Hydrogen Peroxide Interconversion Using Redox Couples

Subscriber access provided by La Trobe University Library

Article

Dioxygen/Hydrogen Peroxide Interconversion Using Redox Couples of Saddle-Distorted Porphyrins and Isophlorins Wataru Suzuki, Hiroaki Kotani, Tomoya Ishizuka, and Takahiko Kojima J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b01038 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 18, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society

Dioxygen/Hydrogen Peroxide Interconversion Using Redox Couples of Saddle-Distorted Porphyrins and Isophlorins Wataru Suzuki, Hiroaki Kotani, Tomoya Ishizuka, and Takahiko Kojima* Department of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba and CREST (JST), 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8571, Japan

ABSTRACT: Interconversion between dioxygen (O2) and hydrogen peroxide (H2O2) has attracted much interest because of growing importance of H2O2 as an energy source. There are many reports on O2 conversions to H2O2; however, no example has been reported on O2/H2O2 interconversion. Herein, we describe successful achievement of a reversible O2/H2O2 conversion based on an N21,N23-dimethylated saddle-distorted porphyrin and the corresponding two electron-reduced porphyrin (isophlorin) for the first time. The isophlorin could react with O2 to afford the corresponding porphyrin and H2O2; conversely, the porphyrin also reacted with excess H2O2 to reproduce the corresponding isophlorin and O2. The isophlorin-O2/porphyrin-H2O2 interconversion was repeatedly proceeded by alternate bubbling of Ar or O2, although no reversible conversion was observed in the case of an N21,N22-dimethylated porphyrin as a structural isomer. Such a drastic change of the reversibility was derived from the directions of inner NH protons in hydrogen-bond formation of the isophlorin core with O2 as well as those of the lone pairs of the inner nitrogen atoms of the porphyrin core to form hydrogen bonds with H2O2. The intriguing isophlorin-O2/porphyrin-H2O2 interconversion was accomplished by introducing methyl groups at the inner nitrogen atoms to minimize the difference of the Gibbs free energy between isophlorin-O2/porphyrin-H2O2 states and the Gibbs activation energy of the interconversion. Based on the kinetic and thermodynamic analysis on the isophlorin-O2/porphyrin-H2O2 interconversion using 1H NMR and UV-Vis spectroscopies and DFT calculations, we propose the formation of a two-point hydrogen-bonding adduct between the N21,N23-dimethylated porphyrin and H2O2 as an intermediate.

INTRODUCTION In Nature, forward and reverse directions in a phenomenon are often convertible in response to the reaction conditions and external stimuli.1-4 For example, the reversible conversion between dioxygen (O2) and peroxide (O22–) play important roles in O2-transport metalloproteins such as hemocyanin4 having a dicopper active site and rechargeable lithium-air batteries.5 So far, the reversible conversion between O2 and O22– has been investigated by redox-active Cu(I) complexes6,7 involving the formation of dinuclear Cu(II) peroxide complexes (CuII2(O22–)) as functional models of hemocyanin, which requires a small Gibbs free energy change (DG˚) and a small Gibbs activation energy change (DG˚‡) between a starting and a product states to be in equilibrium. On the other hand, a reversible O2/O22– conversion has been achieved by employing multi-point hydrogen-bonding sites such as amide N-H groups in a cryptand, without using redox-active metal ions.8 In addition, a redox-inactive zinc(II) complex with amide N-H groups is capable of forming a ZnII2(O22–) complex from hydrogen peroxide (H2O2) as a peroxide source in the presence of a base.9 In these cases, multi-point hydrogen-bonds play an important role in the capture of O2 and the stabilization of the O22– species for reversible O2/O22– conversion.8-10 However, additional strong electron donors and acceptors have been required to interconvert between O2 and O22– because the cryptand or the zinc(II) complex themselves are not redox-active molecules. Thus, if redox-active molecules with hydrogen-bonding sites can act as an electron and proton acceptor for 2e–-oxidation of H2O2, O2 and the corresponding reduced molecules can be produced. Conversely, the reduced molecules are capable of donating electrons and protons to O2 to accomplish O2/H2O2 interconversion (Figure 1a), when the DG˚ value can be made as small as possible. To the best of our knowledge, however, O2/H2O2 interconversion by redox-active molecules has yet to be reported. The challenging O2/H2O2 interconversion beyond the function of metalloproteins can serve the development of new functional materials such as a reduction catalyst using H2O2 as a reductant and a

O2-activation material for a selective generation of H2O2 as an energy source.11 It is expected that polypyrrole macrocycles such as porphyrins are good candidates for a reversible O2/H2O2 conversion because of their redox-active properties including multi-electron oxidation and reduction,12,13 and hydrogen-bonding ability of NH protons and basic imine N atoms of the pyrrole moieties.14-17 Furthermore, redox potentials of porphyrins can be easily modified by introducing substituents or core modifications, allowing us to control the thermodynamic stability of reduction products of porphyrins.18,19 20p-conjugated isophlorins (Figure 1b), which are 2e–-reduced species of porphyrins, 20-22 have been proposed as intermediates in the course of two-electron O2 reduction to H2O2. However, due to the lack of 18p Hückel aromaticity of porphyrins, isophlorins are thermodynamically too unstable to be handled relative to the corresponding porphyrins.20 While core modification such as N-methylation of pyrrole moieties has been demonstrated to stabilize the 20p-conjugated macrocycles, the core-modified isophlorins are too stable to promote the reduction of O2 because of the absence of the NH protons.21

Figure 1. (a) Interconversion between O2 and H2O2 based on redox couples of reduced (Red.) and oxidized (Ox.) molecules. (b) Conversion between 20π isophlorins and 18π porphyrins through proton-coupled two-electron oxidation and reduction.

