Influence of Mesityl and Thiophene Peripheral Substituents on Surface

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Influence of Mesityl and Thiophene Peripheral Substituents on Surface Attachment, Redox Chemistry, and ORR Activity of Molecular Iron Porphyrin Catalysts on Electrodes Robert Götz,† Khoa H. Ly,† Pierre Wrzolek,‡ Arezoo Dianat,§ Alexander Croy,§ Giancarlo Cuniberti,∥ Peter Hildebrandt,⊥ Matthias Schwalbe,*,‡ and Inez M. Weidinger*,† †

Faculty of Chemistry and Food Chemistry, Dresden University of Technology, 01062 Dresden, Germany Institute of Chemistry, Humboldt-Universität zu Berlin, 12489 Berlin, Germany § Institute for Materials Science and Max Bergmann Center of Biomaterials, Dresden University of Technology, 01062 Dresden, Germany ∥ Center for Advancing Electronics, Dresden Center for Computational Materials Science, Dresden University of Technology, 01062 Dresden, Germany ⊥ Institute of Chemistry, Technische Universität Berlin, 10623 Berlin, Germany

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‡

S Supporting Information *

ABSTRACT: Two iron porphyrin complexes with either mesityl (FeTMP) or thiophene (FeT3ThP) peripheral substituents were attached to basal pyrolytic graphite and Ag electrodes via different immobilization methods. By combining cyclic voltammetry and in-operando surface-enhanced Raman spectroscopy along with MD simulations and DFT calculations, their respective surface attachment, redox chemistry and activity toward electrocatalytic oxygen reduction was investigated. For both porphyrin complexes, it could be shown that catalytic activity is restricted to the first (few) molecular layer(s), although electrodes covered with thiophene-substituted complexes showed a better capability to consume the oxygen at a given overpotential even in thicker films. The spectroscopic data and simulations suggest that both porphyrin complexes attach to a Ag electrode surface in a way that maximum planarity and minimum distance between the catalytic iron site and the electrode is achieved. However, due to the distinctive design of the FeT3ThP complex, the thiophene rings are capable of occupying a conformation, via rotation around the bonding axis to the porphyrin, in which all four sulfur atoms can coordinate to the Ag surface. This effect creates a dense and planar surface coverage of the porphyrin on the electrode facilitating a fast (multi) electron transfer via several covalent Ag−S bonds. In contrast, bulky mesityl groups as peripheral substituents, which have been initially introduced to prevent aggregation and improve catalytic behavior in solution, exert a negative effect on the overall electrocatalytic performance in the immobilized state as a less dense coverage and less stable interactions with the surface are formed. Our results underline the importance of rationally designed heterogenized molecular catalysts to achieve optimal turnover, which not only strictly applies to the here discussed oxygen reduction reaction but eventually holds also true for other energy conversion reactions such as carbon dioxide reduction.

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dioxygen,2,12−16 water,17 or protons.18 However, only very few studies have explored this topic systematically. In the past, a variety of tailored porphyrin derivatives were synthesized to enhance ORR, e.g. through incorporation of proton-active functional groups for optimized proton and oxygen shuttling ability to the reaction center.2 Peripheral (meso-)substituents were typically chosen according to their ability to withdraw or increase electron density into the porphyrin macrocycle as well as to prevent aggregation of molecules.19−22

INTRODUCTION

Metal porphyrin complexes are well-known catalysts for a range of chemical energy conversion reactions including the hydrogen evolution and oxygen reduction reaction (HER and ORR1, respectively) as well as the reduction of CO2.2−5 In this respect, the porphyrin’s axial ligation6 and peripheral substituents have been shown to heavily impact their catalytic performance.7−11 In more detail, electron donating axial ligands such as imidazole, pyridine, triphenylphosphine, or thiolates have been shown to increase the electron density (basicity) of the metal center and thus enhance the reactivity of the metal center toward the activation of substrates such as © XXXX American Chemical Society

Received: January 7, 2019

A

DOI: 10.1021/acs.inorgchem.9b00043 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Structure of the investigated porphyrin complexes: FeTMP (A) and FeT3ThP (B).

Specifically, analyzing changes in band positions and relative intensities between Raman spectra in solution and SER/SERR spectra allows for determination of molecule orientation and binding mode on metal surfaces in the immobilized state.39 Furthermore, in combination with electrochemical methods, potential-dependent SERR spectroscopy can be used to analyze catalytic processes under turnover conditions.23,35,40 In the present work, we compare two iron porphyrins with either mesityl or thiophene substituents at the meso-carbons of the porphyrin ring (Figure 1). Porphyrins accommodating mesityl substituents in these positions have been shown to catalyze ORR in organic media.21,41 Due to the bulky nature of the mesityl substituents, however, strong interactions with solid electrodes and formation of a dense electroactive film might be hindered, which would decrease their electrocatalytic performance. Molecular iron porphyrins with thiophene ligands have already been used as sensor for superoxide and other small molecules but, so far, their potential for promoting ORR catalysis has not been investigated in detail.42−45 The thiophene rings are less sterically demanding than the mesityl groups, and the sulfur atoms could principally allow coordination to noble metal surfaces due to the high binding affinity of sulfur.8,46,47 This exchange of substituents may favor adsorption to the electrode surface and enhance the catalytic performance in the adsorbed state whereas its impact on the catalytic processes in solution is likely to be small. In this study, we immobilized both complexes to electrode surfaces via a common dropcast (dc) procedure on a basal pyrolytic graphite (BPG) electrode or via an incubation cast (inc) process on a silver electrode. The respective electrocatalytic performance was subsequently analyzed by electrochemical techniques and correlated with structural properties derived from the analysis of SER/SERR spectra at different applied potentials as well as predictions from MD simulations and DFT calculations.

