Different N-H center dot center dot center dot pi interactions in two indole derivatives

We describe the syntheses and crystal structures of two indole derivatives, namely 6-isopropyl-3-(2-nitro-1-phenylethyl)-1H-indole, C19H20N2O2, (I), and 2-(4-methoxyphenyl)-3-(2-nitro-1-phenylethyl)-1H-indole, C23H20N2O3, (II); the latter crystallizes with two molecules (A and B) with similar conformations (r.m.s. overlay fit = 0.139 angstrom) in the asymmetric unit. Despite the presence of O atoms as potential acceptors for classical hydrogen bonds, the dominant intermolecular interaction in each crystal is an N-H center dot center dot center dot pi bond, which generates chains in (I) and A+A and B+B inversion dimers in (II). A different aromatic ring acts as the acceptor in each case. The packing is consolidated by C-H center dot center dot center dot pi interactions in each case but aromatic pi-pi stacking interactions are absent.