alexa 2-Methyl-1,4-Benzodioxan Substituted Ag(I)-N-Heterocyclic Carbene Complexes and Ru(II)-N-Heterocyclic Carbene Complexes: Synthesis, Crystal Structure and Transfer Hydrogenation of Aromatic Ketones | Open Access Journals
ISSN: 2161-0401
Organic Chemistry: Current Research
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2-Methyl-1,4-Benzodioxan Substituted Ag(I)-N-Heterocyclic Carbene Complexes and Ru(II)-N-Heterocyclic Carbene Complexes: Synthesis, Crystal Structure and Transfer Hydrogenation of Aromatic Ketones

Aydın Aktaş1*, Yetkin Gök1, Mehmet Akkurt2 and Namık Özdemir3
1Department of Chemistry, Faculty of Arts and Sciences, Inönü University, Malatya, Turkey
2Department of Physics, Faculty of Sciences, Erciyes University, Kayseri, Turkey
3Department of Physics, Faculty of Arts and Sciences, Ondokuz Mayis University, Samsun, Turkey
Corresponding Author : Aydın Aktaş
Department of Chemistry
Faculty of Arts and Sciences
Inönü University, 44280
Malatya, Turkey
Tel: +905356891975
Fax: +904222384712
E-mail: [email protected]
Received: August 26, 2015; Accepted: September 08, 2015; Published: September 15, 2015
Citation: Aktaş A, Gök Y, Akkurt M, Özdemir N (2015) 2-Methyl-1,4-Benzodioxan Substituted Ag(I)-N-Heterocyclic Carbene Complexes and Ru(II)-N-Heterocyclic Carbene Complexes: Synthesis, Crystal Structure and Transfer Hydrogenation of Aromatic Ketones. Organic Chem Curr Res 4:149. doi:10.4172/2161-0401.1000149
Copyright: © 2015 Aktaş A, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Abstract

In this study a series of unsymmetrically silver(I)-N-heterocyclic carbene (NHC) and ruthenium(II)-NHC complexes were synthesised. The Ag(I)-NHC complexes (1a-f) were synthesized in dichloromethane at room temperature from the imidazolium salts and Ag2O. The Ru(II)-NHC complexes (2a-f) were prepared from Ag(I)-NHC complexes by using transmetallation method. All compounds were characterized by spectroscopic techniques (NMR and FT-IR) and elemental analyses. The catalytic activities of the Ru(II)-NHC complexes that were investigated in the transfer hydrogenation reactions showed excellent activity. Also, the 2a complex was characterized by single crystal X-ray crystallography. The Ru atom in the 2a complex, [RuCl2(η6-C10H14)(C19H20N2O2)] exhibit a pseudo-octahedral geometry, with the arene occupying three adjacent sites of the octahedron, two Cl atoms and one carbene group. The six-membered ring of the p-cymene is essentially planar [rms deviation=0.008 Å].

Keywords
N-Heterocyclic carbene; Silver and ruthenium complexes; Homogeneous catalysis; Transfer hydrogenation
Introduction
In the last decades, using NHC’s as ligands has increased in homogeneous catalysis [1-3]. Since the free NHC once isolated in 1991 by Arduengo et al. [4-6], these ligands in coordination chemistry and organometallic catalysis have received considerable interest [7]. The structure of NHC ligands is very diverse. However, the most commonly used catalytic complexes: are imidazol-2-ylidenes, imidazolin-2-ylidenes, thiazo- 2-ylidenes and triazo-5-ylidenes. These complexes containe functional ligands (actor ligand, e.g., hydride, alkylidene) and a variety of other ligands are spectator (e.g., ancillary). The protective steric substituents in heterocyclic structure have showed high temperature stability, strong σ-donor and weak π-acceptor character and in very durable metal-NHC bond [8-11]. However, specific features, especially electronic, steric properties, reactivity and catalytic activities of these ligands can be set. Because of the these properties, NHC ligands have been shown as an alternative to phosphines in organometallic catalysis [12]. In recent years, the applications of organic transformations including hydrogenation [13], hydrosilylation [14], copolymerization [15], hydroformylation [16], olefin metathesis [17], C-N [18] and C-C [19] coupling reactions and asymmetric transformations [20] have involved a wide range of metal-NHC complexes as catalysts. The Ag(I)-NHC complexes have attracted continuous attention [21]. Many other metal-NHC complexes have been obtained by using appropriate transfer reagents Ag(I)-NHC complexes. For example, Pd, Ru, Ni, Rh and Ir NHC complexes successfully have been transferred by using this method [22-26]. In the commonly used method in the synthesis of Ag(I)-NHC complexes, a silver base has been used as agent of deprotonation. In this context silver bases (Ag2O, AgOAc, and Ag2CO3) and the solvents (1,2-dichloroethane, acetone, acetonitrile, DMF, DMSO and methanol) have been used [27-30]. Ruthenium complexes, recently playing an influential role in a variety of studies on catalytic processes have made a great progress. After these developments, a lot of ruthenium complexes were synthesized, designed successfully and used in chemical reactions. Ruthenium complexes with organic or inorganic ligands are positioned around the metal core, with a suitable choice of the electronic and steric environment (e.g., cloride, phosphane, NHC and schiff bases) sometimes arene, alkylidene, vinylidene, allenylidene or cumulenydene substitutes have taken place [31-36]. Among these substituents, NHCs are stable to air and moisture, as great as to make durable bond with the metal which is remarkable property [37,38]. The structural motifs of Ru(II)-NHC complexes have found wide application in catalytic processes [39,40]. The positive features of other known of Ru(II)-NHC complexes, ruthenium presents in the coordination geometry and it has a wide range of oxidation [41]. Catalytic transfer hydrogenation in homogeneous phase has become a common tool in synthetic chemistry. Officially, transfer hydrogenation is the reaction of an unsaturated reagent with molecule H2. In most publications, 2-propanol by addition of a suitable base is used as hydrogen donor. Occasionally, system formic acid/triethylamine is used. 2- propanole is oxidized to acetone in the process of transfer hydrogenation [42]. Usually in industrial processes, transfer hydrogenation is used instead of hydrogen because of its safety and low price. Our studies in this paper involve the use of a new series of 2-methyl-1,4- benzodioxan substituted NHC ligands, we here report the synthesis and structures of a number of Ag(I)-NHC and Ru(II)-NHC complexes. The catalytic activities of the Ru(II)-NHC complexes have been investigated as catalysts for the transfer hydrogenation of a range of ketons in the presence of 2-propanole and KOH as base. The moleculer and crystal structure of dichloro-[1-(2-methyl-1,4-benzodioxan)-3-benzylimidazolidin- 2-ylidene](p-cymene)ruthenium(II) complex 2a has been determined by single crystal X ray diffraction technique.
