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In-situ Mechanistic Investigation of O2 Reduction by Iron-porphyrin Electrocatalysts by SERRS-RDE Kushal Sengupta, Sudipta Chatterjee, and Abhishek Dey ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01122 • Publication Date (Web): 01 Aug 2016 Downloaded from http://pubs.acs.org on August 3, 2016

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In-situ Mechanistic Investigation of O2 Reduction by Iron-porphyrin Electrocatalysts by SERRS-RDE

Kushal Sengupta, Sudipta Chatterjee and Abhishek Dey*

Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata, 700032.

*email: [email protected]

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Abstract Heme/porphyrin based electrocatalysts (both synthetic and natural) are known to catalyse electrochemical O2, H+ and CO2 reduction.

Lack of direct spectroscopic

investigations of intermediates formed on the electrodes during these processes limits detailed understanding of the mechanism of these catalysts which is essential for their development. Recently, rotating disk electrochemistry coupled to surface enhanced resonance Raman spectroscopy (SERRS-RDE) has been developed which could be used to study iron porphyrin electrocatalysts which reduce O2 in buffered aqueous solutions. Unlike conventional single turnover intermediate trapping experiments, SERRS-RDE experiments probe the system while it is under steady state. A combination of oxidation and spin state marker bands and metal ligand vibrations allowed identification of O2 derived intermediates formed on the electrode surface in-situ under physiological conditions. This approach which combines dynamic electrochemistry with resonance Raman spectroscopy and it has been used to in understand the role of different axial ligands and distal superstructures in the mechanism of electrochemical O2 reduction.

Keywords: Iron porphyrins, electrocatalysis, oxygen reduction, resonance Raman, SERRS-RDE, steady state


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1. Introduction Electrocatalysis has taken a center stage in today’s research due to its importance in the area of renewable energy. Electrocatalytic O2 reduction, O2 evolution, H2 evolution, H2 oxidation and CO2 reduction are thus areas of great interest and have attracted researchers across several disciplines of science.1-23 Both natural (metalloenzymes like Cytochrome C Oxidase (CcO), Hydrogenase, Glucose oxidase, etc.) and biomimetic (i.e. synthetic/biochemical mimics of the natural enzyme) systems have been demonstrated to function as electrocatalysts.24-31 In particular heme/porphyrin based electrocatalysts (both natural and synthetic) have been known to catalyze electrochemical O2, H+ and CO2 reduction for over five decades.29,32-42 Several excellent metalloporphyrin based O2 reduction electrocatalysts have been reported; some mimicking natural active sites and some inspired by their design.23,29,43-45 Among these catalysts, the iron based catalysts have provided valuable insights into the O2 reduction mechanism and a better understanding of the structure function correlations of key enzymes like CcO; the terminal enzyme in the mitochondrial electron transport chain.1,20,30,46-48 Similarly, several naturally occurring enzymes have been used as electrocatalysts for O2 reduction (e.g. CcO, microperoxidase) and substrate oxidation using O2 (e.g. cytochrome P450).25,49,50 Apart from porphyrins, transition metal complexes of corroles have been extensively used as electrocatalysts for O2 reduction and H2O oxidation.5,6 The electrocatalytic systems are efficient in catalyzing selective O2 activation/reduction reactions where the electrons (i.e. the reducing component) are provided from the electrode and O2 (the oxidizing component) is obtained from the bulk solution. An inherent advantage of this construct is its heterogeneity allowing use of water insoluble catalysts, to catalyze the reaction in an environmentally benign solvent like water. Several groups have conducted elaborate mechanistic investigations on these electrocatalysts. However, these investigations were limited to chemical and electrochemical perturbation.4,51 Moreover, there were no direct spectroscopic data on 3 ACS Paragon Plus Environment

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the intermediates formed during electrocatalysis in any of the studies. This is in sharp contrast to the investigation of O2 activation by iron porphyrin catalysts and heme enzymes in homogeneous solutions. In these cases a varied range of reactive intermediate species, formed during single turnover reactions, are reported. 31,52-61 The limitations for heterogeneous constructs mainly lies in investigating a thin layer of catalyst (a monolayer in many cases) on an electrode and getting good signal to noise ratio. Surface Enhanced Resonance Raman Spectroscopy (SERRS) is particularly suited for investigating metalloporphyrins and metalloenzymes attached to surfaces. The surface enhancement in addition to conventional resonance Raman effect (by exciting the intense absorption bands of these complexes) provides excellent enhancement of signals.62,63 Several symmetric ligand vibrations (intense in Raman spectroscopy) have been identified that reflect the oxidation and spin states of the metals chelated by these macrocycles with remarkable fidelity.64-66 In particular, the combination of the oxidation state marker band, ν4 (around 1330-1375 cm-1), and the spin state marker band, ν2 (around 1540-1575 cm-1), ligand vibrations are routinely used to diagnose the ground state electronic structure of iron porphyrin complexes and enzyme active sites. SERRS have been extensively used to investigate various physical aspects of cytochrome c functionalized electrodes; some of them under different applied potentials on the electrode.67 Studying reaction mechanism of a monolayer of catalyst/protein requires accumulation of good signal to noise ratio in a small time scale. For example, the catalytic cycle of CcO, which goes through several intermediates, takes less than one millisecond at room temperature.68,69 Getting reasonable data in such short time scales, without cryogenically trapping the intermediates (cannot be achieved during electrocatalysis), presents a formidable challenge. Alternatively, reactions can be monitored under steady state conditions. In such a case only the species whose rate of formation are higher than their respective rate of decay, in a catalytic cycle, will accumulate under steady state conditions. In principle, any spectroscopic data obtained 4 ACS Paragon Plus Environment

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will have contribution from all the species present and may be hard to resolve. However, in the case of metalloporphyrins, using a combination of the oxidation and spin state marker bands ν4 and ν2 in the resonance Raman spectrum (which are unique and resolvable, Table 1) and metal ligand vibrations (with appropriate isotopic substitutions), the individual intermediates may be identified. In a related approach the intermediates formed during the electrocatalytic H2O2/O2 reduction by Bi/Pd modified Au surfaces on a static electrode have been identified.70,71 In this perspective, SERRS-RDE, a SERRS set-up modified to accommodate a conventional rotating disc electrode (RDE),72 is described.73 First we present a case of a simple iron porphyrin catalyst, engaged in steady state electrocatalytic O2 reduction. The data obtained using SERRS-RDE provide direct first-hand information about the nature of intermediates involved in steady state O2 reduction and helps to define the mechanism of the catalyst. Then we discuss the role of axial ligands like imidazole and thiolate on the mechanism of O2 reduction where a combination of electrochemical and SERRS-RDE results help in understanding the different mechanism of the catalysis and the possible rate determining step.21,74 Finally we confer about the role of a distal metal center (here it is Cu) on the O2 reduction mechanism by an iron porphyrin, also engaged in steady state oxygen reduction at physiological pH.20

2. Developing SERRS-RDE and studying steady state O2 reduction by an iron porphyrin The experimental station is described in Figure 1. The working disc electrode is mounted at the tip of a conventional RDE shaft b. An air tight, water jacketed, three electrode electrochemical cell c with the counter electrode e and the reference electrode f, fitted through standard joints, is used in the set-up. The electrochemical cell is built with a pyrex optical window g such that both the incident laser and the Raman scattering is collected through this optical window. The scattered light is collected and reflected through a series of optics h and focused on the entrance slit of spectrograph j. The optics h may either be a combination of focusing plano-convex lenses and a reflecting mirror or a microscope. The incident light are marked with i, where i1 is the 5 ACS Paragon Plus Environment

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path when the combination of several optics is used and i2 is the path if an inverted microscope is used.

