H+ Complexes: Air-Stable ... - ACS Publications

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Polyoxometalate-Gold(I)/H+ Complexes: Air-Stable, Efficient, Polyvalent, and Bifunctional Catalysts Damien Hueber,† Marie Hoffmann,† Pierre de Frémont,‡ Patrick Pale,*,† and Aurélien Blanc*,† †

Laboratoire de Synthèse, Réactivité Organiques & Catalyse, Institut de Chimie de Strasbourg, and ‡Laboratoire de Chimie des Ligands à Architecture Contrôlée, Institut de Chimie, UMR 7177−CNRS, Université de Strasbourg, 4 Rue Blaise Pascal, 67070 Strasbourg, France S Supporting Information *

ABSTRACT: Gold(I)/H+-polyoxometalate hybrids, with general formula [POM]x−[H]x−1+[R3PAu(MeCN)]+, were synthesized and fully characterized by 31P and 29Si MAS or liquid NMR, FT-IR, and mass spectroscopy. All these polyoxometalate gold(I) complexes exhibited excellent catalytic activity and selectivity compared to standard homogeneous gold catalysts in a large range of reactions. The variation of the ligand and the polyoxometalate nature around gold(I) also highlighted the flexibility and the multifunctionality of such catalysts, as well as their recyclability.

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INTRODUCTION Polyoxometalates (POMs) refer to polyanionic clusters formed by the assembly of early-transition-metal oxide building blocks [MOx], typically tungsten(IV) or molybdenum(VI). Since their discovery two centuries ago, thousands of structurally different POMs with various properties have been reported,1 impacting all the areas of chemistry and notably catalysis.2 In 2010, this large family of POM compounds have been categorized in three distinct groups according to the polyoxoanion structure:3 clusters without additional heteroatoms constitute the isopolyanion class, clusters including heteroatoms such as P and Si as templates form the heteropolyanion class, and the molybdenum-reduced POM clusters represent a third class. This first classification did not take into account the presence of inorganic (H+, Na+, ...) or organic cations (NR4+, ...) compensating the large negative charge of POMs. The nature of the cationic counterpart is however a crucial factor of the POMs’ physical or chemical properties. Among the numerous possibilities, the combination of the inorganic polyanionic POMs with organic entities as counterion provides a unique and tremendous opportunity to fine-tune the POMs’ properties.4 As a result, the formation of multifunctional monomeric or polymeric organic−inorganic hybrids can join the inherent properties of both interdependent parts and go further toward new physical or chemical properties.5 Inspired by the classification of organic−inorganic materials,6 this latter class of organic-POM-based material has been more recently subdivided in two types.7 In the type I hybrids, the organic and inorganic parts are associated by noncovalent interactions (electrostatic, hydrogen bonding, ...). Type II hybrids involve stronger bonding interactions with covalent bonds and usually require the direct substitution of [MOx] of POMs (Scheme 1). However, in the continuously growing field of POMs, new © XXXX American Chemical Society

Scheme 1. Different Types of Polyoxometalates-Based Inorganic/Organic Hybrids

types of organic−inorganic hybrids based on POMs are emerging, and a third and new class has to be considered, in which the organic part was associated with the inorganic POMs throughout coordination to metallic cations (type III, Scheme 1).8 The combination of POMs with coinage-metal/organic ligand complexes provides good examples of type III hybrid class, and various structures have been formed via, notably, three-component (cationic coinage metal, ligands, and POMs) hydrothermal synthesis.9 Interestingly, the charge-compensating coinage-metal cations could not only serve as structural linkers between the organic ligands and inorganic POMs but also bring their Lewis acid properties to the hybrids.10 In this area, we have recently demonstrated that gold(I) cations were effectively very good candidates in constructing type III organic−inorganic complexes.11,12 Moreover, depending on the applied stoichiometry (x, Scheme 1), excellent and new gold Received: July 31, 2015

