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Ranking the Stability of Transition Metal Complexes by On-Surface Atom Exchange Alexandra Rieger, Stephan Schnidrig, Benjamin Probst, Karl-Heinz Ernst, and Christian Wäckerlin J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02834 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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The Journal of Physical Chemistry Letters

Ranking the Stability of Transition Metal Complexes by On-Surface Atom Exchange Alexandra Rieger†, Stephan Schnidrig‡, Benjamin Probst‡, Karl-Heinz Ernst†,‡, and Christian Wäckerlin†,* † Empa, Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland ‡ Department of Chemistry, University of Zurich, 8057 Zurich, Switzerland AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

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Abstract

Surface adsorbed macrocycles exhibit a number of interesting physical and chemical properties, many of them are determined by their transition metal centers. Here, the hierarchical exchange of the central metal atom in such surface adsorbed complexes is demonstrated, specifically in the porphyrin-like macrocycle pyrphyrin adsorbed on Cu(111). Using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS), we show that Cu as central metal atom is easily exchanged with Ni or Fe atoms supplied in trace amounts to the surface. Atom exchange of Ni centers with Fe atoms also occurs, with moderate yield. These results allow to qualitatively rank the stability of the surface-adsorbed Cu, Ni and Fe complexes. The fact that the atom exchange occurs at 423 K, shows that surface-adsorbed macrocycles can be surprisingly easily transformed.

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Transition metal bearing macrocycles adsorbed at surfaces in ultra-high vacuum have been at the focus of intense studies in the last decade, e.g. for their assembly, coordination chemistry or magnetic properties.1–5 The central metal atom can be introduced into free-base macrocycles directly at the solid – vacuum interface.6,7 Many different porphyrin (P), phthalocyanine (Pc) as well as porphyrin-like macrocycles can be metalated with transition metals ranging from Ti to Zn, as well as many other metals.3–7 In solutions, the exchange of the central metal atom in porphyrins and porphyrin-like macrocycles with another one (M1(II)P + M2(II) M2(II)P + M1(II)) is well known.8,9 This reaction has been also referred to as “transmetalation”8,9, although we prefer to refer to it as “atom exchange”. On a metal surface in vacuum, atom exchange reactions imply the reduction and oxidation of involved metal atoms: M1(II)P + M2(0) M2(II)P + M1(0). For brevity, the reaction denoted here as “M1 M2 atom exchange”. This is in contrast exchange reactions in solution, where counter-ligands are available and thus the oxidation state of the metals is generally not changed. Futhermore, in protic solutions the reaction can proceed via transient protonation of the macrocycle.10 With solvent or without, whether the metal exchange occurs or not depends on the change in Gibbs free energy of the two metal porphyrins with respect to the Gibbs free energy of the metals outside of the complexes and it depends also on whether the reaction barrier can be overcome. For macrocycles adsorbed at the solid – vacuum interface, calculating the energetics of atom exchange is very challenging, because high level ab-initio theory of extended macrocycle – substrate systems is required. However, binding enthalpies from density functional theory (DFT) calculations of the metal bearing macrocycles in vacuum can help to infer if a given atom exchange occurs or not. Such calculations report metal binding energies for M(II)-porphyrins,11– 13

M(II)-phthalocyanines,14 M(II)-porphyrazines14 ranked as {Fe, Co, Ni} > Cu > Zn. Note that


