1-D Polymers with Alternate Cu2 and Ln2 Units (Ln = Gd, Er, Y) and Carboxylate Linkages

Three isostructural Cu(2)Ln(2) 1-D polymers [Cu(2)Ln(2)L(10)(H2O)(4)center dot 3H(2)O](n) where Ln = Gd (1), Er (2), and Y (3) and HL = trans-2-butenoic acid, were synthesized and characterized by X-ray crystallography, electron paramagnetic resonance, and magnetic measurements. Pairs of alternate Cu-2 and Ln(2) dinuclear units are combined into a linear array by a set of one covalent eta(2):eta(1):mu(2) carboxylate oxygen and two H bonds, at Cu center dot center dot center dot Ln distances of ca. 4.5 angstrom. These units exhibit four eta(1):eta(1):mu(2) and two eta(2):eta(1):mu(2) carboxylate bridges, respectively. Magnetic measurements between 2 and 300 K, fields B-0 = mu H-0 between 0 and 9 T, and electron paramagnetic resonance (EPR) measurements at the X-band and room temperature are reported. The magnetic susceptibilities indicate bulk antiferromagnetic behavior of the three compounds at low temperatures. Magnetization and EPR data for 1 and 3 allowed evaluation of the exchange couplings between both Cu and Gd ions in their dinuclear units and between Cu and Gd neighbor ions in the spin chains. The data for the isolated Cu-2 units in 3 yield g(parallel to) = 2.350 and g(perpendicular to) = 2.054, J(Cu-Cu) = -338 (3) cm(-1) for the exchange coupling [H-ex(1,2) = -J(1-2) S-1.S-2], and D-0 = -0.342 (0.003) cm(-1) and E-0 = 0.003 (0.001) cm(-1) for the zero-field-splitting parameters of the triplet state arising from anisotropic spin-spin interactions. Considering tetranuclear blocks Gd-Cu-Cu-Gd in 1, with the parameters for the Cu-2 unit obtained for 3, we evaluated ferromagnetic interactions between Cu and Gd neighbors, J(Cu-Gd) = 13.0 (0.1) cm(-1), and between Gd ions in the Gd-2 units, J(Gd-Gd) = 0.25 (0.02) cm(-1), with g(Gd) = 1.991. The bulk antiferromagnetic behavior of 1 is a consequence of the antiferromagnetic coupling between Cu ions and of the magnitude, vertical bar J(Cu-Gd)vertical bar, of the Cu-Gd exchange coupling. Compound 2 displays a susceptibility peak at 15 K that may be interpreted as the combined result from antiferromagnetic couplings between Er-III ions in Er-2 units and their coupling with the Cu-2 units.