COMPLEXATION PROPERTIES OF A HETERODITOPIC CRYPTAND TOWARDS CU-II ANDNI-II, CRYSTAL-STRUCTURES OF THE CRYPTAND AND ITS NICKEL(II) CASCADE COMPLEX

The crystal structure of the heteroditopic cryptand (L) formed by Schi ff-base condensation of tris(2-aminoethyl)amine and 2,2',2 ''-nitrilot ris(ethyleneoxy)tris(benzaldehyde) followed by reduction with NaBH4 ha s been determined. The molecule has an endo-endo conformation with a p seudo-three-fold symmetry axis passing through the two bridgehead nitr ogens. This symmetry is maintained in chloroform solution as indicated by its H-1 NMR spectrum. The cryptand accepts a metal ion (Cu-II, Zn- II or Ni-II) inside the cavity forming mononuclear cryptates of genera l formula [ML][ClO4](2). The metal ion occupies the N-4 end of the cav ity. The light blue CuN4 chromophore thus formed exhibits an axial EPR spectrum with very small A(parallel to) value (63 x 10(-4) cm(-1)) an d a low-energy ligand-field band. Nickel(II) initially forms a blue [N iL][ClO4](2) complex which on dissolving in moist MeCN solution crysta llizes as purple rectangular parallelopipeds with the formulation [NiL (H2O)(MeCN)][ClO4](2) . H2O . 2MeCN. The crystal structure of this com plex has also been determined. The Ni-II ion is octahedrally co-ordina ted 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 site s are occupied by the bridgehead N and the O atom of the water molecul e which enters the cavity forming a cascade complex.