Structural and Magnetic Studies of Copper(II) and Zinc(II) Coordination Complexes Containing Nitroxide Radicals as Chelating Ligands

The mononuclear transition metal complexes [Cu(CF 3 SO 3 ) 2 1 2 ]( 1a ), [Zn(CF 3 SO 3 ) 2 1 2 ] ( 1b ), [Cu(CF 3 SO 3 ) 2 2 2 ] · 2CH 3 CN ( 2a ), and [Zn(CF 3 SO 3 ) 2 2 2 ] · 2CH 3 CN ( 2b ) have been preparedfrom the newly synthesized doxyl nitroxide ligands 4,4‐dimethyl‐2,2‐di(2‐pyridyl)oxazolidine‐ N ‐oxyl ( 1 ) and 4,4‐dimethyl‐2,2‐bis[2‐(3‐methylpyridyl)]oxazolidine‐ N ‐oxyl ( 2 ) and M(CF 3 SO 3 ) 2 (M = Cu II or Zn II ). These metal–nitroxide complexes have been structurally and magnetically characterized. In all complexes, the four pyridyl groups coordinate in equatorial coordination sites and the two nitroxide groups in axial coordination sites, which means that the central metal ion acquires a distorted N 4 O 2 octahedral configuration. The variable‐temperature magnetic susceptibility data show that complexes 1b and 2b exhibit paramagnetic behavior, and hence a very weak intraligand magnetic interaction could be estimated [ J NO–NO = –0.64 ( 1b ) and 0.14 cm –1 ( 2b )]. The χ m T values of 1a and 2a decrease continuously with decreasing temperature until they reach a nearly constant value at around 50 K, thereby indicating an intramolecular antiferromagnetic interaction between the Cu II ion and the nitroxide ligands for both 1a and 2a [ J Cu–NO = –81.6 ( 1a ) and–78.1 cm –1 ( 2b )]. These magnetic behaviors are supported by density functional theory calculations and are discussed in connection with the specific structural features. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)