A

DOI: 10.1021/acs.organomet.5b00656 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

formula for gold(I)/H+-polyoxometalates 1−6: [POM]x−[H]+x−1[R3PAu(MeCN)]+ (Table 1). This technique also showed the expected molecular peak for each anionic POM species [M − yH+] (Table 1), as well as the presence of remaining protons on these complexes. However, this analysis also allowed the detection of minor amounts of higher order complexes [POM]x−[H]x−2+[R3PAu(MeCN)]2+ (see the Supporting Information). NMR analysis in DMSO-d6 clearly proved the preservation of the different POM structures (bolded chemical shifts in Table 1) because the chemical shift of the central atoms of these complexes exhibited the same value (−86.1 ppm 29Si NMR for H4SiW12O40) for the Si-based POM and very similar ones for the P-based POMs compared to the corresponding initial HPAs (−15.6 vs −15.3 ppm 31P NMR for H3PW12O40, −4.1 vs −3.8 ppm for H3PMo12O40, −13.3 vs −12.8 ppm for H6P2W18O62). 31P NMR also allowed the identification of the cationic gold(I) species [(Ph3P)Au(MeCN)]+, [(Me3P)Au(MeCN)]+, and [(2,4-ditBuPhO)3P)Au(MeCN)]+ with signals at respectively 26.7 ± 0.1, −12.5, and 85.6 ppm, in close analogy with known 31P NMR shifts from the literature.15 It is noteworthy to mention that traces of the inactive [(R3P)2Au]+ species were also observed on each 31P spectra (1−4%). POM-H+/gold(I) 1−6 were further analyzed by ESI-MS in both positive mode and FT-IR, confirming each complex structure (see the Supporting Information). Before our first isolation and characterization of gold(I)polyoxometalates,10 the in situ combination of HPAs and Ph3PAuMe in methanol has been successfully used in hydration and/or hydroamination reactions16 and in the cyclization of 1,6-enynes,17 but without characterization of the actual catalysts. To investigate the nature of the active species in such reactions, we applied our methodology to prepare the complex possibly responsible for these catalyses. This complex might be very similar to or the same as complex 1, obtained from H4SiW12O40 and PPh3Au(I)Me (see Table 1). We thus mixed these two entities in methanol instead of acetonitrile and isolated a new solid. The new complex 1MeOH was then studied as previously. However, solid-state 31P NMR was used to avoid the possible effect of deuterated solvent on the catalyst structure (Figure 1). In contrast to 1 prepared in acetonitrile, for which no differences between solid and liquid 31P NMR could be noticed, 1MeOH exhibited new signals, one at 27.6 ppm, which could be assigned to the [Ph3PAu(MeOH)]+ cation, and another one at 44.4 ppm, which corresponded to the known [(Ph3P)2Au]+ cation.18 The important amount of the latter suggested that a large part of gold(I) was reduced to gold(0) during the reaction.

reactivities have been obtained using such [(Ligand)Gold(I)(Solvent)]+x/POMx− hybrids compared to classical homogeneous gold catalysts (Scheme 2). Scheme 2. POM/Gold(I)-Catalyzed Stereoselective Rearrangement of Propargylic gem-Diesters into (E)-Enones12

In the present contribution, we reported the preparation and characterization of new and stable polyoxometalate-gold(I) complexes and their catalytic activities in various gold-catalyzed reactions. By varying the ligand and polyoxometalate nature around gold, we also demonstrated the flexibility and the multifunctionality of such catalysts.

