A hexanuclear gold cluster supported by three-center-two-electron bonds and aurophilic interactions

Article abstract of DOI:10.1002/anie.201303336

A heart of gold: The first hexanuclear gold cluster formed exclusively by gold(I) centers (see picture; Au-yellow, C-black, P-purple) has been shown to be catalytically active for the activation of alkynes under homogeneous conditions. Copyright

Full text of DOI:10.1002/anie.201303336

Angewandte
Chemie
Communications
Gold Clusters
A heart of gold: The first hexanuclear gold
cluster formed exclusively by gold(I)
centers (see picture; Au yellow, C black,
P purple) has been shown to be catalyti-
cally active for the activation of alkynes
under homogeneous conditions.
E. S. Smirnova,
A. M. Echavarren*
&&&& — &&&&
A Hexanuclear Gold Cluster Supported by
Three-Center–Two-Electron Bonds and
Aurophilic Interactions
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
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DOI: 10.1002/anie.201303336
Gold Clusters
A Hexanuclear Gold Cluster Supported by Three-Center–Two-
Electron Bonds and Aurophilic Interactions**
Ekaterina S. Smirnova and Antonio M. Echavarren*
Gold clusters or nanoparticles have been proposed as
catalytically active species in a variety of homogeneous
processes,[1–5] which might proceed by mechanisms that are
very different from those occurring under homogeneous
conditions.[6,7] The preparation of new types of small gold
clusters has attracted recent attention because of their
structural novelty and potential for the discovery of new
reaction types.[8] Herein we report the ready preparation of
a robust hexagold cluster composed exclusively by AuI
centers that is catalytically active in the activation of
alkynes.[9]
4.[12] Indeed, the reaction of [LAuX] with arylboronic acids
leads to aryl gold(I) derivatives [ArAuL] in a general
manner,[13,14] while by using two equivalents of [Ph3PAuNTf2],
diaurated complexes [(m-aryl)(Ph3PAu)2]NTf2 (5) are
obtained (Scheme 1).[15] Very similar gem-diarurated com-
plexes have been obtained by reaction of arylboronic acids
with two equivalents of [{Au(IPr)}2(m-OH)][BF4].[16]
We decided to synthesize new gold(I) complexes of type
1 in which the ortho-boryl group could enhance the electro-
philicity of the metal center in the catalytic activation of
alkynes, allenes, and alkenes. This catalyst design was inspired
both by the high reactivity displayed by cationic catalysts of
type 2 with sterically hindered Buchwald phosphines[10] and
by the stability of gold(I) boratranes, such as 3 developed by
Bourissou, that display significant donor–acceptor Au!B
interactions.[11]
Scheme 1. Formation of diaurated complexes 5.
Complexes 6 and 7a (Scheme 2) were easily prepared in
90–92% yield by reaction of the corresponding o-borylphos-
phines[17] with [Au(tht)Cl] (tht = tetrahydrothiophene). Inter-
estingly, whereas reaction of 7a with AgOTf or AgNTf2 led to
neutral complexes 7b or 7c by chloride ligand exchange, 6
reacted with AgNTf2 to give cationic [AuL2]+Tf2Nꢀ complex
Complexes of type 1 could undergo bimolecular AuI/B
transmetalation to form known dinuclear complexes of type
[*] E. S. Smirnova, Prof. A. M. Echavarren
Institute of Chemical Research of Catalonia (ICIQ)
Av. Paꢀsos Catalans 16, 43007 Tarragona (Spain)
Prof. A. M. Echavarren
Departament de Quꢁmica Analꢁtica i Quꢁmica Orgꢂnica
Universitat Rovira i Virgili
C/Marcel·li Domingo s/n, 43007 Tarragona (Spain)
E-mail: aechavarren@iciq.es
[**] We thank the MICINN (CTQ2010-16088/BQU), the European
Research Council (Advanced Grant No. 321066), the AGAUR (2009
SGR 47), and the ICIQ Foundation for financial support. We also
thank the ICIQ X-Ray diffraction unit for the X-ray structures.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201303336.
Scheme 2. Formation of hexanuclear gold cluster 9.
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8. Very weak Au!B interactions are present in the solid
structure of 7a–c[18] (3.55–3.62 ꢀ), which are in the range
