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6,6′-Di-(2″-thiophenol)-2,2′-bipyridine

6,6′-Di-(2″-thiophenol)-2,2′-bipyridine Short Note 6,6′-Di-(2″-thiophenol)-2,2′-bipyridine molbank Yan Huang and Lianpeng Tong * Short Note School of Chemistry and Chemical Engineering, Guangzhou University, No. 230 Wai Huan Xi Road, Higher 0 00 0 Education Mega Center, Guangzhou 510006, China; 2111905012@e.gzhu.edu.cn 6,6 -Di-(2 -thiophenol)-2,2 -bipyridine * Correspondence: ltong@gzhu.edu.cn Yan Huang and Lianpeng Tong * Abstract: This short note describes the synthesis of compound 6,6′-di-(2″-thiophenol)-2,2′-bipyri- dine from its methyl phenyl sulfane precursor via deprotection of the methyl groups. The product School of Chemistry and Chemical Engineering, Guangzhou University, No. 230 Wai Huan Xi Road, Higher Education Mega Center, Guangzhou 510006, China; 2111905012@e.gzhu.edu.cn as well as the intermediate in the synthetic route have been characterized by UV-Vis spectroscopy, * Correspondence: ltong@gzhu.edu.cn 1 13 H- and C-NMR spectroscopy, FT-IR spectroscopy, and HR-MS analysis. This work presents a rare example of tetradentate chelators that bears pyridy 0 l back 00 bones and thiophenol dono 0 rs for the Abstract: This short note describes the synthesis of compound 6,6 -di-(2 -thiophenol)-2,2 -bipyridine fr coordination om its methyl with phenyl3d sulfane -transition metal cations. precursor via deprotection of the methyl groups. The product as well as the intermediate in the synthetic route have been characterized by UV-Vis spectroscopy, H- and C-NMR spectroscopy, FT-IR spectroscopy, and HR-MS analysis. This work presents a rare example Keywords: thiophenol; bipyridine; tetradentate ligand; hydrogenase; transition metal; coordination of tetradentate chelators that bears pyridyl backbones and thiophenol donors for the coordination with 3d-transition metal cations. Keywords: thiophenol; bipyridine; tetradentate ligand; hydrogenase; transition metal; coordination 1. Introduction [NiFe]-hydrogenases in nature have the ability of catalyzing the protons to hydrogen (H2) reduction reaction at high rates with a small barrier of activation energy [1,2]. The 1. Introduction active site of [NiFe]-hydrogenases is a bimetallic Ni-Fe cluster, of which the Ni and Fe [NiFe]-hydrogenases in nature have the ability of catalyzing the protons to hydrogen (H ) reduction reaction at high rates with a small barrier of activation energy [1,2]. The me 2 tal centers are bridged by two cysteine residue thiolates (Scheme 1) [3–5]. Syntheses of active site of [NiFe]-hydrogenases is a bimetallic Ni-Fe cluster, of which the Ni and Fe metal metal complexes that mimic the structure and function of the active site of [NiFe]-hydro- centers are bridged by two cysteine residue thiolates (Scheme 1) [3–5]. Syntheses of metal genases have long been an important field of bioinorganic chemistry [6,7], and draw even complexes that mimic the structure and function of the active site of [NiFe]-hydrogenases more attention these days in the context of the development of a hydrogen economy [8]. have long been an important field of bioinorganic chemistry [6,7], and draw even more attention The biomime these days tic Ni inand Fe the context model complexes ca of the developmentn he of a hydr lp us to under ogen economy stand the cat [8]. The alytic mech- biomimetic Ni and Fe model complexes can help us to understand the catalytic mechanism anism of hydrogenases, whilst also inspiring the design of transition metal-based hetero- Citation: Huang, Y.; Tong, L. 6,6 -Di- of hydrogenases, whilst also inspiring the design of transition metal-based heterogeneous Citation: Huang, Y.; Tong, L. 6,6′-Di- geneous hydrogen evolution catalysts. 00 0 (2 -thiophenol)-2,2 -bipyridine. hydrogen evolution catalysts. (2″-thiophenol)-2,2′-bipyridine. Molbank 2022, 2022, M1355. https:// Molbank doi.or 2022 g/10.3390/M1355 , 2022, x. [NiFe]-hydrogenase Synthetic model complex https://doi.org/10.3390/xxxxx Academic Editor: Oleg A. Rakitin active site Academic Editor: Oleg A. Rakitin Received: 4 March 2022 Ph Accepted: 21 March 2022 S S CO Received: 4 March 2022 S Ph Published: 24 March 2022 CO Ni Accepted: 21 March 2022 Ni Fe Fe Publisher’s Note: MDPI stays neutral CN Published: 23 March 2022 S with regard to jurisdictional claims in CN published maps and institutional affil- Ph Publisher’s Note: MDPI stays neu- Ph X = O or OH iations. tral with regard to jurisdictional claims in published maps and institu- Scheme 1. A schematic representation of the active site of [NiFe]-hydrogenase (left) and the molecular Scheme 1. A schematic representation of the active site of [NiFe]-hydrogenase (left) and the molec- N2S2 II II tional affiliations. structure of a synthetic model (L Ni Fe , N2S2 right) of II [NiFe]-hydr II ogenase [9]. ular structure of a synthetic model (L Ni Fe , right) of [NiFe]-hydrogenase [9]. Copyright: © 2022 by the authors. Artero et al. recently reported a heterodinuclear Ni-Fe complex (Scheme 1), namely Licensee MDPI, Basel, Switzerland. N2S2 II II L Ni Fe , that models the active site of [NiFe]-hydrogenases and catalyzes electro- This article is an open access article Artero et al. recently reported a heterodinuclear Ni-Fe complex (Scheme 1), namely chemical H evolution [9–11]. This heterodinuclear complex was developed from the distributed under the terms and 2 N2S2 II II L Ni Fe , that models the active site of [NiFe]-hydrogenases and catalyzes electrochem- 0 0 Copyright: © 2022 by the authors. Li- mononuclear nickel complex with the bipyridine-bisthiolate ligand, 2,2 -(2,2 -bipyridine- conditions of the Creative Commons ical H2 evolution [9–11]. This heterodinuclear complex was developed from the mononu- 6,6 -diyl)bis(1,1-diphenylethanethiolate) [12,13]. Despite the successful preparation of censee Attribution MDPI, Basel, Swit (CC BY) licensezerl (https:// and. N2S2 