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RESULTS AND DISCUSSION Preparation and Characterization of Au(I)-POM Complexes. In order to evaluate and compare the subsequent catalytic activities of Au-POM complexes, typical POM structures with different central atoms (Si, P) and metal oxides (W, Mo) and exhibiting different sizes and charges were selected and combined with gold(I) carrying phosphine or phosphite ligands (scheme in Table 1). Three Keggin- and one Dawson-type POM-H+/PPh3Au(I) complex, 1−4 (entries 1−4), were obtained by simply mixing overnight in acetonitrile at room temperature the corresponding heteropolyacids (HPAs, conjugate acids of POM, respectively H4SiW12O40, H3PW12O40, H3PMo12O40, H6P2W18O62) with an equimolar amount of the commercially available methyl(triphenylphosphine)gold(I) complex.13 To compare the influence of the ligands on the catalysis, two other SiW12O40-H+/Au(I) complexes, 5 and 6 (entries 5 and 6), were synthesized starting from respectively methyl(trimethylphosphine)gold(I) and methyl[tris(2,4-ditert-butylphenyl)phosphite]gold(I) complexes prepared from reported procedures.14 All complexes 1−6 were easily obtained in quantitative yields along with the formation of methane as the sole byproduct. Such complexes remained highly insoluble in classic organic solvents except in DMSO. The unambiguous characterization of each complex was achieved by combining liquid 31P and 29Si NMR and mass spectroscopy using electrospray ionization technique (ESI-MS). In its negative mode, the latter afforded the following general

Table 1. Preparation of Various Gold(I)-Polyoxometalates 1−6 in Acetonitrile

NMR DMSO-d6 chemical shifta Sia

29

complex [H3SiW12O40][Ph3PAu(MeCN)] [H2PW12O40][Ph3PAu(MeCN)] [H2PMo12O40][Ph3PAu(MeCN)] [H5P2W18O62][Ph3PAu(MeCN)] [H3SiW12O40][Me3PAu(MeCN)] [H3SiW12O40][(2,4-ditBuPhO)3PAu(MeCN)] a

1 2 3 4 5 6

−86.1

−86.1 −86.1

31 a

P

26.8 26.6, −15.6 26.6, −4.1 26.7, −13.3 −12.9 85.6

mass peaks M − yH+b 1667.12 1668.60 1040.86 1607.95 1574.13 1859.30

(y (y (y (y (y (y

= = = = = =

2) 2) 2) 3) 2) 2)

yield (%) 99 96 96 98 99 99

In ppm. bm/z observed on the ESI negative mode spectra. B


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Figure 1. Comparison of 31P MAS NMR (rotation = 12 000 Hz) of 1 and 1MeOH, [H3SiW12O40][PPh3Au(Solvent)], prepared respectively in acetonitrile and methanol.

filtration of the remaining residues and then letting the solutions stand for 1 week to 6 months. The X-ray diffractions of the so-obtained crystals furnished six structures, which do not correspond to the expected structures (Figure 2). These structures exhibit the expected POM cores corresponding to the initial complexes 1−5, but their charges are fully balanced by gold(I) cations, except for the Dawson POM-Au complex 4a, which retains one proton under the form H5O2+. They thus represent new complexes. The complexes 1a, 2a, 3a, and 5a are saturated Keggin polyanions with the formula [POM]x−[R3PAu(MeCN)]x+. Surprisingly, another saturated complex, 1b, could also be crystallized from complex 1. The X-ray diffraction structure revealed the association of two [(Ph3P)2Au]+ along with two [Ph3PAu(MeCN)]+ around the anionic POM unit. This unexpected structure could arise only from the formation

The triphenylphosphine liberated in this reduction process could then form a relatively high amount of the stable but inactive [(Ph3P)2Au]+ complex. Moreover, 1MeOH’s spectrum exhibited an extra peak at 39.4 ppm that could presumably be assigned to the formation of higher coordinate complexes [(Ph3P)xAu]+ (x = 3, 4) due to the release of an excess of triphenylphosphine.19 These results, gained from the preparation of complex 1MeOH in methanol, clearly questioned the nature and the amount of active species in some previously reported goldcatalyzed transformations.15,16 X Ray Structures of Au(I)-POM Complexes. Due to the high insolubility of complexes 1−6, various attempts to crystallize them failed, even in DMSO, the solvent used for liquid NMR spectroscopy. However, we were able to form crystals by refluxing complexes 1−5 in acetonitrile, followed by C


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Figure 2. Ellipsoid representations of XRD structures of saturated polyoxometalate-gold(I) complexes 1a, 1b, 2a, 3a, 4a, and 5a (hydrogen atoms have been omitted for clarity).

