Oxygen reduction reaction promoted by manganese porphyrins - ACS

Jul 27, 2018 - A second order dependence in HA is observed for unadorned Mn porphyrin platforms whereas with Mn hangman porphyrin, a proton is ...
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Oxygen reduction reaction promoted by manganese porphyrins Guillaume Passard, Dilek Kiper Dogutan, Mengting Qiu, Cyrille Costentin, and Daniel G. Nocera ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01944 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 27, 2018

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Oxygen reduction reaction promoted by manganese porphyrins Guillaume Passard,a Dilek K. Dogutan,a Mengting Qiu,a Cyrille Costentinb and Daniel G. Noceraa,* a

Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States. b Laboratoire d'Electrochimie Moléculaire, Unité Mixte de Recherche Université - CNRS N° 7591, Bâtiment Lavoisier, Université Paris Diderot, Sorbonne Paris Cité, 15 rue Jean de Baïf, 75205 Paris Cedex 13, France. Supporting Information ABSTRACT: The oxygen reduction reaction (ORR) is catalyzed by manganese(II) porphyrins in the presence of Brønsted acids (HAs). Analyses of the catalytic cyclic voltammetric profiles have permitted the ORR mechanism to be constructed and rate constants to be extracted for both the formation of the initial oxygen adduct and the O–O bond cleavage event for a series of HAs. The dependence of the formation rate constant of the oxygen adduct on reactant concentrations reveals a rate law that is first order in Mn porphyrin and oxygen substrate. A second order dependence in HA is observed for unadorned Mn porphyrin platforms whereas with Mn hangman porphyrin, a proton is provided intramolecular to the oxygen adduct and consequently the HA order is reduced to unity. The stabilization of the oxygen adduct with an additional hydrogen bond from HA engenders a rate-determining step involving O–O bond cleavage, resulting in the rare instance where the activation of the O–O bond is directly observed Keywords: Oxygen Reduction, Catalysis, Manganese, Porphyrin, Electrochemistry

Introduction The activation of small molecules for energy conversion fuels processes requires the coupling of electrons to protons so as to avoid highly energetic intermediates and their associated kinetics penalties.1–3 These conversion reactions invariably entail proton coupling to multielectron transformations attendant to bond cleavage and formation.1,3–6 Exemplar reactions are the cleavage of a C–O bond in the reduction of CO2 to CO,7,8 formation of an O–O bond in the oxidation of H2O to O22,9,10 and the cleavage of an O–O bond in the reverse reaction of the reduction of O2 to H2O11–15 with the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) representing critical half-reactions of the complete O2 ↔ H2O fuels cycle.12 Solar energy may be stored in the chemical bonds of the OER products, oxygen and hydrogen, and then recovered on demand with the ORR, via a hydrogen fuel cell. Whereas the proton-coupled electron transfer (PCET) reaction of OER at a mechanistic level has been explored for a limited number of systems,16–19 the PCET kinetics of ORR are less well defined with the notable exception of recent studies on a Fe porphyrin20–24 and other metallo pyrrole-based macrocycles.25,26 The products of ORR either cleave the O–O bond to produce water or retain the O–O bond to produce H2O2, which is more undesirable ORR product owing to its lower cell voltage. Therefore, the overall ORR selectivity is determined by whether bond cleavage driven by PCET occurs or does not. We have recently shown that the difference in ORR selectivity for H2O vs. H2O2 is determined by the overpotential of the ORR reaction, 27 which is largely dictated by the pKa of the proton donor. Nonetheless, kinetics analysis of the role of the acidic proton activity in determining the ORR mechanism remains largely unknown. A challenge in defining the ORR mechanism in terms of proton activity arises from the difficulty

