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 <i>tert</i>-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