Heterothiometallic clusters as robust and efficient copper(I) catalysts for azide-alkyne [3 + 2] cycloadditions

Article abstract of DOI:10.1016/j.catcom.2015.10.019

We investigated heterothiometallic W(Mo)/S/Cu clusters [NH₄]₂[MS₄Cu_nX_n] (M = W, Mo; X = Br, I; n = 2, 4, 6) as novel copper(I) catalysts for Huisgen 1,3-dipolar cycloadditions. The experiments indicat

Full text of DOI:10.1016/j.catcom.2015.10.019

Catalysis Communications 73 (2016) 103–108
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Short communication
Heterothiometallic clusters as robust and efficient copper(I) catalysts for
azide–alkyne [3 + 2] cycloadditions
Xiao-Fei Gao a, Wen-Mei Sun a, Xiao-Miao Li a, Xiao-Jun Liu a, Long-Sheng Wang b, Zheng Liu a, Jun Guo a,
⁎
a
College of Chemistry, Central China Normal University, Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, CCNU-uOttawa Joint Research Centre, 152 Luoyu Road, Wuhan,
Hubei 430079, PR China
b
School of Chemistry and Chemical Engineering, Hubei University of Technology, Wuhan, Hubei 430068, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 August 2015
Received in revised form 12 October 2015
Accepted 13 October 2015
Available online 22 October 2015
We investigated heterothiometallic W(Mo)/S/Cu clusters [NH4]2[MS4CunXn] (M = W, Mo; X = Br, I; n = 2, 4, 6) as
novel copper(I) catalysts for Huisgen 1,3-dipolar cycloadditions. The experiments indicate that the thiometallic li-
gands can stabilize copper(I) ions and improve their solubility. Interestingly, these cluster compounds have very
different catalytic performances in copper(I)-catalyzed azide–alkyne cycloadditions (CuAACs), which may corre-
late to their different redox potentials. Cluster [NH4]2[MS4Cu4I4] was found to be a very robust and efficient cata-
lyst of CuAACs, and was utilized to synthesize a variety of 1,4-disubstituted 1,2,3-triazoles in excellent yields at
room temperature under polar solvent or solvent-free conditions.
Keywords:
W(Mo)/S/Cu clusters
Copper
© 2015 Elsevier B.V. All rights reserved.
Click chemistry
Solvent-free reaction
1. Introduction
optic properties [16–19]. However, in literature there are only very lim-
ited examples that heterothiometallic clusters or copper thiometallic
Pioneered by Huisgen in the 1960s, the azide-alkyne 1,3-dipolar cy-
cloaddition reactions were brought back into focus by Sharpless and
Medal when they introduced the copper(I) catalyst [1–2], which were
then termed as copper(I)-catalyzed azide-alkyne cycloadditions
(CuAACs). The CuAAC reactions have gained widespread utilization in
materials sciences [3], biological sciences [4], and other fields [5–6]. For
green chemistry, CuAAC reactions are also developed under solvent-
free conditions [7]. It is believed that copper(I) species played an impor-
tant role in these Huisgen azide–alkyne 1,3-dipolar cycloaddition
reactions [8]. However, generally copper(I) species have poor solubility
in organic and aqueous solution, the propensity to disproportionate
and high oxidability in solution, and the tendency to aggregate. To ad-
dress these problems, previous researchers have already made various
efforts [9–10] to improve the efficiency of copper(I)-catalyzed azide-
alkyne cycloadditions: (i) in situ generation of the catalytically active
copper(I) species from reduction of copper(II) salts with sodium
ascorbate, which usually needs large excess amount of reductant [1];
(ii) applying N (or C, O, S, P)-containing ligands [11–12], particularly
N-heterocyclic carbene groups and polytriazoles [7,13–14], to stabilize
copper(I) oxidation state and modulate the reactivity; (iii) immobilizing
copper species on different supports [15].
clusters are used as catalysts in organic reactions [20–21], and
W(Mo)/S/Cu heterothiometallic clusters have not been used as catalysts
in CuAAC reactions yet. Herein, for the first time we explored the
W(Mo)/S/Cu cluster compounds — [NH4]2[MS4CunXn] (M = W, Mo;
X = Br, I; n = 2, 4, 6) as novel catalysts in CuAAC reactions for the syn-
thesis of 1,4-disubstituted-1,2,3-triazoles under both polar solvent and
solvent-free conditions.
2. Experimental section
2.1. General procedure for the synthesis of triazoles in DMF
A solution of catalyst (nCu = 0.05 mmol), Et3N (0.5 mmol), alkyne
(0.5 mmol), and azide (0.5 mmol) in DMF (0.3 mL) was stirred at
room temperature under air for 11 h. Then, the suspension was diluted
with dichloromethane (10 mL), washed with water (3 × 2 mL), dried
over sodium sulfate, concentrated and purified by silica gel flash chro-
matography with petroleum ether and ethyl acetate (10:1–5:1), report-
ed yields are the average of two runs.
2.2. General procedure for the synthesis of triazoles without solvent
Heterothiometallic clusters have been intensively investigated,
which demonstrate versatile structures and interesting electric and
The reaction was carried out using azide (0.5 mmol) and alkyne
(0.5 mmol) in the presence of catalyst (nCu = 0.01 mmol) and Et3N
(0.5 mmol) at room temperature under air. The reaction proceeded on
manual grinding the reagents in an agate mortar with a pestle for
⁎
Corresponding author.
E-mail address: jguo@mail.ccnu.edu.cn (J. Guo).
http://dx.doi.org/10.1016/j.catcom.2015.10.019
1566-7367/© 2015 Elsevier B.V. All rights reserved.

