Noble Metal Corrosion: Halogen Bonded Iodocarbenium Iodides Dissolve Elemental Gold—Direct Access to Gold–Carbene Complexes

Article abstract of DOI:10.1002/chem.201901583

A common method to dissolve elemental gold involves the combination of an oxidant with a Lewis base that coordinates to the gold surface, thus lowering the metal's redox potential. Herein we report the usage of organic iodide salts, which provide both oxidative power and a coordinating ligand, to dissolve gold under formation of organo-gold complexes. The obtained products were identified as Au^III complexes, all featuring Au?C bonds, as shown by X-ray single-crystal analysis, and can be isolated in good yields. Additionally, our method provides direct access to N-heterocyclic carbene (NHC-type) complexes and avoids costly organometallic precursors. The investigated complexes show dynamic behavior in acetonitrile and in the case of the NHC(-type) complexes, the involved species could be identified as a monocarbene [AuI₃(carbene)] and biscarbene complex [AuI₂(carbene)₂]⁺.

Full text of DOI:10.1002/chem.201901583

A Journal of
Accepted Article
Title: Noble Metal Corrosion: Halogen Bonded Iodocarbenium Iodides
Dissolve Elemental Gold – A Direct Access to Gold-Carbene
Complexes
Authors: Jana Holthoff, Elric Engelage, Alexander B Kowsari, Stefan
Matthias Huber, and Robert Weiss
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To be cited as: Chem. Eur. J. 10.1002/chem.201901583
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Noble Metal Corrosion: Halogen Bonded Iodocarbenium Iodides
Dissolve Elemental Gold – A Direct Access to Gold-Carbene
Complexes
Jana M. Holthoff, Elric Engelage, Alexander B. Kowsari, Stefan M. Huber* and Robert Weiss*
Herein, we report the first synthesis of gold(III) carbene
complexes directly from Auelem using organic reagents which
provide both oxidative power and the coordinating ligand. More
generally this procedure also represents the first generation of Au-
C bonds from elemental gold as a stoichiometric reagent, a novel
entry into organo-gold chemistry.
Abstract: A common method to dissolve elemental gold involves the
combination of an oxidant with a Lewis base, coordinating to the gold
surface and thus lowering the metal’s redox potential. Herein we
report the usage of organic iodide salts providing both oxidative power
and coordinating ligand, to dissolve gold under formation of organo-
gold complexes. The obtained products were identified as AuIII
complexes, all featuring Au-C bonds, as shown by X-Ray single
crystal analysis, and can be isolated in good yields. Additionally, our
method is providing a direct access to NHC(-type) complexes and
avoids costly organometallic precursors. The investigated complexes
show dynamic behavior in acetonitrile and in case of the NHC(-type)
complexes, the involved species could be identified as a mono-
[AuI3(carbene)] and bis-carbene complex [AuI2(carbene)2]+.
As such reagents, iodocarbenium iodides 1 were chosen
(Scheme 2), which are salt-like if one of R/R’ is a donor group.[12]
Scheme 2. Two ways of liberation of oxidant and Lewis Base from the
hypervalent adduct [Ox]/[LB] ([Ox]’/[LB]’). Both, 1a and 1c are dissociation
products of [Ox]/[LB].
Gold, the noblest of all metals, is notoriously inert to all but the
strongest redox reagents. Dissolution of elemental gold, thus,
requires either powerful oxidants like aqua regia (category A in
Scheme 1) or strong reductants like elemental potassium
(category B). Most commonly the activation of elemental gold is
achieved by a combination of an oxidant with a Lewis base
(category C), as in the industrial process involving cyanide and
oxygen. Category C is commonly referred to as corrosion.
Their structures may be considered as being in-between two
limiting cases of halogen-bonded adducts: either – as the name
implies – as complexes between an iodocarbenium cation and
iodide (1b), or as the adduct of a carbene to elemental iodine (1a).
Both cases represent the combination of an oxidant with a LB and
thus constitute an ideal combination for the oxidative addition to
elemental gold.
Scheme 3. N,N-dimethyliodomethyleneiminium iodide
iodocarbenium iodide, which can be derived from DMF via Vilsmeier reagent 2.
3
as simplest
Scheme 1. Conceptionally different approaches towards the dissolution of
elemental gold (Ox = oxidant, LB = Lewis base).
