Organometallic Ruthenium Nanoparticles: Synthesis, Surface

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Organometallic Ruthenium Nanoparticles: Synthesis, Surface Chemistry, and Insights into Ligand Coordination Luis M. Martínez-Prieto and Bruno Chaudret* LPCNO, Université de Toulouse, CNRS, INSA, UPS, 135, Avenue de Rangueil, 31077 Toulouse, France CONSPECTUS: Although there has been for the past 20 years great interest in the synthesis and use of metal nanoparticles, little attention has been paid to the complexity of the surface of these species. In particular, the different aspects concerning the ligands present, their location, their mode of binding, and their dynamics have been little studied. Our group has started in the early 1990s an investigation of the surface coordination chemistry of ruthenium and platinum nanoparticles but at that time with a lack of adequate techniques to fulfill our ambition. Over 10 years later, we went back to this problem and could obtain a more precise vision of the surface species. This Account is centered on ruthenium chemistry. This metal has been the most studied in our group, first thanks to the availability of a precursor, Ru(cyclooctadiene)(cyclooctatriene) (Ru(COD)(COT)), which possesses the ability to decompose in very mild conditions without leaving residues on the resulting nanoparticles and second because of the absence of magnetic perturbations (Knight shift, paramagnetism, ferromagnetism, etc.), which has allowed the use of solution and solid state NMR. In this respect, it has been possible to evidence the presence of a high concentration of hydrides on the surface of these particles, to study their dynamics, and to show that since the polarity of the Ru−H bond is similar to that of the C−H bond, a Ru/H NP would behave as a big lipophilic entity. The second point was to characterize the coordination of ancillary ligands. This has been achieved for different ligands, in particular phosphines and carbenes, which made possible the study of the modification of NP reactivity induced by surface ligands. This led to the conclusion that the presence of surface ligands can benefit both the activity of NP catalysts and their selectivity. If it was expected that the selectivity could be modulated, the promoting effect from the presence of ligands on, for example, arene or CO hydrogenation was totally unexpected. Playing with poison atoms (Sn, Fe, etc.) or ligands (CO) may allow us to play with the reactivity of the NPs to make them more selective for selected reactions. Finally, the search for specific ligands for nanoparticles is still in its infancy, but some examples have been found as have specific reactions of nanoparticles. Obviously arene hydrogenation and CO hydrogenation were well-known in heterogeneous catalysis, but we could demonstrate that they can be carried out in very mild conditions on ligand stabilized RuNPs. On the other hand, the enantiospecific C−H activation leading to enantioselective labeling of large organic or biomolecules or the C−C bond cleavage in mild conditions were both unexpected. There is still much work to perform for reaching the degree of control on nanoparticles that is presently achieved in organometallic molecular chemistry, but this work shows that it is possible.

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INTRODUCTION If colloids have been known and studied for over a century, their structure, surface chemistry, and properties, both chemical and physical, started really to attract the attention of the scientific community in the late 1980s. At that time, pioneering studies, in particular by Longoni and Chini,1 had demonstrated the possibility to synthesize molecular metal clusters of very high nuclearity by a controlled chemical approach involving stacking of Pt or Ni triangles, whereas Fenske et al. were interested in very large chalcogenide molecular clusters, en route toward quantum dots.2 Concerning the growth of metal clusters, Schmid was a pioneer with the synthesis in 1981 of the long controversial gold 55 cluster Au55Cl12(PPh3)6.3 In this context, some groups started investigating the stabilization of small nanoparticles (“colloids”) in solution. Among these, Bradley et al. were forerunners who described the formation of Pd nanoparticles in © XXXX American Chemical Society

solution and their surface properties, in particular using CO as a probe molecule for infrared (IR) and nuclear magnetic resonance (NMR) studies.4 One expectation was that the formation of colloids would boost the catalytic activity of molecular complexes in some reactions among which hydrosilylation, studied by Lewis and Lewis.5 Concerning ruthenium, nanoparticles have been first prepared in solution by reduction of RuCl3 by sodium borohydride in the presence of surfactants6 and more recently by reduction of the same precursor by polyol.7,8 However, intrigued by the link between the molecular bonding of ligands in clusters and the stabilization of colloids, we looked at the decomposition of high energy organometallic complexes as a clean and simple route toward high nuclearity Received: August 2, 2017

