Oxygen reduction reactivity of cobalt(II) hangman porphyrins

Abstract

Cobalt(II) hangman porphyrins are delivered from easily available starting materials, in two steps, in good yields, and with abbreviated reaction times. Selected compounds from a library of Co(II) hangman porphyrins immobilized on multiwall carbon nanotubes establish that the four-electron four-proton catalytic reduction of oxygen to water in aqueous solution can be achieved at the single cobalt center of the hangman platform. Reaction trends within the library reveal that the selective reduction of O₂ to H₂O occurs at electron deficient hangman porphyrin platforms possessing a distal group that is capable of proton transfer.

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Introduction

The four-electron four-proton activation of the O–O bond of oxygen at high efficiency is of technological consequence for the replacement of platinum at the cathode of proton exchange membrane fuel cells.^1,2 At a fundamental level, the oxygen reduction reaction (ORR) provides mechanistic insights for promoting the reverse reaction, water splitting, which is central to the storage of solar energy.^3–5 Porphyrins are prominent catalysts for the ORR^6–10 and are known to promote the reaction with appreciable efficiency when another metal is in the secondary coordination sphere of the macrocycle.^6–9,11 Bimetallic porphyrin systems have been proposed to attain their reactivity by fulfilling two general requirements: they (i) bypass one- and two-electron redox transformations through redox cooperativity, and (ii) hinder the protonation and release of two-electron peroxo-type intermediates.^6,12 This reactivity model, however, does not account for all ORR chemistry involving porphyrin macrocycles. Enhanced reactivity is observed for a single porphyrin macrocycle^13–15 or for porphyrins in which the second coordination site is vacant or occupied by a redox-inactive metal ion.^16–19 We have rationalized these observations by showing that the function of the secondary site, whether that be another porphyrin, metal complex or metal-free distal site, is to adjust the pK_a of dioxygen adducts such that proton-coupled multi-electron transfer reactions (PCmET) are favored.²⁰ These PCmET reactions promote O–O bond cleavage and in doing so avert hydrogen peroxide formation. To this end, ORR should be a general reaction for porphyrins with metal-free distal sites. We now provide further support for this contention by examining the ORR properties of hangman porphyrins, which poise an acid/base group over the face of the porphyrin macrocycle²¹ containing a cobalt metal center.²² By constructing a library of meso-substituted cobalt hangman porphyrins (CoHPX), we show that ORR is profoundly affected by the electronic properties of the macrocycle. In addition, we immobilize CoHPX on multiwall carbon nanotubes (MWCNTs) and find that the high surface area and attendant catalyst loadings offered by MWCNTs engender high current densities for ORR.

Experimental

All electrochemistry measurements were performed on CoHPX immobilized on MWCNTs. Thin films of the electrodes were prepared by drop casting the CoHPX-MWCNTs onto the surface of a glassy carbon (GC) electrode as has previously been described.^23,24 For the systems described herein a stock solution (2.5 mM) of catalyst in THF was prepared. An aliquot of stock solution was combined with 4 mg MWCNT, and 45 μL 5% Nafion^® and the final volume adjusted to give 3.9 mL of 0.3 mM catalyst solution in THF. The mixture was sonicated for 20 min and then a 10 μL drop was applied to the surface of the GC. Evaporation of the THF produced a thin film containing a MWCNT loading of 50 μg_c cm⁻² with a catalyst loading of 3 nmol and a thickness of 0.2 μm. The film thickness was calculated based on the amount of carbon deposited on the surface. Under these loading conditions, the reaction of H₂O₂ within the electrode film, which artificially inflates the observed number of electrons transferred (n) during oxygen reduction, is expected to be minimal.²⁵

Cyclic voltammograms (CV) were measured on CoHPX complexes dissolved in acetonitrile solution containing 0.1 M NBu₄PF₆ (tetrabutylammonium hexafluorophosphate). The reference electrode used in the 3-electrode configuration was Ag/AgNO₃ and all potentials were referenced against the ferrocene/ferrocenium couple, which was used as an internal standard.

Electrocatalytic activities of CoHPX-MWCNT films were ascertained by rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) techniques. For ORR activity measurements, the CoHPX-MWCNT working electrode was immersed in 0.5 M H₂SO₄. A 3-electrode configuration was employed in which the working electrode was serviced by a platinum wire counter electrode and saturated calomel reference electrode (SCE) that was calibrated against the reversible hydrogen electrode (RHE) reference potential. In RRDE experiments, the platinum ring was held at 1.2 V vs. SCE. Collection efficiencies were calibrated against an iron ferricyanide solution.²⁶

RDE plots were analyzed according to the Koutecky–Levich (KL) equation to assess the apparent number of electrons transferred at various potentials.

