Visible-Light-Promoted AuI to AuIII Oxidation in Triazol-5-ylidene Complexes

Article abstract of DOI:10.1002/asia.201601499

Reaction of triazolium precursors [MIC(CH₂)_n- H⁺]I^? (n=1–3) with potassium hexamethyldisilazane (KHMDS) and AuCl(SMe₂) generates the gold(I) complexes of the type MIC(CH₂)_n?AuI. Visible light exposure of the latter complexes promotes a spontaneous disproportionation process rendering gold(III) complexes of the type [{MIC(CH₂)_n}₂?AuI₂]⁺I^?. Both the Au^I and Au^III complex series were tested in the catalytic hydrohydrazination of terminal alkynes using hydrazine as nitrogen source.

Full text of DOI:10.1002/asia.201601499

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Visible Light Promoted Au(I) to Au(III) Oxidation in Triazol-5-
ylidene Complexes
Authors: Daniel Mendoza-Espinosa, David Rendón-Nava, Alejandro
Alvarez-Hernández, Deyanira Angeles-Beltrán, Guillermo
Enrique Negrón-Silva, and Oscar Rodolfo Suárez-Castillo
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Visible Light Promoted Au(I) to Au(III) Oxidation in Triazol-5-
ylidene Complexes
Daniel Mendoza-Espinosa,*[a] David Rendón-Nava,[a] Alejandro Alvarez-Hernández,[b] Deyanira
Angeles-Beltrán,[a] Guillermo E. Negron-Silva,*[a] and Oscar R. Suárez-Castillo[b]
Dedicated to Emma Espinosa García
Abstract: Reaction of triazolium precursors [MIC(CH2)n-H+]I- (n = 1-
3) with potassium hexamethyldisilazane (KHMDS) and AuCl(SMe2)
generates the gold(I) complexes of the type MIC(CH2)n·AuI. Visible
light exposure of the latter complexes promotes a spontaneous
disproportionation process rendering gold(III) complexes of the type
[{MIC(CH2)n}2·AuI2]+I-. Both the Au(I) and Au(III) complex series were
tested in the catalytic hydrohydrazination of terminal alkynes using
hydrazine as nitrogen source.
respects to functional group tolerance and cost, while most
examples are also conducted at high reaction temperatures.
Late in the last decade, a new subclass of NHCs namely 1,2,3-
triazol-5-ylidenes was discovered and even isolated in their free
form.11 This new generation of NHCs termed as “mesoionic
carbenes” (MICs) have shown stronger donor capabilities than
classical NHCs,12 opening a new alternative of ligands for highly
stable gold (I-III) complexes. With the goal of developing milder
gold-catalysed coupling reactions excluding external oxidants,
we describe herein the one-pot synthesis of Au(I)-MIC
complexes and their facile visible light conversion to cationic
gold(III) complexes. This unprecedented mild one step redox
transformation will be discussed along with the structural
diversity in solution and solid state. Preliminary catalytic
activities of the Au(I)-MIC and Au(III)-MIC complexes in
hydrohydrazination of terminal alkynes using parent hydrazine
will be discussed.
The preparation of the MIC precursors 1-3 was performed
according to Scheme 1. The first step involves the copper
catalyzed cycloaddition of the proper alkyne with mesityl azide in
ethanol/water to provide triazoles A-C.12d,e Further treatment of
the latter precursors with excess methyl iodide in acetonitrile,
delivers the expected triazolium salts 1-3 obtained after
recrystallization of the crude materials with acetonitrile/diethyl
ether (87-92% yields). All compounds were characterized by
NMR spectroscopy, FT-IR, and elemental analysis.
Gold-based chemistry is recently finding great utility in a variety
of catalytic, synthetic, and biomedical applications.1 Most of the
catalytic systems employed range from commercial inorganic
salts, such as Au(I) and Au(III) halides,2 to more versatile
complexes stabilized with soft σ-donor ligands, usually
phosphines3 or N-heterocyclic carbenes (NHCs).4 The Au
complexes supported by NHCs, in particular, have been widely
studied for their abilities to catalyse C−C, C−O, and C−N bond-
forming reactions.5 While the majority of literature reports
describing the chemistry of Au−NHC complexes involve gold
ions in the +1 oxidation state, recent attention has been directed
toward Au(III)−NHC complexes.6 However, due to the high redox
potential of the Au(I)/Au(III) couple (E0 = +1.41 V),7 the
organometallic chemistry of gold(III) with NHCs appears less
developed than for gold(I) species and is mainly restricted to
neutral halide Au(III) complexes, and there is strong evidence for
an in situ reduction of simple gold(III) catalysts by oxidative C-C
coupling in the intermediates.8 A common methodology to attain
the oxidation of NHC-Au(I) species relies on the addition of
strong oxidants such as fluorinating or hypervalent iodine
reagents,6h or treatment with molecular Cl2, Br2, or I2.6e,g More
recently a few outstanding examples of oxidative addition to C-
Br/I and C-C- bonds onto Au(I) have been reported by Bourissou
and Toste.9 Applying this concept, a range of oxidative homo-
and heterocoupling reactions have been developed giving
Scheme 1. Synthesis of triazoliums 1-3.
