Efficient synthesis of bulky N-Heterocyclic carbene ligands for coinage metal complexes

Article abstract of DOI:10.1016/j.jorganchem.2016.07.023

A facile and efficient route to synthesize sterically bulky N-heterocyclic carbenes with high percent buried volumes has been developed. The method involves three steps from commercial reagents and produced the NHC precursors with approximately 40% overall yield without column chromatography. The free carbenes were isolated as confirmed by X-ray crystallography and¹³C NMR peak at 213?ppm corresponding to the carbene carbons. The Cu(I), Ag(I), and Au(I) chloride complexes supported by the NHC ligands were successfully synthesized and fully characterized including X-ray crystallography.

Full text of DOI:10.1016/j.jorganchem.2016.07.023

Journal of Organometallic Chemistry 820 (2016) 1e7
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Efficient synthesis of bulky N-Heterocyclic carbene ligands for coinage
metal complexes
,
,
, b, c, *
Youngsuk Kim a b, Yonghwi Kim a b, Moon Young Hur a, Eunsung Lee a
a Center for Selfeassembly and Complexity, Institute for Basic Science (IBS), Pohang, 37673, Republic of Korea
b Department of Chemistry, Pohang University of Science and Technology, Pohang, 37673, Republic of Korea
c Division of Advanced Materials Science, Pohang University of Science and Technology, Pohang, 37673, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A facile and efficient route to synthesize sterically bulky N-heterocyclic carbenes with high percent
buried volumes has been developed. The method involves three steps from commercial reagents and
produced the NHC precursors with approximately 40% overall yield without column chromatography.
The free carbenes were isolated as confirmed by X-ray crystallography and 13C NMR peak at 213 ppm
corresponding to the carbene carbons. The Cu(I), Ag(I), and Au(I) chloride complexes supported by the
NHC ligands were successfully synthesized and fully characterized including X-ray crystallography.
© 2016 Elsevier B.V. All rights reserved.
Received 3 April 2016
Received in revised form
17 June 2016
Accepted 29 July 2016
Available online 31 July 2016
Keywords:
N-heterocyclic carbene
Coinage metal complex
Ligand design
Percent buried volume
1. Introduction
synthesis contained three or four steps with 49% and 11% of overall
yields for IPr* and IPr*(2ꢀNp) precursors, respectively [17,18]. The
N-heterocyclic carbenes (NHCs) have been drawing great
attention in transition-metal catalysis due to their great perfor-
mance [1e6]. Since the first isolation of NHC 1,3-bis-(1-adamantyl)
imidazol-2-ylidene (IAd) by Arduengo in 1991 [7], a variety of NHCs
have been synthesized for desired reactivity by adjusting two major
properties: electronic and steric factors [8]. Typically, electronic
properties of NHCs can be easily tuned by introducing electron
donating or withdrawing functional groups to NHCs [9e11] or by
replacing nitrogen atoms of imidazole ring with carbon [12,13] or
other heteroatoms [14]. Steric bulkiness of NHCs can be introduced
by adopting large auxiliary molecules to the N-substituents in NHCs
[15,16]. Thus, with optimal electronic and steric properties, the
NHCs perform greatly as an auxiliary ligand in catalysis.
free carbene IPr* was isolated by Nolan et al. [19] in 2011 and
applied successfully to various catalytic reactions [20e22], while
applications of more sterically bulky IPr*(2ꢀNp) have not been re-
ported yet. Meanwhile, inspired by Buchwald ligands (dialkyl
biphenylphosphine ligands), Lassaletta et al. synthesized bulky
NHCs with a heterobiaryl skeleton and chiral N-substituents for
applications in asymmetric catalysis [23]. The synthesis of the
imidazolium precursor contained five steps with 29% overall yield.
Recently, they also reported the synthesis of axially chiral imida-
zoisoquinolinium salts and their corresponding silver or gold metal
complexes [24].
Although the synthesis of such bulky NHCs has been successfully
demonstrated, it would desirable to develop a more practical and
(2ꢀNp)
shows
ꢀ
ꢀ
For the past five years, Marko and Lassaletta have explored
efficient synthetic route. The synthesis of Marko’s IPr*
ꢀ
highly sterically demanding NHCs (Scheme 1). Marko et al. suc-
low overall yield, while Lassaletta's ligands require multiple reaction
steps with column chromatography. We designed bulky NHC li-
gands (1a and 1b) with an imidazo[1,5-a]pyridin-3-ylidene struc-
ture, since the efficient synthesis of NHCs with the structure are
well-studied [25e28], and several modified NHCs were reported
and adapted for catalysis [29e31]. Regarding the importance of such
NHCs, here we report a facile synthetic route to access highly bulky
NHC ligands 1a and 1b, and their coinage metal complexes.
cessfully demonstrated the synthesis of the imidazolium pre-
cursors of exceedingly bulky IPr* and IPr*(2ꢀNp) by replacing methyl
of isopropyl group in IPr (1,3-bis-(2,6-diisopropylphenyl)imidazol-
2-ylidene) with phenyl and naphthyl groups, respectively. The
* Corresponding author. Center for Selfeassembly and Complexity, Institute for
Basic Science (IBS), Pohang, 37673, Republic of Korea.
