Crystal structures and magnetic properties of metal complexes bearing four nitronyl nitroxide moieties in the same coordination sphere.

Abstract

Mononuclear transition metal complexes of the type [M(2,6-NITpy)2](ClO4)2 x solvent (2,6-NITpy = 2,6-bis-(3'-oxide-1'-oxyl-4',4',5',5'-tetramethylimidazolin-2'-yl)pyridine; M = Ni (1), Co (2), Zn (3), Mn (4), Cu (5)) have been synthesized and characterized by single-crystal X-ray diffraction studies. Crystal data: 1, monoclinic, P2(1)/c, Z = 4, a = 20.946(2) A, b = 12.0633(2) A, c = 21.173(2) A, beta = 113.55(1) degrees; 2, monoclinic, P2(1)/c, Z = 4, a = 20.902(2) A, b = 12.0981(8) A, c = 21.215(2) A, beta = 113.130(9) degrees; 3, triclinic, P1, Z = 2, a = 11.410(1) A, b = 12.932(1) A, c = 21.609(2) A, alpha = 96.040(2) degrees, beta = 102.24(1) degrees, gamma = 114.98(1); 4, monoclinic, P2(1)/n, Z = 4, a = 11.5473(8) A, b = 19.212(1) A, c = 25.236(2) A, beta = 98.772(9) degrees; 5, triclinic, P1, Z = 2, a = 12.1604(9) A, b = 12.6961(9) A, c = 18.103(2) A, alpha = 84.191(8) degrees, beta = 73.392(8) degrees, gamma = 66.072(8). The two 2,6-NITpy biradicals behave as terdentate ligands and bind almost perpendicular to each other in meridional positions. In compounds 1-4, the pyridine rings are axially ligated and four different nitronyl nitroxide radicals bind to the metal center through their O(nitroxyl) atoms, forming the equatorial plane of a distorted octahedron. On the contrary, in the copper(II) complex (5), the two N(pyridyl) atoms are found in equatorial positions. Only two nitroxide groups are then bound to the copper(II) ion in the equatorial plane, the other two being axially ligated. The two axially bound nitronyl nitroxide radicals couple ferromagnetically to the copper center (JCu-rad(ax) = + 10 K), whereas a strong antiferromagnetic coupling between this metal ion and the equatorial nitroxide groups (JCu-rad(eq) = -460 K) is observed. The other complexes exhibit strong antiferromagnetic metal-radical interactions: JNi-rad = -240 K, for 1; JMn-rad = -120 K, for 4. Interestingly, the study of the diamagnetic zinc(II) compound (3) reveals a moderate intramolecular antiferromagnetic interaction between radicals coordinated to the same metal center (Jrad-rad = -27.7 K). This interaction is transmitted through space and is also present in the other complexes: Jrad-rad = -14 K, for 1; Jrad-rad = -10 K, for 4; Jrad-rad = -20.5 K, for 5. Antiferromagnetic intermolecular interactions are also present in all the complexes herein studied.