Formation of the triazoliums was easily monitored by the
1
appearance of a new signal in the H NMR spectrum at ca. δ =
access to
a variety of functionalized products using non
4.5 ppm, indicating methylation of the triazolyl moiety (at N-3).
Most of the other resonances in the 1H NMR spectrum shifted
only slightly, with the exception of the now acidic triazolium
proton, which moves to higher frequency (δ = 8.6-9.0 ppm).
Unambiguous characterization of the salts 1 and 2 was also
achieved by X-Ray crystallography with the molecular structures
displayed in Figure 1.
activated arenes and alkynes as starting materials.10 Despite the
success of these methods, the use of large excess of strong
external oxidants, unavoidably reduces their appeal with
[a]
Dr. D. Mendoza-Espinosa, D. Rendón-Nava, Dr. D. Angeles-Beltrán,
Dr. G. E. Negrón-Silva
Departamento de Ciencias Básicas
Universidad Autónoma Metropolitana -Azcapotzalco
Avenida San Pablo No. 180, Cuidad de México, México, 02200.
E-mail: danielme1982@gmail.com; gns@correo.azc.uam.mx
Dr. A. Alvarez-Hernández, Dr. O. R. Suárez-Castillo.
Área Académica de Química
[b]
Universidad Autónoma del Estado de Hidalgo
Carretera Pachuca-Tulancingo Km. 4.5, Mineral de la Reforma
Hidalgo, México, 42090.
Figure 1. Molecular structures of salts 1 (left) and 2 (right). Ellipsoids shown at
50% of probability.
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Our interest on the preparation of precursors 1-3 was due the
fact that besides the possible formation of the MIC ligands after
deprotonation, the remaining hydroxyl group could also be
available for further functionalization. The initial exploration on
the coordination capabilities of salts 1-3 was carried out by its
one-pot reaction with potassium hexamethyldisilazane and
AuCl(SMe2) under strict exclusion of light (Scheme 2). After work
up and purification, the products were obtained in good yields
(83-91%) as white crystalline powders. NMR spectroscopy
studies and elemental analyses confirmed the formation of the
expected Au(I)-MIC complexes 4a-c by the disappearance of the
almost linear, while the Au-C and Au-I bond distances of
2.024(8) and 2.617(9) Å, respectively, are both similar to
analogue gold(I) complexes supported by MIC ligands.13
Complex 5a features a trans-disposition with the MIC ligands
pointing out to the same direction, and the overall charge is
compensated by the presence of the non-coordinated I(3) atom.
1
acidic CH+ proton in the H NMR, and the observation of a low
field signal around 171 ppm in 13C NMR which is similar to
previously reported mononuclear MIC-gold(I) complexes.13
Figure 2. Molecular structure of 5a. Ellipsoids shown at 50% of probability.
Scheme 2. Synthesis of Au(I) complexes 4a-c.
With the free hydroxyl groups and the presence of sp2-nitrogen
atoms, the possibility of hydrogen bonds in the solid state
structure of 5a was investigated. Figure
3 depicts the
Interestingly, after the preparation of the NMR samples in CDCl3
and exposure of the solutions to visible light (from a 23W
fluorescent light bulb), we observed that complexes 4a-c
underwent a colour change from colourless to deep-red along
with the formation of a small amount of precipitate. After filtration,
the solid was identified as gold(0), which was reused
subsequently for the preparation of AuCl(SMe2). NMR
spectroscopy analysis of the supernatants showed similar 1H
NMR patterns as for those observed in 4a-c, however, the
presence of new carbene peaks at ca. 160 ppm in 13C NMR
suggested a major structural transformation. Purification of the
new species was performed by column chromatography
rendering dark-red solids in moderate yields. Surprisingly,
constructed hydrogen-bonded model showing the different
interactions of the the hydroxyl groups on the MIC ligands. For
instance, the O(1)-H(1A) moiety present short contacts with a
vicinal molecule through the O(1)∙∙∙H(6C) and H(1A)∙∙∙N(3)
interactions with distances of 2.543 and 3.045 Å, respectively. In
case of the O(2)-H(2B) fragment, there is only a weak interaction
with a neighboring N-Me group though the O(2)∙∙∙H(19C) contact
with a distance of 2.694 Å.
elemental analysis of the purified complexes indicated
a
carbene-gold 2:1 ratio and lower carbon content than the
expected for a gold center with a single iodine atom. To gain
further insight into the structural features of the type-5
complexes, single crystals of 5a were grown by cooling down a
THF/DCM solution at -35°C. The crystals characterization by X-
ray diffraction unveiled the formation of Au(III) species with a
general formula of [{MIC(CH2)n}2·AuI2]+I- (Scheme 3).