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DOI: 10.1021/acs.accounts.7b00378 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Figure 1. Organometallic approach: formation of ruthenium nanoparticles (RuNPs) using Ru(COD)(COT) as precursor and polymers, ligands or solvents as stabilizers.

Scheme 1. Polymers Used to Stabilize RuNPs

judicious choice of the precursor, ideally a complex decomposing with the least polluting residues possible. Ru(COD)(COT) is therefore a complex of choice since upon hydrogenation in alkanes or tetrahydrofuran (THF), it gives rise rapidly at room temperature (rt) to metal precipitation. Furthermore, it has been used as early as 1986 for the preparation of a ruthenium heterogeneous catalyst on silica, more dispersed and more active in hydrogenation than one prepared from RuCl3.15 This approach attracts a renewed interest for the preparation of well-defined nanocatalysts.16,17 During this process, the ligands are hydrogenated into cyclooctane, which interacts only very weakly with the metal surface. In order to limit the growth of the particles, a stabilizer must be added which can be a polymer, a ligand, or even a solvent (Figure 1).12

molecular or colloidal species avoiding the use of potential sources of pollution arising from the precursor or from the reducing agent. In this respect, the ruthenium(0) complex Ru(cyclooctadiene)(cyclooctatriene) (Ru(COD)(COT)) seemed ideal since we had shown that it could easily decompose under dihydrogen to liberate cyclooctane and allow the synthesis of polyhydride and dihydrogen complexes,9 among which RuH2(H2)2(PCy3)2 remained for a long time the only known bis(dihydrogen) species.10 The principle was to use either relatively weak ligands or a deficiency of ligands, an approach previously shown by Spencer et al.11 to allow the formation of hydrido clusters of platinum. Then the main question was the nature of the bonds between the ligands and the nanoparticles: are they of the same nature as in molecular chemistry and what methods could be used to characterize their presence and behavior? This is particularly important for the basic ligands active in many chemical transformations, namely, hydrides, CO, olefins, arenes, etc. This led us to develop the synthesis of ruthenium nanoparticles of small size and stabilized initially by a polymer,12 then to extend this chemistry to the use of classical ligands of organometallic chemistry and eventually to the search for specific ligands of nanoparticles. Thus, organometallic and coordination chemistry have experienced a tremendous development during the past 40 years producing very active catalysts in a large variety of domains, such as asymmetric hydrogenation, metathesis, or carbon−carbon coupling, all of which have aroused great interest in the scientific community. This was rendered possible by perfect control of the coordination sphere of the complexes and by detailed mechanistic studies. In the case of nanoparticle catalysis, supported or not, this approach is still in its infancy although it raises new interest in heterogeneous catalysis.13,14 We will describe in this Account our efforts to synthesize and characterize nanoparticles of controlled coordination sphere and to develop their chemistry.

In the Absence of Ligands

The use of a polymer for the stabilization of nanoparticles was already documented when we started this project. Several types of polymer have been used including cellulose derivatives (Scheme 1).18 We will concentrate here on polyvinylpyrrolidone (PVP, Scheme 1) which was the most studied stabilizer. Thus, in the presence of PVP, Ru(COD)(COT) reacts with dihydrogen at rt to produce, depending on the pressure, 1.0− 1.5 nm nanoparticles (Ru/PVP).4,19 In this case, the stabilization of RuNPs is purely steric, which means that the particles have little or no chemical bonds with the polymer functions and that a comparison can be made with particles prepared by physical methods. One relevant and striking example of the quality of the nanoparticles prepared by our method concerns cobalt nanoparticles. Hence, CoNPs stabilized by PVP and of 1.5 or 2.0 nm display the same increase in magnetization compared to the bulk value as particles of same size in ultrahigh vacuum (time-of-flight experiments).20 This demonstrates that the surface of the particles is “clean”, which inter alia means unoxidized. If the stabilization origin is steric, how weak can it be to allow the growth of well-defined nano-objects? When the decomposition of Ru(COD)(COT) occurs in neat pentane or THF, an immediate black metallic precipitate forms. However, if the

