Spontaneous oxidative addition of σ-Si-Si bonds at gold

Article abstract of DOI:10.1002/anie.201103719

Gold can do it! The activation of disilanes at gold was observed experimentally and analyzed theoretically. Upon chelation with two or even only one phosphine donor, the oxidative addition of σ-Si-Si bonds proceeds readily at low temperatures. These results show an unexpected similarity between gold and the other late transition metals towards σ bond activation.

Full text of DOI:10.1002/anie.201103719

Communications
DOI: 10.1002/anie.201103719
Gold Chemistry
À
Spontaneous Oxidative Addition of s-Si Si Bonds at Gold**
Pauline Gualco, Sonia Ladeira, Karinne Miqueu,* Abderrahmane Amgoune,* and
Didier Bourissou*
The past decade has witnessed tremendous development in
homogeneous gold catalysis.[1] Thanks to unique soft and
tunable Lewis acidity, gold(I) and gold(III) complexes have
been shown to efficiently activate most types of p-bonded
substrates. Besides these synthetic achievements, mechanistic
issues have attracted considerable attention, and much
progress has been accomplished recently thanks to the
characterization of several key intermediates.[2]
With the aim of further extending the scope of gold
catalysis, major interest has been devoted over the last few
years to gold-mediated processes involving two-electron
redox cycles. The isolobal analogy between Au(I/III) and
Pd(0/II) complexes is stimulating the search for “gold
versions” of palladium-catalyzed transformations, and in
particular cross-couplings.[3] Accordingly, a variety of intra-
and intermolecular coupling reactions have been shown to be
efficiently catalyzed by gold complexes in the presence of
strong exogeneous oxidants.[4] A few studies have also
suggested the ability of gold complexes to promote cross-
couplings in a similar way to palladium catalysts,[5] but
possible palladium contamination has recently cast doubt on
the involvement of gold complexes in these transformations.[6]
These recent contributions as a whole open a new facet of
gold catalysis, and at the same time, raise fundamental
questions about the properties and behavior of gold com-
plexes. In particular, better knowledge on the basic elemen-
tary steps involved in redox cycles (oxidative addition,
transmetallation, migratory insertion, b elimination, reduc-
tive elimination) is highly desirable to help further develop-
ments into this emerging area of gold catalysis.[7–9] In this
regard, it is striking to note that, despite the stability and
availability of both AuI and AuIII complexes,[10] oxidative
addition reactions at gold remain very elusive. In fact, the
reluctance of gold to undergo oxidative addition is presumed
to be an intrinsic limitation to achieve AuI/AuIII redox cycles
akin to those typically operating with Pd.[4i,11] Rare evidence
for oxidative addition at gold dates back to the pioneering
contribution of Kochi on the reaction of highly polarized sp3
[12]
À
C X bonds with alkyl gold(I) complexes.
Furthermore,
Bachman et al. reported in 2008 the first and to date only
example of activation of an apolar s bond with gold.[13]
Fluorinated disulfides were shown to oxidatively add and
reductively eliminate at gold in a reversible fashion.
