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

The title compound, Cu(CO2NH2CHCH2CO2)(H2O)(2), crystallizes in the sp ace 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), b y Patterson and difference Fourier techniques, and refined by full-mat rix least squares to R = 0.024. The Cu(II) ion is in a distorted tetra gonal pyramidal coordination. The shortest equatorial bonds occur at t he pyramid base with a water oxygen [d(Cu-OW) = 1.946(3) Angstrom], th e 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-ca rboxylic 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]. Th e equatorial bonding causes -asp-Cu-asp-Cu-asp-chains along c. These a re linked by a network of interchain II-bonds involving the water mole cules and the amino groups. Magnetic susceptibility data obtained betw een 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 cou pling constant J/k = 5.3 K between neighboring copper ions on a chain. The intrachain superexchange path is identified with the cr bonds alo ng the skeleton of the aspartic acid molecule. The susceptibility data also suggest ferromagnetic exchange coupling between copper ions in d ifferent 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 ex change interaction of the pair of resonances expected for the two magn etically nonequivalent Cu(II) sites in the unit cell. From the crystal g-tensor we calculate the molecular g-values of individual copper ion s. These values are then related to the electronic structure around Cu (II) and compared with those obtained in related compounds. The magnet ic interactions in the aspartic acid compound are discussed in terms o f the superexchange paths and compared with those observed in other co pper-amino acid complexes.