Inorganic-organic heteropolyacid-gold(I) hybrids: Structures and catalytic applications

Article abstract of DOI:10.1002/chem.201304680

Gold(I)-polyoxometalate hybrid complexes 1-4 ([PPh₃AuMeCN] _xH_4-xSiW₁₂O₄₀, x=1-4) were synthesized and characterized. The structure of the primary gold(I)- polyoxometalate 1 (x=1) was fully ascertained by XRD, FTIR, ³¹P and ²⁹Si magic-angle spinning (MAS) NMR, mass spectroscopy, and SEM-energy dispersive X-ray spectroscopy (EDX) techniques. Moreover, this complex exhibited better catalytic activity and selectivity compared with standard, homogeneous, gold catalysts in the new rearrangement of propargylic gem-diesters. Gold(I)-polyoxometalate hybrid complexes ([AuPPh ₃MeCN]_xH_4-xSiW₁₂O₄₀, x=1-4) were synthesized and characterized. [AuPPh₃MeCN]H ₃SiW₁₂O₄₀ exhibited better catalytic activity and selectivity compared with standard, homogeneous, gold catalysts in the new rearrangement of propargylic gem-diesters (see scheme).

Full text of DOI:10.1002/chem.201304680

DOI: 10.1002/chem.201304680
Communication
&
Gold Catalysis
Inorganic–Organic Heteropolyacid–Gold(I) Hybrids: Structures and
Catalytic Applications
Damien Hueber, Marie Hoffmann, Benoꢀt Louis, Patrick Pale,* and Aurꢁlien Blanc*[a]
hybrids is absolutely necessary, especially with new catalytic
Abstract: Gold(I)-polyoxometalate hybrid complexes 1–4
applications in organic synthesis as a perspective.[7]
([PPh3AuMeCN]xH4ÀxSiW12O40, x=1–4) were synthesized
Although best known for the ability to interact with p
and characterized. The structure of the primary gold(I)–
bonds, gold(I) also exhibits high affinity for N and O donors,[8]
polyoxometalate 1 (x=1) was fully ascertained by XRD,
rendering it a perfect candidate to build POM metal–organic
FTIR, 31P and 29Si magic-angle spinning (MAS) NMR, mass
complexes for further use in gold catalysis. So far, only very
spectroscopy, and SEM–energy dispersive X-ray spectros-
few catalytic applications of Au-derived POMs in organic syn-
copy (EDX) techniques. Moreover, this complex exhibited
thesis have been reported,[9] although Au-containing POMs are
better catalytic activity and selectivity compared with
known.[10,11] In gold-catalyzed organic reactions, the first step is
standard, homogeneous, gold catalysts in the new re-
often the activation of the gold catalyst through the genera-
arrangement of propargylic gem-diesters.
tion of the reactive “[LAu]+” entity. This is usually achieved by
replacing a strongly coordinating ligand (usually ClÀ) by
À
a weakly bound counteranion (OTfÀ, NTf2À, SbF6À, PF6À, BF4
,
Polyoxometalates (POMs) are oxygen-bridged, anionic, metal
clusters with unique structural characteristics and a wide diver-
sity in structures. The properties lead to various applications in
different fields, especially in catalysis, medicine, biology, elec-
trochromism, magnetism, and material science.[1] Recent years
have thus witnessed the development of POM-based molecu-
lar and composite materials.[2]
etc.) by using the driving force of AgCl formation and precipi-
tation [Scheme 1, Eq. (1)]. However, the nature of the counter-
Among the latter, non-covalent organic–inorganic hybrid
materials offer the largest possibilities in catalysis. Indeed,
POMs can act as unusually effective ligands to coordinate vari-
ous (transition) metal ions due to their high electronic density.
Although complexes built from Keggin,[3] Wells–Dawson,[4] An-
derson,[5] and Lindquist[6] type POMs and transition-metals co-
ordinated to organic ligands have been synthesized, rational
design and synthesis of such organic–inorganic hybrid materi-
als have yet to be developed, especially for specific purposes.
Indeed, preparation by hydrothermal synthesis is still a chal-
lenge and many parameters, such as initial constituent ratio of
metal oxide/metal/ligand, pH value, crystallization temperature
and pressure can significantly affect the topological struc-
tures.[3–6] Most importantly, the major drawback of this method
resides in poor isolated yields, which are not desired when
starting from precious transition metals. Therefore, the devel-
opment of an easy, simple, and efficient preparation of such
Scheme 1. Mimetic approach towards POM–gold(I) hybrids based on gold
complexes activation.
