Simple synthesis of neutral and cationic Cu-NHC complexes

Article abstract of DOI:10.1039/c4dt02986k

A direct and practical synthetic route to N-heterocyclic carbene copper complexes of [(NHC)CuX] (X = halide) and [(NHC)₂Cu]PF₆ types using commercially available copper powder is described. A number of copper-NHC complexes have been obtained in a range of yields from 26 to 99%. The reactions take place in air without removal of moisture and oxygen, and the excess of copper powder can be easily removed via simple filtration after completion. The direct reactions of imidazolium salts and copper powder can also be performed in aqueous media avoiding tedious purification processes. The procedure is also suitable for gram-scale preparation.

Full text of DOI:10.1039/c4dt02986k

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ARTICLE
Simple Synthesis of Neutral and Cationic Cu-NHC
Complexes
Cite this: DOI: 10.1039/x0xx00000x
Bo Liu,a,b Xueji Ma,a Feifei Wua and Wanzhi Chen*a
A direct and practical synthetic route to N-heterocyclic carbene copper complexes of
[(NHC)CuX] (X = halide) and [(NHC)2Cu]PF6 types using commercially available copper
powder is described. A number of copper-NHC complexes have been obtained in a range of
yields from 26 to 99%. The reactions take place in air without removal of moisture and
oxygen, and the excess of copper powder can be easily removed via simple filtration after
completion. The direct reactions of imidazolium salts and copper powder can also be
performed in aqueous media avoiding tedious purification processes. The procedure is also
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
suitable for gram-scale preparation
.
addition reactions,5 1,3-dipolar cycloaddition,6 carboxylation,7
allylic substitutions and allylation,8 borylation of unsaturated
hydrocarbons,9 and cross-coupling reactions,10 etc. Besides,
researches on the biological activities11 and photoluminescence
properties12 of Cu-NHCs have extended their potential
applications to medicine and photoactive functional materials.
Recently, it has proven that Cu-NHC complexes are useful
carbene transfer agents for the preparation of Au(I)-, Pd(II)-,
Ni(II)-, Ru(II)-, Rh(I)-, and Cr(III)-NHC complexes besides the
popularly used Ag(I)-NHCs.13
Introduction
Since the first Cu-NHC complex was disclosed in 1993 by
Arduengo and co-workers1, Cu-NHCs have been attracting
continuous attention and a number of copper-NHC complexes
have been synthesized so far2. The structural diversity, ease of
modification as well as low cost of Cu-NHCs offer excellent
opportunities for their versatile applications. The use of Cu-
NHCs such as [(NHC)CuX] (X = halide) type complexes or
their derivatives have proven successful2b,3 in homogeneous
catalysis including hydrosilylation reactions,4 conjugate
N
N
R
R
This work
B5a,14a,15-17
R
N
N
N
N
R
R
R
R
N
Cu powder
Cu
Cu
halide
AgX
CH3CN or H2O
air
N
R
N
N R
R
N
N
R
R
CuX
+
R
R
N
PF6
N
Cu powder
CH3CN, air
N
N
R
R
N
N
R
R
N
N
R
R
R'
(R' = CCl3, CO2, I2)
Scheme 1. Synthetic Routes to [(NHC)CuX] (X = halide) Complexes
Copper often forms [(NHC)CuX] type complexes when the use of strong bases and high sensitivity of free carbenes. A
anions are halide ions.2a The known synthetic routes to these modified one-pot synthetic strategy involves in situ
neutral Cu-NHCs are summarized in Scheme 1. Generally, they deprotonation of an imidazolium salt with cuprous halide in the
can be prepared from substitution reactions of free NHCs and presence of a base such as NaOtBu, K2CO3, etc (method
copper precursors (method
A
).1,14 This method suffers from the
B)
5a,14a,15. This strategy was first reported by Buchwald and co-
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ARTICLE
workers in 2003.5a Recently, Cisnetti and co-workers found that cationic [(NHC)2Cu]PF6 with copper powder in both organic
aqueous ammonia can also be a suitable base.16 Cazin and co- solvent and aqueous media.
workers showed that anhydrous solvent and inert gas protection
are not necessary for the one-pot strategy in some cases.17
Results and Discussion
Although the one-pot strategy simplifies the workup compared
To explore the scope of the synthetic procedure, a number of
to the free carbene route, the bases cannot be avoided.
imidazolylidene and benzimidazolylidene precursors were
[(NHC)CuX] can also be obtained via transmetallation of their
selected for this study possessing saturated or unsaturated C4-
corresponding Ag-NHC complexes (method
C
).13c,14a,18
C5 backbone and various anions, shown in Scheme 2. Ligands
IPr, SIPr, IMes and SIMes are among the most frequently used
ligands in Cu-NHC-catalyzed organic transformations2b,3 (IPr =
However, this protocol sacrifices Ag-NHC reagents and
generates silver halide as by-product. In situ deprotonation of
imidazolium salts with a basic copper precursor such as CuOAc,
CuOtBu, etc represents a convenient route (method
1,3-bis(2,6-diisopropylphenyl)-1
1,3-bis(2,6-diisopropylphenyl)-4,5-dihydro-1
ylidene, IMes = 1,3-dimesityl-1 -imidazol-2-ylidene, SIMes =
1,3-dimesityl-4,5-dihydro-1 -imidazol-2-ylidene). Their
hydrochlorides 1a 1d were obtained according to literature
H
-imidazol-2-ylidene, SIPr =
D
)19. Since
H
-imidazol-2-
reported by Danopoulos and co-workers in 2001,19a basic Cu2O
has been intensively employed for the synthesis of copper
complexes.20 [(NHC)CuX] complexes may also be obtained by
thermal decomposition of carbene adducts of CO2,21
H
H
-
procedures.28 IPr•HBr (1e) was obtained in 68% yield via anion
exchange of its hexafluorophosphate with an excess of
tetrabutylammonium bromide in ethyl acetate/CH3CN.
