Redox active ionic liquid as efficient mediator and solvent for visible light-driven B12 catalytic reactions

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ScienceDirect Green Energy & Environment 4 (2019) 116e120 www.keaipublishing.com/gee

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Redox active ionic liquid as efficient mediator and solvent for visible light-driven B12 catalytic reactions Hisashi Shimakoshi*, Noriyuki Houfuku, Li Chen, Yoshio Hisaeda* Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Fukuoka 819-0395, Japan Received 19 January 2019; revised 27 February 2019; accepted 1 March 2019 Available online 8 March 2019

Abstract The redox active ionic liquid, 1-ethyl-4-(methoxycarbonyl)pyridinium bis(trifluoromethanesulfonyl)amide (RIL), was synthesized from its iodide form by an anion exchange reaction of Li(NTf2) with viscos liquid (h ¼ 122 cP at 25  C) and characterized by NMR, IR, and elemental analysis. The compound showed reversible redox couples at 0.65 V and 1.48 V vs. Ag/AgCl and worked as an electron mediator in the B12 complex/[Ru(bpy)3]Cl2 photosensitizer catalytic system under visible light irradiation. The catalytic efficiency in the RIL was higher than those in DMF, MeOH, and the redox inactive ionic liquid, 1-butyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide. © 2019, Institute of Process Engineering, Chinese Academy of Sciences. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Ionic liquid; Photosensitizer; Redox active; Visible light; Vitamin B12

1. Introduction Organometallic B12 compounds are unique metal complexes having a cobalt-carbon s-bond present in biological systems [1–5]. Cleavage of the metal-organic s-bond initiates many B12-dependent enzymatic reactions. These alkylated cobalt complexes are generally formed by the reaction of the Co(I) state of the complex with electrophiles such as an organic halide in vivo. Therefore, the reductive formation of the Co(I) species followed by formal oxidative addition of the organic halide to form the alkylated cobalt complex are key steps for the development of the B12 catalyzed enzymatic reactions. Our group has reported the photosensitizing formation of the Co(I) species of the B12 derivative, heptamethyl cobyrinate perchlorate, using various photosensitizers (PSs), such as [Ru(bpy)3]Cl2 or the cyclometalated iridium(III) complexes, as shown in Fig. 1 and performed various B12

* Corresponding authors. E-mail addresses: [email protected] (H. Shimakoshi), [email protected] (Y. Hisaeda).

model reactions, such as the 1,2-migration of functional groups, dechlorination of organic chloride, etc [6–11]. The significant advantage of the photochemical system is reducing chemical waste using a clean and abundant light source as the driving-force of the reaction and a simple and facilitated reaction system [12]. In recent years, ionic liquids have also attracted great interest as promising media in organic synthesis because of their unique characteristics such as low melting point, negligible vapor pressure, non-flammability, and their good solubility of many organic and inorganic compounds [13–16]. The recyclability of a catalyst dissolved in an ionic liquid provided an additional advantage of ionic liquids for organic synthesis since the products can be separated from the ionic liquid by extraction, while the catalyst remains in the ionic liquid and can be reused. Based on these advantages, we demonstrated the efficient and recycled catalysis of heptamethyl cobyrinate perchlorate in an ionic liquid with the [Ru(bpy)3]Cl2 photosensitizer for the visible light-driven dechlorination of 1,1,1trichloro-2,2-bis(4-chlorophenyl)ethane (DDT) [8,10]. As development of this study, we designed and synthesized the

https://doi.org/10.1016/j.gee.2019.03.001 2468-0257/© 2019, Institute of Process Engineering, Chinese Academy of Sciences. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

H. Shimakoshi et al. / Green Energy & Environment 4 (2019) 116–120

H3CO2C

+

CO2CH3

2+

CO2CH3

H3CO2C

Ru Co • 2CI-

H3CO2C

[Ru(bpy)3]CI2

CO2CH3

CO2CH3

• CIO4[Cob(II)7C1ester]CIO4 (B12 complex) Fig. 1. Structure of the B12 complex, [Cob(II)7C1ester]ClO4, and photosensitizer, [Ru(bpy)3]Cl2.

