Adducts of aluminum and gallium trichloride with a N-heterocyclic carbene and an adduct of aluminum trichloride with a thione

Article abstract of DOI:10.1002/ejic.200400247

The reactions of AlCl₃ and GaCl₃ with 1,3,4,5-tetramethylimidazole-2-ylidene in toluene at room temperature affords the 1:1 adducts 5 and 6, respectively. The use of a bulky N-heterocyclic carbene (NHC) in toluene/THF and AlCl₃ results in the formation of an imidazolium salt 7 with the tetrachloroaluminate anion. A 1:1 adduct 8 of 1,3,4,5-tetramethylimidazole-2(3H)-thione with AlCl₃ is obtained in toluene. The crystal structures of 5·0.5C₇H₈, 7·THF and 8 were determined. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejic.200400247

FULL PAPER
Adducts of Aluminum and Gallium Trichloride with a N-Heterocyclic Carbene
and an Adduct of Aluminum Trichloride with a Thione
Andreas Stasch,[a] Sanjay Singh,[a] Herbert W. Roesky,*[a] Mathias Noltemeyer,[a] and
Hans-Georg Schmidt[a]
Keywords: Adducts / Aluminum / Carbenes / Gallium / Lewis acids
The reactions of AlCl3 and GaCl3 with 1,3,4,5-tetramethyl- imidazole-2(3H)-thione with AlCl3 is obtained in toluene.
imidazole-2-ylidene in toluene at room temperature affords
the 1:1 adducts 5 and 6, respectively. The use of a bulky N-
heterocyclic carbene (NHC) in toluene/THF and AlCl3 re-
sults in the formation of an imidazolium salt 7 with the tetra-
chloroaluminate anion. A 1:1 adduct 8 of 1,3,4,5-tetramethyl-
The crystal structures of 5·0.5C7H8, 7·THF and 8 were deter-
mined.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2004)
Introduction
Results and Discussion
The use of N-heterocyclic carbenes (NHC)[1,2] as good σ-
donor molecules to stabilize trivalent group 13 compounds
was established with the isolation of the first alane adduct
of an imidazol-2-ylidene 1.[3] The stability of these com-
plexes is demonstrated by the melting point of 1 (246Ϫ247
°C). Compound 1 melts without decomposition, whereas
AlH3·NMe3 decomposes above 100 °C.[4] The properties of
the corresponding gallium and indium compounds allow
their isolation and characterization.[5,6] However, adducts
of a NHC with aluminum or gallium halides have not been
reported. The reaction of a bidentate NHC ligand with
AlCl3 in diethyl ether resulted in an imidazolium salt in
moderate yield and in by-products, although moisture was
excluded from the reaction mixture.[7] Only adducts of a
phosphanylsilylcarbene with MCl3 (M ϭ Al 2, Ga 3, In 4)
were reported and the X-ray single-crystal structure of 3
was determined.[8] From reactions of a NHC with InX3
(X ϭ Cl, Br) under different conditions, carbene adducts
with a composition of 1:1 and 1:2 ratios as well as imidazol-
ium salts of carbene-stabilized indates were obtained and
structurally characterized.[9]
AlCl3 was treated with one equivalent of 1,3,4,5-tetra-
methylimidazole-2-ylidene[10] in toluene at room tempera-
ture; AlCl3 dissolved after 30 minutes. The mixture was fil-
tered and concentrated. Colorless plates of 5 were obtained
1
in 56% yield at low temperature. The H NMR spectrum
of 5 shows two singlets (δ ϭ 1.15 and δ ϭ 3.23 ppm) for
the protons of the methyl groups of the carbene ligand.
They are shifted upfield relative to the free carbene 1,3,4,5-
tetramethylimidazole-2-ylidene (δ ϭ 1.59 and δ ϭ 3.35
ppm). No resonances for an imidazolium salt were detected.
