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

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=N-3 or I-3) 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 [Na-2(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.