Polyoxaalkyl esters of phenylboronic acids as new podands

Three oxaalkyl esters of phenylboronic acid and one oxaalkyl ester of phenyldiboronic acid have been synthesized and their ability to form complexes with Li+ and Na+ cations has been studied using multinuclear NMR, ESI mass spectrometric and PM5 semiempirical methods. It has been demonstrated that the esters of phenylboronic acid form 1:1 complexes with Li+ and Na+ cations whereas the ester of phenyldiboronic acid can additionally form complexes of 1:2 stoichiometry. The stability constants of these complexes have been determined to show that with increasing number of oxygen atoms in the oxaalkyl chains the complexes of increasing stability are formed. The exemplary 1:1 and 1:2 structures of the complexes of esters with Li+ and Na+ cations, respectively, are given. They show that the cations are coordinated within a pseudo-crown structures formed by oxaalkyl chains.