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|Title:||Mechanistic studies on hydrotris(pyrazolyl)borate ruthenium complexes-catalyzed C-H bond activation and nitrile hydration reaction||Authors:||Leung, Chung-wing||Keywords:||Hong Kong Polytechnic University -- Dissertations
|Issue Date:||2009||Publisher:||The Hong Kong Polytechnic University||Abstract:||Metal-catalyzed deuteration of organic compounds using the environmentally benign D₂O as deuterium source is relatively rare. The solvento-hydride complex TpRu(PPh₃)(CH₃CN)H (Tp = hydrotris(pyrazolyl)borate) is found to be a catalyst for H/D exchange between D₂O and some organic solvents. This exchange can be preformed under Ar or H₂. In the former case, the hydride ligand is rapidly deuterated by D₂O forming TpRu(PPh₃)(CH₃CN)D. It is believed that the mechanism is similar to that of our previous work on the TpRu(PPh₃)(CH₃CN)H-catalyzed H/D exchange reactions between deuterated organic molecules and methane. We proposed that TpRu(PPh₃)(CH₃CN)D exchanges the deuteride ligands Ru-D with R-H via the intermediacies of the η²-R-H, η²-H-D, and η²-R-D σ-complexes. In the course of catalysis TpRu(PPh₃)(CH₃CN)D is converted to an aquo-acetamido complex TpRu(PPh₃)(D₂O)(NDC(O)CH₃), which at the later stage of the reaction generates two additional minor species, one of which is the partially deuterated carbonyl hydride complex TpRu(PPh₃)(CO)H (or D). All of these complexes, however, show no catalytic activity for H/D exchange between D₂O and organic solvents. In the catalytic reaction under H₂, dihydrogen-hydride complex TpRu(PPh-₃)(H₂)H and its isotopomers TpRu(PPh₃)(H₃-x)Dx formed are the active species for the H/D exchange reactions. The H-H (or D) ligand can be displaced by R-H, and the mechanism is akin to the one mentioned above. This dihydrogen-hydride complex is less active than the solvento-hydride complex, which, due to the higher lability of the CH₃CN ligand, therefore exchanges more readily with the organic molecule (R-H) to form the η²-R-H σ-complex. The aquo-acetamido complex TpRu(PPh₃)(H₂O)(NHC(O)CH₃) is independently synthesized by refluxing a THF solution of the solvento-hydride complex with water. The complex is formed via hydration of the CH3CN ligand of TpRu(PPh₃)(CH₃CN)H; It is shown by theoretical calculations at the Becke3LYP level of DFT theory that the hydration process is promoted by a Ru-H--H-OH dihydrogen bonding interaction between the hydride ligand and the attacking water molecule. The molecular structure of the aquo-acetamido complex is determined by X-ray crystallography. The aquo-acetamido complex is found to be active for catalytic hydration of nitriles to amides. Common mechanisms for catalytic nitrile hydration involve intramolecular nucleophilic attack of a hydroxo (or aquo) ligand or external attack of a hydroxide ion or (water) at the carbon atom of n¹-coordinated nitrile to form the metal amide intermediate and subsequent protonation of amido ligand by an adjacent aquo ligand or solvent water. Our catalysis, however, proceeds via a new mechanism involving the intermediacy of a relatively stable complex containing a chelating N-imidoylimidato ligand; ring-opening nucleophilic attack of this ligand by water generates the product. Formation of the N-imidoylimidato complex from the aquo-acetamido complex involves several steps, the initial one is displacement of the H₂O ligand by a nitrile molecule to yield the nitrile-acetamido species TpRu(PPh₃)(RCN)(NHC(O)CH₃), this is followed by an unusual linkage isomerization of the N-bonded amido ligand to an O-bonded imido, which then undergoes nucleophilic attack at the carbon atom of the nitrile ligand in the complex; facile 1,3-proton shift between the nitrogen atoms on the resulting ring completes the reaction. The catalytic cycle of the aquo-acetamido complex-catalyzed nitrile hydration reaction has been examined by theoretical calculations at the Becke3LYP level of DFT theory. It is learned that there is a substantially high barrier for the hydrolysis of the highly stable N-imidoylimidato complex, a step involving the ring-opening nucleophilic attack of this ligand by water, and this is probably the reason for the requirement of a relatively high reaction temperature.
More conveniently, the aquo-amido complexes TpRu(PPh₃)(H₂O)(NHC(O)R) (R = Me, Ph) can be prepared by reacting TpRu(PPh₃)(RCN)Cl with NaOH in THF in the presence of water. Different N-imidoylimidato complexes TpRu(PPh3)(K² -N,O-NH=CMeN=CMeO), TpRu(PPh₃)(K²-N,O-NH=CPhN=CPhO), and TpRu(PPh₃)(K²-N,O-NH=CMeN=CPhO) were independently synthesized by heating a 1,4-dioxane solution of TpRu(PPh₃)(H₂O)(NHC(O)R) (R = Me, Ph) with the corresponding nitriles. The molecular structure of TpRu(PPh₃)([K²-N,O-NH=CPhN=CPhO) was determined by X-ray crystallography. The aquo-acetamido complex is found to be a catalyst for the H/D exchange between D₂O and some ketones. The result shows that activated hydrogens of ketones such as a-hydrogens are selectively deuterated by D₂O. It is believed that tautomerization from keto to enol form is a crucial step of the catalytic cycle. In the course of catalysis, the amido hydrogen of the acetamido ligand can be deuterated readily by D₂O to form TpRu(PPh₃)(D₂O)(NDCOCH₃), in which the labile ligand D₂O can be substituted by the enol; H/D exchange between the enolic hydrogen and amido deuterium then proceeds. The cycle is completed by displacement of the deuterated enol with D₂O. The reactivity of the aquo-amido complexes has also been studied. The labile aquo ligand can be readily displaced by various substrates such as alcohol, nitrile, and alkyne. An amido vinyl complex TpRu(PPh₃)(C(NHC(O)CH₃)=CHPh) was synthesized by reacting a THF solution of TpRu(PPh₃)(H₂O)(NHC(O)CH₃) with excess phenlyacetylene at room temperature. Formation of the amido vinyl complex involves coordination of phenylacetylene to form a vinylidene complex and subsequent intramolecular nucleophilic attack of the adjacent amido moiety to the α carbon of the vinylidene ligand. This amido vinyl complex reacted with HBF₄ very readily to give a carbene complex, [TpRu(PPh ₃)(=C(CH₂Ph)NHC(O)CH ₃)] [BF₄].
|Description:||xxxv, 240 leaves : ill. ; 30 cm.
PolyU Library Call No.: [THS] LG51 .H577P ABCT 2009 Leung
|URI:||http://hdl.handle.net/10397/2754||Rights:||All rights reserved.|
|Appears in Collections:||Thesis|
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