benzene rings are bent down towards the surface,supposedly because of chemisorptive interactions. For a commensurate, low-temperature sub-monolayer structure, a mixed “Sn-down” (2.59 Å) and “Sn-up”(4.01 Å) geometry was observed [54]. A different incommensurate monolayerphase was studied by Wooley et al., who reported a distance of 2.31 Å betweenSn ion and Ag(111) surface plane [55]. In contrast to Stadler's work, bending ofthe molecule away from the surface was reported, resulting in a large Sn-Cseparation of 1.3 Å (compared to only 0.76 Å for Stadler's incommensuratephase). Gerlach et al. showed with NI-XSW that F16CuPc adsorbs in a flat geom-etry on Cu(111) and Ag(111), but is significantly distorted because of partialrehybridization (sp2 / sp3) of the peripheral C atoms [56].

3. Formation: In-situ synthesis of adsorbed metallo-tetrapyrrole complexes

In most cases, adsorbates of metallo-tetrapyrroles have been prepared by physicalvapour deposition (PVD) of the intact, ex-situ prepared [57] complexes. An alter-native, recently reported route is the direct metalation of the pre-adsorbed, metal-free ligands with vapour-deposited metal atoms [39, 41–43, 49, 58–63]. This




Fig. 2. Series of STM images after the repeated vapor deposition of 0.010 ML Co (1 MLdefined as one Co atom per Ag surface atom) onto a monolayer of 2HTPP on Ag(111). Themetalated CoTPP molecules appear as protrusions. Considering the stoichiometric amount of0.037 ML Co nominally needed to metalate all 2HTPP molecules, it becomes evident that theyield of the process is close to 100%. In the statistical process of the impinging of the evaporatedCo atoms, it is extremely unlikely that every atom directly hits the right coordination site, i.e.,the center of the porphyrin macrocycle. Therefore, one has to assume that Co reaches the lattersite in a diffusive, i.e., surface mediated process. All images were acquired with tunnelingparameters in the given range, ISet = 43±4 pA and UGap = –1.20±0.05 V.

surface-confined redox reaction involves the oxidation of the metal atoms to therespective dications and the release of the pyrrolic hydrogen as H2, according tothe equation:

(1)in which tetraphenylporphyrin (2HTPP) is used as an example. This in-situ ap-proach is advantageous in the case of temperature sensitive or very reactivemetallo-tetrapyrroles. For example, it allows for the preparation of clean monol-ayers of iron(II)-porphyrin, which is oxidation sensitive and therefore difficultto handle outside the vacuum [39, 41, 42, 49, 58]. In most cases, the stoichiomet-ric amount of the metal was vapour-deposited onto the monolayer of the ligand(Fig. 2), but the reverse order of deposition is also possible (see below) [41, 61].The feasibility of the in-situ metalation has first been demonstrated for Fe [39,41, 42, 49, 58, 63] and Co [59, 60], but has since been extended to other metalssuch as Zn [61, 62] and Ce [43]. For some metals, the metalation reaction pro-




Fig. 3. N 1s X-ray photoelectron spectra monitoring the metalation of a 2HTPP monolayer onAg(111) with Zn. The spectrum of the 2HTPP monolayer (top) shows two distinct componentsfor iminic (-N =) and pyrrolic (-NH-) nitrogen. Since the formation of ZnTPP is slow at 300 K,an intermediate (initial complex) is observed at this temperature, which has a “sitting-atop”structure, according to DFT calculations (see Fig. 4). Heating to 550 K leads to rapid reactionof the intermediate to ZnTPP. The spectrum of a monolayer of directly deposited ZnTPP (bot-tom) confirms that the surface-confined metalation reaction indeed leads to ZnTPP. Line colorsin the signal deconvolution: red: 2HTPP, orange: initial “sitting-atop” complex, green: ZnTPP.Adapted from ref. [60].

ceeds instantaneously at room temperature (Fe, Co), while others require elevatedtemperature (for example Zn, see Fig. 3).

