Structure of 1:1 complex of 1-naphthylmethyl ester of monensin A with sodium perchlorate studied by X-ray, FT-IR and ab initio methods

A new crystalline complex formed between 1-naphthylmethyl ester of the naturally occurring antibiotic – monensin A (MON8) with sodium perchlorate has been obtained and studied using X-ray crystallography and FT-IR spectroscopy. The X-ray data of the complex show that MON8 forms a pseudo-cyclic structure stabilised by one weak intramolecular hydrogen bond and the sodium cation co-ordinated by two oxygen atoms of hydroxyl groups and four etheric oxygen atoms in the hydrophilic sphere. Within this structure the oxygen atoms of the ester groups are not involved in the coordination of sodium cation. In contrast to the solid state structure of the complex, in acetonitrile solution an equilibrium between two structures, in which the oxygen atom of the carbonyl ester group is either involved or not involved in the complexation of the sodium cation, is found. In acetonitrile this equilibrium is shifted towards the latter structure i.e. the structure existing in the solid state. The gas-phase structure of [MON8Na]+ cation as shown the ab initio MO calculations is comparable with the crystal one. Three-dimensional molecular electrostatic potential calculated for the neutral MON8 and [MON8Na]+ molecules is helpful for understanding the structural aspects of the sodium complex formation.