Polyhedral Oligomeric Silsesquioxane (POSS) based catalysts 

Our research activity dealing with POSS-based hybrids started with the synthesis of imidazolium modified POSS nanocages as homogeneous catalyst for the synthesis of cyclic carbonates from CO2 and epoxides (Figure 1). This imidazolium functionalized POSS showed improved activity compared to the unsupported 1-butyl-3-methylimidazolium salt. This behavior was ascribed to a proximity effect related to the higher local concentration of imidazolium active sites surrounding the inorganic silsesquioxanes core. Imidazolium functionalized POSS molecules were tested with styrene oxide, epichlorohydrin and 1-butene oxide under optimized reaction conditions using (isopropanol as co-solvent, 150 °C, 4 MPa of CO2) showing turnover frequency (TOF) values in the 138.7-706 h-1 range. 

The influence of POSS nanocages on the catalytic performance for C–C couplings was the subject of a study on a homogeneous pre-catalyst based on POSS nanocages bearing imidazolium tetrachloropalladate moieties (Figure 2). We made a comparison between the POSS-based catalyst and the corresponding POSS-free 1-butyl-3-methylimidazolium tetrachloropalladate highlighting the role of the POSS nanocages to reach higher yields in the Suzuki–Miyaura reaction. This result was ascribed, also in this case, to a proximity effect of the imidazolium moieties linked to the nanocaged structure.

Highly performing metal-free heterogeneous catalysts based on POSS nanostructures functionalized with imidazolium salts have been developed. As showed in Scheme 1, a broad series of catalytic materials was prepared by using a modular synthesis to obtain imidazolium modified POSS nanocages grafted onto amorphous silica (SiO2) and mesostructured SBA-15. The synthetic procedures were designed to study 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. All the solids were tested as heterogeneous organocatalysts for the conversion of epoxides and CO2 into cyclic carbonates in solvent-free reaction conditions. The effect of the nucleophilic species led to the overall order of activity I- > Br- > Cl-. The main goal of this research was to maintain the high catalytic activity of unsupported imidazolium modified POSS, due to the proximity effect of the imidazolium units linked to the POSS nanocage, with the benefits of heterogeneous catalysis, in terms of recyclability, without the need of other co-catalytic species with Brønsted or Lewis acid functionalities. The proposed materials were easily recyclable as well as highly active toward the formation of cyclic carbonates affording very high turnover numbers (TON) and productivity values up to 7875 and 740, respectively. 

Imidazolium functionalized POSS nanocages grafted onto mesostructured SBA-15 allowing a suitable stabilizing support for Pd (II) pre-catalytic species toward Suzuki and Heck cross-coupling reactions. In both processes, the catalytic performances were extensively evaluated in terms of turnover frequency, versatility and recyclability. The catalyst proved to be highly recyclable up to seven cycles without any decrease in the catalytic activity. The recyclability of the material was checked at 0.07 mol% of Pd for six consecutive runs. Interestingly, the catalyst recovered from the sixth cycle was reused for further recycling experiments with decreased amounts of Pd namely for Suzuki (0.007 and 0.0007 mol%) and Heck (0.007 mol%) reactions. The direct comparison of the abovementioned catalytic tests with the analogous ones performed at 0.007 and 0.0007 mol% of Pd with the as-synthesized material assessed the high recyclability of the material. The presence of the POSS nanocage and the textural properties of SBA, allowed obtaining a uniformly distribution of Pd species within the pore walls, acting as a nanoreactor helping to overcome palladium aggregation and limiting leaching phenomena. 

We prepared two novel hybrid materials based on highly crosslinked imidazolium networks from POSS molecular building blocks in order to be used as heterogeneous catalysts bearing different nucleophilic species (bromide and iodide). The solids were synthesized by using the simple one-pot procedure depicted in Scheme 2. The solids and were tested as the sole catalyst under metal- and solvent-free reaction conditions showing full selectivity toward the formation of cyclic carbonates. High TON and productivity values up to 5502 and 1081, respectively for glycidol at 100 °C and up to 4942 and 1122 for epichlorohydrin at 150 °C after 3 h, were afforded. Such outstanding productivity values were ascribed to the optimal organic/inorganic (i.e. imidazolium moiety/POSS support) weight ratio. The good recyclability of the materials (up to four cycles) allowed considering them as promising candidates for continuous flow technologies.