Nitronyl Nitroxide Biradicals as Tetradentate Chelates: Unusually Large Metal-Nitroxide Ferromagnetic Interactions

A series of mononuclear transition metal complexes of the tetradentate (N2O2) 6,6‘-bis(3‘‘-oxide-1‘‘-oxyl-4‘‘,4‘‘,5‘‘,5‘‘-tetramethylimidazoline-2‘‘-yl)-2,2‘-bipyridine (NIT-bpy) has been prepared and characterized by spectroscopic, magnetic, and single-crystal X-ray diffraction studies. The free NIT-bpy ligand is planar with an anti-conformation, whereas in each complex this ligand is tetracoordinated to the metal in a syn-conformation comprising the equatorial plane of a distorted octahedron. Complexes differ by the nature of the ancillary ligands occupying the trans-axial positions, these being two water molecules in [NiII(NIT-bpy)·2H2O](ClO4)2, 1, one molecule of water and one perchlorate anion in [M(NIT-bpy)·H2O·ClO4]ClO4 (M = Ni(II), 2; Mn(II), 3; Co(II), 4), or two perchlorate anions in [CuII(NIT-bpy)·2ClO4], 5. The ferromagnetic metal−nitroxide coupling found for the nickel complex 1 (J12 = + 39.6 cm-1) arises from near-coplanarity between the π-conjugated radical plane and the equatorial plane of...