Synthesis and characterization of a gold(i) bis(triazolylidene) complex featuring a large [(TpMe2)2K] anion

Article abstract of DOI:10.1039/c8nj03318h

In the quest of heteroleptic gold(i) complexes containing triazolylidenes (MIC) and Tp^Me2 ligands, we discovered that reaction of the MIC-Au-I complex 2 with potassium hydrotris-(3,5-dimethyl-1-pyrazolil) borate (KTp^Me2) produces an

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Synthesis and characterization of a gold(I)
bis(triazolylidene) complex featuring a large
Cite this:DOI: 10.1039/c8nj03318h
[(TpMe2)2K] anion†
Received 3rd July 2018,
Accepted 26th July 2018
Armando Priante-Flores,a Veronica Salazar-Pereda,a Arnold L. Rheingoldb and
´
*
a
Daniel Mendoza-Espinosa
DOI: 10.1039/c8nj03318h
rsc.li/njc
In the quest of heteroleptic gold(I) complexes containing triazolyl-
idenes (MIC) and TpMe2 ligands, we discovered that reaction of the complexes known in the literature is supported by homo-bis-
In general, the basic architecture of cationic dicarbene Au(I)
MIC–Au–I complex 2 with potassium hydrotris-(3,5-dimethyl-1- NHCs ligands (mainly imidazolylidenes) and stabilized with
ꢀ 9
pyrazolil) borate (KTpMe2) produces an unexpected bis(triazolylidene) common anions such as Clꢀ, Brꢀ, Iꢀ, PF6ꢀ, SO3CF3ꢀ and BF4
.
gold(I) complex featuring a large [(TpMe2)2K] anion (3). An optimized The investigation related to Au(I) hetero-bis-carbene complexes
synthesis was achieved by reacting a preformed bis(triazolylidene)Au(I) containing different NHCs or more complex anions have been
complex (2a) with two equivalents of KTpMe2 (81%). Full charac- given less attention.10
terization and preliminary catalytic results demonstrate the high
Our group has been interested in the use of mesoionic 1,2,3-
performance of complex 3 in the formation of several oxazoline triazolylidenes (MICs) as ligands for transition metals as they
derivatives.
have demonstrated enhanced s-donor and weaker p-accepting
properties compared to classical N-heterocyclic carbenes (NHCs).
The use of gold complexes stabilized by N-heterocyclic carbenes The facile access to a variety of substituted 1,2,3-triazolium salts
(NHCs) has experienced an exponential growth over the last two as precursors for mesoionic triazolylidenes via click chemistry,
decades.1 NHC–gold complexes have shown applications in has led to a diverse ligand topology and their respective catalytic
several fields including solar light receptors,2 potential anti- applications with transition metals.11
tumoral agents,3 and also as catalysts in various processes such
Compared to classical NHCs, structurally related 1,2,3-
as C–H bond functionalizations, cyclization of enynes, hydro- triazolylidene ligands have been less studied toward the coor-
aminations, among others.4 Interestingly, the high catalytic dination of the gold center. In fact, only a handful of examples
performance of NHC–gold complexes relies in the capability of bis(triazolylidene) gold complexes in either +1 and +3 oxida-
of the gold center to coordinate multiple C–C bonds (alkynes tion states have been reported and their catalytic potential has
and allenes) and thus activate them toward the intra or inter- only been scarcely investigated.5,6 As part of our ongoing
molecular addition of nucleophiles.5 So far, most of the appli- research in MIC-based gold chemistry, we report in here the
cations of NHC–gold complexes reported involve the use of unexpected synthesis of a bis(triazolylidene) gold(I) containing
mononuclear gold(I) species and in minor extent NHC–Au(III) a large [(TpMe2)2K] anion (3) obtained by the treatment of a
complexes.5,6 In this context, it is surprising that investigation mononuclear MIC–Au–I complex 2 with equimolar amounts of
related to cationic dicarbene gold(I) complexes is still under- KTpMe2. The full characterization of the mono and dicarbene
developed despite their potential applications in the biomedical7 complexes 2 and 2a, the optimized preparation of complex 3,
and photoluminescence fields.8
and its preliminary catalytic performance in the preparation of
a series of oxazolines will be discussed.