Scheme 3. POM/Gold(I) Exchanges Leading to the More Soluble Hybrid Complex 1a from Hybrid 1

of trace amounts of [(Ph3P)2Au]+ (∼1%, see Figure 1) during the preparation of complex 1 in acetonitrile. It is noteworthy that in structures 1a and 1b, the central silicon atom holds a special crystallographic position with eight half-oxygen atoms around it; such disorder is classical for Keggin anions.20 These unexpected results revealed that upon heating for recrystallization, these complexes underwent successive exchanges between POMs and gold cations, ultimately leading to the more soluble hybrid complex in which the POM unit charge

is fully counterbalanced by gold complex cations (Scheme 3). In the particular case of the Dawson complex 4a coming from 4, the steric hindrance due to the accumulation of [Ph3PAu(MeCN)]+ cations around the polyoxoanion [P2W18O62]6− seemed to allow for only five of them and led to the presence of residual acidity from [H5O2]+. To provide more structural details, we have selected characteristic bond lengths, interatomic distances, and angles from the X-ray structures of the gold(I)-POM complexes (Table 2). D


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Organometallics Table 2. Selected Bond Lengths or Distances (Å) and Angles (deg) for Gold(I)-POM Complexes 1a,b−5a complex 4−

[SiW12O40] [Ph3PAu(MeCN)]+4 (1a) [SiW12O40]4−[Ph3PAu(MeCN)]+2[(Ph3P)2Au]+2

(1b)

[PW12O40]3−[Ph3PAu(MeCN)]+3 (2a) [PMo12O40]3−[Ph3PAu(MeCN)]+3 (3a) [P2W18O62]6−[Ph3PAu(MeCN)]+5, 4[MeCN], 2[O] (4a) [SiW12O40]4−[Me3PAu(MeCN)]+4, 2[MeCN] (5a) a

Au−P

Au−N

P−Au−N

Au···Aua

Au···Ob

2.23(0)−2.23(7) 2.22(6),c 2.30

2.02(4)−2.08(3) 2.03(2), −c

3.74(2) 8.16(6)

2.23(4) 2.23(8) 2.22(0)−2.23(6) 2.22(7)−2.23(6)

2.03(9) 2.04(6) 2.01(4)−2.08(1) 2.03(9)−2.06(7)

174.2−178.1 175.0(2) 172.8(9)c 174.1(7) 174.5(9) 169.7(8)−176.9(3) 169.0(7)−170.4(7)

3.14(6) 3.21(4) 5.17(6)c 3.18(1) 3.17 (2) 3.20(4)−3.49(3) 2.90(7)−3.30(8)

6.45(7) 6.44(5) 8.02(8) 3.06(9)d

Shortest gold−gold distances. bShortest gold−oxygen distances. cData for [(Ph3P)2Au]+. dd10−d10 interaction.

