Synthesis, Structure, and Magnetic Properties of an Fe Dimethylarsinate Cluster: The Largest "Ferric Wheel".

The synthesis and characterization of a high-nuclearity Fe/O/arsinate cluster is reported within the salt [FeO(OH)(OAsMe)(OCH)(HO)](NO) (). The compound was prepared from the reaction of Fe(NO)·9HO, dimethylarsinic acid (MeAsOH), and triethylamine in a 1:2:4 molar ratio in acetonitrile. The Fe cation of is an unprecedented structural type consisting of nine Fe butterfly units of two types, three {Fe(μ-O)} units , and six {Fe(μ-O)(μ-OH)} units , linked by multiple bridging MeAsO groups into an Fe triangular wheel/loop with crystallographic and virtual symmetry that looks like a guitar plectrum. The unusual structure has been rationalized on the basis of the different curvatures of units and , the presence of intra-Fe hydrogen bonding, and the tendency of MeAsO groups to favor μ-bridging modes. The cations stack into supramolecular nanotubes parallel to the crystallographic axis and contain badly disordered solvent and NO anions. The cation of is the highest-nuclearity "ferric wheel" to date and also the highest-nuclearity Fe/O cluster of any structural type with a single contiguous Fe/O core. Variable-temperature direct-current magnetic susceptibility data and alternating-current in-phase magnetic susceptibility data indicate that the cation of possesses an = 0 ground state and dominant antiferromagnetic interactions. The Fe pairwise couplings were estimated by the combined use of a magnetostructural correlation for high-nuclearity Fe/oxo clusters and density functional theory calculations using broken-symmetry methods and the Green's function approach. The three methods gave satisfyingly similar values and allowed the identification of spin-frustration effects and the resulting relative spin-vector alignments and thus rationalization of the = 0 ground state of the cation.