The imidazolium salts 4a and 4b, with a similar structural motif
E-mail address: eslee@postech.ac.kr (E. Lee).
http://dx.doi.org/10.1016/j.jorganchem.2016.07.023
0022-328X/© 2016 Elsevier B.V. All rights reserved.

2
Y. Kim et al. / Journal of Organometallic Chemistry 820 (2016) 1e7
Scheme 2. Synthesis of bulky NHC ligands and their coinage metal complexes. (a)
triphenylphosphine (0.15 eq), potassium phosphate (2.5 eq), tris(dibenzylideneace-
tone)dipalladium(0) (0.02 eq), toluene (0.2 M), 100 ꢁC, 3 days, 72%. (b) Sulfuric acid (1
eq), methanol (0.1 M), 25 ꢁC, 6 h, 58e59%. (c) Paraformaldehyde (1 eq), trimethylsilyl
chloride (2 eq), toluene (0.1 M), 25 ꢁC, 6 h, 91e96%. (d) potassium tert-butoxide (1.5
Scheme 1. Bulky NHC ligands.
to that of Lassaletta's NHC D, were synthesized in three steps from
commercial reagents with 40% and 38% of overall yields, respec-
tively (Scheme 2). Moreover, the developed method does not
require harsh reaction conditions or any chromatographic purifi-
cation, which enables the preparation of bulky NHCs more acces-
sible and practical from a synthetic point of view.
eq), tetrahydrofuran (0.1 M), 25 ꢁC,
5 h, 91e93%. (e) Silver(I) oxide (0.5 eq),
dichloromethane (0.05 M), 25 ꢁC, one day, 78e84%. (f) Copper(I) chloride (1 eq),
tetrahydrofuran (0.05 M), 25 ꢁC, one day, 86e96%. (g) Chloro(dimethylsulfide)gold(I) (1
eq), tetrahydrofuran (0.05 M), 25 ꢁC, one day, 73e86%.
steps are identical to the well-known procedure for synthesizing IPr
(1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) [36].
The 13C NMR spectra of the free carbenes 1a and 1b showed
singlet peaks corresponding to C1 carbons at 213.5 and 212.9 ppm,
respectively, which confirms the carbene character (see Supporting
Information). The suitable single crystal of 1b for X-ray crystallog-
raphy was grown by slow diffusion of toluene into saturated THF
solution of 1b. As described in Fig.1 and Table S29, the bond lengths
and bond angles of 1b are in good agreement with theoretical
values obtained by density functional theory (DFT) calculations (see
Supporting Information).
Overall, the streamlined synthesis of the ligand 1 was efficiently
performed. In particular, the mild reaction conditions and simple
purification processes enable the method to be highly suitable for a
large scale synthesis of the bulky NHC ligands.
2. Results and discussion
2.1. Synthesis of ligands
The imidazolium chloride salts (4a and 4b) were synthesized
from the commercial resources in three steps: cross-coupling,
imine condensation, and cyclization reactions (Scheme 2).
Pyridinecarboxaldehyde 2 was synthesized via SuzukieMiyaura
cross-coupling reaction from commercially available 6-bromo-2-
formylpyridine and 2,4,6-triisopropylphenylboronic acid. In gen-
eral, the cross-coupling of sterically hindered substrates such as
2,4,6-triisopropylphenylboronic acid is challenging and requires
special ligands such as Buchwald's ligands [32e34]. The previous
synthesis of aldehyde 2 reported by Groysman et al. [35] employed
expensive
SPhos
(2-dicyclohexylphosphino-20,60-dimethox-
2.2. Synthesis and characterization of coinage metal complexes
ybiphenyl) ligand, required a large amount of Pd(OAc)2 (20 mol%),
and suffered from low yield (30%). With the newly developed con-
dition utilizing low-cost triphenylphosphine, aldehyde 2 was suc-
cessfully synthesized in good yield (72%) without column
chromatography. An acid-catalysed imine condensation with aniline
afforded corresponding imine 3a and 3b in moderate yields (59%
and 58%, respectively) in methanol. The following cyclization of 3a
and 3b using paraformaldehyde furnished the targeted imidazolium
salts 4a and 4b in high yields (96% and 91%, respectively). Finally,
bulky NHC ligands 1a and 1b were obtained via deprotonation of the
imidazolium proton with potassium tert-butoxide. The last two
After the successful synthesis of the bulky ligand 1, their coinage
metal complexes [37e39] were successively synthesized. Copper(I)
and gold(I) complexes of 1 were obtained by reaction of 1 with
corresponding metal sources. Silver(I) complex was also synthe-
sized by addition of silver(I) oxide to the imidazolium chloride 4. All
the complexes were fully characterized by NMR and single crystal
X-ray crystallography (Fig. 2).