Experimental Section
All syntheses of Ag(I)-NHC complexes 1a-f and Ru(II)-NHC complexes 2a-f were carried out under an inert atmosphere in flame-dried glassware using standard Schlenk techniques. Solvents were purified by distillation over the drying agents indicated and were transferred under Ar: Et2O (Na/K alloy), CH2Cl2 (P4O10), hexane (Na). All other reagents were obtained commercially from Aldrich and used without further purification. Melting points were identified in glass capillaries under air with an Electrothermal-9200 melting point apparatus. FT-IR spectra were recorded as KBr pellets in the range 400-4000 cm-1 on an AT, UNICAM 1000 spectrometer. Proton (1H) and Carbon (13C) NMR spectra were recorded using a Varian AS 400 Merkur spectrometer operating at400 MHz (1H) or 100 MHz (13C) in CDCl3 and DMSO-d6 with tetramethylsilane as an internal reference. Products were investigated with an Agilent 6890 N GC system by GC-FID with a HP-5 column of 30 m length, 0.32 mm diameter and 0.25 μm film thickness. Column chromatography was performed using silica gel 60 (70-230 mesh). Elemental analyses were performed by Ä°nönü University Scientific and Technology Center. X-ray diffraction data for 2a were collected X-AREA [43]. Cell refinement: X-AREA [43]. Data reduction: X-RED32 [43]. Program(s) used to solve structure: SHELXS97 [44]. Program(s) used to refine structure: SHELXL97 [44]. Molecular graphics: ORTEP-3 for Windows [45]. Software used to prepare material for publication: WinGX [45]. Details of the crystallographic data and structure refinement for 2a is listed Table 1.
General procedure for the preparation of the silver-NHC complexes, 1a-f
To a solution of 2-methyl-1,4-benzodioxan substituted imidazolidinium salt (1.0 mmol) in dichloromethane (30 mL), Ag2O (0.5 mmol) and activated 4 molecular sieves was added. The reaction mixture was stirred for 24 hours at room temperature in dark condition. The reaction mixture was filtered through celite and the solvent were evaporated under vacuum to afford the product as a white solid. The crude product was recrystallized from dichloromethane/diethyl ether (1:3) at room temperature.
Synthesis of bromo[1-(2-methyl-1,4-benzodioxan)-3-benzylimidazol- 2-ylidene]silver(I), 1a
To a solution of 1-(2-methyl-1,4-benzodioxan)-3-benzylimidazol bromide (0.47 g., 1.2 mmol) in dichloromethane (30 mL), Ag2O (0.139g., 0.6 mmol) and activated 4 molecular sieves was added. The reaction mixture was stirred for 24 hours at room temperature in dark condition. The reaction mixture was filtered through celite and the solvent were evaporated under vacuum to afford the product as a white solid. The crude product was recrystallized from dichloromethane/diethyl ether (1:3) at room temperature. Yield: % 86 (0.51 g)
Analytical data for bromo[1-(2-methyl-1,4-benzodioxan)-3-benzylimidazol- 2-ylidene]silver(I), 1a 1H NMR (300 MHz, DMSO), δ 3.71 and 3.80 (m, 4H, -NCH2CH2N-); 4.00-4.04 (m, 2H, -NCH- 2CHOCH2-); 4.31-4.38 (m, 1H, -CH2CHOCH2-); 4.48-4.50 (m, 2H, -OCHCH2O-); 4.69 (s, 2H, -NCH2C6H5); 6.84-6.90 (m, 4H, -OC6H4O- Ar-H); 7.22-7.40 (m, 5H, -CH2C6H5 Ar-H). 13C NMR (300 MHz, DMSO), δ 47.1 and 47.6 (-NCH2CH2N-); 49.1 (-NCH- 2CHOCH2-); 54.3 (-NCH2C6H5); 66.5 (-CH2CHOCH2-); 71.5 (-OCHCH 2O-); 117.5, 121.5, 121.8, 122.1, 127.0, 127.6, 128.1, 129.1, 136.6, 137.9, 142.7 and 143.4. (Ar-C); 205.3 (C-Ag). m.p.: 153-155°C; ν(CN): 1666.8 cm-1. Anal. Calcd. for C19H20AgBrN2O2: C: 45.95; H: 4.03; N: 5.64. Found: C: 45.98; H: 4.02; N: 5.63.
Synthesis of bromo[1-(2-methyl-1,4-benzodioxan)-3-(2- methylbenzyl)imidazol-2-ylidene]silver(I), 1b
The synthesis of 1b was carried out in the same way as that described for 1a, but 1-(2-methyl-1,4-benzodioxan)-3-(2-methylbenzyl) imidazol bromide (0.48 g., 1.2 mmol) was used instead of 1-(2-methyl- 1,4-benzodioxan)-3-benzylimidazol bromide. Yield: % 85 (0.52 g) Analytical data for bromo[1-(2-methyl-1,4-benzodioxan)-3-(2-methylbenzyl) imidazol-2-ylidene]silver(I), 1b 1H NMR (300 MHz, DMSO), δ 2.31 (s, 3H, -CH2C6H4CH3); 3.79-3.82 (m, 4H, -NCH2CH2N-); 4.01- 4.05 (m, 2H, -NCH2CHOCH2-); 4.30-4.34 (m, 1H, -CH2CHOCH2-); 4.52-4.55 (m, 2H, -OCHCH2O-); 4.71 (s, 2H, -NCH2C6H4); 6.83- 6.88(m, 4H, -OC6H4O- Ar-H); 7.22-7.35 (m, 4H, -CH2C6H4(CH3) Ar- H). 13C NMR (300 MHz, DMSO), δ 19.7 (-CH2C6H4CH3); 47.8 and 48.7 (-NCH2CH2N-); 49.6 (-NCH2CHOCH2-); 52.4 (-NCH2C6H4); 65.4 (CH2CHOCH2-); 71.2 (-OCHCH2O-); 117.5, 121.9, 126.6, 126.8, 128.3, 129.8, 131.0, 132.0, 134.4, 137.4, 142.5 and 143.3. (Ar-C); 205.7 (C-Ag). m.p.: 174-177°C; ν(CN): 1645.9 cm-1. Anal. Calcd for C20H22AgBrN2O2: C: 47.04; H: 4.31; N: 5.49. Found: C: 47.06; H: 4.33; N: 5.47.
