Synthesis and Applications of Ionic Liquid-based Hybrids

Palladium catalysed C–C coupling reactions

The synthesis of new catalytic systems for the C–C coupling reactions, especially Suzuki and Heck reactions, have played a central role in our research activity as witnessed by the large number of heterogenous catalysts we have designed and applied for such purpose. In this section we will take a small journey through our most recent results.

The grafting of a properly modified fullerene C₆₀ onto different support materials (amorphous silica, SBA-15 and Fe₂O₃@SiO₂), followed by the reaction with 1-methylimidazole, allowed us to obtain a series of hybrids in which C₆₀, used as molecular scaffold for imidazolium moieties, was uniformly distributed onto the surface of the support materials. These hybrid materials have been employed as stabilizer of palladium nanoparticles (Pd NPs) formed as a result of anion metathesis with tetrachloropalladate ions and the subsequent reduction with NaBH₄ (Scheme 1). The adopted synthetic approach allowed to optimize the functionalization of the support materials with the imidazolium-based moieties that are not directly linked to the surface of the support, but they are present in an octopus-like spatial arrangement around the fullerene core. The prepared materials have been successfully employed as heterogeneous catalysts in C–C bond forming reactions (Suzuki and Heck couplings), showing outstanding catalytic activity. Turn-over frequency (TOF) value up to 3 640 000 h^-1 has been reached and the silica-based catalyst showed full recyclability even after 10 cycles.

The same synthetic approach was used for the functionalization of SBA-15 with imidazolium modified polyhedral oligomeric silsesquioxanes (POSS) used as platforms for Pd(II) species. The so-obtained material has been tested as pre-catalyst in the Suzuki and Heck reactions proving its efficacy in both catalytic processes. The recyclability was assessed for up to seven cycles showing no loss of catalytic activity. Interestingly, only Pd(II) was detected in the recovered catalyst after its use in the Heck reaction. Very low catalytic loadings down to 0.0007 mol% were used showing the effectiveness of the prepared material. The excellent outcome of the reactions could be ascribed to the combination of some factors, namely the textural properties of the SBA-15 support and the presence of the imidazolium-POSS nanocage within the pores of SBA-15, which make the catalytic material a sort of nanoreactor.

We used two carbon nanoforms (CNFs), namely multi-walled carbon nanotubes (MWCNTs) and carbon nanohorns (CNHs), as support materials for the direct functionalization by means of radical polymerization of a bis-vinylimidazolium salt. In this case, high resolution transmission electron microscopy (HRTEM) analyses revealed CNFs acted as a sort of templating agents for the polymerization process, leading to a cylindrical or spherical coating for MWCNTs and CNHs, respectively (Figures 1b (MWCNT-poly-imi) and 1f (CNH-poly-imi)). Immobilization of Pd NPs has given rise to materials with different morphologic features. As matter of fact, very small Pd NPs uniformly distributed onto the surface of material MWCNT-poly-imi-Pd (Figures 1 c,d) were detected by TEM images. Conversely, Pd NPs of bigger dimensions and with a bimodal size distribution were detected on the outer surface of material CNH-poly-imi-Pd (Figures 1 g,h). The prepared materials have been employed as heterogeneous and recyclable catalysts for the Suzuki and Heck couplings. The high degree of functionalization of CNFs with the cross-linked imidazolium network has improved the dispersion of the prepared catalysts in aqueous medium allowing the use of pure water as solvent to carry out the Suzuki couplings.

Heck reaction under flow conditions

The radical polymerization of two bis-vinylimidazolium salts onto 3-mercaptopropyl modified amorphous silica has given rise to materials belonging to the category of supported ionic liquid-like phases (SILLPs) in which a multi-layer of imidazolium salt covers the surface of the support material. The high content of the imidazolium phase could act as stabilizer for Pd NPs. The collaboration with the Vaccaro group allowed to test these materials as heterogeneous catalysts for the Heck coupling under flow conditions defining a highly sustainable and waste-minimized protocol. Other key points of such approach, along with the high activity of the studied catalysts, have been the choice of the reaction medium, namely acetonitrile/water azeotrope, and the use of a heterogeneous base (diethylaminomethyl-polystyrene) that have been totally recovered and reused contributing to minimize the environmental factor (E-factor) values. Pure products have been isolated chromatography-free in high yields and with very low residual palladium contamination.