A weak and broad 13C NMR resonance (δ ϭ 154 ppm) can
be assigned to the carbene carbon atom. The 27Al NMR
spectrum shows a resonance at δ ϭ 103 ppm, which is typi-
cal for four-coordinate aluminum and in the region of the
resonances for hydride derivatives. EI mass spectrometry
presents a molecular ion peak at m/z ϭ 256 in the correct
isotopic pattern and free 1,3,4,5-tetramethylimidazole-2-yli-
dene as the base peak. Compound 5 crystallizes with half
¯
a molecule of toluene in the triclinic space group P1. It
exhibits the expected assembly (see Figure 1), and selected
bond lengths and angles are given in Table 1. The AlϪC
˚
distance in 5 is 2.009(5) A, which is shorter than the corre-
sponding bond length in the adduct of 1,3,4,5-tetra-
[a]
Institut für Anorganische Chemie der Georg-August-
Universität Göttingen,
Tammannstr. 4, 37077 Göttingen, Germany
E-mail: hroesky@gwdg.de
4052
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejic.200400247
Eur. J. Inorg. Chem. 2004, 4052Ϫ4055

Adducts of Aluminum and Gallium Trichloride with a N-Heterocyclic Carbene
FULL PAPER
methylimidazole-2-ylidene with Al(CϵCtBu)3 [2.051(2) ture determination. Selected bond lengths and angles are
[11]
˚
A].
summarized in Table 2. Only the cationic species for the
corresponding imidazolium salt could be detected in the
FAB mass spectrum at (m/z ϭ 389), and no peak for the
tetrachloroaluminate anion was observed. However, the EI-
mass spectrum of the salt shows the ions for the corre-
sponding AlCl3 carbene adduct with one chlorine less
(m/z ϭ 484) and with only one chlorine (m/z ϭ 451) in the
expected isotopic pattern. It is known that AlCl3 dissociates
in THF[15,16] and other donor solvents[17,18] to form donor-
stabilized salts of the type [AlCl2(Do)x][AlCl4]. Moreover,
we assume that the hydrogen atoms in the backbone (here
on C2 and C3) of the imidazolylidene are responsible for
the protonation and the formation of the imidazolium salt.
Figure 1. Crystal structure of 5·0.5C7H8; toluene omitted for clarity
˚
Table 1. Selected bond lengths (A) and angles (°) of 5·0.5C7H8
Al1ϪC1
Al1ϪCl2
N1ϪC1
N2ϪC1
C2ϪC3
2.009(5)
2.121(5)
1.352(5)
1.353(5)
1.343(6)
Al1ϪCl1
Al1ϪCl3
N1ϪC2
N2ϪC3
2.134(4)
2.131(3)
1.392(5)
1.387(5)
C1ϪAl1ϪCl1
N2ϪC1ϪN1
111.1(2)
104.8(3)
Cl2ϪAl1ϪCl1
C3ϪC2ϪN1
110.43(17)
106.9(4)
Figure 2. Crystal structure of 7·THF; only hydrogen atoms on the
imidazolium ring are shown
The gallium analogue of 5 could be prepared and crys-
tallized by a comparable method in 62% yield. Due to the
dimeric nature of GaCl3 and its good solubility in hydro-
carbon solvents, in contrast to polymeric AlCl3 in the solid
state, GaCl3 was cooled with liquid nitrogen while a solu-
tion of 1,3,4,5-tetramethylimidazole-2-ylidene in toluene
was added, and the mixture was slowly warmed to room
temperature. No single crystals were obtained for 6 but the
1H NMR spectrum shows a ratio of ca.1:0.70 ratio of 6 and
toluene. The GaCl3 adduct 6 is less soluble than 5. The
resonances for the methyl groups (δ ϭ 1.05 and 3.19 ppm)
are in the region of the resonances for 5. A resonance for
the carbene carbon atom was not observed in the 13C NMR
spectrum, as seen before for comparable compounds.[7,9]
The EI mass spectrum shows the molecular ion peak Mϩ
at m/z ϭ 300 with the expected isotopic pattern as well as
Mϩ Ϫ Cl at m/z ϭ 265. A gallium triiodide adduct of a
NHC was very recently prepared in our group from the
corresponding hydride and was structurally charac-
