Colloidal ruthenium catalysts for selective quinaldine hydrogenation: Ligand and solvent effects.

Colloidal Ru nanoparticles (NP) display interesting catalytic properties for the hydrogenation of (hetero)arenes as they proceed efficiently in mild reaction conditions. In this work, a series of Ru based materials was used in order to selectively hydrogenate quinaldine and assess the impact of the stabilizing agent on their catalytic performances. Ru nanoparticles stabilized with polyvinylpyrrolidone (PVP) and 1-adamantanecarboxylic acid (AdCOOH) allowed to obtain 5,6,7,8-tetrahydroquinaldine with a remarkable selectivity in mild reaction conditions by choosing the suitable solvent. The presence of a carboxylate ligand on the surface of the Ru NP led to an increase in the activity when compared to Ru/PVP catalyst. The stabilizing agent had also an impact on the selectivity, as carboxylate ligand modified catalysts promoted the selectivity towards 1,2,3,4-tetrahydroquinaldine, with bulky carboxylate displaying the highest ones.

In situ study of the evolution of NiFe nanocatalysts in reductive and oxidative environments upon thermal treatments.

The conversion of biomass as a sustainable path to access valuable chemicals and fuels is very attractive for the chemical industry, but catalytic conversions still often rely on the use of noble metals. Sustainability constraints require developing alternative catalysts from abundant and low-cost metals. In this context, NiFe nanoparticles are interesting candidates. In their reduced and supported form, they have been reported to be more active and selective than monometallic Ni in the hydrogenation of the polar functions of organic molecules, and the two metals are very abundant. However, unlike noble metals, Ni and Fe are easily oxidized in ambient conditions, and understanding their transformation in both oxidative and reductive atmospheres is an important though seldom investigated issue to be addressed before their application in catalysis. Three types of NiFe nanoparticles were prepared by an organometallic approach to ensure the formation of ultrasmall nanoparticles (<3.5 nm) with a narrow size distribution, controlled composition and chemical order, while working in mild conditions: Ni2Fe1 and Ni1Fe1, both with a Ni rich core and Fe rich surface, and an alloy with a Ni1Fe9 composition. Supported systems were obtained by the impregnation of silica with a colloidal solution of the preformed nanoparticles. Using advanced characterization techniques, such as wide-angle X-ray scattering (WAXS) and X-ray absorption spectroscopy (XAS) in in situ conditions, this study reports on the evolution of the chemical order and of the oxidation state of the metals upon exposure to air, hydrogen, and/or increasing temperature, all factors that may affect their degree of reduction and subsequent performance in catalysis. We show that if oxidation readily occurs upon exposure to air, the metals can revert to their initial state upon heating in the presence of H2 but with a change in structure and chemical ordering.

Synthesis of NiFeOx nanocatalysts from metal-organic precursors for the oxygen evolution reaction.

Production of hydrogen from a renewable source that is water requires the development of sustainable catalytic processes. This implies, among others, developing efficient catalytic materials from abundant and low-cost resources and investigating their performance, especially in the oxidation of water as this half-reaction is the bottleneck of the water splitting process. For this purpose, NiFe-based nanoparticles with sizes ca. 3-4 nm have been synthesized by an organometallic approach and characterized by complementary techniques (WAXS, TEM, STEM-HAADF, EDX, XPS, and ATR-FTIR). They display a Ni core and a mixed Ni-Fe oxide shell. Once deposited onto FTO electrodes, they have been assessed in the electrocatalytic oxygen evolution reaction under alkaline conditions. Three different Ni/Fe ratios (2/1, 1/1 and 1/9) have been studied in comparison with their monometallic counterparts. The Ni2Fe1 nanocatalyst displayed the lowest overpotential (320 mV at j = 10 mA cm-2) as well as excellent stability over 16 h.

Ultrasmall Manganese Ferrites for In Vivo Catalase Mimicking Activity and Multimodal Bioimaging.