Scheme 1 in extracting reliable kinetic information when significant current is partitioned between different ORR products. When this is not the case and the current is largely directed towards H2O formation, the kinetic activity can be low, requiring strong acid in nonaqueous conditions to drive ORR. Under these conditions, the proton activity overwhelms the kinetics and thus the mechanism can only be defined in the limiting case of very strong acid over a limited pKa range. We sought to investigate ORR with the criteria of (i) selective H2O production (ii) at sufficiently high kinetic efficiency to enable us to examine ORR over a wide range of pKas. We have found that Mn(II) porphyrins (Scheme 1), generated from the reduction of Mn(III) chloride porphyrin, promote ORR under these criteria. Manganese(II) tetraphenylporphyrin (MnTPP) and hangman porphyrin xanthene (Mn(HPX-CO2H)) catalyze O2 reduction to H2O in acetonitrile (MeCN) and does so in the presence of proton donors with pKas ranging from 10 to 20 units. We show by cyclic voltammetry that ORR is catalytic with the systematic variation of acid strength and concentration. Quantitative analysis of the CV waveforms furnishes rate constants for O2 binding to the Mn porphyrin platform and reveals that the rate determining step is O–O bond cleavage. Because O–O bond cleavage is able to be isolated, the Mn

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porphyrins provide the rare instance where the mechanism of O–O bond cleavage step in ORR may be explicitly defined.

Experimental Acetonitrile (MeCN) was purified and dried by passing through a neutral alumina column under argon. The supporting electrolyte, TBAPF6, was purchased from Sigma Aldrich (> 99%) and was recrystallized from a water-ethanol mixture and dried. All other chemicals including 5,10,15,20-tetraphenylporphyrinato manganese(III) chloride (Sigma Aldrich, > 95%), acetic acid (Sigma Aldrich, > 99.7%), chloroacetic acid (Sigma Aldrich, > 99%), 2, 4 dinitrobenzoic acid (Lancaster, > 99%), dichloroacetic acid (Sigma Aldrich, > 99%), trifluoroacetic acid (Sigma Aldrich. > 99%), trichloroacetic acid (Sigma Aldrich, 99%), deuterated acetic acid (Cambridge Isotope Laboratories, > 99.5%), deuterated trifluoroacetic acid (Cambridge Isotope Laboratories, > 99.5%), hydrogen peroxide (Sigma Aldrich, 50%) and tetraethylammonium acetate tetrahydrate (Sigma Aldrich, > 99%) were used as received. Varying O2 concentrations were delivered from a pre–mixed tank of nitrogen and oxygen purchased from Airgas. Manganese hangman porphyrins, Mn(II)(HPX-CO2Me) and Mn(II)(HPX-CO2H) and their Mn(III) chloride analogs were prepared by following previously developed methods28 with the introduction of microwave synthetic techniques for metal insertion. The details of the synthesis and characterization of the hangman porphyrins are provided in the Supporting Information (SI). Cyclic voltammograms (CVs) and rotating ring disk electrode (RRDE) experiments were performed using a CH Instrument potentiostat. Mn(III)Cl(TPP) was dissolved in a solution containing n-Bu4NPF6 (0.1 M) as supporting electrolyte. A three-electrode cell configuration was used where the counter electrode was a platinum wire, the reference electrode was a saturated calomel electrode (SCE) separated from the solution by a frit containing a solution of supporting electrolyte, and a glassy carbon working electrode that was meticulously polished before each measurement. The polishing procedure was performed on felt using different diamond pastes (15, 6, 3, and 1 μm), for ~2 min per size. Ethanol was used as a lubricant and electrode rinsed between each size of paste used for polishing. Before using, the electrode was briefly sonicated in ethanol and dried in a stream of compressed air. Between each CV measurement, only the 1 μm paste was used for polishing. The same procedure was used on electrodes employed for RRDE experiments. The RRDE and the rotator (MSRX) were purchased from Pine instruments and a threeelectrode configuration as employed for CV experiments. Ohmic drop was compensated by using the positive feedback compensation implemented in the instrument. All the potentials are referred to NHE by adding 0.241 V to the potential vs. SCE. A two-compartment cell was used for bulk electrolysis experiments with working and the reference electrodes in one compartment. The counter electrode placed in a separate compartment that also contained tetraethylammonium acetate (0.4 M), which served as a sacrificial donor at the counter electrode. The solvent for these experiments was acetonitrile and the supporting electrolyte was nBu4NPF6 (0.1 M). Experiments were performed with the cell under a stream of argon to purge the system of oxygen. The electrochemical cell used in this experiment was coupled to a gas chromatograph (SRI 8610C) to detect any oxygen that was produced.

