Activating azides and alkynes for the click reaction with [Cu(aNHC) 2I] or [Cu(aNHC)2]+ (aNHC = triazole-derived abnormal carbenes): Structural characterization and catalytic properties

Article abstract of DOI:10.1002/ejic.201300150

Neutral, iodido-containing copper(I) complexes [Cu(aNHC)₂I] {aNHC = 1-benzyl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene (for 6) and 3-methyl-1-[2-(methylthio)phenyl]-4-phenyl-1,2,3-triazol-5-ylidene (for 7)} and cationic, halide-free copper(I) c

Full text of DOI:10.1002/ejic.201300150

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Abnormal-Carbene Ligands
S. Hohloch, D. Scheiffele,
B. Sarkar* ...................................... 1–11
Activating Azides and Alkynes for the
Click Reaction with [Cu(aNHC)2I] or
[Cu(aNHC)2]+ (aNHC = Triazole-Derived
Abnormal Carbenes): Structural Charac-
terization and Catalytic Properties
Keywords: Copper / Abnormal carbenes /
Triazolylidenes / Cycloaddition / Homo-
geneous catalysis / 1,2,3-Triazoles
Cationic copper(I) complexes containing
two triazolylidene carbenes of the abnor-
mal type are shown to be excellent catalysts
for the [3+2] cycloaddition reaction be-
tween azides and alkynes. A range of azides
and a couple of alkynes are activated by
these copper(I) complexes towards the
catalytic formation of the corresponding
substituted 1,2,3-triazoles.
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DOI:10.1002/ejic.201300150
Activating Azides and Alkynes for the Click Reaction with
[Cu(aNHC)2I] or [Cu(aNHC)2]+ (aNHC = Triazole-
Derived Abnormal Carbenes): Structural Characterization
and Catalytic Properties
Stephan Hohloch,[a] Damaris Scheiffele,[b] and Biprajit Sarkar*[a]
Keywords: Copper / Abnormal carbenes / Triazolylidenes / Cycloaddition / Homogeneous catalysis / 1,2,3-Triazoles
Neutral, iodido-containing copper(I) complexes [Cu(aNHC)2I]
{aNHC = 1-benzyl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene
(for 6) and 3-methyl-1-[2-(methylthio)phenyl]-4-phenyl-
1,2,3-triazol-5-ylidene (for 7)} and cationic, halide-free cop-
per(I) complexes [Cu(aNHC)2](BF4) {aNHC = 1-benzyl-3-
methyl-4-phenyl-1,2,3-triazol-5-ylidene (for 8), 3-methyl-1,4-
diphenyl-1,2,3-triazol-5-ylidene (for 9), 3-methyl-1-[2-(meth-
ylthio)phenyl]-4-phenyl-1,2,3-triazol-5-ylidene (for 10), and
1-mesityl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene (for 11)},
both containing two monodentate abnormal-carbene ligands
(aNHC), were synthesized from [Cu(CH3CN)4](BF4) and the
corresponding triazolium salts. It was possible to selectively
synthesize both kinds of complexes by simply variing the
counter-anion of the triazolium salts and keeping the metal
precursor the same. All complexes were characterized by el-
emental analysis and spectroscopic methods. 6 and 11 were
studied with single-crystal X-ray diffraction analyses. In 6,
the copper(I) center is tricoordinated, and its geometry is in
between trigonal planar and T-shaped. In halide-free 11, the
copper(I) center is linearly coordinated by two abnormal-
carbene ligands. All complexes were tested as catalysts in
the Huisgen [3+2] cycloaddition reaction between azides and
alkynes, and they showed excellent efficiencies under neat
conditions. A comparison between the efficiencies of the hal-
ide-containing complexes 6 and 7 and the halide-free cases
8–11, shows that the halide-free Cu–aNHC complexes are
significantly more efficient than their halide-containing
counterparts. The best catalyst was used for a substrate
screening by utilizing a variety of azides and a couple of al-
kynes. The efficiency of the catalyst was maintained with
loadings as low as 0.005 mol-%. Mechanistic studies were
carried out as well.
cessibility through the Cu(I)-catalyzed click [3+2] cycload-
dition reaction between azides and alkynes (CuAAC).[5] Li-
gands synthesized through the click reaction have found ex-
tensive use in coordination chemistry in recent years.[6]
Their corresponding metal complexes were studied for their
electron transfer[7] and magnetic properties[8] and used as
metallosupramolecular assemblies[9] and as homogeneous
catalysts.[10] Triazolylidene-based aNHC ligands have been
postulated to surpass even their normal NHC counterparts
in their σ-donor capability,[4a,4f–4h] a property that makes
their metal complexes attractive for homogeneous cataly-
sis.[11]
Cu(I) complexes of NHC ligands are potent catalysts for
the click cycloaddition reaction between azides and alk-
ynes.[12] Furthermore, the addition of additional nitrogen
donor ligands to the Cu(I)–NHC complexes increases their
catalytic activity.[13] In recent years, we[14a] and others[14b]
have shown that copper(I) complexes with triazolylidene-
based aNHC ligands of the type (aNHC)Cu–X (X = halide)
are excellent catalysts for the CuAAC. These catalysts are
also able to catalyze cycloaddition reactions with bulky az-
ides or internal alkynes as substrates, which cannot be cata-
lyzed with the original click recipe of CuSO4 and ascorbate.
