Mechanism of catalytic hydration of nitriles with hydrotris(pyrazolyl) borato (Tp) ruthenium complexes

The aquo-amido complexes TpRu(PPh3)(H2O)(NHQO)R) (R = Me, Ph), which can be prepared by refluxing a THF solution of TpRu(PPh3)(RCN)H containing excess water or more conveniently by reacting TpRu(PPhARCN)CI with NaOH in THF in the presence of water, are found to be active for catalytic hydration of nitriles to amides. The catalysis proceeds via a mechanism that is distinctly different from the common ones involving intramolecular nucleophilic attack of a hydroxo (or aquo) ligand or external attack of a hydroxide ion (or water) at the carbon atom of the eta(1)-coordinated nitrile to form the metal amide intermediate and subsequent protonation of amido ligand by an adjacent aquo ligand or solvent water. The new mechanism involves the intermediacy of a relatively stable complex containing a chelating N-imidoylimidato ligand; ring-opening nucleophilic attack of this ligand by water is the product-generating step. Formation of the N-imidoylimidato complex from TpRu(PPhAH(2)O)(NHC(O)R) involves several steps. The initial one is displacement of the H2O ligand by a nitrile molecule to yield the nitrile-amido species TpRu(PPhARCN)(NHQO)R). 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 N-imidoylimidato complexes TpRu(PPh3)(kappa(2) -NO-NH= CMeN=CMeO), TpRu(PPh3)(kappa(2) -NO-NH=CPhN=CPhO), and TpRu(PPh3)(kappa(2)-N,O-NH=CMeN= CPhO) were independently prepared, and the molecular structure of TpRu(PPh3)kappa(2)-NO-NH=CPhN= CPhO) was determined by X-ray crystallography. To study the feasibility of the proposed mechanism for nitrile hydration with the aquo-amido complexes, theoretical calculations were performed at the Becke3LYP level of DFT theory to examine the whole catalytic cycle. 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

Catalytic H/D exchange between organic compounds and D2O with TpRu(PPh3)(CH3CN)H (Tp = hydro(trispyrazolyl)borate). Reaction of TpRu(PPh3)(CH3CN)H with water to form acetamido complex TpRu(PPh3)(H2O)(NHC(O)CH3)

Deuteration of organic molecules using D2O as the deuterium source is affected with catalytic systems based on the ruthenium solvento hydride complex TpRu(PPh3)(CH3CN)H. The deuteration reactions can be performed under Ar or H-2. In the former case, the hydride ligand is rapidly deuterated by D2O, and in the course of the catalysis, D2O converts TpRu(PPh3)(CH3CN)D into the 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 species TpRu(PPh3)(CO)H(or D). All of these complexes are, however, found to be inactive for the H/D exchange reactions between the organic molecules and D2O. In the exchange reactions under H-2, a mixture of the HD isotopomers, TpRu(PPh3)H3-xDx, of the dihydrogen hydride complex TpRu(PPh3)(H-2)H are the active species. On the basis of our previous work on the TpRu(PPh3)(CH3CN)H-catalyzed H/D exchange reactions between deuterated organic molecules and CH4, it is proposed that TpRu(PPh3)(CH3CN)D and TpRu(PPh3)(H3-x)D-x exchange their deuteride ligands Ru-D with R-H via the intermediacies of the eta(2)-R-H(or D) and eta(2)-H-H(or D) sigma-complexes; the Ru-H bonds thus formed after the exchange are deuterated by D2O to regenerate the metal deuterides. The solvento complex TpRu(PPh3)(CH3CN)D under Ar is suggested to be more active than TpRu(PPh3)(H3-x)D-x under H-2 for the H/D exchange reactions because the former reacts more readily with the organic molecule R-H to generate the eta(2)-R-H sigma-complex due to higher lability of the CH3CN ligand in comparison with the dihydrogen or hydrogen-deuterium ligand of TpRu(PPh3)(H3-x)D-x. The acetamido complex TpRu(PPh3)(H2O)(NHC(O)CH3) was independently prepared by refluxing a THF solution of TpRu(PPh3)(CH3CN)H containing excess water for 24 h, and its molecular structure was determined by X-ray crystallography. Theoretical calculations at the Becke3LYP level of DFT theory have been performed to study the reaction of TpRu(PPh3)(CH3CN)H with H2O that leads to the formation TpRu(PPh3)(H2O)(NHC(O)CH3). It is shown that the hydration reaction is promoted by a Ru-H center dot center dot center dot H-OH dihydrogen-bonding interaction between the hydride ligand and the attacking water molecule. An explanation for the failure of the chloro analogue TpRu(PPh3)(CH3CN)Cl to react with water to form the acetamido complex is also provided