Manganese ferrite nanoparticles display interesting features in bioimaging and catalytic therapies. They have been recently used in theranostics as contrast agents in magnetic resonance imaging (MRI), and as catalase-mimicking nanozymes for hypoxia alleviation. These promising applications encourage the development of novel synthetic procedures to enhance the bioimaging and catalytic properties of these nanomaterials simultaneously. Herein, a cost-efficient synthetic microwave method is developed to manufacture ultrasmall manganese ferrite nanoparticles as advanced multimodal contrast agents in MRI and positron emission tomography (PET), and improved nanozymes. Such a synthetic method allows doping ferrites with Mn in a wide stoichiometric range (Mnx Fe3-x O4 , 0.1 ≤ x ≤ 2.4), affording a library of nanoparticles with different magnetic relaxivities and catalytic properties. These tuned magnetic properties give rise to either positive or dual-mode MRI contrast agents. On the other hand, higher levels of Mn doping enhance the catalytic efficiency of the resulting nanozymes. Finally, through their intracellular catalase-mimicking activity, these ultrasmall manganese ferrite nanoparticles induce an unprecedented tumor growth inhibition in a breast cancer murine model. All of these results show the robust characteristics of these nanoparticles for nanobiotechnological applications.

Covalent Grafting of Ruthenium Complexes on Iron Oxide Nanoparticles: Hybrid Materials for Photocatalytic Water Oxidation.

The present environmental crisis prompts the search for renewable energy sources such as solar-driven production of hydrogen from water. Herein, we report an efficient hybrid photocatalyst for water oxidation, consisting of a ruthenium polypyridyl complex covalently grafted on core/shell Fe@FeOx nanoparticles via a phosphonic acid group. The photoelectrochemical measurements were performed under 1 sun illumination in 1 M KOH. The photocurrent density of this hybrid photoanode reached 20 µA/cm2 (applied potential of +1.0 V vs reversible hydrogen electrode), corresponding to a turnover frequency of 0.02 s-1. This performance represents a 9-fold enhancement of that achieved with a mixture of Fe@FeOx nanoparticles and a linker-free ruthenium polypyridyl photosensitizer. This increase in performance could be attributed to a more efficient electron transfer between the ruthenium photosensitizer and the Fe@FeOx catalyst as a consequence of the covalent link between these two species through the phosphonate pendant group.

Correlation between surface chemistry and magnetism in iron nanoparticles.

To shed light on the factors governing the stability and surface properties of iron nanoparticles, a series of iron nanoparticles has been produced by hydrogenation of two different iron amido complexes: the bis[bis(trimethylsilyl)amido] Fe(ii), [Fe(N(SiMe3)2)2]2, and the bis(diphenylamido) Fe(ii), [Fe(NPh2)2]. Nanostructured materials of bcc structure, or nanoparticles displaying average sizes below 3 nm and a polytetrahedral structure, have been obtained. Depending on the synthesis conditions, the magnetization of the nanoparticles was either significantly lower than that of bulk iron, or much higher as for clusters elaborated under high vacuum conditions. Unexpectedly, hydrogenation of aromatic groups of the ligands of the [Fe(NPh2)2] precursor has been observed in some cases. Confrontation of the experimental results with DFT calculations made on polytetrahedral Fe91 model clusters bearing hydrides, amido and/or amine ligands at their surface, has shown that amido ligands can play a key role in the stabilisation of the nanoparticles in solution while the hydride surface coverage governs their surface magnetic properties. This study indicates that magnetic measurements give valuable indicators of the surface properties of iron nanoparticles in this size range, and beyond, of their potential reactivity.

Chemoselective H/D exchange catalyzed by nickel nanoparticles stabilized by N-heterocyclic carbene ligands.

With this work, we report the synthesis and full characterization of nickel nanoparticles (NPs) stabilized by N-heterocyclic carbene (NHC) ligands, namely 1,3-bis(cyclohexyl)-1,3-dihydro-2H-imidazol-2-ylidene (ICy) and 1,3-bis(2,4,6-trimethylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene (IMes). Although the resulting NPs have the same size, they display different magnetic properties and different reactivities, which result from ligand effects. In the context of H/D exchange on pharmaceutically relevant heterocycles, Ni@NHC shows a high chemoselectivity, avoiding the formation of undesired reduced side-products and enabling a variety of H/D exchange on nitrogen-containing aromatic compounds. Using 2-phenylpyridine as a model substrate, it was observed that deuteration occurred preferably at the α position of the nitrogen atom, which is the most accessible position for the C-H activation. In addition, Ni@IMes NPs are also able to fully deuterate the ortho positions of the phenyl substituents.

When organophosphorus ruthenium complexes covalently bind to ruthenium nanoparticles to form nanoscale hybrid materials.

A hybrid material made of mononuclear organophosphorus polypyridyl ruthenium complexes covalently bonded to ruthenium nanoparticles has been synthesized via a one-pot organometallic procedure and finely characterized. These results open new avenues to access unique hybrid transition metal nanomaterials.

