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Activation of molecular hydrogen and olefins in solution by triphenylphosphine complexes of bivalent ruthenium Markham, Larry Derwood

Abstract

Kinetic and equilibrium studies involving solutions of dichlorotris-(triphenylphosphine)ruthenium(II) and the corresponding hydridochloro complex are described, especially reactions involving molecular hydrogen and olefins. In benzene or in dimethylacetamide (DMA) solution, the dichloro complex dissociates with loss of a triphenylphosphine molecule, [See Thesis for equation] and in DMA solution, further dissociation of chloride ion from the bisphosphine complex occurs: [See Thesis for equation]. DMA solutions of RuCl₂(PPh₃)₃ react rapidly and reversibly with molecular hydrogen at room temperature, for example, [See Thesis for equation]. This hydrogenolysis reaction does not occur in benzene solution, but the basic amide solvent promotes hydride formation by effectively stabilizing the released HCl. The relative reactivity toward H₂ of the species present in solution is as follows: RuCl(PPh₃)₂⁺ > RuCl₂(PPh₃)₂ > RuCl₂(PPh₃)₃. For the reverse reaction between hydride complex and HCl, the reactivity order of species present is RuClH(PPh₃)₂ > RuClH(PPh₃)₃. Thermodynamic and kinetic data are given for equilibria such as (3), and thermodynamic data are presented for equilibria (1) and (2). The hydride complex RuClH(PPh₃)₃ is an extremely effective catalyst for the homogeneous hydrogenation of olefins in DMA solution at 35°. Unfavorable steric and electronic factors in the olefin both play a major role in reducing the rate of hydrogenation; these effects can be correlated with the proposed reaction mechanism, which involves a predissociation of the catalyst and the formation of a σ-alkyl intermediate via a hydrido-olefin species: [See Thesis for equation]. Equilibrium constants for reaction (5) with a variety of olefins, are presented together with rate data for reaction (6). Limited studies have been carried out using the corresponding hydridobromo and hydrido-acetate complexes. The hydride complex RuClH(PPh₃)₃ > isolated as a DMA solvate, is also effective as a catalyst for the polymerization of both ethylene and butadiene in DMA solution. The kinetics of these reactions have been studied and analyzed in terms of a mechanism that involves initial formation of a σ-alkyl complex, and propagation via insertion of coordinated olefin into the Ru-carbon bond, for example, [See Thesis for equation]. The ethylene and butadiene systems show different kinetic dependences which are accounted for by the stronger complexing of the diene.

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