Motivated by the luminescent properties found in hetero-
leptic complexes containing cyclic (alkyl)(amino)carbenes (CAACs)
and trispyrazolyl borate ligands,12 we decided to test a similar
synthetic strategy in order to generate analogue heteroleptic
gold(I) complexes featuring triazolylidenes. Thus, as a first step,
we carried out the synthesis of the monocarbene gold(I) complex
2 by the one-pot treatment of corresponding 1,2,3-triazolium
salt 1 with KHMDS, and AuCl(SMe2) (Scheme 1), according to
established procedures.13
a
´
´
´
´
Area Academica de Quımica, Universidad Autonoma del Estado de Hidalgo,
Carretera Pachuca-Tulancingo Km. 4.5, Mineral de la Reforma, Hidalgo, 42090,
Mexico. E-mail: daniel_mendoza@uaeh.edu.mx
b Department of Chemistry and Biochemistry, University of California,
9500 Gilman Drive, La Jolla, San Diego, California 92093, USA
† Electronic supplementary information (ESI) available: General procedures for
catalytic experiments, sample 1H and 13C NMR spectra of new compounds. CIF
file giving details for the crystal structure determinations of 2, 3 and 30. CCDC
1847129–1847131. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c8nj03318h
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Scheme 1 Synthesis of complex 2.
The successful metalation of the triazolium salt was clearly
indicated by the disappearance of the low-field triazolium
proton resonance (located above 9 ppm) in 1H NMR spectro-
scopy and a slight upfield shift of the N–CH3 group. The new
carbene resonance appears at 170.1 ppm in 13C NMR, consistent
with previously reported triazolylidene–gold complexes.6a,13
Single crystals of complex 2 were obtained by pentane vapour
diffusion into a concentrated dichloromethane solution and
they were analysed by X-ray diffraction. The molecular structure
of complex 2 is depicted in Fig. 1 showing the expected linear
coordination geometry around the gold center and confirming
the monocarbenic nature of the complex.
To further investigate the possibility of preparing hetero-
lpetic complexes, we treated complex 2 with equimolar amounts
of KTpMe2 in THF at room temperature (Scheme 2). After work up
and crystallization from THF, we obtained complex 3 as highly
air-stable colorless crystals in low yield (39%). Unambiguous
characterization of complex 3 was achieved by X-ray diffraction
with rather surprising results; the molecular structure of the new
complex showed no coordination of the TpMe2 ligand to the gold
center, but instead, a cationic bis(triazolylidene) stabilized by a
large [(TpMe2)2K] anion was obtained.
Fig. 2 Molecular structure of complex 3. Ellipsoids are shown at 50% of
probability. Hydrogen atoms are omitted for clarity.
fragment where the two MIC–Au bond distances are almost
equivalent (2.005(3) and 2.006(3) Å) and close to reported MIC
analogues (Fig. 2).14 The linear geometry around the metal center
is maintained with an angle of 176.22(15)1 and the triazolylidene
ligands are located in a perpendicular (83.41) fashion to each
other. The anionic moiety is constructed by two TpMe2 ligands
octa-coordinating a single potassium atom. In both TpMe2 ligands
two of the pyrazole rings coordinate the potassium atom in a
monocoordinated fashion with N–K bond distances in the range
of 2.793(3)–2.860(3) Å. Each of the two remaining pyrazoles
coordinate in an Z2-fashion the potassium center through two
nitrogen atoms, forcing the rings to tilt almost 601 respect to the
monocoordinated pyrazoles.
In solution, the NMR spectroscopic analysis of complex 3
shows, as expected from the solid state, highly symmetric patterns
1
in both H and 13C spectra. For instance, the N-methyl and the
%
Complex 3 crystallized in the triclinic P1 space group with
mesityl moieties in the two MIC ligands are equivalent displaying a
single set of signals. Equivalency is also observed in the two TpMe2
ligands with the pyrazole methyl and CH groups resolved
as singlets and located at 2.19 and 5.77 ppm, respectively. In
13C NMR, there is a single carbene resonance located at
173 ppm showing only a marked difference in comparison with
the mononuclear complex 2 (170.1).
the asymmetric unit comprising a cationic bis(triazolylidene)
The unexpected formation of complex 3 is likely related to
the enhanced basicity of the triazolylidene ligands (compared
to NHCs) as previously observed in the preferred formation
of bis(carbene)gold complexes of the type [MIC–Au–MIC]+Xꢀ
rather than neutral MIC–Au–X species. Additionally, gold(I)
mono-carbene complexes with halido co-ligands (such as iodide
in the current case) have been known to form bis(carbene)
complexes, when treated with halide scavengers.15 Thus, we
propose that after a dehalogenation step takes place from the
reaction with KTpMe2, a cationic ‘‘naked’’ MIC–Au(I) species 20 is
formed. This intermediate then undergoes a fast nucleophilic
attack by a second molecule of 2 forming the [MIC–Au–MIC]
cation and releasing AuI. This observation is consistent with the
presence of a large amount of grey precipitate (AuI) in the
reaction depicted in Scheme 2. Finally, the anionic counterpart
of complex 3 is formed by the coordination of the TpMe2
fragment with a second equivalent of the KTpMe2 salt (Scheme 3).
