Aitor Gual

“Imprinting” Catalytically Active Pd Nanoparticles onto Ionic-Liquid-Modified Al2O3 Supports

A new sputtering chamber that allows the constant mixing of the solid support during sputtering by using an electro-magnetic oscillator was developed for the generation of metal nanoparticles (top down approach) uniformly distributed over the solid supports. By using this new chamber, small and welldistributed Pd nanoparticles (Pd-NPs) of 2.6 or 4.3 nm were produced over Al2O3 and new imidazolium ionic liquids covalently supported on Al2O3 by simple sputtering from a Pd foil. The Pd-NPs uniformly distributed over the solid supports display comparable catalytic performances in the hydrogenation of 1,3-cyclohexadiene and 1,3-cyclooctadiene to that achieved by using a catalyst prepared by conventional chemical methods (bottom up approach). Mechanistic and labelling studies show that the hydrogenation of 1,3-cyclohexadiene catalysed by Pd-NPs occurs via meta-stable p-allyl intermediates, characteristic of homogeneous-like catalytically active sites, and disproportionation through the outer-sphere mechanism. The ratio of hydrogenation to disproportionation is dependent on the Pd-NP size, and the disproportionation products are more pronounced with small NPs because of the higher affinity of dienes for this size of particle. Therefore, the surface structural features of small Pd-NPs facilitate the arrangement of diene molecules in the correct geometry for the transfer of H from the donor to the acceptor sites, typical of poly-metallic catalytically active sites. It is proposed that the reaction of 1,3-cyclohexadiene under H2 can be used to probe the homogeneous/ heterogeneous nature of supported metal NPs. It is demonstrated for the first time that highly active and selective nanocatalysts were obtained by using the sputtering-deposition technique (top down approach) and this opens a new window of opportunity for the preparation of size-controlled metal NPs with clean surfaces. Introduction

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.

Ruthenium‐Catalyzed Hydroformylation of Alkenes by using Carbon Dioxide as the Carbon Monoxide Source in the Presence of Ionic Liquids

The reaction of [BMI⋅Cl] (BMI=1‐butyl‐3‐methylimidazolium) or [BMMI⋅Cl] (BMMI=3‐butyl‐1,2‐dimethylimidazolium) with Ru3(CO)12 generates Ru–hydride–carbonyl–carbene species in situ that are efficient catalysts for a reverse water gas shift/hydroformylation/hydrogenation cascade reaction. The addition of H3PO4 increased the catalytic activity of the first step (i.e., the hydrogenation of CO2 to CO). Under the optimized reaction conditions [120 °C and 6.0 MPa CO2/H2 (1:1) for 17 h], cyclohexene and 2,2‐disubstituted alkenes were easily functionalized to alcohols through sequential hydroformylation/carbonyl reduction.

Ligand effect in the Rh-NP catalysed partial hydrogenation of substituted arenes

The Rh nanoparticles Rh1–Rh4 stabilised by the mono- and bidentate phosphine and phosphite ligands I–IV were synthesised, characterised and applied as catalysts in the partial hydrogenation of substituted arenes. In the case of disubstituted arenes, selectivities for the corresponding cyclohexene derivatives of up to 39% were achieved at ca. 40% conversion. The effect of parameters such as temperature and pressure was also examined. In the hydrogenation of styrene, very high selectivities for ethylbenzene were achieved with TOF values up to ca. 23 500 h−1. All these results show that the catalytic performance of small Rh-NPs can be modulated by the appropriate choice of stabilising agents.

The Partial Hydrogenation of 1,3-Dienes Catalysed by Soluble Transition-Metal Nanoparticles

The partial hydrogenation of 1,3‐dienes is a structure sensitive reaction that is typically catalysed by classical heterogeneous (heterotopic) or homogeneous (homotopic) catalysts. Recently, soluble transition‐metal nanoparticles (M‐NPs), particularly palladium and gold‐based systems, have emerged as an efficient alternative. Here, we review the current state of the techniques for the partial hydrogenation of 1,3‐dienes by M‐NPs and conclude that, from the reactivity point of view, these materials possess heterotopic‐like and homotopic‐like characteristics. They are heterotopic‐like because the relative concentration of the monoalkene with respect to the diene does not affect the product selectivity and their catalytic performance is affected by their physical properties (such as size and shape). Furthermore, they are easily recoverable, with long catalytic lifetimes. Additionally, as homotopic systems, their reactivity can be tuned by using an appropriate organic stabiliser, which displays substrate‐selective levels that are not observed for classical heterotopic catalysts.

Carbon Dioxide Transformation in Imidazolium Salts: Hydroaminomethylation Catalyzed by Ru‐Complexes

The catalytic species generated by dissolving Ru3 (CO)12 in the ionic liquids 1-n-butyl-3-methyl-imidazolium chloride or 1-n-butyl-2,3-dimethyl-imidazolium chloride are efficient multifunctional catalysts for: (a) reverse water-gas shift, (b) hydroformylation of alkenes, and (c) reductive amination of aldehydes. Thus the reaction of alkenes with primary or secondary amines (alkene/amine, 1:1) under CO2 /H2 (1:1) affords the hydroaminomethylations products in high alkene conversions (up to 99 %) and selectivities (up to 96 %). The reaction proceeds under relatively mild reaction conditions (120 °C, 60 bar=6 MPa) and affords selectively secondary and tertiary amines. The presence of amine strongly reduces the alkene hydrogenation competitive pathway usually observed in the hydroformylation of terminal alkenes by Ru complexes. The catalytic system is also highly active for the reductive amination of aldehydes and ketones yielding amines in high yields (>90 %).