novel redox active ionic liquid (RIL) and used it as a solvent and electron mediator in the photosensitizing B12 complex catalytic system. Efficient electron transfer from the photosensitizer to the B12 catalyst could be expected using the RIL as a solvent (Fig. 2). 2. Experimental 2.1. Reagents and materials The solvents and chemicals used in the syntheses were of reagent grade and used without further purification. Heptamethyl cobyrinate perchlorate, [Cob(II)7C1ester]ClO4 (Fig. 1), was synthesized by a previously reported method [17]. The ionic liquid, [C4min][NTf2], composed of 1-butyl-3-methyl imidazolium (C4min) and bis(trifluoromethylsulfonyl)amide (NTf2), was purchased from Kanto Chemical Co., Inc., and dried under reduced pressure for 1 day before use. 2.2. Apparatus and characterization The NMR spectra were recorded by a Bruker Avance 500 spectrometer at the Center of Advanced Instrumental Analysis of Kyushu University. The ESR spectra were obtained using a [Ru(bpy)3]2+* -0.72 V

e-

-0.65 V Et

Vis. light

e-

Co(II)/Co(I) -0.49 V

N+

OMe NTf2-

Ru

[Ru(bpy)3]2+

CO2CH3

H3CO2C

O

RIL Mediator & Solvent

CO2CH3

H3CO2C

H3CO2C CO2CH3

CO2CH3

RBr

RH + Br-

B12 catalyst

Fig. 2. Schematic illustration of electron transfer process occurring in redox active ionic liquid (RIL) solution of [Ru(bpy)3]Cl2 and the B12 complex.

117

Bruker EMX-Plus X-band spectrometer at room temperature. The UV-vis absorption spectra were measured by a Hitachi U3300 spectrophotometer at room temperature. The luminescence spectra were measured by a Hitachi F-4500 spectrophotometer in DMF and ionic liquids at room temperature. The MALDI-Time of Flight (TOF) mass spectra were obtained by a Bruker autoflex II using 6-aza-2-thiothymine as the matrix. The GC-mass spectra were obtained using a Shimadzu GC-QP5050A equipped with a J&W Scientific DB-1 column (length 30 m; ID 0.25 mm, film 0.25 mm thick). The cyclic voltammograms (CV) were obtained using a BAS CV 50W electrochemical analyzer. A three-electrode cell equipped with a 1.6-mm diameter glassy carbon rod and platinum wire as the working and counter electrodes, respectively, was used. An Ag/AgCl (3.0 M NaCl) electrode served as the reference. The E1/2 value of the ferrocene-ferrocenium (Fc/Fcþ) was 0.56 V vs. Ag/AgCl with this setup. The viscosities of the solvents were measured by an Anton Paar, Lovis 200 M/ME. 2.3. Synthesis of redox active ionic liquid (RIL) A 20 mL aqueous solution of Li[NTf2] (20 g, 6.9  102 M) was mixed with a 120 mL aqueous solution of 1-ethyl-4-(methoxycarbonyl)pyridium iodide (21 g, 7.2  102 M) and stirred for 12 h at room temperature. The solution was separated into two layers and the aqueous solution was removed using a separating funnel. The amber colored ionic liquid layer was washed with water and dissolved in acetone. The acetone solution of the ionic liquid was passed through a column equipped with activated carbon. The obtained solution was evaporated to obtain a pale yellow color liquid. Yield: 93%. 1H NMR (500 MHz, CDCl3): d ¼ 1.75 (t, J ¼ 7.57 Hz, 3H), 4.10 (s, 3H), 4.82 (q, J ¼ 7.57 Hz, 2H), 8.53 (d, J ¼ 6.62 Hz, 2H), 8.99 (d, J ¼ 6.62 Hz, 2H). 13C NMR (125 MHz, CDCl3): d ¼ 15.68, 53.70, 57.99, 119.41 (q, CF3), 127.85, 144.58, 145.12, 161.86. Elemental analysis: Found: C, 29.38; H, 2.68; N, 6.32%. Calcd for C11H12N2F6O6S 2: C, 29.60; H, 2.71; N, 6.28%. IR (neat): [n/cm1], 1190, 1360. 2.4. General procedure of photocatalytic reaction A 1 mL ionic liquid solution of the B12 complex (1.0  105 M), [Ru(bpy)3]Cl2 (1.0  105 M), triethanolamine (TEOA) (0.5 M) and phenethyl bromide (3.0  102 M) was degassed by N2 gas, then the solution was stirred at room temperature with visible light irradiation using a 200 W tungsten lamp equipped with a 420-nm cut-off filter (Sigma Koki, 42 L) and a heat cut-off filter (Sigma Koki, 30 H). After 24 h, the solution was extracted with water to remove TEOA, then the products and a small amount of unreacted substrate were obtained by extraction with hexane/ diethyl ether (3:1, v/v). The residue was further passed through a silica gel short column eluting with CHCl3 to remove the metal complexes. The product was analyzed by GC-MS and 1 H NMR. In the Heck-type reaction, B12 complex (1.0  105 M), [Ru(bpy)3]Cl2 (1.0  105 M), TEOA (0.5 M), phenethyl bromide (3.0  102 M), and styrene

