NMR, FT-IR and ESI-MS study of new lasalocid ester with 2-(hydroxymethyl)-12-crown-4 and its complexes with monovalent cations

A new ester of lasalocid with 2-(hydroxymethyl)-12-crown-4 (Las8) was synthesised and its ability to complex formation with monovalent cations was studied by ESI mass spectrometry as well as 1H, 13C NMR and FT-IR spectroscopic methods. The ESI-MS spectra indicate that Las8 formes stable 1:1 complexes with all cations. Interestingly for Li+ additionally the formation of 1:2 complexes was observed. In this case both, the crown ether as well the lasalocid parts take part in the complexation mechanism, explaining the second possible complex stoichiometry. The FT-IR spectra of Las8 in acetonitrile solution indicate the existence of keto and enol forms, where the keto form is predominantly formed. An influence of the cations on this keto-enol equilibrium was not observed indicating that the oxygen atoms of the ester group were not involved in the coordination process of cations. The results of the PM5 semiempirical calculations are in agreement with the spectroscopic data and allow visualisation of the structure of Las8 complexes with monovalent cations. The structures of complexes are discussed in details.