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[(MII)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)[(Mi)(4)(Cit)(4)](.) 8H(2)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 P4(2)/n; a similar to 16.2 angstrom, and c similar to 11.5 angstrom). As was intended, cubane-like [M-4(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 S-4 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, Fell, Coll, 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, J(12) = -0.3 cm(-1) and J(13) = +2.97 cm(-1), correlated with the two Ni-(mu(3)-O)-Ni angles observed in the cluster structure. All members of the 3d-block [M-4(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.