Crystal engineering of neutral N-arylpyrimidinones and their HCl and HNO3 adducts with a C–H⋯O supramolecular synthon. Implications for non-linear optics

Abstract

In a previous crystallographic study of some N-arylpyrimidinones 1, we noted that: (1) C–H⋯O hydrogen bonds connect molecules in a linear array; (2) the charge transfer axis of the chromophore is aligned with the main symmetry operator of point groups 2 or m at ca. 57°, a value that is close to the ideal angle of 54.74°; (3) the methyl and chloro derivatives are isostructural. In this paper, we report the characterisation of chloride and nitrate salt adducts of 1 by X-ray diffraction and the analysis of their packing motifs. Recurrence of the same C–H⋯O supramolecular synthon in three neutral and five HCl and HNO₃ adducts of 1 signifies the robustness of this weak hydrogen bond. The occurrence of a mirror plane m in a family of eight crystal structures (four Pnma, two P2₁/m, one Pbcm, and one Pmn2₁) is unusual because this symmetry operation is generally avoided due to close packing considerations. Ab initio calculations show that the bisected phenyl conformation present in these crystal structures is the most stable conformation of the pyrimidinone molecule. The presence of aryl and pyrimidinone chromophores in 1, the correct alignment of the aromatic ring in the crystal and the occurrence of 2D polar layers in some crystal structures are favourable factors for non-linear optical applications. However, a strategy for the crystallisation of these achiral molecules in non-centrosymmetric space groups is yet to be achieved. This crystal engineering study simplifies the challenge of complete 3D structural control into a modular 2D + 1D problem.