“Pincer” N-Heterocyclic Carbene Complexes of Rhodium
FULL PAPER
rhodium(I) complex 2 (ca. 15 mg) in CD2Cl2 (0.75 mL) led to a
rapid colour change from purple to light yellow. Upon standing,
J = 6.9 Hz, 2 H, CH(CH3)2], 6.66 (d, J = 1.6 Hz, 1 H, imidazole
backbone CH), 6.74 (ddd, J = 1.1, 4.7, 7.4 Hz, 1 H, aromatic CH),
7.19–7.32 (m, 5 H, aromatic CH), 8.28 (d, J = 1.7 Hz, 1 H, imid-
light yellow crystals suitable for X-ray diffraction formed in quanti-
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tative yield. H NMR (300 MHz, CD2Cl2, 25 °C): δ = 0.96 (d, J = azole backbone CH), 8.48–8.56 (m, 3 H, aromatic CH), 8.66 (d, J
7.0 Hz, 6 H, Me), 1.11 (d, J = 6.8 Hz, 6 H, Me), 1.15 (d, J = 6.8 Hz,
= 8.2 Hz, 1 H, aromatic CH) ppm. 13C NMR (75.45 MHz, C6D6,
6 H, Me), 1.21 (d, J = 6.5 Hz, 6 H, Me), 2.55 [sept, J = 6.7 Hz, 25 °C): δ = 22.82 (Me), 23.17 (Me), 27.36 [CH(CH3)2], 113.80 (aro-
2 H, CH(CH3)2], 3.36 [sept, J = 6.7 Hz, 2 H, CH(CH3)2], 7.07 (d, matic CH), 115.13 (aromatic CH), 117.63 (aromatic CH), 119.61
J = 2.4 Hz, 2 H, imidazole backbone), 7.15 (d, J = 7.6 Hz, 2 H,
aromatic CH), 7.19 (d, J = 8.1 Hz, 2 H, aromatic CH), 7.35 (t, J =
7.6 Hz, 2 H, aromatic CH), 7.49 (dm, J = 8.1 Hz, 2 H, pyridyl CH),
7.85 (d, J = 2.4 Hz, 2 H, imidazole backbone), 8.12 (t, J = 8.1 Hz,
(aromatic CH), 121.56 (aromatic CH), 122.55 (aromatic CH),
127.07 (aromatic CH), 127.98 (aromatic CH), 135.19 (aromatic
CH), 138.07 (aromatic CH), 145.01 (aromatic C), 148.27 (aromatic
CH), 152.34 (aromatic C), 153.46 (aromatic C), 154.98 (aromatic
1 H, pyridyl CH) ppm. 13C (75.45 MHz, CD2Cl2, 25 °C): δ = 22.9 C) ppm; carbene carbon signal not observed. X-ray quality colour-
(Me), 23.0 (Me), 26.0 (Me), 26.3 (Me), 28.4 [CH(CH3)2], 28.5 less crystals were obtained by cooling solutions of 7 in toluene/
[CH(CH3)2], 68.1 (CD2Cl) 106.1 (aromatic CH), 116.2 (aromatic
CH), 123.3 (aromatic CH), 124.2 (aromatic CH), 126.3 (aromatic
CH), 130.1 (aromatic CH), 134.4 (aromatic C), 141.3 (aromatic C),
145.2 (aromatic CH), 147.8 (aromatic C), 162.8 (aromatic C) ppm,
carbene not observed.
petroleum ether (50:50) at –30 °C.
[1-(2,2Ј-Bipyridin-6-yl)-3-(2,6-diisopropylphenyl)imidazol-2-ylidene]-
silver(I) Bromide (8): The imidazolium salt 6 (1.00 g, 2.16 mmol),
molecular sieves (4 Å) (ca. 5 g) and Ag2O were suspended in the
dark in CH2Cl2 (30 mL) and the mixture refluxed overnight. After
cooling to room temperature, the solution was filtered through Ce-
lite® and the solvent removed in vacuo. The product was obtained
as a pale brown solid (881 mg, 71%). C25H26AgBrN4 (570.3): calcd.