Most studies focussed on the metalation of 2HTPP [39, 41, 43, 59–62], butalso tetramesitylporphyrin [49], tetrapyridylporphyrin [42], and phthalocyanine[63] have been metalated successfully. In both of the latter cases, the metal atomsmay also coordinate to the peripheral N atoms; however, this side reaction hasonly been observed in the case of tetrapyridylporphyrin at low temperatures [36].The most common substrate in these studies was Ag(111); metalation of meso-tetramesitylporphyrin with Fe was performed on Cu(100) [49].

Regarding the mechanism of the metalation reaction, it is reasonable to as-sume that the metal atoms are trapped anywhere on the (ligand-covered) surfaceand then diffuse, until they are eventually coordinated and oxidized. This impliesthat the atoms are sufficiently mobile on the surface in the presence of the li-gands. Alternatively, steering effects which guide the metal atom directly to the




coordination sites could be taken into account, but appear unlikely consideringthe relatively large distances of ~14 Å between these sites.

If coordination is generally preceded by diffusion of the metal atoms, thenthe tetrapyrrole ligands should also be able to “pick up” pre-adsorbed metalatoms. This is indeed the case, as was shown for metalation of 2HTPP with Znand Fe [41, 61]. STM images show that Fe atoms, when deposited on Ag(111),nucleate and form clusters at the step edges. Subsequent deposition of 2HTPP,in combination with elevated temperatures, caused dissolution of the islands dueto coordination of the Fe atoms. This process was fast at 550 K, but slow at293 K, probably because the 2D vapour pressure of the Fe islands is very lowat 293 K and limits the reaction rate [41].

Interestingly, reaction with atoms of the Ag substrate, resulting in the forma-tion of AgII complexes, has not been observed up to now, probably because the+2 oxidation state (as required by the stoichiometry) is not preferred by Ag [41,61]. However, STM images of tetradodecylporphyrin on Au(111) show a sur-face-induced distortion, which was attributed, based on X-ray photoelectronspectroscopy (XPS) data, to the coordination of the iminic nitrogen atoms to theAu surface [63]. Possibly, this distorted geometry represents an intermediatestate of the metalation reaction, which cannot be completed because oxidationof the Au atom to its dication is energetically disadvantageous.

Metalation with relatively large atoms such as Ce leads to metalloporphyrinsin which the metal ion sits outside the molecular plane [43]. This could possiblyallow for the in-situ synthesis of molecular rotors with a sandwich-type double-decker structure. The feasibility of such structures at surfaces was already dem-onstrated by the controlled deposition of double-decker Eu and Lu complexeswith one naphthocyanine and one porphyrin deck; the complexes were depositedon a graphite.phenyloctane interface and studied by STM [65].

Metalation of porphyrin multilayers is also possible, as has been demon-strated with the formation of FeTPP upon iron deposition onto 2HTPP multilay-ers [41]. The efficiency of the reaction is lower than in the case of the monolayers(for which yields of up to 95% have been reported [63]), because metalationcompetes with the formation of Fe clusters in the multilayers. Procedure optimi-zation such as simultaneous deposition of metal and ligand may lead to improvedyields [41].

Metalation of adsorbed porphyrins has also been observed at the solid-liquidinterface: The coordination of Ag by tetrakis(1-methylpyridyl)porphyrin(2HTMPyP), adsorbed on Ag colloids, was observed with surface-enhanced Ra-man spectroscopy. These findings contrast the aforementioned studies on theAg.vacuum interface, where no reaction with the Ag surface occurred. However,the reaction at the solid.liquid interface is probably assisted by co-adsorbates,because a strong dependency of the kinetics on co-adsorbed anions such as borateor citrate was found [66]. Silica gel with adsorbed Zn(II) ions rapidly metalates2HTPP in various organic solvents under formation of ZnTPP, possibly in aheterogeneous reaction [67]. Electrochemically induced metalation of 2HTMPyP




Fig. 4. Schematic energy diagram of the metalation of porphyrin with Zn, according to gas-phase DFT calculations (adapted from [60]). Energies are given in kJmol–1, bond lengths in Å.The pyrrolic hydrogen atoms are marked in red.

with Cu was observed on a chloride precovered Cu(100) surface at very positivepotentials near the onset of copper dissolution [68].

