The First Low-Spin Nickel Complex with Two Coordinated Water Molecules, [Ni(o-MeO-dppp)(H2O)2](PF6)2 − Synthesis and Structural Characterization

Abstract

A series of nickel complexes of the didentate ligand 1, 3-bis[di(o-methoxyphenyl)phosphanyl]propane (o-MeO-dppp) has been synthesized and characterized. The first low-spin nickel complex containing two coordinated water molecules, [Ni(o-MeO-dppp)(H 2 O) 2 ](PF 6 ) 2 (1), and compounds of the general formula [Ni(o-MeO-dppp)X 2 ] [X = Cl, Br, I, or trifluoroacetate (TFA)] are reported. Complex 1 has been characterized by X-ray crystallography. The nickel ion resides in a square-planar coordination environment with two phosphorus donors and two water molecules in a cis configuration (Ni−P distance 2.19 Å ; Ni−O distance 1.97 Å ). The structure of this complex and its behavior in solution are quite different

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“…This signal is signicantly shied downeld in comparison to the signal of non-coordinated water in CDCl 3 which appears at 1.56 ppm 58. Similar situation, where respective solvents had similar polarity to the solvents used in our system, was observed in square-planar nickel(II) complex with contains diphosphane ligand and coordinated water molecules, where structure was solved with single crystal X-ray analysis 59. NMR study in CD 2 Cl 2 solution determined downeld shi of coordinated water (2.30 ppm) in comparison to non-coordinated water (1.52 ppm) 60.…”

Nickel

“…A comparison of the behavior of the nickel complexes of the ligand o-MeO-dppe with the unsubstituted analogue dppe showed that the latter ligand was more readily oxidized [54]. This possibility of oxidation was found to play a major role in the case of the nickel complexes of the propane-bridged ligands o-MeO-dppp and dppp, as for both of these ligands the formation of bis(ligand) complexes is not possible, but only the substituted ligand o-MeO-dppp yields an active catalyst [53]. It has been reported earlier that Pd(OAc) 2 is able to oxidize tertiary phosphanes [59].…”

“…The catalytic activity of nickel(II) acetate in combination with a number of chelating phosphane ligands in the hydrogenation of 1-octene has been studied extensively during the past decade [51][52][53][54][55][56]. It was found that in-situ-prepared mixtures of nickel(II) acetate with the electron-donating ligands o-MeO-dppe, o-MeO-dppp, dcpe, or dcpp (see Scheme 5.2) achieved quite high activities in the hydrogenation of 1-octene, the nonoptimized catalytic systems showing turnover numbers (TONs) of up to 460 per hour at 25 8C and 50 bar [51].…”

“…5.3) [53]. Attempts to synthesize a bis(ligand) complex using the ligand o-MeO-dppp were unsuccessful.…”

“…Attempts to synthesize a bis(ligand) complex using the ligand o-MeO-dppp were unsuccessful. The impossibility to form a bis(ligand) complex might be the most important reason for the considerably higher hydrogenation activity of catalysts based on this ligand, compared to those based on o-MeO-dppe [53]. 5.3) [53].…”