ACS Paragon Plus Environment

Journal of the American Chemical Society 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

We have recently reported the formation of a thermodynamically stable isophlorin derivative (1)23 bearing four NH protons through the chemical reduction of a saddle-distorted dodecaphenylporphyrin derivative24 (2, Scheme 1, top) bearing trifluoromethyl groups (CF3) at the para positions of the four meso-phenyl groups for stabilization and characterization of 1 in the presence of water. Introducing saddle-distortion to the porphyrin ring structure contributes to the high basicity of imine N atoms and the formation of thermodynamically accessible isophlorin derivatives without core modification. Although 1 is too sensitive to O2, an isolated tetramethylated isophlorin derivative (3) is too stable to react with O2 to form the corresponding porphyrin (4, Scheme 1, middle).23 Thus, it is reasonable to consider that partial methylation of the isophlorin core should be effective to stabilize isophlorin derivatives with a moderate reactivity toward O2 for achieving of a metal-free O2/H2O2 interconversion. Herein, we describe successful construction of an unprecedented reversible O2/H2O2 conversion by using an N21,N23-dimethylated saddle-distorted porphyrin (5) and the corresponding isophlorin derivative (6, Scheme 1, bottom). In addition, we employed an N21,N22-dimethylated saddle-distorted porphyrin (7) as the structural isomer of 5 and the corresponding isophlorin derivative (8) to evaluate the importance of hydrogen-bonding interaction between porphyrinoids and O2 or H2O2. Comparison of the reactivity of the structural isomers allowed us to demonstrate that the directions of inner NH protons are very important for achieving the reversibility of the interconversion between porphyrin/H2O2 and isophlorin/O2 combinations. Scheme 1. O2 and H2O2 Interconversion Porphyrin/Isophlorin-Redox Couples

Based

Page 2 of 9

RESULTS AND DISCUSSION Synthesis and Characterization of N,N’-Dimethylated Porphyrins and the Corresponding Isophlorin Derivatives. Methylation at nitrogen atoms of pyrrole moieties of porphyrinoids have been conducted by using methyl iodide (MeI) (Figure 2a).22,25 In the presence of 3 equivalents of MeI with sodium bis(trimethylsilyl)amide (Na[N(SiMe3)2]) as a base, dimethylation of 2 at neighboring pyrrole moieties proceeded in N,N-dimethylformamide (DMF) to afford an N21, N22-dimethylated porphyrin (7) in 27% yield. 7 shows three kinds of 19F NMR signals due to the CF3 groups at –63.05, –63.19, and –63.22 ppm in acetone-d6 (Figure S1 in Supporting information). The formation of 7 was also (a) Ar

Ph

Ph

NH N Ar

Ar N HN Ph

CoII(Cp)2 DMF

Ph

Ph

NH HN Ar

Ar NH HN

Ph

Ph

Ph

Ar Ph

Ph Ph

Ph

Ar

Ph

Ar

Ph

2

Ar =

Ph Ph

1

CF3

MeI Na[N(SiMe3)2] Ar Ph

Ph

Ph

Ph

N HN Ar

Ar

MeI, Na[N(SiMe3)2] DMF

on

NH N

Ph

Ar

Ph

Ph Ph

6 Air

Ph

Ph

N

N

N

N

Ar Ph Ph

Ar

7

Ph

Ph

Ph

Ar

Ar

Ph

Ph

Ph

Ar

Ph

Ph

Ar

Ph

N

N

N

N

Ar

Ph

Ar

Ph Ph

5

Figure 2. (a) Synthetic scheme for 5 and 7. (b) An ORTEP drawing of 7 (top view). (c) Core structure of 7 (side view). (d) An ORTEP drawing of 5 (top view). (e) Core structure of 5 (side view). Hydrogen atoms except for protons of a water molecule were omitted for clarity. Peripheral aryl groups were also omitted for clarity in (c) and (e).