The mechanism of the metalation reaction was clarified by means of gas-phase density functional theory (DFT) calculations for unsubstituted porphyrin incombination with kinetic and spectroscopic measurements of the surface reaction(Fig. 4) [60]. The DFT calculations predicted that the reaction proceeds in threesteps; the first, barrierless step is the coordination of the neutral metal atom bythe four nitrogen atoms of the intact porphyrin molecule. This results in theformation of an intermediate, in which the metal sits outside the porphyrin plane,because the two opposing pyrrolic nitrogen atoms still carry H atoms (Fig. 4b,see also Fig. 3 for an experimental verification of the intermediate). Thus, thisintermediate resembles the “sitting atop complex” proposed for the metalationof porphyrins in solution with metal ions [69–74]. The main differences betweensurface and solution chemistry concern the different charges of the respectiveintermediates (+2 in solution, neutral at the surface) and their further reactions.In solution, the pyrrolic hydrogen atoms are released as (solvated) H+ ions,whereas the direct metalation is completed by release of H2 [60]. The DFTcalculations show that the H atoms first migrate to the metal center, where theyrecombine (Fig. 4c–f). The H migration proceeds in two separate steps, whichcan be barrierless (Fe), but can also require small (Co) or substantial (Zn) activa-tion energies [60]. The release of H2, which can be detected by mass spectrome-




try, provides a convenient monitor for the reaction progress. This was used todetermine the overall activation energy for the metalation of 2HTPP with Zn.By means of a temperature-programmed reaction experiment, a Gibbs activationenergy of 134 kJ.mol was determined, which agrees with the theoretical valuefor the highest barrier (136 kJ.mol for the first hydrogen transfer step) [60]. Thecomputed energies for the gas phase reaction in Fig. 4 should be representativefor multilayer metalation [41], because the intermolecular van-der-Waals interac-tions are comparatively weak compared to the coordinative bond between metaland porphyrin [60]. Whether the computational results are applicable to the reac-tion in the monolayer, however, depends critically on the strength of the interac-tion between metal atom.ion and surface, which in turn depends on the elec-tronic structure of the metal atom.ion. We will focus on this topic in thefollowing chapter.

4. Electronic interaction of metallo-tetrapyrroles withsurfaces

The electronic interaction of metallo-tetrapyrroles with surfaces has only recentlycome into the focus of interest from both the theoretical and the experimentalside. A major problem for theoretical studies using DFT methods is the (presum-ably) large van-der-Waals contribution to the surface chemical bond, becauseDFT is notorious for its tendency to fail in treating dispersion interactions [75].However, in the local interaction of the coordinated metal center with the surface,covalent contributions may prevail. In such cases, DFT can at least provideinformation about the electronic state of the metal center in the presence of thesurface. With regard to the reactivity of the system, this information may bemore valuable than the total adsorption energy or details of the adsorption geom-etry. For example, DFT investigations of Mn and Pd porphyrins on the Au(111)predicted the existence of a covalent or metallic bond between the Mn ion andthe Au surface, accompanied by a shift of electron density from Mn 3p and 3dorbitals to Au(111) surface. This interaction also changes the spin state and leadsto an out-of-plane displacement of the Mn ion toward the surface by 0.2 Å. Pdporphyrin binds less strongly and less site specific to Au(111) than Mn porphyrin[76]. In other DFT studies, dealing with Pd porphyrins on Au(111) [77] andAl(111) [78], it was assumed that the molecules stand vertically on the surface,a configuration which has experimentally not been verified, at least not for mon-olayer and submonolayer coverages. Several early DFT studies of porphyrinadsorption on Au(111) are summarized in ref. [76]. Recently, the adsorption ofiron(II)-porphyrin (FeP) molecules on Ag(111) was studied with DFT, using afixed molecule-to-surface distance (5.6 Å) at the four meso carbon atoms; thisdistance value was obtained from calculations on meso-tetrapyridylporphyrin onthe same surface [79]. In the adsorbed state of the FeP complex, the Fe ionremained in the porphyrin plane and had a magnetic moment similar to that in




the gas phase, suggesting only little electronic interaction with the surface. Inaddition, STM images of the molecules in vacuum were simulated and comparedto experimental STM data. Because of the neglect of the surface and the use ofa truncated, partly rigid molecular geometry, the agreement was limited. Theexperimental STS data of FeTPyP and 2HTPyP show that insertion of the Fe ioncauses an additional signal at and directly below the Fermi energy (between 0and 0.5 eV) and a shift of the lowest unoccupied levels to higher energy by0.3 eV [79]; both observations suggest that there is indeed substantial electronicinteraction between the Fe ion and the surface.