Synthesis of bromo[1-(2-methyl-1,4-benzodioxan)-3-(3- methylbenzyl)imidazol-2-ylidene]silver(I), 1c
The synthesis of 1c was carried out in the same way as that described for 1a, but 1-(2-methyl-1,4-benzodioxan)-3-(3-methylbenzyl) imidazol bromide (0.48 g., 1.2 mmol) was used instead of 1-(2-methyl- 1,4-benzodioxan)-3-benzylimidazol bromide. Yield: % 87 (0.53 g) Analytical data for bromo[1-(2-methyl-1,4-benzodioxan)-3-(3-methylbenzyl) imidazol-2-ylidene]silver(I), 1c 1H NMR (300 MHz, DMSO), δ 2.29 (s, 3H, -CH2C6H4CH3); 3.76-3.82 (m, 4H, -NCH2CH2N-); 3.98- 4.03 (m, 2H, -NCH2CHOCH2-); 4.31-4.33 (m, 1H, -CH2CHOCH2-); 4.53-4.55 (m, 2H, -OCHCH2O-); 4.65 (s, 2H, -NCH2C6H4); 6.82- 6.88(m, 4H, -OC6H4O- Ar-H); 7.10-7.30 (m, 4H, -CH2C6H4(CH3) Ar- H). 13C NMR (300 MHz, DMSO), δ 21.1 (-CH2C6H4CH3); 49.2 (-NCH- 2CHOCH2-); 50.7 and 51.8 (-NCH2CH2N-); 52.8 (-NCH2C6H4(CH3)); 66.1 (CH2CHOCH2-); 71.5 (-OCHCH2O-); 117.4, 117.5, 121.5, 122.1, 128.4, 129.1, 133.7, 137.4, 142.7, 137.4, 143.0 and 143.5. (Ar-C); 205.2 (C-Ag). m.p.: 182-185°C; ν(CN): 1661.5 cm-1. Anal. Calcd for C20H22AgBrN 2O2: C: 47.04; H: 4.31; N: 5.49. Found: C: 47.02; H: 4.33; N: 5.46.
Synthesis of bromo[1-(2-methyl-1,4-benzodioxan)-3-(4- methylbenzyl)imidazol-2-ylidene]silver(I), 1d
The synthesis of 1d was carried out in the same way as that described for 1a, but 1-(2-methyl-1,4-benzodioxan)-3-(3-methylbenzyl) imidazol bromide (0.48 g., 1.2 mmol) was used instead of 1-(2-methyl- 1,4-benzodioxan)-3-benzylimidazol bromide. Yield: % 91 (0.56 g) Analytical data for bromo[1-(2-methyl-1,4-benzodioxan)-3-(4-methylbenzyl) imidazol-2-ylidene]silver(I), 1d 1H NMR (300 MHz, DMSO), δ 2.26 (s, 3H, -CH2C6H4CH3); 3.76-3.82 (m, 4H, -NCH2CH2N-); 4.12- 4.15 (m, 2H, -NCH2CHOCH2-); 4.30-4.34 (m, 1H, -CH2CHOCH2-); 4.42-4.45 (m, 2H, -OCHCH2O-); 4.66 (s, 2H, -NCH2C6H4); 6.81-6.89 (m, 4H, -OC6H4O- Ar-H); 7.08-7.27 (m, 4H, -CH2C6H4(CH3) Ar-H). 13C NMR (300 MHz, DMSO), δ 21.1 (-CH2C6H4CH3); 47.5 (-NCH- 2CHOCH2-); 49.3 and 50.8 (-NCH2CH2N-); 52.8 (-NCH2C6H4); 66.5 (CH2CHOCH2-); 73.0(-OCHCH2O-); 117.4, 121.5, 121.9, 128.4, 129.0, 129.9, 133.5, 134.9, 136.0, 137.4, 138.6 and 143.6. (Ar-C); 205.7 (C-Ag). m.p.: 193-195°C; ν(CN): 1662.6 cm-1. Anal. Calcd for C20H22AgBrN2O2: C: 47.04; H: 4.31; N: 5.49. Found: C: 47.07; H: 4.33; N: 5.47.
Synthesis of bromo[1-(2-methyl-1,4-benzodioxan)-3-(2,4,6- trimethylbenzyl)imidazol-2-ylidene]silver(I), 1e
The synthesis of 1e was carried out in the same way as that described for 1a, but 1-(2-methyl-1,4-benzodioxan)-3-(2,4,6-trimethylbenzyl) imidazol bromide (0.52 g., 1.2 mmol) was used instead of 1-(2-methyl-1,4-benzodioxan)-3-benzylimidazol bromide. Yield: % 90 (0.58 g) Analytical data for bromo[1-(2-methyl-1,4-benzodioxan)- 3-(2,4,6-trimethylbenzyl)imidazol-2-ylidene]silver(I), 1e 1H NMR (300 MHz, DMSO), δ 2.14 and 2.26 (s, 9H, -CH2C6H2(CH3)3); 3.76-3.83 (m, 4H, -NCH2CH2N-); 3.93-3.96 (m, 2H, -NCH2CHOCH2-); 4.33- 4.36 (m, 1H, -CH2CHOCH2-); 4.48-4.51 (m, 2H, -OCHCH2O-); 4.78 (s, 2H, -NCH2C6H2); 6.81-6.87(m, 4H, -OC6H4O- Ar-H); 7.63-7.66 (m, 2H, -CH2C6H2(CH3)3 Ar-H). 13C NMR (300 MHz, DMSO), δ 15.6 and 19.6 (-CH2C6H2(CH3)3); 47.4 (-NCH2CHOCH2-); 48.3 (-NCH2CH2N-); 49.3 (-NCH2C6H2); 66.5 (CH2CHOCH2-); 72.9 (-OCHCH2O-); 117.4, 121.8, 122.1, 126.7, 128.9, 129.5, 133.6, 136.9, 138.3, 143.5 and 145.1. (Ar-C); 204.9 (C-Ag). m.p.: 182-185°C; ν(CN): 1665.9 cm-1. Anal. Calcd for C22H26AgBrN2O2: C: 49.05; H: 4.82; N: 5.20. Found: C: 49.08; H: 4.85; N: 5.19.