We have also extended our work in this field to study the
Introduction
N-heterocyclic carbenes (NHCs) have come to occupy a
prominent place in the toolbox of chemists working in the
organometallic and catalysis field. Since the seminal discov-
ery of the first stable, crystalline NHC by Arduengo,[1] these
classes of ligands have been used in various fields of chemis-
try, and catalysis is the most prominent example of their
application.[2] In recent years, redox-active NHCs have been
reported.[3] Another new development in this field has been
the introduction of new kinds of carbene ligands, which
have been variously named, for example, abnormal N-het-
erocyclic carbenes (aNHCs) or mesoionic carbenes
(MIC).[4] An emerging class of aNHC ligands are those
based on triazolylidenes.[4a,4f–4h] One reason for the current
popularity of this class of aNHC ligands is their easy ac-
[a] Institut für Chemie und Biochemie, Anorganische Chemie,
Freie Universität Berlin,
Fabeckstraße 34–36, 14195, Berlin, Germany
E-mail: biprajit.sarkar@fu-berlin.de,
Homepage: http://www.bcp.fu-berlin.de/en/chemie/ac/agsarkar/
index.html
[b] Institut für Anorganische Chemie, Universität Stuttgart,
Pfaffenwaldring 55, 70569 Stuttgart, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300150.
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effect of additional donor ligands on the catalytic efficiency
of (aNHC)CuI complexes.[14c] As part of our continuing
interest in developing the coordination chemistry of various
click-derived ligands[6d] and in finding applications of the
corresponding metal complexes,[7d,7e,8c,10] we here present
new types of copper(I) complexes with triazolylidene li-
gands. Neutral complexes of the form [Cu(aNHC)2I]
{aNHC = 1-benzyl-3-methyl-4-phenyl-1,2,3-triazol-5-ylid-
ene (for 6) and 3-methyl-1-[2-(methylthio)phenyl]-4-phenyl-
1,2,3-triazol-5-ylidene (for 7)} and cationic complexes of
the form [Cu(aNHC)2](BF4) {aNHC = 1-benzyl-3-methyl-
4-phenyl-1,2,3-triazol-5-ylidene (for 8), 3-methyl-1,4-di-
phenyl-1,2,3-triazol-5-ylidene (for 9), 3-methyl-1-[2-(meth-
ylthio)phenyl]-4-phenyl-1,2,3-triazol-5-ylidene (for 10), and
1-mesityl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene
(for
11)} are presented below. Structural characterizations are
presented to illustrate the effect of an additional halido li-
gand on the coordination geometry of the copper(I) com-
plexes. All complexes were tested for their efficiency in the
CuAAC. We particularly focused on the comparison be-
tween the halide-containing complexes 6 and 7 and their
halide-free counterparts 8–11, with respect to their catalytic
potency. The best catalyst was used for screening a range of
substrates. Evidence from 1H NMR spectroscopy is used to
shed light on the mechanism of the azide and alkyne acti-
vation and on the CuAAC with this class of catalysts in
general.
Scheme 1. Synthetic route for the reported copper(I) complexes.
1
The complexes were characterized by H and 13C NMR
spectroscopy, mass spectrometry, and elemental analysis. In
1
the H NMR spectra of the triazolium salts, the signal of
the triazolium-ring C–H proton is shifted to low field.
Upon the formation of the Cu(I) complexes, this signal dis-
appears, which is a first indication of the binding of the
triazolylidene ligands through the carbene C atom. In the
ESI mass spectrum of each of the complexes 6–11, the mo-
lecular peak corresponding to the respective monocation
was observed. In the 13C NMR spectra of the complexes,
the signal of the carbene C atom appears in the range 162–
167 ppm. Signals in this range were also observed for the
carbene C atom for the monocarbene complexes of the type
[(aNHC)CuI], which we reported recently.[14a]
Results and Discussion
Synthesis and Characterization
The substituted 1,2,3-triazoles that were used in this
work to generate the triazolylidene ligands were all pre-
pared by literature methods.[7d,7e,14a,14b] The corresponding
triazolium salts with iodide as a counter-anion are also
Crystal Structures
–
known.[14] The triazolium salts with BF4 as the counter-
Single crystals of suitable quality could be obtained for
the newly synthesized triazolium salts 3-methyl-1-[2-(meth-
ylthio)phenyl]-4-phenyl-1,2,3-triazolium tetrafluoroborate
(4) and 1-mesityl-3-methyl-4-phenyl-1,2,3-triazolium tetra-
fluoroborate (5). 4 crystallizes in the monoclinic P21/n
space group and 5 crystallizes in the monoclinic P21/c space
group (Table 6). The bond lengths within both triazolium
salts are in a range similar to those of related triazolium
salts reported recently (Figures S1 and S2 and Table 1).[14a]
anion were prepared by reacting the corresponding 1,2,3-
triazoles with the Meerwein salt (see Exp. Sect.).
The copper(I) complexes with two triazolylidene ligands
bound to the copper center were synthesized by reacting
two equivalents of the triazolium salts with one equivalent
of [Cu(CH3CN)4](BF4) in the presence of excess KOtBu
(Scheme 1 and Exp. Sect.). Both the halide-containing and
the halide-free bistriazolylidene complexes could be selec-
tively synthesized by choosing the nature of the triazolium
salt. Thus, the use of iodide-containing triazolium salts led
to the selective formation of the iodide-containing com-
plexes 6 and 7. On the contrary, the use of the tetra-
fluoroborate-containing triazolium salts selectively led to
the isolation of the halide-free complexes 8–11. We note
here that the copper(I) precursor [Cu(CH3CN)4]-
(BF4) is the same for both sets of reactions (Scheme 1).
Thus, the synthetic method reported here allows us to selec-
tively synthesize the halide-containing and the halide-free
bistriazolylidene complexes in very good yields by simply
choosing the appropriate counter-anion of the triazolium
salts.
Table 1. Selected bond lengths [Å].
Atoms
4
5
6
11
C1–C2
1.374(4)
1.364(4)
1.321(4)
1.326(3)
1.348(4)
1.463(4)
–
1.371(3)
1.362(2)
1.319(2)
1.327(2)
1.350(2)
1.463(2)
–
1.405(4)
1.372(4)
1.324(3)
1.328(4)
1.367(4)
1.466(4)
1.935(3)
1.937(3)
2.810(1)
1.386(5)
1.374(4)
1.320(4)
1.337(4)
1.370(4)
1.467(4)
1.899(3)
1.895(3)
–
C2–N3
N3–N2
N2–N1
N1–C1
N3–C18
C1–Cu1
C1A–Cu1
Cu1–I1
–
–
–
–
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We could crystallize the halide-containing complex Table 2. Selected bond angles [°].
