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Syntheses, structures and photocatalytic properties of ruthenium complexes bearing L-methionine ligands
Journal of Organometallic Chemistry 907 (2020) 121078
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Syntheses, structures and photocatalytic properties of ruthenium complexes bearing L-methionine ligands Jiao Ji, Chong Chen, Ai-Quan Jia, Hua-Tian Shi, Qian-Feng Zhang* Institute of Molecular Engineering and Applied Chemistry, Anhui University of Technology, Ma’anshan, Anhui, 243002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 August 2019 Received in revised form 23 October 2019 Accepted 14 December 2019 Available online 17 December 2019
Treatment of [(h6-C6H6)RuCl2]2 or [(h6-p-cymene)RuCl2]2 with two equiv. of L-methionine in the presence of AgNO3 and NaOMe gave mononuclear ruthenium(II) complexes [(h6-C6H6)Ru(L-met)](NO3) (1) and [(h6-cymene)Ru(L-met)](NO3) (2), respectively. Interactions between equal equiv. of L-methionine and [Ru(CO)2Cl2]n, [Ru(bpy)2Cl2]$2H2O (bpy ¼ 2,20 -bipyridine), [Ru(NO)(PPh3)2Cl3], (Et4N) [Ru(CH3CN)2Cl4] or cis-[Ru(DMSO)4Cl2] (DMSO ¼ dimethylsulfoxide) resulted in isolation of the expected ruthenium complexes [(CO)2Ru(L-met)Cl] (3), [(bpy)2Ru(L-met)]Cl (4), [Ru(L-met)(NO)(PPh3)Cl]Cl (5), [Ru(L-met)(CH3CN)Cl2] (6) and [Ru(L-met)(DMSO)2Cl] (7), respectively. Reaction of [Ru(COD)Cl2]2 (COD ¼ 1,5-cyclooctadiene) with L-methionine gave a dinuclear ruthenium complex [(COD)Ru(m-S-Lmet)(m-Cl)RuCl(L-metNMe2)] (8). Complexes 18 were characterized by microanalyses, IR, NMR and UV evis spectroscopies. The structures of complexes 1, 2, 4 and 8 have been unambiguously established by single-crystal X-ray crystallography. The photocatalytic properties for hydrogen production by water splitting of complexes 1, 2, 4 and 8 under visible light (l > 420 nm) were also investigated. © 2019 Published by Elsevier B.V.
Keywords: Ruthenium L-Methionine Synthesis Photocatalyst Crystal structure
1. Introduction Methionine is an essential amino acid that plays fundamental roles in protein synthesis and a number of other biochemical and cellular processes [1,2]. L-Methionine has obtained great interest possibly due to the strongly inhibitory effects on proliferation of cancer cells and the possibility of developing methionine analogs as new anticancer therapeutic agents [3]. Transition metal complexes bearing L-methionine ligands have proven to exhibit interesting properties in very different fields such as asymmetric synthesis in organic media and in an aqueous solution, cytotoxicity, sol-gelderived coatings, and bio-organometallic and solid-state chemistry [4e6]. As a matter of the fact, several transition metal such as copper(II), platinum(II), silver(I), cobalt(III) and rhenium(V) complexes with L-methionine ligands have been synthesized and structurally characterized [7e14]. For instance, reaction of Ag2O with the D/L-methionine led to dinuclear silver(I) complex {[Ag2(Dmet)(L-met)]}n, in which the D/L-methionine ligand has been coordinated to silver(I) in a distorted tetrahedral geometry [10]. It has been noted that the X-ray crystal structures of ruthenium complexes containing L-methionine and different co-ligands, such as p-
cymene, benzene, norbornadiene, chloride and nitrosyl, were established, generally, the central ruthenium atom was coordinated by L-methionine in a k3-S,N,O-mode by the nitrogen atom, the sulfur atom and carboxylate oxygen donor [15e19]. Hydrogen has been regarded as an ideal energy carrier in future because of its high energy capacity and zero emission of environment pollutants [20]. Analogous ruthenium complexes with pyridyl ligands on the photocatalytic H2 evolution over TiO2 nanoparticles have been reported [20,21]. Hydrogen production by water splitting using single ruthenium complexes as photocatalysts is relatively rarely investigated. Previously, we have reported syntheses and reactions of a series of neutral and anionic ruthenium(II)/(III) complexes with N,O,S-donor ligands and their electrochemical properties [22]. To further understand the ruthenium-sulfur complexes in the electron-rich coordination environment, herein we report syntheses and structures of the related ruthenium complexes bearing L-methionine ligands, the photocatalytic properties of ruthenium complexes using methanol as electron donors are initially investigated in this paper. 2. Experimental section 2.1. General considerations
* Corresponding author. E-mail address: [email protected] (Q.-F. Zhang). https://doi.org/10.1016/j.jorganchem.2019.121078 0022-328X/© 2019 Published by Elsevier B.V.
All manipulations were carried out under nitrogen by standard
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Schlenk techniques. Solvents were purified, distilled and degassed prior to use. [(h6-C6H6)RuCl2]2 [23], [(h6-p-cymene)RuCl2]2 [24], [Ru(CO)2Cl2]n [25], [Ru(bpy)2Cl2]$2H2O (bpy ¼ 2,20 -bipyridine) [26], cis-[Ru(DMSO)4Cl2] (DMSO ¼ dimethylsulfoxide) [27], [Ru(COD)Cl2]2 (COD ¼ 1,5-cyclooctadiene) [28], and [Ru(NO)(PPh3)2Cl3] [29] and (Et4N)[Ru(CH3CN)2Cl4] [30] were prepared according to the literature methods. L-metionine (L-Hmet), silver nitrate, and sodium methoxide were purchased from Alfa Aesar Ltd. and used as received. NMR spectra were recorded on a BrukerALX400 spectrometer operating at 400 and 162 MHz for 1H and 31P, respectively. Chemical shifts (d, ppm) were reported with reference to SiMe4 (1H) and 85% H3PO4 (31P). Infrared spectra were recorded on a Perkin-Elmer 16 PC FT-IR spectrophotometer and positive FAB mass spectra were recorded on a Finnigan TSQ 7000 spectrometer. Elemental analyses were carried out using a Perkin-Elmer 2400 CHN analyzer. Electronic absorption spectra were obtained on a Shimadzu UV-2600 spectrophotometer. Water splitting was performed with a MC-SPH2O-AG photocatalysis analyzer. 2.2. Synthesis of [(h6-C6H6)Ru(L-met)](NO3) (1) AgNO3 (68 mg, 0.4 mmol) was added to a solution of [(h6-C6H6) RuCl2]2 (50 mg, 0.1 mmol) in dry methanol (10 mL) giving an orange-red solution and precipitation of AgCl. The mixture was filtered to remove AgCl. The orange-red solution and L-methionine (30 mg, 0.2 mmol) were heated at reflux in the presence of NaOMe (11 mg, 0.2 mmol) for 4 h, during which time color changed from orange to yellow. The solvent was removed in vacuum, and the residue was washed with diethyl ether (5 mL 3). Recrystallization from methanol/diethyl ether (1:4) gave orange crystals of [(h6C6H6)Ru(L-met)](NO3) (1) in four days. Yield: 58 mg, 75%. IR (KBr disc, cm1): nNH 3448 (s), nC¼O 1641 (s), nCO 1198 (m). 1H NMR (400 MHz, CD3OD, ppm): 5.92 (s, 6H, C6H6), 3.56e3.53 (m, 1H, metCH), 2.77 (s, 3H, SCH3), 2.25e2.22 (t, 2H, SCH2-CH2), 2.09e2.05 (m, 2H, SCH2). MS (FAB): m/z 327 [MNO3]þ. Anal. Calc. for C11H16N2O5SRu: C, 33.93; H, 4.14; N, 7.19%. Found: C, 33.90; H, 4.18; N, 7.22%. 2.3. Synthesis of [(h6-p-cymene)Ru(L-met)](NO3) (2) AgNO3 (68 mg, 0.4 mmol) was added to a solution of [(h6-pcymene)RuCl2]2 (61.6 mg, 0.1 mmol) in dry methanol (15 mL) giving an orange-red solution and precipitation of AgCl. The mixture was filtered to remove AgCl. The orange-red solution and L-methionine (80 mg, 0.20 mmol) were heated at reflux for 4 h. The solvent was removed by vacuum, and the residue was washed with diethyl ether (5 mL 3). Recrystallization from methanol/diethyl ether (1:3) gave yellow crystals of [(h6-p-cymene)Ru(L-met)](NO3) (2) in three days. Yield: 69 mg, 77%. IR (KBr disc, cm1): nNH 3446 (s), nC¼O 1653 (s), nCO 1196 (m). 1H NMR (400 MHz, CD3OD, ppm): 5.95e5.75 (m, 4H, Ar), 3.52e3.47 (m, 1H, met-CH), 2.76 (s, 3H, SCH3), 3.08e3.04 (m, 1H, -CH(CH3)2), 2.23 (s, 3H, C6H4-CH3), 2.21e2.19 (t, 2H, SCH2-CH2), 2.09e2.05 (m, 2H, SCH2),1.36e1.34 (d, 6H, -CH(CH3)2). MS (FAB): m/z 383 [MNO3]þ. Anal. Calc. for C15H26N2O5SRu: C, 40.26; H, 5.86; N, 6.26%. Found: C, 40.23; H, 5.88; N, 6.29%. 2.4. Synthesis of [(CO)2Ru(L-met)Cl] (3) To a slurry of [Ru(CO)2Cl2]n (46 mg, 0.20 mmol) in N,N0 -dimethylformamide (5 mL) was added a solution of L-methionine (30 mg, 0.20 mmol) and NaOMe (11 mg, 0.20 mmol) in methanol (10 mL), and then the mixture was heated at 90 C with stirring overnight, during which time there was a color change from yellow to orange. Addition of 30 mL diethyl ether to the reaction solution
