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.
Amorphous silica plays an important role in heterogeneous catalysis as a support and is frequently presumed to be "inert". The structure of the supported catalyst is key to understanding the stability and reactivity of catalytic systems. To provide vital insights into the surface reactivity of silica, Polyhedral oligomeric silsesquioxanes (POSSs) can act as realistic homogeneous molecular models for silica surfaces. Here, we report novel reactivities associated with the silica surface, derived from our insights obtained by means of such model systems with potentially significant implications in catalysis when employing silica-supported catalysts. In this work, the gas-phase reactivities of two cyclohexyl-substituted POSSs, namely the completely condensed triganol prism [Si6cy6O9] (a6b0), and the incompletely-condensed partial cube [Si7cy7O9(OH)3] (a7b3), with cy = c-C6H11, were studied by using atmospheric pressure chemical ionisation (APCI) and collision-induced decomposition (CID) spectroscopies. Silsesquioxane a6b0, containing three-membered rings, was found to be much more reactive, undergoing novel CH2-insertion on reaction with gas phase molecules-a reaction not observed for a7b3, containing only four-membered rings. Both silsesquioxanes displayed the ability to trap ammonia formed in situ within the mass spectrometer from N2 in the instrument. This work also demonstrates the applicability of APCI and the role of CID in elucidating reactive POSS structures, highlighting novel gas-phase reactivities of POSS.
The calcination of pure amorphous silica at temperatures up to 850 °C results in the formation of strained siloxane rings which are capable of undergoing homolytic cleavage to generate radicals when in the presence of an appropriate substrate.
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