Figure 1. (a) CV of Mn(III)Cl(TPP) (1 mM) in MeCN (0.1 M TBAPF6) on a glassy carbon electrode (d = 3 mm) under Ar (▬), and under O2 (8.1 mM) in presence of ClAcOH (50 mM) (▬) at a scan rate of 0.1 V s–1. (b) CV of Mn(III)Cl(TPP) (1 mM) under Ar at various scan rate of 0.1 (▬) to 5 (▬) V s–1. (c) Cathodic peak currents (circles) as a function of the square root of the scan rate (v). The slope of the solid line yields a diffusion coefficient of D = 8.4 × 10–6 cm2 s–1.

Results Electrochemistry of Mn(III)Cl(TPP) in the Absence of Oxygen. Two reversible reduction waves are observed in the CV of Mn(III)Cl(TPP) in acetonitrile and in the absence of oxygen (Figure S1). The reduction of Mn(III) to Mn(II) is assigned to the reversible wave at Eo[Mn(III/II)] = 0.01 V vs. NHE (Figure 1a). A second reduction at –1.25 V vs. NHE likely involves the porphyrin macrocycle as the reduction of Zn(II)TPP occurs at –1.18 V vs NHE in CH2Cl2.29 The peak current for freely diffusing molecules in the solution is given by, = 0.446

√ √

(1)

where S is the electrode surface area, C is the concentration of the substrate, D the diffusion coefficient of the substrate and ν is the scan rate. At low scan rates, ν < 5 V s–1 (Figure 1b), the peak current increases linearly with √ (Figure 1c) as predicted by eq (1), indicating a diffusion-controlled process with no interference of Mn(III)Cl(TPP) adsorption on the working electrode.30 If the molecule was adsorbed on the electrode, the peak current would vary linearly with ν. As defined by eq (1), the diffusion coefficient (D) may be extracted from the slope of the line in Figure 1c, and is found to be D = 8.4 × 10–6 cm2 s–1. From this value of D, the CV may be simulated if the peak separation between the anodic and cathodic trace is higher than 90 mV to yield kETo;31,32 Figure S2 shows that the CV taken at 5 V s–1 is reproduced for kET° = 0.3 cm s–1. The stability of Mn(III)Cl(TPP)/Mn(II)TPP system toward acid was assessed to define the limiting acid concentrations for ORR studies. Figure 2 shows the CV wave for the Mn(III/II) couple in the presence of acetic and trifluoroacetic acids; CVs for ClAcOH, 2,4-

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Figure 2. CVs of Mn(III)Cl(TPP) (1 mM) in MeCN (0.1 M TBAPF6) on a glassy carbon electrode (d = 3 mm) under Ar at a scan rate of 0.1 V s–1 for (a) AcOH additions from 0 (▬) to 1000 mM (▬) and (b) F3AcOH additions from 0 (▬) to 50 mM (▬).

dinitrobenzoic acid, Cl2AcOH and Cl3AcOH are shown in Figure S3. A small shift in the peak potentials and a marginal increase in the current is observed with the addition of acetic acid (Figure 2a). For the stronger acid, F3AcOH, the Mn(III/II) wave becomes irreversible and a second wave appears toward more negative potential (Figure 2b). The appearance of this second wave suggested a chemical transformation subsequent to the reduction of Mn(III) to Mn(II). We suspected strong acid may promote demetalation, which we confirmed by recording the CV of the free base porphyrin under the same acid conditions. The free base porphyrin exhibits a CV wave of similar potential and shape (Figure S4) as to the new wave observed in the CV of Mn(III)Cl(TPP) shown in Figure 2b. For the case of 2,4-dinitrobenzoic acid, a second wave is also observed after Mn(III) reduction (Figure S3b) but in this case it is due to the direct reduction of the acid (probably the nitro moiety), which we confirmed by measuring the CV of the native acid (Figure S5). From these studies, we determined limiting acid concentrations for ORR studies to be 1000, 1000, 50, 200, 50, and 50 mM for AcOH, ClAcOH, 2,4-dinitrobenzoic acid, Cl2AcOH, F3AcOH and Cl3AcOH, respectively. Electrochemistry of Mn(III)Cl(TPP) in the Presence of Oxygen. The Mn(III/II) reduction wave is extremely sensitive to oxygen and acid. Figure 1a shows that the reversible wave under inert atmosphere (black line) exhibits a catalytic response in presence of O2 and a proton donor (blue line), indicating that ORR is catalyzed by porphyrin in the Mn(II) formal oxidation state. The observation of a shallow peak in the catalytic wave is likely a consequence of substrate-limiting consumption,33 as the solubility of oxygen is low in organic solvents (O2 solubility in MeCN is 8.1 mM).34 The product distribution of the ORR reaction was analyzed by RRDE. The faradaic yield of H2O is often determined arbitrarily at the potential for which the disk current is highest. However, as we have recently discussed,27 a more accurate faradaic yield is obtained by taking into account the average yield throughout the entire potential range of ORR catalysis at different rotation rates according to the following, %H O ( ) =

