Polyhedral oligomeric silsesquioxanes in catalysis and photoluminescence applications

Abstract

Cubic Polyhedral Oligomeric SilSesquioxanes (POSS) of general formula Si₈O₁₂R₈ (R = alkenyl, alkoxy, aryl, hydrogen...) have found applications in various fields ranging from biology to chemistry. Besides the advantage of presenting the characteristic dendritic globular shape at low generation, these three-dimensional molecules, easily modified by organic or inorganic reactions, quickly exhibit multiple end groups at their periphery, thus featuring attractive properties in catalysis and photoluminescence applications.

Various dendritic POSS containing diphenylphosphine moieties at their periphery have been used in the methoxycarbonylation of ethene. Those with a -CH₂CH₂- spacer between the silicon and the phosphorus atoms (G0-8ethylPPh₂ and G1-16ethylPPh₂) only produce methyl propanoate whilst a similar dendrimer with a -CH₂- spacer between Si and P (G1-16methylPPh₂) gives only copolymer. The effect of the molecular architecture is discussed in comparison with the selectivities observed when using small molecule analogues. A wide range of non dendritic monodentate phosphines has also been studied in this reaction showing that low steric bulk and high electron density favours polyketone formation. The poorly active, monodentate SemiEsphos phosphine has been turned into an active ligand for rhodium catalysed vinyl acetate hydroformylation by attachment to the periphery of a Polyhedral Oligomeric Silsesquioxane. Whilst some of these dendritic ligands have shown activity, others precipitated upon mixing with the rhodium precursor. Modelling studies correlating the experimental facts have shown that the former are more compact and rigid in comparison to the latter, which are more flexible and hence more prone to monodentate binding to rhodium and cross-linking. Grubbs cross metathesis has been used to functionalize octavinylsilsesquioxane with fluorescent vinylbiphenyl modified chromophores to design new hybrid organic-inorganic nanomaterials. Those macromolecules have been characterized by NMR, microanalyses, MALDI-TOF mass spectrometry and photoluminescence. This last method was shown to be an interesting tool in the analysis of the purity of the cube derivatives. Reduction of the peripheral 4`-vinylbiphenyl-3,5-dicarbaldehyde groups on a Polyhedral Oligomeric Silsesquioxane (POSS) with NaBH₄ or LiAlH₄ activates the fluorescence of this macromolecule by turning the aldehydic functions into primary alcohols providing novel optical sensors for reducing environments.