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(3.0  102 M). Product is E-1,4-diphenyl but-1-ene (colorless oil), yield (20%); 1H NMR (500 MHz, CDCl3): d ¼ 2.58 (m, 2H), 2.83 (t, J ¼ 15.5 Hz, 2H), 6.32 (dt, J ¼ 15.5 Hz, 6.8 Hz, 1H), 6.44 (d, J ¼ 15.5 Hz, 1H), 7.23–7.38 (m, 10H); 13 C NMR (125 MHz, CDCl3): d ¼ 34.84, 35.86, 125.87, 125.97, 126.91, 128.34, 128.46, 129.96, 130.37, 137.72, 141.74; GC-MS: Mþ ¼ 208. 2.5. ESR measurement of the ionic liquid after photoirradiation The ESR spectra were observed during the photoirradiation using a DMF solution of [Ru(bpy)3]Cl2 (1  104 M), RIL (2  103 M), and TEOA (1.0  101 M) under nitrogen at room temperature. The settings for the ESR measurements were a frequency of 9.87 GHz, power of 1.0 mW, center field of 3375 G, sweep width of 100 G, modulation amplitude of 0.5 G, time constant of 40 ms, and sweep time of 20 s. 2.6. ESR measurement of the B12 complex after photoirradiation The ESR spectra were observed at 100 K after the photoirradiation of an RIL solution containing the B12 complex (1  104 M), [Ru(bpy)3]Cl2 (1  104 M), and TEOA (1.0  101 M) under nitrogen. The settings for the ESR measurements were a frequency of 9.41 GHz, power of 10.1 mW, center field of 3200 G, sweep width of 4000 G, modulation amplitude of 20 G, time constant of 164 ms, and sweep time of 40 s.

reductions of RIL to the RIL radical and RIL anion, respectively, as shown in Scheme 2 [19]. The E (M/M*) of [Ru(bpy)3]Cl and E1/2 (Co(II)/Co(I)) of the B12 complex in DMF are 0.72 V [20] and 0.49 V vs. Ag/AgCl [21], respectively. Therefore, the RIL could mediate the electron between the Ru photosensitizer and the cobalt complex. Actually, the luminescence of [Ru(bpy)3]Cl2 was quenched by the RIL as shown in Fig. 4. In contrast, the redox inactive conventional ionic liquid, [C4min][NTf2], does not quench the luminescence of [Ru(bpy)3]Cl2. This result shows that oxidative quenching of the excited state of the Ru photosensitizer by RIL occurred to form the reduced form of the RIL. The reduced form of the RIL was analyzed by ESR. The ESR signal was observed after photoirradiation as shown in Fig. 5. This signal is similar to the reported ESR signal of the pyridinyl radical [22,23]. Formation of the Co(I) species of the B12 complex using this system was monitored by the ESR spectral change. Continuous light irradiation of the RIL solution of the Co(II) state of the B12 complex in the presence of [Ru(bpy)3]Cl2 and sacrificial reductant, TEOA, reduced the starting ESR signal of the Co(II) state of the B12 complex as shown in Fig. 6a and b, which implied formation of the diamagnetic Co(I) species of the B12 complex. Actually, the ESR signal of Co(II) species reappeared after solution was exposed to air as shown in Fig. 6c. This result shows that the reactive Co(I) species of the B12 complex could be formed by photo-induced electron transfer from the photosensitizer in the RIL in spite of its high viscosity.