6·CH2Cl2 and the halide-free complex 11·CH2Cl2 (see Exp.
Sect.), and we analyzed them by single-crystal X-ray dif-
Atoms
4
5
6
11
C2–C1–N1
C1–Cu1–C1A
C1–Cu1–I1
105.9(3) 106.44(16) 100.4(3)
101.9(3)
173.7(1)
fraction. 6·CH2Cl2 crystallizes in the monoclinic P21/n
–
–
–
–
–
–
148.4(1)
113.5(1)
98.0(1)
¯
space group, and 11·CH2Cl2 crystallizes in the triclinic P1
–
–
space group (Table 6). In 6·CH2Cl2, the copper center is C1A–Cu1–I1
tricoordinated by the two carbene C atoms of the two tri-
azolylidene rings and an iodido ligand (Figure 1). The cop-
per center does not contain a local center of symmetry. The
Cu–C(carbene) distance of 1.93 Å (Table 1) matches well
with the corresponding distance in a similar complex that
we reported recently.[14a] The C–C and C–N distances
(Table 1) within the triazolylidene ring point to a delocal-
ized situation inside that ring. This is in contrast to bonding
situations observed for neutral 1,2,3-triazole ligands, where
a localization of the double bonds and a corresponding ten-
dency to favor an azo-like N=N bond inside the ring have
been observed.[7d] The C2–C1–N1 angle of 100.4(3)°
around the carbene C atom (Table 2) shows the large devia-
tion from the 120° angle that would be expected for an ide-
ally sp2-hybridized carbon atom. The C1–Cu1–C1A angle
of 148.4(1)°, the C1–Cu1–I1 angle of 113.5°, and the C1A–
Cu1–I1 angle of 98.0° all point towards a coordination ge-
ometry around the copper center that is in between a trigo-
nal planar and a T-shaped structure. Accordingly, the iod-
ido ligand is more tilted towards one of the triazolylidene
rings as compared to the other. The phenyl as well as the
benzyl substituents on the triazolylidene ring point in a syn
fashion with respect to each other. The dihedral angle be-
tween the phenyl and the triazolylidene ring is 49.9° for one
ligand and 50.9° for the other. One of the benzyl groups is
more tilted away from its triazolylidene ring than the other.
6·CH2Cl2, the copper center in 11·CH2Cl2 does not contain
any crystallographic center of inversion. The Cu–C(carb-
ene) distance of 1.89 Å is in the expected range and fits well
with reported distances between Cu(I) and triazolylidene C
atoms.[14a] Just as in the case of 6·CH2Cl2, the C–C and C–
N distances within the triazolylidene rings point to a delo-
calized situation within the rings. The C2–C1–N1 angle
around the carbene C center is 101.9(3)° and shows the de-
viation of this value from that of an ideally sp2-hybridized
carbon atom. The C1–Cu1–C1A angle is 173.7(1)° and
points to a near linear coordination geometry around the
copper(I) center. Both the mesityl groups of the two triaz-
olylidene rings and the phenyl groups are oriented in a syn
fashion with respect to each other. The dihedral angle be-
tween the phenyl ring and the central triazolylidene ring is
51.6° for one ligand and 35.5° for the other, and that be-
tween the mesityl ring and the central triazolylidene ring is
69.9° and 85.6°, respectively. The phenyl and mesityl sub-
stituents of one of the triazolylidene rings are more bent
towards the Cu(I) center compared to the substituents of
the other triazolylidene ring (Figure 2).
Figure 2. ORTEP plot of 11·CH2Cl2. Ellipsoids are drawn at 50%
probability. Hydrogen atoms, solvent molecules, and the counterion
have been omitted for clarity.
Catalytic [3+2] Cycloaddition Reactions between Azides
and Alkynes
Figure 1. ORTEP plot of 6·CH2Cl2. Ellipsoids are drawn at 50%
probability. Hydrogen atoms and solvent molecules have been
omitted for clarity.
Copper(I) complexes with NHC ligands are potent cata-
lysts for the [3+2] cycloaddition reaction between azides
and alkynes (CuAAC).[12,13] In recent years, our group has
In contrast to the iodide-containing complex 6·CH2Cl2, depicted the use of copper(I) complexes with aNHC ligands
the halide-free complex 11·CH2Cl2 displays a twofold coor- of the triazolylidene type as catalysts for the aforemen-
dinated copper center, and the two triazolylidene rings are tioned reaction.[14a,14c] In the initial work, we reported on
oriented trans to each other (Figure 2). As in the case of the catalytic activity of complexes of the form [Cu-
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(aNHC)I]. We were then interested in testing copper(I) per–ligand-bond breaking will be required for the tricoordi-
complexes with two abnormal-carbene ligands as catalysts nated halide-containing complexes 6 and 7, in comparison
for the CuAAC. In recent years, complexes containing two to 10, where only one copper–ligand bond needs to be
–
normal NHC ligands, that is, [Cu(NHC)2]X (X = BF4 or broken to generate the active catalyst (see below for a dis-
–
PF6 ) have also been shown to be potent catalysts for the cussion of the mechanism). The presence of additional do-
click reaction.[12c,12f] In the present work, we tested the per- nor atoms in the ligand backbone (SMe for 7) does not
formance of the aNHC counterparts of the bis(NHC) com- seem to positively influence the catalyst activity, as indi-
plexes mentioned above. In designing the catalysts, we chose cated by the worse efficiency of 7 compared to 6, which
to synthesize complexes of two different types: One class of does not contain any additional donor atoms.
complexes are the neutral bis(aNHC) complexes that con-
After having established that the halide-free, cationic
tain an additional iodido ligand (6 and 7). The second class complexes are far more efficient as catalysts in the discussed
are those that are iodide-free and hence cationic in nature cycloaddition reaction, we turned our focus to a compari-
(8–11). Hence, the complexes reported here present us with son among all the halide-free complexes 8–11 discussed
an opportunity of testing the influence of charge as well as here. The cycloaddition reaction between benzyl azide and
the presence or absence of an additional iodido ligand on phenylacetylene did not offer good insights here, because
the catalytic activity of the complexes. As a test reaction for reactions with all the halide-free catalysts were complete
screening of the catalysts, we chose the CuAAC between within 15 min. Hence, we turned our attention to the cyclo-
benzyl azide and phenylacetylene (Scheme 2).
addition reaction between phenyl azide and phenylacetylene
(Scheme 3).