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this order refers to the binding energies with respect to neutral M(0) atoms and that in ref. 12 the binding energies are referenced against M(II) ions. This general trend in binding energies is consistent with the M(II)-porphyrin coordinate bond strengths inferred from infra-red spectroscopy15 and with the stability order found from demetalation experiments.9 The reported stability orders for {Fe, Co, Ni} differ for different macrocycles as well as for the same M(II)macrocycles studied with different DFT approaches. On the other hand, the universal stability order {Fe, Co, Ni} > Cu > Zn can be rationalized by the increasing occupation of the orbital with 3dx2-y2 character within the series square-planar Ni(II) (3d8), Cu(II) (3d9) and Zn(II) (3d10) complexes.11,16 Considering the tremendous scientific and technological interest in hybrid organometallic – inorganic interfaces, it is surprising that atom exchange at solid – vacuum interfaces has not yet been thoroughly studied. In particular, atom exchange has been so far only studied by X-ray spectroscopies, i.e. X-ray photoelectron spectroscopy (XPS) and X-ray absorption (XAS). Doyle et al. have reported the exchange of Ni in Ni-bromo-porphyrins with Cu from Cu(111) upon annealing on the basis of Ni 2p XPS and Ni L3 edge XAS.17 However, the role of the Br atoms present at the surface under reaction conditions18,19 has not been addressed. This may be important, as the difference in enthalpy of formation of NiBr2 with respect to CuBr2 (-212.2 kJ mol-1 vs -141.8 kJ mol-1)20 could be a driving force for the reported Cu Ni exchange, in analogy to similar effects reported for on-surface metalation in presence of an oxygen layer.21 Furthermore, the authors do not explain why despite the excess of Cu the atom exchange is incomplete. Co Cu atom exchange was also reported for CoPc/Cu(111),22 although at significantly higher temperature and only based on a very small shift of the Co 2p3/2 core level, rendering this study also not very convincing. Both the Ni Cu and the Co Cu atom


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exchange were reported to be endothermic,17,22 consistent with the above discussed relative stability orders. At the metal – solvent interface, Zn Cu exchange occurs.23 However, in presence of protic solvents, metal exchange via transient protonation10 cannot be excluded. In this study, the atom exchange of Cu, Ni and Fe in a porphyrin-like molecule adsorbed in monolayer coverage at the metal – vacuum interface is investigated using scanning tunneling microscopy (STM) and XPS .The used molecule pyrphyrin (2HPyr) is a tetra-dentate macrocycle based on two fused bi-pyridine units (Scheme 1).24,25 All experiments are performed in ultra-high vacuum on the atomically clean Cu(111) surface prepared by repeated cycles of sputtering and annealing (cf. Methods section for details). The metal atoms are provided by the Cu substrate or they are added by in-situ electron beam evaporation. Using scanning tunneling microscopy (STM), different central metal atoms coordinated in single molecules are identified and the progress of the atom exchange is determined by mere counting. Scheme 1. Structure of metal pyrphyrin and sketch of atom exchange and direct metalation reactions.

All atom exchange and metalation experiments in this study start from 1 monolayer (ML) of 2HPyr on Cu(111), obtained by thermal sublimation of the molecule onto the sample kept at 295 K. 1 ML CuPyr is obtained by metalation of 2HPyr by annealing to 423 K.26 Under normal tunneling conditions the center of CuPyr appears dark (i.e. low) because the Cu 3d orbitals in this


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square-planar complex lack out-of-plane states around the Fermi energy.26–30 Since out-of-plane 3d states are also absent in low-spin Ni(II) complexes, they are also imaged dark.28–30 In contrast, Mn(II), Fe(II) or Co(II) are imaged bright (i.e. high), due to their out-of-plane 3d states.27–30 In this study two regimes of tunneling conditions are used: i) “normal tunneling conditions” and ii) “low bias voltage tunneling conditions”. Under low bias voltage tunneling conditions with generally high tunnel current set-points and with a suitable STM tip, a shallow protrusion in the center may be seen in CuPyr.26 As shown later, under these conditions, NiPyr can be distinguished by its brighter central contrast from CuPyr. Normal tunneling conditions are used to distinguish FePyr from CuPyr or NiPyr. The exact tunneling conditions are presented in Table S1, typical values for normal tunneling conditions are: Ut –0.9 … –1.2 V and It ~ –100 pA. Typical values for low bias voltage tunneling conditions are: Ut ~ –50 … –200 mV, It ~ –0.2 … –1 nA. Starting from 1 ML CuPyr on Cu(111), the addition of trace amounts of Fe followed by annealing to 423 K leads to the appearance of a bright feature in some of the pyrphyrin molecules under normal tunneling conditions, characteristic for a central metal atom with out-ofplane 3d states (Figure 1a).27–31 Thus, these molecules are identified as FePyr. The abundance of FePyr increases linearly with the amount of supplied Fe (Figure 1c). This shows that the Cu Fe atom exchange is very efficient. An abundance of more than 90% FePyr can be obtained with very little additional Fe cluster formation (Figure 1b).