3. Results and discussion 3.1. Synthesis and properties of redox active ionic liquid We designed the redox active ionic liquid based on the pyridium cation structure with an electron withdrawing substituent and NTf2 as the counter anion. The NTf2 anion decreases the melting point of the ionic liquid [18]. The redox active ionic liquid (RIL) was successfully synthesized by the anion exchange reaction of the commercially-available 1ethyl-4-(methoxycarbonyl)pyridium iodide with Li[NTf2] in water as shown in Scheme 1. A pale yellow viscous liquid was obtained after purification. The viscosity (h) of the RIL was 122 cP at 25  C. The redox behavior of the RIL was investigated by cyclic voltammetry in DMF. Reversible redox couples were observed at 0.65 V and 1.48 V vs. Ag/AgCl as shown in Fig. 3. Each redox couple was assigned to the

RIL1-/0 = -1.48 V

RIL0/1+ = -0.65 V

10 μA CoII/III = 0.44 V CoI/II = -0.49 V

4 μA

-1.8

-1.3

-0.8

-0.3

0.2

Fig. 3. CVs of RIL (1 mM) (red line) and the B12 complex (1 mM) (blue line) in DMF containing of 0.1 M n-Bu4NPF6 under N2 at room temperature; sweep rate: 100 mV s1.

Stirring 12 hrs in H2O

Scheme 2. Redox equations of RIL. Scheme 1. Synthesis of redox active ionic liquid (RIL).

0.7

E (V vs. Ag/AgCl)

Intensity

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119

2500

Table 1 Debromination of phenethyl bromide catalyzed by the B12 complex with [Ru(bpy)3]Cl2 in RILa.

2000

Entry

Reaction conditions

Product yieldb(%)

TONc

1 2 3 4 5

as shown no B12 comolex no [Ru(bpy)3]Cl2 no TEOA no light

10 0 0 0 0

300 0 0 0 0

1500

1000

a Conditions: [B12 complex] ¼ 1  105 M; [Ru photosensitizer] ¼ 1  105 M; [TEOA] ¼ 5  101 M; [phenethyl bromide] ¼ 3  102 under N2 at room temperature, solvent RIL (1 mL), light source: 200 W tungsten lamp with 42 L cutoff filter, reaction time ¼ 24 h. b Yield is based on the initial concentration of the substrate. c TON is turnover number calculated based on B12 complex.

500

0 500

600

700

800

Wavelength (nm) Fig. 4. Emission spectra of [Ru(bpy)3]Cl2 (1  105 M) in [C4min][NTf2] (black line) and RIL (red line) at room temperature; lex ¼ 459 nm.

Table 2 Solvent effects for debromination of phenethyl bromide catalyzed by the B12 complex with [Ru(bpy)3]Cl2a. Entry

Solvent

Viscosityb/h (cP)

Products yieldc(%)

TONd

1 2 3 4

RIL [C4min][NTf2] DMF MeOH

122 52 0.802 0.555

10 1 2.5 2

300 30 75 60

Conditions: [B12 complex] ¼ 1  105 M; [Ru photosensitizer] ¼ 1  105 M; [TEOA] ¼ 5  101 M; [phenethyl bromide] ¼ 3  102 M under N2 at room temperature, solvent 1 mL, light source: 200 W tungsten lamp with 42 L cut-off filter, reaction time ¼ 24 h. b Viscosities (h) were measured at 25  C. c Yield is based on the initial concentration of the substrate. d TON is turnover number calculated based on B12 complex. a

Fig. 5. ESR spectrum of RIL radical (2  103 M) in the presence of [Ru(bpy)3]Cl2 (1  104 M) and TEOA (1  101 M) in DMF measured at room temperature under anaerobic condition.

