The crystal structure and low temperature magnetic susceptibility of square-planar bis(1,5-diazacyclooctane)nickel(II) perchlorate dihydrate

We have carried out low temperature magnetic susceptibility measurements and a redetermination of the crystal and molecular structure of bis(1,5-diazacyclooctane)nickel(II) perchlorate dihydrate from three-dimensional intensity data collected on a CAD4 diffractometer. The structure was refined by full-matrix least-squares to the R value of 6.3% for 1501 observed reflections. The positions of the hydrogen atoms were obtained from the diffraction data. Contrary to our earlier conclusions drawn by inspection of molecular models of bis(diazacyclooctane) complexes, the axial sites in the actual crystal structure are open for ligation. The Ni-N bonds are, however, sterically protected by the coplanar N-H bonds, allowing maximum ligand-field stabilization for the squareplanar geometry around the nickel atom. The observed magnetic transition from a diamagnetic state at room temperature to a paramagnetic state at 78 K is explained on the basis of long-range exchange interactions along sterically unshielded z axis. The crystal structure is compared with other similar structures.