Various investigations of the metal.metalloporphyrin interface with photoe-lectron spectroscopy focused on work function changes and energy-level align-ment [80, 81]. With respect to the local interaction of the coordinated metal ionand the substrate, cobalt(II) complexes have been studied most extensively. STMimages of submonolayers of cobalt(II)-phthalocyanine (CoPc) and cobalt(II)-tet-raphenylporphyrin (CoTPP) on Au(111) show almost identical constant-currentcontours over the central cobalt ions of CoTPP and CoPc [11]. This result indi-cates that the tunneling current at the Co ion, presumably mediated by the 3dz2

orbital, differs by less than a factor of 10 (under comparable conditions) and thattherefore the electronic interaction between this orbital and the surface must besimilar in both cases. This is remarkable, since the Co-Au distance of adsorbedCoTPP was believed to be 1.5 Å larger than the respective distance in the caseof CoPc (because the peripheral phenyl groups on CoTPP act as spacers) [11].On the other hand, tunneling spectra of CoTPP and CoPc are different and allowfor the unambiguous identification of these species [11]. In a related study [9],differences in the STM images of CoTPP and NiTPP on Au(111) were explainedwith the different occupation of the 3dz2 orbital of the metal ion (half occupiedin Co(II), fully occupied in Ni(II)), which again implies that tunneling is partlymediated by this orbital and that an electronic coupling to the substrate exists.In two earlier STM studies of various metallophthalocyanines on Au(111), per-formed in the same laboratory, the different appearances of CoPc and CuPc (d7

and d9) [82] and of FePc and NiPc (d6 and d8) [83] could be explained with thedifferent contributions of the metal d orbitals to the electron density near theFermi edge.

Expanding on the aforementioned studies [11, 82, 83], the interaction ofCoTPP and CoTTBPP with a Ag(111) surface was studied in detail with X-rayand UV photoelectron spectroscopy (XPS and UPS) [15]. The XP spectra ofmultilayers of both complexes show a Co 2p signal with a position typical ofCo(II) and some multiplet splitting because of the open-shell character of theCo(II) ion (d7). At monolayer coverage, however, the respective signals were




Fig. 5. Interaction of CoTPP with a Ag(111) surface. Left: Co 2p3.2 XP spectra of multilayersand monolayer of CoTPP on Ag(111). The signal has multiplet structure due to the open-shellcharacter (d7) of the Co(II) ion. The maximum of the multilayer signal appears at a typicalCo(II) position, whereas the monolayer spectrum is shifted to lower binding energy due to theelectronic interaction with the surface. Right: He-I UP spectra of multilayers and monolayer ofCoTPP on Ag(111). The signal at 2.3 eV in the multilayer spectrum (top) is associated withthe HOMO.SOMO of the molecule. In the monolayer spectrum (center), a new signal appearsat 0.6 eV, which results from the electronic interaction between the Co ion and the Ag surface.Accordingly, this signal is missing in the monolayer spectrum of the metal-free ligand (bottom).Adapted from [15].

