Involvement of conformational isomerism in the complexity of the crystal network of 1-(4-nitrophenyl)-1H-1,3-benzimidazole derivatives driven by C—H…A (A = NO₂, N_py and π) and orthogonal N_py…NO₂ and ONO…Csp² interactions

A detailed structural analysis of the benzimidazole nitroarenes 1-(4-nitrophenyl)-1H-1,3-benzimidazole, C₁₃H₉N₃O₂, (I), 1-(4-nitrophenyl)-2-phenyl-1H-1,3-benzimidazole, C₁₉H₁₃N₃O₂, (II), and 2-(3-methylphenyl)-1-(4-nitrophenyl)-1H-1,3-benzimidazole, C₂₀H₁₅N₃O₂, (III), has been performed. They are nonplanar structures whose crystal arrangement is governed by Csp²—H…A (A = NO₂, N_py and π) hydrogen bonding. The inherent complexity of the supramolecular arrangements of compounds (I) (Z′ = 2) and (II) (Z′ = 4) into tapes, helices and sheets is the result of the additional participation of π– and n–π* (n = O and N_py; π* = Csp² and) interactions that contribute to the stabilization of the equi-energetic conformations adopted by each of the independent molecules in the asymmetric unit. In contrast, compound (III) (Z′ = 1) is self-paired, probably due to the effect of the steric demand of the methyl group on the crystal packing. Theoretical ab initio calculations confirmed that the presence of the arene ring at the benzimidazole 2-position increases the rotational barrier of the nitrobenzene ring and also supports the electrostatic nature of the orthogonal ONO…Csp² and N_py…NO₂ interactions.