Metal and Metal Oxide Nanoparticles: A Lever for C–H

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Metal and metal oxide nanoparticles, a lever for C–H functionalization Daniel Pla, and Montserrat Gómez ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00684 • Publication Date (Web): 26 Apr 2016 Downloaded from http://pubs.acs.org on May 4, 2016

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Metal and metal oxide nanoparticles, a lever for C–H functionalization.

Daniel Pla,†,* Montserrat Gómez.‡ ,* †

Laboratoire de Chimie de Coordination, Centre National de la Recherche Scientifique UPR

8241, 205 Route de Narbonne, F-31077 Toulouse Cedex 4, France. ‡

Laboratoire Hétérochimie Fondamentale et Appliquée, Université de Toulouse 3 – Paul

Sabatier, UPS and CNRS UMR 5069, 118 route de Narbonne, F-31062 Toulouse Cedex 9, France.

ACS Paragon Plus Environment

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ABSTRACT. The last decade has seen the development of a new reactivity concept, exploiting the ubiquity of C–H bonds. The direct activation of this bond type brings forth a straightforward way for the derivatization of organic molecules. Consequently, the design of metal catalyzed C– H bond functionalization reactions has become a flourishing area in organic synthesis. Thus, this review focuses on molecular transformation of “unactivated” C(sp3)–H and C(sp2)–H bonds present in feedstock and readily available hydrocarbons into valuable chemicals by nanoparticleleveraged C–H activation, resulting in more sustainable approaches for industrial applications. The distinctive reactivity properties of these nano-entities lead to remarkable reactivity specificities depending on the type of surface sites and the structure dynamics, directly impacting the process selectivity.

Keywords: C–H activation, metal nanoparticles, catalysis, organic synthesis, heterocycles, methodology development, reaction mechanism.


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INTRODUCTION.

The use of catalysis to control the synthesis of architecturally complex molecules is a key aspect for the future of organic chemistry. More than ever before, it is essential to develop new chemistry that enables the preparation of highly valuable compounds such as new materials, drugs, sensors or sources of energy in an efficient and cost-effective way by minimizing the generation of waste as well as overriding costly steps for the preparation of the prefunctionalized building blocks currently employed in cross-coupling methods (Fig. 1).

Figure 1. Comparison between conventional cross-coupling reactions and direct C–H bond functionalization reactions.

The term activation is usually applied in the sense defined by Shilov and Shul’pin: “when we refer to the activation of a molecule, we mean that the reactivity of this molecule increases due to some action. The main result of the activation of a C–H bond is the replacement of a strong C–H bond with a weaker, more readily functionalized bond”.1 Typically the high bond dissociation energies required to effect direct cleavage of C−H bonds, ranging from 96-105 kcal/mol for branched and linear alkanes, to 113 kcal/mol for arenes, classify ubiquitous hydrocarbon motifs amongst the most challenging to undergo cleavage.2 The term C−H bond activation implicitly


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denotes the elementary step in which a C−H bond is converted into a C−M (by action of a metallic reagent or a metal-based catalyst).3 The emerging area of C–H functionalization provides new synthetic tools which are changing the way molecules are made.4 The direct introduction of a new functionality (C–C, C–heteroatom) via direct C–H bond transformation is a highly attractive strategy in synthesis. The ubiquitous nature of C–H bonds in hydrocarbons (alkanes, arenes and polyarenes), complex organic compounds of small molecular weight, pharmaceuticals and synthetic and biological polymers, makes the range of synthetic possibilities virtually limitless.5 Exploring pathways for C–H bond activations will revolutionize the rules that have governed strategies for assembling molecules in the last century. Many mechanistic modes exist for an overall C–H functionalization process (e.g. radical, ionic, metal insertion); of particular importance is the use of transition metal catalysis, as it enables the activation of C–H bonds through a proximity effect.6 Various transition metals, e.g. Pd, Ru, Rh, Cu, and Fe, have been studied in different types of C−H activation and oxidative coupling reactions. In particular, Pd-catalyzed C−H activation coupling reactions, as well as oxidative amination, acetoxylation and halogenation reactions have emerged as efficient synthetic methods for the construction of C−C and C−heteroatom bonds. As a result, considerable efforts in transition-metal catalysis have culminated in pioneering discoveries over the past decade, such as the use of supported metal nanoparticles (NPs) as immobilized catalysts in the absence of ligands to promote the functionalization of C(sp2)−H bonds.7 Furthermore, a review and perspective on for quasiheterogeneous catalysis in C−H bond functionalization processes involving Pd has recently been reported by Fairlamb.8 However, fewer contributions concerning the activation of non-activated C(sp3)–H bonds have been reported to date.9 Present use of transition metal-NPs for the metalcatalyzed activation of C−H bonds via key metallacycle intermediates still remains in its