Similarly, IPr•HI (1f) and SIPr•HI (1g) were readily formed in
60% and 79% yields by anion exchange of IPr•HCl (1a) and
SIPr•HCl (1b) with 1 equiv. of NaI in acetone/ethanol. Anion
exchange of IMes•HCl (1c) and SIMes•HCl (1d) with NH4PF6
in water near-quantitatively gave their hexafluorophosphates
IMes•HPF6 (1h) and SIMes•HPF6 (1i), respectively.29 Besides,
23
chloroform,22 and I2 in the presence of a suitable copper
precursor (method E). Method E is restricted by the availability
of the carbene precursors. In addition, other improvements
including continuous synthesis, microwave-assisted, or
electrochemical procedures were also presented but require
special equipments.24
Noteworthily, due to the great importance of such kind of Cu-
NHCs,2,3,13 seeking their synthetic methods remains the focus
of research.16,17,20,23,24 Despite above mentioned advances,
further simplification of the synthetic procedure of [(NHC)CuX]
complexes is highly desirable especially for scalable
preparation. We have been interested in exploration of synthetic
methods for metal-NHCs,13e,24c,25 and have shown that Cu-
NHCs can be prepared from imidazolium salts with
commercially available copper powder in air.26 However, this
protocol is restricted to functionalized imidazolium salts
bearing a N-heteroarene such as pyridine or pyrimidine group
with the generation of cationic copper products.26,27 Herein, we
report the facile synthesis of neutral [(NHC)Cu(halide)] and
monoimidazolium
heteroarenes 1j
1l (Lj
imidazol-2-ylidene, Lk
benzo[
triazol-4-yl)methyl)-1
hexafluorophosphates
bearing
N-
-
=
=
1,3-bis(pyridin-2-ylmethyl)-1
1,3-bis(pyridin-2-ylmethyl)-1
H
-
-
H
d
]imidazol-3-ylidene, Ll = 1,3-bis((1-benzyl-1
H
-1,2,3-
and
H
-benzo[d]imidazol-2-ylidene)
bisimidazolium hexafluorophosphates linked by pyridine or
1n (Lm
3,5-dimethylpyrazole 1m 3,3'-(pyridine-2,6-
-imidazol-2-ylidene), Ln
diyl)bis(1-butyl-1 3,3'-((1H-
-
=
H
=
pyrazole-3,5-diyl)bis(methylene))bis(1-(pyridin-2-yl)-1
imidazol-2-ylidene)) were also synthesized.
H-
N
N
N
N
N
N
N
N
Cl
Cl
Br
I
IPr HCl (1a)
IMes HCl (1c)
SIMes HCl (1d)
IPr HI (1f)
SIPr HI (1g)
IPr HBr (1e)
SIPr HCl (1b)
Bn
Bn
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
PF6
PF6
PF6
PF6
HPF6 (1h)
HPF6 (1i)
IMes
[HLj]PF6 (1j)
[HLk]PF6 (1k)
[HLl]PF6 (1l)
SIMes
N
N
N
NH
N
N
N
N
N
2PF6
N
nBu
N
nBu
2PF6
N
N
[H2Lm](PF6)2 (1m)
[H2Ln](PF6)2 (1n)
Scheme 2. Schematic Illustration of Carbene Precursors
Imidazolium chlorides NHC•HCl and their copper complexes
are usually soluble in chlorinated solvents, hence
Synthesis of [(NHC)CuX] (X = halide) type complexes with
copper powder
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DOI: 10.1039/C4DT02986K
dichloromethane was initially chosen as the solvent. After almost completely consumed, and the expected products
IPr•HCl (1a) or SIPr•HCl (1b) was treated with 5 equiv. of [(IPr)CuCl] (2a) and [(SIPr)CuCl] (2b) were afforded in 90%
copper powder in CH2Cl2 at 40 ºC in air for 24 h, [(IPr)CuCl] and 88% yields, respectively, after removal of CH3CN (entries
(
2a) and [(SIPr)CuCl] (2b) were respectively generated in low 4 and 7). Although an excess of copper powder was initially
yields of 24% and 31%, determined by 1H NMR spectrum of an employed to promote the complete conversion of imidazolium
aliquot of the crude reaction mixture (Table 1, entries 1-2). salts, further studies showed that when 1 equiv. of copper
Under the same reaction conditions, [(SIMes)CuCl] (2d) was powder was used the yield is not significantly influenced (entry
isolated in 26% yield (entry 3).
5). In addition, unreacted copper powder can be quantitatively
recovered, and the recovered copper powder showed even
better reactivity than commercial copper powder (entry 6).
Similarly, IMes•HCl (1c) also reacted smoothly with copper
powder, and the desired complex [(IMes)CuCl] (2c) was
isolated in 49% yield after recrystallization of the crude product
in diethyl ether/CH3CN at low temperature (entry 8).
Table 1. Synthesis of [(NHC)CuX] (X = halide) Type
Complexesa
The protocol was also applied to imidazolium bromides and
iodides. IPr and SIPr based copper(I) bromide and iodide
complexes are less soluble in acetonitrile, and cooling the
reaction mixture to room temperature gave colorless crystals.
Hence removal of unreacted copper powder should be
performed while the solution was hot. [(IPr)CuBr] (2e) was
obtained in quantitative yield after removal of the solvent (entry
9). The same yield of 2e was obtained when the reaction was
carried out with the recovered copper powder (entry 10).
[(IPr)CuI] (2f) and [(SIPr)CuI] (2g) were obtained in about
80% yields (entries 11-12). Scale preparation is quite important
for potential future application. We attempted the gram-scale
En
try
T
/ºC
Yield
/%b
Salt
Product
Solvent
CH2Cl2
CH2Cl2
CH2Cl2
CH3CN
IPr•HCl
[(IPr)CuCl]
1
2
3
4
40
40
40
55
24c
31c
26
(
1a
SIPr•HCl
1b
SIMes•HCl
1d
IPr•HCl
)
(
2a
[(SIPr)CuCl]
2b
[(SIMes)CuCl]
2d
[(IPr)CuCl]
)
(
)
(
)
(
)
(
)
90
(
1a
1a
1a
)
(
2a
2a
2a
)
5
6
CH3CN
CH3CN
55
55
88d
91e
preparations of 2a, 2e, and 2f (entries 13-15). The reactions of 1
g of the imidazolium salts IPr•HX with an excess of copper
powder were finished within 24 h, and the corresponding
complexes [(IPr)CuCl] (2a), [(IPr)CuBr] (2e), and [(IPr)CuI]
SIPr•HCl
1b
IMes•HCl
1c
IPr•HBr
[(SIPr)CuCl]
2b
[(IMes)CuCl]
2c
[(IPr)CuBr]
7
8
CH3CN
CH3CN
55
55
88
49
(
)
(
)
(
2f) were isolated in the yields of 82%, 93%, and 75%,
(
)
(
)
respectively.
