Surface-Confined Coordination Chemistry with Porphyrins and Phthalocyanines: Aspects of Formation, Electronic Structure, and Reactivity

Abstract

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

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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, Universität
Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany
Dedicated to Prof. Dr. Klaus Christmann on the occasion of his 65
th
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 element
with an active site. While earlier studies of adsorbed metallo-tetrapyrroles mainly addressed
aspects of molecular self-assembly, the focus of interest has shifted gradually to electronic
structure and chemical reactivity. This article gives an overview of recent advances in the field
of 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 novel
catalysts, sensors, and other devices that are based on the interaction of an active
surface with the surrounding medium. Metalloporphyrins and similar planar
metal complexes are especially suitable for this task, because they combine a
structure forming element (the porphyrin framework) with an active site, usually
the coordinated metal center. In the free complex, this metal center is often
coordinated by only the tetradentate, planar ligand (porphyrin, phthalocyanine,
*
Corresponding author. E-mail: michael.gottfried@chemie.uni-erlangen.de

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54
J. M. Gottfried and H. Marbach
or corrole, in the following referred to as tetrapyrroles), and, thus, coordinatively
unsaturated. This unsaturated character, resulting in two vacant axial coordina-
tion sites, is a central reason for the outstanding importance of this class of
molecules in biological systems, in which they represent the active centers of
many enzymes. For example, the ubiquitous heme-thiolate proteins (with the
cytochrome P450 type enzymes as an important class of representatives) contain
an iron-porphyrin as the active center [1, 2]. The axial thiolate ligand attached
to the Fe center plays an important role in controlling the reactivity of the active
site, especially with respect to its ability to reduce oxygen. Other important
examples 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 the
oxygen 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 nature
of the ligand. This unique combination of global stability and local reactivity is
probably 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 assume
a 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 of
the two axial coordination sites is occupied by the underlying surface, which can
act as an unconventional axial ligand and can influence the electronic structure
(and, thus, the chemical reactivity) of the metal center [15, 16]. The remaining
axial 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 TiO
2
pow-
der proved to be an efficient de-NO
x
catalyst, i.e., it catalyzes the reduction of NO
and NO
2
with CO or H
2
[17, 27–29]. Since neither the unsupported CoTPP nor
TiO
2
are active alone, there must be some synergistic interplay between CoTPP and
TiO
2
, causing the catalytic activity. Based on EPR and UV-VIS data obtained from
powder samples, it was proposed that modification of the electronic structure of the
Co ion, caused by electron transfer from the TiO
2
support, is responsible for the
catalytic activity [17]. Although rather speculative at its time, this approach –to
focus on the local interaction between metal center and support to understand the
electronic structure and reactivity of adsorbed coordination compounds –proved
to 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 of
the work summarized in this article: the control of the reactivity of the metal center
by the interaction with the support. In principle, the strength of this interaction can
be 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

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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 nitrogen
bases such as pyridine [26]. The coordination of the N bases to the metal centers
leads 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 focusing
on 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 relevance
of these fundamental studies for potential applications in catalysis and sensor
technology.
2. Structure: Intramolecular conformation and
supramolecular ordering aspects of tetrapyrroles on
surfaces
The structural characterization of metallo-tetrapyrroles on various surfaces is a
fast-growing field due to their potential applications in functional devices based
on self-assembled monolayers. The scanning tunneling microscope (STM) is by
far the most prominent tool to study the microscopic properties in terms of
topography and local electronic structure of large molecules and extended supra-
molecular structures on flat substrates [5, 6, 30]. If not stated differently, in all
cases discussed below the tetrapyrroles are lying flat on the substrates, i.e., the
plane of the macrocycle is oriented parallel to the surface plane. Two main
features are in the focus of STM investigations of tetrapyrroles: a) the intramo-
lecular conformation.electronic structure of individual molecules and b) the long
range order of self-assembled islands or monolayers.
In a low temperature (LT) STM under ultra-high vacuum conditions, the
direct investigation of isolated adsorbed tetrapyrrole molecules is possible [31–
36]. For zinc(II)-etioporphyrin I (ZnEP), adsorbed on NiAl(110) at 13 K, Qiu et
al. found that this molecule exists in two different conformations and can be
reversibly switched between the two states by applying voltage pulses (1.8 V
and –1.8 V for 100 ms) via the STM tip [34]. The same group investigated
ZnEP on a thin oxide film (Al
2
O
3
.NiAl(110)), where the molecule exhibited six
distinguishable conformations [31]. Here, the STM tip was used to characterize
the local electronic structure by acquiring dI.dV spectra and to locally excite
photons; interestingly, the corresponding conformations are associated with dif-
ferent electronic fingerprints and only two out of the six molecular conformations
luminesced, thus demonstrating that the electronic properties can strongly depend
on the actual molecular conformation [31]. In another set of single molecule

