Some ruthenium hydride, dihydrogen, and dihydrogen-bonded complexes in catalytic reactions

The transfer of the hydrogen atoms from the η²-H₂ ligand to the cis-disposed olefin ligand in a ruthenium olefin-dihydrogen complex is discussed. It is realized that H₂O and NEt₃ exhibit promoting effects in the catalytic hydrogenation of olefins with a couple of hydro(trispyrazolyl)borate (Tp)-supported ruthenium complexes. A reaction mechanism that accounts for the promoting effect has been proposed. A Tp-supported ruthenium solvento hydride complex TpRu(PPh₃)(CH₃CN)H was found to react with H₂ and R₃SiH to form the fluxional dihydrogen-hydride, and η²-silane-hydride complexes, respectively. Although no stable and isolable σ-complex was formed with CH₄, the solvento hydride complex was found to be active in catalyzing H/D reactions of CH₄ with some deuterated common organic solvents. In the catalytic CO₂ hydrogenation reactions in THF/H₂O or alcohol, the complex TpRu(PPh₃)(CH₃CN)H generates the metal-ligand bifunctional catalyst TpRu(PPh₃)(ROH)H (R = H or alkyl) which transfers the hydride and a proton from ROH to the CO₂ molecule in a concerted manner, without coordination of the latter to the metal center. Aminocyclopentadienyl ruthenium complexes, which exhibit intramolecular Ru-H⋯H-N dihydrogen-bonding interactions were synthesized and characterized. These complexes provide good models for the study of heterolytic cleavage of η²-H₂ ligand and its reverse-protonation of metal hydride to form dihydrogen complex. An indenyl ruthenium hydride complex was synthesized and found to be good catalyst for nitrile hydration reactions to give amides; these reactions nicely demonstrate the principle of utilizing dihydrogen bond to promote catalytic reactions. © 2006 Elsevier B.V. All rights reserved

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