gave orange precipitate which was filtered and washed with diethyl ether. Yield: 35 mg, 52%. IR (KBr disc, cm1): nNH 3440 (s), nC≡O 1988(s) and 1970(s), nC¼O 1625 (s), nCO 1210 (m). 1H NMR (400 MHz, CD3OD, ppm): 3.18e3.13 (m, 1H, met-CH), 2.54 (s, 3H, SCH3), 2.31e2.28 (t, 2H, SCH2-CH2), 2.13e2.09 (m, 2H, SCH2). MS (FAB): m/z 341 [Mþ], 313 [MþCO], 283 [Ru(L-met)Cl]þ, 250 [Ru(Lmet)]þ. Anal. Calc. for C7H10NO4SClRu: C, 24.67; H, 2.96; N, 4.11%. Found: C, 24.64; H, 2.99; N, 4.15%. 2.5. Synthesis of [(bpy)2Ru(L-met)]Cl (4) A mixture of L-methionine (15 mg, 0.10 mmol), NaOMe (6 mg, 0.10 mmol) and [Ru(bpy)2Cl2]$2H2O (52 mg, 0.10 mmol) were stirred at reflux in methanol (20 mL) for 6 h, during which the color of solution changed from purple to dark red. After removal of solvents in vacuum, methanol (20 mL) was added and the solution was filtered. The filtrate was concentrated and the residue was washed with diethyl ether (5 mL 2) to give the desired product. Recrystallization from methanol/diethyl ether (1:3) afforded dark red block crystals of [(bpy)2Ru(L-met)]Cl (4) suitable for X-ray diffraction in three days. Yield: 39 mg, 65%. IR (KBr disc, cm1): nNH 3448 (s), nC¼O 1618 (s), nC¼N 1424 (m), nCO 1261 (m). 1H NMR (400 MHz, CD3OD, ppm): d 9.35 (m, 2H, bpy), 9.02 (m, 1H, bpy), 8.69 (d, 2H, bpy), 8.57e8.44 (m, J ¼ 8.6 Hz, 2H, bpy), 8.29e8.13 (m, 2H, bpy), 7.93 (m, 1H, bpy), 7.84 (d, J ¼ 7.6 Hz, 3H, bpy), 7.52 (m, 1H, bpy), 7.30e7.10 (m, 2H, bpy), 3.51 (m, 1H, met-CH), 2.62 (s, 3H, SCH3), 2.35e1.97 (m, 4H, CH2). MS (FAB): m/z 562 [MCl]þ. Anal. Calc. for C25H26N5O2SClRu: C, 50.29; H, 4.39; N, N, 11.73%. Found: C, 50.32; H, 4.35; N, 11.70%. 2.6. Synthesis of [Ru(L-met)(NO)(PPh3)Cl]Cl (5) To a solution of L-methionine (30 mg, 0.20 mmol) and NaOMe (11 mg, 0.20 mmol) in methanol (25 mL) was added [Ru(NO)(PPh3)2Cl3] (153 mg, 0.20 mmol), during which the color of solution changed from yellow-brown to dark brown. After removal of solvents in vacuum, the residue was washed with diethyl ether (5 mL 2) to give the desired product which was recrystallized from methanol/diethyl ether (1:3). Yield: 72 mg, 62%. IR (KBr disc, cm1): nNH 3420 (s), nC¼O 1631 (s), nNO 1560 (s), nCO 1235 (m), nCP 654 (w). 1H NMR (400 MHz, DMSO‑d6, ppm): 9.65 (m, 2H, NH2), 7.45 (m, 15H, PPh3), 3.26e3.23 (m, 1H, met-CH), 2.91 (s, 3H, SCH3), 2.67 (m, 2H, SCH2-CH2), 2.31 (m, 2H, SCH2). 31P NMR (162 MHz, DMSO‑d6, ppm): 39.06. MS (FAB): m/z 577 [Ru(Lmet)(NO)(PPh3)Cl]þ, 542 [Ru(L-met)(NO)(PPh3)]þ, 512 [Ru(Lmet)(PPh3)]þ, 315 [Ru(L-met)(NO)Cl]þ, 250 [Ru(L-met)]þ. Anal. Calc. for C23H25N2O3SCl2PRu: C, 45.10; H, 4.11; N, 4.57%. Found: C, 45.12; H, 4.08; N, 4.58%. 2.7. Synthesis of [Ru(L-met)(CH3CN)Cl2] (6) To a slurry of (Et4N)[Ru(CH3CN)2Cl4] (91 mg, 0.20 mmol) in N,N0 dimethylformamide (5 mL) was added L-methionine (30 mg, 0.20 mmol) and NaOMe (11 mg, 0.20 mmol) in methanol (10 mL), and then the mixture was heated at 90 C with stirring for 8 h, during which time there was a color change from yellow-brown to orange. The solvent was removed in vacuum and the residue was washed with diethyl ether. Recrystallization from methanol/diethyl ether (1:3) afforded the desired product [Ru(L-met)(CH3CN)Cl2] (6). Yield: 41 mg, 58%. IR (KBr disc, cm1): nNH 3434 (s), nC≡N 2241 (m), nC¼O 1623 (s), nCO 1246 (m). MS (FAB): m/z 361 [Mþ], 320 [MþCH3CN], 314 [MþCl], 279 [Mþ2Cl], 250 [Ru(L-met)]þ. Anal. Calc. for C7H13N2O2SCl2Ru: C, 23.28; H, 3.63; N, 7.76%. Found: C, 23.25; H, 3.65; N, 7.73%.
J. Ji et al. / Journal of Organometallic Chemistry 907 (2020) 121078
2.8. Synthesis of [Ru(L-met)(DMSO)2Cl] (7) To a solution of L-methionine (30 mg, 0.20 mmol) and NaOMe (11 mg, 0.20 mmol) in methanol (10 mL) was added a solution of cis-[Ru(DMSO)4Cl2] (97 mg, 0.20 mmol) in methanol (5 mL). The mixture was stirred for 6 h at reflux, during which time there was a color change from yellow to pale yellow. The solvent was removed in vacuum and the residue was washed with diethyl ether. The residue was extracted with methanol (10 mL) and yellow product of [Ru(L-met)(DMSO)2Cl] (7) were obtained after allowing the filtrate layered with diethyl ether (15 mL) at room temperature. Yield: 47 mg, 53%. IR (KBr disc, cm1): nNH 3466 (s), nC¼O 1657 (s), nCO 1401 (m), nS¼O 1091 (s). 1H NMR (400 MHz, CD3OD, ppm): 3.81 (m, 1H, met-CH), 2.98e2.71 (m, 12H, DMSO), 2.49 (s, 3H, SCH3), 2.64e2.49 (m, 2H, SCH2-CH2), 2.39e2.32 (m, 2H, SCH2). MS (FAB): m/z 441 [Mþ], 406 [MþCl], 363 [MþDMSO], 285 [Mþ2DMSO], 250 [Ru(L-met)]þ. Anal. Calc. for C9H22NO4S3ClRu: C, 24.51; H, 5.03; N, 3.18%. Found: C, 24.49; H, 5.07; N, 3.20%. 2.9. Synthesis of [(COD)Ru(m-S-L-met)(m-Cl) RuCl(L-metNMe2)] (8) To a slurry of [Ru(COD)Cl2]2 (56 mg, 0.20 mmol) in N,N0 -dimethylformamide (5 mL) was added L-methionine (30 mg, 0.20 mmol) and NaOMe (11 mg, 0.20 mmol) in methanol (10 mL). The mixture was stirred for 30 h at reflux and the red-brown solution with a little brown precipitate formed. The precipitate was removed by filtration and the solvent was removed in vacuum and the residue was washed with diethyl ether and further recrystallized from methanol/diethyl ether at room temperature. Orange-red block crystals of [(COD)Ru(m-S-L-met)(m-Cl) RuCl(L-metNMe2)] (8) suitable for X-ray diffraction were obtained in a week. Yield: 35 mg, 50%. IR (KBr disc, cm1): nNH 3476 (s), nC¼O 1651 (s), nCO 1330 (m). 1 H NMR (400 MHz, CD3OD, ppm): d 4.07 (m, 2H, olefinic H of COD), 3.84 (m, 2H, olefinic H of COD), 3.69 (2 m, 2H, met-CH), 3.32 (s, 3H, NCH3), 3.30 (s, 3H, NCH3), 2.77 (2s, 6H, SCH3), 2.42e2.23 (m, 8H, met-CH2), 2.17 (m, 8H, aliphatic H of COD). MS (FAB): m/z 706 [Mþ], 671 [MþCl], 594 [MþCOD], 559 [MþClCOD].Anal. Calc. for C20H36N2O4S2Cl2Ru2: C, 34.04; H, 5.14; N, 3.97%. Found: C, 34.07; H, 5.13; N, 3.95%. 2.10. X-ray crystallography Crystallographic data and experimental details for ruthenium(II) complexes [(h6-C6H6)Ru(L-met)](NO3) (1), [(h6-p-cymene)Ru(L-met)](NO3) (2), [(bpy)2Ru(L-met)]Cl (4) and [(COD)Ru(mS-L-met)(m-Cl) RuCl(L-metNMe2)] (8) are summarized in Table 1. Intensity data were collected on a Bruker SMART APEX 1000 CCD diffractometer using graphite-monochromated Mo Ka radiation (l ¼ 0.71073 Å). The collected frames were processed with the software SAINT [31]. The data were corrected for absorption using the program SADABS [32]. Structures were solved by Direct Methods and refined by full-matrix least-squares on F2 using the SHELXTL software package [33,34]. All non-hydrogen atoms were refined anisotropically. The positions of all hydrogen atoms were generated geometrically (Csp3eH ¼ 0.96 Å, Csp2eH ¼ 0.93 Å and NeH ¼ 0.86 Å), assigned isotropic thermal parameters, and allowed to ride on their respective parent carbon atoms before the final cycle of least-squares refinement. 2.11. Photocatalyst testing The H2 production reactions were carried out in an outer irradiation-type photoreactor (Pyrex glass) connected to a closed gas-circulation system. A 300 W Xe lamp was afforded as light source, which was collimated and focalized into 5 cm2 parallel
