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-1H-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 pyrazote ring and to the 0 atom of the NO group is 2 333(2) and 2.451(2) angstrom, 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-hrac 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) angstrom, respectively, and an increase in the two Cu-O-hrac distances from 1.950(2) and 1.955(2) angstrom to 2.263(4) and 2.213(4) angstrom, 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 x 10(-4) GPa, as a result of which ICu(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 center dot-N<} exchange clusters inside the polymer chains. The reversible phase transformation is accompanied by a reversible contrastive change of color from blue to dark brown upon cooling and vice versa from dark brown to blue upon heating. The spin crossover temperature is very sensitive to the change of external pressure: when pressure increases from 1.0 x 10(-4) GPa to 0.48 GPa, it increases to 325 K. The increase of pressure provokes a transition from a high-spin to low-spin state, which again transforms into the high-spin state when the external toad is removed. The ECu(hfac)(2)L-Et]-I crystals are the first examples of polymer-chain breathing crystals whose paramagnetic properties are suppressed by the lowering of temperature and/or increase of pressure.