1-(2,2Ј-Bipyridin-6-yl)-3-(2,6-diisopropylphenyl)-3H-imidazol-1-ium
Bromide (6): 6-Bromo-2,2Ј-bipyridine (1.40 g, 5.95 mmol) and 1-
(2,6-diisopropylphenyl)imidazole (1.46 g, 6.38 mmol) were placed
in a glass ampoule which was sealed under vacuum. This was com-
pletely immersed in an oil bath at 150 °C for 6 d. After cooling to
room temperature, the ampoule was opened and the brown residue
was dissolved in dichloromethane. After evaporation of the solvent,
the product was stirred with diethyl ether at 0 °C for several hours,
the solvent removed by filtration and the solid dried in vacuo. This
gave the product as a brown solid (2.27 g, 82 %). C25H27BrN4
(463.42): calcd. C 64.79, H 5.87, N 12.09; found C 64.31, H 5.98,
N 11.92. 1H NMR (300 MHz, CDCl3, 25 °C): δ = 1.19 (d, J =
6.6 Hz, 6 H, Me), 1.32 (d, J = 6.6 Hz, 6 H, Me), 2.46 [sept, J =
6.6 Hz, 2 H, CH(CH3)2], 7.37 (d, J = 7.5 Hz, 2 H, aromatic CH),
7.42 (br. s, 1 H, pyridyl CH), 7.59 (t, J = 7.9 Hz, 1 H, aromatic
CH), 7.68 (br. s, 1 H, pyridyl CH), 8.25 (t, J = 7.9 Hz, 2 H, pyridyl
CH), 8.62–8.75 (m, 3 H, pyridyl CH), 9.33 (d, J = 7.9 Hz, 1 H,
pyridyl CH), 9.37 (br. s, 1 H, imidazole backbone), 11.30 (s, 1 H,
imidazolium salt) ppm. 13C NMR (74.45 MHz, CDCl3, 25 °C): δ
= 24.0 (Me), 24.1 (Me), 28.6 [CH(CH3)2], 116.9 (aromatic CH),
121.2 (aromatic CH), 123.5 (aromatic CH), 124.5 (aromatic CH),
125.5 (aromatic CH), 125.6 (aromatic CH), 129.8 (aromatic C),
131.9 (aromatic CH), 136.2 (aromatic CH), 142.2 (aromatic CH),
144.8 (aromatic C), 145.6 (aromatic C) ppm. MS (ESI+): m/z =
155.0, 367.3, 170.0, 182.1, 196.1, 213.1, 352.2, 383.3 [M+].
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C 52.65, H 4.59, N 9.82; found C 52.15, H 4.44, N 9.68. H NMR
(300 MHz, CDCl3, 25 °C): δ = 1.21 (d, J = 6.6 Hz, 6 H, Me), 1.34
(d, J = 6.6 Hz, 6 H, Me), 2.57 [sept, J = 6.6 Hz, 2 H, CH(CH3)2],
7.24 (br. s, 1 H, imidazole backbone CH), 7.36 (d, J = 8.0 Hz, 2 H,
aromatic CH), 7.43 (m, 1 H, aromatic CH), 7.54 (t, J = 8.0 Hz,
1 H, central aromatic CH), 8.12 (t, J = 7.4 Hz, 1 H, aromatic CH),
8.26 (br. s, 1 H, imidazole backbone CH), 8.30 (d, J = 8.1 Hz, 2 H,
aromatic CH), 8.61 (m, 2 H, aromatic CH), 8.78 (br. s, 1 H, aro-
matic CH) ppm. 13C NMR (75.45 MHz, CDCl3, 25 °C): δ = 24.40
(Me), 24.50 (Me), 28.40 [CH(CH3)2], 115.07 (aromatic CH), 119.94
(aromatic CH), 121.18 (aromatic CH), 124.38 (aromatic CH),
124.50 (aromatic CH), 128.18 (aromatic C), 128.90 (aromatic C),
130.75 (aromatic CH), 140.36 (aromatic CH), 145.52 (aromatic C),
149.36 (aromatic CH), 172.20 (s, carbene) ppm. MS (ES+): m/z =
871.5 [carbene2Ag+].