104
X.-F. Gao et al. / Catalysis Communications 73 (2016) 103–108
Scheme 1. Topological structures of different copper(I) clusters (type A: [MS4Cu2X2]2−; type B: [MS4Cu4X4]2−; type C: [MS4Cu6X6]2−. X = Br or I; M = W or Mo).
15 min. The product was purified by silica gel flash chromatography
with petroleum ether and ethyl acetate (10:1–5:1), reported yields
are the average of two runs.
redox potential as the CuX (X = I or Br). Further investigation of applying
these clusters and CuX as catalysts indicated that their catalytic reactivity
is very closely related to their redox potentials.
3. Results and discussion
3.2. Copper(I) cluster-catalyzed cycloadditions in DMF
3.1. Study of W(Mo)/S/Cu heterothiometallic clusters
The cycloaddition reactions with various cluster catalysts were car-
ried out by using phenylacetylene and benzyl azide as substrates. As
shown in Table 1, in the presence of 10 mol% of copper(I), type B cluster
catalysts (Table 1, entries 7, 8, 9 and 10) provided the 1,4-triazole with
the highest yields (83% to 91%) within 11 h. A marginal decrease in yield
was observed in reactions with type C cluster catalysts (Table 1, entries
13, 14, 15 and 16). However, it is surprising to find that almost no prod-
uct (b1%) was detected in the reactions catalyzed by type A cluster cat-
alysts (Table 1, entries 3, 4, 5 and 6). The different catalytic ability of the
three types of clusters was also confirmed by kinetic study with high
performance liquid chromatography (HPLC) (Fig. S4). These studies
suggested that the catalytic reactivity of [NH4]2[MS4CunXn] cluster cata-
lysts in CuAAC reactions varied dramatically: type A clusters almost had
no catalytic reactivity, type B clusters had excellent reactivity, and type
C clusters had comparable reactivity. It proved that the reactivity of
[NH4]2[MS4CunXn] (M = Mo or W; X = Br or I) have a variety of
three-dimensional structures in solution and solid state [22]. As a specif-
ic sulfuric ligand of copper(I) ions, thiomolybdate or thiotungstate
anion of [MS4]2− (M = Mo or W) can not only stabilize the oxidation
state of copper(I) in solution under air, but also modulate its catalytic ef-
ficiency of copper(I) through versatile coordination modes, which allow
us to investigate the relationships between structures and catalytic abil-
ities in CuAAC.
In our study, following to the procedures [23–24], three types of
W(Mo)/S/Cu clusters [NH4]2[MS4CunXn] (type A: n = 2; type B: n = 4;
type C: n = 6, topological structures were shown in Scheme 1) were eas-
ily prepared at large scales from the reaction of (NH4)2MS4 (M = W or
Mo) with different equivalent amounts of CuX (X = Br or I) in N,N-
dimethylformamide (DMF). The type A cluster contains two Cu atoms
bound to W(Mo) with an angle Cu–W(Mo)–Cu = 180° [25]. For type B
cluster, the W(Mo) and four Cu atoms are coplanar in each unit [18].
The type C cluster consists of a tetrahedral [MS4]2− core enveloped by
octahedral array of six Cu atoms [26]. In these cluster units, each Cu
atom is coordinated by two S atoms and a terminal ligand. The structures
of these cluster compounds have been determined by single-crystal
X-ray and solution NMR in literature [27–29].
Table 1
Catalysts screening for synthesis of 1,4-disubstituted triazole.a
Entry
Catalyst/mol% of CuI
Time (h)
Yield (%)b
1
2
CuBr/10
CuI/10
11
11
64
74
After the reaction of CuX (X = I or Br) and (NH4)2MS4 (M = W or
Mo), the UV absorption band has significant red shift (Fig. S1), indi-
cating the formation of cluster [NH4]2[MS4CunXn] [25]. Because CuX
(X = I or Br) has poor solubility in either organic or aqueous solution,
which has substantial influence to the catalytic efficiency in CuAAC,
we compared the solubility of three types of clusters with CuI in
DMF (Fig. S2). The results show that all three types of clusters greatly
improved the solubility of Cu in solution.
For different types of W(Mo)/S/Cu clusters, one sulfotungstate or
sulfomolybdate core [MS4]2− (M = W or Mo) coordinates to different
numbers of copper atoms, which will result in different redox potentials.
Herein, we studied the cyclic voltammograms (CVs) of these cluster
compounds (Fig. S3). The cyclic voltammograms of [NH4]2[WS4 Cu4I4]
(type B) and [NH4]2[WS4Cu6I6] (type C) showed a quasi-reversible
redox couple. It is believed that these observed potentials in W(Mo)/S/
Cu cluster are all associated with the Cu atoms [30–32]. The reduction
potentials of Cu atom in [NH4]2[WS4Cu4I4] and [NH4]2[WS4Cu6I6] are
0.54 V and 0.55 V, respectively. However, in the cyclic voltammogram
of [NH4]2[WS4Cu2I2] (type A), no quasi-reversible redox couple was
found. In CuAAC reaction, the reduction potential of the catalytically ac-
tive Cu is +0.52 V [7]. The type B and C copper(I) clusters showed similar
Type A cluster
3
4
5
6
[NH4]2[WS4Cu2Br2]/10
[NH4]2[WS4Cu2I2]/10
[NH4]2[MoS4Cu2Br2]/10
[NH4]2[MoS4Cu2I2]/10
11
11
11
11
b1
b1
b1
b1
Type B cluster
7
8
9
10
11
12
[NH4]2[WS4Cu4Br4]/10
[NH4]2[WS4Cu4I4]/10
[NH4]2[MoS4Cu4Br4]/10
[NH4]2[MoS4Cu4I4]/10
[NH4]2[WS4Cu4I4]/5
[NH4]2[WS4Cu4I4]/1
11
11
11
11
24
72
85
91
83
87
93
90
Type C cluster
13
14
15
16
[NH4]2[WS4Cu6Br6]/10
[NH4]2[WS4Cu6I6]/10
[NH4]2[MoS4Cu6Br6]/10
[NH4]2[MoS4Cu6I6]/10
11
11
11
11
60
52
55
61
a
The reaction was carried out using benzyl azide (0.5 mmol) and phenylacetylene
(0.5 mmol) in the presence of catalyst (nCu = 0.05 mmol) and Et3N (0.5 mmol) in
DMF (0.3 mL) at room temperature under air.
b
Isolated yields after column chromatography.