Figure 1. Schematic diagram of the SERRS-RDE experimental set-up. a is the RDE set-up, b is the shaft, c is the electrochemical cell, d is the taper plug assembly, e is the Pt counter electrode, f is the Ag/AgCl reference electrode, g is the optical window, h is the collecting and reflecting optics of the scattered light, i denoted the incident light paths and j is the entrance slit of spectrograph which in-turn is connected to the CCD.

An iron porphyrin catalysts is used for the investigation; iron α4-tetra-2-(4carboxymethyl-1,2,3-triazolyl)-phenylporphyrin (FeEs4)3 (Figure 2A). This is known to electrocatalytically reduce O2 and has been shown to have a hydrogen bonding distal superstructure75 that can stabilize a Fe-O2 adduct.58


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Figure 2. (A) Iron porphyrin complexes α4-FeEs4 and (B) CV and LSV data of FeEs4 physiabsorbed on C8SH SAM in deoxygenated and air saturated pH 7 buffers respectively at a scan rate of 50 mV/s using Ag/AgCl as reference and Pt wire as counter electrodes. For LSV a 200 rpm rotation was maintained. The dotted lines represent the variable potentials applied during the variable potential SERRS-RDE experiments.

A cyclic voltammetry (CV) scan of the FeEs4 catalysts (Figure 2B, green) immobilized on the octanethiol (C8SH) SAM covered Au disc electrode shows the E1/2 of the FeIII/II process at -100 mV vs Ag/AgCl in a de-aerated pH 7 buffer solution. The integrated charge under this wave indicates a surface coverage of 8 x 10 11 molecules/cm2. Such low surface density is consistent with the formation of a monolayer and not multilayer (typically 1014 molecules/cm2) of catalyst on the SAM.76,77 SERRS-RDE data obtained by applying positive (0 V) and negative (-0.5 V) potentials (i.e. at potentials above and below the E1/2) at 200 rpm rotation rate show the ν4 and ν2 vibrations (Table 1) indicative of oxidized, FeIII high spin (HS), (Figure 3, blue) and reduced, FeII HS, (Figure 3, red) iron porphyrin complexes, respectively. Alternatively, a linear sweep voltammetry (LSV) scan, performed in aerated buffer solutions using a RDE show that as the potential of the working electrode is gradually lowered below the E1/2, a catalytic current gradually rises due to electrocatalytic reduction of O 2 (Figure 2B, red) by the FeII porphyrin produced on the electrodes at these potentials (i.e. below E 1/2).


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This catalytic current becomes potential independent at lower potentials and becomes mass transfer limited. The current is determined by the flow of the substrate, O 2, to the electrode i.e. on the angular rotation rate of the electrode.71

Table 1. Oxidation and Spin state marker bands of Iron porphyrins during steady state conditions at room temperature

Fe(III) porphyrin78,64

Fe(II) Porphyrin64 Fe(IV)=O Porphyrin61

Oxidation State Fe(III)

Coordination Number 5

Spin

ν4 (cm-1)

ν2 (cm-1)

5/2

1364

1555

Fe(III)

6

5/2

1366

1555

6

1/2

1366

1565

Fe(II)

5

2

1344

1540

Fe(II)

6

0

1355

1557

Fe(IV)=O

6

1

1372

1571

Fe(IV)=O.+

6

1

1335

1517

Oxidizeda

FeEs4

Fe(III)

5

5/2

1364

1555

Reduceda

FeEs4

Fe(II)

5

2

1349

1547

Fe(III)

6

1/2

1369

1565

Fe(II)

5

2

1352

1540

Fe(IV)=O

6

-

1371

1571

During Steady Statea

FeEs4

Respective references are given in superscript.a Refer to figures 3 and 4 of this perspective and Reference 73. SERRS-RDE data collected on the electrode held at -0.5 V (i.e., during bulk electrolysis experiment using RDE) in air saturated pH 7 buffer show a different spectrum (Figure 3, green) compared to the one obtained in the absence of O2 (Figure 3, red).71 Several ν4 and ν2 vibrations are observed where a particular set of ν4 and ν2 vibration uniquely represents an iron porphyrin complex having a particular oxidation, spin and ligation state (Table 1). For the α4-FeEs4 complex there is a clear increase in intensity at 1369 cm-1 and 1565 cm-1 suggesting the formation of a low spin (LS) FeIII 8 ACS Paragon Plus Environment

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species (Figure 3, green) during turnover.64 Additionally, peaks are observed at 1352 cm1

and 1540 cm-1 (Figure 3, green) which are characteristic of the reduced FeII species.

There is also an increase in intensity at 1371 cm-1 and 1571 cm-1 which is clearly observed in the difference spectrum (Figure 4B) and are suggestive of formation of high valent FeIV=O species.61 Thus, in addition to the resting HS FeIII species, a HS FeII species, a LS FeIII species and a FeIV=O species are observed during steady state O2 reduction by the FeEs4 catalyst. In the low frequency region of the Raman spectrum, there are increase in intensities at 779 cm-1, 631 cm-1 and 570 cm-1 (Figure 3, green).

Figure 3. SERRS-RDE data of FeEs4 in the low (left) and high (right) frequency regions respectively where the oxidized spectra are shown in blue, reduced spectra under anaerobic condition in red and reduced spectra under aerobic condition in green.

The SERRS-RDE data were collected across the kinetic region by applying different potentials at the working electrode (rotation rate 200 rpm). The difference spectrum (data at a particular potential – resting oxidized), in case of FeEs4, in the low energy region show that as the potential is lowered, peaks at 830 cm-1, 782 cm-1, 631 cm-1 and 570 cm-1 gain intensity (Figure 4A), while the peak at 580 cm-1 region loses intensity, as the electrocatalytic O2 reduction current increases and approaches a potential independent mass transfer controlled limit. In the higher energy region the ν 4 and ν2 vibrations at 1352 cm-1 and 1540 cm-1 (HS FeII), 1369 cm-1 and 1565 cm-1 (LS FeIII) and 1371 cm-1 and 1571 cm-1 (FeIV=O), gain intensity (Figure 4A). Alternatively, the ν4 9 ACS Paragon Plus Environment

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and ν2 vibrations at 1364 cm-1 and 1555 cm-1 (HS FeIII) lose intensity (Figure 4A). Thus, as the catalytic current transitions from potential dependent region to mass transfer limited region, the HS FeII, the LS FeIII and a FeIV species gradually build up at the electrode at the expense of the HS FeIII species (Figure 4B). The data obtained at multiple rotations rates (Figure 4C) show that at rotation rates ≥ 200 rpm the distribution of the species observed remain the same indicating that the reaction is in a steady state and there is a build-up of different intermediate species involved in this multi-step catalytic reaction. Similar results were observed when we performed the similar experiments with Fe-tetraphenyl porphyrin (FeTPP), which is again a simple porphyrin lacking axial ligations.73


