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Coordination Chemistry Reviews 272 (2014) 145–165
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Coordination Chemistry Reviews
journal homepage: www.elsevier.com/locate/ccr
Review
Catalysis by 1,2,3-triazole- and related transition-metal complexes
Deshun Huang a , Pengxiang Zhao a,∗ , Didier Astruc b,∗∗
a
b
Science and Technology on Surface Physics and Chemistry Laboratory, P.O. Box 718-35, Mianyang 621907, Sichuan, China
ISM, Univ. Bordeaux, 351 Cours de la Libération, 33405 Talence Cedex, France
Contents
1.
2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Coordination modes of triazole and triazolyl ligands with transition metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mn-triazole complexes for catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.
Mn-nitrogen coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fe-triazole complexes in catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.
4.1.
Fe-nitrogen coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.
Ni-triazole complexes in catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.
Ni-nitrogen coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.
Cu-triazole complexes in catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.
Cu-nitrogen coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.
Cu-carbene complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.
Ru-triazole complexes in catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.1.
Ru-nitrogen coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2.
Ru-carbenes complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.
Rh/Ir-triazole complexes in catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.1.
Rh/Ir-nitrogen coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2.
Rh-nitrogen complexes with anionic triazoles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.
Pd-triazole complexes in catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.1.
Pd-nitrogen coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.2.
Pd-carbenes complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.3.
Other types of molecular Pd-triazole catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.4.
Pd-nanoparticle catalysts stabilized by 1,2,3-triazole-containing macromolecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.
Au-triazole complexes in catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.1.
Au-nitrogen coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.2.
Au-carbenes complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.3.
Au-nitrogen complexes with anionic triazoles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
a r t i c l e
i n f o
Article history:
Received 10 February 2014
Accepted 7 April 2014
Available online 18 April 2014
Keywords:
Catalysis
Transition metal
Click chemistry
Triazole ligand
Carbene
a b s t r a c t
A short overview of the multiple coordination modes of 1,2,3-triazole- and related transition-metal complexes are provided, then the implication of and catalysis with transition-metal-1,2,3-triazole complexes
are detailed with Mn, Fe, Ni, Cu, Ru, Rh, Ir, Pd, and Au catalysts including various ligand coordination modes
and mechanistic features.
© 2014 Elsevier B.V. All rights reserved.
∗ Corresponding author. Tel.: +86 8163369780.
∗∗ Corresponding author.
E-mail addresses: [email protected] (P. Zhao), [email protected] (D. Astruc).
http://dx.doi.org/10.1016/j.ccr.2014.04.006
0010-8545/© 2014 Elsevier B.V. All rights reserved.
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D. Huang et al. / Coordination Chemistry Reviews 272 (2014) 145–165
1. Introduction
The 1,2,3-triazole heterocycle, known since the end of the 19th
century, is now a common heterocyclic ligand in chemistry and
biology [1]. Relatively few studies have been reported before the
year 2000 due to the limited availability of functional triazole
derivatives when the non-selective Huisgens reaction was used
for their synthesis [1b,1c]. The breakthrough in triazole chemistry
came in the early 2000s with a novel concept, that of “click” chemistry, that was first fully presented by Sharpless’ group [2,3]. The
“click” reactions described chemistry tailored to quickly and reliably generate substances by linking small units together under
“green” conditions. This has proved to be a powerful concept allowing molecule fragments to assemble. Indeed, the most popular
reaction representing the “click” chemistry concept is the Cucatalyzed alkyne–azide (CuAAC) reaction with the regioselective
formation of 1,4-disubstituted 1,2,3-triazoles [4]. Besides, the Rucatalyzed alkyne–azide (RuAAC) reaction was later disclosed to
also regioselectively form 1,2,3-triazoles, but at this time with 1,5disubstitution [5]. Thanks to these modular, facile and high-yield
methods for the generation of a large number of 1,2,3-triazoles
and their derivatives, 1,2,3-triazole heterocyclic chemistry now
appears as a new area with potential applications of 1,2,3-triazolemetal complexes in optics, redox sensing, biomedicine and catalysis
[6]. Transition-metal triazole and triazolyl complexes have recently
present catalytic activity for a number of organic reactions, and the
purpose of this review is to survey these properties and catalytic
reactions.
2. Coordination modes of triazole and triazolyl ligands
with transition metals
1,2,3-Triazoles bearing several donor sites are potentially versatile ligands for metal coordination [7]. Generally, there are mainly
three modes with which triazole ligands combine with transition
metals (Figs. 1, 3 and 4). The first mode is through nitrogen coordination of neutral simple triazoles and chelating triazoles (Fig. 1).
DFT calculations have shown that N3 is a better donor compared
to N2 [8]. The triazole ligand coordinates to a metal through the N3
nitrogen atom either as a monodentate ligand (type A) or as part
of a bi- or poly-dentate chelator (type B), when there are other
Fig. 1. Simple triazoles and chelating triazoles coordinate to transition metals.
Fig. 3. Deprotonated triazolium ligands (NHCs) transition metal complexes.
Fig. 4. Deprotonated NH 4,5-disubstituted triazolates as anionic ligands in transition metal complexes.
donor sites nearby. When the additional donor site is adjacent
to N1, coordination through N2 is possible to form a bi- or polydentate chelator (type C) [9]. Thus, for the metal chelators, five- or
six-membered cycles are usually formed. Besides, bridging coordination modes with two metals coordinating to two of the nitrogen
atoms are possible (types D and E).
The second mode is C5 coordination with deprotonated triazoliums to form N-heterocyclic carbenes (NHCs, Fig. 3). NHCs are a
class of well-known, very useful ligands resulting from the deprotonation of imidazolium salts, but members of the family are also
obtained by deprotonation of triazolium salts. NHCs are stronger
neutral electron donors (␴ donors), have a better oxidation stability and undergo easier modification than tertiary phosphines.
Therefore, they have been widely used as ligands with success
in transition metal catalysis [10]. Imidazolium salts are the most
frequently used carbene precursors with metal bounded at the C2
position. Subsequently, imidazole-based carbenes with the metal
bonded at the C4(5) position were also first reported by Crabtree
and co-workers (Fig. 2) [11]. These carbenes are called “abnormal”
N-heterocyclic carbenes (aNHCs), and they are even stronger ␴
donors than C2-bound “normal” N-heterocyclic carbenes (nNHCs)
[12]. The difference between these two classes of carbenes is that
free nNHCs have a resonance form with all-neutral formal charges,
while the free aNHC are mesoionic (Fig. 2). In 2008, Albrecht
and co-workers used 1,3,4-substituted 1,2,3-triazolium salts as
precursors for the synthesis of new aNHCs with various transition
metals [13]. These abnormal triazolylidene complexes (type F) are
expected to have a great potential for the development of new
catalysts with unprecedented reactivities. Recently, an example
Fig. 2. Imidazole-based nNHCs and aNHCs.
D. Huang et al. / Coordination Chemistry Reviews 272 (2014) 145–165
147
Scheme 1. Syntheses of abnormal triazolylidene complexes (the most common R3 X used so far in the literature is MeI).
of normal 1,2,3-triazolylidene carbene with a 1,2,4-substitution
pattern was also reported (type G) [14]. Interestingly, owing to
its unprecedented substitution pattern, the normal triazolylidenes
exhibit even higher donor strength than the abnormal 1,3,4substituted triazolylidenes [14a]. Similarly, metal centers can also
chelate to both the carbene carbons and the other adjacent donor
sites to form more complicate complexes. Additionally, the relatively acidic C H bond on the 5-position can be directly inserted
by transition metal to form a carbon–metal bond (see Section 9.3).