Concerning the hybrids 1a, 1b, 2a, 3a, and 4a, bearing [Ph3PAu(MeCN)]+ cations, the P−Au (2.22−2.23 Å) and Au−N (2.02−2.08 Å) distances were in agreement with various gold(I)/acetonitrile complexes described in the literature.21 The P−Au−N angles (174.1−176.9°) also followed the general tendency for a standard [Ph3PAu(MeCN)]+ cation, except in the unsaturated Dawson complex 4a, where one of the angle values was smaller than usual (169.0°). Such deviation could be attributed to steric interactions between all [Ph3PAu(MeCN)]+ cations around the POM and was confirmed by the Au−N−C angle of 148.6° on this cation, instead of 170−180° for the others. The Au···Au distances in these complexes were between 3.74 and 8.02 Å, indicating the absence of significant aurophilic interactions.22 On the contrary, the X-ray structure of complex 5a showed an effective d10−d10 interaction between two [Me3PAu(MeCN)]+ (Au···Au = 3.06 Å), probably induced by the smaller size of the PMe3 ligand compared to PPh3. In addition, the P−Au−N angle in this complex deviated significantly from linearity (169−170°) due to this d10−d10 bonding. Interestingly, the distances between the rising oxygen atoms of POMs and gold atoms could be observed between 2.90 and 3.49 Å. These values clearly differ from those reported for covalent Au−O bonds (e.g., Au−OH = 2.078 Å,23 Au−OAc = 2.040 Å24) as well as from those without significant interaction between Au and oxygen (e.g., ≥4.0 Å). They readily compared to the distances observed for a POM-Au4O cluster reported by Nomiya (3.38−3.49 Å).10i Therefore, the present values probably reflect the weakness of POMs ([SiW12O40]4−, [PW12O40]3−, [PMo12O40]3−, and [P2W18O62]6−) as counterions associated with gold atoms compared to those classically used in gold catalysis, such as OTf−,14a SbF6−, and BF4−. Such low interactions between the gold cation and its counterion foreshadow potent catalytic activities of these hybrid polyoxometalate-gold(I) complexes, 1−6.25 Catalytic Activities. We evaluated the catalytic ability of new POM-LAun/Hx−n complexes 1−6 in two representative gold-catalyzed reactions both performed in CH2Cl2: the rearrangement of enyne esters to cyclopentenones26 (Table 3, entries 1−8) and the cyclization of N-propargylcarboxamides27 (Table 3, entries 9−16). In homogeneous conditions, the first transformation led to cyclopentenone 8 in 92% yield starting from enyne acetate 7 with Ph3PAuCl/AgSbF6 as catalyst (Table 3, entry 1), while the cyclization of N-propargyl benzamide 9 catalyzed by Ph3PAuNTf2 afforded dihydrooxazole 10 in 91% yield (Table 3, entry 9). These products and yields were obtained using 2 mol % of gold catalyst. These data served as reference values for our own investigations with our heterogeneous catalysts. With all of them, we were pleased to observe the formation of desired products in the same range of yields (Table 3,

Table 3. Comparison of the Catalytic Activity of [POM]x−[H]+x−1[R3PAu(MeCN)]+ 1−6 vs the Homogeneous Version in Two Representative Gold-Catalyzed Transformations

a

Isolated yields. bStarting material was recovered untouched. Estimated conversion based on the 1H NMR analysis of the crude mixture.

c

entries 2−7 and 10−15), while a simple HPA did not promote transformations (entries 8 and 16). For both transformations, the influence of the POM unit was evaluated with the catalysts 1−4, containing the same triphenylphosphine ligand. Although the basicity of the POM anions ([P2W18O62]6− > [SiW12O40]4− ≥ [PMo12O40]3− > [PW12O40]3−)28 could be linked to their catalytic activity, the obtained results showed a more complex pattern. Indeed, the less basic gold counterion [PW12O40]3− afforded excellent yields, 94% and 85%, in both reactions compared to the more basic silicotungstate anion (respectively 86% and 75%; entries 3 and 11 vs 2 and 10). Interestingly, catalyst 2 provided better yields than the homogeneous versions (Table 3, entries 3 vs 1 and 11 vs 9). The catalyst [PMo12O40]3−, 3, exhibited an ambivalent catalytic activity, affording the highest yield for the N-propargylcarboxamide cyclization (96%) and the lowest amount of 8 (68%) in the enyne ester rearrangement (Table 3, entries 4 and 12). In contrast, the Wells−Dawson complex 4 exhibited an excellent activity, disconnected to its relative basicity. Despite being the more basic anion and thus considered as the less active catalyst, high yields were nevertheless obtained, similar for both reactions and equal to the homogeneous versions. These results may be attributed to the larger size of the Dawson anion compared to the Keggin complexes 1−3 (Table 3, entries 5 and 13 vs entries 1 and 9). The electronic properties of ligands also played a crucial role in gold(I) catalysis.29 We thus studied the effect of the nature of the phosphorus ligands associated with the [SiW12O40]4− anion (complexes 1, 5, and 6). Without surprise, the reactivity E