As described in Fig. 2, the metal centres show a linear geometry
with the coordination number of two. However, the
ligandemetalechloride bond angles (C1eMeCl) are slightly

Y. Kim et al. / Journal of Organometallic Chemistry 820 (2016) 1e7
3
under an ambient atmosphere, magnetically stirred. All air- and
moisture-sensitive manipulations were performed using oven-
dried glassware, including standard Schlenk and glovebox tech-
niques under an atmosphere of nitrogen. For drying of the solvents,
dichloromethane was purged with nitrogen, dried by passage
through activated alumina, and stored over 3 Å molecular sieves.
Benzene, benzene-d6, toluene, pentane and THF were distilled from
deep purple sodium benzophenone ketyl. Chloroform-d1 and all
other chemicals were purchased from commercial sources and
used as received. NMR spectra were recorded on a Bruker DRX 500
spectrometer operating at 500 MHz and 125 MHz for 1H and 13C
acquisitions, respectively. Chemical shifts were referenced to the
residual proton solvent peaks (1H: CDCl3,
vent 13C signals (CDCl3,
77.16; C6D6, 128.06).
d 7.26; C6D6, d 7.16), sol-
d
d
4.2. Synthesis of ligand precursors
4.2.1. 6-(2,4,6-Triisopropylphenyl)-2-pyridinecarboxaldehyde (2)
In a glove box, 6-bromo-2-pyridinecarboxaldehyde (500 mg,
2.69 mmol), 2,4,6-triisopropylphenylboronic acid (767 mg,
3.09 mmol), triphenylphosphine (106 mg, 0.403 mmol), potassium
phosphate (K3PO4, 1.43 g, excess base), and tris(dibenzylideneace-
tone)dipalladium(0) (49.2 mg, 0.054 mmol) were added to a 20 mL
vial. Dry toluene (15 mL) was added to the vial and the reaction
mixture was heated to 100 ꢁC for 3 days to produce a brown
mixture. The reaction mixture was then cooled to room tempera-
ture, filtered through a thin pad of Celite, and subsequently eluted
with diethyl ether (3 ꢂ 10 mL) to extract the remaining product in
the Celite pad. The filtrate (brown solution) was dried in vacuo to
afford a brown solid. The solid was then washed with pentane
(2 ꢂ 10 mL) and dried in vacuo to yield the desired product as an
Fig. 1. Molecular structure of 1b from X-ray crystallography. The thermal ellipsoids are
set at a 30% probability level. Selected bond lengths (Å) and bond angles (ꢁ): C1eN1
1.394(7); C1eN2 1.336(5); C2eN1 1.391(4); C3eN2 1.399(6); C2eC3 1.372(8);
N1eC1eN2 100.6(3); C1eN1eC2 113.4(3); C1eN2eC3 115.7(4).
deviated from 180ꢁ because of the high steric hindrance of the rigid
2,4,6-triisopropylphenyl moiety (Table 1). The C1eMeCl angles of
1aeMCl complexes (M ¼ Cu, Ag or Au) are slightly larger than those
of 1beMCl complexes, indicating that 2,6-diisopropylphenyl group
of 1a is bulkier than mesityl group of 1b. Other bond lengths and
bond angles are almost identical in 1aeMCl complexes and
1beMCl complexes.
ivory solid (595 mg, 72%). 1H NMR (CDCl3, 500 MHz):
d
¼ 10.10 (d,
J ¼ 0.8 Hz, 1 H), 7.97e7.90 (m, 2H), 7.52 (dd, J ¼ 7.4, 1.3 Hz, 1 H), 7.10
(s, 2 H), 2.95 (m, 1 H), 2.44 (m, 2 H), 1.29 (d, J ¼ 7.0 Hz, 6 H), 1.13 (d,
J ¼ 6.9 Hz, 6 H), 1.09 (d, J ¼ 6.9 Hz, 6 H). 13C NMR (CDCl3, 125 MHz):
The bulkiness of the NHC ligand can be measured numerically
by the concept of percent buried volume (%VBur) introduced by
Clavier and Nolan [40], which is defined as the ratio of the volume
occupied by the ligand in the coordination sphere. The bulkiness (%
VBur) values calculated from the synthesized complexes and other
well-known bulky NHC complexes are shown in Table 2. The data
clearly show that the 1aeMCl complexes have larger %VBur than
that of the 1beMCl complexes. In addition, the %VBur data of the
d
194.3 (CAldehyde), 161.2, 152.7, 149.6, 146.4, 136.9, 135.3, 129.6,
121.2, 119.5 (9 ꢂ CAromatic), 34.6, 30.6 (2 ꢂ CBenzyl), 24.3, 24.2, 24.1
(3 ꢂ CAlkyl) ppm. HRMS (EI) m/z calcd for C21H27NO 309.2093, found
309.2091.
