Small Gold(I) and Gold(I)-Silver(I) Clusters by C-Si Auration.

Auration of o-trimethylsilyl arylphosphines leads to the formation of gold and gold-silver clusters with ortho-metalated phosphines displaying 3c-2e Au-C-M bonds (M=Au/Ag). Hexagold clusters [Au6 L4 ](X)2 are obtained by reaction of (L-TMS)AuCl with AgX, whereas reaction with AgX and Ag2 O leads to gold-silver clusters [Au4 Ag2 L4 ](X)2 . Oxo-trigold(I) species [Au3 O]+ were identified as the intermediates in the formation of the silver-doped clusters. Other [Au5 ], [Au4 Ag], and [Au12 Ag4 ] clusters were also obtained. Clusters containing PAu-Au-AuP structural motif display good catalytic activity in the activation of alkynes under homogeneous conditions.

Introduction

Aurophilicity has a fundamental importance on the wide structural diversity of gold complexes and clusters,1 as well as on their photophysical properties2 and their catalytic transformations.3 Small gold Au clusters (n=3–10) can catalyze different reactions4 and it has been proposed that small gold clusters can activate the C−H bond of methane.5 However, polynuclear cationic AuI entities remain nearly unexplored in gold catalysis. To date, only a few catalytically active AuI clusters (n≥3) have been reported. Thus, the group of Toste found that trinuclear oxonium gold cluster [(Ph3PAu)3O](BF4) is a catalyst for the cycloisomerization of 1,5‐allenyenes6 and our group reported that tetranuclear and pentanuclear gold–silver clusters are active in the catalytic carbonylation of amines under homogeneous conditions.7

Ligands play a crucial role in the engineering of gold architectures and the tuning of their properties.8 The strategy of combining neutral phosphines and anionic ligands such as thiolate/alkynyl ligands9 has been gaining considerable attention for the construction of hetero‐ligated gold clusters, because the higher affinity between gold and anionic fragments allows accessing gold systems of high nuclearity.10 However, phosphines with both multi‐coordination ability and anionic properties have been less explored as ligands for the preparation of gold clusters.

We have described the synthesis of a single example of a hexagold cluster by a AuI/B transmetalation (Scheme 1 a), which showed catalytic activity in some cycloisomerization of enynes.11 Now we have developed a more general method by auration‐assisted C−Si bond cleavage12 from ortho‐silylphosphines gold(I) chloride complexes 1, which leads to hexanuclear gold(I) clusters [Au6L4](SbF6)2 (2) bearing different phosphine L ligands (Scheme 1 b). Interestingly, when the transmetalation was carried out in the presence of Ag2O, heteronuclear clusters [Au4Ag2L4](SbF6)2 (3) were obtained. All these hexanuclear clusters show 3c–2e Au−C−M bonds (M=Au/Ag).13 Under slightly different reaction conditions, other polynuclear gold clusters have been also obtained by C−Si auration.

Scheme 1

a) C−B auration to form hexanuclear gold clusters 2 a.11 b) C−Si auration of o‐silylphosphine gold(I) complexes 1 to form hexanuclear gold clusters 2 and gold(I)–silver(I) clusters 3. L=PR2, A=SbF6.

Oxonium trigold clusters [Au3O]+ were found to be the key intermediates in the formation of gold–silver clusters 3. Furthermore, our studies reveal that clusters containing the PAu−Au−AuP structural motif activate alkynes under homogeneous conditions, presumably as a consequence of the presence of coordinatively labile gold(I), similar to gold cavities (pocket‐like sites) that exist in the well‐studied [Au25] cluster.3b, 14

Results and Discussion

The o‐trimethylsilylaryl phosphine gold(I) complexes 1 a–e were prepared in 89–95 % yields from the corresponding o‐trimethylsilylaryl phosphines and [Me2SAuCl] in CH2Cl2 at 23 °C. Complexes 1 a–d react with 1 equivalent of AgSbF6 in MeOH–CH2Cl2 to form known 2 a 11 as well as new hexanuclear gold clusters 2 b–e, which were isolated as yellow or pale‐yellow crystalline solids in 52–81 % yields (Scheme 2).15 The reaction of complex 1 a with different silver salts AgX (X=BF4, OTf, NTf2, NO3) led to the corresponding [Au6(L)4](X)2 clusters. However, other chloride scavengers such as Cu(OTf)2, Zn(OTf)2, In(OTf)3, TMSOTf, or Sc(OTf)3 were not effective. When using NaBArF 4, the corresponding hexanuclear gold species could be observed by 31P NMR.