Synthesis of bromo[1-(2-methyl-1,4-benzodioxan)-3- (2,3,5,6-tetramethylbenzyl)imidazol-2-ylidene]silver(I), 1f
The synthesis of 1f was carried out in the same way as that described for 1a, but 1-(2-methyl-1,4-benzodioxan)-3-(2,3,5,6-tetramethylbenzyl) imidazol bromide (0.53 g., 1.2 mmol) was used instead of 1-(2-methyl-1,4-benzodioxan)-3-benzylimidazol bromide. Yield: % 86 (0.57 g) Analytical data for bromo[1-(2-methyl-1,4-benzodioxan)- 3-(2,3,5,6-tetramethylbenzyl)imidazol-2-ylidene]silver(I), 1f 1H NMR (300 MHz, DMSO), δ 2.18 (s, 12H, -CH2C6H(CH3)4); 3.64- 3.74 (m, 4H, -NCH2CH2N-); 3.94-4.02 (m, 2H, -NCH2CHOCH2-); 4.28-4.38 (m, 1H, -CH2CHOCH2-);4.46-4.51 (m, 2H, -OCHCH2O-); 4.63 (s, 2H, -NCH2C6H); 6.79-6.88(m, 4H, -OC6H4O- Ar-H); 6.96 (s, 1H, -CH2C6H(CH3)4 Ar-H). 13C NMR (300 MHz, DMSO), δ 16.3 and 20.7 (-CH2C6H(CH3)4); 48.2 (-NCH2CHOCH2-); 49.4 and 49.7 (-NCH2CH2N-); 52.1 (-NCH2C6H); 65.4 (CH2CHOCH2-); 71.0 (-OCHCH 2O-); 117.5, 117.6, 121.9, 122.1, 131.7, 132.1, 133.1, 133.8, 134.1, 134.3, 142.7 and 143.2. (Ar-C); 204.6 (C-Ag). m.p.: 105-107°C; ν(CN): 1667.2 cm-1. Anal. Calcd for C23H28AgBrN2O2: C: 49.97; H: 5.07; N: 5.07. Found: C: 49.99; H: 5.09; N: 5.06.
General procedure for the preparation of the ruthenium- NHC complexes, 2a-f
The ruthenium complexes (2a-f) have been prepared from NHCsilver complexes by transmetallation method [46]. To a mixture of Ag(I)-NHC complexes (2 mmol), Di-μ-chloro-bis[chloro(η6-1- isopropyl-4-methylbenzene)ruthenium(II)] (1 mmol) and dichloromethane (30 mL) was stirred for 24 hours at room temperature in dark condition. The mixture was filtered through Celite and the solvent were evaporated under vacuum to afford the product as a red-brown solid. The crude product was recrystallized from dicloromethane:dietylether (1:3) at room temperature.
Synthesis of dichloro[1-(2-methyl-1,4-benzodioxan)-3- benzylimidazolidin-2-ylidene](p- cymene)ruthenium(II), 2a
To a solution of bromo[1-(2-methyl-1,4-benzodioxan)-3-benzylimidazol- 2-ylidene]silver(I) (0.14g., 0.28 mmol) in dichloromethane (30 mL), Di-μ-chloro-bis[chloro(η6-1-isopropyl-4-methylbenzene) ruthenium(II)] (0.086 g, 0.14 mmol) was added. The reaction mixture was stirred for 24 hours at room temperature in dark condition. The reaction mixture was filtered through celite and the solvent were evaporated under vacuum to afford the product as a red-brown solid. The crude product was recrystallized from dicloromethane:dietylether (1:3) at room temperature. Yield: % 70 (0.12 g) Analytical data for dichloro[1-(2-methyl-1,4-benzodioxan)-3-benzylimidazolidin-2-ylidene]( p-cymene)ruthenium(II), 2a 1H NMR (300 MHz, CDCI3); δ 1.32 (d, 6H, J: 6.3 Hz Ru-C6H4CH(CH3)2); 2.18 (s, 3H, Ru-C6H4CH3); 2.91- 2.93 (m, 1H, Ru-C6H4CH(CH3)2); 3.48-3.53 (m, 4H, -NCH2CH2N-); 3.58-3.64 (m, 2H, -NCH2CHO-); 3.94-3.98 (m, 1H, -CH2CHOCH2-); 4.28-4.38 (m, 2H, - OCHCH2O-); 4.73 (s, 2H, -NCH2C6H5); 5.35-5.37 (d, 2H, J: 5.8 Hz Ru-Ar-H); 5.47-5.49 (d, 2H, J: 5.6 Hz Ru-Ar-H); 6.86- 6.90 (m, 4H, -OC6H4O- Ar-H); 7.28-7.39 (m, 5H, -CH2C6H5 Ar-H). 13C NMR ( 300 MHz, CDCI3); δ 18.9 (Ru-C6H4CH(CH3)2); 22.2 (Ru- C6H4CH(CH3)2); 30.7 (Ru-C6H4CH3); 49.2 (-NCH2CHOCH2-); 50.3 and 52.6 (-NCH2CH2N-); 55.8 (-NCH2C6H5 ); 66.1 (-CH2CHOCH2-); 74.6 (-OCHCH2O-); 80.5, 83.4, 85.7, 86.8, 99.0 and 108.7. (Ru-Ar-C); 117.3, 117.4, 121.5, 121.7, 141.9 and 143.3. (-OC6H4O- Ar-C); 127.5, 127.7, 128.8, 129.0 and 137.0.(-NCH2C6H5 Ar-C); 209.0 (C-Ru). m.p.: 112-114°C; (CN): 1492.5 cm-1. Anal. Calcd for RuC29H34Cl2N2O2: C: 56.62; H: 5.53; N: 4.56. Found: C: 56.60; H: 5.50; N: 4.54.
