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.