Crucial Role of Paramagnetic Ligands for Magnetostructural Anomalies in “Breathing Crystals”

Breathing crystals based on polymer-chain complexes of
Cu­(hfac)2 with nitroxides exhibit thermally and light-induced
magnetostructural
anomalies in many aspects similar to a spin crossover. In the present
work, we report the synthesis and investigation of a new family of
Cu­(hfac)2 complexes with tert-butylpyrazolylnitroxides
and their nonradical structural analogues. The complexes with paramagnetic
ligands clearly exhibit structural rearrangements in the copper­(II)
coordination units and accompanying magnetic phenomena characteristic
for breathing crystals. Contrary to that, their structural analogues
with diamagnetic ligands do not undergo rearrangements in the copper­(II)
coordination environments. This confirms experimentally the crucial
role of paramagnetic ligands and exchange interactions between them
and copper­(II) ions for the origin of magnetostructural anomalies
in this family of molecular magnets.