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56
J. M. Gottfried and H. Marbach
experiments performed by Moresco et al., the STM tip was used at ~15 K to
laterally 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 was
found that the dihedral angle between the di-tert-butyl-phenyl (DBP) ligands
(with the attached tert-butyl groups) and the porphyrin plane changes upon lateral
manipulation, demonstrating the ability of the large molecule to conformationally
adapt to the surface topography.
A similar effect had already been described in 1997 by Jung et al., who
reported that the overall conformation of CuTTBPP can be described by an
antisymmetric rotation of two opposite DBP substituents. The authors observed
that the corresponding deformations of the molecule in STM depend on the
actual substrate (Cu(100), Au(110), Au(110), and Ag(110)) [8]. The resulting
dihedral angle of the DBPs is then interpreted as a result of the delicate balance
of an attractive molecule-substrate interaction and the steric repulsion between
the 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 intramolecular
conformations. Recently, Buchner et al. found four different phases (three coex-
isting phases at room temperature and one extremely stable and well ordered
herringbone arrangement formed upon thermal activation, see Fig. 1d–f) for the
similar CoTTBPP molecule on Ag(111) [12]. To achieve a consistent picture of
the deformations of the individual molecules in STM, an additional tilt angle of
the DBP substituents had to be taken into account. Regarding the significantly
different intramolecular conformations of the CoTTBPP molecule on the same
substrate, also intermolecular interactions within the long-range ordered phases
must contribute to the deformations of the individual molecules. The diversity
of monolayer structures found for CuTTBPP and CoTTBPP suggests that the
interactions 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 the
side groups can also be illustrated by comparing results for tetraphenylporphyrins
(TPP) and tetrapyridylporphyrins (TPyP), which differ only in that one carbon
atom in each side-group is replaced by a nitrogen atom. TPP molecules generally
tend 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 also
Fig. 1a–c), whereas TPyP molecules arrange in a herringbone structure on
Ag(111) [37, 42]. From these observations, at least two conclusions can be
drawn: i) the ordering of TPP is independent of the substrates and the central
metal atoms tested in the works cited above, ii) the differences in the molecular
arrangements of TPP and TPyP on the same silver substrate must be solely due
to the slight variation in the corresponding side group. For the generation of
tailored 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) on
Cu(110) after a thermal treatment [44]. A similar reaction was studied by Grill

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Surface-Confined Coordination Chemistry with Porphyrins …
57
Fig. 1. Constant current STM images of self-assembled monolayers of CoTPP (left, a–c, I
Set
=
56 pA, U
Gap
=–1.2 V) and CoTTBPP (right, d–f, I
Set
= 300 pA, U
Gap
= +1.2 V) acquired at
room 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 a
lattice constant of ~1.4 nm, whereas the CoTTBPP molecules arrange in a highly interwoven
herringbone structure [12]. The appearance of the CoTPP is dominated by a saddle shape
deformation of the macrocycle (brighter atoms in c). The four bright spots in the CoTTBPP
image in ecan be attributed to the upper tert-butyl groups (marked black in f), and the four
slightly dimmer spots can be identified with the lower tert-butyl groups. The visibility of both
groups 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 on
Au(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 pores
can either serve as a host system for guest molecules such as C
60
[47] or can act
as bearings for multiposition rotary devices for the latter porphyrin derivates
[46]. This concept can be expanded by linking two zinc-porphyrins (triply fused
di-porphyrins), again with alternatingly attached DBP and CP groups, resulting
in altered pore distances [48]. There are other examples from the infinite possibil-
ities to combine the different building blocks, all yielding individual structural
themes, 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

