Copper(I) complexes of normal and abnormal carbenes and their use as catalysts for the Huisgen [3+2] cycloaddition between azides and alkynes

Article abstract of DOI:10.1002/ejic.201100363

Complexes of general formula [(NHC)Cu(μ-I)₂Cu(NHC)] (NHC = 1-benzyl-3-butylbenzimidazolin-2-ylidene for 7; NHC = 1-benzyl-3- isopropylbenzimidazolin-2-ylidene for 8 NHC = 1-butyl-3-isopropylbenzimidazolin- 2-ylidene for 9) and [(aNHC)CuI] {aNHC = 1-benzyl-3-methyl-4-phenyl-1H-1,2,3- triazol-3-ium-5-ylidene for 10; 3-methyl-1-[2-(methylthio)phenyl]-4-phenyl-1H-1, 2,3-triazol-3-ium-5-ylidene for 11} were synthesized from CuI and the corresponding azolium salts. All complexes were fully characterized by spectroscopic methods, and single-crystal X-ray structural analyses were performed for 7 and 11. Complex 7 exhibits an iodido-bridged dicopper(I) form in the solid state. The copper centers in 7 adopt a trigonal coordination. The intramolecular Cu-Cu distance is 2.896(2) A. In contrast to 7, 11 is mononuclear, and the copper center has a linear coordination. All the complexes were tested for their efficiency as catalysts in the [3+2] cycloaddition reaction between azides and alkynes. The benzimidazolin-2-ylidene-containing complexes 7-9 as well as the triazol-3-ium-5-ylidene-containing complexes 10-11 turned out to be highly efficient catalysts for the 1,3-cycloaddition reaction, both under neat conditions and in water. Short reaction times as well as low catalyst loadings could be achieved, and all the manipulations were carried out under proper "click" conditions with simple product isolation and no purification steps. The efficiency of the catalysts seems to correlate well with the σ-donor ability of the NHCs. Cycloaddition reaction with an internal alkyne was also achieved with these catalysts. Finally a bulky azide was used as a substrate to prepare compound 1-(2,6-dimesityl)phenyl-4-(2-pyridyl)-1,2,3- triazole (14) with the above mentioned copper catalysts. Such bulky azides are usually unreactive towards cycloaddition reactions with alkynes in the presence of the Cu^II/ascorbate/tbta catalytic system.

Full text of DOI:10.1002/ejic.201100363

FULL PAPER
DOI: 10.1002/ejic.201100363
Copper(I) Complexes of Normal and Abnormal Carbenes and Their Use as
Catalysts for the Huisgen [3+2] Cycloaddition between Azides and Alkynes
Stephan Hohloch,[a] Cheng-Yong Su,[b] and Biprajit Sarkar*[a]
Keywords: Copper / Carbenes / Cycloaddition / Homogeneous catalysis / 1,2,3-Triazole / Benzimidazole
Complexes of general formula [(NHC)Cu(μ-I)2Cu(NHC)]
(NHC 1-benzyl-3-butylbenzimidazolin-2-ylidene for 7;
The benzimidazolin-2-ylidene-containing complexes 7–9 as
well as the triazol-3-ium-5-ylidene-containing complexes 10–
11 turned out to be highly efficient catalysts for the 1,3-cyclo-
addition reaction, both under neat conditions and in water.
Short reaction times as well as low catalyst loadings could be
achieved, and all the manipulations were carried out under
proper “click” conditions with simple product isolation and
no purification steps. The efficiency of the catalysts seems to
correlate well with the σ-donor ability of the NHCs. Cycload-
dition reaction with an internal alkyne was also achieved
with these catalysts. Finally a bulky azide was used as a sub-
strate to prepare compound 1-(2,6-dimesityl)phenyl-4-(2-
pyridyl)-1,2,3-triazole (14) with the above mentioned copper
catalysts. Such bulky azides are usually unreactive towards
cycloaddition reactions with alkynes in the presence of the
CuII/ascorbate/tbta catalytic system.
=
NHC = 1-benzyl-3-isopropylbenzimidazolin-2-ylidene for 8
NHC = 1-butyl-3-isopropylbenzimidazolin-2-ylidene for 9)
and [(aNHC)CuI] {aNHC = 1-benzyl-3-methyl-4-phenyl-1H-
1,2,3-triazol-3-ium-5-ylidene for 10; 3-methyl-1-[2-(meth-
ylthio)phenyl]-4-phenyl-1H-1,2,3-triazol-3-ium-5-ylidene for
11} were synthesized from CuI and the corresponding azol-
ium salts. All complexes were fully characterized by spectro-
scopic methods, and single-crystal X-ray structural analyses
were performed for 7 and 11. Complex 7 exhibits an iodido-
bridged dicopper(I) form in the solid state. The copper cen-
ters in 7 adopt a trigonal coordination. The intramolecular
Cu–Cu distance is 2.896(2) Å. In contrast to 7, 11 is mono-
nuclear, and the copper center has a linear coordination. All
the complexes were tested for their efficiency as catalysts in
the [3+2] cycloaddition reaction between azides and alkynes.
out to be highly efficient, subsequent work by other groups
stressing the use of additional donor ligands for catalytic
improvement.[34] NHCs and complexes thereof have be-
come valuable reagents for a wide variety of chemical trans-
formations[35–43] with the relatively recent discovery of their
redox-active character[44–47] as well as of abnormal carb-
enes[48–51] and/or mesoionic compounds (aNHC),[52,53]
which add to the continuous development of this field. We
have been interested in the use of the click method for de-
veloping 1,2,3-triazole-based ligands and their metal com-
plexes and investigating their structural and electronic
properties.[54] In view of this, we looked for efficient ways of
performing the click reaction and possibly finding catalyst
systems that might perform catalytic transformations that
are not possible with the now popular CuII/ascorbate/tbta
system. In view of the importance of strong σ-donor li-
gands on CuI for the improved efficiency of the click reac-
tion,[28,32,34] we chose benzimidazole-based NHCs with
alkyl substituents on the nitrogen atoms as well as 1,2,3-
triazole-based aNHCs. Both of these should be better σ-
donors as compared to aromatic substituted imidazole-
based NHCs. In addition, we used CuI salts as precursors,
because of the more labile nature of the Cu–I bond as com-
pared to the Cu–Br or the Cu–Cl bonds. Activation of this
bond is one of the crucial steps in the click catalytic cycle.
