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·ClO ₄ 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.