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58
J. M. Gottfried and H. Marbach
generate distinct assemblies is to mix different tetrapyrrolic molecules such as
F
16
CoPc and NiTPP on Au(111), which leads to the formation of well defined
assemblies 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 the
nucleation 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 the
molecules is the anchoring of thiol-functionalized tetrapyrrols on gold substrates
[18, 53]. Within this approach, completely different adsorption geometries have
been reported; for example, for a thiol-functionalized cobalt porphyrin anchored
on Au(111), the orientation of the macrocycle is almost perpendicular to the
surface [18].
Even though STM is clearly a powerful tool to study the supramolecular
assembly and intramolecular conformation of tetrapyrroles on various surfaces,
it is of limited accuracy in the determination of the distances between the atoms
in the molecule and the substrate surface. Such distances have been successfully
measured with normal-incidence X-ray standing waves (NI-XSW) [54–56].
Tin(II)-phthalocyanine (SnPc) on Ag(111) was independently studied by Stadler
et al. [54] and Wooley et al. [55]; this complex has a bowl-shaped gas phase
structure, because the Sn ion sits outside the molecular plane [54]. For an incom-
mensurate monolayer phase at room temperature, Stadler et al. found adsorption
in “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 monolayer
phase was studied by Wooley et al., who reported a distance of 2.31 Å between
Sn ion and Ag(111) surface plane [55]. In contrast to Stadler's work, bending of
the molecule away from the surface was reported, resulting in a large Sn-C
separation of 1.3 Å (compared to only 0.76 Å for Stadler's incommensurate
phase). Gerlach et al. showed with NI-XSW that F
16
CuPc adsorbs in a flat geom-
etry on Cu(111) and Ag(111), but is significantly distorted because of partial
rehybridization (sp
2
/sp
3
) 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 physical
vapour 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

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Surface-Confined Coordination Chemistry with Porphyrins …
59
Fig. 2. Series of STM images after the repeated vapor deposition of 0.010 ML Co (1 ML
defined as one Co atom per Ag surface atom) onto a monolayer of 2HTPP on Ag(111). The
metalated CoTPP molecules appear as protrusions. Considering the stoichiometric amount of
0.037 ML Co nominally needed to metalate all 2HTPP molecules, it becomes evident that the
yield of the process is close to 100%. In the statistical process of the impinging of the evaporated
Co 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 latter
site in a diffusive, i.e., surface mediated process. All images were acquired with tunneling
parameters in the given range, I
Set
= 43±4 pA and U
Gap
=–1.20±0.05 V.
surface-confined redox reaction involves the oxidation of the metal atoms to the
respective dications and the release of the pyrrolic hydrogen as H
2
, according to
the 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 reactive
metallo-tetrapyrroles. For example, it allows for the preparation of clean monol-
ayers of iron(II)-porphyrin, which is oxidation sensitive and therefore difficult
to 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 metals
such as Zn [61, 62] and Ce [43]. For some metals, the metalation reaction pro-

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60
J. M. Gottfried and H. Marbach
Fig. 3. N 1s X-ray photoelectron spectra monitoring the metalation of a 2HTPP monolayer on
Ag(111) with Zn. The spectrum of the 2HTPP monolayer (top) shows two distinct components
for 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 reaction
of 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 colors
in 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 elevated
temperature (for example Zn, see Fig. 3).
Most studies focussed on the metalation of 2HTPP [39, 41, 43, 59–62], but
also tetramesitylporphyrin [49], tetrapyridylporphyrin [42], and phthalocyanine
[63] have been metalated successfully. In both of the latter cases, the metal atoms
may also coordinate to the peripheral N atoms; however, this side reaction has
only 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) surface
and then diffuse, until they are eventually coordinated and oxidized. This implies
that 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

