Supramolecular Photocatalyst for the Reduction of Au(III) to Au(I) and High-Turnover Generation of Gold Nanocrystals

Abstract

We report a photocatalyst composed of a diphenylan thracene core appended with two lipophilic thioether side chains that binds gold(III) chloride. Upon excitation using visible light, the Au III ions are smoothly reduced to Au I which, in the presence of water, lead to the formation of crystalline gold nanoparticles of 20−50 nm diameter that are devoid of sulfur containing capping agents. Ultrafast transient absorption spectroscopy shows that the anthracene excited state is quenched with a rate k = 3.5 × 10 10 s −1 , assigned to intramolecular energy transfer to the bound gold ions, which then oxidizes the solvent to produce an intermediate low valency gold(I) species. In the absence of water, the latter is stable and can be used as a homogeneous Au I catalyst. When employed in a biphasic reactor, the photocatalyst shows average turnover numbers of 150 atoms of Au III reduced to Au 0 per molecule of photocatalyst.

Photoreduction of Thioether Gold(III) Complexes: Mechanistic Insight and Homogeneous Catalysis

“…The nature of the dialkyl thioether ligand, that is, the presence of diphenylanthracene, aryl, or alkyl, substituents, does not impact the rate of the photoinduced process. This is in agreement with our previous finding of near‐quantitative energy transfer from the excited DPA to the gold(III) center because, under these experimental conditions, all incident light is absorbed either by the DPA or by the gold(III).…”

“…New ligands L2 – L4 were prepared through straightforward routes in high yields (see the Supporting Information) and ligand L1 was synthesized according to our previous procedure . The complexes between thioethers L n ( n =1–4) and AuCl 3 were prepared by liquid/liquid extraction in the dark by dissolving the desired ligand in the organic phase and shaking with an aqueous solution of KAuCl 4 .…”

“…Previously, we showed that intramolecular energy transfer occurs within the L1⋅ 2AuCl 3 complex upon excitation of the DPA chromophore at 365–400 nm. We proposed that this energy transfer might accelerate the rapid and clean reduction to L1⋅ 2AuCl . To better understand the photoreduction mechanism of gold(III) thioether complexes, we further investigated the role of the diphenylanthracene chromophore, taking into account the plausible reductive elimination pathway ,…”

“…We previously described a 9,10‐diphenylanthracene (DPA) derivative L1 (Figure ) with two thioether groups as coordinating sites that allowed facile preparation of an AuCl 3 complex through liquid/liquid extraction . The corresponding gold complex is directly soluble in hydrophobic solvents such as dichloromethane and toluene, thereby avoiding the two‐step procedure typically used to prepare AuCl 3 complexes with ligands other than pyridine .…”

“…We previously described a9 ,10-diphenylanthracene( DPA) derivative L1 (Figure 1) with two thioether groups as coordinating sites that allowed facile preparation of an AuCl 3 complex through liquid/liquid extraction. [14] The corresponding gold complex is directly soluble in hydrophobic solvents such as dichloromethane and toluene, thereby avoiding the twostep procedure typically used to prepare AuCl 3 complexes with ligandso ther than pyridine. [4] Importantly,t he liquid/liquid extraction procedure guarantees that the L1·2AuCl 3 complex is confined to the organic phase and that it is not exposed to additional reduction processes as observedi nother reported protocols.…”