While we were carrying out these studies, a report on the
Introduction
The Huisgen [3+2] cycloaddition between azides and alk-
ynes,[1,2] which was re-tooled as the copper catalyzed “click
reaction”,[3,4] has turned out to be one of the most efficient
and widely used catalytic reactions for the chemical com-
munity. This reaction has found use in medicinal chemis-
try,[5] biology,[6] material science,[7] anion binding,[8–10] and
sensing studies[11] as well as in building ligands for coordi-
nation and organometallic chemistry.[12–24] After the initial
discovery of the use of tris[(1-benzyl-1H-1,2,3-triazole-4-
yl)methyl]amine (tbta) as an additional component of the
catalytic mixture, which leads to acceleration of reactions
and improvement of yields,[25] efforts have been made to use
other such ligands for improving this method.[26–29] In this
regard, the use of copper(I) complexes of N-heterocyclic
carbene (NHC) ligands as catalysts for the click reaction as
pioneered by Nolan and Diéz-Gonzaléz[30–33] has turned
[a] Institut für Anorganische Chemie, Universität Stuttgart
Pfaffenwaldring 55, 70550, Stuttgart, Germany
Fax: +49-711-68564165
E-mail: sarkar@iac.uni-stuttgart.de
[b] School of Chemistry and Chemical Engineering, Sun Yat-Sen
University,
Guangzhou, 510275, China
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100363.
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S. Hohloch, C.-Y. Su, B. Sarkar
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use of CuI complexes of 1,2,3-triazole-based aNHCs as cat- –20 °C, and 6 was crystallized by layering a dichloromethane
alysts for the “click” reaction appeared in the literature.[55] solution of it with diethyl ether at 8 °C. Salt 1 (Figure 1)
In the following, we have used the salts 1-benzyl-3-butyl- crystallizes in the orthorhombic, Pna21 space group, 3 (Fig-
benzimidazolium iodide (1),[56,57] 1-benzyl-3-isopropyl- ure 2) in the trigonal R3 space group, and 6 (Figure 3) in the
benzimidazolium bromide (2),[57,58] 1-butyl-3-isopropyl- monoclinic P21/c space group. Crystallographic details are
benzimidazolium bromide (3),[56,57] 1-benzyl-3-methyl-4- given in Table 6, and important bond lengths are depicted in
phenyl-1,2,3-triazolium iodide (4),[48] and 3-methyl-1-[2- Tables 1 and 2. The structural parameters are in the expected
(methylthio)phenyl]-4-phenyl-1,2,3-triazolium iodide (6) as range,[58] similar to known examples of related compounds,
carbene precursors. The CuI complexes bis(μ-iodido)bis(1- and hence they do not warrant further comment. The n-butyl
benzyl-3-butylbenzimidazolin-2-ylidene)dicopper(I)
(7), group in 3 is disordered.
bis(μ-iodido)bis(1-benzyl-3-isopropyl-benzimidazolin-2-ylid-
ene)dicopper(I) (8), bis(μ-iodido)bis(1-butyl-3-isopropyl-
benzimidazolin-2-ylidene)dicopper(I) (9), (iodido)(1-benzyl-
3-methyl-4-phenyl-1H-1,2,3-triazol-3-ium-5-ylidene)-
copper(I) (10), and (iodido){3-methyl-1-[2-(methylthio)phen-
yl-4-phenyl-1H-1,2,3-triazol-3-ium-5-ylidene}copper(I)
(11) were synthesized from CuI and the corresponding carb-
ene precursor. All compounds were characterized by NMR
spectroscopy and elemental analyses. X-ray crystal structures
of 1, 3, 6, 7, and 11 are also presented. All the CuI complexes
were tested for their catalytic efficiency in the [3+2] dipolar
cycloaddition reactions between azides and alkynes with spe-
cial emphasis on short reaction times and low catalyst load-
ing under strict click conditions. In order to test the effective-
ness of the catalysts, internal alkynes as well as extremely
bulky azides were used as substrates.
Figure 1. ORTEP plot of 1. Ellipsoids are drawn at 30% prob-
ability. Hydrogen atoms are omitted for clarity.
Results and Discussion
Syntheses of Azolium Salts and X-ray Crystal Structures of
1, 3, and 6
Benzimidazolium salts 1, 2 and 3 (known)[56–58] were pre-
pared in a one-pot reaction of benzimidazole with the step-
wise addition of two different alkyl halides in the presence
of a base by reported procedures.[58,59] The products ob-
tained by this route can be purified by simple extraction and
recrystallization procedures.
Figure 2. ORTEP plot of 3. Ellipsoids are drawn at 30% prob-
ability. Hydrogen atoms are omitted for clarity.
The 1,2,3-triazoles were prepared by starting from the
synthesis of the corresponding azides by standard pro-
cedures. The azides were then converted into the 1,2,3-tri-
azoles by using the CuSO4/ascorbate/TBTA mixture under
click conditions (Scheme 1). These triazoles were then con-
verted into triazolium salts 4 (known)[48] and 6 by using
methyl iodide as an alkylating agent (Scheme 1). Product
1
identity and purity was established with H and 13C NMR
spectroscopy, elemental analyses, and mass spectrometry.
Salt 1 was crystallized from a hot 2-propanol solution by
slow cooling, 3 could be crystallized from a mixture of
Figure 3. ORTEP plot of 6. Ellipsoids are drawn at 50% prob-
dichloromethane/ethyl acetate/petroleum ether (1:10:1) at ability. Hydrogen atoms are omitted for clarity.