Seeking to gain more insight into s-bond activation and
oxidative addition at gold, we became interested in extrap-
olating the strategy we have been developing for some years
to study unusual metal–ligand interactions. Our approach
consists in the use of anchoring phosphine sites, and this has
been applied to support the coordination of Lewis acids[14]
[15]
À
and most recently s-Si Si bonds. Accordingly, the coordi-
[*] P. Gualco, Dr. A. Amgoune, Dr. D. Bourissou
Laboratoire Hꢀtꢀrochimie Fondamentale et Appliquꢀe
du Universitꢀ Paul Sabatier, UPS, LHFA
118 Route de Narbonne, 31062 Toulouse Cedex 09 (France)
and
nation of the diphosphine–disilane ligand 1 to copper and
silver has been explored (Scheme 1), leading to the first
structural characterization of a s complex with a coinage
metal. As a further extension of this work, we were intrigued
by the coordination of 1 to gold, considering three possible
scenarios: 1) Formation of a s-complex upon side-on coordi-
nation of the disilane moiety, as observed with copper;[16]
CNRS, LHFA UMR 5069
31062 Toulouse (France)
E-mail: amgoune@chimie.ups-tlse.fr
dbouriss@chimie.ups-tlse.fr
À
2) no participation of the s-Si Si bond to the coordination, as
Dr. S. Ladeira
À
observed with silver; or 3) oxidative addition of the s-Si Si
Institut de Chimie de Toulouse, Universitꢀ Paul Sabatier
FR2599 Toulouse (France)
bond to give a bis(silyl)gold(III) complex. Oxidative addition
Dr. K. Miqueu
Institut Pluridisciplinaire de Recherche sur l’Environnement et
les Matꢀriaux (UMR 5254), Equipe Chimie Physique
Universitꢀ de Pau et des Pays de l’Adour Hꢀlioparc
2 Avenue du Prꢀsident Angot, 64053 Pau cedex 09 (France)
E-mail: karinne.miqueu@univ-pau.fr
[**] Financial support from the Centre National de la Recherche
Scientifique, the Pꢁle de Recherche et d’Enseignement Supꢀrieur de
l’Universitꢀ de Toulouse (fellowship to P.G.), and the Agence
Nationale de la Recherche (ANR-10-BLAN-070901) is gratefully
acknowledged. The theoretical work was granted access to HPC
resources of Idris under allocation 2011 (i2011080045) made by
GENCI (Grand Equipement National de Calcul Intensif).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201103719.
Scheme 1. Coordination of the diphosphine–disilane 1 to copper and
silver.
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Angew. Chem. Int. Ed. 2011, 50, 8320 –8324

of disilanes at transition metals, especially palladium and
platinum, is a well-known process of great synthetic value.[17]
Comparatively, such an elementary step is unprecedented at
gold, although it has been suggested recently by Klinkham-
mer et al. to account for an unusual s bond metathesis process
observed within hypersilyl gold clusters.[18] Upon coordina-
tion of phosphine–disilanes to gold, we gained evidence for
À
the spontaneous oxidative addition of s-Si Si bonds at gold to
form hitherto unknown bis(silyl) gold(III) complexes.[19,20]
These experimental results are reported herein, along with
some theoretical insights.
The reaction of the diphosphine–disilane 1 with [AuCl-
(SMe2)] in dichloromethane at À788C resulted in the
formation of the bis(silyl) gold(III) complex 2 (Scheme 2).
In marked contrast with that observed with copper and silver,
Figure 1. View of complex 3 in the solid state (ellipsoids set at 50%
probability; hydrogen atoms, counteranion, and solvate molecules
omitted for clarity). Selected bond lengths [ꢂ] and angles [8]: Au–P1
2.4135(6), Au–P2 2.4176(6), Au–Si1 2.4128(6), Au–Si2 2.4213(6); P1-
Au-P2 103.63(2), Si1-Au-Si2 89.95(2), P1-Au-Si1 84.67(2), P2-Au-Si2
82.16(2).
À
silyl skeletons are positioned tail-to-tail. The Au
Si bonds (2.4128(6) and 2.4213(6) ꢀ) are slightly
longer than those observed in the few structurally
authenticated gold(I) silyl complexes (2.365 ꢀ),[24]
but still shorter than the sum of the covalent radii
(2.47 ꢀ).[25] Furthermore, the absence of residual
contact between the two silicon atoms is shown by
À
the long Si Si distance (3.4167(9) ꢀ).
À
Scheme 2. Synthesis of complexes 2 and 3 by oxidative addition of the s-Si Si bond
of the diphosphine–disilane 1 at gold.
The coordination of the diphosphine–disilane
1 to gold thus proceeds with oxidative addition of
À
the s-Si Si bond. Such a behavior has been well-
À
the s-Si Si bond is not retained but oxidatively adds to gold.
