Spotting local environments in self-assembled monolayer-protected gold nanoparticles

Organic-inorganic (O-I) nanomaterials are versatile platforms for an incredible high number of applications, ranging from heterogeneous catalysis, molecular sensing, cell targeting, imaging, cancer diagnosis and therapy, just to name a few. Much of their potential stems from the unique control of organic environments around inorganic sites within a single O-I nanomaterial, which allows for new properties inaccessible using purely organic or inorganic materials. Structural and mechanistic characterization plays a key role in understanding and rationally designing such hybrid nanoconstructs. Here, we introduce a general methodology to identify and classify local (supra)molecular environments in an archetypal class of O-I nanomaterials, i.e. self-assembled monolayer-protected gold nanoparticles (SAM-AuNPs). By using an atomistic machine-learning guided workflow based on the Smooth Overlap of Atomic Positions (SOAP) descriptor, we analyze a collection of chemically different SAM-AuNPs, and detect and compare local environments in a way that is agnostic and automated, i.e. with no need of a-priori information and minimal user intervention. In addition, the computational results coupled with experimental electron spin resonance measurements prove that is possible to have more than one local environment inside SAMs, being thickness of the organic shell and solvation primary factors in determining number and nature of multiple co-existing environments. These indications are extended to complex mixed hydrophilic-hydrophobic SAMs. This work demonstrates that it is possible to spot out and compare local molecular environments in SAM-AuNPs exploiting atomistic machine-learning approaches, establishes ground rules to control them, and holds the potential for rational design of O-I nanomaterials instructed from data

Sintesi, Caratterizzazione e Applicazioni di Nanoparticelle e Nanomateriali con Controllo di Proprietà