II II clear nickel complex with the bipyridine-bisthiolate ligand, 2,2′-(2,2′-bipyridine-6,6′- creativecommons.org/licenses/by/ L Ni Fe as a unique and valuable model complex, artificial mimics for the active This article is an open access article 4.0/). site of [NiFe] hydrogenase, with various structural features, are still very rare. The key diyl)bis(1,1-diphenylethanethiolate) [12,13]. Despite the successful preparation of distributed under the terms and con- N2S2 II II L Ni Fe as a unique and valuable model complex, artificial mimics for the active site ditions of the Creative Commons At- of [NiFe] hydrogenase, with various structural features, are still very rare. The key chal- tribution (CC BY) license (https://cre- ativecomm Molbank ons.2022 org/license , 2022, M1355. s/by/4.0https://doi.or /). lenge g/10.3390/M1355 for reproducing the [NiFe]-hydrogenase https://www s active sit .mdpi.com/j e in a sy ournal/molbank nthetic system lies on the Molbank 2022, 2022, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/molbank Molbank 2022, 2022, x FOR PEER REVIEW 2 of 5 Molbank 2022 ass , 2022 embly o , M1355 f multiple thiolate binding sites within one organic ligand and, at the same 2 of 5 time, in a pre-organized manner. Here, we report the synthesis of a novel organic ligand platform with bisthiophenol chelating donors, which has potential as a chelator of Ni cat- challenge for reproducing the [NiFe]-hydrogenases active site in a synthetic system lies ions in the application of syntheses of model complexes for [NiFe]-hydrogenases’ active on the assembly of multiple thiolate binding sites within one organic ligand and, at the site. same time, in a pre-organized manner. Here, we report the synthesis of a novel organic ligand platform with bisthiophenol chelating donors, which has potential as a chelator of 2. Results and Discussion Ni cations in the application of syntheses of model complexes for [NiFe]-hydrogenases’ active site. We designed the compound 6,6′-di-(2″-thiophenol)-2,2′-bipyridine (2 in Scheme 2) by integrating the following two design features: (i) a rigid backbone that provides coordi- 2. Results and Discussion 0 00 0 nating sites and regulates the coordination configuration at certain extent; (ii) the availa- We designed the compound 6,6 -di-(2 -thiophenol)-2,2 -bipyridine (2 in Scheme 2) by integrating the following two design features: (i) a rigid backbone that provides coordinat- bility of multiple S donors that mimic the coordination environment around the active ing sites and regulates the coordination configuration at certain extent; (ii) the availability site of [NiFe]-hydrogenase (Scheme 1). To the best of our knowledge, 6,6′-di-(2″-thiophe- of multiple S donors that mimic the coordination environment around the active site of nol)-2,2′-bipyridine (2) is the first example of tetradentate ligands that contain both bispyr- 0 00 0 [NiFe]-hydrogenase (Scheme 1). To the best of our knowledge, 6,6 -di-(2 -thiophenol)-2,2 - idine and bisthiophenol chelating moieties. A literature survey returned one hit of com- bipyridine (2) is the first example of tetradentate ligands that contain both bispyridine and bisthiophenol chelating moieties. A literature survey returned one hit of compound 2 in a pound 2 in a patent without synthetic details [14] An analogue of 1 with phenanthroline patent without synthetic details [14] An analogue of 1 with phenanthroline backbone has backbone has been reported before [15]. been reported before [15]. SH N N N N NaH, DMF Pd(PPh ) ,K CO 3 4 2 3 S S SH HS Br Br 53% EtOH, Toluene 73% 1 2 0 00 0 Scheme 2. Synthesis of 6,6 -di-(2 -thiophenol)-2,2 -bipyridine. Scheme 2. Synthesis of 6,6′-di-(2″-thiophenol)-2,2′-bipyridine. The title compound (2) was synthesized in a two-step procedure (Scheme 2) from the 0 0 0 commercially available starting material, 6,6 -dibromo-2,2 -bipyridine. Compound 6,6 -di- The title compound (2) was synthesized in a two-step procedure (Scheme 2) from the 00 0 (2 -methylthiophenyl)-2,2 -bipyridine (1) was prepared under typical Suzuki–Miyaura commercially available starting material, 6,6′-dibromo-2,2′-bipyridine. Compound 6,6′-di- coupling conditions using Pd(PPh ) as the catalyst and potassium carbonate as a base. 3 4 (2″-methylthiophenyl)-2,2′-bipyridine (1) was prepared under typical Suzuki–Miyaura The reaction went well in anaerobic toluene and afforded compound 1 in a yield of coupling conditions using Pd(PPh3)4 as the catalyst and potassium carbonate as a base. 73%. Deprotection of the methyl groups was first performed with NaH and tert-nonyl mercaptan in DMF at 160 C [16]. The conventional heating condition, however, did not The reaction went well in anaerobic toluene and afforded compound 1 in a yield of 73%. effectively remove the thioether substituent. Given a relatively long reaction period, TLC Deprotection of the methyl groups was first performed with NaH and tert-nonyl mercap- analysis of the reaction product revealed a collection of compounds without distinctive tan in DMF at 160 °C [16]. The conventional heating condition, however, did not effec- indication for the formation of 2. The application of a microwave reactor, which allows tively remove the thioether substituent. Given a relatively long reaction period, TLC anal- elevation of the reaction temperature to 200 C, achieved the target compound 2 in a ysis of the reaction re produ asonablect y revea ield (53l% ed ). a collection of compounds without distinctive indi- 1 13 Compounds 1 and 2 were both characterized by H- and C-NMR spectroscopy. cation for the formation of 2. The application of a microwave reactor, which allows eleva- The proton NMR spectrum of 1 in CDCl shows the signal of methyl groups as a singlet tion of the reaction temperature to 200 °C, achieved the target compound 2 in a reasonable at 2.44 ppm with the integration of 6H (Figure S1). This