4.2.2. (2,6-Diisopropylphenyl)[6-(2,4,6-triisopropylphenyl)pyridin-
2-ylmethylene]amine (3a)
Pyridinecarboxaldehyde (2) (100 mg, 0.323 mmol), 2,6-
diisopropylaniline (0.609 mL, 3.23 mmol), sulfuric acid (0.017 mL,
0.32 mmol), and methanol (3 mL) were added to a 4 mL vial. The
reaction mixture was stirred at room temperature and subse-
quently afforded a yellow solid. After 6 h, the yellow solid was
collected on a frit and subsequently washed with methanol
(2 ꢂ 1 mL) and acetone (1 ꢂ 1 mL) to afford 3a as a desired product
(2ꢀNp)
ꢀ
present complexes are comparable to that of Marko’s IPr*
Lassaletta's ligand D.
or
3. Conclusions
In summary, the bulky NHC ligands 1a and 1b were synthesized
in an efficient way without any chromatographic purification. The
isolated free carbenes 1a and 1b were identified by single crystal X-
ray crystallography and 13C NMR, as the data showed clear peaks at
213.5 and 212.9 ppm from the carbene carbon, respectively. Their
Cu(I), Ag(I), and Au(I) chloride complexes were also synthesized
and fully characterized including single crystal X-ray crystallog-
raphy. Percent buried volume data for the complexes show the high
steric effect of the ligand. The newly developed synthetic method
for generating sterically bulky NHCs will facilitate further diversi-
fication of NHCs applicable to various transformations.
(89.1 mg, 59%). 1H NMR (C6D6, 500 MHz):
d
¼ 8.63 (s, 1 H), 8.30 (dd,
J ¼ 7.8, 1.0 Hz, 1 H), 7.23 (t, J ¼ 7.7 Hz, 1 H), 7.22 (s, 2 H), 7.16e7.11 (m,
3H), 7.08 (dd, J ¼ 7.6, 1.1 Hz, 1 H), 3.20 (m, 2 H), 2.88 (m, 1 H), 2.73
(m, 2 H), 1.29 (d, J ¼ 6.9 Hz, 6 H), 1.19e1.16 (m, 24 H) ppm. 13C NMR
(C6D6, 125 MHz):
d 164.2, 160.9, 154.8, 149.4, 146.9, 137.6, 136.8,
136.4, 126.8, 125.0, 123.6, 121.0, 119.0 (13 ꢂ Csp2; some signals were
accidently degenerated or overlapped with solvent signals), 35.0,
31.1, 28.5 (3 ꢂ CBenzyl), 24.6, 24.4, 24.1, 23.7 (4 ꢂ CAlkyl) ppm. HRMS
(EI) m/z calcd for C33H44N2 468.3504, found 468.3502.
4.2.3. (2,4,6-trimethylphenyl)[6-(2,4,6-triisopropylphenyl)pyridin-
2-ylmethylene]amine (3b)
4. Experimental section
Pyridinecarboxaldehyde (2) (1 g, 3.23 mmol), 2,4,6-
trimethylaniline (4.54 mL, 32.3 mmol), sulfuric acid (0.172 mL,
3.23 mmol), and methanol (30 mL) were added to a 40 mL vial. The
reaction mixture was stirred at room temperature and a yellow
4.1. General considerations
All air- and moisture-insensitive reactions were carried out

4
Y. Kim et al. / Journal of Organometallic Chemistry 820 (2016) 1e7
Fig. 2. Molecular structure of metal complexes from X-ray crystallography. The thermal ellipsoids are set at a 50% probability level.
solid was precipitated immediately. After 6 h, the yellow solid was
collected on a frit and subsequently washed with methanol
(2 ꢂ 5 mL) and acetone (1 ꢂ 5 mL) to afford 3b as a desired product
160.7, 155.1, 149.4, 148.7, 146.9, 137.0, 136.3, 133.5, 129.4, 127.2,
126.6, 121.0, 118.9 (14 ꢂ CAromatic), 35.0, 31.0 (2 ꢂ ArCH(CH3)2), 24.6,
24.4, 24.2 (3 ꢂ ArCH(CH3)2), 20.9, 18.5 (2 ꢂ ArCH3) ppm. HRMS (EI)
m/z calcd for C30H38N2 426.3035, found 426.3034.
(802 mg, 58%). 1H NMR (C6D6, 500 MHz):
d 8.59 (s, 1 H), 8.32 (dd,
J ¼ 7.8, 1.0 Hz, 1 H), 7.22 (td, J ¼ 7.6, 0.5 Hz, 1 H), 7.23 (s, 2 H), 7.08
(dd, J ¼ 7.5,1.1 Hz,1 H), 6.79 (s, 2 H), 2.87 (m,1 H), 2.75 (m, 2 H), 2.17
(s, 6 H), 2.16 (s, 3 H), 1.30 (d, J ¼ 7.0 Hz, 6 H), 1.19 (d, J ¼ 6.8 Hz, 6 H),
4.2.4. 2-(2,6-Diisopropylphenyl)-5-(2,4,6-triisopropylphenyl)-2H-
imidazo[1,5-a]pyridin-4-ium chloride (4a)
1.18 (d, J ¼ 6.8 Hz, 6 H) ppm. 13C NMR (C6D6, 125 MHz):
d
164.2,
Iminopyridine (3a) (60 mg, 0.13 mmol), paraformaldehyde

Y. Kim et al. / Journal of Organometallic Chemistry 820 (2016) 1e7
5
Table 1
4.3. Synthesis of free carbenes
Selected bond lengths (Å) and bond angles (ꢁ) of the synthesized complexes.