Synthesis of dichloro[1-(2-methyl-1,4-benzodioxan)- 3-(2-methylbenzyl)imidazolidin-2-ylidene](p-cymene) ruthenium(II), 2b
The synthesis of 2b was carried out in the same way as that described for 2a, but bromo[1-(2-methyl-1,4-benzodioxan)-3-(2-methylbenzyl) imidazol-2-ylidene]silver(I) (0.14 g., 0.28 mmol) was used instead of bromo[1-(2-methyl-1,4-benzodioxan)-3-benzylimidazol-2-ylidene] silver(I). Yield: % 64 (0.11 g) Analytical data for dichloro[1-(2- methyl-1,4-benzodioxan)-3-(2-methylbenzyl)imidazolidin-2-ylidene] (p-cymene)ruthenium(II), 2b 1H NMR (300 MHz, CDCI3); δ 1.32 (d, 6H, J: 6.8 Hz Ru-C6H4CH(CH3)2); 2.10 (s, 3H, Ru-C6H4CH3 ); 2.34 (s, 3H, C6H4CH3 ); 2.79-2.86 (m, 1H, Ru-C6H4CH(CH3)2); 3.46-3.50 (m, 4H, -NCH2CH2N-); 3.60-3.67 (m, 2H, -NCH2CHO-); 3.81-3.84 (m, 1H,-CH2CHOCH2-); 4.25-4.40 (m, 2H, -OCHCH2O-); 4.77 (s, 2H, -NCH2C6H4); 5.30-5.32 (d, 2H, J: 5.7 Hz Ru-Ar-H); 5.40-5.42 (d, 2H, J: 5.4 Hz Ru-Ar-H); 6.85-6.90 (m, 4H,-OC6H4O-Ar-H); 7.25-7.29 (m, 4H, -CH2C6H4(CH3) Ar-H). 13C NMR ( 300 MHz, CDCI3); δ 18.5 and 19.3 (C6H4CH(CH3)2); 21.9 (C6H4CH(CH3)2); 23.4 (C6H4CH3); 30.7 (Ru-C6H4CH3); 49.8 (-NCH2CHOCH2-); 50.5 (-NCH2CH2N-); 52.7 (-NCH2C6H5); 66.1 (-CH2CHOCH2-); 74.7 (-OCHCH2O-); 80.9, 81.1, 86.7, 96.8, 99.0. (Ru-Ar-C ); 117.4, 117.5, 121.5, 121.7, 142.0 and 143.4. (-OC6H4O- Ar-C); 127.2, 127.5, 130.9, 135.7 and 135.8.(-NCH2C6H5Ar-C); 209.9 (C-Ru). m.p.: 203-205°C; (CN): 1493.2 cm-1. Anal. Calcd for RuC30H-36Cl2N2O2: C: 57.27; H: 5.73; N: 4.45. Found: C: 57.25; H: 5.72; N: 4.43.
Synthesis of dichloro[1-(2-methyl-1,4-benzodioxan)- 3-(3-methylbenzyl)imidazolidin-2-ylidene](p-cymene) ruthenium(II), 2c
The synthesis of 2c was carried out in the same way as that described for 2a, but bromo[1-(2-methyl-1,4-benzodioxan)-3-(3-methylbenzyl) imidazol-2-ylidene]silver(I) (0.14 g., 0.28 mmol) was used instead of bromo[1-(2-methyl-1,4-benzodioxan)-3-benzylimidazol-2-ylidene] silver(I). Yield: % 75 (0.13 g) Analytical data for dichloro[1-(2- methyl-1,4-benzodioxan)-3-(3-methylbenzyl)imidazolidin-2-ylidene] (p-cymene)ruthenium(II), 2c 1H NMR (300 MHz, CDCI3); δ 1.31 (d, J: 5.8 Hz 6H, Ru-C6H4CH(CH3)2); 2.17 (s, 3H, Ru-C6H4CH3); 2.36 (s, 3H, C6H4CH3 ); 2.88-2.95 (m, 1H, Ru-C6H4CH(CH3)2); 3.37-3.44 ( m, 4H, -NCH2CH2N-); 3.47-3.50 (m, 2H, -NCH2CHO-); 3.96-3.98 (m, 1H, -CH2CHOCH2-); 4.14-4.24 (m, 2H, -OCHCH2O-); 4.26 (s, 2H, -NCH2C6H4(CH3)); 5.38-5.40 (d, 2H, J: 5.4 Hz Ru-Ar-H); 5.47-5.49 (d, 2H, J: 6.0 Hz Ru-Ar-H); 6.83-6.88 (m, 4H, -OC6H4O- Ar-H); 7.13-7.26 (m, 4H, -CH2C6H4(CH3) Ar-H). 13C NMR (300 MHz, CDCI3), δ 18.8 (C6H4CH(CH3)2); 21.9 (Ru-C6H4CH(CH3)2); 23.3 (C6H4CH3); 30.7 (Ru- C6H4CH3 ); 48.0 (-NCH2CHOCH2-);49.1 and 51.6 (-NCH2CH2N-); 55.5 (-NCH2C6H4(CH3)); 66.1 (-CH2CHOCH2-); 73.0 (-OCHCH2O-); 80.9, 81.5, 82.2, 86.7, 96.8 ve 99.0. (Ru-Ar-C ); 117.3, 117.5, 121.5, 121.8, 142.9 and 143.4 (-OC6H4O- Ar-C); 127.4, 127.9, 129.5, 129.9, 133.8 and 137.4. (-NCH2C6H5Ar-C); 209.7 (C-Ru). m.p.: 204-206°C; ν(CN): 1493.3 cm-1. Anal. Calcd for RuC30H36Cl2N2O2: C: 57.27; H: 5.73; N: 4.45. Found: C: 57.24; H: 5.71; N: 4.44.
