Mechanistic studies on hydrotris(pyrazolyl)borate ruthenium complexes-catalyzed C-H bond activation and nitrile hydration reaction

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Mechanistic studies on hydrotris(pyrazolyl)borate ruthenium complexes-catalyzed C-H bond activation and nitrile hydration reaction


Author: Leung, Chung-wing
Title: Mechanistic studies on hydrotris(pyrazolyl)borate ruthenium complexes-catalyzed C-H bond activation and nitrile hydration reaction
Degree: Ph.D.
Year: 2009
Subject: Hong Kong Polytechnic University -- Dissertations.
X-ray crystallography.
Organometallic chemistry.
Department: Dept. of Applied Biology and Chemical Technology
Pages: xxxv, 240 leaves : ill. ; 30 cm.
Language: English
InnoPac Record:
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 n2-R-H, n2-H-D, and n2-R-D D-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 n2-R-H D-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 n-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 a carbon of the vinylidene ligand. This amido vinyl complex reacted with HBF4 very readily to give a carbene complex, [TpRu(PPh3)(=C(CH2Ph)NHC(O)CH3)] [BF4].

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