Bis(oxofluorenediyl)oxacyclophanes: Synthesis, Crystal Structure and Complexation with Paraquat in the Gas Phase

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Abstract
The first three representatives of the new family of oxacyclophanes incorporating two 2,7‐dioxyfluorenone fragments, connected by [‐CH2CH2O‐]m spacers (m=2–4), have been synthesized. The yield of the smallest oxacyclophane (m=2) is considerably higher with respect to the larger ones (m=3 and m=4), which are formed in comparable yields. Molecular modeling and NMR spectra analysis of the model compounds suggest that an essential difference in oxacyclophanes yields is caused by formation of quasi‐cyclic intermediates, which are preorganized for macrocyclization owing to intramolecular π–π stacking interactions between the fluorenone units. The solid‐state structures of these oxacyclophanes exhibit intra‐ and intermolecular π–π stacking interactions that dictate their rectangular shape in the fluorenone backbone and crystal packing of the molecules with the parallel or T‐shape arrangement. The crystal packing in all cases is also sustained by weak CH⋅⋅⋅O hydrogen bonds. FAB mass spectral analysis of mixtures of the larger oxacyclophanes (m=3 and m=4) and a paraquat moiety revealed peaks corresponding to the loss of one and two PF6− counterions from the 1:1 complexes formed. However, no signals were observed for complexes of the paraquat moiety with the smaller oxacyclophane (m=2). Computer molecular modeling of complexes revealed a pseudorotaxane‐like incorporation of the paraquat unit, sandwiched within a macrocyclic cavity between the almost parallel‐aligned fluorenone rings of the larger oxacyclophanes (m=3 and m=4). In contrast to this, only external complexes of the smallest oxacyclophane (m=2) with a paraquat unit have been found in the energy window of 10 kcal mol−1.

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