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infancy.10 Indeed, in comparison to C(sp2)–H, the C(sp3)–H bond does not possess the empty low energy orbitals or the filled high-energy orbitals to interact with the appropriate orbitals of the metal center. Consequently, the activation of C(sp3)–H bonds persists as a great challenge for organic chemists. The research in this area reveals Pd as the metal of choice for this type of transformation showing that the most successful method involves cyclometallation via 5- and 6membered ring systems like the pioneering works (Fig. 2).11

Figure 2. State-of-the-art C(sp3)–H bond functionalization promoted by Pd-based homogeneous catalysts.

The recently coined term “nanocatalysis” combines colloidal catalysis (metallic nanoparticles dispersed in solution which are responsible for the reactivity induced) and catalysis on engineered nano-objects, where nanoparticles accommodate ligands, which can modify the pathway and selectivity of catalyzed organic transformations.12

The kinetic stabilization of metallic NPs (MNPs) entails the choice of suitable ligands (specific scaffolds adapted to coordination at the metallic surface) and solvents involved in the catalytic process. Both factors play crucial roles towards preserving the nanometric species in the


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medium, by avoiding both agglomeration and leaching of molecular species.13 From a mechanistic point of view, catalysis with MNPs can either take place on the surface of the nanoparticle (heterogeneous mechanism, Fig. 3) or in solution by atoms or ions leached from the nanoparticle surface (homogeneous mechanism, Fig. 3).14 The virtually limitless possibilities of MNPs in terms of tunable reactivity arise not only benefit from the intrinsic properties of zerovalent metal centers and combinations thereof, but also their metal oxides and metal salt structures featuring bond polarization between the oxidized metal centers and the heteroatoms due to their electronegativity differences.15

Figure 3. Kinetically stable metal nanoparticles and related processes in wet phase using stabilizers to avoid the agglomeration phenomena. As stated by Somorjai and co-workers,16 enzymatic, homogeneous (organometallic complexes) and heterogeneous (metallic surfaces) catalysis bear in common the molecular understanding of the observed reactivity. Adapted experimental techniques and theoretical calculations permit the “catalytic” community to rationalize the different behaviors observed. The use of transition metal nanoparticles as catalysts exhibit important structural differences in relation to extended metallic


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surfaces (classical heterogeneous catalysts).17 For MNPs smaller than 1 nm, their reactivity is closer to that observed with molecular species. For larger MNPs (< ca. 5 nm), the giant clusters use to adopt structural arrangements corresponding to regular polyhedra (cuboctahedron, icosahedron…), constituted by terraces, corners, edges and also step sites, and their reactivity is closely related to the nature of sites depending on the type of activation. As introduced, early-on, by Boudart,18 reactions can be structure sensitive or structure insensitive. In particular and on the basis of calculated activation energies of elementary reactions, the dissociation energy for methane (CH4 → CH3 + H) is structure sensitive, observing a rate decrease with the increase of MNPs size. However, the reverse reaction (CH3 + H → CH4) is slightly structure insensitive. This complementary behavior is often observed for reactions involving activation of σ bonds. The recent advances in computational techniques offers tools to understand and rationalize more complex transformations, such as dehydrogenation reactions of alkanes.19 Notably, a comprehensive mechanism of the C–H bond cleavage step by the concerted metallation– deprotonation (CMD) pathway has been demonstrated to play an important role in homogeneous catalytic systems.20

