Dimorphism of 2,3,5,6-tetraphenylpyrazine

Two forms of 2,3,5,6-tetraphenylpyrazine (TPP), C28H20N2, have been crystallized. The first variety (alpha) is primitive monoclinic (P2(1)/c), in which the TPP molecule is centrosymmetric, The second variety (beta) is C-face-centred monoclinic (C2/c) with two symmetry-independent molecules having binary axis symmetry, where in one of the molecules, the binary axis passes through the two N atoms of the pyrazine ring, while in the second molecule, the binary axis passes through the midpoints of the two C-C bonds of the pyrazine ring. In these two compounds, the phenyl rings are differently disposed, showing a wing-like conformation in the alpha form and a propeller-like conformation for the two molecules in the beta form. The rotations of the phenyl rings, given by the dihedral angles between the pyrazine rings and the phenyl rings, are in the range 37.56(8)-49.72 (8)degrees.