characteristic methyl proton yield (53%). peak disappears in the H-NMR spectrum of 2. Instead, a singlet with the integration of 1 13 Compounds 1 and 2 were both characterized by H- and C-NMR spectroscopy. The 2H emerges at 4.57 ppm (Figure S3) and is assigned as the thiophenol protons. Elemental analysis was conducted to verify the purity of compounds 1 and 2. A high-resolution proton NMR spectrum of 1 in CDCl3 shows the signal of methyl groups as a singlet at 2.44 mass spectrometer was also employed to confirm the molecular formula of 1 (Figure S5). ppm with the integration of 6H (Figure S1). This characteristic methyl proton peak disap- Comparing the FT-IR spectra of 1 and 2 (Figure S6) reveals the SH stretching bands pears in the H-NMR spectrum of 2. Instead, a singlet with the integration of 2H emerges at 2506 and 2530 cm , which are close to the SH stretching band of thiophenol at 4.57 ppm (Figure S3) and is assigned as the thiophenol protons. Elemental analysis was (2545 cm ) [17]. The UV-Vis spectra of 1 and 2 were recorded in methanol, as displayed in Figure 1. conducted to verify the purity of compounds 1 and 2. A high-resolution mass spectrome- Both compounds show strong absorbance bands at l = 231 and 303 nm, which derive max ter was also employed to confirm the molecular formula of 1 (Figure S5). Comparing the from the p ! p* electron excitation at the pyridyl and phenyl moieties. The addition of one FT-IR spectra of 1 and 2 (Figure S6) reveals the S−H stretching bands at 2506 and 2530 equivalent nickel acetate in the methanol solution of 2 results in significant change of the −1 −1 cm , which are close to the S−H stretching band of thiophenol (2545 cm ) [17]. UV-Vis absorption profile: the emergence of absorbance bands at l = 263 and 291 nm. max The UV-Vis spectra of 1 and 2 were recorded in methanol, as displayed in Figure 1. Both compounds show strong absorbance bands at λmax = 231 and 303 nm, which derive from the π → π* electron excitation at the pyridyl and phenyl moieties. The addition of one equivalent nickel acetate in the methanol solution of 2 results in significant change of the UV-Vis absorption profile: the emergence of absorbance bands at λmax = 263 and 291 nm. The phenomena suggest coordination of Ni(II) ion by the tetradentate compound 2. Molbank 2022, 2022, x FOR PEER REVIEW 3 of 5 Molbank 2022, 2022, M1355 3 of 5 The phenomena suggest coordination of Ni(II) ion by the tetradentate compound 2. In In contrast, the UV-Vis spectrum of 1 is not affected by the presence of nickel ion, indicat- contrast, the UV-Vis spectrum of 1 is not affected by the presence of nickel ion, indicating ing weak or no interaction between the compound Ni(II) in solution. Synthesis and isola- weak or no interaction between the compound Ni(II) in solution. Synthesis and isolation of tion of 3d-transition metal complexes, particularly Ni and Fe complexes, with compound 3d-transition metal complexes, particularly Ni and Fe complexes, with compound 2 as a 2 as a ligand are being carried out. ligand are being carried out. (a) (b) Figure 1. UV-Vis spectra of 1 (a) and 2 (b) measured in methanol in the absence and presence of one Figure 1. UV-Vis spectra of 1 (a) and 2 (b) measured in methanol in the absence and presence of one equivalent of Ni(II) acetate. equivalent of Ni(II) acetate. 3. Materials and Methods 3. Materials and Methods All air- and moisture-sensitive experiments were performed under a dry argon at- All air- and moisture-sensitive experiments were performed under a dry argon at- mosphere using standard Schlenk techniques. Dry solvents for moisture-sensitive experi- mosphere using standard Schlenk techniques. Dry solvents for moisture-sensitive exper- ments were purchased from commercial sources (water content  10 ppm) and used as iments were purchased from commercial 0 sources (w 0 ater content ≤ 10 ppm) and used as received without further purification. 6,6 -dibromo-2,2 -bipyridine, 4,4,5,5-tetramethyl-2-(2- received without further purification. 6,6′-dibromo-2,2′-bipyridine, 4,4,5,5-tetramethyl-2- (methylthio)phenyl)-1,3,2-dioxaborolane, 2-methyloctane-2-thiol, and other chemicals for (2-(m syntheses ethylthi wer o)p e h commer enyl)-1, cially 3,2-d available ioxaborol and ane, 2-m used aset rh eceived. yloctane- Micr 2-t owave hiol, and syntheses other wer chem e icals carried out using an Anton-Parr Monowave 200 microwave reactor (Anton-Parr, Graz, Aus- for syntheses were commercially available and used as received. Microwave syntheses tria). Water for syntheses and analysis was purified by Milli-Q technique (18.2 MW, Merck, were carried out using an Anton-Parr Monowave 200 microwave reactor. Water for syn- Darmstadt, Germany). Thin Layer Chromatography analyses were performed on silica gel theses and analysis was purified by Milli-Q technique (18.2 MΩ). Thin Layer Chromatog- coated glass plates with fluorescence indicator UV254. Flash column chromatography was raphy analyses were performed on silica gel coated glass plates with fluorescence indica- conducted with silica gel at atmospheric pressure. tor UV254. Flash column chromatography was conducted with silica gel at atmospheric 1 13 H- and C-NMR spectra were recorded on a Bruker (Fällanden, Switzerland) Avance pressure. NEO (600 MHz) spectrometer, operating at a probe temperature of room temperature. 