Arene-Ruthenium(II) Complexes Containing Amino-Phosphine Ligands as Catalysts for Nitrite Hydration Reactions

Three different series of novel mononuclear arene-ruthenium(II) complexes containing amino-phosphine ligands, namely, [RuCl2{kappa(1)(P)-2-Ph2PC6H4CH2NHR}(eta(6)-arene)], [RuCl2{kappa(1)(P)-3-Ph2-PC6H4CH2NHR}(eta(6)-arene)] and [RuCl2{kappa(1)(P)-4-Ph2PC6H4CH2NHR}(eta(6)-arene)] (arene = C6H6, p-cymene, 1,3,5-C6H3Me3, C6Me6; R = 'Pr, 'Bu; all combinations), have been synthesized and fully characterized. These readily accessible species are efficient catalysts for the selective hydration of organonitriles into amides under challenging reaction conditions, i.e., pure aqueous medium in the absence of any cocatalyst, being much more active than their corresponding nonfunctionalized triphenylphosphine counterparts [RuCl2(PPh3)(eta(6)-arene)]. The results obtained in this study indicate that the (amino-phosphine)ruthenium(II) complexes operate through a "bifunctional catalysis" mechanism in which the ruthenium center acts as a Lewis acid, activating the nitrite molecule, and the P-donor ligand acts as a Bronsted base, the pendant amino group generating the real nucleophile of the hydration process, i.e., the OH- group.