1/i_app = 1/i_k + 1/i_lim = 1/i_k + 1/0.062nFC_oD^2/3ω^1/2ν^−1/6 (1)

where i_app is the measured or apparent current density, i_k is the kinetic current density, i_lim is the limiting current density, n is the number of electrons transferred, F = 96485 is Faraday's constant, C_o = 1.8 × 10⁻⁵ M is the concentration of oxygen, D = 1.1 × 10⁻⁶ cm² s⁻¹ is the diffusion coefficient of oxygen, ω is the rotation rate of the electrode in rpm, and ν = 1.0 × 10⁻² V s⁻¹ is the kinematic velocity. Plots of inverse current versus the inverse square of the rotation rate give straight lines, where the number of electrons may be extracted from the slope.

The %H₂O₂ and the number of electrons were calculated from the RRDE data according to the following relationship between the ring and disk currents.

%H₂O₂ = (2I_r/N)/(I_d + (I_r/N)) × 100 (2)

n = 4I_d/(I_d + (I_r/N)) (3)

where I_d is the disk current and I_r is the ring current before correction, in mA cm⁻². The collection efficiency of the ring, N = i_r/i_d, was determined to be 0.25 using the reversible [Fe(CN)₆]^2−/3− redox couple, according to the manufacturer’s instructions. The %H₂O was calculated from (1 − %H₂O₂).

Results

The CoHPX series shown in Scheme 1 was synthesized using a statistical and concise synthetic method that delivers hangman porphyrins from easily available starting materials, in two steps, in good yields, and with abbreviated reaction times (4–16 h).²² The details of the synthesis and compound characterization including elemental analysis, electronic absorption, ESI-MS and LD-MS spectra of CoHPX complexes are provided in the ESI.†

Scheme 1

CoHPX-3 was crystallized by slow evaporation of a dichloromethane–hexanes solution. The structure is shown in Fig. 1. The structural metrics are similar to previously reported freebase structures.^22,28 The hanging group positions a proton above the cobalt atom at a distance of 4.3(1) Å.

Fig. 1 Crystal structure of CoHPX-3 with thermal ellipsoids set at 50% probability. Selected internuclear distances (Å) and angles (°) associated with the metal center: Co(1)–N(1) 1.974(4), Co(1)–N(2) 1.979(4), Co(1)–N(3) 1.959(4), Co(1)–N(4) 1.937(4), Co(1)–O(2) 4.4863(66), Co(1)–N(1)–N(2) 90.33(18), Co(1)–N(2)–N(4) 90.05(18), Co(1)–N(3)–N(1) 89.67(18), Co(1)–N(4)–N(3) 89.83(18).

CVs of CoHPX compounds (Fig. S6†) in acetonitrile exhibit a quasi-reversible reduction wave for the Co(II/I) reduction potential that falls within the range of −1.00 V to −1.35 V vs. Fc/Fc⁺ and an irreversible oxidation from Co(II) to Co(III) between the range of 0.0 and 0.5 V vs. Fc/Fc⁺ (see Table 1). CoHPX-1, with its electron withdrawing perfluorinated phenyl groups, is the easiest hangman porphyrin to reduce. Co(C₆F₅)₄ porphyrin was also included in the series in order to assess the effect of the hanging-xanthene group on the electronic properties of the porphyrin and on the catalytic activity. Replacement of a C₆F₅meso group by the xanthene backbone causes a −200 mV shift of the Co(II/I) reduction potential.

Table 1 Electrochemical properties and ORR data of the CoHPX porphyrins listed in Scheme 1

Compound	E 1/2(Co^II^/I)/V^a	E p(Co^III^/II)/V^b	EORR/mV^c	%H2O^d	n
a Half-wave potential for the one electron reduction of the Co(II) HPX porphyrins in acetonitrile vs. a ferrocene (Fc) internal standard (E(Fc^+/Fc) = 0.348 V vs. NHE^27). b Peak of irreversible anodic wave for one electron oxidation of Co(II) to Co(III). c Calculated as the potential at which the current equals half the limiting current at 100 rpm. d %H2O produced from ORR calculated from RRDE data according to (2Ir/N)/(Id + (Ir/N)) where Ir = ring current, Id = disk current, and N = collection efficiency. e n = number of electrons as calculated from RRDE data according to 4Id/(Id + (Ir/N)).
Co(C6F5)4	−1.00	0.40	453 ± 6.5	48	2.9
CoHPX-1	−1.20	0.48	436 ± 8.5	71	3.4
CoHPX-2	−1.31	0.28	364 ± 7.0	33	2.6
CoHPX-3	−1.32	0.11	444 ± 2.1	49	2.9
CoHPX-4	−1.28	0.07	398 ± 4.2	35	2.6
CoHPX-5	−1.23	0.12	425 ± 3.8	39	2.7