[1-(2,2Ј-Bipyridin-6-yl)-3-(2,6-diisopropylphenyl)imidazol-2-ylidene]-
rhodium(I) Halide [Halide = Br (9a), Cl (9b)]: The salt 6 (93 mg,
0.20 mmol) was suspended in THF (5 mL) and cooled to –78 °C,
and a solution of bis[µ-methoxybis(η2-cyclooctene)rhodium(I)]
(49 mg, 0.10 mmol) in THF (5 mL) at –78 °C was added. The solu-
tion was warmed to room temperature and became red, then brown
and finally dark green. A small amount of solid was filtered off,
and the solution diluted with petroleum ether (15 mL). The re-
sulting precipitate was filtered off, dissolved in THF (5 mL) and
layered with petroleum ether (5 mL). Cooling to –30 °C gave green
crystals of the title compound suitable for X-ray diffraction.
C25H26BrN4Rh (565.32): calcd. C 53.12, H 4.64, N 9.91; found C
52.78, H 4.42, N 9.73. 1H NMR ([D8]THF): δ = 1.13 (d, J = 6.9 Hz,
6 H, Me), 1.21 (d, J = 6.8 Hz, 6 H, Me), 3.00 [sept, J = 6.9 Hz,
2 H, CH(CH3)2], 7.20 (m, 3 H, aromatic CH), 7.35 (t, J = 8.2 Hz,
1 H, aromatic CH), 7.44 (d, J = 8.0 Hz, 1 H, aromatic CH), 7.71
(d, J = 8.0 Hz, 1 H, aromatic CH), 7.76 (“t”, J = 6.0 Hz, 1 H,
aromatic CH), 8.10 (m, 2 H, aromatic CH), 8.21 (d, J = 8.0 Hz,
1 H, aromatic CH), 8.26 (t, J = 8.0 Hz, 1 H, aromatic CH), 9.47
(d, 1 H, imidazole backbone CH) ppm. The chloride analogue 9b
can be obtained as green powder by the reaction of 7 generated in
situ with [Rh(COE)2Cl]2 in THF at room temperature for 2 h. It
exhibits identical NMR spectroscopic data to 9a. C25H26ClN4Rh
(520.9): calcd. C 57.65, H 5.03, N 10.76; found C 57.12, H 4.87, N
10.61.
1-(2,2Ј-Bipyridin-6-yl)-3-(2,6-diisopropylphenyl)imidazol-2-ylidene
(7): The salt 6 (1.02 g, 2.21 mmol) and potassium bis(trimethylsil-
yl)amide (0.56 g, 2.81 mmol) were stirred under nitrogen and co-
oled to –78 °C. Pre-cooled THF (35 mL) was added, and the solu-
tion stirred at low temperature for 1 h. It was warmed to –30 °C,
then left to stand overnight. After evaporation of the solvent, the
product was filtered through Celite® as a solution in toluene
(20 mL). Reduction to a volume of 2 mL was followed by addition
of petroleum ether (10 mL); cooling to –30 °C gave the product as
a brown solid (36 mg, 4%). Better yields are obtained as follows:
Potassium metal (348 mg, 8.9 mmol) was added to 6 (3.00 g,
5.25 mmol) in THF (120 mL) and the mixture heated to reflux
overnight. After cooling to room temperature, the solution was fil-
tered through Celite® and the solvent removed in vacuo. The solid
residue was dissolved in toluene (70 mL), the solution filtered and
the volume reduced to ca. 15 mL. After addition of petroleum ether
(50 mL), the product was crystallised at –30 °C. Filtration and
washing with petroleum ether (20 mL) gave the carbene as a brown
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solid (586 mg, 29%). H NMR (300 MHz, C6D6, 25 °C): δ = 1.13
Oxidative Addition Adduct 10 of 9a with CD2Cl2: Addition of
CD2Cl2 to a solid sample of 9a initially gave a dark green solution,
(d, J = 6.9 Hz, 6 H, Me), 1.22 (d, J = 6.8 Hz, 6 H, Me), 2.94 [sept,
Eur. J. Inorg. Chem. 2006, 4857–4865
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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