Inhibitory Effect of Water on the Oxygen Reduction Catalyzed by

Article pubs.acs.org/JPCA

Inhibitory Effect of Water on the Oxygen Reduction Catalyzed by Cobalt(II) Tetraphenylporphyrin Antonín Trojánek, Jan Langmaier, Hana Kvapilová, Stanislav Záliš, and Zdeněk Samec* J. Heyrovský Institute of Physical Chemistry of ASCR, v.v.i., Dolejškova 3, 182 23 Prague 8, Czech Republic S Supporting Information *

ABSTRACT: Stopped-flow kinetic measurements, UV−vis spectroscopy, rotating disk voltammetry, and quantum chemical calculations are used to clarify the role of water in the homogeneous two-electron reduction of O2 to H2O2 in 1,2-dichloroethane (DCE) using ferrocene (Fc) as an electron donor, tetrakis(pentafluorophenyl)boric acid (HTB) as a proton donor, and [5,10,15,20-tetraphenyl-21H,23Hporphine]cobalt(II) (CoIITTP) as a catalyst. Kinetic analysis suggests that the reaction is controlled by the intramolecular proton coupled electron transfer to the O2 molecule coordinated to the metal center producing the O2H• radical. This ratedetermining step is common to both the O2 reduction by Fc catalyzed by CoIITPP and the O2 reduction by CoIITPP itself. Experimental data point to the competitive coordination of water to the metal center leading to a strong inhibition of the catalytic reaction. In agreement with this finding, quantum chemical calculations indicate that water is bound to the metal center much more strongly than triplet O2. A similar effect is demonstrated also for the O2 reduction catalyzed by the porphyrin free base (H2TPP), though its rate is lower by 2 orders of magnitude.

1. INTRODUCTION Synthetic cobalt porphyrins have been investigated extensively as biomimetic catalysts of the oxygen reduction in the homogeneous solution,1−3 at solid electrodes,4−27 and at polarized liquid−liquid interfaces.28−30 The activation of the molecular oxygen involves the binding of O2 to the metal center, which is followed by the electron delocalization from the metal to O2,31,32 and by protonation of the coordinated O2 making it more susceptible to reduction.33 Monomeric cobalt porphyrins mediate the oxygen reduction mostly to H2O2, with some fraction of O2 being reduced to H2O,4−18 while dicobalt cofacial bisporphyrins were shown to act as efficient catalysts for the selective reduction of O2 to water.3,19−27,34 Scheme 1 depicts the proposed catalytic cycle for the two-electron reduction of O2 with a methyl-substituted ferrocene (Fc) in acetonitrile (MeCN),1,2 or benzonitrile (PhCN),3 catalyzed by the cobalt porphyrin (CoP). The cycle consists of three main steps including the coordination of O2 to CoIIP yielding a superoxide adduct (step 1), the proton coupled electron transfer (PCET) from Fc to the coordinated oxygen species (step 2), and the regeneration of CoIIP from CoIIIP by electron transfer from Fc (step 3).1−3 Kinetic data indicated that the latter reaction is the rate-determining step (RDS) of the cycle.1−3 A similar reaction scheme was anticipated for the catalytic O2 reduction at a polarized water/1,2-dichloroethane (DCE) interface, which serves as a proton pump controlled by the interfacial Galvani potential difference.28−30 The density functional theory (DFT) method was used to calculate the binding energies for various adducts of oxygen species with CoIIP.35,36 Those obtained for the end-on O2 and © 2014 American Chemical Society

Scheme 1. Catalytic Cycle for the Two-Electron Reduction of O2 with Ferrocene (Fc) Catalyzed by a Cobalt Porphyrin (CoP)a

a

Adapted with permission from ref 3. Copyright 2004 American Chemical Society.

H2O adducts are comparable,35 which indicates that these oxygen species could compete for the metal active site. In contrast, the DFT binding energy for H2O2 is small, and the molecular hydrogen peroxide may be easily liberated from the complex with a cobalt porphyrin.36 The O2/H2O competition may influence the function of the biological oxygen carriers, e.g., hemoglobin and myoglobin, and the oxygen reduction catalysts such as cytochrome c oxidase.37 Recently, the molecular water has been shown to inhibit the binding of O2 Received: January 3, 2014 Revised: February 20, 2014 Published: February 24, 2014 2018

dx.doi.org/10.1021/jp500057x | J. Phys. Chem. A 2014, 118, 2018−2028

The Journal of Physical Chemistry A

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to the metal active site in hemoprotein models.38 On the other hand, no experimental data are available indicating or quantifying the effect of molecular water on the O2 reduction catalyzed by a synthetic metalloporphyrin. Here we study the role of water in the homogeneous reduction of O2 in DCE using [5,10,15,20-tetraphenyl21H,23H-porphine]cobalt(II) (CoIITTP) as a catalyst, Fc as an electron donor, and tetrakis(pentafluorophenyl)boric acid (HTB) as a proton donor. By using the stopped-flow kinetic measurements we show that the competitive coordination of water to the metal center leads to a strong inhibition of the O2 reduction. Mechanistic considerations are based on the DFT calculations, the observed effects of the CoIITTP, HTB, Fc, and H2O concentrations on the O2 reduction rate, and the results of the concomitant kinetic measurements of the O2 reduction by CoIITTP itself. These considerations are supported by the UV−vis spectroscopic measurements of the cobalt complex in the air-saturated and deaerated DCE solutions at various acid and water concentrations. Rotating disk electrode (RDE) voltammetry is used to obtain relevant information about the oxidation state of the cobalt complex under various experimental conditions, and to evaluate the corresponding formal redox potential E°′. We show that the value of E°′ for the CoIIITPP+/CoIITPP redox couple is far more positive in DCE than in MeCN.1,2 As a result, the catalytic cycle in DCE has a different RDS. We also compare the catalytic activity of CoIITPP and tetraphenylporphyrin free base (H2TPP), which provides an alternative catalytic route for the reduction of O2 through its coordination to the diprotonated free base H4TPP2+.39,40