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ORR activity measurements were made on CoHPX compounds that were supported on unmodified MWCNTs, which provide a high surface area support for catalysts.^23,24 The effect of CNTs on the oxygen reduction activity has been shown to vary depending on the nature of the porphyrin based catalyst. Unchanged or increased activity of porphyrins adsorbed onto CNTs relative to adsorption on GCE has been suggested to result from an increase in catalyst loading on the surface of the electrode.²³ Moreover, the ORR mechanism can change for a given porphyrin on CNTs. For instance, H₂O₂ is the predominant product of ORR for Co(Mes)₄ adsorbed on GCE whereas H₂O prevails as the ORR product for the same porphyrin on SWCNTs.²³ For the latter, a higher catalyst loading of ×2.25 may explain the alteration in mechanism when the catalyst is immobilized on CNTs.

In order to ensure that immobilization on the MWCNT film did not alter ORR activity of the porphyrin catalysts, we undertook several experiments. First, a cobalt pacman catalyst, Co₂(DPX), for which the ORR activity is known for the complex supported on edge-plane graphite,¹¹ was immobilized on MWCNTs. Under similar experimental conditions, the percent water formed for Co₂(DPX)-MWCNT films is within 5–7% of that previously reported for Co₂(DPX). The KL analysis for both systems is consistent with a four electron transfer; however, the limiting current for Co₂(DPX) supported on MWCNTs is nearly four times higher (∼150 μA) than that for the complex immobilized on edge plane graphite (∼40 μA) (Fig. S13–S14†).¹¹ To ensure that hangman systems behave similarly to pacman systems, CoHPX-3 was investigated on edge-plane graphite. We chose CoHPX-3 as the catalyst because of the previous report that the ORR mechanism changed for the meso-substituted Co porphyrin monomer.²³ As shown in Fig. S15,† the ratio of H₂O : H₂O₂ for CoHPX-3 on MWCNT and edge-plane graphite is the same within the margin of error. We note that the total current for the MWCNT is higher due to higher catalyst loading that results from the greater surface area of the MWCNT as compared to more conventional substrates. Second, we assessed whether catalyst loading altered ORR reactivity. The amount of MWCNT was held constant and the CoHPX-3 and the catalyst loading was varied among 1.5, 3, and 6 nmol and ORR activity was measured. Under these conditions, the amount of water produced was 50 (±5%) for each of the catalyst loadings (Fig. S16†). This result suggests that disproportionation of H₂O₂ and reaction at other monometallic sites is not a significant contribution to activity as the amount of water produced would be expected to increase with catalyst loading. Moreover, this result offers evidence that association of hangman porphyrins on the MWCNT surface is not responsible for H₂O formation. Again, if this were the case, the percentage of H₂O produced would be expected to increase with catalyst loading. Finally, the UV-vis absorption spectra of CoHPX-3 and all porphyrins examined in this study show no signs of aggregation at high solution concentrations (1.6 mM in THF). Given that the HPX porphyrins do not associate owing to the steric barriers imposed by the meso substituents, their intermolecular association on the surface of the MWCNT films is unlikely.

The ORR activity of the CoHPX series was assessed by RDE. Oxygen reduction was measured on CoHPX-MWCNT films dispersed on the GC RDE immersed in an oxygen saturated, 0.5 M H₂SO₄ solution. The onset of oxygen reduction occurs between 550–600 mV vs. RHE. As shown in Fig. 2 for CoHPX-1, the current associated with ORR from the CoHPX-MWCNT film is well above that for the GC electrode and a GC electrode modified with MWCNTs in the absence of the catalyst. The enhanced current is attributed to the presence of the CoHPX catalyst. Similar RDE traces are obtained for the other members of the series (Fig. S7–S10†). The shape of the oxygen reduction wave shows a peak between 300–400 mV, particularly at lower rotation rates. This peak is indicative of higher activity at that potential. The magnitude of the peak decreases with decreasing scan rate, and is likely not present under steady state conditions. A similar phenomenon has been observed for bimetallic porphyrin catalysts and it has been ascribed to a catalyst subset that displays higher transient activity.²⁹

Fig. 2 ORR activity of CoHPX-1 at a RDE at different rotation rates in O₂ saturated 0.5 M H₂SO₄ and background activity of glassy carbon electrode (GCE, purple) and MWCNT, (orange) thin film at 1600 rpm. Scan rate 20 mV s⁻¹. Inset: Koutecky–Levich analysis of CoHPX-1 at different potentials.

