Cymene-Osmium(II) Complexes with Amino-Phosphane Ligands as Precatalysts for Nitrile Hydration Reactions

Three different series of half-sandwich Os(II) complexes, namely [OsCl2(eta(6)-p-cymene){PPh2(NR2)}] [R=Me (5a), Et (5b)], [OsCl2(eta(6)-p-cymene){PPh(NR2)(2)}] [R=Me (6a), Et (6b)] and [OsCl2(eta(6)-p-cymene){P(NR2)(3)}] [R=Me (7a), Et (7b)], have been synthesized. These species proved to be catalytically active for the selective hydration of organonitriles, with compound [OsCl2(eta(6)-p-cymene){PPh2(NMe2)}] (5a) being the most effective. In addition, the catalytic activity of 5a was found to be superior to those shown by related Ru(II), Ru(IV), Rh(I) and Pt(II) species containing the amino-phosphane PPh2(NMe2), i.e. compounds [RuCl2(eta(6)-p-cymene){PPh2(NMe2)}] (10), [RuCl2(eta(3):eta(3)-C10H16){PPh2(NMe2)}] (12), [RhCl(COD){PPh2(NMe2)}] (13) and cis-[PtCl2{PPh2(NMe2)}(2)] (15). Details on the synthesis and characterization of complexes 10, 12 and 15 are also included, along with a discussion on the role played by the ligand PPh2(NMe2) during the catalytic reactions. In this sense, the amino-phosphane does not act as H-bond acceptor for the water molecule, generating instead in the aqueous reaction medium the cooperative phosphinous acid ligand PPh2(OH).