2 ( )/ ( ) + ( )/

(2)

where ir is the ring current, id is the disk current, and N the collection efficiency geometrically given by the dimension of the electrode. This value of N for our electrode was determined to be 0.26 as measured with the Fc+/Fc redox couple. The RRDE experiments were

Figure 3. CV of Mn(III)Cl(TPP) (1 mM) in MeCN (0.1 M TBAPF6) on a glassy carbon electrode (d = 3 mm) under Ar (▬), in the presence of acetic acid (100 mM) (▬), and with the addition of H2O2 (1 mM) (▬); scan rate: 0.1 V s–1. Table 1. Faradaic ORR yields for H2O production by Mn(II)TPP. AH

pKaa

% H2O

AcOH

22.3

75 ± 16

ClAcOH

18.8

82 ± 8

2, 4-(NO2)2-PhCOOH

16.1

89 ± 1

Cl2AcOH

13.2

94± 1

F3AcOH

12.6

98 ± 1

Cl3AcOH

10.6

97 ± 1

a

Ref 35

performed on the six different Brønsted acids of varying pKas listed in Table 1.35 The potential at the disk was scanned through the appropriate catalytic region while the potential at the ring was held at 1.17 V (all potentials are reported vs. NHE) to ensure complete oxidization of H2O2. In applying eq (2), the average was taken over a potential range that gave a reliable measure of the current at the disk (id > 0.05 mA). Control experiments in the absence of catalyst show that there is no current on the ring regardless of the acid and its concentration within the limits of our potential window. RRDE traces and plots of eq (2) at different rotation rates for the ORR by Mn(II)TPP in the presence of the six different acids are shown in Figures S6 – S11 data; the average faradaic yields for H2O production as determined from these plots (id > 0.05 mA) are listed in Table 1. The data in Table 1 assumes that the H2O2 produced at the disk does not dismutate before reaching the ring. To check this assumption, two different experiments were performed to address whether Mn porphyrin catalyzes dismutation of hydrogen peroxide from the time it is generated at the disk and detected at the ring. Firstly, a CV was recorded on a Mn(III)Cl(TPP) solution after addition of a small amount of H2O2 under Ar. A catalytic current would be expected to arise due to Mn(III)Cl(TPP)/Mn(II)TPP catalyzed H2O2 dismutation. However, as shown in Figure 3, only a slight increase in current was observed, the magnitude of which is negligible as compared to the catalytic ORR current. Secondly, bulk electrolysis was performed under a flow of nitrogen linked that was fed into a gas chromatograph to detect O2, which is the product of H2O2 dismutation. The experimental results reveal that only 4 ppm of O2 over background was detected, which was the same amount detected in a control experiment in which no porphyrin was present. These results

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Figure 4. Normalized current by the one electron wave extracted from the CVs of Mn(III)Cl porphyrin (1 mM) in MeCN (0.1 M TBAPF6) on a glassy carbon electrode (d = 3 mm) under O2 (8.1 mM) at a scan rate of 0.1 V s–1 for AcOH additions from 0 (▬) to 1000 mM (▬) for: (a) Mn(III)Cl(TPP), (b) Mn(III)Cl(HPX-CO2H) and (c) Mn(III)Cl(HPX-CO2Me).