anion is far from innocent in terms of reactivity, it often modi-
fies the reaction rate, sometimes reverses selectivity,[12] and can
even act as source of chirality.[13]
From this background, we envisage that POM polyanions
with their intrinsic properties could be used as counteranions
for transition-metal cations,[14] such as gold(I); this would lead
to a new family of stable, efficient, and recyclable catalysts.[15]
Inspired by gold-complexes activation, we planned the prepa-
ration of POMÀAu+ by an acid–base reaction between hetero-
polyacids (HPAs; conjugated acids of POM) and a methyl gold
complex [Scheme 1, Eq. (2)]. This synthesis, through H+/Au+
exchange, could offer several advantages: the structure of the
POM is already known (Keggin unit, for example) and the only
byproduct is volatile (methane), facilitating purification and
characterization. As ligands also play a key role in the reactivity
of gold complexes, due to electronic and steric effects,[16] this
strategy also offers the possibility to efficiently introduce vari-
ous types of ligands on gold–POM entities. This approach
seems unprecedented, although similarities can be found in
the works by Hayashi–Tanaka and Echavarren. Both used vari-
ous organic or inorganic acids, including HPAs, in gold-cata-
[a] D. Hueber, M. Hoffmann, Dr. B. Louis, Prof. Dr. P. Pale, Dr. A. Blanc
Laboratoire de Synthꢀse, Rꢁactivitꢁ Organiques & Catalyse
Institut de Chimie de Strasbourg, associꢁ au CNRS
Universitꢁ de Strasbourg
4 rue Blaise Pascal, 67070 Strasbourg (France)
Fax: (+33)368851517
E-mail: ppale@unistra.fr
ablanc@unistra.fr
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304680.
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Communication
lyzed hydration and/or hydroamination of alkynes,[17] and the
cyclization of 1,6-enynes catalyzed by gold(I).[18] However, the
nature of the actual catalyst and catalytic properties have not
yet been explored.
The investigation of H+/Au+ exchange from HPA led us to
rapidly find that the acid–base reaction between silicotungstic
acid hydrate (H4SiW12O40·25H2O) and methyltriphenylphosphino
gold(I) (PPh3AuMe), both commercially available. This reaction
allowed the simple preparation of four POM–H/Au complexes
(1–4, Scheme 2) in quantitative yields in acetonitrile at room
temperature according to the chosen stoichiometry (see the
Supporting Information).
a 99:1 ratio at 28.8 and 44.4 ppm, respectively (d=48.2 ppm
for PPh3AuMe in acetonitrile), the major one is in accordance
with the expected structure of complex 1. Indeed, Lacꢃte
et al.[9] recently reported the synthesis of a chlorodiphenylphos-
phinogold(I) complex tethered to a Dawson a1-organotin-sub-
stitued polyoxotungstate activated by AgSbF6; this complex
presents two 31P NMR signals at d=18.6 and 27.1 ppm in ace-
tonitrile. The latter signal, which could be attributed to the
Dawson-phosphinogold complex, correlates with the value
found for the expected [PPh3AuMeCN]+ cation of complex 1.
The minor signal at 44.4 ppm was attributed to the inactive
[(PPh3)2Au]+ cation in analogy with literature data.[19] This
cation might result from an in situ reduction of a very small
amount of gold(I) during the hybrid formation.[20]
POM–H/Au 1–4 were further analyzed by FTIR and mass
spectroscopy by the electrospray ionization technique (ESI-MS)
in both positive and negative mode. The solid-state FTIR spec-
tra of 1–4 showed the characteristic vibrational bands of the
silicotungstic Keggin structure (740, 880, 910, 970, 1015 cmÀ1
)
and of the coordinated PPh3 ligand (690, 1100, 1435,
1475 cmÀ1; see the Supporting Information). As expected, all
the ESI positive-mode spectra revealed the presence of
[PPh3AuMeCN]+ and [(PPh3)2Au]+ ions; this confirms the inter-
pretation of the solid-state 31P NMR spectra of 1. The negative-
mode investigation of these organic–inorganic hybrid materials
was more fruitful. Indeed, the presence of anionic POMs spe-
cies in complex 1 [H2(AuPPh3)SiW12O40]À ([MÀH]: m/z=3335.3)
and [H1(AuPPh3)SiW12O40]2À ([MÀ2H]: m/z=1667.1) was detect-
ed along with traces of POM 2. These data clearly show the
presence of remaining protons, consistent with the expected
stoichiometry. The analysis of 2–4 revealed that these materials
were mixtures of different gold–POMs in various ratios. Indeed,
the spectra in negative mode reveals a statistical distribution
in accordance with the stoichiometry used and peak intensity
(x=1 to 4, see the Supporting Information).
Scheme 2. Synthesis of POM-Aux/Hx 1–4 hybrids by acid–base exchange of
counteranions.