The above mentioned neutral [(NHC)CuX] (X = halide) type
complexes are colorless solids, and they have been
characterized by NMR spectra, and further confirmed by
9
CH3CN
CH3CN
CH3CN
55
55
55
99
99e
79
(
1e)
(
2e)
10
11
1e
2e
1
elemental analysis. Their H NMR and 13C NMR data are in
IPr•HI
1f
SIPr•HI
1g
IPr•HCl
1a
IPr•HBr
1e
IPr•HI
1f
[(IPr)CuI]
2f
[(SIPr)CuI]
2g
[(IPr)CuCl]
2a
[(IPr)CuBr]
2e
[(IPr)CuI]
2f
(
)
( )
accordance
with
those
described
in
the
literature.4a,5a,14a,15c,17,24a,d
12
13
14
15
CH3CN
CH3CN
CH3CN
CH3CN
55
55
55
55
83
82f
93f
75f
(
)
(
)
Synthesis of [(NHC)2Cu]PF6 type complexes with copper
powder
(
)
(
)
We subsequently examined the efficiency of the strategy for the
synthesis of homoleptic [(NHC)2Cu]PF6 complexes. Although
imidazolium halides NHC•HX have proven to be very reactive
towards copper powder as mentioned above, the reactions
between their hexafluorophosphate analogues NHC•HPF6 and
copper powder were found to be generally sluggish. It should
be noted that the same behavior was also observed in their
(
)
( )
(
)
( )
a Reaction conditions: imidazolium salt 0.2 mmol, copper powder 1.0 mmol,
b
solvent 5 mL, under air, 40-55 ºC, 24 h. Isolated yields unless otherwise
stated. c Determined by 1H NMR of an aliquot of the crude reaction mixture.
d
e
1.0 equiv. of copper powder was used instead of 5 equiv. 1.0 equiv. of
f
recovered copper powder was used instead of fresh copper powder. 1000
mg of imidazolium was used instead of 0.2 mmol.
reactions with Cu2O.20b,24a The reactions of IMes•HPF6 (1h
)
and SIMes•HPF6 (1i) with copper powder were performed at 80
The yields of Cu-NHC complexes were apparently improved
when the reaction was performed in acetonitrile at higher
ºC for 36 h, and the imidazolium salts were not completely
consumed. The corresponding complexes [(IMes)2Cu]PF6 (2h
)
reaction temperature. After stirring the mixture of IPr•HCl (1a
or SIPr•HCl (1b) and 5 equiv. of copper powder in CH3CN at
55 ºC for 24 h, gratifyingly, the imidazolium chlorides were
)
and [(SIMes)2Cu]PF6 (2i) were isolated in 20% and 30% yields
after recrystallization of the crude product in acetonitrile and
ethyl acetate at low temperature. When the reaction time was
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further prolonged to 4 days, the yields of 2h and 2i could be
(2e) was isolated in 58% yield after recrystallization. Although
accordingly increased to 33% and 42% (Table 2, entries 1-2).
the yields are not quite high, the procedure is quite practical
since imidazolium salts are usually soluble in hot water, and the
purification can be avoided.
Table 2. Synthesis of [(NHC)2Cu]PF6 Type Complexesa
+
R
R
Although picolyl-based imidazolium salt [HLj]PF6
(1j) and
PF6
N
N
5 equiv. Cu powder
benzimidazolium salt [HLk]PF6 (1k) are hardly soluble in water
at room temperature, they dissolved in water at 100 ºC giving
homogeneous solutions. After reacting with copper powder for
3 h, white precipitates of copper complexes appeared. After 24
h, the resulting mixture was cooled, and the resulting solid was
dissolved in CH3CN, from which the trinuclear Cu(I)
N
N
Cu
R
R
CH3CN, air, 80 oC, 4 d
N
N
PF6
R
R
- H2O
Entry
1
Salt
IMes•HPF6
1h
SIMes•HPF6
1i
Product
[(IMes)2Cu]PF6
2h
[(SIMes)2Cu]PF6
2i
Yield/%b
33
(
)
(
)
complexes [Cu3Lj3](PF6)3 2j) and [Cu3Lk ](PF6)3
( (2k) were
3
2
42
(
)
( )
isolated in 98% and 88% yields after simple filtration and
subsequent recrystallization in Et2O/CH3CN. It was found that
complex 2j could even be obtained in 30% yield when the
reaction was performed at room temperature. Under the same
conditions, the reaction between 1,2,3-triazole functionalized
benzimidazolium salt [HLl]PF6 (1l) and copper powder could
also take place smoothly. A novel Cu3 complex [Cu3Ll3](PF6)3
a
Reaction conditions: imidazolium 0.4 mmol, copper powder 2.0 mmol,
CH3CN 5 mL, under air, 80 ºC, 4 days. b Isolated yields.
We have previously shown that
N-heteroarene functionalized
NHC copper complexes can be easily generated with copper
powder even at room temperature.26,27 The dramatically
different reactivities of NHC•HPF6 from NHC•HX or
functionalized imidazoliums should result from the
coordinating abilities of halide ions or N-heteroarene, indicating
that the formation of Cu-halide bond or Cu-N bond is an
important driving force in the reactions.
(
2l) was obtained as a colorless solid in 46% yield. However,
pyridine-bridged dicarbene precursor [H2Lm]PF6 (1m) could be
hardly converted when it was treated with copper powder in
water. To our delight, when the mixed solvent of H2O/CH3CN
(v/v=1/1) was used instead of pure water, a novel binuclear
Cu(I)-NHC complex [Cu2Lm2](PF6)2
yellow solid in 54% yield. The reaction of 3,5-
dimethylpyrazole-linked bisimidazolium salt [H2Ln](PF6)2 (1n
(2m) was isolated as a
Synthesis of Cu-NHCs in aqueous media
Water is a green solvent, and some Cu-NHC-catalyzed organic
transformations have been realized in water.30 Exploration the
synthetic method for Cu-NHCs in water is also attracting. We
found that the reactivity of imidazolium halide NHC•HX
towards copper powder in water is generally lower than that in
CH3CN probably due to the lower solubility in water. To
improve the solubility of imidazolium, the reaction was carried
out at 100 ºC. As shown in Table 3, imidazolium chlorides 1a
1d were treated with copper powder in refluxing water for 30 h.