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SYNTHESIS In this Account, we will concentrate on ruthenium nanoparticles (RuNPs) with some digressions toward the chemistry of other metals when necessary. The synthesis requires first a B


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Accounts of Chemical Research reaction is performed in methanol, a stable solution forms that consists of large polycrystalline particles, homogeneous in size (ca. 70−80 nm). Modification of the solvent either by adding THF to methanol or by using a more lipophilic alcohol leads to a well-controlled size reduction.21 A similar phenomenon was observed in ionic liquids. The decomposition of Ru(COD)(COT) leads here to particles of sizes that depend upon the carbon chain length of the imidazolium ionic liquid.22 In both cases, alcohols and ionic liquids, the size control was attributed to the growth of the RuNPs in a lipophilic environment, droplets of cyclooctane resulting from Ru(COD)(COT) decomposition in alcohol or nonpolar domains resulting from the self-organization of the ionic liquid. This implies a lipophilic nature of the RuNPs, which results from the presence at their surface of hydrides and the polarity of the Ru−H bond, which is similar to that of C−H bonds (vide inf ra). This implies in turn that nanoparticle growth can be limited by very weak forces. More rigid templates could also be used to synthesize particles in mild conditions after impregnation of the precursor into the porous structure, such as mesoporous silica or MOF-5. In this case, the particles size follows the pore size.23,24

Figure 3. Transmission electron micrographs of Ru/HDA (left) and Pt/HDA (right). HDA = hexadecylamine. Reproduced with permission from refs 25 and 26. Copyright 2001 American Chemical Society and 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, respectively.

Scheme 2. Schematic Representation of N-Heterocyclic Carbene Ligands IPr, ItBu, and KSO3PrIPra

In the Presence of Ligands

In this case, the size and shape of the particles is modulated by the nature and the quantity of ligands added. Smaller nanoparticles are formed when using more electron donor ligands (Figure 2). The ligands can be divided into those that

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IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; ItBu = N,Ndi(tert-butyl)imidazol-2-ylidene; KSO3PrIPr = 1-(2,6-diisopropylphenyl)-3-(3-potassium sulfonatopropyl)imidazol-2-ylidene.

We found that these ligands strongly coordinate to RuNPs (Ru/IPr; Ru/ItBu).27,28 Interestingly, this synthesis could be extended recently to water-soluble carbenes, which very efficiently stabilize RuNPs in water (Scheme 2).29 Besides the “weak” and the “strong” ligands, a third category could be the ligands that react at the surface of the particles. A striking example concerns thiol ligands, which are efficient stabilizers of, for example, platinum30 but react at the surface of ruthenium to transform into disulfides and hence form agglomerates.25 Ligands Specific to NPs. Much to our surprise, the reaction of Ru(COD)(COT) with 4-(3-phenylpropyl)pyridine (Figure 4)31 under H2 does not lead to the hydrogenation of

Figure 2. Size-dependence with the ligand donor strength in RuNPs prepared by the organometallic approach. ICy.(p‑tol)NCN = 1,3dicyclohexylimidazolium-2-di-p-tolylcarbodiimide; LC-IMe = 1,3dimethyl-4,5-diundecyl imidazol-2-ylidene; dppb = bis(diphenylphosphino)butane.

