Tuning the electronic properties of N-heterocyclic carbenes

Article abstract of DOI:10.1002/chem.200700228

The electron-donating properties of N-heterocyclic carbenes ([N,N′-bis(2,6-dimethylphenyl)imidazol]-2-ylidene and the respective dihydro ligands) with 4,4′-R-substituted aryl rings (4,4′-R = NEt₂, OC₁₂H₂₅, Me, H, Br, S(4-t

Full text of DOI:10.1002/chem.200700228

FULL PAPER
DOI: 10.1002/chem.200700228
Tuning the Electronic Properties of N-Heterocyclic Carbenes
Steffen Leuthäußer, Daniela Schwarz, and Herbert Plenio*[a]
Abstract: The electron-donating prop-
erties of N-heterocyclic carbenes
([N,N’-bis(2,6-dimethylphenyl)imida-
zol]-2-ylidene and the respective dihy-
dro ligands) with 4,4’-R-substituted
aryl rings (4,4’-R=NEt2, OC12H25, Me,
N
2(NHC)]. Cyclic
R=ÀSO2Ar, DE1/2 =+0.92 V) results
in NHC ligands with virtually the same
electron-donating capacity as a trialkyl-
phosphine in [IrCl
=+0.95 V), while [IrCl
complexes with 4-R=NEt2 (DE1/2
A
ACHTREUNG
A
ACHTREUNG
=
H, Br, S
A
G
U
+0.59 V) show drastically more catho-
dic redox potentials and significantly
enhanced donating properties.
tolyl)) were studied. Twelve new N-
heterocyclic carbene (NHC) ligands
were synthesized as well as the respec-
Keywords:
iridium · N-heterocyclic carbenes ·
redox chemistry
IR spectroscopy
·
tive iridium complexes [IrCl(cod)-
Introduction
that the TEP (TEP=Tolman electronic parameter) of the
most common NHC ligands fall within a 3 cmÀ1 range,
which is much smaller than the 12 cmÀ1 observed for typical
phosphines.[33] While the differences between different types
of NHC ligands can be much larger, this goes along with sig-
nificant variation in space requirements; for a given steric
profile, the electronic variability is only small.[34,35]
For the present work, we wanted to modify in a systemat-
ic manner two of the most commonly applied NHC ligands
(N,N’-bis(2,6-dimethylphenyl)imidazol)-2-ylidene and N,N’-
bis(2,6-dimethylphenyl)-4,5-dihydroimidazol)-2-ylidene), to
allow the systematic tuning of the electron density of a
metal bonded to NHC ligands. We wish to report here on
the synthesis of twelve new NHC ligands, which have the
same core structure and differ only with respect to electron-
releasing capacity. To demonstrate this we have synthesized
28 different Ir complexes, all of which were studied by
means of cyclic voltammetry (DE1/2) and IR spectroscopy
(n(CO)).
Control over the electronic and steric properties of ligands
is of critical importance for the optimization of the catalytic
activity of transition-metal-based complexes.[1] Subtle varia-
tions of steric bulk and electron density at the active site
may have drastic effects on the catalytic efficiency.[2–6] Con-
sequently, the quantification of the steric and electronic ef-
fects of ligands,[7–10] such as through the classic approach by
Tolman,[11] is essential.
N-Heterocyclic carbenes (NHCs) are rivaling the leading
status of phosphines as ligands in homogeneous cataly-
sis.[6,12–18] Thus the fine-tuning of NHC ligands is of prime
importance for the design of tailored catalysts with opti-
mized activities, most of this work has concentrated on the
steric effects of NHC ligands, while the construction of a
series of electronically variable NHC ligands with invariant
bulk has been neglected.[19–21] Nonetheless, numerous studies
have been devoted to determining the electron donating
properties of NHC ligands; this is most often done by study-
ing the n˜(CO) of [Ni(CO)
U
G
Results and Discussion
Synthesis of ligands and metal complexes: The respective
imidazolium and imidazolinium salts were synthesized by
following modified procedures from Arduengo and
Noels.[36,37] These procedures are ideally suited to obtain the
protonated NHC ligands with a variety of 4-R groups
(Table 1). In order to cover a large range of electronic ef-
[a] Dipl.-Ing. S. Leuthäußer, D. Schwarz, Prof. Dr. H. Plenio
Anorganische Chemie im Zintl-Institut
Petersenstrasse 18, 64287 Darmstadt (Germany)
Fax : (+49)6151-166040
E-mail: plenio@tu-darmstadt.de
Chem. Eur. J. 2007, 13, 7195 –7203
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7195

Table 1. Synthesis of imidazolium and imidazolinium salts with variable
mines (7–12) and ring closure leads to the analogous imida-
zolinium salts (13s·H+–18s·H+) with a ClÀ counter ion.
Prior to our work, only the imidazolium and imidazolinium
salts with 4-R=Me (15u·H+, 15s·H+) or H (16u·H+,
16s·H+) were known.[37] Despite the failure of the diimine
formation with electron-deficient anilines, it was possible to
synthesize four NHC precursors with strongly electron-with-
groups R (Ar=4-tolyl).[a]
À
drawing groups by oxidation of sulfur in the SAr-substitut-
ed salts 18u·H+ and 18s·H+. The two substituted heterocy-
cles were treated with stoichiometric amounts of H2O2 to
allow the oxidation to the sulfoxides 19u·H+ and 19s·H+.
The respective sulfones 20u·H+ and 20s·H+ were obtained
by treatment of 18u·H+ and 18s·H+ with a large excess of
H2O2 (Scheme 2).
R
Products (yield [%])
NEt2
OC12H25
Me
H
Br
1 (84)
2 (48)
3
7 (99)
8 (61)
9
10
11 (57)
12 (76)
13u·H+ (62)
14u·H+ (62)
15u·H+
13s·H+ (93)
14s·H+ (54)
15s·H+
4
16u·H+
16s·H+
5 (82)
6 (99)
17u·H+ (79)
18u·H+ (55)
17s·H+ (74)
18s·H+ (19)
SAr
[a] Reagents and conditions: a) glyoxal, EtOH, HCOOH, RT; b) LiAlH4,
THF, RT, HCl/H2O; c) HC
oxane, RT.
A
fects we planned on synthesizing the respective heterocycles
with several electronically diverse groups at the 4-position
Scheme 2. Synthesis of electron-deficient NHC·HCl (Ar=4-tolyl).
a) AcOH, 2.0 equiv H2O2 30%, RT; b) AcOH, 18.0 equiv H2O2 30%, RT
(a denotes either a single or a double bond).
(R=NEt2, OC12H25, Me, H, S
requires the respective 2,6-dimethylanilines with different 4-
R groups. As two of the anilines (4-OC12H25, S(4-tolyl))
A
AHCTREUNG
were not known, they had to be synthesized (Scheme 1). To
Ir complexes of the type [IrCl
G
G
in the reaction of [IrCl(cod)]2 with the respective free car-
AHCTREUNG
bene (Scheme 3) according to a general procedure by Herr-
Scheme 3. Synthesis of the Ir complexes. a) THF, KOtBu, azolium salt,
RT; b) CH2Cl2, CO, RT, quantitative conversions.
+
Scheme 1. Synthesis of the new anilines. a) AcOH, 2,4-(NO2)2C6H3N2
ClÀ, 08C; b) H2, Pd/C, THF/MeOH; c) 1,4-C6H4
HOC2H4OH, CuI, K2CO3, 808C.
(CH3)(SH), iPrOH,
mann.[38] Bubbling of CO through solutions of these com-
obtain the NHC ligands, the anilines were reacted with
glyoxal to form the corresponding diimines. We found that
this procedure works well for all anilines, except for those
with strongly electron-withdrawing groups (4-R=CF3, NO2).