For optimal electrocatalytic applications, however, the complexes have to be adsorbed on electrode surfaces, which in turn requires tailored immobilization strategies to ensure efficient heterogeneous electron transfer (ET) to occur. Accordingly, molecular compounds that have been designed for homogeneous catalysis in solution might not necessarily show optimal performance upon unspecific adsorption onto electrode surfaces.23 To this end, only few studies have addressed the challenge of rational optimizing immobilization of molecular catalysts on interfaces.24−26 The most commonly applied immobilization strategy is simple physiadsorption of molecular complexes on graphite surfaces by immersion or drop casting method.26,27 The advantage of this strategy, besides its simple practicability, is the direct and fast electron transfer from the electrode to the molecule. The major disadvantage is that disordered multilayers are often obtained and information on electronically addressable molecules are hard to obtain making it very challenging to compare results obtained in different laboratories.28,29 In some cases, (multiwalled) carbon nanotubes ((MW)CNTs) exhibiting a high surface area with or without Nafion are applied to bring the complexes onto the surface of an electrode.30−32 Other noncovalent interactions to improve catalyst stability and increase catalytic performance are based on dispersion (π−π) interactions by using, for example, a pyrene linker.4,24,25,33 The immobilization of porphyrinic molecular catalysts on electrode materials through covalent interactions is either pursued using axial ligand interactions or by direct grafting methods or by both. A very nice approach is the application of self-assembled-monolayers (SAMs) that are prepared by chemisorption of long chain alkyl thiols on (typically) gold electrodes.34,35 The electron transfer rate between the molecular catalyst and the electrode can be changed by using a specific type of linker and therefore also influences the catalytic performance.36 A covalent binding of the catalyst to the SAM can be realized in different ways, but the use of the “click” reaction to give a triazole ring is a straightforward method to attach, for example, an imidazole unit that can axially coordinate to a porphyrin. The reduction of aryldiazonium salts is another common approach to covalently attach a linker unit to the electrode directly or MWCNTs.36−38 Electrocatalytic activity of molecular compounds is generally probed via electrochemical methods. While this technique is very well suited to monitor the catalytic throughput via current measurements, it does not provide information on the structure of the catalyst during the reaction and its interaction mode with the electrode surface. This information, however, is essential for rational design of heterogenized catalysts that are optimized for a specific interface. Surface-enhanced Raman (SER) and also surface-enhanced resonance Raman (SERR) spectroscopy selectively probe species at the dielectric/metal interface and, therefore, are ideally suited to investigate the interaction of surface-bound molecules with metallic surfaces.

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MATERIALS AND METHODS

Computational Methods. The investigation of the dynamical behavior of molecular structures with Ag(111) was carried out at the DFT level using the standard implementation in the CP2K package.48 Core electrons have been taken into account by means of Goedecker, Tetfer, and Hutter (GTH) approximation.49 For the exchangecorrelation energy, the generalized gradient approximation (GGA) of Perdue, Burke, and Ernzerhof (PBE) was used.50 Dispersion corrections were included through the standard D2 Grimme parametrization.51 Periodic boundary conditions were applied for all directions with a slab gap of about 30 Å. Born−Oppenheimer quantum molecular dynamics (MD) simulations were performed in the canonical ensemble, with a time step of 0.5 fs using a Nose− Hoover thermostat at T = 300 K. The simulations were performed for 8 ps simulation time in total. Experimental Methods. Electrode Preparation. A basal pyrolytic graphite (BPG) electrode with a surface of 1 cm2 or a silver electrode of ca 0.2 cm2 was used as the working electrode. Prior to the immobilization procedure, both electrodes were polished with B