bind strongly to the surface and those that bind reversibly giving rise to a dynamic exchange between free and coordinated species. This is the case of amines, in the presence of which one can observe at rt under dihydrogen the initial formation of small RuNPs, which coalesce into wormlike shapes (Figure 3, left).25 A similar phenomenon was observed for platinum nanoparticles, which gave rise to platinum nanowires (Figure 3, right).26 In contrast, phosphines and better diphosphines can stabilize very efficiently RuNPs. Thus, addition of 0.1 equiv of bis(diphenylphosphino)butane (dppb; Figure 2) to Ru(COD)(COT) followed by hydrogenation leads to particles of 1.9 nm mean size, very stable both in solution and in the solid state (Ru/dppb).19 Similarly, N-heterocyclic carbenes (NHCs) such as IPr or ItBu (Scheme 2) are widely used ligands for ruthenium in molecular chemistry thanks to excellent σ-donation properties and a reduced sensitivity to oxidation compared to phosphines.

Figure 4. 4-(3-Phenylpropyl) pyridine.

the molecule but to stable nanoparticles of 1.3 nm mean size. NMR and reactivity studies demonstrated that this ligand acts as specific bidentate ligand for nanoparticles, coordinating initially through the nitrogen atom followed by the flat coordination of the two rings. Van Leeuwen et al. designed diphosphines that cannot coordinate to a single atom but are perfectly adapted to the pseudo-spherical surface of nanoparticles. These phosphines (Figure 5) were shown to stabilize perfectly RuNPs and to allow a very high reactivity (vide inf ra).32 Some ligands are not specific to nanoparticles but are more adapted to them than to molecular complexes. This is the case of imidazolium amidinates, which result from an adduct between NHCs and carbodiimides (ICy.(p‑tol)NCN; Figure 2). These amidinates were revealed to be extremely good ligands, C


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technique can give information on the mode of coordination of deuterium (terminal, μ2- or μ3- bridging, η2-D2, or interstitial; Figure 7) and on its dynamics.

Figure 5. Specific diphosphine ligands designed to stabilize RuNPs.

even at low concentration, for synthesizing very small RuNPs of ca. 1.0 nm and well-defined PtNPs.33,34

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SURFACE CHARACTERIZATION

Hydrides Figure 7. Hydrogen species on metal surfaces.

Most of the nanoparticles described here are prepared in the presence of dihydrogen. Knowing the affinity of ruthenium for hydrogen, the problem arose of the characterization of these hydrides. The questions were the following: Are hydrides indeed present and in what quantity? How do they compare to hydrides in molecular species in terms of structure, dynamics, reactivity, and spectroscopic characteristics? For answering the first question, in collaboration with Limbach et al., we looked at H/D exchange at the surface of RuNPs reasoning that if hydrides were present they would probably behave as on a catalyst surface or on molecular complexes and hence undergo a fast exchange with D2 in the gas phase.35 Thus, we found that exposing RuNPs, previously dried by heating in a secondary vacuum, to a D2 atmosphere led to the formation of essentially HD (Figure 6) which allowed

As references, we first studied model compounds: mononuclear complexes containing Ru−D and Ru−D2 bonds36 and clusters containing a μ2-, μ3-, or μ6-bridging deuteride.37 Values of ca. 120 kHz for terminal deuteride, 60−80 kHz for μ2-D, 20 kHz for μ3-D, and no splitting for μ6-D were found.38 The measurements on nanoparticles were carried out below the freezing temperature of the deuterium mobility, which was found to be dependent upon the stabilizer used: 273 K for diphosphines, 200 K for HDA, and 25 K for RuNPs included in a MOF. According to the quadrupolar splitting, all modes of coordination were observed on Ru/HDA (Figure 8),38 but only terminal ones on Ru/MOF.24

Figure 6. RuNPs showing hydrogen−deuterium exchange between surface and ligand sites.