For the other anilines, the normal sequence of diimine for-
mation (products 1–6) and ring closure leads to the respec-
tive imidazolium salts (13u·H+–18u·H+) with a ClÀ counter
ion. The sequence of diimine formation and LiAlH4 reduc-
tion of the imines results in the formation of the related dia-
plexes generated the respective [IrCl(CO)
C
plexes in virtually quantitative conversions.[38]
IR spectroscopy of NHC–iridium complexes: The determi-
nation of n˜(CO) in metal complexes is a useful and widely
applied method for the evaluation of the electron-donating
properties of ligands attached to a metal center.[22] We stud-
ied the n˜(CO) of the two series of [IrCl(CO)
plexes (Table 2), which are nearly the same for the saturated
7196
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Chem. Eur. J. 2007, 13, 7195 –7203

N-Heterocyclic Carbenes
FULL PAPER
Table 2. n˜(CO) of the [IrCl(CO)2
A
Table 3. Redox potentials of the [IrCl
N
E
N
unsaturated
TEP
R=
NHC n˜
[cmÀ1
]
[cmÀ1
[IrCl
(cod)
N
unsaturated
saturated
R=
NEt2
Me
13u
15u
16u
17u
19u
20u
1978/
2064
1980/
2066
1981/
2067
1982/
2069
1984/
2073
1985/
2074
1985/
2074
2052.2
2053.6
2054.3
2055.4
2057.6
2058.6
2058.6
13s
15s
16s
17s
19s
20s
1979/
2065
1981/
2068
1981/
2069
1984/
2071
1985/
2073
1986/
2075
2052.9
2054.7
2055.1
2056.9
2057.9
2059.0
NEt2
OC12H25
Me
H
Br
SOAr
SO2Ar
[IrCl
13u
14u
15u
16u
17u
19u
20u
0.648
0.761
0.765
0.786
0.862
0.870
0.920
0.948
80
80
80
78
78
72
80
94
13s
14s
15s
16s
17s
19s
20s
0.591
0.730
0.735
0.759
0.838
0.846
0.910
68
86
76
77
82
86
76
H
Br
SOAr
SO2Ar
U
G
even though the differences appear to be small. Considering
the more electron-donating character of sp3 versus sp2-
carbon atoms in the saturated versus the unsaturated NHC
ligands, this trend is reasonable. Nonetheless, it is surprising
that a small change in the remote groups, 4-R, effect large
changes in the redox potentials, while the effect of two sp3
versus two sp2-carbon atoms is rather small. To illustrate this
point: the replacement of a 4-CH3 group by a 4-H changes
the redox potential of the Ir complexes by 21–24 mV, while
the difference between the respective saturated and unsatu-
rated complexes is roughly the same at 24–27 mV. It is hard
to see why the transfer of electron density over three bonds
should be less efficient than that over seven bonds. Further-
more the six- and five-membered rings are orthogonal,
which can hardly promote the transfer of electronic informa-
tion.
The validity of the data can be shown by the excellent
correlation of the redox potentials determined by us for the
saturated and the unsaturated Ir complexes with the respec-
tive Hammett parameters[45] which produce excellent linear
fits (R2 >0.98 for both types of complexes, Figure 1).[46]
To place the donating properties of NHC ligands on the
electrochemical scale in a more general context we have de-
termined the redox potential of the phosphine complex
A
E
and the unsaturated complexes, but the saturated NHC
complexes appear to show marginally higher TEP values. It
is interesting that the TEP of [IrCl
(cod)
(NHC)] (NHC=
20u) (2059 cmÀ1) and of [IrCl (PCy3)] (2059 cmÀ1)[39]
U
ACHTREUNG
display the same values. The obvious conclusion is that the
PCy3 ligand has almost the same electron-donating capacity
as the SO2Ar-substituted NHC ligand 20u.
À
Electrochemistry of NHC–iridium complexes: Alternatively,
the determination of the redox potentials of metal com-
plexes may serve as a precise measure for the electron-do-
nating properties of the NHC ligand. The electrochemical
properties of Ru and Fe metal complexes of N-heterocyclic
carbenes have been probed previously and were found to be
sensitive to changes of the substituents at the ligands.[40] De-
monceau, Noels et al. determined the redox potentials of a
few NHC–Ru complexes, which were shown to be correlat-
ed with the catalytic activity of the respective NHC–Ru
complexes in the atom transfer radical (ATR) polymeri-
zation.[41,42] Electrochemical data of Grubbs II and Grubbs–
Hoveyda complexes with variable NHC ligands have been
reported by us.[43,44]
[IrCl
N
N
We first studied complexes of the type [IrCl(CO)2
as such complexes were available from the IR studies. How-
ever, the electrochemistry of [IrCl(CO)2(NHC)] is irreversi-
A
AHCTREUNG
ble. This is probably due to the loss of CO ligands following
the oxidation of the IrI center. Fortunately, the related [IrCl-
A
N
plexes are synthesized, are characterized by a reversible
electrochemistry. The redox potential for the various unsatu-
rated [IrCl
0.920 V for NHC=13u–17u, 19u, and 20u and for the satu-
rated [IrCl(cod)(NHC)] complexes (NHC=13s–17s, 19s,
A
G
A
ACHTREUNG
and 20s) between 0.591–0.910 V (Table 3). On comparing
various Ir complexes, the redox potentials of complexes with
the saturated NHC are consistently lower by between 10–
57 mV. Thus we conclude that saturated NHC ligands are
better electron donors than the related unsaturated ones,
Figure 1. Correlation of Hammett parameters and redox potentials of
&
*
[(IrCl
A
saturated.
Chem. Eur. J. 2007, 13, 7195 –7203
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7197

H. Plenio et al.
Table 4. Crystal data and structure refinement of [IrCl
[IrCl(cod)(16u)].
A
ACHTREUNG
of the SO2Ar-substituted NHC complex 20u. We thus
(again) conclude that the electron-donating properties of
SO2Ar-substituted NHC ligands are almost the same as that
of an electron-rich trialkylphosphine, such as PCy3. Thus it
is possible to close the gap between the electron-donating
NHC ligands and phosphines.
G
ACHTREUNG
A
G
(13s)]
N
G
ACHTREUNG
empirical formula
Fw
C35H52ClIrN4
756.5
C27H34ClIrN2
614.25
T [K]
153
153
crystal system
space group
monoclinic
P21/a
triclinic
P1
On comparing the two different techniques for the deter-
mination of the electron donating properties, cyclic voltam-
metry seems to be the superior approach. For the complexes
studied here, the difference between the most and the least
electron-donating NHC ligand is 319 mV, which is more
than 60 times higher than the precision with which a redox
potential can be determined (approximately Æ5 mV). With
the same series of complexes, the TEP based on the n˜(CO)
differs only by 6 cmÀ1, which is only twelve times the
0.5 cmÀ1 precision of the routine IR experiment, which is
limited by the bandwidth of the respective absorptions and
the spectrometer resolution. The advantage of the electro-
chemical method is not unexpected as the determination of
a physical property of a metal center, which is directly
bonded to the NHC ligand, should result in a higher disper-
¯
unit cell dimensions [pm]
a
1432.6
861.4
b
1421.6
876.8
c
1715.2
1696.2
a, b, g [8]
V [Š3]
Z
90, 109.97, 90
3283.1
4
1.53
4.178
91.26, 96.71, 107.50
1211.1
2
1.684
5.639
1calcd [MgmÀ3
]
m [mmÀ1
(000)
]
F
A
1536
608
crystal size [mm3]
V range [8]
0.15”0.14”0.084
3.8–29.55
À19ꢀhꢀ19
À19ꢀkꢀ19
À23ꢀlꢀ23
58945
9056
numerical
6759/0/365
0.11”0.10”0.06
2.42–29.55
À11ꢀhꢀ11
À11ꢀkꢀ11
À23ꢀlꢀ23
6569
6569
numerical
6569/0/273
index ranges
reflections collected
independent reflections
absorption correction
data/restraints/parameters
final R indices [I>2s(I)],
all data
À
sion than a method which probes the C O vibration of a
carbonyl ligand more remote from the NHC. Furthermore,
the small modifications in the 4-R positions reported here
extend the range of electronic diversity within a given series
of NHC ligands to almost 7 cmÀ1, bringing it much closer to
the diversity reported for phosphines.
R1
wR2
0.0451, 0.0752
0.0692, 0.0750,
1.16, À3.0
0.0487, 0.1359
0.0803, 0.1126
1.21, À2.03
largest diff. peak/hole
[eŠ3]
(near Ir)
G
Crystal structure determinations of [IrCl
N
N
(NHC=13s and 16s): To learn whether changes in the elec-
tronic situation at the ligands have an effect on the NHC–
metal interactions we have determined the crystal structures
of two [IrCl
N
N
4-R=ÀNEt2 and 4-R=H. The Ir–C52 (C in NHC) distances
in both crystal structures are identical within estimated stan-
dard deviation (NHC 13s: 205.0(4) and NHC 16s:
205.7(10) pm). The bond lengths around Ir observed in
[IrCl
A
G
G
N
related [IrCl
A
ACHTREUNG
between 203–208 pm in monodentate,[22,26] abnormal,[27] bi-
dentate,[23] or tripodal[47] NHC ligands. The small variance in
the bond length between rather different NHC ligands un-
derlines the weak influence of electronic and steric proper-
ties of the NHC ligand on the structures of such metal com-
plexes.
Figure 2. X-ray crystal structure of [IrCl
probability).
N
A
plexes recently reported by Grela et al.;[48] a comparison of
three closely related complexes shows the mesityl units to
be the flexible section of the NHC ligands.
