Complexes of Ni(II) with triphosphine-phosphite ligand P(OCH₂PPh₂)₃: syntheses, structures, and electronic properties

Ionic complexes [(& kappa;(4)-PP3)P(OCH2PPh2)(3)NiX]BF4 (2 (X=Cl) and 3 (X=Br)) were formed when P(OCH2PPh2)(3) ligand (1) was reacted with mixtures of salts NiX2/Ni(BF4)(2) (X=Cl or Br). Single crystal X-ray characterizations performed on complexes 2 and 3 established that metal centers are in pseudo-trigonal bipyramidal ligand geometries formed by the phosphorus centers from coordinated 1 and the halide ligand. Reactions of 1 with other nickel(II) salts (acetate, nitrate or tetrafluoroborate) proceeded in 2 : 1 (ligand-salt) ratio yielding complex 4, [(& kappa;(3)-PP2)((O--P)(OCH2PPh2)(2))(& kappa;(2)-PP)(O--P)(OCH2PPh2)(OCH2P(=O)Ph-2)Ni]. The formation of this complex required hydrolysis of one of the P-O ester bonds in ligand P(OCH2PPh2)(3) thus forming O-anionic phosphite/diphosphine fragment [O--P(OCH2PPh2)(2)]. One of such fragments is coordinated to the metal center as a tridentate (& kappa;(3)-PP2) ligand, whereas the other is in bidentate (& kappa;(2)-PP) coordination via phosphorus centers of the O--P-OCH2PPh2 chelating pocket; the remaining -OCH2P(=O)Ph-2 arm is P-oxidized and uncoordinated. Nickel center in 4 is in distorted trigonal-bipyramidal geometry, with O-anionic-phosphite groups coordinated at the axial positions, and the three -PPh2 groups located in the equatorial plane. Compounds 2-4 were characterized by multinuclear NMR spectroscopy, elemental analyses, electron absorption spectroscopy, cyclic voltammetry, and single crystal X-ray crystallography. Their electronic structures were also investigated by DFT methods.