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

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)(CH 3CN)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 eta2-R-H, eta 2-H-D, and eta2-R-D sigma-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(PPh 3)(CO)H (or D). All of these complexes, however, show no catalytic activity for H/D exchange between D2O and organic solvents.;The aquo-acetamido complex TpRu(PPh3)(H2O)(NHC(O)CH 3) 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 aquo-acetamido complex is found to be active for catalytic hydration of nitriles to amides. Our nitrile hydration reaction 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)CH 3), 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.;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 alpha-hydrogens are selectively deuterated by D 2O. It is believed that tautomerization from keto to enol form is a crucial step of the catalytic cycle.;An amido vinyl complex TpRu(PPh3)(C(NHC(O)CH3)=CHPh) was synthesized by reacting a THF solution of TpRu(PPh3)(H 2O)(NHC(O)CH3) with excess phenylacetylene 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 alpha 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)CH 3)][BF4].