The third coordination mode results from the combination of
deprotonated NH 4,5-di-substituted triazolates as anionic ligands
in metal complexes (Fig. 4). The acidic N H protons can be used
to generate anionic ligands. Under basic conditions, benzotriazoles
bind to metal via N1 (type H) while 4,5-disubstituted-NH-1,2,3triazoles bind to metal via N2 (type I). Moreover, additional metal
centers can coordinate to the free neutral nitrogen atoms of the
metal triazolates forming bridged complexes (types J, K, and L) [3].
The nitrogen-coordinated triazole complexes are easily formed
by just combining the triazole with metal complex precursors,
resulting in ligand substitution. Thus the C5-coordinated abnormal
triazolylidene complexes have been generated by using 1,2,3triazolium precursors as follows (Scheme 1) [13]. The neutral
triazole heterocycles 1 are readily available by “click” reactions of
the corresponding alkynes and azides. Selective nitrogen alkylation
of the resulting triazoles 1 at the N3-position yields triazolium salts
2 that are precursors of abnormal triazolylidenes. Thus palladium
(3) and silver (4) abnormal carbene complexes are obtained by
metallation of the triazolium salts 2 via C H bond activation using
Pd(OAc)2 or Ag2 O. Other transition metal carbene complexes 5 are
also obtained by transmetallation of the resulting silver carbene
complexes.
The generation of normal 1,2,4-substituted 1,2,3-triazolylidenes
complexes is described in Scheme 2 [8]. Different from the synthesis of 1,3,4-substituted 1,2,3-triazoles using “click” chemistry,
the precursors 1,2,4-substituted 1,2,3-triazolium chlorides 6 are
synthesized via a ring closing procedure based on hydrazonoyl
chlorides and isocyanides. Then the metal carbene complexes 9 are
generated by either reacting the transition metals with the ammonia adducts 7 [14a] or transmetallation of the corresponding silver
carbene complexes 8 [14b].
3. Mn-triazole complexes for catalysis
3.1. Mn-nitrogen coordination
Following the order of elements in periodic table, Mn is the first
mentioned transition metal for which transition metal catalysts
have been disclosed. For instance, N4 tetradentate ligands have
been recently widely used as metal ligands for many important and
challenging catalytic reactions. However, most of them are based on
a pyridyl framework, which limited the possibilities of their modification. Recently, an efficient construction method using “click”
chemistry has been reported, forming a series of triazole-based
N4 tetradenate ligands (Scheme 3) [15]. Their Mn(II) complexes
Scheme 2. Syntheses of normal triazolylidene complexes. (The difference between “normal” 9 and “abnormal” 5 is that they have different carbene precursors (Fig. 3). The
precursor of “normal” 9 is neutral, while the precursor of “abnormal” 5 is mesoionic.)
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D. Huang et al. / Coordination Chemistry Reviews 272 (2014) 145–165
Scheme 5. Mn(II) complex-catalyzed epoxidation of cyclooctene.
decade, iron complexes have been examined in catalytic procedures that originally required expansive precious transition metals
such as palladium, ruthenium or rhodium. Obvious reasons are the
abundance of iron on earth, possibly lower toxicity than that of
other transition metals, and low cost [16b,16c]. Viewing the rapid
development of iron catalysts in the last decade, various efficient
iron-catalyzed processes have now become recognized.
Scheme 3. Synthesis of triazole-based N4 tetradenate ligands and their Mn(II) complexes.
Scheme 4. Mn(II) complex-catalyzed epoxidation of terminal olefins.
were used in the epoxidation of various terminal aliphatic olefins
with peracetic acid as oxidant, and showed good catalytic activities
with low catalyst loading and short reaction time (Scheme 4). As an
example of epoxidation of internal olefin, the Mn(II) catalyst also
showed good catalytic performance (Scheme 5) [15].
4. Fe-triazole complexes in catalysis
Iron complexes have long been known and widely used in coordination chemistry and organometallic chemistry [16]. In the last
4.1. Fe-nitrogen coordination
A remarkable property of iron catalysts is their dual reactivity,
where Fen+ serves either as a Lewis acid or as a redox center through
single-electron-transfer processes [17]. With their combination of
␴-donor (nitrogen lone-pair electrons) and ␲-receptor properties,
the N-heterocycles are considered to be very useful ligands for
Fen+ catalysis as witnessed in the recent literature [18]. However,
up to now, reports concerning the 1,2,3-triazole-Fe catalysts are
very limited, and little is known concerning the comparison of
these ligands with other N-heterocyclic ligands. Shi’s research has
addressed this aspect, however [19].
As shown in Fig. 5, among other heteroaromatic ligands including pyridine, imidazole, tetrazole, and differently substituted
triazoles with the same binding patterns, only the 1,2,3-triazole
ligands provided good yields of enyne in the propargyl alcohol
dehydration reaction. This property has been attributed to the
required electronic effect of 1,2,3-triazole in adjusting the Lewis
acidity of Fe3+ in order to achieve the needed chemoselectivity
for this transformation [19]. This finding shows an example of the
Fig. 5. Conversions and yields obtained with iron complexes of various ligands for propargyl alcohol dehydration.
D. Huang et al. / Coordination Chemistry Reviews 272 (2014) 145–165
149
6. Cu-triazole complexes in catalysis
Scheme 6. Intermolecular nickel-catalyzed coupling of ␲ components in the presence of a main-group organometallic complex M R.
advantage of 1,2,3-triazoles over other N-heterocycles and should
attract attention for catalysts using 1,2,3-triazole-Fe complexes in
future research.
5. Ni-triazole complexes in catalysis
Nickel-catalyzed coupling and cyclization reactions, including oligomerizations, olefin polymerizations, dimerizations, and
hydrometallations have been applied for more than 50 years [20].
The key role of nickel in these reaction is reductive coupling of two
␲ components with a main-group organometallic reagent or metal
hydride (Scheme 6) [20b].
Copper(I) complexes are frequently used catalysts in organic
synthesis. Cu(I) triazole complexes have essentially been used
with triazoles as nitrogen donors for the popular Cu-catalyzed
azide–alkyne cycloaddition (CuAAC) reaction, the most current socalled “click” reaction. The CuAAC was first introduced using Cu(II)
sulfate with sodium ascorbate as a reducing agent in situ to form
air-sensitive Cu(I) species, then with a large variety of copper salts,
in particular with nitrogen ligands and carbenes that activate the
Cu(I) catalyst toward the CuAAC reaction [2,3]. The search for highly
active Cu(I) catalysts was indeed driven by the need to avoid product contamination that is unacceptable in electronics, or can lead
to toxicity in biomedical applications. To overcome this problem,
various copper catalysts have been applied to the CuAAC reactions,
including nano-sized solid catalysts, dendritic copper catalysts and
Cu(I) complexes of organic ligands [24–29]. Thus, as a member of
both nitrogen donating ligands and ␲-acceptor carbenes, Cu(I) triazole complexes are very active toward CuAAC reactions [30–34]
and other organic reactions [35] in recent progress.
5.1. Ni-nitrogen coordination
6.1. Cu-nitrogen coordination
Among all the above nickel-catalyzed reactions, olefin polymerization is the most important one due to its broad applications
in both academic and industrial areas. The use of catalysts based
on nickel complexes (and other late transition metals) containing
Schiff-base ligands has been a major advance in the development
of this reaction because of the unique N ␴-donor properties of these
ligands [21]. Therefore the related 1,2,3-triazole complexes [22,23]
have also been investigated in this context during the last few years,
and they have exhibited a comparable or even better catalytic activity than those of Schiff-base complexes.
