Silsesquioxanes as molecular analogues of single-site heterogeneous catalysts

Abstract

We discuss herein selected examples of metal complexes of polyhedral oligosilsesquioxanes (POSSs) as models of single-site heterogeneous surface catalysts. The utility of these compounds as such models is illustrated when employed as analogues of single-site titanium species supported on a silica surface. Deep insights into structure-functionality relationships can be gained. In particular, it was possible to probe the relationship between accessibility of the reactive centre and turnover frequencies in a manner that is impossible for a purely heterogeneous catalyst. We also report that the partially dehydroxylated SiO 2 surface alone is an effective radical polymerization initiation catalyst. This surface reactivity is modelled by the solution reactions between the olefin substrate and two POSSs, the completely condensed triganol prism, Si 6 Cy 6 O 9 (a6b0, Cy = Cyclohexyl, C 6 H 11 ), and the incompletely condensed partial cube, Si 7 Cy 7 O 9 (OH) 3 (a7b3). The former, with six-membered Si 3 O 3 rings, is a catalyst. The latter, without this feature, is not. Similar reactivity discrimination is observed in the gas phase reactions of these POSSs with the olefin substrate, examined using atmospheric pressure chemical ionization-and collision-induced decomposition spectroscopies. Silsesquioxane a6b0, containing Si 3 O 3 rings, reacts with the olefin, forming grafted olefin monomers and dimers, while this reactivity is not observed with silsesquioxane a7b3.

Unveiling the Active Surface Sites in Heterogeneous Titanium-Based Silicalite Epoxidation Catalysts: Input of Silanol-Functionalized Polyoxotungstates as Soluble Analogues

“…Modeling such a system at a molecular scale is rather difficult. One should mention the titanium-containing polyhedral oligomeric silsesquioxanes (POSS), probably the most relevant example, which enable the coordination of Ti in a tripodal siloxy coordination site. − Nevertheless, the POSS ligands remain flexible enough to favor the expansion of the coordination shell and to easily promote the formation of dimers and also the generation of small embedded polyoxotitanate clusters [TiOH] 4 when in contact with water . Accordingly, the site-isolated model, as in heterogeneous systems, no longer applies.…”

The societal significance of catalysis and the growing practical importance of single-site heterogeneous catalysts

“…It interacts with the silanol (Si-OH) groups at the mesoporous silica surface- fig. 2 of the article by Ward et al (2012) for an illustration of this step-and, in the presence of an added, simple amine, the titanocene loses its chlorine atoms and ends up as a half-sandwich compound, with just one cyclopentadiene (cp) as shown in figure 6. Because the cp groups still attached to each anchored Ti atom exert mutual, van der Waals repulsion, this ensures that the Ti IV active centres that are finally generated (by gentle oxidation of the C 5 H 5 moieties) are spatially well separated.…”

Organosilicon platforms: bridging homogeneous, heterogeneous, and bioinspired catalysis

“…Nonetheless, silsesquioxane with strained Si 3 O 3 six-member siloxane rings 4 catalyzed radical polymerization of methyl acrylate, similar to a SiO 2 sample that had undergone prolonged dehydration at 600 1C. 58 The silanol groups of incompletely condensed silsesquioxane can be coupled to different organic or organometallic compounds to form many catalytically interesting materials. Furthermore, with a judicious choice of solvents, these materials can be used as homogeneous catalysts and yet recoverable by precipitation or phase segregation by simply changing solvents after reaction.…”

“…Besides being superior homogeneous catalysts, 61 these metallasilsesquioxanes also serve as excellent molecular analogues of single-site heterogeneous catalysts. 58 One interesting property pointed out by Feher is that, whereas the geometric arrangement of ligands around a metal complex is often directed by the metal, the silanols of incompletely condensed POSS structures direct the bonding to the metal, similar to silanols on a silica surface. 55 Metallasilsesquioxanes have been examined as catalysts for many different reactions including epoxidation, Lewis acid catalyzed addition, and polymerization.…”

“…Upon storage in a CDCl 3 solvent, it began to equilibrate with the dimeric 5-coordinated Ti complex 9b. 58,68 Apparently, the monomeric form is the kinetic product, but the dimeric form is also thermodynamically possible. The coexistence of the two forms is a consequence of the flexible Si-O-Si bonds that readily accommodate the changes in the bonding of Ti to the silsesquioxane framework.…”