Tuning the catalytic activity and selectivity of water-soluble bimetallic RuPt nanoparticles by modifying their surface metal distribution.

Bimetallic ruthenium-platinum nanoparticles (RuPt NPs) of different surface distributions and stabilized by using a sulfonated N-heterocyclic carbene ligand (1-(2,6-diisopropylphenyl)-3-(3-potassium sulfonatopropyl)-imidazol-2-ylidene) were prepared from Ru(COD)(COT) (COD = cyclooctadiene and COT = cyclooctatriene), and platinum precursors having various decomposition rates (Pt(NBE)3, NBE = norbornene, Pt(CH3)2(COD) and Pt2(DBA)3, DBA = dibenzylideneacetone). Structural and surface studies by FT-IR and solid-state MAS NMR, using carbon monoxide as a probe molecule, revealed the presence of different structures and surface compositions for different nanoparticles of similar sizes, which principally depend on the decomposition rate of the organometallic precursors used during the synthesis. Specifically, the slower the decomposition rate of the platinum precursor, the higher the number of Pt atoms at the NP surface. The different bimetallic RuPt NPs, as well as their monometallic equivalents (Pt and Ru NPs), were used in isotopic H/D exchange through C-H activation on l-lysine. Interestingly, the activity and selectivity of the direct C-H deuteration were dependent on the NP surface composition at the α position but not on that at the ε position. Chemical shift perturbation (CSP) experiments revealed that the difference in reactivity at the α position is due to a Pt-carboxylate interaction, which hinders the H/D exchange.

Alloyed Pt3M (M = Co, Ni) nanoparticles supported on S- and N-doped carbon nanotubes for the oxygen reduction reaction.

Sulfur- (S-CNT) and nitrogen-doped (N-CNT) carbon nanotubes have been produced by catalytic chemical vapor deposition (c-CVD) and were subject to an annealing treatment. These CNTs were used as supports for small (≈2 nm) Pt3M (M = Co or Ni) alloyed nanoparticles that have a very homogeneous size distribution (in spite of the high metal loading of ≈40 wt % Pt), using an ionic liquid as a stabilizer. The electrochemical surface area, the activity for the oxygen reduction reaction and the amount of H2O2 generated during the oxygen reduction reaction (ORR) have been evaluated in a rotating ring disk electrode experiment. The Pt3M/N-CNT catalysts revealed excellent electrochemical properties compared to a commercial Pt3Co/Vulcan XC-72 catalyst. The nature of the carbon support plays a key role in determining the properties of the metal nanoparticles, on the preparation of the catalytic layer, and on the electrocatalytic performance in the ORR. On N-CNT supports, the specific activity followed the expected order Pt3Co > Pt3Ni, whereas on the annealed N-CNT support, the order was reversed.

Deciphering the Crystal Structure of a Scarce 1D Polymeric Thorium Peroxo Sulfate.

The preparation and structural characterization of an original Th peroxo sulfate dihydrate, crystallizing at room temperature in the form of stable 1D polymeric microfibres is described. A combination of laboratory and synchrotron techniques allowed solution of the structure of the Th(O2 )(SO4 )(H2 O)2 compound, which crystallizes in a new structure type in the space group Pna21 of the orthorhombic crystal system. Particularly, the peroxide ligand coordinates to the Th cations in an unusual µ3 -η2 :η2 :η2 bridging mode, forming an infinite 1D chain decorated with sulfato ligands exhibiting simultaneously monodentate and bidentate coordination modes.

Carboxylic acid-capped ruthenium nanoparticles: experimental and theoretical case study with ethanoic acid.

Given that the properties of metal nanoparticles (NPs) depend on several parameters (namely, morphology, size, surface composition, crystalline structure, etc.), a computational model that brings a better understanding of a structure-property relationship at the nanoscale is a significant plus in order to explain the surface properties of metal NPs and also their catalytic viability, in particular, when envisaging a new stabilizing agent. In this study we combined experimental and theoretical tools to obtain a mapping of the surface of ruthenium NPs stabilized by ethanoic acid as a new capping ligand. For this purpose, the organometallic approach was applied as the synthesis method. The morphology and crystalline structure of the obtained particles was characterized by state-of-the art techniques (TEM, HRTEM, WAXS) and their surface composition was determined by various techniques (solution and solid-state NMR, IR, chemical titration, DFT calculations). DFT calculations of the vibrational features of model NPs and of the chemical shifts of model clusters allowed us to secure the spectroscopic experimental assignations. Spectroscopic data as well as DFT mechanistic studies showed that ethanoic acid lies on the metal surface as ethanoate, together with hydrogen atoms. The optimal surface composition determined by DFT calculations appeared to be ca. [0.4-0.6] H/Rusurf and 0.4 ethanoate/RuSurf, which was corroborated by experimental results. Moreover, for such a composition, a hydrogen adsorption Gibbs free energy in the range -2.0 to -3.0 kcal mol-1 was calculated, which makes these ruthenium NPs a promising nanocatalyst for the hydrogen evolution reaction in the electrolysis of water.

