Fluorocarbon complexes of the platinum group metals
Abstract (Summary)Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. Fluorocarbon compounds have a wide range of applications, from lubricants to pharmaceuticals and as a consequence their synthesis and chemical modification is of great importance. The use of metal complexes as sites for modification of small organic compounds is well established. Transition metal complexes of fluorocarbons should offer the same opportunities. Such complexes have been known for thirty years but, despite this, they are still quite poorly understood. This thesis aims to extend our knowledge of the range and the reactivity of fluorocarbon complexes of the metals rhodium, iridium, ruthenium and osmium. Chapter one is a survey of the synthesis, properties, and reactivity of ?-bonded fluorocarbon complexes. The reactions of fluorinated olefins with transition metals are also examined, along with the synthesis and properties of difluorocarbene complexes. This provides a general introduction to the complexes which are discussed in the following chapters. Chapter two describes the synthesis of new d8 trifluoromethyl complexes of iridium and rhodium. Both Rh(CF3)(CO)(PPh3)2 and Ir(CF3)(CO)(PPh3)2 displayed reactivity consistent with 16 e- d8 complexes. They react with a number of small molecules including O2, X2 and MeI. More interesting was the reactivity of the ?-fluorines in these complexes. Rh(CF3)(CO)(PPh3)2 undergo reactions with aqueous acids resulting in hydrolysis of the trifluoromethyl group. Confirmation that this reaction is proceeding via a difluorocarbene intermediate was found when the reaction was repeated using dry HCl. The product in this instance was RhCl2(CF2H)(CO)(PPh3)2. A careful study of this reaction, using multinuclear NMR spectroscopy and 2H labelling experiments, enabled the proposal of a mechanism for the formation of RhCl(CF2H)(PPh3)2 which involved hydride migration to a cationic difluorocarbene ligand bound to rhodium. The reactivity of the ?-fluorines in Ir(CF3)(CO)(PPh3)2 was less than those in its rhodium analogue, giving only IrHCl(CF3)(CO)(PPh3)2 when Ir(CF3)(CO)(PPh3)2 was treated with dry HCl. The action of stronger Lewis acids did, however, result in abstraction of an ?-fluorine. Treatment of Ir(CF3)(CO)2(PPh3)2 with AlCl3 gave [Ir(CF2)(CO)2(PPh3)2]+ which eventually gave IrCl(CF2)(CO)(PPh3)2. This difluorocarbene complex could also be produced from the thermal decarbonylation of Ir(C[O]CF2Cl)(CO)2(PPh3)2. This is the first synthesis of a difluorocarbene from reverse migration of a substituent on a difluoromethyl group. IrCl(CF2)(CO)(PPh3)2 along with other d8 difluorocarbene complexes of iridium IrI(CF2)(CO)(PPh3)2 and Ir(CF3)(CF2)(CO)(PPh3)2 displayed both nucleophilic and electrophilic behaviour at the carbene carbon. IrI(CF2)(CO)(PPh3)2 and Ir(CF3)(CF2)(CO)(PPh3)2 were produced from the reaction of Cd(CF3)2•DME with IrI(CO)(PPh3)2 or Ir(CF3)(CO)(PPh3)2. Ir(CF3)(CF2)(CO)(PPh3)2 was also produced from the reaction of IrCl(CO)(PPh3)2 or IrCl(CF2)(CO)(PPh3)2 with Cd(CF3)2•DME and was characterised by an X-ray crystal structure analysis. This is the first complex which contains both a difluorocarbene and trifluoromethyl ligand attached to the same metal centre. In the synthesis of Ir(CF3)(CF2)(CO)(PPh3)2, Cd(CF3)2•DME acted as both a trifluoromethyl and a difluorocarbene source. The examination of the structures of a series of complexes Ir(CF3)(L)(CO)(PPh3)2(L=CO, CF2 and C2F4) helped demonstrate the effects that varying the ?-acceptance ability of a ligand has on metal geometry. Chapter three describes the formation of a variety of complexes of ruthenium and osmium which contain electron withdrawing olefins as ligands. As with the iridium series described in Chapter 2, the geometry of the metal complex was influenced by the ?-accepting ability of the olefin. X-ray structural determinations of Ru(C2F4)(CO)2(PPh3)2, Os(maleic_anhydride)(CO)2(PPh3)2 and Os(C2F4)Cl(NO)(PPh3)2 help emphasize this effect. In a number of cases the olefin complexes display isomerization between two or more isomers in solution. A variable temperature NMR investigation of the cis/trans equilibrium for Os(C2F4)(CO)2(PPh3)2 revealed that ?H?15 kJmol-1 and ?S?60 kJmol-1. The X-ray structure of a four electron donor acetylene complex, Os(CS)(Ph-C?C-Ph)(PPh3)2 is described. The reactions of the tetrafluoroethylene complexes isolated in chapter three, along with IrCl(C2F4)(PPh3)2 and RhCl(C2F4)(PPh3)2, are described in Chapter four. ?-Coordinated tetrafluoroethylene proved an excellent source of ?-bonded tetrafluoroethyl and halotetrafluoroethyl ligands. The reaction of HCl with coordinated tetrafluoroethylene complexes such as IrCl(C2F4)(PPh3)2 and Ru(C2F4)(CO)2(PPh3)2 gave tetrafluoroethyl complexes, IrCl2(CF2CF2H)(PPh3)2 and RuCl(CF2CF2H)(CO)2(PPh3)2 respectively. The five coordinate complex IrCl2(CF2CF2H)(PPh3)2, reacted with a number of neutral ligands to give stable six coordinate complexes. The reactions of ?