Imidazolium phenolates

Article abstract of DOI:10.1515/znb-2009-0711

1,3-Diisopropyl-4,5-dimethylimidazolium phenolate (4a), pentafluorophenolate (4b), and pentachlorophenolate (4c) are obtained from 2,3-dihydro-1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (2) and the corresponding phenols 3 as stable crystals in excelle

Full text of DOI:10.1515/znb-2009-0711

Imidazolium Phenolates
Norbert Kuhn, Eyad Mallah, Ca¨cilie Maichle-Mo¨ßmer, and Manfred Steimann
Institut fu¨r Anorganische Chemie der Universita¨t Tu¨bingen, Auf der Morgenstelle 18,
72076 Tu¨bingen, Germany
Reprint requests to Prof. Dr. N. Kuhn. E-mail: norbert.kuhn@uni-tuebingen.de
Z. Naturforsch. 2009, 64b, 835 – 839; received March 20, 2009
Dedicated to Professor Helmut Werner on the occasion of his 75th birthday
1,3-Diisopropyl-4,5-dimethylimidazolium phenolate (4a), pentafluorophenolate (4b), and penta-
chlorophenolate (4c) are obtained from 2,3-dihydro-1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene
(2) and the corresponding phenols 3 as stable crystals in excellent yields. The crystal structure anal-
yses of compounds 4 reveal the presence of ion pairs linked by almost linear C–H···O hydrogen
bonds. With 4-hydroxypyridine (5), the carbene 2 gives 1,3-diisopropyl-4,5-dimethylimidazolium 4-
pyridinolate (6) whose crystal structure analysis indicates the ions to be connected by a C–H···N
bond.
Key words: Heterocycles, Imidazole, Phenol, Hydrogen Bond, Crystal Structure
Introduction
Weak hydrogen bonds have attracted considerable
interest in structural chemistry [1]. In 2H-imidazolium
salts, the ions are commonly linked by hydrogen bond-
ing; these compounds play an important role in the
construction of ionic liquids [2].
The first synthesis of a stable heterocyclic car-
bene proceeded via deprotonation of the correspond-
ing imidazolium ion with in situ generated sodium
tert-butylate [3]. Some years ago, the structure of 1,3-
dimesitylimidazolium 2,4,6-tri-tert-butylphenolate (1)
has been reported, and the results appear to imply the
need of sterical hindrance for the stabilization of this
salt [4]. Continuing our investigations on the structural
chemistry of imidazolium salts [5 – 8] we report on the
syntheses and structures of some stable imidazolium
phenolates.
Results and Discussion
As expected, the carbene 2 reacts readily with phe-
nols 3 in diethyl ether, and from this mixture the imi-
dazolium salts 4 [R = H (a), F (b), Cl (c)] precipitate as
stable colorless salts. The NMR data of their solutions
in polar solvents (see Experimental Section) suggest
the presence of separated ions. The crystal structure
analyses of 4, however, reveal their ions to be linked by
hydrogen bonds in the solid state (Table 1, Figs. 1 – 3).
c
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836
N. Kuhn et al. · Imidazolium Phenolates
Fig. 3. Molecular structure of the ion pair of C17H21Cl5N2O
(4c) in the crystal.
Fig. 1. Molecular structure of the ion pair of C17H26N2O (4a)
in the crystal.
Fig. 4. Molecular structure of the ion pair of C16H25N3O (6)
in the crystal.
Fig. 2. Molecular structure of the ion pair of C17H21F5N2O
(4b) in the crystal.
packing effects seem to predominate. In comparison
with the structure of 1, a marked lengthening of the
O···H distances is observed.
Table 2 contains the structural details of the interionic
hydrogen bonds in 4 which are close to linearity. The
structural parameters inside the ions are in the expected
range (Tables 3 – 5).