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Inorganic Chemistry

Figure 2. (A) CVs of BPG|FeTMP(dc) (blue) and BPG|FeT3ThP(dc) (red), measured under oxygen-free/nonturnover conditions. (B) LSVs of BPG|FeTMP(dc) (blue) and BPG|FeT3ThP(dc) (red) under turnover conditions (i.e., air saturated buffer solution). (C) LSVs of Ag|FeTMP(inc) (blue) and Ag|FeT3ThP(inc) (red) in air saturated buffer solution. Black lines in parts B and C represent the blank BPG and the roughened Ag electrode, respectively. (D) LSVs of the blank Ag electrode (black), Ag|FeT3ThP(dc) (dashed-dotted red line) and Ag|FeT3ThP(inc) (red) under oxygen saturated conditions. All measurements were carried out at 900 rpm with a scan rate of 0.05 V/s in 0.1 M phosphate buffer solution at pH 7. aluminum powder (5 μm, 0.3 μm, 0.05 μm, 30 s each). The silver electrode was processed via an electrochemical roughening procedure to obtain a higher surface area and surface enhancement for spectroscopic analyses.52,53 Immobilization of catalyst on the polished BPG electrode was achieved by dropcasting 100 μL of a FeT3ThP solution (2.3 mM in dichloromethane, DCM) on the electrode surface or 20 μL of a FeTMP solution (8.4 mM in DCM). The obtained catalyst coated electrode systems are named BPG|FeTMP(dc) and BPG|FeT3ThP(dc), respectively. After evaporation of the solvent DCM, the modified electrodes were assembled in a commercial electrochemical cell (Pine Instruments). Immobilization on the electrochemically roughened silver electrode was achieved by immersing the electrode into a FeT3ThP (0.58 mM) DCM solution for 3 h, followed by thoroughly rinsing with dry DCM yielding the system Ag|FeT3ThP(inc). The same procedure for FeTMP was applied using an incubation solution with a concentration of 0.2 mM (DCM) named: Ag|FeTMP(inc). For comparison, 10 μL of a FeT3ThP solution (2.3 mM) in DCM was dropcasted on a roughened Ag electrode yielding Ag|FeT3ThP(dc). Electrochemical Measurements. Spectro-electrochemical experiments were performed using a commercial rotating disk electrode (RDE) and an electrochemical cell from PINE Research instruments.54 An Ag/AgCl (3 M KCl) electrode (DriRef, World Precision Instruments) and a platinum wire served as reference and counter electrode, respectively. All potentials cited in the text refer to the Ag/ AgCl (3 M KCl) reference. The electrochemical experiments were controlled by a potentiostat (CH Instruments). All measurements were carried out in a 0.1 M aqueous phosphate buffer solution (pH 7)

at room temperature. For measurements under oxygen-free conditions, the cell was purged with Argon for 15 min before and during the measurement. For oxygen-saturated measurements, the cell was purged with oxygen with a pressure of 0.2 kp cm−2 for 15 min before and during the measurement. Air-saturated measurements were carried out without purging. The RRDE measurements were performed using a Bipotentiostat from Pine Research Instruments (Wavedriver). During the experiments a platin ring electrode surrounding the disc electrode was set to a constant potential of 0.6 V. The diameter of the BPG and silver disc electrode in case of the RRDE measurements was 5 mm. The immobilized amount of the complexes on the BPG has been adjusted to achieve the same surface concentration as it was for LSV/CV measurements: 20 μL of a FeT3ThP solution (2.3 mM in DCM) and 4 μL of a FeTMP solution (8.4 mM in DCM). (Resonance) Raman and Surface-Enhanced (Resonance) Raman Measurements. Spectro-electrochemical measurements were performed using the 413 and 647 nm lines of a Coherent Innova 300c Krypton laser. A CCD camera (nitrogen cooled) in a Jobin Yvon LabRam 800 spectrometer equipped with a confocal microscope was used to collect Raman signals in a 180° backscattering configuration. The laser beam was focused onto the electrode surface by a 20x Nikon objective (NA 0.35) with an effective working distance of 2 cm. The electrode was rotated during Raman measurements to prevent laser-induced sample degradation. The laser power was adjusted to 1 mW on the sample. All SER/SERR measurements were performed on a RDE electrode with a rotation speed of 900 rpm using a published spectro-electrochemical configuration.23 At each potential the spectra were recorded in the C


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Inorganic Chemistry time frame of 5 to 10 min. As the spectra were stable within different accumulation cycles, equilibrium conditions can be assumed. No oxygen concentrations are given for the SERR measurements since, due to the rotation of the electrode, no complete oxygen free conditions can be achieved. While this effect can be neglected in the electrochemical measurements, even small amounts of oxygen can have a large impact on the SERR spectra. Resonance Raman measurements of the complexes in solution were performed in a rotating cuvette with a concentration of 2.3 mM FeT3ThP in DCM (or 70 μM FeTMP in acetonitrile). All chemicals were of highest purity available (Sigma-Aldrich) and used without further purification.

showed ORR activity. This observation suggests that the soheterogenized FeTMP catalyst alone is not capable of quantitatively catalyzing the transformation of all oxygen present at the electrode surface, and therefore, the BPG electrode contributes to the catalysis at potentials more negative than E = −0.2 V (vide supra). In contrast, such behavior is not noted for BPG|FeT3ThP(dc) (compare Figure 2B, red trace) indicating a significantly more pronounced catalytic activity of the latter system. The oxygen diffusion limit for BPG|FeTMP(dc) was reached at −0.7 V (at 200 rpm) and at even more negative potentials for higher rotation rates (Figure S1A). Similar results were obtained for both complexes in oxygen-saturated solutions (Figure S2). For further comparison, we determined the potentials required to achieve a catalytic current density of −0.18 mA cm−2. These potentials were found to be at E = −0.15 V for BPG|FeT3ThP(dc) and E = −0.30 V for BPG|FeTMP(dc), respectively. The overall weak activity of the dropcasted multilayer systemsconsidering the high amount of the complexes on the surfaceindicates that the catalytic performance is probably dominated by the first (few) layer(s) on the electrode and layers further away from the electrode do not contribute to the catalytic current. To test this hypothesis, a second immobilization method, referred to as incubation cast, was applied, which is described in detail in the material and method section. The so-constructed electrode systems are referred to as Ag|FeTMP(inc) and Ag|FeT3ThP(inc), respectively. Here, long-time incubation allowed for a more defined attachment of the complexes, i.e. more narrow distribution of orientations of the porphyrin plane(s) with respect to the electrode surface. Subsequent washing with organic solvent ensured removal of loosely bound/physiosorbed compounds impairing formation of a large multilayered complex architecture. (Nevertheless, the formation of more than a monolayer cannot be excluded.) For this immobilization strategy, a nanoscopically rough Ag electrode was used as support that offers the advantage of applying SER/SERR spectroscopy for structural analysis of the complex in the adsorbed state and in the very vicinity of the electrode surface. In particular, the surface enhancement provided by the rough Ag electrode allows for sensitive spectroscopic probing of very low sample concentration, which is the case when only a monolayer of catalyst is present on the electrode surface.7,59,60 LSVs of Ag|FeTMP(inc) and Ag|FeT3ThP(inc) were recorded under identical conditions as applied for the systems with BPG support (Figure 2C). The potential range of the LSV was set from +0.1 to −0.4 V to avoid Ag oxidation and porphyrin desorption. In contrast to the BPG support, the Ag electrode is active toward catalyzing ORR, which rendered determination of the catalytic activity by the adsorbed complexes very difficult in the past.23,61 In the present case, however, the onset potential for Ag|FeT3ThP(inc) was found to be at 0.05 V and, thus, is slightly more positive than that of the bare Ag electrode (0 V, Figure 2C, red and black trace, respectively), underlining the significant catalytic activity of FeT3ThP in the immobilized state.62 However, due to Ag oxidation, it was not possible to apply potentials higher than 0.1 V. Therefore, we could only estimate the onset potential and the value of 0.05 V should be considered as a lower limit. The diffusion limit is reached at a potential around −0.3 V (900 rpm). Under the same experimental conditions, an onset potential for Ag| FeTMP(inc) for ORR at 0.04 V was determined. The applied potential required to reach a catalytic current density of −0.80