1 13 H- and C-NMR spectra were recorded on a Bruker (Fällanden, Switzerland) Chemical shifts, , are reported in ppm relative to the peak of SiMe , using H chemical shifts of the residual solvents as references [18]. Electronic absorption spectra were recorded Avance NEO (600 MHz) spectrometer, operating at a probe temperature of room temper- with a compact OTO Photonics (Hsinchu, Taiwan) UV-Vis spectrometer (SE2030-050-FUV). ature. Chemical shifts, δ, are reported in ppm relative to the peak of SiMe4, using H chem- High-resolution MS data were obtained using an Agilent (Santa Clara, CA, USA) 1260-6460 ical shifts of the residual solvents as references [18]. Electronic absorption spectra were Q-TOF mass spectrometer. FT-IR spectra were acquired using the TENSOR II + Hyperion recorded with a compact OTO Photonics (Taiwan) UV-Vis spectrometer (SE2030-050- 2000 spectroscopy (Bruker, Ettlingen, Germany). Elemental analysis (C N H S) was per- FUV). High-resolution MS data were obtained using an Agilent (Santa Clara, CA, United formed on Vario EL Cube (Elementar, Langenselbold, Germany). Stated) 1260-6460 Q-TOF mass spectrometer. FT-IR spectra were acquired using the TEN- 0 00 0 Synthesis of 6,6 -di-(2 -methylthiophenyl)-2,2 -bipyridine (1). SOR II + Hyperion 2000 spectroscopy (Bruker, Ettlingen, Germany). Elemental analysis (C 4,4,5,5-Tetramethyl-2-(2-(methylthio)phenyl)-1,3,2-dioxaborolane (1.0 g, 4.0 mmol) N H S) was performed on Vario EL Cube. (Elementar, Langenselbold, Germany) 0 0 was added to a solution of 6,6 -dibromo-2,2 -bipyridine (313 mg, 1.0 mmol) in a mixture Synthesis of 6,6′-di-(2″-methylthiophenyl)-2,2′-bipyridine (1). of toluene (7 mL) and EtOH (7 mL). After degassing by Ar, K CO (4.14 g, 30 mmol) and 2 3 4,4,5,5-tetramethyl-2-(2-(methylthio)phenyl)-1,3,2-dioxaborolane (1.0 g, 4.0 mmol) Pd(PPh ) (58 mg, 0.05 mmol) were added to this solution and the mixture was heated by a 3 4 was added to a solution of 6,6′-dibromo-2,2′-bipyridine (313 mg, 1.0 mmol) in a mixture microwave reactor to 170 C for 65 min under stirring. The solution was allowed to cool to of toluene (7 mL) and EtOH (7 mL). After degassing by Ar, K2CO3 (4.14 g, 30 mmol) and Pd(PPh3)4 (58 mg, 0.05 mmol) were added to this solution and the mixture was heated by a microwave reactor to 170 °C for 65 min under stirring. The solution was allowed to cool to room temperature and the volatile components were removed under vacuum. The res- idue was extracted with methylene chloride (50 mL × 3) three times. The combined organic layers were washed by saturated sodium chloride solution and then dried by anhydrous Molbank 2022, 2022, M1355 4 of 5 room temperature and the volatile components were removed under vacuum. The residue was extracted with methylene chloride (50 mL  3) three times. The combined organic layers were washed by saturated sodium chloride solution and then dried by anhydrous sodium sulfate. The solid salt was removed by filtration. Removal of the solvent under vacuum afforded compound 1 as an orange powder (292 mg. 73%). H-NMR (600 MHz, Chloroform-d) d 8.61 (dd, J = 7.9, 1.0 Hz, 2H), 7.90 (t, J = 7.8 Hz, 2H), 7.58 (ddd, J = 10.6, 7.5, 1.2 Hz, 4H), 7.43–7.38 (m, 4H), 7.29–7.26 (m, 2H), 2.44 (s, 6H). C-NMR (151 MHz, Chloroform-d) d 157.59, 155.43, 138.23, 137.45, 130.10, 129.02, 126.29, 124.88, 123.77, 120.11, 77.16, 16.92. ESI-HRMS: m/e calcd for C H N S (M + H) 401.1146, found 401.1146. Mp: 24 21 2 2 215–218 C. Anal. Calcd. for 1 (C H N S ): C, 71.97; H, 5.03; N, 6.99; S, 16.01. Found: C, 24 20 2 2 71.65; H, 5.05; N, 6.73; S, 15.77. 0 00 0 Synthesis of 6,6 -di-(2 -thiophenol)-2,2 -bipyridine (2). 2-Methyloctane-2-thiol (640 mg, 4.0 mmol) was added to a solution of NaH (96 mg, 4.0 mmol) in anhydrous DMF (13 mL). Compound 1 (200 mg, 0.5 mmol) was added to this solution and the mixture was stirred under an Ar atmosphere for about 10 min, until the gas bubbling ceased. The mixture was then transferred to a glass tube (designed for microwave reaction) and heated by a microwave reactor to 200 C for 75 min under stirring. The solution was allowed to cool to room temperature, and then diluted hydrochloric acid (25 mL) was slowly dropped into it. The orange precipitate was collected by filtration and purified by column chromatography over silica using CH Cl 2 2 as an eluent. The pure product was obtained as an orange powder (98 mg, 53%). H- NMR (600 MHz, Chloroform-d) d 8.64 (dd, J = 7.8, 1.0 Hz, 2H), 7.95 (t, J = 7.8 Hz, 2H), 7.64–7.58 (m, 4H), 7.48–7.43 (m, 2H), 7.29–7.26 (m, 4H), 4.57 (s, 2H). C-NMR (151 MHz, Chloroform-d) d 138.18, 137.92, 132.21, 131.37, 130.41, 129.01, 125.73, 123.55, 120.03, 77.16. Mp: 179–182 C. Anal. Calcd. for 2 (C H N S ): C, 70.94; H, 4.33; N, 7.52; S, 17.21. 22 16 2 2 Anal. Calcd. for 20.6H O (C H N O S ): C, 68.94; H, 4.52; N, 7.31; S, 16.73. Found: 2 22 17.2 2 0.6 2 C, 68.64; H, 4.35; N, 6.87; S, 16.87. 4. Conclusions 00 0 0 00 Compounds di-(2 -methylthiophenyl)-2,2 -bipyridine (1) and 6,6 -di-(2 -thiophenol)- 2,2 -bipyridine (2) have been prepared and characterized. The deprotection of methyl groups of 1 with tert-nonyl mercaptan was achieved in DMF using a microwave reactor at 200 C. The thiophenol bipyridine compound 2 might be used as a chelator for the Ni cation. Supplementary Materials: The following supporting information can be downloaded: NMR spectra 1 13 1 and HRMS analysis. Figure S1. H-NMR spectrum of compound 1 in CDCl . Figure S2. C{ H} NMR spectrum of compound 1 in CDCl . Figure S3. H-NMR spectrum of compound 2 in CDCl . Figure S4. 3 3 13 1 C{ H} NMR spectrum of compound 2 in CDCl . Figure S5. HRMS spectrum of compound 1. Figure S6. FT-IR spectra of compounds 1 (blue) and 2 (red) as KBr pellets. Author Contributions: Y.H. and L.T. conceived and designed the synthetic route, analyzed the data, and drafted the manuscript. Y.H. conducted the experiments of syntheses and characterization. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Guangzhou University under the funding number RQ2020042. Data Availability Statement: The data are reported in the manuscript and Supplementary Materials. Conflicts of Interest: The authors declare no conflict of interest. References 1. Jones, A.K.; Sillery, E.; Albracht, S.P.; Armstrong, F.A. 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6,6′-Di-(2″-thiophenol)-2,2′-bipyridine

Molbank , Volume 2022 (2) – Mar 24, 2022

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1422-8599
