Citrate, in Collaboration with a Guanidinium Ion, as a Generator of Cubane-like Complexes with a Range of Metal Cations:  Synthesis, Structures, and Magnetic Properties of [C(NH<sub>2</sub>)<sub>3</sub>]<sub>8</sub>[(M<sup>II</sup>)<sub>4</sub>(cit)<sub>4</sub>]·8H<sub>2</sub>O (M = Mg, Mn, Fe, Co, Ni, and Zn; cit = Citrate)

Aqueous reaction mixtures containing citric acid, guanidinium carbonate, and a range of metal cations (Mg<sup>2+</sup>, Mn<sup>2+</sup>, Fe<sup>2+</sup>, Co<sup>2+</sup>, Ni<sup>2+</sup>, and Zn<sup>2+</sup>) at room temperature give crystalline products of composition [C(NH<sub>2</sub>)<sub>3</sub>]<sub>8</sub>[(M<sup>II</sup>)<sub>4</sub>(cit)<sub>4</sub>]·8H<sub>2</sub>O (cit = citrate). In all cases, the crystals are suitable for single-crystal X-ray diffraction studies, which reveal that the compounds are isostructural (space group <i>P</i>4<sub>2</sub>/<i>n</i>; <i>a</i> ∼ 16.2 Å, and <i>c</i> ∼ 11.5 Å). As was intended, cubane-like [M<sub>4</sub>(cit)<sub>4</sub>]<sup>8-</sup> complex anions are present. The individual citrate units are chiral, but each cubane unit contains two of one hand and two of the other, related around an <i>S</i><sub>4</sub> axis. The cubane units are involved in no less than 40 H-bonding interactions with guanidinium cations and lattice water molecules. Detailed susceptibility and magnetization studies show that the intracluster magnetic coupling within the Mn<sup>II</sup>, Fe<sup>II</sup>, Co<sup>II</sup>, and Ni<sup>II</sup> cubanes is very weak in all cases with <i>J</i> values of −0.82, −0.43, and −0.09 cm<sup>-1</sup> for the Mn, Fe, and Co species, respectively. A two-<i>J </i>model gave the best agreement with the susceptibility and high-field magnetization data for the Ni<sup>II</sup> case, over the whole temperature range studied, and the sign of the parameters, <i>J</i><sub>12</sub> = −0.3 cm<sup>-1</sup> and <i>J</i><sub>13</sub> = +2.97 cm<sup>-1</sup>, correlated with the two Ni−(μ<sub>3</sub>-O)−Ni angles observed in the cluster structure. All members of the 3d-block [M<sub>4</sub>(cit)<sub>4</sub>]<sup>8-</sup> family have spin ground states, <i>S</i><sub>T</sub>, of zero, with the higher <i>S</i><sub>T</sub> levels just a few reciprocal centimeters away in energy.