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RESULTS AND DISCUSSION Iron tetramesitylporphyrin (FeTMP) and iron tetrakis(3thienyl)porphyrin (FeT3ThP) were synthesized according to published procedures.21,43,55−57 Briefly, the free base porphyrins were reacted with iron(II) chloride in dimethylformamide under argon atmosphere. Aerobic acidic workup yields the corresponding chloroiron(III) porphyrin complexes, shown in Figure 1. Both complexes were immobilized on basal pyrolytic graphite (BPG) electrodes via a dropcast (dc) method yielding the systems BPG|FeTMP(dc) and BPG|FeT3ThP(dc), respectively. For this, the complex dissolved in DCM was deposited dropwise on the electrode surface and air-dried, affording most likely the formation of multilayers with randomly orientated porphyrin complexes on the electrode.23 As the compounds are not soluble in water, it seems likely to assume that the initially formed multilayered coating will be preserved after inserting the electrode into the electrochemical cell filled with aqueous phosphate buffer solution. Electrochemical Characterization. Cyclic voltammograms (CVs) measured for both complexes, immobilized via dropcast on BPG (i.e., BPG|FeTMP(dc) and BPG|FeT3ThP(dc), respectively) under oxygen free conditions, are shown in Figure 2A. Anodic and cathodic nonturnover redox peaks are clearly visible in both cases, albeit being more pronounced for BPG|FeT3ThP(dc). The redox potential of FeT3ThP was found at E0 = −0.21 V, which is by more than 100 mV more positive than for FeTMP, for which a redox potential of E0 = −0.33 V was determined. These values correlate well with redox potentials reported for similar compounds.9 For example, the CV of Fe-TPP (TPP = tetraphenylporphyrin) physiadsorbed on an EPG surface in degassed pH 7 buffer showed the FeIII/II redox couple at −0.32 V vs Ag/AgCl.58 Linear sweep voltammograms (LSVs) recorded under airsaturated conditions in the absence of the immobilized complexes (Figure 2B, black trace) revealed an onset potential for ORR catalysis of the blank BPG electrode at around −0.11 V. In contrast, LSVs of BPG|FeT3ThP(dc) recorded under turnover conditions indicated an onset potential at ca. 0.16 V (Figure 2B, red trace), which is significantly more positive than the blank; thus, confirming catalytic activity of the immobilized FeT3ThP compounds. Diffusion limit of oxygen during ORR catalysis was reached in the region from −0.6 V (200 rpm) to −0.8 V (2500 rpm, Figure S1B). For details on the onset potential determination, see the Supporting Information, Figure S3. BPG|FeTMP(dc) exhibited a slightly more negative onset potential at ca. 0.13 V than BPG|FeT3ThP(dc). Interestingly, the LSV showed two potential intervals with different current slopes (Figure 2B, blue trace, and Figure S4). The second, steeper, slope is seen for potentials more negative than −0.2 V, which is in a potential region where the blank BPG electrode D


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Inorganic Chemistry

Table 1. Summary of All Measured Electrochemical Data of FeT3ThP and FeTMP Immobilized via Different Methods onto BPG or Ag Electrodes vs. Ag/AgCl (3 M KCl)a redox potential E0/V BPG|FeT3ThP(dc) BPG|FeTMP(dc) Ag|FeT3ThP(inc) Ag|FeTMP(inc)

−0.21 (ca. ± 12%) −0.33 (ca. ± 15%) n.d. n.d.

onset potential Eonset/V 0.16 0.13 0.05 0.04

(ca. (ca. (ca. (ca.

±7%) ±8%) ±7%) ±13%)

potential E/V to achieve a certain current density −0.15 −0.30 −0.15 −0.19

(−0.18 (−0.18 (−0.80 (−0.80

mA mA mA mA

cm−2) cm−2) cm−2) cm−2)

(ca. (ca. (ca. (ca.

