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Cite This: Inorg. Chem. 2017, 56, 13613-13626

Cobalt Tetrabutano- and Tetrabenzotetraarylporphyrin Complexes: Effect of Substituents on the Electrochemical Properties and Catalytic Activity of Oxygen Reduction Reactions Lina Ye,†,‡ Yuanyuan Fang,‡ Zhongping Ou,*,‡,§ Songlin Xue,‡ and Karl M. Kadish*,§ †

School of Computer, Jilin Normal University, Siping 136000, China School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China § Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States Downloaded via FORDHAM UNIV on June 29, 2018 at 20:50:48 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Three series of cobalt tetraarylporphyrins were synthesized and characterized by electrochemistry and spectroelectrochemistry. The investigated compounds have the general formula (TpYPP)Co, butano(TpYPP)CoII, and benzo(TpYPP)CoII, where TpYPP represents the dianion of the mesosubstituted porphyrin, Y is a CH3, H, or Cl substituent on the para position of the four phenyl rings, and butano and benzo are respectively the β- and β′-substituted groups on the four pyrrole rings of the compound. Each porphyrin undergoes one or two reductions depending upon the meso substituent and solvent utilized. Two irreversible reductions are observed for (TpYPP)CoII and butano(TpYPP)CoII in CH2Cl2 containing 0.1 M tetra-n-butylammonium perchlorate; the first leads to the formation of a highly reactive cobalt(I) porphyrin, which can then rapidly react with a solvent to give a CoIIICH2Cl as the product. Only one reversible reduction is seen for benzo(TpYPP)CoII under the same solution conditions, and the one-electron-reduction product is assigned as a cobalt(II) porphyrin π-anion radical. Three oxidations can be observed for each examined compound in CH2Cl2. The first oxidation is metal-centered for the (TpYPP)Co and benzo(TpYPP)CoII derivatives, leading to generation of a cobalt(III) porphyrin with an intact π-ring system, but this redox process is ring-centered in the case of butano(TpYPP)CoII and gives a CoII π-cation radical product. Each porphyrin was also examined as a catalyst for oxygen reduction reactions (ORRs) when adsorbed on a graphite electrode in 1.0 M HClO4. The number of electrons transferred (n) during ORRs is 2.0 for the butano(TpYPP)CoII derivatives, consistent with only H2O2 being produced as a product for the reaction with O2. However, the reduction of O2 using the cobalt benzoporphyrins as catalysts gave n values between 2.6 and 3.1 under the same solution conditions, thus producing a mixture of H2O and H2O2 as the reduction product. This result indicates that the β and β′ substituents have a significant effect on the catalytic properties of the cobalt porphyrins for ORRs in acid media.

■

In contrast, both cobalt tetramethylporphyrin23 and cobalt porphine25 have been shown to catalyze the reduction of O2 via a four-electron-transfer pathway (n = 4), giving H2O as a final reduction product. These two less sterically hindered metallomacrocycles may exhibit strong π−π interactions between the molecules in solution, leading to the formation of porphyrin dimers on the electrode surface and thus enhancing the fourelectron electrocatalytic reduction of O2 as a result of the two cobalt centers being in close proximity to each other. Benzoporphyrins, which possess an extended π-conjugated system,45−50 display high chemical stability, increased basicity, and unique UV−vis spectral and electrochemical properties compared to the parent porphyrins. A number of benzoporphyrins have been synthesized to date, and these compounds may have enormous potential applications in both natural and

INTRODUCTION It has long been known that cobalt porphyrins can be used as catalysts for oxygen reduction reactions (ORR) in acid media to generate H2O2 and/or H2O via a two- or four-electron-transfer pathway,1−4 and a large number of derivatives have been examined for their catalytic properties for ORRs over the last 3 decades.5−41 The catalytic activity and selectivity of the cobalt porphyrins toward the formation of H2O2 or H2O as a product of the ORR will depend upon the type and position of the porphyrin ring substituents and the planarity of the macrocycle.21,39,42−44 For example, meso-ferrocenyl-substituted porphyrins have been shown to be highly selective catalysts toward the two-electron reduction of O2 (n = 2) and lead almost exclusively to the formation of H2O2 as a reaction product. The low value of n is due to steric hindrance from the bulky ferrocene groups, which prevent π−π interaction between macrocycles, thus hindering the formation of porphyrin dimers and making the four-electron reduction of O2 less favorable.6,21 © 2017 American Chemical Society

Received: September 19, 2017 Published: October 24, 2017 13613

DOI: 10.1021/acs.inorgchem.7b02405 Inorg. Chem. 2017, 56, 13613−13626

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Inorganic Chemistry applied sciences.50,51 Among the most studied of the benzoporphyrins are the free-base derivatives and complexes containing nickel, copper, zinc, or platinum central metal ions. In contrast to these transition-metal derivatives, to date only a few cobalt benzoporphyrins have been synthesized and characterized.50,51 It is generally expected that the larger the π system of a porphyrin, the stronger the π−π interaction between the molecules in solution. With this in mind, we wished to know how the catalytic activity of a cobalt porphyrin for the ORR might be changed by expanding the π system of the molecule. To answer this question, three series of cobalt tetraarylporphyrins were synthesized and their catalytic activity toward the ORR was evaluated in the present study by cyclic voltammetry and linear-sweep voltammetry using a rotating disk electrode (RDE) and a rotating ring-disk electrode (RRDE). The structures of the examined benzoporphyrin catalysts are shown in Chart 1, which also includes structures of the two