Page 3 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


confirmed by X-ray crystallographic analysis (Figure 2b, c). As shown in Figure 2c, one N-Me group at the pyrrole moiety was oriented against the direction of the other N-Me group in the saddle-distorted macrocycle. In the dimethylation of 2 to form 7, no formation of 5, which is a structural isomer of 7, was observed under the reaction conditions. On the other hand, 5 was successfully synthesized by methylation of an isophlorin derivative (1) as a starting material (Figure 2a). The chemical reduction of 2 by cobaltocene (CoII(Cp)2; Cp = cyclopentadienyl) in DMF resulted in the formation of 1, of which the NH protons would be derived from protons of water molecules in DMF. Dimethylation of 1 using 2 equivalents of MeI and Na[N(SiMe3)2] selectively afforded the dimethylated isophlorin 6, and subsequent oxidation of 6 by air gave the dimethylated porphyrin 5 in 19% yield together with the formation of H2O2 as described below. In stark contrast to 7, the 19F NMR spectrum of 5 shows only one signal at –63.10 ppm owing to the CF3 groups in acetone-d6 (Figure S2), due to the C2v symmetric structure of 5. In the crystal structure of 5 (Figure 2d, e), the N-Me groups in 5 were oriented in the same direction with a C2v symmetric structure in contrast to the case of 7. In addition, non-methylated two nitrogen atoms in 5 were found to form hydrogen bonds with protons of a water molecule with the N•••(H)O distances of 2.938(2) and 2.994(3) Å, whereas no interaction with water molecules was observed in the crystal structure of 7. It should be noted that no formation of 7 was observed in methylation of 1 and subsequent air oxidation. The selective dimethylation of 1 to form 6 would be derived from the thermodynamic stability of deprotonated monomethylated isophlorin derivatives as shown in Figure S3. Based on DFT calculations, the N23-deprotonated species was estimated to be more stable than the N22-deprotonated one by 5.5 kcal mol–1, indicating methylation of a deprotonated monomethyl isophlorin derivative should occur at the N23 position to afford 6 selectively. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) on 2, 3, 5, and 7 were conducted to compare the redox potential (E1/2) of porphyrinoids with N-methyl groups at different positions (Figure S4). The E1/2 of 2, 3, 5, and 7 were measured in the presence of 2 equivalents of HCl as a proton source according to the scheme in Figure 1b. The CV of each porphyrin (2, 5, 7) and 3 showed reversible two-electron redox waves in the range from –0.25 V vs Fc/Fc+ for 3 to –0.63 V vs Fc/Fc+ for 2 in DMF at 298 K containing 0.1 M [N(n-butyl)4]PF6 (TBAPF6) as an electrolyte. The dimethylated porphyrins (5 and 7) showed a similar E1/2 (–0.43 V for 5 and –0.47 V for 7), indicating the positions of the N-methyl groups hardly affected E1/2 (Figure S4c). Chemical reductions of 5 and 7 were conducted to generate the corresponding isophlorins as shown in Scheme 2. The formations were confirmed by UV-Vis and 1H NMR measurements (Figures 3 and S5). Upon addition of a Na2S2O4 aqueous solution (Na2S2O4aq) as a reductant to a DMF solution containing 5 under Ar, the Soret band at 509 nm and the Q band at 708 nm disappeared completely with a concomitant appearance of a new band at 460 nm in UV-Vis measurements (Figure 3, red traces). These spectral changes indicate the loss of aromaticity of 5 to afford a non-aromatic isophlorin derivative 6 as seen in the previously reported isophlorin 1.23 In the case of reduction of 7 by Na2S2O4aq, the Soret band at 511 nm and the Q bands at 702 nm and 777 nm disappeared completely with a concomitant appearance of a new band at 474 nm (Figure 3, blue traces) to form the corresponding isophlorin derivative (8). It should be noted that the NH protons in isophlorin derivatives derived from protons of water molecules.

Scheme 2. Chemical Reduction of N,N’-Dimethylated porphyrins to form the Corresponding Isophlorin Derivatives N

N N

N

Na2S2O4aq

N

H N

H N

N

DMF

5

6

N N

N

N

Na2S2O4aq

NH

N H N

N

DMF

7

8

Figure 3. UV-Vis spectra of 5 (0.017 mM, red dotted line), 5 in the presence of Na2S2O4aq (6, red solid line), 7 (0.0084 mM, blue dotted line), and 7 in the presence of Na2S2O4aq (8, blue solid line) in deaerated DMF at room temperature.

Reversibility between Porphyrins with H2O2 and Isophlorins with O2. Stoichiometric reduction of O2 was conducted by isophlorin derivatives (1, 3, 6, and 8). Isophlorin derivatives (1, 6, and 8) were generated in situ by the chemical reduction using Na2S2O4aq as a reductant in DMF. 1, 6, and 8 reacted with O2 smoothly to afford the corresponding porphyrins (2, 5, and 7, respectively) and H2O2 as products (Figures S6 and S7). The formation of H2O2 was confirmed by iodometry upon addition of excess amounts of potassium iodide (KI) to the reaction mixture (Figure S7).26 The corresponding porphyrins were formed quantitatively without decomposition to afford byproducts such as an oxidized ring-opening product of porphyrins.27 Furthermore, quantitative amounts of H2O2 based on 5 formed were detected after the reaction of 6 with O2. The yields of H2O2 in the reaction of 8 or 1 with O2 were determined to be 76% and 82%, respectively. On the other hand, 3 showed no reactivity toward O2 even in the presence of trifluoroacetic acid (TFA) (Figure S8), suggesting that 3 should be more stable than the corresponding tetramethylated porphyrin dication (4) in the presence of O2 and protons (see Scheme 1). Reverse reaction of the O2 reduction, oxidation of H2O2 by porphyrins (2, 5, 7) have been investigated in deaerated DMF to confirm whether the corresponding isophlorins can be reproduced through the reaction of porphyrins with H2O2 or not. Among those, 5 was converted to 6 as the sole product in the presence of excess amounts of H2O2 (100 mM) in deaerated DMF (eq 1, Figure 4a), although no spectral change was observed in the reaction of H2O2 with 2 or 7 (eq 2, Figure S9). Furthermore, 6 was converted to 5 quantitatively by bubbling O2 gas to the resulting DMF solution



containing H2O2 as seen in Figure 4a. The interconversion between 5 and 6 was completely reversible in at least ten cycles by bubbling Ar or O2, alternately without remarkable degradation of each species (Figure 4b). The interconversion between 5 and 6 was also confirmed by 1H NMR measurements in DMF-d7 (Figure S10).

5 + H2O2 7 + H2O2

K

6 + O2

(1)

8 + O2

(2)

K=

Page 4 of 9

[6][O2]

(3)

[5][H2O2]

Kinetic Analysis on the Interconversion between 5/H2O2 and 6/O2. For the kinetic analysis on the reaction of 6 with O2, UV-Vis spectral changes of 6 was monitored in the presence of O2 in DMF at 298 K. By introducing O2 to a DMF solution of 6, UV-Vis spectral change from 6 to 5 was observed with isosbestic points (350, 419, 475, 558, and 559 nm) as shown in Figure S6b. In air-saturated DMF,28 the pseudo-first-order rate constant (kobs) of the reaction of 6 with O2 was determined to be (5.43 ± 0.03) ´ 10–3 s–1 at 298 K (Figure S13a). Linear dependence of kobs on the O2 concentration was observed and the second-order rate constant (k) was determined to be (6.4 ± 0.3) M–1 s–1 from the slope (eq 4, Figure 5, red). In addition, the reactivity of 6 with O2 was much higher than that of 8 (k = (1.7 ± 0.1) M–1 s–1, eq 5, Figures 5 and S13b), despite the fact that 6 and 8 exhibited almost the same redox potentials (Figure S4c). The temperature dependence of k values for 6 was observed (Figure S14); the k values in the reaction of 6 with O2 were plotted against the inverse of the reaction temperature (T–1) (Eyring plot) to obtain the activation parameters (Figure S15).