±8%) ±5%) ±14%) ±16%)

saturation potential/V (diffusion limit) ESat −0.7 −0.8 −0.3 −0.3

(900 (900 (900 (900

rpm) rpm) rpm) rpm)

(ca. (ca. (ca. (ca.

±14%) ±13%) ±17%) ±13%)

a

Conditions: Argon purged or air-saturated 0.1 M phosphate buffer at pH 7 and room temperature.

Figure 3. RR spectra of FeTMP and FeT3ThP in solution (Sol.) and potential (vs Ag/AgCl (3 M KCl) dependent SERR spectra of Ag| FeT3ThP(inc) (A) and Ag|FeTMP(inc) (B). Spectra were obtained under 413 nm laser excitation. SERR spectra were recorded in aqueous 0.1 M phosphate buffer at pH 7 at room temperature. (C, D) Corresponding relative intensities of the ν4 vibration of the ferric high spin (HSox), ferric low spin (LSox), ferrous high spin (HSred) and ferrous low spin (LSred) species as a function of potential. When applicable the data were fitted by a sigmoidal fit.

mA cm−2 was found to be E = −0.15 V for Ag|FeT3ThP(inc) and E = −0.19 V for Ag|FeTMP(inc), respectively. LSV measurements without rotation (0 rpm, Figure S5) did show similar onset potentials than the ones determined at 900 rpm. However, instead of a diffusion limited plateau, negative current maxima were observed that are indicative for the diffusion limited saturation potential (ESat). ESat was achieved around −0.55 V for the dropcasted BPG−porphyrin systems and around −0.15 V for the incubated Ag−porphyrin systems. Both saturation potentials were found at ca. 50 mV more positive potential than the ones determined for their respective bare electrodes (ESat(BPG) = −0.6 V, ESat(Ag) = −0.2 V). Hence it can be concluded that, while the porphyrin complexes improve the performance of the electrode, saturation limit can

only be achieved with the help of the underlying electrode support. From RRDE measurements with modified silver electrodes the percentage of produced H2O2 and the electron transfer number (n) could be estimated (Figure S6). At −0.1 V values of n = 3.5 and n = 3.4 were determined for Ag|FeT3ThP(inc) and Ag|FeTMP(inc) respectively. At −0.3 V, the values increased to n = 3.9 for both complexes. At both potentials the porphyrin-modified electrodes performed better regarding the 4-electron reduction to water than the bare Ag electrode for which values of n = 3.2 (−0.1 V) and n = 3.6 (−0.3 V) were determined. LSVs were also recorded for multilayer systems of FeT3ThP complexes adsorbed on the Ag electrode via dropcast E


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Inorganic Chemistry

Even at high overpotential (−0.8 V) both reduced and oxidized species are present. Such incomplete reduction observed in SERR-EC measurements can be attributed either to remaining oxygen in the system or by the presence of redox inactive molecules.64,65 As electrochemical measurements are blind to the portion of redox inactive species, such information is very important for optimizing the efficiency per catalysts of an electrocatalytic system. The different observations by cyclic voltammetry and SERR spectroscopy can readily be reconciled by taking into account the specific detection criteria of the two methods. Cyclic voltammetry is only sensitive to molecules exhibiting sufficiently fast electron transfer with the electrode. SERR signals on the other hand show all molecules close to the surface independent of their electrochemical activity. The signal intensity, however, depends on the orientation of the porphyrin plane with respect to the surface normal. Hence, molecules in higher layers with a more perpendicular orientation will also contribute to the spectra. Interestingly electrochemical RR measurements of dropcasted BPG|FeT3ThP (Figure S8) showed no potential dependent redox changes. On BPG electrodes no surface enhancement effect is present but due to the high Resonance Raman signal intensity of porphyrin modes upon 413 nm laser excitation, good Raman spectra can nevertheless be obtained if the number of layers is high enough.57 In this case, the data gives information on all layers equally and suggests that outer layers are not electronically accessible and thus do not participate in the ORR. For both complexes the ferric HS (HSox) species decreased continuously on going to more negative potentials (Figure 3 C,D). A sigmoidal fit to the data yielded a redox potential of ca. −0.4 V for Ag|FeT3ThP(inc) and −0.35 V for Ag| FeTMP(inc). The overall broad transition can be explained if one assumes that different molecular layers with different electrical accessibility contribute to the redox changes. However, especially for FeTMP the so derived redox potentials are similar to the ones obtained from CV measurements on BPG (Figure 2A). The other redox- and spin state components (LSox, HSred, and LSred) behaved differently for the two complexes. For FeT3ThP, the decrease of the ferric HS species was accompanied by a rise of a corresponding ferrous HS species. Furthermore, a slow decrease of the ferric LS species was observed while going to a more negative potential concomitant with an increase of the ferrous LS species. For FeTMP, the situation was more complicated. Here we observed that the ferrous LS species gained more intensity at the expense of the ferric HS species. As such, it seems more likely that LS formation is not purely restricted to molecules in the first layer but is achieved by an HS to LS transformation subsequent to reduction. LS formation requires binding of a strong ligand to the ferrous iron but without further information its nature remains speculative. In addition, we did not observe a gradual decrease of the oxidized LS species that even gained intensity at very negative potentials. However, due to the severe broadening of the spectral bands at these potentials, interpretation of such features becomes very difficult. One possible scenario that would be in line with the observed data is oxygen binding to the ferrous HS species resulting in the ferric LS−peroxide species seen in the SERR spectra. Due to the slow supply of protons, this peroxide species is not reacting further to H2O2 or water but is instead reduced further forming the ferrous LS species.

