Extended X-ray absorption fine structure, crystal structures at 295 and 173 K, and electron paramagnetic resonance and electronic spectra of bis[tris(2-pyridyl)-methane]copper(II) dinitrate

Abstract

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)Å]. The structure at 173 K is very similar [Cu–N 2.095(3)Å]. 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 Å and two at 2.25 Å 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.