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we successfully obtained the same ketone using the same catalyst loading (2.5 mol %) under the same conditions. Very interestingly, comparable and even better yields were achieved with all our Au-POM catalysts (Table 4, entries 2−7, 70−89%). A control experiment with a simple HPA did not afford the ketone 13 but the N-tosylbuta-2,3-dienamine as a single product in 60% yield coming from the acid-catalyzed aza-Cope rearrangement (Table 4, entry 8).31 The latter set of results clearly highlights the capacity of complexes 1−6 to act as bifunctional catalysts, their gold counterpart ensuring the cyclization process and the protic POM the hydrolysis of the so-formed product. With these results in hand, we focused our attention on complex 1 with the aim to evaluate its flexibility regarding various gold transformations (Table 5). We first evaluated 1 as catalyst for alkynyloxirane32 and Meyer−Schuster33 rearrangements of respectively compounds 14 and 16 (Table 5, entries 1−6). Excellent yields were obtained in both rearrangements: furan 15 was quantitatively isolated from alkynyloxirane 14, and α,β-unsaturated ketone 17 was formed in 73% yield from propargylic alcohol 16 (entries 2 and 5). Both heterogeneous versions were once again as efficient and as rapid as the corresponding homogeneous versions (entry 2 vs 1 and entry 5 vs 4). Acid catalysis was examined in both rearrangements and proved very inefficient, as only minor amounts of products 15 and 17 were formed upon prolonged reaction time (entries 3 and 6). We also engaged the diyne diether 18 in a double hydroarylation−cationic cyclization process, reported to be catalyzed by both gold and acidic medium and providing dioxafluoranthene.34 However, the reported homogeneous (PPh3AuSbF6 5 mol %/triflic acid 30 mol %) as well as our heterogeneous version (POM-Au/H3 1, 5 mol %) failed to furnish the expected compound. Only the product of double hydroarylation (19) was obtained in both cases with similar yields (Table 5, entries 7 and 8). A control experiment showed that only gold cations were responsible for the hydroarylation reaction (entry 9). Finally, we investigated the cycloisomerization−hydroalkoxylation of propargyl alcohols described by Krause.35 In this tandem process, the 4-phenylbut-3-yn-1-ol 20 furnished the tetrahydrofuranyl ether 21 in 67% yield using a mixture of PPh3AuBF4

increased with the electron-withdrawing characters of the considered ligands (P(O-2,4-ditBuPh)3 > PPh3 > PMe3). Complex 5, with a trimethylphosphine ligand, was clearly the less reactive catalyst, requiring a long reaction time for both transformations (Table 3, entries 6 and 14), while the catalyst 6 exhibited faster reaction rates and similar yields compared to 1 (Table 3, entries 7 and 15 vs entries 2 and 10). In an attempt to take benefit of residual Brønsted acidities present on our gold(I)/H+-polyoxometalate catalysts 1−6, we applied them to a sequential gold/acid-catalyzed process. We selected the gold-catalyzed aza-Prins cyclization/enol ether hydrolysis described by Rhee as a benchmark reaction (Table 4).30 In the homogeneous version using the strongly Table 4. Bifunctional Catalytic Activity of [POM]x−[H]+x−1[R3PAu(MeCN)]+ 1−6 in Tandem Gold-Catalyzed Aza-Prins Cyclization/Enol Ether Hydrolysis of 11

entry 1 2 3 4 5 6 7 8

gold(I) catalyst (2.5 mol %) (C6F5)3PAuSbF6 then p-TsOH/H2O (10 mol %) SiW-Ph3PAu/H3 1 PW-Ph3PAu/H2 2 PMo-Ph3PAu/H2 3 P2W-Ph3PAu/H5 4 SiW-Me3PAu/H3 5 SiW-(ArO)3PAu/H3 6 H4SiW12O40·25H2O

time (h)

yielda (%)

2 then 24

80b26

1 0.5 0.5 1 3 0.5 24

70 75 86 67 68 89 −c

a c

Isolated yields. bTwo-step yield using homogeneous conditions. N-Tosylbuta-2,3-dienamine was isolated in 60% yield.