Fig. 6 shows the triazolyl-functionalized Schiff base
bis(imino)acenaphthene (BIAN) as a ligand. When this ligand
was coordinated to Ni(II), and using the classic co-catalyst methylaluminoxane (MAO), it showed better activity for ethylene,
norbornene and styrene polymerization than the BIAN-Ni(II)/MAO
system. In this case, the triazolyl groups were considered as labile
moieties of chelating ligands, which were substituted by the
norbornene and styrene monomers, thus generating 16-electron
active species that accelerated the polymerization. Ni(II)-triazole
catalysis has only been reported after 2010, and thus this finding
may now encourage the development of Ni(II) catalysts.
Very efficient N-coordinated triazole-Cu(I) catalysts for CuAAC
reactions were reported in 2004. In their seminal study, the authors
used tris(benzyltriazolylmethyl)amine (TBTA) as a ligand to stabilize Cu(I) for the “click” reaction between phenylacetylene and
benzyl azide (Scheme 7). The tetradentate nature of TBTA allows
completely encapsulation of the Cu(I) center, with the central basic
tertiary amine supposed to be permanently coordinated during
catalysis, while the pendant triazole groups temporarily dissociate
from the metal center to allow formation of a Cu(I)-acetylide intermediate that is crucial in the catalytic cycle. The tetradentate TBTA
ligand protects the Cu(I) center from oxidation and disproportionation, while enhancing its catalytic activity [30b]. Therefore it has
been considered as much more efficient than mono- or bis-triazolyl
ligands.
Later, Williams et al. reported that Cu(II) also coordinated
TBTA, and in the presence of the proper reducing agent, for
example sodium ascorbate, the Cu(II) complex converted to an
active Cu(I) catalyst. In addition, with the classic copper precursor
[Cu(CH3 CN)4 ][BF4 ], the binuclear dicationic Cu(I) complex represented in Fig. 7 formed and is also a very efficient catalyst for the
CuAAC reaction [32].
In addition, covalently immobilized catalysts for CuAAC reactions have been frequently used in recent years [36], but in these
cases the reactions involve either over-stoichiometric amounts of
catalytic resin or high copper loadings [37]. Therefore, due to their
unique nature, the tris(triazolyl) ligands with apical functional
groups were considered as appropriate ligands for CuAAC reactions
R1
+
R2 N3
Cu(I) (0.25-1 mol%)
TBTA (1 mol%)
t
R1
N
N R2
N
>99%
BuOH:H2O, rt
Bn
N
N
N
TBTA =
N N
N
NBn
N
BnN N
Fig. 6. The structure of BIAN and triazolyl-functionalized BIAN, useful ligands in
Ni(II) polymerization catalysis.
Scheme 7. The TBTA-Cu(I)-catalyzed CuAAC reaction (R1 = Ph, R2 = CH2 Ph).
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D. Huang et al. / Coordination Chemistry Reviews 272 (2014) 145–165
Scheme 8. Synthesis of a copper(I) catalyst on polymer-supported tris(triazolyl)methanol ligand [33b].
and further investigated for the design of polymer-immobilized
copper catalysts.
As shown in Scheme 8, the tris(triazolyl) methanol ligand was
firstly linked to the Merrifield resin by alkylation, then CuCl was
added to obtain the immobilized catalyst. This catalyst exhibited a
very efficient “click” activity toward a series of substrates in aqueous or methanol/water system, and tolerated variations of reaction
parameters. In addition it showed an excellent recyclability and
was easy to separate from the product by filtration [33a]. Later on,
Astruc’s group improved this strategy by anchoring this catalyst
onto SiO2 -coated ␥-Fe2 O3 nanoparticles as supports rather than
on polymers. Due to their insoluble, paramagnetic and nanosized
nature, they display better stability and reusability, more efficient
catalytic activity, lower preparation costs, and lower toxicities in
comparison with other materials that are used as supported heterogeneous catalysts. Moreover, this catalyst can be successfully
extended to various organic azides and alkynes, and this procedure is easy to operate, economical, and environmentally friendly
if the azides are not isolated but just intermediates before “click”
reactions with alkynes in situ [33b].
6.2. Cu-carbene complexes
Although the carbene-metal bond is very popular in transition
metal catalysis, Cu-carbene complexes have rarely been reported
in the last decade for the CuAAC reaction. This might be due to
the formation of the Cu-carbene bond between the product and
the copper catalyst during the CuAAC reaction, competing with
the Cu-carbene bond of the catalyst itself. However, there is one
example showing the possibility of a Cu-carbene catalyst from triazole for the CuAAC reaction in which the authors used copper
with 1,4-diphenyl, 1,4-dimesityl, and 1-(2,6-diisopropylphenyl)4-(3,5-xylyl)-1,2,3-triazol-5-ylidene (aNHC) to prepare the new
CuCl(aNHC) catalyst (Fig. 8). This catalyst efficiently catalyzed
Fig. 8. Triazole-based Cu-carbene catalysts.
CuAAC reactions of azides with alkynes to give 1,4-substituted
1,2,3-triazoles in excellent yields at room temperature with short
reaction times [34].
Besides, in very recent results from Sarkar’s group, the copper(I)
complexes containing two triazolylidene ligands (see Fig. 9) was
proven to be capable of catalyzing the click reaction between bulky
azides (a class of substrates considered as difficult to transform
using the original click recipe) and electronically diverse alkynes
under mild conditions [38].
7. Ru-triazole complexes in catalysis
Ruthenium (Ru) is an important transition metal under various oxidation states from 0 to 6 with a variety of ligands for
catalysis of a large number of homogeneous reactions [39]. However, among all the oxidation states, 1,2,3-triazole Ru catalysts
have essentially been synthesized in the low oxidation state Ru(II).
The Ru(II) complexes show high efficiency in many reactions
such as hydrogenation, reduction via hydrogen transfer, alkyne
transformations via ␩2 -alkyne, ␩1 -vinylidene or allenylidene intermediates and oxidative coupling processes, cyclopropanation, and
olefin metathesis [39b]. In spite of the application of Ru(II), its
limited stability and the low activity toward the substrates are still
sometimes drawbacks. Therefore, the proper ligands for stabilizing
and activating Ru(II) are called for [39e], and 1,2,3-triazoles allowed
to improve catalytic activity as NHC ligands [40–48].
7.1. Ru-nitrogen coordination
Ru complexes with N-coordinated triazole ligands are active
catalysts for alcohols oxidation. The synthetic route to these halfsandwich complexes is shown in Scheme 9, and the X-ray crystal
Fig. 7. ORTEP picture, with ellipsoids at the 20% probability level, of the dicationic
Cu(I) catalyst.
Reprinted with permission from Ref. [32] (White’s group). Copyright 2008 Royal
Chemical Society.
Fig. 9. The two triazolylidene ligand-copper complexes.
D. Huang et al. / Coordination Chemistry Reviews 272 (2014) 145–165
151
Scheme 9. Syntheses of 1,2,3-triazole ligands and chelated N-triazole Ru catalysts.
Fig. 10. ORTEP diagram of catalyst 11 (a) and 13 (b) with ellipsoids given at the 30% probability level. H atoms and the PF6 − anion have been omitted for clarity.
Reprinted with permission from Ref. [40] (Singh’s group). Copyright 2013 American Chemical Society.
structures (Fig. 10) of these Ru catalysts were determined. In the
presence of N-methylmorpholine N-oxide (NMO) these complexes
exhibited a high activity toward the oxidation of a series of primary
and secondary alcohols, and these complexes were also demonstrated to be catalytically efficient in transfer hydrogenation of
ketones to alcohols [40]. In both reactions the species in which N2
is bonded to Ru (catalyst 13 and 14) are more efficient than those
involving N3 (catalysts 11 and 12).