6 + O2 8 + O2

k

k

5 + H2O2

(4)

7 + H2O2

(5)

Figure 4. (a) UV-Vis spectra of mixture of 5 (0.015 mM) and H2O2 (100 mM) in DMF were repeatedly converted between 5 (red) and 6 (purple) depending on the presence of O2. Inset: Photographs of 5 and 6. (b) Plots of absorbance at 511 nm of the reaction mixture containing 5 with H2O2 in DMF at 298 K, repeatedly converted in eleven cycles.

In order to quantify the amount of O2 formed in the reaction of 5 with H2O2, the concentration of O2 in DMF was monitored by a fluorescence O2 sensor (Figure S11a). In the presence of 0.10 mM of 5 with 50 mM of H2O2, the interconversion reached an equilibrium to afford a mixture of 5 and 6 with the ratio of 3:2, as determined on the basis of the absorbance at 705 nm (Figure S11b). The concentration of evolved O2 was determined to be 0.04 mM, which was comparable with the concentration of 6 (0.04 mM). No O2 evolution was observed in the absence of 5 (Figure S12), indicating that O2 was formed by the reaction of 5 with H2O2. Then, the equilibrium constant (K) between 5 with H2O2 and 6 with O2 was determined to be 5.3 ´ 10–4 at 298 K (eq 3). Thus, the Gibbs free energy change (DG) of the equilibrium between 5/H2O2 and 6/O2 was calculated to be +4.4 kcal mol–1 at 298 K.

Figure 5. O2 concentration dependence of the pseudo-first-order rate constant (kobs) in the reaction of 6 (red) and 8 (blue) with O2 in DMF at 298 K. Table 1. Summary of Activation Parameters (DH ‡, DS ‡, DG ‡) for 6/O2 to 5/H2O2 and Activation Parameters (DH’ ‡, DS’ ‡, DG’ ‡) for 5/H2O2 to 6/O2 and Thermodynamic Parameters (DH’, DS’, DG’ ) for an Equilibrium between 5/H2O2 and 5-H2O2. 6/O2 to 5/H2O2 5/H2O2 to 6/O2

DH‡ a 6.5 ± 0.4

DS‡ b –(32 ± 1)

DG‡ = (16.1 ± 0.5)a,c

–


–

DH’‡ a

DS’‡ a

DH’ a

DS’ b

–

–

–

–

19 ± 1

–(8 ± 1)

DG’ = (21 ± 2) ‡

–(5.1 ± 0.3)

–(8.3 ± 1.1)

DG’ = –(2.6 ± 0.4)a,c

a,c

’

–1

a: kcal mol . b: cal K–1 mol–1. c: at 298 K.

Page 5 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


As shown in Table 1, the contribution of the activation entropy term (–TDS‡) to the activation barrier (DG‡) of the reaction of 6 with O2 (+9.6 kcal mol–1 at 298 K) was larger than that of DH‡ (+6.5 kcal mol–1), suggesting an associative mechanism in the reduction of O2 by 6. In addition, kinetic isotope effect (KIE) in the reaction of 6 with O2 was determined to be 2.5 based on the rate constants of reactions of 6 or deuterated 6 with O2 (Figure S16), indicating that proton-coupled electron transfer from 6 to O2 is involved in the rate-determining step of the H2O2 formation. Thus, the difference in the reactivity between 6 and 8 toward O2 should be derived from that of the affinity between isophlorin derivatives and O2. In the case of 6, the arrangement of NH protons in 6 is more suitable for interacting with O2 than 8, judging from the orientation difference of the remaining inner NH protons. Kinetic analysis on the reaction of 5 with H2O2 was also performed to estimate the activation energy of the oxidation of H2O2 with 5 (eq 6, Figures 6 and S17). In a deaerated DMF solution containing 5 and excess H2O2, the formation of 6 obeyed the pseudo-first-order kinetics and the pseudo-first-order rate constant (k’obs) of the reaction of 5 with H2O2 to form 6 and O2 was determined to be (6.23 ± 0.04) ´ 10–4 s–1 at 298 K (Inset of Figure S17). In contrast to the reaction of 6 with O2, H2O2 concentration dependence of k’obs showed a saturation behavior as shown in Figure 6, suggesting a pre-equilibrium involving the formation of an adduct between 5 and H2O2. Curve fitting of the plot based on eq 729 allowed us to determine the equilibrium constants (K’ ) of the pre-equilibrium process and the first-order rate constants (k’ ) to form 6 and O2 to be (0.9 ± 0.1) ´ 102 M–1 and (9.6 ± 0.3) ´ 10–4 s–1, respectively, at 298 K. Then, to clarify the electronic structure of the adduct between 5 and H2O2, UV-Vis titration of 5 with H2O2 was conducted in O2-saturated DMF at 298 K because the formation of 6 should be suppressed in the presence of O2, in light of the equilibrium constant (K = 5.3 × 10–4 at 298 K) mentioned above. K’

5-H2O2

5 + H2O2

k’obs =

k’

k’K’[H2O2]