Figure 3. Hydrogen bonding in the solid state structure of 5a.
Notably, the synthesis of complexes 5a-c relies on a light
induced disproportionation reaction in which the MIC(CH2)n·AuI
complexes undergo oxidation, followed by stabilization with the
coordination of a second triazol-5-ylidene (process which is
probably favored by the small size of the ligand), and finally
accompanied by reduction to Au(0) to complete the redox
process. Is noteworthy mentioning that the disproportionation of
Au(I) is known to be very slow for Au(I) halides,14 because it
requires formation of a bimolecular species involving a gold-gold
bond and takes place only at high temperature in acidic media.
Therefore, examples of visible light promoted self-
disproportionation process have not been reported to date. Only
Scheme 3. Synthesis of Au(III) complexes 4a-c.
The molecular structure of 5a depicted in Figure 2, shows a
cationic gold(III) center coordinated to two MIC ligands and two
iodine atoms in an overall square planar geometry. The C(5)-
Au(1)-C(10) (178.9 °) and I(1)-Au(1)-I(2) (172.9 °) angles are
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a
related example of disproportionation to generate
a
Remarkably, the product formation is highly beneficiated by the
replacement of THF for non-polar solvents (Table 2, entries 1-4)
with toluene as the best choice. Additionally, in an attempt to
improve the catalytic efficiency complex 5c, we carried out the
iodine exchange reaction with several salts (Table 2, entries 5-8)
achieving a maximum yield of 62%.
biscarbenic cationic gold(III) complex was reported by Bielawski
where the oxidation process required of a biscarbenic gold(I)
precursor and stoichiometric amounts of AuCl(SMe2) to
proceed.15
The development efficient routes for the generation of carbon–
nitrogen containing products using accessible bulk materials
such as parent hydrazine is of great appeal. Nevertheless,
hydrazine is a strong reducing agent, which can promote the
formation of inactive metal(0), or lead to the formation of
catalytic inactive Werner-like complexes. Therefore, very few
examples of catalytic reactions involving this reagent have been
reported. For instance, it has been demonstrated that Pd- and
Cu-catalysed cross-coupling of hydrazine with aryl chlorides and
tosylates was successful with the usage of an electron rich bulky
P-ligand.16 Bertrand and Hashmi have shown that the
hydrohydrazination of unactivated allynes and allenes can be
efficiently accomplished by cationic (L)Au(I) complexes
supported by several non-classical NHC ligands.17 Based on the
positive results observed in the hydrohydrazination of
unactivated alkynes promoted by cationic gold complexes, we
decided to test our Au(I) and Au(III) MIC series in the
hydrohydrazination of terminal akynes.
Table 2. Optimization of the catalytic reaction conditions.
Entry
Catalyst
Additive
Solvent
Yield(%)
1
2
3
4
5
6
7
8
9
4c
5c
4c
5c
5c
5c
5c
5c
4c
KB(C6F5)4
---------
Toluene
Toluene
Benzene
Benzene
Toluene
Toluene
Toluene
Toluene
Benzene
97
55
91
52
34
37
49
62
<5
KB(C6F5)4
---------
AgSbF6
AgOTf
AgBF4
KB(C6F5)4
--------
As initial screening tests, we carried out the stoichiometric
o
reaction of parent hydrazine and phenylacetylene, at 80 C for
Having identified precatalysts 4c and 5c (3 mol%) in mixture
with KB(C6F5)4 (3 mol%) as the best of the series, we evaluated
the scope of the reaction with several terminal aryl alkynes and
parent hydrazine to yield the hydrazones 6a–f. According to
Table 3, is easily observable that 4c is more proficient in the
catalytic conversions to hidrazones 6 than the Au(III) complex 5c.
The lower conversions obtained with 5c may be related to the
instability of Au(III) complexes under highly reducing conditions
(given the use of hydrazine) resulting in a reduced lifetime of
such catalytic species.
12 h with 3 mol% of complexes 4a-c and 5a-c. In case of
complexes type-4, AgSbF6 was included as initial additive to
render the cationic metal center (entries 1-3), whilst due to the
cationic nature of complexes type-5 no addition was tested at
first (entries 3-6). As observed in Table 1, the initial results
showed that complexes 4c and 5c achieved the best yields
under the standard reaction conditions. Further investigation of
the weakly coordinating anions effect was accomplished by the
mixing of complex 4c with several potassium and silver salts
(entries 7-9), showing that the usage of KB(C6F5)4 improves the
overall catalytic efficiency.