Cenini and co-workers studied the influence of a triazole ligand (triazoline) on the catalytic aziridination of olefins
[41]. As shown in Fig. 11, they used Ru(TPP)CO (TPP = dianion
of tetraphenylporphyrin) with 1-(p-nitrophenyl)-5-methyl-5phenyl-1,2,3-triazoline to yield a 2 -1,2,3-triazoline Ru(II) porphyrin complex that is responsible for the catalyst deactivation
in the aziridination reaction of ␣-methylstyrene by p-nitrophenyl
azide.
7.2. Ru-carbenes complexes
The complexes (␩6 -arene)Ru-(NHC) are among the best catalysts in the formation of amides by direct coupling between
alcohols and amines [42]. An illustration was provided by Albrecht
and co-workers who introduced Ru(II) complexes comprizing a
1,2,3-triazolium-derived carbene complex as catalyst for the selective oxidation of benzylic alcohols and amines and for the oxidative
coupling of alcohols and amines to form amides [43]. As shown
in Fig. 12, the triazolylidene complexes (top) are more effective
than the imidazolylidene systems (bottom) in the base-free oxidation of alcohols, while the opposite is found for the oxidative
homocoupling of amines and the coupling of amines and alcohols
to form amides. This result illustrates how subtle changes in the
Fig. 11. The triazole ligand for the Ru(II)-catalyzed aziridination of olefins.
Fig. 12. Triazolylidene and imidazolylidene ligands in Ru catalysts.
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D. Huang et al. / Coordination Chemistry Reviews 272 (2014) 145–165
Scheme 10. Postulated activation pathway with formation of intermediates.
Scheme 11. Synthesis of 1,3-diaryl-substituted MICs based ruthenium olefin metathesis catalysts.
electron-donating properties of ancillary ligands may affect the
catalytic activity of Ru(␩6 -arene)(NHC) scaffolds.
Later, the same group expanded studies to triazolylidene complexes as catalysts for the oxidation of benzylic alcohols under
base-free conditions [44], and the very best activity was disclosed
for R = n-Bu and R = n-Bu, or R = n-Hex, R = n-Hex (Fig. 13). Additionally, other primary and secondary benzylic alcohols provided
the corresponding aldehydes and ketones in good to excellent
yields when these catalysts were used. The postulated catalytic
activation pathway is represented in Scheme 10. The absence of
base and oxidant is appealing in terms of atom economy and should
allow wide functional group tolerance [44,45].
Grubbs and co-workers reported the preparation of a variety of
1,3-diaryl-substituted MICs by cycloaddition between 1,3-diaza2-azoniaallene salts and alkynes, followed by deprotonation with
alkoxide bases. The highly stable N-arylated MICs were then
transferred to ruthenium olefin metathesis catalysts 16 by ligand
substitution (Scheme 11). The MICs-bearing ruthenium complexes
Fig. 13. Imidazolylidene-Ru complexes for the oxidation of benzylic alcohols.
Scheme 12. ROMP of cyclic olefins and RC olefin metathesis reactions.
16 are highly active in catalyzing the ring-opening metathesis polymerization (ROMP) of cyclic olefins and ring-closing (RC) olefin
metathesis reactions (Scheme 12) [46].
A ruthenium complex containing an N-heterocylic carbene
(NHC) and an unhindered mesoionic carbene (MIC) is extremely
active for alkene metathesis upon protonation of the Ru–MIC bond
by reaction with a Brønsted acid (Scheme 13) [47].
A bis-Ru(II) complex supported by a pyrrole-containing 1,2,3triazolylidene framework is an active catalyst for the ROMP of
Scheme 13. Protonation of the Ru–MIC bond to generate a metathesis-active
species.
D. Huang et al. / Coordination Chemistry Reviews 272 (2014) 145–165
153
Fig. 14. Rhodium(I) and iridium(I) complexes with pyrazolyl–triazolyl bidentate ligands.
norbornene when activated with (trimethylsilyl)-diazomethane
(Scheme 14) [48].
8. Rh/Ir-triazole complexes in catalysis
Rh and Ir complexes are well-known catalysts, particularly for
alkene hydrogenation, hydroformylation and hydroamination, and
phosphines and NHCs are frequently used as donor ligands in these
reactions. Recently, the versatile 1,2,3-triazoles ligands were also
combined with Rh or Ir to promote these reactions, and the corresponding complexes were good catalysts [49].
8.1. Rh/Ir-nitrogen coordination
Messerle and co-workers synthesized pyrazolyl–1,2,3-triazolyl
bidentate ligands via “click” chemistry and a series of cationic
rhodium and iridium complexes herewith (Fig. 14) [50a]. Singlecrystal X-ray diffraction (Fig. 15) showed that the metal center
coordinates to the N3 atom of the triazolyl moiety and the N2 atom
of the pyrazole moiety, forming six-membered metallacycles. The
triazolyl donating capacity is stronger than that of the pyrazolyl
donor, as illustrated by the slightly shorter M–N(triazole) bonds
compared with the M–N(pyrazole) bonds. These new metal complexes are efficient catalysts for the intramolecular hydroamination
of a series of alkynamines and alkenamines (Scheme 15). The iridium complexes were generally more active for the intramolecular
hydroamination reaction of 4-pentyn-1-amine than their rhodium
analogs, while the rhodium complexes were more active catalysts
than their iridium counterparts for the cyclization of alkenamines. Complexes containing the BArF 4 − (BArF 4 = tetrakis[3,5bis(trifluoromethyl)phenyl]borate) counteranion and dicarbonyl
co-ligands are superior catalysts than the analogous complexes
with the BPh4 − counteranion and the COD co-ligand. The
pyrazolyl–triazolyl rhodium and iridium complexes showed better
efficiency and selectivity than previously reported late transition
metal catalysts for the same reactions [50a].
Fig. 15. ORTEP depictions of the cationic fragments of the solid-state structures of (a) [Rh(PyT)(COD)]BPh4 (19, n = 1) and (b) [Ir(PyT)(COD)]BPh4 (20, n = 1) at 40% thermal
ellipsoids for the non-hydrogen atoms.
Reprinted with permission from Ref. [50a] (Messerle’s group). Copyright 2012 American Chemical Society.
Scheme 14. A bis-Ru(II)-pyrrolyl-1,2,3-triazolylidene catalyst for the ROMP of norbornene.
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D. Huang et al. / Coordination Chemistry Reviews 272 (2014) 145–165
NH2
n
(Fig. 16) [51]. However, these complexes showed limited catalytic
efficiency for the intramolecular hydroamination of 4-pentyn-1amine to form 2-methylpyrroline. These results appear to confirm
the general trend according which complexes with ligands that
strongly bind the metal center decrease the catalytic activity for
the hydroamination reaction.
Asymmetric reduction of ketones was obtained by combining [RhCl2 Cp*]2 with a series of l-amino acid thioamide ligands
functionalized with 1,2,3-triazoles via an asymmetric transfer
hydrogenation (ATH) process [52]. The active reduction catalyst
was formed in situ from [RhCl2 Cp*]2 and the amino acid-triazole
ligands in the presence of sodium isopropoxide and lithium chloride. A series of aryl alkyl ketones underwent ATH in the presence
of the Rh complex to obtain secondary alcohol products with
high conversions and moderate to good enantiomeric excesses
(Scheme 17). Generally, these triazole-functionalized catalysts
showed higher activity but inferior enantioselectivity in comparison with other amino acid-based catalyst systems used in the ATH
of acetophenone.