6 + O2

Upon addition of H2O2 to the solution of 5, the Soret band of 5 was slightly red-shifted from 509 nm to 511 nm with an isosbestic point at 498 nm (Figure S18a), while no formation of 6 was observed in the spectroscopic measurements. In addition, the 1H NMR signal derived from N-Me groups of 5 were shifted from –1.02 ppm to –1.30 ppm by adding H2O2 in DMF-d7, suggesting that the aromaticity of the porphyrin was kept and thus no reduction occurred at the porphyrin core of 5 (Figure 7). These spectroscopic results indicate that the adduct formation between 5 and H2O2 involves a hydrogen peroxide adduct of a porphyrin (5-H2O2). The equilibrium constant (K ’) of association of H2O2 to 5 was determined to be (1.10 ± 0.04) ´ 102 M–1 (Figure S18b), which was in good agreement with the result determined from kinetic analysis ((0.9 ± 0.1) ´ 102 M–1) as mentioned above. In the DFT-optimized structure of the adduct between 5 and H2O2 (Figure 8), the O–O bond distance was estimated to be 1.454 Å, reflecting the single O–O bond character of a peroxide.30 Therefore, the electronic structure of the reaction intermediate should be 5-H2O2 with two-point hydrogen bonding as shown in Scheme 3, ruling out other redox states such as a superoxide (O2•–)–porphyrin radical cation adduct. The temperature dependence of k’ and K’ were observed (Figure S19) and plotted against T–1 to obtain the activation parameters (DH’ ‡, DS’ ‡) for the formation of 6 (Eyring plot, Figure S20a) and thermodynamic parameters (DH’, DS’ ) for the pre-equilibrium (van’t Hoff plot, Figure S20b).

(6)

(7)

1 + K’[H2O2]

Figure 7. (a) 1H NMR spectrum of (a) 5 (0.32 mM) in DMF-d7 and (b) 5 (0.32 mM) with H2O2 (70 mM) to form 5-H2O2 in O2-saturated DMF-d7 at 298 K

Figure 6. H2O2 concentration dependence of the pseudo first-order rate constants (k’obs) in the reaction of 5 (0.010 mM) with H2O2 in deaerated DMF at 298 K.

Figure 8. DFT-optimized structure of 5-H2O2 calculated at the B3LYP/6-31G** level of theory: A side view (a) and a top view (b). Protons except for the protons of H2O2 (white) in (a) and peripheral aryl groups in (b) were omitted for clarity. Atom labels: Black: Carbon, Blue: Nitrogen, Red: Oxygen, Yellow green: Fluorine.



Scheme 3. Interconversion between 6/O2 and 5/H2O2 through the Formation of 5-H2O2.

N

H N

H N

N

+ O2

6

N

O O H H N N

N

5-H2O2

N

N N

N

+ H2O2

5

Page 6 of 9

hydrogen bonding with O2 as well as those of the lone pairs of the inner nitrogen atoms to form hydrogen bonding with H2O2. The structural isomers of 5 and 6, i.e., 7 and 8, respectively, showed no reversibility due to the lack of appropriate multi-point hydrogen-bonding sites of 7 with H2O2, supporting the importance of hydrogen-bonding interaction to achieve the O2/H2O2 interconversion. The reversible O2/H2O2 conversion presented herein is expected to contribute to the development of metal-free organic functional materials and catalysts.

SUPPORTING IMFORMATION The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Experimental details and analytical data, including Figures S1-S20 and Table S1-S4 (PDF), X-ray crystallographic data for 5 and 7 (CIF).

AUTHOR IMFORMATION Corresponding Author Figure 9. An energy diagram of the interconversion between 5/H2O2 and 6/O2 through the formation of 5-H2O2. The numbers in this figure show the values of activation energies or the difference of thermodynamic energies (kcal mol–1) at 298 K. ‡

The activation barrier (DG ’ ) of the reaction from 5/H2O2 to 6/O2 was determined to be (21 ± 2) kcal mol–1 at 298 K, which was comparable with the sum of the activation barrier from 6/O2 to 5/H2O2 (DG‡ = (16.1 ± 0.5) kcal mol–1 at 298 K) and DG (+4.4 kcal mol–1) as shown in Figure 9 and Table 1. In addition, the Gibbs free energy change (DG ’) between 5/H2O2 and 5-H2O2 was determined to be –(2.6 ± 0.4) kcal mol–1 at 298 K based on thermodynamic analysis (Table 1). Thus, the plausible energy diagram of the interconversion between 5/H2O2 and 6/O2 via 5-H2O2 can be summarized as depicted in Figure 9. Considering the principle of the microscopic reversibility,31 the reaction of 6 with O2 should form 5-H2O2 in the course of two-electron reduction of O2 to afford 5 and H2O2 (Scheme 3). On the other hand, no reactivity of 7 with H2O2 should be due to the lack of multi-point hydrogen-bonding sites of 7 to interact with H2O2 in the porphyrin core: Only single-point hydrogen bonding would be possible between 7 and H2O2. Thus, the two-point hydrogen-bonding interaction between 5 and H2O2 should play an important role for achieving the O2/H2O2 interconversion. Furthermore, the different behavior of hydrogen-bonding interaction between 5 and 7 with H2O2 should relate to the different reactivity between isophlorin derivatives (6 and 8) with O2 (Figure 5).

