The First Gold(III) Formate: Evidence for β-Hydride Elimination

Article abstract of DOI:10.1002/anie.201705557

The first stable gold(III) formate and experimental evidence for its β-hydride elimination are described. A catalytic dehydrogenation of formic acid together with mechanistic studies shed light on potential pathways operating in fundamental gold-catalyzed transformations.

Full text of DOI:10.1002/anie.201705557

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201705557
German Edition: DOI: 10.1002/ange.201705557
Gold Complexes
The First Gold(III) Formate: Evidence for b-Hydride Elimination
Roopender Kumar, Jean-Philippe Krieger, Enrique Gꢀmez-Bengoa, Thomas Fox,
Anthony Linden, and Cristina Nevado*
Abstract: The first stable gold(III) formate and experimental
evidence for its b-hydride elimination are described. A catalytic
dehydrogenation of formic acid together with mechanistic
studies shed light on potential pathways operating in funda-
mental gold-catalyzed transformations.
T
he b-hydride elimination from a metal alkyl or alkoxy
complex is an elementary reaction in catalysis that has been
harnessed as a productive step in industrially relevant trans-
formations such as olefin polymerization, Mizoroki–Heck
[
1]
couplings, or the Wacker process, among many others. Metal
formates and carboxylates also undergo b-hydride elimina-
tion to produce CO and metal hydrides (MÀH; Scheme 1A).
2
This reactivity has been efficiently exploited in several
transformations, including the water gas shift reaction
(
(
WGSR) as well as the dehydrogenation of formic acid
FA). Over the past years, in-depth mechanistic insight into
[
2]
b-hydride elimination processes has been gained for transi-
[
1]
tion metals such as Pd, Ru, Ni, Pt, and Ir. In contrast, and
À
Scheme 1. A) General pathways for M H formation by b-hydride
elimination. B–D) Precedents of b-hydride elimination on gold.
E) b-Hydride elimination on a (C^N^C)Au carboxylate. F) Evidence for
b-hydride elimination on a novel gold(III) formate.
[
3]
despite prominent developments in gold catalysis, b-hydride
elimination on gold was long deemed unfeasible, and
reports on gold complexes undergoing b-hydride elimination
are scarce. In pioneering work, Hashmi and co-workers
demonstrated that the (IPr)gold(I) ethyl complex does not
undergo b-hydride elimination even at high temperature
[
4]
[5]
[
10]
acid. Furthermore, these species were used for the hydro-
auration of alkynes in the presence of a radical initiator by
homolytic cleavage of the AuÀH bond to form the corre-
[6]
3
(
Scheme 1B). In a seminal work towards C(sp )ÀF bond-
[11]
forming reductive elimination, Toste and Mankad showed
that (NHC)gold(III) alkyl difluoride complexes form alkenes
via b-hydride elimination along with the corresponding alkyl
sponding trans-vinylgold(III) complexes.
Importantly,
gold(III) carboxylates and formates have been proposed as
intermediates prone to undergo b-hydride elimination in
gold-catalyzed WGSRs and the dehydrogenation of formic
acid, but the isolation and direct characterization of these
species to substantiate these proposals has proven remarkably
challenging. In this context, the resulting gold hydrides have
[7]
[12]
fluorides (Scheme 1C). Recently, Bourissou and co-workers
thoroughly investigated b-hydride elimination on low-coor-
dinate (P^C)-type cyclometalated gold(III) alkyl complexes
Scheme 1D). In 2015, Bochmann and co-workers reported
the insertion of CO into a gold(III) hydroxo complex [k -
[
13]
[
8]
(
3
[14]
also attracted significant attention although experimental
[
9]
(
C^N^C)Au(OH)]. Whereas the corresponding gold(III)
carboxylate could not be isolated, its formation was supported
validation for their participation in catalysis remains scarce.
Our group has recently disclosed the synthesis of stable
(N^C^C)gold(III) fluoride complexes that are nevertheless
III
by the observation of the corresponding Au ÀH species as
[15]
a result of the b-hydride elimination (Scheme 1E). The
stability of this gold(III) hydride proved to be remarkable as
it was stable in the presence of air, moisture, and even acetic
reactive towards nucleophiles (alkynes and boronic acids).
