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Solution reactions of HX molecules (X = SH, Cl, Br) with dinuclear palladium(I) complexes containing bis(diphenylphosphino)methane

University of British Columbia

This thesis describes kinetic and spectroscopic studies on the reaction between Pd₂X₂(dpm)₂, 1 (X = CI, a; Br, b; I, c; dpm = bis(diphenylphosph-ino)methane) and HX molecules (X = SH, CI, Br) in solution. The reaction of 1 with hydrogen sulfide leads to the formation of the corresponding μ-sulfide, 2, and H₂ gas. Kinetic studies carried out in CH₂CI₂ solutions over the temperature range 0 - 35°C reveal a first order dependence on both palladium dimer and hydrogen sulfide concentrations. The value of the bimolecular rate constant, k₂, is 1.71 x 10⁻³ M⁻¹ s⁻¹ at 25°C, and the activation parameters for the reaction are ΔH[sup =++] = 55 ± 5 kJ mole⁻¹ and ΔS[sup =++] = -115 ± 10 J K⁻¹ mole⁻¹. Low temperature ¹H and ³¹P{¹H} nmr spectroscopic investigations complement the kinetic studies, and show that the reaction proceeds via the formation of a hydride intermediate. The observations indicate that the reaction proceeds through oxidative-addition of H₂S across the metal-metal bond and the data are discussed in terms of the following reactions: [See Thesis for Diagrams]. There is no direct evidence for the first reaction listed, however. The analogous reactions between 1 and HX (X = CI, Br) were studied with UV-visible,¹H and ³¹P{¹H} nmr spectroscopy. The reaction between HCl and la in CH₂Cl₂ results in the 'direct' formation of Pd(II) monomer, PdCl₂(dpm), with no intermediates being seen. The corresponding reaction between the bromide, 1b and HBr, however, is seen to take place in several steps. Reaction with one mole of HBr results in the formation of a hydride intermediate; this then reacts with a second mole of HBr to form H₂ gas and a tetrabromo complex, Pd₂Br₄(dpm)₂, which subsequently fragments slowly to the Pd(II) monomer, PdBr₂(dpm). All the intermediate species involved in the bromide/HBr reaction were detected by UV-visible and low temperature ¹H and ³¹P{¹H} nmr spectroscopy. An effective way of abstracting the bound μ-sulfide ligand in order to regenerate the palladium dimer, 1, has been demonstrated using reactions with dpm and with diphenyl; the abstraction by dpm results in formation of dpmS, the monosulfide of dpm, as the only sulfur-containing product. A new and as yet incompletely characterized system that catalytically desulfurizes H₂S was generated by having excess dpm present in the reaction between 1b and H₂S. This catalytic system tranfers sulfur from H₂S to the bisphosphine dpm to form dpmS along with some uncharacterized palladium compounds.

Text

2010-08-14

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http://hdl.handle.net/2429/27390

Chemistry

University of British Columbia

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