The combination of phosphine and “clicked” 1,2,3-triazole ligands have also received much attention in Rh catalysis. The first
new chiral 1,2,3-triazole ferrocenyl-based P,P- and P,N-ligands
(ClickFerrophos) (Fig. 17, 28 and 29) were prepared by Fukuzawa
et al. The Rh complexes of ClickFerrophos 29 were effective catalysts for the hydrogenation of alkenes affording products with up
to 99.7% ee. Catalytic asymmetric hydrogenation of ketones with
up to 98% ee was achieved by Ru complexes of Click Ferrophos 29,
and the palladium complex with ClickFerrophos 28 performed well
in asymmetric allylic alkylation with 79% ee [53].
The asymmetric hydrosilylation of ketones is also a versatile method to synthesize enantiomerically enriched alcohols.
N
[Ir]/[Rh]
n
R1
R1
n = 0, 1; R1 = H, Ph
R2
R2
H
N
[Ir]/[Rh]
NH2
H
R2
R2
R2 = Me, -(CH 2) 5-, Ph
Scheme 15. Rh and Ir catalyzed intramolecular hydroamination of alkynamines and
alkenamines.
Later on, the same group further reported the synthesis of a series of Rh(III)/Ir(III) and Rh(I)/Ir(I) complexes with
pyrazolyl–1,2,3-triazolyl bidentate ligands [50b]. Increasing the
electron-withdrawing strength of the phenyl substituent on the
triazolyl ring led to poorer donating capacity of the triazolyl donor
toward the metal center. Both the Rh(I) and Ir(III) complexes
are effective catalysts for C N bond formation of 2-(hydroxyalk1-ynyl)-anilines yielding the corresponding indoles, the Rh(I)
complexes being the most efficient catalysts. Interestingly, the
Ir(III) complexes showed efficient catalytic activity for the synthesis of tricyclic indoles by tandem C N/C C bond formation
(Scheme 16) [50b].
A series of Rh(I), Rh(III) and Ir(III) complexes with bidentate
NHC–1,2,3-triazolyl ligands in combination with a strong imidazolium NHC-C donor and a weak 1,2,3-triazolyl-N donor were
reported by Messerle and co-workers and were expected to be
desirable catalysts presenting both good stability and selectivity
BArF4
N Ir Cl
N N
N
N
23
5 mol%
BArF4
R
NO2
= tetrakis[(3,5-trifluoromethyl)phenyl]borate
n
R
R
n
HO
HO
C-N Bond Formation
n
C-C Bond Formation
t
NH 2
n = 1; R = H, Me
n = 2; R = H
BuOK
N
H
Scheme 16. Ir(III) complex-catalyzed tricyclic indoles synthesis by tandem C N/C C bond formation.
Fig. 16. Rh(I), Rh(III) and Ir(III) complexes with bidentate imidazolium–1,2,3-triazole ligands.
N
H
D. Huang et al. / Coordination Chemistry Reviews 272 (2014) 145–165
155
Scheme 17. Rhodium-catalyzed asymmetric transfer hydrogenation of aryl alkyl ketones.
Scheme 18. Rhodium-catalyzed hydroformylation of 1-octene.
8.2. Rh-nitrogen complexes with anionic triazoles
Fig. 17. Chiral triazole-based P,P- and P,N ligands.
Several Rh complexes with chiral glycosyl-triazole-based P,N
ligands (Fig. 17, 30) were synthesized and applied to the catalytic
asymmetric hydrosilylation of substituted acetophenones to
obtain optical active alcohols in good conversion with moderate
enantiomeric excesses (up to 72% ee) [54].
In some cases, ligands with strong ␲-accepting P-O-groups are
more efficient in catalysis than phosphines. Takacs and co-workers
prepared a series of chiral diphosphites using “clicked” triazoles
to construct ligand scaffolds [55]. The triazole diphosphites were
coordinated with Rh to form a 16-membered P,P-macrocyclic Rh(I)
chelate (Fig. 18), which showed high enantioselectivity (up to 97%
ee) in rhodium-catalyzed asymmetric hydrogenation of enamide.
Mono- and bidentate phosphite ligands based on 1,2,3-triazole
backbone (Fig. 19) have been prepared and applied to the
Rh-catalyzed hydroformylation of 1-octene, leading to high conversions and up to 87% n-regioselectivities (Scheme 18) [56].
N
N
N
O P
O
O
O
O
P
O
Rh
In 2008, Shi and co-workers reported an efficient synthesis
of 4,5-disubstituted-NH-1,2,3-triazoles (TRIA) through a catalytic
cascade three-component condensation (Scheme 19) [57]. In
comparison with generated “click” N-substituted triazoles, the
NH-triazole possesses acidic N H protons and can potentially be
applied as an anionic ligand. As expected, the NH-triazoles were
applied to coordinate cationic Rh(I) under basic conditions, forming
a new class of triazole-bridged [Rh(COD)(TRIA)]2 complexes as confirmed by X-ray diffraction (Fig. 20) [58]. Prior to this, all reported
efforts regarding triazole anion binding had focused on benzotriazoles. The [Rh(COD)(TRIA)]2 complexes showed great stability
toward air and moisture and exhibited effective catalytic properties
in Pauson–Khand reactions (Scheme 20).
9. Pd-triazole complexes in catalysis
Palladium complexes have received enormous attention from
academic and industrial aspects in the last decades as exceptional
catalysts for a variety of reactions including oxidation, substitution,
allylic alkylation and cross-coupling reactions [59,60], and their
reactivity is greatly dependent on the supporting ligands. During
the last decades of last century, phosphine-based ligands have been
used as the most common ligands in palladium-catalyzed reactions. However, these ligands often have drawbacks such as cost,
air sensitivity, toxicity and instability at high temperatures. Thus,
the search of efficient phosphine-free ligands has become active
during the last 15 years, and among them N-heterocyclic carbenes
31
H
R1
NO 2
R2
O
NaN3, L-Proline
+
R
2
Ar
H
1
R
N
NH
DMSO, rt
Ar
Fig. 18. Triazole diphosphites chelated 16-membered P,P-macrocyclic Rh(I) complex.
N
Scheme 19. Catalytic cascade syntheses of 4,5-disubstituted-NH-1,2,3-triazoles
(TRIA).
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D. Huang et al. / Coordination Chemistry Reviews 272 (2014) 145–165
Fig. 19. 1,2,3-Triazole based mono- and bidentate phosphite ligands.
KOH, [Rh(COD)Cl2 ]
N
Ph
NH
Ph
Ar
Ph
O
N
Ph
N
N
N
N
Ar
35a : Ar = Ph
35b: Ar = p-NO2 Ph
N
MeOH, rt
N
Ar
Rh
Rh
Ph
9.1. Pd-nitrogen coordination
5% 35, 6% dppp
CO, xylene, 130 oC
O
O
yield: 80% (35a), 88% (35b)
Scheme 20. Synthesis of
Pauson–Khand reactions.
corresponding normal NHC-Pd complexes in the Miyaura–Suzuki
and Mizoroki–Heck reactions, probably due to the stronger ␴donor ability of aNHC compared to NHC [62]. The versatile synthesis
of 1,2,3-triazoles has made them appropriate aNHC ligands, and the
1,2,3-triazol-5-ylidene palladium complexes have showed great
potential for catalytic activity in organic reactions. Furthermore, the
flexibility of the nitrogen coordination of 1,2,3-triazoles provides
additional opportunities in palladium catalysis.
