New Spin-Transition-Like Copper(II)−Nitroxide Species

Novel copper(II)−nitroxide complexes exhibiting a spin-transition-like behavior have been prepared and characterized.
They include meso, chiral, and racemic 2-(3-pyridyl)-nitronyl nitroxides differently substituted in positions 4 and/or
5 by ethyl groups and pyrimidyl nitroxides. Depending on the stoichiometry of the reaction, tetranuclear and binuclear
complexes were obtained whose structures are cyclic. The tetranuclear species, which include two intracyclic and
two exocyclic metal sites, are similar to the previously reported complex of the tetramethylated analogue, while the
binuclear complexes involve only endocyclic metal ions and have uncoordinated N-oxyl groups. The tetranuclear
complexes exist as two isomers depending on the temperature of crystallization:  at room temperature, N-oxyl
ligand coordination is axial−axial, while it is axial−equatorial at low temperature. Unexpectedly, this isomerism
concerns N-oxyl bonding to the exocyclic metal centers for the derivatives of 4,5-diethyl-substituted ligands while
it involves the endocyclic metal site in the complex of the monoethylated ligand, which converts reversibly from a
high-spin state to a low-spin state, as observed for the complex of the tetramethylated ligand. Binuclear complexes
are diamagnetic at room temperature but convert to a paramagnetic state on warming (90−110 °C); the transition
is irreversible and sharp.