Scheme 1. Synthesis of 6.
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Copper(I) Complexes of Normal and Abnormal Carbenes
Table 1. Selected bond lengths in Å.
lem of sketching a localized non-charge separated form of
these ligands. The carbene character of most of the ligands in
these complexes was established by 13C NMR spectroscopy
(Experimental Section and Supporting Information).
1
3
7
C1–N1
C1–N2
C7–N1
C2–N2
C2–C3
C3–C4
C4–C5
C5–C6
C6–C7
Cu–I
1.344(7)
1.324(7)
1.388(6)
1.373(6)
1.395(7)
1.369(8)
1.411(9)
1.389(8)
1.396(7)
–
1.35(2)
1.32(1)
1.39(1)
1.37(1)
1.39(2)
1.33(2)
1.43(2)
1.31(2)
1.38(2)
–
1.362(7)
1.343(8)
1.378(8)
1.404(8)
1.377(10)
1.397(12)
1.396(14)
1.379(12)
1.398(8)
2.565(1)
1.927(6)
Complex 7 could be crystallized by diffusion of diethyl
ether into a dichloromethane solution of the compound at
8 °C, and 11 was crystallized by layering a dichloromethane
solution of it with n-hexane at 8 °C. Complex 7 crystallizes
in the monoclinic, C2/c space group and 11 in the monoclinic
P21/c space group. Complex 7 turned out to be an iodido-
bridged dimer in the solid state (Figure 4). Such an iodido-
bridged dimer has been recently observed for related CuI
complexes with other NHC ligands.[32] The copper centers in
7 are trigonally coordinated through one carbon atom (on
each Cu) from the carbene ligands and the two bridging
iodide ligands. The bond lengths inside the benzimidazolin-
2-ylidene group are in the expected range (Table 2).[58] The
Cu–C1 distance of 1.927(6) Å and the Cu–I distance of
2.565(1) Å are as expected. The N2–C1–Cu and N1–C1–Cu
angles of 125.5(5) and 128.3(5)° around the C1 center show
the sp2 hybridized nature of this carbon atom. The Cu–I–Cu
angle is 68.69(4)°, and the Cu–Cu distance is 2.896(2) Å. In
Cu–C1
–
–
Table 2. Selected bond lengths in Å.
6
11
N1–N2
N2–N3
N3–C1
C1–C2
C2–N1
N1–C3
N1–C15
Cu–C1
Cu–I
1.319(3)
1.323(2)
1.354(3)
1.367(3)
1.362(2)
1.463(3)
–
1.32(2)
1.34(1)
1.35(1)
1.39(2)
1.33(2)
–
1.48(2)
1.89(1)
2.236(2)
–
–
Synthesis of Copper Complexes and Crystal Structures of 7
and 11
Copper complexes 7–11 were all prepared by a general route
starting from CuI and the azolium salts with a base used for
deprotonation (Scheme 2). This route provided the desired
copper complexes in high yield and purity after a simple
recrystallization step (see Experimental Section). In the com-
plexes 7–9 the carbenes are based on benzimidazolin-2-ylid-
ene, and these are normal NHCs. Complexes 10 and 11 have
1,2,3-triazol-5-ylidene-based carbenes, and these have been
designated as “abnormal” carbenes (aNHCs)[48] or more re-
Figure 4. ORTEP plot of 7. Ellipsoids are drawn at 30% prob-
cently as mesoionic carbenes,[53] because of the known prob- ability. Hydrogen atoms are omitted for clarity.
Scheme 2. Synthesis of copper complexes.
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S. Hohloch, C.-Y. Su, B. Sarkar
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contrast to 7, 11 turned out to be a mononuclear complex initial studies, and follow-up work showed the importance of
in the solid state (Figure 5). The CuI center is linearly coordi- the donating nature of ligands for the efficiency of the cata-
nated with a Cu–C1 distance of 1.89(1) Å and a Cu–I dis- lytic system.[16,25,27] Pioneering work by Nolan and Diéz-
tance of 2.236(2) Å. The presence of heavy I atoms in the Gonzaléz showed the efficiency of NHC ligands, which are
crystal of 11 causes severe absorption although cylindrical excellent σ-donors, for this reaction.[30–33,41] We argued that
corrections were applied, resulting in large electron residuals NHCs containing alkyl substituents on the nitrogen atoms
around the Cu and I atoms and high R values in the final should be good ligands for this reaction, because of the bet-
refinement. The dihedral angle between the phenyl ring and ter donor capabilities of the alkyl groups compared to their
the triazol-3-ium-5-ylidene ring is 51(2)°, and that between aryl counterparts. Substituted 1,2,3-triazol-5-ylidenes
the thioether-substituted phenyl ring and triazol-3-ium-5- (aNHCs) should also be good ligands for this reaction, be-
ylidene ring is 69(1)°.
cause of their excellent donor capabilities.[48]
As a test reaction we chose the 1,3-dipolar cycloaddition
reaction between benzyl azide and phenylacetylene
(Scheme 3). According to the click laws, no precaution was
taken to exclude moisture or oxygen.[3] All the reactions were
followed by 1H NMR spectroscopy, by the characteristic sin-
glet of benzyl azide at δ = 4.31 ppm. The resulting triazole
has a signal at 5.55 ppm, and the ratio of these two signals
can be used to calculate the percentage conversion. After
completion of the reactions, the isolated yields were calcu-
lated after a simple aqueous workup. Figure 6 shows a repre-
sentative 1H NMR spectroscopic experiment, the conversion
vs. time graphs are shown in Figure 7, and Table 3 summa-
rizes the reactions tested and conditions used.
Figure 5. ORTEP plot of 11. Ellipsoids are drawn at 50% prob-
ability. Hydrogen atoms are omitted for clarity.
