Magneto-structure relationship in copper(II) and nickel(II) complexes chelated with stable tert-butyl 5-phenyl-2-pyridyl nitroxide and related radicals.

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 ML 2-type chelated compounds. The magnetic measurements of [Ni(phpyNO) 2(H 2O) 2].(ClO 4) 2 and [Cu(phpyNO) 2(H 2O)].(ClO 4) 2.C 6H 4Cl 2 revealed that the metal-radical exchange couplings were ferromagnetic with 2 J/ k B = +409(10) and +434(12) K, respectively. The torsion angle (varphi) around M-O-N-C 2py can be regarded as a reliable indicator for the plane geometry of chelates; namely, highly planar chelates defined by small varphi exhibit ferromagnetic coupling. An approximate linear relation was found in the J versus varphi plot using the data of the present complexes and related known compounds. The critical angle of varphi, at which the sign of the metal-radical exchange changes from positive to negative, was 12.6(9) degrees . 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.