Synthesis, characterization and complexation properties of aheteroditopic cryptand L towards Cu^II and Zn^II: crystal structures of L·3H₂O and[ZnL][ClO₄]₂·2H₂O

Abstract

A new heteroditopic cryptand (L) has been synthesized in high yields, at 5 °C and in absence of any templating metal ion, by tripod-tripod Schiff-base condensation of tris(3-aminopropyl)amine and 2,2',2''-nitrilotris- (ethyleneoxy)tris(benzaldehyde) followed by reduction in situ with NaBH₄. The molecule has an endo-endo conformation with a distance of 5.291 Å between the two bridgehead nitrogens. The cryptand forms mononuclear cryptates of the general formula [ML][ClO₄]₂ with copper(II) and zinc(II) perchlorates. The crystal structure of [ZnL][ClO₄]₂·2H₂O was determined. The Zn^II occupies the N₄ end of the cavity in an almost ideal tetrahedral geometry. The blue copper(II) cryptate exhibits ligand-field bands at 875 and 745 nm and a ligand-to-metal charge transfer at 275 nm in acetonitrile solution at 298 K indicative of pseudo-tetrahedral CuN₄ co-ordination. However, its EPR spectral characteristics in the solid state or in solution are typical of a magnetically concentrated complex.