CONCLUSION In conclusion, we have successfully established an unprecedented reversible O2/H2O2 conversion for the first time by using an N21, N23-dimethylated saddle-distorted porphyrin (5) and the corresponding isophlorin derivative (6). The reversible O2/H2O2 conversion by 5 and 6 has been elucidated by thermodynamic analysis on the equilibrium between 5 and 6 and kinetic analysis in the O2 reduction by 6 and the H2O2 oxidation by 5. In addition, the interconversion between 5/H2O2 and 6/O2 was completely reversible at least ten cycles by bubbling Ar or O2, alternately. The O2/H2O2 interconversion was achieved by the fine tuning of thermodynamic stability between 5/H2O2 and 6/O2 by dimethylation of the pyrrole nitrogen atoms of the porphyrin core and the appropriate arrangements of inner NH protons in the isophlorin core to form

[email protected]

ACKNOLEDGEMENTS This work was supported by a Grant-in-Aid (17H03027) from the Japan Society of Promotion of Science (JSPS, MEXT) of Japan. Financial support through CREST (JST) is also appreciated (JPMJCR16P1). W.S. appreciates a support from JSPS Research Fellowship for Young Scientists (18J12184).

References (1)

(2)

(3)

(4)

(5)

(6)

Collman, J. P.; Wagenknecht, P. S.; Hutchison, J. E. Molecular Catalysts for Multielectron Redox Reactions of Small Molecules: The “Cofacial Metallodiporphyrin” Approach. Angew. Chem. Int. Ed. 1994, 33, 1537-1554. Shikama, K. The Molecular Mechanism of Autoxidation for Myoglobin and Hemoglobin: A Venerable Puzzle. Chem. Rev. 1998, 98, 1357-1373. (a) Lubitz, W.; Ogata, H.; Rüdiger, O.; Reijerse, E. Hydrogenases. Chem. Rev. 2014, 114, 4081-4148. (b) DuBois, M. R.; DuBois, D. L. The Roles of the First and Second Coordination Spheres in the Design of Molecular Catalysts for H2 Production and Oxidation. Chem. Soc. Rev. 2009, 38, 62-72. Magnus, K. A.; Tonthat, H.; Carpenter, J. E. Recent Structural Work on the Oxygen Transport Protein Hemocyanin. Chem. Rev. 1994, 94, 727-735. (a) Girishkumar, G.; McCloskey, B.; Luntz, A. C.; Swanson, S.; Wilcke, W. Lithium-Air Battery: Promise and Challenges. J. Phys. Chem. Lett. 2010, 1, 2193-2203. (b) Johnson, L.; Li, C.; Liu, Z.; Chen, Y.; Freunberger, S. A.; Ashok, P. C.; Praveen, B. B.; Dholakia, K.; Tarascon, J.-M.; Bruce, P. G. The Role of LiO2 Solubility in O2 Reduction in Aprotic Solvents and its Consequences for Li–O2 Batteries. Nat. Chem. 2014, 6, 1091-1099. (c) Chen, Y.; Freunberger, S. A.; Peng, Z.; Fontaine, O.; Bruce, P. G. Charging a Li-O2 Battery Using a Redox Mediator. Nat. Chem. 2013, 5, 489-494. (a) Jacobson, R. R.; Tyeklar, Z.; Farooq, A.; Karlin, K. D.; Liu, S.; Zubieta, J. A Copper-oxygen (Cu2-O2) Complex. Crystal Structure and Characterization of a Reversible Dioxygen Binding


Page 7 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(7)

(8)

(9)

(10)

(11)

(12)

(13)

System. J. Am. Chem. Soc. 1988, 110, 3690-3692. (b) Hu, Z.; Williams, R. D.; Tran, D.; Spiro, T. G.; Gorun, S. M. Re-engineering Enzyme-Model Active Sites: Reversible Binding of Dioxygen at Ambient Conditions by a Bioinspired Copper Complex. J. Am. Chem. Soc. 2000, 122, 3556-3557. (c) Kodera, M.; Katayama, K.; Tachi, Y.; Kano, K.; Hirota, S.; Fujinami, S.; Suzuki, M. Crystal Structure and Reversible O2-Binding of a Room Temperature Stable µ-h2:h2-Peroxodicopper(II) Complex of a Sterically Hindered Hexapyridine Dinucleating Ligand. J. Am. Chem. Soc. 1999, 121, 11006-11007. (d) Kodera, M.; Kajita, Y.; Tachi, Y.; Katayama, K.; Kano, K.; Hirota, S.; Fujinami, S.; Suzuki, M. Synthesis, structure, and greatly improved reversible O2 binding in a structurally modulated µ-h2:h2-peroxodicopper(II) complex with room temperature stability. Angew. Chem. Int. Ed. 2004, 43, 334-337. (a) Kindermann, N.; Dechert, S.; Demeshko, S.; Meyer, F. Proton-Induced, Reversible Interconversion of a µ-1,2-Peroxo and a µ-1,1-Hydroperoxo Dicopper(II) Complex. J. Am. Chem. Soc. 2015, 137, 8002-8005. (b) Dahl, E. W.; Dong, H. T.; Szymczak, N. K. Phenylamino Derivatives of Tris(2-pyridylmethyl)amine: Hydrogen-Bonded Peroxodicopper Complexes. Chem. Commun. 2018, 54, 892-895. Lopez, N.; Graham, D. J.; MuGuire Jr, R.; Alliger, G. E.; Shao-Horn, Y.; Cummins, C. C.; Nocera, D. G. Reversible Reduction of Oxygen to Peroxide Facilitated by Molecular Recognition. Science 2012, 335, 450-453. Dahl, E. W.; Kiernicki, J. J.; Zeller, M.; Szymczak, N. K. Hydrogen Bonds Dictate O2 Capture and Release within a Zinc Tripod. J. Am. Chem. Soc. 2018, 140, 10075-10079. Kato, T.; Fujimoto, T.; Tsutsui, A.; Tashiro, M.; Mitsutsuka, Y.; Machinami, T. Identification of a Discrete Peroxide Dianion, O22–, in a Two Sodium(1,6-anhydro-b-maltose)2-peroxide Complex. Chem. Lett. 2010, 39, 136-137. (a) Peigs, M. L.; Wise, C. F.; Martin, D. J.; Mayer, J. M. Oxygen Reduction by Homogeneous Molecular Catalysts and Electrocatalysts. Chem. Rev. 2018, 118, 2340-2391. (b) Rosenthal, J.; Nocera, D. G. Role of Proton-Coupled Electron Transfer in O–O Bond Activation. Acc. Chem. Res. 2007, 40, 543-553. (c) Fukuzumi, S. Production of Liquid Solar Fuels and Their Use in Fuel Cells. Joule 2017, 1, 689-738. (d) Fukuzumi, S.; Lee, Y.-M.; Nam, W. Mechanisms of Two‐Electron versus Four‐Electron Reduction of Dioxygen Catalyzed by Earth‐ Abundant Metal Complexes. ChemCatChem 2018, 10, 9-28. (a) Kadish, K. M.; Morrison, M. M. Solvent and Substituent Effects on the Redox Reactions of para-Substituted Tetraphenylporphyrin. J. Am. Chem. Soc. 1976, 98, 3326-3328. (b) Fang, Y.; Bhyrappa, P.; Ou. Z.; Kadish, K. M. Planar and Nonplanar Free‐Base Tetraarylporphyrins: β‐Pyrrole Substituents and Geometric Effects on Electrochemistry, Spectroelectrochemistry, and Protonation/Deprotonation Reactions in Nonaqueous Media. Chem. Eur. J. 2014, 20, 524-532. (a) Inoue, M.; Osuka, A. Redox-Induced Palladium Migrations that Allow Reversible Topological Changes between Palladium(II) Complexes of Möbius Aromatic [28]Hexaphyrin and Hückel Aromatic [26]Hexaphyrin. Angew. Chem. Int. Ed. 2010, 49, 9488-9491. (b) Gopalakrishna, T. Y.; Anand, V. G. Reversible Redox Reaction between Antiaromatic and Aromatic