As the yield of 3 was rather low using complex 2 due to
the forced loss of at least half equivalent of the gold source,
Fig. 1 Molecular structure of complex 2. Ellipsoids are shown at 50% of
probability. Selected bond distances (Å) and angles (1): C5–Au1 2.028(5),
Au1–I1 2.5549(4), C5–Au1–I1 179.17(14).
Scheme 2 Synthesis of complex 3.
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Table 1 Synthesis of oxazoline 4 using MIC–Au complexes 2–3 as
precatalysts
Entry
[Cat]
[Cat mol%]
[Additive/mol%]
Conv.a [%]
1
2
3
4
5
6
7
8
2
2a
3
2a
3
2
2a
3
2
2a
3
2
2a
3
3
3
3
3
3
2
2
2
1
1
1
1
1
1
AgBF4/3
AgBF4/3
AgBF4/3
—
—
AgBF4/2
—
—
AgBF4/1
—
95
66
56
84
92
93
77
92
93
68
90
93
55
52
Scheme 3 Proposed reaction mechanism for complex 3 formation.
9
10
11
12
13
14
—
AgSbF6/1
AgSbF6/1
AgSbF6/1
Scheme 4 Optimized synthesis of complex 3.
Reaction conditions: N-(2-propyn-1-yl)benzamide (0.1 mmol), dichloro-
a
methane (3 mL), room temperature. Isolated yields as the average of
two runs. Conversion determined by 1H NMR spectroscopy with hexa-
decane as internal standard.
we decided to optimized its synthesis by the use of a preformed
bis(triazolylidene)–Au complex as starting material. Thus,
following a slightly modified reported method,16 we carried
out the synthesis of complex 2a by the reaction the free MIC A
with half equivalent of AuCl(SMe2) according to Scheme 4. The
subsequent treatment of complex 2a with two equivalents of
KTpMe2 in THF produces complex 3 in 81% yield after recrys-
tallization from THF/pentane. The spectroscopic analysis of 3 is
consistent with the previous characterization although, inter-
estingly, the X-ray analysis of the single crystals obtained by this
method generate a slightly different unit cell containing larger
edge lengths and unit volume (see ESI† complex 30).
loading and a different additive. It can be observed that the
gradual decrease to 1 mol% and the exchange of halide scavenger
does not affect the conversion percentages of complexes 2 and 3;
however, at 1 mol% loading, complex 2a decreases its conversion
to a minimum of 68%.
In order to get more insight into the catalytic behaviour of
complexes 2, 2a, and 3, the catalytic profiles for the cyclization
of N-2(propyn-1-yl)benzamide were performed with the reaction
conditions used in the Table 1 entries 9–11 (Fig. 3).
As depicted in Fig. 3, complexes 2 and 3 reach conversions
over 60% after only 24 h of reaction and reach their maximum
after 42 h. In the case of complex 2a, conversions are much slower
reaching its maximum conversion after 36 h and remaining
unchanged after that time. The overall profile results suggest that
the low catalytic performance of 2a may be related to decomposi-
tion of the active species after 36 h of reaction while, in the case of
Owing to their Lewis acidic nature, gold(I) complexes are
known to efficiently catalyse the cyclization of various p-bond
containing systems such as alkynes and enynes.17 In particular,
triazolylidene–gold(I) complexes have shown to be efficient
catalysts in the synthesis of oxazolines either by the cyclization
of propargylic amides18 or the condensation of isocyanoacetate
and aldehydes.19 As relatively few examples of oxazoline syn-
thesis via NHC–Au catalysed processes have been reported in
the literature,15 we were interested in testing our triazolylidene
gold(I) complexes 2, 2a, and 3 as precatalysts in the above-
mentioned reaction. Thus, the catalytic performance of our new
mono- and dicarbenic complexes 2–3 was tested in the cycliza-
tion of N-2(propyn-1-yl)benzamide to produce the oxazoline 4.