immobilization (Figure 2D), i.e., Ag|FeT3ThP(dc). The curves reflected a significantly lower catalytic activity and negatively shifted onset potential (≤0 V). The bare metal electrode exhibited a higher catalytic activity than Ag|FeT3ThP(dc), which indicates that (i) the coating inactivates Ag since oxygen molecules cannot reach the Ag surface anymore and (ii) the catalytic activity of the molecular film cannot overcompensate this effect. As a result, the ORR activity cannot clearly be assigned to the complex(es) immobilized on the surface, and a quantification of the catalytic activity was not pursued. The electrochemical/electrocatalytic data are compared in Table 1. Generally, independent of the adsorption procedure an improved catalytic performance was observed for FeT3ThP compared to FeTMP upon immobilization. Electrochemical SERR Spectroscopy (SERR-EC). Resonance Raman (RR) and surface-enhanced resonance Raman (SERR) spectroscopy using 413 nm laser excitation were performed on both complexes in solution and immobilized on the Ag surface via incubation cast (i.e., for Ag|FeTMP(inc) and Ag|FeT3ThP(inc), respectively, Figure 3). All spectra display the typical vibrational modes ν4, ν3 and ν2 in the spectral region between 1200 and 1700 cm−1 that belong to totally symmetric vibrations of the porphyrin. The spectra either in solution or on the surface at a constant potential of 0.1 V look similar to the oxidation and spin state sensitive ν4/ν2 bands located at 1357/1554 cm−1 and 1361/1552 cm−1 for FeT3ThP and FeTMP, respectively. In accordance with literature, we assign these bands to a ferric (Fe3+) high spin (HS) state.63 A more detailed component analysis (see section 2 in the Supporting Information) of the experimentally observed ν4 and ν2 bands in the SERR spectra at 0.1 V (Figure S7) suggests the presence of a small portion of reduced HS species and a low spin (LS) species in its ferrous and ferric state, respectively. The determined band positions of the different spin and oxidation states are presented in Table 2. Table 2. Experimentally Determined Positions of the ν4 and ν2 Vibrations of the Different Oxidation and Spin State Species Present in the SERR-EC Spectra of Ag| FeT3ThP(inc) (A) and Ag|FeTMP(inc) (B)a FeT3ThP ν4/cm HSox LSox HSred LSred

−1

1357 1363 1344 1352

FeTMP

ν2/cm

−1

1554 1558 1544 1565

ν4/cm

−1

1361 1367 1348 1355

ν2/cm−1 1552 1562 1542 1564

a

HSox: Ferric high spin. LSox: Ferric low spin. HSred: Ferrous high spin. LSred: Ferrous low spin).

The presence of two spin states has been previously observed by us when investigating immobilized hangman porphyrin complexes with mesityl substituents.23 In that work, we have assigned the two species to arise from heterogeneities in adsorption such that the LS species reflects complexes directly attached to the Ag surface (i.e., the first layer) whereas the HS species is indicative for complexes in the second or even more remote layer(s). LS formation was explained by the direct interaction of the iron with the chloride covered Ag electrode that might act as a strong fifth axial ligand (see also theoretical section). F


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Inorganic Chemistry Moreover, additional bands contributing to the broad ν4 envelope (i.e., 1336 cm−1) and the poorly structured region between 1480 and 1540 cm−1 at potentials below −0.6 V were observed. These bands are due to non-totally symmetric porphyrin modes and have been assigned previously by us to be characteristic for porphyrin dimerization.23 Such dimers could be formed via μ-oxo (or even μ-peroxo) bridging, which would strengthen our idea that incomplete oxygen reduction is responsible for the observed spectral pattern. We want to point out that μ-oxo formation is not excluded for FeT3ThP, but since no strong porphyrin deformation is needed to form the oxo bridge for this complex, it would not be visible in the Raman spectra. Upon changing the potential back to +0.1 V, the original oxidized spectral pattern is regained for both complexes albeit with less intensity (Figure S9). We therefore can state that the discussed bands observed at high overpotentials indeed represent electrochemical intermediates and that the overall decrease in signal intensity is mostly rationalized by molecule desorption. Other permanent changes of the porphyrin structure, i.e. by Fe−Ag exchange, would lead to irreversible occurrence of strong bands at 1336, 1533, and 1546 cm−1 (Figure S10).66 As no such bands are visible in the reoxidized species, we exclude such effects at the moment. In the oxidized as well as reduced state, the ν4 frequency of Ag|FeT3ThP(inc) appeared at 3−4 cm−1 lower wavenumbers compared to Ag|FeTMP(inc). The ν4 band is strongly indicative of the electron density in the porphyrin ring.67 Electron-donating ligands or substituents like thiophene increase the electron density in the porphyrin and, thus, partial occupation of the antibonding π* orbitals, which in turn affords a downshift, in particular, of the ν4 mode. Accordingly, it seems reasonable to assume that, among other factors, the more positive redox potential of FeT3ThP compared to FeTMP seen in the CV data is probably related to the increased electron density on the central metal ion due to the thiophene substituents. Different Modes of Adsorption of FeT3ThP and FeTMP. In contrast to the mesityl substituents in FeTMP, the thiophene groups in FeT3ThP display an intrinsic affinity toward Ag, suggesting their involvement in the fixation of the complex to the metal electrode.46,47 To test this hypothesis, we measured SER spectra of the immobilized FeT3ThP complex under off-resonance conditions (i.e., using 647 nm laser excitation). In this way, any molecular resonance enhancement of the porphyrin modes can be largely neglected and the relative intensities of the Raman bands of the porphyrin macrocycle and the thiophene substituents should be mainly controlled by their proximity and orientation with respect to the Ag surface. SER spectra in the spectral range from 600 to 900 cm−1 (low frequency) were measured for FeT3ThP using both immobilization methods on a roughened silver electrode (Figure 4, trace a: Ag|FeT3ThP(inc), trace b: Ag|FeT3ThP(dc)). Vibrational modes localized in the thiophene substituents were identified upon comparison with calculated spectra and experimental resonance Raman spectra with 413 nm excitation as the latter spectrum contains predominantly modes from the porphyrin coordinates. Details of the band assignment can be found in the Supporting Information, section S3 (Figure S11 and Table S2). The SER spectra recorded at 647 nm excitation show two major bands at 833 and 639 cm−1, that can readily be assigned to thiophene breathing modes including the sulfur atoms in agreement with

