Self-Assembly of a Tetranuclear Ni₄ Cluster with an S = 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.