& Gold Catalysis

Inorganic–Organic Heteropolyacid–Gold(I) Hybrids: Structures andCatalytic Applications

Damien Hueber, Marie Hoffmann, Beno�t Louis, Patrick Pale,* and Aur�lien Blanc*[a]

Abstract: Gold(I)-polyoxometalate hybrid complexes 1–4([PPh3AuMeCN]xH4�xSiW12O40, x = 1–4) were synthesizedand 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, massspectroscopy, and SEM–energy dispersive X-ray spectros-copy (EDX) techniques. Moreover, this complex exhibitedbetter catalytic activity and selectivity compared withstandard, homogeneous, gold catalysts in the new re-arrangement of propargylic gem-diesters.

Polyoxometalates (POMs) are oxygen-bridged, anionic, metalclusters with unique structural characteristics and a wide diver-sity in structures. The properties lead to various applications indifferent fields, especially in catalysis, medicine, biology, elec-trochromism, magnetism, and material science.[1] Recent yearshave thus witnessed the development of POM-based molecu-lar and composite materials.[2]

Among the latter, non-covalent organic–inorganic hybridmaterials 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, rationaldesign 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 ofmetal oxide/metal/ligand, pH value, crystallization temperatureand pressure can significantly affect the topological struc-tures.[3–6] Most importantly, the major drawback of this methodresides in poor isolated yields, which are not desired whenstarting from precious transition metals. Therefore, the devel-opment of an easy, simple, and efficient preparation of such

hybrids is absolutely necessary, especially with new catalyticapplications in organic synthesis as a perspective.[7]

Although best known for the ability to interact with p

bonds, gold(I) also exhibits high affinity for N and O donors,[8]

rendering it a perfect candidate to build POM metal–organiccomplexes for further use in gold catalysis. So far, only veryfew catalytic applications of Au-derived POMs in organic syn-thesis have been reported,[9] although Au-containing POMs areknown.[10, 11] In gold-catalyzed organic reactions, the first step isoften the activation of the gold catalyst through the genera-tion of the reactive “[LAu]+” entity. This is usually achieved byreplacing a strongly coordinating ligand (usually Cl�) bya weakly bound counteranion (OTf� , NTf2

� , SbF6� , PF6

� , BF4� ,

etc.) by using the driving force of AgCl formation and precipi-tation [Scheme 1, Eq. (1)] . However, the nature of the counter-

anion is far from innocent in terms of reactivity, it often modi-fies the reaction rate, sometimes reverses selectivity,[12] and caneven act as source of chirality.[13]

From this background, we envisage that POM polyanionswith their intrinsic properties could be used as counteranionsfor transition-metal cations,[14] such as gold(I) ; this would leadto a new family of stable, efficient, and recyclable catalysts.[15]

Inspired by gold-complexes activation, we planned the prepa-ration of POM�Au+ by an acid–base reaction between hetero-polyacids (HPAs; conjugated acids of POM) and a methyl goldcomplex [Scheme 1, Eq. (2)] . This synthesis, through H+/Au+

exchange, could offer several advantages: the structure of thePOM is already known (Keggin unit, for example) and the onlybyproduct is volatile (methane), facilitating purification andcharacterization. As ligands also play a key role in the reactivityof gold complexes, due to electronic and steric effects,[16] thisstrategy also offers the possibility to efficiently introduce vari-ous types of ligands on gold–POM entities. This approachseems unprecedented, although similarities can be found inthe works by Hayashi–Tanaka and Echavarren. Both used vari-ous organic or inorganic acids, including HPAs, in gold-cata-

Scheme 1. Mimetic approach towards POM–gold(I) hybrids based on goldcomplexes activation.

