Photodriven Oxidation of Water by Plastoquinone Analogs with a

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Photodriven Oxidation of Water by Plastoquinone Analogs with a Nonheme Iron Catalyst Young Hyun Hong, Jieun Jung, Tatsuo Nakagawa, Namita Sharma, Yong-Min Lee, Wonwoo Nam, and Shunichi Fukuzumi J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b02517 • Publication Date (Web): 04 Apr 2019 Downloaded from http://pubs.acs.org on April 4, 2019

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Journal of the American Chemical Society

Young Hyun Hong,† Jieun Jung,†,‡ Tatsuo Nakagawa,§ Namita Sharma,† Yong-Min Lee,† Wonwoo Nam,*,†,¶ and Shunichi Fukuzumi*,†,# †

Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea Department of Chemistry, Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan. § Unisoku Co., Ltd, SENTAN, Japan Science and Technology Agency (JST), Hirakata, Osaka 573-0131, Japan ‡

¶

State Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou Research Institute of LICP, Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences, Lanzhou 730000, China #

Faculty of Science and Engineering, Meijo University, SENTAN, Japan Science and Technology Agency (JST), Nagoya, Aichi 4680073, Japan

ABSTRACT: Photoirradiation of an acetonitrile solution containing p-benzoquinone derivatives (X-Q) as plastoquinone analogs, a nonheme iron(II) complex, [(N4Py)FeII]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), and H2O afforded the evolution of O2 and the formation of the corresponding hydroquinone derivatives (X-QH2) quantitatively. During the photodriven oxidation of water by X-Q, [(N4Py)FeII]2+ was oxidized by the excited state of X-Q to produce the iron(IV)-oxo complex ([(N4Py)FeIV(O)]2+) quantitatively. The concentration of [(N4Py)FeIV(O)]2+ remained virtually the same during the repeated cycles of photodriven oxidation of water by X-Q. [(N4Py)FeIV(O)]2+ was further oxidized by the excited state of X-Q to [(N4Py)FeV(O)]3+; this FeV-oxo species is proposed as an active oxidant that effects the water oxidation. The photocatalytic mechanism of the water oxidation by X-Q with [(N4Py)FeII]2+ was clarified by detecting intermediates using various spectroscopic techniques, such as transient absorption and EPR measurements. To the best of our knowledge, the present study reports the first example of a functional model of Photosystem II (PSII) using X-Q as plastoquinone analogs in the photocatalytic oxidation of water.

The oxidizing equivalents generated at the donor side of Photosystem II (PSII) are used to oxidize water, whereas the reducing equivalents accumulated at the acceptor side of PSII are used to reduce the two quinone molecules, QA and QB, which act as one-electron and two-electron gates, respectively.1,2 Finally, electrons with protons are transferred to plastoquinone (PQ) in quinone pool to produce plastoquinol (PQH2).1,2 The overall solar-driven reaction is given by eq 1, where PQ is reduced by H2O to produce O2 and

cesses in PSII as well as in PSI.3-9 However, neither the stoichiometry nor the intermediate of photodriven oxidation of water by PQ or PQ analogs has been established.10-12 We report herein the photodriven water oxidation reaction using p-benzoquinone derivatives (X-Q; Scheme 1 for the chemical structures) as plastoquinone analogs with a nonheme iron(II) catalyst, [(N4Py)FeII]2+, in acetonitrile (MeCN) containing H2O (eq Scheme 1. Schematic illustration of (a) [(N4Py)FeII]2+ and (b) X-Q.

PQH2.1-3 The photocatalytic oxidation of water by PQ as the oxidant in PSII makes it possible to connect PSII with PSI in the socalled Z-scheme, where PQH2 is used as the reductant to reduce NADP+ to NADPH and also to generate an electrochemical gradient for protons, across the thylakoid membrane, which can be used for the phosphorylation of ADP to ATP.1,2 Thus, the PQ/PQH2 redox couple plays an essential role in the Z-scheme for the overall photosynthesis.1,2 Tremendous efforts have so far been devoted to realizing artificial photosynthesis by mimicking charge-separation and redox pro-

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Figure 1. Time courses of O2 evolution by X-Q [0.50 mM; DDQ (black), BQ (blue), CA (green), DQ (orange), and PXQ (red)] with [(N4Py)FeII]2+ (0.20 mM) under photoirradiation (white light) in a deaerated MeCN solution containing H2O (0.50 M) at 298 K.

2). This is the first example of a functional mimic of PSII for the oxidation of water by PQ analogs in producing O2 and the corresponding PQH2 analogs. Moreover, an iron(IV)-oxo complex ([(N4Py)FeIV(O)]2+) has been identified as a catalytic intermediate during the photodriven oxidation of water when [(N4Py)FeII]2+ is fully converted to [(N4Py)FeIV(O)]2+. The photocatalytic mechanism is clarified by detecting intermediates using various spectro-

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Figure 2. Time profiles of mass spectra monitoring O2 isotopes [32 (16O– 16 O; black circles), 34 (16O–18O; blue circles), and 36 (18O–18O; red circles)] produced in the photodriven H2O oxidation by DDQ (0.50 mM) with [(N4Py)FeII]2+ (0.20 mM) under photoirradiation (white light) in a deaerated MeCN solution containing H218O (0.50 M) at 298 K. The amounts of 32O2 (black circles) and 34O2 (blue circles) produced were negligible.

scopic techniques, such as transient absorption and EPR measurements.