Complex 2 is not stable above À608C, but its structure was
unambiguously ascertained by multinuclear NMR spectros-
copy at low temperature. The 29Si NMR resonance signal is
shifted downfield (33.7 ppm for 2 versus À21.2 ppm for 1),
documented with palladium and platinum,[17] but is unprece-
dented with gold.[26] The ensuing bis(silyl)gold(III) complexes
2 and 3 expand the variety of silyl complexes of gold, that, to
the best of our knowledge, was limited to date to mono-
(silyl)gold(I) species.[24]
À
indicating the cleavage of the s-Si Si bond. The associated
coupling pattern (two Si P coupling constants of 121.1 and
At this stage, the paucity of such oxidative additions at
gold prompted us to assess computationally the activation of
the s-Si Si bond by exploring the potential energy surface
(PES) of the model complex 3* featuring methyl instead of
phenyl substituents at the phosphorus atoms (Figure 2).[27]
The optimized geometry for the bis(silyl)gold(III) com-
plex 3*(III) nicely reproduces that determined crystallo-
À
À
9.4 Hz) is very similar to that typically encountered in
diphosphine bis(silyl)palladium and platinum complexes,[21]
suggesting a cis arrangement of the bis(silyl) complex 2. It is
worth noting that NMR monitoring indicated the sponta-
neous formation of the bis(silyl)gold(III) complex 2 at
À808C, with no intermediate being detected along the
oxidative addition process. With the aim of improving thermal
stability and obtaining crystals suitable for X-ray diffraction
analysis, complex 2 was reacted with one equivalent of GaCl3
as a chloride abstractor. Gratifyingly, the ensuing cationic
bis(silyl)gold(III) complex 3 was isolated in 81% yield as a
stable white powder (no sign of decomposition within days at
room temperature). The 31P and 29Si NMR data for complex 3
(31P: d = 62.9 ppm and 29Si: d = 34.4 ppm, dd, JSiÀP = 121.0 and
9.2 Hz) are very close to those of 2, suggesting in both cases
strong ionic character and only a weak interaction, if any,
between the gold center and the counteranion.[22]
À
graphically (with deviations of less than 0.05 ꢀ in the key P
À
À
Au, Au Si, and Si Si distances). Another minimum was
located on the PES for the corresponding AuI isomer 3*(I). It
is less stable in energy by 11.3 kcalmolÀ1 and has a two-
coordinate linear structure without coordination of the
À
À
disilane moiety (Si Si and Au Si distances of 2.404 and
3.360 ꢀ, respectively). A transition state 3*-TS associated
À
with oxidative addition of the s-Si Si bond of 3*(I) leading to
3*(III) was also located. The geometric features indicate a
rather late transition state with advanced cleavage of the s-
Si Si bond (2.855 ꢀ), well-developed Au Si bonds (2.554 ꢀ),
and pronounced bending of the P-Au-P skeleton (1448). The
associated activation barrier is remarkably low (10.4 kcal
molÀ1) and accounts well for the spontaneous oxidative
À
À
The molecular structure of 3 was analyzed by X-ray
diffraction (Figure 1). The ionic nature of complex 3 is
apparent from the absence of significant interaction between
À
addition of the s-Si Si bond at low temperature, as observed
À
the metal center and the GaCl4 counteranion (shortest
experimentally.
Cl···Au distance 3.9759(8) ꢀ). The gold center is surrounded
by the two phosphorus and the two silicon atoms, and adopts
square-planar geometry, as expected for a gold(III) center.[23]
Consistent with the spectroscopic data, the two phosphine–
We then sought to determine how general the unusual s-
Si Si bond activation might be that is observed upon
coordination of 1 to gold and to which degree chelating
assistance facilitates the oxidative addition process. On one
À
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Communications
bound to gold in trans position to
the phosphorus atom (JSiÀP from
4.2 Hz in 4 to 149.2 Hz in 5).
DFT calculations were also per-
formed on the model complex 5* to
gain more insight into the mecha-
À
nism of s-Si Si bond activation.
Overall, the reaction path for the
oxidative addition at gold (Figure 3)
is very similar to that computed for
3*. The square-planar bis(silyl)gold-
(III) complex 5*(III) is more stable
than the linear gold(I) isomer 5*(I)
(in which the disilane moiety
remains pendant) by 13.1 kcalmolÀ1.