Similarly, the salt 6 is obtained from the car-
bene 2 and 4-hydroxypyridine (5). In the crys-
tals of 6 (Table 6, Fig. 4), the imidazolium ions
are linked by C–H···N bonds. In addition, we ob-
serve weak contacts between the phenolate oxygen
atoms and methyl protons of a neighboring cation
(Table 2).
Apparently, in the resonance structures of the an-
ion 7 the pyridonate form predominates [11]. In fact,
the solid state structure of 6 corresponds to the crys-
tal structure of 4-pyridone 6/5 hydrate [12] where also
N–H···O hydrogen bonds are present while in its 4-
nitrophenyl adduct both N–H···O and O–H···O bonds
were detected [13].
13C NMR data of a CD3OD solution reveal the car-
bene 2 to be protonated also by methanol, as indi-
cated by the strong shift of C2 (2: δ = 205.9 [9]) up
to 125 ppm. Interestingly, the carbene 2 may be recov-
ered unchanged on evaporation of the solvent. Thus,
dissolving 2 in methanol offers a useful method to store
the carbene in an air-stable form.
Over all, solid state structures and stabilities of im-
idazolium alcoholates are not correlated directly with
the basicity of their anions. Apparently, the structures
are strongly influenced by packing effects, and the de-
composition of the methylate salt in vacuo is due to the
high volatility of methanol.
Experimental Section
The O···H distances of the salts 4 do not correlate
All reactions were performed in purified solvents un-
with the base strengths of their phenolate ions. Thus, der argon. 2,3-Dihydro-1,3-diisopropyl-4,5-dimethylimidaz-
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N. Kuhn et al. · Imidazolium Phenolates
837
Table 1. Crystal data and structure refinement for C17H26N2O (4a), C17H21F5N2O (4b), C17H21Cl5N2O (4c) and
C16H25N3O (6).
4a
4b
4c
6
Empirical formula
Formula weight
Temperature, K
C17H26N2O
274.4
C17H21F5N2O
364.4
C17H21Cl5N2O
446.6
C16H25N3O
275.4
173
MoKα ; 0.71073
orthorhombic
˚
Wavelength; λ, A
Crystal system
Space group
monoclinic
P21/c
orthorhombic
P212121
8.661(1)
13.833(1)
14.752(2)
orthorhombic
Cmcm
12.029(1)
13.537(1)
9.959(2)
P212121
8.678(1)
10.060(1)
23.425(2)
˚
a, A
12.422(1)
9.771(1)
13.740(2)
107.80(1)
1587.9(3)
4
1.148
0.071
600
˚
b, A
˚
c, A
β, deg
3
˚
Volume, A
1767.4(4)
4
1.369
0.122
760
2045.1(2)
4
1.451
0.718
920
1621.0(4)
4
1.128
0.072
600
Z
Dcalcd., g cm−3
µ (MoKα ), mm−1
F(000), e
θ, deg
3.06 – 26.37
21946/3240
3.10 – 26.37
24963/3619
3.22 – 26.36
24012/4175
3.39 – 26.37
11310/920
Refls. coll./indep.
Refinement method
Param. refined
Full-matrix least-squares on F2
285
307
311
93
Final R1/wR2 [I ≥ 2σ(I)]
Final R1/wR2 (all data)
Goodness-of-fit on F2
Extinction coefficient
0.0613/0.0961
0.0675/0.0992
1.092
0.000(1)
+0.537/−0.311
0.0873/0.1147
0.1196/0.1233
1.219
0.006(1)
+0.225/−0.236
0.0237/0.0556
0.0252/0.0562
1.100
0.001(1)
+0.177/−0.217
0.0578/0.1281
0.0634/0.1312
1.207
0.007(1)
+0.232/−0.149
−3
˚
∆ρfin (max/min), e A
˚
˚
Table 4. Selected bond lengths (A) and angles (deg) for
Table 2. Bond lengths (A) and angles (deg) of the interionic
hydrogen bonds C–H···Ea,b
.
C17H21F5N2O (4b).