Scheme 2

Hexanuclear gold clusters 2 a–e, obtained by C−Si auration from 1 a–e. A=SbF6. Fur: furyl. 1,2‐C10H4 and 2,1‐C10H4 derived from 1‐diphenylphosphino‐2‐trimethylsilylnaphthalene and 2‐diphenylphosphino‐1‐trimethylsilylnaphthalene, respectively. Counteranions and solvent molecules are omitted for clarity.

Complexes 1 a–e derived from triaryl phosphines show singlets in their 31P{1H} NMR spectra at 32.3–37.8 ppm in CD2Cl2, whereas the corresponding signals for dialkyl(o‐trimethylsiliylaryl)phosphine and di(2‐furyl)(o‐trimethylsiliylaryl)phosphine gold(I) complexes 1 d and 1 e were observed at 47.10 and −5.12 ppm, respectively. The resulting hexanuclear clusters showed their 31P signals shifted downfield by 8–24 ppm: 2 a–c (44.6–48.2), 2 d (71.50), and 2 e (3.15).

New clusters 2 b–e were characterized by X‐ray diffraction, showing a pseudo‐octahedral geometry with six gold atoms stabilized by only four formally anionic‐phosphine ligands, different from the hexanuclear gold clusters.16, 17 Compared to aurophilic interactions of 2.706(4)–3.351(4) Å found in 2 a,11 the Au−Au bonds lie in the range of 2.689(7)–3.162(8) Å in 2 b, 2.718(6)–3.352(6) Å in 2 c, 2.726(4)–3.325(4) Å in 2 d, and 2.709(2)–3.341(2) Å in 2 e, respectively. In clusters 2 b–c, two naphthyl groups stabilize the gold atoms in the 3c–2e Au−C−Au bonds, whereas two phenyl groups are involved in the 3c–2e bonds in clusters 2 d–e. The average C‐Au‐C angles in 2 b (159.6(6) °) and 2 c (162.1(4) °) are slightly bigger than those of 2 a (159.1(3) °). In clusters 2 d and 2 e, the average C‐Au‐C angles are 157.8(3) and 162.2 °, respectively.15

Reaction of 1 a with excess of silver(I) salts did not result in the formation of gold–silver clusters. However, addition of excess AgSbF6 to a suspension of 1 a and 1 equivalent of Ag2O in CH2Cl2 led to the formation of heterometallic cluster 4 a (Scheme 3). Notably, cluster 4 a rearranges in the presence of acetone to form cluster 3 a, in which each of the silver atoms coordinates with a molecule of acetone. Similar behavior was observed in acetonitrile or methanol. Upon removal of the coordinating solvent under vacuum, 3 a was slowly converted into 4 a. Two related gold–silver clusters 3 b and 4 c were also obtained and structurally characterized. In the silver‐doped hexanuclear gold–silver clusters 4 a and 4 c, silver atoms substitute the two axial positions, forming 3c–2e Au−C−Ag bonds.15 Due to the argentophilic interactions in 4 a and 4 c (ca. 2.9 Å), the hexanuclear heterometallic cores are distributed as distorted octahedrons with edge‐sharing bi‐tetrahedral geometry, similar to the structure of hexanuclear nanogold cluster [(p‐C6H4MeP)6Au6](NO3)2.17 The ligand coordination in 3 a and 3 b is very different. In cluster 3 a, four gold atoms are linked together by the R2P group and the aryl rings, displaying the same coordinating mode as in hexagold(I) analogues 2 a–e. The structure of 3 b is more distorted and can be viewed as the fusion of two binuclear gold complexes bridged by two silver atoms by Au−Ag interactions.

Scheme 3

Hexanuclear gold–silver clusters 3 a,b, 4 a,c, and 5 a′ by C−Si auration in the presence of Ag2O. A=SbF6, A′=NTf2. Hydrogen atoms in 5 a’, counteranions and solvent molecules are omitted for clarity.

Clusters 4 a and 4 c display doublets at 46.5 and 40.3 ppm, respectively in the 31P{1H} NMR spectra, whereas 3 a and 3 b show triplets at 52.2 and 55.8 ppm, respectively.