Scheme 1. Synthetic Routes of Butano(TpYPP)Co and Benzo(TpYPP)Co

Chart 1. Structures of the Investigated Cobalt Porphyrins

mide (DMF) mixture [1:1 (v/v)] to give the desired benzo(TpYPP)Co derivatives 3a−3c, as shown in Scheme 1b. 1 H NMR and UV−Vis Spectra. The 1H NMR spectrum of each porphyrin was measured in CDCl3 at 298 K. Examples of the obtained spectra are shown in Figure 1 for (TPP)Co 1b, butano(TPP)Co 2b, and benzo(TPP)Co 3b. In the case of (TPP)Co 1b, the pyrrole proton resonances are located at 15.96 ppm, while the meso-phenyl proton resonances are observed at 13.16, 9.95, and 9.74 ppm (o-H, m-H, and p-H, respectively). Resonances of the butano protons of 2b are seen at 12.28 and 3.15 ppm, while the meso-phenyl proton resonances are located at 15.75, 10.49, and 10.23 ppm (o-H, m-H, and p-H, respectively). The benzo proton resonances of 3b are seen at 10.11 and 8.79 ppm, while the phenyl proton resonances are observed at 14.00, 12.97, and 9.80 ppm (o-H, mH, and p-H, respectively). UV−vis spectra in CH2Cl2 for compounds in the three series are illustrated in Figure 2. The (TpYPP)CoII complexes 1a−1c are each characterized by a sharp Soret band at 416−418 nm and a low-intensity Q band at 534 nm (Figure 2a), while the butano(TpYPP)CoII derivatives 2a−2c exhibit red-shifted Soret and Q bands compared to the three (TpYPP)CoII compounds with the same meso substituents as seen in Figure 2b. In addition, the single Q band at 534 nm for 1a−1c is replaced by two Q bands for 2a−2c at 548−581 nm, as seen in the figure. A different spectral pattern is seen for the three benzo(TpYPP)CoII complexes 3a−3c. These porphyrins with four βand β′-fused benzo rings are characterized by a sharp Soret band at 442−446 nm and two Q bands, one of which is relatively intense and located at 638−642 nm. The Soret bands are red-shifted by 16−20 nm compared to (TpYPP)Co, while the highest-intensity Q band is red-shifted by ∼105 nm from the related Q-band absorption of the porphyrin lacking the fused benzo rings. The relatively high intensity of the longest-

related cobalt tetraarylporphyrin macrocycles lacking the four β- and β′-fused rings, i.e., (TpYPP)Co and butano(TpYPP)Co, the latter of which possesses four β- and β′-fused butano rings. The effects of the benzo substituents on the UV−vis spectra, reduction/oxidation potentials, and catalytic activity for the reduction of O2 in acid media were examined, and comparisons were made with data for compounds in the two series of porphyrins lacking the fused benzo rings.

■

RESULTS AND DISCUSSION Synthesis. The (TpYPP)Co complexes 1a−1c were synthesized according to procedures described in the literature.6,44,51 The butano(TpYPP)Co complexes 2a−2c were prepared via a reaction of the free-base butanotetraarylporphyrins with Co(OAc)2 in a mixed CHCl3 and CH3OH solvent [4:1 (v/v)] at room temperature (Scheme 1a). The reaction is facile, and the final products were obtained in yields ranging from 80 to 85%. Zinc(II) and copper(II) benzotetraarylporphyrins have been synthesized by oxidizing the corresponding butano derivatives using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone, as described in the literature.45,52 However, the cobalt benzotetraarylporphyrins 3a−3c could not be obtained by this method starting from the corresponding cobalt porphyrins 2a−2c because of decomposition of the butanoporphyrins, which occurred during the reaction, and a different synthetic procedure was therefore utilized. In this method, the benzo(TpYPP)Zn derivatives were first prepared according to literature methods45,52 and then demetalated to give the free-base benzotetraporphyrins, which were reacted with Co(OAc)2 in a CH2Cl2/N,N-dimethylforma13614

DOI: 10.1021/acs.inorgchem.7b02405 Inorg. Chem. 2017, 56, 13613−13626

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Inorganic Chemistry

Figure 1. 1H NMR spectra of (a) (TPP)Co 1b, (b) butano(TPP)Co 2b, and (c) benzo(TPP)Co 3b in CDCl3 at 298 K.

Figure 2. UV−vis spectra of (a) (TpYPP)Co, (b) butano(TpYPP)Co, and (c) benzo(TpYPP)Co in CH2Cl2.

Oxidation. As seen in Figure 3a, benzo(TPP)CoII 3b undergoes three reversible one-electron oxidations at E1/2 = 0.44, 0.70, and 0.89 V in CH2Cl2. These processes are all easier by 260−270 mV compared to (TPP)CoII 1b, which exhibits three reversible one-electron oxidations at E1/2 = 0.70, 0.97, and 1.15 V under the same solution conditions (Figure 3b). The cyclic voltammogram for oxidation of butano(TPP)CoII 2b (Figure 3c) differs from that of both compounds 1b and 3b in that the first one-electron abstraction is quasi-reversible and located at E1/2 = 0.39 V, while the second and third oxidations

wavelength Q band in Figure 2c results from the increased conjugation between the four benzo groups and macrocycle. This result is consistent with what has been reported for other benzoporphyrins having different central metal ions.33−38 Electrochemistry. The redox properties of (TpYPP)CoII 1a−1c, butano(TpYPP)CoII 2a−2c, and benzo(TpYPP)CoII 3a−3c were examined in CH2Cl2 and PhCN containing 0.1 M tetra-n-butylammonium perchlorate (TBAP). The measured potentials are summarized in Table 1, while examples of the cyclic voltammograms are given in Figures 3 and 4. 13615

DOI: 10.1021/acs.inorgchem.7b02405 Inorg. Chem. 2017, 56, 13613−13626

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Table 1. Half-Wave Potentials (V vs SCE) of (TpYPP)Co, Butano(TpYPP)Co, and Benzo(TpYPP)Co in CH2Cl2 and PhCN Containing 0.1 M TBAP solvent CH2Cl2

β and β′ substituents none

butano

benzo PhCN

none

butano

benzo

Y

cpd

CH3 H Cl CH3 H Cl H Cl CH3 H Cl CH3 H Cl H Cl

1a 1b 1c 2a 2b 2c 3b 3c 1a 1b 1c 2a 2b 2c 3b 3c

oxidation 1.14 1.15 1.22 0.70b 0.80b 0.83b 0.89 0.88 1.37 0.94 0.94 0.92b 0.94 0.93