indicate that Mn porphyrin does not catalyze hydrogen peroxide dismutation on the time scale of the RRDE experiment and thus the faradaic efficiencies listed in Table 1 reliably effect the selectivity of the ORR by the manganese porphyrin. ORR Catalysis of Mn Porphyrins. The CVs of hangman porphyrins Mn(III)Cl(HPX-CO2H) and Mn(III)Cl(HPX-CO2Me) are similar to Mn(III)Cl(TPP). In the absence of oxygen and external acid, reversible reduction wave assigned to the Mn(III/II) redox couple is observed at Eo[Mn(III/II)] = –0.005 V and 0.009 vs. NHE, respectively for Mn(III)Cl(HPX-CO2H) and Mn(III)Cl(HPXCO2Me) (Figure S12). These values are similar to the standard potential of Mn(III)Cl(TPP) as expected since the electronic contribution of the xanthene backbone should be similar to a phenyl ring. A diffusion coefficient of D = 8.6 × 10–6 cm2 s–1 is obtained from the scan rate dependence of the CVs of the hangman porphyrins. The stabilities of the manganese HPX-CO2H and HPX-CO2Me systems under acidic conditions for various HA are similar to Mn(III)Cl(TPP) and the same limiting concentrations of HA were used for both ORR experiments. Figures 4b and 4c display the catalytic CVs of Mn(III)Cl(HPX-CO2H) and Mn(III)Cl(HPXCO2Me) in the presence of O2 with increasing concentration of AcOH, respectively. Both hangman porphyrins exhibit catalytic current responses similar to that of Mn(III)Cl(TPP) (Figure 4a).

Discussion Scheme 2 depicts a mechanism that is consistent with the observed reactant orders of the observed rate law and CV results. The reduced Mn(II)TPP formed at the electrode reacts with O2 and acid

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Figure 5. CVs of Mn(III)Cl porphyrins (a) Mn(III)(Cl)TPP (b) Mn(III)(Cl)(HPX-CO2Me) (c) Mn(III)(Cl)(HPX-CO2H) at: 1 mM in MeCN (0.1 M TBAPF6) on a glassy carbon electrode (d = 3 mm) (▬); and under O2 (8.1 mM) (▬); and in presence of 1 mM of Cl2AcOH (▬). Scan rate of 0.1 V s–1.

Scheme 2 to form an O2 pre-equilibrium complex, accounted for by the apparent rate constant k1. As is typically formulated for reduced porphyrin platforms,36,37 the O2 complex is perhaps better described as a Mn(III)TPP superoxide complex or, at a resonance limit, a Mn(IV) peroxy complex. The peroxy intermediate is stabilized by H bonding and thus two equivalents of HA drives the stabilization of the O2 adduct, which is the pre-catalytic state. The Mn(III) hydroperoxo cleaves to release water and a Mn(V) oxo complex as defined by the apparent rate constant k2, which captures the O–O bond breaking event. The Mn(V) oxo complex facilely returns to Mn(III) catalyst with the release of water under the reducing conditions of ORR. The cleavage of the O–O bond is driven by an additional equivalent of HA, which likely assists in the stabilization of the oxo product. The cyclic voltammetry results and observed reaction orders that

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Figure 6. Normalized current by the one electron wave extracted from the CVs of Mn(III)Cl(HPX-CO2H) (1 mM) in MeCN (0.1 M TBAPF6) on a glassy carbon electrode (d = 3 mm) under O2 (8.1 mM) at a scan rate of 0.1 V s–1 with ClAcOH 500 mV (▬). Fitting according to eqs (3) and (4) (▬). (b) FOW analysis of normalized current by the one electron wave extracted from the CVs of Mn(III)Cl(TPP) (1 mM) in MeCN (0.1 M TBAPF6) on a glassy carbon electrode (d = 3 mm) under O2 (8.1 mM) at a scan rate of 0.1 V s–1 with ClAcOH 1000 mV (▬). Fitting according to eq (5) (▬).

