CHR‐Insertion (RH, CH₃) into Cyclohexyl‐Substituted Silsesquioxanes: Reactivity and Decomposition Studies

Abstract

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.

Strained surface siloxanes as a source of synthetically important radicals

“…6. In addition to the molecular ion at m/z ¼ 811.33900 (not shown), 29 the spectrum suggests that methyl acrylate reacts with this molecular ion, generating several products (see Table 4 and Fig. 7).…”

“…As seen in Table 2, the silsesquioxane a6b0 is capable of catalysing the polymerisation of methyl acrylate in hexane at 70 C. Attempts to isolate a reaction intermediate from this reaction proved unsuccessful, thus, it was decided to interrogate the reaction system using mass spectrometry as previously undertaken for simple reactive molecules. 29 The high resolution APCI mass spectrum of the silsesquioxane a6b0 (R ¼ C 6 H 11 , 1) obtained with the co-injection of methyl acrylate/CH 3 CN is shown in Fig. 6.…”

“…The NH 3 is formed in the mass spectrometer as a result of the reaction of the N 2 carrier gas passing through the discharge in a proton rich environment. 29 ] + ; calculated m/z ¼ 884.39228) is the result of addition of two methyl acrylate molecules to form the unobserved species with m/z ¼ 959 followed by loss two C 2 H 4 units and CO 2 . The proposed structures of the species identied are shown in Fig.…”

“…30 Notably, such addition products are completely absent when the same experiment is performed using the incompletely condensed silsesquioxane a7b3 (R ¼ C 5 H 9 ) in the presence of methyl acrylate, with only the expected elimination products for the silsesquioxane observed. 29…”

“…A detailed mass spectroscopic study 29 shows that Si 3 O 3 rings, indeed, react with the organic substrate, whereas Si 4 O 4 rings do not. Therefore, in the case of strained Si 3 O 3 there is now good evidence of the organic substrate either inducing radical formation or arresting extremely fast radical interconversions within the strained ring structure.…”