simple line with two gas scrubbers in series was employed. The first scrubber was filled with DCE and water in two separate phases through which argon gas was passed. Argon flow from the first scrubber was fed to the other one, which was filled either with the test sample or with a solution of one of the sample components. In the latter case, the solution was finally discharged by employing gas pressure from the scrubber into a bottle with a prepared solid component (CoIITPP), which had been previously dried and deaerated. When the first scrubber was filled with DCE only, the procedure led to a decrease in both the oxygen and water content in the sample, unless the water content was initially lower than approximately 30 ppm. In either case, the water content was always determined after completing the sample treatment. Kinetic Measurements. Stopped-flow kinetic measurements were performed with the help of a single-mixing instrument SFA-20 (TgK Scientific, U.K.) and a USB2000+ UV−vis fiber optic spectrometer (Ocean Optics, U.S.A.) with a sampling period of 1 ms. The air-saturated DCE solution of Fc was mixed with the air-saturated DCE solution of (a) HTB and CoIITPP, (b) HTB, or (c) CoIITPP, HTB, and BATB, in a quartz cuvette (path length 0.2 cm). To prevent the photoexcitation of the porphyrin species, the mixing occurred in the closed chamber of the spectrophotometer. The rate of the Fc to Fc+ conversion was monitored by a rise in the Fc+ absorbance at the wavelength λ = 300 nm (ε = 7938 M−1 cm−1, see Figure S1 in the Supporting Information). In the kinetic measurements of the oxidation of CoIITPP with O2, the airsaturated DCE solution of CoIITPP was mixed with the airsaturated DCE solution of HTB. The rate of the CoIITPP to CoIIITPP+ conversion was monitored by an increase or decrease in absorbance at the wavelength λ = 428 or 411 nm, respectively (ε = 2.1 × 105 M−1 cm−1). Spectroscopy and RDE Voltammetry. UV−vis spectra were measured by using a Perkin-Elmer Lambda 25 spectrophotometer with a quartz cuvette (path length 0.1 cm). RDE voltammetric measurements were performed with a rotating Au disk (diameter 3 mm) electrode (Autolab) by using a three-electrode potentiostat (273A, Princeton Applied Research, U.S.A.). The conventional three-electrode glass cell was equipped with a glassy carbon counter electrode and an Ag/AgCl reference electrode separated from the organic solvent phase by a porous graphite frit. Temperature. All measurements were carried out at the ambient temperature, i.e., 25 ± 2 °C. Quantum Chemical Calculations. The DFT calculations were performed using the Gaussian 09.C01 program package.43 Open shell systems were examined by the unrestricted Kohn− Sham approach (UKS). Geometry optimization was followed by vibrational analysis, which enabled us to characterize stationary states and to evaluate the corresponding free energy changes ΔG. For all systems several possible electronic states were considered, so as to find the state with the lowest energy. Calculations were performed using the Perdew−Burke− Ernzerhof PBE0 hybrid functional (G09/PBE0) (spectral calculations),44,45 or the M05-2X functional46 designed for the study of weak interactions and the calculation of the stabilization energies. The polarized double-ζ basis sets 631G(d)47 were used for the H, C, O, F, N, and B atoms, while the polarized triple-ζ basis sets 6-311G(3df) were used for the Co atom.48 Solvent effects were described by the polarizable continuum model (PCM)49 implemented in G09 using the integral equation formalism.50 Employed solvent parameters of

2. EXPERIMENTAL SECTION Materials. All chemicals were used as received. Bis(triphenylphosphoranylidene)ammonium chloride (BACl, >98%) and bis(η5-cyclopentadienyl)iron(II) (Fc, 98%) were purchased from Sigma-Aldrich. 1,2-Dichloroethane (DCE, puriss. p.a.) and lithium tetrakis(pentafluorophenyl)borate ethyl etherate (LiTB) were purchased from Fluka. Tetrakis(pentafluorophenyl)boric acid (HTB) was synthesized in the crystalline form by methathesis of LiTB and HCl following a procedure similar to that employed for the synthesis of tetrakis[3,5-bis(trifluoromethyl)phenyl]boric acid.41 BATB was prepared by metathesis of LiTB and BACl. CoIITTP was a generous gift from Dr. J.-M. Barbe, Institut de Chimie Moléculaire de l′Université de Bourgogne, Dijon, France. Water Content. Water content (ppm, w/w, 1 ppm H2O = 6.875 × 10−5 M H2O) in the DCE solutions was determined with the help of a Karl Fischer coulometer (DL39, Mettler Toledo). DCE in the freshly opened bottle (Sigma-Aldrich) contained 70 ppm (4.8 mM) H2O. After three weeks of drying over the 3 Å molecular sieve, the water content dropped to 1 ppm (69 μM). Solutions for the stopped-flow kinetic measurements were prepared from the dried solvent with various amounts of highly purified water (Milli-Q Gradient, Millipore) added. The water content was determined in all the prepared solutions separately. Before use, the 3 Å molecular sieve (Sigma-Aldrich) was regenerated by heating in the oven at 300 °C for 24 h. Deaeration. The application of Henry’s law with Henry’s constant for O2 in DCE at 20 °C (1.534 × 104 Pa m3 mol−1)42 and the partial pressure of O2 in air yields the concentration of O2 in the air-saturated DCE (1.39 mmol L−1). For the preparation of the deaerated DCE solutions of CoIITPP, a 2019



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change in shape, and two overlapping Q bands emerge at 590 and 650 nm (Figure 1, panel b, curve 2). The spectrum of the water- and air-saturated solution is dominated by the narrow Soret band at 428 nm and the Q-band at 541 nm (Figure 1, panel c, curve 1). Upon the deaeration of this solution, which also leads to a drop in the water concentration from 1565 ppm to 35 ppm, the Soret band at 401 nm is recovered (Figure 1, panel c, curve 2). It is noteworthy that the latter process is reversible. Figure 2 depicts the change in the UV−vis spectrum during the titration of the air-saturated and acidified solution of

DCE and MeCN were built in the G09 database. Electronic transitions were calculated by the time-dependent DFT (TDDFT) method using the linear response LR-PCM/TD-DFT approach51,52 within the nonequilibrium regime at geometries optimized with the same functional.