[a] D. Hueber, M. Hoffmann, Dr. B. Louis, Prof. Dr. P. Pale, Dr. A. BlancLaboratoire de Synth�se, R�activit� Organiques & CatalyseInstitut de Chimie de Strasbourg, associ� au CNRSUniversit� de Strasbourg4 rue Blaise Pascal, 67070 Strasbourg (France)Fax: (+ 33) 368851517E-mail : ppale@unistra.fr

ablanc@unistra.fr

Supporting information for this article is available on the WWW underhttp ://dx.doi.org/10.1002/chem.201304680.

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CommunicationDOI: 10.1002/chem.201304680

lyzed hydration and/or hydroamination of alkynes,[17] and thecyclization of 1,6-enynes catalyzed by gold(I).[18] However, thenature of the actual catalyst and catalytic properties have notyet been explored.

The investigation of H+/Au+ exchange from HPA led us torapidly find that the acid–base reaction between silicotungsticacid hydrate (H4SiW12O40·25H2O) and methyltriphenylphosphinogold(I) (PPh3AuMe), both commercially available. This reactionallowed the simple preparation of four POM–H/Au complexes(1–4, Scheme 2) in quantitative yields in acetonitrile at roomtemperature according to the chosen stoichiometry (see theSupporting Information).

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-typePOM (see the Supporting Information). However, the powderXRD data confirmed the presence of the Keggin structure andthe elemental composition calculated from the EDX analysisagreed with the expected stoichiometry (see the SupportingInformation). Moreover, an extremely homogenous distributionof Si, W, O, Au, and P elements throughout the material wasobserved by statistical EDX mapping within the material(Figure 1) excluding the formation of gold nanoparticles.

To investigate the organic–inorganic hybrid material in moredetail, we then analyzed the heterogeneous complex 1 bysolid-state NMR (see the Supporting Information). The MAS 29Sispectrum exhibited a single peak at �84.6 ppm, which was un-ambiguously attributed to the preserved Keggin structure (d=

�84.7 ppm for silicotungstic acid hydrate).[10c] More pertinentwas the MAS 31P NMR spectrum that shows two signals in

a 99:1 ratio at 28.8 and 44.4 ppm, respectively (d= 48.2 ppmfor PPh3AuMe in acetonitrile), the major one is in accordancewith the expected structure of complex 1. Indeed, Lac�teet 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 complexpresents two 31P NMR signals at d= 18.6 and 27.1 ppm in ace-tonitrile. The latter signal, which could be attributed to theDawson-phosphinogold complex, correlates with the valuefound 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] Thiscation might result from an in situ reduction of a very smallamount of gold(I) during the hybrid formation.[20]

POM–H/Au 1–4 were further analyzed by FTIR and massspectroscopy 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 thesilicotungstic Keggin structure (740, 880, 910, 970, 1015 cm�1)and of the coordinated PPh3 ligand (690, 1100, 1435,1475 cm�1; see the Supporting Information). As expected, allthe 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 materialswas more fruitful. Indeed, the presence of anionic POMs spe-cies in complex 1 [H2(AuPPh3)SiW12O40]� ([M�H]: m/z = 3335.3)and [H1(AuPPh3)SiW12O40]2� ([M�2 H]: m/z = 1667.1) was detect-ed along with traces of POM 2. These data clearly show thepresence of remaining protons, consistent with the expectedstoichiometry. The analysis of 2–4 revealed that these materialswere mixtures of different gold–POMs in various ratios. Indeed,the spectra in negative mode reveals a statistical distributionin accordance with the stoichiometry used and peak intensity(x = 1 to 4, see the Supporting Information).

To confirm the [MeCNAuPPh3]+[H3SiW12O40]� structure ofPOM 1, considerable efforts were devoted to crystallize thiscomplex in various solvents or mixture of solvents, but thismaterial 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 obtainone 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 newcomplex, 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 thePOM hybrid. Crystallization of 5 with four gold atoms perKeggin unit led us to attempt obtaining an X-ray-diffractionstructure of POM–Au4 hybrid 4. Under the same conditionsused for 5, colorless needles of 4 were indeed obtained. Thestructure received by XRD analysis confirmed the postulatedcomposition of 4 (Figure 2), with the Keggin unit [SiW12O40]4�

surrounded by four [MeCNAuPPh3]+ cations (distances P�Au =

2.23 � and Au�N = 2.02–2.08 �). Interestingly, the

Scheme 2. Synthesis of POM-Aux/Hx 1–4 hybrids by acid–base exchange ofcounteranions.