-coordinated tetrafluoroethylene complexes with halogens, paralleled those with acids, except that the products were halotetrafluoroethyl complexes. The X-ray structures of two complexes, (OsI(CF2CF2I)(CO)2(PPh3)2 and IrCl2(CF2CF2Cl)(CO)(PPh3)2), containing these unusual ligands are described. The relative geometry of the products resulting from halogen addition provided some indication as to the mechanism of these reactions. The ?-fluorines of the tetrafluoroethyl and halotetrafluoroethyl complexes were found to be much less reactive than those of the related trifluoromethyl groups. Complexes such as OsCl(CF2CF2H)(CO)2(PPh3)2 failed to react with BCl3 even when treated for long periods at room temperature. The reactivity of the ?-fluorines was found to be related to the relative electron density at the metal centre. Complexes containing more electron releasing groups such as isocyanide, reacted much more readily with acids, than did their carbonyl containing counterparts. Thus, OsCl(CF2CF2H)(CO)(CNR)(PPh3)2 reacted quickly with aqueous HCl to give the acyl complex OsCl(C[O]CF2H)(CO)(CNR)(PPh3)2. The same reaction with ruthenium proceeded at such a rate that neither RuCl(CF2CF2H)(CO)(CNR)(PPh3)2 nor RuCl(C[O]CF2H)(CO)(CNR)(PPh3)2 could be detected. The product from this reaction (when it was carried out in the presence of methanol) was RuCl2(=CHOMe)(CO)(PPh3)2. This methoxy carbene complex was structurally characterised and clearly showed the delocalization of the double bond, a common feature of heteroatom-substituted carbenes. A mechanism for the formation of RuCl2(=CHOMe)(CO)(PPh3)2 has been proposed. This was supported by a 2H labelling study and the isolation of key intermediates derived from a related complex, Ru(CF2CF2H)(S2NEt2)(CO)(PPh3)2. The formation of IrCl2(CF2H)(CO)(PPh3)2 from the reaction of IrCl2(CF2CF2H)(CH3CN)(PPh3)2 with HCl is also discussed. A related synthesis of tetrafluoroethyl complexes involves the reaction of tetrafluoroethylene with transition metal hydride and alkyl complexes. These reactions were investigated in Chapter five. While a number of ruthenium hydride and alkyl complexes reacted with tetrafluoroethylene, the products isolated did not contain any fluorocarbon ligands. The decomposition of the fluorocarbon complexes under the conditions required for their formation was thought to be responsible. In contrast, RhH(CO)(PPh3)3 inserted tetrafluoroethylene at room temperature to form Rh(CF2CF2H)(CO)(PPh3)2. This reactivity was also observed for the iridium complex IrH(CO)(PPh3)3, but this reaction required much more forcing conditions. In the iridium reaction the intermediate complex IrH(C2F4)(CO)(PPh3)2 was isolated. This complex was then converted to a tetrafluoroethyl complex by treatment at high temperatures in the presence of a coordinating ligand. In this way Ir(CF2CF2H)(C2F4)(CO)(PPh3)2, Ir(CF2CF2H)(CO)2(PPh3)2 and Ir(CF2CF2H)(C2H4)(CO)(PPh3)2 were produced. Ir(CF2CF2H)(C2H4)(CO)(PPh3)2 had an interesting geometry, with the ethylene ligand fixed and not undergoing free rotation. The yellow four coordinate complex Ir(CF2CF2H)(CO)(PPh3)2, was produced from the thermally induced loss of ethylene from Ir(CF2CF2H)(C2H4)(CO)(PPh3)2. Ir(CF2CF2H)(CO)(PPh3)2, in contrast to Ir(CF3)(CO)(PPh3)2, was not stable in solution and rapidly disproportionated to give Ir(CF2CF2H)(CO)2(PPh3)2. This lack of stability found for Ir(CF2CF2H)(CO)(PPh3)2 was attributed to ?-elimination as a possible decomposition pathway. The X-ray structure of Ir(CF2CF2H)(CO)2(PPh3)2 was examined and the metal geometry was found to be different from that found in solution. Chapter six deals with the addition of olefins to a d8 methylene complex, Ru(CH2)Cl(NO)(PPh3)2. The result of addition of electron withdrawing olefins to Ru(CH2)Cl(NO)(PPh3)2 was formation of the ylide complexes Ru(CH2PPh3)(olefin)Cl(NO)(PPh3). These complexes could be detected in solution by NMR spectroscopy, but in most cases were not stable except in the case of the tetrafluoroethylene complex. Both Ru(CH2PPh3)(C2F4)Cl(NO)(PPh3) and Os(CH2PPh3)(C2F4)Cl(NO)(PPh3) were isolated as stable crystalline solids. Ru(CH2PPh3)(C2F4)Cl(NO)(PPh3) has been further characterised by an X-ray structural investigation. The insertion of the methylene fragment into the metal-phosphorus bond, rather than the metal-olefin bond, was unexpected. There was evidence for the formation of a metallacyclobutane when the olefin used was ethylene. The reaction of Ru(CH2)Cl(NO)(PPh3)2 with ethylene is discussed along with an attempt to rationalise the difference between the reaction of ethylene and other olefins. The appendix contains the relevant data on the collection and reduction of the crystal structures described in this thesis along with the atom coordinates and thermal parameters for each structure.
School Location:New Zealand
Source Type:Master's Thesis
Date of Publication:01/01/1989