Photodriven Water Oxidation by p-Benzoquinone Derivatives (X-Q) with [(N4Py)FeII]2+. Photoirradiation of an MeCN solution of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) (0.50 mM), [(N4Py)FeII]2+ (0.20 mM), and H2O (0.50 M) with a Xenon lamp (white light) resulted in O2 evolution as shown in Figure 1, where the O2 yield reaches nearly 100% based on the amount of DDQ added (eq 2). The oxygen yield was confirmed by gas chromatography (GC) as well as oxygen sensor (Foxy) measurements (Supporting Information (SI), Figure S1). The quantum yield for the photodriven water oxidation by DDQ in the presence of [(N4Py)FeII]2+ reached up to 88% (SI, Figure S2) and the turnover number (TON) of 190 ± 10 was achieved based on [(N4Py)FeII]2+ (SI, Figure S3). Isotope-labeling experiments using 18O-enriched water (97.4%) were performed to confirm the source of the evolved O2 product; formation of 36O2 (99% based on the labeled H218O percent) was observed as the sole product (Figure 2). The formation of DDQH2 in the photodriven oxidation of H2O by DDQ (0.50 mM) with [(N4Py)FeII]2+ (0.20 mM) under photoirradiation (eq 2) was also observed by UV-vis spectral change at 350 nm due to DDQH2 (SI, Figure S4a) as well as appearance of a new 1H NMR signal corresponding to DDQH2 (Figures 3a and 3b), demonstrating the quantitative formation of DDQH2 (i.e., ~100% yield). At prolonged photoirradiation time, the concentration of O2 decreased gradually, since DDQH2 was further oxidized by O2 to produce hydrogen peroxide, accompanied by regeneration of DDQ (SI, Figure S5). When the concentration of [(N4Py)FeII]2+ decreased to 0.10 mM, the maximum O2 yield decreased to 77%

Figure 3. (a) 1H NMR spectrum of DDQH2 formed in the photodriven oxidation of H2O (0.50 M) by DDQ (0.50 mM) with [(N4Py)FeII]2+ (0.20 mM) under photoirradiation (white light) for 1.5 h in a deaerated CD3CN at 298 K. (b) Plot of DDQH2 yield vs time for photodriven oxidation of H2O (0.50 M) by DDQ (0.50 mM) with [(N4Py)Fe II]2+ (0.20 mM) under photoirradiation (white light) in a deaerated CD3CN at 298 K. (c) 1H NMR spectrum of QH2 formed in the photodriven oxidation of H2O (0.50 M) by BQ (0.50 mM) with [(N4Py)FeII]2+ (0.20 mM) under photoirradiation (white light) for 1 h in a deaerated CD3CN at 298 K. Peak at 6.80 ppm originated from BQ. Hb is exchangeable in the presence of H2O. (d) Plot of QH2 yield vs time for the photodriven oxidation of H2O (0.50 M) by BQ (0.50 mM) with [(N4Py)FeII]2+ (0.20 mM) under photoirradiation (white light) in a deaerated CD3CN at 298 K. In all the cases, 1, 4-dioxane (1.0 mM) was added as a reference after the photoreaction.

(SI, Figure S6). When the concentration of DDQ decreased to 0.30 mM, the maximum O2 yield also decreased to 47% (Figure S6). Photodriven water oxidation was also achieved by other p-benzoquinone derivatives (X-Q) (0.50 mM), such as 1,4-benzoquinone (BQ), tetrachloro-1,4-benzoquinone (CA), tetramethyl-1,4benzoquinone (DQ), and 2,5-dimethyl-1,4-benzoquinone (PXQ), with [(N4Py)FeII]2+ (0.20 mM) and H2O (0.50 M) as shown in Figure 1; the O2 yields by BQ, CA, DQ, and PXQ were 91, 91, 49, and 90%, respectively (Figure 1). The formation of hydroquinone


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Figure 4. Absorption spectral changes showing the formation of [(N4Py)FeIV(O)]2+ in the photodriven oxidation of H2O by DDQ (1.25 mM) with [(N4Py)FeII]2+ (0.50 mM) under photoirradiation (white light) in a MeCN solution containing H2O (1.4 M) at 298 K. Inset shows time profile of absorbance change at 690 nm due to [(N4Py)FeIV(O)]2+.

(QH2), 2,3,5,6-tetrachloro-1,4-hydroquinone (CAH2), and durohydroquinone (DQH2)) as the products was confirmed by the UV-vis spectral changes (SI, Figure S4b), and the generation of QH2 was also verified by taking 1H NMR spectra (Figures 3c and 3d). Mechanistic Insights into the Photodriven Water Oxidation by DDQ with [(N4Py)FeII]2+. During the photodriven H2O oxidation by DDQ, the two-electron oxidation of [(N4Py)FeII]2+ occurs to produce [(N4Py)FeIV(O)]2+ (eq 3). Time course of the formation of [(N4Py)FeIV(O)]2+ in the photooxidation of [(N4Py)FeII]2+ by DDQ is shown in Figure 4, where the absorption band at 690 nm appeared due to [(N4Py)FeIV(O)]2+.13 The formation of [(N4Py)FeIV(O)]2+ was also confirmed by CSI-MS (SI, Figure S7). Since the formation of DDQ•– is not observed in the photooxidation of [(N4Py)FeII]2+ by DDQ, [(N4Py)FeIII]3+ produced by electron transfer from [(N4Py)FeII]2+ to the triplet excited state of DDQ (3DDQ*) may react with H2O to produce [(N4Py)FeIII(OH)]2+ and H+ that reacts with DDQ•– to form DDQH• (eq 4). The electron transfer from [(N4Py)FeII]2+ to 3 DDQ* is highly exergonic because of the much higher reduction potential of 3DDQ* (3.1 V vs SCE)14,15 than the one-electron oxidation potential of [(N4Py)FeII]2+ (Eox = 1.00 V vs SCE).16 The disproportionation of two DDQH• species yields DDQ and DDQH2 (eq 5).