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Figure 4. Difference spectra of the SERRS-RDE data at different potentials -0.15 V (red), -0.25 V (yellow), -0.35 V (blue) and -0.5 V (green) from 0 V of FeEs4 (A) in the low (left) and high (right) frequency regions respectively. The ν4 and ν2 regions of the spectrum obtained from the difference of spectrum at -0.5 V from the spectrum at 0 V of FeEs4 along with their fits are shown in B. (C) The difference spectra obtained at different rotation rates for FeEs4. Experiments were performed in 18O2 saturated pH 7 buffers to gain insight into the nature of these intermediates. The variable potential SERRS-RDE data (Figure 5A, B) as well as the difference data show significant differences between

16

O2 and 18O2 data

(Figure 5C, D). In particular, the 830 cm-1 band, that gains intensity in 16O2 buffer (Figure 5A and C), loses intensity in the

18

O2 saturated buffer (Figure 5B and D). Similarly, the

631 cm-1 band loses intensity in the 18O2 buffer while it gains intensity in 16O2 saturated buffer, whereas the 580 cm-1 band loses intensity in

16

O2 buffer but not in

18

O2 buffer

(Figure 5). Note that, there is a larger increase in intensity in the 770-780 cm-1 region in the case of 18O2 buffer relative to the increase observed in 16O2 buffer. In particular, this peak, centered at 780.5 cm-1 in

18

O2 buffer, is asymmetric due to the presence of a

shoulder at higher energy which is not observed in 16O2 buffer. This indicates the growth of a new peak in

18

O2 buffer. Further, the intensity of the band at 746 cm-1 increase

more in 18O2 buffer than it does in 16O2 buffer. The differences in the intensities of these vibrations in the low energy region of the spectrum between

16

O2 and 18O2 containing

buffers imply the presence of Fe-Ox species which are derived from O2 during reduction. The 16O2/18O2 difference spectrum (obtained at different potentials) reveals a clear shift from 830 cm-1 to 783 cm-1 (Figure 5E and F). There is an additional shift from 631 cm-1 to 594 cm-1. Note that these differences only arise at potentials where electrocatalytic O2 reduction is observed and their intensities increase as the potential is lowered and hence arise due to incorporation of

18

O2 into intermediates formed during

electrocatalytic O2 reduction.


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The peak at 782 cm-1 that gradually gained intensity in the variable potential SERRS-RDE experiment in both

16

O2 and

18

O2 (Figure 5) possibly arises from an

intermediate formed during O2 reduction as well. However, clear isotopic shift was not observed for these species. Note that a weak peak which grows in at 752 cm-1 in the 18

O2 difference spectrum could be due to isotope shift of this species i.e. 782 cm -1 in 16O2

to 752 cm-1 in 18O2. This possibly arises from a FeIV=O (ferryl) species. Ferryl species are known to undergo fast exchange of the ferryl oxygen with water.79,80 Thus any FeIV=18O formed will quickly exchange in H216O in the medium to form FeIV=16O leading to limited population of FeIV=18O species.81 The 830 cm-1 and 631 cm-1 bands represent the O-O and Fe-O vibrations of a low spin FeIII-OOH species respectively and the 782 cm-1 band represents the Fe-O stretch of a FeIV=O species. These assignments are consistent with the observation of ν4 and ν2 bands characteristic of LS FeIII species and FeIV species in the variable potential SERRS-RDE data.64 In addition to the SERRS-RDE data, the formation of the high-valent FeIV=O species was also evaluated by utilizing this species (E0 ~ 1V) produced during O2 reduction on the electrode to chemically oxidize [Fe(CN)6]4- in situ. The E0 for [Fe(CN)6]3/4-

is 250 mV vs Ag/AgCl at pH 7. The FeIII porphyrin species used here, which have E0 of –

100 mV, are incapable of oxidizing [Fe(CN)6]4- to [Fe(CN)6]3-. In a RRDE set-up any [Fe(CN)6]3- produced on the working electrode is radially diffused to the Pt ring (due to the hydrodynamic current produced by the rotation of the electrode bearing cylindrical shaft) and can be detected by reducing it back to [Fe(CN)6]4- by applying a potential of 0 V on the Pt ring (the Pt ring is independently addressable in a RRDE experiment using a bi-potentiostat). A clear [Fe(CN)6]3- reduction current is observed in the ring and the onset of this current coincides with the onset of the O2 reduction current. This indicates that the [Fe(CN)6]4- present in the solution is getting oxidized to [Fe(CN)6]3- by the FeIV=O species produced on the electrode during O2 reduction.77 It is important to note that the observation of a FeIV=O species on the electrode implies that its reduction to FeIII-OH is slow in spite of a large driving force for ET from the electrode (held at -0.5 V) to the FeIV=O species (Eo = ~1 V). This is the case in the active site of CcO where the ET from the 12 ACS Paragon Plus Environment

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heme a to heme a3 (with marginal driving force) is > 105 s-1 82 whereas the ET from heme a to PR (the FeIV=O species produced during O2 reduction) is ~ 103 s-1 in spite of having much larger driving force.83


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Figure 5. SERRS-RDE data of FeEs4 obtained at various potentials under 16O2 (A) and 18O2 (B) containing pH 7 buffers. The spectra obtained from the difference of various potentials from 0 V for

16

O2 and

18

O2 containing buffers are shown in (C) and (D)


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respectively. E represents the difference spectra of

18

O2 from

16

O2 obtained at

respective potentials. The shifts obtained along with their fits are shown in F.

Based on the intermediates identified, a mechanistic proposal of O2 reduction by this Fe porphyrin has been shown in Scheme 1. The SERRS-RDE data indicate that FeII species, HS FeIII species, LS FeIII-OOH species and FeIV=O species are present during steady state electrocatalytic O2 reduction. Furthermore, the variable potential SERRSRDE data indicate that as the potential is lowered, there is an increase in the population of the HS FeII species, LS FeIII-OOH species and FeIV=O species and a decrease in population of the resting HS FeIII species. Thus, at potentials where the current is potential dependent, the population of the O2 derived species is less as the reduction of the HS FeIII to the catalytically active FeII state (iii in Scheme 1) is the rate determining step leading to a higher population of the resting high spin FeIII species on the electrode. However, as steady state O2 reduction current is obtained at low potentials, the LS FeIIIOOH species is observed along with a HS FeII species and a FeIV=O species. This implies that the decay of these species i.e. reductive cleavage of the O-O bond of the FeIII-OOH species (iv  v, Scheme 1), the O2 binding to FeII (ii  iii, Scheme 1) and reduction of FeIV=O (v  i, Scheme 1), are slow steps compared to their respective formation steps during O2 reduction. At this stage we could not find evidence of a Fe-O2 adduct in these data. Since resonance Raman is not quantitative, the relative population of these species i.e. the relative rates of these steps, cannot be determined using these data. It however establishes, with confidence, the species accumulated under steady state and provides a qualitative description of the rate determining steps in O2 reduction by the electrocatalyst.


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Scheme 1. The oxygen reducing mechanism by an iron porphyrin showing the various possible intermediates. The marker bands of the individual species are given in boxes with the ν(Fe-O) and ν(O-O) values (16O2/18O2) correspond to those observed for FeEs4.

