Synthesis, Structure, and Magnetic Properties of an Fe 36 Dimethylarsinate Cluster: The Largest "Ferric Wheel".

The synthesis and characterization of a high-nuclearity Fe III /O/arsinate cluster is reported within the salt [Fe 36 O 12 (OH) 6 (O 2 AsMe 2 ) 63 (O 2 CH) 3 (H 2 O) 6 ](NO 3 ) 12 ( 1 ). The compound was prepared from the reaction of Fe(NO 3 ) 3 ·9H 2 O, dimethylarsinic acid (Me 2 AsO 2 H), and triethylamine in a 1:2:4 molar ratio in acetonitrile. The Fe 36 cation of 1 is an unprecedented structural type consisting of nine Fe 4 butterfly units of two types, three {Fe III 4 (μ 3 -O) 2 } units A , and six {Fe III 4 (μ 3 -O)(μ 3 -OH)} units B , linked by multiple bridging Me 2 AsO 2 - groups into an Fe 36 triangular wheel/loop with C 3 crystallographic and D 3 virtual symmetry that looks like a guitar plectrum. The unusual structure has been rationalized on the basis of the different curvatures of units A and B , the presence of intra-Fe 36 hydrogen bonding, and the tendency of Me 2 AsO 2 - groups to favor μ 3 -bridging modes. The cations stack into supramolecular nanotubes parallel to the crystallographic c axis and contain badly disordered solvent and NO 3 - anions. The cation of 1 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 1 possesses an S = 0 ground state and dominant antiferromagnetic interactions. The Fe 2 pairwise J i , j couplings were estimated by the combined use of a magnetostructural correlation for high-nuclearity Fe III /oxo clusters and density functional theory calculations using broken-symmetry methods and the Green's function approach. The three methods gave satisfyingly similar J i , j values and allowed the identification of spin-frustration effects and the resulting relative spin-vector alignments and thus rationalization of the S = 0 ground state of the cation.