Palladium catalysts are still undergoing significant research into their applications for important organic reactions, and phosphine ligand derivatives are an area in which much benefit may still be found. Gilheany Thompson, and Walker have demonstrated that reaction of triphenylphosphine and tropylium tetrafluorborate gave cycloheptatriene triphenylphosphonium tetrafluoroborate [C7H7PPh3][BF4]. Such ligands are unable to coordinate through lone pair donation bonding due to the coordinately saturated phosphionium center, but may still coordinate through eta bonding interactions with noble metal catalysts including palladium and platinum. Our group has demonstrated that reaction of tropylium tetrafluoroborate with a range of commercially available bidentate phosphine ligands gives rise to a new series of compounds. The three complexes that have been synthesized have a bis-(diphenylphosphino)ethane (dppe), bis-(diphenylphosphino)propane (dppp), and bis-(diphenylphosphino)ferrocene (dppf) bridge. These systems give well resolved 1H NMR spectra with characteristic signals revealing the formation of the cycloheptatriene (CHT) ring. 31P NMR showed a systematic shift from -20 ppm to ~23 ppm characteristic of conversion to the expected derivatives. X-ray diffraction data has shown variation in the bond length between the phosphorous and the sp3 carbon of the CHT ring, from 1.8381(19) to 1.840(3) to 1.8343(15) Angstroms. Both the dppe and dppp derivatives had an orthorhombic crystal structure, and the dppf derivative had a triclinic crystal structure. The X-ray diffraction data also demonstrated that the aromaticity of the seven-membered ring was lost, and in all cases is present in the sterically favored boat conformation. Our group will test if once palladium is added, the cycloheptatriene groups bind to palladium atoms to form a bidentate ligand. New complexes with different bridge conformations as well as the palladium-containing derivatives are currently under investigation.

Heavy metal for organometallic reactions: Palladium-phosphonium systems and their catalytic potential

251st National Meeting (2016)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.