3
faculae, then translated into uprightness light by a viewfinder. A cutoff filter (L-42; l > 420 nm) was employed to obtain visible light irradiation. The reaction was performed in distilled water (50 mL) and methanol (5 mL) solution containing the photocatalyst ruthenium complexes (5 mg), then the solution was thoroughly degassed to remove air, and the reactor was irradiated from the top with the visible light (l > 420 nm). An online gas chromatography with a thermal conductive detector (TCD) was equipped to the reaction system in order to detect amount of H2 evolution after photocatalytic reaction by using argon (Ar) as carrier gas. 3. Results and discussion 3.1. Preparation and characterization of ruthenium complexes The synthetic route of ruthenium complexes 18 with Lmethionine ligands is summarized in Scheme 1. Mononuclear organo-ruthenium complexes similar to 1 and 2 with the same cation have previously been reported according to a Cambridge Crystallographic Data Centre (CCDC) search [17,18], whereas the anion in complex 1 was NO 3 rather than PF6 . The recrystallization conditions were quite different, the complex 2 was recrystallized from methanol/diethyl ether (1:4) at room temperature in our preparation, which is comparable to that in the reported literature with a harsh addition of diisopropyl ether in 96% ethanol at 20 C [18]. The two chloro ligands in [Ru(bpy)2Cl2]$2H2O were replaced by the nitrogen and oxygen atoms of L-methionine ligand to afford complex 4, possibly due to the steric hindrance presence of the methyl substituent on the sulfur atom. Treatment of L-methionine ligand in the presence of base NaOMe with equal equiv. of [Ru(CO)2Cl2]n, [Ru(NO)(PPh3)2Cl3], (Et4N)[Ru(CH3CN)2Cl4] or cis[RuCl2(DMSO)4] gave accordingly complexes [(CO)2Ru(L-met)Cl] (3), [Ru(L-met)(NO)(PPh3)Cl]Cl (5), [Ru(L-met)(CH3CN)Cl2] (6) and [Ru(L-met)(DMSO)2Cl] (7), respectively, in which L-methionine ligand coordinated to the ruthenium atom in an expected k3-S,N,Omode. Reaction of [Ru(COD)Cl2]2 and L-methionine afforded a dinuclear ruthenium(II) complex [(COD)Ru(m-S-L-met)(m-Cl) RuCl(L-metNMe2)] (8), similar to the reported binuclear ruthenium complex [Ru(dppb)(bipy)Cl(CH3S(CH2)2NH2CHCOO)Ru(dppb)(bipy)](PF6)2 [19]. One of NH2 moieties in complex 8 turned to a NMe2 moiety, possibly due to a nucleophilic attack of the amine on formaldehyde intermediate generated by a hydrogen transfer from methanol in the presences of NaOMe as a base and ruthenium-COD complex as a catalyst [35,36]. The ruthenium-COD complex efficiently activated methanol as N-methylation reagent to give related nitrogen complexes. The positions of the nNH2 (~3440 cm1) and nC¼O (~1635 cm1) bands in the infrared spectra of complexes 18 are typical for coordinated amino and carboxylate functions [15,19]. The IR spectrum of complex 3 have nC≡O at 1988 and 1970 cm1, which are comparable to those in other carbonyl ruthenium(II) complexes [22]. The chemical shifts of benzene groups in complex 1 were found at around 5.92 ppm as a singlet. The eNH2 proton in complex 5 was observed at around 9.65 ppm in the 1H NMR spectrum. Moreover, one 31P signal 39.06 ppm was observed in the 31P NMR spectrum of complex 5 due to the PPh3 ligands. The proton (eSCH3) resonances exhibit as a typical singlet peak at around 2.52 ppm in the ruthenium(II) complexes 15, 7 and 8 compared well with those in other ruthenium complexes containing L-methionine ligands, such as [RuCl(D/L-met)(PPh3)2]$CH3OH (2.20 ppm) [15] and [(h6-pcymene)Ru(L-met)]Cl$0.4H2O (2.71 ppm) [18]. The UVevis absorption spectra of complexes 18 in an aqueous solution at room temperature are shown in Fig. 1. At the first glance, all spectra for complexes 18 show strong intense transitions located at 288 nm, the bands are ascribed to n/p* transition of
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Table 1 Crystallographic data and experimental details for [(h6-C6H6)Ru(L-met)](NO3) (1), [(h6-p-cymene)Ru(L-met)](NO3) (2), [(bpy)2Ru(L-met)]Cl (4) and [(COD)Ru(m-S-L-met)(m-Cl) RuCl(L-metMe2)] (8). complex
1
2
4
8
empirical formula formula weight crystal system a (Å) b (Å) c (Å) b ( ) V (Å3) space group Z Dcalc (g cm3) temperature (K) F(000) m(Mo-Ka) (mm1) total refln independent refln parameters Rint R1a, wR2b (I > 2s(I)) R1, wR2 (all data) GoFc
C11H16N2O5SRu 389.39 orthorhombic 7.6736(18) 9.026(2) 19.470(4)
C15H24N2O5SRu 445.49 orthorhombic 7.9088(10) 13.9909(17) 16.131(2)
C20H36N2O4S2Cl2Ru2 705.67 orthorhombic 7.971(3) 12.421(5) 29.980(11)
1348.6(5) P212121 4 1.918 296(2) 784 1.338 8529 3086 183 0.0611 0.0481, 0.1177 0.0658, 0.1374 0.849
1784.9(4) P212121 4 1.658 296(2) 912 1.022 11439 4135 222 0.0399 0.0316, 0.0707 0.0400, 0.0780 0.922
C25H26N5O2SClRu 597.09 monoclinic 12.363(10) 28.09(2) 9.806(8) 108.962(8) 3221(4) P21/c 4 1.231 296(2) 1216 0.660 15794 5654 344 0.1096 0.1368, 0.2769 0.1925, 0.2951 1.048
a b c
2968(2) P212121 4 1.579 296(2) 1424 1.364 18606 6700 295 0.0705 0.0620, 0.1630 0.0801, 0.1800 1.090
R1 ¼ jjFo j jFc jj=jFo j. 2 2 1=2 wR2 ¼ ½wð F 2o F 2c Þ =w F 2o . GoF ¼ ½wðjFo j jFc jÞ2 =ðNobs Nparam Þ1=2 .
Scheme 1. Syntheses and reactivity of ruthenium(II) complexes 18 with L-methionine ligands. Reagents and conditions: (i) [(h6-C6H6)RuCl2]2, AgNO3, NaOMe, MeOH, at reflux; (ii) [(h6-p-cymene)RuCl2]2, AgNO3, NaOMe, MeOH, at reflux; (iii) [Ru(CO)2Cl2]n, NaOMe, MeOH/DMF, 85 C; (iv) [Ru(bpy)2Cl2]$2H2O, NaOMe, MeOH, at reflux; (v) [Ru(NO)(PPh3)2Cl3], NaOMe, MeOH, 85 C; (vi) (Et4N)[Ru(CH3CN)2Cl4], NaOMe, MeOH/DMF (2:1), at reflux; (vii) [Ru(DMSO)4Cl2], NaOMe, MeOH, at reflux; (viii) [Ru(COD)Cl2]2, NaOMe, MeOH/DMF, 85 C.
eC¼O. The bands at 320370 nm for complexes 1, 2, 6 and 8 may be attributed to the metal-to-ligand charge transfer (MLCT) transition. It is interesting to note that there is a red-shift of about 50 nm (372 nm for 1, 368 nm for 2, 320 nm for 6) observed for the transition depending on the presence of a h6-C6H6 or h6-cymene
ligand. In complex 4, the band at 350 nm may be assigned to a typical spin-allowed pep* transition. The intense transitions for complex 4 located at 493 nm may indicate a metal-to-ligand charge transfer (MLCT) transition, which is comparable to a similar ruthenium complex [Ru(k2-C,N-L1)(phen)2](PF6) (L1 ¼ 1-(2,6-
J. Ji et al. / Journal of Organometallic Chemistry 907 (2020) 121078
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Fig. 1. UV/Vis spectra of the complexes 18 measured in an aqueous solution.
diisopropylphenoxy)- 4-phenylphthalazine, 492 nm) [37]. 3.2. X-ray crystal structures of ruthenium complexes 1, 2, 4 and 8
Fig. 3. Structure of cation [(h6-p-cymene)Ru(L-met)]þ in complex 2 showing the atomlabeling scheme, the NO 3 counterion are omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level.