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Surface-Confined Coordination Chemistry with Porphyrins …
61
coordination sites could be taken into account, but appear unlikely considering
the relatively large distances of ~14 Å between these sites.
If coordination is generally preceded by diffusion of the metal atoms, then
the tetrapyrrole ligands should also be able to “pick up” pre-adsorbed metal
atoms. This is indeed the case, as was shown for metalation of 2HTPP with Zn
and 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 due
to coordination of the Fe atoms. This process was fast at 550 K, but slow at
293 K, probably because the 2D vapour pressure of the Fe islands is very low
at 293 K and limits the reaction rate [41].
Interestingly, reaction with atoms of the Ag substrate, resulting in the forma-
tion of Ag
II
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 photoelectron
spectroscopy (XPS) data, to the coordination of the iminic nitrogen atoms to the
Au surface [63]. Possibly, this distorted geometry represents an intermediate
state of the metalation reaction, which cannot be completed because oxidation
of the Au atom to its dication is energetically disadvantageous.
Metalation with relatively large atoms such as Ce leads to metalloporphyrins
in which the metal ion sits outside the molecular plane [43]. This could possibly
allow 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 complexes
with one naphthocyanine and one porphyrin deck; the complexes were deposited
on 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 metalation
competes with the formation of Fe clusters in the multilayers. Procedure optimi-
zation such as simultaneous deposition of metal and ligand may lead to improved
yields [41].
Metalation of adsorbed porphyrins has also been observed at the solid-liquid
interface: 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 the
Ag.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 borate
or citrate was found [66]. Silica gel with adsorbed Zn(II) ions rapidly metalates
2HTPP in various organic solvents under formation of ZnTPP, possibly in a
heterogeneous reaction [67]. Electrochemically induced metalation of 2HTMPyP

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62
J. M. Gottfried and H. Marbach
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 positive
potentials 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 in
combination with kinetic and spectroscopic measurements of the surface reaction
(Fig. 4) [60]. The DFT calculations predicted that the reaction proceeds in three
steps; the first, barrierless step is the coordination of the neutral metal atom by
the four nitrogen atoms of the intact porphyrin molecule. This results in the
formation 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, this
intermediate resembles the “sitting atop complex” proposed for the metalation
of porphyrins in solution with metal ions [69–74]. The main differences between
surface and solution chemistry concern the different charges of the respective
intermediates (+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 H
2
[60]. The DFT
calculations show that the H atoms first migrate to the metal center, where they
recombine (Fig. 4c–f). The H migration proceeds in two separate steps, which
can be barrierless (Fe), but can also require small (Co) or substantial (Zn) activa-
tion energies [60]. The release of H
2
, which can be detected by mass spectrome-

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Surface-Confined Coordination Chemistry with Porphyrins …
63
try, provides a convenient monitor for the reaction progress. This was used to
determine the overall activation energy for the metalation of 2HTPP with Zn.
By means of a temperature-programmed reaction experiment, a Gibbs activation
energy of 134 kJ.mol was determined, which agrees with the theoretical value
for the highest barrier (136 kJ.mol for the first hydrogen transfer step) [60]. The
computed energies for the gas phase reaction in Fig. 4 should be representative
for multilayer metalation [41], because the intermolecular van-der-Waals interac-
tions are comparatively weak compared to the coordinative bond between metal
and 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 the
following chapter.
4. Electronic interaction of metallo-tetrapyrroles with
surfaces
The electronic interaction of metallo-tetrapyrroles with surfaces has only recently
come into the focus of interest from both the theoretical and the experimental
side. A major problem for theoretical studies using DFT methods is the (presum-
ably) large van-der-Waals contribution to the surface chemical bond, because
DFT 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 provide
information about the electronic state of the metal center in the presence of the
surface. With regard to the reactivity of the system, this information may be
more 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 and
the Au surface, accompanied by a shift of electron density from Mn 3p and 3d
orbitals to Au(111) surface. This interaction also changes the spin state and leads
to an out-of-plane displacement of the Mn ion toward the surface by 0.2 Å. Pd
porphyrin 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] and
Al(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 porphyrin
adsorption on Au(111) are summarized in ref. [76]. Recently, the adsorption of
iron(II)-porphyrin (FeP) molecules on Ag(111) was studied with DFT, using a
fixed molecule-to-surface distance (5.6 Å) at the four meso carbon atoms; this
distance value was obtained from calculations on meso-tetrapyridylporphyrin on
the same surface [79]. In the adsorbed state of the FeP complex, the Fe ion
remained in the porphyrin plane and had a magnetic moment similar to that in

