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Synlett 2010(3): 496-500  
DOI: 10.1055/s-0029-1219187
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Synthesis and Catalytic Activity of Novel Benzimidazolinylidene-Ruthenium(II) Complexes

Serpil Demira, Ismail Özdemir*a, Onur Şahinb, Bekir Çetinkayac, Orhan Büyükgüngörb
a Inönü University, Faculty of Science and Art, Department of Chemistry, 44280 Malatya, Turkey
Fax: +90(422)3410212; e-Mail: iozdemir@inonu.edu.tr;
b OndokuzMayis University, Department of Physics, 55139 Samsun, Turkey
c Ege University, Faculty of Science, Department of Chemistry, 35100 Bornova-Îzmir, Turkey
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Publication History

Received 1 September 2009
Publication Date:
11 January 2010 (online)

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Abstract

The reaction of [RuCl2(p-cymene)]2 with 1,3-dialkyl­benzimidazolium salts 1a-d in the presence of a small excess of cesium carbonate yields chelated η6-arene, η¹-carbene ruthenium complexes 2a-d. All compounds were characterized by elemental analysis, NMR spectroscopy and the molecular structures of two of the complexes were determined by X-ray crystallography. The catalytic activity of RuCl2(η6-arene, η¹-benzimidazolinylidene) complexes was evaluated in the direct arylation of 2-phenylpyridine with bromobenzene derivatives.

Key words

ruthenium - benzimidazolinylidene complexes - N-heterocyclic carbene - C-H bond activation - arylation

