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

A series of mononuclear transition metal complexes of the tetradentate (N 2 O 2 ) 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 [Ni II (NIT-bpy)·2H 2 O](ClO 4 ) 2, 1, one molecule of water and one perchlorate anion in [M(NIT-bpy)·H 2 O·ClO 4 ]ClO 4 (M = Ni(II), 2; Mn(II), 3; Co(II), 4 ), or two perchlorate anions in [Cu II (NIT-bpy)·2ClO 4 ], 5 . The ferromagnetic metal−nitroxide coupling found for the nickel complex 1 ( J 12 = + 39.6 cm - 1 ) arises from near-coplanarity between the π-conjugated radical plane and the equatorial plane of the trans -diaqua{Ni(N 2 O 2 )(H 2 O) 2 } octahedron. Deviations from this geometry lead to antiferromagnetic components that are dominant in complex 1 ( J 23 = − 48.7 cm - 1 ) and tend to reduce the extent of the ferromagnetic interactions in complex 2 ( J 12 = +27.8 cm - 1 and J 23 = +6.9 cm - 1 ). For the Mn 2+ complex 3, variable temperature studies indicate antiferromagnetic behavior J = − 65.3 cm - 1, but with weak intermolecular ferromagnetic interactions zJ ‘ = + 0.22 cm - 1 . For the Co 2+ complex 4, experimental data were best explained as a weak ferromagnetic interaction J 12 = + 0.40 cm - 1 and a relatively strong antiferromagnetic interaction J 23 = − 28.0 cm - 1 . The Cu 2+ complex 5 exhibits strong antiferromagnetic behavior J = −166.8 cm - 1 . The magnetic data of the [M‘(NIT-bpy)](ClO 4 ) 2 (M‘ = Zn(II), 6; Fe(II) low-spin, 7 ) complexes were fit to a Curie−Weiss law (θ = −2.43 cm - 1 for 6 and θ = −0.51 cm - 1 for 7 ). No intramolecular radical−radical interaction is apparent in any of these complexes.