The structures of ruthenium(II) complexes 1, 2, 4 and 8 have been confirmed by single crystal X-ray crystallography. The perspective views of the molecular structures of complexes 1, 2, 4 and 8 are shown in Fig. 25, respectively. Selected bond lengths and bond angles for complexes 1, 2, 4 and 8 are summarized in Table 2 for the comparison. Four complexes 1, 2, 4 and 8 adopt chiral space group P212121 or P21/c owing to the coordination of the chiral ligand L-methionine. Complexes 1 and 2 could be formulated as six-coordinate ruthenium species assuming the benzene or cymene moiety occupied three coordination sites and the sixmembered chelate shows a distorted chair conformation with axial position of the thioether sulfur donors [18]. The RuN, RuO and RuS bond lengths in complexes 1, 2, 4 and 8 are in the ranges
Fig. 4. Structure of cation [(bpy)2Ru(L-met)]þ in complex 4 showing the atom-labeling scheme, the Cl counterion and the water molecule are omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level.
Fig. 2. Structure of cation [(h6-C6H6)Ru(L-met)]þ in complex 1 showing the atomlabeling scheme, the NO 3 counterion is omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level.
of 2.099(8)2.172(1), 2.089(3)2.115(8), and 2.262(4)2.394(2) Å, respectively, similar to those in the structure-related complexes [RuCl(L-metme)2(PPh3)]Cl [15] and [(nbd)Ru(d,l-met)Cl] [16]. The bite NRuO bond angles are in the range of 76.7(4)o80.6(4)o in complex 1, 2, 4 and 8, which are compared with those in the reported cationic ruthenium nbd complexes [RuCl(L-his)(h4-nbd)] (Lhis ¼ L-histidine, NRuO 78.1(3)o) [15]. The SRuN bond angle is 84.8(2)o and the SRuO bond angle is 89.5(2)o in complex 1, similar to those in complex 2 (SRuN 83.35(12)o and SRuO 90.57(11)o). In complex 4, the center ruthenium atom is coordinated to two bpy ligands, oxygen and nitrogen atoms of L-methionine which participates in a five-membered chelate ring. The RuN bond lengths of bpy ligands are in the range of 2.017(11) 2.077(11) Å, which are compared with those in a related ruthenium
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J. Ji et al. / Journal of Organometallic Chemistry 907 (2020) 121078
Fig. 5. Molecular structure of complex 8 showing the atom-labeling scheme. Thermal ellipsoids are drawn at the 30% probability level.
Table 2 Selected bond lengths (Å) and angles ( ) for complexes 1, 2, 4 and 8. complex
RuN
RuO
RuS
RuCl
SRuN
SRuO
NRuO
1 2 4
2.099(8) 2.117(4) 2.077(11) 2.036(11) 2.046(12) 2.077(11) 2.102(13) 2.116(11) 2.172(10)
2.091(6) 2.089(3) 2.089(9)
2.394(2) 2.3801(13) e
e e e
84.8(2) 83.35(12) e
89.5(2) 90.57(11) e
77.2(3) 77.70(15) 80.6(4)
2.115(8) 2.104(9)
2.372(4) 2.300(3) 2.262(4)
2.446(3) 2.413(3) 2.501(3)
85.5(3) 94.6(3)
88.1(3) 91.1(3)
78.6(3) 76.7(4)
8
complex [Ru(bpy)2(L)](PF6)2$2.2MeCN (L ¼ N-phenylpyridin-2ylmethanimine, 2.054(2)2.071(2) Å) [38]. It is interesting to note that complex 8 presents a binuclear structure where two ruthenium atoms are bridged by one chlorine atom and one sulfur atom of L-methionine. One ruthenium center Ru(1) further coordinated to a chelating L-methionine and large h4-COD ligand, the other ruthenium center Ru(2) additionally coordinated to a Lmethionine and a chloride atom. The terminal Ru(2)Cl(2) bond length is 2.413(3) Å, as expected, shorter than the bridging RuCl bond lengths (2.446(3)2.501(3) Å) in complex 8. The bridging RuS bond lengths are 2.300(3) and 2.372(4) Å in complex 8, a little longer than the Ru(2)S(2) bond lengths of the L-methionine ligand (2.262(4) Å). The SRuN bond angles of 85.5(3) and 94.6(3) in complex 8 are normal for the related ruthenium complex [RuCl3(L-metet)](PPh3)] (90.6(2) Å) [15]. 3.3. Photocatalytic H2 production In Fig. 6, the photocatalytic activity of ruthenium complexes 1, 2, 4 and 8 towards the water splitting hydrogen production is shown in intervals of periodic evaluation of 0.5 h up to a total of 5 h of irradiation. The amount of hydrogen gas evolved increase almost linearly with time indicating the high stability of corresponging ruthenium photocatalysts under visible light illumination. In Table 3, the band gap values are indicative of the ability of these
complexes to be active as photocatalysts under the visible light electromagnetic spectrum [39]. The total amount of H2 evolved from complex 2 after 5 h irradiation was 9.32 mmol, which was little lower than those produced from complexes 1 (10.61 mmol), 4 (10.69 mmol) and 8 (10.67 mmol). The results reveal that ruthenium complexes 1, 2, 4 and 8 as photocatalysts enhance light absorption greatly and improve the hydrogen production by water spiltting [40]. Fig. 7 shows the time courses of H2 evolution over different content of ruthenium complex 4. As can be seen, the H2 evolution rate increases upon enhancing the concentration of catalyst 4, in which the best content was 15 mg (2.34 mmol/h). The total amount of H2 evolved was stable when the catalyst was over 20 mg, for example, 11.50 mmol H2 evolved when 20 mg or 25 mg catalyst was used. It is known that the sacrificial reagent is an important parameter for the photocatalytic performance H2 evolution. As a result, four different sacrificial reagents of triethylamine, methanol, triisopropanolamine and triethanolamine were employed for the hydrogen production by water spiltting. As depicted in Fig. 8, the total amounts of H2 produced in the irradiation were 9.61, 10.68, 10.94 and 11.48 mmol for triethylamine, methanol, triisopropanolamine and triethanolamine, respectively. The results reveal that triethanolamine may be a better sacrificial reagent possibly due to its strong alkaline and electron donating properties.
J. Ji et al. / Journal of Organometallic Chemistry 907 (2020) 121078
Fig. 6. Time courses of photocatalytic evolution of H2 using ruthenium complexes 1, 2, 4 and 8 suspended in CH3OH aqueous solution under UV irradiation (l > 420 nm). The curves were measured by a H2 sensor and calibrated by GC analysis. Catalyst: 5 mg, pure water: 50 mL, methanol: 5 mL.
Table 3 Photophysical properties and hydrogen evolution rates for the ruthenium photocatalysts. Photocatalyst
Band gap (eV)a
Total H2 evolved (mmol)b
H2 evolution rate (mmol$h1)b
Complex Complex Complex Complex
2.52 2.54 1.91 2.34
10.61 9.32 10.69 10.67
2.12 1.86 2.14 2.13
1 2 4 8
a
Calculated from the onset of the absorption spectrum. Reaction conditions: 5 mg ruthenium complex was suspended in 50 mL water solution and irradiated by a 300 W Xe lamp (l > 420 nm visible filter) for 5 h. b
7
Fig. 8. Time courses of photocatalytic evolution of H2 using ruthenium complex 4 with different sacrificial reagents under UV irradiation (l > 420 nm). The curves were measured by a H2 sensor and calibrated by GC analysis. Pure water: 50 mL, catalyst: 5 mg.
Afterwards, the photogenerated holes transfer from the valence band, thus preserving the holes can oxidize water to produce Hþ and OH [41,42]. Meanwhile, the photogenerated electrons transfer from the conduction band to ruthenium complexes and take part in reduction reaction, such as reduction of proton to form H2. Additionally, the sacrificial reagent methanol can be oxidized by OH to give methanoic acid, accelerating the decomposition of H2O. Ruthenium(II) center acts as an electron donor, which improves the electron transfer efficiency and the separation of the photogenerated electron-hole pairs (inhibit their recombination), thus enhancing the photocatalytic water splitting behavior of ruthenium. In summary, a series of mono- and di-nuclear neutral and cationic ruthenium(II)/(III) complexes with L-methionine ligands were synthesized and spectroscopically characterized. Structures of the four ruthenium(II) complexes 1, 2, 4 and 8 were unambiguously established by single-crystal X-ray diffraction, presenting the rich coordination modes of L-methionine ligands. Up to now, there are few reported ruthenium complexes bearing L-methionine ligand to our best knowledge, possibly due to their diversity of coordination modes and difficulty in crystallization. Ruthenium complexes 1, 2, 4 and 8 were also tested as photocatalytic catalysts, which improves hydrogen evolution rate (2 mmol h1) of water splitting. Exploration of ruthenium complexes as pre-catalysts for transfer hydrogenation of acetophenone is underway in our laboratory. Supplementary material
Fig. 7. Time courses of photocatalytic evolution of H2 using ruthenium complex 4 with different content in CH3OH aqueous solution under UV irradiation (l > 420 nm). The curves were measured by a H2 sensor and calibrated by GC analysis. Pure water: 50 mL, methanol: 5 mL.
A possible mechanism of photocatalytic splitting water by ruthenium complexes photocatalysts have been proposed. Upon visible light irradiation excitation, ruthenium complexes can be excited and generate photogenerated electron-hole pairs.
Crystallographic data for [(h6-C6H6)Ru(L-met)](NO3) (1), [(h6-pcymene)Ru(L-met)](NO3) (2), [(bpy)2Ru(L-met)]Cl (4) and [(COD) Ru(m-S-L-met)(m-Cl)RuCl(L-metMe2)] (8) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC 1947239-1947242, respectively. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax: (þ44)1233-336-033; email: [email protected]]. Declaration of competing interest The authors declare that they have no known competing
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financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[20]
Acknowledgements [21]
This project was supported by the Natural Science Foundation of China (21372007).
