Industrial Azomethine Nickel Complex Pigments. Four Crystal Structures From X-Ray Powder Diffraction Data

The crystal structures of the azomethine nickel complexes Pigment Orange 68 (P.O.68, C29H18N4O3Ni), Pigment Red 257 (P.R.257, C16H4Cl8N6O2Ni), and Solvent Brown 53 (S.Br.53, C18H10N4O2Ni) were determined from powder diffraction data. The compounds are industrially used for the colouration of plastics and coatings. P.O.68 exists in two polymorphic forms, the commercial one is the alpha-phase. The crystal structures were solved from laboratory data using real-space methods and refined by the Rietveld method. For the Rietveld refinement of alpha-P.O.68, synchrotron data were employed. In all structures, the Ni2+ ion is coordinated by two N atoms and two O atoms in a square-planar geometry. Both phases of P.O.68 crystallise in P2(1)/c, Z = 4. In both structures, the molecules form dimers via an inversion centre, with Ni-to-Ni distances of 3.606 angstrom (alpha-phase) and 3.286 angstrom (beta-phase). The dimers are stacked into columns. Neighbouring columns are connected by hydrogen bonds: one classical N-H center dot center dot center dot O bond, and one N-H center dot center dot center dot pi bond to the naphthalene moiety of a molecule in the neighbouring stack. P.R.257 crystallises in P2(1)/c, Z = 2, with molecules on inversion centres. The molecules show a typical van der Waals packing without close Ni-Ni contacts. S.Br.53 exhibits Pbcn symmetry with Z = 8. The molecules form columns with Ni-to-Ni distances of 3.508 angstrom.