0.96 0.97 0.99 0.70b 0.80b 0.83b 0.70 0.71 1.16d 1.19 1.26d 0.76 0.87 0.92b 0.76 0.74

reduction 0.72 0.70 0.74 0.37c 0.39c 0.44c 0.44 0.47 0.57d 0.58a 0.75d 0.34 0.36 0.39 0.45 0.46

−0.94a −0.87a −0.83a −1.08a −1.02a −1.00a −0.61 −0.55 −0.80d −0.85 −0.76d −1.02 −1.01 −0.93 −0.70 −0.65

−1.48a −1.38a −1.33a −1.72a −1.70a −1.60a >−2.0 −2.00a −1.88d −1.98 −1.79d

a

Irreversible peak potential at a scan rate of 0.10 V/s. bTwo overlapped one-electron oxidations. cQuasi-reversible oxidation. dData measured in DMF and taken from ref 53.

Figure 3. Cyclic voltammograms showing the oxidation of (a) benzo(TPP)CoII 3b, (b) (TPP)CoII 1b, and (c) butano(TPP)CoII 2b in CH2Cl2 containing 0.1 M TBAP.

Figure 4. Cyclic voltammograms showing the oxidation of (a) butano(TpCH3PP)CoII 2a, (b) butano(TPP)CoII 2b, and (c) butano(TpClPP)CoII 2c in PhCN containing 0.1 M TBAP.

are overlapped in potential and located at E1/2 = 0.80 V in CH2Cl2. Overlapping of the second and third oxidations is also seen for the butano derivatives 2a (E1/2 = 0.70 V) and 2c (E1/2 = 0.83 V) in CH2Cl2 (Table 1), but an overlapping of the last two oxidations occurs only for compound 2c in PhCN (Figure 4). It is known that the type of meso substituents on a porphyrin will affect the measured redox potentials, with the magnitude of the effect on E1/2 being dependent in many cases on both the site of oxidation or reduction and the properties of the solvent. Generally, porphyrins having electron-withdrawing substituents are harder to oxidize and easier to reduce, while those having electron-donating substituents are easier to oxidize and harder to reduce.53 As shown in Figure 4, the first two oxidations of the butano derivatives are easiest for the meso-CH3Ph derivative

2a (E1/2 = 0.34 and 0.76 V), while the hardest oxidations are seen for the meso-ClPh porphyrin 2c (E1/2 = 0.39 and 0.92 V). The magnitude of the substituent effect on the redox potential is not the same for these two oxidations, with the difference in E1/2 amounting to 50 mV in the case of the first electron abstraction and 150 mV in the case of the second. This contrasts with the third oxidation of the three butano derivatives, where a virtually identical E1/2 value of 0.92−0.94 V is observed for the three compounds having different substituents on the meso-phenyl rings. Because this third oxidation changes little with the substituent and the second shifts positively upon going from Y = CH3 to Y = Cl, the 13616

DOI: 10.1021/acs.inorgchem.7b02405 Inorg. Chem. 2017, 56, 13613−13626

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Inorganic Chemistry second and third oxidations of 2c become overlapped in this solvent. Figure 5 shows the plots of the measured oxidation potentials (E1/2) for butanoporphyrins in CH2Cl2 and PhCN versus the

Figure 6. Cyclic voltammograms showing the reduction of compounds 1b, 2b, and 3b in (a) CH2Cl2 and (b) PhCN containing 0.1 M TBAP.

more negative potentials.44,54,55 A chemical reaction with CH2Cl2 also occurs for the one-electron-reduced (TpYPP)CoII 1a−1c and butano(TpYPP)CoII 2a−2c, but the first oneelectron reduction of the benzoporphyrins 3a−3c are reversible in both CH2Cl2 and PhCN, as seen from the cyclic voltammogram of 3b in Figure 6a. This indicates the lack of a chemical reaction following the electron-transfer process and the formation of stable reduction products on the cyclic voltammetric time scale. Finally, it should be noted that reversible reductions are seen for each investigated porphyrin in PhCN, as shown in Figure 6b for 1b, 2b, and 3b. UV−Vis Spectroelectrochemistry. It has long been known that the site of oxidation for cobalt(II) porphyrins can occur either at the central metal ion to give cobalt(III) derivatives or at the porphyrin macrocycle to give cobalt(II) porphyrin π-cation radicals, with the exact site of electron transfer dependent on the structure of the compound and the solvent utilized.53,56−60 In order to determine the exact site of the initial oxidation in the currently investigated cobalt(II) butanoporphyrins and benzoporphyrins, thin-layer UV−vis spectroelectrochemistry of (TpYPP)CoII 1a−1c, butano(TpYPP)CoII 2a−2c, and benzo(TpYPP)CoII 3a−3c was carried out in CH2Cl2 containing 0.1 M TBAP, and examples of the spectral changes that occurred during the first controlled potential oxidation are shown in Figure 7. Two different types of spectral changes are observed in Figure 7. One is for (TPP)CoII 1b and benzo(TPP)CoII 3b, and the other is for butano(TPP)CoII 2b. The spectral changes during oxidation of 1b and 3b are diagnostic of a CoII/CoIII conversion in that the Soret and Q bands of the singly oxidized compound are well-defined (at 432 and 539 nm for 1b and at 471 and 651 nm for 3b) and red-shifted with respect to their neutral form in CH2Cl2 containing 0.1 M TBAP. Similar spectral changes are observed for benzo(TpClPP)CoII 3c under the same solution conditions (Figure S1).