X.-F. Gao et al. / Catalysis Communications 73 (2016) 103–108
105
Fig. 1. Synthesis of different substituted triazoles in DMF. Reaction conditions: azides (0.5 mmol), terminal alkynes (0.5 mmol), [NH4]2[WS4Cu4I4] (nCu = 0.05 mmol) and Et3N (0.5 mmol)
were dissolved and stirred in DMF (0.3 mL) at room temperature under air.
copper(I) species was sensitively affected by the subtle coordination
structures in the cluster complexes. For different types of clusters,
they have different coordination structures and different redox
potentials, thus they also have different catalytic activities. Here
[NH4]2[WS4Cu4I4] (n = 4) shows the best balance between the stability
of the copper atom and the catalytic reactivity.
We also found that [NH4]2[WS4Cu4I4] was still very effective when
its loading decreased to 1% loading in the reaction (Table 1, entries 10,
11 and 12), albeit longer reaction time (72 h) was required, indicating
the exceptional stability of this catalyst under air. It is noteworthy that
for the same type of clusters, W/S/Cu cluster and Mo/S/Cu cluster
showed similar catalytic efficiency.
The optimized cluster catalyst [NH4]2[WS4Cu4I4] was then extended
to a range of different terminal alkynes and azides to examine the sub-
strate scope of the reaction. As shown in Fig. 1, we used substrates with
various functional groups, including amino, hydroxyl, carboxyl and hal-
ogen groups, and all reactions proceeded with high efficiency to give
corresponding 1,4-disubstituted triazoles, and the yields ranged from
80% to 97%.
Table 2
Catalyst loading in solvent-free reaction.a
Entry
Catalyst
CuI loading (mol%)
Time (min)
Yield (%)b
1
2
3
4
CuI
2
15
15
30
60
68
99
96
91
[NH4]2[WS4Cu4I4]
[NH4]2[WS4Cu4I4]
[NH4]2[WS4Cu4I4]
2
1
0.5
a
The reaction was carried out using benzyl azide (0.5 mmol) and phenylacetylene
(0.5 mmol) in the presence of copper(I) catalyst and Et3N (0.5 mmol) at room
temperature under air. The reaction proceeded on manual grinding of the reagents
in an agate mortar with a pestle.
b
Isolated yields after column chromatography.