3
We envisioned that the novel k -(N^C^C) pincer template
could enable the synthesis of rather elusive gold(III) inter-
mediates. Herein, we present the synthesis of a stable gold-
(
III) formate together with a study of its reactivity towards
[
*] R. Kumar, Dr. J.-P. Krieger, Dr. T. Fox, Prof. A. Linden, Prof. C. Nevado
Department of Chemistry, University of Zꢀrich
Winterthurerstrasse 180, 8057 Zꢀrich (Switzerland)
E-mail: cristina.nevado@chem.uzh.ch
b-hydride elimination and in the context of a catalytic
dehydrogenation of formic acid (Scheme 1F).
III
Cognizant of the basic nature of [(N^C^C)Au F] spe-
[
15]
cies,
we envisioned their potential use as precursors of
Prof. E. Gꢁmez-Bengoa
Departamento de Quꢂmica Orgꢃnica I
Universidad del Pais Vasco
gold(III) formates. Indeed, after preliminary experiments, the
reaction of gold(III) fluoride 1 in the presence of formic acid
in CH Cl at room temperature furnished the corresponding
formate 2 in quantitative yield (Scheme 2).
formate 2 is stable under ambient conditions as a solid but
Apdo 1072, CP-20080 Donostia—San Sebastiꢃn (Spain)
2
2
[
16]
Gold(III)
Supporting information for this article can be found under:
https://doi.org/10.1002/anie.201705557.
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1
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Au1ÀC33 bond of 2.083(9) ꢀ and a small C33-Au1-C13 angle
of 95.68 (Table 1 and Ref. [16]).
To support the hypothesis of a putative gold(III) hydride
[20]
intermediate
that is able to insert into a triple bond,
alternative methods for the synthesis of these species were
III
sought (Scheme 3). Treatment of [(N^C^C)Au Cl] 5 with
Scheme 2. Synthesis and reactivity of (N^C^C)gold(III) formate 2.
decomposes in solution over prolonged periods of time
1
(
days). H NMR analysis of 2 in CD Cl showed a sharp
2
2
singlet resonance at d = 9.46 ppm, characteristic of the
formate moiety. Furthermore, a single crystal was grown by
slow diffusion of MeOH into a concentrated solution of 2 in
CH Cl , and the proposed structure was confirmed by X-ray
2
2
diffraction analysis (Table 1).
[
a]
Scheme 3. Synthesis of vinyl gold(III) complex 4 and gold(III) esters 2,
Table 1: The solid-state molecular structures of 2, 4, and 6.
6, and 7.
2
4
6
l-selectride or LiHBEt in dry THF-d at À788C followed by
3
8
addition of di-tert-butyl acetylenedicarboxylate furnished the
corresponding trans addition product (Z)-4 in 55% yield, in
line with the results observed in Scheme 2. Interestingly, when
the above-mentioned reaction mixture was exposed to formic,
acetic, and trifluoroacetic acid, the corresponding gold(III)
esters 2, 6, and 7 were cleanly obtained, all of which were
unequivocally characterized by X-ray diffraction analysis (see
Bond lengths
ꢄ]
Au1–O1:
2.102(3)
Au1–C8:
Au1–C33:
2.083(9)
Au1–C8:
2.004(8)
C33-Au1-N1:
104.5(3)
Au1–O1:
2.0873(17)
Au1–C8:
1.952(2)
O1-Au1-N1:
98.87(7)
[
1.961(4)
Bond angles
O1-Au1-N1:
95.25(15)
O1-Au1-C14:
[8]
C33-Au1-C13:
95.6(3)
O1-Au1-C14:
99.46(8)
[21]
Table 1 and the Supporting Information).
103.51(17)
Having established the reactivity of gold(III) formate 2
towards b-hydride elimination, we envisioned the possibility
of designing a homogeneous catalytic system for the dehy-
drogenation of formic acid. Gold-catalyzed dehydrogenations
of FA in a heterogeneous fashion have been described,
[a] Ellipsoids set at 50% probability. Hydrogen atoms have been omitted
for clarity.
Interestingly, the AuÀO bond [2.102(3) ꢀ] is slightly
longer than those previously reported for gold(III) oxo
although typically basic additives or inorganic alkali metal
[
17]
[13,22]
complexes,
most probably owing to the strong trans
salts are required for efficient performances.