3. RESULTS AND DISCUSSION UV−Vis Spectroscopy. Figure 1 shows UV−vis spectra of CoIITPP recorded with the air-saturated and deaerated DCE

Figure 2. Absorption spectra (top panel) of the air-saturated DCE solution containing 20 μM CoIITPP, 0.1 mM HTB, and 4 (black), 16 (red), 180 (blue), 437 (green), 813 (brown), or 1540 (magenta) ppm H2O, and the solution color (bottom panel) containing 4, 16, 180, or 1540 ppm H2O (from left to right). Cell path length 0.1 cm. Figure 1. Absorption spectra of the DCE solutions containing 50 μM CoIITPP and various concentrations of HTB, H2O, and O2: (a) no HTB, 27 ppm H2O, first deaerated (line 1) and then air-saturated (line 2); (b) 0.25 mM HTB, 28 ppm H2O, first deaerated (line 1), and then air-saturated (line 2); (c) 0.25 mM HTB, 1580 ppm H2O, first airsaturated (line 1), and then deaerated and simultaneously dried up to 35 ppm H2O (line 2). Cell path length 0.1 cm.

CoIITPP with water (4−1540 ppm). The addition of water leads to the gradual conversion of the Co complex form characterized by the Soret band at 401 nm to the form characterized by the Soret band at 428 nm, with the single isosbestic point at 412 nm (Figure 2, top panel), and to a color change of the solution from light green to light rose (Figure 2, bottom panel). Like in the absence of the acid, the change in the concentrations of CoIITPP (7.8 × 10−7 to 5 × 10−5 mol L−1), or the neutral TB− salt (0, 5, or 10 mM BATB), in the DCE solution containing 1450 ppm H2O has a negligible effect on the molar absorption coefficient of the solution. RDE Voltammetry. Voltammetric measurements were carried out to identify the oxidation state of cobalt in the spectroscopic experiments above, and to evaluate the formal redox potentials E°′ of the CoIIITPP+/CoIITPP redox couple in DCE. Figure 3 (panel a) shows the RDE voltammograms recorded for the acid-free DCE solutions, which were either deaerated (curve 1) or saturated with air (curve 2). Obviously, the voltammetric behavior of the complex is not much affected by the presence of oxygen in the solution. With a reference to the previous studies,54 the anodic wave at ca. 0.4 V (the reversible half-wave potential E1/2rev = 0.40 V vs Fc+/Fc) could be ascribed to the oxidation of the CoII center yielding CoIIITPP+, and two following anodic waves (E1/2rev = 0.89 and 1.40 V vs Fc+/Fc) to the stepwise oxidation at the porphyrin πring system. High stability of CoIITPP toward O2 in the acidfree solutions has been also reported in previous studies.1,2

solutions at various concentrations of water in the absence or presence of HTB. Owing to the limited solubility of water in DCE the water concentration could be varied only in the range 0−110 mmol L-1 (0−1565 ppm). In the absence of the acid, the spectrum is dominated by the Soret band at 411 nm and the Q-band at 527 nm (panel a), in agreement with literature data.53 Evidently, the UV−vis spectra of the deaerated (curve 1) and air-saturated (curve 2) solutions of CoIITPP are identical. Besides, the change in the water concentration, the addition of BATB (1 mM BATB at 5 ppm H2O), or the CoIITPP concentration (2.9 × 10−7 to 5 × 10−5 mol L−1) has no effect on the molar absorption coefficient of the solution. In the presence of the acid, the spectral behavior of CoIITPP is significantly affected by both oxygen and water presence in the solution. The spectrum of the deareated solution containing 0.25 mM HTB and less than 30 ppm H2O is characterized by the Soret band at 407 nm, which is much less pronounced than that observed for the acid-free solutions (Figure 1, panel b, curve 1). Upon the saturation of the solution with air, a blueshift of the Soret band to 401 nm is observed with a minor 2020



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as in the strongly coordinating solvents like pyridine.55,56 The stability of CoIITPP toward the oxidation by O2 is controlled by the thermodynamic and kinetic factors. The former factor can be related to the difference of the formal redox potentials of the CoIIITPP+/CoIITPP and O2/H2O2 or O2/H2O redox couples, which depend on the solution pH (ΔE°′/ΔpH= −0.059 V). Thermodynamic considerations (see the Supporting Information) provide an estimate of the values E°′ = 0.55 and 1.13 V vs Fc+/Fc at pH 0 for the O2/H2O2 and O2/H2O redox couples, respectively. Consequently, the oxidation of CoIITPP by the O2 yielding H2O is probably an exergonic reaction even at very low acid concentrations, while the oxidation by the O2 yielding H2O2 could be an exergonic reaction only at high acid concentrations (pH < 3). On the other hand, RDE voltammetry of CoIITPP in the air-saturated and acid-free DCE solution demonstrates a high stability of CoIITPP (Figure 3, panel a), though some protons formed by the dissociation of trace water in DCE are probably present. This behavior indicates that the direct reduction of O2 to H2O is kinetically hindered. The formal redox potential E°′ ≈ 0.38 V vs Fc+/Fc of the CoIIITPP+/CoIITPP redox couple in DCE actually represents the reaction free energy −ΔGr° (expressed in eV) for the oxidation of Fc by CoIIITPP+, which is a highly exergonic reaction irrespective of the solution pH. The results of the RDE measurements suggest that the Soret band at 428 nm measured for the air-saturated and acidified DCE solution of CoIITPP at high water concentration, as well as the Soret band at 401 nm measured for the initially airsaturated and then deaerated and acidified DCE solution of CoIITPP at low water concentration, can be ascribed to CoIIITPP+. Conversely, the Soret band at 407 nm measured for the initially deaerated and acidified DCE solution at low water concentration can be ascribed to CoIITPP. Kinetics of the O2 Reduction by Fc. While the reduction of O2 by Fc in DCE proceeds rather slowly in the absence of a catalyst, the addition of a cobalt porphyrin,28−30 or H2TPP,39,40 leads to a significant increase of the oxidation rate. Figure 4