4.3.1. 2-(2,6-Diisopropylphenyl)-5-(2,4,6-triisopropylphenyl)-2H-
1aeCuCl 1beCuCl 1aeAgCl 1beAgCl 1aeAuCl 1beAuCl
imidazo[1,5-a]pyridin-3-ylidene (1a)
C1eM
MeCl
C1eMeCl
N1eC1eM
N2eC1eM
1.870
2.108
175.18
129.55
128.79
1.872
2.109
172.50
130.77
126.40
2.104
2.322
177.14
132.02
122.90
2.057
2.318
172.07
131.04
125.68
1.982
2.273
179.28
129.34
126.35
1.981
2.276
176.88
131.56
124.82
In a glove box, 4a (400 mg, 0.773 mmol), potassium tert-but-
oxide (130 mg, 1.16 mmol) and dry THF (8 mL) were added to a
20 mL vial. The reaction mixture was stirred at room temperature
for 5 h. The mixture was then filtered through dry Celite and sub-
sequently eluted with THF (3 ꢂ 2 mL). The filtrate was dried in
vacuo to afford 1a as a white solid (337 mg, 91%). 1H NMR (C6D6,
Table 2
%VBur of some NHC ligands.
500 MHz):
d
7.29 (d, J ¼ 7.8 Hz, 1 H), 7.26 (s, 2 H), 7.13 (d, J ¼ 7.8 Hz,
2 H), 6.92 (dd, J ¼ 9.2, 1.0 Hz, 1 H), 6.88 (s, 1 H), 6.41 (dd, J ¼ 9.2,
6.4 Hz, 1 H), 6.23 (dd, J ¼ 6.4, 1.1 Hz, 1 H), 2.92e2.87 (m, 3 H), 2.60
(m, 2 H), 1.30e1.27 (m, 18 H), 1.09e1.06 (m, 12 H) ppm. 13C NMR
a
Ligand (L)
Ref
%VBur
LeCuCl
LeAgCl
LeAuCl
(C6D6, 125 MHz):
d 213.5 (CCarbene), 149.6, 147.7, 145.9, 142.1, 139.1,
IPrb
[41e43]
47.6
34.8
50.1
57.1
ed
46.9
36.2
55.1[47]
57.4
53.1
57.0
44.5
36.5
50.4
ed
IMesc
[44e46]
[17,19]
[18]
[23]
e
132.3, 131.5, 129.0, 123.6, 121.9, 120.9, 116.4, 112.9, 112.4
(14 ꢂ CAromatic), 34.9, 31.8, 28.3 (3 ꢂ CBenzyl), 25.2, 24.5, 24.4, 24.3
(4 ꢂ CAlkyl) ppm. Anal. Calcd for C34H44N2: C, 84.95; H, 9.23; N, 5.83.
Found: C, 84.86; H, 9.08; N, 5.71.
B (IPr*)
C (IPr*(2ꢀNp)
)
D
1a
1b
51.1
54.1
51.9
55.5
53.4
e
52.9
4.3.2. 2-(2,4,6-trimethylphenyl)-5-(2,4,6-triisopropylphenyl)-2H-
imidazo[1,5-a]pyridin-3-ylidene (1b)
a
Calculated via SambVca [48,49] with following parameters: Radius of
sphere ¼ 3.5 Å; Distance from sphere 2.0 Å; Mesh step 0.05 Å; H atoms omitted;
Bondi radii scaled by 1.17.
In a glove box, 4b (400 mg, 0.842 mmol), potassium tert-but-
oxide (142 mg, 1.26 mmol) and dry THF (8 mL) were added to a
20 mL vial. The reaction mixture was stirred at room temperature
for 5 h. The mixture was then filtered through dry Celite and sub-
sequently eluted with THF (3 ꢂ 2 mL). The filtrate was dried in
vacuo to afford 1b as a white solid (344 mg, 93%). Crystals suitable
for X-ray crystallography were obtained by slow diffusion of
toluene into saturated THF solution. 1H NMR (C6D6, 500 MHz):
b
IPr ¼ 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene.
c
IMes ¼ 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene.
d
X-ray crystal structure has not been reported.