Use of 7–11 as Catalysts for the [3+2] Cycloaddition
between Azides and Alkynes
After the recognition of the CuII/ascorbate mixture as a
catalytic system for the [3+2] cycloaddition between azides
and alkynes and the definition of this process as a click reac-
Scheme 3. General catalytic reaction.
At first a blank test was done with just CuI as a catalyst
tion,[3,4] efforts have been made to find ligands that accelerate under neat conditions. A look at Figure 6 shows that the
this catalytic reaction or make reactions possible, which cycloaddition reaction is rather slow with only 50% conver-
otherwise do not function without an additional ligand.[26–29] sion after 7 hours (entry 1, Table 3). It is also clear from this
The ligand tbta turned out to be a good candidate during the curve that the CuI-catalyzed reaction has a certain induction
1
Figure 6. Conversion of benzyl azide and phenylacetylene as followed by H NMR spectroscopy with 0.25 mol-% 7 as catalyst; *: signal
from benzyl of formed triazole, §: signal from benzyl azide, #: signal from dichloromethane.
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Copper(I) Complexes of Normal and Abnormal Carbenes
Figure 7. Time vs. conversion curves for the tested catalyst in the reaction between benzyl azide and phenylacetylene.
Table 3. Results of catalytic tests between phenylacetylene and
benzyl azide.
We then turned to carbene complexes 7–11. Using
2.5 mol-% 7 as a catalyst under neat conditions at room tem-
perature in air led to nearly 100% conversion within 5 min
with heat evolution (entry 4, Table 3). Encouraged by these
results, we decided to lower the catalyst loading to 0.25 mol-
%. In this case, full conversion was achieved within 13 min
(entry 5, Table 3). Catalysts 8 and 9 showed results compar-
able to 7 under identical reaction conditions, and the reac-
tion was fastest with 9 (entries 7 and 8, Table 3). This can be
rationalized by the better σ-donor ability of the NHC in 9
as compared to those in 7 or 8. Remarkably, no decomposi-
tion of the catalysts or precipitation of metallic copper was
observed during these reactions, showing the robust nature
of these systems and the ability of NHCs to stabilize CuI
centers. We then tried to further lower the catalyst loading
to 0.05 mol-% for catalyst 7 and we were able to achieve full
conversion in about 3 hours. (Figure S4). The aNHC-based
complexes 10 and 11 were then tested as catalysts for the
same reaction. These “abnormal carbenes”[48] or “me-
soionic”[53] compounds are also excellent σ-donors, and
hence one would intuitively except their CuI complexes to be
highly efficient as catalysts for the cycloaddition reactions.
With 0.05 mol-% catalyst loading, 10 and 11 also achieved
full conversion of the click reaction (entry 9 and 10, Table 3).
The reaction time with 7 as a catalyst is between those for
10 and 11, and this shows the comparable activity of these
complexes and hence their high efficiency. Thus, the CuI
complexes with alkyl-substituted NHCs and aNHCs show
comparable catalytic activity owing to the excellent σ-donor
properties of these ligands (Figure S5).
Entry Catalyst
Solvent
Time Yields[a]
1
2
3
4
5
6
7
8
CuI[b]
–
H2O
–
–
–
–
–
–
7 h
7 h
7 h
48.5%*; 42%#
CuI[b]
55.5%*; 51%#
67%*; 61%#
12[b]
7[b]
5 min 100%*; 99%#
13 min 100*; 99%#
7[c]
7[d]
3 h
100%*; 99%#
8[c]
13 min 100%*; 99%#
9[c]
10 min 100%*; 98%#
9
10[d]
–
–
4 h
2 h
25 h
100%*; 98%#
100%*; 98%#
88%*; 86%#
10
11
12
13
11[d]
8[c]
DCM/H2O/tBuOH
–
CuI/KOtBu/1[b]
3 min 100%*; 99%#
CuI/KOtBu/1[b] DCM/H2O/tBuOH
17 h
94%*; 90%#
[a] *Conversions by NMR spectroscopy; #Isolated yields, deter-
mined over at least two runs. [b] Catalyst: 2.5 mol-%. [c] Catalyst:
0.25 mol-%. [d] Catalyst: 0.05 mol-%.
period. This observation led us to speculate that the product
formed (in minute amounts) in this reaction might form a
complex (12, Figure 8) with CuI, which actually catalyzes the
reaction. To prove this 2.5 mol-% CuI and 1-(benzyl)-4-
(phenyl)-1,2,3-triazole were used as a catalyst mixture, and
this reaction showed a conversion of 70% after 7 hours (en-
try 3, Table 3). This also led to a much reduced induction
period, as expected. Addition of water to the CuI catalyst
also led to increased reaction rates, as would be expected for
a cycloaddition reaction (entry 2, Table 3).
Since the click reaction is highly exothermic, carrying out
such reactions under neat conditions have some obvious dis-
advantages,[34] despite the high yields and short reaction
times. In order to address this problem, we also tested the
catalytic activity of 8 (0.25 mol-%) in a dichloromethane/tert-
BuOH/water (1:2:1) mixture at room temperature: 85% con-
Figure 8. Probable complex formed during catalysis with CuI.
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S. Hohloch, C.-Y. Su, B. Sarkar
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version was achieved under those conditions in 25 hours, the catalyst within 24 hours. We also tried to decrease the
demonstrating the efficiency of the catalyst even in those sol- catalyst loading to 0.25 mol-%. This worked as well, and a
vent mixtures (entry 11, Table 3). Isolation of the catalyst is similar conversion as mentioned above was realized after a
sometimes a problem and in any case is an additional reac- reaction time of 96 hours at 80 °C (Table 5).
tion step. Hence, in some cases in situ formation of the cata-
Table 5. Tested conditions for the conversion of internal alkynes
with 7.
lyst is preferred from simple metal salts and ligand precur-
sors. In order to test this, a mixture of 1/CuI/KOtBu was
Entry
Catalyst loading
Time
Conversion[a]
used under neat conditions at room temperature, and this
lead to a full conversion within 3 min (entry 12, Table 3). The
same mixture was also used in the dichloromethane/tBuOH/
water (1:2:1) mixture, and in that case 94% conversion was
achieved in 17 hours (entry 13, Table 3), thus showing that
the system is efficient even when the catalyst is generated in
situ in a solvent mixture.