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X.-F. Gao et al. / Catalysis Communications 73 (2016) 103–108
Fig. 2. Illustration of ATR-FTIR spectra for the reaction catalyzed by CuI (a) and [NH4]2[WS4Cu4I4] (b). Conditions: the reaction was carried out using benzyl azide (0.5 mmol) and
phenylacetylene (0.5 mmol) in the presence of catalyst (nCu = 0.01 mmol) and Et3N (0.5 mmol) at room temperature under air. The reaction proceeded on manual grinding of the reagents
in an agate mortar with a pestle.
3.3. [NH4]2[Cu4WS4I4]-catalyzed cycloadditions without solvent
environmental friendly solvent-free reactions using [NH4]2[WS4Cu4I4]
as catalyst. As shown in Table 2, for solvent-free reactions, the yield of
reaction using CuI as catalyst is much lower than that using
[NH4]2[WS4Cu4I4] (yield: 68% vs. 99%); when the loading of
After successful synthesis of various 1,4-disubstituted triazoles in
DMF with [NH4]2[WS4Cu4I4], we further investigated economic and
Fig. 3. Synthesis of different substituted triazoles without solvent. The reactions were carried out using azides (0.5 mmol) and terminal alkynes (0.5 mmol) in the presence of catalyst
(nCu = 0.01 mmol) and Et3N (0.5 mmol) at room temperature under air. The reaction proceeded on manual grinding of the reagents in an agate mortar with a pestle.

