Synthesis of a heteroditopic cryptand capable of imposing a distorted coordination geometry onto Cu(II): crystal structures of the cryptand (L), [Cu(L)(CN)](picrate), and [Cu(L)(NCS)](picrate) and spectroscopic studies of the Cu(II) complexes

Abstract

The synthesis of a new macrobicyclic cryptand (L) with heteroditopic receptor sites has been achieved in good yields by the [1 + 1] Schiff base condensation of tris(2-aminoethyl)amine (tren) with the tripodal trialdehyde, tris{[2-(3-(oxomethyl)phenyl)oxy]ethyl}amine at 5 °C temperature. The crystal structure of L (P2₁/c, a = 10.756 (5) Å, b = 27.407(9) Å, c = 12.000(2) Å, β = 116.22(3)°, Z = 4, R = 0.060, R_w = 0.058) shows a pseudo-3-fold symmetry axis passing through the two bridgehead nitrogens. This symmetry is maintained in chloroform solution also, as indicated from its ¹H-NMR spectral data. The cryptand readily forms inclusion complexes with the Cu(II) ion at the tren end of the cavity. The tetracoordinated Cu(II) cryptate (1) thus formed with Cu(picrate)₂ exhibits a very small A_II value (60 × 10^-4 cm^-1) in its EPR spectrum and low-energy ligand field bands in its electronic spectrum in MeCN at room temperature. The bound Cu(II) ion readily accepts the anions CN^-, SCN^-, or N₃^-, forming distorted trigonal bipyramidal cmplexes (2-4). The crystal structure of [Cu(L)(CN)](picrate) (2) (P2₁/C, a = 13.099(1) Å, b = 11.847(8) Å, c = 25.844(7) Å, β = 91.22(1)°, Z = 4, R = 0.056, R_w = 0.054) has been determined. The equatorial coordination is provided by the three secondary amino N atoms of the tren unit in L, while the two axial positions are occupied by the bridgehead N of the tren unit and the C atom of the cyanide group. One of the equatorial Cu-N bond distances is 2.339(6) Å, which is longer than normal values. The crystal structure of [Cu(L)(NCS)](picrate) (3) (C2/c, a = 47.889(10) Å, b = 10.467(5) Å, c = 16.922(2) Å, β = 93.90(2)°, Z = 8, R = 0.054, R_w = 0.055) shows the coordination geometry around the Cu(II) ion to be very similar to that in the case of 2. The electronic spectral and EPR spectral data obtained on 2-4 are characteristic of trigonal bipyramidal Cu(II) complexes. The three meta-substituted benzene rings present in L makes the donor atom somewhat rigid in nature which enforces a distorted geometry around the Cu(II) ion.