Figure 1. EDX mapping of Si, W, O, Au, and P elements in POM 1 solid.

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[MeCNAuPPh3]+ cations are associated two and two by p

stacking, pinching the POM (torsion angle Au–P–P–Au of75.2(2)8). The shorter Au···Au distance between two [SiW12O40]–[MeCNAuPPh3]4 units is 3.74(2) �, indicating the absence ofa significant aurophilic interaction.[21] Moreover, the distancesbetween gold atoms and rising oxygen atoms around thePOM are shorter than in structure 5 (Au–O = 3.14–3.14 � vs.3.21 �) or than reported for the triflate phosphinogold(I) com-plex.[22] In both X-ray structures, the central silicon atom of theKeggin unit holds a special position with eight half-oxygenatoms around it ; this is due to the intrinsic geometry of thistype of POM. This kind of crystallographic disorder has beenencountered several times for Keggin anions.[23]

With these new POM–Au hybrids in hand, we evaluated thecatalytic ability. We selected one representative gold-catalyzedreaction, that is, the gold-catalyzed cascade reaction that con-verts enyne esters to cyclopentenones (Table 1).[24] Applied toenyne acetate 6, this reaction led to cyclopentenone 7 in 92 %yield with PPh3AuSbF6 as catalyst (entry 1). When we adjustedthe amount of the heterogeneous catalysts 1–4 to give thesame Au-loading as above, we observed the formation of 7 incomparable 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

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 catalyststhrough a leaching test. POM–Au/H3 1 was stirred in dichloro-methane for 24 h and eliminated by filtration. Substrate 6 wasthen dissolved in the filtrate and after 24 h of stirring no reac-tion had occurred and the starting material was recovered. Toevaluate the recyclability, we repeated the latter reaction withcatalyst 1 several times (5 mol %; see the Supporting Informa-tion). The catalyst was recovered from the reaction mixtureeither by filtration over a nylon membrane or by decantation.Good yields of 7, namely, 84, 76, and 75 %, were receivedduring three consecutive runs, along with increased reactiontimes. However, the yield dramatically dropped during thefourth run and mass-spectroscopy analysis of the recycled cat-alyst revealed that it isomerizes to the other less reactive andmore soluble hybrids, 2–4, along with formation of the inactive[(PPh3)2Au]+ cation.

Based on the results presented above, we then focused onthe more efficient complex 1, for which specific reactivity wasexpected. For the gold-catalyzed rearrangement of propargylicgem-diesters 8, we found out that this POM–Au/H3 catalyst isable to stereoselectively furnish (E)-3-oxycarbonyl enone deriv-atives 9 more efficiently than a large range of homogenousgold catalysts (see the Supporting Information and Table 2).These unconventional O,O-acetal functions have not previouslybeen studied as nucleophiles in the field of gold catalysis.[25]

Silicotungstic acid alone do not catalyze the rearrangement ofgem-diesters derivatives, or any other reaction.

By taking advantage of the superiority of our catalyst, weevaluated the scope of this transformation (Table 2). From thepivaloyl diester 8 a, the corresponding (E)-3-oxycarbonyl enone9 a was efficiently formed and isolated with 73 % yield, where-as acetyl derivative 8 b afforded the hydrolysis-sensitive com-pound 9 b in a modest isolated yield (entry 1 vs. entry 2). Asexpected from lower migratory ability,[26] benzoyl diester 8 c ex-

Figure 2. Ball and stick representation of XRD structure of 4 revealinga [SiW12O40]4� [MeCNAuPPh3]4

4+ structure.

