Various reactions catalyzed by NP networks have been investigated such as reduction, oxidation, or water-splitting, most of them focusing on Au NP, and also few other metals (Ag, Pd, Pt, Ru). Generally, there is a lack of attention paid on the catalytic application of metal NP covalent assemblies, and this despite of the interesting properties of such assemblies for catalysis, for instance: (i) a confined environment, (ii) the possibility to finely tune the metal/ligand interaction, and (iii) the potential robustness of the structure. Additionally, the formation of reversible covalent networks is also discussed, which allows switching between a covalent network of NP and isolated NP by applying diverse stimuli. Two types of strategies to produce NP assemblies are discussed in this chapter: (i) the direct cross-linking method, which is simple and well-controlled, and involves a chemical reaction between the metal nanoparticle surface and the ligand and (ii) the indirect cross-linking method in which the chemical reaction necessary for network building does not directly involve the surface of the metallic nanoparticle. In these structures, the ligands play a fundamental role on constructing the NP network and defining their chemical environment. Metal nanoparticles’ (NP) covalent assemblies exhibit interesting structural, electronic, and photonic features of interest for applications in catalysis. These nanocatalysts, which as such ease organic products separation, also provide a convenient access for building further polycyclic complexity, owing to their high reactivity and selectivity. Thus, sub-2-nm nanoparticles originating from networks building create convenient conditions for generating reactive Au(I) surface single-sites-in the absence of silver additives-useful for heterogeneous gold-catalyzed enyne cyclization. The modifications at bulky stabilizing ligands allow surface steric decongestion for the alkyne moiety activation but also result in network alteration by overoxidation of sulfurs. Our experimental studies and DFT analyses highlighted the necessary oxidative surface reorganization of individual nanoparticles for an effective enyne cycloisomerization. The analysis of gold NP surfaces and their modification were achieved in joint experimental and theoretical studies, using notably XPS, NMR, and DFT modeling. These NP arrays are organized alongside short interparticular distances ranging from 1.9 to 2.7 nm.
By using 1,1′-bisadamantane-3,3′-dithiol (BAd-SH) and diamantane-4,9-dithiol (DAd-SH), serving both as bulky surface stabilizers and short-sized linkers, we provide a simple method to form uniformly small gold NPs (1.3 ± 0.2 nm to 1.6 ± 0.3 nm) embedded in rigid frameworks. Herein, the control of the synthesis of sub-2-nm gold NPs is achieved by the formation of dense networks, which are assembled in a single step reaction by employing ditopic polymantanethiols. The ability to assemble nanoparticles with controllable sizes and shapes within networks concerns research in sensors, medical diagnostics, information storage, and catalysis applications. This control over selectivity further opened the way to one-pot cascade reaction, as illustrated by the 1,6-enyne cycloisomerization–Diels–Alder reaction of dimethyl allyl propargyl malonate with maleic anhydride. Such reaction usually suffers from selectivity issues with homogeneous catalysts. Ultrasmall gold nanoparticles (NPs) stabilized in networks by polymantane ligands (diamondoids) were successfully used as precatalysts for highly selective heterogeneous gold-catalyzed dimethyl allyl(propargyl)malonate cyclization to 5-membered conjugated diene. Extending reversibility to covalently-bound NC assemblies creates new possibilities in plasmonic sensing and catalysis. This method for reversible self-assembly of covalently cross-linked Au NCs is simple, clean, reproducible, and effective for a variety of linkers. The process is repeatable by adding additional dithiol to re-initiate aggregation, although some irreversible fusion of the NCs occurs in subsequent cycles. Destruction of the linkers results in the complete depolymerization of the NC aggregates/precipitates to re-form stable aqueous colloids of individual Au NCs with retention of the original NC size distribution. Here we demonstrate the reversible assembly and disassembly of dithiolate-linked precipitates and colloidal aggregates of Au NCs by oxidizing the dithiolate linkers with ozone. Left unchecked, dithiol-mediated aggregation results in the uncontrolled formation of polymeric NC-dithiolate precipitates.
Covalent cross-linking of colloidal gold nanocrystals (Au NCs) with dithiol molecules is normally considered to be an irreversible self-assembly process.
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