Bringing Selectivity in H/D Exchange Reactions Catalyzed by Metal Nanoparticles through Modulation of the Metal and the Ligand Shell.

Ru and Rh nanoparticles catalyze the selective H/D exchange in phosphines using D2 as the deuterium source. The position of the deuterium incorporation is determined by the structure of the P-based substrates, while activity depends on the nature of the metal, the properties of the stabilizing agents, and the type of the substituent on phosphorus. The appropriate catalyst can thus be selected either for the exclusive H/D exchange in aromatic rings or also for alkyl substituents. The selectivity observed in each case provides relevant information on the coordination mode of the ligand. Density functional theory calculations provide insights into the H/D exchange mechanism and reveal a strong influence of the phosphine structure on the selectivity. The isotope exchange proceeds via C-H bond activation at nanoparticle edges. Phosphines with strong coordination through the phosphorus atom such as PPh3 or PPh2Me show preferred deuteration at ortho positions of aromatic rings and at the methyl substituents. This selectivity is observed because the corresponding C-H moieties can interact with the nanoparticle surface while the phosphine is P-coordinated, and the C-H activation results in stable metallacyclic intermediates. For weakly coordinating phosphines such as P(o-tolyl)3, the interaction with the nanoparticle can occur directly through phosphine substituents, and then, other deuteration patterns are observed.

Dinuclear Iron Complexes of Iminopyridine-Based Ligands as Selective Cytotoxins for Tumor Cells and Inhibitors of Cancer Cell Migration.

A family of dinuclear iron (II) compounds with iminopyridine-based ligands displays selective cytotoxic activity against cancer cell lines. All compounds have IC50 values 2-6 fold lower than that of cisplatin, and 30-90 fold lower than that of carboplatin for the tumor cell lines assayed. Comparing the IC50 values between tumor and non-tumor cell lines, the selectivity indexes range from 3.2 to 34, compound 10, [Fe2(4)2(CH3CN)4](BF4)4, showing the highest selectivity. Those compounds carrying substituents on the iminopyridine ring show the same cytotoxicity as those without substituents. However, the electronic effects of the substituents on position 6 may be important for the cytotoxicity of the complexes, and consequently for their selectivity. All compounds act over DNA, promoting cuts on both strands in the presence of reactive oxygen species. Since compound 10 presented the highest selectivity, its cytotoxic effect was further characterized. It induces apoptosis, affects cell cycle phase distribution in a cell-dependent manner, and its cytotoxic effect is linked to reactive oxygen species generation. In addition, it decreases tumor cell migration, showing potential antimetastatic effects. These properties make compound 10 a good lead antitumor agent among all compounds studied here.

Hollow PdAg-CeO2 heterodimer nanocrystals as highly structured heterogeneous catalysts.

In the present work, hollow PdAg-CeO2 heterodimer nanocrystals (NCs) were prepared and tested as catalysts for the selective hydrogenation of alkynes. These nanostructures combine for the first time the beneficial effect of alloying Pd with Ag in a single NC hollow domain with the formation of active sites at the interface with the CeO2 counterpart in an additive manner. The PdAg-CeO2 NCs display excellent alkene selectivity for aliphatic alkynes. For the specific case of hydrogenation of internal alkynes such as 4-octyne, very low over-hydrogenation and isomerization products were observed over a full conversion regime, even after prolonged reaction times. These catalytic properties were remarkably superior in comparison to standard catalysts. The promotion of Ag on the moderation of the reactivity of the Pd phase, in combination with the creation of interfacial sites with the CeO2 moiety in the same nanostructure, is pointed as the responsible of such a remarkable catalytic performance.

A General One-Pot Methodology for the Preparation of Mono- and Bimetallic Nanoparticles Supported on Carbon Nanotubes: Application in the Semi-hydrogenation of Alkynes and Acetylene.