Ruthenium Trichloride Catalyst in Water: Ru Colloids versus Ru Dimer Characterization Investigations.

An easy-to-prepare ruthenium catalyst obtained from ruthenium(III) trichloride in water demonstrates efficient performances in the oxidation of several cycloalkanes with high selectivity toward the ketone. In this work, several physicochemical techniques were used to demonstrate the real nature of the ruthenium salt still unknown in water and to define the active species for this Csp3-H bond functionalization. From transmission electron microscopy analyses corroborated by SAXS analyses, spherical nanoobjects were observed with an average diameter of 1.75 nm, thus being in favor of the formation of reduced species. However, further investigations, based on X-ray scattering and absorption analyses, showed no evidence of the presence of a metallic Ru-Ru bond, proof of zerovalent nanoparticles, but the existence of Ru-O and Ru-Cl bonds, and thus the formation of a water-soluble complex. The EXAFS (extended X-ray absorption fine structure) spectra revealed the presence of an oxygen-bridged diruthenium complex [Ru(OH) xCl3- x]2(µ-O) with a high oxidation state in agreement with catalytic results. This study constitutes a significant advance to determine the true nature of the RuCl3·3H2O salt in water and proves once again the invasive nature of the electron beam in microscopy experiments, routinely used in nanochemistry.

Ultrathin Gold Nanowires with the Polytetrahedral Structure of Bulk Manganese.

Despite the intensive interest in thin gold nanowires for a variety of technologically important applications, key details of the mechanism of their formation and atomic-scale structure remain unknown. Here we synthesize highly uniform, very long, and ultrathin gold nanowires in a liquid-phase environment and study their nucleation and growth using in situ high-energy synchrotron X-ray diffraction. By controlling the type of solvents, reducing agents, and gold precursor concentration, it is shown that the nucleation and growth of gold nanowires involve the emergence and self-assembly of transient linear gold complexes, respectively. In sharp contrast with the face-centered-cubic bulk gold, the evolved nanowires are found to possess a tetrahedrally close packed structure incorporating distorted icosahedra and larger size coordination polyhedra of the type observed with the room-temperature phase of bulk manganese. We relate the complexes to synergistic effects between the selected precursor and reducing agents that become appreciable over a narrow range of their molar ratios. We attribute the unusual structural state of gold nanowires to geometrical frustration effects arising from the conflicting tendencies of assemblies of metal atoms to evolve toward attaining high atomic packing density while keeping the atomic-level stresses low, ultimately favoring the growth of cylindrical nanowires with a well-defined diameter and atomically smooth surface. Our work provides a roadmap for comprehensive characterization and, hence, better understanding of 1D metallic nanostructures with an unusual atomic arrangement and may have important implications for their synthesis and performance in practical applications.

Magnetic, Structural, and Chemical Properties of Cobalt Nanoparticles Synthesized in Ionic Liquids.

Cobalt nanoparticles (CoNPs) exhibit quite unique magnetic, catalytic, and optical properties. In this work, imidazolium-based ionic liquids (ILs) are successfully used to elaborate magnetically responsive suspensions of quite monodisperse CoNPs with diameters below 5 nm. The as-synthesized CoNPs adopt the noncompact and metastable structure of ϵ-Co that progressively evolves at room temperature toward the stable hexagonal close-packed allotrope of Co. Accordingly, magnetization curves are consistent with zero-valent Co. As expected in this size range, the CoNPs are superparamagnetic at room temperature. Their blocking temperature is found to depend on the size of the IL cation. The CoNPs produced in an IL with a large cation exhibit a very high anisotropy, attributed to an enhanced dipolar coupling of the NPs, even though a larger interparticle distance is observed in this IL. Finally, the presence of surface hydrides on the CoNPs is assessed and paves the way toward the synthesis for Co-based bimetallic NPs.

Control of reactivity through chemical order in very small RuRe nanoparticles.

