Complexation properties of a heteroditopic cryptand towards Cu^II and Ni^II, crystal structures of the cryptandand its nickel(II) cascade complex

Abstract

The crystal structure of the heteroditopic cryptand (L) formed by Schiff-base condensation of tris(2-aminoethyl)amine and 2,2',2''-nitrilotris(ethyleneoxy)tris(benzaldehyde) followed by reduction with NaBH₄ has been determined. The molecule has an endo-endo conformation with a pseudo-three-fold symmetry axis passing through the two bridgehead nitrogens. This symmetry is maintained in chloroform solution as indicated by its ¹H NMR spectrum. The cryptand accepts a metal ion (Cu^II, Zn^II or Ni^II) inside the cavity forming mononuclear cryptates of general formula [ML][ClO₄]₂. The metal ion occupies the N₄ end of the cavity. The light blue CuN₄ chromophore thus formed exhibits an axial EPR spectrum with very small A value (63 × 10^-4 cm^-1) and a low-energy ligand-field band. Nickel(II) initially forms a blue [NiL][ClO₄]₂ complex which on dissolving in moist MeCN solution crystallizes as purple rectangular parallelopipeds with the formulation [NiL(H₂O)(MeCN)][ClO₄]₂·H₂O·2MeCN. The crystal structure of this complex has also been determined. The Ni^II ion is octahedrally co-ordinated with the equatorial sites occupied by the three secondary amino N atoms of the cryptand and the N atom of the bound MeCN. The axial sites are occupied by the bridgehead N and the O atom of the water molecule which enters the cavity forming a cascade complex.