The overall reaction of the one-electron photooxidation of [(N4Py)FeII]2+ by DDQ is given by eq 6. Then, [(N4Py)FeIII(OH)]2+ is further oxidized by 3DDQ* to produce [(N4Py)FeIV(O)]2+ and DDQH• (eq 7) that disproportionates to yield DDQ and DDQH2 (eq 5), resulting in the overall reaction of two-electron photooxidation of [(N4Py)FeII]2+ by DDQ with H2O to produce [(N4Py)FeIV(O)]2+ (eq 3). It was reported previously

Figure 5. (a) Transient absorption spectral changes after photoexcitation at 355 nm on a deaerated MeCN solution of DDQ (0.50 mM). (b) Decay time profiles at 620 nm due to the decay of 3DDQ* at various concentrations of [(N4Py)FeII]2+ [0 (black), 20 (blue), 40 (green), 80 (orange), and 200 (red) μM]. (c) Plot of kobs vs concentration of [(N4Py)FeII]2+. (d) Plot of kobs vs concentration of [(N4Py)FeIV(O)]2+.

that [(N4Py)FeIV(O)]2+ was produced by the two-electron oxidation of [(N4Py)FeII]2+ by two equivalents of [Ru(bpy)3]3+ with H2O.13,17,18 In addition, 18O-labeled water experiments demonstrated that the oxygen atom in [(N4Py)FeIV(O)]2+ derived from water.13

Photoinduced electron transfer from [(N4Py)FeIV(O)]2+ to the triplet excited state of p-benzoquinone derivatives (X-Q) was examined by nanosecond laser transient absorption measurements (Figure 5; SI, Figures S8 and S9). Laser excitation of a deaerated MeCN solution of X-Q resulted in the formation of 3X-Q* with λmax (620 nm for 3DDQ*, 420 nm for 3BQ*, 510 nm for 3CA*, 470 nm for 3DQ*, and 440 nm for 3PXQ*).14,15 The decay rate constants of 3 X-Q* increased with increasing concentration of [(N4Py)FeII]2+ (Figure 5b; see also SI, Figures S8 and S9). The rate constant of electron transfer from [(N4Py)FeII]2+ to 3DDQ* was determined from the slope of plot of kobs vs concentration of [(N4Py)FeII]2+ to be 1.0 × 1010 M–1 s–1 that is close to the diffusion rate constant in MeCN.19 The rate constants of electron transfer from [(N4Py)FeII]2+ to 3BQ*, 3CA*, 3DQ*, and 3PXQ* were also determined to be 1.2 × 1010, 1.7 × 1010, 1.1 × 1010, and 1.1 × 1010 M–1 s–1, respectively (Table 1).20 The decay rate constant of 3X-Q* also increased with increasing concentration of [(N4Py)FeIV(O)]2+ species, which was independently prepared by reacting [(N4Py)FeII]2+ complex with iodosylbenzene (PhIO).16 The rate constant of electron transfer from [(N4Py)FeIV(O)]2+ to 3DDQ* was determined from the slope of plot of kobs vs concentration of [(N4Py)FeIV(O)]2+ to be 9.4 × 109 M–1 s–1. The rate constants of electron transfer from [(N4Py)FeIV(O)]2+ to 3BQ*, 3CA*, 3DQ*, and 3PXQ*, which are close to the diffusion rate constant, are listed in Table 1. The diffusion limited rate constant of electron transfer



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Table 1. Rate Constants of Electron Transfer from an Iron Complex to the Triplet Excited State of p-Benzoquinone Derivatives (X-Q) in MeCN at 298 K

ket, M–1 s–1 X-Q

[(N4Py)FeII]2+

[(N4Py)FeIV(O)]2+

DDQ

(1.0 ± 0.1) × 1010

(9.4 ± 0.6) × 109

BQ

(1.2 ± 0.1) × 1010

(2.1 ± 0.2) × 1010

CA

(1.7 ± 0.1) × 1010

(9.7 ± 0.8) × 109

DQ

(1.1 ± 0.1) × 1010

(3.2 ± 0.2) × 109

PXQ

(1.1 ± 0.1) × 1010

(5.0 ± 0.4) × 109

Scheme 2. Proposed mechanism of the photodriven water oxidation by DDQ with [(N4Py)FeII]2+ Figure 6. (a, b) UV-vis spectral changes observed in the photocatalytic oxidation of H2O by DDQ (0.50 mM) with [(N4Py)FeII]2+ (0.10 mM) under photoirradiation (white light) of a deaerated MeCN solution containing H2O (0.50 M) at 298 K. The absorbance at 690 nm due to [(N4Py)FeIV(O)]2+ (0.10 mM, 100 % yield) decayed little after photoirradiation for 3000 s (inset of Figure 6a), when DDQ was fully converted to DDQH2 (λmax = 350 nm; blue line in Figure 6b), where the reaction solution was diluted by 1/20 after each irradiation. Then, [(N4Py)FeIV(O)]2+ (100% yield) was fully regenerated by adding DDQ (2nd time; 0.50 mM) to the resulting solution; the absorbance at 690 nm due to [(N4Py)FeIV(O)]2+ decayed little again after photoirradiation from 3000 s to 6600 s (inset of Figure 6a, 2nd), when 2nd added DDQ was fully converted to DDQH2 (λmax = 350 nm; green line in Figure 6b). Then, [(N4Py)FeIV(O)]2+ (100% yield) was fully regenerated again by adding further DDQ (3rd time; 0.50 mM) to the resulting solution; the absorbance at 690 nm due to [(N4Py)FeIV(O)]2+ decayed little after photoirradiation from 6600 s to 10400 s (inset of Figure 6a, 3rd), when 3rd added DDQ was fully converted to DDQH2 (λmax = 350 nm, red line in Figure 6b).