terized.[12] The computed MϪC distance for the adduct of
˚
Table 2. Selected bond lengths (A) and angles (°) of 7·THF
Al1ϪCl2
C1ϪN1
N1ϪC2
2.129(2)
1.334(6)
1.379(7)
Al1ϪCl3
C1ϪN2
N2ϪC3
2.140(2)
1.322(7)
1.391(6)
Cl1ϪAl1ϪCl2
109.34(10)
N2ϪC1ϪN1
108.6(4)
In a comparable route, 1,3,4,5-tetramethylimidazole-
2(3H)-thione, which is a common starting material for the
preparation of 1,3,4,5-tetramethylimidazole-2-ylidene by re-
duction with potassium in THF,[10] reacts with AlCl3 in
toluene to afford adduct 8 (Figure 3). The latter compound
1
is less soluble than 5. The H NMR resonances of the li-
gand exhibit singlets at δ ϭ 1.27 and δ ϭ 3.12 ppm. Only
the free thione was detected by EI mass spectrometry. Com-
pound 8 crystallizes in the orthorhombic space group Pbca.
Selected bond lengths and angles are given in Table 3. The
˚
AlϪS bond length is 2.2661(13) A. The CϭS bond length
˚
MCl3 (M ϭ Al, Ga) with diaminocarbene are 2.060 A for
[13]
˚
Al and 2.079 A for Ga.
These bond lengths are nearly
identical. We also found a very similar bond length for the
carbene adduct of Ga(CϵCtBu)3 with 1,3,4,5-tetrameth-
ylimidazole-2-ylidene [2.057(3) A][14] and the corresponding
˚
˚
aluminum compound [2.051(2) A, see above]. The GaϪC
˚
bond length in 3 is 1.978(10) A.
The reaction of a bulky imidazolylidene with AlCl3 in
THF/toluene afforded the imidazolium salt 7·THF in mod-
erate yield (Figure 2). The very poorly soluble compound
7·THF was characterized by an X-ray single-crystal struc- Figure 3. Crystal structure of 8
Eur. J. Inorg. Chem. 2004, 4052Ϫ4055
www.eurjic.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4053

A. Stasch, S. Singh, H. W. Roesky, M. Noltemeyer, H.-G. Schmidt
In summary, we have prepared the first NHC adducts of
nation with AlCl3 in 8 [1.739(3) A] by nearly 0.05 A. The an aluminum and a gallium trihalide, and report the first X-
AlCl3 moiety is coordinated to one lone pair of the sulfur ray crystal structure of an aluminum halide carbene adduct.
atom to give a C1ϪS1ϪAl1 angle of 98.79(9)°. The adduct Moreover, an imidazolium tetrachloroaluminate and a
8 is a model compound for the activation of a carbonyl thione adduct of AlCl3 were also synthesized and structur-
FULL PAPER
[19]
˚
in the free thione [1.690(2) A] is elongated upon coordi-
˚
˚
function by a Lewis acid.
ally characterized.
˚
Table 3. Selected bond lengths (A) and angles (°) of 8
Experimental Section
Al1ϪCl1
Al1ϪCl3
S1ϪC1
C1ϪN2
2.1242(12)
2.1318(12)
1.739(3)
1.338(4)
1.391(4)
113.48(6)
98.79(9)
106.9(2)
Al1ϪCl2
Al1ϪS1
C1ϪN1
N1ϪC2
C2ϪC3
Cl3ϪAl1ϪS1
N1ϪC1ϪS1
N2ϪC1ϪN1
2.1300(13)
2.2661(13)
1.347(4)
1.399(4)
1.351(4)
110.09(5)
125.7(2)
107.4(2)
All manipulations were performed under a dry and oxygen-free
atmosphere (N2 or Ar) using Schlenk line and glove box tech-
niques. Melting points are not corrected. X-ray single-crystal struc-
ture analyses were performed on a StoeϪSiemens four circle
(5·0.5C7H8 and 8) or a Stoe IPDS2 (7·THF) diffractometer with
N2ϪC3
Cl1ϪAl1ϪCl2
C1ϪS1ϪAl1
C3ϪC2ϪN1
˚
Mo-Kα radiation (λ ϭ 0.71073 A). The structures were solved by
direct methods and refined with full-matrix least-squares on F2
using SHELX-97.[21] Crystallographic data are summarized in
Table 4. CCDC-234360 (5·0.5C7H8), -234361 (7·THF) and -234362
(8) contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge at www.ccdc.cam.ac.uk/
conts/retrieving.html [or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax:
(internat.) ϩ44-1223-336-033; E-mail: deposit@ccdc.cam.ac.uk].
