F. E. Hahn et al. · Pd(II) Complexes of N-Allyl Substituted N-Heterocyclic Carbenes
1521
3
ical trans (17.4 Hz) and cis (10.2 Hz) JHH coupling 4.21 (q, 2 H, NCH2CH3), 1.42 (t, 3 H, NCH2CH3). –
13C NMR (100.6 MHz, [D6]DMSO, ppm): δ = 136.21
constants. The molecular structure of 5 is depicted in
(NCN), 132.13 (NCH2CHCH2), 122.77 (NCHCHN), 120.64
(NCH2CHCH2), 51.13 (NCH2CHCH2), 44.63 (NCH2CH3),
15.47 (NCH2CH3). – C8H13N2Br (217.11): calcd. C 44.26,
H 6.04, N 12.90; found C 43.82, H 6.46, N 12.71.
Fig. 1. Again a square-planar complex in the trans-
configuration is obtained. The steric demand of the
N1,N3-substituents in 5 is slightly larger than in 4.
This leads to an even larger angle of the imidazol-
2-ylidene plane relative to the PdC2Br2 plane (angle
between planes 84.9◦). However, the Pd-Ccarbene bond
1,3-Diallyl-imidazolium bromide 2: A mixture of 1.36 g
(20 mmol) of imidazole and 5.04 g (60 mmol) of sodium
hydrogencarbonate in acetonitrile (90 ml) is treated with
allyl bromide (9.68 g, 6.96 ml, 80 mmol). The reac-
tion mixture is heated under reflux for 12 h and fil-
tered. Removal of all solvents gives 2 as a brown oil.
˚
length in 5 (2.028(5) A) is identical within experimen-
tal error to the value observed in 4.
A slightly different protocol was used for the prepa-
ration of 6. Here a THF solution containing the im-
idazolium salt and Pd(OAc)2 was first sonicated in
an ultrasound bath for 5 min and the stirred at room
temperature for 2 d. Complex 6 was obtained in
good yield as a yellow oil. The 13C NMR spectrum
1
Yield: 3.65 g (15.9 mmol, 80%). – H NMR (200.1 MHz,
[D6]DMSO, ppm): δ = 9.41 (s, 1 H, NCHN), 7.81 (d,
2 H, NCHCHN), 5.89 (m, 2 H, NCH2CHCH2), 5.23
(m, 4 H, NCH2CHCH2), 4.87 (d, 4 H, NCH2CHCH2). –
13C NMR (100.6 MHz, [D6]DMSO, ppm): δ = 137.10
shows two slightly different resonances for the termi- (NCN), 132.66 (NCH2CHCH2), 123.55 (NCHCHN), 121.20
(NCH2CHCH2), 51.77 (NCH2CHCH2).
(229.12): calcd. C 47.18, H 5.72, N 12.23; found C 46.68,
H 5.50, N 12.15.
– C9H13N2Br
nal methyl groups of the N-substituents. Complex 6 ex-
hibits a square-planar coordination geometry (Fig. 1)
with bond lengths and bond angles similar to 4 – 5 (Ta-
ble 1).
The olefinic groups in 4 – 6 do not coordinate un-
der the reaction conditions employed. It was attempted
to enforce this coordination by abstraction of bromide
with silver tetrafluoroborate. However, in all cases this
leads to insoluble black solids.
1,3-Bis(3-methyl-2-butenyl)-imidazolium bromide 3:
Compound 3 was synthesized as described for 2 from
0.37 g (5.5 mmol) of imidazole, 1 g (12 mmol) of sodium
hydrogencarbonate and 1.3 ml (1.639 g, 11 mmol) of
4-bromo-2-methyl-2-butene in acetonitrile (12 ml). Yield
1.34 g (4.7 mmol, 86%) of a brown oil. – 1H NMR
(200 MHz, [D6]DMSO, ppm): δ = 9.32 (s, 1 H, NCHN),
7.75 (s, 2 H, NCHCHN), 5.38 (m, 2 H, NCH2CHC(CH3)2),
4.82 (m, 4 H, NCH2CHC(CH3)2), 1.74 (s, broad, 12 H,
NCH2CHC(CH3)2). – 13C NMR (50.3 MHz, [D6]DMSO,
ppm): δ = 136.53 (NCN), 131.85 (NCH2CHC(CH3)2),
118.73 (NCHCHN), 113.83 (NCH2CHC(CH3)2), 42.99
(NCH2CHC(CH3)2), 21.78, 14.64 (NCH2CHC(CH3)2). –
Satisfactory microanalytical data were not obtained owing
to the hygroscopic behaviour of 3.
Experimental Section
If not noted otherwise, all manipulations were performed
in an atmosphere of dry argon by standard Schlenk tech-
niques. Solvents were dried by standard methods and freshly
distilled prior to use.
1-Ethyl-3-allyl-imidazolium bromide 1: Imidazole
(1.36 g, 20 mmol) is dissolved in THF (50 ml) and cooled to
−78 ◦C. To this solution is added n-butyl lithium (20 mmol,
trans-Di[1-ethyl-3-allyl-imidazolin-2-ylidene]palladium
8 ml of a 2.5 M solution). The reaction mixture is stirred dibromide 4: A sample of 0.304 g (1.4 mmol) of 1 is
for 1 h at −78 ◦C and ethyl bromide (2.18 g, 1.49 ml, dissolved in THF (15 ml) and 0.130 g (0.58 mmol) of
20 mmol) is added. Under stirring the solution is allowed to Pd(OAc)2 is added. The reaction mixture is stirred for
warm up to room temperature over 12 h. Half of the solvent 7 d. A yellow suspension is obtained, which is filtered.
is removed under reduced pressure and dichloromethane Removal of the solvent gives 4 as an air-stable yellow solid.
(20 ml) is added. The solution is washed with water and Yield: 66 mg (0.123 mmol, 21%). Suitable crystals for
the organic phase is dried over MgSO4. The solvent is an X-ray diffraction analysis were obtained by diffusion
removed leaving 1-ethylimidazole as a brown oil. The oil of n-hexane into a concentrated dichloromethane solution
is dissolved in THF (30 ml) and allyl bromide (9.68 g, of 4. 1H NMR (598.9 MHz, [D8]THF, ppm): δ = 7.08
6.96 ml, 80 mmol) is added. The reaction mixture is (d, 2 H, NCHCHN), 6.98 (d, 2 H, NCHCHN), 6.29 (m,
heated under reflux for 20 h. Subsequently the solvent is 2 H, NCH2CHCH2), 5.36 (m, 2 H, NCH2CHCHH), 5.21
removed and 1 is obtained as a brown oil. Yield 1.14 g (m, 2 H, NCH2CHCHH), 5.12 (m, 4 H, NCH2CHCH2),
(5.25 mmol, 26%). – 1H NMR (200.1 MHz, [D6]DMSO, 4.52 (q, 4 H, NCH2CH3), 1.57 (t, 6 H, NCH2CH3). –
ppm): δ = 9.31 (s, 1 H, NCHN), 7.87 (m, 1 H, NCHCHN), 13C NMR (150.6 MHz, [D8]THF, ppm): δ = 171.80 (NCN),
7.76 (m, 1 H, NCHCHN), 6.03 (m, 1 H, NCH2CHCH2), 135.50 (NCH2CHCH2), 121.09, 121.07 (NCHCHN), 118.15
5.32 (m, 2 H, NCH2CHCH2), 4.86 (d, 2 H, NCH2CHCH2), (NCH2CHCH2), 54.12 (NCH2CHCH2), 46.39 (NCH2CH3),
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