Synthesis of cyclic carbonates

Another important part of our Research activity is the production of catalytic materials for the conversion of carbon dioxide into cyclic carbonates. In this context, the fundamental collaboration with Aprile group has given rise to an intense Research focused on this topic, and some of these results will be briefly reviewed in this section.

We have used octakis(3-bromopropyl)-octasilsesquioxane and octakis(3-iodoopropyl)-octasilsesquioxane as molecular building blocks for the preparation of an imidazolium cross-linked network by reacting them with 1,4-bis(imidazol-1-yl)-butane (Scheme 2). This simple synthetic approach have led to two materials employed as heterogeneous catalysts for the conversion of carbon dioxide into cyclic carbonates. The catalytic activity of the hybrid materials containing two different nucleophilic species, namely bromide and iodide ions, was compared using them as the sole catalysts under metal- and solvent-free reaction conditions showing full selectivity toward the formation of cyclic carbonates, reaching high turnover numbers (TON) and productivity values.

Another catalytic system based on POSS nanocage has been designed following the same approach previously reported for the C₆₀-based catalysts active in the C–C couplings. Grafting of a properly modified POSS onto amorphous silica and mesostructured SBA-15, followed by reaction with different N-alkyl substituted imidazoles (Scheme 3), allowed us to obtain a series of heterogeneous catalytic systems active in the transformation of CO₂ into cyclic carbonates. The influence of the solid support (SiO₂ vs SBA-15) and the effect of both nucleophilic species (Cl^–, Br^–, I^–) and imidazolium alkyl side chain length has been studied. The very good results in terms of catalytic activity and recyclability have been ascribed to high local concentration of the imidazolium active sites surrounding the POSS core immobilized onto the solid supports.

CNFs also find application in the conversion of carbon dioxide into cyclic carbonates. Herein we report two examples in which both carbon nanohorns and single walled carbon nanotubes were easily functionalized by the radical polymerization of a series of vinylimidazolium salts resulting in the full coverage of the carbonaceous support. All the catalytic materials revealed to be recyclable and, interestingly, in the case of CNHs-based materials containing a p-xylyl linker and a different functionalization degree, an unprecedented increase of activity after recycling has been shown (Figure 2).

Synthesis of polyamines

The cross-linked imidazolium network arising from the radical polymerization of the bis-vinylimidazolium salts can be exploited as precursor of cross-linked polyamines by a simple reduction process mediated by sodium borohydride in hot ethanol. This synthetic approach allowed us to have easy access to a series of both supported onto 3‐mercaptopropyl‐modified amorphous SiO₂ or γ-Fe₂O₃@SiO₂, and unsupported cross-linked polyamines (secondary and tertiary amines). We tested the prepared materials as catalysts in the Knoevenagel reaction obtaining good results and a high recyclability up to 11 cycles.

Metal scavengers

Another useful application of IL-based hybrids consists of their use as scavengers for removal of traces metal. In this context, a series of imidazolium‐based materials have been prepared through homo‐polymerisation, immobilization onto 3‐mercaptopropyl‐modified silica gel or co‐polymerisation with ethylene glycol dimethylacrylate. All the materials were tested as palladium scavengers with a set of palladium(0) and palladium(II) compounds in different solvents and at different starting amounts of palladium. Efficiency of metal scavenging has been good in many cases with residual amounts of palladium lower than 5 ppm. In addition, one of the prepared materials has been used as palladium scavenger in a Suzuki coupling reaction resulting in a 29-fold abatement of the palladium content in the final product with respect to the control reaction.