Table 2. Scope of the rearrangement of propargylic gem-diesters 8 intoenones 9 catalyzed by POM–Au/H3 1.

Entry R1 R2 Enone 9 Time [h] Yield[b] [%]

1 Ph tBu 9 a 0.5 80 (73)2 Ph Me 9 b 0.5 65 (40)3 Ph Ph 9 c 24 344 o-MeOPh tBu 9 d 15[a] 75 (71)5 p-MeOPh tBu 9 e 15[a] 70 (65)6 o-FPh tBu 9 f 15[a] 75 (72)7 C6H13 tBu 9 g 0.5 65 (57)8 Cyclohexyl tBu 9 h 15[a] 91 (75)9 tBu tBu 9 i 15[a] 90 (70)10 BnO(CH2)3 tBu 9 j 15[a] 95 (82)

[a] Reaction run at 45 8C. [b] Calculated yields from 1H NMR integrationrelative to an internal standard (hexamethylbenzene) and isolated yieldsin brackets.

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 Catalyst [mol %] Time [h] Yield [%]

1 PPh3AuSbF6 (2) 0.5 92[b]

2 POM–Au/H3 1 (2)[a] 4 863 POM–Au2/H2 2 (1)[a] 6 674 POM–Au3/H1 3 (0.7)[a] 7 765 POM–Au4 4 (0.5)[a] 22 656 SiW12O40H4 (2) 24 No conversion

[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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hibited a strongly decreased reactivity, affording 9 c in pooryield (entry 3). An electron-withdrawing or -donating arylgroup in position R1 (8 d–f) requires a slightly higher tempera-ture and prolonged reaction time. Nevertheless, similar yieldswere obtained compared to substrate 8 a (entries 4–6 vs.entry 1). Simple or functionalized alkyl groups proved fullycompatible with our reaction conditions, allowing the forma-tion of enones 9 g–j in good to excellent yields (entries 7–10).

In conclusion, we have developed a simple way to synthe-size non-covalent organic–inorganic hybrids based on the as-sociation of gold complexes and POMs. The obtained, fullycharacterized, stable materials exhibit very efficient and specif-ic catalytic activity in an unprecedented, gold-catalyzed, re-arrangement of propargylic gem-diesters compared with thehomogeneous catalysts. Future catalytic applications of thesePOM hybrids are exceedingly important, because these materi-als could a priori be designed for any expensive transitionmetal and then applied to all known associated organic reac-tions. Further works in this area are under progress in ourgroup.

Experimental Section

General procedure for preparation of POM–gold complexes1–4

A solution of silicotungstic acid hydrate (0.105 mmol for 1;0.053 mmol for 2 ; 0.035 mmol for 3 ; 0.026 mmol for 4) in MeCN(2.5 mL) was added to a suspension of (PPh3)AuMe (50.0 mg,0.105 mmol) in MeCN (2.5 mL) at room temperature . The resultingmixture was stirred at room temperature for 16 h (formation ofgray-purple precipitate). The solvent was then evaporated and theresulting solid was dried at 105 8C for 24 h to give the desired cata-lysts.

General procedure for 1-catalyzed transformation of propargylgem-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 wasstirred at room temperature for 30 min or at 45 8C until completionof the reaction (monitored by TLC). The reaction mixture was thenevaporated and the crude residue was purified by flash columnchromatography over silica gel (cyclohexane/ethyl acetate).

Acknowledgements

We gratefully acknowledge the CNRS and the French Ministryof 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 PhDfellowship. A. B. thanks Dr. Brelot for crystallographic structurerefinements of 4 and 5 and Dr. Tessonnier (Fritz Haber Institute,Berlin) for EDX mapping of 1.

Keywords: catalysis · gold(I) · heteropolyacids ·polyoxometalates · propargyl gem-diesters

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Received: November 29, 2013

Published online on February 26, 2014

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