New Structural Motifs of Lithium NHC Complexes
Organometallics, Vol. 28, No. 12, 2009 3535
DMSO-d6, 298 K): 8.57 (br s, 2H imidazole), 7.78 (br s, 2H
imidazole), 7.37 (br s, 2H imidazole), 1.56 (s, 18H, C(CH3)3), 0.26
(s, 6H, B(CH3)2). 13C{1H} NMR (125.77 MHz, DMSO-d6, 298 K):
133.9, 123.0, 119.8 (all s, imidazole C), 57.9 (s, C(CH3)3), 29.3 (s,
C(CH3)3), 8.47 (br s, B(CH3)2). MS-ESI: m/z (rel intens, %) 145
(100) [(M - 2(t-Bu) - 2Me - Br)+], 289 (90) [(M - Br)+].
[H2BIMtBuBPh2]PF6 (3a). The experiment was performed under
air. To a solution of the chloride 2a (3 g, 6.7 mmol) in MeOH
(100 mL) was added in several portions well-milled NH4PF6 (3.27
g, 20.1 mmol). The reaction mixture was stirred at rt for several
hours, the solids were filtered off and washed on the filter with
MeOH, and the methanol washings were discarded. The remaining
solid was thoroughly washed with CH2Cl2, and the filtrate was
evaporated (the desired salt is very soluble in CH2Cl2, unlike
NH4PF6 and NH4Br). The residue obtained was recrystallized from
CH2Cl2 - MeOH to afford a white microcrystalline solid (2.85 g,
75%), mp 213-214 °C. Samples of 3a suitable for X-ray analysis
were obtained by slow crystallization from the diluted methanol
appears at 198.7 ppm (THF-d8). Carbenes 4a,b and 4b · THF
are stable in the solid state or in solution at room temperature,
but they are extremely sensitive to air.
In conclusion, we describe here a convenient method for the
synthesis of sterically demanding anionic ligands [BIMtBuBR2]
(R ) Ph or Me) which enabled us to structurally characterize
two unprecedented, chelating ML3-type and nonchelating µ2-
κC2:κC2′-type lithium carbene complexes with strikingly dif-
ferent bonding motifs in the solid state. The results presented
in this work add to understanding the properties and composition
of lithium carbene complexes in the solid state and in solution.
The new lithium carbene adducts 4a,b are promising reagents
for carbene transfer reactions which are under way in our
laboratory and will be reported in due course.
Experimental Section
Unless otherwise stated, all the experiments were performed
under an Ar atmosphere using a Braun glovebox or by Schlenk
techniques. THF, pentane, and diethyl ether were distilled over
benzophenone sodium ketyl and kept under Ar. Benzene-d6 and
DMSO-d6 were stirred over CaH2 and then degassed three times
prior to use. THF-d8 was distilled over sodium and also degassed
three times. N-tert-Butylimidazole,21 Ph2BCl,22 and Me2BBr23 were
prepared according to known procedures. NMR spectra were
recorded on a Bruker 500, 300, or 250 MHz spectrometer at
the temperatures specified below. 1H and 13C NMR chemical shifts
are reported in parts per million and are referenced to the deuterated
solvents used.
1
solution at room temperature. H NMR (500.13 MHz, DMSO-d6,
298 K): 8.36 (s, 2H imidazole), 7.90 (s, 2H imidazole), 7.22-7.29
(m, 8H, 2H imidazole + 6H aromatic), 7.06-7.08 (m, 4 aromatic
H). 13C{1H} NMR (125.77 MHz, DMSO-d6, 298 K): 135.7, 133.0,
127.5, 126.7, 125.1, 119.9 (all aromatic and imidazole C), 58.4 (s,
NC(CH3)3), 29.1 (s, NC(CH3)3). Anal. Calcd for C26H34BF6N4P:
C, 55.93; H, 6.14; N, 10.03; P, 5.55. Found: C, 55.81; H, 6.12; N,
10.03; P, 5.51.
[H2BIMtBuBMe2]PF6 (3b). The synthesis was carried out under
air. To a stirred solution of bromide 2b (2.0 g, 5.42 mmol) in MeOH
(150 mL) was added portionwise well-milled NH4PF6 (1.943 g,
11.92 mmol). The reaction mixture was stirred for 4 h and filtered.
