2-Pyridyl-phosphine and -diphosphine complexes of nickel(II), and their aqueous solution chemistry

Abstract

The cis, square planar NiII complexes NiX2[d(py)pe], where d(py)pe is the 2-pyridyl-diphosphine ligand py2P(CH2)2Ppy2, and X=Cl (1a), Br (1b), I (1c), NCS, and NO3, and the corresponding d(py)pcp complexes, where d(py)pcp is ▪ with a cyclopentane ring bridge, are synthesized via use of the NiX2(PPh3)2 precursors, and fully characterized, including five X-ray structures. An X-ray structure of d(py)pcp is also reported.Complexes 1a–c dissolve in water to form cis-{Ni(H2O)2[d(py)pe]}2+, which is isolated as the PF6 salt; the d(py)pcp analogues behave similarly. All the diaquo complexes decompose slowly in H2O to generate Ni(H2O)62+ and the dioxides of the pyridyl-diphosphines, likely due to the presence of trace O2. Known, tetrahedral complexes of the type NiX2(PPh3−npyn)2 (X=Cl, Br; py=2-pyridyl, and n=1–3) also dissolve in H2O, again with generation of 2:1 electrolyte species. An attempted synthesis of Ni(CO)2(Ppy3)2 from Ni(CO)2(PPh3)2 and Ppy3 in CH2Cl2 accidentally gives the known NiCl2(P,N-Ppy3)·H2O via a laboratory light-induced reaction; this species partially converts into [Ni(Ppy3)2]Cl2, and a crystal structure confirms the octahedral, tripodal N,N′,N″-coordination of the cation. The water-soluble systems, studied because of their potential for catalysis in aqueous media, are not effective for catalyzed olefin hydration, or transfer-hydrogenation of cyclohexanone.