Synthesis and reactions of organometallic complexes containing trifluoromethylated phosphines

Abstract

This thesis describes an investigation into the synthesis and coordination properties of trifluoromethylated bisphosphines, and a study of the OH activation properties of iron and ruthenium complexes containing trifluoromethylated bisphosphines.

The first part of this work describes the development of a convenient, one step procedure for the synthesis of tn'fluoromethylated phosphines by the low temperature addition of hexaethylphosphorus triamide to a mixture of trifluoromethyl bromide and an appropriate chlorophosphine. l,2-Bis(bis(trifluoromethyl)phosphino)ethane, (dfmpe), was synthesised in 20% isolated yield by this new method, a significant improvement on previous preparations of this compound.

An investigation of the coordination chemistry of dfmpe with iron was undertaken, however no iron complexes containing dfmpe were obtained. The electron-withdrawing properties of the trifluoromethyl groups on dfmpe make this ligand a poor o—donor, which rationalises the difficulties encountered in forming complexes with early transition metals.

The coordination chemistry of dfmpe with ruthenium was also investigated. The complexes RuHCl(CO)(PPh3)(dfmpe), RuH2(CO)(PPh3)(dfmpe) and RuC12(CO)(PPh3)(dfmpe) were synthesised and characterised, but were not found to be useful precursors to the target compound, RuH2(dfmpe)3. The x-ray crystal structure of RuHCl(CO)(PPh3)(dfmpe) was determined and is presented in this work. RuHCl(dfmpe)2 was synthesised by the reaction of excess dfmpe with RuHCl(PPh3)3, and was structurally analysed by x-ray crystallography. The direct synthesis of cis-Rqu(dfmpe)2 in 55% yield by reduction of RuHCl(dfmpe)2 is reported.

RuH2(dfrnpe)2 is an air-stable white powder which forms RuD2(dfmpe)2 upon u.v. irradiation at room temperature under 1 atm of D2 gas. In contrast to its methylated analogue, RuH2(dmpe)2 (dmpe = l,2—bis(dimethylphosphino)ethane), RuH3(dfmpe)2 does not form products when subjected to u.v. irradiation in the presence of alkenes or arenes, or in the presence of carbon monoxide or tn'ethylsilane. This surprising lack of reactivity is a direct consequence of the substitution of fluorine for hydrogen in this complex. Preliminary laser flash photolysis experiments indicate that the back—reaction of the coordinatively unsaturated complex formed under photochemical conditions with liberated hydrogen occurs too rapidly to allow reaction with any organic substrate.

The final section of this work deals with the synthesis of the new unsymmetrical bisphosphine ligand 2-(dimethylphosphino)ethyl(bis(trifluoromethyl)phosphine, MegPCHzCH2P(CF3)2, bife. Selective sulfurisation of MezPCHzCHzPHz to form Me2P(S)CH2CH2PH2 allowed subsequent chlorination of this compound using triphosgene to form Me2P(S)CH2CH2PC12 in a yield of 70%. Application of the trifluoromethylation methodology developed in this work allowed the conversion of Me2P(S)CH2CH2PClz to Me2P(S)CH2CH2P(CF3)2 in 20% yield. Desulfurisation of this compound using tri-nbutylphosphine gave the required ligand, bife, and data for this compound are reported.