When
2
influence of the central C(sp ) carbon atom of the (N^C^C)
ligand. As transition-metal formates are generally known to
be precursors of hydrides, we next set out to confirm the
ability of complex 2 to engage in a b-hydride elimination
process. No conversion was observed by H NMR spectros-
complex 1 or 2 was heated at 1008C in pure formic acid,
a linear production of H versus time and similar moderate
2
[18]
À1
catalytic activities (TOF = 3.1 Æ 0.3 h for 1 and TOF = 3.5 Æ
À1
0.3 h for 2) were obtained. Interestingly, neither decom-
1
position nor a decrease in efficiency was noticed for a period
of up to 44 h, which translated into a turnover number (TON)
copy after stirring 2 in aliphatic chlorinated solvents (CH Cl ,
2
2
1
1
,2-DCE) at temperatures up to 608C. However, heating 2 at
008C resulted in complete conversion and subsequent
of > 137. The activation energy of the catalytic cycle, E =
a
À1
25.4 Æ 1.9 kcalmol , was calculated using an Arrhenius plot
[
23]
decomposition of the complex. This result suggested the
possible formation of a highly reactive gold(III) hydride
species. Interestingly, the reaction of formate 2 in the
presence of di-tert-butyl acetylenedicarboxylate (3, 5 equiv)
in 1,2-DCE at 1008C afforded (Z)-gold(III) vinyl complex 4
in 64% isolated yield (Scheme 2). The reaction was moni-
(see the Supporting Information). These results are also in
line with the reactive nature of a putative gold(III) hydride
intermediate whose reaction rate with formic acid should be
fast at 1008C compared to the b-hydride elimination of
formate 2. As a result, the mechanism of the dehydrogenation
of formic acid can be pictured as a rate-determining b-hydride
elimination of the gold(III) formate 2 upon heating to give
a highly reactive hydride, which rapidly reacts with formic
acid to release hydrogen and regenerate formate 2, thereby
closing the catalytic cycle (Scheme 4).
1
tored by H NMR spectroscopy, revealing a first-order decay
in 2 as well as first-order formation of 4 at comparable rates,
which suggests a b-hydride elimination process (see the
Supporting Information, Section S3.3). In solution, (Z)-4
slowly isomerized to the E isomer, which could be charac-
terized by X-ray diffraction analysis, revealing an elongated
[
19]
We turned to DFT studies to shed light on the overall
reaction energy profile. We were interested in confirming the
2
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Acknowledgements
We thank the European Research Council (ERC Starting
Grant 307948) and the Forschungskredit of the University of
Zꢁrich for financial support. We acknowledge networking
support by the COST Action CHAOS CA15106. The SGI/
IZO-SGIker UPV/EHU is acknowledged for the allocation of
computational resources. We also thank Dr. Olivier Blacque
for the X-ray diffraction analysis of complex 4 and Benjamin
Probst and Prof. Roger Alberto for technical support on the
quantification of H2.
Conflict of interest
Scheme 4. Proposed catalytic cycle for the dehydrogenation of formic
acid. Structures and DH/DG energies (kcalmol ) of the catalytic cycle
computed at the M06/6-31G**(SDD) level of theory, including solvent
À1
The authors declare no conflict of interest.
effects (IEFPCM, solvent=formic acid).
Keywords: b-hydride elimination · gold complexes ·
gold hydrides · reaction mechanisms · vinyl gold species
feasibility of formate-to-hydride decarboxylation and dehy-
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membered cyclic structure. The step is exergonic (À3.6 kcal
À1
°
mol ) but needs a relatively high activation energy (DH =
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III-II
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1
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1
2
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[
24]
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1
[
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mation. CCDC 1511131 (2), 1511132 (4), 1511135 (6), and
1033771(7) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre.
Manuscript received: May 31, 2017
Version of record online: && &&, &&&&
4
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Communications
Gold Complexes
R. Kumar, J.-P. Krieger, E. Gꢁmez-Bengoa,
T. Fox, A. Linden,
C. Nevado*
&&&& — &&&&
The First Gold(III) Formate: Evidence for
b-Hydride Elimination
An anionic ligand exchange reaction on
hydride elimination and its reactivity in
III
[
(N^C^C)Au F] in the presence of formic
the dehydrogenation of formic acid (FA)
have been experimentally and computa-
tionally demonstrated.
acid delivered the first example of a stable
gold(III) formate. Its ability to undergo b-
Angew. Chem. Int. Ed. 2017, 56, 1 – 5
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!