the quantification of hydrides to ca. 1 H per Ru surface atom. The hydrides on the metal surface were later quantified by their titration with olefins.19 In this case, we found between 1.2 and 1.5 H per surface Ru, depending upon the system. The H/ Rusurface ratio observed in colloidal RuNPs is lower than the one determined in silica supported RuNPs (2 H per Ru surface atom). 16,17 The discrepancy between the two systems presumably results from the presence and nature of ligands at the Ru surface, which reduce the room for hydrides and favor H2 elimination. The next question was the location of these hydrides and first whether they are present inside the particles or on their surface. In collaboration with Limbach, Buntkowsky and Poteau, we investigated the presence of deuterium on the surface of the particles by static solid state 2D NMR. Thanks to the quadrupolar splitting, which is related to the degree of anisotropy experienced by a quadrupolar nucleus, this

Figure 8. Left, subsurface and surface adsorption sites. Right, (a) solidstate 2H NMR spectra of RuNPs stabilized by HDA. (b, c, d) Simulations of the resonances using theoretical quadrupolar parameters. Reproduced with permission from ref 38. Copyright 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

CO

CO coordination has been used as a tool to characterize the surface of nanoparticles. CO coordinates in a terminal (COt) or in a bridging mode (COb), the latter being found preferably on compact faces (Figure 9). Addition of CO to Ru/PVP leads to a single sharp peak in solid state MAS (magic angle spinning) NMR (COt) and no spinning side bands, whereas on Ru/dppb, D


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NHCs coordinate strongly to Ru. To ascertain their mode of coordination, the carbene carbon has been labeled with 13C giving rise to a signal near 200 ppm for RuNPs stabilized with IPr (Figure 11),27 a chemical shift typical of a carbene linked to ruthenium in a molecular complex but also close to the chemical shift of the free carbene. In order to determine whether the carbene is really bonded, we looked at platinum since 195Pt, present with a 33.8% abundance, has a spin of 1/2. Several PtNPs stabilized by carbenes were prepared and shown to display interesting reactivities.41,42 When using a 13C labeled sulfonated NHC,43 the observation of a 13C−195Pt coupling of 940 Hz demonstrated the coordination of the NHC through the carbene carbon. Furthermore, for 3.8 nm PdNPs stabilized by the same water-soluble NHC, the carbene resonance is found near 600 ppm (very broad) as a result of the presence of a conduction band in the particles.44 The presence of this paramagnetic (Knight) shift is a definitive proof of the coordination of the carbene on the particle.

Figure 9. Different coordination modes of CO at metallic surface.

a broad COb band is observed together with several narrow peaks corresponding to COt. This reveals that in Ru/PVP CO is mobile on the surface of ruthenium, even in the solid state, whereas CO is blocked on Ru/dppb in various terminal and bridging environments.39 Addition of CO was further used in other systems as a tool to determine the accessibility and the environment of the surface of NPs.

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REACTIVITY

Hydrogenation of Unsaturated Hydrocarbons

The RuNPs are excellent catalysts for the reduction of arenes (Figure 12), with spectacular turnovers for benzene hydrogenation using diphosphine ligands (60000 h−1, 40 bar, 393 K). On this occasion a very interesting ligand effect was detected since the most active species were those containing mixed aryl/ alkyl phosphines.32 Carbene stabilized RuNPs (Ru/IPr and Ru/ ItBu) were also found to be very active hydrogenation catalysts, also displaying a dependency of the reactivity to the nature of the ligand.45 These results demonstrate that ligands can modulate the reactivity of RuNPs as in molecular chemistry. A size dependency of the reactivity was also evidenced using ICy(p‑tol)NCN ligands. RuNPs of 1.0 and 1.3 nm were synthesized,33 the smaller ones losing their ability to hydrogenate phenyl rings and therefore exhibiting a reactivity at the frontier between the molecular complexes and the solid state. A surface chemistry study on two different RuNPs (Ru/PVP and Ru/dppb) using NMR and IR spectroscopies evidenced the presence of selective sites of reactivity and a clear influence of the ligand for catalytic reactions such as the hydrogenation of styrene (Figure 12).46 COb and arenes compete for faces, whereas COt and hydrides compete for apexes and edges. We could show that hydrides can displace selectively the COt located near the bulky diphosphine ligands, forming partially CO covered NPs (Ru/COb/PVP and Ru/COb/dppb), which are active for alkene hydrogenation but not for the hydrogenation of aromatic rings. This finding allows us to perform selective catalytic reactions upon tuning the surface of the nanoparticles. A similar effect can result from the controlled poisoning of the Ru surface with tin, which enhances the selectivity toward olefins.47 Moreover, the catalytic properties