(3.8 mg, 0.13 mmol), trimethylsilyl chloride (32 mL, 0.26 mmol) and
toluene (1 mL) were added to a 4 mL vial. The mixture was stirred at
room temperature for 6 h. The resulting white solid was then
collected on a frit and washed with ether (1 ꢂ 1 mL) and pentane
(3 ꢂ 1 mL). The solid was further dried in vacuo at 60 ꢁC for over-
night to yield 4a as a desired product (63.5 mg, 96%). 1H NMR
d
7.25 (s, 2 H), 6.93 (dd, J ¼ 9.2, 1.1 Hz,1 H), 6.75 (s, 2 H), 6.67 (s, 1 H),
6.43 (dd, J ¼ 9.2, 6.4 Hz,1 H), 6.22 (dd, J ¼ 6.4,1.2 Hz,1 H), 2.94e2.86
(m, 3 H), 2.14 (s, 3 H),1.95 (s, 6 H),1.29e1.26 (m,18 H) ppm. 13C NMR
(C6D6, 125 MHz):
135.1, 132.4, 131.8, 128.9, 121.6, 121.0, 116.4, 112.5, 110.9
(14 Aromatic), 34.9, 31.8 (2 ArCH(CH3)2), 24.9, 24.4
(2 ꢂ ArCH(CH3)2), 21.0, 17.7 (2 ꢂ ArCH3) ppm. Anal. Calcd for
31H38N2: C, 84.88; H, 8.73; N, 6.39. Found: C, 85.00; H, 8.60; N,
d 212.9 (CCarbene), 149.6, 147.7, 142.3, 1395, 137.4,
(CDCl3, 500 MHz):
d
9.37 (s, 1 H), 9.03 (d, J ¼ 9.2 Hz, 1 H), 7.94 (s,
1 H), 7.53 (t, J ¼ 7.8 Hz, 2 H), 7.29 (d, J ¼ 7.8 Hz, 2 H), 7.17 (s, 2 H), 7.07
(d, J ¼ 6.7 Hz, 1 H), 2.95 (m, 1 H), 2.30 (m, 2 H), 2.04 (m, 2 H), 1.28 (d,
J ¼ 6.9 Hz, 6 H), 1.23 (d, J ¼ 6.6 Hz, 6 H), 1.16 (d, J ¼ 6.7 Hz, 6 H),
ꢂ
C
ꢂ
C
6.30.
1.06e1.03 (m, 12 H) ppm. 13C NMR (CDCl3, 125 MHz):
d 153.2, 148.2,
145.2,132.7,132.7,132.3,130.7,125.8,124.8,124.4,122.7,121.7,121.6,
121.5, 121.1 (15 ꢂ CAromatic), 34.6, 31.5, 29.0 (3 ꢂ CBenzyl), 25.2, 24.8,
24.4, 24.0, 23.9 (5 ꢂ CAlkyl) ppm. HRMS (FAB) m/z calcd for C34H45N2
(M ꢀ Cl) 481.3583, found 481.3581.
4.4. Synthesis of metal complexes
4.4.1. 1aeCuCl
In a glove box, 1a (50 mg, 0.10 mmol), copper(I) chloride (10 mg,
0.10 mmol) and dry THF (2 mL) were added to a 4 mL vial. The
reaction mixture was stirred at room temperature for one day. The
mixture was then filtered through dry Celite and subsequently
eluted with THF (1 ꢂ 0.5 mL). The filtrate was dried in vacuo to
afford 1aeCuCl as a white solid (58 mg, 96%). Crystals suitable for X-
ray crystallography were obtained by slow diffusion of pentane into
4.2.5. 2-(2,4,6-trimethylphenyl)-5-(2,4,6-triisopropylphenyl)-2H-
imidazo[1,5-a]pyridin-4-ium chloride (4b)
Iminopyridine (3b) (800 mg, 1.88 mmol), paraformaldehyde
(56.3 mg, 1.88 mmol), trimethylsilyl chloride (0.476 mL, 3.75 mmol)
and toluene (20 mL) were added to a 40 mL vial. The mixture was
stirred at room temperature for 6 h until the solution became
transparent. The solution was then dried in vacuo to afford a white
solid. The solid was washed with diethyl ether (1 ꢂ 10 mL) and
pentane (3 ꢂ 10 mL) and subsequently dried in vacuo for overnight
to afford 4b as a desired product (811 mg, 91%). 1H NMR (CDCl3,
chloroform solution. 1H NMR (CDCl3, 500 MHz):
d
7.47 (dd, J ¼ 9.3,
1.1 Hz, 1 H), 7.43 (t, J ¼ 7.8 Hz, 1 H), 7.30 (s, 1 H), 7.21 (d, J ¼ 7.8 Hz,
2 H), 7.17 (s, 2 H), 7.08 (dd, J ¼ 9.2, 6.6 Hz, 1 H), 6.59 (dd, J ¼ 6.6,
1.1 Hz, 1 H), 2.97 (m, 1 H), 2.43 (m, 2 H), 2.15 (m, 2 H), 1.34 (d,
J ¼ 7.0 Hz, 6 H), 1.23 (d, J ¼ 7.0 Hz, 6 H), 1.23e1.11 (m, 18 H) ppm. 13
C
500 MHz):
d
9.33 (d, J ¼ 1.6 Hz, 1 H), 8.95 (d, J ¼ 9.4 Hz, 1 H), 7.87 (d,
NMR (CDCl3, 125 MHz):
d 152.5, 146.6, 145.2, 138.9, 135.6, 131.3,
J ¼ 1.5 Hz, 1 H), 7.46 (dd, J ¼ 9.3, 6.9 Hz, 1 H), 7.15 (s, 2 H), 7.03 (d,
J ¼ 6.4 Hz, 1 H), 6.97 (s, 2 H), 2.93 (m, 1 H), 2.30 (s, 3 H), 2.28 (m,
2 H),1.91 (s, 6 H),1.26 (d, J ¼ 6.9 Hz, 6 H),1.13 (d, J ¼ 6.8 Hz, 6 H),1.01
130.6, 128.1, 124.1, 123.6, 122.8, 116.6, 116.1, 113.7 (14 ꢂ CAromatic),
34.8, 31.7, 28.5 (3 ꢂ CBenzyl), 25.3, 24.7, 24.5, 24.3, 24.2 (5 ꢂ CAlkyl
)
(d, J ¼ 6.9 Hz, 6 H) ppm. 13C NMR (CDCl3, 125 MHz):
d 153.2, 148.0,
ppm. (Signal from the carbon adjacent to metal were not detected.)
