| Abstract
| - Considerably large magnetic couplings were found in [Ni(phpyNO)2(H2O)2]·(ClO4)2, [Cu(phpyNO)2(ClO4)2]·2CH2Cl2, [Cu(phpyNO)2(H2O)]·(ClO4)2·C6H4Cl2, and [Cu(phpyNO)2(CF3SO3)2]·0.35H2O, where phpyNO stands for tert-butyl 5-phenyl-2-pyridyl nitroxide. The exchange couplings in equatorially coordinated copper(II)- and nickel(II)-nitroxide complexes are very strongly ferromagnetic by nature.
- We have isolated and characterized a new paramagnetic bidentate ligand, tert-butyl 5-phenyl-2-pyridyl nitroxide (phpyNO). The spin distribution onto the pyridine ring was clarified from the Electron Spin Resonance spectrum. Complexation of phpyNO with nickel(II) and copper(II) ions gave ML2-type chelated compounds. The magnetic measurements of [Ni(phpyNO)2(H2O)2]·(ClO4)2 and [Cu(phpyNO)2(H2O)]·(ClO4)2·C6H4Cl2 revealed that the metal-radical exchange couplings were ferromagnetic with 2J/kB = +409(10) and +434(12) K, respectively. The torsion angle (ϕ) around M−O−N−C2py can be regarded as a reliable indicator for the plane geometry of chelates; namely, highly planar chelates defined by small ϕ exhibit ferromagnetic coupling. An approximate linear relation was found in the J versus ϕ plot using the data of the present complexes and related known compounds. The critical angle of ϕ, at which the sign of the metal-radical exchange changes from positive to negative, was 12.6(9)°. This finding could be almost reproduced by density-functional theory calculation on a model copper(II)-nitroxide dyad. The exchange couplings in equatorially coordinated copper(II)- and nickel(II)-nitroxide complexes are very strongly ferromagnetic by nature.
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