Synthesis of dichloro[1-(2-methyl-1,4-benzodioxan)- 3-(4-methylbenzyl)imidazolidin-2-ylidene](p-cymene) ruthenium(II), 2d
The synthesis of 2d was carried out in the same way as that described for 2a, but bromo[1-(2-methyl-1,4-benzodioxan)-3-(4-methylbenzyl) imidazol-2-ylidene]silver(I) (0.14 g., 0.28 mmol) was used instead of bromo[1-(2-methyl-1,4-benzodioxan)-3-benzylimidazol-2-ylidene] silver(I). Yield: % 69 (0.12 g) Analytical data for dichloro[1-(2- methyl-1,4-benzodioxan)-3-(4-methylbenzyl)imidazolidin-2-ylidene] (p-cymene)ruthenium(II), 2d 1H NMR (300 MHz, CDCI3); δ 1.29 (d, 6H, J: 6.6 Hz Ru-C6H4CH(CH3)2); 2.17 (s, 3H, Ru-C6H4CH3); 2.36 (s, 3H, C6H4CH3 ); 2.76-2.82 (m, 1H, Ru-C6H4CH(CH3)2); 3.37-3.43 ( m, 4H, -NCH2CH2N-); 3.56-3.63 (m, 2H, -NCH2CHO-); 3.87-3.90 (m, 1H, -CH2CHOCH2-); 4.18-4.21 (m, 2H, -OCHCH2O-); 4.26 (s, 2H, -NCH2C6H5); 5.34-5.36 (d, 2H, J: 5.9 Hz Ru-Ar-H); 5.40-5.42 (d, 2H, J: 6.0 Hz Ru-Ar-H); 6.85-6.88 (m, 4H, -OC6H4O- Ar-H); 7.10-7.25 (m, 4H, -CH2C6H4(CH3) Ar-H). 13C NMR ( 300 MHz, CDCI3), δ 18.9 and 19.0 (C6H4CH(CH3)2); 21.2 (Ru-C6H4CH(CH3)2); 22.3 (-C6H4CH3); 30.7 (Ru-C6H4CH3); 49.2 (-NCH2CHOCH2-); 55.5 (-NCH2C6H4(CH3)); 51.3 and 52.6 (-NCH2CH2N-); 65.9 (-CH2CHOCH2-); 73.2 (-OCHCH2O-); 80.5, 81.3, 85.7, 96.8, 99.0 and 101.2. (Ru-Ar-C ); 117.3, 117.5, 121.5, 121.8, 143.1 and 143.4. (-OC6H4O- Ar-C); 127.5, 128.0, 129.3, 129.4, 133.8 and 137.4. (-NCH2C6H5Ar-C); 209.7 (C-Ru). m.p.: 167-169°C; ν(CN): 1493.5 cm-1. Anal. Calcd for RuC30H36Cl2N2O2: 57.27; H: 5.73; N: 4.45. Found: C: 57.23; H: 5.70; N: 4.42.
Synthesis of dichloro[1-(2-methyl-1,4-benzodioxan)- 3-(2,4,6-trimethylbenzyl)imidazolidin-2-ylidene](p-cymene) ruthenium(II), 2e
The synthesis of 2e was carried out in the same way as that described for 2a, but bromo[1-(2-methyl-1,4-benzodioxan)-3-(2,4,6-trimethylbenzyl) imidazol-2-ylidene]silver(I) (15 g., 0.28 mmol) was used instead of bromo[1-(2-methyl-1,4-benzodioxan)-3-benzylimidazol-2-ylidene] silver(I). Yield: % 62 (0.17 g) Analytical data for dichloro[1-(2-methyl- 1,4-benzodioxan)-3-(2,4,6-trimethylbenzyl)imidazolidin-2-ylidene] (p-cymene)ruthenium(II), 2e 1H NMR (300 MHz, CDCI3); δ 1.31 (d, 6H, J: 6.4 Hz Ru-C6H4CH(CH3)2); 2.17 (s, 3H, Ru-C6H4CH3); 2.27 and 2.28 (s, 9H, C6H2(CH3)3); 2.98-3.03 (m, 1H, Ru-C6H4CH(CH3)2); 3.32-3.39 (m, 4H, -NCH2CH2N-); 3.48-3.51 (m, 2H, -NCH2CHO-); 3.78-3.85 (m, 1H, -CH2CHOCH2-); 4.04-4.11 (m, 2H, -OCHCH2O-); 4.32 (s, 2H, -NCH2C6H2); 5.37-5.39 (d, 2H, J: 5.9 Hz Ru-Ar-H); 5.46- 5.48 (d, 2H, J: 6.0 Hz Ru-Ar-H); 6.83-6.87 (m, 4H, -OC6H4O- Ar- H); 7.28-7.30 (m, 2H, -CH2C6H2(CH3)3 Ar-H). 13C NMR ( 300 MHz, CDCI3); δ 18.7 (C6H4CH(CH3)2); 18.9 and 20.4 (C6H2(CH3)3); 21.0 (Ru- C6H4CH(CH3)2); 30.7 (Ru-C6H4CH3); 45.3 (-NCH2CHOCH2-); 48.0 and 48.3 (-NCH2CH2N-); 52.7 (-NCH2C6H5); 65.9 (-CH2CHOCH2-); 72.7 (-OCHCH2O-); 80.6, 81.3, 86.2, 96.9, 99.8 and101.9. (Ru-Ar-C); 117.4, 117,5 121.4, 122.7, 138.0 and 143.4. (-OC6H4O- Ar-C); 126.6, 127.9, 129.0, 129.4, 137.4 and 137.6. (-NCH2C6H5Ar-C); 209.2 (C-Ru). m.p.: 191-193°C; ν(CN): 1493.6 cm-1. Anal. Calcd for RuC32H40Cl2N2O2: C: 58.48; H: 6.09; N: 4.26. Found: C: 58.45; H: 6.07; N: 4.24.
