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

Abstract

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 the trans-diaqua{Ni(N2O2)(H2O)2} octahedron. Deviations from this geometry lead to antiferromagnetic components that are dominant in complex 1 (J23 = − 48.7 cm-1) and tend to reduce the extent of the ferromagnetic interactions in complex 2 (J12 = +27.8 cm-1 and J23 = +6.9 cm-1). For the Mn2+ 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 Co2+ complex 4, experimental data were best explained as a weak ferromagnetic interaction J12 = + 0.40 cm-1 and a relatively strong antiferromagnetic interaction J23 = − 28.0 cm-1. The Cu2+ complex 5 exhibits strong antiferromagnetic behavior J = −166.8 cm-1. The magnetic data of the [M‘(NIT-bpy)](ClO4)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.