In the present review, we focus our attention on the functionalization of unactivated C−H bonds of hydrocarbons (alkanes, alkenes) by metal and metal oxide nanoparticles, to give value-added organic molecules. Combustion processes of hydrocarbons and processes involving activated C−H bonds, such as those from terminal alkynes, are out of the scope of the present contribution. The efficiency of the different processes is discussed with a critical view of the attainable transformations. If experimental evidence is provided, catalyst recycling and mechanistic aspects on the active nature of the catalytic species (heterogeneous versus homogeneous behavior), are discussed as well.


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C(sp3)–H bond activation Since the pioneering works independently reported by Bergman21 and Graham22 concerning the C–H bond activation of unreactive saturated hydrocarbons promoted by Ir-based organometallic complexes under photochemical conditions, many strategies have been devoted to the functionalization of alkanes involving homogeneous catalysts.3-4,19a More recently metal-based nano-catalysts have found sustainable applications for the conversion of easily available feedstocks into high added-value chemicals. The huge progress on modeling and theoretical calculations of nano-clusters23, together with cutting-edge characterization techniques of materials24 permits the rationalization of observed reactivity; in particular for the unreactive C–C and C–H bonds of alkanes.

In this section, we describe an overview of alkane transformations promoted by metal nanoparticles; combustion processes are excluded.

Non-activated alkanes

Metal-catalyzed oxidation processes of non-activated alkanes exhibiting high activity and high selectivity represents a hot current research topic.25 Due to the large activation energy required to break C(sp3)–H bonds, harsh conditions (high temperatures and high pressures) are required, often leading to a poor control of the selectivity. Therefore, the search of efficient catalysts showing remarkable thermal stability and inducing high selectivity becomes a crucial purpose.

Methane, the lightest alkane, comes from natural gas and also is generated from biodegradable resources. Currently, methane is mostly used as a fuel for energy production purposes. However in the last 30 years, much research has been focused on the aim of developing catalytic processes


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for the direct and environmentally friendly conversion of methane into value-added chemicals,26 avoiding the co-production of carbon dioxide. The most relevant actions concern the oxidative coupling of methane to give ethane and/or ethylene and the oxidation of methane into methanol and/or formaldehyde; in this context, nano-catalysts have shown promising applications. Nanosized SrCoOx perovskites were prepared by micro-emulsion templating methodology (Sr:Co:O ratio = 1:1:2.9), affording relatively small nanoparticles with narrow size distribution (ca. 38±2 nm or 44±2 nm depending on the calcination temperature, 1023 K or 1073 K, respectively), in comparison with other synthetic approaches (co-precipitation, impregnation, precipitation deposition).27 Methane oxidation (20% CH4 and 10% O2, diluted in 70% He) catalyzed by these perovskites led to a 7% C2-selectivity (ethylene + ethane), significantly higher than those obtained with SrCoOx perovskites obtained by other synthetic methodologies (less than 2% C2selectivity). These results agree with the molecular theory based on calculated activation energies of elementary reactions; this approach points to a NPs structure sensitivity for activation of C–H bonds of methane: increase of reaction rate with decrease of particle size.17 In consequence, MNPs-catalyzed methane activation processes are clearly dependent on the presence of edge, corner and nearby sites, which strongly bind CHx intermediates inducing a diminution of activation barriers. This structural arrangement cannot be expected when stepped surfaces or (111) single crystals are involved.28

Heavier alkanes can be oxidized to alcohols and/or carbonyl derivatives depending on the catalyst nature. AuNPs intercalated into the walls of mesoporous silica proved highly efficient in the oxidation of n-hexadecane to ketones and alcohols, achieving more than 50% conversion with a total selectivity to ketones and alcohols of 71%, using air as oxidant, at 150 °C for 6h.29 The catalytic system could be recycled up to 3 times showing a slight loss of activity. This


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activity is higher in relation to other gold supported catalysts, probably due to the smaller sizes of AuNPs and the ordered mesoporous structure of the support.