is consistent with the largely exergonic electron transfer from [(N4Py)FeIV(O)]2+ to 3DDQ*, judging from the oxidation peak potential of [(N4Py)FeIV(O)]2+ (ca. 2.3 V vs SCE; SI, Figure S10), which is much less positive than the one-electron reduction potential of 3DDQ* (Ered* = 3.1 V vs SCE).14 The transient absorption was observed at λmax = 450 nm due to DDQH•, which was formed by electron transfer from [(N4Py)FeII]2+ to 3DDQ* in the presence of H2O (SI, Figure S11a). The occurrence of electron transfer from [(N4Py)FeIV(O)]2+ to 3 DDQ* was confirmed by the transient absorption at λmax = 590 nm due to DDQ•– (SI, Figure S11b).21 Unfortunately, the transient absorption due to [(N4Py)FeV(O)]3+ could not be observed probably due to the highly unstable nature of the FeV(O) species. Catalytic Mechanism of the Photodriven Water Oxidation by DDQ with [(N4Py)FeII]2+. The catalytic mechanism of the photodriven water oxidation by DDQ with [(N4Py)FeII]2+ is depicted in Scheme 2. Photoexcitation of DDQ results in the formation of 3 DDQ*, to which electron is transferred from [(N4Py)FeII]2+ to produce [(N4Py)FeIII]3+. The latter species reacts with water to give [(N4Py)FeIII(OH)]2+ (Scheme 2, reaction pathway a; SI, Figure S12), and DDQ•– is protonated to afford DDQH•. Then, electron transfer from [(N4Py)FeIII(OH)]2+ to 3DDQ* occurs rapidly to produce [(N4Py)FeIV(O)]2+ and DDQH• (Scheme 2, reaction

pathway b). [(N4Py)FeIV(O)]2+ is further oxidized by electron transfer from [(N4Py)FeIV(O)]2+ to 3DDQ* to produce [(N4Py)FeV(O)]3+ and DDQ•– (Scheme 2, reaction pathway c) with the diffusion-limited rate constant (Table 1).22 Subsequently, the resulting [(N4Py)FeV(O)]3+ species may react rapidly with H2O to produce an iron(III)-hydroperoxo complex [(N4Py)FeIII(OOH)]2+ and H+ (Scheme 2, reaction pathway d).23 The proton is used to protonate DDQ•– to give DDQH•, followed by the disproportionation of DDQH• to yield DDQ and DDQH2 (eq 5). [(N4Py)FeIII(OOH)]2+ is further oxidized by DDQ via hydrogen atom transfer to afford [(N4Py)FeIII(O2•–)]2+ (Scheme 2, reaction pathway e), followed by a rapid release of O2 from [(N4Py)FeIII(O2•–)]2+ to regenerate [(N4Py)FeII]2+ (Scheme 2, reaction pathway f; see also SI, Figure S13). Indeed, when [(N4Py)FeIII(OOH)]2+ was independently prepared in the reaction of [(N4Py)FeII]2+ and H2O224 and oxidized by DDQ thermally, the yield of O2 derived from [(N4Py)FeIII(OOH)]2+ was determined to be ~100% based on DDQ concentration (SI, Figure S14). In the photocatalytic cycle in Scheme 2, electron transfer from [(N4Py)FeIV(O)]2+ to 3DDQ* is the rate-determining step,25 when [(N4Py)FeIV(O)]2+ is produced at the initial photoirradiation as shown in Figure 4, and the concentration of [(N4Py)FeIV(O)]2+ decayed only little (Figure 6a), when DDQ was fully converted to


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Journal of the American Chemical Society DDQH2 (Figure 6b).26 The catalytic cycle was repeated three times by adding DDQ to the resulting solution to start the photodriven oxidation of water by DDQ to produce O2 and DDQH2, when the concentration of [(N4Py)FeIV(O)]2+ decayed only little again. Thus, [(N4Py)FeIV(O)]2+ acts as a robust homogeneous catalyst for the photodriven oxidation of water by DDQ under the neutral conditions. This shows sharp contrast to the reported nonheme iron complexes, such as [FeII(BQEN)]2+ (BQEN = N,N-dimethylN,N’-bis(8-quinolyl)ethane-1,2-diamine) and [FeII(BQCN)]2+ (BQCN = N,N’-dimethyl-N,N-bis(8-quinolyl)cyclohexane-diamine), which undergo demetallation under acidic conditions and decompose to produce iron hydroxide nanoparticles under basic conditions in catalytic oxidation of water.27 Low temperature EPR measurements were also performed to detect paramagnetic intermediates in the photodriven oxidation of H2O by DDQ with [(N4Py)FeII]2+ in MeCN containing H2O. Before irradiation, no EPR signals were detected (SI, Figure S15, black spectrum). Photoirradiation of a deaerated MeCN solution of [(N4Py)FeII]2+ with DDQ and H2O resulted in the formation of [(N4Py)FeIII(OH)]2+ (Scheme 2, reaction pathway a), which exhibited a high-spin (S = 5/2) signal together with a sharp signal at g = 2.0050 due to DDQ•– (SI, Figure S15, red spectrum).28 Further photoirradiation of the reaction solution resulted in decrease in the EPR intensity due to [(N4Py)FeIII(OH)]2+ and increase in the EPR intensity due to DDQ•– (SI, Figure S15, blue spectrum), indicating that [(N4Py)FeIII(OH)]2+ was oxidized by 3DDQ* to produce [(N4Py)FeIV(O)]2+, which is EPR silent, and DDQ•– with a sharp signal at g = 2.0050 (Scheme 2, reaction pathways b and c).13,16

We have shown the photodriven oxidation of H2O by plastoquinone analogs as an oxidant as the case of PSII in photosynthesis using an nonheme iron(III) complex as a water oxidation catalyst. This is the first time to establish the stoichiometry for the photodriven water oxidation by plastoquinone analogs with an iron(II) complex, thereby evolving O2 and yielding the corresponding hydroquinone derivatives, mimicking the function of PSII. The photocatalytic mechanism of H2O oxidation by DDQ with [(N4Py)FeII]2+ is clarified as shown in Scheme 2, where [(N4Py)FeV(O)]3+, produced by successive photoinduced electron transfer with 3X-Q*, is proposed as the key intermediate for the water oxidation.