electron-withdrawing pentafluorotriphenylphosphine gold(I) complex, the cyclic enol ether 12 was formed from N,O-acetal 11 and was subsequently hydrolyzed by addition of paratoluenesulfonic acid and water to furnish the ketone 13 in 80% after 24 h (Table 4, entry 1). In a one-step procedure,

Table 5. Catalytic Activity of [H3SiW12O40][Ph3PAu(MeCN)] (1) Relative to Various Gold-Catalyzed Transformations

a

Isolated yields. F


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Organometallics and p-TsOH in ethanol (entry 10). With our bifunctional catalyst 1, compound 20 also rearranged to ether 21 in 63% yield, despite the longer reaction time (entry 11). This result once again clearly highlighted the multifunctionality of our catalyst 1, which was confirmed by the test with the silicotungstic acid and gold catalyst alone (entries 12 and 13). Catalyst Recycling. The recyclability was examined by performing several times the cyclization of N-propargylcarboxamide 9, which afforded the 5-methyl-2-phenyloxazole 10 at room temperature with 2 mol % of our heterogeneous gold catalysts (see Table 3).36 The catalysts could be recovered from the reaction mixture either by filtration over a nylon membrane or decantation. To avoid possible ligand and metal oxide effects, we focused on the tungstic polyoxometalates 1, 2, and 4 associated with the [Ph3PAu(MeCN)]+ cation for this study. A clear influence of the POM nature was noticed (Figure 3). Indeed, despite the excellent yield obtained in the first run

multivalent POM hybrids of type III. Moreover, the simple and efficient synthesis of the catalysts described herein offered new insights in the design of polyoxometalate-based inorganic/ organic hybrids.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

General Procedure for the Preparation of PolyoxometalateGold Complexes (1−6). To a suspension of R3PAuMe (1.0 equiv) in acetonitrile (2.5 mL) was added at room temperature a solution of heteropolyacid hydrate (1.0 equiv) in MeCN (2.5 mL). The resulting mixture was stirred at room temperature for 16 h (formation of graypurple precipitate). The solvent was then evaporated and dried under high vacuum for 24 h to give the desired catalyst. [Ph3PAu(MeCN)][H3SiW12O40] (1): 99% (349.5 mg, 0.104 mmol from 0.105 mmol of (PPh3)AuMe and H4SiW12O40); gray-purple solid; 29Si MAS+1H DEC NMR (300 MHz, solid) δ −84.2 ppm; 31P MAS NMR (400 MHz, solid) δ 44.4 (1%), 28.8 (99%) ppm; 29Si NMR (600 MHz, DMSO-d6) δ −86.1; 31P NMR (300 MHz, DMSOd6) δ 42.9 (1%), 26.8 (99%) ppm; mp = 200−205 °C (dec); IR (neat) νmax (cm−1) 686, 737, 877, 906, 967, 1017, 1102, 1435, 1480, 1682. [Ph3PAu(MeCN)][H2PW12O40] (2): 99% (348.2 mg, 0.104 mmol from 0.105 mmol of (PPh3)AuMe and H3PW12O40); gray-purple solid; 31P NMR (300 MHz, DMSO-d6) δ 42.9 (4%), 26.6 (96%), −15.6 ppm; mp = 226−236 °C (dec); IR (neat) νmax (cm−1) 686, 886, 973, 1076, 1104, 1436, 1481, 1715. [Ph3PAu(MeCN)][H2PMo12O40] (3): 99% (244.0 mg, 0.107 mmol from 0.108 mmol of (PPh3)AuMe and H3PMo12O40); gray-purple solid; 31P NMR (300 MHz, DMSO-d6) δ 42.9 (4%), 26.6 (96%), −4.1 ppm; mp = 220−230 °C (dec); IR (neat) νmax (cm−1) 687, 871, 954, 1059, 1104, 1435, 1482, 1610. [Ph3PAu(MeCN)][H5P2W18O62] (4): 98% (505.3 mg, 0.104 mmol from 0.106 mmol of (PPh3)AuMe and H6P2W18O6237); gray-white solid; 31P NMR (300 MHz, DMSO-d6) δ 42.9 (1%), 26.7 (96%), −13.5 ppm; mp = 217−227 °C (dec); IR (neat) νmax (cm−1) 685, 736, 897, 957, 1025, 1088, 1436, 1481. [Me3PAu(MeCN)][H3SiW12O40] (5): 99% (682.4 mg, 0.213 mmol from 0.215 mmol of (PMe3)AuMe and H4SiW12O40); dark gray solid; 29 Si NMR (600 MHz, DMSO-d6) δ −86.1; 31P NMR (300 MHz, DMSO-d6) δ 9.0 (14%), −12.9 (86%) ppm; mp = 215−225 °C (dec); IR (neat) νmax (cm−1) 875, 906, 969, 1019, 1310, 1416, 1619. [(O(2,4-ditBuPh)3PAu(MeCN)][H3SiW12O40] (6): 99% (320.3 mg, 0.086 mmol from 0.087 mmol of (P(OR)3)AuMe and H4SiW12O40); gray-purple solid; 29Si NMR (600 MHz, DMSO-d6) δ −86.1; 31P NMR (300 MHz, DMSO-d6) δ 85.6 ppm; mp = 203−208 °C (dec); IR (neat) νmax (cm−1) 739, 910, 966, 1017, 1074, 1209, 1489, 1615, 2868, 2904, 2956.