(14)

(15)

(16)

(17)

(18)

(19)

(20)

(21)

States of 32p-Expanded Isophlorins. Angew. Chem Int. Ed. 2014, 53, 6678-6682. (c) Xie, D.; Liu, Y.; Rao, Y.; Kim, G.; Zhou, M.; Yu, D.; Xu, L.; Yin, B.; Liu, S.; Tanaka, T.; Aratani, N.; Osuka, A.; Liu, Q.; Kim, D.; Song, J. meso-Triaryl-Substituted Smaragdyrins: Facile Aromaticity Switching. J. Am. Chem. Soc. 2018, 140, 16553-16559. (a) Kim, D. S.; Sessler, J. L. Calix[4]pyrroles: Versatile Molecular Containers with Ion Transport, Recognition, and Molecular Switching Functions. Chem. Soc. Rev. 2015, 44, 532-546. (b) Park, J. S.; Sessler, J. L. Tetrathiafulvalene (TTF)-Annulated Calix[4]pyrroles: Chemically Switchable Systems with Encodable Allosteric Recognition and Logic Gate Functions. Acc. Chem. Res. 2018, 51, 2400-2410. (c) Gale, P. A.; Howe, E. N. W.; Wu, X.; Spooner, M. J. Anion Receptor Chemistry: Highlights from 2016. Coord. Chem. Rev. 2018, 375, 333-372. Kielmann, M.; Senge, M. O. Molecular Engineering of Free-Base Porphyrins as Ligands–The N-H•••X Binding Motif in Tetrapyrroles. Angew. Chem. Int. Ed. 2019, 58, 418-441. (a) Hatay, I.; Su, B.; Méndez, M. A.; Corminboeuf, C.; Khoury, T.; Gros, C. P.; Bourdillon, M.; Meyer, M.; Barbe, J.-M.; Ersoz, M.; Zális, S.; Samec, Z.; Girault, H. H. Oxygen Reduction Catalyzed by a Fluorinated Tetraphenylporphyrin Free Base at Liquid/Liquid Interfaces. J. Am. Chem. Soc. 2010, 132, 13733-13741. (b) Trojánek, A.; Langmaier, J.; Šebera, J.; Zális, S.; Barbe, J.-M.; Girault, H. H.; Samec, Z. Fine Tuning of the Catalytic Effect of a Metal-Free Porphyrin on the Homogeneous Oxygen Reduction. Chem. Commun. 2011, 47, 5446-5448. (c) Wu, S.; Su, B. Metal‐Free‐Porphyrin‐Catalyzed Oxygen Reduction at Liquid–Liquid Interfaces. Chem. Eur. J. 2012, 18, 3169-3173. Mase, K.; Ohkubo, K.; Xue, Z.; Yamada, H.; Fukuzumi, S. Catalytic Two-Electron Reduction of Dioxygen Catalysed by Metal-Free [14]Triphyrin(2.1.1). Chem. Sci. 2015, 6, 6496-6504 . Hiroto, S.; Miyake, Y.; Shinokubo, H. Synthesis and Functionalization of Porphyrins through Organometallic Methodologies. Chem. Rev. 2017, 117, 2910-3043. Roucan, M.; Flanagan, K. J.; O’Brien, J.; Senge, M. O. Nonplanar Porphyrins by N-Substitution: A Neglected Pathway. Eur. J. Org. Chem. 2018, 6432-6446. Reddy, B. K.; Basavarajappa, A.; Ambhore, M. D.; Anand, V. G. Isophlorinoids: The Antiaromatic Congeners of Porphyrinoids. Chem. Rev. 2017, 117, 3420-3443. (a) Pohl, M.; Schmickler, H.; Lex, J.; Vogel, E. Isophlorins: Molecules at the Crossroads of Porphyrin and Annulene Chemistry. Angew. Chem. Int. Ed. 1991, 30, 1693-1697. (b) Weiss, A.; Hodgson, M. C.; Boyd, P. D. W.; Siebert, W.; Brothers, P. J. Diboryl and Diboranyl Porphyrin Complexes: Synthesis, Structural Motifs, and Redox Chemistry: Diborenyl Porphyrin or Diboranyl Isophlorin? Chem. Eur. J. 2007, 13, 5982-5993. (c) Reddy, J. S.; Anand, V. G. Planar meso Pentafluorophenyl Core Modified Isophlorins. J. Am. Chem. Soc. 2008, 130, 3718-3719. (d) Yan, J.; Takakusaki, M.; Yang, Y.; Mori, S.; Zhang, B.; Feng, Y.; Ishida, M.; Furuta, H. Doubly N-Confused Isophlorin: Synthesis, Structure and Copper Coordination. Chem. Commun. 2014, 50, 14593-14596. (e) Panchal, S. P.; Gadekar, S. C.; Anand, V. G. Controlled Core‐Modification of a Porphyrin into an Antiaromatic Isophlorin. Angew. Chem. Int. Ed. 2016, 55, 7797-7800.