Scheme 2. Test reaction used for comparing the efficiency of the
iodido containing complexes 6–7 and iodide free complexes 8–11.
Scheme 3. Test reaction used for probing the efficiency of the hal-
ide-free complexes 8–11.
In the test reaction shown in Scheme 2, a comparison
was made between the catalytic efficiency of the iodide-con-
taining complexes 6 and 7, and the halide-free cationic com-
plex 10, which has the same substituents on the triazolylid-
ene ring as 7. With 10 as a catalyst and 0.1 mol-% of cata-
lyst loading, a full conversion to the substituted 1,2,3-tri-
azole was complete within 15 min. For the same reaction
and with the same catalyst loading, the complexes 6 and 7
require 2 and 3 h, respectively, for full conversion. Under
identical conditions, the precursor complex [Cu(CH3CN)4]-
(BF4) requires 5 h for complete conversion (Table 3). All
With 0.5 mol-% catalyst loading, the reaction shown in
Scheme 3 was monitored for 240 min with 8 and for
250 min with 9, and conversions of 91% and 49%, respec-
tively, to the corresponding 1,2,3-triazole were observed af-
ter the monitored time frame (Figure 3). Under identical
conditions, a full conversion was obtained with 10 as well
as with 11 as catalyst after a total time of 170 min (Table 4).
Thus, the catalysts 10 and 11, which have phenyl and 2-
methylthiophenyl substituents and phenyl and mesityl sub-
stituents, respectively, on the triazolylidene rings, are the
most potent in the group. This effect is probably related to
the steric bulk of the substituents on the 2-methylthio-
phenyl and mesityl rings, which are missing in the substitu-
1
conversions were followed by H NMR spectroscopy with
a focus on the disappearance and appearance of the benz-
ylic CH2 proton of the products and reactants (see below).
These results thus clearly point out the need for the aNHC
ligands on the Cu(I) center to increase the catalytic effi-
ciency of the catalysts. Another interesting observation
made during the catalyst screening is the markedly higher
potency of the cationic, halide-free complex 10 in compari-
son to its halide-containing counterparts 6 and 7. The
reasons for this are probably twofold. The substrate mole-
cules are likely to bind more strongly to the cationic com-
plex as compared to its neutral counterpart. More impor-
tant is perhaps the fact that most likely more than one cop-
Figure 3. Conversion–time plots for catalysts 8–11 for the reaction
depicted in Scheme 3.
Table 3. Comparative results of the catalytic efficiency of iodido-
containing (6 and 7) and iodido-free (10) complexes for the reaction
shown in Scheme 2.
Table 4. Comparison within the iodide-free catalyst series (8–11)
for the test reaction shown in Scheme 3.
Catalyst
Time
Conversion [%]
Catalyst
Time
Conversion [%]
6
2 h
3 h
15 min
5 h
100
100
100
21
8
240 min
250 min
170 min
170 min
91
49
100
100
7
9
10
10
11
[Cu(MeCN)4](BF4)
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ents on the triazolylidene rings in 8 and 9 (benzyl and rich alkynes (for example tert-butylacetylene) and for elec-
phenyl groups, respectively). A likely effect of these bulky tron-poor or sterically demanding azides. The first step of
groups is that they increase the solubility of the catalysts the catalytic cycle is the deprotonation of the alkyne, fol-
and also enhance their stability in air. The comparable effi- lowed by the binding of the acetylide ligand to the copper(I)
ciencies of 10 and 11 preclude the possibility that additional center (see Scheme 6 below). An electron-rich alkyne is ex-
donor atoms in the ligand backbone (SMe in the case of pected to be more difficult to deprotonate in comparison to
10) are advantageous for catalysis.
an electron-poor one. This difficulty would explain the need
Once we had established that 10 and 11 are the most for longer reaction times and the worse catalytic efficiency
potent catalysts of all the complexes tested, we decided to for the reaction with electron-rich alkynes. Substituting the
perform a substrate screening with one of these complexes. phenyl ring of phenylacetylene also leads to longer reaction
Since both complexes showed identical activity in the test times and worse catalytic efficiency (Scheme 4). Internal al-
reaction, we chose 11 (with a 0.5 mol-% catalyst loading) kynes, which are considered to be difficult substrates for the
for substrate screening. As can be seen from Scheme 4, 11 CuAAC, could also be converted into the corresponding
is capable of catalyzing the cycloaddition reaction between triazoles with 11 as the catalyst. However, this reaction re-
a host of substrates. The electronic and steric properties of quired 48 h and heating-up to 80 °C.
the starting alkynes and azides were systematically varied
to decipher the effect of these substrate properties on the coordination of the organic azide to the copper(I) center.
activity of the catalyst. Because the R group of RN3 is bound to the same nitrogen
A subsequent step in the catalytic cycle is probably the
As can be seen from the conversion times listed in atom that coordinates to the copper(I) center, the nature of
Scheme 4, the reaction times are the longest for electron- this group is likely to drastically influence the azide binding
Scheme 4. The range of substrates tested with catalyst 11.
