WEAK FERROMAGNETISM IN THE MOLECULAR-COMPOUND CU(TA)(3-PIC).H2O (TA = TEREPHTHALATO, 3-PIC = 3-PICOLINE) - CRYSTAL-STRUCTURE OF CU(TA)(3-PIC)2.0.5(3-PIC).0.5CH3OH

The two compounds Cu(ta)(3-pic)2.0.5(3-pic).0.5CH3OH (1) and Cu(ta)(3-pic).H2O (2) have been synthesized, ta standing for terephthalato and 3-pic for 3-picoline. The crystal structure of 1 has been determined. 1 crystallizes in the monoclinic system, space group P2(1)/c, with a = 9.703 (4) angstrom, b = 13.184 (8) angstrom, c = 19.825 (4) angstrom, beta = 115.12 (2)-degrees, and Z = 4. The structure consists of Cu(ta) infinite layers, to which the 3-pic molecules are perpendicular, and noncoordinated 3-pic and CH3OH molecules. In the lattice the Cu(ta)(3-pic)2 units are centrosymmetrically related, affording copper(II) pairs with a Cu...Cu separation of 4.414 angstrom. Those pairs are linked together through terephthalate bridges. 1 spontaneously evolves in air to yield 2. The copper(II) ions in 1 are magnetically isolated. In contrast, the magnetic properties of 2 are quite unusual. Magnetic susceptibility measurements together with EPR spectra and low-field (5 G) and high-field (up to 20 T) magnetization data lead to the following description: 2 consists of antiferromagnetically coupled copper(II) pairs with a singlet-triplet energy gap J almost-equal-to -330 cm-1. In addition, antisymmetric interaction gives rise to a residual magnetic moment in the ground state. The pairs weakly couple through the terephthalate bridges in such a way that the pair magnetic moments align along the same direction. 2 exhibits a weak ferromagnetic transition at T(c) = 2.8 K.