[22]
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Syntheses, structures and photocatalytic properties of ruthenium complexes bearing L-methionine ligands Jiao Ji, Chong Chen, Ai-Quan Jia, Hua-Tian Shi, Qian-Feng Zhang* Institute of Molecular Engineering and Applied Chemistry, Anhui University of Technology, Ma’anshan, Anhui, 243002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 August 2019 Received in revised form 23 October 2019 Accepted 14 December 2019 Available online 17 December 2019
Treatment of [(h6-C6H6)RuCl2]2 or [(h6-p-cymene)RuCl2]2 with two equiv. of L-methionine in the presence of AgNO3 and NaOMe gave mononuclear ruthenium(II) complexes [(h6-C6H6)Ru(L-met)](NO3) (1) and [(h6-cymene)Ru(L-met)](NO3) (2), respectively. Interactions between equal equiv. of L-methionine and [Ru(CO)2Cl2]n, [Ru(bpy)2Cl2]$2H2O (bpy ¼ 2,20 -bipyridine), [Ru(NO)(PPh3)2Cl3], (Et4N) [Ru(CH3CN)2Cl4] or cis-[Ru(DMSO)4Cl2] (DMSO ¼ dimethylsulfoxide) resulted in isolation of the expected ruthenium complexes [(CO)2Ru(L-met)Cl] (3), [(bpy)2Ru(L-met)]Cl (4), [Ru(L-met)(NO)(PPh3)Cl]Cl (5), [Ru(L-met)(CH3CN)Cl2] (6) and [Ru(L-met)(DMSO)2Cl] (7), respectively. Reaction of [Ru(COD)Cl2]2 (COD ¼ 1,5-cyclooctadiene) with L-methionine gave a dinuclear ruthenium complex [(COD)Ru(m-S-Lmet)(m-Cl)RuCl(L-metNMe2)] (8). Complexes 18 were characterized by microanalyses, IR, NMR and UV evis spectroscopies. The structures of complexes 1, 2, 4 and 8 have been unambiguously established by single-crystal X-ray crystallography. The photocatalytic properties for hydrogen production by water splitting of complexes 1, 2, 4 and 8 under visible light (l > 420 nm) were also investigated. © 2019 Published by Elsevier B.V.
Keywords: Ruthenium L-Methionine Synthesis Photocatalyst Crystal structure
1. Introduction Methionine is an essential amino acid that plays fundamental roles in protein synthesis and a number of other biochemical and cellular processes [1,2]. L-Methionine has obtained great interest possibly due to the strongly inhibitory effects on proliferation of cancer cells and the possibility of developing methionine analogs as new anticancer therapeutic agents [3]. Transition metal complexes bearing L-methionine ligands have proven to exhibit interesting properties in very different fields such as asymmetric synthesis in organic media and in an aqueous solution, cytotoxicity, sol-gelderived coatings, and bio-organometallic and solid-state chemistry [4e6]. As a matter of the fact, several transition metal such as copper(II), platinum(II), silver(I), cobalt(III) and rhenium(V) complexes with L-methionine ligands have been synthesized and structurally characterized [7e14]. For instance, reaction of Ag2O with the D/L-methionine led to dinuclear silver(I) complex {[Ag2(Dmet)(L-met)]}n, in which the D/L-methionine ligand has been coordinated to silver(I) in a distorted tetrahedral geometry [10]. It has been noted that the X-ray crystal structures of ruthenium complexes containing L-methionine and different co-ligands, such as p-
cymene, benzene, norbornadiene, chloride and nitrosyl, were established, generally, the central ruthenium atom was coordinated by L-methionine in a k3-S,N,O-mode by the nitrogen atom, the sulfur atom and carboxylate oxygen donor [15e19]. Hydrogen has been regarded as an ideal energy carrier in future because of its high energy capacity and zero emission of environment pollutants [20]. Analogous ruthenium complexes with pyridyl ligands on the photocatalytic H2 evolution over TiO2 nanoparticles have been reported [20,21]. Hydrogen production by water splitting using single ruthenium complexes as photocatalysts is relatively rarely investigated. Previously, we have reported syntheses and reactions of a series of neutral and anionic ruthenium(II)/(III) complexes with N,O,S-donor ligands and their electrochemical properties [22]. To further understand the ruthenium-sulfur complexes in the electron-rich coordination environment, herein we report syntheses and structures of the related ruthenium complexes bearing L-methionine ligands, the photocatalytic properties of ruthenium complexes using methanol as electron donors are initially investigated in this paper. 2. Experimental section 2.1. General considerations
* Corresponding author. E-mail address: [email protected] (Q.-F. Zhang). https://doi.org/10.1016/j.jorganchem.2019.121078 0022-328X/© 2019 Published by Elsevier B.V.
All manipulations were carried out under nitrogen by standard
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Schlenk techniques. Solvents were purified, distilled and degassed prior to use. [(h6-C6H6)RuCl2]2 [23], [(h6-p-cymene)RuCl2]2 [24], [Ru(CO)2Cl2]n [25], [Ru(bpy)2Cl2]$2H2O (bpy ¼ 2,20 -bipyridine) [26], cis-[Ru(DMSO)4Cl2] (DMSO ¼ dimethylsulfoxide) [27], [Ru(COD)Cl2]2 (COD ¼ 1,5-cyclooctadiene) [28], and [Ru(NO)(PPh3)2Cl3] [29] and (Et4N)[Ru(CH3CN)2Cl4] [30] were prepared according to the literature methods. L-metionine (L-Hmet), silver nitrate, and sodium methoxide were purchased from Alfa Aesar Ltd. and used as received. NMR spectra were recorded on a BrukerALX400 spectrometer operating at 400 and 162 MHz for 1H and 31P, respectively. Chemical shifts (d, ppm) were reported with reference to SiMe4 (1H) and 85% H3PO4 (31P). Infrared spectra were recorded on a Perkin-Elmer 16 PC FT-IR spectrophotometer and positive FAB mass spectra were recorded on a Finnigan TSQ 7000 spectrometer. Elemental analyses were carried out using a Perkin-Elmer 2400 CHN analyzer. Electronic absorption spectra were obtained on a Shimadzu UV-2600 spectrophotometer. Water splitting was performed with a MC-SPH2O-AG photocatalysis analyzer. 2.2. Synthesis of [(h6-C6H6)Ru(L-met)](NO3) (1) AgNO3 (68 mg, 0.4 mmol) was added to a solution of [(h6-C6H6) RuCl2]2 (50 mg, 0.1 mmol) in dry methanol (10 mL) giving an orange-red solution and precipitation of AgCl. The mixture was filtered to remove AgCl. The orange-red solution and L-methionine (30 mg, 0.2 mmol) were heated at reflux in the presence of NaOMe (11 mg, 0.2 mmol) for 4 h, during which time color changed from orange to yellow. The solvent was removed in vacuum, and the residue was washed with diethyl ether (5 mL 3). Recrystallization from methanol/diethyl ether (1:4) gave orange crystals of [(h6C6H6)Ru(L-met)](NO3) (1) in four days. Yield: 58 mg, 75%. IR (KBr disc, cm1): nNH 3448 (s), nC¼O 1641 (s), nCO 1198 (m). 1H NMR (400 MHz, CD3OD, ppm): 5.92 (s, 6H, C6H6), 3.56e3.53 (m, 1H, metCH), 2.77 (s, 3H, SCH3), 2.25e2.22 (t, 2H, SCH2-CH2), 2.09e2.05 (m, 2H, SCH2). MS (FAB): m/z 327 [MNO3]þ. Anal. Calc. for C11H16N2O5SRu: C, 33.93; H, 4.14; N, 7.19%. Found: C, 33.90; H, 4.18; N, 7.22%. 2.3. Synthesis of [(h6-p-cymene)Ru(L-met)](NO3) (2) AgNO3 (68 mg, 0.4 mmol) was added to a solution of [(h6-pcymene)RuCl2]2 (61.6 mg, 0.1 mmol) in dry methanol (15 mL) giving an orange-red solution and precipitation of AgCl. The mixture was filtered to remove AgCl. The orange-red solution and L-methionine (80 mg, 0.20 mmol) were heated at reflux for 4 h. The solvent was removed by vacuum, and the residue was washed with diethyl ether (5 mL 3). Recrystallization from methanol/diethyl ether (1:3) gave yellow crystals of [(h6-p-cymene)Ru(L-met)](NO3) (2) in three days. Yield: 69 mg, 77%. IR (KBr disc, cm1): nNH 3446 (s), nC¼O 1653 (s), nCO 1196 (m). 1H NMR (400 MHz, CD3OD, ppm): 5.95e5.75 (m, 4H, Ar), 3.52e3.47 (m, 1H, met-CH), 2.76 (s, 3H, SCH3), 3.08e3.04 (m, 1H, -CH(CH3)2), 2.23 (s, 3H, C6H4-CH3), 2.21e2.19 (t, 2H, SCH2-CH2), 2.09e2.05 (m, 2H, SCH2),1.36e1.34 (d, 6H, -CH(CH3)2). MS (FAB): m/z 383 [MNO3]þ. Anal. Calc. for C15H26N2O5SRu: C, 40.26; H, 5.86; N, 6.26%. Found: C, 40.23; H, 5.88; N, 6.29%. 2.4. Synthesis of [(CO)2Ru(L-met)Cl] (3) To a slurry of [Ru(CO)2Cl2]n (46 mg, 0.20 mmol) in N,N0 -dimethylformamide (5 mL) was added a solution of L-methionine (30 mg, 0.20 mmol) and NaOMe (11 mg, 0.20 mmol) in methanol (10 mL), and then the mixture was heated at 90 C with stirring overnight, during which time there was a color change from yellow to orange. Addition of 30 mL diethyl ether to the reaction solution
