Tuning the hydrogen donor/acceptor behavior of ionic liquids in Pd-catalyzed multi-step reactions

Ionic Liquids in Metal, Photo-, Electro-, and (Bio) Catalysis

Ionic liquids (ILs) have unique physicochemical properties that make them advantageous for catalysis, such as low vapor pressure, non-flammability, high thermal and chemical stabilities, and the ability to enhance the activity and stability of (bio)catalysts. ILs can improve the efficiency, selectivity, and sustainability of bio(transformations) by acting as activators of enzymes, selectively dissolving substrates and products, and reducing toxicity. They can also be recycled and reused multiple times without losing their effectiveness. ILs based on imidazolium cation are preferred for structural organization aspects, with a semiorganized layer surrounding the catalyst. ILs act as a container, providing a confined space that allows modulation of electronic and geometric effects, miscibility of reactants and products, and residence time of species. ILs can stabilize ionic and radical species and control the catalytic activity of dynamic processes. Supported IL phase (SILP) derivatives and polymeric ILs (PILs) are good options for molecular engineering of greener catalytic processes. The major factors governing metal, photo-, electro-, and biocatalysts in ILs are discussed in detail based on the vast literature available over the past two and a half decades. Catalytic reactions, ranging from hydrogenation and cross-coupling to oxidations, promoted by homogeneous and heterogeneous catalysts in both single and multiphase conditions, are extensively reviewed and discussed considering the knowledge accumulated until now.

Palladium and Copper: Advantageous Nanocatalysts for Multi-Step Transformations

Metal nanoparticles have been deeply studied in the last few decades due to their attractive physical and chemical properties, finding a wide range of applications in several fields. Among them, well-defined nano-structures can combine the main advantages of heterogeneous and homogeneous catalysts. Especially, catalyzed multi-step processes for the production of added-value chemicals represent straightforward synthetic methodologies, including tandem and sequential reactions that avoid the purification of intermediate compounds. In particular, palladium- and copper-based nanocatalysts are often applied, becoming a current strategy in the sustainable synthesis of fine chemicals. The rational tailoring of nanosized materials involving both those immobilized on solid supports and liquid phases and their applications in organic synthesis are herein reviewed.

Task-Specific Properties and Prospects of Ionic Liquids in Cross-Coupling Reactions

Ionic liquids (ILs) are considered as highly useful materials for potential diverse uses such as greener and more convenient alternatives to volatile organic solvents, reagents, additives, ligands and co-solvents. Thermal stability, negligible vapor pressure and high polarity with ionic environments have possibly conferred some unique physico-chemical properties and a wider electrochemical window on ILs. More importantly, these properties are tuneable, depending on variations in alkyl chains and counter-anions. On the other hand, various transition-metal-catalyzed cross-coupling reactions constitute an important backbone of contemporary organic synthesis. A vast number of C-C and C-heteroatom cross-coupling reactions are reported in the presence of ILs, often showing better performance. The influence of IL on the action of a given catalyst or on the course of a reaction can be relatively complex, and is not understood well enough to be able to draw succinct conclusions. However, there are a few reports in the literature that help understand the role of actual and active catalytic species stabilized in an IL environment. Stabilization, which can be either helpful or detrimental to catalysis depends on specific circumstances. This review article is aimed primarily at summarizing the various applications of ILs during the past decade, focusing as far as possible on the task-specific properties of ILs in transition-metal-catalyzed C-C and C-heteroatom cross-coupling reactions. Several successful achievements and noteworthy progress in this field of research leads to the sensible conclusion that future prospects in this field of research are not only bright but promise new horizons.

Rh nanoparticles from thiolate dimers: selective and reusable hydrogenation catalysts in ionic liquids

Thiolate-capped RhNPs in imidazolium-based ionic liquids were synthesized from [Rh(μ-SR)(COD)]₂ dimmers under H₂ pressure without external addition of ligand stabilizers, preserving thiolate integrity on the nanoparticle surface. This nanoparticulated systems showed a remarkable selectivity that led to their application in the one pot reductive N-alkylation to produce amines.

Ionic liquids in catalysis: molecular and nanometric metal systems

The catalyst immobilization in a liquid phase represents an attractive means to preserve high activities and selectivities, also permitting an easy recycling. To attain this goal, organic products should be extracted in a simple way from the catalytic phase leading to metal-free target compounds; for this reason, ionic liquids exhibiting high affinity for metallic species and low affinity for low polar compounds, turn into a promising medium, in particular for the synthesis of fine chemicals. In the present Accounts, we illustrate this approach through our research involving both molecular organometallic compounds and metallic nanoparticles dispersed in an ionic liquid phase.

Palladium nanoparticles in ionic liquids stabilized by mono-phosphines. Catalytic applications

Palladium nanoparticles generated from organometallic complexes in the presence of functionalized mono-phosphines (L1-L3), in both THF and imidazolium-based ionic liquids (ImILs), were successfully synthesized. Depending on the phosphine and solvent nature, PdNPs with different extent of aggregation were observed. Actually, the ligand L1, P(CH2CH2CH2Ph)3, led to small and well-dispersed nanoparticles in both ILs, [BMI][PF6] and [EMI][HP(O)2OMe], in contrast to more agglomerated PdNPs obtained in THF. PdNPs in ILs were catalytically active and chemoselective in C-C cross-coupling (Suzuki-Miyaura and Heck-Mizoroki) and hydrogenation reactions. Well-defined Pd(0) and Pd(II) organometallic complexes containing L1, [PdCl2(L1)2] and [Pd(ma)(L1)2], were also prepared for comparative purposes.