Trinuclear gold clusters supported by cyclic (alkyl)(amino)carbene ligands: Mimics for gold heterogeneous catalysts

Article abstract of DOI:10.1002/anie.201404665

The synthesis of air- and moisture-stable trinuclear mixed-valence gold(I)/gold(0) clusters is described. They promote the catalytic carbonylation of amines under relatively mild conditions. The synthetic route leading to the trinuclear clusters involves a simple ligand exchange from the readily available μ³-oxo-[(Ph₃PAu)₃O]⁺ complex. This synthetic method paves the way for the preparation of a variety of mixed-valence gold(I)/gold(0) polynuclear clusters. Moreover, the well-defined nature of the complexes demonstrates that the catalytic process involves a rare example of a definite change of oxidation state of gold from Au⁰ ₂Au^I to Au^I₃.

Full text of DOI:10.1002/anie.201404665

Angewandte
Chemie
DOI: 10.1002/anie.201404665
Heterogeneous Catalysis
Hot Paper
Trinuclear Gold Clusters Supported by Cyclic (alkyl)(amino)carbene
Ligands: Mimics for Gold Heterogeneous Catalysts**
Liqun Jin, David S. Weinberger, Mohand Melaimi, Curtis E. Moore, Arnold L. Rheingold, and
Guy Bertrand*
Abstract: The synthesis of air- and moisture-stable trinuclear
mixed-valence gold(I)/gold(0) clusters is described. They
promote the catalytic carbonylation of amines under relatively
mild conditions. The synthetic route leading to the trinuclear
clusters involves a simple ligand exchange from the readily
available m3-oxo-[(Ph3PAu)3O]+ complex. This synthetic
method paves the way for the preparation of a variety of
mixed-valence gold(I)/gold(0) polynuclear clusters. Moreover,
the well-defined nature of the complexes demonstrates that the
catalytic process involves a rare example of a definite change of
Scheme 1. Synthesis of the only known trinuclear gold complex A.[7]
a carbonylation reaction, which had previously been studied
using supported metallic gold, and gold nanoparticles.[10]
One of the most classical routes for the preparation of
small phosphine-supported gold clusters, such as
oxidation state of gold from Au0 AuI to AuI3.
2
G
old nanoparticles have attracted a great deal of interest
[(R3PAu)6]2+ [11] [(R3P)6Au8]2+ [12] and [(R3PAu)4]2+ [13]
, , , is the
owing to their potential applications, not only for nano-
electronic devices[1] and medical science,[2] but also for
catalysis.[3] Interestingly, Corma et al.[4] have recently shown
that very small metallic gold clusters (3 to 10 atoms) could
display catalytic activities that are much higher than those
obtained with gold nanoparticles and mononuclear homoge-
neous gold catalysts.[5] This striking discovery led to the
question of whether very small, well-defined clusters, capped
with ligands, could mimic the behavior of heterogeneous
catalysts. One benefit, among other obvious advantages,
would be the possibility of characterizing intermediates in
catalytic reactions.
reduction of the corresponding m3-oxo complexes
[(R3PAu)3O]+XÀ.[14] Although it has been reported that the
analogous NHC-supported m3-oxo species cannot be pre-
pared,[7] we found that treatment of the (CAACc-Hex)AuCl
complex 1[15] with Ag2O, in presence of NaBF4, afforded the
desired cluster 2 as an off-white powder in 62% yield
(Scheme 2). Subsequent reduction with carbon monoxide[16]
Examination of the literature shows a large number of
mixed-valence gold(I)/gold(0) complexes with the formula
(LnAum)x+.[6] However, there is a single example of the
smallest member of the family, namely the tris(gold) cluster
A, which was recently isolated by Sadighi et al.[7] (Scheme 1).
Herein we report two straightforward syntheses of triangular
gold complexes ligated by three cyclic (alkyl)-
(amino)carbenes (CAAC)[8,9] or two CAACs and a phosphine.
