Design, synthesis and application of silica-grafted bis(diphenylphosphino)amines and related compounds in heterogenised hydroformylation catalysis

This project develops a range of heterogeneous hydroformylation catalysts by immobilisation of discrete rhodium complexes onto a silica support via a bidentate phosphine or phosphine sulfide ligand bearing a pendant silica-tethering functionality. Bidentate phosphine ligands capable of silica binding were prepared with various silica tethering groups (alkoxysilane, silatrane and cyclic azasilane). A range of ligands with different phosphine steric and electronic properties were synthesised to enable investigation into the effect of ligand structure on immobilised hydroformylation catalysis. The synthesis of homogeneous hydroformylation catalyst precursors was achieved by reaction of the aforementioned diphosphine ligands with [Rh(COD)2]BF4, resulting in a [Rh(P^P)2]BF4 or [Rh(L^L)(COD)]BF4 complex (P^P = bidentate phosphine ligand, L^L = bidentate ligand), depending on the ligand used. Immobilisation of the prepared rhodium complexes [Rh(P^P)2]BF4 or [Rh(L^L)(COD)]BF4 was achieved by their reaction with AEROPERL 300/30 silica (calcined at 200 °C), and confirmed by solid-state 31P and 29Si NMR spectroscopic and ICP-OES elemental analyses. However, reaction of the parent phosphines with silica results in significant side reactions between the phosphine moieties and silica surface, leading to the formation of unidentified P(V) species. The prepared homogeneous and heterogeneous pre-catalysts were investigated for the hydroformylation of 1-octene (conditions: 90 °C, 20 bar (1:1 H2:CO), [1-octene]/[Rh] = 1000, toluene solvent). Silica-immobilised [Rh((Ph2PCH2)2N(CH2)3Si(OCH2CH2)3N)2]BF4 was found to provide the best catalyst performance of the heterogenous catalysts investigated, giving a TOF of 682 ± 48 h-1 and an aldehyde l:b ratio of 2.6, catalytic performance exactly mirroring that of its soluble analogue in solution. Leaching studies of silica-immobilised [Rh((Ph2PCH2)2N(CH2)3Si(OCH2CH2)3N)2]BF4 found a 1.4% loss of rhodium from the catalyst after a one hour hydroformylation experiment. However, recycling studies showed no loss in activity after one recycle, followed by a subsequent large decrease in catalytic activity down to 25% of initial activity after five runs.