title: Ferro- and antiferromagnetic coupling switch accompanied by twist deformation around the copper(II) and nitroxide coordination bond.
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.
authors.affiliation: Department of Applied Physics and Chemistry, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan.
authors.forename: Atsushi
authors.initials: A
authors.lastname: Okazawa
authors.forename: Daisuke
authors.initials: D
authors.lastname: Hashizume
authors.forename: Takayuki
authors.initials: T
authors.lastname: Ishida
mesh_terms: D003300:Copper / Q000737:chemistry*; D018360:Crystallography, X-Ray; D005290:Ferric Compounds / Q000737:chemistry*; D008280:Magnetics; D008958:Models, Molecular; D008968:Molecular Conformation; D009569:Nitric Oxide / Q000737:chemistry*; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*
chemical_list: D005290:Ferric Compounds; D009942:Organometallic Compounds; D009569:Nitric Oxide; D003300:Copper