The N1ϪC1ϪN2 angle in the imidazole ring of 5
[104.8(3)°] lies between the corresponding angles of the free
carbene 1,3,4,5-tetramethylimidazole-2-ylidene [101.3(2)°
from X-ray, and 101.9(1)° and 102.0(1)° from neutron dif-
fraction experiments on the perdeuterated compound][20]
and that of an imidazolium salt [e.g. 108.6(4)° in 7·THF].
These findings are in agreement with those of previously
reported compounds.[1,2] For comparison, the N1ϪC1ϪN2
angle in 8 is 107.4(2)°.
Preparation of 5: Toluene (30 mL) was added at room temperature
to a mixture of AlCl3 (0.64 g, 4.80 mmol, 1.12 equiv.) and 1,3,4,5-
tetramethylimidazole-2-ylidene[10] (0.55 g, 4.27 mmol, 1.0 equiv.).
The mixture was stirred for 30 min, and the AlCl3 dissolved. The
mixture was filtered and concentrated in vacuo to 15 mL. A brown
Table 4. Crystallographic data
5·0.5C7H8
7·THF
8
Formula
M
C10.50H16AlCl3N2
303.58
C31H45AlCl4N2O
630.47
C7H12AlCl3N2S
289.58
T
200(2) K
133(2) K
200(2) K
orthorhombic
Pbca
13.022(3)
13.368(4)
15.025(5)
Crystal system
Space group
a (A)
b (A)
c (A)
triclinic
triclinic
¯
¯
P1
P1
˚
˚
˚
8.583(10)
8.903(10)
10.73(3)
9.937(3)
12.702(7)
14.265(5)
α (°)
85.3(2)
79.20(6)
68.57(7)
0.75(1)
82.85(3)
82.25(3)
78.66(4)
1.7404(12)
90
90
90
β (°)
γ (°)
V (nm3)
2.6155(12)
Z
2
2
8
δcalcd. (Mg mϪ3
)
1.344
0.649
314
1.203
0.390
668
1.471
0.894
1184
µ (mmϪ1
F(000)
)
Crystal size (mm)
θ (°)
1.00 ϫ 0.70 ϫ 0.60
3.71Ϫ24.95
Ϫ9 Յ h Յ 10, Ϫ10 Յ k Յ 10,
Ϫ5 Յ l Յ 12
3031/2600
0.0351
0.30 ϫ 0.30 ϫ 0.20
2.08Ϫ24.63
Ϫ11 Յ h Յ 11, Ϫ14 Յ k Յ 14,
Ϫ16 Յ l Յ 16
32460/5821
1.00 ϫ 0.80 ϫ 0.20
3.69Ϫ25.06
Ϫ3Յ h Յ 15, Ϫ15 Յ k Յ 15,
Ϫ17 Յ l Յ 17
3030/2295
Index range
Reflexes, collected/unique
Rint
0.0948
0.0700
Completeness to θ (%)
Data/Restraints/Parameter
Goodness-of-fit on F2
R values [I Ͼ 2 σ(I)]
R values (all data)
99.0
2600/265/170
1.103
R1 ϭ 0.0726, wR2 ϭ 0.2270
R1 ϭ 0.0757, wR2 ϭ 0.2314
1.483, Ϫ0.778
99.0
5821/0/360
1.010
R1 ϭ 0.0801, wR2 ϭ 0.1821
R1 ϭ 0.1094, wR2ϭ 0.1985
0.872, Ϫ0.408
99.5
2295/0/132
1.049
R1 ϭ 0.0540, wR2 ϭ 0.1348
R1 ϭ 0.0584, wR2 ϭ 0.1411
0.737, Ϫ0.550
Largest diff. peak and hole
Ϫ3
˚
[max and min] (e· A
)
4054
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Eur. J. Inorg. Chem. 2004, 4052Ϫ4055

Adducts of Aluminum and Gallium Trichloride with a N-Heterocyclic Carbene
FULL PAPER
oily by-product was also formed and could be separated off during
filtration. The solution was stored for two weeks at Ϫ25 °C to give
