Unsymmetrical tridentate phosphine ligands

Abstract

A general synthetic route, via phosphonium salts and phosphine oxides has been developed for the preparation of unsymmetrical triphosphine ligands, which necessarily contain one central chiral phosphorus atom. The ttparent" tridentate phosphine is (2- diphenylphosphinoethyl)(3-diphenylphosphinopropyl) phenylphosphine, Ph2P(CH2)3PPh(CH2)2PPh2, "eptp". Other tridentates were prepared by varying the substituents at one phosphorus atom to form the analogous ligands with a general formula of R2P(CH2)3PPh(CH2)2PPh2 where R=p-FC6H4, p-C1C6H4, o-CH30C6H4. The ligands were found to complex readily with nickel(Il) and palladium(ll) to form air-stable complexes containing square planar species such as [NiI(eptp)]+ in solution. Complexes were isolated using C104, PF6- and 1 as counterions, and were characterised by elemental analysis, magnetic and conductance measurements, infrared spectroscopy, and especially by 31 P nmr spectroscopy where the three non-equivalent phosphorus nuclei, all coupled to one another, provided a wealth of useful data. Crystals were grown for a single-crystal X-ray crystallographic study of the Ni12 complex of eptp, which showed one weakly bonded iodo ligand at the apex of a square pyramid. This is the site at which a substrate molecule would be expected to coordinate in any asymmetric homogeneous catalytic application; the other side of the square plane was sterically crowded. The unit cell contained four molecules, two of each enantiomer. The nickel(II)-triphosphine complexes were screened for catalytic activity in the hydrogenation of alkenes. In this investigation they were all found to be inactive for hydrogenation at ambient conditions. However addition of a little NaBH4 produced colourless solutions, presumably nickel(0) complexes, which in air were found to be excellent oxidation catalysts, rapidly oxidising added phosphines to phosphine oxides. The phosphine oxide intermediate in the tridentate phosphine ligand synthesis, Ph2P(0)(CH2)3P(0)Ph(CH2)2PPh2, teptp02, provided an attractive stage at which to investigate resolution of the two enantiomers, since the final trichlorosilane reduction stage of the phosphine oxides to phosphines is known to be stereospecific. 31P nmr studies showed strong chiral recognition between eptp02 and (+)-camphorsulphonic acid in CDCI3. The novel mixed phosphine/phosphine oxide ligand eptp02 was found to coordinate exclusively via phosphorus to palladium(Il) in [PdC12(eptpO2)21 or via the phosphine oxide groups to lanthanum(lII). It would also coordinate simultaneously to both, and so is ideally suited to the preparation of very unusual heterobinuclear complexes containing one oxophilic f-block metal ion (e.g. La 3 ) and one 'soft' d-block metal ion (e.g. Pd2+). Such complexes presumably contain macrocyclic rings involving both types of metal ion. Various other routes to unsymmetrical tridentate ligands were investigated which involved some unusual mixed bidentate ligands. A new route to bisphosphine monoxides, Ph2P(CH2)P(0)Ph2, has been developed in this programme. These monoxides provided a simple route to new tn- and tetra- phosphine oxides such as eptp03 and the "2,3,2"-tetraoxide , Ph2P(0)(CH2)2P(0)Ph(CH2)3P(0)Ph(CH2)2P(0)Ph2. In collaboration with The University of Barcelona, (Campus of Tarragona), the bisphosphine monoxides with -(CH2)2- and -(CH2)4- backbones were converted to ligands containing both a phosphine sulphide and a phosphine oxide group e.g. Ph2P(S)(CH2)2P(0)Ph2. This ligand and the bisphosphine monoxide, were found to coordinate to tin(II) chloride through the phosphine oxide only. Whereas various alkyl and aryl substituents at phosphorus have been extensively investigated in polyphosphine ligands by previous workers, the pyridyl substituent has not been used in this way. This would be an interesting substituent as it is sterically similar to phenyl but electronically quite different. New routes to a pyridyl substituent at phosphorus were investigated, producing the previously unknown pyridyltriphenyiphosphonium salt [PPh3(2-py)]Br and the ligand 6-bromo-2(diphenylphosphino)pyridine. When hydrolysed phosphonium salts containing a pyridyl group were found to lose the pyridyl group in preference to phenyl, so elaboration to di- and tri-phosphines was not possible