The number of electrons transferred during ORR was determined from the KL plots. Fig. 2 (inset) shows such a plot for CoHPX-1. KL plots for reduction potentials coincident with the peak in the oxygen reduction wave have similar slopes, which yield an electron equivalency that approaches four. As the potential is decreased, the electron equivalency decreases, suggesting that the two-electron reduction of oxygen to H₂O₂ becomes a competing ORR mechanism at higher overpotential. As shown in Table 1, the electron equivalents determined from the KL plots of CoHPX-2 to CoHPX-5 decrease, thus indicating the greater prevalence of H₂O₂ as the product of the ORR. These results suggest that selectivity for H₂O production in the ORR is enhanced by the presence of electron withdrawing groups in the meso positions of the hangman porphyrin. The presence of the hanging group is also important for the selective production of H₂O in the ORR as indicated by the comparative ORR activities of CoHPX-1 and Co(C₆F₅)₄. Although the electronic properties of the two porphyrin platforms are similar, as indicated by redox potentials and electronic absorption spectra, the number of electrons transferred in the ORR of Co(C₆F₅)₄ is consistently two for potentials spanning the entire reduction wave. This result is consistent with previously reported measurements of Co(C₆F₅)₄-MWCNT films.²³ Together these results suggest that the reduction of O₂ to H₂O at the single metal center of a porphyrin is preferred for hangman porphyrins that are electron deficient and are proximate to a distal site that is capable of proton transfer.

The amount of peroxide formed during catalysis was evaluated directly by using a RRDE. Fig. 3 shows the disk and ring current for CoHPX-1, CoHPX-5, and Co(C₆F₅)₄, normalized by the collection efficiency of the electrode measured using [Fe(CN)₆]²⁻ as outlined in the experimental section. Similar traces are shown for the other CoHPX compounds of the series in Fig. S7–S11†; values for %H₂O production of all compounds are listed in Table 1. The shift in potential between Co(C₆F₅)₄, CoHPX-1, and CoHPX-5 highlights the electronic effect of the substituents on ORR onset. The %H₂O exhibits a potential dependence as determined from the KL plots at various potentials, as well as calculated values from the RRDE data. As such the values listed are taken at 0 mV vs. RHE where the catalysts have reached a plateau and the current is independent of scan rate. At this potential a constant level of water production is achieved. The quantity of H₂O produced in the ORR tracks the electron equivalency as determined from the KL plots. RRDE data confirm the finding that CoHPX-1 favors the four-electron, four-proton reduction of O₂ to H₂O whereas the two-electron, two-proton reduction mechanism is dominant for electron rich porphyrins with hanging groups and electron deficient porphyrins lacking a proton transfer hanging group in the distal position of the porphyrin.

Fig. 3 RRDE voltammograms for oxygen reduction (negative current) and hydrogen peroxide oxidation (positive current) measured in O₂ saturated 0.5 M H₂SO₄, at 100 rpm, scan rate 20 mV s⁻¹.

Conclusions

In summary, a new series of cobalt(II) hangman porphyrin complexes have been synthesized and investigated as oxygen reduction catalysts. The results reported herein further establish that redox cooperativity of two metals, such as that presented by pacman porphyrins, is not required for the selective reduction of O₂ to water. Rather, the ORR chemistry of the CoHPX series establishes that O₂ reduction to H₂O may occur at the metal of a single porphyrin if the macrocycle is electron deficient and possesses a distal group that is capable of proton transfer.

Acknowledgements

This work was performed under the auspices of the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DE-FG02-05ER15745 (D.G.N.) and in part by the MRSEC Program of National Science Foundation under award number DMR 0819762 (Y.S.-H.). T.S.T. is grateful to the Fannie and John Hertz Foundation for a graduate research fellowship.

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Footnote

† Electronic supplementary information (ESI) available: Full experimental details and characterization of CoHPX series including additional spectroscopic and electrochemical data. Additional electrocatalytic data for CoHPX-2-5 including RDE, RRDE and Koutecky–Levich analysis. CCDC reference number 775191. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0sc00281j

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