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

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, η^¹-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, η^¹-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

References and Notes

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21

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 (C_carbene), 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 (C_carbene), 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 (C_carbene), 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-97^²³ and refined by full-matrix least-squares methods on F ^² using SHELXL-97^²³ from within the WINGX^²4 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 mm^³, 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) Å^³, 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 |F^²|, 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 mm^³, 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) Å^³, 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 |F^²|, 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.