shifted to lower binding energy by 1.8 eV (Fig. 5, left), which exceeds the shiftof the C 1s and N 1s signals (–0.2 eV) by far. It was suggested that electrontransfer from the Ag surface to the Co ion, resulting in a partial reduction of theCo ion, is responsible for the shift. This conclusion is in line with earlier studieson CoTPP on TiO2 powder, in which the catalytic activity of the system wasrelated to a modified electronic state of the Co ion, induced by electron transferfrom the TiO2 [17]. UP spectra of monolayers of CoTPP and CoTTBPP onAg(111) show a characteristic signal at 0.6–0.7eV below the Fermi energy, i.e.,approximately 1.0–1.2 eV above the signal of the SOMO of the complexes(Fig. 5, right) [15, 16, 59]. A similar signal was also observed in the tunnelingspectra of CoTPP on Ag(111) [40] and Au(111) [11], as well as for CoPc onAu(111) [84]. However, the signal has never been observed for multilayer cover-ages of the complexes, indicating that it results from the electronic interactionbetween the complexes and the surface [15]. This interaction may be dominatedby the metal center, the porphyrin ligand, or both. The UP spectrum of a monol-ayer of 2HTPP on Ag(111) (Fig. 5), in which the interaction-induced peak isabsent, indicates that the Co ion is strongly involved. It was therefore postulatedthat the signal at 0.6–0.7 eV results from the interaction between the Co 3dorbitals with states of matching energy and symmetry at the Ag surface [15].




Most likely, the 3dz2 orbital participates in this interaction, because it extendstowards the surface and is only half-occupied. It was proposed that the resultingtwo mixed Co 3dz2.Ag 5s MOs are both located below the Fermi energy andcan therefore be filled up with electrons from the Fermi sea. Since these twoMOs originally contain only one electron from the Co 3dz2 orbital, they can inprinciple accommodate up to three additional electrons from the Fermi sea ofthe Ag surface (Fig. 8, left). This electron transfer explains the drastic, surface-induced shift of the Co 2p photoemission signal [15, 16, 59].

The different size of the peripheral substituents in CoTPP and CoTTBPPshould allow for studying distance-dependent effects and indeed, small differen-ces in the UP spectra of these complexes were observed [15]. However, sinceno reliable measurements of the distance between the Co ion and the next-neigh-bour Ag atom are available, the interpretation of these effects remains ratherspeculative. This also holds for the previous discussion of the differences in thetunneling spectra of CoPc and CoTPP [11].

Only recently, the magnetic properties of adsorbed metalloporphyrins havecome into the focus of interest. Wende et al. studied iron-octaethylporphyrin(FeOEP) monolayers on thin ferromagnetic films of Ni and Co on Cu(100) withX-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism(XMCD) measurements and found ferromagnetic ordering of the metal com-plexes as well as ferromagnetic coupling with the underlying metal films [85].Supplementary DFT calculations suggest that the coordinated Fe ions do notdirectly interact with the surface; instead, an indirect super-exchange interactionmechanism is proposed in which the coordinating N atoms mediate the interac-tion with the substrate. These studies were extended to submonolayer and (thin)multilayer coverages, proving that only the complexes in direct contact to theferromagnetic substrate are magnetically ordered [86]. The FeOEP monolayerswere prepared from octaethylporphyrinato-iron(III) chloride, which is believedto lose the Cl atom at some not specified stage of the preparation procedure,resulting in the formation of octaethylporphyrinato-iron(II) [85, 86]. The Fe(II)ion apparently preserves its oxidation state when in contact to the surface [85,86], contrasting studies of iron(II)-tetraphenylporphyrin and iron(II)-phthalocya-nine on Ag(111), where substantial reduction of the oxidation state was observedwith XPS [41, 63].

5. Reactivity: Coordination of axial ligands and surfacetrans effect

Metallo-tetrapyrroles with coordinated M(II) ions usually have two vacant axialcoordination sites [87], which for example play an important role for the bio-chemical functionalities of these complexes [88]. Adsorption with the molecularplane parallel to the surface, as is observed in most cases, occupies one of these




Fig. 6. Surface-confined two-step synthesis of the complex (NH3)ZnTPP on Ag(111) from2HTPP, Zn, and NH3. In the first step, 2HTPP is metalated with Zn; thereafter, NH3 is coordi-nated to the Zn ion. The figure shows N 1s XP spectra of (A) a monolayerof 2HTPP on Ag(111)and (B) after deposition of a monolayer of 2HTPP 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 ofdirectly 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 depositedZnTPP for comparison, conditions as in (D). Line colors: red: 2HTPP, orange: ZnTPP, green:NH3. (Adapted from ref. [62].)

sites (see the previous chapter), while the remaining site is free for the coordina-tion of an additional ligand.