The solid on the filter was washed with MeOH (20 mL), the
methanol fractions were combined, and the solvent was completely
removed. The solid residue was treated with CH2Cl2 (120 mL) and
filtered. The residual solid (presumably a mixture of NH4Br and
NH4PF6) was washed with CH2Cl2 (30 mL), and the combined
dichloromethane solutions were evaporated. The solid obtained was
dissolved in a minimum amount of hot methanol (30-40 mL), and
the solution was slowly cooled to -20 °C and kept at this
temperature overnight. The separated crystals were collected on a
filter, washed with Et2O, and then dried in vacuo to yield 1.333 g
[H2BIMtBuBPh2]Cl (2a). To a solution of N-tert-butylimidazole
(6.1 g, 49.2 mmol) in absolute toluene (25 mL) at 0 °C was added
slowly a solution of Ph2BCl (3.79 g, 18.9 mmol) in absolute toluene
(25 mL), and a white precipitate started to form after 1 min. The
reaction mixture was stirred at room temperature (rt) for 15 min
and then at 110 °C for 48 h. The white suspension was filtered
under Ar, and the white precipitate obtained was washed with dry
toluene (5 × 10 mL) and dry hexane (5 × 10 mL) and was dried
at 10-3 mbar to give a white light powder, 7.60 g (89.5%), mp
227-228 °C. The product is only slightly soluble in THF, Et2O,
and toluene, but easily soluble in DMSO or MeOH. 1H NMR
3
1
(300.13 MHz, DMSO-d6, 305 K): 8.38 (pseudo-t, 2H, JHH ) 1.6
(57%) of crystalline product, mp 149-151 °C. H NMR (500.13
3
Hz, 2H imidazole), 7.94 (pseudo-t, 2H, JHH ) 1.7 Hz, 2H
MHz, DMSO-d6, 298 K): 8.47 (pseudo-t, 2H imidazole), 7.76
(pseudo-t, 2H imidazole), 7.34 (pseudo-t, 2H imidazole), 1.56 (s
+ sat, 1JCH ) 127.6 Hz, 18H, C(CH3)3), 0.26 (s + sat, 1JCH ) 114.5
Hz, 6H, B(CH3)2). 13C{1H} NMR (125.77 MHz, DMSO-d6, 298
K): 133.8, 123.1, 119.9 (all s, imidazole C), 58.0 (s, C(CH3)3), 29.3
(s, C(CH3)3), 8.49 (br s, B(CH3)2). MS-ESI: m/z (rel intens, %)
125.6 (40) [(C7H12N2 + H)+], 289.6 (20) [(M - PF6)+]. Anal. Calcd
for C16H30BF6N4P: C, 44.26; H, 6.96; N, 12.90; P, 7.13. Found: C,
44.12; H, 6.95; N, 12.72; P, 7.18.
imidazole), 7.23-7.32 (m, 6 aromatic H), 7.21 (pseudo-t, 2H, 3JHH
) 1.6 Hz, 2H imidazole), 7.06-7.10 (m, 4 aromatic H), 1.57 (s,
18H, NC(CH3)3). 13C{1H} NMR (75.48 MHz, DMSO-d6, 305 K):
135.7 (s, imidazole C), 132.9 (s, aromatic C), 127.4 (s, aromatic
C), 126.6 (s, aromatic C), 125.0 (s, imidazole C), 119.9 (s, imidazole
C), 58.3 (s, NC(CH3)3), 29.0 (s, NC(CH3)3). MS-ESI: m/z (rel intens,
%) 413.4 (M - Cl)+ (25). Anal. Calcd for C26H34BClN4: C, 69.58;
H, 7.64; N, 12.48. Found: C, 68.81; H, 7.61; N, 12.22.
[H2BIMtBuBMe2]Br (2b). To a solution of N-tert-butylimidazole
(11.98 g, 96.6 mmol) in absolute toluene (160 mL) was added
dropwise at -20 °C a solution of Me2BBr (5.37 g, 44.4 mmol) in
absolute toluene (20 mL). The reaction mixture was allowed to
warm to rt and then was refluxed for 4 h. The white suspension
was filtered, washed three times with toluene and three times with
pentane, and dried at 10-3 mbar to yield 15.4 g (94%) of a white,
very fine powder, mp 225-227 °C dec. Compound 2b turned out
to be neither hygroscopic nor air-sensitive. 1H NMR (500.13 MHz,
[Li(BIMtBuBPh2) · Et2O] (4a). To a suspension of the salt 3a (0.3
g, 0.54 mmol) in Et2O (10 mL) was added at -70 °C a solution of
n-BuLi (1.096 mmol) in hexane (0.62 mL). The reaction mixture
was allowed to warm to ambient temperature and then was stirred
for 3 h to form a clear solution. The solvent was removed at 10-3
mbar, and the residue was extracted with absolute pentane (2 × 10
mL) and evaporated to give 70 mg of the carbene 4a as a white
powder. Crystals of 4a suitable for X-ray analysis were obtained
1
by slow evaporation of its freshly prepared pentane solution. H
3
NMR (500.13 MHz, C6D6, 298 K): 7.58 (d, JHH ) 7.2 Hz, 4H
(21) Arduengo, A. J., III.; Gentry, F. P., Jr.; Taverkere, P. K.; Simmons,
H. E., III. U.S. Patent No. 6,177,575 B1, 2001.
ortho-aromatic), 7.40 (pseudo-t, 3JHH ) 7.7 Hz, 4H meta-aromatic),
7.34 (d, 3JHH ) 1.5 Hz, 2H imidazole), 7.32 (pseudo-t, 3JHH ) 7.5
(22) (a) Letsinger, R. L.; Remes, N. J. Am. Chem. Soc. 1955, 77, 2489.
(b) Chremos, G. N.; Weidmann, H.; Zimmerman, H. K. J. Org. Chem. 1961,
26, 1683. (c) Abel, E. W.; Dandegaonker, S. H.; Gerrard, W.; Lappert,
M. F. J. Chem. Soc. 1956, 4697. (d) Abel, E. W.; Gerrard, W.; Lappert,
M. F. J. Chem. Soc. 1957, 3833.
3
Hz, 2H para-aromatic), 6.81 (d, JHH ) 1.5 Hz, 2H imidazole),
3
3.03 (q, JHH ) 7.2 Hz, 4H, 2OCH2), 1.43 (s, 18H, NC(CH3)3),
3
0.88 (t, JHH ) 7.2 Hz, 6H, CH3). 13C{1H} NMR (125.77 MHz,
(23) No¨th, H.; Vahrenkamp, H. J. Organomet. Chem. 1968, 11, 399.
C6D6, 298 K): 199.4 (br s, Ccarbene), 156.8 (br s), 135.4, 127.0, 126.6,