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Short Note 6,6′-Di-(2″-thiophenol)-2,2′-bipyridine molbank Yan Huang and Lianpeng Tong * Short Note School of Chemistry and Chemical Engineering, Guangzhou University, No. 230 Wai Huan Xi Road, Higher 0 00 0 Education Mega Center, Guangzhou 510006, China; 2111905012@e.gzhu.edu.cn 6,6 -Di-(2 -thiophenol)-2,2 -bipyridine * Correspondence: ltong@gzhu.edu.cn Yan Huang and Lianpeng Tong * Abstract: This short note describes the synthesis of compound 6,6′-di-(2″-thiophenol)-2,2′-bipyri- dine from its methyl phenyl sulfane precursor via deprotection of the methyl groups. The product School of Chemistry and Chemical Engineering, Guangzhou University, No. 230 Wai Huan Xi Road, Higher Education Mega Center, Guangzhou 510006, China; 2111905012@e.gzhu.edu.cn as well as the intermediate in the synthetic route have been characterized by UV-Vis spectroscopy, * Correspondence: ltong@gzhu.edu.cn 1 13 H- and C-NMR spectroscopy, FT-IR spectroscopy, and HR-MS analysis. This work presents a rare example of tetradentate chelators that bears pyridy 0 l back 00 bones and thiophenol dono 0 rs for the Abstract: This short note describes the synthesis of compound 6,6 -di-(2 -thiophenol)-2,2 -bipyridine fr coordination om its methyl with phenyl3d sulfane -transition metal cations. precursor via deprotection of the methyl groups. The product as well as the intermediate in the synthetic route have been characterized by UV-Vis spectroscopy, H- and C-NMR spectroscopy, FT-IR spectroscopy, and HR-MS analysis. This work presents a rare example Keywords: thiophenol; bipyridine; tetradentate ligand; hydrogenase; transition metal; coordination of tetradentate chelators that bears pyridyl backbones and thiophenol donors for the coordination with 3d-transition metal cations. Keywords: thiophenol; bipyridine; tetradentate ligand; hydrogenase; transition metal; coordination 1. Introduction [NiFe]-hydrogenases in nature have the ability of catalyzing the protons to hydrogen (H2) reduction reaction at high rates with a small barrier of activation energy [1,2]. The 1. Introduction active site of [NiFe]-hydrogenases is a bimetallic Ni-Fe cluster, of which the Ni and Fe [NiFe]-hydrogenases in nature have the ability of catalyzing the protons to hydrogen (H ) reduction reaction at high rates with a small barrier of activation energy [1,2]. The me 2 tal centers are bridged by two cysteine residue thiolates (Scheme 1) [3–5]. Syntheses of active site of [NiFe]-hydrogenases is a bimetallic Ni-Fe cluster, of which the Ni and Fe metal metal complexes that mimic the structure and function of the active site of [NiFe]-hydro- centers are bridged by two cysteine residue thiolates (Scheme 1) [3–5]. Syntheses of metal genases have long been an important field of bioinorganic chemistry [6,7], and draw even complexes that mimic the structure and function of the active site of [NiFe]-hydrogenases more attention these days in the context of the development of a hydrogen economy [8]. have long been an important field of bioinorganic chemistry [6,7], and draw even more attention The biomime these days tic Ni inand Fe the context model complexes ca of the developmentn he of a hydr lp us to under ogen economy stand the cat [8]. The alytic mech- biomimetic Ni and Fe model complexes can help us to understand the catalytic mechanism anism of hydrogenases, whilst also inspiring the design of transition metal-based hetero- Citation: Huang, Y.; Tong, L. 6,6 -Di- of hydrogenases, whilst also inspiring the design of transition metal-based heterogeneous Citation: Huang, Y.; Tong, L. 6,6′-Di- geneous hydrogen evolution catalysts. 00 0 (2 -thiophenol)-2,2 -bipyridine. hydrogen evolution catalysts. (2″-thiophenol)-2,2′-bipyridine. Molbank 2022, 2022, M1355. https:// Molbank doi.or 2022 g/10.3390/M1355 , 2022, x. [NiFe]-hydrogenase Synthetic model complex https://doi.org/10.3390/xxxxx Academic Editor: Oleg A. Rakitin active site Academic Editor: Oleg A. Rakitin Received: 4 March 2022 Ph Accepted: 21 March 2022 S S CO Received: 4 March 2022 S Ph Published: 24 March 2022 CO Ni Accepted: 21 March 2022 Ni Fe Fe Publisher’s Note: MDPI stays neutral CN Published: 23 March 2022 S with regard to jurisdictional claims in CN published maps and institutional affil- Ph Publisher’s Note: MDPI stays neu- Ph X = O or OH iations. tral with regard to jurisdictional claims in published maps and institu- Scheme 1. A schematic representation of the active site of [NiFe]-hydrogenase (left) and the molecular Scheme 1. A schematic representation of the active site of [NiFe]-hydrogenase (left) and the molec- N2S2 II II tional affiliations. structure of a synthetic model (L Ni Fe , N2S2 right) of II [NiFe]-hydr II ogenase [9]. ular structure of a synthetic model (L Ni Fe , right) of [NiFe]-hydrogenase [9]. Copyright: © 2022 by the authors. Artero et al. recently reported a heterodinuclear Ni-Fe complex (Scheme 1), namely Licensee MDPI, Basel, Switzerland. N2S2 II II L Ni Fe , that models the active site of [NiFe]-hydrogenases and catalyzes electro- This article is an open access article Artero et al. recently reported a heterodinuclear Ni-Fe complex (Scheme 1), namely chemical H evolution [9–11]. This heterodinuclear complex was developed from the distributed under the terms and 2 N2S2 II II L Ni Fe , that models the active site of [NiFe]-hydrogenases and catalyzes electrochem- 0 0 Copyright: © 2022 by the authors. Li- mononuclear nickel complex with the bipyridine-bisthiolate ligand, 2,2 -(2,2 -bipyridine- conditions of the Creative Commons ical H2 evolution [9–11]. This heterodinuclear complex was developed from the mononu- 6,6 -diyl)bis(1,1-diphenylethanethiolate) [12,13]. Despite the successful preparation of censee Attribution MDPI, Basel, Swit (CC BY) licensezerl (https:// and. N2S2 II II clear nickel complex with the bipyridine-bisthiolate ligand, 