Figure 3. RDE voltammograms (0.1 V s−1, 500 rpm) recorded for the DCE solutions containing 1.1 mM CoIITPP and 15 mM BATB (panel a), or 15 mM HTB (panels b,c), which were (a) deaerated (curve 1) or saturated with air (curve 2); (b) dearated (curve 1) or initially saturated with air and then dearated (curve 2); and (c) initially saturated with air and then dearated (curve 1), or initially dearated and then saturated with air (curve 2). Voltammograms recorded in the absence of CoIITPP are shown by the dotted lines.

Figure 3 (panel b) compares the voltammetric curves of the Co complex in the acidified DCE solution, which was either initially deaerated (curve 1) or initially saturated with air and then deaerated (curve 2). While in the former case the voltammetric behavior is identical with that observed in the absence of HTB, in the latter case the anodic wave at ca. 0.4 V is replaced by the cathodic wave at approximately the same potential (E1/2rev = 0.38 V vs Fc+/Fc), which is due to the reduction of CoIIITPP+ formed by the homogeneous oxidation of CoIITPP with the air oxygen. RDE voltammetry of the airsaturated and acidified solution of CoIITPP shows the catalytic enhancement of the cathodic wave of CoIIITPP+, cf. the curve 2 in Figure 3 (panel c), which could be ascribed to the fast regeneration of CoIIITPP+ by the homogeneous oxidation of CoIITPP with O2 following the electrode reduction step. These results suggest that CoIIITPP+ should be the major oxidation form of the Co complex in the air-saturated and acidified DCE solutions. The behavior of the CoIIITPP+/CoIITPP redox couple in DCE is similar to that observed in MeCN, where CoIITPP exhibits high stability toward the oxidation by O2 in the absence of an acid, the facile oxidation of CoIITPP to CoIIITPP+ by O2 upon addition of HClO4, and the catalytic reduction of CoIIITPP+ at a solid electrode in the presence of O2 in MeCN.1,2 However, the formal redox potentials E°′ of the CoIIITPP+/CoIITPP redox couple in DCE (E°′ ≈ E1/2rev = 0.38 V vs Fc+/Fc) and MeCN (E°′ = −0.02 V vs Fc+/Fc)2 differ significantly, probably due to a dissimilar coordination ability of the two solvents.54 While DCE could be classified as a nonbinding solvent like CH2Cl2,54 the shift in E°′ indicates that the reducing power of CoIITPP is enhanced by the axial coordination of MeCN, which yet is not as much pronounced,2

Figure 4. Time-resolved UV−vis spectra (5 ms integration time, 20 ms step) of the reaction mixture containing 1 mM HTB, 0.5 mM Fc, 20 μM CoIITPP, 201 ppm H2O, and 1.39 mM O2. Cell path length 0.2 cm.

shows the millisecond time-resolved UV−vis spectra of the reaction mixture containing 1 mM HTB, 0.5 mM Fc, 20 μM CoIITPP, 1.39 mM O2, and 201 ppm H2O. The formation of Fc+ is indicated by an increase in the absorbance at 300 or 622 nm (see also Figure S1 in the Supporting Information showing the absorption spectra of Fc and Fc+). During the reaction, the spectral behavior of the catalyst undergoes a sudden change. 2021



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When the rate of the Fc+ formation differs from zero, the spectral behavior of the catalyst is dominated by the Soret band at 411 nm, which was ascribed above to CoIITPP. This is an interesting observation, because in the air-saturated DCE such a complex form exists only in the absence of the acid (Figure 1, panel a). It appears that the regeneration of CoIITPP in the catalytic cycle (step 3 in Scheme 1) is much faster than the activation of O2 through its coordination to CoIITPP (step 1 in Scheme 1), which actually appears to be the RDS of the cycle, vide infra. On the other hand, when the concentration of Fc+ reaches the limit given by the initial concentration of Fc (0.5 mmol L−1), and the rate of the Fc+ formation decreases to zero, cf. the constant absorbance at 300 nm after ca. 600 ms in Figure 4, the spectral behavior of the catalyst is dominated by the Soret at 428 nm, which is characteristic for CoIIITPP+. Figure 5 illustrates the time profiles of the Fc+ formation in the presence of Co II TPP and H 2 TPP at the same

Figure 6. The initial rate of the Fc+ formation, v0c = {d[Fc+]/dt}t→0, vs the concentration of (a) CoIITPP in the presence of 1 mM Fc, 0.25 mM HTB, and 250 ppm H2O, (b) HTB in the presence of 1 mM Fc, 50 μM CoIITPP, and 5 (●) or 810 (○) ppm H2O, and (c) Fc in the presence of 50 μM CoIITPP, 0.25 mM HTB, and 35 (●) or 257(○) ppm H2O, in the air-saturated DCE solution.

