EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE, CRYSTAL-STRUCTURES AT 295-K AND 173-K, AND ELECTRON-PARAMAGNETIC-RESONANCE AND ELECTRONIC-SPECTRA OF BIS[TRIS(2-PYRIDYL)METHANE]COPPER(II) DINITRATE

The crystal structure of bis[tris(2-pyridyl)methane]copper(II) dinitrate, [Cu{(C5H4N)3CH}2][NO3]2 has been determined. At 295 K the Cu atom lies on a special position so that all six Cu-N bonds are crystallographically equivalent [Cu-N 2.103(4) angstrom]. The structure at 173 K is very similar [Cu-N 2.095(3) angstrom]. However, the electronic spectrum suggests that the Cu2+ ion experiences a ligand field of tetragonal symmetry. This has been confirmed by the EXAFS of the compound, which shows four nitrogen atoms at 2.04 angstrom and two at 2.25 angstrom from the copper. The apparent trigonal symmetry revealed by the X-ray analysis is thus due to disorder of the long and short Cu-N bonds about the three-fold axis. The EPR spectrum shows an isotropic signal at 295 K, but a signal characteristic of a tetragonally elongated octahedral complex at 150 K. This suggests that the directions of the long and short bonds interchange rapidly on the EPR time-scale at room temperature, but that the complexes become frozen into particular orientations on cooling.