The only significant deviation of the two structures con-
cerns the orientation of the two pairs of mesityl flaps. In the
crystal of [IrCl
A
ACHTREUNG
clined towards Ir than in [IrCl
N
G
Conclusions
by comparing the average of the two pairs of distances Ir-
C13/C23, which are 527 pm in R=H (NHC: 16s) and
574 pm in R=NEt2 (NHC: 13s). It cannot be excluded that
this is the result of different electronic interactions. Howev-
er, we believe this to be caused by different packing forces
The variation of the 4-R groups in N-arylated NHC ligands
allows the systematic tuning of the electron-donating prop-
erties of the respective ligands, which can thus be adapted
to the specific needs of metal complexes. Specifically, we
have demonstrated that NHC ligands with strong electron-
withdrawing substituents (SO2Ar) have virtually the same
electron-donating properties as PCy3, while a modified
in the crystals of [IrCl
A
(13s)] and [IrCl
G
N
high flexibility of these NHC segments is also obvious from
several crystal structures of Grubbs–Hoveyda-type com-
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Chem. Eur. J. 2007, 13, 7195 –7203

N-Heterocyclic Carbenes
FULL PAPER
À
redissolved in CHCl3 (250 mL), and washed with saturated NaHCO3 so-
lution. The organic layer was dried over MgSO4 and the solvent was
evaporated in vacuo. A sample of the azo dye (7.0 g, 14.5 mmol) was dis-
solved in THF/EtOH (8:1, 200 mL) and hydrogenated by using Pd on
Charcoal (10 wt%, 10 mol% Pd, 1.53 g, 1.4 mmol, p(H2)=5 bar). After
5 h, the reaction mixture was filtered over Celite and the solvent evapo-
rated in vacuo. Purification by column chromatography (silica gel, cyclo-
hexane/ethyl acetate 2:1) gave 2.39 g (54%) of 2,6-dimethyl-4-dodecyloxy-
ligand with similar bulk and NEt2 substituents is an elec-
tron-rich carbene. We have used two different methods
(cyclic voltammetry and IR spectroscopy) to probe the elec-
tronic properties of NHC ligands, observing either the redox
potential of [IrCl
[IrCl(CO)2(NHC)] complexes. We consider the use of [IrCl-
(cod)(NHC)] complexes in conjunction with cyclic voltam-
A
N
ACHTREUNG
A
ACHTREUNG
1
aniline. H NMR (300 MHz, CDCl3): d=0.88 (t, 3H; CH3), 1.20–1.50 (m,
metry as the superior technique for the determination of the
electron-donating properties of NHC ligands. The electro-
chemical response is more precise, in that the dispersion of
the physical property studied is much larger with respect to
the resolution of the respective physical technique. On the
other hand, both approaches (IR, CV) can be easily com-
18H; CH2), 1.72 (m, 2H; CH2), 2.16 (s, 6H; CH3), ca. 3.10–3.50 (brs, 2H;
NH2), 3.86 (t, 2H; OCH2), 6.55 ppm (s, 2H; arom.); 13C NMR
(75.5 MHz, CDCl3): d=13.1, 16.9, 21.7, 25.1, 28.3–28.7, 30.9, 67.6, 113.8,
122.1, 135.2, 150.6 ppm.
2,6-Dimethyl-4-(p-tolylthio)aniline: Following
a general procedure by
Kwong and Buchwald,[55] 2,6-dimethyl-4-iodoaniline (3.00 g, 12.1 mmol,
1.0 equiv), CuI (120 mg, 0.63 mmol, 5 mol%), and K2CO3 (3.31 g,
24.0 mmol, 2.0 equiv) were weighed into a Schlenk flask under an atmos-
phere of Ar. Isopropanol (12 mL, techn. grade), ethyleneglycol (1.3 mL),
and 4-methylthiophenol (2.8 mL, 24.2 mmol, 2.0 equiv) were added. The
reaction mixture was stirred at 808C for 4 d, then poured into water
(50 mL), and extracted with diethyl ether (3”50 mL). The combined or-
ganic layers were dried over MgSO4 and the solvent was evaporated in
vacuo. Purification by column chromatography (silica gel; cyclohexane/
ethyl acetate 5:1) gave 2,6-dimethyl-4-(p-tolylthio)aniline (2.56 g; 88%)
bined as the [IrCl(CO)2(NHC)] complex required for the IR
A
method is synthesized from the respective [IrCl
complexes.
N
N
Experimental Section
1
as a brown solid. H NMR (CDCl3, 300 MHz): d=2.14 (s, 6H; CH3), 2.27
General experimental: All chemicals were purchased as reagent grade
from commercial suppliers and used without further purification, unless
otherwise noted. THF was distilled over potassium and benzophenone
(s, 3H; CH3), 3.64 (s, 2H; NH2), 7.01 (m, 2H; arom. CH), 7.06 (m, 2H;
arom. CH), 7.10 ppm (m, 2H; arom. CH); 13C NMR (CDCl3, 75 MHz):
d=17.5, 20.9, 120.4, 122.6, 128.0, 128.3, 129.6, 134.3, 136.1, 142.6 ppm.
1
under an argon atmosphere. H, 13C, and 31P NMR spectra were recorded
on Bruker DRX 500 at 500, 125.75, and 202.46 MHz, respectively, or on
Bruker DRX 300 at 300 or 75.07 MHz. The chemical shifts are given in
parts per million (ppm) on the delta scale (d) and are referenced to tetra-
methylsilane (1H, 13C NMR=0 ppm; 31P NMR used 65% aq. H3PO4 =
0 ppm). Abbreviations for NMR data: s=singlet, d=doublet, t=triplet,
q=quartet, m=multiplet, brs=broad singlet, and arom.=aromatic pro-
tons. IR spectra of the metal carbonyls were recorded on a Perkin–Elmer
1600 IR spectrometer in CHCl2 solution as 100”10À6 m films between
KBr plates. TLC was performed by using silica gel 60 F254 (0.2 mm) on
aluminum plates. For preparative chromatography, E. Merck silica gel 60
(0.063–0.20 mesh) was used. The following compounds were prepared ac-
cording to literature procedures: 2,6-dimethyl-4-bromoaniline as de-
scribed for the 2,6-diisopropyl derivative,[49] 2,6-dimethyl-4-diethylamino-
aniline,[50] 2,4,6-trimethylphenylimidazolium chloride and 2,4,6-trimethyl-
General procedure for the synthesis of diimines: The corresponding ani-
line (2.0–2.5 equiv) was dissolved in EtOH (2 mLmmolÀ1) and then treat-
ed with aqueous glyoxal solution (40%wt, 1.0 equiv) and three drops of
formic acid. The reaction mixture was stirred overnight. The yellow solid
was filtered off, washed with cold MeOH, and dried in vacuo. The
volume of the mother liquor was halved and the remaining solution kept
at 48C overnight for a second batch of product.
N,N’-Bis(2,6-dimethyl-4-diethylaminophenyl)ethylenediimine (1): Starting
materials used were 2,6-dimethyl-4-(diethylamino)aniline (21.85 g,
85 mmol, 2.5 equiv) and glyoxal (3.9 mL, 34 mmol, 1.0 equiv). Yield:
11.65 g (84%); 1H NMR (300 MHz, CDCl3): d=1.17 (t, 12H; CH2CH3),
2.23 (s, 12H; CH3), 3.34 (q, 8H; CH2CH3). 6.43 (s, 4H; arom.), 8.11 ppm
(s, 2H; CH); 13C NMR (75 MHz, CDCl3): d=12.8, 19.4, 44.3, 112.0,
129.4, 139.8, 145.4, 162.2 ppm.
imidazolinium chloride,[37] and [IrCl(cod)]2.[51]
G
N,N’-Bis(2,6-dimethyl-4-dodecyloxyphenyl)ethylenediimine (2): Starting
materials used were 2,6-dimethyl-4-dodecyloxyaniline (2.39, 7.8 mmol,
2.0 equiv) and glyoxal (0.45 mL, 3.9 mmol, 1.0 equiv). Yield: 1.18 g
X-ray crystal structure determination:
Data collection: Data were recorded on a Stoe IPDS II diffractometer by
using monochromated MoKa radiation.
1
(48%); H NMR (300 MHz, CDCl3): d=0.88 (t, 6H; CH3), 1.20–1.47 (m,
Structure refinement: The structures were solved by using SHELXS-86[52]
and were refined anisotropically on F2 by using SHELXL-97.[53] Hydro-
gen atoms were refined by using a riding model. X-ray crystal data were
processed by using the WINGX shell and ORTEP-3.[54] CCDC-636094
and -636095 contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
36H; CH2), 1.76 (m, 4H; CH2), 2.19 (s, 12H; CH3), 3.93 (t, 4H; OCH2),
6.64 (s, 4H; arom.), 8.09 ppm (s, 2H; CH); 13C NMR (75 MHz, CDCl3):
d=14.1, 18.7, 22.7, 26.1, 29.4, 29.4, 29.6, 29.7, 29.7, 31.9, 68.1, 114.2, 128.6,
143.2, 156.3, 163.4 ppm.