(a) 500 G Yield 20% in RIL 6% in MeOH

(b) *

(c)

Fig. 6. ESR spectra of the B12 complex (1  104 M) in the presence of [Ru(bpy)3]Cl2 (1  104 M) and TEOA (1  101 M) in RIL measured at 100 K under anaerobic condition. (a) Before irradiation of visible light, (b) after 2 h irradiation of visible light, (c) after solution was opened to air. Signal * is TEOA radical cation peak.

3.2. Photocatalytic reactions by B12 complex in RIL Based on the CV and spectroscopic studies, the photocatalytic reaction of the B12 catalyst with [Ru(bpy)3]Cl2 was conducted in the RIL. Debromination of phenethyl bromide

Scheme 3. Heck-type reactions catalyzed by the B12 complex and [Ru(bpy)3] Cl2 in RIL and MeOH.

was catalyzed by the B12 complex to form styrene as shown in entry 1 of Table 1. Several controlled reactions showed that the B12 complex, Ru photosensitizer, TEOA, and visible light irradiation were essential for the reaction (entries 2–5 in Table 1). The turnover number (TON) of the B12 catalyst reached 300 in the RIL. Interestingly, the TONs were only 75 and 60 when a conventional low viscosity solvent, DMF (h ¼ 0.802 cP) or MeOH (h ¼ 0.555 cP), were used as the solvents (entries 2 and 3 in Table 2) [24]. Furthermore, the TON in the redox inactive ionic liquid, [C4min][NTf2] (h ¼ 52 cP), was only 30 (entry 4 in Table 2). A similar enhancement of the B12 catalytic reaction efficiency in the RIL was observed in the photocatalytic Heck-type reaction. The photocatalytic Heck-type reaction of phenethyl bromide with styrene afforded the coupling product, E-1,4-diphenyl but-1-ene, in 20% yield in the RIL while the yield was only 6% in MeOH as shown in Scheme 3. Based on these results, the redox active ionic liquid could efficiently

H. Shimakoshi et al. / Green Energy & Environment 4 (2019) 116–120

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*

2+ Ru

Homolysis

2+

Ru

Ru

Fig. 7. Proposed mechanism for redox active ionic liquid mediated visible light-driven catalytic reaction by the B12 complex.

mediated the electron during the reaction in spite of its high viscosity. To explain these results, we proposed an electron self-exchanging mechanism [25–27] in the RIL as shown in Fig. 7. By assuming this mechanism, the electron was efficiently mediated between the Ru photosensitizer and the B12 complex in the high viscosity ionic liquid. Similar solvent-mediated photoinduced electron transfer in a pyridinium ionic liquid was reported [28]. The redox active ionic liquid should possess multi tasks of good electron mediator as well as green solvent as shown in conventional ionic liquid. 4. Conclusions We have developed for the first time molecular transformations in a redox active ionic liquid. The redox active ionic liquid efficiently mediates an electron in the B12 complex/[Ru(bpy)3]Cl2 catalytic system by visible light irradiation. Ongoing work in our laboratory is focused on the application of this catalytic system to other molecular transformations. Conflict of interest There is no conflict of interest. Acknowledgements We express our gratitude to Professor Masahiro Goto and Dr. Fukiko Kubota (Kyushu Univesity, Japan) for helping to measure the viscosity of the ionic liquid. This study was partially supported by JSPS KAKENHI Grant Number 18H04265 in Precisely Designed Catalysts with Customized Scaffolding and Grant Number JP16H04119, and by JSPS and PAN under the Research Cooperative Program Grant Number AJ180081 (30-0004-04) and a Grant from the Nippon Sheet Glass Foundation (HAKF541800).

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.gee.2019.03.001.

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