Figure 5. Plots of the oxidation potentials (E1/2) versus the sum of the Hammett substituent constants (4σ) for butano(TpYPP)Co 2a−2c in (a) CH2Cl2 and (b) PhCN.

sum of the Hammett substituent constant (4σ) for the para substituents on the meso-phenyl rings of the compounds. A linear relationship is seen for each oxidation in both solvents, but the slopes (ρ) of the ΔE1/2/Δ(4σ) plots differ from each other, indicating a different substituent effect on each oxidation process in the two solvents. The ρ value is 44 mV for the first one-electron oxidation, but it is 78 mV for the second and third overlapped oxidations in CH2Cl2 (see Figure 5a). The ρ values are +31, +97, and −13 mV for the three oxidations in PhCN (see Figure 5b). Thus, in both solvents, the meso substituents have a smaller effect on the first oxidation than the second, independent of the solvent utilized. Reduction. Cyclic voltammograms showing the reductions of compounds 1b, 2b, and 3b in CH2Cl2 and PhCN are illustrated in Figure 6. Like (TPP)CoII 1b, which exhibits two irreversible reductions at Epc = −0.87 and −1.38 V in CH2Cl2, the first of which involves reduction of the original cobalt porphyrin and the second a product of a chemical reaction with CH2Cl2.44 Butanoporphyrin 2b also undergoes two irreversible reductions (Epc = −1.02 and −1.70 V) in this solvent, the second of which is also a product of the first reaction, but the potentials for these processes are shifted negatively by 150−320 mV as compared to potentials for the related redox reactions of compound 1b (Figure 6a). Similar irreversible reductions have been reported for most previously examined meso- and/or βpyrrole-substituted cobalt porphyrins in a CH2Cl2 solvent,53 and this is related to a reaction involving the electrogenerated cobalt(I) and the CH2Cl2 solvent to form a transient σ-bonded cobalt(III) porphyrin, which is then irreversibly reduced at 13617

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cobalt(II) porphyrins are only partially converted to their cobalt(III) form and that a cobalt(II) porphyrin π-cation radical might also be generated in solution and is in equilibrium with the cobalt(III) porphyrin when the oxidation is carried out in PhCN. The Soret band of (TPP)CoII 1b decreases only slightly in intensity during the first one-electron reduction of this porphyrin in CH2Cl2 containing 0.1 M TBAP (Figure 9a). Similar spectral changes are seen for butano(TPP)CoII 2b in the same solvent (Figure 9b), indicating that the first reduction of 2b is metal-centered to give cobalt(I) porphyrin, which then reacts with CH2Cl2 to generate cobalt(III) porphyrin having a Co−C σ band, i.e., (TPP)Co(CH2Cl). This result is consistent with what has been previously reported for other cobalt(II) tetraarylporphyrins under the same solution conditions.44 Different spectral changes were observed for the benzoporphyrin derivatives during the first controlled potential reduction in CH2Cl2. Unlike the changes for 1a−1c and 2a−2c, the initial Soret band at 442−444 nm and the Q band at 638−642 nm both substantially decreased in intensity, while a new Soret band grew in at 416−418 nm (Figures 9c and S2). It should be pointed out that the new Soret band for the singly reduced 3b has an intense band at 416 nm, the same value as that of the neutral 1b (Figure 9), suggesting the formation of a cobalt(II) porphyrin π-anion radical, with the added electron being localized on the fused rings. A similar result was obtained after the first one-electron reduction of cobalt(II) quinoxalinoporphyrins having a fused quinoxaline group.60 The spectrum of the initial benzoporphyrin could be recovered by setting the controlled potential back to 0.0 V, a result consistent with the observed reversible one-electron reduction on the cyclic voltammetric time scale (Figure 6a). This suggests that the singly reduced product of cobalt(II) benzoporphyrin is a cobalt(II) porphyrin π-anion radical, which does not react with the CH2Cl2 solvent under the given experimental conditions. In summary, the butano(TpYPP)CoII complexes undergo metal-centered reductions to give cobalt(I) porphyrins in CH2Cl2, but the electrogenerated products rapidly react with CH2Cl2 to generate cobalt(III) porphyrins with bound CH2Cl, which is irreversibly reduced at more negative potentials. This is consistent with that previously been reported for other cobalt tetraarylporphyrins44 and cobalt triarylcorroles61,62 under the same solution conditions. In the case of the benzo(TpYPP)CoII compounds, the first one-electron reduction is proposed to generate a stable cobalt(II) porphyrin π-anion radical in the CH2Cl2 solvent (Scheme 2). The benzo(TpYPP)CoII 3b and 3c show oxidation behavior similar to that of (TpYPP)CoII 1a−1c, which undergo three reversible oxidations in CH2Cl2. The first of those is a metalcentered electron-transfer process to give the cobalt(III) porphyrins described in eqs 1 and 2. Thin-layer spectroelectrochemical data suggest that cobalt(II) porphyrin π-cation radicals are produced after the first one-electron oxidation of the butano(TpYPP)CoII 2a and 2b in CH2Cl2, but a mixture of cobalt(III) porphyrins and cobalt(II) porphyrin π-cation radical is generated when PhCN is the electrochemical solvent, and the prevailing reaction is then given as shown in Scheme 3.

Figure 7. Thin-layer UV−vis spectral changes of (a) (TPP)CoII 1b, (b) butano(TPP)CoII 2b, and (c) benzo(TPP)CoII 3b during the first controlled potential oxidation at an applied potential in CH2Cl2 containing 0.1 M TBAP.

In contrast to the above, the spectral changes during oxidation of butano(TPP)CoII 2b are diagnostic of cobalt(II) porphyrin π-cation radical generation in that the spectrum of the singly oxidized species exhibits a broad low-intensity Soret band with a broad Q band (Figure 7b). The same types of spectral changes are also observed for the butanoporphyrins 2a and 2c (Figure 8a), indicating formation of the cobalt(II) porphyrin π-cation radical during the first controlled potential oxidation of all three compounds under the given solution conditions. Similar spectral changes have been previously observed for β-pyrrole brominated cobalt(II) porphyrins, (BrxTPP)CoII, where x = 6, 7, or 8, during the first oxidation to generate cobalt(II) porphyrin π-cation radicals in CH2Cl2 containing 0.1 M TBAPF6.56 The spectral changes that occur after the first one-electron oxidation of the cobalt butanoporphyrins in PhCN differ from that observed for oxidation of the same compounds in CH2Cl2. As a comparison, the UV−vis spectral changes of butano(TpYPP)CoII 2a−2c during the first controlled potential oxidation at an applied potential in PhCN are shown in Figure 8b. As seen in the figure, a new Soret band grows in at ∼450 nm upon the first one-electron oxidation in PhCN, but the intensity of this new Soret band, which is assigned to the cobalt(III) derivative, is much lower than that of the initial cobalt(II) porphyrin. The result indicates that the initial