1
2
0.5 mol-%
5 mol-%
96 h
24 h
86.1%
80%
[a] Conversion determined by NMR spectroscopy.
Finally, we wanted to test whether our catalysts are cap-
able of catalyzing the click reaction with a “super” bulky
azide such as 2,6-(dimesityl)-phenyl azide. When we used the
popular CuII/ascorbate/tbta mixture as the catalytic system,
we did not observe any conversion of the aforementioned
azide in the cycloaddition reaction. Gratifyingly, the use of
0.25 mol-% of 7 at 100 °C as catalyst was enough to success-
fully convert a mixture of 2,6-(dimesityl)phenyl azide and
pyridylacetylene to the corresponding 1,2,3-triazole within 2
hours. (Scheme 4). Ligands with such steric bulk might facili-
tate the use of appropriate metal complexes of these ligands
for other catalytic processes such as selective polymerization
of olefins.
We were also interested in testing whether the catalysts are
capable of undergoing multiple catalytic cycles. In order to
check this we first reacted 7 (0.25 mol-%) with 1 mmol each
of the substrates to realize the first full conversion within
13 min. Subsequently, the same amounts of the substrates
were added to this mixture every 25 min, and this was re-
1
peated five times. The changes were followed by H NMR
spectroscopy. An overview of the results is depicted in
Table 4.
Table 4. Conversion after several catalytic cycles with 7.
Cycle [min]
Yields[a]
1 [15]
2 [15]
3 [15]
4 [15]
5 [15]
5 [60]
100%
83%
68%
60%
50%
90%
[a] Determined by NMR spectroscopy.
Scheme 4. The “click” reaction with bulky azides.
As can be seen in Table 4, the activity of the catalyst seems
to decrease starting from the second cycle itself. However,
this probably does not have to do with real catalyst deactiva-
tion, but more with precipitation of the solid products that
Conclusions
We have reported on azolium salts as precursors for syn-
are insoluble in the reaction mixture. Small amounts of the thesizing iodido-bridged dicopper(I) complexes, which then
catalyst probably get embedded on the solid surface, which contain either substituted benzimidazolin-2-ylidenes or sub-
leads to an apparent catalyst deactivation. On stirring the stituted 1,2,3-triazol-5-ylidenes as carbene ligands. Structural
mixture for a longer time (last entry, Table 4) the activity of characterization of 7 shows trigonally coordinated di-CuI
the catalyst increases again, and this would support our centers, which are bridged by iodido ligands. In contrast, 11
theory of apparent catalyst deactivation mentioned above.
shows a linearly coordinated mononuclear CuI center. The
Internal alkynes are considered to be difficult substrates CuI–carbene complexes have been shown to be highly ef-
for the click reaction, because the formation of the Cu–acet- ficient catalysts for the Huisgen [3+2] cycloaddition reaction
ylide complex, critical for the catalytic cycle, is clearly favored between azides and alkynes. Complexes of both the alkyl-
for terminal alkynes. However, it has been recently shown substituted benzimidazolin-2-ylidenes as well as the substi-
that CuI-carbene complexes are capable of catalyzing the tuted 1,2,3-triazol-5-ylidenes are efficient catalysts for the
click reaction even with internal alkynes, and this has been “click” reaction owing to the strong σ-donating power of
attributed to the superb donating power of the NHC ligands these carbene ligands. Very low catalyst loading as well as
and the flexibility of the (NHC)Cu–X (X = halide) frame- short reaction times were sufficient for these reactions. The
work that facilitates their binding to internal alkynes.[30] We catalysts were also shown to be effective for the cycloaddition
wanted to test whether our copper catalysts are capable of reaction of diphenylacetylene, which is an internal alkyne.
performing such transformations as well. Complex 7 was Furthermore, an extremely bulky azide could also be used as
used as catalyst for the cycloaddition reaction between di- a substrate for the cycloaddition reaction with our catalysts.
phenylacetylene and benzyl azide. The reaction, carried out Future efforts will be dedicated to further lowering catalyst
at 80 °C, resulted in a conversion of 80% with 5 mol-% of loadings in order to increase the efficiency of this process. In
3072
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 3067–3075

Copper(I) Complexes of Normal and Abnormal Carbenes
General Procedure for the Copper Complexes: A suspension of the
azolium salt (1 equiv., 0.0002 mol) and copper(I) iodide (1 equiv.,
0.0002 mol, 0.038 g) in dichloromethane was cooled to –78 °C. Af-
terwards, potassium tert-butoxide (1.5 equiv., 0.0003 mol, 0.033 g)
dissolved in dichloromethane (5 mL) was slowly added to the solu-
tion, and the solution was stirred at –78 °C overnight. The next day,
the solution was filtered through a pad of Celite, and the solvent
was evaporated. The white solid was recrystallized from dichloro-
methane/hexane (1:10) to afford colorless crystals.
addition, we would also explore the use of the reported “su-
per” bulky 1,2,3-triazole as a ligand in other catalytic pro-
cesses.