colorless crystals of 5. Colorless plates of 5·0.5C7H8 suitable for X-
ray single-crystal structure analysis were obtained from a concen-
posed; in general decomposition at temperatures above about 100
1
°C). H NMR (300 MHz, C6D6): δ ϭ 1.27 (s, 6 H, NCH3), 3.12 (s,
6 H, CCH3) ppm. 13C NMR (126 MHz, C6D6): δ ϭ 8.4, 31.5,
120.1, 161.6 ppm. MS (EI): m/z ϭ 156 [{MeCCN(Me)}2CϭS, 100].
trated solution in toluene at 4 °C. The crystals were dried in vacuo.
IR (Nujol, KBr plates): ν˜ ϭ 1639, 1262, 1100, 1020, 801, 612 cmϪ1
.
1
Yield 0.62 g (2.41 mmol, 56%). M.p. 130 °C. H NMR (500 MHz, C7H12AlCl3N2S (289.59): calcd. C 29.03, H 4.18, N 9.67; found C
C6D6): δ ϭ 1.15 (s, 6 H, NCH3), 3.23 (s, 6 H, CCH3) ppm. 13C 28.70, H 4.22, N 9.44.
NMR (126 MHz, C6D6): δ ϭ 7.9 [C(4,5)CH3], 34.0 (NCH3), 125.6
[C(4,5)], 154 (very broad, carbene-C) ppm. 27Al NMR (78.2 MHz,
C6D6): δ ϭ 103 ppm. MS (EI): m/z ϭ 256 [Mϩ, 3], 221 (Mϩ Ϫ Cl,
Acknowledgments
13), 124 (C7H12N2, 100). IR (Nujol, KBr plates): ν˜ ϭ 1643, 1580,
This work was supported by the Deutsche Forschungsgemeinschaft
1207, 1100, 1031, 487 cmϪ1. C7H12AlCl3N2·C3.5H4 (257.52): calcd.
and the Göttinger Akademie der Wissenschaften.
C 32.65, H 4.70, N 10.88; found C 33.11, H 4.65, N 10.67.
Preparation of 6: A solution of 1,3,4,5-tetramethylimidazole-2-ylid-
ene (0.40 g, 3.23 mmol, 1.0 equiv.) in toluene (18 mL) was slowly
[1]
Stable Carbenes: D. Bourissou, O. Guerret, F. P. Gabba¨ı, G.
added to cooled (Ϫ196 °C) GaCl3 (0.57 g, 3.24 mmol, ca. 1.0
equiv.). The mixture was slowly warmed to room temperature while
stirring, and stirring was continued for an additional 30 min at this
temperature. The mixture was concentrated to about 12 mL, fil-
tered, and stored at Ϫ25 °C to afford a colorless crystalline material
of 6·0.70C7H8. Small amounts of a brown oily by-product formed
during the reaction could be separated during filtration. Yield
0.73 g (2.00 mmol, 62%) of 6·0.70C7H8. M.p. approximately 110
°C. 1H NMR (300 MHz, C6D6) [toluene resonances are not given]:
δ ϭ 1.05 (s, 6 H, NCH3), 3.19 (s, 6 H, CCH3) ppm. 13C NMR
(126 MHz, C6D6): δ ϭ 7.6 [C(4,5)CH3], 33.8 (NCH3), 125.6
[C(4,5)] ppm. MS (EI): m/z ϭ 300 [Mϩ, 1], 265 (Mϩ Ϫ Cl, 25), 124
(C7H12N2, 100). IR (Nujol, KBr plates): ν˜ ϭ 1645, 1261, 1093,
1031, 848, 800, 570, 370 cmϪ1. C7H12Cl3GaN2·C4.9H5.6 (364.77):
calcd. C 39.18, H 4.68, N 7.68; found C 38.98, H 4.65, N 7.50.
