Barium(II) thiocyanate templating Schiff-base lateral macrobicycles derived from 1,10-diaza-15-crown-5

Abstract

Previously we have proved that barium perchlorate is a highly effective template agent in the formation of the Schiff-base lateral macrobicycles L 1 and L 2 (receptors derived from the condensation of N, N′-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 with 2,6-diformylpyridine and 2,6-diformyl-4-methylphenol, respectively). Aiming to understand the effect that the nature of the counteranion present in the reaction medium has on the barium template potential, we have carried out the corresponding template synthesis in the presence of barium thiocyanate. We have found that considerably longer reaction times are required to obtain both receptors when barium thiocyanate is employed. The barium template remains trapped into the formed macrobicyclic cavity and compounds of formula Ba(L 1)(SCN) 2 · H 2O ( 1) and Ba(L 2)(SCN) 2 · 2H 2O ( 2) were isolated. The X-ray crystal structure of 1 shows the metal ion coordinated to the eight donor atoms of the receptor and to the nitrogen atom of a isothiocyanate group, the coordination polyhedron being best described as a monocapped distorted dodecahedron. In 1, the metal ion is nearly centred inside the macrobicyclic cavity of L 1 , whereas NMR studies in solution indicate that in 2 the barium ion is asymmetrically situated in the macrobicyclic cavity of L 2 . This asymmetric coordination is attributed to the proton transfer from the phenol group to the nitrogen atom of one of the imine groups.