N,N’-Bis(2,6-dimethyl-4-bromophenyl)ethylenediimine (5): Starting mate-
rials used were 2,6-dimethyl-4-bromoaniline (5.0 g, 17.8 mmol, 2.0 equiv)
and glyoxal (1.02 mL, 8.9 mmol, 1.0 equiv). Yield: 3.08 g (82%);
1H NMR (300 MHz, CDCl3): d=2.15 (s, 12H; CH3), 7.16 (s, 4H; arom.),
8.07 ppm (s, 2H; CH); 13C NMR (75 MHz, CDCl3): d=18.2, 117.9, 128.8,
131.1, 148.8, 163.7 ppm.
Synthesis of the anilines
2,6-Dimethyl-4-dodecyloxyaniline: NaOH (8.0 g, 0.2 mol, 1.5 equiv) was
dissolved in water (100 mL), 3,5-dimethylphenol (16.2 g, 133 mmol,
1.0 equiv) was added and the mixture was stirred for 0.5 h. A solution of
dodecylbromide (24.6 mL, 133 mmol, 1.0 equiv) in ethanol (100 mL) was
added and the reaction mixture stirred at 1008C for 14 h. After cooling
to room temperature, the product was extracted with diethyl ether (3”
100 mL). The combined organic layers were dried over MgSO4 and the
solvent was evaporated in vacuo. Yield: 16.1 g (41%) of 3,5-dimethyl-do-
decyloxybenzene. The crude product was dissolved in glacial acetic acid
(500 mL). The solution was cooled to 08C and 2,4-dinitrophenyldiazoni-
um chloride solution, prepared from 2,4-dinitroaniline (15.28 g,
83.4 mmol, 1.5 equiv) and NaNO2 (5.76 g, 83.4 mmol, 1.5 equiv), in aque-
ous HCl (5m, 100 mL) was added. The reaction mixture was stirred for
14 h at room temperature. After this time, The red azo dye was filtrated,
N,N’-Bis(2,6-dimethyl-4-tolylthiophenyl)ethylenediimine (6): Starting ma-
terials used were 2,6-dimethylamino-4-(p-tolylthio)aniline (2.56 g,
10.5 mmol, 2.0 equiv) and glyoxal (0.6 mL, 5.25 mmol, 1.0 equiv). Yield:
2.65 g (99%); 1H NMR (CDCl3, 300 MHz): d=2.12 (s, 12H; CH3), 2.34
(s, 6H; CH3), 7.08 (s, 4H; arom. CH), 7.13 (dd, 4H; arom. CH), 7.27 (dd,
4H; arom. CH), 8.09 ppm (s, 2H; CH=N); 13C NMR (CDCl3, 75 MHz):
d=18.2, 21.8, 127.7, 129.9, 130.8, 131.1, 131.8, 132.5, 137.0, 148.9,
163.4 ppm.
General procedure for the synthesis of diamines: The corresponding eth-
ylenediimine (1.0 equiv) was placed in a Schlenk flask and dissolved in
anhydrous THF (10 mLmmolÀ1) under an atmosphere of Ar. The solu-
tion was cooled to 08C and LiAlH4 pellets (2.0 equiv) were added. The
Chem. Eur. J. 2007, 13, 7195 –7203
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7199

H. Plenio et al.
reaction mixture was stirred overnight at room temperature and then
poured carefully into an excess of an ice/concentrated HCl mixture.
enediimine (420 mg, 1.0 mmol, 1.0 equiv), paraformaldehyde (38 mg,
1.25 mmol, 1.25 equiv) and HCl in dioxane (4m, 0.38 mL, 1.5 mmol,
1.5 equiv). Yield: 370 mg (79%); 1H NMR ([D6]DMSO, 500 MHz): d=
1.97 (s, 12H; CH3), 7.49 (d, 4H; arom. H), 8.15 (s, 2H; HC=CH),
9.65 ppm (s, 1H; CH); 13C NMR ([D6]DMSO, 126 MHz): d=17.1, 124.2,
125.0, 131.8, 133.1, 137.8, 139.1 ppm.
Workup A: The white precipitate was collected by filtration, washed with
cold water, and dried in vacuo.
Workup B: The reaction mixture was basified by using NaOH and ex-
tracted with diethyl ether (3”250 mL). The combined organic layers
were dried over MgSO4 and the solvent was evaporated in vacuo.
N,N’-Bis(2,6-dimethyl-4-tolylthiophenyl)imidazolium chloride (18u·H+):
Starting materials used were N,N’-bis(2,6-dimethyl-4-tolylthiophenyl)e-
thylenediimine (1.21 g, 2.33 mmol, 1.0 equiv), paraformaldehyde (87 mg,
2.9 mmol, 1.25 equiv), and HCl in dioxane (4m, 0.76 mL, 3.0 mmol,
1.3 equiv). Yield: 714 mg (55%); 1H NMR ([D6]DMSO, 300 MHz): d=
2.14 (s, 12H; CH3), 2.34 (s, 6H; CH3), 7.14 (s, 4H; arom. CH), 7.29 (d,
N,N’-Bis(2,6-dimethyl-4-diethylaminophenyl)ethylenediamine (7): Start-
ing materials used were N,N’-bis(2,6-dimethyl-4-diethylaminophenyl)-
ethylenediimine (10.82 g, 26.6 mmol, 1.0 equiv) and LiAlH4 (2.0 g,
53.2 mmol, 2.0 equiv). Workup procedure B was followed. Yield: 10.9 g
1
(99%); H NMR (300 MHz, CDCl3): d)=1.00 (t, 12H; CH2CH3), 2.21 (s,
4H; arom. CH), 7.41 (d, 4H; arom. CH), 8.30 (d, 2H, 4J
C
12H; ArCH3), 2.98 (s, 2H; NH2), 3.17 (q, 8H; CH2CH3), 6.32 ppm (s,
4H; arom.); 13C NMR (75 MHz, CDCl3): d=12.8, 19.2, 44.7, 50.0, 113.5,
132.0, 135.9, 143.9 ppm.
AHCTREUNG
([D6]DMSO, 75 MHz): d=17.0, 20.7, 124.7, 127.7, 128.2, 130.6, 131.4,
133.5, 135.7, 137.3, 138.7, 140.5 ppm.
N,N’-Bis(2,6-dimethyl-4-dodecyloxyphenyl)ethylenediamine (8): Starting
materials used were N,N’-bis(2,6-dimethyl-4-dodecyloxyphenyl)ethylene-
diimine (1.18 g, 1.9 mmol, 1.0 equiv) and LiAlH4 (141 mg, 3.7 mmol,
2.0 equiv). Workup procedure B was followed. Yield: 0.74 g (61%);
1H NMR (300 MHz, CDCl3): d=0.88 (t, 6H; CH3), 1.20–1.50 (m, 36H;
CH2), 1.60–1.70 (m, 4H; CH2), 2.29 (s, 12H; CH3), ca. 2.70–3.30 (brs,
2H; NH), 3.08 (s, 4H; NCH2), 3.88 (t, 4H; OCH2), 6.58 ppm (s, 4H;
arom.); 13C NMR (75 MHz, CDCl3): d=14.1, 18.6, 22.7, 26.1, 29.4, 29.4,
29.6, 29.6, 29.7, 31.9, 49.6, 68.1, 114.7, 131.8, 139.0, 154.5 ppm.
General procedure for the synthesis of imidazolinium chlorides (13s–
18s)·H+
Procedure A: The corresponding diamine (1.0 equiv) and NH4Cl
(1.0 equiv) were suspended in HC(OEt)3 (3.0 equiv) and 3 drops of
R
formic acid were added. The reaction mixture was stirred at 1208C for
4 h and then poured into water (500 mL). The aqueous phase was
washed with diethyl ether (2”300 mL) and extracted with CH2Cl2 (3”
300 mL). The combined organic layers were dried over MgSO4 and the
solvent was evaporated in vacuo.
N,N’-Bis(2,6-dimethyl-4-bromophenyl)ethylenediamine dihydrochloride
(11): Staring materials used were N,N’-bis(2,6-dimethyl-4-bromophenyl)e-
thylenediimine (4.22 g, 10 mmol, 1.0 equiv) and LiAlH4 (760 mg,
20 mmol, 2.0 equiv). Workup procedure A was followed. Yield: 2.84 g
(57%); 1H NMR (300 MHz, [D6]DMSO): d=2.37 (s, 12H; CH3), 3.52 (s,
4H; CH2), 7.23 ppm (s, 4H; arom.), NH2 was not observed; 13C NMR
(75 MHz, [D6]DMSO): d=18.1, 46.3, 118.6, 129.9, 132.1 137.1 ppm.
Procedure B: The corresponding diamine dihydrochloride was suspended
in HC
(OEt)3 (5 mLmmolÀ1), 3 drops of formic acid were added and the
U
reaction mixture was stirred at 1208C overnight. The white precipitate
was filtered off, washed several times with diethyl ether and dried in
vacuo.