As initial screening tests, the reactions were performed at room
temperature with a complex loading of 3 mol% in dichloro-
methane. Addition of 3 mol% of the halogen scavenger (AgBF4)
to render catalytically active cationic Au(I) species was only
beneficial for complex 2 where 95% of conversion was achieved
(Table 1, entry 1). For the cationic dicarbenic–Au(I) complexes 2a
and 3, the conversions without any additive reach 77 and 92%,
respectively, with a marked conversion decrease after AgBF4
addition (Table 1, entries 2–5).
Fig. 3 Cyclization of N-2(propyn-1-yl)benzamide catalyzed by com-
plexes 2–3. Reactions were carried out with CD2Cl2 at room temperature
with 1 mol% catalyst loading and 1 mol% AgBF4 in case of complex 2.
Conversions were determined by 1H NMR spectroscopy using hexadecane
In order to challenge the performance of the gold precatalysts,
several reaction conditions were modified, including the catalyst as internal standard.
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Table 2 Synthesis of substituted oxazolines using MIC–Au complexes 2
and 3 as precatalysts
Notes and references
1 (a) N. Marion and S. P. Nolan, Chem. Soc. Rev., 2008,
´
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Acc. Chem. Res., 2012, 45, 778; (i) A. Johnson and M. C.
Gimeno, Chem. Commun., 2016, 52, 9664; ( j) Y. Tokimizu,
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Entry
1
R
Product
Cat: yielda (%)
2: 93
3: 90
2: 89
3: 87
2
2: 94
3: 92
3
4
2: 85
3: 82
Reaction conditions: N-propargyl amide (0.1 mmol), dichloromethane
(3 mL), room temperature. In case of complex 2, 1 mol% of AgBF4.
a
Isolated yields as the average of two runs.
2 T. S. Teets and D. G. Nocera, J. Am. Chem. Soc., 2009,
131, 7411.
3 Special issue ‘‘Bioinorganic and Biomedical Chemistry of
Gold’’, Coord. Chem. Rev., 2009, 253, 1597.
complex 3, the large [(TpMe2)2K] anion stabilises the Au(I) cationic
fragment enhancing the full catalytic performance.
4 (a) S. P. Nolan, Acc. Chem. Res., 2011, 44, 91; (b) N. Marion
and S. P. Nolan, Chem. Soc. Rev., 2008, 37, 1776.
With the optimized catalytic conditions, we next tested com-
plexes 2 and 3 in the cyclization of several propargylated amines.
According to Table 2, the corresponding oxazolines are obtained
in good to excellent yields with precatalyst 2 showing a slightly
better performance. The catalytic performance of complexes 2
and 3 is comparable to the Sarkar19 and Albrecht20 mono and
digold triazolylidene complexes recently reported.
5 See for example: (a) N. Marion, P. de Fremont, N. M. Scott,
L. Fensterbank, M. Malacria and S. P. Nolan, Chem. Commun.,
2006, 2048; (b) A. Correa, N. Marion, L. Fensterback,
M. Malacria, S. P. Nolan and L. Cavallo, Angew. Chem.,
Int. Ed., 2008, 48, 718.
6 See for example: (a) D. Mendoza-Espinosa, D. Rendon-Nava,
In summary, we have reported the serendipitous synthesis of
a bis(triazolylidene) gold(I) complex containing a large [(TpMe2)2K]
anion (3) obtained by the treatment of a mononuclear MIC–Au–I
complex 2 with KTpMe2. As the yield of 3 was rather low starting from
complex 2 (due to the forced loss of half equivalent of the gold
source), an optimized synthesis was achieved by reacting a pre-
formed bis(triazolylidene)Au(I) complex 2a with two equivalents of
KTpMe2 yielding complex 3 in 81%. In solution, the NMR spectro-
scopy analysis of complex 3 shows highly symmetric patterns in both
1H and 13C spectra, consistent with the observed structure in solid
state. All complexes 2–3 have been fully characterized and preliminary
tested the formation of a series of oxazoline derivatives. The
catalytic results demonstrate a similar performance of the 2/AgBF4
system and complex 3, and suggest that the large [(TpMe2)2K] anion
enhances the robustness of the catalytically active Au(I) cationic
species of 3 compared to complex 2a which contains a smaller
chloride counterion. Further studies in the catalytic applications of
complex 3 is a currently topic of interest in our laboratory.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
´
¨
We are grateful to the CONACYT-Mexico (Grant Catedra-CONACyT-
Chem., 2012, 55, 3713; (e) P. C. Kunz, C. Wetzel, S. Kogel,
2016-222) and CONACYT (project 0223800) for financial support.
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