POM 1 (x=1) was fully characterized by several physico-
chemical techniques. This new organic–inorganic hybrid mate-
rial was stable and insoluble in all classical organic solvents, re-
vealing a heterogeneous nature. SEM images of 1 showed ab-
sence of any specific morphology for this hybrid Keggin-type
POM (see the Supporting Information). However, the powder
XRD data confirmed the presence of the Keggin structure and
the elemental composition calculated from the EDX analysis
agreed with the expected stoichiometry (see the Supporting
Information). Moreover, an extremely homogenous distribution
of Si, W, O, Au, and P elements throughout the material was
observed by statistical EDX mapping within the material
(Figure 1) excluding the formation of gold nanoparticles.
To confirm the [MeCNAuPPh3]+[H3SiW12O40]À structure of
POM 1, considerable efforts were devoted to crystallize this
complex in various solvents or mixture of solvents, but this
material remained highly insoluble in all classical solvents.
However, sonication and heating 1 to reflux in acetonitrile, fol-
lowed by filtration of the residual hybrid allowed us to obtain
one colorless crystal from the filtrate. The X-ray diffraction fur-
nished a structure (5) composed of two cationic gold com-
plexes, [Au(PPh3)2]+ and [MeCNAuPPh3]+, per half Keggin unit
(see the Supporting Information); this corresponds to a new
complex, analogue to 4, with four gold atoms around the POM
(x=4). Once again, the crystallization of 5 confirmed the pres-
ence of [Au(PPh3)2]+ detected in 1 by solid-state NMR analysis,
but mainly it proved the presence of [MeCNAuPPh3]+ in the
POM hybrid. Crystallization of 5 with four gold atoms per
Keggin unit led us to attempt obtaining an X-ray-diffraction
structure of POM–Au4 hybrid 4. Under the same conditions
used for 5, colorless needles of 4 were indeed obtained. The
structure received by XRD analysis confirmed the postulated
composition of 4 (Figure 2), with the Keggin unit [SiW12O40]4À
surrounded by four [MeCNAuPPh3]+ cations (distances PÀAu=
To investigate the organic–inorganic hybrid material in more
detail, we then analyzed the heterogeneous complex 1 by
solid-state NMR (see the Supporting Information). The MAS 29Si
spectrum exhibited a single peak at À84.6 ppm, which was un-
ambiguously attributed to the preserved Keggin structure (d=
À84.7 ppm for silicotungstic acid hydrate).[10c] More pertinent
was the MAS 31P NMR spectrum that shows two signals in
Figure 1. EDX mapping of Si, W, O, Au, and P elements in POM 1 solid.
2.23 ꢄ
and
AuÀN=2.02–2.08 ꢄ).
Interestingly,
the
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Communication
promote the same rearrangement, confirming that the remain-
ing protons on POMs 1–3 were not responsible for the cataly-
sis (entry 6). We also verified the heterogeneity of our catalysts
through a leaching test. POM–Au/H3 1 was stirred in dichloro-
methane for 24 h and eliminated by filtration. Substrate 6 was
then dissolved in the filtrate and after 24 h of stirring no reac-
tion had occurred and the starting material was recovered. To
evaluate the recyclability, we repeated the latter reaction with
catalyst 1 several times (5 mol%; see the Supporting Informa-
tion). The catalyst was recovered from the reaction mixture
either by filtration over a nylon membrane or by decantation.
Good yields of 7, namely, 84, 76, and 75%, were received
during three consecutive runs, along with increased reaction
times. However, the yield dramatically dropped during the
fourth run and mass-spectroscopy analysis of the recycled cat-
alyst revealed that it isomerizes to the other less reactive and
more soluble hybrids, 2–4, along with formation of the inactive
[(PPh3)2Au]+ cation.
Figure 2. Ball and stick representation of XRD structure of 4 revealing
4+
a [SiW12O40]4À [MeCNAuPPh3]4 structure.
[MeCNAuPPh3]+ cations are associated two and two by p
stacking, pinching the POM (torsion angle Au–P–P–Au of
75.2(2)8). The shorter Au···Au distance between two [SiW12O40]–
[MeCNAuPPh3]4 units is 3.74(2) ꢄ, indicating the absence of
a significant aurophilic interaction.[21] Moreover, the distances
between gold atoms and rising oxygen atoms around the
POM are shorter than in structure 5 (Au–O=3.14–3.14 ꢄ vs.
3.21 ꢄ) or than reported for the triflate phosphinogold(I) com-
plex.[22] In both X-ray structures, the central silicon atom of the
Keggin unit holds a special position with eight half-oxygen
atoms around it; this is due to the intrinsic geometry of this
type of POM. This kind of crystallographic disorder has been
encountered several times for Keggin anions.[23]
Based on the results presented above, we then focused on
the more efficient complex 1, for which specific reactivity was
expected. For the gold-catalyzed rearrangement of propargylic
gem-diesters 8, we found out that this POM–Au/H3 catalyst is
able to stereoselectively furnish (E)-3-oxycarbonyl enone deriv-
atives 9 more efficiently than a large range of homogenous
gold catalysts (see the Supporting Information and Table 2).