The selective oxidation of cyclohexane to cyclohexanone and cyclohexanol denotes a particular importance in the challenge of C(sp3)–H bonds activation because of the industrial impact in the production of nylon-6 and nylon-66 polymers. At present, commercial processes involve high temperatures, high pressures, low yields and low selectivity. AuNPs supported on VOHPO4·0.5H2O (VHP) were applied in cyclohexane oxidation under relatively low temperatures (70 °C), using hydrogen peroxide as the oxidant and acetonitrile as the solvent.30 Conversion was up to 68% with a cyclohexanol/cyclohexanone combined selectivity of 84% (cyclohexanol:cyclohexanone ratio ≈ 2:1); the nanosized catalyst could be recycled five times with negligible activity loss. A highly selective process was achieved with photocatalytic composites constituted by AuNPs and carbon quantum dots (CDQs), using H2O2 as the oxidant under solvent-free conditions.31 Cyclohexanone was exclusively obtained at ca. 64% conversion; after 10 runs, Au/CDQs catalyst preserved its catalytic behavior. Analogous Ag- and Cu-based catalytic systems were also active, but less efficient than those constituted by gold (Fig. 4). The selectivity observed can be explained by the proposed mechanism, which involves enhancement of light absorption by surface plasma resonance of AuNPs, generation of oxygen radicals (coming from the H2O2 activation) and the interaction of AuNPs with carbon quantum dots under photochemical conditions.


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Figure 4. (a) Transmission Electron Microscopy (TEM) image, X-ray Diffraction pattern (red overlay at the top of the image) and energy-dispersive X-ray spectrum (red overlay at the bottom of the image) of AuNPs/CQDs composite; (b) High-Resolution TEM image of AuNPs/CQDs composite; (c) Conversion and selectivity of MNPs/CQDs composites in the oxidation of cyclohexane under photochemical conditions. Adapted with permission from reference [31]. Copyright 2014, American Chemical Society.

AuNPs supported on gadolinium-doped titania (AuNPs mean size ca. 3 nm) were active and selective in the oxidation of methylcyclohexane to methylcyclohexan-1-ol and also in alkane/alkene co-oxidations such as for stilbene/methylcyclohexane, using tert-butyl hydroperoxide (TBHP) as the oxidant; in the latter case, a mixture of trans-stilbene oxide and methylcyclohexan-1-ol was obtained.32 The nature of the support triggers an effect on the overall reactivity patterns through mass transfer restrictions.


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Size-controlled Co3O4 nanoparticles were prepared by hydrothermal methodology, giving 6 and 12 nm NPs by tuning the ethanol to water ratio.33 Both types of mixed oxides exhibit spinel structures as proven by HAADF-STEM analyses; they were further supported on alumina. The as-prepared catalytic materials were applied in the oxidative dehydrogenation of cyclohexane. The selectivity was dependent on both reaction temperature and nature of catalyst (Fig. 5). Therefore, at lower temperatures (225 °C) mainly CO2 was obtained using 6 nm Co3O4, while cyclohexene was favored with 12 nm Co3O4. At higher temperatures (300 °C), benzene was preferentially formed with the smaller NPs while the larger ones mainly yielded a mixture of benzene and cyclohexene. Density functional theory (DFT) theoretical studies indicated that the smaller Co3O4NPs (with more edge and corner sites) lead to more strongly held and less basic O sites, resulting in lower dehydrogenation (total oxidation is then favored). However larger Co3O4NPs have more weakly held O placed on terrace sites, which leads to lower C–H activation barriers, in agreement with the reactivity observed.