The oxidative addition proceeds via
a rather late transition state 5*-TS
À
with advanced cleavage of the s-Si
Si bond (2.666 ꢀ), well-developed
AuSi bonds (2.529/2.636 ꢀ), and
pronounced bending of the PAuCl
Figure 2. Energy profile computed at the B3PW91/SDD+f(Au),6-
À
31G**(Si,P,C,H) level for the oxidative addition of the s-Si Si bond in
complex 3*. Free energy DG at 258C including ZPE correction [kcal
molÀ1], interatomic distances [ꢂ], angles [8].
hand, it is clear that the coordination of the two phosphorus
buttresses ideally presents the disilane moiety to the metal
center, but on the other, the structures of the related copper
and silver complexes[15] show that oxidative addition is not
imposed geometrically and strongly
À
Scheme 3. Synthesis of complex 5 by oxidative addition of the s-Si Si
bond of the monophosphine–disilane 4 at gold.
depends on the intrinsic properties of
the metal center. To impart higher
flexibility to the disilane moiety, the
monophosphine–disilane 4 was pre-
pared[27] and its coordination to gold
was investigated. Upon reaction with
[AuCl(SMe2)], the neutral bis-
(silyl)gold(III) complex
5
was
formed quantitatively (Scheme 3),
substantiating that oxidative addi-
À
tion of the s-Si Si bond occurs read-
ily even when supported by a single
phosphine buttress. Again, NMR
spectroscopic monitoring at low tem-
perature revealed that the reaction
proceeds instantaneously at À808C
with no detectable intermediate. The
resulting complex is not stable above
À308C, but its structure was unam-
biguously authenticated by NMR
spectroscopy. The two 29Si NMR res-
onance signals are diagnostically
shifted downfield upon oxidative
addition (d from À22.2 and
À20.1 ppm for the free ligand 4 to
15.0 and 23.8 ppm for complex 5).
Another characteristic feature is the
Figure 3. Energy profile computed at the B3PW91/SDD+f(Au),6-31G**(Si,P,C,H) level for the
À
large Si P coupling constant
À
oxidative addition of the s-Si Si bond in complex 5*. Free energy DG at 258C including ZPE
observed for the silicon atom Sib correction [kcalmolÀ1], interatomic distances [ꢂ], angles [8].
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Angew. Chem. Int. Ed. 2011, 50, 8320 –8324

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skeleton (1368). Most remarkably, the associated activation
barrier is only slightly higher for the monophosphine–disilane
(13.8 kcalmolÀ1) than with the diphosphine–disilane, indicat-
À
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À
addition of s-Si Si bonds at gold. Such a behavior markedly
contrasts with that observed with copper and silver, and
remarkably, the activation of the disilane at gold proceeds
readily at low temperature even when supported by a single
phosphorus buttress. These results indicate that gold is not as
reluctant as assumed to undergo oxidative addition and shows
some unexpected similarity with the other late transition
metals, typically palladium and platinum, towards s bond
activation. The straightforward formation of bis(silyl)gold-
(III) complexes lends credence to the redox pathway invoked
[7] In the gold(I)-catalyzed annulations of salicyladehydes and
À
alkynes, Li proposed oxidative addition of the aldehyde C H
bond assisted by the coordination of the adjacent phenol moiety:
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For example, is catalytic bis(silylation) achievable with gold?
Future work will seek to investigate the reactivity of the new
bis(silyl)gold(III) complexes and to compare further the
behavior of gold (and other coinage-metal) complexes with
those of palladium and platinum in the key elementary steps
typically involved in redox cycles.
Chem. Int. Ed. 2007, 46, 1117 – 1119.
2
À
[8] Oxidative addition of sp C H bonds at gold have been
considered as an alternative pathway to concerted metalation
À
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of arenes: P. Lu, T. C. Boorman, A. M. Z. Slawin, I. Larrosa, J.
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[9] Using triazamacrocyclic ligands, direct observation of oxidative
addition of aryl halides at copper has been recently achieved: A.
Casitas, A. E. King, T. Parella, M. Costas, S. S. Stahl, X. Ribas,
Chem. Sci. 2010, 1, 326 – 330.
Received: May 31, 2011
Published online: July 19, 2011
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Keywords: density functional calculations · gold ·
oxidative addition · silicon · X-ray diffraction
.
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À
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À
[20] Following the same strategy, we recently observed an original C
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