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General Method for the Preparation of Ruthenium Complexes 2a-d: A suspension of benzimidazolium salt 1a-d (2.10 mmol), Cs2CO3 (2.14 mmol)and [RuCl2 (p-cymene)]2 (0.82 mmol) was heated under reflux in degassed toluene (20 mL) for 7 h. The reaction mixture was then filtered while hot, and the volume was reduced to about 10 mL before addition of n-hexane (15 mL). The precipitate formed was crystallized from CH2Cl2-hexane (5 mL/15 mL) to give brown crystals. Crystals suitable for single-crystal X-ray diffraction were obtained from CHCl3-Et2O. RuCl 2 [η ¹ -CN{CH 2 ( η 6 -C 6 H 2 ( OMe) 3 -3,4,5)}C 6 H 4 N(CH 2 C 6 H 4 ( CMe 3 - p )] (2a): Yield: 78% (0.96 g); mp 238-239 ˚C. IR: 1439 (CN) cm-¹. ¹H NMR (399.9 MHz, CDCl3): δ = 7.18, 7.03 (m, 8 H, NC6H4N, CH2C6 H 4CMe3-p), 5.75 [m, 2 H, CH2C6 H 2(OMe)3-3,4,5], 5.07, 4.88 [s, 4 H, CH 2C6H2(OMe)3-3,4,5, CH 2C6H4C(Me)3-p], 4.07, 4.27 [s, 9 H, CH2C6H2(OCMe)3-3,4,5], 1.27 (s, 9 H, CH2C6H4CMe 3-p). ¹³C NMR (100.5 MHz, CDCl3): δ = 185.7 (Ccarbene), 97.2, 97.5, 109.0, 109.3, 109.6, 113.0, 122.9, 123.4, 125.0, 127.6, 133.2, 133.3, 135.1, 138.1, 138.4, 150.0 [CH2 C 6H2(OMe)3-3,4,5, CH2 C 6H4CMe3-p, NC6H4N], 57.9, 58.1 [CH2C6H2(OMe)3-3,4,5], 55.7 [CH2C6H2(OMe)3-3,4,5], 53.4 (CH2C6H4CMe3-p), 34.4 (CH2C6H4 CMe3-p), 31.3 (CH2C6H4CMe 3-p). Anal. Calcd for C28H32N2O3RuCl2: C, 54.55; H, 5.23; N, 4.54. Found: C, 54.58; H, 5.20; N: 4.58.
RuCl 2 [η ¹ -CN{CH 2 ( η 6 -C 6 H 2 ( OMe) 3 -3,4,5)}C 6 H 4 N(CH 2 C 6 H 2 ( OMe) 3 -3,4,5)] (2b): Yield: 82% (1.26 g); mp 196-197 ˚C. IR: 1441 (CN) cm-¹. ¹H NMR (399.9 MHz, CDCl3): δ = 7.09 (s, 4 H, NC6H4N), 6.43 [s, 4 H, CH2C6 H 2(OMe)3-3,4,5], 6.13, 6.04 [s, 4 H, CH 2C6H2(OMe)3-3,4,5], 3.73, 3.81 [s, 18 H, CH2C6H2(OMe)3-3,4,5]. ¹³C NMR (100.5 MHz, CDCl3): δ = 191.7 (Ccarbene), 104.6, 112.0, 123.5, 132.9 (NC6H4N), 135.8, 137.9, 153.8, 154.0 [free CH2 C 6H2(OMe)3-3,4,5], 56.7, 61.2 [CH2C6H2(OMe)3-3,4,5], 53.6 [CH2C6H2(OMe)3-3,4,5]. Anal. Calcd for C27H30N2O6RuCl2˙CHCl3: C, 43.68; H, 4.06; N, 3.64. Found: C, 43.72; H, 4.09; N: 3.68.
RuCl 2 [η ¹ -CN{CH 2( η 6 -C 6 H 2 Me 3 -2,4,6)}C 6 H 4 N(CH 2 CH 2 OEt)] (2d): Yield: 70% (0.69 g); mp 293-294 ˚C. IR: 1436 (CN) cm-¹. ¹H NMR (399.9 MHz, CDCl3): δ = 7.28-7.77 (m, 4 H, C6H4N), 5.59 (s, 2 H, CH2C6 H 2Me3-2,4,6), 5.03 (s, 2 H, CH 2C6H2Me3-2,4,6), 4.63 (t, 2 H, J = 5.0 Hz, CH 2CH2OEt), 3.77 (t, 2 H, J = 5.0 Hz, CH2CH 2OEt), 3.40 (q, 2 H, J = 7.0 Hz, OCH 2Me), 2.13, 2.36 (s, 9 H, CH2C6H2 Me 3-2,4,6), 1.08 (t, 3 H, J = 7.0 Hz, OCH2 Me). ¹³C NMR (100.5 MHz, CDCl3): δ = 184.9 (Ccarbene), 109.4, 113.8, 123.2, 123.8, 132.8, 135.9 (NC6H4N), 89.7, 92.8, 98.9, 101.0 (CH2 C 6H2Me3-2,4,6), 72.1 (CH2 CH2OEt), 66.4 (CH2CH2OEt), 48.1 (CH2CH2OEt), 45.1 (CH2C6H2Me3-2,4,6), 16.9, 17.4 (CH2C6H2Me3-2,4,6), 15.3 (OCH2 CH3). Anal. Calcd for C21H26N2ORuCl2: C, 51.01; H, 5.30; N, 5.67. Found: C, 51.10; H, 5.35; N: 5.65.
General Procedure for the Arylation of 2-Phenyl-pyridine: Ruthenium complex 2 (0.025 mmol), 2-phenylpyridine (0.5 mmol), aryl bromide (1.25 mmol) and Cs2CO3 (1.50 mmol) were stirred in NMP (2 mL) at 120 ˚C for 20 h. H2O and EtOAc were added to the cold reaction mixture. The organic phase was separated, dried over MgSO4, filtered and concentrated under vacuum. The remaining residue was purified by column chromatography on silica gel (pentane-Et2O mixture) to yield the ortho-arylated products. Conversions and product ratios were determined by ¹H NMR and by GC analyses. X-ray Structure Determination of 2b and 2d: Brown single crystals of 2b and 2d suitable for data collection were selected and data collection was performed on a STOE IPDS II diffractometer with graphite monochromated Mo-Kα radiation at 296 K. The structures were solved by direct-methods using SHELXS-97²³ and refined by full-matrix least-squares methods on F ² using SHELXL-97²³ from within the WINGX²4 suite of software. All non-hydrogen atoms were refined with anisotropic parameters. Hydrogen atoms were added to the structure models on calculated positions.
Selected Crystallographic Details for 2b: C28H31Cl5N2O6Ru, M = 769.87, brown crystal, 0.48 × 0.30 × 0.06 mm³, monoclinic, space group P21/c, a = 15.9602 (14), b = 13.9350 (10), c = 16.6111 (15) Å, β = 119.790 (6)˚, V = 3206.2 (5) ų, ρ calcd = 1.595 g cm-³, µ = 0.95 mm-¹, empirical absorption correction (0.723£ T £0.956), Z = 4, 18476 intensities collected (± h, ± k, ± l), 6267 independent intensities (R int = 0.062), refinement of 383 parameters against all unique |F²|, R = 0.047, wR = 0.111 for 4099 contributing intensities [I £ 2σ(I)].
Selected Crystallographic Details for 2d: C21H26Cl2N2ORu, M = 494.41, brown crystal, 0.48 × 0.33 × 0.11 mm³, monoclinic, space group C2/c, a = 30.4767 (18), b = 9.8695 (4), c = 14.8978 (8) Å, β = 108.484 (4)˚, V = 4249.9 (4) ų, ρ calcd = 1.545 g cm-³, µ = 1.00 mm-¹, empirical absorption correction (0.565£ T £0.914), Z = 8, 29964 intensities collected (± h, ± k, ± l), 5022 independent intensities (R int = 0.040), refinement of 247 parameters against all unique |F²|, R = 0.025, wR = 0.063 for 3897 contributing intensities [I £ 2σ(I)].
Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication (CCDC 72906 for 2b and CCDC 72907 for 2d). Copies of the data can be obtained free of charge via www.ccdc.cam.ac.uk/data.request/cif.