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

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 degrees C temperature. The crystal structure of L (P2(1)/c, a=10.756(5) Angstrom, b=27.407(9) Angstrom, c=12.000(2) Angstrom, beta=116.22(3) degrees, 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 H-1-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 x 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 N-3(-), forming distorted trigonal bipyramidal complexes (2-4). The crystal structure of [Cu(L)(CN)](picrate)(2) (P2(1)/C, a=13.099(1) Angstrom, b=11.847(8) Angstrom, c=25.844(7) Angstrom, beta=91.22(1) degrees, 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 distance is 2.339(6) Angstrom, which is longer than normal values. The crystal structure of [Cu(L)(NCS)](picrate)(3) (C2/c, a=47.889(10) Angstrom, b=10.467(5) Angstrom, c=16.922(2) Angstrom, beta=93.90(2) degrees, 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.