C–H
–
0.98(1)
0.93(2)
0.84(2)
0.93(2)
H···E
C–H···E
C(1)–O(7)
C(1)–C(6)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
C(20)–N(24)
N(21)–C(22)
C(23)–N(24)
1.276(5)
1.413(7)
1.376(6)
1.376(7)
1.371(7)
1.368(7)
1.324(5)
1.391(5)
1.388(5)
C(1)–C(2)
C(2)–F(8)
C(3)–F(9)
C(4)–F(10)
C(5)–F(11)
C(6)–F(12)
C(20)–N(21)
C(22)–C(23)
1.409(6)
1.364(5)
1.351(5)
1.357(5)
1.349(6)
1.354(5)
1.328(5)
1.356(6)
1c
1.88(3)
2.00(1)
2.07(1)
2.27(2)
2.23(1)
169(2)
179(1)
172(1)
177(1)
180e
4a
4b
4c
6d
a E = O (1, 4a – c), N (6); b the hydrogen atoms in the structure deter-
minations of 4a – c and 6 were placed in idealized positions and sub-
c
sequently refined with isotropic displacement parameters; ref. [4];
O(7)–C(1)–C(2)
C(2)–C(1)–C(6)
C(4)–C(3)–C(2)
C(6)–C(5)–C(4)
124.1(4)
112.3(4)
120.2(4)
121.0(4)
O(7)–C(1)–C(6)
C(3)–C(2)–C(1)
C(5)–C(4)–C(3)
C(5)–C(6)–C(1)
123.6(4)
124.3(4)
118.2(4)
123.9(4)
O(5)···H(15A) 2.479; e C(10), H(10) and N(4) lie on two perpen-
d
dicular crystallographic mirror planes.
˚
Table 3. Selected bond lengths (A) and angles (deg) for
C17H26N2O (4a).
ol-2-ylidene (2) was obtained according to a published pro-
N(5)–C(1)
N(2)–C(3)
C(1)–N(2)
C(11)–O(17)
C(12)–C(13)
C(14)–C(15)
1.320(2) N(5)–C(4)
1.388(2) C(3)–C(4)
1.322(2) C(11)–C(16)
1.288(2) C(11)–C(12)
1.381(2) C(13)–C(14)
1.374(2) C(15)–C(16)
1.388(2)
1.338(2)
1.415(2)
1.410(2)
1.374(2)
1.375(2)
cedure [9].
C17H26N2O (4a)
To a solution containing 2,3-dihydro-1,3-diisopropyl-4,5-
dimethylimidazol-2-ylidene (2, 0.465 g, 2.58 mmol) in
30 mL of dieth◦yl ether, phenol (3a, 0.243 g, 2.58 mmol) was
added at −50 C. After stirring for 12 h at r. t., the precip-
itate was filtered off, washed with diethyl ether, and dried
in vacuo. Yield after recrystallization from diethyl ether/
acetonitrile: 0.670 g (95 %), colorless crystals. – 1H NMR
C(1)–N(5)–C(4)
C(1)–N(2)–C(3)
C(3)–C(4)–N(5)
109.8(1) N(5)–C(1)–N(2)
110.1(1) C(4)–C(3)–N(2)
106.8(1) O(17)–C(11)–C(12) 122.8(1)
107.2(1)
106.2(1)
O(17)–C(11)–C(16) 122.2(1) C(12)–C(11)–C(16)
115.0(1)
121.8(1)
120.7(1)
C(13)–C(12)–C(11)
C(13)–C(14)–C(15)
C(15)–C(16)–C(11)
121.4(1) C(14)–C(13)–C(12)
118.4(1) C(14)–C(15)–C(16)
122.7(1)
3
(CD2Cl2): δ = 1.58 (d, 12 H, 1,3Im-CHMe2, J = 6.8 Hz),
2.15 (s, 6 H, 4,5Im-Me), 4.41 (sept, 2 H, 1,3Im-CHMe2),
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N. Kuhn et al. · Imidazolium Phenolates
2,3,4,5,6
˚
Table 5. Selected bond lengths (A) and angles (deg) for
137.7 – 142.4 (m, C
[10]), 124.8 (C2Im), 133.2 (CI4m,5),
Ph
C17H21Cl5N2O (4c).