2,2′-(2,2′-bipyridine-6,6′- creativecommons.org/licenses/by/ L Ni Fe as a unique and valuable model complex, artificial mimics for the active This article is an open access article 4.0/). site of [NiFe] hydrogenase, with various structural features, are still very rare. The key diyl)bis(1,1-diphenylethanethiolate) [12,13]. Despite the successful preparation of distributed under the terms and con- N2S2 II II L Ni Fe as a unique and valuable model complex, artificial mimics for the active site ditions of the Creative Commons At- of [NiFe] hydrogenase, with various structural features, are still very rare. The key chal- tribution (CC BY) license (https://cre- ativecomm Molbank ons.2022 org/license , 2022, M1355. s/by/4.0https://doi.or /). lenge g/10.3390/M1355 for reproducing the [NiFe]-hydrogenase https://www s active sit .mdpi.com/j e in a sy ournal/molbank nthetic system lies on the Molbank 2022, 2022, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/molbank Molbank 2022, 2022, x FOR PEER REVIEW 2 of 5 Molbank 2022 ass , 2022 embly o , M1355 f multiple thiolate binding sites within one organic ligand and, at the same 2 of 5 time, in a pre-organized manner. Here, we report the synthesis of a novel organic ligand platform with bisthiophenol chelating donors, which has potential as a chelator of Ni cat- challenge for reproducing the [NiFe]-hydrogenases active site in a synthetic system lies ions in the application of syntheses of model complexes for [NiFe]-hydrogenases’ active on the assembly of multiple thiolate binding sites within one organic ligand and, at the site. same time, in a pre-organized manner. Here, we report the synthesis of a novel organic ligand platform with bisthiophenol chelating donors, which has potential as a chelator of 2. Results and Discussion Ni cations in the application of syntheses of model complexes for [NiFe]-hydrogenases’ active site. We designed the compound 6,6′-di-(2″-thiophenol)-2,2′-bipyridine (2 in Scheme 2) by integrating the following two design features: (i) a rigid backbone that provides coordi- 2. Results and Discussion 0 00 0 nating sites and regulates the coordination configuration at certain extent; (ii) the availa- We designed the compound 6,6 -di-(2 -thiophenol)-2,2 -bipyridine (2 in Scheme 2) by integrating the following two design features: (i) a rigid backbone that provides coordinat- bility of multiple S donors that mimic the coordination environment around the active ing sites and regulates the coordination configuration at certain extent; (ii) the availability site of [NiFe]-hydrogenase (Scheme 1). To the best of our knowledge, 6,6′-di-(2″-thiophe- of multiple S donors that mimic the coordination environment around the active site of nol)-2,2′-bipyridine (2) is the first example of tetradentate ligands that contain both bispyr- 0 00 0 [NiFe]-hydrogenase (Scheme 1). To the best of our knowledge, 6,6 -di-(2 -thiophenol)-2,2 - idine and bisthiophenol chelating moieties. A literature survey returned one hit of com- bipyridine (2) is the first example of tetradentate ligands that contain both bispyridine and bisthiophenol chelating moieties. A literature survey returned one hit of compound 2 in a pound 2 in a patent without synthetic details [14] An analogue of 1 with phenanthroline patent without synthetic details [14] An analogue of 1 with phenanthroline backbone has backbone has been reported before [15]. been reported before [15]. SH N N N N NaH, DMF Pd(PPh ) ,K CO 3 4 2 3 S S SH HS Br Br 53% EtOH, Toluene 73% 1 2 0 00 0 Scheme 2. Synthesis of 6,6 -di-(2 -thiophenol)-2,2 -bipyridine. Scheme 2. Synthesis of 6,6′-di-(2″-thiophenol)-2,2′-bipyridine. The title compound (2) was synthesized in a two-step procedure (Scheme 2) from the 0 0 0 commercially available starting material, 6,6 -dibromo-2,2 -bipyridine. Compound 6,6 -di- The title compound (2) was synthesized in a two-step procedure (Scheme 2) from the 00 0 (2 -methylthiophenyl)-2,2 -bipyridine (1) was prepared under typical Suzuki–Miyaura commercially available starting material, 6,6′-dibromo-2,2′-bipyridine. Compound 6,6′-di- coupling conditions using Pd(PPh ) as the catalyst and potassium carbonate as a base. 3 4 (2″-methylthiophenyl)-2,2′-bipyridine (1) was prepared under typical Suzuki–Miyaura The reaction went well in anaerobic toluene and afforded compound 1 in a yield of coupling conditions using Pd(PPh3)4 as the catalyst and potassium carbonate as a base. 73%. Deprotection of the methyl groups was first performed with NaH and tert-nonyl mercaptan in DMF at 160 C [16]. The conventional heating condition, however, did not The reaction went well in anaerobic toluene and afforded compound 1 in a yield of 73%. effectively remove the thioether substituent. Given a relatively long reaction period, TLC Deprotection of the methyl groups was first performed with NaH and tert-nonyl mercap- analysis of the reaction product revealed a collection of compounds without distinctive tan in DMF at 160 °C [16]. The conventional heating condition, however, did not effec- indication for the formation of 2. The application of a microwave reactor, which allows tively remove the thioether substituent. Given a relatively long reaction period, TLC anal- elevation of the reaction temperature to 200 C, achieved the target compound 2 in a ysis of the reaction re produ asonablect y revea ield (53l% ed ). a collection of compounds without distinctive indi- 1 13 Compounds 1 and 2 were both characterized by H- and C-NMR spectroscopy. cation for the formation of 2. The application of a microwave reactor, which allows eleva- The proton NMR spectrum of 1 in CDCl shows the signal of methyl groups as a singlet tion of the reaction temperature to 200 °C, achieved the target compound 2 in a reasonable at 2.44 ppm with the integration of 6H (Figure S1). This characteristic methyl proton yield (53%). peak disappears in the H-NMR spectrum of 