Crystal structure of Pigment Red 254 from X-ray powder diffraction data

The crystal structure of Pigment Red 254 was successfully solved from laboratory X-ray powder diffraction data by the simulated annealing method followed by Rietveld refinement. The dihedral angle between the benzene and pyrrole rings is 11.1 (2)°. In the crystal, molecules are linked via N—H⋯O hydrogen bonds, forming chains along [110] incorporating (8) rings.

The crystal structure of Pigment Red 254 [P.R. 254, C 18 H 10 Cl 2 N 2 O 2 ; systematic name: 3,6-bis(4-chlorophenyl)-2,5-dihydropyrrolo [3,4-c]pyrrole-1,4-dione] was solved from laboratory X-ray powder diffraction data using the simulated annealing method followed by Rietveld refinement because the very low solubility of the pigment in all solvents impedes the growth of single crystals suitable for X-ray analysis. The molecule lies across an inversion center. The dihedral angle between the benzene ring and the pyrrole ring in the unique part of the molecule is 11.1 (2) . In the crystal, molecules are linked via N-HÁ Á ÁO hydrogen bonds, forming chains along [110] incorporating R 2 2 (8) rings.

Chemical context
Within the range of diketopyrrolo-pyrrole (DPP) pigments presently offered to the market, P.R. 254 plays the most important role (Herbst & Hunger, 2004), this commercially available type the pigment being widely used in industrial paints, for example for automotive finishes, and plastics which are processed at high temperature. P.R. 254 affords medium shades of red in full shades, while reductions made with a white paint are somewhat bluish red. The pigment demonstrates excellent fastness to organic solvents and weatherfastness, as well as good coloristic and fastness properties. It also shows good hiding power and high tinctorial strength.
The pigment exhibits very low solubility in all solvents, impeding the growth of single crystals suitable for X-ray analyses. Pigments are not dissolved in their application media, but finely dispersed. Consequently the final product properties depend on the crystal structure of the pigments. The crystal structure was successfully solved from laboratory X-ray powder diffraction data using the simulated annealing method followed by Rietveld refinement. ISSN 2056-9890

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Rietveld refinement was carried out with TOPAS (Coelho, 2007) using all diffraction data. The TOPAS input file (including all crystallographic constraints and chemical restraints) was generated automatically by the DASH-to-TOPAS link. Simulated annealing method (SA) was used to solve the crystal structure from the powder pattern in direct space. The starting molecular geometry was built from known crystal structure of similar compound from the Cambridge Structural Database (CSD; Groom et al., 2016). The half of the molecule has three flexible torsion angles, which combined with three 508 Ivashevskaya and Ivashevskaja C 18 H 10 Cl 2 N 2 O 2 Acta Cryst. (2017). E73, 507-510 research communications Table 1 Hydrogen-bond geometry (Å , ). (17 Symmetry codes: (i) Àx þ 2; Ày þ 2; Àz þ 1; (ii)

Figure 2
Part of the crystal structure of the title compound (viewed along the a axis). Hydrogen bonds are shown as dashed lines.

Figure 1
The molecular structure of the title compound. Unlabelled atoms are related by the symmetry code (Àx + 1, Ày + 1, Àz + 1). The atoms are represented by spheres of arbitrary size.

Figure 3
Layered arrangement in the crystal structure of the title compound. Numerical values refer to distances in Å .

Figure 4
The three flexible torsion angles and their allowed ranges in the structure solution step. translational and three orientational degrees of freedom corresponds to a total of nine degrees of freedom. The program DASH (David et al., 2006) was used for structure solution. DASH allows the torsion angles to be restricted to intervals that significantly reduce the search space. These three flexible torsion angles and their allowed ranges are shown in Fig. 4. The powder pattern was truncated to a real space resolution of 2.6 Å , which for Cu K 1 radiation corresponds to 34.6 in 2. The background was subtracted with a Bayesian high-pass filter (David & Sivia, 2001). The number of SA runs was increased to 50 to get better statistics regarding reproducibility. The background subtraction, peak fitting, Pawley refinement and SA algorithms were used as implemented in the program DASH.
Accurate peak positions for indexing were obtained by fitting 20 manually selected peaks with an asymmetrycorrected Voigt function. Indexing was done with the program DICVOL91 (Boultif & Louë r, 1991). A triclinic unit cell was determined with M(20) = 40.7 (de Wolff et al., 1968), F(20) = 82.8 (Smith & Snyder, 1979). From volume considerations, the unit cell can contain one molecule of P.R. 254 (Z = 1). The molecule has an inversion centre, which means the asymmetric unit must consist of a one half of the molecule.
Pawley refinement (Pawley, 1981) was carried out for refining the background, unit-cell parameters, zero-point error, peak-width and peak-asymmetry parameters. It allows extracting integrated intensities and their correlations. All intensities were refined without reference to a structural model and the result is the best fit that is theoretically possible: R wp = 13.57, R exp = 11.20, 2 = 1.467.
Suitable chemical restraints were applied for all bond lengths, valence angles and the planarity of the aromatic ring systems (including the five-membered condensed system). Anisotropic peak broadening was included to allow the peak profiles to be described accurately. The discrepancies between the observed and the calculated profile appeared to systematically depend on the hkl indices of the reflections, indicating preferred orientation in the [001] direction. The March-Dollase formula (Dollase, 1986) was used. The diffraction profiles and the differences between the measured and calculated profiles are shown in Fig. 5.   (Coelho, 2007); data reduction: DASH3.1 (David et al., 2006); program(s) used to solve structure: DASH3.1 (David et al., 2006); program(s) used to refine structure: TOPAS-Academic (Coelho, 2007); molecular graphics: Mercury (Macrae et al., 2008).