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Figure 5. Product selectivity for Co-based catalysts in the oxidative dehydrogenation of cyclohexane: Co3O4NPs of 6 nm (a-c) and 12 nm (d-f) at different temperatures (225 °C (a,d) and 300 °C (b-c, e-f)) after 1h of reaction (a,c,d,f) and also initially setting the temperature to 300 °C (b,e). Reprinted with permission from reference [33]. Copyright 2012, American Chemical Society.

PtNPs supported on mesoporous zeolite MFI led to an active and highly selective catalyst for the hydrogenative reforming of methylcyclopentane giving mainly benzene, via ring enlargement followed by dehydrogenation.34 When BEA zeolite was employed instead of MFI, cyclohexane was the main product. For both processes, cyclohexene was identified as an intermediate. However using silica as a support, the ring enlargement process was not favored and the products corresponding to ring-opening and isomerization processes were then preferred. In the absence of PtNPs, MFI and BEA zeolites mainly led to ring-opening/isomerization and dehydrogenation products respectively (Fig. 6). Therefore, well-adjusted assembly of MNPs and supports can lead to the design of outstanding selective catalytic materials, as also proven in the selective isomerization of n-hexane involving PtNPs-based catalytic materials using ordered macroporous oxides (SiO2, Al2O3, TiO2, Nb2O5, Ta2O5, ZrO2).35


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Figure 6. (a) Schematic reaction pathways of catalytic hydrogenative reforming of methylcyclopentane (excluding cracking processes); (b) Product selectivity for different catalysts in the hydrogenative methylcyclopentane reforming. Adapted with permission from reference [35]. Copyright 2014, American Chemical Society.

Alkyl aromatic derivatives

The oxidation of alkyl lateral chains of aromatic compounds leading to benzylic alcohols, benzaldehydes or aromatic esters represents a sustainable methodology for the utilization of feedstocks. Conventional heterogeneous catalysts usually exhibit low selectivity, requiring activated oxygen donors.


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Bimetallic AuPd catalysts have found applications in several oxidation processes, showing high activities and selectivities.36 In this context, Hutchings and co-workers reported AuPdNPs alloy supported on TiO2 as a catalyst for the selective oxidation of toluene to benzyl benzoate.37 Based on these results, Luques’s group described the catalytic synthesis of unsymmetrical aromatic esters by oxidation of methyl aromatics with aliphatic alcohols using dioxygen as the oxidant at high temperatures (120 °C).38 In this work, preformed AuPd alloy nanoparticles supported on MIL-101 (metal-organic framework constituted of chromium terephthalate) were used as efficient and reusable catalysts (up to 3 times), which led to a large scope of esters from moderate to high yields (45-90%, except for long-chain alcohols and bromo-substituted toluene) (Scheme 1).

Scheme 1. Oxidative esterification of methyl aromatic substrates with alkyl alcohols catalyzed by AuNPs on MIl-101.38

Bimetallic Ir3Bi and Ir5Bi3 nanoparticles were obtained from thermal decomposition of welldefined clusters, [Ir3(CO)9(µ3-Bi)] and [Ir5(CO)10(µ3-Bi)2(µ4-Bi)] respectively, in the presence of a mesoporous silica-based support (MCM-41).39 These bimetallic NPs exhibited higher activity and selectivity for the oxidation of 3-picoline to niacin (pyridine-3-carboxylic acid) using acetylperoxyborate as the oxidant (which gives peroxyacetic acid in situ under reaction conditions), than the corresponding monometallic counterparts (Fig. 7). For monometallic BiNPs


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the main product corresponds to the N-oxidation of 3-picoline; for IrNPs, low conversions and selectivity were observed.