Synthesis of dichloro[1-(2-methyl-1,4-benzodioxan)- 3-(2,3,5,6-tetramethylbenzyl)imidazolidin-2-ylidene](p-cymene) ruthenium(II), 2f
The synthesis of 2f was carried out in the same way as that described for 2a, but bromo[1-(2-methyl-1,4-benzodioxan)-3-(3,4,5,6- tetramethylbenzyl)imidazol-2-ylidene]silver(I) (16 g., 0.28 mmol) was used instead of bromo[1-(2-methyl-1,4-benzodioxan)-3-benzylimidazol- 2-ylidene]silver(I). Yield: % 66 (0.12 g) Analytical data for dichloro[1-(2-methyl-1,4-benzodioxan)-3-(2,3,5,6-tetramethylbenzyl) imidazolidin-2-ylidene](p-cymene)ruthenium(II), 2f 1H NMR (300 MHz, CDCI3); 1.34 (d, 6 H, J: 6.3 Hz Ru-C6H4CH(CH3)2); 2.18 (s, 3H, Ru-C6H4CH3); 2.21 and 2.23 (s, 12H, C6H(CH3)4); 2.94-2.98 (m, 1H, Ru-C6H4CH(CH3)2); 3.05-3.17 (m, 4H, -NCH2CH2N-); 3.39-3.48 (m, 2H, -NCH2CHO-); 3.83-3.88 (m, 1H, -CH2CHOCH2-); 4.15-4.20 (m, 2H, -OCHCH2O-); 4.52 (s, 2H, -NCH2C6H); 5.38-5.40 (d, 2H, J: 6.3 Hz Ru-Ar-H); 5.48-5.50 (d, 2H, J: 5.9 Hz Ru-Ar-H); 6.84-6.87 (m, 4H, -OC6H4O- Ar-H); 7.29 (s, 1H, -CH2C6H(CH3)4 Ar-H). 13C NMR (300 MHz, CDCI3); δ 18.4 and 18.6 (C6H4CH(CH3)2); 19.0 and 20.5 (C6H(CH3)4); 22.2 (Ru-C6H4CH(CH3)2); 30.6 (Ru-C6H4CH3); 49.4 and 50.7 (-NCH2CH2N-); 66.0 (-CH2CHOCH2-); 52.3 (-NCH2C6H5); 47.6 (-NCH2CHOCH2-); 73.1 (-OCHCH2O-); 80.5, 81.3, 83.1, 96.8 and 101.2. ( Ru-Ar-C ); 117.3, 117.4, 121.7, 121.8 and 143.1. (-OC6H4OAr- C); 126.4, 127.2, 132.0, 133.5, 134.3 and 134.5. (-NCH2C6H5Ar- C); 209.1 (C-Ru). m.p.: 191-193°C; ν(CN): 1494.3 cm-1. Anal. Calcd for RuC33H42BrN2O2: C: 59.94; H: 6.85; N: 4.08. Found: C: 59.91; H: 6.83; N: 4.10.
General method for the transfer hydrogenation of ketones
The catalytic hydrogen transfer reactions were carried out in a closed Schlenk flask under argon atmosphere. Substrate ketone (1 mmol), catalyst ruthenium(II)-NHC complex (2a-f) (0,01 mmol) and KOH (4 mmol) was heated to reflux in 10 mL of i-PrOH for 1 hr. The solvent was then removed under vacuum. At the conclusion of the reaction, the mixture was coled, extracted with ethylacetate/hexane (1:5), filtered through a pad of silica gel with copious washings, concentrated, and purified by flash chromatography on silica gel. The product distribution was determined by 1H NMR spectroscopy, GC and GC-MS.
Results and Discussion
Synthesis
The synthetic route for unsymmetrically 2-methyl-1,4-benzodioxan substituted Ag(I)-NHC complexes and their corresponding Ru(II)- NHC complexes described in this study is given in Scheme 1. The unsymmetrically substituted Ag(I)-NHC complexes 1a-f were prepared by stirring 1-(2-methyl-1,4-benzodioxan)-3-alkylimidazolidinium salts with 0.5 equivalents of Ag2O in dichloromethane at room temperature for 24 hours. The Ag(I)-NHC complexes as off white solid in 85% to 91% yield. The silver complexes were soluble in halogenated solvent and insoluble in non-polar solvents. The complexes were characterized by spectroscopic techniques (1H, 13C NMR, IR) and elemental analysis. 1H and 13C NMR spectra are consistent with the proposed formulate. In the 1H NMR and 13C NMR spectra in d-DMSO-d6, loss of signals for the imidazolium protons (NCHN) (8.80-9.87 ppm) and imidazolium carbon (NCHN) at (157.9-159.1 ppm) showed the formation of the expected Ag(I)-NHC. The 13C NMR spectra, resonances of the carbene carbon atoms of complexes appeared in the range δ 204.6-205.7 ppm respectively for 1a-f. These signals are shifted downfield compared to the carbene precursors which further demonstrates the formation of expected Ag(I)-NHC. The IR data for Ag(I)-NHC complexes exhibit a characteristic ν(C=N) band at 1666.8, 1645.9, 1661.5, 1662.6, 1665.9 and 1667.2 respectively, for 1a-f. The NMR and FT-IR values are similar to results of other Ag(I)-NHC complexes (Scheme 1). The 2-methyl-1,4- benzodioxan substituted Ru(II)-NHC complexes 2a-f were prepared from synthesized Ag(I)-NHC complexes via transmetallation method (Scheme 1). The air and moisture stable Ru(II)-NHC complexes were soluble in solvents such as chloroform and dichloromethane. The Ru(II)-NHC complexes 2a-f were prepared by stirring bromo[1-(2- methyl-1,4-benzodioxan)-3-alkylimidazol-2-ylidene]silver(I) with 0.5 equivalents of [RuCl2(pcym)]2 in dichloromethane at room temperature for 24 hours. The Ru(II)-NHC complexes as a red-brown solid in 64% to 75% yield. The Ru(II)-NHC complexes were soluble in halogenated solvent and insoluble in non-polar solvents. The Ru(II)-NHC complexes have been characterized by analytical and spectroscopic techniques. In the 1H NMR spectra, resonances for the isopropyl and methyl protons of the p-cymene group of complexes 2a-f in the range 1.29-1.34 (methyl of izopropyl group), 2.76-3.03 (single hydrogen of izopropyl group) and 2.14-2.18 ppm (p-methyl of p-cymene group) respectively showed the formation of the Ru(II)-NHC complexes. The 13C NMR spectra, resonances of the carbene carbon atoms of complexes appeared in the range δ 209.0-209.9 ppm respectively for 2a-f. These signals are shifted downfiel compared to corresponding Ag(I)- NHC complexes of Ru(II)-NHC complexes carbene carbons signal at the range 204.6-205.7 ppm respectively showed the formation of the expected Ru(II)-NHC complexes. The IR data for Ru(II)-NHC complexes exhibit a characteristic ν(C=N) band at 1492.5, 1493.2, 1493.3, 1493.5, 1493.6 and 1494.3 respectively, for 2a-f. The NMR and FT-IR values are similar to results of other Ru(II)-NHC complexes.
