CRYSTAL-STRUCTURE AND MAGNETIC-PROPERTIES OF DIAQUA(L-ASPARTATO)COPPER(II)

The title compound, Cu(CO2NH2CHCH2CO2)(H2O)(2), crystallizes in the space group C2, with a = 9.504(1) Angstrom, b = 10.038(3) Angstrom, c = 7.555(1) Angstrom, beta = 94.01 degrees, and Z = 4. The structure was solved by employing 807 independent reflections with I > 3 sigma(I), by Patterson and difference Fourier techniques, and refined by full-matrix least squares to R = 0.024. The Cu(II) ion is in a distorted tetragonal pyramidal coordination. The shortest equatorial bonds occur at the pyramid base with a water oxygen [d(Cu-OW) = 1.946(3) Angstrom], the nitrogen [d(Cu-N) = 1.998(4) Angstrom], an alpha-carboxylic oxygen [d(Cu-O) = 1.955(3) Angstrom] of one aspartate ion (asp), and a beta-carboxylic oxygen [d(Cu-O) = 1.950(2) Angstrom] of another aspartate ion related to the first by a c translation. The longest bond occurs with a water oxygen at the pyramid apex [d(Cu-OW) = 2.313(3) Angstrom]. The equatorial bonding causes -asp-Cu-asp-Cu-asp-chains along c. These are linked by a network of interchain II-bonds involving the water molecules and the amino groups. Magnetic susceptibility data obtained between 5 K and room temperature show an antiferromagnetic behavior, with a peak value at about 7 K, and no indication of a phase transition to a 3D ordered magnetic phase. These data are interpreted in terms of a linear chain model, with antiferromagnetic exchange interaction of coupling constant J/k = 5.3 K between neighboring copper ions on a chain. The intrachain superexchange path is identified with the cr bonds along the skeleton of the aspartic acid molecule. The susceptibility data also suggest ferromagnetic exchange coupling between copper ions in different chains. Room-temperature EPR measurements at 9.7 and 33.4 GHz in single-crystal samples show a single resonance for any orientation of the applied magnetic field. It results from the collapse due to exchange interaction of the pair of resonances expected for the two magnetically nonequivalent Cu(II) sites in the unit cell. From the crystal g-tensor we calculate the molecular g-values of individual copper ions. These values are then related to the electronic structure around Cu(II) and compared with those obtained in related compounds. The magnetic interactions in the aspartic acid compound are discussed in terms of the superexchange paths and compared with those observed in other copper-amino acid complexes.