Cu( ii ) complex with nitronyl nitroxide whose paramagnetism is suppressed by temperature decrease and/or pressure increase

The reaction of Cu(hfac) 2 with a spirocyclic nitronyl nitroxide 2-(1-ethylpyrazol-4-yl)-4,5-bis(spirocyclopentane)-4,5-dihydro-1 H -imidazole-3-oxide-1-oxyl (L Et ) gave a heterospin complex [Cu(hfac) 2 L Et ]-I, whose solid state is formed by polymer chains with a head-to-tail motif (the bridging function inside the chains is performed by the paramagnetic organic ligand). At 295 K the distance from the Cu atom to the N atom of the pyrazole ring and to the O atom of the NO group is 2.333(2) and 2.451(2) Å, respectively. The cooling of the compound to 110 K leads to a structural transition, as a result of which the unit cell volume decreases by more than 7%. The elongated axis of the octahedron changes from N–Cu–O NO to O hfac –Cu–O hfac in all coordination units as a result of a significant decrease in the Cu–O NO and Cu–N bond lengths to 2.002(4) and 2.034(5) Å, respectively, and an increase in the two Cu–O hfac distances from 1.950(2) and 1.955(2) Å to 2.263(4) and 2.213(4) Å, respectively. The [Cu(hfac) 2 L Et ]-I crystals are elastic and stable during the thermally induced spin crossover. The transition from the high-temperature to low-temperature phase is accompanied by a spin transition at 125 K at 1.0 × 10 −4 GPa, as a result of which [Cu(hfac) 2 L Et ]-I transforms from a paramagnetic compound to a diamagnetic one due to the complete spin coupling in the {Cu( ii )–O˙–N