Anal. Calcd for C34H44ClCuN2: C, 70.44; H, 7.65; N, 4.83. Found: C,
70.80; H, 7.38; N, 4.82.
141.9,134.0,132.8,132.7,131.2,129.8,125.4,124.4,122.7,121.4,121.0,
119.8 (14 ꢂ CAromatic, some signals were accidently degenerated),
34.6, 31.3 (2 ꢂ ArCH(CH3)2), 24.9, 24.1, 23.9 (3 ꢂ ArCH(CH3)2), 21.2,
17.2 (2 ꢂ ArCH3) ppm. HRMS (FAB) m/z calcd for C31H39N2 (M ꢀ Cl)
439.3113, found 439.3117.
4.4.2. 1beCuCl
In a glove box, 1b (50 mg, 0.11 mmol), copper(I) chloride (11 mg,

6
Y. Kim et al. / Journal of Organometallic Chemistry 820 (2016) 1e7
0.11 mmol) and dry THF (2 mL) were added to a 4 mL vial. The
reaction mixture was stirred at room temperature for one day. The
mixture was then filtered through dry Celite and subsequently
eluted with THF (1 ꢂ 0.5 mL). The filtrate was dried in vacuo to
afford 1beCuCl as a yellow solid (53 mg, 86%). Crystals suitable for
X-ray crystallography were obtained by slow diffusion of pentane
to afford 1aeAuCl as a white solid (64 mg, 86%). Crystals suitable for
X-ray crystallography were obtained by slow diffusion of pentane
into dichloromethane solution. 1H NMR (CDCl3, 500 MHz):
d 7.48
(dd, J ¼ 9.3, 1.1 Hz, 1 H), 7.44 (t, J ¼ 7.8 Hz, 1 H), 7.31 (s, 1 H), 7.20 (d,
J ¼ 7.8 Hz, 2 H), 7.15 (s, 2 H), 7.08 (dd, J ¼ 9.3, 6.6 Hz, 1 H), 6.61 (dd,
J ¼ 6.6, 1.2 Hz, 1 H), 2.96 (m, 1 H), 2.41 (m, 2 H), 2.13 (m, 2 H), 1.35 (d,
J ¼ 6.9 Hz, 6 H), 1.27 (d, J ¼ 6.9 Hz, 6 H), 1.20 (d, J ¼ 6.8 Hz, 6 H), 1.14
(d, J ¼ 6.8 Hz, 6 H), 1.11 (d, J ¼ 6.8 Hz, 6 H) ppm. 13C NMR (CDCl3,
into dichloromethane solution. 1H NMR (CDCl3, 500 MHz):
d 7.45
(dd, J ¼ 9.2, 1.0 Hz, 1 H), 7.26 (s, 1 H), 7.17 (s, 2 H), 7.04 (dd, J ¼ 9.2,
6.6 Hz, 1 H), 6.92 (s, 2 H), 6.56 (dd, J ¼ 6.5, 1.1 Hz, 1 H), 2.98 (m, 1 H),
2.43 (m, 2 H), 2.30 (s, 3 H), 1.92 (s, 6 H), 1.34 (d, J ¼ 6.9 Hz, 6 H), 1.23
(d, J ¼ 6.9 Hz, 6 H), 1.15 (d, J ¼ 6.8 Hz, 6 H) ppm. 13C NMR (CDCl3,
125 MHz):
d 166.5 (CCarbene), 152.3, 147.0, 145.2, 138.5, 135.7, 131.3,
130.7, 128.9, 124.2, 123.5, 122.2, 117.2, 116.8, 113.5 (14 ꢂ CAromatic),
34.8, 31.9, 28.6 (3 ꢂ CBenzyl), 25.3, 24.5, 24.5, 24.4, 23.8 (5 ꢂ CAlkyl
)
125 MHz):
d
152.3, 146.6, 139.5, 138.9, 136.3, 134.2, 131.6, 129.5,
ppm. Anal. Calcd for C34H44AuClN2: C, 57.26; H, 6.22; N, 3.93.
Found: C, 56.87; H, 6.13; N, 3.87.