After removal of water under vacuum and washing with water,
)
with copper powder in refluxing water gave a red turbid
mixture, from which tetranuclear Cu(II)-NHC complex
[Cu4(ꢀ3-OH)2Ln ](PF6)4
(
2n) was obtained in 44% yield.
2
Complexes 2j 12a,13e 13e and 2n27a have already been reported
,
2k,
utilizing other synthetic procedures. Novel complexes 2l and
2m have been confirmed by elemental analysis and NMR
spectroscopy. The resonance signal of Cu-Ccarbene was not found
in the 13C NMR spectrum of 2l, while the characteristic signal
of Cu-Ccarbene was observed at 178.2 ppm in the 13C NMR
-
the corresponding complexes 2a
-2d were obtained in a range of
spectrum of 2m
.
yields from 30 to 65%. In the case of IPr•HBr (1e), [(IPr)CuBr]
Table 3. Synthesis of Cu-NHCs in Aqueous Mediaa
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a Reaction conditions: imidazolium 0.2-0.3 mmol, copper powder 5.0 equiv., H2O 10 mL, under air, 100 ºC, 24-36 h. Isolated yields unless otherwise stated.
b At room temperature for 24 h. c H2O/CH3CN (v/v = 1/1) was used instead of pure water.
Single crystal X-ray crystallographic studies
Suitable crystals for X-ray diffraction analysis of 2l and 2m
were respectively obtained by slow diffusion of diethyl ether
into their CH3CN solution at room temperature. The structures
of 2l and 2m were identified by X-ray single-crystal diffraction
analysis. As shown in Figure 1, the cation of 2l consists of a
triangular Cu3 core bridged by three triazole based NHC
ligands. Each copper ion is coordinated by two triazole and two
benzimidazolylidene ligands forming a distorted tetrahedral
geometry. The separations of Cu-Cu are at 2.514(1)-2.530(1) Å
displaying weak metal-metal interaction, which are comparable
to those of other similar trinuclear Cu-NHC complexes (2.46-
2.52 Å).12a,13e,24c,27b,31 The Cu-N bond distances fall in the range
of 2.085(4)-2.111(4) Å. Each benzimidazolylidene forms the
3c-2e bond with two copper ions with Cu-C bond distances of
2.055(5) and 2.084(5) Å, which are within the normal range of
Cu-C bonds (ca. 1.80-2.20 Å).2a Such a bridging ligand in a ꢀ2
mode of NHC is rare and only observed in a few silver, copper,
and nickel complexes.12a,13e,14a,19d,27b,c,32
Figure
1. ORTEP drawing of the cationic section of
[Cu3Ll3](PF6)3 (2l). Thermal ellipsoids are drawn at the 30%
probability level. Hydrogen atoms and counteranions have been
removed for clarity. Selected bond distances (Å) and angles
(deg): Cu1-C11 2.055(5), Cu1-N7 2.085(4), Cu1-Cu2 2.530(1),
Cu2-C38 2.076(5), Cu2-C11 2.084(5), Cu2-N2 2.102(4), Cu2-
N11 2.111(4), Cu2-Cu2A 2.514(1), C11-Cu1-C11A 165.2(3),
C11-Cu1-N7A 98.0(2), C11-Cu1-N7 91.4(2), N7A-Cu1-N7
100.5(2), C38-Cu2-C11 164.6(2), C38-Cu2-N2 98.7(1), C11-
Cu2-N2 92.4(2), C38-Cu2-N11 91.9(1), C11-Cu2-N11 96.7(2),
N2-Cu2-N11 99.6(2). Symmetry code: A -x+1, y, -z+1/2.
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The structure of 2m is depicted in Figure 2. The cation consists suitable for high-yielding scalable preparation which is of
2+
of a [Cu2Lm
]
2
dimer situated on a crystallographic inversion significance for future catalytical applications.
center. Complex 2m is isostructural with previously reported
complexes [Ag2Lm2](PF6)2 and [Au2Lm2](PF6)2.33 The dicarbene
Experimental Section
ligands are arranged in
a
helical fashion with
General Details
pyridine/imidazolylidene ring interplanar angles of 13.1° and
23.2°. Each copper ion is coordinated with two carbenic atoms
from two Lm ligands with the bond distances of 1.907(3) and
1.894(4) Å, showing a bent geometry with the C-Cu-C angle of
160.5(2)°. The long Cu-Npyridine distances of 2.685 and 2.797 Å
indicated weak interaction between copper ion and pyridyl
nitrogen. The intramolecular metal-metal distance of 2.807(1)
Å in 2m is much shorter than that of its Ag(I) and Au(I)
isostructural complexes.33
All reactions were carried out under air. Imidazolium salts
IPr•HCl
(
1a),28 SIPr•HCl
(
1b),28 IMes•HCl
(
1c),28 and
28
SIMes•HCl (1d
)
were prepared according to the reported
procedures. The synthetic procedures for IPr•HBr (1e), IPr•HI
(
(
1f), SIPr•HI (1g), IMes•HPF6 (1h), SIMes•HPF6 (1i), [HLj]PF6
1j), [HLk]PF6 1k), [HLl]PF6 1l), [H2Lm](PF6)2
( ( (1m), and
[H2Ln](PF6)2 (1n) are given in the Supporting Information. All
other chemicals were obtained from commercial suppliers and
used as received without further purification. The copper
powder (200 mesh, with purity
≥99.7 wt %) was purchased
from Sinopharm Chemical Reagent Co., Ltd, China, and was
1
used directly without any pretreatment. H and 13C-{1H} NMR
spectra were recorded at 400 and 100 MHz respectively on a
Bruker Avance-400 (400 MHz) spectrometer. Chemical shifts
(δ) are expressed in ppm and coupling constants (J) are
expressed in Hz. All NMR spectra were recorded at 298 K in
deuterated solvent using the residual solvent peak as reference
(CHCl3 δH = 7.26 ppm, δC = 77.16 ppm; acetone δH = 2.05 ppm,
δC = 29.84, 206.26 ppm; DMSO δH = 2.50 ppm, δC = 39.52
ppm; CH3CN δH = 1.94 ppm, δC = 1.32, 118.26 ppm). Single-
crystal X-ray diffraction data were collected at 293(2) K on a
Siemens Smart/CCD area-detector diffractometer with a Mo Kα
radiation (λ = 0.71073 Å). The elemental analyses were
performed on a Flash EA 1112 instrument.