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to the metal center and hence to determine the eventual its efficiency is still maintained. Hence, catalyst 11 is more
efficiency of the catalyst. Thus, azides with phenyl substitu- efficient for the reaction depicted in Scheme 5 than the [Cu-
ents, which would decrease the electron density at the nitro- (aNHC)I] catalysts we reported previously.[14a]
gen atom they are bound to, are less reactive than benzyl
azides. Azides containing bulky substituents are difficult
substrates for the click (CuAAC) reaction, because the
bulky groups are likely to interfere with the binding of the
organic azide to the copper(I) center. Hence, reactions with
bulky azides are slower (Scheme 4). However, with catalyst
11 and stirring the reaction overnight or for 24 h, a nearly
quantitative conversion is achieved even with bulky azides.
These results display the highly potent nature of the cata-
lysts reported here in comparison to the original click rec-
ipe, which employs CuSO4, ascorbate, and tbta.
Finally, we were interested in downscaling the catalyst
loading. To do this, we chose 11 as the catalyst and the
Figure 5. Conversion–time plot for the downscaling of the reac-
tions with 11 as the catalyst.
CuAAC between benzyl azide and phenylacetylene as the
model reaction (Scheme 5). The reaction was monitored by
Table 5. Results of downscaling experiments for the reaction shown
following the disappearance and appearance of the 1H
in Scheme 5 with 11 as the catalyst.
NMR signals assigned to the benzylic CH2 protons of the
reactants and products (Figure 4).
Loading [mol-%]
Time
Conversion [%]
0.5
0.1
0.05
0.005
13 min
15 min
30 min
180 min
92
100
86
82
Scheme 5. Test reaction used for downscaling the catalyst loading
with 11 as the catalyst.
The catalytic mechanism of the processes described here
is likely the same as that described for analogous bis-
carbene–copper(I) complexes recently described in the lit-
erature (Scheme 6). Most likely, the initial step here is the
deprotonation of the alkyne by one of the copper-bound
triazolylidene ligands and the subsequent formation of a
triazolium salt. The thus formed acetylide ligand then binds
to the copper(I) center to form a copper–acetylide complex.
The organic azide then binds to the copper–acetylide com-
plex, and the cycloaddition reaction takes place at the cop-
per center to yield a copper-bound triazolide ligand. The
eventual protonation of the triazolide by the triazolium salt
(formed in the first step) leads to the release of the 1,2,3-
triazole molecule and the regeneration of the copper(I)–
carbene catalyst. Proof that such a mechanism is opera-
tional in the present case came from noncatalytic studies,
for which we added phenylacetylene to the complex 11 in
an NMR experiment (Figure S3). The addition of phenyl-
acetylene leads to the appearance of a downfield-shifted 1H
NMR signal, which corresponds to the ring C–H proton of
the triazolium salt that is formed through deprotonation
of phenylacetylene and protonation of the copper-bound
triazolylidene ligand. Additional signals of the released
Figure 4. 1H NMR spectra of the benzylic CH2 protons of reactant
and product showing the completion of the reaction shown in
Scheme 5.
As stated above, with a catalyst loading of only 0.1 mol- triazolium salt and the corresponding copper-acetylide
%, a full conversion of the reaction shown in Scheme 5 was complex were also observed in the 1H NMR spectrum (Fig-
achieved within 15 min. Upon lowering the catalyst loading ure S3).
to 0.05 mol-%, a conversion of about 86% was observed
This mechanism would explain the lower efficiencies of
after only 30 min (Figure 5 and Table 5). Eventually, we the iodide-containing catalysts 6 and 7 in comparison to
could decrease the catalyst concentration to 0.005 mol-%. the halide-free complexes 8–11. It also explains the ob-
In this case, mild heating to 50 °C was necessary to obtain a served trends in catalytic efficiency as a function of the
conversion of about 82% within 180 min. Thus, the catalyst steric and electronic properties of the substrates, as has been
loading can be decreased to extremely small amounts, and discussed above.
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Scheme 6. Possible mechanism of catalytic action. Adapted from ref.[12c,f]
conditions. Thus, these results establish the use of the
emerging class of triazolylidene compounds as efficient li-
gands for homogeneous catalysis.[4h]
Conclusions
In conclusion, we have reported here on iodido-contain-
ing as well as halide-free copper(I) complexes containing
two triazolylidene ligands. The structural characterization
shows that the iodide-containing copper(I) complexes are
tricoordinated with a coordination geometry in between
that of a trigonal planar and a T-shaped structure. The hal-
ide-free complexes contain copper centers that are linearly
coordinated through the carbon atoms of the two triazolyl-
idene rings. Both kinds of complexes are efficient catalysts
for the [3+2] cycloaddition reaction between azides and alk-
ynes. The cationic, halide-free complexes are more efficient
than their neutral iodide-containing counterparts. A sub-
strate screening was performed by choosing the most potent
complex as the catalyst. The screening showed that these
copper–aNHC complexes are capable of catalyzing the re-
action of electron-poor, electron-rich, and bulky azides with
electronically diverse alkynes. In downscaling experiments,
it could be shown that the catalysts remain active, even
when their loading is decreased to 0.005 mol-%. Our results
illustrate the high efficiency of copper(I) complexes with
aNHC ligands of the triazolylidene type as catalysts for the
click (CuAAC) reaction. Such catalysts are also capable of
catalyzing reactions with bulky azides (a class of substrates
Experimental Section
Materials and Physical Methods: Potassium tert-butoxide, copper
iodide, [Cu(CH3CN)4](BF4) and trimethyloxonium tetrafluorobor-
ate (Meerwein salt) were purchased from Sigma–Aldrich. All rea-
gents were used as supplied. The solvents used for metal-complex
synthesis and methylations with Meerwein salt were dried and dis-
tilled under argon and degassed by common techniques prior to
use.