Ancillary Ligands

As discussed before, ancillary ligands can be used to stabilize nanoparticles and to control their growth and reactivity. Amines have been shown to undergo a dynamical exchange between the surface of the particles and the solution, which can be observed in solution by 1H and 13C NMR in the case of Ru and Pt nanoparticles.25,26 A more elaborate technique was recently applied to amino acids, which are shown to undergo H/D exchange at the surface of RuNPs in water. The Chemical Shift Perturbations (CSPs) technique is commonly used in biology for the determination of protein/ligand interactions. This NMR spectroscopic technique detects the sites of interactions of a ligand by looking at the variation of chemical shifts. It could be used to determine the sites of the coordination of lysine at the Ru surface as a function of the pH (Figure 10).29 The coordination of phosphines to RuNPs can be detected using 31P CP-MAS solid state NMR. Interestingly, the chemical shift of the coordinated phosphines is similar to that observed on molecular species.19 Another technique, HR-MAS (high resolution-MAS) NMR, has been used for the characterization of the coordination of new diphosphines specially designed for the stabilization of nanoparticles.32 In both cases, NMR demonstrates the hydrogenation of a part or all of the aryl groups initially attached to the phosphines into alkyl moieties during the synthesis. The phosphines are strongly coordinated to the NP surface but nevertheless can dissociate in solution since they are catalytically deuterated through H/D exchange.40

Figure 10. Interaction of L-lysine with Ru NP surface as a function of pH. In red, C−H groups giving rise to H/D exchange. At low pH (3), the H/D exchange is blocked, while at higher pHs (7−13), not only the activity increases but also the selectivity changes. Reproduced with permission from ref 29. Copyright 2017 The Royal Society of Chemistry. E


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Figure 11. 13C MAS (bottom) and CP-MAS (top) NMR spectra of RuNPs stabilized with 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) 13 C labeled on the carbene carbon. Reproduced with permission from ref 27. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

(1.3, 1.9, and 3.1 nm) and stabilizers (PVP and dppb), it was shown that a size increase in the ruthenium core did not affect significantly the catalytic properties of the RuNPs but that the main effect came from the presence of dppb at the surface. These results demonstrate that the presence of ligands is not always detrimental to catalytic reactions and may be useful to tune the surface properties of MNPs. C−H Activation. Apart from the aspect of surface characterization, H/D exchange can be useful in a large field of applications in academic and industrial science, in particular for drug design. Recently, we have reported the selective and enantiospecific labeling of organic molecules of interest using RuNPs as nanocatalysts.29,57,58 In a first study, we found an efficient H/D exchange method to deuterate pyridines, quinolones, indoles, and alkyl amines with D2 in the presence of Ru/PVP NPs with high chemo- and regioselectivity under mild conditions (Figure 13).57

Figure 12. Hydrogenation reactions using Ru and FeRu NPs as catalysts.

of NPs can be modulated combining 2 catalytic metals of different reactivity such as Fe and Ru.48 In collaboration with Leitner et al., we have shown that after deposition onto a supported ionic liquid phase (SILP) FeRu NPs selectively hydrogenate furfural into furfuryl alcohol (Figure 12) and are more efficient for the reduction of carbonyl groups than neat RuNPs.49 An activation of the reactivity of ruthenium due to the presence of iron has hence been evidenced. Asymmetric Hydrogenation

