Self-Assembly of a Tetranuclear Ni₄ Cluster with an <i>S</i> = 4 Ground State: The First 3d Metal Cluster Bearing a μ₄-η²:η²-O,O Carbonate Ligand

Reaction of nickel(II) acetate with H3L (2-(5-bromo-2-hydroxyphenyl)-1,3-bis[4-(5-bromo-2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazolidine) yields [Ni2L(OAc)(H2O)2]·3MeCN·2H2O (1·3MeCN·2H2O), crystallographically characterized. 1 is unstable in solution for a long time and hydrolyzes to give [Ni2L(o-OC6H3BrCHO)(H2O)]·2.25MeCN·H2O (2·2.25MeCN·H2O). In addition, 1 uptakes CO2 from air in a basic methanol/acetonitrile solution, yielding {[Ni2L(MeOH)]2(CO3)}·1.5MeOH·MeCN·H2O (3·1.5MeOH·MeCN·H2O). The X-ray characterization of 3 reveals that it is a tetranuclear nickel cluster, which can be considered as the result of a self-assembly process from two dinuclear [Ni2L]+ blocks, joined by a μ4-η2:η2-O,O carbonate ligand. The coordination mode of the carbonate anion is highly unusual and, to the best of our knowledge, it has not been described thus far for first-row transition metal complexes or magnetically studied until now. Magnetic characterization of 1 and 3 shows net intramolecular ferromagnetic coupling between the metal atoms in both cases, with S = 2 and S = 4 ground states for 1 and 3, respectively.