Molecular Structure and Crystal Packing of Monofluoromethoxyarenes

Abstract

X-ray crystal structures of monofluoromethoxy aryl compounds are presented for the first time. In fluoromethoxybenzene derivatives with at least one unsubstituted ortho-position, the fluoromethoxy group adopts a twisted anisole conformation. The twist is most pronounced in derivatives with the least electron-demanding phenyl unit. The observed structural features in the crystalline state are in good agreement with theoretical predictions for prototypical monofluoromethoxybenzene. In contrast, a 5-fluoromethoxyindole derivative exhibits a conformation differing from that by theoretical predictions. Such a difference may be due to crystal packing, in which the fluorine atom is trapped in a hydrophobic pocket lined with aliphatic C–H bonds. The crystal structures of the simple α-monofluorinated anisoles exhibit a variety of linear and cyclic interaction motifs for the monofluoromethoxy group. Similar motifs are also observed in the crystals of other monofluoromethyl-containing compounds. In all these crystal structures the geometrical characteristics of mutual close contacts between monofluoromethoxy or between monofluoromethyl units are dependent on their surrounding environment in the crystal structure, an observation consistent with relatively weak interactions. A survey of all close C–F···H–C contacts in these crystals and in crystal structures of CH2F-containing compounds from the CSD reveals little evidence for significant H-bonding character in these interactions.