Materials. All solvents and chemicals were of reagent-grade quality, obtained commercially and used without further purification, unless otherwise noted. 2,3-Dichloro-5,6-dicyano-p-benzoquinone (DDQ), 1,4-benzoquinone (BQ), tetrachloro-1,4-benzoquinone (CA), duroquinone (DQ), and hydroquinone (QH2) and perchloric acid (70 wt % in H2O) were obtained commercially from Sigma-Aldrich Chemical Co., and p-xyloquinone (PXQ) was obtained from Tokyo Chemical Industry Co. Tetrabutylammonium hexafluorophosphate (TBAPF6) was obtained from Fluka Co. Deuterated acetonitrile (CD3CN) was obtained from Sigma-Aldrich Chemical Co., and used as received. Solvents, such as acetonitrile (CH3CN; MeCN) and methanol (CH3OH; MeOH), were dried according to the literature procedures and distilled under Ar prior to use.29 Nonheme iron(II) complex ([(N4Py)FeII(MeCN)](ClO4)2) and its corresponding iron(IV)-oxo

complex ([(N4Py)FeIV(O)]2+) were prepared by the literature methods.30,31 Iodosylbenzene (PhIO) was prepared by the literature method.32 Instrumentation. The amount of evolved oxygen was recorded by a commercial fast response fiber optic sensor (Foxy, Ocean Optics Ins.) as well as Gas Chromatography (Acme 6000 GC, YOUNG LIN Ins.). Sub-nanosecond laser-induced transient absorption spectra were collected by a customized measuring system equipped with a picosecondpulse laser for the photoexcitation and a photomultiplier detector developed by UNISOKU Co., Ltd.33 Nanosecond time-resolved transient absorption measurements were carried out using an Nd:YAG laser. Measurements of nanosecond transient absorption spectrum were performed according to the following procedure: A mixture solution in a quartz cell (1.0 cm  1.0 cm) was excited by a Nd:YAG laser (Continuum SLII-10, 4-6 ns fwhm, λex = 355 nm, 80 mJ pulse–1, 10 Hz). The photodynamics were monitored by continuous exposure to a xenon lamp for visible region and halogen lamp for near-IR region as a probe light and a photomultiplier tube (Hamamatsu 2949) as a detector. The kinetic traces at appropriate wavelengths were assembled from the time-resolved spectral data. UV-vis spectra were recorded on a Hewlett Packard 8453 diode array spectrophotometer equipped with a UNISOKU Scientific Instruments Cryostat USP-203A. X-band electron paramagnetic resonance (EPR) spectra were recorded at 77 K using a JEOL X-band spectrometer (JES-FA100). The experimental parameters for EPR measurements by JES-FA100 were as follows: microwave frequency = 9.028 GHz, microwave power = 1.0 mW, modulation amplitude = 1.0 mT, modulation frequency = 100 kHz and time constant = 0.03 s. 1H NMR spectra were measured with Bruker model digital AVANCE III 400 FT-NMR spectrometer. Electrochemical measurements were performed on a CHI630B electrochemical analyzer in a deaerated MeCN solution containing 0.10 M nBu4NPF6 (TBAPF6) as a supporting electrolyte at 298 K. A conventional threeelectrode cell was used with a platinum working electrode (surface area of 0.3 mm2), a platinum wire as a counter electrode and an Ag/Ag(NO3) (0.010 M) electrode as a reference electrode. The platinum working electrode (BAS) was routinely polished with BAS polishing alumina suspension and rinsed with acetone and acetonitrile before use. The measured potentials were recorded with respect to an Ag/Ag + reference electrode. All potentials (vs Ag/Ag+) were converted to values vs SCE by adding 0.29 V.34 Kinetic Studies. Typically, the photoinduced electron transfer from [(N4Py)FeIV(O)]2+ to 3DDQ* were examined by monitoring nanosecond time-resolved transient spectral changes. The kinetic traces were assembled, monitoring the decay at 620 nm due to 3DDQ* (0.50 mM) with various concentration of [(N4Py)FeIV(O)]2+ (0.05 to 0.20 mM) in a deaerated MeCN solution. First-order rate constants were determined under pseudo-first-order conditions by fitting the changes in transient absorbance for the decay of peaks due to the excited X-Q derivatives in the electron transfer from [(N4Py)FeIV(O)]2+ to the excited X-Q derivatives in a deaerated MeCN solution at 298 K. Product Analysis. The photo-driven water oxidation by excited state of p-benzoquinone with a nonheme iron complex was examined in a quartz cell (optical path length 1.0 cm) using a Xe lamp (white light; 300 W) on an ASAHI SPECTRA MAX-302 for irradiation at 298K. Oxygen formed in the oxidation of water (500 mM) by excited state of p-benzoquinone (0.50 mM) with a nonheme iron complex (0.20 mM) in CH3CN after irradiation (white light) was identified by GC and Foxy. Quantum Yield Determination. Quantum yield (QY) of photodriven water oxidation by p-benzoquinone with a nonheme iron complex has been determined under visible light irradiation of monochromatized light using a Xe lamp (300 W) on an ASAHI SPECTRA MAX302 through a band-pass filter transmitting λ = 420 nm at 298 K. Typically, the amount of O2 produced during the photochemical reaction



has been determined under photoirradiation (λ = 420 nm) with irradiation time interval of 1 hour (see Figure S2). The initial slope of the graph is taken to determine the quantum yield. The quantum yield was estimated as QY(%) = R/I× 100, where R (mol s–1) is O2 production rate and I (einstein s–1) is coefficient based on the rate of the number of incident photons absorbed by p-benzoquinone. The total number of incident photons was measured by a standard method using an actinometer (potassium ferrioxalate, K3[FeIII(C2O4)3])6 under photoirradiation (λ = 420 nm) at 298 K where photon flux was determined to be 2.03 ×10–9 einstein s–1.