Experimental Section
General Considerations: All commercially available reagents were
used as received. Solvents were dried and distilled according to stan-
dard procedures. Syntheses of the organic compounds were carried
out in air, and the copper complexes were synthesized under an ar-
gon atmosphere. Salts 1, 2, 3, and 4 were synthesized according to
literature procedures.[48,56–58] Azide 13 was synthesized by following
a literature report.[60,61]
Bis(μ-iodido)bis(1-benzyl-3-butylbenzimidazolin-2-ylidene)dicopper(I)
(7): The product was gained from copper(I) iodide (1 equiv.,
0.0002 mol, 0.038 g), azolium salt 1 (1 equiv., 0.0002 mol, 0.078 g),
and KOtBu (1.5 equiv., 0.0003 mol, 0.033 g) in dichloromethane
(20 mL, c = 0.01 molL–1) under the general reaction conditions. The
product was obtained as colorless crystals in a yield of 80% (0.036 g,
1
0.00008 mol). H NMR (250 MHz, CDCl3; 25 °C, TMS): δ = 7.77–
1
Instrumentation: The H and 13C{1H} NMR spectra were recorded
7.60 (m, 2 H, Aryl-H), 7.52–7.46 (m, 2 H, Aryl-H), 7.39–7.26 (m,
3
with a Bruker AC 250 spectrometer. Elemental analyses were per-
formed with a Perkin–Elmer Analyzer 240. Mass spectrometry ex-
periments were carried out with a Bruker Daltronics Mictrotof-Q
mass spectrometer.
14 H, Aryl-H), 5.69 (s, 4 H, NCH2), 4.51 [t, J(H,H) = 7.25 Hz, 4
H, NCH2], 1.97 [p, 3J(H,H) = 7.5 Hz, 4 H, CH2], 1.43 [sex, 3J(H,H)
3
= 7.25 Hz, 4 H, CH2], 0.99 [t, J(H,H) = 7.5 Hz, 6 H, CH3] ppm.
MS (ESI): m/z = 591.2541 [(C18H20N2)2Cu]+. C36H40Cu2I2N4
(909.64): calcd. C 47.53, H 4.43, N 6.16; found C 47.14, H 4.27, N
6.51.
Syntheses
1-[2-(Methylthio)phenyl]-4-phenyl-1,2,3-triazole (5): (2-Azidophen-
yl)(methyl)sulfane {prepared from 2-(mercaptomethyl)aniline ac-
cording to a literature procedure} (1 equiv., 1.86 g, 0.0113 mol) and
phenylacetylene (1 equiv., 1.15 g, 0.0113 mol, 1.23 mL) were dis-
solved in a mixture of dichloromethane/water/tert-butyl alcohol
(14 mL:28 mL:14 mL). Then copper sulfate (0.05 equiv., 0.14 g,
0.00056 mol), sodium ascorbate (0.2 equiv., 0.437 g, 0.00226 mol),
and tbta (0.01 equiv., 0.06 g, 0.00011 mol) were added to the reaction
mixture, and the reaction mixture was heated to reflux overnight at
55 °C. After heating, water (50 mL) was added, and the product was
extracted with dichloromethane (4ϫ20 mL). The combined organic
phases were dried with sodium sulfate, filtered, and the solvent was
evaporated. The crude product was then purified by flash column
chromatography over silica gel by using a gradient from 0 to 10%
methanol in dichloromethane. The product was obtained as a brown
oil in a yield of 95% (2.85 g, 0.0106 mol). 1H NMR (250 MHz,
CDCl3; 25 °C, TMS): δ = 8.12 (s, 1 H, Triazol-5H), 7.92–7.88 (m, 2
H, Aryl-H), 7.50–7.29 (m, 7 H, Aryl-H), 2.37 (s, 3 H, CH3) ppm.
13C NMR (60 MHz, CDCl3, 25 °C, TMS): δ = 147.6, 135.6, 135.1,
130.2, 128.8, 126.8, 126.0, 125.9, 121.6, 120.0 (all Aryl-C), 16.0
(CH3) ppm.
Bis(μ-iodido)(1-benzyl-3-isopropyl-benzimidazolin-2-ylidene)dicopper-
(I) (8): Compound 8 was prepared by following the general pro-
cedure from copper(I) iodide (1 equiv., 0.0002 mol, 0.038 g), azolium
salt 2 (1 equiv., 0.066 g, 0.0002 mol), and KOtBu (1.5 equiv.,
0.0003 mol, 0.033 g) in dichloromethane (20 mL, c = 0.01 molL–1).
The product was obtained as a white solid in a yield of 80% (0.036 g,
1
0.00008 mol). H NMR (250 MHz, CDCl3; 25 °C, TMS): δ = 7.62–
7.58 (m, 1 H, Aryl-H), 7.42–7.27 (m, 8 H, Aryl-H), 5.70 (s, 2 H,
NCH2), 5.10 [hept, 3J(H,H) = 7 Hz, 1 H, NCH], 1.81 [d, 3J(H,H) =
7 Hz, 6 H, CH3] ppm. C34H36Cu2I2N4 (881.59): calcd. C 46.32, H
4.12, N 6.36; found C 46.11, H 4.19, N 6.20.
Bis(μ-iodido)bis(1-butyl-3-isopropylbenzimidazolin-2-ylidene)dicop-
per(I) (9): Compound 9 was prepared by following the general pro-
cedure from copper(I) iodide (1 equiv., 0.0002 mol, 0.038 g), azolium
salt 3 (1 equiv., 0.060 g, 0.0002 mol), and KOtBu (1.5 equiv.,
0.0003 mol, 0.033 g) in dichloromethane (20 mL, c = 0.01 molL–1).