gave orange precipitate which was filtered and washed with diethyl ether. Yield: 35 mg, 52%. IR (KBr disc, cm1): nNH 3440 (s), nC≡O 1988(s) and 1970(s), nC¼O 1625 (s), nCO 1210 (m). 1H NMR (400 MHz, CD3OD, ppm): 3.18e3.13 (m, 1H, met-CH), 2.54 (s, 3H, SCH3), 2.31e2.28 (t, 2H, SCH2-CH2), 2.13e2.09 (m, 2H, SCH2). MS (FAB): m/z 341 [Mþ], 313 [MþCO], 283 [Ru(L-met)Cl]þ, 250 [Ru(Lmet)]þ. Anal. Calc. for C7H10NO4SClRu: C, 24.67; H, 2.96; N, 4.11%. Found: C, 24.64; H, 2.99; N, 4.15%. 2.5. Synthesis of [(bpy)2Ru(L-met)]Cl (4) A mixture of L-methionine (15 mg, 0.10 mmol), NaOMe (6 mg, 0.10 mmol) and [Ru(bpy)2Cl2]$2H2O (52 mg, 0.10 mmol) were stirred at reflux in methanol (20 mL) for 6 h, during which the color of solution changed from purple to dark red. After removal of solvents in vacuum, methanol (20 mL) was added and the solution was filtered. The filtrate was concentrated and the residue was washed with diethyl ether (5 mL 2) to give the desired product. Recrystallization from methanol/diethyl ether (1:3) afforded dark red block crystals of [(bpy)2Ru(L-met)]Cl (4) suitable for X-ray diffraction in three days. Yield: 39 mg, 65%. IR (KBr disc, cm1): nNH 3448 (s), nC¼O 1618 (s), nC¼N 1424 (m), nCO 1261 (m). 1H NMR (400 MHz, CD3OD, ppm): d 9.35 (m, 2H, bpy), 9.02 (m, 1H, bpy), 8.69 (d, 2H, bpy), 8.57e8.44 (m, J ¼ 8.6 Hz, 2H, bpy), 8.29e8.13 (m, 2H, bpy), 7.93 (m, 1H, bpy), 7.84 (d, J ¼ 7.6 Hz, 3H, bpy), 7.52 (m, 1H, bpy), 7.30e7.10 (m, 2H, bpy), 3.51 (m, 1H, met-CH), 2.62 (s, 3H, SCH3), 2.35e1.97 (m, 4H, CH2). MS (FAB): m/z 562 [MCl]þ. Anal. Calc. for C25H26N5O2SClRu: C, 50.29; H, 4.39; N, N, 11.73%. Found: C, 50.32; H, 4.35; N, 11.70%. 2.6. Synthesis of [Ru(L-met)(NO)(PPh3)Cl]Cl (5) To a solution of L-methionine (30 mg, 0.20 mmol) and NaOMe (11 mg, 0.20 mmol) in methanol (25 mL) was added [Ru(NO)(PPh3)2Cl3] (153 mg, 0.20 mmol), during which the color of solution changed from yellow-brown to dark brown. After removal of solvents in vacuum, the residue was washed with diethyl ether (5 mL 2) to give the desired product which was recrystallized from methanol/diethyl ether (1:3). Yield: 72 mg, 62%. IR (KBr disc, cm1): nNH 3420 (s), nC¼O 1631 (s), nNO 1560 (s), nCO 1235 (m), nCP 654 (w). 1H NMR (400 MHz, DMSO‑d6, ppm): 9.65 (m, 2H, NH2), 7.45 (m, 15H, PPh3), 3.26e3.23 (m, 1H, met-CH), 2.91 (s, 3H, SCH3), 2.67 (m, 2H, SCH2-CH2), 2.31 (m, 2H, SCH2). 31P NMR (162 MHz, DMSO‑d6, ppm): 39.06. MS (FAB): m/z 577 [Ru(Lmet)(NO)(PPh3)Cl]þ, 542 [Ru(L-met)(NO)(PPh3)]þ, 512 [Ru(Lmet)(PPh3)]þ, 315 [Ru(L-met)(NO)Cl]þ, 250 [Ru(L-met)]þ. Anal. Calc. for C23H25N2O3SCl2PRu: C, 45.10; H, 4.11; N, 4.57%. Found: C, 45.12; H, 4.08; N, 4.58%. 2.7. Synthesis of [Ru(L-met)(CH3CN)Cl2] (6) To a slurry of (Et4N)[Ru(CH3CN)2Cl4] (91 mg, 0.20 mmol) in N,N0 dimethylformamide (5 mL) was added L-methionine (30 mg, 0.20 mmol) and NaOMe (11 mg, 0.20 mmol) in methanol (10 mL), and then the mixture was heated at 90 C with stirring for 8 h, during which time there was a color change from yellow-brown to orange. The solvent was removed in vacuum and the residue was washed with diethyl ether. Recrystallization from methanol/diethyl ether (1:3) afforded the desired product [Ru(L-met)(CH3CN)Cl2] (6). Yield: 41 mg, 58%. IR (KBr disc, cm1): nNH 3434 (s), nC≡N 2241 (m), nC¼O 1623 (s), nCO 1246 (m). MS (FAB): m/z 361 [Mþ], 320 [MþCH3CN], 314 [MþCl], 279 [Mþ2Cl], 250 [Ru(L-met)]þ. Anal. Calc. for C7H13N2O2SCl2Ru: C, 23.28; H, 3.63; N, 7.76%. Found: C, 23.25; H, 3.65; N, 7.73%.
J. Ji et al. / Journal of Organometallic Chemistry 907 (2020) 121078
2.8. Synthesis of [Ru(L-met)(DMSO)2Cl] (7) To a solution of L-methionine (30 mg, 0.20 mmol) and NaOMe (11 mg, 0.20 mmol) in methanol (10 mL) was added a solution of cis-[Ru(DMSO)4Cl2] (97 mg, 0.20 mmol) in methanol (5 mL). The mixture was stirred for 6 h at reflux, during which time there was a color change from yellow to pale yellow. The solvent was removed in vacuum and the residue was washed with diethyl ether. The residue was extracted with methanol (10 mL) and yellow product of [Ru(L-met)(DMSO)2Cl] (7) were obtained after allowing the filtrate layered with diethyl ether (15 mL) at room temperature. Yield: 47 mg, 53%. IR (KBr disc, cm1): nNH 3466 (s), nC¼O 1657 (s), nCO 1401 (m), nS¼O 1091 (s). 1H NMR (400 MHz, CD3OD, ppm): 3.81 (m, 1H, met-CH), 2.98e2.71 (m, 12H, DMSO), 2.49 (s, 3H, SCH3), 2.64e2.49 (m, 2H, SCH2-CH2), 2.39e2.32 (m, 2H, SCH2). MS (FAB): m/z 441 [Mþ], 406 [MþCl], 363 [MþDMSO], 285 [Mþ2DMSO], 250 [Ru(L-met)]þ. Anal. Calc. for C9H22NO4S3ClRu: C, 24.51; H, 5.03; N, 3.18%. Found: C, 24.49; H, 5.07; N, 3.20%. 2.9. Synthesis of [(COD)Ru(m-S-L-met)(m-Cl) RuCl(L-metNMe2)] (8) To a slurry of [Ru(COD)Cl2]2 (56 mg, 0.20 mmol) in N,N0 -dimethylformamide (5 mL) was added L-methionine (30 mg, 0.20 mmol) and NaOMe (11 mg, 0.20 mmol) in methanol (10 mL). The mixture was stirred for 30 h at reflux and the red-brown solution with a little brown precipitate formed. The precipitate was removed by filtration and the solvent was removed in vacuum and the residue was washed with diethyl ether and further recrystallized from methanol/diethyl ether at room temperature. Orange-red block crystals of [(COD)Ru(m-S-L-met)(m-Cl) RuCl(L-metNMe2)] (8) suitable for X-ray diffraction were obtained in a week. Yield: 35 mg, 50%. IR (KBr disc, cm1): nNH 3476 (s), nC¼O 1651 (s), nCO 1330 (m). 1 H NMR (400 MHz, CD3OD, ppm): d 4.07 (m, 2H, olefinic H of COD), 3.84 (m, 2H, olefinic H of COD), 3.69 (2 m, 2H, met-CH), 3.32 (s, 3H, NCH3), 3.30 (s, 3H, NCH3), 2.77 (2s, 6H, SCH3), 2.42e2.23 (m, 8H, met-CH2), 2.17 (m, 8H, aliphatic H of COD). MS (FAB): m/z 706 [Mþ], 671 [MþCl], 594 [MþCOD], 559 [MþClCOD].Anal. Calc. for C20H36N2O4S2Cl2Ru2: C, 34.04; H, 5.14; N, 3.97%. Found: C, 34.07; H, 5.13; N, 3.95%. 2.10. X-ray crystallography Crystallographic data and experimental details for ruthenium(II) complexes [(h6-C6H6)Ru(L-met)](NO3) (1), [(h6-p-cymene)Ru(L-met)](NO3) (2), [(bpy)2Ru(L-met)]Cl (4) and [(COD)Ru(mS-L-met)(m-Cl) RuCl(L-metNMe2)] (8) are summarized in Table 1. Intensity data were collected on a Bruker SMART APEX 1000 CCD diffractometer using graphite-monochromated Mo Ka radiation (l ¼ 0.71073 Å). The collected frames were processed with the software SAINT [31]. The data were corrected for absorption using the program SADABS [32]. Structures were solved by Direct Methods and refined by full-matrix least-squares on F2 using the SHELXTL software package [33,34]. All non-hydrogen atoms were refined anisotropically. The positions of all hydrogen atoms were generated geometrically (Csp3eH ¼ 0.96 Å, Csp2eH ¼ 0.93 Å and NeH ¼ 0.86 Å), assigned isotropic thermal parameters, and allowed to ride on their respective parent carbon atoms before the final cycle of least-squares refinement. 2.11. Photocatalyst testing The H2 production reactions were carried out in an outer irradiation-type photoreactor (Pyrex glass) connected to a closed gas-circulation system. A 300 W Xe lamp was afforded as light source, which was collimated and focalized into 5 cm2 parallel
