Stabilisation of sodium complexes of 18-crown-6 by intramolecular hydrogen bonding†

Abstract

Complexation of Na⁺ by 18-crown-6 within an aqueous medium resulted in the formation of the monohydrates [Na(18-crown-6)(H₂O)(X)] (X = ClO₄, NO₃ or ReO₄) in the presence of oxygen donor anions. All three complexes exhibit a significant intramolecular hydrogen bond between the co-ordinated water molecule and the crown ether as well as structure organising C–H· · ·O interactions. In the presence of anions with less affinity for Na⁺, complexes of type [Na(18-crown-6)(H₂O)₂]X (X = N₃ or I₃) were formed. In the case of the azide both water molecules hydrogen bond strongly with the crown ligand giving rise to a highly unsymmetrical complex. In the triiodide more symmetrical intramolecular interactions are observed, as well as intermolecular water–crown hydrogen bonds. Reaction of NaBPh₄ with 18-crown-6 in aqueous ethanol resulted in the formation of [Na₂(18-crown-6)₂(H₂O)₃][BPh₄]₂ in which strong intramolecular hydrogen bonds are observed for both bridging and terminal water ligands in a similar fashion to the azide 4a. The bridging aqua ligands interact with both crown ether hydrogen bond acceptors.