Arguably the simplest such iodocarbenium iodide is N,N-
dimethyliodomethyleneiminium iodide 3, which is accessible by
iodination of Vilsmeier reagent 2.[12,13] The latter is known to
oxidize metal precursors leading to so-called secondary carbene
In recent years, a growing number of type C methods have been
developed, involving oxidants such as iodine,[1–6] hydrogen
peroxide,[7] oxygen,[8] or thionyl chloride[9] and coordinating Lewis
bases like halides,[2–4,10] phosphines[8] or sulfur-bearing organic
ligands.[1,5,11] Almost all of these examples, though, require the
use of two distinct reagents and provide products which need
further modification towards organo-gold products, like e.g.
carbene complexes.
((mono)dimethylamino carbene) complexes, but only with starting
[14,15]
materials like Pd(PPh3)4
and other metals less noble than
gold (see scheme 3).[14–20] To the best of our knowledge, such
carbene complexes have not yet been prepared from elemental
metals[21] and in case of gold not even by more conventional
methods.
Salt 3 has been known for decades[22] but its crystal structure has
remained elusive until now (Figure 1 left). It features a halogen
bond between the cation and the anion, and thus its structure is
best represented by 1b (Scheme 2), as had already been shown
by one of us for a related compound.[12]
When to a suspension of 3 in dichloromethane (DCM) at room
temperature gold powder was added, the metal powder was
dissolved in less than 30 min. 1H-NMR spectroscopy indicated the
formation of several species including an organogold complex,
with the latter being finally confirmed by X-Ray analysis of a single
crystal (Figure 1 right).
[a]
[b]
J. M. Holthoff, E. Engelage, A.B. Kowsari, Prof. Dr. S.M. Huber
Fakultät für Chemie und Biochemie
Ruhr-Universität Bochum
Universitätsstraße 150, 44801 Bochum (Germany)
E-mail: stefan.m.huber@rub.de
Prof. Dr. R. Weiss
Institut für Organische Chemie
Friedrich-Alexander-Universität Erlangen-Nürnberg
Henkestraße 42, 91054 Erlangen (Germany)
Supporting information for this article is given via a link at the end of
the document
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(dimethylamino)methylene-gold(I) 5 (see scheme 5). Alternatively,
ligand scrambling and formation of ionic species like
[AuI2(CHNMe2)2]+[AuI2]- 6 would also explain the spectra (and
would be in line with the findings for the NHC(-type) complexes
described below).
Heating a solution of the crystals in dry acetonitrile to 80 °C leads
to the formation of gold particles. Although dimethylamino
carbene complexes of palladium and platinum are described to
be stable against water,[17,19,26] addition of water to the solution of
4 in acetonitrile immediately leads to the formation of DMF. This
seems to indicate that the gold(III) carbene complexes described
herein feature a more electrophilic carbon centre compared to
other complexes of these ligands, e.g. with Pt(IV) (which are
stable against water).[19]
As the synthesis of 4 suffered from poor yields (18%) and poor
reproducibility, further optimization was required. A possible
mechanism involves formation of the Au(II) complex
[AuI2(CHNMe2)] 7 (see Scheme 7 below) after oxidative addition.
This species is likely unstable with regard to disproportionation
(yielding 4 and the corresponding Au(I) carbene 5) and other
possible decomposition products, which would explain the
observed problems. A potential remedy to this issue could be the
addition of a further oxidant which converts the unstable Au(II)
complex 6 to the stable Au(III) complex 4, and to this end
elemental iodine (in 0.5 equivalents) was chosen (Scheme 6).
Figure 1. Solid state molecular structure of N,N-dimethyliodomethyleneiminium
iodide
3 (left, dI-I = 3.4198(8) Å, ∡ (C1-I1-I2) = 168.580(91) °) and triiodo-
(dimethylamino)methylene-gold(III) 4 (right, ∡(N1-C1-Au-I2) = 91.951(595) °).
Thermal ellipsoids at 50% probability level.
Although classical gold catalysis does not feature oxidative
addition steps, this kind of reaction step has attracted more
interest in recent years. It was shown by Toste, Russel, Hashmi
and others that oxidative additions of e.g. organic halides to Au(I)
species are possible under certain conditions.[23] To the best of
our knowledge an oxidative addition process was not yet
employed for the synthesis of gold carbene complexes.
Scheme 4. Resonance structures of triiodo-((dimethylamino)methylene)-
gold(III) 4.