The product was obtained as a white solid in a yield of 80% (0.033 g,
1
0.00008 mol). H NMR (250 MHz, CDCl3; 25 °C, TMS): δ = 7.84–
7.69 (m, 1 H, Aryl-H), 7.61–7.57 (m, 1 H, Aryl-H), 7.52–7.48 (m, 1
3
H, Aryl-H), 7.41–7.37 (m, 1 H, Aryl-H), 5.02 [hept., J(H,H) =
3
6.75 Hz, 1 H, NCH], 4.48 [t, J(H,H) = 7.25 Hz, 3 H, NCH2], 1.96
3-Methyl-1-[2-(methylthio)phenyl]-4-phenyl-1,2,3-triazolium
Iodide
3
3
[p, J(H,H) = 7.25 Hz, 2 H, CH2], 1.78 [d, J(H,H) = 6.75 Hz, 6 H,
(6): 1-[2-(Methylthio)phenyl]-4-phenyl-1,2,3-triazole (5) (1 equiv.,
0.723 g, 0.0027 mol) was dissolved in acetonitrile (5 mL, c =
0.5 molL–1), and methyl iodide (10 equiv., 3.83 g, 0.027 mol, 1.7 mL)
was added. Then the mixture was heated to reflux for 24 h. The
solvent was removed under vacuum, and the residue was dissolved
in dichloromethane (5 mL) and precipitated by the addition of ethyl
ether (50 mL). The yellow precipitate was collected by filtration and
recrystallized from dichloromethane/diethyl ether (2:1). The product
was obtained in a yield of 55% (0.607 g,0.00149 mol). 1H NMR
(250 MHz, CDCl3; 25 °C, TMS): δ = 8.61 (s, 1 H, Triazol-5H), 8.39
(m, 1 H, Aryl-H), 7.96–7.92 (m, 2 H, Aryl-H), 7.66–7.58 (m, 4 H,
Aryl-H), 7.49–7.41 (m, 2 H, Aryl-H), 4.44 (s, 3 H, NCH3), 2.54 (s,
3 H, SCH3) ppm. 13C NMR (60 MHz, CDCl3, 25 °C, TMS): δ =
143.6, 134.4, 133.1, 132.7, 132.1, 130.3, 129.6, 129.0, 127.8, 127.1,
CH3], 1.46 [sex, J(H,H) = 7.5 Hz, 2 H, CH2], 0.99 [t, 3J(H,H) =
3
7.5 Hz, 3 H, CH3] ppm. 13C NMR (100 MHz, CDCl3; 25 °C, TMS):
δ = 186.1 (Carbene-C), 134.0, 132.8, 130.9, 126.8, 123.5, 123.2, 120.2,
113.2, 111.9, 111.2 (All Aryl-C), 53.9 (NCH2), 52.8 (NCH2), 32.3,
22.8 (all Butyl-C), 20.2 (CHCH3), 13.6 (CH3) ppm. C38H40Cu2I2N4
(933.66): calcd. C 41.34, H 4.96, N 6.89; found C 41.33 H 5.12 N
7.05.
(1-Benzyl-3-methyl-4-phenyl-1H-1,2,3-triazol-3-ium-5-ylidene)(iodido)-
copper(I) (10): Compound 10 was prepared by following the general
procedure from copper(I) iodide (1 equiv., 0.0001 mol, 0.019 g),
azolium salt 4 (1 equiv., 0.038 g, 0.0001 mol), and KOtBu
(1.5 equiv., 0.00015 mol, 0.016 g) in dichloromethane (10 mL, c =
0.01 molL–1). The product was obtained as a white solid in a yield
121.5 (all Aryl-C), 39.7 (NCH3), 16.4 (SCH3) ppm. MS (ESI): m/z of 60% (0.026 g, 0.00006 mol). 1H NMR (250 MHz, CDCl3; 25 °C,
= 282.1052 [C16H16N3S]+. C16H16IN3S (409.29): calcd. C 46.95, H
3.94, N 10.27; found. C 46.77, H 3.94, N 10.32.
TMS): δ = 7.65–7.53 (m, 7 H, Aryl-H), 7.45–7.39 (m, 3 H, Aryl-
H), 5.68 (s, 2 H, NCH2), 4.09 (s, 3 H, NCH3) ppm. 13C NMR
Eur. J. Inorg. Chem. 2011, 3067–3075
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3073

S. Hohloch, C.-Y. Su, B. Sarkar
FULL PAPER
(100 MHz, CD2Cl2; 25 °C, TMS): δ = 165.0 (Carbene-C), 148.7,
134.5, 131.9, 130.0, 129.9, 129.8, 129.7, 129.4, 129.0, 129.0, 128.9,
128.9, 127.4 (all Aryl-C), 59.1 (NCH2), 37.9 (NCH3) ppm.
C16H15CuIN3 (439.77): calcd. C 43.70, H 3.44, N 9.56; found C
43.33 H 3.35 N 9.68.
para), 20.6 (CH3 ortho, meta) ppm. C31H30N4 (458.60): calcd. C
81.19, H 6.59, N 12.22; found C 79.44, H 6.64, N 11.34.
General Procedures for Catalysis: Phenylacetylene (1 equiv.,
0.101 g, 0.001 mol, 0.11 mL) or diphenylacetylene (1 equiv.,
0.178 g, 0.001 mol) and benzyl azide (1equiv., 0.133 g, 0.001 mol)
were mixed, and the corresponding catalyst (2.5–0.05 mol-% in the
case of phenylacetylene; 5–0.25 mol-% in the case of diphenylacety-
lene) was added. The reaction mixture was stirred at room tempera-
ture in the case of phenylacetylene or warmed to 80 °C in the case
of diphenylacetylene. The reaction was monitored by 1H NMR
spectroscopy. After completion or a certain period of time, the re-
action was quenched by dissolving the reaction mixture in dichloro-
methane (10 mL). After quenching, a simple aqueous workup was
carried out, and the white solids were dried under high vacuum for
several hours to gain the isolated yields.
(Iodido){3-methyl-1-[2-(methylthio)phenyl]-4-phenyl-1H-1,2,3-tri-
azol-3-ium-5-ylidene}copper(I) (11): Compound 11 was prepared by
following the general procedure from copper(I) iodide (1 equiv.,
0.0001 mol, 0.019 g), azolium salt 6 (1 equiv., 0.041 g, 0.0001 mol),
and KOtBu (1.5 equiv., 0.00015 mol, 0.016 g) in dichloromethane
(10 mL, c = 0.01 molL–1). The product was obtained as a white
solid in a yield of 75% (0.035 g, 0.000075 mol). 1H NMR
(250 MHz, CDCl3; 25 °C, TMS): δ = 7.72–7.65 (m, 2 H, Aryl-H),
7.60–7.54 (m, 5 H, Aryl-H), 7.46–7.33 (m, 2 H, Aryl-H), 4.22 (s, 3
H, NCH3), 2.47 (s, 3 H, SCH3) ppm. 13C NMR (100 MHz,
CD2Cl2; 25 °C, TMS): δ = 167.1 (Carbene-C), 148.2, 137.5, 134.8,
130.8, 130.1, 130.0, 129.8, 129.5, 129.1, 127.3, 127.2, 126.8, 125.7
(all Aryl-C), 37.8 (NCH3), 16.2 (SCH3) ppm. C16H15CuIN3S
(471.83): calcd. C 40.73, H 3.20, N 8.91; found C 40.90, H 3.46, N
8.79.