3
faculae, then translated into uprightness light by a viewfinder. A cutoff filter (L-42; l > 420 nm) was employed to obtain visible light irradiation. The reaction was performed in distilled water (50 mL) and methanol (5 mL) solution containing the photocatalyst ruthenium complexes (5 mg), then the solution was thoroughly degassed to remove air, and the reactor was irradiated from the top with the visible light (l > 420 nm). An online gas chromatography with a thermal conductive detector (TCD) was equipped to the reaction system in order to detect amount of H2 evolution after photocatalytic reaction by using argon (Ar) as carrier gas. 3. Results and discussion 3.1. Preparation and characterization of ruthenium complexes The synthetic route of ruthenium complexes 18 with Lmethionine ligands is summarized in Scheme 1. Mononuclear organo-ruthenium complexes similar to 1 and 2 with the same cation have previously been reported according to a Cambridge Crystallographic Data Centre (CCDC) search [17,18], whereas the anion in complex 1 was NO 3 rather than PF6 . The recrystallization conditions were quite different, the complex 2 was recrystallized from methanol/diethyl ether (1:4) at room temperature in our preparation, which is comparable to that in the reported literature with a harsh addition of diisopropyl ether in 96% ethanol at 20 C [18]. The two chloro ligands in [Ru(bpy)2Cl2]$2H2O were replaced by the nitrogen and oxygen atoms of L-methionine ligand to afford complex 4, possibly due to the steric hindrance presence of the methyl substituent on the sulfur atom. Treatment of L-methionine ligand in the presence of base NaOMe with equal equiv. of [Ru(CO)2Cl2]n, [Ru(NO)(PPh3)2Cl3], (Et4N)[Ru(CH3CN)2Cl4] or cis[RuCl2(DMSO)4] gave accordingly complexes [(CO)2Ru(L-met)Cl] (3), [Ru(L-met)(NO)(PPh3)Cl]Cl (5), [Ru(L-met)(CH3CN)Cl2] (6) and [Ru(L-met)(DMSO)2Cl] (7), respectively, in which L-methionine ligand coordinated to the ruthenium atom in an expected k3-S,N,Omode. Reaction of [Ru(COD)Cl2]2 and L-methionine afforded a dinuclear ruthenium(II) complex [(COD)Ru(m-S-L-met)(m-Cl) RuCl(L-metNMe2)] (8), similar to the reported binuclear ruthenium complex [Ru(dppb)(bipy)Cl(CH3S(CH2)2NH2CHCOO)Ru(dppb)(bipy)](PF6)2 [19]. One of NH2 moieties in complex 8 turned to a NMe2 moiety, possibly due to a nucleophilic attack of the amine on formaldehyde intermediate generated by a hydrogen transfer from methanol in the presences of NaOMe as a base and ruthenium-COD complex as a catalyst [35,36]. The ruthenium-COD complex efficiently activated methanol as N-methylation reagent to give related nitrogen complexes. The positions of the nNH2 (~3440 cm1) and nC¼O (~1635 cm1) bands in the infrared spectra of complexes 18 are typical for coordinated amino and carboxylate functions [15,19]. The IR spectrum of complex 3 have nC≡O at 1988 and 1970 cm1, which are comparable to those in other carbonyl ruthenium(II) complexes [22]. The chemical shifts of benzene groups in complex 1 were found at around 5.92 ppm as a singlet. The eNH2 proton in complex 5 was observed at around 9.65 ppm in the 1H NMR spectrum. Moreover, one 31P signal 39.06 ppm was observed in the 31P NMR spectrum of complex 5 due to the PPh3 ligands. The proton (eSCH3) resonances exhibit as a typical singlet peak at around 2.52 ppm in the ruthenium(II) complexes 15, 7 and 8 compared well with those in other ruthenium complexes containing L-methionine ligands, such as [RuCl(D/L-met)(PPh3)2]$CH3OH (2.20 ppm) [15] and [(h6-pcymene)Ru(L-met)]Cl$0.4H2O (2.71 ppm) [18]. The UVevis absorption spectra of complexes 18 in an aqueous solution at room temperature are shown in Fig. 1. At the first glance, all spectra for complexes 18 show strong intense transitions located at 288 nm, the bands are ascribed to n/p* transition of
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J. Ji et al. / Journal of Organometallic Chemistry 907 (2020) 121078
Table 1 Crystallographic data and experimental details for [(h6-C6H6)Ru(L-met)](NO3) (1), [(h6-p-cymene)Ru(L-met)](NO3) (2), [(bpy)2Ru(L-met)]Cl (4) and [(COD)Ru(m-S-L-met)(m-Cl) RuCl(L-metMe2)] (8). complex
1
2
4
8
empirical formula formula weight crystal system a (Å) b (Å) c (Å) b ( ) V (Å3) space group Z Dcalc (g cm3) temperature (K) F(000) m(Mo-Ka) (mm1) total refln independent refln parameters Rint R1a, wR2b (I > 2s(I)) R1, wR2 (all data) GoFc
C11H16N2O5SRu 389.39 orthorhombic 7.6736(18) 9.026(2) 19.470(4)
C15H24N2O5SRu 445.49 orthorhombic 7.9088(10) 13.9909(17) 16.131(2)
C20H36N2O4S2Cl2Ru2 705.67 orthorhombic 7.971(3) 12.421(5) 29.980(11)
1348.6(5) P212121 4 1.918 296(2) 784 1.338 8529 3086 183 0.0611 0.0481, 0.1177 0.0658, 0.1374 0.849
1784.9(4) P212121 4 1.658 296(2) 912 1.022 11439 4135 222 0.0399 0.0316, 0.0707 0.0400, 0.0780 0.922
C25H26N5O2SClRu 597.09 monoclinic 12.363(10) 28.09(2) 9.806(8) 108.962(8) 3221(4) P21/c 4 1.231 296(2) 1216 0.660 15794 5654 344 0.1096 0.1368, 0.2769 0.1925, 0.2951 1.048
a b c
2968(2) P212121 4 1.579 296(2) 1424 1.364 18606 6700 295 0.0705 0.0620, 0.1630 0.0801, 0.1800 1.090
R1 ¼ jjFo j jFc jj=jFo j. 2 2 1=2 wR2 ¼ ½wð F 2o F 2c Þ =w F 2o . GoF ¼ ½wðjFo j jFc jÞ2 =ðNobs Nparam Þ1=2 .
Scheme 1. Syntheses and reactivity of ruthenium(II) complexes 18 with L-methionine ligands. Reagents and conditions: (i) [(h6-C6H6)RuCl2]2, AgNO3, NaOMe, MeOH, at reflux; (ii) [(h6-p-cymene)RuCl2]2, AgNO3, NaOMe, MeOH, at reflux; (iii) [Ru(CO)2Cl2]n, NaOMe, MeOH/DMF, 85 C; (iv) [Ru(bpy)2Cl2]$2H2O, NaOMe, MeOH, at reflux; (v) [Ru(NO)(PPh3)2Cl3], NaOMe, MeOH, 85 C; (vi) (Et4N)[Ru(CH3CN)2Cl4], NaOMe, MeOH/DMF (2:1), at reflux; (vii) [Ru(DMSO)4Cl2], NaOMe, MeOH, at reflux; (viii) [Ru(COD)Cl2]2, NaOMe, MeOH/DMF, 85 C.
eC¼O. The bands at 320370 nm for complexes 1, 2, 6 and 8 may be attributed to the metal-to-ligand charge transfer (MLCT) transition. It is interesting to note that there is a red-shift of about 50 nm (372 nm for 1, 368 nm for 2, 320 nm for 6) observed for the transition depending on the presence of a h6-C6H6 or h6-cymene
ligand. In complex 4, the band at 350 nm may be assigned to a typical spin-allowed pep* transition. The intense transitions for complex 4 located at 493 nm may indicate a metal-to-ligand charge transfer (MLCT) transition, which is comparable to a similar ruthenium complex [Ru(k2-C,N-L1)(phen)2](PF6) (L1 ¼ 1-(2,6-
J. Ji et al. / Journal of Organometallic Chemistry 907 (2020) 121078
5
Fig. 1. UV/Vis spectra of the complexes 18 measured in an aqueous solution.
diisopropylphenoxy)- 4-phenylphthalazine, 492 nm) [37]. 3.2. X-ray crystal structures of ruthenium complexes 1, 2, 4 and 8
Fig. 3. Structure of cation [(h6-p-cymene)Ru(L-met)]þ in complex 2 showing the atomlabeling scheme, the NO 3 counterion are omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level.