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Figure 1. Atom exchange of Cu in CuPyr with Fe atoms. The STM images are obtained with normal bias voltage tunneling conditions. (a) FePyr is easily distinguished from CuPyr because its center is imaged bright. (b) Large scale image showing more than 90% of FePyr in presence of very few clusters. (c) Abundance of FePyr and CuPyr in dependence of the supplied amount of Fe atoms. 1 eq. corresponds to 1 Fe atom per molecule.

The Cu Ni atom exchange works analogous to the Cu Fe atom exchange and is also very efficient (Figure 2). The STM images are obtained after addition of trace amounts of Ni to 1 ML of CuPyr/Cu(111) and annealing to 423 K. Because Ni(II) in a square-planar complex like NiPyr does not have significant out-of-plane 3d character at the Fermi level, the center is imaged dark under normal tunneling conditions and is in fact virtually indistinguishable from corresponding Cu(II) complexes.27,28 However, under the low bias voltage tunneling conditions, used in the images shown in Figure 2a and 2b, CuPyr and NiPyr can be distinguished: CuPyr appears darker in the center with respect to the macrocycle around it and NiPyr appears intermediately bright in its center. The magnitude of the contrast depends to some extent on the STM tip condition. Like


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for the Cu Fe atom exchange, a monotonously increasing abundance of NiPyr with increasing amounts of deposited Ni is found (Figure 2c).

Figure 2. Atom exchange of Cu in CuPyr with Ni and supplied by sublimation in vacuum and annealing. Under the used low bias voltage tunneling conditions, a different contrast is observed in the centers of the molecules allowing their identification: The center of NiPyr is imaged brighter than the center of CuPyr. (c) Abundance of NiPyr and CuPyr in dependence of the supplied amount of Ni atoms. 1 eq. corresponds to 1 Ni atom per molecule.

The Cu Ni atom exchange is also observed in the Ni 2p3/2 XP spectrum (Figure 3a) of 1 ML CuPyr annealed in presence of Ni atoms. In that case, the Ni 2p3/2 binding energy (855.8 eV) is 3.0 eV higher than the binding energy of metallic Ni/Cu(111) (852.8 eV). Very similar binding energies have been reported for formal Ni(II) in Ni-bromo-porphyrins on Cu(111)17 and Nitetraphenyl-porphyrin (NiTPP) on Au(111),32 thus confirming the formation of NiPyr. In contrast, the Fe 2p3/2 binding energy of FePyr/Cu(111) produced by atom exchange from CuPyr (707.2 eV) is virtually identical to the binding energy of metallic Fe (707.0 eV) (Figure 3b). Such low binding energies of formal M(II) atom in macrocycles, being very close (< 0.3 eV) to


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the binding energies of the respective metals, have been reported for (sub)ML coverages of e.g. FePc/Ag(111)33, CoTPP/Ag(111)6,34 or CoPc/Cu(111).22 As explained by Lukasczyk et al.,34 these low binding energies are the consequence of electronic interactions between the metal ions and the substrate. In the above cited cases as well as for FePyr obtained in this study, XPS is not helpful to distinguish the metal complex from free metal clusters or atoms. However, in STM FePyr is easily distinguished from CuPyr and NiPyr due to its bright contrast in the center. Furthermore, prolonged annealing of FePyr/Cu(111) (Figure S1) and NiPyr/Cu(111) (Figure 3c) at 473 K does not result in an increase of the abundance of CuPyr, i.e. the reverse atom exchange (Ni Cu) and (Fe Cu) does not occur. This implies that NiPyr and FePyr are significantly more stable than CuPyr.