−e

(Tp YPP)Co II ⇌ [(Tp YPP)Co III]+

(1)

−e

benzo(Tp YPP)Co II ⇌ [benzo(Tp YPP)Co III]+ 13618

(2)

DOI: 10.1021/acs.inorgchem.7b02405 Inorg. Chem. 2017, 56, 13613−13626

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Inorganic Chemistry

Figure 8. Thin-layer UV−vis spectral changes of butano(TpYPP)CoII 2a−2c during the controlled potential oxidations at an applied potential in (a) CH2Cl2 and (b) PhCN containing 0.1 M TBAP.

is the kinematic viscosity of H2O, and ω is the angular rotation speed (rad/s) of the electrode. The value of n and the percentage of H2O2 product in the ORR process using the cobalt porphyrins as catalysts are given in Table 2. Generally, a two-electron-transfer process (n = 2) would correspond to the generation of 100% H2O2, while a four-electron process (n = 4) would give 0% H2O2 and 100% H2O. The Koutecky−Levich plots in Figure 11 showed that the number of electrons transferred (n) is 2.4 for 1c, 2.0 for 2c, and 3.1 for 3c, with the corresponding percentage of H2O2 produced ranging from 45 to 100% (Table 2). It should be pointed out that the values of n for the butanosubstituted cobalt porphyrins 2a−2c are 2.0 in each case (see Table 2), indicating that the ORR catalyzed by the butanosubstituted porphyrins is mainly a two-electron-transfer process, giving exclusively 100% H2O2. However, when the benzo-substituted porphyrins 3b and 3c were used as the catalysts, the number of electrons transferred increased to 2.6 and 3.1, respectively. This is consistent with generating a mixture of H2O2 and H2O as the products under the given solution conditions. The RRDE measurements were also carried out in 1.0 M HClO4 for the ORR, with the disk potential being scanned from +0.5 to −0.1 V at a rotation rate of 400 rpm while the ring potential was held at 1.0 V versus saturated calomel electrode (SCE). Examples of the current−voltage curves observed under these conditions are given in Figure 12 for compounds 1c, 2c, and 3c. As seen in the figure, the disk current begins to increase at about 0.40 V for 1c and 0.30 V for both 2c and 3c, and a current plateau is reached at about 0.20 V for 1c and 0.15 V for 2c and 3c, respectively. The amount of H2O2 generated from

Electrocatalytic Reduction of O2. Each cobalt porphyrin was examined for its catalytic activity for the ORR in 1.0 M HClO4. Figure 10 illustrates the cyclic voltammograms obtained for compounds 1c, 2c, and 3c when adsorbed on the surface of an edge-plane pyrolytic graphite (EPPG) disk electrode in acid media. No reduction is seen under N2, but an irreversible reduction is observed at Epc = 0.15 V for 1c and at 0.20 V for compounds 2c and 3c at a scan rate of 50 mV/s. The voltammetric data indicate that a catalytic ORR has occurred on the surface of the electrode. Each porphyrin was also examined by linear-sweep voltammetry in air-saturated 1.0 M HClO4, and the steadystate limiting currents taken at 0.10 V on the plateau of the current−voltage curve were utilized to determine the number of electrons transferred (n) during the ORR. Examples of the obtained current−voltage curves under these conditions are shown in the top of Figure 11 for compounds 1c, 2c, and 3c. The diagnostic Koutecky−Levich plots,63 which were analyzed on the basis of eq 3, are given in the bottom of Figure 11, where jlim is the measured limiting current density (mA/cm2), jk is the kinetic current, and jlev is the Levich current, which is used to measure the rate of the current-limiting chemical reaction as defined by eq 4. 1/jlim = 1/jlev + 1/jk

(3)

jlev = 0.62nFD2/3cv−1/6ω1/2

(4)

Where n is the number of electrons transferred in the overall electrode reaction, F is the Faraday constant (96485 C/mol), D and c are the diffusion coefficient (cm 2/s) and bulk concentration of O2 (mol/L) in 1.0 M HClO4, respectively, v 13619

DOI: 10.1021/acs.inorgchem.7b02405 Inorg. Chem. 2017, 56, 13613−13626

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Figure 10. Cyclic voltammograms of (a) (TpClPP)CoII 1c, (b) butano(TpClPP)CoII 2c, and (c) benzo(TpClPP)CoII 3c absorbed on an EPPG electrode in 1.0 M HClO4 saturated with N2 (- - -) or air (). Scan rate = 50 mV/s.

the ORR was calculated using eq 5, where ID and IR are the Faradaic currents at the disk and ring electrodes, respectively, and N is the collection efficiency (0.24) of the ring electrode. The calculated value of % H2O2 is 78% for 1c, 100% for 2c, and 54% for 3c under the given experimental conditions. % H 2O2 = 100(2IR /N )/(ID + IR /N )

(5)