3. Investigating O2 reduction mechanism by Fe-porphyrins differing in axial ligation or distal superstructure Axial ligands play an indisputably important role in modulating and determining the electronic structure and reactivity of iron porphyrin active sites found in either naturally occurring heme-based metallo-enzymes or in synthetic models.84-86 The effect of axial ligation can be exquisitely understood if one considers that while hemoglobin (Hb) and myoglobin (Mb), both utilizing the imidazole head group of a histidine residue to bind iron, bind O2 for storage or transport,87,88 cytochrome P450 (cyt P450), which utilizes the thiolate group of a cysteine residue to bind heme, not only binds O 2 but also activates it to hydroxylate inert C-H bonds.89,90 This difference in reactivity towards 16 ACS Paragon Plus Environment

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oxygen can be attributed to different extent of push effect exerted by these two axial ligands.74,77,91 Efforts have been focused on developing functional mimic of active sites that reproduce the structural and/or functional properties of these enzyme active sites. However, a direct comparison between an imidazole bound and a thiolate bound heme and/or iron porphyrin centers is often complicated by changes in the enzyme active site due to point mutation and/or lack of appropriately designed synthetic model complexes. Although we have already seen that the electrochemical O2 reduction by simple Fe-porphyrins (FeEs4) involves various intermediates, how the ORR mechanism changes or affected by axial ligands is not well understood. In order to understand this, we have used imidazole (PIM) and thiolate (PPSR-yne) bound Fe-porphyrin complexes in next two sections where mainly SERRS-RDE along with electrochemical data helps in understanding the ORR mechanisms which intrinsically depends on the nature of axial ligands.21 a. Case of Imidazole Imidazole bound PIM (Figure 6A), physiadsorbed on EPG electrode, shows clear FeIII/II redox couple in pH 7 buffer in absence of O2 (orange trace, Figure 6B) which is found to be pH dependent having a slope of 51 mV/pH, consistent with a 1e -/1H+ concerted proton electron transfer (CPET) pathway. Electrochemical along with isotope sensitive SERRS data proves the presence of Im-FeIII-OH (6C)  Im-FeII (5C) + H2O equilibrium. SERRS data obtained at -0.5 V reflects that the reduced FeII species is 5C HS in nature as well. In the presence of oxygen, PIM shows ORR at potentials similar to formal FeIII/II potentials in pH 7 which implies that the potential determining step (PDS; defined as the most thermodynamically uphill step in the catalytic ORR cycle involving lowest reduction potential during ORR process) of ORR for PIM involves the FeIII to FeII reduction step (blue trace, Figure 6B).


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Figure 6. (A) meso-Mono [o-5-(N-imidazolyl) valeramidophenyl]-triphenylporphyrinato iron(III)bromide (PIM), an imidazole bound complex, used for the study; (B) Overlay of CV (under anaerobic conditions) and LSV (under aerobic conditions) for PIM immobilized on EPG surface.

Electrocatalytic O2 reduction properties have been investigated using rotating disk electrochemistry (RDE) and rotating ring disk electrochemistry (RRDE) techniques in neutral H2O and D2O buffers.21,74 While RDE data yields the number of electrons involved in ORR and overall catalytic rate (kcat) of ORR evaluated from Koutecky-Levich equation, RRDE data yields the selectivity of ORR by measuring the amount of partially reduced oxygen species (PROS) produced due to incomplete reduction of O 2.92,93 The kcat values obtained on edge plane graphite (EPG) surface (kET >105 s-1) in both pH 7 and pD 7 are estimated to be 2.3±0.26 x 106 M-1s-1 and 2.2±0.34 x 106 M-1s-1 and indicate the presence of a minor kinetic solvent isotope effect (KSIE) of 1.04. The Tafel slopes for O 2 reduction under slow scan rates and at various rotation speeds have been found to be close to 120 mV/dec which suggests that a 1e- RDS in the kinetic regime is likely to be involved in ORR.94 The PROS values obtained for PIM show that the isotope effect on the amount of PROS is maximum in EPG (~4.7) which decreases to 2.5 and 2 in case of C8SH (kET ~103 s-1) and C16SH (kET ~6-10 s-1) SAM modified electrode, respectively. The data are suggestive of the fact that the PROS producing step must be H/D isotope sensitive irrespective of ET rate between the electrode and the catalyst.21


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SERRS-RDE data of PIM in the oxidized condition in pH 7 show the presence of FeIII HS species (Figure 7A). In the presence of O2, when the electrode is held at reducing potential, where it is involved in steady state (i.e, at -0.5 V vs Ag/AgCl), the ν2 band at 1565 cm-1 corresponding to a FeIII LS species is found to increase in intensity (Figure 7B) relative to the resting oxidized state. Along with the LS Fe III species, a FeII HS species having ν4 and ν2 bands at 1350 cm-1 and 1548 cm-1, respectively, grows in (Figure 7B). The same species with almost similar intensities can be observed in D 2O under steady state conditions having ν2 bands at 1542 cm-1 and 1565 cm-1 and ν4 bands 1348 cm-1 and 1363 cm-1 corresponding to FeII HS and FeIII LS species, respectively (Figure 7C and 7D).21 This is noteworthy that the accumulation of LS FeIII species is almost similar in intensity in both H2O and D2O buffers when a difference spectrum between H2O and D2O is being considered (Figure 8A). This is consistent with a minor H/D isotope effect (~1.04) on the decay of the LS FeIII species.


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Figure 7. SERRS-RDE data of PIM in the high frequency region under oxidized (orange for pH 7 and blue for pD 7) (0 V vs. Ag/AgCl) and steady state (blue for pH 7 and green for pD 7) (-0.5 V vs. Ag/AgCl) conditions in air saturated pH 7 (A and B) and pD 7 (C and D) buffered solutions at 200 rpm rotation. The ν4 and ν2 bands of the steady state spectra of PIM along with their Lorentzian fits showing different components are presented in B and D.

Figure 8. (A) Overlay spectra of SERRS-RDE data of PIM, physiadsorbed on C8SH modified Ag electrode in the high frequency region, under steady state conditions in air saturated pD 7 (red) and pH 7 (blue) buffers. The corresponding difference spectrum is shown by black dotted line. (B) Simulation of experimental points for the amount of PROS produced for PIM immobilized on EPG at different pH buffers (pKa model fit).

The electrochemical and SERRS-RDE data along with existing knowledge about O2 reduction mechanism by iron porphyrin complexes suggest a O2 reduction mechanism for the imidazole ligated Fe-porphyrin complexes, PIM (Scheme 2) which is slightly different from that proposed for simple Fe-porphyrins like FeEs4. The reduction of the resting FeIII species (i, scheme 2) in PIM follows a CPET pathway (step 1a, scheme 2) which give rise to 5C FeII HS (ii, scheme 2) upon reduction. We have observed that kcat of ORR for PIM does not show significant H/D isotope effect in the mass transfer limited 20 ACS Paragon Plus Environment

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region where the overall kinetics is determined by a step that does not involve ET. Thus the steps 1a, 1c, 1d, 1f and 1i in Scheme 2 cannot be the RDS in this region. The accumulation of a LS FeIII species in the SERRS-RDE data at these potentials along with the absence of H/D isotope effect on the kcat suggest that either step 1j or step 1e are the RDS in the mass transfer limited region. The homolysis of an imidazole bound FeIIIOOH species (step 1j, scheme 2) is calculated to have high energy barriers. However, the 1st order rate in the RDS is found to be 103 times higher than the value evaluated considering homolysis of FeIII-OOH species using Eyring equation.21 O2 binding to imidazole bound iron porphyrins (similar protein active sites and model complexes) has rates > 107 M-1s-1 and is thus unlikely to be the RDS.95 Thus the heterolytic cleavage of the FeII-OOH species (step 1e) is likely to be the RDS for PIM.