2. Instead, a singlet with the integration of 1 13 Compounds 1 and 2 were both characterized by H- and C-NMR spectroscopy. The 2H emerges at 4.57 ppm (Figure S3) and is assigned as the thiophenol protons. Elemental analysis was conducted to verify the purity of compounds 1 and 2. A high-resolution proton NMR spectrum of 1 in CDCl3 shows the signal of methyl groups as a singlet at 2.44 mass spectrometer was also employed to confirm the molecular formula of 1 (Figure S5). ppm with the integration of 6H (Figure S1). This characteristic methyl proton peak disap- Comparing the FT-IR spectra of 1 and 2 (Figure S6) reveals the SH stretching bands pears in the H-NMR spectrum of 2. Instead, a singlet with the integration of 2H emerges at 2506 and 2530 cm , which are close to the SH stretching band of thiophenol at 4.57 ppm (Figure S3) and is assigned as the thiophenol protons. Elemental analysis was (2545 cm ) [17]. The UV-Vis spectra of 1 and 2 were recorded in methanol, as displayed in Figure 1. conducted to verify the purity of compounds 1 and 2. A high-resolution mass spectrome- Both compounds show strong absorbance bands at l = 231 and 303 nm, which derive max ter was also employed to confirm the molecular formula of 1 (Figure S5). Comparing the from the p ! p* electron excitation at the pyridyl and phenyl moieties. The addition of one FT-IR spectra of 1 and 2 (Figure S6) reveals the S−H stretching bands at 2506 and 2530 equivalent nickel acetate in the methanol solution of 2 results in significant change of the −1 −1 cm , which are close to the S−H stretching band of thiophenol (2545 cm ) [17]. UV-Vis absorption profile: the emergence of absorbance bands at l = 263 and 291 nm. max The UV-Vis spectra of 1 and 2 were recorded in methanol, as displayed in Figure 1. Both compounds show strong absorbance bands at λmax = 231 and 303 nm, which derive from the π → π* electron excitation at the pyridyl and phenyl moieties. The addition of one equivalent nickel acetate in the methanol solution of 2 results in significant change of the UV-Vis absorption profile: the emergence of absorbance bands at λmax = 263 and 291 nm. The phenomena suggest coordination of Ni(II) ion by the tetradentate compound 2. Molbank 2022, 2022, x FOR PEER REVIEW 3 of 5 Molbank 2022, 2022, M1355 3 of 5 The phenomena suggest coordination of Ni(II) ion by the tetradentate compound 2. In In contrast, the UV-Vis spectrum of 1 is not affected by the presence of nickel ion, indicat- contrast, the UV-Vis spectrum of 1 is not affected by the presence of nickel ion, indicating ing weak or no interaction between the compound Ni(II) in solution. Synthesis and isola- weak or no interaction between the compound Ni(II) in solution. Synthesis and isolation of tion of 3d-transition metal complexes, particularly Ni and Fe complexes, with compound 3d-transition metal complexes, particularly Ni and Fe complexes, with compound 2 as a 2 as a ligand are being carried out. ligand are being carried out. (a) (b) Figure 1. UV-Vis spectra of 1 (a) and 2 (b) measured in methanol in the absence and presence of one Figure 1. UV-Vis spectra of 1 (a) and 2 (b) measured in methanol in the absence and presence of one equivalent of Ni(II) acetate. equivalent of Ni(II) acetate. 3. Materials and Methods 3. Materials and Methods All air- and moisture-sensitive experiments were performed under a dry argon at- All air- and moisture-sensitive experiments were performed under a dry argon at- mosphere using standard Schlenk techniques. Dry solvents for moisture-sensitive experi- mosphere using standard Schlenk techniques. Dry solvents for moisture-sensitive exper- ments were purchased from commercial sources (water content  10 ppm) and used as iments were purchased from commercial 0 sources (w 0 ater content ≤ 10 ppm) and used as received without further purification. 6,6 -dibromo-2,2 -bipyridine, 4,4,5,5-tetramethyl-2-(2- received without further purification. 6,6′-dibromo-2,2′-bipyridine, 4,4,5,5-tetramethyl-2- (methylthio)phenyl)-1,3,2-dioxaborolane, 2-methyloctane-2-thiol, and other chemicals for (2-(m syntheses ethylthi wer o)p e h commer enyl)-1, cially 3,2-d available ioxaborol and ane, 2-m used aset rh eceived. yloctane- Micr 2-t owave hiol, and syntheses other wer chem e icals carried out using an Anton-Parr Monowave 200 microwave reactor (Anton-Parr, Graz, Aus- for syntheses were commercially available and used as received. Microwave syntheses tria). Water for syntheses and analysis was purified by Milli-Q technique (18.2 MW, Merck, were carried out using an Anton-Parr Monowave 200 microwave reactor. Water for syn- Darmstadt, Germany). Thin Layer Chromatography analyses were performed on silica gel theses and analysis was purified by Milli-Q technique (18.2 MΩ). Thin Layer Chromatog- coated glass plates with fluorescence indicator UV254. Flash column chromatography was raphy analyses were performed on silica gel coated glass plates with fluorescence indica- conducted with silica gel at atmospheric pressure. tor UV254. Flash column chromatography was conducted with silica gel at atmospheric 1 13 H- and C-NMR spectra were recorded on a Bruker (Fällanden, Switzerland) Avance pressure. NEO (600 MHz) spectrometer, operating at a probe temperature of room temperature. 