Electrocatalytic Reduction of H2O2. The electrocatalytic reduction of H2O2 was also examined in 1.0 M HClO4 using a benzo(TPP)CoII 3b or benzo(TpClPP)CoII 3c coated EPPG electrode; the cyclic and linear-sweep voltammograms as well as the corresponding Koutecky−Levich plots under these conditions are shown in Figure 13. The measured reduction potential of H2O2 is located at Epc = −0.03 V for 3b and 0.03 V for 3c and is negatively shifted by 0.17−0.20 V compared to the reduction potential of O2 using the same porphyrin as the catalyst under the same experimental conditions. The cathodic current for reduction of H2O2 is much lower than that of O2, indicating that the catalytic activity of the benzo(TpYPP)CoII derivatives toward the reduction of H2O2 in 1.0 M HClO4 is weaker than that for the reduction of O2. The cathodic currents shown in Figure 13 might be due to the two-electron reduction of H2O2 to generate of H2O, and this is confirmed by the Koutecky−Levich plots shown in Figure 13, where the number of electrons transferred (n = 2.0) is consistent with that previously reported for the reduction of H2O2 catalyzed by structurally related cobalt corroles64 and cobalt porphyrins.6,44 It is worth pointing out that no cathodic current is observed for the reduction of H2O2 when butano(TpYPP)CoII 2a−2c is used as the catalyst in 1.0 M HClO4. As seen in Figure S3, identical cyclic voltammograms are obtained with or without added H2O2 for each of the butano-substituted porphyrin catalysts in N2-saturated 1.0 M HClO4 solutions. This result is

Figure 9. Thin-layer UV−vis spectral changes of (a) (TPP)CoII 1b, (b) butano(TPP)CoII 2b, and (c) benzo(TPP)CoII 3b during the first controlled potential reduction at an applied potential in CH2Cl2 containing 0.1 M TBAP.

Scheme 2. Proposed First Reduction Mechanism of the Investigated Cobalt Porphyrins in CH2Cl2 Containing 0.1 M TBAP

Scheme 3. Proposed First Oxidation Mechanism of Butano(TpYPP)Co in CH2Cl2 and PhCN Containing 0.1 M TBAP

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Figure 11. Current−voltage curves and Koutecky−Levich plots for catalyzed reduction of O2 at a rotating EPPG disk electrode coated with the cobalt porphyrins 1c, 2c, and 3c in 1.0 M HClO4 saturated with air. Values of the electrode rotation rate (ω) are indicated on each curve. Potential scan rate = 50 mV/s.

2a−2c could be used as selective electrocatalysts for the twoelectron reduction of O2 in acid media. The benzo-substituted porphyrins have an extended π system, which could lead to stronger π−π interactions between the porphyrin macrocycles in solution than is observed for the non-benzo-substituted compounds, thus enhancing the electrocatalytic activity for the reduction of O2 and leading to a higher n value than that using butanoporphyrins under the same solution conditions. On other hand, the benzoporphyrin macrocycle is nonplanar,52,67−69 leading to a decreased π−π interaction on the electrode surface and n = 2.6 for benzo(TPP)Co 3b and 3.1 for benzo(TpClPP)Co 3c rather than the desired n = 4.0.

Table 2. Electrocatalytic Reduction of O2 by Cobalt Porphyrins in 1.0 M HClO4 compound

Ep, with O2

E1/2a

% H2O2b

n

(TPP)Co 1b (TpClPP)Co 1c butano(TpCH3PP)Co 2a butano(TPP)Co 2b butano(TpClPP)Co 2c benzo(TPP)Co 3b benzo(TpClPP)Co 3c

0.13 0.15 0.15 0.17 0.20 0.17 0.20

0.15 0.20 0.20 0.20 0.22 0.20 0.22

100 80 100 100 100 70 45

2.0 2.4 2.0 2.0 2.0 2.6 3.1

a

Half-wave potential at i = 0.5imax, where imax is the limiting current measured at 400 rpm of the RDE. bCalculated based on the value of n; see the discussion in the text.

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CONCLUSION Three series of cobalt tetraarylporphyrins with the general formulas (TpYPP)Co, butano(TpYPP)CoII, and benzo(TpYPP)CoII have been prepared and investigated using UV−vis spectroscopy as well as electrochemical and spectroelectrochemical approaches. The β- and β′-butano and -benzo groups have a significant effect on the UV−vis spectra, redox potentials, and site of electron transfer of the compounds. The first one-electron oxidation of the butano(TpYPP)CoII complexes is porphyrin-ring-centered and generates a cobalt(II) porphyrin π-cation radical in CH2Cl2 rather than the expected cobalt(III) porphyrin, which is generated upon oxidation of (TpYPP)CoII or benzo(TpYPP)CoII under the same solution conditions. However, a mixture of the cobalt(III) porphyrin and cobalt(II) porphyrin cation radical is proposed for the first oxidation of butano(TpYPP)Co II in PhCN. Cobalt(I) porphyrin is initially generated for (TpYPP)Co II and butano(TpYPP)CoII after the first one-electron reduction in CH2Cl2 and can rapidly react with the solvent to give a CoIII−C σ-bonded species as the final product. In contrast, the electrochemical and spectroelectrochemical data suggest that a cobalt(II) porphyrin π-anion radical is generated after the first

consistent with the fact that the value of n is 2.0 and the product is 100% H2O2 for the ORR when butano(TpYPP)CoII 2a−2c is used as the catalyst (see the above discussion). This indicates that compounds 2a−2c cannot catalyze the reduction of H2O2 under the given experimental conditions. Effect of the Butano and Benzo Groups on the Catalytic Activity. It is known that the nature of the substituents on the meso and/or β positions of metallocorroles and metalloporphyrins can have a significant effect on the catalytic activity of the compound.6,44,64−66 The same appears to be true for the currently examined cobalt porphyrins with different macrocycles. As discussed above, the non-βsubstituted cobalt porphyrins can catalyze the ORR with a value of n ranging from 2.0 for (TPP)Co to 2.4 for (TpClPP)Co. However, the calculated n is 2.0 for each butano(TpYPP)Co, independent of the meso substituents on the porphyrin macrocycle. This result suggests that the butano substituents may change the planarity of the porphyrin macrocycle, resulting in a change in the site of electron transfer for oxidation. Thus, the butano-substituted cobalt porphyrins 13621