To demonstrate the possibility of using small well-defined
clusters for studying catalytic mechanisms, we investigated
Scheme 2. Synthesis of trinuclear gold clusters 2 and 3.
produced the trinuclear complex 3 as a pale yellow powder
(75% yield). Compound 3 was characterized by 1H and
13C NMR spectroscopy as well as by high-resolution mass
spectrometry. The 13C NMR signal corresponding to the
carbene carbon appears at 262.5 ppm, which is half-way
between those observed for gold(I) complexes 1 and 2 (237.4
and 230.4 ppm, respectively) and the recently reported
À
[*] Dr. L. Jin, D. S. Weinberger, Dr. M. Melaimi, Prof. G. Bertrand
UCSD-CNRS Joint Research Laboratory (UMI 3555)
Department of Chemistry and Biochemistry
University of California, San Diego
neutral
(CAACmenthyl)Au Au(CAACmenthyl
)
complex[17]
La Jolla, CA 92093-0343 (USA)
E-mail: guybertrand@ucsd.edu
Homepage: http://bertrandgroup.ucsd.edu
(287.2 ppm), as is expected for a mixed-valence gold(I)/
gold(0) compound. Cluster 3 was found to be indefinitely
stable in air and moisture both in the solid state and in
solution; its melting point (2408C (dec.)) is another indication
of its high stability.
Dr. C. E. Moore, Prof. A. L. Rheingold
UCSD Crystallography Facility
University of California, San Diego (USA)
Even though the reaction sequence from 1 to 3 proved to
be efficient, the starting material 1 can only be made in 41%
yield.[15] Therefore, we decided to investigate the possibility of
ligand exchange, using the readily available phosphine-
[**] The authors gratefully acknowledge financial support from the DOE
(DE-FG02-13ER16370).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201404665.
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9059

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We were at first very disappointed to observe
that with 2.5 mol% of 7, cyclohexylamine did not
react with carbon monoxide (30 psi) even after four
days at 708C under strict anaerobic conditions
(Table 1, entry 1). However, we quickly found that
under the same experimental conditions, but in air,
N,N’-dicyclohexylurea was formed in 30% yield
after two days (entry 2). Increasing the temperature
to 908C appeared to be beneficial for the conver-
sion (entry 3), but further increase above 908C did
not improve the yields, certainly due to the
degradation of the catalyst. Interestingly, under
10 psi of oxygen, the yield improved drastically
(entry 4). Noteworthy, under the same conditions,
the more bulky cluster 3, and the m3-oxo complex 6
(entry 5 and 6) proved to be less efficient than 7,
Scheme 3. Synthesis of trinuclear complexes 6–9 from the phosphine-supported m3-
oxo complex 4.
supported m3-oxo complex 4[14] as a precursor (Scheme 3).
Addition of 3.5 equivalents of CAACdiEt 5[18] to 4 proceeded
smoothly, and the m3-oxo complex 6 was isolated in 86% yield
as an off-white solid. Subsequent treatment of 6 with carbon
monoxide yielded the desired trinuclear gold complex 7 as
a pale yellow solid in almost quantitative yield. Very
interestingly, this reaction sequence is very flexible. Indeed,
the addition of two equivalents of CAACdiEt 5 to 4 allows,
after reduction with CO, the preparation of the mixed
(CAAC)(phosphine)-supported complex 9.
Single-crystal X-ray diffraction studies of 6, 7, and 9 were
carried out (Figure 1).[19] Similar to the analogous cluster
bearing P(o-Tol)3 ligands,[20] 6 has a pseudo-tetrahedral
geometry, and can be viewed as three [CAAC-Au]+ units,
with strong aurophilic interactions,[21] bound to a central O2À.
and the CAACmenthyl–AuCl complex[22a] failed to produce any
N,N’-dicyclohexylurea.
Table 1: Catalytic carbonylation of cyclohexylamine.
Entry
Catalyst
T [8C]
O2
Yield [%][a]
1
2
3
4
5
6
7 (2.5%)
7 (2.5%)
7 (2.5%)
7 (2.5%)
3 (2.5%)
6 (2.5%)
70
70
90
90
90
90
none
air
air
10 psi
10 psi
10 psi
0
30
51
69
29
34
[a] Yields determined by GC, using biphenyl as internal standard.
