Synthesis, structures, adsorption behaviour and magnetic properties of a new family of polynuclear ironclusters

Abstract

Solvothermal reactions of 1,10-phenanthroline-2,9-dicarbaldehyde dioxime (H₂phendox) with FeCl₃·6H₂O or FeBr₃ under solvothermal conditions yielded two trinuclear iron(III) clusters [Fe^III₃(μ₃-O)(phendox)₃]X·14H₂O (X = Cl 1·14H₂O, Br 2·14H₂O) and three hexanuclear iron(III) and iron(II) clusters, [Fe^III₆(μ₄-O)₂(MeO)₆X₄(phendox)₂]·MeOH (X = Cl 3, Br 4) and (H₃O)[Fe^II₆(μ₆-Cl)(phenda)₆]·6H₂O (5·6H₂O). The phendox²⁻ ligand is very useful in constructing magnetically active and stable high-nuclearity metal clusters in that the phenanthroline rings and the oxime nitrogen atoms grasp the metal ions tightly while the two oxygen atoms on the oximates can link other metal centres in the shortest pairwise magnetic exchange pathway. Adjacent Fe₃(μ₃-O)(phendox)₃⁺ motifs in 1 and 2 are packed by off-set π–π interactions of the aromatic rings on phendox²⁻ to generate a 3D supramolecular architecture in the honeycomb topology and with 1D hexagonal channels in the dimension of 13 × 13 Å along the c-axis. 2 is stable upon the removal of guest molecules and the desolvated compound absorbed considerable amount of N₂, CO₂ and H₂. 3 and 4 are isostructural. Two μ₄-O²⁻ and two phendox²⁻ units link four metal atoms into a coplanar butterfly-shaped unit with the μ₄-O²⁻ slightly above and below the plane (±0.264 Å). The other two Fe(III) ions are capped on the alternate planes via the three bridging μ₂-methoxides and accordingly form an unprecedented hexanuclear Fe(III) cluster. Furthermore, the oximes underwent hydrolysis to yield carboxylate groups and the resulted 1,10-phenanthroline-2,9-dicarboxylate link the iron atoms to form a hexanuclear cluster of 5. Magnetic studies show that the antiferromagnetic interactions are present in the Fe₃O core of 2 and in the (μ₆-Cl)Fe₆(μ-O)₁₂ core of 5.