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64
J. M. Gottfried and H. Marbach
the gas phase, suggesting only little electronic interaction with the surface. In
addition, STM images of the molecules in vacuum were simulated and compared
to experimental STM data. Because of the neglect of the surface and the use of
a truncated, partly rigid molecular geometry, the agreement was limited. The
experimental STS data of FeTPyP and 2HTPyP show that insertion of the Fe ion
causes an additional signal at and directly below the Fermi energy (between 0
and 0.5 eV) and a shift of the lowest unoccupied levels to higher energy by
0.3 eV [79]; both observations suggest that there is indeed substantial electronic
interaction 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 ion
and the substrate, cobalt(II) complexes have been studied most extensively. STM
images of submonolayers of cobalt(II)-phthalocyanine (CoPc) and cobalt(II)-tet-
raphenylporphyrin (CoTPP) on Au(111) show almost identical constant-current
contours 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 3d
z2
orbital, differs by less than a factor of 10 (under comparable conditions) and that
therefore the electronic interaction between this orbital and the surface must be
similar in both cases. This is remarkable, since the Co-Au distance of adsorbed
CoTPP was believed to be 1.5 Å larger than the respective distance in the case
of 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 allow
for 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 explained
with the different occupation of the 3d
z2
orbital of the metal ion (half occupied
in Co(II), fully occupied in Ni(II)), which again implies that tunneling is partly
mediated 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 (d
7
and d
9
) [82] and of FePc and NiPc (d
6
and d
8
) [83] could be explained with the
different contributions of the metal d orbitals to the electron density near the
Fermi edge.
Expanding on the aforementioned studies [11, 82, 83], the interaction of
CoTPP and CoTTBPP with a Ag(111) surface was studied in detail with X-ray
and UV photoelectron spectroscopy (XPS and UPS) [15]. The XP spectra of
multilayers of both complexes show a Co 2p signal with a position typical of
Co(II) and some multiplet splitting because of the open-shell character of the
Co(II) ion (d
7
). At monolayer coverage, however, the respective signals were

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Surface-Confined Coordination Chemistry with Porphyrins …
65
Fig. 5. Interaction of CoTPP with a Ag(111) surface. Left: Co 2p
3.2
XP spectra of multilayers
and monolayer of CoTPP on Ag(111). The signal has multiplet structure due to the open-shell
character (d
7
) of the Co(II) ion. The maximum of the multilayer signal appears at a typical
Co(II) position, whereas the monolayer spectrum is shifted to lower binding energy due to the
electronic interaction with the surface. Right: He-I UP spectra of multilayers and monolayer of
CoTPP on Ag(111). The signal at 2.3 eV in the multilayer spectrum (top) is associated with
the HOMO.SOMO of the molecule. In the monolayer spectrum (center), a new signal appears
at 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 shift
of the C 1s and N 1s signals (–0.2 eV) by far. It was suggested that electron
transfer from the Ag surface to the Co ion, resulting in a partial reduction of the
Co ion, is responsible for the shift. This conclusion is in line with earlier studies
on CoTPP on TiO
2
powder, in which the catalytic activity of the system was
related to a modified electronic state of the Co ion, induced by electron transfer
from the TiO
2
[17]. UP spectra of monolayers of CoTPP and CoTTBPP on
Ag(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 tunneling
spectra of CoTPP on Ag(111) [40] and Au(111) [11], as well as for CoPc on
Au(111) [84]. However, the signal has never been observed for multilayer cover-
ages of the complexes, indicating that it results from the electronic interaction
between the complexes and the surface [15]. This interaction may be dominated
by 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 is
absent, indicates that the Co ion is strongly involved. It was therefore postulated
that the signal at 0.6–0.7 eV results from the interaction between the Co 3d
orbitals with states of matching energy and symmetry at the Ag surface [15].

