Gold-mediated insertion of oxygen into silicon-silicon bond: An original Au(I)/Au(III) redox sequence

Article abstract of DOI:10.1021/om3006905

The diphosphine-disilane iPr₂(o-C₆H ₄)SiMe₂SiMe₂(o-C₆H ₄)PiPr₂ reacts with AuCl(SMe₂) via coordination of the two phosphines and oxidative addition of the σ-Si-Si bond. The ensuing bis(silyl) gold(III) complex has been unequivocally authenticated by NMR spectroscopy at -60 °C. Upon heating, it evolves cleanly to give a disiloxane gold(I) complex that has been fully characterized, including by X-ray diffraction analysis. Oxidation of the disilane proceeds via an original Au(I)/Au(III) redox sequence. According to ¹⁸O labeling experiments, both water and dioxygen are competent oxygen sources. Oxidative addition of the σ-Si-Si bond to form a bis(silyl) gold(III) complex seems to be a prerequisite for the disilane → disiloxane conversion to occur.

Full text of DOI:10.1021/om3006905

Communication
pubs.acs.org/Organometallics
Gold-Mediated Insertion of Oxygen into Silicon−Silicon Bond: An
Original Au(I)/Au(III) Redox Sequence
Pauline Gualco,† Sonia Ladeira,‡ Karinne Miqueu,§ Abderrahmane Amgoune,*,† and Didier Bourissou*,†
†Universite
́
de Toulouse, UPS, LHFA, 118 route de Narbonne, 31062 Toulouse (France) and CNRS, LHFA, UMR 5069, 31062
Toulouse, France
‡Institut de Chimie de Toulouse (FR 2599), 118 route de Narbonne, 31062 Toulouse cedex 9, France
§Institut des Sciences Analytiques et de Physico-Chimie pour l’Environnement et les Mater
́
iaux UMR-CNRS 5254, Universite
́
ident Angot, 64053 Pau Cedex 09, France
́
de Pau
et des Pays de l’Adour, Hel
́
ioparc, 2 avenue du Pres
S
* Supporting Information
ABSTRACT: The diphosphine−disilane iPr2(o-C6H4)-
SiMe2SiMe2(o-C6H4)PiPr2 reacts with AuCl(SMe2) via coor-
dination of the two phosphines and oxidative addition of the σ-
Si−Si bond. The ensuing bis(silyl) gold(III) complex has been
unequivocally authenticated by NMR spectroscopy at −60 °C.
Upon heating, it evolves cleanly to give a disiloxane gold(I) complex that has been fully characterized, including by X-ray
diffraction analysis. Oxidation of the disilane proceeds via an original Au(I)/Au(III) redox sequence. According to 18O labeling
experiments, both water and dioxygen are competent oxygen sources. Oxidative addition of the σ-Si−Si bond to form a bis(silyl)
gold(III) complex seems to be a prerequisite for the disilane → disiloxane conversion to occur.
ver the past decade, homogeneous catalysis with gold
ments have been performed to identify the possible source of
Ocomplexes has attracted considerable interest and it has the oxygen atom, and the role of gold in the disilane →
disiloxane conversion has been probed.
been applied to a wide range of organic transformations with
very high, often unprecedented efficiency and selectivity.1 The
vast majority of these reactions are based on the unique
carbophilic Lewis acidity of gold complexes that is typically
used to activate alkynes, alkenes, allenes, etc. toward
nucleophilic attack. Recently, the scope and versatility of
homogeneous gold catalysis has been further extended, in
particular toward oxidative coupling and addition reactions (in
the presence of external oxidants)2 as well as C−C bond
forming reactions using dual Au−Pd catalysis.3
In parallel with these very rapid and important synthetic
achievements, increasing efforts have been made over the past
few years to shed light into the underlying mechanistic issues.