A facile and straightforward methodology for the preparation of monometallic (copper and palladium) and bimetallic nanocatalysts (NiCu and PdCu) stabilized by a N-heterocyclic carbene ligand is reported. Both colloidal and supported nanoparticles (NPs) on carbon nanotubes (CNTs) were prepared in a one-pot synthesis with outstanding control on their size, morphology and composition. These catalysts were evaluated in the selective hydrogenation of alkynes and alkynols. PdCu/CNTs revealed an efficient catalytic system providing high selectivity in the hydrogenation of terminal and internal alkynes. Moreover, this catalyst was tested in the semi-hydrogenation of acetylene in industrially relevant acetylene/ethylene-rich model gas feeds and showed excellent stability even after 40 h of reaction.

Advances in the preparation of highly selective nanocatalysts for the semi-hydrogenation of alkynes using colloidal approaches.

In the last decade, the semi-hydrogenation of alkynes has experienced significant advances in terms of fine control of alkene selectivity and prevention of the over-hydrogenation reaction. Such advances have been possible to a large extent through the progress in colloidal methods for the preparation of metallic nanoparticles. The present review describes the contributions in the field of the selective hydrogenation of alkynes involving the utilization of colloidal methodologies. These approaches permit the fine modulation of several parameters affecting the catalytic performance of the active phase such as the particle size, the bulk and the surface structure and composition. For the transformation of liquid substrates, the nature of the stabilizers, the reducing agents and the metal precursors employed for the synthesis of the catalysts can be tuned to enhance the alkene selectivity. In contrast, in catalytic transformations of gaseous substrates, the presence of adsorbed species at the metal surface usually gives detrimental results while the interplay between the support and the active phase appears to be a more convincing alternative for catalyst tuning.

Facile synthesis of NHC-stabilized Ni nanoparticles and their catalytic application in the Z-selective hydrogenation of alkynes.

Well defined Ni nanoparticles (NiNPs) stabilized with N-heterocyclic carbenes (NHCs) have been synthesized through a new methodology involving the decarboxylation of a zwitterionic CO2 adduct. Their catalytic performance was tested in the partial hydrogenation of alkynes into (Z)-alkenes under very mild reaction conditions (50 °C and 5 bar H2 pressure), providing excellent activities and selectivities.

Effect of the Polymeric Stabilizer in the Aqueous Phase Fischer-Tropsch Synthesis Catalyzed by Colloidal Cobalt Nanocatalysts.

A series of small and well defined cobalt nanoparticles were synthesized by the chemical reduction of cobalt salts in water using NaBH4 as a reducing agent and using various polymeric stabilizers. The obtained nanocatalysts of similar mean diameters (ca. 2.6 nm) were fully characterized and tested in the aqueous phase Fischer-Tropsch Synthesis (AFTS). Interestingly, the nature and structure of the stabilizers used during the synthesis of the CoNPs affected the reduction degree of cobalt and the B-doping of these NPs and consequently, influenced the performance of these nanocatalysts in AFTS.

Novel iminopyridine derivatives: ligands for preparation of Fe(ii) and Cu(ii) dinuclear complexes.

A series of imino- and amino-pyridine ligands based on dihydrobenzofurobenzofuran (BFBF) and methanodibenzodioxocine (DBDOC) backbones have been synthesized. These ligands form exclusively dinuclear complexes with metals such as iron(II) and copper(II). The structures for complexes 15, 16, 18, 19, 20, 21, 23, and 24 were determined by X-ray crystallography. The complexes show large distances for the metal nuclei and different geometries depending on the nature of the metal. An octahedral geometry was observed for the iron(II) complexes, while copper(II) complex 24 showed a distorted trigonal bipyramidal geometry. The iron(II) complexes showed activity as catalysts in the cycloaddition of CO2 to epoxides, obtaining moderate yields of cyclic carbonates.

Selective catalytic deuteration of phosphorus ligands using ruthenium nanoparticles: a new approach to gain information on ligand coordination.

Phenyl rings in phenyl- or phenyl-alkylphosphines are selectively deuterated at the ortho position using Ru/PVP nanoparticles, while are fully reduced in the case of arylphosphine oxide derivatives and do not react in the case of arylphosphite. This different behavior provides information about the coordination mode of each ligand.

Pd-catalysed mono- and dicarbonylation of aryl iodides: insights into the mechanism and the selectivity.