1
The H and 13C NMR spectra were recorded with a Jeol ECS 400
spectrometer. Elemental analyses were performed with the Perkin–
Elmer Analyzer 240 and with a Elementar Vario EL III. Mass spec-
trometry was performed with an Agilent 6210 ESI-TOF.
Synthesis of Ligands: Ligands 1,[11b,14a] 2,[14a] and 3[14b] and were
synthesized according to the methods described in the literature.
3-Methyl-1-[2-(methylthio)phenyl]-4-phenyl-1H-1,2,3-triazol-3-ium
Tetrafluoroborate (4): The corresponding triazole 4a[14a] (1 equiv.,
2 mmol) and Meerwein salt (1.5 equiv., 3 mmol) were dissolved in
dichloromethane (15 mL) under inert gas atmosphere and stirred
overnight. Afterwards, the reaction was quenched by the addition
of methanol (3 mL) and stirred under air for another 30 min. The
considered difficult for the original click recipe) under mild reaction mixture was then poured into ethyl ether (200 mL), and
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the white precipitate was filtered off, washed several times with
ethyl ether, and dried under air to give a white powder in a good
yield of 81%. H NMR (400 MHz, CDCl3, 25 °C, TMS): δ = 8.42
ide (3 equiv., 0.45 mmol, 0.051 g). The desired product was ob-
tained in of 79% yield. (0.118 mmol, 89 mg). H NMR (400 MHz,
CD2Cl2, 25 °C, TMS): δ = 7.53–7.30 (m, 14 H, aryl-H), 7.26–7.26
1
1
(s, 1 H, triazole-5-H), 7.86–7.82 (m, 2 H, aryl-H), 7.73–7.69 (m, 2
(m, 4 H, aryl-H), 4.15 (s, 6 H, N-CH3), 2.18 (s, 6 H, S-CH3) ppm.
H, aryl-H), 7.62–7.51 (m, 4 H, aryl-H), 7.48–7.44 (m, 1 H, aryl- 13C NMR (100 MHz, CD2Cl2, 25 °C, TMS): δ = 166.1 (carbene-
H), 7.37–7.31 (m, 1 H, aryl-H), 4.33 (s, 3 H, N-CH3), 2.48 (s, 3 H,
S-CH3) ppm. 13C NMR (100 MHz, CDCl3, 25 °C, TMS): δ = 143.7 126.6, 125.6 (all aryl-C), 37.9 (N-CH3), 15.9 (S-CH3) ppm. MS
(triazole-5-C), 134.9, 133.2, 133.1, 132.8, 132.1, 129.9, 129.8, 129.6,
(ESI): m/z = calcd. for [C32H30N6S2Cu+] 625.1269; found 625.1268.
C), 148.3, 137.2, 134.6, 130.8, 130.1, 129.4, 129.1, 127.1, 126.9,
128.1, 128.0, 127.1, 121.7 (all aryl-C), 39.1 (N-CH3), 16.5 (S-CH3) C32H30N6S2CuI: calcd. C 51.03, H 4.01, N 11.16; found C 50.97,
ppm. MS (ESI): m/z = calcd. for [C16H16N3S1+] 282.1059; found
282.1063. C16H16BF4N3S·0.3CH2Cl2: calcd. C 49.35, H 4.23, N
10.57; found C 49.28, H 3.69, N 10.66.
H 4.33, N 10.87.
Syntheses of the Halide-Free Biscarbene Complexes
General Procedure: Tetrakis(acetonitrile)copper(I) tetrafluorobor-
ate and the respective triazolium tetrafluoroborate salt were sus-
pended in dichloromethane (20 mL) and cooled to –78 °C. After
that, potassium tert-butoxide suspended in dichloromethane
(5 mL) was slowly added to the suspension, and the reaction mix-
ture was stirred overnight and was slowly warmed to room tem-
perature. The reaction mixture was filtered through a pad of Celite
and the volatiles were removed under high vacuum until only about
15% of the solvent was left. The remaining dichloromethane solu-
tion was layered with hexanes and allowed to stand at –5 °C for
several hours to induce crystallization of the product. The com-
plexes were obtained as colorless crystals in good yields.
1-Mesityl-3-methyl-4-phenyl-1H-1,2,3-triazol-3-ium Tetrafluorobor-
ate (5): The corresponding triazole 5a[11i] (1 equiv., 2 mmol) and
Meerwein salt (1.5 equiv., 3 mmol) were dissolved in dichlorometh-
ane (15 mL) under inert gas atmosphere and stirred overnight. Af-
terwards, the reaction was quenched by the addition of methanol
(3 mL) and stirred under air for another 30 min. The reaction mix-
ture was then poured into ethyl ether (200 mL), and the white pre-
cipitate was filtered off and washed several times with ethyl ether
1
and dried in air to give a white powder in of 85% yield. H NMR
(400 MHz, CDCl3, 25 °C, TMS): δ = 8.32 (s, 1 H, triazole-5-H),
7.78–7.73 (m, 2 H, aryl-H), 7.57–7.50 (m, 3 H, aryl-H), 7.02 (s, 2
H, mesityl-H), 4.37 (s, 3 H, N-CH3), 2.35 (s,3 H, para-mesityl-
CH3), 2.09 (s, 6 H, ortho-mesityl-CH3) ppm. 13C NMR (100 MHz,
CDCl3, 25 °C, TMS): δ = 144.5 (triazole-5-C), 142.6, 134.7, 132.1,
131.4, 130.0, 129.9, 129.8, 129.6, 121.6 (all aryl-C), 39.2 (N-CH3),
21.3 (para-mesityl-CH3), 17.3 (ortho-mesityl-CH3) ppm. MS (ESI):
m/z = calcd. for [C18H20N3+] 278.1657; found 278.1655.
C18H20N3BF4·0.16CH2Cl2: calcd. C 57.20, H 5.38, N 11.00; found
C 57.20, H 6.03, N 11.07.