The choice of suitable organometallic precursors, [Re2(C3H5)4] and [Ru(Me-Allyl)2COD] or [Ru(COD)(COT)], allows us to synthesize polyvinylpyrrolidone (PVP) stabilized bimetallic RuRe nanoparticles of ca. 1.3 nm with narrow size dispersity, displaying the hcp crystal structure and to control their chemical order: an alloy or Re rich surface. The structural features of these NPs were determined using complementary characterization techniques (TEM, HRTEM, STEM-HAADF, EDX, WAXS, FT-IR, MAS-NMR and ICP). In particular, surface state investigation based on CO adsorption and oxidation reactions provided useful information of the chemical order in these nanoparticles. The RuRe NPs were obtained as stable colloidal solutions or powders. Surface reactivity studies demonstrated that the alloy type RuRe/PVP NPs show better resistance to oxidation than the ones displaying a Re enriched surface and are more active towards CO dissociation than monometallic Re/PVP NPs as a result of the synergic effect between Ru and Re. Interestingly, the dissociation of CO was not observed with RuRe/PVP NPs displaying a Re enriched surface. Besides the synthetic aspect, this work highlights the crucial influence of the chemical order resulting from the choice of the metal sources in the control of the reactivity of ultra-small metal nanoparticles.

Dissimilar catalytic behavior of molecular or colloidal palladium systems with a new NHC ligand.

In this work, we describe the synthesis of a new N-heterocyclic carbene (NHC) ligand, derived from a hybrid pyrazole-imidazolium scaffold, namely 1-[2-(3,5-dimethylpyrazol-1-yl)ethyl]-3-((S)-1-phenylethyl)-3H-imidazol-2-ylidene (L). This ligand has been used as a stabilizer for the organometallic synthesis of palladium(0) nanoparticles (Pd NPs). L presents a better stabilizing effect than its pre-carbenic HLCl counterpart, allowing the formation of isolated Pd NPs while HLCl yields aggregated ones. Additionally, molecular Pd(ii) coordination compounds of L and HLCl were synthesized and characterized to better understand the coordination modes of these ligands. Both molecular and colloidal Pd systems have been further tested in catalytic C-C coupling processes. Three different types of reactions have been observed depending on the catalytic system: (i) the Suzuki-Miyaura reaction takes place with Pd molecular complexes; (ii) a secondary reaction, the dehalogenation of the substrate, is always detected and (iii) the C-C homocoupling between two molecules of bromoarenes is observed with colloidal catalysts.

Mechanistic Investigations of the Synthesis of Size-Tunable Ni Nanoparticles by Reduction of Simple NiII Diamide Precursors.

Herein, we present a detailed study of the conversion of a nickel(II) diamide precursor to size-tunable, monodisperse nickel nanoparticles (NPs). The thermal decomposition of nickel(II) dioleylamide, synthesized either independently or in situ, resulted in the formation of Ni NPs without the coproduction of water. Mechanistic studies were conducted on the stability and reduction pathway of the NiII precursor, and on the consequent particle formation. Variations in the ratio of trioctylphosine (TOP) to nickel allowed size tunability, which resulted in nanoparticles that ranged in size from 4 to 11 nm in diameter. The DFT calculations support a mechanistic pathway that involves nickel reduction by imine formation. This water-free method was extended to the synthesis of water-sensitive M0 NPs (M=Fe, Co).

Hexakis [60]Fullerene Adduct-Mediated Covalent Assembly of Ruthenium Nanoparticles and Their Catalytic Properties.

The C66 (COOH)12 hexa-adduct has been successfully used as a building block to construct carboxylate bridged 3D networks with very homogeneous sub-1.8 nm ruthenium nanoparticles. The obtained nanostructures are active in nitrobenzene selective hydrogenation.

Improved Transversal Relaxivity for Highly Crystalline Nanoparticles of Pure γ-Fe2O3 Phase.

Pure and highly crystalline γ-Fe2O3 nanocrystals (NCs) are obtained when hydrolysis and oxidation of a Fe(II) organometallic precursor are performed in successive steps. Their synthesis in pure alkylamine leads to NCs of about 6 nm. In aqueous solutions of poly(vinyl)pyrrolidone, such pristine NCs form aggregates of about 150 nm that exhibit a high transversal relaxivity (r2 =466 mM(-1) s(-1)) about four times higher than that of a commercial Feridex magnetic resonance imaging (MRI) contrast agent. Consequently, they provide a significant decrease in the NMR signal at very short echo time (8 ms), which is of paramount importance in clinical practice because of the reduced duration of MRI measurements.