N,N’-Bis(2,6-dimethyl-4-diethylaminophenyl)imidazolinium
(13s·H+): Procedure A was followed and the starting materials used
were N,N’-bis(2,6-dimethyl-4-diethylaminophenyl)ethylenediamine
(10.8 g, 26.3 mmol, 1.0 equiv), NH4Cl (1.40 g, 26.3 mmol, 1.0 equiv), and
chloride
N,N’-Bis(2,6-dimethyl-4-tolylthiophenyl)ethylenediamine dihydrochloride
(12): Starting materials used were N,N’-bis(2,6-dimethyl-4-tolylthiophenyl)-
ethylenediimine (15.0 g, 29.5 mmol, 1.0 equiv) and LiAlH4 (2.24 g,
59.0 mmol, 2.0 equiv). Workup procedure A was followed. Yield: 13.3 g
(76%); 1H NMR ([D6]DMSO, 300 MHz): d=2.29 (s, 12H; CH3), 2.34 (s,
6H; CH3), 3.51 (s, 4H; CH2), 5.92 (brs, 4H; NH2), 7.00 (s, 4H; arom.
CH), 7.18–7.23 ppm (m, 8H; arom. CH); 13C NMR ([D6]DMSO,
75 MHz): d=23.5, 25.9, 51.7, 135.5, 136.0, 136.5, 137.5, 142.5 ppm.
HC(OEt)3 (13.1 mL, 78.8 mmol, 3.0 equiv). Yield: 11.2 g (93%);
T
1H NMR (300 MHz, [D6]DMSO): d=1.05 (t, 6H; CH3), 2.29 (s, 12H;
CH3), 3.33 (q, 8H; CH2), 4.33 (s, 4H; CH2), 6.46 (s, 4H; arom. H),
8.87 ppm (s, 1H; CH); 13C NMR ([D6]DMSO, 75 MHz): d=12.4, 17.8,
43.6, 51.2, 110.7, 121.1, 136.0, 147.7, 160.8 ppm.
N,N’-Bis(2,6-dimethyl-4-dodecyloxyphenyl)imidazolinium
(14s·H+): Procedure A was followed and the starting materials used
were N,N’-bis(2,6-dimethyl-4-dodecyloxyphenyl)ethylenediamine
(864 mg, 1.36 mmol, 1.0 equiv), NH4Cl (73 mg, 1.36 mmol, 1.0 equiv), and
HC(OEt)3 (0.677 mL, 4.07 mmol, 3.0 equiv). Yield: 500 mg (54%);
chloride
General procedure for the synthesis of imidazolium chlorides (13u–
18u)·H+: The corresponding diimine (1.0 equiv) was dissolved in anhy-
drous THF (10 mLmmolÀ1) under an atmosphere of Ar. A solution of
paraformaldehyde (1.25–1.40 equiv) in HCl in dioxane (4m, 1.5 equiv)
was prepared and added to the diimine solution at 08C by syringe. The
reaction mixture was stirred at room temperature for 4 h, the white pre-
cipitate was filtrated, washed with diethyl ether, and dried in vacuo.
AHCTREUNG
1H NMR (300 MHz, [D6]DMSO): d=0.84 (t, 6H; CH3), 1.10–1.50 (m,
36H; CH2), 1.60–1.81 (m, 4H; CH2), 2.34 (s, 12H; CH3), 3.96 (t, 4H;
OCH2), 4.41 (s, 4H; NCH2CH2N), 6.81 (s, 4H; arom.), 9.02 ppm (s, 1H;
CH); 13C NMR ([D6]DMSO, 75 MHz): d=13.9, 17.5, 22.1, 25.4, 28.5,
28.7, 29.0, 31.3, 51.1, 67.7, 114.4, 126.1, 137.1, 159.1, 160.7 ppm.
N,N’-Bis(2,6-dimethyl-4-bromophenyl)imidazolinium chloride (17s·H+):
Procedure B was followed and N,N’-bis(2,6-dimethyl-4-bromophenyl)-
ethylenediamine dihydrochloride (2.50 g, 5.0 mmol) was used. Yield:
1.75 g (74%); 1H NMR ([D6]DMSO, 300 MHz): d=2.34 (s, 12H; CH3),
4.48 (s, 4H; CH2), 7.54 (s, 4H; arom.), 9.11 ppm (s, 1H; CH); 13C NMR
([D6]DMSO, 126 MHz): d=17.0, 50.8, 122.9, 131.4, 132.7, 138.5,
160.4 ppm.
N,N’-Bis(2,6-dimethyl-4-tolylthiophenyl)imidazolinium chloride (18s·H+):
Procedure B was followed and N,N’-bis(2,6-dimethyl-4-tolylthiophenyl)-
ethylenediamine dihydrochloride (9.20 g, 15.8 mmol) was used. Yield:
1.6 g (19%); 1H NMR ([D6]DMSO, 300 MHz): d=2.30 (s, 12H; CH3),
2.37 (s, 6H; CH3), 4.45 (s, 4H; CH2), 7.02 (s, 4H; arom. CH), 7.26 (d,
4H; arom. CH), 7.36 (d, 4H; arom. CH), 9.13 ppm (s, 1H; imidazolinium
H); 13C NMR ([D6]DMSO, 75 MHz): d=17.3, 20.7, 50.8, 128.1, 128.6,
130.5, 131.6, 133.2, 136.9, 138.6, 139.2, 160.4 ppm.
N,N’-Bis(2,6-dimethyl-4-diethylaminophenyl)imidazolium
chloride
(13u·H+): Starting materials used were N,N’-bis(2,6-dimethyl-4-diethyl-
aminophenyl)ethylenediimine (1.95 g, 4.8 mmol, 1.0 equiv), paraformal-
dehyde (205 mg, 6.84 mmol, 1.4 equiv), HCl in dioxane (4m, 1.8 mL,
7.11 mmol, 1.5 equiv). Yield: 1.35 g (62%); 1H NMR (300 MHz,
[D6]DMSO): d=1.10 (t, 12H; CH2CH3), 2.06 (s, 12H; CH3), 3.38 (q, 8H;
CH2CH3), 6.56 (s, 4H; arom. H), 8.14 (s, 2H; HC=CH), 9.57 ppm (s, 1H;
CH); 13C NMR ([D6]DMSO, 75 MHz): d=12.4, 17.4, 43.7, 110.5, 121.3,
125.2, 134.9, 139.2, 148.3 ppm.
N,N’-Bis(2,6-dimethyl-4-dodecyloxyphenyl)imidazolium chloride (14u·H+):
Starting materials used were N,N’-bis(2,6-dimethyl-4-dodecyloxyphenyl)-
ethylenediimine (650 mg, 1.03 mmol, 1.0 equiv), paraformaldehyde
(38.5 mg, 1.28 mmol, 1.25 equiv) and HCl in dioxane (4m, 0.385 mL,
1.5 equiv). Yield: 435 mg (62%); 1H NMR (300 MHz, [D6]DMSO): d=
0.85 (t, 6H; alkyl-CH3), 1.20–1.40 (m, 36H; CH2), 1.60–1.80 (m, 4H;
CH2), 2.11 (s, 12H; CH3), 4.02 (t, 4H; OCH2), 6.93 (s, 4H; arom. H),
8.24 (s, 2H; HC=CH), 9.69 ppm (s, 1H; CH); 13C NMR ([D6]DMSO,
75 MHz): d=13.9, 17.4, 22.1, 25.4, 28.5, 28.7, 29.0, 31.3, 67.9, 114.3, 125.0,
126.1, 136.0, 139.0, 159.7 ppm.
Oxidation of N,N’-bis(2,6-dimethyl-4-tolylthiophenyl)azolium salts: N,N’-
Bis(2,6-dimethyl-4-tolylsulfinylphenyl)imidazolium chloride (19u·H+)
(500 mg, 0.894 mmol, 1.0 equiv) was dissolved in glacial acetic acid
N,N’-Bis(2,6-dimethyl-4-bromophenyl)imidazolium chloride (17u·H+):
Starting materials used were N,N’-bis(2,6-dimethyl-4-bromophenyl)ethyl-
7200
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ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 7195 –7203

N-Heterocyclic Carbenes
FULL PAPER
(8 mL). H2O2 (30%, 157 mL, 1.788 mmol, 2.0 equiv) was added and the
reaction mixture was stirred overnight at room temperature. The volatiles
were evaporated and the residue was dried in vacuo. Yield: 507 mg
(96%); 1H NMR ([D6]DMSO, 300 MHz): d=2.18 (s, 12H; CH3), 2.33 (s,
6H; CH3), 7.37 (d, 4H; arom.), 7.68 (d, 4H; arom.), 7.74 (s, 4H; arom.),
8.33 (s, 2H; NCHCHN), 9.75 ppm (s, 1H; CH); 13C NMR ([D6]DMSO,
75 MHz): d=17.2, 20.8, 123.7, 124.2, 124.4, 130.2, 135.0, 136.6, 138.7,
141.6, 142.2, 148.8 ppm.