À
In 7, the Au Au distances (2.6324(8)–2.6706(8) ꢀ) are much
shorter than in 6 (3.1669(3)–3.3149(3) ꢀ), which is indicative
of true covalent bonds; for comparison, a gold–gold distance
Using the optimized conditions, we performed prelimi-
nary experiments to study the scope of this catalytic process.
As shown in Table 2, benzyl amine and 2-phenylethylamine
could be converted into the corresponding urea in good yields
of 74% and 72%, respectively (entries 1 and 3). No decom-
position of catalyst could be observed during the reaction with
benzylamine as the substrate, which encouraged us to
decrease the catalyst loading to 0.5 mol%; the urea was
obtained with 52% yield after extending the reaction time to
four days (entry 2). In the case of more sterically hindered
amines, the conversion was significantly decreased (entry 4–
6), while no reaction was observed with the less nucleophilic
aniline (entry 7).
À
of 2.552 ꢀ was observed for the neutral (CAAC)Au Au-
(CAAC) complex.[17] In 9, the distance between the two gold
atoms bearing CAACs (2.6604(4) ꢀ) is slightly longer than
À
the other Au Au bonds (2.6191(4) and 2.6219(4) ꢀ), arguing
for the stronger s-donor properties of CAACs, compared to
triphenyl phosphine.[9]
Owing to our ongoing interest in the gold-catalyzed
functionalization of amines,[22] we chose to investigate the
behavior of cluster 7 for the carbonylation of this class of
compounds. Such a reaction is the simplest and most environ-
mentally friendly route for the preparation of urea deriva-
tives.[23,24]
Figure 1. Solid-state structures of 6, 7, 9, 10a, and 11b. BF4À anions, solvent molecules, and hydrogen atoms, except on the amine nitrogen of
10a and 11b, are omitted for clarity. Isopropyl substituents on the aryl groups were limited to the connecting carbon atoms.
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Table 2: Catalytic carbonylation of amines.
mechanism remains unclear. During the second step (10 and
11 to 7), two gold centers are reduced, and the carbon
monoxide is oxidized by two electrons.
Gold catalysts are usually viewed as soft carbophilic Lewis
acids,[25] and only a few recent reports postulate redox
processes.[26,27] For example, Toste et al.[28] found a catalytic
Entry
Substrate
7 (mol%)
t [h]
Yield [%][a]
I
II
À
process involving two Au complexes giving a dinuclear Au
AuII intermediate. The carbonylation of amines reported
herein is a very rare example of a gold catalyzed process
involving a definite change of oxidation state of the metal
1
2
3
4
5
6
7
benzylamine
benzylamine
2.5
0.5
2.5
2.5
2.5
2.5
2.5
45
96
45
45
72
45
45
74
52
72
40
55
10
0
2-phenylethylamine
a-methylbenzylamine
a-methylbenzylamine
2-adamantylamine
aniline
0
I
from Au2 AuI to Au3 .
Along with the catalytic interest of this work, the synthesis
of trinuclear complexes by ligand exchange paves the way for
the preparation of a variety of carbene-supported multi-
nuclear gold clusters, using known well-defined mixed-
valence phosphine–gold clusters.
[a] Yields determined by NMR with triphenylmethane as the internal
standard.