Figure 4. Low frequency SER spectra under 647 nm excitation of FeT3ThP immobilized via incubation cast (trace a) and dropcast method (trace b) on a roughened silver electrode vs Ag/AgCl (3 M KCl) in 0.1 M aqueous phosphate buffer solution at pH 7 and room temperature. For details of band assignment see the Supporting Information, section S3.

calculated frequencies at 866 and 635 cm−1 using DFT (Figure S11). Since the 866 and 635 cm−1 bands originate from inplane modes of the thiophene, their intensity enhancement requires not only a close proximity to the surface but also a largely perpendicular orientation.68 Such a configuration can easily be achieved if immobilization takes place via direct interactions of the thiophene sulfur atoms with the Ag surface. We now compare the results obtained for the two immobilization methods: Using the incubation cast method, the porphyrin modes in Figure 4 display lower relative intensities with respect to the thiophene modes than in the dropcast system. The same observation is seen in the high frequency area depicted in Figure S13. These findings indicate different average orientations of the adsorbed FeT3ThP complexes on the electrode. In fact, the attachment to the surface may be accomplished via two or four thiophene sulfur atoms, corresponding to a porphyrin orientation perpendicular or parallel to the surface, respectively, while in each case the thiophene substituents adopt a more perpendicular orientation in regard to the porphyrin macrocycle. Considering the surface selection rules (vide supra), binding via up to four substituents is expected to favor the enhancement of the thiophene relative to the porphyrin in-plane modes as compared to the attachment via “only” two thiophene sulfur atoms. Thus, we conclude that the incubation cast immobilization procedure leads to a larger extent of tetrapodal (i.e., all four thiophenes bound) adsorption than the dropcast method. This interpretation is in line with the observed shift of the 639 cm−1 band (that involves a contribution from the sulfur atom) by 4 cm−1 for incubation casted FeT3ThP and the significantly broader band envelopes of the 833 and 639 cm−1 modes for the incubation cast method that may reflect a mixture of tetrapodal and bipodal porphyrin attachment. In contrast, the narrow band shapes in dropcasted systems point to a predominantly bipodal binding of FeT3ThP. Theoretical Calculations. To obtain better insight into the compound-interface interaction, adsorption of the complexes on Ag electrodes was simulated by first-principle molecular dynamics (FPMD) calculations. For a good effigy of the experimental system, simulations were carried out using a G


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Inorganic Chemistry

Figure 5. FPMD simulations of adsorption dynamics of FeT3ThP and FeTMP on Ag electrodes. Initial structures of FeT3ThP (A) and FeTMP (B) at t = 0 ps. Pictures C and D show the respective complexes adsorbed via chloride ions (in orange color) on a Ag(111) surface after t = 8 ps.

Implications for ORR. The gathered data allow proposing a correlation between the different arrangements of the complexes on the different electrode surfaces and their electrochemical properties: A schematic picture of the proposed adsorption geometry of FeT3ThP and FeTMP on electrodes based on the results from electrochemistry, spectroscopy, and calculations is depicted in Figure 6. For FeT3ThP, we assume that attachment of the first layer to the Ag electrode is performed via covalent Ag−S bonds. The long incubation time of the incubation cast procedure may promote formation of the thermodynamically most stable configuration, i.e., the attachment via all four thiophene sulfur atoms, leading