support the mechanism of Scheme 2 will now be presented. O2 Binding and Subsequent Reduction. The binding of O2 to the Mn(II) porphyrin platforms is revealed by the CVs shown in Figure 5. For all three porphyrins, the potential of the CV wave is invariant upon O2 addition in absence of acid but there is a slight increase of the cathodic wave, indicating that the adduct formed between O2 and Mn(II) porphyrin is more facilely reduced at the electrode than the starting Mn(II) porphyrin. The current of the cathodic wave associated with the O2 adduct remains well below the current expected for a two-electron reduction, thus establishing an overall ECE process. For both MnTPP and Mn(HPX-CO2Me), the peak potential of the anodic wave shifts positively for the complexes in the presence of O2 (red trace (bottom) as compared to black trace (bottom)), indicating that the Mn(III)TPP(O2●–) adduct formed on the forward scan is reoxidized at more positive potential than the oxygen free Mn(II) porphyrin. This suggests the slow formation of an off-pathway product, possibly a side-on peroxy adduct. In the presence of 1 mM of Cl2AcOH (blue trace), this shift in the anodic wave disappears for MnTPP and Mn(HPX-CO2Me) indicating the HA prevents formation of this off pathway adduct. Interestingly, no anodic shift is observed for Mn(HPX-CO2H) in the presence of O2 as the internal hydrogen bond from the hanging group enforces the preference for the on-cycle oxygen adduct via an internal hydrogen bond (vide infra). O–O Bond Cleavage. CVs in the presence of acid suggest that O2 binding is not rate limiting and rather O–O bond cleavage is rate determining step in the proposed catalytic cycle. The catalytic ORR waves at large acid concentrations (shown in Figure 4) exhibit shallow peaks owing to depletion of O2 as a result of fast catalysis, corresponding to pure kinetics conditions, and low solubility of oxygen in acetonitrile.33 At intermediate acid concentrations, the catalytic wave exhibits quasi S-shaped character with the observation of a quasi-plateau, as emphasized by the solid black line in Figure 6a. The rise of the wave (i.e., foot-of-the wave (FOW)) corresponds to a Tafel slope of ca. 60 mV as determined from FOW analysis (Figure 6b), indicating that electron transfer kinetics are not involved in the

Figure 7. Variation of the logarithm of k2 extracted from CV analysis for the ORR with different Brønsted acids (HA) as a function of the logarithm of the acid concentration. (a) with Mn(III)Cl(TPP) and AcOH (●, 0.95), ClAcOH (●, 0.95). (b) with Mn(III)Cl(HPX-CO2H) and AcOH (●, 1), ClAcOH (●, 0.95), Cl2AcOH (●, 0.9). The slope of the line that fits the data is given next to the symbol.

rate limiting step.38 Interestingly, the half-wave potential E1/2, defined as the potential where the current is-half of the plateau current is not equal to the standard potential Eo[Mn(III/II)] but it is anodically shifted. That this shift is larger than the smaller shift observed upon addition of acid in absence of O2 (Figure 2a) indicates that the former shift is significant kinetically. Indeed, deviation of E1/2 from Eo has already been shown to indicate, in the framework of two electron catalytic processes, that an intermediate species accumulates in the diffusion-reaction layer and that the rate determining step is not the first irreversible chemical step of the process.39 As shown in the SI, within this framework of an accumulating intermediate (i.e. k1 > k2), in pure kinetics conditions, and absence of mass transport effects for both O2 and acid, the observed overall current ( ) normalized by the current of the one electron Mn(III/II) reduction wave ( ) without acid and oxygen is given by: R 16 F = F 1 + exp R − 2.24

(3) /

with

/

=

[Mn(III/II)] +

R ln 1 + 8 F

(4)

The factors 16 and 8 in eqs (3) and (4), respectively, reflects the four-electron process associated with ORR (i.e., the Mn(III) reduction) where the first electron transfer occurs at the electrode and subsequent redox events occur in homogeneous solution owing to the high concentration of Mn(II)TPP proximate to the electrode surface (though the analysis is indifferent to a heterogeneous electron transfer). Because the E1/2 is positive to the standard potential Eo[(Mn(III/II)], from eq (4), we can infer that k1 >> k2, which is established by fitting the catalytic S-curves to eqs (3) and (4) (vide infra). A fit of eq (3) to the plateau of the quasi-S-shaped normalized CV gives k2 (Figure 6a) and a fit of the wave position (i.e. E1/2) with eq (4) allows k1 to be extracted (Figure S13). The values of k2 for O–O bond cleavage are presented on Figure 7 where data was analyzed

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Figure 9. (a) CVs of Mn(III)Cl(TPP) from 0.25 mM (▬) to 1 mM (▬) in MeCN (0.1 M TBAPF6) on a glassy carbon electrode (d = 3 mm) under O2 (8.1 mM) and in presence of 500 mM of AcOH at a scan rate of 0.1 V s–1. (b) Variation of the log k1, as extracted from the FOW analysis as a function of the logarithm of catalyst concentration. The observed data fits a line of slope 0, indicating a first order reaction with respect to the catalyst according to eq (5).