[Rh(COD)(TRIA)]2
complexes
and
catalysis
of
have emerged as the most versatile ligands for palladium catalyzed cross-coupling reactions [61]. The majority of NHC ligands
are based on imidazol-2-ylidenes and 1,2,4-triazol-5-ylidenes as
normal NHCs. The Pd-aNHC complex were more effective than the
NHC ligands generally demonstrate higher donating capability
but less labile properties than P- and N-donor ligands. Thus, mixed
tridentate donor ligands that comprise the strong carbene ligand
and relatively labile nitrogen donor atoms are effective in achieving both high catalytic activity and stability. A novel ligand that
features a carbene and a labile triazolyl donor has been synthesized via a copper catalyzed click reaction by Chen and co-workers
[63], and silver, palladium, and platinum complexes of this ligand
have been synthesized and characterized. For instance, the pincer palladium complex represented in Fig. 21 shows high activity
Fig. 20. Crystal structure of 35a.
Reprinted with permission from Ref. [58] (Shi’s group). Copyright 2009 American Chemical Society.
D. Huang et al. / Coordination Chemistry Reviews 272 (2014) 145–165
157
Fig. 21. A pincer palladium complex with a labile triazolyl donor.
N
N N
N
R
Pd
AgBF4
Pd
Cl
-AgCl
N
N
N
R
BF 4-
N
R = Ph, 36a
R = n Pr, 36b
Scheme 21. Syntheses of triazole chelated cationic palladium allyl complexes.
for Suzuki–Miyaura cross coupling reactions of aryl bromides and
1,1-dibromo-1-alkenes in neat water under air.
Two triazole chelated cationic palladium allyl complexes have
been synthesized by Scrivanti and co-workers (Scheme 21) [64],
and single-crystal X-ray diffraction analysis of the solid-state
structure revealed that the triazole ligand chelates the palladium
through N2 nitrogen on the triazole heterocycle and N4 nitrogen on
the pyridine substituent (Fig. 22). These complexes have also are
active catalysts in the Suzuki–Miyaura coupling of phenyl boronic
acid with aryl bromides (Scheme 22).
Very recently, Wang et al. reported the use of 2-pyridyl-1,2,3triazole (pyta) as a bidentate ligand to coordinate with palladium
via N3 of 1,2,3-triazole and the nitrogen atom of the pyridinyl
attached to the C4 of the 1,2,3-triazole. Various mono- and polymetallic palladium complexes containing a 2-pyridyl-1,2,3-triazole
Fig. 22. ORTEP view of the cation of 36a. The hydrogen atoms and the tetrafluoroborate anion have been omitted for clarity. Thermal ellipsoids drawn at the 40%
probability level. The position of the C(16A) atom with refined site occupancy = 0.41
has been drawn with white bonds.
Reprinted with permission from Ref. [64a] (Scrivanti’s group). Copyright 2011 Elsevier B.V.
R
Br + (HO)2 B
36a or 36b
0.01-0.1 mol%
K2 CO3, DMF/H 2 O
R
R = CH 3C(O) 36a, 68% conv; 36b, 98% conv
36a, 28% conv; 36b, 22% conv
R = CH 3
R = CH 3CO 36a, 40% conv; 36b, 10% conv
Scheme 22. Catalytic activities of complexes 36 in Miyaura–Suzuki coupling reactions.
ligand and a nonabranch-derived ligand have been synthesized
(Scheme 23) and are excellent homogeneous or heterogeneous
catalysts for Suzuki–Miyaura, Sonogashira and Heck reactions
[65].
Scheme 23. Synthesis of mono palladium complexes containing 2-pyridyl-1,2,3-triazole ligands (37) and polymetallic palladium complexes containing a nonabranch-derived
ligand (38).
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D. Huang et al. / Coordination Chemistry Reviews 272 (2014) 145–165
Scheme 24. The applications of benzotriazole palladium complex in various coupling reactions.
Fig. 23. A fluoroalkylated triazole ligand.
Verma et al. have designed a robust N,N-type of bidentate ligand
based on benzotriazole with N3 being substituted by 2-pyridyl,
for the palladium-catalyzed C C (including Miyaura–Suzuki, Heck,
Fujiwara–Moritani, and Sonogashira), C N and C S coupling reactions (Scheme 24) [66]. The bidentate ability of the ligand is
considered to be enhanced by the donor ability of the N N bond of
the benzotriazole ring and the lone pair of electrons on the nitrogen atom of the pyridyl ring. The ligand is inexpensive, thermally
stable, and easy to synthesize. Along with the tolerance of a variety of functionalized reactants, this ligand is expected to find great
applications in organic synthesis.
A recyclable fluoroalkylated 1,2,3-triazole (Fig. 23) was
prepared as an efficient ligand for the palladium-catalyzed
Suzuki–Miyaura and Mizoroki–Heck reactions [67]. The ligand
could be recovered by fluorous solid-phase extraction with only
slightly decreased activity.
In recent years, the combination of phosphine and “clicked”
1,2,3-triazole have emerged and acted as ligands in catalysis
(Fig. 24). Zhang and co-workers have prepared triazole-based
monophosphine ligands (ClickPhos) (39), and their palladium complexes have been demonstrated to be effective catalysts in the
amination and Miyaura–Suzuki coupling reactions of unactivated
aryl chlorides [68]. ClickPhine ligands (40) have also been synthesized by click cyclization of propynyldiphenylphosphine as
reported by van Maarseveen and co-workers. These ligands were
efficient in the Pd-catalyzed allylic alkylation reaction [69].
9.2. Pd-carbenes complexes
Since the first synthesis and characterization of 1,2,3-triazol5-ylidene-metal (Pd, Rh, and Ir) complexes by Albrecht in 2008,
their catalytic properties have subsequently been investigated. In
2009, Sankararaman and co-workers [70] reported the first chiral palladium aNHC complex 41 and the first palladium bis-aNHC
chelated pincer complex 42, both of which derived from 1,2,3triazolylidene (Fig. 25). The palladium complexes were obtained by
Fig. 24. ClickPhos ligands 39 and ClickPhine ligands 40.
Fig. 25. Several palladium aNHC complexes developed by Sankararaman.
D. Huang et al. / Coordination Chemistry Reviews 272 (2014) 145–165
COOEt
43 (0.5 mol%)
+
COOE t
159
COOEt
TFA, CH 2Cl2
+
44%
COOEt
13%
Scheme 25. Hydroarylation of alkynes.
Mes
Me
Mes
Cl N
N
N
Pd
N
N N
Cl
Me
Mes Mes
44
Me
Dipp
Cl
N N
Pd
N
Dipp
Ph
Dipp = 2,6-diisopropylphenyl
45
Fig. 27. Palladium aNHC complexes prepared by Fukuzawa.
Fig. 26. ORTEP drawing of 43 with thermal ellipsoids at the 30% probability level.
Reprinted with permission from Ref. [71] (Sankararaman’s group). Copyright 2011
American Chemical Society.
transmetallation of the corresponding silver-carbene complexes.
Both complexes are effective in the catalysis of the Suzuki coupling reaction for the synthesis of biphenyl derivatives. However,
they failed to catalyze the formation of binaphthyl derivatives,
instead leading to a deboronation reaction. Later on, the same
group synthesized a 1,2,3-triazolylidene-based binuclear palladacycle complex 43 with bridging acetate ligands (Figs. 25 and 26)
[71]. Both palladium atoms are attached to both the carbene carbon
and the carbon atom located at the ortho position of the phenyl ring
on the nitrogen atom of the triazole ring. The complex showed moderate catalytic activity in the hydroarylation reactions of alkynes
in the presence of TFA (Scheme 25). Several electron-rich arenes
underwent hydroarylation to form the corresponding vinyl derivatives in a stereoselective manner.
Fukuzawa and co-workers [72a,72b] prepared a palladium
aNHC complex 44 derived from 1,4-dimesityl-1,2,3-tirazole
(Fig. 27). This catalyst was successfully used in C C cross-coupling
reactions such as the Miyaura–Suzuki, Mizoroki–Heck, and Sonogashira reactions. It was demonstrated that this bis-TMes-Pd
complex was even more efficient than the bis-IMes-Pd analog complex that is known as an effective imidazole carbene complex
due to electronically-rich and sterically-hindered mesityl groups.
