Crystal and molecular structures of diazapyrenes and a study of pi\cdotspi interactions.

Abstract

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 reported here and are the first representatives of diazapyrenes. Crystal data: (1) monoclinic, P2(1)/c, a = 4.0246 (5), b = 15.5147 (5), c = 9.1453 (9) Å, beta = 101.23 (1) degrees, V = 560.1 (1) Å(3), Z = 2, R = 0.043; (2) monoclinic, C2/m, a = 12.4968 (3), b = 11.4751 (4), c = 3.9615 (5) Å, beta = 96.80 (1) degrees, V = 564.09 (5) Å(3), Z = 2, R = 0.0405. The experimental bond lengths are compared with those calculated by molecular mechanics (MM3), semi-empirical methods (MOPAC6.0-PM3, AM1, MNDO) and values predicted by valence-bond and variable-electronegativity self-consistent field (VESCF) methods. pi.pi interactions in (1), (2) and seventeen other 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 interactions have been calculated: the shortest crystallographic axis, the offset parameter, the interplanar angle, the interactive volume and the overlapping surfaces. 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 atomic charges of the ring non-H atoms and the number of aromatic skeleton electrons are used as parameters for classification. MOPAC-PM3 was used to calculate the partial atomic charges in (1), (2) and pyrene analogues. Correlations between geometrical and electronic structure parameters reveal an analogy between the beta-type structures and the crystal structure of graphite.