Scheme 2. Proposed mechanism of oxygen reduction by imidazole bound Fe-pophyrin, PIM immobilized on surface under physiological conditions.


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The pKa of proximal oxygen for an imidazole bound FeIII-OOH species is estimated to be 7.15±0.05 obtained from the amount of PROS produced at different pH buffers (Figure 8B).21 It is suggested that reduction of the central metal can lead to an increase of 36 units in the pKa of bound –OH ligands for iron systems.96 Thus it is quite likely that the pKa of FeII-OOH species is >40 which is significantly higher than the pH range evaluated and hence no pH dependence on kcat is observed. Protonation of the proximal O-atom of the FeIII-OOH species (v, scheme 2) will produce FeIII-H2O2 species (viii, step 1g, scheme 2) that will release H2O2 in the solution to be detected in the RRDE experiments. In fact, a single protonation equilibrium having pK a of (7.15±0.05) is consistent with the isotope effect of 4.7 observed for the PROS production step here as observed in the active sites of heme and non-heme enzymes where similar protonation step is associated with reasonable H/D kinetic isotope effects (~2-4.2).97-99 In the case of PIM, the pKa of FeIII-OOH species is low and it does not undergo fast O-O bond cleavage as generally observed for O2 reducing enzymes and other synthetic Fe-porphyrins. Rather it has to be reduced to LS FeII-OOH before the O-O bond can be cleaved in the RDS of 4e-/4H+ O2 reduction in the mass transfer limited region. In the kinetic region, where the ET from the electrode is slow, protonation of the proximal oxygen of LS FeIII-OOH species is found to be the RDS for the 2e-/2H+ O2 reduction process.

b. Case of Thiolate Thiolate bound PPSR-yne complex (Figure 9A), physiadsorbed on surface, show FeIII/II redox couple in pH 7 buffer in absence of O2 (red trace, Figure 9B) similar to PIM. Unlike imidazole bound complex, CV for thiolate bound complex show very little pH dependence between pH 7-10 consistent either with a RS-FeIII  RS-FeII [both 5C HS species] or RS-FeIII-OH2  RS-FeII-OH2 [both 6C LS species] redox equilibrium in this pH range.21,77 The possibilities of the presence of either 5C FeIII or 6C FeIII under resting state can be delineated by SERRS-RDE data which will be discussed in the following section. In the presence of O2, PPSR-yne shows ORR, obtained from LSV data, at 22 ACS Paragon Plus Environment

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potentials (EORR) which are distinctly more negative than the formal Fe III/II potentials suggesting that the FeIII → FeII reduction step is not the PDS in pH 7 (green trace, Figure 9B).

Figure 9. (A) Thiolate bound complex, PPSR-yne used for this study; (B) Overlay of CV (under anaerobic conditions) and LSV (under aerobic conditions) for PPSR-yne immobilized on EPG surface in pH 7 buffer.

The O2 reduction activity of PPSR-yne was evaluated using RDE and RRDE t pH 7 and RT.21 The kcat values are estimated to be 1.1±0.2 x 107 M-1s-1and 0.61±0.07 x 106 M1 -1

s in H2O and D2O buffers, respectively under fast ET rate which yielded H/D isotope

effect on overall kcat ~18. The Tafel slopes of PPSR-yne for O2 reduction under similar conditions have been found to be very similar to those of PIM which apparently suggests that a 1e- RDS is involved during ORR in the kinetic regime. The PROS data obtained for PPSR-yne at various ET rate in both H2O and D2O follow similar trend with smaller isotope effects compared to PIM. However, the H/D isotope effect on kcat of 2ereduction process is markedly different for these two complexes. RRDE data obtained for this thiolate bound complex indicate that upon lowering the potential i.e. increasing the driving force, both the 2e-/2H+ current responsible for O2 → H2O2 conversion and the 4e-/4H+ current responsible for O2 → H2O conversion increases (vide infra) which, in turn, indicate that a) the RDS for both the processes involve ET in the kinetic regime and 23 ACS Paragon Plus Environment

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b) the rate of former is enhanced relative to the rate of latter on increasing the driving force. This observation is completely different in case of imidazole bound complex PIM suggesting that PPSR-yne must follow a different ORR mechanism. SERRS-RDE experiments during steady-state O2 reduction has therefore found to be the ‘central dogma’ in determining probable intermediates, supplementing the information obtained from the electrochemical analyses. SERRS-RDE data of PPSR-yne in pH 7 under resting state show the ν4 band at 1363 cm-1, 1354 cm-1 and the ν2 band at 1555 cm-1, 1559 cm-1 (Figure 10A) corresponding to a FeIII HS and FeII LS species, respectively.21 Lorentzian fits of the ν2 and ν4 region of the data indicate the presence of a peak at 1567 cm -1 corresponding to LS FeIII species.21 In the presence of O2 in pH 7 buffer under steady state conditions the ν2 band at 1564 cm-1 corresponding to a FeIII LS species is found to increase in intensity (Figure 10B). Along with the FeIII species some FeII HS species as well as some FeII LS, corresponding to ν4 bands at 1340 cm-1, and 1353 cm-1 and ν2 bands at 1542 cm-1, and 1557 cm-1, respectively, have been found to increase in intensity (Figure 10B). SERRSRDE data in pD 7 buffer show the presence of the same species under steady state conditions (Figure 10C and 10D). Interestingly, the accumulation of LS FeIII species (shown by black arrow in Figure 11) is higher in D2O buffer compared to H2O buffer for PPSR-yne when a difference spectrum between H2O and D2O is being considered (Figure 11A). The higher accumulation of LS species in D2O for PPSR-yne implies a significant H/D isotope effect on the decay of the LS FeIII species which is not the case for PIM, as discussed in the previous section.


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Figure 10. SERRS-RDE data of PPSR-yne, physiadsorbed on C8SH modified Ag electrode, in the high frequency region under oxidized (orange for pH 7 and blue for pD 7) (0V vs. Ag/AgCl) and steady state (blue for pH 7 and green for pD 7) (-0.5 V vs. Ag/AgCl) conditions in air saturated pH 7 (A and B) and pD 7 (C and D) buffered solutions at 200 rpm rotation. The ν4 and ν2 bands of the steady state spectra of PPSR-yne along with their Lorentzian fits showing different components are presented in B and D.


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Figure 11. (A) Overlay spectra of SERRS-RDE data of PPSR-yne in the high frequency region, under steady state conditions in air saturated pH 7 (blue) and pD 7 (red)buffers. The corresponding difference spectrum is shown by black dotted line. (B) Plot of the ratio of ring current to disk current versus the potential to the disk applied for PPSR-yne in both pH 7 (blue dice) and pD 7 (green squares) buffers.

