Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - PGOLDCAT (Polynuclear Gold Cluster Catalysis)

Teaser

Gold catalysis has been undergoing a prosperous development in the last decades, and it has been considered as one of the most effective methodologies for constructing carbon-carbon or carbon-heteroatom bonds. Due to the strong fact that gold complexes can be used as effective...

Summary

Gold catalysis has been undergoing a prosperous development in the last decades, and it has been considered as one of the most effective methodologies for constructing carbon-carbon or carbon-heteroatom bonds. Due to the strong fact that gold complexes can be used as effective π-acids for the activation of unsaturated C-C bonds, there are numerous gold(I)-catalyzed transformations have been reported. However, the main challenge in the development of enantioselective transformations by gold catalysis is due to the traditional linear two-coordinated mode of Au(I) complexes, thus making the π-activation substrates much closer to the chiral center.
With a greater awareness of the size and geometric effects in catalytic behaviours, the atomically precise nanogold clusters such as Au25 naloclusters have been growing into a promising class of model heterogeneous catalysts. An enhancement of catalytic activities of gold nanoparticles by decreasing the size has been reported. Several experimental reports have shown that small gold clusters Aun (n = 3–10) can efficiently catalyse different reactions, such as the hydration of alkynes, the bromination of arenes. Well–defined trinuclear gold species Au02AuI can catalyse the carbonylation of amines. Also, the selective activation of the C–H bond of methane by small gold clusters has been theoretically proven. However, the polynuclear cationic AuI entities still remain less explored in gold catalysis. To date, only a few of catalytically active AuIn clusters (n ≥ 3) have been reported. The trinuclear oxonium gold cluster [(Ph3PAu)3O](BF4) catalysed the cycloisomerization of 1,5-allenyenes, as reported by the group of Toste. Tetranuclear and pentanuclear gold(I)–silver(I) clusters reported by our group have been proven to be active in the carbonylation of amines, and also a hexagold(I) cluster was found to catalyse the cycloisomerization of enynes.
Herein, instead of pursuing the novel monogold complex from the view of ligand, based on the potential application of gold clusters in catalysis, we were trying to take advantage of the unique coordination mode of a hexanuclear gold clusters in the possible activation of alkynes in enyne chemistry. Therefore, our efforts were focused on developing novel gold(I) and gold(I)–silver(I) clusters with new designed TMS-phosphine ligands via AuI/Si transmetallaiton, studying their catalytic reactivity in gold-catalysed systems and applying these clusters in the exploration for further enantioselective gold catalysis.

Work performed

Firstly, as we proposed, we have synthesized a series of o-trimethylsilylphosphines (as shown in Scheme 2) and the corresponding monogold chloride complexes in Scheme 3. Secondly, by using this well-designed LAuCl complexes (L = o-TMS phosphines), we have developed a method in constructing a class of hexanuclear gold(I) clusters via a AuI/Si transmetallation methodology (Scheme 4). On the other hand, we also successfully obtained a range of hexanuclear gold(I)-silver(I) clusters through a slow AuI/Si transmetallation methodology (Scheme 5), in which an essential intermediate was found as oxonium trigold complexes (Scheme 1). This slow AuI/Si transmetallation method also led to different nuclear gold(I) and gold(I)-silver(I) clusters (Scheme 6). All these gold(I) and gold(I)-silver(I) clusters have been analysed by X-ray diffraction analysis. With the known and traditional AuI-catalyzed model reactions (Table 1 and 2), we have observed interesting structure-dependent reactivity with different size with these atomically precise gold and gold-silver clusters, and a cluster-mediated catalytic process was proposed through the kinetic studies as shown in Figure 2. These results would give insights into the discovery of new gold or gold-silver catalytic systems and shed light on the structure-property relationship in gold catalysed reactions.
We have presented our results of this research in our group seminars and also three renowned and scientific conferences:
1) XVI Biennial Meeting - Spanish Royal Society of Chemistry (RSEQ), June 25-29, 2017, Sitges, Spain. In this XVI Biennial conference, we have presented our research titled as ¨Silver Doping of Hexanuclear Gold(I) Clusters Stabilized by Hemilabile Phosphine¨ in the poster section.
2) GOLD 2018, July 15-18, 2018, Paris, France. In this GOLD 2018 conference, we have presented our work titled as ¨Anionic-Arylphosphines-Stabilized Small Gold and Gold-Silver Clusters: From Mono- to Bi-metallic Catalysts¨ in the oral presentation section.
3) ICIQ-INTECAT School, December 11-13, 2018, Tarragona, Spain. In this ICIQ-INTECAT School, we have presented our work titled as ¨ From Mono- to Bi-metallic Catalysts: Anionic-Arylphosphines-Stabilized Small Gold and Gold-Silver Clusters¨ in the oral presentation section and also poster section.
Our results on “Synthesis of Catalytically Active Anionic–aryl–phosphine–protected Hexanuclear Gold/Gold–Silver Clusters via AuI/Si Transmetallation” have attracted several specialists’ attention and recognition during those conferences, and the manuscript on this topic is under preparation.

Final results

Our project on ¨Polynuclear gold cluster catalysis¨ showed methodology of AuI/Si transmetallation is efficient in constructing novel gold(I) and gold(I)–silver(I) clusters protected with anionic-aryl-phosphines, and their interesting structure-dependent catalytic activities have been demonstrated in the gold catalysed transformations, revealing an important cluster–mediated catalytic process as indicated by the detailed kinetic studies (see in Figure 1). This work would promote the development of new thinking way of designing gold catalysts, that is not only simple gold complexes but also polynuclear gold clusters could have chance, considering that homometallic and heterometallic gold or gold-M clusters are emerging in a large of numbers these decades due to their potentially structure diversities.

Website & more info

More info: http://www.iciq.org/research/research_group/prof-antonio-m-echavarren/.