On the distorted {NiN₆} octahedron in hexakis(imidazole)nickel(II) bis(4-nitrobenzoate) dihydrate

Abstract

181-188The reaction of [Ni(H₂O)₆]Cl₂ with the sodium salt of 4-nitrobenzoic acid (4-nbaH) in the presence of imidazole results in the formation of the title compound hexakis(imidazole)nickel(II) bis(4-nitrobenzoate) dihydrate (1). Compound (1) is dehydrated to hexakis(imidazole)nickel(II) bis(4-nitrobenzoate) (2) by heating at 100oC. Both compounds are characterized by elemental analysis, infrared spectra, X-ray powder pattern and thermal studies. The title compound [Ni(Im)₆](4-nba)₂.2H₂O (1) crystallizes in the centrosymmetric triclinic space group P₁⁻ with the Ni(II) situated on an inversion center. The structure of (1) consists of a distorted octahedral hexakis(imidazole)nickel(II) cation, a free uncoordinated 4-nba anion and a lattice water with half of the molecule accounting for the asymmetric unit. In the complex cation the central metal is bonded to six neutral terminal Im ligands. The distortion of the {NiN6}octahedron in (1) is discussed in terms of the difference between the longest and shortest Ni-N bonds. A comparative study of several [Ni(Im)6]2+ compounds in different structural environments is described. In the crystal structure, the cation, anion and lattice water are linked by three types of H-bonding interactions comprising two O-H•••O, three N-H•••O and three C-H•••O interactions. Each hexacoordinated Ni(II) complex cation is linked to eight symmetry related 4-nba anions and four different lattice water molecules via N-H•••O and C-H•••O interactions, while each 4-nba anion is H-bonded to four complex cations and two symmetry related lattice water molecules. Pairs of [Ni(Im)₆]²⁺ cations and 4-nba anions are linked to lattice water molecules via O-H•••O and C-H•••O interactions. As a result of the hydrogen bonding interactions, the cations and anions are organized into alternating layers