(69%); 1H NMR (CDCl3, 500 MHz): d=1.17–1.35 (m, 4H; cod CH2),
1.20 (t, 12H; CH3), 1.66–1.76 (m, 4H; cod CH2), 2.11 (s, 6H; CH3), 2.34
(s, 6H; CH3), 3.08 (m, 2H; cod CH), 3.32–3.44 (m, 8H; NCH2), 4.12 (m,
2H; cod CH), 6.42 (s, 2H; arom.), 6.46 (s, 2H; arom.), 6.92 ppm (s, 2H;
NCHCHN); 13C NMR (CDCl3, 125 MHz): d=11.6, 17.8, 19.3, 28.0, 32.6,
43.4, 50.0, 80.2, 109.1, 110.6, 122.6, 126.5, 134.3, 137.0, 146.6, 180.7 ppm.
A
(cod)
U
U
U
0.027 mmol, 1.0 equiv), KOtBu (6.1 mg, 0.054 mmol, 2.0 equiv), and N,N’-
bis(2,6-dimethyl-4-dodecyloxyphenyl)imidazolinium chloride (34 mg,
0.049 mmol, 1.8 equiv). Workup procedure A was followed. Yield: 30 mg
N,N’-Bis(2,6-dimethyl-4-tolylsulfinylphenyl)imidazolinium
chloride
(19s·H+): N,N’-Bis(2,6-dimethyl-4-tolylthiophenyl)imidazolium chloride
(293 mg, 0.524 mmol, 1.0 equiv) was dissolved in glacial acetic acid
(10 mL). H2O2 (30%, 110 mL, 1.074 mmol, 2.0 equiv) was added and the
reaction mixture was stirred overnight at room temperature. The volatiles
were evaporated and the residue dried in vacuo. Yield: 312 mg (99%);
1H NMR ([D6]DMSO, 300 MHz): d=2.30 (s, 12H; CH3), 2.33 (s, 6H;
CH3), 4.44 (s, 4H; NCH2CH2N), 7.05 (s, 4H; arom.), 7.26 (d, 4H; arom.),
7.35 (d, 4H; arom.), 9.07 ppm (s, 1H; CH); 13C NMR ([D6]DMSO,
75 MHz): d=17.3, 20.7, 50.8, 124.2, 128.1, 128.6, 130.5, 131.6, 13.2, 136.9,
138.6, 139.2 ppm.
1
(61%); H NMR (CDCl3, 500 MHz): d=0.88 (t, 6H; CH3), 1.24–1.36 (m,
36H; CH2), 1.44–1.46 (m, 4H; cod CH2), 1.64–1.65 (m, 4H; CH2), 1.76–
1.79 (m, 4H; cod CH2), 2.34 (s, 6H; CH3), 2.55 (s, 6H; CH3), 3.08–3.09
(m, 2H; cod CH), 3.88 (s, 4H; NCH2CH2N), 3.94–3.98 (t, 4H; OCH2),
4.11 (m, 2H; cod CH), 6.66 (d, 2H; arom.), 6.68 ppm (d, 2H; arom.);
13C NMR (CDCl3, 125 MHz): d=13.1, 17.7, 19.3, 21.7, 25.0, 27.7, 28.3,
28.3, 28.4, 28.6, 28.6, 28.6, 28.7, 30.9, 32.5, 50.3, 50.9, 67.0, 82.6, 112.8,
113.1, 130.7, 135.7, 138.6, 157.3, 207.0 ppm.
A
(cod)
U
G
N
N,N’-Bis(2,6-dimethyl-4-tolylsulfonylphenyl)imidazolium
chloride
(20u·H+): N,N’-Bis(2,6-dimethyl-4-tolylthiophenyl)imidazolium chloride
(1.00 mg, 1.79 mmol, 1.0 equiv) was dissolved in glacial acetic acid
(40 mL). H2O2 (30%, 3.2 mL, 31.4 mmol, 17.6 equiv) was added and the
reaction mixture was stirred for 2 d at room temperature. The volatiles
were evaporated and the residue dried in vacuo. Yield: 1.11 g (99%);
1H NMR ([D6]DMSO, 300 MHz): d=2.22 (s, 12H; CH3), 2.38 (s, 6H;
CH3), 7.46 (d, 4H; arom.), 7.92 (d, 4H; arom.), 8.01 (s, 4H; arom. CH),
8.36 (d, 2H; NCHCHN), 9.88 ppm (t, 1H; CH); 13C NMR ([D6]DMSO,
75 MHz): d=17.2, 21.0, 124.3, 127.3, 127.7, 130.3, 137.1, 137.2, 137.5,
138.7, 143.4, 144.9 ppm.
0.067 mmol, 1.8 equiv). Workup procedure A was followed. Yield: 40 mg
(61%); H NMR (CDCl3, 500 MHz): d=0.88 (t, 6H; CH3), 1.25–1.37 (m,
1
36H; CH2), 1.45–1.50 (m, 4H; cod CH2), 1.64–1.76 (m, 4H; CH2), 1.77–
1.83 (m, 4H; cod CH2), 2.15 (s, 6H; CH3), 2.36 (s, 6H; CH3), 2.97 (m,
2H; cod CH), 3.97–4.01 (m, 2H; cod CH), 4.16 (t, 4H; OCH2), 6.69 (s,
2H; arom.), 6.71 (s, 2H; arom.), 7.26 ppm (s, 2H; NCHCHN); 13C NMR
(CDCl3, 125 MHz): d=13.1, 17.5, 19.1, 21.7, 25.0, 25.9, 28.0, 28.3, 28.3,
28.4, 28.6, 28.6, 28.7, 30.9, 32.6, 50.3, 67.1, 81.5, 112.5, 112.9, 122.5, 130.5,
134.9, 137.9, 157.9, 180.5 ppm.
A
(cod)
U
G
A
N,N’-Bis(2,6-dimethyl-4-tolylsulfonylphenyl)imidazolinium
chloride
(20s·H+): N,N’-Bis(2,6-dimethyl-4-tolylthiophenyl)imidazolinium chlo-
ride (1.00 mg, 1.79 mmol, 1.0 equiv) was dissolved in glacial acetic acid
(40 mL). H2O2 (30%, 3.2 mL, 31.4 mmol, 17.6 equiv) was added and the
reaction mixture was stirred for 2 d at room temperature. The volatiles
were evaporated and the residue dried in vacuo. Yield: 1.11 g (99%);
1H NMR ([D6]DMSO, 300 MHz): d=2.36 (s, 6H; CH3), 2.45 (s, 12H;
CH3), 4.50 (s, 4H; NCH2CH2N), 7.43 (d, 4H; arom.), 7.86–7.89 (m, 8H;
arom.), 9.14 ppm (s, 1H; CH); 13C NMR ([D6]DMSO, 75 MHz): d=17.5,
21.2, 50.7, 127.3, 127.6, 130.3, 137.4, 137.7, 138.3, 142.5, 144.7, 160.3 ppm.
0.075 mmol, 1.0 equiv), KOtBu (17 mg, 0.15 mmol, 2.0 equiv), and N,N’-
bis(2,4,6-trimethylphenyl)imidazolinium chloride (46 mg, 0.135 mmol,
1.8 equiv). Workup procedure A was followed. Yield: 62 mg (72%).
1H NMR (CDCl3, 300 MHz): d=1.18–1.34 (m, 4H; cod CH2), 1.59–1.62
(m, 4H; cod CH2), 2.31 (s, 6H; CH3), 2.34 (s, 6H; CH3), 2.55 (s, 6H;
CH3), 3.07 (m, 2H; cod CH), 3.90 (s, 4H; NCH2CH2N), 4.09 (m, 2H; cod
CH), 6.94 (s, 2H; arom.), 6.97 ppm (s, 2H; arom.); 13C NMR (CDCl3,
75 MHz): d=17.4, 18.8, 20.0, 27.6, 32.4, 50.4, 50.8, 82.7, 127.3, 128.8,
134.2, 135.2, 136.7, 137.0, 206.3 ppm.
A
(cod)
U
U
U
General procedure for the synthesis of [IrCl
A
G
0.075 mmol, 1.0 equiv), KOtBu (17 mg, 0.15 mmol, 2.0 equiv), and N,N’-
bis(2,4,6-trimethylphenyl)imidazolium chloride (46 mg, 0.135 mmol,
1.8 equiv). Workup procedure A was followed. Yield: 80 mg (93%);
1H NMR (CDCl3, 300 MHz): d=1.18–1.34 (m, 4H; cod CH2), 1.61–1.70
(m, 4H; cod CH2), 2.16 (s, 6H; CH3), 2.36 (s, 6H; CH3), 2.36 (s, 6H;
CH3), 2.97 (m, 2H; cod CH), 4.15 (m, 2H; cod CH), 6.95 (s, 2H;
NCHCHN), 6.98 (s, 2H; arom.), 7.04 ppm (s, 2H; arom.); 13C NMR
(CDCl3, 75 MHz): d=17.2, 18.6, 20.1, 27.9, 32.5, 50.4, 81.5, 122.3, 127.1,
128.5, 133.4, 135.1, 136.3, 137.6, 179.8 ppm.