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desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) spectra were taken on a Bruker BIFLEX III ultrahighresolution instrument using α-cyano-4-hydroxycinnamic acid as the matrix. Cyclic voltammetry was carried out at 298 K by using an EG&G Princeton Applied Research (PAR) 173 potentiostat/galvanostat or a CHI-730C electrochemical workstation. A homemade three-electrode cell was used for all electrochemical measurements. A three-electrode system was used in each case and consisted of a glassy carbon or graphite working electrode (model MT134, Pine Instrument Co.) for cyclic voltammetry and voltammetry at an RDE and a platinum ring− graphite disk electrode combination for voltammetry at the RRDE. A platinum wire served as the auxiliary electrode and SCE as the reference electrode, which was separated from the bulk of the solution by means of a salt bridge of low porosity that contained the solvent/ supporting electrolyte mixture. The RRDE was purchased from Pine Instrument Co. and consisted of a platinum ring and a removable EPPG disk (A = 0.196 cm2). A Pine Instrument MSR speed controller was used for the RDE and RRDE experiments. The platinum ring was first polished with 0.05 μm α-alumina powder and then rinsed successively with H2O and acetone before being activated by cycling the potential between +1.20 and −0.20 V in 1.0 M HClO4 until reproducible voltammograms were obtained. The catalysts were irreversibly adsorbed on the electrode surface by means of a dip-coating procedure described in the literature.23 The freshly polished electrode was dipped in a 1.0 mM catalyst solution of CH2Cl2 for 5 s, transferred rapidly to pure CH2Cl2 for 1−2 s, and then exposed to air, where the adhering solvent rapidly evaporated, leaving the porphyrin catalyst adsorbed on the electrode surface. All experiments were carried out under room temperature. Chemicals. Reagents and solvents (Sigma-Aldrich, Fluka, or Sinopharm Chemical Reagent Co.) for the synthesis and purification were of analytical grade and were used as received. Dichloromethane (CH2Cl2; 99.8%) was purchased from EMD Chemicals Inc. and used as received. Tetra-n-butylammonium perchlorate (TBAP) was purchased from Sigma Chemical or Fluka Chemika Co., recrystallized from ethyl alcohol, and dried under vacuum at 40 °C for at least 1 week prior to use. Synthesis of (TpYPP)Co. About 50 mg of free-base tetraarylporphyrins, (TpYPP)H2, were dissolved in 100 mL of CHCl3/ MeOH [4/1 (v/v)], which contains excess Co(OAc)2·2H2O (100 mg), the solution was stirred for 2 h at room temperature, and the progress of the reaction was monitored by TLC and UV−vis spectra. The reaction solution was then evaporated to dryness and freshly chromatographed with neutral alumina (200−300 mesh) using CH2Cl2 as the eluent. The red fraction was collected and evaporated to dryness to obtain the product with a yield of 80−85%. (TpCH3PP)Co 1a. Yield: 46.1 mg, 85%. UV−vis (CH2Cl2): λmax = 418, 534 nm. 1H NMR (400 MHz, CDCl3, 298 K): δ 16.00 (s, 8H, pyrrole H), 13.09 (s, 8H, ph-ortho H), 9.76 (s, 8H, ph-meta H), 4.17 (s, 12H, group H). MS (MALDI-TOF). Calcd for C48H36CoN4: m/z 727.227. Obsd: m/z 727.339. (TPP)Co 1b. Yield: 44.8 mg, 82%. UV−vis (CH2Cl2): λmax = 416, 534 nm. 1H NMR (400 MHz, CDCl3, 298 K): δ 15.96 (s, 8H, pyrrole H), 13.16 (s, 8H, ph-ortho H), 9.95 (s, 8H, ph-meta H), 9.74 (s, 4H, ph-para H). MS (MALDI-TOF). Calcd for C44H28CoN4: m/z 671.165. Obsd: m/z 671.243. (TpClPP)Co 1c. Yield: 43.0 mg, 80%. UV−vis (CH2Cl2): λmax = 416, 534 nm. 1H NMR (400 MHz, CDCl3, 298 K): δ 15.75 (s, 8H, pyrrole H), 12.88 (s, 8H, ph-ortho H), 9.82 (s, 8H, ph-meta H). MS (MALDITOF). Calcd for C44H24Cl4CoN4: m/z 809.006. Obsd: m/z 809.343. Synthesis of Butano(TpYPP)Co. Butano(TpYPP)H2 (50 mg) was dissolved in 150 mL of CHCl3/MeOH [4:1 (v/v)] containing excess Co(OAc)2·2H2O (100 mg). The mixture was stirred for 2 h at room temperature, and the progress of the reaction was monitored by thin-layer and UV−vis spectra. After the starting compound was completely consumed, the mixture was evaporated to dryness and then freshly chromatographed with neutral alumina (200−300 mesh) using CH2Cl2 as the eluent. The red fraction was collected and evaporated to dryness.

Figure 12. RRDE voltammograms of (a) (TpClPP)CoII 1c, (b) butano(TpClPP)CoII 2c, and (c) benzo(TpClPP)CoII 3c in HClO4 saturated with air. The potential of the disk electrode was scanned in a negative direction from +0.5 to −0.1 V, while the potential of the ring electrode was held at 1.0 V. Rotation rate = 400 rpm, and scan rate = 10 mV/s.

reduction of benzo(TpYPP)CoII in CH2Cl2, with the proposed site of electron addition being the fused rings. The electrocatalytic properties of the investigated butano-substituted cobalt porphyrins are highly selective in the two-electron electrocatalytic reduction of O2 and lead exclusively to the formation of H2O2 as a reaction product. However, the number of electrons transferred for the ORR using cobalt benzoporphyrins ranges from 2.6 to 3.1 in acid media, probably because an extension of the conjugated π system in the benzoporphyrins may lead to stronger π−π interactions between the porphyrin macrocycles on the electrode surface.