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Supporting Information. Figures S1 – S15. This material is available free of charge via the Internet at http://pubs.acs.org.

[email protected]; [email protected]

This work was supported by the NRF of Korea through CRI (NRF2012R1A3A2048842 to W.N), GRL (NRF-2010-00353 to W.N.), and Basic Science Research Program (2017R1D1A1B03029982 to Y.-M.L. and 2017R1D1A1B03032615 to S.F.). This work was also supported by Grants-in-Aid (no. 16H02268 to S.F.) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), SENTAN project (to S.F.) from Japan Science and Technology Agency (JST).

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(1) (a) Najafpour, M. M.; Renger, G.; Hołyńska, M.; Moghaddam, A. N.;

Aro, E.-M.; Carpentier, R.; Nishihara, H.; Eaton-Rye, J. J.; Shen, J.-R.; Allakhverdiev, S. I. Manganese Compounds as Water-Oxidizing Catalysts: From the Natural Water-Oxidizing Complex to Nanosized Manganese Oxide Structures. Chem. Rev. 2016, 116, 2886-2936. (b) Najafpour, M. M.; Heidari, S.; Balaghi, S. E.; Hołyńska, M.; Sadr, M. H.; Soltani, B.; Khatamian, M.; Larkum, A. W.; Allakhverdiev, S. I. Proposed Mechanisms for Water Oxidation by Photosystem II and Nanosized Manganese Oxides. Biochim. Biophy. Acta 2017, 1858, 156-174. (c) Yano, J.; Yachandra, V. Mn4Ca Cluster in Photosynthesis: Where and How Water is Oxidized to Dioxygen. Chem. Rev. 2014, 114, 4175-4205. (2) (a) Suga, M.; Akita, F.; Sugahara, M.; Kubo, M.; Nakajima, Y.; Nakane, T.; Yamashita, K.; Umena, Y.; Nakabayashi, M.; Yamane, T.; Nakano, T.; Suzuki, M.; Masuda, T.; Inoue, S.; Kimura, T.; Nomura, T.; Yonekura, S.; Yu, L.-J.; Sakamoto, T.; Motomura, T.; Chen, J.-H.; Kato, Y.; Noguchi, T.; Tono, K.; Joti Y.; Kameshima, T.; Hatsui, T.; Nango, E.; Tanaka, R.; Naitow, H.; Matsuura, Y.; Yamashita, A.; Yamamoto, M.; Nureki, O.; Yabashi, M.; Ishikawa, T.; Iwata, S.; Shen, J.-R. Light-Induced Structural Changes and the Site of O=O Bond Formation in PSII Caught by XFEL. Nature 2017, 543, 131-135. (b) Shen, J.-R. The Structure of Photosystem II and the Mechanism of Water Oxidation in Photosynthesis. Annu. Rev. Plant Biol. 2015, 66, 23-48. (3) (a) Zobnina, V.; Lambreva, M. D.; Rea, G.; Campi, G.; Antonacci, A.; Scognamiglio, V.; Giardi, M. T.; Polticelli, F. The Plastoquinol-Plastoquinone Exchange Mechanism in Photosystem II: Insight from Molecular Dynamics Simulations. Photosynth. Res. 2017, 131, 15-30. (b) Müh, F.; Glöckner, C.; Hellmich, J.; Zouni, A. Light-Induced Quinone Reduction in Photosystem II. Biochim. Biophys. Acta 2012, 1817, 44-65. (4) (a) Paul, S.; Neese, F.; Pantazis, D. A. Structural Models of the Biological Oxygenevolving Complex: Achievements, Insights, and Challenges for Biomimicry. Green Chem. 2017, 19, 2309-2325. (b) PérezNavarro, M.; Neese, F.; Lubitz, W.; Pantazis, D. A.; Cox, N. Recent

(9)

(10)

(11)

(12)

(13)

(14)

(15)