The mechanism of the experimentally reported phosphine-free palladium-catalysed carbonylation of aryl iodides with amines in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as base was investigated at the DFT level. Paths were identified for both di- and monocarbonylation, and the calculated selectivity for three different substrates was in agreement with experiment. In dicarbonylation yielding α-ketoamides, formation of the second carbon-carbon bond occurs through reductive elimination in the Pd acyl amide intermediate after DBU-assisted nucleophilic attack of an amine at a terminal CO ligand. This path yields the major product with iodobenzene and the almost exclusive product with p-methoxyiodobenzene. Two different possible pathways yield the monocarbonylated amide product. In one of them, which affords the minor product for iodobenzene, base-assisted nucleophilic attack of the amine takes place on a Pd-bound acyl ligand. For substrates with electron-withdrawing substituents, such as p-cyanoiodobenzene, aryl migration to the CO ligand is disfavoured, and this allows base-assisted amine attack at a terminal CO ligand early in the catalytic cycle. From the resulting Pd amide aryl complex, the subsequent reductive elimination occurs easily, and monocarbonylation becomes favoured.

Asymmetric hydroformylation.

Rhodium is currently the metal of choice to achieve high enantioselectivities in the hydroformylation of a relatively wide variety of alkene substrates. The elucidation of the different steps of the catalytic cycle and the characterization of the resting state, together with the discovery of several types of ligands that are able to provide high enantioselectivities, have made the rhodium-catalyzed hydroformylation a synthetically useful tool. For years, ligands containing phosphite moieties such as diphosphites and phosphine-phosphites were considered the most successful ligands to achieve high enantioselectivities for classical substrates such as styrene and vinyl acetate. In fact, the phosphite-phosphine BINAPHOS (43) and its derivatives are still today the most successful ligands in terms of selectivity and scope. For more substituted substrates, general trends can be extracted. However, recent studies showed that these general trends can be sometimes reversed by the use of the appropriate catalyst and choice of reaction conditions, clearly showing that these trends are only indicative and that there are still many challenges to be tackled in this area.

Interplay between cationic and neutral species in the rhodium-catalyzed hydroaminomethylation reaction.

The reactivity of [Rh(CO)(2){(R,R)-Ph-BPE}]BF(4) (2) toward amine, CO and/or H(2) was examined by high-pressure NMR and IR spectroscopy. The two cationic pentacoordinated species [Rh(CO)(3) {(R,R)-Ph-BPE}]BF(4) (4) and [Rh(CO)(2)(NHC(5)H(10)){(R,R)-Ph-BPE}]BF(4) (8) were identified. The transformation of 2 into the neutral complex [RhH(CO)(2){(R,R)-Ph-BPE}] (3) under hydroaminomethylation conditions (CO/H(2), amine) was investigated. The full mechanisms related to the formation of 3, 4 and 8 starting from 2 are supported by DFT calculations. In particular, the pathway from 2 to 3 revealed the deprotonation by the amine of the dihydride species [Rh(H)(2)(CO)(2){(R,R)-Ph-BPE}]BF(4) (6), resulting from the oxidative addition of H(2) on 2.

Room temperature asymmetric Pd-catalyzed methoxycarbonylation of norbornene: highly selective catalysis and HP-NMR studies.

Palladium complexes bearing monodentate and bidentate phosphine ligands (1-7) were synthesised and used as catalyst precursors in the methoxycarbonylation of norbornene. The catalytic systems bearing ligands 1, 3 and 4 afforded excellent conversions (>99%) and selectivity of the ester (>99%). NMR investigations showed that using complex 1a as the precursor resulted in the protonated phosphine, 1-H(+), being formed under catalytic conditions and thus the addition of acid is not required for the activation of this system since the reaction involving the precursor with methanol under CO pressure produces 2 equivalents of HCl and leads to the formation of the active species. The protonation of ligand 4 under methoxycarbonylation conditions was also observed and the diprotonated diphosphine was isolated and characterised. This compound was tested as a ligand and acid source in a catalysis and provided excellent conversion and high selectivity to the ester.

