Synthesis and Characterization of Symmetrically versus Unsymmetrically Proton-Bridged Hexa-Iron Clusters

[Image: see text] Syntheses and magnetic and structural characterization of hexa-iron complexes of derivatized salicylaldoximes are discussed. Complexation of Fe(BF(4))(2)·6H(2)O with each ligand (H(2)L1 and H(4)L2) in a methanolic-pyridine solution resulted in hexa-iron compounds (C1 and C2, respectively), which each contain two near-parallel metal triangles of [Fe(3)–μ(3)-O], linked by six fluoride bridges and stabilized by a hydrogen-bonded proton between the μ(3)-O groups. Within each metal triangle of C2, Fe(III) ions are connected via the amine “straps” of (H(4)L2-2H). Variable-temperature magnetic susceptibility and Mössbauer data of C1 and C2 indicate the presence of dominant antiferromagnetic interactions between the high-spin (S = 5/2) Fe(III) centers. For C1, two quadrupole doublets are observed at room temperature and 5 K, consistent with structural data from which discrete but disordered [Fe(3)–μ(3)-O] and [Fe(3)–μ(3)-OH] species were inferred. For C2, a single sharp quadrupole doublet with splitting intermediate between those determined for C1 was observed, consistent with the symmetric [Fe(3)–μ(3)-O···H···μ(3)-O–Fe(3)] species inferred crystallographically from the very short μ(3)-O···μ(3)-O separation. The differences in the physical properties of the complexes, as seen in the Mössbauer, X-ray, and magnetic data, are attributed to the conformational flexibility imparted by the nature of the linkages between the closely related ligands.