Oxidative Access via Aqua Regia to an Electrophilic, Mesoionic Dicobaltoceniumyltriazolylidene Gold(III) Catalyst

Article abstract of DOI:10.1021/acs.organomet.9b00616

A gold(III) complex with the hitherto most electron poor mesoionic carbene ligand is presented. Aqua regia was the oxidizing agent of choice for the synthesis of this unusual organometallic compound. The Au^III complex is redox-rich and also act

Full text of DOI:10.1021/acs.organomet.9b00616

Communication
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Oxidative Access via Aqua Regia to an Electrophilic, Mesoionic
Dicobaltoceniumyltriazolylidene Gold(III) Catalyst
Stefan Vanicek,† Julia Beerhues,‡ Tobias Bens,‡ Volodymyr Levchenko,† Klaus Wurst,§
Benno Bildstein,*,§ Mats Tilset,*,† and Biprajit Sarkar*,‡,∥
†Department of Chemistry, University of Oslo, Sem Sælands vei 26, 0315 Oslo, Norway
‡Institut fu
̈
r Chemie und Biochemie, Anorganische Chemie, Freie Universitat Berlin, Fabeckstraße 34-36, 14195 Berlin, Germany
̈
§Institute of General, Inorganic and Theoretical Chemistry, Center for Chemistry and Biomedicine, University of Innsbruck, Innrain
80-82, 6020 Innsbruck, Austria
∥Institut fu
̈
r Anorganische Chemie, Universitat Stuttgart, Pfaffenwaldring 55, D-70568 Stuttgart, Germany
̈
S
* Supporting Information
ABSTRACT: A gold(III) complex with the hitherto most
electron poor mesoionic carbene ligand is presented. Aqua
regia was the oxidizing agent of choice for the synthesis of this
unusual organometallic compound. The AuIII complex is
redox-rich and also acts as a catalyst for oxazole formation,
delivering selectively a completely different isomer in
comparison to its AuI congener.
he use of gold complexes in homogeneous catalysis has
Tbeen a dormant field of research for decades. However,
gold catalysis has emerged from a mere curiosity to one of the
most powerful means for the activation of C−C multiple bonds
toward a variety of complexity-oriented transformation
reactions.1−6 Whereas the expertise in gold(I) catalysts has
attained a progressive level to date,7,8 comparatively less
information is available regarding gold(III) catalysts.9,10
However, gold(III) catalysts have an advantage over their
gold(I) congeners, as they are usually relatively cheap materials
and, more importantly, they are in general insensitive to air,
moisture, and light with the exclusion of AuCl3, AuBr3, and
HAuCl4·nH2O.1,9,11 For this reason gold(III) catalysts have a
broad spectrum of applications in organic syntheses under mild
conditions. Square-planar gold(III) complexes are diamagnetic
16-electron systems frequently used in regio- and stereo-
selective intra- and intermolecular addition reactions as well as
in multicomponent reactions (MCRs), with their established
areas still expanding.12,13 A prerequisite to using well-defined
gold(III) complexes as catalysts is the development of facile
synthetic routes to gain access to these materials.
triazol-4-ylidene (MIC),23 can be synthesized. Furthermore,
we wish to compare and contrast the catalytic activity of the
resulting Au(III) complex with that of its Au(I) congener,
which we reported recently. The preparation of the
corresponding tricationic triazolium salt (1) of its MIC24,25
gold(I) complex, and the catalytic activity thereof were
reported in our previous work.23 Taking these investigations
further toward the envisaged MIC gold(III) complex (3), a
straightforward approach would be deprotonation of the
triazolium precursor with Ag2O26 followed by transmetalation
to K[AuCl4]. However, all our attempts via this synthetic route
led to the target compound persistently accompanied by its
homoleptic gold(III) congener, with the dimeric silver(I)
intermediate and the substrate as byproducts, each embodying
multicationic species that proved impossible to separate. For
this reason, an alternative oxidative route was pursued
(Scheme 1), starting from the readily accessible gold(I)
complex 223 as a precursor of the desired MIC gold(III)
complex (3). Interestingly, the use of various standard
oxidizing agents, e.g. Br2, CsBr3, etc,. failed27,28 or, in the
case of iodobenzene dichloride,29,30 lead again to inseparable
mixtures. Finally and remarkably, aqua regiaa very
uncommon oxidant in organometallic chemistryproved to
be the reagent of choice.
