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.

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.

Table 1: Crystallographic data and structure refinement for 2a.

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 RuC₃₀H₃₆C_l2N₂O₂: 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 RuC₃₀H₃₆C_l2N₂O₂: 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 RuC₃₂H₄₀C_l2N₂O₂: 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.

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.

Scheme 1: Synthesis of Ag(I)-NHC 1a-f and Ru(II)-NHC 2a-f 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 Ru^II 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 Ru^II 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 U_iso(H)=1.2 or 1.5U_eq(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.

Figure 1: The molecular structure of 2a with the atom numbering scheme. Displacement ellipsoids for non-H atoms are drawn at the 30% probability level.

Figure 2: View of the hydrogen bonding and molecular packing of 2a along a-axis.

Table 2: Hydrogen-bond parameters for 2a (Å, °). Symmetry codes: (i) x, 1/2-y, -1/2+z; (ii) 1-x, 1-y, 2-z.

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.

Table 3: Transfer hydrogenation of ketones catalyzed by 2a-f. ^aDetermined by GCMS and yields are based on ketones. ^b2h, 80°C.

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).

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.

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).