Figure
[Cu2Lm2](PF6)2 (2m). Thermal ellipsoids are drawn at the 30%
probability level. Hydrogen atoms, -butyl groups, and
2. ORTEP drawing of the cationic section of
General synthetic procedure for [(NHC)CuX] (X = halide)
type complexes
n
counteranions have been removed for clarity. Selected bond
distances (Å) and angles (deg): Cu1-C2 1.894(4), Cu1-C1
1.907(3), Cu1-Cu1A 2.807(1), C2-Cu1-C1 160.5(2). Symmetry
code: A -x+1, y, -z+3/2.
A solution of NHC•HX (X = halide) (1.0 equiv.) and copper
powder (5.0 equiv.) in CH3CN (5 mL) was vigorously stirred at
55 ºC for 24 hours. The reaction conversion was determined by
1H NMR spectrum in CDCl3 of an aliquot of the crude reaction
mixture. The resulting solution was filtered through silica to
remove the excess of copper powder while it was hot. The
desired product was obtained by removal of the solvent under
vacuum. When necessary, the solid was further washed with
hot water or recrystallized with proper solvent.
Conclusions
In summary, we described a direct approach for the synthesis of
neutral [(NHC)CuX] (X = halide) and cationic [(NHC)2Cu]PF6
type complexes in a range of yields from imidazolium salts and
commercially available copper powder as the simplest copper
source. The procedure avoids addition of bases, handling highly
sensitive free carbene intermediates, or sacrificing Ag-NHC
complexes. The reaction workups are obviously simplified as
the reactions take place in air without removal of moisture and
oxygen, and the solvent does not require any purification prior
to use. In addition, water is the only byproduct, and the excess
of copper powder can be easily removed via simple filtration.
Further simplification of the procedure is realized in water
since imidazolium salts are usually soluble in hot water. This
straightforward route is quite practical and suitable for various
neutral and cationic Cu complexes of unsaturated and saturated
NHCs with different substituents. Especially, the procedure is
[(IPr)CuCl], 2a. This complex was synthesized from IPr•HCl
(
1a) (0.2 mmol, 85 mg) and copper powder (1.0 mmol, 64 mg).
Removal of the solvent under vacuum gave the desired product.
Yield: 88 mg (90%), colorless solid. 1H NMR (400 MHz,
CDCl3):
4H, Ar-
δ
H
7.49 (t,
), 7.14 (s, 2H, NC
= 6.8 Hz, each 12H, CH3). 13
180.7 (Cu- ), 145.6, 134.5, 130.6,
J
= 7.6 Hz, 2H, Ar-
H), 7.29 (d, J = 7.6 Hz,
HCH
N), 2.48-2.59 (m, 4H,
CH
(CH3)2), 1.29, 1.23 (both d,
J
C
NMR (100 MHz, CDCl3):
δ
C
124.3, 123.3, 28.8, 24.9, 24.0. The 1H NMR and 13C NMR data
are in accordance with those described in the
literature.5a,15c,17,24a,d Anal. Calcd for C27H36ClCuN2: C, 66.51;
H, 7.44; N, 5.75. Found: C, 66.60; H, 7.48; N, 5.67.
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 7 of 11
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DOI: 10.1039/C4DT02986K
[(SIPr)CuCl], 2b
. This complex was synthesized from Anal. Calcd for C27H36CuIN2: C, 56.00; H, 6.27; N, 4.84.
SIPr•HCl (1b) (0.2 mmol, 86 mg) and copper powder (1.0 Found: C, 55.89; H, 6.27; N, 4.75.
mmol, 64 mg). Removal of the solvent under vacuum gave the [(SIPr)CuI], 2g. This complex was synthesized from SIPr•HI
1
desired product. Yield: 86 mg (88%), colorless solid. H NMR
(
1g) (0.2 mmol, 104 mg) and copper powder (1.0 mmol, 64 mg).
(400 MHz, CDCl3):
(d, = 8.0 Hz, 4H, Ar-
3.10 (m, 4H, C (CH3)2), 1.34, 1,33 (both d,
12H, CH3). 13C NMR (100 MHz, CDCl3):
146.7, 134.5, 129.9, 124.6, 53.8, 29.0, 25.6, 24.0. The 1H NMR
and 13C NMR data are in accordance with those described in NMR (100 MHz, CDCl3):
the literature.15c,17,34 Anal. Calcd for C27H38ClCuN2: C, 66.24; 124.6, 53.8, 29.0, 25.6, 24.1. The H NMR and 13C NMR data
H, 7.82; N, 5.72. Found: C, 66.28; H, 7.78; N, 5.63.
are in accordance with those described in the literature.14a Anal.
[(IMes)CuCl], 2c This complex was synthesized from Calcd for C27H38CuIN2: C, 55.81; H, 6.59; N, 4.82. Found: C,
δ
7.38 (t,
), 4.00 (br. s, 4H, NCH2
= 7.2 Hz, each CDCl3):
203.0 (Cu- ), 4H, Ar-
J
= 8.0, 7.6 Hz, 2H, Ar-
H
), 7.23 Removal of the solvent under vacuum gave the desired product.
1
J
H
CH2N), 3.00- Yield: 97 mg (83%), colorless crystals. H NMR (400 MHz,
H
J
δ
H
7.40 (t,
), 4.02 (s, 4H, NCH2
= 7.2 Hz, each 12H, CH3). 13
204.4 (Cu- ), 146.7, 134.2, 129.9,
J
= 7.6 Hz, 2H, Ar-
H), 7.24 (d, J = 8.0 Hz,
δ
C
C
H2N), 3.02-3.10 (m, 4H,
CH
(CH3)2), 1.36, 1.33 (both d,
J
C
δ
C
1
.