Page 6 of 8

Developments in Biological Water Oxidation. Curr. Opin. Chem. Biol. 2016, 31, 113-119. (a) Llansola-Portoles, M. J.; Gust, D.; Moore, T. A.; Moore, A. L. Artificial Photosynthetic Antennas and Reaction Centers. C. R. Chimie 2017, 20, 296-313. (b) El-Khouly, M. E.; El-Mohsnawy, E.; Fukuzumi, S. Solar Energy Conversion: From Natural to Artificial Photosynthesis. J. Photochem. Photobiol. C: Photochem. Rev. 2017, 31, 36-83. (c) Mulfort, K. L.; Utschig, L. M. Modular Homogeneous Chromophore−Catalyst Assemblies. Acc. Chem. Res. 2016, 49, 835-843. (a) Rudolf, M.; Kirner, S. V.; Guldi, D. M. A Multicomponent Molecular Approach to Artificial Photosynthesis – The Role of Fullerenes and Endohedral Metallofullerenes. Chem. Soc. Rev. 2016, 45, 612630. (b) KC, C. B.; D’Souza, F. Design and Photochemical Study of Supramolecular Donor–Acceptor Systems Assembled via Metal–Ligand Axial Coordination. Coord. Chem. Rev. 2016, 322, 104-141. (c) Hammarström, L. Accumulative Charge Separation for Solar Fuels Production: Coupling Light-Induced Single Electron Transfer to Multielectron Catalysis. Acc. Chem. Res. 2015, 48, 840-850. (d) Fukuzumi, S.; Ohkubo, K.; Suenobu, T. Long-Lived Charge Separation and Applications in Artificial Photosynthesis. Acc. Chem. Res. 2014, 47, 1455-1464. (e) Gust, D.; Moore, T. A.; Moore, A. L. Solar Fuels via Artificial Photosynthesis. Acc. Chem. Res. 2009, 42, 1890-1898. (a) Nocera, D. G. Solar Fuels and Solar Chemicals Industry. Acc. Chem. Res. 2017, 50, 616-619. (b) Sakimoto, K. K.; Kornienko, N.; Yang, P. Cyborgian Material Design for Solar Fuel Production: The Emerging Photosynthetic Biohybrid Systems. Acc. Chem. Res. 2017, 50, 476-481. (c) Su, J.; Vayssieres, L. A Place in the Sun for Artificial Photosynthesis? ACS Energy Lett. 2016, 1, 121-135. (a) Ashford, D. L.; Gish, M. K.; Vannucci, A. K.; Brennaman, M. K.; Templeton, J. L.; Papanikolas, J. M.; Meyer, T. J. Molecular Chromophore−Catalyst Assemblies for Solar Fuel Applications. Chem. Rev. 2015, 115, 13006-13049. (b) Berardi, S.; Drouet, S.; Francàs, L.; Gimbert-Suriñach, C.; Guttentag, M.; Richmond, C.; Stoll, T.; Llobet, A. Molecular Artificial Photosynthesis. Chem. Soc. Rev. 2014, 43, 7501-7519. (a) Yuan, Y.-J.; Yu, Z.-T.; Chen, D.-Q.; Zou, Z.-G. Metal-Complex Chromophores for Solar Hydrogen Generation. Chem. Soc. Rev. 2017, 46, 603-631. (b) Zhou, H.; Yan, R.; Zhang, D.; Fan, T. Challenges and Perspectives in Designing Artificial Photosynthetic Systems. Chem.–Eur. J. 2016, 22, 9870-9885. (c) Fukuzumi, S. Artificial Photosynthetic Systems for Production of Hydrogen. Curr. Opinion Chem. Biol. 2015, 25, 18-26. (d) Yu, Z.; Li, F.; Sun, L. Recent Advances in Dye-Sensitized Photoelectrochemical Cells for Solar Hydrogen Production Based on Molecular Components. Energy Environ. Sci. 2015, 8, 760-775. (a) Asmawy, F. M.; McAuliffe, C. A.; Parish, R. V.; Tames, J. The Photolysis of Coordinated Water in [Mn(saltm)(H2O)]2(ClO4)2 [saltm = N, N′-porpylenebis (salicylideneaminato)]. J. Chem. Soc., Chem. Commun. 1984, 14-16. (b) Watkinson, M.; Whiting, A.; McAuliffe, C. A. Synthesis of a Bis-Manganese Water Splitting Complex. J. Chem. Soc., Chem. Commun. 1994, 2141-2142. Awad, M. K.; Anderson, A. B. Photoactivation of Water by p-Benzoquinone and the Role of Manganese(III) Complexes in Oxygen Evolution: Molecular Orbital Theory. J. Am. Chem. Soc. 1989, 111, 802806. González-Riopedre, G.; Fernández-García, M. I.; González-Noya, A. M.; Vázquez-Fernández, M. Á .; Bermejo, M. R.; Maneiro, M. Manganese-Schiff base complexes as catalysts for water photolysis. Phys. Chem. Chem. Phys. 2011, 13, 18069-18077. Kotani, H.; Suenobu, T.; Lee, Y.-M.; Nam, W.; Fukuzumi, S. Photocatalytic Generation of a Non-Heme Oxoiron(IV) Complex with Water as an Oxygen Source. J. Am. Chem. Soc. 2011, 133, 3249-3251. Ohkubo, K.; Fujimoto, A.; Fukuzumi, S. Visible-Light-Induced Oxygenation of Benzene by the Triplet Excited State of 2,3-Dichloro-5,6dicyano‑ p‑ benzoquinone. J. Am. Chem. Soc. 2013, 135, 5368-5371. (a) Fukuzumi, S.; Ohkubo, K. One-Step Selective Hydroxylation of Benzene to Phenol. Asian J. Org. Chem. 2015, 4, 836-845. (b)


Page 7 of 8 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


(16)

(17) (18)

(19)

(20)

(21)

(22)