A mono-triazolylidene aNHC-palladium complex 45 (Fig. 27) with
a cinnamyl ligand was also synthesized and is a highly active catalyst for the room-temperature Suzuki–Miyaura coupling reaction
R
Cl
Cl
N
N
Pd N
Me N
Cl
R'
R = Me, Et, nBu, Mes
R' = Ph, nBu, Mes
46
[72c]. The reactions proceeded with aryl chlorides, regardless of
the electronic and steric properties of the substituents, as well as
with sterically crowded arylboronic acids. The remarkably superior catalytic activity of these triazolylidene NHC complexes may
be attributed to the stronger donor properties of the ligands than
those of the imidazole-derived NHCs.
In 2012, Albrecht and co-workers [73] reported the synthesis of a series of 1,2,3-triazolylidene-derived pyridine enhanced
precatalyst preparation stabilization and initiation (PEPPSI) palladium complexes 46 with the 3-chloropyridine ligand as an
easily cleavable ligand (Fig. 28). The activity of these complexes
in Suzuki–Miyaura cross-coupling can be adjusted by varying the
substituents on the triazolylidene ring. In contrast to imidazol-2ylidene, less bulky substituents induce better catalytic activity than
the bulkier, sterically congested mesityl substituents. Experimental
evidence indicates that palladium nanoparticles are generated as
the resting state of the catalyst in a heterogeneous manner, and palladium atoms are leached from the nanoparticles as active species
to catalyze the reaction under relatively mild conditions. However,
these complexes appear to be less effective than the analogous NHC
derivatives reported by Organ [74].
Nearly in the same time, Crudden and co-workers [75] reported
similar mono- and bimetallic 1,2,3-triazol-5-ylidene abnormal carbene complexes of palladium with pyridine as ligand (Fig. 28,
47 and 48). These new PEPPSI complexes were tested in the
Mizoroki–Heck reaction, and high conversion was observed with
methyl acrylate in the case of aryl iodides and electron-deficient
bromides. Consistent with Albrecht’s observation, the reaction
likely proceeds via palladium nanoparticles as suggested by the
mercury-poisoning test.
A series of hetero-bis(carbene) complexes bearing i Pr2 -bimy
and mesoionic 1,2,3-triazolin-5-ylidenes with various substituents
have been reported by Huynh and co-worker (Fig. 29) [76a]. A study
of the 13 C NMR spectra shows that the 1,2,3-triazolin-5-ylidenes
are generally stronger donors than normal NHCs but weaker than
some abnormal NHCs such as pyrazolin-3-ylidenes and mesoionic
imidazolin-4-ylidenes. The trifluoroacetato analogs have been
Dipp
N
N
Cl
N
Cl Pd
Cl
N
Pd
Cl
Pd N
Cl
N
Me
Cl
N Dipp
Dipp N
Ph
N N
N
N
Dipp = 2,6-diisopropylphenyl
Me Me
47
48
Fig. 28. PEPPSI palladium complexes.
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D. Huang et al. / Coordination Chemistry Reviews 272 (2014) 145–165
Scheme 26. Direct arylation of pentafluorobenzene catalyzed by a trifluoroacetato complex.
Scheme 27. Synthesis of a normal 1,2,3-triazolylidene carbene Pd complex and its X-ray structure.
Reprinted with permission from Ref. [14b] (Kuhn’s group). Copyright 2013 American Chemical Society.
[14b]. During the reaction, formation of a precipitate was observed,
which indicated that complex 51 may be not stable under oxidative
conditions.
9.3. Other types of molecular Pd-triazole catalysts
Fig. 29. Hetero-bis(carbene) palladium complexes.
synthesized through salt metathesis of the bromides with
AgO2 CCF3 . Both of them were used as catalysts in the direct
arylation of pentafluorobenzene, and trifluoroacetato complexes
showed better reactivity than their bromo analogs (Scheme 26).
Generally, complexes bearing less donating triazole ligands perform better in this catalysis.
Although the majority of 1,2,3-triazolylidenes form abnormal
carbenes exhibiting a 1,3,4-substitution pattern, an example of
normal 1,2,3-triazolylidene carbene with a 1,2,4-substitution pattern was reported very recently [14a]. The Pd complex shown in
Scheme 27 was applied to Suzuki–Miyaura coupling of aryl bromides and chlorides. Mercury-poisoning experiments suggested
that activity of the catalytic species was at least partly due to
Pd nanoparticles. Moderate performances with aryl chloride substrates were observed probably due to the instability of the
catalysts [14b]. The Mo complex was used to catalyze the epoxidation of olefins and showed moderate catalytic activity (Scheme 28)
Beside the above modes of triazole bonding with palladium, the
relatively acidic C H bond on the 5-position can be inserted by
palladium to form a carbon–metal bond. The use of pincer complexes as catalysts has been extensively investigated because of
their unique coordination environment and adjustable steric and
electronic properties. However, the synthesis of various related
pincer ligands still remains a challenge. Gandelman and co-workers
[76b] successfully synthesized a hetero-tridentate triazole-based
pincer complex in which palladium is bonded to the ligand through
two donor groups in the 1,4-positions and the carbanion in the 5position by directed insertion of the relatively acidic C H bond
(Scheme 29). And the structure was confirmed by X-ray structure analysis (Fig. 30). The resulting triazole-based pincer complex
exhibits extremely high catalytic efficiency in the Heck reaction.
The application of “clicked” triazole rings in ligand synthesis should
allow access to a broad range of tailor-made pincer ligands.
9.4. Pd-nanoparticle catalysts stabilized by
1,2,3-triazole-containing macromolecules
In recent years, interest in nanoparticle (NP) catalysts has considerably increased because of their high reaction efficiency and
environmentally benign conditions [77]. Ornelas and co-workers
have synthesized Pd nanoparticles stabilized by 1,2,3-triazole-
N N
N
D1
N N
N
[PdCl2 (tmeda)]
D2
Et 3N, DMF, 70 oC
D1
Pd
Cl
D2
D 1 = PPh2 , SPh
D 2 = pyridine, PPh2 , P(o-MeOC 6H 4) 2
Scheme 28. Mo catalyst for the epoxidation of cyclooctene.
Scheme 29. Synthesis of hetero-tridentate triazole-based pincer complexes.
D. Huang et al. / Coordination Chemistry Reviews 272 (2014) 145–165
161
Fig. 30. X-ray structure of a pincer complex (D1 = D2 = PPh2 ) in Scheme 29.
Reprinted with permission from Ref. [76b] (Gandelman’s group). Copyright 2008 Wiley-VCH.
containing dendrimers with ferrocenyl and other termini (Fig. 31)
and related polymers. These PdNPs showed good catalytic activity in Miyaura–Suzuki reactions and hydrogenation of olefins
[77a–77d]. Water-soluble sulphonated 1,2,3-triazole-containing
dendrimer-stabilized palladium nanoparticles also demonstrated
to be highly effective to catalyze allylic alcohol hydrogenation and
Suzuki coupling reactions in aqueous media under ambient conditions [77e].
Clicked dendrimers that are terminated by triethylene glycol
(TEG) termini are stabilizers of palladium nanoparticles (PdNPs)
that show excellent catalytic activity for Miyaura–Suzuki reactions
of bromoaromatics at 80 ◦ C with Pd amounts down to sub-ppm
level. These PdNPs are stable in air for months and retain their
catalytic activity in the case of small “clicked” zeroth-generation
arene-cored dendrimers surrounding 1.4-nm-sized PdNPs [77f].