IMes•HCl (2c) (0.2 mmol, 69 mg) and copper powder (1.0 55.92; H, 6.63; N, 4.72.
mmol, 64 mg). Recrystallization in acetonitrile and diethyl
Scale synthesis of [(IPr)CuX] (X = halide)
ether at -15 ºC led to isolation of the desired product. Yield: 39
1
mg (49%), colorless crystals. H NMR (400 MHz, CDCl3):
7.06 (s, 2H, NC N), 7.00 (s, 4H, Ar- ), 2.35 (s, 6H, CH3),
2.10 (s, 12H, CH3). 13C NMR (100 MHz, CDCl3):
179.0 (Cu-
δ
A solution of IPr•HX (1000 mg) and copper powder (5.0 equiv.)
in CH3CN (25 mL) was vigorously stirred at 55 ºC for 24 hours.
The resulting solution was quickly filtered through silica when
it was hot, followed by elution with CH3CN. The solvent was
H
C
H
H
δ
1
C
), 139.5, 135.1, 134.6, 129.5, 122.4, 21.1, 17.8. The H NMR
and 13C NMR data are in accordance with those described in
the literature.15c,17,24d,34 Anal. Calcd for C21H24ClCuN2: C, 62.52;
H, 6.00; N, 6.94. Found: C, 62.50; H, 6.06; N, 6.89.
removed under vacuum and washed with
was dried in vacuo to give the desired product.
[(IPr)CuCl], 2a 940 mg (82%), Anal. Calcd for
n-hexane. The solid
.
[(SIMes)CuCl], 2d. This complex was synthesized from
SIMes•HCl (1d) (0.2 mmol, 70 mg) and copper powder (1.0
mmol, 64 mg) in CH2Cl2 at 40 ºC for 24 hours. The solid was
C27H36ClCuN2: C, 66.51; H, 7.44; N, 5.75. Found: C, 66.65; H,
7.49; N, 5.59.
[(IPr)CuBr], 2e
.
1054 mg (93%), Anal. Calcd for
washed several times with water and dried in vacuo. Yield: 21
C27H36BrCuN2: C, 60.95; H, 6.82; N, 5.27. Found: C, 60.77; H,
6.79; N, 5.04.
[(IPr)CuI], 2f. 840 mg (75%), Anal. Calcd for C27H36CuIN2: C,
56.00; H, 6.27; N, 4.84. Found: C, 55.84; H, 6.27; N, 4.70.
1
mg (26%), colorless solid. H NMR (400 MHz, CDCl3):
δ
6.95
H2N), 2.31 (s, 12H, CH3),
2.30 (s, 6H, CH3). 13C NMR (100 MHz, CDCl3):
202.7 (Cu-
(s, 4H, Ar-H), 3.95 (br. s, 4H, NCH2C
δ
1
C
), 138.7, 135.5, 135.1, 129.8, 51.1, 21.1, 18.1. The H NMR
and 13C NMR data are in accordance with those described in
the literature.4a,15c,24a Anal. Calcd for C21H26ClCuN2: C, 62.21;
H, 6.46; N, 6.91. Found: C, 62.03; H, 6.59; N, 6.74.
General synthetic procedure for [(NHC)2Cu]PF6
A solution of NHC•HPF6 (0.4 mmol) and copper powder (2.0
mmol) in CH3CN (5 mL) was vigorously stirred at 80 ºC for 4
days. The resulting solution was filtered to remove the excess
of copper powder. The filtrate was concentrated, and
recrystallized with ethyl acetate at -15 ºC to isolate the desired
products.
[(IPr)CuBr], 2e. This complex was synthesized from IPr•HBr
(
1e) (0.2 mmol, 94 mg) and copper powder (1.0 mmol, 64 mg).
Removal of the solvent under vacuum gave the desired product.
1
Yield: 105 mg (99%), colorless crystals. H NMR (400 MHz,
CDCl3):
Hz, 4H, Ar-
(CH3)2), 1.30, 1.22 (both d,
NMR (100 MHz, CDCl3): 181.5 (Cu-
δ
7.49 (t,
), 7.14 (s, 2H, NC
= 6.8 Hz, each 12H, CH3). 13
), 145.7, 134.4, 130.7,
J
= 7.6, 8.0 Hz, 2H, Ar-
H), 7.29 (d, J = 8.0
[(IMes)2Cu]PF6, 2h. This complex was synthesized from the
reaction of IMes•HPF6 (1h) (0.4 mmol, 181 mg) and copper
powder (2.0 mmol, 128 mg). Yield: 54 mg (33%), colorless
H
HCH
N), 2.52-2.61 (m, 4H,
CH
J
C
δ
C
solid. 1H NMR (400 MHz, acetone-d6):
NC N), 7.05 (s, 8H, Ar-
24H, CH3). 13C NMR (100 MHz, acetone-d6):
δ
7.43 (s, 4H,
), 2.45 (s, 12H, CH3), 1.74 (s,
178.3 (Cu- ),
124.3, 123.2, 28.8, 24.9, 24.0. The 1H NMR and 13C NMR data
are in accordance with those described in the literature.14a,17
Anal. Calcd for C27H36BrCuN2: C, 60.95; H, 6.82; N, 5.27.
Found: C, 60.84; H, 6.76; N, 5.15.
H
CH
H
δ
C
1
140.2, 135.8, 135.4, 130.0, 123.9, 21.2, 17.2. The H NMR and
13C NMR data are in accordance with those described in the
literature.16,35 Anal. Calcd for C42H48CuF6N4P: C, 61.72; H,
5.92; N, 6.85. Found: C, 61.56; H, 5.82; N, 6.97.
[(SIMes)2Cu]PF6, 2i. This complex was synthesized from the
reaction of SIMes•HPF6 (1i) (0.4 mmol, 181 mg) and copper
[(IPr)CuI], 2f. This complex was synthesized from IPr•HI (1f
)
(0.2 mmol, 104 mg) and copper powder (1.0 mmol, 64 mg).
Removal of the solvent under vacuum gave the desired product.