bare and a chloride-covered silver electrode. In the latter case, the chloride ions were adsorbed on the fcc sites of Ag(111) surface with a coverage of Θ = 0.25. As initial structures, the porphyrin molecules were located approximately 6 Å from the surface (Figure S12). During the time evolution of the systems, the complexes approached the surfaces and after about 4 ps adsorption was achieved. In presence of chloride ions, the central ion of the porphyrin complex was found to bind to the Ag surface via the chloride ions yielding an average Fe−Ag distance of about 3 and 4 Å for FeT3ThP and FeTMP, respectively (Figure 5). In the absence of chloride, a weak interaction between the iron centers and silver atoms on the surface with a slightly longer Fe−Ag distance could be determined (not shown). The Fe−Ag interaction on surfaces was already investigated and analyzed in the literature previously.6 Besides coordination of the iron centers to the electrode surface, both complexes tended to adopt a preferentially flat porphyrin structure in the adsorbed state, which was better achieved for FeT3ThP than for FeTMP because of steric requirements of the meso-substituents. Independent of the presence of chloride ions, up to four sulfur atoms of FeT3ThP coordinated directly to the Ag surface resulting in an almost parallel orientation of the porphyrin plane to the metal surface and a slightly less perpendicular (28°) orientation of the thiophene rings in comparison to the gas phase. Albeit second layer formation and interaction with water molecules was neglected in these simulations, the results clearly show different orientational geometries of FeT3ThP and FeTMP on the Ag surface. FeT3ThP has a preferred flat adsorption geometry resulting in a smaller Ag−(Cl−)Fe distance and a nondistorted porphyrin macrocycle. In contrast, the bulky mesityl substituents in FeTMP afford an increased Ag− (Cl−)Fe distance concomitant with a larger displacement of the Fe atom out of the heme plane toward the Ag surface as the preferred adsorption geometry for the first adsorption layer.

Figure 6. Schematic representation of the proposed first layer structures of FeT3ThP via dropcast (A), incubation cast (B), and FeTMP (C, dropcast and incubation cast) on electrodes. H


Inorganic Chemistry

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to a largely parallel orientation of the porphyrin plane with respect to the electrode surface (Figure 6B). For dropcast systems, e.g., Ag|FeT3ThP(dc), the thermodynamically stable configuration may not be fully achieved such that, in this case, the bipodal binding may be favored (Figure 6A). In case of FeTMP, interactions with the Ag electrode are not sufficiently strong such that largely unordered layers are expected for incubation as well as dropcast method (Figure 6C). FeT3ThP and FeTMP show a similar onset potential for ORR on both Ag and BPG electrodes, but a higher overpotential, needed to achieve a certain catalytic current, is observed for FeTMP. For both complexes the activity for dropcasted systems is lower than the one for incubated systems leading to the conclusion that only the first layer of catalysts in direct contact with the electrode contributes predominantly to the ORR. This assumption is supported by the electrochemical resonance Raman measurements that only show potential spectral changes when silver electrodes are used that selectively enhance the Raman signal of the first molecular layers. Furthermore, from the SERR spectra, it becomes evident that a portion of the complexes is already reduced at +0.1 V and thus could participate in the ORR at the experimentally observed onset potential. Whether these are the molecules closest to the electrode cannot be unambiguously said from the spectra as both HS and LS species show partial reduction. In particular, the LS species could represent molecules in direct contact with the electrode. But since the measurements also suggest that molecules in higher layers can adapt a LS conformation, no definite conclusions can be drawn. Fast (multiple) ET from the electrode to the porphyrin is a crucial factor for ORR activity. In contrast to FeTMP, FeT3ThP can bind to metal surfaces via metal−sulfur bonds. An interaction of the porphyrin with the electrode via tetrapodal Ag−S bonds (Figure 6B) would allow better multielectron transfer, needed to fully reduce oxygen to water, than bipodal Ag−S bond attachment (Figure 6A). As this tetrapodal arrangement seems to be more promoted by the incubation cast procedure (Figure 5) we propose that longer incubation times of such catalysts on noble metal electrodes are favorable for ORR catalysis. These conclusions are supported by our initial RDE measurements. In contrast to the thiophene substituents, the bulky mesityl groups of FeTMP are designed to prevent interaction of the catalytically active Fe sites with other molecules or surfaces. While this property has been shown to improve homogeneous catalysis in solution, it prevents formation of a stable interaction with the electrode and leads to a less dense multilayer arrangement (Figure 6C). Furthermore, the occurrence of additional intermediate species at high overpotentials (characteristic for porphyrin vibrations at 1336, 1480, and 1540 cm−1, which were previously assigned to FeTMP dimerization),23 is a further hint that a significant fraction of catalysts even close to the surface is not sufficiently active for ORR. This explains why higher overpotentials are needed to achieve a certain catalytic current as in the case of FeT3ThP and why at higher overpotentials the electrode itself contributes to the ORR activity. Our results underline the importance of specifically designed heterogenized molecular catalysts possessing, if possible, suitable anchor groups for electrocatalytic applications.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00043.

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LSV curves of FeTMP and FeT3ThP, RR and SERR spectra with 413 nm excitation, and calculated and experimental Raman spectra with 647 nm excitation (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(M.S.) E-mail: [email protected]. *(I.M.W.) E-mail: [email protected]. ORCID

Alexander Croy: 0000-0001-9296-9350 Giancarlo Cuniberti: 0000-0002-6574-7848 Peter Hildebrandt: 0000-0003-1030-5900 Matthias Schwalbe: 0000-0003-4209-1601 Inez M. Weidinger: 0000-0001-9316-6349 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We want to address special thanks to Jana Staffa. The Project was financed by DFG (Cluster of Excellences UniCat (EXC 314) and UniSysCat (EXC 2008/1 - 390540038), SFB 1078 and SCHW1454/10-1). Additionally, we acknowledge the Center for Information Services and High Performance Computing (ZIH) at TU Dresden for computational resources.

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REFERENCES

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