Unexpected beauty and diversity in the structures of three homologous 4,5-dialkoxy-1-ethynyl-2-nitrobenzenes: the subtle interplay between intermolecular C-H...O hydrogen bonds and alkyl chain length.

The synthesis, 1 H and 13 C NMR spectra, and X-ray structures are described for three dialkoxy ethynylnitrobenzenes that differ only in the length of the alkoxy chain, namely 1-ethynyl-2-nitro-4,5-dipropoxybenzene, C14 H17 NO4 , 1,2-dibutoxy-4-ethynyl-5-nitrobenzene, C16 H21 NO4 , and 1-ethynyl-2-nitro-4,5-dipentoxybenzene, C18 H25 NO4 . Despite the subtle changes in molecular structure, the crystal structures of the three compounds display great diversity. Thus, 1-ethynyl-2-nitro-4,5-dipropoxybenzene crystallizes in the trigonal crystal system in the space group R-3, with Z = 18, 1,2-dibutoxy-4-ethynyl-5-nitrobenzene crystallizes in the monoclinic crystal system in the space group P21 /c, with Z = 4, and 1-ethynyl-2-nitro-4,5-dipentoxybenzene crystallizes in the triclinic crystal system in the space group P-1, with Z = 2. The crystal structure of 1-ethynyl-2-nitro-4,5-dipropoxybenzene is dominated by planar hexamers formed by a bifurcated alkoxy sp-C-H...O,O' interaction, while the structure of the dibutoxy analogue is dominated by planar ribbons of molecules linked by a similar bifurcated alkoxy sp-C-H...O,O' interaction. In contrast, the dipentoxy analogue forms ribbons of molecules alternately connected by a self-complementary sp-C-H...O2 N interaction and a self-complementary sp2 -C-H...O2 N interaction. Disordered solvent was included in the crystals of 1-ethynyl-2-nitro-4,5-dipropoxybenzene and its contribution was removed during refinement.