Figure 8. Variation of the logarithm of k1 extracted from CV analysis for the ORR with different Brønsted acids (HA) as a function of the logarithm of the acid concentration with: (a) Mn(III)Cl(TPP) and AcOH (●, 1.95), ClAcOH (●, 1.94), 2,4-dinitrobenzoic acid (●, 1.98), Cl2AcOH (●, 1.94), F3AcOH(●, 2.02) and Cl3AcOH (●, 1.99); (b) Mn(III)(Cl)(HPX-CO2H) and AcOH (●, 2), ClAcOH (●, 1, 2.1 ), Cl2AcOH (●, 1); and, (c) Mn(III)(Cl)(HPX-CO2Me) and AcOH (●, 1.85), Cl2AcOH (●, 1.6). The slope of the line that fits the data is given next to the symbol.

for acid concentrations that gave rise to an observed plateau. In the absence of quasi-plateau (i.e. at large acid concentration or with the strongest acids), kinetics analysis is restricted to the FOW where E >> E1/2. Under this condition, eq (3) reduces to,38 = 2.24

R F ( − 2 exp − R F

[Mn(III/II)])

(5)

allowing for k1 to be directly extracted from the foot of the catalytic wave, as shown in Figures 6b and Figures S14-S19 for Mn(III)TPP. Figure 8 summarizes the k1s obtained from the FOW analysis for the Mn porphyrins with selected acids. Comparing k1 and k2 for a given acid concentration in Figures 8 and 7, respectively, we find that k2 > k2 is not valid under low O2 concentration, which results from the low solubility of O2 in MeCN; accordingly, eqs (3) to (5) are not appropriate for FOW analysis for the

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system in the presence of HA and O2 as k2 >> k1. For k2 >> k1, the foot of the wave analysis (see SI) leads to,

= 2.24

F R ( − 4 exp − R F

[Mn(III/II)])

(6)

A plot of k1 (Figure 10b), obtained from a FOW analysis of the CV under varying concentrations of O2 (Figure S21), shows ORR to be first order in O2 (slope = 1.07). The HA reaction order has two different parentages: its role in stabilizing the O2 adduct and its role in ORR catalysis. The stabilization of the initially formed O2 adduct at the reduced porphyrin platform is important to ORR conversion,2,37 suggesting that HA forms a hydrogen bond(s) to the O2 adduct (Scheme 2). The consistent fits of a slope of 2 for Mn(II)TPP (Figure 8a) indicate that the initially formed Mn(III) superoxide-Mn(IV) peroxide intermediate is stabilized by two equivalents of HA. The order two in HA suggests that hydrogen bonding from two HAs drives the system to the peroxy intermediate, which is subsequently reduced by one-electron to form the Mn(III) peroxy pre-catalytic state. To further assess the HA equivalency, the same experiment was undertaken with the hangman porphyrin, Mn(II)(HPX-CO2H) (Figure S22). In the hangman construct, a pendant acid functionality within the secondary coordination sphere is poised above the porphyrin to allow an internal hydrogen bond to be formed to the oxygen substrate. Consistent with this contention, the reaction order of HA in k1 is reduced from 2 for Mn(II)TPP (Figure 8a) to 1 for Mn(II)(HPX-CO2H) at low acid concentrations ([HA] < 0.1 M, Figure 8b). At higher acid concentration ([HA] > 0.1 M, Figure 8b), this internal stabilization is overwhelmed and a second order in acid is recovered with Mn(II)(HPX-CO2H). When that internal hydrogen bond is removed in the hangman system by esterification of the acid group, Mn(II)(HPX-CO2Me) (Figure S23), a second order dependence is again observed in HA at all acid concentrations (Figure 8c). These results support that the superoxide is stabilized by multiple hydrogen bonding to HA. Based on the reaction orders, the k1 rate constant for oxygen adduct formation is given by, =

[O ][HA]

