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UBC Theses and Dissertations

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UBC Theses and Dissertations

Characterization of chlorohydridobis (tertiaryphosphine) ruthenium (II) complexes, and their use as homogeneous hydrogenation catalysts Thorburn, Ian Stuart

Abstract

The thesis describes a study of the catalytic hydrogenation of an olefinic substrate by the complex hydridochlorobis(triphenylphosphine) ruthenium(II) and an investigation of the complex in the solid state and in solution. The visible spectra of the complex, (HRuCl(PPh₃)₂)₂, at a series of concentrations showed that Beer's law is not obeyed, and that in solution a dissociative equilibrium exists: (1) (HRuCl(PPh₃)₂)₂ K ⇆ (2HRuCl(PPh₃)₂ The complex in N,N-dimethylacetamide solution was found to be an effective catalyst for the homogeneous hydrogenation of hex-l-ene. A detailed kinetic study on this system revealed a mechanism involving initial formation of a σ-alkyl intermediate which then reacts with molecular hydrogen to produce the saturated product and regenerate the catalyst: (2) HRuCl(PPh₃)₂ + olefin k₁ ⇆ k₋₁ RuCl(PPh₃)₂(alkyl) (3) HRuCl(PPh₃)₂(alkyl) + H₂ k₂→ HRuCl(PPh₃)₂ + alkane. The mechanism is quite different from that reported for the same catalyst system but using acrylamide as substrate, thereby showing that the nature of the substrate can have a pronounced affect on the course of hydrogenation. Addition of triphenylphosphine and lithium chloride to the (HRuCl(PPh₃)₂)₂- hex-l-ene system were found to decrease and increase the rate of hydrogenation, retrospectively. The added phosphine likely competes with the olefinic substrate for a coordination site; the role of the chloride ion is more uncertain, but a more active catalyst containing more than one chloride ligand is the most obvious rationale. To enhance the solubility of this hydridophosphine type catalyst the tri-p-tolylphosphine analogue of the triphenylphosphine complex was prepared; the variable temperature ¹H and ³¹P{¹H}-solution n.m.r. of the (HRuCl(P(p-tolyl)₃)₂)₂ complex showed the presence of both monomer and fluxional dimer. Addition of dimethyl maleate to the complex in order to obtain a Ru-alkyl species (equation (2)) gave very complex spectra which could not be interpreted in terms of a single species, but there was some evidence for a hydrido(olefin) species rather than an alkyl. An x-ray analysis of the p-tolyl complex confirmed the expected chloro-bridged dimeric structure of these hydridochlorobis(phosphine) species. There is a square pyramidal coordination geometry about each ruthenium atom, and two such centres share a basal edge, but the molecule has no symmetry as a result of the small hydride ligands allowing distortion.

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