Figure 5. Time profiles of the formation of Fc+ at 300 nm in the reaction mixture containing 0.125 mM HTB, 1 mM Fc, 200 ppm H2O, 1.39 mM O2, and 50 μM CoIITPP (1) or 50 μM H2TPP (2).

v0c = {d[Fc+]/dt }t → 0 = 2v01 = −2{d[CoIITPP]/dt }t → 0 = 2k1[CoIITPP][HTB]

concentrations of the other components of the reaction mixture. Obviously, the rate of the O2 reduction catalyzed by the metal-free porphyrin is considerably lower than that measured in the presence of the Co porphyrin. A comparison of the initial rates shows that the ratio of the two rates makes about 2 orders of magnitude. The formation of Fc+ was also monitored by following the absorbance at 622 nm for the reaction mixture containing 10 mM HTB, 10 mM Fc, 20 μM CoIITPP, 1.39 mM O2, and 50 ppm H2O, in a hermetically sealed glass cuvette (path length 1 cm). After ca. 15 min, the reaction rate decreased to zero, presumably due to the complete reduction of O2. The final Fc+ concentration was found to be 2.75 mmol L−1 (see Figure S2 in the Supporting Information), which is exactly two times the initial concentration of O2. Consequently, the summary reaction is given by eq 1: 2Fc + O2 + 2H+ → 2Fc+ + H 2O2

(2)

where k1 is the pseudo-second-order rate constant including the concentration of O2. The conclusion about the RDS is supported by the effect of water. As it can be seen from Figure 6, the presence of water leads to a remarkable deceleration of the catalytic O2 reduction; cf. the solid and empty points in panel b and panel c. At a given concentration of CoIITPP, HTB, and Fc, the initial rate decreases sharply with the concentration of water (Figure 7, panel a). At water concentrations higher than ca. 50 ppm, v0c is inversely proportional to the water concentration [H2O]; at lower water concentration this dependence has a much lower slope (Figure S3 in the Supporting Information). Qualitatively the same effect is observed for the reaction catalyzed by H2TPP, cf. the solid and empty points in Figure 7 (panel a), though a comparison of the initial rates indicates that the ratio of the two rates makes about 2 orders of magnitude. The effect has been previously ascribed to a competition between O2 and H2O for the same reaction site,40,57 which could be also the origin of the inhibitory effect of water on the reaction catalyzed by CoIITPP. Figure 7 (panel b) demonstrates a similar, yet much less pronounced, inhibitory effect of the acid anion TB− pointing to a weak interaction of TB− with CoIITPP. Kinetics of the O2 Reduction by CoIITPP. If the above conclusion about the RDS is correct, the O2 reduction by CoIITPP itself should follow the same kinetic law as that described by eq 2. RDE voltammetric measurements confirmed that O2 is reduced rapidly by CoIITPP in the acidified DCE solution (Figure 3, panels b and c). The homogeneous reaction could be monitored by measuring the decrease of the

(1)

The formation of H2O2 as the main product of the catalytic O2 reduction in DCE has been previously confirmed by iodometric titration.28 Figure 6 depicts the effects of the concentrations of CoIITPP (panel a), HTB (panel b), and Fc (panel c) on the initial rate of the Fc+ formation, v0c = {d[Fc+]/dt}t→0. The initial rate is independent of the Fc concentration, when this concentration exceeds that of HTB (panel c), but it is proportional to the concentration of CoIITPP (panel a) and HTB (panel b). These results suggest that the RDS of the catalytic cycle is the protonassisted coordination of O2 to CoIITPP (step 1 in Scheme 1), the initial rate v01 of which could be related to v0c by eq 2: 2022



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Figure 7. The initial rate of the Fc+ formation, v0c = {d[Fc+]/dt}t→0, vs the concentration of (a) water in the presence of 1 mM Fc and either 0.25 mM HTB + 50 μM CoIITPP (●) or 0.125 mM HTB + 50 μM H2TPP (v0c values multiplied by 102) (○) and (b) BATB in the presence of 1 mM Fc, 0.25 mM HTB, 50 μM CoIITPP, and 30 ppm H2O, in the air-saturated DCE solution. Figure 9. The initial rate of the O2 reduction with CoIITPP, v01 = {d[CoIIITPP+]/dt}t→0 = −{d[CoIITPP+]/dt}t→0, vs the concentration of (a) CoIITPP in the presence of 0.5 mM HTB and 190 ppm H2O, (b) HTB in the presence of 50 μM CoIITPP and 815 ppm H2O, and (c) water in the presence of 50 μM CoIITPP and 0.25 mM HTB, in the air-saturated DCE solution. The reaction was followed by measuring the decrease of absorbance at 411 nm (●) or increase of absorbance at 428 nm (○), cf. Figure 11. In panel c, the rate of the catalyzed reaction divided by 2, v0c/2, is shown for comparison (dashed line).

absorbance at 411 nm, which is related to the decreasing concentration of CoIITPP, or by measuring the increase of the absorbance at 428 nm, which is related to the increasing concentration of CoIIITPP+ (Figure 8)

Figure 8. Time-resolved UV−vis spectra of the reaction mixture containing 50 μM CoIITPP, 0.25 mM HTB, 1.39 mM O2, and 186 ppm H2O recorded at 0 ms (dashed black), 20 ms (black), 40 ms (red), 60 ms (blue), 80 ms (brown), 100 ms (magenta), and 120 ms (green) from mixing the air-saturated DCE solutions of CoIITPP and HTB. Cell path length 0.2 cm.

The initial reaction rate v01 = {d[CoIIITPP+]/dt}t→0 = −{d[CoIITPP+]/dt}t→0 is proportional both to the concentration of CoIITPP (Figure 9, panel a) and to the concentration of HTB (Figure 9, panel b). Besides, the rate v01 decreases sharply with the concentration of water, like the initial rate of the catalyzed O2 reduction divided by 2, v0c/2 (Figure 9, panel c), thereby pointing to the same kinetic law. Quantum Chemical Calculations. The DFT method was used to investigate the electronic structures of the CoIITPP, CoIIITPP+ complexes and their interaction with various ligands including O2, H2O, (H2O)4, O2H•, H2O2, TB−, and H+. The tetramer (H2O)4 is considered here as a representative of the general water aggregate (H2O)n. The optimized structures of the complexes {CoIITPP(O 2)}, {Co IITPP(H 2 O) 4 }, and {CoIIITPP+(TB−)} are shown in Figure 10 and Figure 11; analogous structures were found for CoIITPP and CoIIITPP+ aducts with H2O, O2H•, and H2O2 (Figures S4 to S6 in the

Figure 10. DFT/M05-2X/PCM optimized structures of the complexes {(CoIITPP)(O2)} (top) and {(CoIITPP)(H2O)4} (bottom); the averaged Co−O distances, cf. the dashed lines, were calculated to be 2.506 Å and 2.204 Å, respectively.