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66
J. M. Gottfried and H. Marbach
Most likely, the 3d
z2
orbital participates in this interaction, because it extends
towards the surface and is only half-occupied. It was proposed that the resulting
two mixed Co 3d
z2
.Ag 5s MOs are both located below the Fermi energy and
can therefore be filled up with electrons from the Fermi sea. Since these two
MOs originally contain only one electron from the Co 3d
z2
orbital, they can in
principle accommodate up to three additional electrons from the Fermi sea of
the 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 CoTTBPP
should allow for studying distance-dependent effects and indeed, small differen-
ces in the UP spectra of these complexes were observed [15]. However, since
no reliable measurements of the distance between the Co ion and the next-neigh-
bour Ag atom are available, the interpretation of these effects remains rather
speculative. This also holds for the previous discussion of the differences in the
tunneling spectra of CoPc and CoTPP [11].
Only recently, the magnetic properties of adsorbed metalloporphyrins have
come into the focus of interest. Wende et al. studied iron-octaethylporphyrin
(FeOEP) monolayers on thin ferromagnetic films of Ni and Co on Cu(100) with
X-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 not
directly interact with the surface; instead, an indirect super-exchange interaction
mechanism 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 the
ferromagnetic substrate are magnetically ordered [86]. The FeOEP monolayers
were prepared from octaethylporphyrinato-iron(III) chloride, which is believed
to 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 observed
with XPS [41, 63].
5. Reactivity: Coordination of axial ligands and surface
trans effect
Metallo-tetrapyrroles with coordinated M(II) ions usually have two vacant axial
coordination sites [87], which for example play an important role for the bio-
chemical functionalities of these complexes [88]. Adsorption with the molecular
plane parallel to the surface, as is observed in most cases, occupies one of these

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Surface-Confined Coordination Chemistry with Porphyrins …
67
Fig. 6. Surface-confined two-step synthesis of the complex (NH
3
)ZnTPP on Ag(111) from
2HTPP, Zn, and NH
3
. In the first step, 2HTPP is metalated with Zn; thereafter, NH
3
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 (θ
Zn
=
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) (NH
3
)ZnTPP on Ag(111) at 140 K,
NH
3
background pressure 1 !10
–8
mbar. (E) (NH
3
)ZnTPP produced with directly deposited
ZnTPP for comparison, conditions as in (D). Line colors: red: 2HTPP, orange: ZnTPP, green:
NH
3
. (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 ligand
to 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 on
Ag(100), studied with STM [89]. Later, it was shown by XPS that NH
3
coordi-
nates to ZnTPP on Ag(111) below 130 K [62]. The NH
3
-Zn bond energy of
40 kJmol
–1
was determined by a temperature-programmed decomposition experi-
ment. (Note that the NH
3
coordination was also used as a reaction step in the
surface-confined two-step synthesis of (NH
3
)ZnTPP from 2HTPP, Zn, and NH
3
,
see Fig. 6 [62]). Axial coordination was also employed for the realization of a
surface-anchored, porphyrin-based molecular pinwheel, which was briefly men-
tioned in Section 2. Its rotor consists of a ZnTTBPP molecule, which is pinned
to the surface by 4-methoxypyridine. It is believed that this ligand binds to the
Zn ion through the pyridine N atom, while it is bound to the Ag(100) surface

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68
J. M. Gottfried and H. Marbach
Fig. 7. Interaction of (NO)CoTPP with Ag(111). The axial NO ligand reduces the electronic
interaction between the Co ion and the Ag surface due to the surface trans effect. The Co
2p
3.2
XP spectra (left) show only a small shift of –0.4 eV between multilayer and monolayer
signal, much less than for CoTPP without the NO ligand (–1.8 eV, see Fig. 5). Note that the
multiplet structure seen in the case of CoTPP (Fig. 5) has vanished, because (NO)CoTPP is a
closed-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 desorption
at 550 K restores the signal. (Adapted from ref. [16].)
through the -OCH
3
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 et
al. 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 especially
important for an atomistic understanding of the catalytic activity of supported
CoTPP in the NO
x
reduction [17]. In a combined XPS.UPS study [16], it was
found 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 for
CoTPP without NO ligand, 1.8 eV (see chapter 4). This result indicates that the
surface has less influence on the electronic structure of the Co ion if this ion
carries 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 arises
from the electronic interaction between the Co ion and the Ag surface [15]. If
NO was attached to the Co ion, this signal disappeared, but was restored when
the NO ligand was thermally desorbed (Fig. 7, right). To explain these findings,
it was proposed that the interaction between the Co 3d
z2
orbital and the Ag
surface, which is present in adsorbed CoTPP, is replaced by a stronger interaction
between the Co 3d
z2
orbital and the π* orbital of NO. This interaction leads to
a larger energetic separation between bonding and antibonding level, with the