Based on the assimilated electrochemical and spectro-electrochemical data, an ORR catalytic cycle can be proposed (Scheme 3) for the thiolate bound PPSR-yne complex. The electrochemical data suggest that in the mass transfer limited region there is a KIE ~18 on the overall kcat. Therefore, the steps 2c, 2e, 2f, 2g and 2h in Scheme 3 cannot be the RDS in this mass transfer limited region rather the protonation of a LS FeIII-OOH species (iv, scheme 3) is likely to be the RDS in this region. The observation of LS FeIII species and its higher population in D2O in the SERRS-RDE data during steady state (Figure 11A) is consistent with this proposal. The 2e-/2H+ O2 reduction step shows an H/D isotope effect of ~47, estimated from H/D isotope effect values on 4e- kcat and amount of PROS.21 Further a plot of the ratio of 2e-/2H+ and 4e-/4H+ current (Figure 11B) indicates that the 2e- current increases monotonously with increasing driving force even at high over-potentials in H2O where the 4e- current is mass transfer limited. The monotonous increase in the 2e- current with increase in applied over-potential is limited in D2O relative to H2O reflecting the large KIE on the 2e-/2H+ reduction current. Both the above facts (i.e. the increase in 2e-/2H+ current with applied over-potential and the large 26 ACS Paragon Plus Environment

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H/D KIE) suggest that the step resulting in 2e-/2H+ oxygen reduction is CPET. Protonation of proximal oxygen of a FeIII-O2-. species (iii, scheme 3), formed upon binding of oxygen to FeII (step 2b and 2i, scheme 3), can be a CPET step resulting in a FeIII-(HO)O- species (vii, scheme 3) which has the neutral oxygen of the hydroperoxide anion bound to Fe III and is thus prone to hydrolysis. The hydrolysis of the above species will result in production of H2O2 (step 2h, scheme 3), the 2e-/2H+ reduction product of O2 which is detected as PROS in RRDE experiments. The greater KIE in the CPET to the proximal oxygen relative to the distal oxygen is likely due to a greater barrier in transferring a proton (likely from the solvent) and an electron simultaneously to the sterically protected bound proximal O-atom relative to the unbound distal O-atom, making this step to be the RDS for 2e-/2H+ O2 reduction cycle.

Scheme 3. Proposed mechanism of oxygen reduction by thiolate bound Fe-pophyrin, PPSR-yne immobilized on surface under physiological conditions.

Our investigations of analogous thiolate bound iron porphyrin complexes have indicated that while the resting FeIII species exists mainly in the 5C HS state in these 27 ACS Paragon Plus Environment

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complexes, the reduced FeII species is a mixture of 5C HS and 6C LS states.74 It has also been observed that the LS FeII state is associated with weaker O2 binding kinetics compared to its HS analogue. Thus we believe, in this case, the accumulation of a LS FeII species along with the LS FeIII species in the SERRS-RDE data for PPSR-yne under steady state is observed for the aforesaid reason. In the thiolate bound PPSR-yne, the selectivity of O2 reduction is determined by the site of proton transfer to the bound superoxide species. The proximal oxygen, which is bound to the iron in the porphyrin ring, is difficult to access by the proton donor solvent relative to the distal oxygen. Thus the donor acceptor distance in the CPET transition state (TS) is likely to be higher in the former case resulting in larger KIE for the 2e- process.21 c. Case of axial imidazole and distal copper In order to investigate the role of both axial ligand and distal superstructure to obtain significant knowledge and mechanistic insight in electrocatalytic ORR by Feporphyrin complexes, synthetic mimic of cytochrome c oxidase (CcO) might be the most appropriate to discuss about (Figure 12). In nature, CcO catalyses the selective and efficient 4e−/4H+ reduction of O2 to H2O in the terminal step of the electron transport chain during respiration, avoiding the formation of detrimental reactive oxygen species (ROS).47,68,100,101 The X-ray structures of CcO reveal the active site responsible for O2 binding and reduction, to be a binuclear site comprising of heme-a3, with a proximal histidine (bearing imidazole) along with a distal side histidine (three imidazoles) bound copper (CuB).68,102,103 While the formation of bridging peroxide FeIII-(O2)-2-CuII (either side-on or end-on depending on the strength of the external ligand) intermediates preceding PM (ferryl FeIV=O and CuII−OH/H2O), formed after the O−O bond cleavage during O2 reduction cycle in CcO, have been characterized crystallographically or spectroscopically in some CcO active sites and invoked in a few investigations, their relevance in catalytic O2 reduction is not clear.56,104-106 The investigation of synthetic heme-Cu systems has greatly assisted our understanding of the reaction mechanism of O2 reduction and factors that lead to selective 4e−/4H+ reduction of O2 avoiding formation of detrimental PROS.1,29,43,56,107 28 ACS Paragon Plus Environment

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While Collman and co-workers have reported the initial formation of an Fe-O2 adduct followed by PM-like intermediate formation with their functional CcO model under rate limiting conditions,1,29 Karlin’s group have stressed on the formation of a bridging peroxide as a possible intermediate during O2-reduction under single turnover conditions.56,108,109 These bridging peroxo intermediates can also be interconverted, in some cases, from a side-on to an end-on product in synthetic myoglobin110,111 or CcO models107,108 upon incorporation of an external strong field ligand like imidazole or its analogues. In spite of obtaining significant knowledge and insight into the O 2 reduction mechanism under single turnover conditions in organic solvents, the mechanistic details in an aqueous environment remains unresolved due to the lack of direct spectroscopic evidence.

Figure 12. 6L-Fe (1), 6L-FeCu (2) and 6L-Fe(Im)Cu (3) are the synthetic model complexes used for this case. In this case, the electrocatalytic O2-reduction of the 6L-FeCu complex (2, Figure 12) and its imidazole adduct 6L-Fe(Im)Cu (3, Figure 12) have been investigated at physiological pH using SERRS-RDE under steady state conditions. Results from study of the Fe-only complex (1, Figure 12) serve as a control to evaluate the role of the copper. Although the presence of Cu ion was initially thought to be essential for selective 4e – reduction of O2, it has now been established that the presence of Cu is only necessary when the ET from the electrode is slow.29,112


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SERRS data have been obtained for these three catalysts immobilized on C 8SH SAM modified surfaces in both their respective resting/oxidizing and reducing conditions in aqueous pH 7 buffer. While SERRS data for 6L-Fe complex shows the presence of a 5C FeIII HS species on the surface under resting conditions, the formation of a HS FeII species is observed upon reduction. SERRS data for 6L-FeCu and 6L-Fe(Im)Cu indicate the presence of 6C FeIII LS species under resting state.20 Similarly, when these two complexes have been reduced, 6L-FeCu leads to the formation of a HS FeII species, while 6L-Fe(Im)Cu mainly gives rise LS FeII species along with minor HS FeII component as well. In the case of complex 6L-FeCu, the resting oxidized state is observed to be LS when immobilized on the electrode unlike in an organic solvent where it is high spin. The change in spin state in 6L-FeCu is likely because that complex 6L-FeCu bind water (from bulk solvent) as an axial ligand (Scheme 4). However, for complex 6L-Fe(Im)Cu the sixth position has already been occupied by the external imidazole ligand before immobilization and it is LS in both organic solvent and aqueous medium (Scheme 4). Upon reduction, FeII HS and FeII LS species have been obtained as the major components for complex 6L-FeCu and 6L-Fe(Im)Cu, respectively. Hence it may be proposed that complex 6L-FeCu and complex 6L-Fe(Im)Cu exist as 5C water bound FeII HS and Im-water bound

6C

FeII

LS

,

respectively (Scheme

4) when reduced.20


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Scheme 4. Possible co-ordinations of FeCu complexes under oxidizing and reducing conditions.

