Inmaculada Angurell

A general approach to fabricate fe3O4 nanoparticles decorated with Pd, Au, and Rh: Magnetically recoverable and reusable catalysts for Suzuki C-C cross-coupling reactions, hydrogenation, and sequential reactions

A facile strategy has been explored for loading noble metals onto the surface of ferrite nanoparticles with the assistance of phosphine-functionalized linkers. Palladium loading is shown to occur with participation of both the phosphine function and the surface hydroxyl groups. Hybrid nanoparticles containing simultaneously Pd and Au (or Rh) are obtained by successive loading of metals. Similarly, ferrite nanoparticles decorated with Pd, Au, and Rh have also been formed by using the same strategy. The catalytic properties of the new nanoparticles are evidenced in processes such as reduction of 4-nitrophenol or hydrogenation of styrene. Besides, the sequential process involving a cross-coupling reaction followed by reduction of 1-nitrobiphenyl has been successfully achieved by employing Pd/Au decorated nanoferrite particles.

Magnetite-supported palladium single-atoms do not catalyse the hydrogenation of alkenes but small clusters do

The activity of supported noble metal catalysts strongly depends on the particle size. The ultimate small-size limit is the single-atom catalyst (SAC), which maximizes the catalytic efficiency in the majority of the examples. Here, we investigate the catalytic behavior of Pd SACs supported on magnetite nanoparticles and we unambiguously demonstrate that Pd SACs are absolutely inactive in the hydrogenation of various alkene substrates. Instead, Pd clusters of low atomicity exhibit outstanding catalytic performance.

Antisymbiotic Self-Assembly and Dynamic Behavior of Metallamacrocycles with Allylic Corners

The design and synthesis of discrete supramolecular architectures by metal–ligand self-assembly is an area of current interest. The application of the directional bonding approach has proved to be very versatile, allowing the synthesis of many molecular triangles, squares, rectangles, pentagons, hexagons, and cages. Among them, homometallic molecular squares, formed by the reaction of cis-protected square planar complexes with linear symmetric ditopic ligands, have been the most widely studied. The assembly of metallamacrocycles containing different metal corners and/or nonsymmetric ligands using this method presents additional difficulties due to the possible generation of a mixture of isomers that are difficult to separate. To overcome this, they are generally obtained through modular self-assembly methodology. This strategy requires the initial synthesis of mononuclear complexes with strong covalently bound ligands that have additional donor sites available for coordination to further acceptor building blocks. Nevertheless, several molecular triangles and squares have been recently assembled following the directional bonding approach by using nonsymmetric linkers and identical metal corners. Interestingly, in some cases, unexpected self-selection of a single linkage isomer was observed. To analyze the large quantity of metallamacrocycles reported to date, the following classification is proposed, according to the nature of the assembled units: 1) species containing identical metal corners and symmetric linkers; 2) species containing identical metal corners and nonsymmetric linkers; and 3) species containing different metal corners and nonsymmetric linkers. Herein, we report the use for the first time of the directional bonding approach for the synthesis of metallamacrocycles displaying two different metal corners connected by a nonsymmetric ditopic linker. By the correct choice of the different units, that is, metal corners and ambidentate ligand, we have been able to promote its self-assembly producing molecular square architectures and, more interestingly, achieving the self-selection of a single linkage isomer. Evidently, the correct choice of a heteroditopic ligand in which the different (stereo)electronic characteristics of the donor groups could control the reactivity at the metal site is decisive. Thus, we focused our attention on the 4-PPh2py ligand (py=pyridine), given its hard-soft character. Furthermore, the electronic nature of the ancillary ligands in the metal corners on the proposed assembly is also crucial for the self-selection process. As a result, complex [Pd(h-2Me-C3H4) ACHTUNGTRENNUNG(cod)] ACHTUNGTRENNUNG(CF3SO3) was chosen as the source of {Pd(h-2-Me-C3H4)} + , a potential metal corner that could provide a varied reactivity to the system, as expected for the allyl palladium species. On the other hand, the complex [PdACHTUNGTRENNUNG(H2O)2 ACHTUNGTRENNUNG(dppp)] ACHTUNGTRENNUNG(CF3SO3)2 was chosen as the second metal corner supplier, {Pd ACHTUNGTRENNUNG(dppp)}2+ (dppp=bis(diphenylphosphino)propane). The selected metal fragments were expected to coordinate selectively to each of the donor atoms of the heteroditopic 4-PPh2py ligand. Following Pearson s antisymbiotic effect, the greatest stability is expected when the softest ligands are bonded trans to the hardest ones. With these three components in our hands, that is, [Pd(h2-Me-C3H4) ACHTUNGTRENNUNG(cod)] ACHTUNGTRENNUNG(CF3SO3), [Pd ACHTUNGTRENNUNG(H2O)2ACHTUNGTRENNUNG(dppp)] ACHTUNGTRENNUNG(CF3SO3)2, and 4-PPh2py, the self-assembly was explored in solution in dichloromethane at room temperature (Scheme 1). After several minutes, the color of the solution faded away and the P NMR spectrum (Supporting Information, Figure S1) showed two signals associated with the P atoms of the dppp ligand and those bonded to the allylpalladium fragment, respectively, indicating the presence of only one species in solution. This species, the targeted macrocycle 1, was obtained [a] Dr. I. Angurell, Dr. M. Ferrer, Dr. A. Guti rrez, Prof. M. Mart nez, Dr. L. Rodr guez, Prof. O. Rossell Departament de Quimica Inorg nica, Universitat de Barcelona Mart i Franqu s 1-11, 08028 Barcelona (Spain) Fax: (+34)934907725 E-mail : montse.ferrer@qi.ub.es [b] Dr. M. Engeser Kekul -Institut f r Organische Chemie und Biochemie der Universit t, Gerhard-Domagk-Str.1, 53121 Bonn (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201002605.