A
ACHTREUNG
tube, dissolved in THF (5 mL) under an atmosphere of Ar and stirred for
10 min at room temperature. To this mixture was added the correspond-
ing azolium salt (1.8 equiv). The reaction mixture was stirred for 2 h at
room temperature and the solvent was evaporated in vacuo.
Workup A: The residue was dissolved in diethyl ether and purified by
column chromatography using diethyl ether as an eluent. The product
was obtained as a yellow powder.
Workup B: The residue was washed with diethyl ether (3 mL), dissolved
in CH2Cl2, and filtrated over Celite. The solvent was evaporated in vacuo
leaving a yellow powder.
A
G
(16s)]: Starting materials used were [Ir
G
E
0.075 mmol, 1.0 equiv), KOtBu (17 mg, 0.15 mmol, 2.0 equiv), and N,N’-
bis(2,6-dimethylphenyl)imidazolinium chloride (43 mg, 0.135 mmol,
1.8 equiv). Workup procedure A was followed. Yield: 75 mg (90%);
1H NMR (CDCl3, 300 MHz): d=1.18–1.33 (m, 4H; cod CH2), 1.54–1.63
(m, 4H; cod CH2), 2.40 (s, 6H; CH3), 2.59 (s, 6H; CH3), 3.05 (m, 2H;
cod CH), 3.94 (s, 4H; NCH2CH2N), 4.09 (m, 2H; cod CH), 7.23 (s, 6H;
arom.), 7.25 ppm (s, 6H; arom.); 13C NMR (CDCl3, 75 MHz): d=17.6,
19.0, 27.6, 32.4, 50.5, 50.6, 83.1, 126.7, 127.2, 128.1, 134.6, 137.5, 137.6,
206.2 ppm.
A
(cod)
U
U
U
0.075 mmol, 1.0 equiv), KOtBu (17 mg, 0.15 mmol, 2.0 equiv), and N,N’-
bis(2,6-dimethyl-4-diethylaminophenyl)imidazolinium chloride (62 mg,
0.135 mmol, 1.8 equiv). Workup procedure A was followed. Yield: 78 mg
(76%); 1H NMR (CDCl3, 300 MHz): d=1.14–1.28 (m, 4H; cod CH2),
1.14–1.19 (m, 12H; CH3), 1.61–1.66 (m, 4H; cod CH2), 2.32 (s, 6H; CH3),
2.53 (s, 6H; CH3), 3.17 (m, 2H; cod CH), 3.30–3.41 (q, 8H; NCH2), 3.87
(s, 4H; NCH2CH2N), 4.06 (m, 2H; cod CH), 6.40 (d, 2H; arom.),
6.43 ppm (d, 2H; arom.); 13C NMR (CDCl3, 75 MHz): d=11.6, 18.2, 19.4,
27.7, 32.5, 43.4, 50.0, 51.2, 81.2, 109.6, 111.0, 126.6, 135.0, 137.9, 146.1,
207.0 ppm.
G
G
E
1.8 equiv). Workup procedure A was followed. Yield: 69 mg (82%);
1H NMR (CDCl3, 500 MHz): d=1.23–1.27 (m, 2H; cod CH2), 1.32–1.35
(m, 2H; cod CH2), 1.63–1.70 (m, 4H; cod CH2), 2.21 (s, 6H; CH3), 2.40
(s, 6H; CH3), 2.95 (m, 2H; cod CH), 4.15 (m, 2H; cod CH), 7.00 (s, 2H;
NCHCHN), 7.18 (m, 6H; arom.), 7.31 ppm (s, 6H; arom.); 13C NMR
A
(cod)
U
U
U
0.075 mmol, 1.0 equiv), KOtBu (17 mg, 0.15 mmol, 2.0 equiv) and N,N’-
bis(2,6-dimethyl-4-diethylaminophenyl)imidazolium chloride (61 mg,
0.135 mmol, 1.8 equiv). Workup procedure A was followed. Yield: 70 mg
Chem. Eur. J. 2007, 13, 7195 –7203
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7201

H. Plenio et al.
(CDCl3, 125 MHz): d=17.3, 18.8, 27.9, 32.5, 50.5, 81.9, 122.2, 126.5, 127.8,
127.9, 133.8, 136.8, 137.4, 179.6 ppm.
vent was evaporated and the residue washed with pentane (10 mL) to
obtain the products as yellow powders. The conversions for all reactions
are quantitative, the lower isolated yields result from small losses (several
mg) of material during workup.
A
(cod)
U
G
N
A
G
A
1
0.135 mmol, 1.8 equiv). Workup procedure A was followed. Yield: 71 mg
(68%); 1H NMR (CDCl3, 300 MHz): d=1.18–1.43 (m, 4H; cod CH2),
1.58–1.75 (m, 4H; cod CH2), 2.35 (s, 6H; CH3), 2.57 (s, 6H; CH3), 2.98
(m, 2H; cod CH), 3.89 (s, 4H; NCH2CH2N), 4.20 (m, 2H; cod CH),
7.31 ppm (d, 4H; arom.); 13C NMR (CDCl3, 75 MHz): d=17.4, 18.9, 27.6,
used. Yield: 52 mg (95%); H NMR (300 MHz, CDCl3): d=1.18 (t, 12H;
CH3), 2.40 (s, 12H; Ar-CH3), 3.35 (q, 8H; CH2), 3.94 (s, 4H; CH2CH2),
6.38 ppm (s, 4H; ArH); 13C NMR (75 MHz, CDCl3): d=12.8, 19.3, 44.2,
52.3, 111.0, 125.8, 136.8, 147.6, 169.0, 180.9, 202.3 ppm; HRMS: m/z:
calcd for C29H40N4O2ClIr: 704.2465; found: 704.24694.
À
32.4, 50.5, 50.8, 84.4, 120.8, 129.6, 130.9, 136.6, 136.7, 139.7 ppm, Ir C
could not be observed.
A
G
T
N
1
used. Yield: 39 mg (80%); H NMR (300 MHz, CDCl3): d=1.21 (t, 12H;
CH3), 2.18 (s, 12H; Ar-CH3), 3.38 (q, 8H; CH2), 6.41 (s, 4H; ArH),
7.04 ppm (s, 2H; CHCH); 13C NMR (75 MHz, CDCl3): d=12.7, 19.1,
44.2, 110.6, 124.0, 126.0, 136.0, 148.0, 168.7, 176.8, 180.7 ppm; HRMS:
m/z: calcd for C29H38N4O2ClIr: 702.2308; found: 702.22938.
A
(cod)
U
G
N
0.135 mmol, 1.8 equiv). Workup procedure A was followed. Yield: 65 mg
(63%); 1H NMR (CDCl3, 300 MHz): d=1.18–1.46 (m, 4H; cod CH2),
1.63–1.85 (m, 4H; cod CH2), 2.18 (s, 6H; CH3), 2.37 (s, 6H; CH3), 2.89
(m, 2H; cod CH), 4.25 (m, 2H; cod CH), 6.97 (s, 2H; NCHCHN), 7.26
(s, 2H; arom.), 7.35 ppm (s, 2H; arom.); 13C NMR (CDCl3, 75 MHz): d=
17.2, 18.7, 27.9, 32.5, 50.7, 83.2, 121.7, 122.3, 129.4, 130.7,136.0, 136.4,
138.9, 180.2 ppm.
[IrCl(CO)
G
used. Yield: 36 mg (78%); 1H NMR (300 MHz, CDCl3): d=2.32 (s, 6H;
Ar-CH3), 2.43 (s, 12H; Ar-CH3), 4.00 (s, 4H; CH2CH2), 6.96 ppm (s, 4H;
ArH); 13C NMR (75 MHz, CDCl3): d=18.6, 21.1, 51.8, 129.6, 134.7,
135.9, 138.7, 168.6, 180.2, 201.8 ppm; HRMS: m/z: calcd for
C23H26N2O2ClIr: 590.1308; found: 590.12688.
A
(cod)
U
U
U
A
G
G
G
0.075 mmol, 1.0 equiv), KOtBu (17 mg, 0.15 mmol, 2.0 equiv), and N,N’-
bis(2,6-dimethyl-4-tolylsulfinylphenyl)imidazolinium chloride (79 mg,
0.135 mmol, 1.8 equiv). Workup procedure B was followed. Yield: 45 mg
(40%); 1H NMR (CDCl3, 500 MHz): d=1.08–1.44 (m, 8H; cod CH2),
2.38 (m, 12H; CH3), 2.58 (s, 6H; CH3), 2.82–2.84 (m, 2H; cod CH), 3.96
(m, 4H; NCH2CH2N), 4.01–4.06 (m, 2H; cod CH), 7.24–7.27 (m, 5H;
arom.), 7.37 (m, 1H; arom.), 7.34 (m, 1H; arom.), 7.52–7.56 ppm (s, 5H;
arom.); 13C NMR (CDCl3, 125 MHz): d=18.7, 19.8, 20.8, 27.8, 32.5, 51.7,
51.1, 85.7, 124.4, 124.9, 128.0, 128.5, 129.5, 137.1, 139.3, 141.1, 141.8,
1
used. Yield: 48 mg (87%); H NMR (300 MHz, CDCl3): d=2.21 (s, 12H;
Ar-CH3), 2.36 (s, 6H; Ar-CH3), 7.01 (s, 4H; ArH), 7.10 ppm (s, 2H;
CHCH); 13C NMR (75 MHz, CDCl3): d=18.5, 21.2, 123.6, 129.3, 134.8,
135.1, 139.5, 168.4, 176.1, 180.0 ppm; HRMS: m/z: calcd for
C23H24N2O2ClIr: 588.1151; found: 588.11485.
