An orthorhombic polymorph of pyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile

The title compound, C16H6N6, is a polymorph of the previously reported structure [Kozlov & Goldberg (2008 ▶). Acta Cryst. C64, o498–o501]. Unlike the previously reported monoclinic polymorph (space group P21/c, Z = 8), the title compound reveals orthorhombic symmetry (space group Pnma, Z = 4). The molecule shows crystallographic mirror symmetry, while the previously reported structure exhibits two independent molecules per asymmetric unit. In the title compound, adjacent molecules are essentially parallel along the c axis and tend to be vertical along the b axis with dihedral angles of 72.02 (6)°. However, in the reported polymorph, the entire crystal structure shows an antiparallel arrangement of adjacent columns related by inversion centers and the two independent molecules are nearly parallel with a dihedral angle of 2.48 (6)°.

Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to elemental solids. When polymorphism exists as a result of difference in crystal packing, it is called packing polymorphism. The different crystal types probably are the result of hydration, solvation or the effect of synthesis methods. There are recently reported examples of organic polymorphs: 2-(pyrimidin-2-ylsulfanyl) acetic acid, which is able to form monoclinic and orthorhombic crystals (Ramos Silva et al., 2011;Pan & Chen, 2009). 3,4,5-Trihydroxybenzoic acid monohydrate is found to form three polymorphs (Demirtaş et al., 2011;Jiang et al., 2000;Okabe et al., 2001). We herein report a new orthorhombic polymorph of DICNQ (1).
ORTEPIII (Burnett & Johnson, 1996) representation of the title compound (1) is shown in Fig. 1. The title compound belongs to orthorhombic system, space group Pnma, consisting of one symmetric unit of single molecule, unlike the reported monoclinic polymorph (space group P2 1 /c), (2), , which consists of two crystallographically independent molecules.
The phenanthroline fragment of both polymorphs is nearly planar, and a similar bending of the cyano groups from the plane of the phenanthroline residues can be observed. The deviating distance of the cyano groups from the plane in (1) (0.182 (7) Å) is obviously bigger than the corresponding deviating distances in (2) (0.0345 Å and 0.0374 Å).
The packing pattern differs from the reported polymorph. In the title compound, there are two packing arrangements ( Fig. 2), adjacent molecules are essentially parallel along the c axis and tend to be vertical along the b axis with dihedral angles of 72.02 (6)°. Howerver, in the reported polymorph the entire crystal structure shows an antiparallel arrangement of the adjacent columns related by inversion centers, and the two independent molecules are nearly parallel with a dihedral angle of 2.48 (6)°.
Various methods and conditions of the crystallization process are the main reason responsible for the formation of different polymorphic forms (Thallapally et al., 2004). The two polymorphs (1) and (2)

Refinement
H atoms were positioned geometrically and refined using a riding model with C-H = 0.93 Å and with U iso (H) = 1.2U eq (C). Fig. 1. Perspective view of a basic unit of the title complex. Atoms are displayed as elliposoids at the 50% probability level at ca 293 K.