Bertrand, Chem. Rev. 2000, 100, 39Ϫ91.
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[4]
[5]
[6]
[7]
[8]
[9]
Preparation of 7: Toluene (50 mL) and THF (50 mL) were added to
a mixture of AlCl3 (0.56 g, 4.20 mmol, 1.1 equiv.) and 1,3-bis(2,6-
diisopropylphenyl)imidazole-2-ylidene (1.47 g, 3.80 mmol, 1.0
equiv.). The mixture was stirred for 1 h and subsequently filtered.
Yellow crystals of 7·THF were obtained at 2 °C. Yield 0.98 g
(1.56 mmol, 41%). M.p. 260 °C: 1H NMR (300 MHz, CD3OD)
[THF resonances are not given]: δ ϭ 1.22 [d, J ϭ 6.9 Hz, 12 H,
CH(CH3)2], 1.34 [d, J ϭ 6.9 Hz, 12 H, CH(CH3)2], 2.45 [sept, J ϭ
6.9 Hz, 4 H, CH(CH3)2], 7.50 (d, J ϭ 7.6 Hz, 4 H, m-ArH), 7.67
(t, J ϭ 7.6 Hz, 2 H, p-ArH), 8.27 (d, J ϭ 2.5 Hz, 2 H, CH), 9.97
(t, J ϭ 2.5 Hz, 1 H, N2CH) ppm. 13C NMR (126 MHz, CD3OD):
[10]
[11]
[12]
[13]
[14]
[15]
[16]
δ ϭ 23.9, 24.7, 30.4, 126.0, 127.6, 131.5, 133.4, 141.1, 146.5 ppm.
[17]
[18]
[19]
[20]
ϩ
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6812Ϫ6822.
MS (EI): m/z
ϭ
484 [AlCl2C3N2H(Dipp)2
,
15], 451
[AlClC3N2H(Dipp)2ϩ, 2], 389 [C3N2H3(Dipp)2ϩ, 100]. MS (FAB)
cations: m/z ϭ 389 [C3N2H3(Dipp)2ϩ, 100]. IR (Nujol, KBr plates):
ν˜ ϭ 1643, 1600, 1540, 1331, 1206, 1062, 953, 804, 756, 683, 616,
491 cmϪ1. C27H37AlCl4N2·C4H8O (630.49): calcd. C 59.05, H 7.19,
N 4.44; found C 58.55, H 7.03, N 4.68.
Preparation of 8: Toluene (25 mL) was added to a mixture of AlCl3
(0.50 g, 3.75 mmol, 1.0 equiv.) and 1,3,4,5-tetramethylimidazole-
2(3H)-thione[10] (0.585 g, 3.75 mmol, 1.0 equiv.), stirred for 15 h
at room temperature, 30 min at 50 °C, and finally filtered at this
temperature. Storing the solution at 4 °C afforded crystals of 8.
Yield 0.63 g (2.18 mmol, 58%). M.p. approximately 165 °C (decom-
[21]
G. M. Sheldrick, SHELXL-97, Program for Crystal Structure
Refinement, University of Göttingen, Germany, 1997.
Received March 29, 2004
Early View Article
Published Online August 3, 2004
Eur. J. Inorg. Chem. 2004, 4052Ϫ4055
www.eurjic.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4055