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EXPERIMENTAL SECTION

Instrumentation. Thin-layer UV−vis spectroelectrochemical experiments were performed with a home-built thin-layer cell that has a light transparent platinum net working electrode. Potentials were applied and monitored with an EG&G PAR model 173 potentiostat. Time-resolved UV−vis spectra were recorded with a Hewlett-Packard model 8453 diode-array spectrophotometer. High-purity N2 was used to deoxygenate the solution and kept over the solution during each electrochemical and spectroelectrochemical experiment. 1 H NMR spectra were recorded on a Bruker Avanc II 400 MHz instrument. Chemical shifts (δ ppm) were determined with tetramethylsilane as the internal reference. Matrix-assisted laser 13622

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Figure 13. (i) Cyclic voltammograms under N2 without (- - -) or with 0.2 mM H2O2 (), (ii) current−voltage curve for catalyzed H2O2 in 1.0 M HClO4, and (iii) a Koutecky−Levich plot. A rotating EPPG disk electrode coated with (a) benzo(TPP)CoII 3b and (b) benzo(TpClPP)CoII 3c was employed. Scan rate = 50 mV/s. Butano(TpCH3PP)Co 2a. Yield: 42.6 mg, 80%. UV−vis (CH2Cl2): λmax = 426, 548, 582 nm. 1H NMR (400 MHz, CDCl3, 298 K): δ 15.69 (s, 8H, ph-ortho H), 12.70 (s, 16H, butano H), 10.27 (s, 8H, ph-meta H), 4.35 (s, 12H, group H), 3.11 (s, 16H, butano H). MS (MALDITOF). Calcd for C64H60CoN4: m/z 943.415. Obsd: m/z 943.817. Butano(TPP)Co 2b. Yield: 43.8 mg, 82%. UV−vis (CH2Cl2): λmax = 426, 548, 580 nm. 1H NMR (400 MHz, CDCl3, 298 K): δ 15.75 (s, 8H, ph-ortho H), 12.28 (s, 16H, butano H), 10.49 (s, 8H, ph-meta H), 10.23 (m, 4H, ph-para H), 3.15 (s, 16H, butano H). MS (MALDITOF). Calcd for C60H52CoN4: m/z 887.352. Obsd: m/z 887.774. Butano(TpClPP)Co 2c. Yield: 45.0 mg, 85%. UV−vis (CH2Cl2): λmax = 424, 550, 581 nm. 1H NMR (400 MHz, CDCl3, 298 K): δ 15.63 (s, 8H, ph-ortho H), 11.55 (s, 16H, butano H), 10.49 (s, 8H, ph-meta H), 3.16 (s, 16H, butano H). MS (MALDI-TOF). Calcd for C60H48Cl4CoN4: m/z 1025.194. Obsd: m/z 1025.760. Synthesis of Benzo(TpYPP)Co. The benzo(TpYPP)Zn complexes (50 mg) were dissolved in 100 mL of CHCl3 containing 3 mL of concentrated HCl. The mixture was stirred for 1 h at room temperature, then washed with H2O, and dried over Na2SO4. After that, the mixture was freshly chromatographed with neutral alumina (200−300 mesh) using CHCl3 as the eluent. The green fraction was collected and evaporated to dryness, and benzo(TpYPP)H2 was obtained with a yield of ∼80%. Benzo(TpYPP)H2 (20 mg) was dissolved in 80 mL of 1:1 CH2Cl2/ DMF containing Co(OAc)2·2H2O (50 mg). The reaction mixture was refluxed for 2 h, and the progress of the reaction was monitored by UV−vis spectra. After the starting compound was completely consumed, the mixture was evaporated to remove CH2Cl2 and washed with H2O. After filtration, the solid was dissolved in CHCl3, dried over Na2SO4, and evaporated to dryness. The green fraction was crystallized from CHCl3/hexanes to obtain benzo(TpYPP)Co.

Benzo(TpCH3PP)Co 3a. Yield: 9.6 mg, 45%. UV−vis (CH2Cl2): λmax = 446, 587, 638 nm. 1H NMR (400 MHz, CDCl3, 298 K): δ 13.89 (s, 8H, ph-ortho H), 13.03 (s, 8H, ph-meta H), 9.87 (s, 8H, benzo H), 8.81 (s, 8H, benzo H), 4.16 (s, 12H, group H). MS (MALDI-TOF). Calcd for C64H44CoN4: m/z 927.290. Obsd: m/z 927.406. Benzo(TPP)Co 3b. Yield: 11.3 mg, 53%. UV−vis (CH2Cl2): λmax = 442, 586, 638 nm. 1H NMR (400 MHz, CDCl3, 298 K): δ 14.00 (s, 8H, ph-ortho H), 12.97 (s, 8H, ph-meta H), 10.11 (s, 8H, benzo H), 9.80 (s, 4H, ph-para H), 8.79 (s, 8H, benzo H). MS (MALDI-TOF). Calcd for C60H36CoN4: m/z 871.227. Obsd: m/z 871.377. Benzo(TpClPP)Co 3c. Yield: 10.0 mg, 47%. UV−vis (CH2Cl2): λmax = 444, 586, 642 nm. 1H NMR (400 MHz, CDCl3, 298 K): δ 13.91 (s, 8H, ph-ortho H), 12.92 (s, 8H, ph-meta H), 10.06 (s, 8H, benzo H), 8.82 (s, 8H, benzo H). MS (MALDI-TOF). calcd for C60H32Cl4CoN4: m/z 1007.071. Obsd: m/z 1006.649.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02405. UV−vis spectral changes and cyclic voltammograms (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 13623

DOI: 10.1021/acs.inorgchem.7b02405 Inorg. Chem. 2017, 56, 13613−13626

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Karl M. Kadish: 0000-0003-4586-6732 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from the National Natural Science Foundation of China (Grants 21501070 and 21071067), Jiangsu University Foundation (Grants 15JDG131 and 05JDG051), China Postdoctoral Science Foundation (Grant 2017M611707), and Robert A. Welch Foundation (to K.M.K.; Grant E-680).

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DOI: 10.1021/acs.inorgchem.7b02405 Inorg. Chem. 2017, 56, 13613−13626

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DOI: 10.1021/acs.inorgchem.7b02405 Inorg. Chem. 2017, 56, 13613−13626

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DOI: 10.1021/acs.inorgchem.7b02405 Inorg. Chem. 2017, 56, 13613−13626