Yield: 92 mg (79%), colorless solid. 1H NMR (400 MHz,
CDCl3):
Hz, 4H, Ar-
(CH3)2), 1.29, 1.22 (both d,
NMR (100 MHz, CDCl3): 182.9 (Cu-
δ
7.49 (t,
), 7.14 (s, 2H, NC
= 6.8 Hz, each 12H, CH3). 13
), 145.6, 134.2, 130.5,
J
= 8.0, 7.6 Hz, 2H, Ar-
H), 7.29 (d, J = 7.6
powder (2.0 mmol, 128 mg). Yield: 69 mg (42%), colorless
H
HCH
N), 2.50-2.60 (m, 4H,
1
solid. H NMR (400 MHz, acetone-d6):
δ
6.99 (s, 8H, Ar-
H),
CH
J
C
4.00 (s, 8H, NCH2
C
136.7, 135.8, 130.3, 51.7, 21.2, 17.6. The H NMR and 13C
CH2N), 2.40 (s, 12H, CH3), 1.92 (s, 24H,
δ
C
H3). 13C NMR (100 MHz, acetone-d6):
δ 202.0 (Cu-C), 139.4,
124.1, 123.2, 28.7, 24.9, 24.0. The 1H NMR and 13C NMR data
1
are in accordance with those described in the literature.14a,17
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 7

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DOI:D10a.l1t0o3Vn9ie/TCwr4AaDrtniTc0slea2O9cn8tl6iinoKens
ARTICLE
NMR data are in accordance with those described in the NMR (100 MHz, DMSO-d6):
δ
175.5 (Cu-C), 153.6, 147.8,
1
literature.35 Anal. Calcd for C42H52CuF6N4P: C, 61.41; H, 6.38; 140.0, 134.3, 125.0, 124.9, 124.5, 111.0, 50.2. The H NMR
N, 6.82. Found: C, 61.23; H, 6.45; N, 6.60.
and 13C NMR data are in accordance with those described in
the literature.13e Anal. Calcd for C57H48Cu3F18N12P3: C, 44.85;
H, 3.17; N, 11.01. Found: C, 44.76; H, 3.17; N, 11.08.
General synthetic procedure of Cu-NHCs in water
[Cu3Ll3](PF6)3, 2l. This complex was synthesized from the
A solution of imidazolium salt (1.0 equiv.) and copper powder
(5.0 equiv.) in water (10 mL) was vigorously stirred at 100 ºC
for 24-36 hours. The aqueous solution was then dried under
vacuum (for neutral Cu-NHCs) or cooled to room temperature
to give the solid (for cationic Cu-NHCs), which was collected
and washed several times with water. The solid was dissolved
in acetonitrile and filtered through silica. The desired product
was obtained after removal of the solvent under vacuum. When
necessary, the solid was further recrystallized.
reaction of [HLl]PF6
(1l) (0.3 mmol, 182 mg), and copper
powder (1.5 mmol, 96 mg) for 24 hours. The desired product
was isolated via recrystallization from diethyl ether/acetonitrile.
1
Yield: 92 mg (46%), white solid. H NMR (400 MHz, DMSO-
d6):
benzimidazole-
(both d, = 14.8 Hz, each 6H, NCH2), 5.00 (d,
NC H), 4.81 (d, = 15.2 Hz, 6H, NCH
). 13C NMR (100
MHz, CD3CN): 142.2, 135.6, 135.2, 130.2, 130.0, 129.1,
125.4, 124.7, 111.3, 55.5, 41.8 (resonance signal of Cu- was
δ
8.34 (s, 6H, triazole-
H
), 7.36-7.56 (m, 30H, Ph-
= 7.2 Hz, 12H, Ph- ), 5.41, 5.30
= 15.6 Hz, 6H,
H +
H
), 7.20 (d,
J
H
J
J
H
J
H
δ
[(IPr)CuCl], 2a. This complex was synthesized from the
reaction of IPr•HCl (1a) (0.2 mmol, 85 mg), and copper powder
(1.0 mmol, 64 mg) for 30 hours. Yield: 33 mg (34%).
C
not observed). Anal. Calcd for C81H72Cu3F18N24P3: C, 48.47; H,
3.62; N, 16.75. Found: C, 48.33; H, 3.70; N, 16.89. Single
crystals of 2l were obtained by slow diffusion of diethyl ether
to its CH3CN solution at room temperature.
[Cu2Lm2](PF6)2, 2m. This complex was synthesized from the
reaction of [H2Lm](PF6)2 (1m) (0.2 mmol, 123 mg), and copper
[(SIPr)CuCl], 2b. This complex was synthesized from the
reaction of SIPr•HCl (1b) (0.2 mmol, 86 mg), and copper
powder (1.0 mmol, 64 mg) for 30 hours. Yield: 29 mg (30%).
[(IMes)CuCl], 2c. This complex was synthesized from the
reaction of IMes•HCl (1c) (0.2 mmol, 69 mg), and copper
powder (1.0 mmol, 64 mg) for 30 hours. Yield: 52 mg (65%).
[(SIMes)CuCl], 2d. This complex was synthesized from the
reaction of SIMes•HCl (1d) (0.2 mmol, 69 mg), and copper
powder (1.0 mmol, 64 mg) for 30 hours. Yield: 39 mg (48%).
[(IPr)CuBr], 2e. This complex was synthesized from the
reaction of IPr•HBr (1e) (0.2 mmol, 94 mg), and copper powder
(1.0 mmol, 64 mg) for 36 hours. The desired product was
powder (1.0 mmol, 64 mg) at 100 ºC in
6 mL of
water/acetonitrile (v/v = 1/1) for 24 hours. The desired product
was isolated via recrystallization from diethyl ether/acetonitrile.
Yield: 57 mg (54%), yellow solid. 1H NMR (400 MHz, DMSO-
d6):
δ
8.51 (t,
J
= 7.6, 8.0 Hz, 2H, Py-
H
), 8.33, 7.63 (both br. s,
), 3.61 (t,
each 4H, NC
H
C
H
N), 8.07 (d, = 8.0 Hz, 4H, Py-
J
H
J
= 7.6, 7.2 Hz, 8H, NCH2CH2CH2CH3), 1.10-1.32 (m, 8H,
NCH2CH2CH2CH3), 0.81-0.92 (m, 8H, NCH2CH2CH2CH3),
isolated via recrystallization from
Yield: 62 mg (58%).
n-hexane/dichloromethane.
