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Activation of molecular hydrogen in solution by complexes of univalent, divalent, and trivalent ruthenium Hui, Benjamin Ching-Yue

Abstract

Kinetic studies of a number of interesting and significant reactions involving reaction of molecular hydrogen, olefins and carbon monoxide with solutions of ruthenium chloride complexes are described. Ruthenium trichloride trihydrate, "RuCl₃.3H₀0", which is a mixture of ruthenium(III) and ruthenium(IV), was found to react with molecular hydrogen in dimethylacetamide (DMA) solution under mild conditions, to produce ruthenium(II) and ruthenium(I) in successive steps involving activation of the hydrogen by ruthenium(III) and ruthenium(II): [forumulae omitted] In aqueous acid solution, the reverse of reaction (1) prevents reduction of ruthenium(III); in DMA, a more basic solvent, the released proton is stabilized and reduction is observed all the way to the univalent state. Convincing evidence was found for the existence of ruthenium(I) in DMA, although no well-characterized ruthenium(I) solid complexes were isolated. The present studies are the first reported on the solution chemistry of ruthenium(I) chlorides. Ruthenium(I) chloride complexes in DMA (80°) were found to activate molecular hydrogen through dihydride formation for the catalyzed reduction of olefins. The following mechanism is indicated: [formulae omitted] Accompanying olefin isomerization and some deuterium isotope studies suggest that reaction (5) goes through an ϭ-alkyl hydride intermediate, the hydrogen transfer process involving two consecutive single hydrogen atom transfers to a coordinated olefin. Addition of triphenylphosphine (PPh₃) to the ruthenium(I) catalyst solution decreases the hydrogenation rate. However, reaction of hydrogen with a ruthenium(I) solution containing PPh₃ and no substrate gave evidence for the formation of a hydride species. In the presence of PPh₃, reaction of H₂ with ruthenium(II) chloride in DMA does not produce ruthenium(I). The ruthenium(II) hydride intermediate is stabilized by the phosphine ligand yielding the well-known complex RuHCl(PPh₃)₃ which has been found to be extremely active in catalyzing the hydrogenation of olefins. An extremely simple method for the preparation of the catalyst "in situ" is demonstrated, again utilising the basic properties of DMA. A mechanism involving a predissociation of the catalyst, and formation of an ϭ-alkyl intermediate is thought to be operative in the catalyzed hydrogenation of olefins: [formulae omitted] Both ruthenium(I) and ruthenium(II) chlorides in DMA were found to absorb carbon monoxide readily at ambient temperatures, producing Ru[superscript I](CO) and Ru[superscript I](CO)₂, and Ru[superscript II](CO) and Ru[superscript II](CO)₂respectively. The introduction of carbonyl groups into these ruthenium complexes was found to inhibit catalytic activity for the hydrogenation of olefins. The anion [RuCI₄(bipyridine)]²⁻ , in 3 M HC1, was found to be a hydrogenation catalyst for olefin reduction, though not a very efficient one. A mechanism similar to the RuHCl(PPh₃)₃ catalyzed system seems to be involved, and is quite different to that reported for a corresponding system involving the tetrachlororuthenate(II) complex, [RuCI₄]²⁻.

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