The crystal structure of triiodo-((dimethylamino)methylene)-
gold(III)
4 indicates that the electronic structure of gold
dimethylamino carbene complexes is best described by the
iminium-like resonance structure 4.2: The C-N-double bond in
iodocarbenium iodide (dC-N = 1.272(5) Å) is identical in length to
the corresponding bond in complex 4 (dC-N = 1.272(8) Å). This is
also confirmed by 1H-NMR-spectroscopy, which indicates
hindered rotation around the C1-N bond, and is in agreement to
data presented for the corresponding carbene complexes with
other metal centers.[18–20]
Scheme 6. Synthesis of the secondary carbene complex triiodo-
((dimethylamino)methylene)-gold(III) 4. a) DCM, r.t., 1. (COCl)2, 1 h, 2. Me3SiI
(2 eq), 2h, b) Au, I2 (0.5 eq), r.t. DCM or acetonitrile, overnight.
1H-NMR spectroscopy in CD2Cl2 of the crude product mixture
indicated complete conversion under formation of complex 4 and
a second species (e.g. 5 or 6, see above) with a ratio of
AuI3(CHNMe2) 4:second species of ≈ 5:4 (reaction in acetonitrile)
and of 5:1 (performed in DCM) respectively. The crude product
mixture containing 4 in about 55% yield, according to NMR, was
dissolved in acetonitrile and stored at -30 °C for several days to
crystallize complex 4. The product was isolated in up to 73% yield,
confirming the existence of an equilibrium between 4 and the
second species.
1H-NMR spectroscopy also indicates that an equilibrium exists in
some solvents: while the spectrum of dissolved crystals of 4 in
DCM showed only one set of signals, a second species is
apparently formed in acetonitrile. When the solvent is changed
back to DCM, the signals of 4 are found again together with those
of the newly formed species.
Since triiodide has been reported to dissolve elemental gold,[2–4]
additional mechanistic pathways are feasible beside the one
indicated above, the direct reaction between gold particles (or AuI-,
which is only a formal particle, representing the iodidecoordinated
to the gold surface, see below) and the carbenium ion to form
Au(II) species 7 with subsequent oxidation. Gold could also be
-
first oxidized to Au(I) species like AuI or AuI2 , which would then
undergo oxidative addition with the organic cation to form the
targeted Au(III) complex 4 (Scheme 7).
Scheme 5. The gold(I) carbene complex 5 and the bis-carbene complex 6 are
possible species triiodo-((dimethylamino)methylene)-gold(III) 4 is in equilibrium
with.
Since it is described in the literature that AuI3(L) complexes tend
to be in equilibrium with their corresponding gold(I) species,[24] it
is possible that complex 4 undergoes a similar reaction. High-field
shifts in the proton spectra of open-chain aminocarbene
complexes when changing the oxidation state of gold from AuIII to
AuI are reported in the literature.[25] This is in accordance with the
observed high-field shift of C1-H, which is also expected due to
the
more
electron-rich
gold
center
in
iodo-
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[(Me2N)2CI][AuI2] 8b, a possible reaction intermediate (see SI).
For the NHC-compound, the simultaneous formation of two
products could be observed, which were identified by single
crystal analyses as mono- and bis-NHC-Au(III) complexes 11-
mono and 11-bis (see Figure 4). The latter is in an equilibrium
with the former, in which the more prevalent 11-bis is formed by
partial reduction of 11-mono under release of elemental iodine
(see SI).
Scheme 7. Possible pathways for the reaction between the iodocarbenium
iodide 3, gold and elemental iodine.
While it is outside the scope of this report to fully elucidate the
mechanism, comparison experiments were performed to illustrate
the role of the individual reaction partners. Firstly, Vilsmeier
reagent 2 does not dissolve gold under identical conditions,
demonstrating the superior reactivity of the iodo iodide. Secondly,
coordination of a LB to the gold surface seems to be crucial for
the oxidation process (as also shown in silico by Repo et al.[7]):
the analogous triflate salt of 3, obtained by anion exchange with
silver triflate, does not undergo any reaction with gold (even after
addition of elemental iodine). This clearly illustrates that
coordination of iodide to the gold (surface) is a prerequisite for the
dissolution process. Thirdly, the organic cation is not decisive for
the oxidation of gold (but of course for the formation of the
carbene complex): replacing it with tetrabutylammonium still lead
to complete dissolution at room temperature in both acetonitrile
(as reported by Nakao)[2–4] and DCM, demonstrating that the
Figure 4. Crystal structures of NHC(-like) carbene complexes: the urea-derived
complexes 10-mono (top, left, dAu-C = 2.041(7) Å) and 10-bis (bottom, left,
dAu-C = 2.085(4) Å) and the corresponding NHC-complexes 11-mono (top, right,
dAu-C = 2.074(15) Å) and 11-bis (bottom, right, dAu-C = 2.021(7) Å). Thermal
ellipsoids at 50% probability.
