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Ferro- and antiferromagnetic coupling switch accompanied by twist deformation around the copper(II) and nitroxide
Atsushi Okazawa1, Daisuke Hashizume, Takayuki Ishida
1Department of Applied Physics and Chemistry, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan.
Abstract:
Two novel copper(II) complexes with tert-butyl 2-pyridyl nitroxide (2pyNO(*)), [Cu(2+)(2pyNO(-))(2pyNO(*))](2)(BF(4)(-))(2) (1 x BF(4)) and [Cu(2+)(2pyNO(-))(2pyNO(*))](2)(ClO(4)(-))(2) (1 x ClO(4)), 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 [Cu(2+)(2pyNO(-))(2pyNO(*))] unit was dimerized by mu-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 x ClO(4) showed ferromagnetic copper-radical coupling in all temperature ranges investigated here. On the other hand, 1 x BF(4) 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 d sigma and nitroxide pi* 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.
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