Figure 3. Atom exchange of 1 ML metal-Pyr on Cu(111) studied by XPS. The spectra are normalized with respect to the substrate signal and offset for clarity. (a) Ni 2p3/2 XP spectra of NiPyr compared with metallic Ni. Ni in NiPyr is clearly distinguished by its binding energy (855.8 eV) from metallic Ni (852.8 eV). (b) Fe 2p3/2 XP spectra of FePyr compared with metallic Fe. The binding energy of Fe in FePyr


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(707.2 eV) is very close to the binding energy of metallic Fe (707.0 eV). Therefore FePyr on Cu(111) cannot be distinguished from metallic Fe. (c) NiPyr on Cu(111) before and after annealing for 1 h at 473 K. The Ni 2p3/2 remains at 855.8 eV with constant intensity and no metallic Ni is present. Thus Ni in NiPyr is not replaced with Cu. (d) Ni 2p3/2 spectra showing the exchange of Ni in NiPyr with Fe. The initial NiPyr sample has a slight excess of Ni and exhibits therefore a small metallic Ni signal. Addition of Fe and annealing to 423 K leads to a decreased NiPyr signal and to an increased intensity of metallic Ni, evidencing that Ni in NiPyr has been partly replaced by Fe.

NiPyr and FePyr can be directly obtained from 2HPyr via direct metalation, i.e. by annealing of 1 ML 2HPyr/Cu(111) (Figure S2) to 423 K in presence of Ni or Fe atoms. This is no surprise in view of the extensive literature on on-surface metalation. Because of the above discussed facile Cu Ni and Cu Fe atom exchange reactions, it cannot excluded that NiPyr and FePyr are produced via CuPyr as an intermediate. However, metalation with a range of transition metals, including Ni and Fe, also proceeds on Ag or Au surfaces.3–7,31–33,35 In order to rank the stability of FePyr with respect to NiPyr, atom exchange experiments where Fe is added to NiPyr/Cu(111) and Ni is added to FePyr/Cu(111) are performed (Figure 4). After addition of 0.9 Fe atoms per molecule to a layer consisting of 84% NiPyr and 16% CuPyr (Figure 4a) and annealing to 423 K, 50% of the molecules are identified as FePyr by their bright protrusion in the center under normal tunneling conditions (Figure 4c). Since the abundance of FePyr is significantly higher than the abundance of CuPyr in the original sample, Ni in NiPyr must have been partly replaced by Fe. In the Ni 2p3/2 XP spectrum, obtained on a NiPyr sample annealed in presence of Fe atoms (Figure 3d), metallic Ni (852.8 eV) is found, confirming that Ni was expelled from NiPyr due to atom exchange with Fe. If an excess of Fe (3 eq.) is used, an abundance of ~50% of FePyr is found even before annealing (Figure S3). Annealing to 423 K


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leads to ~60% of FePyr (Figure S3). These results show that atomic Fe from the electron beam evaporator is significantly more reactive then Fe at the surface. Whether or not a given atom is exchanged may also depend on where the incoming Fe atom lands with respect to the metal center. Similar behavior has been found for metalation reactions of porphyrins: metalation at room temperature is observed if metal atoms are deposited on a molecular layer, the reverse procedure requires annealing to < 500 K to induce the reaction.32,35 Note that also the Cu Fe atom occurs partially already at room temperature. Specifically, 0.9 eq. of Fe on CuPyr resulted in ~30% of FePyr (Figure S4). However, starting from FePyr and adding Ni does not yield a observable atom exchange of Fe with Ni after annealing to 423 K (Figure 4b,d). This shows that FePyr is more stable than NiPyr, even though the difference might be smaller than in case of FePyr and NiPyr vs CuPyr. The facts that a complete Cu Fe atom exchange occurs after annealing and that the Ni Fe exchange does not proceed completely, indicate that the barrier for Ni Fe atom exchange is significantly larger than the one for Cu Fe atom exchange.


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Figure 4. Atom exchange experiments (a c) NiPyr + Fe and (b d) FePyr + Ni. The STM images are obtained with low (a) and normal bias voltage (b-d) tunneling conditions to distinguish (a) NiPyr from CuPyr and (b-d) FePyr from CuPyr or NiPyr. (ac) Ni Fe atom exchange occurs with moderate yield. For the reverse experiment (bd) no visible change in the abundance of FePyr is observed.