Au(I) complexes of mesoionic carbenes (MICs) of the 1,2,3-
triazol-4-ylidene type have been used in homogeneous catalysis
in the past few years, and a few examples of Au(III) complexes
with these ligands were reported as well.14−22 Within this
communication, we would like to address the question whether
the Au(III) complex of the hitherto electron poorest NHC
ligand, represented by 1,4-dicobaltoceniumyl-3-methyl-1,2,3-
Received: September 10, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00616
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Communication
Scheme 1. Synthetic Route toward MIC Gold(III)
Complexes 3a,b
Figure 1. Molecular structure of 3b with key atoms labeled. Selected
bond lengths (Å): Au(1)−C(12) = 2.019(5), Au(1)−Cl(1) =
2.307(2), Au(1)−Cl(2) = 2.286(2), Au(1)−Cl(3) = 2.267(2).
Selected angles (deg): C(12)−Au(1)−Cl(1) = 178.68(14), Cl(3)−
Au(1)−Cl(2) = 174.69(7).
spectrum of 3a, wherein one chlorido ligand was readily
replaced by a methoxide, formed by the solvent. When the
aforementioned elongated bond length is taken into account, it
is only reasonable to attribute this reactivity to the loss of the
trans-chlorido ligand.
In detail, 2 was stirred in 5 mL of freshly prepared aqua regia
at room temperature overnight. After evaporation of excess of
aqua regia in vacuo, the product was washed with water to
remove the remaining amounts of acid, dissolved in acetone,
and precipitated by addition of Et2O, thereby affording air-
stable, yellow 3a in 68% isolated yield. The clean chemo-
selectivity of this reactionno formation of undesired
homoleptic complexesis most likely due to the very high
concentration of available chlorido ligands that form
intermediate chlorido−aurate complexes, which are less
prone to further substitution. Quite remarkably, the
carbene−gold bond remains intact under these harsh
conditions. Synthetically, from an organometallic chemist’s
point of view, aqua regia is a very convenient new oxidizing
agent in gold coordination chemistry, potentially simplifying
the synthesis of new trichlorido(NHC)gold(III) catalysts in
future work.
As reported by us earlier, the triazolium salt 1 displays two
close-lying one-electron-reduction steps that can be assigned to
the successive reduction of the formally CoIII centers to CoII.
Those reduction steps are followed by two further close-lying
one-electron-reduction processes that are assigned to the
reduction of the CoII centers to CoI.23 We have now
performed UV−vis−NIR spectroelectrochemistry on 1, and
the measurements prove that the four aforementioned
reduction steps are also reversible on the spectroelectrochem-
istry time scale (Figures S9 and S10 and Tables S7 and S8). A
further one-electron-reduction step assignable to the triazolium
ring was also observed for 1. This process is also reversible in
the spectroelectrochemical measurements. Similar redox events
were also observed for the AuI complex 2.23 The AuIII complex
3a displays reduction steps at −0.24, −0.98, −1.08, −1.93,
−2.02, and −2.25 V in CH3CN/0.1 M Bu4NPF6 (Figure 2).
Except for the redox wave at −0.24 V, all other redox events
observed for 3a are very similar to those observed for 1 and 2.
The position of the reduction peak at −0.24 V was found to be
dependent on the experimental conditions. Additionally, as can
1
Complex 3a was fully characterized by H/13C/19F NMR,
IR, and HRMS (see the Supporting Information). The 13C
NMR signal of the carbene carbon was observed at 137.5 ppm,
significantly shifted upfield in comparison to its Au(I)
precursor 2 (δcarbene 162.2 ppm).23 Interestingly, the weakly
coordinating triflates remain as the present counterions under
these harsh conditions, as could be corroborated by 13C/19F
NMR, IR, and PXRD (see the Supporting Information). A
single-crystal X-ray analysis of 3a provided a data set of poor
quality; therefore the counterions were changed to hexafluor-
idophosphates (compare the Supporting Information) and
good-quality single crystals of 3b suitable for X-ray analysis
(R1 = 3.64%, Supporting Information) were obtained. Figure 1
shows the molecular structure of 3b. Overall, the dication has a
regular square-planar gold center with a perpendicular
triazolylidene ligand containing regular, undistorted cobalto-
ceniumyl substituents that are positioned in an anti
conformation, clearly due to electrostatic repulsion. Further-
more, complex 3b shows a strong trans effect with a Au−Cltrans
bond length of 2.307(2) Å, slightly elongated in comparison to
its gold(I) congener, evincing a bond length of 2.284(1) Å.23
This trans effect was also palpable in the HRMS (ESI positive)
Figure 2. Cyclic voltammogram of 3a in CH3CN and 0.1 M NBu4PF6
at 100 mV/s with a glassy-carbon working electrode.
