Nonclassical ruthenium silyl dihydride complexes and their catalytic activities : catalytic reactions with electrophilic bipyridine ruthenium complexes

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Nonclassical ruthenium silyl dihydride complexes and their catalytic activities : catalytic reactions with electrophilic bipyridine ruthenium complexes

 

Author: Lee, Ting-yan
Title: Nonclassical ruthenium silyl dihydride complexes and their catalytic activities : catalytic reactions with electrophilic bipyridine ruthenium complexes
Degree: Ph.D.
Year: 2011
Subject: Ruthenium compounds.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Applied Biology and Chemical Technology
Pages: xxiii, 200 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2456232
URI: http://theses.lib.polyu.edu.hk/handle/200/6203
Abstract: The X-ray crystallographic study showed that it is more appropriate to describe the ruthenium-silane complexes TpRu(PPh₃)"H₂SiR₃" as TpRu(PPh₃)(η³-HSiR₃H), a static structure containing H...Si...H bonding rather than a highly fluxional pair of σ-silane hydride species TpRu(PPh₃)(Ha)(η²-HbSiR₃H) ⇄ TpRu(PPh₃)(Hb)(η²-HaSiR₃H). One of the complexes, TpRu(PPh₃)(η³-HSiPhMe₂H), was used for the catalytic hydrolytic oxidation of organosilanes to silanols. A mechanism, which does not involve the usual oxidative addition of silane to the metal center to form the silyl hydride species, is proposed; it is supported by theoretical calculations. The ruthenium-silane complex TpRu(PPh₃)(η³-HSiPhMe₂H) was also found to effect catalytic reduction of carbon dioxide by PhMe₂SiH to give methoxide as the ultimate reduction product. Proton NMR study of the reaction revealed that various reduced products of carbon dioxide including formoxysilane, bis(silyl)acetal, and silyl methoxide were formed during the course of the catalysis, the identities of which were further confirmed by ¹³C NMR through the use of ¹³CO₂. Concurrent ³¹P NMR monitoring indicated that the starting complex was first converted to the carbonyl-hydride species TpRu(PPh₃)(CO)H, probably due to decarbonylation of the initially formed formoxysilane. The carbonyl-hydride complex was then partially converted to a new species, which we suspect to be the formate complex TpRu(PPh₃)(CO)(η¹-OCHO) generated by protonation of the hydride complex by formic acid in the reaction. Slow formation of the dicarbonyl-hydride complex TpRu(CO)2H from the formate complex was observed at the latter stage of the monitoring experiment. A mechanism for the reduction of carbon dioxide to the various silicon-containing products was proposed after taking into consideration the NMR monitoring results. The air stable, dicationic bipyridine ruthenium diaquo complex cis-[Ru(6,6’-Cl₂bpy)₂(H₂O)₂](OTf)₂ was found to be an efficient catalyst for the β-alkylation of secondary alcohols with primary alcohols. On the other hand, the dimethyl analog cis-[Ru(6,6’-Me₂bpy)₂(H₂O)₂](OTf)₂ was found to be an efficient catalyst for the transfer hydrogenation of carbonyl compounds with 1,4-butanediol. The diol, acting as both solvent and hydrogen donor, was converted to the more thermodynamically stable γ-butyrolactone upon its release of hydrogen. For each catalytic reaction a mechanistic pathway was proposed. It should be noted that both catalytic systems are insensitive to oxygen and therefore do not require the use of nitrogen or argon atmosphere for protection.

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