In questo lavoro di tesi, l’approccio della nano-catalisi è stato impiegato nello sviluppo di nanomateriali catalitici a base di nanoparticelle e nano-cluster dei metalli di transizione. Nanoparticelle di Pt porose e sfaccettate sono state concepite e sintetizzate attraverso l’impiego di tecniche chimiche in fase liquida, atte a risaltare la porosità delle nanoparticelle e, nel contempo, a promuovere l’esposizione di determinati piani cristallini sulla superficie. La peculiare struttura porosa delle nanoparticelle, decorata da piccoli cristalliti sfaccettati, è stata sviluppata a partire dall’evoluzione di piccoli nuclei cristallini. Tale evoluzione durante la sintesi è stata monitorata tramite esperimenti di microscopia elettronica. Esperimenti di controllo sono stati svolti a sostegno della scelta delle condizioni di reazione adottate e a supporto della necessità di un appropriato rapporto OLAM/OLAC per garantire l’effettiva riuscita della sintesi. L’effettivo controllo delle proprietà, ottenute attraverso l’ingegnerizzazione della superficie delle nanoparticelle, è stato corroborato da test catalitici sull’elettro-riduzione dell’ossigeno e l’ossidazione di metanolo, da cui è risultata una performance superiore delle nanoparticelle porose rispetto ad un catalizzatore commerciale ed uno non poroso. (Pd8/Al2O3) è stato preparato e studiato tramite un approccio di progettazione razionale a partire dalla sintesi del complesso Pd8(SCH2COOMe)16. Il complesso di Pd(II), avente una nuclearità altamente definita, può essere fonte di cluster non protetti estremamente attivi dopo il processo di riduzione atto alla rimozione dei leganti organici e dello zolfo. In seguito alla deposizione del complesso, eseguita per irraggiamento di luce UV e con l’uso di basse concentrazioni di Pd in modo da escludere fenomeni di aggregazione, il pre-catalizzatore è stato in seguito ridotto ad alte temperature in idrogeno favorendo la generazione di cluster metallici di palladio. Approfondite caratterizzazioni, lungo tutto il processo di preparazione del catalizzatore, sono state svolte tramite l’utilizzo di tecniche TEM, XPS ed EXAFS, confermando la ridotta dimensione dei cluster e la loro natura metallica. Il catalizzatore nanostrutturato è stato sottoposto a reazioni di cross coupling di Suzuki, con bromuri arilici e acido fenilboronico, mostrando una spiccata attività e selettività. . In aggiunta, il catalizzatore si è dimostrato tollerante verso vari gruppi funzionali durante l’analisi del campo di applicabilità della reazione e molto stabile nei test di riciclo e di disattivazione causato da rilascio di specie metalliche in fase liquida. Infine, cluster di oro ad atomicità precisa sono stati applicati nella preparazione di foto-catalizzatori atti alla produzione di idrogeno da foto-ossidazione di etanolo. Questi cluster di oro a diversa atomicità sono stati impiegati in uno studio di correlazione attività/dimensione. Al livello sub-nanometrico, le proprietà elettroniche dei cluster di oro non hanno una variazione lineare con la dimensione e di conseguenza questo influisce notevolmente sulle performance catalitiche. Ne consegue che studi correlanti l’attività con la dimensione dei cluster sono prioritari in determinare il potere catalitico dei cluster metallici. In questo studio, i cluster Au11, Au18, Au23 and Au25 protetti da leganti tiolati o fosfinici, sono stati preparati ed ancorati su TiO2 utilizzando due diversi approcci basati su irraggiamento con luce UV e impregnazione diretta. I due set di catalizzatori, generati dalle diverse preparazioni, sono stati infine testati per la foto-generazione di idrogeno risultando in superiori attività e stabilità catalitiche rispetto ad un analogo catalizzatore foto-deposto usato come riferimento.In this thesis work, the nano-catalysis philosophy was applied for the development of engineered catalytic nanomaterials based on transition metal nanoparticles (NPs) and nanoclusters (NCs). Porous multifaceted Pt NPs were designed and synthesised via wet-chemical approach meant for the expression of porous features characterized by extended regular planes as surface features. The peculiar porous structure of platinum NPs decorated with faceted crystallites arose from controlled growth of small seeds through progressive evolution that could be monitored by TEM experiments. Control experiments were performed to support the selected reaction conditions as the optimal in delivering the desired features and to corroborate the needful use of a defined surfactant mixture composition for the successful NPs formation. The property control on these NPs, through surface modification, was confirmed by the catalytic tests performed in electrochemical ORR and MOR achieving superior performances compared to a commercial and a non-porous benchmark catalysts. Concerning the control of properties through size engineering, Pd8/Al2O3 catalyst was prepared and studied through a rational design approach, starting from the synthesis of Pd8(SCH2COOMe)16 complex. Unlike the conventional preparation of small metal clusters, in this work the atomicity control was attained by the use of organometallic chemistry. The Pd(II) complex, having a defined nuclearity, can provide highly active unprotected clusters once removal of ligands and reduction of Pd is performed. After complex deposition onto alumina by UV irradiation, working at low Pd concentration in order to preserve the atomicity, the pre-catalyst was reduced at high temperature with hydrogen to deliver metallic Pd clusters. Extensive characterization, through the various steps of catalyst preparation, performed by TEM, XPS and EXAFS, confirmed the small size of the clusters and their metallic nature. The nanostructured catalyst was used in Suzuki coupling reactions of aryl bromides and phenylboronic acid, resulting in very high activity and selectivity. Additionally, the catalyst was able to deliver substituted biphenyls with great tolerance toward the use of various substituted bromobenzenes and demonstrated enhanced stability as evinced by the recycling and leaching tests. Lastly, atomically precise Au NCs were applied in the preparation of photocatalysts to exploit their potential in the production of hydrogen from ethanol. Au NCs with different atomicity were employed in order to study their size influence on the activity. At the sub-nanometric level, the electronic properties of Au NCs have a non-monotonic oscillation that consequently can influence the performance of a catalyst, thus studies on the size/activity relationship are pivotal to provide a deeper understanding of the behaviour of such metal nanoobjects in catalysis. In this study Au11, Au18, Au23 and Au25 thiolate and phosphine protected clusters were synthesised and dispersed on TiO2 by two different approaches based on UV irradiation and in situ impregnation. The two sets of catalysts were tested in the photocatalytic hydrogen production resulting superior in activity and stability toward a photodeposited Au/TiO2 benchmark catalyst

Regio and Stereoselective Nickel Catalyzed Coupling of Boronic Acids with Allenoates

The [Ni(II)]-catalyzed cross-coupling of aryl boronic acids and allenoates is documented. The high regio- and stereoselectivity of the process enables a wide range of \uf062-aryl \uf062\uf02c\uf067-unsaturated esters to be prepared in good to excellent yields (up to 95%) and high E:Z-selectivity. Additionally, [3+2]-cascade sequence was observed when the 2-formyl boronic acid 2p was employed

Catalytic a-Allylation of Enones with Alcohols via [Gold(I)]- Mediated [3,3]-Sigmatropic Rearrangement of Propargylic Carboxylates

The site-selective a-allylic alkylation of enones with alcohols via gold-triggered formation of nucleophilic allenoates by means of [3,3]-sigma- tropic rearrangements of propargylic carboxylates is reported. A range of a-alkylated enones is obtained in high yields in the presence of the gold complex [JohnPhosAu(ACN)]SbF6 (2 mol%) as the promot- ing agent. Examples of allylation/Friedel\u2013Crafts al- kylation sequences are also provided, delivering densely functionalized dihydroindenes and dihydro- benzo[7]annulenes in a one-pot procedure in mod- erate yields. The role of the Br\uf8nsted acidity (i.e., pivalic acid) delivered during the reaction course in assisting the formation of carbocationic intermedi- ates is documented