Chemical Transformations of Phosphine and Phosphido Ruthenium Complexes

Abstract

Primary phosphines contain two reactive P-H bonds, providing the opportunity for further derivatisation. However, due to their reputation for being toxic, malodorous, and air-sensitive their chemistry is underdeveloped. Phosphido (PR2) groups exhibit curious behaviour as transition metal ligands, with the phosphorus atom binding in either pyramidal and nucleophilic, or planar and electrophilic forms. In the pyramidal case, the phosphorus lone pair shows increased nucleophilicity and basicity. The work described in this thesis endeavours to combine the phosphorus-based reactivity of phosphido complexes with the potential functionalisability of primary phosphines.

Initial efforts focused on utilising PH2Cy in substitution reactions of [RuCl(PPh3)2(Tp)] (Tp = hydrotris(pyrazolyl)borate), yielding the products [RuCl(PPh3)(PH2Cy)(Tp)], [Ru(PPh3)(PH2Cy)2(Tp)]+ and [Ru(PH2Cy)(dppf)(Tp)]+ (dppf = 1,1'-bis(diphenylphosphino)ferrocene). The deprotonation of [RuCl(PPh3)(PH2Cy)(Tp)] resulted in the cyclometallation of the PPh3 ligand and experiments were conducted to gain insight into this process. Deprotonating the cations [Ru(PPh3)(PH2Cy)2(Tp)]+ and [Ru(PH2Cy)(dppf)(Tp)]+ appeared to form the respective phosphido complexes, but the products were extremely basic and were not isolated. The in situ reactivity of putative [Ru(PHCy)(dppf)(Tp)] with AuCl(SMe2) was also investigated.

The introduction of the pi-acidic CO co-ligand was expected to mitigate the high basicity of the phosphorus lone pair, yielding more conveniently-handled phosphido complexes. Beginning from the MeCN-substituted octahedral cation [RuCl(NCMe)(CO)(PPh3)2(PH2Cy)]+ the new complexes [RuCl(CO)2(PPh3)2(PH2Cy)]+, [RuCl(CO)(CNMes)(PPh3)2(PH2Cy)]+, [Ru(CO)(PPh3)2(PH2Cy)(S2CNEt2)]+ and [Ru(CO)(PPh3)(PH2Cy)(Tp)]+ were obtained. The latter product could also be synthesised from [RuH(CO)(PPh3)(Tp)]. The attempted deprotonation of [RuCl(CO)2(PPh3)2(PH2Cy)]+ and [Ru(CO)(PPh3)2(PH2Cy)(S2CNEt2)]+ resulted in an intractable mixture of products while the deprotonation of [Ru(CO)(PPh3)(PH2Cy)(Tp)]+ yielded the phosphido complex [Ru(CO)(PPh3)(PHCy)(Tp)]. The ambiphilic nature of phosphido complexes was also demonstrated through the synthesis of the complex [Ru(CO)2(PPh3)2{PH(OMe)Cy}]. The reactivity of this complex bearing the unusual PH(OMe)Cy ligand was also investigated, mainly focused on ligand substitution and the Ru(0) centre.

The properties of the phosphido complex [Ru(CO)(PPh3)(PHCy)(Tp)] were studied. The complex forms as a mixture of two diastereomers and the kinetics of the exchange process were investigated by NMR spectroscopy. Treating [Ru(CO)(PPh3)(PHCy)(Tp)] with MeI resulted in a mixture of the cations [Ru(CO)(PPh3)(PH2Cy)(Tp)]+, [Ru(CO)(PPh3)(PHMeCy)(Tp)]+ and [Ru(CO)(PPh3)(PMe2Cy)(Tp)]+. The PMe2Cy complex could be obtained from the reaction of [Ru(CO)(PPh3)(PH2Cy)(Tp)]+ with excess base and MeI. The addition of BH3.SMe2 to [Ru(CO)(PPh3)(PHCy)(Tp)] resulted in the formation of [Ru(CO)(PPh3){PH(BH3)Cy}(Tp)], which could also be obtained from [Ru(CO)(PPh3)(PH2Cy)(Tp)]+ and NaBH4. Treating [Ru(CO)(PPh3)(PHCy)(Tp)] with CS2 gave the adduct [Ru(CO)(PPh3){PH(CS2)Cy}(Tp)]. Adduct formation was reversible, and the product could only be isolated following methylation as [Ru(CO)(PPh3){PH(CS2Me)Cy}(Tp)]+.

Treating [Ru(CO)(PPh3)(PHCy)(Tp)] with an appropriate chalcogen source resulted in the complexes [Ru(CO)(PPh3){PH(E)Cy}(Tp)] (E = O, S, Se, Te), but only the sulfide and selenide products were isolated and fully characterised. These two complexes are nucleophilic at the chalcogen atom, reacting with MeOTf to give the cations [Ru(CO)(PPh3){PH(EMe)Cy}(Tp)]+ (E = S, Se). The sulfide complex may also be reversibly protonated to give [Ru(CO)(PPh3){PH(SH)Cy}(Tp)]+. The cation [Ru(CO)(PPh3){PH(SMe)Cy}(Tp)]+ reacts with base to give the transient phosphido complex [Ru(CO)(PPh3){P(SMe)Cy}(Tp)] for which the in situ reactions with MeI, BH3.SMe2 and S8 were explored.