The structures of ruthenium(II) complexes 1, 2, 4 and 8 have been confirmed by single crystal X-ray crystallography. The perspective views of the molecular structures of complexes 1, 2, 4 and 8 are shown in Fig. 25, respectively. Selected bond lengths and bond angles for complexes 1, 2, 4 and 8 are summarized in Table 2 for the comparison. Four complexes 1, 2, 4 and 8 adopt chiral space group P212121 or P21/c owing to the coordination of the chiral ligand L-methionine. Complexes 1 and 2 could be formulated as six-coordinate ruthenium species assuming the benzene or cymene moiety occupied three coordination sites and the sixmembered chelate shows a distorted chair conformation with axial position of the thioether sulfur donors [18]. The RuN, RuO and RuS bond lengths in complexes 1, 2, 4 and 8 are in the ranges
Fig. 4. Structure of cation [(bpy)2Ru(L-met)]þ in complex 4 showing the atom-labeling scheme, the Cl counterion and the water molecule are omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level.
Fig. 2. Structure of cation [(h6-C6H6)Ru(L-met)]þ in complex 1 showing the atomlabeling scheme, the NO 3 counterion is omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level.
of 2.099(8)2.172(1), 2.089(3)2.115(8), and 2.262(4)2.394(2) Å, respectively, similar to those in the structure-related complexes [RuCl(L-metme)2(PPh3)]Cl [15] and [(nbd)Ru(d,l-met)Cl] [16]. The bite NRuO bond angles are in the range of 76.7(4)o80.6(4)o in complex 1, 2, 4 and 8, which are compared with those in the reported cationic ruthenium nbd complexes [RuCl(L-his)(h4-nbd)] (Lhis ¼ L-histidine, NRuO 78.1(3)o) [15]. The SRuN bond angle is 84.8(2)o and the SRuO bond angle is 89.5(2)o in complex 1, similar to those in complex 2 (SRuN 83.35(12)o and SRuO 90.57(11)o). In complex 4, the center ruthenium atom is coordinated to two bpy ligands, oxygen and nitrogen atoms of L-methionine which participates in a five-membered chelate ring. The RuN bond lengths of bpy ligands are in the range of 2.017(11) 2.077(11) Å, which are compared with those in a related ruthenium
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J. Ji et al. / Journal of Organometallic Chemistry 907 (2020) 121078
Fig. 5. Molecular structure of complex 8 showing the atom-labeling scheme. Thermal ellipsoids are drawn at the 30% probability level.
Table 2 Selected bond lengths (Å) and angles ( ) for complexes 1, 2, 4 and 8. complex
RuN
RuO
RuS
RuCl
SRuN
SRuO
NRuO
1 2 4
2.099(8) 2.117(4) 2.077(11) 2.036(11) 2.046(12) 2.077(11) 2.102(13) 2.116(11) 2.172(10)
2.091(6) 2.089(3) 2.089(9)
2.394(2) 2.3801(13) e
e e e
84.8(2) 83.35(12) e
89.5(2) 90.57(11) e
77.2(3) 77.70(15) 80.6(4)
2.115(8) 2.104(9)
2.372(4) 2.300(3) 2.262(4)
2.446(3) 2.413(3) 2.501(3)
85.5(3) 94.6(3)
88.1(3) 91.1(3)
78.6(3) 76.7(4)
8
complex [Ru(bpy)2(L)](PF6)2$2.2MeCN (L ¼ N-phenylpyridin-2ylmethanimine, 2.054(2)2.071(2) Å) [38]. It is interesting to note that complex 8 presents a binuclear structure where two ruthenium atoms are bridged by one chlorine atom and one sulfur atom of L-methionine. One ruthenium center Ru(1) further coordinated to a chelating L-methionine and large h4-COD ligand, the other ruthenium center Ru(2) additionally coordinated to a Lmethionine and a chloride atom. The terminal Ru(2)Cl(2) bond length is 2.413(3) Å, as expected, shorter than the bridging RuCl bond lengths (2.446(3)2.501(3) Å) in complex 8. The bridging RuS bond lengths are 2.300(3) and 2.372(4) Å in complex 8, a little longer than the Ru(2)S(2) bond lengths of the L-methionine ligand (2.262(4) Å). The SRuN bond angles of 85.5(3) and 94.6(3) in complex 8 are normal for the related ruthenium complex [RuCl3(L-metet)](PPh3)] (90.6(2) Å) [15]. 3.3. Photocatalytic H2 production In Fig. 6, the photocatalytic activity of ruthenium complexes 1, 2, 4 and 8 towards the water splitting hydrogen production is shown in intervals of periodic evaluation of 0.5 h up to a total of 5 h of irradiation. The amount of hydrogen gas evolved increase almost linearly with time indicating the high stability of corresponging ruthenium photocatalysts under visible light illumination. In Table 3, the band gap values are indicative of the ability of these
complexes to be active as photocatalysts under the visible light electromagnetic spectrum [39]. The total amount of H2 evolved from complex 2 after 5 h irradiation was 9.32 mmol, which was little lower than those produced from complexes 1 (10.61 mmol), 4 (10.69 mmol) and 8 (10.67 mmol). The results reveal that ruthenium complexes 1, 2, 4 and 8 as photocatalysts enhance light absorption greatly and improve the hydrogen production by water spiltting [40]. Fig. 7 shows the time courses of H2 evolution over different content of ruthenium complex 4. As can be seen, the H2 evolution rate increases upon enhancing the concentration of catalyst 4, in which the best content was 15 mg (2.34 mmol/h). The total amount of H2 evolved was stable when the catalyst was over 20 mg, for example, 11.50 mmol H2 evolved when 20 mg or 25 mg catalyst was used. It is known that the sacrificial reagent is an important parameter for the photocatalytic performance H2 evolution. As a result, four different sacrificial reagents of triethylamine, methanol, triisopropanolamine and triethanolamine were employed for the hydrogen production by water spiltting. As depicted in Fig. 8, the total amounts of H2 produced in the irradiation were 9.61, 10.68, 10.94 and 11.48 mmol for triethylamine, methanol, triisopropanolamine and triethanolamine, respectively. The results reveal that triethanolamine may be a better sacrificial reagent possibly due to its strong alkaline and electron donating properties.
J. Ji et al. / Journal of Organometallic Chemistry 907 (2020) 121078
Fig. 6. Time courses of photocatalytic evolution of H2 using ruthenium complexes 1, 2, 4 and 8 suspended in CH3OH aqueous solution under UV irradiation (l > 420 nm). The curves were measured by a H2 sensor and calibrated by GC analysis. Catalyst: 5 mg, pure water: 50 mL, methanol: 5 mL.
Table 3 Photophysical properties and hydrogen evolution rates for the ruthenium photocatalysts. Photocatalyst
Band gap (eV)a
Total H2 evolved (mmol)b
H2 evolution rate (mmol$h1)b
Complex Complex Complex Complex
2.52 2.54 1.91 2.34
10.61 9.32 10.69 10.67
2.12 1.86 2.14 2.13
1 2 4 8
a
Calculated from the onset of the absorption spectrum. Reaction conditions: 5 mg ruthenium complex was suspended in 50 mL water solution and irradiated by a 300 W Xe lamp (l > 420 nm visible filter) for 5 h. b
7
Fig. 8. Time courses of photocatalytic evolution of H2 using ruthenium complex 4 with different sacrificial reagents under UV irradiation (l > 420 nm). The curves were measured by a H2 sensor and calibrated by GC analysis. Pure water: 50 mL, catalyst: 5 mg.
Afterwards, the photogenerated holes transfer from the valence band, thus preserving the holes can oxidize water to produce Hþ and OH [41,42]. Meanwhile, the photogenerated electrons transfer from the conduction band to ruthenium complexes and take part in reduction reaction, such as reduction of proton to form H2. Additionally, the sacrificial reagent methanol can be oxidized by OH to give methanoic acid, accelerating the decomposition of H2O. Ruthenium(II) center acts as an electron donor, which improves the electron transfer efficiency and the separation of the photogenerated electron-hole pairs (inhibit their recombination), thus enhancing the photocatalytic water splitting behavior of ruthenium. In summary, a series of mono- and di-nuclear neutral and cationic ruthenium(II)/(III) complexes with L-methionine ligands were synthesized and spectroscopically characterized. Structures of the four ruthenium(II) complexes 1, 2, 4 and 8 were unambiguously established by single-crystal X-ray diffraction, presenting the rich coordination modes of L-methionine ligands. Up to now, there are few reported ruthenium complexes bearing L-methionine ligand to our best knowledge, possibly due to their diversity of coordination modes and difficulty in crystallization. Ruthenium complexes 1, 2, 4 and 8 were also tested as photocatalytic catalysts, which improves hydrogen evolution rate (2 mmol h1) of water splitting. Exploration of ruthenium complexes as pre-catalysts for transfer hydrogenation of acetophenone is underway in our laboratory. Supplementary material
Fig. 7. Time courses of photocatalytic evolution of H2 using ruthenium complex 4 with different content in CH3OH aqueous solution under UV irradiation (l > 420 nm). The curves were measured by a H2 sensor and calibrated by GC analysis. Pure water: 50 mL, methanol: 5 mL.
A possible mechanism of photocatalytic splitting water by ruthenium complexes photocatalysts have been proposed. Upon visible light irradiation excitation, ruthenium complexes can be excited and generate photogenerated electron-hole pairs.
Crystallographic data for [(h6-C6H6)Ru(L-met)](NO3) (1), [(h6-pcymene)Ru(L-met)](NO3) (2), [(bpy)2Ru(L-met)]Cl (4) and [(COD) Ru(m-S-L-met)(m-Cl)RuCl(L-metMe2)] (8) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC 1947239-1947242, respectively. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax: (þ44)1233-336-033; email: [email protected]]. Declaration of competing interest The authors declare that they have no known competing
8
J. Ji et al. / Journal of Organometallic Chemistry 907 (2020) 121078
financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[20]
Acknowledgements [21]
This project was supported by the Natural Science Foundation of China (21372007).
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