corresponding to the sum of their van der Waals radii
(3.58 ꢀ), whereas in the case of 6 and 8 no interaction was
observed in the solid state (Figure 1).
Figure 2. Structure of dicationic [Au6]2+ cluster 9. ORTEP plot (ellip-
ꢀ
soids set at 50% probability). Hydrogen atoms, SbF6 anions, and
solvate molecules are omitted for clarity.
3.05 ꢀ, with a closest Au···Au interaction of 2.71 ꢀ, which is
the shortest bond distance between gold atoms in structurally
characterized hexanuclear gold clusters. This distance is
within the range observed for homoleptic mesitylgold com-
plex (AuMes)5 (2.69–2.71 ꢀ) with a five-pointed star struc-
ture.[21]
Cluster 9 is unique among hexanuclear gold clusters as it
bears only four phosphines to stabilize six AuI centers and
features four ipso-carbon–digold interactions.[22] In contrast to
the carbon-centered hexagold cluster [CAu6(dppy)6](BF4)2
(dppy = diphenylphosphino-2-pyridine), which is non-emis-
sive in solution,[23] complex 9 shows emission at room
temperature in CH2Cl2 solution with the maximum at circa
460 nm that is due to an intraligand and/or metal-to-ligand
transitions. In the solid state, 9 displays more intense
emission, which is substantially red-shifted with the maximum
at ca. 550 nm.
Although 9 is quite robust and does not react at 238C with
nitriles, isonitriles, or pyridine, it is conceivable that the two
gold atoms Au3 and Au5 bonded to the ortho carbons of the
aryl phosphines by relatively weak three-center–two-electron
bonds could act as electrophilic centers to activate alkynes by
complexes 10 (Scheme 3).
Accordingly, treatment of 1,6-enyne 11 with gold cluster 9
as the catalyst led to dienes 12a,b[24] (Scheme 4). Cluster 9 was
quantitatively recovered from the reaction mixture. Further-
more, no induction period was observed by monitoring the
cycloisomerization of 11a to 12a,b in CD2Cl2 at 0–238C.[25]
Catalyst 7c with a NTf2 ligand is a more reactive catalyst
than 9. The more demanding [4+2] cycloaddition[10a,26] of 1,6-
enyne 13 bearing a disubstituted alkyne with a o-tolyl
substituent (Table 1). For this cycloisomerization, catalyst
Figure 1. Structure of complexes 6 (a), 7a (b), 7b (c), 7c (d), and 8
(e). ORTEP plot (ellipsoids set at 50% probability). Hydrogen atoms,
solvate molecules, and the Tf2Nꢀ anion for 8 are omitted for clarity.
In an attempt at preparing a cationic AuI complex of type
2, neutral complexes 6 and 7a were treated with AgSbF6 at
238C in the presence of acetonitrile, benzonitrile, or 2,4,6-
trimethoxybenzonitrile. Surprisingly, hexanuclear cluster
[AuL4](SbF6)2 9 was obtained instead as a yellow solid. The
best yield (45%) was achieved by treatment of 7a with
AgSbF6 in CH2Cl2 at 238C. An analogous hexanuclear cluster
9’ was obtained using AgBF4.
The X-ray crystal structure of the dicationic hexanuclear
cluster 9 shows a pseudoctahedral geometry with two types of
gold atoms: four AuI centers (Au1, Au2, Au4, and Au6)
bonded to the carbon and phosphorous atoms of the L ligands
and two AuI centers (Au3 and Au5) bonded to carbon atoms
through three-center–two-electron bonds (Figure 2). Owing
to the distortion of planarity formed by the four gold atoms,
the cluster is C2v-symmetric instead of D4h-symmetric. Dicat-
ionic hexanuclear clusters [Au6(PR3)6]2+(Aꢀ)2 have distorted
octahedral[19] or edge-sharing bitetrahedral structures[20] with
ꢀ
average Au Au bond distances of 3.02 ꢀ and 2.76 ꢀ,
ꢀ
respectively. In cluster 9, the average Au Au bond length is
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Keywords: aurophilic interactions · borylphosphanes ·
.
cycloisomerizations · gold catalysis · gold clusters
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t [h]
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1
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7c
2a
12
0.75
1.5
73
71
77
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Received: April 19, 2013
Revised: May 31, 2013
Published online: && &&, &&&&
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