Kinetico-mechanistic insights on the assembling dynamics of allyl-cornered metallacycles: the Pt-Npy bond is the keystone.

The square-like homo- and heterometallamacrocycles [{Pd(η(3) -2-Me-C3 H4 )(L(n) )2 }2 {M(dppp)}2 ](CF3 SO3 )6 (dppp=1,3-bis(diphenylphosphino)propane) and [{Pd(η(3) -2-Me-C3 H4 )(L(1) )2 }2 {M(PPh3 )2 }2 ](CF3 SO3 )6 [py=pyridine, M=Pd, Pt, L(n=) 4-PPh2 py (L(1) ), 4-C6 F4 PPh2 py (L(2) )] containing allyl corners were synthesised by antisymbiotic self-assembly of the different palladium and platinum metallic corners and the ambidentate N,P ligands. All the synthesised assemblies displayed a complex dynamic behaviour in solution, the rate of which is found to be dependent on the electronic and/or steric nature of the different building blocks. A kinetico-mechanistic study by NMR line shape analysis of the dynamics of some of these assemblies was undertaken in order to determine the corresponding thermal activation parameters. Both an enhanced thermodynamic stability and slower dynamics were observed for platinum-pyridine-containing species when compared with their palladium analogues. Time-dependent NMR spectroscopy in combination with ESI mass spectrometry was used to study the exchange between the assemblies and their building blocks, as well as that occurring between different metallamacrocycles. Preliminary studies were carried out on the activity of some of the metallamacrocyclic compounds as catalytic precursors in the allylic substitution reaction, and the results compared with that of the monometallic allylic corner [Pd(η(3) -2-Me-C3 H4 )(L(1) )2 ](+) .

Improved thermal stability of oxide-supported naked gold nanoparticles by ligand-assisted pinning

We report a method to improve the thermal stability, up to 900 °C, of bare-metal (naked) gold nanoparticles supported on top of SiO(2) and SrTiO(3) substrates via ligand-assisted pinning. This approach leads to monodisperse naked gold nanoparticles without significant sintering after thermal annealing in air at 900 °C. The ligand-assisted pinning mechanism is described.