,

(7)

where KO2 accounts for the equilibrium for O2 binding to the Mn(II) porphyrin, and k1,0 accounts for the composite kinetics for the role of HA driving the system irreversibly to the Mn(III) hydroperoxy adduct, i.e., the species prior to the rate-determining step of O–O bond cleavage. For a given concentration of O2, eq (7) may be reduced to, log( ) = log

,

[O ] + 2 log[HA]

(8)

Figure 8a is indeed a plot of eq (8) where log(k1) vs pKa gives rise to a line of slope 2 with an intercept of log 1,0 O2 [O2 ] . The intercepts of Figure 8a contain information on how HA is involved in the overall k1 process, as it forms the basis for a Brønsted plot, which is a linear free energy relation between the kinetic rate constant and reaction driving force formulated in terms of pKa. A Brønsted plot

Figure 11. Plot of the intercept of eq (8) (i.e., intercepts of Figure 8a) as a function of the pKa of the acid for Mn(III)ClTPP. The linear fit of the data yields Brønsted slopes of α = –0.3.

Figure 12. Plot of the intercept of eq (9) (i.e., intercepts of Figure 7) as a function of the pKa of the acid for Mn(III)(Cl)TPP (●) and Mn(III)(Cl)(HPX-CO2H) (●). The linear fit of the data yields Brønsted slopes of α = –0.14 and –0.13 respectively.

thus provides a direct measure that the role that HA plays in O2 activation. Figure 11 presents the Brønsted plots for the O2 binding to Mn(II)TPP in the presence of the HAs listed in Table 1. The attenuated Brønsted slope of α = –0.31 is consistent with a reaction with a large driving force and thus with an irreversible step (formation of the hydroperoxide in Scheme 2) characterized by an early transition state. Similar Brønsted slopes of –0.3 have been observed for the dissociative activation of the C–O bond of CO2 by Fe porphyrins41,42 and the O–O bond cleavage of organic peroxides. Both reactions have been shown to couple electron transfer to chemical steps at large driving force.43 This suggests that the HA equivalency of two drives an electron transfer that is not rate determining to deliver the pre-catalytic Mn(III) hydroperoxy. The preparation of the pre-catalytic state together with the small k2 poises the system immediately prior to the O–O bond breaking step of ORR. Accordingly, analysis of the CVs furnishes the O–O bond cleavage rate constant. For both MnTPP and Mn(HPXCO2H) catalysts, the k2 rate constant is zero-order in porphyrin and O2 and first order in HA (Figure 7), giving rise to the following overall rate expression for O–O bond cleavage, log(

) = log

,

+ log[HA]

(9)

, accounts for the composite kinetics for the role of HA in the O– O bond breaking. We believe that the participation of HA in O–O

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bond cleavage is to stabilize the manganese oxo product. Figure 12 presents a Brønsted plot for the O–O bond cleavage rate constant. The very small Brønsted slope of α = –0.13 is consistent with HA participating as an active chemical promoter of bond cleavage as opposed to simply acting as a H-bond donor. ORR may inform on OER and vice versa as these reactions are ideally the microscopic reverse of each other. Especially important are intermediates unobservable in one reaction that may be captured in the reverse reaction. Such correlations between OER and ORR were recognized by Babcock,44 who showed that the associative O2 bond forming chemistry from water at the •YZ / OEC (Y = tyrosine, OEC = oxygen evolving complex) active site in Photosystem II is remarkably similar to the dissociative O2 bond breaking chemistry of •Y/ Cu-Fe heme active site in cytochrome c oxidase. As is the case for cytochrome c oxidase,45 the two-electron peroxy intermediate is critical to O–O bond cleavage and accordingly, the results reported herein point to metal bound peroxy intermediates as the linchpin for managing associative and dissociative O–O bond activation in driving energy conversion processes involving oxygen.

ASSOCIATED CONTENT

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Supporting Information Synthetic details for Mn hangman porphyrins, additional CV and RRDE experiment data, CV analysis, formal kinetics derivation.

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge Dr. Chong Liu for assistance with the bulk electrolysis system coupled to the gas chromatography. This material is based upon work supported under the Solar Photochemistry Program of the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences of the U. S. Department of Energy DE-SC0017619.

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