X-ray Crystallography: Compound 1 was crystallized from a solu-
tion in hot 2-propanol by slow cooling, 3 could be crystallized from
a mixture of dichloromethane/ethyl acetate/petroleum ether (1:10:1)
at –20 °C, and 6 was crystallized by layering a dichloromethane
solution of it with ethyl ether at 8 °C. Complex 7 could be crys-
tallized by diffusion of diethyl ether into a dichloromethane solu-
tion of the compound at 8 °C, and 11 was crystallized by layering
a dichloromethane solution of it with n-hexane at 8 °C. Data collec-
tion was performed by using an OXFORD XCALIBUR-S CCD,
or a Kappa CCD, or a Stoe IPDS II, or a Bruker Kappa Apex 2
duo diffractometer. The measurements were carried out at 173, or
100, or 133 K by using a Mo-Kα radiation (graphite monochroma-
tor). The structures were solved and refined by full-matrix least-
1-(2,6-Dimesityl)phenyl-4-(2-pyridyl)-1,2,3-triazole (14): Terphenyl-
azide 13 (1 equiv., 0.0005 mol, 0.178 g) was mixed with 2-ethinylpy-
ridine at room temperature, and 7 (0.25 mol-%) was added to the
reaction vessel. The mixture was then capped and heated under
vigorous stirring to 100 °C for 2 h. After cooling to room tempera-
ture, the solids were dissolved in dichloromethane and purified by
silica gel column chromatography by using an eluent of 0 to 10%
acetone in dichloromethane. The product was gained as a yellow
fluffy solid in a yield of 95% (0.220 g,0.00048 mol). 1H NMR
(250 MHz, CDCl3; 25 °C, TMS): δ = 8.44–8.42 (m, 1 H, Aryl-H), squares techniques on F2 by using the SHELX-97 program.[62] The
7.93–7.89 (m, 1 H, Aryl-H), 7.66–7.57 (m, 3 H, Aryl-H), 7.28 (m,
2 H, Aryl-H), 7.11–7.03 (m, 1 H, Aryl-H), 6.73 (s, 4 H, Aryl-H),
2.14 (s, 6 H, CH3 para), 2.05 (s, 12 H, CH3 meta, ortho) ppm. 13C
n-butyl group in 3 is disordered (Table 6). CCDC-790995,
-790996, -790997, 804974, and 819819 contain the supplementary
crystallographic data for this paper. These data can be obtained
NMR (60 MHz, CDCl3, 25 °C, TMS): δ = 148.8, 148.3, 139.2, free of charge from The Cambridge Crystallographic Data Centre
137.2, 135.7, 135.7, 133.9, 130.1, 127.9 (all Aryl-C), 20.9 (CH3,
via www.ccdc.cam.ac.uk/data_request/cif.
Table 6. Crystallographic details.
1
3
6
7
11
Chemical formula
Mr
Crystal system
Space group
a (Å)
b (Å)
c (Å)
C18H20IN2
392.17
orthorhombic
Pna21
12.7971(3)
10.5105(5)
8.0033(2)
90.00
C14H21BrN2
297.24
trigonal
R3
20.83(1)
20.83(1)
8.72(1)
90.00
C16H16N3SI
409.28
monoclinic
P21/c
10.5679(4)
15.6146(6)
11.3519(4)
90.00
C36H30Cu2I2N4
899.52
monoclinic
C2/c
24.7422(10)
8.9604(5)
19.0640(6)
90.00
C16H15CuIN3S
471.81
monoclinic
P21/c
10.4241(6)
16.3560(9)
10.7096(6)
90.00
α (°)
β (°)
90.00
90.00
1690.99(8)
4
1.537
90.00
117.583(3)
90.00
1660.31(11)
4
1.637
808
124.827(2)
90.00
3469.4(3)
4
1.722
1752
115.3370(10)
90.00
1650.30(16)
4
1.899
920
γ (°)
120.00
3278(3)
9
1.355
1386
V (Å)3
Z
Density (gcm–3)
F(000)
780
Radiation type
μ (mm–1)
Crystal size (mm)
Measured refl.
Independent refl.
Mo-Kα
1.890
0.28ϫ0.23ϫ0.18
7818
Mo-Kα
2.804
0.25ϫ0.22ϫ0.16
7048
2446
1893
Mo-Kα
2.051
0.5ϫ0.49ϫ0.47
23362
3517
3479
Mo-Kα
3.036
0.26ϫ0.23ϫ0.16
6896
4045
3194
Mo-Kα
3.319
0.28ϫ0.18ϫ0.14
12524
3221
2927
Obsd. [IϾ2σ(I)] refl. 2590
2853
Rint
0.022
0.029
0.070
0.081
0.078
0.200
0.0195
0.0199
0.0439
0.033
0.057
0.1461
0.1485
0.1577
0.3563
R [F2 Ͼ 2σ(F2)]
wR(F2)
S
1.110
1.638; –0.841
1.095
0.622; –0.864
1.245
0.5009; 0.3805
1.069
1.135; –1.931
1.070
5.224; –5.060
Δρmax; Δρmin (eÅ–3)
3074
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 3067–3075

Copper(I) Complexes of Normal and Abnormal Carbenes
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Received: April 5, 2011
Published Online: June 1, 2011
Eur. J. Inorg. Chem. 2011, 3067–3075
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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