Bis(1-benzyl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene)copper(I)
Tetrafluoroborate (8): Synthesized from tetrakis(acetonitrile)cop-
per(I) tetrafluoroborate (1 equiv., 0.15 mmol, 47 mg), triazolium
tetrafluoroborate salt 1 (2 equiv., 0.3 mmol, 113 mg), and potas-
sium tert-butoxide (3 equiv., 0.45 mmol, 51 mg). The desired prod-
uct was obtained in 71% yield (0.108 mmol, 69 mg). 1H NMR
(400 MHz, CD2Cl2, 25 °C, TMS): δ = 7.51–7.17 (m, 20 H, aryl-H),
5.43 (s, 4 H, N-CH2-Ph), 4.03 (s, 6 H, N-CH3) ppm. 13C NMR
(100 MHz, CD2Cl2, 25 °C, TMS): δ = 162.6 (Carbene-C), 149.1,
134.5, 130.1, 129.4, 129.1, 129.0, 128.6, 127.4 (all aryl-C), 59.1 (N-
Syntheses of the Halogenated Biscarbene-Complexes
General Procedure: Tetrakis(acetonitrile)copper(I) tetrafluorobor-
ate and the respective triazolium iodide were suspended in dichlo-
romethane (20 mL) and cooled to –78 °C. After that, potassium
CH2-Ph), 37.4 (N-CH3) ppm. MS (ESI): m/z
=
calcd. for
561.1823.
[C32H30N6Cu+]
561.1822; found
tert-butoxide suspended in dichloromethane (5 mL) was slowly C32H30N6CuBF4·3CH2Cl2: calcd. C 46.51, H 4.01, N 9.30; found
added to the suspension, and the reaction mixture was stirred over-
night and was slowly warmed to room temperature. The reaction
mixture was filtered through a pad of Celite, and the volatiles were
removed under high vacuum until only about 15% of the solvent
was left. The remaining dichloromethane solution was layered with
hexanes and allowed to stand at –5 °C for several hours to induce
crystallization of the product. The complexes were then obtained
as colorless crystals in good yields.
C 46.19, H 3.75, N 9.67.
Bis(3-methyl-1,4-diphenyl-1,2,3-triazol-5-ylidene)copper(I) Tetra-
fluoroborate (9): Synthesized from tetrakis(acetonitrile)copper(I)
tetrafluoroborate (1 equiv., 0.15 mmol, 47 mg), triazolium tetra-
fluoroborate salt 3 (2 equiv., 0.3 mmol, 97 mg), and potassium tert-
butoxide (3 equiv., 0.45 mmol, 51 mg). The desired product was ob-
tained in 86 % yield (0.13 mmol, 82 mg). 1H NMR (400 MHz,
CD2Cl2, 25 °C, TMS): δ = 7.87–7.75 (m, 4 H, aryl-H), 7.56–7.49
(m, 6 H, aryl-H), 7.47–7.38 (m, 6 H, aryl-H), 7.35–7.24 (m, 4 H,
aryl-H), 4.17 (s, 6 H, N-CH3) ppm. 13C NMR (100 MHz, CD2Cl2,
25 °C, TMS): δ = 162.0 (carbene-C), 149.4, 139.6, 130.3, 130.2,
129.5, 129.2, 123.0 (all aryl-C), 37.7 (N-CH3) ppm. MS (ESI): m/z
= calcd. for [C3 0 H2 6 N6 Cu+ ] 533.1509; found 533.1502.
Bis(1-benzyl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene)copper(I)
Iodide (6): Synthesized from tetrakis(acetonitrile)copper(I) tetra-
fluoroborate (1 equiv., 0.15 mmol, 47 mg), triazolium salt 1
(2 equiv., 0.3 mmol, 113 mg), and potassium tert-butoxide (3 equiv.,
0.45 mmol, 51 mg). The desired product was obtained in 85% yield.
1
(0.127 mmol, 88 mg). H NMR (400 MHz, CD2Cl2, 25 °C, TMS): C30H26N6CuBF4: calcd. C 58.03, H 4.22, N 13.53; found C 58.51,
δ = 7.53–7.47 (m, 4 H, aryl-H), 7.42–7.37 (m, 2 H, aryl-H), 7.36–
7.30 (m, 4 H, aryl-H), 7.27–7.16 (m, 10 H, aryl-H), 5.56 (s, 4 H, N-
CH2-Ph), 4.04 (s, 6 H, N-CH3) ppm. 13C NMR (100 MHz, CD2Cl2,
25 °C, TMS): δ = 165.1, 148.4, 134.9, 129.8, 129.5, 129.0, 128.9,
128.7, 128.5, 127.8 (all aryl-C), 58.8 (N-CH2-Ph), 37.5 (N-CH3)
ppm. MS (ESI): m/z = calcd. for [C32H30N6Cu+] 561.1822; found
561.1835. C32H30CuIN6·0.3CH2Cl2: calcd. C 53.36, H 4.27, N
11.49; found C 53.78, H 4.67, N 11.01.
H 4.34, N 13.19.
Bis{3-methyl-1-[2-(methylthio)phenyl]-4-phenyl-1,2,3-triazol-5-
ylidene}copper(I) Tetrafluoroborate (10): Synthesized from tetrakis-
(acetonitrile)copper(I) tetrafluoroborate (1 equiv., 0.15 mmol,
47 mg), triazolium salt 4 (2 equiv., 0.3 mmol, 110 mg), and potas-
sium tert-butoxide (3 equiv., 0.45 mmol, 51 mg). The desired prod-
uct was obtained in 78 % yield (0.117 mmol, 83 mg). 1H NMR
(400 MHz, CD2Cl2, 25 °C, TMS): δ = 7.52–7.31 (m, 15 H, aryl-H),
7.24–7.18 (m, 3 H, aryl-H), 4.13 (s, 6 H, N-CH3), 2.18 (s, 6 H, S-
CH3) ppm. 13C NMR (100 MHz, CD2Cl2, 25 °C, TMS): δ = 166.0
(carbene-C), 148.3, 137.2, 134.6, 130.8, 130.1, 129.4, 129.1, 127.2,
126.9, 126.6, 125.6 (all aryl-C), 37.6 (N-CH3), 15.8 (S-CH3) ppm.