Figure 3. Recycling of [POM]x−[H]+x−1[Ph3PAu(MeCN)]+ 1, 2, and 4 catalysts in the cyclization of N-(propargyl)benzamide 9.

(>90%), catalyst 2, with the [PW12O40]3− polyanion, did not express any catalytic activity in the second run. Switching to catalyst 1, having the [SiW12O40]4− anion, allowed performing three runs with correct yields (respectively 75%, 72%, 54%) before being almost deactivated (10% for run 4). Finally, the best recycling ability was obtained with the Dawson catalyst 4 ([P2W18O62]6−). In this case, the catalytic activity was maintained during the fourth run, although the reaction temperature needed to be slightly increased to 45 °C to reach full conversion (80, 78, 80, and 71 for runs 1−4). 31P NMR and mass analysis in positive mode of catalyst 4 recovered after the fourth run showed almost exclusively the presence of the inactive cation [(Ph3P)2Au]+. Thus, the increase of polyoxometalate charge and size seems to stabilize gold cations and be beneficial for the recycling.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00656. Complete characterization data and experimental procedures of organic compounds (PDF) Crystal data and CIF file for 1a (CCDC 1407459) (CIF) Crystal data and CIF file for 1b (CCDC 1407460) (CIF) Crystal data and CIF file for 2a (CCDC 1407461) (CIF) Crystal data and CIF file for 3a (CCDC 1407462) (CIF) Crystal data and CIF file for 4a (CCDC 1407463) (CIF) Crystal data and CIF file for 5a (CCDC 1407464) (CIF)

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CONCLUSION In the present study, we disclosed a simple and efficient synthesis of new gold-polyoxometalate hybrids. We were able to fully characterize them. Furthermore, we also have shown that these new gold-polyoxometalate hybrids are reliable and efficient catalysts for various gold-catalyzed transformations, such as rearrangement, cyclization, and hydroarylation reactions. In all these reactions, the Au-POM catalysts behave as effectively, and sometimes more effectively, as the corresponding homogeneous catalysts. Their bifunctionality (Au+/H+) has also been highlighted and opens the way toward more sophisticated

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. G


Article

Organometallics

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ACKNOWLEDGMENTS We gratefully acknowledge the CNRS and the French Ministry of Research. A.B. and D.H. thank the Agence Nationale de la Recherche (Grant ANR-11-JS07-001-01 SyntHetAu), for respectively financial support and a Ph.D. fellowship. M.H. thanks the French Ministry of Research for a Ph.D. fellowship.

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