Since ligands used as stabilizers can modify the catalytic properties of MNPs, in the last years chiral ligands were tested in some asymmetric catalytic reactions.50−52 With chiral NHC50 and 1,3-diphosphite ligands,51 the enantiomeric excesses were very low or negligible. Using chiral N-donor ligands,52 the enantioselectivity remained very modest. However, when an asymmetric secondary phosphine oxide was used to stabilize chiral iridium nanoparticles,53 some activity in asymmetric hydrogenation of ketones was evidenced.

Figure 13. Scope of deuteration reactions and labeling positions.

Oxidation/Hydrogenation Reactions

This method was extended to enantiospecific C−H activation/deuteration of amino acids and peptides. Here the deuteration of stereogenic centers, located in α-position of a heteroatom, was applied to an extensive number of synthetically and biologically important compounds under mild conditions in both water and organic solvents.58 Experimental evidence and theoretical calculations suggest that the C−H deuteration mechanism involves both coordination of a nitrogen atom acting as directing group and the presence of two nearby Ru atoms forming a four-membered dimetallacycle as a novel key intermediate. (Figure 14). C−C bond cleavage. Upon exposure of RuNPs to ethylene, an unusual reaction was discovered. Thus, at rt, we could observe the dissociation of ethylene to give rise to surface methyl groups unambiguously characterized by solid state and gas phase NMR, in particular using 12C2D4 and 13C2H4, as well as by desorption at 180 °C, which yielded methane as the main product.19 The mechanism is still unknown, but this reaction

RuNPs stabilized by long-chain NHCs are effective catalysts both for hydrogenation and for oxidation reactions. Remarkably, they were used in a one-pot oxidation/hydrogenation process, which evidenced the importance of these NPs as versatile air-stable catalysts for different organic transformations.54 Some Specific Reactivity of Nanoparticles

Hydrogenation of CO. CO hydrogenation has been attempted on molecular systems but was never found to be efficient. Ruthenium is one of the most efficient Fischer− Tropsch Synthesis (FTS) catalysts, and the mechanism of this reaction is still an object of debates.55 In collaboration with Salmeron et al., our group used RuNPs as model catalyst to study FTS in very mild conditions (150−180 °C, 3 bar of syngas, 1:1 molar mixture of H2 and CO) by gas phase NMR and ambient pressure X-ray photoelectron spectroscopy (APXPS).56 Here, using well-defined RuNPs with different sizes F


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asymmetric catalysis, although progress has been made in this domain.53 Finally, it is worth mentioning that these NPs can be deposited on a support leading to nanocatalysts, the reactivity of which could be modulated through ligands. This Account is centered on organometallic Ru NPs, which have demonstrated great activity in some important reactions (hydrogenations, C−H activation, Fischer−Tropsch synthesis, etc.) and could be extended to other ones as significant as water splitting, functionalization of C−H bonds, or hydrogenation of CO2 to formic acid. However, it largely ignores the work carried out by others in important applications of ruthenium (FTS, OER, HER, etc.) or on other systems (e.g., magnetic NPs, CuNPs, etc.) and does not mention our efforts to prepare bimetallic systems and to control their chemical order and, in turn, their chemical and physical properties. The same techniques of synthesis and of characterization apply for oxides and compounds such as quantum dots or iron carbides, which have not been mentioned here. Finally, it does not address the use of the knowledge of surface chemistry for controlling the growth of the particles to obtain specific shapes (cubes, rods, wires, urchins, etc.). However, as a conclusion, this work clearly demonstrates the interest in controlling the surface of nanoparticles in terms of mechanistic knowledge and reactivity control and opens the door to the design of a complex chemistry taking profit of the richness of the surface in terms of geometry and of the nature of metal sites to perform more and more elaborate and selective catalytic transformations.