(22) Liu, C.; Shen, D.-M.; Chen, Q.-Y. Synthesis and Reactions of

(23)

(24)

(25)

(26)

(27)

(28)

(29)

(30) (31)

20π-Electronβ-Tetrakis(trifluoromethyl)-meso-tetraphenylporp hyrins. J. Am. Chem. Soc. 2007, 129, 5814-5815. Suzuki, W.; Kotani, H.; Ishizuka, T.; Shiota, Y.; Yoshizawa, K.; Kojima, T. Formation and Isolation of a Four‐Electron‐Reduced Porphyrin Derivative by Reduction of a Stable 20π Isophlorin. Angew. Chem. Int. Ed. 2018, 57, 1973-1977. (a) Medforth, C. J.; Senge, M. O.; Smith, K. M.; Sparks, L. D.; Shelnutt, J. A. Nonplanar Distortion Modes for Highly Substituted Porphyrins. J. Am. Chem. Soc. 1992, 114, 9859-9869. (b) Harada, R.; Kojima, T. A Porphyrin Nanochannel: Formation of Cationic Channels by a Protonated Saddle-Distorted Porphyrin and its Inclusion Behavior. Chem. Commun. 2005, 716-718. (c) Kojima, T.; Nakanishi, T.; Harada, R.; Ohkubo, K.; Yamauchi, S.; Fukuzumi, S. Selective Inclusion of Electron-Donating Molecules into Porphyrin Nanochannels Derived from the Self-assembly of Saddle-Distorted, Protonated Porphyrins and Photoinduced Electron Transfer from Guest Molecules to Porphyrin Dications. Chem. Eur. J. 2007, 13, 8714-8725. (d) Fukuzumi, S.; Honda, T.; Kojima, T. Structures and Photoinduced Electron Transfer of Protonated Complexes of Porphyrins and Metallophthalocyanines. Coord. Chem. Rev. 2012, 256, 2488-2502. Furusho, Y.; Kawasaki, H.; Nakanishi, S.; Aida, T.; Takata, T. Design of Multidentate Pyrrolic Ligands by N-Modification: Synthesis of N-Monomethyl, Monoethyl, Dimethyl, Trimethyl, and Tetramethylporphyrinogens. Tetrahedron Lett. 1998, 39, 3537-3540. (a) Fukuzumi, S.; Kuroda, S.; Tanaka, T. Flavin Analog-Metal Ion Complexes Acting as Efficient Photocatalysts in the Oxidation of p-Methylbenzyl alcohol by Oxygen under Irradiation with Visible Light. J. Am. Chem. Soc. 1985, 107, 3020-3027. (b) Mase, K.; Ohkubo, K.; Fukuzumi, S. Efficient Two-Electron Reduction of Dioxygen to Hydrogen Peroxide with One-Electron Reductants with a Small Overpotential Catalyzed by a Cobalt Chlorin Complex. J. Am. Chem. Soc. 2013, 135, 2800-2808. Sandell, J. P. L.; Kakeya, K.; Mizutani, T. Ring-Opening with One Dioxygen Molecule in the Coupled Oxidation of Iron Tetraarylporphyrins. Tetrahedron Lett. 2014, 55, 1532-1535. Machacek, M.; Demuth, J.; Cermak, P.; Vavreckova, M.; Hruba, L.; Jedlickova, A.; Kubat, P.; Simunek, T.; Novakova, V.; Zimcik, P. Tetra(3,4-pyrido)porphyrazines Caught in the Cationic Cage: Toward Nanomolar Active Photosensitizers. J. Med. Chem. 2016, 59, 9443-9456. Ishizuka, T.; Ohzu, S.; Kotani, H.; Shiota, Y.; Yoshizawa, K.; Kojima. T. Hydrogen Atom Abstraction Reactions Independent of C–H Bond Dissociation Energies of Organic Substrates in Water: Significance of Oxidant–Substrate Adduct Formation. Chem. Sci. 2014, 5, 1429-1436. Wallen, C. M.; Bacsa, J.; Scarborough, C. C. Hydrogen Peroxide Complex of Zinc. J. Am. Chem. Soc. 2015, 137, 14606-14609. Blackmond, D. G. “If pigs could fly” Chemistry: A Tutorial on the Principle of Microscopic Reversibility. Angew. Chem. Int. Ed. 2009, 48, 2648-2654.


Page 8 of 9

Page 9 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC


Hydrogen Peroxide Interconversion Using Redox Couples

Recommend Documents