Bis{3-methyl-1-[2-(methylthio)phenyl]-4-phenyl-1,2,3-triazol-5-
ylidene}copper(I) Iodide (7): Synthesized from tetrakis(acetonitrile)-
copper(I) tetrafluoroborate (1 equiv., 0.15 mmol, 0.047 g), triazol-
ium salt 2 (2 equiv., 0.3 mmol, 0.123 g), and potassium tert-butox-
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Table 6. Crystallographic details.
4
5
6·CH2Cl2
11·CH2Cl2
Formula
Mr
Crystal system
Space group
a [Å]
b [Å]
c [Å]
C16H16N3SBF4
C18H20N3BF4
365.18
monoclinic
P21/c
7.9155(17)
10.153(2)
21.707(5)
90
92.726(5)
90
C33H32N6ICuCl2
773.99
monoclinic
P21/n
14.0666(8)
14.3507(9)
17.3045(10)
90
111.519(2)
90
C37H40N6CuBF4Cl2
790.00
369.19
monoclincic
P21/n
10.730(3)
15.643(4)
11.467(3)
90
116.946(5)
90
1715.7(8)
4
triclinic
¯
P1
11.493(3)
13.516(4)
13.775(4)
103.323(6)
94.257(6)
111.738(6)
1903.7(9)
2
α [°]
β [°]
γ [°]
V [Å3]
1742.6(6)
4
1.392
3249.7(3)
4
1.582
Z
Densitiy [gcm–3]
F(000)
1.429
760
1.378
816
760
1552
Radiation type
μ [mm–1]
Crystal size
Meas. reflections
Indep. reflections
Obsvd. [IϾ2σ(I)] refl.
Rint
Mo-Kα
0.233
0.20ϫ0.15ϫ0.15
16019
3130
2360
Mo-Kα
0.113
0.38ϫ0.10ϫ0.10
13389
2969
2247
0.0378
0.0356, 0.909, 1.025
0.228, –0.226
Cu-Kα
10.176
0.22ϫ0.10ϫ0.08
20267
5482
4963
0.0475
0.0340, 0.0872, 1.060
1.110, –0.645
Mo-Kα
0.769
0.25ϫ0.20ϫ0.08
21920
7381
5943
0.0462
0.0572, 0.1613, 1.050
0.991, –0.643
0.0522
R [F2 Ͼ 2σ(F2)], wR(F2), S 0.589, 0.1603, 1.049
Δρmax, Δρmin [eÅ–3]
0.558, –0.418
MS (ESI): m/z = calcd. for [C32H30N6S2Cu+] 625.1269; found
625.1266. C32H30N6S2CuBF4·0.3CH2Cl2: calcd. C 51.66, H 4.14, N
10.06; found C 51.91, H 4.26, N 10.29.
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Supporting Information (see footnote on the first page of this arti-
cle): ORTEP plots for compounds 4 and 5 and NMR spectrum
showing the formation of the triazolium salt upon adding phenyl-
acetylene to complex 11.
Bis(1-mesityl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene)copper(I)
Tetrafluoroborate (11): Synthesized from tetrakis(acetonitrile)cop-
per(I) tetrafluoroborate (1 equiv., 0.15 mmol, 47 mg), triazolium
salt 5 (2 equiv., 0.3 mmol, 110 mg), and potassium tert-butoxide
(3 equiv., 0.45 mmol, 0.051 g). The desired product was obtained
in 85% yield (0.12 mol, 84.6 mg). 1H NMR (400 MHz, CD2Cl2,
25 °C, TMS): δ = 7.64–7.48 (m, 4 H, aryl-H), 7.45–7.38 (m, 6 H,
aryl-H), 6.94 (s, 4 H, mesityl-H), 4.16 (s, 6 H, N-CH3), 2.40 (s, 6
H, para-mesityl-CH3), 1.83 (s, 12 H, ortho-mesityl-CH3) ppm. 13C
NMR (100 MHz, CD2Cl2, 25 °C, TMS): δ = 165.0 (carbene-C),
149.2, 140.3, 136.1, 134.0, 130.1, 129.3, 129.2, 129.1, 127.0 (all aryl-
C), 37.7 (N-CH3), 21.1 (ortho-mesityl-CH3), 17.1 (para-mesityl-
CH3) ppm. MS (ESI): m/z = calcd. for [C36H38N6Cu+] 617.2454;
found 617.2452. C36H38N6CuBF4·2CH2Cl2: calcd. C 52.16, H 4.84,
N 9.61; found C 52.23, H 4.65, N 9.80.
Acknowledgments
The authors are grateful to the Fonds der Chemischen Industrie
(FCI) for financial support. Paul Kubella is very kindly acknowl-
edged for experimental help.
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vial, and the catalyst (0.5–0.005 mol-%) was added. (Caution: reac-
tions on a large scale, with more than 5 mmol, under neat conditions
are highly exothermic!) The reaction was stirred at room tempera-
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heated up to 50 °C. The reaction was monitored by 1H NMR spec-
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further reaction was taking place. Conversions were determined by
1
using H NMR spectroscopy.
X-ray Crystallography: Data were collected by using either a Bruker
Kappa Apex 2 duo or a Bruker Smart AXS diffractometer (Table
6). The measurements were carried out at 100 K by using Mo-Kα
radiation (graphite monochromator). The structures were solved
and refined by full-matrix least-squares techniques on F2 by using
the SHELX-97 program.[15]
CCDC-913981 (for 4), -911943 (for 5), -906190 (for 6), and -906191
(for 11) contain the supplementary crystallographic data for this
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Received: January 30, 2013
Published Online: ᭿
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