1 13 H- and C-NMR spectra were recorded on a Bruker (Fällanden, Switzerland) Chemical shifts, , are reported in ppm relative to the peak of SiMe , using H chemical shifts of the residual solvents as references [18]. Electronic absorption spectra were recorded Avance NEO (600 MHz) spectrometer, operating at a probe temperature of room temper- with a compact OTO Photonics (Hsinchu, Taiwan) UV-Vis spectrometer (SE2030-050-FUV). ature. Chemical shifts, δ, are reported in ppm relative to the peak of SiMe4, using H chem- High-resolution MS data were obtained using an Agilent (Santa Clara, CA, USA) 1260-6460 ical shifts of the residual solvents as references [18]. Electronic absorption spectra were Q-TOF mass spectrometer. FT-IR spectra were acquired using the TENSOR II + Hyperion recorded with a compact OTO Photonics (Taiwan) UV-Vis spectrometer (SE2030-050- 2000 spectroscopy (Bruker, Ettlingen, Germany). Elemental analysis (C N H S) was per- FUV). High-resolution MS data were obtained using an Agilent (Santa Clara, CA, United formed on Vario EL Cube (Elementar, Langenselbold, Germany). Stated) 1260-6460 Q-TOF mass spectrometer. FT-IR spectra were acquired using the TEN- 0 00 0 Synthesis of 6,6 -di-(2 -methylthiophenyl)-2,2 -bipyridine (1). SOR II + Hyperion 2000 spectroscopy (Bruker, Ettlingen, Germany). Elemental analysis (C 4,4,5,5-Tetramethyl-2-(2-(methylthio)phenyl)-1,3,2-dioxaborolane (1.0 g, 4.0 mmol) N H S) was performed on Vario EL Cube. (Elementar, Langenselbold, Germany) 0 0 was added to a solution of 6,6 -dibromo-2,2 -bipyridine (313 mg, 1.0 mmol) in a mixture Synthesis of 6,6′-di-(2″-methylthiophenyl)-2,2′-bipyridine (1). of toluene (7 mL) and EtOH (7 mL). After degassing by Ar, K CO (4.14 g, 30 mmol) and 2 3 4,4,5,5-tetramethyl-2-(2-(methylthio)phenyl)-1,3,2-dioxaborolane (1.0 g, 4.0 mmol) Pd(PPh ) (58 mg, 0.05 mmol) were added to this solution and the mixture was heated by a 3 4 was added to a solution of 6,6′-dibromo-2,2′-bipyridine (313 mg, 1.0 mmol) in a mixture microwave reactor to 170 C for 65 min under stirring. The solution was allowed to cool to of toluene (7 mL) and EtOH (7 mL). After degassing by Ar, K2CO3 (4.14 g, 30 mmol) and Pd(PPh3)4 (58 mg, 0.05 mmol) were added to this solution and the mixture was heated by a microwave reactor to 170 °C for 65 min under stirring. The solution was allowed to cool to room temperature and the volatile components were removed under vacuum. The res- idue was extracted with methylene chloride (50 mL × 3) three times. The combined organic layers were washed by saturated sodium chloride solution and then dried by anhydrous Molbank 2022, 2022, M1355 4 of 5 room temperature and the volatile components were removed under vacuum. The residue was extracted with methylene chloride (50 mL  3) three times. The combined organic layers were washed by saturated sodium chloride solution and then dried by anhydrous sodium sulfate. The solid salt was removed by filtration. Removal of the solvent under vacuum afforded compound 1 as an orange powder (292 mg. 73%). H-NMR (600 MHz, Chloroform-d) d 8.61 (dd, J = 7.9, 1.0 Hz, 2H), 7.90 (t, J = 7.8 Hz, 2H), 7.58 (ddd, J = 10.6, 7.5, 1.2 Hz, 4H), 7.43–7.38 (m, 4H), 7.29–7.26 (m, 2H), 2.44 (s, 6H). C-NMR (151 MHz, Chloroform-d) d 157.59, 155.43, 138.23, 137.45, 130.10, 129.02, 126.29, 124.88, 123.77, 120.11, 77.16, 16.92. ESI-HRMS: m/e calcd for C H N S (M + H) 401.1146, found 401.1146. Mp: 24 21 2 2 215–218 C. Anal. Calcd. for 1 (C H N S ): C, 71.97; H, 5.03; N, 6.99; S, 16.01. Found: C, 24 20 2 2 71.65; H, 5.05; N, 6.73; S, 15.77. 0 00 0 Synthesis of 6,6 -di-(2 -thiophenol)-2,2 -bipyridine (2). 2-Methyloctane-2-thiol (640 mg, 4.0 mmol) was added to a solution of NaH (96 mg, 4.0 mmol) in anhydrous DMF (13 mL). Compound 1 (200 mg, 0.5 mmol) was added to this solution and the mixture was stirred under an Ar atmosphere for about 10 min, until the gas bubbling ceased. The mixture was then transferred to a glass tube (designed for microwave reaction) and heated by a microwave reactor to 200 C for 75 min under stirring. The solution was allowed to cool to room temperature, and then diluted hydrochloric acid (25 mL) was slowly dropped into it. The orange precipitate was collected by filtration and purified by column chromatography over silica using CH Cl 2 2 as an eluent. The pure product was obtained as an orange powder (98 mg, 53%). H- NMR (600 MHz, Chloroform-d) d 8.64 (dd, J = 7.8, 1.0 Hz, 2H), 7.95 (t, J = 7.8 Hz, 2H), 7.64–7.58 (m, 4H), 7.48–7.43 (m, 2H), 7.29–7.26 (m, 4H), 4.57 (s, 2H). C-NMR (151 MHz, Chloroform-d) d 138.18, 137.92, 132.21, 131.37, 130.41, 129.01, 125.73, 123.55, 120.03, 77.16. Mp: 179–182 C. Anal. Calcd. for 2 (C H N S ): C, 70.94; H, 4.33; N, 7.52; S, 17.21. 22 16 2 2 Anal. Calcd. for 20.6H O (C H N O S ): C, 68.94; H, 4.52; N, 7.31; S, 16.73. Found: 2 22 17.2 2 0.6 2 C, 68.64; H, 4.35; N, 6.87; S, 16.87. 4. Conclusions 00 0 0 00 Compounds di-(2 -methylthiophenyl)-2,2 -bipyridine (1) and 6,6 -di-(2 -thiophenol)- 2,2 -bipyridine (2) have been prepared and characterized. The deprotection of methyl groups of 1 with tert-nonyl mercaptan was achieved in DMF using a microwave reactor at 200 C. The thiophenol bipyridine compound 2 might be used as a chelator for the Ni cation. Supplementary Materials: The following supporting information can be downloaded: NMR spectra 1 13 1 and HRMS analysis. Figure S1. H-NMR spectrum of compound 1 in CDCl . Figure S2. C{ H} NMR spectrum of compound 1 in CDCl . Figure S3. H-NMR spectrum of compound 2 in CDCl . Figure S4. 3 3 13 1 C{ H} NMR spectrum of compound 2 in CDCl . Figure S5. HRMS spectrum of compound 1. Figure S6. FT-IR spectra of compounds 1 (blue) and 2 (red) as KBr pellets. Author Contributions: Y.H. and L.T. conceived and designed the synthetic route, analyzed the data, and drafted the manuscript. Y.H. conducted the experiments of syntheses and characterization. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Guangzhou University under the funding number RQ2020042. Data Availability Statement: The data are reported in the manuscript and Supplementary Materials. Conflicts of Interest: The authors declare no conflict of interest. References 1. Jones, A.K.; Sillery, E.; Albracht, S.P.; Armstrong, F.A. 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Journal

MolbankMultidisciplinary Digital Publishing Institute

Published: Mar 24, 2022

Keywords: thiophenol; bipyridine; tetradentate ligand; hydrogenase; transition metal; coordination

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