A
G
G
G
1
used. Yield: 36 mg (83%); H NMR (300 MHz, CDCl3): d=2.48 (s, 12H;
Ar-CH3), 4.05 (s, 4H; CH2CH2), 7.15 (d, 4H; ArH), 7.25 ppm (t, 2H;
ArH); 13C NMR (75 MHz, CDCl3): d=18.8, 51.6, 129.9, 129.1, 136.4,
137.1, 168.5, 180.0, 201.8 ppm; HRMS: m/z: calcd for C21H22N2O2ClIr:
562.0995; found: 562.09575.
À
144.6 ppm, Ir C could not be observed.
E
N
G
G
E
A
G
G
G
1
used. Yield: 20 mg (69%); H NMR (300 MHz, CDCl3): d=2.27 (s, 12H;
Ar-CH3), 7.16 (s, 4H; ArH), 7.21 (d, 4H; ArH), 7.33 ppm (t, 2H; ArH);
13C NMR (75 MHz, CDCl3): d=18.6, 123.5, 129.6, 129.7, 135.6, 136.2,
168.3, 176.0, 179.8 ppm; HRMS: m/z: calcd for C21H20N2O2ClIr:
560.0838; found: 560.08177.
0.135 mmol, 1.8 equiv). Workup procedure B was followed. Yield: 76 mg
(63%); 1H NMR (500 MHz): d=0.83–0.89 (m, 2H; cod CH2), 1.54 (m,
6H; cod-CH2), 2.19 (m, 6H; CH3), 2.36–2.39 (m, 12H; CH3), 2.69–2.89
(m, 2H; cod CH), 4.01–4.07 (m, 2H; cod CH), 7.00–7.57 ppm (m, 14H;
arom.); 13C NMR (CDCl3, 125 MHz): d=17.5, 19.0, 20.4, 27.6, 32.3, 50.6,
83.1, 121.7, 122.3, 123.2, 124.1, 129.2, 131.4, 138.5, 139.2, 140.9, 141.4,
145.3, 180.1 ppm.
A
(19s)]: Complex [IrCl
G
N
used. Yield: 37 mg (95%); 1H NMR (CDCl3, 500 MHz): d=2.39 (s, 6H;
CH3), 2.46 (s, 6H; CH3), 2.48 (s, 6H; CH3), 4.01 (s, 4H; NCH2CH2N),
7.26 (s, 2H; arom.), 7.29 (d, 4H; arom.), 7.46 (s, 2H; arom.), 7.55 ppm
(d, 4H; arom.); 13C NMR (CDCl3, 125 MHz): d=18.0, 20.4, 27.0, 50.5,
123.9, 124.0, 124.3, 127.7, 128.8, 129.2, 129.3, 138.1, 141.0, 145.0, 167.2,
178.4, 201.1 ppm; HRMS: m/z: calcd for C35H34N2O4ClIrS2: 838.1278;
found: 838.12569.
A
(cod)
U
G
N
bis(2,6-dimethyl-4-tosylphenyl)imidazolinium
0.135 mmol, 1.8 equiv). Workup procedure
chloride
was followed. Yield:
(84 mg,
B
110 mg (89%); 1H NMR (CDCl3, 500 MHz): d=1.06–1.26 (m, 8H; cod
CH2), 2.39 (s, 6H; CH3), 2.40 (s, 6H; CH3), 2.60 (s, 6H; CH3), 2.75 (m,
2H; cod CH), 3.94 (s, 4H; NCH2CH2N), 3.94–3.96 (m, 2H; cod CH),
7.30 (d, 4H; arom.), 7.68 (s, 2H; arom.), 7.72 (s, 2H; arom.), 7.81 ppm
(d, 4H; arom.); 13C NMR (CDCl3, 125 MHz): d=19.2, 20.6, 22.0, 28.6,
33.4, 51.5, 52.1, 91.3, 127.0, 128.0, 128.5, 130.3, 137.7, 139.0, 141.0, 141.6,
142.8, 144.6, 208.2 ppm.
A
2(19u)]: Complex [IrCl
G
N
used. Yield: 17 mg (93%); 1H NMR (500 MHz): d=2.25 (s, 6H; CH3),
2.27 (s, 6H; CH3), 2.40 (s, 6H; CH3), 7.14 (s, 2H; NCHCHN), 7.31 (d,
4H; arom.), 7.38 (s, 2H; arom.), 7.52 (s, 2H; arom.), 7.57 ppm (d, 4H;
arom.); 13C NMR (CDCl3, 125 MHz): d=17.9, 20.5, 27.0, 122.5, 123.5,
123.7, 124.3, 127.7, 129.2, 136.2, 138.0, 140.9, 141.1, 146.0, 167.0, 175.2,
178.2 ppm; HRMS: m/z: calcd for C35H32N2O4ClIrS2: 836.11216; found:
836.11202.
A
(cod)
U
U
U
0.075 mmol, 1.0 equiv), KOtBu (17 mg, 0.15 mmol, 2.0 equiv), and N,N’-
bis(2,6-dimethyl-4-tosylphenyl)imidazolium chloride (83 mg, 0.135 mmol,
1.8 equiv). Workup procedure B was followed. Yield: 115 mg (93%);
1H NMR (CDCl3, 500 MHz): d=1.06–1.11 (m, 2H; cod CH2), 1.16–1.30
(m, 4H; cod CH2), 1.32–1.40 (m, 2H; cod CH2), 2.20 (s, 6H; CH3), 2.40
(s, 12H; CH3), 2.67 (m, 2H; cod CH), 4.01 (m, 2H; cod CH), 7.01 (s,
2H; NCHCHN), 7.32 (d, 4H; arom.), 7.76 (s, 4H; arom.), 7.83 ppm (d,
4H; arom.); 13C NMR (CDCl3, 125 MHz): d=17.5, 19.0, 20.6, 27.5, 32.1,
50.6, 83.7, 122.1, 125.4, 126.7, 126.9, 129.0, 135.6, 137.4, 138.8, 140.9,
141.2, 143.4, 180.2 ppm.
A
2(20s)]: Complex [IrCl
G
N
used. Yield: 22 mg (96%); 1H NMR (CDCl3, 500 MHz): d=2.43 (s, 6H;
CH3), 2.48 (s, 12H; CH3), 4.00 (s, 4H; NCH2CH2N), 7.25 (s, 2H; arom.),
7.34 (d, 4H; arom.), 7.72 (s, 2H; arom.), 7.81 ppm (d, 4H; arom.);
13C NMR (CDCl3, 125 MHz): d=19.0, 21.6, 28.0, 51.4, 127.7, 127.9, 128.0,
128.7, 130.1, 138.0, 138.2, 140.9, 142.2, 144.6, 168.2, 179.3, 201.9 ppm;
HRMS: m/z: calcd for C35H34N2O6ClIrS2: 870.1176; found: 870.11335.
A
G
G
G
1
General procedure for the synthesis of [IrCl(CO)2
The corresponding [IrCl(cod)(NHC)] complex was dissolved in CH2Cl2
(10 mL) and CO was bubbled through this solution for 15 min. The sol-
A
used. Yield: 51 mg (98%); H NMR (CDCl3, 500 MHz): d=2.27 (s, 12H;
CH3), 2.44 (s, 6H; CH3), 7.14 (s, 2H; NCHCHN), 7.36 (d, 4H; arom.),
7.75 (s, 4H; arom.), 7.87 ppm (d, 4H; arom.); 13C NMR (CDCl3,
A
ACHTREUNG
7202
www.chemeurj.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 7195 –7203

N-Heterocyclic Carbenes
FULL PAPER
125 MHz): d=17.2, 17.8, 20.6, 122.6, 126.7, 127.0, 129.1, 136.4, 136.5,
136.6, 136.9, 139.7, 142.1, 143.7, 167.0, 175.0, 178.0 ppm; HRMS: m/z:
calcd for C35H32N2O6ClIrS2: 868.10199; found: 868.10494.
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Received: February 7, 2007
Published online: June 25, 2007
Chem. Eur. J. 2007, 13, 7195 –7203
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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