The recent characterization of some putative intermediates
involved in gold activation of π systems4 (side-on complexes,5
vinyl gold complexes,6 geminal digold complexes7) has
provided valuable information and contributed to refine
mechanistic proposals. Comparatively, the precise factors
involved in controlling two-electron redox and dual catalysis
with gold complexes remain less understood, but some insight
has started to be gained into key transformations (oxidative
addition/reductive elimination, transmetalation, etc.)8,9
As previously reported,10 the diphosphine−disilane ligand 1a
readily reacts with [AuCl(SMe2)] in dichloromethane at −78
°C via coordination of the two phosphines and oxidative
addition of the σ-Si−Si bond at gold (Scheme 1). The ensuing
bis(silyl) gold(III) complex 2a can be detected and
characterized by NMR at low temperature, but it rapidly
decomposes above −60 °C, giving a mixture of unidentified
products. Addition of GaCl3 to complex 2a significantly
improves the stability and allows for the structural character-
−
ization of the cationic bis(silyl) gold(III) complex with GaCl4
as the counteranion.10 At this point, we were intrigued about
the role of the phosphorus substituents in the oxidative
addition process and the stability of the resulting complex. We
thus replaced the Ph groups by more electron donating iPr
substituents. The corresponding diphosphine−disilane ligand
1b12 was reacted with [AuCl(SMe2)] at low temperature in an
NMR tube. Similarly to 1a, spontaneous formation of the
bis(silyl) gold(III) complex 2b was unequivocally established
by 31P and 29Si NMR spectroscopy (δ(31P) s, 86.7 ppm; δ(29Si)
dd, 36.9 ppm, JSiP = 112.8 and 8.4 Hz). Disappointingly,
complex 2b is stable only below −60 °C, but in marked
contrast with 2a, it evolves cleanly upon warming to give the
new complex 3b. A single 31P NMR resonance signal is
observed at 60 ppm for 3b, indicating that the two phosphorus
atoms remain symmetrically coordinated to gold. Upon
transformation of 2b into 3b, the 29Si NMR signal shifts to
In that general context, we have recently shown, thanks to
phosphine chelating assistance, that gold can undergo
unexpectedly facile oxidative addition of σ-Si−Si bonds,
affording original bis(silyl) gold(III) complexes.10,11 Upon
varying the substituents at the phosphorus anchors, we have
incidentally found that the disilane moiety can be converted
into a disiloxane moiety (oxygen insertion) via a Au(I)/Au(III)
sequence. These results are reported herein. Labeling experi-
Received: July 23, 2012
Published: August 14, 2012
© 2012 American Chemical Society
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Organometallics
Communication
Scheme 1. Coordination of the Diphosphine−Disilane Ligands 1 to Gold: Formation and Evolution of the Bis(silyl) Gold(III)
Complexes 2
high field (Δδ = 34.2 ppm) to appear in the typical range of
organic siloxane derivatives (δ 2.7 ppm for 3b, compared with δ
−2.2 ppm for PhMe2SiOSiPhMe2).13 The coupling pattern of
the 29Si NMR signal also changes drastically. The large JSiP
coupling constant characteristic of bis(silyl) gold(III) com-
plexes 2 (112.8 Hz for 2b) is no longer present for 3b. Instead,
we observe a second-order AA′XX′ system (A, A′ = 29Si, X, X′
(2.854(2) Å) exceeding the sum of covalent radii (2.02 Å)15 by
more than 41%.
Thus, the bis(silyl) gold(III) complex 2b spontaneously
evolves into the diphosphine gold(I) complex 3b with insertion
of an oxygen atom between the two silicon atoms. The overall
reaction scheme involves oxidation of the disilane fragment of
1b and proceeds via a Au(I)/Au(III) redox sequence.
=
31P) with a small value of N = 1/2(JAX + JA′X) = 3.6 Hz. High-
We tried to gain some insight into the source of oxygen atom
and the role of gold in the formation of 3b. Apparently, the
reaction conditions strongly influence the transformation of 2b
into 3b, and if extreme precautions are taken to exclude any
traces of air and moisture, the process can be significantly
slowed down. In contrast, the addition of small amounts of
water to a dichloromethane solution of complex 2b
considerably increases the rate of formation of 3b. Typically,
while only 20% of the gold(III) complex 2b is converted into
gold(I) complex 3b within 30 h under standard conditions,
complete conversion is observed within 20 h upon addition of
1−3 equiv of water. Similarly, the transformation of 2b into 3b
is speeded up when a dioxygen atmosphere (1 bar) is applied to
complex 2b under anhydrous conditions. To ascertain the
origin of the oxygen atom incorporated in the disiloxane
moiety, 18O labeling experiments were performed. In a first set
of experiments, a few equivalents of H218O were added to a
dichloromethane solution of 2b under dioxygen-free con-
ditions. Another experiment was carried out with 18O2 gas (0.7
bar) and a dichloromethane solution of 2b under anhydrous
conditions. In both cases, 31P NMR spectroscopy indicated the
complete transformation of 2b into 3b, and ESI-MS
spectrometry revealed up to 80% of 18O enrichment within
the disiloxane fragment.12 Both water and dioxygen are thus
competent oxygen sources for the formation of 3b. The partial
18O enrichment of 3b (50−80%) suggests that traces of
unlabeled water and/or dioxygen also contribute to the
formation of 3b. The exact stoichiometry of these reactions
can hardly be determined and the fate of the oxidant remains
unclear at this stage. A priori, dihydrogen or water can be
envisioned as byproducts. Overall, these observations are
consistent with the known ability of gold to promote oxidation
reactions with water or dioxygen as oxidants.16
resolution mass spectrometry measurements (HR ESI-MS) also
provide valuable information. The signal observed at m/z 715
amu for 3b and associated isotopic distribution correspond to
the mass of the starting material 2b plus one oxygen atom. It is
most likely that this mass increment results from disilane →
disiloxane conversion, in accordance with the 29Si NMR data.