Figure 7. Schematic representation of the different catalytic behaviors depending on the catalyst nature. Reprinted with permission from reference [39]. Copyright 2013, American Chemical Society.

Zinc and cadmium peroxide nanoparticles (ZnO2 and CdO2) have been used as oxidants for the selective oxidation of toluene into benzaldehyde, taking advantage of their decomposition temperatures which are comparable to the reaction conditions (160-180 °C).40 When hydrogen peroxide was used instead of the metal peroxides, nearly equal proportions of benzyl alcohol, benzaldehyde and benzoic acid were obtained. ZnO2NPs were also efficient in cyclohexane oxidation, giving up to 60% conversion and a mixture of cyclohexanol and cyclohexanone in a 4:1 ratio, respectively. Toluene derivatives have been adeptly involved in iron-catalyzed alkenylation of C(sp3)–H bond activation through decarboxylation of cinnamic acids.41 Both ferrocene organometallic


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complexes and Fe3O4 nanoparticles (70-80 nm) were active in the decarboxylative coupling using di-tert-butyl peroxide as the oxidant at high temperatures (120 °C); in the case of Fe3O4NPs, the catalyst was reused up to 7 times without loss in activity or selectivity (Scheme 2).

Scheme 2. Decarboxylative coupling of cinnamic acids with toluene derivatives catalyzed by Fe3O4NPs (DTBP = di-tert-butyl peroxide).41 α-Functionalization of C(sp3)–H bonds adjacent to N- and O-heteroatoms.

Given the relative easier achievability of C–H bond functionalization in α-positions to heteroatoms due to the low bond dissociation energy values (e.g. 92.1 kcal/mol for THF αC(sp3)–H, and 104.3 kcal/mol for isoquinoline α-C(sp2)–H),2,42 we limited the scope of this review to those literature reports describing novel activation pathways.

Pieters, Chaudret and Rousseau have recently reported the first general method for enantiospecific C–H activation, which can be applied to a wide range of synthetically and biologically relevant compounds under moderate heating conditions (55 °C). This transformation provides new reactivity modes towards direct C–H activation in the α-position of a heteroatom using RuNPs,43 which is otherwise hampered with other catalytic systems due to limiting factors such as β-H elimination oxidative processes (Fig. 8).


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Figure 8. Scope of the C(sp3)–H deuteration reaction and structures of labeled biologically relevant compounds. Labeling positions are highlighted in cyan, together with isotopic enrichment in percentual values.43

Late-stage deuteration by H/D exchange of complex molecules of biological interest can be achieved with D2 in the presence of RuNPs stabilized by polyvinylpirrolidone (PVP) under mild conditions and water as a compatible solvent. The reactivity scope of this method has been tested with more than 20 heterocyclic substrates, as well as 8 biologically privileged compounds featuring piperidine, morpholine, pyridine, quinoline, indole, and alkyl amine moieties. This study reveals a rich chemistry on the surface of Ru nanocatalysts, yielding high chemo- and regioselectivity together with unchanged enantiomeric purity, even when labeling occurs in the


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vicinity of stereogenic centers. A four-membered dimetallacycle has been proposed as the key intermediate by theoretical studies, and suggested that a collective motion of surface species can optimize the C–H activation step by modulating the local electronic structure (Fig. 8).43

The unprecedented reactivity observed on the surface of the nanocatalysts arising from such a four-membered dimetallacycle intermediate enables α-functionalization to heteroatoms present in the substrate. This represents a new reaction mode in comparison to homogeneous phase catalysts which have been described to undergo formation of four- or five-membered metallacycles containing only one metal atom, and thus furnishing β- or γ-functionalizations.44