147.6 (t, C1Ph
,
2J = 14 Hz). – 19F NMR (CD3CN, CCl3F
ext.): δ = −173.5 (C4-F), −196.8 (C2,6-F), −196.9 (C3,5
-
C(1)–N(2)
1.334(2) C(1)–N(5)
1.386(2) C(3)–C(4)
1.390(2) C(20)–O(2)
1.434(2) C(20)–C(21)
1.384(2) C(21)–Cl(27)
1.393(2) C(22)–Cl(28)
1.399(2) C(23)–Cl(29)
1.385(2) C(24)–Cl(30)
1.731(2)
1.334(2)
1.354(2)
1.263(2)
1.436(2)
1.737(2)
1.731(2)
1.728(2)
1.727(2)
N(2)–C(3)
C(4)–N(5)
F). – MS (FAB neg.): m/z (%) = 183 (100) [C6F5O]. – Ele-
mental analysis for C17H21F5N2O (364.35): calcd. C 56.04,
H 5.81, N 7.69; found C 55.87, H 5.51, N 7.21.
C(20)–C(25)
C(21)–C(22)
C(22)–C(23)
C(23)–C(24)
C(24)–C(25)
C(25)–Cl(31)
C
H Cl N O (4c)
17 21 5 2
To a solution containing 2,3-dihydro-1,3-diisopropyl-4,5-
dimethylimidazol-2-ylidene (2, 0.560 g, 3.11 mmol) in
30 mL of diethyl ether, pentac◦hlorophenol (3c, 0.828 g,
3.10 mmol) was added at −50 C. After stirring for 12 h
at r. t., the precipitate was filtered off, washed with diethyl
ether, and dried in vacuo. Yield after recrystallization from
diethyl ether/acetonitrile: 1.27 g (91 %), colorless crystals. –
N(2)–C(1)–N(5)
C(4)–C(3)–N(2)
C(1)–N(5)–C(4)
O(26)–C(20)–C(21) 123.5(2) C(25)–C(20)–C(21)
C(22)–C(21)–C(20)
C(22)–C(23)–C(24)
108.4(2) C(1)–N(2)–C(3)
107.2(1) C(3)–C(4)–N(5)
108.8(1) O(26)–C(20)–C(25) 123.6(2)
108.8(1)
106.8(1)
112.9(1)
120.7(2)
120.8(2)
123.7(2) C(21)–C(22)–C(23)
118.3(2) C(25)–C(24)–C(23)
3
1H NMR (CDCl3): δ = 1.49 (d, 12 H, 1,3Im-CHMe2, J =
˚
Table 6. Selected bond lengths (A) and angles (deg) for
6.8 Hz), 2.15 (s, 6 H, 4,5Im-Me), 4.41 (sept, 2 H, 1,3Im
-
C17H26N2O (6)a.
CHMe2), 10.37 (s, 1 H, 2Im-H). – 13C NMR (CDCl3): δ =
8.7 (4,5Im-Me), 22.4 (1,3Im-CHMe2), 51.2 (1,3Im-CHMe2),
115.6 (C4Ph), 122.1 (CP2,h6), 125.8 (C2Im), 129.2 (CP3,h5), 134.3
(C4Im,5), 161.4 (CP1h). – MS (FAB neg.): m/z (%) = 265
(100) [C6Cl5O]. – Elemental analysis for C17H21Cl5N2O
(446.62): calcd. C 45.72, H 4.74, N 6.27; found C 45.87,
H 4.51, N 6.21.