The crystal structures of the iodocarbenium iodides 8a and 9 and
the mono-carbene complexes 10-mono and 11-mono show
similar features to the ones of 3 and 4, respectively (Figure 1).
The trans arrangement of the two carbene ligands in 10-bis and
11-bis is in agreement with other gold bis-carbene complexes
-
formation of AuI or AuI2 salts could also play a role in the
mechanism. To further investigate this, AuI was treated with 3,
which lead to precipitation of gold (~19%) but also to the formation
of complex 4 (isolated yield 20%). Obviously, though, our initial
experiments had also shown that direct oxidation of Au(0) (or AuI-)
by the iodocarbenium iodide is a viable competing pathway.
Finally, the scope of the method was extended towards less
electrophilic carbene precursors, which are expected to be more
challenging substrates. Indeed, no reaction was observed for the
bis-donor-substituted iodocarbenium iodide 8a and 9 (see Figure
3) at room temperature under our established conditions.
1
found in literature.[28] In H-NMR spectroscopy, only one set of
signals is obtained for the methyl groups in both systems.
The formation of the bis-carbene complexes, which could be the
product of a disproportion reaction of the corresponding Au(II)-
species [AuI(carbene)]I, suggests that the reactions of the bis-
donor substituted salts 8a and 9 with gold do not require any
additive at all. Indeed, the best yields for 10-bis and 11-bis could
be achieved when refluxing the starting materials with gold
powder only. Notably, under these reaction conditions, the
formation of the bis-carbene complexes is preferred over the
mono-carbene complexes (see Scheme 9), and in case of 11-bis,
the yield could be increased significantly to 69%.
Obviously, the conversion of gold towards the carbene complexes
using additional iodine is somewhat disappointing, which is
puzzling since a combination of tetraalkylammonium iodide and
iodine (0.5 eq) is sufficient to completely dissolve the metal. In
contrast to this noncoordinating cation, however, both 8 and 9 are
halogen bond donors (halogen-based Lewis acids), and their
interaction with iodide would weaken the crucial coordination of
iodide to the gold surface. As a counterpoise, we added 0.5
equivalents of tris(dimethylamino)cyclopropenium iodide (TDAI),
a source of “naked” iodide,[29] resulting in a significant acceleration
of the reaction and a complete consumption of the starting
material. In this case 11-mono is not formed and 11-bis can be
isolated in 54% yield (for more details see SI).[30]
Figure
3.
Crystal
structure
iodide
of
8a
urea-derived
(left, dI-I = 3.3897(4) Å,
bis-(N,N’-
dimethylamino)iodomethylene
∡(C-I-I) = 170.231(71) °) and 2-iodo-1,3-dimethylimidazolium iodide 9 (right, dI-
I = 3.3298(4) and 3.3096(4) Å, ∡ (C-I-I) = 178.018(101) and 176.689(109) °).
Thermal ellipsoids at 50% probability.
Therefore, the reaction mixture was stirred in refluxing acetonitrile
for 3-4 days, resulting in incomplete dissolution of the gold powder.
Nevertheless, in case of the urea-derived salt 8a, the expected
Au(III)-complex 10-mono (see Figure 4) could be isolated in 37%
yield.[27] The mother liquor also yielded crystals of
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Keywords: oxidative addition • gold complexes • carbenes •
noble metal • transition metal • halogen bonding
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Acknowledgements
We thank André Hennig and Dr. Jan-Philipp Gliese for preliminary
experimental contributions and Dr. Bert Mallick for measuring the
X-ray structure of salt 3. Jana M. Holthoff is grateful for financial
support by the Studienstiftung des deutschen Volkes.
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can be observed, even though the bis-carbene complex was not found
as a reaction product (see SI). Currently, we cannot explain these
findings.
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[30] A second species in ca. 19% yield was observed via NMR, but its
structure is currently unknown.
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Entry for the Table of Contents
COMMUNICATION
Jana M. Holthoff, Elric Engelage,
Alexander B. Kowsari, Stefan M. Huber,*
Robert Weiss*
Page No. – Page No.
Noble Metal Corrosion: Halogen
Bonded Iodocarbenium Iodides
Dissolve Elemental Gold – A Direct
Access to Gold-Carbene Complexes
Organic iodide salts, providing both oxidative power and coordinating ligand, dissolve
elemental gold under formation of organo-gold complexes. The obtained products
were identified as AuIII complexes, all featuring Au-C bonds, as shown by X-ray single
crystal analyses. This method also provides a direct access to NHC(-type) complexes
and avoids costly organometallic precursors.
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