These unconventional O,O-acetal functions have not previously
been studied as nucleophiles in the field of gold catalysis.[25]
Silicotungstic acid alone do not catalyze the rearrangement of
gem-diesters derivatives, or any other reaction.
With these new POM–Au hybrids in hand, we evaluated the
catalytic ability. We selected one representative gold-catalyzed
reaction, that is, the gold-catalyzed cascade reaction that con-
verts enyne esters to cyclopentenones (Table 1).[24] Applied to
enyne acetate 6, this reaction led to cyclopentenone 7 in 92%
yield with PPh3AuSbF6 as catalyst (entry 1). When we adjusted
the amount of the heterogeneous catalysts 1–4 to give the
same Au-loading as above, we observed the formation of 7 in
comparable yields with, as expected for heterogeneous cata-
lysts, longer reaction times, especially for hybrid 4 (entries 2–
5). Control experiments revealed that silicotungstic acid do not
By taking advantage of the superiority of our catalyst, we
evaluated the scope of this transformation (Table 2). From the
pivaloyl diester 8a, the corresponding (E)-3-oxycarbonyl enone
9a was efficiently formed and isolated with 73% yield, where-
as acetyl derivative 8b afforded the hydrolysis-sensitive com-
pound 9b in a modest isolated yield (entry 1 vs. entry 2). As
expected from lower migratory ability,[26] benzoyl diester 8c ex-
Table 2. Scope of the rearrangement of propargylic gem-diesters 8 into
enones 9 catalyzed by POM–Au/H3 1.
Table 1. Comparison of the catalytic activity of POM–Aux/Hx 1–4 to a ho-
mogeneous catalyst in a representative gold-catalyzed transformation.
Entry
R1
R2
Enone 9
Time [h]
Yield[b] [%]
1
2
3
4
5
6
7
8
9
Ph
Ph
Ph
tBu
Me
Ph
tBu
tBu
tBu
tBu
tBu
tBu
tBu
9a
9b
9c
9d
9e
9 f
9g
9h
9i
0.5
0.5
24
15[a]
15[a]
15[a]
0.5
15[a]
15[a]
15[a]
80 (73)
65 (40)
34
75 (71)
70 (65)
75 (72)
65 (57)
91 (75)
90 (70)
95 (82)
Entry
Catalyst [mol%]
Time [h]
Yield [%]
o-MeOPh
p-MeOPh
o-FPh
C6H13
Cyclohexyl
tBu
1
2
3
4
5
6
PPh3AuSbF6 (2)
0.5
4
6
7
22
24
92[b]
86
67
76
POM–Au/H3 1 (2)[a]
POM–Au2/H2 2 (1)[a]
POM–Au3/H1 3 (0.7)[a]
POM–Au4 4 (0.5)[a]
SiW12O40H4 (2)
65
10
BnO(CH2)3
9j
No conversion
[a] Reaction run at 458C. [b] Calculated yields from 1H NMR integration
relative to an internal standard (hexamethylbenzene) and isolated yields
in brackets.
[a] The catalytic loading of POM–Au/H used in this reaction was propor-
tional to the amount of gold(I) present in each hybrid. [b] From refer-
ence [24].
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Experimental Section
General procedure for preparation of POM–gold complexes
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General procedure for 1-catalyzed transformation of propargyl
gem-diesters 8 into derivatives 9
Catalyst 1 (33.4 mg, 2 mol%) was added to a solution of 8
(100 mg, 5 mmol) in CH2Cl2 (5 mL). The resulting mixture was
stirred at room temperature for 30 min or at 458C until completion
of the reaction (monitored by TLC). The reaction mixture was then
evaporated and the crude residue was purified by flash column
chromatography over silica gel (cyclohexane/ethyl acetate).
Acknowledgements
We gratefully acknowledge the CNRS and the French Ministry
of Research. D. H. thanks the Agence Nationale de la Recher-
che (Grant ANR-11-JS07–001–01 SyntHetAu) for a PhD fellow-
ship. M. H. thanks the French Ministry of Research for a PhD
fellowship. A. B. thanks Dr. Brelot for crystallographic structure
refinements of 4 and 5 and Dr. Tessonnier (Fritz Haber Institute,
Berlin) for EDX mapping of 1.
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Keywords: catalysis
polyoxometalates · propargyl gem-diesters
·
gold(I)
·
heteropolyacids
·
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Published online on February 26, 2014
Chem. Eur. J. 2014, 20, 3903 – 3907
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ꢂ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim