| Summary: | A series of heterometallic Gd<sup>III</sup>-V<sup>IV</sup> compounds were synthesized by the reaction of VOSO<sub>4</sub>·3H<sub>2</sub>O with cyclobutane-1,1-dicarboxylic acid salts M<sub>2</sub>(cbdc) (M = Na, Rb, Cs). The new compounds were formed by [Gd(VO)<sub>2</sub>(cbdc)<sub>4</sub>(H<sub>2</sub>O)<sub>8</sub>]<sup>−</sup> trinuclear anionic units that were similar in composition but differed in structure, depending on the nature of the alkali metal cation incorporated in the crystal structure of the compound. In the case of Na<sup>+</sup>, the {GdV<sub>2</sub>}<sup>−</sup> units were characterized by identical V···Gd distances and were linked into the 1D-polymeric chain [NaGd(VO)<sub>2</sub>(cbdc)<sub>4</sub>(H<sub>2</sub>O)<sub>10</sub>]<i><sub>n</sub></i> (<b>1</b>). In the systems with Rb<sup>+</sup> and Cs<sup>+</sup>, the V···Gd distances were different, and the {GdV<sub>2</sub>}<sup>−</sup> units were linked into the 3D-framework {[RbGd(VO)<sub>2</sub>(cbdc)<sub>4</sub>(H<sub>2</sub>O)<sub>10</sub>]·2.5H<sub>2</sub>O}<i><sub>n</sub></i> (<b>2</b>) and the octanuclear molecule {[CsGd(VO)<sub>2</sub>(cbdc)<sub>4</sub>(H<sub>2</sub>O)<sub>11</sub>]·5H<sub>2</sub>O}<sub>2</sub> (<b>3</b>), respectively. According to <i>dc</i>-magnetic measurements, the V<sup>IV</sup> and Gd<sup>III</sup> ions were ferromagnetically coupled in compound <b>1</b> (<i>J</i><sub>VGd</sub> = 0.163 ± 0.008 cm<sup>−1</sup>), while in compounds <b>2</b> and <b>3</b>, ferro- and weak antiferromagnetic exchange interactions were observed (<i>J</i><sub>VGd</sub> = 0.989 ± 0.028 and −0.089 ± 0.008 cm<sup>−1</sup> for <b>2</b>, 0.656 ± 0.009 and −0.050 ± 0.004 cm<sup>−1</sup> for <b>3</b>). Analysis of the EPR spectra of <b>1</b> revealed the presence of weak magnetic anisotropy of Gd<sup>III</sup> ions (<i>D</i> ~ 0.08 cm<sup>−1</sup> and <i>E</i>/<i>D</i> ~ 0.1–0.15). <i>Ac</i>-susceptibility measurements showed an occurrence the field-induced slow relaxation of magnetization in <b>1</b>–<b>3</b>.
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