With related polymers containing both TEG and triazole polymers,
the same reactions are faster with a few ppm Pd, but the PdNPs are
less stable [77g]. The parameters are consistent with an “homeopathic” and leaching mechanism, i.e. the catalytic reactions work
all the better as the Pd concentration is lower, because the leached
active Pd atoms are less quenched by the mother PdNPs as the Pd
concentration decreases down to the ppm level [78a–78c].
10. Au-triazole complexes in catalysis
For a long time, Au was considered to be inert in catalysis. The
pioneer of Au-catalyzed chemistry was Haruta who discovered in
the 1980s that small oxide-supported Au nanoparticles (<5 nm)
catalyze the CO oxidation by O2 to CO2 even at temperatures
down to −78 ◦ C [79a–79d]. From then on, heterogeneous Au
catalysis became a hot topic. However, in the first decade of 21st
century, the area of homogeneous Au(I) and Au(III) catalysts has
also been rapidly growing, especially regarding the catalytic electrophilic activation of multiple carbon–carbon bonds, especially
with alkynes and allenes [79e,79f]. In most reactions of this type,
cationic Au(I) complexes are paired with a weakly coordinating
anion such as OTf− , BF4 − , PF6 − , NTf2 − or B(C6 F5 )4 − in the catalysts
Fig. 31. Two examples of triazolyl dendrimers stabilized PdNPs.
Reprinted with permission from Ref. [77c] (Astruc’s group). Copyright 2008 Wiley-VCH.
162
D. Huang et al. / Coordination Chemistry Reviews 272 (2014) 145–165
Scheme 30. Simple cationic gold catalysts for propargyl ester/ether 3,3-rearrangements.
Scheme 31. Triazolyl-Au catalysts for propargyl ester/ether 3,3-rearrangements.
Scheme 32. The triazole-AuNP nanocatalyst for efficient p-nitrophenol reduction.
[79g–79i]. Most of these cationic catalysts are remarkably active,
but they have the disadvantage of a lack of stability at high
temperature. Therefore, the chemists began to use triazolestabilized Au(I) complexes to solve this problem.
efficiency of triazole-stabilized AuNP nanocatalysts beyond other
AuNPs catalysts results from the compromise between stability
and activity of the weakly-ligated AuNP surface through the Aunitrogen coordination.
10.1. Au-nitrogen coordination
10.2. Au-carbenes complexes
Shi’s group has investigated the cationic triazolyl-Au (TA-Au)
complexes with Au-nitrogen coordination that were obtained upon
treatment of the neutral catalyst with HOTf to catalyze propargyl ester/ether 3,3-rearrangements. This new catalyst showed an
effective chemoselectivity, yielding highly reactive allenes [80,81],
and promoted this transformation with much improved efficiency,
broader substrate scope, and more practical reaction conditions
that could not be achieved by simple cationic gold catalysts (see
Schemes 30 and 31) [82,83].
Au-nitrogen coordination is also useful in catalysis, as for
instance with triazole-Au(0) nanoparticle (AuNP) complexes [84]
that have been used by Zhao et al. as catalysts for the NaBH4 reduction of nitrophenol to aminophenol (Scheme 32) [84b]. The high
In addition, 1,2,3-triazolylidene ligands were also reported to
coordinate with Au(I) to form homogeneous catalyst containing
Au-carbene bond in which the carbene is in the singlet state as in
classic NHC carbenes. The neutral Au(I) triazolylidene complexes
Scheme 33. General procedure for the synthesis of neutral Au(I) triazolylidene
complexes.
D. Huang et al. / Coordination Chemistry Reviews 272 (2014) 145–165
163
Scheme 34. Au(I)-triazolylidene catalyzed carbene transformation from a series of ethyl diazoacetate.
Scheme 36. Syntheses of triazolyl-Au complexes.
Scheme 35. Au(I)-triazolylidene catalyzed allene hydroalkoxylation.
were synthesized from the corresponding 1,2,3-triazolium salts
and Ag2 O and subsequent transmetalation with AuCl(SMe2 )
(Scheme 33) [85–89].
For instance, Crowley and co-workers synthesized the neutral 1,2,3-triazolylidene Au(I) chloride complex with R = benzyl
and R = phenyl and investigated the catalysis of carbene transformation from a series of ethyl diazoacetates (Scheme 34) and
allene hydroalkoxylation in the presence of AgSbF6 (Scheme 35).
The results showed a high reactivity of this catalyst, and it was
also demonstrated that analogs with different electronic and steric
properties also contributed to the development and optimization
of Au(I)-catalyzed reactions [87].
The presence of the Ag salts in these reactions is required
for Cl− abstraction with the aim to form a highly active cationic
triazolylidene-Au(I) center [85–88]. In a very recent report, however, Albrecht’s group provided experimental evidence showing
that the carbene ligands of [AuCl(carbene)] complexes are easily
dissociated in the presence of Ag salts, providing ligandless Au
centers that impart high catalytic activity. This discovery may be
relevant beyond Au-centered N-heterocyclic carbenes chemistry
and warrant a cautionary note when assuming a “strong” or “kinetically inert” bonding of N-heterocyclic carbenes to metal centers,
especially to coinage metals [89].
10.3. Au-nitrogen complexes with anionic triazoles
The first synthesis of Au(I)-triazolyl complexes with anionic triazoles was described by Nomiya et al. in 1998 [79j], and one decade
later Shi and co-workers used the 1,2,3-triazolyl-Au(I) complexes
as a new catalyst [79k,90]. In their study, the authors just mixed
the ligands and Au(I) cation in methanol suspension with several
hours of stirring to prepare the 1,2,3-triazolyl-Au(I) complexes. For
instance, mixing [PPh3 AuCl] with the 1,2,3-triazole under basic
conditions leads to the formation of the neutral Au(I) catalyst in
which the triazolyl ligand is thus “anionic” (Scheme 36).
This Au(I) catalyst showed significantly improved thermal stability with the triazolyl ligand, which provided highly effective
catalysis of intermolecular alkyne hydroamination at high temperature (80 ◦ C) without agglomeration [90]. This research opened a
temperature range for homogeneous Au(I) catalysis.
11. Conclusion
1,2,3-Triazoles have recently become popular, because they are
formed by the well-known catalyzed and selective “click” reactions
between alkynes and azides, but they can also form by reactions
between hydrazonoyl chlorides and isocyanides. Their complexes
with transition metals adopt preferred coordination at the N3 site
and often show unique catalytic activity for a variety of reactions. They are also easily deprotonated and form C- or N-bonded
complexes of the virtually “anionic” triazolyl ligand that often
equally show desirable catalytic properties. In summary, a variety
of coordination modes and catalytic properties have been disclosed
for 1,2,3-triazole and triazolyl complexes in the last decade, i.e.
since the advent of the “click” concept that has provided the large
majority of these ligands. The similarity and relationship with the
illustrious imidazole and imidazolium ligands provides a richness
and sophistication of possibilities in catalysis by both transition
metal complexes and transition-metal nanoparticles that will grow
at a fast rate in the near future. Besides classic noble-metal catalysts
that show excellent catalytic activities (including the fast-growing
Au catalysis), recent research is increasingly focusing on biocompatible and abundant first-row transition metal complexes for the
sake of “green chemistry”.
Acknowledgements
Financial supports from the Science and Technology on Surface Physics and Chemistry Laboratory, item no. TP201302-1, the
164
D. Huang et al. / Coordination Chemistry Reviews 272 (2014) 145–165
Discipline Development Foundation of the Laboratory (XK201305),
the funding 2103B0302047 from China Academy of Engineering
Physics and the University of Bordeaux and the CNRS are gratefully
acknowledged.
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