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 = -2JS(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.

1D Manganese(II) Derivatives of an Imidazole-Substituted Nitronyl Nitroxide. An Approach toward Molecular Magnetic Materials of High Dimensionality.

Extended linear complexes of manganese(II) with a bis-chelating nitronyl nitroxide ligand, 2-(2-imidazolato)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-3-oxide-1-oxy (NITIm), have been prepared where metallic and organic spin carriers alternate. Depending on the deprotonating agent, the solvent, and the counteranion, the species [Mn(NITIm)(H(2)O)(2)]CH(3)COO, 1, [Mn(NITIm)(DMSO)(2)]BPh(4), 2, [Mn(NITIm)(H(2)O)(ImH)]NO(3), 3, and [Mn(NITIm)(NITImH)]ClO(4), 4, have been obtained, which differ by the additional ligands completing the metal coordination sphere. Complexes 1-3 are cis isomers and 4 is found as the mer modification; in all compounds, one observes a regular alternation of Lambda and Delta metal environments. Their magnetic properties are similar, with Mn(II)-nitroxide interactions J approximately -45 cm(-)(1) (H = -2JS(i)().S(j)()), and they display weak ferromagnetic properties below 5 K. Canting of the manganese ions is responsible for these properties. Relevant crystallographic parameters are as follows: 1, space group Fdd2, a = 16.713(1), b = 40.111(3), c = 9.735(1), Z = 16; 2, space group Pca2(1), a = 30.328(3), b = 13.422(1), c = 9.589(1), Z = 4; 3, space group P2(1)/c, a = 9.787(2), b = 22.973(5), c = 9.671(2), beta = 117.32(3) degrees, Z = 4; 4, space group P2(1)/c, a = 9.761(2), b = 28.668(5), c = 9.941(2), beta = 96.07(3) degrees, Z = 4.

Two-Dimensional Nitroxide-Based Molecular Magnetic Materials.

Manganese(II) complexes of imidazolyl-substituted chelating nitronylnitroxides are characterized by a stereoselective arrangement of the metal centers and the ligands to give a honeycomblike layer structure (shown on the right). The magnetic properties at high temperature indicate the presence of a ferrimagnetic structure within the layers. At low temperatures, ferromagnetic ordering is observed.