To shed light on the precise structure of complex 3b,
colorless crystals suitable for X-ray diffraction analysis were
grown from a dichloromethane/pentane solution at −30 °C
(Figure 1).12 Accordingly, 3b was unambiguously identified as a
Figure 1. X-ray crystal structure of complex 3b. Thermal ellipsoids are
drawn at the 50% probability level, and hydrogen atoms, lattice solvent
molecules, and the chloride counteranion are omitted for clarity.
Selected bond lengths (Å) and angles (deg): P1−Au, 2.313(1); P2−
Au, 2.310(1); Si1−O, 1.646(2); Si2−O, 1.640(2); P1−Au−P2,
171.13(2); Si1−O−Si2, 147.81(12).
The role of gold in the disilane → disiloxane conversion was
then investigated. First, the coordination of the diphosphine−
disilane ligand to gold was found to be a prerequisite for the
oxidation of the disilane moiety. In contrast with the bis(silyl)
gold(III) complex 2b, no evolution of the disilane fragment is
detected when a solution of 1b is left without precaution under
air, indicating that the diphosphine−disilane ligand is much
more stable toward hydrolysis and oxidation. Then, we
wondered whether oxidative addition of the σ-Si−Si bond at
gold is necessary or not for the oxidation of the disilane moiety.
To address this question, the diphosphine−disilane ligand 4
was prepared and coordinated to gold.12 The short methylene
cationic diphosphine gold(I) complex featuring a pendant
disiloxane moiety. Its ionic nature is apparent from the long
distance between the metal center and chloride counteranion
(shortest Au···Cl distance 6.434(1) Å). The gold(I) center
adopts a classical dicoordinate arrangement14 with the two
phosphorus atoms coordinated in a quasi-linear fashion (P−Au
= 2.313(1) and 2.310(1) Å, P−Au−P = 171.13(2)°). The two
silicon atoms are no longer bound to gold and remain far away
from the metal center (Au···Si distances 3.556(6) and 3.436(7)
Å). The oxygen atom inserted between the two silicon atoms
also stands far away from the gold center, the Au···O distance
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Organometallics
Communication
bridge between the P and Si atoms was envisioned to maintain
the disilane moiety in a position remote from gold and thereby
prevent geometrically the oxidative addition of the σ-Si−Si
bond. Upon reaction of 4 with [AuCl(SMe2)] in dichloro-
methane at −78 °C, complex 5 was isolated as a white powder
in 87% yield (Scheme 2).
transformations. The group 10 metals, in particular Pd and Pt,
have been shown to efficiently promote the activation and
functionalization of σ-Si−Si, σ-Sn−Sn, σ-Si−Sn, ... bonds,18−21
while to the best of our knowledge, such transformations
remain unprecedented with the coinage metals. Future work
will seek to further generalize the ability of gold and the other
coinage metals to participate in redox sequences combining
oxidative addition of σ-bonds and subsequent functionalization.
Scheme 2. Synthesis and Behavior of the Diphosphine−
Disilane Gold(I) Complex 5
ASSOCIATED CONTENT
■
S
* Supporting Information
Text, figures, tables, and CIF files giving detailed experimental
conditions and procedures, analytical data for new compounds
1b, 2b, and 3−5, theoretical details for 5, and crystal data for 3b
and 5. This material is available free of charge via the Internet at
http://pubs.acs.org.
31P and 29Si NMR spectroscopy indicate both the symmetric
coordination of the two phosphines and the retention of the
disilane moiety. Colorless crystals of 5 were obtained from a
dichloromethane/pentane solution at −30 °C, and its
molecular structure was analyzed by X-ray diffraction (Figure
2). The gold center is surrounded by the two phosphorus
AUTHOR INFORMATION
■
Corresponding Author
*Fax: (+33) 5 61 55 82 04. E-mail: amgoune@chimie.ups-tlse.
fr (A.A.); dbouriss@chimie.ups-tlse.fr (D.B.).
Author Contributions
All authors have given approval to the final version of the
manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support from the Centre National de la Recherche
■
Scientifique, the Universite
́
de Toulouse, and the Agence
Nationale de la Recherche (ANR-10-BLAN-070901) is grate-
fully acknowledged. Idris is acknowledged for computer
facilities. Part of the theoretical work was granted access to
HPC resources of Idris under allocation 2012 (i2012080045)
made by GENCI (Grand Equipement National de Calcul
Intensif).
Figure 2. X-ray crystal structure of complex 5. Thermal ellipsoids are
drawn at the 50% probability level, and hydrogen atoms are omitted
for clarity. Selected bond lengths (Å) and angles (deg): P1−Au,
2.311(2); P2−Au, 2.315(2); Si1−Au, 3.673(2); Si2−Au, 3.726(2);
Si1−Si2, 2.347(2); P1−Au−P2, 143.53(6).
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