128.2, 123.3, 122.8, 116.6, 115.8, 112.5 (14 ꢂ CAromatic), 34.8, 31.6
(2 ꢂ ArCH(CH3)2), 25.2, 24.3, 24.2 (3 ꢂ ArCH(CH3)2), 21.2, 17.6
(2 ꢂ ArCH3) ppm. (Signal from the carbon adjacent to metal were
not detected.) Anal. Calcd for C31H38ClCuN2: C, 69.25; H, 7.12; N,
5.21. Found: C, 69.46; H, 6.93; N, 5.15.
4.4.6. 1beAuCl
In a glove box, 1b (50 mg, 0.11 mmol), chloro(dimethylsulfide)
gold(I) (34 mg, 0.11 mmol) and dry THF (2 mL) were added to a 4 mL
vial. The reaction mixture was stirred at room temperature for one
day. The mixture was then filtered through dry Celite and subse-
quently eluted with THF (1 ꢂ 0.5 mL). The filtrate was dried in vacuo
to afford 1beAuCl as a white solid (56 mg, 73%). Crystals suitable for
X-ray crystallography were obtained by slow diffusion of pentane
4.4.3. 1aeAgCl
In a glove box, 4a (50 mg, 0.097 mmol), silver(I) oxide (11 mg,
0.048 mmol) and dry dichloromethane (2 mL) were added to a 4 mL
vial. The reaction mixture was stirred at room temperature for one
day. The mixture was then filtered through dry Celite and subse-
quently eluted with dichloromethane (1 ꢂ 0.5 mL). The filtrate was
dried in vacuo to afford 1aeAgCl as a white solid (51 mg, 84%).
Crystals suitable for X-ray crystallography were obtained by slow
diffusion of pentane into dichloromethane solution. 1H NMR
into dichloromethane solution. 1H NMR (CDCl3, 500 MHz):
d 7.45
(dd, J ¼ 9.3, 1.1 Hz, 1 H), 7.27 (s, 1 H), 7.15 (s, 2 H), 7.05 (dd, J ¼ 9.2,
6.6 Hz, 1 H), 6.91 (s, 2 H), 6.58 (dd, J ¼ 6.7, 1.2 Hz, 1 H), 2.97 (m, 1 H),
2.41 (m, 2 H), 2.29 (s, 3 H), 1.92 (s, 6 H), 1.35 (d, J ¼ 6.9 Hz, 6 H), 1.28
(d, J ¼ 6.9 Hz, 6 H), 1.13 (d, J ¼ 6.8 Hz, 6 H) ppm. 13C NMR (CDCl3,
(CDCl3, 500 MHz):
d
7.51 (d, J ¼ 9.3 Hz, 1 H), 7.43 (t, J ¼ 7.8 Hz, 1 H),
125 MHz):
d 165.5 (CCarbene), 152.2, 146.9, 139.7, 138.6, 136.4, 134.3,
7.39 (d, J ¼ 1.8 Hz, 1 H), 7.21 (d, J ¼ 7.8 Hz, 2 H), 7.19 (s, 2 H), 7.11 (dd,
J ¼ 9.3, 6.7 Hz, 1 H), 6.64 (dd, J ¼ 6.6, 1.2 Hz, 1 H), 2.99 (m, 1 H), 2.43
(m, 2 H), 2.12 (m, 2 H), 1.36 (d, J ¼ 6.9 Hz, 6 H), 1.21 (d, J ¼ 6.9 Hz,
131.6, 129.5, 129.0, 123.2, 122.1, 117.0, 116.8, 112.3 (14 ꢂ CAromatic),
34.8, 31.8 (2 ꢂ ArCH(CH3)2), 25.3, 24.5, 23.8 (3 ꢂ ArCH(CH3)2), 21.3,
17.7 (2 ꢂ ArCH3) ppm. Anal. Calcd for C31H38AuClN2: C, 55.48; H,
5.71; N, 4.17. Found: C, 55.32; H, 5.80; N, 4.01.
6 H), 1.16e1.11 (m, 18 H) ppm. 13C NMR (CDCl3, 125 MHz):
d 152.6,
146.7,145.2,138.7, 135.8,131.9, 130.8,127.7,124.3,123.5,122.7,116.8,
116.6, 114.3 (14 ꢂ CAromatic), 34.7, 31.7, 28.5 (3 ꢂ CBenzyl), 25.3, 24.7,
24.6, 24.3, 24.3 (5 ꢂ CAlkyl) ppm. (Signal from the carbon adjacent to
metal were not detected.) Anal. Calcd for C34H44AgClN2: C, 65.44; H,
7.11; N, 4.49. Found: C, 65.23; H, 6.90; N, 4.41.
Acknowledgements
This work was supported by Institute for Basic Science (IBS)
[IBS-R007-D1]. X-ray diffraction experiment with synchrotron ra-
diation was performed at the Pohang Accelerator Laboratory
(Beamline 2D).
4.4.4. 1beAgCl
In a glove box, 4b (50 mg, 0.11 mmol), silver(I) oxide (12 mg,
0.053 mmol) and dry dichloromethane (2 mL) were added to a 4 mL
vial. The reaction mixture was stirred at room temperature for one
day. The mixture was then filtered through dry Celite and subse-
quently eluted with dichloromethane (1 ꢂ 0.5 mL). The filtrate was
dried in vacuo to afford 1beAgCl as a white solid (48 mg, 78%).
Crystals suitable for X-ray crystallography were obtained by slow
diffusion of pentane into chloroform solution. 1H NMR (CDCl3,
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2016.07.023.
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