Supporting Information). All calculations preserved the spin state of the nonbonded ligands in the complex. The geometry optimization on the complex {CoIITPP(O2)} containing triplet oxygen (4A state) leads to the end-on structure (Figure 10) with Co−O bond length of 2.506 Å. The O−O bond is slightly polarized; the Mulliken charge at the O 2023



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Table 2. DFT/M05-2X Stabilization Energy ΔE(L) and the Corresponding Free Energy ΔG(L) for the Extraction of the Ligand L from the Complex {CoIITPP(L)(H2O)} in DCE Calculated with the PCM Solvent Correction

a

energy/L

O2a

H2O

ΔE (L)/eV ΔG (L)/eV

0.139 −0.202

0.439 −0.037

O2 in the 3A state.

Table 3. DFT/M05-2X Stabilization Energy ΔE(L) and the Corresponding Free Energy ΔG(L) for the Extraction of the Ligand L from the Complex {CoIIITPP+(TB−)(L)} in DCE Calculated with the PCM Solvent Correction Figure 11. DFT/M05-2X optimized structure of the complex {(CoIIITPP+)(TB−)}. Stabilization energy ΔE(TB−) and the corresponding free energy ΔG(TB−) for the extraction of the TB− ion from the complex calculated with the PCM solvent correction: ΔE(TB−) = 0.267 eV, ΔG(TB−) = −0.521 eV.

atom attached to Co is −0.03, and the charge on the remote O atom is 0.01. The comparative geometry optimization on {CoIITPP(O2)} in the 2A state yielded the Co−O bonding distance of 1.82 Å, which is close to the published value.58 The calculation gives higher energy for 2A than for 4A, which corresponds to the bonded triplet O2. The optimized structures of the two possible protonated forms of the complexes CoIITPP and CoIIITPP+ were also calculated. In the first form (A) H+ is attached to the nitrogen atom, and in the second form (B) an outer carbon atom of porphyrin ring is protonized (see Figures S7 and S8 in the Supporting Information). The latter form has a slightly lower free energy (ΔG = 0.049 eV). The stabilization energies of the ligand L in the complexes {CoIITPP(L)}, {CoIITPP(L)(H2O)}, and {CoIIITPP+(TB−)(L)} in DCE are given in Table 1, Table 2, and Table 3, respectively. Both the stabilization and free energy of extraction of the water aggregate in {CoIITPP(L)}are significantly higher than those for the extraction of the triplet O2, H2O, or H2O2. Table 1 also includes the results of the DFT calculations for MeCN. Apparently, the free energies of the extraction of the ligands L = O2, H2O, and (H2O)4 from the complex {CoIITPP(L)}, as well as their difference determining the equilibrium constant Kex of the exchange reaction (eq 4), do not exhibit a significant change when DCE is replaced with MeCN. The data in Table 2 suggest that the additional coordination of H2O to {CoIITPP(L)} (L = H2O or O2) yielding {CoIITPP(L)(H2O)} leads to a destabilization of the ligand L. Analogously, the data in Table 3 indicate that the coordination of TB− leads to even larger destabilization of the ligands L = O2, O2H•, H2O, (H2O)4, and H2O2 in the complex {CoIIITPP+(TB−)(L)}. Simulated spectra of CoIITPP, CoIIITPP+, and {CoIITPP(H+)}+ complexes are depicted in Figure 12; Table S1 in the Supporting Information summarizes the calculated wavelengths and oscillator strengths for these complexes. TD-DFT

energy/L

O2

O2H•

H2O

(H2O)4

H2O2


0.117 −0.355

0.498 −0.111

0.557 0.074

0.825 0.227

0.226 −0.374

Figure 12. TD-DFT/PBE0/PCM simulated spectra of CoIITPP (1), CoIIITPP+ (2), and {(CoIITPP)(H+)}+ (3).

calculations reproduce qualitatively well both the spectrum of the parent complex CoIITPP and the redshift of the Soret band due to the oxidation of CoIITPP to CoIIITPP+. Spectra of {CoIITPP(O2)} and {CoIITPP(H2O)4} are virtually identical with that of CoIITPP. On the other hand, the protonization results in a geometry perturbation of the {CoIITPP(H+)}+ complex, which has a strong effect on the π−π transitions giving rise to the Soret band. As a result, the Soret band of {CoIITPP(H+)}+ should undergo a blueshift and a decrease of intensity as compared with CoIITPP, in agreement with experimental data (Figure 1). A comparison of the simulated spectra of CoIIITPP+, {(CoIIITPP)(H+)}2+, and {CoIIITPP(H2O)}+ (Figure S9 in the Supporting Information) indicates that the protonization of the oxidized porphyrin also causes the perturbation in the π-system and a deformation of the simulated spectrum, while the coordination of water has only a minor effect. On taking into account the positive values of the free energies of the extraction of H2O or (H2O)4 from the complex {CoIIITPP+(TB−)(L)} (Table 3), it can be concluded

Table 1. DFT/M05-2X Stabilization Energy ΔE(L) and the Corresponding Free Energy ΔG(L) for the Extraction of the Ligand L from the Complex {CoIITPP(L)} in DCE and MeCN Calculated with the PCM Solvent Correction

a

energy/L

O2a

H2O

(H2O)4

H2O2


0.174 (0.164)b −0.206 (−0.215)

