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

Abstract

Three isostructural Cu2Ln2 1-D polymers [Cu2Ln2L10(H2O)4 · 3H2O]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 Cu2 and Ln2 dinuclear units are combined into a linear array by a set of one covalent η2:η1:μ2 carboxylate oxygen and two H bonds, at Cu · · · Ln distances of ca. 4.5 Å. These units exhibit four η1:η1:μ2 and two η2:η1:μ2 carboxylate bridges, respectively. Magnetic measurements between 2 and 300 K, fields B0 ) μ0H 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 Cu2 units in 3 yield g|| ) 2.350 and g⊥ ) 2.054, JCu-Cu ) -338 (3) cm-1 for the exchange coupling [Hex(1,2) ) -J1-2 S1 · S2], and D0 ) -0.342 (0.003) cm-1 and E0 ) 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 Cu2 unit obtained for 3, we evaluated ferromagnetic interactions between Cu and Gd neighbors, JCu-Gd ) 13.0 (0.1) cm-1, and between Gd ions in the Gd2 units, JGd-Gd ) 0.25 (0.02) cm-1, with gGd ) 1.991. The bulk antiferromagnetic behavior of 1 is a consequence of the antiferromagnetic coupling between Cu ions and of the magnitude, |JCu-Gd|, 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 ErIII ions in Er2 units and their coupling with the Cu2 units.