Our findings on the metalation and the exchange of the central metal atom in the pyrphyrin (Pyr) macrocycles adsorbed in monolayer coverage on Cu(111) allow the ranking the stabilities of surface adsorbed CuPyr, FePyr and NiPyr. The Cu Ni and Cu Fe atom exchange reactions proceed at low temperatures (433 K) and with high yield. No evidence for the reverse reaction, i.e. Ni Cu or Fe Cu atom exchange, is found. Furthermore, annealing of NiPyr in presence of Fe gives FePyr with a moderate yield. The reverse reaction, i.e. replacement of Fe in FePyr with Ni, is not observed. From the above observed atom exchange reactions, the ෥ NiPyr > CuPyr is established, consistent with the knowledge qualitative stability order FePyr > from porphyrin solution chemistry and DFT calculations.9,11 The detailed mechanism of atom exchange at metal – vacuum interfaces is unknown, but is likely to involve an intermediate metal – metal dimer where the coordinative bond switches from the weaker binding to the stronger binding metal atom. The structure of such an intermediate state may be similar to the one of surface adsorbed metal-porphyrins decorated at cryogenic temperatures with hetero metal atoms.36 However, the second metal atom may arrive from below the macrocycle where it is further stabilized by interactions with the metal surface. The fact that the atom exchange proceeds at rather low temperatures is important for research and applications of metal bearing macrocycles at surfaces. It highlights that special care must be taken to ensure that the composition of the surface adsorbed complex has not changed, in particular if metal centers of low stability (e.g. Cu, Zn, Ag) are used in presence of stronger


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binding metals such as Ni, Co or Fe. Our study shows that the relative stabilities of metalmacrocycles can be used to rationalize whether on-surface atom exchange proceeds or not. Note however, that important thermodynamic and kinetic effects are yet to be explored. For example, it is unclear if the activation barriers in tetrapyrrols are similar or if the pyrphyrin ligand presents exceptionally low barriers. Too high activation barriers may prohibit atom exchange before decomposition. Furthermore, the free enthalpy and entropy of the metal atoms being embedded and also of the ones being expelled have to considered.17,22 Very recently, Pt Ag atom exchange has been reported for non-macrocyclic ligands.37 Therefore such atom exchange processes in transition metal complexes adsorbed at surfaces may be quite ubiquitous. Methods All experiments were performed in ultra-high vacuum (base pressure < 5×10-10 mbar). The Cu(111) crystal was cleaned by several cycles of Ar+ sputtering and annealing to 723 K. 2HPyr (purified by resublimation in vacuum) was evaporated from a Knudsen cell kept at 623 K onto the sample kept at room temperature. Unless specified, the samples were annealed for 10 min. XPS measurements (Specs, PHOIBOS 100 electron analyzer) were performed using nonmonochromatic Al Kα X-rays. The background obtained on the clean Cu(111) sample was appropriately scaled and subtracted from the shown spectra. The spectra are normalized with respect to the substrate signal (here Cu 2p3/2) and offset for clarity. The binding energy scale is calibrated using the Cu 2p3/2 signal (932.7 eV) and the Fermi level (0.0 eV). The used measurement parameters result in a full width at half maximum of 1.2 eV for C 1s of in-situ grown graphene/Cu(111). The STM images were obtained with a Specs Aarhus 150 at room temperature using a mechanically cut and in-situ sputtered PtIr (90% Pt) tip. The images were


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calibrated on atomically resolved Cu(111). The imaging conditions are to be found in Table S1. All the STM images were processed using the WSxM software.38 ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Imaging parameters, additional STM images (PDF) Notes The authors declare no competing financial interests. ACKNOWLEDGMENT Financial support by the University Research Priority Program LightChEC of the University of Zürich, Switzerland, the Swiss National Science Foundation (R’Equip, Grant 200021_152559), and the Competence Centre for Materials Science and Technology (CCMX) is gratefully acknowledged. We thank L. Zoppi for fruitful discussions.

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