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Surface-Confined Coordination Chemistry with Porphyrins …
69
Fig. 8. Molecular orbital (MO) model of the interaction of the half-occupied 3d
z
2 orbital of the
coordinated Co ion with the Ag(111) surface (left) and with the π* orbital of NO (right). In
the 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 E
F
(Fig. 8, right). Therefore, it cannot
be occupied with electrons from the Fermi sea and the electron transfer from the
surface to the Co ion is suppressed, in agreement with the experiment [16].
This interpretation implies the existence of a competition between the two
axial coordination bonds in which the Co ion is involved, the Co-surface and the
Co-NO bond. Similar competitive effects are well known in molecular coordina-
tion chemistry as “trans effect” and are especially manifest during substitution
reactions [90, 91]. For example, the substitution of a ligand (T2) in trans-position
to another ligand (T1) is accelerated, if T1 is a stronger σ-donor or π-acceptor
ligand than T2. As a result, T1 and T2 direct ligands entering the complex
according to the strength of their trans effects in the trans-position. The relative
strength of the trans-directing influence is fairly constant and decreases in the
following order: CO > NO > PR
3
>NO
2
> SCN
–
>I
–
>CH
3
>Br
–
>Cl
–
>NR
3
>
H
2
O [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 that
their 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 a
πacceptor ligand, then the M-T1 σ-donor .π-acceptor bond withdraws electron
density from the M-T2 bond in trans position. This weakens the M-T2 bond and
activates this position for a nucleophilic substitution [91]. In the case of the
surface trans effect on adsorbed metalloporphyrins, the situation is probably anal-
ogous: Similar to CO and O
2
, NO is known for its strong π-accepting ability

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70
J. M. Gottfried and H. Marbach
in metalloporphyrins, resulting for example in the low-spin character of these
complexes. 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 a
stronger trans effect than the metal surface. It remains to be seen whether systems
with the opposite behaviour (i.e., a surface with a strong trans effect weakening
the 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 on
the metal ion nor the axial molecular ligand on the ion-surface interaction is
(NH
3
)ZnTPP on Ag(111) [61]. This shows that the electron configurations of
metal ion, molecular ligand, and surface play an important role. Hence, generali-
zations about the character of the metal ion-to-surface bond should be avoided
before a detailed understanding at the atomistic level has been achieved for a
larger number of systems.
6. Summary
Metalloporphyrins, metallophthalocyanines, and other tetrapyrrole complexes
have found increasing attention for the functionalization of surfaces, especially
with respect to catalytic activity, sensor functionality, and applications in spin-
tronic devices. This general, potential technologic interest has stimulated a wide
range of research activities during the recent years, which resulted in a detailed
understanding of the molecular ordering and intramolecular conformation of ad-
sorbed metallotetrapyrroles, their electronic interaction with the surface, and their
reactivity towards atoms and molecules. It has been shown that the surface can
strongly influence the electronic structure of the coordinated metal centers and
that the strength of this interaction depends critically on the electronic structure
of the metal center and its further ligands. In addition, novel routes for the in-
situ preparation of metallo-tetrapyrroles and their complexes with axial ligands
have been described, which allow for the preparation of temperature, air, or
moisture sensitive complexes directly on the surface in ultra-high vacuum. Most
of the previous work was limited to metal surfaces, while interesting systems
with catalytic activity or sensor functionality consist of metallo-tetrapyrroles on
oxide surfaces. To obtain a similarly detailed knowledge of these systems as has
previously been obtained for metal surfaces is the major challenge on this field
in 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

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Surface-Confined Coordination Chemistry with Porphyrins …
71
during August.September 2008, when this manuscript was written. We thank
Prof. Dr. Hans-Peter Steinrück for stimulating discussions and Yun Bai, Florian
Buchner, Dr. Ken Flechtner, and Martin Schmid for providing the figures. This
work was supported by the Deutsche Forschungsgemeinschaft through Sonder-
forschungsbereich 583.
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