0.57 (t,
MHz, DMSO-d6):
J
= 7.2 Hz, 12H, NCH2CH2CH2CH3). 13C NMR (100
δ
178.2 (Cu-C), 148.4, 144.3, 123.5, 118.4,
[Cu3Lj3](PF6)3, 2j. This complex was synthesized from the
112.8, 50.7, 31.9, 18.6, 12.9. Anal. Calcd for
C38H50Cu2F12N10P2: C, 42.90; H, 4.74; N, 13.17. Found: C,
42.72; H, 4.70; N, 13.41. Single crystals of 2m were obtained
by slow diffusion of diethyl ether to its CH3CN solution at
room temperature.
reaction of [HLj]PF6
(1j) (0.3 mmol, 119 mg), and copper
powder (1.5 mmol, 96 mg) for 24 hours. The desired product
was isolated via recrystallization from diethyl ether/acetonitrile.
Yield: 135 mg (98%), white solid. 1H NMR (400 MHz, DMSO-
d6):
+NC
4.8 Hz, 6H, Py-
= 14.8 Hz, 6H, NCH
168.3 (Cu-
δ
8.03 (t,
N), 7.31 (t,
), 5.07 (d,
). 13C NMR (100 MHz, DMSO-d6):
J
= 7.6 Hz, 6H, Py-
= 6.0, 6.4 Hz, 6H, Py-
= 14.4 Hz, 6H, NC
H), 7.64-7.71 (m, 12H, Py-
[Cu4(µ3-OH)2Ln ](PF6)4, 2n 27a
This complex was synthesized
.
2
H
H
CH
J
H
), 6.43 (d,
J
=
J
from the reaction of [H2Ln](PF6)2 (1n) (0.2 mmol, 135 mg), and
copper powder (1.0 mmol, 64 mg) for 24 hours. The desired
product was isolated via recrystallization from diethyl
ether/acetonitrile. Yield: 72 mg (44%), red solid. Anal. Calcd
for C42H36Cu4F24N16O2P4: C, 30.93; H, 2.22; N, 13.74. Found:
C, 30.71; H, 2.40; N, 13.73. The compound is paramagnetic,
and thus no NMR spectrum was obtained.
H
J
HH), 4.97 (d,
H
δ
C), 153.9, 148.1, 139.5, 124.7, 124.3, 124.1, 53.3.
The H NMR and 13C NMR data are in accordance with those
described in the literatures.12a,13e Anal. Calcd for
C45H42Cu3F18N12P3: C, 39.27; H, 3.08; N, 12.21. Found: C,
39.32; H, 3.10; N, 12.15.
1
[Cu3Lk ](PF6)3, 2k. This complex was synthesized from the
3
Acknowledgements
We thank the National Natural Science Foundation of China
(No. 21352002) for financial support.
reaction of [HLk]PF6 (1k) (0.3 mmol, 134 mg), and copper
powder (1.5 mmol, 96 mg) for 24 hours. The desired product
was isolated via recrystallization from diethyl ether/acetonitrile.
Yield: 134 mg (88%), white solid. 1H NMR (400 MHz, DMSO-
Notes and references
a
Department of Chemistry, Zhejiang University, Xixi Campus,
d6):
6H, Py-
7.51 (dd,
6.4 Hz, 6H, Py-
15.6 Hz, 6H, NC
δ
7.97 (t,
), 7.77 (dd,
= 3.2, 6.0 Hz, 6H, benzimidazole-H), 6.99 (t,
), 6.36 (d, = 8.4 Hz, 6H, Py- ), 5.41 (d,
H), 5.17 (d, = 16.0 Hz, 6H, NCH
). 13C
J
= 6.8, 8.0 Hz, 6H, Py-
= 2.8, 5.6 Hz, 6H, benzimidazole-
= 5.6,
H
), 7.84 (d,
J
= 8.0 Hz,
H
J
H
),
Hangzhou 310028, P. R. China. E-mail: chenwzz@zju.edu.cn; Tel & fax:
(+) 86-571-88273314.
J
J
H
J
H
J =
b
Shanghai Research Institute of Petrochemical Technology, Sinopec,
H
J
H
Shanghai 201208, P. R. China.
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 9 of 11
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View ArticAle ROnTliInCeLE
DOI: 10.1039/C4DT02986K
†
Electronic Supplementary Information (ESI) available: Synthetic
M. F. Haddow, H. Othman and G. R. Owen, Organometallics, 2012,
31, 6595. (i) E. K. Bullough, M. A. Little and C. E. Willans,
Organometallics, 2013, 32, 570. (j) J. Mormul, M. Steimann and U.
Nagel, Eur. J. Inorg. Chem., 2014, 2014, 1389.
procedures for IPr•HBr (1e), IPr•HI (1f), SIPr•HI (1g), IMes•HPF6 (1h),
SIMes•HPF6
[H2Lm](PF6)2
(
1i), [HLj]PF6
(
1j), [HLk]PF6
( (1l),
1k), [HLl]PF6
(
1m), and [H2Ln](PF6)2
(
1n). Copies of 1H and 13C-{1H}
NMR spectra of imidazolium salts and Cu(I)-NHC products. CCDC No.
1025967 (2l), 1025968 (2m) contain the supplementary crystallographic
data for this paper. See DOI: 10.1039/b000000x/
14 (a) S. Díez-González, E. C. Escudero-Adán, J. Benet-Buchholz, E. D.
Stevens, A. M. Z. Slawin and S. P. Nolan, Dalton Trans., 2010, 39
,
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Vacher, R. Brousses, N. Lugan and G. Lavigne, Dalton Trans.
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2
A. J. Arduengo III, H. V. R. Dias, J. C. Calabrese and F. Davidson,
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View Article Online
DOI: 10.1039/C4DT02986K
Graphical Abstract
Simple Synthesis of Neutral and Cationic Cu-NHC Complexes
Bo Liu, Xueji Ma, Feifei Wu and Wanzhi Chen*
1. Color Graphic
2. Text (no more than 20 words)
Simple synthesis of [(NHC)CuX] and [(NHC)2Cu]PF6 complexes with cheap copper
powder in both organic solvent and water is described.