B
DOI: 10.1021/acs.organomet.9b00616
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Communication
be seen from Figure 2, this wave is irreversible, and a
comparison of the current height of this wave with those of the
following waves indicates that this redox wave is likely a two-
electron-transfer event. We thus assign the redox wave at
−0.24 V to the reduction of the AuIII center to AuI. This step
will certainly be accompanied by the dissociation of two
chloride ions from the Au center, making this process
irreversible. It has been previously reported that the redox
potential of such Au-centered processes are very sensitive to
the experimental conditions.31 The waves at higher negative
potentials are thus assigned to the reduction of both the CoIII
centers first to CoII and then to CoI. The peak at −2.25 V is
assigned to the reduction of the triazolylidene ring. The
oxidation wave observed at 0.73 V is a follow-up of the
reduction peak at −0.24 V. Even though the irreversible nature
of the redox events on the spectroelectrochemical time scale
prohibits a direct assignment of the redox events, the
comparison of the redox processes and their potentials with
1 and 2 does lend credibility to the assignments made above.
We recently reported that the AuI complex 2 selectively
catalyzes the conversion of substrate A to the corresponding
oxazoline B (Scheme 2) without the need of any additional
selectively form the isomer B by using the AuI complex 2 as a
catalyst or the isomer C by using the AuIII complex 3 as a
catalyst. However, it should be borne in mind that the
mechanisms of action of Au(I) and Au(III) catalysts are
usually different. The formation of isomer C through AuIII
catalysis has also been reported previously.32
Summarizing, we have presented here the first example of a
AuIII complex with the hitherto most electron-poor carbene
ligand. This complex is redox-rich and displays gold-,
cobaltoceniumyl-, and triazolylidene-based reductions. The
AuIII complex is an active catalyst, delivering selectively a
different isomer as a product in comparison to its AuI
congener.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00616.
Experimental procedures, analytical, electrochemical,
and catalytic data, and spectra (PDF)
Cartesian coordinates for the calculated structure (XYZ)
Scheme 2. Catalytic Activity of the Electron-Poor Gold(III)
Carbene Complex 3 in Comparison to the Reactivity of the
Previously Investigated Corresponding Gold(I) Complex
223
Accession Codes
CCDC 1938205 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail for B.B.: benno.bildstein@uibk.ac.at.
*E-mail for M.T.: mats.tilset@kjemi.uio.no.
*E-mail for B.S.: biprajit.sarkar@iacuni-stuttgart.de.
ORCID
Benno Bildstein: 0000-0003-1969-1334
Mats Tilset: 0000-0001-8766-6910
Biprajit Sarkar: 0000-0003-4887-7277
Notes
The authors declare no competing financial interest.
additives.23 Unfortunately, under identical conditions no
conversion was observed with the AuIII complex 3. We then
decided to use AgSbF6 as an additional additive in the catalytic
reaction. The use of the additive delivered only about 5% B in
2 h when 1 mol % of 3 was used. During these experiments we
observed that the catalytic mixture was not well soluble in
CD2Cl2. We thus decided to change to CD3CN and increased
both the catalyst and the additive loading to 3 mol % each.
Under those conditions in CD3CN at room temperature we
observed a conversion of 4% after 3 h and 17% after 24 h.
Furthermore, during this reaction, we started observing the
formation of a different product, the other isomer, 5-methyl-2-
phenyloxazole (C). To investigate whether the addition of the
additive was necessary for the conversion, we decided to
perform the catalytic reaction with 3 mol % of 3 in CD3CN at
70 °C without any additional silver salts. Gratifyingly, under
these conditions more than 90% conversion was observed after
24 h, and we also observed the exclusive formation of the
isomer C (Figures S11 and S12). Thus, it is seen that, through
optimization of the reaction conditions, it is possible to
ACKNOWLEDGMENTS
■
We acknowledge David Wragg and Sigurd Øien-Ødegaard
from the Department of Chemistry, University of Oslo, for
measuring and solving the initial crystal structure of compound
3a, albeit the crystal quality was not sufficient for publication.
This work was supported by the Austrian Science Fund
(FWF), grants J-4158 and P 30221, and by the Research
Council of Norway through grant no. 250795 “Confine”
(stipend to V.L.).
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