C(1)–O(5)
C(2)–C(3)
1.268(4)
1.371(4)
1.329(2)
1.353(4)
C(1)–C(2)
C(3)–N(4)
N(11)–C(12)
1.422(4)
1.341(3)
1.387(3)
C(10)–N(11)
C(12)–C(12)#2
O(5)–C(1)–C(2)
C(3)–C(2)–C(1)
C(3)–N(4)–C(3)#1
123.5(2)
120.7(3)
113.4(3)
C(2)–C(1)–C(2)#2
N(4)–C(3)–C(2)
113.0(3)
126.0(3)
a Symmetry transformations used to generate equivalent atoms: #1 x,
y, −z+1/2; #2 −x, y, z.
C16H25N3O (6)
To a solution containing 2,3-dihydro-1,3-diisopropyl-4,5-
dimethylimidazol-2-ylidene (2, 0.680 g, 3.77 mmol) in
30 mL of diethyl ether, 4-hyd◦roxypyridine (5, 0.360 g,
3.79 mmol) was added at −50 C. After stirring for 12 h
at r. t., the precipitate was filtered off, washed with diethyl
ether, and dried in vacuo. Yield after recrystallization from
diethyl ether/acetonitrile: 0.550 g (55 %), colorless crystals. –
1H NMR (CD3CN): δ = 1.55 (d, 12 H, 1,3Im-CHMe2, 3J =
7.01 – 7.22 (m, 5 H, Ph), 10.58 (s, 1 H, 2Im-H). – 13C NMR
(CD2Cl2): δ = 7.7 (4,5Im-Me), 21.9 (1,3Im-CHMe2), 50.2
(1,3Im-CHMe2), 115.6 (C4Ph), 121.6 (C2P,h6), 124.8 (CI2m),
128.8 (C3P,h5), 133.2 (C4,5), 156.2 (CP1h). – MS (FAB neg.):
Im
m/z (%) = 93 (100) [C6H5O]. – Elemental analysis for
C17H26N2O (274.40): calcd. C 74.41, H 9.55, N 10.21; found
C 75.25, H 10.07, N 10.21.
6.7 Hz), 2.22 (s, 6 H, 4,5Im-Me), 4.48 (sept, 2 H, 1,3Im
CHMe2), 6.00 – 7.64 (m, 4 H, C6H4NO), 9.86 (s, 1 H, 2Im
H). – 13C NMR (CD3CN): δ = 7.3 (4,5Im-Me), 21.5 (1,3Im
-
-
-
C17H21F5N2O (4b)
To a solution containing 2,3-dihydro-1,3-diisopropyl-4,5-
dimethylimidazol-2-ylidene (2, 0.547 g, 3.03 mmol) in
30 mL of diethyl ether, pentafluorophenol (3b, 0.560 g,
CHMe2), 49.8 (1,3Im-CHMe2), 116.2 (C3,5), 125.7 (CI2m),
Py
131.8 (C4Im,5), 148.9 (C2,6), 176.0 (CP4y). – MS (FAB neg.):
Py
◦
m/z (%) = 94 (100) [C5H4NO]. – Elemental analysis for
C16H25N3O (275.39): calcd. C 69.78, H 9.15, N 15.26; found
C 67.77, H 9.63, N 14.77.
3.04 mmol) was added at −50 C. After stirring for 12 h
at r. t., the precipitate was filtered off, washed with diethyl
ether, and dried in vacuo. Yield after recrystallization from
diethyl ether/acetonitrile: 0.920 g (84 %), colorless crystals. –
1H NMR (CD3CN): δ = 1.50 (d, 12 H, 1,3Im-CHMe2, 3J =
CCDC 721443 (4b), CCDC 721444 (4c), CCDC 721445
(6), and CCDC 721446 (4a) contain the supplementary crys-
tallographic data for his paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data request/cif.
6.7 Hz), 2.23 (s, 6 H, 4,5Im-Me), 4.49 (sept, 2 H, 1,3Im
-
CHMe2), 9.34 (s, 1 H, 2Im-H). – 13C NMR (CD3CN): δ =
7.24 (4,5Im-Me), 21.4 (1,3Im-CHMe2), 49.9 (1,3Im-CHMe2),
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N. Kuhn et al. · Imidazolium Phenolates
839
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