Immobilization of alkoxylated phosphine ligands and their Rh complexes to a silica surface coated with an organic mono- or multilayer

Abstract

A new methodology for the immobilization of homogeneous catalysts on a modified silica surface is suggested. Phosphine ligands with polar substituted groups as well as their rhodium carbonyl complexes are attached to modified silica substrates. The modified silica substrates were prepared by either depositing a crosslinked poly(hydroxyethyl methacrylate) network on the surface of porous silica, or by transforming the surface-pendant hydroxyl groups of silica into [3-(siloxysilyl)propyl]-ethylenediamine groups. They are denoted as P(HEMA-MBAM) /SiO 2 and SSPEDA/SiO 2, respectively. The following phosphine ligands were attached to the modified silica: (1) α,ω-bis(diphenylphosphino)-poly(ethylene glycol) (DPP-PEG); (2) 2-methoxy-methylenephenyl-diphenylphosphine (MPDPP); and (3) 1,2-bis{[(2-methoxyacetylphenyl)-phenyl-(diphenyl)]phosphino}ethane (MAPDPPE). The FT-IR investigations of these immobilized catalysts revealed hydrogen bonding between the surface-pendant hydroxyl or amino groups and the oxygen atoms of the phosphine ligands. The SEM and EDS surface analyses showed that the phosphine rhodium carbonyl complexes were uniformly distributed over the polymer coated silica. Hydroformylation reactions of several olefins were employed for the assessment of the efficiency of immobilization, and both hydrophobic (cyclohexane) and hydrophilic (water) reaction media have been employed. The time dependence of the hydroformylation of 2-(±)-ethylhexyl acrylate (EHA) in a hydrophobic medium indicated that the modified silica surfaces are more compatible with the hydrophobic reactant molecules than the pure silica surface, and that SSPEDA/SiO 2 is more effective than P(HEMA-MBAM)/SiO 2. The much higher hydroformylation rate for sodium 10-undecenoate (UDNa) than for 1-vinylimidazole when water was used as reaction medium is due to the higher ability of the former surface active molecules to be adsorbed on the catalyst surface. Successive reuses of the immobilized catalysts in the hydroformylation of ethyl 10-undecenoate in cyclohexane have demonstrated that MAPDPPE provides stronger immobilization than MPDPP.