0.566 (0.550) 0.091 (0.047)

0.898 (0.868) 0.226 (0.184)

0.632 0.081

O2 in the 3A state. bData for MeCN are given in parentheses. 2024



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c), on the initial rate of the O2 reduction, indicate that the first PCET (eq 5) is the RDS of the sequence. Assuming that the exchange reaction (eq 4) is at equilibrium, the initial rate v0c is given by eq 2, where the pseudo-second-order rate constant k1 is given by eq 8:

that the Soret band at 428 nm is associated with the CoIII complex coordinating water. Mechanistic Considerations. Present results elucidate the mechanism of catalysis of the O2 reduction by CoIITPP. Quantum chemical calculations indicate that the complex {CoIITPP(H2O)n} should prevail in the acid-free solution containing CoIITPP, O2, and H2O, cf. the high values of both the stabilization and free energy for the extraction of (H2O)4 from the complex {CoIITPP(H2O)4} (Table 1). Therefore, the equilibrium position of the coordination of a water aggregate (H2O)n to CoIITPP according to eq 3, COII TPP + (H 2O)n ⇆ {COII TPP(H 2O)n },

K ass

k1 = kKex[O2 ]/[(H 2O)n ]

Equation 8 provides an explanation for the hyperbolic dependence of the initial rate on the concentration [(H2O)n] (Figure 7, panel a). In the absence of Fc, the role of the electron donor should be played by the CoII species,2 here mainly by {CoIITPP(H2O)n} participating in the terminating PCET step, as described by eq 9:

(3)

is far to the right, and the corresponding equilibrium constant Kass ≫ 1. The coordination of a second ligand in the axial position is much less probable due to its considerably lower stabilization energy (Table 2). A large difference between the stabilization energies of (H2O)4 and O2 (Table 1) suggests that only a small fraction of {CoIITPP(H2O)n} is converted into the complex {CoIITPP(O2)} according to eq 4:

{CoIIITPP(O2 H•)(TB−)} + {CoIITPP(H 2O)n } + H+ + TB− → 2{CoIIITPP(TB−)} + H 2O2 + (H 2O)n (9)

However, because the RDS is the same (eq 5), the reaction rate should equal one-half of the rate of the Fc+ formation in the O2 reduction by Fc catalyzed by CoIITPP, which indeed is the case (Figure 9, panel c). Time-resolved spectra of the reaction mixture shown in Figure 4 do not provided any evidence for the participation of the protonized complexes like {CoIITPP(H+)(TB−)} or {CoIIITPP+(H+)(TB−)2} in the catalytic cycle. On the other hand, UV−vis spectroscopy of the deaerated DCE solution containing CoIITPP point to the presence of either {CoIITPP(H+)(TB−)} or {CoIITPP(H2O)4(H+)(TB−)} (Figure 1, panel b, full line). Analogously, the UV−vis spectrum of the airsaturated DCE solution containing the CoIII species (Figure 3, panel b, dashed line) points also to the presence of the protonized CoIII complexes like {CoIIITPP+(H+)(TB−)2} characterized by the Soret band at 401 nm (Figure 1, panel b, dashed line). However, such a spectrum can only be recorded at low water concentrations, while increased water concentration leads to the dissociation of proton, and to the formation of {CoIIITPP+(TB−)}, which is characterized by the Soret band at 428 nm (Figure 2, top panel). Several additional aspects of the reaction mechanism are worth considering, including the nature of H+ in the PCET steps (eqs 5, 6, and 9), the effects of the nature of the electron donor and solvent, and the demetalation of the metalloporphyrin. First, HTB was prepared in the form of the crystalline oxonium acid with two molecules of diethyl ether (OEt2) coordinated to the hydrogen ion yielding [H(OEt2)2]+.42 Since the concentration of water in DCE in the present study was typically more than 1 order of magnitude higher than that of HTB, [H(OEt2)2]+ was probably converted into the [H(OEt2)(H2O)]+ cation with the mixed coordination shell,59 or the solvated [H(H2O)n]+ cation.60 IR spectroscopic measurements provided evidence that the solvated [H(H2O)3]+ ion is a good representation of H+ in a wet organic solvent like DCE.60 The energy associated with the destruction of the proton solvation shell should contribute to the activation barrier of the PCET steps. Second, the effect of the nature of the electron donor on the oxygen reduction catalyzed by CoIITPP has been previously studied in a biphasic system consisting of an air-saturated aqueous solution of HCl and an air-saturated DCE solution containing LiTB, CoIITPP and Fc, dimethylferrocene (DFc), or

{COII TPP(H 2O)n } + O2 ⇆ {COII TPP(O2 )} + (H 2O)n , Kex

(4)

where Kex ≪ 1 represents the equilibrium constant of the exchange reaction. The UV−vis spectrum of both complex forms is dominated by the Soret band at 411 nm (Figure 1, panel a). Upon the addition of HTB and Fc, the complex {CoIITPP(O2)} enters the sequence of the consecutive irreversible steps converting the coordinated O2 into H2O2, and the complex {CoIITPP(H2O)n} into {CoIIITPP+(TB−)}, where the acid anion TB− is included to compensate the positive charge of the complex. On refining Scheme 1, the exchange reaction (eq 4) could be the first step of the catalytic cycle, which is followed by the proton coupled electron transfer (PCET), as described by eq 5: {CoIITPP(O2 )} + H+ + TB− → {CoIIITPP(O2 H•) (TB−)},

k

(5)

where k is the corresponding rate constant, and by two fast electron transfer reactions including the second PCET and the regeneration of the catalyst as described by eq 6 and eq 7, respectively: {CoIIITPP(O2 H•)(TB−)} + H+ + Fc → {CoIIITPP(TB−)} + H 2O2 + Fc+ {CoIIITPP(TB−)} + Fc → CoIITPP + Fc+ + TB−