Figure 14. Four-membered dimetallacycle intermediate on a deuterated Ru55 nanoparticle (1 nm) with 1.6 D atoms per Ru surface atom; deuterium yellow, ruthenium green. Reproduced with permission from ref 58. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

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AUTHOR INFORMATION

Corresponding Author

demonstrates the ability of nanoparticles to activate strong bonds.

*E-mail: [email protected].

CONCLUSIONS AND PERSPECTIVES This long-term study has demonstrated that there are no fundamental differences between the behaviors of ligands coordinated in molecular complexes and those at the surface of nanoparticles. Hydrides are present when the synthesis involves the use of H2. They coordinate and display a dynamic very similar to what can be seen in molecular chemistry. The same is true for CO, the emblematic ligand of the early years of organometallic chemistry, the stretching frequency of which depends on the electron density on the metal in both systems. Ancillary ligands such as phosphines or carbenes not only stabilize NPs through coordination but can enrich their reactivity and allow some reactions, which are definitely not possible for a NP or an heterogeneous catalyst alone (e.g., one pot oxidation/reduction of selected products). However, obviously the difference between molecular complexes and NPs lies in the number of surface atoms accessible for a given reaction, which explains 3 specific reactions of nanoparticles, namely, CO hydrogenation, arene reduction, enantiospecific HD exchange in amino-acids, and presumably many others. Thus, these NPs seem to be prepared for more complex reactivity, the first example of it is a one-pot oxidation followed by reduction. However, cascade reactions using islands of one metal deposited on another one are now accessible and should develop in the coming years. Furthermore, the ease of preparation of these organometallic NPs should make them accessible as synthetic tools in a similar way to the development of the use of organometallic compounds in organic chemistry. One point that is still specific to organometallic chemistry is

Bruno Chaudret: 0000-0001-9290-6421

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ORCID Notes

The authors declare no competing financial interest. Biographies Luis M. Martı ́nez-Prieto received his Ph.D. degree in Organometallic Chemistry from the University of Seville in 2012 at the “Instituto de ́ Investigaciones Quimicas; IIQ” (CSIC). After graduation, he moved to the “Laboratoire de Chimie de Coordination; LCC” (CNRS), in Toulouse, France, for a postdoctoral stay. During this period, his research was centered on the synthesis, characterization, and study of the surface chemistry of metal nanoparticles. Since July 2015 he has been working in the “Laboratoire de Physique et Chimie des NanoObjets; LPCNO” (INSA) in Toulouse, exploring new metal nanoparticles as efficient catalysts. Bruno Chaudret graduated from École Nationale Supérieure de Chimie de Paris in 1975. He received his Ph.D. from Imperial College London in 1977 with Sir Geoffrey Wilkinson and the degree of a “Docteur és Sciences” from the University of Toulouse in 1979. He is now “Director of Research CNRS” and Director of the “Laboratoire de Physique et Chimie des Nano-Objets” in Toulouse as well as President of the Scientific Council of CNRS. He is a coauthor of 480 publications and 17 patents. He received several awards among which Humboldt-Gay-Lussac (AvH foundation), the Wilkinson Prize and lectureship (RSC), the silver Medal of CNRS, the Catalan−Sabatier Award (RSEQ), the Wittig-Grignard Prize (GDCH), and the Pierre Sue Prize (SCF). He is a member of the French Academy of Science and of Academia Europeae. His present research interests concern the G


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synthesis and surface chemistry of nanoparticles using an organometallic approach and their applications in the fields of catalysis, magnetism, optics, micro- and nanoelectronics.

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ACKNOWLEDGMENTS The authors thank CNRS, UPS and INSA Toulouse, and the University of Toulouse through the Chair UFTMIP: 2015-193CIF-D-DRD. Part of this project has received support from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement No. 246763 - NANOSONWINGS and Grant Agreement No. GA694159 - MONACAT).

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Organometallic Ruthenium Nanoparticles: Synthesis, Surface

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