Hexadecanuclear heterometallic cluster 5 a was obtained as a byproduct in the preparation of 4 a (Scheme 3). This cluster could also be accessed from a complex [(L)Au(NEt3)](SbF6) 1 a′, prepared by reaction of 1 a with AgSbF6 in the presence of excess NEt3. Treatment of 1 a′ in CH2Cl2 containing excess water with AgSbF6 (2 equiv) led to 5 a in 47 % yield. Cluster 5 a′ was prepared similarly using AgNTf2 instead of AgSbF6. The structure of 5 a' shows 3c–2e Au−C−Au and Au−C−Ag bonds, as well as an interesting μ4‐O2− coordinating mode, which to the best of our knowledge, corresponds to the highest‐nuclei oxo‐bridged gold–silver assembly among the known oxo‐gold clusters.15, 18

Interestingly, treatment of 1 c with NaSbF6 and Ag2O led to pentanuclear gold(I) cluster 6 in 69 % yield,19 along with pentanuclear gold(I)–silver(I) 7 as a minor product (ca. 5 %) (Scheme 4). Cluster 7 could be obtained from 1 c in 57 % yield using 0.25 equivalents of AgSbF6.15 Moreover, digold complex15, 20 8 was obtained by reaction of 1 c with AgOAc through the initial formation of neutral [(L‐TMS)Au(OAc)]. The addition of excess AgSbF6 to a solution of 8 in CH2Cl2 led quantitively to the formation of cluster 4 c.

Scheme 4

Clusters 6–8 obtained from gold(I) complex 2 c. A=SbF6. Counteranions and solvent molecules are omitted for clarity.

We confirmed that the reaction of [Ph3PAuCl] with Ag2O and AgSbF6 gives oxonium gold cluster [O(AuPPh3)3](SbF6),21 which suggests that similar oxonium gold complexes might be involved as intermediates in the formation of the gold–silver clusters. Indeed, upon addition of AgSbF6 to a mixture of 1 a and Ag2O in CD2Cl2, a new species corresponding to 9 a was formed, which reacted further to finally form 4 a (Scheme 5). Oxonium gold cluster 9 a was isolated as a white solid and its structure was confirmed by mass spectrometry (m/z 1609.2049). Starting from 1 d, a similar oxonium trigold(I) complex 9 d was obtained, whose structure was determined by X‐ray diffraction.15, 22

Scheme 5

Oxonium gold intermediates 9 a,b from 1 a,d. A=SbF6. Counteranions and solvent molecules are omitted for clarity.

Presumably, reaction of 1 a–d complexes with AgX salts leads to complexes [(L‐TMS)Au(S)]+X−, which immediately evolve to form hexanuclear gold(I) clusters 2 a–d. On the other hand, when the reactions are performed in the presence of Ag2O, oxonium trigold complexes 9 are formed as intermediates, which are less reactive in the C−Si auration and, as a result, can incorporate silver(I) leading to the formation of gold(I)–silver(I) clusters.

Hexagold clusters 2 a–d are stable in solution. Thus, cluster 2 a was recovered unchanged upon recrystallization in acetonitrile as well as after being heated at 80 °C in 1,2‐dichloroethane for 24 h. Thermogravimetric analysis shows that clusters 2 a, 2 e, 3 a, and 4 a only undergo decomposition in the solid state at high temperatures (230–270 °C).23 However, treatment of 2 a with PPh3 in CH2Cl2 led to the formation of a known digold complex similar to 8.19 In the case of pentagold cluster 6, the 31P NMR signals became broad in acetonitrile solution, although 6 was recovered unchanged after crystallization in this solvent.

The catalytic activity of the new clusters was studied in the addition of 1,3,5‐trimethoxybenzene, indole, and N‐methylindole to 1,6‐enyne 10 to give cycloadducts 11 a–b regio‐ and stereoselectively (Table 1).24 Clusters 2 a–e showed activities in the order 2 e>2 c>2 a≈2 b>2 d (Table 1, entries 1–9). These results correlate with the electronic and steric properties of the ligands, since 2‐furyl group in 1 e is the most electron‐withdrawing and less bulky phosphine substituent. Remarkably, 1 mol % of cluster 2 e led to 11 a in 95 % yield in about 3 h (entry 7), showing a catalytic activity comparable to that displayed by a bulky phosphite gold(I) complex (5 mol %, 2 h, 66 % yield),24 which is one of the most reactive gold(I) complexes used routinely in the activation of alkynes. The catalyst loading with 2 a and 2 d could be decreased to 0.05 mol % maintaining good conversions.23 Cluster 2 e was also found to be the most reactive23 for the formation of indenes from 7‐phenylethynyl cycloheptatriene,25 and for the formal [4+2] intramolecular cycloaddition of arylalkynes with alkenes.26

Table 1

Addition of aromatic and heteroaromatic nucleophiles to 1,6‐enyne 10 to form 11 a–c catalyzed by gold or gold–silver clusters.