Interception of a Rh(I)-Rh(III) dinuclear trihydride complex revealing the dihydrogen activation by [Rh(CO)2{(R,R)-Ph-BPE}].

Reaction of [Rh(CO)(2){(R,R)-Ph-BPE}][BF(4)] 1 under 7 bar H(2) provides the dihydride [Rh(H)(2)(CO)(2){(R,R)-Ph-BPE}][BF(4)] 3, which reacts with the neutral hydride [Rh(H)(CO){(R,R)-Ph-BPE}] 2 arising from 3 in THF. The resulting complex is the dimeric monocationic Rh((I))-Rh((III)) complex [Rh(H)(2)(CO)(2){(R,R)-Ph-BPE}][BF(4)] 4.

A phosphine-free Pd catalyst for the selective double carbonylation of aryl iodides.

The first phosphine-free Pd-catalysed double carbonylation of aryl iodides is reported as a general and practical method, giving excellent conversions and selectivities for a wide range of aryl iodides and amine nucleophiles under atmospheric CO pressure.

Pd nanoparticles for C-C coupling reactions.

The catalytic formation of C-C bonds is one of the most useful transformations in organic synthesis. Over the last decade, the use of transition metal nanoparticles (NPs) in catalysis has attracted much interest and their use in C-C bond formation reactions constitutes one of their most important applications, including the Suzuki, Heck, and Sonogashira reactions. This tutorial review highlights recent work in this active area, considering the stabilising agents used to prepare the NPs, the catalytic results and the recycling possibilities.

Efficient recycling of a chiral palladium catalytic system for asymmetric allylic substitutions in ionic liquid.

Chiral carbohydrate-based diphosphites were used for Pd-catalysed asymmetric allylic substitution (alkylation, amination, phosphination) in neat ionic liquids (ILs). Pyrrolidinium-based IL led to the best activities, allowing an efficient catalyst immobilization. In the allylic amination (TOF > 3100 h(-1)), the catalyst could be recycled nine times preserving both activity and enantioselectivity.

C1-symmetric carbohydrate diphosphite ligands for asymmetric Pd-allylic alkylation reactions. Study of the key Pd-allyl intermediates.

A series of C(1)-symmetrical 1,3-diphosphite ligands with a furanoside backbone have been applied in the Pd-catalysed asymmetric allylic alkylation of mono- and disubstituted linear substrates. These diphosphite ligands were designed by selective modification of the successful diphosphite ligand L1a with the 6-deoxy-1,2-O-isopropylidene-glucofuranose backbone in order to study the effect of the ligand structure on the catalytic performance. The effect of the solvent, the substrate/metal ratio and ligand/metal ratio were also investigated. The results in the Pd-allylic alkylation of rac-1,3-diphenyl-3-acetoxyprop-1-ene showed that the ligand structure and the reaction conditions had a considerable effect on enantioselectivity and on the kinetics of the reaction producing the kinetic resolution of the substrate. The alkylated product 2 was therefore obtained in 95% ee at 53% conversion and the enantiopure substrate 1 was recovered in 99.9% ee. Furthermore, the effect of the ligand structure and solvent were also observed in the Pd-allylic alkylation of monosubstituted 1-phenyl-3-acetoxyprop-1-ene. The use of a pro-chiral nucleophile was also explored in this reaction leading to excellent regioselectivities but moderate enantioselectivities. Finally, in order to determine how the ligand structure affected the chiral pocket of the Pd-π-allyl intermediates, the complexes [Pd(η(3)-C(15)H(13))(L)]PF(6), where L = L1a, L5-L8a, were synthesised and characterised by NMR spectroscopy.

Soluble transition-metal nanoparticles-catalysed hydrogenation of arenes.

Over the last decade, the hydrogenation of arenes catalysed by soluble nanoparticles has attracted much interest from both academic and industrial research groups due to the milder conditions and the interesting selectivities achieved when compared to those obtained with classical heterogeneous catalysts. When substituted arenes are used as substrates in this reaction, the stereoselectivity is a key objective, and high levels of enantioselectivity are yet to be achieved.