The electrocatalytic O2 reduction properties have been investigated for these CcO mimics in air saturated pH 7 buffer using RDE and RRDE techniques. The RDE results indicated that while 6L-Fe and 6L-FeCu complexes reduce O2 almost selectively to H2O by 4e- process under fast ET rate, 6L-Fe(Im)Cu does not do so under similar condition (reduces O2 by 3.4e- process). RRDE data show that the amount of PROS produced at pH7 by 6L-Fe under fast (EPG), moderate(C8SH SAM) and very slow (C16SH SAM) electron fluxes are 6±1%, 9±0.5%and 23±3%, respectively. We find that with the “extra” redox center, the copper ion in 6L-FeCu, PROS quantities decrease, compared to 6L-Fe which is more prominent under very slow electron flux. The estimated PROS generated by 6LFeCu are 6.3±0.1%, 7±0.1% and 13.5±1% when physiabsorbed on EPG, C8SH SAM and C16SH SAM respectively. As observed for other CcO mimics and porphyrin complexes, 6LFe and 6L-FeCu show the same general trend of increasing PROS generation with a decrease in the rate of electron transfer from the electrode. However, 6L-Fe(Im)Cu complex shows the opposite trend. While on EPG the amount of PROS produced is 25±2%, i.e. only 75% of the O2 was fully reduced by a 4e–/4H+ process to give H2O and consistent with 3.4e- process, on C8SH and C16SH SAM modified Au electrode, the magnitude of PROS is 10±1% and 4±1%, respectively. The diminished amount of 2e –/2H+ reduction product (i.e., H2O2) with a decrease in ET rate is unprecedented for synthetic heme-Cu systems. Better understanding of this phenomenon were reached when SERRS-RDE experiments under steady state conditions were performed for these catalysts. SERRS-RDE data, collected on C8SH SAM modified Ag disks, of 6L-Fe complex during steady state O2 reduction i.e., catalytic turnover shows the presence of four species in the high frequency region. The major species has ν4 and ν2 at 1362 and 1556 cm-1, respectively, corresponding to HS FeIII species (Figure 13D, red). There are two 31 ACS Paragon Plus Environment

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additional minor components having the ν4 and ν2 bands at 1350 and 1545 cm-1 (Figure 13D, green)and at 1375 and 1572 cm-1 (Figure 13D, black), respectively. The first minor species correspond to a HS FeII species and the other may correspond to a FeIV=O species as observed for other Fe-porphyrin complexes.61,64,73 Note that a very weak FeIII LS component in the ν2 band could also be observed at 1565 cm-1. Under similar steady state conditions for 6L-FeCu a mixture of species is observed in the high frequency region (Figure 13B) which could be resolved to three components with Lorentzian fits of the ν4 and ν2 bands (Figure 13E). The major species is a LS FeIII complex with ν4 and ν2 bands at 1366 and 1567 cm-1.113,114 The other components correspond to a HS FeII species having ν4 and ν2 bands at 1349 and 1542 cm-1, respectively, and a HS FeIII species with the ν4 and ν2 bands at 1359 and 1555 cm-1, respectively (Figure 13E).20 SERRS-RDE data for 6L-Fe(Im)Cu complex during steady state O2 reduction shows the presence of LS FeIII, LS FeII and HS FeII species (Figure 13C and 13F). The major component is the LS FeIII species with the ν4 and ν2 bands at 1368 and 1569 cm-1 (Figure 13F, red). Fits to the ν4 and ν2 region clearly show the presence of bands at 1345 and 1544 cm -1 corresponding to a HS FeII species whereas the bands at 1357 and 1559 cm-1 correspond to a LS FeII species (Figure 13F).64,73 Note that we observed a larger intensity of the LS FeII species relative to the HS component that can be due to the higher resonance enhancement and/or a greater population, the latter being more likely.20


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Figure 13. SERRS-RDE data for 6L-Fe (A), 6L-FeCu (B) and 6L-Fe(Im)Cu (C) in the high frequency region at oxidized (red) and steady state conditions (blue) in air saturated pH 7 buffer under aerobic conditions at a constant rotation rate of 200 rpm. The difference spectra are shown in green. The ν4 and ν2 bands of the spectra obtained during steady state O2 reduction of these complexes along with their Lorentzian fits showing different components are given in (D), (E) and (F), respectively. 33 ACS Paragon Plus Environment

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SERRS-RDE data for 6L-FeCu in the low frequency region shows the presence of an iron-peroxo intermediate (either bridged or terminal), with an O-O stretching vibration (νO-O) at 819 cm-1 in 16O2, shifting to 759 cm-1 in 18O2 (Figure 14A).20 Unlike for simple Fe-porphyrin systems like 6L-Fe or FeEs4, a FeIV=O vibration was not observed. The SERRS-RDE experiment, performed in air saturated pD7 buffer, shows no shift in the νO-O indicating that the species is a peroxo bridged complex, (i.e., Fe-O22--Cu), and not an iron hydroperoxide (FeIII-OOH) complex.20 Unfortunately, the low frequency region of the spectrum does not reveal corresponding Fe-O/Cu-O vibrations for the bridgedperoxo 6L-FeCu complex.20 However, the νO-O absolute value and its observed 60 cm-1 16/18

O shift (vide supra) are consistent with the O-O bond stretching frequency of Fe-

peroxo species, excluding the possible formation of any high-valent FeIV=O species where a 25-30 cm-1 downshift is generally observed on

18

O substitution. Please note

these data are collected in an aqueous medium and one may expect complications/variabilities arising out of hydrogen bonding and high dielectric constant of water.

Figure 14. SERRS-RDE data in the low frequency region under steady state conditions in the presence of air and 18O2 saturated pH 7 buffer for 6L-FeCu (A) and 6L-Fe(Im)Cu (B) on C8SH modified Ag surfaces. The difference spectra are shown in dotted line (black).