More recently, the same authors have studied the reactivity scope, providing excellent results towards enantiospecific C–H activation/deuteration at stereogenic centers of a number of amino acids under moderate heating conditions (55 °C).45 Their calculations show that C–H bond scission is the rate-limiting step (Fig. 9). Moreover, the grafting of the N and Cα atoms onto two vicinal Ru atoms on the surface proceeds with retention of configuration due to the rigidity of the dimetallacycle, which explains both the experimentally observed regioselectivity and enantiospecificity (Fig. 10).45


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Figure 9. Enantiospecific C–H activation/deuteration of amino acids and derivatives. Isotope incorporation yields at the Cα position are given in green. Additional isotopic enrichment at other sites is highlighted in grey and yields are given in parentheses.45


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Figure 10. Energy diagram for the Langmuir–Hinshelwood‐type H/D exchange mechanism as investigated with 1 nm Ru55Dn clusters; energies given in kcal mol−1. Reprinted with permission from reference [45]; license number 3821380955895. Copyright 2015, Wiley-VCH Verlag GmbH & Co. KGaA.

A different mechanism involving C–H bond activation under heterogeneous conditions has been reported by Libuda and co-workers for the dehydrogenation of dodecahydro-N-ethylcarbazole promoted by PdNPs/Al2O3 catalysts.46 The adsorption and thermally induced surface reactions was monitored by infrared reflection absorption spectroscopy (IRAS) and high-resolution X-ray photoelectron spectroscopy (HR-XPS), proving that the initial activation of the molecule occurs through α-C–H bond scission at the 8a- and 9a-positions of the carbazole scaffold at temperatures above 170 K, followed by successive dehydrogenation events at higher temperatures to gain full aromatization of the molecule (Scheme 3).


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Scheme 3. Dehydrogenation of dodecahydro-N-ethylcarbazole on PdNPs/Al2O3.46 β- and γ-Functionalization of C(sp3)–H bonds adjacent to N- and O-heteroatoms.

Despite the success of C–H activation in the last decades under homogeneous conditions, little effort has been addressed to the utilization of well-defined transition metal nanocatalysts for remote β- and γ-functionalization. The translation of this reactivity concept from assumedly homogeneous complexes to nanocatalytic systems remains fairly unexplored. Following initial reports by β-C–H activation assisted by 8-aminoquinoline amide directing groups promoted in solution by Corey47 and Daugulis,48 Arisawa and Chatani have been developed a sulfurmodified, gold-support with PdNPs on its surface (SAuPd).49 This nano-sized catalytic material was applied in direct ethynylation of aliphatic carboxylic acid derivatives via C(sp3)–H bond activation at the β positions of amide at high temperatures (135 °C). Furthermore, the low leaching properties of SAuPd enabled it to be recycled and reused up to 10 times. This is the first example of PdNPs catalysis of unactivated C(sp3)–H bond of functionalizated amides (Fig. 11).


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ACS Catalysis

Figure 11. Direct ethynylation of amides using SAuPd, via C(sp3)–H bond functionalization. Figures indicate isolated yields; in brackets, yields after the 10th run of SAuPd recycling.49 C(sp2)–H

Since Muetterties’ seminal studies on noble-metal arene hydrogenation catalyzed by polynuclear clusters and surfaces,50 a number of colloidal and nanoparticulate catalysts have emerged. As suggested by Piers and co-workers,51 in situ produced MNPs in the zero-valent state can act as faster nanocatalysts, and even superseding the homogeneous reaction manifold. Thus, variable induction times corresponding to the formation of the NPs might just be first evidence that further studies on the complexity of C–H bond activation with cutting-edge characterization techniques could help progress advancements in the field. As discussed below, we would like to stress the importance on the thorough study of the active catalytic species and regimes operating during C(sp2)–H bond activation events. C(sp2)–H hydrogenation/deuteration and oxidation of arenes.


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Claver, Godard and co-workers have recently reported a hydrogenation of arenes employing RhNPs stabilized with triphenylphosphine or triphenylphosphite and exhibiting spherical and small NPs (

Metal and Metal Oxide Nanoparticles: A Lever for C–H

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