Structural characterization of 2a
Suitable crystal for X-ray crystallography to determine the molecular structure of 2a was obtained in a saturated dichloromethane solution with slow infusion of diethyl ether. The molecular structure of 2a is shown in Figure 1. Figures 1 and 2 were drawn using the PLATON program. In the p-cymene ligand of the 2a in Figure 1, the arene ring (C21–C26) has a planar configuration [rms deviation=0.008 Å]. The C-C bond lengths within the p-cymene ring are similar, except from a shortening of the C24-C25 bond of 1.362(13) Å. The Ru-carbene distance in 2a is 2.052(8) Å. The distance of 1.690(3) Å between the centroid of the p-cymene ring and ruthenium is very close to that reported in other RuII arene complexes [47,48]. The Ru-Cl1 and Ru-Cl2 bond lengths [2.4323(19) and 2.407(2) Å, respectively] are similar to those in other RuII complexes [47,48]. In the crystal, molecules are connected by intermolecular C-H...Cl hydrogen bonds (Table 2, Figure 2), forming 3D network. H atoms were positioned geometrically and were refined using a riding model with Uiso(H)=1.2 or 1.5Ueq(C). The highest residual peak lies 1.22 Å from Ru1. The small proportion of reflections observed is a result of the rather poor quality of the very thin crystals obtained. Figure 2 shown a view of the crystal packing for 2a.
Catalytic transfer hydrogenation of ketones
Ruthenium complexes have been used as active catalysts for transfer hydrogenation using 2-propanol as a hydrogen source. The reaction conditions for this transformation are economic, partly mild and environmentally benign friendly. 2-propanol using as a hydrogen source is a popular reactive solvent for the catalytic transfer hydrogenation since it is easy to handle and is relatively non-toxic, environmentally benign, and inexpensive. The volatile acetone by-product can also be easily removed. We have investigated and compared the catalytic properties of 2-methyl- 1,4-benzodioxan substituted Ru(II)-NHC complexes 2a-f in the transfer hydrogenation of various methyl aryl ketones. The reduction of acetophenone with 2-propanol to 1-phenylethanol was chosen as a model reaction. The catalytic transfer hydrogenations of ketones were carried out using Ru(II)-NHC precatalyst (0.01 mmol), KOH (4 mmol) and substrate ketone (1.00 mmol) in 2-propanol at 80°C. The conversion was monitored by GC and NMR. It is well known that catalytic transfer hydrogenation is sensitive to the nature of the base. We surveyed K2CO3, Cs2CO3, NaOH, KOH, t-BuOK and NaOAc for the choice of base. The highest rate was observed when KOH was employed. A variety of ketones by 2-propanol were transformed to the corresponding secondary alcohols. Typical results are summerized in Table 3.
Is examined in general, all complexes 2a-f is seen to be reasonably active in hydrogen transfer reactions. Under the reaction conditions complex 2c turned out to be the active catalyst in comparison with 2a, 2b, 2d, 2e and 2f. The reduction of p-methoxyacetophenone with 2c was completed within 1 hr reaching 81%. In contrast, p-methoxacetophenone was reduced within 1 hr using 2a, 2b, 2d, 2e and 2f with 80, 70, 78, 74 and 68% conversion, respectively (Table 3).
A variety of ketones were converted to be corresponding secondary alcohols. Typical results is illustrated in Table 3. Under those conditions p-methoxyacetophenone and p-fluoroacetophenone react neatly and in good yields with 2-propanole (Table 3). The existence of electron withdrawing (F) or electron donating (OCH3) substituents on acetophenone (Table 3) has effect on the reduction of major of ketones to their corresponding alcohols. The more conversion of p-methoxyacetophenone to secondary alcohole was obtained at a time 1 h (Table 3). The transformation of ketones with bulky substituents was not shown or mildly decreased. We tried this reaction with benzophenone at 1 hr. But, we have achieved low yields. Therefore, we have extended the duration of experiments for benzophenone to 2 hr. The benzophenone was reduced within 1 hr using 2a and 2b with 21% and 23% conversion, respectively. However, the yields lower than 2 hr, for example the reduction of benzophenone with 2a and 2b was completed within 94% and 97% respectively (Table 3, Scheme 1).
Conclusions
As a result, we have described the preparation and catalytic activities of well-defined 2-methyl-1,4-benzodioxan substituted Ru(II)- NHC complexes 2a-f. Via the Ag(I)-NHC 1a-f transmetallation route, Ru(II)-NHC complexes were readily accessible and are effective catalyst precursors for the transfer hydrogenation of ketones. The catalytic activities of these six Ru(II)-NHC complexes have been examined for the transfer hydrogenation of ketones.
Acknowledgements
This work was financially supported by Inönü University Research Fund (IUBAP 2011/25) and the Faculty of Arts and Sciences, Ondokuz Mayis University, Turkey, for the use of the Stoe IPDS 2 diffractometer (purchased under grant F.279 of the University Research Fund).
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