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For 6L-Fe(Im)Cu with imidazole axial ligand, a prominent peak at 847 cm-1 appears, under steady state conditions in an air saturated pH 7 buffer, in the SERRS-RDE data which is observed to shift to 786 cm-1 (61 cm-1 16/18O2 downshift) in 14B). Note that the magnitudes of the

16/18

18

O2 (Figure

O isotope shifts (>60 cm-1) for both the

complexes are larger than the calculated values for the harmonic O-O diatomic oscillators which is likely due to the mixing or mode-coupling of the fundamental O-O stretching mode with porphyrin vibrations. A weak 16/18O2 sensitive νFe-O band was also observed.20 Here too no high-valent FeIV=O intermediates could be detected. This relatively high energy of νO-O in 6L-Fe(Im)Cu compared to 6L-FeCu is likely due to the formation of an end-on (µ-1,2) bridged peroxo complex. This is because of the presence of a stronger σ-donor imidazole ligand in 6L-Fe(Im)Cu which facilitates the formation of an end-on bridging peroxo moiety and not a side-on coordination, resulting in lower νO-O stretching vibrations, as also known from literature reports on similar CcO model complexes in organic media.108,115,116 We thus understood that the vibration observed for 6L-FeCu at 819 cm-1, which shifts to 759 cm-1 in

18

O2 but does not shift on

deuteration, indicates that a) a bridging peroxide is involved in the mechanism and b) the O-O bond cleavage of this peroxy species is likely to be a slow step in the mass transfer limited region of electrocatalysis. In contrast to the 6L-FeCu complex, which produces only 6% PROS, the imidazole bound analogue 6L-Fe(Im)Cu (imidazole bound trans to the bridging hydroxo ligand in the resting oxidized state), produces significant amounts of PROS (25%) during O2 reduction on an EPG electrode. However, SERRS-RDE data on this complex shows the presence of a 16/18O sensitive O-O vibration at 847 cm-1 (786 cm-1) and Fe-O vibration at 545 cm-1 (524 cm-1). Thus a bridging peroxide ligand is an intermediate in the O2 reduction cycle even with an imidazole axial ligand and the heterolytic O-O bond cleavage is a slow step as well. Note that the presence of end-on peroxide species has been demonstrated in X-ray structures of CcO104,106 and their involvement in O2 reduction has been proposed for “mixed valence” CcO (i.e. where just the FeCu active site is reduced).117


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Since SERRS-RDE is performed at a potential where catalytic current is mass transfer limited i.e. no change in the catalytic current with increase in driving force and the current depends only on the supply of species from the aqueous phase, as discussed in previous section any species whose decay involves an electron transfer cannot be the rds and will accumulate during steady state at these potentials. Consistently, it is observed that FeII species (both HS and LS) which decay by binding oxygen (derived from bulk solvent) for both 6L-FeCu and 6L-Fe(Im)Cu accumulate in the SERRS-RDE data. This is because of the formation of FeII (a, Schemes 5 and 6) via reduction of FeIII (via ET from electrode) is faster at these potentials than O2 binding. For 6L-FeCu, while O2 binding in the distal pocket (i.e. containing the Cu) will lead to O-O cleavage and subsequent O2 reduction, O2 binding to the proximal site, replacing the axial water ligand, will likely result in the production of PROS (c, Scheme 5). We believe the FeIII LS species observed for both the complexes is the bridging peroxide (b, Scheme 5 and c, Scheme 6) (the metal ligand region in the SERRS-RDE spectra also reflects the same) because its decay involves proton (from bulk) with subsequent O-O bond cleavage leading to the formation of high-valent compound I (d, Schemes 5 and 6). However, compound I cannot be isolated as an intermediate during steady state may be because of the immediate reduction of this high valent species by electron transfer from the electrode which is held at -0.5 V vs Ag/AgCl during SERRS-RDE experiments i.e. its decay is faster than its formation precluding its accumulation during steady state. Thus the data indicate that both O2 binding and heterolytic cleavage of O-O bond are slow. However, from the greater intensity LS FeIII component in the ν4 and ν2 region relative to the FeII component, it may be proposed that decay of the bridging peroxide species via protonation is slower than decay of FeII resulting from O2 binding. The relative values of the O-O suggest that the peroxo intermediate formed under steady state conditions in the 6L-FeCu complex has a side-on coordination, whereas the one formed in the 6L-Fe(Im)Cu complex is bound in an end-on geometry. These assignments are in good agreement with the previously reported homogeneous data where similar shifts are observed upon switching a side-on to end-on 36 ACS Paragon Plus Environment

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intermediate.108,109,116 Considering the fact that the peroxo adduct of the 6L-FeCu complex is 6C LS (based on the 4 and 2 values), the binding motif of the bridging sideon peroxo to the Fe and Cu centers is likely be µ-η1:η2 (η2 at Cu and η1 at Fe; b, Scheme 5) in an aqueous environment and not µ-η2:η1 as observed in organic solvents.108,109,116 The O-O observed in the SERRS-RDE data, here, in an aqueous environment, are ~30 cm1

higher than the values observed in structurally characterized high-spin Fe-(µ-η2:η1-

peroxo)-Cu model compounds in organic solution. We believe, these differences may arise from differences in polarity of the medium or hydrogen bonding from water, or due to the change from high-spin to low-spin, or of course due to proposed change to being side-on to Cu (Scheme 5).

Scheme 5. Plausible mechanistic pathway during steady state O2 reduction by 6L-FeCu complex.


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Scheme 6. Plausible mechanistic pathway during steady state O2 reduction by 6LFe(Im)Cu complex. 6

L-Fe(Im)Cu shows selective 4e–/4H+ O2 reduction under slow ET flux (kO2 for the LS FeII center), the distal Cu reduces O2 to produce PROS (b, Scheme 6). As the ET rate is slowed down and is comparable to or even slower than the O 2 binding rate to LS FeII (i.e. kET≈ kO2 or kET 47. Previously, apart from enhanced rate of ORR, not much difference in their reactivities could be identified.120 Similarly, the role of Cu in reducing PROS could be established by the bridging peroxide identified. Unusual species identified indicate unusual side reactions which were not invoked in erstwhile investigations. The LS Fe II species responsible for PROS generation by outer sphere ET is a good example of that. As a newly developed approach, SERRS-RDE has shown promise as a way to understand the mechanistic details of metalloporphyrinoid complexes in general.121 Over the last two years integration of a microscope has enhanced the signal to noise ratio by more than an order of magnitude. As a result good quality spectra can currently be obtained within 30-60 seconds (e.g. Fig. 14) whereas the original set up required 600 s to get reasonable data (e.g. Fig. 4). SERRS-RDE is still a work in progress and much of its scope is yet to be explored. So far all the investigations have been limited to porphyrins and corroles. Porphyrins offers an advantage to the approach as the 40 ACS Paragon Plus Environment

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oxidation and spin state marker bands help distinguish the species. However such calibrations need to be developed for metallocorroles before SERRS-RDE can be used to its full advantage in understanding the reaction mechanism of metallocorrole based electrocatalysts in-situ. The requirement of Ag surfaces to perform these SERRS-RDE experiments in contrast to Au surfaces which are more commonly used for RDE and RRDE experiments, is a current limitation. These surfaces are not stable at potentials where oxygen evolution or hydrogen evolution generally occur. The SERS intensity drops as the thiol chain length increases making investigations under slow ET fluxes inaccessible for now. Catalysis on graphite electrodes, the more common approach in electrocatalysis, needs to be investigated using SERRS-RDE. To achieve that a way to obtain SERS enhancements can be affected on these surfaces needs to be discovered. Another current limitation of the approach is the requirement of large volume of solvent in the current set-up, which renders any investigations with labelled isotope an expensive enterprise. Thus spectroelectrochemistry cells requiring smaller volumes of solvent yet allowing to achieve steady state needs to be developed. However the advantage of the approach is clear as it allows unprecedented insight into the details of electrocatalysts in operando. Along with using SERRS-RDE on different interesting systems, effort towards eliminating the above limitations are underway.

Acknowledgements The works presented here has been supported by Department of Science and Technology, India (Grant SB/S1/IC-25/2013). KS and SC acknowledge Council of Scientific and Industrial Research, India and IACS for fellowship.

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