Proximate Nitroxide Ligands in the Coordination Spheres of Manganese(II) and Nickel(II) Ions. Precursors for High-Dimensional Molecular Magnetic Materials

The chelating nitroxide ligands 2-(2-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-3-oxide-1-oxy (NITPy, 1), 2-(2-imidazolyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-3-oxide-1-oxy (NITImH, 2), and 2-(2-benzimidazolyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-3-oxide-1-oxy (NITBzImH, 3) and some of their transition metal complexes (Mn(II), Ni(II), Zn(II)) have been prepared and characterized by X-ray diffraction techniques and magnetic susceptibility measurements. All complexes are four- (or three-) spin systems where the metal coordination sphere is free of ancillary ligands because of the chelate effect which enforces the coordination of the oxyl group. The fac or mer nature of these species depends on the metal ions and on the steric demand of the ligand. It has been found that crystal packing is an important driving force toward the fac modification when steric requirements are not important. Crystal packing is probably also the cause of the noncentrosymmetric space group observed for the derivatives of NITPy. For the Zn(II) complex of NITImH, a moderate inter-nitroxide interaction within the metal coordination sphere of −14 cm-1 is estimated. However, due to the modification of the spin distribution upon complexation, this interaction does not play a major role in the other complexes, where strong antiferromagnetic metal−nitroxide interactions (H = −2J S i·S j, −111 < J < −53 cm-1) are operative. The derivatives of NITImH are precursors of extended species which would be obtained by deprotonation of the ligand.