Synthesis and characterization of tridentate phosphine ligands incorporating long methylene chains and ethoxysilane groups for immobilizing molecular rhodium catalysts

Abstract

The new tridentate phosphine ligands EtOSi((CH2)nPPh2)3 and O[Si((CH2)nPPh2)3]2 (n = 4, 7, 11), as well as their precursor ethoxysilanes EtOSi((CH2)mCH = CH2)3 and disiloxanes O[Si((CH2)mCH = CH2)3]2 (m = 2, 5, 9) and EtOSi((CH2)11PPh2RhClCOD)3 (COD = cyclooctadiene) have been synthesized and fully characterized. The ethoxysilane- and disiloxane-containing phosphine ligands have been immobilized on silica via covalent siloxane bonds. The immobilized linkers have been characterized by 31P and 29Si CP/MAS and HRMAS NMR spectroscopy. The covalent siloxane bonds between the linkers and the silica surface prevent translational mobility of the phosphines. Immobilized Wilkinson-type Rh hydrogenation catalysts have been obtained by ligand exchange of ClRh(PPh3)3 and ClRhpyCOD (py = pyridine) with the surface-bound ligands. The single crystal X-ray structure of ClRhpyCOD has been reported. The activities and lifetimes have been studied for the hydrogenation of 1-dodecene. The new catalysts are highly active and they can be recycled up to 15 times without major loss of activity. Within the first hours of the catalytic reaction the initially molecular complexes form Rh nanoparticles with a narrow size distribution on the surface. The nanoparticles do not leach into the supernatant solution and are not air-sensitive.