Examination of the literature shows that several other
transition-metal catalysts are efficient for this transformation, Experimental Section
Synthesis of m3-oxo complex 6: A THF solution (30 mL) of CAACdiEt
but generally at higher CO pressures and temperatures.[10,23,24]
More importantly, the well-defined nature of 7 allowed for an
investigation of the intermediates involved in the catalytic
process. We first found that 7 does not react with carbon
monoxide (30 psi) at 908C. Similarly, in the absence of air, no
reaction occurs with cyclohexylamine at 908C. In contrast, in
aerobic condition, or under 10 psi of oxygen, 7 reacts with
cyclohexylamine, leading after two days at 708C to the
dinuclear complex 10a, which was isolated in 63% yield
(Scheme 4). Interestingly, under the same experimental con-
ditions, but using ammonia, a mixture of the dinuclear and
trinuclear complexes 10b and 11b was obtained. The
structures of 10a/b and 11b were ascertained by X-ray
(1.1 g, 3.5 mmol)Àwas added slowly to a THF solution (90 mL) of
(PPh3Au)3O+BF4 (1.5 g, 1.0 mmol). After 20 min, volatiles were
removed under vacuum and the residue was washed with diethyl
ether (3 ꢁ 10 mL). After drying, complex 6 was obtained as an off-
white solid (1.42 g, 86% yield, m.p. 2208C (dec.)). 1H NMR (CDCl3,
500 MHz): d = 7.36 (t, J = 7.7 Hz, 3H), 7.18 (d, J = 7.7 Hz, 6H), 2.67
(sept., CHCH3, J = 7.0 Hz, 6H), 2.00 (s, 6H), 1.76–1.63 (m, 6H), 1.61–
1.49 (m, 6H), 1.29–1.23 (m, 36H), 1.23–1.17 (m, 18H), 0.96 ppm (t,
J = 7.0 Hz, 18H); 13C{1H} NMR (CDCl3, 125 MHz): d = 231.0 (Ccarb),
145.1 (Cortho), 134.1 (Cipso), 129.7 (Cpara), 124.7 (Cmeta), 80.6 (Cquat), 62.8
(Cquat), 40.3 (CH2), 32.1 (CH2), 29.3, 29.2, 27.7, 22.6, 9.71 ppm; HRMS
(ESI-TOFMS): m/z calculated for C66H105Au3N3O+ 1546.7254, found
1546.7226. Single crystals of 6 were obtained by vapor diffusion of
diethyl ether into a saturated THF solution.
À
Synthesis of trinuclear cluster 7: A THF solution (15 mL) of 6
(1.4 g, 0.87 mmol) was stirred under 30 psi of CO for 12 h. The solvent
was removed under vacuum and 7 was obtained as a yellow solid
diffraction (Figure 1). In all of these complexes, the Au Au
bond distances (3.16 to 3.31 ꢀ) are typical for gold(I)–gold(I)
aurophilic interactions,[21] and are similar to those observed in
the m3-oxo complex 6.
1
(1.28 g, 91% yield, m.p. 2328C (dec.)). H NMR (CDCl3, 500 MHz):
d = 7.37 (t, J = 7.8 Hz, 3H), 7.18 (d, J = 7.8 Hz, 6H), 2.68 (sept.,
CHCH3, J = 6.6 Hz, 6H), 1.90 (s, 6H), 1.64–1.53 (m, 6H), 1.45–1.35
(m, 6H), 1.28–1.19 (m, 36H), 1.09 (d, J = 6.5 Hz, 18H), 0.89 ppm (t,
J = 7.3 Hz, 18H); 13C{1H} NMR (CDCl3, 125 MHz) = 264.8 (Ccarb),
145.0 (Cortho), 134.0 (Cipso), 129.5 (Cpara), 124.5 (Cmeta), 81.7 (Cquat), 63.6
(Cquat), 41.9 (CH2), 31.5 (CH2), 29.2, 29.1, 28.0, 22.4, 9.80 ppm; HRMS
+
(ESI-TOFMS): m/z calculated for C66H105Au3N3 1530.7305, found
1530.7312. Single crystals of 7 were obtained by vapor diffusion of
diethyl ether into a saturated THF solution.
Received: April 24, 2014
Revised: May 15, 2014
Published online: July 1, 2014
Scheme 4. Addition of amines to trinuclear cluster 7 to afford dinu-
clear and trinuclear complexes 10a,b and 11b, and subsequent
reaction with CO to return 7, along with isocyanates.
Keywords: amine carbonylation · carbenes · gold clusters ·
.
heterogeneous catalysis
The next question was whether CO was able to displace
the nitrogen fragment of complexes 10 and 11. Indeed, when
10a, 10b, and 11b were treated with carbon monoxide (30 psi)
at 708C, the trinuclear cluster 7 was recovered. In the case of
10a, we were also able to observe the formation of the
cyclohexyl isocyanate, along with traces of urea.
During the first step of the catalytic process (7 to 10 and
11), two gold centers are oxidized. We hypothesize that
oxygen is necessary to scavenge the hydrogen atoms, but the
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