(8)

(6) (7)

Equation 6 assumes that H2O2 is easily relased from the {CoIIITPP(H2O2)(TB−)} complex. This assumption is supported by the very negative free energy of the H2O2 extraction (Table 3). As noted above, the regeneration of the catalyst (eq 7) is much faster than its consumption in the cycle, so that only CoIITPP and/or {CoIITPP(H2O)n} or {CoIITPP(O2)} complexes are detected until all Fc is consumed, cf. the timeresolved spectra in Figure 4 dominated by the Soret band at 411 nm. The effects of concentrations of CoIITPP (Figure 6, panels a), HTB (Figure 6, panel b), and H2O (Figure 7), as well as the absence of the effect of the Fc concentration (Figure 6, panel 2025



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decamethylferrocene (DMFc). 28 The quantity of H 2 O 2 generated in a shake flask experiment after 30 min followed the order DMFc > DFc > Fc with a molar ratio 4:1.3:1.28 This order corresponds to the increasing formal redox potential in the sequence DMFc (E°′ = −0.542 V vs Fc+/Fc) < DFc (E°′ = −0.125 V vs Fc+/Fc) < Fc (E°′ = 0 vs Fc+/Fc),61 i.e., to the decreasing driving force for the O2 reduction. On this basis the conclusion was made that the regeneration of CoIITPP (eq 7) by Fc, DFc, or DMFc is the RDS of the catalytic cycle, in agreement with the study of the reaction in MeCN,2 and at variance with the present results. However, the rate of the catalytic production of H2O2 has not been apparently corrected for the rate of the production of H2O2 by the direct (noncatalyzed) reduction of oxygen, which could be also described by eq 1. Kinetic measurements of the noncatalyzed oxygen reduction by the methyl-substituted ferrocenes showed that the initial reduction rate decreases by about 1 order of magnitude, when the formal redox potential of ferrocene increases by 0.1 V. 61 Consequently, the noncatalyzed production of H2O2 itself should follow the order DMFc > DFc > Fc with a molar ratio 3 × 105:18:1. From this point of view, the reported effect28 of the nature of the electron donor on the rate of the production of H2O2 is rather weak, if not negligible, as it could be expected on the basis of the present results. Third, the effect of the solvent could be considered by comparing the present results with those reported for the catalytic O2 reduction in MeCN.2 The difference betweeen the formal redox potentials E°′ of the CoIIITPP+/CoIITPP redox system in DCE (E°′ ≈ E1/2rev = 0.38 V vs Fc+/Fc) and MeCN (E°′ = −0.02 V vs Fc+/Fc)2 implies that the reaction free energy ΔGr° for the oxidation of Fc by CoIIITPP+ in DCE is lower by 0.4 eV. The decrease in ΔGr° should lead to an acceleration of the Fc oxidation by a factor of exp(βΔΔGr°/ RT) = 2.4 × 103, assuming that the Brønsted coefficient β = 0.5. The considerable acceleration of the regeneration step (eq 7) could explain why the reduction of CoIIITPP+ by Fc, which was identified as the RDS of the catalytic cycle in MeCN,2 does not control the catalytic rate in DCE. The DFT calculations indicate that the equilibrium constant Kex of the exchange reaction (eq 4) is not much affected when DCE is replaced by MeCN. Finally, in the presence of an acidic reagent, the metal ions like Fe3+, Cu2+, Co2+, or Ni2+ can be removed from a metalloporphyrin complex yielding the corresponding metalfree porphyrin.62 Since the catalytic effect of the metal-free tetraphenylporphyrin (H2TPP) on the oxygen reduction is about 2 orders of magnitude weaker (Figure 7, panel a), an extensive demetalation of the CoIITPP catalyst would lead to a significant change in the reaction rate. However, such a change was not observed on the time scale of the present kinetic measurements and, hence, the effect of demetalation can be excluded.

the intramolecular proton coupled electron transfer to the O2 molecule coordinated to the metal center leading to the formation of the O2H• radical. This step is common both to the O2 reduction by Fc catalyzed by CoIITPP and to the O2 reduction by CoIITPP itself. The change in the RDS is associated with a significant increase of rate of the regeneration of CoIITPP caused by a decrease in the corresponding reaction free energy. As a result, CoIITPP remains the sole form of the catalyst in the reaction course. Kinetic data point to the competitive coordination of water to the metal center leading to a strong inhibition of the catalytic reaction. In agreement with this finding, quantum chemical calculations indicate that water is bound to the metal center much more strongly than triplet O2. A similar effect is demonstrated in the O2 reduction catalyzed by the metal-free porphyrin (H2TPP), though in this case the catalytic rate is lower by 2 orders of magnitude under comparable experimental conditions. These investigations are relevant to studies of synthetic porphyrins in electrocatalytic applications and biomimetic models.

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ASSOCIATED CONTENT

S Supporting Information *

UV−vis spectra of Fc and Fc+ (Figure S1). Stoichiometry of the catalytic O2 reduction with Fc (Figure S2). Thermodynamic considerations. Effect of water concentration on the catalytic O2 reduction (Figure S3). DFT calculated structures of CoIITPP and CoIIITPP+ aducts with various ligands (Figures S4 to S8, Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support from the Grant Agency of the Czech Republic (Grant No. P208/11/0697) is acknowledged.

■

REFERENCES

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4. CONCLUSIONS Kinetic measurements of the O2 reduction by Fc catalyzed by CoIITPP in the air-saturated DCE containing HTB and water shed light on the catalytic mechanism, particularly on the previously ignored role of water. The amount of Fc+ formed during the total conversion of O2 indicated that O2 is reduced by two electrons yielding H2O2. In contrast to MeCN, where the reaction is controlled by electron transfer from Fc to CoIIITPP+ thereby regenerating CoIITPP,1 the RDS in DCE is 2026



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