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Ohkubo, K.; Hirose, K.; Fukuzumi, S. Solvent-Free One-Step Photochemical Hydroxylation of Benzene Derivatives by the Singlet Excited State of 2,3-Dichloro-5,6-dicyano-p-benzoquinone Acting as a Super Oxidant. Chem.–Eur. J. 2015, 21, 2855-2861. (c) Ohkubo, K.; Fujimoto, A.; Fukuzumi, S. Aromatic Monochlorination Photosensitized by DDQ with Hydrogen Chloride under Visible‑Light Irradiation Chem.–Asian J. 2016, 11, 996-999. Lee, Y.-M.; Kotani, H.; Suenobu, T.; Nam, W.; Fukuzumi, S. Fundamental Electron-Transfer Properties of Non-heme Oxoiron(IV) Complexes. J. Am. Chem. Soc. 2008, 130, 434-435. Fukuzumi, S. Electron-Transfer Properties of High-Valent MetalOxo Complexes. Coord. Chem. Rev. 2013, 257, 1564-1575. Fukuzumi, S.; Kojima, T.; Lee, Y.-M.; Nam, W. High-Valent MetalOxo Complexes Generated in Catalytic Oxidation Reactions Using Water as an Oxygen Source. Coord. Chem. Rev. 2017, 333, 44-56. Fukuzumi, S.; Ohkubo, K.; Suenobu, T.; Kato, K.; Fujitsuka, M.; Ito, O. Photoalkylation of 10-Alkylacridinium Ion via a Charge-Shift Type of Photoinduced Electron Transfer Controlled by Solvent Polarity. J. Am. Chem. Soc. 2001, 123, 8459-8467. Electron transfer from [(N4Py)FeII]2+ to 3DDQ* to produce [(N4Py)FeIII]3+ and DDQ•– is highly exergonic to be located in the Marcus inverted region. In such a case, however, electron transfer from [(N4Py)FeII]2+ to 3DDQ* may produce an excited state of [(NePy)FeIII]3+ and DDQ•– to keep the electron transfer in the Marcus normal region, where electron transfer occurs at the diffusion-limited rate as reported for photoinduced electron transfer from electron donors to flavin derivatives. See: Murakami, M.; Ohkubo, K.; Fukuzumi, S. Inter‑ and Intramolecular Photoinduced Electron Transfer of Flavin Derivatives with Extremely Small Reorganization Energies. Chem.–Eur. J. 2010, 16, 7820-7832. Iida, Y. The Electronic Spectra of the Anion Radical Salts Derived from 2,3-Dicyano-1,4-benzoquinone and 2,3-Dichloro-5,6-dicyano1,4-benzoquinone. Bull. Chem. Soc. Jpn. 1971, 44, 1777-1780. It should be noted that no electron transfer occurs from [(N4Py)FeII]2+ to [(N4Py)FeIV(O)]2+ to produce the μ-oxo dimer, [(N4Py)FeIII-O-FeIII(N4Py)]4+, because the one-electron oxidation of [(N4Py)FeII]2+ (Eox vs. SCE = 1.00 V, ref. 23) is higher than the one-electron reduction potential of [(N4Py)FeIV(O)]2+ (Ered vs. SCE = 0.51 V, ref. 16). In contrast to the case of [(N4Py)FeV(O)]3+, cis binding sites are required for water oxidation by iron(IV)-oxo intermediates; see: (a) Kottrup, K. G.; D’Agostini, S.; van Langevelde, P. H.; Siegler, M. A.; Hetterscheid, D. G. H. Catalytic Activity of an Iron-Based Water Oxidation Catalyst: Substrate Effects of Graphitic Electrodes. ACS Catal. 2018, 8, 1052-1061. (b) Kundu, S.; Matito, E.; Walleck, S.; Pfaff, F. F.; Heims, F.; Rábay, B.; Luis, J. M.; Company, A.; Braun, B.; Glaser, T.;

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(27)

(28)

(29) (30)

(31)

(32) (33)

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Ray K. O-O Bond Formation Mediated by a Hexanuclear Iron Complex Supported on a Stannoxane Core. Chem. – Eur. J. 2012, 18, 2787-2791. (c) Kundu, S.; Schwalbe, M.; Ray, K. Metal-Oxo-Mediated O-O Bond Formation Reactions in Chemistry and Biology. Inorg. React. Mech. 2012, 8, 41-57. (d) Gamba, I.; Codolà, Z.; LloretFillol, J.; Costas, M. Making and Breaking of the O-O Bond at Iron Complexes. Coord. Chem. Rev. 2017, 334, 2-24. Hong, S.; Lee, Y.-M.; Shin, W.; Fukuzumi, S.; Nam, W. Dioxygen Activation by Mononuclear Nonheme Iron(II) Complexes Generates Iron-Oxygen Intermediates in the Presence of an NADH Analogue and Proton. J. Am. Chem. Soc. 2009, 131, 13910-13911. Although the rate of electron transfer from FeIV=O is diffusion-limited, as soon as FeV=O is produced, it reacts with H2O. In such a case, the electron transfer from FeIV=O to 3DDQ* is the rate-determining step for the catalytic water oxidation when FeIV=O is observed as an intermediated during the reaction. When DDQ was fully converted to DDQH2 (Figure 6b), only a trace amount of [(N4Py)FeIV(O)]2+ decayed (Figure 6a). Therefore, the reaction of [(N4Py)FeIV(O)]2+ with DDQH2 is negligible under the conditions. (a) Hong, D.; Mandal, S.; Yamada, Y.; Lee, Y.-M.; Nam, W.; Llobet, A.; Fukuzumi, S. Water Oxidation Catalysis with Nonheme Iron Complexes under Acidic and Basic Conditions: Homogeneous or Heterogeneous? Inorg. Chem. 2013, 52, 9522-9531. (b) Fukuzumi, S.; Lee, Y.-M.; Nam, W. Kinetics and Mechanisms of Catalytic Water Oxidation. Dalton Trans. 2019, 48, 779-798. Fukuzumi, S.; Nishizawa, N.; Tanaka, T. Mechanism of Hydride Transfer from an NADH Model Compound to p-Benzoquinone Derivatives. J. Org. Chem. 1984, 49, 3571-3578. Armarego, W. L. F.; Chai, C. L. L. Purification of Laboratory Chemicals, 6th ed.; Pergamon Press: Oxford, 2009. Kaizer, J.; Klinker, E. J.; Oh, N. Y.; Rohde, J.-U.; Song, W. J.; Stubna, A.; Kim, J.; Münck, E.; Nam, W.; Que, L., Jr. Nonheme FeIVO Complexes That Can Oxidize the C-H Bonds of Cyclohexane at Room Temperature. J. Am. Chem. Soc. 2004, 126, 472-473. Lubben, M.; Meetsma, A.; Wilkinson, E. C.; Feringa, B.; Que, L., Jr. Nonheme Iron Centers in Oxygen Activation: Characterization of an Iron(III) Hydroperoxide Intermediate. Angew. Chem., Int. Ed. 1995, 34, 1512-1514. Saltzman, H.; Sharefkin, J. G., Eds.; Organic Syntheses; Wiley: New York, 1973; Vol. V, p 658. Nakagawa, T.; Okamoto, K.; Hanada, H.; Katoh, R. Probing with Randomly Interleaved Pulse Train Bridges the Gap between Ultrafast Pump-Probe and Nanosecond Flash Photolysis. Opt. Lett. 2016, 41, 1498-1501. Mann, C. K.; Barnes, K. K. In Electrochemical Reactions in Nonaqueous Systems; Mercel Dekker: New York, 1970.



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