Highly water-dispersible magnetite-supported Pd nanoparticles and single atoms as excellent catalysts for Suzuki and hydrogenation reactions

The molecule 4-(diphenylphosphino)benzoic acid (dpa) anchored on the surface of magnetite nanoparticles permits the easy capture of palladium ions that are deposited on the surface of the magnetite nanoparticles after reduction with NaBH4. Unexpectedly, a significant fraction of dpa is removed in this process. Samples of Fe3O4dpa@Pdx containing different Pd loadings (x = 0.1, 0.3, 0.5 and 1.0 wt%) were prepared, and their catalytic efficiency for the Suzuki C–C coupling reaction was studied. The best catalyst was Fe3O4dpa@Pd0.5, which gave the highest TOF published to date for the reaction of bromobenzene with phenylboronic acid in a mixture of ethanol/water (1/1). Interestingly, the same reaction carried out in water also produced excellent yields of the resulting C–C coupling product. The behaviour of other bromide aryl molecules was also investigated. The best catalytic results for the aqueous phase reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) were obtained using Fe3O4dpa@Pd0.1. The presence of Pd SACs (single atom catalysts) seems to be responsible for this performance. In contrast, the same Fe3O4dpa@Pd0.1 catalyst is absolutely inactive for the hydrogenation of styrene in ethanol.

4-Mercaptophenyldiphenylphosphine as linker to immobilize Pd onto the surface of magnetite nanoparticles. Excellent catalytic efficiency of the system after partial linker removal

The molecule 4-mercaptophenyldiphenylphosphine (Sdp) has been synthesized for use as a linker to immobilize Pd nanoparticles onto the surface of magnetite nanoparticles. The high loading shown by Sdp was attributed to the presence of one only anchoring (S) atom. The treatment of the resulting nanoparticles after Pd deposition with an aqueous solution of hydrogen peroxide permitted the removal of a large fraction (70%) of Sdp. Aberration corrected high-resolution scanning transmission electron microscopy images of the final nanocomposite showed that the Pd dispersion is constituted by very small nanoparticles along with a few isolated metal atoms. The partial loss of linker protection makes the Pd nanoparticles one of the most efficient catalysts for the Suzuki–Miyaura C–C cross coupling reaction and for hydrogenation of 4-nitrophenol and styrene.

Macroporous silicon microreactor for the preferential oxidation of CO

A macroporous silicon micromonolith containing ca. 40,000 regular channels of 3.3 μm in diameter per square millimeter has been successfully functionalized with an Au/TiO2 catalyst for CO preferential oxidation (CO-PrOx) in the presence of hydrogen. The functionalization of the silicon microchannels has been accomplished by growing a SiO2 layer on the channel walls, followed by exchange with a titanium alkoxyde precursor and decomposition into TiO2 and, finally, by anchoring carbosilanethiol dendron protected pre-formed Au nanoparticles. Catalytically active centers at the Au-TiO2 interface have been obtained by thermal activation. With this method, an excellent homogeneity and adherence of the catalytic layer over the microchannels of the macroporous silicon micromonolith has been obtained, which has been tested for CO-PrOx at 363-433 K and λ=2 under H2/CO=0-20 (molar). The macroporous silicon micromonolith converts ca. 3 NmL of CO per minute and mL of micro reactor at 433 K under H2/CO=20, suggesting that it could be particularly effective for hydrogen purification in low-temperature microfuel cells for portable applications.

Remarkable carbon dioxide hydrogenation to ethanol on a palladium/iron oxide single-atom catalyst

The hydrogenation of CO2 into value‐added chemicals is one of the most investigated methods to reduce CO2 emissions in the atmosphere and thereby contributes to a sustainable chemical industry. Whereas the catalytic hydrogenation of CO2 into methanol and synthetic hydrocarbons is well established, the effective and selective transformation of CO2 into higher alcohols is still challenging. Here, we show that Pd single atoms anchored on the surface of Fe3O4 are very active for the hydrogenation of CO2 to ethanol at 300 °C, even at atmospheric pressure. By comparing various Pd/MOx catalysts, we conclude that the metal–oxide interface has a strong influence on catalytic behavior.

NH2- or PPh2-functionalized linkers for the immobilization of palladium on magnetite nanoparticles?

Immobilization of palladium on magnetite nanoparticles has been carried out with the assistance of two differently functionalized linkers containing phosphino- or amino-terminated groups. The linkers have been anchored to the magnetite surface by means of catechol, mercapto or carboxylate groups. The nature of the resulting Pd nanoparticles deposited has been examined by HAADF-STEM images and XPS electron spectroscopy. The efficiency of the two kinds of catalysts has been checked and compared for the Suzuki–Miyaura reaction, 4-nitrophenol reduction and styrene hydrogenation. The results evidence that the nanoparticles equipped with the phosphino fragment are better catalysts than those functionalized with the amino group and, in some processes, they are among the most active catalysts reported in the literature.