Ferro- and Antiferromagnetic Coupling Switch Accompanied by Twist Deformation around the Copper(II) and Nitroxide Coordination Bond

Two novel copper(II) complexes with tert-butyl 2-pyridyl nitroxide (2pyNO•), [Cu2+(2pyNO−)(2pyNO•)]2(BF4−)2 (1·BF4) and [Cu2+(2pyNO−)(2pyNO•)]2(ClO4−)2 (1·ClO4), were prepared and structurally characterized. They contained mixed-valent ligands from 2pyNO, whose oxygen atoms were located at equatorial positions of the copper ion. The [Cu2+(2pyNO−)(2pyNO•)] unit was dimerized by μ-oxo bridges of the anion ligand, giving a zigzag linear spin system involving four paramagnetic S = 1/2 centers. The two compounds are isomorphous in an orthorhombic Pbca space group. Magnetic study revealed that 1·ClO4 showed ferromagnetic copper−radical coupling in all temperature ranges investigated here. On the other hand, 1·BF4 exhibited a structural phase transition at 64 K, where the magnetic susceptibility was drastically dropped on cooling. The copper−radical magnetic couplings were characterized as ferro- and antiferromagnetic for the high- and low-temperature phases, respectively. The crystallographic analysis clarified that the nitroxide oxygen atom remained at the equatorial position throughout the single-crystal-to-single-crystal phase transition, while the previously known spin-transition-like copper−radical compounds showed conversion of the roles of equatorial and axial positions. The orthogonal arrangement between the copper dσ and nitroxide π* orbitals is essential for the ferromagnetic coupling, and a slight dislocation of the radical oxygen atom from the chelate plane leads to violation of the orthogonal orbital arrangement, giving a practically diamagnetic low-temperature phase.