X.-F. Gao et al. / Catalysis Communications 73 (2016) 103–108
107
Scheme 2. Applications of [NH4]2[WS4Cu4I4] to functional material and biomolecule.
[NH4]2[WS4Cu4I4] is lower to 0.5 mol%, the yield of reaction is still good
(yield: 91%). The kinetic of solvent-free system was investigated by
attenuated total reflection Fourier transform infrared (ATR-FTIR)
substrate is liquid. As can be seen from the Fig. 3, for substances of ter-
minal alkynes and azides with different functional groups, all gave the
desired products in 15 min in excellent yields.
spectroscopy (Fig. 2). Phenylacetylene (weak signal at 3291 cm−1
)
and benzyl azide (strong signal at 2094 cm−1) consumed much faster
in [NH4]2[WS4Cu4I4]-catalyzed system than that of CuI. The two
reactants were not detected at 15 min in [NH4]2[WS4Cu4I4]-catalyzed
system, while benzyl azide was still remained after 30 min in
3.4. Applications and plausible catalytic pathway of [NH4]2[WS4Cu4I4]-
catalyzed cycloadditions
Compared with the reported copper(I) catalysts for CuAAC [11–12,
21], the heterothiometallic cluster [NH4]2[WS4Cu4I4] is air-stable and
soluble, and can perform well without the need of inert gas or reduc-
tant. Encouraged by the success of cluster [NH4]2[WS4Cu4I4] as catalyst
for CuAAC reactions, we applied it to modify functional material and
biomolecule. As shown in Scheme 2, the biocompatible polyethylene
glycol (PEG) polymer MeO-PEG2000-N3 was carried out in solution
conditions, and the unprotected biomolecule mannose derivative was
carried out in solvent-free conditions, both reactions provided the con-
jugated products in excellent yields.
Based on our research, the catalytic efficiency of W(Mo)/S/Cu
heterothiometallic clusters in CuAAC is subtly influenced by coordination
structures and redox potentials of copper atoms. The type B and C
copper(I) clusters show similar redox potential as the CuX (X = I or Br),
and have excellent catalytic activities, between them [NH4]2[WS4Cu4I4]
shows the best catalytic performances.
CuI-catalyzed system. The signal of triazole product at 3124 cm−1
,
attributed to the stretch of C-H in triazole ring, can be detected obvious-
ly at 5 min in [NH4]2[WS4Cu4I4]-catalyzed reaction. The plausible
explanation for the observation of different reactivities is that: for
CuI-catalyzed reaction, the resulting copper(I) complex with terminal
alkyne has the propensity to aggregate indiscriminately [33], resulting
highly insoluble polymeric products, thus cannot react with azide com-
pounds efficiently; while for [NH4]2[WS4Cu4I4]-catalyzed reaction,
[WS4]2− can be regarded as a suitable “capping ligand” to protect pe-
riphery of copper(I) acetylide, preventing copper(I) acetylide from ag-
gregating, thus copper(I) acetylide can react with azide compounds
more efficiently.
We next continued to conduct solvent-free experiments of a range
of terminal alkynes and azides with 2 mol% catalyst loading. It is note-
worthy that the solvent-free reactions were more efficient than those
run in DMF, providing products in similar yields but completing within
much shorter time (15 min vs. 11 h) in the presence of less catalyst
(2 mol% vs. 10 mol% Cu(I)). To guarantee the necessary contact between
two substances, the solvent-free reaction requires that at least one
Previous research on the mechanism of CuAAC based on kinetic ex-
periments and crossover experiments with isotopically pure 63Cu has il-
lustrated that two active copper(I) atoms are involved in the
cycloaddition step [8]. When [NH4]2[WS4Cu4I4] is applied as CuAAC
Scheme 3. Plausible catalytic pathway of [NH4]2[WS4Cu4I4]-catalyzed cycloaddition (B = base).

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X.-F. Gao et al. / Catalysis Communications 73 (2016) 103–108
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complex, and has the advantage of synergic effect. A plausible
mechanism of [NH4]2[WS4Cu4I4]-catalyzed azide-alkyne cycloaddition
is outlined in Scheme 3. In this proposed mechanism, the
copper(I) cluster is initially coordinated to acetylene derivative to
form π-complex and to lower the pKa of terminal alkyne C–H by up to
9.8 units [34]. When the terminal alkyne is deprotonated, the σ-com-
plex of copper(I) acetylide species is formed [8], and the electron densi-
ty of alkyne is reduced by the copper(I) atom [35]. Subsequently, the
copper(I) acetylide complex undergoes nucleophilic attack by azide
group coordinated on the neighbor copper(I) atom to form six-
membered intermediate, and the final product of 1,4-disubstituted tri-
azole is formed after release of the copper(I) cluster.
4. Conclusions
For the first time W(Mo)/S/Cu cluster compounds were successfully
explored as copper(I) catalyst in copper(I)-catalyzed azide–alkyne cy-
cloadditions (CuAACs). These three types of cluster compounds have
very different catalytic performances (B N C N A) in CuAACs, and their
different catalytic activities may be correlated to their different redox
potentials. Compared with CuI, [NH4]2[WS4Cu4I4] cluster was found to
have significant advantage in good stability and high catalytic efficiency.
CuAAC using [NH4]2[WS4Cu4I4] cluster as catalyst under both polar so-
lution and solvent-free conditions all gave the 1,4-disubstituted triazole
compounds in good yields with high tolerance of various functional
groups. This facile and efficient CuAAC reaction was also applied in the
modification of biomolecule and polymer material. The successful ex-
ploration of W(Mo)/S/Cu clusters as catalyst may give a new insight of
applying thiometallic clusters as homogeneous metallic catalyst for or-
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Acknowledgments
We are grateful to the National Natural Science Foundation of China
(No. 21272084, No. 21402058 and No. 21101062) and Central China
Normal University for support of this research.
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Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2015.10.019.
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