Table 3. Scope of the hydrohydrazination of terminal alkynes with hydrazine.
Table 1. Catalytic screening of 4a-c and 5a-c in the synthesis of 6a.
Entry
Catalyst
Additive
Solvent
Yield(%)
1
2
3
4
5
6
7
8
9
4a
4b
4c
5a
5b
5c
4c
4c
4c
AgSbF6
AgSbF6
AgSbF6
---------
THF
THF
THF
THF
THF
THF
THF
THF
THF
57
52
74
41
38
49
69
62
85
---------
---------
AgOTf
Reaction conditions: Alkyne (1.0 mmol), hydrazine (1.2 mmol), toluene 3 mL.
Isolated yields.
AgSbF6
KB(C6F5)4
During the course of studies, we noted the formation of trace
amounts of products resulting from a bis-hydrohydrazination
reaction. Under the standard conditions with 4c, but using half
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equivalent of parent hydrazine, we were able to obtain the
corresponding azines 7a–c in good yields (Table 4) after a
simple purification by means of chromatographic column.
Acknowledgements
We are grateful to the CONACYT (project 181488) and
PRODEP (UAM-PTC-475) for financial support. D.M.E., G.N.S.,
D.A.B., A.A.H., and R.S.S. wish to acknowledge the SNI for the
distinction and the stipend receivedAcknowledgements Text.
Table 4. (Bis)hydrohydrazination of terminal alkynes with hydrazine.
(4c, 3%mol)
N
R
+
H2N-NH2
2 R
R
N
KB(C6F5)4
Keywords: Mesoionic carbenes •gold(I) •gold(III) •
hydrohydrazination • oxidation
Toluene, 80 oC, 12 h
7a-c
MeO
N
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Reaction conditions: Alkyne (1.0 mmol), hydrazine (0.49 mmol), toluene 3 mL.
Isolated yields.
The possibility of formation of heterogeneous gold as the active
catalyst, prompted us to perform mercury poisoning tests in the
formation 6a (ESI-Figure S1).18 Hence, the formation process of
6a was disturbed by the addition of excess of elemental mercury
to the reaction mixture using the precatalysts 4c and 5c.
According to GC monitoring during reaction time, no significant
changes in the product yields were noticed (Fig S1-ESI). These
results suggest that at 80oC the catalytic conversions are
performed essentially by molecular species. Further analysis of
the gold catalysts nature were tested by following Crabtree´s
test for the selective homogeneous catalysts poisoning by the
addition of slight excess of dibenzo[a,e]cyclooctatriene (DCT).19
In this case the activity for the pre-catalysts 4c and 5c remains
similar until the fourth hour of reaction, however as the time
progresses and the DCT start the inhibition process, the
conversion to products decreases drastically to 46% (for 4c) and
27% (for 5c) (Fig S2-ESI). These conversion data complements
well the mercury poisoning tests, suggesting the homogeneous
nature of the gold catalysts at the process reaction condition.
In summary, we report the facile preparation of a series of
MIC(CH2)n·AuI complexes by the one-pot reaction of the
triazolium precursors [MIC(CH2)n-H+]I- (n = 1-3) with potassium
hydride (KHMDS) and AuCl(SMe2). Visible light exposure of
chloroform solutions of the latter gold(I) complexes results in a
spontaneous disproportionation process rendering gold(III)
complexes of the type [{MIC(CH2)n}2·AuI2]+I-. All complexes have
been characterized in solution and solid state and both the Au(I)
and Au(III) complex series were tested in the catalytic
hydrohydrazination of terminal alkynes using hydrazine as
nitrogen source. From the catalytic conversions, we can observe
that although both the Au(I) and Au(III) complexes series are
successful to generate the hydrohydrazination products under
mild conditions, the combination of complexes 4c with KB(C6F5)4
as additive, display the best performance of the series. Further
exploration of the catalytic potential of complexes 4 and 5 is
currently being explored in our laboratory.
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10.1002/asia.201601499
Chemistry - An Asian Journal
COMMUNICATION
Layout 2:
COMMUNICATION
Daniel Mendoza-Espinosa*, David
Rendón-Nava, Alejandro Alvarez-
Hernández, Deyanira Angeles-Beltrán,
Guillermo E. Negrón-Silva*, Oscar R.
Suárez-Castillo.
((Insert TOC Graphic here))
Page No. – Page No.
We report the visible light promoted disproportionation of AuI complexes (4) which
produces AuIII complexes of type 5. Both the AuI and AuIII series were tested in the
hydrohydrazination of terminal alkynes using hydrazine as nitrogen source.
Visible Light Promoted Au(I) to Au(III)
Oxidation in Triazol-5-ylidene
Complexes
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.