The first example for the controlled reversible attachment of an axial ligandto an adsorbed porphyrin was the coordination of the nitrogen base DABCO(1, 4-diazabicyclo[2.2.2]octane) to the metal center of ZnTTBPP molecules onAg(100), studied with STM [89]. Later, it was shown by XPS that NH3 coordi-nates to ZnTPP on Ag(111) below 130 K [62]. The NH3-Zn bond energy of40 kJmol–1 was determined by a temperature-programmed decomposition experi-ment. (Note that the NH3 coordination was also used as a reaction step in thesurface-confined two-step synthesis of (NH3)ZnTPP from 2HTPP, Zn, and NH3,see Fig. 6 [62]). Axial coordination was also employed for the realization of asurface-anchored, porphyrin-based molecular pinwheel, which was briefly men-tioned in Section 2. Its rotor consists of a ZnTTBPP molecule, which is pinnedto the surface by 4-methoxypyridine. It is believed that this ligand binds to theZn ion through the pyridine N atom, while it is bound to the Ag(100) surface




Fig. 7. Interaction of (NO)CoTPP with Ag(111). The axial NO ligand reduces the electronicinteraction between the Co ion and the Ag surface due to the surface trans effect. The Co2p3.2 XP spectra (left) show only a small shift of –0.4 eV between multilayer and monolayersignal, much less than for CoTPP without the NO ligand (–1.8 eV, see Fig. 5). Note that themultiplet structure seen in the case of CoTPP (Fig. 5) has vanished, because (NO)CoTPP is aclosed-shell system. In the UP spectra (right), the interaction-induced signal at 0.6 eV disap-pears, when NO is coordinated to the Co ion. Removal of the NO ligand by thermal desorptionat 550 K restores the signal. (Adapted from ref. [16].)

through the -OCH3 group. On this molecular support, the porphyrin can spin,resulting in a circular symmetry as observed by STM [52].

Axial coordination on NO to CoTPP on Ag(111) was used by Flechtner etal. to study the influence of an axial ligand on the interaction between the coordi-nated metal center and the surface [16]. This particular system is especiallyimportant for an atomistic understanding of the catalytic activity of supportedCoTPP in the NOx reduction [17]. In a combined XPS.UPS study [16], it wasfound that the Co 2p binding energies of multilayers and monolayers of(NO)CoTPP differ by only 0.4 eV (Fig. 7, left) and not, as was the case forCoTPP without NO ligand, 1.8 eV (see chapter 4). This result indicates that thesurface has less influence on the electronic structure of the Co ion if this ioncarries an axial NO ligand, a conclusion that was corroborated by UP spectra:In the valence region, CoTPP monolayers show a signal at 0.6 eV, which arisesfrom the electronic interaction between the Co ion and the Ag surface [15]. IfNO was attached to the Co ion, this signal disappeared, but was restored whenthe NO ligand was thermally desorbed (Fig. 7, right). To explain these findings,it was proposed that the interaction between the Co 3dz2 orbital and the Agsurface, which is present in adsorbed CoTPP, is replaced by a stronger interactionbetween the Co 3dz2 orbital and the π* orbital of NO. This interaction leads toa larger energetic separation between bonding and antibonding level, with the




Fig. 8. Molecular orbital (MO) model of the interaction of the half-occupied 3dz2 orbital of thecoordinated Co ion with the Ag(111) surface (left) and with the π* orbital of NO (right). Inthe presence of the NO ligand, the coordinative bond between the Co ion and the surface (left)is weakened due to the surface trans effect and largely replaced by a Co-NO σ-donor . π-acceptor bond (right).

result that the latter is now located above EF (Fig. 8, right). Therefore, it cannotbe occupied with electrons from the Fermi sea and the electron transfer from thesurface to the Co ion is suppressed, in agreement with the experiment [16].

