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(NH₂)₃]₈[(M^II)₄(cit)₄]·8H₂O (M = Mg, Mn, Fe, Co, Ni, and Zn; cit = Citrate)

Aqueous reaction mixtures containing citric acid, guanidinium carbonate, and a range of metal cations (Mg²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, and Zn²⁺) at room temperature give crystalline products of composition [C(NH₂)₃]₈[(M^II)₄(cit)₄]·8H₂O (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 P4₂/n; a ∼ 16.2 Å, and c ∼ 11.5 Å). As was intended, cubane-like [M₄(cit)₄]^8- 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 S₄ 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^II, Fe^II, Co^II, and Ni^II cubanes is very weak in all cases with J values of −0.82, −0.43, and −0.09 cm^-1 for the Mn, Fe, and Co species, respectively. A two-J model gave the best agreement with the susceptibility and high-field magnetization data for the Ni^II case, over the whole temperature range studied, and the sign of the parameters, J₁₂ = −0.3 cm^-1 and J₁₃ = +2.97 cm^-1, correlated with the two Ni−(μ₃-O)−Ni angles observed in the cluster structure. All members of the 3d-block [M₄(cit)₄]^8- family have spin ground states, S_T, of zero, with the higher S_T levels just a few reciprocal centimeters away in energy.