Crystal and molecular structures of diazapyrenes and a study of pi center dot center dot center dot pi interactions

Two diazapyrenes, 5,10-dimethyl-4,9-diazapyrene (1) and novel 2,7-dimethyl- 4,9-diazapyrene (2) have been synthesized. Their crystal structures are rep orted here and ale the first representatives of diazapyrenes. Crystal data: (1) monoclinic, P2(1)/c, a = 4.0246(5), b = 15.5147(5), c = 9.1453(9) Angs trom, beta = 101.23(1)degrees, V = 560.1 (1) Angstrom, Z = 2, R = 0.043; (2 ) monoclinic, C2/m, a = 12.4968(3), b = 11.4751 (4), c = 3.9615 (5) Angstro m, beta = 96.80 (1)degrees, V = 564.09 (5) Angstrom(3), Z = 2, R = 0.0405. The experimental bond lengths are compared with those calculated by molecul ar mechanics (MM3), semiempirical methods (MOPAC6.0-PM3, AM1, MNDO) and val ues predicted by valence-bond and variable-electronegativity self-consisten t field (VESCF) methods. pi ...pi interactions in (1), (2) and seventeen ot her pyrene and pyrene-like molecules selected from the Cambridge Structural Database [Allen & Kennard (1993). Chem. Des. Autom. News, 8, 131-137] have been studied. The following quantitative parameters of pi ...pi interactio ns have been calculated: the shortest crystallographic axis, the offset par ameter, the interplanar angle, the interactive volume and the overlapping s urfaces. They are used for the classification of crystal-packing motifs; a high predominance of beta and a few cases of gamma and sandwich-herringbone types are observed. In addition, electronegativity, the sum of partial ato mic charges of the ring non-H atoms and the number of aromatic skeleton ele ctrons are used as parameters for classification. MOPAC-PM3 was used to cal culate the partial atomic charges in (1), (2) and pyrene analogues. Correla tions between geometrical and electronic structure parameters reveal an ana logy between the beta-type structures and the crystal structure of graphite .