Critical role of weak [C–H⋯O] hydrogen bonds in the assembly of benzo[1,2-d:4,5-d′]bisoxazole cruciforms into supramolecular sheets

Abstract

This paper explores the structural patterns adopted by a class of eight conjugated cruciforms based on a benzo[1,2-d:4,5-d′]bisoxazole (BBO) nucleus. Within the series, the lengths of cruciforms' x- and y-axes, as well as their terminal substituents, were varied. Two new cruciforms have been prepared using microwave-assisted Sonogashira couplings. Examination of molecular structures of BBO cruciforms revealed the expected cross-shaped geometries, with ∼90° crossing angles between the two conjugation circuits. Most significant deviations from the idealized planar structures were caused by the distortion of benzene rings positioned along the cruciforms' y-axis from the plane of the central benzobisoxazole ring, by as much as 82.1°. Deviations from linearity within carbon–carbon triple bonds were mild, but additively resulted in notably bent y-axes of the two largest examined cruciforms. Most significantly, the packing patterns of benzobisoxazole cruciforms revealed three distinct organizational motifs. Cruciforms with carbonyl groups on either the horizontal or the vertical axis (or both) all organized into two-dimensional infinite sheets, apparently connected chiefly through [C–H⋯O] contacts that varied in length between 2.24 and 2.60 Å, and could be interpreted as weak hydrogen bonds. The parallel layers of these two-dimensional sheets then assembled into three-dimensional structures through [π⋯π] stacking between the layers. The packing patterns of the two cruciforms with 4-(N,N-dimethylamino)phenyl groups on the vertical axis are dominated by one-dimensional columnar or tape-like assemblies that are formed through [π⋯π] stacking; these then organize into two-dimensional sheets, which in turn layer into a three-dimensional structure, but with a ∼60° angle between the adjacent sheets. Finally, in the parent tetraphenyl-substituted cruciform, the sp² nitrogen on the BBO core plays the key role in the assembly. It establishes 2.64 Å-long [C–H⋯N] contacts with the phenyl groups, mediating the formation of a unique packing pattern in which six tilted columnar assemblies of [π⋯π] stacked cruciforms surround a three-fold rotation axis in the crystal.