This interpretation implies the existence of a competition between the twoaxial coordination bonds in which the Co ion is involved, the Co-surface and theCo-NO bond. Similar competitive effects are well known in molecular coordina-tion chemistry as “trans effect” and are especially manifest during substitutionreactions [90, 91]. For example, the substitution of a ligand (T2) in trans-positionto another ligand (T1) is accelerated, if T1 is a stronger σ-donor or π-acceptorligand than T2. As a result, T1 and T2 direct ligands entering the complexaccording to the strength of their trans effects in the trans-position. The relativestrength of the trans-directing influence is fairly constant and decreases in thefollowing order: CO > NO > PR3 > NO2> SCN–> I–> CH3 > Br–> Cl–> NR3 >H2O [90]. Obviously, the NO molecule exerts a relatively strong trans effect.Although it is still unclear where metal surfaces rank in this list, it is likely thattheir trans-directing influence is smaller than that of NO, as the strong suppress-ing effect of the NO ligand on the Co-surface interaction shows. From a mecha-nistic point of view, the trans effect is usually interpreted as follows: If T1 is aπ acceptor ligand, then the M-T1 σ-donor . π-acceptor bond withdraws electrondensity from the M-T2 bond in trans position. This weakens the M-T2 bond andactivates this position for a nucleophilic substitution [91]. In the case of thesurface trans effect on adsorbed metalloporphyrins, the situation is probably anal-ogous: Similar to CO and O2, NO is known for its strong π-accepting ability




in metalloporphyrins, resulting for example in the low-spin character of thesecomplexes. The metal surface, on the other hand, acts more like a donor ligand,as can be seen from the surface-induced XPS peak shifts to lower binding ener-gies. Therefore, it is not surprising that NO as an acceptor ligand exerts astronger trans effect than the metal surface. It remains to be seen whether systemswith the opposite behaviour (i.e., a surface with a strong trans effect weakeningthe bond to the axial molecular ligand) can be observed experimentally.

The trans competition between axial ligand and surface is not always ob-served: An example for which neither the surface has a substantial influence onthe metal ion nor the axial molecular ligand on the ion-surface interaction is(NH3)ZnTPP on Ag(111) [61]. This shows that the electron configurations ofmetal ion, molecular ligand, and surface play an important role. Hence, generali-zations about the character of the metal ion-to-surface bond should be avoidedbefore a detailed understanding at the atomistic level has been achieved for alarger number of systems.

6. Summary

Metalloporphyrins, metallophthalocyanines, and other tetrapyrrole complexeshave found increasing attention for the functionalization of surfaces, especiallywith respect to catalytic activity, sensor functionality, and applications in spin-tronic devices. This general, potential technologic interest has stimulated a widerange of research activities during the recent years, which resulted in a detailedunderstanding of the molecular ordering and intramolecular conformation of ad-sorbed metallotetrapyrroles, their electronic interaction with the surface, and theirreactivity towards atoms and molecules. It has been shown that the surface canstrongly influence the electronic structure of the coordinated metal centers andthat the strength of this interaction depends critically on the electronic structureof the metal center and its further ligands. In addition, novel routes for the in-situ preparation of metallo-tetrapyrroles and their complexes with axial ligandshave been described, which allow for the preparation of temperature, air, ormoisture sensitive complexes directly on the surface in ultra-high vacuum. Mostof the previous work was limited to metal surfaces, while interesting systemswith catalytic activity or sensor functionality consist of metallo-tetrapyrroles onoxide surfaces. To obtain a similarly detailed knowledge of these systems as haspreviously been obtained for metal surfaces is the major challenge on this fieldin the near future [92].

Acknowledgement

JMG gratefully acknowledges Prof. Dr. Klaus Christmann's inspirational mentor-ship and support during his doctoral studies at the Freie Universität Berlin (1999–2003) and sincerely thanks Prof. Dr. Charles T. Campbell for his kind hospitality




during August.September 2008, when this manuscript was written. We thankProf. Dr. Hans-Peter Steinrück for stimulating discussions and Yun Bai, FlorianBuchner, Dr. Ken Flechtner, and Martin Schmid for providing the figures. Thiswork was supported by the Deutsche Forschungsgemeinschaft through Sonder-forschungsbereich 583.

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