Entry	NuH	Catalyst	Time [h]	11 a–c (yield, %)[a]
1	ArH	2 a	20	11 a (76)
2	IndH	2 a	20	11 b (40)
3	MeIndH	2 a	16	11 c (76)
4	ArH	2 b	12	11 a (71)
5	ArH	2 c	12	11 a (87)
6	ArH	2 d	12	11 a (3)
7	ArH	2 e	3.3	11 a (98, 95[b])
8	IndH	2 e	8	11 b (79)
9	MeIndH	2 e	8	11 c (67)
10	ArH	4 a	4	11 a (98)
11	ArH	3 a	4.5	11 a (97)
12	ArH	3 b	9	11 a (99)
13	ArH	4 c	6.5	11 a (96)
14	ArH	6	12	11 a (<1)
15[c]	ArH	6+NaBArF 4	12	11 a (99)
16	ArH	7	12	– [d]
17[c]	ArH	7+NaBArF 4	12	–[d]
18	ArH	8	24	–[d]
19	ArH	5 a [e]	3.5	11 a (93)

[a] Yields determined by 1H NMR using 1,3,5‐tris(trifluoromethyl)benzene as internal standard. [b] Isolated yield. [c] Reaction in the presence of NaBArF 4 (10 mol %). [d]<1 % yield. [e] Catalyst loading 0.05 %.

Clusters 3 a,b and 4 a,c display higher catalytic activity than the corresponding hexagold congeners probably due to the structure effect by silver doping27 (Table 1, entries 10–11). In contrast, the reaction with cluster 6 led only to traces of 11 a (entry 14), although the reactivity could be restored in the presence of NaBArF 4 (entry 15), whereas NaBArF 4 by itself does not promote this transformation. However, cluster 7 was unreactive even in the presence of NaBArF 4 (entries 16 and 17). Digold complex 8 showed no reactivity (entry 18). Hexadecanuclear cluster 5 a [Au12Ag4] also displays good activity (0.05 mol %, 3.5 h, 93 % yield of 20 a) (entry 19). Similar catalytic activity was found with enynes bearing internal alkynes.23

Homometallic gold clusters 2 a–c, 2 d, and 6 did not undergo decomposition when the corresponding reactions in Table 1 were monitored by 31P NMR spectroscopy.23 On the other hand, the gold(I)–silver(I) clusters underwent slow decomposition, which might explain why heteronuclear clusters 4 a and 4 c, which present only PAu−Ag−AuP motifs, are also catalytically active (entries 10 and 13). In these cases, as suggested by 31P NMR, decomposition or structural rearrangement of 4 a and 4 c to generate an active gold(I) species probably takes place in solution.

To probe our hypothesis that the catalytically active site is the central gold atom in the PAu−Au−AuP structural motifs, we also examined the reactivity of known full‐phosphine‐protected gold and gold–silver clusters [Au6C],16a [Au6Ag4C],16b nanogold clusters nano‐[Au6]19 and [Au13].28 However, none of these species displayed catalytic activity in the addition of 1,3,5‐trimethoxybenzene to 1,6‐enyne 10 at 25 °C.23

Conclusions

We have found that the auration of trimethylsilyl phosphines leads to the formation of well‐defined small gold and gold–silver clusters containing 3c–2e Au−C−M (M=Au/Ag) bonds. On the other hand, when the chloride abstraction of complexes [(L−TMS)AuCl] was performed with AgSbF6 in the presence of Ag2O, hexanuclear gold–silver clusters [Au4Ag2]2+ were obtained. Trinuclear oxonium gold species [Au3O]+ acts as the intermediate in this silver‐doping process, which takes place due to a slower C−Si auration process. Other clusters [Au5], [Au4Ag], [Au2] and [Au12Ag4] have also been obtained. The activity of these small gold clusters has been studied in typical AuI‐catalyzed reactions of enynes. Remarkably, hexanuclear gold cluster 2 e with difurylphosphine ligand displays a reactivity similar or even higher than other commonly used mononuclear gold catalysts.

Conflict of interest

The authors declare no conflict of interest.

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